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A superstructure on {111} faces of AgBr
Surface Science Letters 261 (1992) L39-L43. North-Holland
surface science letters
Surface Science Letters
A superstructure
on { 111) faces of AgBr
H. Haefke Instituteof Physics, University of Basel, Klingelbergstrasse
82, CH-4056 Basel, Switzerland
and M. Krohn Institute of Solid State Physics and Electron Microscopy,
Weinberg 2, O-4050 Halle (Saale), Germany
Received 28 August 1991; accepted for publication 7 October 1991
We have studied the surfaces of discontinuous AgBr films epitaxially grown on LiF(OO1).The 3D AgBr islands are preferentially (111) oriented with [llO] AgBr 11[1101LiF and [llO] AgBr II[ilO] LiF, respectively. By using the gold decoration technique a superstructure is revealed on the (111) top surfaces of the AgBr islands. The decorating gold nuclei are aligned along the (110) directions of the AgBr lattice. Optical diffraction exhibits sixfold symmetry of the superstructure with spacings of about 8.3 nm.
1. Introduction The morphology of silver halide surfaces, i.e., their structure on a scale of nanometer to micrometer, is of particular interest in the field of photography. Photographic phenomena are strongly affected by certain surface properties of the silver halides [ll. In the last decades many studies have been attempted to examine the structure of (100) and (111) surfaces, because they are the prevail faces at silver halide microcrystals found in photographic materials. For (100) faces of AgBr and AgCl the gold decoration technique reveals the complete step structure, including steps of monatomic height [2-41. Very recently atomic force microscopy @FM) enabled the resolution of individual atoms at 1100) surfaces of AgBr [5,6]. Up to now, for Illl] faces of NaCl-type silver halides the surface structure has not yet been well-characterized on a nanometer scale. Electrostatic arguments suggest that the 1111) faces can only exist if substantial reconstruction or adsorption of species occur at the surface [7]. Hamilton and Brady [S], using 0039-6028/92/$05.00
electron microscopy and low-angle electron diffraction of platinum-palladium nuclei deposited on (111) oriented AgBr films, observed a hexagonal array of the metal nuclei. They proposed a model based on the premise that the (111) surface layer consists of a half layer of either bromine ions or silver ions. Recently Baetzold et al. [93 have investigated theoretically the configurations of AgBr surfaces by employing simulation techniques. Their equilibrium configurations, obtained for {ill} faces, could not confirm the specific model of Hamilton and Brady [81. However, Surface-extended-X-ray-absorption-fine-structure (SEXAFS) measurements [lo] agree, within the limits of experimental uncertainty, with the theoretical calculations of Baetzold et al. [91. Moreover, we found no references on facetting of AgBrIlll] faces into 1100) planes similar to that observed on (111) surfaces of NaCl [11,12] and MgO [13,14]. In order to study (111) surfaces of AgBr we have used the decoration technique. Discontinuous AgBr films were produced on LiF(OO1) to determine the shapes and epitaxial relationship
0 1992 - Elsevier Science Publishers B.V. All rights reserved
H. Haefke, M. Krohn / A superstructure on { 11 I} faces of AgBr
and to investigate the superstructure surface of AgBr(ll1) islands.
on the top
2. Experimental The AgBr films were grown by vapour deposition on LiF(001) faces freshly cleaved in an oilfree vacuum. During the AgBr evaporation the background pressure was always better than 5 X lop7 mbar. The AgBr films were deposited at a substrate temperature of about 300 o C measured by a NiCr-Ni thermocouple cemented in the LiF crystal. Nominal film thicknesses between 100 and 300 nm were deposited at a rate of 0.2 nm s-l. The AgBr evaporator, particularly designed
because of the high reactivity of the silver halides, has been described previously [4]. The decoration technique [15,16] applied to study the surface structure of the AgBr island films was carried out (in the same vacuum cycle) at 50°C by depositing a quantity of gold, equivalent to a film thickness of 0.2 nm, at a rate of 0.001 nm s-‘. Film thicknesses and deposition rate were monitored by a quartz crystal oscillator. Following the deposition of gold, a platinum/ carbon layer was evaporated from an oblique direction to provide the supporting layer for the gold nuclei and to give shadow contrasts of the gross structure. An accurate adjustment of the Pt/C source [17,18] towards the LiF crystal allowed the epitaxial orientation as well as the
Fig. 1. Electron micrograph of the replica of an epitaxial AgBr film (nominal thickness 200 nm) grown by vapour deposition on LiF(001) at 300°C. The islands are predominantly (111) oriented with triangular or hexagonal shapes. Occasionally square or rectangular AgBr(001) islands occur (arrows; left-hand ED pattern). The right-hand ED pattern reflects the (111) orientation of the hexagonally shaped island in the image centre. A schematic contour drawing (right-hand inset) illustrates the growth faces of this island.
H. Haejlce, M. Krohn /A
superstructure on {Ill)
height of the AgBr islands to be determined from electron micrographs. The replica with the adherent gold nuclei was separated from the LiF crystal and the AgBr film by floating it off in a 10% solution of Na,S,O,. Having been washed, the replica was mounted on a grid and was examined in a transmission electron microscope in bright field and dark field modes and by electron diffraction (ED). The optical diffractometer used for the analysis of electron micrographs has been described elsewhere [191. -
3. Results and discussion Vapour growth of AgBr on LiF(001) at 300°C proceeds via birth and spreading of 3D islands. Fig. 1 shows a replica of an epitaxially grown AgBr island film (nominal thickness 200 nm). The large variety in dimensions of the islands originates in the liquid-like coalescence occurring during the growth process. This kind of coalescence is characterized by merging of neighbouring islands and by secondary nucleation in bare substrate areas which results from the liquid-like regression of the circumferences of the islands. At the first growth stage 3D islands mostly have a triangular pyramidal shape. On further film growth, these islands predominantly extend in lateral directions and their pyramidal tips appear more or less truncated. In order to determine the orientation as well as the epitaxial relationship of the AgBr islands, we made use of an epitaxy effect already known for platinum-palladium [S] and gold [18] on AgBr. It has turned out that the deposited gold, under sufficiently clean conditions 1201, grows in perfect epitaxy on the AgBr islands. Consequently, the ED patterns of the gold nuclei (insets in fig. 1) reflect perfectly the orientation of the AgBr islands. In general the islands are (111) oriented (right-hand ED pattern in fig. 1); only occasionally (001) oriented islands with square or rectangular shape were observed (see arrows in fig. 1; left-hand ED pattern). By comparing significant directions in the ED patterns with the thoroughly adjusted direction of
faces of AgBr
the Pt/C shadow, the following epitaxial relationships have been identified: [llO]( 111) AgBr II[ llO](OOl) LiF, [llO](lll)
AgBr II[ilO](OOl) LiF,
[lOO](OOl) AgBr Il[lOO](OOl) LiF, [lOO](OOl) AgBr Il[llO](OOl) LiF. Similar epitaxial relationships have been reported for thick AgBr films melt-grown on LiF cleavage faces [21]. A very large truncated AgBr island is seen in the centre of fig. 1. By means of an accurate adjustment of the Pt/C source the height of this hexagonal island was determined to be about 250 nm. Because of their high surface-to-volume ratio can be concluded that such islands are quite similar in shape to the well-known tabular microcrystals (or “T-grains”) as used in highly sensitive photographic emulsions [22,231. The side walls are formed alternately by (100) and (111) faces, as found by determining their angles to the (111) basal plane which amount to 55 k 4” for the {loo} faces and 70 + 4 o for the {ill} faces. A schematic contour drawing of the hexagonally shaped--- island is insetted in fig. 1; as basal plane the (111) face was chosen. The (111) oriented islands show a cube-octahedra morphology with rounded edges which represent the equilibrium crystal habitus of AgBr. At a sufficiently high magnification the surface fine structure of individual AgBr islands, revealed by gold decoration, becomes visible. Fig. 2a shows and characteristic micrograph of the top surface of a truncated AgBr(ll1) island. Although the gold nuclei do not decorate typical surface steps being known of {loo} surfaces of silver halides 12-41, they appear to grow at preferential sites. Often the nuclei form shevron-like arrays with short sizes being mostly less than 0.1 pm. Obviously this decoration pattern reflects a superstructure due to the surface reconstruction. By looking at the micrograph in certain directions oblique to the image plane it can be seen that the gold nuclei are highly ordered along three directions. These directions are indicated by solid lines in fig. 2a. Taking advantage of the epitaxy effect the ED analysis permits to identify the significant
H. Haefke, M. Krohn /A
superstructure on {Ill}
directions as the (110) directions of the AgBr lattice. Our result is in marked contrast to that reported by Hamilton and Brady [8]. They found that the metal nuclei are aligned along the (642) directions. In addition to ED, optical diffraction is a powerful technique for studying preferred orientations and mean spacings of the gold nuclei. Fig. 2b shows an optical diffractogram of the decorated surface structure. There are six prominent spots, lying on a ring, at a radial distance corresponding to an 8.3 nm spacing of the rows of gold nuclei. The spots are equally spaced on the ring and exhibit a sixfold symmetry in accordance with the honeycomb-like arrangement of the gold nuclei. By way of contrast Hamilton and Brady [8] have obtained low-angle ED patterns with twelve spots having a 7.0 nm spacing. We attribute this difference to the lower substrate temperatures ranging between room temperature and lOo”C, which were applied by them. It should be noted that the arrangements of the decorating gold
faces of AgBr
nuclei can also be influenced by the residual gas of the vacuum [20]. In an earlier paper [24] we have reported that at a slight gas adsorption, as it still occurs at 100 OC in a high vacuum, the AgBr{ 111) faces appear as structure-less and can then be distinguished from the (100) faces by a smaller density of gold nuclei.
4. Conclusion The surface structure and epitaxial relationships of discontinuous AgBr films were studied by means of surface decoration technique. On the 1111) top surfaces of 3D AgBr islands gold decoration reveals a superstructure of sixfold symmetry and with mean spacings of about 8.3 nm. Comparing of our results with those of others authors [81 we suppose that the superstructure formation is induced by surface reconstruction and is dependent on the substrate temperature.
Fig. 2. (a) Gold decoration of a superstructure on an (111) face of AgBr. The gold nuclei are arranged along the (110) directions the AgBr lattice. The relevant three directions are indicated by solid lines. (b) The optical diffractogram reveals a sixfold symmet of the superstructure with periodicities of about 8.3 nm.
H. Haefke, hi. Krohn / A superstructure on {Ill}
This could be an evidence for micro-facetting along (100) planes as it has been observed for (111) faces of NaCl [11,12] and MgO [13,14]. The nature of the AgBr(ll1) faces still remains an open question for further experiments and calculations. Studies with the atomic force microscope are planned to reveal the atomic structure of these surfaces.
Acknowledgements
We wish to thank R. Hillebrandt for cal diffraction measurements and U.D. for many interesting discussions. The Laboratory of Photoprocesses in Sofia is edged for providing the AgBr material.
the optiSchwarz Central acknowl-
References
[II J.F. Hamilton, 121H. Haetke, M.
Adv. Phys. 37 (1988) 359. Krohn and A. Panov, J. Cryst. Growth
49 (1980) 7. and M. Krohn, J. 131H. Haetke, A. Panov, H. Hofmeister Photogr. Sci. 32 (1984) 8. [41H. Haefke, R. Mattheis and M. Krohn, Thin Solid Films 195 (1991) 225. H. Haetlce, G. Gerth and El E. Meyer, H.-J. Giintherodt, M. Krohn, Europhys. Lett. 15 (1991) 319.
faces of AgBr
[61 H. Haetke, E. Meyer, H.-J. Giintherodt, G. Gerth and M. Krohn, J. Imaging Sci. 35 (1991) 290. [7] P.W. Tasker, J. Phys. C (Solid State Phys.) 12 (1979) 4977. [8] J.F. Hamilton and L.E. Brady, Surf. Sci. 23 (1970) 389. [9] R.C. Baetzold, Y.T. Tan and P.W. Tasker, Surf. Sci. 195 (1988) 579. [lo] P. Tangyunyong, T.N. Rhodin, R. Ozer, D. Batchelor, Y.T. Tan and K.J. Lushington, Phys. B 158 (1989) 637. [ll] D. Knoppik and A. Liisch, J. Ctyst. Growth 34 (1976) 332. [12] K.W. Keller, Metall. Trans. A 22 (1991) 1299. [13] V.E. Henrich, Surf. Sci. 57 (1976) 385. [14] H. Onishi, C. Egawa, T. Aruga and Y. Iwasawa, Surf. Sci. 191 (1987) 479. [15] G.A. Bassett, Philos. Mag. 3 (1958) 1042. [16] M. Krohn, Vacuum 37 (1987) 67. 1171 A. Panov, H. Haefke and M. Krohn, J. Cryst. Growth 58 (1982) 452. 1181 H. Hofmeister, H. Haefke and A. Panov, J. Ctyst. Growth 58 (1982) 500. [19] W. Neumann, R. Hillebrandt and T. Krajewski, in: Electron Microscopy in Solid State Physics, Eds. H. Bethge and J. Heydenreich (Elsevier, Amsterdam, 1987) p. 265. [20] H. Haefke, H. Hofmeister and M. Krohn, J. Inform. Rec. Mater. 15 (1987) 123. [21] V.I. Saunders and W. West, Mater. Res. Bull. 4 (1969) 203. 1221 J.E. Maskasky, J. Imaging Sci. 31 (1987) 15. 1231 J.E. Maskasky, J. Imaging Sci. 31 (1987) 93. [24] H. Haefke and M. Krohn, J. Photogr. Sci. 34 (1986) 25.
surface science letters
Surface Science Letters
A superstructure
on { 111) faces of AgBr
H. Haefke Instituteof Physics, University of Basel, Klingelbergstrasse
82, CH-4056 Basel, Switzerland
and M. Krohn Institute of Solid State Physics and Electron Microscopy,
Weinberg 2, O-4050 Halle (Saale), Germany
Received 28 August 1991; accepted for publication 7 October 1991
We have studied the surfaces of discontinuous AgBr films epitaxially grown on LiF(OO1).The 3D AgBr islands are preferentially (111) oriented with [llO] AgBr 11[1101LiF and [llO] AgBr II[ilO] LiF, respectively. By using the gold decoration technique a superstructure is revealed on the (111) top surfaces of the AgBr islands. The decorating gold nuclei are aligned along the (110) directions of the AgBr lattice. Optical diffraction exhibits sixfold symmetry of the superstructure with spacings of about 8.3 nm.
1. Introduction The morphology of silver halide surfaces, i.e., their structure on a scale of nanometer to micrometer, is of particular interest in the field of photography. Photographic phenomena are strongly affected by certain surface properties of the silver halides [ll. In the last decades many studies have been attempted to examine the structure of (100) and (111) surfaces, because they are the prevail faces at silver halide microcrystals found in photographic materials. For (100) faces of AgBr and AgCl the gold decoration technique reveals the complete step structure, including steps of monatomic height [2-41. Very recently atomic force microscopy @FM) enabled the resolution of individual atoms at 1100) surfaces of AgBr [5,6]. Up to now, for Illl] faces of NaCl-type silver halides the surface structure has not yet been well-characterized on a nanometer scale. Electrostatic arguments suggest that the 1111) faces can only exist if substantial reconstruction or adsorption of species occur at the surface [7]. Hamilton and Brady [S], using 0039-6028/92/$05.00
electron microscopy and low-angle electron diffraction of platinum-palladium nuclei deposited on (111) oriented AgBr films, observed a hexagonal array of the metal nuclei. They proposed a model based on the premise that the (111) surface layer consists of a half layer of either bromine ions or silver ions. Recently Baetzold et al. [93 have investigated theoretically the configurations of AgBr surfaces by employing simulation techniques. Their equilibrium configurations, obtained for {ill} faces, could not confirm the specific model of Hamilton and Brady [81. However, Surface-extended-X-ray-absorption-fine-structure (SEXAFS) measurements [lo] agree, within the limits of experimental uncertainty, with the theoretical calculations of Baetzold et al. [91. Moreover, we found no references on facetting of AgBrIlll] faces into 1100) planes similar to that observed on (111) surfaces of NaCl [11,12] and MgO [13,14]. In order to study (111) surfaces of AgBr we have used the decoration technique. Discontinuous AgBr films were produced on LiF(OO1) to determine the shapes and epitaxial relationship
0 1992 - Elsevier Science Publishers B.V. All rights reserved
H. Haefke, M. Krohn / A superstructure on { 11 I} faces of AgBr
and to investigate the superstructure surface of AgBr(ll1) islands.
on the top
2. Experimental The AgBr films were grown by vapour deposition on LiF(001) faces freshly cleaved in an oilfree vacuum. During the AgBr evaporation the background pressure was always better than 5 X lop7 mbar. The AgBr films were deposited at a substrate temperature of about 300 o C measured by a NiCr-Ni thermocouple cemented in the LiF crystal. Nominal film thicknesses between 100 and 300 nm were deposited at a rate of 0.2 nm s-l. The AgBr evaporator, particularly designed
because of the high reactivity of the silver halides, has been described previously [4]. The decoration technique [15,16] applied to study the surface structure of the AgBr island films was carried out (in the same vacuum cycle) at 50°C by depositing a quantity of gold, equivalent to a film thickness of 0.2 nm, at a rate of 0.001 nm s-‘. Film thicknesses and deposition rate were monitored by a quartz crystal oscillator. Following the deposition of gold, a platinum/ carbon layer was evaporated from an oblique direction to provide the supporting layer for the gold nuclei and to give shadow contrasts of the gross structure. An accurate adjustment of the Pt/C source [17,18] towards the LiF crystal allowed the epitaxial orientation as well as the
Fig. 1. Electron micrograph of the replica of an epitaxial AgBr film (nominal thickness 200 nm) grown by vapour deposition on LiF(001) at 300°C. The islands are predominantly (111) oriented with triangular or hexagonal shapes. Occasionally square or rectangular AgBr(001) islands occur (arrows; left-hand ED pattern). The right-hand ED pattern reflects the (111) orientation of the hexagonally shaped island in the image centre. A schematic contour drawing (right-hand inset) illustrates the growth faces of this island.
H. Haejlce, M. Krohn /A
superstructure on {Ill)
height of the AgBr islands to be determined from electron micrographs. The replica with the adherent gold nuclei was separated from the LiF crystal and the AgBr film by floating it off in a 10% solution of Na,S,O,. Having been washed, the replica was mounted on a grid and was examined in a transmission electron microscope in bright field and dark field modes and by electron diffraction (ED). The optical diffractometer used for the analysis of electron micrographs has been described elsewhere [191. -
3. Results and discussion Vapour growth of AgBr on LiF(001) at 300°C proceeds via birth and spreading of 3D islands. Fig. 1 shows a replica of an epitaxially grown AgBr island film (nominal thickness 200 nm). The large variety in dimensions of the islands originates in the liquid-like coalescence occurring during the growth process. This kind of coalescence is characterized by merging of neighbouring islands and by secondary nucleation in bare substrate areas which results from the liquid-like regression of the circumferences of the islands. At the first growth stage 3D islands mostly have a triangular pyramidal shape. On further film growth, these islands predominantly extend in lateral directions and their pyramidal tips appear more or less truncated. In order to determine the orientation as well as the epitaxial relationship of the AgBr islands, we made use of an epitaxy effect already known for platinum-palladium [S] and gold [18] on AgBr. It has turned out that the deposited gold, under sufficiently clean conditions 1201, grows in perfect epitaxy on the AgBr islands. Consequently, the ED patterns of the gold nuclei (insets in fig. 1) reflect perfectly the orientation of the AgBr islands. In general the islands are (111) oriented (right-hand ED pattern in fig. 1); only occasionally (001) oriented islands with square or rectangular shape were observed (see arrows in fig. 1; left-hand ED pattern). By comparing significant directions in the ED patterns with the thoroughly adjusted direction of
faces of AgBr
the Pt/C shadow, the following epitaxial relationships have been identified: [llO]( 111) AgBr II[ llO](OOl) LiF, [llO](lll)
AgBr II[ilO](OOl) LiF,
[lOO](OOl) AgBr Il[lOO](OOl) LiF, [lOO](OOl) AgBr Il[llO](OOl) LiF. Similar epitaxial relationships have been reported for thick AgBr films melt-grown on LiF cleavage faces [21]. A very large truncated AgBr island is seen in the centre of fig. 1. By means of an accurate adjustment of the Pt/C source the height of this hexagonal island was determined to be about 250 nm. Because of their high surface-to-volume ratio can be concluded that such islands are quite similar in shape to the well-known tabular microcrystals (or “T-grains”) as used in highly sensitive photographic emulsions [22,231. The side walls are formed alternately by (100) and (111) faces, as found by determining their angles to the (111) basal plane which amount to 55 k 4” for the {loo} faces and 70 + 4 o for the {ill} faces. A schematic contour drawing of the hexagonally shaped--- island is insetted in fig. 1; as basal plane the (111) face was chosen. The (111) oriented islands show a cube-octahedra morphology with rounded edges which represent the equilibrium crystal habitus of AgBr. At a sufficiently high magnification the surface fine structure of individual AgBr islands, revealed by gold decoration, becomes visible. Fig. 2a shows and characteristic micrograph of the top surface of a truncated AgBr(ll1) island. Although the gold nuclei do not decorate typical surface steps being known of {loo} surfaces of silver halides 12-41, they appear to grow at preferential sites. Often the nuclei form shevron-like arrays with short sizes being mostly less than 0.1 pm. Obviously this decoration pattern reflects a superstructure due to the surface reconstruction. By looking at the micrograph in certain directions oblique to the image plane it can be seen that the gold nuclei are highly ordered along three directions. These directions are indicated by solid lines in fig. 2a. Taking advantage of the epitaxy effect the ED analysis permits to identify the significant
H. Haefke, M. Krohn /A
superstructure on {Ill}
directions as the (110) directions of the AgBr lattice. Our result is in marked contrast to that reported by Hamilton and Brady [8]. They found that the metal nuclei are aligned along the (642) directions. In addition to ED, optical diffraction is a powerful technique for studying preferred orientations and mean spacings of the gold nuclei. Fig. 2b shows an optical diffractogram of the decorated surface structure. There are six prominent spots, lying on a ring, at a radial distance corresponding to an 8.3 nm spacing of the rows of gold nuclei. The spots are equally spaced on the ring and exhibit a sixfold symmetry in accordance with the honeycomb-like arrangement of the gold nuclei. By way of contrast Hamilton and Brady [8] have obtained low-angle ED patterns with twelve spots having a 7.0 nm spacing. We attribute this difference to the lower substrate temperatures ranging between room temperature and lOo”C, which were applied by them. It should be noted that the arrangements of the decorating gold
faces of AgBr
nuclei can also be influenced by the residual gas of the vacuum [20]. In an earlier paper [24] we have reported that at a slight gas adsorption, as it still occurs at 100 OC in a high vacuum, the AgBr{ 111) faces appear as structure-less and can then be distinguished from the (100) faces by a smaller density of gold nuclei.
4. Conclusion The surface structure and epitaxial relationships of discontinuous AgBr films were studied by means of surface decoration technique. On the 1111) top surfaces of 3D AgBr islands gold decoration reveals a superstructure of sixfold symmetry and with mean spacings of about 8.3 nm. Comparing of our results with those of others authors [81 we suppose that the superstructure formation is induced by surface reconstruction and is dependent on the substrate temperature.
Fig. 2. (a) Gold decoration of a superstructure on an (111) face of AgBr. The gold nuclei are arranged along the (110) directions the AgBr lattice. The relevant three directions are indicated by solid lines. (b) The optical diffractogram reveals a sixfold symmet of the superstructure with periodicities of about 8.3 nm.
H. Haefke, hi. Krohn / A superstructure on {Ill}
This could be an evidence for micro-facetting along (100) planes as it has been observed for (111) faces of NaCl [11,12] and MgO [13,14]. The nature of the AgBr(ll1) faces still remains an open question for further experiments and calculations. Studies with the atomic force microscope are planned to reveal the atomic structure of these surfaces.
Acknowledgements
We wish to thank R. Hillebrandt for cal diffraction measurements and U.D. for many interesting discussions. The Laboratory of Photoprocesses in Sofia is edged for providing the AgBr material.
the optiSchwarz Central acknowl-
References
[II J.F. Hamilton, 121H. Haetke, M.
Adv. Phys. 37 (1988) 359. Krohn and A. Panov, J. Cryst. Growth
49 (1980) 7. and M. Krohn, J. 131H. Haetke, A. Panov, H. Hofmeister Photogr. Sci. 32 (1984) 8. [41H. Haefke, R. Mattheis and M. Krohn, Thin Solid Films 195 (1991) 225. H. Haetlce, G. Gerth and El E. Meyer, H.-J. Giintherodt, M. Krohn, Europhys. Lett. 15 (1991) 319.
faces of AgBr
[61 H. Haetke, E. Meyer, H.-J. Giintherodt, G. Gerth and M. Krohn, J. Imaging Sci. 35 (1991) 290. [7] P.W. Tasker, J. Phys. C (Solid State Phys.) 12 (1979) 4977. [8] J.F. Hamilton and L.E. Brady, Surf. Sci. 23 (1970) 389. [9] R.C. Baetzold, Y.T. Tan and P.W. Tasker, Surf. Sci. 195 (1988) 579. [lo] P. Tangyunyong, T.N. Rhodin, R. Ozer, D. Batchelor, Y.T. Tan and K.J. Lushington, Phys. B 158 (1989) 637. [ll] D. Knoppik and A. Liisch, J. Ctyst. Growth 34 (1976) 332. [12] K.W. Keller, Metall. Trans. A 22 (1991) 1299. [13] V.E. Henrich, Surf. Sci. 57 (1976) 385. [14] H. Onishi, C. Egawa, T. Aruga and Y. Iwasawa, Surf. Sci. 191 (1987) 479. [15] G.A. Bassett, Philos. Mag. 3 (1958) 1042. [16] M. Krohn, Vacuum 37 (1987) 67. 1171 A. Panov, H. Haefke and M. Krohn, J. Cryst. Growth 58 (1982) 452. 1181 H. Hofmeister, H. Haefke and A. Panov, J. Ctyst. Growth 58 (1982) 500. [19] W. Neumann, R. Hillebrandt and T. Krajewski, in: Electron Microscopy in Solid State Physics, Eds. H. Bethge and J. Heydenreich (Elsevier, Amsterdam, 1987) p. 265. [20] H. Haefke, H. Hofmeister and M. Krohn, J. Inform. Rec. Mater. 15 (1987) 123. [21] V.I. Saunders and W. West, Mater. Res. Bull. 4 (1969) 203. 1221 J.E. Maskasky, J. Imaging Sci. 31 (1987) 15. 1231 J.E. Maskasky, J. Imaging Sci. 31 (1987) 93. [24] H. Haefke and M. Krohn, J. Photogr. Sci. 34 (1986) 25.