Current Research projects:

Direct Methods

Direct Methods were originally developed as techniques aimed at solving the phase problem encountered in bulk X-ray diffraction. While the phases of diffracted beams cannot be directly measured, it was shown starting in the early 1950's that the phases of strong beams can be estimated from their measured intensities. The process of estimating phase information from diffraction intensity information has become known as Direct Methods and has enjoyed widespread success in modern bulk X-ray crystallography. In our lab we are optimizing Direct Methods and exploring new areas for their use including applications to surface diffraction in both two and three dimensions and to transmission electron diffraction.

Surface Structures

As solid state electronic devices continue to shrink, a larger percentage of the atoms in the devices reside at a surface or interface, and therefore surfaces and interfaces are playing a role of increasing importance in determining the behavior of microelectronic structures. However, the general level of understanding of surface and interface physics is noticeably lacking in many cases of technological interest. We mantain an active program in the area of surface structure determination. Using high-resolution electron microscopy and/or direct methods applied to either grazing incidence X-ray diffraction or to transmission electron diffraction data, we have analyzed the surface structures of semiconductor substrates and submonolayer metal films on semiconductors.

Dry Cutting

At the heart of the dry machining idea is that the natural reaction product of the dry cutting-tool/work-piece interaction is one or more metal oxides. It is thus, logical to work with, rather than against nature by using a tool coating that produces its own lubricious oxide when the tool tip reaches a certain temperature. Lubricious materials such as MoS2 have limited temperature stability in oxidizing environments and are generally too weak to incorporate inside coatings designed for cutting metals. This research work anticipates a coating that is multicomponent and multifunctional. It will have high strength that will persist to high temperature, excellent toughness to resist cracking and delaminating, low wear and optimized friction.

Quantum Electron Crystallography

Surface crystallography is the study of surface crystal structure and is important to the understanding of interface phenomena. Quantum surface crystallography takes this one step further in an attempt to experimentally measure the structure of the electrons themselves, which is of greater importance than atomic positions in determining material properties. The electron charge density governs almost all of the relevant properties of a material, such as magnetism, electronegativity, optical behavior, and even physical properties such as hardness. Using a powerful combination of transmission electron diffraction, surface x-ray diffraction, and ab initio Density Functional Theory quantum mechanics calculations, our group has been able to successfully measure the charge density on single crystal surfaces, which has never before been accomplished.

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Figure 1 First reported experimental measurement of surface charge density. Data collected on the MgO [111] surface via transmission electron diffraction. The contour interval is 1.5 x 10⁴ e^-/Å² (dashed lines are negative).
Figure 2 This is a plot of the (a) calculated) and (b) measured charge density of the Si(001)-2x1H surface projected onto the (110) plane. Color bar in units of e^-/Å².

Surface Charge Density

X-ray and electron structure factors are essential for electron image simulations. It is well known that both X-ray structure factors are determined by the total ground-state charge density. Electron structure factors are also related to charge density by the Poisson equation. However, structure factors are usually calculated from linear superposition of atomic charge densities, which neglect charge redistribution due to crystal bonding. Current project is to use Density Functional Theory (DFT) to get the real crystal charge density and structure factors after charge redistribution. Then multislice method is used to simulate high-resolution electron microscope images with different structure factors. We are interested in what kind of microscope can ¡°see¡± charge redistribution effects directly. Aberration-corrected microscope reaches sub-angstrom resolution and may be a potential tool to detect charge redistribution.

Difference images from structure factors before and after charge redistribution.

Enviromental Catalysis

As a member of the Institute for Environmental Catalysis, we work to study the molecular science of oxidation on an atomic scale. The scientific focus for IEC derives from a recognition that the understanding and improvement of one type of reaction, the selective catalytic oxidation of organic compounds, could reduce the environmental impact of a broad range of industrial activities. Using high resolution electron microscopy in tandem with other analytical techniques such as XPS we hope to elucidate the factors that control selectivity in catalytic oxidation as well as understand the atomic structure of these oxide surfaces and how the adsorption of reactant species affect the surface structure and properties. Works have been done on various oxide surface in this group.

Precession Electron Diffraction

Precession is an innovative data collection technique for bulk crystallography. In a precession diffraction experiment, the electron beam is conically scanned above the sample and subsequently descanned below the specimen. This innovative technique avoids the strong dynamical scattering of a zone axis condition and allows for instantaneous collection and integration of hundreds of off-zone diffraction experiments. The acquired intensities from a precession diffraction experiment are of higher quality, which means one is more likely to find the true structure. Details are here

Tribology

Tribology encompasses the study of contacting bodies in relative motion (i.e. sliding, rolling, slip). Despite its ubiquitous nature, surprisingly little is known about the fundamental mecahnisms of friction. Of particular interest to our group are the atomic-scale contributions to friction. How does a frictional force arise from purely elastic atomic interactions? How do interfacial dislocations play a role in sliding contacts? Do relative crystallographic orientations affect friction behavior? Experimentally, we are probing tribological contacts at the single-asperity scale inside the TEM with a specialized STM-TEM(TM) holder. This gives us very fine motion and electronic control over a small probe inside the TEM to investigate dynamic interactions, electrical properties, tunneling spectroscopy, as well as topographic features. Theoretical models have also been developed to tackle friction from a standpoint of dislocation drag and grain boundary models.

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Figure 1: A tungsten STM probe nearing contact with a gold film. Contact and sliding dynamics are investigated with an in-situ STM-TEM(TM) probe.
Figure 2: TEM bright field image of an in situ fabricated STM tip via melting a tungsten tip to a gold sample. A series of voltage pulses and pull-off manipulation created this extremely sharp tip (sub-5nm radius of curvature). Knowledge of the tip size and shape are very important when interpreting, among other things, STM output and tribological contacts.

Nano-Scale Characterization of Cementitious Materials

The fundamental properties of concrete are affected by material properties at the nano-scale. However, the nano and microstructure of the most important hydration product in cement paste, calcium silicate hydrates (C-S-H, with C = CaO, S = SiO2, and H = H2O) is not well understood. Our research is to characterize the properties of cementitious materials using different imaging techniques namely optical microscope, Scanning Electron Microscope and Atomic Force microscope and evaluate the local mechanical properties of different phases of hydrated cement paste. This will provide information from a length scale of micrometer to nanometer. For the determination of local mechanical properties, a nanoindenter with unique advantage of in-situ SPM imaging that allows pre and post-test observation of the sample is being used. We are also investigating the changes in nano-scale properties with the addition of different commonly used Supplementary Cementitious Materials like silica fume and fly ash.

Figure 1: 4.7 mm x 4.7 mm AFM topography image of C-S-H gel
Figure 2: 10 mm x 10 mm image of C-S-H gel close to unhydrated cement particle after indentation

Solid Oxide Fuel Cell

In recent years, much development has focused on solid oxide fuel cells (SOFC), both because they are able to convert a wide variety of fuels and because they do so with such high efficiency (40-60% unassisted, up to 70% in pressurized hybrid system) compared to engines and modern thermal power plants (30-40% efficient). SOFC technology dominates competing fuel cell technologies because of the ability of SOFCs to use currently available fossil fuels, thus reducing operating costs. They are also attractive as energy sources because they are clean, reliable, and almost entirely nonpolluting. Because there are no moving parts and the cells are therefore vibration-free, the noise pollution associated with power generation is also eliminated.

Limited by the processing methods, it is often difficult to achieve desired SOFC microstructures, since these structues are very difficult to keep at the lelatively high firing temperatures used to densify ceramic electrolyes and to produce desired highly-interconnected electrode microstructures. A new method has been developed to form catalytic metal (Ru, Ni)nanoparticles on anode surface. These nanoparticles are produced without extra processing steps under SOFC operation condition, accompanied by a dramatic improvement in SOFC performance.

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Figure 1: Ru nanoparticles (2-5 nm) formed after annealing (La0.8Sr0.2)(Cr0.82Ru0.18)O3 SOFC anode material at 800 degree in hydrogen for 15 minutes
Figure 2: Ru nanoparticles (2-5 nm) populated without sintering after annealing (La0.8Sr0.2)(Cr0.82Ru0.18)O3 SOFC anode material at 800 degree in hydrogen for 311 hours