Surface glassy dynamics and single-molecule absorption of quantum dots detected by scanning tunneling microscopy

Using optically assisted scanning tunneling microscopy (STM), I investigated surface glassy dynamics, absorption spectroscopy and intermolecular energy transfer between single quantum dots (QDs) and carbon nanotubes (CNTs). Glass is an exotic state of matter which is not in equilibrium, but can be stable for billions of years. However, the microscopic understanding of glasses, the glass transition and glass dynamics remains a highly debated area. In addition to the bulk glass, studying surface glass dynamics likely contributes equally towards understanding the connection between theories of glass and experiments to test these theories. In the first part of this thesis, I studied surface glassy dynamics by making STM movies of various amorphous surfaces at room temperature in a wide temporal range from 10-3 s to ~105 s. STM movies reveal that the surface of metallic glasses and amorphous materials consists of a disordered network of cooperatively rearranged regions (CRRs or clusters). On all investigated surfaces, CRRs have an average size of ~4-5 glass forming units and mostly relax in a two-state fashion. Dynamics of single CRRs was also monitored by parking the STM tip on the top and measuring the tunneling as a function of time. CRRs on the glass surfaces show excess fluctuations in tunneling current compared to measurements on crystalline surfaces. By quantifying this fluctuations, I was able to reconstruct the energy landscape of two-state hopping. At sufficient low temperature in the supercooled liquid regime, glass dynamics is split into two modes of relaxation, primary α-relaxation which is responsible for the glass transition and secondary β-relaxation which is thought to be the precursor of the α -relaxation. In the glassy regime, β-relaxation is the main relaxation mode which is important for mechanical properties of glasses. I investigated a system of La-based metallic glasses with distinct β-relaxation characteristics, ranging from a pronounced peak to a shoulder relative to the α-relaxation peak. This study allows us to correlate the atomic mobility with nanoscale hopping of surface CRRs. Glass dynamics is strongly influenced by external perturbation including mechanical, thermal and optical stresses. I investigated surface glassy dynamics under optical stress by making movies on amorphous silicon carbide surface irradiated with above-bandgap light. The glass surface relaxes faster under light illumination, mainly by recruiting previously immobile clusters to hop. This photoinduced enhancement of surface dynamics follows an athermal electronic mechanism, which could underlie photoinduced aging and relaxation in glasses. Interaction and energy transfer between QDs and other molecules are important for applications in biological imaging, solar cells and photocatalysis. In the second part of this thesis, optically assisted STM was employed to investigate absorption and intermolecular energy transfer between single QDs and CNTs. Single PbS, CdSe, CdSe/ZnS QDs and CNTs were deposited onto gold, crystalline silicon carbide and amorphous silicon carbide surfaces by matrix-assisted dry contact transfer. Adsorbed molecules were excited with modulated 532 nm light and the modulated tunneling current proportional to the optical absorption signal was detected by STM with a lock-in amplifier. Absorption of individual QDs varies significantly on all investigated surfaces. Single QD absorption shape and intensity are strongly dependent on the sample bias voltage, reflecting different excited states. Using the STM tip, QDs can be moved on the surface, and three-dimensional absorption shapes were imaged. In arrays of QDs, absorbed energy is funneled to one or a few QDs. Evidence of energy transfer between single QDs and single CNTs was also observed