Spatial control of coherent anti-stokes emission with height-modulated gold zig-zag nanowires

Intrinsic coherent anti-Stokes emission is observed in lithographically patterned gold nanowires. Polarization dependent measurements reveal that the nanostructure's anti-Stokes response is polarized in the direction of the transverse surface plasmon resonance of the wire. We have used specially fabricated gold nanozigzag wires that are modulated in height between 20 and 80 nm to demonstrate tuning of the plasmon polarizability through control of wire height. Stronger anti-Stokes emission is shown to correlate with structures that support higher plasmon polarizability, underlining the primary role of the transverse plasmon resonance in the generation of anti-Stokes radiation from gold nanostructures. Our results also point out that a potential surface-enhanced coherent anti-Stokes Raman scattering (CARS) assay for detecting the vibrational response of surface-tethered molecules needs to include a mechanism for separating the molecular response from the strong intrinsic anti-Stokes emission of the metallic nanosubstrate.     

Tuning the optical properties of metamaterials based on gold nanowire arrays embedded in alumina

We fabricated a series of gold nanowires/alumina composite films with different wire lengths. Optical transmission measurements confirmed that the composite films exhibit transverse and longitudinal surface plasmon resonances. We show that the wavelength of the longitudinal resonance is sensitive to nanowire length, while that of the transverse resonance is not. The experimental results are in agreement with the modeled results based on the Maxwell Garnett effective medium theory. Moreover, the window for negative refraction of the samples can be tuned in synchronism with the longitudinal resonance by the nanowire length.     

One-step construction of silver nanowires in hexagonal mesoporous silica using the polyol process

An original one-step preparation of single-crystal silver nanowires in hexagonal mesoporous silica is presented. The silver precursor, silver nitrate, is reduced in ethylene glycol (EG). This procedure avoids thermal treatments which can lead to phase segregation. The absorbance spectrum of the resulting hybrid material exhibits a transverse resonance plasmon peak near 360 nm whereas the longitudinal oscillation is shifted to the near-infrared region at about 1500 nm.     

Calculations of light scattering from isolated and interacting metallic nanowires of arbitrary cross section by means of Green's theorem surface integral equations in parametric form

We study theoretically the light scattering from metal wires of arbitrary cross section, with emphasis on the occurrence of plasmon resonances. We make use of the rigorous formulation of the Green's theorem surface integral equations of the electromagnetic wave scattering, written for an arbitrary number of scatterers described in parametric form. We have investigated the scattering cross sections for nanowires of various shapes (circle, triangles, rectangles, and stars), either isolated or interacting. The relationship between the cross sectional shape and the spectral dependence of the plasmon resonances is studied, including the impact of nanoparticle coupling in the case of interacting scatterers. Near-field intensity maps are also shown that shed light on the plasmon resonance features and the occurrence of local field enhancements.     

Raman spectroscopy and structure of crystalline gallium phosphide nanowires

Gallium phosphide nanowires with a most probable diameter of approximately 20.0 nm and more than 10 microns in length have been synthesized by pulsed laser vaporization of a heated GaP/5% Au target. The morphology and microstructure of GaP nanowires have been investigated by scanning electron microscopy and transmission electron microscopy. Twins have been observed along the crystalline nanowires, which have a growth direction of [111]. Raman scattering shows a 4 cm-1 downshift of the longitudinal optical phonon peak in the nanowire with respect to the bulk; the transverse optical phonon frequency and line width are, however, the same as in the bulk. The quantum confinement model first proposed by Richter et al. cannot explain the observed behavior of the Raman modes.     

Effect of molecular adsorption on the electrical conductance of single au nanowires fabricated by electron-beam lithography and focused ion beam etching

Metal nanowires are one of the potential candidates for nanostructured sensing elements used in future portable devices for chemical detection; however, the optimal methods for fabrication have yet to be fully explored. Two routes to nanowire fabrication, electron-beam lithography (EBL) and focused ion beam (FIB) etching, are studied, and their electrical and chemical sensing properties are compared. Although nanowires fabricated by both techniques exhibit ohmic conductance, I-V characterization indicates that nanowires fabricated by FIB etching exhibit abnormally high resistivity. In addition, the resistivity of nanowires fabricated by FIB etching shows very low sensitivity toward molecular adsorption, while those fabricated by EBL exhibit sensitive resistance change upon exposure to solution-phase adsorbates. The mean grain sizes of nanowires prepared by FIB etching are much smaller than those fabricated by EBL, so their resistance is dominated by grain-boundary scattering. As a result, these nanowires are much less sensitive to molecular adsorption, which mediates nanowire conduction through surface scattering. The much reduced mean grain sizes of these nanowires correlate with Ga ion damage caused during the ion milling process. Thus, even though the nanowires prepared by FIB etching can be smaller than their EBL counterparts, their reduced sensitivity to adsorption suggests that nanowires produced by EBL are preferred for chemical and biochemical sensing applications.     

Plasmon Modes Induced by Anisotropic Gap Opening in Au@Cu2O Nanorods

Integration of semiconductors with noble metals to form heteronanostructures can give rise to many interesting plasmonic and electronic properties. A number of such heteronanostructures have been demonstrated comprising noble metals and n‐type semiconductors, such as TiO₂, ZnO, SnO₂, Fe₃O₄, and CuO. In contrast, reports on heteronanostructures made of noble metals and p‐type semiconductors are scarce. Cu₂O is an unintentional p‐type semiconductor with unique properties. Here, the uniform coating of Cu₂O on two types of Au nanorods and systematic studies of the plasmonic properties of the resultant core–shell heteronanostructures are reported. One type of Au nanorods is prepared by seed‐mediated growth, and the other is obtained by oxidation of the as‐prepared Au nanorods. The (Au nanorod)@Cu₂O nanostructures produced from the as‐prepared nanorods exhibit two transverse plasmon peaks, whereas those derived from the oxidized nanorods display only one transverse plasmon peak. Through electrodynamic simulations the additional transverse plasmon peak is found to originate from a discontinuous gap formed at the side of the as‐prepared nanorods. The existence of the gap is verified and its formation mechanism is unraveled with additional experiments. The results will be useful for designing metal–semiconductor heteronanostructures with desired plasmonic properties and therefore also for exploring plasmon‐enhanced applications in photocatalysis, solar‐energy harvesting, and biotechnologies.     

Single particle plasmon spectroscopy of silver nanowires and gold nanorods

The excitation of surface plasmons in individual silver nanowires and gold nanorods is investigated by means of high-resolution electron energy loss spectroscopy in a transmission electron microscope. The transverse and longitudinal modes of these nanostructures are resolved, and the size variation of the plasmon peaks is studied. The effect of electromagnetic coupling between closely spaced nanoparticles is also observed. Finally, the relation between energy-loss measurements and optical spectroscopy of nanoparticle plasmon modes is discussed.     

Analytical solutions to light scattering by plasmonic nanoparticles with nearly spherical shape and nonlocal effect

We derive analytical solutions for the scattering of electromagnetic waves by a nanoparticle with nearly spherical shape and nonlocal dielectric function by using an extended Mie scattering theory with additional boundary conditions. A perturbation method is used to treat the correction due to deviation from the spherical shape. A surface characteristic function is introduced to describe the non-spherical surface profile of the nanoparticle, and it plays an important role in our analytical formulation. Complex surface plasmon modes are obtained. It is found that not only the transverse but also the longitudinal surface plasmon modes of the nanoparticle are excited due to the nonlocal effect. Our analytical formulation provides an alternative method for investigating the optical behaviors of the surface plasmon of nanoparticles with nearly spherical shape and nonlocal effect.     

High‐order DGTD methods for dispersive Maxwell's equations and modelling of silver nanowire coupling

A high‐order discontinuous Galerkin time‐domain (DGTD) method for Maxwell's equations for dispersive media of Drude type is derived and then used to study the coupling of 2D silver nanowires, which have potential applications in optical circuits without the restriction of diffraction limits of traditional dielectric waveguides. We have demonstrated the high accuracy of the DGTD for the electromagnetic wave scattering in dispersive media and its flexibility in modelling the plasmon resonant phenomena of coupled silver nanowires. Specifically, we study the cross sections of coupled nanowires, the dependence of the resonance on the number of nanowires with more resolved resonance information than the traditional FDTD Yee scheme, time‐domain behaviour of waves impinging on coupled silver nanowires of a funnel configuration, and the energy loss of resonant modes in a linear chain of circular and ellipse nanowires. Copyright © 2006 John Wiley & Sons, Ltd.     

Surface-enhanced Raman scattering of rhodamine 6G on nanowire arrays decorated with gold nanoparticles

We investigate the surface-enhanced Raman scattering (SERS) of rhodamine 6G (R6G) adsorbed on Au nanoparticles attached to InP nanowires. We find that nanowire arrays act as frameworks for effective SERS substrates with a significantly higher Raman signal sensitivity than a planar framework of Au nanoparticles adsorbed two-dimensionally on a flat surface. The SERS signal displays a clear polarization-dependent effect when the nanowires are arranged in a row. We also find that the SERS signal increases with time during continuous laser illumination. The plasmon-enhanced optical forces between Au nanoparticles may either move pairs of nanoparticles closer together or attract adsorbed molecules by moving them to the junctions of Au nanoparticle aggregates. Such effects by plasmon optical forces may cause the observed increase of the SERS signal with continuous laser illumination.     

Silver nanowire layer-by-layer films as substrates for surface-enhanced Raman scattering

In this paper, the fabrication of highly stable, surface-enhanced Raman scattering (SERS) active dendrimer/silver nanowire layer-by-layer (LBL) films is reported. Ag nanowires, approximately 100 nm in diameter, were produced in solution and transferred, using the LBL technique, onto a single fifth-generation DAB-Am dendrimer layer on a glass substrate. The Ag nanowires, and the resulting LBL films were characterized using UV-visible surface plasmon absorbance, while the LBL films were further characterized by atomic force microscopy measurements and surface-enhanced Raman and resonance Raman scattering of several analytes. The dendrimer was found to effectively immobilize the Ag nanowires with increased control over spacing and aggregation of the particles. These films are shown to be excellent substrates for SERS/SERRS measurements, demonstrating significant enhancement, and trace detection capability. Several trial analytes were tested using a variety of excitation energies, and results confirmed effective enhancement of Raman signals throughout the visible range (442-785 nm) with different molecules. Analytes were deposited onto the enhancing Ag nanowire LBL films surface using both casting and Langmuir-Blodgett monolayer transferring techniques.     

Interaction of antiparallel transverse domain walls in ferromagnetic nanowires

The interaction of antiparallel transverse domain walls in ferromagnetic nanowires was investigated via micromagnetic simulation with systematic variations of the external field strength as well as the wire thickness. The interaction of antiparallel transverse walls after domain wall collision exhibited damped multiple collisions due to the rigid structure of the antiparallel transverse walls. The detailed process during the multiple collisions was analyzed via the Fast Fourier Transform technique, along with a careful examination of the inner spin structures of the colliding domain walls. It was found that a frequency peak of multiple collisions shifted to a higher peak position as the external field strength increases. With a stronger field strength of around a few hundred mT, it was found that two antiparallel transverse walls were finally annihilated with formation of complex antivortex structures.     

Silicon nanowire Esaki diodes

We report on the fabrication and characterization of silicon nanowire tunnel diodes. The silicon nanowires were grown on p-type Si substrates using Au-catalyzed vapor-liquid-solid growth and in situ n-type doping. Electrical measurements reveal Esaki diode characteristics with peak current densities of 3.6 kA/cm(2), peak-to-valley current ratios of up to 4.3, and reverse current densities of up to 300 kA/cm(2) at 0.5 V reverse bias. Strain-dependent current-voltage (I-V) measurements exhibit a decrease of the peak tunnel current with uniaxial tensile stress and an increase of 48% for 1.3 GPa compressive stress along the <111> growth direction, revealing the strain dependence of the Si band structure and thus the tunnel barrier. The contributions of phonons to the indirect tunneling process were probed by conductance measurements at 4.2 K. These measurements show phonon peaks at energies corresponding to the transverse acoustical and transverse optical phonons. In addition, the low-temperature conductance measurements were extended to higher biases to identify potential impurity states in the band gap. The results demonstrate that the most likely impurity, namely, Au from the catalyst particle, is not detectable, a finding that is also supported by the excellent device properties of the Esaki diodes reported here.     

Tailoring Optical and Plasmon Resonances in Core-shell and Core-multishell Nanowires for Visible Range Negative Refraction and Plasmonic Light Harvesting:A Review

Semiconductor nanowires(NWs) are sub-wavelength structures which exhibit strong optical(Mie)resonances in the visible range.In addition to such optical resonances,the localized surface plasmon resonances(LSPRs) in metal-semiconductor core-shell(CS) and core-multishell(CMS) NWs can be tailored to achieve novel negative-index metamaterials(N1M),extreme absorbers,invisibility cloaks and sensors.Particularly,in this review,we focus on our recent theoretical studies which highlight the versatility of CS and CMS NWs for:1) the design of negative-index metamaterials in the visible range and2) plasmonic light harvesting in ultrathin photocatalyst layers for water splitting.Utilizing the LSPR in the metal layer and the magnetic dipole(Mie) resonance in the semiconductor shell under transverse electric(TE) polarization,semiconductor-metal-semiconductor CMS NWs can be designed to exhibit spectrally overlapping electric and magnetic resonances in the visible range.NWs exhibiting such double resonances can be considered as meta-atoms and arrayed to form polarization dependent,low-loss NIM.Alternatively,by tuning the LSPR in the TE polarization and the optical resonance in the transverse magnetic(TM) polarization of metal-photocatalyst CS and semiconductor-metal-photocatalyst CMS NWs,the absorption within ultrathin(sub-50 nm) photocatalyst layers can be substantially enhanced.Notably,aluminum and copper based NWs provide absorption enhancement remarkably close to silver and gold based NWs,respectively.Further,such absorption is polarization independent and remains high over a large range of incidence angles and permittivity of the medium.Therefore,due to the tunability of their optical properties,CS and CMS NWs are expected to be vital components for the design of nanophotonic devices.     

Valence excitations and dopant distribution of Al doped ZnO nanowires analyzed by electron energy loss spectroscopy

Valence electron energy loss spectroscopy (VEELS) with scanning transmission electron microscopy (STEM) has been employed to probe the valence excitations and dopant distribution of Al doped ZnO nanowires. The results reveal that while the typical Al concentration is on the order of 1020 1/cm3, Al tends to segregate at the surface leading to an Al-rich sheath. In VEEL spectra, O-2p, Zn-3d, Al-3p, O-2s, interband transitions as well as bulk plasmon have been identified. The bulk plasmon peak is blue-shifted, and the projected interband transition decreases from 2.14 to 1.88 eV as the doping concentration increases from 0.83 x 10(20) to 2.18 x 10(20) 1/cm3.     

Synthesis and optical properties of silver nanobars and nanorice

Silver nanobars with rectangular side facets and an average aspect ratio of 2.7 have been synthesized by modifying the concentration of bromide added to a polyol synthesis. Subsequent rounding of nanobars transformed them into nanorice. Due to their anisotropy, nanobars and nanorice exhibit two plasmon resonance peaks, scattering light both in the visible and in the near-infrared regions. With a combination of discrete-dipole approximation calculations and single-nanoparticle spectroscopy, we explored the effect of nanostructure aspect ratio and corner sharpness on the frequency of plasmon resonance. Near-field calculations and surface-enhanced Raman scattering measurements on single particles were performed to show how local field enhancement changes with both the wavelength and polarization of incident light.     

Raman spectra of intrinsic and doped hydrogenated nanocrystalline silicon films

Raman scattering characteristics of intrinsic and doped hydrogenated nanocrystalline silicon films which prepared by a plasma-enhanced chemical vapor deposition system are investigated. Results indicate that Raman spectra depend intensively on microstructure and impurity in the films. Taking into account phonon confinement effect and tensile strain effect in Si nanocrystals, peak redshift of measured transverse optical modes in Raman spectra of intrinsic films can be well interpreted. With respect to Raman scattering from doped samples, besides phonon confinement effect, the peak of experimental transverse optical mode further downshifts with heightening doping level, which can be primarily assigned to impurity effect from doping. In addition, the increase in relative integral intensity ratio of transverse acoustic branch to transverse optical mode and that of longitudinal acoustic branch to transverse optical mode with decreasing mean dimension of nanocrystals and heightening doping ratio, respectively, can be ascribed to disorder. Furthermore, at the same doping level, incorporation of boron can induce higher disorder than incorporation of phosphorus in nc-Si:H films.     

Luminescence quantum yield of single gold nanorods

We study the luminescence quantum yield (QY) of single gold nanorods with different aspect ratios and volumes. Compared to gold nanospheres, we observe an increase of QY by about an order of magnitude for particles with a plasmon resonance >650 nm. The observed trend in QY is further confirmed by controlled reshaping of a single gold nanorod to a spherelike shape. Moreover, we identify two spectral components, one around 500 nm originating from a combination of interband transitions and the transverse plasmon and one coinciding with the longitudinal plasmon band. These components are analyzed by correlating scattering and luminescence spectra of single nanorods and performing polarization sensitive measurements. Our study contributes to the understanding of luminescence from gold nanorods. The enhanced QY we report can benefit applications in biological and soft matter studies.     

Wurtzite ZnSe nanowires: growth, photoluminescence, and single-wire Raman properties

Wurtzite ZnSe nanowires were prepared on GaAs substrates in a metal-organic chemical vapour deposition system. Electron microscopy shows that they are smooth and uniform in size. Both transmission electron microscopy and x-ray diffraction reveal the wurtzite structure of the nanowires, which grows along the [Formula: see text] direction. Raman scattering studies on individual nanowires were performed in the back-scattering geometry at room temperature. Besides the commonly observed longitudinal and transverse optical phonon modes, a possible surface mode located at 233?cm(-1) is also observed in the Raman spectrum. A peak located at 2.841?eV was clearly observed in the photoluminescence spectra of the nanowires, which can be assigned to near band edge emissions of wurtzite ZnSe.