Field emission electron sources

In order to increase the intensity and focus of electrons, a field emission source may be used. This consists of a single crystal tungsten or LaBeClOO) wire that is sharpened to a tip diameter of ca. 100 nm — 1 j,m. For crystalline tungsten, the axis is suitably aligned with respect to the optical axis of the microscope. For example, a beam with a diameter < 5 nm is possible from alignment of the filament planes perpendicular to (310) and (111). In addition to W and LaBe, a number of other materials are proposed for field emission applications, such as silicon, single-walled nanotubes,and ultrananocrystalline diamond (UNCD) or Cu/Li alloy films deposited onto sharpened tips.t ... 

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... 

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. 

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... 

The STEM instrument itself can produce highly focused high-intensity beams down to 2 A if a field-emission source is used. Such an instrument provides a higher spatial resolution compositional analysis than any other widely used technique, but to capitalize on this requires very thin samples, as stated above. EELS and EDS are the two composition techniques usually found on a STEM, but CL, and even AES are sometimes incorporated. In addition simultaneous crystallographic information can be provided by diffraction, as in the TEM, but with 100 times better spatial resolution. The combination of diffraction techniques and analysis techniques in a TEM or STEM is termed Analytical Electron Microscopy, AEM. A well-equipped analytical TEM or STEM costs well over 1,000,000. 

There are three types of instruments that provide STEM imaging and analysis to various degrees the TEM/STEM, in which a TEM instrument is modified to operate in STEM mode the SEM/STEM, which is a SEM instrument with STEM imaging capabilities and dedicated STEM instruments that are built expressly for STEM operation. The STEM modes of TEM/STEM and SEM/STEM instruments provide useful information to supplement the main TEM and SEM modes, but only the dedicated STEM with a field emission electron source can provide the highest resolution and elemental sensitivity. 

Future developments of this instrumentation include field emission electron sources at 200-300 kV that will allow better elemental detectability and better spatial resolution. Multiple X-ray detectors having large collection angles will also improve elemental detectability in X-ray microanalysis. The higher accelerating... 

Future trends will include studies of grain-dependent surface adsorption phenomena, such as gas-solid reactions and surface segregation. More frequent use of the element-specific CEELS version of REELM to complement SAM in probing the conduction-band density of states should occur. As commercially available SAM instruments improve their spot sizes, especially at low Eq with field emission sources, REELM will be possible at lateral resolutions approaching 10 nm without back scattered electron problems. 

Scanning Auger Electron Spectroscopy (SAM) and SIMS (in microprobe or microscope modes). SAM is the most widespread technique, but generally is considered to be of lesser sensitivity than SIMS, at least for spatial resolutions (defined by primary beam diameter d) of approximately 0.1 im. However, with a field emission electron source, SAM can achieve sensitivities tanging from 0.3% at. to 3% at. for Pranging from 1000 A to 300 A, respectively, which is competitive with the best ion microprobes. Even with competitive sensitivity, though, SAM can be very problematic for insulators and electron-sensitive materials. 

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. 

Combined Electron Impact and Field Emission Source. ... 

Fig 7 Combined electron impact and field emission source. 

Field Emission Electron Sources. The strength of an electric field E is increased at sharp points of radius r, and if a voltage V is applied to such a spherical point, the field at the surface is given by ... 

Specimens for field emission sources are of a very fine needle shape, usually in the form of tungsten wire with a tip radius of <0.1 pm (Figure 5.4). Application of a potential of lkV thus generates a field of 106V/m which lowers the work function barrier sufficiently for electrons to tunnel out of the tungsten. FEG electron microscopes usually employ a gun potential of 3-4 keV. 

An electron gun (see Section 5.1.3) provides the requisite electron source for AES, and may consist of a tungsten or a LaB6 cathode, or a Field Emission source. The latter provide the brightest beams, and beam widths as narrow as lOnm permit... 

Field emission displays are VFDs that use field emission cathodes as the electron source. The cathodes can be molybdenum microtips,33-35 carbon films,36,37 carbon nanotubes,38" 16 diamond tips,47 or other nanoscale-emitting materials.48 Niobium silicide applied as a protective layer on silicon tip field emission arrays has been claimed to improve the emission efficiency and stability.49 ZnO Zn is used in monochrome field emission device (FED) displays but its disadvantage is that it saturates at over 200 V.29... 

De Heer, W. A., Chatelain, A., and Ugarte, D. 1995. A carbon nanotube field-emission electron source. Science 270 1179-1180. 

The second result relates to an electronation procedure in which field emission into liquids provides a source of low energy electrons which are capable of effecting selective chemical changes (Noda et al., 1979). Thus dissociative electron capture by alkyl halides, and electron capture followed by protonation, afford alternative routes to specific spin adducts, as exemplified in Scheme 8. 

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