USE OF FIELD EMISSION ELECTRON MICROSCOPE 923 



That the surface has changed detectably is shown by the fact that at room 

 T the emission decreased from 31 to 28 ^la even though V increased from 8840 

 to 8870 volts. This means that the abnormal "bumps" have decreased 

 sHghtly. These effects get progressively more pronounced as the glowing 

 T was increased to 1200°K. The 611 region is now the brightest; the small 

 310 and HI planes are still poor emitters. A continuation of the test showed 

 that the HI plane became normal after one hour at 1300°K. The 310 plane 

 became normal after one hour at 1500°K. After one hour at 1600°K, the 

 pattern looked like normal clean W except that the 100 and 211 planes were 

 larger than in photo a of Fig. 6; it was similar to Fig. 4, photo d, after 

 glowing at 1600°K. 



PART II: EMISSION AND ADSORPTION PROPERTIES 

 OF Ba ON W 



Field Emission from Ba on W 



Figure 7 shows a series of photographs in which successive units or "shots" 

 of Ba were vaporized onto the W point. The geometry of the tube was such 

 that the greatest rate of deposition occurred on the upper right 611 region 

 (Fig. 3b) and tapered off to zero on an "arc" which passes slightly to the 

 left of the upper left 211, central 110, and lower right 211 planes. (Photo f 

 of Fig. 7) In this series a "shot" of Ba was produced by heating the Ba coil 

 with 2.4 amps for one minute. Later calculations will show that one "shot" 

 deposited about 0.5 to 0.7 of a monolayer in the 611 region so that 7 shots 

 deposited 3 to 5 layers in this region and deposited about 1 layer near the 

 "arc" region. 



The first photo shows clean tungsten treated so as to enlarge the 100 and 

 211 planes and to modify the shape of the 110 plane; the remainder of the 

 surface is approximately on a paraboloid. In these latter regions the emis- 

 sion density is nearly uniform. In the 110 plane on the negative, there is a 

 clear but faint ellipse. This elUpse is enhanced by the Ba in photos b, c, and 

 d. We believe this ellipse to be due to the edge of a 110 plane which extends 

 over only part of the larger underlying 110 plane. At this edge the local field 

 is larger than in nearby regions and hence produces slightly greater emis- 

 sions even on clean W. When Ba is deposited on this plane the edge serves as 

 a nucleation center for Ba clusters even at 300°K. Prominent clusters also 

 appear on the edges of the 211 planes in photos b, c, and d. Clusters also 

 appear on the paraboloidal surfaces. The existence of these clusters shows 

 that Ba atoms can move over a short distance — about 200 A — even at room 

 temperature. 



