Field electron emission measurements as a complementary technique to assess carbon nanotube quality

Carbon nanotubes (CNTs) can be used in many different applications. Field emission (FE) measurements were used together with Raman spectroscopy to show a correlation between the microstructure and field emission parameters. However, field emission characterization does not suffer from fluorescence noise present in Raman spectroscopy. In this study, Raman spectroscopy is used to characterize vertically aligned CNT forest samples based on their D/G band intensity ratio (I_D/I_G), and FE properties such as the threshold electric field, enhancement coefficient, and anode to CNT tip separation (ATS) at the outset of emission have been obtained. A relationship between ATS at first emission and the enhancement factor, and, subsequently, a relationship between ATS and the I_D/I_G are shown. Based on the findings, it is shown that a higher enhancement factor (∼3070) results when a lower I_D/I_G is present (0.45), with initial emissions at larger distances (∼47 μm). For the samples studied, the morphology of the CNT tips did not play an important role; therefore, the field enhancement factor (β) could be directly related to the carbon nanotube structural properties such as breaks in the lattice or amorphous carbon content. Thus, this work presents FE as a complementary tool to evaluate the quality of CNT samples, with the advantages of a larger probe size and an averaging over the whole nanotube length. Correspondingly, one can find the best field emitter CNT according to its I_D/I_G.