Ian M. Anderson

Electron Vortex Beams with High Quanta of Orbital Angular Momentum

Diffraction holograms are used to create electron vortex beams that should enable higher-resolution imaging. Electron beams with helical wavefronts carrying orbital angular momentum are expected to provide new capabilities for electron microscopy and other applications. We used nanofabricated diffraction holograms in an electron microscope to produce multiple electron vortex beams with well-defined topological charge. Beams carrying quantized amounts of orbital angular momentum (up to 100ℏ) per electron were observed. We describe how the electrons can exhibit such orbital motion in free space in the absence of any confining potential or external field, and discuss how these beams can be applied to improved electron microscopy of magnetic and biological specimens.

Transmission electron microscopy with a liquid flow cell.

The imaging of microscopic structures at nanometre‐scale spatial resolution in a liquid environment is of interest for a wide range of studies. Recently, a liquid flow transmission electron microscopy (TEM) holder equipped with a microfluidic cell has been developed and shown to exhibit flow of nanoparticles through an electron transparent viewing window. Here we demonstrate the application of the flow cell system for both scanning and conventional transmission electron microscopy imaging of immobilized nanoparticles with a resolution of a few nanometres in liquid water of micrometre thickness. The spatial resolution of conventional TEM bright field imaging is shown to be limited by chromatic aberration due to multiple inelastic scattering in the water, and we demonstrate that the liquid in the cell can be displaced by a gas phase that forms under intense electron irradiation. Our data suggest that under appropriate conditions, TEM imaging with a liquid flow cell is a promising method for understanding the in situ behaviour of nanoscale structures in a prescribed and dynamically changing chemical environment.

Designed Synthesis, Structure, and Properties of a Family of Ferecrystalline Compounds [(PbSe)1.00]m(MoSe2)n

The targeted synthesis of multiple compounds with specific controlled nanostructures and identical composition is a grand challenge in materials chemistry. We report the synthesis of the new metastable compounds [(PbSe)1.00]m(MoSe2)n using precursors each designed to self-assemble into a specific compound. To form a compound with specific values for m and n, the number of atoms within each deposited elemental layer was carefully controlled to provide the correct absolute number of atoms to form complete layers of each component structural unit. On low-temperature annealing, these structures self-assemble with a specific crystallographic orientation between the component structural units with atomically abrupt interfaces. There is rotational disorder between the component structural units and between MoSe2 basal plane units within the MoSe2 layers themselves. The lead selenide constituent has a distorted rock salt structure exactly m bilayers thick leading to peaks in the off-axis diffraction pattern as a result of the finite size of and rotational disorder between the crystallites. The in-plane lattice parameters of the PbSe and MoSe2 components are independent of the value of m and n, suggesting little or no strain caused by the interface between them. These compounds are small band gap semiconductors with carrier properties dominated by defects and exhibit extremely low thermal conductivity as a result of the rotational disorder. The thermal conductivity can be tuned by varying the ratio of the number of ordered PbSe rock salt layers relative to the number of rotationally disordered MoSe2 layers. This approach, based on controlling the local composition of the precursor and low temperature to limit diffusion rates, provides a general route to the synthesis of new compounds containing alternating layers of constituents with designed nanoarchitecture.

Size-Dependent Structural Distortions in One-Dimensional Nanostructures†

Nanoscale materials have been intensely studied since the discovery that the optical properties of semiconductor nanoparticles are size dependent. This and subsequent research has demonstrated that a given physical property of a particle exhibits a size dependence when the size becomes comparable to its characteristic length scale. Examples of relevant length scales include the de Broglie wavelength and/or the mean free path of electrons, phonons, and elementary excitations, all of which typically range from one to a few hundred nanometers. The ability to tune a wide variety of properties by controlling the particle size has spurred the development of novel chemistries for preparing nanostructured elements and compounds with goals of precisely controlling size, shape, and ligand shell. As the size of a nanocrystal decreases, the ratio of bulk to surface atoms decreases. This progression increases the relative contribution of the surface free-energy relative to the volume free-energy of the bulk structure, such that distortions from bulk equilibrium structures might be expected as the nanoparticle size decreases. Unfortunately, while researchers have demonstrated the ability to prepare ordered lattices of nanoparticles, the isolation of lattices of nanoparticles with long-range atomic periodicity is rare. Hence detailed atomic structures and, in turn, the size-structure-property relationships of most nanoparticle systems cannot readily be determined. Recently we reported that the intergrown compounds [(MSe)1+y]m(TSe2)n, with M= {Pb, Bi, Ce} and T= {W, Nb, Ta} self-assemble from designed precursors. The values of m and n represent, respectively, the number of MSe and TSe2 structural units of the unit cell of the superstructure and y describes the misfit between these structural units. As reported herein, the long-range structural order along the modulation direction permits us to determine the atomic structure of these precisely defined one-dimensional (1D) nanolaminate structures as a function of m and n using a combination of scanning transmission electron microscopy (STEM) high-angle annular dark-field (HAADF) imaging and X-ray diffraction (XRD) with Rietveld refinement. STEM-HAADF images of the first five [(PbSe)1.00]m(MoSe2)n compounds in the family where m= n are shown in Figure 1 along with aggregate intensity plots used to quantify the PbSe intraand inter-pair distances. All have a regular periodic structure along the modulated axis with well-defined layers of PbSe and MoSe2. The STEM images show ordered domains of PbSe with characteristic dimensions of a single structural unit along the layering direction and tens of nanometers perpendicular to the layering direction, with random in-plane rotational variants both within a layer and between layers. The orientations of the MoSe2 domains are more difficult to discern from the STEM images, but rotational variants have been observed between individual MoSe2 structural units. The STEM-HAADF images reveal a distortion of the PbSe layers, with the atomic planes grouped into pairs rather than being evenly spaced as expected for the equilibrium (bulk) rock salt structure. The distortion is most evident in the structural variant (m, n)= (2, 2) and decreases in magnitude until it can no longer be observed for (5, 5).

Electron Laguerre-Gaussian beams

We use nanofabricated diffraction holograms to demonstrate electron Laguerre-Gaussian beams. These beams are analogous to optical vortices but are composed of charged particle wavefunctions possessing mass.

Electron Beams Carrying Quantized Orbital Angular Momentum

We use nanofabricated diffraction holograms to demonstrate electron vortex beams carrying quantized orbital angular momentum. These beams are analogous to optical vortices but are composed of charged particle wavefunctions possessing mass.

Event-Streamed Spectral Imaging in an Aberration-Corrected AEM: A Robust Approach to High Spatial Resolution XEDS Elemental Mapping

The development of aberration-correcting, multi-pole electron optics has greatly increased the resolving power of the analytical electron microscope (AEM). This technology has rendered the acquisition of atomic-resolution images nearly routine, and atomic-resolution chemical and bondstate mapping via electron energy-loss spectroscopy (EELS) have been achieved and are becoming widespread [1,2]. Nevertheless, a host of applications that could benefit immensely from atomicresolution elemental-mapping remain inaccessible via EELS analysis due to the low signal-tobackground of the characteristic inner-shell ionization edges, and thus poor EELS sensitivity, of many elements. In contrast, X-ray energy dispersive spectroscopy (XEDS) exhibits high and roughly equal sensitivity for all elements with Z > 4. However, this advantage is counteracted by the poor signal-collection efficiency of XEDS in the AEM due to the limited solid angle subtended by standard detectors within the limited space afforded by the high-resolution objective lens pole piece. Recent advances in detector design have shown promise for improving collection efficiency by overcoming these geometric constraints [3], but such detectors are not yet commercially available and their implementation in the aberration-corrected AEM does not appear to be imminent. The poor collection efficiency of current generation XEDS detectors can be somewhat mitigated by longer acquisition times; however, this solution inevitably exacerbates the problems of beam damage and spatial drift, which are particularly acute for characterization at ultra-high spatial resolution.

Characterization of Thin Film CuCr2Se4 Synthesized by A Modulated Elemental Reactant Deposition

For several decades, spinel-structured compounds of chemical formula MCr2Se4 (M = first row transition metal) have been of interest for their exceptional intrinsic magnetic properties [1-3]. As opposed to the traditional ferrite spinels, which tend towards ferrimagnetism, these materials typically demonstrate ferromagnetism in addition to magneto-optical and ferroelectric behaviors. An example of particular interest is the compound CuCr2Se4, both because of its high Curie temperature and for the changes in magnetization the material undergoes during electromagnetic stimulation. Studies have also suggested that the magnetic properties of these materials are affected by the bulk morphology of the material, with the Curie temperature for nanocrystalline materials differing from those of polycrystalline and thin film materials [3,4]. This work details the XRD, EPMA, and TEM characterization of a CuCr2Se4 thin film synthesized using a novel single step synthetic method, elucidating a number of previously unexplained aspects of CuCr2Se4 thin films. Thin films were deposited by evaporating elemental Cu, Cr, and Se in stoichiometric proportion on wafers of {001} silicon. Films were annealed at 600°C for 1 hour in a nitrogen atmosphere. Samples were initially analyzed using X-ray reflectivity and diffraction. Samples were then cross sectioned and prepared for TEM analysis using the small angle cleavage technique (SACT) [5]. High-resolution (HR) TEM imaging was performed at 300 kV operating voltage with a Philips CM300FEG TEM (Cs = 1.2 mm) using a 10.8 mrad objective aperture semi-angle. Thin-film composition was measured by electron microprobe analysis (EPMA) using a Cameca SX-100 microprobe running Probe for Windows and StrataGem data postprocessing. The films were found by X-ray diffraction analysis to be single phase and highly textured, as shown in Figure 1. Rocking curves suggest that the film has a preferred growth orientation with the <111> planes parallel to the substrate. EPMA analysis verified the composition of CuCr2Se4, as shown in Table 1. HRTEM analysis yielded a lattice image consistent with the spinel structure as shown in Figure 2.

Synthesis of new ferecrystals (SnSe) y (TSe 2 ) where T = V and Ta

(SnSe)<inf>1.09</inf>(VeSe<inf>2</inf>) and (SnSe)<inf>1.16</inf>(TaSe<inf>2</inf>) ferecrystals were synthesized using the modulated elemental reactant (MER) technique. The materials composition, in-plane crystalline structure, and layering characteristics were characterized through electron microprobe, x-ray diffraction, and transmission electron microscope analysis.

In Vitro Transmission Electron Microscopy of Water-Borne Dendrimer-Encapsulated Gold Nanoparticles

Due to the growing applications, especially in health-related fields, of functional nanoparticles comprised of both the hard and soft material components, there is rising demand for characterization techniques with the high spatial resolution necessary to image individual such nanostructures in their relevant aqueous (“in vitro”) environment. This research addresses the above challenge using a prototype liquid flow cell specimen holder for the transmission electron microscope (TEM) [1]. The high signal and consequent rapid acquisition of TEM bright-field imaging, coupled with the capacity to flow liquid through the probed region, also enables the performance of dynamic experiments, which are currently inaccessible by alternative techniques such as cryo-TEM, and thus allow a unique in situ approach to study particle behavior in a prescribed chemical environment.