Local Electric Field Enhancement Research Articles

Marangoni Convection Assisted Single Molecule Detection with Nanojet Surface Enhanced Raman Spectroscopy.

Many single-molecule (SM) label-free techniques such as scanning probe microscopies (SPM) and magnetic force spectroscopies (MFS) provide high resolution surface topography information, but lack chemical information. Typical surface enhanced Raman spectroscopy (SERS) systems provide chemical information on the analytes, but lack spatial resolution. In addition, a challenge in SERS sensors is to bring analytes into the so-called "hot spots" (locations where the enhancement of electromagnetic field amplitude is larger than 103). Previously described methods of fluid transport around hot spots like thermophoresis, thermodiffusion/Soret effect, and electrothermoplasmonic flow are either too weak or detrimental in bringing new molecules to hot spots. Herein, we combined the resonant plasmonic enhancement and photonic nanojet enhancemnet of local electric field on nonplanar SERS structures, to construct a stable, high-resolution, and below diffraction limit platform for single molecule label-free detection. In addition, we utilize Marangoni convection (mass transfer due to surface tension gradient) to bring new analytes into the hotspot. An enhancement factor of ∼3.6 × 1010 was obtained in the proposed system. Rhodamine-6G (R6G) detection of up to a concentration of 10-12 M, an improvement of two orders of magnitude, was achieved using the nanojet effect. The proposed system could provide a simple, high throughput SERS system for single molecule analysis at high spatial resolution.

Achieving Strong Field Enhancement and Light Absorption Simultaneously with Plasmonic Nanoantennas Exploiting Film-Coupled Triangular Nanodisks

Plasmonic nanoantennas (PNs) comprised of film-coupled or in-plane coupled nanoparticles have been exploited for light confinement or field enhancement. However, achieving strong local electric field enhancements (|Eloc|/|E0| > 100) and near-perfect absorption (>95%) simultaneously remains a challenge, although it will benefit a wide range of applications. Here Ag/Al2O3/Au PNs are proposed by introducing high-density triangular nanodisks into film-coupled systems, which can produce dense “hot spots” with a large |Eloc|/|E0| of 211 and a near-unity absorbance. Due to the combination of the strong lightning rod effect and the out-of-plane coupling, the MIM structure combined with the triangular nanodisks effectively enhances the coupling strength and thereby the electric field confinements along the x-, y-, and z-directions in film-coupled PNs, showing a lateral resolution as small as 4 nm. The highest |Eloc|/|E0| is more than three times higher than for their circular and square counterparts and more than ...

Laser writing of single-crystalline gold substrates for surface enhanced Raman spectroscopy

Surface enhanced Raman scattering (SERS) spectroscopy, a powerful contemporary tool for studying low-concentration analytes via surface plasmon induced enhancement of local electric field, is of utility in biochemistry, material science, threat detection, and environmental studies. We have developed a simple, fast, scalable, and relatively low-cost optical method of fabricating and characterizing large-area, reusable and broadband SERS substrates with long storage lifetime. We use tightly focused, intense infra-red laser pulses to write gratings on single-crystalline, Au (1 1 1) gold films on mica which act as SERS substrates. Our single-crystalline SERS substrates compare favourably, in terms of surface quality and roughness, to those fabricated in poly-crystalline Au films. Tests show that our SERS substrates have the potential of detecting urea and 1,10-phenantroline adulterants in milk and water, respectively, at 0.01 ppm (or lower) concentrations.

Comparative Study of Surface Interaction and Excitation

Extensive research has found nano-particles, for many years subject of unanimous concern because of their anomalous electromagnetic properties originating from the resonant interaction between light and collective conduction electron oscillations so called surface Plasmons. These properties in particular local electric field enhancements enable applications such as single molecule detection using surface enhanced Raman scattering and the synthesis of composite materials exhibiting an enhanced nonlinear optical response. Since all these applications rely on the resonant behavior of localized surface Plasmons, they are restricted to a limited frequency range determined by the dielectric function of the involved media, the size and shape of the particles and the electromagnetic interaction between them. Thus those properties of the Plasmons and Polaritons on the Carbon Nanotube have been studied in different size with help of theoretical study. The surface frequency of Plasmon varies with propagation constant on the surface of Carbon Nanotube when it is placed in dielectric medium, so this new property of Carbon Nanotube have been seen in other dielectric medium. This property of surface Plasmon can be used in telecommunication, satellite media, biomedical sciences etc.

Enhancement of multipolar Fano resonances by nanocrescent elliptical disk structures

Multipolar Fano resonance is important for sensing, nonlinear optics, and enhanced spectroscopy at short wavelength. A symmetry breaking nanocrescent elliptical disk (NCED) structure is employed to demonstrate enhanced multipolar Fano resonances. The local-electric-field enhancement (LFE) factor of hexapole and octupole modes in NCED reaches approximately 1000. This is due to the strong coupling of nanocrescent and the nanogap modes formed by a nanocrescent and elliptical disk. The multipolar resonance wavelengths can be tuned by the short axis of the ellipse and is sensitive to the surrounding refractive index in an ultraviolet and visible region. This work presents the promising applications of the multipolar Fano resonance in surface Raman spectroscopy, bio-chemistry sensing, and nonlinear optical response.

Controllable synthesis of Au@SnO2 core–shell nanohybrids with enhanced photocatalytic activities

Combination of semiconductors with plasmonic nanostructures is an effective route to promote the solar light harvesting as well as the efficiency of photocatalysis. In the present work, the Au@SnO2 hybrid nanostructures with Au nanorods as the cores and highly crystallized SnO2 nanoparticles as the shells were fabricated by a facile hydrothermal method. A critical factor, which influences the coating state of the SnO2 shells over Au NRs, was found to be the concentration of CTAB agent in the system and the corresponding mechanism was also proposed. The photocatalytic activities of the Au@SnO2 nanohybrids were examined by degradation of rhodamine B (RhB) dyes at room temperature. The Au@SnO2 nanohybrids exhibited much higher catalytic activities than that of the commercial SnO2 NPs, which could be attributed to the localized electric field enhancement effect of Au nanorods plasmon and charges transfer between the Au nanorods and SnO2.

Fabrication of Gas Ionization Sensor to Recognize Gases and Measure Pressure and Humidity

Single Ionic gas sensor (IGS) was fabricated to measure pressure, humidity and recognizes the gases. Anodic method was used to create TiO2 nanotubes (TNTs) (with internal diameter from 45 to 90 nm) on the surfaces of pure Ti and Ti-6Al-7Nb alloy. Covered surface with TNTs was operated as negative electrode to build high localized electric field and minimize breakdown voltage (Vbd). The distance between the electrodes was fixed at 60 μm. Local electric field enhancement β factor was calculated by Fowler–Nordheim plot. Its value was 857 for anodized Ti (ATi). After annealing at 800 °C; TNTs on pure Ti surface transformed to parallel rods with diameters (from 95 to 550 nm). But on alloy surface, TNTs were transformed to protrusions. Sensitivity of measuring pressure for anodized samples decreased with increasing mean free path of testing gases. For all gases; there was increment in Vbd with pressure increasing. For each gas there was specific Vbd at definite pressure and humidity; this value represented a “fingerprint” for each gas recognized it from other gases. Sensitivity of humidity for both anodized and annealed IGSs increased with increasing of relative humidity (%RH). (Vbd, Humidity) curve was approximately linear for annealed Ti and its alloy.

Theory of the optical-rectification effect in metallic thin films with periodic modulation

We conducted theoretical and numerical investigations of the optical rectification (OR) effect in metallic structures with periodic modulation. A new formulation of the OR effect is presented, and the mechanism by which the OR effect is generated, which has been a controversial issue in previous studies, is clarified. We reveal that the OR effect is strongly enhanced by a combination of spatial variation of the metallic structure and local electric field enhancement. Our theory was numerically evaluated and agreed fairly well with experiment.

Light-triggered liposomal cargo delivery platform incorporating photosensitizers and gold nanoparticles for enhanced singlet oxygen generation and increased cytotoxicity.

We developed light-triggered liposomes incorporating 3–5 nm hydrophobic gold nanoparticles and Rose Bengal (RB), a well-known photosensitizer used for photodynamic therapy. Singlet oxygen generated by these liposomes with 532 nm light illumination was characterized for varying the molar ratio of lipids and gold nanoparticles while keeping the amount of RB constant. Gold nanoparticles were found to enhance the singlet oxygen generation rate, with a maximum enhancement factor of 1.75 obtained for the molar ratio of hydrogenated soy l-α-phosphatidylcholine:1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(hexanoylamine):gold of 57:5:17 compared with liposomes loaded with RB alone. The experimental results could be explained by the local electric field enhancement caused by gold nanoparticles. We further assessed cellular cytotoxicity of gold-loaded liposomes by encapsulating an antitumor drug, doxorubicin (Dox); such Dox-loaded liposomes were applied to human colorectal cancer cells (HCT116) and exposed to light. Gold-loaded liposomes containing RB and Dox where Dox release was triggered by light were found to exhibit higher cytotoxicity compared with the liposomes loaded with RB and Dox alone. Our results indicate that gold-loaded liposomes incorporating photosensitizers may serve as improved agents in photodynamic therapy and chemotherapy.

米状银纳米颗粒对局域电场增强的理论模拟

米状银纳米颗粒对局域电场增强的理论模拟

米状银纳米颗粒对局域电场增强的理论模拟

米状银纳米颗粒对局域电场增强的理论模拟

Resistive switching of Sn-doped In2O3/HfO2 core-shell nanowire: geometry architecture engineering for nonvolatile memory.

Core-shell NWs offer an innovative approach to achieve nanoscale metal-insulator-metal (MIM) heterostructures along the wire radial direction, realizing three-dimensional geometry architecture rather than planar type thin film devices. This work demonstrated the tunable resistive switching characteristics of ITO/HfO2 core-shell nanowires with controllable shell thicknesses by the atomic layer deposition (ALD) process for the first time. Compared to planar HfO2 thin film device configuration, ITO/HfO2 core-shell nanowire shows a prominent resistive memory behavior, including lower power consumption with a smaller SET voltage of ∼0.6 V and better switching voltage uniformity with variations (standard deviation(σ)/mean value (μ)) of VSET and VRESET from 0.38 to 0.14 and from 0.33 to 0.05 for ITO/HfO2 core-shell nanowire and planar HfO2 thin film, respectively. In addition, endurance over 103 cycles resulting from the local electric field enhancement can be achieved, which is attributed to geometry architecture engineering. The concept of geometry architecture engineering provides a promising strategy to modify the electric-field distribution for solving the non-uniformity issue of future RRAM.

Synergistic effect of Au-Ag nano-alloying: intense SEIRA and enhanced catalysis.

The well-known AuAg nanohybrid system was effectively synthesized, utilizing polyvinyl pyrrolidone (PVP) as a multiple metal precursor reducing agent (based solely on their reduction potentials), thereby drastically decreasing their activation energy of diffusion and readily facilitating the homogenous alloy nanoparticle formation, duly confirmed by Vegard's law. Furthermore, the crucial parameters in determining the local electric field enhancement in the vicinity of the alloy nanoparticles such as the quality factor Q, dephasing time, T2 and the homogenous line width Γ were also evaluated in a systematic manner, in correlation with their unique visible optical absorption spectra. Optical band gap calculations of this robust AuAg/PVP nanocomposite system demonstrate the formation of trap levels between the HOMO and LUMO energy states with increasing Urbach's energy for decreasing Au content. The surface enhanced infrared (SEIRA) measurements with the analyte bromophenol, not only highlight the differential metal-PVP-analyte binding, but also corroborate the optical constants of the nanohybrids. Moreover, the catalytic conversion of 4-nitro-aniline (4-NA) to p-phenylenediamine (p-PDA) in the presence of AuAg nanohybrids visibly distinguishes their excellent stability and reproducibility along with their moderate rate constant in direct contrast to the poor recycling ability of their individual metal counterparts.

Exploring the coherent interaction in a hybrid system of hollow gold nanoprisms and cyanine dye J-aggregates: role of plasmon-hybridization mediated local electric-field enhancement.

In this work, we probed the possibility of observing strong plasmon-exciton interactions in hollow gold nanoprism-J-aggregate nanocomposites. Several different hollow gold nanoprisms (HGNs) with different aspect ratios were synthesized. This allowed us to systematically tune the LSPR energies through the exciton energy of the PIC-J-aggregate, which in turn allowed us to have direct determination of the coupling strength of HGN-J-aggregate composites. Hybrid nanosystems were prepared by adsorbing and assembling 1,1'-diethyl-2,2'-cyanine (pseudoisocyanine or PIC) iodide onto the surface of hollow gold nanoprisms. Plasmon-exciton interactions were studied using extinction spectroscopy. The experimental results were analysed, and complemented by the results obtained from numerical simulations. Our results reveal that the HGN-PIC-J-aggregate hybrid nanosystem shows coherent coupling between the localized surface plasmons of the HGN and excitons of the PIC-J-aggregate, as obvious from the observation of a clear transparency dip and the formation of two new hybrid plexcitonic modes in the plexcitonic spectra. Anti-crossing behaviour of the plexcitonic modes, together with large Rabi splitting and coupling constant, asserts strong coupling between the plasmon and the exciton, overwhelming the decoherence effects, in our hybrid nanosystem. Analysis of the calculated near-field distribution establishes that the plasmon-hybridization mediated large electric-field enhancement holds the key to the strong coupling.

Solar cycle variation of ionospheric parameters over the low latitude station Hainan, China, during 2002–2012 and its comparison with IRI-2012 model

The low latitude ionospheric data observed by digisonde at Hainan station (19.5°N, 109.1°E) in a whole solar activity cycle period from 2002 to 2012 within Ap<20 have been analyzed to explore the diurnal, seasonal, annual variations and solar activity dependences of the ionospheric peak parameters (foF2, hmF2, and Chapman scale height Hm), as well as some quantitative comparison with IRI-2012 modeling predictions. The results show that the winter anomaly in the daytime foF2 appears at different levels of solar activity. The semiannual anomaly in the daytime and nighttime foF2 with two maxima in equinox seasons is present. The foF2 have a close correlation with a solar activity factor F107P=(F107+F107A)/2 and the correlation coefficients (r) in their diurnal variation are around 0.7. The slope of foF2 varying with F107P in daytime is usually smaller than in nighttime. The afternoon and evening hmF2 show good correlation with F107P (their r values exceed 0.6), but hmF2 at other time are low or poor related to F107P. The prominent character of hmF2 in equinox and summer seasons is its strong increase at sunset in high solar activity period, which may be due to pre-reversal enhancement (PRE) of local electric field. We also note that hmF2 values around midnight slightly decrease with increasing F107P index in equinox seasons. The diurnal variation of Hm usually has two peaks around noontime and pre-sunrise. The daytime Hm has an annual variation with maximum in summer and minimum in winter. Moreover, the dependence of the daytime Hm on solar activity is not strong due to meridional wind and other factors. The above results over Hainan are considerably different from those reported over Millstone Hill, which is attributed to their different geomagnetic locations. The quantitative results compared between IRI-2012 model predictions and observations show that the predicted foF2 values are basically underestimated and the magnitude of their deviations obviously increases with increasing solar activity. The predicted hmF2 obtained with measured M(3000)F2 inputs in low and moderate solar activity agree well with the observed ones. However, their deviations in high solar activity are significantly magnified.

Effect of space charge layers on the electric field enhancement at the physical interfaces in power cable insulation

This paper intends to investigate the effect of two vented water trees w 1 and w 2 growing simultaneously from both conductor and insulation screens of medium voltage cable in the presence or not of space charge, on the electric field distribution. +12 kV DC voltage has been applied. Such simulation has been implemented under Comsol Multiphysics® environment using the finite element method. Particular interest is assigned to the effect of the polarity, density and layers thickness of the space charge as well as to the behavior of this charge initially accumulated at water trees tips. Note that for the polarity influence, the four polarities configurations related to the space charge accumulated at w 1 and w 2 have been considered. As results, the electric field enhancement at the semiconductor layers especially close to the water tree defect is strongly dependent not only upon the relative permittivity distribution inside water trees, but also on the amount, density and layers thickness of the space charge. In the absence of space charge, we show that, with regard to the inhomogeneous (linear and parabolic) distribution of the permittivity, the homogeneous one causes much more local enhancement of electric field at the trees tips. Moreover, the electric field distribution depends on the geometrical parameters (length and width) and shape of trees; the spherical form engenders a higher electric field than elliptical one. In addition, the association of the space charge could lead to the initiation of the electric trees even at lower voltages. Furthermore, the initiation of electrical trees from the physical interfaces is more probable in the case of opposite polarities especially for negative-positive configuration. Such electrical trees start not only from the tip but also from the water tree root (interface of treed region, un-treed one and semiconductor) which is particularly more vulnerable than inside.

Plasmonic enhancement of random lasing from dye-doped polymer film by bristled Ag/TiO_2 composite nanowires

We report on the enhanced random lasing from an organic gain medium in a planar waveguide, with the assistance of bristled Ag/TiO2 composite nanowires. Tris(8-hydroxyquinolinato)aluminum (Alq3) and 4-(dicyanomethylene)-2-tert-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) are used as donor-acceptor lasing dyes. Comparing to the gain medium with silver nanowires, the lower lasing threshold is achieved utilizing bristled Ag/TiO2 nanowires, and the threshold is reduced by 3.7 times than that of the neat gain medium. It is attributed to the broader localized surface plasmon resonance spectrum of Ag/TiO2 nanowires which could sufficiently overlap with both absorption and emission spectra of donor-acceptor lasing dyes, the stronger localized electric field enhancement effect and scattering effect. In addition, the unique plasmonic waveguide effect could also contribute to the enhanced lasing and lead to the lower lasing threshold. This method is expected to be a potential metal-modified technology for improving the lasing performance.

Local electric field enhancement at the heterojunction of Si/SiGe axially heterostructured nanowires under laser illumination

We present a phenomenon concerning electromagnetic enhancement at the heterojunction region of axially heterostructured Si/SiGe nanowires when the nanowire is illuminated by a focused laser beam. The local electric field is sensed by micro Raman spectroscopy, which allows the enhancement of the Raman signal arising from the heterojunction region to be revealed; the Raman signal per unit volume increases at least ten times with respect to the homogeneous Si and SiGe nanowire segments. In order to explore the physical meaning of this phenomenon, a three-dimensional solution of the Maxwell equations of the interaction between the focused laser beam and the nanowire was carried out by finite element methods. A local enhancement of the electric field at the heterojunction was deduced. However, the magnitude of the electromagnetic field enhancement only approaches the experimental one when the free carriers are considered, showing enhanced absorption at the carrier depleted heterojunction region. The existence of this effect promises a way of improving photon harvesting using axially heterostructured semiconductor nanowires.

Strong electric field enhancements in asymmetric metallic nanostructures and high-order harmonic generation.

Numerical investigation of high-order harmonic generation (HHG) is carried out in noble gases near metal nano-dimers. The effect of geometry, shape, and gap of the dimers in plasmon resonance and local electric field enhancement has been investigated numerically by using finite-difference time-domain methods. It is shown that a lack of symmetry in dimer shapes plays an important role in the HHG process, producing appreciable modifications to the energy-resolved photoelectron spectra.

A high-yield, one-step synthesis of surfactant-free gold nanostars and numerical study for single-molecule SERS application

We report a high-yield synthesis of star-shaped gold nanostructures in one step, using a new surfactant-free wet chemistry method. Compared to the existing reports, these nanostars were found to have longer and sharper spikes anchored uniformly on the surface of the spherical core, allowing at least a few hot spots irrespective of the incident light polarization. The average experimental values of core radius and spike length were found to be 88.5 and 72 nm, respectively. Using these values in numerical simulations, the local electric field enhancement (η) and localized surface plasmon resonance (LSPR) spectrum were obtained. Moreover, the single-molecule surface-enhanced Raman scattering (SERS) enhancement factor was found to vary from 1010 to 1013 depending on the excitation wavelengths. Our theoretical calculations suggest that these nanostructures can be used to fabricate efficient SERS-based biosensors for the detection of single molecules in real time and for predicting structural information of single molecules.