  A single nanoparticle (NP) mass spectrometry method was used to measure
sublimation rates as a function of nanoparticle temperature (TNP) for a number
of individual graphite and graphene NPs. Initially, the NP sublimation rates
were ca. 400 times faster than that for bulk graphite, and there were large
NP-to-NP variations. Over time, the rate slowed substantially, though remaining
well above the bulk rate. The initial activation energies (Eas) were
correspondingly low and doubled as a few monolayer's worth of material were
sublimed from the surfaces. The high initial rates and low Eas are attributed
to large numbers of edge and other low coordination sites on the NP surfaces,
and the changes are attributed to atomic-scale "smoothing" of the surface by
preferential sublimation of the less stable sites. The emissivity of the NPs
also changed after heating, most frequently increasing. The emissivity and
sublimation rates were anti-correlated, leading to the conclusion that high
densities of low-coordination sites on the NP surfaces enhances sublimation but
suppresses emissivity.
