FIKLD EMISSION MICROSCOPY 



Field electron microscopy of clean metal 

 surfaces also provides information on the 

 rate of surface migration as a function of 

 temperature, so that activation energies for 

 various surface sites can be derived from 

 Arrhenius plots. Changes in tip geometry- 

 due to surface migration have been observed 

 at temperatures as low as one quarter of the 

 absolute melting temperature. 



Adsorption Studies 



Adsorption plays a basic role in many sur- 

 face phenomena, such as gas-solid reactions, 

 catalysis and corrosion. Both general types 

 of adsorption, physical adsorption and 

 chemisorption can be studied with the 

 methods of field electron microscopy, ^^'hich 

 provide the following special features: The 

 specificity of various crystal planes is immedi- 

 ately recognized. Low degrees of coverage 

 down to a small fraction of a monoatomic 

 layer are detected, and the particular stabil- 

 ity of monolayers or multilayers is readily 

 observed. The same surface can be studied 

 in an extremely wide temperature range, 

 from liquid helium temperature to above 

 2000°K. 



j\Iost of the adsorption studies have been 

 made with tungsten tips. The alkali and 

 earth alkali metals develop dipole layers 

 which lower the work function considerably, 

 thereby increasing the current density at a 

 gi^'en field strength. Surface migration of 

 these adsorption films can be measured as a 

 function of tip temperature, crystallographic 

 direction, degree of coverage, and applied 

 field. The conventional technique is to con- 

 dense the adsorbate on one side of the tip 

 from an evaporation source, and then to ob- 

 serve the spreading of the film under various 

 conditions. Physically adsorbed single and 

 sometimes multilayer films of the rare gases, 

 easily observed at low temperatures, also 

 lower the work function. 



The common gases all decrease the elec- 

 tron emission by forming dipole layers with 

 the negative end directed away from the sur- 



FiG. 2. Field electron microscope pattern of an 

 iron tip in 001 orientation with an adsorption film 

 of CO on it. 



face. Oxygen has the greatest effect, a 

 monolayer of which may reduce the emission 

 at a fixed field strength by more than .six 

 orders of magnitude. Since the inception of 

 field emission microscopy oxygen on tung- 

 sten has been the subject of a considerable 

 number of investigations, as the adsorption 

 of oxygen and the initial stages of oxidation 

 are of such great importance. If the micro- 

 scope is immersed in a bath of liquid helium, 

 (2) oxygen can be condensed on one side of 

 the tip only, just as the deposits of electro- 

 positive metals at room temperature, and the 

 same type of surface migration experiments 

 can be made by heating the tip gently. A 

 very thin film of oxygen shows mobility only 

 above 450°K, while a film thicker than a 

 monolayer can migrate already at 40°K. 

 Heating of an oxygen covered tungsten tip 

 to 700 or 1000°K produces small oxide 

 crystals', which spread out again at higher 

 temperature. In the range from 1000 to 

 2000°K the field emission pattern of oxygen 

 on tungsten goes through several quite dis- 

 tinct stages, while desorption is taking place. 

 Concurrent with the desorption is a deforma- 

 tion of the timgsten surface. Rearrangement 

 of tungsten atoms takes place because of the 

 crystallographic specificity of the free surface 



327 



