Field Emission Devices Research Articles

ELEKTROMANYETİK ALAN MARUZİYETİNİN KAS DOKUSUNDA OLUŞTURDUĞU ETKİNİN MODELLENMESİ VE ANALİZİ

Bu çalışmada günlük hayatta maruziyetinden sakınılamayan farklı frekanslardaki cihazların oluşturduğu elektromanyetik yayılım için kas eşdeğer modelleri oluşturulmuştur. Günümüzde elektromanyetik alan yayan cihazların kullanımının giderek yaygınlaşması maruz kalınan frekans ve güç seviyelerinin çeşitliliğini artırmaktadır. Elektromanyetik dalgalar farklı güçlerde ve farklı frekanslarda, insan vücudunda değişik etkilerde bulunmaktadır. Bu etkiler vücutta dokuları ısıtmakta veya kimyasal yoldan değişimlere yol açmaktadır. Bu çalışmada, 900 MHz ve 1800 MHz frekanslarındaki cep telefonu, 2.45 GHz frekansındaki kablosuz haberleşme cihazlarının ve ayrıca 27.12 MHz frekansındaki tıbbi fizyoterapi tedavi cihazlarının (kısa dalga diatermi) insan kas dokusu üzerindeki etkilerini belirlemek amaçlı kas eşdeğer sıvıları oluşturulmuştur. Bu oluşturulan fantom model üzerinde elektromanyetik maruziyetin neden olduğu ısıl değişimler ölçülmüştür.

Plasma-Etched Pattern Transfer of Sub-10 nm Structures Using a Metal-Organic Resist and Helium Ion Beam Lithography.

Field-emission devices are promising candidates to replace silicon fin field-effect transistors as next-generation nanoelectronic components. For these devices to be adopted, nanoscale field emitters with nanoscale gaps between them need to be fabricated, requiring the transfer of, for example, sub-10 nm patterns with a sub-20 nm pitch to substrates like silicon and tungsten. New resist materials must therefore be developed that exhibit the properties of sub-10 nm resolution and high dry etch resistance. A negative tone, metal-organic resist is presented here. It can be patterned to produce sub-10 nm features when exposed to helium ion beam lithography at line doses on the order of tens of picocoulombs per centimeter. The resist was used to create 5 nm wide, continuous, discrete lines spaced on a 16 nm pitch in silicon and 6 nm wide lines on an 18 nm pitch in tungsten, with line edge roughness of 3 nm. After the lithographic exposure, the resist demonstrates high resistance to silicon and tungsten dry etch conditions (SF6 and C4F8 plasma), allowing the pattern to be transferred to the underlying substrates. The resist's etch selectivity for silicon and tungsten was measured to be 6.2:1 and 5.6:1, respectively; this allowed 3 to 4 nm thick resist films to yield structures that were 21 and 19 nm tall, respectively, while both maintained a sub-10 nm width on a sub-20 nm pitch.

Dependence of enhancement factor on electrode size for field emission current from carbon nanotube on silicon wafer

This work studies the enhancement factor associated with a current emitted from a multi-wall carbon nanotube to an extremely small counter electrode. The experimental data show that the field enhancement factor increases by 1.15 times when the width of the counter electrode increases from 50 to 200 nm. To better understand this enhancement effect, field intensities at the emitter surface are numerically simulated. The experimental work and simulations demonstrate that the observed field enhancement results from increases in the capacitance between the emitter and counter electrode. In addition, corrugated counter electrodes are found to greatly affect both the capacitance and enhancement factor. This is because the corrugation of the anode surface raises the capacitance and thus provides a higher current. We experimentally show that an effective surface area enlargement of 1.67 times due to the corrugation provides a 1.06-fold increase of the enhancement factor. These results should assist in the future development of field emission devices based on semiconductor fabrication processes.

One-step synthesis of tunable nitrogen-doped graphene from graphene oxide and its high performance field emission properties

Simultaneous reduction, repairing and doping of graphene oxide has been realized by the chemical vapor deposition method, using acetonitrile as a nitrogen source. A step-wise increase of acetonitrile partial pressure from 15 to 90 Pa results in nitrogen doped graphene (NG) with gradually tuning N-doping concentration from 0.38 to 0.66 at% and systematically rising graphite-N ratio from 23.25 to 45.39%, which in turn modulates field emission performance and enhance the stability. Raman spectroscopy and X-ray photoelectron spectroscopy suggest pyrrolic-N and pyridinic-N doping bring defects, while defects decrease as N-doping concentration increases due to the rising of graphite-N ratio. Proper defects may increase emission site density and N-doping can reduce work function. When N-doping concentration is controlled at 0.42 at%, NG exhibits the considerable decreasing of turn-on field from 3.35 to 2.18 V/μm at the emission current of 10 μA/cm2 and increasing of field enhancement factor from 1835 to 2967. It also reveals a good field emission stability with no degradation, which is superior to pristine reduced GO emitters. It is suggested the NG with tuning concentration of three type nitrogen emitter is a widely candidate for various field emission devices and applications.

Field electron emission induced glow discharge in a nanodiamond vacuum diode

The present paper extends the prior findings on self-induced heating of solid state field emission devices. It was found that a vacuum diode (base pressure ∼10−9 Torr), that makes use of graphite-rich polycrystalline diamond as cathode material, can switch from a diode regime to a resistor regime to a glow discharge plasma regime without any external perturbation, i.e. all transitions are self-induced. Combined results of in situ field emission microscopy, ex situ electron microscopy and Raman spectroscopy suggest that it is the nanodiamond cathode of the diode heated to about 3000 K which causes self-induced material evaporation, ionization and eventually micro-plasma formation. Our results confirm that field emission, commonly called cold emission, is a very complex phenomenon that can cause severe thermal load. Thermal load and material runaway could be the major factors causing vacuum diode deterioration, i.e. progressive increase in turn-on field, decrease in field enhancement factor, and eventual failure.

Electron emission characteristics of wet spun carbon nanotube fibers

Wet spun carbon nanotube fibers were characterized using both field emission and electron energy distribution measurements. Fowler-Nordheim analysis of the field emission results showed that the carbon fibers demonstrated a large effective emission area, 2 x 10-12 m2, which resulted in a reduced brightness of 1.84 x 1010 A/m2/sr/V. By considering the emission and number of carbon nanotube emitters it can be shown that the brightness is consistent with previous reports for single nanotube emitters. Additionally, using the effective emission area determined from the Fowler-Nordheim analysis an emittance value around 0.70 μm was found. These characteristics are useful metrics in determining the applicability of using wet spun carbon nanotube fibers for field emission devices.

Facile synthesis of α‐Si3N4 nanoneedles and their photoluminescence properties

AbstractNeedle‐like α‐Si3N4 with unique structures has potential application in both field emission devices and tips for atomic force microscopes. Single‐crystalline, α‐Si3N4 nanoneedles have been prepared by using an improved chemical vapor deposition (CVD) method at 1200°C for 3 hours. The phases, chemical composition, and microstructure of the as‐prepared products were determined by X‐ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM) equipped with EDS. The as‐synthesized products show a needle‐like morphology with hundreds of micrometers in length and nanometer‐featured tips. The nanoneedles show a α‐Si3N4@SiOx core‐shell heterostructure as characterized by TEM, with single‐crystalline α‐Si3N4 core and an insulating amorphous silicon oxide shell. The growth of α‐Si3N4 nanoneedles corresponds to vapor‐liquid‐solid cap‐growth and base‐growth mechanism. Photoluminescence (PL) properties of the products were also characterized. An obvious emission peak at 400 nm under 254 nm UV excitation was observed. The nanoneedle morphology and photoluminescence properties of the products have potentials to be used in future optoelectronic nanodevices.

Monolayer GaN functionalized with alkali metal and alkaline earth metal atoms: A first-principles study

Abstract Based on first-principles calculations, we have carried out a systematic study on the geometric, electronic and magnetic properties of free-standing monolayer GaN (ML GaN) functionalized with Lithium (Li), Sodium (Na), Beryllium (Be) or Calcium (Ca) atoms. We consider three different levels of concentrations i.e. θ = 1 / 8 , 1 / 18 and 1 / 32 . Within the tested θ , metallization of ML GaN only occurs with the adsorption of Li or Na atoms. The adsorption of Be or Ca atoms preserves the semiconducting characteristics of ML GaN. The ML GaN remains non-magnetic with the adsorption of Be or Ca atom. In contrast, the total magnetization of the Li-adsorbed ML GaN decreases as Li atoms concentration increases. For the case of Na adsorption, the ML GaN exhibits ferromagnetism only at θ = 1 / 18 . In addition, we found the work function of the functionalized ML GaN can be controlled by varying the concentrations of the adatoms. Our findings here suggest that by selective adsorption of Group I and Group II element, ML GaN is a promising material for the development of spintronic and field emission devices.

Fabrication of large-area ZnO nanowire field emitter arrays by thermal oxidation for high-current application

Large-area ZnO nanowire field emitter arrays have important applications in flat-panel displays, light sources, photodetectors and X-ray sources, where it is crucial to realize a high field emission current and current density over a large-area. In this paper, large-area ZnO nanowire arrays with various array spacings were prepared by the thermal oxidation technique from patterned Zn film arrays, and their field emission properties were studied. The results show that the Zn array density is related to the nanowire length and population density. A mechanism is proposed wherein the stress-induced process and mass transport process both play key roles in the growth. Herein, ZnO nanowire arrays in a 4.7 × 4.7 cm2 area exhibited a sufficient quantity of emission sites and diminished field screening effect and a high current of 12.9 mA with a current density of 0.58 mA/cm2. This work provides useful guidance for optimizing ZnO nanowires and enhancing field emission current performance for large-area ZnO nanowire field emission devices.

Large Scale Synthesis of Nanopyramidal-Like VO₂ Films by an Oxygen-Assisted Etching Growth Method with Significantly Enhanced Field Emission Properties.

The present investigation reported on a novel oxygen-assisted etching growth method that can directly transform wafer-scale plain VO2 thin films into pyramidal-like VO2 nanostructures with highly improved field-emission properties. The oxygen applied during annealing played a key role in the formation of the special pyramidal-like structures by introducing thin oxygen-rich transition layers on the top surfaces of the VO2 crystals. An etching related growth and transformation mechanism for the synthesis of nanopyramidal films was proposed. Structural characterizations confirmed the formation of a composite VO2 structure of monoclinic M1 (P21/c) and Mott insulating M2 (C2/m) phases for the films at room temperature. Moreover, by varying the oxygen concentration, the nanocrystal morphology of the VO2 films could be tuned, ranging over pyramidal, dot, and/or twin structures. These nanopyramidal VO2 films showed potential benefits for application such as temperature−regulated field emission devices. For one typical sample deposited on a 3-inch silicon substrate, its emission current (measured at 6 V/μm) increased by about 1000 times after the oxygen-etching treatment, and the field enhancement factor β reached as high as 3810 and 1620 for the M and R states, respectively. The simple method reported in the present study may provide a protocol for building a variety of large interesting surfaces for VO2-based device applications.

Field emission properties of single crystalline W5O14 and W18O49 nanowires

Single crystalline tungsten oxides in a form of W5O14 and W18O49 nanowires were synthesized by iodine transport method. The morphology, work functions and field emission properties of these nanowires were investigated. Work functions of the W5O14 (4.20–4.34 eV) and W18O49 (4.55–4.57 eV) nanowires (NWs) have been measured by Kelvin probe force microscopy (KPFM) in ultra-high vacuum. Field emission (FE) measurements of individual nanowires were performed in ultra-high vacuum at microscopic and macroscopic distances between the emitter and electron collector. The obtained FE curves at microscopic distances were analyzed in the framework of the Fowler–Nordheim (F–N) theory. Field enhancement factors of W5O14 at the emitter-collector distance of 2, 4 and 5 μm were calculated to be 110 ± 10, 180 ± 25 and 210 ± 30, respectively, and 125 ± 15 for W18O49 at 2 μm. At macroscopic distances, the F–N theory revealed unrealistic high field enhancement factors: for W5O14 at 1 mm it was 17,000 ± 500, and for W18O49, the field enhancement factors were 5050 ± 30 and 6450 ± 30 at 600 μm and 800 μm emitter-collector distance, respectively. Therefore, more realistic model was discussed. The lower work function and typically smaller diameter of the W5O14 nanowires in comparison with the W18O49 wires, range the W5O14 nanowires to the promising sources of electrons in field emission devices.

Field Emitters Using Inverse Opal Structures

AbstractElectronics to be used in space must often perform in high temperature or radiation hard environments that render conventional solid‐state technologies unable to meet mission requirements. As a result, microscale and nanoscale field emission devices are being explored as fundamental components of electronics capable of operating in these harsh environments. Wide scale implementation of these devices is hindered by the difficulty of fabricating large, mechanically stable, uniform arrays of sharp emitting tips. This work presents a scalable method to produce uniform arrays of field emitting tips. Polystyrene spheres are applied as a template for electrochemical deposition. An electrochemical etching process is developed to sharpen tips to a radius of curvature of 5–10 nm, optimizing them for field emission applications. The flexibility of the fabrication process allows for device optimization in terms of tip geometry, density, and constituent material to achieve high field enhancement factors, exceeding 100. Miniaturized field emitting diode and gated triode devices are fabricated. Finally, the electrochemically deposited material is used as a scaffold for the deposition of a refractory, low work function emitting layer, and the hybrid cathode is characterized as a field emitter at temperatures up to 300 ºC.

Updating the Role of Reduced Graphene Oxide Ink on Field Emission Devices in Synergy with Charge Transfer Materials.

Hydroiodic acid (HI)-treated reduced graphene oxide (rGO) ink/conductive polymeric composites are considered as promising cold cathodes in terms of high geometrical aspect ratio and low field emission (FE) threshold devices. In this study, four simple, cost-effective, solution-processed approaches for rGO-based field effect emitters were developed, optimized, and compared; rGO layers were coated on (a) n+ doped Si substrate, (b) n+-Si/P3HT:rGO, (c) n+-Si/PCDTBT:rGO, and (d) n+-Si/PCDTBT:PC71BM:rGO composites, respectively. The fabricated emitters were optimized by tailoring the concentration ratios of their preparation and field emission characteristics. In a critical composite ratio, FE performance was remarkably improved compared to the pristine Si, as well as n+-Si/rGO field emitter. In this context, the impact of various materials, such as polymers, fullerene derivatives, as well as different solvents on rGO function reinforcement and consequently on FE performance upon rGO-based composites preparation was investigated. The field emitter consisted of n+-Si/PCDTBT:PC71BM(80%):rGO(20%)/rGO displayed a field enhancement factor of ~2850, with remarkable stability over 20 h and low turn-on field in 0.6 V/μm. High-efficiency graphene-based FE devices realization paves the way towards low-cost, large-scale electron sources development. Finally, the contribution of this hierarchical, composite film morphology was evaluated and discussed.

Thickness Effect on Field-Emission Properties of Carbon Nanotube Composite Cathode

Although low work function materials have been widely used to coat carbon nanotube (CNT) cathodes for improving field-emission performance, there have been few reports on the effect of coated layer thickness except for a few examples, in which some contradictory observations were reported. Hence, in this paper, we employed a low work function material of LiF/Al to coat CNT cathodes with the capability to precisely control the coated film thickness and systematically investigated the film thickness effect on field-emission properties of the CNT composite field-emission devices (FEDs). A nonmonotonic relationship between field-emission performance and cathode film thickness was observed and the state-of-the-art performance was optimally demonstrated for the CNT composite cathode with Al layer thickness of 1 nm. Based on experimental data and analysis, the nonmonotonic tendency was found to correspond well with the behavior of thickness-dependent resistance and thus was mainly assigned to the conductance of composite cathodes. In addition, the improved stability and lighting uniformity for the FEDs with an Al layer were attributed to the increased effective emission sites and the robust emitters being protected. The superior performance of the CNT cathode composited with a low work function material of LiF/Al demonstrates potential applications in vacuum electronic devices and an optimized thickness of the coated layer is important for field emission enhancement of CNT composite cathodes.

Numerical study on the field-emission properties of a graphene–C60 composite

A new model of a graphene–C60 composite is constructed to explore its electronic structure and field-emission characteristics. We investigate the structural stability, energy levels, local electron density distribution, work function, ionization potential, and Mulliken, Hirshfeld, and electrostatic potential fitting (ESP) charges of this composite by using first-principles methods. The results indicate that the electronic structure of the composite can be modulated obviously by applying an external electric field. The energy gap decreases as the electric field is increased, and the change in the local electron density distribution plays the role of a tip emission point. We find that the binding energy increases as the electric field is increased, which confirms that the assembly of graphene and C60 directly improves the stability of the compound and results in excellent semiconducting properties. With the increasing electric field, we also find that the work functions and ionization potentials of this composite decrease linearly, and the Mulliken, Hirshfeld and ESP charge move efficiently. All of those and the change of energy gap and the local electron density distribution show the improvement of graphene–C60 composite’s field emission properties. We conclude that the graphene–C60 composite may be a promising candidate for use in field-emission devices.

Exposure to cell phone radiofrequency changes corticotrophin hormone levels and histology of the brain and adrenal glands in male Wistar rat.

Objective(s):Nowadays, the electromagnetic field-emitting devices are used routinely in our lives. Controversial reports exist concerning the effects of mobile radiofrequency (RF) on different parts of the body, especially stress hormones. The main goal of the present work was to study the long-term effects of mobile RF900 MHz exposure with special focus on the adrenal gland pathophysiology and function. Materials and Methods:Adult male Wistar rats were exposed to mobile RF 6 hr daily for 4–8 weeks. Intact and switched-off exposed animals were considered as controls. Plasma ACTH and cortisol levels were measured by the ELISA method. At the end of the experiment, a histological study was done on adrenal gland and brain tissues by hematoxylin and eosin staining. The thickness of the fasciculate layer of the adrenal gland, and its cell count and perimeter were measured using the Fiji software. Results:Enhanced plasma ACTH and cortisol levels were found after prolonged exposure to mobile RF. The fasciculata layer of adrenal cortex eventually thickened following mobile RF radiation. While the number of cells in zona fasciculata remained constant, the cell size and perimeter increased during RF exposure. Finally, we found that vacuolization in brain tissue and the number and size of vacuoles considerably increased during two months of RF exposure. Conclusion:Cell phone RF exposure induced significant hormonal and structural changes in adrenal gland and brain tissues. Therefore, the public should be aware and limit their exposure as much as possible.

Width dependent structural and electrical properties of zigzag ZnTe nanoribbons

We carry out density functional theory based investigation to understand the structural and electrical properties such as atomic structure, edge energy, band gap, and work function of zigzag ZnTe nanoribbons. It is found that the zigzag nanoribbons may be stabilized by passivating the edge atoms with Hydrogen, Oxygen or Fluorine atoms. Our study reflects that zigzag ZnTe nanoribbons with smaller width behave like semiconductor. However, they exhibit a transition from semiconducting phase to a metallic phase as width increases. A wide variation of band gap is obtained with respect to the choice of edge passivating elements. Work functions of all the nanoribbons are also estimated in order to assess the utility of these nanoribbons in various field emission devices.

A parametric study to unravel the alignment mechanism of carbon nanotubes during its plasma-assisted growth

The alignment and catalyst-assisted growth mechanism of carbon nanotubes (CNTs) in a hydrogen-diluted methane plasma are demonstrated in the present work. The consequence of plasma parameters (plasma density and temperature) and bias potential of the substrate on the sheath electric field is examined. The role of the electric field in the plasma sheath to provide the necessary electrostatic force and its repercussion on the carbon nanotubes alignment in the plasma are studied by taking into contemplation the steady state fluid equations, kinetics of the plasma sheath, charged species dynamics (positive ions and electrons) and neutral atoms in a reactive plasma, kinetics of the catalyst particle, and creation of atomic species of carbon and hydrogen over the surface of catalyst nanoparticle and incorporating various processes vital for the growth of carbon nanotube in plasma. Taking into deliberation the initial conditions and the glow discharge parameters determined experimentally for the model, the numerical solutions for the equations are obtained. The outcomes of the studies revealed that the alignment of the nanotube depends on the force exerted at the tip of CNTs, i.e., the catalyst nanoparticle. This force is a consequence of the plasma sheath electric field which is dependent on the plasma parameters and the bias voltage at the substrate. This electrostatic force is also dependent upon the dimensions of the nanotube growing in the plasma environment and modifies itself continuously conferring to the instantaneous nanotube measurement and alignment during the growth. The present study can be apprehended to manufacture carbon nanotubes aligned vertically in plasma for better applications in the areas of field emission devices. The results of the present study have been assessed and compared with the existing experimental observations which accredit the proposed mechanisms.

Novel Nanofabricated Mo Field-Emitter Array for Low-Cost and Large-Area Application

This paper demonstrates a novel nanofabrication technology for producing high-density gated molybdenum (Mo) field-emitter array (FEA) based on modified nanosphere lithography (NSL). The arc-shaped side wall of gate apertures introduced by the conventional NSL process put obstacles in the formation of sharp cone-type emitters. In this paper, a two-method solution was employed to facilitate the realization of gate apertures with sharper cutting side wall which are beneficial for the formation of sharper gated Mo emitters. One method was softening nanosphere to enlarge the contact area between nanosphere and substrate. The other was optimizing gate film deposition conditions to dramatically reduce the diffraction effects. Due to the improvements in the geometrical shape of gated Mo emitters, a low-voltage working high-density gated Mo FEA was demonstrated in this paper. This is the first observation of remarkable electron emission from high-density gated Mo FEA fabricated using such as low-cost, high-throughput, and high-efficiency technique which is expected to substantially overcome the challenges in the traditional fabrication process and promotes the low-cost and large-area application of high-density gated field-emitter devices. In addition, the novel nanofabricating method is expected to offer a simple and scalable route which allows fabrication of many gated field-emission devices.

Controlled Hetero‐Architectures of Au‐Nanoparticles‐Decorated ZnO Nanowires for Enhanced Field Electron Emission Displays

AbstractThe spitzer shaped pointed ZnO nanowires of diameter 120 nm and length ∼5‐6 μm synthesized via hydrothermal techniques were decorated with Au nanoparticles of diameter < 20 nm along their textural boundaries. The turn‐on field (Eon) of 1.56 V/μm required to draw current density of 10 μA/cm2 from ZnO nanowires was reduced further to 0.96 V/μm after controlled decoration of Au nanoparticle, is significantly lower than pristine/doped ZnO nanostructures, Ag@ZnO, Au@ZnO, Au@CuO, and MoS2@TiO2 heterostructure based field emission (FE) devices. The orthodoxy test confirmed the feasibility of field‐enhancement factor (βFE) of 4477 for Au/ZnO/ITO emitters. The enhanced FE behavior are attributed to pointed nanotips, individual dispersion of Au nanoparticles along the textural boundaries of ZnO nanowires, and enhanced energy well formation at their interface. Moreover, the density of state increases enormously due to the hetero‐structured interface of Au@ZnO and a significant amount of electrons from both Au and ZnO contribute for enhanced emission current density (2.1 mA/cm2 @ 1.92 V/μm), and greatly stable electron emission. Our experiments suggest that the tunable hetero‐architectures of Au nanoparticles@ZnO nanowires hold the promises for applications in display screens and intense electron source.