Articles you may be interested inLocal solid phase growth of few-layer graphene on silicon carbide from nickel silicide supersaturated with carbon
This research explores the surface chemistry of halogen based plasmas on silicon carbide and is aimed at the synthesis of large area graphene-on-insulator films. In these studies, 6H-SiC (0001) substrates were etched using either CF4 and Cl2 based plasmas and then thermally annealed. The resulting surfaces were analyzed using x-ray photoelectron spectroscopy, reflection high energy electron diffraction, atomic force microscopy, and Raman spectroscopy. The analyses showed that the etching process selectively etched silicon to produce carbon rich surface layers on the silicon carbide substrate, and when annealed, these carbon rich layers formed graphene films with halogen-and oxygen-based defects. The thickness of the graphene was controlled by the plasma etch parameters. Two point current-voltage measurements were used to characterize the electrical properties of the films. The current-voltage plots exhibited back-to-back Schottky behavior which suggests that the defects open a band gap in these films. Current-voltage data were used to determine the Schottky barrier height, carrier density, and the upper limit for the film resistivity.
Halogen based (CF4 and Cl2) inductively coupled reactive ion etching (ICP-RIE) has been used to selectively etch silicon from 6H-SiC to produce a controlled number of carbon layers. After annealing at temperatures in the range of 550 °C to 1100 °C to reconstruct the near surface layers, x-ray photoelectron spectroscopy has been used to characterize the composition of the films. For the Cl2 based ICP-RIE, two carbon species are observed. One is due to carbon bound as SiC in the substrate and a second which can be attributed to graphene. In the case of CF4 based etching the situation is similar except the second peak is most closely aligned with p-type graphene. This is most likely due to electron transfer from the graphene to the trace levels of fluorine remaining on the surface after annealing.
Buckled graphene produced by the halogen based etching of 6H-SiC provides a new route for the functionalization of the graphene surface. This surface provides an important new stepping off point in the development of molecular electronics and sensors. While the graphene surface is relatively inert, the fluorinated defect sites inherent in the buckled graphene surface yield an excellent location for chemical reactions such as nucleophilic substitution. This thesis shows the utility of the fluorinated defect sites through the well characterized diazonium reaction. Buckled graphene films were prepared on silicon carbide substrates using inductively coupled plasma and reactive ion etching, and annealed at 1000º C to coalesce the BG. The films were reacted with benzene, nitrobenzene, acetonitrile, or a nitrophenyl diazonium salt solution. The diazonium salt was chosen due to its known reaction with graphene produced by other methods. Consequently, reaction of the diazonium with buckled graphene would provide a basis for comparing the reactivity of the surface with these other forums of graphene. The interactions of buckled graphene with the other species were investigated as they represent either constituent parts of the diazonium salt or the solvent. The reacted surfaces were analyzed by X-ray photoelectron spectroscopy, which reveals changes in the surface chemical state due to the functionalization of the buckled graphene by each species. Each reaction yielded significant π-π bonding, while the diazonium salt reaction produced additional covalently bonded phenyl groups on the buckled graphene surface. The covalent reaction site was shown to be the surface fluorinated defect site. This observation illustrates the utility of the buckled graphene surface in the functionalization of graphene. Moreover, it provides additional confirmation of the nature of the buckled graphene surface.
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