Field Enhancement Effect Research Articles

Highly Excretable Gold Supraclusters for Translatable In Vivo Raman Imaging of Tumors.

Raman spectroscopy provides excellent specificity for in vivo preclinical imaging through a readout of fingerprint-like spectra. To achieve sufficient sensitivity for in vivo Raman imaging, metallic gold nanoparticles larger than 10 nm were employed to amplify Raman signals via surface-enhanced Raman scattering (SERS). However, the inability to excrete such large gold nanoparticles has restricted the translation of Raman imaging. Here we present Raman-active metallic gold supraclusters that are biodegradable and excretable as nanoclusters. Although the small size of the gold nanocluster building blocks compromises the electromagnetic field enhancement effect, the supraclusters exhibit bright and prominent Raman scattering comparable to that of large gold nanoparticle-based SERS nanotags due to high loading of NIR-resonant Raman dyes and much suppressed fluorescence background by metallic supraclusters. The bright Raman scattering of the supraclusters was pH-responsive, and we successfully performed in vivo Raman imaging of acidic tumors in mice. Furthermore, in contrast to large gold nanoparticles that remain in the liver and spleen over 4 months, the supraclusters dissociated into small nanoclusters, and 73% of the administered dose to mice was excreted during the same period. The highly excretable Raman supraclusters demonstrated here offer great potential for clinical applications of in vivo Raman imaging.

Terahertz absorber based on vanadium dioxide with high sensitivity and switching capability between ultra-wideband and ultra-narrowband.

The terahertz perfect absorber can be applied in the control, sensing and modulation of optical fields in micro- and nanostructures. However, they are only single function, complex device structure and low sensing sensitivity. Based on this, by introducing the bound state in the continuum (BIC) with infinite quality factor and field enhancement effect, and taking advantage of the phase transition characteristics of vanadium dioxide (VO2), we designed a terahertz perfect absorber device which can actively switch between ultra-wideband and ultra-narrowband. The absorption mechanism is explained by multipole analysis theory, impedance matching theory and electromagnetic field distribution. The broadband absorption is mainly due to the electric dipole resonance on metallic VO2 materials, and the absorption is more than 99% across 3.64-6.96 THz, and it has excellent characteristics such as robustness. Ultra-narrowband perfect absorption has a quality factor greater than 2200 due mainly to the implementation of symmetrically protected BIC with a sensing sensitivity of 2.575 THz per RIU. Therefore, this research could be widely used in the fields of integrated optical circuits, optoelectronic sensing and perceptual modulation of energy, as well as providing additional design ideas for the design of terahertz multifunctional devices.

Synergistic coupling of interface ohmic contact and LSPR effects over Au/Bi24O31Br10 nanosheets for visible-light-driven photocatalytic CO2 reduction to CO.

The challenge of synergistically optimizing different mechanisms limits the further improvement of plasmon-mediated photocatalytic activities. In this work, an Au/Bi24O31Br10 composite, combining an interface ohmic contact and localized surface plasmon resonance (LSPR), is prepared by a thermal reduction method. The LSPR effect induces the local resonance energy transfer effect and the local electric field enhancement effect, while the interface ohmic contact forms a stronger interface electric field. The novel synergistic interaction between the interface ohmic contact and LSPR drives effective charge separation and provides more active sites for the adsorption and activation of CO2 with improved photocatalytic efficiency. The optimized 0.6 wt% Au (5.7 nm) over Bi24O31Br10 nanosheets showed an apparently improved photocatalytic activity without any sacrificial reagents, specifically CO and O2 yields of 44.92 and 17.83 μmol g-1 h-1, and demonstrated superior stability (only lost 6%) after continuous reaction for 48 h, nearly 5-fold enhanced compared to Bi24O31Br10 and a great advantage compared with other bismuth-based photocatalysts.

Interaction between gold nanoclusters and gold nanoparticles encapsulated in polymer nanoparticles

Gold nanoclusters (AuNCs) feature size-dependent photoluminescence, with an extremely high photostability and a large Stokes shift. However, they can have limited colloidal stability and low luminescence quantum yields that hinder promising application in bioimaging, sensing and catalysis. Here we report the encapsulation of AuNCs with 25 gold atoms stabilized with 6-mercaptohexanoic acid ligands (Au25(MHA)18) into methacrylate polymer nanoparticles (PNPs) with c.a. 50 nm diameter. The time evolution of these structures in aqueous dispersion shows the formation of gold nanoparticles (AuNPs) with c.a. 4 nm diameter, leading to a 2-fold emission amplification (EA) of the AuNCs due to the field enhancement effect of the plasmon. The EA can be controlled by the annealing time of the PNPs in dispersion, the temperature and irradiation time. The optical properties of the PNPs are frozen upon drying because the diffusion of AuNCs inside the PNPs is negligible in the absence of solvent.

Controllable synthesis of flower-like AuNFs@ZIF-67 core-shell nanocomposites for ultrasensitive SERS detection of histamine in fish

Histamine is a significant biomarker to assess the freshness of fish products. In this study, a novel MOF-based SERS sensor for histamine determination was synthesized by wrapping PVP-capped Au nanoflowers with a ZIF-67 shell (Au NFs@ZIF-67). The highly branched Au NFs core exhibited a strong electromagnetic field enhancement effect and provided an ultra-sensitive SERS fingerprint spectrum, while ZIF-67 shell was the contributor to enrich the target and stabilize the substrate. The morphology of the core-shell structures can be easily controlled by the concentrations of the capping agent PVP and MOF precursor Co ion. Consequently, 4-MBA pre-grafted on the optimized SERS substrate can act as the Raman internal standard (IS) to eliminate signal fluctuations through standardizing all spectra against its peak at 1074 cm−1. Moreover, as the specific receptor for histamine molecules, 4-MBA helped reach the low detection sensitivity, where the SERS intensity ratio, I1172/I1074 presented a good linear relationship towards the histamine concentrations (10−3-10−7 M) with the LOD of 0.87 × 10−7 M (R2 = 0.9930). Furthermore, the application in monitoring fish spoilage process demonstrated the feasibility and reliability of the developed sensor. This work provided a facile strategy to construct MOF-based SERS substrate as a potential platform for the shelf-life prediction of fish products.

Enhanced mechanism and experimental research of laser cavitation degradation of methylene blue by AuNPs

As a novel degradation method, laser cavitation degradation suffers from low degradation efficiency in the absence of medium gain. An enhancement method of laser cavitation using Au nanoparticles (AuNPs) is proposed for degrading organic dye wastewater. Methylene blue (MB) aqueous solution was employed as the simulated organic dye wastewater. The strong absorption characteristics of AuNPs and the mechanism underlying AuNPs-enhanced laser cavitation degradation were analyzed. The local field enhancement effect and shock wave signal were obtained. Therelationshipsamong the AuNPs, laser energy, the number of irradiations number and the degradation effect were investigated. The results show that the local field enhancement coefficient of AuNPs in Methylene blue (MB) aqueous solution reached 1.9, and the degradation degree increased by three times compared with that of pure solution because that thermal decomposition and oxidation of hydroxyl (OH) radicals were enhanced by AuNPs. With the laser energy levels increasing (70–110 mJ), the intensity of shock wave increased by 2–4 times and the degree of degradation increased by three times. The degradation rate reached 37.7 % within 3 min and the irradiation number reached 1600. Compared with other methods, the AuNPs-enhanced laser cavitation approach has exhibited remarkable controllability together with a higher degradation efficiency.

Electric Field Improvement for High-Voltage Bushings

Resin-impregnated paper (RIP) bushing has gained significant interest due to its extended application in Extra High Voltage (EHV) and Ultra High Voltage (UHV) electricity transmission systems. However, the design criterion of its overall structure, the geometry parameters of the condenser layers, and stress release devices, etc., are still not fully understood. This article proposes a unique electric field optimization technique to integrate both the analytical and the numerical methods. The charge simulation method (CSM) is employed to create the overall equipotential surface, within which the finite element analysis (FEA) is adapted to study the localized field enhancement effects, taking into consideration the multi-physics coupled fields. A case study is performed on an actual UHV bushing. The results are compared to the traditional methods, to demonstrate the benefit of the hybrid method.

Plasmon guided assembly of nanoparticles in solids

Light manipulation of metallic nanoparticles (NPs) with outstanding plasmonic properties attracts broad interest in multiple areas owing to the significant field enhancement and localization effect mediated by plasmons. So far, optical migration of NPs has only been achieved on platforms such as gases, solutions, and mesoporous thin films (TiO2), while not been realized for NPs inside large macroscopic transparent materials due to the high density and viscosity of a host matrix. The high localization of light intensity by plasmons allows the laser pulses to deposit energy via near-field at the nanoscale at reduced thermal modification and ablation over extended regions inside a host material. This makes it possible to achieve directed and guided migration of NPs within large macroscopic transparent materials. Here, we propose a plasmon-enhanced method using ultrafast laser direct writing to manipulate metal NPs inside a glass. The optical potential between closely spaced NPs increases significantly under laser irradiation, leading to guided energy deposition by ionization, and light interference which leads to the self-assembly of NPs into nanostructures under irradiation of subsequent pulses. In this work, ordered subwavelength NP gratings are fabricated, which exhibit polarization dependence and show promise for applications in information encoding and optical storage. This study provides a new physical case for ultrafast laser-driven guided self-assembly of NPs, where the dynamic evolution via their fragmentation and migration is observed, contributing to the understanding of the origin of self-assembled gratings and opening up new research directions for light manipulation of NPs.

Optical Trapping of a Single Molecule of Length Sub-1 nm in Solution

Optical Trapping of a Single Molecule of Length Sub-1 nm in Solution

Amplified plasmonic emission enhancement of PbS quantum dots via Al-oxide/PMMA heterostructures

We study the impact of Al oxide/Poly(methyl methacrylate) (PMMA) interface on plasmonic emission enhancement of infrared semiconductor quantum dots (QDs). For this, PbS QDs embedded in PMMA matrix are deposited on the top of heterostructures consisting of a Au thin film, a dielectric spacer, and an ultrathin layer of Al oxide. Our results suggest that such structures can support an emission enhancement far more than what can be reached in the cases when the QDs/PMMA films are placed on Au thin film/dielectric spacer directly, i.e. in the absence of the Al oxide. We also demonstrate that Au/Si/Al oxide/PMMA heterostructures can increase the photo-induced fluorescence enhancement of PbS QDs, making them brighter as they are irradiated with a laser field. We discuss these results in terms of combined effects of plasmonic field enhancement (Purcell effect) and the carboxylate anion bonds formed at the Al oxide/PMMA interface.

Reporter Molecules Embedded Au@Ag Core-Shell Nanospheres as SERS Nanotags for Cardiac Troponin I Detection

Rapid and accurate detection of acute myocardial infarction can improve patients' chances of survival. Cardiac troponin I (cTn I) is an important diagnostic biomarker for acute myocardial infarction. However, current immunoassays are insufficient to accurately measure cTn I, as they have limited detection sensitivity and are time-consuming. Surface-enhanced Raman scattering (SERS) is a brilliant fingerprints diagnostic technique characterised by ultrasensitivity, fast response, and qualitative and quantitative analysis capabilities. In this study, reporter molecules (4-Mercaptobenzoic acid, 4-MBA) embedded Au@Ag core-shell nanospheres as SERS nanotags were prepared for the detection of cTn I. As the Raman reporters were embedded between the core and the shell, they could be protected from the external environment and nanoparticle aggregation. Excellent SERS performances were obtained due to the enhanced local electromagnetic field in the gap of core and shell metals. In a standard phosphate buffered saline (PBS) environment, the limit of detection for cTn I was 0.0086 ng mL-1 (8.6 ppt) with a good linear relationship. The excellent Raman detection performance was attributed to the localized surface plasmon resonance effect and strong electromagnetic field enhancement effect produced by the gap between the Au core and the Ag shell. The SERS nanotags we prepared were facile to synthesize, and the analysis procedure could be completed quickly (15 min), which made the detection of cTn I faster. Therefore, the proposed SERS nanotags have significant potential to be a faster and more accurate tool for acute myocardial infarction diagnostics.

Advances in Photovoltaic Technologies from Atomic to Device Scale

The question of how energy resources can be efficiently used is likewise of fundamental and technological interest. In this opinion, we give a brief overview on developments of harvesting solar energy across different length scales and address some strategies to tackle economic and ecological challenges, in particular with a view to sustainability and toward a circular economy. On the mesoscopic scale, the emergence of thermodynamic laws in open quantum systems is of central importance and how they can be employed for efficient quantum thermal machines and batteries. The broad tunability of band gaps in quantum dot systems makes them attractive for hybrid photovoltaic devices. Complementary, machine learning-aided band gap engineering and the high-throughput screening of novel materials assist with improving absorption characteristics. On the device scale, hybrid concepts of optical control via metasurfaces enable a multitude of functionalities such as a directed re-emission of embedded photoluminescent materials or field enhancement effects from nanostructures. Advanced techniques in computational nanophotonics concern a topology optimization of nanostructured layers together with multiobjective optimization toward specific light management tasks. On the industrial level, modern manufacturers explore 3D printing and flexible solar cell platforms obtained from roll-to-roll technologies. The remote control of solar parks through applications via the Internet of Things opens up new strategies to expand to difficult terrain where human interaction is only required to a limited extent.

Ponderomotive forces in the system of two nanoparticles

Mechanical consequences of electromagnetic interaction of two nanoparticles have been modeled and simulated. It has been shown that the local field enhancement effect in the studied system causes the appearance of the local field gradients. As a consequence, the local field gradients can lead to ponderomotive forces acting on the nanoparticles near their surface. In the work, the model describing the phenomena has been developed. The model is based on the near-field interaction in the self-consistent system and the effective susceptibility concept. Using the model distribution of the local field in the system of two different-sized nanoparticles has been calculated and ponderomotive forces directions and values were simulated. It has been shown that in the system of two different-sized nanoparticles the forces act mainly on the surface of the bigger nanoparticle and for some systems, the value of its density per volume unit may acquire up to several tens of nano newtons. Possible application of the results to the study of biological systems has been also discussed.

Particle Generation, Their Behavior and Effect on Post-Arc Breakdowns After Capacitive-Current Vacuum Arc Interruption

Post-arc breakdowns of vacuum circuit breakers during capacitive operation can be possibly caused by particles. In this work, we experimentally observe the generation and dynamics of particles in capacitive interruptions and their resultant effects on post-arc breakdowns. We further constructed a synthetic test circuit for the capacitive current switching and adopted the laser-shadow technique to observe the dynamics behaviors of particles, formed by different electrode materials and inrush currents. CuCr10 electrodes tend to generate particles with high probability and quantity, owing to their weak refractoriness and lower mechanical strength. Furthermore, we find that the higher inrush current facilitates the formation of particles. Additionally, under the influence of recovery voltages and variable charges, particles show complex and diverse dynamic behavior, such as bouncing, oscillating, levitating, lifting, and rotating. Essentially, all particles relevant to breakdowns moved at a low speed (below 0.5 m/s), besides, almost all non-sustained disruptive discharges and restrikes are concerned with the particles closed to electrodes under near-peak recovery voltage. Therefore, we think neither energy accumulation nor high-speed bombardment of particles can explain post-arc breakdowns, conversely, the joint effect of particle-induced field enhancement and resulting field electron emission is responsible for breakdowns.

Exploiting total internal reflection geometry for deep broadband terahertz modulation using a GaAs Schottky diode with integrated subwavelength metal microslits.

We developed a GaAs Schottky diode with integrated periodic subwavelength metal microslits with total internal reflection (TIR) geometry to achieve deep broadband THz modulation at high frequency with low insertion loss. The non-resonant electric field enhancement effect in the subwavelength microslits intensifies the evanescent wave in TIR, which increases broadband absorbance of THz light signals by free carriers in the GaAs Schottky diode. Devices with various microslit spatial periods and gap widths were fabricated and measured. Among the devices, that with a microslit period of 10 µm and gap width of 2 µm produced ∼70% modulation depth at frequencies of 0.2 to 1.2 THz, while in the range of 0.25 to 0.4 THz, ∼90% modulation depth was achieved. By encapsulating the device in high refractive index material, ∼100% modulation depth was achieved in the range of 0.4 to 0.6 THz, the 3 dB bandwidth operational frequency was ∼160 kHz, and the insertion loss introduced by the device was less than 8 dB, which is much lower than existing metasurface-based THz modulators. In general, our first-generation device has improved modulation depth, operational bandwidth, insertion loss, and operational frequency. Optimization of the metal microslits, TIR geometry, and doped layer could further improve the performance of our design.

Breakdown behavior of SiC photoconductive switch with transparent electrode

The breakdown behavior of a V-doped 4H silicon carbide photoconductive switch with a transparent electrode under a high electric field is studied. The device is triggered by a laser pulse below the bandgap wavelength with a repetition rate of 100 Hz. The light peak-power of the laser pulse reaches hundreds of kW, and the bias voltage is increased from 10 to 20 kV. With the accumulation of the number of pulses, the device shows breakdown behavior. Through the microscopic diagnosis of damaged and breakdown devices, according to the analysis of theoretical and simulation results, the breakdown of the device is due to the interaction between the laser and the aluminum doped zinc oxide transparent electrode on the device surface, resulting in the melting of the transparent electrode. The direct illuminate of the laser can form a field enhancement effect at the junction of the substrate and the transparent electrode, resulting in the damage of the device and the formation of pre-breakdown. Finally, the breakdown of the device inevitably occurs.

Dopamine-mediated self-assembled anisotropic Au nanoworms conjugated with MoS2 nanosheets for SERS-based sensing

Surface-enhanced Raman scattering (SERS) utilizing plasmonic metals coupled with transition metal dichalcogenide (TMD) has received increasing attention in bio-applications due to the synergistically enhanced detection sensitivity. However, there have been several issues to be addressed such as strong SERS background signals and low Raman intensities of analytes. In this study, we report a novel approach for high SERS performance based on anisotropic hybrid plasmonic nanostructures of gold nanoworms (Au NWs) conjugated with 2D Molybdenum disulfide (MoS2) nanosheets. To significantly enhance the SERS enhancement factor (EF), we optimized the gap between Au NWs and MoS2 to ~2 nm by controlling the thickness of polydopamine (PDA). Thus, giant localized electric field enhancement and synergistic coupling effect occurred in the hybrid plasmonic nanostructures, generating abundant hot spots. Consequently, SERS EF of the order of ~107 was achieved, which was 104 times higher than that of colloidal Au nanoparticles. Further, we showcase that our AuNW@MoS2 can detect the 10−10 M level thiram, a typical pesticide, under soil-pollutant muddy condition, evidencing the powerful multimodal sensory platform of our hybrid substrates. This work provides new insight into the rational design of plasmon-coupled 2D nanohybrids for high-performance SERS substrates with a premise for the practical and viable biosensing applications.

Microplasma emission performances dependent on silicon nanowires morphologies

Silicon nanowires (SiNWs) are introduced into microdischarge to improve microplasma properties due to its field emission electrons and field enhancement effect. The geometrical arrangement and dimensional features of SiNWs have desicive influence on field emission properties, thus the dependence of microplasma emission performances on the SiNWs morphologies is investigated in this paper. The different morphologies of SiNWs can be prepared by electrocatalytic metal-assisted chemical etching with varied etching currents. With the increase of etching current from 3 mA to 30 mA (AgNO3:HF:H2O2 = 0.02:4.6:0.1 mol l−1, deposition time 1 min and etching time 10 min), the field emission current density J of the SiNWs prepared at 20 mA etching current is the largest ∼0.28 mA cm−2 at a field 4.5 V μm−1, and turn-on field is the lowest of 3.52 V μm−1. Accordingly, the microplasma in the device fabricated on the SiNWs-decorated substrate (etching current at 20 mA) has the strongest average emission intensity of ∼11 565 a.u., the minimal relative standard deviation of emission intensity 4.9% and the fastest propagation velocity of 471 km s−1. The field emission electrons of SiNWs could inject more seed electrons into microcavity which causes higher electron collision probability, and the field enhancement effect at tips of SiNWs can provide more energy for the charged particles, which are helpful to the microdischarge. The most difficulty is to balance the distance of emitters and the percentage of SiNWs in entire emission region because the shielding effect will reduce while the surface emitter numbers will decrease when the distance of emitters increases. Here, a ‘proper percentage of SiNWs’ of 19.3% is obtained what indicates that if SiNWs percentage is greater than the threshold, field enhancement factor β eff is weakened by the decrease of aspect ratio and the increase of percentage. When SiNWs percentage is less than 19.3%, β eff will increase and be dominated by the percentage of SiNWs. The results are significant for the application of SiNWs in microdischarge devices.

Cap‐Like Ellipsoid Deposition on the ZnO Nanowires for Large Increase of Fluorescence Induced by Localized Field Enhancement Effect (Crystal Research and Technology 7/2022)

Cap‐Like Ellipsoid Deposition on the ZnO Nanowires for Large Increase of Fluorescence Induced by Localized Field Enhancement Effect (Crystal Research and Technology 7/2022)

Numerical Study on Rolled Strip Pulse Forming Line with Combined Insulation Structure

Rolled strip pulse forming line (RSPFL) with combined insulation structure can effectively increase the breakdown electric strength and energy storage density of pulse forming line by optimization of the edge field enhancement effect. In this paper, a combined insulation (which consists of polyimide and polyvinylidene fluoride films) based RSPFL is investigated theoretically and numerically. Firstly, parameters of the pulse forming line was designed theoretically. Secondly, the finite element software was employed to study the influence of insulation film combination and structural parameters on the electric field distribution and the transmission characteristics. The parameters are then optimized. Finally, combined insulation based RSPFL was successfully used as a single pulse forming line, and a 200 ns quasi-square pulse was obtained on a 2.5 Ω dummy load by simulation. The simulation shows that it has an energy storage density of approximately 19.57 J/L when the PFL is charged to 60 kV.