Metallic Nanorods Research Articles

Tuning the Optical Properties of Hyperbolic Metamaterials by Controlling the Volume Fraction of Metallic Nanorods.

Porous films of anodic aluminum oxide are widely used as templates for the electrochemical preparation of functional nanocomposites containing ordered arrays of anisotropic nanostructures. In these structures, the volume fraction of the inclusion phase, which strongly determines the functional properties of the nanocomposite, is equal to the porosity of the initial template. For the range of systems, the most pronounced effects and the best functional properties are expected when the volume fraction of metal is less than 10%, whereas the porosity of anodic aluminum oxide typically exceeds this value. In the present work, the possibility of the application of anodic aluminum oxide for obtaining hyperbolic metamaterials in the form of nanocomposites with the metal volume fraction smaller than the template porosity is demonstrated for the first time. A decrease in the fraction of the pores accessible for electrodeposition is achieved by controlled blocking of the portion of pores during anodization when the template is formed. The effectiveness of the proposed approach has been shown in the example of obtaining nanocomposites containing Au nanorods arrays. The possibility for the control over the position of the resonance absorption band corresponding to the excitation of collective longitudinal oscillations of the electron gas in the nanorods in a wide range of wavelengths by controlled decreasing of the metal volume fraction, is shown.

Strong and tunable plasmonic field coupling and enhancement generating from the protruded metal nanorods and dielectric cores

Abstract Numerical investigation of resonant peak by modifying the number of protruded metal nanorods (MNRDs) and the core material in metal nanoshells for near field intensity are explored by means of the finite element method. The surface plasmon effect arising from the core cases with/without the protruded MNRDs was compared along with that arose from their solid counterparts. A strong and tunable field coupling and enhancement effects corresponding to the transverse surface plasmon resonance (SPR) and cavity plasmon resonance (CPR) modes are observed. It is found that the near field intensity spectra obtained from the silver and silver-shell nanorods with the protruded silver nanorods is approximately 2.5–3.0 times larger than that of the silver nanorods without the protruded silver nanorods. The peak resonance wavelength shows a red shift as the increasing of the permittivity in the core regions and the number of the protruded MNRDs. This indicates that the core regions and the protruded MNRDs can significantly mediate the EM waves coupling and enhancing the plasmonic effects and play a key role to tune the peak resonance wavelengths position. The investigated nanostructures offer multiple design freedoms to modulate the peak resonant wavelength and electromagnetic wave properties.

Design of an achromatic projection system on a curved surface for enlarging view using polarization control metasurface

Image projection systems are widely used in modern applications. To enlarge the field of view (FOV), especially for curved-surface projection, multiple projectors are used to display different images, and the images are spliced with increasing system volume. For a single projector, chromatic aberration caused by the edge of the complex optical system is unavoidable during the enlargement of view angle. We propose and designed a projection system using a metallic nanorod antenna array metasurface with polarization control to enlarge the FOV with a slim structure and realize achromatic multicolor display on curved surfaces. With far-field computational simulations, three output images were stitched into a picture on a hemispherical surface, and the field diffraction angle was found to be three times as large as direct projection, approaching 270 deg theoretically. With the derived formula, the achromatic effect can be realized by optimizing incident angle parameters at different wavelengths, with an angular dispersion of <6.6 × 10 − 5 deg / nm, which is sufficient for multicolor display. The system greatly reduces the complexity of optical design with multicolor display and shows potential for the miniaturization of curved-surface projection with the metasurface.

Finite-element method for obtaining the regularized photon Green function in lossy material

The photon Green function (GF) is a vital and decisive factor in the field of quantum light-matter interaction. It is divergent with two equal space arguments in lossy structure and should be regularized. We introduce a finite-element method (FEM) for calculating the regularized GF in arbitrary-shaped lossy structure, which is expressed by the averaged radiation electric field over the finite size of a photon emitter. For an emitter located in a homogeneous lossy material, excellent agreement with the analytical results is found for both real and virtual cavity models. For an emitter located in a metal nano-sphere, the regularized scattered GF, which is the difference between the regularized GF and the analytical regularized one in homogeneous space, agrees well with the analytical scattered GF. Applying this method for an emitter located in a metal nano-rod where there is no analytical solution, we find that the scattering contribution to the enhancement of the spontaneous emission rate is nearly unrelated to the cavity radius while the homogeneous part depends heavily. Our results should be significant for novel photon sources within lossy structures.

Tunable metasurface refractive index plasmonic nano-sensor utilizing an ITO thin layer in the near-infrared region.

In this paper, a refractive index plasmonic nano-sensor with an array of metallic nano-rods has been designed in the near-infrared region. The tunable sensor is based on a perfect absorber, and an indium tin oxide (ITO) layer has been utilized as an active metasurface to improve sensor performance. By applying the voltage to the ITO layer, a perfect absorption has been obtained. Also, the sensitivity rises from 625 to 1125 nm/RIU (refractive index unit), and the figure of merit is 10 times higher. Moreover, the sensor application in the fields of medicine and hematology has been shown. The proposed sensor can be used to diagnose and determine anemia and malaria parasites.

Tapered arrangement of metallic nanorod chains for magnified plasmonic nanoimaging

Plasmonic nanolens, a 3-dimensional tapered arrangement of metallic nanorod chains, holds a great promise as a new plasmonics-based optical nano-imaging technique. While multiple nanorod chains can transfer the near-field signal originating from a sample to an image at a distance larger than a micro-meter, where each nanorod chain contributes in forming one pixel in the image, the tapered arrangement of the nanorod chains with a certain taper angle allows image magnification. We experimentally demonstrate the feature of image formation and magnification in a nanolens by fabricating a tapered arrangement of two silver nanorod chains, which were separated by a distance smaller than the diffraction limit at one end and larger than the diffraction limit at the other end. We placed two nano-sized optical sources of quantum dots near the first ends of the chains, which served as two subwavelength objects. In the optical measurement, we demonstrated that the unresolved subwavelength optical sources could be imaged at the other ends of the chains and were well resolved in accordance with the magnification feature of a nanolens. This verification is an experimental proof of the image magnification, and an important step toward the realization of plasmonic nanolens.

Supercluster-coupled crystal growth in metallic glass forming liquids

While common growth models assume a structure-less liquid composed of atomic flow units, structural ordering has been shown in liquid metals. Here, we conduct in situ transmission electron microscopy crystallization experiments on metallic glass nanorods, and show that structural ordering strongly affects crystal growth and is controlled by nanorod thermal history. Direct visualization reveals structural ordering as densely populated small clusters in a nanorod heated from the glass state, and similar behavior is found in molecular dynamics simulations of model metallic glasses. At the same growth temperature, the asymmetry in growth rate for rods that are heated versus cooled decreases with nanorod diameter and vanishes for very small rods. We hypothesize that structural ordering enhances crystal growth, in contrast to assumptions from common growth models. The asymmetric growth rate is attributed to the difference in the degree of the structural ordering, which is pronounced in the heated glass but sparse in the cooled liquid.

Active Control of Chiral Optical near Fields on a Single Metal Nanorod

Chiral optical fields (typified by circularly polarized light) localized on the nanoscale enhance the chiral light–matter interaction, which may provide novel potential applications. This property enables the development of an ultrasensitive method for characterization of chiral molecules and nanoscale magnetic control realized by an all-optical method to interconnect spintronic nano-optical devices. A local chiral light source with switchable handedness or controllable chirality is indispensable for building such applications for practical use. In the current major method used for local chiral light generation, the handedness of the light is controlled by the handedness of the nanomaterial, which is not convenient when we need to change the handedness of the light. We experimentally achieve here generation and active control of a highly chiral local optical field by using a combination of an achiral gold nanorod and achiral linearly polarized optical field. By tilting the azimuth angle for the incident l...

Plasmon-tuned upconversion luminescence in silanized NaYF4:Yb,Er nanocrystals via ultra-thin gold film

Plasmonic enhancement of the upconversion luminescence (UCL) via ultra-thin gold film was rarely investigated so far and the photophysical mechanism was not completely clear, although the noble metal nanoparticles or nanorods have been used to enhance the UCL of upconverting nanophosphors. Two types of sandwich geometry involving gold film and silanized nanophosphors were designed and facilely fabricated. Both intensified and quenched UCL were observed under the excitation of 980 nm NIR laser. Based on the steady state spectra and the UCL decay data, the coupling of the excitation energy with the surface plasmon resonance (SPR) absorption band of ultrathin gold film with significant roughness surface could be responsible for the UCL enhancement. Results indicated that employing the SPR effect on the rough surface of gold film could be a promising strategy to achieve efficient and tunable UCL, which makes it more practical for the widespread applications.

Study of cavity-enhanced dipole emission on a hyperbolic metamaterial slab

We theoretically study strong coupling of a dipole emitter positioned close to a hyperbolic metamaterial (HMM) slab. Enhanced dipole emission with a large Purcell factor originated from the superposition of p-wave resonant coupling and critical coupling to the high-k waveguide modes in a HMM slab is addressed, and the dependence of the slab thickness and the fill ratio of metallic nanorods in HMM on the emission strength is explicitly summarized. Via optimizing the mode coupling condition, we point out a general design rule for the HMM structure to yield a large Purcell factor. Furthermore, by incorporating a Si bullseye grating on the HMM slab, 18-fold far-field radiation enhancement owing to extraction of the mainly coupled resonant mode in HMM is presented.

Controlled fabrication of hierarchical metal nanostructures

Hierarchical metal nanostructures (HMNS) have attracted great interests for applications in electronics, catalysis, and sensors. Here, we report a novel method to prepare hierarchical metal nanorods arrays by combining superplastic nanomolding with dealloying. The fabricated sample possesses size as large as centimeters and with uniform hierarchical metal nanorods chemically attached in its surface, which guarantees the structural stability of the HMNS. This is distinct with other techniques, where assembling nanostructures on the macroscale with good uniformity and maintaining good mechanical stability has general challenging. We further show that the microstructures in HMNS can be tuned through independent ways, which provides the possibility to study microstructure-property relationship.

Investigations on the surface plasmonic effect of pulsed laser dispersed (PLDi) NiTi and CuAl alloy nanoparticles on hydrothermally grown ZnO nanorods

In this work, we have demonstrated the photoluminescence enhancement of metal alloy (NiTi and CuAl) coated ZnO nanorods. The ZnO nanorods were grown by the conventional hydrothermal process on which the alloy nanoparticles (NiTi/ CuAl) were dispersed through pulsed laser deposition technique. An enhancement in photoluminescence was observed because of metal alloy nanoparticles were dispersed on ZnO nanorods. The maximum achieved UV enhancement with a factor of ∼8 and defect suppression with a factor of ∼2.4 were observed for NiTi coated ZnO nanorods. However, CuAl coated ZnO nanorods showed relatively less enhancement (∼4.3) and improved suppression (∼4.1) compared to NiTi coated ZnO nanorods. Such emission enhancement and defect suppression can be attributed to the resonant coupling between electron-hole pair and carrier transfer between metal and ZnO nanorods. Hence, the metal-ZnO interface plays an important role towards the enhancement factors. Therefore, we can claim that the Earth-abundant metal alloy coated ZnO nanorods show comparable surface plasmon resonance effect in comparison to the noble metals coated ZnO nanostructures.

The Generalized Analytical Expression for the Resonance Frequencies of Plasmonic Nanoresonators Composed of Folded Rectangular Geometries

A robust generalized analytical expression for resonance frequencies of plasmonic nanoresonators, which consists of folded rectangular structures, is proposed based on a circuit route. The formulation is rigorously derived from the lumped circuit analogue of the plasmon resonance in a rectangular metallic nanorod. Induced by the nonhomogeneous charge distributions in the plasmonic resonators of rectangular end-caps, the electromagnetic forces drive the harmonic oscillations of free electrons in the plasmonic nanoresonators, generating intrinsically nonlinear shape-dependent LC resonance responses. Even for the plasmonic nanoresonators with much larger structure sizes than the skin depths, the significant frequency deviations due to the phase-retardation behavior can still be adequately described by the generalized expression. Moreover, for a large range of plasmonic nanoresonators with various folded rectangular geometries, sizes and materials, the generalized analytical expression gives the underlining physics and provides accurate predictions, which are perfectly verified by a series of numerical simulations. Our studies not only offer quantitative insights of nearly any plasmonic nanoresonators based on folded rectangular geometries, but also reveal potential applications to design complex plasmonic systems, such as periodic arrays with embedded rectangular nanoresonators.

Multisegmented Metallic Nanorods: Sub-10 nm Growth, Nanoscale Manipulation, and Subwavelength Imaging.

Multisegmented metallic nanorods (MS-M-NRs) have attracted increasing attention thanks to their integrated structures and complex functions. The integration of nanoscale segments in 1D enables maximum exposure of each segment and enhanced interaction between adjacent segments. Such structural integration will induce functional complexity in the nanorods, leading to superior properties for the individual components. Herein, recent progress on the development of MS-M-NRs is reviewed. Their precise fabrication, nanoscale manipulation, and subwavelength imaging, as well as simultaneous manipulation and imaging are discussed, respectively. Specifically, precise fabrication of MS-M-NRs focuses on porous anodic alumina (PAA) templated electrodeposition, which enables sub-10 nm growth of the segments and their interfaces/fronts. Nanoscale manipulation of MS-M-NRs introduces the fundamental methods that are employed for delicate movement control on the nanorods through internal or external stimulations. Subwavelength imaging of MS-M-NRs highlights the achievements on identification and location of constituent nanoscale segments/gaps based on their differences and interactions. Simultaneous manipulation and imaging of MS-M-NRs addresses the significance and potential applications of the nanorods with magnetic-plasmonic dual modulation. The development of MS-M-NRs will greatly contribute to materials science and nanoscience/nanotechnology.

When is Lonely Adatom Model valid?

The Lonely Adatom Model (LAM) provides a description of nucleation during crystal growth. LAM is valid when the adatom number on the nucleating surface is sufficiently low. However, how low is sufficiently low, or when will the adatom number be sufficiently low? This paper reports an analytical theory that defines the critical deposition conditions when LAM is valid for nanorod growth. For metallic nanorods, LAM is valid for mode II growth over nearly the entire range of substrate temperatures, but it is valid for mode I growth only below room temperatures.

Highly conductive and metallic cobalt-nickel selenide nanorods supported on Ni foam as an efficient electrocatalyst for alkaline water splitting.

Water splitting has long been considered as a promising chemical reaction that can produce clean hydrogen fuel to relieve the energy crisis and environmental pollution. Herein, we report that Co0.75Ni0.25Se/NF formed by two-step growth of metallic cobalt-nickel selenide nanorods on porous nickel foam was used as a bifunctional electrocatalyst. Ni foam serves as a slow-releasing nickel source together with a Co source to form a special proportional cobalt-nickel selenide. Due to its unique rough nanostructure, bimetallic cooperative effects and intrinsic metallic character, the obtained Co0.75Ni0.25Se/NF electrode exhibits a low overpotential of 269 mV (50 mA cm-2) for the oxygen evolution reaction and an overpotential of 106 mV (10 mA cm-2) for the hydrogen evolution reaction. Furthermore, this bifunctional electrocatalyst requires a cell voltage of 1.60 V to achieve a current density of 10 mA cm-2. Besides, based on theoretical calculation, it is further shown that the synergy between Co and Ni elements is beneficial for improving the internal structure of the catalyst, resulting in a high electrical conductivity, and low HER Gibbs free-energy and water adsorption energy. The present results indicate that Co0.75Ni0.25Se/NF exhibits advanced electrocatalytic activity for overall water splitting. This work offers an appropriate methodology and theoretical guidance to synthesize a bimetal-selenide electrocatalyst for water splitting.

横向激励下金属纳米棒聚合体的Fano共振

横向激励下金属纳米棒聚合体的Fano共振

Highly efficient and tunable selective absorber design based on metallic nanoparticles in a graded index dielectric

We propose a novel solar selective absorber design based on transverse localized surface plasmon resonances of infinite metallic nanorods embedded in a graded index dielectric slab. The physics principles on which the design is based are explained, and decent results are obtained by numerical simulations; solar absorptance values exceeding 0.99 are reached together with a near-zero infrared emittance. The proposed structure design offers a flexible tunability of thermal emission, and this spectral control over thermal emission promises advances not only in solar energy harvesting efficiency, but also in sensing, camouflage, and other thermal management applications.

Nanorattle Au@PtAg encapsulated in ZIF-8 for enhancing CO2 photoreduction to CO

Imidazolate-based ZIF-8 catalysts M@ZIF-8 (M = Au NR, Au@Ag NR, or Au@PtAg NRT; NR = nanorod, NRT = nanorattle), were assembled. Au NRs acted as the core for the epitaxial growth of the Ag shell, and oxidative etching of Au@Ag NRs led to Au@PtAg NRTs with K2PtCl4 aqueous solution. All metal nanorods (MNRs) and metal nanorattles (MNRTs) were well dispersed and fully encapsulated in ZIF-8. Au@PtAg NRTs encapsulated in ZIF-8 could lead to enhanced stability and selectivity for catalytic applications, combining the advantages of ZIF-8 (tailorable porosity) with the high surface area and improved optical sensitivity of rod-shaped NRTs. The catalyst Au@PtAg@ZIF-8 exhibited efficient catalytic activity and CO selectivity for the gas-phase photoreduction of CO2 with H2O.

Crystal structure dependence of the breathing vibration of individual gold nanodisks induced by the ultrafast laser.

The ultrafast laser-excited breathing vibrations of gold nanodisks with different crystal structures have been studied via atomistic simulations. The vibrational periods and damping rates of nanodisks are obtained by the analysis of the simulated transient responses of nanodisks. It is shown that the breathing period of nanodisks is considerably dependent on their crystal structure, which is contrary to the cases for the breathing vibration of metal nanospheres and nanorods. Furthermore, single-crystal nanodisks exhibit much lower intrinsic damping as compared with polycrystalline nanodisks, for which the additional energy dissipation by the grain boundaries in the polycrystalline nanodisks could be one major factor.