Nanoscale Geometry Research Articles

Tunable and large plasmonic field enhancement in core-shell heterodimer/trimer

ABSTRACTHeterodimer/trimer are two/three closely adjacent nanoparticles dissimilar in shape or size, and represent a simple nanoscale geometry which gives rise to a remarkably rich set of properties. The core-shell nanostructure has advantages of tunability and stability over a similar homogeneous particle. We theoretically demonstrate tunable and significant field enhancement in both heterodimer and heterotrimer systems using core-shell geometry with a metal shell and dielectric core and compared our results with core-shell homodimer and homotrimer. The analytical modeling is based on the Rayleigh theory for small particles and Foldy–Lax multi-scattering theory for multiple particles. It is shown that the E-field can be enhanced in core-shell dimer/trimer due to the plasmonic resonance in the metal shell, which can be easily tunable by changing the core-shell ratio, composition of the core and shell, relative size and distance between the particles, and wavelength of the incident light. We report significant maximum field intensity enhancement near the particles. Moreover, this tunable nanostructures with high field intensity can be useful as potential bio-sensing applications.

Characterization of Dynamic and Nanoscale Materials and Metamaterials with Continuously Referenced Interferometry

AbstractThe development of new optical materials and metamaterials has seen a natural progression toward both nanoscale geometries and dynamic performance. The development of these materials, such as optical metasurfaces which impart discrete, spatially dependent phase shifts on incident light, often benefits from the measurement of transmitted or reflected phase. Careful measurement of phase typically proves difficult to implement, due to high measurement sensitivity to practically unavoidable environmental sources of noise and drift. To date, no characterization technique has yet emerged as a frontrunner for these applications. This challenge is addressed using a custom‐designed three‐beam Mach–Zehnder interferometer capable of continuously referenced measurement of both phase and transmittance, resulting in a 10× reduction of noise and drift and phase measurement standard deviation over 10 min of 0.56° and over 16 h of 2.8°. High measurement stability provided by this method enables samples to be easily characterized under dynamic conditions. Temperature‐dependent measurements are demonstrated with phase‐change material vanadium dioxide (VO2), and with wavelength‐dependent measurements of a dielectric Huygens metasurface supporting a characteristic resonant reflection peak. A Fourier‐based signal filtering technique is applied, reducing measurement uncertainty to 0.13° and enabling discernment of monolayer thickness variations in 2D material MoS2.

Dictating Nanoparticle Assembly via Systems-Level Control of Molecular Multivalency.

Nanoparticle assembly can be controlled by multivalent binding interactions between surface ligands, indicating that more precise control over these interactions is important to design complex nanoscale architectures. It has been well-established in natural materials that the arrangement of different molecular species in three dimensions can affect the ability of individual supramolecular units to coordinate their binding, thereby regulating the strength and specificity of their collective molecular interactions. However, in artificial systems, limited examples exist that quantitatively demonstrate how changes in nanoscale geometry can be used to rationally modulate the thermodynamics of individual molecular binding interactions. As a result, the use of nanoscale design features to regulate molecular bonding remains an underutilized design handle to control nanomaterials synthesis. Here we demonstrate a polymer-coated nanoparticle material where supramolecular bonding and nanoscale structure are used in conjunction to dictate the thermodynamics of their multivalent interactions, resulting in emergent bundling of supramolecular binding groups that would not be expected on the basis of the molecular structures alone. Additionally, we show that these emergent phenomena can controllably alter the superlattice symmetry by using the mesoscale particle arrangement to alter the thermodynamics of the supramolecular bonding behavior. The ability to rationally program molecular multivalency via a systems-level approach therefore provides a major step forward in the assembly of complex artificial structures, with implications for future designs of both nanoparticle- and supramolecular-based materials.

Ultrasensitive Three-Dimensional Orientation Imaging of Single Molecules on Plasmonic Nanohole Arrays Using Second Harmonic Generation.

Recently, fluorescence-based super-resolution techniques such as stimulated emission depletion (STED) and stochastic optical reconstruction microscopy (STORM) have been developed to achieve near molecular-scale resolution. However, such a super-resolution technique for nonlinear label-free microscopy based on second harmonic generation (SHG) is lacking. Since SHG is label-free and does not involve real-energy level transitions, fluorescence-based super-resolution techniques such as STED cannot be applied to improve the resolution. In addition, due to the coherent and non-isotropic emission nature of SHG, single-molecule localization techniques based on isotropic emission of fluorescent molecule such as STORM will not be appropriate. Single molecule SHG microscopy is largely hindered due to the very weak nonlinear optical scattering cross sections of SHG scattering processes. Thus, enhancing SHG using plasmonic nanostructures and nanoantennas has recently gained much attention owing to the potential of various nanoscale geometries to tightly confine electromagnetic fields into small volumes. This confinement provides substantial enhancement of electromagnetic field in nanoscale regions of interest, which can significantly boost the nonlinear signal produced by molecules located in the plasmonic hotspots. However, to date, plasmon-enhanced SHG has been primarily applied for the measurement of bulk properties of the materials/molecules, and single molecule SHG imaging along with its orientation information has not been realized yet. Herein, we achieved simultaneous visualization and three-dimensional (3D) orientation imaging of individual rhodamine 6G (R6G) molecules in the presence of plasmonic silver nanohole arrays. SHG and two-photon fluorescence microscopy experiments together with finite-difference time-domain (FDTD) simulations revealed a ∼106-fold nonlinear enhancement factor at the hot spots on the plasmonic silver nanohole substrate, enabling detection of single molecules using SHG. The position and 3D orientation of R6G molecules were determined using the template matching algorithm by comparing the experimental data with the calculated dipole emission images. These findings could enable SHG-based single molecule detection and orientation imaging of molecules which could lead to a wide range of applications from nanophotonics to super-resolution SHG imaging of biological cells and tissues.

Cellulose elementary fibril orientation in the spruce S1-2 transition layer

The tight organization of major wood cell wall polymers limits the swellability, solubility and reactivity of cellulose fibers during the production of regenerated textile fibers, nanocellulose, bioethanol, and many other value-added products. However, the ultrastructural assembly of cellulose elementary fibrils (EF) and matrix materials in one of the outer layers, i.e. S1-2 transition layer of wood cell wall, is far from being understood. Here, single-axis electron tomography on ultrathin spruce sections was applied to observe the three-dimensional (3D) structure of the S1-2 layer. The nanoscale geometries of the EFs were further quantitatively modeled through mathematical fitting of the tomographic subvolumes by suitable parametric space curves. The results showed that crisscross, bundled and parallel EF organizations are all present in this layer; the former two exhibit a denser structure. Several quantitative measures such as distances and angles were obtained for the analyzed structures. The result obtained in this study suggests that the S1-2 transition layer differs in structure than the principal cell wall layers. The structural differences and its possible role in wood cell wall have been discussed. These results will enhance our understanding of the swellability, accessibility and solubility of woody biomass for its conversion into the aforementioned value-added products.

Seeded Lateral Solid-Phase Crystallization of the Perovskite Oxide SrTiO3

Crystallization from an amorphous precursor presents a new route to control the properties of complex oxides by selecting their nanoscale morphology. A key challenge in crystal growth from the amorphous form is to select the locations of nucleation and the crystallographic orientation of the resulting crystals. Nucleation sites for crystallization of the prototypical perovskite complex oxide SrTiO3 (STO) from its amorphous form can be reproducibly introduced using nanoscale seed crystals. The results of two seeding strategies are reported here: (i) SrRuO3 (SRO) (001)-oriented nanomembranes and (ii) STO nanocrystalline seeds. Amorphous STO crystallizes laterally over distances of several microns from the seeds before encountering separately nucleated crystals. The lateral crystallization rates for both types of seeding methods are close to the values measured in solid-phase epitaxy (SPE) of STO on single-crystal substrates. The lateral crystallization distances along different crystallographic orientations are equal, indicating that the lateral crystallization rate is isotropic. The isotropic crystallization suggests that the rate-limiting steps for crystallization occur within the amorphous STO away from the amorphous/crystalline interface. In addition to the lateral crystallization, STO crystallizes on top of the planar SRO nanomembrane seeds via SPE, forming partially relaxed [001]-oriented heteroepitaxial STO layers. The in-plane orientation of laterally crystallized STO near the SRO nanomembrane seeds exhibits overall polycrystallinity with regions in which micron-scale grains of laterally crystallized STO share the same in-plane orientation as the SRO nanomembrane seeds. Crystallization from seed crystals provides opportunities to create a wide range of other perovskite oxides in nanoscale geometries.

Artificial ceramic diatoms with multiscale photonic architectures via nanoimprint lithography for CO2 photoreduction

AbstractThe elaborated photonic architectures and unique light manipulation ability of diatoms provide inspirations for the design of efficient artificial photosynthetic systems. However, the biomimetic synthesis of artificial diatoms with accurate control over nanoscale geometry for scalable production has been a great challenge. Herein, diatom Coscinodiscus sp. is served as a model for the biomimetic production of artificial ceramic diatoms with multiscale photonic architectures for CO2 photoreduction. Optical finite‐difference time‐domain simulations are conducted to investigate the optical mechanisms on enhanced light harvesting and to establish a modified structural model. Electron Beam Lithography and Nanoimprint Lithography are applied to fabricate artificial TiO2 diatoms with elaborated periodic photonic structures and high surface areas (169.4 m2 g−1). Artificial photosynthesis via CO2 reduction enhances CO and CH4 evolution on the artificial diatoms by up to 2.75‐ and 2.3‐fold, respectively, compared with the corresponding powder sample. Furthermore, gas diffusion behaviors, closely related to the gas‐phase reaction, are investigated by theoretical simulation to reveal the hierarchical structural effects on catalytic efficiency. This work provides a new pathway to design and biomimetic synthesis of artificial structures for enhanced performances.

Nanoplasmonic Sensor Detects Preferential Binding of IRSp53 to Negative Membrane Curvature.

Biosensors based on plasmonic nanostructures are widely used in various applications and benefit from numerous operational advantages. One type of application where nanostructured sensors provide unique value in comparison with, for instance, conventional surface plasmon resonance, is investigations of the influence of nanoscale geometry on biomolecular binding events. In this study, we show that plasmonic “nanowells” conformally coated with a continuous lipid bilayer can be used to detect the preferential binding of the insulin receptor tyrosine kinase substrate protein (IRSp53) I-BAR domain to regions of negative surface curvature, i.e., the interior of the nanowells. Two different sensor architectures with and without an additional niobium oxide layer are compared for this purpose. In both cases, curvature preferential binding of IRSp53 (at around 0.025 nm−1 and higher) can be detected qualitatively. The high refractive index niobium oxide influences the near field distribution and makes the signature for bilayer formation less clear, but the contrast for accumulation at regions of negative curvature is slightly higher. This work shows the first example of analyzing preferential binding of an average-sized and biologically important protein to negative membrane curvature in a label-free manner and in real-time, illustrating a unique application for nanoplasmonic sensors.

A label-free aptamer-based nanogap capacitive biosensor with greatly diminished electrode polarization effects.

A significant impediment to the use of impedance spectroscopy in bio-sensing is the electrode polarization effect that arises from the movement of free ions to the electrode-solution interface, forming an electrical double layer (EDL). The EDL screens the dielectric response of the bulk and its large capacitance dominates the signal response at low frequency, masking information particularly relevant for biological samples, such as molecular conformation changes and DNA hybridization. The fabrication of nanogap capacitors with electrode separation less than the EDL thickness can significantly reduce electrode polarization effects and provide enormous improvement in sensitivity due to better matching of the sensing volume with the size of the target entities. We report on the fabrication of a horizontal thin-film nanogap capacitive sensor with electrode separation of 40 nm that shows almost no electrode polarization effects when measured with water and ionic buffer solutions, thereby allowing direct quantification of their relative permittivity at low frequencies. Surface modification of the electrodes with thiol-functionalized single strand DNA aptamers transforms the device into a label-free biosensor with high sensitivity and selectivity towards the detection of a specific protein. Using this approach, we have developed a biosensor for the detection of human alpha thrombin. In addition, we also examine frequency dependent permittivity measurements on high ionic strength solutions contained within the nanogap and discuss how these support recent experimental observations of large Debye lengths. A large shift in the Debye relaxation frequency to lower frequency is also found, which is consistent with water molecules being in a rigid-like state, possibly indicating the formation of an ordered "ice-like" phase. Altogether, this work highlights the need for better understanding of fluids in confined, nanoscale geometries, from which important new applications in sensing may arise.

Detecting Selective Protein Binding Inside Plasmonic Nanopores: Toward a Mimic of the Nuclear Pore Complex.

Biosensors based on plasmonic nanostructures offer label-free and real-time monitoring of biomolecular interactions. However, so do many other surface sensitive techniques with equal or better resolution in terms of surface coverage. Yet, plasmonic nanostructures offer unique possibilities to study effects associated with nanoscale geometry. In this work we use plasmonic nanopores with double gold films and detect binding of proteins inside them. By thiol and trietoxysilane chemistry, receptors are selectively positioned on the silicon nitride interior walls. Larger (~150 nm) nanopores are used detect binding of averaged sized proteins (~60 kg/mol) with high signal to noise (>100). Further, we fabricate pores that approach the size of the nuclear pore complex (diameter down to 50 nm) and graft disordered phenylalanine-glycine nucleoporin domains to the walls, followed by titration of karyopherinβ1 transport receptors. The interactions are shown to occur with similar affinity as determined by conventional surface plasmon resonance on planar surfaces. Our work illustrates another unique application of plasmonic nanostructures, namely the possibility to mimic the geometry of a biological nanomachine with integrated optical sensing capabilities.

Shining New Light on the Structural Determinants of Cardiac Couplon Function: Insights From Ten Years of Nanoscale Microscopy.

Remodelling of the membranes and protein clustering patterns during the pathogenesis of cardiomyopathies has renewed the interest in spatial visualisation of these structures in cardiomyocytes. Coincidental emergence of single molecule (super-resolution) imaging and tomographic electron microscopy tools in the last decade have led to a number of new observations on the structural features of the couplons, the primary sites of excitation-contraction coupling in the heart. In particular, super-resolution and tomographic electron micrographs have revised and refined the classical views of the nanoscale geometries of couplons, t-tubules and the organisation of the principal calcium handling proteins in both healthy and failing hearts. These methods have also allowed the visualisation of some features which were too small to be detected with conventional microscopy tools. With new analytical capabilities such as single-protein mapping, in situ protein quantification, correlative and live cell imaging we are now observing an unprecedented interest in adapting these research tools across the cardiac biophysical research discipline. In this article, we review the depth of the new insights that have been enabled by these tools toward understanding the structure and function of the cardiac couplon. We outline the major challenges that remain in these experiments and emerging avenues of research which will be enabled by these technologies.

Effective Modal Volume in Nanoscale Photonic and Plasmonic Near-Infrared Resonant Cavities

We survey expressions of the effective modal volume, Veff, commonly used in the literature for nanoscale photonic and plasmonic cavities. We apply different expressions of Veff to several canonical cavities designed for nanoscale near-infrared light sources, including metallo-dielectric and coaxial geometries. We develop a metric for quantifying the robustness of different Veff expressions to the different cavities and materials studied. We conclude that no single expression for Veff is universally applicable. Several expressions yield nearly identical results for cavities with well-confined photonic-type modes. For cavities with poor confinement and a low quality factor, however, expressions using the proper normalization method need to be implemented to adequately describe the diverging behavior of their effective modal volume. The results serve as a practical guideline for mode analysis of nanoscale optical cavities, which show promise for future sensing, communication, and computing platforms.

Spintronic memristor synapse and its RWC learning algorithm

As one of the most widely used memristor models, the spintronic memristor has become a promising candidate for the electronic synapse. Non-volatility, nanoscale geometries, binary data and multi-level information storage make the circuit simpler and consume less electricity. In this study, a new spintronic memristor synaptic circuit is proposed which can realise positive, zero and negative weights successfully. Furthermore, the circuits of the presented synaptic-based neuron and compact neural network are designed and an improved random weight change (RWC) algorithm is proposed. Compared with the traditional RWC algorithm, it has faster training speed and less training error. In addition, the neural network is applied to data prediction, the result of which is closer to real data. Finally, the correctness and effectiveness of the proposed network are verified via a series of computer simulations.

Mannan Molecular Substructures Control Nanoscale Glucan Exposure in Candida

Cell wall mannans of Candida albicans mask β-(1,3)-glucan from recognition by Dectin-1, contributing to innate immune evasion. Glucan exposures are predominantly single receptor-ligand interaction sites of nanoscale dimensions. Candida species vary in basal glucan exposure and molecular complexity of mannans. We used super-resolution fluorescence imaging and a series of protein mannosylation mutants in C.albicans and C.glabrata to investigate the role of specific N-mannan featuresin regulating the nanoscale geometry of glucan exposure. Decreasing acid labile mannan abundance and α-(1,6)-mannan backbone length correlated most strongly with increased density and nanoscopic size of glucan exposures in C.albicans and C.glabrata, respectively. Additionally, a C.albicans clinical isolate with high glucan exposure produced similarly perturbed N-mannan structures and elevated glucan exposure geometry. Thus, acid labile mannan structure influences the nanoscale features of glucan exposure, impacting the nature of the pathogenic surface that triggers immunoreceptor engagement, aggregation, and signaling.

Programmable Multivalent DNA-Origami Tension Probes for Reporting Cellular Traction Forces.

Mechanical forces are central to most, if not all, biological processes, including cell development, immune recognition, and metastasis. Because the cellular machinery mediating mechano-sensing and force generation is dependent on the nanoscale organization and geometry of protein assemblies, a current need in the field is the development of force-sensing probes that can be customized at the nanometer-length scale. In this work, we describe a DNA origami tension sensor that maps the piconewton (pN) forces generated by living cells. As a proof-of-concept, we engineered a novel library of six-helix-bundle DNA-origami tension probes (DOTPs) with a tailorable number of tension-reporting hairpins (each with their own tunable tension response threshold) and a tunable number of cell-receptor ligands. We used single-molecule force spectroscopy to determine the probes' tension response thresholds and used computational modeling to show that hairpin unfolding is semi-cooperative and orientation-dependent. Finally, we use our DOTP library to map the forces applied by human blood platelets during initial adhesion and activation. We find that the total tension signal exhibited by platelets on DOTP-functionalized surfaces increases with the number of ligands per DOTP, likely due to increased total ligand density, and decreases exponentially with the DOTP's force-response threshold. This work opens the door to applications for understanding and regulating biophysical processes involving cooperativity and multivalency.

Nanometer Scale Confined Growth of Single-Crystalline Gold Nanowires via Photocatalytic Reduction.

Single-crystalline gold nanowires (Au NWs) are directly synthesized by the photocatalytic reduction of an aqueous HAuCl4 solution inside high-aspect-ratio TiO2 nanotubes (NTs). Crystalline TiO2 (anatase) NTs are prepared by the template-assisted atomic layer deposition technique with a subsequent annealing. Under the irradiation of ultraviolet light, photoexcited electrons are formed on the surfaces of TiO2 NTs and could reduce Au ions to create nuclei without using any surfactant, reducing agent, and/or seed. Once nucleation occurred, high-aspect-ratio Au NWs are grown inside the TiO2 NTs in a diffusion-controlled manner. As the solution pH increased, the nucleation/growth rate decreased and twin-free (or not observed), single-crystalline Au NWs are formed. At a pH above 6, the nucleation/growth rates increased and Au nanoparticles are observed both inside and outside of the TiO2 NTs. The confined nanoscale geometries of the interior of the TiO2 NTs are found to play a key role in the controlled diffusion of Au species and in determining the crystal morphology of the resulting Au NWs.

MoS2 Decorated on Different Metal Oxide Nanotubular Structures with a High Density of Reactive Sites for HER Reactions

MoS2 is intensively investigated for decades owing to some unique chemical and physical properties. 1 For many applications, the use of nanoscale geometries is highly advantageous owing to the high specific surface area and short carrier and ion diffusion pathways that can be established.2 This is to a large extent due to the weak van der Waals force between the stacked S-Mo-S units giving it a graphene-like layer structure. 3 This is not only the origin of outstanding lubrication properties but also the key to the insertion and extraction of small foreign ions into the free space of the S-Mo-S layers.4 In this study, we report the self-organized formation of anodic molybdenum oxide nanotube arrays.5 The amorphous tubes can be crystallized to MoO2 or MoO3 and be converted fully or partially into molybdenum sulfide. Vertically aligned MoOx/MoS2 nanotubes can be formed with defined MoS2 sheets in a layer by layer arrangement. These provide a high density of reactive stacking misalignments (defects). These core–shell nanotube arrays consist of a conductive suboxide core and a functional high defect density MoS2 coating. Such structures are investigated for applications in electrocatalysis (hydrogen evolution). Furthermore, nanoscaled MoS2 has a gap broadens up to 1.8 eV when reaching monolayer thickness with a conduction band more negative to the conduction band of TiO2. In our experiments, MoS2 can be decorated on the top of anodic anatase TiO2 nanotubes (TiNTs) site-selectively. 6 The layers can be used as co-catalysts for photocatalytic hydrogen evolution. A strongly enhanced H2 evolution activity can be observed using only a nominal 1 nm thick MoS2 decoration on top of a 6 μm thick TiNT layer. We ascribe this strong beneficial effect to two factors: (i) the thin molybdenum sulfide on the top acts as an electron transfer mediator, i.e. as an H2 evolution co-catalyst; and (ii) the underlying tube layer acts as a light-to-electron harvester.

Role of the Carbon Support on the Oxygen Reduction and Evolution Activities in LaNiO3 Composite Electrodes in Alkaline Solution

Metal–air batteries and fuel cells show a great deal of promise in advancing low-cost, high-energy-density charge storage solutions for sustainable energy applications. To improve the activities and stabilities of electrocatalysts for the critical oxygen reduction and evolution reactions (ORR and OER, respectively), a greater understanding is needed of the catalyst/carbon interactions and carbon stability. Herein, we report how LaNiO3 (LNO) supported on nitrogen-doped carbon nanotubes (N-CNT) made from a high-yield synthesis lowers the overpotential for both the OER and ORR markedly to enable a low bifunctional window of 0.81 V at only a 51 μg cm–2 mass loading. Furthermore, the addition of LNO to the N-CNTs improves the galvanostatic stability for the OER by almost 2 orders of magnitude. The nanoscale geometries of the perovskites and the CNTs enhance the number of metal–support and charge transfer interactions and thus the activity. We use rotating ring disk electrodes (RRDEs) combined with Tafel slope ...

Nanoscale Relative Emission Efficiency Mapping Using Cathodoluminescence g(2) Imaging.

Cathodoluminescence (CL) imaging spectroscopy provides two-dimensional optical excitation images of photonic nanostructures with a deep-subwavelength spatial resolution. So far, CL imaging was unable to provide a direct measurement of the excitation and emission probabilities of photonic nanostructures in a spatially resolved manner. Here, we demonstrate that by mapping the cathodoluminescence autocorrelation function g(2) together with the CL spectral distribution the excitation and emission rates can be disentangled at every excitation position. We use InGaN/GaN quantum wells in GaN nanowires with diameters in the range 200–500 nm as a model system to test our new g(2) mapping methodology and find characteristic differences in excitation and emission rates both between wires and within wires. Strong differences in the average CL intensity between the wires are the result of differences in the emission efficiencies. At the highest spatial resolution, intensity variations observed within wires are the result of excitation rates that vary with the nanoscale geometry of the structures. The fact that strong spatial variations observed in the CL intensity are not only uniquely linked to variations in emission efficiency but also linked to excitation efficiency has profound implications for the interpretation of the CL data for nanostructured geometries in general.

Lead-Free Perovskite Nanocrystals for Light-Emitting Devices

Lead halide perovskites with nanoscale geometries have received recent attention due to the defect-tolerant high photoluminescence quantum yield at tunable emission wavelengths and the possibility of room-temperature synthesis that does not compromise the physical properties of the materials. These characteristics offer opportunities to advance displays that cover the widest perceivable color. However, lead toxicity obstructs the commercialization of this technology. Therefore, recent efforts have investigated lead-free halide perovskite nanocrystals. Here, we provide our perspectives on the most exciting achievements in the materials design and photophysical properties of lead-free perovskite nanocrystals, particularly for applications in light-emitting devices. This Perspective includes a short summary on the characteristic features of halide perovskite nanocrystals; discussion on the candidate elements to replace lead; methods to prepare colloidal lead-free perovskite nanocrystals; methods to control and enhance the optical properties; a recent demonstration of utilizing lead-free perovskite nanocrystals in light-emitting devices; and an outlook on the field.