Preparation Of Graphene And Its Dendrites On SiO₂/Si Substrates By CVD Method

Graphene is a two-dimensional crystal composed of sp2 hybridized carbon atoms arranged in a hexagonal lattice with only a single atomic layer thickness.The unique crystal structure of graphene determines its unique properties,which makes it promising for many applications,such as solar cells,field-effect transistors and energy storage materials.To date,many methods for preparing graphene have been developed.Among them,chemical vapor deposition(CVD)has become an effective way to prepare large-area and high-quality graphene because of its merits of low cost and simple process.At present,the preparation of graphene by chemical vapor deposition on metal substrates has gained great progress.However,graphene on the metal substrates cannot be directly applied to the field of electronic devices,and the graphene needs to be transferred to the insulating substrates.Graphene is prone to form defects such as wrinkles,breakage and organic contamination during the transfer process.Therefore,it is significant to prepare graphene directly on insulating substrates.There are many problems in preparing graphene on an insulating substrate,for example small size and low growth rate etc.For these reasons,we combined the characteristics of graphene growth with traditional crystal growth processes,and proposed a new method:induced growth of graphene by "point seeds".The "point seeds" are generally selected to be with the same or similar crystal structure as the graphene.In this paper,a method for preparing heterogeneous "point seeds"(h-BN)on SiO2/Si substrates to induce graphene growth was designed.This method could effectively increase the size of graphene.In addition,the force of between layer and layer of two-dimensional graphene films is strong,and graphene film is difficult to disperse,resulting in a reduction in effective specific surface area,which limits its application in microelectronic devices and catalytic fields.In order to avoid the above problems,modification work has been carried out on graphene materials.Graphene dendrite is one of the representative materials of graphene materials.The three-dimensional dendritic graphene has good stability and large specific surface area.Moreover,the mechanical and electrical properties of graphene dendrites are better than those of graphene films.For this reason,the graphene dendrites were prepared on SiO2/Si substrates by chemical vapor deposition,and the electrical and mechanical properties were studied.The research contents and main results of this thesis are as follows:1)Study on growth of graphene induced by h-BNIn view of the shortcomings of the current technology for the preparation of graphene on insulating substrates,h-BN is used as the heterogeneous "point seeds" to induce graphene growth because of the low lattice mismatch between h-BN and graphene.The h-BN "point seeds" with low density and small size can be obtained on the substrate by mechanical exfoliation method.The quality and the layer of graphene can be effectively controlled by the regulation of methane flow rate and growth temperature.Graphene can continue to grow along the lateral outward edge of the h-BN "point seeds",demonstrating that h-BN has the activity for inducing graphene growth.2)Study on preparation of graphene micro-wafersThe experiments of graphene growth were carried out by using h-BN as "point seeds",and the SiO2/Si substrate was directly placing on the copper foil.By controlling the experimental conditions such as methane flow rate,growth time and growth temperature,graphene wafers with diameter of?63 ?m can be obtained.Graphene is also grown along the side of h-BN.Under the same experimental conditions,the diameter of graphene wafers induced by h-BN is larger than carbon spontaneous nucleation graphene wafers,and the growth rate is significantly improved.This method provides a new way for the subsequent preparation of large size single crystal graphene on insulating substrates.3)Study on preparation of graphene dendrites from SiO2/Si substratesAt present,the preparation of graphene dendrites mainly relies on the use of graphene oxide as raw materials,resulting in poor electrical properties.The substrate is usually made of metal materials,so graphene cannot be directly applied to the field of electronic devices.We successfully prepared graphene dendrites on the SiO2 and Si surfaces of SiO2/Si substrates by chemical vapor deposition.The length of graphene dendrites can be regulated by growth temperature and growth time.The length of graphene dendrites can reach up to 20 ?m,and the diameters of the primary and secondary graphene dendrites are about 1 ?m and 50 nm,respectively.The EDS mapping test confirmed that the content of the elements of the dendrites on both surfaces was comparable to the elemental content of graphene on this substrate.Raman spectroscopy and TEM characterization further confirmed that the dendrites were graphene.The graphene dendrites prepared by this method had a conductivity of about 286 Scm-1 and the Young's modulus of up to 2.26 GPa,which were comparable to other graphene-derived materials.This method avoids the complex post-growth transfer processes,thus promising for use in electronic device filed.