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Habit of gold particles vapour-deposited onto silver bromide films
Journal of Crystal Growth 58 (1982) 507—516 North-Holland Publishing Company
507
HABIT OF GOLD PARTICLES VAPOUR-DEPOSITED ONTO SILVER BROMIDE FILMS H. HOFMEISTER, H. HAEFKE and M. KROHN Institute of Solid State Physics and Electron Microscopy, Academy of Sciences of the GDR, Weinberg 2, DDR-4010 Halle (Saale), German Dem. Rep.
Received 15 April 1982
Structure and habit of individual Au particles on AgBr single crystal films have been analysed by combined electronoptical methods. Besides epitaxial particles, non-epitaxial particles of various kinds have been specified. The epitaxial particles show octahedral habit and some other forms derived from octahedra by truncations of corners and edges. Among the non-epitaxial particles multiply-twinned icosahedra and decahedra as well as special tetrahedral and rhombic particles have been identified. The latter ones prove to be composed of different subunits. With increasing temperature an increasing number of particles with planar defects are found which are regarded to result from coalescence processes.
1. Introduction By condensation of metal vapour on certain substrate surfaces (e.g., on ionic crystals) under vacuum conditions three-dimensional metal particles arise. The orientation, habit and structure of these particles can be investigated by transmission electron diffraction (TED) and transmission electron microscopy (TEM) [1—10]. The complete characterization often requires selected area diffraction (SAD) for individual particles. Limited by constructive conditions (diameter of the minimum selected area: about 300 nm), for individual partides SAD is only possible with relatively low particle densities (about 1 X l0~cm~2).The partide systems in most substrate—deposit combinations investigated up to now are characterized by larger densities. Thus from the TEM images and the corresponding SAD patterns only integral information on groups of particles can be gained (for exceptions see refs. [11,12]). In applying the epitaxial AgBr evaporation layer a substrate was found [13] on which, by deposition of silver or gold, particle systems with sufficiently low density can be prepared, enabling a complete characterization of individual particles. While in the preceding paper [14] results are given from investigations of the influence of vacuum condi0022-0248/82/0000—0000/$02.75
©
tions, substrate temperature and surface structure on growth and habit of the particles, in this paper habit and structure of individual particles shall be considered more in detail.
2. Experimental The determination of orientation, shape and structure of the particb~s required to combine several electron microscopical methods. By conventional bright field TEM (BF), TED and dark field TEM (DF) information could be obtained on the particle profile, on the orientation and crystallographic structure of particles as well as on the particle construction (single-crystalline or composed). Additionally weak-beam dark field (WBDF) [15—17]and lattice plane imaging [18,19] were used to determine the topography of singlecrystalline particles and the construction of composed particles, respectively. For improving the outlined particle models, it was of special impOrtance that the particle height could be determined by platinum/carbon shadow casting [20]. For details of the substrate layer preparation and the gold deposition we refer to the experimental part given in the preceding paper [14]. The investigations were mainly carried out with the
1982 North-Holland
508
H. Hofmeister ef al.
/
Habit of Au particles vapour-deposited onto AgBr
particle systems obtained at 250°C substrate ternperature because here particle density and establishment of the particle habit were most convenient. In electron microscopic examinations a highresolution electron microscope (JEM IOOC) was used.
3. Habit and structure of the particles 3.1. Octahedral particles
The elementary form of the epitaxially onented * particles in BF images exhibits a quadratic or rectangular profile bounded by (110) edges (figs. Ia and 2a). Other forms diverging from this additionally contain <100) edges in their profiles (figs. 2b—2d). WBDF images of particles with quadratic profiles taken with one of the (200) reflections in the SAD pattern (fig. lc) reveal the topography of these particles by means of thickness fringes. From this, it follows that the particles have a pyramidal or double-pyramidal shape. Some of these partides (above all at 150 and 200°Csubstrate ternperature, respectively) in WBDF images show more or less extended plateaus without thickness fringes in the particle centre (fig. Ib) indicating truncations by (001) faces. The slope of the particle side faces with respect to the substrate can be determined from particle height (obtained from the shadow length) and edge length in the particle profile. By supposing a double-pyramidal shape a value of about 55° follows corresponding to the slope of ~111) faces to a (001) plane (54.7°). With that, the particles are indirectly characterized as octahedra and a schematic construction of octahedral particles with quadratic profiles can be set up (fig. ld). Characteristic of this are the eight octahedral side faces and in the case of truncations (fig. le) the quadratic (001) basis and top face. Analogous investigations for particles with rectangular profiles and for particles diverging from *
The epitaxial orientation of the particles is established by means of the shadow direction [14]; it implies corrcspondence of the directions of all crystallographic axes of gold with those of AgBr.
the elementary form result in corresponding schematic constructions (fig. 2). The particle with rectangular profile (fig. 2a), besides eight octahedral side faces and rectangular basis and top faces, additionally exhibits slight truncations by { 1 10} faces as they occur frequently at such particles. In the case of particles with distinctly formed (100) edges in their profiles (figs. 2b and 2c) one and two pairs of octahedral side faces, respectively, are replaced by two (fig. 2b) and four (fig. 2c) pairs of {llO} side faces, respectively. In a special form of octahedral particles (fig. 2d) besides eight octahedral side faces strong truncations by { 1 l0} faces are occurring at two particle edges facing each other. This kind of particle has been observed at the ledges of hollows in non-cornpletely continuous AgBr layers (see fig. 4 in ref. [14]). Directly, the octahedron particle habit can be recognized from the profile of particles which (caused by the preparation for electron microscopical examination) are tipped to (110) orientation or a (Ill) orientation with respect to the electron beam. In fig. 3a, an octahedral particle in (111) orientation (see SAD pattern, fig. 3c) is shown close to the “empty” impression with “shadow cone” originating from the previous (001) orientation of the particle. The scheme (fig. 3b) shows the connection between impression and partide following from the correspondence of the edge length a. Considerations of the same the kind are valid for the octahedral particle in (110) orientation (figs. 3d and 3e) where the fully established octahedron particle habit can be seen rather distinctly. In order to complete the characterization of octahedral particles, in addition we will deal with planar defects observed in some of them. The planar defects are most clearly observed in WBDF images of such particles which are characterized by a specially arranged system of fringes additionally to the thickness fringes (compare figs. 4a and 4b). The schematic representation in fig. 4c illustrates the connection between spatial arrangement of the planar defect in the particle and its appearance in the particle profile. If the particle with planar defect is situated in a (Ill) orientation with respect to the electron beam (fig. 4d), because
/
H. Hofmeister et al.
a
_________________________
/
509
Habit of Au particles vapour-deposited onto AgBr
_______
_____
[api]
d
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~ lGV7nm
C
c\ [001]
/
__
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ex
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____
b Fig. 1. Elementary form of octahedral gold particles~~. (a) BF image; (h) (200) WBDF image; (c) SAD pattern; (d) octahedron particle model; (e) truncated octahedron particle model.
of the changed particle position a modified arrangement of the additional fringes is occurring in the WBDF image (fig. 4e). From the amount of the angles -y’ (fig. 4c) and -y” (fig. 40, respectively, the slope of the inner boundary face can be determined supposing octahedral side faces. In both particle positions investigated this slope is equal to that of a (Ill) plane. The SAD patterns of the particles concerned show them to be always single-crystalline and they correspond to the SAD patterns of partides without such defects. Consequently, twin boundaries or grain boundaries can be excluded as inner boundary faces. It is obvious to assume that the observed planar defects are stacking faults resulting from the coalescence of particles [21].
3.2. Tetrahedral and rhombic particles
In the group of non-epitaxial particles one can distinguish particles having trigonal, rhombic, pentagonal and hexagonal profile, respectively. The habit of these particles, also known from investigations of other substrate—deposit combinations (e.g., metals on alkali halides [1,3,7—9,19]), has been extensively examined in particular in the case of pentagonal decahedra and hexagonal icosahedra. Decahedra and icosahedra are called “multiply twinned particles” (MTPs) because they are thought to be composed of 5 tetrahedra (decahedron) and 20 tetrahedra (icosahedron), respectively, which are situated in twin positions to each
510
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J
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~
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d Fig. 2. Different forms of octahedral particles and corresponding models: (a) rectangular profile; (h) and (c) (111) side faces replaced by pairs of { 110) side faces; (d) additional { 110) side faces.
H. Hofmeister et al.
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Habit of Au particles vapour-deposited onto AgBr
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Fig. 3. Octahedral gold particles with changed Orientation caused by preparation: (a) particle in (lID orientation close to the place of its previous (001) orientation; (b) schematic representation of (a); (c) SAD pattern of the particle in (a); (d) a particle in (110) orientation; (e) SAD pattern of the particle in (d).
other [2,3,71. (In spite of the upper size limit (about 40 nm) due to a loss of stability of such MTPs, as discussed by mo and Ogawa [221, the MTPs found on AgBr films have size as large as 100 nm. Because of the limited amount of gold deposited an experimental upper size limit of MTPs cannot be given here.) Usually, one basis tetrahedron is situated in (111) orientation to the substrate. For the particles with rhombic profiles also a decahedron model has been proposed in which the basis tetrahedron is situated in (001) orientation to the substrate [8]. In the particle systems obtained on AgBr films isolated tetrahedra (trigonal profile) are occurring
very seldom. Mostly they exhibit strong truncations by { Ill } faces parallel to their contact planes to the substrate. Tetrahedra having not this platelike shape (fig. Sb) are commonly associated with a (001) oriented (octahedral) building unit. The existence of such octahedral units can be documented by the SAD pattern (fig. Sc) if they cannot be recognized in the particle profiles. Besides the reflections of the (001) oriented unit here (111) and (222) reflections occur which are arranged in a [110] direction with regard to the (001) unit. The imaging of [200) lattice planes of the (001) unit permits to line off both units in the profile of the particle (fig. 5a).
512
H. Hofnie,ster et al.
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of Au particles vapour-deposited onto AgBr
a
_______
b
UX~1J
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f 220
d
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Fig. 4. Planar defects in octahedral gold particles. (a) BF image of a particle in (001) orientation; (b) (200) WBDF image of the particle in (a); (c) schematic representation of the particle in (a); (d) BF image of a particle in (Ill) orientation; (e) (220) WBDF image of the particle in (d); (f) schematic representation of the particle in (d).
Fig. 5d shows a tetrahedral particle with a more distinctly established octahedral unit. The topography of its single building units is revealed by WBDF images taken with a (200) reflection (fig. Sf) and with a (222) reflection (fig. Se), respectively. From the results given in figs. 5a—5f a model of the habit of these particles and their composition of two units can be concluded (fig.
5h). In the spatial representation, the contact area of the particle to the substrate ((001) face of the unit 1) is marked by dark hatching. In the particle projection (fig. 5g), unit 1 is accentuated by light hatching. One can suppose that these particles are resulting from the coalescence of always one octahedral particle and one isolated tetrahedral partide and that they represent precursors of the rhombic particles.
H. Hofmeister et al.
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Habit of Au particles vapour-deposited onto AgBr
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1 Fig. 5. Tetrahedral gold particles: (a) particle with (200) lattice fringes (by tilted illumination) lining off the octahedral unit; (b) BF image of the particle in (a); (c) SAD pattern of the particle in (a); (d) a tetrahedral particle with more distinctly established octahedral unit. BF image; (e) (222) WBDF image of the particle in (d); (f) (200) WBDF image of the particle in (d); (g) projection of a model particle; (h) model of tetrahedral particles.
514
H. Hofme,.stcr etiji.
habit of Au particles vapour-deposited onto A gBr
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0 Fig. 6. Ithombic gold particle: (a) BF image of a rhombic particle; (b) (200) WBDF image; (c) (Ill) WBDF image; (d) projection of a model particle; (e) model of rhombic particles; (f) parallel moire fringes (by axial illumination) in the region of the octahedral unit; (g) (111 } lattice fringes (by tilted illumination); (h) SAD pattern.
H. Hofmeister ci at
/
Habit of Au particles vapour-deposited onto AgBr
The rhombic particles (fig. 6a) are likewise to be characterized as composed particles. Here in the SAD pattern (fig. 6h) also besides the reflections of a (001) oriented unit (111) and (222) reflections occur which are arranged in direction of the longitudinal axis of the particle. Position and topography of the (001) unit are illustrated by the WBDF image taken with a (200) reflection (fig. 6b). This building unit is embedded in a body composed of tetrahedral units, the topography of which is revealed by the WBDF image taken with a (111) reflection (fig. 6c). Lattice plane imaging for the whole particle profile shows [111) lattice fringes resulting from tetrahedral units (fig. 6g). In the region of the (001) unit parallel moire fringes can be observed (fringe spacing 0.37 nm, fig. 6f) resulting from electron beam interaction with (111) lattice planes of tetrahedral units and [220) lattice planes of the octahedral unit, With these results a model of the habit of the rhombic particles and their composition of several building units can be set up which in principle agrees with the suggestion of a decahedron having a (001) oriented basis tetrahedron [81. Due to the knowledge on the topography of the different units from WBDF images as well as from measurements of the particle height certain modifications of the mentioned model can be inferred (fig. 6e). From the five building units composing the particle the tetrahedra 2, 3, 4 and 5 are truncated by [111) faces and the tetrahedron I is truncated by a (001) face (contact plane to the substrate marked by dark hatching). In the particle projection (fig. 6d) the unit I is accentuated by light hatching. Note that in fig. 6e in comparison to fig. 6d the longitudinal axis of the particle is turned by 90°. For imaging the [111) lattice planes, one set of [Ill) planes of the units 4 and 5 must be arranged nearly parallel to the electron beam. Also for imaging the parallel moire observed, one set of [111) planes of the units 4 and 5 must be arranged nearly parallel to the electron beam and additionally parallel to one set of (220) planes of the unit 1. This is in contrast to the gap in the spatial representation between the units 4 and S resulting from the composition of the model particle of fcc tetrahedra. Neither in the lattice plane and moire images nor in the WBDF images such a gap or
515
indications of non-uniform lattice distortions to accommodate the geometrical misfit could be observed in the rhombic particles investigated. These findings support the particle models of Heinemann, Poppa and coworkers [9] who found uniform distortions of the fcc lattice in MTPs.
~ Summary On epitaxial AgBr films particle systems can be obtained by deposition of gold which enables a complete characterization of three-dimensional particles by combination of several electron microscopical methods. The elementary form of epitaxial particles exhibits an octahedral habit with eight [111) side faces and more or less extended (001) basis and top faces. In other forms diverging from this, two or four of the [111) side faces are replaced by always a pair of [1 lO} side faces, or [110) faces additionally occur as a result of truncations. The investigation of the internal structure of these partides reveals planar defects (stacking faults on [Ill) planes) in some of them. The non-epitaxial particle forms with trigonal and rhombic profile are shown to be composed of several building units. Tetrahedral particles in most cases occur associated with an octahedral unit. They are supposed to be precursors of the rhombic particles. The rhombic particle appears composed of five tetrahedra, four of them having truncations by [111 } faces and the basis tetrahedron being truncated by a (001) face which is the contact plane of the particle to the substrate. This particle model is a modification of the one given by Fukaya, Ino and Ogawa [8].
Acknowledgements The authors would like to thank Professor J. Malinowski of the Sofia Institute of Photographic Processes for kindly providing the AgBr materials used for the preparation of the substrate layers. They are deeply indebted to Professor H. Bethge for advice and suggestions promoting this work.
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H. Hofmeister et al.
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Habit of Au particles vapour-deposited onto AgBr
References [I] S. Ogawa et al., J. Phys. Soc. Japan 21(1966)1963. [2] J.G. Alipress and J.V. Sanders, Surface Sci. 7(1967)1. [3] S. mo, J. Electron Microsc. 18 (1969) 237. [4] H. Sato and S. Shinozaki, J. Appl. Phys. 41(1970) 3165; H. Sato and S. Shinozaki. J. Vacuum Sci. Technol. 8
(1971) 159. 151
[10] K. Heinemann and H. Poppa, Optik 56 (1980) 183. [11] S. Ino, S. Owaga, T. Taoka and H. Akahori, Japan. i. AppI. Phys. 11(1972)1859. 112] RH. Geiss, AppI. Phys. Letters 27 (1975) 174. [13] H. Haefke, M. Krohn and A. Panov. J. Crystal Growth 49 (1980) 7. [14] H. Hofmeister, H. Haefke and A. Panov, J. Crystal Growth
58(1982)500.
T.P. Darhy and CM. Wayman. J. Crystal Growth 28
[15] D. Cockayne, I.L.F. Day and M.J. Whelan. Phil. Mag. 20
G.M. Wayman and T.P. Darby. J. Crystal Growth 28 (1975) 53 [6] K. Yagi. K. Takayanagi, K. Kobayashi and G. Honjo. J. Crystal Growth 28(1975)117. [7] M. Gillet, Surface Sci. 67 (1977) 139. [8] K. Fukaya, S. lno and S. Ogawa, Trans. Japan Inst. Metals 19 (1978) 445. [9] K. Heinemann, M.J. Yacamán, C.Y. Yang and H. Poppa. J. Crystal Growth 47 (1979) 177; M.J. Yacamân, K. Heinemann, C.Y. Yang and H. Poppa, J. Crystal Growth 47 (1979) 187.
[16] M.J. Yacamán et al.. Phil. Mag. A40 (1979) 645. [17] H. Heinemann, M.J. Yacamán and H. Poppa. Phys. Status Solidi (a) 54 (1979) 675. [18]J.W. Menter, Proc. Roy. Soc. (London) A236 (1956) 119. [19] T. Komoda. Japan. J. AppI. Phys. 7(1968) 27. [20] A. Panov, H. Haefke and M. Krohn, J. Crystal Growth 58 (1982) 452. [21] M.J. Stowell, in: Epitaxial Growth, Part B, Ed. .1W. Matthews (New York, 1975) p. 437. [221S. mo, J. Phys. Soc. Japan 26 (1969) 1559; S. Ogawa and S. Ino. J. Crystal Growth 13/14 (1972) 48.
(1975) 41;
(1968) 1265.
507
HABIT OF GOLD PARTICLES VAPOUR-DEPOSITED ONTO SILVER BROMIDE FILMS H. HOFMEISTER, H. HAEFKE and M. KROHN Institute of Solid State Physics and Electron Microscopy, Academy of Sciences of the GDR, Weinberg 2, DDR-4010 Halle (Saale), German Dem. Rep.
Received 15 April 1982
Structure and habit of individual Au particles on AgBr single crystal films have been analysed by combined electronoptical methods. Besides epitaxial particles, non-epitaxial particles of various kinds have been specified. The epitaxial particles show octahedral habit and some other forms derived from octahedra by truncations of corners and edges. Among the non-epitaxial particles multiply-twinned icosahedra and decahedra as well as special tetrahedral and rhombic particles have been identified. The latter ones prove to be composed of different subunits. With increasing temperature an increasing number of particles with planar defects are found which are regarded to result from coalescence processes.
1. Introduction By condensation of metal vapour on certain substrate surfaces (e.g., on ionic crystals) under vacuum conditions three-dimensional metal particles arise. The orientation, habit and structure of these particles can be investigated by transmission electron diffraction (TED) and transmission electron microscopy (TEM) [1—10]. The complete characterization often requires selected area diffraction (SAD) for individual particles. Limited by constructive conditions (diameter of the minimum selected area: about 300 nm), for individual partides SAD is only possible with relatively low particle densities (about 1 X l0~cm~2).The partide systems in most substrate—deposit combinations investigated up to now are characterized by larger densities. Thus from the TEM images and the corresponding SAD patterns only integral information on groups of particles can be gained (for exceptions see refs. [11,12]). In applying the epitaxial AgBr evaporation layer a substrate was found [13] on which, by deposition of silver or gold, particle systems with sufficiently low density can be prepared, enabling a complete characterization of individual particles. While in the preceding paper [14] results are given from investigations of the influence of vacuum condi0022-0248/82/0000—0000/$02.75
©
tions, substrate temperature and surface structure on growth and habit of the particles, in this paper habit and structure of individual particles shall be considered more in detail.
2. Experimental The determination of orientation, shape and structure of the particb~s required to combine several electron microscopical methods. By conventional bright field TEM (BF), TED and dark field TEM (DF) information could be obtained on the particle profile, on the orientation and crystallographic structure of particles as well as on the particle construction (single-crystalline or composed). Additionally weak-beam dark field (WBDF) [15—17]and lattice plane imaging [18,19] were used to determine the topography of singlecrystalline particles and the construction of composed particles, respectively. For improving the outlined particle models, it was of special impOrtance that the particle height could be determined by platinum/carbon shadow casting [20]. For details of the substrate layer preparation and the gold deposition we refer to the experimental part given in the preceding paper [14]. The investigations were mainly carried out with the
1982 North-Holland
508
H. Hofmeister ef al.
/
Habit of Au particles vapour-deposited onto AgBr
particle systems obtained at 250°C substrate ternperature because here particle density and establishment of the particle habit were most convenient. In electron microscopic examinations a highresolution electron microscope (JEM IOOC) was used.
3. Habit and structure of the particles 3.1. Octahedral particles
The elementary form of the epitaxially onented * particles in BF images exhibits a quadratic or rectangular profile bounded by (110) edges (figs. Ia and 2a). Other forms diverging from this additionally contain <100) edges in their profiles (figs. 2b—2d). WBDF images of particles with quadratic profiles taken with one of the (200) reflections in the SAD pattern (fig. lc) reveal the topography of these particles by means of thickness fringes. From this, it follows that the particles have a pyramidal or double-pyramidal shape. Some of these partides (above all at 150 and 200°Csubstrate ternperature, respectively) in WBDF images show more or less extended plateaus without thickness fringes in the particle centre (fig. Ib) indicating truncations by (001) faces. The slope of the particle side faces with respect to the substrate can be determined from particle height (obtained from the shadow length) and edge length in the particle profile. By supposing a double-pyramidal shape a value of about 55° follows corresponding to the slope of ~111) faces to a (001) plane (54.7°). With that, the particles are indirectly characterized as octahedra and a schematic construction of octahedral particles with quadratic profiles can be set up (fig. ld). Characteristic of this are the eight octahedral side faces and in the case of truncations (fig. le) the quadratic (001) basis and top face. Analogous investigations for particles with rectangular profiles and for particles diverging from *
The epitaxial orientation of the particles is established by means of the shadow direction [14]; it implies corrcspondence of the directions of all crystallographic axes of gold with those of AgBr.
the elementary form result in corresponding schematic constructions (fig. 2). The particle with rectangular profile (fig. 2a), besides eight octahedral side faces and rectangular basis and top faces, additionally exhibits slight truncations by { 1 10} faces as they occur frequently at such particles. In the case of particles with distinctly formed (100) edges in their profiles (figs. 2b and 2c) one and two pairs of octahedral side faces, respectively, are replaced by two (fig. 2b) and four (fig. 2c) pairs of {llO} side faces, respectively. In a special form of octahedral particles (fig. 2d) besides eight octahedral side faces strong truncations by { 1 l0} faces are occurring at two particle edges facing each other. This kind of particle has been observed at the ledges of hollows in non-cornpletely continuous AgBr layers (see fig. 4 in ref. [14]). Directly, the octahedron particle habit can be recognized from the profile of particles which (caused by the preparation for electron microscopical examination) are tipped to (110) orientation or a (Ill) orientation with respect to the electron beam. In fig. 3a, an octahedral particle in (111) orientation (see SAD pattern, fig. 3c) is shown close to the “empty” impression with “shadow cone” originating from the previous (001) orientation of the particle. The scheme (fig. 3b) shows the connection between impression and partide following from the correspondence of the edge length a. Considerations of the same the kind are valid for the octahedral particle in (110) orientation (figs. 3d and 3e) where the fully established octahedron particle habit can be seen rather distinctly. In order to complete the characterization of octahedral particles, in addition we will deal with planar defects observed in some of them. The planar defects are most clearly observed in WBDF images of such particles which are characterized by a specially arranged system of fringes additionally to the thickness fringes (compare figs. 4a and 4b). The schematic representation in fig. 4c illustrates the connection between spatial arrangement of the planar defect in the particle and its appearance in the particle profile. If the particle with planar defect is situated in a (Ill) orientation with respect to the electron beam (fig. 4d), because
/
H. Hofmeister et al.
a
_________________________
/
509
Habit of Au particles vapour-deposited onto AgBr
_______
_____
[api]
d
/1110]
~ lGV7nm
C
c\ [001]
/
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____
b Fig. 1. Elementary form of octahedral gold particles~~. (a) BF image; (h) (200) WBDF image; (c) SAD pattern; (d) octahedron particle model; (e) truncated octahedron particle model.
of the changed particle position a modified arrangement of the additional fringes is occurring in the WBDF image (fig. 4e). From the amount of the angles -y’ (fig. 4c) and -y” (fig. 40, respectively, the slope of the inner boundary face can be determined supposing octahedral side faces. In both particle positions investigated this slope is equal to that of a (Ill) plane. The SAD patterns of the particles concerned show them to be always single-crystalline and they correspond to the SAD patterns of partides without such defects. Consequently, twin boundaries or grain boundaries can be excluded as inner boundary faces. It is obvious to assume that the observed planar defects are stacking faults resulting from the coalescence of particles [21].
3.2. Tetrahedral and rhombic particles
In the group of non-epitaxial particles one can distinguish particles having trigonal, rhombic, pentagonal and hexagonal profile, respectively. The habit of these particles, also known from investigations of other substrate—deposit combinations (e.g., metals on alkali halides [1,3,7—9,19]), has been extensively examined in particular in the case of pentagonal decahedra and hexagonal icosahedra. Decahedra and icosahedra are called “multiply twinned particles” (MTPs) because they are thought to be composed of 5 tetrahedra (decahedron) and 20 tetrahedra (icosahedron), respectively, which are situated in twin positions to each
510
H. Hofmeister et al. / Habit of Au particles vapour-deposited
J
.4,gBr
I~l?1]
lOOnm
(~
05110
[001]
[100]
~
~
SOnm ______
[100]
[001] [110]
____
/
/
~—~00]
d Fig. 2. Different forms of octahedral particles and corresponding models: (a) rectangular profile; (h) and (c) (111) side faces replaced by pairs of { 110) side faces; (d) additional { 110) side faces.
H. Hofmeister et al.
/
Habit of Au particles vapour-deposited onto AgBr
511
lOOnm
(1WJAg~ C
220
4.
& d
\ •
b
lOOnm
. ~.
\\
~ (001)
‘::j
° ~ ~ii)
e
Fig. 3. Octahedral gold particles with changed Orientation caused by preparation: (a) particle in (lID orientation close to the place of its previous (001) orientation; (b) schematic representation of (a); (c) SAD pattern of the particle in (a); (d) a particle in (110) orientation; (e) SAD pattern of the particle in (d).
other [2,3,71. (In spite of the upper size limit (about 40 nm) due to a loss of stability of such MTPs, as discussed by mo and Ogawa [221, the MTPs found on AgBr films have size as large as 100 nm. Because of the limited amount of gold deposited an experimental upper size limit of MTPs cannot be given here.) Usually, one basis tetrahedron is situated in (111) orientation to the substrate. For the particles with rhombic profiles also a decahedron model has been proposed in which the basis tetrahedron is situated in (001) orientation to the substrate [8]. In the particle systems obtained on AgBr films isolated tetrahedra (trigonal profile) are occurring
very seldom. Mostly they exhibit strong truncations by { Ill } faces parallel to their contact planes to the substrate. Tetrahedra having not this platelike shape (fig. Sb) are commonly associated with a (001) oriented (octahedral) building unit. The existence of such octahedral units can be documented by the SAD pattern (fig. Sc) if they cannot be recognized in the particle profiles. Besides the reflections of the (001) oriented unit here (111) and (222) reflections occur which are arranged in a [110] direction with regard to the (001) unit. The imaging of [200) lattice planes of the (001) unit permits to line off both units in the profile of the particle (fig. 5a).
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Fig. 5d shows a tetrahedral particle with a more distinctly established octahedral unit. The topography of its single building units is revealed by WBDF images taken with a (200) reflection (fig. Sf) and with a (222) reflection (fig. Se), respectively. From the results given in figs. 5a—5f a model of the habit of these particles and their composition of two units can be concluded (fig.
5h). In the spatial representation, the contact area of the particle to the substrate ((001) face of the unit 1) is marked by dark hatching. In the particle projection (fig. 5g), unit 1 is accentuated by light hatching. One can suppose that these particles are resulting from the coalescence of always one octahedral particle and one isolated tetrahedral partide and that they represent precursors of the rhombic particles.
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The rhombic particles (fig. 6a) are likewise to be characterized as composed particles. Here in the SAD pattern (fig. 6h) also besides the reflections of a (001) oriented unit (111) and (222) reflections occur which are arranged in direction of the longitudinal axis of the particle. Position and topography of the (001) unit are illustrated by the WBDF image taken with a (200) reflection (fig. 6b). This building unit is embedded in a body composed of tetrahedral units, the topography of which is revealed by the WBDF image taken with a (111) reflection (fig. 6c). Lattice plane imaging for the whole particle profile shows [111) lattice fringes resulting from tetrahedral units (fig. 6g). In the region of the (001) unit parallel moire fringes can be observed (fringe spacing 0.37 nm, fig. 6f) resulting from electron beam interaction with (111) lattice planes of tetrahedral units and [220) lattice planes of the octahedral unit, With these results a model of the habit of the rhombic particles and their composition of several building units can be set up which in principle agrees with the suggestion of a decahedron having a (001) oriented basis tetrahedron [81. Due to the knowledge on the topography of the different units from WBDF images as well as from measurements of the particle height certain modifications of the mentioned model can be inferred (fig. 6e). From the five building units composing the particle the tetrahedra 2, 3, 4 and 5 are truncated by [111) faces and the tetrahedron I is truncated by a (001) face (contact plane to the substrate marked by dark hatching). In the particle projection (fig. 6d) the unit I is accentuated by light hatching. Note that in fig. 6e in comparison to fig. 6d the longitudinal axis of the particle is turned by 90°. For imaging the [111) lattice planes, one set of [Ill) planes of the units 4 and 5 must be arranged nearly parallel to the electron beam. Also for imaging the parallel moire observed, one set of [111) planes of the units 4 and 5 must be arranged nearly parallel to the electron beam and additionally parallel to one set of (220) planes of the unit 1. This is in contrast to the gap in the spatial representation between the units 4 and S resulting from the composition of the model particle of fcc tetrahedra. Neither in the lattice plane and moire images nor in the WBDF images such a gap or
515
indications of non-uniform lattice distortions to accommodate the geometrical misfit could be observed in the rhombic particles investigated. These findings support the particle models of Heinemann, Poppa and coworkers [9] who found uniform distortions of the fcc lattice in MTPs.
~ Summary On epitaxial AgBr films particle systems can be obtained by deposition of gold which enables a complete characterization of three-dimensional particles by combination of several electron microscopical methods. The elementary form of epitaxial particles exhibits an octahedral habit with eight [111) side faces and more or less extended (001) basis and top faces. In other forms diverging from this, two or four of the [111) side faces are replaced by always a pair of [1 lO} side faces, or [110) faces additionally occur as a result of truncations. The investigation of the internal structure of these partides reveals planar defects (stacking faults on [Ill) planes) in some of them. The non-epitaxial particle forms with trigonal and rhombic profile are shown to be composed of several building units. Tetrahedral particles in most cases occur associated with an octahedral unit. They are supposed to be precursors of the rhombic particles. The rhombic particle appears composed of five tetrahedra, four of them having truncations by [111 } faces and the basis tetrahedron being truncated by a (001) face which is the contact plane of the particle to the substrate. This particle model is a modification of the one given by Fukaya, Ino and Ogawa [8].
Acknowledgements The authors would like to thank Professor J. Malinowski of the Sofia Institute of Photographic Processes for kindly providing the AgBr materials used for the preparation of the substrate layers. They are deeply indebted to Professor H. Bethge for advice and suggestions promoting this work.
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References [I] S. Ogawa et al., J. Phys. Soc. Japan 21(1966)1963. [2] J.G. Alipress and J.V. Sanders, Surface Sci. 7(1967)1. [3] S. mo, J. Electron Microsc. 18 (1969) 237. [4] H. Sato and S. Shinozaki, J. Appl. Phys. 41(1970) 3165; H. Sato and S. Shinozaki. J. Vacuum Sci. Technol. 8
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[10] K. Heinemann and H. Poppa, Optik 56 (1980) 183. [11] S. Ino, S. Owaga, T. Taoka and H. Akahori, Japan. i. AppI. Phys. 11(1972)1859. 112] RH. Geiss, AppI. Phys. Letters 27 (1975) 174. [13] H. Haefke, M. Krohn and A. Panov. J. Crystal Growth 49 (1980) 7. [14] H. Hofmeister, H. Haefke and A. Panov, J. Crystal Growth
58(1982)500.
T.P. Darhy and CM. Wayman. J. Crystal Growth 28
[15] D. Cockayne, I.L.F. Day and M.J. Whelan. Phil. Mag. 20
G.M. Wayman and T.P. Darby. J. Crystal Growth 28 (1975) 53 [6] K. Yagi. K. Takayanagi, K. Kobayashi and G. Honjo. J. Crystal Growth 28(1975)117. [7] M. Gillet, Surface Sci. 67 (1977) 139. [8] K. Fukaya, S. lno and S. Ogawa, Trans. Japan Inst. Metals 19 (1978) 445. [9] K. Heinemann, M.J. Yacamán, C.Y. Yang and H. Poppa. J. Crystal Growth 47 (1979) 177; M.J. Yacamân, K. Heinemann, C.Y. Yang and H. Poppa, J. Crystal Growth 47 (1979) 187.
[16] M.J. Yacamán et al.. Phil. Mag. A40 (1979) 645. [17] H. Heinemann, M.J. Yacamán and H. Poppa. Phys. Status Solidi (a) 54 (1979) 675. [18]J.W. Menter, Proc. Roy. Soc. (London) A236 (1956) 119. [19] T. Komoda. Japan. J. AppI. Phys. 7(1968) 27. [20] A. Panov, H. Haefke and M. Krohn, J. Crystal Growth 58 (1982) 452. [21] M.J. Stowell, in: Epitaxial Growth, Part B, Ed. .1W. Matthews (New York, 1975) p. 437. [221S. mo, J. Phys. Soc. Japan 26 (1969) 1559; S. Ogawa and S. Ino. J. Crystal Growth 13/14 (1972) 48.
(1975) 41;
(1968) 1265.