Field-emission

Contrary to other field emission displays that consist of arrays of tips generated by mechanical means, carbon nanotubes are stable up to much higher field intensities, while they emit already at a rather low field of 1V(xm . They can also stand high current densities of more than 1 Acm l Both SWNTs and MWNTs are suitable for the production of field emission devices. The actual absence of defects is a more decisive parameter here. It correlates well with the efficiency of emission. MWNTs are more robust on long-term use, whereas for SWNTs, the small diameter of individual tubes is an attractive feature. 

Application of a voltage, U, to a diode consisting of a tip electrode and a plane electrode produces an inhomogeneous electric field, the strength of which increases quickly as we approach r, the radius of curvature of the tip. If the electrode separation R is large compared to r, the two electrodes can be considered as sections of two concentric spheres and the electric field strength E(r) is given by 

Fine tips can be manufactured by electrochemical etching of tungsten wire. Typically, tip radii of 1 pm can be obtained with no difficulty at all. Application of 100 V leads to an electric field strength at the tip of 1 MV/cm. This is the order of magnitude where field emission from metals becomes noticeable. In the initiation of electric breakdown, this process is of paramount importance. 

Field emission of electrons into vacuum is described by the Fowler-Nordheim equation which gives the current density, j (A/cm ), as a function of the electric field strength at the metal surface, E (V/cm), such that 

Substitution of Equations 55 and 56 into Equation 54 yields, for the emission current. 

Here E is in V/cm, O in eV, A in cm, and i in A. Usually, log (iem/U ) is plotted as a function of (1/U) which yields a straight line, indicating that the functions g(y) and f(y) are slowly varying with E. g(y) is practically unity. f(y) describes the influence of the image charge on the emission process it decreases with increasing E. In liquids, the influence of the image force is reduced by l/e,. 

It is empirically known that as grown diamond films do not exhibit a charge-up under SEM observation, while it is not so for single erystal diamonds. This is most likely because of the effect of surface conduction by H-termination. It is also known that H-terminated undoped and B-doped diamonds exhibit negative electron affinity (NEA), where the vacuum level is energetically lower than the conduction band. It is thus expected that onee electrons are excited to the conduction band of diamond, they are spontaneously emitted to the vacuum presumably across a small barrier at the surface. 

Field emission from a heteroepitaxial undoped (lOO)-oriented diamond film of 20-[rm thickness deposited on Ir(lOO) substrate was investigated in Ref [441] using a W needle as an anode. As compared with a polycrystalline diamond film also deposited on Ir(lOO), the heteroepitaxial film emitted electrons at lower voltage, roughly 1/3 of the voltage needed for the polycrystalline film. The emission was uniform over the entire area of 3 mm in diameter. The threshold voltage when the emission current was 10 A was estimated to be 40 V/pm, which was fairly high, because the film surface was flat and the film was thick. 

Field emission measurements were also done using a B-doped, heteroepitaxial, (lOO)-oriented diamond film grown on Ir(lOO) [442]. The observed results are 

Among diamond films with various morphologies, HOD and heteroepitaxial diamond films will provide us with the best material for highest value-added, high performance applications. In this sense, the works reviewed in this monograph have given a great contribution. 

However, we still have to establish several technologies on heteroepitaxial growth of diamond, as listed below  

The density of current flowing from the semiconductor to the metal is proportional to the product of the transmission coefficient T( ), the occupation probability in the semiconductor s, and the unoccupation probability in the metal, 1 —fm  

The total density of current is simply the sum of the density of current flowing in both directions and can be approximated by 

1) The occupation probability depicts the likelihood of a state being occupied by an electron, and one minus the occupation probability exhibits that to be free of electrons. 

Padovani and Stratton [14] presented an analytical expression for the forward current for direct tunneling as 

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Nelson et al. [34] determined from void shapes that the ratio 7100/7110 was 1.2, 0.98 and 1.14 for copper at 600°C, aluminum at 550°C, and molybdenum at 2000°C, respectively, and 1.03 for 7100/7111 for aluminum at 450°C. Metal tips in field emission studies (see Section VIII-2C) tend to take on an equilibrium faceting into shapes agreeing fairly well with calculations [133]. 

The field emission microscope (FEM), invented in 1936 by Muller [59, 60], has provided major advances in the structural study of surfaces. The subject is highly developed and has been reviewed by several groups [2, 61, 62], and only a selective, introductory presentation is given here. Some aspects related to chemisorption are discussed in Chapter XVII. 

While field ion microscopy has provided an effective means to visualize surface atoms and adsorbates, field emission is the preferred technique for measurement of the energetic properties of the surface. The effect of an applied field on the rate of electron emission was described by Fowler and Nordheim [65] and is shown schematically in Fig. Vlll 5. In the absence of a field, a barrier corresponding to the thermionic work function, prevents electrons from escaping from the Fermi level. An applied field, reduces this barrier to 4> - F, where the potential V decreases linearly with distance according to V = xF. Quantum-mechanical tunneling is now possible through this finite barrier, and the solufion for an electron in a finite potential box gives... 

FEM Field emission microscopy [62, 101, 102] Electrons are emitted from a tip in a high field Surface structure... 

R. Gomer, Field Emission and Field Ionization, Harvard University Press, Cambridge, MA, 1961. 

Mobility of this second kind is illustrated in Fig. XVIII-14, which shows NO molecules diffusing around on terraces with intervals of being trapped at steps. Surface diffusion can be seen in field emission microscopy (FEM) and can be measured by observing the growth rate of patches or fluctuations in emission from a small area [136,138] (see Section V111-2C), field ion microscopy [138], Auger and work function measurements, and laser-induced desorption... 

A catalyst may play an active role in a different sense. There are interesting temporal oscillations in the rate of the Pt-catalyzed oxidation of CO. Ertl and coworkers have related the effect to back-and-forth transitions between Pt surface structures [220] (note Fig. XVI-8). See also Ref. 221 and citations therein. More recently Ertl and co-workers have produced spiral as well as plane waves of surface reconstruction in this system [222] as well as reconstruction waves on the Pt tip of a field emission microscope as the reaction of H2 with O2 to form water occurred [223]. Theoretical simulations of these types of effects have been reviewed [224]. 

Figure Bl.17.8. Iron oxide particles coated with 4 nm of Pt in an m-planar magnetron sputter coater (Hennann and Mtiller 1991). Micrographs were taken in a Hitachi S-900 in-lens field emission SEM at 30,000 primary magnification and an acceleration voltage of 30 kV. Image width is 2163 nm.

Crewe A V, Eggenberger D N, Wall J and Welter L M 1968 Electron gun using a field emission source Rev. Sol. Instrum. 39 576-86... 

A low-energy electron beam can also be obtained using a field emission tip and used in the field emission retarding-potential method. This combination provides an absolute measure of the sample work fiinction and the resolution is excellent [52]. 

Gomer R 1994 Field emission, field ionization, and field desorption Surf. Sc/299/300 129-52... 

Todd C J and Rhodin T N 1974 Adsorption of single alkali atoms on tungsten using field emission and field desorption Surf. Sc/. 42 109-21... 

One important sem source that is not based on thermionic emission is the field emission (fe) source. Fe-sem systems typically give images of much higher resolution than conventional sems due to the much narrower energy distribution (on the order of 0.25 eV) of the primary electron beam. A fe source is a pointed W tip from which electrons tunnel under the influence of a large electric field. This different mechanism of electron generation also results in a brightness comparable to a conventional thermionic source with much less current. 

The source requited for aes is an electron gun similar to that described above for electron microscopy. The most common electron source is thermionic in nature with a W filament which is heated to cause electrons to overcome its work function. The electron flux in these sources is generally proportional to the square of the temperature. Thermionic electron guns are routinely used, because they ate robust and tehable. An alternative choice of electron gun is the field emission source which uses a large electric field to overcome the work function barrier. Field emission sources ate typically of higher brightness than the thermionic sources, because the electron emission is concentrated to the small area of the field emission tip. Focusing in both of these sources is done by electrostatic lenses. Today s thermionic sources typically produce spot sizes on the order of 0.2—0.5 p.m with beam currents of 10 A at 10 keV. If field emission sources ate used, spot sizes down to ca 10—50 nm can be achieved. 

Hafnium carbide [12069-85-1] can be used as surface coating on cemented-carbide cutting tools, shows promise as a stable field emission cathode... 

Fig. 2. Behavior of electron-field emission at room temperature from Spindt-type arrays of 5000 tips per mm, beginning and ending with ultrahigh vacuum (UHV), eg, ultracontrol (UC) (a) water (b) hydrogen and (c) oxygen, where the dashed line indicates noise. To convert Pa to torr, divide by 133.3.

Figure 3 Photograph of a modern field emission SEM. (Courtesy of AMRAY Inc., Bedford, MA)...

There are three major types of electron sources thermionic tungsten, LaBg, and hot and cold field emission. In the first case, a tungsten filament is heated to allow... 

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