Pillar Density Research Articles

Behavioral transition of a fish school in a crowded environment.

In open water, social fish gather to form schools, in which fish generally align with each other. In this work, we study how this social behavior evolves when perturbed by artificial obstacles. We measure the behavior of a group of zebrafish in the presence of a periodic array of pillars. When the pillar density is low, the fish regroup with a typical interdistance and a well-polarized state with parallel orientations, similarly to their behavior in open-water conditions. Above a critical density of pillars, their social interactions, which are mostly based on vision, are screened and the fish spread randomly through the aquarium, orienting themselves along the free axes of the pillar lattice. The abrupt transition from natural to artificial orientation happens when the pillar interdistance is comparable to the social distance of the fish, i.e., their most probable interdistance. We develop a stochastic model of the relative orientation between fish pairs, taking into account alignment, antialignment, and tumbling, from a distribution biased by the environment. This model provides a good description of the experimental probability distribution of the relative orientation between the fish and captures the behavioral transition. Using the model to fit the experimental data provides qualitative information on the evolution of cognitive parameters, such as the alignment or the tumbling rates, as the pillar density increases. At high pillar density, we find that the artificial environment imposes its geometrical constraints to the fish school, drastically increasing the tumbling rate.

Post-growth tuning of detachable Ge membranes adhesion strength via porous Ge transformation

Single crystal germanium (Ge) membranes have recently gained increasing interest for lightweight and low-cost solar cells and flexible optoelectronic devices. These membranes achieved similar material quality as bulk Ge substrate. However, the control of the membrane detachment is still challenging. In this work, we explore post-growth engineering of the adhesion strength of a Ge membrane on a porous germanium (PGe) substrate by inducing morphological transformations in the separation layer through Thermal Budget (TB) control. Indeed, the pillars formed through PGe sintering during epitaxy are found to evolve with post-growth thermal annealing. Scanning electron microscopy (SEM) based analysis of the residue of the post-detachment broken pillars has been performed showing that the pillar's diameter and density can be tuned by thermal annealing. Depending on the post-growth annealing temperature, the membrane adhesion strength can be successively tailored from 0.5 to up to 3.5 MPa while ensuring 100 % detachment yield. The experimental results have been correlated with Finite Element Modeling (FEM) considering realistic pillar distribution revealing that pillar size and density are the dominant factors influencing the membrane adhesion strength.

A DFT-based kinetic Monte Carlo simulation of multiphase oxide-metal thin film growth

Functional thin films of nanoscale metal pillars in oxide or nitride matrices known as vertically aligned nanocomposite (VAN) have gained much interest owing to their unique strain-coupled and highly anisotropic properties. So far, the deposition of these films has been explored mostly experimentally. In this work, a density functional theory (DFT)-based kinetic Monte Carlo simulation model using Bortz–Kalos–Lebowitz algorithm was developed to understand the growth of VAN films deposited by pulsed laser technique on mismatching substrates. The model has been parameterized and applied to understand the kinetics of growth thin films consisting of Au pillars in CeO2 matrix deposited on SrTiO3 substrates. The effects of pulsed laser deposition (PLD) conditions including the pulse frequency, deposition flux, and substrate temperature were explored. The simulations indicate that the Au pillar size and shape exhibit significant dependence on the PLD conditions. Namely, increasing the temperature increases the average pillar size and lowers the pillar density, and vice versa. In addition, the simulations revealed that increasing the deposition rate results in lowering the average pillar size and increasing the density. Particularly, the DFT results suggest that Au pillar size can be tuned during the initial growth of the first monolayer due to the significantly low activation barrier. Our analysis showed that the relationship between the average pillar size and pillar density is influenced by the kinetics. Furthermore, autocorrelation analysis showed that pillars self-organize in quasi-ordered patterns at certain windows of the deposition conditions, which is attributed to the complex nature of the chemical interactions in the system, the kinetics, and the deposition parameters.

Soft wetting: an analytical model for pillar topography- and softness-dependent droplet depinning force.

The extent to which a droplet pins on a textured substrate is determined by the dynamics of the contact line and the liquid-vapor interface. However, the synergistic contribution of contact line sliding and interface distortion to the droplet depinning force remains unknown. More strikingly, current models fail to predict the depinning force per unit length of droplets on soft pillar arrays. Therefore, we fabricate soft pillar arrays with varying geometrical dimensions and mechanical properties and measure the depinning forces per unit length by allowing droplets to evaporate on such substrates. We then analyze the decrease in excess Gibbs free energy of the apparent droplet caused by the detachment of the droplet boundary from the previously pinned pillars. In contrast to prior notions, based on the measured decreases in excess Gibbs free energy, we find that the coefficient, that governs the ratio of interface distortion's contribution to the depinning force to that of the sliding contact line, increases with a decrease in pillar packing density. By considering the combined contribution from contact line sliding, liquid-vapor interface distortion, and pillar deflection, we introduce an analytical model to predict the droplet depinning force per unit length and corroborate the model using experimental data reported in this and prior studies.

Effects of donor-specific microvascular anatomy on hemodynamic perfusion in human choriocapillaris

Evidence from histopathology and clinical imaging suggest that choroidal anatomy and hemodynamic perfusion are among the earliest changes in retinal diseases such as age-related macular degeneration (AMD). However, how inner choroidal anatomy affects hemodynamic perfusion is not well understood. Therefore, we sought to understand the influences of choroidal microvascular architecture on the spatial distribution of hemodynamic parameters in choriocapillaris from human donor eyes using image-based computational hemodynamic (ICH) simulations. We subjected image-based inner choroid reconstructions from eight human donor eyes to ICH simulation using a kinetic-based volumetric lattice Boltzmann method to compute hemodynamic distributions of velocity, pressure, and endothelial shear stress. Here, we demonstrate that anatomic parameters, including arteriolar and venular arrangements and intercapillary pillar density and distribution exert profound influences on inner choroidal hemodynamic characteristics. Reductions in capillary, arteriolar, and venular density not only reduce the overall blood velocity within choriocapillaris, but also substantially increase its spatial heterogeneity. These first-ever findings improve understanding of how choroidal anatomy affects hemodynamics and may contribute to pathogenesis of retinal diseases such as AMD.

Microfluidic Assessment of T Cell Deformability and Capillary Network Occlusion

Introduction: Adoptive T cell therapies offer a new frontier of cancer treatment that disrupts the conventional chemotherapy paradigm. Naïve T cells can be genetically modified to directly target cancerous cells and have proven to be aggressively effective in fighting cancers resistant to chemotherapies. A major challenge that adoptive T cell therapies face is successfully delivering the cell products to the location of the target cells, which is particularly challenging in the case of solid tumors. A complete characterization of the biomechanical properties of T cell products would help researchers gain insight into the capacity of these engineered cells to navigate and extravasate through vasculature and tumor microenvironments. Previous studies have found that T cell volume, cortical tension, and dynamic resistance increase with activation (Waugh et al. Front Bioeng Biotechnol. 2023). A simple and reproducible method to easily quantify and assess such cellular properties has yet to be established. Presently, we describe a microfluidic assay for the rapid and robust characterization of the mechanical changes of T cells upon activation. Our assay models the microcapillaries through a gradient of micropillar arrays, which allows for the evaluation of T cells perfused through the network. The spacing between pillars is 12 µm at the first array, similar to small arterioles, and incrementally decreases to 3 µm spacing in the last array, mimicking small capillaries ( Figure 1A). Trafficking of adoptive T cell therapies through the microvasculature networks of this scale into their target tissues represents a persistent and major challenge facing these therapies (Marofi et al. Stem Cell Res. Ther. 2021). Methods: The microfluidic devices were fabricated by casting polydimethylsiloxane (PDMS) on a silicone wafer fabricated through photolithographic protocols. Devices contain arrays of pillars measuring 20 µm x 10 µm x 12 µm with gradient spacing beginning at 12 µm at the inlet and 3 µm at the outlet. Jurkat T cells were cultured in RPMI 1640 medium with 10% fetal bovine serum and used in a naïve state or activated via antibody stimulation of CD3 and CD28. T cells were then suspended in growth media at 1x10 6 cells/mL. Microchannels were wetted and cleaned with 100% ethanol perfused at 10 µL/min for 15 minutes followed by cell culture media at 10 µL/min for 15 minutes to block the non-specific binding sites on the PDMS and to create a biocompatible environment for the cell sample. The devices were connected to a digital pneumatic pressure-based flow pump system. The cell sample was perfused at 100 mBar for 7 minutes and a subsequent wash process with growth media at 100 mBar for 7 minutes was performed to flush non-occluded cells. Upon completion of the experiment, the channel was scanned using Phase and DAPI filters with an Olympus IX83 inverted motorized microscope, and occluded T cells were quantified using ImageJ. Results: Our assay showed distinct levels of T cell occlusion upon activation through CD3 and CD28 stimulation. Naïve T cells showed lower levels of occlusion in each of the first five arrays compared to activated T cells, while both groups showed high levels of occlusion in the final 3 µm array. Activated T cells showed approximately three times the amount of occlusion in the second through fifth arrays. To assess overall occlusion, we use the Occlusion Index (OI), which considers the density of pillars and the number of cells caught in each array. We found that activated T cells had a significantly higher OI when compared to non-activated T cell samples (p < 0.05, unpaired t-test). Naïve T cells had an OI mean of 24.4 ± 5.0 while activated T cells had a mean of 42.5 ± 5.6 ( Figure 1B). Conclusions: Our results show that there is a significant increase in resistance of activated T cells through a microvasculature-modeled environment compared to their naïve counterparts. As adoptive T cell therapies increase in popularity, a complete understanding of the biomechanical properties of these cell products will be necessary for effective design. Our microfluidic assay mimics the narrowing channels of the microvasculature and permits the study of how naïve and engineered T cells travel through these mechanically challenging spaces.

Soft Wetting: Droplet Receding Contact Angles on Soft Superhydrophobic Surfaces.

Despite intensive investigations on the droplet receding contact angle on superhydrophobic surfaces, i.e., a key parameter characterizing surface wettability and adhesion, the quantitative correlation between the surface structure mechanical properties (softness) and the droplet receding contact angles remains vague. By systematically varying the geometric dimensions and mechanical properties of soft pillar arrays, we find that the droplet receding contact angles decrease with the decrease in the pillar spring constant. Most surprisingly, the densely packed pillar arrays may result in larger receding contact angles than those on sparsely packed pillars, opposing the understanding of rigid pillar arrays, where the receding contact angles increase with a decrease in the packing density of pillars. This is attributed to the collective effects of capillarity and elasticity, where the energy consumed by the sliding contact line, the energy stored in the distorted liquid-vapor interface, and the energy stored in the deflected pillar contribute to the droplet depinning characteristics. We develop an analytical model to predict the droplet receding contact angles on soft superhydrophobic pillar arrays with knowledge of the material intrinsic receding contact angle, the pillar geometry, and the pillar mechanical properties. The predictions are corroborated by the experimental data measured in this and prior studies.

Vertical nanoscale strain-induced electronic localization in epitaxial La2/3Sr1/3MnO3 films with ZrO2 nanopillar inclusions

Unusual electrical transport properties associated with weak or strong localization are sometimes found in disordered electronic materials. Here, we report experimental observation of a crossover of electronic behavior from weak localization to enhanced weak localization due to the spatial influence of disorder induced by ZrO2 nanopillars in (La2/3Sr1/3MnO3)1−x:(ZrO2)x (x = 0, 0.2, and 0.3) nanocomposite films. The spatial strain regions, identified by scanning transmission electron microscopy and high-resolution x-ray diffraction, induce a coexistence of two-dimentional (2D) and three-dimentional (3D) localization and switches to typical 2D localization with increasing density of ZrO2 pillars due to length scale confinement, which interestingly accords with enhancing vertically interfacial strain. Based on the excellent agreement of our experimental results with one-parameter scaling theory of localization, the enhanced weak localization exists in metal range close to the fixed point. These films provide a tunable experimental model for studying localization in particular the transition regime by appropriate choice of the second epitaxial phase.Graphical

Deformability and collision-induced reorientation enhance cell topotaxis in dense microenvironments

In vivo, cells navigate through complex environments filled with obstacles such as other cells and the extracellular matrix. Recently, the term “topotaxis” has been introduced for navigation along topographic cues such as obstacle density gradients. Experimental and mathematical efforts have analyzed topotaxis of single cells in pillared grids with pillar density gradients. A previous model based on active Brownian particles (ABPs) has shown that ABPs perform topotaxis, i.e., drift toward lower pillar densities, due to decreased effective persistence lengths at high pillar densities. The ABP model predicted topotactic drifts of up to 1% of the instantaneous speed, whereas drifts of up to 5% have been observed experimentally. We hypothesized that the discrepancy between the ABP and the experimental observations could be in 1) cell deformability and 2) more complex cell-pillar interactions. Here, we introduce a more detailed model of topotaxis based on the cellular Potts model (CPM). To model persistent cells we use the Act model, which mimics actin-polymerization-driven motility, and a hybrid CPM-ABP model. Model parameters were fitted to simulate the experimentally found motion of Dictyostelium discoideum on a flat surface. For starved D. discoideum, the topotactic drifts predicted by both CPM variants are closer to the experimental results than the previous ABP model due to a larger decrease in persistence length. Furthermore, the Act model outperformed the hybrid model in terms of topotactic efficiency, as it shows a larger reduction in effective persistence time in dense pillar grids. Also pillar adhesion can slow down cells and decrease topotaxis. For slow and less-persistent vegetative D. discoideum cells, both CPMs predicted a similar small topotactic drift. We conclude that deformable cell volume results in higher topotactic drift compared with ABPs, and that feedback of cell-pillar collisions on cell persistence increases drift only in highly persistent cells.

Tunable Magnetic and Optical Anisotropy in ZrO2‐Co Vertically Aligned Nanocomposites

AbstractMetamaterials have gained great research interest in recent years owing to their potential for property tunability, multifunctionality, and property coupling. As a new group of self‐assembled thin films, vertically aligned nanocomposite (VAN)‐based hybrid metamaterials have been demonstrated with significant anisotropic physical properties and a broad range of property tailorability, such as optical anisotropy, magnetic anisotropy, hyperbolic dispersion, and enhanced second harmonic generation properties. Herein, self‐assembled ZrO2‐Co nanocomposite films, with high epitaxial quality and ultra‐fine vertically aligned Co nanopillars (with an average diameter of ≈2 nm) embedded in a ZrO2 matrix, are fabricated using a pulsed laser deposition (PLD) method. The Co pillar density can be effectively tuned by varying the Co concentration in the target, which results in tunable optical properties and magnetic properties. Specifically, a high saturation magnetization of 100 emu cm−3, strong out‐of‐plane magnetic anisotropy and tailorable magnetization properties are achieved via tuning the Co nanopillar density. Coupled with hyperbolic dispersion of dielectric constant from 950 to 1500 nm in wavelength, plasmonic Co metal nanopillars, and the unique dielectric ZrO2 matrix, this new nanoscale hybrid metamaterial shows great potential for future integrated optical and magnetic device designs.

Spreading, Breakup, and Rebound Behaviors of Compound Droplets Impacting on Microstructured Substrates.

In this study, we numerically investigate the dynamic behaviors of micron-scale compound droplets impacting onto superhydrophobic surfaces patterned by micropillar arrays using a three-dimensional free-energy-based lattice Boltzmann method. We address how the interplay between physical parameters (i.e., Weber number) and geometric parameters (i.e., pillar density and spacing and the droplet core-shell size ratio) affects the spreading, breakup, and rebound behaviors of compound droplets, which remains unknown and unquantified. We identify three flow regimes in which the interfacial morphology between the core and shell evolves and breaks up in distinct ways: namely, hole nucleation at the substrate, rupture of the film at the apex of the shell, and toroidal formation of the core droplet before its detachment from the pillars. We demonstrate that the transition between the three regimes and the maximum spreading factor of compound droplets can be changed by varying the core-shell size ratio, the pillar density, and the Weber number. The non-wetting behavior of the pillar structures eventually forms a new suspended pure droplet or a new suspended compound droplet, which can be characterized by the core-shell size ratio, pillar density, and Weber number.

Use of Polymer Micropillar Arrays as Templates for Solid-Phase Immunoassays.

We investigate the use of periodic micropillar arrays produced by high-fidelity microfabrication with cyclic olefin polymers for solid-phase immunoassays. These three-dimensional (3D) templates offer higher surface-to-volume ratios than two-dimensional substrates, making it possible to attach more antibodies and so increase the signal obtained by the assay. Micropillar arrays also provide the capacity to induce wicking, which is used to distribute and confine antibodies on the surface with spatial control. Micropillar array substrates are modified by using oxygen plasma treatment, followed by grafting of (3-aminopropyl)triethoxysilane for binding proteins covalently using glutaraldehyde as a cross-linker. The relationship between microstructure and fluorescence signal was investigated through variation of pitch (10–50 μm), pillar diameter (5–40 μm), and pillar height (5–57 μm). Our findings suggest that signal intensity scales proportionally with the 3D surface area available for performing solid-phase immunoassays. A linear relationship between fluorescence intensity and microscale structure can be maintained even when the aspect ratio and pillar density both become very high, opening the possibility of tuning assay response by design such that desired signal intensity is obtained over a wide dynamic range compatible with different assays, analyte concentrations, and readout instruments. We demonstrate the versatility of the approach by performing the most common immunoassay formats—direct, indirect, and sandwich—in a qualitative fashion by using colorimetric and fluorescence-based detection for a number of clinically relevant protein markers, such as tumor necrosis factor alpha, interferon gamma (IFN-γ), and spike protein of severe acute respiratory syndrome coronavirus 2. We also show quantitative detection of IFN-γ in serum using a fluorescence-based sandwich immunoassay and calibrated samples with spike-in concentrations ranging from 50 pg/mL to 5 μg/mL, yielding an estimated limit of detection of ∼1 pg/mL for arrays with high micropillar density (11561 per mm2) and aspect ratio (1:11.35).

Laser Desorption/Ionization Mass Spectrometry Using TiO2 Nanopillar Array Substrates with Tunable Surface Roughness and Wettability

The ability to sensitively detect trace quantities of substances is important in many practical applications. Herein, the use of TiO2 nanopillar arrays, formed using oblique electron-beam deposition and thermal treatment at 800 °C, as substrates for laser desorption/ionization mass spectrometry (LDI-MS) is reported. The TiO2 nanopillar arrays effectively assist ionization of various analytes, including small amino acids, a sugar, pesticides, peptides, and proteins with molecular weights of up to 24,000. The fabricated surface nanostructures were analyzed by measuring contact angles and applying the Cassie–Baxter and Wenzel wetting models. The TiO2 nanopillar surface density was shown to significantly affect the thermal properties of the substrate and the wettability of analyte-containing droplets. A TiO2 nanopillared substrate with a lower surface density exhibited more intense signals for the detection of small (∼1.2 kDa) analytes. The use of the benzyl pyridinium ion ([BP]+), as a thermometer chemical, revealed that TiO2 substrates with the lower pillar density exhibited effective heat confinement due to the pillar morphology, which plays a significant role in increasing the signal intensities of small molecules. The wetting behavior of the droplet on the substrate revealed that analyte enrichment, facilitated by surface hydrophobization, is another factor that contributes to enhancing small-molecule LDI performance. In contrast, the homogeneous adsorption of target molecules onto the nanopillared surface is likely to be a dominant factor for the detection of proteins. In this work, it is demonstrated that the appearance and disappearance of “coffee rings” during the evaporation of droplets on the surface are closely related to analyte distribution and, consequently, the intensities of the detected LDI-MS signals. The heat confinement effect of the TiO2 nanopillars, along with suitable sample wettability, delivers significant LDI performance.

Epitaxial pillar–matrix nanocomposite thin films of Bi–Ti–Fe–O and CoFe2O4 grown on SrTiO3 (110)

Controlling nanostructures and crystallographic orientations in epitaxial nanocomposite thin films are important for tuning their physical properties. Here, we present epitaxial nanocomposite thin films of (110) oriented CoFe2O4 (CFO) and (1110) oriented Bi5Ti3FeO15 (BTFO15) grown on SrTiO3 (110) substrates with a vertically aligned pillar–matrix type structure. The size and density of CFO pillars embedded in the BTFO matrix were controlled by the growth temperature and CFO concentration. Moreover, BTFO takes intergrowth phases with the general formula of Bi4Ti3O12 • nBiFeO3 (n = 1–∼1.5) depending on the growth temperature. Scanning probe analysis on the ferroelectric properties of BTFO15–CFO nanocomposite thin films suggested that the BTFO has a switchable out-of-plane polarization component originating from the tilted orientation of its a–b plane polarization. For high CFO pillar density, however, a non-negligible number of conductive paths which might be formed at the pillar–matrix interface are likely to prevent the polarization reversal.

3D enabled facile fabrication of substrates with human tongue characteristics for analysing the tribological behaviour of food emulsions

The mouth-feel or oral perception of a food may vary in different oral surfaces or tongue-palate environment depending on the physical characteristics of human tongue. Tribological measurements emulating the characteristics of tongue-palate environment will assist in the evaluation of oral perception of food materials. This study reports a 3D printing assisted approach for rapid fabrication of the soft elastomer substrates with different surface (roughness) and bulk characteristics (elastic modulus and thickness) for tribological measurements. The influence of substrates with various regularly patterned surfaces, elastic modulus, and thickness on the lubrication properties of four food emulsions (full-fat and skim milk, high-fat and low-fat yoghurt) has been investigated. It is observed that friction coefficient of food emulsions increases with a decrease in height or an increase in density of rectangular micro-scale pillars representing papillae on the surface of soft substrate. Interestingly, a decrease in the elastic modulus of the substrates causes an increase in the friction coefficient for the milk samples, whereas for yoghurt samples it causes a decrease. The variations in the thickness of the substrates have little effect on the friction coefficients of the food emulsions.

Non-planar platinum group metal-free fuel cell cathodes for enhanced oxygen transport and water rejection

Proton exchange membrane fuel cells (PEMFC) with cathodes using platinum group metal-free (PGM-free) catalysts could significantly reduce costs, but the lower volumetric oxygen reduction reaction (ORR) activity requires thick electrodes that suffer from liquid water flooding and increased oxygen transport resistance. To address these challenges, we developed a 3D gas diffusion electrode (GDE) architecture to enhance liquid water removal through the diffusion media and reduce cathode saturation. The cathode features a uniform catalyst layer adjacent to the membrane for high ORR activity and then pillars of the hydrophilic catalyst layer that pass through the microporous layer (MPL), providing a low capillary pressure barrier pathway to the carbon fiber paper layer and channel. The non-planar cathode also increases the interfacial area between the catalyst layer and hydrophobic MPL for a greater fraction of the cathode with high O2 concentration. Our studies included parametric experimental study of the pillar density to identify the optimum pitch between pillars. Our measurements show significant improvements in the mass transport region of the polarization curve with 3D structured electrodes leading to an 8% increase in maximum current density, 19% increase in maximum power density, and 16% increase in current density at 0.67 V with air.

Self‐Assembled Metal–Dielectric Hybrid Metamaterials in Vertically Aligned Nanocomposite Form with Tailorable Optical Properties and Coupled Multifunctionalities

Metal–dielectric hybrid metamaterials have attracted increasing research interest in recent years because of their novel optical properties and promising applications in the fields of electronic and photonic devices. Dielectric permittivity is a key parameter that strongly influences the optical properties of materials. By self‐assembling the metallic and dielectric components in a pillar‐in‐matrix structure, a strong anisotropic structure forms and results in opposite signs of permittivity components (i.e., ε|| > 0 and ε⊥ < 0, or vice versa) and exotic optical responses including hyperbolic dispersion in the visible to near‐infrared region. Herein, the main approaches of tuning the permittivity in self‐assembled metal–dielectric vertically aligned nanocomposite (VAN) thin films are reviewed, including tuning the metal pillar density and geometry, film strain state and background pressure during growth, and seeking other metal‐free and complex structure designs. Future research directions are also proposed, including unique approaches to improve their thermal stability, integrate on flexible substrates toward wearable device fabrication, and explore real‐time tunable metamaterials.

Selective-area growth study of GaN micropillars for quasi-vertical Schottky diodes

In this work, metal organic vapor phase epitaxy (MOVPE) is employed for selective-area growth (SAG) of undoped and lightly n-doped GaN micropillars on masked GaN-on-sapphire templates. In micropillar geometry, the limits of GaN drift layer thickness in hetereoepitaxial Schottky diodes are expected to be significantly pushed upwards. This is an important step towards the realization of GaN-based quasi-vertical power devices with high breakdown voltage (V br) on low-cost foreign substrates. Micropillar growth evolution and the impact of growth rate and V/III ratio on micropillar morphology and parasitic GaN deposition on AlOx hard masks are investigated. By using a combination of low growth rate and high V/III ratio, 12–22 µm high micropillars with planar pillar tops are grown in circular mask openings with radii of 35–100 µm. The threading dislocation density of the highest pillars is (1.4 ± 0.3) × 107 cm−2. Schottky diodes on micropillars with lowest net doping concentration (N D − N A) of 1.3 × 1016 cm−3 exhibit excellent forward characteristics with ideality factors , on/off-ratios up to 1010, and on-resistances R on 1 m cm−2.

Prediction of superconductivity in bilayer borophenes.

Borophenes and related two-dimensional materials have exhibited many exotic properties, especially for superconductivity, although the superconductivity of single-layer borophene is suppressed by the strains or doping from its substrates. Intriguingly, bilayer (BL) borophenes can be stabilized by appropriate pillar density and hexagonal holes density, rather than being supported by Ag(111) or Cu(111) substrates. Thus, we studied the two most stable structures, namely BL-B8 and BL-B30, stabilized by the above-mentioned two methods. Within density functional theory and Bardeen–Cooper–Schrieffer theory framework, their stability, electron structures, and phonon properties, as well as possible superconductivity are systematically scrutinized. The metallic BL-B8 and BL-B30 exhibit intrinsic superconducting features with superconductivity transition temperatures (Tc) of 11.9 and 4.9 K, respectively. The low frequency (below 400 cm−1) consisting of out-of-plane vibrations of boron atoms plays crucial rule in their superconductivity. In particular, a Kohn anomaly appears at the Γ point in BL-B8, leading to substantial electron–phonon coupling. Here, our findings will provide instructive clues for experimentally determining the superconductivity of borophene and will broaden the two-dimensional superconductor family.

Modelling analyses of the thermal property and heat transfer performance of a novel compositive PV vacuum glazing

This paper proposes an integrated photovoltaic vacuum glazing unit with an intermediate air cavity and a calibrated modelling approach to quantify its thermal properties and evaluate the heat transfer performance. Theoretical analyses of the heat transfer process are conducted with reasonable hypotheses and traceable boundary conditions. Three-dimensional heat transfer models are then established and cross-validated against previous publications. The detailed validation demonstrates the reliability of the developed complex models under different circumstances. Furthermore, four photovoltaic vacuum glazing configurations are compared in terms of the temperature distribution and overall heat transfer coefficient (i.e. U-value). Simulation results show that the photovoltaic vacuum double glazing can achieve the optimum performance among the four configurations based on simultaneous consideration of the PV module temperature and U-value. Sensitivity analyses of glazing design factors are also conducted for the U-value, which is found to be greatly reduced by decreasing the density and diameter of vacuum pillars as well as the glass thermal conductivity. A lowest U-value of 0.23 W/(m2·K) is achieved for the photovoltaic-vacuum double glazing and can be further improved with future design optimizations. This research can provide guidance to design improvement of PV vacuum glazing systems and promote their integration with building modelling tools.