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We have studied the RHEED resonance condition on the GaAs(110) surface near the (880) and (620) Bragg conditions. The resonance intensity has been measured, using an energy filtering device which excludes inelastically scattered electrons involving energy losses greater than 2.5 eV. These measured intensities are compared with simulated ones using the Bloch wave method. It is found that the anomalous intensity enhancement at the resonance condition is due to the multiple scattering in the top surface atomic layers, and the surface potential barrier may effect the magnitude of the intensity enhancement. Resonance occurs when there is a Bloch wave that has a very small penetration depth into the bulk crystal, and this Bloch wave is strongly excited.

High-resolution images can be obtained by collecting high-angle scattered electrons with an annular detector in a scanning transmission electron microscope (STEM). For a crystal, with incident electron beam along a principle axis, the high-angle annular dark-field (HAADF) signal may be mainly generated by quasi-elastic (thermal diffuse) scattering (TDS). The angular distribution of TDS is determined by the Fourier transform of the differential of the atomic potential. The collected signal is thus dependent on the atomic number Z, the shape of the atom, the amplitude of the thermal vibration and the total number of atoms encountered by the electron probe. This imaging mode reveals high Z sensitivity with atomic resolution. Angular filtered ADF imaging is also possible by selecting the desired angular window to extract information about thick specimens in transmission geometry or information about bulk specimens in reflection geometry. Subnanometer-resolution secondary electron (SE) images can be obtained simultaneously with HAADF images, providing topographic and material contrast of surfaces of the materials studied. In the UHV STEM MIDAS (microscope for imaging, diffraction and analysis of surfaces) both the entrance and the exit surfaces of the sample can be imaged with secondary electrons. Surface steps can be observed with high contrast. Applications of HAADF and high-resolution SE imaging to the study of several kinds of materials are discussed.

A high-angle annular detector provides a useful imaging signal for the detection of small metal catalyst clusters in light-element supports. The high-angle annular-dark-field (HAADF) signal is mainly generated by quasi-elastic scattering. The collected signal intensity depends on the atomic number of the scatterer, its amplitude of vibration and the total number of atoms encountered by the probe. This imaging mode can reveal high- Z sensitivity and eliminate most of the diffraction contrast present in low-angle ADF images. In a STEM instrument, secondary electron images with a resolution approaching the probe size ( ∼ 0.5 nm in diameter) can be obtained simultaneously with HAADF images, providing topographic information about the metal catalyst-support system. The combination of a variety of imaging and diffraction techniques available in a STEM instrument can yield important information about the distribution of small metal clusters in the supports, crystallographic relationships between the metal clusters and the support, and morphology as well as chemical compositions of the metal catalyst-support system. Samples of Pt/carbon, Pt/γ-alumina and Pt/NaY zeolite catalysts have been examined. Preliminary results reveal useful information about these catalyst systems.

Microscopists have done combined space and frequency decompositions optically since long before coining of the word “wavelet” [1], under the name “darkfield imaging” [2]. Now integrative views of lattice information in digital images yield new challenges for mathematical harmonic analysis [3]. Such challenges are illustrated by a series of notes [4-7] on the uses and shortcomings of “weakly convergent” Fourier transform window techniques. Here, we show how robust (if pedestrian) digital darkfield techniques help to characterize: (i) an Antimony-doped Tin Oxide nanoparticle screw dislocation, and (ii) twinning and strain in a gold decahedral twin.

The nucleation and evolution of Ni nanoparticles during reduction of a Ni(NO3)2.6H2O precursor supported on a commercial titania substrate have been studied in situ at atomic resolution using environmental transmission electron microscopy. An incipient wetness technique was used to prepare the starting unreduced material (10 wt % Ni precursor on titania). The Ni precursor, before reduction, shows a nonuniform distribution over the titania support. It is observed that upon reduction, the initial Ni "seed" crystal nucleates within the precursor patch. The distribution and size of the Ni nanoparticles thus generated are influenced by the distribution and size of the precursor patches. In this system, we see no evidence of preferential nucleation of Ni particles on anatase or rutile. At 350 degrees C with CO as the reducing agent, the {111} surface facets of the Ni nanoparticles are predominant during the initial stage of nucleation and growth. However, the {111} facets are partially consumed with time, indicating that they are not thermodynamically favored in the CO atmosphere. In CO and H2 atmospheres, Ni particles show a nonwetting morphology on titania, while in a mild oxidizing environment, a thin layer of NiOx is formed, thus giving rise to a morphology that is indicative of wetting of the support. This work provides fundamental information on understanding and controlling the important parameters involved in the preparation of a well-designed supported Ni catalyst using the incipient wetness technique.

Scanning transmission electron microscopy (STEM) techniques can provide imaging, diffraction and spectroscopic information, either simultaneously or in a serial manner, of the specimen with an atomic or a sub-nanometer spatial resolution. High-resolution STEM imaging, when combined with nanodiffraction, atomic resolution electron energy-loss spectroscopy and nanometer resolution X-ray energy dispersive spectroscopy techniques, is critical to the fundamental studies of importance to nanoscience and nanotechnology. The availability of sub-nanometer or sub-angstrom electron probes in a STEM instrument, due to the use of a field emission gun and aberration correctors, ensures the greatest capabilities for studies of sizes, shapes, defects, crystal and surface structures, and compositions and electronic states of nanometer-size regions of thin films, nanoparticles and nanoparticle systems. The various imaging, diffraction and spectroscopy modes available in a dedicated STEM or a field emission TEM/STEM instrument are reviewed and the application of these techniques to the study of nanoparticles and nanostructured catalysts is used as an example to illustrate the critical role of the various STEM techniques in nanotechnology and nanoscience research.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Heterogeneous catalysis is one of the oldest nanosciences. Although model catalysts can be designed, synthesized, and, to a certain degree, characterized, industrial heterogeneous catalysts are often chemically and physically complex systems that have been developed through many years of catalytic art, technology, and science. The preparation of commercial catalysts is generally not well controlled and is often based on accumulated experiences. Catalyst characterization is thus critical to developing new catalysts with better activity, selectivity, and/or stability. Advanced electron microscopy, among many characterization techniques, can provide useful information for the fundamental understanding of heterogeneous catalysis and for guiding the development of industrial catalysts. In this article, we discuss the recent developments in applying advanced electron microscopy techniques to characterizing model and industrial heterogeneous catalysts. The importance of understanding the catalyst nanostructure and the challenges and opportunities of advanced electron microscopy in developing nanostructured catalysts are also discussed.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

In this paper, we will describe the experimental processes involved in analytical atomic-resolution scanning transmission electron microscopy (STEM) of supported nano-scale systems. We show that the combination of high-resolution Z-contrast imaging and electron energy loss spectroscopy (EELS) provides an analytical tool with unprecedented chemical and spatial sensitivity that is vital for studying interfaces in heterogeneous catalyst systems. We apply the described methods to study two example heterogeneous catalyst systems: Pt/SiO2, and Cu/Al2O3. In particular, the presence of a few monolayers of platinum oxide in Pt/SiO2 can be clearly seen, and changes in the chemistry of the SiO2 support within ~1 nm of the metal-oxide interface can be characterized as a function of the catalyst preparation conditions. The Cu/Al2O3, reduced at various temperatures, exhibits an increasing oxidation of the Cu-particles upon higher temperature reduction.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

The use of a high-brightness field emission gun and novel secondary electron detection systems makes it possible to acquire nanometer-resolution surface images of bulk materials, even at low electron beam voltages. The advantages of low-voltage SEM include enhanced surface sensitivity, reduced sample charging on non-conducting materials, and significantly reduced electron range and interaction volume. High-resolution images formed by collecting the backscattered electron signal can give information about the size and spatial distribution of metal nanoparticles in supported catalysts. Low-voltage XEDS can provide compositional information of bulk samples with enhanced surface sensitivity and significantly improved spatial resolution. High-resolution SEM techniques enhance our ability to detect and, subsequently, analyze the composition of nanoparticles in supported metal catalysts. Applications of high-resolution SEM imaging and microanalysis techniques to the study of industrial supported catalysts are discussed.

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2006

The results of the breakdown of microwave pulses in air have been described in this paper. The experimental study has beendone in a waveguide, with the frequency of 9.37GHz, the peak power up to 200kW, pulse width from O.3ts to 2.Ots, singleshot and repetition rate up to 970Hz, and the processes of the breakdown of repetitive pulse were also been recorded for tenpulses. The analytic methods for threshold power density are presented also. The comparisions between the theoretical andthe experimental results for the breakdown threshold are presented, and in good agreement. Keywords: high power microwave, propagation, breakdown, plasma, ionization, attachment, recombination, pulse, threshold, repetition rate 1. INTRODUCTION The propagation of HPM through the air is a process that strongly concerns on such factors as air breakdown efffect, the airabsorption and scattering, and reflection, diffraction and scattering of HPM from all kinds of earth obstacles. The high powerdensity ofHPM makes air breakdown effect the most significent phenomina for the study of the propagation of HPM throughatmosphere. There are several events may arise while the 11PM air breakdown happens. These events are: the decay of HPMpulse, tail-erosion, and the power reflection from the air breakdown point. The decay and tail-erosion of the HPM pulselimits the power can be transmitted to targets, and consquently decrease the efficiency of the HPM source; at the same time,the reflected power from the air breakdown plasma may be dangerous to the HPM source. Especially for the design of HPMsystems, it is very important for the designers to have in-depth understanding of the physics of HPM air breakdown andrelated experimental data, including the breakdown thresholds for different HPM parameters(frequency, pulse duration,repetition rate, and power).A waveguide system was employed to study HPM air breakdown, at the same time, theoretical analysis were made tounderstand the physics of air breakdown. The experimental and theoretical results were compared and some conclusionswere drawn.The high density plasma generated by HPM air breakdown results not only in the reflection, absorption, and tail-erosion ofHPM, but also in such phenomina as enhancement of amplitude, widening of spectrum, and self-focus of HPM if certainconditions could be satisfied. The enhancement ofthe 11PM amplitude may due to the dispersion characteristic of the plasma,sometimes the amplitude of the field can be higher than that of the injected 11PM pulse. As to the widening of HPM spectrum,the main reason is that the pulse duration of the incident HPM is "shorted" by the breakdown plasma. When the density ofthe breakdown plasma satisfies some conditions, the transmitted HPM pulse will focus. As the first step, the work reported in