3D Au Research Articles

Trans-Scale 2D Synthesis of Millimeter-Large Au Single Crystals via Silk Fibroin Templates

The largest possible planar surfaces are highly sought for two-dimensional (2D) Au single-crystals applicable in catalysis, biomedicine, electronics, sensing, and optics. However, conventional chemical methods of synthesis are currently limited to planar area of 104 μm2. Here, the trans-scale and 2D growth of Au single-crystals is successfully achieved in a “kinetically controlled” pathway, taking advantage from natural fibrous proteins with different amino sequences and supramolecular architectures. It was found that silk fibroin (SF) has a uniquely suited amino sequence and supramolecular assembly ability to fulfill this task, enabling mild reducing ability and strong capping effects. By the synergetic aid of Cl– ions, 2D Au single-crystals were synthesized with an unprecedented lateral dimension exceeding 2 mm, yet with atomically smooth surfaces. When growing nanotips on their large planar surfaces, these 2D Au crystals stand as ideal platforms for single-molecule sensing (e.g., surface enhanced Raman...

Enhancing Upconversion from NaYF₄:Yb,Er@NaYF₄ Core-Shell Nanoparticles Assembled on Metallic Nanostructures.

We report a simple method for the fabrication of a three-layered plasmonic structure of silicon substrate-Au nanospheres-upconversion particles (UCNPs) that displays up to 101-fold fluorescence enhancement. Monodispersed pure hexagonal-phase NaYF4:Yb,Er core and NaYF4:Yb,Er@NaYF4 core@shell nanocrystals were prepared by a solvothermal method. Two dimensional (2D) assembled Au spheres were prepared on a Si substrate, and then, 2D arrays of UCNPs were deposited on the grown 2D monolayered Au spheres by a self-organizing process. The distance between plasmonic Au NPs and rare-earth (RE) core was finely adjusted by changing the undoped NaYF4 shell thickness. The UC emission enhancement shows a pronounced shell thickness dependence. For the non-plasmonic structured samples, a single peak in upconversion luminescence (UCL) enhancement was observed as the undoped NaYF4 shell thickness increases from 0 nm to 23.0 nm. In contrast, for the plasmonic structured samples, multi-oscillations in UCL enhancement were observed in the undoped NaYF4 shell thickness range of 0-23.0 nm, where the UCL enhancement factors of three bands (521 nm, 540 nm and 654 nm) are high up to 65, 101 and 61, respectively, at 19.6 nm-thick NaYF4 shell. The multi-oscillations in UCL enhancement in the plasmonic samples can be associated with plasmonic coupling between arrays of core-shell UCNPs with various sizes and the underlying 2D Au spheres. The related mechanisms of the UCL enhancements are discussed.

Regulating Surface Facets of Metallic Aerogel Electrocatalysts by Size-Dependent Localized Ostwald Ripening.

It is well known that the activity and stability of electrocatalysts are largely dependent on their surface facets. In this work, we have successfully regulated surface facets of three-dimensional (3D) metallic Au m- n aerogels by salt-induced assembly of citrate-stabilized gold nanoparticles (Au NPs) of two different sizes and further size-dependent localized Ostwald ripening at controlled particle number ratios, where m and n represent the size of Au NPs. In addition, 3D Au m- n-Pd aerogels were further synthesized on the basis of Au m- n aerogels and also bear controlled surface facets because of the formation of ultrathin Pd layers on Au m- n aerogels. Taking the electrooxidation of small organic molecules (such as methanol and ethanol) by the resulting Au m- n and Au m- n-Pd aerogels as examples, it is found that surface facets of metallic aerogels with excellent performance can be regulated to realize preferential surface facets for methanol oxidation and ethanol oxidation, respectively. Moreover, they also indeed simultaneously bear high activity and excellent stability. Furthermore, their activities and stability are also highly dependent on the area ratio of active facets and inactive facets on their surfaces, respectively, and these ratios are varied via the mismatch of sizes of adjacent NPs. Thus, this work not only demonstrates the realization of the regulation of the surface facets of metallic aerogels by size-dependent localized Ostwald ripening but also will open up a new way to improve electrocatalytic performance of 3D metallic aerogels by surface regulation.

Time-dependent optical response of three-dimensional Au nanoparticle arrays formed on silica nanowires

We present stationary and transient absorption measurements on 3D Au nanoparticle (NP)-decorated $\mathrm{Si}{\mathrm{O}}_{2}$ nanowire arrays. The 3D NP array has been produced by the dewetting of a thin Au film deposited on silica nanowires produced by oxidation of silicon nanowires. The experimental behaviors of the spectral and temporal dynamics observed in the experiment are accurately described by a two-step, three-temperature model. Using an arbitrary set of Au NPs with different aspect ratios, we demonstrate that the width of the experimental spectra, the energy shift of their position with time, and the asymmetry between the two positive wings in the dynamical variation of absorption can all be attributed to the nonuniform shape distribution of the Au NPs in the sample.

Controlling the electric charge of gold nanoplatelets on an insulator by field emission nc-AFM

Charging of 2D Au nanoplatelets deposited on an insulating SiO2 substrate to or from the tip of a non-contact atomic force microscope (nc-AFM) is demonstrated. Charge transfer is controlled by monitoring the resonance frequency shift Δf(V) during the bias voltage ramp V applied to the tip-back electrode junction. The onset of charge transfer is revealed by a transition from a capacitive parabolic behavior to a constant Δf(V) region for both polarities. An analytical model, based on charging by electron field emission, shows that the field-emitted current saturates shortly after the onset of the charging, due to the limiting effect of the charge-induced rise of the Au platelet potential. The value of this current plateau depends only on the rate of the bias voltage ramp and on the value of the platelet/SiO2/back electrode capacitance. This analysis is confirmed by numerical simulations based on a virtual nc-AFM model that faithfully matches the experimental data. Our charging protocol could be used to tune the potential of the platelets at the single charge level.

A general soft-enveloping strategy in the templating synthesis of mesoporous metal nanostructures

Metal species have a relatively high mobility inside mesoporous silica; thus, it is difficult to introduce the metal precursors into silica mesopores and suppress the migration of metal species during a reduction process. Therefore, until now, the controlled growth of metal nanocrystals in a confined space, i.e., mesoporous channels, has been very challenging. Here, by using a soft-enveloping reaction at the interfaces of the solid, liquid, and solution phases, we successfully control the growth of metallic nanocrystals inside a mesoporous silica template. Diverse monodispersed nanostructures with well-defined sizes and shapes, including Ag nanowires, 3D mesoporous Au, AuAg alloys, Pt networks, and Au nanoparticle superlattices are successfully obtained. The 3D mesoporous AuAg networks exhibit enhanced catalytic activities in an electrochemical methanol oxidation reaction. The current soft-enveloping synthetic strategy offers a robust approach to synthesize diverse mesoporous metal nanostructures that can be utilized in catalysis, optics, and biomedicine applications.

Coupling effects in 3D plasmonic structures templated by Morpho butterfly wings.

This paper presents the study of the coupling effects of three dimensional (3D) plasmonic nanostructures templated by Morpho butterfly wings. Different from the random deposition of metallic nanoparticles (NPs) or conformal coating of metallic layers on butterfly wings reported previously, the 3D plasmonic nanostructures studied in this work consist of gold (Au) nanostrips quasi-periodically arranged in 3D, which allows us to investigate the plasmonic coupling effects. Through refractive index (RI) matching, the plasmonic coupling can be differentiated from the optical contribution of butterfly wings. By tuning the deposition thickness of Au from 30 to 90 nm, the plasmonic coupling effects between the 3D Au nanostrips are gradually enhanced. In particular, the near-field coupling results in two resonant modes and enhances the surface-enhanced Raman scattering (SERS) signals.

Formation of Au Nanoparticles in Liquid Cell Transmission Electron Microscopy: From a Systematic Study to Engineered Nanostructures.

In this work, a systematic study of the effect of electron dose rate, solute concentration, imaging mode (broad beam vs scanning probe mode), and liquid cell setup (static vs flow mode) on the growth mechanism and the ultimate morphology of Au nanoparticles (NPs) was performed in chloroauric acid (HAuCl4) aqueous solutions using in situ liquid-cell TEM (LC-TEM). It was found that a diffusion limited growth dominates at high dose rates, especially for the solution with the lowest concentration (1 mM), resulting in formation of dendritic NPs. Growth of 2D Au plates driven by a reaction limited mechanism was only observed at low dose rates for the 1 mM solution. For the 5 mM and 20 mM solutions, reaction limited growth can still be induced at higher dose rates, due to abundance of the precursor available in the solutions, leading to formation of 2D plates or 3D faceted NPs. As a proof-of-concept, an Au nanostructure with a 3D faceted particle core and a dendritic shell can be in situ produced by simply tuning the electron dose in the 1 mM solution irradiated in a flow cell setup in the STEM mode. This work paves the way to study the growth of complex heteronanostructures composed of multiple elements in LC-TEM.

Simulation and analysis of 2D material (MoS2/MoSe2) based plasmonic sensor for measurement of organic compounds in infrared

A plasmonic sensor with enhanced performance based on chalcogenide (ChG) substrate with 2D nanomaterial (transition metal dichalcogenides: TMDC) and Au layer in NIR regime is proposed for the diagnostics of organic compounds. MoS2 and MoSe2 in their monolayer forms have been considered as the 2D materials. Detailed simulation and analysis of sensing properties of surface plasmons (SPs) supported by Au-MoS2/MoSe2 interface are carried out. The sensing performance is evaluated in terms of the figure of merit (FOM) that takes both sensitivity as well as accuracy of the sensor into account. Further, the effect of the number of MoSe2 and MoS2 layers is investigated. It is concluded that longer wavelength and smaller number of MoS2/MoSe2 (preferably monolayer) exhibit significantly enhanced FOM. Also, the strong chemical and thermal stability of ChG and 2D materials lead to highly steady sensing performance.

Tuning the Band Gap in Titanium Dioxide Thin Films by Surfactant-Mediated Confinement and Patterning of Gold Nanoparticles

Au nanoparticle (NP)-decorated titanium dioxide (TiO2) thin films (Au-TO), prepared by a unique surfactant-assisted 2D self-assembling technique of Au NP layering onto a titanium dioxide matrix (TO) with molecular-level control, in conjunction with a classic sol–gel route, showed a significant decrease in the optical band gap (ΔEg = ∼0.6 eV) of Au-TO films compared to the conventional sol–gel-prepared pristine counterpart. Strong dependence on surfactant type and deposition temperature of the 2D Au NP layer was observed upon a band gap decrease of the films. Unlike spin-coated Au NP overlayers on TiO2, which resulted in Au NP agglomeration, in this modified inverted Langmuir–Schaefer (MILS) technique, the organic surfactant induced 2D patterned confinement of Au NP layers on TiO2, causing an increase in the active surface area of the Au NP–TiO2 interface. Results of X-ray diffraction and near-edge X-ray absorption fine structure spectroscopy indicated changes in crystal structure as well as in electronic ...

Enhanced photocatalytic hydrogen production on three-dimensional gold butterfly wing scales/CdS nanoparticles

Abstract Photocatalytic water splitting via utilizing various semiconductors is recognized as a promising way for hydrogen production. Plasmonic metals with sub-micrometer textures can improve the photocatalytic performance of semiconductors via a localized surface plasmon resonance (LSPR) process. Moreover, arrays of multilayer metallic structures can help generate strong LSPR. However, artificial synthesis has difficulties in constructing novel multilayer metallic arrays down to nanoscales. Here, we use three dimensional (3D) scales from Morpho didius forewings (M) to prepare 3D Au-wings with intact hierarchical bio-structures. For comparison, we use Troides helena forewings (T) which are known for their antireflection quasi-honeycomb structures resulting in strong light absorbing ability. Results show that multilayer rib structures of Au-M can significantly amplify the LSPR of 3D Au and thus can efficiently help the photocatalytic process (9-fold increase). This amplification effect is obviously more superior to the straightforward enhancement of the absorption of incident light (Au-T, 5-fold increase). Thus, our study provides the possibility to prepare highly efficient plasmonic photocatalysts (possessing 3D multilayer rib structures) via an easy method. This work will also be revealing for plasmonic applications in other fields.

A facile synthesis of a 3D high-index Au NCs@CuO supported on reduced graphene oxide for glucose sensing

A novel electrochemical sensor based on 3D reduced graphene oxide supported high-index Au NCs@CuO (RGO-Au NCs@CuO) for detection of glucose was reported in this work. A facile one-pot hydrothermal synthetic method was employed at room temperature with electrostatic self-assembly technology for the synthesis of reduced graphene oxide supported high-index Au nanocrystals. Morphology, structure and composition of RGO-Au NCs@CuO were characterized by various technologies including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Significantly, with the combination of large active surface area and high conductivity, electrocatalytic activity toward the oxidation of glucose was markedly enhanced. Under the best experimental conditions, the steady-state current response was linearly proportional to glucose concentration in the range from 0.1μM to 3mM with a low detection limit (30nM). Meanwhile, the sensor was of excellent selectivity and stability and successfully applied to the detection of glucose in human serum.

Constructing sub-10-nm gaps in graphene-metal hybrid system for advanced surface-enhanced Raman scattering detection

We prepared a three-dimensional (3D) hybrid structure by assembling dense gold nanoparticles (Au NPs) on graphene supported on electron beam lithography-fabricated silver nanodisk arrays (Ag NDAs). Using the strategy of modulating the structure parameters via EBL system to sufficiently narrow the distances between adjacent disks, we successfully constructed sub-10-nm gaps between the horizontally patterned Ag NDs. Moreover, uniform nanometer-scale graphene gaps were obtained between Au NPs and Ag NDAs. Finite element numerical simulations revealed that the multi-dimensional plasmonic couplings in the 3D Au NP-graphene-Ag NDA system led to an electric field enhancement up to 112 times in graphene defined gaps. As demonstrated by our SERS measurements, the well-designed and fabricated 3D Au NP-graphene-Ag NDA hybrid structure exhibits 3200-fold enhancement of the Raman response of graphene, and sensitive SERS detection with a limit of 0.1 pM for crystal violet molecules, which can be attributed to the extremely strong electric field enhancement and chemical enhancement of graphene. This work represents a step towards high-sensitivity and strong-reproducibility SERS substrate fabrication, and opens a new avenue for rationally designing graphene-plasmonic hybrid structures for SERS sensing.

Eggshell membrane-templated synthesis of 3D hierarchical porous Au networks for electrochemical nonenzymatic glucose sensor

Sensitive and accurate test of blood glucose levels is necessary to monitor and prevent diabetic complications. Herein, we developed a novel and sensitive non-enzymatic glucose sensing platform by employing 3D hierarchical porous Au networks (HPANs) as electrocatalyst for glucose oxidization. The HPANs were prepared through a bio-inspired synthesis method, in which the natural eggshell membrane (ESM) was introduced as template. The structure and properties of the as-prepared HPANs were characterized by a set of techniques, including scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (XRD) and cyclic voltammetry (CV). The HPANs showed high catalytic activity towards glucose oxidization due to the unique structure. Inspiringly, the HPANs-based electrochemical glucose sensor could be driven at low potential (+0.1V) and showed an outstanding performance for glucose determination with two linear ranges of 1–500μM and 4.0–12mM, a limit of detection (LOD) of 0.2μM (3σ), and fast response time (less than 2s). Moreover, the stability and anti-interference performance of developed sensor was also excellent, enabling its preliminary application in clinical sample (human serum) test. Significantly, this work offered an environmentally friendly method for fabricating 3D nanostructure by using ESM (a biowaste) as template, setting up a typical example for producing new value-added nanomaterials with sensing application.

Synthesis of three-dimensional honeycomb-like Au nanoporous films by laser induced modification and its application for surface enhanced Raman spectroscopy

We report on the fabrication of three-dimensional (3D) Au nanoporous films with pores the size of ~200nm by laser-induced modification of as-prepared Au flat films. The construction of honeycomb-like Au porous structures should be attributed to a model of subsurface micro-explosive boiling followed by laser irradiating of Au flat films. The 3D honeycomb-like Au nanoporous films with unique interconnected frameworks exhibit ultrahigh surface enhanced Raman scattering (SERS) activity with an enhancement factor up to ~109 and excellent stability/reusability even after 300 cycles of repeated SERS analyses. The project of wielding laser light as a versatile tool for sculpting stable nanoporous films will prompt the renewed interest in the multi-functional nanomaterials.

Determination of the excitation rate of quantum dots mediated by momentum-resolved Bloch-like surface plasmon polaritons.

When light emitters are being placed in close proximity to a plasmonic system, not only the emission but also the excitation can be strongly enhanced and both yield the surface plasmon polariton (SPP) mediated fluorescence enhancement. Here, we combine the rate equation model and coupled mode theory to formulate the excitation rate of light emitters located on a periodic metallic array. The rate is expressed in terms of quantities that can be measured by angle- and polarization-resolved reflectivity and photoluminescence spectroscopy. As a demonstration, we have studied the excitation rate of CdSeTe quantum dots deposited on a 2D Au nanohole array as a function of the propagation direction of the (-1,0) Bloch-like SPPs. At the excitation wavelength of 633 nm, we find the rate remains almost constant at ~44 ps-1 regardless of the propagation direction of SPPs, which move from the Γ-X towards the Γ-M direction in the first Brillouin zone, and the crossing of the (-1,0) and (0,-1) SPPs along the Γ-M direction where two bright and dark modes are formed. The results are supported by the finite-difference time-domain simulations. We conclude the excitation rate is an intrinsic parameter and the enhanced excitation of the quantum dots arises entirely from field enhancement.

Material-assisted metamaterial: a new dimension to create functional metamaterial

A high Q-value reflective type metasurface consisting of 1D Au nanorods, a SiO2 spacer and a Au back reflector is demonstrated. It is shown that the sideband of the resonant mode can be suppressed as the resonant wavelength close to the phonon absorption of SiO2. By combining both designed structured resonance and inherent property of the based materials, a low angle-dependent metasurface with a Q-value of 40 has been demonstrated. The proposed structure will be useful for high sensitivity sensing and narrow band thermal emitter.

二维金纳米阵列制备及其光学响应

二维金纳米阵列制备及其光学响应

Observing the dynamic "hot spots" on two-dimensional Au nanoparticles monolayer film.

Interparticle spacing was controlled by evaporating water on 2D Au nanoparticles arrays. Relationships among SERS effect, SPR catalysis, and gap distance were experimentally and theoretically studied.

Toward highly sensitive surface-enhanced Raman scattering: the design of a 3D hybrid system with monolayer graphene sandwiched between silver nanohole arrays and gold nanoparticles.

We report a novel graphene-metal hybrid system by introducing monolayer graphene between gold nanoparticles (Au NPs) and silver nanohole (Ag NH) arrays. The design incorporates three key advantages to promote the surface-enhanced Raman scattering (SERS) sensing capacity: (i) making full use of the single-atomic feature of graphene for generating uniform sub-nanometer spaces; (ii) maintaining the bottom layer of Ag nanoarrays with an ordered manner for facilitating the transfer of graphene films and assembly of the top layer of Au NPs; (iii) integrating the advantages of the strong plasmonic effect of Ag, the chemical stability of Au, as well as the mechanical flexibility and biological compatibility of graphene. In this configuration, the plasmonic properties can be fine-tuned by separately optimizing the horizontal or vertical gaps between the metal NPs. Exactly, sub-20 nm spaces between the horizontally patterned Ag tips constructed by adjacent Ag NHs, and sub-nanometer scale graphene gaps between the vertically distributed Au NP-Ag NH have been achieved. Finite element numerical simulations demonstrate that the multi-dimensional plasmonic couplings (including the Au NP-Au NP, Au NP-Ag NH and Ag NH-Ag NH couplings) promote for the hybrid platform an electric field enhancement up to 137 times. Impressively, the as-prepared 3D Au NP-graphene-Ag NH array hybrid structure manifests ultrahigh SERS sensitivity with a detection limit of 10-13 M for R6G molecules, as well as good reproducibility and stability. This work represents a step towards high-performance SERS substrate fabrication, and opens up a new route for graphene-plasmonic hybrids in SERS applications.