Doping Graphene Sheets Grown by Chemical Vapor Deposition Synthesis and the Influence of Nitrogen, Oxygen and Bromine Dopants on Electrical Resistance

Abstract

The ultrathin two dimensional hexagonal lattice of sp2 hybridized carbon layers also known as graphene possesses distinctive properties and high performance chemical stability, surface area, mechanical strength, thermal and electrical conductivity. However, pristine graphene being a zero-band gap material is difficult to be used in applications requiring a band gap. There are various methods to tune the band gap of graphene. However, heteroatom doping of graphene can effectively tailor its band gap and electro-chemical properties. Moreover, doping can endow versatile products, as graphene can be doped with a wide range of heteroatoms either singly or in combination of two or more. Besides, the nature of the obtained product can further vary depending upon the synthetic process being employed. Present work is specifically based on doping of graphene with bromine, and oxygen and nitrogen species using low pressure chemical vapor deposition (CVD) process. This came out using single solid precursor of 1,2,5,6,9,10-Hexabromocyclododecane and nitrophenyl ferrocene respectively. Moreover, the CVD growth accomplished was of short duration (less than 5 minutes), facile single step and required low amount of precursor. Pure H2 was used as a carrier gas during the heating and growth steps while pure Ar was used during the cooling step. This process produced doped graphene sheets that are crystalline with large area. Furthermore, the effects of different CVD parameters and properties of doped graphene were investigated in details studying (i) the CVD reactor tube configuration (half closed inner quartz tube and fully open inner quartz tube) and (ii) precursor amounts (0.5, 1, 5, 10 mg) (iii) reaction duration (3, 5, 7 minutes) (iv) and precursor heating temperature (300, 380, 500 ⁰C) to better understand the growth mechanisms involved and provide doping and layer number control. The results are characterized using Raman spectroscopy, scanning electron microscopy, Fourier-transform infrared spectroscopic (FTIR), high resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS) techniques. In addition, to evaluate the electronic properties of doped graphene sheets, four-probe sheet resistance method has been utilized. The characterization results showed that the graphene sheets were successfully doped with the bromine, nitrogen and oxygen atoms. The doping percentage acquired was 1.8 % for bromine covalently bonded in graphene (C-Br bond), around 2 % for bromine molecularly adsorbed on graphene. The sheet resistance increased with the increasing precursor heating temperature in the explored temperature range of 200, 250 and 300 ⁰C. The atomic percentage of O and N was determined as 8.4 and 2.9 % respectively in nitrogen and oxygen doped graphene sheets. The doped graphene sheets were tested for their electronic property of sheet resistance using four-probe sheet resistance measurements. The results revealed increasing sheet resistance as the precursor amount increased that led to increased layer number of the doped graphene sheets. Moreover, the increasing growth times lead to an increase in the amount of defects. For the different precursor heating zone temperatures, the corresponding sheet resistance was highest at temperature of 300 °C where there was incomplete coverage of the hetero-doped graphene, while for higher temperature at 380 °C with complete coverage the sheet resistance was reduced. The oxygen and nitrogen doped graphene macrostructure synthesized here can find applications in environmental applications such as greenhouse gas mitigation. Moreover, their synergistic interactions with ionic liquids can be explored for the ultrafast removal of toxic industrial contaminants as well as membrane separation technologies. Electrostatic forces, π−π interactions and hydrogen bond between the ionic liquid and doped graphene architectures can play a major role for the exceptional separation and adsorption capacity of molecules. Keywords: Hetero-doped graphene based macromolecular architecture, low-pressure CVD synthesis, precursor powder, rapid growth, bromine doped graphene, nitrogen and oxygen doped graphene, electrical sheet resistance, functionalization with ionic liquids and environmental applications.