Search

The performance of a direct detection camera (DDC) is evaluated in the context of off-axis electron holographic experiments in a transmission electron microscope. Its performance is also compared directly with that of a conventional charge-coupled device (CCD) camera. The DDC evaluated here can be operated either by the detection of individual electron events (counting mode) or by the effective integration of many such events during a given exposure time (linear mode). It is demonstrated that the improved modulation transfer functions and detective quantum efficiencies of both modes of the DDC give rise to significant benefits over the conventional CCD cameras, specifically, a significant improvement in the visibility of the holographic fringes and a reduction of the statistical error in the phase of the reconstructed electron wave function. The DDC's linear mode, which can handle higher dose rates, allows optimisation of the dose rate to achieve the best phase resolution for a wide variety of experimental conditions. For suitable conditions, the counting mode can potentially utilise a significantly lower dose to achieve a phase resolution that is comparable to that achieved using the linear mode. The use of multiple holograms and correlation techniques to increase the total dose in counting mode is also demonstrated.

We describe a new approach for preparing organic-inorganic perovskite solar cells for electron beam-induced current (EBIC) measurements in plan-view geometry. This method substantially reduces sample preparation artefacts, provides good electrical contact and keeps the preparation steps as close as possible to those for real devices. Our EBIC images were acquired simultaneously with annular dark-field scanning transmission electron microscopy images using a home-made highly sensitive EBIC amplifier. High-angle annular dark-field images and energy dispersive X-ray maps were recorded from the same area immediately afterwards. This allowed the EBIC contrast to be correlated with regions containing N and a deficiency of O. The EBIC contrast was also found to be similar to secondary electron contrast recorded with a scanning electron microscope. By identifying the generation and absorption electron processes, we determine that EBIC cannot be separated from the secondary electron and absorbed currents. This means that careful analysis needs to be performed before conclusions can be made on the origin of the current measured across p-n or p-i-n junctions. Nanyang Technological University We are grateful to Thomas Duden for the fabrication and operation of the amplifier during the EBIC experiments. M.D. acknowledges fi- nancial support from Nanyang Technological University start-up grant M4081924

The technique of double exposure electron holography, which is based on the superposition of two off-axis electron holograms, was originally introduced before the availability of digital image processing to allow differences between electron-optical phases encoded in two electron holograms to be visualised directly without the need for holographic reconstruction. Here, we review the original method and show how it can now be extended to permit quantitative studies of phase shifts that oscillate in time. We begin with a description of the theory of off-axis electron hologram formation for a time-dependent electron wave that results from the excitation of a specimen using an external stimulus with a square, sinusoidal, triangular or other temporal dependence. We refer to the more general method as continuous exposure electron holography, present preliminary experimental measurements and discuss how the technique can be used to image electrostatic potentials and magnetic fields during high frequency switching experiments.

Electron holography is a powerful technique for characterizing electrostatic potentials, charge distributions, electric and magnetic fields, strain distributions and semiconductor dopant distributions with sub-nm spatial resolution. Mapping internal electrostatic and magnetic fields within nanoparticles and other low-dimensional materials by TEM requires both high spatial resolution and high phase sensitivity. Carrying out such an analysis fully quantitatively is even more challenging, since artefacts such as dynamical electron scattering may strongly affect the measurement. In-line electron holography, one of the variants of electron holography, features high phase sensitivity at high spatial frequencies, but suffers from inefficient phase recovery at low spatial frequencies. Off-axis electron holography, in contrast, can recover low spatial frequency phase information much more reliably, but is less effective in retrieving phase information at high spatial frequencies when compared to in-line holography. We investigate gold nanoparticles using hybrid electron holography at both atomic-resolution and intermediate magnification. Hybrid electron holography is a novel technique that synergistically combines off-axis and in-line electron holography, allowing the measurement of the complex wave function describing the scattered electrons with excellent signal-to-noise properties at both high and low spatial frequencies. The effect of dynamical electron scattering is minimized by beam tilt averaging.

Quantitative materials characterization using electron holography frequently requires knowledge of the mean inner potential, but reported experimental mean inner potential measurements can vary widely. Using density functional theory, we have simulated the mean inner potential for materials with a range of different surface conditions and geometries. We use both “thin-film” and “nanowire” specimen geometries. We consider clean bulk-terminated surfaces with different facets and surface reconstructions using atom positions from both structural optimization and experimental data and we also consider surfaces both with and without adsorbates. We find that the mean inner potential is surface-dependent, with the strongest dependency on surface adsorbates. We discuss the outlook and perspective for future mean inner potential measurements.

Off-axis electron holography provides a direct means of retrieving the phase of the wavefield in a transmission electron microscope, enabling measurement of electric and magnetic fields at length scales from microns to nanometers. To maximise the accuracy of the technique, it is important to acquire holograms using experimental conditions that optimise the phase resolution for a given spatial resolution. These conditions are determined by a number of competing parameters, especially the spatial coherence and the instrument instabilities. Here, we describe a simple, yet accurate, model for predicting the dose rate and exposure time that give the best phase resolution in a single hologram. Experimental studies were undertaken to verify the model of spatial coherence and instrument instabilities that are required for the optimisation. The model is applicable to electron holography in both standard mode and Lorentz mode, and it is relatively simple to apply.

It is demonstrated how an electron-optical arrangement consisting of two electron biprisms can be used to generate three-wave vortex lattices with effective lattice spacings between 0.1 and 1 nm. The presence of vortices in these lattices was verified by using a third biprism to perform direct phase measurements via off-axis electron holography. The use of three-wave lattices for nanoscale electromagnetic field measurements via vortex interferometry is discussed, including the accuracy of vortex position measurements and the interpretation of three-wave vortex lattices in the presence of partial spatial coherence.

Quantitative comparisons of experimentally obtained and simulated high-resolution electron microscope images have shown that the contrast in experimental images is usually much less than is predicted by simulations. The aim here is to investigate this loss of contrast as a function of image spatial frequency using high-resolution images of amorphous carbon. It seems that experimental images of amorphous carbon have an unexpectedly high contrast for the low spatial frequencies but that the loss of contrast is constant for frequencies above 0.5 nm −1 .

The methods used to extract the momentum dependence of the energy loss near edge structure are compared. The improved counting statistics of the 45° method relative to the axial method for equivalent momentum resolution are noted. The experimental conditions required for the successful application of the method are also investigated and, examining Cu L edge data for YBa 2 Cu 3 O 7−δ as obtained in a number of different manners, it is concluded that it is essential to modify the exact orientation for the 45° method so as to minimise the effects of elastic scattering.

The lattice image for the centre of a carbon nanotube is sensitive to variations in the parameters of the tube such as the helical angle, the cross-sectional shape and the crystal tilt. We have obtained HREM images of tubes which show (002) fringes at the edges of the tube and {100} fringes in the centre. From these images we have deduced much useful information about the structure of the tubes. Simulations of the {100} fringes for a typical carbon nanotube demonstrate the sensitivity of the image to differences in helical pitch, large angles of crystal tilt and the presence of amorphous carbon in the centre of the tube. Small crystal tilts and random translational disorder between the shells are shown to have little effect on the image. The expected variations in the images as a function of cross-sectional shape are discussed.

In order to investigate the reproducibility of measurements of transmission electron microscope detector modulation transfer functions (MTFs) we measure the MTF of a charge-coupled device (CCD) camera using five different methods. MTFs derived from a sharp edge, a circular aperture and electron holographic interference fringes are found to agree closely with one other. The difficulty of obtaining accurate measurements of MTFs and the potential of using focused electron probes to make direct measurements of MTFs is discussed. We highlight the sensitivity of image contrast after deconvolution to small differences in the measured MTF.

The possibility of using Fresnel (or phase) contrast in a scanning transmission electron microscope as a means of measuring the heights of steps on the surface of MgO smoke cubes is examined. It is unknown that once the microscope parameters are known it is possible to match the intensities of the Fresnel fringes quantitatively with multislice simulations for step heights of the order of a few monolayers on a cube of thickness 44 nm.

We describe a simple technique for measuring the primary energy of a transmission electron microscope (TEM) using an electron energy-loss spectrometer. The technique is checked against a measurement based on simulating a convergent beam diffraction pattern, and found to be accurate to better than 0.1%.

The amount of phonon scattering as a function of specimen thickness is determined for a clean silicon sample, free from amorphous surface layers, by measuring the diffuse scattering in energy-filtered convergent-beam diffraction patterns. It is found that for a 25 nm thick sample, only 7.5% of the intensity scattered to less than 18 nm −1 is phonon scattered. This means that in a typical high-resolution sample most of the diffuse scattering is caused by surface amorphous layers rather than phonon scattering.

The prediction of the properties of modern multilayered systems provides a challenge to the accuracy to which the structure of a boundary can be characterised using the transmission electron microscope (TEM). A variety of new ways of using the TEM have recently been developed in this context, and here we describe some of the approaches which are now being applied. Particular emphasis is given to a description of the strengths and weaknesses of the Fresnel method in its potentially broad application to compositional analysis.

In an analysis of the contributions which inelastically scattered electrons can make to the high resolution details of an electron microscope image, comparisons are made between experimental images with and without a centre-stop aperture and simulations which both neglect and include the expected inelastically scattered electrons. The simulations incorporating contributions from electrons which have undergone energy losses are, as described, only first-order approximations but of sufficient accuracy to demonstrate the importance of including such effects generally.

The effect of inelastic scattering on high resolution [110] images of GaAs/AlAs is analysed by comparing experimental and simulated images, with and without a centre-stop aperture. Inelastic scattering from the specimen alone is found to be insufficient to account either for the lack of compositionally dependent contrast or for the low visibility of the half spacings in the experimental images. It is shown using energy loss spectroscopy that surface contamination and ion beam damage layers contribute a sufficient proportion of both elastic and inelastically scattered electrons to centre-stop images that the associated weakly defocus-dependent lattice resolution detail can account for the overall form of the experimental images seen.

The technique of double exposure electron holography, which is based on the superposition of two off-axis electron holograms, was originally introduced before the availability of digital image processing to allow differences between electron-optical phases encoded in two electron holograms to be visualised directly without the need for holographic reconstruction. Here, we review the original method and show how it can now be extended to permit quantitative studies of phase shifts that oscillate in time. We begin with a description of the theory of off-axis electron hologram formation for a time-dependent electron wave that results from the excitation of a specimen using an external stimulus with a square, sinusoidal, triangular or other temporal dependence. We refer to the more general method as continuous exposure electron holography, present preliminary experimental measurements and discuss how the technique can be used to image electrostatic potentials and magnetic fields during high frequency switching experiments.