Chemical vapor deposition and applications of vertical graphene:Progress and prospects
Vertical graphene(VG),also known as carbon nanowalls,carbon nanosheets,standing-up graphene,vertically aligned graphene arrays,etc.,is a unique vertically aligned graphene material.On the one hand,VG possesses many of fundamental characteristics of graphene materials,for instance,high electrical conductivity,high thermal conductivity,structural and chemical stability,large specific surface area,chemically active edges and defects.On the other hand,due to its vertically aligned structure perpendicular to the substrate,non-stacking morphology,abundant exposed sharp edges,open channels,larger specific surface area and porous structure,VG exhibits special excellent properties,showing enormous application potential in various fields such as new energy,gas sensors,electrochemical biosensors,photothermal conversion,flexible sensing,electromagnetic shielding,field emission and semiconductors,etc.Various techniques have been developed for the synthesis of VG in a stable and reliable way,for example,directional freezing,plasma enhanced chemical vapor deposition(PECVD),KOH assisted hydrothermal and substrate sputtering.Among all these methods,plasma enhanced chemical vapor deposition gradually becomes the principal method used for VG fabrication currently due to its intriguing merits of large-area synthesis,low substrate temperature,catalyst-free substrates,high growth rate and controllable process.In addition,introducing plasma into the chemical vapor deposition system enables to regulate the growth of VG.A widely accepted growth mechanism of VG including 3 primary growth stages is proposed as follows:The formation of buffer layers,nucleation,and vertical growth stages.Although this three-stage mechanism has been proved by numerous experimental observations during the PECVD process,there is still a lack of unified and systematic understanding which can guide VG growth precisely for rational structure design.Thus,controllable growth of VG with desirable characteristics for specific applications remains a challenge and it is significant and worthwhile to provide guidelines on the design of every parameter.The vertically aligned structure of VG is a complex result of the actions of carbon materials under conditions such as electric field,temperature field,energetic plasma and substrate catalysis during the PECVD process.The morphology and structure of VG are strongly influenced by a series of operation parameters including carbon sources,etching gas,temperature,pressure and growth time.So far,successful applications of VG include various sensors,stretchable electronic devices,protective coatings,electromagnetic shielding,heating devices,electrode batteries,bio-based materials and other fields.However,there are some limitations left in synthesis methods nowadays,and the application development of VG films in scenarios such as wearables and flexible electronics progressed slowly.Because wearables electronics are mostly based on the flexible materials as substrates considered to be high-temperature sensitive,we assume that the direct growth of VG on flexible substrates at low temperature is likely to become the key for the technological breakthroughs in the future.Furthermore,more efforts are needed in controllable morphology engineering.This paper reviews the progress of VG from four aspects:Structure and properties,formation mechanism,synthesis technology,and applications.Finally,the challenges and future prospects of VG materials are discussed.
vertical graphene(VG)plasma enhanced chemical vapor deposition(PECVD)mechanism of growth of VGflexible sensing
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维普
