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Morphology of SnO2 whiskers
602
Journal of Crystal Growth 83 (1987) 602—605 North-Holland, Amsterdam
LETTER TO THE EDITORS MORPHOLOGY OF Sn02 WHISKERS H. IWANAGA. A. TOMIZUKA and N. SHIBATA Faculty of Liberal Arts, Nagasaki University, Nagasaki 852, Japan
and T. MATSUMOTO, H. KATSUKI and M. EGASHIRA Department of Materials Science and Engineering, Faculty of Engineering, Nagasaki University, Nagasaki 852, Japan
Received 10 November 1986; manuscript received in final form 15 March 1987
Straight and bent 5n02 whiskers were grown by oxidizing Sn. Straight whiskers obtained are classified into three types growing in the [011], [100] and [111] directions. Bent whiskers are composed of two coherent element whiskers among three types of straight whiskers. Sn02 whiskers always have a (011) surface on which isosceles triangular mounds and many steps are seen. The growth direction of the whiskers is easily identified from the orientation of the triangular mound and of the step on the (011) plane.
Sn02 crystals have the rutile structure which is reactor was opened so that a small amount of a tetragonal crystal with a 4.737 A and c 3.185 oxygen migrated back into the reaction zone [5]. A [1]. They are grown by the vapor phase reaction SnO2 whiskers with various shapes and growth of Sn or SnO with 02 [2—4].Reed et al. [2] have directions were grown on the edge of the mullite reported that many SnO2 rod crystals were grown boat. They are 1-2 mm in length and 0.2—S itm in in the [001] and some of them perpendicular to the thickness. [100]. Nagano [3] has described that Sn02 whiskers The morphology of Sn02 whiskers was obgrew normal to the (100) direction of the rod served by optical microscope and scanning eleccrystal by the VLS mechanism. Matsushita et al. tron microscope. The X-ray oscillation and Laue [4] have indicated that ribbon-like whiskers were methods were applied to determine the growth grown in the [100] direction. However, detailed axis of the whiskers. The surface indices of the study on the morphology of Sn02 crystals has not whiskers were determined from interfacial angles yet been reported. between adjacent surfaces measured by stereoIn this paper, the morphology of various types graphic observation of the crystals on a goniostage of Sn02 whiskers will be reported. Results of our of an optical microscope. experiments reveal that the crystallographic direcSn02 straight whiskers obtained in our experition of the whiskers is easily identified by the ments are classified into three types with respect shape and orientation of isosceles triangular to their growth axis. The majority of the whiskers mounds and the direction of growth steps develgrew in the [011] or [100] direction and fewer in oped on the (011) surface of the whisker. the [111] direction. Crystals growing in the [001] Sn02 whiskers were grown by oxidation of direction were not found at all, dissimilar to the metallic tin at 1100°C in a flow tube reactor of experiment of Reed et al. [2]. This is due to the mullite. The source materials were placed in a difference in the temperature of the growth zone. mullite boat. The atmosphere nitrogen Fig. axis 1 a shows a ribbon-like whose 3/min, and the was outlet end offlowthe growth is the [011] direction. whisker It is called the ing at 30 cm 0022-0248/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) =
=
H. Iwanaga et al.
b
(100)
/
603
Morphology of SnO, whiskers
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Fig. 1. (a) Ribbon-liki, whisker growing in the [0111 direction. (b) Schematic drawing of the whisker in (a). (c) Triangular mounds found on the (011) surface of the [0111whisker.
p S2
[011] whisker. Its cross section is rectangular and the side faces are composed of two wide (011) planes (ribbon surface) and two narrow (100) planes, as shown schematically in fig. lb. Many steps are found on the narrow (100) plane. On the other hand, the (Oil) ribbon surface is generally smooth, but trigonal growth mounds are often observed on it as seen in fig. ic. A typical example of [100] whiskers is iilustrated in fig. 2a. The four side surfaces of the whisker are nearly parallel to the {0ll } type planes, though they are not flat. A schematic drawing of the whisker is shown in fig. 2b. Angles between adjacent surfaces are measured to be 68° and 112°,that is, the cross sections of the whisker normal to its growth axis are always parallelograms with the same two interior angles of 68° and 112°. However, the shape of the parallelograms is not similar at different points (A, B and C in fig. 2a) of the whisker because of the change in the whisker thickness. In other words, the surfaces of the whisker have high Miller indices, The roundness of these faces is due to the layer growth by the two-dimensional nucleation on the surface, where many steps perpendicular to the growth direction are left as seen in an optical micrograph (fig. 2c).
e
0
80° (Oh)
S1 R [111]
—~
_____________
Fig. 2. (a) Whisker growing in the [100] direction. (b) schematic drawing of the whisker with its cross section of parallelograms. (c) Optical micrograph of the [100] whisker with many steps on the surface of high Miller indices. (d) Two isosceles triangular mounds on the (011) surface. (e) Schematic drawing of the mound showing its shape and orientation. (0 SEM photograph when the image of two planes S~and S2 disappears.
Sometimes growth mounds similar to those observed on the (011) plane of the [011] whisker (fig. Ic) were also found in the [100] whisker as illustrated in fig. 2d. The shape and orientation of the mounds are schematically shown in fig. 2e. The isosceles triangle PQR (the flat mound surface) is parallel to the (011) ribbon surface. The vertical angle made by the two isosceles sides (PQ and PR) is nearly 80°.PQ and PR are in the [111] and [111], respectively, because the base side QR is parallel to the whisker axis [100]. The triangular mound is very useful for the identification of the
604
H. Iwanaga eta!.
/
Morphology of 5n0
crystallographic orientation of the whisker, since it appears only on the (Oil) plane and two isosceles sides and the bases are always parallel to the [111], [111] and [100], respectively. In the SEM observation of the [100] whisker, first, the mound face is set parallel to the SEM image screen. After a rotation of the whisker around the side QR of the mound by 55°, the image of two planes S1 and S2 disappears (fig. 2f). This implies that these two planes are perpendicular to the SEM image screen and the interfacial angle between S1 and S2 is estimated to be 112°. This angle agrees with the calculated value of the angle between the (101) and (101)_planes. Therefore, the S1 and S2 planes are (101) and (101) planes, respectively. The growth process of the isosceles triangular mound on the (Oil) is clearly seen in fig. 3. A small mound nucleates at an edge of the (011) surface, indicated by an arrow in fig. 3a. The mound becomes larger and its apex reaches the opposite edge (in fig. 3b), then the triangular mound changes into the trapezoid PP’QR as seen in fig. 3c. The thickness of the whisker becomes larger as the result of the spreading of the mound. Figs. 4a and 4b are a SEM and an optical microscope photograph of a whisker which is single though its appearance is slightly complicated. A
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________________________________________ Fig. 3. Series of micrographs showing the growth process of the isosceles triangular mound on the (011) surface of [100] whiskers.
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111 -_~--
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~ Fig. 4. (a) SEM photograph of a [1111 whisker. (h) Optical micrograph of the whisker in (a) with an isosceles triangular mound and many steps on the (011) surface.
triangular mound PQR in fig. 4b is found to be the same orientation as that on the (011) surfaces of the [100] whisker as mentioned above. Therefore, the isosceles side PQ is oriented in the [111]. This fact shows that the growth direction of the whisker is the [111]. In addition, many steps found on the (Oil) surface are parallel to the [011] direction inclined to the whisker axis by 40°,i.e. the direction of the step is also the same as that in the [100] whisker. In [111] whisker we can identify their growth direction and their surface using the orientation of the mound and of the growth step without taking the X-ray oscillation photograph. Several types of bent SnO2 whiskers were found in the growth of Sn02 as well as straight whiskers tn the same run of the growth expenment Three types of them are illustrated in fig. 5. The first type of a bent whisker changing three times its growth direction by 90° is shown in fig. 5a. A (211) diffraction spot of the X-ray oscillation photograph of the whisker is illustrated in the lower right part of the figure. The shape of the diffraction spot is similar to that of the whisker. It is clear that four element whiskers of the bent whisker are crystallographically coherent each other. A triangular mound is found at the boundary between two element whiskers which spreads coherently across their boundary. This —
fact also suggests that two element whiskers are coherent, as seen in the X-ray diffraction spot. From the orientation of the mound, the element
H. Iwanaga et a!.
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/
Morphology of Sn0
2 whiskers
605
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Fig. 5. (a) Bent whisker of two [1001and two [0111element whiskers; (211) diffraction spot is in the lower right part. (b) Bent whisker composed of the [1001and [1111element whiskers. (c) Bent whisker composed of the [0111and [111] element whiskers.
whiskers A and B are identified as the [011] and [100] straight whiskers. The second type of the bent whisker is shown in fig. 5b, which is composed of two element whiskers A and B. The element whisker B is determined to be the [100] whisker from the orientation of the mound on its (011) surface, indicated by an arrow. The whisker A should be the [111] whisker since it makes an angle of 130° with B. Similarly the third type of the bent whisker with an angle of 140°Cis composed of the [011] and [111] element whiskers as shown in fig. Sc.
References [1] W.H. Baur, Acta Cryst. 9 (1956) 515. [21 TB. Reed J.T. Roddy and AN. Mariano, J. AppI. Phys. 33 (1962) 1014 [3] M. Nagano, J. Crystal Growth 66 (1984) 377. [4] T. Matsushita, H. Kataoka, K. Kodaira and A. Tsunashima, Yogyo-Kyokai Shi 84 (1976) 147 (in Japanese). [5] M. Egashira, Y. Yoshida and S. Kawasumi, Sensors and Actuators 9 (1986) 147.
Journal of Crystal Growth 83 (1987) 602—605 North-Holland, Amsterdam
LETTER TO THE EDITORS MORPHOLOGY OF Sn02 WHISKERS H. IWANAGA. A. TOMIZUKA and N. SHIBATA Faculty of Liberal Arts, Nagasaki University, Nagasaki 852, Japan
and T. MATSUMOTO, H. KATSUKI and M. EGASHIRA Department of Materials Science and Engineering, Faculty of Engineering, Nagasaki University, Nagasaki 852, Japan
Received 10 November 1986; manuscript received in final form 15 March 1987
Straight and bent 5n02 whiskers were grown by oxidizing Sn. Straight whiskers obtained are classified into three types growing in the [011], [100] and [111] directions. Bent whiskers are composed of two coherent element whiskers among three types of straight whiskers. Sn02 whiskers always have a (011) surface on which isosceles triangular mounds and many steps are seen. The growth direction of the whiskers is easily identified from the orientation of the triangular mound and of the step on the (011) plane.
Sn02 crystals have the rutile structure which is reactor was opened so that a small amount of a tetragonal crystal with a 4.737 A and c 3.185 oxygen migrated back into the reaction zone [5]. A [1]. They are grown by the vapor phase reaction SnO2 whiskers with various shapes and growth of Sn or SnO with 02 [2—4].Reed et al. [2] have directions were grown on the edge of the mullite reported that many SnO2 rod crystals were grown boat. They are 1-2 mm in length and 0.2—S itm in in the [001] and some of them perpendicular to the thickness. [100]. Nagano [3] has described that Sn02 whiskers The morphology of Sn02 whiskers was obgrew normal to the (100) direction of the rod served by optical microscope and scanning eleccrystal by the VLS mechanism. Matsushita et al. tron microscope. The X-ray oscillation and Laue [4] have indicated that ribbon-like whiskers were methods were applied to determine the growth grown in the [100] direction. However, detailed axis of the whiskers. The surface indices of the study on the morphology of Sn02 crystals has not whiskers were determined from interfacial angles yet been reported. between adjacent surfaces measured by stereoIn this paper, the morphology of various types graphic observation of the crystals on a goniostage of Sn02 whiskers will be reported. Results of our of an optical microscope. experiments reveal that the crystallographic direcSn02 straight whiskers obtained in our experition of the whiskers is easily identified by the ments are classified into three types with respect shape and orientation of isosceles triangular to their growth axis. The majority of the whiskers mounds and the direction of growth steps develgrew in the [011] or [100] direction and fewer in oped on the (011) surface of the whisker. the [111] direction. Crystals growing in the [001] Sn02 whiskers were grown by oxidation of direction were not found at all, dissimilar to the metallic tin at 1100°C in a flow tube reactor of experiment of Reed et al. [2]. This is due to the mullite. The source materials were placed in a difference in the temperature of the growth zone. mullite boat. The atmosphere nitrogen Fig. axis 1 a shows a ribbon-like whose 3/min, and the was outlet end offlowthe growth is the [011] direction. whisker It is called the ing at 30 cm 0022-0248/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) =
=
H. Iwanaga et al.
b
(100)
/
603
Morphology of SnO, whiskers
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Fig. 1. (a) Ribbon-liki, whisker growing in the [0111 direction. (b) Schematic drawing of the whisker in (a). (c) Triangular mounds found on the (011) surface of the [0111whisker.
p S2
[011] whisker. Its cross section is rectangular and the side faces are composed of two wide (011) planes (ribbon surface) and two narrow (100) planes, as shown schematically in fig. lb. Many steps are found on the narrow (100) plane. On the other hand, the (Oil) ribbon surface is generally smooth, but trigonal growth mounds are often observed on it as seen in fig. ic. A typical example of [100] whiskers is iilustrated in fig. 2a. The four side surfaces of the whisker are nearly parallel to the {0ll } type planes, though they are not flat. A schematic drawing of the whisker is shown in fig. 2b. Angles between adjacent surfaces are measured to be 68° and 112°,that is, the cross sections of the whisker normal to its growth axis are always parallelograms with the same two interior angles of 68° and 112°. However, the shape of the parallelograms is not similar at different points (A, B and C in fig. 2a) of the whisker because of the change in the whisker thickness. In other words, the surfaces of the whisker have high Miller indices, The roundness of these faces is due to the layer growth by the two-dimensional nucleation on the surface, where many steps perpendicular to the growth direction are left as seen in an optical micrograph (fig. 2c).
e
0
80° (Oh)
S1 R [111]
—~
_____________
Fig. 2. (a) Whisker growing in the [100] direction. (b) schematic drawing of the whisker with its cross section of parallelograms. (c) Optical micrograph of the [100] whisker with many steps on the surface of high Miller indices. (d) Two isosceles triangular mounds on the (011) surface. (e) Schematic drawing of the mound showing its shape and orientation. (0 SEM photograph when the image of two planes S~and S2 disappears.
Sometimes growth mounds similar to those observed on the (011) plane of the [011] whisker (fig. Ic) were also found in the [100] whisker as illustrated in fig. 2d. The shape and orientation of the mounds are schematically shown in fig. 2e. The isosceles triangle PQR (the flat mound surface) is parallel to the (011) ribbon surface. The vertical angle made by the two isosceles sides (PQ and PR) is nearly 80°.PQ and PR are in the [111] and [111], respectively, because the base side QR is parallel to the whisker axis [100]. The triangular mound is very useful for the identification of the
604
H. Iwanaga eta!.
/
Morphology of 5n0
crystallographic orientation of the whisker, since it appears only on the (Oil) plane and two isosceles sides and the bases are always parallel to the [111], [111] and [100], respectively. In the SEM observation of the [100] whisker, first, the mound face is set parallel to the SEM image screen. After a rotation of the whisker around the side QR of the mound by 55°, the image of two planes S1 and S2 disappears (fig. 2f). This implies that these two planes are perpendicular to the SEM image screen and the interfacial angle between S1 and S2 is estimated to be 112°. This angle agrees with the calculated value of the angle between the (101) and (101)_planes. Therefore, the S1 and S2 planes are (101) and (101) planes, respectively. The growth process of the isosceles triangular mound on the (Oil) is clearly seen in fig. 3. A small mound nucleates at an edge of the (011) surface, indicated by an arrow in fig. 3a. The mound becomes larger and its apex reaches the opposite edge (in fig. 3b), then the triangular mound changes into the trapezoid PP’QR as seen in fig. 3c. The thickness of the whisker becomes larger as the result of the spreading of the mound. Figs. 4a and 4b are a SEM and an optical microscope photograph of a whisker which is single though its appearance is slightly complicated. A
lw~u ________________________________________ ____
U
______
_____________________________________
~ _____
________________________________________ Fig. 3. Series of micrographs showing the growth process of the isosceles triangular mound on the (011) surface of [100] whiskers.
2 whiskers
_____
-~
r
1
111 -_~--
--
~
~ Fig. 4. (a) SEM photograph of a [1111 whisker. (h) Optical micrograph of the whisker in (a) with an isosceles triangular mound and many steps on the (011) surface.
triangular mound PQR in fig. 4b is found to be the same orientation as that on the (011) surfaces of the [100] whisker as mentioned above. Therefore, the isosceles side PQ is oriented in the [111]. This fact shows that the growth direction of the whisker is the [111]. In addition, many steps found on the (Oil) surface are parallel to the [011] direction inclined to the whisker axis by 40°,i.e. the direction of the step is also the same as that in the [100] whisker. In [111] whisker we can identify their growth direction and their surface using the orientation of the mound and of the growth step without taking the X-ray oscillation photograph. Several types of bent SnO2 whiskers were found in the growth of Sn02 as well as straight whiskers tn the same run of the growth expenment Three types of them are illustrated in fig. 5. The first type of a bent whisker changing three times its growth direction by 90° is shown in fig. 5a. A (211) diffraction spot of the X-ray oscillation photograph of the whisker is illustrated in the lower right part of the figure. The shape of the diffraction spot is similar to that of the whisker. It is clear that four element whiskers of the bent whisker are crystallographically coherent each other. A triangular mound is found at the boundary between two element whiskers which spreads coherently across their boundary. This —
fact also suggests that two element whiskers are coherent, as seen in the X-ray diffraction spot. From the orientation of the mound, the element
H. Iwanaga et a!.
~
/
Morphology of Sn0
2 whiskers
605
~1 I. U~ni_
Fig. 5. (a) Bent whisker of two [1001and two [0111element whiskers; (211) diffraction spot is in the lower right part. (b) Bent whisker composed of the [1001and [1111element whiskers. (c) Bent whisker composed of the [0111and [111] element whiskers.
whiskers A and B are identified as the [011] and [100] straight whiskers. The second type of the bent whisker is shown in fig. 5b, which is composed of two element whiskers A and B. The element whisker B is determined to be the [100] whisker from the orientation of the mound on its (011) surface, indicated by an arrow. The whisker A should be the [111] whisker since it makes an angle of 130° with B. Similarly the third type of the bent whisker with an angle of 140°Cis composed of the [011] and [111] element whiskers as shown in fig. Sc.
References [1] W.H. Baur, Acta Cryst. 9 (1956) 515. [21 TB. Reed J.T. Roddy and AN. Mariano, J. AppI. Phys. 33 (1962) 1014 [3] M. Nagano, J. Crystal Growth 66 (1984) 377. [4] T. Matsushita, H. Kataoka, K. Kodaira and A. Tsunashima, Yogyo-Kyokai Shi 84 (1976) 147 (in Japanese). [5] M. Egashira, Y. Yoshida and S. Kawasumi, Sensors and Actuators 9 (1986) 147.