Anisotropic structural, vibrational, electronic, optical, and elastic properties of single-layer hafnium pentatelluride: an <i>ab initio</i> study

Abstract

Motivated by the highly anisotropic nature of bulk hafnium pentatelluride (HfTe<INF>5</INF>), the structural, vibrational, electronic, optical, and elastic properties of single-layer two-dimensional (2D) HfTe<INF>5</INF> were investigated by performing density functional theory (DFT)-based first-principles calculations. Total energy and geometry optimizations reveal that the 2D single-layer form of HfTe<INF>5</INF> exhibits in-plane anisotropy. The phonon band structure shows dynamic stability of the free-standing layer and the predicted Raman spectrum displays seven characteristic Raman-active phonon peaks. In addition to its dynamic stability, HfTe<INF>5</INF> is shown to exhibit thermal stability at room temperature, as confirmed by quantum molecular dynamics simulations. Moreover, the obtained elastic stiffness tensor elements indicate the mechanical stability of HfTe<INF>5</INF> with its orientation-dependent soft nature. The electronic band structure calculations show the indirect-gap semiconducting behavior of HfTe<INF>5</INF> with a narrow electronic band gap energy. The optical properties of HfTe<INF>5</INF>, in terms of its imaginary dielectric function, absorption coefficient, reflectance, and transmittance, are shown to exhibit strong in-plane anisotropy. Furthermore, structural analysis of several point defects and their oxidized structures was performed by means of simulated STM images. Among the considered vacancy defects, namely , , V<INF>Te<INF>out</INF></INF>, V<INF>Te<INF>in</INF></INF>, , and V<INF>Hf</INF>, the formation of V<INF>Te<INF>out</INF></INF> is revealed to be the most favorable defect. While and V<INF>Hf</INF> defects lead to local magnetism, only the oxygen-substituted V<INF>Hf</INF> structure possesses magnetism among the oxidized defects. Moreover, it is found that all the bare and oxidized vacant sites can be distinguished from each other through the STM images. Overall, our study indicates not only the fundamental anisotropic features of single-layer HfTe<INF>5</INF>, but also shows the signatures of feasible point defects and their oxidized structures, which may be useful for future experiments on 2D HfTe<INF>5</INF>.