Phonon Transport And Thermomechanics In The Novel Two-dimensional Materials From Numerical Simulations

Since the successful isolation of graphene, two-dimensional (2D) materials have attracted intensive studies. Due to their unique physical properties,2D materials show great potential applications, such as catalysis, energy storage, field-effect transistor, logical circuit, optical detector and sensing, etc. Compared with the abundant researches on their electronic, magnetic and optical properties, the studies of the thermal properties of 2D materials are just beginning. Phonon transport and thermomechanics are important thermal properties, because they are not only the fundamental physical properties of materials, but also crucial in practical applications at finite temperature. In this paper, we study the phonon transport and thermomechanical properties of some 2D materials from first-principles calculations.The thermomechanical properties of crystals mainly include thermal expansion coefficients and thermal elastic moduli. The thermal expansion is determined by the anharmonic potential in the crystals. Based on quasi-harmonic approximation (QHA), we obtain the temperature and lattice parameters dependent free energy from which we get the thermal expansion coefficients and thermal elastic moduli.The main mission of the study on phonon transport is calculating the lattice conductivity. Similar to the lattice thermal expansion, the finite lattice thermal conductivity also results from the anharmonic potential in crystals. From the mechanism of three-phonon scattering, we obtain the lattice thermal conductivity, the accumulative thermal conductivity and phonon life time by iteratively solving the linearized Boltzmann equation.In this paper, we first introduce the background of 2D materials, then we present the theories of thermomechanics and phonon transport, and finally we use these methods to study some novel 2D materials. The overview of each chapter is listed as follows:In chapter 1, we introduce the background of 2D materials and their thermal studies.In chapter 2, we give the theory of thermomechanics, including strain tensor, stress tensor, elastic moduli, the general Hooke's rule, the equation of state of crystals, the general gruneisen theory and the computational methods of thermomechanics based on QHA.In chapter 3, we give the theory of phonon transport. Based on the three-order potential and Fermi-Golden rule, we deduce the expressions of phonon distributions and scattering rates, and then linearize the Boltzmann equation to a form that can be iteratively solved, finally we get the expressions of lattice thermal conductivity and phonon lifetime. In addition, we also introduce the computational methods of phonon transport.In chapter 4, based on the QHA and first-principles calculations, we study the thermomechanical properties of single-layer black and blue phosphorus. We find the thermomechanics of black phosphorus is very different with that of blue phosphorus. The thermal expansion coefficient of black phosphorus is positive while that of blue phosphorus is negative. In addition, we find the in-plane stiffness of black phosphorus is strong anisotropic (a factor of 4 to 5).In chapter 5, we investigate the thermal properties of borophene. We find the thermal properties of borophene are unique. Strong phonon-phonon scattering is found in borophene, which results in an unexpectedly low thermal conductivity. Furthermore, we find the thermal expansion coefficients along two directions are all negative, and the elastic moduli will be stiffened when temperature increases. In addition, we also found borophene has negative Poisson' ratio in certain temperature range.In chapter 6, we study the thermal properties of single-layer Td-WTe2. We find the phonon transport is anisotropic and low (10 Wm-1K-1), but the anisotropic thermal expansion coefficients are not observed until at high temperature range. Furthermore, we find the phonon excitation can stiffen the in-plane stiffness while weaken the in-plane strain-strain correlation, showing opposite temperature effect.In chapter 7, we make a brief summary on this paper and give an outlook.