[Paper Review] Doping effect on thermoelectric properties of MoS$_2$

Huaihong Guo, Teng Yang+2 more|arXiv (Cornell University)|Dec 14, 2012

Advanced Thermoelectric Materials and Devices1 references3 citations

TL;DR

This study uses first-principles calculations and Boltzmann transport theory to investigate the thermoelectric properties of MoS₂ under p-type doping. It identifies an optimal doping level of ~10¹⁹ cm⁻³, achieving a ZT of 0.3 at 700 K in the in-plane direction, with strong anisotropy in electrical and electronic thermal conductivity but negligible anisotropy in thermopower, favoring in-plane applications.

ABSTRACT

We systematically study thermoelectric properties of layered MoS$_2$ by doping, based on Boltzmann transport theory and first-principles calculations. We obtain optimal doping region (around 10$^{19}$ cm$^{-3}$) by looking closely to the temperature and doping level dependent thermopower, electrical conductivity, power factor (PF) and ultimately figure of merit (ZT) coefficient along in-plane and cross-plane directions. MoS$_2$ has a vanishingly small anisotropy of thermopower but a big anisotropy of electrical conductivity and electronic thermal conductivity in optimal doping region. $κ_e$ is comparable to $κ_l$ in the plane while $κ_l$ dominates over $κ_e$ across the plane. ZT can reach as high as 0.3 at around 700 K. In-plane direction is demonstrated to be more preferable for thermoelectric applications of MoS$_2$ by doping.

Motivation & Objective

• To systematically investigate the thermoelectric performance of MoS₂ under p-type doping using first-principles and Boltzmann transport theory.
• To identify the optimal doping level and temperature range that maximize the figure of merit ZT.
• To analyze the anisotropic transport behavior of thermopower, electrical conductivity, and electronic thermal conductivity along in-plane and cross-plane directions.
• To evaluate the role of electronic scattering time anisotropy in determining directional differences in transport properties.
• To assess the potential for enhancing ZT by reducing lattice thermal conductivity through microstructural engineering such as random stacking.

Proposed method

• Ab initio electronic band structure calculations using the WIEN2K package with the generalized gradient approximation (GGA-PBE) for exchange-correlation functional.
• Boltzmann transport theory implemented via the BOLTZTRAP code to compute thermopower, electrical conductivity, power factor, and electronic thermal conductivity.
• Assumption of energy-independent electron scattering time to avoid adjustable parameters in transport calculations.
• Use of experimental lattice thermal conductivity data (κₗ) from Kim et al. [29], fitted as κ = 183.103/T + 0.412671, to estimate total thermal conductivity.
• Calculation of ZT using κₑ from the Wiedemann-Franz law and κₗ from experimental fit, with ZT = (S²σT)/κ.
• Systematic variation of hole doping from 10¹⁵ to 10²⁰ cm⁻³ and temperature from 100 K to 700 K to map ZT and transport functions.

Experimental results

Research questions

• RQ1 What is the optimal doping level for maximizing the thermoelectric figure of merit ZT in MoS₂?
• RQ2 How does the directional anisotropy of thermopower, electrical conductivity, and electronic thermal conductivity vary with doping and temperature?
• RQ3 What is the relative contribution of electronic and lattice thermal conductivity in the in-plane and cross-plane directions?
• RQ4 To what extent does the anisotropy in electron scattering time influence the directional transport properties?
• RQ5 Can the ZT of MoS₂ be enhanced beyond 0.3 through microstructural engineering such as random stacking to reduce lattice thermal conductivity?

Key findings

• The thermopower exceeds 200 μV/K over a wide doping range (10¹⁷–10²⁰ cm⁻³) and shows negligible anisotropy between in-plane and cross-plane directions at doping >10¹⁷ cm⁻³.
• Electrical conductivity and electronic thermal conductivity exhibit strong anisotropy, with σₓₓ and κₑˣˣ being two orders of magnitude larger than σ_zz and κₑzz due to anisotropic scattering time.
• In the in-plane direction, electronic thermal conductivity (κₑˣˣ) is comparable to lattice thermal conductivity (κₗˣˣ), while in the cross-plane direction, κₗᶻᶻ dominates over κₑᶻᶻ.
• The maximum ZT reaches 0.3 at 700 K for in-plane transport at a carrier density of ~10¹⁹ cm⁻³, with ZT remaining saturated at this value.
• The in-plane direction is significantly more favorable for thermoelectric applications than the cross-plane direction due to higher power factor and ZT.
• Further reduction of lattice thermal conductivity via microstructural control (e.g., random stacking) could potentially increase ZT beyond 0.3, though this requires validation of its impact on electrical transport.