Thermal Conductivity Reduction of Polycrystalline CVD Graphene by Grain Boundary and Hole Defect

Abstract

Graphene, a two-dimensional material consisting of carbon atoms, has attracted great interest due to its novel physical properties, such as high electrical mobility, optical transparency, and superior mechanical strength. Especially, graphene has exceptionally outstanding thermal properties owing to long mean free path of phonon in two-dimensional lattices. The extraordinarily high thermal conductivity of graphene can be remarkably reduced by some defects, such as grain boundary, vacancy, and foreign atom. Therefore, Grain boundary effects on thermal transport in CVD grown graphene that has many grain boundaries should be investigated. <br/> In this study, graphene samples with different grain sizes were synthesized by chemical vapor deposition (CVD) system to investigate the grain boundary effect on thermal transport in polycrystalline graphene. Manipulation of the CVD synthesis conditions allowed for synthesis of polycrystalline single-layered graphene with controlled grain sizes. Thermal conductivities of 3 graphene with different grain sizes were measured by the optothermal Raman technique, using the temperature-dependent 2D peak shift in Raman spectra. At first, the optical absorption was measured to define the absorbed power of defected graphene by Raman laser irradiation. Then, the temperature coefficient of Raman 2D peak positions and 2D peak shifts as the absorbed power were measured. Finally, thermal conductivities of defected graphene were obtained by solving the heat diffusion equation where experimental data were used. Furthermore, the effect of air convection losses and the hole edge temperature of suspended graphene were analyzed by numerically solving the full energy balance. Thermal conductivities of the single-crystal graphene as temperature were estimated by extrapolation/fitting of the experimental data. And the temperature discrepancies between the measured temperature from G peak and 2D peak shifts were compared to confirm noneuqilibrium phonons.<br/> Although defects in graphene have a detrimental effect on thermal conductivity of graphene, the thermal conductivity suppression of graphene by defects can be useful for various applications. For the quantitative study of thermal conductivity of graphene as defects concentrations and for the more effective thermal conductivity suppressions, thermal conductivity reductions of holey graphene (HG) as porosity were measured by optothermal Raman method. HG samples were obtained by focused ion beam (FIB) processing for the study. The hole defects were precisely fabricated and controlled by direct FIB milling which does not require any masking or post processing. After graphene samples were transferred on the substrate by PMMA, all samples were characterized by diverse technique (e.g. Raman spectroscopy, SEM, optical microscope) Finally, the measured thermal conductivity reductions of holey graphene as porosity were compared to other reference experimental data and calculation results.