Thermal-field emitters

Such brightness is available from thermal-field emitters, and workers are using them in prototype high throughput systems (39), but the ir reliability at the high total currents needed to produce a final beam current of several microamps is not proven for production environments. 

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-... 

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],... 

Directly across the deposition chamber from the FEM tip was a fluorescent screen and viewport with which the field emission pattern from the tip could be observed during cluster deposition. The tip was first cleaned by joule heating. The two fold symmetry characteristic of a clean W(110) field emitter was observed and the voltage necessary to observe this pattern was recorded. The voltage was then reduced to about 1/2 of the original value and the tip was exposed to clusters that had been slowed to thermal speeds in the gas cell. Clusters landing near the apex of the tip appear as bright dots on the fluorescent screen. This procedure was repeated until an individual cluster positioned near the apex of the tip was obtained. 

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]. 

The electron field emission photograph of a tungsten point formed by field evaporation at low temperatures, shown in Fig. 58, is dominated by the extensive nonemitting (110) plane. The emission comes primarily from the higher index planes around the 100 s with a much smaller contribution from the triangular llljs. The 100 poles, as well as the (211 s, are only ill defined. This is in contrast to the usual thermally shaped emitters, in which the 100, as well as 211 poles appear as definite and prominent areas of low emission. 

Thermionic, thermal-field, and FE zone boundary for an emitter with the WS of 4.5 eV. In addition, FE zones with WSs of 4.0 and 5.0 eV are presented. The down arrow on the x axis indicates the field at which the I-T curve was measured. (Reprinted from Carbon, 43(13), Lim, S.C. et al.. Extracting independently the work function and field enhancement factor from thermal-field emission of multi-walled carbon nanotube tips, 2801-2807, copyright (2005), with permission from Elsevier.)... 

Similarly, the field emitter can be adapted by reducing the work function. The practical device is called the Schottky field emission gun, but it is really a field-enhanced thermionic emitter. The sharp tip is coated with zirconium oxide. The field is reduced by flattening the tip and acts to reduce the potential barrier, but the electrons are thermally excited over the low barrier, not sucked through it. 

For nonvolatile or thermally labile samples, a solution of the substance to be examined is applied to the emitter electrode by means of a microsyringe outside the ion source. After evaporation of the solvent, the emitter is put into the ion source and the ionizing voltage is applied. By this means, thermally labile substances, such as peptides, sugars, nucleosides, and so on, can be examined easily and provide excellent molecular mass information. Although still FI, this last ionization is referred to specifically as field desorption (FD). A comparison of FI and FD spectra of D-glucose is shown in Figure 5.6. 

Field desorption. The formation of ions in the gas phase from a material deposited on a solid surface (known as an emitter) that is placed in a high electrical field. Field desorption is an ambiguous term because it implies that the electric field desorbs a material as an ion from some kind of emitter on which the material is deposited. There is growing evidence that some of the ions formed are due to thermal ionization and some to field ionization of material... 

As field desorption (FD) refers to an experimental procedure in which a solution of the sample is deposited on the emitter wire situated at the tip of the FD insertion probe, it is suited for handling lubricants as well as polymer/additive dissolutions (without precipitation of the polymer or separation of the additive components). Field desorption is especially appropriate for analysis of thermally labile and high-MW samples. Considering that FD has a reputation of being difficult to operate and time consuming, and in view of recent competition with laser desorption methods, this is probably the reason that FD applications of polymer/additive dissolutions are not frequently being considered by experimentalists. 

Hunt, D.F. Shabanowitz, J. Botz, F.K. CI-MS of Salts and Thermally Labile Organics With Field Desorption Emitters as Solids Probes. Anal. Chem. 1977, 49, 1160-1163. 

Example D-Glucose may be evaporated into the ion source without complete decomposition as demonstrated by its FI spectrum (Fig. 8.13). FD yields a spectrum with a very low degree of fragmentation that is most probably caused by the need for slight heating of the emitter. The occurrence of ions, m/z 180, and [Mh-H]" ions, m/z 181, in the FD spectrum suggests that ion formation occurs via field ionization and field-induced proton transfer, respectively. However, thermal... 

There are of course many other similarities and differences, and some of them are listed in Table 5.1 without further explanations. In general, STM is very versatile and flexible. Especially with the development of the atomic force microscope (AFM), materials of poor electrical conductivity can also be imaged. There is the potential of many important applications. A critically important factor in STM and AFM is the characterization of the probing tip, which can of course be done with the FIM. FIM, with its ability to field evaporate surface atoms and surface layers one by one, and the capability of single atom chemical analysis with the atom-probe FIM (APFIM), also finds many applications, especially in chemical analysis of materials on a sub-nanometer scale. It should be possible to develop an STM-FIM-APFIM system where the sample to be scanned in STM is itself an FIM tip so that the sample can either be thermally treated or be field evaporated to reach into the bulk or to reach to an interface inside the sample. After the emitter surface is scanned for its atomic structure, it can be mass analyzed in the atom-probe for one atomic layer,... 

---