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Electron microscopic studies of surface structures and some relations to surface phenomena
SURFACE
SCIENCE 3 (1964) 33-41;
ELECTRON SURFACE
o North-Holland
MICROSCOPIC
STRUCTURES SURFACE
AND
Publishing
STUDIES
SOME
PHENOMENA
Co., Amsterdam
OF
RELATIONS
TO
*
H. BETHGE German Academy of Sciences at Berlin, Inst. of Electron Microscopy, Halle (Saale) Received 29 June 1964 Surface structures on NaCl crystals detectable by the electron microscopic image with step heights down to one interplanar spacing, are described. The corresponding forms of producing the structures are discussed. Structures obtained by evaporation in a high vacuum are of special interest, as they are well reproducible. The conditions for the surface decoration method used for the image formation are discussed. First investigations to form images of monoatomic steps by means of the decoration method also on metal crystals show that this is possible under certain conditions.
1. Introduction
The surface of a macroscopic crystal given by a lattice plane of definite indication will be smooth in an atomic scale only in smallest regions. Due to corresponding pretreatment or formation of the surface mainly atomic steps occur, and these structures must be investigated for exactly describing a surface. The atomic steps of some ion crystals can electron microscopically form images by means of the surface decoration method. NaCl was investigated to a large extent, and this certainly is the crystal the surface structure of which is best known at present l). 2. The process of surface decoration
For carrying out surface decoration, described as Au-decoration first by Bassetts), the crystal is evaporated with a very thin film of Au, Pd, or Pt. On the crystal heated only slightly (to about 100 “C) metal atoms, hitting the crystal, do not form a homogeneous film, but they coalesce to small nuclei which preferably arrange themselves along the steps. But in regions which are not disturbed these nuclei are statistically distributed. A carbon film subsequently evaporated encloses the nuclei, and after stripping the film can * Delivered at the International Surfaces, Providence, 1964.
Conference
33
on the Physics and Chemistry
of Solid
34
H. BETHGE
be investigated under the electron microscope. The step structures crystal surface can be seen in the image of the gold-nuclei distribution. shows a picture
of manifold
magnification
than in the case of Au-evaporation
of an Au-decorated
Pd and Pt decoration
of the Fig. 1
step. Other
resp. under other-
wise similar conditions (rate of evaporation, temperature of NaCl crystal) results in a smaller diameter of the metal nuclei. The achievable lateral resolving power is determined by the size of the metal nuclei. From the surface structures studied the requisite for the resolving power is derived, and correspondingly it must be decided which method could be used advantageously. Fig. 2 shows an example of Pd decorations). On the surface of a
Fig. 1. Au nuclei arranged in statistical distribution and along a step.
Fig. 2. Slip structures on a NaCl crystal, deformed in a vacuum by bending, which are represented by palladium decoration.
deformed NaCl crystal slip traces of individual up to 30 A are reproduced fairly well separated.
dislocations
with intervals
3. Evaporation structures on crystal surfaces Evaporation structures on a (lOO)-plane of a NaCl crystal, created by annealing in a high vacuum (10e5 Torr) can easily be seen*). Experimentally for this purpose the crystal is so mounted in a furnace that the plane to be investigated lies open towards the vacuum. In order to avoid back condensation as far as possible, there shall possibly be no increased partial pressure of NaCl vapour on the evaporating plane. Starting from the emergence points of the dislocations lamellar structures form, which completely determine the surface structure in the regions free from higher cleavage steps. This is shown in fig. 3. Starting from the emergence point of screw dislocations we each time find a step the height of which is one or two interplanar spacings according to the corresponding Burgers vector. Because of the lower bond energy of the molecules in the step the decomposition of the crystal begins at these steps, and in the course of this
ELECTRON
MICROSCOPIC
process the step forms a spiral. The round
STUDIES
35
spirals show a step height of one
interplanar spacing, i.e. 2.82 A. The square spirals have twice this height, and it can be seen that always two curved steps enter one straight. In the region of the emergence point of edge dislocations there is an excess charge of an ion which effects a preferred surface nucleation. In the further course of evaporation annular lamellae also with a height of only 2.82 8, are produced. The structures are in agreement with the Burgers vectors to be expected in the NaCl lattice. For simplicity we above spoke of edge and screw dislocations only. To be more exact, the formation of spiral lamellae or annular lamellae each time depends on the fact, whether the dislocations
Fig. 3.
Evaporation structures around screw dislocations (left) and around edge dislocation (right). Annealing condition: 450 “C, 30 min.
have a component of the Burgers vector perpendicular to the (I OO)-plane or whether the Burgers vector lies in the cubic plane. The formation of the structures can be well explained by the crystal decomposition in repeatable steps during evaporation according to the theories of Kossel5) and Stranskis) and Frank and CabreraT), who extended the earlier concepts by the decisive action of the dislocations. For monoatomic curved steps can be concluded that the evaporation rate is an isotropic one. For the formation of the square spirals with twice the step height can be concluded that the molecules in the [ 1 IO]-steps are bound more weakly, as it is the increased decomposition at these steps which makes possible the formation of the more extended [lOO]-steps. This can also be shown by the effect of impurity ions with higher bond energy on the advance of the steps. While the evaporation structures of fig. 3 are taken from sufficiently pure crystals (a few ppm of bivalent impurity ions), fig. 4 shows the jagged shape
36
of the lamellar
H. BETHGE
steps on a crystal doped with 15 ppm Ca+ +. In comparison
to the Na ions the Ca ions have a higher bond energy and hamper
the ad-
vance by a “sticking” of the steps. The jags agree well with the expected mean spacing of the Ca ions in this concentration and the present atomic distribution. But it must be stressed that the atomic distribution deduced from the jag distance, is certainly valid for the temperature during evaporation only. At room temperature a segregation of Ca ions to complexes is probable. The higher bond energy of the Ca ions does not manifest itself in straight [ lOO]-steps of twice the height. But as fig. 5 shows, jags are produced
Fig. 4. Jagged shape of steps on a crystal doped with 15 ppm Ca. Annealing condition as in fig. 3.
Fig. 5. Effect of impurities with higher bond energy on the shape of biatomic steps with different direction and bond energy.
in the steps with [ 1IO]-direction, which occur in a somewhat longer distance from the centre of the spiral. In the same way striking is the fact that the gold decoration at the [ 1 lo]-steps is more dense than at the [ lOO]-steps. The different bond energy of both the steps manifests itself directly in the condition of jag formation and in the different gold nuclei density as well, The ions in the [lOO]-steps have a stronger bond, and there the higher bond energy of the Ca ions has no effect, whereas it is effective in the [ 1 IO]-steps with the weaker bound ions. As said before, an evaporated crystal basically shows the characteristic structures represented in fig. 3. The extension of the lamellar structures produced by dislocations depends on the density of the dislocations. In the case of wide distances between the dislocations, lamellar systems can be seen, which can consist of some hundreds of ledges. After sufficiently evaporating the surface the structures are independent of the initial structure. A first
ELECTRON
MICROSCOPIC
STUDIES
37
evaporation begins by surface nucleation. On the surface numerous flat “holes” surrounded by atomic steps are formed - as shown in fig. 6. Beside the holes also the initial state of individual evaporation spirals are perceptible (indicated by an arrow). A still earlier state is shown in fig. 7. Many Au-nuclei arranged in squares with four nuclei each, can be seen. We assume that this arrangement is effected by a disc-shaped monqatomic hole (“unit pit”) of extremely small diameter, as shown in the schematic illustration. Certainly the unit pits will be produced at vacancies of the lattice plane, which form the surface. But statistically only a small fraction of the surface vacan-
Fig. 6. Structures formed at the beginning of evaporation. Annealing conditions: 450 “C, 10 min.
Fig. 7. First formation of discshaped monoatomic holes on a cleavage surface. Annealing conditions: 300 “C, 40 min.
ties can grow to the critical size of a unit pit, from which the ledge advance takes place, as shown in fig. 6. When eventually the evaporation is determined only by the dissociation of motecules from lamellar ledges released by the dislocations, no formation of unit pits takes place. As the molecules dissociated from the steps, at first diffuse on the surface between the steps before they enter the vapour sphere, statistically formed surface vacancies can quickly be occupied again and there is little probability for a formation of critical further expanding nuclei. 4. Cleavage structures The initial structure, still perceptible in fig. 7, is the typical cleavage structure, which we call ‘“elementary cleavage structure” 8). Characteristical for this structure are the ~ashshaped cleavage steps, the step height of which also
38
H. BETHGE
amounts to a single interplanar spacing only. In regions free from coarser cleavage steps, i.e. those which are detectable by means of an optical microscope, the elementary cleavage structure is the determining surface structure, besides that sporadic slip steps can be seen which were formed by dislocations moved during cleavage. It is supposed that the point angle formed by the elementary cleavage structure depends on the velocity of rupture during cleavage. It also can be observed that the angle is rather sharp and the two steps practically run parallel (compare discussion to fig. 8). In order to detect the steps of the elementary cleavage structure it is practicable when cleavage occurs in a vacuum and preparation is carried out in the same vacuum. A cleavage in ambient air instantly causes the crystal to be covered with water vapour and the crystal surface layer is slightly dissolved. 5. Surface layer dissolving-growth effect structures* When a NaCl crystal exposed to ambient air is set into a vacuum and heated, the water evaporates and the ions, previously dissolved, recrystallize and form growth structures on the surface. It was observed that the structures created in this “surface layer dissolving-growth effect” are determined in their step shape by line tension. This is clearly shown in fig. 8. The original
Fig. 8. Annular structures in the region of an original step structure: formed by slightly dissolving the surface layer and subsequent growth. The illustration schematically shows the “contraction” effected by marginal stresses.
cleavage structure consisted of two nearly parallel steps which run through the whole picture and ended in a point. By covering with water vapour the surface was at first slightly dissolved. ** During evaporating the water and during the recrystallization a multitude of rings, i.e. flat monoatomic discs, were produced on the surface, thus reducing the line tension. This process * A comprehensive paper on this subject will be published shortly in Z. Kristaliographie (H. Bethge and M. Krohn). ** The crystal previously cleft in a vacuum was momentarily exposed to ambient air.
ELECTRON
MICROSCOPIC
STUDIES
39
proceeded from right to left under successively tieing up the growth discs, as shown by the schematic illustration. In the case of a somewhat more intense surface layer dissolving individual droplets are formed from the dissolved layer, because of the surface tension. These preferably form at distinct centres on the surface, particularIy around the emergence points of screw dislocations. In the course of the growth from these droplets by evaporating the water in a vacuum, hills arise, built up of
Fig. 9.
Growth structures around a screw dislocation formed after slight surface-layer dissolving and subsequent crystallization.
monoatomic steps, which in the case of a screw dislocation show a spiral growth. In contradistinction to the regular step shape of the evaporation spirals (see fig. 3), the step shape now is predetermined by the line tension, and the co~~gurations observed differ from each other widely. This is illustrated in fig. 9. 6, Surface decoration as a problem of surface physics The method of surface decoration used for this study contains suppositions which directly touch problems of interface physics. For the decoration effect it is obviously necessary to have atomically smooth surfaces between the steps, and that the prerequisite surface diffusion of the metallic atoms is not hampered by ~sturbing surface impurities. It could be shown that evaporation structures can be achieved not only on NaCl but also on KCl, NaBr, KBr, NaI, and KI crystals, and that they can be made visible by Au decoration. Investigations on fluorides (NaF and LiF) did not yet show decoration effects corresponding to those of the chlorides and bromides. Sup-
40
H. BEl’HGE
posing the evaporated crystal surface of the fluorides to have a degree of purity comparable with that of the chlorides and bromides, the higher binding energy of the fluorides could be the reason for hampering the surface mobility of the deposited metal atoms, which is necessary for diffusion. To apply the decoration method on.metallic crystals means to produce atomically smooth surfaces on the crystal - at least in small regions. Such surfaces can be obtained only by growing. First investigations were done on Ag and Cu crystals. In a high vacuum (lo-’ to 1O-6 Torr) small, globular crystals (4 mm (ZI) were formed from molten dropletss). Suitable surface
Fig. 10.
Evaporation structures and slip lines on a Ag single crystal globe. Au decoration.
regions are found in the range of the poles of cubic faces. Immediately after solidification the crystals were evaporated at temperatures close below the melting point, and after cooling down to about 300 “C Au evaporation took place for decoration. Fig. 10 shows evaporation structures and decorated slip lines of individual dislocations on an Ag crystal. Structures corresponding to those in fig. 10 could also be found on Cu crystals. Not yet solved is the question whether the surface must be absolutely clean in order to achieve sufficient surface mobility of the decorated metal atoms, or whether the surface diffusion is promoted by a monoatomic gas layer. At present we are carrying out corresponding studies with ultra high vacuum conditions. 7. Conclusion It was shown that in case of comprehensive description of atomic height. Details on how in this paper. For this purpose
a NaCl crystal it was possible to give a rather the surface structures formed by steps of to determine the step height were not given we refer to reference No. 1. It may be added
ELECTRON
that
not only a statement
MICROSCOPIC
can be given
STUDIES
on the step height,
41
but it is also
possible to say on which step side the higher level lies. This is done by means of double decorationrO). It is the representation of the atomic step structures which makes possible detailed explanations on the processes of surface structure formation in dependence on suitable pretreatment. Evaporation structures can be produced in a form which is extremely well reproducible. Because of the structure, which then is supposed to be known, the application of surfaces produced by evaporation is recommended for a number of problems in interface physics, e.g. epitaxies, for which the NaCl crystal or also some other alkali halide crystal serves best as a subject of investigation.* Electron microscopic observation of decoration effects appears to be of importance not only for obtaining images of surface structures, but also as a method of surface physics. In the decoration effect the surface diffusion and its dependence on the state of the surface is immediately expressed. A further study of the behaviour of various suitable heavy metals on surfaces of crystals with different surface energies seems to be of particular importance. For the investigation of metals obviously crystals showing sufficiently extended growth planes as surfaces are required. In the case of the spherical crystals used for our first investigations this requirement was fulfilled only in a rather insufficient way. But by electrolytic growth at Ag globules, planes of different indication, spreading over l-2 mm, can be produced, and first electron microscopic tests with conventionally oblique shadowed replicas show that the (lOO)- and (1 lO)-planes are disturbed only weakly. With such planes as a starting point suitable surfaces should be produced by evaporation. References 1) 2) 3) 4) 5) 6) 7) 8) 9)
H. Bethge, Phys. Stat. Sol. 2 (1962) 3 and 775. G. A. Bassett, Phil. Mag. 3 (1958) 1042. H. Bethge and G. Klstner, to be published in Z. Naturf. H. Bethge and K. W. Keller, Z. Naturf. 1Ja (1960) 271. W. Kossel, Leipziger Vortriige 1928 (Leipzig, 1928). I. N. Stranski, Z. Phys. Chem. A 136 (1928) 259. W. K. Burton, N. Cabrera and F. C. Frank, Phil. Trans. Roy. Sot. A 243 (1951) 299. H. Bethge, G. Klstner and M. Krohn, Z. Naturf. 16a (1961) 321. E. Menzel, W. St&se1 and M. Otter, Z. f. Physik 142 (1955) 241; H. Bethge and 0. Schaffer, Naturwissenschaften 41 (1954) 573. 10) H. Bethge and K. W. Keller, to be published in Optik.
* First use of this was made in the thesis by R. Niedermayer, Bergakademie Clausthal, 1963. Acknowledgements are due to Prof. Mayer for his kind permission for my reading the paper which is not yet published.
SCIENCE 3 (1964) 33-41;
ELECTRON SURFACE
o North-Holland
MICROSCOPIC
STRUCTURES SURFACE
AND
Publishing
STUDIES
SOME
PHENOMENA
Co., Amsterdam
OF
RELATIONS
TO
*
H. BETHGE German Academy of Sciences at Berlin, Inst. of Electron Microscopy, Halle (Saale) Received 29 June 1964 Surface structures on NaCl crystals detectable by the electron microscopic image with step heights down to one interplanar spacing, are described. The corresponding forms of producing the structures are discussed. Structures obtained by evaporation in a high vacuum are of special interest, as they are well reproducible. The conditions for the surface decoration method used for the image formation are discussed. First investigations to form images of monoatomic steps by means of the decoration method also on metal crystals show that this is possible under certain conditions.
1. Introduction
The surface of a macroscopic crystal given by a lattice plane of definite indication will be smooth in an atomic scale only in smallest regions. Due to corresponding pretreatment or formation of the surface mainly atomic steps occur, and these structures must be investigated for exactly describing a surface. The atomic steps of some ion crystals can electron microscopically form images by means of the surface decoration method. NaCl was investigated to a large extent, and this certainly is the crystal the surface structure of which is best known at present l). 2. The process of surface decoration
For carrying out surface decoration, described as Au-decoration first by Bassetts), the crystal is evaporated with a very thin film of Au, Pd, or Pt. On the crystal heated only slightly (to about 100 “C) metal atoms, hitting the crystal, do not form a homogeneous film, but they coalesce to small nuclei which preferably arrange themselves along the steps. But in regions which are not disturbed these nuclei are statistically distributed. A carbon film subsequently evaporated encloses the nuclei, and after stripping the film can * Delivered at the International Surfaces, Providence, 1964.
Conference
33
on the Physics and Chemistry
of Solid
34
H. BETHGE
be investigated under the electron microscope. The step structures crystal surface can be seen in the image of the gold-nuclei distribution. shows a picture
of manifold
magnification
than in the case of Au-evaporation
of an Au-decorated
Pd and Pt decoration
of the Fig. 1
step. Other
resp. under other-
wise similar conditions (rate of evaporation, temperature of NaCl crystal) results in a smaller diameter of the metal nuclei. The achievable lateral resolving power is determined by the size of the metal nuclei. From the surface structures studied the requisite for the resolving power is derived, and correspondingly it must be decided which method could be used advantageously. Fig. 2 shows an example of Pd decorations). On the surface of a
Fig. 1. Au nuclei arranged in statistical distribution and along a step.
Fig. 2. Slip structures on a NaCl crystal, deformed in a vacuum by bending, which are represented by palladium decoration.
deformed NaCl crystal slip traces of individual up to 30 A are reproduced fairly well separated.
dislocations
with intervals
3. Evaporation structures on crystal surfaces Evaporation structures on a (lOO)-plane of a NaCl crystal, created by annealing in a high vacuum (10e5 Torr) can easily be seen*). Experimentally for this purpose the crystal is so mounted in a furnace that the plane to be investigated lies open towards the vacuum. In order to avoid back condensation as far as possible, there shall possibly be no increased partial pressure of NaCl vapour on the evaporating plane. Starting from the emergence points of the dislocations lamellar structures form, which completely determine the surface structure in the regions free from higher cleavage steps. This is shown in fig. 3. Starting from the emergence point of screw dislocations we each time find a step the height of which is one or two interplanar spacings according to the corresponding Burgers vector. Because of the lower bond energy of the molecules in the step the decomposition of the crystal begins at these steps, and in the course of this
ELECTRON
MICROSCOPIC
process the step forms a spiral. The round
STUDIES
35
spirals show a step height of one
interplanar spacing, i.e. 2.82 A. The square spirals have twice this height, and it can be seen that always two curved steps enter one straight. In the region of the emergence point of edge dislocations there is an excess charge of an ion which effects a preferred surface nucleation. In the further course of evaporation annular lamellae also with a height of only 2.82 8, are produced. The structures are in agreement with the Burgers vectors to be expected in the NaCl lattice. For simplicity we above spoke of edge and screw dislocations only. To be more exact, the formation of spiral lamellae or annular lamellae each time depends on the fact, whether the dislocations
Fig. 3.
Evaporation structures around screw dislocations (left) and around edge dislocation (right). Annealing condition: 450 “C, 30 min.
have a component of the Burgers vector perpendicular to the (I OO)-plane or whether the Burgers vector lies in the cubic plane. The formation of the structures can be well explained by the crystal decomposition in repeatable steps during evaporation according to the theories of Kossel5) and Stranskis) and Frank and CabreraT), who extended the earlier concepts by the decisive action of the dislocations. For monoatomic curved steps can be concluded that the evaporation rate is an isotropic one. For the formation of the square spirals with twice the step height can be concluded that the molecules in the [ 1 IO]-steps are bound more weakly, as it is the increased decomposition at these steps which makes possible the formation of the more extended [lOO]-steps. This can also be shown by the effect of impurity ions with higher bond energy on the advance of the steps. While the evaporation structures of fig. 3 are taken from sufficiently pure crystals (a few ppm of bivalent impurity ions), fig. 4 shows the jagged shape
36
of the lamellar
H. BETHGE
steps on a crystal doped with 15 ppm Ca+ +. In comparison
to the Na ions the Ca ions have a higher bond energy and hamper
the ad-
vance by a “sticking” of the steps. The jags agree well with the expected mean spacing of the Ca ions in this concentration and the present atomic distribution. But it must be stressed that the atomic distribution deduced from the jag distance, is certainly valid for the temperature during evaporation only. At room temperature a segregation of Ca ions to complexes is probable. The higher bond energy of the Ca ions does not manifest itself in straight [ lOO]-steps of twice the height. But as fig. 5 shows, jags are produced
Fig. 4. Jagged shape of steps on a crystal doped with 15 ppm Ca. Annealing condition as in fig. 3.
Fig. 5. Effect of impurities with higher bond energy on the shape of biatomic steps with different direction and bond energy.
in the steps with [ 1IO]-direction, which occur in a somewhat longer distance from the centre of the spiral. In the same way striking is the fact that the gold decoration at the [ 1 lo]-steps is more dense than at the [ lOO]-steps. The different bond energy of both the steps manifests itself directly in the condition of jag formation and in the different gold nuclei density as well, The ions in the [lOO]-steps have a stronger bond, and there the higher bond energy of the Ca ions has no effect, whereas it is effective in the [ 1 IO]-steps with the weaker bound ions. As said before, an evaporated crystal basically shows the characteristic structures represented in fig. 3. The extension of the lamellar structures produced by dislocations depends on the density of the dislocations. In the case of wide distances between the dislocations, lamellar systems can be seen, which can consist of some hundreds of ledges. After sufficiently evaporating the surface the structures are independent of the initial structure. A first
ELECTRON
MICROSCOPIC
STUDIES
37
evaporation begins by surface nucleation. On the surface numerous flat “holes” surrounded by atomic steps are formed - as shown in fig. 6. Beside the holes also the initial state of individual evaporation spirals are perceptible (indicated by an arrow). A still earlier state is shown in fig. 7. Many Au-nuclei arranged in squares with four nuclei each, can be seen. We assume that this arrangement is effected by a disc-shaped monqatomic hole (“unit pit”) of extremely small diameter, as shown in the schematic illustration. Certainly the unit pits will be produced at vacancies of the lattice plane, which form the surface. But statistically only a small fraction of the surface vacan-
Fig. 6. Structures formed at the beginning of evaporation. Annealing conditions: 450 “C, 10 min.
Fig. 7. First formation of discshaped monoatomic holes on a cleavage surface. Annealing conditions: 300 “C, 40 min.
ties can grow to the critical size of a unit pit, from which the ledge advance takes place, as shown in fig. 6. When eventually the evaporation is determined only by the dissociation of motecules from lamellar ledges released by the dislocations, no formation of unit pits takes place. As the molecules dissociated from the steps, at first diffuse on the surface between the steps before they enter the vapour sphere, statistically formed surface vacancies can quickly be occupied again and there is little probability for a formation of critical further expanding nuclei. 4. Cleavage structures The initial structure, still perceptible in fig. 7, is the typical cleavage structure, which we call ‘“elementary cleavage structure” 8). Characteristical for this structure are the ~ashshaped cleavage steps, the step height of which also
38
H. BETHGE
amounts to a single interplanar spacing only. In regions free from coarser cleavage steps, i.e. those which are detectable by means of an optical microscope, the elementary cleavage structure is the determining surface structure, besides that sporadic slip steps can be seen which were formed by dislocations moved during cleavage. It is supposed that the point angle formed by the elementary cleavage structure depends on the velocity of rupture during cleavage. It also can be observed that the angle is rather sharp and the two steps practically run parallel (compare discussion to fig. 8). In order to detect the steps of the elementary cleavage structure it is practicable when cleavage occurs in a vacuum and preparation is carried out in the same vacuum. A cleavage in ambient air instantly causes the crystal to be covered with water vapour and the crystal surface layer is slightly dissolved. 5. Surface layer dissolving-growth effect structures* When a NaCl crystal exposed to ambient air is set into a vacuum and heated, the water evaporates and the ions, previously dissolved, recrystallize and form growth structures on the surface. It was observed that the structures created in this “surface layer dissolving-growth effect” are determined in their step shape by line tension. This is clearly shown in fig. 8. The original
Fig. 8. Annular structures in the region of an original step structure: formed by slightly dissolving the surface layer and subsequent growth. The illustration schematically shows the “contraction” effected by marginal stresses.
cleavage structure consisted of two nearly parallel steps which run through the whole picture and ended in a point. By covering with water vapour the surface was at first slightly dissolved. ** During evaporating the water and during the recrystallization a multitude of rings, i.e. flat monoatomic discs, were produced on the surface, thus reducing the line tension. This process * A comprehensive paper on this subject will be published shortly in Z. Kristaliographie (H. Bethge and M. Krohn). ** The crystal previously cleft in a vacuum was momentarily exposed to ambient air.
ELECTRON
MICROSCOPIC
STUDIES
39
proceeded from right to left under successively tieing up the growth discs, as shown by the schematic illustration. In the case of a somewhat more intense surface layer dissolving individual droplets are formed from the dissolved layer, because of the surface tension. These preferably form at distinct centres on the surface, particularIy around the emergence points of screw dislocations. In the course of the growth from these droplets by evaporating the water in a vacuum, hills arise, built up of
Fig. 9.
Growth structures around a screw dislocation formed after slight surface-layer dissolving and subsequent crystallization.
monoatomic steps, which in the case of a screw dislocation show a spiral growth. In contradistinction to the regular step shape of the evaporation spirals (see fig. 3), the step shape now is predetermined by the line tension, and the co~~gurations observed differ from each other widely. This is illustrated in fig. 9. 6, Surface decoration as a problem of surface physics The method of surface decoration used for this study contains suppositions which directly touch problems of interface physics. For the decoration effect it is obviously necessary to have atomically smooth surfaces between the steps, and that the prerequisite surface diffusion of the metallic atoms is not hampered by ~sturbing surface impurities. It could be shown that evaporation structures can be achieved not only on NaCl but also on KCl, NaBr, KBr, NaI, and KI crystals, and that they can be made visible by Au decoration. Investigations on fluorides (NaF and LiF) did not yet show decoration effects corresponding to those of the chlorides and bromides. Sup-
40
H. BEl’HGE
posing the evaporated crystal surface of the fluorides to have a degree of purity comparable with that of the chlorides and bromides, the higher binding energy of the fluorides could be the reason for hampering the surface mobility of the deposited metal atoms, which is necessary for diffusion. To apply the decoration method on.metallic crystals means to produce atomically smooth surfaces on the crystal - at least in small regions. Such surfaces can be obtained only by growing. First investigations were done on Ag and Cu crystals. In a high vacuum (lo-’ to 1O-6 Torr) small, globular crystals (4 mm (ZI) were formed from molten dropletss). Suitable surface
Fig. 10.
Evaporation structures and slip lines on a Ag single crystal globe. Au decoration.
regions are found in the range of the poles of cubic faces. Immediately after solidification the crystals were evaporated at temperatures close below the melting point, and after cooling down to about 300 “C Au evaporation took place for decoration. Fig. 10 shows evaporation structures and decorated slip lines of individual dislocations on an Ag crystal. Structures corresponding to those in fig. 10 could also be found on Cu crystals. Not yet solved is the question whether the surface must be absolutely clean in order to achieve sufficient surface mobility of the decorated metal atoms, or whether the surface diffusion is promoted by a monoatomic gas layer. At present we are carrying out corresponding studies with ultra high vacuum conditions. 7. Conclusion It was shown that in case of comprehensive description of atomic height. Details on how in this paper. For this purpose
a NaCl crystal it was possible to give a rather the surface structures formed by steps of to determine the step height were not given we refer to reference No. 1. It may be added
ELECTRON
that
not only a statement
MICROSCOPIC
can be given
STUDIES
on the step height,
41
but it is also
possible to say on which step side the higher level lies. This is done by means of double decorationrO). It is the representation of the atomic step structures which makes possible detailed explanations on the processes of surface structure formation in dependence on suitable pretreatment. Evaporation structures can be produced in a form which is extremely well reproducible. Because of the structure, which then is supposed to be known, the application of surfaces produced by evaporation is recommended for a number of problems in interface physics, e.g. epitaxies, for which the NaCl crystal or also some other alkali halide crystal serves best as a subject of investigation.* Electron microscopic observation of decoration effects appears to be of importance not only for obtaining images of surface structures, but also as a method of surface physics. In the decoration effect the surface diffusion and its dependence on the state of the surface is immediately expressed. A further study of the behaviour of various suitable heavy metals on surfaces of crystals with different surface energies seems to be of particular importance. For the investigation of metals obviously crystals showing sufficiently extended growth planes as surfaces are required. In the case of the spherical crystals used for our first investigations this requirement was fulfilled only in a rather insufficient way. But by electrolytic growth at Ag globules, planes of different indication, spreading over l-2 mm, can be produced, and first electron microscopic tests with conventionally oblique shadowed replicas show that the (lOO)- and (1 lO)-planes are disturbed only weakly. With such planes as a starting point suitable surfaces should be produced by evaporation. References 1) 2) 3) 4) 5) 6) 7) 8) 9)
H. Bethge, Phys. Stat. Sol. 2 (1962) 3 and 775. G. A. Bassett, Phil. Mag. 3 (1958) 1042. H. Bethge and G. Klstner, to be published in Z. Naturf. H. Bethge and K. W. Keller, Z. Naturf. 1Ja (1960) 271. W. Kossel, Leipziger Vortriige 1928 (Leipzig, 1928). I. N. Stranski, Z. Phys. Chem. A 136 (1928) 259. W. K. Burton, N. Cabrera and F. C. Frank, Phil. Trans. Roy. Sot. A 243 (1951) 299. H. Bethge, G. Klstner and M. Krohn, Z. Naturf. 16a (1961) 321. E. Menzel, W. St&se1 and M. Otter, Z. f. Physik 142 (1955) 241; H. Bethge and 0. Schaffer, Naturwissenschaften 41 (1954) 573. 10) H. Bethge and K. W. Keller, to be published in Optik.
* First use of this was made in the thesis by R. Niedermayer, Bergakademie Clausthal, 1963. Acknowledgements are due to Prof. Mayer for his kind permission for my reading the paper which is not yet published.