A Computational Study of 2D Materials: Thermoelectric and Magnetic Properties

Abstract

This thesis is a collection of works aimed at exploring the physical properties of novel two-dimensional (2D) materials using advanced first-principles calculations in the framework of density functional theory (DFT) for environmental and energy conversion (thermoelectric energy), spintronics, and valleytronics applications. This work includes two sections of introduction and methodology and three sections outlining novel results. In the first part, we place the present work in the context of earlier studies of 2D materials. Then, we tackled the used theoretical approaches including the first principle theory and DFT tool as well as the Wannier functions for calculating maximally-localised Wannier functions (MLWFs) from a set of Bloch states. Also, the transport properties through the Boltzmann transport theory were described for the transport properties calculations. In the results part (sections: 3, 4, and 5), we presented the physical properties of several newly discovered or proposed 2D materials. First, we addressed the flexibility features of the 2D graphene-based material for wearable next-generation thermoelectric applications, where we found that the ZnO/graphene heterostructure displayed the highest ZT ~ 2.4 in the n-doped ZnO/graphene heterostructure at 500 K. Our findings may stimulate further studies to confirm our results as well as the development of flexible TE generators based on graphene for the Internet of Things (IoT) thermoelectric applications. Also, we investigated the thermoelectric (TE) performance of the GaSe0.5Te0.5 alloy monolayer. The n-type system showed better TE performance than the p-type system. For instance, the n-type maximum ZT was 2.05 at 700 K, and this high ZT had weak carrier concentration dependence. For instance, it was changed by only ~ 10% in the range of carrier concentration of 1 ~ 10 x 1019 hole/cm3. We proposed that the n-type GaSe0.5Te0.5 alloy shows outstanding thermoelectric performance, and this result will provoke further experimental investigations to verify our prediction. Then, we investigated the spin dependent Seebeck effect (SSE), where the SSE is a key factor in the spin caloritronics field. Despite many studies of the SSE in bulk materials, investigations on pure two-dimensional ferromagnetic materials are scarce. Therefore, we here investigated the SSE of the ferromagnetic CrI3 monolayer using the Boltzmann transport approach allowing diffusive scattering. We obtained a giant effective spin Seebeck effect of 1450 μV/K, and this value is at least 4 ~ 5 times larger than previously reported values in bulk systems. Moreover, the generation of the transverse electric current by a longitudinal charge or heat current is receiving extensive research efforts because of its potential applications in information-processing devices. Therefore, we investigated the electric field dependent anomalous Hall conductivity (AHC), and anomalous Nernst conductivity (ANC) of the 2H-MoTe2/1T-VSe2 heterostructure. Interestingly, we obtained the electric field induced switching of the AHC in the electron doped system. Also, we found that the electric field dependence of the ANC was more prominent in the electron doping system. We obtained a large ANC of 2.3 A/K.m when the electric field was applied from VSe2 to the MoTe2 layer, and this was switched to -0.6 A/K.m with an opposite electric field. Besides, we investigated also the electric field effect on the Curie temperature (TC). We found that the Curie temperature of MoTe2/VSe2 heterostructure was substantially increased to 355 K under the electric field. Therefore, we mainly found that the 2D MoTe2/VSe2 heterostructure can be used for potential applications in energy conversion and spintronic devices. Also, we explored the optical transparency of the 2D ferromagnetic WSe2/VSe2 heterostructure. The Curie temperature of the 1T-VSe2 monolayer was 250 K, and this was substantially enhanced to 380 K. The optical transparency of the heterostructure reached more than 75 % in the visible range with a very large refractive index (2.85). Overall, we find that the WSe2/VSe2 heterostructure can be used for potential device applications as an optically transparent magnetic system at room temperature. The last section outlines the electric field-induced valley polarization in the WSe2/CrSnSe3 heterostructures. We found a valley polarization of 9 meV, and this could be further enhanced to a giant valley polarization of 67 meV if an electric field is applied from CrSnSe3 to the WSe2 layer with 0.6 V/Å intensity. We attribute this electric field dependency to the dipolar effect. Overall, we proposed that the WSe2/CrSnSe3 heterostructure can be a potential structure for obtaining a giant valley polarization.