1. Vacuum Ultraviolet Photo-oxidation of Carbon Nanotubes M. R. Rasmussen, D. D. Allred Nanotube Etching Results—Scanning Electron Micrographs 0 min15 min30 min60 min120 min180 min 30,000x Alignment 15,000x Alignment From 0 to 30 minutes, the nanotubes don’t appear significantly etched.Black dots are particles of iron catalyst. When nanotubes cross them, they conduct, appearing white. This gives the nanotubes the appearance of a white string with bright beads. 4 Theory papers suggest nanotubes oxidize preferentially at endcaps and defect sites. 5 The dashed line suggests that nanotube etching started at a defect in the middle, and was stopped at intersections with other nanotubes, creating T-junctions. At 60 minutes, there are markedly fewer nanotubes. T-junctions again suggest that etching is stopped where nanotubes cross. The top photo shows a residue of unknown composition left by incomplete etching. Not shown in these photos are several patches of bare substrate where the nanotubes were completely etched. At 120 minutes, only small patches of nanotubes remain, sparsely distributed on the substrate. At 180 minutes, it was difficult to find any nanotubes to photograph. The black square is unwanted carbon, deposited by the SEM. A similar square of carbon was deposited on a bare part of the substrate; 15 minutes of oxidation was sufficient to remove it. 15 min180 min 5,000x Alignment The dust particle provides shelter from VUV exposure and a different site where atomic oxygen can adsorb. Nanotubes are etched everywhere but under this shelter. This suggests that photolithography could be used to create nanotube devices. Residue after etching ? Similar densities SEM Deposited Carbon One nanotube remains A small patch is left Decreased density T-Junctions Similar densities Dust acts as a mask Streaks due to charging Only the sheltered nanotubes remain Department of Physics and Astronomy Brigham Young University A mat of single-walled nanotubes was exposed to 172 nm vacuum ultraviolet light from a xenon excimer lamp, in air. The sample was taken out at intervals to characterize it; it was oxidized for a total of 3 hours. X-ray photoelectron spectroscopy (XPS) was used to determine the atomic composition of the sample, and the fractions of different carbon functional groups Scanning electron microscopy (SEM) was done after XPS, because the SEM deposits additional carbon on the substrate. This is removed when the nanotubes go back under the lamp. Experiment Introduction Ultraviolet light has been found to functionalize carbon nanotubes by creating ozone, which reacts with the nanotube surface to create ether, carbonyl, and carboxyl functional groups. 1 We used a 172 nm vacuum ultraviolet lamp, normally used to clean optical surfaces. We hoped this lamp would functionalize nanotubes more effectively, since it creates atomic oxygen, and the higher energy photons (7.2 eV) would also affect photo- oxidation. 2,3 We also hoped that with long enough exposure we could fully oxidize the carbon to etch the nanotubes completely away. This technique has applications for composite materials, as nylon molecules could be bonded to functionalized sites on the nanotubes. This technique could also be applied with photolithography to create electronic devices, by etching away nanotubes in unwanted areas. We fit curves under the carbon 1s peak to determine the fractions of carbon, ether/carbonyl and carboxyl moieties. After 15 minutes, the nanotubes appear to have a much larger fraction of carboxyl groups. Unfortunately, much of this may be due to adventitious carbon, rather than functionalization of the nanotubes The XPS antechamber is dirty and deposits adventitious carbon. Exposure to the lamp reduces carbon, as seen in the survey scans, but the carbon seen in the 120 minute scan is almost entirely from the XPS system rather than from nanotubes. CarbonEther/Carbonyl Carboxyl CarbonEther/Carbonyl Carboxyl O Auger O 1s C 1s Si 2s Si 2p O 2s O Auger O 1s C 1s Si 2s Si 2p O 2s Results—XPS Scans Conclusions This is an effective method to fully oxidize and etch carbon nanotubes. They do not etch homogeneously; the density of nanotubes is patchy after 1 hour, but complete etching can be done in 3 hours. Adventitious carbon can also be fully removed with 15 minutes or less of oxidation. Sheltered areas effectively mask the nanotubes from being etched, suggesting possibilities with photolithography. XPS results suggest that at shorter exposure times, nanotubes can be functionalized, but further study is needed to conclusively demonstrate this. Abstract Photo-oxidation of carbon nanotubes occurred with exposure to 172 nm vacuum ultraviolet light from a xenon excimer lamp, in air. Characterization with X-ray photoelectron spectroscopy (XPS) suggests that nanotubes are functionalized with ether, carbonyl, and carboxyl groups after a few minutes exposure. This has not yet been fully demonstrated due to contamination in our XPS system. Scanning electron microscopy (SEM) shows the structure of these nanotubes remains unchanged; it also shows that at longer exposures of one or more hours, the nanotubes can be etched away through complete oxidation. Acknowledgements Stephanie Getty, of NASA Goddard Space Flight Center, provided this nanotube sample. The excimer lamp was provided by Resonance, Ltd Jeff Farrer and Bryan Hicks provided access and training for SEM; access and training for XPS was provided by Matthew Linford and Lei Pei. Elisabeth Strein tracked down the source of the impurities in the XPS. Funding was provided by the College of Physical and Mathematical Sciences at Brigham Young University. References 1 B. Parekh et al., Mater. Res. Soc. Symp. Proc. 887 (Q01), 07.1-07.6 (2006). 2 Jun-Ying Zhang and Ian W. Boyd, Appl. Phys. Lett. 71 (20), 2964-2965 (1997). 3 Koji Asano et al., Jpn. J. Appl. Phys 45 (4B), 3573-3576 (2006). 4 Johnathan Goodsell et al., The Journal of the Utah Academy of Sciences, Arts, and Letters 84, 130-140 (2007). 5 S. Dag et al., Phys. Rev. B 67 (16), 165424-5 (2003).