Μm Resolution Research Articles

Flash Scanning Volumetric Optoacoustic Tomography for High Resolution Whole‐Body Tracking of Nanoagent Kinetics and Biodistribution

AbstractTracking of biodynamics across entire living organisms is essential for understanding complex biology and disease progression. The presently available small‐animal functional and molecular imaging modalities remain constrained by factors including long image acquisition times, low spatial resolution, limited penetration or poor contrast. Here flash scanning volumetric optoacoustic tomography (fSVOT), a new approach for high‐speed imaging of fast kinetics and biodistribution of optical contrast agents in whole mice that simultaneously provides reference images of vascular and organ anatomy with unrivaled fidelity and contrast, is presented. The imaging protocol employs continuous overfly scanning of a spherical matrix array transducer, accomplishing a 200 µm resolution 3D scan of the whole mouse body within 45 s without relying on signal averaging. This corresponds to an imaging speed gain of more than an order of magnitude compared with existing state‐of‐the‐art implementations of comparable resolution performance. Volumetric tracking and quantification of gold nanoagent and near infrared (NIR)‐II dye kinetics and their differential uptake in various organs are demonstrated. fSVOT thus offers unprecedented capabilities for multiscale imaging of pharmacokinetics and biodistribution with high contrast, resolution, and speed.

Using µCT in live larvae of a large wood-boring beetle to study tracheal oxygen supply during development

How respiratory structures vary with, or are constrained by, an animal’s environment is of central importance to diverse evolutionary and comparative physiology hypotheses. To date, quantifying insect respiratory structures and their variation has remained challenging due to their microscopic size, hence only a handful of species have been examined. Several methods for imaging insect respiratory systems are available, in many cases however, the analytical process is lethal, destructive, time consuming and labour intensive. Here, we explore and test a different approach to measuring tracheal volume using X-ray micro-tomography (µCT) scanning (at 15 µm resolution) on living, sedated larvae of the cerambycid beetle Cacosceles newmannii across a range of body sizes at two points in development. We provide novel data on resistance of the larvae to the radiation dose absorbed during µCT scanning, repeatability of imaging analyses both within and between time-points and, structural tracheal trait differences provided by different image segmentation methods. By comparing how tracheal dimension (reflecting metabolic supply) and basal metabolic rate (reflecting metabolic demand) increase with mass, we show that tracheal oxygen supply capacity increases during development at a comparable, or even higher rate than metabolic demand. Given that abundant gas delivery capacity in the insect respiratory system may be costly (due to e.g. oxygen toxicity or space restrictions), there are probably balancing factors requiring such a capacity that are not linked to direct tissue oxygen demand and that have not been thoroughly elucidated to date, including CO2 efflux. Our study provides methodological insights and novel biological data on key issues in rapidly quantifying insect respiratory anatomy on live insects.

Direct U–Pb dating of carbonates from micron-scale femtosecond laser ablation inductively coupled plasma mass spectrometry images using robust regression

Abstract. Uranium–lead (U–Pb) dating of carbonates by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) spot analysis is an increasingly used method in the field of geosciences, as it brings very strong constraints over the geological history of basins, faults or reservoirs. Most ages currently published are based on the measurement of U and Pb ratios on spot ablations, using nanosecond lasers coupled to sector field or multi-collector ICP-MS. Here, we test a new strategy for the U–Pb dating of carbonates from 2D isotopic ratio maps, based on the use of a robust regression approach in the data reduction workflow. The isotopic maps, with a minimum area of 0.65 mm2 (∼ 1000 pixels of 13×25 µm resolution), are obtained using a 257 nm femtosecond laser ablation system at a high repetition rate (500 Hz) coupled to a high-resolution ICP-MS. The maps commonly show significant variations in isotope ratios at the pixel scale, allowing the plotting of pixel U–Pb ratios in concordia or isochron diagrams and the calculation of U–Pb ages. Due to the absence of individual ratio uncertainties, the ages are calculated using MM-robust linear regression rather than the more commonly used York-type regression. The goodness of fit to the data is assessed by the calculation of the residual standard error (RSE) of the regression and by the calculation of a mean square of weight deviates (MSWD) on discretised data. Several examples are provided that compare the ages calculated by robust regression with those obtained by other techniques (e.g. isotope dilution, LA-ICP-MS spot analyses and the pixel-pooling approach). For most samples, characterised by high U concentrations (> 1 ppm), robust regression allows for the calculation of ages and uncertainties similar to those obtained with the other approaches. However, for samples with lower U concentrations (< 0.5 ppm), the ages obtained are up to 10 % too young due to pixels with high U ∕ Pb acting as leverage points for the regression. We conclude that the U–Pb ages calculated by the regression method tested here, although statistically robust, should be critically analysed before validation, especially for samples with low U concentrations.

Operando µ-Raman study of the membrane water content in the polymer electrolyte membrane fuel cell: Effects of gas flow-field geometry and temperature

The energy conversion efficiency of the polymer electrolyte membrane fuel cell needs improved thermal and water managements. Here, the actual water concentration of the NafionⓇ membrane in the fuel cell working at constant stoichiometry and relative humidity is measured by operando Raman microspectroscopy. Through-plane water content profiles with μm resolution are obtained across the membrane at locations corresponding to the gas distribution channel and under the lands for the current collection, at the center of the active surface. The influence of the cell working temperature, of the current density and of the geometry of the gas feed channel (serpentine vs. parallel) are investigated. The combined measurement of the cell resistance and mass balance allow to establish relationships between the local water distribution throughout the cell and electrochemical performances. The membrane water content lowers with the increase of both current density and temperature regardless to the flow-field channel geometry. The membrane dehydration with current is ascribed to the concomitant raise of pressure losses and the spontaneous increase of the fuel cell inner temperature, with the last prevailing when the cell is managed at low temperatures. In that case, the distribution of water at the channel/lands scale has a distinct effect on the electrochemical behavior and performance of the cell. The parallel geometry exhibits easier accumulation of water at the under-lands location, which induces detrimental local water condensation but limits the membrane dehydration with current. The larger pressure losses generated by the serpentine geometry allow less inhomogeneous water distribution at the channel/lands scale, lower mass-transport over-voltage and, thus, a more efficient use of the active surface. The cell electrochemical behavior results from the interplay between the hydration of the membrane and the repartition of water, in such a way that the parallel geometry exhibits better performances at high current density when the cell is managed at low temperature. The serpentine design shows better performances for a large number of temperature and current conditions, namely when the cell is operated at the standard t = 80 °C.

A data fusion approach to optimize compositional stability of halide perovskites

Summary Search for resource-efficient materials in vast compositional spaces is an outstanding challenge in creating environmentally stable perovskite semiconductors. We demonstrate a physics-constrained sequential learning framework to subsequently identify the most stable alloyed organic-inorganic perovskites. We fuse data from high-throughput degradation tests and first-principle calculations of phase thermodynamics into an end-to-end Bayesian optimization algorithm using probabilistic constraints. By sampling just 1.8% of the discretized CsxMAyFA1−x−yPbI3 (MA, methylammonium; FA, formamidinium) compositional space, perovskites centered at Cs0.17MA0.03FA0.80PbI3 show minimal optical change under increased temperature, moisture, and illumination with >17-fold stability improvement over MAPbI3. The thin films have 3-fold improved stability compared with state-of-the-art multi-halide Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3, translating into enhanced solar cell stability without compromising conversion efficiency. Synchrotron-based X-ray scattering validates the suppression of chemical decomposition and minority phase formation achieved using fewer elements and a maximum of 8% MA. We anticipate that this data fusion approach can be extended to guide materials discovery for a wide range of multinary systems.

Aerosol Spray Deposition of Liquid Metal and Elastomer Coatings for Rapid Processing of Stretchable Electronics.

We report a spray deposition technique for patterning liquid metal alloys to form stretchable conductors, which can then be encapsulated in silicone elastomers via the same spraying procedure. While spraying has been used previously to deposit many materials, including liquid metals, this work focuses on quantifying the spraying process and combining it with silicones. Spraying generates liquid metal microparticles (~5 μm diameter) that pass through openings in a stencil to produce traces with high resolution (~300 µm resolution using stencils from a craft cutter) on a substrate. The spraying produces sufficient kinetic energy (~14 m/s) to distort the particles on impact, which allows them to merge together. This merging process depends on both particle size and velocity. Particles of similar size do not merge when cast as a film. Likewise, smaller particles (<1 µm) moving at the same speed do not rupture on impact either, though calculations suggest that such particles could rupture at higher velocities. The liquid metal features can be encased by spraying uncured silicone elastomer from a volatile solvent to form a conformal coating that does not disrupt the liquid metal features during spraying. Alternating layers of liquid metal and elastomer may be patterned sequentially to build multilayer devices, such as soft and stretchable sensors.

Spatial organization of soil microaggregates

The physical arrangement of compounds in soil microaggregates is controlling many ecosystem functions such as soil stability and C sequestration. However, little is known about the spatial arrangement of organic and inorganic compounds in soil microaggregates, due to the lack of in-situ analyses of undisturbed material. We hypothesized that microaggregates are spatially organized due to interactions between organic matter and mineral phases. To test this, we separated the water stable, occluded, large and small microaggregate fractions (250–53 and 53–20 µm, 60 J ml−1 dispersion energy) from Ap horizons of a toposequence of sandy to loamy Luvisols (Germany) with increasing clay contents (19 to 35% clay), and subjected in total 60 individual aggregates to elemental mapping by electron probe micro analysis (EPMA), which recorded C, N, P, Al, Fe, Ca, K, Cl, and Si contents at 1 × 1 µm resolution. Stoichiometric element ratios characteristic for organic and inorganic matter were used to define discrete components of the aggregates and analyze their spatial arrangement. We found a pronounced heterogeneity in content and arrangement of discrete aggregate components, which was not reproducible for different specimens from the same soil microaggregate fraction, and thus largely independent of clay content in soil. However, nearest neighbor analyses revealed close spatial correlations between plant detritus (C:N app. 100:10) and microbial organic matter (C:N app. 10:1) indicating a spatial relationship between source and consumer. There was no systematic relationship between soil minerals and organic matter, suggesting that well-established macroscale correlations between contents of pedogenic oxides and clay minerals with soil organic matter storage do not apply to soil microaggregates.

Simultaneous and continuous particle separation and counting via localized DC-dielectrophoresis in a microfluidic chip.

A novel microfluidic method of counting the number of particles while they are separated by a localized DC-dielectrophoresis force was presented. Liquid flow from a wide microchannel forces the to-be-detected particles to pass over a small orifice on a side wall of the sample input channel. A direct current (DC) voltage applied across the small orifice and a strong electric field gradient is generated at the corners of the orifice for dielectrophoretic particle separation. Particle counting is achieved by detecting the electric current change caused by the being-separated particle near the sensing orifice. In this way, particles can be separate and in situ counted at the same time. Numerical simulations show that particle separation is achieved at the edge of the sensing orifice where the strength of the electric field gradient is the largest. Separation and counting of polystyrene particles of two and three different sizes with 1 μm resolution were demonstrated experimentally.

Real-Time High-Resolution MRI Endoscopy at up to 10 Frames per Second.

Objective. Atherosclerosis is a leading cause of mortality and morbidity. Optical endoscopy, ultrasound, and X-ray offer minimally invasive imaging assessments but have limited sensitivity for characterizing disease and therapeutic response. Magnetic resonance imaging (MRI) endoscopy is a newer idea employing tiny catheter-mounted detectors connected to the MRI scanner. It can see through vessel walls and provide soft-tissue sensitivity, but its slow imaging speed limits practical applications. Our goal is high-resolution MRI endoscopy with real-time imaging speeds comparable to existing modalities. Methods. Intravascular (3 mm) transmit-receive MRI endoscopes were fabricated for highly undersampled radial-projection MRI in a clinical 3-tesla MRI scanner. Iterative nonlinear reconstruction was accelerated using graphics processor units connected via a single ethernet cable to achieve true real-time endoscopy visualization at the scanner. MRI endoscopy was performed at 6-10 frames/sec and 200-300 μm resolution in human arterial specimens and porcine vessels ex vivo and in vivo and compared with fully sampled 0.3 frames/sec and three-dimensional reference scans using mutual information (MI) and structural similarity (3-SSIM) indices. Results. High-speed MRI endoscopy at 6-10 frames/sec was consistent with fully sampled MRI endoscopy and histology, with feasibility demonstrated in vivo in a large animal model. A 20-30-fold speed-up vs. 0.3 frames/sec reference scans came at a cost of ~7% in MI and ~45% in 3-SSIM, with reduced motion sensitivity. Conclusion. High-resolution MRI endoscopy can now be performed at frame rates comparable to those of X-ray and optical endoscopy and could provide an alternative to existing modalities, with MRI's advantages of soft-tissue sensitivity and lack of ionizing radiation.

A simple method defines 3D morphology and axon projections of filled neurons in a small CNS volume: Steps toward understanding functional network circuitry

BackgroundFundamental to understanding neuronal network function is defining neuron morphology, location, properties, and synaptic connectivity in the nervous system. A significant challenge is to reconstruct individual neuron morphology and connections at a whole CNS scale and bring together functional and anatomical data to understand the whole network. New MethodWe used a PC controlled micropositioner to hold a fixed whole mount of Xenopus tadpole CNS and replace the stage on a standard microscope. This allowed direct recording in 3D coordinates of features and axon projections of one or two neurons dye-filled during whole-cell recording to study synaptic connections. Neuron reconstructions were normalised relative to the ventral longitudinal axis of the nervous system. Coordinate data were stored as simple text files. ResultsReconstructions were at 1 μm resolution, capturing axon lengths in mm. The output files were converted to SWC format and visualised in 3D reconstruction software NeuRomantic. Coordinate data are tractable, allowing correction for histological artefacts. Through normalisation across multiple specimens we could infer features of network connectivity of mapped neurons of different types. Comparison with Existing MethodsUnlike other methods using fluorescent markers and utilising large-scale imaging, our method allows direct acquisition of 3D data on neurons whose properties and synaptic connections have been studied using whole-cell recording. ConclusionsThis method can be used to reconstruct neuron 3D morphology and follow axon projections in the CNS. After normalisation to a common CNS framework, inferences on network connectivity at a whole nervous system scale contribute to network modelling to understand CNS function.

Precision Medicine in Ossiculoplasty.

Long term results of ossiculoplasty surgery are considered poor with displacement and extrusion amongst the common reasons for failure. Application of 3Dimensional (3D) printing may help overcome some of these barriers, however digital methods to attain accurate 3D morphological studies of ossicular anatomy are lacking, exacerbated by the limitation of resolution of clinical imaging. 20 human cadaveric temporal bones were assessed using micro computed tomography (CT) imaging to demonstrate the lowest resolution required for accurate 3D reconstruction. The bones were then scanned using conebeam CT (125 μm) and helical CT (0.6 mm). 3D reconstruction using clinical imaging techniques with microCT imaging (40 μm resolution) as a reference was assessed. The incus was chosen as the focus of study. Two different methods of 3D printing techniques were assessed. A minimum resolution of 100 μm was needed for adequate 3D reconstruction of the ossicular chain. Conebeam CT gave the most accurate data on 3D analysis, producing the smallest mean variation in surface topography data relative to microCT (mean difference 0.037 mm, p < 0.001). Though the incus varied in shape in between people, paired matches were identical. Thus, the contralateral side can be used for 3D printing source data if the ipsilateral incus is missing. Laser based 3D printing was superior to extrusion based printing to achieve the resolution demands for 3D printed ossicles. Resolution of modern imaging allows 3D reconstructions and 3D printing of human ossicles with good accuracy, though it is important to pay attention to thresholding during this process.

Enhancement of oil recovery by emulsion injection: A pore scale analysis from X-ray micro-tomography measurements

The efficiency of emulsion injection as an enhanced oil recovery (EOR) method is investigated in a synthetic porous medium consisting of sintered bi-disperse glass beads, which emulates the porosity and permeability of real oil-bearing rocks. Synthetic seawater and an oil-in-water emulsion are successively used to displace a mineral oil which initially saturates the porous space. Micro-CT images with a 4 μm resolution are acquired at the end of the different stages of the process; the water phase is doped with KI to optimize the contrast between the liquid phases. Thus, three-dimensional (3D) images showing the beads, doped water and residual oil present a 3-modal histogram. After denoising with a non-local means filter, alignment and segmentation, the 3D images provide the spatial distributions of the water and oil phases, and thus allow comparing the populations of residual oil ganglia prior to and after the injection of the emulsion. Visual comparison of the spatial phase distributions show that the oil droplets of the oil-water emulsion divert the water path, mobilizing previously trapped oil ganglia. The probability density functions (PDFs) of different geometrical properties of the trapped oil ganglia (104 ganglia with volumes spanning 6 orders of magnitude) after water injection show well defined power law behaviors between a size corresponding to the typical pore throat and that typical of 10 typical pore volumes, and a few very large clusters of sizes between 10 and 5×105 typical pore volumes. The largest of them alone accounts for 97% of the trapped oil. The same PDFs after emulsion injection demonstrate successful fragmentation of these few very large ganglia, which in this case is the key to efficient EOR procedure through diversion of the water flow by emulsion oil droplets to less permeable regions.

MicroCT for Scanning and Analysis of Mouse Bones.

The purpose of this Chapter is to present a detailed description of methods for performing bone Micro-Computed Tomography (microCT) scanning and analysis. MicroCT is an x-ray imaging method capable of visualizing bone at the micro-structural scale, that is, 1-100 µm resolution. MicroCT is the gold-standard method for assessment of 3D bone morphology in studies of small animals. As applied to the small bones of mice or rats, microCT can efficiently and accurately assess bone structure (e.g., cortical bone area [Ct.Ar]) and micro-structure (e.g., trabecular bone volume fraction [Tb.BV/TV]). The particular application described herein is for post mortem mouse femur specimens. The material presented should be generally applicable to many commercially available laboratory microCT systems, although some details are specific to the system used in our lab (Scanco mCT 40; SCANCO Medical AG, Bruttisellen, Switzerland).

Electric field measurements in a He:N2 nanosecond pulsed discharge with sub-ns time resolution

The electric field in the He:N2 nanosecond atmospheric pressure plasma jet is studied using the electric-field induced second harmonic generation technique. It is shown that the calibration obtained with a DC voltage applied to the discharge cell may lead to incorrect results in the electric field measurements. It is proposed to use nanosecond high voltage pulses at low repetition rates for the calibration instead of a DC voltage. The temporal development of the electric field in the discharge at different distances from the cathode is measured with high temporal (100 ps) and spatial (50 µm) resolution. An electric field profile structure similar to the one in streamers or ionization fronts is observed. The velocity of the propagation of the falling edge of the ionization front is determined to be 0.85 × 106 m s−1. The validity of the local field approximation, important for modeling of these kind of discharges, is confirmed for the present conditions based on time and space derivatives of the measured electric field. The temporal evolution of the electron density is obtained by the measured electrical current and the time resolved electric field measurement combined with the electron mobility calculated with BOLSIG+.

A shape-switch-block method for confocal light-sheet microscopy with sectioned Bessel beams and stimulated emission depletion

Microscopy seeks to simultaneously maximize optical resolution, contrast, speed, volume size, and probe tolerability, which is possible by combining different complementary imaging techniques with their specific strengths. Here, we show how to combine three physical concepts to increase resolution and contrast in light-sheet microscopy by making the effective light-sheet thinner through phase shaping, fluorophores-switching, and dynamic blocking of fluorescence. This shape-switch-block principle is realized by illumination with two holographically shaped, sectioned Bessel beams. Second, by switching off fluorophores in the proximity of the excitation center using continuous-wave stimulated emission depletion (STED). And third, by blocking fluorescence outside the switching region by confocal line detection. Thereby, we reduce the light-sheet thickness by 35%, achieving an isotropic resolution with beads in a 300 × 70 × 50 µm³ volume. Without STED, we obtain 0.37 µm resolution in cell clusters at improved sectioning and penetration depth. The shape-switch-block concept promises high potential, also for other microscopy techniques.

Characterization of the Microstructure of the Cement/Casing Interface Using ESEM and Micro-CT Scan Techniques

Summary Integrity of the cement/casing interface is an essential element of establishing an effective barrier system in cased and cemented wellbores. Failure to establish and maintain an effective barrier can result in negative environmental and economic impacts, such as leakage of formation fluids into the environment or loss of production and costly remediation. The main focus of this paper is defined as the characterization of the cement/casing interface microstructure, which is critical for better understanding of the requirements for establishing effective zonal isolation and the long-term integrity of cemented wellbore sections of active wells, as well as the integrity of the abandoned wells. The primary objectives of this study were: first, to confirm the most suitable methods for preparing cement/casing samples and characterize the microstructure of the cement/casing interface (i.e., quantify the size of the microchannel due to possible debonding (gap) at the cement/casing interface and its progression along the wellbore axis); and second, to understand how different test methods can affect the microstructure, and once we have a firm baseline, analyze the effect of different cement compositions on the performance. More specifically, we have investigated effects of the cement composition and preparation, environmental conditions [i.e., relative humidity (RH) of the storage and testing conditions], cement shrinkage, and the expansion additive on the integrity of the cement/casing interface at the micrometer scale by using environmental scanning electron microscope (ESEM, 0.05 µm resolution) and microcomputed tomography (μ-CT, 11.92 µm resolution) scanning techniques. Cemented casing samples were prepared by using Class G and special abandonment cement blends with and without expansion additives and stainless-steel pipes. Results showed that any significant change in the RH of the environment during the cement preparation, curing, and testing process significantly affects the size of the gap at the cement/casing interface in test samples. Analyses of the ESEM images have shown that the 2D nonuniform gap size between the cement and the casing is inversely proportional to the change in the RH of the environment. Results suggested that cement slurries set and cured under downhole conditions with relatively constant RH may not undergo significant shrinkage and yield only minimal debonding effect. The 3D gap model reconstructed from the µ-CT images confirmed that the gap between cement and casing mostly occurred at the cement polished surface and the gap didn’t show any significant connectivity below the cement polished surface, indicating that common sample preparation methods can significantly affect the near-surface interface. Cement blends prepared with expansion additives have shown smaller gap size. The use of expansion additives enhances the cement/casing interface integrity by effectively reducing the gap size at the cement/casing interface. We conclude that as long as the RH of the environment does not change significantly, the cement is expected to undergo a limited shrinkage, and the gap between the cement and the casing may not induce any significant leakage pathway because the gap is only locally distributed at the cement polished surface without showing any significant connectivity along the wellbore axis.

Dynamic Neutron Imaging and Modeling of Cationic Impurities in Polymer Electrolyte Water Electrolyzer

The high operation and capital costs of polymer electrolyte water electrolyzers (PEWE) are the major obstacles that have to be tackled for hydrogen to penetrate the market as a solution for renewable energy storage. Commercial stacks often suffer from cationic contamination of catalyst-coated membranes (CCMs) that comes from impure feed water and corrosion of system components, which can result in increasing operation costs and lowered lifetime. This study describes the behavior of the contaminants in the CCMs and their impact on performance under various operating conditions using Gd3+ as model contaminant imaged with neutron with a combination of high effective temporal (2 s) and spatial (30 μm) resolutions. The presence of electric field directly affects the position of cations in the CCM leading to their accumulation near the cathode catalyst layer. The cationic impurities trigger multiple loss mechanisms, as the ohmic resistance increase does not scale linearly with the amount of occupied exchange groups in the membrane (10% resistance increase caused by 2.5% exchange groups occupation). A model has been developed that predicts the movement of the ions in the CCM under intermittently operating PEWE and was used as a basis to explain the hysteresis observed in the polarization curve of contaminated PEWEs.

Single-pulse laser ablation–inductively coupled plasma–mass spectrometry U–Pb dating of thin zircon rims: An application to metamorphic rocks from Mount Everest, eastern Nepal

This study describes an improved method for U–Pb dating of external zircon surfaces using laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS). Single-pulse LA of zircons mounted in Teflon by Excimer and femtosecond lasers produced age–depth profiles with 0.1 μm resolution. Application of this technique to samples from the Higher Himalayan Crystallines, Mount Everest region, eastern Nepal, revealed outer rims with ages of 15 ± 1 Ma. Most of these rims were not visible on cathodoluminescence images of polished cross-sections of the grains.

Development of control mechanisms for a laser wakefield accelerator-driven bremsstrahlung x-ray source for advanced radiographic imaging

A high power laser was used to accelerate electrons in a laser-driven wakefield accelerator. The high energy electrons were then used to generate an x-ray beam by passing them through a converter target. This bremsstrahlung source was characterised and used to perform penetrative imaging of industrially relevant samples. The photon spectrum had a critical energy in excess of 100 MeV and a source size smaller than the resolution of the diagnostic (150 µm). Simulations indicate a significantly smaller source is achievable. Variations in the x-ray source characteristics were realised through changes to the plasma and converter parameters while simulations confirm the adaptability of the source. Imaging of high areal density objects with 150 µm resolution was performed, demonstrating the unique advantages of this novel source.

Intermediate scale bandgap fluctuations in ultrathin Cu(In,Ga)Se2 absorber layers

Ultrathin single- and three-stage Cu(In,Ga)Se2 absorber layers were analyzed with room temperature photoluminescence (PL) spectra. An anomalous blueshift was observed upon increasing carrier injection for both samples. This blueshift was attributed to the presence of bandgap fluctuations that are of the same order as the minority carrier diffusion length. From time resolved measurements, a diffusion length of a few 100 nms was deduced. The single-stage spectrum consists of two peaks, and the sample was, therefore, also analyzed by hyperspectral imaging, providing lateral PL and reflectance data with 1 μm resolution. Marginal variations were observed in the PL yield and spectra. This homogeneity could again be attributed to an intermediate scale of the bandgap fluctuation with an upper limit of 1 μm for the scale of the lateral bandgap fluctuations. The two peaks in the PL spectra of the single-stage sample could be attributed to interference, and correction methods were applied. The bandgap fluctuations were extracted for the three-stage and single-stage sample and were 45 meV and 72 ± 3 meV, respectively. It is suggested that this difference is attributed to the smaller grains and larger amount of grain boundaries in the single-stage sample.