Concave Pyramids Research Articles

Unidirectional transport of both wettable and nonwettable liquids on an asymmetrically concave structured surface

Unidirectional liquid transport (UDLT) has been widely used in various fields as an important process for transferring both mass and energy. However, UDLT driven by a structural gradient has been witnessed for a long time only in wettable liquids. For nonwettable liquids, UDLT can hardly proceed merely by a structural gradient. Herein, we propose an asymmetrically concave structured surface (AMC-surface), featuring tip-to-base periodically arranged pyramid-shaped concave structures with a certain degree of overlap, which enables the UDLT of both wettable and nonwettable liquids. For wettable liquids, the capillary force along each corner leads to the UDLT pointing toward the base side of the concave pyramid, while for nonwettable liquids, the UDLT is attributable to the static liquid pressure overwhelming the repulsive Laplace pressure induced by the asymmetric grooves and overlapping part. As a result, both wettable and nonwettable liquids transport spontaneously and unidirectionally on the AMC-surface with no energy input. Moreover, the concave structure endows good mechanical stability and can be easily prepared using a facile nail-punching approach over a large area. We also demonstrated its application in a continuous chemical reaction in a confined area. We envision that the unique UDLT behavior on the as-developed AMC-surface will shed new light on the programmable manipulation of various liquids.

Reshapable MXene/Graphene Oxide/Polyaniline Plastic Hybrids with Patternable Surfaces for Highly Efficient Solar‐Driven Water Purification

AbstractAlthough many solar‐driven water evaporators are developed for solar steam generation, most solarthermal energy conversion materials cannot be used repeatedly for constructing solarthermal water evaporators with variable shapes and patternable surfaces. Herein, reshapable Ti3C2Tx MXene/graphene oxide (GO)/polyaniline (PANI) (MGP) hybrids with variable shapes and patternable surfaces are fabricated by PANI‐assisted assembly of GO and MXene for efficient solar‐driven purifications of both seawater and wastewater. The variable shapes, patternable surfaces, and reusability of the plastic MGP hybrids are attributed to the strong interactions of PANI with both GO and MXene. Benefiting from the excellent solarthermal energy conversion of hydrophilic GO and MXene, the variable shapes and patternable surfaces of the MGP, and the reduced water vaporization enthalpy, the patternable MGP evaporators with flat and concave pyramid surfaces exhibit average water evaporation rates of as high as 2.89 and 3.30 kg m−2 h−1 under 1‐sun irradiation, respectively. When the plastic MGP is molded to a flower‐shaped evaporator, an outstanding evaporation rate of ≈3.94 kg m−2 h−1 with an exceptional evaporation efficiency of ≈135.6% is achieved under 1‐sun irradiation. The reusable MGP evaporators are highly efficient in generating clean water from both seawater and wastewater with satisfactory ion rejection rates of nearly 100%.

Silk fibroin-decorated with tunable Au/Ag nanodendrites: A plastic near-infrared SERS substrate with periodic microstructures for ultra-sensitive monitoring of lactic acid in human sweat

The flexible and plastic near-infrared surface-enhanced Raman spectroscopy (NIR-SERS) substrate, with efficient tunable surface microstructures for probe molecular detection on irregular targets, has great potential for ultra-sensitive monitoring in real-world scenarios. Herein, we propose an interesting 785 nm laser-excited plastic SERS substrate based on the silk fibroin fibers (SFF) decorated with multiple-branched Au/Ag nanodendrites (NDs). Compared with the original smooth flexible substrate, the further constructed plastic SFF-Au/Ag NDs with concave pyramid surface microstructures provides a relatively higher NIR-SERS activity at bottom of the inward concave region. Moreover, the enhanced NIR-SERS activity is also highly related to the content of Au/Ag NDs in the hybrid flexible substrate. The maximal NIR-SERS signal intensity can be realized at the content of 0.15 wt% Au/Ag NDs in the SFF-Au/Ag NDs, which is about 8-fold enhancement compared to that obtained at the content of 0.01 wt%. Then, the molecular detection limits of crystal violet (CV) and 4-mercaptobenzoic acid (4-MBA) are calculated about 10−15 and 10−13 M, respectively, reaching the ultra-sensitive requirements of NIR-SERS trace detection. Moreover, the substrate can detect lactic acid (LA) molecules with a concentration as low as 10−9 M, and can effectively reflect slight LA fluctuation in human sweat, providing a basis for the realization of ultra-sensitive NIR-SERS health monitoring.

Konstruktivno geometrijsko generisanje konkavnih piramida četvrte vrste

The paper presents the results of the study of the new set of polihedra, the Concave pyramids of the fourth sort, the construction procedures for generating them and their possible application. Correspondingly to the method of generating the Concave cupolae of fourth sort, the Concave pyramids of fourth sort have the similar logic of origination, and their counterpart in regular faced convex pyramids. They are characterised by the polygonal base, deltahedral surface net, obtained by folding the planar net of unilateral triangles, the polar distribution of the unit space cells with common apex - the top of the Concave pyramid. Polihedral surface of the planar net of Concave pyramids is produced by polar distribution of unit cells, consisting of a spatial sexagon and spatial pentagon - six, or five, unilateral triangles grouped around the common vertex. In the deltahedral surface, the two neighbouring unit cells are joined by means of a unilateral triangle in the zone of the polygonal base and a spatial quadrangle with which they share common sides. The criterion of face regularity is respected, as well as the criterion of multiple axial symmetry. The sort of the Concave pyramids is determined by the number of equilateral triangle rows in thus obtained polyhedron's net. The parameters of the solids were determined constructively by geometric methods.

A downshifting Eu3+ doped glass embedded with concave pyramid microstructure to improve the efficiency of silicon solar cell

The average photoelectric conversion efficiency (PCE) of a bare mono crystalline silicon solar cell is 14.71% ± 0.03% under AM1.5. It decreases to 14.20% ± 0.005% when covering an un-doped flat glass on the solar cell, and it goes down to 14.10% ± 0.005% by using a 5 wt% Eu3+ doped glass. The absorptions of the Eu3+ doped CPM glass one-to-one match the excitation spectra at 362, 381, 393, 400, 413 and 464 nm, which are related to the transitions of 7F0→(5D4, 5G2, 5L6, 5D3), 7F1→5D3, and 7F0→5D2, respectively. In addition, a concave pyramid microstructure (CPM) is embedded in the glass surface to increase light transmittance. The average PCE increases to 14.61% ± 0.07% when a 5 wt% Eu3+ doped CPM glass covers on the silicon solar cell. Compared with the un-doped flat glass, a net increase of the PCE is 0.41%, where the 0.16% increment of PCE is from the lighting trapping of the CPM structure, and the downshifting of near ultraviolet (NUV) light by Eu3+ ion donates the other 0.25% increment. It confirms that the as-prepared Eu3+ doped CPM glass has a good downshifting and antireflection function.

Enhanced Transmission and Self-Cleaning of Patterned Sapphire Substrates Prepared by Wet Chemical Etching Using Silica Masks.

Highly transparent and superhydrophilic sapphire with surface antireflective subwavelength structures were prepared by wet etching using colloidal monolayer silica masks. The film thicknesses of the silica masks were adjusted by the volume concentrations of polystyrene spheres. The evolution of etching morphologies of sapphire was studied, and antireflective concave pyramid nanoarrays on sapphire substrates were designed by calculation and were then prepared. The transmission and wettability of as-obtained patterned sapphire substrates were also investigated. As for sapphire with optimum surface concave micropyramid arrays, average visible transmittance can reach 91.7%, which is apparently higher than that of flat sapphire (85.5%). Moreover, the concave pyramid arrays can significantly increase the surface hydrophilicity of sapphire, exhibiting a water contact angle of 12.6° compared with 62.7° of flat sapphire. The proposed method can be an excellent strategy for preparing antireflective and self-cleaning concave micropyramid subwavelength structures on sapphire without complicated equipment and expensive raw materials.

Searching for imperfection insensitive externally pressurized near-spherical thin shells

This paper studies the buckling behavior and imperfection sensitivity of geodesic and stellated shells subject to external pressure. It is shown that these structures can completely eliminate the severe imperfection sensitivity of spherical shells and can achieve buckling pressure and mass efficiency higher than the perfect sphere. Key results of this paper are as follows. First, a shell with the shape of an icosahedron can carry external pressure significantly higher than a spherical shell, when the effects of geometric imperfections are considered. Second, stellated shells are generally insensitive to imperfections. For pyramids with height-to-radius ratios greater than 35% the buckling pressure is greater than for a perfect sphere. The specific ratio 45% gives the highest buckling pressure, 28% higher than the perfect sphere. Third, stellated icosahedra with concave pyramids have higher mass efficiency than the perfect sphere. Fourth, in terms of volume efficiency, geodesic shells are comparable to spherical shells with a knockdown factor of 0.2 and convex stellated shells are comparable to spherical shells with a knockdown factor of 0.65.

Conformal Electroless Nickel Plating on Silicon Wafers, Convex and Concave Pyramids, and Ultralong Nanowires.

Nickel (Ni) plating has garnered great commercial interest, as it provides excellent hardness, corrosion resistance, and electrical conductivity. Though Ni plating on conducting substrates is commonly employed via electrodeposition, plating on semiconductors and insulators often necessitates electroless approaches. Corresponding plating theory for deposition on planar substrates was developed as early as 1946, but for substrates with micro- and nanoscale features, very little is known of the relationships between plating conditions, Ni deposition quality, and substrate morphology. Herein, we describe the general theory and mechanisms of electroless Ni deposition on semiconducting silicon (Si) substrates, detailing plating bath failures and establishing relationships between critical plating bath parameters and the deposited Ni film quality. Through this theory, we develop two different plating recipes: galvanic displacement (GD) and autocatalytic deposition (ACD). Neither recipe requires pretreatment of the Si substrate, and both methods are capable of depositing uniform Ni films on planar Si substrates and convex Si pyramids. In comparison, ACD has better tunability than GD, and it provides a more conformal Ni coating on complex and high-aspect-ratio Si structures, such as inverse fractal Si pyramids and ultralong Si nanowires. Our methodology and theoretical analyses can be leveraged to develop electroless plating processes for other metals and metal alloys and to generally provide direction for the adaptation of electroless deposition to modern applications.

Oriented Attachment Is a Major Control Mechanism To Form Nail-like Mn-Doped ZnO Nanocrystals.

Here, we present a controlled synthesis of Mn-doped ZnO nanoparticles (NPs) with predominantly nail-like shapes, whose formation occurs via tip-to-base-oriented attachment of initially formed nanopyramids, followed by leveling of sharp edges that lead to smooth single-crystalline "nails". This shape is prevalent in noncoordinating solvents such as octadecene and octadecane. Yet, the double bond in the former promotes oriented attachment. By contrast, Mn-doped ZnO NP synthesis in a weakly coordinating solvent, benzyl ether, results in dendritic structures because of random attachment of initial NPs. Mn-doped ZnO NPs possess a hexagonal wurtzite structure, and in the majority of cases, the NP surface is enriched with Mn, indicating a migration of Mn2+ ions to the NP surface during the NP formation. When the NP formation is carried out without the addition of octadecyl alcohol, which serves as a surfactant and a reaction initiator, large, concave pyramid dimers are formed whose attachment takes place via basal planes. UV-vis and photoluminescence spectra of these NPs confirm the utility of controlling the NP shape to tune electro-optical properties.

Polyhedral forms obtained by combinig lateral sheet of CP II-10 and truncated dodecahedron

The paper analyzes the possibility of obtaining polyhedral shapes formed by combining polyhedral surfaces based on the segment surface of elongated concave pyramids of the second sort (CeP II-10, type A and type B). In previous research, CP II type A and CP II type B were elaborated in detail. In this paper we discuss further potential of these polyhedral surfaces, on the example of combining them with Archimedean solid - Truncated dodecahedron (U26). The faces of this solid consist of 12 decagons and 20 triangles. On the decagonal faces, decagonal polygons of the CeP II segments (CP II-10) can be added, which provides the new polyhedral composite forms that are, furthermore, concave deltahedra. There are considered possibilities of obtaining polyhedral shapes by combining sheet segments CP II-10-A, as well as of CP II-10-B with U26. Finally, a couple of new shape suggestions are given: compound polyhedra, obtained by intersection of paired composite concave polyhedra originated in the described manner.

Improved performance of GaN-based light emitting diodes with nanopatterned sapphire substrates fabricated by wet chemical etching

The authors present a facile approach to the fabrication of nanopatterned sapphire substrates (NPSS) with concave pyramid structure by a wet etching method. The silica wet-etching mask on the sapphire substrate was prepared by using a colloidal monolayer templating strategy. NPSS with two different periods were prepared by employing 660 and 900 nm polystyrene colloidal spheres. Light emitting diode (LED) structure was epitaxially grown on the NPSS by the metal-organic chemical vapor deposition method. Both structural and optoelectronic properties of LEDs grown on flat sapphire substrate and NPSS were demonstrated and compared in detail. The calculations based on x-ray rocking curves revealed that the threading dislocation densities (TDDs) were decreased and the crystalline quality of epitaxial GaN film was improved when NPSS was used. Furthermore, the decrease in the edge TDDs was more prominent than that in the screw TDDs, which could be explained by the dislocation annihilation mechanism. The LEDs grown on 660- and 900-nm-NPSS exhibited 48% and 56% enhancement in light output power compared to that of a conventional LED on a flat sapphire substrate, arising from the nanoscale patterns in the sapphire which allowed more light to reflect from sapphire substrate onto the top direction.

The Contribution of Planes, Vertices, and Edges to Recombination at Pyramidally Textured Surfaces

We present a methodology by which one may distinguish three key contributors to enhanced recombination at pyramidally textured silicon surfaces. First, the impact of increased surface area is trivial and equates to a √3-fold increase in <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff,UL</sub> . Second, the presence of {1 1 1}-oriented facets drives a fivefold increase in <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff,UL</sub> at SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -passivated surfaces but a small (1.5-fold) increase for SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> passivation. A third factor, which is often proposed to relate to stress at convex and concave pyramids and edges, is shown to depend on pyramid period (and, hence, vertex/ridge density). This third factor impacts least on <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff,UL</sub> when the pyramid period is 10 μm. At this period, it results in a negligible increase in <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff,UL</sub> at SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -passivated textured surfaces but causes at least a sevenfold increase at the Si/SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> interface. Finally, we found that <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff,UL</sub> is 1.5-2.0 times higher at inverted pyramid texture than at surfaces featuring a random arrangement of upright pyramids. The results of this study, particularly for the Si/SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> system, likely depend on process conditions, but the methodology is universally applicable. We believe this to be the first study to distinguish the impact of {1 1 1} facets from those of vertices and edges. Further, we find that {1 1 1} surfaces, rather than vertices and edges, are chiefly responsible for the poor-quality passivation achieved by thick oxides on textured surfaces.

Pyramidal micromirrors for microsystems and atom chips

Concave pyramids are created in the (100) surface of a silicon wafer by anisotropic etching in potassium hydroxide. High quality micromirrors are then formed by sputtering gold onto the smooth silicon (111) faces of the pyramids. These mirrors show great promise as high quality optical devices suitable for integration into micro-optoelectromechanical systems and atom chips. We have shown that structures of this shape can be used to laser-cool and hold atoms in a magneto-optical trap.

Effect of biquadratic exchange coupling on magnetoresistance and magnetization process in magnetic bilayer systems

An improved model is proposed to deal with the magnetic bilayer systems taking into account the contribution of the anisotropy energy and biquadratic exchange coupling to elaborate on the evolution of the magnetoresistance (MR) ratio and magnetization process. The results indicate that the characteristic behavior of the MR ratio depends distinctly on both the biquadratic coupling constant and the layer thickness. The profile of the MR ratio was found to vary from an inverted bell shape to a concave pyramid with increasing biquadratic coupling strength, and decays sharply with the layer thickness. This model calculation helps us to provide a venue for further understanding the MR or giant magnetoresistance behavior of the magnetic multilayer system.