Francis Sweeney

Direct observation of individual hydrogen atoms at trapping sites in a ferritic steel

Heavy hydrogen gets frozen in place Hydrogen embrittlement contributes to the failure of steel in a wide variety of everyday applications. Various strategies to mitigate hydrogen embrittlement, such as adding carbides into the steel, are hard to validate because we are unable to map the hydrogen atoms. Chen et al. combined fluxing steel samples with deuterium and a cryogenic transfer protocol to minimize hydrogen diffusion, allowing for detailed structural analysis (see the Perspective by Cairney). Their findings revealed hydrogen trapped in the cores of the carbide precipitates. The technique will be applicable to a wide range of problems, including corrosion, catalysis, and hydrogen storage. Science, this issue p. 1196; see also p. 1128 The combination of deuteration and a cryogenic transfer protocol reveals hydrogen locations in high-strength steel. The design of atomic-scale microstructural traps to limit the diffusion of hydrogen is one key strategy in the development of hydrogen-embrittlement–resistant materials. In the case of bearing steels, an effective trapping mechanism may be the incorporation of finely dispersed V-Mo-Nb carbides in a ferrite matrix. First, we charged a ferritic steel with deuterium by means of electrolytic loading to achieve a high hydrogen concentration. We then immobilized it in the microstructure with a cryogenic transfer protocol before atom probe tomography (APT) analysis. Using APT, we show trapping of hydrogen within the core of these carbides with quantitative composition profiles. Furthermore, with this method the experiment can be feasibly replicated in any APT-equipped laboratory by using a simple cold chain.

Quantitative phase contrast optimised cancerous cell differentiation via ptychography

This paper shows that visible-light ptychography can be used to distinguish quantitatively between healthy and tumorous unstained cells. Advantages of ptychography in comparison to conventional phase-sensitive imaging techniques are highlighted. A novel procedure to automatically refocus ptychographic reconstructions is also presented, which improves quantitative analysis.

Probe position recovery for ptychographical imaging

Ptychographical iterative phase retrieval is a promising new transmission imaging technique which uses a set of measured intensities from the consecutive illumination of overlapping regions of the specimen to form an image of the transmission function in phase and amplitude. Although the technique has been shown to work very effectively in both the light-optical and X-ray domains, electron ptychography poses significant difficulties, not least of which is the uncertainty in probe position due to drift and other instabilities. We demonstrate three methods for deriving the relative positions of the illumination spot on the specimen a-posteriori.

Ptychography: a novel phase retrieval technique, advantages and its application

This paper is intended to introduce ptychography, a novel and very promising phase retrieval technique. It is based on the lens-less recording of a series of diffraction patterns caused by coherent object illumination. In the visible region of light, ptychography has successfully been implemented for visible light microscopy and optical metrology. Ptychography has also successfully been applied to X-ray microscopy where it is difficult to manufacture good quality lenses and where, at high X-ray energies, absorption contrast is low but where phase contrast is significant. In the course of this paper theoretical fundamentals of ptychography are explained, advantages in comparison to traditional optical techniques are represented and applications are shown.

Noise limit on practical electron ptychography

Ptychography is a wavelength-limited phase retrieval method that promises to provide sub-0.5A resolution in the electron microscope, even if the lens employed (which is required only to form an illumination spot) can itself only achieve rather modest resolution. The fidelity of a practical specimen reconstruction using ptychography hinges on various experimental factors, the major one of which is the source brightness. Achieving a high degree of coherence of an electron beam requires source demagnification which reduces the electron counts reaching the detector for a given exposure time. Specimen drift, damage and contamination also limits the practical exposure time. In this paper we investigate the effect of the counting statistics required for a good quality ptychographic reconstruction

Ptychography: a powerful phase retrieval technique for biomedical imaging

This paper discusses ptychography, a coherent diffraction imaging technique. Advantages of ptychography with respect conventional imaging techniques for cell visualisation are highlighted and demonstrated using unstained healthy and tumorous mouse cells as the object under investigation. A novel procedure to automatically refocus a possible slightly out of focus ptychographic data set will be discussed, by which an improved quantitative analysis and discrimination is enabled.

Solving for the phase of STEM probes in real space

The combination of a Gerchberg-Saxton phase retrieval algorithm with a genetic algorithm is used to retrieve the phase of a TEM or STEM probe and pinpoint its defocus. Results from modelled data and initial experimental work are included.

Measuring coherence in an electron beam for imaging

Transmission electron microscopy applies either parallel or focused electron beams for illuminating the object, and all high resolution methods such as lattice imaging, electron holography or diffractive imaging rely on interference effects which necessitate coherent electrons. In lattice imaging the effects of partial spatial and partial temporal coherence are usually dealt with by applying Gaussian envelopes to the contrast transfer function to describe the dampening of higher spatial frequencies. This means the coherence is only modelled indirectly, for partial spatial coherence by a convergence angle and for temporal coherence by a defocus spread due to chromatic broadening. To measure the coherence function would enable a full quantification of the contrast observed in these interference experiments and can be done by the following methods: 1. Young’s slits experiment: interference of electrons that pass fine holes nanomachined into a thick specimen using e.g. a focused ion beam instrument [1] 2. electron diffraction: interference between Bragg diffracted discs in convergent beam electron diffraction of thin crystalline specimens [2] 3. electron holography: interference of two phase-related electron beams created by splitting the electron beam by an electrostatic bi-prism [3] 4. comparison of the defocused far-field ring pattern formed behind an illuminated aperture with simulations

STEM probe characteristics at large defoci for use in ptychographical imaging

A possible configuration for undertaking diffractive imaging microscopy in the conventional scanning transmission electron microscope (STEM) is to defocus the probe by a large distance in order to minimise the number of diffraction patterns required to reconstruct the ptychographical image. Although the characteristics of STEM probes have been well explored and measured near the beam crossover, they are rarely observed (or calculated) at very large defoci. In this paper we compare probes calculated under a variety of approximations with measured data from a JEOL 2010F in STEM mode. When the illumination aperture is narrow and the defocus is large (near parallel illumination useful for ptychographical imaging) the Ronchigram cannot be used easily to characterize aberration, We develop an alternative method of estimating the effective aperture size and condenser lens spherical aberration.