V. J. Keast

Mapping surface plasmons at the nanometre scale with an electron beam

The optical response from metal nanoparticles and nanostructures is dominated by surface plasmon generation and is critically dependent on the local structure and geometry. Electron energy-loss spectroscopy (EELS), combined with recent developments in spectrum imaging and data processing, has been used to observe the energy and distribution of surface plasmons excited by fast electrons. The energy of the plasmon responses is consistent with the optical response and with calculations. For gold and silver rods and ellipsoids, longitudinal, transverse and distinct cluster modes were readily identified and mapped. The spatial resolution of the presented maps is one order of magnitude better than that achievable with scanning near-field optical microscopy (SNOM)-based techniques.

Electron energy‐loss near‐edge structure – a tool for the investigation of electronic structure on the nanometre scale

Electron energy‐loss near‐edge structure (ELNES) is a technique that can be used to measure the electronic structure (i.e. bonding) in materials with subnanometre spatial resolution. This review covers the theoretical principles behind the technique, the experimental procedures necessary to acquire good ELNES spectra, including potential artefacts, and gives examples relevant to materials science.

Quantitative compositional mapping of Bi segregation to grain boundaries in Cu

The segregation of impurities and subsequent embrittlement of grain boundaries in metallic alloys is an important and extensively studied phenomenon. X-ray compositional mapping in the analytical electron microscope (AEM) can identify, quantify and determine the distribution of the segregating elements. This approach offers the advantage over surface sensitive techniques that the sample does not have to be fractured, which permits a more complete description of the distribution of the segregant to be obtained. Optimization of a dedicated 300 kV, field-emission gun, ultra-high vacuum scanning transmission electron microscope allows the acquisition of compositional maps at high spatial resolution and high sensitivity. Bismuth segregation to grain boundaries in Cu has been mapped with a spatial resolution better than 2 nm and a sensitivity better than one tenth of a monolayer. Some of the advantages of mapping over traditional fixed probe or profile approaches have been demonstrated and a survey of segregation levels in a number of boundaries has illustrated the large degree of anisotropy in segregation levels beyond that revealed by surface techniques.

Optimizing EELS acquisition.

A method for spectral acquisition, called binned gain averaging, will be described and tested. Systematic or correlated noise is efficiently suppressed with this method by averaging the gain over a series of CCD pixels. As a result, improved signal-to-noise ratios are obtained that allow the detection of very weak signals. At the same time, the spectral energy resolution is not degraded--even for long acquisition periods. It will be demonstrated that with this method, it is possible to significantly enhance the acquisition speed and quality of electron energy-loss (EEL) spectra and EELS maps. Examples will be given of double ionic scattering (i.e. the detection of the second boron K-edge) and the mapping of gold surface plasmons in the near-infrared and visible energy range.

Quantification of boundary segregation in the analytical electron microscope

When studying equilibrium grain‐boundary segregation using the small (~1 nm) electron probe of the scanning transmission electron microscope and X‐ray energy dispersive spectroscopy, the assumptions made about the size and shape of the beam–specimen interaction volume may introduce errors in quantification of up to a factor of five. Comparisons between experimental segregation profiles and different theoretical models have shown that a Gaussian model for the electron distribution will provide the best description for the interaction volume. Calculations of minimum detectable segregation levels have shown that optimum sensitivity is not achieved in the thinnest samples or even with the smallest probe sizes. In addition, operating at 300 keV (rather than 100 keV) will halve the minimum detectable segregation level, assuming all other experimental conditions are equal. Rastering the electron probe over a fixed area while acquiring spectra improves the accuracy of quantification but at the price of reducing sensitivity by at least a factor of three.

Chemistry and bonding changes associated with the segregation of Bi to grain boundaries in Cu

Grain-boundary embrittlement, caused by the segregation of impurity and alloying elements, occurs in many systems and has been the focus of a large amount of research owing to its technological importance. However, the exact mechanism by which the segregating elements cause embrittlement remains unclear. In this paper the localized changes in the electronic structure in the classical embrittling system of Bi in Cu have been studied. Experimental results were obtained by examining the fine structure in the electron energy loss spectrum which was then compared to calculations using the layer Korringa-Kohn-Rostoker (LKKR) method. A change in the d density of states has been observed for the Cu atoms at the grain boundary, associated with Bi, and an electronic model to explain embrittlement is described.

High-resolution imaging of nanoparticle bimetallic catalysts supported on mesoporous silica

Although conventional high‐resolution transmission electron microscopy is a powerful method for the elucidation of the structure of mesoporous solids (diameter of pores from 1.5 to 20 nm), it is far less capable than high‐resolution scanning transmission electron microscopy in identifying the spatial distribution of nanocrystals of catalysts encapsulated within the mesopores. Using high‐angle annular dark‐field imaging (either in a 100 or 300 keV STEM system), it is possible to locate precisely individual bimetallic nanoparticles (Ag3Ru10, Cu4Ru12 and Pd6Ru6 hydrogenation catalysts) supported on mesoporous silica, to determine their size distribution, and to record their characteristic X‐ray emission maps. It is also established that there is little tendency for elemental fragmentation of the bimetallic catalysts, all of which were prepared by decarbonylating, by thermolysis, precursor cluster carbonylate anions: [Ag3Ru10C2(CO)28Cl]−, [Ru6C(CO)16Cu2Cl]2− and [Ru6Pd6(CO)24]2−.

The structure of boron-, phosphorus- and nitrogen-doped tetrahedral amorphous carbon deposited by cathodic arc

Some properties of amorphous carbon films prepared by filtered cathodic arc deposition were measured using electron diffraction and electron energy loss spectroscopy. Pure carbon films prepared in this way have more than 85% sp3 carbon. The effects on structure produced by doping the films with boron, phosphorus and nitrogen are determined. It was found that levels of these substances of up to 1% do not change the tetrahedral structure of the carbon network. At 15% nitrogen, the structure becomes a carbon nitrogen alloy consisting of a sp2-bonded network in which nitrogen substitutes for carbon. No evidence is found for a β-Si3N4-type structure for the carbon-nitrogen alloys.

Light splitting in nanoporous gold and silver.

Nanoporous gold and silver exhibit strong, omnidirectional broad-band absorption in the far-field. Even though they consist entirely of gold or silver atoms, these materials appear black and dull, in great contrast with the familiar luster of continuous gold and silver. The nature of these anomalous optical characteristics is revealed here by combining nanoscale electron energy loss spectroscopy with discrete dipole and boundary element simulations. It is established that the strong broad-band absorption finds its origin in nanoscale splitting of light, with great local variations in the absorbed color. This nanoscale polychromaticity results from the excitation of localized surface plasmon resonances, which are imaged and analyzed here with deep sub-wavelength, nanometer spatial resolution. We demonstrate that, with this insight, it is possible to customize the absorbance and reflectance wavelength bands of thin nanoporous films by only tuning their morphology.

Quantitative TEM-based phase retrieval of MgO nano-cubes using the transport of intensity equation.

Through focus series of images are collected from MgO nano-cube crystals in the transmission electron microscope (TEM). The experimental data is used to solve the transport of intensity equation (TIE) to retrieve phase maps, which portray the morphology of the cubes and are quantified by the mean inner potential V(0). Particular attention is given to the practical difficulties associated with TIE phase retrieval of non-conducting polyhedron particles.