Enhancing thermoelectric properties of Janus WSSe monolayer by inducing strain mediated valley degeneracy

https://doi.org/10.1016/j.jallcom.2020.157304Get rights and content

Highlights

  • Onset of valley degeneracy with biaxial strain in WSSe at valence and conduction band edges.

  • Enhanced power factor with valley degeneracy under strain in WSSe monolayer.

  • TE efficiency of biaxial stained monolayer based device improved about 29% at 1500 K.

Abstract

The Janus WSSe monolayer is a new 2D direct band gap material attracting attention due to its unique physical and chemical properties. The lattice thermal conductivity is calculated using the self-consistent approach as well as single-mode relaxation time approximation. We notice that acoustic modes contribute mainly to the lattice thermal conductivity. Thermoelectric properties are investigated using the Seebeck coefficient, electrical conductivity, thermal conductivity, power factor, and figure of merit for pristine WSSe monolayer. The effect of strain on thermoelectric properties is also investigated. Interestingly, the tensile strain resulted in a lowering of the lattice thermal conductivity from 25.37 to 9.90 W m−1K−1. The valley degeneracy with biaxial strain in the valence and conduction band edges enhances the power factor. Thus, strain ultimately, improves the figure of merit from 0.72 (0.73) to 1.06 (1.08) under biaxial strain for n(p) carriers at 5 × 1020 cm3 and 7 × 1020 cm3, respectively. The thermoelectric efficiency of biaxial stained monolayer based device improved about 29% at 1500 K. These studies provide an avenue to engineer the thermoelectric properties using strain mediated valley degeneracy as the external stimuli.

Introduction

Thermoelectric generators are very promising with huge potential in thermal management, which can be used to convert the waste thermal heat into electrical energy. Additionally, these can be used in reverse to design thermoelectric coolers without consisting of any movable components. The developments will rely on efficient thermoelectric materials and thus attracted attention to innovate suitable materials. There are numerous materials showing the potential for thermoelectric applications. CoSb3 is one such early thermoelectric material, and various engineering approaches are used to improve its thermoelectric properties [1,2]. The performance of the thermoelectric material is evaluated using dimensionless quantity ZT = S2σT/κ, also known as a figure of merit for a thermoelectric material, where S is Seebeck coefficient, σ is electrical conductivity, κ is thermal conductivity and T is temperature. The quantity S2σ is known as the power factor, which determines the electrical performance of the material. The total thermal conductivity of the material is κ = κellat, where, κel is electronic thermal conductivity and κlat is lattice thermal conductivity. Thus, to achieve the optimal thermal performance, ZT should be the maximum, i.e., at a given temperature; the power factor should be maximum together with the minimal thermal conductivity [1]. These parameters are optimized for bulk CoSb3, and the best-reported figure of merit (ZT) is around 0.52 at 600 K for the heavily doped, i.e., n-type CoSb3 material [3]. Further, bismuth and lead based chalcogenides are widely explored thermoelectric materials, showing good thermoelectric response [4,5]. The other widely used thermoelectric material is bulk Bi2Te3, showing relatively high ZT ∼0.8 with respect to other contemporary materials. Hicks and Dresselhaus [6,7] showed the enhanced thermoelectric properties for nano Bi2Te3 with record ZT ∼ 2.4 with respect to 0.8 for bulk Bi2Te3 material. This study provided an avenue to engineer the thermoelectric properties by reducing the geometrical dimensions. Further, there are both experimental and theoretical evidences that by reducing the thickness of 2D materials, thermoelectric performance can be improved significantly [8]. Here in low dimensional materials, the enhancement in thermoelectric properties is attributed to the enhanced density of the states near the valence and conduction band edges. It increases the power factor (S2σ) for a low-dimensional material with respect to the bulk. This motivated the research community to look for nanomaterials with enhanced thermoelectric properties.

The two dimensional (2D) materials are widely explored for their unusual physical, chemical, and optoelectronic properties [9]. The TMDs based Mo(S/Se)2 and W(S/Se)2 in their bulk and monolayers are widely explored theoretically and experimentally because of their exceptional valley dependent electronic, optical, and thermoelectric properties [[10], [11], [12], [13]]. The improvement in the power factor can enhance the thermoelectric response. The electrical conductivity and Seebeck coefficient are improved by numerous ways like doping of heteroatoms [[14], [15], [16], [17]], alloying [18], distorting electronic density of states near band edges [19], strain engineering [[20], [21], [22], [23], [24]], heterostructures [25], electric field [26], and even inducing defects [27]. Among these, the valley degeneracy and heterostructures are two important ways, used in engineering the thermoelectric performance of TMDs based monolayers [28].

Recently, a derivative of Mo(S/Se)2 and W(S/Se)2 monolayers i.e., Janus (Mo/W)SSe monolayers are invented and attracting attention as a new class of 2D material where out of plane symmetry is broken intrinsically because of two different chalcogens [29]. Further, Janus MoSSe is also realized experimentally by sulfurization/selenization of MoSe2/MoS2 monolayer [30,31]. Janus WSSe system shows close resemblance in the density of states near to valence and conduction band edges with WS2/WSe2 heterostructure [32]. Janus transition metal dichalcogenides monolayers are explored for various applications like solar cell, gas sensor, water splitting, electronic devices and many more [[33], [34], [35]]. WSSe monolayer showed stability within the range of biaxial tensile (8%) to compressive (6%) strains [32]. The modulation of valleys of the valence band and conduction band edge in the band structure can improve electrical conductivity and Seebeck coefficient. Interestingly, the electronic band structure of WSSe Janus monolayer shows the valley degeneracy in the conduction band and valence band edge under biaxial strains. Strain can also reduce the lattice thermal conductivity due to enhanced scattering and results an improved the thermoelectric performance.

Interestingly, WSSe monolayer exhibits the valley degeneracy and the close resemblance with heterostructure like density of states, suggesting the possibility of enhancing the thermoelectric performance by manipulating the valley degeneracy and density of states by external stimuli such as strain. However, to the best of author’s knowledge, there is no study on the thermoelectric properties of Janus WSSe monolayer, showing the impact of strain in manipulating valley degeneracy for enhancing the thermoelectric response. This motivated us to compute the thermoelectric properties of the Janus WSSe monolayer. We also used biaxial compressive and tensile strains to understand its impact on the thermoelectric properties in terms of Seebeck coefficient, electrical conductivity, thermal conductivity, power factor, and figure of merit. We notice a large enhancement in the thermoelectric figure of merit for Janus WSSe monolayer with strain.

Section snippets

Computational details

We designed the Janus WSSe monolayer from WS2 monolayer together with a 25 Å vacuum along the z-axis by replacing the top sulfur with selenium atom and relaxed this structure without any constrain using the density functional theory (DFT) as implemented in Quantum ESPRESSO (QE) [36]. Generalized gradient approximation (GGA) together with the Perdew-Burke-Ernzerhof (PBE) approach used as an exchange-correlation functional [37]. The ultrasoft pseudopotentials are used for core-shell potentials.

Thermoelectric properties of unstrained monolayer

The optimized structure of the pristine WSSe monolayer with detailed optoelectronic properties are described by Chaurasiya et al. [32,42]. We have computed the phonon band dispersion along with the high symmetry Γ-Μ-Κ-Γ points of BZ, showing nine vibrational modes. The first three vibrational modes, at the Γ point of BZ, correspond to acoustic mode, whereas the other six vibrational modes are the optical modes. All the acoustic and optical modes show real frequencies values, confirming the

Conclusion

We have systematically investigated the thermoelectric properties in terms of Seebeck coefficient, electrical conductivity, electrical thermal conductivity, lattice thermal conductivity, power factor, and figure of merit for unstrained Janus WSSe monolayer. The lattice thermal conductivity reduces with increasing the temperature. The thermoelectric response of the unstrained Janus WSSe monolayer is found close to that of WS2 and WSe2 monolayers. The thermodynamic stability of biaxial strained

CRediT authorship contribution statement

Rajneesh Chaurasiya: Methodology, and computations, draft preparation. Shubham Tyagi: Computations and initial draft preparation. Nirpendra Singh: Writing - review & editing, Validation. Sushil Auluck: Writing - review & editing, Validation. Ambesh Dixit: Conceptualization, Supervision, Manuscript, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Rajneesh Chaurasiya and Shubham Tyagi contributed equally in the manuscript. Authors acknowledge Mr. Abhijeet J. Kale, Mr. Sumit Kukreti, Mr. Chandra Prakash and Mr. Kuldeep Kaswan for useful discussions. Author Ambesh Dixit acknowledges Department of Science and Technology, Government of India, through project DST/INT/Mexico/P-02/2016 and IIT Jodhpur for providing computational resources for this work.

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