Spectroscopy and Microscopy at Atomic Level Laboratory (SMALL) is a research group lead by Dr. Shaowei Li in the department of chemistry and biochemistry. We combine optical techniques with scanning probe microscopy to explore the quantum properties of single molecules and low dimensional materials in both real and temporal spaces.
In general, the light-induced physical and chemical processes lead to the dynamic reorganization of a large number of atoms and molecules. One persistent goal in the scientific community is to be able to visualize and manipulate individual molecules and track their motions in real-time. The coupling of photon excitation with electron tunneling at the junction of a scanning tunneling microscope (STM) can combine the femtosecond sensitivity of a laser and the Angstrom resolution of tunneling electrons. The joint fs-Å resolution provides a new window for viewing the unique properties of individual nano-scale objects.
We utilize the ultra-high spatial resolution of STM to induce and control chemical reactions that involve low-dimensional quantum systems such as atoms, molecules, and 2D materials. The structural, electronic, and vibrational information of both the reactants and products can be probed in real space with STM. Our experimental approach can distinguish the spatial inhomogeneity in chemical reactions which is not possible in the ensemble measurements.
Due to the quantum confinement effect that occurs at the nanometer scale, low dimensional catalysis such as single atoms/molecules and plasmonic nanoclusters have shown outstanding catalytic properties on reactions including hydrogenation, dehydrogenation, oxidation. The catalytic properties of the low-dimensional catalysis depend sensitively on its local chemical environment. For example, the catalytic properties of graphene-supported single metal atoms vary on their adsorption sites and geometry. We aim to understand these inhomogeneous chemical properties to design effective and low-cost catalysts.
Inducing electronic or vibrational excitations of molecules using photons is the most fundamental process of photochemical reactions. At "SMALL", we excite the molecules in the STM junction with photons and probe the photoinduced transition using electron tunneling. The electronic excitation such as HOMO-LUMO transitions can be achieved with visible/near-infrared light, the vibrational excitation can be induced using mid-far infrared light.
Being able to excite the molecule electronically and vibrationally provides an opportunity for controlling the molecular motion. Photoexcitation of molecules can often trigger molecular actions including diffusions, rotations, and structural transitions. Such molecular actions can be controlled by tuning the photon energy and monitored in real-space with STM. With this unique ability, we aim to provide fundamental understandings of light-matter interaction at the subwavelength level, and design nano-scale electro-optical devices such as molecular motors and transistors.
Phys. Rev. Lett.122, 077401 (2019)
Coherent chemistry involves the idea of controlling the population of different excited states of a quantum system with light. When a molecule with discrete orbitals is excited by a broadband femtosecond laser pulse, it can be driven into a coherent state consists of the superposition of multiple excited states. Being able to control the population of these discrete quantum states of individual molecules opens the possibility of using them as molecular qubits for quantum computation.
Through the combination of femtosecond laser and STM, we can investigate the ultrafast dynamics of these molecular excited states. The joint spatial-temporal resolution of the laser couple STM allows us to visualize the inhomogeneous dynamics as exemplified by the effects of the interaction between the molecule and its local environment. Such knowledge helps us to design a method to control the molecular coherence at the single-molecule level.
Phys. Rev. Lett. 119, 176002 (2017)
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