AVS 47th International Symposium
    Surface Science Monday Sessions
       Session SS3-MoM

Paper SS3-MoM5
Nanocrystal Surface Melting -- A UHV X-ray Diffraction Study on n-Pb

Monday, October 2, 2000, 9:40 am, Room 210

Session: Surface Science Opportunities and New Applications
Presenter: K.F. Peters, Northwestern University
Authors: K.F. Peters, Northwestern University
Y.-W. Chung, Northwestern University
Correspondent: Click to Email
Discovery of a nanocrystal surface phase transition is reported. The surface influence on the melting of nanoparticles has been debated since seminal concepts of surface melting and of a size-dependent melting temperature were given by Faraday and Thompson. On macroscopic Pb crystals, surface melting occurs on certain surface orientations yet not others. On highly curved surfaces of nanoparticles, 3 competing melting theories are based on liquid skin formation before melting, during melting, or never. Each theory also predicts a depression of the melting temperature Tm(D) as a function of crystallite diameter D. To measure the surface melting and melting temperature of Pb nanocrystals, we applied x-ray powder diffraction in situ in UHV.@footnote 1,2@ An array of Pb nanoparticle islands of 5-50 nm diameter was fabricated on a silica substrate by evaporation and annealing. The Pb 111/200 powder diffraction peaks were collected versus temperature. Narrowing of the diffraction peak revealed the melting of smaller particles at lower temperatures. Fourier analysis of the peak shape shows the evolution of the size distribution of 'crystallites' with increasing temperature. The size-dependent melting temperature was found to be Tm(D)=1-0.62/D, in close agreement with the Liquid Skin Melting Model. To measure directly the liquid skin growth on the crystallites, we rely on the undercooling of liquid particles without solidification. First the sample is heated to within 3K of the melting temperature of the largest particles in the sample (to melt the smaller particles). Upon slight cooling, we observed a reversible increase in the diffraction intensity and a narrowing of the diffraction peak shape. These changes in the crystallite volume and size are due to small changes in the liquid skin thickness (up to 0.5nm over a 20K temperature excursion for crystallites of 20 and 50nm diameter). @FootnoteText@ @footnote 1@Appl.Phys.Lett., 71 (16), 1997, 2391-2393. @footnote 2@Phys.Rev.B, 57, 21, 1998, 13430-13438.