High Field Enhancement Factor Research Articles

Synthesis and field emission properties of Cu-filled vertically aligned carbon nanotubes

A highly conductive metal core modifies the electronic properties of a carbon shell, offering the possibility of enhancing its field emission (FE) behavior. Herein, a method has been devised to synthesize copper-filled vertically aligned carbon nanotubes (Cu@VACNTs) directly on Cu disks without extra metal catalysts via plasma-enhanced chemical vapor deposition. An ensemble of Cu particles formed on the surface of Cu disks due to surface reconstruction in a reducing environment plays a crucial role in the nucleation and growth of Cu@VACNTs. The filling of Cu inside the VACNTs can be controlled by tuning the growth temperature. The study of FE properties revealed that a conductive Cu-core extending throughout the entire length of the VACNTs could significantly enhance the FE properties of the VACNTs. Excellent FE properties including low turn-on field (ETo = 1.57 V/µm), low threshold field (ETh = 2.43 V/µm), high field enhancement factor (β = 3061), and high FE stability were observed for the Cu@VACNTs. The enhanced FE properties of the Cu@VACNTs can be accredited to low field screening due to bundled morphology and improved electrical and thermal conductivities offered by the encapsulation of highly conductive Cu nanowires inside the cores of VACNTs.

Direct synthesis of micropillars of vertically aligned carbon nanotubes on stainless-steel and their excellent field emission properties

Vacuum micro/nano-electronics can greatly benefit from the improved emission current density and temporal stability of the electron beam due to the morphological control and direct contact of cold field-emitters on a conducting substrate. In this work, high-performance vertically aligned carbon nanotube (VACNT) pillar-based field-emitter arrays (VACNT-P-FEAs) were synthesized directly on stainless-steel (SS) via plasma-enhanced chemical vapor deposition. Nanosphere lithography was employed to limit the growth sites of VACNTs and form VACNT-P-FEAs of different sizes on the SS substrate. The field emission (FE) properties of the VACNT-P-FEAs were dependent on the size of the pillar emitters. Remarkable FE properties, such as low turn-on electric field (ETo = 1.57 V/μm), low threshold electric field (ETh = 2.94 V/μm), high field enhancement factor (β = 4977), and high emission current density (∼33 mA/cm2 at a field of 4 V/μm) were observed from the VACNT-P-FEAs grown using polystyrene microspheres (450 nm diameter) for lithography. The excellent FE performance can be ascribed to the inherent properties of the sample, such as the high conductivity of the SS substrate, low contact resistance between the SS substrate and VACNTs, the suitable number of active VACNT pillar field-emitters, and the reduced screening effect between the VACNT pillars.

Novel strategy for fabrication of field-emission Ti5Si3 nanowires via casting-extraction method

In this work, a novel casting-extraction method is proposed for fabrication of Ti5Si3 nanowires on a large scale. The Ti5Si3 nanowires were firstly synthesized in brass melt using Ti and Si as raw materials, and then extracted by electrochemical reaction. Analysis of microstructure reveals the as-prepared Ti5Si3 nanowires possess a single-crystal D88 hexagonal crystal structure with a high aspect ratio of 250 ± 70 and grow along the [0001] direction. Moreover, the Ti5Si3 nanowires exhibit promising superior field-emission performance with a low turn-on field of 1.8 V/μm and threshold field of 2.7 V/μm, and a high field enhancement factor (β) of 1556.

Excellent field emission from ultrafine vertically aligned nanorods of NdB6 on silicon substrate

Rare earth hexaboride based display devices have attracted significant research interest because of low work function and interesting field emission properties. In the present manuscript, the synthesis of nanostructured neodymium hexaboride (NdB6) has been carried out via low-temperature process using ultrafine nanorods of Nd(OH)3 and NaBH4 as starting material. The Nd(OH)3 were reacted with NaBH4 at 1000 °C in an inert atmosphere at a variable heating rate (40 °C/h and 250 °C/h). The low heating rate (40 °C/h) leads to the formation of ultrafine nanorods of NdB6 (10 nm × 100 nm), while at the high heating rate ultrafine nanocubes (15–20 nm) were obtained. Further, the as-obtained nanocubes and nanorods were deposited on Si- substrate using the spin-coating process to fabricate NdB6 film. Surface analysis of deposited NdB6 film shows that nanorods of NdB6 were vertically aligned. Field emission study of vertically aligned NdB6 nanorods shows low turn-on field (<4.5 V/μm) and the highest enhancement factor (β = 2263) ever reported for NdB6. The reason behind the high field enhancement factor (two fold higher than the maximum reported (β = 1037)) is due to ultrafine vertical orientated nanorods.

Synthesis and field emission of ZrC nanowire

〈100〉 Oriented zirconium carbide (ZrC) nanowires have been synthesized by chemical vapor deposition (CVD). A single ZrC nanowire emitter has been prepared for investigations of its field emission properties. A high field enhancement factor of 8.23 × 106 m−1 was obtained, attributed to the unique structure of the ZrC nanowire. Furthermore, the emitter exhibited a high emission current density of 6.92 × 1010 A/m2 at a low extraction voltage of 555 V, which is much lower than the voltage required for traditional cone shaped ZrC emitters. The results demonstrated that ZrC nanowires are of great advantages for field emission applications.

Growth and Field Emission of Single‐Crystalline Hafnium Carbide Nanowires

Hafnium Carbide Nanowires Single-crystalline hafnium carbide (HfC) nanowires are prepared by chemical vapor deposition (CVD) and their growth morphology as well as electric field-induced emission of electrons is investigated. A high field enhancement factor of 6 × 106 m−1 is obtained from a HfC nanowire due to its unique strucutre, indicating great performance as a potential material for cold field-emission applications. More details can be found in article number 2000007 by Ta-Wei Chiu, Jie Tang, and co-workers.

Growth and Field Emission of Single‐Crystalline Hafnium Carbide Nanowires

&lt;100&gt;‐oriented hafnium carbide (HfC) nanowires are synthesized on the graphite substrate by chemical vapor deposition. The dependence of temperature, HfCl4content, and flow rate of CH4gas on the morphology of nanowires is investigated herein. The results indicate that the HfC nanowire often kinks to another growth direction at a high HfCl4content or a low temperature due to the destabilization of the catalyst droplet induced by either a faster growth rate or slower thermal activation. Reduced nanowire density and length are observed at the sample synthesized at a lower CH4flow rate. Based on the experimental results, high‐crystallinity nanowires with high aspect ratios and high densities are prepared at a lower HfCl4content, higher temperature, and high CH4flow rate. The field‐emission properties of a single HfC nanowire are also characterized by assembling a single HfC nanowire emitter. An extremely high field enhancement factor of 5.57 × 106 m−1is obtained herein.

Comparison of field emission performances and durability of three nanocarbon materials

The performances and operation lifetime of field emission (FE) cathodes made by spray-coating three different nanocarbon materials—carbon nanotubes (CNTs), graphene (GE), and carbon nanocoils (CNCs)—were compared. Though CNTs had the benefits of the lowest turn-on field, CNCs exhibited the most stable cycle-to-cycle repeatability and a significantly longer life than CNTs and GE under a continuous high-current-density operation. The microscopic morphology and Raman spectra of these nanocarbon emitters before and after the 50-h life tests were examined in order to understand the damage mechanism and the cause of their distinct performance decay behaviors. The results indicated that CNCs, whose diameters are significantly larger than those of CNTs and the thickness of GE, can withstand the thermal effects of high current density with the least degree of damage in the morphology and microstructure. According to our performance and life tests, CNTs are most suitable for low-voltage small-current FE applications because of their highest field enhancement factor and lowest turn-on field, while CNCs are a more favorable candidate for high-voltage long-term applications such as FE x-ray tubes.

(Cu2O-Au) \u2013 Graphene - Au layered structures as efficient near Infra - Red SERS substrates

Near Infra-Red Surface Enhanced Raman Spectroscopy (NIR SERS) has gained huge attention in recent years as the conventional visible SERS suffers from overwhelming fluorescence background from the fluorophore resulting in the masking of Raman signals. In this paper, we propose a novel multi-layered SERS substrate- (Cu2O - Au) - Graphene – Au - for efficient NIR SERS applications. The proposed structure has a monolayer of Cu2O - Au core-shell particles on a Au substrate with 1 nm thick graphene spacer layer. Mie simulations are used to optimize the aspect ratios of core-shell particles to shift their plasmon resonances to NIR region using MieLab software. Further, Finite Difference Time Domain (FDTD) simulations using Lumerical software are used for the design of the multiparticle layered SERS substrate as MieLab software works only for single particle systems. Designed structure is shown to provide high field enhancement factor of the order of 108 at an excitation of 1064 nm thus ensuring the possibility of using the proposed structure as efficient NIR SERS substrate which could probably be used for various NIR sensing applications.

Template-free one-step synthesis of the multi-layer carbon or stacked graphene sheets coessentially coating N-doped graphene tubes and their field emission and photoluminescence properties

Graphene nanotubes (GNTs), as a rising member among the carbon nanomaterials family, has drawn significant research interest. The fabrication of the GNTs-based coessential composites could be a more facile and effective strategy to promote GNTs performances. Herein, based on the self-prepared N-doped graphene tubes (N-GNTs), multi-layer carbon wrapped N-doped GNTs (MLC@N-GNTs) and numerous stacked wrinkled graphene sheets encapsulated N-doped GNTs (GS@N-GNTs) are constructed via a facile free-template, one-step CVD method. The growth mechanism of the two types of coessential carbon composites was investigated in detail. The products, especially GS@N-GNTs display the most excellent field emission properties with a very low Eto of 0.33 V/mm, Ethr of 0.96 V/mm and a high field enhancement factor of 50237. The excellent field emission properties is attributed to the synergistic effect of the improved electronic conductivity, the high density of structure defects, the abundance of effective electron emission sites along with the high field enhancement factor and the electronic properties changes induced by the winkled graphene. In addition, MLC@N-GNTs and GS@N-GNTs show the more intense photoluminescence emission than that of pristine N-GNTs, because the coessential coatings induces the interfaces and large amounts of structure defects, which can act as trapping sites for the excitation energy.

Growth of Large-Scale Praseodymium Hexaborides (PrB6) Nanowires and Enhanced Field-Emission Performance

Large-scale PrB6 nanowires were fabricated by an effective, catalyst-free, and a simple low-pressure chemical vapor deposition (LPCVD) process. These nanowires, characterized in detail by various analytical instruments, demonstrated the large aspect ratio and high single-crystalline grown along the [001] crystal direction perpendicular to the (001) crystal plane. The field electron emission equipment tests manifest that the asgrown PrB6 products have a low turn-on field (Eto, 2.32 V/μm), a threshold field (Ethr, 4.28 V/μm), a high field enhancement factor (β, 2336), as well as a stable current-density (J) of field-emission. The relationships of the field electron emission parameters, such as J, Eto, and β versus cathode gap (d), have been established when d is increased from 500 μm to 800 μm. The outstanding properties suggest that the PrB6 products may be promising emitters in the cold-field-emission cathode application.

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.

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.

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.

Field Emission Properties of Molybdenum Nanoparticles Decorated ZnO Nanorod Arrays.

Precisely controlled dimensions of heterostructured ZnO nanorod arrays were grown on micropatterned Au films supported by Si substrate using chemical vapor deposition (CVD). The field emission properties were attributed to pointed nanorods, thickness of catalyst, preferential growth, density, morphology of ZnO and Molybdenum (Mo) decorated ZnO nanorod arrays (Mo/ZnO). The selective restrained heterostructure approach resulted in excellent control over periodicity, location and density of ZnO nanorod arrays. Overall, field emission properties of bare ZnO nanorod arrays showed a low turn-on field of ~4.7 V/μm and a high field enhancement factor (β) ~1686 to 7.3 V/μm and (β) ~807 for Mo/ZnO. It was also found that the field emission properties were significantly influenced by densely decorated Mo nanoparticles on as-grown ZnO nanorod arrays.

A model of gas desorption and radiation during initial high voltage conditioning in vacuum

Large quantities of gas are desorbed from the anode electrode during initial high-voltage conditioning of broad-area electrodes in a vacuum, equivalent to many monolayers and consist mostly of neutral molecules with an ionic component of a few percent. The ions are accelerated to the cathode, producing secondary electrons from the cathode and x-rays. There is still no reasonable explanation of these phenomena. Experiments have been performed to try to understand the source of the gas and radiation by using a residual gas analyzer to examine the desorption products from copper electrodes during initial high-voltage conditioning. The desorption products produced during initial high-voltage conditioning were then compared with those from thermal desorption of the same electrodes and surface preparation as they were heated in vacuum to about 600 °C, both with and without a high electric field present. Thermal desorption with an applied field showed a significant difference in the desorption spectrum compared with no applied field and produced a modest source of radiation that did not appear to be produced by field emission. At electrode temperatures beyond 450 °C, radiation production increased exponentially with temperature, likely produced by surface ionization of minor contaminants with a low ionization potential. These observations have been used as input to propose that the radiation and gas desorption observed during initial high-voltage conditioning is initiated by surface ionization from the many projections with high field enhancement factors on the anode surface.

Density control of vertically aligned carbon nanotubes and its effect on field emission properties

Vertically aligned carbon nanotubes (VACNTs) were synthesized directly on stainless steel substrate and their surface densities were tuned by an order of magnitude via growing them at different temperatures. The surface density of the VACNTs was correlated with the surface evolution of the substrate, a process in which catalyst nano hills were generated at different growth temperatures in the presence of reducing gas environment. Enhanced field emission (FE) properties including low turn-on electric field (ETurn-on = 0.66 V μm-1), low threshold electric field (ETh = 1.07 V μm-1), high field enhancement factor (β = 4084), and high FE stability for 600 min were obtained from VACNT arrays synthesized at 650 °C. It is speculated that the inherent features of our sample such as the conductive substrate, the low contact resistance between the substrate and the VACNTs, the uniform length and the appropriate spacing between the VACNTs have contributed to the excellent field emission properties. Simulation on the electrostatic field of various VACNT arrays was performed and the result is in good agreement with the experimental data.

Enhanced field electron emission of single-walled carbon nanotubes prepared by imprinting technique

In this work, single-walled carbon nanotubes (SWCNTs) field electron emitters were fabricated by imprinting SWCNTs on substrates with a silver buffer layer. It was found that the field emission properties of imprinted SWCNTs were significantly improved with turn-on field at 0.62 V/μm and threshold field at 1.04 V/μm, comparing to screen-printed SWCNTs emitters who had turn-on field at 1 V/μm and threshold field at 2 V/μm. These improvements of SWCNTs emitters were owing to the directly imprinting method which made SWCNTs have clean surface and more emitters on the substrates. The lower turn-on electric field of field emission was attributed to no impurities encapsulate on the emitters and the large aspect ratio of SWCNTs resulting in high field enhancement factor of 15800. The emission performance of the imprinted cathode was almost unchanged after many times repeated experiments. This simple and low cost technique can fabricate high performance field emission cathodes efficiently and also demonstrates potential applications in many electronic devices.

Growth of Wafer-Scale ReS2 with "Tunable" Geometry toward Electron Field-Emission Application.

Despite high potential, the promise of 2D materials has not been realized practically because of limits of tiny grown size and difficult manipulation of the active spot. The utilization of 2D layers is the ultimate approach, which should be supported by large-scale production. In this very first report, we demonstrate the wafer-scale production of ReS2 using the conventional sputtering method. The controllability of ReS2 geometry has been investigated to form typical thin films or vertically aligned layers that are further applied for field emission. The vertically aligned ReS2 layers exhibit ultralow turn-on electric field (0.6 V μm-1) with the current density (0.6 mA cm-2) and significantly low threshold electric field (0.8 V μm-1), respectively, along with outstanding emission stability. The results are attributed to weakly coupled ReS2 layers and the high geometrical field enhancement factor (∼1.08 × 105). Further, Kelvin probe force microscopy measurements confirm that lowering the work function is not solely responsible to achieve the ultralow operative field. Moreover, finite element simulation suggests that not only the length, width, and separation of the nanostructures but also the local slope plays an important role in suppressing screening effects.

Graphene nanosheets assisted carbon hollow cylinder for high-performance field emission applications

Electron sources are critical constituents for myriad many electronic applications including electron imaging, mass spectroscopy etc. Here, we report the realization of high-performance field emission from graphene assisted carbon hollow cylinders (Gr-CHCs) composed of multi-stacked radially aligned CNTs. These CHCs have built-in synthesis related Fe nanoparticles (NPs). This engineered nano-structure exhibits excellent field emission properties such as one of the ultra-low turn-on field (0.64 V μm−1 at 10 μA cm−2), low threshold field (0.74 V μm−1 at 100 μA cm−2), very high current density (15.49 mA cm−2 at 1.32 V μm−1) which is nearly double the current density obtained in our previous study. High field enhancement factor (0.72 × 104) with highly stable emission current at 100 μA was observed for more than 3 hrs at the base pressure of ∼1 × 10−8 mbar. This study suggests an approach to enhance the current density using the proposed innovative nanostructure and forms the basic theme of this communication. Highly efficient and stable field emissions observed are attributed to the geometry of cylinder and the production of the high density of sharp protrusions within the graphene sheets which enhance the local electric field and dramatically enhance field emission. This innovative cylindrical geometry associated with graphene assisted on Fe bearing aligned CNTs along with the periphery of the bulk cylinder provides an easy injection of electrons from the conduction band of CHCs into the vacuum in the presence of an external electric field. The tunability of field emission properties of these CHCs can be easily achieved by tailoring their diameter (10 and 20 mm) and different concentrations of the precursor. This new approach of the graphene assisted cylindrical geometry-based field emitter source provides enormous prospects and demand for next-generation high resolutions display devices.