Rüdiger R. Meyer

Characterisation of the signal and noise transfer of CCD cameras for electron detection.

Methods to characterise the performance of CCD cameras for electron detection are investigated with particular emphasis on the difference between the transfer of signal and noise. Similar to the Modulation Transfer Function MTF, which describes the spatial frequency dependent attenuation of contrast in the image, we introduce a Noise Transfer Function NTF that describes the transfer of the Poisson noise that is inevitably present in any electron image. A general model for signal and noise transfer by an image converter is provided. This allows the calculation of MTF and NTF from Monte‐Carlo simulations of the trajectories of electrons and photons in the scintillator and the optical coupling of the camera. Furthermore, accurate methods to measure the modulation and noise transfer functions experimentally are presented. The spatial‐frequency dependent Detection Quantum Efficiency DQE, an important figure of merit of the camera which has so far not been measured experimentally, can be obtained from the measured MTF and NTF. The experimental results are in good agreement with the simulations and show that the NTF at high spatial frequencies is in some cases by a factor of four higher than the MTF. This implies that the noise method, which is frequently used to measure the MTF, but in fact measures the NTF, gives over‐optimistic results. Furthermore, the spatial frequency dependent DQE is lower than previously assumed. Microsc. Res. Tech. 49:269–280, 2000.

The effects of electron and photon scattering on signal and noise transfer properties of scintillators in CCD cameras used for electron detection

Abstract The detection properties of scintillators used in charge-coupled device cameras suitable for electron microscopy are examined with particular emphasis on the statistics of electron scattering and photon generation in the scintillator. We show that the root of the power spectrum of an evenly illuminated white noise image is in general not equal to the modulation transfer function (MTF) of the scintillator, as the former corresponds to the statistical properties of the detectable light intensity caused by single incident electrons while the latter corresponds to the statistical properties of the detected intensity caused by many electrons incident on the same point. A difference between these statistical properties leads to over-optimistic estimations of the MTF when the noise method is used and furthermore deteriorates the detection quantum efficiency (DQE) at high spatial frequencies. Monte Carlo simulations are used to calculate the true MTF, the expected outcome of a noise method measurement (MTFnoise) and the spatial frequency dependent DQE for various scintillator thicknesses and acceleration voltages.

Indirect high-resolution transmission electron microscopy: aberration measurement and wavefunction reconstruction.

Improvements in instrumentation and image processing techniques mean that methods involving reconstruction of focal or beam-tilt series of images are now realizing the promise they have long offered. This indirect approach recovers both the phase and the modulus of the specimen exit plane wave function and can extend the interpretable resolution. However, such reconstructions require the a posteriori determination of the objective lens aberrations, including the actual beam tilt, defocus, and twofold and threefold astigmatism. In this review, we outline the theory behind exit plane wavefunction reconstruction and describe methods for the accurate and automated determination of the required coefficients of the wave aberration function. Finally, recent applications of indirect reconstruction in the structural analysis of complex oxides are presented.

Direct Electron Detector

Direct electron detectors have played a key role in the recent increase in the power of single-particle electron cryomicroscopy (cryoEM). In this chapter, we summarize the background to these recent developments, give a practical guide to their optimal use, and discuss future directions.

A composite method for the determination of the chirality of single walled carbon nanotubes

An approach to the unambiguous determination of the conformation of individual single walled nanotubes utilizing high‐resolution transmission electron microscopy and digital image processing is described. The exit plane wave of single walled nanotubes restored from a focal series of images is used in a stepwise characterization procedure utilizing both the phase of the real space restoration and its Fourier transform. A combination of these complementary characterization steps yields an accurate measurement of the chiral vector for an individual nanotube.

Local Measurement and Computational Refinement of Aberrations for HRTEM

Methods for accurate and automated determination of the coefficients of the wave aberration function are compared with particular emphasis on measurements of higher order coefficients in corrected instruments. Experimental applications of aberration measurement to the determination of illumination isoplanicity and high precision local refinement of restored exit waves are also described.

Complete characterisation of a Sb2O3/(21,−8)SWNT inclusion composite

The structure of a one-dimensional crystal of Sb2O3 encapsulated within a single-walled carbon nanotube and conformation of the latter have been solved simultaneously by high resolution transmission electron microscopy.

Observation of octahedral cation coordination on the {111} surfaces of iron oxide nanoparticles

High-resolution (scanning) transmission electron micrographs taken on both aberration corrected and uncorrected microscopes, and indirect reconstructed images of mixed phase magnetite—maghemite nanoparticles all show the presence of {111} facets that terminate with enhanced contrast. This enhanced contrast is shown to be a real effect caused by the presence of additional octahedrally coordinated iron cations occupying the {111} terminating layers of these nanoparticle surfaces.

LaI2@(18,3)SWNT: the unprecedented structure of a LaI2 "Crystal," encapsulated within a single-walled carbon nanotube.

The novel crystallization properties of nano-materials represent a great challenge to researchers across all disciplines of materials science. Simple binary solids can be found to adopt unprecedented structures, when confined into nanometer-sized cavities, such as the inner cylindrical bore of single-walled carbon nanotubes (SWNT). Lanthanum iodide was encapsulated within SWNTs and the resulting encapsulation composite was analyzed using energy-dispersive X-ray microanalysis (EDX) and high-resolution transmission electron microscopy (HRTEM) imaging techniques, to reveal a one-dimensional crystal fragment, with the stoichiometry of LaI2, crystallizing in the structure of LaI3 with one third of the iodine positions unoccupied. A complete characterization of the encapsulation composite was achieved using an enhanced image restoration technique, which restores the object wave from a focal series of HRTEM images, providing information about the precise structural data of both filling material and host SWNT, and thereby enabling the identification of the SWNT chirality.

Band-Gap Modification Induced in HgTe by Dimensional Constraint in Carbon Nanotubes: Effect of Nanotube Diameter on Microstructure

A new tubular form of HgTe grown in narrow single walled carbon nanotubes is described with Hg and Te in reduced coordination. Two unique projections obtained by HRTEM from two separate crystal fragments enabled reconstruction of the atomic arrangement of the new form. DFT confirmed the stability of the new structure and that it has a modified band gap, transforming HgTe from a semimetal to a semiconductor (band gap +1.3eV). HRTEM shows that as the nanotube diameter increases, the new form is no longer obtained and for diameters of 1.6-2 nm, disordered HgTe is obtained, for diameters >2 nm, sphalerite HgTe is obtained.