Electron field emission from single-walled carbon nanotube nonwoven

Field emission from single-walled carbon nanotube (SWNT) nonwoven has been investigated under high vacuum with different vacuum gaps. A low turn-on electric field of 1.05\,V/$\mu $m is required to reach an emission current density of 10 $\mu $A/cm$^{2}$. An emission current density of 10 mA/cm$^{2}$ is obtained at an operating electric field of 1.88\,V/$\mu $m. No current saturation is found even at an emission current of 5\,mA. With the vacuum gap increasing from 1 to 10 mm, the turn-on field decreases monotonically from 1.21 to 0.68\,V/$\mu $m, while the field amplification is augmented. The good field-emission behaviour is ascribed to the combined effects of the intrinsic field emission of SWNT and the waved topography of the nonwoven.     

Electron field emission from magnetic nanomaterial encapsulated multi-walled carbon nanotubes

The present work describes the field emission characteristics of nanoscale magnetic nanomaterial encapsulated multi-walled carbon nanotubes (MWNTs) fabricated over flexible graphitized carbon cloth. Ni/MWNTs, NiFe/MWNTs and NiFeCo/MWNTs have been synthesized by catalytic chemical vapor decomposition of methane over Mischmetal (Mm)-based AB₃ (MmNi₃, MmFe_1.5Ni_1.5 and MmFeCoNi) alloy hydride catalysts. Metal-encapsulated MWNTs exhibited superior field emission performance than pure MWNT-based field emitters over the same substrate. The results indicate that a Ni-filled MWNT field emitter is a promising material for practical field emission application with a lowest turn-on field of 0.6 V/μm and a high emission current density of 0.3 mA/cm² at 0.9 V/μm.     

Efficient light emission from crystalline and amorphous silicon nanostructures

Optical and electronic properties of crystalline silicon (c-Si) and amorphous silicon (a-Si) nanostructures are reviewed. The photoluminescence (PL) peak energies of c-Si and a-Si nanostructures are blueshifted from those of bulk c-Si and a-Si. The temperature dependence of the PL intensity is drastically improved in c-Si and a-Si nanostructures, and efficient luminescence from c-Si and a-Si nanostructures is observed at room temperature. The quantum confinement, spatial confinement, and surface effects on luminescence properties are summarized and the PL mechanism of silicon nanostructures is discussed.     

Field emission from nano-patterned amorphous carbon

Electron emission from nano-patterned amorphous carbon is realized in this paper. The patterned carbon consists of islands with size of tens of nanometers, and is formed by etching uniform carbon film in oxygen plasma using a bismuth island-like film as the mask. Uniform and stable electron emission is reproducibly obtained, and the emission efficiency is above 2% at an anode voltage of 3 kV. Small carbon particles between large islands are supposed to be necessary for stable electron emission.     

Electron field emission properties of closed carbon nanotubes

Recent experiments have shown that carbon nanotubes exhibit excellent electron field emisson properties with high current densities at low electric fields. Here we present theoretical investigations that incorporate geometrical effects and the electronic structure of nanotubes. The electric field is dramatically enhanced near the cap of a nanotube with a large variation of local field distribution. It is found that deviation from linear Fowler-Nordheim behavior occurs due to the variation of the local field in the electron tunneling region. The maximum current per tube is of the order of 10 microA. Local and microscopic aspects of field emission from nanotubes are also presented.     

Determination of temperature difference in carbon nanostructures in field emission

It is shown that a large temperature difference between the emitting surface and the substrate exists in the case of field emission of electrons from nanosize carbon structures. This makes it possible to explain small values of the effective electron work function in the case of field emission from such structures.     

Electron field emission from broad-area electrodes

In this review, measurements are surveyed for both dc and microwave electric fields. These show significant emission at applied fields typically 100 times smaller than those expected theoretically or measured experimentally for ideal microtip cathodes. Recent work is reviewed which studies on a microscopic scale this emission and the localized sites which produce it. Rather than forming sharp field enhancing projections, these sites often appear flat and are frequently associated with grain boundaries or insulating inclusions. Following an examination of various techniques for changing the emission of a given cathode, several theoretical models are considered which may explain some aspects of the observed emission characteristics.     

The effect of hydrogen dilution on the field emission from hydrogenated amorphous carbon films

The microstructures and field emission characteristics of hydrogenated amorphous carbon films prepared by using different hydrogen dilution ratio were investigated. It was found that a very low threshold electric field emission could be achieved for samples with moderated hydrogen dilution ratio. However, the field emission characteristics became worse for samples with high hydrogen dilution ratio. The change of field emission can be attributed to the change of electronic structures due to the hydrogen dilution in addition to the increase of the hydrogen surface termination.     

Electron field emission properties of highly dense carbon nanotube arrays

In this paper, we have studied field emission properties of highly dense arrays of multi-walled carbon nanotubes (CNTs) used as cathodes in diode-type field emission devices with a phosphor screen. For the high-density CNT emitters it is demonstrated that the emission sites are located on the CNT-cathode edges, which is direct experimental evidence of the ‘edge effect’. The results of computer simulations (using ‘ANSYS Electromagnetic’ software) are presented to confirm the experimental data and to analyze the effect of patterning on the electric field distribution for high-density CNT arrays. It is shown that selective-area removal of nanotubes in the arrays leads to the formation of additional edges characterized by the high field enhancement factor and enhanced emission from the CNT cathodes. In addition, scanning probe microscopy techniques are employed to examine surface properties of the high-density CNT arrays. For CNT arrays of ‘short’ nanotubes, the work function distribution over the sample surface is detected using a scanning Kelvin microscopy method.     

Comparative study on modification of multi-walled carbon nanotubes by a hydrophilic polymer with different approaches

The multi-walled carbon nanotubes (MWNTs) modified by a hydrophilic polymer were prepared with polymerization and blending approaches. The differences of both modified MWNTs were compared by Fourier transform infrared and Raman spectroscopies, thermogravimetric analysis and transmission electron microscopy. Chemical grafting reaction had occurred between MWNTs and polyvinyl pyrrolidone (PVP) after modification with polymerization and blending approaches. Polymerization modification can graft more PVP on the surface of MWNTs compared with blending modification. Polymerization modification of MWNTs belongs to the “grafting from” mechanism, while blending modification belongs to the “grafting to” mechanism. Modified MWNTs exhibit remarkable solubility in water, ethanol and dimethyl formamide.     

Electron emission degradation of nano-structured sp^2-bonded amorphous carbon films

The initial field electron emission degradation behaviour of original nano-structured sp^2-bonded amorphous carbon films has been observed, which can be attributed to the increase of the work function of the film in the field emission process analysed using a Fowler-Nordheim plot. The possible reason for the change of work function is suggested to be the desorption of hydrogen from the original hydrogen termination film surface due to field emission current-induced local heating. For the explanation of the emission degradation behaviour of the nano-structured sp2-bonded amorphous carbon film, a cluster model with a series of graphite （0001） basal surfaces has been presented, and the theoretical calculations have been performed to investigate work functions of graphite （0001） surfaces with different hydrogen atom and ion chemisorption sites by using first principles method based on density functional theory-local density approximation.     

Spatial versus quantum confinement in porous amorphous silicon nanostructures

Light-emitting porous amorphous silicon has been produced by anodization in HF of hydrogenated amorphous silicon films. The maximal thickness of the porous films is limited by the onset of an instability which results in the formation of large channels short-circuiting the amorphous layer. This is due to the high resistivity of the amorphous silicon films as compared to that of the electrolyte. Confinement effects on the electron wavefunction are analyzed in situ using photoluminescence measurements in hydrofluoric acid and compared to those observed in porous crystalline silicon. For crystalline silicon, a huge blue shift of the photoluminescence is observable upon reducing the size of the structures by photo-etch, showing clear evidence of quantum confinement effects in this material. No shift has been observed when carrying out the same experiment with amorphous silicon. This indicates that the extent of the wavefunction in the bandtail states involved in luminescence is too small to be sensitive to confinement down to the minimum sizes of our porous material ( 3 nm). Measurements of the width and the temperature dependence of the photoluminescence demonstrate that the Urbach energy does not change upon increasing the porosity, i.e., upon decreasing the size of the a-Si:H nanostructures, in contradiction with what has been reported in ultrathin a-Si:H multilayers. Received: 3 August 1998     

Electron field emission from boron doped microcrystalline diamond

Field emission properties of hot filament chemical vapor deposited boron doped polycrystalline diamond have been studied. Doping level (N_B) of different samples has been varied by the B/C concentration in the gas feed during the growth process and doping saturation has been observed for high B/C ratios. Threshold field (E_th) for electron emission as function of B/C concentration has been measured, and the influences of grain boundaries, doping level and surface morphology on field emission properties have been investigated. Carrier transport through conductive grains and local emission properties of surface sites have been figured out to be two independent limiting effects in respect of field emission. Emitter current densities of 500 nA cm⁻² were obtained using electric fields less than 8 V/μm.     

Electron transport in double-walled carbon nanotubes

The transport properties of finite length double-walled carbon nanotubes subject to the influences of a transverse electric field and a magnetic field with varying polar angles are investigated theoretically. The electrical conductance, thermal conductance and Peltier coefficient dependences on the external fields and symmetric configuration are studied in linear response regime. Prominent peak structures of the electrical conductance are predicted when varying the electric field strength. The features of the conductance peaks are found to be strongly dependent on the external fields and the intertube interactions. The heights of the electrical and thermal conductance peaks display the quantized behavior, while those of the Peltier coefficient do not. The conductance peaks are found to be broadened by the finite temperature.     

Electron field emission from a patterned array of diamond-like carbon on anodic aluminum oxide template

A patterned array of diamond-like carbon (DLC) was grown on anodic aluminum oxide (AAO) template by filtered cathodic arc plasma (FCAP) technique at room temperature. The diameters of patterned array of DLC were ∼150 nm, and the patterned array density was estimated to ∼10⁹ cm⁻². A broad asymmetric band ranging from 1000 cm⁻¹ to 2000 cm⁻¹ was detected by Raman spectrum attributed to characteristic band of DLC. The fraction of sp³ bonded carbon atoms of the patterned array of DLC was measured by X-ray photoelectron spectrum (XPS) and the ratio was about 62.4%. Field emission properties of the patterned array of DLC were investigated. A low turn-on field of 3.4 V/μm at 10 μA/cm² with an emission area of 3.14 mm² was achieved. The results indicated that the electrons were emitted under both the effect of enhanced field because of the geometry and the work function of the DLC sample. Based on Fowler-Nordheim plot, the values of work function for the patterned array of DLC were estimated in range of 0.38 to 1.75 from a linearity plot.     

Mechanism of field emission from carbon materials

Field emission in diamond and graphite-like polycrystalline films is investigated experimentally. It is shown that the emission efficiency increases as the nondiamond carbon phase increases; for graphite-like films the threshold electric field is less than 1.5 V/μm, and at 4 V/μm the emission current reaches 1 mA/cm², while the density of emission centers exceeds 10⁶ cm⁻². A general mechanism explaining the phenomenon of electron field emission from materials containing graphite-like carbon is proposed. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 56–60 (10 July 1998)     

ZnO nanostructures with different morphologies and their field emission properties

Zinc oxide (ZnO) products with the morphologies of balls, nunchakus and belts have been synthesized from aqueous solutions by adjusting the reagent concentration and reaction time. The X-ray diffraction (XRD) peaks of the products were indexed to ZnO materials, but exhibited different relative intensities for the (0 0 2) diffraction peak. Field emission (FE) measurements showed that the turn-on and threshold field for the ZnO nanonunchakus were 3.01 ± 0.005 and 5.47 ± 0.005, 3.71 ± 0.005 and 6.43 ± 0.005 V/μm, respectively, for the ZnO nanobelts, revealing that the products have comparable FE properties with those of the reported ZnO nanowires and carbon nanotubes (CNTs).     

Model calculation and empirical investigation of enhanced field emission behavior of branched carbon nanostructures

In this paper, we propose the novel branched carbon nanotubes (B-CNTs) as efficient candidate for field emission applications. We believe that the double-stage structure of B-CNTs, beside formation of multiple thin branches at the apex of each vertical CNT, is responsible for the observed enhanced field emission behavior in B-CNTs. In this regard, we have derived an analytical model to evaluate the field enhancement factor (β) of the B-CNTs in comparison with CNTs, as the most popular cathode for field emission applications in the scientific society. The presented model also allows investigating the effect of different structural parameters on the field emission characteristic. We have also, compared the field emission characteristics of the B-CNTs with vertical CNTs experimentally. We observed a β value for B-CNTs which was around three times higher than CNTs. The observed enhancement in the experimental data was in good agreement with the presented analytical model.     

Electron field emission properties of conducting polymer coated multi walled carbon nanotubes

The present work describes the field emission characteristics of conducting polymer coated multi walled carbon nanotubes (MWNTs) field emitters fabricated over flexible graphitized carbon cloth. Nanocomposites involving the combination of MWNTs and conducting polymers polyaniline (PANI) and polypyrrole (PPy) have been prepared by in-situ polymerization method and have been characterized using scanning electron microscopy and transmission electron microscopy. Using spin coating method, field emitters based on PANI/MWNTs and PPy/MWNTs over flexible graphitized carbon cloth have been prepared. The field emission characteristics have been studied using an indigenously fabricated set up in a vacuum chamber with a base pressure of 2 × 10⁻⁵ Pa and the results are discussed. Our results display that the field emission performance of the emitters depends strongly on the work function of the emitting material. Low turn on emission field of 2.12 V/μm at 10 μA/cm² and high emission current density of 1 mA/cm² at 3.04 V/μm have been observed for PANI/MWNTs field emitter.     

Electron emission from arrays of carbon nanotubes/fibres

The overall aim of this work is to produce arrays of field emitting microguns, based on carbon nanotubes, which can be utilised in the manufacture of large area field emitting displays, parallel e-beam lithography systems and electron sources for high frequency amplifiers. This paper will describe the work carried out to produce patterned arrays of aligned multiwall carbon nanotubes (MWCNTs) using a dc plasma technique and a Ni catalyst. We will discuss how the density of the carbon nanotube/fibres can be varied by reducing the deposition yield through nickel interaction with a diffusion layer or by direct lithographic patterning of the Ni catalyst to precisely define the position of each nanotube/fibre. Details of the field emission behaviour of the different arrays of MWCNTS will also be presented.