Superlattice Arrays Research Articles

Superlattice‐based Plasmonic Catalysis: Concentrating Light at the Nanoscale to Drive Efficient Nitrogen‐to‐Ammonia Fixation at Ambient Conditions

AbstractPlasmonic catalysis promises green ammonia synthesis but is limited by the need for co‐catalysts and poor performances due to weak electromagnetic field enhancement. Here, we use two‐dimensional plasmonic superlattices with dense electromagnetic hotspots to boost ambient nitrogen‐to‐ammonia photoconversion without needing co‐catalyst. By organizing Ag octahedra into a square superlattice to concentrate light, the ammonia formation is enhanced by ≈15‐fold and 4‐fold over hexagonal superlattice and disorganized array, respectively. Our unique catalyst achieves superior ammonia formation rate and apparent quantum yield up to ≈15‐fold and ≈103‐fold, respectively, better than traditional designs. Mechanistic investigations reveal the abundance of intense plasmonic hotspots is crucial to promote hot electron generation and transfer for nitrogen reduction. Our work offers valuable insights to design electromagnetically hot plasmonic catalysts for diverse chemical and energy applications.

Superlattice-based Plasmonic Catalysis: Concentrating Light at the Nanoscale to Drive Efficient Nitrogen-to-Ammonia Fixation at Ambient Conditions.

Plasmonic catalysis promises green ammonia synthesis but is limited by the need of co-catalysts and poor performances due to weak electromagnetic field enhancement. Here, we use two-dimensional plasmonic superlattices with dense electromagnetic hotspots to boost ambient nitrogen-to-ammonia photoconversion without needing co-catalyst. By organizing Ag octahedra into a square superlattice to concentrate light, the ammonia formation is enhanced by ~15-fold and 4-fold over hexagonal superlattice and disorganized array, respectively. Our unique catalyst achieves superior ammonia formation rate and apparent quantum yield up to ~15-fold and ~103-fold, respectively, better than traditional designs. Mechanistic investigations reveal the abundance of intense plasmonic hotspots is crucial to promote hot electron generation and transfer for nitrogen reduction. Our work offers valuable insights to design electromagnetically hot plasmonic catalysts for diverse chemical and energy applications.

Printable assemblies of perovskite nanocubes on meter-scale panel.

Hierarchical assemblies of functional nanoparticles can have applications exceeding those of individual constituents. Arranging components in a certain order, even at the atomic scale, can result in emergent effects. We demonstrate that printed atomic ordering is achieved in multiscale hierarchical structures, including nanoparticles, superlattices, and macroarrays. The CsPbBr3 perovskite nanocubes self-assemble into superlattices in ordered arrays controlled across 10 scales. These structures behave as single nanoparticles, with diffraction patterns similar to those of single crystals. The assemblies repeat as two-dimensional planar unit cells, forming crystalline superlattice arrays. The fluorescence intensity of these arrays is 5.2 times higher than those of random aggregate arrays. The multiscale coherent states can be printed on a meter-scale panel as a micropixel light-producing layer of primary-color photon emitters. These hierarchical assemblies can boost the performance of optoelectronic devices and enable the development of high-efficiency, directional quantum light sources.

Preparation of Graded Photonic Structure by using multi-beam interfering method based on dual-parameter hexagonal prism

In this paper, a double-cone interfering system is constructed by using a dual-parameter top-cut hexagonal prism (TCHP), which is independently designed by our group. By using the system, a large area of SU-8 photoresist material is subjected to a single holographic exposure to prepare the gradient photonic crystal (GPC) superlattice structures and array. Through the AFM measurement of the sample, a double-period hexagonal GPC structure with a small lattice of sub-micron size is observed. Compared with the structures obtained by CCD microscope camera system monitoring and theoretical design, the experiment has achieved a relatively good consistency. The method is simple and easy to prepare GPC structures and array in a large area through a single exposure, which provides a new way for the rapid preparation of GPC.

Mid-Infrared Detector Array Technologies for SOFIA and Sub-Orbital Observatory Instruments

The status of various photovoltaic, photoconductive, BIB/IBC, superlattice, TES and KID technologies to produce arrays sensitive from 15 to [Formula: see text]m (Mid-infrared) will be reviewed to assess where reasonable investments should be made for SOFIA and other sub-orbital observatory instruments. These include HgCdTe, Si:As IBC, Si:Sb BIB, Ge:Ga, type 2 superlattice arrays of both III–V and II–VI materials and both TES and KID arrays. KID technologies for this wavelength region show promise, although to date there are no published experimental data on KID arrays for the mid-infrared.

An active tunable Fano switch in a plasma-filled superlattice array

We propose a Fano switch arising from the superlattice array of a plasma-filled quartz tube, which can be tuned and reconfigured by the plasma density in the tube. The generation of the switch depends on a Fano band that is induced by the interference between the Mie resonance in an isolated cylinder and Bragg scattering in a periodic array. The underlying dispersion characteristics reveal that a localized tunable flat band corresponding to the Mie resonance plays an important role in the appearance of the Fano resonance. This active tunable switch can be potentially applied to microwave communications as a single-pole multi-throw switch and to monitor environmental variables that impact the plasma density.

TiO2 film supported by vertically aligned gold nanorod superlattice array for enhanced photocatalytic hydrogen evolution

Photocatalytic hydrogen generation from water as a renewable non-polluting technique for converting solar energy to chemical energy has recently attracted worldwide attention. However, low hydrogen production efficiency of traditional photocatalysis still remains as a challenge. Here, we report a large area TiO2 film supported by vertically ordered Au nanorods superlattice array (O-AuNRs/TiO2), which demonstrates excellent photocatalytic performances of hydrogen evolution from water under solar and visible light irradiation. The O-AuNRs/TiO2 architecture enables the significant localized surface plasmon resonance (LSPR) enhancement, including both local electromagnetic field effect and hot electron transfer effect, which promotes the photocatalytic hydrogen evolution rate of the TiO2 film by 58 times. The photocatalytic efficiency of O-AuNRs/TiO2 exceeds that of the TiO2 film supported by randomly oriented AuNRs by over five times. Finite difference time domain (FDTD) modeling results support that the strong coupling of O-AuNRs enhances the electromagnetic field intensity along the longitudinal axis in the gaps between adjacent AuNRs and the average electric field enhancement factors at the interface between the AuNRs and TiO2 of O-AuNRs/TiO2. This work demonstrates the substantial performance boost of conventional photocatalyst by LSPR enhancement, thus provides a promising tactic to devise highly efficient photocatalytic system for solar energy conversion.

Quantitative Surface-Enhanced Raman Spectroscopy Analysis through 3D Superlattice Arrays of Au Nanoframes with Attomolar Detection.

This paper reports a methodology for synthesizing and ordering gold nanoframes into three-dimensional (3D) arrays with a controlled thickness, leading to homogeneous plasmonic superstructures, with which quantitative analysis via surface-enhanced Raman spectroscopy (SERS) has been successfully demonstrated. Because this preparation method allows for systematic control of nanoframe film thickness and the resulting 3D plasmonic superstructure, which exhibits a unique nanoporous network of hot-spots, detection limits down to 10-18 M, corresponding to ≈6000 molecules, have been measured. Compared to analogous solid nanoparticle superstructures, the nanoframe superstructures with their unique nanoporous architecture effectively dissipate the heat inevitably generated by laser excitation during measurement, effectively suppressing the formation of carbonaceous materials and therefore their accompanying fluorescence interference, especially important for low concentration detection.

Freeze‐Facilitated Ligand Binding to Plasmonic Gold Nanorods

AbstractGold nanorods (AuNRs) are an important class of advanced plasmonic materials, but unlike gold nanospheres (AuNSs) their surface modification is difficult due to the specific surfactant layer of cetyltrimethylammonium bromide (CTAB) resulted from the synthesis. In this paper, a freeze‐induced surface modification strategy is proposed for AuNRs. Freezing AuNRs right after a simple wash step can facilitate binding of surface ligands and allow assembly of functional biointerface in a fast time scale. This strategy is simple, fast, versatile, and robust, allowing attachments of different ligand molecules, including organic dyes, poly(ethylene glycol)s (PEGs), and DNAs onto AuNRs without additional reagent. An optimal condition for DNA loading is determined with a matrix driven approach. It is shown that the attached ligand molecules are functional, allowing formation of core–satellite type structures using DNAs linking to AuNSs, or high‐density vertical superlattice arrays using PEGylated AuNRs.

Design and characterization of asymetrical super-lattice Si/4H-SiC pin photo diode array: a potential opto-sensor for future applications in bio-medical domain

Photo-sensors are integral part of different bio-medical diagnostic equipment. Each type of bio-molecules possess unique spectral fingerprint in visible wavelength region of electro-magnetic spectrum. Now-a-days, the enhancement of quantum-efficiency and photo-responsivity of such bio-medical opto-sensors, for accurate identification of virus/anti-bodies in blood by optical means, is a big challenge to medical-device Engineers. The authors have addressed this issue in this research paper by proposing a novel structure of asymmetrical Si/4H-SiC super-lattice pin diode array for the development of high-sensitive visible light (300–800 nm) sensor on native substrate. The simulation experiment is carried out by developing a generalized large-signal quantum modified drift–diffusion simulator incorporating three different modes of carrier generation-recombination under light and dark conditions: avalanching, tunneling and photo-irradiation. The validity of the model has been established by comparing the simulation results with those of experimental observations. A good agreement between theory and experiment, under similar biasing conditions, establishes the validity of the developed model. The characteristics analysis depicts that the quantum efficiency of the designed single photo-sensor is ~ 65% within 400–700 nm wavelength region, whereas, the same enhances to nearly ~ 90% with a 3 × 3 photo-sensor array based on asymmetrical super-lattice single pin devices. In visible wavelength region, the simulated photo-sensors (both single and array) have demonstrated significant photo responsivity. The photo-responsivity values, at 500 nm wavelength of incident radiation, are observed to be 0.65 A/W for a single photo-diode and 0.85 A/W for a 3 × 3 combination of photo-diode array. This clearly establishes the potentiality of the asymmetrical super-lattice pin-array as a visible photo-sensor for future application in developing medical instruments. A comparative analysis of Si and Si/4H-SiC asymmetrical super-lattice photo-sensors establishes the superiority of the later as a high-sensitive visible light-sensor in terms of better photo-responsivity and quantum efficiency. To the best of authors’ knowledge, this is the first report on Si/4H-SiC super-lattice pin photo-sensor array in the visible range of optical irradiation. The experimental feasibility of the device and a proposed circuit for future bio-medical implementation are also incorporated in the present research-paper for further development.

Gold Nanoparticle Plasmonic Superlattices as Surface-Enhanced Raman Spectroscopy Substrates

Metal colloids are of great interest in the field of nanophotonics, mainly due to their morphology-dependent optical properties, but also because they are high-quality building blocks for complex plasmonic architectures. Close-packed colloidal supercrystals not only serve for investigating the rich plasmonic resonances arising in strongly coupled arrangements but also enable tailoring the optical response, on both the nano- and the macroscale. Bridging these vastly different length scales at reasonable fabrication costs has remained fundamentally challenging, but is essential for applications in sensing, photovoltaics or optoelectronics, among other fields. We present here a scalable approach to engineer plasmonic supercrystal arrays, based on the template-assisted assembly of gold nanospheres with topographically patterned polydimethylsiloxane molds. Regular square arrays of hexagonally packed supercrystals were achieved, reaching periodicities down to 400 nm and feature sizes around 200 nm, over areas up to 0.5 cm2. These two-dimensional supercrystals exhibit well-defined collective plasmon modes that can be tuned from the visible through the near-infrared by simple variation of the lattice parameter. We present electromagnetic modeling of the physical origin of the underlying hybrid modes and demonstrate the application of superlattice arrays as surface-enhanced Raman scattering (SERS) spectroscopy substrates which can be tailored for a specific probe laser. We therefore investigated the influence of the lattice parameter, local degree of order, and cluster architecture to identify the optimal configuration for highly efficient SERS of a nonresonant Raman probe with 785 nm excitation.

Self-Assembly of CoPt Magnetic Nanoparticle Arrays and its Underlying Forces.

Nanoparticles covered with surfactants are often used to study particle motion patterns and self-assembly processes in solutions. Surfactants have influence on the interparticle interactions and therefore on the particle motion tracks and final patterns. In this study, CoPt nanoparticles are synthesized in aqueous solution without any surfactant. In situ transmission electron microscopy observation is performed to monitor the self-assemble process. Two types of magnetic nanoparticle superlattice arrays are formed: hexagonal equal distance superlattice arrays when particle size is 3 nm, and tight unequal distance superlattice arrays when particle size is 4.5 nm. It is interesting to observe that two small arrays merge into a large one through rotational and translational movements. A Monte Carlo simulation is carried out which successfully restores the whole process. It is identified that the underlying forces are van der Waals and magnetic dipolar interactions. The latter is responsible for orientation of each particle during the whole process. This investigation leads to a better understanding of the formation mechanism of magnetic nanoparticle superlattice arrays.

On-surface synthesis of superlattice arrays of ultra-long graphene nanoribbons.

We report the on-surface synthesis of graphene nanoribbon superlattice arrays directed by the herringbone reconstruction of the Au(111) surface. The uniaxial anisotropy of the zigzag pattern of the reconstruction defines a one dimensional grid for directing the Ullmann polymerization and inducing periodic arrays of parallel ultra-long nanoribbons (>100 nm), where the periodicity is varied with coverage at discrete values following a hierarchical templating behavior.

Large-Scale, Long-Range-Ordered Patterning of Nanocrystals via Capillary-Bridge Manipulation.

Deterministic assembly of nanoparticles with programmable patterns is a core opportunity for property-by-design fabrication and large-scale integration of functional materials and devices. The wet-chemical-synthesized colloidal nanocrystals are compatible with solution assembly techniques, thus possessing advantages of high efficiency, low cost, and large scale. However, conventional solution process suffers from tradeoffs between spatial precision and long-range order of nanocrystal assembly arising from the uncontrollable dewetting dynamics and fluid flow. Here, a capillary-bridge manipulation method is demonstrated for directing the dewetting of nanocrystal inks and deterministically patterning long-range-ordered superlattice structures. This is achieved by employing micropillars with programmable size, arrangement, and shape, which permits deterministic manipulation of geometry, position, and dewetting dynamics of capillary bridges. Various superlattice structures, including one-dimensional (1D), circle, square, pentagon, hexagon, pentagram, cross arrays, are fabricated. Compared to the glassy thin films, long-range-ordered superlattice arrays exhibit improved ferroelectric polarization. Coassembly of nanocrystal superlattice and organic functional molecule is further demonstrated. Through introducing azobenzene into superlattice arrays, a switchable ferroelectric polarization is realized, which is triggered by order-disorder transition of nanocrystal stacking in reversible isomerization process of azobenzene. This method offers a platform for patterning nanocrystal superlattices and fabricating microdevices with functionalities for multiferroics, electronics, and photonics.

Efficient Enrichment and Self-Assembly of Hybrid Nanoparticles into Removable and Magnetic SERS Substrates for Sensitive Detection of Environmental Pollutants.

A structure consisting of a low surface energy substrate and low surface tension liquid is designed and prepared by taking advantage of perfluorinated fluid infusion into the porous Teflon membrane. This slippery platform allows efficient enrichment and self-assembly of hybrid nanoparticles and the assembled structure can be detached from the membrane. A macroscale superlattice array of Au nanorods doped with magnetic Fe3O4 nanoparticles is obtained by suppressing the outward capillary flow and coffee-ring effect during evaporative self-assembly. In SERS (surface enhanced Raman scattering) detection of environmental pollutants including thiram, diquat and polycyclic aromatic hydrocarbons, the removable plasmonic superlattice array with magnetic properties enables rapid separation of analytes from the solution resulting in excellent sensitivity and detection limits down to the nanomolar level. The self-assembly strategy shows great potential in the fabrication of removable 3D plasmonic superlattice arrays for SERS detections.

Bimetallic PtAu superlattice arrays: Highly electroactive and durable catalyst for oxygen reduction and methanol oxidation reactions

Abstract Superlattice arrays, an important type of nanomaterials, have wide applications in catalysis, optic/electronics and energy storage for the synergetic effects determined by both individual metals and collective interactions. Herein, a simple one-pot solvothermal coreduction approach is developed for facile preparation of bimetallic PtAu alloyed superlattice arrays (PtAu SLAs) in oleylamine, with the assistance of urea via hydrogen bonding induced self-assembly. Urea is essential in morphology-controlled process and prevents PtAu nanoparticles from the disordered aggregation. The characterization and formation mechanism of PtAu SLAs are investigated in details. The as-synthesized hybrid nanocrystals exhibit enhanced electrocatalytic performances for oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) in alkaline electrolyte in comparison with commercial Pt-C (50%, wt.%) and Pt black catalysts.

Evaporative Self-Assembly of Gold Nanorods into Macroscopic 3D Plasmonic Superlattice Arrays.

Millimeter-scale 3D superlattice arrays composed of dense, regular, and vertically aligned gold nanorods are fabricated by evaporative self-assembly. The regular organization of the gold nanorods into a macroscopic superlattice enables the production of a plasmonic substrate with excellent sensitivity and reproducibility, as well as reliability in surface-enhanced Raman scattering. The work bridges the gap between nanoscale materials and macroscopic applications.

Variation of energy density of states in quantum dot arrays due to interparticle electronic coupling.

Subnanometer-resolved local electron energy structure was measured in PbS quantum dot superlattice arrays using valence electron energy loss spectroscopy with scanning transmission electron microscopy. We found smaller values of the lowest available transition energies and an increased density of electronic states in the space between quantum dots with shorter interparticle spacing, indicating extension of carrier wave functions as a result of interparticle electronic coupling. A quantum simulation verified both trends and illustrated the wave function extension effect.

Spin filter effect in iron oxide nanocrystal arrays

Integrating nanocrystals (NCs) into magnetic tunnel structures is of considerable interest due to expectation of novel properties from their spin selective transport and single electron features. Superstructures by colloidal NCs having translational and orientational order and interesting collective magnetic properties can be prepd. by soln. casting through sensitive interparticle and particle-substrate interactions. In this work, we discuss the study on magnetic field induced assembly of mono-dispersed iron oxide NCs to obtain spin filter effect across the superlattice array, when sandwiched between gold electrodes. The deposition of mixed phase Fe3O4@γ-Fe2O3 NCs on SiO2/Au surface proceeds through slow solvent evapn. and are studied for controlled interparticle spacing. For specific NC concn., the ordering depends on the substrate chem. and the ligands passivating NC surface, which affects the concn. of cluster nuclei formed. In presence of a magnetic field, the tunnel structure exhibits enhanced pos. tunnel magnetoresistance at low temps., which could be related to their ferromagnetism and the attempts by electrons to percolate NC superlattice with preserved spin. A sign reversal for magnetoresistance is exhibited by the vertical tunnel junctions on raising the temp.

Poly(N-isopropylacrylamide) Surfactant-Functionalized Responsive Silver Nanoparticles and Superlattices

Metal nanoparticles exhibit unique optical characteristics in visible spectra produced by local surface plasmon resonance (SPR) for a wide range of optical and electronic applications. We report the synthesis of poly(N-isopropylacrylamide) surfactant (PNIPAM-C18)-functionalized metal nanoparticles and ordered superlattice arrays through an interfacial self-assembly process. The method is simple and reliable without using complex chemistry. The PNIPAM-C18-functionalized metal nanoparticles and ordered superlattices exhibit responsive behavior modulated by external temperature and relative humidity (RH). In situ grazing-incidence small-angle X-ray scattering studies confirmed that the superlattice structure of PNIPAM-C18 surfactant-functionalized nanoparticle arrays shrink and spring back reversibly based on external thermal and RH conditions, which allow flexible manipulation of interparticle spacing for tunable SPR. PNIPAM-C18 surfactants play a key role in accomplishing this responsive property. The ease of fabrication of the responsive nanostructure facilitates investigation of nanoparticle coupling that depends on interparticle separation for potential applications in chemical and biological sensors as well as energy storage devices.