Double Atomic Layer Research Articles

(Invited) Catalytic Growth of Width-Controlled Single Atomic Layer MoS2 Sub-10nm Nanoribbons

For the next generation materials for optoelectronics, quantum computing, communication and sensing, single atomic layer semiconducting nanoribbons offer additional degree of freedoms for engineering their properties. However, direct width-controlling synthesis of TMD nanoribbons, especially for the appealing width range below 30 nm, remains challenge. Therefore, developing facile methods for their direct and controllable synthesis is of central importance. We present a new growth method for single and double atomic layer of MoS2 nanoribbons with width down to sub-10 nm. Nanoribbons are grown via precipitation from the Ni-contained composite nanoparticles via vapor-liquid-solid mechanism in which the nanoparticle diameter also controls the width of nanoribbon. We also present preliminarily studies on width-dependent properties of the nanoribbons, which evidence their potential application in the quantum electronics, particularly as a single electron transistor and single photon emitter.

Direct methane activation by atomically thin platinum nanolayers on two-dimensional metal carbides

Efficient and direct conversion of methane to value-added products has been a long-term challenge in shale gas applications. Here, we show that atomically thin nanolayers of Pt with a single or double atomic layer thickness, supported on a two-dimensional molybdenum titanium carbide (MXene), catalyse non-oxidative coupling of methane to ethane/ethylene (C2). Kinetic and theoretical studies, combined with in-situ spectroscopic and microscopic characterizations, demonstrate that Pt nanolayers anchored at the hexagonal close-packed sites of the MXene support can activate the first C–H bond of methane to form methyl radicals that favour desorption over further dehydrogenation and thus suppress coke deposition. At 750 °C and 7% methane conversion, the catalyst runs for 72 hours of continuous operation without deactivation and exhibits >98% selectivity towards C2 products, with a turnover frequency of 0.2–0.6 s−1. Our findings provide insights into the design of highly active and stable catalysts for methane activation and create a platform for developing atomically thin supported metal catalysts. The challenge in non-oxidative coupling of methane lies in the activation of the first C–H bond while avoiding further dehydrogenations, which lead to the formation of coke. Here, atomically thin platinum nanolayers on two-dimensional molybdenum titanium carbides are reported as a superior catalyst for this reaction owing to reduced coke formation.

Electrical and Optical Characteristics of Two-Dimensional MoS₂ Film Grown by Metal-Organic Chemical Vapor Deposition.

Atomically thin molybdenum disulfide (MoS₂) films were synthesized on a SiO₂/Si substrate by metal-organic chemical vapor deposition (MOCVD). Raman spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy studies reveal the double-atomic-layer structure and the surface element composition of the MOCVD-grown MoS₂ films. The photoluminescence measurement demonstrates a strong emission peak with a bandgap of 685.1 nm, attributed to highly efficient radiative transition at the double atomic layer. The contact resistance between the doubleatomic-layer MoS₂ film and metal electrode was measured using the transmission-line modeling method. A Ti/Au electrode forms an ohmic contact with the double-atomic-layer MOCVD-grown MoS₂ film, exhibiting a resistivity of 100 kΩ. The field-effect transistor based on the double-atomiclayer MoS₂ film exhibits an electron mobility of 1.3×10-4 cm²/V·s and an on/off ratio of 6.5×10² at room temperature.

Self-Limited and Forming-Free CBRAM Device With Double Al2O3 ALD Layers

Self-limited and forming-free Cu-based CBRAM devices with a double Al2O3 atomic layer deposition layer (D-ALD) structure were developed. The proposed structure offers desirable properties such as forming free ( ${V}_{\textsf {forming}}={V}_{\textsf {SET}}$ ), self-limited resistive switching with very low programming current ~10 nA), high ON/ OFF ratio (> 100), and nonlinear ${I}$ – ${V}$ (NL $_{\textsf {read}}$ and NL $_{\textsf {SET}}$ ~ 10) at low resistance state. The outstanding performance of D-ALD devices was due to the HT-ALD layer that operates as both a diffusion barrier preventing over injection of Cu ions and a tunneling barrier enabling self-limited programming.

Double-atomic layer of Tl on Si(111): Atomic arrangement and electronic properties

Metastable double-atomic layer of Tl on Si(111) has recently been found to display interesting electric properties, namely superconductivity below 0.96 K and magnetic-field-induced transition into an insulating phase intermediated by a quantum metal state. In the present work, using a set of experimental techniques, including low-energy electron diffraction, scanning tunneling microscopy, angle-resolved photoelectron spectroscopy, in a combination with density-functional-theory calculations, we have characterized atomic and electronic properties of the Tl double layer on Si(111). The double Tl layer has been concluded to contain ∼ 2.4 monolayer of Tl. A top Tl layer has a ‘1 × 1’ basic structure and displays 6 × 6 moiré pattern which originates from various residence sites of Tl atoms. Upon cooling below ∼ 140 K, the 6 × 6 moiré pattern changes to that having a 63×63 periodicity. However, the experimentally determined electron band dispersions show a 1 × 1 periodicity. The calculated band structure unfolded into the 1 × 1 surface Brillouin zone reproduces well the main features of the photoelectron spectra.

Superconductivity in thallium double atomic layer and transition into an insulating phase intermediated by a quantum metal state

We report on the first observation of superconductivity in a double atomic layer of Tl on Si(1 1 1) using in situ electrical resistivity measurements in ultrahigh vacuum. The structure of the Tl bilayer was characterized by a set of techniques, including scanning tunneling microscopy, electron diffraction and photoemission spectroscopy, which confirmed the metastability and metallic nature of the Tl bilayer. The epitaxial growth of atomically thin ‘soft’ metallic film over the entire surface of substrate enabled us to find a macroscopic superconducting transition at 0.96 K, accompanied by thermal and quantum fluctuations of order parameter. The system also demonstrates a perpendicular-magnetic-field-induced superconductor-insulator transition, together with an intermediate metallic state. We have found that the magnetoresitivity at the lowest temperature is consistent with the Bose metal picture, which is a consequence of strong quantum fluctuations.

Structural and physical properties of a novel misfit-layered cobalt oxide (CaOH)1.14CoO2

We have studied crystal structure and physical properties of a new layered cobalt oxide (CaOH)1.14CoO2. The compound is a composite crystal which consists of two interpenetrating subsystems of the CdI2-type CoO2 layer and the rock-salt-type double CaOH atomic layer. The two subsystems have incommensurate periodicity along the a-axis, resulting in modulated crystal structure due to the inter-subsystem interaction. We found that the observed physical properties originate in the electronic state with a “low-carrier-density limit” in the cobalt t2g band.

Dynamics and metastable surface structure of double atomic layer of water molecules and ions at the interface between KBr(c) and water

Abstract The metastable surface structure and dynamics of water molecules, cations, and anions at the interface between KBr(001) and water have been demonstrated from the images in situ observed in atomic resolution using atomic force microscopy. The vertical motion of potassium ions, which means their own transfer from the equilibrium sites to the upper height right on the underlying bromide ions, has been observed at the interface. They are used to be located in some steady state stabilized by their interaction with water molecules in the double atomic layer at the interface. The observed water molecules bridge two bromide ions by hydrogen bond; the water molecules are sandwiched by the potassium ions and vice versa.

Schottky barrier at the Al/Si(111) doped and double-doped interfaces: a local-density cluster study

Using the local-density approximation method we have investigated a cluster model of the Al/Si(111) interface. P atoms were placed in the fifth silicon double-atomic layer (DAL) counting from the interface. The second DAL was doped with either Ga or As. The Schottky barrier height was found to be 0.75 eV at the Al/Si(intrinsic) system and 0.73 eV at the Al/Si(P-doped) case. The additional doping of the near-interface layer by Ga increased the barrier to 0.90 eV while the As doping decreased it to 0.5 eV.

Influence of P, As and Ga doping on electronic structure of [formula omitted] interface

Using the cluster version of the local-density approximation method, we have studied the dopant's influence on the electronic structure of the Al Si (111) interface. P atoms were placed in the fifth silicon double-atomic layer (DAL) counting from the interface. The second DAL was doped with either Ga or As. The Schottky barrier height was found to be 0.75 eV in the Al Si (pure) cluster system and 0.73 eV in the Al Si (P-doped) case. The additional doping of the near-interface layer by Ga increased the barrier to 0.90 eV, while the As doping decreased it to 0.5 eV.

GaAs heteroepitaxy on an epitaxial Si surface with a low-temperature process

This letter reports the growth of an anti-phase-free GaAs layer on a (100) epitaxial Si substrate offset by 0.5° without high-temperature treatment prior to growth. Atomic force microscopy shows that the epitaxial Si surface has regular steps prior to growth. The average terrace width is 25.6 nm, and the step height is calculated to be 0.22 nm from the terrace width and offset angle, which approximately corresponds to a double atomic layer height of (100) Si of 0.27 nm. This single domain structure of the epitaxial Si surface gives the heteroepitaxial GaAs a high crystalline quality. Secondary ion mass spectrometry profiles show an abrupt GaAs-Si interface. Laser diodes on epitaxial Si substrates show room-temperature cw operation which confirms device applicability.

HREM studies of surface chemistry of superconducting YBa2Cu3O7- delta

The authors have investigated the surface chemistry of superconducting YBa2Cu3O7- delta using high-resolution electron microscopy. Specimens crushed in air showed a thick 'amorphous' coating, which would not recrystallize into the bulk structure under electron beam annealing. Instead oxide microcrystals formed within the coating. The amorphous coating layers were reduced to less than half of their original thickness when the specimens were crushed in argon, and were found to recrystallize into the YBa2Cu3O7- delta structure under the same conditions, with atomically clean surfaces produced and observed directly in the profile images. It was also found that the (001) surface terminated with the Cu-O chain. Surface reconstruction with a double atomic layer of Cu covering the (113) and (112) surfaces was confirmed by computer simulation of the profile images. Possible mechanisms of formation of the coating and clean surfaces are discussed.

Scanning tunneling microscopy/scanning tunneling spectroscopy observation of step structures of Si (001) and (111) surfaces

The step-edge structures of the Si (001) and (111) surfaces roughened by heating at 1300 °C were studied utilizing the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The bunches of 7–8 double layer steps and the 200–300 Å wide terraces were observed on the Si (111) surface. An unexpected finding is the interminglement of single and double atomic layer steps on the vicinal Si (001) surfaces tilted to any orientation. The double layer steps split into two single layer steps at the kink sites of the step edge. The dimer rows on the lower terraces shift in the direction of the rotation of the vicinal surface around the normal axis of the surface. The tunneling spectra at the single step exhibits several peaks at the sample bias voltages 0.6, −0.6, −1.4, −1.7, −2.1, and −2.4 V. The peaks at −1.4 and −1.7 V are found to be particularly sharp and high at the step edge.