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At least 73 records · Page 4

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NASA NTRS2012

Modeling Transverse Cracking in Laminates With a Single Layer of Elements Per Ply

The objective of the present paper is to investigate the ability of mesolevel X-FEM models with a single layer of elements per ply to capture accurately all aspects of matrix cracking. In particular, we examine whether the model can predict the insitu ply thickness effect on crack initiation and propagation, the crack density as a function of strain, the strain for crack saturation, and the interaction between delamination and transverse cracks. Results reveal that the simplified model does not capture correctly the shear-lag relaxation of the stress field on either side of a crack, which leads to an overprediction of the crack density. It is also shown, however, that after onset of delamination many of the inserted matrix cracks close again, and that the density of open cracks becomes similar to the density predicted by the detailed model. The degree to which the spurious cracks affect the global response is quantified and the reliability of the mesolevel approach with a single layer of elements per ply is discussed.

Van Der Meer, Frans P.↗

DOE OSTI2015

Investigation of single-layer/multilayer self-assembled InAs quantum dots on GaAs{sub 1-x}Sb{sub x}/GaAs composite substrates

The structure-performance properties of single-layered and multi-layered InAs/GaAs{sub 1−x}Sb{sub x} quantum dot (QD) system, grown by molecular beam epitaxy on GaAs (001) substrates, have been investigated as a function of Sb concentration. Electron microscopy observations showed no significant crystalline defects for the single-layered InAs QDs (Sb 20%). X-ray diffraction analysis revealed that the increase of Sb concentration from 7.3% to 10.2% for the multi-layered QDs increased the strain relaxation from 0% to ∼23% and the dislocation density of GaAsSb layers went up to 3.6 × 10{sup 9 }cm{sup −2}. The peak energy of QD luminescence was red-shifted with increasing Sb concentration due to reduced strain inside QDs. Moreover, the carrier lifetime of the QDs was highly improved from 1.7 to 36.7 ns due to weak hole confinement as the Sb concentration was increased from 7.3% to 10.2%. These structures should be highly promising as the basis for photovoltaic solar-cell applications. Finally, the increased Sb concentration increased the thermal activation energy of electrons confined in the QDs from 163.7 to 206.8 meV, which was indicative of the improved thermal stability with Sb concentration.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2017

Analysis of current-driven oscillatory dynamics of single-layer homoepitaxial islands on crystalline conducting substrates

Here we report results of a systematic study on the complex oscillatory current-driven dynamics of single-layer homoepitaxial islands on crystalline substrate surfaces and the dependence of this driven dynamical behavior on important physical parameters, including island size, substrate surface orientation, and direction of externally applied electric field. The analysis is based on a nonlinear model of driven island edge morphological evolution that accounts for curvature-driven edge diffusion, edge electromigration, and edge diffusional anisotropy. Using a linear theory of island edge morphological stability, we calculate a critical island size at which the island’s equilibrium edge shape becomes unstable, which sets a lower bound for the onset of time-periodic oscillatory dynamical response. Using direct dynamical simulations, we study the edge morphological dynamics of current-driven single-layer islands at larger-than-critical size, and determine the actual island size at which the migrating islands undergo a transition from steady to time-periodic asymptotic states through a subcritical Hopf bifurcation. At the highest symmetry of diffusional anisotropy examined, on {111} surfaces of face-centered cubic crystalline substrates, we find that more complex stable oscillatory states can be reached through period-doubling bifurcation at island sizes larger than those at the Hopf points. We characterize in detail the island morphology and dynamical response at the stable time-periodic asymptotic states, determine the range of stability of these oscillatory states terminated by island breakup, and explain the morphological features of the stable oscillating islands on the basis of linear stability theory.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2018

Observation of topologically protected states at crystalline phase boundaries in single-layer WSe2

Abstract Transition metal dichalcogenide materials are unique in the wide variety of structural and electronic phases they exhibit in the two-dimensional limit. Here we show how such polymorphic flexibility can be used to achieve topological states at highly ordered phase boundaries in a new quantum spin Hall insulator (QSHI), 1 T ′-WSe 2 . We observe edge states at the crystallographically aligned interface between a quantum spin Hall insulating domain of 1 T ′-WSe 2 and a semiconducting domain of 1 H -WSe 2 in contiguous single layers. The QSHI nature of single-layer 1 T ′-WSe 2 is verified using angle-resolved photoemission spectroscopy to determine band inversion around a 120 meV energy gap, as well as scanning tunneling spectroscopy to directly image edge-state formation. Using this edge-state geometry we confirm the predicted penetration depth of one-dimensional interface states into the two-dimensional bulk of a QSHI for a well-specified crystallographic direction. These interfaces create opportunities for testing predictions of the microscopic behavior of topologically protected boundary states.

36 MATERIALS SCIENCE↗

DOE OSTI2017

Gold Dispersion and Activation on the Basal Plane of Single-Layer MoS 2

Gold islands are typically associated with high binding affinity to adsorbates and catalytic activity. In this paper, we present the growth of dispersed nanoscale gold islands on single layer MoS 2 , prepared on an inert SiO 2 /Si support by chemical vapor deposition. This study offers a combination of growth process development, optical characterization, photoelectron spectroscopy at submicron spatial resolution, and advanced density functional theory modeling for detailed insight into the electronic interaction between gold and single-layer MoS 2 . In particular, we find the gold density of states in Au/MoS 2 /SiO 2 /Si to be far less well-defined than Au islands on other 2-dimensional materials such as graphene, for which we also provide data. We attribute this effect to the presence of heterogeneous Au adatom/MoS 2 -support interactions within the nanometer-scale gold cluster. Theory predicts that CO will exhibit adsorption energies in excess of 1 eV at the Au cluster edges, where the local density of states is dominated by Au 5d z 2 symmetry.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Photoluminescence Induced by Substitutional Nitrogen in Single-Layer Tungsten Disulfide

The electronic and optical properties of two-dimensional materials can be strongly influenced by defects, some of which can find significant implementations, such as controllable doping, prolonged valley lifetime, and single-photon emissions. Here, we demonstrate that defects created by remote N 2 plasma exposure in single-layer WS 2 can induce a distinct low-energy photoluminescence (PL) peak at 1.59 eV, which is in sharp contrast to that caused by remote Ar plasma. This PL peak has a critical requirement on the N 2 plasma exposure dose, which is strongest for WS 2 with about 2.0% sulfur deficiencies (including substitutions and vacancies) and vanishes at 5.6% or higher sulfur deficiencies. Both experiments and first-principles calculations suggest that this 1.59 eV PL peak is caused by defects related to the sulfur substitutions by nitrogen, even though low-temperature PL measurements also reveal that not all the sulfur vacancies are remedied by the substitutional nitrogen. The distinct low-energy PL peak suggests that the substitutional nitrogen defect in single-layer WS 2 can potentially serve as an isolated artificial atom for creating single-photon emitters, and its intensity can also be used to monitor the doping concentrations of substitutional nitrogen.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2019

Comparison of Antarctic and Arctic Single-Layer Stratiform Mixed-Phase Cloud Properties Using Ground-Based Remote Sensing Measurements

Ground-based remote sensing measurements from the Atmospheric Radiation Measurement Program (ARM) West Antarctic Radiation Experiment (AWARE) campaign at the McMurdo station and the ARM North Slope of Alaska (NSA) $Utqia\dot{g}vik$ site are used to retrieve and analyze single-layer stratiform mixed-phase cloud macro- and microphysical properties for these different polar environments. Single-layer stratiform mixed-phase clouds have annual frequencies of occurrence of ~14.7% at $Utqia\dot{g}vik$ and ~7.3% at McMurdo, with the highest occurrences in early autumn. Compared to $Utqia\dot{g}vik$, stratiform mixed-phase clouds at McMurdo have overall higher and colder cloud-tops, thicker ice layer depth, thinner liquid- dominated layer depth, and smaller liquid water path. These properties reflect clear seasonal variations. Supercooled liquid fraction at McMurdo is greater than at $Utqia\dot{g}vik$ because, at a given temperature, McMurdo clouds have comparable liquid water paths but smaller ice water paths. Assessment of retrieved cloud microphysical properties show that, compared to $Utqia\dot{g}vik$, stratiform mixed-phase clouds at McMurdo have greater liquid droplet number concentration, smaller layer-mean effective radius, and smaller ice water content and ice number concentration at a given cloud-top temperature. These relationships may be related to different aerosol loading and chemical composition, and environment dynamics. Findings introduced in this report can be used as observational constraints for model representations of stratiform mixed-phase clouds.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2019

Single-layer tensor network study of the Heisenberg model with chiral interactions on a kagome lattice

Here, we study the antiferromagnetic kagome Heisenberg model with an additional scalar-chiral interaction by using the infinite projected entangled-pair state (iPEPS) ansatz. We discuss in detail the implementation of the optimization algorithm in the framework of the single-layer tensor network based on the corner transfer matrix technique. Our benchmark based on the full-update algorithm shows that the single-layer algorithm is stable, which leads to the same level of accuracy as the double-layer ansatz but with much less computation time. We further apply this algorithm to study the nature of the kagome Heisenberg model with a scalar-chiral interaction by computing the bond dimension scaling of magnetization, bond energy difference, chiral order parameter, and correlation length. In particular, we find that for strong chiral coupling the correlation length, which is extracted from the transfer matrix, saturates to a finite value for large bond dimension, representing a gapped spin-liquid state. Further comparison with density matrix renormalization group results supports that our iPEPS faithfully represents the time-reversal symmetry breaking chiral state. Our iPEPS simulation results shed new light on constructing PEPS for describing gapped chiral topological states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2019

Single-Layer Graphene Sandwiched between Proton-Exchange Membranes for Selective Proton Transmission

Proton transmission through single-layer CVD graphene in graphene/proton-exchange-membrane (PEM) sandwich structures is found to be more than 100 times faster than for any other cation. Ion transmission rates were measured for protons and a series of other cations including Li + , Na + , K + , Rb + , Cs + , and NH 4 + using a four-electrode method in which two platinum electrodes drive ionic current through the membrane and two reference electrodes installed in Luggin capillaries sense the transmembrane potential difference induced by the forced ion flow. Characterization studies including confocal Raman microscopy and X-ray photoelectron spectroscopy for graphene on Nafion, and defect visualization by etching through defects for graphene on copper, are also reported. All findings are consistent with a defect-based mechanism for transmission through graphene of all cations except protons, which likely follow a different mechanism, perhaps involving high-rate transmission through sites at which transmission of other ions is forbidden. Electrochemical impedance spectroscopy (EIS) was also used to study ion transmission rates through graphene in PEM sandwich structures. EIS gave much lower resistances for ion transmission through graphene than were obtained using the four-electrode method. Here, this latter finding is thought to reflect a capacitive coupling of mobile ions with/through graphene at the high frequencies (up to 100 kHz) used in the EIS measurement. Near-steady-state dc methods are thus necessary to evaluate true ion transmission rates through graphene.

Ion transmission↗

DOE OSTI2020

Electrical and Elastic Properties of Individual Single-Layer Nb 4 C 3 T x MXene Flakes

2D carbides and nitrides (MXenes) are widely recognized for their exceptional promise for numerous applications. However, physical property measurements of their individual monolayers remain very limited despite their importance for revealing the intrinsic physical properties of MXenes. The first mechanical and electrical measurements of individual single-layer flakes of Nb 4 C 3 T x MXene, which are prepared via an improved synthetic method are reported. Characterization of field-effect transistor devices based on individual single-layer Nb 4 C 3 T x flakes shows an electrical conductivity of 1024 ± 165 S cm -1 , which is two orders of magnitude higher than the previously reported values for bulk Nb 4 C 3 T x assemblies, and an electron mobility of 0.41 ± 0.27 cm 2 V -1 s -1 . Atomic force microscopy nanoindentation measurements of monolayer Nb 4 C 3 T x membranes yield an effective Young's modulus of 386 ± 13 GPa, assuming a membrane thickness of 1.26 nm. This is the highest value reported for nanoindentation measurements of solution-processable 2D materials, revealing the potential of Nb 4 C 3 T x as a primary component for various mechanical applications. In conclusion, the agreement between the mechanical properties of 2D Nb 4 C 3 T x MXene and cubic NbC suggests that the extensive experimental data on bulk carbides could be useful for identifying new MXenes with improved functional characteristics.

36 MATERIALS SCIENCE↗

DOE OSTI2018

A charge-transfer resistance model and Arrhenius activation analysis for hydrogen ion transmission across single-layer graphene

Transmission rates for protons and deuterons across single-layer graphene embedded in Nafion | graphene | Nafion sandwich structures are measured as a function of temperature in electrochemical hydrogen pump cells. Rates of ion transmission through graphene are obtained in the form of area-normalized ion-transfer resistances, and are interpreted in terms of ion-exchange current densities and standard heterogeneous ion-transfer rate constants. An encounter pre-equilibrium model for the ion-transfer step is then used to provide rate constants for the fundamental microscopic step of ion (proton or deuteron) transmission across graphene. Application of this rate model to interpret variable-temperature data on proton and deuteron transmission rates provides values for the activation energy and pre-exponential factor for the fundamental ion transmission step across graphene. Activation energies obtained from the Arrhenius plots for proton and deuteron transmission are as follows; for proton, E act = 48 ± 2 kJ/mole (0.50 ± 0.02 eV) and for deuteron, E act = 53 ± 5 kJ/mole (0.55 ± 0.05 eV). The difference between these two values of approximately 5 kJ/mole is in good agreement with the expected difference in vibrational zero-point energies for O—H and O-D bonds, albeit with some uncertainty given the uncertainties in the activation energy values. Pre-exponential frequency factor values of 8.3 ± 0.4 × 10 13 s -1 and is 4.7 ± 0.5 × 10 13 s -1 were obtained for proton and deuteron transmission respectively across graphene. These pre-factor values are both quite large, on the order of the values predicted from the Eyring – Polanyi equation with a transmission coefficient near one. The ratio of 1.8 for the rate pre-factors (H/D) is in reasonable agreement with the value of 1.3 for the ratio of bond vibrational frequencies for O—H and O-D stretching, respectively. Taken together, these data support a model in which proton and deuteron transmission across graphene are largely adiabatic processes for which the differences in transmission rate at room temperature are due largely to differences in activation energies.

2D materials↗

DOE OSTI2020

Quantification of defects engineered in single layer MoS 2

Atomic defects are controllably introduced in suspended single layer molybdenum disulfide (1L MoS 2 ) using helium ion beam. Vacancies exhibit one missing atom of molybdenum and a few atoms of sulfur. Quantification was done using a Scanning Transmission Electron Microscope (STEM) with an annular detector. Experimentally accessible inter-defect distance was employed to measure the degree of crystallinity in 1L MoS 2 . A correlation between the appearance of an acoustic phonon mode in the Raman spectra and the inter-defect distance was established, which introduces a new methodology for quantifying defects in two-dimensional materials such as MoS 2 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2018

Tuning Electronic Structure of Single Layer MoS 2 through Defect and Interface Engineering

Transition-metal dichalcogenides (TMDs) have emerged in recent years as a special group of two-dimensional materials and have attracted tremendous attention. Among these TMD materials, molybdenum disulfide (MoS 2 ) has shown promising applications in electronics, photonics, energy, and electrochemistry. In particular, the defects in MoS 2 play an essential role in altering the electronic, magnetic, optical, and catalytic properties of MoS 2 , presenting a useful way to engineer the performance of MoS 2 . The mechanisms by which lattice defects affect the MoS 2 properties are unsettled. In this work, we reveal systematically how lattice defects and substrate interface affect MoS 2 electronic structure. We fabricated single-layer MoS 2 by chemical vapor deposition and then transferred onto Au, single-layer graphene, hexagonal boron nitride, and CeO 2 as substrates and created defects in MoS 2 by ion irradiation. Here, we assessed how these defects and substrates affect the electronic structure of MoS 2 by performing X-ray photoelectron spectroscopy, Raman and photoluminescence spectroscopies, and scanning tunneling microscopy/spectroscopy measurements. Molecular dynamics and first-principles based simulations allowed us to conclude the predominant lattice defects upon ion irradiation and associate those with the experimentally obtained electronic structure. We found that the substrates can tune the electronic energy levels in MoS 2 due to charge transfer at the interface. Furthermore, the reduction state of CeO 2 as an oxide substrate affects the interface charge transfer with MoS 2 . The irradiated MoS 2 had a faster hydrogen evolution kinetics compared to the as-prepared MoS 2 , demonstrating the concept of defect controlled reactivity in this phase. Our findings provide effective probes for energy band and defects in MoS 2 and show the importance of defect engineering in tuning the functionalities of MoS 2 and other TMDs in electronics, optoelectronics, and electrochemistry.

42 ENGINEERING↗

DOE OSTI2016

Mutual Photoluminescence Quenching and Photovoltaic Effect in Large-Area Single-Layer MoS 2 –Polymer Heterojunctions

Two-dimensional transition metal dichalcogenides (TMDCs) have recently attracted attention due to their superlative optical and electronic properties. Specifically, their extraordinary optical absorption and semiconducting band gap have enabled demonstrations of photovoltaic response from heterostructures composed of TMDCs and other organic or inorganic materials. Yet, these early studies were limited to devices at the micrometer scale and/or failed to exploit the unique optical absorption properties of single-layer TMDCs. In this work, we present an experimental realization of a large-area type-II photovoltaic heterojunction using single-layer molybdenum disulfide (MoS 2 ) as the primary absorber, by coupling it to the organic π-donor polymer PTB7. This TMDC–polymer heterojunction exhibits photoluminescence intensity that is tunable as a function of the thickness of the polymer layer, ultimately enabling complete quenching of the TMDC photoluminescence. The strong optical absorption in the TMDC–polymer heterojunction produces an internal quantum efficiency exceeding 40% for an overall cell thickness of less than 20 nm, resulting in exceptional current density per absorbing thickness in comparison to other organic and inorganic solar cells. Moreover, this work provides insight into the recombination processes in type-II TMDC–polymer heterojunctions and thus provides quantitative guidance to ongoing efforts to realize efficient TMDC-based solar cells.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2016

Bioelectronic interfaces by spontaneously organized peptides on 2D atomic single layer materials

Self-assembly of biological molecules on solid materials is central to the “bottom-up” approach to directly integrate biology with electronics. Inspired by biology, exquisite biomolecular nanoarchitectures have been formed on solid surfaces. We demonstrate that a combinatorially-selected dodecapeptide and its variants self-assemble into peptide nanowires on two-dimensional nanosheets, single-layer graphene and MoS 2 . The abrupt boundaries of nanowires create electronic junctions via spatial biomolecular doping of graphene and manifest themselves as a self-assembled electronic network. Furthermore, designed peptides form nanowires on single-layer MoS 2 modifying both its electric conductivity and photoluminescence. The biomolecular doping of nanosheets defined by peptide nanostructures may represent the crucial first step in integrating biology with nano-electronics towards realizing fully self-assembled bionanoelectronic devices.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Manipulation of Exciton Dynamics in Single-Layer WSe 2 Using a Toroidal Dielectric Metasurface

Recent advances in emerging atomically thin transition metal dichalcogenide semiconductors with strong light–matter interactions and tunable optical properties provide novel approaches for realizing new material functionalities. Coupling two-dimensional semiconductors with all-dielectric resonant nanostructures represents an especially attractive opportunity for manipulating optical properties in both the near-field and far-field regimes. In this work, by integrating single-layer WSe 2 and titanium oxide (TiO 2 ) dielectric metasurfaces with toroidal resonances, we realized robust exciton emission enhancement over 1 order of magnitude at both room and low temperatures. Furthermore, we could control exciton dynamics and annihilation by using temperature to tailor the spectral overlap of excitonic and toroidal resonances, allowing us to selectively enhance the Purcell effect. Our results provide rich physical insight into the strong light–matter interactions in single-layer TMDs coupled with toroidal dielectric metasurfaces, with important implications for optoelectronics and photonics applications.

36 MATERIALS SCIENCE↗

DOE OSTI2022

High accuracy single-layer free-space diffractive neuromorphic classifiers for spatially incoherent light

Free-space all-optical diffractive systems have shown promise for neuromorphic classification of objects without converting light to the electronic domain. While the factors that govern these systems have been studied for coherent light, the fundamental properties for incoherent light have not been addressed, despite the importance for many applications. Here we use a co-design approach to show that optimized systems for spatially incoherent light can achieve performance on par with the best linear electronic classifiers even with a single layer containing few diffractive features. This performance is limited by the inherent linear nature of incoherent optical detection. We circumvent this limit by using a differential detection scheme that achieves greater than 94% classification accuracy on the MNIST dataset and greater than 85% classification accuracy for Fashion-MNIST, using a single layer metamaterial.

Léonard, François (ORCID:0000000249736743)↗

DOE OSTI2023

Atomic-scale electronic inhomogeneity in single-layer iron chalcogenide alloys revealed by machine learning of STM/S data

Chemical pressure from the isovalent substitution of Se by a larger Te atom in the epitaxial film of iron chalcogenide FeSe can effectively tune its superconducting, topological, and magnetic properties. However, such substitution during epitaxial growth inherently leads to defects and structural inhomogeneity, making the determination of alloy composition and atomic sites for the substitutional Te atoms challenging. Here, we utilize machine learning to distinguish between Se and Te atoms in scanning tunneling microscopy images of single-layer FeSe1−xTex on SrTiO3(001) substrates. Defect locations are first identified by analyzing spatial-dependent dI/dV tunneling spectra using the K-means clustering method. After excluding the defect regions, the remaining dI/dV spectra are further analyzed using the singular value decomposition method to determine the Se/Te ratio. Our findings demonstrate an effective and reliable approach for determining alloy composition and atomic-scale electronic inhomogeneity in superconducting single-layer iron chalcogenide films.

Materials Science↗