Research and lab
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We use high-pressure scanning tunneling microscopy to investigate the model single crystal catalyst surface reconstructions at gas pressures as high as 1 bar. Flat and stepped single metal surfaces are used as model catalysts. Ambient-pressure X-ray photoelectron spectroscopy (see below) and infrared reflection-adsorption spectroscopy are employed to identify the chemical states and adsorbate coverages relevant to the restructuring processes.
Ambient-pressure X-ray photoelectron spectroscopy is now a widely used technique in many labs around the world. Using a multi-stage differential pumping system with electrostatic lenses is an approach first taken in our group that drastically increased its capabilities and use. In this project, we employ this technique together with NEXAFS to identify the chemical states relevant to the restructuring processes (see HPSTM section left). Furthermore, we use this technique to measure reaction activation energies on different crystal faces in order to relate them to different macroscopic phenomena like adsorption energies, activation energies, etc. Currently, we are working on carbon monoxide adsorption on copper surfaces as well as its reaction with oxygen to produce carbon dioxide, and its reaction with water to produce carbon dioxide and hydrogen.
We are performing systematic experimental and theoretical studies of the atomic scale surface structure and electronic defects of nanoscale semiconductors, with a focus on colloidal quantum dots. We found that random distribution of defects generally induce heterogeneity in the energy landscape of disordered semiconductor systems, by studying a model system of artificial quantum dot solid. Counterintuitively, certain defects themselves can assist charge transport, paving the way for high-mobility percolative transport in heterogeneous artificial solids. We expect the discovered defect physics, percolation transport phenomena and the invented ultrasensitive photodetectors to have significant impacts on both the fundamental understanding of nanoscale semiconductors and their large-area device applications
This project aims to develop a fundamental understanding of the nanometer-scale processes occurring at solid-liquid interface under realistic conditions. We use ultra-thin film, such as 2D materials, as model electrodes. The photon transparency of graphene enables IR spectroscopy studies of its interface with liquids. This project also includes complementary techniques to probe their chemical (XPS, Auger), and structural (AFM) evolution during electrochemical cycling. We thereby examine the key mechanisms involved in the formation of the solid-electrolyte interphase.
Electron Yield-soft X-ray Absorption Near-Edge Spectroscopy was an old technique for probing elemental and chemical information on surface. Now we use it to investigate some physical and chemical processes on solid-liquid interface. The flow liquid cell is capped by ultra-thin silicon nitride membrane, which can seal liquid inside the cell and serve as substrate for deposition of material of interest. Water structure near electrode and chemical information of noble metal in contact with electrolyte could be identified.
Strongly determined by symmetry broken of species at interface, Sum Frequency Generation is ultra-sensitive to the very few layers of molecules near electrode. Here we use this technique combined with Electrochemical STM to investigate some electrochemical processes, such as corrosion, adsorption and water rearranging.
Synthesis and Catalysis
We study the reactivity of metal nanoparticle catalysts of varying size, shape, and composition (synthesized by Judit) under relevant conditions of pressure and temperature. We follow changes in the electronic, chemical, and physical structure of the particles as they are exposed to reactive gases using ambient pressure x-ray photoelectron spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) at the Advanced Light Source (ALS), the Berkeley Lab synchrotron. Typically these x-ray techniques require ultra high vacuum environments, but our group has developed unique tools for performing experiments near atmospheric pressures. Therefore, we can study the catalyst reactivity in situ, under realistic conditions. We are also in collaboration with Prof. Omar Yaghi's group and Prof. Alexis Bell's group for the XPS analysis of their samples