Monolayer Of Atoms Research Articles

Point defect formation near the epitaxial Ge(001) growth surface and the impact on phosphorus doping activation

First-principles calculations are used to systematically investigate the impact of surface effects on the energetic cost to incorporate point defects near the growing surface [Ge(001)] and its consequence on the dopant activation in P-doped Ge layers. We illustrate the formation enthalpy ΔHf for the incorporation of a mono-vacancy, which is the dominant point defect responsible for the electrically inactive dopant incorporation in P-doped Ge. The calculated values point toward a significant lowering of ΔHf near the growing surface in comparison to the bulk cubic Ge supercell. The impact of the surface almost vanishes beyond the sixth atomic monolayer located below the surface and the calculated ΔHf values overlap with the ones computed for a bulk cubic Ge supercell. For epitaxial Ge:P layers, grown at low growth temperatures (<450°C) using the deposition method such as the Chemical Vapor Deposition, the dopant-vacancy clusters are formed within the first few monolayers close to the growing surface. The low ΔHf values for the incorporation of a mono-vacancy near the growing surface suggest that the concentration of vacancies can be significantly enhanced with respect to the bulk Ge, which can subsequently trap more dopants and deactivate them at the surface itself. Our first-principles calculation results are in line with previously reported experimental positron annihilation spectroscopy analysis on Ge:P layers grown at 440 °C using GeH4 as gas precursor. At P concentrations above 1×1020cm−3, the mono-vacancy sized open-volume defects are passivated by at least three P atoms.

Graphene on glassy carbon microelectrodes demonstrate long-term structural and functional stability in neurophysiological recording and stimulation

Objective. There is a growing interest in the use of carbon and its allotropes for microelectrodes in neural probes because of their inertness, long-term electrical and electrochemical stability, and versatility. Building on this interest, we introduce a new electrode material system consisting of an ultra-thin monoatomic layer of graphene (Gr) mechanically supported by a relatively thicker layer of glassy carbon (GC). Approach. Due to its high electrical conductivity and high double-layer capacitance, Gr has impressive electrical and electrochemical properties, two key properties that are useful for neural recording and stimulation applications. However, because of its two-dimensional nature, Gr exhibits a lack of stiffness in the transverse direction and hence almost non-existent flexural and out-of-plane rigidity that will severely limit its wider use. On the other hand, GC is one of carbon’s important allotropes and consists of three-dimensional microstructures of Gr fragments with a natural molecular similarity to Gr. Further, GC has exceptional chemical inertness, good electrical properties, high electrochemical stability, purely capacitive charge injection, and fast surface electrokinetics coupled with lithography patternability. This makes GC an ideal candidate for addressing Gr’s lack of out-of-plane rigidity through providing a matching sturdier and robust mechanical backing. Combining the strengths of these two allotropes of carbon, we introduce a new neural probe that consists of ∼1 nm thick layer of patterned Gr microelectrodes supported by another layer of 3–5 μm thick patterned GC. Main results. We present the fabrication technology for the new Gr on GC (graphene on glassy carbon) microelectrodes and the accompanying pattern transfer technology on flexible substrate and report on the bond between these two allotropes of carbon through FTIR, surface morphology through SEM, topography through atomic force microscopy, and microstructure imaging through scanning transmission electron microscopy. A long-term (18 weeks) in vivo study of the use of these Gr on GC microelectrodes assessed the quality of the electrocorticography-based neural signal recording and stimulation through electrophysiological measurements. The probes were demonstrated to be functionally and structurally stable over the 18 week period with minimal glial response—the longest reported so far for Gr-based microelectrodes. Significance. The Gr on GC microelectrodes presented here offers a compelling case for expanding the potentials of Gr-based technology in the broad areas of neural probes.

Design and realization of topological Dirac fermions on a triangular lattice

Large-gap quantum spin Hall insulators are promising materials for room-temperature applications based on Dirac fermions. Key to engineer the topologically non-trivial band ordering and sizable band gaps is strong spin-orbit interaction. Following Kane and Mele’s original suggestion, one approach is to synthesize monolayers of heavy atoms with honeycomb coordination accommodated on templates with hexagonal symmetry. Yet, in the majority of cases, this recipe leads to triangular lattices, typically hosting metals or trivial insulators. Here, we conceive and realize “indenene”, a triangular monolayer of indium on SiC exhibiting non-trivial valley physics driven by local spin-orbit coupling, which prevails over inversion-symmetry breaking terms. By means of tunneling microscopy of the 2D bulk we identify the quantum spin Hall phase of this triangular lattice and unveil how a hidden honeycomb connectivity emerges from interference patterns in Bloch px ± ipy-derived wave functions.

Effect of heat treatment on the diffusion intermixing and structure of the Cu thin film on Si (111) substrate: a molecular dynamics simulation study

ABSTRACT This work is devoted to the study of the diffusion process at the interface between copper films with a thickness of 2, 3, 4, 7 and 10 atomic monolayers and silicon substrate by molecular dynamics simulation method. For this purpose, the variation of the concentration of copper and silicon along the perpendicular direction to the interface was investigated. An analysis of the density profile along this direction made it possible to determine the melting point of the interface between copper and silicon. The atomic structure of the diffusion layer was compared with that of the bulk Cu3Si compound. Using the formalism pair correlation functions find partial coordination numbers distribution it was revealed the possibility of nucleation centres formation with the structure of the compound in the liquid state. This work will allow expanding knowledge about the process of atomic diffusion at the metal–semiconductor interface.

Fabrication and characterization of large-area suspended MoSe2 crystals down to the monolayer

Many layered materials, such as graphene and transition metal dichalcogenides, can be exfoliated down to atomic or molecular monolayers. These materials exhibit exciting material properties that can be exploited for several promising device concepts. Thinner materials lead to an increased surface-to-volume ratio, with mono- and bi-layers being basically pure surfaces. Thin crystals containing more than two layers also often behave as an all-surface material, depending on the physical property of interest. As a result, flakes of layered materials are typically highly sensitive to their environment, which is undesirable for a broad range of studies and potential devices. Material systems based on suspended flakes overcome this issue, yet often require complex fabrication procedures. Here, we demonstrate the relatively straightforward fabrication of exfoliated MoSe2 flakes down to the monolayer, suspended over unprecedentedly large holes with a diameter of 15 µm. We describe our fabrication methods in detail, present characterization measurements of the fabricated structures, and, finally, exploit these suspended flakes for accurate optical absorption measurements.

Change of Bandgap Energy in Quantum System of Nanolayer on Silicon

In the quantum system of nanolayer (NL) on silicon, the bandgap energy obviously increases with the decrease of NL thickness, where the quantum confinement (QC) effect plays the main role as the thickness of Si NL changes along with (100), (110) and (111) directions, respectively. And the simulation result demonstrated that the direct bandgap can be obtained as the NL with (001) direction is thinner than 10 nm on Si surface. However, it is discovered in the simulated calculation that the QC effect disappears as the NL thickness arrives at the size of the monoatomic layer, in which its bandgap sharply decreases, where the abrupt change effect in bandgap energy occurs near-ideal 2D-layer. In the experiment, we fabricated the Si NL structure by using electron beam irradiation and laser deposition methods, in which a novel way was used to control the NL thickness by modulating irradiation time of the electron beam. The new effect should have a good application on a photonic-electronic chip of silicon.

Kink propagation and solute partitioning in an atomic monolayer on a substrate.

When a monolayer of Lennard-Jones atoms is driven by an external force over an atomically spaced lattice, the atoms do not move in the direction of the force. By considering monolayers containing a solvent and two different solutes, we show that the different atomic species follow distinct directions and so partition from one another and from the solvent. The strength of the driving force is chosen so that at any instant, most atoms are stationary while only a small fraction propagates as solitary waves. In this regime, the mean velocity of the layer is due to the nonzero contribution from merely a few atoms. We also present a simple theory, based on the probability that an atom in the monolayer will hop from one equilibrium location to the next, that explains the distinct directions of atomic migration.

Picometer sensitivity metrology for EUV absorber phase

With growing interest in EUV attenuated phase-shift masks due to their superior image quality for applications such as dense contact and pillar arrays, it is becoming critical to model, measure, and monitor the intensity and relative phase of multilayer and absorber reflections. We present a solution based on physical modeling of reflectometry data, which can achieve single picometer phase precision and sensitivity to changes in average film thickness below one atomic monolayer. We measure absorber and multilayer reflectivity to determine thin-film parameters with a multidimensional optimization and then acquire a new measurement of either multilayer or absorber to determine perturbations in surface contamination thickness. While it is difficult to assess the accuracy of the first step, the simplicity of the second step allows us to characterize our sensitivity to changes in contamination thickness. We apply this analysis using an initial set of measurements and repeated measurements after a period of storage. For the multilayer, the total contamination growth was 1068 pm, which occurred almost exclusively during storage (1085 pm) and decreased very slightly during repeated measurements (−17 pm). For the absorber, the behavior was quite different, with a total growth of 126 pm, which occurred much less during storage (28 pm) and primarily during repeated measurements (98 pm). Ultimately, the change in relative phase (absorber minus multilayer) was −0.86 deg for the multilayer and −1.12 deg for the absorber. We estimate the precision of the surface contamination measurement to be 3σ < 6 pm for measuring thickness and 3σ < 0.2 deg for measuring phase.

Etching Mechanism of Monoatomic Aluminum Layers during MXene Synthesis

Etching Mechanism of Monoatomic Aluminum Layers during MXene Synthesis

Giant Photoluminescence Enhancement and Carrier Dynamics in MoS2 Bilayers with Anomalous Interlayer Coupling.

Fundamental researches and explorations based on transition metal dichalcogenides (TMDCs) mainly focus on their monolayer counterparts, where optical densities are limited owing to the atomic monolayer thickness. Photoluminescence (PL) yield in bilayer TMDCs is much suppressed owing to indirect-bandgap properties. Here, optical properties are explored in artificially twisted bilayers of molybdenum disulfide (MoS2). Anomalous interlayer coupling and resultant giant PL enhancement are firstly observed in MoS2 bilayers, related to the suspension of the top layer material and independent of twisted angle. Moreover, carrier dynamics in MoS2 bilayers with anomalous interlayer coupling are revealed with pump-probe measurements, and the secondary rising behavior in pump-probe signal of B-exciton resonance, originating from valley depolarization of A-exciton, is firstly reported and discussed in this work. These results lay the groundwork for future advancement and applications beyond TMDCs monolayers.

Enhancing spontaneous emission using structural resonances of self-assembled monolayers

Self-assembled monolayers made using sub-micron dielectric spheres are analogous to atomic monolayers that have proved to be a modern workhorse in condensed matter physics. Here, we discuss the origin of optical resonances associated with sub-micron dielectric monolayers through experiments that are further supported by theory and simulation. These resonances are used to enhance the emission rate of nitrogen-vacancy centers in nanodiamonds. Emission decay rate measurements confirm a 14% decrease in the excited state lifetime, which implies an enhanced emission rate at the resonances. The increase in measured decay rates is supported by the calculated local density of optical states. Our results provide a platform to tailor light transport and emission for applications in photonics and quantum technology using these monolayers.

Angular dependence of nanofriction of mono- and few-layer MoSe2

The frictional properties of two-dimensional (2D) materials are strongly dependent on the crystallographic orientation and number of layers. Although friction anisotropy caused by crystallographic orientations has been reported for various 2D materials, the flexural deformations and different defects complicate the insight into the mechanism of the in-plane friction anisotropy of these materials. Here, the anisotropic friction behavior between an atomic force microscopy tip and monolayer (ML) and few-layer (FL) MoSe2 flakes grown by CVD was performed by nanofriction measurements at different crystallographic directions, applied loads (FA), and tip scanning velocities. Our results reveal that the angular dependence of the friction forces is highly anisotropic for both types of MoSe2 flakes, and the anisotropy decreases with FA. Importantly, the anisotropies demonstrate opposite angular dependence for ML and FL MoSe2 flakes. As confirmed by the friction scanning velocity dependences, this difference seems to originate from different friction mechanisms for ML and FL MoSe2 flakes. The friction of FL flakes was predominantly influenced by atomic stick–slip motion. In contrast, ML MoSe2 is characterized by a coexistence of deformation-induced and atomic stick–slip motion. The experimental results presented here extend the understanding of the tribological properties of dry lubricants operating at the nanoscale.

An Insight into Chemistry and Structure of Colloidal 2D-WS2 Nanoflakes: Combined XPS and XRD Study.

The surface and structural characterization techniques of three atom-thick bi-dimensional 2D-WS2 colloidal nanocrystals cross the limit of bulk investigation, offering the possibility of simultaneous phase identification, structural-to-morphological evaluation, and surface chemical description. In the present study, we report a rational understanding based on X-ray photoelectron spectroscopy (XPS) and structural inspection of two kinds of dimensionally controllable 2D-WS2 colloidal nanoflakes (NFLs) generated with a surfactant assisted non-hydrolytic route. The qualitative and quantitative determination of 1T’ and 2H phases based on W 4f XPS signal components, together with the presence of two kinds of sulfur ions, S22− and S2−, based on S 2p signal and related to the formation of WS2 and WOxSy in a mixed oxygen-sulfur environment, are carefully reported and discussed for both nanocrystals breeds. The XPS results are used as an input for detailed X-ray Diffraction (XRD) analysis allowing for a clear discrimination of NFLs crystal habit, and an estimation of the exact number of atomic monolayers composing the 2D-WS2 nanocrystalline samples.

Electrochemical reduction of CO2 to CH4 over transition metal atom embedded antimonene: First-principles study

Electrocatalytic CO2 reduction (ECR) represents a promising way for the utilization of renewable energy such as wind and solar to produce value-added fuels or chemicals. The process mainly suffers from the sluggish CO2 reaction kinetics and the lack of efficient electrocatalysts. Herein, we investigated a wide range of transition metal (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ru, Rh, Pd, Ag, Cd, Ir, Pt and Au) atoms anchored into monovacancy of antimonene (Sb monolayer) as single-atom catalysts (SACs) for ECR by first- principles calculation. Interestingly, non-precious TM atoms from the 3d group (TM = Sc, V, Cr, Mn, Fe, Co, Zn) supported on Sb monolayer show higher ECR selectivity than hydrogen evolution reaction selectivity. Moreover, the primary ECR product of these non-precious metal-based SACs is CH4, except for Zn which produces HCOOH. Co@Sb monolayer exhibits the lowest overpotential 0.50 V, which is comparable to reported excellent electrocatalysts. The interaction between TM atom and Sb monolayer greatly affects the intrinsic activity of SACs. Meanwhile, the interaction between TM atoms and intermediates of the potential determining steps (PDS) will determine the overpotential and final products of ECR. This work opens a new family of stable and selective SACs based on antimonene monolayer and provides new insights for screening SACs by charge transfer of bonding atoms.

Nanoscale Turing patterns in a bismuth monolayer

Turing’s reaction–diffusion theory of morphogenesis has been very successful for understanding macroscopic patterns within complex objects ranging from biological systems to sand dunes. However, Turing patterns on microscopic length scales are extremely rare. Here we show that a strained atomic bismuth monolayer assembled on the surface of NbSe2—and subject to interatomic interactions and kinetics—displays Turing patterns. Our reaction–diffusion model produces stripe patterns with a period of five atoms (approximately 2 nm) and domain walls with Y-shaped junctions that bear a striking resemblance to what has been experimentally observed. Our work establishes that Turing patterns can occur at the atomic scale in a hard condensed-matter setting.

Micro-cracking pattern recognition of hybrid CNTs/GNPs cement pastes under three-point bending loading using acoustic emission technique

The generation of microcracks has an important influence on the behaviour of concrete structures. In this study, the acoustic emission (AE) technique was used to investigate the fracture phenomena and micro-cracking behavior of hybrid carbon nanotubes (CNTs, the 1-D allotrope of carbon atoms) and graphene nanoplatelets (GNPs, 2D monolayer of sp2-hybridized carbon atoms), cement composites under three-point bending loading. In addition, dispersion level by ultraviolet visible spectroscopy (UV–Vis), mechanical performance, and micro-structure properties by scanning electron microscopy (SEM) of hybrid cement composites were studied. Different typical AE parameters like AE hits, rise time, frequency and amplitude distribution, and b-value were used for the AE analysis. The cracking pattern and failure mechanism of the specimens were observed into three stages: initial elastic stage, pre-fracture stage, and post-fracture stage. The conclusions were drawn as follows: (1) through the UV–Vis spectrometry, it is found that GNPs act as dispersion agents for CNTs; (2) hybrid specimens experienced large deformation with the increasing wt.% of CNTs + GNPs during post fracture phase, thus showing more ductile behavior as compared to the control sample; (3) based on the average frequency (AF) and RA value (rise time divided by the amplitude), the addition of CNTs + GNPs content in cement composites shifted the failure mode from shear to tensile; (4) the AE parameters are feasible to analyze and differentiate the cracking pattern between micro and macrocracks; (5) the field emission scanning electron microscopy (FESEM) image evidence that hybrid CNTs + GNPs particles are well dispersed within cement composites. Based on the obtained results, it was concluded that the acoustic emission technique is a favorable approach to monitor and identify the microcracking pattern in brittle concrete structures.

Shell tailoring of core–shell nanoparticles for heterogeneous catalysis

In this study, platinum core–palladium shell nanoparticles have been synthesized by a chemical method with the shell thickness controlled to submonolayer precision. A single palladium monolayer has been prepared on platinum nanoparticles by chemically-driven underpotential deposition in a batch reactor with a redox buffer. The platinum core–palladium shell nanoparticles studied exhibit the highest electrocatalytic properties toward formic acid oxidation when the shell is only one monolayer thick, which underlines the need for precise control of the shell thickness for applications in heterogeneous catalysis. In this study we demonstrate that chemical methods allow for easy preparation of large quantities of core–shell nanoparticles in batch reactors, where the shell thickness can be tailored from 1 to 10 monoatomic layers.

Relaxation Mechanisms and Strain-Controlled Oxygen Vacancies in Epitaxial SrMnO3 Films.

SrMnO3 has a rich epitaxial strain-dependent ferroic phase diagram, in which a variety of magnetic orderings, even ferroelectricity, and thus multiferroicity, are accessible by gradually modifying the strain. Different relaxation processes, though, including the presence of strain-induced oxygen vacancies, can severely curtail the possibility of stabilizing these ferroic phases. Here, we report on a thorough investigation of the strain relaxation mechanisms in SrMnO3 films grown on several substrates imposing varying degrees of strain from slightly compressive (−0.39%) to largely tensile ≈+3.8%. First, we determine the strain dependency of the critical thickness (tc) below which pseudomorphic growth is obtained. Second, the mechanisms of stress relaxation are elucidated, revealing that misfit dislocations and stacking faults accommodate the strain above tc. Yet, even for films thicker than tc, the atomic monolayers below tc are proved to remain fully coherent. Therefore, multiferroicity may also emerge even in films that appear to be partially relaxed. Last, we demonstrate that fully coherent films with the same thickness present a lower oxygen content for increasing tensile mismatch with the substrate. This behavior proves the coupling between the formation of oxygen vacancies and epitaxial strain, in agreement with first-principles calculations, enabling the strain control of the Mn3+/Mn4+ ratio, which strongly affects the magnetic and electrical properties. However, the presence of oxygen vacancies/Mn3+ cations reduces the effective epitaxial strain in the SrMnO3 films and, thus, the accessibility to the strain-induced multiferroic phase.

High Osmotic Power Generation via Nanopore Arrays in Hybrid Hexagonal Boron Nitride/Silicon Nitride Membranes.

Nanopores embedded in two-dimensional (2D) nanomaterials are a promising emerging technology for osmotic power generation. Here, coupling our new AFM-based pore fabrication approach, tip-controlled local breakdown (TCLB), with a hybrid membrane formed by coating silicon nitride (SiN) with hexagonal boron nitride (hBN), we show that high osmotic power density can be obtained in systems that do not possess the thinness of atomic monolayers. In our approach, the high osmotic performance arises from charge separation induced by the highly charged hBN surface rather than charge on the inner pore wall. Moreover, exploiting TCLB's capability of producing sub 10 nm pore arrays, we investigate the effects of pore-pore interaction on the overall power density. We find that an optimum pore-to-pore spacing of ∼500 nm is required to maintain an efficient selective transport mechanism.

Low-temperature synthesis of pure-phase Ti3(Al, Fe)C2 solid solution with magnetic monoatomic layers by replacement reaction

Low-temperature synthesis of pure-phase Ti3(Al, Fe)C2 solid solution with magnetic monoatomic layers by replacement reaction