Vladimir P. Oleshko

Use of plasmon spectroscopy to evaluate the mechanical properties of materials at the nanoscale.

Relationships between volume plasmon excitations and mechanical properties of various materials are considered. Based on systematic evaluation of available data, correlations between the volume plasmon energy, Ep, Youngs modulus, Ym, bulk modulus, Bm, shear modulus, Gm, and microhardness, Hm, are established. The resulting correlations indicate that plasmon energies potentially can be used to predict and/or determine the local mechanical properties of technologically important materials, such as metal alloys, semiconductors, and ceramics at the nanometer level.

In situ determination and imaging of physical properties of metastable and equilibrium precipitates using valence electron energy-loss spectroscopy and energy-filtering transmission electron microscopy

The physical (elastic, cohesive, and electronic) properties of precipitates are important in determining factors such as their equilibrium shape, coarsening, and strengthening behavior in alloys. In this work, we use valence electron energy-loss spectroscopy (VEELS) and energy-filtering transmission electron microscopy (EFTEM) to determine quantitatively and image the elastic moduli, cohesive energy, and interstitial electron density of both metastable and equilibrium precipitates in two different metal alloys. We show that the elastic properties of θ′ and θ precipitates in Al–Cu alloys can be measured in situ as a function of temperature and during transformation from θ′ and θ phases. We also measure and image in situ the elastic and cohesive properties of metastable TiHx precipitates in a Ti–H alloy. These results demonstrate the capability of VEELS∕EFTEM for real-time nanoscale determination and imaging of multiple physical properties of precipitates in solid-solid phase transformations.

Electron Tweezers as a Tool for In Situ Manipulation and Processing of Individual Metal Nanoparticles in a Two-Phase Partially Molten Alloy

Vladimir P. Oleshko and James M. Howe University of Virginia, Department of Materials Science & Engineering, Charlottesville, VA 22904 Understanding physical principles for controlling the dynamic behavior of nanoparticles in inhomogeneous gas (liquid) – solid systems is becoming increasingly important for nanoscience and technology. Nondestructive trapping of micron- and nano-sized dielectric and metal particles in a liquid using a laser beam, which is refracted by the particle and transfers momentum to it, is known as a single-beam gradient force optical trap, or optical tweezers [1]. Recent experiments utilizing partially molten submicron-sized Al-Si alloy spheres and a focused electron beams in a medium-voltage TEM [2] and theoretical calculations of linear and angular momentum transfer from an electron beam to small particles [3] indicate that optical trapping of particles using electron beams may occur. Such thermally assisted “electron tweezers” potentially can be utilized for manipulation and processing of individual nanoobjects and fabrication of assembled nanodevices with up to atomic level sensitivity and lateral resolution provided by modern electron optical systems. In this work, we demonstrate electron-beam trapping of solid nanoparticles inside opaque submicron-sized Al-Si alloy spheres using energy filtering TEM (EFTEM) and parallel electron energy-loss spectroscopy (PEELS). Thermally assisted motion of crystalline Al nanospheres confined within the partially molten Al-Si particles was initiated by the electron beam, which was used to control and to observe the trapping in real-time by plasmon spectroscopic imaging (Fig. 1). Such approaches enable

CTEM, HRTEM and FE-AEM Investigation of the Metastable Tetragonal Phase Stabilization in Undoped, Sol-Gel Derived, Nanocrystalline Zirconia

Stabilizing the metastable t-phase in ZrO2 powders is a major challenge and dopants are usually used [1]. In this paper, the mechanisms underlying stabilization of the t-phase in undoped, sol-gel derived nanocrystalline ZrO2 are studied by conventional and high-resolution transmission electron microscopy (CTEM/HRTEM) combined with field-emission analytical electron microscopy (FEAEM) utilizing parallel electron energy-loss spectroscopy (PEELS). ZrO2 nanopowders were synthesized by hydrolysis of zirconium (IV) n-propoxide in an alcohol solution at two ratios of molar concentrations of water to zirconium n-propoxide, R=5 and 60, in the presence of 1.0 g/L hydroxylpropyl cellulose. The sol was subsequently dried at 80C following calcination of a gel for 2 h at 400C in air. Samples were examined using bright-field (BF) and dark-field (DF) TEM, selected-area electron diffraction (SAED) and HRTEM in a JEOL 4000EX TEM operating at 400 kV and in a JEOL 2010F FE-AEM operating at 200 kV and equipped with a Gatan Model 678 Imaging Filter. The as-precipitated ZrO2 processed with R=5 contained highly aggregated clusters and elongated denser particles 200-500 nm in diameter. HRTEM examination, however, revealed nanocrystals 5-11 nm in size, randomly distributed in the amorphous matrix (Fig. 1a) that could serve as nuclei for growing crystalline phases during calcination. In contrast, calcinated ZrO2 particles 400-600 nm in size, were found to be highly crystalline as indicate distinct Bragg reflections assigned to the t-phase in a SAED pattern (insert in Fig. 1b). DF TEM using the 1 0 1 reflection showed 10-100 nm–sized crystalline regions of various shapes throughout the particles. Multiple lattice fringes with spacings ~0.25-0.64 nm were observed by HRTEM (Fig. 1c). The ZrO2 powder precipitated at R=60 consisted of 4-11 nm-sized particles forming aggregates (~50-100 nm) with an amorphous structure (Fig. 2a). The SAED pattern (insert in Fig. 2a) shows a broad diffuse ring with an intensity maximum corresponding to the most probable interatomic spacing ~ 0.3 nm. BF TEM (Fig. 2b) and HRTEM of the calcinated powder (Fig. 2b) revealed aggregates of randomly oriented 8-100 nm-sized nanocrystals of various shapes with two families of lattice fringes of ~0.3 nm and 0.71-0.99 nm. The SAED pattern (insert in Fig. 2b) displays discrete rings and spot reflections, which were indexed according to both m and t-phase spacings. The net PEEL intensity (Fig. 2c) satisfactorily fits to the expected position of a direct band gap for ZrO2 (solid curve) between 4-5 eV energy loss [2]. For the as-precipitated nanopowder (dash curve), the intensity threshold is clearly less pronounced, probably due to a number of defect states in the gap. The peaks below 30 eV are primarily associated with plasmons and interband transitions, while those above 30 eV are related to the Zr-N2,3 edge with an onset at ~32 eV energy loss. An unresolved peak at ~7.4 eV is likely due to excitation of valence electrons into unoccupied d-states in the conduction band. The bulk plasmon at 13.4 eV for the nanocrystalline material is reduced in intensity and shifted to 14.7 eV for the as-precipitated sample. The broad peak at 25-26 eV is at least partly due to additional unexhausted collective excitation of all (16) valence electrons of O and Zr per ZrO2 unit. Fingerprints of the ZrO2 band structure in the low-loss PEEL spectra allow differentiation between the amorphous-like and nanocrystalline powders. Stabilization of the t-phase with much larger 410 Microsc Microanal 9(Suppl 2), 2003 Copyright 2003 Microscopy Society of America DOI: 10.1017/S1431927603442050

Imaging of Electric Fields at the GaN/Ni Interface Using Electron Beam Induced Current in a Scanning Transmission Electron Microscope

Gallium nitride is a promising wide bandgap material for demanding applications such as hightemperature and high-power electronics, as well as for space applications where its higher resistance to high fluxes of proton and electron radiation compared to Si represents an important advantage [1]. Electron irradiation of GaN is believed to create both N and Ga vacancies, as well as to induce threading dislocation glide. Typically, the creation or modification of structural defects in GaN are studied in a scanning electron microscope (SEM) retrofitted with cathodoluminescence (CL) or electron beam induced current (EBIC) tools, which identify locations of high defect concentration as dark regions due to increased (non-radiative) recombination rates. Unfortunately, the spatial resolution of these methods is limited due to the relatively large interaction volumes generated by primary electrons. While scanning transmission electron microscopy (STEM) enables the requisite spatial resolution necessary to understand the mechanisms of how defects are created and/or propagated under electron beam irradiation, it is still rarely combined with CL or EBIC, which can identify electronically active defects. Here we report the first STEM based EBIC characterization of Schottky diodes consisting of Ni contacts to free-standing hydride vapor phase epitaxy (HVPE) grown GaN with a threading dislocation density of ~10/cm.

Study of Direct Lithiation of Thin Si Membranes with Spatially-Correlative Low Energy Focused Li Ion Beam and Analytical Electron Microscopy Techniques

Understanding and controlling defect interactions and transport properties of Li ions in silicon is crucial for the development of emerging technologies in energy storage and microelectronics [1-4]. With a theoretical energy storage capacity of ~4200 mAhg, which is more than 10 times over than that of graphite (372 mAhg), Si is an attractive anode material for high-performance Li-ion batteries [1, 4]. The practical use of Si-based anodes is, however, hampered by the large volumetric changes (up to ~400%) that occur during electrochemical charge/discharge cycling and the concomitant capacity fading. Further insights into fundamental mechanisms of Li-Si reactions are needed to address these problems. Here, we report on controlled low dose Li ion implantation into 9 nm-thick amorphous (a-Si) membranes and 35 nm-thick single crystalline <100>-oriented (c-Si) membranes using a low-energy focused lithium ion beam ((LiFIB), FIG. 1a). With probe sizes of a few tens of nanometers at energies ranging from 500 eV to 6 keV and beam currents of a few pA, the LiFIB enables surface topography and composition sensitive imaging using ion-induced secondary electrons (iSE) and backscattered ions (BSI), respectively [5]. Furthermore, the LiFIB has a unique ability to implant certain amounts of Li ions into any material with nanoscale precision, making it a potentially powerful tool for advanced battery materials research and material design via defect engineering and surface modification. In this work, we analyze the spatial distributions of lithium in the selected regions of the thin Si membranes where the low dose Li ions were implanted through a combination of correlative low-energy LiFIB implantation/imaging, conventional and phase-contrast high-resolution transmission electron microscopy (CTEM/HRTEM), selected-area electron diffraction (SAED), scanning TEM (STEM), and electron energy-loss spectroscopic STEM imaging (EELS-STEM SI). This reveals information about the Li–Si interactions, bonding and lateral distributions of low Li concentrations within the membranes in the range from 1×10 ion/nm to 13×10 ion/nm or from 1.1×10 g/nm to 14.6×10 g/nm, respectively (FIGs. 1 and 2). It is noteworthy that this approach avoids the inherent electrochemical complications from the formation of an unstable solidelectrolyte interface (SEI) with poor conductivity. Through this approach we directly reveal the series of overlapping nanoscale physico-chemical processes that occur during the early stages of Li implantation, including: (a) selective etching, amorphization and cracking of the Si membranes (FIGs. 1b and 2b); (b) the creation of interstitial-rich extended stepped and v-shaped defects (FIG. 2b); (c) clustering of mobile interstitials in tetragonal sites of the c-Si matrix (FIG. 2c) as well as (d) concentration gradient driven local diffusion of Li ions followed by (e) the formation of various mixed composition Si-Li alloy phases.

In Situ Quantitative Plasmon Spectroscopic Determination and Imaging of Multiple SolidState Properties at the Nanoscale: a New Capability for Material Research

Measuring material properties is critical to understanding the behavior of contemporary nanostructured materials. In this paper, we show that as a consequence of the universal binding energy relation (UBER), universal features and strong scaling correlations exist between the volume plasmon energy and cohesive energy, valence electron density, elastic constants and hardness of various materials with metallic and covalent bonding. Based on these relations, we propose novel techniques that allow direct measurement and imaging of material properties in situ using valence electron energy-loss spectroscopy combined with energy-filtering transmission electron microscopy. This is illustrated by evaluation of elastic and cohesive properties of individual metastable nanoprecipitates in structural alloys and hardness of diesel-engine soot particles. The results demonstrate that new plasmon spectro-microscopic techniques have the potential to determine quantitatively and image multiple solid-state properties at the nanoscale, establishing a new capability for material research.

Solid State Property - Bonding Electron Density – Volume Plasmon Energy Scaling Relationships: Novel AEM Technique for In Situ Diagnostics of Material Properties at the Nanoscale

Quantized high-frequency (~10 16 Hz) correlated longitudinal electron excitations (plasmons) generated in the energy-loss range 0-50 eV by fast electrons passing through any solid enable one to probe various states of matter. Their energy, Ep, is directly related to the density of valence electrons, thus allowing determination of solid-state properties that are governed by ground-state densities. Universal features and scaling in relations between Ep and the cohesive energy per atomic volume, bonding electron density and elastic constants have been established. The resulting correlations follow the universal binding energy relationship, thus providing new insights into the fundamental nature of structure-property relationships. They allow direct in situ determination of local material properties in an analytical electron microscope, as illustrated by examples utilizing Al- and Ti-based structural alloys.

Cold-Atom Ion Sources for Focused Ion Beam Applications

1Center for Nanoscale Science and Technology, 2Material Measurement Laboratory, and 3Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899 USA 4Theiss Research, La Jolla, CA 92037, USA 5Maryland Nanocenter, University of Maryland, College Park, MD 20742, USA 6Ilmenau University of Technology, Ilemnau, Germany 7zeroK NanoTech, Gaithersburg, MD 20879 USA

In-Situ Analytical Transmission Electron Microscopy Study of Electrochemical Lithiation of a Sulfur - Carbon Nanotube Composite Cathode

Rechargeable Li-S batteries have the potential to meet the high power demands of next generation lightweight, low-cost, and environmentally friendly batteries useful in both small-scale portable devices and large-scale applications such as electric vehicles. Lithium-sulfur (Li-S) batteries offer a high theoretical capacity of 1,672mAh/g and a theoretical energy density of 2567Wh/kg, roughly five times larger than that of currently utilized carbon-based Li-ion batteries. [1] In addition, sulfur is light weight, earthly abundant, and nontoxic. Despite its promise, Li-S batteries still suffer from poor cycling performance caused by the polysulfide shuttle process which occurs during the multistep reduction of sulfur to Li2S. Due to the insulating nature of sulfur, this reaction relies on the incorporation of sulfur into a conductive carbon host structure. Various carbons serve as both an electrically conductive pathway, as well as a structural network to accommodate the volumetric expansions associated with cycling. Carbon nanotubes (CNTs), possessing a high electrical conductivity and mechanical strength, are considered a prospective material to serve this role. In spite of extensive efforts, significant gaps still remain in the understanding of the behavior of lithium and reduced sulfur species at the carbon-sulfur interface during working conditions. Therefore a deeper understanding of the fundamental electrochemical reaction mechanisms and kinetics of this system is required to develop the next generation of ultrafast, long-life, high-energy density Li-S batteries. [2]