Large Emission Current Density Research Articles

Insights into the Solution Processing of Non-van der Waals Boron Carbide.

Exfoliation of non-layered materials is crucial to unleash their enormous potential in wide range of applications. However, the presence of strong non-van der Waals interactions in all three dimensions makes exfoliation challenging. Boron carbide (B4C), known for its high hardness, holds great potential for diverse applications. In this work, B4C is exfoliated in 29 solvents to determine its Hansen solubility parameters (HSP) and understand the dispersion stability. The experimentally determined HSP values are 19.6, 15.3 and 15.1 MPa1/2 for dispersive interactions (δD), polar interactions (δP) and hydrogen bonding interactions (δH), respectively. Subsequently, in situ dispersion stability is analyzed qualitatively and quantitatively from the space-time resolved extinction profiles employing an analytical centrifuge. The sedimentation kinetics of B4C dispersions are analyzed using instability index, integral extinction and sedimentation velocity. B4C dispersions in solvents like ε-caprolactone, isopropyl alcohol, propylene carbonate and formamide exhibited excellent stability. Additionally, B4C nanosheets are utilized to investigate their field emission properties. These nanosheets exhibit a turn-on electric field of 3.35 V/μm at an emission current density of 1 μA cm-2, and it delivered a large emission current density of ~ 375 μA cm-2 at an applied electric field of 5.7 V/μm.

Improved field electron emission behavior of ultrathin lanthanum hexaboride-coated copper oxide nanowires

A simple approach dealing with thermal oxidation at atmospheric pressure was used to synthesize copper oxide nanowires (CuO-NWs) on copper substrate. The vertically oriented CuO-NWs were used for further subjected to deposition of ultrathin layer (∼20[Formula: see text]nm) of lanthanum hexaboride (LaB[Formula: see text] using the electron beam evaporation technique. The observation of the characteristic vibrational mode of LaB6 in the Raman spectrum of LaB6-coated CuO-NWs (LaB6@CuO-NWs) revealed the existence of LaB6 in the deposited sample. Furthermore, confirmation of LaB6 coating on CuO-NWs was corroborated by X-ray photoelectron spectroscopy (XPS) analysis. The field electron emission (FEE) characteristics of pristine CuO-NWs and LaB6@CuO-NWs were investigated in a planer diode geometry at a base pressure of [Formula: see text][Formula: see text]mbar. The values of turn-on and threshold fields (defined at emission current density of 1 and 10[Formula: see text][Formula: see text]A/cm2, respectively) were found to be 5.4[Formula: see text]V/[Formula: see text]m and 1.6[Formula: see text]V/[Formula: see text]m (for pristine CuO-NWs emitter) and 6.75, and 2.85[Formula: see text]V/[Formula: see text]m (for LaB6@CuO-NWs emitter). Interestingly, a noticeably large emission current density of [Formula: see text][Formula: see text][Formula: see text]A/cm2 has been extracted from the LaB6@CuO-NWs emitter at a relatively lower applied field of 5.8[Formula: see text]V/[Formula: see text]m. The Fowler–Nordheim (FN) plots of both the emitters showed nonlinear behavior, indicative of their semiconductive nature. Furthermore, both the emitters showed relatively stable emission behavior tested at a pre-set value of 1[Formula: see text][Formula: see text]A more than 3[Formula: see text]h. The enhanced FEE behavior of LaB6@CuO-NWs emitter is attributed to the modulation of electronic properties due to LaB6-CuO interface-induced effects. Although the promising FEE behavior of LaB6@CuO-NWs emitter warrants its potential for practical applications, further improvement can be achieved via fabricating an array emitter due to CuO NWS and optimizing the thickness of LaB6 coating.

Field electron emission performance of Janus MoSSe and MoSSe-MWCNTs composite: Corroboration by Hall measurement and DFT simulation

Janus MoSSe and its composite with multiwalled carbon nanotubes (MWCNTs) have been synthesized using a facile one pot hydrothermal route. Formation of Janus MoSSe has been confirmed from the X-ray Diffraction, Raman, and X-ray Photoelectron spectroscopic analysis. Furthermore, field electron emission (FEE) behaviour of the as-synthesized Janus MoSSe and MoSSe-MWCNTs composite has been investigated at base pressure of ∼ 1 × 10−8 mbar in a planar diode configuration. Interestingly, the MoSSe-MWCNTs emitter exhibited enhanced FEE characteristics in terms of lower values of turn-on and threshold fields, competency to deliver large emission current density, in contrast to the pristine Janus MoSSe and MWCNTs emitters. An attempt has been made to reveal the underlying physics facilitating improvement in FEE behavior using computational analysis. In this regard, density functional theory (DFT) simulation was carried out to find electronic structure of the as-synthesized emitters. The DFT results indicate charge transfer from C 2p to Mo 4d orbitals, thereby enhancing electrical conductivity and reduction in work function of MoSSe-MWCNTs composite. Moreover, the experimental validation due to Hall measurements undoubtedly revealed noticeable increase in carrier concentration. The observed results clearly exemplify modulation electronic properties of Janus materials thereby highlighting their potential for various applications, including vacuum microelectronic devices.

Systematic synthesis of Bi2S3 nanoplates by template free solid-state approach and its field emission behaviour

We have demonstrated herewith controlled synthesis of Bi2S3 nanoplates by simple, template free, one step solid-state reaction at moderate temperature. The process parameters viz. precursor ratio, reaction temperature and duration were optimized. Nanostructured Bi2S3 synthesized at optimum conditions was further characterized by XRD, FESEM and FETEM, to study their phase, morphology and structure. The microscopic and XRD study reveals the formation of nanoplates like structures of orthorhombic Bi2S3. An attempt has been made to provide with a plausible growth mechanism as a function of reaction temperature and duration. In order to exploit the high 'aspect ratio' of the Bi2S3 nanoplates and explore its feasibility as potential electron source, field emission (FE) study has been performed at base pressure of ∼1 × 10−8 mbar. The results obtained herein reveal the morphology dependent FE behaviour of Bi2S3 nanoplates. The emitter comprised of Bi2S3 nanoplates synthesized at 180 °C showed better FE properties, lower turn-on field value, delivery of large emission current density (∼1670 µA/cm2 at 6.9 V/µm) and excellent emission current stability which are comparable to alike emitters due to semiconducting nanostructures. Highly crystalline nature and high aspect ratio of Bi2S3 nanoplates might be responsible for their good FE performance. These Bi2S3 nanostructures may have potential for applications in other optoelectronics devices.

Field electron emission behavior of pristine and Au nanoparticles decorated Sb2Te3-rGO heterostructure emitters

A facile one-pot hydrothermal method was used to synthesize pristine Sb2Te3 and its heterostructure with reduced graphene oxide (rGO). A novel approach dealing with high energy (6 MeV) electron irradiation has been efficiently used for synthesis of gold nanoparticles (Au- NPs) and their in-situ decoration on Sb2Te3-rGO heterostructure. From an application point of view, we have investigated the field electron emission characteristics of pristine Sb2Te3 and Sb2Te3-rGO heterostructures, with and without Au-NPs decoration. The Au-NPs decorated Sb2Te3-rGO heterostructure emitter showed enhanced electron emission behavior in terms of reduced value of turn-on field, delivery of large emission current density (∼1171 µA/cm2) at relatively lower applied field (∼3.1 V/µm), and excellent emission stability. The improvement in emission characteristics upon decoration of Au-NPs is attributed to noticeable increase in the number of emission sites, higher value of field enhancement factor (β = 4562), and betterment in the electronic properties due to Au-NPs-Sb2Te3-rGO interface effects. The obtained results suggest that Au-NPs decorated Sb2Te3-rGO heterostructure emitter has potential towards practical applications in various vacuum microelectronic devices. Furthermore, the present approach dealing with high energy electron irradiation for synthesis and in-situ decoration of nanoparticles can extended to other inorganic heterostructures towards improving their functionality.

Titanium Trisulfide Nanosheets and Nanoribbons for Field Emission-Based Nanodevices

The field emission (FE) properties of TiS3 nanosheets and nanoribbons, synthesized by direct sulfuration of bulk titanium, are investigated. The nanosheets show an enhanced FE behavior with a low turn-on field of ∼0.3 V/μm, required for drawing an emission current density of ∼10 μA/cm2. Interestingly, the TiS3 nanosheet emitter delivered a large emission current density of ∼0.9 mA/cm2 at a relatively low applied electric field of ∼0.4 V/μm. We have estimated the values of the field enhancement factor (β), which are found to be ∼5 × 104 for the TiS3 nanosheet emitter and ∼4 × 103 for the nanoribbon emitter. We attribute the superior FE performance to the presence of atomically sharp edges and the reduced thickness of TiS3, as reflected in the high value of β. In fact, the nanosheet sample presents a higher density of ultrathin layers (∼12 nm-thick), and thus, they have a larger edge to volume ratio than the nanoribbon samples (which are ∼19 nm-thick). The superior FE behavior of TiS3 nanosheets over nanoribbons makes them a propitious field emitter and can be utilized for various FE-based applications, demanding large emission currents and lower operational voltages. Moreover, the FE current stability recorded on these samples confirms their promising performance. Thus, the present investigation brings out a great promise of TiS3 nanosheets and nanoribbons as field emitters for vacuum nanoelectronics devices.

A primary understanding of field emission from superhigh specific surface area activated carbon with interconnection structure

In this study, activated carbon (AC) with the specific surface area up to 3152 m2 g−1 is synthetized and structural-characterized. The AC powder displays the interconnective framework structure composed of numerous carbon nanosheets and the sharp edges act as the active sites to achieve efficient electron emission. The screen-printed AC cold cathode shows large emission current density of 15 mA cm−2 under relatively low electrical field of 6.0 V μm−1, with stable emission of up to 280 min. In conjunction to its uniform emission pattern, robustness, and vacuum-compatibility test, the feasibility of AC cold cathode in field emission devices have been demonstrated. This work provides a primary understanding of field emission performance for the AC electron emitter and expands the application fields of superhigh specific surface area AC.

Fabrication of carbon nanotube on nickel–chromium alloy wire for high-current field emission

The application of field emission (FE) devices requires emitters having a low threshold field (Eth, applied field at 10 mA/cm2) and an excellent FE stability at a high emission current density. Herein, we report on the fabrication and field emission characteristics of a carbon nanotube (CNT) emitter formed as a CNT film grown directly on nickel–chromium (Ni80Cr20) alloy wire using microwave plasma enhanced chemical vapour deposition (PECVD). The FE performance of the emitter has a low threshold field of 0.76 V/µm, a large field enhancement factor (8311.3 ± 53.1) and an extremely large emission current density (Jmax) of 7.65 A/cm2 at a relatively low electric field of 2.13 V/µm. The great increase in Jmax is ascribed to the reinforced adhesion of CNTs to the Ni80Cr20 alloy wire substrates. The direct synthesis of CNTs on the nickel–chromium wire substrate without any catalyst layers and the wrapping of the CNT roots with a carbon nanoflake layer deposited on the surface of the wire substrate are responsible for the enhanced adhesion. The tip-type CNT emitter presents an excellent field emission stability at high emission current densities (3.06 A/cm2), and could potentially be applied to field emission devices.

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.

A moderate synthesis route of 5.6 mA-current LaB6 nanowire film with recoverable emission performance towards cold cathode electron source applications

The as-grown LaB6 nanowire arrays still remain a very large and stable emission current density over 16.7 mA cm−2 at high temperature as well as recoverable emission performances, which should have promising future in cold cathode electron sources.

High-efficiency field emission from pressed nickel foam–flat graphene–vertical graphene hybrids

Vertical graphene (VG) was prepared on flat graphene (FG) coated pressed Ni foams by using plasma enhanced chemical vapor deposition. The VG is typical few-layer graphene and is rich in defects. The field emission performance of Ni–FG–VG hybrids with optimal morphology is found to outperform that of VG prepared on pristine Ni foams and Si wafers. It has a low turn-on electric field of 2.03V/μm and a large maximum emission current density (Jmax) of 5.91mA/cm2 and also exhibits excellent field emission stability at ~50% and ~75% Jmax over a period of 30h.

Large field emission current from Si-doped AlN film grown by MOCVD on n-type (001) 6H-SiC

A large field emission current density of 2.55A/cm2 at 20.9V and a low turn-on voltage of 7.28V is obtained from the Si-doped 50nm-thick AlN film, synthesized by metalorganic chemical vapor deposition on the n-type SiC substrates, which is the best result reported for AlN film. Accompanying with atomic force surface micro-images, it is found that this current is achieved owing to a blunting process under a high voltage of 95V, which can lead to a decrease of the root mean square roughness from 4.23 to 1.03nm.

Vapor–solid preparation of densely distributed and small-sized graphene nanoflakes on one-dimensional nanomaterials for low-field and highly stable field emission

Differently shaped two-dimensional (2-D) graphene nanoflakes (GNFs) were prepared on one-dimensional (1-D) carbon nanotubes (CNTs) and silicon nanowires (SiNWs) forming 2-D–1-D composites by using microwave plasma enhanced chemical vapor deposition. The GNFs are vertically aligned on 1-D nanomaterials with their sharp edges (less than 10 layers) unfolded outside and are rich in defects. GNF–CNT (or SiNW) composites with densely distributed and small-sized GNFs and not obviously thickened CNTs (or SiNWs) are found to have superior FE properties. The optimal FE performance obtained from the GNF–CNT composites shows a low threshold field of 1.54 V/μm and an extremely large maximum emission current density of 75.46 mA/cm2, far better than 1.77 V/μm and 27.90 mA/cm2 for the pristine CNTs, respectively. The FE improvement is ascribed to the significant increase of active emission sites and also the preservation of the high aspect ratio of CNTs. Furthermore, longtime (30 h) stable FE is achieved from GNF–CNT composites with optimal shapes at a high mean emission current density of 45.47 mA/cm2 and a low operation field of 1.766 V/μm, showing promising prospects in high-performance vacuum electronic device applications.

Patterned growth of single-crystal 3, 4, 9, 10-perylenetetracarboxylic dianhydride nanowire arrays for field-emission and optoelectronic devices

3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) organic nanostructures possess extraordinary electronic and optoelectronic properties. However, it remains a challenge to achieve patterned growth of PTCDA nanowire (NW) arrays for integrated device applications. Here, we demonstrated the high-density, large-area, uniform, and cross-aligned growth of single-crystalline PTCDA NW arrays by using Au nanoparticles (NPs) as the growth templates. The high surface energy of Au NPs led to the cross-aligned growth of organic NWs, enabling the growth of PTCDA NW arrays with any desirable patterns by pre-patterning the Au films on a Si substrate. The PTCDA NW arrays as field emitters show good performance with a large emission current density and high emission stability. Furthermore, photodetectors based on PTCDA NW arrays were constructed via a simple in-situ growth approach, which exhibited high sensitivity to a wideband light ranging from 400–800 nm and surpassed the individual NW-based photodetectors in terms of higher photocurrent and faster response speed. Successful applications of PTCDA NW arrays in field emission and photodetectors show a great potential application of organic NW arrays in future efficient electronic and optoelectronic devices.

Ultralow field emission from thinned, open-ended, and defected carbon nanotubes by using microwave hydrogen plasma processing

Ultralow field emission is achieved from carbon nanotubes (CNTs) by using microwave hydrogen plasma processing. After the processing, typical capped CNT tips are removed, with thinned, open-ended, and defected CNTs left. Structural analyses indicate that the processed CNTs have more SP3-hybridized defects as compared to the pristine ones. The morphology of CNTs can be readily controlled by adjusting microwave powers, which change the shape of CNTs by means of hydrogen plasma etching. Processed CNTs with optimal morphology are found to have an ultralow turn-on field of 0.566V/μm and threshold field of 0.896V/μm, much better than 0.948 and 1.559V/μm of the as-grown CNTs, respectively. This improved FE performance is ascribed to the structural changes of CNTs after the processing. The thinned and open-ended shape of CNTs can facilitate electron tunneling through barriers and additionally, the increased defects at tube walls can serve as new active emission sites. Furthermore, our plasma processed CNTs exhibit excellent field emission stability at a large emission current density of 10.36mA/cm2 after being perfectly aged, showing promising prospects in applications as high-performance vacuum electron sources.

Pulsed laser-deposited MoS₂ thin films on W and Si: field emission and photoresponse studies.

We report field electron emission investigations on pulsed laser-deposited molybdenum disulfide (MoS2) thin films on W-tip and Si substrates. In both cases, under the chosen growth conditions, the dry process of pulsed laser deposition (PLD) is seen to render a dense nanostructured morphology of MoS2, which is important for local electric field enhancement in field emission application. In the case of the MoS2 film on silicon (Si), the turn-on field required to draw an emission current density of 10 μA/cm(2) is found to be 2.8 V/μm. Interestingly, the MoS2 film on a tungsten (W) tip emitter delivers a large emission current density of ∼30 mA/cm(2) at a relatively lower applied voltage of ∼3.8 kV. Thus, the PLD-MoS2 can be utilized for various field emission-based applications. We also report our results of photodiode-like behavior in (n- and p- type) Si/PLD-MoS2 heterostructures. Finally we show that MoS2 films deposited on flexible kapton substrate show a good photoresponse and recovery. Our investigations thus hold great promise for the development of PLD MoS2 films in application domains such as field emitters and heterostructures for novel nanoelectronic devices.

Structural, nanomechanical, field emission and ammonia gas sensing properties of nitrogenated amorphous carbon films deposited by filtered anodic jet carbon arc technique

This paper reports the effect of substrate bias on the structural, nanomechanical, field emission and ammonia gas sensing properties of nitrogenated amorphous carbon films embedded with nanocrystallites (a-C: N: nc) deposited by a filtered anodic jet carbon arc (FAJCA) technique. The films are characterized by X-ray diffraction, high resolution transmission electron microscopy, energy dispersive X-ray spectroscopic analysis, Raman spectroscopy, nanoindentation, field emission and ammonia gas sensing measurements. The properties of the films obtained are found to depend on the substrate bias. The maximum hardness (H)=42.7GPa, elastic modulus (E)=330.4GPa, plastic index parameter (H/E)=0.129 and elastic recovery (% ER)=74.4% have been obtained in a-C: N: nc films deposited at −60V substrate bias which show the lowest ID/IG=0.43, emission threshold (ET)=4.9V/µm accompanied with the largest emission current density (Jmax)=1mA/cm2 and field enhancement factor (β)=1805.6. The gas sensing behavior of the a-C: N: nc film has been tested by measuring the change in electrical resistance of the sample in ammonia environment at room temperature with the fast response and recovery time as 29 and 66.9s, respectively.

Improved field emission of few-layer graphene–carbon nanotube composites by high-temperature processing

Self-assembled few-layer graphene–carbon nanotube (FLG–CNT) composites were prepared and their field emission (FE) properties before and after high-temperature (H-T) processing were investigated. It was found that their FE performance deteriorated slightly when they were processed at 1273K for 5h before the FE tests; the applied field increased ~0.05V/μm at 10mA/cm2. X-ray photoelectron spectroscopy analysis indicates that the decreased amount of SP3-hybridized defects from 39.91% to 23.87% after the H-T processing directly leads to this FE degradation. However, the largest emission current density (Jmax) of the FLG–CNT composites increased from 33.48 to 64.00mA/cm2 after the H-T processing. This improved FE performance was attributed to the enhanced adhesion between the CNT and substrates. We consider that the catalyst wrapping at the CNT foot after the H-T processing is responsible for this CNT–substrate reinforcement.

Large emission current density of LaxCe1−xB6 high quality single crystals grown by floating zone technique

The high quality LaxCe1−xB6 (x=0, 0.2, 0.4, 0.6, 1) single crystals with large emission current density have been successfully grown by floating zone method. The crystal structure, crystal quality and emission properties of LaxCe1−xB6 single crystals were characterized by 360° Phi scanning single crystal diffractometer, X-ray Laue diffraction and the thermal electron emission measurements. It is found that the as-grown crystals have a good crystal quality without subgrains and twin crystal phenomenon. The thermionic emission measurement results show that the maximum emission current density on (100) surface of La0.2Ce0.8B6 single crystal is 71.2A/cm2 at 1873K under an applied voltage of 800V, which is larger than the reported values in the literatures up to now. Furthermore, the crystals with different components also show a higher emission current density such as 53.8A/cm2 for La0.4Ce0.6B6 and 53.3A/cm2 for CeB6 single crystals. The field emission single tip made by LaB6 single crystal shows a superior performance than the polycrystalline LaB6 field emission tip and its field enhancement factor (β) is 36,231cm−1. Thus, the LaxCe1−xB6 single crystals will have promising application prospects as a hot or cold cathode.

Field emission properties of silicon doped AlGaN thin film

Aluminum gallium nitride (AlGaN) thin films are prepared by pulsed laser deposition with different silicon doping concentrations from 0.5% to 2%. The field emission measurement shows that the 1% Si-doped AlGaN film has the best field emission property. Compared with undoped film, the Si-doped film has a large emission current density and a low threshold field. The increase of doping concentration can increase the carrier concentration, which will add a number of supply electrons, thereby improving greatly the FE property of AlGaN film. With the doping concentration further increasing, the defect of film increases and the electron mobility reduces. The reduction of the internal supply electron is greater than the contribution of the increase of electrons concentration, which induces the field emission performance to deteriorate. This study will provide a reliable basis for designing high-performance field emission devices.