We study a wide range of topics involving primarily surfaces, interfaces, and ultrathin films by using a combination of optical and electron diffraction/microscopy techniques . These topics include quantum dissipation effect in atom diffusion on solids, fundamentals of mass transport on solids, mechanisms of thin film growth, biophysics and biochemstry on hard and soft substrates, and development of novel bioimaging techniques. The electron microscopy and diffraction techniques enable atomic-level observation, while the optical techniques provide versatility and capability of in-situ observation under physical conditions beyond ultrahigh vacuum. Specifically the research activities of our group are in four general areas:
Quantum dissipation effect and collective behaviors in atom diffusion on solids: We investigate diffusion of atoms and molecules on surfaces of crystalline solids over a wide range of temperature, density, and length scale. We use a combination of linear optical diffraction (LOD) and scanning tunneling microscopy (STM) techniques that enable measurements of diffusion rates over 16 order of magnitude. A wide accessible temperature range allows us to distinguish and examine quantum dissipation effects from various low energy excitations in a solid on classical over-barrier hopping and under-barrier tunneling motion of adatoms. A large accessible length scale of observation and a large variable adatom density allow us to examine collective behaviors in mass transport on solids (concerted or otherwise), namely, how the presence and motion of adatoms on the motion of other adatoms.
Kinetics, morphology, and mechanisms in thin film growth under a wide range of physical conditions: We investigate mechanisms in growth of thin films with desirable morphology (crystalline orientation, sharpness of interfaces, smoothness, uniformity of self-assembled structures, texture in cases of polycrystalline materials), chemical makeup, and novel physical/functional properties. We use a combination of oblique-incidence optical reflectivity difference technique (OI-RD), optical sum-frequency generation (SFG), reflection high energy electron diffraction (RHEED), and low energy electron diffraction (LEED) techniques that enable in-situ examination of surface structure and composition of a thin film under physical conditions ranging from ultrahigh vacuum (MBE), high-pressure gaseous ambient (PLD), to electrochemical environment (electrodeposition). The capability of in-situ observation allows us to study kinetic-limited growths away from thermodynamic equilibria. Widely variable physical conditions provide us with opportunities for comprehensive examination of interplay of various physical processes that control the morphology, composition, and resultant properties of a thin fim or a stack of thin films.
Macromolecular interactions (e.g., between protein and DNA orbetween protein sand membranes) under label-free conditions: In this post-genome era, we study macromolecular interaction (soft condensed matters) at protein and membrane level under conditions as native as is relevant to their properties. Conventionally macromolecular interactions are investigated and characterized by labeling at least one of the reactants with fluorescent molecule(s) as a marker. Since proteins and membrane are soft condensed matter and yet fucntion in a complex way that may involve subtle overall change of the whole molecules, attachment of foreign labeling molecule(s) can alter the very interaction either quantitatively or qualitatively, and such an alteration is often difficult to specify or characterize without independent label-free measurements. We recently developed an oblique-incidence reflectivity difference (OI-RD) technique for surface science study into a label-free optical microscope (OI-RD Microscope) for detection of macromolecules on hard and soft substrates (e.g., membranes). Such a technique and others in development enable us to investigate macromoleuclar interactions, particularly the phyiscs and chemistry of proteins and membranes under label-free conditions.
Light transport through highly scattering media and its application in diffuse-photon tomographic imaging: We study propagation of light in homogeneous and heterogeneous turbid media and develop mathematical tools to analyze physical information in diffuse photons that emerge from the surface of such a medium. We further explore how such a study and analysis may lead to useful diffuse photon tomographic techniques for biomedical imaging.
For published work and other research activities in our group, please visit http://www.physics.ucdavis.edu/xdzhu
E-mail zhu@physics.ucdavis.edu
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