Field emitter

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. 

Eigure 4.30 is an example of X-ray mapping of an (In,Ga)As quantum wire structure using a TEM/STEM Philips GM20 equipped with a thermally-assisted field-emitter and a Ge EDXS detector (Tracor Northern) [4.124]. The cross-section STEM bright-... 

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. 

AIN, GaN, and InN are attractive materials for applications such as blue lasers and field emitters single-source precursors for these of formula [R2MNR 2]2 (R = alkyl, R = alkyl or H) have been reported.236 The reaction of alkylamines with group 13 trialkyl metal compounds affords oligomeric or polymeric ring and cage structures of metal amides and imides (see section on nitrides). 

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... 

Helical (carbon) nanocoils have received attention recently255-257 for their properties as field emitter and thus for the fabrication of flat panel field emission displays. The properties depend on the electrical field generated due to the helical nanostructure, and to the characteristics of the tip of the carbon nanocoil. In fact, carbon... 

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. 

A final point about CNT field emitters is that MWNTS are preferred to SWNTs [25]. Nanotubes do have a very high Young s modulus in tension. But their small diameter of SWNTs means that they are quite flexible and can bend easily. Their stiffness varies as the moment of inertia, or diameter to the fourth power. MWNTs are much stiffer, and this is more useful for emitter structures. It is not useful, if CNTs bend over under various influences such as applied fields or vibrations, and touch adjacent emitters. They should remain rigidly in their designed position. 

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],... 

Fig. 13.7 Carbon nanotube field emitter grown on metal tip (courtesy M. Mann).

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]... 

Note The field anode is usually referred to as field emitter, FI emitter, or FD emitter. The properties of the field emitter are of key importance for FI- and FD-MS. The electrode on the opposite side is called field cathode or simply counter electrode. 

Note In FI-MS, the ionization efficiency is very low, because of the low probability for a neutral effusing from any inlet system towards the field emitter to come close enough to the whiskers. Consequently, FI-MS produces very low ion currents. The application of FI-MS is therefore restricted to samples that are too volatile for FD-MS or require gas chromatographic separation before. 

Furthermore, the radicals formed upon field-induced hydrogen abstraction can lead to polymerization products on the emitter surface. The mechanism of this field polymerization helped to elucidate the phenomenon of activation of field emitters, i.e., the growth of microneedles on the emitter surface under the conditions of field ionization of certain polar organic compounds. [59]... 

Swanson, L. W., and Grouser, L. C. (1966). Anomalous total energy distribution for a tungsten field emitter. Phys. Rev. Lett. 16, 389-392. 

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%. 

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