Maximum Emission Current Density Research Articles

Synthesis, physico-chemical characterization, DFT simulation, and field electron behaviour of 2D layered Ti3C2Tx MXene nanosheets

Nanostructures of Ti3C2Tx, one of the members of the MXenes family, have been successfully prepared by chemical etching of Al from Ti3AlC2 (MAX phase) using Hydrofluoric Acid (HF) for various etching durations at room temperature. The phase, morphological, structural, and chemical analysis was performed using XRD, FESEM, TEM, Raman, and x-ray photoelectron spectroscopy. The surface morphology of as-synthesized Ti3C2Tx (MXene) phase is characterized by stacks of layered sheets like structures. Field electron emission (FEE) behaviour was investigated at the base pressure of 1 × 10−8 mbar. The pristine Ti3AlC2 (MAX) and Ti3C2Tx (MXene) nanosheets emitters showed values of turn-on field (defined at current density ∼ 1 μA cm−2) as 4.18 and 1.67 V μm−1, respectively. Furthermore, maximum emission current density of ∼ 825 μA cm−2 was extracted from the MXene nanosheets emitter at an applied field of 3.60 V μm−1, in contrast to ∼71 μA cm−2 drawn at 7.31 V μm−1 from the pristine MAX emitter. The MXene nanosheets emitter exhibited good emission current stabilities at pre-set values ∼ 10 and 100 μA over 3 h duration. Work function values of the MAX and MXene nanosheets emitters were measured using a retarding field analyzer, and found to be 4.4 and 3.6 eV, respectively. Extensive ab-initio simulations have been performed to provide structural and electronic properties, as well as for estimating the work function of Ti3C2 layered material. The estimated electronic density of states revealed its metallic character. The improved FEE performance exhibited by the 2D layered Ti3C2Tx (MXene) nanosheets emitter is attributed to its unique morphology characterized by high aspect ratio, metallic electronic properties and relatively lower work function.

Study on field emission performance of SrTiO3 film enhanced by LiF film

A series of monolayer SrTiO3 films and LiF/SrTiO3 bilayer films were prepared on n-Si substrates by using vacuum electron beam vapor deposition (EBVD) technology. The morphologies and thicknesses of the SrTiO3 film and LiF film were controlled through adjustments and optimization of the growth conditions as well as deposition time. The experimental results show that the field emission performance of SrTiO3 film is significantly improved after LiF film is added. Under the research conditions in our laboratory, a large number of LiF/SrTiO3 bilayer film field emission devices have been tested and analyzed in the wide range of LiF film thickness (10–300 nm), and the effective field emission LiF film thickness (50–90 nm) was initially determined. Further study shows that the LiF/SrTiO3 bilayer film has the best field emission performance when the optimal LiF film thickness is 70 nm, and its maximum field emission current density can reach 305.48 μA/cm2. The field emission current density of the LiF/SrTiO3 bilayer thin film device is nearly 4 times compared to that of the monolayer SrTiO3 film when the applied electric field intensity reaches 9.05 V/μm. All the field emission devices exhibit excellent functioning stability and experimental reproducibility.

Improved field emission properties of reduced graphene oxide decorated with indium nanoparticles

Low work function metals decoration of graphene is a effective method for the improvement of electron field emission properties. Indium, a relatively lower work function metal, is used to decorate reduced graphene oxide (RGO) films for the first time by simple and efficient solution process combined with magnetron sputtering method and the field emission properties are investigated. At continue voltage mode, the indium decorated RGO films display lower turn-on and threshold field as well as better emission stability than those of pristine RGO films. Moreover, the field emission performance of indium decorated RGO films is affected by the spacing between the cathode and anode. At pulse voltage mode, the indium decorated RGO films show approximately 10 times higher maximum emission current density compared to that at continue voltage mode. The calculation results by density functional theory indicate that the indium decoration of RGO significantly increases the density of states (DOS) near Fermi level and reduces the work function.

Mechanism analysis of field electron emission of titanium

Field electron emission (FEE) is generally considered to be closely correlated with radio frequency (RF) breakdown in accelerating structures and high-power microwave (HPM) devices. In this study, the field emission characteristics of titanium are investigated by using a field emission test system. With the increase of the number of field emission tests and stability tests, the repeatability of the field emission characteristic curve of titanium tends to be better, and the amplitude of the current oscillation in the stability tests gradually decreases, indicating that the field emission performance of titanium gradually becomes stable. Subsequently, the field emission characteristic curve of titanium is analyzed by adopting the field emission theory of metal microprotrusions. Combined with the analysis of the electric field enhancement effects caused by the surface morphology, the dominant effect of the metal microprotrusions on the FEE of titanium is excluded from the two aspects of the electric field enhancement factor and local maximum emission current density. At last, the field emission theory of dielectric micropoints is introduced to analyze the field emission characteristics of titanium. The electric field enhancement factor of 102 ∼ 103 are explained theoretically. Simultaneously, the reasonable effective emission area and local emission current density are given, which can better explain the field emission phenomena of titanium, such as the sharp decrease in emission current and repeatability of the field emission curve. Hence, it is revealed that the key factor that dominates the FEE of titanium is the dielectric impurities on the surface, rather than the metal microprotrusions.

Thickness dependent field emission study of LaB6 coated Si nanowire arrays

A simple physical metal mask is efficiently employed to obtain an array of silicon (Si) nanowires (Si-NWs) on a Si substrate grown using the hot filament chemical vapor deposition route. Well adhered and uniform coating of different thicknesses (20 and 50 nm) of lanthanum hexaboride (LaB6) on Si-NWs was obtained using electron beam evaporation technique. The thickness of LaB6 coating was estimated from ellipsometry measurement. Structural, morphological, and chemical properties of the LaB6 coated Si-NWs (LaB6@Si-NWs) arrays were revealed using x-ray diffraction, field emission scanning electron microscope, transmission electron microscope, Raman spectroscopy, and x-ray photoelectron spectroscopy. Field electron emission characteristics of pristine Si-NW array and LaB6 coated Si-NWs array emitters were studied in planar diode configuration at a base pressure of 1 × 10−8 mbar. The values of turn-on field (current density ∼1 μA/cm2) were observed as ∼2.2, 1.2, and 1.6 V/μm for pristine Si-NWs, LaB6@Si-NWs_20, and LaB6@Si-NWs_50 array emitters, respectively. Furthermore, maximum emission current densities of ∼1276.81, 2763.64, and 2231.81 μA/cm2 have been extracted from the pristine Si-NWs, LaB6@Si-NWS_20, and LaB6@Si-NWS_50 array emitters at an applied field of 3.1, 2.7, and 2.7 V/μm, respectively. The LaB6@Si-NWS_20 array emitter demonstrated superior FEE properties as compared to the pristine Si-NWs and LaB6@Si-NWS_50 emitters. Furthermore, LaB6@Si-NWS_20 emitter depicted very good emission current stability tested at a preset value of 1 μA over a duration of 3 h. The enhanced FEE performance exhibited by the LaB6@Si-NWs_20 array emitter is attributed to reduction in effective work function and enhanced electron tunneling probability across the LaB6–Si interface.

Facile synthesis of nanocomposites of CNF-Sn and C-Sn microspheres: Prospective field emitter

We report synthesis of Carbon nanofibers (CNFs) and hybrid nanocomposites namely, CNF-Sn and C-Sn microspheres using simple electrospinning technique, followed by annealing in controlled atmosphere. The as-prepared materials were characterized using X-raydiffraction (XRD), Field emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM), Ultra-violet photoelectron spectroscopy (UPS), and Raman spectroscopy to reveal their physico-chemical properties. As carbon family members are potential materials for field emission (FE) based applications, owing to their high aspect ratio FE characteristics of the synthesized materials were explored at base pressure of 1 × 10−8 mbar. Interestingly, the hybrid nanocomposite CNF-Sn and C-Sn emitters showed improved FE behavior (with the turn-on field of 3.4 and 1.36 V/μm, respectively) in contrast to the pristine CNFs emitter (turn-on field of 5.4 V/μm). Furthermore, the maximum emission current density is substantially enhanced, 7.75 and 4.6 mA/cm2 for CNF-Sn and C-Sn emitters, respectively. The improvement in the FE behavior of nanocomposite emitters is attributed to the combined effect of morphology and modulation of electronic properties at the interface of nanocomposites. The results confirm that FE characteristics of pristine nanostructures can be greatly improved upon formation of their nanocomposites and this approach can be extended to other nanostructures for improving their multi-functionalities.

Nanocrystalline high-entropy hexaboride ceramics enable remarkable performance as thermionic emission cathodes

The development of high-entropy borides with combined structural and functional performance holds untold scientific and technological potential, yet relevant studies have been rarely reported. In this work, we report nanocrystalline (La0.25Ce0.25Nd0.25Eu0.25)B6 high-entropy rare-earth hexaboride (HEReB6-1) ceramics fabricated through the high-pressure sintering of self-synthesized nanopowders for the first time. The as-fabricated samples exhibited a highly dense (96.3%) nanocrystalline (94 nm) microstructure with major (001) fiber textures and good grain boundaries without any impurities, resulting in a remarkable mechanical, electrical, and thermionic emission performance. The results showed that the samples possessed outstanding comprehensive mechanical properties and a high electrical resistivity from room temperature to high temperatures; these were greater than the average values of corresponding binary rare-earth hexaborides, such as a Vickers hardness of 23.4 ± 0.6 GPa and a fracture toughness of 3.0 ± 0.4 MPa•m1/2 at room temperature. More importantly, they showed high emission current densities at elevated temperatures, which were higher than the average values of the corresponding binary rare-earth hexaborides. For instance, the maximum emission current density reached 48.3 A•cm−2 at 1873 K. Such superior performance makes the nanocrystalline HEReB6-1 ceramics highly suitable for potential applications in thermionic emission cathodes.

Protruded graphene oxide sheets on nickel cobalt oxide nanostructures for enhanced field emission

The current study demonstrates field emission response of nickel cobalt oxide (NCO) nanostructures-graphene oxide (GO) hybrid. NCO nanostructures were synthesized directly on Ni substrates by hydrothermal method and GO was synthesized by modified Hummer's method. The as-synthesized nanostructures were characterized to determine phase, morphology, composition, and work function. The bare, one-dimensional (1D) needle-like nanostructure of NCO showed a low field emission response. Turn-on fields for NCO were 5.8 V/µm and 6.0 V/µm for foam and foil substrates, respectively. By making a hybrid of GO layers on top of the NCO nanostructure, the field emission response was found to enhance abruptly. The turn-on field was reduced down to 2.9 V/µm the hybrid structure from a high value of 6.1 V/µm. Maximum emission current density of the NCO foil and NCO foam was 90 and 132 µA/cm2, respectively. After depositing GO on these samples, maximum current increased to 1090 and 2360 µA/cm2, respectively. The combination of 1D and 2D nanostructure offers a modified surface, leading to a change of local field concentration and enhanced field emission. Surface modification by the deposition of GO on the NCO nanostructure led to enhancement of field emission response.

Copper Nanowire Arrays: Growth and Properties

Cu nanowire arrays of three different diameters have been synthesized via template assisted electrode position technique. Morphological, structural, optical, electrical and field emission properties were examined for these three nanowire arrays. Morphological study reveals the nanowire arrays are of uniform diameter throughout the length and of desired dimensions. Structural study shows the face centred cubic structure of nanowires. The crystallite size has been calculated using the Debye-Scherrer relation and micro strain has been calculated using Williamson-Hall analysis. The surface plasmon resonance absorption peak shifts towards red end with increase in diameter of nanowires. I-V characteristics show the ohmic behaviour. Electron field emission properties of Cu nanowire arrays have been studied using Fowler Nordheim theory. The field emission parameters like maximum emission current density, turn on field and field enhancement factor have been calculated for nanowire arrays of different diameters. Cu nanowire arrays of diameter 50 nm exhibit better field emission properties, indicating that thin nanowire arrays have great potential to be used as field emitters.

Synthesis, physico-chemical characterization and field emission behaviour of 3D chrysanthemum like pristine ReS2, and ReS2-rGO nanocomposite

3D chrysanthemum like pristine ReS2 and nanocomposite of ReS2 with reduced graphene oxide (rGO) have been synthesized using facile one step hydrothermal method, followed by physico-chemical characterizations to reveal their phase, structural and electronic properties. Efforts have been made to reveal the influence of process parameters on morphology and growth of the as-synthesized products. From application point of view, field emission (FE) behavior of pristine ReS2 and ReS2-rGO nanocomposite emitters has been investigated at base pressure of 1 × 10–8 torr. The ReS2 and ReS2-rGO nanocomposite emitters showed values of turn-on field (corresponding to emission current density of 1μA cm−2) as 2.10 and 1.66 V μm−1, and field enhancement factor (β) as ∼965 and 1176, Furthermore, the ReS2-rGO nanocomposite emitter delivered maximum emission current density of ∼1472 μA cm−2 at applied field of 3.1 V μm−1. Both the emitters exhibited good emission current stability at pre-set value ∼5 μA over more than 3 h duration. The superior FE properties of the ReS2-rGO nanocomposite emitter are attributed to optimized morphology offering high field enhancement factor coupled with modulation of electronic properties reflected as lowering of the work function. The value of work function of ReS2-rGO nanocomposite, measured using a retarding field analyzer, is found to be 4.29, lower than that of the pristine sample (4.49 eV). The results signify that functionality of pristine nanostructures is greatly improved via formation of nanocomposites and desirable nanocomposites possessing unique morphology can be synthesized under optimized experimental conditions using a facile and inexpensive hydrothermal route.

Controlled decoration of palladium (Pd) nanoparticles on graphene nanosheets and its superior field emission behavior

Development in nanoscience and nanotechnology has inspired scientists to continuously explore new cathode material for field emission applications. In the present article, different weight (wt) % of Pd nanoparticles (Pd NPs) ensemble on graphene (Gr) nanosheets (Pd-Gr) were prepared via one step simple chemical route. The field emission (FE) properties of Pd decorated Gr nanosheets were studied. Structural and morphological analysis of as synthesized Pd-Gr composites were carried out by using X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscope (TEM). The FE properties of the Pd-Gr changed according to the variation in the wt% of Pd NPs. The Pd3-Gr field emitter showed better performance among all the emitters with lower turn on field, threshold field and maximum emission current density (J). The enhanced FE properties of Pd-Gr are due to the decoration of low work function Pd NPs. The FE results demonstrated herein suggest that, Pd-Gr based field emitters can open up many opportunities for their potential utilization in large area field emitters in various vacuum micro-nanoelectronic devices such as flexible field emission displays, portable X-ray and microwave tubes.

Systematic growth of carbon nanotubes on aluminum substrate for enhanced field emission performance

For more than two decades, carbon nanotubes (CNTs) have shown great potential for a wide range of applications. Several methods are known to synthesize CNTs, though only a few of them are able to produce good quality and economically available CNTs. Chemical vapor deposition (CVD) is one of those methods that produce economically feasible and good quality CNTs onto specific substrates, even with nanopatterning. However, growing CNTs by CVD at temperatures below 700 °C remained a long-time challenge, as this meant keeping a host of low-melting materials out of bounds for direct CNT growth on them. In this work, CNTs have been synthesized directly onto a low-melting, conducting substrate, aluminum, by thermal CVD, at a temperature as low as 550 °C and up to as high as 650 °C (just below the melting point of aluminum). The diameters of the grown CNTs were observed to be influenced by process parameters, e.g., temperature and pressure. The effect of synthesis parameters on CNT diameters was verified by scanning electron microscopy and transmission electron microscopy. The quality of the CNTs was checked by Raman spectroscopy, selected area electron diffraction pattern of transmission electron microscopy, and XPS. It was observed that an increase in temperature and pressure had a significant effect on the diameters of the CNTs. Randomly entangled CNTs were measured to have an average diameter of 28 nm at 550 °C and one atmospheric (760 Torr) pressure, whereas it was observed to be 78 nm at a temperature of 650 °C and pressure of 0.01 Torr. The field emission response, i.e., the turn-on field (2.5 V/μm) and the maximum emission current density (2.17 mA/cm2) of the CNTs synthesized at the temperature of 550 °C and pressure of 1 atm (760 Torr) was found to be excellent.

Stable electron field emission from graphene/hexagonal boron nitride hybrid structure

Graphene/h-BN hybrid structure is constructed using simple and efficient solution process and the field emission properties of the hybrid structure are examined. It is found that the hybrid structure exhibits much more stable electron field emission properties compared to the pure graphene films. A Significantly higher maximum emission current density is obtained from the hybrid structure. In addition, the hybrid structure displays much better emission stability, whose current fluctuation is less than 4.5%. These results indicate that the graphene/h-BN hybrid structure is a promising candidate of reliable electron emission source in vacuum nanoelectronics devices.

Evaluation of field electron emission properties of ZnIn2S4 nanopetals grown on conducting carbon substrate

The hierarchical structure of nanometric thick Zinc Indium Sulfide (ZnIn2S4) has been prepared on electrically conductive carbon cloth using a single step hydrothermal method. The surface morphology of the as-synthesized ZnIn2S4 (ZIS) is characterized with well interconnected criss cross-network of vertically oriented nanopetals. Owing to the unique morphology offering high aspect ratio, field emission (FE) characteristics of the hierarchical ZIS structure have been investigated at a base pressure of 1x10−8 mbar. Interestingly, the ZIS emitter exhibited ultralow values of turn-on and threshold fields, 0.84 and 0.95 V/μm, defined at 1 and 10 μA/cm2, respectively. Furthermore, maximum emission current density of 1.9 mA/cm2 has been obtained from the emitter at relatively lower applied field of 2.07 V/μm. The emission current stability tested at a pre-set value of ~5 and 10 μA over 4 h duration is observed to be very good. The superior FE characteristics are attributed to the high aspect ratio of the vertically oriented nanopetals, their excellent mechanical and chemical sturdiness, and intimate contact with the electrically conducting carbon fibers. The outcome of FE properties indicates the potential applicability of hierarchical ZIS nanostructure emitter, grown on carbon cloth and possessing unique morphology grown on carbon cloth in the practical application in FE based vacuum micro/nano-electronic devices.

Perovskite-type SrTiO3 thin film preparation and field emission properties

By using vacuum electron beam vapor deposition (EBVD) technology, perovskite-type SrTiO3 was deposited on indium tin oxide (ITO) conductive glass. The SrTiO3 orientation and its film thickness were well-controlled by appropriately modifying the growth conditions and deposition time. The elemental composition, microstructure, surface morphology and field emission character of SrTiO3 film were characterized by energy dispersive spectrum (EDS), X-ray diffraction (XRD) spectrum, field emission scanning electron microscope (FE-SEM) and field emission (FE) detecting system. The FE test results showed that the SrTiO3 film showed good field emission characteristics. The maximum field emission current density reach 88.96 μA/cm2 at 10.45V/μm applied electric field. And the experimental results show that the SrTiO3 film has certain potential in FE field.

Superior field emission and alternating current conduction mechanisms for grains and grain boundaries in an NiO-[CdO]2 nanocomposite

A hierarchical NiO-[CdO]2 nanocomposite has been synthesized by a sol–gel auto-combustion route and characterized with a view to studying the electric field emission and conduction mechanism therein. The structural features, surface morphologies, and elemental compositions of the as-prepared samples have been characterized by XRD, Raman, FESEM, and TEM techniques. A low turn-on field (4.50 V/μm) and threshold field (5.04 V/μm) were found to be sufficient to draw emission current densities of 1 μA/cm2 and 10 μA/cm2 from NiO-[CdO]2-modified cathodes. A maximum emission current density of 121 μA/cm2 at a low applied electric field of 6.5 V/μm and long emission current stability were achieved at a preset value of 5 μA. The field enhancement factor (β) was determined as 1854 in the high-field region by computing the local work function (φ) through density functional theory (DFT), and the entire field emission (FE) performances have been compared with those of various pristine compounds. The temperature-dependent electrical conduction mechanism has been further explained with the help of impedance analysis over the temperature range 323–623 K and a wide frequency range from 5 Hz to 1 MHz. The grain and grain boundary contributions were well distinguished by impedance and a modulus formalism, with respective activation energies of Eg = 0.25–0.26 eV and Egb = 0.31–0.32 eV. The temperature-dependent frequency exponents for grains (n1) and grain boundaries (n2) demonstrate two different conduction mechanisms, namely quantum mechanical tunneling for grains, and correlated barrier hopping for grain boundaries. Maxwell–Wagner-type dielectric polarizations are explained by our experimental results, and the highest real dielectric constant (εr) 1893 was calculated at 623 K.

Highly Monochromatic Electron Emission from Graphene/Hexagonal Boron Nitride/Si Heterostructure.

In this work, a planar electron emission device based on a graphene/hexagonal boron nitride (h-BN)/n-Si heterostructure is fabricated to realize highly monochromatic electron emission from a flat surface. The h-BN layer is used as an insulating layer to suppress electron inelastic scattering within the planar electron emission device. The energy spread of the emission device using the h-BN insulating layer is 0.28 eV based on the full-width at half-maximum (FWHM), which is comparable to a conventional tungsten field emitter. The characteristic spectral shape of the electron energy distributions reflected the electron distribution in the conduction band of the n-Si substrate. The results indicate that the inelastic scattering of electrons at the insulating layer is drastically suppressed by the h-BN layer. Furthermore, the maximum emission current density reached 2.4 A/cm2, which is comparable to that of a conventional thermal cathode. Thus, the graphene/h-BN heterostructure is a promising material for planar electron emission devices to obtain a highly monochromatic electron beam and a high electron emission current density.

Micro Flowers of SrS/Bi2S3 Nanocomposite and Its Field Emission Properties

The three-dimensional hierarchical SrS/Bi2S3 heterostructures were synthesized by a template-free single-step hydrothermal method. The structural and morphological studies revealed the formation of a single crystalline orthorhombic heterostructure with rod-like morphologies possessing a high aspect ratio. The field emission properties of SrS/Bi2S3 nanorods were investigated. J–E and the Fowler–Nordheim (F–N) plot, as well as long-term field emission (FE) stability, were studied. SrS/Bi2S3 nanoflowers have enhanced the FE properties more than the virgin Bi2S3. The observed values of the re-producible turn-on field for SrS/Bi2S3 defined to draw an emission current density of ca. 1 µA/cm2 were found to be ca. 2.50 V/µm, and of the threshold field to draw a current density of ca. 10 µA/cm2 were found to be ca. 3.00 V/µm (without visible light illumination). A maximum emission current density of ca. 527 μA/cm2 was drawn without light and a current density of ca. 1078 μA/cm2 with light, which is higher than that of pristine Bi2S3.

Nanosecond laser induced surface micro-structuring of Tantalum targets for improved field emission

In this paper we report our results on nanosecond laser based surface micro (μ)-structuring of Tantalum (Ta) samples and their field emission behavior. Surface micro-structuring has been carried out using a typical laser fluence of 0.9 J cm−2 and varying number of incident laser pulses in the range of 3000 to 9000. Laser treated samples have been characterized in terms of surface morphology, chemical phase and field emission behavior. Nanosecond laser irradiation resulted in formation of self assembled surface μ-protrusions in the laser treated region. Peak height of the generated μ-protrusions and average roughness of the laser treated region increased with increasing number of irradiating laser pulses. Laser treated specimens have shown enhanced field emission in comparison to pristine Ta specimen. Specimens μ-structured using 3000, 6000 and 9000 laser pulses per spot have shown field emission with turn on field (Eon) of 6.6 V−1 μm−1, 4.8 V −1μm−1 and 3.7 V −1μm−1, respectively. Also, maximum emission current density delivered by these laser modified specimens was found to increase with increasing number of laser pulses. Although, Ta specimen μ-structured with 9000 laser pulses has shown highest emission current density, emission current stability of this sample was poorer in comparison to specimens modified using 3000 and 6000 laser pulses.

Simple and Efficient Preparation of a Single-Crystal LaB6 Field Emission Tip

Single-crystal LaB6 field emission tips have strong anti-poisoning ability, good emission stability, high emission current density and mature applications. As such, they have attracted much attention in field emission research. In this paper, an electrochemical corrosion device is designed to enable the simple and efficient preparation of single-crystal-LaB6 tips with different radius of curvature values. Computer simulation technology is used to study the field emission behavior of the single-crystal LaB6 tip and provides the theoretical basis for the calculation of field emission current density. The results show that the preparation process is automated, more efficient and more controllable than previous methods. When the applied voltage was 2250 V, the single-crystal LaB6 tip achieved maximum emission current density of 2.05 × 104 A/cm2 at a radius of curvature of 0.5 μm.