Large Enhancement Factor Research Articles

Engineering electric and magnetic dipole coupling in arrays of dielectric nanoparticles

Dielectric nanoparticles with both strong electric and magnetic dipole (ED and MD) resonances offer unique opportunities for efficient manipulation of light-matter interactions. Here, based on numerical simulations, we show far-field diffractive coupling of the ED and MD modes in a periodic rectangular array. By using unequal periodicities in the orthogonal directions, each dipole mode is separately coupled and strongly tuned. With this method, the electric and magnetic response of the dielectric nanoparticles can be deliberately engineered to accomplish various optical functionalities. Remarkably, an ultra-sharp MD resonance with sub-10 nm linewidth is achieved with a large enhancement factor for the magnetic field intensity on the order of ∼103. Our results will find useful applications for the detection of chemical and biological molecules as well as the design of novel photonic metadevices.

Local probing of the enhanced field electron emission of vertically aligned nitrogen-doped diamond nanorods and their plasma illumination properties

A detailed conductive atomic force microscopic investigation is carried out to directly image the electron emission behavior for nitrogen-doped diamond nanorods (N-DNRs). Localized emission measurements illustrate uniform distribution of high-density electron emission sites from N-DNRs. Emission sites coupled to nanographitic phases at the grain boundaries facilitate electron transport and thereby enhance field electron emission from N-DNRs, resulting in a device operation at low turn-on fields of 6.23 V/μm, a high current density of 1.94 mA/cm2 (at an applied field of 11.8 V/μm) and a large field enhancement factor of 3320 with a long life-time stability of 980 min. Moreover, using N-DNRs as cathodes, a microplasma device that can ignite a plasma at a low threshold field of 390 V/mm achieving a high plasma illumination current density of 3.95 mA/cm2 at an applied voltage of 550 V and a plasma life-time stability for a duration of 433 min was demonstrated.

Field emission property of vertically aligned nitrogen-doped multi-walled carbon nanotubes produced by chemical vapor deposition

Vertically aligned nitrogen-doped multi-walled carbon nanotubes (N-MWNTs) were synthesized by the chemical vapor deposition (CVD) at 900 °C using imidazole mixed with ferrocene as carbon and nitrogen sources, and catalyst, respectively. The effects of ammonia (NH3) and hydrogen (H2) flow rates on the growth of N-MWNTs were investigated (hereafter referred to A-N-MWNTs and H-N-MWNTs, respectively). Transmission electron microscopy (TEM) revealed the bamboo-like structure of the N-MWNTs, in which the separation between individual bamboo compartments decreased with increasing nitrogen concentration in N-MWNTs. X-ray photoelectron spectroscopy (XPS) analysis results supported that the nitrogen concentrations in N-MWNTs is 0.55 at.%, whereas A-N-MWNTs and H-N-MWNTs with the flow rate at 10 standard cubic centimeters per minute (sccm) (10A-N-MWNTs and 10H-N-MWNTs, respectively) showed 1.14 and 4.06 at.%, respectively. We found that the optimal conditions for the highest nitrogen-doped multi-walled carbon nanotubes (MWNTs) was a flow rate of NH3 and H2 at 10 sccm. Results from field emission measurements indicated that the turn-on fields of N-MWNTs, 10A-N-MWNTs and 10H-N-MWNTs were 6.7, 4.3 and 3.1 V/µm, respectively, while the field enhancement factors (β) were 5230, 10,805 and 20,390, respectively. Furthermore, the current density of N-MNWTs increased with increasing the nitrogen atoms in MWNTs. The results support that field emission based on N-MWNTs is a good emitter with low turn-on field and large field enhancement factor. Nitrogen doping in MWNTs makes them attractive candidates as high-performance field emitters.

High-current field emission from “flower-like” few-layer graphene grown on tip of nichrome (8020) wire

ABSTRACTWe report a novel tip-type field emission (FE) emitter by synthesizing the few-layer graphene (FLG) flakes on tip of nichrome (8020) wire (ϕ≈80 μm) by microwave plasma enhanced chemical vapor deposition(PECVD). These resultant random arrays of free-standing FLG flakes are aligned vertically to the substrate surface in a high-density and stacked to each other to form several larger “flower-like” agglomerates in spherical shapes. The FE performance of the tip-type FLG flakes emitter shows a low threshold field of 0.55 V/μm, a large field enhancement factor of 9455 ± 46, a large field emission current density of 22.18 A/cm2 at 2.70 V/μm, and an excellent field emission stability at high emission current densities (6.93 A/cm2). It can be used in variety of applications that include cathode-ray tube monitors, X-ray sources, electron microscopes, and other vacuum electronic applications.

Self-organized multi-layered graphene-boron-doped diamond hybrid nanowalls for high-performance electron emission devices.

Carbon nanomaterials such as nanotubes, nanoflakes/nanowalls, and graphene have been used as electron sources due to their superior field electron emission (FEE) characteristics. However, these materials show poor stability and short lifetimes, which prevent their use in practical device applications. The aim of this study was to find an innovative nanomaterial possessing both high robustness and reliable FEE behavior. Herein, a hybrid structure of self-organized multi-layered graphene (MLG)-boron doped diamond (BDD) nanowall materials with superior FEE characteristics was successfully synthesized using a microwave plasma enhanced chemical vapor deposition process. Transmission electron microscopy reveals that the as-prepared carbon clusters have a uniform, dense, and sharp nanowall morphology with sp3 diamond cores encased by an sp2 MLG shell. Detailed nanoscale investigations conducted using peak force-controlled tunneling atomic force microscopy show that each of the core-shell structured carbon cluster fields emits electrons equally well. The MLG-BDD nanowall materials show a low turn-on field of 2.4 V μm-1, a high emission current density of 4.2 mA cm-2 at an applied field of 4.0 V μm-1, a large field enhancement factor of 4500, and prominently high lifetime stability (lasting for 700 min), which demonstrate the superiority of these materials over other hybrid nanostructured materials. The potential of these MLG-BDD hybrid nanowall materials in practical device applications was further illustrated by the plasma illumination behavior of a microplasma device with these materials as the cathode, where a low threshold voltage of 330 V (low threshold field of 330 V mm-1) and long plasma stability of 358 min were demonstrated. The fabrication of these hybrid nanowalls is straight forward and thereby opens up a pathway for the advancement of next-generation cathode materials for high brightness electron emission and microplasma-based display devices.

Synthesis and field electron emission properties of multi-walled carbon nanotube films directly grown on catalytic stainless steel substrate

The multi-walled CNT films were directly grown on stainless steel substrates using an oxidation-reduction treatment with chemical vapor deposition. Microstructure and field emission properties of the as-received CNT films were detailedly investigated. The results indicate that the CNTs have originally random orientation with relative high density, good crystalline perfection and large diameter. Besides, the CNT films exhibit superior field electron emission performances with low threshold electric field (∼4.2 V/μm) and very high emission current density (∼24 mA/cm2). Importantly, the CNTs directly grown on stainless steel substrates have remarkable long-term stability with ∼3.4% fluctuation over 50 h continuous measurement under high current density, ∼10 mA/cm2. The perfect graphitization, large field enhancement factor, low contact resistance, good adhesion and high thermal conductivity are believed to be principally responsible for their superior field emission properties, which make them very promising application as an electron source in microthrust space electric propulsion neutralizer.

Concentric toroids as a wideband and efficient substrate for surface-enhanced Raman spectroscopy applications

ABSTRACTDesigning a structure having high local field enhancement, wideband resonance, and large hot spot area is the key element to obtain a large enhancement factor for surface-enhanced Raman spectroscopy applications. Here, the concentric toroid structures in dimer configuration is proposed, which shows a large local field intensity in a wide spectral range and the region that leads to a high-surface-enhanced Raman scattering signal intensity. Calculations show that the average surface-enhanced Raman scattering enhancement is up to 60 times more compared to the conventional dimer toroid structures with similar size.

Lifetime advantage and failure mechanism of a metal-ferroelectric cathode

The lifetime of explosive emission cathodes is important for high power microwave generators operating in the repetitive regime. For normal metallic cathodes, micropoints on the cathode surface with large field enhancement factors may be gradually consumed in explosive electron emission, which will lead to a limited lifetime. In this paper, a metal-ferroelectric cathode made of stainless steel and BaTiO3 is manufactured. Under voltage close to 1 MV and current near 10 kA, this cathode presents a much longer lifetime than the normal stainless steel cathode, demonstrating the lifetime advantage of the metal-ferroelectric cathode. Nevertheless, in the lifetime experiment of 1.28 × 105 pulses, this metal-ferroelectric cathode also presents obvious lifetime phenomena, one of which is the microwave duration generated by a relativistic backward wave oscillator decreasing from 27 ns to 19 ns. Observation of the cathode surface morphology shows that the emission property deterioration of the metal-ferroelectric ca...

Superior B-Doped SiC Nanowire Flexible Field Emitters: Ultra-Low Turn-On Fields and Robust Stabilities against Harsh Environments.

Low turn-on fields together with boosted stabilities are recognized as two key factors for pushing forward the implementations of the field emitters in electronic units. In current work, we explored superior flexible field emitters based on single-crystalline 3C-SiC nanowires, which had numbers of sharp edges, as well as corners surrounding the wire body and B dopants. The as-constructed field emitters behaved exceptional field emission (FE) behaviors with ultralow turn-on fields (Eto) of 0.94-0.68 V/μm and current emission fluctuations of ±1.0-3.4%, when subjected to harsh working conditions under different bending cycles, various bending configurations, as well as elevated temperature environments. The sharp edges together with the edges were able to significantly increase the electron emission sites, and the incorporated B dopants could bring a more localized state close to the Fermi level, which rendered the SiC nanowire emitters with low Eto, large field enhancement factor as well as robust current emission stabilities. Current B-doped SiC nanowires could meet all essential requirements for an ideal flexible emitters, which exhibit their promising prospect to be applied in flexible electronic units.

Optimizing silver‐capped silicon nanopillars to simultaneously realize macroscopic, practical‐level SERS signal reproducibility and high enhancement at low costs

AbstractThe ideal surface‐enhanced Raman spectroscopy (SERS) substrate should fulfil the following: (a) predictable SERS enhancement, (b) macroscale SERS signal uniformity, and (c) suitability for mass production at low costs. Macroscale SERS uniformity and reproducibility at practical levels are big obstacles, which have been preventing most SERS substrates from reliable sensing applications. We have previously shown that SERS‐active nanopillar structures, fabricated by lithography‐free processes, exhibit high average SERS enhancements and are mass producible. Here, we report an optimized process and show that the improved structures exhibit unrivalled macroscale SERS uniformities (RSD: ∼2.5% in millimeter scale, ∼7% in wafer scale) and reproducibility (RSD: ∼1.5% across 3 wafers), while at the same time exhibiting a very large average SERS enhancement factor of >108. The obtained SERS uniformity (~2.5% RSD in millimeter scale) is the best to date measured on large‐area solid SERS substrates. Fast and reproducible SERS analysis of trans‐1,2‐bis (4‐pyridyl) ethylene down to 4 × 10−13 mol is demonstrated using the optimized structures. We emphasize that achieving simultaneously macroscopic, practical‐level SERS signal reproducibility and high enhancement via a lithography‐free process is a notable advance towards industrialization of substrate‐based SERS sensors.

Gold nanorod/nanosphere clustering by split-GFP fragment assembly for tunable near-infrared SERS detections

We describe the near-field properties of plasmonic nanoclusters made of gold nanorod (AuNR) and gold nanosphere (AuNS) colloids that are assembled using self-complementary split-green fluorescence protein (sGFP) fragments. Numerical modeling of the optical responses and field enhancement characteristics for these hybrid AuNR/AuNS heteroclusters were performed (i) as a function of AuNS binding locations along the edges or at the tips of AuNRs, (ii) as a function of the size and number of AuNS per AuNR, and (iii) as a function of cluster geometry and orientation with respect to the major polarization states of light. We show that near-infrared (NIR)-active plasmonic hot spots that provide large SERS enhancement factors of the vibrational fingerprints from the reconstructed GFP-chromophore are consistently obtained for longitudinally polarized-excitation of the AuNR/AuNS nanoassemblies. A set of clusters having sufficiently flexible geometry and good spectral resonance with traditional NIR laser excitations at 785 nm is proposed for NIR SERS detection of the GFP chromophore with enhancement factors in the range of 107-108 folds. This study provides grounds to improve the assembly of hot spot AuNR/AuNS SERS nanoprobes for NIR biosensing using GFP as a Raman reporter.

Lifetime of Metallic Explosive Emission Cathodes and Microscopic Explanations

Metallic cathodes can be used to generate intense relativistic electron beams for high-power microwave devices. Many previous papers have demonstrated that metallic cathodes usually have a limited lifetime. In this paper, three metallic cathodes, namely, stainless steel cathode, copper cathode, and aluminum cathode, are researched with a repetitive-operation accelerator and an X-band relativistic backward-wave oscillator. The cathode surface morphologies are observed with a scanning electron microscope before and after lifetime test. The experimental results demonstrate that the stainless steel cathode has a very limited lifetime ( $ pulses), the copper cathode has a moderate lifetime ( $\sim 10^{4}$ pulses), and the aluminum cathode has a surprisingly long lifetime (at least $10^{5}$ pulses). The different lifetimes of the three cathodes may originate from the different situations of regeneration of microprotrusions on the cathode surface. The large number of regenerated microprotrusions which possess large microscopic field enhancement factors supports the long lifetime of the aluminum cathode.

A new metal-dielectric cathode with long lifetime

Explosive emission cathodes are widely used in high power microwave generation. Conventional metallic cathodes have the disadvantages of bad emission uniformity and short lifetime. In order to improve the emission property of metallic cathodes, a new metal-dielectric cathode is fabricated with the plasma spraying technology. Unlike previous metal-dielectric cathodes, our metal-dielectric cathode adopts a ferroelectric ceramic which possesses a large permittivity. The results of lifetime experiments show the metal-dielectric cathode presents just slight performance deterioration within 2.5 × 105 pulses and thus has a much longer lifetime than the normal copper cathode. The morphology observation demonstrates that the good emission property of the metal-dielectric cathode may benefit from the appearance of irregularities on the dielectric surface, which will have large microscopic field enhancement factors with the help of large permittivity of the ferroelectric material.

Electric dipole moment of C13

We calculate for the first time the electric dipole moment (EDM) of $^{13}$C generated by the isovector CP-odd pion exchange nuclear force in the $\alpha$-cluster model, which describes well the structures of low lying states of the $^{13}$C nucleus. The linear dependence of the EDM of $^{13}$C on the neutron EDM and the isovector CP-odd nuclear coupling is found to be $d_{^{13}{\rm C}} = -0.33 d_n - 0.0020 \bar G_\pi^{(1)}$. The linear enhancement factor of the CP-odd nuclear coupling is smaller than that of the deuteron, due to the difference of the structure between the $1/2^-_1$ state and the opposite parity ($1/2^+$) states. We clarify the role of the structure played in the enhancement of the EDM. This result provides good guiding principles to search for other nuclei with large enhancement factor. We also mention the role of the EDM of $^{13}$C in determining the new physics beyond the standard model.

Vertically aligned diamond-graphite hybrid nanorod arrays with superior field electron emission properties

A “patterned-seeding technique” in combination with a “nanodiamond masked reactive ion etching process” is demonstrated for fabricating vertically aligned diamond-graphite hybrid (DGH) nanorod arrays. The DGH nanorod arrays possess superior field electron emission (FEE) behavior with a low turn-on field, long lifetime stability, and large field enhancement factor. Such an enhanced FEE is attributed to the nanocomposite nature of the DGH nanorods, which contain sp2-graphitic phases in the boundaries of nano-sized diamond grains. The simplicity in the nanorod fabrication process renders the DGH nanorods of greater potential for the applications as cathodes in field emission displays and microplasma display devices.

Versatile Gap Mode Plasmon under ATR Geometry towards Single Molecule Raman, Laser Trapping and Photocatalytic Reactions.

We have investigated various aspects of a gap mode plasmon to establish it as an analytical tool. First, markedly large (107 - 109) enhancement factors for the Raman scattering intensity from a thiophenol (TP) monolayer sandwiched by Ag films on a prism and silver nanoparticles (AgNPs) were obtained under attenuated total reflection (ATR) geometry. Second, AgNPs with a radius of ∼20 nm were optically trapped and immobilized on TP-covered Ag films under a gap mode resonance with extremely weak laser power density of ∼1 μW/μm2 at 532 nm. The observed optical trapping and immobilization were theoretically rationalized using a dipole-dipole coupling and van der Waals interaction between AgNPs and Ag films. Third, p-alkyl TP molecules such as p-methyl TP, p-ethyl TP, p-isopropyl TP, and p-tertiary butyl TP were photocatalytically oxidized into p-carboxyl TP, whereas o- and m-methyl TP did not show such reactions.

Design of Hybrid Nanostructural Arrays to Manipulate SERS-Active Substrates by Nanosphere Lithography.

An easy-handling and low-cost method is utilized to controllably fabricate nanopattern arrays as the surface-enhanced Raman scattering (SERS) active substrates with high density of SERS-active areas (hot spots). A hybrid silver array of nanocaps and nanotriangles are prepared by combining magnetron sputtering and plasma etching. By adjusting the etching time of polystyrene (PS) colloid spheres array in silver nanobowls, the morphology of the arrays can be easily manipulated to control the formation and distribution of hot spots. The experimental results show that the hybrid nanostructural arrays have large enhancement factor, which is estimated to be seven times larger than that in the array of nanocaps and three times larger than that in the array of nanorings and nanoparticles. According to the results of finite-difference time-domain simulation, the excellent SERS performance of this array is ascribed to the high density of hot spots and enhanced electromagnetic field.

A zeolitic imidazolate framework based nanoporous carbon as a novel fiber coating for solid-phase microextraction of pyrethroid pesticides

A high-surface-area nanoporous carbon (NPC) has been successfully synthesized by using the metal-organic framework ZIF-90 as both the template and precursor together with furfuryl alcohol as a secondary carbon source. The prepared ZIF-90 templated NPC (ZIF-90-NPC) was then coated onto a stainless steel wire by a simple physical adhesion approach to prepare solid-phase microextraction (SPME) fiber. By coupling the ZIF-90-NPC coated fiber-based SPME with gas chromatography-microelectron capture detection (GC-μECD), the developed method gave a large enhancement factor (984-2869), low limit of detection (0.1–0.5ngg−1) and good linearity (0.3–50ngg−1) for the determination of some pyrethroid pesticides (bifenthrin, fenpropathrin, cyhalothrin, permethrin, cyfluthrin, flucythrinate, fenvalerate and deltamethrin) from different fruits and vegetables. The relative standard deviations (RSDs) for six replicate extractions of the pyrethroids by the ZIF-90-NPC coated fiber ranged from 4.3% to 8.0%. The recoveries of the spiked pyrethroids (5ngg−1 and 20ngg−1) from fruit and vegetable samples were in the range of 88.0–104.0% and 86.0–103.5% with the RSDs ranging from 4.8% to 12.9% and 5.0–10.8%, respectively. Besides, the ZIF-90-NPC coated fiber was stable enough for 100 extraction cycles without a significant loss of extraction efficiency. The method was successfully applied to the determination of the pyrethroids in fruit and vegetable samples.

Enhanced effect of plasma on catalytic reduction of CO2 to CO with hydrogen over Au/CeO2 at low temperature

In terms of the reaction of CO2 reduction to CO with hydrogen, CO2 conversion is very low at low temperature due to the limitation of thermodynamic equilibrium (TE). To overcome this limitation, plasma catalytic reduction of CO2 to CO in a catalyst-filled dielectric barrier discharge (DBD) reactor is studied. An enhanced effect of plasma on the reaction over Au/CeO2 catalysts is observed. For both the conventionally catalytic (CC) and plasma catalytic (PC, Pin=15 W) reactions under conditions of 400°C, H2/CO2=1, 200 SCCM, GHSV=12,000 mL•g−1cat•h−1, CO2 conversions over Au/CeO2 reach 15.4% and 25.5% due to the presence of Au, respectively, however, those over CeO2 are extremely low and negligible. Moreover, CO2 conversion over Au/CeO2 in the PC reaction exceeds 22.4% of the TE conversion. Surface intermediate species formed on the catalyst samples during the reactions are determined by in-situ temperature-programmed decomposition (TPD) technique. Interestingly, it disclosed that in the PC reaction, the formation of formate intermediate is enhanced by plasma, and the acceleration by plasma in the decomposition of formate species is much greater than that in the formation of formate species on Au/CeO2. Enhancement factor is introduced to quantify the enhanced effect of plasma. Lower reactor temperature, higher gas hourly space velocity (GHSV), and lower molar ratio of H2/CO2 would be associated with larger enhancement factor.

Enhancement of plasma illumination characteristics of few-layer graphene-diamond nanorods hybrid

Few-layer graphene (FLG) was catalytically formed on vertically aligned diamond nanorods (DNRs) by a high temperature annealing process. The presence of 4–5 layers of FLG on DNRs was confirmed by transmission electron microscopic studies. It enhances the field electron emission (FEE) behavior of the DNRs. The FLG-DNRs show excellent FEE characteristics with a low turn-on field of 4.21 V μm−1 and a large field enhancement factor of 3480. Moreover, using FLG-DNRs as cathode markedly enhances the plasma illumination behavior of a microplasma device, viz not only the plasma current density is increased, but also the robustness of the devices is improved.