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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Computational simulations of solvation force and squeezing out of dodecane chain molecules in an atomic force microscope

Understanding the squeeze out behaviors of liquid films at nanometer scale in an atomic force microscope (AFM) has been a significant interest since the 1990s. We carry out all-atom static-mode AFM simulations in a liquid-vapor molecular dynamics ensemble to investigate the solvation force oscillation and squeeze out mechanisms of a confined linear dodecane fluid between a gold AFM tip and a mica substrate. Solvation force oscillations are found to be associated with the layering transition of the liquid film and unstable jumps of the AFM tip. Detailed structural analyses and molecular animations show that the local permeation of chain molecules and the squeeze out of molecules near the edge of contact promote the layering transition under compression. The confinement-induced slow down dynamics is manifested by the decrease in diffusivity and increase in rotational relaxation times. Furthermore, the persistent diffusive behavior of dodecane chain molecules even in the single-monolayer film is attributed to the chain sliding motions in the film due to the substantial vacancy space and thermal fluctuations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Placing single atoms in graphene with a scanning transmission electron microscope

We demonstrate that the sub-atomically focused beam of a scanning transmission electron microscope (STEM) can be used to controllably manipulate individual dopant atoms in a 2D graphene lattice. We demonstrate the manipulation of adsorbed source materials and the graphene lattice with the electron beam such that individual vacancy defects can be controllably passivated by Si substitutional atoms. We further demonstrate that these Si defects may be directed through the lattice via e-beam control or modified (as yet, uncontrollably) to form new defects which can incorporate new atoms into the graphene lattice. Furthermore, these studies demonstrate the potential of STEM for atom-by-atom nanofabrication and fundamental studies of chemical reactions in 2D materials on the atomic level.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Mesoscopic-microscopic spatial stochastic simulation with automatic system partitioning

The reaction-diffusion master equation (RDME) is a model that allows for efficient on-lattice simulation of spatially resolved stochastic chemical kinetics. Compared to off-lattice hard-sphere simulations with Brownian dynamics or Green’s function reaction dynamics, the RDME can be orders of magnitude faster if the lattice spacing can be chosen coarse enough. However, strongly diffusion-controlled reactions mandate a very fine mesh resolution for acceptable accuracy. It is common that reactions in the same model differ in their degree of diffusion control and therefore require different degrees of mesh resolution. This renders mesoscopic simulation inefficient for systems with multiscale properties. Mesoscopic-microscopic hybrid methods address this problem by resolving the most challenging reactions with a microscale, off-lattice simulation. However, all methods to date require manual partitioning of a system, effectively limiting their usefulness as “black-box” simulation codes. In this paper, we propose a hybrid simulation algorithm with automatic system partitioning based on indirect a priori error estimates. We demonstrate the accuracy and efficiency of the method on models of diffusion-controlled networks in 3D.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2018

Imaging nanoscale spatial modulation of a relativistic electron beam with a MeV ultrafast electron microscope

Here, an accelerator-based MeV ultrafast electron microscope (MUEM) has been proposed as a promising tool to the study structural dynamics at the nanometer spatial scale and the picosecond temporal scale. Here, we report experimental tests of a prototype MUEM where high quality images with nanoscale fine structures were recorded with a pulsed ~3 MeV picosecond electron beam. The temporal and spatial resolutions of the MUEM operating in the single-shot mode are about 4 ps (FWHM) and 100 nm (FWHM), corresponding to a temporal-spatial resolution of 4 × 10 –19 sm, about 2 orders of magnitude higher than that achieved with state-of-the-art single-shot keV UEM. Using this instrument, we offer the demonstration of visualizing the nanoscale periodic spatial modulation of an electron beam, which may be converted into longitudinal density modulation through emittance exchange to enable production of high-power coherent radiation at short wavelengths. Our results mark a great step towards single-shot nanometer-resolution MUEMs and compact intense x-ray sources that may have widespread applications in many areas of science.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

One-minute nano-tomography using hard X-ray full-field transmission microscope

Full field transmission X-ray microscope (TXM) is a powerful technique for non-destructive 3D imaging with nanometer-scale spatial resolution. However, to date, the typical acquisition time with the hard X-ray TXM at a synchrotron facility is > 10 minutes for a 3D nano-tomography data set with sub-50 nm spatial resolution. This is a significant limit on the types of 3D dynamics that can be investigated using this technique. Here, we present a demonstration of one-minute nano-tomography with sub-50 nm spatial resolution. This achievement is made possible with an in-house designed and commissioned TXM instrument at the Full-field X-ray Imaging (FXI) beamline at the National Synchrotron Light Source-II (NSLS-II) at Brookhaven National Laboratory. This capability represents an order of magnitude decrease in the time required for studying sample dynamics with 10s of nm spatial resolution.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Cryogen-free variable temperature scanning SQUID microscope

Scanning Superconducting QUantum Interference Device (SQUID) microscopy is a powerful tool for imaging local magnetic properties of materials and devices, but it requires a low-vibration cryogenic environment, traditionally achieved by thermal contact with a bath of liquid helium or the mixing chamber of a “wet” dilution refrigerator. We mount a SQUID microscope on the 3 K plate of a Bluefors cryocooler and characterize its vibration spectrum by measuring SQUID noise in a region of sharp flux gradient. By implementing passive vibration isolation, we reduce relative sensor-sample vibrations to 20 nm in-plane and 15 nm out-of-plane. A variable-temperature sample stage that is thermally isolated from the SQUID sensor enables measurement at sample temperatures from 2.8 K to 110 K. We demonstrate these advances by imaging inhomogeneous diamagnetic susceptibility and vortex pinning in optimally-doped YBCO above 90 K.

47 OTHER INSTRUMENTATION↗
DOE OSTI2019

Design, characterization, and performance of a hard x-ray transmission microscope at the National Synchrotron Light Source II 18-ID beamline

A transmission X-ray microscope (TXM) has been designed and commissioned at the 18-ID Fullfield X-ray Imaging (FXI) beamline at the National Synchrotron Light Source-II (NSLS-II). This instrument operates in the 5-11 keV range, and, with the current set of optics, is capable of 30 nm spatial resolution imaging, with a field of view (FOV) of about 40 microns. For absorption contrast, the minimum exposure time for a single projection image is about 20 ms and an entire 3D tomography data set can be acquired in under one minute. The system enables tomographic reconstructions with sub-50 nm spatial resolution without the use of markers on the sample or corrections for rotation run-outs.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Imaging microscopic electronic contrasts at the interface of single-layer WS 2 with oxide and boron nitride substrates

The electronic properties of devices based on two-dimensional materials are significantly influenced by interactions with the substrate and electrode materials. In this work, we use photoemission electron microscopy to investigate the real- and momentum-space electronic structures of electrically contacted single-layer WS 2 stacked on hBN, SiO 2 , and TiO 2 substrates. Furthermore, using work function and X-ray absorption imaging, we single-out clean microscopic regions of each interface type and collect the valence band dispersion. We infer the alignments of the electronic bandgaps and electron affinities from the measured valence band offsets of WS2 and the three substrate materials using a simple electron affinity rule and discuss the implications for vertical band structure engineering using mixed three- and two-dimensional materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

The microscopic Einstein-de Haas effect

The Einstein-de Haas (EdH) effect, where the spin angular momentum of electrons is transferred to the mechanical angular momentum of atoms, was established experimentally in 1915. While a semiclassical explanation of the effect exists, modern electronic structure methods have not yet been applied to model the phenomenon. In this paper, we investigate its microscopic origins by means of a noncollinear tight-binding model of an O2 dimer, which includes the effects of spin-orbit coupling, coupling to an external magnetic field, and vector Stoner exchange. By varying an external magnetic field in the presence of spin-orbit coupling, a torque can be generated on the dimer, validating the presence of the EdH effect. The avoided energy level crossings and the rate of change of magnetic field determine the evolution of the spin. We also find that the torque exerted on the nuclei by the electrons in a time-varying B field is not only due to the EdH effect. In conclusion, the other contributions arise from field-induced changes in the electronic orbital angular momentum and from the direct action of the Faraday electric field associated with the time-varying magnetic field.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Microscopic origin of multiple exciton emission in low-dimensional lead halide perovskites

Low-dimensional halide perovskites exhibit intriguing excitonic properties and emerge as an important class of self-activated luminescent materials. However, the ability to manipulate and optimize their luminescent properties is limited by the lack of the microscopic understanding of the exciton relaxation and emission and the inconsistency in the theoretical results in the literature. In this work, based on first-principles calculations, we studied excitons in 1D lead halide perovskites, C 4 N 2 H 14 PbBr 4 and C 4 N 2 H 14 PbCl 4 , which are both bright visible-light emitters. We find that, in both compounds, the polaron-pair exciton (EX-PP) is more stable than the onsite exciton (EX-OS) and only the EX-PP emission energy from the calculation is close to the main photoluminescence (PL) peak observed in the experiment. We note the EX-OS is found to emit UV light in both compounds. Therefore, the EX-PP is responsible for the experimentally observed visible light emission in both C 4 N 2 H 14 PbBr 4 and C 4 N 2 H 14 PbCl 4 . Furthermore, the calculated small energy difference between the EX-PP and EX-OS in C 4 N 2 H 14 PbBr 4 suggests that the metastable EX-OS can be thermally populated at room temperature (RT); the calculated EX-OS emission energy agrees well with the energy of a minor PL peak observed at RT but not at 77 K. The validity our approach in the exciton calculation is supported by the benchmark of the calculated exciton emission energies against the experimental results in 13 0D and 1D metal halides. The discrepancies between this work and a recent theoretical study in the literature are also discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Microscopic model of the doping dependence of linewidths in monolayer transition metal dichalcogenides

A fully microscopic model of the doping-dependent exciton and trion linewidths in the absorption spectra of monolayer transition metal dichalcogenides in the low temperature and low-doping regime is explored. The approach is based on perturbation theory and avoids the use of phenomenological parameters. In the low-doping regime, we find that the trion linewidth is relatively insensitive to doping levels, while the exciton linewidth increases monotonically with doping. On the other hand, we argue that the trion linewidth shows a somewhat stronger temperature dependence. The magnitudes of the linewidths are likely to be masked by phonon scattering for T ≥ 20 K in encapsulated samples in the low-doping regime. We discuss the breakdown of perturbation theory, which should occur at relatively low-doping levels and low temperatures. Our work also paves the way toward understanding a variety of related scattering processes, including impact ionization and Auger scattering in clean 2D samples

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

A large field-of-view high-resolution hard x-ray microscope using polymer optics

Here, we present an effective approach using a matched pair of polymer-based condenser-objective lenses to build a compact full-field X-ray microscope with a high spatial resolution. A unique condenser comprising arrays of high-aspect-ratio prisms with equilateral cross-section is used for uniformly illuminating samples over a large field of view (FoV) from all angles, which match the acceptance of an objective made of interdigitated orthogonal rows of one-dimensional lenses. State-of-the-art Talbot grating interferometry used to characterize these lenses for the first time revealed excellent focusing properties and minimal wave-front distortions. Using a specific lens-pair designed for 20 keV X-rays, short-exposure times, and image registration with a cross-correlation technique, we circumvent vibrational instabilities to obtain distortion-free images with a uniform resolution of 240 nm (smallest resolvable line pair) over a large FoV, 80 × 80 µm 2 in extent. The results were contrasted with those collected under illumination using commercial two-dimensional parabolic lenses with a smaller FoV. This approach implemented on a diffractometer would enable diffraction-contrast or dark-field microscopy for fast observations of ‘mesoscopic’ phenomena in real space complementing reciprocal-space studies using diffraction on the same instrument.

47 OTHER INSTRUMENTATION↗
DOE OSTI2021

Hysteresis curves reveal the microscopic origin of cooperative CO 2 adsorption in diamine-appended metal–organic frameworks

Diamine-appended metal-organic frameworks (MOFs) of the form Mg 2 (dobpdc)(diamine) 2 adsorb CO 2 in a cooperative fashion, exhibiting an abrupt change in CO 2 occupancy with pressure or temperature. This change is accompanied by hysteresis. While hysteresis is suggestive of a first-order phase transition, we show that hysteretic temperature-occupancy curves associated with this material are qualitatively unlike the curves seen in the presence of a phase transition; they are instead consistent with CO 2 chain polymerization, within one-dimensional channels in the MOF, in the absence of a phase transition. Here, our simulations of a microscopic model reproduce this dynamics, providing a physical understanding of cooperative adsorption in this industrially important class of materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Microscopic piezoelectric behavior of clamped and membrane (001) PMN-30PT thin films

Bulk single-crystal relaxor-ferroelectrics, like Pb(Mg 1/3 Nb 2/3 )O- 3 -PbTiO 3 (PMN-PT), are widely known for their large piezoelectricity. This is attributed to polarization rotation, which is facilitated by the presence of various crystal symmetries for compositions near a morphotropic phase boundary. Relaxor-ferroelectric thin films, which are necessary for low-voltage applications, suffer a reduction in their piezoelectric response due to clamping by the passive substrate. To understand the microscopic behavior of this adverse phenomenon, we employ the AC electric field driven in-operando synchrotron x-ray diffraction on patterned device structures to investigate the piezoelectric domain behavior under an electric field for both a clamped (001) PMN-PT thin film on Si and a (001) PMN-PT membrane released from its substrate. In the clamped film, the substrate inhibits the field-induced rhombohedral (R) to tetragonal (T) phase transition resulting in a reversible R to Monoclinic (M) transition with a reduced longitudinal piezoelectric coefficient d 33 < 100 pm/V. Releasing the film from the substrate results in recovery of the R to T transition and results in a d 33 > 1000 pm/V. Here, using diffraction with spatial mapping, we find that lateral constraints imposed by the boundary between the active and inactive materials also inhibit the R to T transition. Phase-field calculations on both clamped and released PMN-PT thin films simulate our experimental findings. Resolving the suppression of thin film piezoelectric response is critical to their application in piezo-driven technologies.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Digital autofocusing of a coded-aperture Laue diffraction microscope

To provide optimal depth resolution with a coded-aperture Laue diffraction microscope, an accurate position of the coded-aperture and its scanning geometry need to be known. However, finding the geometry by trial and error is a time-consuming and often challenging process because of the large number of parameters involved. In this paper, we propose an optimization approach to automate the focusing process after data is collected. Here we demonstrate the robustness and efficiency of the proposed approach with experimental data taken at a synchrotron facility.

47 OTHER INSTRUMENTATION↗
DOE OSTI2024

Atom-resolved imaging with a silicon tip integrated into an on-chip scanning tunneling microscope

Limited throughput is a shortcoming of the Scanning Tunneling Microscope (STM), particularly when used for atomically precise lithography. To address this issue, we have developed an on-chip STM based on Microelectromechanical-Systems (MEMS) technology. The device reported here has one degree of freedom, replacing the Z axis in a conventional STM. The small footprint of the on-chip STM provides a great opportunity to increase STM throughput by incorporating a number of on-chip STMs in an array to realize parallel STM. The tip methodology adopted for the on-chip STM presented here, which is a batch-fabricated Si tip, makes our design conducive to this goal. In this work, we investigate the capability of this on-chip STM with an integrated Si tip for STM imaging. We integrate the on-chip STM into a commercial ultrahigh-vacuum STM system and perform imaging with atomic resolution on par with conventional STMs but at higher scan speeds due to the higher sensitivity of the MEMS actuator relative to a piezotube. The results attest that it is possible to achieve a parallel and high-throughput STM platform, which is a fully batch-fabricated MEMS STM nanopositioner capable of performing atomic-resolution STM imaging.

Instruments & Instrumentation↗
DOE OSTI2024

Wavefront distortion correction in scanning tunneling microscope image

Here, we report an algorithm to identify and correct distorted wavefronts in atomic resolution scanning tunneling microscope images. This algorithm can be used to correct nonlinear in-plane distortions without prior knowledge of the physical scanning parameters, the characteristics of the piezoelectric actuator, or individual atom positions. The 2D image is first defined as a sum of sinusoidal plane waves, where a nonlinear distortion renders a curve for an otherwise ideal linear wavefront. Using the Fourier transforms of local areas of the image, the algorithm generates a wavefront vector field. The identified wavefronts are subsequently linearized for each plane wave without changing lattice orders, giving rise to distortion corrections. Our algorithm is complementary to conventional post-processing algorithms that require prior detection of real space features, which can also be used to correct nonlinear distortions in 2D images acquired by other microscopy techniques.

47 OTHER INSTRUMENTATION↗
DOE OSTI2024

Quick-connect scanning tunneling microscope head with nested piezoelectric coarse walkers

To meet changing research demands, new scanning tunneling microscope (STM) features must constantly evolve. We describe the design, development, and performance of a modular plug-in STM, which is compact and stable. The STM head is equipped with a quick-connect socket that is matched to a universal connector plug, enabling it to be transferred between systems. This head can be introduced into a vacuum system via a load-lock and transferred to various sites equipped with the connector plug, permitting multi-site STM operation. Its design allows for reliable operation in a variety of experimental conditions, including a broad temperature range, ultra-high vacuum, high magnetic fields, and closed-cycle pulse-tube cooling. The STM’s compact size is achieved by a novel nested piezoelectric coarse walker design, which allows for large orthogonal travel in the X, Y, and Z directions, ideal for studying both bulk and thin film samples ranging in size from mm to μm. Its stability and noise tolerance are demonstrated by achieving atomic resolution under ambient conditions on a laboratory desktop with no vibrational or acoustic isolation. The operation of the nested coarse walkers is demonstrated by successful navigation to a μm-sized 2D sample.

Instruments & Instrumentation↗