The Spectrum Properties Of Graphene Oxide Under External Electric Field

Graphene has attracted abundant researches based on the unique physical and chemical properties by the international scientists since the production in 2004. Due to the six carbon atoms of benzene rings in graphene have contributed an unpaired valence electron which could move freely in the grid of carbon atoms, thus, the conduction band and valence band are overlapping in the Fermi energy which is attributed to those unique structures in graphene. However, the graphene oxide, a derivative of graphene, has the similar structure feature. Graphene oxide has attracted a considerable amount of attention in recent years, due to the main precursor for graphene synthesis and the huge potential applications of GO itself in biosensing, energy storage and nonlinear optics. The conductivity of graphen oxide is decreased because of the existence of functional groups. It is considered that the graphene oxide demonstrate the feature of insulator which confined the applications in electronic and mechanical technology. However, the sp1 clusters formed by graphite carbon atoms and the sp3 clusters consisted of functional groups are contributed to the experimental and theoretical applications which located in the optical properties. Based on the excellent and unique optical features of graphene oxide, the fluorescence-sensor, supercapacitor, laser absorption medium, energy storage and photovoltaic cell have been reported, to date.The main work in this thesis is to investigate the optical properties of monolayer graphene oxide, and to manipulate the transmission and emission features under external electric field. Based on the laser scanning confocal microcopy, it is demonstrated that both the spectra and images of the transmission and emission in graphene oxide have been changed under electric stimulate. It is clear that all the functional groups in graphene oxide are polar groups, thus, we present the explanation of those phenomena based on the polarity effects of the polar clusters which is contributed to the variation of the density distribution of localized states. Furthermore, the optical properties are changed by the external electric field. In addition, the changes demonstrate the reversible feature during the whole period of the electric stimulate. We have performed the calculations for DOLS by utilizing first-principles density function theory (DFT). The results show that after moving the O and OH groups from their equilibrium sites along the [010] and [0-10] direction, which shows the electronic field induced polarity effect, the localized states in CB are successfully manipulated. The tunable DDLS is beneficial to improve and control the mGO’s optoelectronic properties.The innovations of this paper:1. We study the reversible changes in spectra and imaging both of the transmission and emission in graphene oxide based on the laser scanning confocal microcopy. The two-terminal electrode with 2 mm space is consisted of thin aluminum which is stick to the substrate of the samples. Due to lots of small sp3 clusters isolated by the various graphitic sp2 clusters, the fluorescence images revealed the mGO’s anisotropic density distribution of localized states. Because of the oxygen functional groups in these selected spots may have different type, number and coverage, the enhanced and quenched fluorescence trajectories demonstrated the different optical responses between selected spots under external electric field.2. First-principles density-functional theory (DFT) calculations are performed by means of the Vienna ab initio simulation package. Based on the composition of graphene oxide which is used in our experiment, we build a simplified model are used to qualitatively simulate the polarity effect of functional groups leading to the variation of density distribution of localized states. Finally, the optical properties of graphene oxide are manipulated in the electric field.3. It is believed that the variations in ration (ID/IG) of GO network revealed the structural change of the GO sheet. We measure the Raman spectra of graphene oxide under electric field, and our Raman results can be understood as the GO’s structure varied under electric field. These observations confirmed that electrical stimulus induced the polarity effects of polar clusters which consisted of oxidized functional groups result in the variations of absorption properties and emission spectrum.