Oxide Monolayer Research Articles

Harnessing room-temperature ferroelectricity in metal oxide monolayers for advanced logic devices

Harnessing room-temperature ferroelectricity in metal oxide monolayers for advanced logic devices

Growth and Structure of Ultrathin Iron Silicate and Iron Germanate Films.

The growth and structure of two-dimensional iron silicate and iron germanate films on Ru(0001) are studied. We investigate in detail the temperature-dependent film formation of ultrathin layers of iron silicate and iron germanate. These two-dimensional films can be seen as model systems for more complex catalytically active structures, such as zeolites, which can be used as selective catalysts or molecular sieves. The experimental methods of XPS, LEED, LEEM, LEEM-IV, and XPEEM are applied for correlated chemical and physical characterization in situ and in real time, and DFT is applied for theoretical consideration. We show that both systems can be considered as two-layered systems, with a monolayer of iron oxide at the Ru interface and a monolayer of silica or germania on top, respectively. The Fe-Fe distance in the iron oxide layer is influenced by the Si-O-Si or Ge-O-Ge bond length, in agreement with those of unstrained silicates or germanates. Moreover, iron silicate can be prepared using different preparation methods. The actual loading of Fe atoms is three per unit cell for FeGeOx and only two for FeSiOx.

Design of an Oxide Monolayer with High ZT by a Strong Anharmonicity Unit.

In this paper, a new strategy to obtain a transition-metal oxide (TMO) thermoelectric monolayer is demonstrated. We show that the TMO thermoelectric monolayer can be achieved by the replacement of a transition-metal atom with a cluster, which is composed of heavy transition atoms with abundant valence electrons. Specifically, the transition-metal atom in the XO2 (X = Ti, Zr, Hf) monolayer is replaced by the [Ag6]4+ cluster and a stable structure Ag6O2 is achieved. Due to the abundant valence electrons in the [Ag6]4+ cluster unit, n-type Ag6O2 has high electrical conductivity, which leads to a satisfactory power factor. More importantly, Ag6O2 has an extremely low phonon thermal conductivity of 0.16 W·m-1·K-1, which is one of the lowest values in thermoelectric materials. An in-depth study reveals that the extremely low value originates from the strong phonon anharmonicity and weak metal bond of the [Ag6]4+ cluster unit. Due to the satisfactory power factor and ultralow phonon thermal conductivity, Ag6O2 has high ZT at 300-700 K, and the maximum ZT is 3.77, corresponding to an energy conversion efficiency of 22.24%. Our results demonstrate that replacement of the transition-metal atom by an appropriate cluster is a good way to obtain a TMO thermoelectric monolayer.

Green and solid state reduction of GO monolayers sandwiched between Arachidic acid LB layers

A novel, single step and environment friendly solid state approach for reduction of graphene oxide (GO) monolayers has been demonstrated, in which, arachidic acid/GO/arachidic acid (AA/GO/AA) sandwich structure obtained by Langmuir-Blodgett (LB) technique was heat treated at moderate temperatures to obtain RGO sheets. Heat treatment of AA/GO/AA sandwich structure at 200 °C results in substantial reduction of GO, with concurrent removal of AA molecules. Such developed RGO sheets possess sp2-C content of ∼69 %, O/C ratio of ∼0.17 and significantly reduced I(D)/I(G) ratio of ∼1.1. Ultraviolet photoelectron spectroscopy (UPS) studies on RGO sheets evidenced significant increase in density of states in immediate vicinity of Fermi level and decrease in work function after reduction. Bottom gated field effect transistors fabricated with isolated RGO sheets displayed charge neutrality point at a positive gate voltage, indicating p-type nature, consistent with UPS and electrostatic force microscopy (EFM) measurement results. The RGO sheets obtained by heat treatment of AA/GO/AA sandwich structure exhibited conductivity in the range of 2–7 S/cm and field effect mobility of 0.03–2 cm2/Vs, which are comparable with values reported for RGO sheets obtained by various chemical/thermal reduction procedures. The extent of GO reduction is determined primarily by proximity of AA molecules and found to be unaltered with either escalation of heat treatment temperature or increase of AA content in sandwich structure. The single-step GO reduction approach demonstrated in this work is an effective way for development of RGO monolayers with high structural quality towards graphene-based electronic device applications.

Immersion ellipsometry for the uncorrelated determination of ultrathin film thickness and index of refraction: Theory and examples

Immersion ellipsometry can break the well-known correlation between optical constants and thicknesses of ultrathin (<5–10 nm) films, allowing both to be determined. In immersion ellipsometry, ellipsometric data is acquired in air and liquid ambients, and the data sets are combined in the analysis. The contrast in index between the liquid and film adds information to the analysis that breaks the correlation between the film thickness and refractive index that exists for air-only measurements. We describe the theory and practice of immersion ellipsometry. We also discuss the use of multiwavelength immersion ellipsometry to measure the thicknesses and optical constants of two thin films: native oxide on silicon and an alkyl monolayer on that native oxide. The average thicknesses of the native oxide and chloro(dimethyl)octadecylsilane (CDMOS) monolayer were 1.526 ± 0.027 nm and 1.968 ± 0.057 nm, and their average indices of refraction at 633 nm were 1.519 ± 0.005 and 1.471 ± 0.004, respectively. The native oxide and CDMOS monolayer were also characterized with x-ray photoelectron spectroscopy (XPS) and contact angle goniometry. Both the XPS C 1 s peak and the water contact angle increased substantially after monolayer deposition. While immersion ellipsometry has been known for decades, its use has been limited, maybe due to a lack of awareness of the technique and/or the need to immerse the sample surface in a liquid that could be destructive if the sample is not compatible with the liquid. As ultrathin films become widely used in science technology, immersion ellipsometry should increase in importance.

Tailoring Work Functions of Heterostructures by Varying the Depth of a Buried Monolayer

AbstractThe work function is a critical parameter for device design and understanding of materials properties, but contemporary experimental literature on tailored work functions is rather scarce. This paper presents a promising new approach to work function design that uses pulsed laser deposition (PLD) growth with unit cell precision. It is shown that a monolayer buried within a material can influence the work function as a function of the layer depth. This method can preserve properties of the original material's surface, and furthermore, a sensitive material can be used as the monolayer, both of which are beneficial for device design. Proof of concept is demonstrated by inserting a nominal monolayer of barium oxide (BaO) into strontium titanate (SrTiO3) (001) at different depths, revealing a previously unknown work function trend. Density functional theory (DFT) calculations, surface characterization, and scanning transmission electron microscopy (STEM) studies verify the quality of the heterostructures and help to understand how charge transfer, slight barium diffusion, and termination effects account for the observed work function trends. This work function tunability is a promising approach for future devices that require a tailored work function, which can easily be extended to a wider variety of materials.

AgO and PdO monolayers on the Mo(112) surface: A DFT study

With performed DFT calculations it is shown that AgO as well as PdO monolayer is bound to the Mo(112) surface with Ag or Pd atoms lying in surface furrows and O atoms situated in sites on Mo surface rows. The intrinsic corrugation of the surface facilitates the placement of oxide monolayers on the surface by effectively compensating for the difference in the sizes of O and Ag or Pd atoms, resulting in the adsorbed AgO and PdO layers becoming almost flat, which is typical of free monolayers. The surface states associated with the oxide layers on Mo(112) are either significantly above or below the EF, which does not lead to any noticeable changes in the band structures and densities of states in the vicinity of the EF of these layered systems. Nevertheless, it can be expected that adsorbed oxide monolayers can exhibit useful catalytic properties due to the adsorption-modified structure of the layers.

Морфология и электрофизические свойства модифицированных ленгмюровских пленок оксида графена

The work devoted to study of the electrical properties and morphology of Langmuir-Blodgett films of modified graphene oxide (GO). The Langmuir-Blodgett method was used for studying of formation process of Langmuir monolayers formed on the subphase contained dissolved ions of tin chloride salt. The compression isotherms of monolayers formed in the presence of tin ions shifted towards larger occupied areas. Langmuir monolayers of graphene oxide formed under various conditions were transferred on solid substrates and studied by atomic force microscopy and voltammetry. It has been established that the presence of tin ions in the aqueous subphase leads to changes in the morphology of the formed films. Thus, the average roughness and surface area of such films increased from 0.2 nm and 0.04 μm2 to 5.4 nm and 0.46 μm2, respectively. The presence of tin ions dissolved in the aqueous subphase influenced the type of current-voltage characteristics of the films under study. The maximum reverse current increases from 075 nA to -0.3 nA and the resistance increases from 11 GOhm to 700 GOhm. Temperature annealing of the graphene oxide film at 300°C leads to a decrease in the maximum forward current and an increase in the reverse current from 0.0407 nA and -0.071 nA to 0.0065 nA and 0.0616 nA, respectively. For a film formed in the presence of ions in the subphase, annealing does not affect the maximum forward current, and the current of the reverse branch of the current-voltage characteristic increases from -0.1669 nA to 0.0468 nA.

Remarkable enhancement of the adsorption and diffusion performance of alkali ions in two-dimensional (2D) transition metal oxide monolayers via Ru-doping

Transition metal oxides (TMO) are the preferred materials for metal ion battery cathodes because of their high redox potentials and good metal-ion intercalation capacity, which serve as an outstanding replacement for layered sulphide. In this work, using first-principles calculations based on Density functional theory approach, we explored the structural and electronic properties which comprise of adsorption and diffusion behaviour along with the analysis of voltage profile and storage capacity of Ru doped two-dimensional transition metal oxide MnO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$MnO_{2}$$\\end{document}, CoO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$CoO_{2}$$\\end{document}, and NiO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$NiO_{2}$$\\end{document} monolayers. The adsorption of alkali ions (Li, Na) to the surface of TMOs is strengthened by Ru-atom doping. Ru doping enhanced the adsorption energy of Li/Na-ion by 25%/11% for MnO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$MnO_{2}$$\\end{document}, 8%/13% for CoO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$CoO_{2}$$\\end{document}, and 10%/11% NiO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$NiO_{2}$$\\end{document} respectively. The open circuit voltage (OCV) also increases due to the high adsorption capacity of doped Monolayers. Ru doping makes the semiconducting TMOs conduct, which is suitable for battery application. As alkali ion moves closer to the dopant site, the adsorption energy increases. When alkali ions are close to the vicinity of doping site, their diffusion barrier decrease and rises as they go further away. Our current findings will be useful in finding ways to improve the storage performance of 2D oxide materials for application in energy harvesting and green energy architecture.

Enhanced Ferromagnetism in Atomically Thin Oxides Achieved by Interfacial Reconstruction

AbstractDiscoveries of ferromagnetic materials with ultrathin thickness are of great importance for both fundamental science and technological applications. Transition metal oxides (TMOs) provide promising candidates in the context of next‐generation spintronics, despite the severe decay of ferromagnetism as the thickness reduces to the nanometer regime. Here, an efficient strategy to eliminate the magnetic dead layer in atomically thin oxides is presented, by using the epitaxial interface of 3d and 5d oxide monolayers that reconciles both strong exchange interaction and large uniaxial magnetic anisotropy. Combining multiple experimental methods, a ferromagnetic transition in an ultrathin oxide heterostructure comprised of only one La0.2Sr0.8MnO3 monolayer sandwiched by SrIrO3 monolayer (total thickness of three unit‐cells) is unambiguously demonstrated. Remarkably, a largely enhanced saturation magnetization (2 µB Mn−1) and Curie temperature (80 K) are observed for the single manganite monolayer, as compared to previously reported ferromagnetic monolayer oxides. The results demonstrate a general strategy for creating robust ferromagnetism in ultrathin TMOs, potentially enabling novel oxide spin‐orbitronic devices.

Large second-order susceptibility from a quantized indium tin oxide monolayer.

Due to their high optical transparency and electrical conductivity, indium tin oxide thin films are a promising material for photonic circuit design and applications. However, their weak optical nonlinearity has been a substantial barrier to nonlinear signal processing applications. In this study, we show that an atomically thin (~1.5 nm) indium tin oxide film in the form of an air/indium tin oxide/SiO2 quantum well exhibits a second-order susceptibility χ2 of ~1,800 pm V-1. First-principles calculations and quantum electrostatic modelling point to an electronic interband transition resonance in the asymmetric potential energy of the quantum well as the reason for this large χ2 value. As the χ2 value is more than 20 times higher than that of the traditional nonlinear LiNbO3 crystal, our indium tin oxide quantum well design can be an important step towards nonlinear photonic circuit applications.

Graphenylene oxide: A new two-dimensional material

We investigated the oxidation and sulfurization of the graphenylene monolayer. Although this structure is unstable when it is 100 % covered with oxygen, obtaining a stable form of oxidized graphenylene is possible for 50 % of coverage. In this structure, the oxygen atoms are added on alternating sides of the monolayer, adopting an epoxy-like structure. It is stable up to 1635 K and is a minimum on the potential surface energy. It is an insulator with an indirect band gap of 5.3 eV, a value much larger than the band gap of the pristine graphenylene. Its sulfur counterpart is stable up to 1256 K, presenting a smaller indirect band gap of 3.5 eV.

Fabrication of a novel biomaterial of mono-layer of halloysite nanotube-magnesium oxide/ β-tricalcium phosphate composite on implant substrate: Structural and biological properties

In the present study, a novel composite coating consisting of magnesium oxide on halloysite nanotube (HNT-MgO) and doping of zinc and strontium ions in beta-tricalcium phosphate (Mn+-β-TCP). The monolayers of biomaterials were developed by coating HNT-MgO-Mn+-β-TCP on Ti–6Al–4V alloy using the electrophoretic method. The physicochemical properties such as functional groups, phase composition, surface morphology, cross-section, and chemical composition of biomaterials were characterized by adopted analytical techniques such as Fourier transform infrared spectroscopy, X-ray diffraction analysis, Scanning electron microscopy-Energy dispersive spectroscopy, Atomic force microscopy, High-resolution transmission electron microscopy. The cytotoxic activity of the prepared composite was tested against osteosarcoma MG-63 cells using MTT assay, and AO/EB staining the results clearly showed, that by increasing the composite concentration (5–100 μg/ml) the cell viability was reduced. Further, a cell adhesion study was performed by FE-SEM analysis, which showed that the MG-63 cells could inhibit and restrict the HNT-MgO-Mn+-β-TCP monolayer composite. The composite coating confirms the significant anticancer activity against osteosarcoma cells suggesting it is a promising candidate for further studies for practical orthopedic applications.

Graphene-Based Transparent Flexible Strain Gauges with Tunable Sensitivity and Strain Range.

Monolayers of graphene oxide, assembled into densely packed sheets at an immiscible hexane/water interface, form transparent conducting films on polydimethylsiloxane membranes after reduction in hydroiodic acid (HI) vapor to reduced graphene oxide (rGO). Prestraining and relaxing the membranes introduces cracks in the rGO film. Subsequent straining opens these cracks and induces piezoresistivity, enabling their application as transparent strain gauges. The sensitivity and strain range of these gauges is controlled by the cracked film structure that is determined by the reducing conditions used in manufacture. Reduction for 30 s in HI vapor leads to an array of parallel cracks that do not individually span the membrane. These cracks do not extend on subsequent straining, leading to a gauge with a usable strain range >0.2 and gauge factor (GF) at low strains ranging from 20 to 100, depending on the prestrain applied. The GF reduces with increasing applied strain and asymptotes to about 3, for all prestrains. Reduction for 60 s leads to cracks spanning the entire membrane and an increased film resistance but a highly sensitive strain gauge, with GF ranging from 800 to 16,000. However, the usable strain range reduces to <0.01. A simple equivalent resistor model is proposed to describe the behavior of both gauge types. The gauges show a repeatable and stable response with loading frequencies >1 kHz and have been used to detect human body strains in a simple e-skin demonstration.

Femtomolar detection of the heart failure biomarker NT-proBNP in artificial saliva using an immersible liquid-gated aptasensor with reduced graphene oxide

Measuring NT-proBNP biomarker is recommended for preliminary diagnostics of the heart failure. Recent studies suggest a possibility of early screening of biomarkers in saliva for non-invasive identification of cardiac diseases at the point-of-care. However, NT-proBNP concentrations in saliva can be thousand time lower than in blood plasma, going down to pg/mL level. To reach this level, we developed a label-free aptasensor based on a liquid-gated field effect transistor using a film of reduced graphene oxide monolayer (rGO-FET) with immobilized NT-proBNP specific aptamer. We found that, depending on ionic strength of tested solutions, there were different levels of correlation in responses of electrical parameters of the rGO-FET aptasensor, namely, the Dirac point shift and transconductance change. The correlation in response to NT-proBNP was high for 1.6 mM phosphate-buffered saline (PBS) and zero for 16 mM PBS in a wide range of analyte concentrations, varied from 1 fg/mL to 10 ng/mL. The effects of transconductance and Dirac point shift in PBS solutions of different concentrations are discussed. The biosensor exhibited a high sensitivity for both transconductance (2 µS/decade) and Dirac point shift (2.3 mV/decade) in diluted PBS with the linear range from 10 fg/mL to 1 pg/mL. The aptasensor performance has been also demonstrated in undiluted artificial saliva with the achieved limit of detection down to 41 fg/mL (∼4.6 fM).

Prediction of nonlayered oxide monolayers as flexible high-κ dielectrics with negative Poisson’s ratios

During the last two decades, two-dimensional (2D) materials have been the focus of condensed matter physics and material science due to their promising fundamental properties and (opto-)electronic applications. However, high-κ 2D dielectrics that can be integrated within 2D devices are often missing. Here, we propose nonlayered oxide monolayers with calculated exfoliation energy as low as 0.39 J/m2 stemming from the ionic feature of the metal oxide bonds. We predict 51 easily or potentially exfoliable oxide monolayers, including metals and insulators/semiconductors, with intriguing physical properties such as ultra-high κ values, negative Poisson’s ratios and large valley spin splitting. Among them, the most promising dielectric, GeO2, exhibits an auxetic effect, a κ value of 99, and forms type-I heterostructures with MoSe2 and HfSe2, with a band offset of ~1 eV. Our study opens the way for designing nonlayered 2D oxides, offering a platform for studying the rich physics in ultra-thin oxides and their potential applications in future information technologies.

Aqueous Radical Initiated Oxidation of an Organic Monolayer at the Air-Water Interface as a Proxy for Thin Films on Atmospheric Aerosol Studied with Neutron Reflectometry.

Neutron reflectometry has been used to study the radical initiated oxidation of a monolayer of the lipid 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) at the air-solution interface by aqueous-phase hydroxyl, sulfate, and nitrate radicals. The oxidation of organic films at the surface of atmospheric aqueous aerosols can influence the optical properties of the aerosol and consequently can impact Earth's radiative balance and contribute to modern climate change. The amount of material at the air-solution interface was found to decrease on exposure to aqueous-phase radicals which was consistent with a multistep degradation mechanism, i.e., the products of reaction of the DSPC film with aqueous radicals were also surface active. The multistep degradation mechanism suggests that lipid molecules in the thin film degrade to form progressively shorter chain surface active products and several reactive steps are required to remove the film from the air-solution interface. Bimolecular rate constants for oxidation via the aqueous phase OH radical cluster around 1010 dm3 mol-1 s-1. Calculations to determine the film lifetime indicate that it will take ∼4-5 days for the film to degrade to 50% of its initial amount in the atmosphere, and therefore attack by aqueous radicals on organic films could be atmospherically important relative to typical atmospheric aerosol lifetimes.

Promising cathode material MnO2/CoO2 heterostructure for the Li and Na ion battery: A computational study

Although two-dimensional (2D) transition metal oxide monolayers have shown potential for applications in metal-ion batteries, the heterostructures of this family are yet to be studied in details for energy storage applications. In this work, we have made the heterostructure by taking half-metallic ferromagnetic 2D transition metal oxide CoO2 and semiconducting MnO2 monolayers and demonstrated its potential application as a cathode material in lithium and sodium-ion batteries by performing first-principles calculations using density functional theory approach. We have systematically studied the electronic structure and stability of the MnO2/CoO2 heterostructure. We have carefully examined the adsorption and diffusion behavior of metal ions (lithium and sodium). Our structure has offered a maximum adsorption energy of −3.84 eV, which is greater than the adsorption energy of individual monolayers. We found that the lowest diffusion barrier is 0.4 eV for lithium ion and 0.32 eV for sodium ion. Also, our system has shown a maximum open circuit voltage of 2.18 V for lithium ion battery and 0.32 V for Na-ion battery. The specific capacity is found to be 584 mAh g−1 for lithium ion and 529 mAh g−1 for sodium ion battery. These findings can serve as a proof that the MnO2/CoO2 heterostructure should be considered as a potential cathode for lithium- and sodium-ion batteries.

Growth and Electronic Structure of Copper Oxide Monolayer Epitaxial Films.

Copper-based high-temperature superconductors share a common feature in their crystal structure, which is the presence of a CuO2 plane, where superconductivity takes place. Therefore, important questions arise as to whether superconductivity can exist in a single layer of the CuO2 plane and, if so, how such superconductivity in a single CuO2 plane differs from that in a bulk cuprate system. To answer these questions, studies of the superconductivity in cuprate monolayers are necessary. In this study, we constructed a heterostructure system with a La2-xSrxCuO4 (LSCO) monolayer containing a single CuO2 plane and measured the resulting electronic structures. Monolayer LSCO has metallic and bulk-like electronic structures. The hole doping ratio of the monolayer LSCO is found to depend on the underlying buffer layer due to the interface effect. Our work will provide a platform for research into ideal two-dimensional cuprate systems.

Study of the structure, structural transition, interface model, and magnetic moments of CrN grown on MgO(001) by molecular beam epitaxy

Structural phase transition is studied in high quality CrN thin films grown by molecular beam epitaxy on MgO(001) substrates. Cross-sectional transmission electron microscopy and x-ray diffraction reveal that the epitaxial relationship between CrN film and MgO substrate is [100]CrN/[100]MgO, [110]CrN/[110]MgO, and [001]CrN/[001]MgO. The films show tensile strain/compression at the CrN/MgO(001) interface, which relaxes gradually with the film growth. Temperature dependent x-ray diffraction measurements show a first-order structural phase transition. In addition to the experimental measurements, first-principles theoretical calculations have been carried out for finding a stable model for the CrN/MgO interface. These calculations determine two possible models for the interface, where a monolayer of chromium oxide is formed between the CrN and MgO layers.