Chai, X. D., Tian, K., Chen, H. J., Tang, X. Y., Li, T. J., Zhu, Z. Q., et al. (1989). X-ray photoelectron spectroscopy study of lipid-free multilayer systems of copper phthalocyanine tetrasulphonate. Thin Solid Films, 178, 221-225. doi:10.1016/0040-6090(89)90306-4.