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Reflection electron microscopy (REM) of vicinal surfaces of fcc metals
Ultramicroscopy North-Holland
167
11 (1983) 167- 172 Publishing Company
REFLECTION
ELECTRON
MICROSCOPY
(REM) OF VICINAL
SURFACES
OF fee METALS
Tung HSU Department Received
of Ph.ysics, Arizona State Umverstty, 12 January
1983; presented
Tempe, Arizona 85287. USA
at Workshop
January
1983
REM has been applied to study the low index facets and their vicinal regions on Au and Pt spheres solidified from the melt. Areas of regularly spaced steps have caused splitting of diffraction spots, and the splitting in the diffraction pattern agrees with the periodicity of steps as measured in the REM image. Resolution limits for REM images are discussed and compared to the 9 A resolution experimentally achieved.
1. Introduction Since the re-introduction of REM in 1975 emphasizing the high resolution possible with diffraction contrast [ 1,2], work has been done using this technique on various specimens [3-81. We have been exploring the potential of REM using standard commercial electron microscopes and specimens prepared either in atmosphere or in vacuum but transferred through the atmosphere into the microscope. We found that crystals of noble metals provide rich information on surface crystallography and the nature of contrast under REM imaging conditions [8]. This paper covers observations made on the vicinal planes which were naturally formed when gold or platinum was cooled in atmosphere from the molten state. Vicinal planes have been thoroughly studied by LEED and RHEED methods [9-121. These methods are limited mainly in three ways: (1) Their resolutions have upper limits in real space. That is, any periodicity greater than the limit - usually a few hundreds A - cannot be resolved in LEED or RHEED patterns. (2) The information contained in LEED or RHEED patterns is an average over fairly large areas, of the order of microns in diameter, of the specimen surface. Local variations cannot be measured. (3) There is no adequate dynamic theory for 0304-3991/83/0000-0000/$03.00
0 1983 North-Holland
comparison with the intensity measurements. REM is just the right method to overcome the first two of these limitations. While capable of imaging a surface step one atom high, REM is obviously not limited by the large spacing between steps. A single step on an otherwise atomically smooth surface can be imaged. This method can also provide information on the distribution of steps and dislocations on the surface. The irregularity in heights, spacings and senses (up or down) of steps is directly observed. Results of REM observations are presented in the following section.
2. Experimental
and results
2.1. Fine structures
of the vicinal surfaces
Method of preparing small spheres of metal for TEM observation has been described elsewhere [8]. Essentially, they are solidified in atmosphere from the molten state. When they solidify, flat (111) facets develop. Areas between facets are in general rounded. Ideally, the atomic steps on the curved surface should be distributed systematically as: t, = d/tan where
0,
t, = d/sin
B is the angle
8,
between
the normal
to the
Tung Hsu / REM of vicrnal .surfaces of /cc metals
(111) facet and the normal to the locally averaged surface plane, d is the height of the steps, t, is the width of the terrace, or the distance between two adjacent steps measured in the (Ill) plane, and t, is the distance between two adjacent steps measured in a direction tangential to the sphere (fig. 11. Fig. I. Contour of an idealized sphere with facet made of cubic crystal. Terrace widths 1, and t, are measured along (111) plane and tangential plane, respectively: t, = I, when B is small.
Fig. 2. (a) A low magnification view of the (111) facet and its vicinal region on a Pt sphere. (b)-(f) A sequence at higher magnification showing varied step and band distributions as the distance from the facet increases.
of images of Au crystal
Tung Hsu / REM of vicinnl surJ&es OJJCCmetals
However, it was found that in the region near the facet, the step distribution function was not monotonic. Instead of gradually decreasing terrace widths and thus gradually increasing slope, the surface showed alternating bands of two distinctive slopes. The shallow slope dominated the immediate neighborhood of the facet. Further away from the facet, narrow bands of the steeper slope began to show up. Farther from the facet, the steep bands became wider and with shorter distances in between. Finally, the steep slope dominated the surface (fig. 2). On one Pt specimen, individual steps were resolved along these alternating bands (fig. 3). The 32 and 58 A terrace widths correspond to 4.1” and 2.2” slopes respectively. On some specimens, variations of this basic banding produced images that are aesthetically beautiful but scientifically difficult to explain (fig. 4). It appears that the particles on the surfaces
Fig. 3. Individual respectively.
steps in both light and dark bands of this Pt specimen
169
were impurities arising either from the specimen or from the procedures of specimen preparation. These particles were arranged in orderly patterns during solidification through a process yet to be explored. 2.2. Resolution Resolution of REM was analyzed by Haine and Hirst in 1953 [ 131 and by Hojlund Nielsen and Cowley in 1976 [2]. Both analyses were based on standard methods used in TEM, taking into account the diffraction from the objective aperture, spherical aberration, and chromatic aberration. However, chromatic aberration is difficult to evaluate because electrons diffracted from surfaces are known to have higher energy spread than those transmitted through thin specimens. Therefore, in the REM case, the energy spread due to the specimen should be added to the energy spread due to
are resolved.
Terrace
widths are measured
to be 58 and 32 A,
--
Fig. 4. Examples
of varied vicinal suriaces.
171
the instrument, and consequently it becomes difficult to estimate. Also, due to the high intensity of diffuse scattering, in general the optimum aperture size cannot be used as in the TEM case. A smaller aperture may be necessary for better depth of focus and contrast. Taking data provided by the manufacturer, the resolution of REM images using a 30 pm aperture in the Philips-400T electron microscope is 7.2 A, if the chromatic aberration term is neglected. Astigmatism is another factor that limits the resolution. In REM it is in general more difficult to correct astigmatism than in TEM. Fig. 5 shows an image of a vicinal surface of Au with very good resolution. The area near the foot of a steep slope (darker band) shows 9 A fringes. This is near the theoretically estimated limit of a chromatic aberration-free and astigmatism-free image as stated before.
Fig. 5. (a) A high resolution image ot Au surface. Steps are clearly resolved in light bands. The area in the box is enlarged in (b) to show fringes approximately 9 ;\ apart. Fringes can be seen better by looking at the picture at low angle along the length of the page.
2.3. Periodical steps The area in fig. 5 is obviously not flat, because fringes are not uniform in direction and spacing over a large area. Flatter areas were found in
Fig. 6. (a) This relatively flat area on the Au crystal has steps of approximately 35 A periodicity, corresponding to a 3.9” slope below the (111) plane. (b) The diffraction pattern from the area in (a). Spot splitting corresponds to 34 b, periodicity and the 4’ tilt of the streaks indicates that the surface is tilted 4” below the (111) plane. A sketch of the surface with terrace width t,, step height d,,, and tilt angle 0 is shown in (b). Vector N is perpendicular to the surface.
172
Tung Hsu / REM of Cud
regions near the facet. Fig. 6a shows such an area on an Au crystal where steps are nearly periodically spaced and show only very small curvature along the surface. (When foreshortening is compensated in the vertical direction by a factor of about 20, the steps are nearly straight over a distance of several microns.) A selected area diffraction pattern from this area is in fig. 6b. The split spots and the slightly tilted streaks are observed. Measurements from this diffraction pattern show that the splitting corresponds to a periodicity of 34 A and a slope of the surface 4” away from the (111) plane. On the other hand, the periodicity of steps as measured in fig. 6a is about 35 A. From this measured periodicity and the step height d,,, = 2.36 A, the slope of the surface relative to the (111) plane is calculated to be 3.9”. The agreement is therefore good.
surfaces
offcc
metds
stepped surfaces, such as periodic step-up and step-down, randomly distributed steps of different step heights, etc., equally well with the REM technique. In addition to verifying theories put out by LEED and RHEED workers, results presented here provide other strong evidence that these steps are indeed one atom high.
Acknowledgements This work was supported by NSF grant DMR8015785 and made use of the resources of the Facility for High Resolution Microscopy supported by grant CHE-7916098 from the NSF Regional Instrumentation Facilities Program. The author also wishes to thank Dr. J.M. Cowley for his advice and support.
References 3. Conclusion 111J.M. Cowley and P.E. Hejlund Nielsen, Ultramicroscopy
These experiments have revealed the fine and varied structures of vicinal surfaces around the (111) facets of Au and Pt. The alternating band structure certainly deserves an explanation which is not available at the present. Resolution of REM was pushed to 9 A which is close to the instrumental limit. This allows REM to be employed in studying surface structures and defects in considerable detail. It is also a further demonstration that contamination under the ordinary vacuum in a commercial electron microscope does not obstruct the strong contrast from the atomic steps on crystal surfaces. Periodically distributed steps previously deduced from diffraction data were shown in real space. Comparing the REM image and the corresponding diffraction pattern, good agreement in spot splitting and terrace width has been obtained. It should be possible to observe other types of
1 (1975) 145. 121P.E. Hlajlund Nielsen and J.M. Cowley, Surface Sci. 54 (1976) 340. [31 N. Osakabe, Y. Tanishiro, K. Yagi and G. Honjo, Surface Sci. 97 (1980) 393. [41 Z.C. Kang, J. Microsc. Spectrosc. Electron. 7 (1982) 33. [51 Tung Hsu and S. Iijima, in Proc. 40th Annual EMSA Meeting, Washington, DC, 1982 (Claitor’s, Baton Rouge, LA, 1982) p. 450. [61 S. IiJima and Tung Hsu, in: Proc. 10th Intern. Congr. on Electron Microscopy, Hamburg, 1982, Vol. 2, p. 293. [71 N. Yamamoto and J.C.H. Spence, Thin Solid Films, to be published. PI Tung Hsu and J.M. Cowley, to be published. [91 B. Lang, R.W. Joyner and G.A. Somorjai, Surface Sci. 30 (1972) 440. Berlin, [lOI M.G. Lagally, in: Proc. ISISS, 1981 (Springer, 1982). [ill M. Henzler, Appl. Phys. 9 (1976) 11. [121 F. Hottier, J.B. Theeten, A. Mason and J.L. Domange. Surface Sci. 65 (1977) 563. [I31 M.E. Haine and W. Hirst, Brit. J. Appl. Phys. 4 (1953) 239.
167
11 (1983) 167- 172 Publishing Company
REFLECTION
ELECTRON
MICROSCOPY
(REM) OF VICINAL
SURFACES
OF fee METALS
Tung HSU Department Received
of Ph.ysics, Arizona State Umverstty, 12 January
1983; presented
Tempe, Arizona 85287. USA
at Workshop
January
1983
REM has been applied to study the low index facets and their vicinal regions on Au and Pt spheres solidified from the melt. Areas of regularly spaced steps have caused splitting of diffraction spots, and the splitting in the diffraction pattern agrees with the periodicity of steps as measured in the REM image. Resolution limits for REM images are discussed and compared to the 9 A resolution experimentally achieved.
1. Introduction Since the re-introduction of REM in 1975 emphasizing the high resolution possible with diffraction contrast [ 1,2], work has been done using this technique on various specimens [3-81. We have been exploring the potential of REM using standard commercial electron microscopes and specimens prepared either in atmosphere or in vacuum but transferred through the atmosphere into the microscope. We found that crystals of noble metals provide rich information on surface crystallography and the nature of contrast under REM imaging conditions [8]. This paper covers observations made on the vicinal planes which were naturally formed when gold or platinum was cooled in atmosphere from the molten state. Vicinal planes have been thoroughly studied by LEED and RHEED methods [9-121. These methods are limited mainly in three ways: (1) Their resolutions have upper limits in real space. That is, any periodicity greater than the limit - usually a few hundreds A - cannot be resolved in LEED or RHEED patterns. (2) The information contained in LEED or RHEED patterns is an average over fairly large areas, of the order of microns in diameter, of the specimen surface. Local variations cannot be measured. (3) There is no adequate dynamic theory for 0304-3991/83/0000-0000/$03.00
0 1983 North-Holland
comparison with the intensity measurements. REM is just the right method to overcome the first two of these limitations. While capable of imaging a surface step one atom high, REM is obviously not limited by the large spacing between steps. A single step on an otherwise atomically smooth surface can be imaged. This method can also provide information on the distribution of steps and dislocations on the surface. The irregularity in heights, spacings and senses (up or down) of steps is directly observed. Results of REM observations are presented in the following section.
2. Experimental
and results
2.1. Fine structures
of the vicinal surfaces
Method of preparing small spheres of metal for TEM observation has been described elsewhere [8]. Essentially, they are solidified in atmosphere from the molten state. When they solidify, flat (111) facets develop. Areas between facets are in general rounded. Ideally, the atomic steps on the curved surface should be distributed systematically as: t, = d/tan where
0,
t, = d/sin
B is the angle
8,
between
the normal
to the
Tung Hsu / REM of vicrnal .surfaces of /cc metals
(111) facet and the normal to the locally averaged surface plane, d is the height of the steps, t, is the width of the terrace, or the distance between two adjacent steps measured in the (Ill) plane, and t, is the distance between two adjacent steps measured in a direction tangential to the sphere (fig. 11. Fig. I. Contour of an idealized sphere with facet made of cubic crystal. Terrace widths 1, and t, are measured along (111) plane and tangential plane, respectively: t, = I, when B is small.
Fig. 2. (a) A low magnification view of the (111) facet and its vicinal region on a Pt sphere. (b)-(f) A sequence at higher magnification showing varied step and band distributions as the distance from the facet increases.
of images of Au crystal
Tung Hsu / REM of vicinnl surJ&es OJJCCmetals
However, it was found that in the region near the facet, the step distribution function was not monotonic. Instead of gradually decreasing terrace widths and thus gradually increasing slope, the surface showed alternating bands of two distinctive slopes. The shallow slope dominated the immediate neighborhood of the facet. Further away from the facet, narrow bands of the steeper slope began to show up. Farther from the facet, the steep bands became wider and with shorter distances in between. Finally, the steep slope dominated the surface (fig. 2). On one Pt specimen, individual steps were resolved along these alternating bands (fig. 3). The 32 and 58 A terrace widths correspond to 4.1” and 2.2” slopes respectively. On some specimens, variations of this basic banding produced images that are aesthetically beautiful but scientifically difficult to explain (fig. 4). It appears that the particles on the surfaces
Fig. 3. Individual respectively.
steps in both light and dark bands of this Pt specimen
169
were impurities arising either from the specimen or from the procedures of specimen preparation. These particles were arranged in orderly patterns during solidification through a process yet to be explored. 2.2. Resolution Resolution of REM was analyzed by Haine and Hirst in 1953 [ 131 and by Hojlund Nielsen and Cowley in 1976 [2]. Both analyses were based on standard methods used in TEM, taking into account the diffraction from the objective aperture, spherical aberration, and chromatic aberration. However, chromatic aberration is difficult to evaluate because electrons diffracted from surfaces are known to have higher energy spread than those transmitted through thin specimens. Therefore, in the REM case, the energy spread due to the specimen should be added to the energy spread due to
are resolved.
Terrace
widths are measured
to be 58 and 32 A,
--
Fig. 4. Examples
of varied vicinal suriaces.
171
the instrument, and consequently it becomes difficult to estimate. Also, due to the high intensity of diffuse scattering, in general the optimum aperture size cannot be used as in the TEM case. A smaller aperture may be necessary for better depth of focus and contrast. Taking data provided by the manufacturer, the resolution of REM images using a 30 pm aperture in the Philips-400T electron microscope is 7.2 A, if the chromatic aberration term is neglected. Astigmatism is another factor that limits the resolution. In REM it is in general more difficult to correct astigmatism than in TEM. Fig. 5 shows an image of a vicinal surface of Au with very good resolution. The area near the foot of a steep slope (darker band) shows 9 A fringes. This is near the theoretically estimated limit of a chromatic aberration-free and astigmatism-free image as stated before.
Fig. 5. (a) A high resolution image ot Au surface. Steps are clearly resolved in light bands. The area in the box is enlarged in (b) to show fringes approximately 9 ;\ apart. Fringes can be seen better by looking at the picture at low angle along the length of the page.
2.3. Periodical steps The area in fig. 5 is obviously not flat, because fringes are not uniform in direction and spacing over a large area. Flatter areas were found in
Fig. 6. (a) This relatively flat area on the Au crystal has steps of approximately 35 A periodicity, corresponding to a 3.9” slope below the (111) plane. (b) The diffraction pattern from the area in (a). Spot splitting corresponds to 34 b, periodicity and the 4’ tilt of the streaks indicates that the surface is tilted 4” below the (111) plane. A sketch of the surface with terrace width t,, step height d,,, and tilt angle 0 is shown in (b). Vector N is perpendicular to the surface.
172
Tung Hsu / REM of Cud
regions near the facet. Fig. 6a shows such an area on an Au crystal where steps are nearly periodically spaced and show only very small curvature along the surface. (When foreshortening is compensated in the vertical direction by a factor of about 20, the steps are nearly straight over a distance of several microns.) A selected area diffraction pattern from this area is in fig. 6b. The split spots and the slightly tilted streaks are observed. Measurements from this diffraction pattern show that the splitting corresponds to a periodicity of 34 A and a slope of the surface 4” away from the (111) plane. On the other hand, the periodicity of steps as measured in fig. 6a is about 35 A. From this measured periodicity and the step height d,,, = 2.36 A, the slope of the surface relative to the (111) plane is calculated to be 3.9”. The agreement is therefore good.
surfaces
offcc
metds
stepped surfaces, such as periodic step-up and step-down, randomly distributed steps of different step heights, etc., equally well with the REM technique. In addition to verifying theories put out by LEED and RHEED workers, results presented here provide other strong evidence that these steps are indeed one atom high.
Acknowledgements This work was supported by NSF grant DMR8015785 and made use of the resources of the Facility for High Resolution Microscopy supported by grant CHE-7916098 from the NSF Regional Instrumentation Facilities Program. The author also wishes to thank Dr. J.M. Cowley for his advice and support.
References 3. Conclusion 111J.M. Cowley and P.E. Hejlund Nielsen, Ultramicroscopy
These experiments have revealed the fine and varied structures of vicinal surfaces around the (111) facets of Au and Pt. The alternating band structure certainly deserves an explanation which is not available at the present. Resolution of REM was pushed to 9 A which is close to the instrumental limit. This allows REM to be employed in studying surface structures and defects in considerable detail. It is also a further demonstration that contamination under the ordinary vacuum in a commercial electron microscope does not obstruct the strong contrast from the atomic steps on crystal surfaces. Periodically distributed steps previously deduced from diffraction data were shown in real space. Comparing the REM image and the corresponding diffraction pattern, good agreement in spot splitting and terrace width has been obtained. It should be possible to observe other types of
1 (1975) 145. 121P.E. Hlajlund Nielsen and J.M. Cowley, Surface Sci. 54 (1976) 340. [31 N. Osakabe, Y. Tanishiro, K. Yagi and G. Honjo, Surface Sci. 97 (1980) 393. [41 Z.C. Kang, J. Microsc. Spectrosc. Electron. 7 (1982) 33. [51 Tung Hsu and S. Iijima, in Proc. 40th Annual EMSA Meeting, Washington, DC, 1982 (Claitor’s, Baton Rouge, LA, 1982) p. 450. [61 S. IiJima and Tung Hsu, in: Proc. 10th Intern. Congr. on Electron Microscopy, Hamburg, 1982, Vol. 2, p. 293. [71 N. Yamamoto and J.C.H. Spence, Thin Solid Films, to be published. PI Tung Hsu and J.M. Cowley, to be published. [91 B. Lang, R.W. Joyner and G.A. Somorjai, Surface Sci. 30 (1972) 440. Berlin, [lOI M.G. Lagally, in: Proc. ISISS, 1981 (Springer, 1982). [ill M. Henzler, Appl. Phys. 9 (1976) 11. [121 F. Hottier, J.B. Theeten, A. Mason and J.L. Domange. Surface Sci. 65 (1977) 563. [I31 M.E. Haine and W. Hirst, Brit. J. Appl. Phys. 4 (1953) 239.