A Study of Thermoelectric Performance of TlGaSe₂ Layered Dichalcogenides from First‐Principles Calculations: Vacancy Defects Modeling and Engineering

Abstract

Herein, the thermoelectric performance of TlGaSe2 ternary layered dichalcogenides is evaluated by applying ab initio density functional theory calculations combined with Boltzmann's transport equation. A novel approach to design the intrinsic structural defects via Se‐anion vacancies in unit cell has been developed. Two kinds of Se‐vacancy defects in host TlGaSe2 crystal lattice are engineered: the single vacancy defect induced intrinsically in the unit cell (1×1×1) and in the supercell lattice (1×1×4). It is found that the electrical transport properties and thermoelectric efficiency of this semiconductor could be significantly altered by introducing Se‐vacancy states into crystalline structure. In addition, simulation shows that inclusion of Se‐vacancy defects significantly improves the thermoelectric efficiency as well as the thermoelectric power factor and figure of merit (ZT) values of this compound. Additionally, the thermoelectric performance of TlGaSe2 is estimated by means of the electronic fitness function calculations in the valence and conduction edges. The results demonstrate that TlGaSe2 with introduced Se‐vacancies may be a perspective material for thermoelectric applications.

Modification of the optical and elastic properties of TlGaSe2 layered semiconductor produced by the memory effect

“…2 It should be noted that chalcogen vacancies are the most common type of intrinsic defects in the layered A B C III III VI 2 dichalcogenides [31,32]. According to our ab initio calculations [31], the presence of a single Se atom vacancy in the unit cell as well as ( ) 1 1 2 ´´supercell of TlGaSe 2 could induce the formation the conduction band with lowest minimums located along the Y -Γ -X symmetric direction of TlGaSe 2 Brillouin zone.…”

“…The stoichiometric TlGaSe 2 bulk sample is a brick -red compound. The first -principles electronic band structure calculations demonstrate that the perfect monolayer of TlGaSe 2 must exhibit a direct energy band gap at G -point of the Brillouin zone [4,[31][32][33][34][35][36][37]. It is also important to mention that the experimentally evaluated band gap values for undoped TlGaSe 2 samples are large enough than expected for a brick -red colored compound.…”

“…According to our recent findings [31,58], the high quality (ideal) TlGaSe 2 single crystal is a direct bandgap semiconductor despite some debate in the previous literature [1][2][3][4][5][6]. Electronic transitions from the no degenerate upper valence -band maximum to the slightly flattened conduction band bottom located at the G -point of TlGaSe 2 Brillouin zone govern the direct optical absorption edge of the pristine TlGaSe 2 layered monocrystal [31,32,[36][37][38]. Our electronic structure calculation based on the first -principles density functional theory has demonstrated that, the vacancy type intrinsic defect in the Se atomic sites of TlGaSe 2 lattice structure can significantly alter the optical and electronic properties of TlGaSe .…”

“…It is also important to mention that the experimentally evaluated band gap values for undoped TlGaSe 2 samples are large enough than expected for a brick -red colored compound. Recently, our detailed theoretical and experimental investigations of the optical properties of TlGaSe 2 in terms of the electronic band structure have shown that the presence of crystallographic defects as intrinsic selenium vacancies can induce the appearance conduction band minimum located along the highsymmetry directions of the Brillouin zone, away from Γ -point [31,32]. In contrast to the indirect energy band gap model, we suggest that crystal imperfections related to Se vacancies can introduce the defect electronic states in the dispersion of the conduction band of TlGaSe 2 with minimum energetically located away from of Γ -point of the Se vacancy modified Brillouin zone which is looked like as an indirect band gap.…”

The Franz–Keldysh effect in the optical absorption spectrum of a TlGaSe2 layered semiconductor caused by charged native defects

“…The studies of native defects in pristine Tl-based ternary dichalcogenides, such as TlGaSe 2 layered semiconductors, have been started only recently [12,23,25,30]. According to the literature, the TlGaSe 2 layered crystal (as well as other low-dimensional transition-metal and Tl-based metal dichalcogenides) is always a p-type semiconductor with hole conduction behavior.…”