Sub-nanometer Length Scale Research Articles

Unveiling short-range magnetic correlations: The development of magnetic pair distribution function method at CSNS

Recent advancements in the magnetic pair distribution function (mPDF) analysis of neutron total scattering data provide a powerful approach to investigate local magnetic correlations in materials. This technique is promising for revealing short-range magnetic correlations at the sub-nanometer length scale directly in real space. It is particularly suitable for strongly correlated electron systems and geometrically frustrated magnets. In this study, the mPDF experiment is conducted at the multi-physics instrument (MPI) of the China Spallation Neutron Source, one of the latest neutron total scattering diffractometers in the world. We systematically benchmarked a series of important parameters related to the experimental setup and data processing for mPDF experiments at the MPI and similar instruments. This method not only advances the magnetic structure determination of fundamental materials but also opens the door for extending mPDF studies to more complicated and frontier magnetic systems that are challenging for conventional diffraction methods.

Nanocrystal residual strains and density layers enhance failure resistance in the cleithrum bone of evolutionary advanced pike fish

Failure-resistant designs are particularly crucial for bones subjected to rapid loading, as is the case for the ambush-hunting northern pike (Esox lucius). These fish have slim and low-density osteocyte-lacking bones. As part of the swallowing mechanism, the cleithrum bone opens and closes the jaw. The cleithrum needs sufficient strength and damage tolerance, to withstand years of repetitive rapid gape-and-suck cycles of feeding. The thin wing-shaped bone comprises anisotropic layers of mineralized collagen fibers that exhibit periodic variations in mineral density on the mm and micrometer length scales. Wavy collagen fibrils interconnect these layers yielding a highly anisotropic structure. Hydrated cleithra exhibit Young's moduli spanning 3-9 GPa where the yield stress of ∼40 MPa increases markedly to exceed ∼180 MPa upon drying. This 5x observation of increased strength corresponds to a change to brittle fracture patterns. It matches the emergence of compressive residual strains of ∼0.15% within the mineral crystals due to forces from shrinking collagen layers. Compressive stresses on the nanoscale, combined with the layered anisotropic microstructure on the mm length scale, jointly confer structural stability in the slender and lightweight bones. By employing a range of X-ray, electron and optical imaging and mechanical characterization techniques, we reveal the structure and properties that make the cleithra impressively damage resistant composites. Statement of significanceBy combining structural and mechanical characterization techniques spanning the mm to the sub-nanometer length scales, this work provides insights into the structural organization and properties of a resilient bone found in pike fish. Our observations show how the anosteocytic bone within the pectoral gridle of these fish, lacking any biological (remodeling) repair mechanisms, is adapted to sustain natural repeated loading cycles of abrupt jaw-gaping and swallowing. We find residual strains within the mineral apatite nanocrystals that contribute to forming a remarkably resilient composite material. Such information gleaned from bony structures that are different from the usual bones of mammals showcases how nature incorporates smart features that induce damage tolerance in bone material, an adaptation acquired through natural evolutionary processes.

Subnanometer Scale Mapping of Hydrogen Doping in Vanadium Dioxide.

Hydrogen donor doping of correlated electron systems such as vanadium dioxide (VO2) profoundly modifies the ground state properties. The electrical behavior of HxVO2 is strongly dependent on the hydrogen concentration; hence, atomic scale control of the doping process is necessary. It is however a nontrivial problem to quantitatively probe the hydrogen distribution in a solid matrix. As hydrogen transfers its sole electron to the material, the ionization mechanism is suppressed. In this study, a methodology mapping the doping distribution at subnanometer length scale is demonstrated across a HxVO2 thin film focusing on the oxygen-hydrogen bonds using electron energy loss spectroscopy (EELS) coupled with first-principles EELS calculations. The hydrogen distribution was revealed to be nonuniform along the growth direction and between different VO2 grains, calling for intricate hydrogenation mechanisms. Our study points to a powerful approach to quantitatively map dopant distribution in quantum materials relevant to energy and information sciences.

Nanoscale Kirkendall shift in thin films studied using x-ray waveguide structures

Kirkendall effect has played an important role in developing the understanding of atomic diffusion and mass transport in solids. In thin films and multilayers, in which the microstructure may be far from thermal equilibrium, behavior of a system can be very different from that of the bulk. Therefore, there is a great need to understand the peculiarities of Kirkendall shift occurring at nanometer or sub-nanometer length scale in such systems. In the present work, Kirkendall shift in thin films has been measured with sub-nanometer accuracy, by making use of x-ray planar waveguide structures. Using this technique, we report an apparently counterintuitive result of Kirkendall shift associated with self-diffusion in nanocrystalline iron. The cavity of the waveguide consists of two layers of iron having different microstructures, separated by a thin marker layer of Ag. Thermal annealing results in shift of the Ag marker towards the Fe layer having a coarser grain structure. The effect is established unambiguously by studying two samples with the order of the two Fe layers reversed, and a third sample with the two Fe layers having the same microstructure. The result can be understood in terms of the difference in the microstructure of the two Fe layers; difference in the grain size and hence different concentration of the grain boundaries in the two layers results in a net mass transfer from one region to the other.

Subnanometer Visualization of Spatially Varying Local Field Resonances that Drive Tip-Enhanced Optical Spectroscopy.

Our knowledge of the electromagnetic fields that power modern nanoscale optical measurements, including (non)linear tip-enhanced Raman and photoluminescence, chiefly stems from numerical simulations. Aside from idealized in silico vs heterogeneous (nano)structures in the laboratory, challenges in quantitative descriptions of nanoscale light-matter interactions more generally stem from the very nature of the problem, which lies at the interface of classical and quantum theories. This is particularly the case in ultrahigh spatial resolution measurements that are sensitive to local optical field variations that take place on subnanometer length scales. This work approaches this challenge through extinction-based spectral nanoimaging experiments. We demonstrate <1 nm spatial resolution in hyperspectral extinction measurements that track spatially varying plasmon resonances. We describe the principles behind our experiments and highlight more general implications of our observations.

Quantum hydrodynamic model for noble metal nanoplasmonics

The quantum hydrodynamic model (QHDM) has become a versatile and efficient tool for plasmonics at nanometer and even subnanometer length scales, but the theory is principally applicable only to simple metals. For the most common plasmonic materials, i.e., noble metals, QHDM has not been duly justified to describe their optical responses. In particular, when noble metal structures interface with dielectric materials, electron spillover plays an important role. Although a background optical permittivity is usually invoked in Amp\`ere's law to encompass the $d$-band electrons' effect on optical responses in a direct manner, their indirect effects, e.g., acting via conduction electrons, still remain elusive. Here we identify these indirect effects and develop a near-field potential ${U}_{\mathrm{aff}}$ refined QHDM (U-QHDM), in which the $d$-band electrons' effects on the conduction electrons are recognized and the electrostatic properties of dielectric are incorporated. We report the detailed procedures of calibrating U-QHDM. Material-independent model parameters are fitted with density functional theory (DFT) calculations for simple metals. Last, the calibrated U-QHDM is benchmarked against DFT and time-dependent DFT within stabilized jellium model for Ag, Au, and Al nanostructures interfacing with dielectrics. We expect U-QHDM will be a valuable asset for the rapidly developing field of extreme nanophotonics.

Structural rationale for boson peak in metallic glass informed by an interpretable neural network model

In amorphous solids, there exists a universal phenomenon of vibrational anomaly, i.e., an excess of vibrational modes over the Debye level in the low-terahertz-frequency regime, which is termed a boson peak. Although the origin of the boson peak has been studied extensively for decades, quantitative prediction of its intensity remains elusive. In this paper, we try to quantify the intensity of the boson peak via an interpretable machine-learning strategy based on a dataset consisting of &gt;1600 glass samples prepared with cooling rates spanning four orders of magnitude. We have attempted to extract information from a pure static structure and vibrational localization through four different feature inputs, among which the partial pair distribution function (PDF) yields the best predictive performance. It is found that the first several neighbor shells corresponding to a characteristic subnanometer length scale are important for capturing the structural genes of the boson peak in amorphous solids, regardless of the size of the glasses. Moreover, a higher boson peak relates to a more disordered atomic arrangement with lower peaks and shallower basins in the PDF. The obtained knowledge sheds light on rationalization of the boson peak in amorphous solids from pure structural information.

Characterizing Transient Protein-Protein Interactions by Trp-Cys Quenching and Computer Simulations.

Transient protein-protein interactions occur frequently under the crowded conditions encountered in biological environments. Tryptophan-cysteine quenching is introduced as an experimental approach with minimal labeling for characterizing such interactions between proteins due to its sensitivity to nano- to microsecond dynamics on subnanometer length scales. The experiments are paired with computational modeling at different resolutions including fully atomistic molecular dynamics simulations for interpretation of the experimental observables and to gain molecular-level insights. This approach is applied to model systems, villin variants and the drkN SH3 domain, in the presence of protein G crowders. It is demonstrated that Trp-Cys quenching experiments can differentiate between overall attractive and repulsive interactions between different proteins, and they can discern variations in interaction preferences at different protein surface locations. The close integration between experiment and simulations also provides an opportunity to evaluate different molecular force fields for the simulation of concentrated protein solutions.

The dielectric function profile across the water interface through surface-specific vibrational spectroscopy and simulations

The dielectric properties of interfacial water on subnanometer length scales govern chemical reactions, carrier transfer, and ion transport at interfaces. Yet, the nature of the interfacial dielectric function has remained under debate as it is challenging to access the interfacial dielectric with subnanometer resolution. Here we use the vibrational response of interfacial water molecules probed using surface-specific sum-frequency generation (SFG) spectra to obtain exquisite depth resolution. Different responses originate from water molecules at different depths and report back on the local interfacial dielectric environment via their spectral amplitudes. From experimental and simulated SFG spectra at the air/water interface, we find that the interfacial dielectric constant changes drastically across an ∼1 Å thin interfacial water region. The strong gradient of the interfacial dielectric constant leads, at charged planar interfaces, to the formation of an electric triple layer that goes beyond the standard double-layer model.

Elevating Surface-Enhanced Infrared Absorption with Quantum Mechanical Effects of Plasmonic Nanocavities

Plasmonic nanocavities, with the ability to localize and concentrate light into nanometer-scale dimensions, have been widely used for ultrasensitive spectroscopy, biosensing, and photodetection. However, as the nanocavity gap approaches the subnanometer length scale, plasmonic enhancement, together with plasmonic enhanced optical processes, turns to quenching because of quantum mechanical effects. Here, instead of quenching, we show that quantum mechanical effects of plasmonic nanocavities can elevate surface-enhanced infrared absorption (SEIRA) of molecular moieties. The plasmonic nanocavities, nanojunctions of gold and cadmium oxide nanoparticles, support prominent mid-infrared plasmonic resonances and enable SEIRA of an alkanethiol monolayer (CH3(CH2)n-1SH, n = 3-16). With a subnanometer cavity gap (n < 6), plasmonic resonances turn to blue shift and the SEIRA signal starts a pronounced increase, benefiting from the quantum tunneling effect across the plasmonic nanocavities. Our findings demonstrate the new possibility of optimizing the field enhancement and SEIRA sensitivity of mid-infrared plasmonic nanocavities.

Discriminating protein tags on a dsDNA construct using a Dual Nanopore Device

We report Brownian dynamics simulation results with the specific goal to identify key parameters controlling the experimentally measurable characteristics of protein tags on a dsDNA construct translocating through a double nanopore setup. First, we validate the simulation scheme in silico by reproducing and explaining the physical origin of the asymmetric experimental dwell time distributions of the oligonucleotide flap markers on a 48 kbp long dsDNA at the left and the right pore. We study the effect of the electric field inside and beyond the pores, critical to discriminate the protein tags based on their effective charges and masses revealed through a generic power-law dependence of the average dwell time at each pore. The simulation protocols monitor piecewise dynamics at a sub-nanometer length scale and explain the disparate velocity using the concepts of nonequilibrium tension propagation theory. We further justify the model and the chosen simulation parameters by calculating the Péclet number which is in close agreement with the experiment. We demonstrate that our carefully chosen simulation strategies can serve as a powerful tool to discriminate different types of neutral and charged tags of different origins on a dsDNA construct in terms of their physical characteristics and can provide insights to increase both the efficiency and accuracy of an experimental dual-nanopore setup.

Size-Dependent Order-Disorder Crossover in Hydrophobic Hydration: Comparison between Spherical Solutes and Linear Alcohols.

Theory and computer simulation studies have predicted that water molecules around hydrophobic molecules should undergo an order–disorder transition with increasing solute size around a 1 nm length scale. Some theories predict the formation of a clathrate-like ordered structure around smaller hydrophobic solutes (<1 nm) and the formation of disordered vapor–liquid interfaces around larger solutes (>1 nm) and surfaces. Experimental validation of these predictions has often been elusive and contradictory. High-resolution Raman spectroscopy has detected that water around small hydrophobic solutes shows a signature similar to that of bulk water at lower temperature (increased ordering and a stronger hydrogen-bonded network). Similarly, water around larger solutes shows an increasing population of dangling OH bonds very similar to higher temperature bulk water. Thus, the solute size dependence of the structure and dynamics of water around hydrophobic molecules seems to have an analogy with the temperature dependence in bulk water. In this work, using atomistic classical molecular dynamics (MD) simulations, we have systematically investigated this aspect and characterized this interesting analogy. Structural order parameters including the tetrahedral order parameter (Q), hydrogen bond distribution, and vibrational power spectrum highlight this similarity. However, in contrast to the experimental observations, we do not observe any length-dependent crossover for linear hydrophobic alcohols (n-alkanols) using classical MD simulations. This is in agreement with earlier findings that linear alkane chains do not demonstrate the length-dependent order–disorder transition due to the presence of a sub-nanometer length scale along the cross section of the chain. Moreover, the collapsed state of linear hydrocarbon chains is not significantly populated for smaller chains (number of carbons below 20). In the context of our computational results, we raise several pertinent questions related to the sensitivity of various structural and dynamical parameters toward capturing these complex phenomena of hydrophobic hydration.

Structural Information on Bacterial Amyloid and Amyloid-DNA Complex Obtained by Small-Angle NeutronorX-Ray Scattering.

Small-angle scattering is a powerful technique to obtain structural information on biomacromolecules in aqueous solution at the sub-nanometer and nanometer length scales. It provides the sizes and overall shapes of the scattering particles. While small-angle X-ray scattering (SAXS) has often been used for structural analysis of a single-component system, small-angle neutron scattering (SANS) has been used to reveal the internal organization of a multicomponent system such as protein-protein and protein-DNA complexes. This is due to the fact that the neutron scattering length is largely different between hydrogen and deuterium, and thus it allows to make a specific component in complexes "invisible" to neutrons by changing the H2O/D2O ratio in the solvent with or without molecular deuteration. In this chapter, we describe a method to characterize the biomolecular structures using SANS and SAXS, in particular, focusing on fibrillar proteins such as bacterial amyloids and their complexes with nucleic acids.

Novel Ultrabright and Air-Stable Photocathodes Discovered from Machine Learning and Density Functional Theory Driven Screening.

The high brightness, low emittance electron beams achieved in modern X-ray free-electron lasers (XFELs) have enabled powerful X-ray imaging tools, allowing molecular systems to be imaged at picosecond time scales and sub-nanometer length scales. One of the most promising directions for increasing the brightness of XFELs is through the development of novel photocathode materials. Whereas past efforts aimed at discovering photocathode materials have typically employed trial-and-error-based iterative approaches, this work represents the first data-driven screening for high brightness photocathode materials. Through screening over 74 000 semiconducting materials, a vast photocathode dataset is generated, resulting in statistically meaningful insights into the nature of high brightness photocathode materials. This screening results in a diverse list of photocathode materials that exhibit intrinsic emittances that are up to 4x lower than currently used photocathodes. In a second effort, multiobjective screening is employed to identify the family of M2 O (M = Na, K, Rb) that exhibits photoemission properties that are comparable to the current state-of-the-art photocathode materials, but with superior air stability. This family represents perhaps the first intrinsically bright, visible light photocathode materials that are resistant to reactions with oxygen, allowing for their transport and storage in dry air environments.

Electron tomography for functional nanomaterials

Abstract

Nucleation Effects in the Atomic Layer Deposition of Nickel–Aluminum Oxide Thin Films

Atomic layer deposition (ALD) has become an important technique to synthesize a wide variety of materials on the sub-nanometer length scale. Expanding the library of ALD into ternary materials is vital for applications in which ternary materials allow for tuning of physical, optical, and electronic properties. In this work, we demonstrate the first report of nickel-aluminum oxide (Ni-Al-O) films deposited by atomic layer deposition (ALD), using nickelocene-ozone and trimethylaluminum-water as reactants. While deposition of a wide range of compositions is achieved, the observed growth per cycle (GPC) did not follow the simple combination of GPCs observed for the binary ALD processes. Nucleation studies performed to better understand this behavior reveal that the deposition of aluminum oxide is greatly enhanced on a NiO surface prepared by nickelocene and ozone. A model is developed that considers nucleation effects to predict composition and thickness as a function of supercycle recipe. Characterization of...

Insights into the Emerging Networks of Voids in Simulated Supercooled Water.

The structural evolution of supercooled liquid water as we approach the glass transition temperature continues to be an active area of research. Here, we use molecular dynamics simulations of TIP4P/ice water to study the changes in the connected regions of empty space within the liquid, which we investigate using the Voronoi-voids network. We observe two important features: supercooling enhances the fraction of nonspherical voids and different sizes of voids tend to cluster forming a percolating network. By examining order parameters such as the local structure index (LSI), tetrahedrality and topological defects, we show that water molecules near large void clusters tend to be slightly more tetrahedral than those near small voids, with a lower population of under- and overcoordinated defects. We show further that the distribution of closed rings of water molecules around small and large void clusters maintain a balance between 6 and 7 membered rings. Our results highlight the changes of the dual voids and water network as a structural hallmark of supercooling and provide insights into the molecular origins of cooperative effects underlying density fluctuations on the subnanometer and nanometer length scale. In addition, the percolation of the voids and the hydrogen bond network around the voids may serve as useful order parameters to investigate density fluctuations in supercooled water.

Recording interfacial currents on the subnanometer length and femtosecond time scale by terahertz emission.

Electron dynamics at interfaces is a subject of great scientific interest and technological importance. Detailed understanding of such dynamics requires access to the angstrom length scale defining interfaces and the femtosecond time scale characterizing interfacial motion of electrons. In this context, the most precise and general way to remotely measure charge dynamics is through the transient current flow and the associated electromagnetic radiation. Here, we present quantitative measurements of interfacial currents on the subnanometer length and femtosecond time scale by recording the emitted terahertz radiation following ultrafast laser excitation. We apply this method to interlayer charge transfer in heterostructures of two transition metal dichalcogenide monolayers less than 0.7 nm apart. We find that charge relaxation and separation occur in less than 100 fs. This approach allows us to unambiguously determine the direction of current flow, to demonstrate a charge transfer efficiency of order unity, and to characterize saturation effects.

Dumbbell gold nanoparticle dimer antennas with advanced optical properties.

Plasmonic nanoantennas have found broad applications in the fields of photovoltaics, electroluminescence, non-linear optics and for plasmon enhanced spectroscopy and microscopy. Of particular interest are fundamental limitations beyond the dipolar approximation limit. We introduce asymmetric gold nanoparticle antennas (AuNPs) with improved optical near-field properties based on the formation of sub-nanometer size gaps, which are suitable for studying matter with high-resolution and single molecule sensitivity. These dumbbell antennas are characterized in regard to their far-field and near-field properties and are compared to similar dimer and trimer antennas with larger gap sizes. The tailoring of the gap size down to sub-nanometer length scales is based on the integration of rigid macrocyclic cucurbituril molecules. Stable dimer antennas are formed with an improved ratio of the electromagnetic field enhancement and confinement. This ratio, taken as a measure of the performance of an antenna, can even exceed that exhibited by trimer AuNP antennas composed of comparable building blocks with larger gap sizes. Fluctuations in the far-field and near-field properties are observed, which are likely caused by distinct deviations of the gap geometry arising from the faceted structure of the applied colloidal AuNPs.

Light-cured dimethacrylate dental restorative composites under a prism of annihilating positrons.

Breakthrough resolutions in current biopolymer engineering rely on reliable diagnostics of atomic-deficient spaces over the finest sub-nanometer length scales. One such diagnostic is positron annihilation lifetime spectroscopy, which probes space-time continuum relationships for the interaction between electrons and their antiparticle (positrons) in structural entities like free-volume defects, vacancies, vacancy-like clusters, interfacial voids and pores, etc. This paper is intended to highlight the possibilities of positron annihilation lifetime spectroscopy as an informative instrumentation tool to parameterize free-volume evolution in light-cured dimethacrylate dental restorative composites exemplified by Charisma® (Heraeus Kulzer GmbH, Hanau, Germany) and Dipol® (Oksomat-AN Ltd, Kyiv, Ukraine). The subjects of the study were the commercially available dimethacrylate-type dental restorative composites Charisma® and Dipol®. The analysis used a fast-fast coincidence system of 230 ps resolution based on 2 photomultiplier tubes coupled to BaF2 scintillator detectors and ORTEC® (ORTEC, Oak Ridge, USA) electronics to register lifetime spectra in normal-measurement statistics evolving ~1 million coincidences. The annihilation process in both composites is identified as mixed positron-Ps (positronium) trapping, where ortho-Ps decaying is caused entirely by free-volume holes in the polymer matrix, and the 2nd component is defined mainly by interfacial free-volume holes between filler nanoparticles and the surrounding polymer. The most appropriate model-independent estimation of photopolymerization volumetric shrinkage in dental restorative composites can be done using averaged positron annihilation lifetime. Partiallyconstrained x4-term analysis of lifetime spectra is less efficient, giving greater scatter of variance with an additional artifact of fixed shortest lifetime allowing unresolved mixing in the 2nd component. A meaningful phenomenological description of transformations in Ps and positron-trapping sites under light curing, which occurs more efficiently in Charisma® than in Dipol® nanocomposites, can be developed at the basis of a semi-empirical model exploring a x3-x2-coupling decomposition algorithm. A deep understanding of void-evolution processes in dimethacrylate dental composites employing positron annihilation lifetime spectroscopy makes it possible to diagnose, characterize and engineer novel biomaterials for advanced use in medical practice.