Measurements and control of thermal transport across interfaces of two-dimensional materials

Abstract

This dissertation presents the experimental studies of measurements and control of thermal transport that occurs through the interfaces of two-dimensional materials in response to an engineered imperfection in the materials. Especially, advancing the understanding of thermal transport properties of junction interfaces between different materials (i.e., Al/Graphene/Cu, Al/MoS2/SiO2), where one side is comprised of atomically thin two-dimensional material, is the focus of this dissertation. Time-domain thermoreflectance (TDTR), which is an optical pump-probe technique, has been employed for the measurements of thermal conductivity of involved materials and interfacial thermal conductance (G) between materials. I carried out modifying processes to generate structural or chemical disorder in two-dimensional materials, either using ion irradiation or UV-ozone (O3) treatment. Following the characterization of produced defects using several spectroscopic techniques, measurements and analyses of changes in the thermal transport properties revealed how such disorder modified vibrational states of two-dimensional materials and influences heat transfer. In chapter 2, I present the investigation of the thermal energy transport across the interfaces of physically and chemically modified graphene with two metals (i.e., Al and Cu). I could monitor changes in the thermal conductance in response to varying degrees of disorder. The measured conductance increases as the density of the physical disorder increases, but undergoes abrupt modulation with increasing degrees of chemical modification, which decreases at first and the increases considerably. The bimodal results of thermal conductance are attributed to an interplay between the distinct effects on graphene’s vibrational modes exerted by graphene modification and by the scattering of modes. In chapter 3, I will discuss how structural anisotropy in few-layer MoS2 can be related to the vibrational states of the material and their quantization and how we can describe the propagation of thermal energy along the reduced dimension. MoS2 is a highly anisotropic material due to the difference in the chemical bonds between ab-plane and cross-plane. I measured thermal conductance across a junction including MoS2 layer before and after introducing imperfection by using ion-irradiation at a temperature range between 80 K and 300 K, as well as bulk single crystal specimen. The transport of thermal energy along the cross-plane could be accounted for by the propagation of phonon modes that are quantized along the nanoscale thickness of MoS2 films and hybridized between different polarizations. These results suggest a nanoscale origin of phonon focusing effect previously suggested for highly anisotropic bulk materials and thus open an engineering possibility to control the heat transport in nanoscale layered materials.