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By using temperature-variable scanning tunneling microscopy, we studied two-dimensional vacancy islands on Pt͑111͒ between 593 and 713 K. Due to the difference in the step free energies of A and B steps, the equilibrium shape of the vacancy islands has a threefold symmetry. From the analysis of the equilibrium shape of the vacancy islands, we calculate the angular dependence of the step free energy on Pt͑111͒. The absolute values for the step free energies per atom of A and B steps are determined from the equilibrium shape fluctuations to be ␤ A = 348± 16 meV/atom and ␤ B = 300± 14 meV/atom, respectively. Furthermore, we derive the temperature dependence of the step-edge stiffness ␤ with ␤ A between 510 and 453 meV/atom and ␤ B between 1974 and 1104 meV/atom in the temperature range considered. From the stiffness, we obtain the kink formation energies for A and B steps, A = 143± 9 meV and B = 206± 9 meV, respectively.

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Obliquely deposited amorphous Ge x Se 100−x thin films at several compositions in the 15% Ͻ x Ͻ 33.3% range and at several obliqueness angles in the 0 Ͻ ␣ Ͻ 80°range at each x were evaporated on Si and glass substrates. Here ␣ designates the angle between film normal and direction of vapor transport. Raman scattering, IR reflectance, and optical absorption measurements were undertaken to characterize the vibrational density of states and optical band gaps. Edge views of films in scanning electron microscopy ͑SEM͒ confirm the columnar structure of obliquely ͑␣ = 80°͒ deposited films. Films, mounted in a cold stage flushed with N 2 gas, were irradiated to UV radiation from a Hg-Xe arc lamp, and photocontraction ͑PC͒ of oblique films were examined. Compositional trend of PC exhibit a bell-shaped curve with a rather large effect ͑0.25 m͒ centered in the 20% Ͻ x Ͻ 25% range, the intermediate phase ͑IP͒ with the PC decreasing at x Ͼ 25%, the stressed rigid phase, and at x Ͻ 20%, the flexible phase. IR reflectance confirmed absence of photo-oxidation of films under these conditions. The IP represents a range of compositions across which stress-free networks form. Columns observed in SEM reveal a high aspect ratio, with typical lengths in the 1 -2-m range and a lateral width in the 50-nm range. We observe a blueshift ͑up to 0.38 eV͒ in the optical band gap of oblique films ͑␣ = 80°͒ in relation to normally deposited ͑␣ =0°͒ ones, a result we identify with carrier confinement in nanofilaments ͑Ͻ10 nm͒ that form part of columns observed in SEM. In the IP, the large PC results due to the intrinsically stress-free character of filaments, which undergo facile photomelting resulting in film densification. Ge-rich films ͑25% Ͻ x Ͻ 33.3%͒ are intrinsically nanoscale phase separated and consist of nanofilaments ͑ϳGe 25 Se 75 ͒ that demix from a Ge-rich ͑ϳGe 40 Se 60 ͒ phase that fills the intercolumnar space. Loss of PC in such films is traced to the growth of the Ge-rich phase, which is intrinsically stressed and photoinactive. In contrast, Se-rich films are homogeneous, and loss of PC as x decreases below 20% is traced to the accumulation of network stress in the Se-rich nanofilaments. The microscopic origin of the giant PC effect in amorphous semiconducting thin films can be traced, in general, to three conditions being met: ͑i͒ growth of a columnar structure leading to porous films, ͑ii͒ formation of columns that are rigid but intrinsically stress free, and ͑iii͒ an appropriate flux of pair-producing radiation leading to photomelting of columns. These findings lead us to predict that PC will, in general, be maximized in obliquely deposited films of semiconducting networks glasses residing in their IP when irradiated with supra-band-gap radiation.

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