G. G. Hembree

SELF-ORGANIZED FE NANOWIRE ARRAYS PREPARED BY SHADOW DEPOSITION ON NACL(110) TEMPLATES

Iron nanowire arrays have been grown by shadow deposition on a self-organized grating template produced by annealing the sodium chloride (110) surface. The typical wire size as measured using transmission electron microscopy is 45 nm×13 nm×10 μm. The typical wire array period is 90 nm. The magnetic properties were dominated by a strong in-plane shape anisotropy. The hysteresis loops examined by magneto-optical Kerr effect measurements indicated coherent switching, even though the individual wires were isolated from one another.

Growth of nanometer‐size metallic particles on CaF2(111)

Fe, Co, and Ag particles grown on various CaF2 substrates have been studied using ultrahigh vacuum scanning electron microscopy with nanometer resolution. Fe and Co show a very high nucleation density which is remarkably independent of deposition temperature in the range 20<T<300 °C, on both bulk CaF2(111), and on thin CaF2(111) films grown on Si(111). This feature is characteristic of nucleation at defect sites with a high trapping energy. An atomistic nucleation model has been extended to cover this case. The comparison with experiment requires adsorption, pair binding, and defect trapping energies all to be around 1 eV. The trapping sites occupy 1% of the surface, and are thought to be chemical (F‐vacancy, oxide, or hydroxide) in nature. In contrast, the growth of Ag on the same substrates shows a more usual nucleation and growth pattern, though the growth of Ag on Fe islands shows interesting features which are discussed. A self‐similar coalescence model is tested using the data obtained. The agreemen...

Biassed secondary electron imaging in a UHV-STEM

Abstract A new dedicated ultra-high-vacuum scanning transmission electron microscope (UHV-STEM) has been developed for the NSF HREM facility at Arizona State University, in conjunction with VG Microscopes Ltd. This instrument is fitted with a specimen preparation chamber in which vacua of better than 5x10-10 mbar can be routinely achieved; the pressure inside the column is better than 10-10 mbar. The initial performance in various imaging modes is reported. Several techniques have been incorporated to obtain information from surfaces. Here we demonstrate the usefulness of biassed secondary electron imaging from both sides of thin transmission samples. In conjunction with conventional STEM imaging and analysis techniques, these methods can be used to correlate surface and subsurface information on samples with complex surface topography. Examples typical of catalytic and semiconductor applications are given.

High resolution Auger electron spectroscopy and microscopy of a supported metal catalyst

High spatial resolution Auger electron spectra and scanning Auger microscope (SAM) images of supported metal catalysts have been obtained in a UHV scanning transmission electron microscope. Ag/α-Al2O3 was used as a model catalyst system, where silver was evaporated, in situ, onto polycrystalline alumina carriers. Silver particles, as small as 2 nm in diameter, were clearly revealed in SAM images with high contrast. On large islands, an edge resolution < 3 nm was achieved. Information about surface and bulk properties of supported catalysts can be extracted from images formed with different signals generated from the same area which are obtained simultaneously.

Powder diffraction from a continuous microjet of submicrometer protein crystals.

Atomic-resolution structures from small proteins have recently been determined from high-quality powder diffraction patterns using a combination of stereochemical restraints and Rietveld refinement [Von Dreele (2007), J. Appl. Cryst. 40, 133-143; Margiolaki et al. (2007), J. Am. Chem. Soc. 129, 11865-11871]. While powder diffraction data have been obtained from batch samples of small crystal-suspensions, which are exposed to X-rays for long periods of time and undergo significant radiation damage, the proof-of-concept that protein powder diffraction data from nanocrystals of a membrane protein can be obtained using a continuous microjet is shown. This flow-focusing aerojet has been developed to deliver a solution of hydrated protein nanocrystals to an X-ray beam for diffraction analysis. This method requires neither the crushing of larger polycrystalline samples nor any techniques to avoid radiation damage such as cryocooling. Apparatus to record protein powder diffraction in this manner has been commissioned, and in this paper the first powder diffraction patterns from a membrane protein, photosystem I, with crystallite sizes of less than 500 nm are presented. These preliminary patterns show the lowest-order reflections, which agree quantitatively with theoretical calculations of the powder profile. The results also serve to test our aerojet injector system, with future application to femtosecond diffraction in free-electron X-ray laser schemes, and for serial crystallography using a single-file beam of aligned hydrated molecules.

Nanometer-resolution scanning Auger electron microscopy

Abstract New instrumental developments are described, based on STEM optics and through-the-lens electron detection, which allow scanning Auger electron microscopy (SAM) to be performed at the nanometer level. Example Auger images with 3 nm resolution are shown of silver islands deposited on Si(100). A consistent definition of SAM edge resolution is given with emphasis placed on the distinction between image and analytical resolution. The structure of ideal samples for testing resolution, at the level where localization effects may become important, is discussed.

Diffraction and imaging from a beam of laser-aligned proteins: resolution limits.

The effect of the limited alignment of hydrated molecules is considered in a laser-aligned molecular beam, on diffraction patterns taken from the beam. Simulated patterns for a protein beam are inverted using the Fienup-Gerchberg-Saxton phasing algorithm, and the effect of limited alignment on the resolution of the resulting potential maps is studied. For a typical protein molecule (lysozyme) with anisotropic polarizability, it is found that up to 1 kW of continuous-wave near-infrared laser power (depending on dielectric constant), together with cooling to liquid-nitrogen temperatures, may be needed to produce sufficiently accurate alignment for direct observation of the secondary structure of proteins in the reconstructed potential or charge-density map. For a typical virus (TMV), a 50 W continuous-wave laser is adequate for subnanometre resolution at room temperature. The dependence of resolution on laser power, temperature, molecular size, shape and dielectric constant is analyzed.

Self-organized mesoscopic magnetic structures

Three types of mesoscopic magnetic microstructure have been formed using self-organization: linear arrays of nanometer diameter islands, nanometer width lines, and undulating, continuous films. These structures were produced by annealing NaCl (110) and (111) surfaces in situ to produce patterned templates with 10–100 nm periodicity. Growth parameters such as groove spacing, substrate temperature and total deposit thickness can be varied in order to define specific mesoscopic magnetic structures. The microstructural evolution during growth is discussed in the context of nucleation and coalescence kinetics. The resulting magnetic properties are described, and their connection to the underlying microstructure elucidated.

Experimental lensless soft-X-ray imaging using iterative algorithms: phasing diffuse scattering.

Images of randomly placed two-dimensional arrays of gold balls have been reconstructed from their soft-X-ray transmission diffraction patterns. An iterative hybrid input-output (HiO) algorithm was used to solve the phase problem for the continuous distribution of diffuse X-ray scattering. Knowledge of the approximate size of the clusters was required. The images compare well with scanning electron microscope (SEM) images of the same sample. The use of micrometre-sized silicon nitride window supports is suggested, and absorption filters have been used to allow collection of low spatial frequencies often obscured by a beam stop. This method of phasing diffuse scattering may have application to scattering from individual inorganic nanostructures or single macromolecules.

Dose, exposure time and resolution in serial X‐ray crystallography

The resolution of X-ray diffraction microscopy is limited by the maximum dose that can be delivered prior to sample damage. In the proposed serial crystallography method, the damage problem is addressed by distributing the total dose over many identical hydrated macromolecules running continuously in a single-file train across a continuous X-ray beam, and resolution is then limited only by the available molecular and X-ray fluxes and molecular alignment. Orientation of the diffracting molecules is achieved by laser alignment. The incident X-ray fluence (energy/area) is evaluated that is required to obtain a given resolution from (i) an analytical model, giving the count rate at the maximum scattering angle for a model protein, (ii) explicit simulation of diffraction patterns for a GroEL-GroES protein complex, and (iii) the spatial frequency cut-off of the transfer function following iterative solution of the phase problem, and reconstruction of an electron density map in the projection approximation. These calculations include counting shot noise and multiple starts of the phasing algorithm. The results indicate counting time and the number of proteins needed within the beam at any instant for a given resolution and X-ray flux. An inverse fourth-power dependence of exposure time on resolution is confirmed, with important implications for all coherent X-ray imaging. It is found that multiple single-file protein beams will be needed for sub-nanometer resolution on current third-generation synchrotrons, but not on fourth-generation designs, where reconstruction of secondary protein structure at a resolution of 7 A should be possible with relatively short exposures.