Strain-induced electronic phase transition and strong enhancement of thermopower ofTiS2

Abstract

Using first principles density functional theory calculations, we show a semimetal to semiconducting electronic phase transition for bulk TiS2 by applying uniform biaxial tensile strain. This electronic phase transition is triggered by charge transfer from Ti to S, which eventually reduces the overlap between Ti-(d) and S-(p) orbitals. The electronic transport calculations show a large anisotropy in electrical conductivity and thermopower, which is due to the difference in the effective masses along the in-plane and out of plane directions. Strain induced opening of band gap together with changes in dispersion of bands lead to three-fold enhancement in thermopower for both p-and n-type TiS2. We further demonstrate that the uniform tensile strain, which enhances the thermoelectric performance, can be achieved by doping TiS2 with larger iso-electronic elements such as Zr or Hf at Ti sites. [7]. Among the TMDs, TiS 2 has been in focus of extensive research due to its potential applications as cathode materials for lithium-ion batteries [8][9][10][11]. Previous experimental studies [12] have reported that nearly stoichiometric TiS 2 shows a large power factor value of 37.1 µW/K 2 -cm at room temperature that is comparable with the best thermoelectric material Bi 2 Te 3 [13]. The large power factor originates from a sharp increase in the density of states just above the Fermi energy as well as the inter-valley scattering of charge carriers [12,14,15]. However, the semimetallic nature of TiS 2 gives rise to bipolar effects which are not desirable for thermoelectric applications [16].The electronic structure of TiS 2 is very unique and even a slight change can significantly influence thermoelectric properties. It has been shown that 0.04% Mg doping at Ti-site in TiS 2 causes a 1.6 times increase in thermopower at 300 K [17]. Recently, it has been discovered that the electronic structures of semiconducting TMDs (MX 2 (M = Mo, W; X=S, Se, Te)) are very sensitive to the applied pressure/strain, which causes an electronic phase transition from semiconductor to metal [18][19][20][21]. Also few layers and mono-layer TMDs show wide range of tunability in electronic and magnetic properties by application of strain [22][23][24][25][26]. Contrary to that, TiS 2 remains semimetallic up to a compressive hydrostatic pressure of 20 GPa [27]. So far there has been no studies on effect of uniform tensile strain on the electronic properties of TiS 2 . Here, we carry out a study of the electronic structure of TiS 2 as a function of applied uniform biaxial tensile strain (BTS). The material undergoes electronic phase transition from semimetal to a small band gap (< 0.15 eV) semiconductor. Most interestingly, the semiconducting strained TiS 2 exhibits nearly a four-fold enhancement in thermopower compared to the unstrained phases. We also explore the possibility of generating such a strain by doping with larger atoms at Ti sites. Iso-electronic Hf and Zr turn out to be the best dopants to generate the 2% tensile strain producing the same...

Designing high-performance layered thermoelectric materials through orbital engineering

“…All of these compounds possess crystal field split p orbital characteristics at the band edges, similar to the CaAl 2 Si 2 -type Zintl compounds. The results of biaxial strain engineering in chalcopyrites, metal dichalcogenides and lithium intercalated metal dichalcogenides from references (CuGaTe 2 and TiS 2 )3839 and our own data (ZrS 2 and LiZrSe 2 ; see Supplementary Fig. 13 for details) confirm orbital engineering.…”

High potential thermoelectric figure of merit in ternary La3Cu3X4 (X = P, As, Sb and Bi) compounds

“…Also the dispersion of these first three bands near VBM is dramatically different; two bands being relatively heavy and one band being highly dispersive (nearly parabolic). Usually the lower splitting and a combination of heavy and light bands results in higher thermoelectric performance 14,22,23,57 . This suggests that the thermoelectric performance of La 3 Cu 3 P 4 will be higher than that of the other compounds.…”

“…Additionally nanostructuring, which reduces the lattice thermal conductivity via enhanced phonon scattering can provide a route to achieve high ZT . Various theoretical and experimental studies have discussed the role of mass anisotropy 14,16,18 , quantum confinement 19,20 , and band structure engineering 14,21–23 in designing materials with enhanced thermoelectric properties.…”

Strategies on Phase Control in Transition Metal Dichalcogenides

“…Finally, we would like to point out that although here we elaborate the phase transition largely based on group‐6 TMDs, the same mechanisms actually can be adopted to understand the phase transition in other groups of TMDs …”