Thermal Conductivity and Pressure-Dependent Raman Studies of Vertical Graphene Nanosheets

Thermal and mechanical properties of graphene sheet are of significant importance in the areas of thermal and stress management, respectively. Here, we report the thermal conductivity and high-pressure behaviors of unsupported vertical graphene nanosheets (VGNs) grown by electron cyclotron resonance-plasma enhanced chemical vapor deposition method. Structural morphology of the as-grown VGNs on SiO₂/Si substrate suggests a homogeneous, uniformly interconnected network of graphene sheets standing vertically on a basal nanographitic layer. On examination of edges of exfoliated sheets using transmission electron microscopy, seven layers of graphene is estimated. The frequency of the G-band (E_2g-in plane mode) is found to vary linearly with temperature. The first-order temperature coefficient for G-band is found to be 1.47(1) × 10^–2 cm^–1 K^–1. Using the G-band temperature coefficient and its position dependence on excitation laser power, the thermal conductivity of the VGNs at room temperature is estimated to be 250 (19) W m^–1 K^–1. The effect of pressure (<i>P</i>) on the G-mode frequency (ω) of unsupported VGNs is investigated by in situ Raman spectroscopic studies up to 40 GPa using a diamond anvil cell. Above 16 GPa, discontinuity in the ω versus <i>P</i> curve suggests a disruption of long-range order in the graphene layers resulting in a deviation from two-dimensional layer structure. Persistence of local sp²-hybridization up to 40 GPa is evident from the presence of G-band at this highest pressure. Upon decompression, VGN is found to recover its original ordered structure

Role of Surface Polarity in Self-Catalyzed Nucleation and Evolution of GaN Nanostructures

Self-catalytic growth of GaN nanotips and nanoparticles, grown by chemical vapor deposition technique, are investigated. Three important parameters, comprised of incubation time, anisotropy of diffusion, and rate-limiting factors of Ga and N adatoms migration over polar and nonpolar surfaces, are found to play significant roles in determining the final morphology of these nanostructures. Nucleation of GaN nanotips takes place under Ga-rich conditions. As the reaction proceeds, the stochiometry changes occur as a result of a shift in Ga-rich to N-rich conditions on the surface. In all of these cases, the growth continues to be in vapor–solid mode. The conical shape of the nanotips is explained in terms of differential growth in the reduced surface diffusion of Ga under N-rich conditions on polar surfaces (0001) relative to nonpolar surfaces (101̅0). Nanoparticles are grown initially in N-rich conditions with significantly shorter incubation times. A mechanistic approach that expounds evolution of nanotips and nanoparticles is elucidated in details using crystallographic and electronic structural studies