Measurement of the electronic density of states of graphene oxide using capacitive photocurrent spectroscopy.

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

Tanesh Bansal

University:University of Louisville

Department:Electrical and Computer Engineering

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

Others  :  https://ir.library.louisville.edu/cgi/viewcontent.cgi?article=1067&context=etd

美国|英语

来源: The Universite of Louisville's Institutional Repository

【 摘 要 】

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