Control of valley degeneracy inMoS2by layer thickness and electric field and its effect on thermoelectric properties

Abstract

We have investigated the valley degeneracy of MoS 2 multilayers and its effect on thermoelectric properties. By modulating the layer thickness and external electric field, the hole valleys at Γ and K points in the highest energy valence band and the electron valleys at K and Σ min points in the lowest energy conduction band are shifted differently. The hole valley degeneracy is observed in MoS 2 monolayer, while that of electron valley is in MoS 2 bilayer and monolayer under the external electric field. By tuning the valley degeneracy, the Seebeck coefficient and electrical conductivity can be separately controlled, and the maximum power factor can be obtained in n-type (p-type) MoS 2 monolayer with (without) the external electric field. We suggest that the transition metal dichalcogenides are good example to investigate the role of valley degeneracy in the thermoelectric and optical properties with the control of interlayer interaction and external electric field strength.

Enhanced thermoelectric power in two-dimensional transition metal dichalcogenide monolayers

“…A significant enhancement in the 2D monolayer forms results in a power factor that is one order of magnitude larger than that in the 3D bulk forms. theoretical predictions [19][20][21]. These results directly clarify the low-dimensional effect in layered TMDCs, opening up a route for high-performance thermoelectric applications.…”

“…It is already established that 2D materials exhibit higher thermoelectric properties than those of three-dimensional (3D) bulks because, according to the Mott equation, S is proportional to the energy derivative of the DOS at around E F , and the stairlike energy dependence of the DOS of 2D materials leads to a larger S. This motivated our study in ideal quantum wells of TMDC monolayers [5][6][7][8][9]. In particular, several theoretical studies predict the enhancement of thermoelectric properties in TMDCs as decreasing layer thickness [19][20][21]. Owing to the increased valley degeneracy at the band edge, mono-and/or few-layer TMDCs show significantly increased σ , resulting in the enhancement of the power factor at |n 2D | up to 1.0 × 10 13 cm −2 .…”

“…(iv) Most importantly, although the previous studies reported the high thermoelectric properties, the experimental evaluation of the enhancement still remains a challenge. Very recently, several theoretical studies predicted that high thermoelectric performance can be achieved in ultrathin TMDCs, which encourage the enhancement due to the low-dimensional effect [19][20][21].…”

Recent Progress of Two-Dimensional Thermoelectric Materials

“…These quantities have been widely studied in the case of 2D materials. Owing to their characteristic quasi-two-dimensional crystal structures, TMDCs exhibit high Seebeck coefficient [71][72][73]. By considering MoS 2 as a reference compound, a brief introduction about the thermoelectric properties of 2D TMDCs is presented below.…”

Pressure-induced enhancement in the thermoelectric properties of monolayer and bilayer SnSe 2

“…As for layered transition-metal dichalcogenides, the figure of merit of these compounds is limited by the moderate PF due to their intrinsically low electronic conductivity [ 21 ], suggesting that a strategy to enhance thermoelectric performance ZT of layered transition-metal dichalcogenides is to increase their electrical transport properties. Recently, it has been found that higher electrical conductivity can be obtained for the monolayer and bilayer transition metal dichalcogenides when compared with bulk materials, and such phenomenon can be explained by the valley degeneracy at the band edge [ 21 ]. On the other hand, the thermoelectric properties of n -type layered transition-metal dichalcogenides can be enhanced under either normal compressive strain or biaxial compressive strain [ 22 ].…”