Thermal evolution of cobalt nanocrystals embedded in silica

Materials Science and Engineering C 27 (2007) 193 – 196 www.elsevier.com/locate/msec

Thermal evolution of cobalt nanocrystals embedded in silica☆ C. Maurizio a,⁎, G. Mattei b , P. Canton c , E. Cattaruzza b , C. de Julián Fernández b , P. Mazzoldi b , F. D'Acapito a , G. Battaglin d , C. Scian b , A. Vomiero e a

INFM-OGG European Synchrotron Radiation Facility BP 220 F-38043 Grenoble-Cedex, France INFM and Dipartimento di Fisica, Università di Padova, via Marzolo 8, 35131 Padova, Italy c Dipartimento di Chimica Fisica, via Torino 155/b, I-30172 Venezia-Mestre, Italy INFM and Dipartimento di Chimica Fisica, Università di Venezia, Dorsoduro 2137, 30123 Venezia, Italy e INFN-Laboratori Nazionali di Legnaro, Via dell' Università 2, 35020 Legnaro (PD), Italy b

d

Available online 12 July 2006

Abstract The structural evolution of cobalt nanoclusters synthesized in silica glass by ion implantation has been investigated upon thermal annealing. The samples were characterized by in-situ grazing incidence X-ray diffraction, exploiting a synchrotron radiation beam and following their evolution during thermal treatments in vacuo up to T = 800 °C. Before heating, the system is composed of hcp Co nanocrystals; we have not detected the transition from hcp to fcc structure that in the bulk phase occurs around 420 °C; nevertheless, the differences in the diffraction pattern recorded at T = 800 °C with respect to the corresponding one at room temperature suggest the presence of a second crystalline phase. © 2006 Elsevier B.V. All rights reserved. Keywords: Crystalline phase transition; Metal nanoclusters; Ion implantation

1. Introduction Composites formed by metal nanoparticles embedded in dielectric matrices draw much interest for their application in magnetic recording as well as in optoelectronic technology [1–3]. Basic questions concerning the composite formation and stability are far from being understood and modelized. In particular, Co nanoparticles can occur in multiple crystalline phases, which can result in large differences in the magnetic properties such as the crystalline anisotropy, so affecting the possible application in magnetic recording [4–6]. The bulk equilibrium phase of Co at room temperature is hcp, whereas the fcc structure is more stable at T > 420 °C (at normal pressure). On the other hand, Co nanoparticles generated by high pressure sputtering have been evidenced to be either hcp- or fcc-ordered [7]: in that case the transition from hcp to fcc was ascribed only to a size effect (the crystals were fcc below 20 nm, hcp above about 40 nm) i.e., to the lower surface energy of the fcc phase. It has been shown that the ☆

This article is part of the EMPS 2004 Symposium G Current Trend in Nanoscience. ⁎ Corresponding author. E-mail address: [email protected] (C. Maurizio). 0928-4931/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2006.06.001

fcc → hcp phase transition of fine Co particles on cooling is considerably suppressed [8]. Anyway, it is still not clear if the main physical property that drives the phase transition is the cluster size, or if other physical effects can determine the crystalline structure, especially when the Co clusters are embedded in a dielectric matrix. In fact, it has been shown [9,10] that 6-nmsized Co particles in silica synthesized by sol–gel process exhibit fcc crystalline structure, while Co nanoparticles fabricated by ion implantation have hcp structure in a wide range of sizes [11,12]. Moreover, magnetron sputtering co-deposition of cobalt and silica can originate composite glass in which both fcc and hcp Co nanoparticles are embedded together in the silica matrix [6,13]. To investigate the physical processes that determine the crystalline structure of Co clusters in silica and possible hcp–fcc phase transition, we have performed an in-situ X-ray diffraction experiment by heating in vacuo (up to 800 °C) cobalt nanoclusters formed in silica glass by ion implantation. The experiment exploited the X-ray synchrotron radiation from a bending magnet: a grazing incidence geometry was used [14,15] to enhance the scattering signal from the nanoclusters with respect to that one from the amorphous silica matrix. Before heating, the system is composed of hcp Co nanocrystals; the transition from hcp to fcc structure that in the bulk phase occurs around T = 420 °C was not

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detected; the differences in the diffraction patterns recorded at high temperatures with respect to the corresponding one at room temperature are discussed; the beginning of the formation of a fcc crystalline phase is evidenced for the sample heated at T = 800 °C. 2. Experimental One fused silica slide (Herasil I by Heraeus) was implanted with 180 keV Co+ ions using a 200 keV high-current Danfysik 1090 implanter at the INFN–INFM Implantation Laboratory (Legnaro, Italy); the current density was kept below 2 μA/cm2 to keep the sample at room temperature. The nominally implanted fluence was 2 × 1017 Co+/cm2: in this condition the Co concentration depth profile simulated with the SRIM code [16] has a maximum at a depth of about 150 nm with a full width at half maximum of about 90 nm. The maximum atomic concentration of dopant is about 30% near the projected range. The actual fluence, measured by the Rutherford backscattering spectrometry is (1.97 ± 0.09) × 1017 Co/cm2, compatible with the nominal one. In agreement with previous results [9,11,12], this kind of preparation is able to promote the aggregation of Co atoms in crystals with size in the nanometer range, that are confined in the implanted layer below the sample surface. The in-situ X-ray diffraction experiment was performed on the Italian beamline GILDA of the European Synchrotron Radiation Facility, exploiting the X-ray beam from a bending magnet. The X-ray energy was selected with a (511) Si double crystal monochromator and the beam was horizontally focused on the sample by bending the second crystal of the monochromator. The stability of the X-ray energy, set to E = 18,000 eV, was monitored during the experiment by measuring the absorption coefficient from a Zr foil in transmission mode: being the energy of the 1s state from the metallic Zr equal to 17,998 eV, any fluctuation in the energy of the monochromatic X-ray beam is measured as variation in the absorption coefficient from the standard foil. The energy of the monochromatic X-ray beam was stable within 0.2 eV. The sample was mounted in a cylindrical oven equipped with a window transparent to the X-rays; the lower surface of the sample was clipped onto a sample holder whose temperature can be risen up to 1150 °C and controlled with a K-type thermocouple. Since the Co clusters are confined in a submicrometer thick layer below the sample surface, the X-ray

Fig. 1. Sketch of the experimental set-up used for the in-situ X-ray diffraction experiment. The sample-to-detector distance is about 45 cm.

Fig. 2. Diffraction pattern measured during heating at T = 554 °C. The recording area is selected by the X-ray transparent window of the oven; the direct beam – completely absorbed by a beam stopper – would be recorded near the lower edge of the imaging plate.

diffraction pattern from the sample was recorded in grazing incidence mode to enhance the signal from the Co clusters: the incidence angle (α ≅ 0.15°) was chosen by measuring the reflectivity curve from the sample, as explained in Ref. [14]; a sketch of the experimental setup is reported in Fig. 1. The diffraction pattern was recorded with a FUJIFILM imaging plate (height = 400 mm, width = 200 mm, pixel size = 100 μm × 100 μm), placed at about 45 cm from the sample [17]. During heating, the sample was re-aligned to compensate for the temperature-induced drift of the sample holder. The diffraction pattern was first measured at room temperature; then, the sample was heated in vacuum (p ∼ 10− 4 mbar) at different increasing temperatures up to 800 °C and for each one, after waiting a few minutes to reach the thermal equilibrium, the diffraction pattern was recorded. The diffraction patterns were then integrated with the Fit2D software [18]. 3. Results and discussion A typical diffraction pattern recorded by the imaging plate is shown in Fig. 2, where the recording area is selected by the Be window of the oven. The first intense wide ring on the lower part of the image is the signal from the silica matrix, while the narrower ones are from the Co nanocrystals. The signal obtained upon a radial integration from the diffraction image recorded for the as-implanted sample (i.e. before heating) is reported in Fig. 3. Besides the signal from the matrix (peaked at 2θ ≅ 10°), the diffraction peaks from the hexagonal closed packed (hcp) structure of the nanocrystals are pretty evident. The scattering signal from the matrix was measured in the same way from the backside of the sample. To single out the signal of the Co clusters from that one of the matrix, we have used the experimental scattering pattern of the silica matrix, whose intensity has been properly adjusted to fit the signal from Co-implanted silica in the

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Fig. 3. Radially integrated diffraction profiles of Co-implanted silica (measured at room temperature) and of SiO2 matrix (dashed line); the difference between the two signals is also reported (solid line) and labeled according to the Co hcp phase.

regions where the signal from the nanocrystals is not present. In Fig. 3 the integrated XRD pattern is shown before and after the silica background subtraction: the five most intense reflections from the hcp Co nanocrystals are clearly visible and labeled. Starting from this point, the sample was heated progressively at higher temperatures, up to T = 800 °C. The procedure followed was to heat the sample at a selected temperature, to wait a few minutes for thermalization and to record a diffraction pattern after checking the geometrical alignment; after that the temperature was increased of 10–20 °C for the next measurement. In Fig. 4 two of the radially integrated spectra measured at high temperatures are compared to the one recorded at room temperature: the signal from the nanocrystals is always clearly visible. The comparison with the reflections of pure hcp and fcc Co bulk crystals indicates that most of the nanocrystals are in the hcp phase, even during heating at T = 800 °C. This behavior is completely different from the one typical of the bulk Co, that undergoes a hcp → fcc crystalline phase transition at T = 420 °C [19]. The reasons for this behavior are still under investigation: the

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Fig. 5. Diffraction profiles (after background subtraction) in the range 2θ = 16– 25 deg from the signals reported in Fig. 4. The reflections from hcp and fcc Co bulk in the same 2θ-range are reported for comparison.

finite cluster size could likely have a determinant role, but also the cluster–matrix interaction could affect possible phase transitions, since a change in the crystal structure should probably re-arrange the atomic structure of the cluster surface. In Fig. 5 the diffraction signal is reported, after the background subtraction, for the three cases in the region 2θ = 16–25 deg and it is compared with the reflections for the Co hcp and fcc bulk phases. It is worth noting that for the measurements performed at room temperature and at T = 554 °C the second peak (corresponding to the reflection <002> of the hcp phase) is lower than the third one (reflection <101> of the hcp phase), as in the pure hcp Co phase. On the other hand, when the same sample undergoes a heating up to T = 800 °C, the top of the second peak is at the same height of the third one; the <111> reflection for the fcc Co phase (Fig. 5) is located exactly in the region of the second peak, so that this modification could be related to the presence of a second crystal fcc phase that starts to appear at a temperature considerably higher than for the corresponding bulk material. Other measurements are in progress to clarify this point, on one hand monitoring the diffraction pattern at T = 800 °C for a longer time, and on the other by heating the sample at higher temperatures. Preliminary in-situ investigations performed with transmission electron microscope are in agreement with this result. As a last remark, from the inspection of Figs. 4 and 5 it emerges that, despite the thermal vibrations, the sample heated at T = 800 °C exhibits the most intense signal from the Co nanocrystals; this effect is probably a consequence of the temperature-induced Co diffusion in the matrix, that is known to both reduce the fraction of dopant dispersed in the matrix and to promote the aggregation of larger clusters [20–22]. 4. Conclusion

Fig. 4. Radially integrated diffraction profiles of Co-implanted silica measured at room temperature, at T = 554 °C and at T = 800 °C. The reflections from hcp and fcc Co bulk are reported for comparison.

We have investigated the crystalline structure of Co nanoclusters in the silica matrix formed by ion implantation: the experiment was performed by heating in-situ the system up to T = 800 °C and recording the diffraction pattern in grazing incidence

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geometry to enhance the cluster signal; the crystal phase of the asimplanted sample is hcp. The hcp → fcc phase transition that occurs in the Co bulk at T = 420 °C has not been detected: this result could be likely related to the finite cluster size as well as to the cluster matrix interaction. Some modification in the diffraction pattern recorded at T = 800 °C could be related to the beginning of the fcc nanocluster formation. Work is in progress to investigate this effect at higher temperatures. Acknowledgements The technical support of Fabio D'Anca and Fabrizio La Manna (staff of the Italian Institute for the Physics of Matter-INFM in Grenoble) is gratefully acknowledged. References [1] F. Gonella, P. Mazzoldi, in: H.S. Nalwa (Ed.), Handbook of Nanostructured Materials and Nanotechnology, vol. 4, Academic Press, S. Diego, 2000, p. 81. [2] G. Hadjipanayis, G.A. Prinz (Eds.), Science and Technology of Nanostructures Magnetic Materials, Plenum Press, New York, 1991. [3] F.J. Himpsel, J.E. Ortega, G.J. Mankey, R.F. Willis, Adv. Phys. 47 (1998) 511. [4] R.H. Kodama, J. Magn. Magn. Mater. 200 (1999) 359. [5] V. Skumryev, S. Stoyanov, Y. Zhang, G. Hadjipanayis, D. Givord, J. Nogués, Nature 423 (2003) 850. [6] E. Cattaruzza, G. Battaglin, P. Canton, C. de Julian Fernandez, M. Ferroni, F. Gonella, C. Maurizio, P. Riello, C. Sada, C. Sangregorio, B.F. Scremin, Appl. Surf. Sci. 226 (2004) 62.

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