Monolayer Of Atoms Research Articles

A silicene-based plasmonic electro-optical switch in THz range

The first design and simulation of a silicene-based optical switch in the frequency range of 20 to 50 THz are presented. Silicene is a monolayer of silicon atoms stacked together in the shape of a honeycomb that has outstanding features such as compatibility with current technology, strong spin-orbital coupling, and attractive optical properties. The proposed switch consists of a silicene waveguide encapsulated between layers of which transmits the input signal by emitting surface plasmon polaritons. To control the ON and OFF switching modes using impurities and DC voltage, we change the chemical potential of silicene. The main advantages of the proposed switch over similar graphene types are fewer losses, broadband frequency range, more compatibility with current technology, and also due to the encapsulation of silicene between the waveguide is isolated from the environment. Also, Q-factor, insertion loss, and extinction ratio for the proposed switch are 189, 0.39 dB, and 51 dB, respectively.

Electronic structures and magnetic properties of two-dimensional honeycomb-kagome structured V[formula omitted]O[formula omitted] monolayer and V[formula omitted]O[formula omitted]/M(0001) (M=Zr and Hf) system

Transition metal oxides exhibit a variety of atomic structures with associated exotic electronic and magnetic properties, being expected to act an important role in two-dimensional spintronics applications. Based on density functional theory, the electronic structures and magnetic properties of V2O3 monolayer with a two-dimensional honeycomb-kagome configuration were systemically studied by first-principles calculations. The atomic thin V2O3 monolayer was found to be an intrinsic half-metallic ferromagnetism with a Curie temperature up to 450 K evaluated by Monte Carlo simulations, exhibiting an excellent mechanical flexibility and an exceptional dynamical stability. Dominated by exchange interactions between the nearest neighboring V ions in a high spin state of d2↑, some Dirac cones and flat bands emerge around the Fermi level, which were demonstrated by a tight-binding approach in further. The Dirac electrons exhibit a high Fermi velocity about 3.39×105 m/s and a rather large gap about 23.5meV opened with the account of spin–orbital coupling. A structure-polarization and a sizable band gap can be introduced into V2O3 monolayer by heavy bi-axial compressive strains. Then the electronic and magnetic properties of V2O3/M(0001) (M=Zr and Hf) interface system were investigated in a series of configurations. It was found that the V2O3 monolayer transforms into antiferromagnetic with a structure-polarization and a modified electronic structure caused by the strong interactions between the monolayer and substrates. Our investigations provide a theoretical guidance and a direct interest for the two-dimensional magnetic materials on metal substrates.

Magnetic properties of transition-metal atomic monolayer in nickel supercell: Density functional theory and Monte Carlo simulation

An extensive description of the magnetic properties of the 3d, 4d, and 5d transition-metal (TM) atomic monolayers sandwiched in pure nickel Ni/TM/Ni is presented using density functional theory (DFT) and Monte Carlo simulation (MC). The supercell method is used to model the Ni/TM/Ni structure where the unit cell is repeated along the z-axis. The magnetic properties of nickel can be modified by adding different TM monolayers. The perpendicular magnetocrystalline anisotropy energy (EMCA) has the greatest value for Ni/Pt/Ni sandwich followed by Ni/W/Ni, while the highest in-plane EMCA belongs to Ni/Ir/Ni and Ni/La/Ni sandwiches. The effect of substitution the TM monolayer on the exchange interactions extends up to the third layer from the TM monolayer. The resulted Curie temperature (Tc) relative to pure Ni is increased for some of the Ni/3d/Ni sandwiches and decreased for all Ni/4d/Ni and Ni/5d/Ni sandwiches.

Gold Interlayer Promotes Superconductivity in Single and Double Atomic Pb Layers on Si(100).

Introducing an atomic Au monolayer between a Pb film and a Si(100) substrate allows us to fabricate Pb films with single- and double-atom thicknesses. The Pb films have a 2D square-lattice structure with the 1D atomic chains of Pb adatoms on their top, forming Si(100)1 × 7-(Pb, Au) and Si(100)5 × 1-(Pb, Au) superstructures for single and double atomic Pb layers, respectively. Their common characteristic feature is the occurrence of bundles of quasi-1D metallic bands. Transport measurements showed that samples with a Au interlayer demonstrate enhanced superconductor properties, as compared to Pb layers grown on the bare Si(100) surface. Toward improved superconductor properties, the (Pb, Au)/Si(100) system successively avoids risks associated with possible intermixing between adsorbate layers and substrate, as well as with possible Peierls transition into an insulator state, typical for the 1D systems. This finding opens new ways to control low-dimensional superconductivity at the atomic-scale limit.

Controlling the electronic and magnetic properties in epitaxial LaMnO3/LaScO3 superlattices

Perovskite superlattices (SLs) have attracted considerable interest owing to their rich and diverse physical properties. In this study, epitaxial LaMnO3/LaScO3 SLs were grown using pulsed laser deposition (PLD) by controlling the number of atomic monolayers. Raman spectroscopy combined with x-ray diffraction reciprocal space maps confirmed that the introduction of LaScO3 suppressed Jahn–Teller distortion in the SLs. Electrical transport measurements revealed a thermally activated single-gap behavior which mainly depended on the thickness of LaMnO3 layers. Magnetic measurements indicated that the magnetic properties of the SLs were related to the proportions of LaMnO3 and LaScO3 layers. These results are beneficial for the further understanding of the electronic and magnetic properties of LaMnO3-based SLs.

High-Performance Pockels Effect Modulation and Switching in Silicon-Based GaP/Si, AlP/Si, ZnS/Si, AlN/3C-SiC, GaAs/Ge, ZnSe/GaAs, and ZnSe/Ge Superlattice-On-Insulator Integrated Circuits.

We propose new a Si-based waveguided Superlattice-on-Insulator (SLOI) platforms for high-performance electro-optical (EO) 2 × 2 and N × M switching and 1 × 1 modulation, including broad spectrum and resonant. We present a theoretical investigation based on the tight-binding Hamiltonian of the Pockels EO effect in the lattice-matched undoped , , , , , , and wafer-scale short-period superlattices that are etched into waveguided networks of small-footprint Mach-Zehnder interferometers and micro-ring resonators to yield opto-electronic chips. The spectra of the Pockels r33 coefficient have been simulated as a function of the number of the atomic monolayers for “non-relaxed” heterointerfaces. The large obtained r33 values enable the SLOI circuit platforms to offer a very favorable combination of monolithic construction, cost-effective manufacturability, high modulation/switching speed, high information bandwidth, tiny footprint, low energy per bit, low switching voltage, near-IR-and-telecom wavelength coverage, and push-pull operation. By optimizing waveguide, clad, and electrode dimensions, we obtained very desirable values of the VπL performance metric, in the range of 0.062 to 0.275 V·cm, portending a bright future for a variety of applications, such as sensor networks or Internet of Things (IoT).

Large-gap quantum anomalous Hall states induced by functionalizing buckled Bi-III monolayer/ Al2O3

Chiral edge modes inherent to the topological quantum anomalous Hall (QAH) effect are a pivotal topic of contemporary condensed matter research aiming at future quantum technology and application in spintronics. A large topological gap is vital to protecting against thermal fluctuations and thus enabling a higher operating temperature. From first-principle calculations, we propose Al$_{2}$O$_{3}$ as an ideal substrate for atomic monolayers consisting of Bi and group-III elements, in which a large-gap quantum spin Hall effect can be realized. Additional half-passivation with nitrogen then suggests a topological phase transition to a large-gap QAH insulator. By effective tight-binding modelling, we demonstrate that Bi-III monolayer/Al$_{2}$O$_{3}$ is dominated by $p_{x}, p_{y}$ orbitals, with subdominant $p_z$ orbital contributions. The topological phase transition into the QAH is induced by Zeeman splitting, where the off-diagonal spin exchange does not play a significant role. The effective model analysis promises utility far beyond Bi-III monolayer/Al$_{2}$O$_{3}$, as it should generically apply to systems dominated by $p_{x}, p_{y}$ orbitals with a band inversion at $\Gamma$.

Adsorption of atomic and molecular monolayers on Pt-supported graphene

Adsorption energies of monolayers of noble gases (Ar, Kr, Xe) and small molecules (N2, O2, CO, CH4, C2H6, and C3H8) on graphene supported by Pt(111) surface were computed by DFT, corrected for temperature and zero-point effects, and, compared to the recent temperature-programmed desorption experiments (Smith et al., 2016). The results indicate that some of the considered DFT exchange–correlation functionals reasonably well describe molecule–graphene/Pt(111) noncovalent interactions. For selected models, fixed-node diffusion Monte Carlo computations were used for ultimate cross-validation of interaction energies between the molecule and free-standing graphene.

Perpendicularly magnetized epitaxial Co/Ni multilayers grown on Ru(0001) layers by alternate monoatomic layer deposition

The hcp stacked Bh-ordered CoNi film was a candidate material to simultaneously have large uniaxial magnetic anisotropy energy (Ku) and a low Gilbert damping constant, which were previously predicted by a first-principles calculation. In this paper, we investigated the film growth condition to form the hcp stacked CoNi layers. [Co(1 ML)/Ni(1 ML)]N multilayers were epitaxially grown on Al2O3(11-20)/Ru(0001) underlayers by molecular beam epitaxy using an alternate monoatomic layer deposition technique with changing growth temperature (Ts) and repetition number (N). As the results of structural characterizations by x-ray diffraction measurements, it was found that the fcc- and hcp-CoNi phases were mixed in the samples, and the volume fraction of the fcc phase was larger than that of the hcp phase. However, the formation of the hcp-CoNi phase was promoted near the interface with the Ru layer having the hcp structure. The low growth temperature and the small N increased the volume fraction of the hcp phase. The Ku value of 5.1 Merg/cm3 was achieved for the [Co(1 ML)/Ni(1 ML)]20 multilayer grown at room temperature. In addition, a perpendicularly magnetized film was realized even for the thin sample with N = 5 (2 nm). This means that the Co/Ni multilayers are a thin ferromagnetic material suitable for application to spintronic devices.

First-principles calculations of Schottky barrier height at barium titanate/metal interface

The Schottky barrier (SB) height at insulator/metal interfaces is important for a wide variety of electronic devices. We performed first-principles analysis of the SB formed between BaTiO3 (BTO) and metals with a cubic crystal structure. We found that the barrier height strongly depends on the contact metal and the BTO termination surface. These results were quantitatively understood by analyzing the contributions of the atomic and charge rearrangement at the interfaces. It was also found that when a different metal is substituted for one of the monoatomic Ni layers at the BTO/Ni interface, the SB is significantly influenced by the nature of the substituting metal.

Graphene, phosphorene and silicene coatings on the (0001) surfaces of hcp metals: Structural stability and hydrophobicity

Single atomic layer or monolayer (ML) 2D materials have unique features that can be relevant for protection and functionalization of high-performance surfaces. Here we report a combined density functional theory and ab initio molecular dynamics study on the stability of graphene (Gr), phosphorene (Pp) and silicene (Sl) coatings at the most stable (0001) surface of the metals that have hcp structure at room temperature: Hf, Mg, Os, Re, Ru, Sc, Ti, Zn and Zr. We found that the three 2D materials are stable at the surfaces of many of these metals: Gr is stable at Os, Re, Ti, Zn and Zr; Pp is stable in the blue form at Hf, Mg, Ti and Zr; Sl is stable in the 2D zigzag structure at Hf, Mg and Ti and in the planar form at the Zr surface. For the remaining metals Gr, Pp and Sl do not form van der Waals bonded MLs. In those cases we found structures such as 2D arrangements of mixed hexagons and pentagons, trigonal pyramidal structures and 2 half-monolayers. Many of these structures are very stable coatings covalently bound to the surfaces with considerable adsorption energies that can reach − 2 eV per atom of the coating. Gr imparts hydrophobicity to the surfaces of all materials here studied and increases the distance between these and the first H2O layer by as much as 75 % for Gr coated Ti. These H2O layers have increased rigidity despite being located further apart from the surfaces. These effects could be explored for the protection or functionalization of high-performance systems such as metallic electrodes for example.

Transition volumes from multiply twinned particles to single crystals of supported Ag and Au nanoparticles

Shape changes of Ag and Au nanoparticles supported on single crystal reconstructed SrTiO3(001) and (111) substrates were investigated using scanning tunneling microscopy. Both metals nucleate as multiply twinned particles (MTPs) and transform into face-centered-cubic single crystals (SCs) beyond a critical volume. On SrTiO3(001)-c(4 × 2) the critical volumes are measured as 141 ± 51 nm3 for Ag and 107 ± 23 nm3 for Au, whereas on SrTiO3(111)–(4 × 4)+(6 × 6) the critical volumes are 53 ± 26 nm3 for Ag and 26 ± 40 nm3 for Au. A much larger transition volume was observed on SrTiO3(001)–(2 × 1), where Ag remains as MTPs up to 3400 nm3, while Au nucleates as atomic monolayers instead of MTPs. This work demonstrates the significant impact of small variations of the surface structure of the substrate on the MTP–SC transition volume.

Experimental Realization of Atomic Monolayer Si9 C15.

Monolayer Six Cy constitutes an important family of2Dmaterials that is predicted to feature a honeycomb structure and appreciable bandgaps. However, due to its binary chemical nature and the lack of bulk polymorphs with a layered structure, the fabrication of such materials has so far been challenging. Here, the synthesis of atomic monolayer Si9 C15 on Ru (0001) and Rh(111) substrates is reported. A combination of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and density functional theory (DFT) calculations is used to infer that the 2D lattice of Si9 C15 is a buckled honeycomb structure. Monolayer Si9 C15 shows semiconducting behavior with a bandgap of ≈1.9eV. Remarkably, the Si9 C15 lattice remains intact after exposure to ambient conditions, indicating good air stability. The present work expands the 2D-materials library and provides a promising platform for future studies in nanoelectronics and nanophotonics.

High‐Harmonic Generation Enhancement with Graphene Heterostructures

AbstractHigh‐harmonic generation (HHG) in graphene heterostructures, consisting of metallic nanoribbons separated from a graphene sheet by either a few‐nanometer layer of aluminum oxide or an atomic monolayer of hexagonal boron nitride, is investigated. The nanoribbons amplify the near‐field at the graphene layer relative to the externally applied pumping, thus allowing the observation of third‐ and fifth‐harmonic generation in the carbon monolayer at modest pump powers in the mid‐infrared. The dependence of the nonlinear signals on the ribbon width and spacer thickness, as well as pump power and polarization are studied, and enhancement factors (EF) relative to bare graphene reaching 1600 and 4100 for third‐ and fifth‐harmonic generation, respectively, are demonstrated. The work supports the use of graphene heterostructures to selectively enhance specific nonlinear processes of interest, an essential capability for the design of nanoscale nonlinear devices.

Sound Velocities in Graphene-Based Epoxy Nanocomposites

Elastic properties of epoxy-based nanocomposites (ENCs) filled with bare and TiO2-deposied multi-layered graphene nanoplatelets (MLG) have been tested by using a phase-frequency continuous-wave ultrasound probing (USP). The dian epoxy CHS-EPOXY 520 curried with diethylenetriamine (DETA) was the polymer matrix for the nanocomposites. The nanoplatelets of the specific surface area Sf ~ 790 m2/g consist of several dozen loosely bound monoatomic graphene layers with an area of about least 5×5 μm2. MLG-mass-loading (φf,m) of the nanocomposites varied from 0.1 % to 5.0 % by weight. Anatase- TiO2 particles, being of about 50 nm in diameter and of Sf ~ 1500 m2/g, have been deposited on MLG in mass concentration of about 1 %. Elastic moduli of the ENCs (namely, the Lame’s constants, the Young’s module, the compression module, and the Poisson’s ratio) have demonstrated negligible variation with φf,m varying regardless the type of filling particles. However, MLG:TiO2-hybrid nanoparticles have proven to impact stronger on the moduli as compared to bare MLG. This result shows a capability to modify molecular structure of epoxy resins by controlling surface reactivity of MLG embedded in the resin.

Light Isotope Separation through the Compound Membrane of Graphdiyne

The separation of isotopes of one substance is possible within the framework of the quantum mechanical model. The tunneling effect allows atoms and molecules to overcome the potential barrier with a nonzero probability. The membranes of two monoatomic layers enhance the differences in the components’ passage through the membrane, thereby providing a high separation degree of mixtures. The probability of overcoming the potential barrier by particles is found from the solving of the Schrödinger integral equation. Hermite polynomials are used to expand all the terms of the Schrödinger integral equation in a series to get a wave function. A two-layer graphdiyne membrane is used to separate the mixture.

Classical and quantum photonic sources based upon a nonlinear GaP/Si-superlattice micro-ring resonator

We present a theoretical investigation, based on the tight-binding Hamiltonian, of efficient second- and third-order nonlinear optical processes in the lattice-matched undoped (GaP)N/(Si2)M short-period superlattice that is waveguide-integrated in a microring resonator on an opto-electronic chip. The nonlinear superlattice structures are situated on the optically pumped input area of a heterogeneous “XOI” chip based on silicon. The spectra of χzzz(2)(2ω,ω,ω), χxzx(2)(2ω,ω,ω), χxxxx(3)(3ω,ω,ω,ω) and the Kerr refractive index (n2), have been simulated as a function of the number of the atomic monolayers for “non-relaxed” heterointerfaces; These nonlinearities are induced by transitions between valence and conduction bands. The large obtained values make the (GaP)N/(Si2)M short-period superlattice a good candidate for future high-performance XOI photonic integrated chips that may include Si3N4 or SiC or AlGaAs or Si. Near or at the 810-nm and 1550-nm wavelengths, we have made detailed calculations of the efficiency of second- and third-harmonic generation as well as the performances of entangled photon-pair quantum sources that are based upon spontaneous parametric down conversion and spontaneous four-wave mixing. The results indicate that the (GaP)N/(Si2)M short-period superlattice is competitive with present technologies and is practical for classical and quantum applications.

Surface oxygen versus native oxide on tungsten: contrasting effects on deuterium retention and release

We performed a direct comparison of deuterium retention and release from tungsten in presence or in absence of oxygen impurities. A single crystal of W(110) was used to prepare tungsten with four different surface states: with its native oxide, atomically clean, covered with half a monolayer of oxygen atoms, and covered with three fourths of a monolayer of oxygen atoms. For a D ion fluence of 3 × 1021 D+ m−2 implanted at 300 K, deuterium retention was highest with the native oxide, lowest with three fourths of a monolayer of oxygen atoms at the surface and intermediate for the clean surface. This counterintuitive result is explained by a different localization of deuterium retention in these samples. For tungsten with its native oxide, deuterium retention occurs solely in the bulk, i.e. below the first atomic plane of the surface. For clean tungsten, deuterium retention occurs in part at the surface and sputtering should play a role. For tungsten with a sub-monolayer surface coverage of oxygen atoms, a transition from surface to bulk retention is observed above half a monolayer of adsorbed oxygen. Striking differences in desorption peak(s) temperature(s) are observed between D ion-implanted samples and D2 molecules-exposed samples. These results highlight the importance of the (near-) surface localization of oxygen and deuterium on the temperature dependence of deuterium desorption rate i.e. on the fusion fuel recycling coefficient.

Recent advances in graphene and other 2D materials

The isolation of the first two-dimensional material, graphene – a monolayer of carbon atoms arranged in a hexagonal lattice - opened new exciting opportunities in the field of condensed matter physics and materials. Its isolation and subsequent studies demonstrated that it was possible to obtain sheets of atomically thin crystals and that these were stable, and they also began to show its outstanding properties, thus opening the door to a whole new family of materials, known as two-dimensional materials or 2D materials. The great interest in different 2D materials is motivated by the variety of properties they show, being candidates for numerous applications. Additionally, the combination of 2D crystals allows the assembly of composite, on-demand materials, known as van der Waals heterostructures, which take advantage of the properties of those materials to create functionalities that otherwise would not be accessible. For example, the combination of 2D materials, which can be done with high precision, is opening up opportunities for the study of new challenges in fundamental physics and novel applications. Here we review the latest fundamental discoveries in the area of 2D materials and offer a perspective on the future of the field.

Monolayer Graphitic Carbon Nitride as Metal-Free Catalyst with Enhanced Performance in Photo- and Electro-Catalysis

HighlightsThe g-C3N4 monolayer in the perfect 2D limit was successfully realized, for the first time, by the well-defined chemical strategy based on the bottom-up process.The most striking evidence was made from Cs–high resolution transmission electron microscopy measurements by observing directly the atomic structure of g-C3N4 unit cell, which was again supported by the corresponding high resolution transmission electron microscopy image simulation results.We demonstrated that the newly prepared g-C3N4 monolayer showed outstanding photocatalytic activity for H2O2 generation as well as excellent electrocatalytic activity for oxygen reduction reaction.The exfoliation of bulk graphitic carbon nitride (g-C3N4) into monolayer has been intensively studied to induce maximum surface area for fundamental studies, but ended in failure to realize chemically and physically well-defined monolayer of g-C3N4 mostly due to the difficulty in reducing the layer thickness down to an atomic level. It has, therefore, remained as a challenging issue in two-dimensional (2D) chemistry and physics communities. In this study, an “atomic monolayer of g-C3N4 with perfect two-dimensional limit” was successfully prepared by the chemically well-defined two-step routes. The atomically resolved monolayer of g-C3N4 was also confirmed by spectroscopic and microscopic analyses. In addition, the experimental Cs-HRTEM image was collected, for the first time, which was in excellent agreement with the theoretically simulated; the evidence of monolayer of g-C3N4 in the perfect 2D limit becomes now clear from the HRTEM image of orderly hexagonal symmetry with a cavity formed by encirclement of three adjacent heptazine units. Compared to bulk g-C3N4, the present g-C3N4 monolayer showed significantly higher photocatalytic generation of H2O2 and H2, and electrocatalytic oxygen reduction reaction. In addition, its photocatalytic efficiency for H2O2 production was found to be the best for any known g-C3N4 nanomaterials, underscoring the remarkable advantage of monolayer formation in optimizing the catalyst performance of g-C3N4.