(54b) Measuring the Surface Energy of Unsupported Nanocrystals

A novel method to study the surface energy of unsupported nanocrystals is developed. The experiment consists of two different ion-mobility schemes in series. The first mobility characterization is to size select particles with a differential mobility analyzer (DMA). The second mobility characterization employs an aerosol particle mass analyzer (APM) and measures changes in mass resulting for a controlled evaporation of the nanocrystals. The experiment shows that the onset temperature of evaporation for nanocrystals decreases as the particle size decreases. This gives direct evidence of the Kelvin effect. A simple kinetic model is used to relate the evaporation rate to the temperature dependent surface energy. We report the first measurements for Zn nanocrystal surface energies of 9.0 J/m2 and 13.6 J/m2 at 375 0C and 350 0C, respectively. Such high surface energies can be attributed as an intrinsic property of free (unsupported) nanocrystals. We also observed the edge effect which leads to the evaporation anisotropy in different surfaces of Zn nanocrystals using TEM and SEM analysis.