Michelle A. Hekmaty

Towards better 3-D reconstructions by combining electron tomography and atom-probe tomography

Scanning transmission electron microscope tomography and atom-probe tomography are both three-dimensional techniques on the nanoscale. We demonstrate here the combination of the techniques by analyzing the very same volume of an Al-Ag alloy specimen. This comparison allows us to directly visualize the theoretically known artifacts of each technique experimentally, providing insight into the optimal parameters to use for reconstructions and assessing the quality of each reconstruction. The combination of the techniques for accurate morphology and compositional information in three dimensions at the nanoscale provides a route for a new level of materials characterization and understanding.

Thermal expansion coefficients of Bi2Se3 and Sb2Te3 crystals from 10 K to 270 K

Lattice constant of Bi2Se3 and Sb2Te3 crystals is determined by x-ray powder diffraction measurement in a wide temperature range. Linear thermal expansion coefficients (α) of the crystals are extracted, and considerable anisotropy between α|| and α⊥ is observed. The low temperature values of α can be fit well by the Debye model, while an anomalous behavior at above 150 K is evidenced and explained. Gruneisen parameters of the materials are also estimated at room temperature.

Temperature dependence of Raman-active optical phonons in Bi2Se3 and Sb2Te3

Inelastic light scattering spectra of Bi2Se3 and Sb2Te3 single crystals have been measured over the temperature range from 5 K to 300 K. The temperature dependence of dominant A1g2 phonons shows similar behavior in both materials. The temperature dependence of the peak position and linewidth is analyzed considering the anharmonic decay of optical phonons and the material thermal expansion. This work suggests that Raman spectroscopy can be used for thermometry in Bi2Se3- and Sb2Te3-based devices in a wide temperature range.

Nanoporous Pd alloys with compositionally tunable hydrogen storage properties prepared by nanoparticle consolidation

Nanoporous palladium and palladium alloys are expected to have improved mass transport rates and cycle life compared to bulk materials for energy storage and other applications due to high ratios of surface area to metal volume. Preparation of such materials with high thermal stability and well-controlled metal composition, however, remains a challenge. This work describes a scalable, bottom-up technique for preparing nanoporous palladium alloys based on partial consolidation of dendrimer-encapsulated nanoparticles (DEN). Destabilization of a colloidal suspension of DEN and purification yields high surface area material (60–80 m2 g−1) with a broad pore size distribution centered between 20 and 50 nm. This approach allows for precise tuning of product composition through adjustment of the composition of the precursor DEN. Nanoporous Pd0.9Rh0.1 alloys with uniform composition or with Rh enrichment at pore walls and grain boundaries have been prepared and these structures have been confirmed with high-spatial resolution, aberration corrected quantitative STEM-EDS. Compared to bulk alloys of the same nominal composition, the nanoporous bimetallics show much faster hydrogen uptake kinetics, and store hydrogen at much lower pressure. Pore structure remains intact to temperatures above 300 °C, suggesting that these materials will have long lifetimes at the temperatures used for hydrogen storage applications.

Synthesis of mesoporous palladium with tunable porosity and demonstration of its thermal stability by in situ heating and environmental transmission electron microscopy

Palladium and its alloys have high-value applications as materials for high-performance hydrogen storage, chromatographic separation of hydrogen isotopes, electrocatalysis and catalysis. These materials can be formed by chemical or electrochemical reduction in a lyotropic liquid crystalline template that constrains their growth on the nanometer scale. This approach works for a variety of metals, but Pd presents special challenges due to the autocatalytic nature of its growth, which can disrupt the template structure, resulting in disordered pores. Presented herein is a scaleable synthesis that overcomes these challenges, yielding mesoporous Pd powder having pore diameters of 7 or 13 nm. Pore size control is effected by varying the size of the molecular template, polystyrene-block-polyethylene oxide. We have used heated-stage TEM for in situ observation of the materials in vacuum and in the presence of H2 gas, demonstrating that both pore diameter and the chemical state of the surface play important roles in determining thermal stability. Improved stability compared to previously reported examples facilitates preparation of scalable quantities of regularly mesoporous Pd that retains porosity at the elevated temperatures required for applications in hydrogen charge/discharge and catalysis.

Resist substrate studies for LIGA microfabrication with application to a new anodized aluminum substrate

Resist substrates used in the LIGA process must provide high initial bond strength between the substrate and resist, little degradation of the bond strength during x-ray exposure, acceptable undercut rates during development and a surface enabling good electrodeposition of metals. Additionally, they should produce little fluorescence radiation and give small secondary doses in bright regions of the resist at the substrate interface. To develop a new substrate satisfying all these requirements, we have investigated secondary resist doses due to electrons and fluorescence, resist adhesion before exposure, loss of fine features during extended development and the nucleation and adhesion of electrodeposits for various substrate materials. The result of these studies is a new anodized aluminum substrate and accompanying methods for resist bonding and electrodeposition. We demonstrate the successful use of this substrate through all process steps and establish its capabilities via the fabrication of isolated resist features down to 6 µm, feature aspect ratios up to 280 and electroformed nickel structures at heights of 190 to 1400 µm. The minimum mask absorber thickness required for this new substrate ranges from 7 to 15 µm depending on the resist thickness.

Ion beam modification of topological insulator bismuth selenide

We demonstrate chemical doping of a topological insulator Bi2Se3 using ion implantation. Ion beam-induced structural damage was characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Ion damage was reversed using a simple thermal annealing step. Carrier-type conversion was achieved using ion implantation followed by an activation anneal in Bi2Se3 thin films. These two sets of experiments establish the feasibility of ion implantation for chemical modification of Bi2Se3, a prototypical topological insulator. Ion implantation can, in principle, be used for any topological insulator. The direct implantation of dopants should allow better control over carrier concentrations for the purposes of achieving low bulk conductivity. Ion implantation also enables the fabrication of inhomogeneously doped structures, which in turn should make possible new types of device designs.

TEM Analyses of Bismuth-Antimony Thermoelectric Nanowires

Thermoelectric materials have diverse applications for solid-state cooling and power generation, but at present have poor energy conversion efficiencies. Theory has long predicted that Bi-based nanowires could achieve dramatic improvements in thermoelectric performance through quantum confinement to create peaks in the electronic density-of-states that enhance the thermoelectric power factor [1,2]. However, achieving this promise has been difficult, in part because of the experimental challenges of controlling the materials growth in these systems. Key issues include control of composition and crystallinity. In this presentation, we will discuss our electron microscopic investigation of nanowires based on alloys of Bi and Sb, with the goal of understanding the underlying compositional and microstructural issues necessary for improving nanowire growth and quality.

Compositional Ordering and Stability in Nanostructured, Bulk Thermoelectric Alloys

Thermoelectric materials have many applications in the conversion of thermal energy to electrical power and in solid-state cooling. One route to improving thermoelectric energy conversion efficiency in bulk material is to embed nanoscale inclusions. This report summarize key results from a recently completed LDRD project exploring the science underpinning the formation and stability of nanostructures in bulk thermoelectric and the quantitative relationships between such structures and thermoelectric properties.

An aluminum resist substrate for microfabrication by LIGA.

Resist substrates used in the LIGA process must provide high initial bond strength between the substrate and resist, little degradation of the bond strength during x-ray exposure, acceptable undercut rates during development, and a surface enabling good electrodeposition of metals. Additionally, they should produce little fluorescence radiation and give small secondary doses in bright regions of the resist at the substrate interface. To develop a new substrate satisfying all these requirements, we have investigated secondary resist doses due to electrons and fluorescence, resist adhesion before exposure, loss of fine features during extended development, and the nucleation and adhesion of electrodeposits for various substrate materials. The result of these studies is a new anodized aluminum substrate and accompanying methods for resist bonding and electrodeposition. We demonstrate successful use of this substrate through all process steps and establish its capabilities via the fabrication of isolated resist features down to 6 {micro}m, feature aspect ratios up to 280 and electroformed nickel structures at heights of 190 to 1400 {micro}m. The minimum mask absorber thickness required for this new substrate ranges from 7 to 15 {micro}m depending on the resist thickness.