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On the crystal structure of small gold crystals and large gold clusters
Surface
256
ON THE CRYSTAL STRUCTURE LARGE GOLD CLUSTERS L.R. WALLENBERG
OF SMALL
Science 156 (1985) 2566264 North-Holland. Amsterdam
GOLD CRYSTALS
AND
and J.-O. BOVIN
Inorganic Chemistry 2, Chemical Center, P. 0. Box 124, S-221
00 Lund, Sweden
and G. SCHMID Inorganic Germany
Chemistry,
University
Received
16 July 1984; accepted
of Essen, Universitijtssirasse
for publication
29 August
5- 7, D-4300
Essen I, Fed. Rep.
of
1984
A large gold cluster with the formula Au,,[(C,H,)~P],,CI,, was imaged with a high-resolution transmission electron microscope (HRTEM) and a proposed model was confirmed. Growth of gold crystals of approximately 4.0 nm size could be followed, row by row of atoms, with the use of a low light level TV camera and on-line image processing.
1. Introduction Recent advances in preparation techniques of very large metallic clusters [l] together with instrumental improvements on high-resolution transmission electron microscopes (HRTEM) have made it possible to image the structure of clusters and microcrystals directly. This has attracted the attention from both practical as well as theoretical sides. Practical problems, as spreading an expensive catalyst efficiently on a support with a maintained narrow size distribution and sintering, which may change the activity and other properties of a catalyst, may be studied using HRTEM. More theoretical aspects, as how the large number of defects and faults in microcrystals are formed, and how strain caused by these defects is accommodated in the crystal as it grows, can be illuminated with this technique. Resolution limits for modern routine instruments is approaching 0.2 nm and, as shown here, the crystal structure of particles down to even 1.0 nm can be studied, using heavier elements as gold for maximum contrast. Supplying the transmission electron microscope with TV cameras makes it possible to follow dynamic events as crystal growth, virtually row by row of atoms, and rearrangements of atoms within the crystals in real time. 0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
L..R. Wallenberg et al. / Small gold crystals and large gold clusters
251
Earlier HRTEM work by Marks and Smith [2] on gold and silver crystals with sizes around 20 nm showed a great deal of complexity. Multiply twinned and lamellar twinned particles with mainly icosahedral and decahedra1 geometrical shape were described, as well as partial dislocations which were found in particles larger than about 15 nm. Cowley and Monosmith [3] have published electron microdriffraction pattern with the use of the very narrow electron beam in a STEM instrument, both for decahedra1 and cubeoctahedral particles ranging from 1.5 to 2.0 nm in size, together with theoretical calculations, although no crystal structure images could be obtained.
2. Experimental method Gold clusters containing 55 atoms were manufactured by reducing (C,H,),PAuCl with B,H, in benzene [l]. This gave a black precipitate which is stable in air. The chemical composition was found to be Au,,z[P(C,H,),],Cl. Molecular weight determination by means of an ultracentrifuge gave the formula Au,,[P(C,H,),],,Cl,. In different kinds of solvents, though with different rate, gold crystals are inevitably formed which can account for the small stoichiometric excess of gold. A model was proposed from Mossbauer spectra by Schmid et al. with cubic close-packing of the 55 gold atoms in a cubeoctahedron as shown in fig. 1. The cluster is almost completely isolated from the surroundings by the twelve triphenylphosphane groups situated in each corner, and the chlorine atoms on
Fig. 1. Model of S-atom
cluster, where atoms are ccp in a cubeoctahedron.
258
L. R. Wallenberg et al. / Small gold crystals and large gold clusters
the centre of the 3 X 3 square surfaces. This gives four groups of gold atoms, depending on the different types of surroundings. The number of atoms of each kind fits the interpretation of the Mossbauer spectra, based on the model. This is an extremely large cluster molecule. A JEOL 200CX transmission electron microscope equipped with a top-entry high-resolution goniometer stage was used for imaging, capable of 0.23 nm crystal structure resolution at 200 kV operating voltage. Recently, a low light level TV camera was installed, interfaced with the microscope through a fluorescent screen and a lead glass window in the bottom. The signal from the camera is put through an on-line video processor for noise reduction in real time. Two different types of video processors have been tested, both with 512 X 512 pixels resolution and 256 grey levels. The less expensive model used “pixel incrementing” for averaging where the value of every stored pixel is compared to the incoming value from the camera. A fixed increment of the difference is added to the stored value and the stored digital image can then continuously be viewed on a TV monitor. The other type of processor, which we found to give less tailing of moving objects, uses what is known as “true averaging” or “sliding average”. The displayed pixel value in this case is an exponentially weighted average of the values from up to 128 of the preceding frames. The processed image is then recorded on a video taperecorder. While replaying the tape, it is possible to freeze the image at any time and use the 50 Hz scanning frequency of the taperecorder as an accurate timestep for “frameby-frame” viewing. A drop of the sample, dispersed in methanol, was placed on an amorphous holey carbon film, the alcohol was evaporated at 25°C and the sample was then inserted into the microscope. Computer-simulated images using the SHRLI multislice program (M.A. O’Keefe) for the gold structure along the (110) direction with the actual instrumental parameters were performed. The simulations showed that, at Scherzer defocus and for crystal thicknesses up to 8.0 nm, black spots could be interpreted as atom positions.
3. Observations Mainly, three different sizes of particles were observed; (1) a few large, spherical or ellipsoidal, about 250-350 nm in diameter; (2) roughly spherical, lo-30 nm and (3) a large number of very small particles, about 1.0-5.0 nm, covering the carbon film and larger particles. All three are represented in figs. 2a and 2b. The medium-sized particles are on the surface of a large particle, 300 nm. The size of the particle indicated by an arrow in fig. 2a was determined to 26.0 nm by lattice spacings at higher magnification and in fig. 2b it is shown how it is covered by microcrystals approximately 1.0-2.0 nm.
L. R. Wallenberg et al. / Small gold crystals and large gold clusters
259
Electron diffraction images of the large particle showed only a few weak spots in a ring pattern, presumably arising from the medium-size crystals on the surface. This is due to the fact that the largest particles are assembled by a large number of the smallest clusters/microcrystals which are too small to give rise to a detectable diffraction pattern. This was confirmed by the fact that no ED patterns were obtainable from areas with only microcrystals whereas areas with crystals of category (2) present, showed clear ring patterns. This also explains the grainy appearance of the large spheres at higher magnification and the random orientation of lattice planes which can be resolved in minor areas. Medium-size particles (lo-30 nm) was found to consist mainly of multiplyand lamellar-twinned particles as described by Marks and Smith (fig. 3). The electron diffraction pattern from complex particles this size is shown in fig. 4. measuring the radius of the rangs by means of a calibrated aperture showed good agreement with data given in the literature for gold (marked in the figure). Gold is face-centered cubic with a = 0.408 nm. To obtain images of the smallest microcrystals, fresh samples and minimum intensity on the electron beam had to be used, since particles this size are not stable in the beam; they immediately start growing. A crystal, approximately 1.2 nm across is imaged in figs. 5a where the crystal habitus and structure image shows a good resemblance with the proposed model viewed along (110)
Fig. 2. (a) Spherical particle, about 300 nm in diameter with smaller crystals, lo-30 nm, on the surface. The edge of the carbon film is seen in the lower part. (b) Enlargement of crystal indicated by an arrow in (a). Microcrystals, 1.0-2.0 nm, are covering the crystal and carbon film. Top left is seen the edge of the large particle, built by microcrystals.
260
L R. Wailenberg et oi. / Small goid crystak and large gold clusters
Fig. 3. Multiply-
and tamelIar-twinned
particles.
about
10 nm in diameter.
Fig. 4. Electron diffraction pattern from particles up to 20 nm. Ring radii indicated in the fig;ure correspond to lattice spacings given in the literature. To the right the area selected by the diffraction aperture is seen.
I&R. Wailenberg et al. / Smufi goid crystals and large gold clusters
Fig. 5. (a) Microcrystal, approximately 1.2 nm cubeoctahedron. Lattice spacings between crossing 55-atom cluster, viewed along (110).
261
across, showing cross section similar to a (111) planes is 0.235 nm. (b) Model of the
with Fig. 6. Left micrograph is recorded IS-20 s before the one to the right. Crystal A has grown 1 approximately 300 atoms to 2.6 nm in diameter during this time. in crystal B the twin plane has disappeared.
262
LR.
Wallenberg et al. / Small gold crystals and large gold clusters
as in fig. 5b. The spacings between the crossing (111) planes is 0.235 nm in this projection and the Au-Au distance is 0.250 nm. Even crystals as small as these show a large amount of disorder. The lower crystal in the left micrograph in fig. 6 is showing evidence of a twin plane. The right micrograph is exactly the same area, approximately 15-20 s later. Here the twin plane has disappeared but a dislocation has appeared instead (an extra atom plane is inserted in the upper half of the crystal). The upper crystal has grown during this time with approximately 300 atoms to 2.6 nm diameter, assuming a spherical shape. The growth rate seems to be rather uniform in all directions since the habitus of the crystal is roughly maintained. A 1.4 nm crystal was also seen changing its perfect close-packing into a twinned structure while growing (figs. 7a and 7b). In fig. 8 a partial dislocation in an extremely small crystal (about 7 atom planes) is seen. The crystal is slightly tilted which is why we did not get two-dimensional resolution. Focusing was made on the carbon film or on larger gold crystals, since the fringes of crystals smaller than 10 nm were invisible to the eye on the fluorescent screen. Astigmatism correction was made on the amorphous carbon film and could easily be confirmed on the micrograph, since the clusters had a completely random orientation. The installation of the low-light TV camera and image processor provided useful help for viewing the microcrystals. Magnification could easily be increased to 24000000 X on a 9” TV screen with the use of contrast enhancement facilities. Resolution was not quite as good (about 0.28 nm) as on photographs. A lot of motion and dynamic events occurred within the sample, caused by the electron beam. Microcrystals could be seen aligning the lattices and fusing
Fig. 7. (a) Microcrystal into a twin.
with perfect close-packing
of atoms. (b) 30 s later the crystal has grown
L. R. Wallenberg et al. / Small gold crystals and large gold clusters
Fig. 8. Gold crystal with dislocation,
263
slightly titled from the (110) direction.
together, necks were formed between particles, crystals etc., and were recorded on video tape. crystals causing them to be out of orientation contrast gave the illusion of pulsating crystals. The image in fig. 9a shows a growing crystal, the lower (111) surface a new incomplete layer,
rearranging of the shape of the The constant movement of the and thereby changing their about 4.0 nm at the widest. On 1 atom high, has been formed.
Fig. 9. (a) crystal where new layer is formed on the lower (111) surface. (b) 0.35 s later one more row of atoms has been added towards the comer between the two (111) surfaces. (c) A few seconds later the new layer has been completed. Photographs were taken straight off the TV screen.
264
L. R. Wallenberg et al. / Small gold crystals and large gold clusters
Three atomic rows are still missing toward the corner with the other (111) surface. In fig. 9b one more atom row has been added. The time difference between the images is 0.35 s. An image recorded 45 s later shows the completed layer (fig. SC). The growth does not proceed in a straightforward manner. Atoms added to the site formed by the step are frequently removed again, or possibly the incomplete surface layer is rearranged. Motion is speeded up considerably when larger apertures were used as more electrons hit the sample, and also raising the temperature.
4. Conclusions The proposed model for the 55-atom gold cluster shows good agreement with the micrographs, both for crystal structure and crystal habitus. High-resolution transmission electron microscopy is the onljr way to confirm the model, since no single crystals can be obtained for X-ray analysis. Not even X-ray powder diffraction reveals any lines. The organic ligands and chlorine atoms are presumably evaporated and removed by the vacuum system, since EDX analysis only showed gold peaks and trace amounts of chlorine. Phosphor peaks, if present, were obscured by the Au Mcll peaks. Possibilities for enhancing contrast and higher magnification made the TV system a very useful tool, since details were revealed which were not visible to the human eye on the normal fluorescent viewing screen. Also dynamic events were recorded that would have been far to fast for the normal 3-8 s exposure time for photographic film. No time-consuming darkroom work is needed before seeing the results. Nucleation and growth processes could be studied with the 55-atom clusters as known starting point. Determination of the actual nucleation time and position is otherwise very difficult because of lack of contrast. New atoms were added at the site formed by the atomic step in the surface, that is, new layers were usually completed before the next layer was started.
References [l] G. S&mid et al., Chem. Ber. 114 (1981) 3634. [2] L.D. Marks and D.J. Smith, J. Crystal Growth 54 (1981) 425. [3] W.B. Monosmith and J.M. Cowley, Ultramicroscopy 12 (1984) 177.
256
ON THE CRYSTAL STRUCTURE LARGE GOLD CLUSTERS L.R. WALLENBERG
OF SMALL
Science 156 (1985) 2566264 North-Holland. Amsterdam
GOLD CRYSTALS
AND
and J.-O. BOVIN
Inorganic Chemistry 2, Chemical Center, P. 0. Box 124, S-221
00 Lund, Sweden
and G. SCHMID Inorganic Germany
Chemistry,
University
Received
16 July 1984; accepted
of Essen, Universitijtssirasse
for publication
29 August
5- 7, D-4300
Essen I, Fed. Rep.
of
1984
A large gold cluster with the formula Au,,[(C,H,)~P],,CI,, was imaged with a high-resolution transmission electron microscope (HRTEM) and a proposed model was confirmed. Growth of gold crystals of approximately 4.0 nm size could be followed, row by row of atoms, with the use of a low light level TV camera and on-line image processing.
1. Introduction Recent advances in preparation techniques of very large metallic clusters [l] together with instrumental improvements on high-resolution transmission electron microscopes (HRTEM) have made it possible to image the structure of clusters and microcrystals directly. This has attracted the attention from both practical as well as theoretical sides. Practical problems, as spreading an expensive catalyst efficiently on a support with a maintained narrow size distribution and sintering, which may change the activity and other properties of a catalyst, may be studied using HRTEM. More theoretical aspects, as how the large number of defects and faults in microcrystals are formed, and how strain caused by these defects is accommodated in the crystal as it grows, can be illuminated with this technique. Resolution limits for modern routine instruments is approaching 0.2 nm and, as shown here, the crystal structure of particles down to even 1.0 nm can be studied, using heavier elements as gold for maximum contrast. Supplying the transmission electron microscope with TV cameras makes it possible to follow dynamic events as crystal growth, virtually row by row of atoms, and rearrangements of atoms within the crystals in real time. 0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
L..R. Wallenberg et al. / Small gold crystals and large gold clusters
251
Earlier HRTEM work by Marks and Smith [2] on gold and silver crystals with sizes around 20 nm showed a great deal of complexity. Multiply twinned and lamellar twinned particles with mainly icosahedral and decahedra1 geometrical shape were described, as well as partial dislocations which were found in particles larger than about 15 nm. Cowley and Monosmith [3] have published electron microdriffraction pattern with the use of the very narrow electron beam in a STEM instrument, both for decahedra1 and cubeoctahedral particles ranging from 1.5 to 2.0 nm in size, together with theoretical calculations, although no crystal structure images could be obtained.
2. Experimental method Gold clusters containing 55 atoms were manufactured by reducing (C,H,),PAuCl with B,H, in benzene [l]. This gave a black precipitate which is stable in air. The chemical composition was found to be Au,,z[P(C,H,),],Cl. Molecular weight determination by means of an ultracentrifuge gave the formula Au,,[P(C,H,),],,Cl,. In different kinds of solvents, though with different rate, gold crystals are inevitably formed which can account for the small stoichiometric excess of gold. A model was proposed from Mossbauer spectra by Schmid et al. with cubic close-packing of the 55 gold atoms in a cubeoctahedron as shown in fig. 1. The cluster is almost completely isolated from the surroundings by the twelve triphenylphosphane groups situated in each corner, and the chlorine atoms on
Fig. 1. Model of S-atom
cluster, where atoms are ccp in a cubeoctahedron.
258
L. R. Wallenberg et al. / Small gold crystals and large gold clusters
the centre of the 3 X 3 square surfaces. This gives four groups of gold atoms, depending on the different types of surroundings. The number of atoms of each kind fits the interpretation of the Mossbauer spectra, based on the model. This is an extremely large cluster molecule. A JEOL 200CX transmission electron microscope equipped with a top-entry high-resolution goniometer stage was used for imaging, capable of 0.23 nm crystal structure resolution at 200 kV operating voltage. Recently, a low light level TV camera was installed, interfaced with the microscope through a fluorescent screen and a lead glass window in the bottom. The signal from the camera is put through an on-line video processor for noise reduction in real time. Two different types of video processors have been tested, both with 512 X 512 pixels resolution and 256 grey levels. The less expensive model used “pixel incrementing” for averaging where the value of every stored pixel is compared to the incoming value from the camera. A fixed increment of the difference is added to the stored value and the stored digital image can then continuously be viewed on a TV monitor. The other type of processor, which we found to give less tailing of moving objects, uses what is known as “true averaging” or “sliding average”. The displayed pixel value in this case is an exponentially weighted average of the values from up to 128 of the preceding frames. The processed image is then recorded on a video taperecorder. While replaying the tape, it is possible to freeze the image at any time and use the 50 Hz scanning frequency of the taperecorder as an accurate timestep for “frameby-frame” viewing. A drop of the sample, dispersed in methanol, was placed on an amorphous holey carbon film, the alcohol was evaporated at 25°C and the sample was then inserted into the microscope. Computer-simulated images using the SHRLI multislice program (M.A. O’Keefe) for the gold structure along the (110) direction with the actual instrumental parameters were performed. The simulations showed that, at Scherzer defocus and for crystal thicknesses up to 8.0 nm, black spots could be interpreted as atom positions.
3. Observations Mainly, three different sizes of particles were observed; (1) a few large, spherical or ellipsoidal, about 250-350 nm in diameter; (2) roughly spherical, lo-30 nm and (3) a large number of very small particles, about 1.0-5.0 nm, covering the carbon film and larger particles. All three are represented in figs. 2a and 2b. The medium-sized particles are on the surface of a large particle, 300 nm. The size of the particle indicated by an arrow in fig. 2a was determined to 26.0 nm by lattice spacings at higher magnification and in fig. 2b it is shown how it is covered by microcrystals approximately 1.0-2.0 nm.
L. R. Wallenberg et al. / Small gold crystals and large gold clusters
259
Electron diffraction images of the large particle showed only a few weak spots in a ring pattern, presumably arising from the medium-size crystals on the surface. This is due to the fact that the largest particles are assembled by a large number of the smallest clusters/microcrystals which are too small to give rise to a detectable diffraction pattern. This was confirmed by the fact that no ED patterns were obtainable from areas with only microcrystals whereas areas with crystals of category (2) present, showed clear ring patterns. This also explains the grainy appearance of the large spheres at higher magnification and the random orientation of lattice planes which can be resolved in minor areas. Medium-size particles (lo-30 nm) was found to consist mainly of multiplyand lamellar-twinned particles as described by Marks and Smith (fig. 3). The electron diffraction pattern from complex particles this size is shown in fig. 4. measuring the radius of the rangs by means of a calibrated aperture showed good agreement with data given in the literature for gold (marked in the figure). Gold is face-centered cubic with a = 0.408 nm. To obtain images of the smallest microcrystals, fresh samples and minimum intensity on the electron beam had to be used, since particles this size are not stable in the beam; they immediately start growing. A crystal, approximately 1.2 nm across is imaged in figs. 5a where the crystal habitus and structure image shows a good resemblance with the proposed model viewed along (110)
Fig. 2. (a) Spherical particle, about 300 nm in diameter with smaller crystals, lo-30 nm, on the surface. The edge of the carbon film is seen in the lower part. (b) Enlargement of crystal indicated by an arrow in (a). Microcrystals, 1.0-2.0 nm, are covering the crystal and carbon film. Top left is seen the edge of the large particle, built by microcrystals.
260
L R. Wailenberg et oi. / Small goid crystak and large gold clusters
Fig. 3. Multiply-
and tamelIar-twinned
particles.
about
10 nm in diameter.
Fig. 4. Electron diffraction pattern from particles up to 20 nm. Ring radii indicated in the fig;ure correspond to lattice spacings given in the literature. To the right the area selected by the diffraction aperture is seen.
I&R. Wailenberg et al. / Smufi goid crystals and large gold clusters
Fig. 5. (a) Microcrystal, approximately 1.2 nm cubeoctahedron. Lattice spacings between crossing 55-atom cluster, viewed along (110).
261
across, showing cross section similar to a (111) planes is 0.235 nm. (b) Model of the
with Fig. 6. Left micrograph is recorded IS-20 s before the one to the right. Crystal A has grown 1 approximately 300 atoms to 2.6 nm in diameter during this time. in crystal B the twin plane has disappeared.
262
LR.
Wallenberg et al. / Small gold crystals and large gold clusters
as in fig. 5b. The spacings between the crossing (111) planes is 0.235 nm in this projection and the Au-Au distance is 0.250 nm. Even crystals as small as these show a large amount of disorder. The lower crystal in the left micrograph in fig. 6 is showing evidence of a twin plane. The right micrograph is exactly the same area, approximately 15-20 s later. Here the twin plane has disappeared but a dislocation has appeared instead (an extra atom plane is inserted in the upper half of the crystal). The upper crystal has grown during this time with approximately 300 atoms to 2.6 nm diameter, assuming a spherical shape. The growth rate seems to be rather uniform in all directions since the habitus of the crystal is roughly maintained. A 1.4 nm crystal was also seen changing its perfect close-packing into a twinned structure while growing (figs. 7a and 7b). In fig. 8 a partial dislocation in an extremely small crystal (about 7 atom planes) is seen. The crystal is slightly tilted which is why we did not get two-dimensional resolution. Focusing was made on the carbon film or on larger gold crystals, since the fringes of crystals smaller than 10 nm were invisible to the eye on the fluorescent screen. Astigmatism correction was made on the amorphous carbon film and could easily be confirmed on the micrograph, since the clusters had a completely random orientation. The installation of the low-light TV camera and image processor provided useful help for viewing the microcrystals. Magnification could easily be increased to 24000000 X on a 9” TV screen with the use of contrast enhancement facilities. Resolution was not quite as good (about 0.28 nm) as on photographs. A lot of motion and dynamic events occurred within the sample, caused by the electron beam. Microcrystals could be seen aligning the lattices and fusing
Fig. 7. (a) Microcrystal into a twin.
with perfect close-packing
of atoms. (b) 30 s later the crystal has grown
L. R. Wallenberg et al. / Small gold crystals and large gold clusters
Fig. 8. Gold crystal with dislocation,
263
slightly titled from the (110) direction.
together, necks were formed between particles, crystals etc., and were recorded on video tape. crystals causing them to be out of orientation contrast gave the illusion of pulsating crystals. The image in fig. 9a shows a growing crystal, the lower (111) surface a new incomplete layer,
rearranging of the shape of the The constant movement of the and thereby changing their about 4.0 nm at the widest. On 1 atom high, has been formed.
Fig. 9. (a) crystal where new layer is formed on the lower (111) surface. (b) 0.35 s later one more row of atoms has been added towards the comer between the two (111) surfaces. (c) A few seconds later the new layer has been completed. Photographs were taken straight off the TV screen.
264
L. R. Wallenberg et al. / Small gold crystals and large gold clusters
Three atomic rows are still missing toward the corner with the other (111) surface. In fig. 9b one more atom row has been added. The time difference between the images is 0.35 s. An image recorded 45 s later shows the completed layer (fig. SC). The growth does not proceed in a straightforward manner. Atoms added to the site formed by the step are frequently removed again, or possibly the incomplete surface layer is rearranged. Motion is speeded up considerably when larger apertures were used as more electrons hit the sample, and also raising the temperature.
4. Conclusions The proposed model for the 55-atom gold cluster shows good agreement with the micrographs, both for crystal structure and crystal habitus. High-resolution transmission electron microscopy is the onljr way to confirm the model, since no single crystals can be obtained for X-ray analysis. Not even X-ray powder diffraction reveals any lines. The organic ligands and chlorine atoms are presumably evaporated and removed by the vacuum system, since EDX analysis only showed gold peaks and trace amounts of chlorine. Phosphor peaks, if present, were obscured by the Au Mcll peaks. Possibilities for enhancing contrast and higher magnification made the TV system a very useful tool, since details were revealed which were not visible to the human eye on the normal fluorescent viewing screen. Also dynamic events were recorded that would have been far to fast for the normal 3-8 s exposure time for photographic film. No time-consuming darkroom work is needed before seeing the results. Nucleation and growth processes could be studied with the 55-atom clusters as known starting point. Determination of the actual nucleation time and position is otherwise very difficult because of lack of contrast. New atoms were added at the site formed by the atomic step in the surface, that is, new layers were usually completed before the next layer was started.
References [l] G. S&mid et al., Chem. Ber. 114 (1981) 3634. [2] L.D. Marks and D.J. Smith, J. Crystal Growth 54 (1981) 425. [3] W.B. Monosmith and J.M. Cowley, Ultramicroscopy 12 (1984) 177.