Raman spectroscopy and field-emission properties of CVD-grown carbon-nanotube films

Carbon-nanotube films are very efficient cathodes for field-emission device s. This study presents a comprehensive comparison between structural, spect roscopic and field-emission properties of films of aligned and non-aligned multi-wall nanotubes (MWNTs) which are grown by thermal chemical vapour dep osition. Three types of films are investigated: vertically aligned MWNTs wi th clean and coated nanotube side walls as well as non-aligned MWNT films. Raman spectra taken on the aligned MWNT films consist of many lines of firs t-, second- and third-order signals. Several lines are reported here far th e first time for MWNTs. The presence of the surface coating leads to a decr ease and broadening of the higher-order signals as well as an increase in t he disorder-induced contributions in the first-order regime. The aligned MW NT films have excellent field-emission properties with very high emission c urrent densities and low turn-on and threshold fields. The presence of a su rface coating has no impact on the efficiency of the field-emission process . Films of non-aligned MWNTs show considerably reduced electron-emission cu rrent densities and larger critical fields.