Fresnel Contrast Research Articles

Observation of Glass Nanophase Separation from TEM Fresnel Contrast Images

Properties of phase-separated glasses, such as mechanical strength, chemical durability, and optical transmission, change drastically with the sizes and types of phase separation. Characterizing glass phase separated structures at the nanometer scale is vital for establishing the structure-property relation of various glass systems. This work will first briefly review the “staining” technique previously used to reveal the nanodroplet phase separation in Pyrex glass and its limitations. It will then introduce the TEM Fresnel contrast technique for studying glass structures. The theoretical background, practical application procedures, and examples of the method are described.

High Resolution STEM Images of the Human Tooth Enamel Crystals

High-resolution scanning transmission electron microscopy (STEM) images of human tooth enamel crystals, mainly in the high-angle annular dark-field (STEM-HAADF) mode, are presented in this work along the [1000], [10-11]. and [1-210] directions. These images allow knowing some structural details at the nanometric level of the human tooth enamel crystals and of the central dark line (CDL) observed at their centers. The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of the CDL showed the Fresnel contrast. In the STEM bright-field (STEM-BF) and annular-dark-field (STEM-ADF) images, the CDL was observed as an unstrain hydroxyapatite (HAP)-like zone but surrounded by a strained zone. In the STEM-HAADF images, the CDL appeared with a weak contrast, and its contrasts’ thickness was registered between 3 and 8 Å. The arrangement obtained in the STEM-HAADF images by identifying the bright points with the Ca atoms produced the superposition of the HAP atomic sites, mainly along the [0001] direction. The findings provide further information on the structure details at the center of enamel crystals, which favors the anisotropic carious dissolution at the CDL.

Tensile deformations of the magnetic chiral soliton lattice probed by Lorentz transmission electron microscopy

We consider the case of a chiral soliton lattice subjected to uniaxial elastic strain applied perpendicular to the chiral axis and derive through analytical modelling the phase diagram of magnetic states supported in the presence of an external magnetic field. The strain induced anisotropies give rise to three distinct non-trivial spin textures, depending on the nature of the strain, and we show how these states may be identified by their signatures in Lorentz transmission electron microscopy (TEM). Experimental TEM measurements of the Fresnel contrast in a strained sample of the prototypical monoaxial chrial helimagnet CrNb$_3$S$_6$ are reported and compare well with the modelled contrast. Our results demonstrate an additional degree of freedom that may be used to tailor the magnetic properties of helimagnets for fundamental research and applications in the areas of spintronics and the emerging field of strain manipulated spintronics.

Indentation cracking behaviour and structures of nanophase separation of glasses

Author(s): Cheng, S; Song, C; Ercius, P; Cionea, C; Hosemann, P | Abstract: Correlative microscopy is used to compare the performance and nanophase structures of soda-lime-silica, fused silica and borosilicate glasses using Vickers indentation crack analysis and the transmission electron microscopy (TEM) Fresnel contrast method. It is found that the observed indentation cracking behaviour is correlated to the nanophase separation structure of these glasses. The so-called normal cracking behaviour of soda-lime-silica glass is influenced by its spinodal nanophase separation; while the anomalous cracking behaviour of fused silica is due to the uniform single phase structure. Borosilicate glass has a droplet nanophase separation and shows intermediate cracking behaviour. These results indicate that in order to produce low britleness glasses it is important to control nanophase separation structure of a glass.

An iterative method for near-field Fresnel region polychromatic phase contrast imaging

We present an iterative method for polychromatic phase contrast imaging that is suitable for broadband illumination and which allows for the quantitative determination of the thickness of an object given the refractive index of the sample material. Experimental and simulation results suggest the iterative method provides comparable image quality and quantitative object thickness determination when compared to the analytical polychromatic transport of intensity and contrast transfer function methods. The ability of the iterative method to work over a wider range of experimental conditions means the iterative method is a suitable candidate for use with polychromatic illumination and may deliver more utility for laboratory-based x-ray sources, which typically have a broad spectrum.

Using MDECR‐PECVD to study the impact of ion bombardment energy on microstructural properties of μc‐Si:H thin film grown from an SiF4/H2 chemistry

AbstractThe matrix‐distributed electron cyclotron resonance plasma‐enhanced chemical vapor deposition (MDECR‐PECVD) technique has been shown to achieve high deposition rates for hydrogenated microcrystalline silicon (μc‐Si:H) thin film. Due to the fact that plasma is sustained by a microwave discharge, by biasing the substrate holder with additional power supply, one can achieve independent control over the plasma density and the maximum ion bombardment energy (IBE). In this work, we present studies of the impact of IBE on the microstructural properties of the μc‐Si:H film deposited by MDECR‐PECVD. Insufficient ion bombardment is found to be responsible for the substantial presence of nano‐porous regions within the material, resulting in significant post‐deposition oxidation. Good agreement between transmission electron microscopy (TEM) Fresnel contrast analysis and the results of infrared absorption and hydrogen effusion measurements for the deposited films suggest that moderate IBE is of vital importance to achieve high quality μc‐Si:H. In doing so, denser films with significantly decreased nano‐porous regions and better stability are obtained, which is of great interest to optimize the process parameters for solar cell applications. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structure of ion tracks in ceria irradiated with high energy xenon ions

Structure and accumulation behavior of ion tracks in CeO2 irradiated with 200 MeV Xe ions were examined by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) to obtain fundamental knowledge on the microstructure evolution induced by fission fragments in nuclear fuels and transmutation targets, which is of importance for the development of advanced fuel/target materials at high burn-up conditions. Bright-field (BF) TEM images of ion tracks from an inclined direction showed Fresnel contrast along penetrating path of incident ions. The signal intensity of high-angle annular dark-field (HAADF) STEM images was decreased at the core damage region of ion tracks along the path of ions, revealing the reduction of atomic density inside the ion track. Preferential formation of smaller and larger ion tracks was observed at a high ion fluence of 1 × 1014 cm−2 compared to a low ion fluence of 1 × 1011 cm−2. Results were discussed due to the coalescences and incomplete recovery of the core damage regions during the overlap of high density electronic excitation damage, which is induced during the repetition of the formation and recovery of ion tracks within an influence region.

Aberration-Corrected X-Ray Spectrum Imaging and Fresnel Contrast to Differentiate Nanoclusters and Cavities in Helium-Irradiated Alloy 14YWT

Helium accumulation negatively impacts structural materials used in neutron-irradiated environments, such as fission and fusion reactors. Next-generation fission and fusion reactors will require structural materials, such as steels, that are resistant to large neutron doses yet see service temperatures in the range most affected by helium embrittlement. Previous work has indicated the difficulty of experimentally differentiating nanometer-sized cavities such as helium bubbles from the Ti-Y-O rich nanoclusters (NCs) in radiation-tolerant nanostructured ferritic alloys (NFAs). Because the NCs are expected to sequester helium away from grain boundaries and reduce embrittlement, experimental methods to study simultaneously the NC and bubble populations are needed. In this study, aberration-corrected scanning transmission electron microscopy (STEM) results combining high-collection-efficiency X-ray spectrum images (SIs), multivariate statistical analysis (MVSA), and Fresnel-contrast bright-field STEM imaging, have been used for such a purpose. Fresnel-contrast imaging, with careful attention to TEM-STEM reciprocity, differentiates bubbles from NCs. MVSA of X-ray SIs unambiguously identifies NCs. Therefore, combined Fresnel-contrast STEM and X-ray SI is an effective STEM-based method to characterize helium-bearing NFAs.

Multislice simulation of transmission electron microscopy imaging of helium bubbles in Fe

Formation of nanoscale helium (He) bubbles in reduced activation ferritic/martensitic steels may lead to degradation of mechanical properties of materials. Transmission electron microscopy (TEM) has commonly been used to image the Fresnel contrast of He bubbles, using an underfocus of 0.5-1µm. This paper presents our study of multislice simulation of the size correlation between imaged Fresnel rings and the actual He bubbles. It was found that for bubbles equal to or >3nm in diameter, the imaged bubble size, represented by its inner diameter of the first dark Fresnel ring (D(in)) in underfocused imaging conditions, increases with increasing electron-beam incoherency, but decreases with increasing underfocus. The electron-beam accelerating voltage, bubble size, bubble position and TEM sample thickness were found to have no significant influence on the deviation of D(in) from the actual bubble size (D(0)). However, for bubbles equal to or <2nm, D(in)/D(0) increases dramatically with increasing underfocus when it is above a threshold limit (e.g. Δf=-1µm for a 2-nm bubble). The results of this study also suggested that He bubbles can be differentiated from argon (Ar) bubbles by contrast differences.

Multi-scale characterisation of oxide on zirconium alloys

Multi-scale characterisation using TEM, EELS and 3D FIB reconstruction was carried out on both pre-transition and post-transition oxides grown in high-temperature water. 3-D characterisation of cracks using high resolution FIB sequential sectioning gave a quantitative measure of crack density over time. There is no sudden burst of cracking occurring at the kinetic transition. On the nanometre scale, Fresnel contrast was used to reveal porosity of 1 – 3 nm in size in the oxide, which develops during growth from isolated pores to a connected network. Quantification of oxygen concentration using STEM-EELS has identified a suboxide, ZrO, layer tens of nanometres thick at the metal – oxide interface.

Micromagnetic study of flux-closure states in Fe dots using quantitative Lorentz microscopy

A micromagnetic study of epitaxial micron-sized iron dots is reported through the analysis of Fresnel contrast in Lorentz Microscopy. Their use is reviewed and developed through analysis of various magnetic structures in such dots. Simple Landau configuration is used to investigate various aspects of asymmetric Bloch domain walls. The experimental width of such a complex wall is first derived and its value is discussed with the help of micromagnetic simulations. Combination of these two approaches enables us to define what is really extracted when estimating asymmetric wall width in Lorentz Microscopy. Moreover, quantitative data on the magnetization inside the dot is retrieved using phase retrieval as well as new information on the degrees of freedom of such walls. Finally, it is shown how the existence and the propagation of a surface vortex can be characterized and monitored. This demonstrates the ability to reach a magnetic sensitivity a priori hidden in Fresnel contrast, based on an original image treatment and backed-up by the evaluation of contrasts obtained from micromagnetic simulations.

Localization of intracrystalline organic macromolecules in mollusk shells

As a crucial first step for understanding the organic–inorganic interaction in biomineralization of mollscan shells, localization of the intracrystalline organic macromolecules in biogenic calcium carbonate crystals of the nacreous, prismatic ( Pinctada fucata) and foliated ( Cellana toreuma) microstructures were investigated using Fresnel contrast analysis in a transmission electron microscope. Spherular Fresnel contrasts in the crystals correspond to organic substances, which was confirmed by the detection of 1s→π ⁎ (CC) transition peak at 284 eV in electron energy loss spectroscopy. Nano-sized (5–10 nm) spherules in the aragonite tablets constituting the nacreous layer of P. fucata specifically concentrate in the vicinity of the interlamellar membrane between the aragonite tablets. The dominant sizes of the organic macromolecules extracted by dissolving the aragonite tablets in the nacreous layer of P. fucata were estimated using the gel-filtration analysis to be roughly 10 and 4 nm, which dimensionally corresponds to the sizes observed by Fresnel contrast imaging in the tablets. These results will serve for understanding the functions of intracrystalline organic macromolecules in mollusk shells.

Studies Regarding Corrosion Mechanisms in Zirconium Alloys

Abstract Understanding the key corrosion mechanisms in a light water reactor primary water environment is critical to developing and exploiting improved zirconium alloy fuel cladding. In this paper, we report recent research highlights from a new collaborative research programme involving 3 U.K. universities and 5 partners from the nuclear industry. A major part of our strategy is to use the most advanced analytical tools to characterise the oxide and metal/oxide interface microstructure, residual stresses, as well as the transport properties of the oxide. These techniques include three-dimensional atom probe (3DAP), advanced transmission electron microscopy (TEM), synchrotron X-ray diffraction, Raman spectroscopy, and in situ electro-impedance spectroscopy. Synchrotron X-ray studies have enabled the characterisation of stresses, tetragonal phase fraction, and texture in the oxide as well as the stresses in the metal substrate. It was found that in the thick oxide (here, Optimized-ZIRLO, a trademark of the Westinghouse Electric Company, tested at 415°C in steam) a significant stress profile can be observed, which cannot be explained by metal substrate creep alone but that local delamination of the oxide layers due to crack formation must also play an important role. It was also found that the oxide stresses in the monoclinic and tetragonal phases grown on Zircaloy-4 (autoclave testing at 360°C) first relax during the pre-transition stage. Just before transition, the compressive stress in the monoclinic phase suddenly rises, which is interpreted as indirect evidence of significant tetragonal to monoclinic phase transformation taking place at this stage. TEM studies of pre- and post-transition oxides grown on ZIRLO, a trademark of the Westinghouse Electric Company, have used Fresnel contrast imaging to identify nano-sized pores along the columnar grain boundaries that form a network interconnected once the material goes through transition. The development of porosity during transition was further confirmed by in situ electrochemical impedance spectroscopy (EIS) studies. 3DAP analysis was used to identify a ZrO sub-oxide layer at the metal/oxide interface and to establish its three-dimensional morphology. It was possible to demonstrate that this sub-oxide structure develops with time and changes dramatically around transition. This observation was further confirmed by in situ EIS studies, which also suggest thinning of the sub-oxide/barrier layer around transition. Finally, 3DAP analysis was used to characterise segregation of alloying elements near the metal/oxide interface and to establish that the corroding metal near the interface (in this case ZIRLO) after 100 days at 360°C displays a substantially different chemistry and microstructure compared to the base alloy with Fe segregating to the Zr/ZrO interface.

Medipix2 as a highly flexible scanning/imaging detector fortransmission electron microscopy

The transmission electron microscope (TEM) is an indispensable tool for the study of magnetic micro- and nano-structures. High resolution digital detectors have largely replaced film as the primary detection method for electron microscopy. In this study we assess the benefits of using the Medipix2 chip, bump bonded to a 300 μm Si detector, as a flexible dual purpose detector for scanning/imaging mode operation on a Philips CM20 FEG TEM/STEM at the University of Glasgow. Specifically, the magnetic imaging modes of Fresnel and Differential phase contrast (DPC) were utilised and the performance of the Medipix chip compared to that of the CCD and the segmented photodiode normally used for these techniques. The Medipix detector matched the performance of the segmented detector for DPC imaging while offering greater flexibility in the approach. It also shown an improvement of > 60 in the signal to noise performance over the CCD used for Fresnel imaging.

Quantitative characterization of xenon bubbles in silicon: Correlation of bubble size with the damage generated during implantation

Making use of Fresnel fringe contrast under different focusing conditions in transmission electron microscopy (TEM), we present a detailed evaluation of the depth dependent size distribution of gas bubbles contained in a stationary profile of 40 keV Xe implanted in Si. Voids generated during sample preparation by ion milling were also characterized carefully. The largest bubbles, with mean and maximum sizes of 5 and 7 nm, respectively, were observed at depths <22 nm. However, the first 2 nm of the sample did not contain any bubbles. Towards the end of range the bubble size decreased rapidly. No bubbles were found beyond 45 nm (the minimum size of detectable bubbles was estimated to be about 1.8 nm). Some observations suggest that the bubbles were over-pressurized. The derived data could be converted to a depth dependence of the Xe concentration contained in bubbles, n Xe,b. Comparison with the previously reported depth distribution of Xe measured by Rutherford backscattering spectrometry (RBS), n Xe,b turned out to be depth dependent, with a maximum of ∼28% in the region of maximum bubble size. n Xe,b is shown to correlate closely with the damage density generated during Xe implantation. The findings lead to a model of bubble formation which involves the idea that the redistribution and transport processes initiated by ion impact take place mostly during the lifetime of the collision cascade.

Image simulations of kinked vortices for transmission electron microscopy

We present an improved model of kinked vortices in high- T c superconductors suitable for the interpretation of Fresnel or holographic observations carried out with a transmission electron microscope. A kinked vortex is composed of two displaced half-vortices, perpendicular to the film plane, connected by a horizontal flux-line in the plane, resembling a connecting Josephson vortex (JV) segment. Such structures may arise when a magnetic field is applied almost in the plane, and the line tension of the fluxon breaks down under its influence. The existence of kinked vortices was hinted in earlier observations of high- T c superconducting films, where the Fresnel contrast associated with some vortices showed a dumbbell like appearance. Here, we show that under suitable conditions the JV segment may reveal itself in Fresnel imaging or holographic phase mapping in a transmission electron microscope.

The measurement and interpretation of electrostatic potential profiles across grain boundaries in strontium titanate

Results from a systematic Fresnel contrast (in-line electron holography) study of a large number of grain boundary potentials from undoped, donor-doped and acceptor-doped polycrystalline strontium titanate samples are summarised. An assessment is made of whether the measured variations in potential can be used to provide information about space charge distributions at the boundaries, or whether they are instead dominated by local changes in composition, density and specimen thickness. The effects on the measured contrast of both specimen preparation for transmission electron microscopy and irradiation by the high-energy electron beam are also discussed.

Intergranular glassy films: An overview

In certain ceramics like Si 3N 4, SiC, SrTiO 3, Al 2O 3, etc., the grain boundary (GB) region can have an amorphous film of about 1–2 nm thickness. These intergranular glassy films (IGFs) are characterized by a nearly constant thickness which is basically independent of the orientation of the bounding grains, but is dependent on the composition of the ceramic. The IGF is resistant to crystallization and is thought to represent an equilibrium configuration. The presence of the IGF, along with its structure, plays an important role in determining the properties of the ceramic as a whole. Important amongst these properties, keeping in mind the system based specificities, are fracture, creep, oxidation and electrical behaviour. Depending on the system, various synthetic routes like liquid phase sintering, solid-state activated sintering, crystallization of glass surrounding the crystal, etc., have led to the formation of IGFs. Equilibrium thickness amorphous films on surfaces have also been synthesized which are considered to be the surface analogue of IGFs. Important advances in the microscopy techniques have provided invaluable insights into the structure of IGFs, along with its interface with the bounding crystals. These techniques include: high-resolution microscopy, Fresnel contrast imaging, diffuse dark field imaging, diffraction analysis, electron holography, high-angle annular dark field imaging, energy-dispersive X-ray analysis and electron energy loss spectroscopy. It is now being progressively realized that the composition and structure within the IGF is graded, i.e., it has a diffuse interface with the bounding crystals and that the amorphous material in the IGF is different from the bulk glass forming in that system. The order induced by the bounding crystals on the IGF is seen as a contributing factor to the gradation. In spite of the achievements, a lot of open questions remain regarding the formation of IGFs, its behaviour with temperature, its dependence on crystallography of the bounding grains, etc. This overview aims at introducing IGFs to a non-specialist audience, summarizing the important advances in the field and outlining the outstanding issues.

Aspects regarding measurement of thickness of intergranular glassy films

Materials such as Si(3)N(4), SiC and SrTiO(3) can have grain boundaries characterized by the presence of a thin intergranular amorphous film of nearly constant thickness, in some cases (e.g. Si(3)N(4)) almost independent of the orientation of the bounding grains, but dependent on the composition of the ceramic. Microscopy techniques such as high-resolution lattice fringe imaging, Fresnel fringe imaging and diffuse dark field imaging have been applied to the study of intergranular glassy films. The theme of the current investigation is the use of Fresnel fringes and Fourier filtering for the measurement of the thickness of intergranular glassy films. Fresnel fringes hidden in high-resolution micrographs can be used to objectively demarcate the glass-crystal interface and to measure the thickness of intergranular glassy films. Image line profiles obtained from Fourier filtering the high-resolution micrographs can yield better estimates of the thickness. Using image simulation, various kinds of deviation from an ideal square-well potential profile and their effects on the Fresnel image contrast are considered. A method is also put forth to objectively retrieve Fresnel fringe spacing data by Fourier filtering Fresnel contrast images. Difficulties arising from the use of the standard Fresnel fringe extrapolation technique are outlined and an alternative method for the measurement of the thickness of intergranular glassy films, based on zero-defocus (in-focus) Fresnel contrast images is suggested. The experimental work is from two ceramic systems: Lu-Mg-doped Si(3)N(4) and SrTiO(3) (stoichiometric and nonstoichiometric). Further, a comparison is made between the standard high-resolution lattice fringe technique, the standard Fresnel fringe extrapolation technique and the methods of analyses introduced in the current work, to illustrate their utility and merits. Taking experimental difficulties into account, this work is intended to be a practical tool kit for the study of intergranular glassy films.

Variation of domain-wall structures and magnetization ripple spectra in permalloy films with controlled uniaxial anisotropy

We have studied domain-wall structures and magnetization ripple in permalloy films with widely varying values of anisotropy field and thickness. Both the Fresnel and differential phase contrast imaging modes of Lorentz microscopy have been used, the latter proving particularly valuable for providing reliable quantitative data. Cross-tie walls were frequently observed but the density of the cross ties increased markedly with anisotropy field while the cross-tie length decreased. In accord with expectation, the mean ripple wavelength and magnitude of the ripple decreased with increasing anisotropy field, albeit the variation was more rapid than could be accounted for using simple theories. Generally the root-mean-square dispersion angle was found to be greater than that in earlier experimental work, a factor that was ascribed to the use of an incomplete analysis procedure in the past.