Controllable Synthesis And Doping Effects Of Graphene Films And Reduced Graphene Oxide

Graphene is a two-dimensional carbon-based new material, which is formed by closed packing mono- or multi-layer carbon atoms with honeycomb structure. Due to its excellent electrical and optical properties, graphene films have wide applications in microelectronic and photoelectronic devices. Due to its high specific surface area and high electrochemical activity, reduced graphene oxide(rGO) powders have wide applications in new energy devices. Doping heteroatoms into graphene could effectively modulate its energy band, electrical and electrochemical properties. Currently, the investigations on the controllable synthesis and doping effect of the graphene become the international frontier and hot spots. The present dissertation focuses on the controllable synthesis and doping effect of graphene films and rGO powders. Firstly, controllable synthesis of pristine and nitrogen-, sulfur- and silicon-doped graphene films was systematically investigated. The effect and physical mechanism of doped heteroatoms on the electronic structure and electrical properties of graphene were studied. Then, controllable synthesis of pristine rGO and sulfur-, phosphorus- and silicon-doped rGO was systematically investigated. The effect and physical mechanism of doped heteroatoms on the electronic structure, electrical and electrochemical properties of r GO were studied. The main results are as follows.1. After the controllable synthesis of large-area graphene films by chemical vapor depositon(CVD) method was investigated, the properties of graphene-based alternating current electroluminescence(ACEL) devices and field-effect transistors(FETs) were studied.(1) After the effects of pretreatment of the copper foil substrates, the growth temperature, and the gas flux on the structure and electrical properties of graphene films were systematically investigated, the controllable process of large-area, high-quality monolayer graphene grown by CVD was obtained. On this basis, for the first time, large-area, flexible, graphene-based ACEL was fabricated. The results show that the luminance of ACEL is mainly influenced by the transmittance of graphene, and the best photoelectrical performance was obtained for the monolayer graphene transparent electrode. At 480 V, the luminance and luminous efficiency are 1140 cd/m2 and 5.0 lm/W, respectively. Furthermore, it demonstrates that the graphene-based ACEL device is highly flexible and can work very well even under a very large strain of 5.4%, suggesting great potential applications in flexible optoelectronics.(2) The effects and mechanism of dielectric on the electrical properties of graphene FETs were investigated. Because Si3N4 films could not only prevent graphene FETs from H2 O or O2, but also dope electrons into graphene, the graphene FETs with Si3N4 film shows an air-stable n-type behavior. However, the charged impurities on the surface of SiO2 could not be screened by Si3N4 film. Furthermore, it shows that high-κ organic dielectric can effectively weaken the scattering of charged impurities and the mobility of graphene FETs increases from 1050 cm2/Vs to 5.18x104 cm2/Vs after introducing high-κ organic dielectric dimethyl sulfoxide(DMSO).2. Nitrogen-, sulfur-, and silicon-doped graphene films were synthesized by using solid doping sources, and the effects and mechanism of doped heteroatoms on the electronic structure, electrical properties of graphene were investigated.(1) Controllable synthesis and electrical properties of nitrogen-doped graphene films were systematically investigated. The results show that doping source can obviously influence the doped configuration, and the doped configuration can significantly influence the electrical properties. Compared with pyridinic-N configuration, nitrogen atoms with pyrrolic-N configuration have stronger donor ability and lower carrier scattering.(2) Controllable synthesis of sulfur-doped graphene films was obtained by CVD employing benzyl disulfide(C14H14S2) as a single solid source simultaneously providing carbon and sulfur. The highest sulfur-doping concentration is up to 2.36 at%, and the electrical properties of sulfur-doped graphene shows n-type behavior. Its mobility is as large as ~800 cm2/Vs, which is one order higher than those reported by other groups.(3) For the first time, controllable synthesis of silicon-doped graphene films was obtained by CVD employing triphenylsilane(C18H16Si) as a single solid source simultaneously providing carbon and silicon. The silicon-doping concentration could be modulated ranging from 0 to 6 at%. Silicon-doped graphene exhibits p-type behavior and its mobility is as large as ~660 cm2/Vs.3. For the first time, controllable synthesis of pure thiophene-sulfur doped rGO(S-rGO) was obtained by ethanol solvent-thermal method, and the highest sulfur-doping concentration is up to 1.2 at%. The results show that sulfur doping could not only facilitate the reduction of graphene oxide, but also increase the electron concentration of rGO. Compared with pristine rGO, the conductivity of thiophene-sulfur doped rGO is increased by 321%, in which 192% is resulted from the higher reduction degree and 129% is resulted from the doped sulfur atoms. On this basis, TiO2 mixed with S-rGO, was employed as photoelectrode in dye-sensitivity solar cells(DSSCs). The photoelectric conversion efficiency of DSSCs with S-rGO/TiO2 photoelectrode increased by 49 % compared to that with conventional TiO2 photoelectrode.4. The synthesis process of phosphorus- and silicon-doped rGO was investigated, and then the effects and mechanism of doped heteroatoms on the electronic structure, electrochemical properties of rGO were studied.(1) Phosphorus-doped rGO(P-rGO) was synthesized by high-temperature thermal-exfoliation method. For the first time, effects and mechanism of various phosphorus doped configurations on the electrocatalytic properties of rGO counter electrode were investigated. The results show that compared with P-O configuration, the P-C configuration has higher electrocatalytic activity; in addition, the electrocatalytic activity will increase with increasing phosphorus doping concentration. When the doping concentration of P is 1.27 at%, the photoelectric conversion efficiency is 6.04 %, which reaches 90% of that of Pt counter electrode. It suggests the P-rGO is very promising to be used as the counter electrode for DSSCs with low cost and high efficiency.(2) For the first time, silicon-doped rGO(Si-rGO) was synthesized by high-temperature thermal-exfoliation method. On this basis, DSSCs are fabricated, in which Si-rGO is employed as counter electrode materials. The results show when the Si doping concentration is 5.64 at%, Si-rGO has the highest electrocatalytic activity, and the photoelectric conversion efficiency reaches 4.95 %.