Spectroscopic studies of large sheets of graphene oxide and reduced graphene oxide monolayers prepared by Langmuir-Blodgett technique

Graphene oxide (GO) sheets prepared by chemical exfoliation were spread at the air-water interface and transferred to silicon substrates by Langmuir-Blodgett technique as closely spaced monolayers of 20-40 μm size. Hydrazine exposure followed by annealing in vacuum and argon ambient results in the formation of reduced graphene oxide (RGO) monolayers, without significantly affecting the overall morphology of the sheets. The monolayer character of both GO and RGO sheets was ascertained by atomic force microscopy. X-ray photoelectron spectroscopy supported by Fourier transform infrared spectroscopy revealed that the reduction process results in a significant decrease in oxygen functionalities, accompanied by a substantial decrease in the ratio of non-graphitic to graphitic (sp² bonded) carbon in the monolayers from 1.2 to 0.35. Raman spectra of GO and RGO monolayers have shown that during the reduction process, the G-band shifts by 8-12 cm^− 1 and the ratio of the intensities of D-band to G-band, I(D)/I(G) decreases from 1.3 ± 0.3 to 0.8 ± 0.2, which is in tune with the smaller non-graphitic carbon content of RGO monolayers. The significant decrease in I(D)/I(G) has been explained by assuming that substantial order is present in precursor GO monolayers as well as RGO monolayers obtained by solid state reduction.     

In diffusion and electronic energy structure in polymer layers on In tin oxide

We have investigated the effect of In diffusion on the electronic structure of a polyethylenedioxythiophene-poly (styrenesulfonate) layer deposited on InSn_xO_1 − x substrate by photoelectron spectroscopy. We observe additional states near the highest occupied molecular orbital of the polymer layer that are generated by In releases from the substrate. Also, we observe that In diffusion continues into an overlayer of a mixture of poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester. On the basis of the results of the numerical line shape analysis of In 3d_5/2 core level emission, we rule out the presence of metallic In clusters within the organic layers and suggest instead that In/sulfur compounds are present within the organic layers.     

Formation and characterization of cobalt oxide layers on polyimide films via surface modification and ion-exchange technique

Polyimide (PI) films with thin cobalt oxide (Co₃O₄) layers on both film sides have been prepared via a surface modification and ion-exchange technique. The method works by hydrolyzing the PI film surfaces in aqueous potassium hydroxide solution and incorporating Co²⁺ into the hydrolyzed layers of PI film via subsequent ion exchange, and followed by thermal treatment in ambient atmosphere. The PI composite films were characterized by Attenuated total reflection-Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffractions, scanning electron microscopy, transmission electron microscopy and thermogravimetric analyses, as well as surface resistance and mechanical measurements. By varying the absorbed cobalt ion content, a series of PI/Co₃O₄ composite films with insulative to semiconductive surfaces were obtained. The room temperature surface resistances of the semiconductive composite films reached to about 10⁷ Ω. The Co₃O₄ particle formed on PI film surfaces was in the range of 10-40 nm. The final composite films maintained the essential mechanical properties and thermal stability of the pristine PI films. The adhesion between surface Co₃O₄ layers and PI matrix was acceptable.     

Multilayer Graphene:A Potential Anti-oxidation Barrier in Simulated Primary Water

Multilayer graphene as a potential anti-oxidation barrier to protect nickel foils from oxidation was studied in simulated primary water of pressurized water reactors （PWRs）. The results show that after immersion for 1000 h, the structure of the multilayer graphene remains unchanged and no obvious oxide film formed on the graphene coated nickel foils, indicating multilayer graphene can effectively act as the anti-oxidation barrier to protect the substrate from oxidation and hence can improve the heat transfer efficiency of the substrate in simulated primary water of PWRs.     

Size distribution-controlled preparation of graphene oxide nanosheets with different C/O ratios

A convenient and efficient preparation method for separation graphene oxide with well-defined size distribution is developed using a centrifugation technique. The graded profile of graphene oxide nanosheets with narrow size distribution is effectively controlled by varying the centrifugation speed. The results show that the oxygen content of graphene oxide is highly dependent on their size distribution. Graphene oxide nanosheet with large size shows a red-shift in UV–vis absorption spectra, compared to graphene oxide with small size. This phenomenon is interpretation by a density functional theory calculation. The present work will provide a simple method to prepare graphene oxide nanosheets with controllable size distribution and C/O ratio, which will be valuable for the functionalization of graphene-based hybrids and the fabrication of graphene nano-devices.     

Ultrathin silicon oxide films deposited by synchrotron irradiation of condensed layers of silanes and water

Silicon oxide and carbide ultrathin films (less than 50 Å thick) were grown at rates of up to 1 Å s⁻¹ using a previously developed technique. To form silicon oxide, a mixture of silane or tetramethylsilane and water was condensed onto a metal surface, which was then exposed to either broad-band or monochromatized synchrotron radiation. Characterization by soft X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure showed that clean, near-stoichiometric films of self-limited thickness were grown. The results also suggested that the reactions leading to film growth were predominantly excited by electrons produced by photon absorption in the substrate.     

Unusual toughening effect of graphene oxide on the graphene oxide/nylon 11 composites prepared by in situ melt polycondensation

Graphene oxide/nylon 11 composites were prepared by in situ melt polycondensation. These composites displayed better mechanical properties including stiffness and toughness than the pure nylon 11 matrix. The enhanced toughness was ascribed to the change of crystal form of nylon 11, namely the triclinic α crystal form to the pseudo-hexagonal δ′ crystal form transition trend with the incorporation of GO. Scanning electron microscopy and transmission electron microscopy images showed that GO bundles and stacks with an average thickness of 20 nm are homogeneously dispersed over the nylon 11 matrix with almost no large agglomerates.     

Native oxide consumption during the atomic layer deposition of TiO2 films on GaAs (100) surfaces

The consumption of the surface native oxides is studied during the atomic layer deposition of TiO₂ films on GaAs (100) surfaces. Films are deposited at 200 °C from tetrakis dimethyl amido titanium and H₂O. Transmission electron microscopy data show that the starting surface consists of ~2.6 nm of native oxide and X-ray photoelectron spectroscopy indicates a gradual reduction in the thickness of the oxide layer as the thickness of the TiO₂ film increases. Approximately 0.1-0.2 nm of arsenic and gallium suboxide is detected at the interface after 250 process cycles. For depositions on etched GaAs surfaces no interfacial oxidation is observed.     

Structural and opto-electrical properties of the tin-doped indium oxide thin films fabricated by the wet chemical method with different indium starting materials

Tin-doped indium oxide (ITO) thin films were fabricated by the sol-gel spin-coating method with different indium precursor solutions synthesized from In(NO₃)₃ or InCl₃ (denoted as N-ITO and Cl-ITO, respectively). For both N-ITO and Cl-ITO thin films, the increase of mobility/conductivity and the reduction of carrier concentration with increasing annealing temperatures from 400 to 700 °C are related to the increase of crystallization/densification and the annihilation of oxygen vacancies. The refractive index (1.84 at λ = 550 nm), packing density (0.83), conductivity [(234 (Ω-cm)^− 1], and optical band gap (3.95 eV) of N-ITO thin films are higher than that of Cl-ITO thin films, which can be attributed to the higher densification, lower crystallinity, and more free charge carriers of N-ITO thin films. These properties make the indium nitrate-derived ITO thin films have better potential applications for some commercial products.