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Title: The energy landscape of glassy dynamics on the amorphous hafnium diboride surface

Direct visualization of the dynamics of structural glasses and amorphous solids on the sub-nanometer scale provides rich information unavailable from bulk or conventional single molecule techniques. We study the surface of hafnium diboride, a conductive ultrahigh temperature ceramic material that can be grown in amorphous films. Our scanning tunneling movies have a second-to-hour dynamic range and single-point current measurements extend that to the millisecond-to-minute time scale. On the a-HfB{sub 2} glass surface, two-state hopping of 1–2 nm diameter cooperatively rearranging regions or “clusters” occurs from sub-milliseconds to hours. We characterize individual clusters in detail through high-resolution (<0.5 nm) imaging, scanning tunneling spectroscopy and voltage modulation, ruling out individual atoms, diffusing adsorbates, or pinned charges as the origin of the observed two-state hopping. Smaller clusters are more likely to hop, larger ones are more likely to be immobile. HfB{sub 2} has a very high bulk glass transition temperature T{sub g}, and we observe no three-state hopping or sequential two-state hopping previously seen on lower T{sub g} glass surfaces. The electronic density of states of clusters does not change when they hop up or down, allowing us to calibrate an accurate relative z-axis scale. By directly measuring and histogramming single cluster vertical displacements,more » we can reconstruct the local free energy landscape of individual clusters, complete with activation barrier height, a reaction coordinate in nanometers, and the shape of the free energy landscape basins between which hopping occurs. The experimental images are consistent with the compact shape of α-relaxors predicted by random first order transition theory, whereas the rapid hopping rate, even taking less confined motion at the surface into account, is consistent with β-relaxations. We make a proposal of how “mixed” features can show up in surface dynamics of glasses.« less
Authors:
;  [1] ;  [2] ;  [1] ; ;  [3] ;  [4] ;  [2] ;  [1] ;  [2] ;  [2]
  1. Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 (United States)
  2. (United States)
  3. Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 (United States)
  4. Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 (United States)
Publication Date:
OSTI Identifier:
22413247
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 141; Journal Issue: 20; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; DENSITY OF STATES; ELECTRIC POTENTIAL; FREE ENERGY; GLASS; HAFNIUM; HAFNIUM BORIDES; MOLECULES; RELAXATION; RESOLUTION; SOLIDS; SPECTROSCOPY; SURFACES; TRANSITION TEMPERATURE; TUNNEL EFFECT; TUNNELING