Off-normal Angles Of Incidence Research Articles

Tailoring adaptive bioresorbable Mg-based scaffolds with directed plasma nanosynthesis for enhanced osseointegration and tunable resorption

Directed plasma nanosynthesis (DPNS) is a plasma-based surface modification process used to provide high-fidelity bioactive and bioresorbable interfaces for Mg-based foams having an average 500-μm pore size and containing main components of Al, Zn and Ca at bal., 3.3%, 1.11%, and 0.21%, respectively. Correlations of incident particle energies of 400–700 eV and room temperature, normal and off-normal incidence angles of 0° and 60°, respectively, and high-ion fluence conditions are combined to elicit a bioreactive Mg-foam surface. H2 evolution and pH levels of irradiated and non-irradiated Mg-foams were examined and correlated to the DPNS parameters. In situ X-ray photoelectron spectroscopy and focused ion-beam results have shown that energies of ~ 400–700 eV can control surface topography and composition, which, in turn, controls the foam-corrosion mechanism. Samples are immersed in Dulbecco’s modified eagle media, and a synergistic reaction is found in which the irradiated samples enhance the formation of calcium–phosphate (CaP) phases to CaP ratios close to the hydroxylapatite phase that enhances bone-tissue regeneration. These results lead to a surface modification strategy that adjusts the interaction of the material and the environment without using a coating that could affect the geometry and the bulk properties of the porous material.

Nanopatterning of the (001) surface of crystalline Ge by ion irradiation at off-normal incidence: Experiment and simulation

Intricate topographical patterns can form on the surface of crystalline Ge(001) subject to low-energy ion irradiation in the reverse epitaxy regime, i.e., at elevated temperatures which enable dynamic recrystallization. We compare such nanoscale patterns produced by irradiation from varied polar and azimuthal ion incidence angles with corresponding calculated surface topographies. To this end, we propose a continuum equation including both anisotropic erosive and anisotropic diffusive effects. Molecular dynamics simulations provide the coefficients of angle-dependent sputter erosion for the calculations. By merely changing these coefficients accordingly, the experimentally observed surface morphologies can be reproduced, except for extreme ion incidence angles. Angle-dependent sputter erosion is thereby identified as a dominant mechanism in ion-induced pattern formation on crystalline surfaces under irradiation from off-normal incidence angles.

Terahertz Van Atta Retroreflecting Arrays

As an initial step in developing steerable mirrors for terahertz (THz) wireless communication systems, this paper describes the development of a passive planar terahertz retroreflectors based on the Van Atta array. The retroreflector is optimized and simulated using finite-element modeling (FEM) software, fabricated via a low-cost additive manufacturing method, and characterized using THz time-domain spectroscopy. Comparison to a flat metal plate shows a significant increase in retroreflectivity for off-normal angles of incidence.

Incident Angle Dependence of CHD3 Dissociation on the Stepped Pt(211) Surface.

The dissociation of methane on transition metal surfaces is not only of fundamental interest but also of industrial importance as it represents a rate-controlling step in the steam-reforming reaction used commercially to produce hydrogen. Recently, a specific reaction parameter functional (SRP32-vdW) has been developed, which describes the dissociative chemisorption of CHD3 at normal incidence on Ni(111), Pt(111), and Pt(211) within chemical accuracy (4.2 kJ/mol). Here, we further test the validity of this functional by comparing the initial sticking coefficients (S0), obtained from ab-initio molecular dynamics calculations run using this functional, with those measured with the King and Wells method at different angles of incidence for CHD3 dissociation on Pt(211). The two sets of data are in good agreement, demonstrating that the SRP32-vdW functional also accurately describes CHD3 dissociation at off-normal angles of incidence. When the direction of incidence is perpendicular to the step edges, an asymmetry is seen in the reactivity with respect to the surface normal, with S0 being higher when the molecule is directed toward the (100) step rather than the (111) terrace. Although there is a small shadowing effect, the trends in S0 can be attributed to different activation barriers for different surface sites, which in turn is related to the generalized co-ordination numbers of the surface atom to which the dissociating molecule is adsorbed in the transition state. Consequently, most reactivity is seen on the least co-ordinated step atoms at all angles of incidence.

Experimental study of EUV mirror radiation damage resistance under long-term free-electron laser exposures below the single-shot damage threshold.

The durability of grazing- and normal-incidence optical coatings has been experimentally assessed under free-electron laser irradiation at various numbers of pulses up to 16 million shots and various fluence levels below 10% of the single-shot damage threshold. The experiment was performed at FLASH, the Free-electron LASer in Hamburg, using 13.5 nm extreme UV (EUV) radiation with 100 fs pulse duration. Polycrystalline ruthenium and amorphous carbon 50 nm thin films on silicon substrates were tested at total external reflection angles of 20° and 10° grazing incidence, respectively. Mo/Si periodical multilayer structures were tested in the Bragg reflection condition at 16° off-normal angle of incidence. The exposed areas were analysed post-mortem using differential contrast visible light microscopy, EUV reflectivity mapping and scanning X-ray photoelectron spectroscopy. The analysis revealed that Ru and Mo/Si coatings exposed to the highest dose and fluence level show a few per cent drop in their EUV reflectivity, which is explained by EUV-induced oxidation of the surface.

MD and BCA simulations of He and H bombardment of fuzz in bcc elements

We present results of MD simulations of low energy He ion bombardment of low density fuzz in bcc elements. He ions can penetrate several micrometers into sparse fuzz, which allows for a sufficient He flux through it to grow the fuzz further. He kinetic energy falls off exponentially with penetration depth. A BCA code was used to carry out the same ion bombardment on the same fuzz structures as in MD simulations, but with simpler, 10 million times faster calculations. Despite the poor theoretical basis of the BCA at low ion energies, and the use of somewhat different potentials in MD and BCA calculations, the ion penetration depths predicted by BCA are only ∼12% less than those predicted by MD. The MD-BCA differences are highly systematic and trends in the results of the two methods are very similar. We have carried out more than 200 BCA calculation runs of ion bombardment of fuzz, in which parameters in the ion bombardment process were varied. For most parameters, the results show that the ion bombardment process is quite generic. The ion species (He or H), ion mass, fuzz element (W, Ta, Mo, Fe) and fuzz element lattice parameter turned out to have a modest influence on ion penetration depths at most. An off-normal angle of incidence strongly reduces the ion penetration depth. Increasing the ion energy increases the ion penetration, but the rate by which ion energy drops off at high ion energies follows the same exponential pattern as at lower energies.

Ultra-broadband long-wavelength infrared metamaterial absorber based on a double-layer metasurface structure

In this paper, we report a metamaterial absorber design that achieves a broad absorption band encompassing the whole long-wavelength infrared (LWIR) region. The structure consists of two parallel metasurfaces buried in an amorphous silicon dielectric layer, where the minimum size for all possible planar details does not go below 1 μm, making the use of standard optical lithography possible for fabrication. The dielectric layer of the structure is placed over a metallic ground plane that inhibits the transmission of incident waves. A substrate underneath the ground plane may also be needed for the purpose of mechanical support. This structure achieves a minimum absorptivity of about 90% in almost the full LWIR band in the case of normal incidence in a polarization-insensitive manner due to the four-fold symmetry of the structural geometry. The absorber also shows reduced sensitivity to off-normal incidence angles, satisfying a minimum absorption level of approximately 80% up to the incidence angle of 45 deg. This broadband metamaterial absorber design is anticipated to find applications in thermal emitters/coolers and in thermal infrared sensors.

Viscous surface flow induced on Ti-based bulk metallic glass by heavy ion irradiation

Ti-based bulk metallic glass was irradiated by a 20MeV Cl4+ ion beam under liquid-nitrogen cooling, which produced remarkable surface smoothing and roughening that respectively correspond to normal and off-normal incidence angles of irradiation. Atomic force microscopy confirms two types of periodic ripples distributed evenly over the rough glass surface. In terms of mechanism, irradiation-induced viscosity agrees with the theoretical prediction for metallic glasses near glass transition temperature. Here, a model is introduced, based on relaxation of confined viscous flow with a thin liquid-like layer, that explains both surface smoothing and ripple formation. This study demonstrates that bulk metallic glass has high morphological instability and low viscosity under ion irradiation, which assets can pave new paths for metallic glass applications.

Ion-Beam-Directed Self-Ordering of Ga Nanodroplets on GaAs Surfaces.

Ordered nanodroplet arrays and aligned nanodroplet chains are fabricated using ion-beam-directed self-organization. The morphological evolution of nanodroplets formed on GaAs (100) substrates under ion beam bombardment is characterized by scanning electron microscopy and atomic force microscopy. Ordered Ga nanodroplets are self-assembled under ion beam bombardment at off-normal incidence angles. The uniformity, size, and density of Ga nanodroplets can be tuned by the incident angles of ion beam. The ion beam current also plays a critical role in the self-ordering of Ga nanodroplets, and it is found that the droplets exhibit a similar droplet size but higher density and better uniformity with increasing the ion beam current. In addition, more complex arrangements of nanodroplets are achieved via in situ patterning and ion-beam-directed migration of Ga atoms. Particularly, compared to the destructive formation of nanodroplets through direct ion beam bombardment, the controllable assembly of nanodroplets on intact surfaces can be used as templates for fabrication of ordered semiconductor nanostructures by droplet epitaxy.

Light trapping design for low band-gap polymer solar cells.

We demonstrate numerically a 2-D nanostructured design for light trapping in a low band-gap polymer solar cell. Finite element method simulations are used to study the effect of varying nanostructure periodicity, height, and shape on active layer absorption. Maintaining a constant active layer thickness of 100 nm we observe an enhancement in solar absorption of almost 40% relative to a planar cell. Improvements of this magnitude enable single-junction, low-band-gap cells to achieve power conversion efficiencies of 11.2% and perform competitively with even state-of-the-art tandem cells. Our design is also shown to significantly outperform tandem cells at off-normal angles of incidence.

Depth profile investigation of the incorporated iron atoms during Kr+ ion beam sputtering on Si (001)

We investigate the incorporation of iron atoms during nano-patterning of Si surfaces induced by 2keV Kr+ ion beam erosion under an off-normal incidence angle of 15°. Considering the low penetration depth of the ions, we have used X-ray reflectivity (XRR) and X-ray absorption near edge spectroscopy (XANES) under grazing-incidence angles in order to determine the depth profile and phase composition of the incorporated iron atoms in the near surface region, complemented by secondary ion mass spectrometry and atomic force microscopy. XRR analysis shows the accumulation of metallic atoms within a near surface layer of a few nanometer thickness. We verify that surface pattern formation takes place only when the co-sputtered Fe concentration exceeds a certain limit. For high Fe concentration, the ripple formation is accompanied by the enhancement of Fe close to the surface, whereas no Fe enhancement is found for low Fe concentration at samples with smooth surfaces. Modeling of the measured XANES spectra reveals the appearance of different silicide phases with decreasing Fe content from the top towards the volume.

Modal analysis of enhanced absorption in silicon nanowire arrays

We analyze the absorption of solar radiation by silicon nanowire arrays, which are being considered for photovoltaic applications. These structures have been shown to have enhanced absorption compared with thin films, however the mechanism responsible for this is not understood. Using a new, semi-analytic model, we show that the enhanced absorption can be attributed to a few modes of the array, which couple well to incident light, overlap well with the nanowires, and exhibit strong Fabry-Pérot resonances. For some wavelengths the absorption is further enhanced by slow light effects. We study the evolution of these modes with wavelength to explain the various features of the absorption spectra, focusing first on a dilute array at normal incidence, before generalizing to a dense array and off-normal angles of incidence. The understanding developed will allow for optimization of simple SiNW arrays, as well as the development of more advanced designs.

Energetic deposition of metal ions: observation of self-sputtering and limited sticking for off-normal angles of incidence

The deposition of films under normal and off-normal angles of incidence has been investigated in order to explore the relevance of non-sticking and self-sputtering of energetic ions. Non-sticking and self-sputtering lead to the formation of neutral atoms which return to the plasma and affect its properties. The flow of energetic ions was obtained using a filtered cathodic arc system in high vacuum. The range of materials included Cu, Ag, Au, Pt, Ti and Ni. Consistent with molecular dynamics simulations published in the literature, the experiments showed that the combined effects of non-sticking and self-sputtering appear to be significant, although the relatively large error range of the experimental method would not allow us to derive quantitative data. It was shown that modest heating of the substrate and intentional introduction of oxygen background gas considerably affected the results.

Ion beam sharpening of diamond tools having small apex angle without facet and ripple formations

The sharpening of diamond tools with small apex angles by low energy (1.0 keV) ion beam faces great challenges because of facet formation at the cutting edge of the tools. Adding to the problem is the formation of ripple, also appearing at the cutting edge of the tools that occurs when the ion beam bombardment is done at some off-normal angle of incidence. In this work, the authors investigated the dependence of the facet angles on the ion beam energy. They also studied the phenomenon of ripple formation as influenced by ion dose, ion beam energy, and ion incidence angles. Based on the experimental results they then developed a method for sharpening the tool with 45° apex without facet formation. They also studied ripple formation using 1.0 keV Ar+ ion beam at a tilt angle β of 30°. The work also used simulations to predict the changes in the profile of diamond tools during ion beam machining at a fixed tilt angle β. They found that simulation results on the profile of the diamond knife matched quite well with the experimental results. They conclude that it is possible to make sharp diamond knife with an apex angle of 45° and that sharpening time can be predicted applying simulations.

Thin AMC Structure for Radar Cross-Section Reduction

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A thin artificial magnetic conductor (AMC) structure is designed and breadboarded for radar cross-section (RCS) Reduction applications. The design presented in this paper shows the advantage of geometrical simplicity while simultaneously reducing the overall thickness (for the current design <formula formulatype="inline"><tex>$\lambda/16$</tex></formula>). The design is very pragmatic and is based on a combination of AMC and perfect electric conductor (PEC) cells in a chessboard like configuration. An array of Sievenpiper's mushrooms constitutes the AMC part, while the PEC part is formed by full metallic patches. Around the operational frequency of the AMC-elements, the reflection of the AMC and PEC have opposite phase, so for any normal incident plane wave the reflections cancel out, thus reducing the RCS. The same applies to specular reflections for off-normal incidence angles. A simple basic model has been implemented in order to verify the behavior of this structure, while Ansoft-HFSS software has been used to provide a more thorough analysis. Both bistatic and monostatic measurements have been performed to validate the approach. </para>

A Silicon-Based, Sequential Coat-and-Etch Process to Fabricate Nearly Perfect Substrate Surfaces

For many thin-film applications substrate imperfections such as particles, pits, scratches, and general roughness, can nucleate film defects which can severely detract from the coating's performance. Previously we developed a coat-and-etch process, termed the ion beam thin film planarization process, to planarize substrate particles up to approximately 70 nm in diameter. The process relied on normal incidence etching; however, such a process induces defects nucleated by substrate pits to grow much larger. We have since developed a coat-and-etch process to planarize approximately 70 nm deep by 70 nm wide substrate pits; it relies on etching at an off-normal incidence angle, i.e., an angle of approximately 470 degrees from the substrate normal. However, a disadvantage of this pit smoothing process is that it induces defects nucleated by substrate particles to grow larger. Combining elements from both processes we have been able to develop a silicon-based, coat-and-etch process to successfully planarize approximately 70 nm substrate particles and pits simultaneously to at or below 1 nm in height; this value is important for applications such as extreme ultraviolet lithography (EUVL) masks. The coat-and-etch process has an added ability to significantly reduce high-spatial frequency roughness, rendering a nearly perfect substrate surface.

Berreman Effect in Amorphous and Crystalline WO3 Thin Films

Thin films of tungsten oxide deposited by hot filament metal oxide deposition (HFMOD) were thermally annealed up to 800 °C and investigated by means of XRD, Raman spectroscopy, and infrared reflection−absorption spectroscopy (IRRAS). As clearly shown by the XRD and Raman spectroscopy data, the deposited films were amorphous and crystallized by thermal annealing. The monoclinic WO3 phase was formed in all annealed samples. The IRRAS spectra were obtained using the IR beam with p-polarization and an off-normal incidence angle. In this condition, absorptions due to the longitudinal optical (LO) modes (Berreman effect) can be observed in the spectra. Absorptions due to LO modes are not detected by the standard infrared absorption spectroscopy, in which an unpolarized IR beam is used at normal incidence, and thus are not frequently reported in the literature. To analyze the experimental IRRAS spectra, the LO and TO functions were calculated from the transmission spectra of the as-deposited sample, using the Kr...

Anisotropic magnetic properties of obliquely deposited Ni films

Magnetic properties of evaporated nickel films, deposited onto 75 μm thick 300 H Kapton substrates by evaporation at oblique off-normal angles of incidence, α were investigated by SQUID magnetometry. We found that, in the film plane, the direction of easy magnetization lay perpendicular to the incidence plane for films deposited at α < 50°. At large α s, the easy axis changed to the direction parallel to the incidence plane. The anisotropy, coercivity and squareness of the hysteresis loops increased with an increase in α. The results may be qualitatively understood from the presence of an inclined columnar structure with shape anisotropy governing the demagnetization of the magnetic fields.

Diffractive and reactive scattering of (v=0, j=0) HD from Pt(111): Six-dimensional quantum dynamics compared with experiment

We present results of (v=0, j=0) HD reacting on and scattering from Pt(111) at off-normal angles of incidence, treating all six molecular degrees of freedom quantum mechanically. The six-dimensional potential energy surface (PES) used was obtained from density functional theory, using the generalized gradient approximation and a slab representation of the metal surface. Diffraction and rotational excitation probabilities are compared with experiment for two incidence directions, at normal incidence energies between 0.05–0.16 eV and at a parallel translational energy of 55.5 meV. The computed ratio of specular reflection to nonspecular in-plane diffraction for HD+Pt(111) is lower than found experimentally, and lower for HD+Pt(111) than for H2+Pt(111) for both incidence directions studied. The calculations also show that out-of-plane diffraction is much more efficient than in-plane diffraction, underlining that results from experiments that solely attempt to measure in-plane diffraction are not sufficient to show the absence of surface corrugation. Discrepancies in rotational excitation and diffraction probabilities between theory and experiment are discussed, as well as possible future improvements in the dynamical model and in the calculation of the PES.

Nanopatterning of silicon surfaces by low-energy ion-beam sputtering: dependence on the angle of ion incidence

We report on the production of nanoscale patterning onSi substrates by low-energy ion-beam sputtering. Thesurface morphology and structure of the irradiated surface werestudied by atomic force microscopy (AFM) and high-resolutiontransmission electron microscopy (HRTEM). Under ion irradiationat off-normal incidence angle (~50°), AFM images showthe formation of both nanoripple and sawtooth-like structuresfor sputtering times longer than 20 min. The latter featurecoarsens appreciably after 60 min of sputtering, inducing alarge increase in the surface roughness. This behaviour isattributed to the preferential direction determined on the substrate by the ion beam for this incidence angle, leading toshadowing effects among surface features in the sputteringprocess. Under irradiation at normal incidence, the formationof an hexagonal array of nanodots is induced for irradiationtimes longer than 2 min. The shape and crystallinity of thenanodots were determined by HRTEM. At this incidence angle, the surface roughness is very low and remains largely unchanged evenafter 16 h of sputtering. For the two angle conditionsstudied, the formation of the corresponding surface structurescan be understood as the interplay between an instability dueto the sputtering yield dependence on the local surfacecurvature and surface smoothing processes such as surfacediffusion.