Daniel Alberto Patino Villagomez
Field Emission Microscope (FEM)
The phenomenon of field emission was used to develop a microscope on the basis of the difference in
workfunction of the various crystal planes on the surface. The emitter is made in the form of a sharp
"tip" producing intense electric field around it. The electric field at the apex of the tip is inversely
proportional to the radius if the tip.
The Field emitted electrons travel along the field lines and producing bright and dark patches on the
flouroscent screen giving a one-to-one correspondence with the crystal planes of the hemispherical
emitter. In short, the workfunction anisotropy of the crystal planes are mapped onto the screen as
intensity variations. This acts as a microscope without any lenses. This microscope having a
magnification of 105 and a resolving power 30 Angstroms is known as Field Emission Microscope
(FEM). This apparatus is suitable for studying adsorption, Surface and volume diffusion and
desorption. This has played a major role in understanding the structure and properties of solid surfaces
since 1940s
.
(Field emission Micrographs of a clean tungsten single crystal surface
and a surface with a grain boundary)
Probe-Hole Field Emission Microscope (FEM)
This is a modification of the field emission microscope to study the electron emission from selected
crystallographic planes. The anode screen on which the field emission pattern is observed is provided
with a hole of 2mm diameter with a Faraday cup collector behind it. With magnetic deflection of the
pattern one can bring the single crystal planes of interest to coincide with the hole. The electrons thus
falling on the collector are necessarily the ones coming from the probed area. Since the tip to screen
distance is about 4-5 cm and the magnification is roughly 106, the probed area on the surface will be
few nanometers, which is well within the size of most of the crystal planes observed in the field
emission pattern.
We have been working on all-metal and all-glass field emission setups.
Probe-hole field emission tube fabricated for the use of our laboratory.
Applications
Surface Science (Electronic and Structural aspects)
Field emission has been extensively used in the characterization of surface structures and electronic
properties. This technique has given a wealth of information and understanding of surfaces, metal and
gas interface systems before the advent of many other techniques for surface analysis. Adsorption of
atoms in the submonolayer, monolayer amounts of adatoms, desorption and surface diffusion
measurements can be carried out using field emission techniques. The analysis of field emission current
fluctuation can yield quantitative information regarding the surface phenomena occurring on the
emitter
When the metal under study is the cathode for field emission, its surface condition particularly its
surface cleanliness can be judged from the emission pattern. At this point, it becomes clear that role of
Ultra High Vacuum is extremely prominent in field emission experiments, as it has an extreme surface
sensitivity to change the emission pattern even in the submonolayer regime. A base pressure less than
10-10 mbar needs to be maintained during the experiments. Also since, field emission occurs from all
single crystal facets, field emission microscopy is best suited to study these facets (crystal planes) and
to compare the results under identical conditions in single experiments. These features make the
relevance of results of field emission microscopy studies important and unique among other standard
surface analytical tools.
Technologocal Applications (Field Emitters as Cathodes)
Apart from the point of view of the surface physics, field emission at present, has gained a different
importance in technology. Field emitters can be used as cathodes for electron emission applications
because of its superior emission properties. The constraints arising from the stringent requirements of
vacuum and electric field kept this field behind the forefront over a long period. Currently with the
advent of the latest technologies, the problems have been overcome with suitable modification of the
emitters. Nickel deposited tungsten has been found to be a better emitter from the recently reported
research work.
Fuentes
http://physics.unipune.ernet.in/~fem/intro-fem.htm