Monolayer Of Atoms Research Articles

Van der Waals screening by graphenelike monolayers

The ubiquitous van der Waals (vdW) interaction determines the fundamental properties of material surfaces. Here, we show that graphene can partly screen the vdW energy by up to 53%, as tested in a graphene trilayer and a sandwiched BN/graphene/BN multilayer using the random phase approximation and density functional theory based many-body-dispersion method. The vdW screening turns weaker in semiconducting and insulating graphenelike monoatomic layers, displaying a strong correlation with the band gap and the dielectric constant of materials, but depends less on the screened materials. The revealed relationship between the vdW screening and the fundamental electronic structure of the graphenelike material will be instrumental in the rational control of surface interactions.

A Reduction in Particle Size Generally Causes Body-Centered-Cubic Metals to Expand but Face-Centered-Cubic Metals to Contract.

From a careful analysis of existing data as well as new measurements, we show that the size dependence of the lattice parameters in metal nanoparticles with face-centered cubic (fcc) and body-centered cubic (bcc) symmetries display opposite trends: nanoparticles with fcc structure generally contract with decreasing particle size, while those with bcc structure expand. We present a microscopic explanation for this apparently puzzling behavior based on first-principles simulations. Our results, obtained from a comparison of density functional theory calculations with experimental data, indicate that the nanoparticles are capped by a surface monolayer of oxygen atoms, which is routinely detected by surface-sensitive techniques. The bcc- and fcc-based nanoparticles respond in contrasting fashion to the presence of the oxygen capping layer, and this dictates whether the corresponding lattice parameter would increase or decrease with size reduction. The metal-oxygen bonds at the surface, being shorter and stronger than typical metal-metal bonds, pull the surface metal atoms outward. This outward movement of surface atoms influences the core regions to a larger extent in the relatively open bcc geometry, producing a rather large overall expansion of the cluster, compared to the bulk. In case of fcc clusters, on the other hand, the outward movement of surface metal atoms does not percolate too far inside, resulting in either a smaller net expansion or contraction of the cluster depending on the extent of surface oxygen coverage. Our study therefore provides a convincing physicochemical basis for the correlation between the underlying geometry and the nature of change of the lattice parameters under size reduction.

Developing seedless growth of atomically thin semiconductor layers: Application to transition metal dichalcogenides

Controlled growth of atomic monolayers of IV-VII transition metal dichalcogenides (TMDs) has provided unprecedented opportunities to fabricate modern optoelectronic nanodevices. However, synthesis of large-area and high quality two-dimensional TMDs is still challenging. We have synthesized WS2 and MoS2 nanosheets by atmospheric pressure chemical vapor deposition (APCVD) at wide-range of processing conditions. The nanostructures were analyzed by optical and confocal microscopy, atomic force microscopy, Raman spectroscopy, and X-ray diffraction to determine the thickness, lateral size and structure of the deposits. Through designing and performing of a set of controlled experiments as well as comparing the attained results with others, we present a general roadmap for APCVD of atomically thin WS2 and MoS2 nanosheets. The appropriate working windows for the controlled synthesis are established. For the controlled growth of 2D WS2 crystals, relatively low growth temperature (750 °C) can be utilized. It is shown that an oxide mass to the gas flow rate of 0.004 favorably results in the formation of 2D WS2. Monolayers and few layers might be processed at higher temperatures (≥ 800 °C) at an oxide mass/gas flow rate ratio of 2–3. The seedless growth of MoS2 nanosheets is more complicated than WS2 possessing with narrower working window. A minimum temperature of 575 °C is required to provide enough oxide vapor pressure for deposition; otherwise, no deposits are formed. Above 650 °C, achieving 2D nanolayers is tricky due to massive deposition of the metal oxide from the vapor phase forming thick layers. In the intermediate range, however, there is a possibility to attain 2D nanostructures through fine controlling of the oxide mas/gas flow rate. Finally, it is shown that through precise control of the processing parameters, a controllable growth of the nanosheets without pre-seeding at lower temperatures and shorter times is feasible.

High-Definition Nanoimprint Stamp Fabrication by Atomic Layer Etching

Nanoimprint lithography (NIL) has the potential for low-cost and high-throughput nanoscale fabrication. However, the NIL quality and resolution are usually limited by the shape and size of the nanoimprint stamp features. Atomic layer etching (ALE) can provide a damage-free pattern transfer with ultimate etch control for features of all length scales, down to the atomic scale, and for all feature geometries, which is required for good quality and high-resolution nanoimprint stamp fabrication. Here, we present an ALE process for nanoscale pattern transfer and high-resolution nanoimprint stamp preparation. This ALE process is based on chemical adsorption of a monoatomic layer of dichloride (Cl2) on the silicon surface, followed by the removal of a monolayer of Cl2-modified silicon by argon bombardment. The nanopatterns of different geometries, loadings, and pitches were fabricated by electron beam lithography on a silicon wafer, and ALE was subsequently performed for pattern transfer using a resist as an etc...

Morphological and Radio Frequency Characterization of Graphene Composite Films

Graphene is a monolayer of carbon atoms which exhibits remarkable electronic and mechanical properties. Graphene based nano-materials have gained a lot of interest for many applications. In this paper, inks with three different graphene concentrations (12.5, 25, and 33 wt % in graphene) were prepared and deposited by screen printing. A detailed investigation of films’ surface morphology using Scanning Electron Microscope (SEM) and Atomic Force Microscopy (AFM) revealed that the graphene films present a homogeneous dispersion of the filler with a comparatively lower surface roughness at higher concentrations and negligible agglomerates. The films were then printed between copper electrodes on FR-4 substrate (trade name for glass-reinforced epoxy laminate material), commonly used in Radio Frequency (RF) circuits, and the measured scattering parameters analyzed. Finally, the reflection coefficient of a patch antenna, fabricated on FR-4 substrate with and without a stub loaded by a thin film were measured. The difference of the resonant frequency due to the different interaction between graphene flakes and polymer binders shows attractive features of functionalized graphene films for chemical and bio-sensing applications.

Spin valley and giant quantum spin Hall gap of hydrofluorinated bismuth nanosheet

Spin-valley and electronic band topological properties have been extensively explored in quantum material science, yet their coexistence has rarely been realized in stoichiometric two-dimensional (2D) materials. We theoretically predict the quantum spin Hall effect (QSHE) in the hydrofluorinated bismuth (Bi2HF) nanosheet where the hydrogen (H) and fluorine (F) atoms are functionalized on opposite sides of bismuth (Bi) atomic monolayer. Such Bi2HF nanosheet is found to be a 2D topological insulator with a giant band gap of 0.97 eV which might host room temperature QSHE. The atomistic structure of Bi2HF nanosheet is noncentrosymmetric and the spontaneous polarization arises from the hydrofluorinated morphology. The phonon spectrum and ab initio molecular dynamic (AIMD) calculations reveal that the proposed Bi2HF nanosheet is dynamically and thermally stable. The inversion symmetry breaking together with spin-orbit coupling (SOC) leads to the coupling between spin and valley in Bi2HF nanosheet. The emerging valley-dependent properties and the interplay between intrinsic dipole and SOC are investigated using first-principles calculations combined with an effective Hamiltonian model. The topological invariant of the Bi2HF nanosheet is confirmed by using Wilson loop method and the calculated helical metallic edge states are shown to host QSHE. The Bi2HF nanosheet is therefore a promising platform to realize room temperature QSHE and valley spintronics.

Attosecond-resolution Hong-Ou-Mandel interferometry.

When two indistinguishable photons are each incident on separate input ports of a beamsplitter, they "bunch" deterministically, exiting via the same port as a direct consequence of their bosonic nature. This two-photon interference effect has long-held the potential for application in precision measurement of time delays, such as those induced by transparent specimens with unknown thickness profiles. However, the technique has never achieved resolutions significantly better than the few-femtosecond (micrometer) scale other than in a common-path geometry that severely limits applications. We develop the precision of Hong-Ou-Mandel interferometry toward the ultimate limits dictated by statistical estimation theory, achieving few-attosecond (or nanometer path length) scale resolutions in a dual-arm geometry, thus providing access to length scales pertinent to cell biology and monoatomic layer two-dimensional materials.

Direct Measurement of the Intrinsic Sharpness of Magnetic Interfaces Formed by Chemical Disorder Using a He+ Beam.

Using ion beams to locally modify material properties and subsequently drive magnetic phase transitions is rapidly gaining momentum as the technique of choice for the fabrication of magnetic nanoelements. This is because the method provides the capability to engineer in three dimensions on the nanometer length scale. This will be animportant consideration for several emerging magnetic technologies (e.g., spintronic devices and racetrack and random-access memories) where device functionality will hinge on the spatial definition of the incorporated magnetic nanoelements. In this work, the fundamental sharpness of a magnetic interface formed by nanomachining FePt3 films using He+ irradiation is investigated. Through careful selection of the irradiating ion energy and fluence, room-temperature ferromagnetism is locally induced into a fractional volume of a paramagnetic (PM) FePt3 film by modifying the chemical order parameter. A combination of transmission electron microscopy, magnetometry, and polarized neutron reflectometry measurements demonstrates that the interface over which the PM-to-ferromagnetic modulation occurs in this model system is confined to a few atomic monolayers only, while the structural boundary transition is less well-defined. Using complementary density functional theory, the mechanism for the ion-beam-induced magnetic transition is elucidated and shown to be caused by an intermixing of Fe and Pt atoms in antisite defects above a threshold density.

Large magnetoresistance by Pauli blockade in hydrogenated graphene

We report the observation of a giant positive magnetoresistance in millimetre scale hydrogenated graphene with magnetic field oriented in the plane of the graphene sheet. A positive magnetoresistance in excess of 200\% at a temperature of 300 mK was observed in this configuration, reverting to negative magnetoresistance with the magnetic field oriented normal to the graphene plane. We attribute the observed positive, in-plane, magnetoresistance to Pauli-blockade of hopping conduction induced by spin polarization. Our work shows that spin polarization in concert with electron-electron interaction can play a dominant role in magnetotransport within an atomic monolayer.

Mechanical and Viscoelastic Properties of In-situ Amine Functionalized Multiple Layer Grpahene /epoxy Nanocomposites

Introduction: Graphene is flat monolayer of carbon atoms (one atom thick), covalently bonded to three other atoms in tightly packed two-dimensional (2D) hexagonal single layer stable crystalline honeycomb lattice structure. In this paper, In-situ amine functionalized exfoliated graphene with multiple layers (3-6) with low defect contents and average aspect ratio upto 10 microns (average X and Y dimensions) and thickness upto 2-3 nm (average Z-direction) which have been produced with the combined effort of chemical vapor deposition (CVD) and chemical graphite exfoliation method. Methods: This paper also focuses on the effect of the reinforcement of amine functionalized multiple graphene layers (AF-MGL) on the mechanical and visco-elastic properties of epoxy composites. AFMGL/ epoxy composites (AF-MGL/EpC) were prepared with graphene fractions ranging from 0.5 to 2.0 wt%. The four different samples were prepared using an amount of graphene as 0.0, 0.5, 1.5, and 2.0. A series of tensile three point bend tests were performed on the different AFMGL/epoxy composites. Optical and scanning electron microscopy (SEM) was used to examine the micro structural features and fractured surfaces of AF-MGL/EpC. Results: Increased graphene content results in improved tensile strength and the modulus of an epoxy matrix when compared with the pure epoxy matrix. The 1.5 wt% AF-MGL/EpC showed an increase in tensile strength and modulus by 50.2 and 52.8% respectively. However, a shrink was noticed beyond 1.5 wt.% samples of AF-MGL/EpC composite. Moreover, an improvement of 28.8% in the storage modulus was also recorded when compared with epoxy composites. Conclusion: The effect of the amine functional group on the mechanical and viscoelastic properties was also explored using molecular dynamics (MD) simulations and predicted results were then compared with experimental results.

Photoelectrochemical Analysis of Tin Selenide (SnSex) Thin Films Formed Using Electrochemical Atomic Layer Deposition (E-ALD)

Tin selenide is an attractive material for photovoltaic devices as an absorber layer.1 There are two derivatives: SnSe and SnSe2. SnSe is a p-type material that exhibits a band gap of 1.0 eV. SnSe2 is a n-type material with a band gap of 1.6 eV.1 In this study, tin selenide thin films have been electrochemically deposited using a process known as electrochemical atomic layer deposition (E-ALD). E-ALD uses the phenomenon known as underpotential deposition to deposit an atomic monolayer of each element separately. This process is known as a cycle and is repeated until the desired thickness is achieved.2 These films need further optimization for the selection of one species over the other. Initial photolectrochemical studies have shown the current thin film is p-type and is an option for a photocathode for hydrogen production in a photoelectrochemical cell. Further studies will include improving the deposition cycle used to increase the photoresponse of the material and to gain control over the appearance of SnSe or SnSe2. Subramanian, B; Snajeeviraja, C.; Jayaychandran, M. Review on materials properties Sn(S,Se) compound semiconductors useful for photoelectrochemical solar cells. Electrochem. 2002, 18 (8), 349-366.Stickney, J. L.; Villegas, I.; Suggs, D. W.; Gregory, B. W., Electrochemical Atomic Layer Epitaxy (ECALE). Abstr Pap Am Chem S 1991, 201, 289-Coll.

Spontaneous Emission Enhancement in Strain-Induced WSe2 Monolayer-Based Quantum Light Sources on Metallic Surfaces

Atomic monolayers of transition metal dichalcogenides represent an emerging material platform for the implementation of ultracompact quantum light emitters via strain engineering. In this framework, we discuss experimental results on creation of strain induced single photon sources using a WSe2 monolayer on a silver substrate, coated with a very thin dielectric layer. We identify quantum emitters that are formed at various locations in the sample. Their emission is highly linearly polarized, stable in linewidth, and decay times down to 100 ps are observed. We provide numerical calculations of our monolayer-metal device platform to assess the strength of the radiative decay rate enhancement by the presence of the plasmonic structure. We believe that our results represent a crucial step toward the ultracompact integration of high performance single photon sources in nanoplasmonic devices and circuits.

From Salt to Germanene: A Cookbook for Electrochemical Formation of 2D Materials (Inspired by R. Adžić)

2D materials with honeycomb lattices are increasingly studied due to their unique electronic and mechanical properties. The typical preparation techniques, such as chemical vapor deposition and cleavage of bulk crystals, are either complicated or limited to only certain classes of materials. Here we discuss basic ordering principles in 2D monoatomic layers and show that electrochemical deposition can also result in 2D films with desired honeycomb structures. The principles are first studied on model ionic 2D layers prepared in ultrahigh vacuum. Two ordering possibilities are identified: a bilayer structure for covalently bonded films and a mixed monolayer structure for ionically bonded films. Typically, the most energetically favorable configuration in mixed layers is when ions with one polarity are closely surrounded by ions of the opposite polarity. This translates into a layer with honeycomb structure on substrates with p6/p3 symmetry. This phenomenon can be used to prepare germanene, a layer with such a structure.

Towards spontaneous parametric down conversion from monolayer MoS2

We present a detailed study of the second order nonlinearity of 2D (mono-atomic layer) dichalcogenide MoS2, both in the visible and in the IR regime, and test its potential for spontaneous parametric down-conversion (SPDC), the amplification of vacuum fluctuations mediated by optical nonlinearity. We develop a model of SPDC from a deeply subwavelength nonlinear medium, where phase matching conditions are completely relaxed, and make predictions about the rate of emitted photons, their momentum, polarisation and spectrum. We show that detection in the visible spectral region is hindered by the strong photoluminescence background. Moving to the IR regime we observe indications of SPDC by performing polarization, power dependence and lifetime measurements around 1560 nm. We show that the signal from a single monolayer is qualitatively different from that generated by multi-layer MoS2. Finally, we characterize the latter as a new kind of photo-luminescence emission which is enhanced at the edges of multi-layer MoS2.

Multi-Mode Elastic Peak Electron Microscopy (MM-EPEM): A new imaging technique with an ultimate in-depth resolution for surface analysis

A non-destructive new imaging technique called Multi-Mode Elastic Peak Electron Microscopy (MM-EPEM), hypersensitive to surface chemistry and with an in-depth resolution of one atomic monolayer was developed. This method consists on performing several MM-EPEM images containing n × n pixels associated to an intensity of the elastic backscattered electrons by varying the incident electron energy in the range 200–2000 eV. This approach allows obtaining depth sampling information of the analyzed structures. Furthermore, MM-EPEM is associated with Monte-Carlo simulations describing the electron pathway in materials in order to obtain very precise quantitative information, for instance the growth mode and the organization of ultra-thin layers (2D materials) or nanoparticules. In this work, we used this new method to study the deposition of very small amount of gold down to one monolayer. Example of 3D reconstruction is also provided.

Thermodynamics of dilute 3He–4He solid solutions with hcp structure

To interpret the anomalies in heat capacity CV(T) and temperature-dependent pressure P(T) of solid hexagonal close-packed (hcp) 4He we exploit the model of hcp crystalline polytype with specific lattice degrees of freedom and describe the thermodynamics of impurity-free 4He solid as superposition of phononic and polytypic contributions. The hcp-based polytype is a stack of 2D basal atomic monolayers on triangular lattice packed with arbitrary long (up to infinity) spatial period along the hexagonal c axis perpendicular to the basal planes. It is a crystal with perfect ordering along the layers, but without microscopic translational symmetry in perpendicular direction (which remains, nevertheless, the rotational crystallographic axis of third order, so that the polytype can be considered as semidisordered system). Each atom of the hcp polytype has twelve crystallographic neighbors in both first and second coordination spheres at any arbitrary packing order. It is shown that the crystal of such structure behaves as anisotropic elastic medium with specific dispersion law of phonon excitations along c axis. The free energy and the heat capacity consist of two terms: one of them is a normal contribution [with CV(T) ∼ T3] from phonon excitations in an anisotropic lattice of hexagonal symmetry, and another term (an “excessive” heat) is a contribution resulted by packing entropy from quasi-one-dimensional system of 2D basal planes on triangular lattice stacked randomly along c axis without braking the closest pack between neighboring atomic layers. The excessive part of the free energy has been treated within 1D quasi-Ising (lattice gas) model using the transfer matrix approach. This model makes us possible to interpret successfully the thermodynamic anomaly (heat capacity peak in hcp 4He) observed experimentally.

Scattering of electromagnetic waves by a graphene-coated thin cylinder of left-handed metamaterial

In this paper, we explored the scattering behavior of thin cylinders made of a left-handed material (LHM) and coated by a monoatomic graphene layer. A spectral tunability of the resonance peaks is evidenced by altering the chemical potential of the graphene coating, a fact that occurs at any state of polarization of the incident plane wave in opposition to the case of scatterers of dielectric core. On the contrary, no invisibility condition can be satisfied for dielectric environments. A singular performance is also found for cylinders with permittivity and permeability near zero. Practical implementations of our results can be carried out in sensing and wave manipulation driven by metamaterials.

TiO₂ Films Modified with Au Nanoclusters as Self-Cleaning Surfaces under Visible Light.

In this study, we applied cluster beam deposition (CBD) as a new approach for fabricating efficient plasmon-based photocatalytic materials. Au nanoclusters (AuNCs) produced in the gas phase were deposited on TiO2 P25-coated silicon wafers with coverage ranging from 2 to 8 atomic monolayer (ML) equivalents. Scanning Electron Microscopy (SEM) images of the AuNCs modified TiO2 P25 films show that the surface is uniformly covered by the AuNCs that remain isolated at low coverage (2 ML, 4 ML) and aggregate at higher coverage (8 ML). A clear relationship between AuNCs coverage and photocatalytic activity towards stearic acid photo-oxidation was measured, both under ultraviolet and green light illumination. TiO2 P25 covered with 4 ML AuNCs showed the best stearic acid photo-oxidation performance under green light illumination (Formal Quantum Efficiency 1.6 × 10−6 over a period of 93 h). These results demonstrate the large potential of gas-phase AuNCs beam deposition technology for the fabrication of visible light active plasmonic photocatalysts.

A comprehensive study of catalytic, morphological and electronic properties of ligand-protected gold nanoclusters using XPS, STM, XAFS, and TPD techniques.

Ultra-small gold nanoclusters were synthesized via a ligand exchange method and deposited onto different TiO2 supports to study their properties. STM imaging revealed that the as-synthesized gold nanoclusters had 2-D morphology consisting of monolayers of gold atoms. Subsequent XPS, XAFS, and CO oxidation TPD results indicated that heat treatments of gold clusters at different temperatures significantly altered their electronic and catalytic properties due to ligand deprotection and cluster agglomeration.

Predictive modeling of intrinsic strengths for several groups of chemically related monolayers by a reference model.

The linear correlation between the ratios of linear and nonlinear intrinsic mechanical properties allows the prediction of largely unknown nonlinear fracture properties of brittle two-dimensional (2D) solids using a reference model. It is demonstrated that from accurately known Young's moduli unknown theoretical strengths can be derived. The predictive power of the reference model for the systematic evaluation of the fundamental mechanical behavior of whole groups of compounds with chemically-related configurations is shown. This is revealed for graphene-like atomic monolayers, various derivatives of graphene, transition-metal dichalcogenide (TMDC), and transition-metal monochalcogenide (TMMC) molecular monolayers of the fast growing family of 2D solids. The estimated unknown intrinsic strengths represent the upper limits for the mechanical performance of 2D compounds and can be used to judge the actual deviations between real monolayers and their ideal counterparts. A detailed comparison of the reference model with existing experiments and density functional theory (DFT) calculations confirmed that the ratios of Young's moduli and intrinsic strengths reach a factor of ∼6 for strongly bonded TMMCs, the bulk value of ∼9 for graphene-like monolayers, ∼11 for graphene derivatives, whereas for transition-metal dichalcogenide (TMDC) monolayers the ratio is ∼13.5. With its minimal requirement of mechanical input data and its versatility the reference model provides a unique tool to predict unknown nonlinear mechanical properties.