High contrast imaging and thickness determination of graphene with in-column secondary electron microscopy

We report a new method for quantitative estimation of graphene layer thicknesses using high contrast imaging of graphene films on insulating substrates with a scanning electron microscope. By detecting the attenuation of secondary electrons emitted from the substrate with an in-column low-energy electron detector, we have achieved very high thickness-dependent contrast that allows quantitative estimation of thickness up to several graphene layers. The nanometer scale spatial resolution of the electron micrographs also allows a simple structural characterization scheme for graphene, which has been applied to identify faults, wrinkles, voids, and patches of multilayer growth in large-area chemical vapor deposited graphene. We have discussed the factors, such as differential surface charging and electron beam induced current, that affect the contrast of graphene images in detail. (C) 2011 American Institute of Physics. doi:10.1063/1.3608062]

Microstructure and mechanical properties of electron beam weld joints of a Zr41Ti14Cu12Ni10Be23 bulk metallic glass with Zr

The electron beam welding technique was used to join Zr41Ti14Cu12Ni10Be23 bulk metallic glass (BMG) to crystalline pure Zr. Compositional, microstructural, and mechanical property variations across the welded interface were evaluated. It is shown that a crystalline layer develops close to the welding interface. Transmission electron microscopy of this layer indicates the crystalline phase to be tetragonal with lattice parameters close to that reported for Zr2Ni. However, the composition of this phase is different as it contains other alloying additions. The interface layer close to the bulk metallic glass side contains nanocrystalline Zr2Cu phase embedded in the glassy matrix. Nanoindentation experiments indicate that the hardness of the crystalline layer, although less than the bulk metallic glass, is more than the Zr itself. Commensurately, tensile tests indicate that the failure of the welded samples occurs at the Zr side rather than at the weld joint.

Growth of rutile TiO2 nanorods on TiO2 seed layer deposited by electron beam evaporation

Dense rutile TiO2 nanorods were grown on anatase TiO2 seed layer coated glass substrate by solution technique. The crystalline nature of nanorods has confirmed by transmission electron microscopy. The band gap of the TiO2 seed layer and nanorods were calculated using the UV-vis absorption spectrum and the band gap value of the anatase seed layer and rutile nanorods were 3.39 eV and 3.09 eV respectively. Water contact angle measurements were also made and showed that the contact angle of rutile nanorods was (134 degrees) larger than the seed layer contact angle (93 degrees). The RMS surface roughness of the TiO2 seed layer (0.384 nm) and nanorods film (18.5 nm) were measured by an atomic force microscope and correlated with their contact angle values. (C) 2011 Elsevier B.V. All rights reserved.

Bismuth catalyzed growth of silicon nanowires by electron beam evaporation

Silicon nanowires (NWs) have been grown in the vapor phase for the first time with bismuth (Bi) as a catalyst using the electron beam evaporation method at a low substrate temperature of 280 degrees C. The grown Si nanowires were randomly oriented on the substrate with an average length of 900 nm for a deposition time of 15 min. Bi faceted nanoparticles (crowned) at the end of the grown Si nanowires have been observed and attributed to the Vapor-Liquid-Solid (VLS) growth mechanism. Transmission Electron Microscopy analysis on the nanowires revealed their single crystalline nature and interestingly bismuth particles were observed in Si nanowires. The obtained results have shown a new window for Si nanowires growth with bismuth as a catalyst. (C) 2012 Elsevier B.V. All rights reserved.

Formation and Thermal Stability of Gold-Silica Nanohybrids: Insight into the Mechanism and Morphology by Electron Tomography

Gold-silica hybrids are appealing in different fields of applications like catalysis, sensorics, drug delivery, and biotechnology. In most cases, the morphology and distribution of the heterounits play significant roles in their functional behavior. Methods of synthesizing these hybrids, with variable ordering of the heterounits, are replete; however, a complete characterization in three dimensions could not be achieved yet. A simple route to the synthesis of Au-decorated SiO2 spheres is demonstrated and a study on the 3D ordering of the heterounits by scanning transmission electron microscopy (STEM) tomography is presentedat the final stage, intermediate stages of formation, and after heating the hybrid. The final hybrid evolves from a soft self-assembled structure of Au nanoparticles. The hybrid shows good thermal stability up to 400 degrees C, beyond which the Au particles start migrating inside the SiO2 matrix. This study provides an insight in the formation mechanism and thermal stability of the structures which are crucial factors for designing and applying such hybrids in fields of catalysis and biotechnology. As the method is general, it can be applied to make similar hybrids based on SiO2 by tuning the reaction chemistry as needed.

Electron microscopy of Ag-(Ni-O) core-shell nanowires

This report provides information about an electrodeposition based two-step synthesis methodology for producing core-shell Ag-(Ni-O) nanowires and their detailed structural and compositional characterization using electron microscopy technique. Nanowires were produced by employing anodic alumina templates with a pore diameter of 200 nm. In the first step of the synthesis process, nanocrystalline Ni-O was electrodeposited in a controlled manner such that it heterogeneously nucleated and grew only on the template pore walls without filling the pores from bottom upwards. This alumina template with pore walls coated with Ni-O was then utilized as a template during the electrodeposition of Ag in the second step. Electrodeposited Ag filled the template pores to finally produce Ag-(Ni-O) core-shell nanowires with an overall diameter of 200 nm.

Synthesis and Electron Microscopy of Superalloy Nanowires

An electrodeposition based methodology for synthesizing Ni-Cr-Fe nanowires is provided. As-synthesized nanowires were 200 nm in diameter and more than 5 mu m in length. Detailed characterization of the nanowires using electron microscopy technique revealed an amorphous microstructure for the nanowires with uniform distribution of Ni, Fe and Cr atoms. Annealing of the nanowire using the electron beam inside electron microscope resulted in gradual crystallization of amorphous microstructure into a nanocrystalline one which illustrated the potential for microstructural engineering of the nanowires. (C) 2014 The Electrochemical Society. All rights reserved.

Synthesis, electron microscopy and anti-microbial properties of Fe3O4-Ag nanotubes

Electrodeposition was used for synthesizing 200 nm diameter Fe3O4-Ag nanotubes. Compositional analysis at the single nanotube level revealed a fairly uniform distribution of component elements in the nanotube microstructure. As-synthesized Fe3O4-Ag nanotubes were superparamagnetic in nature. Electron diffraction revealed the ultrafine nanocrystalline microstructure of the nanotubes. The effect of Ag on the anti-microbial response of the nanotubes was investigated by comparing the effect of sulphate reducing bacteria (SRB) on Fe3O4-Ag and Fe3O4 nanotubes. Fe3O4 nanotubes were also electro-deposited in the present study. It was observed that the Fe3O4-Ag nanotubes exhibited good resistance to sulphate reducing bacteria which revealed the anti-microbial nature of the Fe3O4-Ag nanotubes.

Integrating AlGaN/GaN high electron mobility transistor with Si: A comparative study of integration schemes

AlGaN/GaN high electron mobility transistor stacks deposited on a single growth platform are used to compare the most common transition, AlN to GaN, schemes used for integrating GaN with Si. The efficiency of these transitions based on linearly graded, step graded, interlayer, and superlattice schemes on dislocation density reduction, stress management, surface roughness, and eventually mobility of the 2D-gas are evaluated. In a 500 nm GaN probe layer deposited, all of these transitions result in total transmission electron microscopy measured dislocations densities of 1 to 3 x 10(9)/cm(2) and <1 nm surface roughness. The 2-D electron gas channels formed at an AlGaN-1 nm AlN/GaN interface deposited on this GaN probe layer all have mobilities of 1600-1900 cm(2)/V s at a carrier concentration of 0.7-0.9 x 10(13)/cm(2). Compressive stress and changes in composition in GaN rich regions of the AlN-GaN transition are the most effective at reducing dislocation density. Amongst all the transitions studied the step graded transition is the one that helps to implement this feature of GaN integration in the simplest and most consistent manner. (C) 2015 AIP Publishing LLC.

Synthesis of 3-dimensional porous graphene nanosheets using electron cyclotron resonance plasma enhanced chemical vapour deposition

Microwave plasma driven chemical vapour deposition was used to synthesize graphene nanosheets from a mixture of acetylene and hydrogen gas molecules. In this plasma, acetylene decomposes to carbon atoms that form nanostructures in the outlet plasma stream and get deposited on the substrate. The GNS consists of a few layers of graphene aligned vertically to the substrate. Graphene layers have been confirmed by high-resolution transmission electron microscopy, and Raman spectral studies were conducted to observe the defective nature of the sample. The growth of nanosheets in a vertical direction is assumed to be due to the effect of electric field and from the difference in the deposition rate in the axial and parallel directions. These vertical graphene sheets are attractive for various applications in energy storage and sensors.

Synthesis and electron microscopy of high entropy alloy nanoparticles

This work provides a methodology for synthesizing isolated multi-component, high entropy alloy nanoparticles. Wet chemical synthesis technique was used to synthesis NiFeCrCuCo nanoparticles. As synthesized nanoparticles were spherical with an average size of 26.7 +/- 3.3 nm. Average composition of the as-synthesized nanoparticle dispersion was 26 +/- 2 at% Cr, 14 +/- 2 at% Fe, 10 +/- 0.6 at% Co, 25 +/- 0.1 at% Ni and 25 +/- 1.1 at% Cu. Compositional analysis of the nanoparticles conducted using the compositional line profile analysis and compositional mapping on a single nanoparticle level revealed a fairly uniform distribution of all the five component elements within the nanoparticle volume. Electron diffraction analysis clearly revealed that the structure of as-synthesized nanoparticles was face centered cubic. (C) 2015 Elsevier B.V. All rights reserved.

Eggs of British Meniscus midges (diptera;dixidae) observed by scanning electron microscopy

The egg of Dixella martinii is described for the first time. The eggs of the Dixidae are placed in three morphological groups: bulbous and meshed; streamlined and smooth; streamlined and minutely spiculated. Ten of the fourteen species known from Britain are placed in these groups. After a detailed description of the egg of D. martinii, the three morphological groups are described and scanning electron micrographs are provided.