Step Orientation Research Articles

On the dependence between the step orientation and the received direct solar radiance of a PV panel. Part II: the pseudo-equatorial orientation

This second part of the paper approaches a problem that is referring to the energetic efficiency maximization of a tracked PV panel, which consists in maximizing the received solar radiation and minimizing the driving energy. The objective is to optimize the “received direct solar radiation” – “number of daytime tracking steps” correlation for a pseudo-equatorially tracked PV panel (Fig. 1). The solution presented is a graphical one and an analytic model will be developed in the future.

Tracking the little ones: use of fluorescent powder to follow a leaf litter lizard (Coleodactylus meridionalis, Squamata: Sphaerodactylidae) in the Atlantic Forest of southern Bahia, Brazil.

The ecology of movement is an expanding area, marked by the diversity of analytical methods and protocols, which enables this integrative reading. We investigated movement ecology aspects of Coleodactylus meridionalis in southern Bahia, northeastern Brazil, using fluorescent powder with mineral oil to track individuals. We monitored 69 individuals of C. meridionalis that walked an average distance of 148 cm in 2h. We identified this movement as foraging due to the orientation of the step sequence and microenvironments used. We find no significant differences between walking distance and weight. However, we found a decrease in activity over the follow-up period. Most of the lizard's movements were directed north, while south, east, and west were followed equally. The individuals stayed predominantly on the ground (leaf litter), but it was possible to observe the use of other surfaces, such as trunks and burrows on the ground. Therefore, we studied the movement in three dimensions (ground height, distance traveled, and orientation of steps). We observed the lizard's foraging, one of the most common and least investigated movements in small lizards like C. meridionalis. This involves not only the species' activity schedule but other intrinsic and extrinsic factors that shape the movement decisions of individuals.

Bi‐Oriented Step Guided Nucleation and Epitaxy of Twist Bilayer Graphene with Precisely Controlled Twist Angle

AbstractTwist bilayer graphene (tBLG) with a small magic angle deviated from trivial stacking exhibits fantastic electronic properties. However, the growth of large‐area tBLG with precisely controlled twist angle remains a grand challenge due to the thermodynamically unfavorable nucleation. Here, a bi‐oriented step‐guided strategy is theoretically proposed for the nucleation and epitaxial growth of tBLG, where the twist angle can be predetermined by the orientations of adjacent steps owing to the covalent alignment between the steps and graphene zigzag edges. The presence of bi‐oriented steps can greatly reduce the free energy of the tBLG with matched twist angle, thus promoting the nucleation priority of tBLG. Importantly, it is shown that the 28 sets of bi‐oriented steps with twist angles of 1.5°–30° can be intentionally constructed via two‐step miscutting from bulk crystal of catalytic metals by exploring all potential combinations of bi‐steps on different epitaxial surfaces. The employment of either low‐temperature CVD growth with certain precursors or the high‐melting‐point metal substrates is suggested to against the reconstruction of bi‐oriented steps during growth. This work demonstrates an efficient scheme for the epitaxial growth of large‐area tBLG with pre‐designed twist angle, which can be potentially extended to the growth of other twist 2D materials.

Atomistic simulation for initiation of crystal slip deformation from surface of nanoscale copper single-crystal nanowires

In nano/micrometer-scale materials, dislocations accounting for crystal slip deformation are introduced from the surface because of lack of pre-existing dislocations inside the materials. The dislocation emission from the surface must play an important role in the formation of dislocation network structures in the nano/micrometer-scale materials under cyclic loading, which should differ from that in macroscale materials. The purpose of this study is to investigate the effect of the atomic-level structure of surfaces on the initiation of crystal slip deformation. We create copper single-crystal nanowire models with different surface structures by varying the cross-section shape and crystal orientation. To observe the initiation of the slip deformation from the surface, we conduct atomistic simulations of tensile deformation for these models. We also discuss the effect of the surface structure on the critical shear stress for slip occurrence from the surface. Our analyses reveal that the orientation of surface steps and the normal stress on the active slip plane are important factors affecting the slip occurrence. Thus, the slip occurs in a slip system so as to reduce the height of a surface step. The active slip system is affected by the cross-section shape and crystal orientation because the slip system is dependent on the relationship between slip planes and the orientation of the surface step. Our results also suggest that the cross-section shape and crystal orientation affect the critical shear stress because of the difference of the normal stress on the active slip plane according to these factors.

Effect of basal plane bending on the atomic step morphology of the 4H–SiC substrate surface

The atomic step morphology of the SiC (0001) surface is determined by the crystal structure of SiC and reflects the real crystal morphology of the SiC substrate surface. The atomic step morphology can be used as feedback in final processing of the substrate and can reflect the characteristics and defects of the crystal itself. However, the widths and orientations of the atomic steps in different regions of the same wafer surface are often quite different. In this paper, atomic force microscopy (AFM) was used to systematically study the atomic steps of the 4H–SiC substrate (0001) surface without surface damage. The results show that the distributions of the atomic step morphology on the surfaces of wafers obtained from the same crystal are consistent. Based on a comparison of high-resolution X-ray diffraction (HRXRD) results, basal plane bending is the fundamental reason for the change in the atomic step morphology in different regions of the substrate. The results of this paper deepen the understanding of the surface morphology of SiC substrates and reveal the two-dimensional distribution of the bending characteristics of the basal plane throughout the entire substrate.

Basics of surface reconstruction during selective area metalorganic chemical vapour-phase epitaxy of GaAs films in the stripe-type ultra-wide window

The evolution of the surface morphology and growth steps of the GaAs film during selective area epitaxy has been studied. It has been shown that during the selective area epitaxy three regions with different growth mode and the atomic-height step orientation can be formed. A correlation between the local misorientation and the specific features of these areas has been identified. The layer surface’ local misorientation angle causes the transition between growth modes. A simulation model of the change in the growth steps’ front in the window area is proposed. It is shown that the local rotation angle of the growth steps’ front is governed by the growth rate change across the window. It was experimentally and theoretically shown that a small change in the orientation of the growth steps in the central area of the window and a rotation of the front by nearly 90°for the outermost window areas are specific features due to the symmetric mask, whereas the size of the central area decreases sharply with increasing layer thickness.

3D topology optimization of sandwich structures with anisotropic shells

The development of a density-based 3D minimum compliance topology optimization approach for sandwich structures with anisotropic shells is presented. A constant thickness shell is enforced at the interface of a base structure through a two-step filtering approach. Homogenous effective properties of a cellular material are assigned to the base structure while the shell is treated as a constant stiffness laminate with anisotropic properties. Local shell normal vectors are obtained from the density gradient introduced in the second filtering step and local material orientations in the shell’s tangent plane are approximated based on the fabrication process of the structure. Simple approximating strategies using the Gauss map are presented for a fabric draping process and the fused filament fabrication additive manufacturing process. The effect of material anisotropy on the optimal topology is studied on the Messerschmitt Bölkow Blohm beam benchmark problem. An application to composite structures is then presented where a square, simply supported laminate is reinforced on one face by a topology optimized sandwich structure. Though manufacturability cannot be ensured in the current implementation, careful consideration of the optimization parameters allow to constrain the problem towards manufacturable solutions using additively manufactured cores.

Stability of single-atomic-layer-height disconnections on [formula omitted] twin boundary in Mg

Four different configurations of single-atomic-layer-height disconnections on (101¯2) twin boundary in magnesium are investigated using molecular dynamics simulations. It is found that a prismatic-basal (PB) single-atomic-layer-height disconnection is more stable than a basal-prismatic (BP) single-atomic-layer-height disconnection when each of them is connected with an I1 stacking fault associated with a stacking sequence change of BABCB. In contrast, a BP single-atomic-layer-height disconnection is more stable than a PB single-atomic-layer-height disconnection when each of them is connected with an I1 fault associated with a stacking sequence change of ABACA. A stable BP (or PB) single-atomic-layer-height disconnection can transform to a stable PB (or BP) single-atomic-layer-height disconnection when its step orientation changes with a change in the lying plane of the connected I1 fault.

Maximisation of installed net resulting force through multi-level optimisation of an ultra-high bypass ratio engine nacelle

Engine/wing interference in podded installations can introduce relevant changes to their isolated behaviour, especially for large turbofans with an ultra-high bypass ratio. With such coupled flow field, the interaction is best described by the integrated force acting on the whole aircraft. Rather than focusing on drag or thrust metrics separately, in this paper we maximise the net resulting force on the NASA CRM wing body with underwing-installed turbofans through a multi-level optimisation procedure of the engine nacelle. The first stage involves two-dimensional axisymmetric geometries for cowl and exhaust ducts shapes optimisation. The solutions are transformed into three-dimensional nacelles with pylon, featuring a non-axisymmetric inlet. The impact of the pylon on the exhaust flow is evaluated through a design of experiment and a successive surrogate-based optimisation. In the last step, nacelle/wing offset, intake angles, and engine orientation are assessed in the installed configuration at fixed lift. The final optimised design is compared with a baseline geometry initially defined, showing a significant improvement of the net vehicle thrust equal to 3% of the reference force. The result highlights a counteracting effect of individual force components, in a way that the installed resulting force can only be improved when considering them globally.

Quantifying trade‐offs between butterfly abundance and movement in the management of agricultural set‐aside strips

Abstract Agri‐environment schemes (AES) create small areas of habitat within agricultural landscapes to support biodiversity. Here, we study butterfly flight behaviour within linear AES features and examine whether differences in resource availability affect the speed, linearity, or directionality of local movements, thereby affecting their contribution to landscape connectivity. We surveyed butterflies within three basic (naturally regenerating) and three wildflower‐sown linear field margin strips (0.09–0.15 ha) on a farm in North Yorkshire, UK, and mapped butterfly flight paths to quantify local displacement (movement speed), efficiency (linearity, turning angles), directionality (step orientation), and behaviour (time spent flying, nectaring). Butterfly species richness was similar between margin types {estimated asymptotic species richness of 21.9 [confidence interval (CI): 15.0–77.7] for basic margins and 14.2 (CI: 14.0–18.7) for wildflower‐sown margins}, but abundance was 78% higher in wildflower‐sown margins. For the three most common species [meadow brown, Maniola jurtina (L.); ringlet, Aphantopus hyperantus (L.); and small white, Pieris rapae (L.); n = 233 paths], movements within both margin types were highly linear (median turning angle 45°) and generally oriented along the length of the margin strip (median step orientation 27°). Movements in basic margins were slightly more orientated along the length of the margin but we found no differences between margin types in speed, path linearity, turning angles or the proportion of time spent flying or nectaring. We found strong channelling of movements along field margin strips regardless of management type, potentially aiding landscape connectivity. Providing field margin strips with additional foraging resources through wildflower‐sowing increases butterfly abundance without impeding local movement rates or efficiency.

Nanostructured Graphene on β-SiC/Si(001): Atomic and Electronic Structures, Magnetic and Transport Properties (Brief Review)

The studies of the properties of graphene synthesized on the surface of epitaxial films of cubic single-crystal silicon carbide preliminarily grown on Si(001) wafers have been reviewed. These studies were supported by the Russian Foundation for Basic Research, project no. 17-02-01139. The results of these studies demonstrate that graphene layers synthesized on β-SiC/Si(001) substrates have the atomic structure and electronic properties of a quasi-freestanding graphene sheet. Continuous graphene layers with a preferential direction of nanodomain boundaries, which is determined by the orientation of steps on the initial surface, can be synthesized on vicinal SiC(001) substrates. The possibility of controlled growth of mono-, bi-, and trilayer graphene on β-SiC/Si(001) wafers has been demonstrated. The studies have shown the opening of a transport gap and a high positive magnetoresistance in a parallel magnetic field in an ordered system of graphene nanoribbons on the vicinal SiC(001) surface. It has been shown that the functionalization of graphene with organic compounds changes the electronic properties of graphene on SiC(001), modifying it to a semiconductor with given properties, which allows applications in modern micro- and nanoelectronics.

Polar zinc oxide surface in electrolyte solutions: an atomic view of reconstruction, hydration and surface states.

The stabilization mechanism of the Zn-terminated (Zn-) ZnO(0001) surface in electrolyte solutions has been investigated by using atomic-resolution liquid-environment atomic force microscopy (AFM) and an electrochemical method. The electrochemically measured pH dependence of the flat band potential of the Zn-ZnO(0001) surface indicated the adsorption of OH groups onto the (0001) surface in the wide pH range of 1-13. Atomic-scale AFM images of the Zn-ZnO(0001) surface showed a well-ordered hydroxide superstructure in an alkaline solution but a disordered structure in an acidic solution, which is probably attributed to the rapid diffusion of the adsorbed OH groups. Furthermore, the density of the O-terminated step edge on the Zn-ZnO(0001) surface in an acidic solution was higher than that in an alkaline solution. From these findings, we concluded that the excess positive charges of the Zn-ZnO(0001) surface are compensated by the adsorbed OH groups and the O-terminated step edges. In acidic solutions, a higher density of the O-terminated step edge is required for charge compensation. In addition, it was found that potential-dependent reversible surface reconstruction occurs in the local transition area with disordered step orientation by electrochemical AFM. We concluded that the reconstruction compensates the excess surface charges of the local transition area which are induced and varied by potential-dependent local surface states.

Controllable synthesis and electronic structure characterization of multiple phases of iron telluride thin films

We present an investigation on the controlled growth of epitaxial iron telluride films of multiple phases by molecular beam epitaxy. By optimizing the substrate temperature, we fabricate different phases of \ensuremath{\alpha}-FeTe, \ensuremath{\beta}-FeTe, and $\mathrm{Fe}{\mathrm{Te}}_{2}$, respectively, whose crystalline morphologies are determined by means of in situ scanning tunneling microscopy/spectroscopy and ex situ scanning transmission electron microscopy. While both \ensuremath{\alpha}- and \ensuremath{\beta}-FeTe films are metallic, we uncover a \ensuremath{\sim}185- and 65-mV semiconducting band gap for the (100) and (011) facets of $\mathrm{Fe}{\mathrm{Te}}_{2}$ film, respectively, with the former one being compatible with the first principles calculations. Moreover, for $\mathrm{Fe}{\mathrm{Te}}_{2}$, we observe reduced gaps with enhanced conductance in the vicinity of edge boundaries, which are dependent on the geometries of step orientation and possibly in correlation to the magnetic anisotropy with structural distortion. Our study provides insight into the controllable synthesis of Fe-Te compounds with variation of stoichiometries and surface terminations, which may generalize to other epitaxial Fe chalcogenide films.

Polishing tool with phyllotactic distributed through-holes for photochemically combined mechanical polishing of N-type gallium nitride wafers

In this work, we further developed the photochemically combined mechanical polishing (PCMP) method for finishing N-type gallium nitride (GaN) wafers. A core improvement is to design a novel polishing tool with phyllotactic distributed through-holes, through which the wafer surface underneath through-holes can receive ultraviolet (UV)-light for the photochemical oxidation, while the rest parts undergo mechanical polishing. During PCMP, the co-rotation of the wafer and polishing tool allows the wafer surface to undergo the uniform and high-frequency conversion of oxidation and polishing. Based on the designed PCMP system and apparatus, the fundamental issues arising from such an alternate processing mode, which is different from the parallel mode of conventional chemical mechanical polishing (CMP), were investigated. Results show that the technical features of PCMP depend on the nature of the photochemical oxidation of wafers themselves if the mechanical polishing procedure can sufficiently remove oxides in time. The material removal rate (MRR) is inversely proportional to the dislocation density of wafers. Under acidic conditions, the oxidation proceeds by the GaN monocrystal step orientation, allowing PCMP to clear surface/subsurface damages (SSDs) and to prepare step-terrace structures on the wafer surface. When the polishing solution (pH = 1.5) includes 0.1 M K2S2O8 oxidants and 10 wt% SiO2 abrasives, the surface roughness Sa attains 0.21 nm in 10 × 10 μm2, and the MRR reaches 275.3 nm/h. The present study shows that the phyllotactic distributed through-hole array structure designed for polishing tools offers rich possibilities for the innovation of polishing technologies combining with various oxidation approaches.

Adsorbate-Driven Electrode Processes, Part 2: Order to Disorder Transitions

Adsorbates on metal electrodes change the step formation energies and preferred step orientations relative to those of clean electrodes. We apply previously developed approximate density functional theory potential energy surfaces from Behler-Parrinello neural networks. We investigate the adsorbate order to disorder transition on terraces and monolayer and bilayer steps. We identify how the adlattice disorders with changing adsorbate coverage for these surfaces.

Atomic scale step structure and orientation of a curved surface ZnO single crystal

We have investigated the surface structure of a curved ZnO-crystal, going from the (0001)-facet at 0° miscut to the (101¯4)-facet at a miscut of 24.8° using scanning tunneling microscopy and low energy electron diffraction. We find that the surface separates locally into (0001)-terraces and (101¯4)-facets, where the ratio between the facets depends on the miscut angle. In X-ray photoemission spectroscopy (XPS) the intensity of an O 1s component scaling with the step density of the surface is observed. No other facets were observed and the surface maintains a high degree of order over all angles. Such a curved ZnO crystal can be used for systematic studies relating the step density to the chemical reactivity using XPS to probe the curved surface at different positions.

Monitoring of CO Binding Sites on Stepped Pt Single Crystal Electrodes in Alkaline Solutions by in Situ FTIR Spectroscopy.

The site geometry preference of CO binding on stepped Pt single crystals in alkaline solution was investigated by in situ FTIR spectroscopy. The surfaces of the Pt single crystals consisted of different width (111) terraces, interrupted by (110) or (100) monatomic steps. Experiments carried out with CO adsorbed exclusively on the top of the steps revealed that only linearly bonded CO formed on the (110) steps, while two CO binding geometries (linear and bridge) were observed on the (100) steps. On one hand, for CO adsorbed only on the steps, the positions of the bands corresponding to linearly bonded CO were similar, regardless of the density of steps, suggesting the existence of an interaction between COads only along the line of the steps. On the other hand, for full CO coverage, the CO stretching frequencies and the geometry of bound CO were sensitive to the width of the (111) terraces and the step orientations. Consequently, the CO binding sites favored linearly bonded CO for surfaces consisting of shorter (111) terraces and (110) steps. Bridge-bonded CO was favored on surfaces consisting of shorter (111) terraces interrupted by (100) steps. In order to understand the origin of the preference of CO binding sites, the results were compared to the corresponding behavior in acid media, which revealed that, in addition to the effect inherent to the Pt surface, the charge on the metal side in an aqueous environment should be taken into consideration. The analysis suggested that the CO adlayers formed at full coverage in acidic and alkaline media had different structures. On the other hand, the structure of the layer of CO adsorbed only at the steps was independent of pH.

Characteristics of medial-lateral postural control while exposed to the external perturbation in step initiation

Controllability of posture in the medial-lateral direction is critical for balance maintenance, particularly in step initiation. The objective of the current study was to examine the effects of external perturbation and landing orientation on medial-lateral control stability in step initiation. Eleven young healthy participants stood on the force platform and waited for the instruction of taking a step while experiencing a pendulum perturbation applied at the lateral side of the right shoulder. Eight experimental conditions were conducted by two levels of step side (right or left), two levels of perturbation (with or without), and two levels of landing orientation (forward or diagonal). The center of pressure (COP), pelvic movements, and muscle activities were recorded and analyzed as the onset of COP and pelvic movement, the COP displacement, and cocontraction and reciprocal muscle activation pattern. The temporal events of COP and pelvic movement were not significantly different in all experimental conditions. However, COP and pelvic movement were significantly later in the diagonal condition. Most of the segments showed reciprocal muscle activation patterns in relation to the perturbation released time. Subsequently, all segments showed cocontraction muscle activation patterns, which was significantly affected by step side, perturbation, and orientation. The results suggest that how the CNS initiated a step was identical with the COP then pelvic movement. The outcome highlights the importance of external perturbation and foot landing orientation effects on postural adjustments, which may provide a different approach to help step initiation.

Stoichiometric edges during the intrinsic growth of hexagonal boron nitride on Ir(111)

The growth of hexagonal boron nitride, hBN, on Ir(111) at 1150–1200 K by thermal decomposition of borazine has been monitored in situ by means of low energy electron microscopy, LEEM. A major and unexpected hBN growth induced transformation of the step morphology on Ir(111) is observed. The dominant orientation of the steps evolves persistently from 〈1–10〉 during the initial experiment(s), towards 〈11–2〉 after numerous experiments. The latter orientation was established directly from an involved analysis of the profile of the specular beam in a selected area diffraction pattern. Images of the hBN islands grown on surface regions with dominant 〈1–10〉 steps reveal that the hBN edges are oriented along 〈1–10〉 and 〈11–2〉 directions in about equal abundance. Using the direct ascertainment of the step orientations we are able to identify unequivocally the nature of the edges of individual hBN islands on areas with predominant 〈11–2〉 steps as solely of the armchair type. This assessment consolidates both the major evolution of the step morphology from 〈1–10〉 into dominant 〈11–2〉 steps during hBN growth on Ir(111) and the earlier reported alignment of the moiré pattern with the substrate.

Influence of interface steps on the buckle delamination of thin films

The mechanics of thin film buckle delamination has been widely studied, with the assumption of perfect (flat) interfaces. This work is reporting a study of the evolution of an initially straight-sided buckle propagating in a thin film lying on a substrate in the presence of a step at the film/substrate interface, resulting from the plastic deformation of the substrate. Assuming the interface toughness depends on the mode mixity, it has been found using finite elements calculations that depending on the step orientation and height, the initially straight propagation of the buckle can be perturbed so that the buckle eventually propagates along the step. The critical parameters (orientation and height) of the step have been then determined, which characterizes the transition from a straight propagation perpendicular to the compression axis to a propagation along the inclined step. A “behavior” diagram has been finally presented for the buckle, where the propagation, either perpendicularly to the step or along it have been displayed versus the step height and orientation.