Imaging the surface dynamics of glasses and photoexcited molecules

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With support from the Chemical Measurement and Imaging (CMI) Program in the Division of Chemistry, and the Ceramics (CER) Program in the Division of Materials Research, Professor Martin Gruebele at the University of Illinois at Urbana-Champaign and his group will develop two novel implementations of scanning tunneling microscopy (STM). The first aim is to examine quantum dots, carbon nanotubes, and light-harvesting dendrimers on surfaces specifically designed to study single molecule/nanostructure absorption under the influence of a laser coupled to the STM. The researchers will develop ultraflat (1 nm rms), transparent (15% transmission) and conductive (100 kiloOhm point contact resistance) surfaces out of gold/platinum, graphenized silicon carbide, as well as graphene-coated sapphire. On these surfaces, they will study single molecule absorption and energy transfer of quantum dots to carbon nanotubes, and within light-harvesting synthetic organic dendrimers and derivatized nucleic acids. The second aim is to study the dynamics on glass surfaces, expanding our repertoire from simple metal alloys to ceramics such as hafnium diboride, and semimetals (metalloids) such as antimony or selenium-antimony alloys. In these studies, time-lapse movies of glassy dynamics at the surface are produced for comparison with models of glass dynamics. A major instrumentation goal is to improve the current time resolution from minutes to milliseconds. The greater dynamic range affords more rigorous testing of dynamical glass theories.Nanomaterials have great promise in miniaturization, reduced power requirements, speed (e.g. in computing) and portability (e.g. in sensing applications). Much engineering work is done on these novel materials already, but in many cases the physical underpinnings of their function is not well understood. This proposal tackles the fundamental physical understanding of two areas. The first question to be answered is how energy from natural light (e.g. for human-engineered photosynthesis) or artifical light (e.g. laser excitation), once the energy has been deposited in a nanomaterial, moves around within that material. Moving energy from one place to another on a nanoscale without major losses and without damage to the nanomaterial is critical if the researchers are to scale conventional microdevices to smaller size and energy consumption. The proposed experiments can image with near-atomic resolution where the energy is located within a nanostructure, and how it spreads among various interconnected parts of that structure, or even how it hops from one structure to another. The second question to be answered is how glasses move. Glasses, or supercooled liquids, are not ordered like crystals, but they seem rigid like crystals on the 'human time scale' of hours to years. Nonetheless, they flow very slowly, and the mechanism of how they flow is poorly understood. The researchers will make time-lapse movies of glass surfaces with atomic resolution that show directly what's moving, how far, and how often. Direct visualization is the most direct way to answer the mystery of how glasses flow. The irregularity at the atomic level and flow processes actually make glasses potentially more robust than crystals, which fracture at regular crystal boundaries. Tailoring glasses at the microscopic level may allow chemists and ceramic engineers to improve their material properties to make new materials such as superior heat insulation tiles or touch-responsive glasses. Students from all levels will be involved in the proposed research. Professor Gruebele also plans to teach introductory chemistry to Vietnamese students through international collaborations.