high resolution tem studies of gold and palladium nano-particles

Acta metall, mater. Vol. 40, No. 7, pp. 1663-1674, 1992 0956-7151/92 $5.00 + 0.00 Printed in Great Britain. All rights reserved Copyright © 1992 Pergamon Press Ltd

HIGH RESOLUTION TEM STUDIES OF GOLD A N D PALLADIUM NANO-PARTICLES

S. TEHUACANERO, R. HERRERA, M. AVALOS and M. JOSI~ Y A C A M A N

Instituto de Fisica, Universidad Nacional Aut6noma de M~xico, Apartado Postal 20-364, Delegacirn Alvaro Obreg6n, 01000 Mrxico, D.F. Mrxico

(Received 4 November 1991)

Abstract--In the present work we analyze nano-particles of gold and palladium using a combination of High Resolution Electron Microscopy (HREM) and image processing techniques. The experimental results are compared with calculations of images using dynamical diffraction theory. We focus mainly on the decahedral (Dh) and icosahedral (Ic) Particles. We found that the decahedral particles correspond well with the model for a perfect MPT. The icosahedral particle on the other hand appears to be much more distorted than the MPT model predicts. Images of polycrystalline and amorphous particles are also shown. No significant differences between Au and Pd were found.

Rrsum~-Dans ce travail, on analyse des nanoparticules d'or et de palladium en combinant la microscopie 6lectronique en haute rrsolution et des techniques de traitement d'image. Les rrsultats exprrimentaux sont comparrs aux calculs des images par la throrie dynamique. On s'intrresse en particulier aux particutes drcardriques (Dh) et icosardriques (It). On trouve que les particules drcardriques correspondent bien au modrle de particules multimaclres parfaites. D'autre part, la particule icosardrique semble plus distordue que ne le pr6voit ce modrle. Des images de particules polycristallines et amorphes sont aussi montrres. Aucune diffrrence significative n'est trouvre entre l'or et le palladium.

Zusammenfassung~old- und Palladium-Nanoteilchen werden im HochauflSsungselektronen- mikroskop unter Verwendung eines Bildverarbeitungsverfahrens analysiert. Die experimentellen Bilder werden mit den mittels der dynamischen Beugungstheorie berechneten Bildern verglichen. Der Schwerpunkt liegt bei dekaedrischen (Dh) und ikosaedrischen (Ic) Teilchen. Wir finden, dab die dekaedrischen Teilchen sehr gut dem Modell fiir ein perfektes MPT entsprechen. Andererseits scheinen die ikosaedrischen Teilchen viel st/irker verzerrt zu sein, als das MPT-Modell voraussagt. Des weiteren werden Bilder yon polykristallinen und amorphen Teilchen gezeigt. Au und Pd unterscheiden sich nicht wesentlich.

1. INTRODUCTION

Ever since the work of Ino [1] the structure of noble metal particles with five-fold symmetry have been extensively studied [2-5]. High Resolution Electron Microscopy ( H R E M ) of icosahedral (Ic) and decahedral (Dr) particles was reported by Marks and Smith [6]. Computed H R E M images of icosahedral clusters were reported by Barry et al. [7] and more recently by Flueli et al. [8]. Some other works have added image processing and Fourier analysis to the H R E M images [9]. Despite of all the work several aspects of the structure of five-fold particles are not totally resolved.

In this work we present the systematic studies of H R E M of gold and palladium particles grown by vacuum evaporation. We will present evidence that there are Ic particles whose images do not correspond to the perfect M P T model, and therefore correspond to a more distorted state. We also found other structures not reported before such as polycrystalline and amorphous particles.

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2. EXPERIMENTAL

In the present work samples were prepared by vacuum evaporation from a filament, with a vacuum of 10 -7 Pa, in an oil free vacuum system. A NaCI crystal, cleaved in vacuum, was used as the substrate, the temperature of the substrate was about 300°C during the deposition. Films were then carbon covered, stripped by flotation and mounted in T E M grids for observation. In other cases the films were de- posited in a vacuum chamber adjacent to the microscope using a carbon grid as substrate and then transferred without breaking the vacuum to the spec- imen holder area of the microscope.

Observations were carried out in a JEOL-4000EX Microscope with top entry configuration and Cs = 1 mm or in a JEOL-4000EX Microscope adapted with a vacuum chamber for evaporat ion (with a vacuum pressure of 10 -7 Pa). Pictures were obtained most of the time in the opt imum focus condit ion (black columns of atoms) that correspond to the first maximum on the contrast transfer

1663

function. However Flueli [13] has shown that in many cases it is more convenient to image the particles in the second maximum of the contrast transfer function (white columns of atoms). In Fig. 1 the transfer contrast characteristics for the 4000-EX vs the defocus of the objective lenses is plotted. The two maxima conditions are clearly shown. We however obtained some of the images in a condition close to the second maximum because in this condition it is easier to image at the same time (111) and (200) planes.

Pictures were digitized using a CCD camera or a microdensitometer. The Fourier transform of the image was computer calculated using an IBM-PS-2. Then different filters were applied in order to process the pictures to minimize the noise and reconstruct the image using different frequencies [10-11].

Theoretical images were computed using a soft- ware package developed by Herrera and Grmez [12] which admits an arbitrary shaped object and it is specially well suited for icosahedral and decahedral particle image computations.

3. RESULTS

3.1. Decahedral particles

In both samples, gold and palladium, we found frequently decahedral particles (Dh). Typical examples along with its fourier transform are shown in Fig. 2(a-d). The fact that F F T can be indexed as diffraction patterns [10] allows us to analyze the crystal structure of the particles. The FFT agrees quite well with the diffraction pattern expected for Dh particles [2]. The experimental images also agree quite

AU (111 200 220)

well with the calculated images for the decahedral particles. We should note however that in many cases the five-twin boundaries do not join at the center of the particle. In particles with a diameter of ~ 20-30/~ the asymmetric boundaries seem to be the most common case.

Figure 3 shows the case for a Pd Dh particle with five-fold symmetry. It should be noted that in this case a larger degree of disorder is seen along the rows of atoms than in the case of gold shown in Fig. 2.

3.2. Icosahedral particles

3.2.1. Three fold symmetry particles. A particle which is very often found is the icosahedral particle oriented with a three-fold axis parallel to the electron beam. A typical example of this is shown in Fig. 4(a). The F F T Fig. 4(b) shows the three-fold symmetry. Theoretical images of this kind of particle are shown in Fig. 5(a-c) where the first and second maximum of contrast are shown along with the projected potential. The agreement between the calculated and experimental images is quite good. The fourier transform shows arched spots which indicates that the particle has been strained to fit the icosahedral structure. This type of arching is consistent with both homogeneous or inhomogeneous strain [2-3]. However not all the particles show the same type of FFT. Figure 6(a~l) shows two other gold particles and their corresponding FFT. In one case the spots are very sharp and in the other there is a split of spots as would be expected for a f.c.c, structure.

The kind of behavior described above strongly suggests that there are a number of possible struc-

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1 6 6 4 TEHUACANERO et al.: HIGH RESOLUTION TEM OF NANO-PARTICLES

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Fig. 1. Contrast transfer characteristics of the JEOL-4000EX Microscope vs defocus of the objective lenses.

TEHUACANERO et al.: HIGH RESOLUTION TEM OF NANO-PARTICLES 1665

Fig. 2. (a, b) Images of decahedral gold particles along the five-fold axis of symmetry and (c, d) their corresponding FFT showing ten-spots.

tures between the f.c.c, and the icosahedral structure. A similar result was reported by Avalos and Ponce in large decahedral particles [14].

3.2.2. Two-fold symmetry particles. Another particle which is found very often in both Au and Pd is that oriented along a two-fold axis. A typical particle

Fig. 3. (a) Image of decahedral particle in Pd (note the larger distortion on the atomic planes) and (b) its corresponding FFT.


Fig. 4. (a) Image of an icosahedral gold particle oriented along a three-fold axis, (b) its corresponding FFT and (c) processed image to enhance the central portion of the

particle.

is shown in Fig. 7(a) along with its F F T in Fig. 7(b). The calculated image for the two principal contrast maxima along with the projected potential

is shown in Fig. 8(a-c). Again the agreement between experimental and theoretical images is quite good.

3.2.3. Five-foM oriented particles. A surprising fea- ture was found in the particles oriented along the five-fold axis. This type of particle was found in a much lower proportion than the two previous orien- tations described above. This is also in contrast with the decahedral particles in which the five-fold orientation is the most prominent.

A typical example of the contrast along the five- fold axis for a Pd particle is shown in Fig. 9(a, b) along with its corresponding FFT. In Fig. 10(a-c) we show the corresponding calculated image for the two maxima of contrast and the projected potential. A similar example for gold particles is shown in Fig. 1 l(a-c).

As can be observed on the figures there is no agreement between the calculated and experimental images for either gold or palladium particles. This fact was previously found by Flueli [13] who explains the disagreement using a model of distorted icosahedron.

In order to rule out the possibility that the lack of agreement was due to misorientations on the particle, we have made a calculation of the contrast of an icosahedron in five-fold orientation as the function of tilt. The calculations are shown in Fig. 12(a~l). The tilt was made away from the direction of the electron beam parallel to the five-fold axis. As can be seen, a tilt of 2 ° is enough to blurr the contrast and 3 ° to destroy it completely. Therefore we can conclude that the particle in Fig. 9 is in a low index orientation and the observed change in the contrast is not due to misorientations.

3.2.4. Other kinds of particles. We found that there are a number of particle structures which have not been discussed before. An interesting example is shown in Fig. 13(a,b). The HREM image shows a complicated polycrystalline contrast. This is confirmed by the FFT spectrum. No model exists at the present to explain this kind of particle.

Finally a type of particle which is very often seen in both gold and palladium are the single twinned ones from which an example is shown in Fig. 14(a,b). The FFT can be readily indexed as corresponding to a single twinned structure.

3.2.5. Amorphous Pd particles. In the case of the Pd particles in one experiment we deposit the metal onto a carbon covered grid at room temperature. The particles were observed directly without breaking the vacuum. In this case we obtained amorphous particles. Examples of those are shown in Fig. 15(a~l) which corresponds to the images and its fourier transform. The FFT clearly demonstrates the amorphous structure of the particle.

In the Fig. 16(a) we show the processed image of the particle in Fig. 15(c). In this case the border of the particle is not well defined because of the fourier


Fig. 5. Calculated images of a 585 atom gold icosahedral particle along a three-fold axis. (a) Af = -405 and aperture size of 0.4 1/A. (b) Af = -705/~ and aperture size of 0.4 1//~. (c) Projected potential of the

particle.

transforming. However the central portion of the particle reproduces the two contrasts. As it is clearly seen, some icosahedral-like features can be observed on the particle.

4. DISCUSSION

Our results show that the general trends are the same for both Pd and Au. The main difference being small changes on the population of different kinds of particles.

The decahedral particle (Dh) appears to be the most stable one in both cases. However we found that the most common case is the asymmetrical Dh nano-particle. However the faceting on the D h particles can be observed in many cases. This indicates that they

correspond well with the Wulff polyhedron described by Marks [15], in which only (111) and (100) faces are present. However the most common case is the asymmetric polyhedron. This can be explained in terms of the Marks model which has a local minimum on the total energy (elastic strain plus surface energy) for the case of the asymmetric Dh.

In the case of the icosahedral (Ic) particles it is clear that the most convenient orientation for structural observation is the three-fold one. In this case only (111) type of planes contributed to the image. It is clear that despite the agreement between calculated and experimental images, the FFT indicates that the particle is strained beyond the prediction of the MPT model. Moreover the fact that the FFT spots vary from arching to splitting indicates that between the


i

Fig. 6. Images of particles of gold oriented along a three-fold axis, (a, b), along with their corresponding FFT in (c, d). Note the difference in the two FFT patterns.

Fig. 7. (a) Images of a gold icosahedral particle oriented along the two-fold axis and (b) its corresponding FFT.

TEHUACANERO e t al.: HIGH RESOLUTION TEM OF NANO-PARTICLES 1669

Fig. 8. Calculated images of icosahedral gold particle with 585 atoms oriented along a two-fold axis. The aperture used had a diameter of 0.4 1//~. (a) A f = - 4 0 5 ~ . (b)

Af = -705 A. (c) Projected potential. Fig. 9. (a) Experimental image of an icosahedral palladium particle along the five-fold axis, (b) amplified image of the

previous particle and (c) its corresponding FFT.


Fig. 10. Calculated images of a five-fold oriented Pd particle with 585 atoms with an aperture of 0.41[A. (a) Af = -405 A. (b) Af = -705 .~,. (c) Projected potential.

pure f.c.c. (split) to the pure rhombohedral distortion (arching) there are a continuous of structures.

In the case of the five-fold particles the image is formed by a stacking of four (111) planes. Therefore images on this orientation are the most sensitive to strain on the particle. In the case of a perfect icosahedral particle the image has a ten-fold symmetry around the center of the particle, as shown in Fig. 10. This is never achieved in the experimental images. Moreover if we observe carefully the FFT of Fig. 9(c), we note that the ten-fold symmetry of the pattern is broken i.e. the spots are forming angles different from 36 ° . This indicates that the structure does not correspond to a pure icosahedron model. We have made a detailed analysis of many particles in five-fold symmetry and found in all cases that the angles are not the correct ones. This was also reported by Flueli [13].

The fact that the symmetry is close to ten-fold suggest that the particle probably corresponds to a heavily distorted icosahedron. We should point out that the observed distortion goes beyond that re- quired to strain the f.c.c, structure into a MPT. Since this distortion was already included into the calculations of the images of Fig. 10. It is irrelevant whether the distortion is introduced through a discli- nation [3] or through a change in structure of the tetrahedral units [2].

Flueli e t al. (8) attempted an explanation of this disagreement by introducing an extra distortion by rotating the upper portion of the icosahedron with respect to the lower portion and then calculating the image. The resulting image looks closer to the experimental image. The work of these authors clearly shows the fact that we are dealing with an icosahedral particle much more distorted than previously thought.

In the case of the two-fold and three-fold images the observed planes (111) and (110) do not overlap as much and these images are less sensitive to strain. Therefore the calculated images are more in agreement with the experiment. However the FFT also indicates that we have a structure which deviates from the perfect icosahedron.

We can then conclude that the models used so far to explain the MPT model structures are approxi- mants to the real structure. Our results are in agreement with the notion that the particles are in the state of quasimelting at the proper temperature [16]. The main idea comes from the observation that small particles go into a state of continual fluctuation under the effect of the electron beam of the TEM [17]. The implication is that there are a number of different locally stable structures with very small energy bar- tiers between them. The particle can therefore jump from one state to another when the proper thermal energy is delivered. Our observations will correspond to states stabilized by the substrate, and therefore any state might be possible. The fact that we have a large number of different types of F F T indicates the


m Fig. 11. (a, b) Images of a gold icosahedral particle oriented along the five-fold axis and (c, d) its

corresponding FFT.

possibility of having a continuous set of structures between the f.c.c, case and the perfect icosahedral case, if this idea is valid it strongly supports the concept of Ajayan and Marks [18] that quasimelting is happening in small particles unless it is stabilized by the substrate.

In the case of complex polycrystalline particles there are no models that can explain the observed structure. However at the present time molecular dynamics simulations are being attempted in order to reproduce the observed contrast.

5. CONCLUSIONS

The following aspects can be concluded:

• The observed images of decahedral particles are in agreement with the theoretical calculations corresponding to a perfect Wulff polyhedron described by Marks [15]. The most common observed case is that in which the five boundaries do not join at the center of the particle. This

might be the result of minimizing the elastic strain energy and the surface of the particle.

• The icosahedral particles produced images which are not in agreement with the model of a perfect icosahedra. These icosahedral MPT structures are therefore more distorted than previously assumed.

• We are observing in the case of Ic nano-particles a large number of different structures that sup- port the idea of quasimelting.

• There are structures different from the Ic and D h or the single crystal such as a poly-crystalline. No models exist at the present to explain such particles.

• There are also a large number of single twinned particles.

• In the case of Palladium we observed amorphous structures when deposits were made at room temperature.

• No significant differences were found between the cases of gold and palladium.

1672 TEHUACANERO et al.: HIGH RESOLUTION TEM OF NANO-PARTICLES

Fig. 12. Calculation of the change on the contrast of an icosahedral particle with image conditions of aperture of 0.4 1//~ and A f = -405 /~ but misoriented with respect to the electron beam by (a) I °, (b) 2 °,

(c) 3 ° and (d) 5 °.

t

Fig. 13. (a) HREM image of a complex polycrystalline particle and (b) its corresponding FFT.


Fig. 14. (a) HREM image of a single twinned gold particle and (b) its corresponding FFT.

Fig. 15. Images of an amorphous Palladium particle, (a, c), and its corresponding FFT transform (b, d).

1674 TEHUACANERO et al.: HIGH RESOLUTION TEM OF NANO-PARTICLES

REFERENCES

Fig. 16. Computer processed image of the particle in Fig. 15(c). Note the features which are similar to the

icosahedral-like particles.

Acknowledgements--The authors are indebted to C. Zorrilla for the calculations of the images of nano-particles and to Mr L. Rend6n for technical assistance with the TEM. We thank Dr Romeu for his model of MPT particles that we used on the image calculations. This work was financed by DGAPA-UNAM and CONACYT through a grant of the DAIC.

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10. M. Tomita, H. Hashimoto, H. Ikuta, T. Endoh and Y. Yokota, Ultramicrose. 16, 9 (1985).

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high resolution tem studies of gold and palladium nano-particles

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