Short Length Scales Research Articles

Cross-plane Thermoelectric and Thermionic Transport across Au/h-BN/Graphene Heterostructures

The thermoelectric voltage generated at an atomically abrupt interface has not been studied exclusively because of the lack of established measurement tools and techniques. Atomically thin 2D materials provide an excellent platform for studying the thermoelectric transport at these interfaces. Here, we report a novel technique and device structure to probe the thermoelectric transport across Au/h-BN/graphene heterostructures. An indium tin oxide (ITO) transparent electrical heater is patterned on top of this heterostructure, enabling Raman spectroscopy and thermometry to be obtained from the graphene top electrode in situ under device operating conditions. Here, an AC voltage V(ω) is applied to the ITO heater and the thermoelectric voltage across the Au/h-BN/graphene heterostructure is measured at 2ω using a lock-in amplifier. We report the Seebeck coefficient for our thermoelectric structure to be −215 μV/K. The Au/graphene/h-BN heterostructures enable us to explore thermoelectric and thermal transport on nanometer length scales in a regime of extremely short length scales. The thermoelectric voltage generated at the graphene/h-BN interface is due to thermionic emission rather than bulk diffusive transport. As such, this should be thought of as an interfacial Seebeck coefficient rather than a Seebeck coefficient of the constituent materials.

Imaging the Hydrogen Absorption Dynamics of Individual Grains in Polycrystalline Palladium Thin Films in 3D.

Defects such as dislocations and grain boundaries often control the properties of polycrystalline materials. In nanocrystalline materials, investigating this structure-function relationship while preserving the sample remains challenging because of the short length scales and buried interfaces involved. Here we use Bragg coherent diffractive imaging to investigate the role of structural inhomogeneity on the hydriding phase transformation dynamics of individual Pd grains in polycrystalline films in three-dimensional detail. In contrast to previous reports on single- and polycrystalline nanoparticles, we observe no evidence of a hydrogen-rich surface layer and consequently no size dependence in the hydriding phase transformation pressure over a 125-325 nm size range. We do observe interesting grain boundary dynamics, including reversible rotations of grain lattices while the material remains in the hydrogen-poor phase. The mobility of the grain boundaries, combined with the lack of a hydrogen-rich surface layer, suggests that the grain boundaries are acting as fast diffusion sites for the hydrogen atoms. Such hydrogen-enhanced plasticity in the hydrogen-poor phase provides insight into the switch from the size-dependent behavior of single-crystal nanoparticles to the lower transformation pressures of polycrystalline materials and may play a role in hydrogen embrittlement.

Electron Accumulation and Emergent Magnetism in LaMnO_{3}/SrTiO_{3} Heterostructures.

Emergent phenomena at polar-nonpolar oxide interfaces have been studied intensely in pursuit of next-generation oxide electronics and spintronics. Here we report the disentanglement of critical thicknesses for electron reconstruction and the emergence of ferromagnetism in polar-mismatched LaMnO_{3}/SrTiO_{3} (001) heterostructures. Using a combination of element-specific x-ray absorption spectroscopy and dichroism, and first-principles calculations, interfacial electron accumulation, and ferromagnetism have been observed within the polar, antiferromagnetic insulator LaMnO_{3}. Our results show that the critical thickness for the onset of electron accumulation is as thin as 2 unit cells (UC), significantly thinner than the observed critical thickness for ferromagnetism of 5 UC. The absence of ferromagnetism below 5 UC is likely induced by electron overaccumulation. In turn, by controlling the doping of the LaMnO_{3}, we are able to neutralize the excessive electrons from the polar mismatch in ultrathin LaMnO_{3} films and thus enable ferromagnetism in films as thin as 3 UC, extending the limits of our ability to synthesize and tailor emergent phenomena at interfaces and demonstrating manipulation of the electronic and magnetic structures of materials at the shortest length scales.

Lithium self-diffusion in a model lithium garnet oxide Li5La3Ta2O12: A combined quasi-elastic neutron scattering and molecular dynamics study

In this work lithium self-diffusion in the model lithium garnet oxide Li5La3Ta2O12 was investigated by combining quasi-elastic neutron scattering experiments and molecular dynamics simulation. The Q-dependence of the quasi-elastic broadening measured experimentally and of the mean relaxation rate of lithium calculated by molecular dynamics simulations were both well described by the Singwi-Sjölander diffusion model. This model describes lithium nuclei that undergo diffusion via a jump-type mechanism consisting of a distribution of jumps with a mean square jump distance and residence time. The extracted mean jump length is consistent with a jump between tetrahedral (24d) to octahedral (48g and 96h) sites within the Ia3¯d symmetry structure and the residence time of Li at these sites obeys an Arrhenius relation from ~ps timescales at 1100K to the ~ns range at 400K. This result supports a lithium diffusion mechanism in which jumps are more frequent (smaller residence time) and shorter at higher temperature. The self-diffusivities of Li from both experiment and calculation were in good agreement, but deviations from those previously measured using nuclear magnetic resonance and muon spin relaxation were observed and discussed. Analysis of stretching parameter describing the relaxation of lithium calculated by molecular dynamics indicated that Li motion is more cooperative at shorter length scales, below ~4.4Å, which corresponds to the distance between octahedral sites across the tetrahedral site.

Beyond Ordered Materials: Understanding Catalytic Sites on Amorphous Solids

Amorphous materials are widely used as components of solid catalysts and have been the subject of much applied research. In some instances, their catalytic performance is demonstrably superior to that of their crystalline counterparts, due in part to their greater flexibility. Amorphous or disordered phases can also be generated from crystalline phases under reaction conditions, and thus, ex situ observations of long-range order may provide an incomplete or misleading picture. Until recently, theorists and experimentalists have mostly neglected these important materials in fundamental studies, preferring instead to study “well-defined” (often crystalline) catalysts that are potentially more tractable and amenable to computational modeling of their structure–activity relationships. The amorphous materials were assumed to be simply nonuniform versions of compositionally similar materials with long-range order, having the same key features at short and medium length scales. In this Perspective, shortcomings ...

Assimilating Conventional and Doppler Radar Data with a Hybrid Approach to Improve Forecasting of a Convective System

A hybrid ensemble adjustment Kalman filter—three-dimensional ensemble—variational (EAKF-En3DVar) system is developed to assimilate conventional and radar data, and is applied to a convective case in Colorado and Kansas, USA. The system is based on the framework of the Weather Research and Forecasting model’s three-dimensional variational (3DVar) and Data Assimilation Research Testbed. A two-step assimilation procedure with a shorter length scale and analysis cycle is used to reduce analysis noise in radar data assimilation. Results show that the hybrid experiment assimilating only conventional data improves the quantitative precipitation forecast (QPF) and quantitative reflectivity forecast over those of a 3DVar experiment, and the improvements are also evident after assimilating radar data. The assimilation of radar data substantially improves the QPF up to seven hours, with either the 3DVar or hybrid method. The hybrid experiment assimilating both conventional and radar data forecasts a more accurate convective system in terms of structure, spatial extent and intensity and produces increased low-level cooling and mid-level warming in the convective region. These improvements are attributable to an improved forecast background field of wind, temperature and water vapor mixing ratio, with maximum root mean square error reduction at the tropopause and near the surface.

Reproducible ultra-long FBGs in phase corrected non-uniform fibers

Ultra-long fiber Bragg gratings (FBGs), i.e., FBGs of several tens of cm in length, have attracted much attention in the last few decades for their potential applications in advanced devices. Although numerous fabrication methods as well as ultra-long functionalized FBGs have been proposed and demonstrated successfully, such devices are difficult to reproduce. We have recently found that specialty optical fibers of the type required for these applications are highly non-uniform on a short length scale, severely affecting the characteristics of ultra-long FBGs. We propose here a new production technique that can be adapted to any non-uniform fiber for ultra-long FBG fabrication. This technique involves a fiber characterization prior to FBG inscription followed by the writing of a phase corrected ultra-long FBG. This technique has no limitations in terms of correction amplitude or FBG length. The results are quite astonishing, as near-perfect 1-m-length-scale FBGs are possible in fibers in which it was impossible to write uniform period gratings prior to phase correction.

Discovery and implications of the chaotic attractor from zoned pyroxene in Qixiangzhan comenditic lava, Tianchi volcano, NE China

Mineral compositional analysis is a powerful tool for identifying both volcanic and petrogenetic processes. This property is observed because spatial variations in the composition of magmatic minerals record chemical and physical changes in the magma from which they crystallized. Thus, these variations can also be used to decipher the history of a magmatic system and its triggering mechanism prior to eruption. The Tianchi volcano, located in the Changbaishan volcanic region, is one of the most active and dangerous volcanoes in East Asia; however, the eruptional mechanism of most recent eruptions are not well-resolved, especially the comenditic lava flow in the Qixiangzhan stage, Tianchi volcano. We focused on the zoned patterns in pyroxene phenocrysts of the comenditic lava to decipher the magma process and triggering mechanism of this silicic lava eruption. We analyzed major and minor element concentrations by point measurement and line-scan across a zoned pyroxene grain using the electron probe micro-analyzer (EPMA) equipped with the energy-dispersive (ED) spectrum. We also extracted a compositional time series by considering the variation of zone thickness across oscillatory zoning and reconstructed the phase space of the compositional time series using the optimal time delay method. First, we calculated the optimal time delay of phase space reconstruction obtained from the first minimum method of mutual information, and we later determined the embedded dimension. Using two parameters (i.e., time delay and embedded dimension), we subsequently reconstructed the phase space. We also present the Poincare section of the reconstructed system. The results of EPMA point measurement (Table 1 and Figure 4) showed that the zoned pyroxenes have higher MgO (2 wt%–2.12 wt%) and lower Na2O (1.26 wt%–1.39 wt%) in the core compared with the rim (MgO, 1.22 wt%–1.54 wt%; Na2O, 1.71 wt%–1.92 wt%). The results of the line-scan of EPMA-ED showed that the normalized concentration of each minor element (MgO, Na2O, MnO) exhibits an intense oscillatory variation (Figure 4). Meanwhile, in the reconstructed phase space (Figure 5(c)) and the Poincare section (Figure 5(d)), the compositional time series of Mg exhibits a chaotic attractor. This attractor is different from the periodic oscillation attractor and the completely random oscillatory dynamic system (Figure 3). These features indicated that the pyroxene phenocrysts grow in a system with a high degree of freedom and complexity and the magma composition around pyroxene changed notably and quickly during crystallization. These features lead us to believe that the oscillatory zoning observed in the Qixiangzhan comenditic lava is not the result of a simple fractional crystallization process, which generally occurred in the stable magma chamber. Although such a process may lead to compositional zoning in the pyroxenes, it cannot form oscillatory zoning. A simple, closed-system fractional crystallization process will produce a periodic oscillation attractor instead of a strange attractor in their reconstructed phase space. A feasible process that may account for the oscillatory zoning features in the Qixiangzhan comenditic lava is an interaction between two (or more) compositionally distinct magmas (magma mixing). This process can create strong heterogeneities within magma bodies in a short time and on a very short length scale. More importantly, this process can produce a strange attractor in the reconstructed phase space of a zoning mineral. These features are all consistent with what we have obtained from the zoned pyroxene in the Qixiangzhan comenditic lava. Thus, we believe that the Qixiangzhan comenditic lava results from magma mixing or the magma recharge process. The mixing and convection processes of infilled primary magma and evolved magma can accelerate the vesiculation of the magma chamber, thereby causing an eruption. Combined with the frequent occurrence of magma recharge or magma mixing evidence in the latest eruptions of the Tianchi volcano, we believe that the infilling of primitive magmas plays a key role in the triggering mechanism of the Tianchi volcano.

Identifying Defects with Guided Algorithms in Bragg Coherent Diffractive Imaging

Crystallographic defects such as dislocations can significantly alter material properties and functionality. However, imaging these imperfections during operation remains challenging due to the short length scales involved and the reactive environments of interest. Bragg coherent diffractive imaging (BCDI) has emerged as a powerful tool capable of identifying dislocations, twin domains, and other defects in 3D detail with nanometer spatial resolution within nanocrystals and grains in reactive environments. However, BCDI relies on phase retrieval algorithms that can fail to accurately reconstruct the defect network. Here, we use numerical simulations to explore different guided phase retrieval algorithms for imaging defective crystals using BCDI. We explore different defect types, defect densities, Bragg peaks, and guided algorithm fitness metrics as a function of signal-to-noise ratio. Based on these results, we offer a general prescription for phasing of defective crystals with no a priori knowledge.

Flow-through microfluidic immunosensors with refractive index-matched silica monoliths as volumetric optical detection elements

A sensitive and rapid absorbance based immunosensor that utilizes ex situ functionalized porous silica monoliths as volumetric optical detection elements are demonstrated in this study. The porous monolith structure facilitates high capture probe density and short diffusion length scales, enabling sensitive and rapid assays. Silica monoliths, synthesized and functionalized with immunocapture probes off-chip before integration into a sealed thermoplastic microfluidic device, serve to capture target antigens during perfusion through the porous structure. Gold nanoparticle immunoconjugates are combined with silver enhancement to create microscale silver clusters, followed by perfusion of an aqueous sucrose solution to limit light scattering and enhance optical signal. Using this approach, detection limits as low as 1ng/mL are achieved for a sandwich assay, with a dynamic range of at least 4 logs. The results confirm that the combination of on-chip index matching with functionalized porous silica monoliths can enable simple and practical flow-through immunoassays for the sensitive and rapid detection of target antigens.

An integrated approach for the installation of a wave farm

The installation of a wave farm involves the appropriate selection of a location and a wave energy converter (WEC) for harnessing the energy resource. There is typically a considerable number of options both regarding the site and the technology, and choosing the optimum WEC-site combination is not straightforward. An integrated approach is demonstrated in this work by means of a case study in Burela (Galicia, NW Spain)—an area that has been proposed for wave energy exploitation. First, three possible locations are defined with a view to avoiding potential environmental impacts and the interference of the wave farm with existing uses of the marine space. Then, the power performance of six technologies at the selected locations is computed by means of a comprehensive methodology based on high-resolution numerical modelling, which guarantees accurate and reliable results. Significant spatial variability of the resource over short length scales is found, which translates into large variations in the performance of the various WECs considered. On this basis, the most appropriate site and WEC for the area are determined, thereby the integrated approach for installing a wave farm—which may be used in any other region of interest—is demonstrated.

Investigation of effective impact parameters in electron–ion temperature relaxation via Particle–Particle Coulombic molecular dynamics

Aiming at a high simulation accuracy, a Particle–Particle (PP) Coulombic molecular dynamics model is implemented to study the electron–ion temperature relaxation. In this model, the Coulomb's law is directly applied in a bounded system with two cutoffs at both short and long length scales. By increasing the range between the two cutoffs, it is found that the relaxation rate deviates from the BPS theory and approaches the LS theory and the GMS theory. Also, the effective minimum and maximum impact parameters (bmin⁎ and bmax⁎) are obtained. For the simulated plasma condition, bmin⁎ is about 6.352 times smaller than the Landau length (bC), and bmax⁎ is about 2 times larger than the Debye length (λD), where bC and λD are used in the LS theory. Surprisingly, the effective relaxation time obtained from the PP model is very close to the LS theory and the GMS theory, even though the effective Coulomb logarithm is two times greater than the one used in the LS theory. Besides, this work shows that the PP model (commonly known as computationally expensive) is becoming practicable via GPU parallel computing techniques.

Eruptive and shallow conduit dynamics during Vulcanian explosions: insights from the Episode IV block field of the 1912 eruption of Novarupta, Alaska

The study of ~1300 juvenile and lithic blocks from a Vulcanian phase of the 1912 eruption of Novarupta provides new insight into the state of the magma as an eruption passes from sustained Plinian to dome growth. Blocks that were predominantly ballistically ejected were measured and sampled within an ~2–3-km radius from vent and supply a picture of a dynamic and complex shallow conduit prior to magma fragmentation in repeated small explosions. Extreme conduit heterogeneity is expressed in the diverse range of dacitic block types, including pumiceous, dense, banded, and variably welded breccia clasts, all with varied degrees of surface breadcrusting. We present new maps of block lithology and size, making Episode IV the most thoroughly mapped Vulcanian deposit to date. Sectorial regions rich in specific lithologies together with the block size data suggest multiple, small explosions. Modeling of block trajectories to reproduce the field data indicates that ejection velocities range from 50 to 124 m/s with a median of ~70 m/s. We propose that individual explosions originated from a heterogeneous shallow conduit characterized both by the juxtaposition of magma domains of contrasting texture and vesiculation state and by the intimate local mingling of different textures on short vertical and horizontal length scales at the contacts between these domains. In our model, each explosion disrupted the conduit to only shallow depths and tapped diverse, localized pockets within the conduit. This contrasts with existing models for repetitive Vulcanian explosions, and suggests that the dynamics of Episode IV were more complex than a simple progressive top-down evacuation of a horizontally stratified conduit.

Efficient Multiscale Modeling and Validation of Residual Stress Field in Cutting

Residual stress (RS) has significant impact on cutting process optimization. However, conventional process modeling approaches are limited to only single cutting pass on very short length and time scales due to the exceedingly high computational cost. This work provides a new concept of equivalent loading which enables an efficient modeling approach to predict RS in an actual machined surface by incorporating multiple cutting passes and crossing different length and time scales. The predicted residual stress profiles are validated in turning Inconel 718 superalloy under different edge geometries and process conditions.

Are optical fibers really uniform? Measurement of refractive index on a centimeter scale.

Many applications of optical fiber, such as specialized fiber Bragg gratings (FBGs), require high uniformity of a fiber's refractive index (RI) along its length. We show here that the mode effective index of most fibers is not constant, even on a short length scale. To help improve fiber manufacturing and selection, we demonstrate a technique for characterizing a meter length single-mode optical fiber's effective RI over a centimeter scale with a precision of 3×10-6 RI units (RIUs) and an absolute accuracy of 2×10-4 RIU. By writing several weak probe FBGs as frequency references and then measuring the frequency deviation of these probe FBGs along the length of the fiber with an optical frequency domain reflectometer, the RI distribution of the effective mode index may be found. We validate our measurements on reference and fibers under test with theoretical simulations.

Wound-induced Ca2+wave propagates through a simple release and diffusion mechanism

Damage-associated molecular patterns (DAMPs) are critical mediators of information concerning tissue damage from damaged cells to neighboring healthy cells. ATP acts as an effective DAMP when released into extracellular space from damaged cells. Extracellular ATP receptors monitor tissue damage and activate a Ca2+ wave in the surrounding healthy cells. How the Ca2+ wave propagates through cells after a wound is unclear. Ca2+ wave activation can occur extracellularly via external receptors or intracellularly through GAP junctions. Three potential mechanisms to propagate the Ca2+ wave are source and sink, amplifying wave, and release and diffusion. Both source and sink and amplifying wave regulate ATP levels using hydrolysis or secretion, respectively, whereas release and diffusion relies on dilution. Here we systematically test these hypotheses using a microfluidics assay to mechanically wound an epithelial monolayer in combination with direct manipulation of ATP hydrolysis and release. We show that a release and diffusion model sufficiently explains Ca2+-wave propagation after an epithelial wound. A release and diffusion model combines the benefits of fast activation at short length scales with a self-limiting response to prevent unnecessary inflammatory responses harmful to the organism.

SPH-[formula omitted] simulation of 2D turbulence driven by a moving cylinder

The aim of this paper is to describe how effectively a turbulence model for the particle method SPH (Monaghan 2011 denoted by SPH-ϵ) simulates 2D turbulence driven by a moving cylinder inside a square box with no-slip walls. The model makes few assumptions, conserves linear and angular momentum, satisfies a discrete version of Kelvin’s circulation theorem and is computationally efficient. Furthermore, the results from the model for decaying two dimensional turbulence are in good agreement with the experimental and computational results. The SPH turbulence model is based on a Lagrangian similar to that used for the Lagrangian averaged Navier–Stokes (LANS) turbulence model, but with a different smoothed velocity. The smoothed velocity preserves the shape of the spectrum of the unsmoothed velocity for short length scales, but reduces its magnitude by an amount which depends on a parameter ϵ, where 0≤ϵ≤1. When ϵ=0, there is no smoothing, and when ϵ=1 the smoothing is a maximum. We compare calculations with resolution given by the Reynolds length ℓR and those using SPH-ϵ with ϵ=0.8, and a coarser resolution 2ℓR. The second and fourth order velocity correlation functions, the energy spectrum and the mixing calculated using SPH-ϵ and the coarse resolution 2ℓR, are in good agreement with those using the resolution ℓR, but requires 8 times less work to integrate to the same time. The mixing induced by the turbulence is in good agreement with high resolution calculations.

Implementing structural slow light on short length scales: the photonic speed bump

One-dimensional (1D) infinite periodic systems exhibit vanishing group velocity and diverging density of states (DOS) near band edges. However, in practice, systems have finite sizes and inevitably this prompts the question of whether helpful physical quantities related to infinite systems, such as the group velocity that is deduced from the band structure, remain relevant in finite systems. For instance, one may wonder how the DOS divergence can be approached with finite systems. Intuitively, one may expect that the implementation of larger and larger DOS, or equivalently smaller and smaller group velocities, would critically increase the system length. Based on general 1D-wave-physics arguments, we demonstrate that the large slow-light DOS enhancement of periodic systems can be observed with very short systems, whose lengths scale with the logarithm of the inverse of the group velocities. The understanding obtained for 1D systems leads us to propose a novel sort of microstructure to enhance light-matter interaction, a sort of photonic speed bump that abruptly changes the speed of light by a few orders of magnitude without any reflection. We show that the DOS enhancements of speed bumps result from a classical electromagnetic resonance characterized by a single resonance mode and also that the nature and the properties of the resonance are markedly different from those of classical defect-mode photonic-crystal cavities.

Advances in Tip-Enhanced Near-Field Raman Microscopy Using Nanoantennas.

Tip-enhanced near-field Raman microscopy spectroscopy is a scanning probe technique that is capable of providing vibrational spectroscopic information on single nanoobjects and surfaces at (sub-) nanometer spatial resolution and high detection sensitivity. In this review, we first illustrate the physical principle of optical nanoantennas used in tip-enhanced near-field Raman microscopy and tip-enhanced Raman scattering (TERS) to efficiently couple light to Raman excitations on nanometer length scales. Although the antennas' electric near-field distributions are commonly understood to determine the spatial resolution, recent experiments showing subnanometer-resolved optical images put this understanding into question. This is because such images enter a regime in which classical electrodynamical descriptions might no longer be applicable and quantum plasmonic and atomistic effects could become relevant. After summarizing the current understanding of plasmonic phenomena at extremely short length scales, we discuss the different mechanisms contributing to the signal enhancement. In addition to the known contributions from electric-field and chemical enhancement, several new models have been proposed very recently that could provide important guidelines for the optimization of TERS experiments. We then review recent developments in the areas of antenna design, fabrication, and characterization. Finally, we briefly highlight recent applications to illustrate future directions of tip-enhanced near-field Raman microscopy and TERS.

Observation of Flexibility Reversal in DNA Bending

Several experiments on DNA looping show that DNA looping probability at short length scales is higher than the prediction of the wormlike chain model. This observation suggests that DNA becomes more flexible at large bending angles and hints at the possibility that sequence dependence of flexibility may not be universal across different bending regimes. Using a FRET-based DNA looping assay, we measured flexibilities of various DNA sequences (some of which include base pair mismatches) from the looping and unlooping rates. Surprisingly, we find a strong correlation between the measured looping and unlooping rates, which points to an apparent flexibility reversal: more flexible sequences in the unlooped state are more rigid in the looped state. To explain this counterintuitive finding, we present a few hypotheses that challenge our coarse-grained level understanding of DNA.