Charge transport in graphene oxide.
Rasanayagam Sivasayan Kajen.
Date of Issue2013
School of Electrical and Electronic Engineering
The objective of this thesis is to study charge injection across the gold/graphene-oxide (Au/GO) interface and elucidate the charge transport mechanisms in graphene oxide (GO) and lightly reduced GO (RGO). Ballistic electron emission microscopy (BEEM) studies show the Au/GO interface to be non-homogeneous at the nanoscale. In addition, the distribution of electron/hole injection barriers across the Au/GO interface was extracted via ballistic electron/hole emission spectroscopy (BHES/BEES). The heterogeneity of the interface is believed to arise due to the different functional groups on GO, which are known to create defects/traps. Trap energy levels in GO were extracted via thermally stimulated current (TSC) measurements on graphene oxide devices, fabricated by e-beam lithography. In addition, TSC measurements carried out on RGO showed a decrease in the overall trap density compared to that of GO. However, several states close to the Fermi level are shown to emerge with reduction. Conventional temperature dependent current-voltage (I-V) measurements on RGO measured down to 12K, showed various charge transport mechanisms such as tunneling, hopping, space charge limited current with exponentially distributed traps (SCLC-EDT), and Poole-Frenkel limited transport, depending on the applied field and temperature. The transport gap in GO/RGO is shown to be of the order of a few meV, resembling an ohmic contact. Overall, the I-V, BEEM and TSC experiments give a correlated view of the transport in GO/RGO, supporting a multi-energy bandgap. In terms of technology, we find that several issues related to trap states have to be first resolved before GO/RGO could be used either as a dielectric or as a channel material.