Electronic and optical properties of zinc based hybrid organic-inorganic compounds

There is significant interest in hybrid organic-inorganic (HOI) compounds since these materials offer multiple functionalities and properties that can be tailored at the mesoscopic and nanoscale levels. HOIs investigated for photovoltaic applications typically contain lead or mercury. There is considerably less work done on Zn-based HOIs. These could potentially be considered in biomedical applications due to presence of organic components and the biocompatibility of Zn cations. Using a systematic materials selection approach, we have carried out a detailed search of Zn-HOI compounds in two comprehensive experimental crystallographic repositories: Inorganic Crystal Structure Database and American Mineralogist Crystal Structure Database. Thirteen Zn-HOI compounds are discovered: CuZnO2(CO3), Zn(C2O4), ((CH3)(2)NH2)Zn-3(PO4)(HPO4)(2), (CH3NH3)Zn-4(PO4)(3), Zn(N(CH2PO3H)(3))(H2O)(3), (CH3NH3)Zn(HCO2)(3), Zn-4(CO3)(2), Zn-8(HPO4)(16)(C2H8N)(8), Zn-5(CO3)(2), (Mg2Zn)(8)(CO3)(2)(OH), Zn-7(CO3)(2)(OH)(10), Ca3Zn2(PO4)CO3(OH).2H(2)O, and Zn(CO3). We have then performed first principles calculations via density functional theory with hybrid functional treatment to determine the electronic band gap and optical response of these materials. Our computations show that eleven of the thirteen compounds have insulating properties with band gaps ranging from 2.8 eV to 6.9 eV. Ten of these are found to have a high absorbance in the far ultra-violet (FUV) region of 200-112 nm wavelength. For example, the absorption coefficient of (CH3NH3)Zn(HCO2)(3) is similar to 0.75 x 10(5) cm(-1) for F-2 excimer laser energy (wavelength similar to 157 nm) which is more than three orders higher than the average tissue absorbance (similar to 10(1.5) cm(-1)) and the refractive index of 1.85 is larger than typical biological matter which is in the range 1.36-1.49. These results suggest that Zn-HOIs could potentially find applications in photothermolysis and UV protection.