$\Gamma$-phonons in ZnSe(C$_{2}$H$_{8}$N$_{2}$)$_{1/2}$ and ZnTe(C$_{2}$H$_{8}$N$_{2}$)$_{1/2}$ hybrid materials

Artificially synthesized organic/inorganic hybrid materials with semiconductor component are of growing interest due to potential applications in flexible microelectronics. Among them, hybrid ZnSe(C$_{2}$H$_{8}$N$_{2}$)$_{1/2}$ and ZnTe(C$_{2}$H$_{8}$N$_{2}$)$_{1/2}$ are two examples capable of tuning electronic and optical properties on a wide range. Engineering of these materials is difficult without deeper understanding of their fundamental physical characteristics, including electronic band structure and phonon spectra. Recent experimental investigations [1] performed on $\beta$--ZnTe(C$_{2}$H$_{8}$N$_{2}$)$_{1/2}$ samples and showed multiple sharp phonon modes in frequencies very different from the LO($\Gamma$) phonon of the binary semiconductor ZnTe. Motivated by these results, we have performed density-functional calculations of the $\Gamma$ phonon modes for both $\alpha$-- and $\beta$--phases of ZnSe(C$_{2}$H$_{8}$N$_{2}$)$_{1/2}$ and ZnTe(C$_{2}$H$_{8}$N$_{2}$)$_{1/2}$. We found, in particular, that light hydrogen atoms not only define the high frequency motion (1500--3000 cm$^{-1}$), but also dominate in some low frequency modes in the region of 100--150 cm$^{-1}$ where they move mostly perpendicular to the superlattice stacking direction. Based on symmetry of the materials, the Raman modes at the $\Gamma$ point are identified. \newline [1] X. Huang, J. Li, Y. Zhang, and A. Mascarenhas, J. Am. Chem. Soc. {\bf 125}, 7049 (2003).