Metal and metal oxide nanoparticles produced by ion implantation in silica: A microstructural study using HRTEM

NIM B Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B 257 (2007) 99–103 www.elsevier.com/locate/nimb

Metal and metal oxide nanoparticles produced by ion implantation in silica: A microstructural study using HRTEM O. Pen˜a, J.C. Cheang-Wong *, L. Rodrı´guez-Ferna´ndez, J. Arenas-Alatorre, A. Crespo-Sosa, V. Rodrı´guez-Iglesias, A. Oliver Instituto de Fı´sica, Universidad Nacional Auto´noma de Me´xico, A.P. 20-364, Me´xico, D.F. 01000, Me´xico Available online 3 January 2007

Abstract Characterization of the morphology of nanoparticles of Ag and Cu and their oxides obtained by ion implantation in silica and a subsequent thermal annealing in a reducing or in an oxidizing atmosphere, was carried out by high-resolution transmission electron microscopy. The studied nanoparticle size range was 2–23 nm. Typical structures such as fcc cuboctahedra have been obtained in the all the size range, for both, the metal and oxide nanoparticles. The truncated decahedron structure was frequently present in Ag and Cu nanoparticles in the 4–6 nm range size. The nanoparticle size distribution, as well as the particle composition, depends on the annealing atmosphere.  2007 Elsevier B.V. All rights reserved. PACS: 81.07. b; 78.70. g; 78.67. n; 61.82.Rx Keywords: Metal nanoparticles; Microstructure; HRTEM; Ion implantation; Silica

1. Introduction Nanometer-sized metal particles embedded in a glass matrix exhibit quite interesting optical properties, and are particularly promising candidates for applications in the fields of non-linear integrated optics and photonics. Shape, size, spatial distribution and ambient conditions are crucial parameters to control their optical behavior. Nanoparticles (NPs) have peculiar properties because they are finite small objects. To finite objects, the constraint of translation invariance on a lattice does not apply. For this reason, NPs can present non-crystalline structures, icosahedra and decahedra being the most known. An important issue in nanoscience is to understand whether crystalline or noncrystalline structures prevail for a given size and composition. NP’s atomic arrangement may be different from that of the bulk crystal, provided that the energy cost of internal

*

Corresponding author. Tel.: +52 55 5622 5164; fax: +52 55 5622 5009. E-mail address: cheang@fisica.unam.mx (J.C. Cheang-Wong).

0168-583X/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.12.138

strain is offset by a favorable arrangement of surface atoms, and the final arrangement is the result of a competition between thermodynamic and kinetic factors [1]. This implies that one structure will be preferred to others over specific size ranges. It is expected that icosahedral structures will be the most stable at small sizes (1–3 nm), decahedral ones will be favorable at intermediate sizes (4–7 nm) and finally crystalline structures are recovered in the limit of large objects [2]. Theoretical studies support these general predictions for several materials [3–6]. Nevertheless, large Ag icosahedra (above 5 nm) have been observed in inert-gas aggregation sources experiments of free NPs [7], and sodalime silicate float glass implanted with Ag ions [8]. The synthesis method of metal NPs embedded in a glass matrix is quite different from the free NPs growth by inertgas aggregation sources. Moreover, in addition of the hydrostatic pressure caused by the large surface-to-volume ratio, there is an interface stress between both materials, the glass and the metal, which are necessarily in contact. This fact must affect the variety of NP shapes in relation

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to those grown by inert-gas aggregation. So, a study of such NPs is very important, not only from the point of view of basic research, but also from the applications one. There are several experimental methods of synthesis of metal NPs in a glass matrix. Among them, ion implantation plus thermal processing is a very useful method to obtain large volume fractions of NPs in a region well defined depending on the ion energy [9]. In this work, we present a microstructural study of Cu and Ag NPs grown in silica by ion implantation and subsequent thermal annealing in different atmospheres. The NPs were characterized by high-resolution transmission electron microscopy (HRTEM), and computational techniques such as digital processing and image simulation using the multislice method. 2. Experimental High-purity silica glass plates (20 · 20 · 1 mm) with an OH content less than 1 ppm, no individual impurity greater than 1 ppm and a total impurity content less than 20 ppm, were implanted at room temperature with 2 MeV Ag and Cu ions. The ion fluences were in the range of 4–

8 · 1016 ions/cm2. The fluences and the Cu and Ag depth profile distributions were determined by Rutherford backscattering spectrometry (RBS). The ion range was 0.9 lm for Ag and 1.6 lm for Cu ions. In order to nucleate the Ag and Cu NPs, a subsequent thermal annealing in a reducing (RA = 50% N2 + 50% H2) or in an oxidizing atmosphere (OA = air) was carried out at 600 C for Ag and at 900 C for Cu. HRTEM analysis was performed in a JEOL-2010 FEG instrument with a point-to-point resolution of 0.19 nm. Mechanical polishing and ion milling techniques with an Ar+ ion beam were used for the TEM sample preparation. The measurements of lattice-fringe spacing and of the angles recorded in digital HRTEM micrographs were made using digital image analysis of reciprocal space parameters, according to the de Ruijter et al. method [10]. With this method, the precision is 0.0001 nm for lattice spacing and 0.1 for lattice planes angles. This analysis was carried out with the aid of the Digital Micrograph software. About 200 particles were analyzed for structural characterization by HRTEM. In some cases digital image processing were used for the background subtraction and to resolve the nanoparticle shape. The nanoparticle size distribution was determined by means

Fig. 1. Ag2O3 NP of about 4 nm showing a cuboctahedral shape: (a) original HRTEM image, (b) FFT of the HRTEM micrograph, (c) model used in the simulation of the HRTEM image and (d) simulated HRTEM micrograph, obtained using the multi-slice method.

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Fig. 2. Ag NP of about 5 nm showing a truncated decahedral shape: (a) original HRTEM image, (b) FFT of the HRTEM micrograph, (c) model used in the simulation of the HRTEM image and (d) simulated HRTEM micrograph, obtained using the multi-slice method.

of the ANAlysis software. Nanoparticle image simulation was performance with the SIMULATEM software [11]. 3. Results TEM micrographs containing about 200 NPs in Cu or Ag as-implanted samples exhibit NP sizes in the range 2– 23 nm. After the thermal annealing in RA, the NP presented a narrower size distribution, typically in the 2– 9 nm range, as compared to that obtained in the OA. Moreover, the samples annealed in OA contained large metal oxide NPs. Metal, as well as metal oxide NPs, were present in all the samples. The formation of metal oxide NPs was more frequently observed when the samples were annealed in OA. When metal oxide NPs were found, they presented only one crystalline phase. Cu samples showed more oxide NPs than the Ag samples, and Cu oxide NPs were present even when the sample was annealed in RA. In contrast, almost no Ag oxide NPs were found when the samples were annealed in RA. In samples implanted with Ag ions, we found Ag2O3 and AgO NPs that exhibited cuboctahedral shapes (Fig. 1), with sizes ranging from 5 nm to 12 nm. For the metallic

Ag NPs, the prevailing shapes were truncated decahedra (Fig. 2) and cuboctahedra for the size range 4–6 nm. In the case of samples implanted with Cu ions, both large and scarce unshapely CuO crystalline NPs, 20 nm, and small Cu4O3 NPs, 3 nm, with cuboctahedral shape can be grown in OA (Fig. 3), appearing also, in less number, in the samples annealed in RA. In the 4–9 nm size range, numerous Cu particles exhibited cuboctahedral shapes (Fig. 4), and truncated decahedra were also present, for samples annealed in either atmospheres, RA or OA.

4. Discussion The NP size distribution depends on the annealing atmosphere. This fact concerns the defects created by the ion implantation, which apparently behave like nucleation centers, and the surrounding ions [12]. The chemical interaction strength among the host ions, the diffusing atoms of the annealing atmosphere in the silica, and the implanted metal ions are a determining factor that influences the nature of the formed NPs. In both implantations, Cu or Ag ions, metal NPs as well as metal oxide NPs can be formed.

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Fig. 3. Cu4O3 NP of about 3 nm showing a cuboctahedral shape: (a) original HRTEM image, (b) FFT of the HRTEM micrograph, (c) model used in the simulation of the HRTEM image and (d) simulated HRTEM micrograph, obtained using the multi-slice method.

The processes governing the chemical and physical interaction between the implanted ions and the ions of the host matrix are not completely understood and are still under debate [13]. The subsequent heat treatment, depending on the annealing atmosphere, also influences the chemical composition during the NP nucleation, being Cu more susceptible to be oxidized, as can be seen from our results. It is interesting to point out that neither in a previous work [14], nor in this one, it was found a conclusive evidence of the existence of the Cu2O phase. Concerning the NP shapes, the truncated octahedron is the fcc crystalline structure that optimizes the surface energy in order to have no internal strain. However, as we have shown, a variety of shapes can be formed by ion implantation of either Cu or Ag ions in silica with a subsequent annealing. Besides single crystalline particles (fcc for Cu and Ag), non-crystalline structures like decahedra have been observed in Ag and Cu NPs embedded in silica. Neither the small-sized particles nor the big ones exhibit icosahedral shapes. This result differs from those obtained in inert-gas aggregation source experiments of free NPs [7], and sodalime silicate float glass implanted with Ag ions [8].

The important presence of truncated decahedra in contrast of no evidence of the existence of icosahedra shapes in our work, can be explained from the point of view of surface energy. The decahedra, which have a single fivefold axis, are less spherical than the icosahedra, but have less internal strain, and they can optimize quite well the surface energy [2]. On the other hand, icosahedra present six fivefold symmetry axes and are limited by close-packed distorted (1 1 1)-like facets. They are able to minimize efficiently the NP surface energy, but at the expense of the internal strain [15]. 5. Conclusions The NPs size distribution, as well as the particle composition, depends on the annealing atmosphere, for NP growth by either Cu or Ag ion implantation in high-purity silica. Single crystalline NPs with a fcc structure could be observed for metal and metal oxide particles in the size range of 2–20 nm. Cuboctahedral shapes for both, metallic Cu and Ag NPs, and their oxides were exhibited by numerous particles. The main non-crystalline structure present in

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Fig. 4. Cu NP of about 5 nm showing a cuboctahedral shape: (a) original HRTEM image, (b) FFT of the HRTEM micrograph, (c) model used in the simulation of the HRTEM image and (d) simulated HRTEM micrograph, obtained using the multi-slice method.

our samples was the truncated decahedra, and no evidence for the presence of particles with icosahedral shape was observed. These results are in better agreement with the theoretical calculations [3–6], than with those obtained in inert-gas aggregation source experiments of free NPs [7], and sodalime silicate float glass implanted with Ag ions [8]. Acknowledgements The authors would like to thank S. Tehuacanero, J.G. Morales, R. Herna´ndez, and L. Rendo´n for technical assistance in the HRTEM studies, K. Lo´pez and F.J. Jaimes for the Pelletron accelerator operation, and DGEP for providing a scholarship for one of the authors (O. Pen˜a). This work was partially supported by DGAPA-UNAM IN119706-3 and IN-104303 projects, and by the CONACyT 42626 and 50504 grants.

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