Research on quantum efficiency and photoemission characteristics of exponential-doping GaN nanowire photocathode

Aimed at improving the actual photoemission performance of nanowire photocathode, an axial exponential-doping GaN nanowire photocathode is proposed. Based on two-dimensional continuity equation and finite difference method, the quantum efficiency of this exponential-doping GaN nanowire photocathode is obtained. The simulation results suggest that this structure of GaN nanowire photocathode can effectively obviate the difficulty in collecting the electrons escaping from side faces because a large part of carriers will escape from top surface under the built-in electric field. Besides, it is discovered that the optimal height of nanowires is 300 nm when the doping concentration of top surface is 1 × 10¹⁸ cm^?3 and that of back interface is 1 × 10¹⁹ cm^?3. Then, when the nanowires are arranged as array, the optimal light angle of incidence is approximately 60° by analyzing the electrons flow density of the array. By comparison of collection proportion of photoelectrons, the optimal nanowire spacing is 231 nm. This study demonstrates potential application value of exponential-doping GaN nanowire photocathode. The results can direct the preparation of this kind of photocathode.     

Photoinduced enhancement of optical second harmonic generation in LiB3O5 nanocrystallites embedded between the Ag/ITO electrodes

It is reported that there is substantial enhancement of the optical second harmonic generation (SHG) at 1064 nm Nd:YAG laser wavelength for LiB₃O₅ nanocrystatllites embedded into the electric field aligned photopolymer oligoetheracrylate matrices. The borate nanocomposite was put between the electrodes containing Ag/ZnO NP with silver sizes 20, 40 and 60 nm. We study an influence of the Ag NP sizes on the output SHG. It is clearly seen that only excitation by the green continuous wave 532 nm laser with power about 350–400 mW with beam diameter about 4 mm give significant effect. The latter confirms a principal role of the surface plasmon resonances spectrally overlapped with the nonlinear excitations responsible for the observed changes of the SHG.     

High Aspect Ratio Microcolumns to Manipulate Single Electrons on a Liquid Helium Surface for Quantum Logic Bits

Electrons are bound to the surface of liquid helium by the image potential due to the induced polarization of the helium. The potential varies as the reciprocal of the distance of the electron from the surface so that the energy levels for the electron motion perpendicular to the surface are those of a one-dimensional hydrogen atom with very small charge. In order to make quantum logic elements (qubits) using the ground state and first excited state in this potential, individual electrons must be confined over microelectrodes which are individually connected to variable voltage sources so that the electron energy levels can be shifted in and out of resonance with applied microwave radiation. The electrodes consist of an array of gold columns, about 1.2$muhbox m$tall and 200 nm in diameter, separated by 500 nm. These are grown by electroplating on the ends of leads deposited on a silicon substrate by e-beam lithography. The leads are covered by a dielectric layer and then a metal ground plane, so that the electric field from the leads is screened. We describe, here, our technique for fabricating this system and present the numerical computations of the electric fields from the electrodes.     

Improvement and characteristics of conical silicon emitters employing wet-dry etching

A number of new technologies require conical and sharp tips to serve as electron emitters in the vacuum microelectronics. In this paper, we improved radius of curvature, height and cone angle of emitters in order to reach the enhancement result of field enhancement factor (β). We developed a fabrication process to improve geometry of emitter by employing isotropic dry etching in pure SF₆ and a mixture of SF₆ and O₂ followed by thermal oxidation technique. We successfully achieved excellent conical emitters with 5–10 nm radius of curvature, 4.4 μm height, and 30° cone angle. The conical silicon emitters current–voltage characteristics shows that E_to = 4.8 V/μm (turn-on electric field) with current density of 10 μA/cm², and maximum current density J = 60.4 μA/cm² at E = 8.14 V/μm. This study may provide a practical guideline for design and fabrication of a high-performance silicon emitter used in various industrial applications.     

Influence of electric field on the quenched-in vacancy and solute clustering during early stage ageing of Al-Cu alloy

The effects of electric field on the evolution of excess quenched-in vacancy as well as solute clustering in Al-4wt%Cu alloy, and on the vacancy migration and formation enthalpy of pure aluminum were investigated, using positron annihilation lifetime spectroscopy, high-angle annular dark-field scanning transmission electron microscopy, transmission electron microscopy, hardness measurement and four-probe electrical resistivity measurement. The results showed that the electric field improved age hardening response obviously and postponed the decay of excess vacancies for 30 min during the early stage ageing of Al-4wt%Cu alloy. A large number of 2–4 nm GP zones with dense distribution were observed after 1 min ageing with an electric field applied. The electric field-assisted-aged sample owned a lower coarsening rate of GP zone, which was about three fifths of that in the aged sample without an electric field, from 1 min to 120 min ageing. The electric field contributed 8% increase of the vacancy migration enthalpy(0.663 ± 0.021 e V) of pure Al, comparing with that(0.611 ± 0.023 e V) of pure Al without an electric field. The increase of vacancy migration enthalpy, induced by the electric field, was responsible for the difference on evolution of quenched-in vacancy, rapid solute clustering and age hardening improvement during the early stage ageing of Al-4wt%Cu alloy.     

Second harmonic generation using an electrically controlled asymmetric plasmonic waveguide

ABSTRACT

In this study, it was shown that field enhancement in the nonlinear metal-insulator-metal (MIM) plasmonic waveguides can result in a large enhancement of the second harmonic generation (SHG) magnitude as compared with values reported in the literature. The proposed structure has two metals at the top and two metals at the bottom of the crystal. In this structure, a voltage is applied on metals to produce a SHG electrically. Hence, the metals that define the cavity also serve as electrodes capable of generating high direct current electric fields across the nonlinear material. The frequency of a fundamental wave at 458 nm was doubled and modulated in intensity by applying a moderate external voltage to the electrodes, yielding a voltage-dependent nonlinear generation with a higher coupling efficiency. All the simulations here have been calculated by using the finite-element-based commercial COMSOL software.     

Hydrothermal synthesis of Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:Eu<Superscript>3+</Superscript> nanorods and its growth mechanism and luminescence properties

Homogeneous Y₂O₃:Eu³⁺ nanorods with the lengths of several micrometres were successfully synthesised on a large scale by using a urea-assisted hydrothermal method and a post-calcining process. In this study, the influences of urea content and NaOH concentration on the oriented growth, photoluminescence (PL) and electroluminescence (EL) intensity enhancement of Y₂O₃:Eu³⁺ were investigated. As a precipitant for isotropic growth, urea can counteract the effect of NaOH on oriented growth along the c-axis during hydrothermal treatment. The Y₂O₃:Eu³⁺ powders exhibited a strong red emission centred at 613 nm under either 245 nm UV excitation or the direct current high electric field. The PL intensity of the Y₂O₃:Eu³⁺ phosphor prepared with 0.3 g of urea reached 141 % that of the sample prepared under the same conditions but without urea. The strategy for controlling the oriented growth, PL and EL enhancement of Y₂O₃:Eu³⁺ can be extended to the synthesis of other inorganic nano/micromaterials.     

Suppressing electrostatic screening in nanostructured electrode arrays

An individual nanostructure provides very high electric field enhancement because the sharp curvature of the nanostructure tip amplifies the local electric field near the apex tip. However any practical nanostructured electrode is comprised of an ensemble (array) of nanostructures. In such systems, mutual electrostatic shielding (or screening) severely limits the maximum achievable electric field enhancement. In this paper, we discuss three approaches for suppression of shielding. These include--(1) reducing anode-to-cathode distance to less than the nanostructure-to-nanostructure spacing, (2) increasing length of selected individual nanostructures within the array, and (3) design of electrodes with multistage amplification. We show that these approaches are effective in alleviating electrostatic shielding and that the enhancement factor of the electrode array (ensemble) can be engineered to match that of the individual (isolated) nanostructure.     

The fabrication and properties of field emission display based on ZnO tetrapod-liked nanostructure

ZnO tetrapod-liked nanostructure with fine crystalline structure and high purity was synthesized via CVD method. Each branch of the nanotetrapod was 50-200 nm in diameter and the nanotetrapod structure exhibited a high aspect ratio. Cathode emission materials with this nanostructure were employed to fabricate field emission display with 72 × 72 pixel array. The as-obtained device showed an ideal field emission property with a threshold electric field of 4.1 V/μm and an emission current density of 1 mA/cm² when the electric field reached 11.5 V/μm. The field enhancement factor of the nanotetrapods was calculated to be 1852. Using proper circuit drive, dynamic Chinese characters can be successfully displayed in the device.     

Enhanced electrical properties of multiwalled carbon nanotube/poly(vinylidenefluoride) films through a rolling process

Poly(vinylidene fluoride) (PVDF) films containing multiwalled carbon nanotubes (MWCNTs) were prepared by solution casting method, followed by a rolling process. With the presence of MWCNTs (up to 0.3 wt%), the influence of rolling process on the crystal structure and the electrical properties of PVDF was studied. It is shown that rolling can induce phase transition as well as increase in the crystallinity of PVDF, leading to enhancement of ferroelectric and dielectric properties. In addition, MWCNTs may promote the phase transition of PVDF, while impeding the increase of crystallinity during the rolling process. With a higher breakdown field and a lower coercive electric field in comparison with the original films, the rolled films can be efficiently poled to obtain good piezoelectricity. Moreover, with the presence of MWCNTs, electric field-induced changes in crystallinity can be obviously observed in the poled MWCNTs/PVDF films. With the highest crystallinity of β-phase, the rolled films containing 0.075 wt% MWCNTs possess the highest piezoelectric coefficient d₃₃ of ~33 pC/N while that of pure PVDF films is only ~22 pC/N.     

Dielectric,ferroelectric, and field-induced strain properties of Ta-doped 0.99Bi0.5(Na0.82K0.18)0.5TiO3–0.01LiSbO3 ceramics

Ta-doped 0.99Bi_0.5(Na_0.82K_0.18)_0.5TiO₃–0.01LiSbO₃ (BNKTT–LS) ceramics were prepared through a conventional mixed oxide solid-state sintering route. Partial substitution of Ta for Ti decreased the dielectric constant and depolarization temperature. The dielectric curves, polarization and strain hysteresis loops demonstrated that the incorporation of Ta stabilized the canonical relaxor phase of BNKT–LS ceramics leading to the degradation of piezoelectric and ferroelectric responses. The destabilization of field-induced ferroelectric order at x = 0.013 was accompanied by substantial enhancement in strain level. A unipolar field-induced strain of 0.39 % with a normalized strain (S _max/E _max =  $ d_{33}^{*} $ ) of 650 pm/V was achieved at a driving field of 6 kV/mm. The observed large strain can be attributed to the non-ergodic relaxor phase at zero electric field that transformed into an ergodic relaxor phase under the influence of the applied electric field.     

Spin Wave Excitations and Winter’s Magnons in Vertically Coupled Vortex State Permalloy Dots

Circular soft magnetic dots are the main elements of many proposed novel spintronics devices, capable of fascinating spin-based electronics applications, from extremely sensitive magnetic field sensors, to current-tunable microwave vortex oscillators. Here, we investigate static and broadband dynamic magnetization responses of vertically coupled Permalloy (Py) magnetic dots in the vortex state in layered nanopillars (experiment and simulations), which were explored as a function of in-plane magnetic field and interlayer separation. Under reduction of magnetic field from saturation for the field range just above vortex-vortex ground state. We observe a metastable double vortex state for each of the dots. In this state, novel kinds of spin waves (Winter’s magnons along domain walls between vortex cores and half-edge antivortex) are excited. For dipolarly coupled circular Py(25 nm)/Cu(20 nm)/Py(25 nm) trilayer nanopilars of diameter 600 nm, a small in-plane field splits the eigenfrequencies of azimuthal spin wave modes inducing an abrupt transition between acoustic (in-phase) and optic (out-of-phase) kinds of the low-lying coupled spin wave modes. Qualitatively similar changes (although more gradual and at higher values of in-plane fields) occur in the exchange coupled Py(25 nm)/Cu(1 nm)/Py(25 nm) trilayer nanopillars. These findings are in qualitative agreement with micromagnetic dynamic simulations.     

Electric-field thermopower modulation in SrTiO3-based field-effect transistors

Electric-field thermopower modulation method is demonstrated in detail using SrTiO₃-based field-effect transistor structure as an example. Using water-infiltrated nanoporous 12CaO·7Al₂O₃ glass “CAN” as the gate insulator, carrier electrons up to ~10¹⁵ cm^?2 can accumulate within an extremely narrow 2D electron gas (~2 nm), leading to an unusually large enhancement of thermopower. Our electric field-effect approach should be applicable to fully verify the performance of thermoelectric materials with complicated crystal structures. This approach may accelerate the development of nanostructures of high performance thermoelectric materials.     

Origin of the large strain response in tenary SrTi0.8Zr0.2O3 modified Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3 lead-free piezoceramics

The perovskite oxides (1 ? x)Bi_0.5(Na_0.9K_0.1)_0.5TiO₃–xSrTi_0.8Zr_0.2O₃ (SZT1000x, x = 0, 0.2, 0.4, 0.6, 0.8, and 1 %) were prepared via the conventional solid-state reaction method. The room temperature ferroelectric P–E loops coordinate with polarization current density J–E curves illustrated the changes of ferroelectric domains and polar nanoregions under different driving fields exhaustively. The composition and electric field dependent strain behavior of this system were investigated to develop a lead-free piezoelectric material with a large strain response at a lower electric field. A large strain of 0.44 % (S _max/E _max = 744 pm/V) at an applied field of 50 kV/cm was obtained at the composition of 0.6 mol% SZT. Temperature-dependent hysteresis measurements reveal the primary origin of the large strain is due to the presence of a nonpolar phase at a zero field. Upon the application of an electric field, the nonpolar phase that can easily transform into a long-range ferroelectric phase, and then brings the system back to its unpoled state once the applied electric field is removed. Notably, the electric field required to deliver large strains is reduced to 40 kV/cm while the S _max/E _max reached up to 717 pm/V, indicating that the developed material is highly promising for actuator applications.     

Enhancing electrical stability for flexible ZnO nanowire UV sensor by textured substrates

The major problem in the fabrication of electronic devices on plastic substrate arises from the mismatch and weak physical bonding between inorganic semiconductor crystal and the organic plastics, so the electrical performance stability under mechanical stress is an essential factor affecting the practical application of flexible electronic sensors. In this paper, a flexible ZnO nanowire (NW) UV sensor is presented as an exemplary verification for investigating the effects of substrate morphology on reliability of flexible electronic devices. Sensors on ordinary smooth polyimide (PI) substrate have cracked during the device fabrication process due to the residual stress caused by temperature, humidity, etc. These cracks were short pieces and randomly distributed, so they may not significantly affect the device performance, but after mechanical bending, cracks that penetrating the electrodes were generated and caused electric contact failure. Although improving the ZnO seed layer quality could reduce the cracking and buckling that formed during device fabrication, it would not prevent cracking during mechanical bending. In a 3?×?3 sensor array, only one sensor survived after bending. Device failed when the bending angle larger than ??40° or 60°. On the contrary, device fabricated on textured PI substrates exhibited much better electrical stability. All the sensors remained their original performances after mechanical bending in a 3?×?3 sensor array. The light–dark current ratios kept in the level of 10⁵ under the 365 nm UV light of 15 mW/cm². The bending angle varied from ??80° to 80°. The enhancement of electrical stability was because that textured substrate could tolerate more stress than smooth substrate to prevent the film from cracking and at the same time it could increase the contact area between ZnO film and PI substrate, which improved the interfacial bonding strength and delayed the local strain of the film.     

Engineering optical response through gold bowtie nanoantennas array

We propose the coupled gold bowtie nanoantennas array and investigate its plasmonic properties theoretically. The bowtie antenna consists of a pair of opposing truncated cones. We calculate the transmission spectra and the electric field distributions. The evolution of the transmission spectrum with the gap width of the bowtie, the diameter of the tip of the cone and the distance between adjacent bowties is directly visualized. Furthermore, the sensitivity of the antennas array to dielectric constant changes of the environment is also investigated in detail. We show the electric field distribution of this nanostructure and find that E_x is mainly located at the corners of the cross section, especially at the extremity of the cone. As for the E_y, the electric field enhancement localizes at the external edges and the gap of the bowtie. Our work elucidates further the plasmonic interactions can be useful in the design of optimized, sensitive optical sensors, and the enhancement of the fluorescence of molecules.     

Photoluminescence Enhancement of CuInS2 Quantum Dots in Solution Coupled to Plasmonic Gold Nanocup Array

A strong plasmonic enhancement of photoluminescence (PL) decay rate in quantum dots (QDs) coupled to an array of gold‐coated nanocups is demonstrated. CuInS₂ QDs that emit at a wavelength that overlaps with the extraordinary optical transmission (EOT) of the gold nanocup array are placed in the cups as solutions. Time‐resolved PL reveals that the decay rate of the QDs in the plasmonically coupled system can be enhanced by more than an order of magnitude. Using finite‐difference time‐domain (FDTD) simulations, it is shown that this enhancement in PL decay rate results from an enhancement factor of ≈100 in electric field intensity provided by the plasmonic mode of the nanocup array, which is also responsible for the EOT. The simulated Purcell factor approaches 86 at the bottom of the nanocup and is ≈3–15 averaged over the nanocup cavity height, agreeing with the experimental enhancement result. This demonstration of solution‐based coupling between QDs and gold nanocups opens up new possibilities for applications that would benefit from a solution environment such as biosensing.     

Densification of La0.6Sr0.4Co0.2Fe0.8O3 ceramic by flash sintering at temperature less than 100 °C

The effect of DC electric field on sintering and electrical conductivity of La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF), considered as highly promising cathode material for solid oxide fuel cell, is investigated in the present work. It is shown that sintering can be carried out at (furnace) temperature <100 °C under electric field ranging from 7.5 to 12.5 V/cm; such extraordinary effect is associated with the high electrical conductivity of LSCF through a peculiar mechanism. Microstructural analysis suggests similar morphology and enhanced grain growth compared to traditional sintering; with the proper choice of processing parameters (electric field and current density) during flash sintering, homogeneous porous microstructure for cathodic application can be obtained in very short time. The role of electric field and specimen temperature in flash sintering is analyzed for the understanding of observed outstanding event. The conductivity is found to be a coupled response of electric field and temperature; 2–3 V/cm and 15–25 °C are sufficient for dense LSCF specimen to stimulate the electric field effect on sintering. Electric field controls the conductivity in the same way as temperature does suggesting that under flash effect conductivity is increased by usual mechanism. On the same basis, flash sintering is proposed to be accelerated by the “polaron hopping” phenomenon.     

Enhancement of antibacterial properties of Ag nanorods by electric field

Abstract

The effect of an electric field on the antibacterial activity of columnar aligned silver nanorods was investigated. Silver nanorods with a polygonal cross section, a width of 20–60 nm and a length of 260–550 nm, were grown on a titanium interlayer by applying an electric field perpendicular to the surface of a Ag/Ti/Si(100) thin film during its heat treatment at 700 °C in an Ar+H₂ environment. The optical absorption spectrum of the silver nanorods exhibited two peaks at wavelengths of 350 and 395 nm corresponding to the main surface plasmon resonance bands of the one-dimensional silver nanostructures. It was found that the silver nanorods with an fcc structure were bounded mainly by {100} facets. The antibacterial activity of the silver nanorods against Escherichia coli bacteria was evaluated at various electric fields applied in the direction of the nanorods without any electrical connection between the nanorods and the capacitor plates producing the electric field. Increasing the electric field from 0 to 50 V cm^?1 resulted in an exponential increase in the relative rate of reduction of the bacteria from 3.9×10^?2 to 10.5×10^?2 min^?1. This indicates that the antibacterial activity of silver nanorods can be enhanced by applying an electric field, for application in medical and food-preserving fields.     

Directing Gold Nanoparticles into Free‐Standing Honeycomb‐Like Ordered Mesoporous Superstructures

2D mesoporous materials fabricated via the assembly of nanoparticles (NPs) not only possess the unique properties of nanoscale building blocks but also manifest additional collective properties due to the interactions between NPs. In this work, reported is a facile and designable way to prepare free‐standing 2D mesoporous gold (Au) superstructures with a honeycomb‐like configuration. During the fabrication process, Au NPs with an average diameter of 5.0 nm are assembled into a superlattice film on a diethylene glycol substrate. Then, a subsequent thermal treatment at 180 °C induces NP attachment, forming the honeycomb‐like ordered mesoporous Au superstructures. Each individual NP connects with three neighboring NPs in the adjacent layer to form a tetrahedron‐based framework. Mesopores confined in the superstructure have a uniform size of 3.5 nm and are arranged in an ordered hexagonal array. The metallic bonding between Au NPs increases the structural stability of architected superstructures, allowing them to be easily transferred to various substrates. In addition, electron energy‐loss spectroscopy experiments and 3D finite‐difference time‐domain simulations reveal that electric field enhancement occurs at the confined mesopores when the superstructures are excited by light, showing their potential in nano‐plasmonic applications.