Nano-graphene growth and texturing by Nd:YAG pulsed laser ablation of graphite on Silicon

Abstract

Pulsed laser ablation from a rotating graphite target operating both in vacuum (∼10-5 Pa) and in He sustaining gas (∼10 Pa) has been used to grow thin carbon films on Si <100>substrates kept at temperatures from RT to 900 °C. Synchrotron and laboratory X-ray diffraction (XRD), performed at grazing incidence, established the formation of nano-sized graphene structures at higher deposition temperatures (∼800 ÷ 900°C). When the carbon plume was expanding in vacuum, these structures resulted to be formed by few parallel graphene layers, characterised by an oriented growth along the graphene planes, with the axis parallel to the substrate. High resolution (HR) cross section TEM images of C nano-structures on grids or in film/Si substrate confirmed both size and orientation of the graphene nano-particles. The presence of He atmosphere in the reaction chamber changes basically the particle nucleation process: a clustering phenomenon of aromatic structures is promoted in the gas phase also at low temperature. The nano-structured particles, however, are characterised by a round shaped morphology and random orientation. The mass density of deposited films, measured by X-ray reflectivity, is also strongly dependent on the experimental settings: films grown in the inert gas show lower density compared to the vacuum deposited ones. The preferential vertically oriented growth of graphene layers in vacuum and high temperature can be explained as a combined effect of different processes under a fast kinetic mode: thermal surface diffusion, in-plane growth of graphene sheets and line source direction of activated carbon species of the laser plume. He deposited samples are characterised by a different nucleation and growth process and a more complex structure.