Monolayer Edge Research Articles

Direct Quantifying Charge Transfer by 4D-STEM: A Study on Perfect and Defective Hexagonal Boron Nitride.

Four-dimensional scanning transmission electron microscopy (4D-STEM) offers an attractive approach to simultaneously obtain precise structural determinations and capture details of local electric fields and charge densities. However, accurately extracting quantitative data at the atomic scale poses challenges, primarily due to probe propagation and size-related effects, which may even lead to misinterpretations of qualitative effects. In this study, we present a comprehensive analysis of electric fields and charge densities in both pristine and defective h-BN flakes. Through a combination of experiments and first-principle simulations, we demonstrate that while precise charge quantification at individual atomic sites is hindered by probe effects, 4D-STEM can directly measure charge transfer phenomena at the monolayer edge with sensitivity down to a few tenths of an electron and a spatial resolution on the order of a few angstroms.

Molybdenum Disulfide Nanoribbons with Enhanced Edge Nonlinear Response and Photoresponsivity.

MoS2 nanoribbons have attracted increased interest due to their properties, which can be tailored by tuning their dimensions. Herein, the growth of MoS2 nanoribbons and triangular crystals formed by the reaction between films of MoOx (2<x<3)grown by pulsed laser deposition and NaF in a sulfur-rich environment is demonstrated. The nanoribbons can reach up to 10µm in length, and feature single-layeredges, forming a monolayer-multilayer junction enabled by the lateral modulation in thickness. The single-layer edges show a pronounced second harmonic generation due to the symmetry breaking, in contrast to the centrosymmetric multilayer structure, which is unsusceptible to the second-order nonlinear process. A splitting of the Raman spectra is observed in MoS2 nanoribbons arising from distinct contributions from the single-layer edges and multilayer core. Nanoscale imaging reveals a blue-shifted exciton emission of the monolayer edge compared to the isolated MoS2 monolayers due to built-in local strain and disorder. Wefurther report on an ultrasensitive photodetector made of a single MoS2 nanoribbon with a responsivity of 8.72×102 AW-1 at 532nm, among the highest reported up-to-date for single-nanoribbon photodetectors. These findings can inspire the design of MoS2 semiconductors with tunable geometries for efficient optoelectronic devices.

Glucose and Inositol Transporters, SLC5A1 and SLC5A3, in Glioblastoma Cell Migration.

(1) Background: The recurrence of glioblastoma multiforme (GBM) is mainly due to invasion of the surrounding brain tissue, where organic solutes, including glucose and inositol, are abundant. Invasive cell migration has been linked to the aberrant expression of transmembrane solute-linked carriers (SLC). Here, we explore the role of glucose (SLC5A1) and inositol transporters (SLC5A3) in GBM cell migration. (2) Methods: Using immunofluorescence microscopy, we visualized the subcellular localization of SLC5A1 and SLC5A3 in two highly motile human GBM cell lines. We also employed wound-healing assays to examine the effect of SLC inhibition on GBM cell migration and examined the chemotactic potential of inositol. (3) Results: While GBM cell migration was significantly increased by extracellular inositol and glucose, it was strongly impaired by SLC transporter inhibition. In the GBM cell monolayers, both SLCs were exclusively detected in the migrating cells at the monolayer edge. In single GBM cells, both transporters were primarily localized at the leading edge of the lamellipodium. Interestingly, in GBM cells migrating via blebbing, SLC5A1 and SLC5A3 were predominantly detected in nascent and mature blebs, respectively. (4) Conclusion: We provide several lines of evidence for the involvement of SLC5A1 and SLC5A3 in GBM cell migration, thereby complementing the migration-associated transportome. Our findings suggest that SLC inhibition is a promising approach to GBM treatment.

Methodology for comprehensive cell-level analysis of wound healing experiments using deep learning in MATLAB

BackgroundEndothelial healing after deployment of cardiovascular devices is particularly important in the context of clinical outcome. It is therefore of great interest to develop tools for a precise prediction of endothelial growth after injury in the process of implant deployment. For experimental investigation of re-endothelialization in vitro cell migration assays are routinely used. However, semi-automatic analyses of live cell images are often based on gray value distributions and are as such limited by image quality and user dependence. The rise of deep learning algorithms offers promising opportunities for application in medical image analysis. Here, we present an intelligent cell detection (iCD) approach for comprehensive assay analysis to obtain essential characteristics on cell and population scale.ResultsIn an in vitro wound healing assay, we compared conventional analysis methods with our iCD approach. Therefore we determined cell density and cell velocity on cell scale and the movement of the cell layer as well as the gap closure between two cell monolayers on population scale. Our data demonstrate that cell density analysis based on deep learning algorithms is superior to an adaptive threshold method regarding robustness against image distortion. In addition, results on cell scale obtained with iCD are in agreement with manually velocity detection, while conventional methods, such as Cell Image Velocimetry (CIV), underestimate cell velocity by a factor of 0.5. Further, we found that iCD analysis of the monolayer movement gave results just as well as manual freehand detection, while conventional methods again shows more frayed leading edge detection compared to manual detection. Analysis of monolayer edge protrusion by ICD also produced results, which are close to manual estimation with an relative error of 11.7%. In comparison, the conventional Canny method gave a relative error of 76.4%.ConclusionThe results of our experiments indicate that deep learning algorithms such as our iCD have the ability to outperform conventional methods in the field of wound healing analysis. The combined analysis on cell and population scale using iCD is very well suited for timesaving and high quality wound healing analysis enabling the research community to gain detailed understanding of endothelial movement.

Energetics and kinetics of monolayer formation in vapor-liquid-solid nanowire growth

The vapor-liquid-solid growth of semiconductor nanowires proceeds via the sequential addition of individual atomic or biatomic monolayers at the interface between the nanowire stem and a liquid catalyst droplet. Each monolayer growth cycle comprises a period of nucleation and extension of the monolayer followed by a waiting time before the next nucleation. The cyclic evolution of the liquid-solid system and the duration of these periods affect the properties of individual nanowires and the statistics of nanowire ensembles. Based on recent in situ growth experiments, we investigate theoretically the thermodynamics and kinetics of monolayer formation, with emphasis on nanowires of III-V materials. We first study the thermodynamics of the process in a quasistatic approximation. We calculate and minimize the work of formation of a partial monolayer, accounting for the depletion and refill of the liquid phase and all relevant interface energies. We focus on the regime where the liquid does not contain enough nanowire material at nucleation to form a full monolayer and where growth comprises a fast stage where a partial monolayer forms rapidly using the available material and a slower stage during droplet refill. We explore the effect of the nonmonotonous variation of the total monolayer edge energy on these two growth stages and determine the partition between liquid and monolayer of the atoms provided during refill. We then develop a kinetic model that accounts in simple terms for the competition between delivery and attachment of atoms to the monolayer and droplet refill. We perform fully analytical calculations that describe a continuum between the pulsed regime explored in our thermodynamic study and a smooth regime where the monolayer extends at the pace of refill. The various characteristic times of the monolayer cycle are calculated in presence of desorption of the volatile group V species from the liquid droplet. Simplified asymptotic formulas corresponding to fast attachment or low desorption are derived. Desorption is found to affect weakly the monolayer formation time but strongly the waiting time.

Polarization‐dependent anisotropic Raman response of CVD‐grown vertically stacked MoS2 layers

AbstractWe report on the polarized Raman scattering results of vertically stacked few‐layer MoS2 grown by chemical vapor deposition. Results of monolayer MoS2 showed that the polarization‐angle‐resolved intensity profiles of both out‐of‐plane A1g and in‐plane phonon modes followed the Raman polarization selection rules. In contrast, the polarization‐angle dependence of the phonon intensity in the multilayer region showed a deviation from the polarization selection rules, whereas that of the A1g phonon intensity remained unchanged regardless of the layer number. Furthermore, the polarization anisotropy of the phonon intensity increased with an increase in layer number. The vertically stacked MoS2 multilayers had oblique, deformed edge boundaries that were not parallel to the basal monolayer edge, suggesting the presence of slight interlayer twisting and/or sliding between neighboring layers. Therefore, we suggest that the polarization‐dependent anisotropic behavior of the phonon in the multilayer region is attributed to stacking disorder between MoS2 layers.

In vitro cell migration quantification method for scratch assays.

The scratch assay is an in vitro technique used to assess the contribution of molecular and cellular mechanisms to cell migration. The assay can also be used to evaluate therapeutic compounds before clinical use. Current quantification methods of scratch assays deal poorly with irregular cell-free areas and crooked leading edges which are features typically present in the experimental data. We introduce a new migration quantification method, called 'monolayer edge velocimetry', that permits analysis of low-quality experimental data and better statistical classification of migration rates than standard quantification methods. The new method relies on quantifying the horizontal component of the cell monolayer velocity across the leading edge. By performing a classification test on in silico data, we show that the method exhibits significantly lower statistical errors than standard methods. When applied to in vitro data, our method outperforms standard methods by detecting differences in the migration rates between different cell groups that the other methods could not detect. Application of this new method will enable quantification of migration rates from in vitro scratch assay data that cannot be analysed using existing methods.

Atomic Step Flow on a Nanofacet.

Crystal growth often proceeds by atomic step flow. When the surface area available for growth is limited, the nucleation and progression of the steps can be affected. This issue is particularly relevant to the formation of nanocrystals. We examine the case of Au-catalyzed GaAs nanowires, which we grow in a transmission electron microscope. Our insitu observations show that atomic layers nucleate at the periphery of the interface between the nanowire and the catalyst droplet. From this starting location, the atomic step flows within a restricted area of hexagonal shape. At specific partial coverages, the monolayer configuration changes abruptly. A simple model based on the geometry of the system and its edge energies explains these observations. In particular, we observe an inversion of the step curvature which reveals that the effective energy per unit length of monolayer edge is much lower at the interface periphery than inside the catalyst droplet.

Energy-Resolved Photoconductivity Mapping in a Monolayer-Bilayer WSe2 Lateral Heterostructure.

Vertical and lateral heterostructures of van der Waals materials provide tremendous flexibility for band-structure engineering. Because electronic bands are sensitively affected by defects, strain, and interlayer coupling, the edge and heterojunction of these two-dimensional (2D) systems may exhibit novel physical properties, which can be fully revealed only by spatially resolved probes. Here, we report the spatial mapping of photoconductivity in a monolayer-bilayer WSe2 lateral heterostructure under multiple excitation lasers. As the photon energy increases, the light-induced conductivity detected by microwave impedance microscopy first appears along the heterointerface and bilayer edge, then along the monolayer edge, inside the bilayer area, and finally in the interior of the monolayer region. The sequential emergence of mobile carriers in different sections of the sample is consistent with the theoretical calculation of local energy gaps. Quantitative analysis of the microscopy and transport data also reveals the linear dependence of photoconductivity on the laser intensity and the influence of interlayer coupling on carrier recombination. Combining theoretical modeling, atomic-scale imaging, mesoscale impedance microscopy, and device-level characterization, our work suggests an exciting perspective for controlling the intrinsic band gap variation in 2D heterostructures down to a regime of a few nanometers.

DFT Studies of Single Lithium Adsorption on Coronene

Abstract Density functional theory (DFT) calculations were performed to study the adsorption of neutral and cationic forms of single lithium (Li) on representative original and each of oxygen/sulfur-terminated coronene monolayer surfaces. First, the monolayers of coronene structures were prepared. Next, Li/Li+ adsorptions were investigated on the surfaces of the already optimised coronene models. The results indicate that the singular coronene models can be considered as appropriate surfaces for Li/Li+ adsorption, with stronger Li+ adsorption. Localisations of LI/Li+ species were carefully examined at the central carbon zone of the monolayer surface; however, only one model showed discrepancy by getting localised at the monolayer edge. Energy levels and distribution patters for the molecular orbitals indicate the effects of atomic terminations and Li/Li+ adsorptions, in which the singular and Li+-adsorbed models reveal identical results. Atomic-scale nuclear quadrupole resonance (NQR) properties were also evaluated, with the results indicating that the atomic properties can determine the major electronic properties for applying the coronene structure for specific applications.

Calcium oscillations in wounded fibroblast monolayers are spatially regulated through substrate mechanics

The maintenance of tissue integrity is essential for the life of multicellular organisms. Healing of a skin wound is a paradigm for how various cell types localize and repair tissue perturbations in an orchestrated fashion. To investigate biophysical mechanisms associated with wound localization, we focus on a model system consisting of a fibroblast monolayer on an elastic substrate. We find that the creation of an edge in the monolayer causes cytosolic calcium oscillations throughout the monolayer. The oscillation frequency increases with cell density, which shows that wound-induced calcium oscillations occur collectively. Inhibition of myosin II reduces the number of oscillating cells, demonstrating a coupling between actomyosin activity and calcium response. The spatial distribution of oscillating cells depends on the stiffness of the substrate. For soft substrates with a Young’s modulus E ~ 360 Pa, oscillations occur on average within 0.2 mm distance from the wound edge. Increasing substrate stiffness leads to an average localization of oscillations away from the edge (up to ~0.6 mm). In addition, we use traction force microscopy to determine stresses between cells and substrate. We find that an increase of substrate rigidity leads to a higher traction magnitude. For E < ~2 kPa, the traction magnitude is strongly concentrated at the monolayer edge, while for E > ~8 kPa, traction magnitude is on average almost uniform beneath the monolayer. Thus, the spatial occurrence of calcium oscillations correlates with the cell–substrate traction. Overall, the experiments with fibroblasts demonstrate a collective, chemomechanical localization mechanism at the edge of a wound with a potential physiological role.

Seeds of Locally Aligned Motion and Stress Coordinate a Collective Cell Migration

We find how collective migration emerges from mechanical information transfer between cells. Local alignment of cell velocity and mechanical stress orientation—a phenomenon dubbed “plithotaxis”—plays a crucial role in inducing coordinated migration. Leader cells at the monolayer edge better align velocity and stress to migrate faster toward the open space. Local seeds of enhanced motion then generate stress on neighboring cells to guide their migration. Stress-induced motion propagates into the monolayer as well as along the monolayer boundary to generate increasingly larger clusters of coordinately migrating cells that move faster with enhanced alignment of velocity and stress. Together, our analysis provides a model of long-range mechanical communication between cells, in which plithotaxis translates local mechanical fluctuations into globally collective migration of entire tissues.

Colloidal membranes of hard rods: unified theory of free edge structure and twist walls.

Monodisperse suspensions of rod like chiral fd viruses are condensed into a rod-length thick colloidal monolayers of aligned rods by depletion forces. Twist deformations of the molecules are expelled to the monolayer edge as in a chiral smectic A liquid crystal, and a cholesteric band forms at the edge. Coalescence of two such isolated membranes results in a twist wall sandwiched between two regions of aligned rods, dubbed π-walls. By modeling the membrane as a binary fluid of coexisting cholesteric and chiral smectic A liquid-crystalline regions, we develop a unified theory of the π-walls and the monolayer edge. The mean-field analysis of our model yields the molecular tilt profiles, the local thickness change, and the crossover from smectic to cholesteric behavior at the monolayer edge and across the π-wall. Furthermore, we calculate the line tension associated with the formation of these interfaces. Our model offers insights regarding the stability and the detailed structure of the π-wall and the monolayer edge.

Geometrical edgeactants control interfacial bending rigidity of colloidal membranes

A colloidal membrane is a one rod-length thick monolayer of aligned rods which spontaneously assembles in a mixture of rod-like viruses and non-adsorbing polymer. The complex structure of the monolayer edge modifies its fluctuations, effectively smoothing out the interface. We demonstrate that long filaments such as F-actin or flagella preferentially dissolve in the membrane's edge and can be used to control its interfacial properties. This effect is not driven by energetic interactions, but is rather a direct consequence of the intrinsic geometry of the constituent particles; hence we call such filaments geometrical edgeactants. Using optical manipulation techniques we adsorb individual filaments onto the edge of the colloidal membrane and measure their influence on the edge fluctuations. Edgeactants stiffen the interface, increasing its bending rigidity by up to an order of magnitude. Furthermore, they also locally suppress a polymorphic transition into twisted ribbons inherent to colloidal membranes. These results demonstrate new ways to control soft materials in which the final structure is only determined by the geometry of the constituent objects.

Acoustic analog of monolayer graphene and edge states

Acoustic analog of monolayer graphene has been designed by using silicone rubber spheres of honeycomb lattices embedded in water. The dispersion of the structure has been studied theoretically using the rigorous multiple-scattering method. The energy spectra with the Dirac point have been verified and zigzag edge states have been found in ribbons of the structure, which are analogous to the electronic ones in graphene nanoribbons. The guided modes along the zigzag edge excited by a point source have been numerically demonstrated. The open cavity and “Z” type edge waveguide with 60° corners have also been realized by using such edge states.

Monolayer spreading on a chemically heterogeneous substrate

We study the spreading kinetics of a monolayer of hard-core particles on a semi-infinite, chemically heterogeneous solid substrate, one side of which is coupled to a particle reservoir. The substrate is modeled as a square lattice containing two types of sites -- ordinary ones and special, chemically active sites placed at random positions with mean concentration $\alpha$. These special sites temporarily immobilize particles of the monolayer which then serve as impenetrable obstacles for the other particles. In terms of a mean-field-type theory, we show that the mean displacement $X_0(t)$ of the monolayer edge grows with time $t$ as $X_0(t) = \sqrt{2 D_{\alpha} t \ln(4 D_{\alpha} t/\pi a^2)}$, ($a$ being the lattice spacing). This time dependence is confirmed by numerical simulations; $D_{\alpha}$ is obtained numerically for a wide range of values of the parameter $\alpha$ and trapping times of the chemically active sites. We also study numerically the behavior of a stationary particle current in finite samples. The question of the influence of attractive particle-particle interactions on the spreading kinetics is also addressed.

Dewetting, partial wetting, and spreading of a two-dimensional monolayer on solid surface

We study the behavior of a semi-infinite monolayer, which is placed initially on a half of an infinite in both directions, ideal crystalline surface, and then evolves in time due to random motion of the monolayer particles. Particles dynamics is modeled as the Kawasaki particle-vacancy exchange process in the presence of long-range attractive particle-particle interactions. In terms of an analytically solvable mean-field-type approximation we calculate the mean displacement $X(t)$ of the monolayer edge and discuss the conditions under which a monolayer spreads $[X(t)&gt;0],$ partially wets $[X(t)=0],$ or dewets from the solid surface $[X(t)&lt;0].$

Reaction of NO and N2O gases with graphite in TGA

Thermal analyses were conducted in a thermogravimetric analyzer by isothermal techniques in order to characterize the carbon-nitrogen oxide reaction. The carbon samples employed in the present study were SP-1 graphite and Micro 450 graphite. Carbon-NO and carbon-N2O reactions were carried out in a temperature range of 550–900 °C and 5–20 kPa of the partial pressure of reactant. In the NO reaction, reaction orders with respect to NO concentration and activation energy were 0.46-0.92 and 85–102 kJ/mol, respectively. The rate on the monolayer edge was higher than the rate on the multilayer edges. In the N2O reaction, reaction orders with respect to N2O concentration and activation energy were 0.55–1.35 and 167–190 kJ/mol, respectively.

Microscopic model for spreading of a two-dimensional monolayer

We study the behavior of a monolayer, which occupies initially a bounded region on an ideal crystalline surface and then evolves in time due to random hopping of the monolayer particles. In the case when the initially occupied region is the half-plane X ≤ 0, we determine explicitly, in terms of an analytically solvable mean-field-type approximation, the mean displacement X(t) of the monolayer edge. We find that x(t) ∼ A D0t, in which law D0 denotes the bare diffusion coefficient and the prefactor A is a function of the temperature and of the particle-particle interactions parameters. We show that A can be greater, equal or less than zero, and specify the critical parameter which distinguishes between the regimes of spreading (A > 0), partial wetting (A = 0) and dewetting (A < 0).

The Active Surface Species in Alkali-Catalyzed Carbon Gasification: Phenolate (COM) Groups vs Clusters (Particles)

The activities and catalytic roles of different species of potassium in the gasification reactions of graphite by H 2O and CO 2 are investigated. TEM techniques are used to measure the rates of monolayer edge recession (uncatalyzed and COK catalyzed) and rates of monolayer channeling (catalyzed by particles) on the basal plane of graphite reacting with 21 Torr H 2O at 700°C. The turnover frequencies for carbon gasification are: 0.08 s −1 (uncatalyzed), 0.15 s −1 (catalyzed by COK groups), and 7.8 s −1 (catalyzed by particles). Thus the particles have a high activity, whereas the COK groups have only a small activity. TGA and literature results using mixtures of carbon and alkali salts show a sigmoidal dependence of gasification rates on catalyst loading. This is the result of catalyst dispersion and competition between the COK groups and alkali particles. A CNDO semi-empirical molecular orbital calculation is performed on model graphite substrates with -O and -OK groups bonded to the {10 1 l} zigzag face and {11 2 l} armchair face. On the zigzag face, the carbon atom bridging two COK groups gains a large negative charge (−0.486) and hence is a favorable site for binding an O atom. The surface CC bonds in this structure are substantially weakened by adding O atoms on the bridging C atoms, leading to CO release. The O atoms are supplied by the dissociation of H 2O and CO 2. The possible reason for the alkali particles being more active than the COM (where M = alkali) groups is that the particles can dissociate H 2O and CO 2 at higher rates, by providing either more active sites or higher activities. The CNDO results also predict that the COK groups have an inhibiting effect on the armchair face; an inhibiting effect has indeed been observed earlier in our laboratory.