Multi-scale dynamics at the glassy silica surface

Abstract

Silica-based glass is a household name, providing insulation for windows to microelectronics. The debate over the types of motions thought to occur in or on SiO2 glass well below the glass transition temperature continues. Here, we form glassy silica films by oxidizing the Si(100) surface (from 0.5 to 1.5 nm thick, to allow tunneling). We then employ scanning tunneling microscopy in situ to image and classify these motions at room temperature on a millisecond to hour time scale and 50-pm to 5-nm length scale. We observe two phenomena on different time scales. Within minutes, compact clusters with an average diameter of several SiO2 glass-forming units (GFUs) hop between a few (mostly two) configurations, hop cooperatively (facilitation), and merge into larger clusters (aging) or split into smaller clusters (rejuvenation). Within seconds, Si–O–Si bridges connect two GFUs within a single cluster flip, providing a vibrational fine structure to the energy landscape. We assign the vibrational fine structure using electronic structure calculations. Calculations also show that our measured barrier height for whole cluster hopping at the glass surface (configurational dynamics) is consistent with the configurational entropy predicted by thermodynamic models of the glass transition and that the vibrational entropy for GFU flipping and configurational entropy for cluster hopping are comparable (on a per GFU basis).

Glass Dynamics Deep in the Energy Landscape

“…The enhanced surface mobility predicted by RFOT theory also means that when the glass is heated above the glass transition, the surface of the glass will become mobile well before the interior of a glass. This leads to the phenomenon of rejuvenation fronts emanating from the surface, which has been observed for ultrastable glasses. − Splitting of cooperatively rearranging regions with enhanced mobility on the surface has also been observed directly on 1.5 nm SiO 2 films …”

“…Glasses are disordered solids, and their structural disorder ultimately manifests itself at the atomic level, whether the glass forming units are organic molecules, segments of a polymer, or small inorganic units. − While it is difficult to apply techniques such as X-ray diffraction to bulk glasses due to the diffuse signals, glass surfaces provide a structural window into glass dynamics down to the atomic level by using techniques such as scanning tunneling microscopy, atomic force microscopy, or electron microscopy. , As discussed for organic-based glasses described above, the observed surface dynamics is more weakly temperature-dependent and much faster than that of the bulk, allowing molecular motion to be observed hundreds of Kelvin below the bulk glass transition temperature T g , deep in the energy landscape. The surfaces that have been studied have been formed by rapid quenching, vapor deposition, ion bombardment, and fracture .…”

“…Telegraph-like signals reflecting two-state transitions have been observed at polymer surfaces and can be monitored from milliseconds to a day down to atomic resolution in some cases by scanning tunneling microscopy movies (Figure ). , On a large variety of substrates including metallic glasses, , semiconductors, ceramics, and insulating films such as SiO 2 , the cooperatively rearranging regions that hop between two states are seen to be large compact clusters, containing about one hundred glass-forming units, so that their diameter averages N 1/3 = 4–5 glass forming units (Figure A shows data for the hafnium boride ceramic). The more immobile clusters tend to have larger diameters (Figure B), so the activation barrier increases rapidly with cluster size.…”

“…Experimentally, the distinction has been made quantitative for bulk metallic glasses by combining neutron scattering measuring the vibrations, and calorimetry measuring the total entropy; the conclusion is that most of the excess of the entropy of the bulk glass over that of the crystal is configurational. On silica surfaces, vibrational and configurational motions have been imaged directly by collecting STM movies over a millisecond to hours dynamic range . Experiments reveal two length scales of dynamics, one fast (∼1 s –1 ), with 50 picometer displacements, and one slow (2 × 10 –3 s –1 , with nanometer displacements of entire silica clusters from which the surface is composed.…”

“…Concerted events where one two-state transition induces a transition in its neighboring region (somewhat like an earthquake rearranging the landscape to cause an afterquake somewhere nearby) lead to more diffusion-like behavior on glass surfaces (Figure A shows an example) . The correlated motion of nearby clusters indicates that one cluster’s state influences the energy landscape of nearby clusters Figure B shows the effect of correlated motion on single cluster dynamics on a MetGlass SA1 (85% Fe, 10% Si, 5% B) surface at 295 K: when the local energy landscape of a cluster is altered due to a nearby collective rearrangement, the cluster’s rearrangement rate intermittently fluctuates but then returns to its original value.…”