Single Atomic Layer Research Articles

Nonlinear optical effect and nonlinear optical mass sensor based on graphene optomechanical system

Graphene, atomically thin two-dimensional (2D) nanomaterial consisting of a single layer of carbon atoms, has received tremendous attention in the past few decades. Graphene may be considered as an excellent nanomaterial for fabricating nanomechanical resonator systems to investigate the quantum behavior of the motion of micromechanical resonators because of its unique properties of low mass density, high frequency, high quality-factor, and intrinsically small size. Additionally, graphene optomechanics based on a bilayer graphene resonator coupled to a microwave on-chip cavity, where light and micromechanical motion interact via the radiation pressure, has been demonstrtated experimentally recently. In this work, we demonstrate theoretically the nonlinear optical effect including optical bistability and four-wave mixing under the regimes woth different parameters and detunings in a graphene resonator-microwave cavity system. When the graphene optomechanics is driven by one strong pump laser beam, we find that the optical bistability can be controlled by tuning the power and the frequency of the pump beam. The four-wave mixing (FWM) phenomenon is also investigated and we find that sharp peaks in the FWM spectrum exactly are located at the resonant frequency of graphene resonator. Therefore, a straight nonlinear optical means for determining the resonant frequency of the graphene resonator is presented. Setting the cavity field resonating with pump field, and then scanning the probe frequency across the cavity frequency, one can easily and exactly obtain the resonant frequency of the resonator from the FWM spectrum. We further theoretically propose a mass sensor based on the graphene optomechanical system. The mass of external nanoparticles deposited onto the graphene resonator can be measured conveniently by tracking the shift of resonant frequency due to mass changing in the FWM spectrum. Compared with optomechanical mass sensors in linear regime, the nonlinear optical mass sensor may be immune to the detection noise. The system may have potential applications in communication networks for frequency conversion and provide a new platform for high sensitive sensing devices.

Структура и гранулометрический состав углеродсодержащей добавки в тяжелых бетонах

The discovery of a two-dimensional carbon form – graphene, consisting of a single layer of atoms connected by a structure of chemical bonds, gave impetus to developing a new wide field of its application in construction. An urgent task is to improve the performance of heavy cement concretes on dense boulders. One of the most effective ways is the use of a new generation of carbon-containing modifiers. The physical and mechanical properties of cement concrete should be regulated at the micro-level by optimizing the process of forming a cement matrix structure. A promising direction is the use of graphene as a carbon-containing modifier. Data from research studies devoted to oxidized thermally expanding graphite are presented. The results of microscopic and laser diffraction granulometric investigations are discussed. The conducted studies allowed the properties of the heavy concrete under study to be assessed. The granulometric composition was analysed using a Fritsch NanoTeс laser particle size measurement device equipped with a dispersion unit in a liquid medium with a total measurement range from 0.01 to 2100 µm. This analysis showed a reduction in size from 574 µm to 22.2 µm. Microscopic examination was performed using a scanning electron microscope TESCAN Mira 3 with a magnification range of x1–x1,000,000. The conducted experimental studies proved the effectiveness of using a carbon-containing additive as a modifier of heavy cement concretes on dense boulders. The use of oxidized thermo expandable graphite allows for a denser packing of the structure and provides an opportunity to create additional cites for new formations. This allows the physical, mechanical and operational characteristics of the final material to be improved.

Thermal conductance bottleneck of a three dimensional graphene-CNT hybrid structure: a molecular dynamics simulation.

Three dimensional (3D) graphene-CNT hybrid structures (GCNTs) are promising materials for applications including capacitors and gas storage and separation devices, however until now their thermal conductance mechanism has scarcely been studied. These hybrid nanomaterials are particularly suitable as next-generation thermal interface materials due to the excellent thermal properties of carbon nanotubes and single atomic layer graphene. In this paper, the out-of-plane thermal conductivities of GCNTs, graphene nanomesh (GNM), and graphene sheets are investigated using molecular dynamics (MD) simulations which apply the Green-Kubo method. Distinct from GNMs and graphene sheets, the GCNTs exhibit a relatively high out-of-plane thermal conductivity, stemming from the CNTs' ability to accelerate the energy flow. However, the GCNT out-of-plane thermal conductivity is still far lower than that of pristine graphene due to extreme phonon localizations, which are concentrated on the graphene-CNT junction regions as evidenced by the participation ratio, phonon vibrational density of states, and overlap energy. This study provides microscopic insight into the GCNT heat transfer mechanism and offers design guidelines for application of GCNTs in thermal management devices.

单原子层薄膜热传导性质的晶格动力学研究(II)——数值计算与结果分析

单原子层薄膜热传导性质的晶格动力学研究(II)——数值计算与结果分析

单原子层薄膜热传导性质的晶格动力学研究(I)——声子线宽和热传导系数公式

单原子层薄膜热传导性质的晶格动力学研究(I)——声子线宽和热传导系数公式

The wetting characteristics of copper droplets on tungsten surfaces on atomic scale: A molecular dynamics simulation

In recent years, the tungsten-copper composite has attracted much attention due to many excellent performances. In the present work, the wetting behaviors of Cu droplets on W (1 0 0), (1 1 0) and (1 1 1) surfaces had been investigated by molecular dynamics (MD) simulations. Results show that a precursor film with single atomic layer will form on W surfaces. The precursor films showed the anisotropic diffusion due to the difference of diffusion barrios along different directions. Only on the W (1 1 0) surface, the precursor film presents a circle. The precursor film is an oval shape on the W (1 0 0) surface and that is similar to the shape of a triangle on W (1 1 1) surface but the shape of Cu droplet on the W (1 1 1) surface is a circle. The Cu droplet on the W (1 1 1) has the best wettability than others due to the effects of both precursor films and the alloying effect. The Cu droplet on the W (1 1 0) surface has the worst wettability. In addition, the potential of mean force (PMF) analysis was applied to further character the wettability. Besides, the effect of both temperature and substrate structure on wettability was also studied, which shows that the wettability increases with the increased temperature. Moreover, the grooved surface can greatly advance the wetting kinetics along the grooves direction, but in the perpendicular direction, the effect is opposite.

Enhancement of magnetoelectric coupling by insertion of Co atomic layer into Fe3Si/BaTiO3(001) interfaces identified by first-principles calculations

The insertion of a Co single atomic layer into multiferroic interfaces is examined from first principles taking interfaces between BaTiO3 and Fe3Si as an example. We demonstrate that insertion of a Co atomic layer strongly prevents Si from being exposed to the interface. This protecting effect of Co makes the interface magnetoelectric constant of Fe3Si/Co/BaTiO3(001) remarkably large with 1.4 as, which makes contrast with the reduction of the interface multiferroicity by Si at the Fe3Si/BaTiO3(001) interface. The stability of the Co monolayer and the origin of the large magnetoelectric effect are clarified.

An analytical method to determine the complex refractive index of an ultra-thin film by ellipsometry

We proposed a completely analytical method to determine the complex refractive index of an ultra-thin film on an arbitrary substrate by ellipsometry. The method is based on directly solving the polynomial equation derived from the 2nd-order Taylor expansion of ellipsometric functions without any prior knowledge about electronic transitions of the material. Numerical simulations demonstrate that the proposed method can be applied to deal with ultra-thin films with a wide thickness range from a single atomic layer to 5 nm over an ultra-broad spectral range from deep ultraviolet to infrared regions. With the proposed method, complex refractive indices of typical two-dimensional (2D) materials, including the monolayer graphene and 1–3 layer WSe2, were experimentally investigated over an ultra-wide spectral range (193–1690 nm). The proposed method shows great potential in the accurate determination of complex refractive indices of ultra-thin films especially the emerging 2D materials.

Atomic Layer Deposition ZnO Over-Coated Cu/SiO2 Catalysts for Methanol Synthesis from CO2 Hydrogenation

Cu-ZnO-based catalysts are of importance for CO2 utilization to synthesize methanol. However, the mechanisms of CO2 activation, the split of the C=O double bond, and the formation of C-H and O-H bonds are still debatable. To understand this mechanism and to improve the selectivity of methanol formation, the combination of strong electronic adsorption (SEA) and atomic layer deposition (ALD) was used to form catalysts with Cu nanoparticles surrounded by a non-uniform ZnO layer, uniform atomic layer of ZnO, or multiple layers of ZnO on porous SiO2. N2 adsorption, H2 temperature-programmed reduction (H2-TPR) X-ray diffraction (XRD), transmission electron microscope (TEM), energy-dispersive X-ray spectroscopy (EDX), CO-chemisorption, CO2 temperature-programmed desorption (CO2-TPD), X-ray adsorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) were used to characterize the catalysts. The catalyst activity was correlated to the number of metallic sites. The catalyst of 5 wt% Cu over-coated with a single atomic layer of ZnO exhibited higher methanol selectivity. This catalyst has comparatively more metallic sites (smaller Cu particles with good distribution) and basic site (uniform ZnO layer) formation, and a stronger interaction between them, which provided necessary synergy for the CO2 activation and hydrogenation to form methanol.

Effect of tool edge radius on material removal mechanism in atomic and close-to-atomic scale cutting

For mechanical cutting, when cutting depth is decreased to atomic and close-to-atomic scale(ACS), the material removal mechanism would be dominated by dislocation motion, different from conventional cutting and nanocutting. It is is greatly influenced by cutting edge radius effect in the aspects of chip formation, cutting forces, and stress distribution, etc. There appear three deformation zones in ACS cutting, including dislocation slip zone, chip formation zone and elastic deformation zone. As cutting-edge radius increases, both of dislocation slip zone and chip formation zone are suppressed while the elastic deformation zone tends to continually grow. When cutting edge radius is increased to be larger than a threshold, the elastic deformation zone is further transformed into one elastic and plastic deformation zone, while a very small chip formation zone, and dislocation zone is remained ahead of rounded cutting edge. Consequently, though the minimum cutting depth is decreased to single atomic layer, the material removal behaviour is also dominated by extrusion action of cutting tool, following the mechanism of nanocutting. The research findings would provide theoretical guidelines to the cutting tool design for atomic and close-to-atomic scale cutting.

Magnon-Mediated Indirect Exciton Condensation through Antiferromagnetic Insulators.

Electrons and holes residing on the opposing sides of an insulating barrier and experiencing an attractive Coulomb interaction can spontaneously form a coherent state known as an indirect exciton condensate. We study a trilayer system where the barrier is an antiferromagnetic insulator. The electrons and holes here additionally interact via interfacial coupling to the antiferromagnetic magnons. We show that by employing magnetically uncompensated interfaces, we can design the magnon-mediated interaction to be attractive or repulsive by varying the thickness of the antiferromagnetic insulator by a single atomic layer. We derive an analytical expression for the critical temperature T_{c} of the indirect exciton condensation. Within our model, anisotropy is found to be crucial for achieving a finite T_{c}, which increases with the strength of the exchange interaction in the antiferromagnetic bulk. For realistic material parameters, we estimate T_{c} to be around 7K, the same order of magnitude as the current experimentally achievable exciton condensation where the attraction is solely due to the Coulomb interaction. The magnon-mediated interaction is expected to cooperate with the Coulomb interaction for condensation of indirect excitons, thereby providing a means to significantly increase the exciton condensation temperature range.

Dimensionally driven crossover from semimetal to direct semiconductor in layered SbAs

Two-dimensional (2D) materials have attracted a great deal of attention because they exhibit intriguing physical and chemical properties with great potential applications in electronic and optoelectronic devices, and even energy conversion. Due to the high anisotropy and unique crystal structure of layered materials, the properties can be effectively tuned by simply reducing dimensions to 2D. In this work, a unique 2D semiconductor, namely, monolayered SbAs, with high stability and indirect band gap, is predicted on the basis of first-principles calculations together with cluster expansion and Monte Carlo simulations. Interestingly, although the bulk antimony arsenide compound SbAs is known to exhibit semimetallic behavior, our calculations find that it is transformed into a direct semiconductor with a band gap of 1.28 eV when thinned down to a single atomic layer. The monolayer with antisite defects is transformed from indirect into a direct band-gap semiconductor. Such dramatic changes in the electronic structure could pave the way for SbAs to play a role in electronic device applications. Moreover, we find that the interlayer interactions in SbAs lead to a higher exfoliation energy than typical transition metal dichalcogenides such as $\mathrm{Mo}{\mathrm{S}}_{2}$, and hence we suggest that chemical deposition methods might be better than mechanical exfoliation methods for obtaining monolayer samples.

Mechanism of atomic and close-to-atomic scale cutting of monocrystalline copper

Mechanical cutting is one of promising subtractive machining processes to enable the atomic and close-to-atomic scale (ACS) manufacturing of the processed surface with atomic scale form accuracy or functional feature size. When cutting depth is decreased to ACS, minimum chip thickness could be decreased to several atomic layers and even single atomic layer. Consequently, there arises one disruptive cutting technology towards atomic and close-to-atomic scale manufacturing, i.e. ACS cutting, the next-generation cutting technology different from conventional cutting, microcutting and nanocutting. In this paper, one ACS cutting model is proposed to study the material removal mechanism at close-to-atomic scale and even atomic scale. It is postulated that chip formation in ACS cutting is conducted by shear stress-driven dislocation movement, significantly different from the shearing-dominated chip formation in conventional cutting and the extrusion-dominated chip formation in micro/nano cutting. Moreover, two kinds of sizing effects, namely, cutting edge radius effect and atomic sizing effect, would greatly influence the surface generation and material removal in ACS cutting.

Nonstoichiometric Phases of Two-Dimensional Transition-Metal Dichalcogenides: From Chalcogen Vacancies to Pure Metal Membranes.

Two-dimensional (2D) membranes consisting of a single layer of Mo atoms were recently manufactured [ Adv. Mater. 2018 , 30 , 1707281 ] from MoSe2 sheets by sputtering Se atoms using an electron beam in a transmission electron microscope. This is an unexpected result as formation of Mo clusters should energetically be more favorable. To get microscopic insights into the energetics of realistic Mo membranes and nonstoichiometric phases of transition-metal dichalcogenides (TMDs) MaXb, where M = Mo and W and X = S, Se, and Te, we carry out first-principles calculations and demonstrate that the membranes, which can be referred to as metallic quantum dots embedded into a semiconducting matrix, can be stabilized by charge transfer. We also show that an ideal neutral 2D Mo or W sheet is not flat but a corrugated structure, with a square lattice being the lowest-energy configuration. We further demonstrate that several intermediate nonstoichiometric phases of TMDs are possible as they have lower formation energies than pure metal membranes. Among them, the orthorhombic metallic 2D M4X4 phase is particularly stable. Finally, we study the properties of this phase in detail and discuss how it can be manufactured by the top-down approaches.

P-wave superconductivity in iron-based superconductors

The possibility of p-wave pairing in superconductors has been proposed more than five decades ago, but has not yet been convincingly demonstrated. One difficulty is that some p-wave states are thermodynamically indistinguishable from s-wave, while others are very similar to d-wave states. Here we studied the self-field critical current of NdFeAs(O,F) thin films in order to extract absolute values of the London penetration depth, the superconducting energy gap, and the relative jump in specific heat at the superconducting transition temperature, and find that all the deduced physical parameters strongly indicate that NdFeAs(O,F) is a bulk p-wave superconductor. Further investigation revealed that single atomic layer FeSe also shows p-wave pairing. In an attempt to generalize these findings, we re-examined the whole inventory of superfluid density measurements in iron-based superconductors and show quite generally that single-band weak-coupling p-wave superconductivity is exhibited in iron-based superconductors.

GaN nanowires as probes for high resolution atomic force and scanning tunneling microscopy

GaN nanowires are potential candidates for use in scanning probe microscopy due to their well-defined, reproducible, geometric shapes, their hardness, and their light guiding properties. We have developed and investigated probes for high resolution atomic force microscopy and scanning tunneling microscopy utilizing GaN nanowires as probes. The nanowires are n-doped and the morphology of the nanowires has been tailored for scanning probe microscopy by growing them with a sharp tip for measurements and high thickness for robustness. The individual GaN nanowires were removed from their growth substrate and attached onto commercial atomic force microscopy cantilevers or etched tungsten wires for scanning tunneling microscopy. A standard scanning electron microscope equipped with a nanoprobe, a focused ion beam column and a gas injection system was used to locate, transfer, and attach the nanowires. We evaluated the properties of the GaN probes on different substrates including HOPG, Au, SiO2, InAs, and GaAs. We demonstrate both atomic force microscopy and scanning tunneling microscopy measurements with single atomic layer resolution and evaluate the robustness of the tips by monitoring them before and after scanning. Finally, we explore the use of the tips for scanning tunneling spectroscopy demonstrating that reliable results, which can reveal information on the electronic properties of the surface-tip system, are obtainable. The fundamental properties of these probes, which are demonstrated in this work, show promise for future use of the probes in exploring semiconductor-semiconductor tunneling junctions at the nanoscale as well as for other scanning probe techniques where high resolution is required.

High-Rate Spinel LiMn2O4 (LMO) Following Carbonate Removal and Formation of Li-Rich Interface by ALD Treatment

Recent work has identified enhanced charge-storage capacity in the spinel lithium manganese oxide (LiMn2O4; LMO) lithium-ion battery cathode upon a single atomic layer deposition (ALD) cycle compos...

Fabrication of Graphene Oxide/Calcium Carbonate/Chitosan Nanocomposite Film with Enhanced Mechanical Properties

Nanocomposites are an attractive field of materials providing novel performance owing to their remarkable properties at low filler loading. Currently, they are being increasingly used by industry whilst displacing the use of conventional filler materials. A lot of nanoscale fillers are capable to enhance mechanical and thermal properties of nanocomposites. Graphene, a single layer of carbon atoms in a hexagonal lattice, has recently attracted much attention due to their novel electronic and mechanical properties. Graphene and graphene oxide show very high mechanical properties with well biocompatibility, and they have potential application as biomaterials. Therefore, 2D graphene or graphene oxide sheets are expected to offer promising nanoscale filler for the next generation of nanocomposite materials. Here, Graphene Oxide/Calcium Carbonate/Chitosan (GO/CaCO3/CS) Nanocomposite Film has been fabricated by the blending aqueous solution. The graphene oxide used in this study was synthesized by chemical way from pure graphite using modified hummer’s technique. The graphene oxide resulted from hummer’s method was characterized by IR, XRD and SEMEDX. The measurement result of the mechanical properties of the nanocomposite film showed that with adding GO content in the composite enhanced the tensile strength and peaks at the content of GO 5 % due to good dispersion of the graphene oxide as nano-scale fillers.

Surface Reorganization and Rate Enhancement of Spinel LiMn2O4 (LMO) on First Trimethylaluminum/Water Exposure

Recent work has identified enhanced charge storage capacity in the spinel lithium manganese oxide (LiMn2O4;LMO) lithium ion battery cathode upon a single atomic layer deposition (ALD) cycle comprised of one chemical exposure of trimethylaluminum (TMA) and one exposure of water (H2O). Here, we report further study of the rate capability following one TMA/H2O exposure and identify enhanced rate capability versus pristine LMO. To understand this effect, we experimentally probe the surface composition of LMO with TMA/H2O treatment using X-ray photoelectron spectroscopy (XPS) measurements with inert-transfer. This includes a study of the LMO surface properties following TMA exposure before exposure to H2O. We identify the removal of a surface carbonate layer from LMO upon TMA exposure, and the formation of a Li-rich aluminum oxide surface layer upon subsequent H2O exposure. We also observe a previously undescribed phenomenon of Li ions depleting from the LMO surface upon TMA exposure and returning upon H2O exposure. Ab initio materials modeling is used to complement our measurements and provides fundamental understanding for the effects we observe. The chemical effects we observe are connected with the enhanced rate capability of ALD-coated LMO and are related to a range of emerging studies on carbonate surface layers in battery cathodes, as well as the use of ALD to stabilize battery interfaces. Figure 1

Laminar Graphene Oxide Membranes Towards Selective Ionic and Molecular Separations: Challenges and Progress.

Resolution of resources and environmental crises requires an efficient separation technologies, consequently, scientists and engineers are working vigorously for ideal separation materials. Laminar graphene oxide (GO) is a two-dimensional (2D) material offers considerable interest in this field due to its single atomic layer thickness, good stability, chemical inertness, and variety of functional groups. Recently, GO have emerged as a novel membrane material for molecular and ionic separation of gases, solvent, water, and desalination applications. This tutorial review aims to discuss the various approaches used to control the stacking of GO-based membrane with emphasis of advantages and drawbacks associated with each approach. Further, attention will also be given to describe the recent progress in GO based membranes for ionic and molecular separations. Meanwhile, challenges and opportunities will also be discussed in detail. We hope this review expected to provide a compact source of information that will be of great interest to chemists, material scientists, physicists, and engineers working or planning to work in GO based membranes for separation applications.