Lightning Rod Effect Research Articles

Optical Properties of Plasmonic Mirror-Image Nanoepsilon.

We propose a novel mirror-image nanoepsilon (MINE) structure to achieve highly localized and enhanced near field at its gap and systematically investigate its plasmonic behaviors. The MINE can be regarded as a combination of two fundamental plasmonic nanostructures: a nanorod dimer and nanoring. By adapting a nanoring surrounding a nanorod dimer structure, the nanorod is regarded as a bridge pulling the charges from the nanoring to the nanorod, which induces stronger plasmon coupling in the gap to boost local near-field enhancement. Two resonance peaks are identified as the symmetric and anti-symmetric modes according to the symmetries of the charge distributions on the ring and rod dimer in the MINE. The symmetric mode in the MINE structure is preferred because its charge distribution leads to stronger near-field enhancement with a concentrated distribution around the gap. In addition, we investigate the influence of geometry on the optical properties of MINE structures by performing experiments and simulations. These results indicate that the MINE possesses highly tunable optical properties and that significant near-field enhancement at the gap region and rod tips can be realized by the gap and lightning-rod effects. The results improve understanding of such complex systems, and it is expected to guide and facilitate the design of optimum MINE structures for various plasmonic applications.

Extending nanoscale spectroscopy with titanium nitride probes

We present a means of controlling the stoichiometry of titanium nitride (TiN) coatings on probes for tip‐enhanced Raman spectroscopy measurements made using sputtering so that outstanding enhancements can be obtained. This provides a more robust alternative to gold‐coated tips that also has potential for tuning the plasmon resonance and working in new environments. Proof of concept measurements on poly(3,4‐ethylenedioxythiophene)/poly(styrenesulfonate) thin films demonstrate increases in the observed intensity with contrast values up to 3.1. TiN is mechanically, chemically, and thermally robust. When deposited under appropriate conditions it has optical properties, including a plasmon resonance, very similar to those of gold. However, the spontaneous formation of a surface TiNxOy layer with relatively high values of y has prevented attaining enhancements in tip‐enhanced Raman spectroscopy beyond that provided by the lightning rod effect. Depositing a thin layer of aluminum to form a passivating Al2O3 layer over the TiN plasmonic structure allows the stoichiometry achieved in the vacuum deposition to be maintained. Copyright © 2016 John Wiley & Sons, Ltd.

Template-Stripped Multifunctional Wedge and Pyramid Arrays for Magnetic Nanofocusing and Optical Sensing.

We present large-scale reproducible fabrication of multifunctional ultrasharp metallic structures on planar substrates with capabilities including magnetic field nanofocusing and plasmonic sensing. Objects with sharp tips such as wedges and pyramids made with noble metals have been extensively used for enhancing local electric fields via the lightning-rod effect or plasmonic nanofocusing. However, analogous nanofocusing of magnetic fields using sharp tips made with magnetic materials has not been widely realized. Reproducible fabrication of sharp tips with magnetic as well as noble metal layers on planar substrates can enable straightforward application of their material and shape-derived functionalities. We use a template-stripping method to produce plasmonic-shell-coated nickel wedge and pyramid arrays at the wafer-scale with tip radius of curvature close to 10 nm. We further explore the magnetic nanofocusing capabilities of these ultrasharp substrates, deriving analytical formulas and comparing the results with computer simulations. These structures exhibit nanoscale spatial control over the trapping of magnetic microbeads and nanoparticles in solution. Additionally, enhanced optical sensing of analytes by these plasmonic-shell-coated substrates is demonstrated using surface-enhanced Raman spectroscopy. These methods can guide the design and fabrication of novel devices with applications including nanoparticle manipulation, biosensing, and magnetoplasmonics.

What is the Key Structural Parameter for Infrared Absorption Enhancement on Nanostructures?

The key structural parameter for greater enhancement of infrared absorption on metal nanostructures was identified by systematic and exhaustive surface-enhanced infrared absorption (SEIRA) measurements for well-defined periodic gold square column (SC) arrays on silicon wafers fabricated using electron beam lithography. The SC arrays have size parameters compatible with those of vacuum-evaporated thin metal films, which are used conventionally for SEIRA. The crucially important parameter determining the enhancement factor for infrared absorption is the ratio of the gap size to the nanoparticle size. An electrostatic SC model calculation based on a nonresonant-type electromagnetic mechanism shows good quantitative agreement with experimental observations of enhanced infrared absorption of adsorbed species. Furthermore, results show that the electrostatic SC model contains the previously proposed lightning-rod effect’s case. The clarified enhancement mechanism for infrared absorption of molecules adsorbed on...

Narrow dark resonance modes in concentric ring/disk cavities

Localized surface plasmon resonances (LSPRs) in metallic nanostructures have been studied intensely in the last decade. Fano interference is an important way to decrease the resonance linewidth and enhance the spectral detection resolution. The concentric ring/disk cavities (CRDCs) support multipolar dark resonance modes and are desired nanostructures for realizing super-narrow, high-contrast-ratio Fano resonances. In this paper, the optical properties of CRDCs are analyzed by conducting systematic numerical studies. The multipolar dark modes can be excited directly by horizontal incident light and also by the bright dipolar mode of an external nanodisk. The relationship between the dark modes and the resulting Fano resonances are discussed, and the formation mechanism of the narrow dark modes is illustrated by the electrostatic lightning rod effect that causes large field enhancement in the ring gap of CRDCs.

A novel PLED architecture containing biologically synthesized gold nanoparticles and ultra thin silver layer

The influences of biologically synthesized gold nanoparticles (bio-GNPs) and ultra thin silver layer (UTSL) on the polymer light emitting diodes (PLEDs) are investigated for the first time. The performance of the fabricated PLEDs is enhanced by embedding of bio-GNPs into the hole transport layer (HTL). Furthermore, the tailored device architecture containing UTSL increases the electron transport through lightning rod effect. Bio-GNPs are successfully produced as spherical shape with size of circa 10.4nm. Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene (MEH-PPV) and [poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate); (PEDOT:PSS)/bio-GNPs blends are used as an active layer and HTL, respectively. Novel PLEDs fabricated with 0.125wt% bio-GNPs/PEDOT:PSS and 0.5nm UTSL exhibit nearly 2.5-fold enhancement in the device efficiency.

Improve the surface enhanced Raman scattering of gold nanorods decorated graphene oxide: The effect of CTAB on the electronic transition

The surface-enhanced Raman scattering (SERS) activity of the graphene oxide (GO) and Au nanorod (AuNR) hybrid (GO–AuNR) has been investigated by using the rhodamine 6G (R6G) as Raman active probe. Because of the existence of CTAB in the GO–AuNR substrate, the electronic transition efficiency has been greatly increased, which results in the significant improvement of SERS activity. A non-monotonous intensity change of SERS as a function of longitudinal surface plasmon resonance (SPR) wavelength has also been observed, which has been attributed to the aspect ratio-dependent competition between SPR and lightning rod effect.

Radiative transition, local field enhancement and energy transfer microcosmic mechanism of tellurite glasses containing Er3+, Yb3+ ions and Ag nanoparticles

Er3+-doped, Er3+/Yb3+ co-doped tellurite glass with and without Ag NPs were synthesized by melt-quenching method. The high resolution transmission electron microscopy (HR-TEM) and selected area electron diffractions (SAED) manifest growth of Ag NPs. The UV–vis–NIR absorption spectroscopy and fluorescence spectroscopy were measured. The optical band gap and multiphonon relaxation rate constants were calculated. The electronic band structure and local density of state (DOS) of Ag NPs are calculated. The fluorescence emission and enhancement mechanism including localized surface plasmon resonance (LSPR) and energy transfer (ET) microcosmic mechanism were discussed. The electric field distributions of Ag NPs are emulated by FDTD solutions software. Local field enhancement (LFE) induced by LSPR and lightning rod effect was found to be responsible for the fluorescence enhancement while energy transfer from Ag NPs to rare-earth was considered ignorable in the samples without photoluminescent emission.

Improve the surface-enhanced Raman scattering from rhodamine 6G adsorbed gold nanostars with vimineous branches

The surface-enhanced Raman scattering (SERS) activity of the gold nanostars with vimineous branches has been investigated by using rhodamine 6G (R6G) as the Raman active probe. The colloidal gold nanostars have two intense localized surface plasmon resonance (LSPR) peaks in the visible and infrared ranges, respectively. Besides the visible LSPR dependent local field effect induced Raman signal enhancement, the SERS ability also greatly depends on the infrared absorption from the plasmon resonance along the aligned branches. Whether increasing the peak intensity or wavelength of the infrared absorption leads to the efficient improvement of SERS. These correlations between plasmonic absorption and SERS indicate that the lightning rod effect and creation of hot spots have been enhanced with the length and number of gold branches.

Enhanced Efficiency in Dye-Sensitized Solar Cells with Shape-Controlled Plasmonic Nanostructures

In this work, we demonstrate enhanced light harvesting in dye-sensitized solar cells (DSSCs) with gold nanocubes of controlled shape. Silica-coated nanocubes (Au@SiO2 nanocubes) embedded in the photoanodes of DSSCs had a power conversion efficiency of 7.8% relative to 5.8% of reference (TiO2 only) devices, resulting in a 34% improvement in DSSC performance. Photocurrent behavior and incident photon to current efficiency spectra revealed that device performance is controlled by the particle density of Au@SiO2 nanocubes and monotonically decreases at very high nanocube concentration. Finite difference time domain simulations suggest that, at the 45 nm size regime, the nanocubes predominantly absorb incident light, giving rise to the lightning rod effect, which results in intense electromagnetic fields at the edges and corners. These intense fields increase the plasmonic molecular coupling, amplifying the carrier generation and DSSC efficiency.

Laser fabrication of gold nanoparticle clustered tips for use in apertureless near-field scanning optical microscopy.

A laser fabrication method was developed to make gold nanoparticle clustered (GNC) tips for apertureless near-field scanning optical microscopes (ANSOMs) and tip-enhanced Raman spectroscopy (TERS). The near-field Rayleigh and Raman scattering of samples are highly enhanced when a gold nanoparticle cluster is synthesized on the end of the tip. This is due to the lightning rod effect in the sharp tips. The localized electromagnetic field enhancement and the spatial resolution (~30 nm) of the fabricated GNC tip were verified by TERS and ANSOM measurements of carbon nanotubes.

Optimized Lightning-Rod Effect to Overcome Trade-Off Between Switching Uniformity and On/Off Ratio in ReRAM

For uniform switching of resistive random access memory, narrower physical switching gap between an electrode and remained conducting filament can be an effective method, which also leads to degradation of ON/OFF ratio. To overcome a trade-off between the switching uniformity and the ON/OFF ratio, an additional layer was intentionally inserted. Consequently, improved uniformity of switching parameters was achieved without degradation of the ON/OFF ratio.

Analysis and design of a cross dipole nanoantenna for fluorescence-sensing applications

This paper introduces the novel concept of a cross dipole nanoantenna for use in fluorescence based sensing applications. The dual-arm nature of the cross nanoantenna allows a dual resonant structure to be designed such that the shorter arm resonates with the pump wavelength and the longer arm with the emission wavelength. This is expected to further enhance emission from any fluorescent molecule that can couple to both nanoantenna arms when compared with a singly resonant structure. The paper uses the finite-difference time-domain method to first analyze the two-arm nanoantenna case and then shows how intensity enhancement depends on the antenna geometry and tapering of arms in the antenna gap. The results show that smaller gap sizes always produce larger enhancement compared with lightning rod effects due to tapering. A four-arm cross nanoantenna is then studied, highlighting differences from the two-arm case. Finally, the effect of a diagonally aligned molecule transiting the central gap region is studied. The results show that two hotspots occur on either side of the central gap region when the molecule is aligned perpendicular to the transit direction and only a single central hotspot occurs when the alignment is parallel to the transit direction.

Large-volume hot spots in gold spiky nanoparticle dimers for high-performance surface-enhanced spectroscopy.

Hot spots with a large electric field enhancement usually come in small volumes, limiting their applications in surface-enhanced spectroscopy. Using a finite-difference time-domain method, we demonstrate that spiky nanoparticle dimers (SNPD) can provide hot spots with both large electric field enhancement and large volumes because of the pronounced lightning rod effect of spiky nanoparticles. We find that the strongest electric fields lie in the gap region when SNPD is in a tip-to-tip (T-T) configuration. The enhancement of electric fields (|E|(2)/|E0|(2)) in T-T SNPD with a 2 nm gap can be as large as 1.21 × 10(6). And the hot spot volume in T-T SNPD is almost 7 times and 5 times larger than those in the spike dimer and sphere dimer with the same gap size of 2 nm, respectively. The hot spot volume in SNPD can be further improved by manipulating the arrangements of spiky nanoparticles, where crossed T-T SNPD provides the largest hot spot volume, which is 1.5 times that of T-T SNPD. Our results provide a strategy to obtain hot spots with both intense electric fields and large volume by adding a bulky core at one end of the spindly building block in dimers.

Ag-nanoparticles-decorated NiO-nanoflakes grafted Ni-nanorod arrays stuck out of porous AAO as effective SERS substrates

NiO-nanoflakes (NiO-NFs) grafted Ni-nanorod (Ni-NR) arrays stuck out of the porous anodic aluminum oxide (AAO) template are achieved by a combinatorial process of AAO-confined electrodeposition of Ni-NRs, selectively etching part of the AAO template to expose the Ni-NRs, wet-etching the exposed Ni-NRs in ammonia to obtain Ni(OH)2-NFs grafted onto the cone-shaped Ni-NRs, and annealing to transform Ni(OH)2-NFs in situ into NiO-NFs. By top-view sputtering, Ag-nanoparticles (Ag-NPs) are decorated on each NiO-NFs grafted Ni-NR (denoted as NiO-NFs@Ni-NR). The resultant Ag-NPs-decorated NiO-NFs@Ni-NR (denoted as Ag-NPs@NiO-NFs@Ni-NR) arrays exhibit not only strong surface-enhanced Raman scattering (SERS) activity but also reproducible SERS-signals over the whole array. It is demonstrated that the strong SERS-activity is mainly ascribed to the high density of sub-10 nm gaps (hot spots) between the neighboring Ag-NPs, the semiconducting NiO-NFs induced chemical enhancement effect, and the lightning rod effect of the cone-shaped Ni-NRs. The three-level hierarchical nanostructure arrays stuck out of the AAO template can be utilized to probe polychlorinated biphenyls (PCBs, a kind of global environmental hazard) with a concentration as low as 5 × 10(-6) M, showing promising potential in SERS-based rapid detection of organic environmental pollutants.

Solution‐based SERS method to detect dithiocarbamate fungicides in different real‐world matrices

Surface‐enhanced Raman scattering (SERS) has become a valuable tool for the characterization of trace quantities of environmental toxins. Utilizing established wet chemical synthetic protocols, dogbone‐shaped colloidal gold nanoparticle substrates with sharp features were prepared with regions that exhibit significant SERS enhancement due to the lightning rod effect. These highly enhancing substrates were utilized for the quantitative determination of two dithiocarbamate fungicides by SERS in several complex matrices such as tap water, apple juice, and vegetable juice. Limits of detection and quantitation are reported and compared with Environmental Protection Agency mandated maximum allowable concentrations in tap water. In the case of tap water, limits of detection of 13.39 ± 3.89 nM for thiram and 1.78 ± 0.20 nM for ferbam was achieved. The sensitivity of the solution‐based SERS method decreased with increasing complexity of the matrix in which the limit of detection achieved in apple juice is 47.22 nM for thiram and 11.88 ± 1.38 nM for ferbam and that for vegetable juice is 87.01 ± 2.88 nM for thiram and 36.72 ± 2.90 nM for ferbam. It was found that using the solution‐based SERS method results in sensitivities that are greater than that required by Environmental Protection Agency mandated maximum allowable concentrations for complex matrices such as apple and vegetable juice. Copyright © 2013 John Wiley & Sons, Ltd.

Lightning rod effect in surface work function of semiconductor nanomaterials

The inherent correlation between the work function and surface microstructures of semiconductor nanomaterials (SNMs) was investigated. It was found that the work function of SNMs decreased with the increase of the degree of order of SNMs. The well-aligned SNMs corresponded to the lowest work function, which was just as the lightning rod effect being of advantage to the electron transport in SNMs. Such a simple and intuitive analogy to the change trend of work function is of help to enhance the understanding of electron transport of SNMs.

Giant optical nonlinearity of a single plasmonic nanostructure

We realize giant optical nonlinearity of a single plasmonic nanostructure which we call a split hole resonator (SHR). The SHR is the marriage of two basic elements of nanoplasmonics, a nanohole and a nanorod. A peak field intensity in the SHR occurs at the single tip of the nanorod inside the nanohole. The peak field is much stronger than those of the nanorod and nanohole, because the SHR field involves contributions from the following two field-enhancement mechanisms: (1) the excitation of surface plasmon resonances and (2) the lightning-rod effect. Here, we demonstrate the use of the SHR as a highly efficient nonlinear optical element for: (i) the generation of the third harmonic from a single SHR; (ii) the excitation of intense multiphoton luminescence from a single SHR.

Understanding of the correlation between work function and surface morphology of metals and alloys

Abstract The relationships between material behaviors and its structures are extremely complicated, and the understanding of these relationships is of much significance for revealing the physical, chemical and mechanical properties of various materials. In this study, the change of the work function (WF) of metals and alloys as a function of surface roughness was investigated by scanning Kelvin probe, with the aim of understanding the inherent correlation between the WF and surface morphology using a simple and intuitive way. It was demonstrated that at the rough surface of Cu and Ag, the sharp micro/nanostructures induced a lower WF, just as the lightning rod effect providing a direct and fast path for electron transport. While for Al and Mg alloys, the rough surface resulted in an increase of the WF owing to the effect of surface oxide layers, just as the anti-lightning rod effect providing a protected layer to confine the electron transport.

Nanooptical Studies on Physical and Chemical Characteristics of Noble Metal Nanostructures

Abstract Studies on physical and chemical properties of noble metal nanostructures, mainly by near-field optical microscopy and spectroscopy, are described. Near-field optical microscopy provides optical observation methodology with a spatial resolution in nanometer regime beyond the diffraction limit of light. We developed near-field imaging systems equipped with various light sources including ultrashort pulsed lasers, which enables advanced nonlinear and ultrafast near-field measurements as well as conventional near-field imaging. In particular, near-field two-photon excitation imaging was shown to provide a convenient tool to visualize the enhanced optical fields in the vicinities of metal nanostructures. With these methods we demonstrated that nanoscale optical field structures in metal nanostructures can be directly visualized. For single noble metal nanoparticles, such as gold nanorods as typical examples, plasmon standing wave functions were visualized. Ultrafast imaging revealed that sub-picosecond relaxation was reflected on the plasmon wave function images through thermally induced dielectric function changes of the metal. In some cases optical field distribution features arising from the lightning rod effects were observed, depending on the resonance conditions of the incident wavelengths with the plasmon modes. We also found anomalous near-field transmission phenomenon for nanoapertures blocked by nanodisks near the plasmon resonance wavelengths, which arise from the efficient near-field to propagating-field conversion ability of the nanodisks. In assembled nanoparticles, enhanced optical fields at the gap sites between the particles were visualized, which elucidates experimentally the mechanism of surface-enhanced Raman scattering. The characteristic field distributions in many particle assemblies were also observed and analyzed. Through these studies, we established a valuable methodology to investigate optical and spectroscopic properties of metal nanostructures, and the information obtained is available only by optical measurements with high spatial resolution.