Edges Of Flakes Research Articles

Application of the Knife-Edge Technique on Transition Metal Dichalcogenide Monolayers for Resolution Assessment of Nonlinear Microscopy Modalities.

We report application of the knife-edge technique at the sharp edges of WS2 and MoS2 monolayer flakes for lateral and axial resolution assessment in all three modalities of nonlinear laser scanning microscopy: two-photon excited fluorescence (TPEF), second- and third-harmonic generation (SHG, THG) imaging. This technique provides a high signal-to-noise ratio, no photobleaching effect and shows good agreement with standard resolution measurement techniques. Furthermore, we assessed both the lateral resolution in TPEF imaging modality and the axial resolution in SHG and THG imaging modality directly via the full-width at half maximum parameter of the corresponding Gaussian distribution. We comprehensively analyzed the factors influencing the resolution, such as the numerical aperture, the excitation wavelength and the refractive index of the embedding medium for the different imaging modalities. Glycerin was identified as the optimal embedding medium for achieving resolutions closest to the theoretical limit. The proposed use of WS2 and MoS2 monolayer flakes emerged as promising tools for characterization of nonlinear imaging systems.

Plasmon-assisted spatially selective grafting of Ti3C2TX flakes for prevention of MXene oxidation and stability increase

The MXene flakes are considered excellent materials for a range of applications and device construction. Especially attractive is the utilization of MXenes as the catalysts or co-catalysts for water splitting, including both half-reactions – hydrogen (HER) and oxygen (OER) evolution. However, the MXenes exhibit a strong tendency to oxidize during utilization, especially in water media and at oxygen presence. The oxidation of the MXene flakes commonly starts from the edges and then proceeds to the basal planes. Since the catalytic activity of flakes is mostly related to the basal plane, strong covalent blocking of flake edges can significantly protect MXene in oxidative conditions and increase the flake’s lifetime without the loss of their catalytic activity. In this work, we demonstrated the combination of plasmon-assisted chemistry and electrochemistry for spatial-selective and precise modification of MXene (Ti3C2TX) flake edges. Our approach is based on the spatial distribution of plasmon energy under the flake’s illumination with infrared (IR) light. Plasmon local excitation leads to homolysis of the modification agent – iodonium salt, the formation of highly reactive radicals, and their immediate grafting to the edges of MXene flakes in the neighborhood. As a result, the edges of the flakes are protected, and the initial stage of their oxidation is prevented. After optimization of plasmon-assisted grafting, the modified flakes were tested in HER and OER and a significant increase in flakes stability was validated.

Morphology and structural evolution of fine beach gold in comparison to detrital platinum, southern New Zealand

AbstractBeach placer gold has been mined around the world historically, but extraction of fine (~ 100 µm) gold particles is notoriously difficult. This study illustrates morphological and mineralogical changes that transform fine gold during aeolian processes on windy beaches and contribute to mine concentration inefficiencies. Sandblasting on exposed beaches in southern New Zealand has caused extreme attenuation of edges of gold flakes that were previously transported in rivers for > 200 km. Flakes have been transformed into complex but compact toroids and spheroids with thin (~ 20 µm) internal and external strands of attenuated gold. Most of the gold within the attenuated strands has recrystallised to fine (micron-scale) undeformed grains with little or no Ag (< 1 wt%). Some coarse (> 40 µm) gold grains remain from the precursor fluvial particles, and these retain original Ag contents (1–10 wt%). These coarse grains show substantial internal crystallographic deformation and sub-grain formation, although some of these strain effects may have been inherited from fluvial transport. Co-existing detrital platinum minerals are much less malleable than gold during sandblasting and have only minor (10-µm scale) toroidal deformation on edges of fluvial flakes. The complex frameworks of the fine toroidal and spheroidal gold particles can include air, water, and clay, which lowers their average density and so they commonly float on water and are readily entrained with other heavy minerals. The fine particle size, compact shapes, and clay coatings also resist mercury amalgamation.

Heteroatomic stitching of broken WS2 monolayer with enhanced surface potential.

Sub-nanometre thick two-dimensional (2D) materials suffer from severe cracking during the high temperature chemical vapour deposition growth process. The cracking can be utilised to generate more active edges. These active edges can be stitched with a homo- or hetero-material. While the direct growth of 2D-heterostructures is mostly limited to a small fraction of outer edges of monolayer flakes, the cracked monolayers can be utilised to produce a large fraction of heterostructures. Heterostructures are important for developing multifunctional components for nanoscale electronics and optoelectronics. In this work, we demonstrate the formation of WS2-MoS2 heterostructures in large fractions by atomic stitching of cracked WS2 monolayers with the sequential growth of MoS2. Aberration-corrected scanning transmission electron microscopy and Raman spectroscopy have been utillised to probe fine details on the stitched interface between WS2 and MoS2. Growth of MoS2 domains were observed to take place at the terminating edge of cracked WS2, which then merged to form MoS2 multilayers at the cracked site. Growth of MoS2 at the opposite edges of WS2 eventually results in a MoS2-MoS2 junction with a Σ = 7 tilt boundary. Kelvin probe force microscope (KPFM) measurement from the stitched region revealed that the work function of WS2 is ∼32.46 meV higher than that of MoS2, which also closely matches with the Fermi energy difference between these two materials. With the aid of an atomic force microscope and KPFM, the surface potential width at the stitched region was found to be ∼5 times higher than the actual width of the interface, confirming the modulation of properties near the interface.

Nanoimaging of the Edge-Dependent Optical Polarization Anisotropy of Black Phosphorus.

The electronic structure and functionality of 2D materials is highly sensitive to structural morphology, not only opening the possibility for manipulating material properties but also making predictable and reproducible functionality challenging. Black phosphorus (BP), a corrugated orthorhombic 2D material, has in-plane optical absorption anisotropy critical for applications, such as directional photonics, plasmonics, and waveguides. Here, we use polarization-dependent photoemission electron microscopy to visualize the anisotropic optical absorption of BP with 54 nm spatial resolution. We find the edges of BP flakes have a shift in their optical polarization anisotropy from the flake interior due to the 1D confinement and symmetry reduction at flake edges that alter the electronic charge distributions and transition dipole moments of edge electronic states, confirmed with first-principles calculations. These results uncover previously hidden modification of the polarization-dependent absorbance at the edges of BP, highlighting the opportunity for selective excitation of edge states of 2D materials with polarized light.

Rich activated edges of hexagonal boron nitride flakes in-situ triggered by nickel nanoparticles to achieve efficient reduction of friction and wear

The application of hexagonal boron nitride (h-BN) nanosheets as lubrication additives has attracted tremendous attention due to their excellent stability and friction performance. However, the exfoliation and functionalization of h-BN nanosheets are difficult which appear to limit the practical application of them. Herein, we demonstrate that commercial h-BN flakes (h-BNFs) can be directly used to fabricate antifriction and antiwear coatings through a simple spraying process after mixing with nickel nanoparticles (Ni NPs). The experiment results and density functional theory calculations demonstrated that Ni NPs could in-situ promote nitrogen atoms to break down from the edges of h-BNFs, forming electron deficient h-BNFs and Ni NPs, and then the electron deficient Ni NPs would adsorb oxygen to generate superoxide radicals during friction process. The as-obtained superoxide radicals can react with the electron deficient h-BNFs, and these functionalized h-BNFs can stably adsorb poly-alpha-olefins (PAO6) molecules to form antifriction and antiwear tribo-films on the stainless steel (SS) substrates. Besides, the metal-edge interaction (SMEI) between Ni NPs and h-BNFs can accelerate PAO6 molecules converting into carbon based films on the counterpart. Fantastically, the antifriction and antiwear performance of SS have been greatly improved after coated by Ni/h-BNF composite coatings with PAO6, with friction coefficients (COFs) as low as 0.07 and wear rates less than 1.5 × 10−13 m3/(N·m), respectively. This finding opens up a new way for the practical application of commercial h-BNFs as low friction lubricants.

Effect of annealing temperature on electrical properties of RGO thin films deposited by Atomized Spray Pyrolysis

The modified Hummers method is an ideal method for the bulk production of reduced graphene oxide (RGO) using Sri Lankan natural vein graphite. Deposition of RGO as a thin film (TF) on a substrate is essential for electronic applications in various industries. A novel, improved spray pyrolysis technique called Atomized Spray Pyrolysis deposition method was used to deposit RGO-TFs at a substrate temperature of 150 °C. Thermal annealing (TA) was carried out after deposition of RGO-TFs since post-deposition treatment is essential to enhance its intrinsic properties. Then TA was done for 30 min at different temperatures from 230 to 430 °C. Electrical, morphological and structural characterizations suggested that TA at 330 °C yields optimum intrinsic properties of RGO-TFs. Annealing temperature up to 330 °C is adequate to eliminate most defects such as fabrication residues, residual oxygen functional groups, and folds at edges of RGO flakes. TA at 330 °C retains maximum possible hexagonal lattice structure and carbon to oxygen atomic ratio. However, TA beyond 330 °C exhibits doping of oxygen atoms to vacant carbon atoms at the hexagonal structure of RGO, increasing bandgap since oxygen atoms localize some electrons. At higher annealing temperatures, the formation of buckling, ripples, and wrinkles on the RGO sheets due to induced compressive strain was observed. This results in hindering charge transportation since they behave as scattering sites.

Enhanced dispersion and antibacterial activity of mechanically exfoliated graphite flakes in the presence of n-hexane and NaCl

We report on the high dispersion and antibacterial performance (AP) of graphite flakes prepared by the mechanical exfoliation of bulk graphite materials using wet high energy ball milling (BM) in the presence of NaCl and n-hexane. Synergic effects of functionalized surfaces and sharp edges of well-dispersed carbon flakes offer high dispersion, and promising AP against Escherichia coli (E. coli) through damaging its cell membrane. This paper reports on the facile and green preparation of exfoliated graphitic flakes with enhanced AP.

Effectiveness of fresh cement kiln dust as a soil stabilizer and stabilization mechanism of high swelling clays

The expansive clay soils, which cover most of the new urbanized areas in Egypt, create major damages in most of the engineering structures and infrastructures such as buildings, roads, bridges, pipelines and others due to bad human activities, e.g. lack of maintenance for sewage and drinking lines, and flood irrigation of golf areas which causes fluctuation of water content through the foundation clay layers associated with shrink-swell behavior. These geotechnical challenges and their treatment solutions consume most of the urbanized project budget and delay the development plan. Therefore, in this study the high plasticity and soft to firm clay soils were stabilized using various dosages of fresh cement kiln dust (CKD) as an economical alternative to the other expensive stabilizing agents (stabilizers or binders) such as lime, cement and other chemical stabilizers. The effectiveness of this type of CKD as a stabilizing agent, soil stabilization mechanism, and changes in mineralogical and microstructural characteristics of stabilized soils due to this stabilization process were evaluated through Atterberg limits, unconfined compressive strength (UCS), stiffness (stress–strain behavior, Es), pH, electrical conductivity (EC), total dissolved solids (TDS), X-Ray diffraction (XRD) and scanning electron microscope (SEM) investigations. The present study revealed that by adding CKD, the workability of the treated clays was increased by decreasing the liquid limit and plasticity index as well as increasing the bearing capacity and decreasing the settlement by increasing UCS and Es. This study also showed that by adding CKD, the initial pH, EC, and TDS of CKD-clay system increased, creating favorable conditions for dissolution of edges and surfaces of clay flakes. The active montmorillonite clay flakes adsorbed the soluble calcium cation and sulfate anion on their surfaces that are rich in soluble silica and alumina and then the cation exchange and flocculation–agglomeration processes began. With curing, the pozzolanic products grew on the surfaces and edges of clay flakes and extended into the voids of the agglomerated inter-clay particles resulting in the filling of these micro-voids. The present investigation also found that, the progressive decreasing in pH, EC, and TDS values of CKD-clay systems through different curing periods is a good indicator for pozzolanic activity.

Li intercalation into multilayer graphene with controlled defect densities

We studied the influence of defect density in multilayer graphene sheets on the intercalation of Li ions by means of x-ray photoelectron spectroscopy. The multilayer graphene was prepared by chemical vapor deposition on Ni substrates. The deposition of Li at room temperature results in immediate intercalation, while additional annealing at 300 °C partially removes Li from the sample surface. We found that varying the defect density over several orders of magnitude does not influence the intercalation process. We propose that the edges of the graphene flakes are the dominant sites that facilitate Li intercalation in multilayer graphene.

Maximum volume simplex method for automatic selection and classification of atomic environments and environment descriptor compression

Fingerprint distances, which measure the similarity of atomic environments, are commonly calculated from atomic environment fingerprint vectors. In this work, we present the simplex method that can perform the inverse operation, i.e., calculating fingerprint vectors from fingerprint distances. The fingerprint vectors found in this way point to the corners of a simplex. For a large dataset of fingerprints, we can find a particular largest simplex, whose dimension gives the effective dimension of the fingerprint vector space. We show that the corners of this simplex correspond to landmark environments that can be used in a fully automatic way to analyze structures. In this way, we can, for instance, detect atoms in grain boundaries or on edges of carbon flakes without any human input about the expected environment. By projecting fingerprints on the largest simplex, we can also obtain fingerprint vectors that are considerably shorter than the original ones but whose information content is not significantly reduced.

Atomically Asymmetric Inversion Scales up to Mesoscopic Single-Crystal Monolayer Flakes.

Symmetry is highly relevant with various quantities and phenomena in physics. While the translational symmetry breaks at the edges of two-dimensional hexagonal crystalline flakes, it is usually associated with the breaking of central inversion symmetry that is yet to be observed in terms of physical properties. Here, we report an experiment-theory joint study on in-plane compressed single-crystal monolayer WS2 flakes. Although the flakes show a hexagonal appearance with a C6 symmetry, our density functional theory calculations predict that their in-plane strain, geometric structure, work-function, energy bandgap, and mechanical modulus are nonequivalent among the triangular regions with different edge terminations at the atomic scale, and the flakes exhibit self-patterns with a C3 symmetry. Such nonequivalence of physical properties and concomitant self-patterns persist even in a 50 μm-sized monolayer WS2, observed using atomic force microscopy. This indicates that the symmetry arising from the atomic geometry could preserve up to tens of microns for both geometric and properties of the flake, regardless of its mesoscopic geometry, i.e., C6 here. Such a detectable mesoscopic scale and symmetric nano- to mesoscale patterns provide promising building blocks for 2D materials and devices and also allow edge terminations of 2D flakes to be directly distinguished.

Revealing the Role of Methylammonium Chloride for Improving the Performance of 2D Perovskite Solar Cells.

Layered perovskite films, composed of two-dimensional (2D) Ruddlesden-Popper perovskites (RPPs), show improved stability compared to their conventional three-dimensional (3D) counterparts in perovskite solar cells (PSCs). However, 2D PSCs exhibit a lower power conversion efficiency (PCE), which has been attributed to compositional inhomogeneity and nonuniform alignment of the 2D perovskite phases. Methylammonium chloride (MACl) has been adopted as an additive to improve the PCE and the operational stability of 2D PSCs, although the role of MACl in performance enhancement is unclear. In this work, time- and spatially resolved fluorescence and absorption techniques have been applied to study the composition and charge carrier dynamics in MACl-doped BA2MA4Pb5I16 (⟨n⟩ = 5) layered perovskite films. The inhomogeneous phase orientation distribution in the direction orthogonal to the substrate for undoped layered perovskite films undergoes reorganization upon MACl doping. Based on structural and crystallographic analyses, it is revealed that MACl can facilitate the crystallization of small-n 2D perovskite phases at the cost of consuming an increased amount of BA cations. Consequently, an increase in the thickness of large-n 2D perovskite phases accompanies their enhanced perpendicular alignment ([101] crystalline orientation) to the substrate, which facilitates charge carrier transport and collection by electrodes. The defect passivation of the MACl-doped layered perovskite film provided by the small-n phase is also beneficial to the photovoltaic performance of the PSC device. A maximum PCE (∼14.3%) was achieved at 6 mol % MACl doping, with this optimum level influenced by the increased interfacial roughness of the layered perovskite film caused by the edges of small-n perovskite flakes emerging on the front surface.

Hollow carbon microbox from acetylacetone as anode material for sodium-ion batteries

Abstract Carbon-based materials have attracted much interest as one of the promising anodes for sodium-ion batteries. However, low utilization of electrolyte and slow ion-transfer rate during electrochemical process hinder the further application of traditional bulk carbon. In order to enhance the diffusion kinetics and maintain the reversibility, hierarchical hollow carbon microbox was successfully prepared through a tunable bottom-up self-template routine for sodium-ion batteries. During annealing process, the morphology construction and activation happened synchronously. Based on that, a range of cross-linked porous nanosheet and hollow microbox were attained by manipulating reactant condition. The generation of texture and physical property are analyzed and are established linkages related to the electrochemical behavior. As results depicted in kinetic exploration and simulation based on cyclic voltammetry, the surface-controlled electrochemical behavior gradually turns to be the diffusion-controlled behavior as the hollow microbox evolves to porous nanosheet. The probable reason is that the rational microstructure/texture design leads to the accelerated diffusion kinetic procedure and the reduced concentration difference polarization. Sodium storage mechanism was deduced as reversible binding of Na-ions with local defects, including vacancies on sp2 graphitic layers, at the edges of flakes and other structural defects instead of intercalation. Bestowed by the morphology design, the broad pore width distribution, abundant defects/active sites and surface functionality, hollow microbox electrode delivers great electrochemical performances. This work is expected to propose a novel and effective strategy to prepare tunable hierarchical hollow carbon microbox and induce the fast kinetic of carbon anode material.

Edge Capping of 2D‐MXene Sheets with Polyanionic Salts To Mitigate Oxidation in Aqueous Colloidal Suspensions

AbstractMXenes have shown promise in myriad applications, such as energy storage, catalysis, EMI shielding, among many others. However, MXene oxidation in aqueous colloidal suspensions when stored in water at ambient conditions remains a challenge. It is now shown that by simply capping the edges of individual MXene flakes, Ti3C2Tz and V2CTz, by polyanions such as polyphosphates, polysilicates or polyborates, it is possible to quite significantly reduce their propensity for oxidation even when held in aerated water for weeks. This breakthrough resulted from the realization that the edges of MXene sheets are positively charged. It is thus an example of selectively functionalizing the edges differently from the MXene sheet surfaces.

Edge Capping of 2D-MXene Sheets with Polyanionic Salts To Mitigate Oxidation in Aqueous Colloidal Suspensions.

MXenes have shown promise in myriad applications, such as energy storage, catalysis, EMI shielding, among many others. However, MXene oxidation in aqueous colloidal suspensions when stored in water at ambient conditions remains a challenge. It is now shown that by simply capping the edges of individual MXene flakes, Ti3 C2 Tz and V2 CTz , by polyanions such as polyphosphates, polysilicates or polyborates, it is possible to quite significantly reduce their propensity for oxidation even when held in aerated water for weeks. This breakthrough resulted from the realization that the edges of MXene sheets are positively charged. It is thus an example of selectively functionalizing the edges differently from the MXene sheet surfaces.

In-situ visualization of hydrogen evolution sites on helium ion treated molybdenum dichalcogenides under reaction conditions

Nanostructured 2D transition metal dichalcogenides play an increasingly important role in heterogeneous catalysis. These materials are abundant (co-)catalysts with tunable properties to catalyze a number of key reactions related to energy provision, for instance the hydrogen evolution reaction (HER). It is vital to understand which surface sites are active in order to maximize their number and to improve the overall (photo-)catalytic behavior of those materials. Here, we visualize these active sites under HER conditions at the surface of molybdenum dichalcogenides (MoX2, X = Se, S) with lateral resolution on the nanometer scale by means of electrochemical scanning tunneling microscopy. The edges of single MoX2 flakes show high catalytic activity, whereas their terraces are inactive. We demonstrate how the inert basal planes of these materials can be activated towards the HER with the help of a focused beam of a He-ion microscope. Our findings demonstrate that the He-ion induced defects contribute at lower overpotentials to the HER, while the activity of the edges exceeds the activity of the basal defects for sufficiently high overpotentials. Given the lithographic resolution of the helium ion microscope, our results show the possibility to generate active sites in transition metal dichalcogenides with a spatial resolution below a few nanometers.

The Role of Oxygen Atoms on Excitons at the Edges of Monolayer WS2.

We clarify that the chemisorption of oxygen atoms at the edges is a key contributor to the frequently observed edge enhancement and spatial non-uniformities of photoluminescence (PL) in WS2 monolayers. Here we have investigated with momentum- and real-space nanoimaging of the chemical and electronic density inhomogeneity of WS2 flakes. Our finding from a large panoply of techniques together with density functional theory calculation confirms that the oxygen chemisorption leads to the electron accumulation at the edges. This facilitates the trion dominance of PL at the edges of WS2 flakes. Our results highlight and unravel the significance of chemisorbed oxygen at the edges in the PL emission and electronic structure of WS2, providing a viable path to enhance the performance of transition-metal-dichalcogenide-based devices.

Atomic Structure and Local Electronic States of Single Pt Atoms Dispersed on Graphene

In single-atom catalysis, the atomic structure and local electronic states of single atoms on a supporting material remain a fundamental question. We experimentally and theoretically solved these problems for single Pt atoms dispersed on freestanding graphene using plasma sputtering. Electron microscopy revealed the atomic arrangements of Pt atoms binding to carbon atoms at the step edges of nanographene flakes. X-ray photoelectron spectroscopy revealed a large binding energy shift of the Pt 4f state of a single Pt atom. First-principles calculations elucidated that the Pt 5dxy-orbital in the step edge plays a crucial role in the formation of chemical bonds to C atoms and in the considerable charge transfer from Pt to C atoms, resulting in the large binding energy shift of the Pt 4f state.

Electrical Transport in Devices Based on Edge‐Fluorinated Graphene

AbstractThe conductivity of few‐ and monolayer graphene with covalently bound moieties is a key‐point in the potential application of these materials in any electrical and optoelectronic device. In particular, fluorination of such graphene‐based systems is of interest, as fluorine is expected to have a strong influence on the charge‐carrier density due to its high electronegativity, and therefore modify the electrical transport properties significantly. Here it is shown that, depending on the device architecture, the electrical properties of fluorinated graphene‐based devices are significantly different. It is found that the conductivity of thin films of few‐layer graphene decreases by several orders of magnitude with fluorine content increasing from 2.4 to 16.6 at%, whereas individual flakes show a significant increase in both conductivity and charge carrier mobility. This observation, combined with Raman microscopy study, points toward the fact that the edges of the flakes are primary sites for fluorine within the experimental range of fluorine content. The strong decrease in conductivity in the film devices is therefore associated with the high contact resistance between the fluorine saturated edges of the individual flakes.