Atomic Length Scales Research Articles

Towards surface diffusion potential mapping on atomic length scale

The surface diffusion potential landscape plays an essential role in a number of physical and chemical processes such as self-assembly and catalysis. Diffusion energy barriers can be calculated theoretically for simple systems, but there is currently no experimental technique to systematically measure them on the relevant atomic length scale. Here, we introduce an atomic force microscopy based method to semiquantitatively map the surface diffusion potential on an atomic length scale. In this proof of concept experiment, we show that the atomic force microscope damping signal at constant frequency-shift can be linked to nonconservative processes associated with the lowering of energy barriers and compared with calculated single-atom diffusion energy barriers.

(Invited) Atomic-Scale Exploration of Synthetic Low Dimensional Materials

Low-dimensional materials functioning at the nanoscale are a critical component for a variety of current and future technologies. From the optimization of light harvesting solar technologies to large-scale catalytic processes, key physical phenomena are occurring at the nanometer and atomic length-scales and predominately at interfaces. For instance, graphene is a nearly ideal two-dimensional conductor that is comprised of a single sheet of hexagonally packed carbon atoms. In order fully realize the potential of graphene for novel electronic applications, large-scale synthesis of high quality graphene and the ability to control the electronic properties of this material on a nanometer length scale are key challenges. In addition to graphene, we are interested in exploring the synthesis of low-dimensional materials that do not occur in nature. This talk will highlight how scanning probe microscopy presents a series of powerful experimental tools that can overcome several challenges and allow for the direct characterization of several advanced materials. This talk will also cover our most recent discovery and synthesis of new two-dimensional (2D) boron allotropes (borophenes). This discovery of metallic 2D sheets of boron presents almost ideal example of the synergy between predictive modeling resulting in the experimental realization of a designer materials.

Molecular modelling of epoxy resin crosslinking experimentally validated by near-infrared spectroscopy

Reliable simulation of polymers on an atomistic length scale requires a realistic representation of the cured material. A molecular modelling method for the curing of epoxy systems is presented, which is developed with respect to efficiency while maintaining a well equilibrated system. The main criterion for bond formation is the distance between reactive groups and no specific reaction probability is prescribed. The molecular modelling is studied for three different mixing ratios with respect to the curing evolution of reactive groups and the final curing stage. For the first time, the evolution of reactive groups during the curing process predicted by the molecular modelling is validated with near-infrared spectroscopy data, showing a good agreement between simulation results and experimental measurements. With the proposed method, deeper insights into the curing mechanism of epoxy systems can be gained and it allows us to provide reliable input data for molecular dynamics simulations of material properties.

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.

Investigation of Triple-Helix Collagen Hydroxylation by Solid-State NMR Spectroscopy.

Solid-state nuclear magnetic resonance spectroscopy (ssNMR) is an emerging technique in structural methods of studying collagen proteins, capable of identifying features on an atomic length scale in tissues and protein samples without extensive extraction or purification. Hydroxylation is a key posttranslational modification of collagen that gives rise to distinctive signals in the ssNMR spectrum of collagen proteins. Here we outline the type of information that ssNMR can provide and describe the procedures involved in a ssNMR structural study, with particular focus on using dynamic nuclear polarization to enhance sensitivity for detecting hydroxylysine residues by ssNMR.

Effect of processing and end groups on the crystal structure of polypeptoids studied by cryogenic electron microscopy at atomic length scales.

Cryogenic electron microscopy at atomic length scales was used to study the structure of self-assembled crystalline nanosheets obtained from a series of polypeptoids with the same chain architecture but with different end groups. While long-range order is enhanced by slowing down the self-assembly process, the dominant crystalline motif was found to be a sensitive function of both processing details and end group chemistry. In some cases, adjacent rows of polypeptoid molecules adopt anti-parallel V-shaped side chain conformations. In other cases, adjacent rows of polypeptoid molecules adopt parallel V-shaped side chain conformations. Interestingly, the unit cell is rectangular in both cases with dimensions a = 4.5 Å and c = 50 Å. In all cases, long-range order, quantified by the average number of concatenated unit cells of the same type, is more prevalent along the a direction.

Kinetics of the Topochemical Transformation of (PbSe) m(TiSe2) n(SnSe2) m(TiSe2) n to (Pb0.5Sn0.5Se) m(TiSe2) n.

Solid-state reaction kinetics on atomic length scales have not been heavily investigated due to the long times, high reaction temperatures, and small reaction volumes at interfaces in solid-state reactions. All of these conditions present significant analytical challenges in following reaction pathways. Herein we use in situ and ex situ X-ray diffraction, in situ X-ray reflectivity, high-angle annular dark field scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy to investigate the mechanistic pathways for the formation of a layered (Pb0.5Sn0.5Se)1+δ(TiSe2) m heterostructure, where m is the varying number of TiSe2 layers in the repeating structure. Thin film precursors were vapor deposited as elemental-modulated layers into an artificial superlattice with Pb and Sn in independent layers, creating a repeating unit with twice the size of the final structure. At low temperatures, the precursor undergoes only a crystallization event to form an intermediate (SnSe2)1+γ(TiSe2) m(PbSe)1+δ(TiSe2) m superstructure. At higher temperatures, this superstructure transforms into a (Pb0.5Sn0.5Se)1+δ(TiSe2) m alloyed structure. The rate of decay of superlattice reflections of the (SnSe2)1+γ(TiSe2) m(PbSe)1+δ(TiSe2) m superstructure was used as the indicator of the progress of the reaction. We show that increasing the number of TiSe2 layers does not decrease the rate at which the SnSe2 and PbSe layers alloy, suggesting that at these temperatures it is reduction of the SnSe2 to SnSe and Se that is rate limiting in the formation of the alloy and not the associated diffusion of Sn and Pb through the TiSe2 layers.

In-situ visualizing atomic structural evolution during crystallization in ternary Zr[sbnd]Cu[sbnd]Al bulk metallic glasses

A well-designed experimental method has been presented to in-situ visualize the amorphous-to-crystalline phase transformation of two glass formers at the atomic-length scale in the supercooled liquid region using a high voltage electron microscopy (HVEM). Analysis of the HVEM high-resolution images, supported by the in-situ synchrotron diffraction, further confirms previous observations of distinctively different crystallization pathways in the two ZrCuAl alloys. Moreover, the HVEM results illustrate that isolated distributed nanocrystals with mutual orientation easily grew up from the Zr56Cu36Al8, an average glass former, which follows a classical crystallization pathway; while density population poorly ordered atomic clusters with large misorientation suspended the growth in the Zr46Cu46Al8, a good glass former, which might follow an unusual crystallization pathway. In addition, in-situ synchrotron diffraction measurements confirm that the Zr56Cu36Al8 alloy finally crystallized into an extended structure, in contrast, the final crystalline product of Zr46Cu46Al8 alloy possesses a damped structure. Our study provides a detailed microscopic understanding of the crystallization behaviors in the supercooled liquids, showing that the density population nucleation site with large misorientation and the confined final crystalline structure contribute to the stability of ZrCuAl supercooled liquids.

Influence of solvent granularity on the effective interactions between two overall neutral surfaces with quenched charge heterogeneity

By using classical density functional theory of an electrolyte solvent primitive model (SPM), we investigate influences of solvent granularity on the effective interactions between two overall neutral planar surfaces with quenched charge heterogeneity over atomic length scale. Main conclusions are summarized as follows: (i) with consideration of the solvent granularity, the potential energy curves can (but not always) be highly oscillatory with a period approximately equal to the particle diameter. (ii) The present uelec∗ curve can change obviously with the bulk electrolyte concentration of the SPM in contrast with the homogeneously charged surfaces, but the concentration dependence of the uelec∗ curve will decrease as the charge separation extent decreases. (iii) Surface charge distributions, electrolyte types and bulk concentrations influence on the uelec∗ curve synergistically. For asymmetrical surface charge distributions, the electrolyte types and charge separation extent do not influence the overall uelec∗ curve shape evidently except that the first valley moves down with the electrolyte valency; by contrast, the overall morphologies of the uelec∗ curves for symmetrical surface charge distributions show features: first, for high valency salt and low bulk concentrations the uelec∗ is purely repulsive; only after the concentration becomes high enough, the alternating repulsive peaks and attractive valleys start appearing but still with weak strength. Second, in the presence of only low valency salt, the uelec∗ curves approximately as a whole (particularly the main valley) move up at low concentrations, and then move down at higher concentrations. Third, the concentration effect in the symmetrical surface charge distributions is not so obvious as that in the asymmetrical distributions. Using liquid theory arguments, all of these observations can be explained logically. Significance1.Influence of solvent granularity on the potential energy curve is investigated.2.Influence of quenched charge heterogeneity on effective interaction is analyzed.3.Concentration dependence of potential curve increases with charge separation extent.

Efficient Charge Separation in 2D Janus van der Waals Structures with Built-in Electric Fields and Intrinsic p–n Doping

Janus MoSSe monolayers have been recently synthesized by replacing S by Se on one side of MoS2 (or vice versa for MoSe2). Due to the different electronegativities of S and Se, these structures carry a finite out-of-plane dipole moment. As we show here by means of density functional theory calculations, this intrinsic dipole leads to the formation of built-in electric fields when the monolayers are stacked to form N-layer structures. For sufficiently thin structures (N < 4), the dipoles add up and shift the vacuum level on the two sides of the film by ∼N·0.7 eV. For thicker films, the vacuum level shift saturates at around 2.2 eV due to compensating surface charges, which in turn leads to the formation of atomically thin n- and p-doped electron gasses at the surfaces. The doping concentration can be tuned between 5 × 1012 and 2 × 1013 e/cm2 by varying the film thickness. On the basis of band structure calculations and the Mott–Wannier exciton model, we compute the energies of intra- and interlayer excitons as a function of film thickness, suggesting that the Janus multilayer films are ideally suited for achieving ultrafast charge separation over atomic length scales without chemical doping or applied electric fields. Finally, we explore a number of other potentially synthesizable two-dimensional Janus structures with different band gaps and internal dipole moments. Our results open new opportunities for ultrathin optoelectronic components, such as tunnel diodes, photodetectors, or solar cells.

Imaging Unstained Synthetic Polymer Crystals and Defects on Atomic Length Scales Using Cryogenic Electron Microscopy

Properties of soft crystalline materials such as synthetic polymers are governed by locations of constituent atoms. Determining atomic-scale structures in these materials is difficult because they degrade rapidly when studied by electron microscopy, and techniques such as X-ray scattering average over volumes much larger than coherent blocks of the unit cells. We obtained cryo-electron microscopy images of self-assembled nanosheets of a peptoid polymer, made by solid-phase synthesis, in which we see a variety of crystalline motifs. A combination of crystallographic and single-particle methods, developed for cryo-electron microscopy of biological macromolecules, was used to obtain high-resolution images of the crystals. Individual crystals contain grains that are mirror images of each other with concomitant grain boundaries. We have used molecular dynamic simulations to build an atomic model of the crystal structure to facilitate the interpretation of electron micrographs. Direct visualization of crystalli...

Absolute timing of the photoelectric effect.

Photoemission spectroscopy is central to understanding the inner workings of condensed matter, from simple metals and semiconductors to complex materials such as Mott insulators and superconductors1. Most state-of-the-art knowledge about such solids stems from spectroscopic investigations, and use of subfemtosecond light pulses can provide a time-domain perspective. For example, attosecond (10-18 seconds) metrology allows electron wave packet creation, transport and scattering to be followed on atomic length scales and on attosecond timescales2-7. However, previous studies could not disclose the duration of these processes, because the arrival time of the photons was not known with attosecond precision. Here we show that this main source of ambiguity can be overcome by introducing the atomic chronoscope method, which references all measured timings to the moment of light-pulse arrival and therefore provides absolute timing of the processes under scrutiny. Our proof-of-principle experiment reveals that photoemission from the tungsten conduction band can proceed faster than previously anticipated. By contrast, the duration of electron emanation from core states is correctly described by semiclassical modelling. These findings highlight the necessity of treating the origin, initial excitation and transport of electrons in advanced modelling of the attosecond response of solids, and our absolute data provide a benchmark. Starting from a robustly characterized surface, we then extend attosecond spectroscopy towards isolating the emission properties of atomic adsorbates on surfaces and demonstrate that these act as photoemitters with instantaneous response. We also find that the tungsten core-electron timing remains unchanged by the adsorption of less than one monolayer of dielectric atoms, providing a starting point for the exploration of excitation and charge migration in technologically and biologically relevant adsorbate systems.

Force induced removal of an encapsulated semi-flexible polymer from single walled carbon nanotube

Encapsulation of a variety of atoms, molecules, polymers, bio-polymers into a Single Wall Carbon Nanotube (SWCNT) depict unexpected properties such as unique morphology, physical properties, high thermal stability etc. The encapsulation and pull-out processes are very important parts of synthesis and characterization of new materials at atomic length scale, which controls the overall desirable properties. Here, we investigate the encapsulation and the pullout process of a long-polymer chain into/from a SWCNT by molecular dynamics (MD). However, during externally applied force induced pull-out process of polymer chain from SWCNT depict a significant transition in an organization with the creation of additional surface area. We have observed that the amount of applied force highly influences the organization and creation of additional surface area of polymer chain during pull-out process.

Fast Na ion transport triggered by rapid ion exchange on local length scales

The realization of green and economically friendly energy storage systems needs materials with outstanding properties. Future batteries based on Na as an abundant element take advantage of non-flammable ceramic electrolytes with very high conductivities. Na3Zr2(SiO4)2PO4-type superionic conductors are expected to pave the way for inherently safe and sustainable all-solid-state batteries. So far, only little information has been extracted from spectroscopic measurements to clarify the origins of fast ionic hopping on the atomic length scale. Here we combined broadband conductivity spectroscopy and nuclear magnetic resonance (NMR) relaxation to study Na ion dynamics from the µm to the angstrom length scale. Spin-lattice relaxation NMR revealed a very fast Na ion exchange process in Na3.4Sc0.4Zr1.6(SiO4)2PO4 that is characterized by an unprecedentedly high self-diffusion coefficient of 9 × 10−12 m2s−1 at −10 °C. Thus, well below ambient temperature the Na ions have access to elementary diffusion processes with a mean residence time τNMR of only 2 ns. The underlying asymmetric diffusion-induced NMR rate peak and the corresponding conductivity isotherms measured in the MHz range reveal correlated ionic motion. Obviously, local but extremely rapid Na+ jumps, involving especially the transition sites in Sc-NZSP, trigger long-range ion transport and push ionic conductivity up to 2 mS/cm at room temperature.

Reprint of Multiscale model approaches to the design of advanced permanent magnets

We describe the process of multiscale modelling of magnetic materials, based on atomistic models coupled parametrically to micromagnetic calculations. At the atomistic lengthscale we use Spin Dynamics (SD) to study switching mechanisms, using structures predicted by Molecular Dynamics. The process is completed using SD to calculate the cell size and temperature dependent parameters for micromagnetic calculations. We demonstrate an unusually weak cell size scaling for Nd2Fe14B and demonstrate numerically the existence of atomic scale Barkhausen jumps during magnetization switching. Scaling of magnetic properties is shown to be important in micromagnetic calculations of hysteresis, especially considering variation in micromagnetic cell size.

Antiferromagnetic order without recourse to staggered fields

In the theory of antiferromagnetism, the staggered field---an external magnetic field that alternates in sign on atomic length scales---is used to select the classical N\'eel state from a quantum magnet, but justification is missing. This work examines, within the decoherence framework, whether repeated $local$ measurement can replace a staggered field. Accordingly, the conditions under which local decoherence can be considered a continuous measurement are studied. The dynamics of a small magnetic system is analysed to illustrate that local decoherence can lead to (symmetry-broken) order similar to order resulting from a staggered field.

Integration of external electric fields in molecular dynamics simulation models for resistive switching devices

Resistive switching devices emerged a huge amount of interest as promising candidates for non-volatile memories as well as artificial synapses due to their memristive behavior. The main physical and chemical phenomena which define their functionality are driven by externally applied voltages and the resulting electric fields. Although molecular dynamics simulations are widely used in order to describe the dynamics on the corresponding atomic length and time scales, there is a lack of models which allow for the actual driving force of the dynamics, i.e., externally applied electric fields. This is due to the restriction of currently applied models to solely conductive, non-reactive, or insulating materials, with thicknesses on the order of the potential cutoff radius, i.e., 10 Å. In this work, we propose a generic model, which can be applied in particular to describe the resistive switching phenomena of metal-insulator-metal systems. It has been shown that the calculated electric field and force distribution in case of the chosen example system Cu/a-SiO2/Cu are in agreement with the fundamental field theoretical expectations.

Mesoscopic friction and network morphology control the mechanics and processing of carbon nanotube yarns

Carbon nanotube (CNT) yarns are synthetic nanomaterials of interest for diverse applications. To bring fundamental understanding into the yarn formation process, we perform mesoscopic scale distinct element method (mDEM) simulations for the stretching of CNT networks. The parameters used by mDEM, including the mesoscale friction, are based on full atomistic results. By bridging across the atomistic and mesoscopic length scales, our model predicts accurately the mechanical response of the network over a large deformation range. At small and moderate deformations, the microstructural evolution is dominated by zipping relaxations directed along the applied strain direction. At larger deformations, the occurrence of energetic elasticity promotes yarn densification, by lowering CNT waviness and eliminating squashed pores. Next, by varying the mesoscopic dissipation as well as the network structure, we reveal that the mesoscale friction and film morphology are key factors for the yarn formation: While lack of friction compromises the strain-induced alignment process, phononic and polymeric friction promote CNT alignment by enabling load transfer and directed zipping relaxations, especially in networks containing long and entangled CNTs. Yarns drawn from cellular networks are shown instead to maintain high porosity, even with enhanced polymeric friction.

Probing Long-Range Neutrino-Mediated Forces with Atomic and Nuclear Spectroscopy.

The exchange of a pair of low-mass neutrinos between electrons, protons, and neutrons produces a "long-range" 1/r^{5} potential, which can be sought for in phenomena originating on the atomic and subatomic length scales. We calculate the effects of neutrino-pair exchange on transition and binding energies in atoms and nuclei. In the case of atomic s-wave states, there is a large enhancement of the induced energy shifts due to the lack of a centrifugal barrier and the highly singular nature of the neutrino-mediated potential. We derive limits on neutrino-mediated forces from measurements of the deuteron binding energy and transition energies in positronium, muonium, hydrogen, and deuterium, as well as isotope-shift measurements in calcium ions. Our limits improve on existing constraints on neutrino-mediated forces from experiments that search for new macroscopic forces by 18 orders of magnitude. Future spectroscopy experiments have the potential to probe long-range forces mediated by the exchange of pairs of standard-model neutrinos and other weakly charged particles.

Rapid Li Ion Dynamics in the Interfacial Regions of Nanocrystalline Solids.

Diffusive processes are ubiquitous in nature. In solid state physics, metallurgy and materials science the diffusivity of ions govern the functionality of many devices such as sensors or batteries. Motional processes on surfaces, across interfaces or through membranes can be quite different to that in the bulk. A direct, quantitative description of such local diffusion processes is, however, rare. Here, we took advantage of 7Li longitudinal nuclear magnetic relaxation to study, on the atomic length scale, the diffusive motion of lithium spins in the interfacial regions of nanocrystalline, orthorhombic LiBH4. Magnetization transients and free induction decays revealed a fast subset of Li ions having access to surface pathways that offer activation barriers (0.18 eV) much lower than those in the crystalline bulk regions (0.55 eV). These observations make orthorhombic borohydride a new nanostructured model system to study disorder-induced enhancements in interfacial diffusion processes.