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Direct write with a liquid precursor using an ion beam in situ, allows fabrication of nanostructures with higher purity than using gas phase deposition. Specifically, positively charged helium ions, when compared to electrons, localize the reaction zone to a single-digit nanometer scale. However, to control the interaction of the ion beam with the liquid precursor, as well as enable single digit fabrication, a comprehensive understanding of the radiolytic process, and the role of secondary electrons has to be developed. Here, we demonstrate an approach for directly writing platinum nanostructures from aqueous solution using a helium ion microscope, and discuss possible mechanisms for the beam-induced particle growth in the framework of Born-Oppenheimer and real-time electron dynamics models. We illustrate the nanoparticle nucleation and growth parameters through data analysis of in situ acquired movie data, and correlate these results to a fully encompassing, time-dependent, quantum dynamical simulation that takes into account both quantum and classical interactions. Finally, sub-15 nm resolution platinum structures generated in liquid are demonstrated.

Rapid advances in nanoscience rely on continuous improvements of material manipulation at near-atomic scales. Currently, the workhorse of nanofabrication is resist-based lithography and its various derivatives. However, the use of local electron, ion, and physical probe methods is expanding, driven largely by the need for fabrication without the multistep preparation processes that can result in contamination from resists and solvents. Furthermore, probe-based methods extend beyond nanofabrication to nanomanipulation and to imaging which are all vital for a rapid transition to the prototyping and testing of devices. In this work we study helium ion interactions with the surface of bulk copper indium thiophosphate CuM(III)P2X6 (M = Cr, In; X= S, Se), a novel layered 2D material, with a Helium Ion Microscope (HIM). Using this technique, we are able to control ferrielectric domains and grow conical nanostructures with enhanced conductivity whose material volumes scale with the beam dosage. Compared to the copper indium thiophosphate (CITP) from which they grow, the nanostructures are oxygen rich, sulfur poor, and with virtually unchanged copper concentration as confirmed by energy-dispersive X-ray spectroscopy (EDX). Scanning electron microscopy (SEM) imaging contrast as well as scanning microwave microscopy (SMM) measurements suggest enhanced conductivity in the formed particles, whereas atomic force microscopy (AFM) measurements indicate that the produced structures have lower dissipation and are softer as compared to the CITP.

Before attempting automatic or manual peak identification, it is critical that the EDS system be properly calibrated to ensure that accurate energy values are measured for the characteristic X-ray peaks. Follow the vendor’s recommended procedure to rigorously establish the calibration. The calibration procedure typically involves measuring a known material such as copper that provides characteristic X-ray peaks at low photon energy (e.g., Cu L3-M5 at 0.928 keV) and at high photon energy (Cu K-L3 at 8.040 keV). Alternatively, a composite aluminum-copper target (e.g., a copper penny partially wrapped in aluminum foil and continuously scanned so as to excite both Al and Cu) can be used to provide the Al K-L3 (1.487 keV) as the low energy peak and Cu K-L3 for the high energy peak. After calibration, peaks occurring within this energy range (e.g., Ti K-L3 at 4.508 keV and Fe K-L3 at 6.400 keV) should be measured to confirm linearity. A well-calibrated EDS should produce measured photon energies within ±2.5 eV of the ideal value. Low photon energy peaks below 1 keV photon energy should also be measured, for example, O K (e.g., from MgO) and C K. For some EDS systems, non-linearity may be encountered in the low photon energy range. Figure 18.1 shows an EDS spectrum for CaCO3 in which the O K peak at 0.523 keV is found at the correct energy, but the C K peak at 0.282 keV shows a significant deviation below the correct energy due to non-linear response in this range.

The nanoscale structure of the ionomer film located in the catalyst layer of polymer exchange membrane fuel cells (PEMFCs) is of vital importance to proton transport and catalyst utilization. Classical molecular dynamic simulations are conducted to explore the molecular-level structure as well as the structure–property relationships in the ionomer film. Twenty-four systems are simulated to investigate the effect of (i) hydration, (ii) ionomer film thickness, (iii) oxidation of the carbon support surface, and (iv) the presence of catalyst nanoparticles on film adhesion and morphology. The ionomer does not form a continuous film on the carbon surface; rather, the ionomer forms irregular patches through which proton transport from the catalyst to the membrane must occur. These ionomer films are not able to retain water to the same extent as bulk ionomer membranes. However, thicker films retain proportionally more water than thinner films, allowing for a larger and better connected aqueous domain required for...

Carbon supported platinum (Pt/C) catalyst remains among the most preferable catalyst materials for Proton Exchange Membrane (PEM) fuel cells. However, platinum (Pt) particles suffer from poor durability and encounter electrochemical surface area (ESA) loss under operation with the accompany of Pt nanoparticle coarsening. Several proposed mechanisms have involved the Pt detachment from its carbonate support as an initial step for the deactivation of Pt nanoparticles. In this study, we investigated the detachment mechanism from the nano-adhesion point of view. Classic molecular dynamics simulations are performed on systems contain Pt nanoparticles of different sizes and shapes. A thin Nafion film (1 nm) at different hydration levels is also included in the system to study the environmental effect on nanoparticle adhesion. We found that the adhesion force strengthens as the Pt size goes up. Pt nanoparticles of tetrahedral shape exhibit relatively stronger connection with the carbon substrate due to its unique ‘anchor-like’ structure. Adhesion is enhanced with the introduction of a Nafion. The humidity level in the Nafion film has a rather complicated effect on the strength of nanoparticle adhesion. The binding energies and maximum adhesive forces are reported for all systems studied.

Summary Electron microscopy is an essential tool for the evaluation of microstructure and properties of the catalyst layer (CL) of proton exchange membrane fuel cells (PEMFCs). However, electron microscopy has one unavoidable drawback, which is radiation damage. Samples suffer temporary or permanent change of the surface or bulk structure under radiation damage, which can cause ambiguity in the characterization of the sample. To better understand the mechanism of radiation damage of CL samples and to be able to separate the morphological features intrinsic to the material from the consequences of electron radiation damage, a series of experiments based on high-angle annular dark-field–scanning transmission scanning microscope (HAADF-STEM), energy filtering transmission scanning microscope (EFTEM), and electron energy loss spectrum (EELS) are conducted. It is observed that for thin samples (0.3–1 times λ), increasing the incident beam energy can mitigate the radiation damage. Platinum nanoparticles in the CL sample facilitate the radiation damage. The radiation damage of the catalyst sample starts from the interface of Pt/C or defective thin edge and primarily occurs in the form of mass loss accompanied by atomic displacement and edge curl. These results provide important insights on the mechanism of CL radiation damage. Possible strategies of mitigating the radiation damage are provided. SCANNING 36:338–346, 2014. © 2013 Wiley Periodicals, Inc.

Development of devices and structures based on the layered 2D materials critically hinges on the capability to induce, control and tailor the electronic, transport and optoelectronic properties via defect engineering, much like doping strategies have enabled semiconductor electronics and forging enabled introduction the of iron age. Here, we demonstrate the use of a scanning helium ion microscope (HIM) for tailoring the functionality of single layer MoSe2 locally and decipher associated mechanisms at the atomic level. We demonstrate He+ beam bombardment that locally creates vacancies, shifts the Fermi energy landscape and increases the Young’s modulus of elasticity. Furthermore, we observe for the first time, an increase in the B-exciton photoluminescence signal from the nanoforged regions at the room temperature. The approach for precise defect engineering demonstrated here opens opportunities for creating functional 2D optoelectronic devices with a wide range of customizable properties that include operating in the visible region.

Because the ability to perform some form of chemical microanalysis has become an essential feature for any microscope, it is necessary to investigate what options are available in the new “ORION” helium ion microscope (HIM). The HIM has the ability to visualize local variations in specimen chemistry in both the ion induced secondary electron and the Rutherford backscattered imaging modes, but this provides only limited and qualitative information. Quantitative, elementally specific, microanalysis could be performed in the HIM using secondary electron spectroscopy, Rutherford backscattered ion spectroscopy, or secondary ion mass spectroscopy, but while each of these options has promise, none of them can presently guarantee either reliable element identification or quantitative analysis across the periodic table.

Interest in engineered nanostructures has risen in recent years due to their use in energy conservation strategies and biomedicine. To ensure prudent development and use of nanomaterials, the fate and effects of such engineered structures on the environment should be understood. Interactions of nanomaterials with environmental microorganisms are inevitable, but the general consequences of such interactions remain unclear, due to a lack of standard methods for assessing such interactions. Therefore, we have initiated a multianalytical approach to understand the interactions of synthesized nanoparticles with bacterial systems. These efforts are focused initially on cerium oxide nanoparticles and model bacteria in order to evaluate characterization procedures and the possible fate of such materials in the environment. The growth and viability of the Gram-negative species Escherichia coli and Shewanella oneidensis , a metal-reducing bacterium, and the Gram-positive species Bacillus subtilis were examined relative to cerium oxide particle size, growth media, pH, and dosage. A hydrothermal synthesis approach was used to prepare cerium oxide nanoparticles of defined sizes in order to eliminate complications originating from the use of organic solvents and surfactants. Bactericidal effects were determined from MIC and CFU measurements, disk diffusion tests, and live/dead assays. For E. coli and B. subtilis , clear strain- and size-dependent inhibition was observed, whereas S. oneidensis appeared to be unaffected by the particles. Transmission electron microscopy along with microarray-based transcriptional profiling was used to understand the response mechanism of the bacteria. Use of multiple analytical approaches adds confidence to toxicity assessments, while the use of different bacterial systems highlights the potential wide-ranging effects of nanomaterial interactions in the environment.

Adhesion of spores of Bacillus thuringiensis (Bt) and spherical silica particles on surfaces was experimentally and theoretically investigated in this study. Topography analysis via atomic force microscopy (AFM) and electron microscopy indicates that Bt spores are rod shaped, approximately 1.3 mum in length and approximately 0.8 mum in diameter. The adhesion force of Bt spores and silica particles on gold-coated glass was measured at various relative humidity (RH) levels by AFM. It was expected that the adhesion force would vary with RH because the individual force components contributing to the adhesion force depend on RH. The adhesion force between a particle and a planar surface in atmospheric environments was modeled as the contribution of three major force components: capillary, van der Waals, and electrostatic interaction forces. Adhesion force measurements for Bt spore (silica particle) and the gold surface system were comparable with calculations. Modeling results show that there is a critical RH value, which depends on the hydrophobicity of the materials involved, below which the water meniscus does not form and the contribution of the capillary force is zero. As RH increases, the van der Waals force decreases while the capillary force increases to a maximum value.

A combination of the 'semi-empirical' model for secondary electron production and the TRIM routines which describe ion stopping power, scattering, and transport, has been used to construct a Monte Carlo simulation (IONiSE) that can quantitatively interpret the generation of secondary electrons (SE) from materials by fast helium ions. This approach requires that the parameters of the semi-empirical model be determined by fitting to experimental yield data but has the merit that, unlike more fundamental models, it can be applied with equal ease to both pure elements and complex compounds. The application of the model to predict the topographic yield variation of helium generated SE as a function of energy and material, and to investigate the ratio between SE generated by incident and backscattered ions, is demonstrated.

Electron beam-induced deposition (EBID) provides a simple way to fabricate submicron- or nanometer-scale structures from various elements in a scanning electron microscope (SEM). The growth rate and shape of the deposits are influenced by many factors. We have studied the growth rate and morphology of EBID-deposited nanostructures as a function of the tungsten hexafluoride (WF6) and tetraethylorthosilicate (TEOS) precursor gas pressure and growth time, and we have used Monte Carlo simulations to model the growth of tungsten and silicon oxide to elucidate the mechanisms involved in the EBID growth. The lateral radius of the deposit decreases with increasing pressure because of the enhanced vertical growth rate which limits competing lateral broadening produced by secondary and forward-scattered electrons. The morphology difference between the conical SiO(x) and the cylindrical W nanopillars is related to the difference in interaction volume between the two materials. A key parameter is the residence time of the precursor gas molecules. This is an exponential function of the surface temperature; it changes during nanopillar growth and is a function of the nanopillar material and the beam conditions.

Empirical forms for electron/atom scattering cross sections predict backscattering factors that compare well with those calculated using tabulated Mott data from 0.1 to 30 keV. The form of the empirical total cross section is similar to the screened Rutherford cross section. The fit to the tabulated differential Mott cross sections is decomposed into two parts, one part being of the same mathematical form as the screened Rutherford cross section (σR), and the second part being an isotropic distribution (σI). The ratio of the total cross sections (σR/σI) between the screened Rutherford part of the differential scattering cross section and the isotropic part of the distribution is fitted to give the same ratio of forward to backscattered currents as the tabulated Mott differential cross sections. The three equations, one for the total elastic cross section and two describing the differential cross section—one for the Rutherford screening parameter and one for the ratio σR/σI—give backscattering results covering all the major trends with energy and atomic number compared with the backscattering coefficients calculated using tabulated Mott cross sections. However, agreement with experiment is poor for some well-researched examples such as Au. Monte Carlo calculations using the empirical cross sections show that surface effects may be critical in interpreting experimental results.

Monte Carlo modeling of electron-solid interactions requires a detailed and accurate supply of experimental data on which to base its physics and against which to test its predictions. To meet this need, a collection of data—comprising measurements of secondary and backscattered electron yields, electron stopping powers, and x-ray ionization cross sections, as a function of energy—has been assembled from published sources. The quality and quantity of the compilation varies widely, with little or no data being available for the majority of elements in the periodic table, while results for complex materials of current technologic interest are also almost nonexistent. To meet the needs of Monte Carlo simulation in areas such as dimensional metrology or microanalysis, a program of systematic measurements is required.

Charge accumulation in insulating or semiconducting samples due to electron beam irradiation is one of the key problems in electron microscopy. One of the most promising techniques for reducing the severity of such charging is to surround the sample with a low-pressure atmosphere of a gas. The charging behavior of a number of materials, surrounded by a variety of gases, has been determined to identify the important factors which control charging under these conditions. The magnitude of the surface potential was deduced from an analysis of x-ray spectra from the surface. The relationship between surface charge, gas pressure, and gas type are measured, and the charging reduction efficiency (CRE) is compared.

Experimental measurements are presented of the total scattering cross-section of four gases: He, Air, CH 4 , and Argon as a function of electron beam energy. The method used was that of Gauvin. A comparison of theoretical estimates of the total scattering cross-section with the experimental data indicates that the model used gives generally reliable results. The data confirm that the gases tend to be molecular rather than atomic in nature in this pressure range (1-300 Pascal). The factors affecting the accuracy of these results are examined.

A study of the temperature dependence of the electron beam induced deposition (EBID) of tungsten was carried out in a temperature range of −30–75°C under two conditions of beam energy and beam current using WF6 as the precursor gas. An Arrhenius relationship was found for the temperature dependence of the deposition rate which is consistent with an electron stimulated desorption mechanism for this process. In addition, the mechanism for EBID under different conditions was discussed.

Low voltage scanning electron microscopy (LVSEM) has become the imaging technique of choice for many applications because it offers surface specific information, and both control of charging and a reduction of damage to labile specimens. However fundamental limitations such as the electron wavelength, lens aberrations, and gun brightness, severely limit the performance of the LVSEM, and in addition some capabilities such as microanalysis are difficult or impossible. New technologies, including aberration correction and brighter electron sources are now available, and novel spectroscopic approaches are in development. Together these advances could significantly enhance the performance of the LVSEM. The advent of ultra-high performance ion column imaging systems offers interesting competition to the LVSEM for both imaging and analysis. [DOI: 10.1380/ejssnt.2006.369]

A new and thorough examination of secondary electron (SE) yield as a function of primary energy (EPE) and atomic number Z for the 44 elements in the database1 is made. The principles of the semiempirical universal law for the SE yield are described and a template for Monte Carlo (MC) simulation is produced accordingly. Both universal curve fitting and MC simulation are made for the 44 elements. The resulted maximum SE yield δm, corresponding primary energy , SE excitation energy e, and effective escape depth λ are tabulated and plotted as a function of atomic number Z. It is found that similarities exist in the profiles of e and λ, δm and , and all of these parameters seem to have characteristics associated with atomic shell filling. Copyright © 2005 John Wiley & Sons, Ltd.

The absolute efficiency of generation of a selection of K, L, and M- x-ray lines has been measured as a function of the incident electron beam energy. At an overvoltage U=2 this efficiency falls within the range 1E-4 to 1E-7, with K-lines being highest and L-Lines usually being the lowest. It is shown that for all three families of lines the efficiency has a functional variation which has the form A. (U- 1)n, as first suggested by Compton and Allison, where A and n are constants. Values of A and n for the K, L, and M shells are tabulated. The smoothly varying behavior of the efficiency makes it well suited for analytical use and spectrum simulation purposes.

A simple method is described to determine the effective gas path length when incident electrons scatter in the gas above the specimen. This method is based on the measurement of a characteristic x-ray line emitted from a region close to the incident beam. From various experimental measurements performed on various microscopes, it is shown that the effective gas path length may increase with the chamber pressure and that it is also often dependent of the type of x-ray bullet.

Except in the idealized case where the unknown and the standard are essentially identical obtaining accurate matrix corrections requires a knowledge of the absolute value and functional behavior of a number of quantities such the mass absorption coefficient, the electron stopping power, ionization cross sections and fluorescent yields. This talk examines the accuracy of the data available in each category, and the range of elements and compounds for which values have been published. It is found that the quality and quantity of data available varies greatly and that, with only a few exceptions, much more work is required to produce the reliable and inclusive data base required for accurate matrix corrections.

Specimens that charge under electron beam irradiation in the scanning electron microscope (SEM) can be stabilized by choosing the beam energy to be such a value that the sum of the secondary and backscatter electron yields is unity, as this establishes a dynamic charge balance. We show here that for pure elements, the energies El and E2, for which charge balance occurs, are related directly to the atomic number of the material. Although generally there is no comparable relation for compounds, we also show that for polymers, the E2 energy is related both to the ratio of the number of valence electrons to molecular weight and to the electro-negativity of the monomer units that form the polymer.

Secondary electron imaging is not possible in the variable pressure scanning electron microscope because the mean free path of the secondaries in the gas is too short to permit them to reach the detector. This paper therefore investigates an alternative strategy for producing an image containing significant amounts of secondary electron contrast. This involves modifying the microscope by the addition of a biased electrode above the sample and then collecting a specimen current signal. This system, originally described by Farley and Shah (1988), is found to produce true secondary electron detail over a wide range of conditions.

Concentration-dependent bimodal size distributions (comprised of single-molecule particles and multimolecule clusters) observed by microscopic examination of particles collected during electrospray (ES) of dilute solutions of high molecular weight polymers suggest that chain entanglement can interfere with the droplet subdivisions believed to be intrinsic to the electrospray process. The feasibility of such interference is discussed in the context of the spray model of Kebarle, along with its potential impact on the ES mass spectrometry of macromolecules.

We describe a novel, all dry approach that uses direct epitaxial growth for nanostructure fabrication. The two major requirements for achieving direct epitaxial growth are the ability to generate and to subsequently maintain and control spatial and chemical selectivity in the film growth process. The spatial selectivity is generated by pattering a surface adsorption layer on Si(100) using scanning electron beam lithography. This artificial lateral variation in surface reactivity is used as a template in subsequent epitaxy. Selective epitaxial growth on the resulting patterns is achieved by supersonic molecular jet epitaxy. Systematic investigation of the effects of various patterning and growth parameters on spatial and chemical selectivity at a sub- 100-nm feature scale using hydrogen terminated and nitrogen terminated growing Si(100) surfaces are presented.

Based on the one-band tight-binding model, we systematically investigate the interlayer exchange coupling and the angular dependence of the coupling energy in a magnetic sandwich covered on both sides by nonmagnetic films. Our results show that (i) the thickness of magnetic and outer nonmagnetic films influence significantly the oscillatory behavior of exchange coupling, (ii) the appearance of noncollinear exchange coupling is very sensitive to the thickness of magnetic and outer nonmagnetic layers, (iii) the nonoscillatory component of the coupling varies generally with the thickness of magnetic or outer nonmagnetic films, and (iv) the results in the case where the thickness of both magnetic or both outer nonmagnetic films vary simultaneously are significantly different from that in the case where the thickness of one of the two magnetic or outer nonmagnetic films is fixed while the other is varied. These results are qualitatively in agreement with the experimental measurements. (C) 1998 American Institute of Physics. [S0021-8979(98)02302-0].

As noted in the previous section, the present ALIS helium ion source is a descendant of the original work based on FIM technology (for a historical overview see Muller and Tsong 1993) although important research in this area has also been carried out by several other prominent groups (e.g. Orloff and Swanson 1977). In order to be suitable for application, in a high-performance particle beam microscope, the source should ideally not only be bright, but also be as compact as possible to ensure mechanical stability, provide highly stable emission over time periods of several hours, be capable of operating at energies at least in the 10–50 keV range, and be capable of being re-formed and then reused multiple times without a significant change in performance. An overview of history of the helium ion microscope can be found in the literature (Economou 2011), while other technical details can be found in the published patents listed at the end of the bibliography.

The scanning electron microscope (SEM) has become the most widely used high-performance microscope. However because of the fundamental limitations of electron beams the new technology of ion beam microscopy is being developed

It has to be expected that both ions and electrons will damage specimens under examination to a greater or lesser degree. Electrons are low in mass but can travel at velocities which are a significant fraction of the speed of light. In general, electrons do not significantly damage metallic or inorganic specimens, but even relatively low doses of electrons can be expected to chemically alter or destroy organic materials such as polymers and biological samples.

Dynamic charging is the condition in which the magnitude of the charge on the surface of a non-conductor, such as a polymer, under electron irradiation appears to change with the incident current density. Measurements confirm that this effect does occur and show that the magnitude of the effect depends on the relationship between the incident beam energy and the energy (E2) at which the sample has a total electron yield of unity. The effect arises from changes in the leakage resistance of the sample, and two possible explanations for this behavior are suggested.

Electron-beam-stimulated deposition and etching has been investigated as a clean, alternative method for nanoscale selective processing. Depositions using W(CO)6 and hydrocarbon sources have yielded efficient and selective electron-beam deposits. Primarily fluorine-based precursors have been used to etch a variety of materials. Initial results regarding the selective etching of silicon and silicon dioxide suggest that inelastic scattering of the primary electron beam with the gas occurs and is more severe at lower beam energies. The etch rate increases linearly with decreasing electron-beam energy, however, it is not clear if this is due to enhanced primary- or secondary-electron-stimulated processes. Feature sizes as small as 55 nm have been selectively processed.

A model is presented regarding the mechanistic properties associated with the interaction of hydrogen with nanoporous palladium (np-Pd) films prepared using a spontaneous galvanic displacement reaction (SGDR), which involves PdCl(2) reduction by atomic Ag. Characterization of these films shows both chemical and morphological factors, which influence the performance characteristics of np-Pd microcantilever (MC) nanomechanical sensing devices. Raman spectroscopy, uniquely complemented with MC response profiles, is used to explore the chemical influence of palladium oxide (PdO). These combined techniques support a reaction mechanism that provides for rapid response to H(2) and recovery in the presence of O(2). Post-SGDR processing via reduction of PdCl(2)(s) in a H(2) environment results in a segregated nanoparticle three-dimensional matrix dispersed in a silver layer. The porous nature of the reduced material is shown by high resolution scanning electron microscopy. Extended grain boundaries, typical of these materials, result in a greater surface area conducive to fast sorption/desorption of hydrogen, encouraged by the presence of PdO. X-ray diffraction and inductively coupled plasma-optical emission spectroscopy are employed to study changes in morphology and chemistry occurring in these nanoporous films under different processing conditions. The unique nature of chemical/morphological effects, as demonstrated by the above characterization methods, provides evidence in support of observed nanomechanical response/recovery profiles offering insight for catalysis, H(2) storage and improved sensing applications.

Empirical forms have been found for the total and differential elastic scattering cross sections for electron/atom scattering. The cross sections are valid over the range 0.1–30 keV and across the periodic table. The empirical forms of the cross sections are derived from trends in tabulated Mott scattering cross sections. The form of the total cross section is similar to a previously published cross section and is based on the screened Rutherford cross section. The fit to the differential Mott cross sections is decomposed into two parts, one part being of the same mathematical form as the screened Rutherford cross section σR, and the second part being an isotropic distribution σI. These two mathematical forms were chosen because they give a straightforward generation of random scattering angles. The screened Rutherford part of the differential scattering cross section is first fitted to the half‐angle of the Mott cross sections. This fit of the differential screened Rutherford is in turn reduced to a fit ...

Ion induced secondary electrons (iSE) can produce high-resolution images ranging from a few eV to 100keV over a wide range of materials. The interpretation of such images requires knowledge of the secondary electron yields (iSE δ) for each of the elements and materials present and as a function of the incident beam energy. Experimental data for helium ions are currently limited to 40 elements and six compounds while other ions are not well represented. To overcome this limitation, we propose a simple procedure based on the comprehensive work of Berger et al. Here we show that between the energy range of 10-100keV the Berger et al. data for elements and compounds can be accurately represented by a single universal curve. The agreement between the limited experimental data that is available and the predictive model is good, and has been found to provide reliable yield data for a wide range of elements and compounds.

Single-wall carbon nanotubes (SWNT) were grown to micron lengths by laser-annealing nanoparticulate soot containing short (∼50 nm long) nanotube “seeds.” The “seeded” nanoparticulate soot was produced by restricting the time spent by an ablation plume inside an 800 °C oven following laser vaporization of a C–Ni–Co target. The soot collected from the laser vaporization apparatus was placed inside graphite crucibles under argon, and heated by a CO2 laser. In situ pyrometry was used to estimate the sample temperature. Length distributions of SWNT bundles in the unannealed and annealed samples were measured by transmission electron microscopy and field emission scanning electron microscopy. Annealing treatments exceeding 1600 °C produced no increase in nanotube length, while lower temperatures in the 1000–1300 °C range were optimal for growth. These experiments indicate that SWNT grow by the conversion of condensed phase nanomaterial during annealing, a similar mechanism to that proposed for growth during normal laser–vaporization production.

Microorganisms have long been known to develop resistance to metal ions either by sequestering metals inside the cell or by effluxing them into the extracellular media. Here we report the biosynthesis of extracellular silver-based single nanocrystallites of well-defined composition and homogeneous morphology utilizing the gamma-proteobacterium, Shewanella oneidensis MR-1, upon incubation with aqueous silver nitrate solution. Further characterization of these particles revealed that the crystals consist of small, reasonably monodispersed spheres in the 2-11 nm size range (average of 4 +/- 1.5 nm). The bactericidal effect of these nanoparticles (biogenic-Ag) is compared to chemically synthesized silver nanoparticles (colloidal-Ag and oleate capped silver nanoparticles, oleate-Ag) and assessed using Gram-negative (E. coli and S. oneidensis) and Gram-positive (B. subtilis) bacteria. Relative toxicity was based on the diameter of inhibition zone in disk diffusion tests, minimum inhibitory concentrations, live/dead assays, and atomic force microscopy. From a toxicity perspective, strain-dependent inhibition depended on the synthesis procedure and the surface coat. Biogenic-Ag was found to be of higher toxicity compared to colloidal-Ag for all three strains tested, whereas E. coli and S. oneidensis were found to be more resistant to either of these nanoparticles than B. subtilis. In contrast, oleate-Ag was not toxic to any of the bacteria. These findings have implications for the potential uses of Ag nanomaterials and for their fate in biological and environmental systems.

Recent advances in instrumentation have produced the first commercial examples of what can justifiably be called high-resolution scanning electron microscopes. The key components of such instruments are a cold field emission gun, a small-gap immersion probe-forming lens, and a clean dry-pumped vacuum. The performance of these microscopes is characterized by several major features including a spatial resolution, in secondary electron mode on solid specimens, which can exceed 1 nm on a routine basis; an incident probe current density of the order of 10 6 A/cm 2 ; and the ability to maintain these levels of performance over an accelerating voltage range of from 1 to 30 keV. This combination of high resolution, high probe current, low contamination and flexible electron-optical conditions provides many new opportunities for the application of the SEM to materials science, physics, and the life sciences.

SUMMARY Current scanning electron microscopes, equipped with field emission guns and high-performance immersion lenses, can achieve spatial resolutions of the order of 1 nm in both secondary and backscattered imaging modes over a wide range of operating energies. The generation and interpretation of images with nanometre-scale resolution relies on a detailed knowledge, and application, of electron-solid interactions. This paper develops the practical steps required to produce a high-resolution image, and discusses the principles which govern image interpretation. Attention is focused primarily on materials which are low in atomic number and density, such as biological tissue, but the results apply after appropriate scaling of the physical parameters to most other materials.

The diffusion matrix model proposed by Desai and Reimer (1990) has been investigated. After some modifications it appeared to be a powerful tool for rapid determination of the SE and BSE signals from two-dimensional surface topographies. Results for low atomic number specimens show good agreement between experimental and theoretical data.

Electron-beam-induced deposition (EBID) and etching (EBIE) provides a simple way to fabricate or etch submicron or nanoscale structures of various materials in a direct-write (i.e.nonlithographic) fashion. The growth rate or the etch rate are influenced by many factors such as beam energy, beam current, temperature of the substrate material, pressure of the chamber, and geometry of the gas injector etc. The mechanism of EBID and EBIE involves the interaction of the incident electron beam or emitted electron from the target material. The role of these electrons is still not completely understood although the contribution of low energy secondary electrons (SE) has been assumed to be the dominant contributor of EBID and EBIE based on its overlap with the dissociation cross section. We have studied the growth and etching phenomenon under various biasing conditions to investigate how low voltage biasing of the substrate affects secondary electron trajectories and subsequently modifies electron-beam-induced deposition and etching.SCANNING 29: 000–000, 2007. © 2007 Wiley Periodicals, Inc.

To examine the practical limits and effects of low voltage operation, studies of electron beam lithography (EBL) in the low (few keV) to ultra-low (E < 500eV) energy range, employing commonly used resists such as PMMA was done, and the results were compared to those from conventional high voltage processing. The direct writing was performed at low energies by our homemade scan generator and a Schottky field emission gun scanning electron microscope (SEM), used in cathode-lens mode for ultra-low voltage operation. The exposure characteristics and sensitivity of the system at these energies have been investigated using an advanced Monte Carlo simulation method. Our modeling of the lithographic process showed a significant increase in resolution and process latitude for thinner resists.

Summary The electron backscattering coefficient of liquid water has been determined for electrons in the energy range 15–30 keV using QuantomixTM capsules. Values of the mean atomic number for water estimated from a fit to the backscatter yield, the mean ionization potential of water and from Monte Carlo simulations, show that the scattering behaviour of water is not anomalous despite the effects of hydrogen bonding. Computations of the electron range, and of the mean depth for backscattering, in water as a function of incident beam energy show that water and vitreous ice are good media for imaging purposes.

In the variable pressure scanning electron microscope (VP-SEM) the incident electrons pass through a gaseous environment and the beam is scattered by these interactions. We show here that the experimental intensity profile of the scattered beam can be described as Gaussian in form to a high level of accuracy. This provides a rapid means of accounting for the effects of beam scatter in imaging and microanalysis because the standard deviation of the Gaussian is a simple function of parameters such as working distance, beam energy, gas type and pressure.