Research On The Growth Mechanism And Physical Properties Of Vertical Graphene By PECVD

In recent years,graphene has become a research hotspot in the field of two-dimensional（2D）materials due to its excellent properties in the field of light,electricity,heat,etc.However,2D graphene is easy to self-stacking in practical applications,thus losing the unique properties of 2D graphene,resulting in a drop in the performance of materials is far below the theoretical value.Constructing graphene nanoflakes into interconnected three-dimensional（3D）structures can avoid the stack,and improve efficiency in the application.Compared with traditional 2D graphene,3D graphene not only retains the intrinsic properties of 2D graphene,but also obtains the characteristics of low density,high mechanical properties,high electrical conductivity,high specific surface area,and more active edges from 3D structure.Further research on the preparation method,growth mechanism,and excellent properties of 3D graphene could promote the efficient use of graphene in energy storage and conversion,electrochemical catalysis,photocatalysis,sensing,environmental remediation,etc.At present,plasma-enhanced chemical vapor deposition（PECVD）is the main method to grow 3D graphene,which has the advantage in no catalyst,low temperature,high grpwth rate,etc.3D graphene by PECVD is also called vertical graphene because of its unique vertically oriented structure.However,the structure of 3D graphene prepared by PECVD has the unitary structure,which can hardly meet the structure and properties requirements of multiple applications.Moreover,the growth mechanism of VG by PECVD is important to investigate the morphology control of VG,which is still unclear and has set an obstacle to the ability to control the morphology and optimization of VG in applications.Therefore,it is urgent to conduct systematic and in-depth research on the growth mechanism and characteristics of VG by PECVD.In this thesis,the growth mechanism of VG is investigated,which is prepared by the PECVD process,oxygen-assisted PECVD process,and electric-and magnetic-field-assisted PECVD process.In addition,the optical and electrical properties and applications of VG are further studied.The main research contents of this thesis include:（1）By controlling the growth position of the VG in PECVD,VG with different morphology is obtained.By increasing the distance between the substrate and the plasma source antenna,the etching effect of active particles in the plasma is higher than the deposition effect,and the morphology of VG continuously changes from a dense structure to an independent wall-like structure.Compared with the other growth conditions,such as temperature,plasma power,ect.,the morphology control of VG can be easily realized by adjusting the growth position.（2）An ultrahigh VG is successfully prepared by oxygen-assisted PECVD process,which solved the problem of high saturation of vertical graphene with increasing growth time.After 12 h growth with the oxygen-assisted process,the height of VG reaches 80μm.The growth rate is greatly enhanced by oxygen-assisted PECVD process,reaching a 45μm after 6 h.At the same time,the height of VG without oxygen-assisted is only 13μm.In the oxygen-assisted trimming process,the amorphous carbon is etched away so that the growth rate is maintained.Meanwhile,during the growth the flakes of VG are trimmed to keep the exposed sharp edges,and these exposed edges could provide more nucleation sites for the growth of graphene nanosheets.Thus,with the help of oxygen,sturcture of VG is trimmed and showing high heights and growth rates.（3）The electic field of sheath in plasma around the substrate was isolated by ion lens,and utilize electric and magnetic s to control the PECVD process to control the movement and energy of the active particle.Combined with growth conditions,such as growth temperature,growth time,and plasma power,the density of active particles,deposition of active particles and etching effects were effectively controlled.（4）A VG/laser-etched nickel structure is fabricated for light absorption based on intrinsic light absorption and nano-and micro-structure synergistic light absorption mechanisms.The structure has an omnidirectional low reflectivity of 0.25%in the visible range,an absorption rate of up to 99.75%,which is independent of the direction of incidence light.The large spacing between vertical graphene nanowalls is conducive to the formation of light traps and higher light absorption performance.Besides,laser etching technology is used to form a“cauliflower-like”microstructure on the surface of the nickel substrate,which further enhances the light capture ability.（5）The ultrahigh VG has excellent electrochemical properties.The cohesive and uniform structure of ultrahigh VG provides a smooth channel for electrolyte ion transportation,which enables the ultrahigh VG electrode to maintain high surface capacitance and stable volume capacitance at high thickness.At a current density of 0.1m A cm^-2,the surface capacitance and volume capacitance of 80μm thick ultrahigh VG reach241.35 m F cm^-2 and 30.17 m F cm^-3respectively,with a good cyclic stability.（6）A forest-like VG is fabricateed on a quartz substrate,which is composed of unique tree-like graphene sheets.And the tree-like graphene sheets are interconnected to form a 3D conductive network so that VG has low film resistance.And the tree-like graphene sheet is also beneficial for VG to contact with other electrodes.Then it is suitable for friction nanogenerators as an electrode,based on which the nanogenerators obtains a voltage response of 20 V and a current response of 0.75μA.