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Synthesis of graphene sheets and their application for transparent conductors

Authors Zheng, Qingbin
Issue Date 2011
Summary Graphene, a monolayer of sp2-bonded carbon atoms or one monolayer of graphite, has a special atomically thick two dimensional structure and possesses unique mechanical, electrical, thermal and optical properties. These properties make graphene a good candidate material for transparent conductors. Monolayer graphene oxide (GO) sheets with sizes ranging from a few to ~200 μm are synthesized based on a chemical method. In order to obtain ultra-large graphene oxide (UL-GO), three main modifications were made in our experiments: i) using the natural graphite with a large lateral size (up to ~800 μm) as starting material; ii) using intercalation and thermal shock to perform exfoliation, avoiding the destructive process of ultrasonication; iii) using a three-step centrifugation to sort the GO by sheet size. New thermal and chemical schemes, which include (i) a modified thermal treatment, (ii) acid treatment in a HNO3 bath and (iii) doping by immersing in a SOBr2 solution, are developed to treat graphene films to improve the electrical conductivity and transparency. It is shown that a longer thermal treatment at 1100 ⁰C as well as additional acid and doping treatments reduce the sheet resistance by about 20–50% with improved transmittance. The final product has a sheet resistance of 1600 Ω/sq and a transparency of 82%, which is quite sufficient to replace the transparent conducting films made from indium tin oxide for many existing applications in photovoltaic cells and optoelectronics. The transmittance and sheet resistance measured after 3 months of exposure to air confirms the stability of the improved characteristics after the additional treatments. Transparent conductive films are produced using the ultra-large graphene oxide (UL-GO) sheets that are deposited layer-by-layer on a substrate using the Langmuir-Blodgett (L-B) assembly technique. The density and degree of wrinkling of the UL-GO monolayers are turned from dilute, close-packed flat UL-GO to graphene oxide wrinkles (GOWs) and concentrated graphene oxide wrinkles (CGOWs) by varying the LB processing conditions. The method demonstrated here opens up a new avenue for high-yield fabrication of GOWs or CGOWs that are considered promising materials for hydrogen storage, supercapacitors, and nanomechanical devices. The films produced from UL-GO sheets with a close-packed flat structure exhibit exceptionally high electrical conductivity and transparency after thermal reduction and chemical doping treatments. A remarkable sheet resistance of ~500 Ω/sq at 90% transparency is obtained, which outperforms the graphene films grown on a Ni substrate by chemical vapor deposition. The technique used in this work to produce transparent conductive UL-GO thin films is facile, inexpensive, and tunable for mass production. Regarding the theoretical part, the effects of the degree of functionalization, molecular structure and molecular weight of functional groups on the Young’s modulus of graphene sheets were investigated through molecular dynamics and molecular mechanics simulations. The dependence of shear modulus, strength and critical wrinkling strain of graphene sheets on the chemical functionalization was also examined. It is found that Young’s modulus depends greatly on the degree of functionalization and molecular structure of the functional groups, while the molecular weight of the functional groups plays a minor role in determining Young’s modulus. The chemical functionalization also reduces the shear modulus and critical wrinkling strain. The binding energy between the functional groups and the graphene sheets is mainly responsible for these findings.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2011
Language English
Format Thesis
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