Functionalization Of Graphene And Its Optoelectronic Properties

Graphene receives much research attention due to its remarkable properties since the discovery in 2004. As a gapless semiconductor, graphene exhibits ultrahigh carrier mobility and fascinating transport phenomena that qualify it for applications in field effect transistors, photovoltaic cells and liquid crystal devices. In this thesis, we were systematically focused on the functionalization of graphene oxide (GO) and reduced graphene oxide (RGO). Results illustrated the relationship between the functionalization and the optoelectronic properties, which are experimentally and theoretically important to potential applications.GO produced by modified Hummers method was presented almost entirely as individual sheets in polar solvents. Wrinkled GO sheets with thickness of 0.9 ~ 1.1 nm were observed with lateral dimensions of several hundred nanometers for a fully exfoliated GO sheet. Fourier transformation infrared (FT-IR) spectra and X-ray photoelectron spectroscopy measurements demonstrated that GO was a layered material bearing oxygen functional groups such as carboxyl, hydroxyl and epoxide on its basal planes and edges. The interlayer distance became smaller after the controlled reduction using hydrazine, in spite of the different from graphite. The conductivity of RGO was measured to be 0.15 S/m with the increased conjugation after the reduction.A naphthalocyanine (NPc) derivative and an azobenzene derivative were immobilized on GO, respectively through a supermolecular self-assembly method. Spectroscopic evidence indicated the strongπ-πinteraction between GO and NPc (azobenzene), and the stoichiometry of the GO-NPc hybrid was a mass ratio of 1:6 (GO: NPc). The fluorescence quenching indicated the electron or energy transfer from NPc (azobenzene) to GO. A reversible rise/decay of photocurrent in response to the on/off illumination step was obtained for the GO-NPc hybrid. Upon ultraviolet (UV) irradiation, the azobenzene moieties on RGO underwent a rapid and reversible photoisomerization. The enhanced photoconductivity under UV light implied the fast and efficient electron transfer within the hybrid material.The azobenzene derivatives were covalently attached on GO through an amide linkage. After the covalent functionalization, the electrostatic repulsion balance existed among the GO lattices was disturbed, and thus the GO sheets tended to aggregate, resulting in an internal short-range ordered crystalline structure. The red-shift in absorption, the fluorescence quenching and the shortened life time testified the strong electronic interactions between GO and azobenzene derivatives. The azobenzene moieties on GO underwent a reversible trans-cis photoisomerization upon UV irradiation, but the isomerization rate constant decreased compared with that of pristine azobenzene. An optical modulated conductance of the GO-AZO film induced by the photoisomerization of the azobenzene chromophores was obtained when the isomerization rate was slow. In contrast, the hybrid film with a fast isomerization rate showed an enhanced reversible photoswitching performance with high on/off ratio of 8 and fast response time less than 500 ms. The high sensitivity of GO-AZO switch arises from the intramolecular donor-acceptor architecture with efficient charge transfer.Nanocomposite films of bacterial cellulose (BC) and GO with layered structures were prepared by vacuum filtration of the aqueous mixture of both components. Strong interactions between BC and GO were proved by FT-IR spectra. The integration of GO changed the intra- and inter-molecular hydrogen bonding of BC, resulting in a decrease in the crystallinity. The films were mechanically strong and showed an increase in both Young's modulus and tensile strength. The electrical conductivity of the composite film containing 1 wt% of GO after in situ reduction enhanced by six orders compared with that of pristine BC.