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

Abstract

Electronic, optical, and thermoelectric properties of germanium tellurides (GeTe) were investigated through a series of first-principles calculations of band structures, absorption coefficients, and thermoelectric transport coefficients. We consider bulk GeTe to consist of cubic and rhombohedral phases, while the two-dimensional (2D) GeTe monolayers can form as a 2D puckered or buckled honeycomb crystals. All of the GeTe variants in the bulk and monolayer shapes are excellent light absorbers in a wide frequency range: (1) bulk cubic GeTe in the near-infrared regime, (2) bulk rhombohedral GeTe and puckered monolayer GeTe in the visible-light regime, and (3) buckled monolayer GeTe in the ultraviolet regime. We also found specifically that the buckled monolayer GeTe exhibits remarkable thermoelectric performance compared to the other GeTe phases due to a combination of electronic band convergence, a moderately wide band gap, and unique 2D density of states from the quantum confinement effect.

The thermoelectric properties of XTe (X = Ge, Sn and Pb) monolayers from first-principles calculations

“…For example, Zhang et al reported that puckered GeTe monolayer can reach a ZT value of 4.23 at 900 K, which is much larger compared to bulk GeTe [29]. Sinambela et al showed a ZT value of 1.6 for buckled GeTe monolayer at 500 K [30], and the ZT value of n-type PbTe monolayer can reach up to 1.96 at 700 K [31].…”

Thermoelectric properties of monolayer and bilayer buckled XTe (X = Ge, Sn, and Pb)

“…2.7, while bilayer PbTe has ZT e of 0.5. In the case of twodimensional buckled GeTe, although the maximum ZT e of bilayer GeTe is smaller than that of monolayer GeTe, bilayer GeTe still gives better TE performance than bulk counterpart [28]. In short, it is strongly attributed to the quantum confinement effect [10].…”

“…For a piece of additional evidence to the enhancement of ZT e , the confinement length of low-dimensional materials must be smaller than thermal wavelength Λ of bulk materials [7]. The confinement lengths of monolayer (L=1.56Å) and bilayer (L≈7Å) GeTe are obtained to be smaller than thermal wavelength Λ of the bulk counterparts (36Λ215Å for cubic GeTe and 41Λ215Å for rhombohedral GeTe [28]). It is expected that similar conditions exist for the enhancement in ZT e for SnTe and PbTe materials.…”

“…puckered and buckled structures. Here, we focus on the buckled structure since (1) it should be thinner than the puckered counterpart, (2) it should be naturally stable at high temperature, and (3) buckled monolayer XTe is expected to have a better TE performance than that of puckered counterpart as inspired from other theoretical works in antimonene [22] and monolayer GeTe [28]. We perform comprehensive calculations on the electronics and thermoelectric properties of group-IV XTe (X=Ge, Sn, and Pb) by using the density functional theory (DFT) and the linearised Boltzmann transport equation (BTE) with a constant relaxation time approximation (CRTA).…”

Advances in flexible inorganic thermoelectrics

“…Monolayer 2D thermoelectric materials generally exhibit unique thermoelectric properties due to their quantum confinement effects. [199][200][201][202][203][204] Although there is still a significant gap between the theory and practical applications of monolayer 2D thermoelectric materials, theoretical calculations enable us to study the structural and performance evolution of these materials under certain strains along specific directions. This knowledge can guide the preparation and structural optimization of actual materials.…”