Synthesis Of Single-crystal Graphene By Chemical Vapor Deposition And Its Gas-sensor Applications

Graphene has excellent optical,electrical,mechanical properties and structural characteristics.Due to its unique academic and potential application value,it has attracted wide attention.Since the successful growth of graphene by chemical vapor deposition(CVD)on copper foil in 2009,the controlled growth of graphene has entered a new chapter.Based on the current research of CVD-grown graphene,synthesis of single-crystal graphene meets with more challenges compared to that of polycrystalline graphene.Nowadays,large-area high-quality polycrystalline graphene films have been gradually industrialized.However,large-area single-crystal graphene is still in a laboratory stage,and there is still a huge gap from practical applications.We investigate the synthesis of single-crystal graphene without grain boundaries and the surface modification of polycrystalline graphene for a good gas-sensitivity performance,Cu(111)single-crystal on the a-Al2O3(0001)substrate(also named C-plane sapphire)was successfully prepared by DC magnetron sputtering following an annealing process.Single-crystal graphene with uniform orientation and regular shape was synthesized on the Cu single-crystal by an atmospheric pressure chemical vapor deposition(APCVD)process,and no grain boundary was observed at the joint of two graphene domains.In addition,we carried out surface modification of high quality single-layer polycrystalline graphene film by ultraviolet ozone(UVO)treatment.The properties of wettability,sheet resistance,defect degree and functional group on the UVO treated graphene were studied.Resistive gas sensors based on UVO-graphene were prepared to study the gas sensing properties towards ammonia.The main research is concluded as follows:Cu(111)/a-Al2O3(0001)substrate was prepared by magnetron sputtering following an annealing process.We investigated the effects of substrate treatment method,sputtering time,sputtering temperature and annealing temperature on the surface morphology and preferred orientation of the single-crystal copper film.A fast growth rate of the copper film about 0.37 nm/s was obtained.The surface of the prepared Cu(111)/sapphire substrate is flat with a low surface roughness of 0.778 nm.The substrate shows only one orientation index,corresponding to Cu(111),and no twin boundaries were observed.The optimal preparation parameters of Cu(111)/sapphire are as follows:firstly,choosing a hydroxy-terminated c-plane sapphire substrate;secondly,sputtering at 100 W,1 Pa,20 sccm Ar for 40 min at 180?;thirdly,annealing at 1050 C for 120 mim with mixed gas of Hz and Ar(30 sccm:300 sccm).Then,we studied the growing parameters of single-crystal graphene by APCVD on annealed copper foil.Single-crystal graphene with regular hexagonal shapes and the same orientation was synthesized on the Cu(111)/sapphire substrate w.What's more,there is no grain boundary between the connected graphene grains.Large-area high-quality single-layer polycrystalline graphene was prepared by low pressure chemical vapor deposition(LPCVD).After the UVO modification,the wettability of the graphene film was improved,which converted to hydrophilic from hydrophobic.With increasing UVO treating time,the sheet resistance of the graphene film firstly decreases and then increases,while the graphene defect degree shows a linearly increase.Through the modification process,the surface of the graphene is further cleaned and also more oxygen-containing functional groups,such as carboxyl group(-COOH),C-O bond and hydroxyl group(-OH),were introduced.Resistive gas sensors were prepared to study the gas-sensitivity performance of UVO-modified graphene.UVO modification can effectively improve the sensing performance of graphene towards ammonia gas.The sensors based on the UVO-treated graphene for 18 min performed the best with a highest response,a fastest response speed and a shortest desorption time.For 20 ppm ammonia,the response of gas sensor based on UVO-modified graphene is 2.08 times higher than that of gas sensor based on pristine graphene.The optimal operating temperature of UVO-graphene based sensors is 20°C.At 50 ppm,the UVO treated graphene sensor can achieve a response of 17.0%at 20?,but only 10.7%at 50°C.Towards 200 ppm test gas,the responses of the UVO treated graphene sensors to ammonia gas at 40? were 11.9 times,47.4 times,and 78.6 times higher than that of ethanol,methanol,and acetone,respectively,indicating a good gas selectivity.