CVD and applications of standing, dendritic and continuous graphene and their hybrids

Plasma Enhanced Chemical Vapor Deposition (PECVD) and thermal CVD of graphene films, single-domain graphene dendrites, standing graphene structures, hybrid graphene-diamond nanoplatelets, and hybrid graphite-diamond coatings and their properties and applications will be reported. Grain-boundary engineering of CVD thin-film graphene, diamond, and their hybrids allows novel functions of these nanoscale carbon materials to be tailored for practical applications. Under the influence of electric field in the plasma sheath and the impinging ions and neutral radicals, standing nanocrystalline multi-layer graphene microstructures are grown on graphene, diamond, and non-carbon substrates [1-3]. These standing graphene structures develop into interconnected wall-like microstructures, also known as carbon nanowall, which exhibits a porous surface morphology with a large effective surface area and sharp edges. By properly controlling the synthesis conditions, such standing multi-layer graphene nanowalls can be incorporated with nanocrystalline diamond to form hybrid graphene-diamond films of high electrical conductivity while preserving essential part of diamond’s chemical and electrochemical properties. Incorporated nanodiamond in the hybrid graphene-diamond structure serves as diamond seeds to allow the coating on it a continuous nanodiamond film. This allows a biochemically inert nanodiamond coated electrical circuit to be built as an all-carbon structure such as an electrosurgical tool. The embedded low resistance multi-layer graphene provides a low-resistance and, therefore, allowing low applied voltage for delivering higher power to a resistive heater. Low-voltage and high-power-density are desirable for special applications such as electrosurgical tools. In the first part of this presentation, the fabrication of an all-carbon and nanodiamond encapsulated resistive heater for biomedical applications will be discussed. By controlling orientation and shape dependent anisotropic growth rate of graphene by competitive graphene growth and etching processes, single-domain monolayer graphene dendrites of excellent crystalline quality have been achieved [4]. These graphene dendrites exhibit branch aspect ratios higher than 100 and very large ratios of the length of edge lines to their surface areas. Unique electrochemical properties of the long graphene edges and the high electronic quality graphene branches without domain boundaries provide opportunities for special applications of graphene. The atom scale sharp edges allows electron field emission at a low applied voltage. By shifting the competitive growth and etching processes towards a higher growth rate and a lower etch rate, the width of graphene branches is allowed to increase and eventually leading to the merger of neighboring graphene branches. The high growth rate of graphene primary branches, the high crystalline quality of graphene, and the controllable gap spacing between graphene branches provide us a novel route to the growth of high quality graphene films of large domain sizes. In the second part of this presentation, the synthesis process and the mechanisms for the formation of high-aspect-ratio graphene dendrites will be discussed along with their properties and potential applications. Continuous graphene films synthesized by conventional CVD or by the merger of branches of dendritic graphene possesses excellent properties for electronic and optoelectronic applications. In the third part of this presentation, effects of graphene and hydrogenated graphene on surface enhance Raman scattering of molecules and photo-induced conductivity change in graphene and hydrogenated graphene [5] will be presented. Hydrogenated graphene is among the thinnest possible electrical insulators which is stable in the ambient atmosphere. It, therefore, serves as an excellent encapsulation layer to protect metallic nanoparticles such as silver from undesirable environmental reactions without significantly attenuating the SERS effects for sensor applications. On the other hand, intrinsic graphene is a zero-gap semimetal. Hot carrier effects and carrier multiplication in graphene result in special optoelectronic effects such as reduced conductivity upon light illumination, which will be discussed as negative photoconductivity in this part of presentation. The basic mechanisms and methods of enhancing the photoresponse will be presented. Environmental effects on photoresponse are discussed [6]. Applications of such negative photoconductivity including photodetectors will be discussed.