The initial size (equivalent spherical diameter) of the Hf NP was ~41 nm, compared to ~35 nm, ~50 nm, and ~57 nm for the graphite, Si, and carbon black NPs, respectively. Prior to the start of the data records Fig. shown, each NP was trapped and cleaned by preheating above 1800 K in 2 mTorr of Ar to desorb contaminants and sublime a few monolayers from the NP surfaces, also melting the NP in the case of silicon. As discussed below, 1800 K is hot enough to destabilize and desorb oxide layers that might be present from air exposure during feedstock handling, i.e., the NPs should have been free of significant oxide after pre-heating.