Emission current

Electric drive systems have been tried as a means of achieving propulsion without harmful emissions. Currently, most battery-operated vehicles use... 

As mentioned above, the interpretation of CL cannot be unified under a simple law, and one of the fundamental difficulties involved in luminescence analysis is the lack of information on the competing nonradiative processes present in the material. In addition, the influence of defects, the surface, and various external perturbations (such as temperature, electric field, and stress) have to be taken into account in quantitative CL analysis. All these make the quantification of CL intensities difficult. Correlations between dopant concentrations and such band-shape parameters as the peak energy and the half-width of the CL emission currently are more reliable as means for the quantitative analysis of the carrier concentration. 

Fig. I. Field emission dala from a mounted nanotube. An activated nanotube emits a higher current when heated by the laser than when the laser beam is bloeked (a). When aetivated by exposing the nanotube to oxygen while heating the tip, this behavior is reversed, and the emission current increases dramatically when the laser is blocked. The activated state can also be achieved by laser heating while maintaining a bias voltage of —75 V. Note that the scale of the two plots is different the activated current is always higher than the inactivated current. As discussed in the text, these dala led to the conclusion that the emitting feature is a chain of carbon atoms pulled from a single layer of the nanotube —an atomic wire.

The latter mainly results from the thermal emission current. The dark current is apparent mainly in the long-wavelength range of the spectrum when the photocurrent is appropriately small [53, 54, 131]. It is relatively small for alloy cathodes (e.g. Sb-Cs cathodes), but not small enough to be negligible. 

Fig. 13. Electric-field dependence of the emission current obtained for a carefully aligned MWCNT film [38], Inset Fowler-Nordheim plot, where y is the field-enhancement factor.

It is difficult to measure metal/polymer Schottky energy barriers smaller than about 0.5 eV using internal pholoemission. Small Schotiky energy barriers lead to thermal emission currents produced by the absorption of light in the metal which are difficult to separate from true photocurrents 134]. If the structure is cooled to try to improve this contrast, it is often found that the significant decrease in the electrical transport properties of the polymer [27 [ makes it difficult to measure the internal photoemission current. To overcome this limitation, internal photoemission and built-in potential measurements are combined to measure small Schottky energy barriers, as described below. 

The most likely cause of problems in any MS is associated with the ion source. When a MS problem is discovered, a good rule is to clean the source and check for shorts or a burned-out filament. For most instruments, it is relatively simple to determine that a filament has burned out because the emission current will be zero. Sometimes, the filament will short to the block. In this case, the emission current will read high when in the Cl mode where emission current is read between the filament and the ion source block. In either case, the source should be cleaned and the filament inspected. If the filament is sagging, it is a good idea to replace it. 

FIG. 40. Electron emission current density as a function of applied electric field, for a-C(N) H films with several N contents. (Reproduced from [1. ]. 

For the results of measurements, see Fig. 5.22. Curve 1 describes the electron emission current as a function of the Au concentration on the ZnO surface at a constant concentration of He atoms in the gaseous phase. This dependence is of extremal nature with a linear rise and a maximum peak at the Au concentration of 0.9-10 atoms/cm. A similar curve having its maximum at the same point resulted for Ne atoms. It can be seen from curve 3 that near the emission current maximum, an... 

The dependence of the initial velocity of changes in the electrical conductivity of Au/ZnO being acted upon by He on the surface concentration of Au (curve 2) also has its clear maximum at some point of 2 10 atoms/cm. The fact that peaks of curves / and 2 are abscissaspaced apart almost an order of magnitude engages our attention, i.e. we see that there is no direct correlation between the secondary emission currents and the sensitivity of Au/ZnO film to metastable atoms. 

Field emission is the emission of electrons from a solid under an intense electric field, usually at ambient temperatures. It occurs by the quantum mechanical tunneling of electrons through a potential barrier (Fig. 13.1). This leads to an exponential dependence of emission current density J on the local electric field, as given by the Fowler Nordheim equation,... 

Nilsson et al. [26,27] showed how the total emission current from a less than perfect array of CNT emitters will depend on the distribution of the field enhancement factors, p. The p probability distribution follows a roughly bell-shaped distribution. Clearly, the high p values matter most, so that these follow an effectively exponential decreasing probability distribution (Fig. 13.4),... 

CNTs are also valuable as field emitters because they have a small virtual source size [30], a high brightness, and a small positive temperature coefficient of resistance [31]. The latter means that they can run hot under high emission currents, but not go into thermal runaway. Emission from nanotubes can be visualized by electron holography in a TEM [32],... 

To achieve a more stable mode of operation, in modem instruments the heating current for the filament is emission-controlled, i.e., the current of the electron trap is used to keep emission comparatively independent from actual ion source conditions. Typical emission currents are in the range of 50-400 pA. 

Note The lifetime of a filament is several weeks under proper operating conditions. However, harsh conditions such as aggressive analytes or reagent gases in chemical ionization, too high emission current, and in particular sudden breakdown of the high vacuum have devastating consequences. 

The number of ions i" produced from a gas in the ion source is proportional to the emission current i, to the specific ionization Sggj, to a geometry factor f representing the ionization path inside the ionization source, to the relative ionization probability RlPgaj. and to the partial pressure Pggj. This number of ions produced is, by definition, made equal to the sensitivity Egas times the partial pressure Pg ... 

Fig. 1.36. The topografiner. An instrument developed by Young, Ward, and Scire in the late 1960s, which is the closest ancestor of the STM. (a) The tip is driven by the x and y piezos, and the sample is mounted on the z piezo. By applying a high voltage between the tip and the sample, a field-emission current is induced. Using the field-emission current as the feedback signal, topography of the sample surface is obtained, (b) Close-up of the tip and the sample. The end of the tip has a small radius, typically a few hundred A. The typical tip-sample distance is a few thousand A. (After Young, 1971.)...

The standard method for treating field-emission data is to make a plot of the logarithm of field-emission current In / versus MV or 1/F, then fit the data points with a straight line. This plot is called a Fowler-Nordheim plot (Good and Muller, 1956). 

As seen from Fig. 1.36 and Eq. (1.31), the field-emission current is not very sensitive to the tip-sample distance xq. Using the topografiner, the best vertical resolution is found to be 30 A, and the best horizontal resolution is 4000 A (Young 1971, Young, Ward, and Scire, 1972). 

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