Hyperthermal atomic hydrogen and oxygen etching of vertically oriented graphene sheets

Abstract

Carbon nanosheets have previously been shown to be promising high current field emission cathodes for a variety of potential applications. The vertically oriented planar sp2 carbon nanosheets grown by rf plasma-enhanced chemical vapor deposition terminate with one to seven graphene sheets and grow to ∼1 μm in height. High current field emission, Je∼0.15 mA mm−2 (8 V μm−1), conducted within an ultrahigh vacuum system in a diode configuration in line-of-sight to a mass spectrometer, shows that CH4, CO2, and CO are generated as a result of cathode bombardment by hyperthermal oxygen and hydrogen neutrals and ions generated by electron stimulated desorption at the Cu anode. Confirmation of the mechanism was achieved by repeating the experiments using a Au anode. Simultaneous acquisition of I-V data and the partial pressures of reaction products in the mass spectrometer have shown repeatable, sustained CH4, CO2, and CO production. As these hyperthermal atomic hydrogen and oxygen species impinge on the sidewalls and edges of the carbon nanosheets, they bond to various sites throughout the sp2 carbon array. Progressively, as further hydrogen and oxygen arrive, CH4, CO2, and CO are formed and desorbed, thereby etching the film. Raman spectroscopy has confirmed a corresponding increase in defect sites (ID/IG increased from 0.57 to 0.81) over the test interval. Scanning electron microscopy cross sections of carbon nanosheet cathodes before and after high current lifetime testing (>200 h) show the average etching rate to be ∼1.7×10−3 nm/s.