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Light-microscopic and electronmicroscopic investigations of whisker-like AgCl crystals grown from the vapour phase
330
Journal of Crystal Growth 58 (1982) 330—334 North-Holland Publishing Company
LIGHT-MICROSCOPIC AND ELECTRONMICROSCOPIC INVESTIGATIONS OF WHISKER-LIKE AgCI CRYSTALS GROWN FROM THE VAPOUR PHASE E. STUCKENSCHMIDT and F. GRANZER Institutfür Angewandte Physik, Robert-Mayer-Strasse 2
—
4, D-6000 Frankfurt/Main, Fed. Rep. of Germany
Received 4 January 1982; manuscript received in final form 15 February 1982
Whisker-like AgC1 microcrystals were grown by vapour deposition on different substrates and investigated in the light-microscope and in the electronmicroscope.
1. Introduction Despite the fact that silver halide crystals, especially those with rocksalt structure like AgCI and AgBr play an important role in classical photography and have been used frequently as transparent model systems for studying the mechanical properties of metals (see, e.g., ref. [1]), there exists at least to our knowledge! up till now no publication concerning the growth and properties of silver halide whiskers. It is therefore worthwhile to grow perfect, whisker-like silver halide crystals for two reasons: (a) A comparison of the strength and the plastic behaviour of small perfect, silver halide crystals free from dislocations with the corresponding properties of large crystals, containing a high density of dislocations, would reveal the role played by dislocations in plastic deformation which, like in most of the bcc metals, is governed by pencil glide in AgCI and AgBr. (b) The formation of the latent image in the interior of the silver halide microcrystals of a photographic emulsion during exposure and the photolytic precipitation of silver upon prolonged illumination is markedly facilitated by the presence of dislocations [2]. Thus, in order to study systematically the influence of imperfections on the photographic sensitivity, it is necessary, to replace a commercial photographic emulsion of silver halide microcrystals grown from solution by a —
—
—
0022-0248/82/0000
model emulsion consisting of perfect silver halide crystals of comparable size, e.g. whiskers grown from the vapour phase, embedded in gelatine. The first step in realizing such a program is to develop a method for growing perfect AgC1 microcrystals from the vapour phase and to control their properties by light-microscopic and electronmicroscopic investigations.
—
—
0000/$02.75
©
2. Experimental 2.1. Apparatus The AgC1 microcrystals were grown by vapour deposition on different substrates in a rather simple manner. As sketched schematically in fig. 1, the AgCl powder (Johnson Matthey Specpure, purity 99.999%) was heated above the melting point of AgCI (TM = 455%C) in a porcelain crucible and the geometrical arrangement inside the Simon Muller furnace (type SM 3a) was such as to maintam a well defined temperature gradient between the AgC1 melt and the substrate lying upon the crucible. Optimal and reproducible results were obtained when the temperature of the AgCl melt was held between 460 and 500°C, leading to temperatures between 420 and 440°C on the substrate. The evaporation time varied from 3 to 21 h in order to get crystals of different sizes which then were
1982 North-Holland
—
E. Stuckenschmidt, F. Granzer
/
LM and EM investigations of whisker-like AgCI
‘— ‘/
the carbon film carrying the AgC1 microcrystals with it from the substrate. The most successful
A /10
Substrate
°
‘1°
°
-‘
\
_________
/Jo
/
p
°
o
oo
°
—
AgCl
~Simon
/~///
/
/~/t.
-
melt
/ /
/ ~ /
331
-Muller furmce
/
I Fig. I. Geometrical arrangement inside the Simon— Muller fur. nace.
appropriate for examination in the light-microscope (LM) as well as in the transmission electronmicroscope (TEM). Besides their vapour deposition inside the Simon Muller furnace, AgCl microciystals were also grown by sublimation in the heating stage of the Leitz-Ortholux microscope (1350). Several materials: freshly cleaved NaCl, KCI, and mica crystals and quartz glass were used as substrates. As the high mobility of the AgC1 molecules on the substrate being one of the main conditions for the growth of isolated AgCl crystals is opposed by the solubility of AgCl in NaCl and KC1, the best results were obtained with “inert, non-reacting” substrate materials, like mica and quartz glass. —
—
—
method was toand support thebreak carbon a layer of Technovit then to offfilm the by Technovit together with the carbon film from the substrate. In the next step, the Technovit layer was dissolved in aceton, and the carbon film with the AgC1 microcrystals still on it was inserted into the object holder of the cooling stage of the TEM (JEOL JEM- I OOB) to avoid thermal assisted photolysis of the AgC1 crystals. In a similar procedure, after shadow casting with platinum, carbon replicas were taken from the AgC1 crystals and examined in the TEM (Siemens-Elmiskop I) in order to reveal the morphology and details of the surface structure of the microcrystals. For more details of the preparation technique, see ref. [3]. 3. Results 3.1. Investigations with the LM There was no significant difference in the shape of the AgCl microcrystals and their orientation with respect to the substrate, whether they have been vapour deposited inside the Simon Muller furnace or in the heating stage of the LM. As shown in figs. 2 5, the AgCl crystals grow mainly in a needle- or plate-like shape, or as more or less regular pyramides (corners of a cube). While the —
—
2.2. Preparation and investigation of the AgC1 microcrystals in the LM and TEM With the LM (Leitz-Ortholux and Zeiss-Photomikroskop) it was always possible to examine in reflected light the vapour deposited crystals “in situ” on the substrate. For examination in the TEM (Siemens-Elmiskop I and JEOL JEM- 1 OOB), however, the AgCl microcrystals had to be separated from the substrate in the following way: at first the substrate (mica or quartz glass) with the AgC1 crystals on it was covered with a thin carbon film by the usual high vacuum evaporation techmque. Several methods were then tried to separate
Fig. 2. AgCI crystals on quartz glass evaporated for 19 h in the heating stage of the LM.
332
E. Stuckenschmidt, F. Granzer / LM and EM investigations of whisker-like A
5CJ
—
Fig. 3. Like fig. 2, at higher magnification.
Fig. 6. Electron micrograph of Ag( I crystals on mica, evaporated for 4 h in the heating stage of the LM. Carbon replica, shadow cast with platinum.
~
-
-‘~.
Fig. 4. AgCI crystals on Muller furnace for II h.
mica
e~aporatedinside the SimonFig. 7. Like fig. 6, evaporated for 19 h inside the Simon— Muller furnace on quartz glass.
,~ Fig. 5. Like fig. 4 (evaporated for 21 h), at higher magnification.
Fig. 8. Electron micrograph of a needle-like A8CI crystal.
E. Stuckenschmidr, F. Granzer
/ LM
and EM investigations of whisker-like AgCl
333
~P4~
___
b
Fig. 10. Electron micrograph of a plate-like AgCl crystal(a) and the corresponding electron diffraction diagram (b).
Fig. 9. Electron micrograph of a plate-like AgCl crystal before (a) and after (b) tilting of about 10°in the TEM.
orientations of the thin rods are random on the quartz glass substrate (fig. 2), they seem to grow epitaxially on cleaved mica, enclosing angles of 60°(fig. 4). 3.2. Investigations with the TEM 3.2.1. Carbon replicas The typical shapes of the AgCI microcrystals already observed in the LM were found again in the TEM at higher magnifications (figs. 6 and 7). 3.2.2. Direct transmission ,and electron diffraction Even at the highest voltage (100 kV) available in the TEM, most of the AgCl crystals were only partly transmissible. This is demonstrated in the electron micrographes (figs. 8 and 9). Fig. 9b shows the same crystal as fig. 9a after the specimen
has been tilted in the TEM about 10°. While the extinction contours are migrating and changing their shape, no contrast typical of dislocations could be observed, when the orientation of the AgC1 crystals was altered during the tilting of the specimen. Fig. 10 again shows a perfect AgC1 crystal and the corresponding electron diffraction diagram indicates that the crystal is oriented with a {l00) plane parallel to the object plane. Due to the way the crystals have been prepared, their orientations with respect to the plane of the substrate, however, must not necessarily be the same.
4. Discussion The variety of the shapes of the AgCI microcrystals observed in the LM and after replication in the TEM may be interpreted in a geometric or crystallographic sense as resulting from cuts per-
334
E. Stuckenschmidt, F. Granzer
/
LM and EM investigations of whisker-like AgCI
formed along different planes of a cube. The cut along a (III) plane, separating exactly the corner of the cube, corresponds to the pyramides of regular shape in fig. 6. The cut along the (110) plane, separating the edge of the cube, corresponds to the rod-like shaped crystal in fig. 6. All the other AgC1 microcrystals are intermediate in shape between these two extremes (fig. 7). From this simple geometric consideration and the evaluation of a series of electron diffraction diagrams, it follows that the axis of the rod- or needle-like AgCl microcrystals, and thus their preferred growth direction, is a!ways (l00~. The investigations of crystals thin enough for TEM did not show any diffraction contrast attributable to dislocations. Therefore, it may be concluded that the transmissible regions of the crystals were free from dislocations. The lack of dislocations, or at the utmost a very low dislocation density in the thicker crystals, is confirmed by the absence of any glide steps on the surface of the crystals, demonstrating their resistance against plastic deformation even when they have been -
heavily stressed by breaking them off from the substrate (see section 2.2). From all these observations, it becomes obvious that the needle-like AgCl microcrystals not only resemble in shape to, but also behave like AgCl whiskers.
Acknowledgements We are indebted to Prof.Dr. H. Fuess for providing us with the facilities of the Institute for Crystallography and Mineralogy of the University Frankfurt/Main and to Mrs.Dr. J. Töpel of the same Institute for performing the electron microscopial investigation with the JEOL JEM- 1OOB.
References [l} J.F, Nye, Proc. Roy. Soc. (London) A198 (1949) 191. (2J J.W. Mitchell, Dislocations and Mechanical Properties of Crystals (Wiley, New York, 1956) p. 69. . . . 131 E. Stuckenschmidt, Diplom Thesis, Frankfurt/Main (1981).
Journal of Crystal Growth 58 (1982) 330—334 North-Holland Publishing Company
LIGHT-MICROSCOPIC AND ELECTRONMICROSCOPIC INVESTIGATIONS OF WHISKER-LIKE AgCI CRYSTALS GROWN FROM THE VAPOUR PHASE E. STUCKENSCHMIDT and F. GRANZER Institutfür Angewandte Physik, Robert-Mayer-Strasse 2
—
4, D-6000 Frankfurt/Main, Fed. Rep. of Germany
Received 4 January 1982; manuscript received in final form 15 February 1982
Whisker-like AgC1 microcrystals were grown by vapour deposition on different substrates and investigated in the light-microscope and in the electronmicroscope.
1. Introduction Despite the fact that silver halide crystals, especially those with rocksalt structure like AgCI and AgBr play an important role in classical photography and have been used frequently as transparent model systems for studying the mechanical properties of metals (see, e.g., ref. [1]), there exists at least to our knowledge! up till now no publication concerning the growth and properties of silver halide whiskers. It is therefore worthwhile to grow perfect, whisker-like silver halide crystals for two reasons: (a) A comparison of the strength and the plastic behaviour of small perfect, silver halide crystals free from dislocations with the corresponding properties of large crystals, containing a high density of dislocations, would reveal the role played by dislocations in plastic deformation which, like in most of the bcc metals, is governed by pencil glide in AgCI and AgBr. (b) The formation of the latent image in the interior of the silver halide microcrystals of a photographic emulsion during exposure and the photolytic precipitation of silver upon prolonged illumination is markedly facilitated by the presence of dislocations [2]. Thus, in order to study systematically the influence of imperfections on the photographic sensitivity, it is necessary, to replace a commercial photographic emulsion of silver halide microcrystals grown from solution by a —
—
—
0022-0248/82/0000
model emulsion consisting of perfect silver halide crystals of comparable size, e.g. whiskers grown from the vapour phase, embedded in gelatine. The first step in realizing such a program is to develop a method for growing perfect AgC1 microcrystals from the vapour phase and to control their properties by light-microscopic and electronmicroscopic investigations.
—
—
0000/$02.75
©
2. Experimental 2.1. Apparatus The AgC1 microcrystals were grown by vapour deposition on different substrates in a rather simple manner. As sketched schematically in fig. 1, the AgCl powder (Johnson Matthey Specpure, purity 99.999%) was heated above the melting point of AgCI (TM = 455%C) in a porcelain crucible and the geometrical arrangement inside the Simon Muller furnace (type SM 3a) was such as to maintam a well defined temperature gradient between the AgC1 melt and the substrate lying upon the crucible. Optimal and reproducible results were obtained when the temperature of the AgCl melt was held between 460 and 500°C, leading to temperatures between 420 and 440°C on the substrate. The evaporation time varied from 3 to 21 h in order to get crystals of different sizes which then were
1982 North-Holland
—
E. Stuckenschmidt, F. Granzer
/
LM and EM investigations of whisker-like AgCI
‘— ‘/
the carbon film carrying the AgC1 microcrystals with it from the substrate. The most successful
A /10
Substrate
°
‘1°
°
-‘
\
_________
/Jo
/
p
°
o
oo
°
—
AgCl
~Simon
/~///
/
/~/t.
-
melt
/ /
/ ~ /
331
-Muller furmce
/
I Fig. I. Geometrical arrangement inside the Simon— Muller fur. nace.
appropriate for examination in the light-microscope (LM) as well as in the transmission electronmicroscope (TEM). Besides their vapour deposition inside the Simon Muller furnace, AgCl microciystals were also grown by sublimation in the heating stage of the Leitz-Ortholux microscope (1350). Several materials: freshly cleaved NaCl, KCI, and mica crystals and quartz glass were used as substrates. As the high mobility of the AgC1 molecules on the substrate being one of the main conditions for the growth of isolated AgCl crystals is opposed by the solubility of AgCl in NaCl and KC1, the best results were obtained with “inert, non-reacting” substrate materials, like mica and quartz glass. —
—
—
method was toand support thebreak carbon a layer of Technovit then to offfilm the by Technovit together with the carbon film from the substrate. In the next step, the Technovit layer was dissolved in aceton, and the carbon film with the AgC1 microcrystals still on it was inserted into the object holder of the cooling stage of the TEM (JEOL JEM- I OOB) to avoid thermal assisted photolysis of the AgC1 crystals. In a similar procedure, after shadow casting with platinum, carbon replicas were taken from the AgC1 crystals and examined in the TEM (Siemens-Elmiskop I) in order to reveal the morphology and details of the surface structure of the microcrystals. For more details of the preparation technique, see ref. [3]. 3. Results 3.1. Investigations with the LM There was no significant difference in the shape of the AgCl microcrystals and their orientation with respect to the substrate, whether they have been vapour deposited inside the Simon Muller furnace or in the heating stage of the LM. As shown in figs. 2 5, the AgCl crystals grow mainly in a needle- or plate-like shape, or as more or less regular pyramides (corners of a cube). While the —
—
2.2. Preparation and investigation of the AgC1 microcrystals in the LM and TEM With the LM (Leitz-Ortholux and Zeiss-Photomikroskop) it was always possible to examine in reflected light the vapour deposited crystals “in situ” on the substrate. For examination in the TEM (Siemens-Elmiskop I and JEOL JEM- 1 OOB), however, the AgCl microcrystals had to be separated from the substrate in the following way: at first the substrate (mica or quartz glass) with the AgC1 crystals on it was covered with a thin carbon film by the usual high vacuum evaporation techmque. Several methods were then tried to separate
Fig. 2. AgCI crystals on quartz glass evaporated for 19 h in the heating stage of the LM.
332
E. Stuckenschmidt, F. Granzer / LM and EM investigations of whisker-like A
5CJ
—
Fig. 3. Like fig. 2, at higher magnification.
Fig. 6. Electron micrograph of Ag( I crystals on mica, evaporated for 4 h in the heating stage of the LM. Carbon replica, shadow cast with platinum.
~
-
-‘~.
Fig. 4. AgCI crystals on Muller furnace for II h.
mica
e~aporatedinside the SimonFig. 7. Like fig. 6, evaporated for 19 h inside the Simon— Muller furnace on quartz glass.
,~ Fig. 5. Like fig. 4 (evaporated for 21 h), at higher magnification.
Fig. 8. Electron micrograph of a needle-like A8CI crystal.
E. Stuckenschmidr, F. Granzer
/ LM
and EM investigations of whisker-like AgCl
333
~P4~
___
b
Fig. 10. Electron micrograph of a plate-like AgCl crystal(a) and the corresponding electron diffraction diagram (b).
Fig. 9. Electron micrograph of a plate-like AgCl crystal before (a) and after (b) tilting of about 10°in the TEM.
orientations of the thin rods are random on the quartz glass substrate (fig. 2), they seem to grow epitaxially on cleaved mica, enclosing angles of 60°(fig. 4). 3.2. Investigations with the TEM 3.2.1. Carbon replicas The typical shapes of the AgCI microcrystals already observed in the LM were found again in the TEM at higher magnifications (figs. 6 and 7). 3.2.2. Direct transmission ,and electron diffraction Even at the highest voltage (100 kV) available in the TEM, most of the AgCl crystals were only partly transmissible. This is demonstrated in the electron micrographes (figs. 8 and 9). Fig. 9b shows the same crystal as fig. 9a after the specimen
has been tilted in the TEM about 10°. While the extinction contours are migrating and changing their shape, no contrast typical of dislocations could be observed, when the orientation of the AgC1 crystals was altered during the tilting of the specimen. Fig. 10 again shows a perfect AgC1 crystal and the corresponding electron diffraction diagram indicates that the crystal is oriented with a {l00) plane parallel to the object plane. Due to the way the crystals have been prepared, their orientations with respect to the plane of the substrate, however, must not necessarily be the same.
4. Discussion The variety of the shapes of the AgCI microcrystals observed in the LM and after replication in the TEM may be interpreted in a geometric or crystallographic sense as resulting from cuts per-
334
E. Stuckenschmidt, F. Granzer
/
LM and EM investigations of whisker-like AgCI
formed along different planes of a cube. The cut along a (III) plane, separating exactly the corner of the cube, corresponds to the pyramides of regular shape in fig. 6. The cut along the (110) plane, separating the edge of the cube, corresponds to the rod-like shaped crystal in fig. 6. All the other AgC1 microcrystals are intermediate in shape between these two extremes (fig. 7). From this simple geometric consideration and the evaluation of a series of electron diffraction diagrams, it follows that the axis of the rod- or needle-like AgCl microcrystals, and thus their preferred growth direction, is a!ways (l00~. The investigations of crystals thin enough for TEM did not show any diffraction contrast attributable to dislocations. Therefore, it may be concluded that the transmissible regions of the crystals were free from dislocations. The lack of dislocations, or at the utmost a very low dislocation density in the thicker crystals, is confirmed by the absence of any glide steps on the surface of the crystals, demonstrating their resistance against plastic deformation even when they have been -
heavily stressed by breaking them off from the substrate (see section 2.2). From all these observations, it becomes obvious that the needle-like AgCl microcrystals not only resemble in shape to, but also behave like AgCl whiskers.
Acknowledgements We are indebted to Prof.Dr. H. Fuess for providing us with the facilities of the Institute for Crystallography and Mineralogy of the University Frankfurt/Main and to Mrs.Dr. J. Töpel of the same Institute for performing the electron microscopial investigation with the JEOL JEM- 1OOB.
References [l} J.F, Nye, Proc. Roy. Soc. (London) A198 (1949) 191. (2J J.W. Mitchell, Dislocations and Mechanical Properties of Crystals (Wiley, New York, 1956) p. 69. . . . 131 E. Stuckenschmidt, Diplom Thesis, Frankfurt/Main (1981).