Sheet Of Atoms Research Articles

Non-destructive characterization of thin layer resonant tunneling diodes

We present an advanced nondestructive characterization scheme for high current density AlAs/InGaAs resonant tunneling diodes pseudomorphically grown on InP substrates. We show how low-temperature photoluminescence spectroscopy (LT-PL) and high-resolution X-ray diffractometry (HR-XRD) are complementary techniques to increase the confidence of the characterized structure. The lattice-matched InGaAs is characterized and found to be of high quality. We discuss the inclusion of an undoped “copy” well (C-well) in terms of enhancements to HR-XRD and LT-PL characterization and quantify the improved precision in determining the structure. As a consequence of this enhanced precision in the determination of physical structure, the AlAs barriers and quantum well (QW) system are found to contain nonideal material interfaces. Their roughness is characterized in terms of the full width to half-maximum of the split LT-PL emission peaks, revealing a ±1 atomic sheet variance to the QW width. We show how barrier asymmetry can be detected through fitting of both optical spectra and HR-XRD rocking curves.

Fabrication of graphite by pulsed light irradiation of network silicon bearing anthryl groups

We report structural changes of spin-coated network silicon bearing anthryl groups, poly(9-anthrylsilylene) (PAS), by intense pulsed light (IPL) irradiation. The structural changes are achieved in a few hundred microseconds resulting in the formation of graphite films with sheet resistance up to 50 Ω/sq. The conditions of IPL were optimized in order to tune the formation of graphite films as a function of exposure energy and film thickness. The effect of IPL conditions on the morphology and the electrical properties of PAS films were investigated by UV–visible, X-ray photoelectron (XPS), and Raman spectroscopies, as well as atomic force microscopy and sheet resistance measurements. The elimination of network silicon is evidenced in the changes of XPS. The correlation between Raman spectroscopy analysis and sheet resistance exhibit the formation of graphite films.

Microwave-Assisted Catalytic Degradation of Brilliant Green by Spinel Zinc Ferrite Sheets.

Microwave (MW)-assisted catalytic degradation, being an emerging technique, can potentially fill in the technological gap which promises on-demand, prompt, and efficient catalysis, and therefore, suitable MW catalysts are curiously being hunted. Candidature of spinel zinc ferrite (SZFO) atomic sheets as a MW catalyst has thoroughly been investigated in this article. Analytical techniques prove SZFO atomic sheets to be highly crystalline, thermally stable, good dielectric, and superparamagnetic, which render it a potentially strong MW catalyst. Brilliant green (BG) has been demonstrated to be chemisorbed on the SZFO atomic sheets, which upon MW irradiation gets mineralized within 5 min, and the overall efficiency has been observed to be >99%. Total organic carbon removal of ∼80% has been obtained. Ionic chromatography proves the formation of SO42– and NO3– anions which increase with MW exposure time. Liquid chromatography mass spectroscopy studies have established intermediate formations during catalysis. SZFO, established as a uniquely suited and highly efficient MW catalyst for BG, is expected to broaden the horizons of MW-assisted catalytic degradation and lead it toward its broader applications.

Manipulation of structural, electronic and transport properties of hydrogen-passivated graphene atomic sheet through vacancy defects: first-principles numerical simulations based on density-functional-theory along with tight-binding approximation

Using the first-principles procedure of density-functional-theory within tight-binding approximation and nonequilibrium Green’s function formalism, this paper reports on the impact of vacancy defects on the structural, electronic and transport properties of hydrogen-passivated graphene atomic sheet. After the introduction of vacancy defects in graphene atomic sheet passivated with hydrogen atoms, apart from increase in band gap, a suppression is noted in the intensity of transmission channels and density of states arising from the long array deformations of the graphene sheet and a corresponding shift of the Fermi level. This in turn decreases the conductance of the defected graphene atomic sheet. In case of slow-ion bombardment method, the conductance of the sheet decreases slowly and its value of the order 10−6 S before vanishing the percolation drops to the order 10−10 as the percolation of the sheet is destroyed. But in case of fast bombardment the conductance of the sheet shows a linear drop before vanishing of the percolation of the sheet, and its value of the order 10−6 S before vanishing the percolation drops to the order 10−10 as the percolation of the sheet is destroyed. Furthermore, it is found that the atomic vacancy defects effectively terminate the original smooth sp2-hybrid network of 2D graphene atomic sheet that leads to modify its electronic and transport properties, especially a decrease in its electrical conductance. Interestingly, transmission spectrum of graphene atomic wire with large vacancy defects of 143 attains identical shape to that of a molecular benzene ring.

Origin of Extended UV Stability of 2D Atomic Layer Titania-Based Perovskite Solar Cells Unveiled by Ultrafast Spectroscopy.

The inherent instability of UV-induced degradation in TiO2-based perovskite solar cells was largely improved by replacing the anatase-phase compact TiO2 layer with an atomic sheet transport layer (ASTL) of two-dimensional (2D) Ti1-δO2. The vital role of microscopic carrier dynamics that govern the UV stability of perovskite solar cells was comprehensively examined in this work by performing time-resolved pump-probe spectroscopy. In conventional perovskite solar cells, the presence of a UV-active oxygen vacancy in compact TiO2 prohibits current generation by heavily trapping electrons after UV degradation. Conversely, the dominant vacancy type in the 2D Ti1-δO2 ASTL is a titanium vacancy, which is a shallow acceptor and is not UV-sensitive. Therefore, it significantly suppresses carrier recombination and extends UV stability in perovskite solar cells with a 2D Ti1-δO2 ASTL. Other carrier dynamics, such as electron diffusion, electron injection, and hot hole transfer processes, were found to be less affected by UV irradiation. Quantitative pump-probe data clearly show a correlation between the carrier dynamics and UV aging of perovskite solar cells, thus providing a profound insight into the factors driving UV-induced degradation in perovskite solar cells and the origin of its performance.

Freestanding Borophene and Its Hybrids.

Borophene, an elemental metallic Dirac material is predicted to have unprecedented mechanical and electronic character. Need of substrate and ultrahigh vacuum conditions for deposition of borophene restricts its large-scale applications and significantly hampers the advancement of research on borophene. Herein, a facile and large-scale synthesis of freestanding atomic sheets of borophene through a novel liquid-phase exfoliation and the reduction of borophene oxide is demonstrated. Electron microscopy confirms the presence of β12 , X3 , and their intermediate phases of borophene; X-ray photoelectron spectroscopy, and scanning tunneling microscopy, corroborated with density functional theory band structure calculations, validate the phase purity and the metallic nature. Borophene with excellent anchoring capabilities is used for sensing of light, gas, molecules, and strain. Hybrids of borophene as well as that of reduced borophene oxide with other 2D materials are synthesized, and the predicted superior performance in energy storage is explored. The specific capacity of borophene oxide is observed to be ≈4941 mAh g-1 , which significantly exceeds that of existing 2D materials and their hybrids. These freestanding borophene materials and their hybrids will create a huge breakthrough in the field of 2D materials and could help to develop future generations of devices and emerging applications.

(Invited) Atomic Level Investigation of Resistance Switching in 2D Memory Devices

The last few years have witnessed significant developments in non-volatile resistance switching (NVRS) devices which employ layered 2D materials such as hexagonal Boron Nitride (hBN) and Transitional Metal Dichalcogenides (TMDs) as the active medium. Among these developments, the demonstration of NVRS in vertical metal-insulator-metal (MIM) devices with single layer atomic sheets used as the insulating layer, is particularly intriguing. While it was long believed that leakage currents would prevent observation of this phenomenon for atomically thin insulating layers our previous studies on mesoscopic scale devices demonstrated stable unipolar and bipolar operations. Furthermore, these studies suggest that localized defects and other atomic-scale multiphysics mechanisms are likely to have a key role in NVRS observed in these 2D devices. Here, we present our current research on the atomic level details of the relationship between the NVRS and heterogeneities in 2D materials. For these investigations, we utilize both single probe and multi-probe scanning electron microscopy (STM) methods, first, to identify the atomic level details of the heterogeneity and then to conduct transport experiments in the vicinity of specific heterogeneities. These studies aim to provide one to one correlation between the structural properties of the heterogeneity and its role on the transport properties of the material. Through these studies, we seek to deduce which particular defect structures are most desirable for NVRS and utilize this information for the development of extremely high-density data storage devices as well as neuromorphic applications and low power RF switches.

How hydrogen sticks to graphene

The 2-D sheet of sp2-hybridized carbon atoms in graphene helps grant the material unique and useful chemical, material, and electronic properties. Adding hydrogen atoms to the carbons can improve those properties—for example, by turning graphene into a semiconductor. New laboratory and computational experiments reveal how energy dissipates through a graphene sheet when hydrogen atoms strike it, making C–H bond formation possible (Science 2019, DOI: 10.1126/science.aaw6378). Alec. M. Wodtke of the University of Gottingen and the Max Planck Institute for Biophysical Chemistry was experimenting with hydrogen scattering from metal surfaces when his group observed that graphene grown on platinum behaved differently. Hydrogen and metal atoms collide like billiard balls, he says, but when hydrogen hits a graphene carbon it doesn’t bounce off, it forms a bond. Wodtke’s collaborator, Thomas F. Miller III of the California Institute of Technology, used computational models to better understand what happens. When st...

Out-of-plane excitons in two-dimensional crystals

Two-dimensional materials are ideal platforms to explore new physical phenomena and reveal unique optical properties. Excitonic and quasiparticle effects are strongly enhanced by the reduced screening. The response to a perturbing electromagnetic field is dominated by in-plane light polarization. The quantum confinement of the atomic sheets in normal direction also allows out-of-plane pair excitations, which are ruled by selection rules and local-field effects. We apply the GW approximation and the Bethe-Salpeter equation to investigate optical transitions for light polarization both parallel and normal to the atomic planes. Prototypical systems range from semimetals to large-gap insulators such as graphene, graphene oxide, boron nitride, and molybdenum disulfide. For the considered materials, the out-of-plane absorption edge and the first van Hove singularities are blueshifted in dependence of the effective sheet thickness, while the intensity of the optical conductivity is reduced by orders of magnitude.

Structure determination of the Si(111)−7×3−In atomic-layer superconductor

The atomic structure of a superconducting atomic sheet of indium grown on Si(111) surface, i.e., $\mathrm{In}/\mathrm{Si}(111)\text{\ensuremath{-}}\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$, is determined by using surface x-ray diffraction (SXRD) and low-energy electron diffraction (LEED). The structure consists of double layers of In atoms with a rectangular arrangement, verifying a theoretical prediction [J. W. Park and M. H. Kang, Phys. Rev. Lett. 109, 166102 (2012)]. For the so-called hexagonal phase, which has been considered to be a different $\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ structure and to undergo a transition to the $\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7}$ structure below 265 K, LEED data indicate that the structural basis is $\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7}$ even at room temperature. The SXRD data are well reproduced by single-layer In with the In coverage of 10/7 In atoms per Si surface atom, proposed by a theory [J. W. Park and M. H. Kang, Phys. Rev. Lett. 117, 116102 (2016)].

Graphene Utilization for Water Desalination Process

Though above 70% of the Earth is covered by water, most of the seas and oceans are unusable for drinking. Freshwater lakes, rivers and underground aquifers imply 2.5 percent of the global’s whole freshwater supply. Unfortunately, in addition to being scarce, fresh water is dreadfully unevenly spread. Enhanced demand for freshwater is a global concern. In many countries demanding is further than regular reserves. Sensible use of water, reducing spreading losses and upgraded treatment of recycled water to mitigate the concern, though, water scarcity is still presented consequently desalination of seawater is highly required. Graphene, a single sheet of carbon atoms, possibly will deliver the principal for a novel category of extremely permeable membranes for water purification and desalination. Though, a one atom thickness graphene reveals both brilliant mechanical strength and impermeability to atoms as small as helium. High-density, subnanometer pores within graphene have the potential for ultra-fast water permeance and high solute rejection as the atomic thinness makes slight resistance to stream which deters the transfer of solutes bigger than the pores. The two-dimensional, nanoporous membrane is expected to display orders-of-magnitude permeability and selectivity enhancement over current separation membranes for processes such as brackish water, water softening, or nanofiltration. This study is aimed that the existing desalination methods are not adequate to upgrade water sources unless the desalination technologies are improved significantly. Nanotechnology and utilizing graphene will deliver desalination technology to meet the requirements in the near future. Lately, novel procedures have been technologically progressed by means of nanotechnology and applying graphene for water desalination. This research will emphasize the concept of water desalination for the near futures.

A Simple Model for Relative Energies of All Fullerenes Reveals the Interplay between Intrinsic Resonance and Structural Deformation Effects in Medium-Sized Fullerenes.

Fullerenes are sheets of sp2 carbon atoms wrapped around to form spheres. With this simple consideration, we have in the present study devised and (with over 3600 DFT data points) successfully validated a simple model, termed R+D, for estimating the relative energies of fullerenes. This model contains a resonance component to account for the intrinsic differences between the π-energies of different fullerenes, and a deformation component for treating the distortions from planarity. Notably, we find that both terms (and they alone) are required to obtain good relative energies, which lends support to the formulation of the R+D model. An interesting finding is that for some medium-sized IPR fullerenes, their isomers show similar variations in the two components. We deduce that these fullerenes may represent a good opportunity for tuning molecular properties for practical applications. We hope that the promising results of the present study will encourage further investigations into fullerenes from a fundamental perspective.

Biomedical Potential of Graphene oxide based Nanoformulations: An Overview

Graphene oxide is graphene-based two dimensional material and structure based on single sheet of carbon atoms arranged in hexagonal lattice or honeycomb framework. Graphene oxide is better than graphene in terms of dispersion in polar solvents due to oxygen-containing functional groups attached with carbon sheets, results as researchers to get indulged in this molecule. Graphene oxide has been attracted researchers in different fields like chemistry, physics and materials science. Graphene oxide has endless potential applications in building materials, environmental protection, electronics, medicine, pharmaceutical industries etc. In medicinal industry, graphene oxide used as antibacterial, antimicrobial and antifungal agent. Its potential activity with other nanoparticles as antifungal agent has come in light and became the main focus of going on works on graphene oxide. The antifungal property of GO-Ag composites was investigated. Furthermore, nanoparticles show great potential in the fields of biomedical, because of their antifungal properties. The GO-Ag composites act as alternative antifungal material. Moreover, antifungal activity of Reduced Graphene Oxide (rGO) nanosheets is also researched.

Interfacial transmetallation synthesis of a platinadithiolene nanosheet as a potential 2D topological insulator.

The construction of two-dimensional metal complex materials is fascinating because of the structural and functional diversity of these materials. Previously, we have reported the synthesis of electroconductive nickelladithiolene (NiDT) and palladadithiolene (PdDT) nanosheets using benzenehexathiol (BHT). Down the group from Ni, Pd to Pt, there is a distinct positive shift in the reduction potential; as a result, it becomes synthetically more challenging to stabilize Pt2+ than to form metallic Pt(0) in the presence of BHT as a reducing agent. Herein, a novel synthetic strategy for the preparation of platinadithiolene nanosheet (PtDT) using a dibutyltin-protected BHT ligand is reported, leading to transmetallation in the presence of dioxygen. Both free-standing stacked sheets and atomic layer sheets were obtained and characterized by microscopic techniques such as AFM, SEM, and TEM. To study the morphology of the sheets and determine their charge neutrality, X-ray photoelectron (XP) and infrared (IR) spectroscopic techniques were used. Powder X-ray diffraction analysis of the multilayer PtDT indicates a half-way slipped hexagonal configuration in the P3[combining macron]1m space group. The band structure of this PtDT exhibits a band gap at the Fermi level, which is different from that of NiDT in the staggered configuration, and a Dirac gap, indicating the possibility of 2D topological insulation at room temperature. PtDT is insulating but chemically activated by oxidation with I2 to increase the conductivity by more than 106 folds up to 0.39 S cm-1. The MDT sheets exhibit electrocatalytic activity for the hydrogen evolution reaction, and the activity order is NiDT < PdDT < PtDT.

Two-Dimensional Pnictogen for Field-Effect Transistors.

Two-dimensional (2D) layered materials hold great promise for various future electronic and optoelectronic devices that traditional semiconductors cannot afford. 2D pnictogen, group-VA atomic sheet (including phosphorene, arsenene, antimonene, and bismuthene) is believed to be a competitive candidate for next-generation logic devices. This is due to their intriguing physical and chemical properties, such as tunable midrange bandgap and controllable stability. Since the first black phosphorus field-effect transistor (FET) demo in 2014, there has been abundant exciting research advancement on the fundamental properties, preparation methods, and related electronic applications of 2D pnictogen. Herein, we review the recent progress in both material and device aspects of 2D pnictogen FETs. This includes a brief survey on the crystal structure, electronic properties and synthesis, or growth experiments. With more device orientation, this review emphasizes experimental fabrication, performance enhancing approaches, and configuration engineering of 2D pnictogen FETs. At the end, this review outlines current challenges and prospects for 2D pnictogen FETs as a potential platform for novel nanoelectronics.

Ultrahigh-Sensitive Broadband Photodetectors Based on Dielectric Shielded MoTe2 /Graphene/SnS2 p-g-n Junctions.

2D atomic sheets of transition metal dichalcogenides (TMDs) have a tremendous potential for next-generation optoelectronics since they can be stacked layer-by-layer to form van der Waals (vdW) heterostructures. This allows not only bypassing difficulties in heteroepitaxy of lattice-mismatched semiconductors of desired functionalities but also providing a scheme to design new optoelectronics that can surpass the fundamental limitations on their conventional semiconductor counterparts. Herein, a novel 2D h-BN/p-MoTe2 /graphene/n-SnS2 /h-BN p-g-n junction, fabricated by a layer-by-layer dry transfer, demonstrates high-sensitivity, broadband photodetection at room temperature. The combination of the MoTe2 and SnS2 of complementary bandgaps, and the graphene interlayer provides a unique vdW heterostructure with a vertical built-in electric field for high-efficiency broadband light absorption, exciton dissociation, and carrier transfer. The graphene interlayer plays a critical role in enhancing sensitivity and broadening the spectral range. An optimized device containing 5-7-layer graphene has been achieved and shows an extraordinary responsivity exceeding 2600 A W-1 with fast photoresponse and specific detectivity up to ≈1013 Jones in the ultraviolet-visible-near-infrared spectrum. This result suggests that the vdW p-g-n junctions containing multiple photoactive TMDs can provide a viable approach toward future ultrahigh-sensitivity and broadband photonic detectors.

Flat Monolayer Graphene Cathodes for Li-Oxygen Microbatteries.

Miniature batteries can accelerate the development of mobile electronics by providing sufficient energy to power small devices. Typical microbatteries commonly use thin-film inorganic electrodes based on Li-ion insertion reaction. However, they rely on the complicated thin-film synthesis method of inorganics containing many elements. Graphene, one atomic layer thick carbon sheet, has diverse physical and chemical properties and is compatible with conventional micron-scale device fabrication. Here, we study the use of chemical vapor deposition (CVD) grown monolayer graphene in a two-dimensional configuration, as a future Li-oxygen microbattery cathode. By maximizing the dissolution of discharge intermediates, we obtain 2610 Ah/ggraphene of capacity corresponding to 20% higher areal cathode energy density and 2.7 times higher cathode specific energy than that can be derived from the same volume or mass of conventional Li-ion battery cathode material. Furthermore, a clear observation on the discharge reaction on composite electrodes and their role in the charging reaction was made, thanks to the two-dimensional monolayer graphene Li-oxygen battery cathode. We demonstrate an easy integration of two-dimensional CVD graphene cathode into microscale devices by simply transferring or coating the target device substrate with flexible graphene layers. The ability to integrate and use monolayer graphene on arbitrary device substrates as well as precise control over a chemical derivation of the carbon interface can have a radical impact on future energy-storage devices.

Non-thermal and low-destructive X-ray induced graphene oxide reduction

Large-scale graphene fabrication by thermal and chemical reductions of graphene oxide has faced the problem of defect formation. To solve the problem, we have considered a physically alternative reduction process including electronic excitation followed by the oxygen group detachment from the carbon sheet without capturing a sheet of carbon atoms. Single-layer graphene oxide films were studied by photoemission spectroscopy in the course of monochromatic synchrotron X-ray radiation with in situ control of the layer thickness, chemical composition, atomic ordering, and defect concentration exactly in the modified area. The radiation flux was too low to heat the film. A non-thermal and low-destructive effect of X-ray induced graphene oxide reduction has been revealed. Transformation of the sp3 σ bonds into sp2 π ordered bonds, bandgap closing, and significant diminishing of the oxygen content (below 5 at. %) have been observed without any signs of defects in the photoemission spectra. The effective cross section of the oxygen group detachment induced by a soft X-ray photon (hν = 130 eV) was estimated to be σ* ∼ 3 × 10−18 cm2. A reduced single-layer graphene oxide with a narrow bandgap (0.4 – 0.8 eV) attractive for many applications was obtained, as well as almost pure graphene.

Lateral Graphene p–n Junctions Realized by Nanoscale Bipolar Doping Using Surface Electric Dipoles and Self‐Organized Molecular Anions

AbstractLateral p–n junctions take the unique advantages of 2D materials, such as graphene, to enable single‐atomic layer microelectronics. A major challenge in fabrication of the lateral p–n junctions is in the control of electronic properties on a 2D atomic sheet with nanometer precision. Herein, a facile approach that employs decoration of molecular anions of bis‐(trifluoromethylsulfonyl)‐imide (TFSI) to generate p‐doping on the otherwise n‐doped graphene by positively polarized surface electric dipoles (pointing toward the surface) formed on the surface oxygen‐deficient layer “intrinsic” to an oxide ferroelectric back gate is reported. The characteristic double conductance minima V Dirac− and V Dirac + illustrated in the obtained lateral graphene p–n junctions can be tuned in the range of −1 to 0 V and 0 to +1 V, respectively, by controlling the TFSI anions and surface dipoles quantitatively. The unique advantage of this approach is in adoption of polarity‐controlled molecular ion attachment on graphene, which could be further developed for various lateral electronics on 2D materials.

Upper and Lower Bounds on Infinite Summation of Pair Potential Energy between Atom and Single Graphene Sheet

The infinite pairwise summation of the potential energy between an atom above a graphene sheet and carbon atoms arranged in a hexagonal lattice is considered. Its magnitude depends on the minimum d...