Face centered cubic indium nano-particles studied by UHV-transmission electron microscopy

Structures of indium nano-particles of 3–20 nm were observed by ultra-high vacuum transmission electron microscopy. Structural fluctuation of such small particles was suppressed by cooling the temperature down to 120 K. Indium nano-particles were found to have face centered cubic (fcc) structure below 5 nm in diameter. The larger particles had the body centered tetragonal (bct) structure of the bulk crystal. The particles of fcc and bct structures had different morphology. Wulff’s criterion for their morphologies showed equalization of the surface energies of the (1 1 1), (1 0 0) and (0 0 1) facets for the fcc nano-particles. Theoretical estimation of the elastic energy and free energy suggested that structure change to fcc can occur at 5 nm, if the 2% expansion of the a-axis (4% compression of the c-axis) of the bct lattice results in the 4% increase of the {1 1 1} surface energy.