In situ determination of the shape of supported silver clusters during growth

The dependence of the shape of metal nanoparticles generated on the surface of wide bandgap materials has been investigated in situ during growth. For this purpose, silver dusters with diameters ranging from 0.8 to 10 nm were fabricated in ultrahigh vacuum on quartz surfaces by deposition of silver atoms and subsequent Volmer-Weber growth. During sample preparation the sub strate temperature was chosen to be either T = 300 or 700 K. The size and s hape of the particles were determined by an approach combining non-contact scanning force microscopy (SFM) and optical spectroscopy. Made at room temp erature, clusters with mean diameters below 1 nm are found to be almost sph erical whereas larger particles become more and more oblate during growth, with the axial ratio decreasing from unity to 0.4. In contrast, if the subs trate temperature is held at 700 K during cluster growth, the particles tur n out to be almost spherical irrespective of their size. The results are th e explained by size- and temperature-dependent relaxation times the particl es need to reach their equilibrium shape. (C) 1999 Elsevier Science B.V. Al l rights reserved.