The Studies Of Anisotropic Thermal Transport Properties In Graphene And WTe₂

With continuous and significant increasement of integration,the size and surface effects induced by correspondingly reduce of dimension and size become serious challenge in electronics,leading to the enhancement of phonon-interface scattering and thus giving rise to poor energy dissipation.Considering the ultrahigh thermal conductivity in graphene,it has gained much attention in term of addressing the poor heat dissipation issue.Nevertheless,the large interfacial thermal resistance between graphene and substrate greatly limits the application and development of graphene-based microelectronics devices.Taking advantage of the lattice structure related anisotropic thermal transport in two-dimensional（2D）materials,the self-heat dissipation of anisotropic substrate can be utilized in graphene devices to solve the large surface roughness in Si O₂ substrate induced higher interface thermal resistance of graphene/Si O₂ in our dissertation.Furthermore,the Joule heat inside the graphene device can also dissipate quickly along the direction of CrOCl substrate with higher thermal conductivity to guarantee the stability of performance and working environment in graphene-based electronic devices.The research object focuses on graphene,WTe₂ and graphene/CrOCl heterojunction.Additionally,the influence of layer number and anisotropic substrate on thermal transport in graphene has been systematically analyzed.Especially,we also investigate the relationship of crystal structure on the thermal transport characteristics of WTe₂.The detailed studies are as follows:（1）The thickness-independent energy dissipation of graphene has been investigated in details.The representative two Raman modes（G and 2D bands）of graphene shift towards the low frequency when applied with bias voltage,originated from the phonon softening during the electric heating process.In addition,the Joule heat accumulates at the center of graphene channel,forming hot spot.The temperature at the center of the graphene channel is higher than that close to metal electrodes,ascribed to the efficient heat dissipation of metal electrode.Furthermore,the influence of graphene layer on energy dissipation has been systematically investigated via Raman spectroscopy.Surprisingly,the energy dissipation of monolayer graphene is comparable to that of other layer graphene devices,breaking the traditional understanding of the relationship between energy dissipation and dimensions.The heat dissipation model has been established to reveal the mechanism of interface heat transport in graphene device.The result of thermal infrared reveals the quick temperature response of monolayer graphene,indicating the efficient energy dissipation of graphene.（2）The thermal transport of WTe₂ is revealed via Raman method with electric heating.The crystal structure of few-layer WTe₂ was firstly identified via polarized Raman spectrum.Interestingly,the thermal conductivity of WTe₂ significantly depends on its crystal structure orientation,in which the thermal conductivity along zigzag direction is 1.6 times compared with that along armchair direction.Such behavior can be ascribed to the discrepancy of the phonon mean free path along these two axes.Take advangetage of the superior heat transport direction of WTe₂ opposite to the preferable electrical property,the heat manipulation model was proposed in anisotropic WTe₂devices.The waste heat in WTe₂ devices can be easily dissipated along the favorable thermal conductivity（zigzag direction）,effectively relieving the local temperature rise and preventing the devices from degradation or breakdown.（3）The crystal structural influence of low-symmetric material（CrOCl）on interface thermal transport has been investigated in graphene/CrOCl heterojunction devices.Based on the characterization of angular-dependent Raman spectrum in graphene on CrOCl,graphene displays the polarization angle-induced Raman property on G and 2D modes,indicating the obvious twofold symmetry.Especially,D band can be observed,attributed to the lattice mismatch-induced in-plane polarization in graphene.Additionally,there exists discrepancy in the temperature coefficient of G mode along zigzag and armchair directions of CrOCl flake through the temperature-dependent Raman spectrum,exhibiting the diversity of phonon behavior in graphene on CrOCl substrate.Furthermore,the conductance of graphene also exhibits strong dependence on the crystal structure.Based on the properties mentioned above,the interface thermal transport of graphene on CrOCl was investigated,where zigzag direction of CrOCl delivered the much favorable thermal transport compared to armchair direction.Consequently,the heat dissipation model was also established to reveal its corresponding interface heat transport mechanism.