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Field-ion microscopy of osmium
199
FIELD-ION
MICROSCOPY
A. J. MELMED
and J. J. CARROLL
OF OSMIUM
N~rtiotiul Bwrat~ of Srundtrrc/s,Imstifutr for Muterirrls
Research.
Wd~inyton.
D.C.
20234 ( Li.S..4. I
(Received June 13, 197’)
SUMMARY
Osmium has been successfully imaged in the field-ion microscope using helium ions at temperatures of about 87 K and lower. Specimen preparation procedures and details of image characteristics are described.
INTRODUCTION
Osmium is a metal which is usually available only in the form of powder or sponge. Therefore, although its high melting temperature (3318 K)’ and high heat of sublimation (187 kcal~mole)2 have made it apparently well suited for study in the field-ion microscope3, it has not been field-ion imaged previously. Kollmar and Stark4 were successful, however, in making OS field electron emission specimens from irregular pieces, We have had an abiding interest in the field-ion microscopy of h.c.p. metals, and have been concerned over the fact that certain differences exist amongst the h.c.p. metals that have already been imaged, regarding the visibility of several crystallographic regions. Of the three h.c.p. metals that have thus far been imaged well enough clearly to define the situation, namely Re5, Ru6, and Hf’, only Ru images have displayed all of the crystal planes expected to develop and appear upon field-evaporation and imaging in the field-ion microscope. Since no rigorous theoretical explanation has been produced to clarify this situation, we are attempting to collect further experimental data on a variety of h.c.p. metals. Thus. we have been motivated to explore the field-ion microscopy of osmium. EXPERIMENTAL
We obtained a solid button of arc-melted osmium (99.89;) from a commercial vendor and also obtained some osmium powder (99.90,,) which was arc-melted into the form of a solid button at NBS. Arc melting was carried out in a Ti gettered argon atmosphere at about 12 Torr. Slabs about 0.635 mm thick were cut from the buttons by spark erosion, and then slices, about 0.635 mm wide, 5 .to 10 mm in length, were made in the slabs, also by spark erosion, yielding rods of approximately square cross-section. These were then eiectrolyticalIy polished in dilute HCIS (about 100,; HCl in water) or in approximately one normal aqueous KOH solution, using about 40-20 volts ac., to give approximately circular cross-section
200
A. J. MELMED,
J. J. CARROLL
rods of about 0.127 mm diam. In order to produce a specimen with a tip sufficiently sharp for field-ion microscopy, a rod was first spotwelded to a short length of 0.254 mm diameter Pt wire for support (this was later spotwelded to the actual tip assembly used in the field-ion microscope), and then polished further, usually in KOH solution. We found it useful at this stage to polish the specimen with the KOH contained in a small loop of 0.127 mm diam. Pt wire (with a loop diameter of about 7 mm), manually moving the loop back and forth along the length of the OS rod during the polishing. During this stage the polishing voltage used was between 15 and 10. Final polishing was done under an optical microscope, at x 100 magnification, operated horizontally, in a manner previously reported CL*. Since certain real hazards potentially exist in working with 0s9 we .took precautionary safety steps during specimen preparation. The oil bath used in the spark erosion process was isolated from most of the spark erosion machine and was discarded after use and the eyes of personnel involved in the specimen preparation were always shielded from possible contact with oxide vapors. Two field-ion microscopes were used in this work. One was a simple, liquid nitrogen-cooled He field-ion microscope with no image amplification, and the other was a gaseous helium cooled field-ion microscope operated at temperatures between about 87 K and 20 K, using He or Ne imaging gases and micro-channel plate image intensification. RESULTS
Good helium field-ion images of OS were obtained at liquid nitrogen temperature, and at other temperatues within the 87-20 K range. Imaging in neon at a temperature of about 30 K produced a less regular endform, possibly due to surface interactions with gaseous impurities in the microscope. Illustrative He FIM images are shown in the micrographs of Fig. 1. All specimens used were successfully field-ion imaged. A large cap-like region, centered on the basal (001) planes and extending nearly to the center of the (101) planes, developed as a result of field evaporation into smooth, well ordered areas at relatively low voltages. However, most specimens did not develop much short range atomic surface order in other areas until they were sufficiently blunt to image at 15 kV or higher, even though initial imaging of surface atoms occurred at voltages as low as 3 kV. The specimen roughness in parts of the surface other than the basal plane cap may have been due to the electropolish used in the preparation procedure or perhaps due to some subsequent non-uniform oxidation. Some of the specimens were therefore annealed in the FIM vacuum, about 10e7 Tort-, at 50@ 700°C for 3-4 min in attempts to improve the situation, but no noticeable differences occurred. The ratio of field evaporation voltage to helium ion imaging voltage was generally high. For most of the specimens used this ratio was about 1.25. One sharp specimen had a ratio of 1.10. Figure 2(a) shows a pattern, having very little bright-zone contrast, which occurred occasionally during the process of field evaporating an OS specimen. Compared to the He field-ion endform in Fig. l(a), there is increased visibility of the atoms in a roughly triangular area bounded at the vertexes by (lOl), (112) and
201
400
-
110
(b)
Fig. 1. Helium field-ion micrographs of osmium. (a) Field evaporation and imaging done at liquid nitrogen temperature. Imaging voltage 17,300 V; (b) crystallographic map of (a); (c) Field evaporation and imaging done at 24 K. Imaging voltage 9,100 V. (Some photographic dodging done, but essential bright-zone contrast unchanged.)
Fig. 2. He field-ion micrographs of OS field evaporated and imaged at 24 K. (a) 16.000 V; fb) 16,700 V. (Some photographic dodging done, but essential bright-zone contrast unchanged.)
FIELD-ION
MICROSCOPY
OF OSMIUM
203
(212) plane regions. Figure 2(b) shows the same tip at a later stage of field evaporation. The alternation of image intensity around the plane edges in the basal plane region’ O observed for all of the h.c.p. metals thus far imaged well, was readily observed in the images of OS. In all of the other low index crystallographic regions where, due to the h.c.p. stacking sequence two types of planes of alternating different interplanar spacing are possible6. 11, two alternating plane edge intensities (relatively bright, A and dim, B) were observed. These were routinely observed in the ( IOO), (102) (103) and their crystallographically equivalent regions, and only inconsistently observed in the (101) and equivalent regions, of the He field-ion endform (Figs. 1 and 2(b)). Th us, OS exhibits the expected A, B plane visibility in the low index regions. Illustrations of these image intensity effects are shown in the micrographs of Fig. 3.
Fig. 3. He field-ion micrographs showing alternating image intensity, dim, of successive plane edges of (a) OS (100) region. 87 K, 9,900 10,100 V; (c) OS (102) region, liquid nitrogen cooled, 15,200 V.
A-relatively bright, B-relatively V; (b) OS (101) region, 87 K.
204
A. J. MELMED, J. J. CARROLL
There still remain some small crystallographic regions where the imaging is less than optimum. This may be due to inherent image contrast effects specific to these regions, or to inherent field-evaporation specificities, or to inhomogeneities caused by the metallurgical sample preparation, electropolishing, or oxidation (or other chemical interactions). Further experimentation with OS should clarify the situation, ACKNOWLEDGMENTS
The authors gratefully acknowledge the U. S. Atomic Energy Commission for providing the OS howder, and Mr. David P. Fickle of NBS for arc-melting the powder into a solid button. REFERENCES 1 A. G. Knapton, J. Savill and R. Siddall, J. Less-Common Metals, 2 (1960) 357. 2 R. E. Honig, RCA Rev., 23 (1962) 567. 3 E. W. Miiller and T. T. Tsong, Field Ion Microscopy, Elsevier, New York, 1969. 4 W. Kollmar and D. Stark, Z. Physik, 178 (1964) 39. 5 E. W. Miiller, J. Appl. Phys., 28 (1957) 1. 6 A. J. Melmed, Sur. Sci., 8 (1967) 191. 7 T. Reisner, 0. Nishikawa and E. W. Miiller, Sur. Sci., 20 (1970) 163. 8 A. J. Melmed, J. Chem. Phys., 38 (1963) 607. 9 For example, Clifford A. Hamped (ed.), The Encyclopedia of the Chemical Elements, Reinhold, New York, 1968. 10 E. W. Miiller, Sur. Sci., 2 (1964) 484. 11 A. J. Melmed, Sur. Sci., 5 (1966) 359.
FIELD-ION
MICROSCOPY
A. J. MELMED
and J. J. CARROLL
OF OSMIUM
N~rtiotiul Bwrat~ of Srundtrrc/s,Imstifutr for Muterirrls
Research.
Wd~inyton.
D.C.
20234 ( Li.S..4. I
(Received June 13, 197’)
SUMMARY
Osmium has been successfully imaged in the field-ion microscope using helium ions at temperatures of about 87 K and lower. Specimen preparation procedures and details of image characteristics are described.
INTRODUCTION
Osmium is a metal which is usually available only in the form of powder or sponge. Therefore, although its high melting temperature (3318 K)’ and high heat of sublimation (187 kcal~mole)2 have made it apparently well suited for study in the field-ion microscope3, it has not been field-ion imaged previously. Kollmar and Stark4 were successful, however, in making OS field electron emission specimens from irregular pieces, We have had an abiding interest in the field-ion microscopy of h.c.p. metals, and have been concerned over the fact that certain differences exist amongst the h.c.p. metals that have already been imaged, regarding the visibility of several crystallographic regions. Of the three h.c.p. metals that have thus far been imaged well enough clearly to define the situation, namely Re5, Ru6, and Hf’, only Ru images have displayed all of the crystal planes expected to develop and appear upon field-evaporation and imaging in the field-ion microscope. Since no rigorous theoretical explanation has been produced to clarify this situation, we are attempting to collect further experimental data on a variety of h.c.p. metals. Thus. we have been motivated to explore the field-ion microscopy of osmium. EXPERIMENTAL
We obtained a solid button of arc-melted osmium (99.89;) from a commercial vendor and also obtained some osmium powder (99.90,,) which was arc-melted into the form of a solid button at NBS. Arc melting was carried out in a Ti gettered argon atmosphere at about 12 Torr. Slabs about 0.635 mm thick were cut from the buttons by spark erosion, and then slices, about 0.635 mm wide, 5 .to 10 mm in length, were made in the slabs, also by spark erosion, yielding rods of approximately square cross-section. These were then eiectrolyticalIy polished in dilute HCIS (about 100,; HCl in water) or in approximately one normal aqueous KOH solution, using about 40-20 volts ac., to give approximately circular cross-section
200
A. J. MELMED,
J. J. CARROLL
rods of about 0.127 mm diam. In order to produce a specimen with a tip sufficiently sharp for field-ion microscopy, a rod was first spotwelded to a short length of 0.254 mm diameter Pt wire for support (this was later spotwelded to the actual tip assembly used in the field-ion microscope), and then polished further, usually in KOH solution. We found it useful at this stage to polish the specimen with the KOH contained in a small loop of 0.127 mm diam. Pt wire (with a loop diameter of about 7 mm), manually moving the loop back and forth along the length of the OS rod during the polishing. During this stage the polishing voltage used was between 15 and 10. Final polishing was done under an optical microscope, at x 100 magnification, operated horizontally, in a manner previously reported CL*. Since certain real hazards potentially exist in working with 0s9 we .took precautionary safety steps during specimen preparation. The oil bath used in the spark erosion process was isolated from most of the spark erosion machine and was discarded after use and the eyes of personnel involved in the specimen preparation were always shielded from possible contact with oxide vapors. Two field-ion microscopes were used in this work. One was a simple, liquid nitrogen-cooled He field-ion microscope with no image amplification, and the other was a gaseous helium cooled field-ion microscope operated at temperatures between about 87 K and 20 K, using He or Ne imaging gases and micro-channel plate image intensification. RESULTS
Good helium field-ion images of OS were obtained at liquid nitrogen temperature, and at other temperatues within the 87-20 K range. Imaging in neon at a temperature of about 30 K produced a less regular endform, possibly due to surface interactions with gaseous impurities in the microscope. Illustrative He FIM images are shown in the micrographs of Fig. 1. All specimens used were successfully field-ion imaged. A large cap-like region, centered on the basal (001) planes and extending nearly to the center of the (101) planes, developed as a result of field evaporation into smooth, well ordered areas at relatively low voltages. However, most specimens did not develop much short range atomic surface order in other areas until they were sufficiently blunt to image at 15 kV or higher, even though initial imaging of surface atoms occurred at voltages as low as 3 kV. The specimen roughness in parts of the surface other than the basal plane cap may have been due to the electropolish used in the preparation procedure or perhaps due to some subsequent non-uniform oxidation. Some of the specimens were therefore annealed in the FIM vacuum, about 10e7 Tort-, at 50@ 700°C for 3-4 min in attempts to improve the situation, but no noticeable differences occurred. The ratio of field evaporation voltage to helium ion imaging voltage was generally high. For most of the specimens used this ratio was about 1.25. One sharp specimen had a ratio of 1.10. Figure 2(a) shows a pattern, having very little bright-zone contrast, which occurred occasionally during the process of field evaporating an OS specimen. Compared to the He field-ion endform in Fig. l(a), there is increased visibility of the atoms in a roughly triangular area bounded at the vertexes by (lOl), (112) and
201
400
-
110
(b)
Fig. 1. Helium field-ion micrographs of osmium. (a) Field evaporation and imaging done at liquid nitrogen temperature. Imaging voltage 17,300 V; (b) crystallographic map of (a); (c) Field evaporation and imaging done at 24 K. Imaging voltage 9,100 V. (Some photographic dodging done, but essential bright-zone contrast unchanged.)
Fig. 2. He field-ion micrographs of OS field evaporated and imaged at 24 K. (a) 16.000 V; fb) 16,700 V. (Some photographic dodging done, but essential bright-zone contrast unchanged.)
FIELD-ION
MICROSCOPY
OF OSMIUM
203
(212) plane regions. Figure 2(b) shows the same tip at a later stage of field evaporation. The alternation of image intensity around the plane edges in the basal plane region’ O observed for all of the h.c.p. metals thus far imaged well, was readily observed in the images of OS. In all of the other low index crystallographic regions where, due to the h.c.p. stacking sequence two types of planes of alternating different interplanar spacing are possible6. 11, two alternating plane edge intensities (relatively bright, A and dim, B) were observed. These were routinely observed in the ( IOO), (102) (103) and their crystallographically equivalent regions, and only inconsistently observed in the (101) and equivalent regions, of the He field-ion endform (Figs. 1 and 2(b)). Th us, OS exhibits the expected A, B plane visibility in the low index regions. Illustrations of these image intensity effects are shown in the micrographs of Fig. 3.
Fig. 3. He field-ion micrographs showing alternating image intensity, dim, of successive plane edges of (a) OS (100) region. 87 K, 9,900 10,100 V; (c) OS (102) region, liquid nitrogen cooled, 15,200 V.
A-relatively bright, B-relatively V; (b) OS (101) region, 87 K.
204
A. J. MELMED, J. J. CARROLL
There still remain some small crystallographic regions where the imaging is less than optimum. This may be due to inherent image contrast effects specific to these regions, or to inherent field-evaporation specificities, or to inhomogeneities caused by the metallurgical sample preparation, electropolishing, or oxidation (or other chemical interactions). Further experimentation with OS should clarify the situation, ACKNOWLEDGMENTS
The authors gratefully acknowledge the U. S. Atomic Energy Commission for providing the OS howder, and Mr. David P. Fickle of NBS for arc-melting the powder into a solid button. REFERENCES 1 A. G. Knapton, J. Savill and R. Siddall, J. Less-Common Metals, 2 (1960) 357. 2 R. E. Honig, RCA Rev., 23 (1962) 567. 3 E. W. Miiller and T. T. Tsong, Field Ion Microscopy, Elsevier, New York, 1969. 4 W. Kollmar and D. Stark, Z. Physik, 178 (1964) 39. 5 E. W. Miiller, J. Appl. Phys., 28 (1957) 1. 6 A. J. Melmed, Sur. Sci., 8 (1967) 191. 7 T. Reisner, 0. Nishikawa and E. W. Miiller, Sur. Sci., 20 (1970) 163. 8 A. J. Melmed, J. Chem. Phys., 38 (1963) 607. 9 For example, Clifford A. Hamped (ed.), The Encyclopedia of the Chemical Elements, Reinhold, New York, 1968. 10 E. W. Miiller, Sur. Sci., 2 (1964) 484. 11 A. J. Melmed, Sur. Sci., 5 (1966) 359.