Anexos PUBLICACIONES RELACIONADAS CON EL TRABAJO DE TESIS

1. Larios-Rodriguez, E.; Lilián Calderón, Karen Guerrero, Emanuel Pinedo,

Amir Maldonado and Judith Tanori. Synthesis of Core-shell (Pd-Au)

bimetallic nanoparticles in microemulsions, (2010) Journal of Dispersion

Science and Technology.

2. E. Larios-Rodríguez, Z. Molina-Arenas, A. Maldonado, Maryse Lancin, and

J. Tánori-Córdova. Characterization of Coppercore-Goldshell

nanoparticles synthesized in self-assembling colloidal systems. (2010)

Journal of Dispersion Science and Technology.

3. El Bouayadi, Rachid; Regula, Gabrielle; Lancin, Maryse; Larios, Eduardo;

Pichaud, Bernard; Ntsoenzok, Esidor. Silver nanocrystals at cavities

created by high energy helium implantation in bulk silicon. Materials

Research Society Symposium Proceedings (2007).

----- Forwarded message from johan.sjoblom@chemeng.ntnu.no ----- Date: Thu, 16 Jul 2009 14:57:08 +0200 From: johsj <johan.sjoblom@chemeng.ntnu.no> Reply-To: johan.sjoblom@chemeng.ntnu.no Subject: Ms JDST 2010 / 191 To: judith.tanori@mrl.ucsb.edu, "VanSciver, Catherine" <catherine.VanSciver@taylorandfrancis.com> Dear Dr Tanori Your ms "Synthesis of Core-Shell... will be accepted for publication in J Dispersion Science Technology. Your ms will appear in JDST (2010). Before we can start the editing process your ms must be written in accordance with "Instructions to Author", i.e. 1) All papers must be double spaced, with 1-inch margins (on all sides), preferably in a 12-point font. 2) All figures and tables should be placed at the end of the document (or placed in a separate document). They should NOT be placed between paragraphs within the text of the article. 3) Documents that have been written on a non-standard keyboard, should be converted (e.g., documents written in an Arabic language are automatically flush right; these documents become un-editable and errors are often introduced, especially where equations are concerned). 4) PDFs are not acceptable. Word documents should be submitted. If an authors is worried about figures, equations, etc. being accidentally moved in an editable program, they may include a PDF for reference. 5) Please be sure all citations appear as numbered references. References should be cited in the order they appear in the reference list. Samples of references may also be found on the Instructions to Authors page." You will be contacted by our Production Editor Catherine van Sciver if some stylistic corrections are needed in your ms. Thank you for this most valuable contribution to JDST !!! Before starting the editing process you should fill in the copyright transfer form (attached) and send it (properly signed) as a PDF file to Production Editor Catherine VanSciver. Please clearly indicate on the form the name of the journal (JDST/volume/issue/ms nr) as specified in this email of acceptance. Email: catherine.VanSciver@taylorandfrancis.com Thank you very much for your collaboration !

Synthesis of Core-shell (Pd-Au) bimetallic nanoparticles in microemulsions Eduardo Lariosa,c, Lilián Calderónc, Karen Guerreroc, Emanuel Pinedoc, Amir Maldonadob and Judith Tanoria*

aDepto. de Investigación en Polímeros y Materiales, Universidad de Sonora. Apdo. Postal 130, 83 000, Hermosillo, Sonora, México. bDepto. de Fisica, Universidad de Sonora, Apdo. Postal 1626, 83000, Hermosillo, Sonora, México. cDepartamento de Ingeniería Química y Metalurgia, Universidad de Sonora. Apdo. Postal 130, 83000, Hermosillo, Sonora, México. ∗ Corresponding author. Tel.: 52 + (662) 259-2161; fax: 52 + 662 259-2216 E-mail address: jtanori@polimeros.uson.mx Abstract Palladium-gold core-shell nanoparticles were synthesized in the aqueous domains of water in oil microemulsions by the sequential reduction of H2PdCl4 and HAuCl4. The nanoparticles were characterized by UV-Vis spectroscopy and Transmission Electron Microscopy (TEM). The UV-Vis spectra confirm the presence of palladium nanoparticles after reducing H2PdCl4. These particles have been used as seeds for the core-shell particles. UV-Vis spectra show that, after reducing HAuCl4, the surface plasmon absorption of the nanoparticles is dominated by gold, revealing the encapsulation of the palladium seeds. These results agree with crystallographic analysis performed with High-Resolution TEM pictures, as well as with Selected Area Electron Diffraction. The TEM pictures show the core-shell nanoparticles with an average diameter of 9.1 nm, as compared with 5 nm for the palladium seeds, in good agreement with the used Pd:Au molar ratio. Keywords: microemulsions, palladium-gold, nanoparticles, core-shell. I. Introduction In the present days, the synthesis and the manipulation of nanometric entities has special relevancy in the so-called nanotechnology revolution1. The interest in nanometric-scale materials stems from their size and from the different physical properties they have, as compared to the same bulk materials. In this context, the synthesis of nanoparticles of different materials (metallic, semiconductor, dielectrics) has attracted a lot of attention due to their applications in catalysis, medicine, electronics, etc. Material scientists continuously look for methods that allow the control of the size and/or shape of the nanoparticles, as required by their applications. In this respect, the preparation of core-shell nanoparticles2-6 is of significance in nanotechnology due to the fact that their physical and chemical properties can be tuned by controlling parameters such as the size of the core, the thickness of the shell, as well as the overall shape of the particle. In some applications, it is desirable to have at the same time properties given by the material in the core (magnetism, density, etc) as well as properties provided by the material in the external surface, the shell (catalytic ability, optical properties, conductivity, etc). In this way, nanotechnology has the potential to create many new materials and devices with a wide spectrum of applications in fields like medicine7,8, catalysis9,11, photo-electronics12,13, energy production, and so forth. A number of methods have been used to prepare metallic nanoparticles. Physical methods involve processes such as vapor deposition14,15 and laser ablation16. The chemical methods start with the reduction of the metal ions to metal atoms. They are followed by the controlled aggregation of the material9. These methods perform the

synthesis in different environments: microemulsions17-22, solution10,13,23, a solid matrix24, etc. They use a number of reducing agents, including alcohols, hydrazine, polyols, borohydrides, light, sound waves, etc. Particularly, methods of preparation of bimetallic nanoparticles can be divided into two categories: sequential and simultaneous reduction. The first one involves the consecutive addition of two different metallic precursors in the reaction system3,22. Nanoparticles of one material are created in the initial times of the reaction to form the core of the nanoparticle. Over this core, a shell of a different material is deposited in the second part of the reaction. In the case of a simultaneous reduction, the two metallic precursors are present in the reactor at the same time2,9. In this work, we study the preparation of palladium-gold (Pd-Au) core-shell nanoparticles in water-in-oil microemulsions. We chose these materials because palladium has applications in catalysis and gold has a lot of applications in nanotechnology. For instance, it has been shown that Pd displays catalytic activity towards trichloroethene (TCE), a common hazardous organic contaminant of ground water.25 Furthermore, this activity increases as much as 70 times when palladium is supported on gold nanoparticles.10,25 Pd-on-Au nanoparticles have been successfully applied to the aqueous phase catalysis of TCE. This is a promising technology for ground water treatment.26 It is worth noting that gold-palladium alloy nanoparticles have been used in the direct synthesis of hydrogen peroxide (H2O2). The nanoparticles switch off the decomposition of H2O2, an undesirable effect of other catalysts.27 On the other hand, gold nanoparticles on their own have a lot of applications. For instance, in biology and nanomedicine, they are used as contrast agents for tissue visualization, for immunostaining, single particle tracking, gene delivery, etc. Recent reviews of these applications can be found in references.28,29

For all these reasons, it is interesting to device a system with the properties of gold and some, even if residual, catalytic activity given by palladium. As a first step towards this goal, in this paper we use the microemulsion method to synthesize Pd-Au core-shell nanoparticles. Our aims are to assess if this method is suitable for the preparation of this kind of material, and to characterize the obtained nanoparticles. The microemulsions are created in the ternary system water/Aerosol OT/isooctane. The nanoparticles are prepared by the sequential reduction of the tetrachloropalladate (II) and tetrachloroaurate (III) ions with hydrazine. The size, optical properties and core-shell formation were characterized by transmission electron microscopy (TEM), high resolution TEM (HRTEM), selected area electro diffraction (SAED) and UV/Vis spectroscopy. The remainder of the manuscript is divided as follows. In the next section we give the details of our experiments. In section III we present and discuss our results. Finally, in section IV we draw some conclusions. II. Experimental section Microemulsions The core-shell nanoparticles have been synthesized in the aqueous domains of microemulsions of the ternary system water-aerosol OT-isooctane. The phase diagram of the system can be found elsewhere.30 Aerosol OT (AOT or sodium bis(2-ethylhexyl) sulfosuccinate) is a commercially available surfactant. It was purchased from Sigma- Aldrich. The oil phase in the microemulsion is isooctane, from Fluka. In addition, we used ultrapure water from a Milli Q system (Millipore). The nanoparticles were prepared in the reactor environment defined by the aqueous domains of reverse micelles. These micelles are water droplets suspended in isooctane, stabilized by an

AOT monolayer. We performed the synthesis in micelles with a surfactant concentration of 0.1 M (in isooctane). Water was added to form the microemulsion, until a water-surfactant concentration ratio, w ≡ [H2O]/[AOT] = 5 was obtained. Preparation of Palladium seeds (metallic core). For the synthesis of Palladium seeds, metallic ions were introduced as an aqueous solution (0.2 M) of H2PdCl4, prepared by dissolving PdCl2 in a 0.2 N HCl solution 31,32. 1 μL of this solution was added per 1 mL of microemulsion. The reducing agent was hydrazine (1 μL per 3 mL of microemulsion). The reaction proceeded in the water pool after hydrazyne addition. The metallic precursor was reduced to its zero-valence state, and then the palladium nanoparticles formed by aggregation. These palladium particles have served as seeds for the core-shell nanostructures. Addition of a Gold layer (metallic shell). The obtained palladium nanoparticles were used as preformed cores for the preparation of core-shell bimetallic Pd-Au nanoparticles. An aqueous 0.2 M solution of HAuCl4 (2 μl per 1 ml of microemulsion) was added into the reaction system containing the Pd seeds under stirring. Afterwards, hydrazine was added to the system under vigorous stirring. The bimetallic nanoparticles were obtained as a purple-brown suspension. The Pd:Au molar ratio was 1:2. Preparation of Passivated Core-shell Bimetallic Palladium-Gold Nanoparticles. We passivated the surface of the Pd-Au nanoparticles in order to stabilize them against coagulation driven by attractive van der Waals forces. For this, we adsorbed the functional -SH thiol group of dodecanethiol onto the gold surface of the nanoparticles. When the palladium-gold particles were obtained, we added to the solution 9 μL of dodecanethiol per mL of microemulsion. Characterization of Pd and Pd-Au nanoparticles. The nanoparticles were characterized in a JEOL 2010F Transmission Electron Microscope. A drop of each sample was placed on a carbon film supported by a copper grid in order to obtain TEM micrographs. We have further analyzed the core-shell structures by means of High Resolution TEM images and Selected Area Electron Diffraction (SAED). The optical properties of the samples were investigated by UV-Vis spectroscopy. The UV-Vis spectra were obtained with a Perkin- Elmer Lambda 2 spectrophotometer with the samples placed in 1x1x3 cm rectangular quartz cells. Results The prepared H2O/AOT/Isooctane microemulsions are homogeneous, transparent liquids. Their UV-Vis spectra do not show any absorption band. These features allow to monitoring the nanoparticle growth by the change in color and by the UV-Vis spectra of the microemulsions used as chemical reactors. In these microemulsions we performed the sequential reduction of palladium and gold in order to obtain the core-shell nanoparticles. Palladium Nanoparticles For the synthesis of the palladium nanoparticles, which served as seeds for the core-shell structures, Pd ions were reduced to metallic Pd in the presence of hydrazine. As the reaction proceeded, the color of the solution slowly turned from a subtle brown to a dark brown. We let the reaction proceed for a week. The UV-Vis spectrum of the Pd nanoparticles in suspension is shown in figure 1. The strong surface plasmon resonance band around 280 nm is indicative of the presence of palladium nanoparticles.33 A representative TEM image of the Pd particles is shown in figure 2. The reaction resulted in well dispersed nanoparticles with a relatively narrow size distribution. Their average diameter was 5.3 nm (standard deviation of 2.5 nm, with a

coefficient of variation of 47 %). In order to characterize the palladium structure, we performed electron diffraction experiments. In figure 3(b), we present an electron diffraction pattern obtained for these nanoparticles. The diffractogram corresponds to the crystallographic structure of palladium nanoparticles oriented along different planes of the Pd atomic lattice. The diffractogram displays several reflections. The spots on rings 1, 2 and 3 are related to the lattice spacings 2.24 Å, 1.945 Å, and 1.375 Å, which correspond to the (111), (200), and (220) planes of the fcc structure of palladium (JCPDS file no. 46-1043). In figure 3(a) we present a HRTEM image where the lattice spacing is 2.24 Å, corresponding to the (111) plane. As we explained before, these small Pd nanoparticles where used as seeds in order to deposit on them a gold shell. It is worth noting that Pd nanoparticles remained in suspension, without aggregation between particles. This behavior is probably due to the adsorption of the surfactant molecules on the surface of the nanoparticles. Core-shell Palladium-Gold Nanoparticles The microemulsions containing the suspended Pd nanoparticles were used as reactors for the reduction of gold. When this new reaction took place, the color of the suspension changed from the previously attained brown to a purple-brown color. This modification was almost instantaneous. The UV-Vis spectrum of the resulting nanoparticle suspension is shown in figure 4. The observed plasmon resonance band around 530 nm is indicative of the presence of metallic gold. That is, it indicates the formation of a gold shell around the palladium nanoparticles. Even if the palladium seeds were obtained as isolated nanoparticles and had a relatively narrow size distribution, the particles obtained after adding gold, had different characteristics. As can be observed in figure 5a, the bimetallic nanoparticles formed large agglomerates. The coalescence is due to attractive Van der Waals forces between the gold-covered surfaces. The average diameter of the observed particles is 9 nm (standard deviation of 3.6 nm, with a coefficient of variation = 40 %). The coalescence observed in figure 5a is also very clearly revealed in the HRTEM images. In figure 6a we show two fused nanoparticles. The diffractograms obtained for the particles of figure 6b exhibit the Debye-Scherrer rings that correspond to the fcc structure of gold. The rings in the diffractogram can be indexed and correspond to what is expected for gold. In fact, spots on rings 1, 2, 3, 4, and 5 are related to the lattice spacings 2.35 Å, 2.039 Å, 1.442 Å, 1.230 Å, and 1.177 Å which correspond to the (111), (002), (022), (113) and (004) planes of the fcc structure of gold (JCPDS file no. 4-784). It is worth noting that some spots of diffraction in the vicinity of the first ring are rather related to the lattice spacing 2.24 Å, which corresponds to the (111) planes of the fcc structure of palladium. This means that the diffraction experiment is evidencing the presence of gold, with a residual signal for palladium, as expected for core-shell particles of these materials. Passivated Core-shell Bimetallic Palladium-Gold Nanoparticles. As shown in figure 5a, the gold-covered particles coalesce, due to van der Waals forces. In order to stabilize a suspension of isolated, individual nanoparticles, we covered their surface with thiol groups. Figure 8a shows TEM image of passivated bimetallic Pd-Au nanoparticles with an average diameter of 9.1 nm (standard deviation of 3.03 nm with a coefficient of variation = 33 %). This image shows that the particles disperse better as compared to the case of the bimetallic particles with the bare gold surface (figure 5a). Thus, the strategy of covering the particle surface with thiol groups prevents agglomeration and coalescence of the particles. The UV-Vis spectra for this passivated Pd-Au bimetallic system, figure 7, display a slight shift and a strong damping in the surface plasmon resonance band, due to the presence of the thiol groups in the

surface. In figure 9a we show an HRTEM image for a passivated nanoparticle. We can appreciate one of the lattices spacings associated with gold: 2.039 Å. Discussion. Our experiments confirm that the sequential reduction of Pd and Au in the aqueous domains of the AOT-isooctane-water microemulsions effectively leads to the formation of core (Pd) – shell (Au) nanoparticles. The first synthesized palladium seeds are successfully covered with a gold shell. From the UV-Vis spectroscopy experiments we confirm the modification occurring in the surface of the palladium seeds. It is known that Pd and Au nanoparticles have surface plasmon resonance bands around 280 and 520 nm, respectively. The formation of core-shell Pd-Au nanoparticles is deduced from the position of the adsorption band in the spectrum. The plasmon maximum is shifted towards the red region. This band was initially located at 280 nm, as expected for palladium, but after the addition of HAuCl4 and hydrazine, it shifted to 530 nm. This reveals the encapsulation of the palladium seeds in gold shells, since the first absorption is expected for Pd while the latter is near of that of gold. This result is in agreement with the prediction of Mulvaney et al5. It is expected that even one monolayer of gold is sufficient to mask the palladium plasmon resonance band completely. On the other hand, the absorption spectra of the passivated Pd-Au nanoparticles shows a drastic damping of the plasmon band. This damping is associated with the SH- chemisorption onto the gold surface, which involves changes in the free electron concentration.12 This implies demetalization of the metal nanoparticles, which causes the damping of the plasmon band. Note that the electron diffraction patterns support the conclusions obtained with UV-Vis spectroscopy. The electron diffraction experiments performed with the Pd-Au nanoparticles yielded results compatible mostly with the gold structure. However, the patterns show some spots related with the palladium diffraction. In figure 9b, we superimpose and compare the spectrum of the palladium seeds (left) with that of the Pd- Au nanoparticles (right). In the second case we can distinguish the diffraction from both materials: gold and palladium. And this diffraction pattern is in agreement with what one would expect of a core-shell nanoparticle. The diffraction from gold is clear, but the palladium diffraction is less intense due to the fact that it is masked by the gold shell. In figure 10 we present a schematic representation of the core-shell nanoparticle, based on the actual sizes of the Pd and Pd-Au nanoparticles, as well as on the Pd and Au atomic radii. In this idealistic atomic model, the core-shell nanoparticle is composed of 7 atomic layers of palladium and 13 layers of gold. Conclusions Core-shell Pd-Au nanoparticles have been synthesized by sequential reduction of H2PdCl4 and HAuCl4 in the presence of hydrazine. Both UV-Vis spectroscopy and Transmission Electron Microscopy revealed the formation of core-shell bimetallic nanoparticles. For the used Pd:Au molar proportion, the core-shell nanoparticles display a diameter around 8 – 10 nm. The electron diffraction patterns display the characteristics of gold, with a residual signal for palladium, in agreement with the UVVis results. Acknowledgments.- E. R.-L. acknowledges a fellowship from Conacyt (Mexico). The TEM experiments were performed in the Laboratorio de Microscopía Electrónica of Universidad de Sonora. References 1. Wang, Z. L. J. Phys. Chem. B, (2000) 104:1153-1175. 2. Ferrer, D., Torres-Castro, A., Gao, X., Sepúlveda-Guzman, S., Ortiz-Mendez,

U., Yacaman, M.J., Nano Letters (2007) 7:1701-1705. 3. Lee, W.R., Kim, M.G., Choi, J.R., Park, J.I., Ko, S.J., Oh, S.J., Cheon, J. J. Am. Chem. Soc. (2005) 127:16090-16097. 4. Wang, Y., Toshima, N. J. Phys. Chem. B (1997) 101:5301-5306. 5. Mulvaney, P., Giersig, M., Henglein, A. J. Phys. Chem. (1993) 97:7061-7064. 6. Teng, X., Black, D., Watkins, N.J., Gao, Y., Yang, H. Nano Lett. (2003) 3:261- 264. 7. Jain, P.K., El-Sayed I.H., El-Sayed, M.A. Nano Today (2007) 2:18-29. 8. El-Sayed, I.H., Huang, X., El-Sayed, M.A. Nano Lett. (2005) 5:829-834. 9. Toshima, N., Yonezawa, T. New J. Chem. (1998) 1179-1201. 10. Nutt, M.O., Hughes, J.B., Wong, M.S. Environ Sci Technol. (2005) 39:1346- 1353. 11. Schmid, G., West, H., Mehles, H., Lehnert, A. Inorg. Chem. (1997) 36:891-895. 12. Mulvaney, P. Langmuir (1996) 12:788-800. 13. Liz-Marzan, L.M. Materials Today (2004) 26-31. 14. Tsen, S.C.Y., Crozier, P.A., Liu, J. Ultramicroscopy (2003) 98:63-72. 15. Koga, K., Sugawara, K. Surface Science (2003) 529:23-35. 16. Maylavantham, G., O´Brien, D.T., Becker, M.F., Keto, J.W., Kovar, D. J. Nanopart. Research (2004) 6:661-664. 17. Boutonnet M., Kizling J., Stenius P., Maire G. Colloids and Surfaces (1982) 5:209-255. 18. Eastoe, J., Warne, B. Current Opinion in Colloid & Interface Science (1996) 1:800-805. 19. Tanori, J., Pileni M.P. Langmuir (1997) 13:639-646. 20. Chen, D.H., Chen, C.J. J. Mat. Chem. (2002) 12:1557-1562. 21. López-Quintela, M.A. Current Opinion in Colloid and Interface Science (2003) 8:137-144. 22. Del Castillo –Castro, T., Larios-Rodriguez, E., Castillo-Ortega, M.M., Tánori, J. Composites:Part A (2007) 38:107-113. 23. Slistan-Grijalva, A., Herrera-Urbina, R., Rivas-Silva, J.F., Ávalos-Borja, M., Castillon-Barraza, F.F., Posadas-Amarillas, A. Physica E (2005) 25:438-448. 24. El Bouayadi, R., Regula, G., Lancin, M., Larios, E., Pichaud, B., Ntsoenzok, E. Materials Reserch Society Symposium Proceedings (2007) 994:131. 25. Nutt, M.O., Heck, K.N., Alvarez, P., Wong, M.S. Applied Catalysis B (2006) 69:115-125. 26. Wong, M.S., Alvarez, P.J., Fang, Y., Akcin, N., Nutt, M.O., Miller, J.T., Heck, K.N. J. Chem Technol Biotechnol (2009) 84:158-166. 27. Edwards, J.K., Solsona, B., Ntainjua, E., Carley, A.F., Herzing, A.A., Kiely, C.J., Hutchings, G.J. Science (2009) 323:1037-1041. 28. Sperling, R.A.; Rivera, G.P.; Zhang, F.; Zanella, M.; Parak, W.J. Chem. Soc. Rev. (2008) 37:1869-1908. 29. Salata, O.V. Journal of Nanobiotechnology (2004) 2:3. 30. Tamamushi, B., Watanabe, N. Colloid & Polymer Sci. (1980) 258:174-178. 31. Schmid, G., Lehnert, A., Malm, J. O. Bovin, J. O. Angew. Chem. Int. Ed. Engl. (1991) 30:874-876. 32. Wu, M., Chen, D., Huang, T. Journal of Colloid and Interface Science (2001) 243:102–108. 33. Creighton, J.A., Eadon, D. G. J. Chem. Soc. Faraday Trans. (1991) 87:3381- 3391.

Figure captions Figure 1.- UV-Vis spectra of the palladium nanoparticles. AOT-water/H2PdCl4- isooctane system, [AOT=0.1 M], [H2PdCl4] = 2x10-4 M, W= 5. The adsorption band around 280 nm corresponds to metallic palladium nanoparticles. Figure 2.- TEM micrograph (a) and particle size distribution (b) of the palladium nanoparticles. AOT-water/H2PdCl4-isooctane system, [AOT=0.1 M], [H2PdCl4] = 2x10-

4 M, W= 5. These particles are used as seeds for the core-shell nanoparticles. Figure 3.- High Resolution TEM image of a palladium nanoparticle (a) and Selected Area Electron Diffraction pattern of one palladium particle. The crystallographic data obtained from the analysis of both pictures agrees with the fcc structure of palladium. Figure 4.- UV-Vis spectra of the core-shell palladium-gold nanoparticles Pd:Au 1:2. The curve displays a shoulder around 530 nm, very close to the expected value for gold (520 nm). The inset shows and amplification of the region around the adsorption maximum. Figure 5.- TEM micrograph (a) and particle size distribution (b) of the core-shell palladium-gold nanoparticles. Pd:Au 1:2. Note the strong tendency to agglomeration, due to the presence of gold in the external surface of the particle. Figure 6.- High Resolution TEM image of a core-shell nanoparticle (a) and corresponding Selected Area Electron Diffraction (b). The crystallographic data obtained from the analysis of both pictures agrees with the fcc structure of gold, except for some spots in the SAED picture corresponding to palladium. Figure 7.- UV-Vis spectra of the passivated core-shell nanoparticles. Note that the adsorption around 530 nm is damped (see figure 4) due to the presence of the thiol molecules in the external surface of the particles. Figure 8.- TEM micrograph (a) and particle size distribution (b) of the passivated coreshell nanoparticles. The picture shows that the thiol molecules effectively protect the particles against aggregation. Figure 9.- High Resolution TEM image of a passivated core-shell nanoparticle (a) and corresponding Selected Area Electron Diffraction pattern. As in the case of nonpassivated core-shell nanoparticles, the crystallographic data correspond to the fcc structure of gold, with a residual signal for palladium. Figure 10.-. Schematic rendering of the structure of a core-shell palladium-gold nanoparticle.

From: johsj <johan.sjoblom@chemeng.ntnu.no> To: judith.tanori@mrl.ucsb.edu, camild@chemeng.ntnu.no, "VanSciver, Catherine" <catherine.VanSciver@taylorandfrancis.com> Date: Fri, 28 May 2010 08:15:24 +0200 Subject: Ms JDST 2011 / 109 Dear Dr Tanori Your ms "Synthesis and Characterization of Bi... will be accepted for publication in J Dispersion Science Technology. Your ms will appear in JDST (2011). Before we can start the editing process your ms must be written in accordance with "Instructions to Author", i.e. 1) All papers must be double spaced, with 1-inch margins (on all sides), preferably in a 12-point font. 2) All figures and tables should be placed at the end of the document (or placed in a separate document). They should NOT be placed between paragraphs within the text of the article. 3) Documents that have been written on a non-standard keyboard, should be converted (e.g., documents written in an Arabic language are automatically flush right; these documents become un-editable and errors are often introduced, especially where equations are concerned). 4) PDFs are not acceptable. Word documents should be submitted. If an authors is worried about figures, equations, etc. being accidentally moved in an editable program, they may include a PDF for reference. 5) Please be sure all citations appear as numbered references. References should be cited in the order they appear in the reference list. Samples of references may also be found on the Instructions to Authors page." You will be contacted by our Production Editor Catherine van Sciver if some stylistic corrections are needed in your ms. Thank you for this most valuable contribution to JDST !!! Before starting the editing process you should fill in the copyright transfer form (attached) and send it (properly signed) as a PDF file to Production Editor Catherine VanSciver. Please clearly indicate on the form the name of the journal (JDST/volume/issue/ms nr) as specified in this email of acceptance. Email: catherine.VanSciver@taylorandfrancis.com Thank you very much for your collaboration !

Synthesis and Characterization of Bimetallic Copper-Gold Nanoparticles

Eduardo Lariosa,c, Zulema Molinaa, Amir Maldonadob and Judith Tanoria*

aDepartamento de Investigación en Polímeros y Materiales, Universidad de Sonora. Apdo.

Postal 130, 83000, Hermosillo, Sonora, México.

bDepartamento de Física, Universidad de Sonora, Apdo. Postal 1626, 83000, Hermosillo,

Sonora, México.

cDepartamento de Ingeniería Química y Metalurgia, Universidad de Sonora, Hermosillo,

Sonora, México.

∗ Corresponding author. Tel.: 52-(662)259-2161; fax: 52-(662)259-2216

E-mail address: jtanori@polimeros.uson.mx

Abstract. We have synthesized copper-gold, core-shell nanoparticles by the microemulsion

method. The particles were prepared in two steps, by first reducing copper ions and then

gold ions in the aqueous domains of anionic microemulsions. Two surfactants have been

used as emulsifiers: AOT and Cu(AOT)2. The latter is the source of copper ions. Gold ions

come from aqueous solutions of HAuCl4. UV-Vis Spectroscopy experiments show that

copper nanoparticles are created in the first step of the synthesis, and that a gold layer

covers them in the second step. Transmission Electron Microscopy and related techniques

confirm the formation of copper (core)-gold (shell) nanocrystals.

Keywords: copper-gold; nanoparticles; core-shell; microemulsion; bimetallic.

1. - Introduction

In recent years there has been a continuous and growing interest among materials

scientists for the synthesis and characterization of nanomaterials [1, 2, 3, 4, 5]. This is due

to the fact that materials at the nanometer scale display properties different from those in

the bulk scale. In many applications such as electronics, medicine and the designing of

smart materials, it is necessary to prepare nanoparticles of different physical properties

(metallic, dielectric, magnetic, semiconducting, etc).

In particular, copper nanoparticles have applications in many areas. They are being

investigated as major constituents of conductive inks and pastes for printing electronic

components [6]. They have also a catalytic activity, just like bulk copper but with a higher

efficiency due to their large surface-to-volume ratio [7]. However, a difficulty for some

applications is the propensity of copper to oxidation. For this reason, it is desirable to cover

the copper surface with a non-oxidizable material. One way of doing this is to synthesize

core-shell nanoparticles that retain some of the properties of copper (low cost, electrical

conductivity) while protecting it from oxidation. This objective can be attaining by

covering the Cu nanoparticles with layers of a noble metal such as gold. In a previous work

covered Cu nanoparticles have been used as inclusions in order to increase the electrical

conductivity while keeping mechanical strength of conductive polymer composite materials

[8]. However, no characterization has been reported of these Cu-Au nanoparticles. On the

other hand, it is worth mentioning that gold nanoparticles on their own have a lot of

applications. For instance, in biology and nanomedicine [9,10] they are used as contrast

agents for tissue labeling and visualization; they are also conjugated with antibodies in

order to allow for the immunostaining of molecules of interest or the single particle

tracking of cell elements; they are also studied as gene delivery agents; etc. Gold

nanoparticles can also be used in catalysis. Unlike bulk gold, small gold nanoparticles (< 10

nm) become very active for many reactions such as the CO oxidation [11].

In this paper we report the synthesis of Cu-Au core-shell nanoparticles by the

microemulsion method. This procedure has been successfully used in order to control the

size and shape of nanoparticles of several metals [12,13,14,15,16,17,18]. The particles are

prepared by performing the chemical reactions in the aqueous or oil domains of

microemulsions. These complex fluids are thermodynamically stable dispersions of water

droplets in oil (w/o microemulsion) or of oil droplets in water (o/w microemulsion)

stabilized by a surfactant. Some of the most commonly used surfactants are Sodium Bis (2-

ethylhexyl)sulfosuccinate (AOT or NaAOT) and its derivatives.

The aim of the present study is to use the microemulsion method in order to

synthesize copper nanoparticles covered with a gold shell. The metallic particles were

prepared using anionic microemulsions as chemical reactors. Two surfactants were used as

emulsifiers: NaAOT and Cu(AOT)2. The oil phase was isooctane and the aqueous phase

was ultra pure water. The growth of the metallic nanoparticles was monitored with UV-Vis

spectroscopy and the nanoparticles were characterized by Transmission Electron

Microscopy (TEM) and related techniques: HRTEM and EDS.

The remainder of the manuscript is divided as follows. In section 2 we give the

experimental details of our work. In section 3 we present and discuss our results, and in

section 4 we draw some conclusions.

2. - Experimental section

Chemicals.

Sodium bis (2-ethylhexyl)sulfosuccinate (AOT), hydrazine hydrate (64%, N2H4+

H2O), tetrachloroauric acid trihydrate (HAuCl4.3H2O) and 1-dodecanethiol were purchased

from Sigma-Aldrich. Isooctane was supplied by Fluka. All chemicals were used without

further purification. Copper(II) bis(2-ethylhexyl)sulfosuccinate, Cu(AOT)2, has been

prepared as described elsewhere [19]. Ultrapure water was prepared by purifying single-

distilled water through an Autostill WA33 system (Yamato Scientific LTD) until its

resistivity reached 18.2 MΩ-cm.

Microemulsions

The core-shell nanoparticles were synthesized in the aqueous domains of mixed

microemulsions of the water-AOT/Cu(AOT)2-isooctane system. The preparation of the

microemulsions was carried out in a controlled atmosphere (nitrogen) glove box. The AOT

microemulsions consisted of a mixture of copper bis(2-ethylhexyl)sulfosuccinate

surfactant, ([Cu(AOT)2] = 4x10-3 M) and sodium bis(2-ethylhexyl)sulfosuccinate

surfactant, ([AOT] = 92 x 10-3 M) in isooctane. To this solution we added ultrapure water

until the desired water content was reached. We define the water content as the ratio w =

[water]/[surfactant].

Nanoparticle synthesis

The synthesis of the Cu-Au nanoparticles was carried out in a controlled atmosphere

(nitrogen) glove box. The copper cores of the nanoparticles were prepared by chemical

reduction of the copper ions within the aqueous phase of the microemulsion with hydrazine

as reducing agent. The Cu(AOT)2 concentration was 4 x 10-3 M. The reaction was allowed

to proceed under nitrogen atmosphere for two hours. After this time, an aqueous solution of

HAuCl4 was added to the system under stirring in order to form the gold shell. The Cu:Au

molar ratio was 10:1. The water content in the reaction system was 10 before addition of

the HAuCl4 solution. In order to prevent the coalescence of the gold-covered nanoparticles,

their surface was protected with 1-dodecanethiol. For this, 9 μL of this substance, per mL

of microemulsion, were added to the reacting system.

UV Visible Spectroscopy

The growth of the nanoparticles was followed with UV-Vis spectroscopy. We used a Perkin

Elmer Lambda 2 Spectrophotometer in the wavelength range of 300-900 nm. Samples were

placed in 1 cm x 1 cm x 3 cm rectangular quartz cells.

Transmission Electron Microscopy (TEM) Experiments

Transmission Electron Microscopy pictures and High Resolution Electron Microscopy

(HRTEM) measurements were obtained using a JEOL 2010F Transmission Electron

Microscope operating a 200kV. Energy Dispersive X-ray (EDS) measurements were

obtained with a Philips Tecnai Twin 22 Electron Microscope operated a 120 kV. For TEM

characterization, a drop of the sample was placed on an amorphous carbon film supported

by a nickel or copper grid. The core-shell structure was analyzed by different

complementary techniques, including HRTEM and EDS.

3. - Results and Discussion

The strategy we follow to synthesize the core-shell Cu-Au nanoparticles has two

main steps. In the first one we create copper nanoparticles (cores) by reducing the Cu2+ ions

in the aqueous domains of the microemulsions. In a second step, we cover the copper

nanoparticles with a gold layer (shell) by reducing the Au ions in the system.

The starting system was a microemulsion prepared with a mixture of AOT and

Cu(AOT)2 as surfactant, as described in the experimental section. This liquid was

homogenous and transparent. It had a light blue color due to the Cu2+ ions. After the

addition of hydrazine the system turned from light blue to a pale brown almost

instantaneously, indicating the reduction of the copper ions. The intensity of the color in the

system increased gradually, suggesting a slow growth (in size and number) of the copper

nanocrystals [20]. This growth was followed by UV-Vis spectroscopy. A typical UV-Vis

spectrum of the colloidal dispersion of copper metallic nanoparticles is shown in figure 1a.

The curve exhibits an absorption band around 565 nm that is characteristic of spherical

copper nanoparticles due to the plasmon resonance in their surface [20,21]. In the next step,

these copper nanoparticles were covered with a gold layer.

For this, two hours after the copper reduction started, tetrachloroauric ions were

added to the system. The microemulsion turned from its brown color to a reddish tonality

almost instantaneously, suggesting the rapid formation of a gold shell. The UV-Vis

spectrum for the dispersion of the copper-gold nanoparticles is shown in figure 1b. We can

appreciate that the absorption band of the system shifted to a value very close, within

experimental error, to that expected for gold (520 nm). This value is characteristic of the

plasmon absorption peak of gold nanoparticles [21,22,23].

The shift in the absorption band is evidence that a gold shell effectively covered the

surface of the previously prepared copper nanoparticles. The observed absorption band

corresponds to nanoparticles with gold in their external surface. Note that one possible

result of the described reactions would have been a mixed dispersion of independent copper

and gold nanoparticles. This is so because it might be possible that the gold atoms do not

nucleate over the previously formed copper nanoparticles, leaving them unmodified.

However, our UV-Vis data allow us to rule out this possibility since two absorption bands

(520 nm and 565 nm) would be expected in the same spectrum for the case of a mixture of

separate gold and copper nanoparticles [22]. We never observed the two bands at the same

time in any sample. Note that another possibility for the outcome of our two-step synthesis

approach is to have particles composed of a copper-gold alloy. In principle, the addition of

HAuCl4 could partially disintegrate the copper seeds, leading to a simultaneous nucleation

of Cu and Au atoms in an alloy structure. However, this effect can also be ruled out since in

that case, the UV-Vis peak would shift to a value intermediate between that of copper (565

nm) and that of gold (520 nm). In our spectra, we only observe the band corresponding to

gold, an indication that this material is actually in the surface of the nanoparticles.

We have characterized the core-shell nanoparticles by Transmission Electron

Microscopy. Bright field TEM images and the corresponding size distribution of the

copper-gold nanoparticles are shown in figure 2. TEM images of the obtained Cu-Au

nanoparticles show that the system consists of cuasi-spherical nanoparticles with an

average diameter of 7.8 ± 2.2 nm. In comparison, the copper particles before adding the

gold ions had a diameter around 3 nm. The coefficient of variation [24] for the obtained

copper-gold nanoparticles is 28.5%, indicating that the particles are relatively polydisperse.

Energy-dispersive X-ray spectroscopy (EDS) was used to determine the

composition of a single bimetallic nanoparticle. Positioning and focusing the electron beam

on a single nanoparticle acquired the EDS spectra. In this situation, the interaction between

the electrons in the beam and the atoms in the sample produces X-rays whose

characteristics depend on the elements forming the nanoparticle. All the obtained spectra

displayed the peaks related to copper and gold, revealing the presence of both materials,

which indicate that the nanoparticles are effectively bimetallic. A typical spectrum is

shown in figure 3, where the main observed contributions are those of gold and copper.

The Ni signal (around 7.5 keV) is originated from the nickel grid used to hold the sample.

The presence of the two metals in the nanoparticles, a copper core and a gold shell,

has also been confirmed through a digital Fourier transform analysis and filtering of the

HRTEM images. In Figure 4 we show the results of this procedure when applied to two of

the obtained nanoparticles (Figure 4A). The central, bigger nanoparticle presents a

polyhedral structure. The observed pattern in the fast Fourier transform (FFT) spot diagram

(Figure 4B) confirms the crystallinity of the nanoparticles observed in image 4A. We

analyzed each pair of complementary reflections in the FFT spot diagram separately and

applied a filter to the FFT image in order to determine the lattice spacings (Figures 4 C-H).

The distances obtained with this procedure are: 2.345 Å, 1.445 Å, 1.2386 Å, 1.7357 Å,

2.347 Å, and 2.0817Å. The first three distances correspond to the (111), (220), and (311)

planes of the face centered cubic (fcc) lattice of gold (JCPDS file 4-0784), while the 1.7357

Å distance corresponds to the (200) plane of the fcc lattice of copper (JCPDS file 4-0836).

The last distance, 2.0187 Å, corresponds either to the (111) or (200) plane of the fcc

structures of copper or gold, respectively. Since all the lattice distances correspond to

copper or/and gold, the data confirm the presence of both metals in each single

nanoparticle.

Furthermore, the location of both metals on the volume of the nanoparticles can be

delineated from the determination of the different lattice spacings on distinct points of the

nanoparticle image. By performing the Fourier transform analysis and filtering on several

points in the nanoparticle, we have found that reflections (or, inversely, lattice spacings)

corresponding to gold are observed in the entire nanoparticle image; on the contrary,

reflections corresponding to copper are only obtained from points located in the center of

the image. This observation confirms that the bimetallic nanoparticles are formed by a

copper core surrounded by a gold shell.

4. - Conclusions

In this work we have prepared core-shell bimetallic nanoparticles by a two step

reduction of copper and gold ions in aqueous microemulsions. The initial formation of

copper core nanoparticles was followed by UV-Vis spectroscopy and TEM. Over this core,

a gold layer was deposited, as assessed also by UV-Vis experiments. EDS qualitatively

confirmed the presence of both copper and gold in the nanoparticles while high resolution

TEM experiments confirmed that the outer layer (shell) of the particles is formed by gold

while the core of the particles is formed by copper. These bimetallic nanoparticles can be

useful in applications where some properties of non-oxidizable copper nanoparticles are

needed as well as in catalysis. The employed methodology can also be used to synthesize

other core-shell nanomaterials.

Acknowledgements. - E.L.R. acknowledges a fellowship from CONACYT-Mexico. J. T.

thanks Professor Phil Pincus, Professor Joe Zasadzinski and the MRL-UCSB for their

hospitality during a sabbatical leave as well as CONACYT-Mexico for financial support.

The TEM experiments were performed in the Laboratorio de Microscopía Electrónica of

Universidad de Sonora and Centre Pluridisciplinaire de Microscopie Électronique et

de Microanalyse, Faculté des Sciences et Techniques de St-Jérôme, Aix-Marseille III.

References

1. Eastoe, J., Hollamby, M.J., and Hudson, L. (2006) Adv. Colloid Interface Sci.128-

130:5-15.

2. Wilcoxon, J. P., and Abrams, B. L. (2006) Chem. Soc. Rev. 35:1162-1194.

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5. Bracey, C.L., Ellis, P.R. and Hutchings, G.J. (2009) Chem. Soc. Rev. 38:2231-2243.

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8. Del Castillo–Castro, T., Larios-Rodriguez, E., Castillo-Ortega, M.M., and Tanori, J.

(2007) Composites, Part A 38:107-113.

9. Sperling, R., Rivera Gil, P., Zhang, F., Zanella, M., and Parak, W. (2008) Chem.

Soc. Rev. 37:1896-1908.

10. Salata, O. (2004) Journal of Nanobiotechnology 2:3.

11. Haruta, M. (2003) The Chemical Record 3:75–87

12. Boutonnet M., Kizling J., Stenius P., and Maire G. (1982) Colloids and Surfaces

5:209-255.

13. Tanori, J., and Pileni, M.P. (1995) Adv. Mater. 7:862-864.

14. Esumi, K., Matsuhisa, K., and Torigoe, K. (1995) Langmuir 11:3285-3287.

15. Eastoe, J., and Warne, B. (1996) Current Opinion in Colloid & Interface Science

1:800-805.

16. Tanori, J., Pileni M.P. (1997) Langmuir 13:639-646.

17. López-Quintela, M.A. (2003) Current Opinion in Colloid and Interface Science

8:137-144.

18. Sanchez-Dominguez, M., Boutonnet, M., and Solans, C. (2009) Journal of

Nanoparticle Research 11:1823–1829.

19. Petit, C., Lixon, P., and Pileni, M.P. (1991) Langmuir 7:2620-2625.

20. Lisiecki, I., and Pileni, M.P. (1993) J. Am. Chem. Soc. 115:3887-3896.

21. Creighton, J.A., and Eadon, D.G. (1991) J. Chem. Soc. Faraday Trans. 87:3881-

3891.

22. Link, S., Wang, Z.L., and El-Sayed, M. A. (1999) J. Phys. Chem. B, 103:3529-

3533.

23. Logunov, S. L., Ahmadi, T. S., El-Sayed, M. A., Khoury, J. T., and Whetten, R. L.

(1997) J. Phys. Chem. B, 101:3713-3719.

24. Hunter, R.J. (1987) Foundations of Colloid Science; New York: Clarendon Press.

Figure captions

Figure 1.- (A) UV-Vis spectrum of the copper nanoparticles. The absorption peak at 565

nm corresponds to the surface plasmon resonance of copper. (B) UV-Vis Spectrum after

gold reduction on the previously formed copper nanoparticles. The absorption peak has

shifted to a value close to that expected for gold (520 nm), indicating that the external

surface is covered by this material.

Figure 2.- (A) TEM picture of the copper-gold, core-shell nanoparticles. (B) Histogram of

the size distribution of the bimetallic particles.

Figure 3.- EDS spectra of the bimetallic particles. Most of the peaks correspond to gold

(shell of the particle), but some copper (core of the particle) peaks are also observed. The

Ni peak is due to the used grid.

Figure 4.- Typical high resolution TEM analysis of the Cu-Au nanoparticles. (A) HRTEM

image of two nanoparticles. (B) The fast Fourier Transform of area 1 in (A) displays

several complementary reflections corresponding to the planes of the fcc lattices of either

gold or copper. The observed lattice distances and planes corresponding to gold are: C)

2.345 Å, (111); D) 1.445 Å, (220); and E) 1.2386 Å (311). Data in (F) correspond to

copper: 1.7357 Å, (200). Finally, data in (G) correspond either to the (111) plane of the fcc

structure of copper or to the (200) plane of the fcc structure of gold: 2.0187 Å.

Figure 5.- Location of copper and gold on the volume of the nanoparticle are delineated

from the Fourier transform analysis, filtering and determination of the lattice spacings on

different points of the nanoparticle. Lattice spacings corresponding to gold are in the entire

nanoparticle image while lattice spacings from copper come from points located in the

center of the nanoparticle. The inset shows the studied nanoparticle.

Microscopy-Related-Analyses of Silver Trapped at Cavities Created by High Energy Helium Implantation in Single Crystals of Silicon Rachid El Bouayadi1, Gabrielle Regula2, Maryse Lancin2, Eduardo Larios3, Bernard Pichaud2, and Esidor Ntsoenzok4

1LES, University of Oujda, Faculté des sciences, B.P. 717, Oujda, 60000, Morocco 2TECSEN, Paul Cézanne University, Aix-Marseille III, Avenue de l’Escadrille Normandie Niemen, Marseille, 13397, France 3DIPM, University of Sonora, Blvd Transversal y Rosales, Hermosillo, Sonora, 83 000, Mexico 4CERI-CNRS, University of Orléans, 3A rue de la Férollerie, Orléans, 45071, France ABSTRACT High resolution transmission electron microscopy observations show for the first time the presence of two phases of silver which precipitate in nanocavities induced by implantation at 1.6 MeV with a dose of 5×1016 He+ cm-2 and a two hour annealing at 1050°C. These phases were called A and B to refer to the two well-known nickel silicide (NiSi2) precipitates or Ag films on a {111} silicon surface. Thus, the A phase is due to a growth of silver precipitates on {111} cavity walls in epitaxy with the Si matrix with an orientation relationship Ag(-111)[211]||Si(-111)[211]. The B phase develops on a {111} plane parallel to a {111} cavity wall as well, but in a twin orientation with respect to the Si matrix defined by Ag(-111)[211]||Si(-111)[-2-1-1]. The precipitates have a size ranging from a few nm to 50 nm. Most of them have the faceted-shape characteristic of .clean. cavities. They often contain bands of A and B phases alternatively in good agreement with the low stacking fault energy of silver. Some silver precipitates were also found at dislocations located at the He+ projection range, but these trapping sites were found thermally unstable as compared to the cavity ones. During a second identical annealing, the precipitates grow in cavities whereas they fade at dislocations. INTRODUCTION As the device features are being down-scaled in ultra-large-scale integrated (ULSI) technology, the purity of the wafers should increase to avoid any electrical degradation of the device. Silver is a potential candidate of advanced interconnect materials due to its superior conductivity and its better resistance to oxidation than copper. Moreover, Ag like Au does not make stable silicide. Silver is

indeed already used as back contact material in solar cells. Nevertheless, Si wafers can be contaminated not only by direct mechanical contact with metal silver but also by silver re-plating from chemical solutions which contain silver as an impurity. Due to its high diffusivity in SiO2

even at low temperature, silver can rapidly penetrate the protecting oxide layer of electronic device in MOS technology and precipitate at the SiO2/Si interface causing detrimental effects [1]. Despite of all these possible contamination sources, only little work about silver diffusion or precipitation in silicon has been carried out so far, thus the knowledge concerning silver properties in silicon is still very poor [2- 5]. Beside, cavities created in silicon by gas-ion implantation (mainly H+ and He+) have been recently found of great interest to remove transition metal impurities even if the latter are in very small amount from active regions of the devices in order to improve their electrical properties [6-9]. Theses impurities are assumed to be trapped at cavity by chemisorption, covering the walls of cavities up to a monolayer. The trapping process is ascribed to reactive silicon bonds at internal cavity walls. The .clean. cavities have a cuboctahedral shape (tetrakaidecahedron) formed by (100) and (111) planes[10]. Assuming there is a finite number of trapping sites per unit surface of cavity wall (7×1018

[11]) the final amount of impurities trapped at cavities is expected to increase with the radius of cavities and thus with the dose of the implanted gas ion [12], provided both the impurity does not form stable silicide and no other competing trapping sites exist (like the damages near the sample surface due to metal ion implantation). It was shown [4] that implantations of high doses of negative silver 2 2 ions in a sample containing a cavity layer induced by H+ implantation cannot saturate the trapping sites available at cavity walls though there is a threshold of platinum dose at around 1014cm-2 to reach the saturation. Indeed, beyond this threshold dose, the authors found precipitates supposed to be made up of Ag located at cavities and at the damaged-near surface as well. However, these precipitates were only characterized by their black contrast in TEM micrographs without any detail about their crystalline structure or chemical composition. The aim of this work is to clarify the trapping mechanism of Ag by cavities. It consists in localizing

the silver precipitates and determining their chemistry and their structural phase by cross section transmission electron microscopy (XTEM) imaging and energy dispersive X-Rays spectroscopy (EDS) coupled with secondary ion mass spectrometry (SIMS). To avoid any Ag precipitation at the damage-near surface, silver was plated on the back surface of the samples. EXPERIMENTAL DETAILS (111) p-type Czochralski (Cz) silicon wafers were implanted at room temperature with 1.6 MeV and 5⋅1016 He+ cm-2. The corresponding projection range predicted by transport range of ions in matter simulation (TRIM) is Rp=5.6µm. Silver was then spread on the back surface of the sample using silver contact paint. The samples were annealed at 1050°C for 2h under argon gas flow in a quartz tube using a conventional furnace and cooled down (- 4.5 °C/s) to room temperature, the furnace being pulled along the tube far from the samples. To check silver trapping ability of the defects induced by implantation, SIMS profiles were carried out with a +

2 O primary source for a better ionic yield. The precipitates, cavities and other implantation-related defects were localized and characterized by XTEM. Electronic diffraction patterns (EDP), high resolution imaging (HRTEM) coupled with power spectrum and EDS performed with a 5 nm probe diameter were realized using a Field Emission Gun Microscope (Jeol 2010F URP22) equipped with a Slow Scan CCD camera (Gatan) and an ultra thin window Si-Li detector (Kevex super Quantum). The thermal stability of the trapping sites was tested by a post-annealing at 1050°C for two more hours according the here above described-conditions. RESULTS AND DISCUSSION Chemical evidence and location of trapped silver Precipitates are revealed on bright field images thanks to their Z-absorption dark contrast (see Fig. 1(a) as an example). Generally, their shape is very similar to that of empty cavities [10, 13] but some of them only found on dislocations exhibit a clearly different morphology (Fig. 1(b)). The precipitate distribution seems to be very heterogeneous in figure 2 but it is likely due to the thin foil preparation. Indeed, the thinner the foil, the less numerous the dark precipitates which makes tricky HRTEM observations to be achieved. All of the precipitates are located nearby Rp, the He+

projection-range. Figure 1: Bright field XTEM along [110]Si of a sample

implanted at 1.6 MeV and 5⋅1016 He+ cm-2, Ag plated and diffused at 1050°C for 2 hours; a) general view of the damaged area (and zoom on two precipitates) and b) silver precipitate on a dislocation (white arrows). The location and width (1µm) of the zone which contains the precipitates are in good agreement with those obtained by SIMS (compare Fig.1(a) and Fig.3(a)). Indeed, the maximum of the SIMS peak on the Ag depth profile corresponds to Rp. Thus, the dark precipitates in figure 1 are assumed to be silverrich precipitates. EDS analyses (Fig 3(b)) on a large number of dark precipitates confirmed the (a) (b) 3 3 presence of Ag while those on white cavities which remain among the dark ones only reveal Si, copper being due to contamination. However, it is unlikely that the Ag concentration profile follows the asymmetry of the SIMS peak which extends over more than one micron beyond its maximum. Such a tail is probably an artifact due to ion mixing and recoil implantation of Ag in the matrix by the primary ions. The Ag level out of the damaged-zone is about the Ag solubility limit at 1050°C (dotted line in Fig.3). Figure 2: Brigth field XTEM image along <110> of the cavity chain obtained after an annealing extension at 1050°C for 2 more hours: a) thin foil (100±50nm); b) thick foil (260±50nm). Figure 3: Chemical analyses: a) SIMS depth profile of silver on a sample implanted at 1.6 MeV (Rp=5.6µm) and 5⋅1016 He+ cm-2. Silver was plated on the back surface of the sample and diffused at 1050°C for 2 hours. b) EDS spectrum taken either on the Si matrix or on a dark cavity. Silver phase identification To obtain structural information on the Ag phases, nanodiffraction patterns were taken on precipitates. Figure 4(a) depicts an example of them obtained using the [110] zone axis of the silicon wafer. A face-centered cubic phase grown in epitaxy on silicon can be observed (compare Fig. 5 (a) and (b)). Note the presence of double diffraction at each Si spot. Taking the reflection spots of the Si lattice as reference, the {111} and {002} interreticular distances of the precipitates are found to be Figure 4: Electron diffraction patterns of Ag precipitates taken along the [110] Si axis. The {111}Si are

related by a grey solid line, and the {111}Ag are linked by a dark dotted line: a) Experimental EDP taken on a silver precipitate; (b) and (c) simulated JEMS EDP (acceleration voltage 200kV, deviation 0.8nm-1, half convergence 0) of A and B phases respectively. Light and dark big spots correspond to Si and Ag respectively. Note that the small black dots in (b) (and the black circled-white spots in (c)) are added manually and correspond to a matrix double diffraction. (a) (b) 0 1 2 3 4 5 6 7 8 9 1013

1014

1015

1016

1017

1018

1019

1020

1021

CAg1

profondeur (µm) Ag (at.cm-3) 1 μm depth (µm) (a) (b) (a) (b) (c) 4 4 (0.236 ± 0.2) nm and (0.205± 0.2) nm respectively. Theses values correspond to the Ag interreticular distances which are 0.235 nm and 0.204 nm for {111} and {200} planes respectively. This Ag phase corresponds to a perfect epitaxy of Ag on {111} silicon planes described for instance by the orientation relationship Ag(-111)[211]||Si(-111)[211]. We label it A to refer to Ag growth on {111}Si surfaces [14-17]. This epitaxial growth is favored by the simple relationship between lattice constants of Si (aSi) and Ag (aAg): aAg ~ ¾ aSi. Diffraction patterns on more than fifty precipitates were always similar to the one displayed in figure 4 (a). The corresponding A phase is thus the major one. However, the bright field images often reveal a black and white contrast in the precipitates. An analysis of power spectra (Fig.5) of high resolution images realized on some of these precipitates show that Ag grow in the cavity according two possible orientation relationships with respect to the silicon lattice. We called the second phase B still following the analogy with A and B NiSi2/Si{111} or

Ag/Si{111} phases [12,14-17]. Indeed, the B phase is in epitaxy with respect to the silicon lattice according to the orientation Figure 5: Two power spectra (a) and (b) of precipitates and matrix viewed along the [110]Si

computed from HRXTEM images, (c) and (d) respectively. Slight vertical lines on the power spectra are due to the square shaped sampling box of the HR images. The {111}Ag spots are highlighted in dark and the {111}Si are in grey. (a) and (c) correspond to a A type Ag precipitate; (b) and (d) correspond to a B type. In b) the spots of the common plane are related with a solid line, and the two other {111} spots are related with dotted lines (dark for Ag and grey for Si). relationship Ag(-111)[211]||Si(-111)[-2-1-1]. Note that B orientation is related to A orientation by the rotation ([1-12],180°), the (-111) plane being the twinning plane (the corresponding spots are related by a black solid line called T) as indicated in figure 4(c). Of course each of the four {111}Si

planes can be a twinning plane, but only the ones containing the electron beam direction can induce Bragg contrast. Both phases were often observed on a same precipitate (Fig. 6(a)) where bands of each phase are stacked alternatively. However, the percentage of B phase is always too low to be detected by EDP, as it is usually observed when Ag is deposited on a (111)Si substrate at room temperature [15,16]. (c) (d) (a) (b) 5 5 Nucleation and growth of Ag precipitates at cavities To discuss the precipitate formation, it is important to note that Ag-filled-cavities have very similar shapes than .clean. ones (see empty cavities in Fig.6(a)). The cavity facets are mainly parallel to {111}, {100} and more rarely to {110} planes. The sections of the large cavities in the {110} plane used for the XTEM observations appear as sections of tetrakaidecahedrons (octahedrons with {100} truncations at 2/3 of the pyramid height) as drawn in figure 6(b). According to the model of solid precipitation, as long as the mean Ag coverage of the cavities is much lower than the unit, the shape of Ag-contaminated-cavities should first become quasi spherical [13,18]. Even if locally the vicinal surfaces have a coverage higher than the unit, the cavity shape should have turned spherical, since the

surface specific energy of Ag (©Ag =1.25 J.m-2) is lower then the mean surface specific energy of silicon (©Si =1.36 J.m-2, corresponding to the © value of vicinal surfaces). Then, when Ag continues to fill the cavity its morphology should remain spherical. The Ag-filled-cavities morphology obviously proves that the model design for solid precipitation does not apply. Ag atoms gathered in the cavity must be in a liquid phase during the annealing performed at a temperature (1050°C) greater than the melting temperature of Ag (961°C). This implies that no astride effect [19] takes place in the cavities. The nucleation of the precipitates does not occur on vicinal surfaces as it would do during solid precipitation. The nuclei of this first order liquid/solid Ag transformation are located on {111} surfaces leading to the orientation relationships above mentioned. The high frequency of variants in the precipitates (Fig. 6(a)) may have two origins. On one hand, it could be favored by the low stacking fault energy (0.008Jm-2 [20].) which eases the growth of A on B or vice versa. In that case, the rapid growth of precipitates during cooling makes easier the disorder stacking. On the other hand, the twin band formation may occur during cooling below the melting temperature of Ag due to the different thermal expansion of Si and Ag (2.5 10-6 to 4.5 10-6

K-1 for Si at room and high temperature respectively versus 2.0 10-5 K-1 at room temperature). Such a transformation is probably favored by the presence of extended defects in precipitates. The latter were found to be numerous when Ag is grown on (111)Si at room temperature [15-17]. Figure 6 : a) Bright field XTEM image viewed along the [110]Si showing the similar shape of both silver precipitates and empty cavities. and (00-1)Si respectively. Note the presence of the two phases A and B having a different diffraction contrast in the cavities; Note that both (-111)Si

and (-11-1)Si parallel to the electron beam can be common twinning planes. b) volume of a .clean. cavity and its view along [110] to compare with XTEM observed-shape. Thermal stability of cavities and dislocations The evolution of the defects and precipitates distribution due to a second two hours annealing at 1050°C is obvious when comparing figures 1 and 2. The damaged area considerably shrinks in width by a factor of five. Moreover, it is free of dislocations which were previously pinned or not by Ag precipitates. At Rp only a chain of cavities is eventually left. The precipitates formed at dislocations

during the first annealing have disappeared whereas those at cavities have grown further. Thus, the (a) (b) 6 6 dislocation-Ag binding energy is lower than the cavity-Ag one. It is noteworthy that the cavities are thermally more stable than the dislocations. It is interesting to underline that the presence of Ag diffusing at such temperature via the kick out mechanism slows down the cavity growth. Indeed, in clean silicon, the cavities created with the same He+ implantation conditions reach their equilibrium after a two hours annealing at 1050°C. They form a narrow band of cavities similar but the contrast to the one obtained in Ag contaminated Si (Fig. 2) after a four hours annealing at 1050°C. CONCLUSION Silver is trapped both at dislocations (thermally unstable) and at cavities created in the vicinity of the He+ projection range. There, this impurity precipitates in the Si bulk under two pure silver phases. They consist of two variants grown by epitaxy on the {111} silicon walls of cavities, with or without an angle of 70° between the {111} planes of the Ag precipitates and the Si matrix. They probably grow via a relaxation mechanism. Ag precipitation does not modify the cavity shape since the atoms gather in a liquid phase at cavities, but it slackens the reaching of the equilibrium configuration of the cavity band. To confirm such hypothesis, further in situ X-Ray experiments during annealing will provide new frames of the precipitation process. ACKNOWLEDGEMENTS The authors are very grateful to Christiane Dubois for performing the SIMS measurements and to all the members of the CP2M of Paul Cézanne University, for their technical support. References 1. S. Kar and R. Varghese, J. Appl. Phys. 53, 4435-4440 (1982) 2. Linghui Chen, Yuxiao Zeng, Phucanh Nyugen, T.L. Alford, Materials Chemistry and Physics 76 224.227 (2002) 3. K. Graff, Metal Impurities in Silicon-Device Fabrication, Springler, Berlin (1995) 4. A. Kinomura, J. S. Williams, J. Wong-Leung, and M. Petravic, Appl. Phys. Lett. 72, 2713 (1998) 5. F. Rollert, N. A. Stolwijk, H. Mehrer, J. Phys. D: Appl. Phys. 20, 1148-1155 (1987) 6. J. Wong-Leung, C.E Ascheron, M. Petravic, R. G. Elliman and J. S. Williams, Appl. Phys. Lett. 67,

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