Theoretical investigation of 2D MXenes for thermoelectric application

Abstract

The increasing demand for sustainable, eco-friendly, affordable, and renewable energy resources has become a primary focus for researchers to overcome the energy requirements of society. A significant portion of the energy is dissipated as thermal energy in industrial and household operations, and thermoelectric materials can convert the waste heat into electricity and vice-versa. The performance of thermoelectric materials, determined by the figure of merit (ZT), depends on their electrical and thermal transport properties. A new class of 2D materials known as MXenes with the general formula Mn+1XnTx [where M represents early transition metals such as Sc, Zr, Hf, Mo, Ta, Ti, Hf, V, Nb; X is C and/or N and Tx are surface terminating groups such as O, OH, F, S, Se, Te attracted significant attention for the renewable energy applications due to their high electrical conductivity, excellent structural and chemical stabilities. This thesis investigates the thermoelectric performance of 2D MXenes by considering electron and phonon transport, combining Density functional theory (DFT) with a semi-classical model based on the Boltzmann transport equation (BTE). We have investigated the thermoelectric performance of Janus monolayer MoWCO2, observing a significant impact of surface scattering on phonon transport. Surface scattering reduced the lattice thermal conductivity (κl) by approximately 80% for a ribbon width (L) of 1µm, while the electrical conductivity remains unchanged at room temperature. This reduction in thermal conductivity leads to the enhancement of ZT to 0.33 for p-type and 0.08 for n-type at 700 K for L=10 nm, compared to 0.04 for p-type and 0.01 for n-type. Furthermore, the mechanical and piezoelectric coefficients of MoWCO2 were determined, finding Young's modulus of 244 N/m and Poisson's ratio of 0.55, indicating the material's ability to deform under small strain. The in-plane piezoelectric coefficients, e11 = 268 pC/m and d11= 1.6 pm/V, suggest that MoWCO2 is suitable for wearable thermoelectric devices and sensor applications. Extensive research has been conducted to explore the thermoelectric properties of MXenes functionalized with O, F, and OH. Additionally, MXenes terminated with Cl, Br, S, Se, and Te have been synthesized experimentally. Hence, investigating the thermoelectric properties of MXenes terminated with these and other functional groups apart from F, O, and OH could provide intriguing insights. In this study, we have explored the thermoelectric properties of Janus monolayer Zr2COS and Hf2COS, and findings suggest that optimizing thermoelectric properties is more effective through n-type doping compared to p-type. The lattice thermal conductivity (κl) values of Zr2COS and Hf2COS are obtained to be 21.8 W/m K and 27 W/m K, respectively, at 300K, and the values of κl are significantly lower than those of oxygen-functionalized MXenes Zr2CO2 (61.9 W/m K) and Hf2CO2 (86.3 W/m K). The projected ZT value can reach 0.27 and 0.23 at 700K for n-type Zr2COS and Hf2COS, respectively. These findings reveal the promising application prospects of n-type Zr2COS and Hf2COS in the field of thermoelectrics. Various studies have shown that the transport properties of materials can be tuned by applying strain. Previous research has focused on the electronic properties of monolayer Lu CF under strain, ₂ C F ₂

u n d e r

s t r a i n , ₂ C F ₂

u n d e r

s t r a i n , highlighting its high carrier mobility and stability under biaxial strain. The thermoelectric performance of a monolayer Lu CF under biaxial tensile strain has been investigated. Applying ₂ C F ₂

u n d e r

s t r a i n , ₂ C F ₂

u n d e r

s t r a i n , ₂ C F ₂

u n d e r

s t r a i n , biaxial strain enhances the electron-phonon relaxation time, leading to elevated electrical conductivity and consequently increasing the thermoelectric power factor (PF). Specifically, the PF for n-type Lu CF triples under a 4% biaxial tensile strain at 700 K, while for p-type, it becomes ₂ C F ₂

u n d e r

s t r a i n , more than double. The lattice thermal conductivity (κl) also decreases under tensile strain. At 300 K, κl drops to 47 W/m·K, 24.4 W/m·K, and 16.2 W/m·K for tensile strains of 2%, 4%, and 6%, respectively, from 88 W/m·K, showing an approximate 81.5% reduction at 6% tensile strain. As a result, the ZT of Lu CF becomes approximately ten times, rising from 0.07 to 0.68 for n-type under ₂ C F ₂

u n d e r

s t r a i n , ₂ C F ₂

u n d e r

s t r a i n , 4% biaxial strain and from 0.06 to 0.63 for p-type under 6% biaxial strain, indicating that biaxial tensile strain can significantly enhance the thermoelectric efficiency of Lu CF monolayer. ₂ C F ₂

u n d e r

s t r a i n , ₂ C F ₂

u n d e r

s t r a in,This thesis theoretically investigates the thermoelectric performance of 2D MXenes, demonstrating their promising potential in the field of thermoelectrics. This highlights the need for more experimental studies of MXenes for thermoelectric application