Electron field-emission mechanism in nanostructured carbon films: A quest

Abstract

An open question to the community about the general consensus on the field-emission mechanism in carbon-based materials led to this study. By applying the Fowler–Nordheim (FN) model for carbon-based films, despite the fact that the microstructure and the resulting physical properties of the films can be tuned by scanning various process parameters, providing, in turn, from almost insulating (less defective) to semiconducting (highly defective) films and even a mixture of the two, the material can be categorized as electrically heterogeneous nanostructured carbon. The electrical heterogeneity arises from the different carbon hybridizations (sp2- versus sp3-bonded carbon). In an attempt to tackle these issues, we have performed a comprehensive analysis of I–V data obtained from filament-assisted chemical-vapor-deposition-grown sulfur-incorporated nanocomposite carbon thin films with different microstructures. Studies of the augmentation of the field-emission properties in this material indicated various roles of sulfur in modifying the film properties [Gupta et al., Appl. Phys. Lett. 80, 3446 (2002)]. The I–V data were fitted to various mathematical forms: I=AV2 exp(−B/V) [FN model], I=C exp(aV1/2/kT) [Schottky model], and I=Vn (n>1, for high fields) [space-charge-limited current (SCLC) model]. The goodness of fit along with the theoretical justification(s) on the electron field-emission results were taken into consideration to provide favorable indications for accepting or discarding any particular model. These findings suggest that there is an apparent crossover from SCLC to FN behavior as a function of film microstructure occurring due to the impurity incorporation as the microstructure transits smoothly from microcrystalline to nanocrystalline carbon. Other evidence in support of the aforementioned suggestion is based on the concept of percolation occurring in this nanocomposite carbon (a mix of conducting–insulating/semiconducting) material, whereby the electrons are allowed to tunnel from one conductive cluster to another separated by an insulating matrix, which is demonstrated through electrical conductivity measurements.