Electron field emitters

Electron Field Emitters and Field-Effect Transistors... 

In principle, energy-analyzer systems can be designed such that their electron-optical properties do not limit the energy resolution attainable, i. e. their intrinsic energy resolution is much better than the energy width of the primary electron beam, which is of the order of approximately 1.5-2.5 eV for a tungsten hairpin cathode, approximately 1 eV for a LaBg cathode, approximately 0.7 eV for a Schottky field emitter, and 0.3-0.5 eV for a pure cold-field emitter. 

These observations consummated in a growth model that confers on the millions of aligned zone 1 nanotubes the role of field emitters, a role they play so effectively that they are the dominant source of electron injection into the plasma. In response, the plasma structure, in which current flow becomes concentrated above zone 1, enhances and sustains the growth of the field emission source —that is, zone 1 nanotubes. A convection cell is set up in order to allow the inert helium gas, which is swept down by collisions with carbon ions toward zone 1, to return to the plasma. The helium flow carries unreacted carbon feedstock out of zone 1, where it can add to the growing zone 2 nanotubes. In the model, it is the size and spacing of these convection cells in the plasma that determine the spacing of the zone 1 columns in a hexagonal lattice. 

As mentioned above, employment of MWCNT for field emitter will be one of the most important applications of MWCNT. For this purpose, MWCNT is prepared by the chemical purification process [30,38], in which graphite debris and nanoparticles are removed by oxidation with the aid of CuCl2 intercalation [38]. Purified MWCNT is obtained in the form of black and thin "mat" (a flake with thickness of ca. a few hundreds of [im). Figure 7 shows a typical transmission electron microscope (TEM) picture of MWCNT with an open end, which reveals that a cap is etched off and the central cavity is exposed. 

Neutral and charged gold carbonyl species have also been observed on gold field emitter tips upon interaction with CO gas at room temperature in the presence of high electrostatic fields. The adsorbed complexes and the desorption pathways were identified using time-of-flight mass spectroscopy. [(CO)Au] species are more abundant than [Au(CO)2] species. The product distribution was rationalized by DF calculations of the electronic structure of the complexes.291... 

The standard MAT 90 ion source is used for optimized FD/FI mode by means of the newly designed FD/FI probe. Conversion from electron impact (El), chemical ionization (Cl) or fast ion bombardment (FAB) to FD/FI operation does not require the exchange of the ion source. The FD/FI probe accommodates both the field emitter and the extraction electrodes, mounted at the probe tip. Both are introduced as a unit into the ion source through the ionization volume exchange lock without breaking vacuum. The fast and simple changeover illustrates the versatility of the Finnigan MAT 90 with no compromise on the performance. 

Molecules can lose an electron when subjected to a high electric potential resulting in field ionization (FI) [366,534,535]. High fields can be created in an ion source by applying a high voltage between a cathode and an anode called a field emitter. A field emitter consists of a wire covered with microscopic carbon dendrites, which greatly amplify the effective field at the carbon points. 

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],... 

The field emission display held a particular fascination for this field, because of its potentially large market [36,44-49]. Field emission displays (FEDs) are flat panel displays, which are a flat panel equivalent of the cathode ray tube (CRT), but in which each pixel is addressed by its own electron beam from a field emitter, rather than having a beam scanned across it as in the CRT (Fig. 13.8) [44]. The emitters can be diode or triode type. The triode type is the most elegant, the diode type is lower cost. 

To increase printing rates an idea is to develop parallel e-beam lithography [36,50-56]. This would use electrically addressable two-dimensional arrays of electron sources. Each source would be a field emitter inside a CMOS control element. The electron sources in this case are quite complex, having not only grids but also focusing electrodes [36],... 

Cl in conjunction with a direct exposure probe is known as desorption chemical ionization (DCI). [30,89,90] In DCI, the analyte is applied from solution or suspension to the outside of a thin resistively heated wire loop or coil. Then, the analyte is directly exposed to the reagent gas plasma while being rapidly heated at rates of several hundred °C s and to temperatures up to about 1500 °C (Chap. 5.3.2 and Fig. 5.16). The actual shape of the wire, the method how exactly the sample is applied to it, and the heating rate are of importance for the analytical result. [91,92] The rapid heating of the sample plays an important role in promoting molecular species rather than pyrolysis products. [93] A laser can be used to effect extremely fast evaporation from the probe prior to CL [94] In case of nonavailability of a dedicated DCI probe, a field emitter on a field desorption probe (Chap. 8) might serve as a replacement. [30,95] Different from desorption electron ionization (DEI), DCI plays an important role. [92] DCI can be employed to detect arsenic compounds present in the marine and terrestrial environment [96], to determine the sequence distribution of P-hydroxyalkanoate units in bacterial copolyesters [97], to identify additives in polymer extracts [98] and more. [99] Provided appropriate experimental setup, high resolution and accurate mass measurements can also be achieved in DCI mode. [100]... 

In many field emission and field ionization experiments, field strength is a basic parameter which has to be known accurately before a lot of experimental data can be interpreted properly. Determination of field strength at the field emitter surface and field distribution above the field emitter surface in field electron and field ion emission, however, is not an easy task because of the complicated geometry of the tip. In field emission, the validity of the Fowler-Norheim theory has been established experimentally to within about 15%, and the current density as a function of the field has been tabulated.26 Thus it is possible to determine the field strength simply from the field emission current density. The field strength so determined cannot of course be more accurate than 15%. 

Electric fields at field emitter tips, as discussed before, are typically on the order of 10 V ran-1. This is in the same range as the electrostatic fields that are present in zeolite cages (see below) and at the interface of electrode and electrolyte interface. Since these fields are all of the same order as the fields inside atoms and molecules, they are strong enough to induce the rearrangement of electronic orbitals of atoms and molecules. It is therefore expected that it should be feasible to stimulate chemistry with such electric fields. 

ZnO is a wide band gap semiconductor, which is used for various applications. Based on textured ZnO films one can build highly effective piezo field emitters. On the other hand ZnO is a very effective electron-excited phosphor. ZnO films easily withstand electron fluence more than 1 W/cm. ZnO films doped with Al, Ga, or In have a low resistivity of about 10 " Qcm and a high transparency of about 90%. This is sufficient for applications as a front contact in solar cells, liquid crystal displays etc. Dielectric ZnO films have a high electromechanical coupling factor that allow using ZnO in various surface acoustic wave (SAW) devices such as delay lines, delay-line filters, resonators, transducers and SAW convolvers. 

Two classifications of field emitters are used in electron microscopes, cold and Schottky sources (Table 7.2). Whereas the former operates through electron... 

The interaction of CO2 with group VIII metals was reviewed by Solymosi (29). At 80 K, the adsorption on a Rh field emitter exhibits an interesting crystal face dependency 30). In addition to molecular adsorption, dissociation occurs at an appreciable rate on the stepped surfaces around (111) and (100) at temperatures higher than 220 K. The field electron microscopy (FEM) patterns suggest that the surface structure of Rh has a striking influence on the ability of the metal to dissociate the CO2 molecule. From... 

Apart from the promising electrochemical properties that will be exhaustively discussed through this chapter, carbon nanotubes have become a hot research topic due to their outstanding electronic, mechanical, thermal, optical and chemical properties and their biocompatibility. Near- and long-term innovative applications can be foreseen including nanoelectronic and nanoelectromechanical devices, field emitters, probes, sensors and actuators as well as novel materials for mechanical reinforcement, fuel cells, batteries, energy storage, (bio)chemical separation, purification and catalysis [20]. 

Menzel and Corner 147) observed desorption of gases from a tungsten field-emitter tip under slow-electron bombardment. They note that the process is in general inefficient, with a cross section about 10 of that for ionization or excitation in the gas phase. This desorption under bombardment with slow electrons was used by Zingerman and Ishchuk 148) as an analytical technique in a study of adsorption of oxygen on tungsten. Desorption by electron bombardment from copper and stainless steel was reported by Auslender and Minchenkov 148a). 

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