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Effect of 100 MeV Au irradiation on embedded Au nanoclusters in silica glass
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 256 (2007) 659–664 www.elsevier.com/locate/nimb
Effect of 100 MeV Au irradiation on embedded Au nanoclusters in silica glass B. Joseph a, J. Ghatak a, H.P. Lenka a, P.K. Kuiri a, G. Sahu a, N.C. Mishra b, D.P. Mahapatra a,* a b
Institute of Physics, P.O. Sainik School, Bhubaneswar, Orissa 751 005, India Department of Physics, Utkal University, Bhubaneswar, Orissa 751 004, India Received 9 October 2006; received in revised form 17 December 2006 Available online 18 January 2007
Abstract Embedded Au nanoclusters (NCs) in silica glass were formed by ion beam synthesis using 32 keV Au implantation to a fluence of 4 · 1016 ions cm 2. Subsequently the NCs were irradiated with swift heavy ions in the form of 100 MeV Au8+ at 100 K in the fluence range of 5 · 1012–1 · 1014 ions cm 2. Samples were analyzed using Rutherford backscattering spectrometry, transmission electron microscopy and optical absorption spectroscopy. At lower irradiation fluence the high energy heavy ion irradiation has been found to result in a loss in Au due to an outward movement of the NCs together with a growth in size. At the highest irradiation fluence, almost 80% Au was lost, with only few large NCs seen which had moved to the surface. These were found to be of a deformed non-spherical shape. The amount of Au lost has been found to linearly increase with irradiation fluence indicating the movement of Au to be not dominated by diffusion. Results indicate the importance of nuclear energy loss in sputtering at very high energies. 2007 Elsevier B.V. All rights reserved. PACS: 41.75.Cn; 81.07 Ta; 68.37.Lp Keywords: Metal nanoparticles; Ion irradiation; Transmission electron microscopy
1. Introduction Recently a lot of attention has been paid to study swift heavy ion (SHI) irradiation induced modification of nanoclusters (NCs). This is because SHI could be an ideal tool to modify the physical and chemical properties under controlled conditions on a nanometric scale. At MeV/amu energies, an ion looses its energy almost entirely by electronic excitation and ionization of the target atoms. The energy loss in such a situation involving inelastic interactions with target electrons is given by the electronic stopping power, Se. If Se exceeds a threshold value for a SHI traversing in a medium, an amorphized latent track, a few nm in diameter
*
Corresponding author. Tel.: +91 674 2301058; fax: +91 674 2300142. E-mail address: [email protected] (D.P. Mahapatra).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.12.183
and several lm in length, can be created in the matrix [1]. SHI induced tracks are effective in tailoring the shape and size of embedded NCs. For example, embedded spherical Co nanoparticles were found to be elongated under SHI irradiation resulting in anisotropic magnetic properties [2,3]. 30 MeV Se irradiation of metallic dielectric colloids with an Au core and silica shell structure has been found result in different effects on the core and the shell. The irradiation turned the spherical silica shells into oblate ellipsoids while the spherical metal cores transformed into prolate ellipsoids [4]. Interestingly, spherical Au particles remained spherical under above irradiation. Similarly, 2–14 MeV Au irradiation to a fluence of 1014 ions cm 2 has been found to induce an anisotropic plastic deformation in spherical silica colloids of diameter 1.0 lm transforming them into oblate ellipsoids [5]. Plastic deformation has also been seen in colloids consisting of a silica core (diameter 300–500 nm) and
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Au shell (thickness 20–60 nm) structure upon irradiation with 30 MeV Cu ions. The shape of the spherical colloids changed into oblate ellipsoids, with the final degree of anisotropy depending upon the ion fluence [6]. Distinct nanoparticle self-organization in nanocomposites has been observed in 100 nm of Teflon AF containing Au and Ag clusters upon SHI irradiation, with 120 MeV Au, for SHI fluence in the range of 1 · 1011–3 · 1012 ions cm 2 [7]. Decoration of the SHI tracks with Au atoms has been observed in a study where a 2 nm thick Au marker layer, embedded in a 500 nm thick NiO layer, was irradiated with 260 MeV Kr and 230 MeV Xe ions [8]. In yet another study involving 30 MeV Si irradiation of Ag ion exchanged soda lime glass samples, alignment of Ag NCs has been reported [9]. In most of these studies, thermal spike concepts [1] are invoked to explain the observed results. In the thermal spike model, due to the passage of a SHI, the electronic subsystem is assumed to be excited first which thermalises in a very short time (10 13 s) and then the excitation energy is transferred to the lattice (in about 10 12 s) by electron–phonon coupling. Due to a large energy deposition in a short time, the material undergoes deformation leading to chemical and physical changes in nanometer sized regions. The above inelastic thermal spike model also explains enhanced sputtering from surfaces. Recently it has been shown that elastic nuclear collision cascades, together with inelastic effects coming from electronic energy loss, also play a dominant role regarding enhanced sputtering [10]. Such a model is very effective regarding enhanced surface evaporation. However, its applicability to cases with modification or outward movement of buried NCs is still to be tested. In view of the above works we have carried out some studies on SHI induced modifications of embedded spherical Au NCs in silica glass. Metal NCs embedded in glass have stimulated the interest of scientists because of their linear and non-linear optical properties [11]. The embedded spherical Au NCs were produced in silica glass via a high fluence implantation of 32 keV Au ions. In the present work SHI irradiation, using 100 MeV Au8+ ions, has been used for modifications of these NCs. For lower irradiation fluence, our results indicate an out flow and growth of Au NCs to larger sizes in the near surface region. Further increase in the irradiation fluence causes a shape deformation of the spherical clusters with the Au concentration shifting towards the surface. This has also been found to result in Au loss from an enhanced sputtering.
using a uniformly scanned 32 keV Au beam on the sample surface. All the implantations were performed at normal incidence in a chamber maintained at a base pressure of 7 · 10 7 mbar. Here after all the above Au implanted samples will be referred as ‘‘as-implanted’’ samples. One of the as-implanted samples was annealed at 850 C for 1 h in air (using a conventional quartz furnace) for a further growth of Au NCs in the matrix. Three of the as-implanted samples were irradiated with SHI in the form of 100 MeV Au8+ ions with fluence of 5 · 1012, 2 · 1013 and 1 · 1014 ions cm 2, respectively. The annealed sample was also irradiated with the same beam at a fluence of 1 · 1014 ions cm 2. All the SHI irradiations were carried out using the 15 MV Pelletron accelerator facility at the Inter University Accelerator Centre, New Delhi. During the SHI irradiation the ion beam was magnetically scanned over sample surface for a uniform exposure to ion irradiation. The samples were mounted on a copper target holder, cooled to around 100 K using liquid nitrogen. The chamber pressure during irradiation was around 5 · 10 6 mbar. Following the SHI irradiation, the samples were characterized by optical absorption (OA) spectroscopy, Rutherford backscattering spectrometry (RBS) and transmission electron microscopy (TEM). The Au concentration profiles were measured by RBS with 3 MeV a particles using the 3 MV Pelletron accelerator facility at IOP. Cross-sectional TEM (X-TEM) measurements were also carried out at IOP with a 200 kV (JEOL 2010) high resolution transmission electron microscope with a point to point resolution of 0.19 nm. The OA spectra were recorded in the transmission mode with a pure silica glass in the reference line using a Shimadzu PC3101 UV–VIS–NIR dual beam spectrophotometer. 3. Results Results of OA measurements carried out on an asimplanted sample before and after annealing at 850 C
2. Experiment Ion beam synthesis is an established way to produce embedded NCs, specially for that of noble metals [11–13]. We have used this technique to produce the Au NCs. For this purpose, high purity silica glass samples (area 1 cm · 1 cm), were implanted with 4 · 1016 Au cm 2 using a low energy negative ion implanter facility at the Institute of Physics (IOP), Bhubaneswar. The above Au implantations for the NCs synthesis were carried out at room temperature
Fig. 1. OA spectra of an as-implanted (32 keV 4 · 1016 Au cm 2 implanted), silica sample measured before (open circle) and after annealing in air at 850 C for 1 h (filled circle).
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a
Fig. 2. X-TEM image of as-implanted (32 keV 4 · 1016 Au cm 2 implanted) silica sample together with the SAED pattern from the Au NC region. Dotted line in the X-TEM image indicates the sample surface.
for 1 h are shown in Fig. 1. The spectra show a surface plasmon resonance (SPR) peak around 540 nm corresponding to embedded Au NCs in the silica matrix. Compared to the as-implanted sample, the annealed sample shows an enhancement in the SPR peak intensity with a reduced full width at half maximum (FWHM), which is an indication of the growth of NCs to larger sizes. The FWHM of the SPR for the as-implanted sample was 102 nm, which reduced to 78 nm after annealing. In an earlier study on a similar system (high fluence Au implanted into an oxidized Si matrix), we had seen a similar growth in size of Au NCs after high temperature annealing [12]. TEM measurements carried out on the as-implanted sample clearly showed Au NCs formation confirming the OA results. A typical X-TEM micrograph for the above sample (Fig. 2) shows 1–6 nm size Au clusters with almost spherical shape to be present in the silica matrix close to the surface. NCs are clearly seen up to a depth of 40 nm from the surface. A selected area electron diffraction (SAED) pattern from these NCs (Fig. 2) shows a ring pattern with dots indicating the crystalline nature of the NCs. The appearance of an SPR peak 540 nm in the OA spectrum (Fig. 1), is clearly due to these NCs shown in Fig. 2. The variation of the Au concentration in the samples at various SHI irradiation fluence were determined through an analysis of RBS data taken on the samples. In Fig. 3(a) we show some typical RBS spectra corresponding to an as-implanted sample and a sample irradiated with SHI at a fluence of 1 · 1014 ions cm 2. In Fig. 3(b) we show the variation of Au content inside the silica matrix (as determined from the RBS measurements), as a function of the SHI irradiation fluence. As can be seen from the figure, there was a gradual decrease in the amount of Au present in the silica matrix with increase in the irradiation fluence. For a SHI irradiation fluence of 1 · 1014 ions cm 2 it was found that almost 80% of the Au earlier present in the as-implanted sample was lost. In Fig. 4 we show the OA spectra corresponding to the SHI irradiated samples together with the same for the as-implanted sample. Different spectra are shifted in Y-axis for clarity in presentation. Comparing the spectra for the as-implanted and the irradiated sample for a SHI fluence of 5 · 1012 ions cm 2, we can clearly see, the later has a
b
Fig. 3. RBS spectra of Au as-implanted sample together with the same irradiated with 100 MeV Au8+ for fluence 1 · 1014 ions cm 2 (a) and reduction in the retained Au content in silica matrix as a function of 100 MeV Au8+ irradiation fluence (b).
Fig. 4. OA spectra of the SHI irradiated samples. The irradiation fluence in each case is shown as legends in the figure. The spectrum for the asimplanted sample is also included in the figure for comparison.
more intense SPR peak with slightly lower FWHM. These results are quite similar to those presented in Fig. 1 which also shows a growth in NC size induced by annealing. It is also important to note, from RBS data (Fig. 3(b)), that
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there is not much reduction in the Au content in the matrix at this fluence of irradiation. These observations imply that NCs in the as-implanted samples grow either on annealing or on irradiation at a fluence of 5 · 1012 ions cm 2. At a SHI irradiation fluence of 2 · 1013 ions cm 2, there was a broadening of the SPR peak (FWHM 110 nm) with a slight redshift in its position. In this case, using a careful fit, the SPR peak position was found to be red-shifted by about 5 nm (to a position 549 nm) as compared to the as-implanted sample. Such an increase in FWHM of the SPR peak and the redshift in its position is known to be due to an increase in the size of the NCs [14]. However, the OA spectrum for a SHI irradiation fluence of 1 · 1014 ions cm 2 has been found to be almost flat (filled circles in Fig. 4). This is primarily due to a much reduced volume fraction of the Au NCs in the matrix as can be seen in Fig. 3(b). In order to have a look at the micro-structure of the Au NCs after SHI irradiation, X-TEM measurements were carried out on samples irradiated with 2 · 1013 and 1 · 1014 ions cm 2. One such X-TEM micrograph for the sample with SHI fluence of 2 · 1013 ions cm 2 is shown in Fig. 5. Comparing this image with that shown in Fig. 2, one can clearly see a growth in the near surface NCs. One can also observe a reduction in the width of the Au NCs layer. Width of the Au NCs layer in this case is around 25 nm. The observed red-shift in the SPR peak for this sample is due to the growth of Au NCs to larger sizes. As mentioned earlier, the OA spectrum for a sample with a SHI irradiation fluence of 1 · 1014 Au ions cm 2, as shown in Fig. 4, looks almost flat. A magnified view of the same spectrum is shown in Fig. 6. The figure shows two peak-like structures, one near 525 nm and the other near 600 nm. A similar OA spectrum has also been obtained for the annealed sample after SHI irradiation to an identical fluence. This has also been included in Fig. 6 for comparison. An X-TEM micrograph corresponding
Fig. 5. X-TEM image of Au as-implanted sample irradiated with 100 MeV Au8+ for a fluence of 2 · 1013 ions cm 2. The dotted line in the X-TEM image indicates the surface.
Fig. 6. OA spectrum measured at an Au irradiation fluence of 1 · 1014 ions cm 2 (open circles). Filled circles correspond to that of the annealed sample as shown in Fig. 1.
Fig. 7. X-TEM image of Au as-implanted sample irradiated with 100 MeV Au8+ for a fluence of 1 · 1014 ions cm 2. Dotted line in the figure indicates the surface. The inset shows an SAED pattern taken on a single large particle.
to the SHI irradiated, unannealed sample, is shown in Fig. 7. One can clearly see, after a high dose SHI irradiation (1 · 1014 ions cm 2) there were only a few isolated Au particles in the system corresponding to about 20% of the total initial Au content in the as-implanted samples. But the observed particles have been found to be clearly larger than the Au NCs seen in the samples irradiated at a lower fluence. A SAED pattern obtained on one such particle is also included in Fig. 7. The figure shows diffraction spots which correspond to the fcc Au structure. The X-TEM image taken on the sample with the highest SHI irradiation fluence clearly shows that the Au NCs were no longer spherical in shape. There has been an earlier SHI irradiation study on Au-core-silica-shell particles using 30 MeV Cu ions with a fluence of 2 · 1014 ions cm 2. The measured optical extinction spectrum, showed a broad peak in the long-wavelength region (600–650 nm) with the main SPR peak slightly blue-shifted to 521 nm [4]. It must be mentioned that the extinction spectrum with very
B. Joseph et al. / Nucl. Instr. and Meth. in Phys. Res. B 256 (2007) 659–664
small contribution from scattering effects from NCs, is almost the same as that obtained from absorption measurements. The feature viz a blue shift of main SPR peak and a broad hump in the long wavelength region, in the absorption spectrum is very clearly seen in Fig. 6. The above two peak structure in OA spectrum has been shown to be a characteristic of elongated Au NCs [14]. The OA spectrum for the annealed sample, after SHI irradiation with a fluence of 1 · 1014 ions cm 2 (Fig. 6), also shows identical features. Based on this, even without any X-TEM data, we conclude that the sample contained elongated Au NCs as in case of the highest fluence SHI irradiated sample. 4. Discussions As pointed out in [1], the Se threshold for track formation in SiO2 is 1 keV/nm and for Se = 8 keV/nm, latent tracks, 3 nm in radius, could be formed. From a SRIM calculation [15] one can see that the electronic energy loss, Se, of 100 MeV Au ions in silica glass is about 13.5 keV/nm which is much higher than that required for creation of a latent track of 3 nm in radius. We thus expect SHI induced tracks much wider than 3 nm to be created in the matrix. In this situation, at a SHI irradiation fluence of 2 · 1013 ions cm 2 overlapping of tracks would occur and NCs embedded in the matrix (in the as-implanted sample) are expected to be affected by more than one SHI. However, at lower irradiation fluence whatever happens is mostly due to single SHI impact only. D’Orle´ans et al. [3] have carried out a systematic study of SHI (with 200 MeV I) irradiation induced effects on ion beam synthesized Co NCs in SiO2 where embedded Co NCs were prepared through high fluence Co implantation at 160 keV at various temperatures. SHI irradiations of samples prepared at 295 K and 77 K, has been found to result only in a growth in size of NCs, from 3–4 nm to double the size, with increase in SHI fluence up to 1 · 1014 ions cm 2. However, for samples prepared at 873 K, with initial average size of NCs 10 nm, there was growth in NC size with fluence, up to a fluence of 5 · 1012 ions cm 2. Increase in SHI fluence to 1 · 1013 ions cm 2, resulted in modifications in shape of some of the NCs. At a SHI fluence of 1 · 1014 ions cm 2, there was a drastic change in shape of NCs which were found to be elongated along the direction of the propagation of ions. Most importantly, the total Co content in the matrix was found to remain intact without any out diffusion or sputtering. The Se values, calculated using SRIM [15], are almost similar for both 100 MeV Au and 200 MeV I in SiO2. However, in contrast with the above we did not observe any significant deformation of the Au NCs even at an irradiation fluence of 2 · 1013 ions cm 2 although there was an outward movement and loss of Au (Fig. 3(b) and Fig. 6). In fact the outward movement of Au with increase in irradiation fluence, as observed in the present case, is quite similar to that observed in case of a Au marker layer in NiO under SHI (230 MeV Xe) irradiation [8]. One can
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see, Se for 100 MeV Au is slightly smaller than Se for 230 MeV Xe in NiO. Even then there was a lot of Au ‘‘sputtering’’ in the present case quite contrary to what has been found with 230 MeV Xe irradiation of embedded Au NCs in NiO. This shows nuclear stopping also plays a dominant role even though it has a three orders lower value. This goes in line with the findings of Mieskes et al. [10] who have shown that a synergy between Se and Sn can play a significant role resulting in enhanced sputtering within the thermal spike model. It is quite interesting to look into the nature of the transport of Au to the surface and its eventual loss from the matrix. The linear variation of the amount of Au lost from the matrix as a function of SHI irradiation fluence (Fig. 3(b)), clearly rules out diffusion as the dominating transport mechanism. As suggested by Bolse and Wiesner [8], a SHI may produce a molten track. Then because of the density reduction in the molten phase, a high pressure will be present which, in a continuous track, may squeeze the melt along the track axis towards the pressure free surface. This can lead to the observed out-flow of the Au and its eventual loss from the silica matrix. Fig. 7 also shows a flaring type feature on the outer face of the Au ‘‘nano droplets’’ which seem to suggest a flowing out tendency at the surface. In their SHI irradiation study involving Co nanoparticles, D’Orle´ans et al. [2] have suggested that the mechanisms involved in the track formation could also be responsible for the SHI induced anisotropic growth of NCs. Using the thermal spike model, they have shown that under SHI irradiation there can be a growth in size of small NCs mainly through fragmentation and ripening until the NCs reach a critical diameter. After this the initial temperature inside the nanoparticle is lower than the temperature at the center of the ion track in the SiO2 matrix. This causes differences in volume expansion and compressibility, due to which there could be a large over pressure in the molten NCs leading to a deformation. In the present case, at a SHI irradiation fluence of 2 · 1013 ions cm 2, there would be spatially overlapping but temporally separated ion tracks which effectively produce a latent tracks larger than 3 nm radius. In these, evaporation of small NCs through SHI impact would result in the growth of larger NCs. However, at still higher fluence, the larger NCs would melt resulting in a change in density and volume. In that case, as suggested earlier, the molten material can be squeezed out to the surface. This would result in deformation in shape associated with outward motion. It is also important to mention here that the contribution from elastic (nuclear) interactions is dependent on the track diameter. In case the track diameter is small and the energy loss from elastic interactions is confined to this region then molten NCs can get elongated during the thermal spike formation. This does not seem to have happened for Au NCs in silica glass matrix. In addition, the observed movement of the Au NCs along latent tracks created by the SHI cannot be understood within the framework of a thermal spike model
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which does not include density and pressure effects [10]. One needs to consider the pressure pulse coming from fast density changes in the ion track. The shape of the Au NCs in case of a SHI irradiation fluence of 1 · 1014 ions cm 2 (Fig. 7), is indicative of a pressure spike induced push out of a molten metal NC close to the surface. 5. Conclusions We have studied the effect of 100 MeV Au8+ irradiation on embedded Au NCs of 1–6 nm, produced by implantation of 4 · 1016 32 keV Au cm 2 into silica glass. For an irradiation fluence of 5 · 1012 ions cm 2, a growth in size of the Au NCs has been observed. Increasing the irradiation fluence to 2 · 1013 ions cm 2, resulted in a significant growth in size along with an outward movement of NCs. At this irradiation fluence, about 15% of the Au content was found to have been sputtered out from the matrix. Increasing the irradiation fluence to 1 · 1014 ions cm 2, the amount of Au retained in the silica matrix dropped to about 20% of its initial value. There were only a few isolated larger Au NCs with non-spherical shapes situated close to the surface. Changing the initial size distribution of NCs in the starting sample by annealing did not have any marked effect on the final state of the Au NCs. Present results on the growth in size of the Au NCs for lower irradiation fluence and their deformation from spherical shape at higher fluence are in line with the thermal spike model. Acknowledgements Authors wish to thank D. Kanjilal and A. Tripathi for important suggestions and helps during the swift heavy ion irradiation. We also thank S.N. Sahu and S. Sarangi for the optical absorption measurements and all the opera-
tors of the Ion Beam Laboratory, at IOP for their helps in ion beam experiments. References [1] M. Toulemonde, C. Trautmann, E. Balanzat, K. Hjort, A. Weidinger, Nucl. Instr. and Meth. B 216 (2004) 1. [2] C. D’Orle´ans, J.P. Stoquert, C. Estourne´s, C. Cerruti, J.J. Grob, J.L. Guille, F. Haas, D. Muller, M. Richard-Plouet, Phys. Rev. B 67 (2003) 220101. [3] C. D’Orle´ans, J.P. Stoquert, C. Estourne´s, J.J. Grob, D. Muller, J.L. Guille, M. Richard-Plouet, C. Cerruti, F. Haas, Nucl. Instr. and Meth. B 216 (2004) 372. [4] S. Roorda, T. van Dillen, A. Polman, C. Graf, A. van Blaaderen, B.J. Kooi, Adv. Mater. 16 (2004) 235. [5] T. van Dillen, A. Polman, W. Fukarek, A. van Blaaderen, Appl. Phys. Lett. 78 (2001) 910. [6] J.J. Penninkhof, C. Graf, T. van Dillen, A.M. Vredenberg, A. van Blaaderen, A. Polman, Adv. Mater. 17 (2005) 1484. [7] A. Biswas, D.K. Avasthi, D. Fink, J. Kanzow, U. Schu¨rmann, S.J. Ding, O.C. Aktas, U. Saeed, V. Zaporojtchenko, F. Faupel, R. Gupta, N. Kumar, Nucl. Instr. and Meth. B 217 (2004) 39. [8] W. Bolse, J. Wiesner, Nucl. Instr. and Meth. B 218 (2004) 479. [9] J.J. Penninkhof, A. Polman, L.A. Sweatlock, S.A. Maier, H.A. Atwater, A.M. Vredenberg, B.J. Kooi, Appl. Phys. Lett 83 (2003) 4137. [10] H.D. Mieskes, W. Assmann, F. Gru¨ner, H. Kucal, Z.G. Wang, M. Toulemonde, Phys. Rev. B 67 (2003) 155414; M. Toulemonde, W. Assmann, C. Trautmann, F. Gru¨ner, H.D. Mieskes, H. Kucal, Z.G. Wang, Nucl. Instr. and Meth. B 212 (2003) 346. [11] F. Gonella, P. Mazzoldi, in: H.S. Nalwa (Ed.), Handbook of Nanostructured Materials and Nanotechnology, Vol. 4, Academic Press, CA, 2000, p. 81, Chapter 2. [12] B. Jospeh, S. Mohapatra, B. Satpati, H.P. Lenka, P.K. Kuiri, D.P. Mahapatra, Nucl. Instr. and Meth. B 227 (2005) 559. [13] B. Jospeh, S. Mohapatra, H.P. Lenka, P.K. Kuiri, D.P. Mahapatra, Thin Solid Films 492 (2005) 35. [14] S. Link, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 8410. [15] SRIM 2003 – A version of the TRIM program, J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Matter, Pergamon Press, New York, 1995.
Nuclear Instruments and Methods in Physics Research B 256 (2007) 659–664 www.elsevier.com/locate/nimb
Effect of 100 MeV Au irradiation on embedded Au nanoclusters in silica glass B. Joseph a, J. Ghatak a, H.P. Lenka a, P.K. Kuiri a, G. Sahu a, N.C. Mishra b, D.P. Mahapatra a,* a b
Institute of Physics, P.O. Sainik School, Bhubaneswar, Orissa 751 005, India Department of Physics, Utkal University, Bhubaneswar, Orissa 751 004, India Received 9 October 2006; received in revised form 17 December 2006 Available online 18 January 2007
Abstract Embedded Au nanoclusters (NCs) in silica glass were formed by ion beam synthesis using 32 keV Au implantation to a fluence of 4 · 1016 ions cm 2. Subsequently the NCs were irradiated with swift heavy ions in the form of 100 MeV Au8+ at 100 K in the fluence range of 5 · 1012–1 · 1014 ions cm 2. Samples were analyzed using Rutherford backscattering spectrometry, transmission electron microscopy and optical absorption spectroscopy. At lower irradiation fluence the high energy heavy ion irradiation has been found to result in a loss in Au due to an outward movement of the NCs together with a growth in size. At the highest irradiation fluence, almost 80% Au was lost, with only few large NCs seen which had moved to the surface. These were found to be of a deformed non-spherical shape. The amount of Au lost has been found to linearly increase with irradiation fluence indicating the movement of Au to be not dominated by diffusion. Results indicate the importance of nuclear energy loss in sputtering at very high energies. 2007 Elsevier B.V. All rights reserved. PACS: 41.75.Cn; 81.07 Ta; 68.37.Lp Keywords: Metal nanoparticles; Ion irradiation; Transmission electron microscopy
1. Introduction Recently a lot of attention has been paid to study swift heavy ion (SHI) irradiation induced modification of nanoclusters (NCs). This is because SHI could be an ideal tool to modify the physical and chemical properties under controlled conditions on a nanometric scale. At MeV/amu energies, an ion looses its energy almost entirely by electronic excitation and ionization of the target atoms. The energy loss in such a situation involving inelastic interactions with target electrons is given by the electronic stopping power, Se. If Se exceeds a threshold value for a SHI traversing in a medium, an amorphized latent track, a few nm in diameter
*
Corresponding author. Tel.: +91 674 2301058; fax: +91 674 2300142. E-mail address: [email protected] (D.P. Mahapatra).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.12.183
and several lm in length, can be created in the matrix [1]. SHI induced tracks are effective in tailoring the shape and size of embedded NCs. For example, embedded spherical Co nanoparticles were found to be elongated under SHI irradiation resulting in anisotropic magnetic properties [2,3]. 30 MeV Se irradiation of metallic dielectric colloids with an Au core and silica shell structure has been found result in different effects on the core and the shell. The irradiation turned the spherical silica shells into oblate ellipsoids while the spherical metal cores transformed into prolate ellipsoids [4]. Interestingly, spherical Au particles remained spherical under above irradiation. Similarly, 2–14 MeV Au irradiation to a fluence of 1014 ions cm 2 has been found to induce an anisotropic plastic deformation in spherical silica colloids of diameter 1.0 lm transforming them into oblate ellipsoids [5]. Plastic deformation has also been seen in colloids consisting of a silica core (diameter 300–500 nm) and
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Au shell (thickness 20–60 nm) structure upon irradiation with 30 MeV Cu ions. The shape of the spherical colloids changed into oblate ellipsoids, with the final degree of anisotropy depending upon the ion fluence [6]. Distinct nanoparticle self-organization in nanocomposites has been observed in 100 nm of Teflon AF containing Au and Ag clusters upon SHI irradiation, with 120 MeV Au, for SHI fluence in the range of 1 · 1011–3 · 1012 ions cm 2 [7]. Decoration of the SHI tracks with Au atoms has been observed in a study where a 2 nm thick Au marker layer, embedded in a 500 nm thick NiO layer, was irradiated with 260 MeV Kr and 230 MeV Xe ions [8]. In yet another study involving 30 MeV Si irradiation of Ag ion exchanged soda lime glass samples, alignment of Ag NCs has been reported [9]. In most of these studies, thermal spike concepts [1] are invoked to explain the observed results. In the thermal spike model, due to the passage of a SHI, the electronic subsystem is assumed to be excited first which thermalises in a very short time (10 13 s) and then the excitation energy is transferred to the lattice (in about 10 12 s) by electron–phonon coupling. Due to a large energy deposition in a short time, the material undergoes deformation leading to chemical and physical changes in nanometer sized regions. The above inelastic thermal spike model also explains enhanced sputtering from surfaces. Recently it has been shown that elastic nuclear collision cascades, together with inelastic effects coming from electronic energy loss, also play a dominant role regarding enhanced sputtering [10]. Such a model is very effective regarding enhanced surface evaporation. However, its applicability to cases with modification or outward movement of buried NCs is still to be tested. In view of the above works we have carried out some studies on SHI induced modifications of embedded spherical Au NCs in silica glass. Metal NCs embedded in glass have stimulated the interest of scientists because of their linear and non-linear optical properties [11]. The embedded spherical Au NCs were produced in silica glass via a high fluence implantation of 32 keV Au ions. In the present work SHI irradiation, using 100 MeV Au8+ ions, has been used for modifications of these NCs. For lower irradiation fluence, our results indicate an out flow and growth of Au NCs to larger sizes in the near surface region. Further increase in the irradiation fluence causes a shape deformation of the spherical clusters with the Au concentration shifting towards the surface. This has also been found to result in Au loss from an enhanced sputtering.
using a uniformly scanned 32 keV Au beam on the sample surface. All the implantations were performed at normal incidence in a chamber maintained at a base pressure of 7 · 10 7 mbar. Here after all the above Au implanted samples will be referred as ‘‘as-implanted’’ samples. One of the as-implanted samples was annealed at 850 C for 1 h in air (using a conventional quartz furnace) for a further growth of Au NCs in the matrix. Three of the as-implanted samples were irradiated with SHI in the form of 100 MeV Au8+ ions with fluence of 5 · 1012, 2 · 1013 and 1 · 1014 ions cm 2, respectively. The annealed sample was also irradiated with the same beam at a fluence of 1 · 1014 ions cm 2. All the SHI irradiations were carried out using the 15 MV Pelletron accelerator facility at the Inter University Accelerator Centre, New Delhi. During the SHI irradiation the ion beam was magnetically scanned over sample surface for a uniform exposure to ion irradiation. The samples were mounted on a copper target holder, cooled to around 100 K using liquid nitrogen. The chamber pressure during irradiation was around 5 · 10 6 mbar. Following the SHI irradiation, the samples were characterized by optical absorption (OA) spectroscopy, Rutherford backscattering spectrometry (RBS) and transmission electron microscopy (TEM). The Au concentration profiles were measured by RBS with 3 MeV a particles using the 3 MV Pelletron accelerator facility at IOP. Cross-sectional TEM (X-TEM) measurements were also carried out at IOP with a 200 kV (JEOL 2010) high resolution transmission electron microscope with a point to point resolution of 0.19 nm. The OA spectra were recorded in the transmission mode with a pure silica glass in the reference line using a Shimadzu PC3101 UV–VIS–NIR dual beam spectrophotometer. 3. Results Results of OA measurements carried out on an asimplanted sample before and after annealing at 850 C
2. Experiment Ion beam synthesis is an established way to produce embedded NCs, specially for that of noble metals [11–13]. We have used this technique to produce the Au NCs. For this purpose, high purity silica glass samples (area 1 cm · 1 cm), were implanted with 4 · 1016 Au cm 2 using a low energy negative ion implanter facility at the Institute of Physics (IOP), Bhubaneswar. The above Au implantations for the NCs synthesis were carried out at room temperature
Fig. 1. OA spectra of an as-implanted (32 keV 4 · 1016 Au cm 2 implanted), silica sample measured before (open circle) and after annealing in air at 850 C for 1 h (filled circle).
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a
Fig. 2. X-TEM image of as-implanted (32 keV 4 · 1016 Au cm 2 implanted) silica sample together with the SAED pattern from the Au NC region. Dotted line in the X-TEM image indicates the sample surface.
for 1 h are shown in Fig. 1. The spectra show a surface plasmon resonance (SPR) peak around 540 nm corresponding to embedded Au NCs in the silica matrix. Compared to the as-implanted sample, the annealed sample shows an enhancement in the SPR peak intensity with a reduced full width at half maximum (FWHM), which is an indication of the growth of NCs to larger sizes. The FWHM of the SPR for the as-implanted sample was 102 nm, which reduced to 78 nm after annealing. In an earlier study on a similar system (high fluence Au implanted into an oxidized Si matrix), we had seen a similar growth in size of Au NCs after high temperature annealing [12]. TEM measurements carried out on the as-implanted sample clearly showed Au NCs formation confirming the OA results. A typical X-TEM micrograph for the above sample (Fig. 2) shows 1–6 nm size Au clusters with almost spherical shape to be present in the silica matrix close to the surface. NCs are clearly seen up to a depth of 40 nm from the surface. A selected area electron diffraction (SAED) pattern from these NCs (Fig. 2) shows a ring pattern with dots indicating the crystalline nature of the NCs. The appearance of an SPR peak 540 nm in the OA spectrum (Fig. 1), is clearly due to these NCs shown in Fig. 2. The variation of the Au concentration in the samples at various SHI irradiation fluence were determined through an analysis of RBS data taken on the samples. In Fig. 3(a) we show some typical RBS spectra corresponding to an as-implanted sample and a sample irradiated with SHI at a fluence of 1 · 1014 ions cm 2. In Fig. 3(b) we show the variation of Au content inside the silica matrix (as determined from the RBS measurements), as a function of the SHI irradiation fluence. As can be seen from the figure, there was a gradual decrease in the amount of Au present in the silica matrix with increase in the irradiation fluence. For a SHI irradiation fluence of 1 · 1014 ions cm 2 it was found that almost 80% of the Au earlier present in the as-implanted sample was lost. In Fig. 4 we show the OA spectra corresponding to the SHI irradiated samples together with the same for the as-implanted sample. Different spectra are shifted in Y-axis for clarity in presentation. Comparing the spectra for the as-implanted and the irradiated sample for a SHI fluence of 5 · 1012 ions cm 2, we can clearly see, the later has a
b
Fig. 3. RBS spectra of Au as-implanted sample together with the same irradiated with 100 MeV Au8+ for fluence 1 · 1014 ions cm 2 (a) and reduction in the retained Au content in silica matrix as a function of 100 MeV Au8+ irradiation fluence (b).
Fig. 4. OA spectra of the SHI irradiated samples. The irradiation fluence in each case is shown as legends in the figure. The spectrum for the asimplanted sample is also included in the figure for comparison.
more intense SPR peak with slightly lower FWHM. These results are quite similar to those presented in Fig. 1 which also shows a growth in NC size induced by annealing. It is also important to note, from RBS data (Fig. 3(b)), that
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there is not much reduction in the Au content in the matrix at this fluence of irradiation. These observations imply that NCs in the as-implanted samples grow either on annealing or on irradiation at a fluence of 5 · 1012 ions cm 2. At a SHI irradiation fluence of 2 · 1013 ions cm 2, there was a broadening of the SPR peak (FWHM 110 nm) with a slight redshift in its position. In this case, using a careful fit, the SPR peak position was found to be red-shifted by about 5 nm (to a position 549 nm) as compared to the as-implanted sample. Such an increase in FWHM of the SPR peak and the redshift in its position is known to be due to an increase in the size of the NCs [14]. However, the OA spectrum for a SHI irradiation fluence of 1 · 1014 ions cm 2 has been found to be almost flat (filled circles in Fig. 4). This is primarily due to a much reduced volume fraction of the Au NCs in the matrix as can be seen in Fig. 3(b). In order to have a look at the micro-structure of the Au NCs after SHI irradiation, X-TEM measurements were carried out on samples irradiated with 2 · 1013 and 1 · 1014 ions cm 2. One such X-TEM micrograph for the sample with SHI fluence of 2 · 1013 ions cm 2 is shown in Fig. 5. Comparing this image with that shown in Fig. 2, one can clearly see a growth in the near surface NCs. One can also observe a reduction in the width of the Au NCs layer. Width of the Au NCs layer in this case is around 25 nm. The observed red-shift in the SPR peak for this sample is due to the growth of Au NCs to larger sizes. As mentioned earlier, the OA spectrum for a sample with a SHI irradiation fluence of 1 · 1014 Au ions cm 2, as shown in Fig. 4, looks almost flat. A magnified view of the same spectrum is shown in Fig. 6. The figure shows two peak-like structures, one near 525 nm and the other near 600 nm. A similar OA spectrum has also been obtained for the annealed sample after SHI irradiation to an identical fluence. This has also been included in Fig. 6 for comparison. An X-TEM micrograph corresponding
Fig. 5. X-TEM image of Au as-implanted sample irradiated with 100 MeV Au8+ for a fluence of 2 · 1013 ions cm 2. The dotted line in the X-TEM image indicates the surface.
Fig. 6. OA spectrum measured at an Au irradiation fluence of 1 · 1014 ions cm 2 (open circles). Filled circles correspond to that of the annealed sample as shown in Fig. 1.
Fig. 7. X-TEM image of Au as-implanted sample irradiated with 100 MeV Au8+ for a fluence of 1 · 1014 ions cm 2. Dotted line in the figure indicates the surface. The inset shows an SAED pattern taken on a single large particle.
to the SHI irradiated, unannealed sample, is shown in Fig. 7. One can clearly see, after a high dose SHI irradiation (1 · 1014 ions cm 2) there were only a few isolated Au particles in the system corresponding to about 20% of the total initial Au content in the as-implanted samples. But the observed particles have been found to be clearly larger than the Au NCs seen in the samples irradiated at a lower fluence. A SAED pattern obtained on one such particle is also included in Fig. 7. The figure shows diffraction spots which correspond to the fcc Au structure. The X-TEM image taken on the sample with the highest SHI irradiation fluence clearly shows that the Au NCs were no longer spherical in shape. There has been an earlier SHI irradiation study on Au-core-silica-shell particles using 30 MeV Cu ions with a fluence of 2 · 1014 ions cm 2. The measured optical extinction spectrum, showed a broad peak in the long-wavelength region (600–650 nm) with the main SPR peak slightly blue-shifted to 521 nm [4]. It must be mentioned that the extinction spectrum with very
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small contribution from scattering effects from NCs, is almost the same as that obtained from absorption measurements. The feature viz a blue shift of main SPR peak and a broad hump in the long wavelength region, in the absorption spectrum is very clearly seen in Fig. 6. The above two peak structure in OA spectrum has been shown to be a characteristic of elongated Au NCs [14]. The OA spectrum for the annealed sample, after SHI irradiation with a fluence of 1 · 1014 ions cm 2 (Fig. 6), also shows identical features. Based on this, even without any X-TEM data, we conclude that the sample contained elongated Au NCs as in case of the highest fluence SHI irradiated sample. 4. Discussions As pointed out in [1], the Se threshold for track formation in SiO2 is 1 keV/nm and for Se = 8 keV/nm, latent tracks, 3 nm in radius, could be formed. From a SRIM calculation [15] one can see that the electronic energy loss, Se, of 100 MeV Au ions in silica glass is about 13.5 keV/nm which is much higher than that required for creation of a latent track of 3 nm in radius. We thus expect SHI induced tracks much wider than 3 nm to be created in the matrix. In this situation, at a SHI irradiation fluence of 2 · 1013 ions cm 2 overlapping of tracks would occur and NCs embedded in the matrix (in the as-implanted sample) are expected to be affected by more than one SHI. However, at lower irradiation fluence whatever happens is mostly due to single SHI impact only. D’Orle´ans et al. [3] have carried out a systematic study of SHI (with 200 MeV I) irradiation induced effects on ion beam synthesized Co NCs in SiO2 where embedded Co NCs were prepared through high fluence Co implantation at 160 keV at various temperatures. SHI irradiations of samples prepared at 295 K and 77 K, has been found to result only in a growth in size of NCs, from 3–4 nm to double the size, with increase in SHI fluence up to 1 · 1014 ions cm 2. However, for samples prepared at 873 K, with initial average size of NCs 10 nm, there was growth in NC size with fluence, up to a fluence of 5 · 1012 ions cm 2. Increase in SHI fluence to 1 · 1013 ions cm 2, resulted in modifications in shape of some of the NCs. At a SHI fluence of 1 · 1014 ions cm 2, there was a drastic change in shape of NCs which were found to be elongated along the direction of the propagation of ions. Most importantly, the total Co content in the matrix was found to remain intact without any out diffusion or sputtering. The Se values, calculated using SRIM [15], are almost similar for both 100 MeV Au and 200 MeV I in SiO2. However, in contrast with the above we did not observe any significant deformation of the Au NCs even at an irradiation fluence of 2 · 1013 ions cm 2 although there was an outward movement and loss of Au (Fig. 3(b) and Fig. 6). In fact the outward movement of Au with increase in irradiation fluence, as observed in the present case, is quite similar to that observed in case of a Au marker layer in NiO under SHI (230 MeV Xe) irradiation [8]. One can
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see, Se for 100 MeV Au is slightly smaller than Se for 230 MeV Xe in NiO. Even then there was a lot of Au ‘‘sputtering’’ in the present case quite contrary to what has been found with 230 MeV Xe irradiation of embedded Au NCs in NiO. This shows nuclear stopping also plays a dominant role even though it has a three orders lower value. This goes in line with the findings of Mieskes et al. [10] who have shown that a synergy between Se and Sn can play a significant role resulting in enhanced sputtering within the thermal spike model. It is quite interesting to look into the nature of the transport of Au to the surface and its eventual loss from the matrix. The linear variation of the amount of Au lost from the matrix as a function of SHI irradiation fluence (Fig. 3(b)), clearly rules out diffusion as the dominating transport mechanism. As suggested by Bolse and Wiesner [8], a SHI may produce a molten track. Then because of the density reduction in the molten phase, a high pressure will be present which, in a continuous track, may squeeze the melt along the track axis towards the pressure free surface. This can lead to the observed out-flow of the Au and its eventual loss from the silica matrix. Fig. 7 also shows a flaring type feature on the outer face of the Au ‘‘nano droplets’’ which seem to suggest a flowing out tendency at the surface. In their SHI irradiation study involving Co nanoparticles, D’Orle´ans et al. [2] have suggested that the mechanisms involved in the track formation could also be responsible for the SHI induced anisotropic growth of NCs. Using the thermal spike model, they have shown that under SHI irradiation there can be a growth in size of small NCs mainly through fragmentation and ripening until the NCs reach a critical diameter. After this the initial temperature inside the nanoparticle is lower than the temperature at the center of the ion track in the SiO2 matrix. This causes differences in volume expansion and compressibility, due to which there could be a large over pressure in the molten NCs leading to a deformation. In the present case, at a SHI irradiation fluence of 2 · 1013 ions cm 2, there would be spatially overlapping but temporally separated ion tracks which effectively produce a latent tracks larger than 3 nm radius. In these, evaporation of small NCs through SHI impact would result in the growth of larger NCs. However, at still higher fluence, the larger NCs would melt resulting in a change in density and volume. In that case, as suggested earlier, the molten material can be squeezed out to the surface. This would result in deformation in shape associated with outward motion. It is also important to mention here that the contribution from elastic (nuclear) interactions is dependent on the track diameter. In case the track diameter is small and the energy loss from elastic interactions is confined to this region then molten NCs can get elongated during the thermal spike formation. This does not seem to have happened for Au NCs in silica glass matrix. In addition, the observed movement of the Au NCs along latent tracks created by the SHI cannot be understood within the framework of a thermal spike model
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which does not include density and pressure effects [10]. One needs to consider the pressure pulse coming from fast density changes in the ion track. The shape of the Au NCs in case of a SHI irradiation fluence of 1 · 1014 ions cm 2 (Fig. 7), is indicative of a pressure spike induced push out of a molten metal NC close to the surface. 5. Conclusions We have studied the effect of 100 MeV Au8+ irradiation on embedded Au NCs of 1–6 nm, produced by implantation of 4 · 1016 32 keV Au cm 2 into silica glass. For an irradiation fluence of 5 · 1012 ions cm 2, a growth in size of the Au NCs has been observed. Increasing the irradiation fluence to 2 · 1013 ions cm 2, resulted in a significant growth in size along with an outward movement of NCs. At this irradiation fluence, about 15% of the Au content was found to have been sputtered out from the matrix. Increasing the irradiation fluence to 1 · 1014 ions cm 2, the amount of Au retained in the silica matrix dropped to about 20% of its initial value. There were only a few isolated larger Au NCs with non-spherical shapes situated close to the surface. Changing the initial size distribution of NCs in the starting sample by annealing did not have any marked effect on the final state of the Au NCs. Present results on the growth in size of the Au NCs for lower irradiation fluence and their deformation from spherical shape at higher fluence are in line with the thermal spike model. Acknowledgements Authors wish to thank D. Kanjilal and A. Tripathi for important suggestions and helps during the swift heavy ion irradiation. We also thank S.N. Sahu and S. Sarangi for the optical absorption measurements and all the opera-
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