Experimental and Simulation Study of Electron and Phonon Properties in Crystalline Materials

The continuous increase in demand of electrical energy has become a major concern for many countries. The world has seen a constant search for new alternative energy resources mainly focusing on resources that are environmentally friendly. One technology that goes in that direction is thermoelectrics.

Thermoelectrics can directly convert a temperature difference into electrical current or vice versa. The mechanism behind such devices is the thermoelectric effect, which is referred to as the Seebeck or Peltier effect. A good thermoelectric materials needs the simultaneous optimization of three key properties to reach a high efficiency, which can usually be characterized by the figure of merit (ZT): high Seebeck coefficient, high electrical conductivity, and low thermal conductivity. However, maximizing one property while minimizing the other is a difficult task, because materials tend either to conduct both electricity and heat well, or both poorly.

The study of thermal properties plays important roles in many applications as thermoelectricity and the thermal management of electronics devices. While phonons dominate the thermal transport in crystalline semiconductors, electrons are the major thermal and charge carriers in metals.

The aim of this thesis is to use both simulation and experimental tools to study the electron and phonon transport properties of crystalline materials such as SnSe, SnS, GeS, GeSe, SnS2, SnSe2, and NbSe2. Regarding the experimental study, we used the structural and morphological characterization of bulk SnSe, to determine its microstructure and crystallinity. In addition, thermoelectric properties were measured, where the ZT reaches a value of 0.11 at 772 K. Moreover, we studied electron-phonon interaction from first- principles calculations using density functional theory and Boltzmann transport equation to calculate electrical and thermal properties of monolayer SnSe, SnS, GeS, GeSe, SnS2, and SnSe2, at different doping levels, helping evaluate their potential as thermoelectric materials. SnSe was found with the largest ZT. Regarding phonon transport, we applied first-principles lattice dynamics and semi-empirical Boltzmann transport to study the phonon properties and lattice thermal conductivity of NbSe2. Our estimation of thermal conductivity of 13.5 W/mK, are in good agreement with the in-plane thermal conductivity obtained from experiments, where the thermal conductivity is measured to be 14 ± 5 W/mK at room temperature.