Anisotropic to Isotropic Transition in Monolayer Group-IV Tellurides

Abstract

Monolayer group-IV tellurides with phosphorene-derived structures are attracting increasing research interest because of their unique properties. Here, we systematically studied the quasiparticle electronic and optical properties of two-dimensional group-IV tellurides (SiTe, GeTe, SnTe, PbTe) using the GW and Bethe–Salpeter equation method. The calculations revealed that all group-IV tellurides are indirect bandgap semiconductors except for monolayer PbTe with a direct gap of 1.742 eV, while all of them are predicted to have prominent carrier transport ability. We further found that the excitonic effect has a significant impact on the optical properties for monolayer group-IV tellurides, and the predicted exciton binding energy is up to 0.598 eV for SiTe. Interestingly, the physical properties of monolayer group-IV tellurides were subject to an increasingly isotropic trend: from SiTe to PbTe, the differences of the calculated quasiparticle band gap, optical gap, and further exciton binding energy along different directions tended to decrease. We demonstrated that these anisotropic electronic and optical properties originate from the structural anisotropy, which in turn is the result of Coulomb repulsion between non-bonding electron pairs. Our theoretical results provide a deeper understanding of the anisotropic properties of group-IV telluride monolayers.

Chemical Reactions and Phase Stabilities in the Si–Te System at High Pressures and High Temperatures

“…We found no evidence for the formation of any phases of SiTe and Si 2 Te 3 reported earlier from theoretical predictions. 5,[10][11][12]14 The F-centered clathrate Te 7+x Si 20−x (cF) was identified for the compositional mix of Si 38 /Te 16 , where it formed simultaneously with the Te 8 @(Si 38 Te 8 )-type clathrate after the hexagonal phase disappeared (Figure S2). At higher temperatures, the Te 8 @(Si 38 Te 8 )-type clathrate is favored and is retained on cooling.…”

“…11 Altogether, the bulk IV−VI compounds have structures derived from a black phosphorene, which is a distorted variant of the NaCl structure. 12 Quantum-chemical calculations suggested a plausible synthetic approach to silicon monotelluride since silicon can be incorporated into the crystal structure of Si 2 Te 3 to yield SiTe at pressures of about 7 GPa. 5 Several candidates for the hypothetical high-pressure phase of SiTe were proposed and their formation enthalpies were obtained from density functional theory (DFT) calculations.…”

Electronic, Optical, and Thermoelectric Properties of Bulk and Monolayer Germanium Tellurides

“…All these GeTe phases are dynamically stable according to the literature, e.g., Reference [18] for cubic and rhombohedral GeTe, Reference [19] for puckered GeTe, and Reference [20] for buckled GeTe. There have also been earlier DFT calculations for the electronic band structures of GeTe monolayers in the puckered lattice [19,[21][22][23] and buckled lattice [23,24], but the results seem to be inconsistent with a recent experiment [25]. In Reference [25], Zhang et al only observed buckled GeTe thin films in argon and oxygen environments with band gaps of about 1.89-2.08 eV, while References [23,24] obtained band gaps of about 2.35 eV.…”

“…However, since we are interested in the qualitative comparison between optical spectra of the bulk and monolayer GeTe to judge their possible absorption performance, it is sufficient to consider the independent particle and dipole approximation. For GeTe, the relative absorption coefficients calculated using this approach should not be much different from that calculated with the GW method [22].…”

“…In other words, the above formulation is equivalent to calculating one third of each trace of S, σ, and κ e tensors. We noticed that GeTe may possess small anisotropic transport features [9,21,22], where a particular transport axis gives slightly larger coefficients than the others; however, we preferred to focus our attention to the average comparison of the thermoelectric properties of bulk and monolayer GeTe at certain temperatures to identify which form of GeTe has the best thermoelectric performance.…”