Enhanced Thermoelectric Conversion Efficiency of CVD Graphene with Reduced Grain Sizes

Abstract

The grain size of CVD (Chemical Vapor Deposition) graphene was controlled by changing the precursor gas flow rates, operation temperature, and chamber pressure. Graphene of average grain sizes of 4.1 µm, 2.2 µm, and 0.5 µm was synthesized in high quality and full coverage. The possibility to tailor the thermoelectric conversion characteristics of graphene has been exhibited by examining the grain size effect on the three elementary thermal and electrical properties of σ, S, and k. Electrical conductivity (σ) and Seebeck coefficients (S) were measured in a vacuum for supported graphene on SiO2/Si FET (Field Effect Transistor) substrates so that the charge carrier density could be changed by applying a gate voltage (VG). Mobility (µ) values of 529, 459, and 314 cm2/V·s for holes and 1042, 745, and 490 cm2/V·s for electrons for the three grain sizes of 4.1 µm, 2.2 µm, and 0.5 µm, respectively, were obtained from the slopes of the measured σ vs. VG graphs. The power factor (PF), the electrical portion of the thermoelectric figure of merit (ZT), decreased by about one half as the grain size was decreased, while the thermal conductivity (k) decreased by one quarter for the same grain decrease. Finally, the resulting ZT increased more than two times when the grain size was reduced from 4.1 µm to 0.5 µm.

Raman Study of Strain Relaxation from Grain Boundaries in Epitaxial Graphene Grown by Chemical Vapor Deposition on SiC

“…Although exfoliated graphene offers a large (10–100 μm) and high-quality single domain, the film size is too small for wafer-scale applications. Alternatively, chemical vapor deposition (CVD) on metal substrates and sublimation of silicon atoms from silicon carbide are promising routes for producing wafer-size graphene film [8,9]. However, polycrystalline graphene exists in the films grown by these two methods and the grain size is much smaller than exfoliated graphene, which results in the inevitable formation of grain boundaries [10].…”

Carbon Nanotubes for Active Refrigeration and Cooling in Micro and Mesoscale Systems

“…The density of free charge carriers passing across a unit area per unit time is characterized by the transport rate (θ), and the energy sensitivity of carrier transport (s) is represented as the relative change of transport rate per unit relative change of carrier energy (ε) counting from the corresponding band edge s=(dθ/θ)/(dε/ε). The value of such an energy sensitivity is determined by the specific materials system and the temperature, and are measured to be around ~1.00 for electrons and holes in graphene and carbon nanotubes at 300 K [67,69,70] and around ~0.50 at 150 K. [67,68] The asymmetry ratio between the transport rates of holes and electrons is measured to be in the range of 1.50~2.00 at 150~300 K. [67,69,70] The electrical conductivity and the thermopower can be generally calculated as [67] 𝜎…”

Actively Tunable Terahertz Switches Based on Subwavelength Graphene Waveguide

“…Noting that during the experimental fabrication, the chemical treatment and crystal growth direction may result in imperfections in the graphene, such as the formation of polycrystalline graphene from folding defects. This would improve the light absorption of graphene [ 35 , 36 ]. However, we only numerically investigate the ideal single layer graphene for simplicity.…”