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Nano-Particle-Resolved Studies

Periodic Report Summary 3 - NANOPRS (Nano-Particle-Resolved Studies)

Amorphous solids (glasses) have novel properties, which make them important in emerging technologies. Metallic glass, for example, does not possess grain boundaries unlike normal crystalline metal. Since these are common failure points, compared to conventional materials, metallic glass has superior mechanical properties, such as hardness and tensile strength. A second example is so-called phase change materials, which are alloys based around the element tellurium. Prized for its unusual bonding, these tellurium alloys exhibit strong optical and electronic contrast between amorphous and crystalline forms, which is exploited in a number of emerging materials. Optical contrast underpins re-writeable optical media, while electronic contrast is the key ingredient in phase change memory, a potential replacement technology for current hard disks and flash memory. Such rewriteable memory requires fast switching between amorphous and crystalline forms, which necessitates control of the delicate balance between crystallisation and glass formation.

At a more fundamental level, the glass transition and crystal nucleation are major challenges in condensed matter. The behavior of the macroscopic material depends on the precise details of the behavior of small groups of atoms and molecules, which may “nucleate” to initiate the process of crystallization, or, in viscous liquids, local regions exhibit very different dynamic behavior. So some regions of the liquid appears almost solid while others appear runny (low viscosity).

Understanding these microscopic processes is crucial to the development of the materials mentioned above, and also to our fundamental understanding of the process by which a liquid becomes a solid without crystallization (to form a glass) or by crystallization to form a crystal. It isn’t possible to study these fast-moving microscopic details of most materials, but in suspensions of micron-sized colloids, which behave in the same way, is it possible to resolve the individual particles (which behave as atoms).

However this technique of particle-resolved studies has an Achilles heel: micron-sized colloids large enough to resolve optically are rather sluggish compared to atoms. Thus rare events such as crystal nucleation are unlikely to be observed, and timescales in viscous liquids become unmanageable. Here we solve this limitation of sluggish motion of the colloids. The resolution of Stimulated Emission Depletion (STED) “nanoscopy” is around 20 nm, which brings particle-resolved studies to the nanoscale. At a sweep, the experimental limitation is largely eliminated: the mobility of nanoparticles much smaller than the colloids previously used, which can be nevertheless resolved by STED, is up to 100,000 times faster than current particle-resolved studies. In this way, it is possible to effectively rescale time, so that our “nano-particle resolved studies” technique runs 100,000 times faster than previous studies and thus allows major advances in our understanding of crystal nucleation and the glass transition.