Field Enhancement Factor Research Articles

Tuning the structural, optical, local work function and field emission properties of molybdenum oxide thin films with oxygen partial pressures

We report a simple method involving optimization of oxygen partial pressure (OPP) in the after-growth annealing process of MoOx nanostructured thin films that results in a better field emitter with a low turn-on field and a high field enhancement factor. A ≈250 nm thick molybdenum oxide thin film has been synthesized on Si(100) substrates by the physical vapor deposition technique. The as-deposited thin films are annealed at 600 °C for 30 min in different OPPs, such as ≈2.1 × 10−9 Pa, ≈2.1 × 10−1 Pa, and ≈2.1 × 104 Pa. The effect of postannealing on the morphology, crystal structure, stoichiometry, elemental concentration, optical and local work function of the samples has been investigated using scanning electron microscopy, atomic force microscopy, resonant Rutherford backscattering spectrometry, and X-ray diffraction methods. As-deposited amorphous thin films transformed to granular MoO2 and layeredlike α-MoO3 nanostructures after annealing at 600 °C for 30 min in the OPP of ≈2.1 × 10−9 Pa and ≈2.1 × 104 Pa, respectively. The optical bandgap and the local work function are increased from 2.60 eV to 3.22 eV and 5.12 eV to 5.76 eV, whereas the field emission turn-on field is decreased from 6.6 V/μm to 4.7 V/μm for 10 μA/cm2 with increasing OPP from ≈2.1 × 10−9 Pa to ≈2.1 × 104 Pa.

Nanotip Contacts for Electric Transport and Field Emission Characterization of Ultrathin MoS2 Flakes.

We report a facile approach based on piezoelectric-driven nanotips inside a scanning electron microscope to contact and electrically characterize ultrathin MoS2 (molybdenum disulfide) flakes on a SiO2/Si (silicon dioxide/silicon) substrate. We apply such a method to analyze the electric transport and field emission properties of chemical vapor deposition-synthesized monolayer MoS2, used as the channel of back-gate field effect transistors. We study the effects of the gate-voltage range and sweeping time on the channel current and on its hysteretic behavior. We observe that the conduction of the MoS2 channel is affected by trap states. Moreover, we report a gate-controlled field emission current from the edge part of the MoS2 flake, evidencing a field enhancement factor of approximately and a turn-on field of approximately at a cathode–anode separation distance of .

Fowler–Nordheim Law Correlated with Improved Field Emission in Self‐Assembled NiCr2O4 Nanosheets

Electric field emission (FE) properties are measured on self‐assembled NiCr2O4 nanosheets in a planner “diode” arrangement at a base pressure of ≈1.0 × 10−8 mbar. The turn‐on field at FE current density 1 μA cm−2 and the threshold field at FE current density 10 μA cm−2 are observed as 4.10 and 4.94 V μm−1, respectively. The local work function (Φ) is calculated as 4.358 eV, using density functional theory (DFT) in Quantum Espresso code and field enhancement factor (β) intensified up to 2074 from the sharp edges of the NiCr2O4 nanosheet arrays’ emitter surface. An exemplary FE current stability is observed from the current–time plot over a period of 4.5 h. The field enhancement factor (β), inferior turn‐on field, and superior stability compared with any other pristine nanosheet compound recommend its potential application in flat panel display and vacuum micro‐/nanoelectronics.

Growth evolution of self-catalytic tin-doped indium oxide nanowires

Self-catalytic tin-doped indium oxide nanowires were prepared via thermal-chemical vapor deposition. The growth evolution of these nanowires with respect to the growth time was investigated. Furthermore, the effect of oxygen gas flow on the growth was determined by comparing nanowires grown with/without the flow. In the early growth stages, the nanowires crawled to the substrate after droplet formation. Based on the Ellingham diagram and the difference between the energy barriers, Sn remained in the liquid phase on the head of the nanowires, whereas In2O3 formed as the stem of the wires. After 30 s of growth, the head of the crawling nanowires separated from the substrate, regardless of the droplet size. The droplets were almost completely eliminated from the end of the ITO nanowires grown under the oxygen gas flow but were present at the tips of the wires grown without the flow. The ITO nanowires grown under the flow were characterized by a turn-on field and field enhancement factor of ∼3.4 V/μm and 1632, respectively. The present work provides the mechanism for the growth of nanowires and the design architecture of materials used in various applications.

Strong field enhancement in individual Φ-shaped dielectric nanostructures based on anapole mode resonances.

Due to their ability to produce high electric field enhancements in relatively large nanoscale volumes with minimum absorption and nonradiating properties, anapole modes excited in high index dielectric nanostructures have attracted considerable attentions in these years. We propose a design strategy to simultaneously excite the anapole mode efficiently and maintain its resonant wavelength, which has been remained as a challenge in the conventional dielectric nanostructures. Based on analyzing the relationship between the field enhancement factor and scattering intensity of the electric and toroidal dipoles, we introduce two and four nanocuboids into the nil field intensity areas in the silicon disk system, respectively. The geometric volume of the system can be increased effectively and the electric field enhancement is boosted to be 190% and 250% while the resonant wavelength of the anapole mode is almost maintained constant. The systems combined with a slot in the strongest field intensity area also follow the same law, revealing that the design strategy can be easily extended to other geometric, material and frequency systems. Different from the design strategy to add new components into the areas with strong field intensity, the incorporations occurring at the minimum intensity area is another design scheme to engineer the properties of the resonant systems and can find broad applications in nano-device designs.

Анализ эмиссии электронов с одиночного кремниевого катода в квазивакуумную (воздушную) среду методом атомно-силовой микроскопии

Using atomic force microscopy (AFM), we experimentally examined the features of field-electron emission from a single point-type silicon cathode into a quasi-vacuum (air) medium. In the non-contact AFM operating mode, the current – voltage characteristics (CVCs) of a single cathode with a nanometer radius of curvature of the tip were measured at distances of 10 nm and 20 nm between the cathode tip and the top of the measuring probe. The electric field distribution was simulated both on the surface of the tip of a single cathode and on the surface of the tips of individual cathodes within the array, based on which a theoretical estimate of the field enhancement factor as a function of the cathode-probe distance was made. The field-enhancement factor calculated from the experimental CVCs in the Fowler-Nordheim coordinates is several orders of magnitude higher than its value obtained from theoretical calculations. Such a mismatch between the experimental data and the simulation results indicates the need to take into account additional quantum-size effects, which play an important role in the formation of the field-electron emission current in the nanoscale gap. In particular, deformation of the silicon emitter tip can occur at this scale due to the penetration of a strong electric field into its surface region, which, in turn, causes the distortion of the potential barrier at the interface with the quasi-vacuum medium.

Design of a femtosecond electron diffractometer with adjustable gaps

One of the grand challenges in ultrafast science is real-time visualization of the microscopic structural evolution on atomic time and length scales. A promising pump-probe technique using a femtosecond laser pulse to initiate the ultrafast dynamics and another ultrashort electron pulse to probe the resulting changes has been developed and widely used to study ultrafast structural dynamics in chemical reactions, phase transitions, charge density waves, and even biological functions. In the past three decades, a number of different ultrafast electron guns have been developed to generate ultashort electron sources, mainly including hybrid electron gun with radio-frequency (RF) cavities for compressing the pulse broadening, relativistic electron gun for suppressing the coulomb interaction, single-electron pulses without space charge effect and compact direct current (DC) electron gun for minimizing the electron propagation distance. At present, these developments with different final electron energy and available total charge have improved the time response of ultrafast electron diffraction (UED) setups to a new frontier approaching to 100 fs regime. Although enormous efforts have been made, the superior capabilities and potentials of ultrafast electron diffraction (UED) are still hindered by space-charge induced pulse broadening. Besides, the penetration depth of electrons increases with the electron energy, while the scattering probability of electrons has the opposite consequence. Thus, in addition to the temporal resolution enhancement, it is also important that the electron energy should be tunable in a wide range to meet the requirements for samples with different thickness. Here in this work, we design a novel ultra-compact electron gun which combines a well-designed cathode profile, thereby providing a uniform field and a movable anode configuration to achieve a temporal resolution on the order of 100 fs over an accelerating voltage range from 10 kV to 125 kV. By optimizing the design of the high-voltage electrode profile, the field enhancement factor on the axis and along the cathode surface are both less than ~4% at different cathode-anode spacings, and thus the maximum on-axis field strength of ~10 MV/m is achieved under various accelerating voltages. This effectively suppresses the space charge broadening effect of the electron pulse. Furthermore, the anode aperture is designed as a stepped hole in which the dense sample grid can be placed, and the sample under study is directly supported by the grid and located at the anode, which reduces the cathode-to-sample distance, thus minimizing the electron pulse broadening from the cathode to sample. Moreover, the defocusing effect caused by the anode hole on the electron beam can be effectively reduced, therefore improving the lateral focusing performance of the electron beam.

Tuning optical force of dielectric/metal core-shell placed above Au film

Manipulating the core-shell structure with the optical force has been extensively studied, giving birth to applications such as particle sorting, biomarkers and drug delivery. Tailoring the optical force exerted on the core-shell above the metallic film remains unexplored, despite the obvious benefits for both fundamental research and applications including strong coupling, surface enhanced spectroscopy, nanolaser, and nanoscale sensing. In this work, we systematically investigate the optical force exerted on a dielectric/metal core-shell above a gold film by utilizing the Maxwell stress tensor formalism. It is found that at the present gold substrate, the optical force on the core-shell can be one order of magnitude larger than that on the individual core-shell due to the strong coupling between the core-shell and the gold film. Interestingly, the direction of the optical force can be reversed from positive to negative by distributing the local field from the upside of core-shell to the structure gap through changing the excitation wavelength. Furthermore, we demonstrate that the magnitude and peak wavelength of the optical force can be well controlled by altering the structure gap, the size and refractive index of the core. More specifically, it is found that the coupling strength between the core-shell and the gold film decreases with the gap size increasing. As a result, we observe the blue shift of bonding mode and the decrease of local field in the gap, which leads the force peak wavelength to be blue-shifted and the force peak magnitude to decrease, respectively. Also, by increasing the radius and refractive index of the core, a red shift of force peak is accompanied with the red shift of the bonding mode. In addition, the force peak magnitude follows the same trend as the total local field enhancement factor when the radius and refractive index of the core change. We hope that our results open the way to control the cavity size of particle on film structure, which would be beneficial for tailoring the light matter interaction even down to single molecular level and promises to have the applications in novel functional photonic devices.

Field emission behaviors of CsPbI3 nanobelts

We report the field emission behaviors of CsPbI3 nanobelts, which had a low turn-on field of ∼2.62 V μm−1 with a high field enhancement factor of 3553.

Effect of core size/shape on the plasmonic response of spherical ZnO@Au core-shell nanostructures embedded in a passive host-matrices of MgF<sub>2</sub>

In this paper, we investigated the effect of shape and size of core on the plasmonic response of spherical ZnO@Au core-shell nanostructures embedded in a passive host matrices of MgF_<i>2</i> within the framework of the qausistatic approximation. The absorption cross-section and local field enhancement factor of spherical ZnO@Au core-shell nanostructures are effectively studied by optimizing the parameters for a fixed composite diameter of 20 nm. In this two-layered core-shell nanostructures, four plasmonic resonances are found; the first two resonances associated with ZnO/Au and Au/MgF₂ interfaces, whereas the third and fourth resonances are associated with the transverse and longitudinal modes, respectively. The peaks position and intensity of these resonances are varied by optimizing the shape and size of the core material. The tunability of the plasmon resonances of the composite systems enables it to exhibit very interesting material properties in a variety of applications extending from the visible to infrared spectral regions.

Zinc oxide nanoflowers synthesized by sol-gel technique for field emission displays (FEDs)

ZnO has been adopted as preferred material in transparent transistors and can be very useful in products such as field emission displays (FEDs). In this work, Zinc Oxide nanoflowers/nanoparticles have been synthesized by using sol–gel technique. During synthesis, zinc acetate and distilled water were used precursors. The characterization of synthesized zinc oxide nanoparticles was done by field emission scanning electron microscope (FESEM), X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM). FESEM results revealed the nanoflower type structure of zinc oxide nanoparticles. Further, field emission studies of the grown ZnO nanoflowers were studied and it was observed that due to flower type structure, ZnO nanoparticles have high current density of 30.5 mA/cm2 with low turn on field at 1.2 Vμm−1 and good field enhancement factor.

Bowtie Nanoantenna Array Integrated With Artificial Impedance Surfaces for Realizing High Field Enhancement and Perfect Absorption Simultaneously

High field enhancement and near-perfect absorption in nanoantennas were realized by using in-plane (between nanoantenna arms), out-of-plane (between nanoantenna and reflector), and array coupling (between nanoantennas in an array); however, it was challenging to satisfy both conditions at the same time. In this paper, we show that a bowtie nanoantenna array integrated with an artificial impedance surface can simultaneously satisfy both high field enhancement and perfect absorption. The artificial impedance surface is implemented as a metallic patch array on a grounded 50 nm-thick SiO2 substrate with reactive impedance surface (RIS) or high impedance surface (HIS) characteristic. Through the proposed design methodology, we designed a bowtie nanoantenna array on an optimum RIS patch array and achieved a high field enhancement factor (E/E0) of 228 and a nearly perfect absorption rate of 98% at 230 THz. This novel design outperforms the previously reported nanoantenna structures and the same bowtie nanoantenna array designed using a conventional grounded SiO2. We also show that the HIS-integrated bowtie antenna array cannot realize both goals at the same time because the highly reflective HIS cannot guarantee perfect absorption. The proposed RIS-combined nanoantenna array with high field enhancement and near-perfect absorption can be used for efficient infrared (IR) and optical detectors, sensors, and energy harvesting devices.

Highly enhanced field emission from copper oxide nanoparticle decorated vertically aligned carbon nanotubes: Role of interfacial electronic structure

Field emission (FE) properties of Copper (3 nm and 10 nm) decorated vertically aligned carbon nanotubes (VACNTs) grown on Si substrate before and after oxidation are investigated. The current density is found to be the maximum (20 mA/cm2) for 3 nm thick Cu decorated VACNTs after oxidation. The variation in conventionally monitored FE parameters like work function and field enhancement factor cannot explain the experimentally determined high FE current density and its variation. A critical analysis of the electronic structure reveals that the ratio of CuO/Cu2O on VACNTs plays an important role in controlling the FE current density. The highly enhanced FE current density and its temporal stability of 3 nm Cu decorated VACNTs after oxidation indicate that it may be used as a potential material for a FE electron source in future vacuum microelectronic devices.

Analytic model of a compound thermal-field emitter and its performance

A methodology for implementing the recently developed reformulated general thermal-field equation describing simultaneous thermal and field emission contributions to electron emission is developed, with modifications directed to enhancing speed and accuracy of computation as demanded by emitter characterization and electron beam simulations. An accurate factor to correct both thermal-field (TF) and high field (Fowler-Nordheim or FN) predictions based on a rapid Lorentzian fit model is given. The analytic protrusion model is constructed from modifications to a point dipole model that allow surface elements and field enhancement factors to be rapidly evaluated. The model is applied to an analytical model of a protrusion on a bump in a diode configuration to characterize how current-voltage I(V) relations are affected by TF emission conditions. In addition, it is also shown (i) how aggressively Schottky’s conjecture is undermined as the protrusion dimensions become larger; (ii) how the total current in the TF regime can be substantially larger than predicted by canonical [FN and Richardson-Laue-Dushman (RLD)] formulations; (iii) how an optimal protrusion size may exist; and (iv) how the inference of field enhancement, notional emission area, and work function are poorly predicted using conventional methods relying on the canonical FN and RLD equations even outside the thermal-field regime, even though data can be linear on FN and RLD plots.

Influence of the distribution of local field enhancement factors on the shape of the current-voltage characteristics of carbon-nanotube-based large-area emitters

Experimental current-voltage characteristics (IVCs) of carbon nanotube (CNT)-based large-area field emitters (LAFEs) were measured and analyzed using a “two-class model”, in which some emitters had a set of local field enhancement factors (FEFs) significantly larger than those of the other emitters. An online measurement of the experimental IVCs was carried out and, interestingly, showed the presence of strong nonlinearity in the form of “upward bends” in the corresponding orthodox Fowler-Nordheim (FN) plots. Our studies were conducted over a wide range of currents, and excellent agreement between our experimental results and the proposed theory confirmed the general validity of the two-class FEF model of orthodox field emission. The results presented here were also supported by a bimodal distribution of the brightness level observed by an analysis of the glow patterns, since the brightness was expected to depend on the site's emitted current. We believe this is an important step for LAFE characterization when strong nonlinearites are present in the FN plots, since an FEF similar to the classically defined FEF was recently proven to be applicable to CNTs in the context of full quantum ab initio calculations.

Boosting field emission performance of TiO2 nanoarrays with designed architectures

As the “heart” of vacuum electronic device, the low turn-on field (Eto) of field emitters plays a crucial role for their practical application. In current work, we reported the designed fabrication of TiO2 field emitters with boosted field emission (FE) behaviors, by growing sharp branches and decorating Au nanoparticles (Au NPs) around the surfaces of preformed TiO2 nanorod arrays. It was found that the obtained nanoarrays with designed architectures could make a balance between the increased emission sites and limited field screening effect. The field enhancement factor (β) of as-fabricated TiO2 emitters was ~2 times to that the bare emitters. Moreover, their turn-on field and current density could be of 1.61 V/μm and ~4.95 mA/cm2, respectively, both of which were state-of-the-art ones among the TiO2 field emitters ever reported.

Designing a broadband terahertz plasmonic field enhancer with a homojunction of semiconductors

We proposed a broadband THz plasmonic field enhancer taking advantage of the spatially varying charge carrier concentration in a homojunction of semiconductors. The theoretical simulation indicates that the structure has a broadband resonance response and shows a field enhancement factor greater than 10 in the majority of the THz waveband. Near 6 THz, a maximum of 30-fold field enhancement is predicted. We believe that the proposed structures are ideal candidates for broadband field enhancement and may help realize more sophisticated THz devices.

Modeling the Field Emission Enhancement Factor for Capped Carbon Nanotubes Using the Induced Electron Density.

In many field electron emission experiments on single-walled carbon nanotubes (SWCNTs), the SWCNT stands on one of two well-separated parallel plane plates, with a macroscopic field FMapplied between them. For any given location"L" on the SWCNT surface, a field enhancement factor (FEF) is defined as FL/FM, where FL is a local field defined at "L". The best emission measurements from small-radii capped SWCNTs exhibit characteristic FEFs that are constant (i.e., independent of FM). This paper discusses how to retrieve this result in quantum-mechanical (as opposed to classical electrostatic) calculations. Density functional theory (DFT) is used to analyze the properties of two short, floating SWCNTs, capped at both ends, namely, a (6,6) and a (10,0) structure. Both have effectively the same height (∼5.46 nm) and radius (∼0.42 nm). It is found that apex values of local induced FEF are similar for the two SWCNTs, are independent of FM, and are similar to FEF values found from classical conductor models. It is suggested that these induced-FEF values are related to the SWCNT longitudinal system polarizabilities, which are presumed similar. The DFT calculations also generate "real", as opposed to "induced", potential-energy (PE) barriers for the two SWCNTs, forFM values from 3 V/μm to 2 V/nm. PE profiles along the SWCNT axis and along a parallel "observation line" through one of the topmost atoms are similar. At low macroscopic fields, the details of barrier shape differ for the two SWCNT types. Even for FM = 0, there are distinct PE structures present at the emitter apex (different for the two SWCNTs); this suggests the presence of structure-specific chemically induced charge transfers and related patch-field distributions.

Boron-Doped Nanocrystalline Diamond-Carbon Nanospike Hybrid Electron Emission Source.

Electron emission signifies an important mechanism facilitating the enlargement of devices that have modernized large parts of science and technology. Today, the search for innovative electron emission devices for imaging, sensing, electronics, and high-energy physics continues. Integrating two materials with dissimilar electronic properties into a hybrid material is an extremely sought-after synergistic approach, envisioning a superior field electron emission (FEE) material. An innovation is described regarding the fabrication of a nanostructured carbon hybrid, resulting from the one-step growth of boron-doped nanocrystalline diamond (BNCD) and carbon nanospikes (CNSs) by a microwave plasma-enhanced chemical vapor deposition technique. Spectroscopic and microscopic tools are used to investigate the morphological, bonding, and microstructural characteristics related to the growth mechanism of these hybrids. Utilizing the benefits of both the sharp edges of the CNSs and the high stability of BNCD, promising FEE performance with a lower turn-on field of 1.3 V/μm, a higher field enhancement factor of 6780, and a stable FEE current stability lasting for 780 min is obtained. The microplasma devices utilizing these hybrids as a cathode illustrate a superior plasma illumination behavior. Such hybrid carbon nanostructures, with superb electron emission characteristics, can encourage the enlargement of several electron emission device technologies.

Effect of electric field uniformity on positive streamer and breakdown characteristics of transformer liquids

Streamer development in liquid has different dominant mechanisms under uniform and divergent electric fields, respectively initiation and propagation. This paper presents a study on the influence of electric field uniformity on the pre-breakdown and breakdown characteristics of transformer insulating liquids. Streamers were investigated experimentally in different needle-plane non-uniform field and plane-needle-plane semi-uniform field configurations under standard positive lightning impulses. For the same gap distance, streamers under non-uniform field propagate with a more side-branched structure whereas streamer propagation under semi-uniform field is more constrained to the axial direction. Field enhancement factor was used to indicate the degree of inhomogeneity of the field configuration. Streamers in fields with higher field enhancement factors occupy larger areas. As field enhancement factor reduces, the streamer withstanding probability distribution gets closer to the breakdown probability distribution and hence the breakdown happens at a lower normalized voltage level. Furthermore, a transition phase from a propagation-dominated to an initiation-dominated process of streamer development was evident as the field enhancement factor decreases. The field enhancement factor at which this transition happens was quantitatively derived. It is 54.1 for the investigated mineral oil under positive lightning impulses.