A field emission gun is a type of electron gun in which a sharply-pointed Müller-type emitter is held at several kilovolts negative potential relative to a nearby electrode, so that there is sufficient potential gradient at the emitter surface to cause field electron emission. Emitters are either of cold-cathode type, usually made of single crystal tungsten sharpened to a tip radius of about 100 nm, or of the Schottky type, in which thermionic emission is enhanced by barrier lowering in the presence of a high electric field. Schottky emitters are made by coating a tungsten tip with a layer of zirconium oxide, which has the unusual property of increasing in electrical conductivity at high temperature.
In electron microscopes, a field emission gun is used to produce an electron beam that is smaller in diameter, more coherent and with up to three orders of magnitude greater current density or brightness than can be achieved with conventional thermionic emitters such as tungsten or lanthanum hexaboride (LaB6)-tipped filaments. The result in both scanning and transmission electron microscopy is significantly improved signal-to-noise ratio and spatial resolution, and greatly increased emitter life and reliability compared with thermionic devices.
Famous quotes containing the words field, emission and/or gun:
“Vigil strange I kept on the field one night;
When you my son and my comrade dropt at my side that day,
One look I but gave which your dear eyes return’d with a look I
shall never forget,
One touch of your hand to mine O boy, reach’d up as you lay on the ground,”
—Walt Whitman (1819–1892)
“Approximately 80% of our air pollution stems from hydrocarbons released by vegetation, so let’s not go overboard in setting and enforcing tough emission standards from man-made sources.”
—Ronald Reagan (b. 1911)
“As for fowling, during the last years that I carried a gun my excuse was that I was studying ornithology, and sought only new or rare birds. But I confess that I am now inclined to think that there is a finer way of studying ornithology than this. It requires so much closer attention to the habits of the birds, that, if for that reason only, I have been willing to omit the gun.”
—Henry David Thoreau (1817–1862)