The schematics of the TEP measurement system. (a) Schematic diagram of the quartz reactor containing the sample located inside a tube furnace and connected to a turbo molecular pump and a gas handling system. (b) Graphene sample with two K-type thermocouples attached ∼2 mm apart with a platinum resistive heater anchored closer to one thermocouple. Voltages V1 , V2 , and ΔV = V1–V2 used to determine the thermo-emf (ΔV) and the temperature difference (ΔT) are also shown.
Simultaneous TEP and R measurements of multilayer epitaxial graphene (MEG) on C-face SiC. The time evolution of the (a) TEP (left axis) and the corresponding temperature profile (right axis) are plotted during the vacuum-annealing process. (b) Simultaneous behavior of R and temperature during the degassing process. (c) Simultaneous TEP and R of the vacuum-annealed MEG on C-face SiC during annealing plotted in an expanded scale.
Simultaneous measurement of TEP and R of the annealed MEG upon (a) exposure to ambient air at 300 K plotted in an expanded scale. (b) exposure to ambient air and ammonia.
The time evolution of TEP and R the corresponding temperature profile during the vacuum annealing of the few layers of graphene (1-3 layers) on C-face SiC. The temperature profile is shown on the right axis. The observed behavior is similar to what was observed for the MEG sample shown in Figure 1 . Note that the TEP measurement on few layers of C-face graphene is performed on the same sample that had been previously used for TEP measurement of MEG.
Energy level diagram of the water/oxygen redox couple (blue lines) compared to the theoretical band diagrams of epitaxial graphene/SiC from our simulations (the zero of energy corresponds to the vacuum level of the system). Shaded areas are the projected bulk bands from the SiC substrate. (a) (left panel) Water-free surface: the EF is pinned by an interface state located in the conduction band just above the Dirac point causing electron doping of graphene. These states are due to dangling bonds of the C rest atom on the SiC (2 × 2)C surface reconstruction (see panel c) and Supplemental Information). (right panel) P-type doping of graphene when the Fermi energy, EF, is pinned at the chemical potential μ determined by the redox potential of oxygen dissolved in the mildly acidic water adsorbed on the SiC surface. Bands calculated for a (positive) surface charge corresponding to a depletion of ∼4.5 × 1014 electrons/cm2. (b) Difference between the Fermi and Dirac energy as a function of surface charge. The neutrality point (EDirac = EFermi) coincides with a charge depletion of ∼1 × 1014 electrons/cm2. (c) Geometry of the graphene/SiC interface (Si, purple; C, cyan; Si adatoms, darker purple). For a detailed account of the interface structure see Supplemental Information. The electron density of the dangling bond state localized on the interface C rest atoms at the Dirac point is shown with the red isosurface (0.15 × 10−3 electrons.)
Temperature dependence of (a) S and (b) R of the vacuum-annealed and air-exposed multilayer epitaxial graphene (MEG) on C-face SiC. (1) Heating portion of the experiment and (2) cooling portion of the experiment. Data obtained during desorption are excluded from both plots.
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