The 1s-State Analysis Applied to High-Angle, Annular Dark-Field Image Interpretation - When Can We Use It?

A small probe centered on an atomic column excites the bound and unbound states of the two-dimensional projected potential of the column. It has been argued that, even when several states are excited, only the 1s state is sufficiently localized to contribute a signal to the high-angle detector. This article shows that non-1s states do make a significant contribution for certain incident probe profiles. The contribution of the 1s state to the thermal diffuse scattering is calculated directly. Sub-Ångstrom probes formed by Cs-corrected lenses excite predominantly the 1s state and contributions from other states are not very large. For probes of lower resolution when non-1s states are important, the integrated electron intensity at the column provides a better estimate of image intensity

Light splitting in nanoporous gold and silver

Figure Persented: 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. © 2011 American Chemical Society

Optical response of nanostructured metal/dielectric composites and multilayers

The homogeneous optical response in conducting nanostructured layers, and in insulating layers containing dense arrays of self assembled conducting nanoparticles separated by organic linkers, is examined experimentally through their effective complex indices (n*, k*). Classical effective medium models, modified to account for the 3-phase nanostructure, are shown to explain (n*, k*) in dense particulate systems but not inhomogeneous layers with macroscopic conductance for which a different approach to homogenisation is discussed, (n*, k*) data on thin granular metal films, thin mesoporous gold, and on thin metal layers containing ordered arrays of voids, is linked to properties of the surface plasmon states which span the nanostructured film. Coupling between evanescent waves at either surface counterbalanced by electron scattering losses must be considered. Virtual bound states for resonant photons result, with the associated transit delay leading to a large rise in n* in many nanostructures. Overcoating n-Ag with alumina is shown to alter (n*, k*) through its impact on the SP coupling. In contrast to classical optical homogenisation, effective indices depend on film thickness. Supporting high resolution SEM images are presented

Default perception of high-speed motion

When human observers are exposed to even slight motion signals followed by brief visual transients—stimuli containing no detectable coherent motion signals—they perceive large and salient illusory jumps. This novel effect, which we call “high phi”, challenges well-entrenched assumptions about the perception of motion, namely the minimal-motion principle and the breakdown of coherent motion perception with steps above an upper limit. Our experiments with transients such as texture randomization or contrast reversal show that the magnitude of the jump depends on spatial frequency and transient duration, but not on the speed of the inducing motion signals, and the direction of the jump depends on the duration of the inducer. Jump magnitude is robust across jump directions and different types of transient. In addition, when a texture is actually displaced by a large step beyond dmax, a breakdown of coherent motion perception is expected, but in the presence of an inducer observers again perceive coherent displacements at or just above dmax. In sum, across a large variety of stimuli, we find that when incoherent motion noise is preceded by a small bias, instead of perceiving little or no motion, as suggested by the minimal-motion principle, observers perceive jumps whose amplitude closely follows their own dmax limits

Calculation of phase changes of electron waves near excited nanoparticles

When a metallic nanoparticle is illuminated by electromagnetic radiation of the appropriate frequency, strong electric and magnetic fields are generated close to the surface of the particle by the valence electrons of the metal. The energy in these surface excitations can be transferred to external electrons passing near the particle (energy-gain spectral spectroscopy) and they change the phase of the zero-loss component of the electron wave. This paper provides an estimate of how the phase of the wave at the image plane of a 100-kV transmission electron microscope varies with position in the image plane. For a 16-nm radius gold sphere illuminated by the light of wavelength 500nm and irradiance 10MW/cm2, the phase in the image plane changes by 0.002rad/nm along a radial line outside the sphere. Changes of similar magnitude have been measured in previous studies. © 2009 Elsevier B.V

Imaging point defects using a transmission electron microscope with controllable spherical aberration

Computer simulations are utilized to show how to use a transmission electron microscope which has an objective lens with an adjustable coefficient of spherical aberration to determine the three spatial coordinates of a single heavy atom embedded in a crystal. This information can be obtained by forming an image with only those electrons that have been scattered through a large angle by the crystal. By using a high-angle annular dark-field aperture the atoms can be considered as independent scatterers, in contrast with imaging with low-angle coherent scattering. In addition, by reducing the aberration coefficients of the lens, the effective outer radius of the aperture can be made large, thereby leading to a small depth of focus. Calculations show that this form of imaging produces detectable contrast with currently available aberration correctors. sources and detectors. © 2001 Taylor & Francis Group, LLC

Limitations on the s-state approach to the interpretation of sub-angstrom resolution electron microscope images and microanalysis

The s-state approach is useful for analysing transmission electron microscope images of a thin crystalline foil consisting of well-separated atomic columns. It assumes that the signal collected (e.g., the annular dark-field image, the EELS spectrum) can be attributed to only the lowest-energy bound eigenstate of the two-dimensional projected potential of a single column. When, however, columns are close, the form of the bound states depends on more than one column, which implies that interpretation of the signal may not be so simple. For closely spaced columns we show that the simple s-state approach fails for the case of a sub-Angstrom probe initially centred on one column of a pair, because two bound states are excited. The energy in the probe is almost completely transferred to the neighbouring column after it has propagated some tens of nanometres through the foil and then is transferred back. Signals which relate directly to the local probe intensity (e.g. annular dark-field formed by thermal diffuse scattering, EELS) must be analysed in terms of the two bound states. Accurate calculations of bound states of pairs of columns are more demanding than for a single column but sufficient accuracy can be achieved from knowledge of the 1s-states of isolated columns. We provide formulae for the bound states of a column pair. These can be used to determine if image analysis requires the extension to the s-state approach described in this paper. © 2002 Elsevier Science B.V. All rights reserved

Analysis of Power Spectra of High-Resolution Electron-Micrographs

Power spectra of a series of high resolution electron microscope images of a thin foil of crystalline silicon are compared with theoretical predictions based on the standard theory for scattering and imaging of electrons. The differences between theoretical and experimental power spectra appear to be due to inexact knowledge of experimental parameters rather than limitations of the theory. Examination of a number of theoretical power spectra leads to an estimate of the optical parameters of the microscope although not with the same precision that can be achieved using an amorphous region of the specimen