Size-dependent melting of Bi nanoparticles

Nanocalorimetry was used to investigate the melting of Bi nanoparticles. The particles were formed by evaporating Bi onto a silicon nitride substrate, which was then heated. The particles self-assemble into truncated spherical particles. Below 5 - nm average film thickness, mean particle sizes increased linearly with deposition thickness but increased rapidly for 10 - nm -thick films. As expected, small particles were found to exhibit size-dependent melting temperatures less than the bulk melting temperature (e.g., Δ T = 67 K for a 3 - nm radius particle). The measured melting temperatures for particles below ∼ 7 nm in radius, however, were ∼ 50 K above the value predicted by the homogeneous melting model. We discuss this discrepancy in terms of a possible size-dependent crystal structure change and the superheating of the solid phase.