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.
ABSTRACT
Manipulation of the chemical vapor deposition graphene synthesis conditions, such as operating P, T, heating/cooling time intervals, and precursor gas concentration ratios (CH4/H2), allowed for synthesis of polycrystalline single-layered graphene with controlled grain sizes. The graphene samples were then suspended on 8 µm diameter patterned holes on a silicon-nitride (Si3N4) substrate, and the in-plane thermal conductivities k(T) for 320 K < T < 510 K were measured to be 2660-1230, 1890-1020, and 680-340 W/m·K for average grain sizes of 4.1, 2.2, and 0.5 µm, respectively, using an opto-thermal Raman technique. Fitting of these data by a simple linear chain model of polycrystalline thermal transport determined k = 5500-1980 W/m·K for single-crystal graphene for the same temperature range above; thus, significant reduction of k was achieved when the grain size was decreased from infinite down to 0.5 µm. Furthermore, detailed elaborations were performed to assess the measurement reliability of k by addressing the hole-edge boundary condition, and the air-convection/radiation losses from the graphene surface.
ABSTRACT
Reliable determination of the complex refractive index (RI) of graphene inherently requires two independent measurement realizations for two independent unknowns of the real (nG) and imaginary (kG) components, i.e., RI = nG + i kG. Thus, any single set of measurement realization provides only one constraint that is insufficient to uniquely determine the complex RI of graphene. Tandem uses of two independent measurement techniques, namely the surface plasmon resonance (SPR) angle detection and the attenuated total reflection (ATR) intensity measurement, allow for the unique determination of the complex RI of CVD-synthesized graphene. The presently measured graphene RI is determined to be 2.65 + 1.27i for the E-field oscillating parallel to graphene at 634â nm wavelength, with variations for different numbers of L (1, 3 and 5) remaining within ±3%. Thus, our demonstration results for the specified wavelength serve as an impetus to suggest the need for two independent measurement techniques in determining both the real and imaginary RI values for graphene. Additional efforts have been made to characterize graphene layers using the density function theory (DFT): this calculation provides RIG = 2.71 + 1.41i.
