FIELD EMISSION MICROSCOPY 



energy of the oxygen covered substrate. Be- 

 ginning at 1500°K the 111 plane is built up 

 to assume a locally smaller radius of curva- 

 ture. A greatly enhanced electron emission 

 is the result of the local field increment rather 

 than a lowering of work function. Only by 

 heating above 2100°K in high vacuum can 

 all the oxygen be desorbed from a tungsten 

 surface. 



The interpretation of the crystallographic 

 specificity of adsorption patterns has often 

 been attempted on the basis of matching the 

 atomic size of the adsorbate with the spac- 

 ings on the various substrate lattice sites, 

 and also with the number of bonds to next 

 nearest neighbors that can be made (3). 

 \\Tiile this procedure is often successful, it is 

 also found that chemical differences, e.g., 

 the electron structure of the metal plays a 

 role. For instance, tungsten and molybde- 

 num differ only by .6 % in their lattice con- 

 stants. Their clean state field emission pat- 

 terns are identical. Nevertheless their 

 adsorption patterns with oxygen or other 

 adsorbates differ considerably, even when 

 compared at relative equal temperatures 

 matched to their melting points. To make 

 such observations unambiguous, special 

 field emission microscopes have been used 



Fig. 3. Field electron microscope images of in- 

 dividual phthalocyanine molecules adsorped on a 

 tungsten tip in Oil orientation. 



with the two different tips mounted in one 

 vacuum vessel, so that both metals are ex- 

 posed to the same adsorbate gas under 

 identical conditions of pressure, time, tem- 

 perature and possible contaminations. 



The chemisorption of hydrogen is crystal- 

 lographically less specific than of oxygen, 

 probably because of the small size of the 

 adsorbate. Surface migration is possible at 

 temperatures as low as 20°K. The dipole 

 moment of a monolayer at room temperature 

 is such as to make the work function of 

 tmigsten rise by approximately .5 e-volt. At 

 licjuid nitrogen temperature the formation of 

 a second layer with lower work function can 

 be seen. Similar obsen^ations can be made 

 with nitrogen or carbon monoxide on tung- 

 sten. 



Some adsorbates form very specific and 

 characteristic adsorption layers of low elec- 

 tron emission and with ciuite sharp borders 

 when the substrate temperature is raised to 

 allow a rearrangement of the substrate 

 atoms, or maybe the formation of epitaxed 

 compounds. Most characteristic are the 33-1 

 planes caused by carbon on tungsten above 

 1000°K, and the 256 planes developed when 

 a nitrogen film is present above 800°K. Very 

 specific patterns are also obtained with zirco- 

 nium, or the oxides of copper, strontium, 

 beryllium, aluminum, and uranium evapo- 

 rated onto tungsten. 



It appears quite possible that in spite of 

 the limited resolution of the field electron 

 microscope large atoms may become visible 

 individually as blurred scattering disks (1). 

 The granulation of very thin barium films 

 was explained as being due to individual 

 atoms. It is definitely established that some 

 flat organic molecules can be seen individu- 

 ally, although their patterns do not neces- 

 sarily represent the actual shape of the 

 molecule. The image formation mechanism 

 is not yet fully understood. The fourfold 

 symmetric patterns of the 10 by 10 ang- 

 stroms large phthalocyanine molecule, for 

 instance, appear only on tungsten, molybde- 



328 



