Progress in ultrahigh energy resolution EELS.

Electron energy loss spectroscopy (EELS) in the electron microscope has progressed remarkably in the last five years. Advances in monochromator and spectrometer design have improved the energy resolution attainable in a scanning transmission electron microscope (STEM) to 4.2 meV, and new applications of ultrahigh energy resolution EELS have not lagged behind. They include vibrational spectroscopy in the electron microscope, a field that did not exist 5 years ago but has now grown very substantially. Notable examples include vibrational mapping with about 1 nm spatial resolution, analyzing the momentum dependence of vibrational states in very small volumes, determining the local temperature of the sample from the ratio of energy gains to energy losses, detecting hydrogen and analyzing its bonding, probing radiation-sensitive materials with minimized damage by aloof spectroscopy and leap-frog scanning, and identifying biological molecules with different isotopic substitutions. We review the instrumentation advances, provide a summary of key applications, and chart likely future directions.

Magnetic films on nanoperforated templates: a route towards percolated perpendicular media.

We present a study on the magnetization reversal in Co/Pt multilayer films with an out-of-plane easy axis of magnetization deposited onto substrates with densely distributed perforations with an average period as small as 34 nm. Deposition of magnetic Co/Pt multilayers onto the nanoperforated surface results in an array of magnetic nanodots surrounded by a continuous magnetic film. Following the evolution of the magnetic domain pattern in the system, we suggest that domain walls are pinned on structural inhomogeneities given by the underlying nanoperforated template. Furthermore, a series of micromagnetic simulations was performed in order to understand the modification of the pinning strength of domain walls due to the magnetic interaction between nanodots and the surrounding film. The results of the simulations show that magnetic exchange coupling between the nanodots and the surrounding film strongly influences the pinning behavior of the magnetic domain walls which can be optimized to provide maximal pinning.

Vacancy clusters, dislocations and brown colouration in diamond.

Following on from the idea that clusters of vacancies are the origin of the featureless absorption and brown colouration in natural diamond, dislocations are shown to exhibit sub-bandgap absorption also. The vacancy cluster idea has arisen from theoretical predictions of π-bonded chains reconstructing the cluster surfaces and has been confirmed by energy loss studies. In contrast, bandgap states at dislocations are observed in brown and colourless diamonds alike, giving rise to weak absorption, which resembles that theoretically predicted from shuffle dislocation segments. This, however, would not account for the degrees of brownness in the diamonds, but it suggests that if such shuffle segments exist, vacancies must have been present and moved to dislocations to create these configurations in the first place. The question arises, what happens to the vast number of vacancy clusters upon high pressure high temperature (HPHT) annealing, which renders the diamonds colourless. Our observations on natural brown diamonds after HPHT treatment suggest that vacancy clusters, trapped in the strain fields of dislocations, grow in size accompanied by a decrease in their numbers; this leads to much reduced optical absorption.

Contrast reversal in atomic-resolution chemical mapping.

We report an unexpected result obtained using chemical mapping on the new, aberration corrected Nion UltraSTEM at Daresbury. Using different energy windows above the L2,3 edge in 011 silicon to map the position of the atomic columns we find a contrast reversal which produces an apparent and misleading translation of the silicon columns. Using simulations of the imaging process, we explain the intricate physical mechanisms leading to this effect.

Three-dimensional atomic-scale structure of size-selected gold nanoclusters.

An unambiguous determination of the three-dimensional structure of nanoparticles is challenging. Electron tomography requires a series of images taken for many different specimen orientations. This approach is ideal for stable and stationary structures. But ultrasmall nanoparticles are intrinsically structurally unstable and may interact with the incident electron beam, constraining the electron beam density that can be used and the duration of the observation. Here we use aberration-corrected scanning transmission electron microscopy, coupled with simple imaging simulation, to determine with atomic resolution the size, three-dimensional shape, orientation and atomic arrangement of size-selected gold nanoclusters that are preformed in the gas phase and soft-landed on an amorphous carbon substrate. The structures of gold nanoclusters containing 3096 atoms can be identified with either Ino-decahedral, cuboctahedral or icosahedral geometries. Comparison with theoretical modelling of the system suggests that the structures are consistent with energetic considerations. The discovery that nanoscale gold particles function as active and selective catalysts for a variety of important chemical reactions has provoked much research interest in recent years. We believe that the detailed structure information we provide will help to unravel the role of these nanoclusters in size- and structure-specific catalytic reactions. We note that the technique will be of use in investigations of other supported ultrasmall metal cluster systems.

Microscopy in solid state science.

The Microstructural Physics group at the Cavendish Laboratory is actively involved in a considerable number of research projects which cover a broad range of materials science. In this paper, we describe briefly several such projects, with particular emphasis given to the application of parallel-detection electron energy loss spectroscopy (PEELS) on a scanning transmission electron microscope (STEM) to the analysis of materials such as stainless steels, catalysts, and high temperature superconductors. In addition, we describe a number of related projects that are currently being carried out in the group, particularly those which utilise and develop novel STEM imaging and analytical techniques.