Electron Source Brightness and Illumination Semi-Angle Distribution Measurement in a Transmission Electron Microscope

Temporal coherence envelope function of field emission in electron microscopy

Imaging in electron microscopy is adversely affected by partial electron spatial and temporal coherence. Temporal coherence has been treated theoretically in the past using the method pioneered fifty years ago by Hanßen and Trepte, who assumed a Gaussian energy distribution. However, state-of-the-art instruments employ field emission (FE) sources that emit electrons with a non-Gaussian energy distribution. We have updated the treatment of temporal coherence to describe the effects of an arbitrary energy distribution on image formation. The updated approach is implemented in Fourier optics simulations to explore the effect of FE on image formation in conventional, non-aberration-corrected (NAC) and aberration-corrected (AC) low energy electron microscopy. It is found that the resolution that can be achieved for the FE distribution is only slightly degraded compared to a Gaussian distribution with the same energy spread. FE also produces a focus offset. These two effects are weaker for AC than for NAC microscopy. These and other insights may be relevant to the selection of the aperture size that optimizes resolution and to analyses that make use of focal image series. The approach developed here is also applicable to transmission electron microscopy.

Near-Monochromatic Tuneable Cryogenic Niobium Electron Field Emitter

Creating, manipulating, and detecting coherent electrons is at the heart of future quantum microscopy and spectroscopy technologies. Leveraging and specifically altering the quantum features of an electron beam source at low temperatures can enhance its emission properties. Here, we describe electron field emission from a monocrystalline, superconducting niobium nanotip at a temperature of 5.9 K. The emitted electron energy spectrum reveals an ultranarrow distribution down to 16 meV due to tunable resonant tunneling field emission via localized band states at a nanoprotrusion's apex and a cutoff at the sharp low-temperature Fermi edge. This is an order of magnitude lower than for conventional field emission electron sources. The self-focusing geometry of the tip leads to emission in an angle of 3.7°, a reduced brightness of 3.8×10^{8}  A/(m^{2} sr V), and a stability of hours at 4.1 nA beam current and 69 meV energy width. This source will decrease the impact of lens aberration and enable new modes in low-energy electron microscopy, electron energy loss spectroscopy, and high-resolution vibrational spectroscopy.

High order phase contrast and source divergence in low energy electron microscopy

We present experimental observations of high order phase contrast in aberration corrected low energy electron microscopy (AC-LEEM). Phase contrast produced by atomic steps on a Ag (111) surface exhibits prominent high order interference fringes, which have not been reported before. These phase contrast features depend upon defocus and incident electron energy, similar to the prominent first order fringes observed previously and in agreement with Fourier optics (FO) model predictions. The comparison of experimental results and FO model simulations demonstrates that fringe amplitudes are strongly affected at large defocus by the source divergence. This effect is exploited to quantitatively determine the divergence, 0.055 ± 0.005 mrad, of the field emission source in AC-LEEM under the imaging conditions used. Although the divergence determines the spatial coherence of the illumination in microscopy, it has not been possible to characterize this key instrumental parameter in LEEM before.

Defocus-dependent Thon-ring fading

The brightness of modern Schottky field-emission guns can produce electron beams that have very high spatial coherence, especially for the weak-illumination conditions that are used for single-particle electron cryo-microscopy in structural biology. Even so, many users have observed defocus-dependent Thon-ring fading that has led them to restrict their data collection strategy to imaging with relatively small defocus values. In this paper, we reproduce the observation of defocus-dependent Thon-ring fading and produce a quantitative analysis and clear explanation of its causes. We demonstrate that a major cause is the delocalization of high-resolution Fourier components outside the field of view of the camera. We also show that, to correctly characterize the phenomenon, it is important to make a correction for linear magnification anisotropy. Even when the anisotropy is quite small, it is present at all defocus values before circular averaging of the Thon rings, as is also true before merging data from particles in many orientations. Under the conditions used in this paper, which are typical of those used in single-particle electron cryomicroscopy, fading of the Thon rings due to source coherence is negligible. The principal conclusion is that much higher values of defocus can be used to record images than is currently thought to be possible, keeping in mind that the above-mentioned delocalization of Fourier components will ultimately become a limitation. This increased understanding should give electron microscopists the confidence to use higher amounts of defocus to allow, for example, better visibility of their particles and Ewald sphere correction.

High-resolution analyses of the mineralisation process at bio/inorganic interfaces

To gain insight into bio-inspired mineralisation processes that may be directed or influenced by the protein coat of tobacco mosaic virus (TMV), microscopic studies are crucial. The authors elucidate key steps towards the understanding of the mineralisation mechanism by high-resolution transmission electron microscopy imaging of the organic/inorganic interfaces. Wild-type TMV and mutants with distinct surface properties are used to investigate the processes of nucleation and mineral formation based on ZnO and ZnS deposition. The development of suitable sample preparation methods is central in the study, as well as finding the optimal imaging conditions in combination with the use of a latest-generation high-end microscope. The mineralisation is to be analysed with respect to the underlying nucleation process: Correlation of structure findings with mechanical properties will pave the way to the goal of the design of materials with enhanced mechanical properties.

CryoEM at 100 keV: a demonstration and prospects

100 kV is investigated as the operating voltage for single-particle electron cryomicroscopy (cryoEM). Reducing the electron energy from the current standard of 300 or 200 keV offers both cost savings and potentially improved imaging. The latter follows from recent measurements of radiation damage to biological specimens by high-energy electrons, which show that at lower energies there is an increased amount of information available per unit damage. For frozen hydrated specimens around 300 Å in thickness, the predicted optimal electron energy for imaging is 100 keV. Currently available electron cryomicroscopes in the 100-120 keV range are not optimized for cryoEM as they lack both the spatially coherent illumination needed for the high defocus used in cryoEM and imaging detectors optimized for 100 keV electrons. To demonstrate the potential of imaging at 100 kV, the voltage of a standard, commercial 200 kV field-emission gun (FEG) microscope was reduced to 100 kV and a side-entry cryoholder was used. As high-efficiency, large-area cameras are not currently available for 100 keV electrons, a commercial hybrid pixel camera designed for X-ray detection was attached to the camera chamber and was used for low-dose data collection. Using this configuration, five single-particle specimens were imaged: hepatitis B virus capsid, bacterial 70S ribosome, catalase, DNA protection during starvation protein and haemoglobin, ranging in size from 4.5 MDa to 64 kDa with corresponding diameters from 320 to 72 Å. These five data sets were used to reconstruct 3D structures with resolutions between 8.4 and 3.4 Å. Based on this work, the practical advantages and current technological limitations to single-particle cryoEM at 100 keV are considered. These results are also discussed in the context of future microscope development towards the goal of rapid, simple and widely available structure determination of any purified biological specimen.