Grazing Incidence Reflection Research Articles

Multislice forward modeling of coherent surface scattering imaging on surface and interfacial structures.

To study nanostructures on substrates, surface-sensitive reflection-geometry scattering techniques such as grazing incident small angle X-ray scattering are commonly used to yield an averaged statistical structural information of the surface sample. Grazing incidence geometry can probe the absolute three-dimensional structural morphology of the sample if a highly coherent beam is used. Coherent surface scattering imaging (CSSI) is a powerful yet non-invasive technique similar to coherent X-ray diffractive imaging (CDI) but performed at small angles and grazing-incidence reflection geometry. A challenge with CSSI is that conventional CDI reconstruction techniques cannot be directly applied to CSSI because the Fourier-transform-based forward models cannot reproduce the dynamical scattering phenomenon near the critical angle of total external reflection of the substrate-supported samples. To overcome this challenge, we have developed a multislice forward model which can successfully simulate the dynamical or multi-beam scattering generated from surface structures and the underlying substrate. The forward model is also demonstrated to be able to reconstruct an elongated 3D pattern from a single shot scattering image in the CSSI geometry through fast-performing CUDA-assisted PyTorch optimization with automatic differentiation.

A Search for the 3.5 keV Line from the Milky Way’s Dark Matter Halo with HaloSat

Abstract Previous detections of an X-ray emission line near 3.5 keV in galaxy clusters and other dark-matter-dominated objects have been interpreted as observational evidence for the decay of sterile neutrino dark matter. Motivated by this, we report on a search for a 3.5 keV emission line from the Milky Way’s galactic dark matter halo with HaloSat. As a single pixel, collimated instrument, HaloSat observations are impervious to potential systematic effects due to grazing incidence reflection and CCD pixelization, and thus may offer a check on possible instrumental systematic errors in previous analyses. We report nondetections of a ∼3.5 keV emission line in four HaloSat observations near the Galactic center. In the context of the sterile neutrino decay interpretation of the putative line feature, we provide 90% confidence level upper limits on the 3.5 keV line flux for a field centered 18.6 degrees from the Galactic center and the corresponding 7.1 keV sterile neutrino mixing angle: F ≤ 0.077 ph cm−2 s−1 sr−1 and sin 2 ( 2 θ ) ≤ 4.25 × 10 − 11 . The HaloSat mixing angle upper limit was calculated using a modern parameterization of the Milky Way’s dark matter distribution, and in order to compare with previous limits, we also report the limit calculated using a common historical model. The HaloSat mixing angle upper limit places constraints on a number of previous mixing angle estimates derived from observations of the Milky Way’s dark matter halo and galaxy clusters, and excludes several previous detections of the line. The upper limits cannot, however, entirely rule out the sterile neutrino decay interpretation of the 3.5 keV line feature.

Supersymmetry, half-bound states, and grazing incidence reflection

Electromagnetic waves at grazing incidence onto a planar medium are analogous to zero energy quantum particles incident onto a potential well. In this limit waves are typically completely reflected. Here we explore dielectric profiles supporting optical analogues of ‘half–bound states’, allowing for zero reflection at grazing incidence. To obtain these profiles we use two different theoretical approaches: supersymmetric quantum mechanics, and direct inversion of the Helmholtz equation, showing that discretized approximations to these profiles exhibit low reflectivity close to grazing incidence.

Characterization of Dislocations in 6H-SiC Wafer Through X-Ray Topography and Ray-Tracing Simulations

Silicon carbide (SiC) is an important semiconductor material for a variety of electronic and optoelectronic applications owing to the unique combination of its superior electronic and physical properties. In order to continuously improve the crystal quality and improve device performance, obtaining a clear understanding of the defect types and their distribution and potential influence on device operation is of great importance. In this study, 6H-SiC crystals grown by physical vapor transport (PVT) have been characterized by synchrotron monochromatic beam x-ray topography (SMBXT). By recording six different $$11{\bar{2}},12$$ grazing incidence reflections and analyzing the contrast patterns of threading screw dislocations (TSDs), threading edge dislocations (TEDs), threading mixed dislocations (TMDs), and basal plane dislocations (BPDs) observed in conjunction with ray-tracing simulation, the Burgers vectors of these dislocations have been determined. This successfully demonstrated a direct Burgers vector determination approach for each type of dislocation. Understanding these dislocation types and their distributions in 6H-SiC wafers can provide crucial feedback for pursuing crystal quality enhancement during growth process. High-resolution x-ray diffraction (HRXRD) has been performed on the wafer to carry out the rocking curve analysis of areas with different degrees of lattice distortion.

Tailoring electric field standing waves in reflection–absorption infrared spectroscopy to enhance absorbance from adsorbates on ice surfaces

The spectroscopic detection of molecules adsorbed onto ice surfaces at coverages similar to those encountered under typical environmental conditions requires high surface selectivity and sensitivity that few techniques can afford. An experimental methodology allowing a significant enhancement in the absorbance from adsorbed molecules is demonstrated herein. It exploits Electric Field Standing Wave (EFSW) effects intrinsic to grazing incidence Reflection-Absorption Infrared (RAIR) spectroscopy, where film thickness dependent optical interferences occur between the multiple reflections of the IR beam at the film-vacuum and the substrate-film interfaces. In this case study, CH4 is used as a probe molecule and is deposited on a 20 ML coverage dense amorphous solid water film adsorbed onto solid Ar underlayers of various thicknesses. We observe that, at thicknesses where destructive interferences coincide with the absorption features from the CH stretching and HCH bending vibrational modes of methane, their intensity increases by a factor ranging from 10 to 25. Simulations of the RAIR spectra of the composite stratified films using a classical optics model reproduce the Ar underlayer coverage dependent enhancements of the absorbance features from CH4 adsorbed onto the ice surface. They also reveal that the enhancements occur when the square modulus of the total electric field at the film's surface reaches its minimum value. Exploiting the EFSW effect allows the limit of detection to be reduced to a coverage of (0.2 ± 0.2) ML CH4, which opens up interesting perspectives for spectroscopic studies of heterogeneous atmospheric chemistry at coverages that are more representative of those found in the natural environment.

Single-channel method for measuring the reflectance spectra of grazing incidence mirrors in the extreme ultraviolet range.

This Letter presents a one-channel method of measurements of a grazing incidence reflection coefficient. The idea is to use the quasi-flat field extreme ultraviolet spectrometer to produce direct and reflected from the sample images of the spectral lines simultaneously at the same detector matrix. The analysis of a set of spectral lines in the working spectral range of the spectrometer gives the spectral dependence of the reflection coefficient. The results of the refection coefficient measurements for a plane ruthenium mirror for grazing angles of 6° and 10° in the spectral range 15-40nm are presented.

Suppression of higher diffraction orders in the extreme ultraviolet with a trapezoidal nano-mirror array

ABSTRACTHigher diffraction orders from a grating introduce harmonic contamination in ‘monochromatic’ output beams processed by grating monochromators at synchrotron radiation facilities, resulting in imprecise calibration of optical elements and detectors. Suppressing these orders can be achieved with a quasi-random nano-hole array, but its fabrication can be a significant limiting factor for implementation. Here, an advanced type of grating is demonstrated that contains a periodic array of several hundred million trapezoidal nano-mirrors with a grazing-incidence reflection geometry that suppresses higher diffraction orders. Moreover, it reduces the difficulty of fabrication to the level of a traditional grating. It has great potential for harmonic suppression in synchrotron radiation, spectral diagnostics of plasmas, and astrophysics.

Multislice does it all-calculating the performance of nanofocusing X-ray optics.

We describe an approach to calculating the optical performance of a wide range of nanofocusing X-ray optics using multislice scalar wave propagation with a complex X-ray refractive index. This approach produces results indistinguishable from methods such as coupled wave theory, and it allows one to reproduce other X-ray optical phenomena such as grazing incidence reflectivity where the direction of energy flow is changed significantly. Just as finite element analysis methods allow engineers to compute the thermal and mechanical responses of arbitrary structures too complex to model by analytical approaches, multislice propagation can be used to understand the properties of the real-world optics of finite extent and with local imperfections, allowing one to better understand the limits to nanoscale X-ray imaging.

Optimized Model Surfaces for Advanced Atomic Force Microscopy Studies of Surface Nanobubbles.

The formation of self-assembled monolayers (SAMs) of binary mixtures of 16-mercaptohexadecanoic acid (MHDA) and 1-octadecanethiol (ODT) on ultraflat template-stripped gold (TSG) surfaces was systematically investigated to clarify the assembly behavior, composition, and degree of possible phase segregation in light of atomic force microscopy (AFM) studies of surface nanobubbles on these substrates. The data for SAMs on TSG were compared to those obtained by adsorption on rough evaporated gold, as reported in a previous study. Quartz crystal microbalance and surface plasmon resonance data acquired in situ on TSG indicate that similar to SAM formation on conventional evaporated gold substrates ODT and MHDA form monolayers and bilayers, respectively. The second layer on MHDA, whose formation is attributed to hydrogen bonding, can be easily removed by adequate rinsing with water. The favorable agreement of the grazing incidence reflection Fourier transform infrared (GIR FTIR) spectroscopy and contact angle data analyzed with the Israelachvili-Gee model suggests that the binary SAMs do not segregate laterally. This conclusion is fully validated by high-resolution friction force AFM observations down to a length scale of 8-10 nm, which is much smaller than the typical observed surface nanobubble radii. Finally, correspondingly functionalized TSG substrates are shown to be valuable supports for studying surface nanobubbles by AFM in water and for addressing the relation between surface functionality and nanobubble formation and properties.

Design and test of a broadband split-and-delay unit for attosecond XUV-XUV pump-probe experiments.

We present the design of a split-and-delay unit for the production of two delayed replicas of an incident extreme ultraviolet (XUV) pulse. The device features a single grazing incidence reflection in combination with attenuation of remaining infrared light co-propagating with the XUV beam, offering a high throughput without the need of introducing additional optics that would further decrease the XUV flux. To achieve the required spatial and temporal stabilities, the device is controlled by two PID-controllers monitoring the delay and the beam pointing using an optical reference laser beam, making collimation of the beam by additional optics unnecessary. Finally, we demonstrate the stability of the split-and-delay unit by performing all-reflective autocorrelation measurements on broadband few-cycle laser pulses.

Structural characterization and low-temperature properties of Ru/C multilayer monochromators with different periodic thicknesses.

Ru/C multilayer monochromators with different periodic thicknesses were investigated using X-ray grazing-incidence reflectivity, diffuse scattering, Bragg imaging, morphology testing, etc. before and after cryogenic cooling. Quantitative analyses enabled the determination of the key multilayer structural parameters for samples with different periodic thicknesses, especially the influence from the ruthenium crystallization. The results also reveal that the basic structures and reflection performance keep stable after cryogenic cooling. The low-temperature treatment smoothed the surfaces and interfaces and changed the growth characteristic to a low-frequency surface figure. This study helps with the understanding of the structure evolution of multilayer monochromators during cryogenic cooling and presents sufficient experimental proof for using cryogenically cooled multilayer monochromators in a high-thermal-load undulator beamline.

Investigation of defects in sputtered W/B4C multilayers

Sputtered W/B4C multilayers were determined by X-ray photoelectron spectroscopy, Raman scattering spectroscopy, scanning electron microscopy and atomic force microscopy synthetically. Two defect modes were observed in multilayers: buckle delamination and oxidation. This paper compares the chemical composition varies along multilayer depth and at different regions and tries to interpret the mechanism of defect evolution. The X-ray grazing incidence reflection profiles were compared to the theoretical value to estimate the influences from different defects.

Thickness dependence of the porosity of PPy/DDS films

Polypyrrole (PPy) films doped with large, low mobile ions (dodecylsulfate – DDS) were galvanostatically electrosynthesized. The effect of film thickness on its porosity was determined by a grazing incidence reflection small-angle X-ray scattering (GISAXS) technique. The Particle-/Pore-size analysis package of NANO-Solver v3.6 (Rigaku™) software was employed to quantify the volume distribution function of the pore sizes. The dopant concentration of the PPy/DDS films was determined by an Energy Dispersive X-ray microanalysis (EDX), with the morphology of these films investigated with atomic force microscopy (AFM). Our experimental results showed that the film thickness had quite a significant effect on the roughness and porosity of PPy film. The thicker PPy/DDS films were found to be significantly rougher than the thinner ones. Concerning porosity, whereas the 0.04μm thick film possesses a predominantly microporous structure (≈57% of total porosity), the thicker films (0.2μm and 1μm) are mainly mesoporous (≈75%), or in the case of 5μm thick film, almost entirely (≈98%) mesoporous. The GISAXS analysis showed that the average pore diameter (dAV) was smallest for the thinnest (0.04μm) film – 2.3nm. It increased slightly for thicknesses ranging from 0.2 to 1μm – a few under 3nm, but increased significantly for the thickest (5μm) film – 7.9nm. The results were analyzed on the basis of the correlation between PPy film thickness and its doping level, as well as its density.

Conception of broadband stigmatic high-resolution spectrometers for the soft X-ray range

We formulate an approach to the development of stigmatic high-resolution spectral instruments for the soft X-ray range , which is based on the combined operation of normal-incidence multilayer mirrors (including broadband aperiodic ones) and grazing-incidence reflection gratings with nonequidistant grooves (so-called VLS gratings). A concave multilayer mirror serves to produce a slightly astigmatic image of the radiation source (for instance, an entrance slit), and the diffraction grating produces a set of its dispersed stigmatic spectral images. The width of the operating spectral region is determined by the aperiodic structure of the multilayer mirror and may range up to an octave in wavelength.

Wave propagation of induced radiation in microcapillary holes of a glass microchannel plate

Soft x-ray reflection and transmission spectra of radiation propagating inside microchannel plates have been analyzed in the framework of the wave approximation. X-ray and fluorescence yields due to the grazing incidence reflection phenomenon have been investigated with respect to the geometry of the channel walls. We also discuss x-ray absorption near-edge structure spectra collected at the exit of microcapillaries under the total x-ray reflection condition, taking into account a surface transition layer at the sample surface. The fine structures and the angular distribution of the transmitted radiation have been interpreted in the energy range of the anomalous dispersion of the Si L2,3 edges.

Abnormal anti-Stokes Raman scattering and surface-enhanced infrared absorption spectroscopy studies of carbon nanotubes electrochemically functionalized with poly(2,2′-bithiophene-co-pyrene)

Abstract A new composite material was synthesized via the electrochemical oxidation of 2,2′-bithiophene (BTh) and pyrene (Py) monomers dissolved in a solution of LiClO 4 in CH 3 CN onto a working electrode of Au coated with a film of single-walled carbon nanotubes (SWNTs). The interaction of the poly(2,2′-bithiophene-co-pyrene) (PBTh–Py) with the SWNTs and the orientation of the copolymer on the working electrode were studied by anti-Stokes and Stokes Raman scattering and FTIR spectroscopy in the grazing-incidence angle reflection geometry. The Raman lines at 1464 and 1435 cm −1 , attributed to the symmetric stretching modes of the C C bond in the quinoid and aromatic rings of the PBTh–Py copolymer, were observed when the working electrode is either a blank Au film or a Au support coated with a SWNT film. In the latter case, charge transfer occurs at the interface of the two constituents. This charge transfer leads to functionalization of the carbon nanotubes with the copolymer molecules. The presence of enhanced anti-Stokes Raman lines at −1461, −1435 and −1187 cm −1 indicates a resonant optical excitation process of the PBTh–Py/SWNT composite. The FTIR spectra acquired under polarized light for the PBTh–Py copolymer electrosynthesized on a bare Au support reveal significant changes in the absorption bands situated in the spectral ranges of 1000–1150 and 1550–1650 cm −1 that originate in the surface-enhanced IR absorption (SEIRA) processes. The functionalization of the SWNTs with the PBTh–Py copolymer is evidenced by the enhancement of the absorption of the FTIR bands at 793 and 846 cm −1 , which are assigned to the C S C deformation vibrational mode and the substituted benzene ring, respectively, due to the effects of steric hindrance that are induced for the copolymer molecules by the binding of carbon nanotubes.

Analyzing the Molecular Orientation of Ultrathin Organic Films by Polarized Transmission and Grazing Incidence Reflection Absorption IR Spectroscopy

Analyzing the Molecular Orientation of Ultrathin Organic Films by Polarized Transmission and Grazing Incidence Reflection Absorption IR Spectroscopy

Interface characterization of B4C-based multilayers by X-ray grazing-incidence reflectivity and diffuse scattering

B4C-based multilayers have important applications for soft to hard X-rays. In this paper, X-ray grazing-incidence reflectivity and diffuse scattering, combining various analysis methods, were used to characterize the structure of B4C-based multilayers including layer thickness, density, interfacial roughness, interdiffusion, correlation length, etc. Quantitative results for W/B4C, Mo/B4C and La/B4C multilayers were compared. W/B4C multilayers show the sharpest interfaces and most stable structures. The roughness replications of La/B4C and Mo/B4C multilayers are not strong, and oxidations and structure expansions are found in the aging process. This work provides guidance for future fabrication and characterization of B4C-based multilayers.

Covalently cross-linked poly(acrylamide) brushes on gold with tunable mechanical properties via surface-initiated atom transfer radical polymerization

Versatile surface coatings, which exhibit anti-fouling properties and allow one to tune surface functionality and the state of internal (macro)molecular organization, are of central importance as platforms for the study of cell–surface interactions. To enable studies of various cell characteristics, such including cell migration, proliferation and adhesion, we investigated how the mechanical properties of poly(acrylamide) (PAAm) brushes on gold can be controlled by the varying the cross-link density. Brushes and covalently cross-linked hydrogel brushes were synthesized using self-assembled monolayers (SAMs) of ω-mercaptoundecyl bromoisobutyrate (MUBiB) on gold to initiate the surface-initiated atom transfer radical polymerization (SI-ATRP) of AAm with different fractions of (N,N′-methylenebis(acrylamide)) bisAAm in the feed. The polymer layers prepared with varied polymerization times or cross-linking densities were characterized by water contact angle measurements, grazing incidence reflection FTIR spectroscopy, ellipsometry and atomic force microscopy (AFM). It was found that during PAAm brush synthesis the film thickness increased markedly in early stages of the reaction, whereas after 60min a plateau of approximately 50nm (dry) film thickness was reached. The introduction of bisAAm into the feed resulted in reduced film thicknesses. AFM force–displacement measurements revealed that the variation of cross-linking density of polymer brush films influenced both the surface morphology and the films’ nanomechanical properties. The stiffness of the polymer brush films increased with increasing fraction of bisAAm, while the swelling ratio decreased. Hence the elastic modulus of the cross-linked PAAm brushes, as determined by AFM nanoindentation, could be varied between 20 and 700kPa, which will find application in the study of cell–surface interactions.

Conversion of Basal Plane Dislocations to Threading Edge Dislocations by Annealing 4H-SiC Epilayers at High Temperatures

Basal plane dislocations (BPDs) converting to threading edge dislocations (TEDs) has been observed in 4H-SiC epilayers after thermal annealing at high temperatures. Grazing incidence reflection synchrotron X-ray topography was used to investigate the dislocation behaviors. It is argued that the conversion is achieved by constricted BPD segments cross-slipping to the prismatic plane and TED glide on its slip plane. Higher conversion ratio and better surface morphology were achieved by performing ion implantation and annealing before epitaxial growth.