Controllable Synthesis And Growth Mechanism Of Graphene On Dielectric Substrates

With excellent physical and chemical properties,graphene has broad application prospects in microelectronics,environmental and energy fields.Large-scale production is the prerequisite for the development of emerging materials.Over the past ten years,controllable growth of high-quality graphene on transition metals by chemical vapor deposition(CVD)has made appealing achievements.However,graphene grown on metal substrates needs to be transferred onto dielectric substrates for the fabrication of electronic devices.The complicated transfer process is easy to damage and contaminate graphene.Direct growth of graphene on dielectric substrate,which is compatible with silicon process technology,is able to avoid the difficulties caused by transfer and simplify the fabrication of devices,becoming a significant research field.At present,high reaction temperature and poor quality of graphene are critical issues to be addressed.Herein,we present the direct synthesis of various graphene materials on different substrates(silicon nitride,silica,etc)at lower temperatures by using new carbon sources and introducing metal catalysts.Due to the poor catalytic capability of dielectrics,high reaction temperature(> 1100 oC)is required for graphene growth,leading to damages on substrates.Using methanol as a labile carbon source,we demonstrate a gallium(Ga)-assisted atmosphere pressure CVD to synthesize few-layer graphene films on Si3N4/Si O2/Si at temperature as low as 800 oC.Methanol molecules can be dissociated on the surface of Ga.The formed carbon atoms diffuse and precipitate at the interface between Ga and Si3N4,resulting in graphene films.Graphene films grown on Si3N4/Si O2/Si are directly used to fabricate field-effect transistors.The average carrier mobility is 125.3 cm2 V-1 s-1.Liquid Ga could be directly dropped on the substrate and roll off after growth,achieving graphene on Si3N4/Si O2/Si.The catalyst can be reused five times,but the quality and uniformity of graphene are comparable to that prepared using fresh Ga,revealing the reusability of Ga catalysts.Chemical doping is an efficient way to regulate the electronic property of graphene for different devices.Controllable N-doping is commonly used to prepare graphene with n-type transport behaviors.However,C-N bond is easy to break at high temperature,making it difficult to synthesize N-doped graphene on dielectric substrates.Here,by using pyridine as the sole source of carbon and nitrogen,we propose a metal-free thermal CVD to synthesize high-quality N-doped graphene on various dielectrics(Si O2/Si,quartz,rock crystal,and sapphire).At 1000 oC,increasing H2 flow could promote the quality of graphene and the proportion of graphitic-N.When the flow rate of H2 is 200 sccm,graphitic-N is confirmed to be the only bonding type in the product.Impressively,despite the low doping level(?1.1%),the few-layer N-doped graphene films grown on Si O2/Si exhibit strong n-type semiconducting behaviors with electron mobilities of up to 283 cm2 V-1 s-1 in air,rivaling heavily doped graphene grown on metals.Compared to N-doped graphene with multiple types of nitrogen atoms,single graphitic-N doping leads to more prominent n-type transport behaviors,which is demonstrated by density functional theory calculations.This is owing to the absence of p-type pyrrolic-N and pyridinicN.In addition to parallel growth,out-plane growth of graphene can take place on different substrates,leading to vertical graphene nanosheets(VGs)consisting of a multilayer film and abundant nano-walls.At present,high temperature and plasma generator equipment are usually demanded,which limits the large-scale production and application of VGs.We design a copper vapor assisted conventional CVD to synthesize VGs on Si O2/Si at a temperature of 1000 oC.In this process,graphene buff-layer with cracks is formed,and out-plane growth occurred subsequently,resulting in VGs.The concentration of CH4 plays a critical role in the growth of VGs.By increasing the CH4/H2 ratio,the products change from multilayer graphene film to VGs.But amorphous carbon would be achieved with excessive high CH4/H2 ratios.Furthermore,using Cu Cl2 as the catalyst precursor,VGs are directly grown on submicron hollow silicon spheres,achieving the growth of VGs on three-dimensional(3D)surface.Cu Cl2 pre-loaded in templates can be reduced by hydrogen,then the generated copper nanoparticles provide copper vapors to promote the decomposition of CH4 and the growth of graphene.Graphene yield per milligram of templates is up to 4 mg.After the removal of templates,novel hollow graphene flowers with massive graphene petals are synthesized.This work offers a facile way to grow VGs on 3D non-metallic templates.