- Email: [email protected]
A method for the continuous observation of the oxygen adsorption phenomena in the field ion microscope
SURFACE
SCIENCE
A METHOD
24 (1971) 663-666
8 North-Holland
FOR THE CONTINUOUS
THE OXYGEN
ADSORPTION
Publishing Co.
OBSERVATION
PHENOMENA
OF
IN THE
FIELD ION MICROSCOPE Received 4 September 1970
The oxygen adsorption phenomena on a tungsten surface in a Field Ion Microscope were continuously observed with He or Ne as imaging gases and recorded in a motion picture. The FIM tube was a demountable all-glass tube and oxygen was introduced through a silver diffusion leak. After several times of oxygen flushing at a pressure of about 10T3 Torr, the tube was evacuated to a pressure of 8.0 x lo-’ Torr and then oxygen was introduced to a pressure of 2.5 x lo-’ Torr. The tip temperature was kept at 77°K. For continuous observation, an image intensifier which reduced the exposure time by a factor of about 10’ was used. As the image was dim, however, it could not be recorded in a motion picture with a usual tip. To obtain the bright image and at the same time to keep the resolution good, it was found to be practical to use the tip whose top apex was torn off by applying a strong electric field. By correcting the shape of this tip with a field evaporation method, the bright and finely resolved image could be obtained at a high best image voltage, e.g. 20 kV. The motion picture was taken at the rate of 12 frames per set and the exposure time for a frame was & sec. When the applied voltage was adjusted suitably, we could observe the bright spots resulted from the oxygen adsorption. Though it was not clear what species was the origin of the bright spots, some of the bright spots showed the interesting motion as shown in fig. 1: they changed their sizes, migrated, separated or combined. The separated spots also moved and changed their sizes. These may indicate the interaction between the adsorbates other than the interaction between the adsorbate and the substrate. In some cases, a bright spot became dim as if it dispersed, and afterwards in the neighborhood a spot or two appeared. The other spots stayed at the same place for a short time and then disappeared. These bright spots were observed on the planes on the [llO] zone lines except on the (110) plane and its terraces. The bright spots on the (110) plane and its terraces were observed for a short time and hardly moved. This corresponds to the small sticking probability of oxygen on the (110) planer). 663
C
d
e
f
Fig. 1 a-f
CONTINUOUS
OBSERVATION
OF THE OXYGEN
ADSORPTION
PHENOMENA
665
Fig. 1 g-i
Fig. 1. The motion of the bright spots on a tungsten surface. These photographs were taken from a motion picture and show the images observed at intervals of r/la set in order. The imaging gas was Ne. The interesting changes can be seen on the (111) plane and on the region between the (111) and the (211) planes. Among the three spots on the (111) plane shown in (c), the size of the two lower spots reversed in (d) and after s/12 set a large spot grew at the same place as shown in (i). On the region between the (111) and the (211) planes, the migration and the coalescence of the spots were observed.
The motion of the bright spots was affected by the electric field on the surface. When it was strengthened adequately, the bright spots became to cease the motion and were observed for a longer time because the adsorbed oxygen atoms were prevented to desorb spontaneously. This may be due to the increase of the adsorption energy by the polarization effect2). Whatever was the species which made the bright spots, these images imply the local changes of the field strength and/or the electron distribution on the
666
EIZI
SUGATA
AND
SAKAE
ISHII
surface3). Since it is reportedd) that the predominant species that gave rise to individual spots were identified to be single tungsten atoms, the bright spots in the present paper should be examined by a mass spectrometer41 5) for the further discussion. When comparing the image of the substrate tungsten atoms obtained after the disappearance of the bright spots which resulted from the adsorption with the image before the adsorption by a color superposition technique, we found no significant changes, so that the disappearance of the bright spots did seldom mean the field evaporation of the tungsten atoms. The authors are grateful to Toray Science Foundation support to make an image intensifier.
for the financial
EIZI SUGATA and SAKAE ISHII
Department of Electronics, Faculty of Engineering, Yamada-kami, Suita, Osaka, Japan
Osaka University,
References 1) T. Engel and R. Gomer, J. Chem. Phys. 52 (1970) 1832; C. Kohrt and R. Gomer, J. Chem. Phys. 52 (1970) 3283. 2) E. W. Mtiller and T. T. Tsong, Field Zon Microscopy (Elsevier, New York, 1969) p. 68. 3) T. T. Tsong, Surface Sci. 10 (1968) 303; T. Reisner, 0. Nishikawa and E. W. Miiller, Surface Sci. 20 (1970) 163. 4) S. S. Brenner and J. T. McKinney, Surface Sci. 20 (1970) 411. 5) S. S. Brenner and J. T. McKinney, Appl. Phys. Letters 13 (1968) 29; E. W. Mtiller, J. A. Panitz and S. B. McLane, Rev. Sci. Instr. 39 (1968) 83; E. W. Mtiller, S. B. McLane and J. A. Panitz, Surface Sci. 17 (1969) 430.
SCIENCE
A METHOD
24 (1971) 663-666
8 North-Holland
FOR THE CONTINUOUS
THE OXYGEN
ADSORPTION
Publishing Co.
OBSERVATION
PHENOMENA
OF
IN THE
FIELD ION MICROSCOPE Received 4 September 1970
The oxygen adsorption phenomena on a tungsten surface in a Field Ion Microscope were continuously observed with He or Ne as imaging gases and recorded in a motion picture. The FIM tube was a demountable all-glass tube and oxygen was introduced through a silver diffusion leak. After several times of oxygen flushing at a pressure of about 10T3 Torr, the tube was evacuated to a pressure of 8.0 x lo-’ Torr and then oxygen was introduced to a pressure of 2.5 x lo-’ Torr. The tip temperature was kept at 77°K. For continuous observation, an image intensifier which reduced the exposure time by a factor of about 10’ was used. As the image was dim, however, it could not be recorded in a motion picture with a usual tip. To obtain the bright image and at the same time to keep the resolution good, it was found to be practical to use the tip whose top apex was torn off by applying a strong electric field. By correcting the shape of this tip with a field evaporation method, the bright and finely resolved image could be obtained at a high best image voltage, e.g. 20 kV. The motion picture was taken at the rate of 12 frames per set and the exposure time for a frame was & sec. When the applied voltage was adjusted suitably, we could observe the bright spots resulted from the oxygen adsorption. Though it was not clear what species was the origin of the bright spots, some of the bright spots showed the interesting motion as shown in fig. 1: they changed their sizes, migrated, separated or combined. The separated spots also moved and changed their sizes. These may indicate the interaction between the adsorbates other than the interaction between the adsorbate and the substrate. In some cases, a bright spot became dim as if it dispersed, and afterwards in the neighborhood a spot or two appeared. The other spots stayed at the same place for a short time and then disappeared. These bright spots were observed on the planes on the [llO] zone lines except on the (110) plane and its terraces. The bright spots on the (110) plane and its terraces were observed for a short time and hardly moved. This corresponds to the small sticking probability of oxygen on the (110) planer). 663
C
d
e
f
Fig. 1 a-f
CONTINUOUS
OBSERVATION
OF THE OXYGEN
ADSORPTION
PHENOMENA
665
Fig. 1 g-i
Fig. 1. The motion of the bright spots on a tungsten surface. These photographs were taken from a motion picture and show the images observed at intervals of r/la set in order. The imaging gas was Ne. The interesting changes can be seen on the (111) plane and on the region between the (111) and the (211) planes. Among the three spots on the (111) plane shown in (c), the size of the two lower spots reversed in (d) and after s/12 set a large spot grew at the same place as shown in (i). On the region between the (111) and the (211) planes, the migration and the coalescence of the spots were observed.
The motion of the bright spots was affected by the electric field on the surface. When it was strengthened adequately, the bright spots became to cease the motion and were observed for a longer time because the adsorbed oxygen atoms were prevented to desorb spontaneously. This may be due to the increase of the adsorption energy by the polarization effect2). Whatever was the species which made the bright spots, these images imply the local changes of the field strength and/or the electron distribution on the
666
EIZI
SUGATA
AND
SAKAE
ISHII
surface3). Since it is reportedd) that the predominant species that gave rise to individual spots were identified to be single tungsten atoms, the bright spots in the present paper should be examined by a mass spectrometer41 5) for the further discussion. When comparing the image of the substrate tungsten atoms obtained after the disappearance of the bright spots which resulted from the adsorption with the image before the adsorption by a color superposition technique, we found no significant changes, so that the disappearance of the bright spots did seldom mean the field evaporation of the tungsten atoms. The authors are grateful to Toray Science Foundation support to make an image intensifier.
for the financial
EIZI SUGATA and SAKAE ISHII
Department of Electronics, Faculty of Engineering, Yamada-kami, Suita, Osaka, Japan
Osaka University,
References 1) T. Engel and R. Gomer, J. Chem. Phys. 52 (1970) 1832; C. Kohrt and R. Gomer, J. Chem. Phys. 52 (1970) 3283. 2) E. W. Mtiller and T. T. Tsong, Field Zon Microscopy (Elsevier, New York, 1969) p. 68. 3) T. T. Tsong, Surface Sci. 10 (1968) 303; T. Reisner, 0. Nishikawa and E. W. Miiller, Surface Sci. 20 (1970) 163. 4) S. S. Brenner and J. T. McKinney, Surface Sci. 20 (1970) 411. 5) S. S. Brenner and J. T. McKinney, Appl. Phys. Letters 13 (1968) 29; E. W. Mtiller, J. A. Panitz and S. B. McLane, Rev. Sci. Instr. 39 (1968) 83; E. W. Mtiller, S. B. McLane and J. A. Panitz, Surface Sci. 17 (1969) 430.