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Anomalous structures in nanophases
NanoSTRUCTURED MATERIALS VOL. 1, PP. 241-244, 1992 COPYRIGHT©1992 PERGAMONPRESSLtd. ALL RIGHTSRESERVED
0965-9773/92 $5.00 + .00 PRINTED IN THE USA
ANOMALOUS STRUCTURES IN NANOPHASES M. Jos6 -Yacam~n Instituto de Fisica, Universidad Nacional Aut6noma de M6xico Apartado Postal 20-364. Delegaci6n Alvaro Obreg6n 01000 Mexico, D.F. MEXICO Introduction In the present work, we describe a number of anomalous structures that are produced in nanostructures of the noble metals Pd and Au, and Bi-Mn alloys. The main characterization techniques used were high resolution electron microscopy (HREM), electron diffraction analysis and image processing. The hardware used was a Jeol 4000-Ex microscope, a Jeol 100-Cx STEM microscope and an image processing Innovion system coupled to a Micro-VAX-III. Palladium Clusters Palladium particles were grown in an UHV-Chamber fitted to a Jeol 4000-Ex microscope (1). The particles produced had a size of approximately 50rim. A HREM of such particles is shown in Fig. la) along with its corresponding power spectrum (FFT), which is equivalent to an optical diffraction pattern. The figure shows that the particle is amorphous (2). However, when the metal is annealed at higher temperatures, or when the deposition flux is increased, the clusters show a completely different structure, as shown in fig. 2a). This image shows mainly "two-dimensional" islands but now the structure is defined. An examination of the cluster, using image processing, indicates that most of the clusters have a decagonal structure with several defects Fig. 2b), as was reported by Herrera et al (3). No icosahedral structures were found. It is interesting to note that the path to crystallization goes from the amorphous to the decagonal structures with boundary defects. This latter type of cluster has also been reported by Gao and Gleiter (4) for the case of gold. A point to be made is that our samples were grown in UHV, so that contamination should play no role. Multiply Twinned Gold Particles Nanophase gold particles have been widely studied, but despite that, many aspects of their structures are unclear. One of the most intriguing aspects is the formation of the so called MPT's i.e., the decahedral and the icosahedral structures. We have observed in many samples, prepared under several growth conditions, that the HREM of decahedral particles always have images that match with the calculated ones. In the case of the icosahedral particles, the situation is very different, as shown in Fig. 3), where experimental HREM images of icosahedral particles are compared with calculated ones. Both the images and calculations correspond to the second maximum of the contrast (5), where better conditions for observing gold particles are achieved (6). As can be seen from the figure, no agreement exists between the experimental and theoretical images, particularly in the central portion of the image. This effect is basically due to mistakes in the perfect icosahedral packing (7) during the growth of the particle. These defects are produced mainly when, in addition to the central atom, a second atom acts as a growing center. This breaks up the icosahedral packing but retains the overall ten-fold symmetry. This kind of anomaly requires further study in order to fully understand it.
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Fig. 1.a) High resolution images of Pd particles :grown in-situ in UHV. b) Power spectrum of a particle indicating its amorphous character.
Fig. 2. a) HREM image of a Pd film grown in situ in a UHV environment, b) Image of a decogonal particle taken from Fig. la); image processing was used to enhance the contrast.
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Fig. 3. Experimental high resolution images of a) two-fold, b) three-fold oriented icosahedron, c) calculated image of a icosahedron in two-fold orientation at a defocus of -400A. d) Calculated image of a three-fold oriented icosahedron calculated at a -400A defocus.
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Another kind of structure reported before is shown in Figure 4), where the HREM and its corresponding FFT power spectrum show rings like an amorphous fully polycrystalline film. This type of anomalous structure might be the result of multiple centers of nucleation during growth. It is remarkable that the intensity of the diffuse rings is not symmetric and that two rings appear more prominent. Bi-Mn Alloys It is well documented in the literature that quasicrystals, which are systems that have order but no periodicity, are formed in a number of alloys (8-9). In some cases, such as AlCuCo, a stable decagonal phase has been found (10). A new phase with dodecagonal symmetry has been recently reported in Bi-Mn alloys (11). This phase is only present in particle sizes of -50nm, and represents another example of how nanophases can have crystal structures that are not allowed in "oulk" or micrometer-sized materials. Acknowledgements We are indebted to Mr. L. Rendon, S. Tehuacanero and C. Zorrilla for technical support of this work. This work was supported by CONACYT P228CCOX881228. The Palladium Micrographs were obtained by Dr. M. Avalos in the Xerox Corporation Laboratory in Palo Alto, California. References 1.
2. 3. 4. 5. 6.
7. 8. 9.
10. 11.
F. Ponce; Personal Communication. M. Josr-Yacam~in, M. Avalos and F. Ponce; to be published. R. Herrera, Scripta Metallurgica. 23, 1555 (1989). P. Gao and H. Gleiter, in Physics and Chemistry of Small Clusters, edited by P. Jena et al; Plenum Press, New York (1987). O. Krivanek, in High Resolution Transmission Electron Microscopy and Associated Techniques. Edited by P. Buseck, J. Cowley and L. Egring, Oxford University Press, New York (1988). M. Fluie, Ph.D. Thesis, EPL Lausanne (1989). D. Romeu, Acta Metallurgica. Quasicrystals and Incommensurate Structures, edited by M. Josr-YacamCm, D. Romeu, A. G6mez and V. Casta~o, World Scientific Singapore (1990). Aperiodicity and Order, Vol. I, edited by M. J~iric, Academic Press, New York (1989). H. Chen, Dx. Li and K.H. Kuo, Phys. Rev. Lett. 60, 1645 (1988). J. Reyes-Gasga, R. Hermindez and M. Josr-Yacam~n, Scripta Metallurgica; submitted for publication.
Fig. 4. Image of a Bi-Mn alloy showing a dodecagonal quasicrystalline nanophase.
0965-9773/92 $5.00 + .00 PRINTED IN THE USA
ANOMALOUS STRUCTURES IN NANOPHASES M. Jos6 -Yacam~n Instituto de Fisica, Universidad Nacional Aut6noma de M6xico Apartado Postal 20-364. Delegaci6n Alvaro Obreg6n 01000 Mexico, D.F. MEXICO Introduction In the present work, we describe a number of anomalous structures that are produced in nanostructures of the noble metals Pd and Au, and Bi-Mn alloys. The main characterization techniques used were high resolution electron microscopy (HREM), electron diffraction analysis and image processing. The hardware used was a Jeol 4000-Ex microscope, a Jeol 100-Cx STEM microscope and an image processing Innovion system coupled to a Micro-VAX-III. Palladium Clusters Palladium particles were grown in an UHV-Chamber fitted to a Jeol 4000-Ex microscope (1). The particles produced had a size of approximately 50rim. A HREM of such particles is shown in Fig. la) along with its corresponding power spectrum (FFT), which is equivalent to an optical diffraction pattern. The figure shows that the particle is amorphous (2). However, when the metal is annealed at higher temperatures, or when the deposition flux is increased, the clusters show a completely different structure, as shown in fig. 2a). This image shows mainly "two-dimensional" islands but now the structure is defined. An examination of the cluster, using image processing, indicates that most of the clusters have a decagonal structure with several defects Fig. 2b), as was reported by Herrera et al (3). No icosahedral structures were found. It is interesting to note that the path to crystallization goes from the amorphous to the decagonal structures with boundary defects. This latter type of cluster has also been reported by Gao and Gleiter (4) for the case of gold. A point to be made is that our samples were grown in UHV, so that contamination should play no role. Multiply Twinned Gold Particles Nanophase gold particles have been widely studied, but despite that, many aspects of their structures are unclear. One of the most intriguing aspects is the formation of the so called MPT's i.e., the decahedral and the icosahedral structures. We have observed in many samples, prepared under several growth conditions, that the HREM of decahedral particles always have images that match with the calculated ones. In the case of the icosahedral particles, the situation is very different, as shown in Fig. 3), where experimental HREM images of icosahedral particles are compared with calculated ones. Both the images and calculations correspond to the second maximum of the contrast (5), where better conditions for observing gold particles are achieved (6). As can be seen from the figure, no agreement exists between the experimental and theoretical images, particularly in the central portion of the image. This effect is basically due to mistakes in the perfect icosahedral packing (7) during the growth of the particle. These defects are produced mainly when, in addition to the central atom, a second atom acts as a growing center. This breaks up the icosahedral packing but retains the overall ten-fold symmetry. This kind of anomaly requires further study in order to fully understand it.
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Fig. 1.a) High resolution images of Pd particles :grown in-situ in UHV. b) Power spectrum of a particle indicating its amorphous character.
Fig. 2. a) HREM image of a Pd film grown in situ in a UHV environment, b) Image of a decogonal particle taken from Fig. la); image processing was used to enhance the contrast.
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Fig. 3. Experimental high resolution images of a) two-fold, b) three-fold oriented icosahedron, c) calculated image of a icosahedron in two-fold orientation at a defocus of -400A. d) Calculated image of a three-fold oriented icosahedron calculated at a -400A defocus.
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Another kind of structure reported before is shown in Figure 4), where the HREM and its corresponding FFT power spectrum show rings like an amorphous fully polycrystalline film. This type of anomalous structure might be the result of multiple centers of nucleation during growth. It is remarkable that the intensity of the diffuse rings is not symmetric and that two rings appear more prominent. Bi-Mn Alloys It is well documented in the literature that quasicrystals, which are systems that have order but no periodicity, are formed in a number of alloys (8-9). In some cases, such as AlCuCo, a stable decagonal phase has been found (10). A new phase with dodecagonal symmetry has been recently reported in Bi-Mn alloys (11). This phase is only present in particle sizes of -50nm, and represents another example of how nanophases can have crystal structures that are not allowed in "oulk" or micrometer-sized materials. Acknowledgements We are indebted to Mr. L. Rendon, S. Tehuacanero and C. Zorrilla for technical support of this work. This work was supported by CONACYT P228CCOX881228. The Palladium Micrographs were obtained by Dr. M. Avalos in the Xerox Corporation Laboratory in Palo Alto, California. References 1.
2. 3. 4. 5. 6.
7. 8. 9.
10. 11.
F. Ponce; Personal Communication. M. Josr-Yacam~in, M. Avalos and F. Ponce; to be published. R. Herrera, Scripta Metallurgica. 23, 1555 (1989). P. Gao and H. Gleiter, in Physics and Chemistry of Small Clusters, edited by P. Jena et al; Plenum Press, New York (1987). O. Krivanek, in High Resolution Transmission Electron Microscopy and Associated Techniques. Edited by P. Buseck, J. Cowley and L. Egring, Oxford University Press, New York (1988). M. Fluie, Ph.D. Thesis, EPL Lausanne (1989). D. Romeu, Acta Metallurgica. Quasicrystals and Incommensurate Structures, edited by M. Josr-YacamCm, D. Romeu, A. G6mez and V. Casta~o, World Scientific Singapore (1990). Aperiodicity and Order, Vol. I, edited by M. J~iric, Academic Press, New York (1989). H. Chen, Dx. Li and K.H. Kuo, Phys. Rev. Lett. 60, 1645 (1988). J. Reyes-Gasga, R. Hermindez and M. Josr-Yacam~n, Scripta Metallurgica; submitted for publication.
Fig. 4. Image of a Bi-Mn alloy showing a dodecagonal quasicrystalline nanophase.