@article{wang_gupta_garguilo_liu_qin_nemanich_2005, title={Growth and field emission properties of small diameter carbon nanotube films}, volume={14}, ISSN={["1879-0062"]}, DOI={10.1016/j.diamond.2004.10.003}, abstractNote={Vertically aligned carbon nanotube films with diameters smaller than 5 nm, high densities up to 1012/cm2, and lengths of ∼ 5–8 μm were deposited by microwave plasma-assisted chemical vapor deposition. Experiments show that, by continuous reduction in the thickness of the iron film (i.e., ∼0.3–0.5 nm), small diameter carbon nanotubes can be achieved with diameters that ranged from 1–5 nm, and the films are comprised of both single- and double-wall nanotubes. The electron field emission properties of the films were investigated by variable distance field emission and temperature-dependent field electron emission microscopy (T-FEEM). The films showed an emission site density of ∼104/cm2 and a threshold field of 2.8 V/μm similar to multiwalled nanotubes (1.9 V/μm). In addition, they also exhibited a temperature dependence of the emission site intensity.}, number={3-7}, journal={DIAMOND AND RELATED MATERIALS}, author={Wang, YY and Gupta, S and Garguilo, JM and Liu, ZJ and Qin, LC and Nemanich, RJ}, year={2005}, pages={714–718} }
 @article{gupta_wang_garguilo_nemanich_2005, title={Imaging temperature-dependent field emission from carbon nanotube films: Single versus multiwalled}, volume={86}, ISSN={["1077-3118"]}, DOI={10.1063/1.1850616}, abstractNote={Field emission properties of vertically aligned single- and multiwalled carbon nanotube films at temperatures up to 1000°C are investigated by electron emission microscopy, enabling real-time imaging of electron emission to provide information on emission site density, the temporal variation of the emission intensity, and insight into the role of adsorbates. The nanotube films showed an emission site density of 104∼105∕cm2, which is compared to the areal density (from 1012–1013∕cm2to108–109∕cm2). At ambient temperature, the emission indicated temporal fluctuation (∼6%–8%) in emission current with minimal changes in the emission pattern. At elevated temperatures, the emission site exhibited an increase in emission site intensity. From the experimental observations, it is proposed that the chemisorbed molecules tend to desorb presumably at high applied electric fields (field-induced) in combination with thermal effects (thermal-induced) and provide a contrasting comparison between semiconducting (single-walled) and metallic (multiwalled) nanotubes.}, number={6}, journal={APPLIED PHYSICS LETTERS}, author={Gupta, S and Wang, YY and Garguilo, JM and Nemanich, RJ}, year={2005}, month={Feb} }
 @article{wang_gupta_liang_nemanich_2005, title={Increased field-emission site density from regrown carbon nanotube films}, volume={97}, ISSN={["1089-7550"]}, DOI={10.1063/1.1897836}, abstractNote={Electron field-emission properties of as-grown, etched, and regrown carbon nanotube thin films were investigated. The aligned carbon nanotube films were deposited by the microwave plasma-assisted chemical vapor deposition technique. The surface of the as-grown film contained a carbon nanotube mat of amorphous carbon and entangled nanotubes with some tubes protruding from the surface. Hydrogen plasma etching resulted in the removal of the surface layer, and regrowth on the etched surface displayed the formation of a new carbon nanotube mat. The emission site density and the current–voltage dependence of the field emission from all of the samples were analyzed. The results showed that the as-grown sample had a few strong emission spots and a relatively high emission current density (∼20μA∕cm2 at 1V∕μm), while the regrown sample exhibited a significantly increased emission site density.}, number={10}, journal={JOURNAL OF APPLIED PHYSICS}, author={Wang, YY and Gupta, S and Liang, M and Nemanich, RJ}, year={2005}, month={May} }
 @article{gupta_morell_weiner_2004, title={Electron field-emission mechanism in nanostructured carbon films: A quest}, volume={95}, ISSN={["1089-7550"]}, DOI={10.1063/1.1737045}, abstractNote={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.}, number={12}, journal={JOURNAL OF APPLIED PHYSICS}, author={Gupta, S and Morell, G and Weiner, BR}, year={2004}, month={Jun}, pages={8314–8320} }
 @article{wang_gupta_nemanich_2004, title={Role of thin Fe catalyst in the synthesis of double- and single-wall carbon nanotubes via microwave chemical vapor deposition}, volume={85}, ISSN={["0003-6951"]}, DOI={10.1063/1.1796529}, abstractNote={Synthesis of vertically aligned small diameter (single- and double-wall) carbon nanotube films on thermally oxidized n+-Si(001) wafers, with acetylene diluted with ammonia gas mixture using a microwave plasma-assisted chemical vapor deposition technique, is reported. Experiments show that by continuous reduction in the thickness of the iron catalyst film to ∼0.3–0.5nm, or alternately, smaller catalyst particles produces hollow concentric tubes with a fewer number of walls. Double- and single-wall carbon nanotubes with diameters ranging from 1 to 5nm were identified using transmission electron microscopy and Raman spectroscopy. A relatively higher deposition temperature (∼850°C) in conjunction with a controlled catalyst and rapid growth (<40s) allowed for the growth of well-graphitized, high areal density (∼1012-1013∕cm2) nanotubes with reduced amorphous carbon and iron. Our results also indicate that the base growth is the most appropriate model to describe the growth mechanism for the nanotube films.}, number={13}, journal={APPLIED PHYSICS LETTERS}, author={Wang, YY and Gupta, S and Nemanich, RJ}, year={2004}, month={Sep}, pages={2601–2603} }