Study and modulation of band structure of graphene materials by soft X-ray and visible light

• Main Authors: Lin, Chi-Yuan, 林志遠
• Other Authors: Chang, Chen-Shiung
• Format: Others
• Language: en_US
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/51273532772039339568

博士 === 國立交通大學 === 光電工程研究所 === 104 === The band structure and electronic properties of graphene are influenced by number of layer (NL), oxidized level and doping level itself. In this thesis, we would like to use soft X-ray and visible light to study and modulate the band structure of graphene under the three aspects of NL, oxidized level and doping level. In the aspect of NL, we began characterizing the dependence of photoelectron spectra of mechanical exfoliation method (MEM) produced graphene on NL by the scanning photoelectron microscopy (SPEM). The work function of graphene with various NL was measured by electrostatic force microscopy (EFM). According the results of SPEM and EFM, we found that the C1s core-level energy (E1s, core-level to vacuum level) of graphene decreases with the increase of NL. The E1s s of single layer and multilayer graphene (MLG) are ~289.42 eV and ~289.13 eV, respectively. Based on the bond-order-length-strength correlation theory (BOLS), we derived the relationship between E1s and effect coordination number of carbon atom, and obtained the energies of a free carbon atom (288.17 eV) and diamond (288.91 eV). According the evolution of XPS with various NL of graphene and the coordination-resolved E1s, we supported the existence of surface core-level shift (SCLS) on graphite. The magnitude of SCLS of MLG is 0.09 eV. Reduced graphene oxide (RGO) from the reduction of graphene oxide (GO) is one of ways to produce graphene hugely. We are interesting in the influence of oxidized level to the electronic band structure of graphene, and also would like to know if the soft X-ray exposure can induce the reduction of GO. Therefore, we demonstrate that X-ray irradiation is able to induce the reduction of graphene oxide (GO) first by the characterization of time-resolved X-ray photoelectron spectroscopy (XPS). The number of CO bonds of GO exhibits an exponential decay with exposure time. The X-ray reduction rate constant of GO is positively correlated with the intensity of low-energy secondary electrons excited from substrates by soft X-ray exposure. The reduction of GO is due to the secondary electrons induce the dissociation of the CO bonds, not X-ray itself. According the results of XPS, valence band maximum and EFM measurements, we obtained the variation in band structure from GO to RGO. The energy band of C1s core level to valence band maximum from GO to RGO is rigid, and E1s decreases with reduction of GO. Because of the unique band structure of graphene at Dirac point, the resistance of graphene is obviously influenced by the doping level, i.e. the magnitude of work function. To efficaciously modulate the doping level of graphene, we fabricated a graphene/DR1-PMMA field-effect transistor (FET), where the DR1-PMMA is an azobenzene molecule copolymer. The polarization of DR1 molecules can be built and erased by photo-assisted poling (PAP) and photo-deploying (PD) operations with visible light, respectively. The ternary–logic memory of graphene/DR1-PMMA FET can be performed in the room temperature by PAP and PD operations. The PAP operation writes “1” resistance states into the graphene/DR1-PMMA FET. The PD operation erases “1” resistance states to “0” resistance state. The resistance change ratio between“1” state and “0” state is 60%. Under the room temperature, the fixed DR1 molecules result the graphene/DR1-PMMA FET device maintaining in the written states under an external electric field as high as 0.2 MV/cm.