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Professor George Nazin's group is developing novel scanning probe techniques for atomic-resolution spectroscopic studies of physics and chemistry in molecular and nanoscale materials.

The Nazin group investigates the connection between the chemical structure and properties of nanoscale materials and devices. We are particularly interested in real-space experimental approaches that provide spectroscopic information on the atomic and molecular scales. We have constructed a state-of-the-art Ultra-High Vacuum Scanning Tunneling Microscope (STM), an instrument that allows direct imaging and spectroscopic measurements of individual atoms and molecules (Fig. 1), as well as construction of artificial nanostructures in-situ (Fig. 2).

Fig. 1: (A) The topography of a surface of interest is determined by using the quantum-mechanical tunneling of electrons between the surface and an atomically-sharp metallic tip located within a few Angstroms above the surface. This tunneling electron current is sensitive to the surface structure on the atomic scale. (B) Energy-diagram of electronic states in an STM junction with a molecule on the sample surface. On the diagram, electrons from occupied states (shown in blue) in the tip tunnel into the Lowest Unoccupied Molecular Orbiltal (LUMO) of the molecule. Current through individual molecules is enhanced by the presence of molecular electronic and vibrational states, which produce distinct features in the current-voltage characteristics. This is useful for identification of the local chemical structure. (C) By injecting high-energy electrons into individual adsorbates, one can induce and observe fluorescence-like radiation that contains spectroscopic information about individual adsorbates.

Fig. 2: STM image (bottom) with a schematic (top) of a model molecular junction consisting of two gold atomic chains and a CuPc molecule bridging the gap between the two chains. This molecular junction was assembled by manipulating the CuPc molecule and individual gold atoms with an STM tip. In this work (done in the group of Professor Wilson Ho at UC Irvine) the molecular junction was used to visualize the chemistry of molecular electronic devices.

At the nanoscale, the properties of materials become strongly size-dependent and sensitive to the surface chemistry. One striking example of this is the gold atomic wires seen in Fig. 2: instead of being metallic, as one would expect a gold wire to be, these wires have a semiconductor-like electronic structure, which changes with addition (or extraction) of a single gold atom. These gold wires effectively are all-surface (no bulk), and adsorption of even a single molecule on such a wire (the molecule doesn't have to be CuPc) can disrupt the one-dimensional particle-in-a box electronic states in such chains. Similar effects are also important in other nanoscale objects, such as quantum dots, nanowires, nanotubes and thin films, with dramatic consequences for their electronic structure, charge and energy transport, optical properties, etc. The powerful combination of atomic-scale imaging, spectroscopy and manipulation afforded by our STM allows us to gain unique insights into the relationship between the chemical structure and properties of novel nanoscale materials. In future, by combining STM experiments with characterization of devices made using such nanomaterials, we will be able to evaluate how the spectroscopic properties determined using STM methods relate to the device behavior, which is essential for laying out the groundwork required for a rational design of future nanoscale devices.

Some of the research directions are described below:

1) Nanoscale inorganic semiconductors: In collaboration with our industrial partners, we explore strategies for surface passivation and functionalization of semiconducting nanocrystals, with potential applications in optoelectronics and photovoltaics. The properties of devices based on such nanocrystals are strongly affected by localized sub-bandgap states associated with surface imperfections. To understand the nature of such surface states, a correlation between their properties and the atomic-scale structure of chemical imperfections responsible for their appearance must be established. To achieve this, we use Scanning Tunneling Spectroscopy to visualize electronic states in individual nanocrystals (Fig. 3).

Fig. 3: Spatial mapping of electronic states in individual PbS nanocrystals.

2) Organic semiconductors: Research in organic semiconductors is aimed at understanding the impacts of the molecular structure, composition and packing in molecular solids on the processes associated with energy and charge transfer, which are central to the operation of electronic and photovoltaic devices based on organic semiconductors.

3) Carbon nanotubes: Our group is applying advanced STM-based spectroscopic techniques to understand the fundamental properties of electronic and excitonic states in carbon nanotubes (CNTs). The unique photophysics of semiconducting single-wall carbon nanotubes is a result of a complex interplay among several factors arising from their low-dimensionality:  (a) the primary photoexcitations in CNTs are described by one-dimensional excitonic bands formed via strongly correlated electron-hole interactions;  (b) variations of the excitonic bandgap are caused by environmental inhomogeneities and CNT defects; (c) photo-generated excitons tend to diffuse along the nanotube axis and can become trapped in regions with smaller excitonic bandgaps, or on individual defects;  (d) relatively low luminescence yields are typically observed, a consequence of defect-induced quenching, and the intrinsic hierarchy of the excitonic manifold containing dark excitonic bands. Our STM-based spectroscopic approach enables direct visualization of these effects providing information inaccessible to any other measurement technique.

Fig. 4: STM image of a carbon nanotube.

Our research is at the intersection of several disciplines, including surface science, molecular spectroscopy, materials science and solid state physics. Students working in our group have the opportunity to participate in construction of novel instrumentation and learn such techniques as ultra high vacuum technology, scanning probe microscopy, nanoscale device fabrication and optical spectroscopy.

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