Four-dimensional STEM-EELS: enabling nano-scale chemical tomography.

Advances in electron-based instrumentation have enabled the acquisition of multidimensional data sets for exploring the unique structure-property relationship of nanomaterials. In this manuscript, we report a technique for directly probing and analyzing the three-dimensional (3D) electronic structure of a material at the nano-scale. This technique, referred to here as 4D STEM-EELS, utilizes a rotation holder and pillar-shaped samples to allow STEM mode high-angle annular dark-field (HAADF) and EELS spectrum images to be recorded over a complete 180 degrees rotation to minimize artifacts. The end result is a four-dimensional data set, containing two spatial dimensions, rotation angle and energy-loss information I(x, y, theta, DeltaE), which can then be processed to extract any EELS signal as a rotation or "tilt-series" map. If the extracted properties satisfy the linear projection criteria, these maps can then be used for tomographic reconstruction to yield volumetric maps of the corresponding properties. Hence by combining STEM HAADF and energy-loss information from such a series of spectrum images, it is possible to map not only the microstructure, but also the elemental, physical and chemical state information of a material in three dimensions. Two examples are reported here to demonstrate the potential of this technique. To illustrate chemical tomography, 4D STEM-EELS was used to directly probe the 3D electronic structure of a W-to-Si contact from a semiconductor device. Core-loss data were used to reconstruct and render the composition of the W-to-Si contact in three dimensions. The fine structure of the 99eV Si edge was analyzed with MLLS fitting to map the variations in Si bonding in 3D. To illustrate the direct probing of intrinsic material anisotropy, 4D STEM-EELS was used to probe a ZnO thin film. Subtle but systematic changes in low-loss structure were observed as a function of electron-beam orientation with respect to the ZnO crystallographic axes. Together these examples illustrate how the 4D STEM-EELS technique reported here can be used to probe the elemental, physical and chemical state information of a material in three dimensions and extend our knowledge of nano-scale structures.

Three-dimensional electron microscopy of individual nanoparticles.

The characterization of nanomaterials with complex three-dimensional (3D) geometries is required to further research and enable the continuing development of nanotechnology. In this manuscript, we report a protocol which combines focused ion beam (FIB) milling, thin film deposition and solution chemistry to optimize a rotation holder for 3D structural and chemical analysis of nanoparticles. This protocol is used to customize the geometry, surface and chemistry of a scanning transmission electron microscope (STEM) or transmission electron microscope (TEM) rotation holder for the nanoparticle system of interest. To illustrate this concept, rotation holder stubs were optimized to facilitate the 3D STEM imaging and analysis of core-shell nanoparticles used for DNA detection. Using this approach, it was possible to characterize the morphology, optoelectronic properties and chemical composition of individual core-shell nanoparticles in 3D. STEM images were captured at regular angular intervals over a complete 360 degrees rotation to eliminate missing wedge artifacts. Electron energy-loss spectroscopy (EELS) spectrum images were acquired intermittently for comparative chemical analysis. This approach allows the 3D STEM/TEM analysis to be performed with the nanoparticle of interest cantilevered over vacuum to minimize substrate effects. Standard tomography techniques were used to reconstruct the 3D structure of the individual nanoparticles from the STEM HAADF rotation series. EELS spectrum imaging was used to determine the local material properties such as composition, band-gap and plasmon energy. The nanoparticle analysis protocol reported here can easily be adapted to facilitate 3D TEM/STEM analysis of other nanomaterial systems.

The development and characteristics of a high-speed EELS mapping system for a dedicated STEM.

A new EELS (electron energy loss spectroscopy) real-time elemental mapping system has been developed for a dedicated scanning transmission electron microscope (STEM). The previous two-window-based jump-ratio system has been improved by a three-window-based system. It is shown here that the three-window imaging method has less artificial intensity in elemental maps than the two-window-based method. Using the new three-window system, the dependence of spatial resolution on the energy window width was studied experimentally and also compared with TEM-based EELS. Here it is shown experimentally that the spatial resolution of STEM-based EELS is independent of the energy window width in a range from 10 eV to 60 eV.

Hyperbranched lead selenide nanowire networks.

Lead chalcogenide nanostructures are good potential candidates for applications in multiexciton solar cells, infrared photodetectors, and electroluminescence devices. Here we report the synthesis and electrical measurements of hyperbranched PbSe nanowire networks. Hyperbranched PbSe nanowire networks are synthesized via a vapor-liquid-solid (VLS) mechanism. The branching is induced by continuously feeding the PbSe reactant with the vapor of a low-melting-point metal catalyst including In, Ga, and Bi. The branches show very regular orientation relationships: either perpendicular or parallel to each other. The diameter of the individual NWs depends on the size of the catalyst droplets, which can be controlled by the catalyst vapor pressure. Significantly, the hyperbranched networks can be grown epitaxially on NaCl, a low-cost substrate for future device array applications. Electrical measurements across branched NWs show the evolution of charge carrier transport with distance and degree of branching.

Fast, completely reversible li insertion in vanadium pentoxide nanoribbons.

Layered-structure nanoribbons with efficient electron transport and short lithium ion insertion lengths are promising candidates for Li battery applications. Here we studied at the single nanostructure level the chemical, structural, and electrical transformations of V2O5 nanoribbons. We found that transformation of V2O5 into the omega-Li3V2O5 phase depends not only on the width but also the thickness of the nanoribbons. Transformation can take place within 10 s in thin nanoribbons, suggesting a Li diffusion constant 3 orders of magnitude faster than in bulk materials, resulting in a significant increase in battery power density (360 C power rate). For the first time, complete delithiation of omega-Li3V2O5 back to the single-crystalline, pristine V2O5 nanoribbon was observed, indicating a 30% higher energy density. These new observations are attributed to the ability of facile strain relaxation and phase transformation at the nanoscale. In addition, efficient electronic transport can be maintained to charge a Li3V2O5 nanoribbon within less than 5 s. These exciting nanosize effects can be exploited to fabricate high-performance Li batteries for applications in electric and hybrid electric vehicles.

Synthesis and characterization of phase-change nanowires.

Phase-change memory materials have stimulated a great deal of interest although the size-dependent behaviors have not been well studied due to the lack of method for producing their nanoscale structures. We report the synthesis and characterization of GeTe and Sb(2)Te(3) phase-change nanowires via a vapor-liquid-solid growth mechanism. The as-grown GeTe nanowires have three different types of morphologies: single-crystalline straight and helical rhombohedral GeTe nanowires and amorphous curly GeO(2) nanowires. All the Sb(2)Te(3) nanowires are single-crystalline.