Large‐area,polarisation‐sensitive plasmonic materials from colloidal lithography

The development of simple to prepare, polarization‐sensitive plasmonic apertures with two plasmonic modes, is described. To achieve these results, monocrystalline nanosphere lithography masks of 438 nm polystyrene spheres are modified with reactive ion etching before silver is subsequently evaporated through the mask at varied angles. As the angle of evaporation increases, round apertures, elliptical apertures or lines with bow‐tie like features between two lines are produced. A primary plasmon mode is shown at 570 nm, while a tunable plasmon mode is demonstrated between 700 nm and 900 nm. Finite‐difference time‐domain calculations agree with the observed results and predict that this method of fabrication can produce tunable plasmonic features throughout the NIR optical telecommunication wavelength range. Lastly, the excitation polarization angle is compared with that of plasmonic nanorods and asymmetric nano‐apertures systems to describe why the excitation polarization of the low energy mode is orthogonal to the long axis of the apertures. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Formation of three‐dimensional GaAs microstructures by combination of wet and metal‐assisted chemical etching

Previously, plasma‐enhanced dry etching has been used to generate three‐dimensional GaAs semiconductor structures, however, dry etching induces surface damages that degrade optical properties. Here, we demonstrate the fabrication method forming various types of GaAs microstructures through the combination etching process using the wet‐chemical solution. In this method, a gold (Au)‐pattern is employed as an etching mask to facilitate not only the typical wet etching but also the metal‐assisted chemical etching (MacEtch). High‐aspect‐ratio, tapered GaAs micropillars are produced by using [HF]:[H₂O₂]:[EtOH] as an etching solution, and their taper angle can be tuned by changing the molar ratio of the etching solution. In addition, GaAs microholes are formed when UV light is illuminated during the etching process. Since the wet etching process is free of the surface damage compared to the dry etching process, the GaAs microstructures demonstrated to be well formed here are promising for the applications of III–V optoelectronic devices such as solar cells, laser diodes, and photonic crystal devices. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of surface topology of printed nanoparticle layers using wide‐angle low‐Q scattering

A new small‐angle scattering technique in reflection geometry is described which enables a topological study of rough surfaces. This is achieved by using long‐wavelength soft X‐rays which are scattered at wide angles but in the low‐Q range normally associated with small‐angle scattering. The use of nanometre‐wavelength radiation restricts the penetration to a thin surface layer which follows the topology of the surface, while moving the scattered beam to wider angles preventing shadowing by the surface features. The technique is, however, only applicable to rough surfaces for which there is no specular reflection, so that only the scattered beam was detected by the detector. As an example, a study of the surfaces of rough layers of silicon produced by the deposition of nanoparticles by blade‐coating is presented. The surfaces of the blade‐coated layers have rough features of the order of several micrometers. Using 2 nm and 13 nm X‐rays scattered at angular ranges of 5°≤θ≤ 51° and 5°≤θ≤ 45°, respectively, a combined range of scattering vector of 0.00842 Å^?1≤Q≤ 0.4883 Å^?1 was obtained. Comparison with previous transmission SAXS and USAXS studies of the same materials indicates that the new method does probe the surface topology rather than the internal microstructure.     

Improvement of reproducibility in grazing‐exit EPMA (GE‐EPMA)

In grazing exit electron probe microanalysis (GE‐EPMA), characteristic x‐rays emitted from only near surface regions of a specimen are detected at extremely low exit angles near 0°. Therefore, GE‐EPMA is useful for localized surface analysis. However, there is a practical problem with GE‐EPMA, namely, reproducibility of angle adjustment. Therefore, we developed a new instrument, a ‘laser beam and four‐separated photodetector system’, to adjust the sample inclination. It was found that the reproducibility of angle adjustment was improved by about one‐tenth by applying this system. Copyright © 2005 John Wiley & Sons, Ltd.     

A three‐crystal spectrometer for high‐energy resolution fluorescence‐detected X‐ray absorption spectroscopy and X‐ray emission spectroscopy at SSRF

A Johann‐type spectrometer for the study of high‐energy resolution fluorescence‐detected X‐ray absorption spectroscopy, X‐ray emission spectroscopy and resonant inelastic X‐ray scattering has been developed at BL14W1 X‐ray absorption fine structure spectroscopy beamline of Shanghai Synchrotron Radiation Facility. The spectrometer consists of three crystal analyzers mounted on a vertical motion stage. The instrument is scanned vertically and covers the Bragg angle range of 71.5–88°. The energy resolution of the spectrometer ranges from sub‐eV to a few eV. The spectrometer has a solid angle of about 1.87 × 0^?3 of 4π sr, and the overall photons acquired by the detector could be 10⁵ counts per second for the standard sample. The performances of the spectrometer are illustrated by the three experiments that are difficult to perform with the conventional absorption or emission spectroscopy. Copyright © 2016 John Wiley & Sons, Ltd.     

Using an in‐vacuum CCD detector for simultaneous small‐ and wide‐angle scattering at beamline X9

The implementation of simultaneous small‐ and wide‐angle X‐ray scattering at beamline X9 of the National Synchrotron Light Source is described. By utilizing an in‐vacuum CCD detector with a truncated cone‐shaped head and positioned at ～20° off‐axis from the direct beam, the overlap of the scattering angle coverage between the wide‐angle detector and the conventional small‐angle detector is maximized. The combined q‐range for measurements in transmission geometry is typically 0.006–2.0 Å^?1 at 13.5 keV, with overlapping data within the range ～0.1–0.2 Å^?1. Simultaneous data collection can also be performed in grazing‐incident measurements of flat substrate‐supported samples, in which case the wide‐angle detector can collect the scattering data along the sample normal as well as parallel to the sample surface without blocking the direct beam. Data processing and correction procedures will be discussed and examples presented.     

Can the HVL help the mechanical X‐ray tube characterization?

High‐intensity X‐ray beams are usually characterized by their kVp (kilovoltage peak) value and half‐value layer (HVL). While the first parameter is reasonably well known (apart from accelerating potential fluctuations), on the second, there is a greater deal of uncertainty. The HVL depends on the used filtration, the effective kVp value and on some of the X‐ray tube mechanical features, such as the anode angle. This last parameter is not always provided by the tube manufacturer, so we may question if the HVL dependence on the anode angle can be used to extract information on this angle. We tried to give an answer to this question using two different numerical models and a full Monte Carlo (MC) program to simulate the photon field produced by the X‐ray tube for several anode angles. One of the numerical models was developed by the Institute of Physics and Engineering in Medicine and gives X‐ray spectra and HVL values for a wide range of kVp values and anode angles. The other model, named SpekCalc, is based on a theoretical work developed by Gavin Poludniowski and Phil Evans. The MC simulation was done using the PENELOPE code for coupled electron‐photon transport. Using the computed photon spectra, HVLs were obtained and compared with experimental HVL values obtained with a Philips PW 2184/00 X‐ray tube with a 26° tungsten anode and accelerating potentials in the range of 40–90 kVp. We are now able to show the PENELOPE simulation can deliver the correct anode angle value. Copyright © 2011 John Wiley & Sons, Ltd.     

Silicon nitride transmission X‐ray mirrors

Transmission X‐ray mirrors have been fabricated from 300–400 nm‐thick low‐stress silicon nitride windows of size 0.6 mm × 85 mm. The windows act as a high‐pass energy filter at grazing incidence in an X‐ray beam for the beam transmitted through the window. The energy cut‐off can be selected by adjusting the incidence angle of the transmission mirror, because the energy cut‐off is a function of the angle of the window with respect to the beam. With the transmission mirror at the target angle of 0.22°, a 0.3 mm × 0.3 mm X‐ray beam was allowed to pass through the mirror with a cut‐off energy of 10 keV at the Cornell High Energy Synchrotron Source. The energy cut‐off can be adjusted from 8 to 12 keV at an angle of 0.26° to 0.18°, respectively. The observed mirror transmittance was above 80% for a 300 nm‐thick film.     

Selective formation of silicon nanowires on pre-patterned substrates

In this paper, the selective growth of silicon nanowires (SiNWs) was studied. With the aid of photolithography, the vertically aligned silicon nanowires were selectively formed on the patterned substrates via an electroless metal deposition (EMD) method under normal conditions (room temperature, 1 atm). Low-pressure chemical vapor deposition (LPCVD) silicon nitride was used as the masking layer for SiNWs preparation. The scanning electron microscope was used to examine the etching results. Both the patterned and the unpatterned silicon substrates were used for study. The results indicated that the growth rates of the SiNWs upon the patterned and the unpatterned substrates are different. For the patterned substrates, the growth rate of SiNWs is dependent upon the pattern shape. The influence of length-to-width ratio for the rectangular-shaped patterns was studied. It is concluded that by designing the proper length-to-width ratio, the nanowires with different lengths can be fabricated simultaneously on the same substrate.     

Transparent and Superamphiphobic Surfaces from One‐Step Spray Coating of Stringed Silica Nanoparticle/Sol Solutions

Transparent and superamphiphobic coatings that repel both water and low‐surface‐tension oil offer energy and environmental benefits to building windows, solar cell panels, electronic displays, and other optical equipment. Here, we developed a solution consisting of stringed amphiphilic silica nanoparticles (NPs) and an amphiphilic sol, which could be spray coated onto a variety of planar and curved substrates in one step. High transparency, water/oil contact angles greater than 150° and roll‐off angles less than 10° were demonstrated. The stringed NPs formed a fractal‐like nanoporous network, while the sol acted as a surface modifier and binder to enhance the coating robustness against heating (up to 400 °C), water jetting, and sand abrasion.     

The effect of differently sized Ag catalysts on the fabrication of a silicon nanowire array using Ag-assisted electroless etching

A simple and low cost method to generate single-crystalline, well-aligned silicon nanowires (SiNWs) of large area, using Ag-assisted electroless etching, is presented and the effect of differently sized Ag catalysts on the fabrication of SiNWs arrays is investigated. The experimental results show that the size of the Ag catalysts can be controlled by adjusting the pre-deposition time in the AgNO₃/HF solution. The optimum pre-deposition time for the fabrication of a SiNWs array is 3 min (about 162.04 ± 38.53 nm Ag catalyst size). Ag catalysts with smaller sizes were formed in a shorter pre-deposition time (0.5 min), which induced the formation of silicon holes. In contrast, a large amount of Ag dendrites were formed on the silicon substrate, after a longer pre-deposition time (4 min). The existence of these Ag dendrites is disadvantageous to the fabrication of SiNWs. Therefore, a proper pre-deposition time for the Ag catalyst is beneficial to the formation of SiNWs.SiNWs were synthesized in the H₂O₂/HF solution system for different periods of time, using Ag-assisted electroless etching (pre-deposition of the Ag catalyst for 3 min). The length of the SiNWs increases linearly with immersion time. From TEM, SAED and HRTEM analysis, the axial orientation of the SiNWs is identified to be along the [001] direction, which is the same as that of the initial Si wafer. The use of HF may induce Si–H_x bonds onto the SiNW array surface. Overall, the Ag-assisted electroless etching technique has advantages, such as low temperature, operation without the need for high energy and the lack of a need for catalysts or dopants.     

Wettability of Polyethylene Micropatterns with Aligned One-Dimensional Nanostructures

To analyze the influence of one-dimensional nanostructures on the wettabilities of polymer micropatterned surfaces, water contact angles of polyethylene micropatterned surfaces with aligned nanofibers and nanotubes were examined systematically. The results indicate that polyethylene micropatterned surfaces with aligned nanofibers and nanotubes exhibit hydrophobicity with the water contact angle ranging from 136° to 161°. Aligned nanotubes micropatterns with either honeycomb-like or labyrinth-like topographies exhibit higher hydrophobicity than those with aligned nanofibers when the topographies are similar. Results also indicate honeycomb-like micropatterns show higher hydrophobicities than the labyrinth-like micropatterns composed of the same one-dimensional nanostructures.     

Characteristics of Friedel pairs and diffraction contrast tomography with non‐perpendicular rotation axis

The characteristics of Friedel pairs in diffraction contrast tomography (DCT) are studied in the condition that the rotation axis of the sample is not exactly perpendicular to the incident X‐ray direction. For the rotation axis approximately aligned along the vertical direction, the Friedel pairs close to the horizontal plane are insensitive to the non‐perpendicularity of the rotation axis, and can be used to refine the sample‐to‐detector distance and X‐ray energy, while the Friedel pairs close to the vertical direction are sensitive to the non‐perpendicularity of the rotation axis, and can be used to determine the rotation axis orientation. The correct matching proportion of Friedel pairs decreases with increasing non‐perpendicularity of the rotation axis. A method of data processing considering rotation axis misalignment is proposed, which significantly increases the correct matching and indexing proportions of the diffraction spots. A pure aluminium polycrystalline sample is investigated using DCT at beamline 4W1A of Beijing Synchrotron Radiation Facility. Based on the analysis of Friedel pairs, the sample‐to‐detector distance and X‐ray energy are refined to be 8.67 mm and 20.04 keV, respectively. The non‐perpendicular angle of the rotation axis is calculated to be 0.10°. With these refined geometric parameters, the matching proportion of the spatial position of diffraction spots is 90.62%. Three‐dimensional reconstruction of the sample with 13 grains is realised using the algebraic reconstruction technique. It is demonstrated that the precise correction of the orientation of the sample rotation axis is effective in DCT suffering from rotation axis misalignment, and the higher accuracy in determining the rotation axis is expected to improve the reconstruction precision of grains.     

A theoretical practice on grazing‐exit energy dispersive X‐ray spectroscopy as a surface analysis strategy to investigate BiVO4 nano‐films

In the current work, a thin film of bismuth vanadate was defined over a silicon substrate, and a calculative Monte Carlo approach was followed to achieve the best grazing‐exit angle to acquire compositional data from top few nanometers of surface. This strategy is very beneficial in order to increase X‐ray signals originated from surface and diminish the background X‐ray signals started off from the substrate. In this regard, grazing‐exit energy dispersive X‐ray spectroscopy can be considered as an accessible and economical analytical tool to investigate thin films and nano‐layers. The major advantage of this method is that just by applying a re‐arrangement in a scanning electron microscope, it can be used to study compositional properties of thin layers. In this contribution, a theoretical approach using Monte Carlo models was used to simulate the behavior of electron beams impinging onto BiVO₄ nano‐layers with thickness of 50 nm and electron trajectories inside the film. Characteristic X‐rays and spatial energy distribution of the backscattered electrons were also calculated. Under grazing‐exit angle of around 0.5°, the best surface signal/background noise ratio was achieved. Copyright © 2014 John Wiley & Sons, Ltd.     

Interference phenomena of synchrotron radiation in TEY spectra for silicon‐on‐insulator structure

The general matrix theory of the photoelectron/fluorescence excitation in anisotropic multilayer films at the total reflection condition of X‐rays has been developed. In a particular case the theory has been applied to explain the oscillation structure of L_2,3 XANES spectra for a SiO₂/Si/SiO₂/c‐Si sample in the pre‐edge region which has been observed by a sample current technique at glancing angles of synchrotron radiation. Remarkably the phase of the oscillations is reversed by a ～2° angle variation. The observed spectral features are found to be a consequence of waveguide mode creation in the middle layer of strained Si, which changes the radiation field amplitude in the top SiO₂ layer. The fit of the data required the correction of the optical constants for Si and SiO₂ near the Si L_2,3‐edges.     

Inelastic and elastic scattering differential cross‐sections of 59.5 keV photons for Cu and Zn targets

Differential cross‐sections for incoherent and coherent scattering of 59.54 keV photons were measured for Cu and Zn targets at scattering angles of 40–135° in a reflection geometry setup with a graded shielding arrangement. The ratio of these cross‐sections, which can be determined with higher precision than absolute values, is given. A method was used to determine differential incoherent and coherent scattering cross‐sections of elements. This method is based on simultaneous measurement of fluorescence radiation and scattered radiation, thus avoiding problems with measuring source strength and source‐to‐detector solid angle. The measurements were performed using a point source of Am‐241 radioisotope and an Si(Li) detector. The experimental results, which complement earlier results, were compared with the theories of incoherent scattering function and form factor. Copyright © 2004 John Wiley & Sons, Ltd.     

Multiple‐element spectrometer for non‐resonant inelastic X‐ray spectroscopy of electronic excitations

A multiple‐analyser‐crystal spectrometer for non‐resonant inelastic X‐ray scattering spectroscopy installed at beamline ID16 of the European Synchrotron Radiation Facility is presented. Nine analyser crystals with bending radii R = 1 m measure spectra for five different momentum transfer values simultaneously. Using a two‐dimensional detector, the spectra given by all analysers can be treated individually. The spectrometer is based on a Rowland circle design with fixed Bragg angles of about 88°. The energy resolution can be chosen between 30–2000 meV with typical incident‐photon energies of 6–13 keV. The spectrometer is optimized for studies of valence and core electron excitations resolving both energy and momentum transfer.     

Lift‐off protocols for thin films for use in EXAFS experiments

Lift‐off protocols for thin films for improved extended X‐ray absorption fine structure (EXAFS) measurements are presented. Using wet chemical etching of the substrate or the interlayer between the thin film and the substrate, stand‐alone high‐quality micrometer‐thin films are obtained. Protocols for the single‐crystalline semiconductors GeSi, InGaAs, InGaP, InP and GaAs, the amorphous semiconductors GaAs, GeSi and InP and the dielectric materials SiO₂ and Si₃N₄ are presented. The removal of the substrate and the ability to stack the thin films yield benefits for EXAFS experiments in transmission as well as in fluorescence mode. Several cases are presented where this improved sample preparation procedure results in higher‐quality EXAFS data compared with conventional sample preparation methods. This lift‐off procedure can also be advantageous for other experimental techniques (e.g. small‐angle X‐ray scattering) that benefit from removing undesired contributions from the substrate.     

Thermal deactivation of lifetime‐ limiting grown‐in point defects in n‐type Czochralski silicon wafers

In this study, we uncover a recombination‐active grown‐in defect reducing the minority carrier lifetime of Czochralski grown n‐type silicon from 5 ms to below 2 ms. We also demonstrate that the defect can be de‐activated by annealing between 300 °C and 360 °C. Our experimental findings suggest that vacancy‐related pairs incorporated during ingot growth may be responsible for the decreased minority carrier lifetime. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Simultaneous small‐ and wide‐angle scattering at high X‐ray energies

Combined small‐ and wide‐angle X‐ray scattering (SAXS/WAXS) is a powerful technique for the study of materials at length scales ranging from atomic/molecular sizes (a few angstroms) to the mesoscopic regime (～1 nm to ～1 µm). A set‐up to apply this technique at high X‐ray energies (E > 50 keV) has been developed. Hard X‐rays permit the execution of at least three classes of investigations that are significantly more difficult to perform at standard X‐ray energies (8–20 keV): (i) in situ strain analysis revealing anisotropic strain behaviour both at the atomic (WAXS) as well as at the mesoscopic (SAXS) length scales, (ii) acquisition of WAXS patterns to very large q (>20 Å^?1) thus allowing atomic pair distribution function analysis (SAXS/PDF) of micro‐ and nano‐structured materials, and (iii) utilization of complex sample environments involving thick X‐ray windows and/or samples that can be penetrated only by high‐energy X‐rays. Using the reported set‐up a time resolution of approximately two seconds was demonstrated. It is planned to further improve this time resolution in the near future.