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Field emission properties of vertically grown Carbon Nanotubes, Nanoflakes and mechanically exfoliated Highly Oriented Pyrolitic Graphite: A comparison

Field emission properties of vertically grown Carbon Nanotubes (CNTs), Nanoflakes (CNFs), and mechanically exfoliated Highly Oriented Pyrolitic Graphite (ex-HOPG) has been investigated using digital Field Emission Microscopy (FEM), Current-Voltage (I-V), and Current-time (I-t) measurements. Prior to the field emission experiments, surface morphology of the samples was studied by using Scanning Electron Microscopy (SEM), Scanning Tunneling Microscopy (STM)/ Spectroscopy (STS), and Atomic Force Microscopy (AFM). The number of emitting spots in FEM images was found to be: nHOPG>nCNT>nCNT in contrast to NCNT> NCNF> NHOPG sequence observed for the potential emitters. The turn-on voltage for 10μA field emission current for CNT, CNF, and ex-HOPG was 400V, 1200V, 1300V respectively. The plot of ln(I/V2) versus 1/V beyond the turn-on voltage was linear for CNF and ex-HOPG but was markedly nonlinear at low voltage for CNT. The I-t characteristics for all the samples exhibited random noise with intermittent spikes for CNT and ex-HOPG. The spikes were totally absent in CNF data. The spectral density of CNT,CNF and HOPG exhibited P(f)=A(Iα/fξ) behaviour with the exponent α=1.6, 1.3, 2 and ξ=1.7, 1.8, 1.8 respectively.The magnitude of current fluctuation for CNT, CNF, and HOPG was 24%, 5%, 28% respectively.