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Microscopic Manipulation of Ferroelectric Domains in SnSe Monolayers at Room Temperature

  • Kai Chang*
    Kai Chang
    Max Planck Institute of Microstructure Physics, Weinberg 2, Halle 06120, Germany
    *Email: [email protected] (K.C.).
    More by Kai Chang
    Orcidhttp://orcid.org/0000-0002-4965-4537
    • , 
  • Felix Küster
    Felix Küster
    Max Planck Institute of Microstructure Physics, Weinberg 2, Halle 06120, Germany
    More by Felix Küster
    • , 
  • Brandon J. Miller
    Brandon J. Miller
    Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, United States
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    • , 
  • Jing-Rong Ji
    Jing-Rong Ji
    Max Planck Institute of Microstructure Physics, Weinberg 2, Halle 06120, Germany
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    • , 
  • Jia-Lu Zhang
    Jia-Lu Zhang
    Max Planck Institute of Microstructure Physics, Weinberg 2, Halle 06120, Germany
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    • , 
  • Paolo Sessi
    Paolo Sessi
    Max Planck Institute of Microstructure Physics, Weinberg 2, Halle 06120, Germany
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    • , 
  • Salvador Barraza-Lopez
    Salvador Barraza-Lopez
    Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, United States
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    • , and 
  • Stuart S. P. Parkin*
    Stuart S. P. Parkin
    Max Planck Institute of Microstructure Physics, Weinberg 2, Halle 06120, Germany
    *Email: [email protected] (S.S.P.P.).
    More by Stuart S. P. Parkin
    Orcidhttp://orcid.org/0000-0003-4702-6139
Cite this: Nano Lett. 2020, 20, 9, 6590–6597
Publication Date (Web):August 18, 2020
https://doi.org/10.1021/acs.nanolett.0c02357
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Abstract

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Two-dimensional (2D) van der Waals ferroelectrics provide an unprecedented architectural freedom for the creation of artificial multiferroics and nonvolatile electronic devices based on vertical and coplanar heterojunctions of 2D ferroic materials. Nevertheless, controlled microscopic manipulation of ferroelectric domains is still rare in monolayer-thick 2D ferroelectrics with in-plane polarization. Here we report the discovery of robust ferroelectricity with a critical temperature close to 400 K in SnSe monolayer plates grown on graphene and the demonstration of controlled room-temperature ferroelectric domain manipulation by applying appropriate bias voltage pulses to the tip of a scanning tunneling microscope (STM). This study shows that STM is a powerful tool for detecting and manipulating the microscopic domain structures in 2D ferroelectric monolayers, which are difficult for conventional approaches such as piezoresponse force microscopy, thus facilitating the hunt for other 2D ferroelectric monolayers with in-plane polarization with important technological applications.

KEYWORDS:
As the research of ferroelectrics moves toward the 2D limit, the existence of ferroelectricity in monolayer (ML)-thick materials has been confirmed in both van der Waals MLs(1−3) and epitaxial perovskites.(4) Compared with traditional perovskite ferroelectric thin films, 2D ferroelectrics bring the freedom of fabricating functional van der Waals heterostructures without the restriction of lattice matching. Many layered ferroelectrics, such as CuInP2S6,(5,6) SnTe(1,7)–,(9) SnS,(3,10) In2Se3,(11−16) WTe2,(17) MoTe2,(2) BA2PbCl4,(18,19) and Bi2O2Se(20) have been discovered in the past few years. In order to gain a deeper understanding of these novel materials, microscopic studies of the ferroelectric domain structures and switching mechanisms become timely tasks. Among common approaches for ferroelectric studies, scanning probe microscopy (SPM) methods are the only category that can simultaneously realize the microscopic characterization and manipulation of ferroelectric domains. Other methods, such as X-ray diffraction, transmission electron microscopy, Raman spectroscopy, and second harmonic generation, cannot execute domain manipulation, while devices based on fabricated electrodes can only reveal collective ferroelectric switching behaviors. Currently, piezoresponse force microscopy (PFM) is the most widely applied SPM method for the microscopic studies of 2D ferroelectrics. In PFM experiments, out-of-plane and in-plane polarization components are detected by measuring the deflection and torsion of a cantilever, as responses to the material deformation induced by the vertical electric field applied through a tip fixed at the end of the cantilever.(21) Microscopic imaging and controlled manipulation of the out-of-plane polarization in 2D ferroelectric materials via PFM have been demonstrated in MoTe2 MLs,(2) freestanding perovskite thin films,(22) as well as several-layer thick van der Waals ferroelectrics.(5,11,13,15−17) However, although PFM studies of in-plane ferroelectricity have been reported in several nanometer thick layered materials,(13−15,18−20) to the best of our knowledge none of the current PFM studies of in-plane polarized ferroelectrics has reached the thickness limit of a single van der Waals ML.
As another important SPM method, scanning tunneling microscopy (STM) provides a different viewpoint for the studies of in-plane polarized ferroelectric MLs. Since STM is extremely sensitive to the surface electronic structures of atomically flat samples, in-plane polarization can be detected through measuring the electronic band bending induced by the bound charges at the edges and ferroelectric domain walls of the films. Therefore, the imaging of in-plane polarization in 2D ferroelectric MLs is relatively easy for STM, while the microscopic manipulation of ferroelectric domains turns out to be more difficult than PFM because the in-plane component of the electric field between the STM tip and the sample is much weaker than the out-of-plane one. Uncontrolled switching through STM tip induced domain motion in the in-plane polarized SnTe MLs at 4.7 K was demonstrated in 2016.(1) Although the first controlled switching experiment of SnS MLs through coplanar electrodes was reported very recently,(3) microscopic manipulation of the ferroelectric domains in these in-plane polarized 2D ferroelectric MLs is still absent. Furthermore, the ferroelectric transition temperatures of these group-IV monochalcogenide MLs(23−28) need to be determined in a reliable way because these values are important for applications. Here we employed a variable temperature STM to controllably switch the in-plane polarization via domain manipulation in SnSe MLs at room temperature and demonstrated its ferroelectric transition temperature to be as high as 380–400 K, close to that of BaTiO3.(29) These results have demonstrated the ability of STM in characterizing and manipulating these in-plane polarized 2D ferroelectric MLs and open the door to the potential fabrication of subnanometer-thick ultrathin memories,(30,31) nonlinear optic,(32,33) spintronic,(34−36) and valleytronic(37) devices that operate at room temperature based on group-IV monochalcogenide MLs.
We first establish the conformation of ultrathin SnSe nanoplates grown by molecular beam epitaxy (MBE) on graphitized 6H-SiC(0001) surfaces. Belonging to the noncentrosymmetric space group Pnm21, a SnSe ML contains two strongly bonded atomic layers vertically separated by 2.8 Å. The lattice vectors a1 and a2 are parallel and perpendicular to the in-plane spontaneous polarization P, respectively. As seen in Figure 1a, the edges of rectangular SnSe ML plates occur along the ⟨11⟩ directions with two edges hosting positive bound charges and the other two having negative bound charges. The SnSe plates display three relative crystalline orientations where a1 (a2) aligns with the graphene’s zigzag (armchair) direction. Accordingly, their in-plane polarization points along one of the six directions shown in Figure 1b given that a crystalline orientation allows two conformations with antiparallel polarizations. Such highly oriented growth, which is not seen for SnTe MLs on this substrate,(1,8,38) is further confirmed by reflection high energy electron diffraction (RHEED) patterns and large-scale dI/dV mapping images (Figure S1 in Supporting Information). By reproducing the moiré pattern between the SnSe MLs and graphene through a lattice simulation, we determine the lattice parameters for these SnSe MLs at room temperature to be a1 = 4.35 ± 0.02 Å and a2 = 4.26 ± 0.02 Å (Figures S2–S4). The lattice parameter a2 agrees with graphene’s periodicity along its armchair direction ( 2.46 Å = 4.26 Å), which is the reason for the highly oriented growth of SnSe MLs. For comparison, SnTe MLs have a significant lattice mismatch with graphene (a1 = 4.58 Å and a2 = 4.44 Å, 4.2% mismatch along the armchair direction), thus the relative orientation of SnTe MLs on graphene was found to be random.(1)

Figure 1

Figure 1. In-plane spontaneous polarization in SnSe monolayer plates. (a) Lattice structure of SnSe monolayer. The solid rectangle indicates a unit cell. Dashed lines indicate the preferred edge orientations of these plates. The signs of bound charges at opposite edges are labeled. (b) Schematic diagram of three possible crystalline orientations of SnSe monolayers. The rhomboids and rectangles represent the unit cells of graphene and SnSe, respectively. Arrows in the rectangles indicate the directions of polarization allowed in this configuration. (c) Spatially resolved dI/dV spectra along the dashed arrow across the SnSe monolayer plate shown in the inset, obtained at 1.8 K. Set points: Vs = 3.0 V, It = 50 pA for positive Vs; Vs = −1.0 V, It = 50 pA for negative Vs. The red dashed curves are exponential fittings of the band bending profiles. (d,e) Spatially resolved dI/dV spectra acquired from the same SnSe monolayer plate, along the directions parallel (d) and perpendicular (e) to the polarization, as the inset shows. The conditions of measurements are the same as in (c).

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The spatially resolved dI/dV spectra in Figure 1c, approximately proportional to the scanned surface’s local density of states (LDOS),(39,40) uncover an in-plane polarization by displaying band bending toward opposite directions at opposite edges of a single-domain SnSe ML plate, distinct from the band bending toward the same directions at opposite edges in nonpolar semiconductors. The edges of a SnSe plate are roughly 45° away from the direction of polarization, generating a bound charge density of at the edges. The valence band maximum (VBM) lies at −0.44 eV and the conduction band minimum (CBM) lies at 1.69 eV at the center of this 50 nm wide SnSe ML plate, yielding an electronic band gap of 2.13 eV at 1.8 K. The CBM and VBM show +0.24 eV (upward) and −0.31 eV (downward) band bending at opposite edges in Figure 1c. Fitting these band edge profiles with an exponential function V(x) = A exp[−(x – x0)/L0] + V0, we obtain decay lengths of L0 = 4.22 ± 0.23 nm for upward bending, and L0 = 5.89 ± 0.36 nm for downward bending (Figure S5). Such an exponential decay is due to screening from the metallic graphene substrate. Without screening, the decay would take a much slower logarithmic form in a 2D ferroelectric with in-plane polarization, in analogy with the electric potential generated by charged wires. To further confirm that the band bending is consistent with the direction of polarization, we also acquired spatially resolved dI/dV spectra along directions parallel and perpendicular to the polarization of a SnSe ML plate (Figure 1d,e). As expected, no significant band bending is resolved along the direction perpendicular to the polarization.
Though still resolvable, band edges become less clear on dI/dV spectra acquired at room temperature (Figure S6). In that case, and as seen in Figure 2a,b, topographic and dI/dV mapping images still clearly display opposite charge accumulation at opposite edges resulting from band bending. The tunneling current is roughly proportional to the sample’s LDOS integrated between eVs and EF, where Vs is the sample bias voltage and EF is the Fermi level set at 0 eV. The tip moves away from (closer to) the surface at a place with a higher (lower) integrated LDOS when the STM is operated in a constant current mode (keeping tunneling current It unchanged in the scan), generating a higher (lower) apparent height z. Therefore, on an atomically flat surface, an edge with upward (downward) band bending shows a higher (lower) apparent height at a negative Vs close to VBM. At Vs = −0.2 V and It = 2 pA, the topographic image of the SnSe ML plate in Figure 2a displays a contrast of 1.0 Å at opposite edges (zup/zdown = 1.2), while the dI/dV image in Figure 2b shows a contrast of 0.6 pS [(dI/dV)up/(dI/dV)down = 2.3], both confirming an in-plane polarization parallel to a1.

Figure 2

Figure 2. Ferroelectric domains in SnSe monolayer plates at room temperature. (a,b) Room-temperature topography and simultaneously recorded dI/dV images of a SnSe monolayer plate. Set point: Vs = −0.2 V, It = 2 pA. (c,d) dI/dV images of SnSe monolayer plates with 180° straight (c) and zigzag (d) domain walls. The domain walls are indicated by the white dashed lines. Set points: Vs = −0.2 V, It = 2 pA for (c), Vs = −0.35 V, It = 2 pA for (d).

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The highly oriented growth prohibits the existence of 90° domain walls that were predominantly seen in SnTe MLs.(1) Only 180° domain walls are observed in SnSe MLs, either in a straight (Figure 2c) or zigzag shape (Figure 2d). An atom-resolved image reveals identical lattice vectors in neighboring domains (Figure S7) in agreement with the 180° domain orientation. The straight wall in Figure 2c is 12° away from a1, yielding a bound charge density λP = 0.42P, where P is the in-plane polarization estimated to be 1.5 × 10–10 C/m from our first-principles calculations. The walls in Figure 2d have domains joining “tail-to-tail”, and the 34° zigzag angle leads to a bound charge density of λP = 0.58P. Interestingly, the 180° domain walls in Figure 2c,d exhibit high dI/dV intensity comparable to that at the edges with upward band bending, even though the expected bound charge density is lower at the domain walls (λP = 0.71P at the edges). This is probably because of the existence of a bound state at the VBM of the 180° domain walls, according to our first-principles calculations (Figure S8). Another intriguing fact is that the 180° domain walls are usually mildly negatively charged (tail-to-tail) with most of the angles between the domain wall and the polarization below 30°. Further studies are needed to unveil the mechanism behind this phenomenon.
Now that the SnSe ML plates are characterized, we discuss the principal contribution of the present work: the controlled ferroelectric domain manipulation at room temperature, including moving, creating, and eliminating the 180° domains, achieved by applying bias voltage pulses with an STM tip situated at a lateral distance d0 away from the SnSe plate as illustrated in Figure 3a. The width of the uprising edge of these pulses is 0.14 ms (Figure 3b), much longer than the relaxation time of the carriers in graphene,(41) thus the electric field induced by the pulse can be regarded as quasi-static. The pulse voltage VP lasts for tP = 50 ms, unless otherwise specified. In Figure 3c–f, the domain manipulation process is demonstrated as a series of pulses were applied with the STM tip placed at d0 = 20 nm away from a corner of the plate. There were two domains and a 180° domain wall across the plate at the onset; the domain with P parallel to E// expanded and the one with P antiparallel to E// shrunk as the first VP = −5 V pulse was applied (Figure 3d). This manipulation not only induced a change of domain sizes but also caused the rotation of the domain wall. Specifically, the angle between the domain wall and the polarization was changed from 28° to 7°, reducing λP from 0.94P to 0.24P, which is consistent with the suppressed dI/dV intensity at the domain wall after the manipulation. Such domain wall rotations were common in the domain manipulations that we performed this way. The whole plate turned into a single domain with P parallel to E// (Figure 3e) after a second −5 V pulse took place. A new domain was created as a +8 V pulse was applied, and part of the polarization in this plate was switched to the opposite direction (Figure 3f). Finally, another +8 V pulse set the whole plate into a single domain again but inverting P from its initial direction (Figure 3g). The polarization of a SnSe ML plate can be repeatedly and controllably switched this way, and additional experiments are given in Figure S9. In Supporting Information, we also present a video to show back-and-forth domain wall motion by applying bias voltage pulses of opposite signs with the STM tip staying at the same location.

Figure 3

Figure 3. Controllable ferroelectric switching of SnSe monolayer plates. (a) Schematic of ferroelectric switching achieved by applying a bias voltage pulse VP at a point on the graphene substrate close to a SnSe monolayer plate. The corner of SnSe closest to the STM tip was set as d = 0, and the upper surface of graphene was set as z = 0. (b) Rising edges of the bias voltage pulses. Dashed lines indicate the onset and maximum values of the bias voltage. (c–g) Consecutive dI/dV images of a ferroelectric switching sequence in a SnSe monolayer plate. Set points: Vs = −0.35 V, It = 2 pA. The pulses were applied at the same point indicated in (c). The widths of all the pulses were 50 ms. The directions of the in-plane components of tip-induced electric fields are indicated by the white arrows. All of the data in this figure were collected at room temperature.

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It should be noted that the dI/dV images acquired after bias voltage pulses were applied are only suitable for inspections of the distribution of ferroelectric domains, while quantitative analysis of band bending profiles should be based on the data acquired right after the STM tip calibration (in our case, on a Au(111) surface), such as in Figures 1c and 2b. This is because the strong electric field during the pulse could modify the tip state and the corresponding tunneling matrix elements, consequently inducing quantitative deviations for the dI/dV images. The interaction between the STM tip and the ferroelectric MLs during the switching process likely contains rich physics for future studies.
Successful manipulation operations require pulses to be applied on the graphene substrate and away from the SnSe ML plate, because the STM-tip-induced electric field has an out-of-plane component Ez several orders of magnitude larger than its in-plane component E// (Figure 4a). Such a huge Ez can lead to the electric breakdown when the tip is above of a SnSe ML plate. For instance, assuming VP = 5 V and a tip–sample distance of 5 Å, Ez can easily reach 108 V/cm beneath the STM tip.

Figure 4

Figure 4. Quantitative studies of the domain wall motion induced by bias voltage pulses. (a) Simulated out-of-plane and in-plane electric fields along a horizontal line along the d-axis (see Figure 2a) at the height of z = 0.3 nm. (b) Statistics of a series of bias voltage pulse experiments with different VP and d0. Each dot represents a single pulse applied. Those pulses that successfully moved a domain wall are shown in red, and those that did not induce domain wall motion are in blue. (c) Simulated in-plane electric fields with VP fixed while varying d0. (d) The experimental d0 dependence of the critical pulse voltages |VP,c| of domain wall motion (inset) and the corresponding E//,c at the closest corner of SnSe.

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Judging from Figure 4b, which shows statistics of many single bias voltage pulse experiments, a pulse with shorter d0 and larger |VP| has a higher probability of inducing domain wall motion. The uncertainty of the effect of a single pulse might be a result of the complicated shape of the STM tip and the environment of the SnSe ML plates; domain wall pinning could happen due to the defects, wrinkles, or atomic steps on the substrate. In order to quantitatively determine the critical field of domain wall motion, E//,c, we carried out a series of experiments in which |VP| was gradually increased until domain wall motion was observed (see Figure S8 for details). Comparing the experimental critical pulse voltages |VP,c| with the E// obtained from numerical simulations in Figure 4c (see also Figure S10 for details of the simulations), we derived the corresponding critical fields for different d0 in Figure 4d. Our data suggest that for d0 ≥ 30 nm, E//,c converges to 1.4 ± 0.2 × 105 V/cm, which we consider to be the intrinsic critical field for domain wall motion. The E//,c for shorter d0 is higher, probably because of nonlinear electrostrictive effects induced by the very large Ez at these distances.
We now determine the transition temperature Tc above which the spontaneous polarization is suppressed. In Figure 5, we used a variable temperature STM to study the temperature dependence of band bending at the edges of the SnSe ML plate shown in Figure 2a. The contrast of both apparent heights (Figure 5a–f) and dI/dV (Figures 5g–l) at opposite edges, both depending on the magnitude of band bending, decreases as the temperature is increased and becomes totally indistinguishable at 400 K (Figure 5e,k,q). Band bending reappears as the temperature is decreased to 308 K, regaining the magnitudes registered before heating (compare Figure 5 panels a,g,m to panels f,l,r). This implies that the Tc of SnSe MLs on graphene is between 380 and 400 K, similar to that of bulk BaTiO3 (396 K),(29) a well-known perovskite ferroelectric. For comparison, SnTe MLs on graphene substrates have a Tc = 270 K(1) which is a value below room temperature that may hinder practical applications. The larger value of Tc obtained for SnSe MLs on graphene is consistent with a larger energy required to turn the ferroelectric phase into the paraelectric phase as the chalcogen atom (Se versus Te) becomes lighter.(23,42)

Figure 5

Figure 5. Temperature dependence of ferroelectricity in a SnSe monolayer plate. (a–f) Topography images of a SnSe monolayer plate at temperature T increasing from 310 to 400 K from (a) to (e), and decreasing to 308 K in (f). Set points: Vs = −0.2 V, It = 2 pA. (g–l) Simultaneously recorded dI/dV images corresponding to (a–f). (m–r) Apparent height and dI/dV profiles along the dashed arrows in (a,g), extracted from (a–f) and (g–l), respectively. (s) Evolution of the difference of apparent height and dI/dV between the two opposite edges with upward and downward band bending directions, extracted from m–q.

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In the end, despite the preferential orientation during growth discussed previously, SnSe ML plates can indeed be controllably moved by the STM tip on the graphene substrate without observable damage (Figure S11), still implying a weak interaction between SnSe and the substrate. This makes the creation of heterostructures containing SnSe MLs by transferring and stacking techniques, or even by in situ scanning probe manipulations, likely to be feasible.
This ferroelectric domain characterization and manipulation method by STM is a useful complementary technique for the traditional coplanar electrode experiments on in-plane polarized 2D ferroelectrics, such as the recently reported ferroelectric switching of SnS MLs.(3) On the one hand, measuring the collective ferroelectric behaviors, coplanar electrode experiments are more defined and quantitatively precise; on the other hand, STM has the unique ability of microscopically imaging and manipulating the fine domain structures in these ferroelectric MLs. The combination of these two approaches should be beneficial for future comprehensive studies.
The discovery that highly oriented SnSe MLs on graphene are 2D ferroelectrics whose ferroelectric domains can be controllably manipulated at room temperature brings predicted effects and device concepts based on group-IV monochalcogenide MLs, such as linearly polarized-light-controlled valley selective excitations,(24,43,37) shift current photovoltaics,(32) intrinsic valley Hall effects,(37) in-plane ferroelectric tunneling junctions,(30,31) and nonlinear photocurrent switching devices,(33) one step closer to reality. Additionally, we envision that the coupling between existing 2D ferromagnets and 2D ferroelectrics via vertical or horizontal heterojunctions may lead to an eventual deployment of layered artificial multiferroics.

Methods

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Sample Preparation

SnSe monolayer plates were grown on graphene substrates via van der Waals molecular beam epitaxy in an ultrahigh vacuum (UHV) chamber with a base pressure of 1 × 10–10 mbar. Graphene substrates were prepared following a sequential direct current annealing procedure of nitrogen doped 6H-SiC(0001) (Si face). The substrate temperature was monitored by a high-precision infrared pyrometer. The 2 mm × 10 mm sized SiC substrate was first degassed at 500 °C overnight, then annealed at 900 °C in Si flux for 15 min to form a Si-rich 3 × 3 reconstruction. Finally, the substrate was annealed at 1400 °C for 10 min to graphitize the surface. SnSe molecules were evaporated from a home-built thermal evaporator with an h-BN crucible containing 99.999% SnSe granules from Alfa Aesar. During growth, the SnSe evaporator was kept at ∼450 °C and the substrate temperature varied from 70 to 220 °C, depending on the specific plate shape desired. During growth, the substrate was heated by radiation from tungsten filaments, and temperature was read from a thermocouple. Higher substrate temperatures generate thicker plates with uniform thicknesses and straight edges, while plates grown at lower substrate temperatures are more irregular and have more ∼6 Å high steps at their surfaces. Rectangular SnSe monolayer plates were grown in a two-step process: (i) by initially depositing irregularly shaped monolayer plates with a coverage lower than 0.05 monolayers at a substrate temperature of 70 °C and (ii) a subsequent annealing up to 240 °C for 1 h to turn the plates rectangular. If larger monolayer plates are desired, one can deposit SnSe again at the substrate temperature of 240 °C, at which SnSe plates tend to grow horizontally.

Variable-Temperature Scanning Tunneling Microscopy (VT-STM)

As-grown samples were transferred into an Omicron VT-STM-XT system connected to the growth chamber without leaving the UHV environment. Mechanically sharpened Pt/Ir (80/20) alloy tips calibrated on a Au(111) standard sample were used for scanning. For the measurement of dI/dV spectra and mappings, a sinusoidal modulation voltage of 30 mV with a 713 Hz frequency was added to the bias voltage. In the ferroelectric switching experiments, the tip was first stabilized at the parameters for scanning (typically −0.2 to −0.4 V, 2 pA) and then the feedback loop was turned off when a pulse voltage (typically ±3 to ±8 V) was applied. The feedback loop was turned on immediately at the end of a pulse. A Lakeshore 335 temperature controller was used for the variable temperature experiments. The temperature ramping speed was limited to <5 °C/min, and the STM kept scanning during the ramping process in order to track the thermal drift of the sample. At each temperature set point, the temperature was stabilized for at least 30 min to reduce thermal drift. For collecting the dI/dV spectra at room temperature, the bias voltage was scanned both forward and backward to compensate the drifting of tunneling junction width. Acquiring each spectrum took only 3–4 s. Twenty spectra were averaged at each point in order to reduce the noise.

Low-Temperature Scanning Tunneling Microscopy (LT-STM)

Low-temperature measurements were performed using a cryostat (Oxford Instruments) equipped with an UHV insert hosting the STM head (Sigma Surface Science). A steady state temperature of 1.8 K is achieved by continuously pumping the 4He cooling circuit. Samples were transferred from the MBE system to the LT-STM using a vacuum suitcase with a pressure in the 10–10 mbar range, thus always preserving UHV conditions. STM data were obtained using electrochemically etched W tips. Before measurements, the tips were conditioned on an Ag(111) single crystal. dI/dV spectra were acquired by a lock-in technique, using a bias voltage sinusoidal modulation of 30 mV at a frequency of 736 Hz. To avoid any drift, line spectra across SnSe monolayer plates were taken after scanning the very same area for several hours. Each line consists of 200 points and was measured in approximately 6 h. Positive and negative energy ranges were acquired separately.

Ab Initio Calculations

Ab initio calculations with the VASP code(44,45) that employ the self-consistent van der Waals exchange correlation functional as implemented by Hamada were carried out. In these calculations, 17 unit cells of the SnSe monolayer were almost commensurate with 30 rectangular graphitic cells (each containing 8 atoms) down to less than 0.5% strain. In addition, periodic 180° domains 15 nm wide were created, using methods similar to those employed in refs (23,26,and46), by the vertical stacking of 35 rectangular unit cells with polarization parallel to lattice vector a1, and 35 additional unit cells with polarization antiparallel to a1. The antiparallel unit cells were displaced horizontally until a minimum energy was found, and a subsequent structural optimization employing 25 k-points along the horizontal direction was displaced until forces were smaller than 0.01 eV/Å. The projected density of states was produced employing 100 k-points along the horizontal direction, and spin orbit coupling was included in these calculations.

Numerical Simulations of Electric Field

The numerical simulations were performed with the COMSOL software. The electric field was set as static in the simulations. The diagram and parameters of the model and the calculated results are included in Figure S9 of Supporting Information.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.0c02357.

  • Additional data containing Figures S1–S11 (PDF)

  • Video showing the consecutive motion of a domain wall induced by a series of pulses (MP4)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Authors
  • Authors
    • Felix Küster - Max Planck Institute of Microstructure Physics, Weinberg 2, Halle 06120, Germany
    • Brandon J. Miller - Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, United States
    • Jing-Rong Ji - Max Planck Institute of Microstructure Physics, Weinberg 2, Halle 06120, Germany
    • Jia-Lu Zhang - Max Planck Institute of Microstructure Physics, Weinberg 2, Halle 06120, Germany
    • Paolo Sessi - Max Planck Institute of Microstructure Physics, Weinberg 2, Halle 06120, Germany
    • Salvador Barraza-Lopez - Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, United States
  • Author Contributions

    K.C. and S.S.P.P. designed the experiments. K.C. and J.-R.J. prepared the samples and performed the VT-STM experiments. F.K. and P.S. conducted the LT-STM experiments. B.J.M. and S.B.-L. carried out the ab initio calculations. J.-L.Z. performed the numerical simulations of electric fields. K.C., P.S., S.B.-L., and S.S.P.P. wrote the manuscript. All coauthors read and commented on the manuscript.

  • Notes

    The authors declare no competing financial interest.

Acknowledgments

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We thank Z. K. Liu for providing the SiC substrates, and J. D. Villanova, S. P. Poudel, and Y. Zhuang for technical assistance. K.C., F.K., J.-R.J., P.S., and S.S.P.P. were supported by Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), Project number PA 1812/2-1. B.J.M. and S.B.L. were funded by an Early Career Grant from the U.S. Department of Energy, Office of Basic Energy Sciences (Award DE-SC0016139). Calculations were performed at University of Arkansas’ Trestles, funded by the U.S. National Science Foundation (Grants 0722625, 0959124, 0963249, and 0918970), a grant from the Arkansas Economic Development Commission, the Office of the Vice Provost for Research and Innovation, and at Cori at NERSC, a U.S. DOE Office of Science User Facility operated under Contract No. DE-AC02-05CH11231.

ABBREVIATIONS
MBE

molecular beam epitaxy

STM

scanning tunneling microscopy

2D

two-dimensional

ML

monolayer

PFM

piezoresponse force microscopy

LDOS

local density of states

RHEED

reflective high energy electron diffraction

VBM

valence band maximum

CBM

conduction band minimum.

References

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This article references 46 other publications.

  1. 1
    Chang, K.; Liu, J.; Lin, H.; Wang, N.; Zhao, K.; Zhang, A.; Jin, F.; Zhong, Y.; Hu, X.; Duan, W.; Zhang, Q.; Fu, L.; Xue, Q.-K.; Chen, X.; Ji, S.-H. Discovery of robust in-plane ferroelectricity in atomic-thick SnTe. Science 2016, 353, 274,  DOI: 10.1126/science.aad8609
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    Discovery of robust in-plane ferroelectricity in atomic-thick SnTe
    Chang, Kai; Liu, Junwei; Lin, Haicheng; Wang, Na; Zhao, Kun; Zhang, Anmin; Jin, Feng; Zhong, Yong; Hu, Xiaopeng; Duan, Wenhui; Zhang, Qingming; Fu, Liang; Xue, Qi-Kun; Chen, Xi; Ji, Shuai-Hua
    Science (Washington, DC, United States) (2016), 353 (6296), 274-278CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Stable ferroelectricity with high transition temp. in nanostructures is needed for miniaturizing ferroelec. devices. Here, the authors report the discovery of the stable in-plane spontaneous polarization in at.-thick tin telluride (SnTe), down to a 1-unit cell (UC) limit. The ferroelec. transition temp. Tc of 1-UC SnTe film is greatly enhanced from the bulk value of 98 K and reaches as high as 270 K. Moreover, 2- to 4-UC SnTe films show robust ferroelectricity at room temp. The interplay between semiconducting properties and ferroelectricity in this 2-dimensional material may enable a wide range of applications in nonvolatile high-d. memories, nanosensors, and electronics.
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    Yuan, S.; Luo, X.; Chan, H. L.; Xiao, C.; Dai, Y.; Xie, M.; Hao, J. Room-temperature ferroelectricity in MoTe2 down to the atomic monolayer limit. Nat. Commun. 2019, 10, 1775,  DOI: 10.1038/s41467-019-09669-x
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    Room-temperature ferroelectricity in MoTe2 down to the atomic monolayer limit
    Yuan Shuoguo; Luo Xin; Chan Hung Lit; Xiao Chengcheng; Hao Jianhua; Luo Xin; Dai Yawei; Xie Maohai
    Nature communications (2019), 10 (1), 1775 ISSN:.
    Ferroelectrics allow for a wide range of intriguing applications. However, maintaining ferroelectricity has been hampered by intrinsic depolarization effects. Here, by combining first-principles calculations and experimental studies, we report on the discovery of robust room-temperature out-of-plane ferroelectricity which is realized in the thinnest monolayer MoTe2 with unexploited distorted 1T (d1T) phase. The origin of the ferroelectricity in d1T-MoTe2 results from the spontaneous symmetry breaking due to the relative atomic displacements of Mo atoms and Te atoms. Furthermore, a large ON/OFF resistance ratio is achieved in ferroelectric devices composed of MoTe2-based van der Waals heterostructure. Our work demonstrates that ferroelectricity can exist in two-dimensional layered material down to the atomic monolayer limit, which can result in new functionalities and achieve unexpected applications in atomic-scale electronic devices.
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  3. 3
    Higashitarumizu, N.; Kawamoto, H.; Lee, C.-J.; Lin, B.-H.; Chu, F.-H.; Yonemori, I.; Nishimura, T.; Wakabayashi, K.; Chang, W.-H.; Nagashio, K. Purely in-plane ferroelectricity in monolayer SnS at room temperature Nat. Nat. Commun. 2020, 11, 2428,  DOI: 10.1038/s41467-020-16291-9
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    Purely in-plane ferroelectricity in monolayer SnS at room temperature
    Higashitarumizu, Naoki; Kawamoto, Hayami; Lee, Chien-Ju; Lin, Bo-Han; Chu, Fu-Hsien; Yonemori, Itsuki; Nishimura, Tomonori; Wakabayashi, Katsunori; Chang, Wen-Hao; Nagashio, Kosuke
    Nature Communications (2020), 11 (1), 2428CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    2D van der Waals ferroelecs. have emerged as an attractive building block with immense potential to provide multifunctionality in nanoelectronics. Although several accomplishments have been reported in ferroelec. switching for out-of-plane ferroelecs. down to the monolayer, a purely in-plane ferroelec. has not been exptl. validated at the monolayer thickness. Herein, an in-plane ferroelectricity is demonstrated for micrometer-size monolayer SnS at room temp. SnS has been commonly regarded to exhibit the odd-even effect, where the centrosymmetry breaks only in the odd-no. layers to exhibit ferroelectricity. Remarkably, however, a robust room temp. ferroelectricity exists in SnS below a crit. thickness of 15 layers with both an odd and even no. of layers, suggesting the possibility of controlling the stacking sequence of multilayer SnS beyond the limit of ferroelectricity in the monolayer. This work will pave the way for nanoscale ferroelec. applications based on SnS as a platform for in-plane ferroelecs.
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  4. 4
    Wang, H.; Liu, Z. R.; Yoong, H. Y.; Paudel, T. R.; Xiao, J. X.; Guo, R.; Lin, W. N.; Yang, P.; Wang, J.; Chow, G. M.; Venkatesan, T.; Tsymbal, E. Y.; Tian, H.; Chen, J. S. Direct observation of room-temperature out-of-plane ferroelectricity and tunneling electroresistance at the two-dimensional limit. Nat. Commun. 2018, 9, 3319, (2018).  DOI: 10.1038/s41467-018-05662-y
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    Direct observation of room-temperature out-of-plane ferroelectricity and tunneling electroresistance at the two-dimensional limit
    Wang H; Yoong H Y; Xiao J X; Guo R; Lin W N; Wang J; Chow G M; Venkatesan T; Chen J S; Liu Z R; Tian H; Paudel T R; Tsymbal E Y; Yang P; Venkatesan T
    Nature communications (2018), 9 (1), 3319 ISSN:.
    Out-of-plane ferroelectricity with a high transition temperature in nanometer-scale films is required to miniaturize electronic devices. Direct visualization of stable ferroelectric polarization and its switching behavior in atomically thick films is critical for achieving this goal. Here, ferroelectric order at room temperature in the two-dimensional limit is demonstrated in tetragonal BiFeO3 ultrathin films. Using aberration-corrected scanning transmission electron microscopy, we directly observed robust out-of-plane spontaneous polarization in one-unit-cell-thick BiFeO3 films. High-resolution piezoresponse force microscopy measurements show that the polarization is stable and switchable, whereas a tunneling electroresistance effect of up to 370% is achieved in BiFeO3 films. Based on first-principles calculations and Kelvin probe force microscopy measurements, we explain the mechanism of polarization stabilization by the ionic displacements in oxide electrode and the surface charges. Our results indicate that critical thickness for ferroelectricity in the BiFeO3 film is virtually absent, making it a promising candidate for high-density nonvolatile memories.
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    Liu, F.; You, L.; Seyler, K. L.; Li, X.; Yu, P.; Lin, J.; Wang, X.; Zhou, J.; Wang, H.; He, H.; Pantelides, S. T.; Zhou, W.; Sharma, P.; Xu, X.; Ajayan, P. M.; Wang, J.; Liu, Z. Room-temperature ferroelectricity in CuInP2S6 ultrathin flakes. Nat. Commun. 2016, 7, 12357,  DOI: 10.1038/ncomms12357
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    Room-temperature ferroelectricity in CuInP2S6 ultrathin flakes
    Liu, Fucai; You, Lu; Seyler, Kyle L.; Li, Xiaobao; Yu, Peng; Lin, Junhao; Wang, Xuewen; Zhou, Jiadong; Wang, Hong; He, Haiyong; Pantelides, Sokrates T.; Zhou, Wu; Sharma, Pradeep; Xu, Xiaodong; Ajayan, Pulickel M.; Wang, Junling; Liu, Zheng
    Nature Communications (2016), 7 (), 12357CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    Two-dimensional (2D) materials have emerged as promising candidates for various optoelectronic applications based on their diverse electronic properties, ranging from insulating to superconducting. However, cooperative phenomena such as ferroelectricity in the 2D limit have not been well explored. Here, we report room-temp. ferroelectricity in 2D CuInP2S6 (CIPS) with a transition temp. of ∼320 K. Switchable polarization is obsd. in thin CIPS of ∼4 nm. To demonstrate the potential of this 2D ferroelec. material, we prep. a van der Waals (vdW) ferroelec. diode formed by CIPS/Si heterostructure, which shows good memory behavior with on/off ratio of ∼100. The addn. of ferroelectricity to the 2D family opens up possibilities for numerous novel applications, including sensors, actuators, non-volatile memory devices, and various vdW heterostructures based on 2D ferroelectricity.
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    Deng, J.; Liu, Y.; Li, M.; Xu, S.; Lun, Y.; Lv, P.; Xia, T.; Gao, P.; Wang, X.; Hong, J. Thickness-dependent in-plane polarization and structural phase transition in van der Waals ferroelectric CuInP2S6. Small 2020, 16, 1904529,  DOI: 10.1002/smll.201904529
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    Thickness-dependent in-plane polarization and structural phase transition in van der Waals ferroelectric CuInP2S6
    Deng, Jianming; Liu, Yanyu; Li, Mingqiang; Xu, Sheng; Lun, Yingzhuo; Lv, Peng; Xia, Tianlong; Gao, Peng; Wang, Xueyun; Hong, Jiawang
    Small (2020), 16 (1), 1904529CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Van der Waals (vdW) layered materials have rather weaker interlayer bonding than the intralayer bonding, therefore the exfoliation along the stacking direction enables the achievement of monolayer or few layers vdW materials with emerging novel phys. properties and functionalities. The ferroelectricity in vdW materials recently attracts renewed interest for the potential use in high-d. storage devices. With the thickness becoming thinner, the competition between the surface energy, depolarization field, and interfacial chem. bonds may give rise to the modification of ferroelectricity and cryst. structure, which has limited investigations. In this work, combining the piezoresponse force microscope scanning, contact resonance imaging, the existence of the intrinsic in-plane polarization in vdW ferroelecs. CuInP2S6 single crystals is reported, whereas below a crit. thickness between 90 and 100 nm, the in-plane polarization disappears. The Young's modulus also shows an abrupt stiffness at the crit. thickness. Based on the d. functional theory calcns., these behaviors are ascribed to a structural phase transition from monoclinic to trigonal structure, which is further verified by transmission electron microscope technique. These findings demonstrate the foundational importance of structural phase transition for enhancing the rich functionality and broad utility of vdW ferroelecs.
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    Chang, K.; Kaloni, T. P.; Lin, H.; Bedoya-Pinto, A.; Pandeya, A. K.; Kostanovskiy, I.; Zhao, K.; Zhong, Y.; Hu, X.; Xue, Q.-K.; Chen, X.; Ji, S.-H.; Barraza-Lopez, S.; Parkin, S. S. P. Enhanced spontaneous polarization in ultrathin SnTe films with layered antipolar structure. Adv. Mater. 2019, 31, 1804428,  DOI: 10.1002/adma.201804428
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    Chang, K.; Parkin, S. S. P. The growth and phase distribution of ultrathin SnTe on graphene. APL Mater. 2019, 7, 041102  DOI: 10.1063/1.5091546
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    The growth and phase distribution of ultrathin SnTe on graphene
    Chang, Kai; Parkin, Stuart S. P.
    APL Materials (2019), 7 (4), 041102/1-041102/7CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
    Recently, a monolayer of SnTe was discovered to be a two-dimensional ferroelec. with an in-plane polarization, and, most dramatically, it exhibits a significant enhancement of the ferroelec. phase transition temp. compared to its bulk counterpart. This phenomenon is due to a structural phase transition from bulk-like α/β-SnTe, a topol. cryst. insulator, to layered γ-SnTe as the thickness is decreased to a few at. layers. A detailed understanding of the growth mechanism and phase distribution of ultrathin SnTe films are of great interest for potential applications. Here, we report detailed studies of the mol. beam epitaxial growth and in situ scanning tunneling microscopy characterization of ultrathin SnTe films on graphene substrates. By varying the growth conditions, SnTe can be prepd. as either a continuous film or in the form of large rectangular plates. The rate of nucleation of SnTe was found to be highly sensitive to the substrate temp. The coexistence and competition between the β and γ phases formed at room temp. was studied, and the phase diagram with respect to the av. thickness of SnTe and the substrate temp. during growth is drawn. (c) 2019 American Institute of Physics.
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    Chang, K.; Miller, B. J.; Yang, H.; Lin, H.; Parkin, S. S. P.; Barraza-Lopez, S.; Xue, Q.-K.; Chen, X.; Ji, S.-H. Standing waves induced by valley-mismatched domains in ferroelectric SnTe monolayers. Phys. Rev. Lett. 2019, 122, 206402,  DOI: 10.1103/PhysRevLett.122.206402
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    9
    Standing Waves Induced by Valley-Mismatched Domains in Ferroelectric SnTe Monolayers
    Chang, Kai; Miller, Brandon J.; Yang, Hao; Lin, Haicheng; Parkin, Stuart S. P.; Barraza-Lopez, Salvador; Xue, Qi-Kun; Chen, Xi; Ji, Shuai-Hua
    Physical Review Letters (2019), 122 (20), 206402CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
    Two-dimensional (2D) quasiparticle standing waves originate from the interference of coherent quantum states and are usually created by the scattering off edges, at. steps, or adatoms that induce large potential barriers. We report standing waves close to the valence band max. (EV), confined by elec. neutral domain walls of newly discovered ferroelec. SnTe monolayers, as revealed by spatially resolved scanning tunneling spectroscopy. Ab initio calcns. show that this novel confinement arises from the polarization lifted hole valley degeneracy and a ∼90° rotation of the Brillouin zones that render holes' momentum mismatched across neighboring domains. These results show a potential for polarization-tuned valleytronics in 2D ferroelecs.
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    Bao, Y.; Song, P.; Liu, Y.; Chen, Z.; Zhu, M.; Abdelwahab, I.; Su, J.; Fu, W.; Chi, X.; Yu, W.; Liu, W.; Zhao, X.; Xu, Q.-H.; Yang, M.; Loh, K. P. Gate-Tunable In-Plane Ferroelectricity in Few-Layer SnS. Nano Lett. 2019, 19, 5109,  DOI: 10.1021/acs.nanolett.9b01419
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    10
    Gate-tunable in-plane ferroelectricity in few-layer SnS
    Bao, Yang; Song, Peng; Liu, Yanpeng; Chen, Zhihui; Zhu, Menglong; Abdelwahab, Ibrahim; Su, Jie; Fu, Wei; Chi, Xiao; Yu, Wei; Liu, Wei; Zhao, Xiaoxu; Xu, Qing-Hua; Yang, Ming; Loh, Kian Ping
    Nano Letters (2019), 19 (8), 5109-5117CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Ultrathin ferroelecs. hold great promise for modern miniaturized sensors, memories and optoelectronic devices. However, in most ferroelec. materials, polarization is destabilized in ultrathin films by the intrinsic depolarization field. Here, the authors report robust in-plane ferroelectricity in few-layer tin sulfide (SnS) 2D crystals that is coupled anisotropically to lattice strain. Specifically, the intrinsic polarization of SnS manifests as nanoripples along the armchair direction due to a converse piezoelec. effect. Most interestingly, such nanoripples show an odd-and-even effect in terms of its layer dependence, indicating that it is highly sensitive to changes in inversion symmetry. Ferroelec. switching is demonstrated in field effect transistor devices fabricated on ultrathin SnS films, in which a stronger ferroelec. response is achieved at neg. gate voltages. The work shows the promise of 2D SnS in ultrathin ferroelec. field-effect transistors as well as nanoscale electromech. systems.
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    Zhou, Y.; Wu, D.; Zhu, Y.; Cho, Y.; He, Q.; Yang, X.; Herrera, K.; Chu, Z.; Han, Y.; Downer, M. C.; Peng, H.; Lai, K. Out-of-plane piezoelectricity and ferroelectricity in layered α-In2Se3 nanoflakes. Nano Lett. 2017, 17, 5508,  DOI: 10.1021/acs.nanolett.7b02198
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    11
    Out-of-plane piezoelectricity and ferroelectricity in layered α-In2Se3 nanoflakes
    Zhou, Yu; Wu, Di; Zhu, Yihan; Cho, Yujin; He, Qing; Yang, Xiao; Herrera, Kevin; Chu, Zhaodong; Han, Yu; Downer, Michael C.; Peng, Hailin; Lai, Keji
    Nano Letters (2017), 17 (9), 5508-5513CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Piezoelec. and ferroelec. properties in the two-dimensional (2D) limit are highly desired for nanoelectronic, electromech., and optoelectronic applications. Here we report the first exptl. evidence of out-of-plane piezoelectricity and ferroelectricity in van der Waals layered α-In2Se3 nanoflakes. The noncentrosym. R3m symmetry of the α-In2Se3 samples is confirmed by scanning transmission electron microscopy, second-harmonic generation, and Raman spectroscopy measurements. Domains with opposite polarizations are visualized by piezo-response force microscopy. Single-point poling expts. suggest that the polarization is potentially switchable for α-In2Se3 nanoflakes with thicknesses down to ∼10 nm. The piezotronic effect is demonstrated in two-terminal devices, where the Schottky barrier can be modulated by the strain-induced piezopotential. Our work on polar α-In2Se3, one of the model 2D piezoelecs. and ferroelecs. with simple crystal structures, shows its great potential in electronic and photonic applications.
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    Ding, W.; Zhu, J.; Wang, Z.; Gao, Y.; Xiao, D.; Gu, Y.; Zhang, Z.; Zhu, W. Prediction of intrinsic two-dimensional ferroelectrics in In2Se3 and other III2-VI3 van der Waals materials. Nat. Commun. 2017, 8, 14956,  DOI: 10.1038/ncomms14956
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    12
    Prediction of intrinsic two-dimensional ferroelectrics in In2Se3 and other III2-VI3 van der Waals materials
    Ding, Wenjun; Zhu, Jianbao; Wang, Zhe; Gao, Yanfei; Xiao, Di; Gu, Yi; Zhang, Zhenyu; Zhu, Wenguang
    Nature Communications (2017), 8 (), 14956CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    Interest in two-dimensional (2D) van der Waals materials has grown rapidly across multiple scientific and engineering disciplines in recent years. However, ferroelectricity, the presence of a spontaneous elec. polarization, which is important in many practical applications, has rarely been reported in such materials so far. Here we employ first-principles calcns. to discover a branch of the 2D materials family, based on In2Se3 and other III2-VI3 van der Waals materials, that exhibits room-temp. ferroelectricity with reversible spontaneous elec. polarization in both out-of-plane and in-plane orientations. The device potential of these 2D ferroelec. materials is further demonstrated using the examples of van der Waals heterostructures of In2Se3/graphene, exhibiting a tunable Schottky barrier, and In2Se3/WSe2, showing a significant band gap redn. in the combined system. These findings promise to substantially broaden the tunability of van der Waals heterostructures for a wide range of applications.
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    Xiao, J.; Zhu, H.; Wang, Y.; Feng, W.; Hu, Y.; Dasgupta, A.; Han, Y.; Wang, Y.; Muller, D. A.; Martin, L. W.; Hu, P.; Zhang, X. Intrinsic Two-dimensional ferroelectricity with dipole locking. Phys. Rev. Lett. 2018, 120, 227601,  DOI: 10.1103/PhysRevLett.120.227601
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    13
    Intrinsic Two-Dimensional Ferroelectricity with Dipole Locking
    Xiao, Jun; Zhu, Hanyu; Wang, Ying; Feng, Wei; Hu, Yunxia; Dasgupta, Arvind; Han, Yimo; Wang, Yuan; Muller, David A.; Martin, Lane W.; Hu, PingAn; Zhang, Xiang
    Physical Review Letters (2018), 120 (22), 227601CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
    Out-of-plane ferroelectricity with a high transition temp. in ultrathin films is important for the exploration of new domain physics and scaling down of memory devices. However, depolarizing electrostatic fields and interfacial chem. bonds can destroy this long-range polar order at two-dimensional (2D) limit. Here we report the exptl. discovery of the locking between out-of-plane dipoles and in-plane lattice asymmetry in atomically thin In2Se3 crystals, a new stabilization mechanism leading to our observation of intrinsic 2D out-of-plane ferroelectricity. Through second harmonic generation spectroscopy and piezoresponse force microscopy, we found switching of out-of-plane elec. polarization requires a flip of nonlinear optical polarization that corresponds to the inversion of in-plane lattice orientation. The polar order shows a very high transition temp. (∼700 K) without the assistance of extrinsic screening. This finding of intrinsic 2D ferroelectricity resulting from dipole locking opens up possibilities to explore 2D multiferroic physics and develop ultrahigh d. memory devices.
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  14. 14
    Zheng, C.; Yu, L.; Zhu, L.; Collins, J. L.; Kim, D.; Lou, Y.; Xu, C.; Li, M.; Wei, Z.; Zhang, Y.; Edmonds, M. T.; Li, S.; Seidel, J.; Zhu, Y.; Liu, J. Z.; Tang, W.-X.; Fuhrer, M. S. Room temperature in-plane ferroelectricity in van der Waals In2Se3. Sci. Adv. 2018, 4, eaar7720,  DOI: 10.1126/sciadv.aar7720
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    Cui, C.; Hu, W.-J.; Yan, X.; Addiego, C.; Gao, W.; Wang, Y.; Wang, Z.; Li, L.; Cheng, Y.; Li, P.; Zhang, X.; Alshareef, H. N.; Wu, T.; Zhu, W.; Pan, X.; Li, L.-J. Intercorrelated in-plane and out-of-olane ferroelectricity in ultrathin two-dimensional layered semiconductor In2Se3. Nano Lett. 2018, 18, 1253,  DOI: 10.1021/acs.nanolett.7b04852
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    15
    Intercorrelated in-plane and out-of-plane ferroelectricity in ultrathin two-dimensional layered semiconductor In2Se3
    Cui, Chaojie; Hu, Wei-Jin; Yan, Xingxu; Addiego, Christopher; Gao, Wenpei; Wang, Yao; Wang, Zhe; Li, Linze; Cheng, Yingchun; Li, Peng; Zhang, Xixiang; Alshareef, Husam N.; Wu, Tom; Zhu, Wenguang; Pan, Xiaoqing; Li, Lain-Jong
    Nano Letters (2018), 18 (2), 1253-1258CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Enriching the functionality of ferroelec. materials with visible-light sensitivity and multiaxial switching capability would open up new opportunities for their applications in advanced information storage with diverse signal manipulation functions. The authors report exptl. observations of robust intralayer ferroelectricity in 2-dimensional (2D) van der Waals layered α-In2Se3 ultrathin flakes at room temp. Distinct from other 2D and conventional ferroelecs., In2Se3 exhibits intrinsically intercorrelated out-of-plane and in-plane polarization, where the reversal of the out-of-plane polarization by a vertical elec. field also induces the rotation of the in-plane polarization. On the basis of the in-plane switchable diode effect and the narrow band gap (∼1.3 eV) of ferroelec. In2Se3, a prototypical nonvolatile memory device, which can be manipulated both by elec. field and visible light illumination, is demonstrated for advancing data storage technologies.
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    Poh, S. M.; Tan, S. J. R.; Wang, H.; Song, P.; Abidi, I. H.; Zhao, X.; Dan, J.; Chen, J.; Luo, Z.; Pennycook, S. J.; Castro Neto, A. H.; Loh, K. P. Molecular-beam epitaxy of two-dimensional In2Se3 and its giant electroresistance switching in ferroresistive memory junction. Nano Lett. 2018, 18, 6340,  DOI: 10.1021/acs.nanolett.8b02688
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    16
    Molecular-Beam Epitaxy of Two-Dimensional In2Se3 and Its Giant Electroresistance Switching in Ferroresistive Memory Junction
    Poh, Sock Mui; Tan, Sherman Jun Rong; Wang, Han; Song, Peng; Abidi, Irfan H.; Zhao, Xiaoxu; Dan, Jiadong; Chen, Jingsheng; Luo, Zhengtang; Pennycook, Stephen J.; Castro Neto, Antonio H.; Loh, Kian Ping
    Nano Letters (2018), 18 (10), 6340-6346CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Ferroelec. thin film has attracted great interest for nonvolatile memory applications and can be used in either ferroelec. Schottky diodes or ferroelec. tunneling junctions due to its promise of fast switching speed, high on-to-off ratio, and nondestructive readout. Two-dimensional α-phase indium selenide (In2Se3), which has a modest band gap and robust ferroelec. properties stabilized by dipole locking, is an excellent candidate for multidirectional piezoelec. and switchable photodiode applications. However, the large-scale synthesis of this material is still elusive, and its performance as a ferroresistive memory junction is rarely reported. Here, we report the low-temp. mol.-beam epitaxy (MBE) of large-area monolayer α-In2Se3 on graphene and demonstrate the use of α-In2Se3 on graphene in ferroelec. Schottky diode junctions by employing high-work-function gold as the top electrode. The polarization-modulated Schottky barrier formed at the interface exhibits a giant electroresistance ratio of 3.9 × 106 with a readout c.d. of >12 A/cm2, which is more than 200% higher than the state-of-the-art technol. Our MBE growth method allows a high-quality ultrathin film of In2Se3 to be heteroepitaxially grown on graphene, thereby simplifying the fabrication of high-performance 2D ferroelec. junctions for ferroresistive memory applications.
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  17. 17
    Fei, Z.; Zhao, W.; Palomaki, T. A.; Sun, B.; Miller, M. K.; Zhao, Z.; Yan, J.; Xu, X.; Cobden, D. H. Ferroelectric switching of a two-dimensional metal. Nature 2018, 560, 336,  DOI: 10.1038/s41586-018-0336-3
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    17
    Ferroelectric switching of a two-dimensional metal
    Fei, Zaiyao; Zhao, Wenjin; Palomaki, Tauno A.; Sun, Bosong; Miller, Moira K.; Zhao, Zhiying; Yan, Jiaqiang; Xu, Xiaodong; Cobden, David H.
    Nature (London, United Kingdom) (2018), 560 (7718), 336-339CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    A ferroelec. is a material with a polar structure whose polarity can be reversed (switched) by applying an elec. field. In metals, itinerant electrons screen electrostatic forces between ions, which explains in part why polar metals are very rare. Screening also excludes external elec. fields, apparently ruling out the possibility of ferroelec. switching. However, in principle, a thin enough polar metal could be sufficiently penetrated by an elec. field to have its polarity switched. Here, the authors show that the topol. semimetal WTe2 provides an embodiment of this principle. Although monolayer WTe2 is centro-sym. and thus non-polar, the stacked bulk structure is polar. The authors find that 2- or 3-layer WTe2 exhibits spontaneous out-of-plane elec. polarization that can be switched using gate electrodes. They directly detect and quantify the polarization using graphene as an elec.-field sensor. Moreover, the polarization states can be differentiated by cond. and the carrier d. can be varied to modify the properties. The temp. at which polarization vanishes is above 350 K, and even when WTe2 is sandwiched between graphene layers it retains its switching capability at room temp., demonstrating a robustness suitable for applications in combination with other 2-dimensional materials.
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  18. 18
    Liao, W.-Q.; Zhang, Y.; Hu, C.-L.; Mao, J.-G.; Ye, H.-Y.; Li, P.-F.; Huang, S. D.; Xiong, R.-G. A lead-halide perovskite molecular ferroelectric semiconductor. Nat. Commun. 2015, 6, 7338,  DOI: 10.1038/ncomms8338
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    18
    A lead-halide perovskite molecular ferroelectric semiconductor
    Liao Wei-Qiang; Zhang Yi; Ye Heng-Yun; Li Peng-Fei; Hu Chun-Li; Mao Jiang-Gao; Huang Songping D; Xiong Ren-Gen
    Nature communications (2015), 6 (), 7338 ISSN:.
    Inorganic semiconductor ferroelectrics such as BiFeO3 have shown great potential in photovoltaic and other applications. Currently, semiconducting properties and the corresponding application in optoelectronic devices of hybrid organo-plumbate or stannate are a hot topic of academic research; more and more of such hybrids have been synthesized. Structurally, these hybrids are suitable for exploration of ferroelectricity. Therefore, the design of molecular ferroelectric semiconductors based on these hybrids provides a possibility to obtain new or high-performance semiconductor ferroelectrics. Here we investigated Pb-layered perovskites, and found the layer perovskite (benzylammonium)2PbCl4 is ferroelectric with semiconducting behaviours. It has a larger ferroelectric spontaneous polarization Ps=13 μC cm(-2) and a higher Curie temperature Tc=438 K with a band gap of 3.65 eV. This finding throws light on the new properties of the hybrid organo-plumbate or stannate compounds and provides a new way to develop new semiconductor ferroelectrics.
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  19. 19
    You, L.; Liu, F.; Li, H.; Hu, Y.; Zhou, S.; Chang, L.; Zhou, Y.; Fu, Q.; Yuan, G.; Dong, S.; Fan, H. J.; Gruverman, A.; Liu, Z.; Wang, J. In-plane ferroelectricity in thin flakes of van der Waals hybrid perovskite. Adv. Mater. 2018, 30, 1803249,  DOI: 10.1002/adma.201803249
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  20. 20
    Ghosh, T.; Samanta, M.; Vasdev, A.; Dolui, K.; Ghatak, J.; Das, T.; Sheet, G.; Biswas, K. Ultrathin free-standing nanosheets of Bi2O2Se: room temperature ferroelectricity in self-assembled charged layered heterostructure. Nano Lett. 2019, 19, 5703,  DOI: 10.1021/acs.nanolett.9b02312
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    20
    Ultrathin free-standing nanosheets of Bi2O2Se: Room temperature ferroelectricity in self-assembled charged layered heterostructure
    Ghosh, Tanmoy; Samanta, Manisha; Vasdev, Aastha; Dolui, Kapildeb; Ghatak, Jay; Das, Tanmoy; Sheet, Goutam; Biswas, Kanishka
    Nano Letters (2019), 19 (8), 5703-5709CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Ultrathin ferroelec. semiconductors with high charge carrier mobility are much coveted systems for the advancement of various electronic and optoelectronic devices. However, in traditional oxide ferroelec. insulators, the ferroelec. transition temp. decreases drastically with decreasing material thickness and ceases to exist below certain crit. thickness owing to depolarizing fields. Herein, we show the emergence of an ordered ferroelec. ground state in ultrathin (∼2 nm) single cryst. nanosheets of Bi2O2Se at room temp. Free-standing ferroelec. nanosheets, in which oppositely charged alternating layers are self-assembled together by electrostatic interactions, are synthesized by a simple, rapid, and scalable wet chem. procedure at room temp. The existence of ferroelectricity in Bi2O2Se nanosheets is confirmed by dielec. measurements and piezoresponse force spectroscopy. The spontaneous orthorhombic distortion in the ultrathin nanosheets breaks the local inversion symmetry, thereby resulting in ferroelectricity. The local structural distortion and the formation of spontaneous dipole moment were directly probed by at. resoln. scanning transmission electron microscopy and d. functional theory calcns.
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  21. 21
    Soergel, E. Piezoresponse force microscopy (PFM). J. Phys. D: Appl. Phys. 2011, 44, 464003,  DOI: 10.1088/0022-3727/44/46/464003
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    21
    Piezoresponse force microscopy (PFM)
    Soergel, Elisabeth
    Journal of Physics D: Applied Physics (2011), 44 (46), 464003/1-464003/17CODEN: JPAPBE; ISSN:0022-3727. (Institute of Physics Publishing)
    A review. Piezoresponse force microscopy (PFM) detects the local piezoelec. deformation of a sample caused by an applied elec. field from the tip of a scanning force microscope. PFM is able to measure deformations in the sub-picometre regime and can map ferroelec. domain patterns with a lateral resoln. of a few nanometers. These 2 properties have made PFM the preferred technique for recording and investigating ferroelec. domain patterns. In this review we shall describe the tech. aspects of PFM for domain imaging. Particular attention will be paid to the quant. anal. of PFM images.
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  22. 22
    Ji, D.; Cai, S.; Paudel, T. R.; Sun, H.; Zhang, C.; Han, L.; Wei, Y.; Zang, Y.; Gu, M.; Zhang, Y.; Gao, W.; Huyan, H.; Guo, W.; Wu, D.; Gu, Z.; Tsymbal, E. Y.; Wang, P.; Nie, Y.; Pan, X. Freestanding crystalline oxide perovskites down to the monolayer limit. Nature 2019, 570, 87,  DOI: 10.1038/s41586-019-1255-7
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    22
    Freestanding crystalline oxide perovskites down to the monolayer limit
    Ji, Dianxiang; Cai, Songhua; Paudel, Tula R.; Sun, Haoying; Zhang, Chunchen; Han, Lu; Wei, Yifan; Zang, Yipeng; Gu, Min; Zhang, Yi; Gao, Wenpei; Huyan, Huaixun; Guo, Wei; Wu, Di; Gu, Zhengbin; Tsymbal, Evgeny Y.; Wang, Peng; Nie, Yuefeng; Pan, Xiaoqing
    Nature (London, United Kingdom) (2019), 570 (7759), 87-90CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Two-dimensional (2D) materials such as graphene and transition-metal dichalcogenides reveal the electronic phases that emerge when a bulk crystal is reduced to a monolayer1-4. Transition-metal oxide perovskites host a variety of correlated electronic phases5-12, so similar behavior in monolayer materials based on transition-metal oxide perovskites would open the door to a rich spectrum of exotic 2D correlated phases that have not yet been explored. Here we report the fabrication of freestanding perovskite films with high cryst. quality almost down to a single unit cell. Using a recently developed method based on water-sol. Sr3Al2O6 as the sacrificial buffer layer13,14 we synthesize freestanding SrTiO3 and BiFeO3 ultrathin films by reactive mol. beam epitaxy and transfer them to diverse substrates, in particular cryst. silicon wafers and holey carbon films. We find that freestanding BiFeO3 films exhibit unexpected and giant tetragonality and polarization when approaching the 2D limit. Our results demonstrate the absence of a crit. thickness for stabilizing the cryst. order in the freestanding ultrathin oxide films. The ability to synthesize and transfer cryst. freestanding perovskite films without any thickness limitation onto any desired substrate creates opportunities for research into 2D correlated phases and interfacial phenomena that have not previously been tech. possible.
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  23. 23
    Mehboudi, M.; Dorio, A. M.; Zhu, W.; van der Zande, A.; Churchill, H. O. H.; Pacheco-Sanjuan, A. A.; Harriss, E. O.; Kumar, P.; Barraza-Lopez, S. Two-Dimensional Disorder in Black Phosphorus and Monochalcogenide Monolayers. Nano Lett. 2016, 16, 1704,  DOI: 10.1021/acs.nanolett.5b04613
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    23
    Two-Dimensional Disorder in Black Phosphorus and Monochalcogenide Monolayers
    Mehboudi, Mehrshad; Dorio, Alex M.; Zhu, Wenjuan; van der Zande, Arend; Churchill, Hugh O. H.; Pacheco-Sanjuan, Alejandro A.; Harriss, Edmund O.; Kumar, Pradeep; Barraza-Lopez, Salvador
    Nano Letters (2016), 16 (3), 1704-1712CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Ridged, orthorhombic two-dimensional at. crystals with a bulk Pnma structure such as black phosphorus and monochalcogenide monolayers are an exciting and novel material platform for a host of applications. Key to their crystallinity, monolayers of these materials have a 4-fold degenerate structural ground state, and a single energy scale EC (representing the elastic energy required to switch the longer lattice vector along the x- or y-direction) dets. how disordered these monolayers are at finite temp. Disorder arises when nearest neighboring atoms become gently reassigned as the system is thermally excited beyond a crit. temp. Tc that is proportional to EC/kB. EC is tunable by chem. compn. and it leads to a classification of these materials into two categories: (i) Those for which EC ≥ kBTm, and (ii) those having kBTm > EC ≥ 0, where Tm is a given material's melting temp. Black phosphorus and SiS monolayers belong to category (i): these materials do not display an intermediate order-disorder transition and melt directly. All other monochalcogenide monolayers with EC > 0 belonging to class (ii) will undergo a two-dimensional transition prior to melting. EC/kB is slightly larger than room temp. for GeS and GeSe, and smaller than 300 K for SnS and SnSe monolayers, so that these materials transition near room temp. The onset of this generic atomistic phenomena is captured by a planar Potts model up to the order-disorder transition. The order-disorder phase transition in two dimensions described here is at the origin of the Cmcm phase being discussed within the context of bulk layered SnSe.
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  24. 24
    Hanakata, P. Z.; Carvalho, A.; Campbell, D. K.; Park, H. S. Polarization and valley switching in monolayer group-IV monochalcogenides. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 94, 035304  DOI: 10.1103/PhysRevB.94.035304
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    24
    Polarization and valley switching in monolayer group-IV monochalcogenides
    Hanakata, Paul Z.; Carvalho, Alexandra; Campbell, David K.; Park, Harold S.
    Physical Review B (2016), 94 (3), 035304/1-035304/7CODEN: PRBHB7; ISSN:2469-9950. (American Physical Society)
    Group-IV monochalcogenides are a family of two-dimensional puckered materials with an orthorhombic structure that is comprised of polar layers. In this article, we use first principles calcns. to show the multistability of monolayer SnS and GeSe, two prototype materials where the direction of the puckering can be switched by application of tensile stress or elec. field. Furthermore, the two inequivalent valleys in momentum space, which are dictated by the puckering orientation, can be excited selectively using linearly polarized light, and this provides an addnl. tool to identify the polarization direction. Our findings suggest that SnS and GeSe monolayers may have observable ferroelectricity and multistability, with potential applications in information storage.
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  25. 25
    Fei, R.; Kang, W.; Yang, L. Ferroelectricity and Phase Transitions in Monolayer Group-IV Monochalcogenides. Phys. Rev. Lett. 2016, 117, 097601  DOI: 10.1103/PhysRevLett.117.097601
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    25
    Ferroelectricity and phase transitions in monolayer group-IV monochalcogenides
    Fei, Ruixiang; Kang, Wei; Yang, Li
    Physical Review Letters (2016), 117 (9), 097601/1-097601/6CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Ferroelectricity usually fades away as materials are thinned down below a crit. value. We reveal that the unique ionic-potential anharmonicity can induce spontaneous in-plane elec. polarization and ferroelectricity in monolayer group-IV monochalcogenides MX (M = Ge, Sn; X = S, Se). An effective Hamiltonian has been successfully extd. from the parametrized energy space, making it possible to study the ferroelec. phase transitions in a single-atom layer. The ferroelectricity in these materials is found to be robust and the corresponding Curie temps. are higher than room temp., making them promising for realizing ultrathin ferroelec. devices of broad interest. We further provide the phase diagram and predict other potentially two-dimensional ferroelec. materials.
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  26. 26
    Mehboudi, M.; Fregoso, B. M.; Yang, Y.; Zhu, W.; van der Zande, A.; Ferrer, J.; Bellaiche, L.; Kumar, P.; Barraza-Lopez, S. Structural Phase Transition and Material Properties of Few-Layer Monochalcogenides. Phys. Rev. Lett. 2016, 117, 246802,  DOI: 10.1103/PhysRevLett.117.246802
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    26
    Structural phase transition and material properties of few-layer monochalcogenides
    Mehboudi, Mehrshad; Fregoso, Benjamin M.; Yang, Yurong; Zhu, Wenjuan; van der Zande, Arend; Ferrer, Jaime; Bellaiche, L.; Kumar, Pradeep; Barraza-Lopez, Salvador
    Physical Review Letters (2016), 117 (24), 246802/1-246802/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    GeSe and SnSe monochalcogenide monolayers and bilayers undergo a two-dimensional phase transition from a rectangular unit cell to a square unit cell at a crit. temp. Tc well below the m.p. Its consequences on material properties are studied within the framework of Car-Parrinello mol. dynamics and d.-functional theory. No in-gap states develop as the structural transition takes place, so that these phase-change materials remain semiconducting below and above Tc. As the in-plane lattice transforms from a rectangle into a square at Tc, the electronic, spin, optical, and piezoelec. properties dramatically depart from earlier predictions. Indeed, the Y and X points in the Brillouin zone become effectively equiv. at Tc, leading to a sym. electronic structure. The spin polarization at the conduction valley edge vanishes, and the hole cond. must display an anomalous thermal increase at Tc. The linear optical absorption band edge must change its polarization as well, making this structural and electronic evolution verifiable by optical means. Much excitement is drawn by theor. predictions of giant piezoelectricity and ferroelectricity in these materials, and we est. a pyroelec. response of about 3 × 10-12 C/K m here. These results uncover the fundamental role of temp. as a control knob for the phys. properties of few-layer group-IV monochalcogenides.
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  27. 27
    Wang, H.; Qian, X. Two-dimensional multiferroics in monolayer group IV monochalcogenides. 2D Mater. 2017, 4, 015042  DOI: 10.1088/2053-1583/4/1/015042
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    Wu, M.; Zeng, X. C. Intrinsic ferroelasticity and/or multiferroicity in two-dimensional phosphorene and phosphorene analogues. Nano Lett. 2016, 16, 3236,  DOI: 10.1021/acs.nanolett.6b00726
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    28
    Intrinsic Ferroelasticity and/or Multiferroicity in Two-Dimensional Phosphorene and Phosphorene Analogues
    Wu, Menghao; Zeng, Xiao Cheng
    Nano Letters (2016), 16 (5), 3236-3241CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Phosphorene and phosphorene analogs such as SnS and SnSe monolayers are promising nanoelectronic materials with desired bandgap, high carrier mobility, and anisotropic structures. Here, we show first-principles calcn. evidence that these monolayers are potentially the long-sought two-dimensional (2D) materials that can combine electronic transistor characteristic with nonvolatile memory readable/writeable capability at ambient condition. Specifically, phosphorene is predicted to be a 2D intrinsic ferroelastic material with ultrahigh reversible strain, whereas SnS, SnSe, GeS, and GeSe monolayers are multiferroic with coupled ferroelectricity and ferroelasticity. Moreover, their low-switching barriers render room-temp. nonvolatile memory accessible, and their notable structural anisotropy enables ferroelastic or ferroelec. switching readily readable via elec., thermal, optical, mech., or even spintronic detection upon the swapping of the zigzag and armchair direction. In addn., it is predicted that the GeS and GeSe monolayers as well as bulk SnS and SnSe can maintain their ferroelasticity and ferroelectricity (anti-ferroelectricity) beyond the room temp., suggesting high potential for practical device application.
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  29. 29
    Sakayori, K.; Matsui, Y.; Abe, H.; Nakamura, E.; Kenmoku, M.; Hara, T.; Ishikawa, D.; Kokubu, A.; Hirota, K.; Ikeda, T. Curie temperature of BaTiO3. Jpn. J. Appl. Phys. 1995, 34, 5443,  DOI: 10.1143/JJAP.34.5443
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    29
    Curie temperature of BaTiO3
    Sakayori, Ken-ichi; Matsui, Yasunori; Abe, Hiroyuki; Nakamura, Eiji; Kenmoku, Mikihiko; Hara, Tomoyuki; Ishikawa, Daisuke; Kokubu, Akihiro; Hirota, Ken-ichi; Ikeda, Takuro
    Japanese Journal of Applied Physics, Part 1: Regular Papers, Short Notes & Review Papers (1995), 34 (9B), 5443-5CODEN: JAPNDE; ISSN:0021-4922. (Japanese Journal of Applied Physics)
    The Curie point (TC) of BaTiO3 was earlier reported to be 120° and has been recently believed to be 130°. Some expts. have been performed here to reconfirm the TC. Firing conditions used for prepg. the BaTiO3 were examd. first with the use of pure BaCO3 or Ba(NO3)2 and TiO2. The x-dependence of TC in (BaO)1-x(TiO2)1+x solid soln. was measured. Data were scattered and suffered from individual variations. According to probability considerations, the TC of BaTiO3 was evaluated from the intercept at x = 0. On the other hand, the compn. dependence of TC in some related solid soln. systems, (Ba1-yPby)TiO3, (Ba1-ySry)TiO3, and (BaTiO3)1-y(KF)y, was examd., and the TC of BaTiO3 was estd. by extrapolation toward the limit y → 0. In conclusion, the Curie point of BaTiO3 is detd. as 123.0 ± 0.6°.
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    Shen, X.-W.; Fang, Y.-W.; Tian, B.-B.; Duan, C.-G. Two-dimensional ferroelectric tunnel junction: the case of monolayer In:SnSe/SnSe/Sb:SnSe homostructure. ACS Appl. Electron. Mater. 2019, 1, 1133,  DOI: 10.1021/acsaelm.9b00146
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    30
    Two-dimensional ferroelectric tunnel junction: the case of monolayer In:SnSe/SnSe/Sb:SnSe homostructure
    Shen, Xin-Wei; Fang, Yue-Wen; Tian, Bo-Bo; Duan, Chun-Gang
    ACS Applied Electronic Materials (2019), 1 (7), 1133-1140CODEN: AAEMBP; ISSN:2637-6113. (American Chemical Society)
    Ferroelec. tunnel junctions, in which ferroelec. polarization and quantum tunneling are closely coupled to induce the tunneling electroresistance (TER) effect, have attracted considerable interest due to their potential in nonvolatile and low-power consumption memory devices. The ferroelec. size effect, however, has hindered ferroelec. tunnel junctions from exhibiting a robust TER effect. Here, the study proposes doping engineering in a 2-dimensional in-plane ferroelec. semiconductor as an effective strategy to design a 2-dimensional ferroelec. tunnel junction composed of homostructural p-type semiconductor/ferroelec./n-type semiconductor. Because the in-plane polarization persists in the monolayer ferroelec. barrier, the vertical thickness of 2-dimensional ferroelec. tunnel junction can be as thin as a monolayer. The authors show that the monolayer In:SnSe/SnSe/Sb:SnSe junction provides an embodiment of this strategy. Combining d. functional theory calcns. with nonequil. Green's function formalism, they investigate the electron transport properties of In:SnSe/SnSe/Sb:SnSe and reveal a giant TER effect of 1460%. The dynamical modulation of both barrier width and barrier height during the ferroelec. switching is responsible for this giant TER effect. These findings provide an important insight into the understanding of the quantum behaviors of electrons in materials at the 2-dimensional limit and enable new possibilities for next-generation nonvolatile memory devices based on flexible 2-dimensional lateral ferroelec. tunnel junctions.
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    Shen, H.; Liu, J.; Chang, K.; Fu, L. In-plane ferroelectric tunneling junction. Phys. Rev. Appl. 2019, 11, 024048  DOI: 10.1103/PhysRevApplied.11.024048
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    31
    In-Plane Ferroelectric Tunnel Junction
    Shen, Huitao; Liu, Junwei; Chang, Kai; Fu, Liang
    Physical Review Applied (2019), 11 (2), 024048CODEN: PRAHB2; ISSN:2331-7019. (American Physical Society)
    Ferroelec. materals are an important platform for the realization of nonvolatile memories. So far, existing ferroelec. memory devices have utilized out-of-plane polarization in ferroelec. thin films. In this paper, we propose a type of random-access memory (RAM) based on ferroelec. thin films with in-plane polarization, called an "in-plane ferroelec. tunnel junction." Apart from nonvolatility, lower power usage, and a faster writing operation compared with traditional dynamic RAMs, our proposal has the advantage of a faster reading operation and a nondestructive reading process, thus overcoming the write-after-read problem that exists widely in current ferroelec. RAMs. The recent discovered room-temp. ferroelec. IV-VI semiconductor thin films are a promising material platform for the realization of our proposal.
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    Xu, L.; Yang, M.; Wang, S. J.; Feng, Y. P. Electronic and optical properties of the monolayer group-IV monochalcogenides MX (M = Ge, Sn; X = S, Se, Te). Phys. Rev. B: Condens. Matter Mater. Phys. 2017, 95, 235434,  DOI: 10.1103/PhysRevB.95.235434
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    32
    Electronic and optical properties of the monolayer group-IV monochalcogenides MX (M = Ge, Sn; X = S, Se, Te)
    Xu, Lei; Yang, Ming; Wang, Shi Jie; Feng, Yuan Ping
    Physical Review B (2017), 95 (23), 235434/1-235434/9CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    By using d.-functional theory and many-body perturbation theory based first-principles calcns., we have systematically investigated the electronic and optical properties of monolayer group-IV monochalcogenides MX (M = Ge, Sn; X = S, Se, Te). All MX monolayers are predicted to be indirect gap semiconductors, except the GeSe monolayer, which has a direct gap of 1.66 eV. The carrier mobilities of MX monolayers are estd. to be on the order of 103 to 105 cm2 V-1 s-1, which is comparable to, and in some cases higher than, that of phosphorene using a phonon-limited scattering model. Moreover, the optical spectra of MX monolayers obtained from GW-Bethe-Salpeter equation calcns. are highly orientation dependent, esp. for the GeS monolayer, suggesting their potential application as a linear polarizing filter. Our results reveal that the GeSe monolayer is an attractive candidate for optoelectronic applications as it is a semiconductor with a direct band gap, a relatively high carrier mobility, and an onset optical absorption energy in the visible light range. Finally, based on an effective-mass model with nonlocal Coulomb interaction included, we find that the excitonic effects of the GeSe monolayer can be effectively tuned by the presence of dielec. substrates. Our studies provide an improved understanding of electronic, optical, and excitonic properties of group-IV monochalcogenides monolayers and might shed light on their potential electronic and optoelectronic applications.
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    Intrinsic persistent spin helix state in two-dimensional group-IV monochalcogenide MX monolayers (M=Sn or Ge and X=S, Se, or Te)
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    Physical Review B (2019), 100 (11), 115104CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    Energy-saving spintronics are believed to be implementable on systems hosting the persistent spin helix (PSH) since they support an extraordinarily long spin lifetime of carriers. However, achieving the PSH requires a unidirectional spin configuration in the momentum space, which is practically nontrivial due to the stringent conditions for fine-tuning the Rashba and Dresselhaus spin-orbit couplings. Here, we predict that the PSH can be intrinsically achieved on a two-dimensional (2D) group-IV monochalcogenide MX monolayer, a new class of the noncentrosym. 2D materials having in-plane ferroelectricity. Due to the C2v point-group symmetry in the MX monolayer, a unidirectional spin configuration is preserved in the out-of-plane direction and thus maintains the PSH that is similar to the [110] Dresselhaus model in the [110]-oriented quantum well. Our first-principle calcns. on various MX (M= Sn, Ge; X= S, Se, Te) monolayers confirmed that such typical spin configuration is obsd., in particular, at near the valence-band max. where a sizable spin splitting and a substantially small wavelength of the spin polarization are achieved. Importantly, we observe reversible out-of-plane spin orientation under opposite in-plane ferroelec. polarization, indicating that an elec. controllable PSH for spintronic applications is plausible.
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    Applied Physics Letters (2020), 116 (2), 022411CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    A non-vanishing elec. field inside a non-centrosym. crystal transforms into a momentum-dependent magnetic field, namely, a spin-orbit field (SOF). SOFs are of great use in spintronics because they enable spin manipulation via the elec. field. At the same time, however, spintronic applications are severely limited by the SOF, as electrons traversing the SOF easily lose their spin information. Here, we propose that in-plane ferroelectricity in (001)-oriented SnTe thin films can support both elec. spin controllability and suppression of spin dephasing. The in-plane ferroelectricity produces a unidirectional out-of-plane Rashba SOF that can host a long-lived helical spin mode known as a persistent spin helix (PSH). Through direct coupling between the inversion asymmetry and the SOF, the ferroelec. switching reverses the out-of-plane Rashba SOF, giving rise to a maximally field-tunable PSH. Furthermore, the giant out-of-plane Rashba SOF seen in the SnTe thin films is linked to the nano-sized PSH, potentially reducing spintronic device sizes to the nanoscale. We combine the two ferroelec.-coupled degrees of freedom, longitudinal charge and transverse PSH, to design intersectional electro-spintronic transistors governed by non-volatile ferroelec. switching within nanoscale lateral and at.-thick vertical dimensions. (c) 2020 American Institute of Physics.
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    Monolayer group-IV monochalcogenides (MX, M = Ge, Sn, Pb; X = S, Se, Te) are a family of novel two-dimensional (2D) materials that have at. structures closely related to that of the staggered black phosphorus lattice. The structure of most monolayer MX materials exhibits a broken inversion symmetry and many of them exhibit ferroelectricity with a reversible in-plane elec. polarization. A further consequence of the noncentrosym. structure is that when coupled with strong spin-orbit coupling, many MX materials are promising for the future applications in non-linear optics, photovoltaics, spintronics, and valleytronics. Nevertheless, because of the relatively large exfoliation energy, the creation of monolayer MX materials is not easy, which hinders the integration of these materials into the fast-developing field of 2D material heterostructures. In this Perspective, we review recent developments in exptl. routes to the creation of the monolayer MX, including mol. beam epitaxy and two-step etching methods. Other approaches that could be used to prep. the monolayer MX are also discussed, such as liq. phase exfoliation and soln.-phase synthesis. A quant. comparison between these different methods is also presented. (c) 2020 American Institute of Physics.
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    Physical Review Materials (2019), 3 (12), 124004CODEN: PRMHBS; ISSN:2475-9953. (American Physical Society)
    We performed d. functional theory calcns. with self-consistent van der Waals cor. exchange-correlation (XC) functionals to capture the structure of black phosphorus and twelve monochalcogenide monolayers and find the following results, which are independent of XC choice: (a) The in-plane unit cell changes its area in going from the bulk to a monolayer. Such structural behavior is unlike the one seen in more traditional two-dimensional materials such as graphene or MoS2, in which monolayers keep their structure upon exfoliation. The change of in-plane distances implies that bonds weaker than covalent or ionic ones are at work within the monolayers themselves and may require corrections beyond PBE XC. This observation is relevant for the prediction of the crit. temp. Tc and important for that reason. (b) There exists a hierarchy of independent parameters that uniquely define a ground state ferroelec. unit cell (and square and rectangular paraelec. unit cells as well): only 5 optimizable parameters are needed to establish the unit cell vectors and the four basis vectors of the ferroelec. ground state unit cell, while square and rectangular paraelec. structures are defined by only three or two independent optimizable variables, resp. (c) The reduced no. of independent structural variables correlates with larger elastic energy barriers on a rectangular paraelec. unit cell when compared to the elastic energy barrier of a square paraelec. structure. This implies that Tc obtained on a structure that keeps the lattice parameters fixed (for example, using an NVT ensemble) should be larger than the transition temp. on a structure that is allowed to change in-plane lattice vectors (for example, using the NPT ensemble). (d) Surprisingly, the dissocn. energy (bulk cleavage energy) of these materials is similar to the energy required to exfoliate graphite and MoS2. (e) There exists a linear relation among the square paraelec. unit cell lattice parameter and the lattice parameters of the rectangular ferroelec. ground state unit cell. These results highlight the subtle atomistic structure and chem. bond of these novel 2D ferroelecs.
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    Tuning the ferroelectric-to-paraelectric transition temperature and dipole orientation of group-IV monochalcogenide monolayers
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    Physical Review B (2018), 97 (2), 024110CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    Coordination-related, two-dimensional (2D) structural phase transitions are a fascinating facet of two-dimensional materials with structural degeneracies. Nevertheless, a unified theor. account of these transitions remains absent, and the following points are established through ab initio mol. dynamics and 2D discrete clock models here: Group-IV monochalcogenide (GeSe, SnSe, SnTe,...) monolayers have four degenerate structural ground states, and a phase transition from a threefold coordinated onto a fivefold coordinated structure takes place at finite temp. On unstrained samples, this phase transition requires lattice parameters to evolve freely. A fundamental energy scale J permits understanding this transition, and numerical results indicate a transition temp. Tc of about 1.41J. Numerical data provides a relation among the exptl. (rhombic) parameter 〈Δα〉 and T of the form 〈Δα〉=Δα(T=0)1-T/Tcβ, with a crit. exponent β≃1/3 that coincides with expt. It is also shown that 〈Δα〉 is temp. independent in another theor. work, and thus incompatible with expt. Tc and the orientation of the in-plane intrinsic elec. dipole can be controlled by moderate uniaxial tensile strain, and a modified discrete clock model describes the transition on strained samples qual. An anal. of out-of-plane fluctuations and a discussion of the need for van der Waals corrections to describe these materials are given too. These results provide an exptl. compatible framework to understand structural phase transitions in 2D materials and their effects on material properties.
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    Figure 1. In-plane spontaneous polarization in SnSe monolayer plates. (a) Lattice structure of SnSe monolayer. The solid rectangle indicates a unit cell. Dashed lines indicate the preferred edge orientations of these plates. The signs of bound charges at opposite edges are labeled. (b) Schematic diagram of three possible crystalline orientations of SnSe monolayers. The rhomboids and rectangles represent the unit cells of graphene and SnSe, respectively. Arrows in the rectangles indicate the directions of polarization allowed in this configuration. (c) Spatially resolved dI/dV spectra along the dashed arrow across the SnSe monolayer plate shown in the inset, obtained at 1.8 K. Set points: Vs = 3.0 V, It = 50 pA for positive Vs; Vs = −1.0 V, It = 50 pA for negative Vs. The red dashed curves are exponential fittings of the band bending profiles. (d,e) Spatially resolved dI/dV spectra acquired from the same SnSe monolayer plate, along the directions parallel (d) and perpendicular (e) to the polarization, as the inset shows. The conditions of measurements are the same as in (c).

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    Figure 2

    Figure 2. Ferroelectric domains in SnSe monolayer plates at room temperature. (a,b) Room-temperature topography and simultaneously recorded dI/dV images of a SnSe monolayer plate. Set point: Vs = −0.2 V, It = 2 pA. (c,d) dI/dV images of SnSe monolayer plates with 180° straight (c) and zigzag (d) domain walls. The domain walls are indicated by the white dashed lines. Set points: Vs = −0.2 V, It = 2 pA for (c), Vs = −0.35 V, It = 2 pA for (d).

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    Figure 3

    Figure 3. Controllable ferroelectric switching of SnSe monolayer plates. (a) Schematic of ferroelectric switching achieved by applying a bias voltage pulse VP at a point on the graphene substrate close to a SnSe monolayer plate. The corner of SnSe closest to the STM tip was set as d = 0, and the upper surface of graphene was set as z = 0. (b) Rising edges of the bias voltage pulses. Dashed lines indicate the onset and maximum values of the bias voltage. (c–g) Consecutive dI/dV images of a ferroelectric switching sequence in a SnSe monolayer plate. Set points: Vs = −0.35 V, It = 2 pA. The pulses were applied at the same point indicated in (c). The widths of all the pulses were 50 ms. The directions of the in-plane components of tip-induced electric fields are indicated by the white arrows. All of the data in this figure were collected at room temperature.

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    Figure 4

    Figure 4. Quantitative studies of the domain wall motion induced by bias voltage pulses. (a) Simulated out-of-plane and in-plane electric fields along a horizontal line along the d-axis (see Figure 2a) at the height of z = 0.3 nm. (b) Statistics of a series of bias voltage pulse experiments with different VP and d0. Each dot represents a single pulse applied. Those pulses that successfully moved a domain wall are shown in red, and those that did not induce domain wall motion are in blue. (c) Simulated in-plane electric fields with VP fixed while varying d0. (d) The experimental d0 dependence of the critical pulse voltages |VP,c| of domain wall motion (inset) and the corresponding E//,c at the closest corner of SnSe.

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    Figure 5

    Figure 5. Temperature dependence of ferroelectricity in a SnSe monolayer plate. (a–f) Topography images of a SnSe monolayer plate at temperature T increasing from 310 to 400 K from (a) to (e), and decreasing to 308 K in (f). Set points: Vs = −0.2 V, It = 2 pA. (g–l) Simultaneously recorded dI/dV images corresponding to (a–f). (m–r) Apparent height and dI/dV profiles along the dashed arrows in (a,g), extracted from (a–f) and (g–l), respectively. (s) Evolution of the difference of apparent height and dI/dV between the two opposite edges with upward and downward band bending directions, extracted from m–q.

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    This article references 46 other publications.

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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXit12itbjM&md5=f3a1e14b200c00d94c9f00a6984ecffd
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      Recently, a monolayer of SnTe was discovered to be a two-dimensional ferroelec. with an in-plane polarization, and, most dramatically, it exhibits a significant enhancement of the ferroelec. phase transition temp. compared to its bulk counterpart. This phenomenon is due to a structural phase transition from bulk-like α/β-SnTe, a topol. cryst. insulator, to layered γ-SnTe as the thickness is decreased to a few at. layers. A detailed understanding of the growth mechanism and phase distribution of ultrathin SnTe films are of great interest for potential applications. Here, we report detailed studies of the mol. beam epitaxial growth and in situ scanning tunneling microscopy characterization of ultrathin SnTe films on graphene substrates. By varying the growth conditions, SnTe can be prepd. as either a continuous film or in the form of large rectangular plates. The rate of nucleation of SnTe was found to be highly sensitive to the substrate temp. The coexistence and competition between the β and γ phases formed at room temp. was studied, and the phase diagram with respect to the av. thickness of SnTe and the substrate temp. during growth is drawn. (c) 2019 American Institute of Physics.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhtFKlsbjF&md5=c7daabdad933bec4bbf98d7d42c369aa
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXht1OqsbrM&md5=3a750a17d1019f06566ae33c2e1fe934
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhtl2lsrbI&md5=8012592362a929984e95a2b504cb62e3
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      Interest in two-dimensional (2D) van der Waals materials has grown rapidly across multiple scientific and engineering disciplines in recent years. However, ferroelectricity, the presence of a spontaneous elec. polarization, which is important in many practical applications, has rarely been reported in such materials so far. Here we employ first-principles calcns. to discover a branch of the 2D materials family, based on In2Se3 and other III2-VI3 van der Waals materials, that exhibits room-temp. ferroelectricity with reversible spontaneous elec. polarization in both out-of-plane and in-plane orientations. The device potential of these 2D ferroelec. materials is further demonstrated using the examples of van der Waals heterostructures of In2Se3/graphene, exhibiting a tunable Schottky barrier, and In2Se3/WSe2, showing a significant band gap redn. in the combined system. These findings promise to substantially broaden the tunability of van der Waals heterostructures for a wide range of applications.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXlvVCis7s%253D&md5=9bebbf63b08e9d17b7681c7552a48cd6
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      Out-of-plane ferroelectricity with a high transition temp. in ultrathin films is important for the exploration of new domain physics and scaling down of memory devices. However, depolarizing electrostatic fields and interfacial chem. bonds can destroy this long-range polar order at two-dimensional (2D) limit. Here we report the exptl. discovery of the locking between out-of-plane dipoles and in-plane lattice asymmetry in atomically thin In2Se3 crystals, a new stabilization mechanism leading to our observation of intrinsic 2D out-of-plane ferroelectricity. Through second harmonic generation spectroscopy and piezoresponse force microscopy, we found switching of out-of-plane elec. polarization requires a flip of nonlinear optical polarization that corresponds to the inversion of in-plane lattice orientation. The polar order shows a very high transition temp. (∼700 K) without the assistance of extrinsic screening. This finding of intrinsic 2D ferroelectricity resulting from dipole locking opens up possibilities to explore 2D multiferroic physics and develop ultrahigh d. memory devices.
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    15. 15
      Cui, C.; Hu, W.-J.; Yan, X.; Addiego, C.; Gao, W.; Wang, Y.; Wang, Z.; Li, L.; Cheng, Y.; Li, P.; Zhang, X.; Alshareef, H. N.; Wu, T.; Zhu, W.; Pan, X.; Li, L.-J. Intercorrelated in-plane and out-of-olane ferroelectricity in ultrathin two-dimensional layered semiconductor In2Se3. Nano Lett. 2018, 18, 1253,  DOI: 10.1021/acs.nanolett.7b04852
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      15
      Intercorrelated in-plane and out-of-plane ferroelectricity in ultrathin two-dimensional layered semiconductor In2Se3
      Cui, Chaojie; Hu, Wei-Jin; Yan, Xingxu; Addiego, Christopher; Gao, Wenpei; Wang, Yao; Wang, Zhe; Li, Linze; Cheng, Yingchun; Li, Peng; Zhang, Xixiang; Alshareef, Husam N.; Wu, Tom; Zhu, Wenguang; Pan, Xiaoqing; Li, Lain-Jong
      Nano Letters (2018), 18 (2), 1253-1258CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Enriching the functionality of ferroelec. materials with visible-light sensitivity and multiaxial switching capability would open up new opportunities for their applications in advanced information storage with diverse signal manipulation functions. The authors report exptl. observations of robust intralayer ferroelectricity in 2-dimensional (2D) van der Waals layered α-In2Se3 ultrathin flakes at room temp. Distinct from other 2D and conventional ferroelecs., In2Se3 exhibits intrinsically intercorrelated out-of-plane and in-plane polarization, where the reversal of the out-of-plane polarization by a vertical elec. field also induces the rotation of the in-plane polarization. On the basis of the in-plane switchable diode effect and the narrow band gap (∼1.3 eV) of ferroelec. In2Se3, a prototypical nonvolatile memory device, which can be manipulated both by elec. field and visible light illumination, is demonstrated for advancing data storage technologies.
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    16. 16
      Poh, S. M.; Tan, S. J. R.; Wang, H.; Song, P.; Abidi, I. H.; Zhao, X.; Dan, J.; Chen, J.; Luo, Z.; Pennycook, S. J.; Castro Neto, A. H.; Loh, K. P. Molecular-beam epitaxy of two-dimensional In2Se3 and its giant electroresistance switching in ferroresistive memory junction. Nano Lett. 2018, 18, 6340,  DOI: 10.1021/acs.nanolett.8b02688
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      16
      Molecular-Beam Epitaxy of Two-Dimensional In2Se3 and Its Giant Electroresistance Switching in Ferroresistive Memory Junction
      Poh, Sock Mui; Tan, Sherman Jun Rong; Wang, Han; Song, Peng; Abidi, Irfan H.; Zhao, Xiaoxu; Dan, Jiadong; Chen, Jingsheng; Luo, Zhengtang; Pennycook, Stephen J.; Castro Neto, Antonio H.; Loh, Kian Ping
      Nano Letters (2018), 18 (10), 6340-6346CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Ferroelec. thin film has attracted great interest for nonvolatile memory applications and can be used in either ferroelec. Schottky diodes or ferroelec. tunneling junctions due to its promise of fast switching speed, high on-to-off ratio, and nondestructive readout. Two-dimensional α-phase indium selenide (In2Se3), which has a modest band gap and robust ferroelec. properties stabilized by dipole locking, is an excellent candidate for multidirectional piezoelec. and switchable photodiode applications. However, the large-scale synthesis of this material is still elusive, and its performance as a ferroresistive memory junction is rarely reported. Here, we report the low-temp. mol.-beam epitaxy (MBE) of large-area monolayer α-In2Se3 on graphene and demonstrate the use of α-In2Se3 on graphene in ferroelec. Schottky diode junctions by employing high-work-function gold as the top electrode. The polarization-modulated Schottky barrier formed at the interface exhibits a giant electroresistance ratio of 3.9 × 106 with a readout c.d. of >12 A/cm2, which is more than 200% higher than the state-of-the-art technol. Our MBE growth method allows a high-quality ultrathin film of In2Se3 to be heteroepitaxially grown on graphene, thereby simplifying the fabrication of high-performance 2D ferroelec. junctions for ferroresistive memory applications.
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    17. 17
      Fei, Z.; Zhao, W.; Palomaki, T. A.; Sun, B.; Miller, M. K.; Zhao, Z.; Yan, J.; Xu, X.; Cobden, D. H. Ferroelectric switching of a two-dimensional metal. Nature 2018, 560, 336,  DOI: 10.1038/s41586-018-0336-3
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      17
      Ferroelectric switching of a two-dimensional metal
      Fei, Zaiyao; Zhao, Wenjin; Palomaki, Tauno A.; Sun, Bosong; Miller, Moira K.; Zhao, Zhiying; Yan, Jiaqiang; Xu, Xiaodong; Cobden, David H.
      Nature (London, United Kingdom) (2018), 560 (7718), 336-339CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      A ferroelec. is a material with a polar structure whose polarity can be reversed (switched) by applying an elec. field. In metals, itinerant electrons screen electrostatic forces between ions, which explains in part why polar metals are very rare. Screening also excludes external elec. fields, apparently ruling out the possibility of ferroelec. switching. However, in principle, a thin enough polar metal could be sufficiently penetrated by an elec. field to have its polarity switched. Here, the authors show that the topol. semimetal WTe2 provides an embodiment of this principle. Although monolayer WTe2 is centro-sym. and thus non-polar, the stacked bulk structure is polar. The authors find that 2- or 3-layer WTe2 exhibits spontaneous out-of-plane elec. polarization that can be switched using gate electrodes. They directly detect and quantify the polarization using graphene as an elec.-field sensor. Moreover, the polarization states can be differentiated by cond. and the carrier d. can be varied to modify the properties. The temp. at which polarization vanishes is above 350 K, and even when WTe2 is sandwiched between graphene layers it retains its switching capability at room temp., demonstrating a robustness suitable for applications in combination with other 2-dimensional materials.
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    18. 18
      Liao, W.-Q.; Zhang, Y.; Hu, C.-L.; Mao, J.-G.; Ye, H.-Y.; Li, P.-F.; Huang, S. D.; Xiong, R.-G. A lead-halide perovskite molecular ferroelectric semiconductor. Nat. Commun. 2015, 6, 7338,  DOI: 10.1038/ncomms8338
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      18
      A lead-halide perovskite molecular ferroelectric semiconductor
      Liao Wei-Qiang; Zhang Yi; Ye Heng-Yun; Li Peng-Fei; Hu Chun-Li; Mao Jiang-Gao; Huang Songping D; Xiong Ren-Gen
      Nature communications (2015), 6 (), 7338 ISSN:.
      Inorganic semiconductor ferroelectrics such as BiFeO3 have shown great potential in photovoltaic and other applications. Currently, semiconducting properties and the corresponding application in optoelectronic devices of hybrid organo-plumbate or stannate are a hot topic of academic research; more and more of such hybrids have been synthesized. Structurally, these hybrids are suitable for exploration of ferroelectricity. Therefore, the design of molecular ferroelectric semiconductors based on these hybrids provides a possibility to obtain new or high-performance semiconductor ferroelectrics. Here we investigated Pb-layered perovskites, and found the layer perovskite (benzylammonium)2PbCl4 is ferroelectric with semiconducting behaviours. It has a larger ferroelectric spontaneous polarization Ps=13 μC cm(-2) and a higher Curie temperature Tc=438 K with a band gap of 3.65 eV. This finding throws light on the new properties of the hybrid organo-plumbate or stannate compounds and provides a new way to develop new semiconductor ferroelectrics.
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    19. 19
      You, L.; Liu, F.; Li, H.; Hu, Y.; Zhou, S.; Chang, L.; Zhou, Y.; Fu, Q.; Yuan, G.; Dong, S.; Fan, H. J.; Gruverman, A.; Liu, Z.; Wang, J. In-plane ferroelectricity in thin flakes of van der Waals hybrid perovskite. Adv. Mater. 2018, 30, 1803249,  DOI: 10.1002/adma.201803249
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    20. 20
      Ghosh, T.; Samanta, M.; Vasdev, A.; Dolui, K.; Ghatak, J.; Das, T.; Sheet, G.; Biswas, K. Ultrathin free-standing nanosheets of Bi2O2Se: room temperature ferroelectricity in self-assembled charged layered heterostructure. Nano Lett. 2019, 19, 5703,  DOI: 10.1021/acs.nanolett.9b02312
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      20
      Ultrathin free-standing nanosheets of Bi2O2Se: Room temperature ferroelectricity in self-assembled charged layered heterostructure
      Ghosh, Tanmoy; Samanta, Manisha; Vasdev, Aastha; Dolui, Kapildeb; Ghatak, Jay; Das, Tanmoy; Sheet, Goutam; Biswas, Kanishka
      Nano Letters (2019), 19 (8), 5703-5709CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Ultrathin ferroelec. semiconductors with high charge carrier mobility are much coveted systems for the advancement of various electronic and optoelectronic devices. However, in traditional oxide ferroelec. insulators, the ferroelec. transition temp. decreases drastically with decreasing material thickness and ceases to exist below certain crit. thickness owing to depolarizing fields. Herein, we show the emergence of an ordered ferroelec. ground state in ultrathin (∼2 nm) single cryst. nanosheets of Bi2O2Se at room temp. Free-standing ferroelec. nanosheets, in which oppositely charged alternating layers are self-assembled together by electrostatic interactions, are synthesized by a simple, rapid, and scalable wet chem. procedure at room temp. The existence of ferroelectricity in Bi2O2Se nanosheets is confirmed by dielec. measurements and piezoresponse force spectroscopy. The spontaneous orthorhombic distortion in the ultrathin nanosheets breaks the local inversion symmetry, thereby resulting in ferroelectricity. The local structural distortion and the formation of spontaneous dipole moment were directly probed by at. resoln. scanning transmission electron microscopy and d. functional theory calcns.
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    21. 21
      Soergel, E. Piezoresponse force microscopy (PFM). J. Phys. D: Appl. Phys. 2011, 44, 464003,  DOI: 10.1088/0022-3727/44/46/464003
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      21
      Piezoresponse force microscopy (PFM)
      Soergel, Elisabeth
      Journal of Physics D: Applied Physics (2011), 44 (46), 464003/1-464003/17CODEN: JPAPBE; ISSN:0022-3727. (Institute of Physics Publishing)
      A review. Piezoresponse force microscopy (PFM) detects the local piezoelec. deformation of a sample caused by an applied elec. field from the tip of a scanning force microscope. PFM is able to measure deformations in the sub-picometre regime and can map ferroelec. domain patterns with a lateral resoln. of a few nanometers. These 2 properties have made PFM the preferred technique for recording and investigating ferroelec. domain patterns. In this review we shall describe the tech. aspects of PFM for domain imaging. Particular attention will be paid to the quant. anal. of PFM images.
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    22. 22
      Ji, D.; Cai, S.; Paudel, T. R.; Sun, H.; Zhang, C.; Han, L.; Wei, Y.; Zang, Y.; Gu, M.; Zhang, Y.; Gao, W.; Huyan, H.; Guo, W.; Wu, D.; Gu, Z.; Tsymbal, E. Y.; Wang, P.; Nie, Y.; Pan, X. Freestanding crystalline oxide perovskites down to the monolayer limit. Nature 2019, 570, 87,  DOI: 10.1038/s41586-019-1255-7
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      22
      Freestanding crystalline oxide perovskites down to the monolayer limit
      Ji, Dianxiang; Cai, Songhua; Paudel, Tula R.; Sun, Haoying; Zhang, Chunchen; Han, Lu; Wei, Yifan; Zang, Yipeng; Gu, Min; Zhang, Yi; Gao, Wenpei; Huyan, Huaixun; Guo, Wei; Wu, Di; Gu, Zhengbin; Tsymbal, Evgeny Y.; Wang, Peng; Nie, Yuefeng; Pan, Xiaoqing
      Nature (London, United Kingdom) (2019), 570 (7759), 87-90CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Two-dimensional (2D) materials such as graphene and transition-metal dichalcogenides reveal the electronic phases that emerge when a bulk crystal is reduced to a monolayer1-4. Transition-metal oxide perovskites host a variety of correlated electronic phases5-12, so similar behavior in monolayer materials based on transition-metal oxide perovskites would open the door to a rich spectrum of exotic 2D correlated phases that have not yet been explored. Here we report the fabrication of freestanding perovskite films with high cryst. quality almost down to a single unit cell. Using a recently developed method based on water-sol. Sr3Al2O6 as the sacrificial buffer layer13,14 we synthesize freestanding SrTiO3 and BiFeO3 ultrathin films by reactive mol. beam epitaxy and transfer them to diverse substrates, in particular cryst. silicon wafers and holey carbon films. We find that freestanding BiFeO3 films exhibit unexpected and giant tetragonality and polarization when approaching the 2D limit. Our results demonstrate the absence of a crit. thickness for stabilizing the cryst. order in the freestanding ultrathin oxide films. The ability to synthesize and transfer cryst. freestanding perovskite films without any thickness limitation onto any desired substrate creates opportunities for research into 2D correlated phases and interfacial phenomena that have not previously been tech. possible.
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    23. 23
      Mehboudi, M.; Dorio, A. M.; Zhu, W.; van der Zande, A.; Churchill, H. O. H.; Pacheco-Sanjuan, A. A.; Harriss, E. O.; Kumar, P.; Barraza-Lopez, S. Two-Dimensional Disorder in Black Phosphorus and Monochalcogenide Monolayers. Nano Lett. 2016, 16, 1704,  DOI: 10.1021/acs.nanolett.5b04613
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      23
      Two-Dimensional Disorder in Black Phosphorus and Monochalcogenide Monolayers
      Mehboudi, Mehrshad; Dorio, Alex M.; Zhu, Wenjuan; van der Zande, Arend; Churchill, Hugh O. H.; Pacheco-Sanjuan, Alejandro A.; Harriss, Edmund O.; Kumar, Pradeep; Barraza-Lopez, Salvador
      Nano Letters (2016), 16 (3), 1704-1712CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Ridged, orthorhombic two-dimensional at. crystals with a bulk Pnma structure such as black phosphorus and monochalcogenide monolayers are an exciting and novel material platform for a host of applications. Key to their crystallinity, monolayers of these materials have a 4-fold degenerate structural ground state, and a single energy scale EC (representing the elastic energy required to switch the longer lattice vector along the x- or y-direction) dets. how disordered these monolayers are at finite temp. Disorder arises when nearest neighboring atoms become gently reassigned as the system is thermally excited beyond a crit. temp. Tc that is proportional to EC/kB. EC is tunable by chem. compn. and it leads to a classification of these materials into two categories: (i) Those for which EC ≥ kBTm, and (ii) those having kBTm > EC ≥ 0, where Tm is a given material's melting temp. Black phosphorus and SiS monolayers belong to category (i): these materials do not display an intermediate order-disorder transition and melt directly. All other monochalcogenide monolayers with EC > 0 belonging to class (ii) will undergo a two-dimensional transition prior to melting. EC/kB is slightly larger than room temp. for GeS and GeSe, and smaller than 300 K for SnS and SnSe monolayers, so that these materials transition near room temp. The onset of this generic atomistic phenomena is captured by a planar Potts model up to the order-disorder transition. The order-disorder phase transition in two dimensions described here is at the origin of the Cmcm phase being discussed within the context of bulk layered SnSe.
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    24. 24
      Hanakata, P. Z.; Carvalho, A.; Campbell, D. K.; Park, H. S. Polarization and valley switching in monolayer group-IV monochalcogenides. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 94, 035304  DOI: 10.1103/PhysRevB.94.035304
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      24
      Polarization and valley switching in monolayer group-IV monochalcogenides
      Hanakata, Paul Z.; Carvalho, Alexandra; Campbell, David K.; Park, Harold S.
      Physical Review B (2016), 94 (3), 035304/1-035304/7CODEN: PRBHB7; ISSN:2469-9950. (American Physical Society)
      Group-IV monochalcogenides are a family of two-dimensional puckered materials with an orthorhombic structure that is comprised of polar layers. In this article, we use first principles calcns. to show the multistability of monolayer SnS and GeSe, two prototype materials where the direction of the puckering can be switched by application of tensile stress or elec. field. Furthermore, the two inequivalent valleys in momentum space, which are dictated by the puckering orientation, can be excited selectively using linearly polarized light, and this provides an addnl. tool to identify the polarization direction. Our findings suggest that SnS and GeSe monolayers may have observable ferroelectricity and multistability, with potential applications in information storage.
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    25. 25
      Fei, R.; Kang, W.; Yang, L. Ferroelectricity and Phase Transitions in Monolayer Group-IV Monochalcogenides. Phys. Rev. Lett. 2016, 117, 097601  DOI: 10.1103/PhysRevLett.117.097601
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      25
      Ferroelectricity and phase transitions in monolayer group-IV monochalcogenides
      Fei, Ruixiang; Kang, Wei; Yang, Li
      Physical Review Letters (2016), 117 (9), 097601/1-097601/6CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Ferroelectricity usually fades away as materials are thinned down below a crit. value. We reveal that the unique ionic-potential anharmonicity can induce spontaneous in-plane elec. polarization and ferroelectricity in monolayer group-IV monochalcogenides MX (M = Ge, Sn; X = S, Se). An effective Hamiltonian has been successfully extd. from the parametrized energy space, making it possible to study the ferroelec. phase transitions in a single-atom layer. The ferroelectricity in these materials is found to be robust and the corresponding Curie temps. are higher than room temp., making them promising for realizing ultrathin ferroelec. devices of broad interest. We further provide the phase diagram and predict other potentially two-dimensional ferroelec. materials.
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    26. 26
      Mehboudi, M.; Fregoso, B. M.; Yang, Y.; Zhu, W.; van der Zande, A.; Ferrer, J.; Bellaiche, L.; Kumar, P.; Barraza-Lopez, S. Structural Phase Transition and Material Properties of Few-Layer Monochalcogenides. Phys. Rev. Lett. 2016, 117, 246802,  DOI: 10.1103/PhysRevLett.117.246802
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      26
      Structural phase transition and material properties of few-layer monochalcogenides
      Mehboudi, Mehrshad; Fregoso, Benjamin M.; Yang, Yurong; Zhu, Wenjuan; van der Zande, Arend; Ferrer, Jaime; Bellaiche, L.; Kumar, Pradeep; Barraza-Lopez, Salvador
      Physical Review Letters (2016), 117 (24), 246802/1-246802/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      GeSe and SnSe monochalcogenide monolayers and bilayers undergo a two-dimensional phase transition from a rectangular unit cell to a square unit cell at a crit. temp. Tc well below the m.p. Its consequences on material properties are studied within the framework of Car-Parrinello mol. dynamics and d.-functional theory. No in-gap states develop as the structural transition takes place, so that these phase-change materials remain semiconducting below and above Tc. As the in-plane lattice transforms from a rectangle into a square at Tc, the electronic, spin, optical, and piezoelec. properties dramatically depart from earlier predictions. Indeed, the Y and X points in the Brillouin zone become effectively equiv. at Tc, leading to a sym. electronic structure. The spin polarization at the conduction valley edge vanishes, and the hole cond. must display an anomalous thermal increase at Tc. The linear optical absorption band edge must change its polarization as well, making this structural and electronic evolution verifiable by optical means. Much excitement is drawn by theor. predictions of giant piezoelectricity and ferroelectricity in these materials, and we est. a pyroelec. response of about 3 × 10-12 C/K m here. These results uncover the fundamental role of temp. as a control knob for the phys. properties of few-layer group-IV monochalcogenides.
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    27. 27
      Wang, H.; Qian, X. Two-dimensional multiferroics in monolayer group IV monochalcogenides. 2D Mater. 2017, 4, 015042  DOI: 10.1088/2053-1583/4/1/015042
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    28. 28
      Wu, M.; Zeng, X. C. Intrinsic ferroelasticity and/or multiferroicity in two-dimensional phosphorene and phosphorene analogues. Nano Lett. 2016, 16, 3236,  DOI: 10.1021/acs.nanolett.6b00726
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      28
      Intrinsic Ferroelasticity and/or Multiferroicity in Two-Dimensional Phosphorene and Phosphorene Analogues
      Wu, Menghao; Zeng, Xiao Cheng
      Nano Letters (2016), 16 (5), 3236-3241CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Phosphorene and phosphorene analogs such as SnS and SnSe monolayers are promising nanoelectronic materials with desired bandgap, high carrier mobility, and anisotropic structures. Here, we show first-principles calcn. evidence that these monolayers are potentially the long-sought two-dimensional (2D) materials that can combine electronic transistor characteristic with nonvolatile memory readable/writeable capability at ambient condition. Specifically, phosphorene is predicted to be a 2D intrinsic ferroelastic material with ultrahigh reversible strain, whereas SnS, SnSe, GeS, and GeSe monolayers are multiferroic with coupled ferroelectricity and ferroelasticity. Moreover, their low-switching barriers render room-temp. nonvolatile memory accessible, and their notable structural anisotropy enables ferroelastic or ferroelec. switching readily readable via elec., thermal, optical, mech., or even spintronic detection upon the swapping of the zigzag and armchair direction. In addn., it is predicted that the GeS and GeSe monolayers as well as bulk SnS and SnSe can maintain their ferroelasticity and ferroelectricity (anti-ferroelectricity) beyond the room temp., suggesting high potential for practical device application.
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      Sakayori, K.; Matsui, Y.; Abe, H.; Nakamura, E.; Kenmoku, M.; Hara, T.; Ishikawa, D.; Kokubu, A.; Hirota, K.; Ikeda, T. Curie temperature of BaTiO3. Jpn. J. Appl. Phys. 1995, 34, 5443,  DOI: 10.1143/JJAP.34.5443
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      29
      Curie temperature of BaTiO3
      Sakayori, Ken-ichi; Matsui, Yasunori; Abe, Hiroyuki; Nakamura, Eiji; Kenmoku, Mikihiko; Hara, Tomoyuki; Ishikawa, Daisuke; Kokubu, Akihiro; Hirota, Ken-ichi; Ikeda, Takuro
      Japanese Journal of Applied Physics, Part 1: Regular Papers, Short Notes & Review Papers (1995), 34 (9B), 5443-5CODEN: JAPNDE; ISSN:0021-4922. (Japanese Journal of Applied Physics)
      The Curie point (TC) of BaTiO3 was earlier reported to be 120° and has been recently believed to be 130°. Some expts. have been performed here to reconfirm the TC. Firing conditions used for prepg. the BaTiO3 were examd. first with the use of pure BaCO3 or Ba(NO3)2 and TiO2. The x-dependence of TC in (BaO)1-x(TiO2)1+x solid soln. was measured. Data were scattered and suffered from individual variations. According to probability considerations, the TC of BaTiO3 was evaluated from the intercept at x = 0. On the other hand, the compn. dependence of TC in some related solid soln. systems, (Ba1-yPby)TiO3, (Ba1-ySry)TiO3, and (BaTiO3)1-y(KF)y, was examd., and the TC of BaTiO3 was estd. by extrapolation toward the limit y → 0. In conclusion, the Curie point of BaTiO3 is detd. as 123.0 ± 0.6°.
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      Shen, X.-W.; Fang, Y.-W.; Tian, B.-B.; Duan, C.-G. Two-dimensional ferroelectric tunnel junction: the case of monolayer In:SnSe/SnSe/Sb:SnSe homostructure. ACS Appl. Electron. Mater. 2019, 1, 1133,  DOI: 10.1021/acsaelm.9b00146
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      30
      Two-dimensional ferroelectric tunnel junction: the case of monolayer In:SnSe/SnSe/Sb:SnSe homostructure
      Shen, Xin-Wei; Fang, Yue-Wen; Tian, Bo-Bo; Duan, Chun-Gang
      ACS Applied Electronic Materials (2019), 1 (7), 1133-1140CODEN: AAEMBP; ISSN:2637-6113. (American Chemical Society)
      Ferroelec. tunnel junctions, in which ferroelec. polarization and quantum tunneling are closely coupled to induce the tunneling electroresistance (TER) effect, have attracted considerable interest due to their potential in nonvolatile and low-power consumption memory devices. The ferroelec. size effect, however, has hindered ferroelec. tunnel junctions from exhibiting a robust TER effect. Here, the study proposes doping engineering in a 2-dimensional in-plane ferroelec. semiconductor as an effective strategy to design a 2-dimensional ferroelec. tunnel junction composed of homostructural p-type semiconductor/ferroelec./n-type semiconductor. Because the in-plane polarization persists in the monolayer ferroelec. barrier, the vertical thickness of 2-dimensional ferroelec. tunnel junction can be as thin as a monolayer. The authors show that the monolayer In:SnSe/SnSe/Sb:SnSe junction provides an embodiment of this strategy. Combining d. functional theory calcns. with nonequil. Green's function formalism, they investigate the electron transport properties of In:SnSe/SnSe/Sb:SnSe and reveal a giant TER effect of 1460%. The dynamical modulation of both barrier width and barrier height during the ferroelec. switching is responsible for this giant TER effect. These findings provide an important insight into the understanding of the quantum behaviors of electrons in materials at the 2-dimensional limit and enable new possibilities for next-generation nonvolatile memory devices based on flexible 2-dimensional lateral ferroelec. tunnel junctions.
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    31. 31
      Shen, H.; Liu, J.; Chang, K.; Fu, L. In-plane ferroelectric tunneling junction. Phys. Rev. Appl. 2019, 11, 024048  DOI: 10.1103/PhysRevApplied.11.024048
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      In-Plane Ferroelectric Tunnel Junction
      Shen, Huitao; Liu, Junwei; Chang, Kai; Fu, Liang
      Physical Review Applied (2019), 11 (2), 024048CODEN: PRAHB2; ISSN:2331-7019. (American Physical Society)
      Ferroelec. materals are an important platform for the realization of nonvolatile memories. So far, existing ferroelec. memory devices have utilized out-of-plane polarization in ferroelec. thin films. In this paper, we propose a type of random-access memory (RAM) based on ferroelec. thin films with in-plane polarization, called an "in-plane ferroelec. tunnel junction." Apart from nonvolatility, lower power usage, and a faster writing operation compared with traditional dynamic RAMs, our proposal has the advantage of a faster reading operation and a nondestructive reading process, thus overcoming the write-after-read problem that exists widely in current ferroelec. RAMs. The recent discovered room-temp. ferroelec. IV-VI semiconductor thin films are a promising material platform for the realization of our proposal.
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      Xu, L.; Yang, M.; Wang, S. J.; Feng, Y. P. Electronic and optical properties of the monolayer group-IV monochalcogenides MX (M = Ge, Sn; X = S, Se, Te). Phys. Rev. B: Condens. Matter Mater. Phys. 2017, 95, 235434,  DOI: 10.1103/PhysRevB.95.235434
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      Electronic and optical properties of the monolayer group-IV monochalcogenides MX (M = Ge, Sn; X = S, Se, Te)
      Xu, Lei; Yang, Ming; Wang, Shi Jie; Feng, Yuan Ping
      Physical Review B (2017), 95 (23), 235434/1-235434/9CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      By using d.-functional theory and many-body perturbation theory based first-principles calcns., we have systematically investigated the electronic and optical properties of monolayer group-IV monochalcogenides MX (M = Ge, Sn; X = S, Se, Te). All MX monolayers are predicted to be indirect gap semiconductors, except the GeSe monolayer, which has a direct gap of 1.66 eV. The carrier mobilities of MX monolayers are estd. to be on the order of 103 to 105 cm2 V-1 s-1, which is comparable to, and in some cases higher than, that of phosphorene using a phonon-limited scattering model. Moreover, the optical spectra of MX monolayers obtained from GW-Bethe-Salpeter equation calcns. are highly orientation dependent, esp. for the GeS monolayer, suggesting their potential application as a linear polarizing filter. Our results reveal that the GeSe monolayer is an attractive candidate for optoelectronic applications as it is a semiconductor with a direct band gap, a relatively high carrier mobility, and an onset optical absorption energy in the visible light range. Finally, based on an effective-mass model with nonlocal Coulomb interaction included, we find that the excitonic effects of the GeSe monolayer can be effectively tuned by the presence of dielec. substrates. Our studies provide an improved understanding of electronic, optical, and excitonic properties of group-IV monochalcogenides monolayers and might shed light on their potential electronic and optoelectronic applications.
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      Wang, H.; Qian, X. Ferroicity-driven nonlinear photocurrent switching in time-reversal invariant ferroic materials. Sci. Adv. 2019, 5, eaav9743,  DOI: 10.1126/sciadv.aav9743
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      Absor, M. A. U.; Ishii, F. Intrinsic persistent spin helix state in two-dimensional group-IV monochalcogenide MX monolayers (M = Sn or Ge and X = S, Se, or Te). Phys. Rev. B: Condens. Matter Mater. Phys. 2019, 100, 115104,  DOI: 10.1103/PhysRevB.100.115104
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      Intrinsic persistent spin helix state in two-dimensional group-IV monochalcogenide MX monolayers (M=Sn or Ge and X=S, Se, or Te)
      Absor, Moh. Adhib Ulil; Ishii, Fumiyuki
      Physical Review B (2019), 100 (11), 115104CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      Energy-saving spintronics are believed to be implementable on systems hosting the persistent spin helix (PSH) since they support an extraordinarily long spin lifetime of carriers. However, achieving the PSH requires a unidirectional spin configuration in the momentum space, which is practically nontrivial due to the stringent conditions for fine-tuning the Rashba and Dresselhaus spin-orbit couplings. Here, we predict that the PSH can be intrinsically achieved on a two-dimensional (2D) group-IV monochalcogenide MX monolayer, a new class of the noncentrosym. 2D materials having in-plane ferroelectricity. Due to the C2v point-group symmetry in the MX monolayer, a unidirectional spin configuration is preserved in the out-of-plane direction and thus maintains the PSH that is similar to the [110] Dresselhaus model in the [110]-oriented quantum well. Our first-principle calcns. on various MX (M= Sn, Ge; X= S, Se, Te) monolayers confirmed that such typical spin configuration is obsd., in particular, at near the valence-band max. where a sizable spin splitting and a substantially small wavelength of the spin polarization are achieved. Importantly, we observe reversible out-of-plane spin orientation under opposite in-plane ferroelec. polarization, indicating that an elec. controllable PSH for spintronic applications is plausible.
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      Lee, H.; Im, J.; Jin, H. Emergence of the giant out-of-plane Rashba effect and tunable nanoscale persistent spin helix in ferroelectric SnTe thin films. Appl. Phys. Lett. 2020, 116, 022411  DOI: 10.1063/1.5137753
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      Emergence of the giant out-of-plane Rashba effect and tunable nanoscale persistent spin helix in ferroelectric SnTe thin films
      Lee, Hosik; Im, Jino; Jin, Hosub
      Applied Physics Letters (2020), 116 (2), 022411CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      A non-vanishing elec. field inside a non-centrosym. crystal transforms into a momentum-dependent magnetic field, namely, a spin-orbit field (SOF). SOFs are of great use in spintronics because they enable spin manipulation via the elec. field. At the same time, however, spintronic applications are severely limited by the SOF, as electrons traversing the SOF easily lose their spin information. Here, we propose that in-plane ferroelectricity in (001)-oriented SnTe thin films can support both elec. spin controllability and suppression of spin dephasing. The in-plane ferroelectricity produces a unidirectional out-of-plane Rashba SOF that can host a long-lived helical spin mode known as a persistent spin helix (PSH). Through direct coupling between the inversion asymmetry and the SOF, the ferroelec. switching reverses the out-of-plane Rashba SOF, giving rise to a maximally field-tunable PSH. Furthermore, the giant out-of-plane Rashba SOF seen in the SnTe thin films is linked to the nano-sized PSH, potentially reducing spintronic device sizes to the nanoscale. We combine the two ferroelec.-coupled degrees of freedom, longitudinal charge and transverse PSH, to design intersectional electro-spintronic transistors governed by non-volatile ferroelec. switching within nanoscale lateral and at.-thick vertical dimensions. (c) 2020 American Institute of Physics.
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      Sławińska, J.; Cerasoli, F. T.; Wang, H.; Postorino, S.; Supka, A.; Curtarolo, S.; Fornari, M.; Nardelli, M. B. Giant spin Hall effect in two-dimensional monochalcogenides. 2D Mater. 2019, 6, 025012  DOI: 10.1088/2053-1583/ab0146
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      Giant spin Hall effect in two-dimensional monochalcogenides
      Slawinska, Jagoda; Cerasoli, Frank T.; Wang, Haihang; Postorino, Sara; Supka, Andrew; Curtarolo, Stefano; Fornari, Marco; Nardelli, Marco Buongiorno
      2D Materials (2019), 6 (2), 25012CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
      One of the most exciting properties of two dimensional materials is their sensitivity to external tuning of the electronic properties, for example via elec. field or strain. Recently discovered analogs of phosphorene, group-IV monochalcogenides (MX with M = Ge, Sn and X = S, Se, Te), display several interesting phenomena intimately related to the in-plane strain, such as giant piezoelectricity and multiferroicity, which combine ferroelastic and ferroelec. properties. Here, using calcns. from first principles, we reveal for the first time giant intrinsic spin Hall conductivities (SHC) in these materials. In particular, we show that the SHC resonances can be easily tuned by combination of strain and doping and, in some cases, strain can be used to induce semiconductor to metal transition that makes a giant spin Hall effect possible even in absence of doping. Our results indicate a new route for the design of highly tunable spintronics devices based on two-dimensional materials.
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      Rodin, A. S.; Gomes, L. C.; Carvalho, A.; Castro Neto, A. H. Valley physics in tin (II) sulfide. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 045431  DOI: 10.1103/PhysRevB.93.045431
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      Valley physics in tin (II) sulfide
      Rodin, A. S.; Gomes, Lidia C.; Carvalho, A.; Castro Neto, A. H.
      Physical Review B (2016), 93 (4), 045431/1-045431/5CODEN: PRBHB7; ISSN:2469-9950. (American Physical Society)
      Tin (II) sulfide (SnS) is a layered mineral found in nature. In this paper, we study the two-dimensional (2D) form of this material using a combination of ab initio calcn. and k · p theory. In particular, we address the valley properties and the optical selection rules of 2D SnS. Our study reveals SnS as an extraordinary material, where there are two pairs of valleys, each placed along the two perpendicular axes, which can be selected exclusively with linearly polarized light, and can be sepd. using nonlocal elec. measurements.
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      Chang, K.; Parkin, S. S. P. Experimental formation of monolayer group-IV monochalcogenides. J. Appl. Phys. 2020, 127, 220902,  DOI: 10.1063/5.0012300
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      Experimental formation of monolayer group-IV monochalcogenides
      Chang, Kai; Parkin, Stuart S. P.
      Journal of Applied Physics (Melville, NY, United States) (2020), 127 (22), 220902CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      Monolayer group-IV monochalcogenides (MX, M = Ge, Sn, Pb; X = S, Se, Te) are a family of novel two-dimensional (2D) materials that have at. structures closely related to that of the staggered black phosphorus lattice. The structure of most monolayer MX materials exhibits a broken inversion symmetry and many of them exhibit ferroelectricity with a reversible in-plane elec. polarization. A further consequence of the noncentrosym. structure is that when coupled with strong spin-orbit coupling, many MX materials are promising for the future applications in non-linear optics, photovoltaics, spintronics, and valleytronics. Nevertheless, because of the relatively large exfoliation energy, the creation of monolayer MX materials is not easy, which hinders the integration of these materials into the fast-developing field of 2D material heterostructures. In this Perspective, we review recent developments in exptl. routes to the creation of the monolayer MX, including mol. beam epitaxy and two-step etching methods. Other approaches that could be used to prep. the monolayer MX are also discussed, such as liq. phase exfoliation and soln.-phase synthesis. A quant. comparison between these different methods is also presented. (c) 2020 American Institute of Physics.
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      Selloni, A.; Carnevali, P.; Tosatti, E.; Chen, C. D. Voltage-dependent scanning-tunneling microscopy of a crystal surface: Graphite. Phys. Rev. B: Condens. Matter Mater. Phys. 1985, 31, 2602,  DOI: 10.1103/PhysRevB.31.2602
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      Voltage-dependent scanning-tunneling microscopy of a crystal surface: graphite
      Selloni, A.; Carnevali, P.; Tosatti, E.; Chen, C. D.
      Physical Review B: Condensed Matter and Materials Physics (1985), 31 (4), 2602-5CODEN: PRBMDO; ISSN:0163-1829.
      The possible application of the scanning-tunneling microscope (STM) to surface electronic spectroscopy is discussed, with an explicit calcn. of the voltage-dependent tunneling current for an ideal STM expt. performed on graphite. A study was made on how surface and bulk electronic states are reflected in the tunneling current-voltage spectra. Empty surface states of graphite can be well discriminated against bulk-like structures by considering STM spectra at different tip-surface sepns.
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      Ukraintsev, V. A. Data evaluation technique for electron-tunneling spectroscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 53, 11176,  DOI: 10.1103/PhysRevB.53.11176
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      Data evaluation technique for electron-tunneling spectroscopy
      Ukraintsev, Vladimir A.
      Physical Review B: Condensed Matter (1996), 53 (16), 11176-85CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      A systematic study of local-d.-of-states (LDOS) deconvolution from tip-surface tunneling spectra is reported. The one-dimensional Wentzel-Kramers-Brillouin approxn. is used to simulate the process. A technique for DOS deconvolution from the electron-tunneling spectroscopy data is proposed. The differential cond. normalized to its fit to the tunneling probability function is used as a method of recovering sample DOS. This explicit procedure does not use unconstrained parameters and reveals a better DOS deconvolution in comparison with other techniques. The advantage of this method is its feasibility for extg. two important phys. parameters from exptl. tunneling spectra: (1) local surface potential, and (2) tip-sample distance. These values are the parameters used in the proposed fitting procedure. The local surface potential and the tip-sample distance retrieval are demonstrated by means of numerical simulations. Comparative scanning tunneling spectroscopy is proposed as an approach to eliminate the influence of the tip condition on the surface LDOS recovery.
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      Breusing, M.; Ropers, C.; Elsaesser, T. Ultrafast carrier dynamics in graphite. Phys. Rev. Lett. 2009, 102, 086809  DOI: 10.1103/PhysRevLett.102.086809
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      Ultrafast Carrier Dynamics in Graphite
      Breusing, Markus; Ropers, Claus; Elsaesser, Thomas
      Physical Review Letters (2009), 102 (8), 086809/1-086809/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Optical pump-probe spectroscopy with 7-fs pump pulses and a probe spectrum wider than 0.7 eV reveals the ultrafast carrier dynamics in freestanding thin graphite films. The authors discern for the 1st time a rapid intraband carrier equilibration within 30 fs, leaving the system with sepd. electron and hole chem. potentials. Phonon-mediated intraband cooling of electrons and holes occurs on a 100 fs time scale. The kinetics are in agreement with simulations based on Boltzmann equations.
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      Poudel, S. P.; Villanova, J. W.; Barraza-Lopez, S. Group-IV monochalcogenide monolayers: two-dimensional ferroelectrics with weak intralayer bonds and a phosphorenelike monolayer dissociation energy. Phys. Rev. Materials 2019, 3, 124004,  DOI: 10.1103/PhysRevMaterials.3.124004
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      Group-IV monochalcogenide monolayers: Two-dimensional ferroelectrics with weak intralayer bonds and a phosphorenelike monolayer dissociation energy
      Poudel, Shiva P.; Villanova, John W.; Barraza-Lopez, Salvador
      Physical Review Materials (2019), 3 (12), 124004CODEN: PRMHBS; ISSN:2475-9953. (American Physical Society)
      We performed d. functional theory calcns. with self-consistent van der Waals cor. exchange-correlation (XC) functionals to capture the structure of black phosphorus and twelve monochalcogenide monolayers and find the following results, which are independent of XC choice: (a) The in-plane unit cell changes its area in going from the bulk to a monolayer. Such structural behavior is unlike the one seen in more traditional two-dimensional materials such as graphene or MoS2, in which monolayers keep their structure upon exfoliation. The change of in-plane distances implies that bonds weaker than covalent or ionic ones are at work within the monolayers themselves and may require corrections beyond PBE XC. This observation is relevant for the prediction of the crit. temp. Tc and important for that reason. (b) There exists a hierarchy of independent parameters that uniquely define a ground state ferroelec. unit cell (and square and rectangular paraelec. unit cells as well): only 5 optimizable parameters are needed to establish the unit cell vectors and the four basis vectors of the ferroelec. ground state unit cell, while square and rectangular paraelec. structures are defined by only three or two independent optimizable variables, resp. (c) The reduced no. of independent structural variables correlates with larger elastic energy barriers on a rectangular paraelec. unit cell when compared to the elastic energy barrier of a square paraelec. structure. This implies that Tc obtained on a structure that keeps the lattice parameters fixed (for example, using an NVT ensemble) should be larger than the transition temp. on a structure that is allowed to change in-plane lattice vectors (for example, using the NPT ensemble). (d) Surprisingly, the dissocn. energy (bulk cleavage energy) of these materials is similar to the energy required to exfoliate graphite and MoS2. (e) There exists a linear relation among the square paraelec. unit cell lattice parameter and the lattice parameters of the rectangular ferroelec. ground state unit cell. These results highlight the subtle atomistic structure and chem. bond of these novel 2D ferroelecs.
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      Cook, A. M.; Fregoso, B. M.; de Juan, F.; Coh, S.; Moore, J. E. Design principles for shift current photovoltaics. Nat. Commun. 2017, 8, 14176,  DOI: 10.1038/ncomms14176
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      Design principles for shift current photovoltaics
      Cook, Ashley M.; Fregoso, Benjamin M.; de Juan, Fernando; Coh, Sinisa; Moore, Joel E.
      Nature Communications (2017), 8 (), 14176CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      While the basic principles of conventional solar cells are well understood, little attention has gone towards maximizing the efficiency of photovoltaic devices based on shift currents. By analyzing effective models, here we outline simple design principles for the optimization of shift currents for frequencies near the band gap. Our method allows us to express the band edge shift current in terms of a few model parameters and to show it depends explicitly on wavefunctions in addn. to std. band structure. We use our approach to identify two classes of shift current photovoltaics, ferroelec. polymer films and single-layer orthorhombic monochalcogenides such as GeS, which display the largest band edge responsivities reported so far. Moreover, exploring the parameter space of the tight-binding models that describe them we find photoresponsivities that can exceed 100 mA W-1. Our results illustrate the great potential of shift current photovoltaics to compete with conventional solar cells.
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      Kresse, G.; Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169,  DOI: 10.1103/PhysRevB.54.11169
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      Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set
      Kresse, G.; Furthmueller, J.
      Physical Review B: Condensed Matter (1996), 54 (16), 11169-11186CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      The authors present an efficient scheme for calcg. the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrixes will be discussed. This approach is stable, reliable, and minimizes the no. of order Natoms3 operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special "metric" and a special "preconditioning" optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calcns. It will be shown that the no. of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order Natoms2 scaling is found for systems contg. up to 1000 electrons. If we take into account that the no. of k points can be decreased linearly with the system size, the overall scaling can approach Natoms. They have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large no. of different systems (liq. and amorphous semiconductors, liq. simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
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      Kresse, G.; Jouber, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758,  DOI: 10.1103/PhysRevB.59.1758
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      From ultrasoft pseudopotentials to the projector augmented-wave method
      Kresse, G.; Joubert, D.
      Physical Review B: Condensed Matter and Materials Physics (1999), 59 (3), 1758-1775CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blochl's projector augmented wave (PAW) method is derived. The total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addn., crit. tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed-core all-electron methods. These tests include small mols. (H2, H2O, Li2, N2, F2, BF3, SiF4) and several bulk systems (diamond, Si, V, Li, Ca, CaF2, Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
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      Barraza-Lopez, S.; Kaloni, T. P.; Poudel, S. P.; Kumar, P. Tuning the ferroelectric-to-paraelectric transition temperature and dipole orientation of group-IV monochalcogenide monolayers. Phys. Rev. B: Condens. Matter Mater. Phys. 2018, 97, 024110  DOI: 10.1103/PhysRevB.97.024110
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      Tuning the ferroelectric-to-paraelectric transition temperature and dipole orientation of group-IV monochalcogenide monolayers
      Barraza-Lopez, Salvador; Kaloni, Thaneshwor P.; Poudel, Shiva P.; Kumar, Pradeep
      Physical Review B (2018), 97 (2), 024110CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      Coordination-related, two-dimensional (2D) structural phase transitions are a fascinating facet of two-dimensional materials with structural degeneracies. Nevertheless, a unified theor. account of these transitions remains absent, and the following points are established through ab initio mol. dynamics and 2D discrete clock models here: Group-IV monochalcogenide (GeSe, SnSe, SnTe,...) monolayers have four degenerate structural ground states, and a phase transition from a threefold coordinated onto a fivefold coordinated structure takes place at finite temp. On unstrained samples, this phase transition requires lattice parameters to evolve freely. A fundamental energy scale J permits understanding this transition, and numerical results indicate a transition temp. Tc of about 1.41J. Numerical data provides a relation among the exptl. (rhombic) parameter 〈Δα〉 and T of the form 〈Δα〉=Δα(T=0)1-T/Tcβ, with a crit. exponent β≃1/3 that coincides with expt. It is also shown that 〈Δα〉 is temp. independent in another theor. work, and thus incompatible with expt. Tc and the orientation of the in-plane intrinsic elec. dipole can be controlled by moderate uniaxial tensile strain, and a modified discrete clock model describes the transition on strained samples qual. An anal. of out-of-plane fluctuations and a discussion of the need for van der Waals corrections to describe these materials are given too. These results provide an exptl. compatible framework to understand structural phase transitions in 2D materials and their effects on material properties.
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