Date on Master's Thesis/Doctoral Dissertation

5-2011

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Electrical and Computer Engineering

Committee Chair

Alphenaar, Bruce William

Author's Keywords

Graphene; Capacitive photocurrent spectroscopy; Graphene oxide; Photoluminescence; Reduced graphene oxide; Density of states

Subject

Spectrum analysis; Graphite

Abstract

Understanding of the band structure of the new nano materials is essential to have a deeper insight into their optical and electrical properties. Along with the existing techniques used for characterization of nano-materials, it is also important to develop new techniques and methods which can reveal previously unobserved details. This dissertation describes an innovative measurement technique, capacitive photocurrent spectroscopy (CPS), which is used to probe the density of states (DOS) of graphene, graphene oxide (GO) and reduced graphene oxide (rGO). In contrast to the standard photocurrent measurement technique, CPS technique uses a floating electrode (electrodes separated by an insulator) configuration. Floating electrode eliminates the existence of any background current, which in turn increases the sensitivity towards photo generated electron/hole pairs revealing new features in the photocurrent spectrum. Also, this method eliminates the need for locating individual pieces on the substrate to make two contacts for current transport as required by the standard photocurrent method. The increase in sensitivity achieved by this technique can be exploited to gather absorption information from individual nano sized pieces, which is not possible with standard absorption spectroscopy. Capacitive photocurrent spectroscopy measurements on graphene samples reveal a monotonic increase in the photo current spectrum with increasing incident photon energy (photon energy scanned from 0.5 eV to 4 eV) together with an evidence of three reproducible peaks. The monotonous increase in the CPS spectrum is as expected from the semi-metallic graphene DOS. The presence of three peaks is attributed to unintentional oxidation of graphene samples. X-ray photoelectron spectroscopy (XPS) measurements of graphene samples confirm the presence of oxygen functional groups attached to the carbon atoms in graphene. For GO, the monotonic background disappears, and the three peaks become the dominant features in the spectrum. Two of these peaks match well with published photoluminescence data, as transitions between the GO states and the p/p* grapheme levels match the peak luminescence energy of GO. X-ray photoelectron spectroscopy measurements confirm that identical oxygen functional groups (as seen in grapheme samples) exist (in varying quantities) for all the samples measured. In the case of rGO samples, the background density of states due to the graphene reemerges along with additional states due to defects. The three peaks due to the GO also remain, indicating the continued presence of oxidized regions. X-ray photoelectron spectroscopy measurements confirm the presence of the identical oxygen functional groups, along with an extra feature indicating the presence of a carbon-nitrogen group (introduced during the reduction process). Density of state calculations for graphene samples, based on the experimental results, indicate the presence of three small peaks near the vicinity of Fermi energy superimposed on the graphene DOS. For GO samples, DOS calculations revealed five peaks separated by hard gaps (no electron energy state), three of those are in the vicinity of Fermi energy (as seen in the case of graphene samples) while the remaining two resemble the p/p* peaks of graphene DOS. For rGO samples, many peaks with no hard gaps between them were revealed. The five peaks, three near Fermi energy and two resembling p/p* peaks (as seen in GO samples) are still present, along with more peaks. In summary, a direct experimental probe of the GO density of states as a function of oxygen coverage using capacitive photocurrent spectroscopy is described. Three intense additional peaks in the DOS of GO appear near the Fermi energy (attributed to the presence of oxygen functional groups) along with standard p/p* peaks present in graphene DOS. Reduced graphene oxide (rGO) shows a distribution of peaks for the entire energy range probed in the experiment. Three similar peaks (less intense as compared to peaks seen in GO) with no hard gaps near Fermi energy were seen for the rGO sample providing the evidence of some residual oxidation. Extra peaks seen in rGO sample are attributed to defect creation during the reduction process. XPS measurements confirm the presence of similar oxygen functional groups in all the samples (Chemical Vapor Deposition, CVD grown graphene, GO and rGO) with varying intensity dictating the oxidation content percentage.

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