Preparation And Optical Properties Of Oxidized Graphene Materials

Graphene is one atom thick 2D honeycomb carbon plane which has outstanding optical and electronic properties and has attracted great attention of researchers around the world in recent years. Graphene can absorb photons from the visible to the infrared range with strong intra-band transition and ultrafast response. It makes graphene attractive for photo-detection application especially in the middle infrared range. The needed tuning band-gap of grapheme-based materials still remains unsolved though it has unique highest carrier mobility and ultrafast optical response. Graphene oxide (GO) has an elusive chemical formula in the form of epoxy, hydroxyl, carbonyl and carboxyl on carbon atoms, and may have different composition of carbon, oxygen and hydrogen. The controlled reduction graphene oxide as an attractive alternative to graphene provides the new way which can possibly tune the band-gap through studying the optical properties of graphene at different spectral range. This not only will make the band structure be tunable accessing to a zero band-gap graphene structure via complete removal of the functional groups on carbon atoms, but also will provide a potential option to tune the optical, electronic and mechanical properties of the chemical-derived graphene. Meanwhile, it presents the possibility to study the optical response of graphene with different functional groups since in terms of applications, up to date, a systematical experimental study of optical response of graphene with different functional groups is still lacking. To clarify the relationship between the functional groups and reduction of graphene is a crucial and feasible step to controll the band-gap-tuable process. Theoretical works have also been carried out in various ways to understand the role of functional groups by studying optical and electric properties of graphene. Although apparently there exists numbers of works focused on the preparation and characterization of different reduction level of GO, the optical properties and especially band-gap structure determination of GO in the reduction process are still unsolved issues to be studied more in the future.In this work, we investigated the optical properties of chemical-derived graphene oxide and reduced graphene oxide by analyzing the spectra of Raman scattering and transmission electron microscopy. Furthermore, we use spectroscopic ellipsometry to analyze the thickness and band structure of grapheme-based samples by using Lorentz oscillator model to fit the data.Then, we made a series of graphene oxide samples with different reduction level and find that the different absorption of few layer graphene oxide with different reduction levels is originated from different inter-band and intra-band transitions by using infrared-visible spectroscopy, and the band-gap of graphene oxide can be tunable from 2 eV down to 0.02 eV, which agrees well with the result predicted from the density functional theory (DFT). Comprehensive studies of band-gap tuning and optical properties of the evolution from few-layer reduced graphene oxide (rGO) to graphene oxide are reported from various aspects. We give GO with different reduction level a complete optical investigation and propose a nondestructive and feasile method to study the band-gap structure based on the spectroscopic ellipsometry (SE) data. This fast and reliable energy band-gap determination and modeling method has been reported for the first time and it can offer technical support to the mass production of grapheme-based materials. We use Lorentz oscillator model to describe the band-gap tuning of rGO and illustrate the Dirac fermion inter-band transitions and intra-band transitions in the graphene with different functional groups. The DFT calculations were carried out to investigate the optical response and band-gap tuning of graphene with different functional group in visible and infrared range which agrees well with the experimental result in the work. These investigations are believed to be significant to give a nondestructive in-situ monitor of graphene in large scale production. It indicates that these reduced graphene oxide hold great potential for a substitute for photo-detection in mid-infrared range.In the work, we also used chemical reduced graphene oxide (rGO) to fabricate Field-effect transistor (FET) for investigating the performance of grapheme-based devices. The contaminations and residues on the rGO surface will influence the performance of the device. An inductively coupled plasma plasma with very low density is applied to etch the surface of rGO. Eventually we make the properties of rGO more like that of pristine graphene after plasma etching which is a Si-compatible cleaning process for rGO.