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We report that heating chemical vapor deposition grown monolayer MoS2 in air at temperatures as low as 285 °C for 2 h results in rapid degradation of the monolayer within 2.5 weeks of ambient air exposure after heating. We find that the rapid degradation proceeds via the growth of dendrites on the basal plane that have a fractal dimension close to that of diffusion-limited aggregation. We also observe dendrites in unheated samples that have been in ambient air for a year. We explain the rapid degradation after heating to an increase in MoO3. We propose that the mechanism for dendrite growth involves the diffusion of H2O to oxide sites. This results in the liquefication of the oxides. The liquefied oxides do not protect the surface from further oxidation. Putting heated samples in a dry box for 2 weeks immediately after heating prevents the rapid degradation from occurring.

Ecton, Philip A. Low-Energy Electron Irradiation of Preheated and Gas-Exposed Single-Wall Carbon Nanotubes. Doctor of Philosophy (Physics), December 2016, 110 pp., 53 figures, 40 numbered references, bibliography.We investigate the conditions under which electron irradiation of single-walled carbon nanotube (SWCNT) bundles with 2 keV electrons produces an increase in the Raman D peak.We find that an increase in the D peak does not occur when SWCNTs are preheated in situ at 600 C for 1 h in ultrahigh vacuum (UHV) before irradiation is performed. Exposing SWCNTs to air or other gases after preheating in UHV and before irradiation results in an increase in the D peak. Small diameter SWCNTs that are not preheated or preheated and exposed to air show a significant increase in the D and G bands after irradiation. X-ray photoelectron spectroscopy shows no chemical shifts in the C1s peak of SWCNTs that have been irradiated versus SWCNTs that have not been irradiated, suggesting that the increase in the D peak is not due to chemisorption of adsorbates on the nanotubes.

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The authors investigate the mechanism for etching of exfoliated graphene multilayers on SiO 2 by low-energy (50 eV) electron irradiation using He plasma systems for electron sources. A mechanism for this etching has been previously proposed in which the incident electrons traverse the graphene and dissociate oxygen from the SiO 2 substrate at the graphene/SiO 2 interface. The dissociated oxygen reacts with carbon defects formed by the electron irradiation and thereby etches the graphene from below. They study etching using graphene flakes of various thicknesses on SiO 2 , low and higher resistivity Si, indium tin oxide (ITO), and silicon carbide (SiC). They find that thicker layer graphene on SiO 2 does not etch less than thinner layers, contrary to the previously proposed model. They find that etching does not occur on low-resistivity Si and ITO. Etching occurs on higher resistivity Si and SiC, although much less than on SiO 2 . This is attributed to He ion sputtering and vacancy formation. From these observations, they propose that oxygen etches graphene from above rather than below. In addition, they propose He ions instead of incident electrons cause the defects that oxygen reacts with and etches.