Field Emission Patterns Research Articles

Field emission patterns with atomic resolution of single-walled carbon nanotubes by field emission microscopy

Electron emission properties of single-walled carbon nanotubes (SWCNTs) assembled on a tungsten tip were investigated using field emission microscopy (FEM). The transmission electron microscopy (TEM) micrograph confirmed the existence of an SWCNT bundle on the W tip. Under appropriate experimental conditions, a series of FEM patterns with atomic resolution were obtained. These patterns arose possibly from the field emission of the open end of an individual (16,0) SWCNT protruding from the SWCNT bundle. The magnification factor and the resolution under our experimental conditions were calculated theoretically. If the value of the compression factor β was set at β=1.76, the calculated value of the magnification factor was in agreement with the measured value. The resolving power of FEM was determined by the resolution equation given by Gomer. The resolution of 0.277 nm could be achieved under the typical electric field of 5.0×107 V/cm, which was close to the interatomic separation 0.246 nm between carbon atoms along the zigzag edge at the open end for the (16, 0) SWCNT. Consequently, our experimental results were further supported by our theoretical calculation.

Coherent electron emission from carbon nanotubes: asymmetric young's interference fringes

AbstractWe have calculated the Young's interference of two electron beams that are asymmetrical about the emission current and/or emission area and compared them with the experimental results of field emission patterns from multi‐walled carbon nanotubes. Most of the fringes observed from the clean surface were similar to those produced with the symmetric slit system, which corresponds to the fact that asymmetry in intensity does not change the fringe shape. In addition, no interference fringes of the two asymmetrical beams about the area were detected. Molecular chemisorption enhanced the emission current at an emission site, but the electron waves were incoherent to the others. Copyright © 2003 John Wiley & Sons, Ltd.

Work function of single-walled carbon nanotubes determined by field emission microscopy

The field emission characteristics of a single micro-bundle of single-walled carbon nanotubes (SWCNTs) were investigated using field emission microscopy (FEM). Fowler–Nordheim plots revealed that the work function of the SWCNTs was reduced with increasing heating temperature, and reached a minimum value around 1000 °C, assuming that the β factor was constant during the heating process. Field emission patterns also demonstrated fine structures that were believed to be images of the cap of a SWCNT, which was in a clean state. The radius of the SWCNT micro-bundle was measured by transmission electron microscopy (TEM), and the β factor was calculated using two empirical formulae. Then, the work function of the SWCNT was determined from the slope, K, of its Fowler–Nordheim plot. The work function values were Φ1=4.76 eV and Φ2=4.88 eV, respectively.

Watching carbon nanotubes grow

The growth of carbon nanotubes by chemical vapor deposition (CVD) is followed and characterized by performing field-emission microscopy directly in a modified CVD reactor, where the hydrocarbon gas is introduced at a partial pressure below 10−2 mbar and a high voltage is applied between the heated substrate and a phosphor screen. This allows us not only to detect a field-emission current that increases with the length of the nanotubes, but also to observe the growth of the individual emitters by following the evolution of their field-emission patterns on the phosphor screen. Nanotubes grow after an activation time of a few s and reach a length of 5–10 μm in typically 10 s. We deduce a growth rate in the range 1–10 μm/s that increases with the gas pressure.

Atomically resolved field emission patterns of single-walled carbon nanotubes

The electron emission and structural properties of single-walled carbon nanotubes (SWCNTs) were investigated by using field emission microscopy (FEM). The transmission electron microscopy micrograph confirmed the existence of an SWCNT bundle on the W tip. Under appropriate experimental conditions, an FEM image with an elliptic ring-like structure composed of separated bright dots was obtained, a reasonable interpretation of it is that it was produced from the open end for a zigzag (16,0) SWCNT protruding from the SWCNT bundle, each bright dot corresponding to a single atom at the open end. And, if true, this means that the FEM demonstrated 0.2 nm resolution, which was theoretically possible for the assumed geometry. The calculated value of the magnification of the pattern was also consistent with the measured value if the value of the compression factor β was set at 1.76.

Field emission from open ended aluminum nitride nanotubes

This letter reports the field emission measurements from the nanotubes of aluminum nitride which were synthesized by gas phase condensation using the solid-vapor equilibria. A dc arc plasma reactor was used for producing the vapors of aluminum in a reactive nitrogen atmosphere. Nanoparticles and nanotubes of aluminum nitride were first characterized by transmission electron microscope and tube dimensions were found to be varying from 30 to 200 nm in diameter and 500 to 700 nm in length. These tubes were mixed with nanoparticles of size range between 5 and 200 nm in diameter. Tungsten tips coated with these nanoparticles and tubes were used as a field emitter. The field emission patterns display very interesting features consisting of sharp rings which were often found to change their shapes. The patterns are attributed to the open ended nanotubes of aluminum nitride. A few dot patterns corresponding to the nanoparticles were also seen to occur. The Fowler–Nordheim plots were seen to be nonlinear in nature, which reflects the semi-insulating behavior of the emitter. The field enhancement factor is estimated to be 34 500 indicating that the field enhancement due to the nanometric size of the emitter is an important cause for the observed emission.

Coherent electron emission from carbon nanotubes.: Young's interference in the emission patterns

In field emission patterns of multi-walled carbon nanotubes, we have observed interference fringes of electrons emitted through different sites. Main features of the emission patterns are reproduced from Young's interference formula of optics. We point out reasons to observe clear Young's interference fringes in the electron emission patterns.

Young's interference of electrons in field emission patterns.

We have observed interference fringes of electrons in field emission patterns from multiwalled carbon nanotubes at 60 K. The observed fringe pattern is reproduced by calculations based on the formula of Young's interference of two beams. Three-beam interference has also been detected over short time periods. We discuss the reason why Young's interference appears in the electron emission pattern in accelerating fields.

Study on electronic properties of single-wall carbon nanotubes

Single-wall carbon nanotubes (SWCNTs) have been synthesized and purified. Using a water colloid, SWCNTs have been assembled at the end of a tungsten tip that can be used as the scanning tunneling microscope (STM) tip, and atomic resolution images of highly oriented pyrolytic graphite (HOPG) and gold film surfaces have been obtained. The field emission properties of SWCNTs have also been studied exploiting the field emission microscopy (FEM). A field emission pattern of an SWCNT at atomic resolution was observed, which was consistent with the calculated results of the (9,9) armchair SWCNT. Electron transport properties along SWCNTs were investigated, negative differential conductance was observed.

Electron field emission from polymer films treated by a pulsed ultraviolet laser

The poly(phenylcarbyne) polymer films were coated on silicon substrates and then irradiated by a pulsed ultraviolet laser (λ=248 nm) with various fluences (1–60 mJ/cm2) at an atmospheric pressure of nitrogen. The structures of the resulted films were investigated by Raman spectroscopy. The morphologies of the films were examined by scanning electron microscopy (SEM). The electron field emission properties of the films as cathodes were studied. Raman spectrum analysis and SEM results indicate that the polymer film is converted to nanoparticle carbon film with the laser fluence from 10 to 60 mJ/cm2. The conversion mechanism from the polymer to nanoparticle carbon and electron field emission mechanism from the converted carbon film is discussed. The converted carbon film showed better field emission properties, i.e., lower turn-on threshold emission field, higher emission current density, and higher emission light spot density with increasing laser fluence from 10 to 50 mJ/cm2. By increasing the laser fluence from 50 to 60 mJ/cm2, the emission property showed a slight degradation. A turn-on threshold emission field of 1.8 V/μm (at 1 nA/cm2) and emission current density of 30 mA/cm2 with an emission light spot density of 104/cm2 (at 14 V/μm) were observed for the polymer film irradiated with a fluence of 50 mJ/cm2. Circular and multiple-line field emission patterns are demonstrated using the laser irradiation of the poly(phenylcarbyne) polymer.

Faceting of Pt-covered W, Pt-covered Ir, and Pd-covered Mo field emitter tips

The adsorption of ultrathin films of Pt on W, Pt on Ir, and Pd on Mo field emitters has been studied with an emphasis on faceting processes, by means of field electron microscopy (FEM) under UHV conditions. The film thicknesses ranged from fractions of an atomic monolayer to several monolayers. From the analysis of field emission patterns and current–voltage measurements we have found that the steady-state shape of the emitter microcrystal (emitter tip) can be changed by the presence of the metallic adsorbate film (in general a transition occurs from a nearly hemispherical shape to a nearly polyhedron shape). The transition is connected with the faceting of the substrate, which is manifested by changes in size and in electron emission of some planes. The effect is particularly pronounced for the Pt/W and Pd/Mo systems at adsorbate coverages Θ>1ML in the substrate temperature range, roughly, 700–1400K depending on the adsorption system. Under such conditions, planes of the zone type (110)-(101) of the W and Mo crystals (bcc structure) appear as facets, e.g. {112}, {123} and {178}, whereas for Ir (fcc), planes of the (001)-(111) zone appear, e.g. {116} or {115}.

Structural change at the carbon-nanotube tip by field emission

Carbon-nanotube tips are plastically deformed during field emission. High-resolution transmission electron microscopy and structural simulations suggest that the deformed structure of the closed nanotube is explained by heterogeneous nucleation of the pentagonal and heptagonal carbon ring pairs, and that of the opened one is represented by sp3-like line defects in the hexagonal carbon network. It is considered that the changing of the inclination of the Fowler–Nordheim plots corresponds to the structural change in which a tip becomes sharp. The field ion microscope image and the corresponding field-emission pattern suggest that the electron emission from a closed nanotube is not necessarily from pentagonal carbon rings, but from the protrudent carbon network sites on the tip.

Temperature Dependence of Field-Emission Characteristics from a p-Type Si Single Emitter with Real Surface

The temperature dependence of field-emission current from a p-type Si single emitter with a real surface was investigated. The field emission pattern of the emitter did not show any symmetry which indicates a clean crystal structure of the surface; however, the highly nonlinear characteristics in log (I/V2) vs 1/V plots (F-N plots) commonly observed from a clean p-type Si emitter are obtained. The emission current at the gentle slope region of F-N plots (Isat) shows strong temperature dependence at a fixed applied voltage. The slope of log Isat vs 1/T plots gives an activation energy of 0.58 eV. The most probable conclusion is that the Fermi level is pinned at the middle of the band gap of the surface and electrons generated from the surface state to the conduction band are emitted.

Field emission from polymer-converted carbon films by ultraviolet radiation

A polymer, poly(phenylcarbyne) (PPC), was converted to carbon film by ultraviolet pulsed-laser (KrF, λ=248 nm) irradiation in nitrogen atmosphere at room temperature. The irradiated PPC consisted of fine graphitic carbon particles of several tens of nm in size, and exhibited good field emission properties. Low turn-on emission field of 2 V/μm (at 1 nA/cm2), high emission current density of 20 mA/cm2 (at 12 V/μm), and high emission luminescent spots density of 104/cm2 were observed for the polymer film irradiated by a pulsed-laser fluence of 40 mJ/cm2. We demonstrate multiple-lines and 2×5 array field emission patterns using the laser irradiation of PPC.

Effect of non-diamond carbon etching on the field emission property of highly sp 2 bonded nanocrystalline diamond films

The effect of the non-diamond carbon etching on the electron field emission behavior of highly sp 2 bonded nanocrystalline diamond films has been systematically investigated by performing hydrogen treatments. Field emission properties and emission patterns of the films are initially enhanced and then degraded with an increase in the hydrogen treatment time. The most uniform distribution of the emission site is obtained for a 1-min hydrogen-treated film, and the reason for this is explained in terms of the increase of triple junction site due to etching of the sp 2 bonded carbon by atomic hydrogen. On the other hand, the degraded emission property of the films treated for more than 10 min is due to the decrease of triple junction, which is indirectly confirmed by Raman and Auger electron spectroscopy.

A STUDY OF FIELD EMISSION OF SINGLE-WALLED CARBONNANOTUBES USING FIELD EMISSION MICROSCOPY

Using Field Emission Microscopy (FEM), the electron field emission from a bundle of single-walled carbon nanotubes (SWCNT's) was studied under vacuum better than 4.0×10-7Pa after being thermo-desorbed at different temperatures.Work function of the SWCNT's decreases as the temperature of desorption goes up.After heat treatment at 1000℃, the SWCNT's exhibits the lowest work function and their electron field emission pattern also shows a fine structure.As the temperature for thermo-desorption is raised further, the SWCNT's field emission power deteriorates and finally collapse of SWCNT's occurs.

Current saturation mechanisms in carbon nanotube field emitters

Recent studies have shown a current limiting effect in the field emission behavior of carbon nanotubes. In this letter, we demonstrate that an individual nanotube exhibits current saturation above 100 nA of emission current, and we show that this current saturation is a direct result of an adsorbate-enhanced field emission mechanism. Current saturation results from the displacement of adsorbates from configurations of tunneling enhancement as electric field and current are increased. Saturation is concurrent with rapid fluctuations in emission current and distinctive changes in the field emission patterns. At high fields, the adsorbate states are completely removed from the nanotube. A single, clean single-walled nanotube shows no evidence of current saturation for emission currents reaching 2 μA.

Field Emission from Carbon Nanofibers

Carbon nanofibers possess the following properties favorable for field emitters: (1) high aspect ratio, (2) small radius of curvature at their tips, (3) high chemical stability and (4) excellent mechanical strength. Field emission microscopy (FEM) study was carried out using various types of nanofibers including multiwall nanotubes (MWNTs), single-wall nanotubes (SWNTs) and vapor grown carbon fibers (VGCFs). Field emission patterns and current versus voltage characteristics are discussed. As an application of nanotube field emitters, field emission displays (FED) equipped with nanotube cathodes are now attracting considerable attention. Fabrication and performance of nanotube-based FED elements are briefly reviewed.

New Application of Carbon Nanotubes. Field Emission of Electrons and Field Evaporation of C20 from Carbon Nanotubes.

Electron emission from various types of nanotubes, e.g., multiwall nanotubes (MWNTs) with capped and open ends, and single-wall nanotubes (SWNTs), was investigated by field emission microscopy (FEM). Field emission patterns originating from the pentagons placed on a cap of a nanotube were observed for the capped MWNTs with clean surfaces. Emission current versus voltage characteristics revealed that open MWNTs possess the most excellent properties as field emitters among the nanotubes investigated. As an application of nanotube field emitters, field emission displays (FED) equipped with nanotube cathodes are now attracting considerable attention. Fabrication and performance of nanotube-based FED elements are also reviewed briefly. When a positive electric potential, being an opposite potential to the case of electron emission, was applied to nanotubes, carbon clusters were directly emitted from nanotubes through field evaporation. We found that C20+ were selectively emitted from both MWNTs and SWNTs under an extremely high field over 10 V/nm.

Field emission from carbon nanotubes and its application to electron sources

Carbon nanotubes possess the following properties favorable for field emitters: (1) high aspect ratio, (2) small radius of curvature at their tips, (3) high chemical stability and (4) high mechanical strength. A field emission microscopy (FEM) study was carried out for both multiwall nanotubes (MWNTs) and single-wall nanotubes (SWNTs) produced by arc discharge between carbon electrodes. Field emission patterns as well as current versus voltage characteristics and Fowler–Nordheim plots are discussed. As an application of nanotube field emitters, we manufactured cathode-ray tube (CRT) type lighting-elements and vacuum-fluorescence display (VFD) panels. In both display elements, conventional thermionic cathodes were replaced with MWNT field emitters. Stable electron emission, adequate luminance, and long life of the emitters was demonstrated.