Aligned Edges Research Articles

A mechanism for thickness-controllable single crystalline 2D materials growth

Recent efforts in growing two-dimensional (2D) multilayers have enabled the synthesis of single crystalline 2D multilayers in a wafer scale through the seamless stitching of multiple epitaxial 2D islands. Unlike previously observed wedding-cake or inverted-wedding-cake structures, these multilayer islands have the same size and shape in each layer with aligned edges. In this study, we investigated the underlying growth mechanisms of synchronic 2D multilayers growth and have showed that a heterogenous layer on a crystalline substrate is critical for maintaining the synchronic growth of 2D multilayers. During growth, the heterogenous layer passivates the edges of multilayer 2D island and thus prevents the coalescence of these active edges, while the high interfacial energy between the heterogenous surface layer and the substrate stabilizes the synchronic structure. Based on this model, we have successfully explained the previously observed synchronic growth of graphene and hexagonal boron nitride multilayers (Nat Nanotech 2020, 15: 861; Nature 2022, 606: 88). The deep understanding on the mechanism paves a way towards the synthesis of wafer-scale single-crystal 2D multilayers with a uniform thickness.

Interpolating Graph Pair to Regularize Graph Classification

We present a simple and yet effective interpolation-based regularization technique, aiming to improve the generalization of Graph Neural Networks (GNNs) on supervised graph classification. We leverage Mixup, an effective regularizer for vision, where random sample pairs and their labels are interpolated to create synthetic images for training. Unlike images with grid-like coordinates, graphs have arbitrary structure and topology, which can be very sensitive to any modification that alters the graph's semantic meanings. This posts two unanswered questions for Mixup-like regularization schemes: Can we directly mix up a pair of graph inputs? If so, how well does such mixing strategy regularize the learning of GNNs? To answer these two questions, we propose ifMixup, which first adds dummy nodes to make two graphs have the same input size and then simultaneously performs linear interpolation between the aligned node feature vectors and the aligned edge representations of the two graphs. We empirically show that such simple mixing schema can effectively regularize the classification learning, resulting in superior predictive accuracy to popular graph augmentation and GNN methods.

Multimodal Biofilm Inactivation Using a Photocatalytic Bismuth Perovskite-TiO2-Ru(II)polypyridyl-Based Multisite Heterojunction.

Infectious bacterial biofilms are recalcitrant to most antibiotics compared to their planktonic version, and the lack of appropriate therapeutic strategies for mitigating them poses a serious threat to clinical treatment. A ternary heterojunction material derived from a Bi-based perovskite-TiO2 hybrid and a [Ru(2,2'-bpy)2(4,4'-dicarboxy-2,2'-bpy)]2+ (2,2'-bpy, 2,2'-bipyridyl) as a photosensitizer (RuPS) is developed. This hybrid material is found to be capable of generating reactive oxygen species (ROS)/reactive nitrogen species (RNS) upon solar light irradiation. The aligned band edges and effective exciton dynamics between multisite heterojunctions are established by steady-state/time-resolved optical and other spectroscopic studies. Proposed mechanistic pathways for the photocatalytic generation of ROS/RNS are rationalized based on a cascade-redox processes arising from three catalytic centers. These ROS/RNS are utilized to demonstrate a proof-of-concept in treating two elusive bacterial biofilms while maintaining a high level of biocompatibility (IC50 > 1 mg/mL). The in situ generation of radical species (ROS/RNS) upon photoirradiation is established with EPR spectroscopic measurements and colorimetric assays. Experimental results showed improved efficacy toward biofilm inactivation of the ternary heterojunction material as compared to their individual/binary counterparts under solar light irradiation. The multisite heterojunction formation helped with better exciton delocalization for an efficient catalytic biofilm inactivation. This was rationalized based on the favorable exciton dissociation followed by the onset of multiple oxidation and reduction sites in the ternary heterojunction. This together with exceptional photoelectric features of lead-free halide perovskites outlines a proof-of-principle demonstration in biomedical optoelectronics addressing multimodal antibiofilm/antimicrobial modality.

Higher-order adaptive virtual element methods with contraction properties

<abstract><p>The realization of a standard Adaptive Finite Element Method (AFEM) preserves the mesh conformity by performing a completion step in the refinement loop: In addition to elements marked for refinement due to their contribution to the global error estimator, other elements are refined. In the new perspective opened by the introduction of Virtual Element Methods (VEM), elements with hanging nodes can be viewed as polygons with aligned edges, carrying virtual functions together with standard polynomial functions. The potential advantage is that all activated degrees of freedom are motivated by error reduction, not just by geometric reasons. This point of view is at the basis of the paper [L. Beirão da Veiga et al., "Adaptive VEM: stabilization-free a posteriori error analysis and contraction property", SIAM Journal on Numerical Analysis, vol. 61, 2023], devoted to the convergence analysis of an adaptive VEM generated by the successive newest-vertex bisections of triangular elements without applying completion, in the lowest-order case (polynomial degree $ k = 1 $). The purpose of this paper is to extend these results to the case of VEMs of order $ k\ge2 $ built on triangular meshes. The problem at hand is a variable-coefficient, second-order self-adjoint elliptic equation with Dirichlet boundary conditions; the data of the problem are assumed to be piecewise polynomials of degree $ k-1 $. By extending the concept of global index of a hanging node, under an admissibility assumption of the mesh, we derive a stabilization-free a posteriori error estimator. This is the sum of residual-type terms and certain virtual inconsistency terms (which vanish for $ k = 1 $). We define an adaptive VEM of order $ k $ based on this estimator, and we prove its convergence by establishing a contraction result for a linear combination of (squared) energy norm of the error, (squared) residual estimator, and (squared) virtual inconsistency estimator.</p></abstract>

The SWSe-BP vdW Heterostructure as a Promising Photocatalyst for Water Splitting with Power Conversion Efficiency of 19.4.

Hydrogen generation by photocatalytic water splitting has drawn enormous research attention for converting sunlight and water into clean and green hydrogen fuel. However, the search for a high efficiency photocatalyst for water splitting is a key challenge. Two dimensional (2D) van der Waals (vdW) heterostructures as photocatalysts exhibit many advantages over the stacked original materials. In this article, we designed two novel 2D vdW heterostructures composed of WSSe and blue phosphorene (BP) monolayers, SWSe-BP and SeWS-BP, which are thermodynamically stable at room temperature. Using first-principles calculations, we found that the SWSe-BP vdW heterostructure can act as a potential photocatalyst for water splitting due to its robust stabilities, type-II band alignment, moderate bandgap, and suitable band edge positions for the redox reactions of water splitting, strong optical absorption, and excellent power conversion efficiency (PCE). Remarkably, the PCE of the SWSe-BP vdW heterostructure can achieve approximately 19.4% under a 3% biaxial tensile strain.

The Intrinsic Thermodynamic Difficulty and a Step-Guided Mechanism for the Epitaxial Growth of Uniform Multilayer MoS2 with Controllable Thickness.

Multilayer MoS2 shows superior performance over the monolayer MoS2 for electronic devices while the growth of multilayer MoS2 with controllable and uniform thickness is still very challenging. It is revealed by calculations that monolayer MoS2 domains are thermodynamically much more favorable than multilayer ones on epitaxial substrates due to the competition between surface interactions and edge formation, leading accordingly to a layer-by-layer growth pattern and non-continuously distributed multilayer domains with uncontrollable thickness uniformity. The thermodynamics model also suggests that multilayer MoS2 domains with aligned edges can significantly reduce their free energy and represent a local minimum with very prominent energy advantage on a potential energy surface. However, the nucleation probability of multilayer MoS2 domains with aligned edges is, if not impossible, extremely rare on flat substrates. Herein, a step-guided mechanism for the growth of uniform multilayer MoS2 on an epitaxial substrate is theoretically proposed. The steps with proper height on sapphire surface are able to guide the simultaneous nucleation of multilayer MoS2 with aligned edges and uniform thickness, and promote the continuous growth of multilayer MoS2 films. The proposed mechanism can be reasonably extended to grow multilayer 2D materials with uniform thickness on epitaxial substrates.

Classical-Contextual Interactions in V1 May Rely on Dendritic Computations

A signature feature of the neocortex is the dense network of horizontal connections (HCs) through which pyramidal neurons (PNs) exchange “contextual” information. In primary visual cortex (V1), HCs are thought to facilitate boundary detection, a crucial operation for object recognition, but how HCs modulate PN responses to boundary cues within their classical receptive fields (CRF) remains unknown. We began by “asking” natural images, through a structured data collection and ground truth labeling process, what function a V1 cell should use to compute boundary probability from aligned edge cues within and outside its CRF. The “answer” was an asymmetric 2-D sigmoidal function, whose nonlinear form provides the first normative account for the “multiplicative” center-flanker interactions previously reported in V1 neurons (Kapadia et al., 1995, 2000; Polat et al., 1998). Using a detailed compartmental model, we then show that this boundary-detecting classical-contextual interaction function can be computed by NMDAR-dependent spatial synaptic interactions within PN dendrites – the site where classical and contextual inputs first converge in the cortex. In additional simulations, we show that local interneuron circuitry activated by HCs can powerfully leverage the nonlinear spatial computing capabilities of PN dendrites, providing the cortex with a highly flexible substrate for integration of classical and contextual information.

Excited-State Properties of CuInP2S6 Monolayer as Photocatalyst for Water Splitting.

In recent years, two-dimensional (2D) materials of ferroelectricity have been illustrated to have great potential in solar energy conversion processes such as photocatalytic water splitting, although the optical properties of such materials are rarely discussed. In combination with the first-principles calculations, many-body Green's function method was used to obtain the excited-state properties of the representative CuInP2S6 to unravel the ingredients affecting the photocatalytic behavior. In particular, quasiparticle (QP) band gap correction and bound exciton binding energy are 1.25/1.38 and 0.93/0.87 eV for paraelectric/ferroelectric CuInP2S6, respectively. In addition to facilitating the charge carrier recombination, here we emphasize that the large exciton binding energy reduces the reduction potential of the photoexcited electrons. In bilayer structures, the improved photocatalytic performance should be ascribed to the type-II band alignment and large band edge offsets (0.44 and 0.33 eV for CuInP2S6), rather than the increased light absorption due to the reduced band gap.

Synthesis and Characterization of an α-Fe2O3-Decorated g-C3N4 Heterostructure for the Photocatalytic Removal of MO.

This study describes the preparation of graphitic carbon nitride (g-C3N4), hematite (α-Fe2O3), and their g-C3N4/α-Fe2O3 heterostructure for the photocatalytic removal of methyl orange (MO) under visible light illumination. The facile hydrothermal approach was utilized for the preparation of the nanomaterials. Powder X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy dispersive X-ray (EDX), and Brunauer–Emmett–Teller (BET) were carried out to study the physiochemical and optoelectronic properties of all the synthesized photocatalysts. Based on the X-ray photoelectron spectroscopy (XPS) and UV-visible diffuse reflectance (DRS) results, an energy level diagram vs. SHE was established. The acquired results indicated that the nanocomposite exhibited a type-II heterojunction and degraded the MO dye by 97%. The degradation ability of the nanocomposite was higher than that of pristine g-C3N4 (41%) and α-Fe2O3 (30%) photocatalysts under 300 min of light irradiation. The formation of a type-II heterostructure with desirable band alignment and band edge positions for efficient interfacial charge carrier separation along with a larger specific surface area was collectively responsible for the higher photocatalytic efficiency of the g-C3N4/α-Fe2O3 nanocomposite. The mechanism of the nanocomposite was also studied through results obtained from UV-vis and XPS analyses. A reactive species trapping experiment confirmed the involvement of the superoxide radical anion (O2•−) as the key reactive oxygen species for MO removal. The degradation kinetics were also monitored, and the reaction was observed to be pseudo-first order. Moreover, the sustainability of the photocatalyst was also investigated.

Ultrahigh Rate Supercapacitor based on Self-Standing Carbon Nanotubes Supported Vertically Aligned MoS2 Sheets

Abstract Recently, two-dimensional layered structures, especially MoS2 has come out as the most investigated electrode material for batteries and supercapacitors, possessing well preserved in-plane covalent bonding, leading to extraordinary mechanical elasticity within the layers as well as outstanding firmness along the c-axis. The present work is aimed to fabricate vertically aligned edge exposed molybdenum disulfide nanoflakes on the surface of the self-standing hydrophilic carbon nanotubes, using a two-step process involving a chemical route and magnetron sputtering techniques for flexible supercapacitor application. These hybrid heterostructures have been characterized using XRD, FESEM, and cyclic voltammetry. In the aqueous electrolyte of 1M Li2SO4, the symmetric device revealed very high areal capacitance of 182.5 mF/cm2 and 155 mF/cm2, at scan rates of 2 and 5 mV/s, respectively. The device shows high energy density of 365 mWh/cm2.

Bismuth sulphide decorated ZnO nanorods heterostructure assembly via controlled SILAR cationic concentration for enhanced photoelectrochemical cells

The current study investigates Bi2S3 thin films coated on ZnO NRAs with varying cationic concentrations through ionic layer adsorption and reaction (SILAR) technique. XRD patterns reveal that Bi2S3 is successfully synthesised and exhibits orthorhombic structure on the wurtzite ZnO NRAs. The band gap energy (Eg) of Bi2S3/ZnO NRAs shows a notable red shift with increasing cationic concentration. The photocurrent density increases significantly with increasing concentration from 1 mM to 3 mM before decreases at higher concentration due to agglomeration of Bi2S3 NPs and formation of recombination centres. The hybrid photoanode Bi2S3/ZnO NRAs at 3 mM exhibits the highest photocurrent value (1.92 mA cm−2), which is about six times greater than that of plain ZnO NRAs (0.337 mA cm−2). The high photoconversion efficiency value of 1.65% versus 0.5 V Ag A−1g−1C−1l−1 is obtained by Bi2S3/ZnO NRAs (3 mM) in comparison with pristine ZnO NRs, mainly due to the stepwise band alignment edge and significant enhancement of morphological and optical properties. The study reveals that controlling the cationic concentration can potentially improve the photoconversion efficiency.

Edge morphology effect on field emission properties of graphene thin films

Field emission current density from the edge of graphene thin films is calculated in terms of their edge morphology, using density functional theory combined with the effective screening medium method and the Fowler-Nordheim theory. The field emission current from edges of the bilayer graphene is higher than that from the monolayer graphene edges. Edge displacement of the bilayer graphene causes a higher emission current than the aligned edges for both zigzag and armchair edges. Bilayer graphene with closed edges further enhances the emission current compared with those with open clean edges, owing to the decrease of their work functions and the potential barrier outside them.

A constrained localization algorithm for improving the efficiency and accuracy of forming curved hull plates in shipbuilding

ABSTRACT Localization with edges constrained is an effective method for improving the efficiency of fabricating curved plates. The iterative closest point (ICP) algorithm is one of the most widely used methods for localization, while the original ICP method cannot directly be applied to the localization under the edges constrained. In this paper, a two-stage localization approach (the edges are aligned first, and then the whole curved plate is aligned under the edges constrained) is proposed. In the first stage, only the constrained edges are aligned for achieving the optimised localization of them, which can accelerate the convergence of the whole algorithm and improve the accuracy of the aligned edges. In the second stage, the whole curved plate is aligned under the edges constrained. In this stage, a point-to-projection technique is used for efficiently searching correspondences in the ICP frame. Furthermore, a novel double-transformations technique for computing the transformation matrix is also proposed in this stage for ensuring the accuracy of the constrained edges when aligning the whole plate. The results of extensive experiments show that the proposed algorithm has high accuracy and efficiency, and outperforms the existing methods by a large margin.

Strain-Energy Release in Bent Semiconductor Nanowires Occurring by Polygonization or Nanocrack Formation.

Strain engineering of semiconductors is used to modulate carrier mobility, tune the energy bandgap, and drive growth of self-assembled nanostructures. Understanding strain-energy relaxation mechanisms including phase transformations, dislocation nucleation and migration, and fracturing is essential to both exploit this degree of freedom and avoid degradation of carrier lifetime and mobility, particularly in prestrained electronic devices and flexible electronics that undergo large changes in strain during operation. Raman spectroscopy, high-resolution transmission electron microscopy, and electron diffraction are utilized to identify strain-energy release mechanisms of bent diamond-cubic silicon (Si) and zinc-blende GaAs nanowires, which were elastically strained to >6% at room temperature and then annealed at an elevated temperature to activate relaxation mechanisms. High-temperature annealing of bent Si-nanowires leads to the nucleation, glide, and climb of dislocations, which align themselves to form grain boundaries, thereby reducing the strain energy. Herein, Si nanowires are reported to undergo polygonization, which is the formation of polygonal-shaped grains separated by grain boundaries consisting of aligned edge dislocations. Furthermore, strain is shown to drive dopant diffusion. In contrast to the behavior of Si, GaAs nanowires release strain energy by forming nanocracks in regions of tensile strain due to the weakening of As-bonds. These insights into the relaxation behavior of highly strained crystals can inform the design of nanoelectronic devices and provide guidance on mitigating degradation.

Unsupervised Video Object Foreground Segmentation and Co-Localization by Combining Motion Boundaries and Actual Frame Edges

In this article the authors proposed a fast and fully unsupervised approach for a foreground object co-localization and segmentation of unconstrained videos. This article first computes both the actual edges and motion boundaries of the video frames, and then aligns them by the proposed HOG affinity map approach. Then, by filling the occlusions generated by the aligned edges, the paper obtained more precise masks about the foreground object. With an accumulation process, these masks could be derived as the motion-based likelihood, which is used as a unary term in the proposed graph model. Another unary term is called color-based likelihood, which is computed by the color distribution of foreground and background. Experiment results shows the method is fast and effective to detect and segment foreground objects.

(Invited) Oxide Nanowires Array Based CdSe/CdS/TiO2 1-D Heterojunction Photoelectrode for Enhanced Solar Fuel Conversion

Production of hydrogen by photoelectrochemical system has received considerable interest as one of the promising route without generation any undesired pollutants. A major component of a photoelectrochemical (PEC) water splitting system is the photoelectrode where critical processes such as light harvesting, generation of charge carriers, minority carrier migration towards its interface with the electrolyte, as well as majority carrier transportation towards the external circuit for water splitting. Co-sensitization by double-shell of QDs (e.g., CdSe/CdS) for the TiO2 or ZnO nanostructures demonstrated a large PEC performance improvement due to their extension of light absorption range. A sequential coating of two different light absorbers on the electrode (e.g. CdS/CdSe multilayer structure on TiO2) forms terraces in their band diagram (i.e, type II heterojunction), thus the aligned band edge promotes separation of electrons and holes and cascade-like carrier transport at their interface, and thereby increases the photocurrent The deposition of these QDs on one-dimensional (1-D) nanostructures of semiconductor including nanowires/nanorods (NWs/NRs) and nanotubes (NTs) increases the charge transport properties by providing a “highway” for electron travel. Although a few recent studies have shown that the ITO NWs improves the charge transport/collection for a single layer light absorber, their overall PEC performance is still lower than theoretically expected value because the photoelectrode absorb only a part of visible light (by a single layer absorber) and/or photo-generated charge carriers are largely recombined in the light absorber layer due to a low charge separation efficiency. In the present study, we employed In2O3 and SnO2 NWs as a support for the CdSe/CdS/TiO2 multi-shelled heterojunction photoelectrode to demonstrate a largely enhanced PEC hydrogen production by improving the charge transport property. PEC performance of the resultant CdSe/CdS/TiO2/ITO NWs photoelectrode for the hydrogen production was investigated and they are compared with those of a planar structured and m-TiO2 nanoparticles based CdSe/CdS heterojunction photoelectrodes. The resultant 1-D heterojunction photoelectrode (CdSe/CdS/TiO2/ITO NWs) shows improved light absorption and charge transport properties in comparison to the planar structured photoelectrode (without the ITO NWs) and mesoporous TiO2 nanoparticles film based photoelectrode. We found that the ITO NWs based CdSe/CdS/TiO2 multi-shelled photoelectrode exhibits photocurrent density of ~16.2 mA/cm2, which is 2.4 times higher than that of m-TiO2 nanoparticles based photoelectrode and even outperforms previously reported TiO2 or ZnO NWs based photoelectrodes.

Image Registration and Fusion of Visible and Infrared Integrated Camera for Medium-Altitude Unmanned Aerial Vehicle Remote Sensing

This study proposes a novel method for image registration and fusion via commonly used visible light and infrared integrated cameras mounted on medium-altitude unmanned aerial vehicles (UAVs).The innovation of image registration lies in three aspects. First, it reveals how complex perspective transformation can be converted to simple scale transformation and translation transformation between two sensor images under long-distance and parallel imaging conditions. Second, with the introduction of metadata, a scale calculation algorithm is designed according to spatial geometry, and a coarse translation estimation algorithm is presented based on coordinate transformation. Third, the problem of non-strictly aligned edges in precise translation estimation is solved via edge–distance field transformation. A searching algorithm based on particle swarm optimization is introduced to improve efficiency. Additionally, a new image fusion algorithm is designed based on a pulse coupled neural network and nonsubsampled contourlet transform to meet the special requirements of preserving color information, adding infrared brightness information, improving spatial resolution, and highlighting target areas for unmanned aerial vehicle (UAV) applications. A medium-altitude UAV is employed to collect datasets. The result is promising, especially in applications that involve other medium-altitude or high-altitude UAVs with similar system structures.

Restoration for Out-of-Focus Color Image Based on Gradient Profile Sharpness

In color images, out-of-focus problems often occur when different wavelengths of rays are focused at different positions in the focal plane. This occurs because of lenses that have different refractive indices for different wavelengths of light. These color images in turn become blurred, and noticeable colored edges appear around objects. These misaligned edges thus degrade the overall quality of the images. In this study, we propose a restoration algorithm for misaligned edges in color images. This algorithm is based on the assumption that the gradients of color channels are highly correlated such that all edges spatially overlap in the same manner as the desired gradient. The constraint term in least squares optimization is proposed to align edges to match the desired gradient based on the transformation theory of gradient profile sharpness. The proposed constraint term adaptively uses the gradients of the channels with different weights to estimate sharp edges. We also design a new measurement to compute the energy of aligned edges in a color image. The proposed algorithm can be applied to images captured by various sensors in different environments. Experimental results show that the proposed algorithm performs effectively when estimating high-quality color images.

Controllable nanoscale engineering of vertically aligned MoS2 ultrathin nanosheets by nitrogen doping of 3D graphene hydrogel for improved electrocatalytic hydrogen evolution

Three-dimensional (3D) porous molybdenum disulfide/nitrogen-doped reduced graphene oxide (MoS2/N-rGO) hydrogels were fabricated through a facile and controllable one-pot hydrothermal method. The nanosized MoS2 ultrathin nanosheets were uniformly and vertically dispersed on the rGO framework after nitrogen incorporation. The incorporated nitrogen in rGO played a key role for nano-scaling of MoS2 due to the protonation at pyridinic N-doping sites on carbon surface. The vertically aligned edge of nanosized MoS2 sheets, nitrogen incorporation of rGO and 3D network structure made the MoS2/N-rGO highly efficient for hydrogen evolution reaction, with improved double-layer capacitance and turnover frequency, small onset overpotential of 119 mV, low Tafel slope of 36 mV·decade−1 and superior long-time catalytic stability.

Photocatalytic Properties of g-C6N6/g-C3N4 Heterostructure: A Theoretical Study

As a promising photocatalytic material in water splitting and organic degradation, the polymeric graphitic g-C3N4 has attracted intensive research interest during the past decade due to the visible light response, nontoxicity, abundance, easy preparation, as well as high thermal and chemical stability. However, the low efficiency owing to the fast charge recombination limits its practical applications. In the present work, we systematically investigated the electronic structure and photocatalytic properties of layered g-C6N6/g-C3N4 heterostructure on the basis of first-principles calculations. The results show that the type-II heterojunction can be established between g-C6N6 and g-C3N4 monolayers due to a perfect lattice match and aligned band edges, facilitating the separation of photogenerated carriers. In addition, it is worthwhile to note that hole effective masses of g-C6N6/g-C3N4 heterostructure can be significantly reduced compared to pristine g-C3N4 due to orbital hybridization between the two mon...