Enhanced Electron Emission Research Articles

Effects of surfactants on the microstructures of electrospun polyacrylonitrile nanofibers and their carbonized analogs

ABSTRACTIn this study, the influence of surfactants on the processability of electrospun polyacrylonitrile (PAN) nanofibers and their carbonized analogs was investigated. The surfactants employed in this effort are Triton X‐100 (nonionic surfactant, SF‐N), sodium dodecyl sulfate (SDS) (anionic surfactant, SF‐A), and hexadecyltrimethylammonium bromide (HDTMAB) (cationic surfactant, SF‐C). Interactions between electrospun PAN and the surfactants, reflected in effects on as‐spun and carbonized nanofiber morphologies and microstructures, were explored. The results show that uniform nanofibers are obtained when cationic and anionic surfactants (surfactant free and nonionic surfactants) are utilized in the preparation of electrospun PAN. In contrast, a bead‐on‐a‐string morphology results when the aniconic and cationic surfactants are present, and defect structure is enhanced with cationic surfactant addition. Moreover, fiber breakage is observed when the nonionic surfactant Triton X‐100 is employed for electrospinning. After carbonizaition, the PAN polymers were observed to have less ordered structures with addition of any type of surfactant used for electrospinning and the disorder becomes more pronounced when the anionic surfactant is utilized. Owing to the fact that microstructure defects create midband gap states that enable more electrons to be emitted from the fiber, an enhancement of electron emission is observed for PAN electrospun in the presence of the anionic surfactant. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3726–3735, 2013

Vibrational enhancement of electron emission in CO (a3Π) quenching at a clean metal surface

We have combined a Stark decelerator with a molecular beam-surface scattering setup to accurately measure the absolute electron emission yield, γ, of CO (a(3)Π) quenching at a Au(111) surface with quantum state resolution. We clearly observe an enhanced probability of electron emission when scattering vibrationally excited states of CO from the surface.

Enhancement of electron emission and long-term stability of tip-type carbon nanotube field emitters via lithium coating

Carbon nanotubes (CNTs) were deposited on conical tip-type substrates via electrophoresis and coated with lithium (Li) thin films with diverse thicknesses via electroplating. For the as-deposited (i.e., without Li coating) CNT, the turn-on (or triggering) electric field was 0.92V/μm, and the emission current, which was generated at an applied field of 1.2V/μm was 56μA. In the case of the 4.7nm-thick Li-coated CNT, the turn-on field decreased to 0.65V/μm and the emission current at the same applied field increased more than ten times to 618μA. The analysis based on the Kelvin probe measurement and Fowler–Nordheim theory indicated that the coating of Li caused a loss in the structural-aspect-ratio of the CNTs and it reduced their effective work functions from 5.36eV to 4.90eV, which led to a great improvement of their electron emission characteristics. The results obtained in this study also showed that the long-term emission stability could be enhanced by the coating of thin Li films on CNTs.

Three region model and quantum enhancement of thermionic and photoelectric electron emission from negatively charged metallic surfaces

In this paper the authors have evaluated the transmission coefficient of electrons across a negatively charged plane metallic surface on the basis of a three region model for variation of the poten...

Enhanced electron emission from tetrahedral amorphous carbon capped carbon nanotube core-shelled structure

Core-shelled composites have been shown to enhance material properties, notably for chemical sensing and biomolecular applications. In this comprehensive study, we show that a simple core–shell structure, made up of tetrahedral amorphous carbon (ta-C) tip-capped onto an aligned dense array of carbon nanotubes (CNT), exhibits superior electron emission properties, having a distinct enhancement in the turn-on field of < 1 V/μm without any lithography patterning. This is compared to ~ 2 V/μm for pristine CNT samples. A KrF pulsed laser deposition process was used to deposit high (> 70%) sp 3 content non-hydrogenated ta-C films of varying thickness for the tip-capping process. Combining scanning electron microscopy images and electron emission testing results, our systematic study shows that first, the thickness of the ta-C can range from 20 to 100 nm and is sufficient to achieve the enhancement effect. However, thicker coatings > 200 nm will degrade the electron emission. Second, a simple tip-capping is sufficient to achieve the enhancement without the need to encapsulate the entire carbon nanotube.

Structural modification and enhanced electron emission from multiwalled carbon nanotubes grown on Ag/Fe catalysts coated Si-substrates

Multiwalled carbon nanotubes and carbon nano-filaments were grown using Fe as the main catalyst and Ag as a co-catalyst by microwave plasma enhanced chemical vapour deposition. In this work we demonstrate the growth behaviour of carbon nanotubes (CNTs) grown on pure Fe-film and Ag–Fe films. We find that using Ag film beneath Fe film significantly abate the catalyst–substrate interactions by acting as a barrier layer as well as enhances the nucleation sites for the growth of CNTs due to the limited solubility with Fe and silicon. Scanning electron microscopy and transmission electron microscopy studies were carried out to image the microstructures of the samples. It was observed that the length of Fe catalyzed CNTs was ∼500 nm and Ag–Fe catalyzed CNTs varied from ∼600 nm to 1.7 μm. Micro Raman spectroscopy confirmed the improved crystalline nature of Ag–Fe CNTs. It was found that I D/ I G ratio for Fe catalyzed CNTs was ∼1.08 and for Ag–Fe catalyzed CNTs was ∼0.7. The Ag–Fe catalyzed CNTs were found to be less defective as compared to Fe catalyzed CNTs. Field emission measurements using diode configuration, showed that electron emission from Ag–Fe catalyzed CNTs was much stronger as compared to Fe catalyzed CNTs. The threshold field for Ag–Fe catalyzed CNTs was (2.6 V μm −1) smaller as compared to Fe catalyzed CNTs (3.8 V μm −1) and thus shows better emission properties. This enhancement in electron emission mechanism as a result of introduction of Ag underlayer is attributed to the increased emitter sites and improved crystallinity.

Effect of titanium interlayer on the microstructure and electron emission characteristics of multiwalled carbon nanotubes

The influence of the titanium (Ti) interlayer thickness on the growth and electron emission characteristics of carbon nanotubes (CNTs) deposited on silicon (Si) coated with an iron (Fe) catalyst layer was investigated. Ti films 5 nm, 10 nm, and 15 nm in thickness were deposited beneath the Fe catalyst layers. Multiwalled carbon nanotubes (MWCNTs) were deposited via microwave plasma enhanced chemical vapor deposition. The Ti interlayer hinders the diffusion of Fe into the silicon substrate and thus helps in the growth of MWCNTs. In addition, the role of Ti as a sacrificial layer on the catalytic diffusion, surface morphology, microstructure, and, thus, the growth of MWCNTs was probed through scanning and high resolution transmission electron microscope studies. The enhanced electron emission mechanism as a result of the introduction of a Ti interlayer is explained on the basis of a double barrier model and the formation of a conducting channel between the substrate and the CNTs. The intensified micro-Raman characteristics of the MWCNTs were directly related with the thickness of the Ti interlayer in the three layered system (Ti/Fe/CNT) as compared to a two layer system (Fe/CNT) deposited on a Si substrate.

Direct observation of sub-threshold field emission from silicon nanomembranes

Excited by an energetic electron beam, field enhanced electron emission from a silicon nanomembrane is examined by using a local current probe. The experiments reveal clear transitions from conventional secondary electron emission (SEE) to sub-threshold field electron emission (SFE) and then to conventional field electron emission (FEE). An electron yield of more than 104 is obtained and a model of sub-threshold emission is verified. Ultra sensitive charge/particle detectors could be realized by implementing sub-threshold field emission in nano scale structures, where electron excitations occur within the screen depth of surface electric field.

Effect of ultraviolet light irradiation on electron field emission from titanium-oxide nanostructures

Field-emission current from titanium-oxide nanostructures is increased by ultraviolet (UV) light irradiation. The field-emission current after UV irradiation was tens of times larger than that before irradiation. The enhancement in electron emission by UV at 365 nm is larger than that at 405 nm. The increased emission current did not decrease after the irradiation was stopped but did decrease after the sample was exposed to air. This suggests that surface cleaning by photon induced reaction is the origin of the observed current enhancement.

Effect of annealing on field emission properties of nanodiamond coating

Field electron emission of detonation nanodiamond (ND) coated on a titanium substrate by electrophoretic deposition is investigated. It is found that thermal annealing can significantly improve the field emission properties of the ND layer, which can be mainly attributed to the formation of the TiC phase between diamond and Ti. The first-principles calculated results show that the formation of transition layers can lower the interface barrier and enhance the field electron emission of ND coating. Besides, the transformation of diamond to graphite after annealing has been revealed by Raman spectra. This transformation also benefits the electron emission enhancement.

Enhanced electron emission from titanium coated multiwalled carbon nanotubes

Enhanced field emission properties and improved crystallinity of titanium (Ti) coated multiwalled carbon nanotubes (MWCNTs), prepared by microwave plasma enhanced chemical vapour deposition have been observed. Ti films of extremely low thicknesses (0.5 nm, 1.0 nm and 1.5 nm) were coated over carbon nanotubes (CNTs) and their field emission behaviour was investigated. The turn on field of Ti coated CNTs was found to be low (~ 0.8 V/μm) as compared to pristine CNTs (~ 1.8 V/μm). The field enhancement factor for Ti coated CNTs was quite large (~ 1.14 × 10 4) as compared to pristine CNTs (~ 6 × 10 3). This enhancement in electron emission is attributed to the passivation of defects and improved crystallinity of CNTs. Surface morphological and microstructural studies were carried out to investigate the growth of pristine and Ti coated CNTs. It was observed that Ti nanoclusters adsorb on the edges of MWCNTs and increase their crystallinity. This increase is directly correlated with the thickness of Ti film deposited. Micro Raman spectroscopy confirmed the improved crystallanity of Ti coated CNTs.

Ab initioinvestigation of barium-scandium-oxygen coatings on tungsten for electron emitting cathodes

Microwave, x-ray, and radio-frequency radiation sources require a cathode emitting electrons into vacuum. Thermionic B-type dispenser cathodes consist of ${\text{Ba}}_{x}{\text{O}}_{z}$ coatings on tungsten (W), where the surface coatings lower the W work function and enhance electron emission. The new and promising class of scandate cathodes modifies the B-type surface through inclusion of Sc, and their superior emissive properties are also believed to stem from the formation of a low work function surface alloy. In order to better understand these cathode systems, density-functional theory (DFT)-based ab initio modeling is used to explore the stability and work function of ${\text{Ba}}_{x}{\text{Sc}}_{y}{\text{O}}_{z}$ on W(001) monolayer-type surface structures. It is demonstrated how surface depolarization effects can be calculated easily using ab initio calculations and fitted to an analytic depolarization equation. This approach enables the rapid extraction of the complete depolarization curve (work function versus coverage relation) from relatively few DFT calculations, useful for understanding and characterizing the emitting properties of novel cathode materials. It is generally believed that the B-type cathode has some concentration of Ba-O dimers on the W surface, although their structure is not known. Calculations suggest that tilted Ba-O dimers are the stable dimer surface configuration and can explain the observed work function reduction corresponding to various dimer coverages. Tilted Ba-O dimers represent a new surface coating structure not previously proposed for the activated B-type cathode. The thermodynamically stable phase of Ba and O on the W surface was identified to be the ${\text{Ba}}_{0.25}\text{O}$ configuration, possessing a significantly lower $\ensuremath{\Phi}$ value than any of the Ba-O dimer configurations investigated. The identification of a more stable ${\text{Ba}}_{0.25}\text{O}$ phase implies that if Ba-O dimers cover the surface of emitting B-type cathodes, then a nonequilibrium steady state must dominate the emitting surface. The identification of a stable and low work function ${\text{Ba}}_{0.25}{\text{Sc}}_{0.25}\text{O}$ structure suggests that addition of Sc to the B-type cathode surface could form this alloy structure under operating conditions, leading to improved cathode performance and stability. Detailed comparison to previous experimental results of ${\text{Ba}}_{x}{\text{Sc}}_{y}{\text{O}}_{z}$ on W surface coatings are made to both validate the modeling and aid in interpretation of experimental data. The studies presented here demonstrate that ab initio methods are powerful for understanding the fundamental physics of electron emitting materials systems and can potentially aid in the development of improved cathodes.

Electronic structure of nanopolycrystalline pulsed laser deposited LaB6 films and single crystals: The boron perspective

We report an investigation of the electronic structure of LaB6 nanopolycrystalline pulsed laser deposited (PLD) film and single crystal with x-ray absorption near edge structures (XANES) spectroscopy at the B K-edge. The experimental results are compared with theoretical calculations using density functional theory, and real space multiple scattering. It is found that (i) the LaB6 PLD film is of high quality and metallic albeit it is a polycrystalline phase of nanocrystallites and (ii) the B K-edge XANES for the single crystal specimens exhibit well defined spectral features corresponding to the theoretical partial densities of states of B p character and the threshold energy is relatively low and free electronlike, revealing its metallic character. The implications of these observations to the enhancement in electron emission from the PLD films are discussed.

Position-controlled AlN/ZnO coaxial nanotube heterostructure arrays for electron emitterapplications

We studied the fabrication and field-emission characteristics of position-controlledAlN/ZnO nanotube heterostructure arrays. AlN layers with various thicknesses from 20 to52 nm were deposited coaxially on the position-controlled ZnO nanotube arrays. Thefield-emission properties of the coaxial AlN/ZnO nanotube arrays were controlled using theAlN thickness and the nanotube interdistance. As compared to the bare ZnO nanotubearrays, the AlN-coated coaxial nanotube arrays exhibited enhanced electron emission, andthe optimum AlN coating layer thickness on the nanotube tips was 26 nm. The improvedfield emission from the coaxial nanotube heterostructures is attributed to the lowelectron affinity and the thickness modulation effect of the AlN coating layer.

Electrophoretic Deposition of Oxide Nanoparticles for Electron Emission Enhancement

Four oxide nanoparticles, such as yttrium oxide, lanthanum oxide, cerium oxide and zirconium oxide were coated on rhenium filaments by an electrophoretic deposition method. The oxide coatings were examined by scanning electron microscopy, and electron emission characteristics of the oxide-coated rhenium were studied to find alternative oxide materials for replacing the hazardous thorium oxide. Among four oxides tested in this work, yttrium oxide and lanthanum oxide coated rhenium showed enhanced electron emission compared with an uncoated rhenium. However, lanthanum oxide coating evaporated too quickly in a vacuum chamber, and hence yttrium oxide was found to be the only alternative material for use in a routine mass spectrometry. An optimum coating thickness of the yttrium oxide and lanthanum oxide was around 20 μm.

Nanodiamod lateral device field emission diode fabricated by electron beam lithography

Nitrogen incorporated nanodiamond film is known to aid in promoting enhanced electron emission via the induced graphitic behavior both in the bulk material and also the surface of the film. Since electron emission current is inversely proportional to the cathode to anode inter-electrode distance; it is necessary to implement electron beam lithography (EBL) to obtain a small emission gap. To achieve high resolution from EBL, a thinner nanodiamond film is required. In this work, we fabricated lateral field emitters on a 0.65 µm nanodiamond film. The nanodiamond film was deposited onto a silicon-on-insulator (SOI) substrate in CH 4/H 2/N 2 plasma ambient by microwave chemical vapor deposition. The SOI was prepared for diamond nucleation using mechanical abrasion and ultrasonication in nanodiamond powder. Electron beam lithography (EBL) was used to delineate a 10 emitter tipped diode with a 2 µm anode-to- anode emission gap.

Temperature enhancement of secondary electron emission from hydrogenated diamond films

The effect of temperature on the stability of the secondary electron emission (SEE) yield from ∼100-nm-thick continuous diamond films is reported. At room temperature, the SEE yield was found to decay as a function of electron irradiation dose. The SEE yield is observed to increase significantly upon heating of the diamond surface. Furthermore, by employing moderate temperatures, the decay of the SEE yield observed at room temperature is inhibited, showing a nearly constant yield with electron dose at 200 °C. The results are explained in terms of the temperature dependence of the electron beam-induced hydrogen desorption from the diamond surface and surface band bending. These findings demonstrate that the longevity of diamond films in practical applications of SEE can be increased by moderate heating.

Enhanced Field Emission from Argon Plasma-Treated Ultra-sharp α-Fe2O3Nanoflakes

Hematite nanoflakes have been synthesized by a simple heat oxide method and further treated by Argon plasmas. The effects of Argon plasma on the morphology and crystal structures of nanoflakes were investigated. Significant enhancement of field-induced electron emission from the plasma-treated nanoflakes was observed. The transmission electron microscopy investigation shows that the plasma treatment effectively removes amorphous coating and creates plenty of sub-tips at the surface of the nanoflakes, which are believed to contribute the enhancement of emission. This work suggests that plasma treatment technique could be a direct means to improve field-emission properties of nanostructures.

12.1: Invited Paper: Enhanced Electron Emission of Carbon‐Nanotube Emitters with Post‐Growth Treatment

AbstractWe used a resist‐assisted patterning (RAP) process for the growth of CNTs as electron emitters. The electron emission sites were easily controlled by simple resist patterning process. To obtain stabilized and enhanced electron emission, post growth treatment process was adopted. The electron emission current strongly depended on the post growth treatment conditions. The electron emission current was increased with increasing acid treatment time up to 20 second. The enhanced electron emission current was more enhanced with consecutive post growth temperature and acid treatment. After 20 sec post growth treatment on robust CNTs, we can obtain 1.62 mA/cm2 current density at 4 V/μm field. The controlled electron emission current of CNT emitter array with RAP process and structural improvement of CNT‐FEAs after HF treatment for enhanced electron emission was discussed.

12.2: Enhanced Electron Emission of the Cone‐Shaped Emitters with Carbon Nanostructures

AbstractWe developed novel cone‐shaped emitters with individual carbon nanotips using the resist‐assisted patterning process. The novel cone‐shaped nanotip emitters were arranged in an array with many nano emitters in pre‐patterned diameters of 3 μm. The cone‐shaped nanotip emitters consist of a number of carbon‐based nanostructures such as nanotubes and/or nanofibers with silicon and nitrogen elements at the periphery of each element. The cone‐shaped nano‐emitters strongly adhered to the substrate due to the large area bottom contact which is covered with diffused silicon elements. The growth mechanism and enhanced electron emission of cone‐shaped nanotip emitters was discussed.