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SiO2 interface by 10 MeV Si3+ irradiation
Optical Materials 35 (2013) 1315–1319
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Au nanoparticles formation in ZnO/SiO2 interface by 10 MeV Si3+ irradiation D.R. Hernández-Socorro ⇑, L. Rodríguez-Fernández, H.G. Silva Pereyra Instituto de Física, Universidad Nacional Autónoma de México, Ciudad Universitaria, México, Coyoacan, D.F. 04510, Mexico
a r t i c l e
i n f o
Article history: Received 6 May 2012 Received in revised form 31 December 2012 Accepted 5 January 2013 Available online 25 March 2013 Keywords: ZnO SiO2 SPR Optical properties Au nanoparticles
a b s t r a c t This research proposes a method to obtain Au nanoparticles (NPs) at a ZnO/SiO2 interface. Initially a well defined Au thin film was grew between of ZnO film and SiO2 glass layers. Post-treatment of 10 MeV Si3+ irradiation or/and thermal annealing treatments were used. Samples were analyzed by Optical Absorption Spectroscopy, Rutherford Backscattering Spectrometry, X-Ray Diffraction and Transmission Electron Microscopy. Results showed the pulverization of initial Au thin film and the formation of NPs in both sides of ZnO/SiO2 interface and in between. In general the formations of Au NPs sizes inside both matrices were between 3 and 8 nm. Last thermal treatment was important because caused the ZnO film recovery from irradiation damage. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The starting point of ion induced radiation damage studies was aged 50 years ago with the advent of nuclear energy. Most of interest in this early period had been devoted to the modification of metallic compounds under neutron irradiation. Later the use of implantation for doping semiconductors extended massively. Finally the formation of new phases by ion irradiation, ion implantation, atomic mixing of multilayers and by dynamic mixing during implantation or deposition, constituted an active research area [1]. The inclusion of metal nanoparticles (NPs) embedded in transparent semiconductor and dielectric matrices have many different important applications due to the new optical, electrical and magnetically properties that nanostructure materials can offer in these matrices. The applications of these properties have special interest in opto-electronics field [2–5]. They have been meaningful as very promising materials for sensor application, for example as NH3 sensor [6]. Another sample is the optical nonlinearities providing for Au nanoparticles embedded in these kinds’ matrices [7]. For example, a material with positive sign of c can be used for soliton formation in optical waveguides [7,8] and a material characterized by a negative sign of c and a low efficiency of nonlinear absorption can be applied as an optical limiter of laser radiation due to the divergence produced by self-defocusing [9]. This paper presents the mixing of interfaces by two treatments: annealing and ion beam. As resulted of these treatments, we obtained metallic nanoparticles formation inside semiconductor and insulator matrix. The use of annealing treatment for this ⇑ Corresponding author. Tel.: +52 55 56225160; fax: +52 55 56225009. E-mail address: [email protected] (D.R. Hernández-Socorro). 0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.01.004
purpose is very well known [10], however; the use of ion beam or the combination of both method for modification in solids, thin films, surface and interfaces is novel and actual in the nanoscience field. In recent years, the injection of material into a target specimen in the form of an accelerated ion beam offers a most valuable tool for altering its physical properties in a controlled manner [1,2]. This is particularly important for substances as semiconductors and insulator. In the semiconductor the electrical behavior is determined by extremely small concentrations of certain impurities and in the insulator are possible observing large optical nonlinearity [11], ultrafast response [12], and single electron transport [13]. Most of the research conducted today in this topic has been focus on the metal/Si, metal/metal, metal–oxide/Si, metal/ceramic systems, while a lacks of investigation in insulator/metal/semiconductor systems is present [14–16]. In this investigation a multilayer systems with form SiO2/Au/ ZnO was irradiated with 10 MeV Si3+ ions. This kind of ion and energy were chosen because they do not produce a large ballistic damage in the films and a small surface sputtering effect is expected. Also, Si ions are finally implanted far away from the ZnO/ SiO2 interface. Previous researches with low dose of Si ions implanted in SiO2-glass do not show evidence of effects in light absorption properties of glass [17]. Post-thermal treatments are applied in irradiated multi-layers systems in order to generate Au NPs nucleation in the interface region. 2. Experimental detail Fig. 1 show samples preparation process to generate Au NPs at interface of ZnO film and SiO2-glass substrate. Au thin films were deposited on clean SiO2 substrates. The technique used was direct
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3. Results and discussions 3.1. Optical Absorption Spectroscopy Fig. 2 shows OAS spectrum obtained for three families of samples with initial Au layer thickness (a-1.6 nm, b-2.9 nm, and c3.8 nm) and each family with their four treatments G1–G4. All samples showed declines at around 400 nm corresponding with a
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current (DC) magnetron sputtering with a 99.9% purity Au target. Three different Au film thicknesses: 1.6 nm, 2.9 nm and 3.8 nm were grown under a constant deposition rate of 3 ± 0.4 Å/s. In addition, a 160 nm of ZnO thin film was deposited over an Au layer. The technique used was radio frequency (RF) magnetron sputtering with a 117 W of power and a constant deposition rate of 1.34 ± 0.07 Å/s [18]. The target was ZnO with 99.99% purity located at a distance of 15 cm to samples. After deposition, samples were thermally annealed (TA) at 773.15 K in air for 1 h and irradiated by 10 MeV Si3+ ions. After irradiation, selected samples received a second TA treatment under same conditions. TA was carried out in a Thermolyne furnace model 1400. Irradiations were conducted at a fluence of 1.0 1016 Si3+ ions/cm2 using the 3 MV Pelletron tandem accelerator at Physics Institute, UNAM. According to SRIM (Stopping and Range of Ions in Matter) the Si ions implanted in the glass were located at 4 lm beneath Au thin film [19]. Four groups of samples were obtained for each thickness of Au. The treatments performed were as-growth (G1), as-growth plus TA (G2), as-growth plus TA plus irradiation (G3), and asgrowth plus TA plus irradiation plus TA (G4). Samples were characterized using Optical Absorption Spectroscopy (OAS), Rutherford Backscattering Spectrometry (RBS), X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). UV–Vis optical absorption spectra (range 350–800 nm) were obtained with a Varian Cary 5000 UV–Vis–IR spectrophotometer with double beam. Structural properties of films were analyzed using XRD in Bruker Advance D-8 equipment using Cu Ka radiation. Data were obtained through 2h configuration (Bragg–Brentano geometry) between 30° and 50° with a step of 0.02°. RBS measurements were performed at Pelletron accelerator described above and using 2 MeV 4He2+ ion beams. Backscattered ions were registered by a detector located at angle of 165°. RBS spectra were analyzed using RUMP (Rutherford Universal Manipulation Program) simulation software [20,21]. The TEM characterization were carried out using JEOL 2010FasTem type FEG transmission electron microscope operating at 200 kV (point-to-point resolution 1.9 Å).
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Fig. 1. Schematic diagram illustrating of Au NPs formation process at interface between ZnO film and SiO2 substrate.
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Wavelength (nm) Fig. 2. OAS results of each family of samples with initial Au layer thickness: (a) 1.6 nm, (b) 2.9 nm, and (c) 3.8 nm.
light absorption of SiO2 substrate and ZnO thin film. As-growth samples (circles-dashed in Fig. 2G1) presented an absorption increment like a hump in range 550–800 nm, which is due to absorption in a simple Au layer. These humps increases directly proportional to initial thickness of Au film as expected. After first thermal annealing (continuous-line in Fig. 2G2) this hump disappears and new peaks at around 600 nm are shown. These peaks are related with formation of Au NPs in ZnO thin films. Indeed, Mishra et al. reported SPR band at red-shift; which was associated with Au
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r¼
mf Dw1=2
ð1Þ
where mf is electron velocity corresponding to Fermi energy of metal, (for Au mf = 1.39 108 cm s1), and Dw1=2 ¼ 2pcðDk=k2p Þ. Where Dk is the full width at half maximum (FWHM) wavelength, and kp is the peak wavelength for the SPR. From optical absorption spectrum is possible to determinate both. The kp depends of metal in NPs formation and Dk is related with the size of NPs. As a result of this approximation, Au NPs sizes into ZnO matrix were between 4 to 8 nm and into SiO2 glass ranged from 3 to 6 nm.
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NPs in ZnO matrix for annealing treatment in range of 473.15– 873.15 K [22]. This fact confirms that TA treatments allow diffusion of initial Au thin films into a ZnO film, (Fig. 1G2). Also, TA treatments by themselves are unable to induce Au diffusion into a SiO2 substrate. The Si3+ ion irradiation processes resulted in reduction of peak at around 600 nm and a new band showed around 522 nm. This new band is associated with SPR of Au NPs in SiO2 (dashed-lines in Fig. 2G3c) [23]. Ion irradiation leads to diffusion of Au into SiO2 substrate with subsequent formation of NPs. It is well known that ion-irradiation damage creates defects and causes changes in its optical response during irradiation process. Fukuoka et al. and Kaschny et al. reported evidence that non-equilibrium dynamics along the ion track creates conditions that promote diffusion [24,25]. These changes could be from different type and can be treated like a statistical process along the Si ion track. SPR located at 520 nm with previous treatment was reported by Ila et. al. [26]. Post-irradiation with few MeV of He and B ions of SiO2 matrix implanted with Au induced formation of Au NPs in SiO2 matrix. The second TA treatment (short-dashed-lines in Fig. 2G4) caused NPs nucleation in both sides of the interface. This effect is more notorious for the thicker layer of initial Au film (Fig. 2G4c). The overlap of two peaks suggests the presence of Au NPs in both matrices. Previous publications demonstrated that optical properties have strong dependency on the size and geometry of NPs formation [22,24,26]. Early in the last century, Maxwell’s equations were used to forecast absorption of an electromagnetic wave which travels through a solid matrix with suspending metal spheres inside. Therefore, different authors estimate precipitate size from broadening of SPR according with Mie theory [2,27,28]. This theory considers spherical shape for metallic NPs inside the matrix. The average radius r could be calculated according to Doyle’s formula [29]:
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Energy (keV) Fig. 3. (a) Typical RBS spectrum obtained. The arrows indicate location corresponding to O and Au on surface. (b) RBS spectra for a sample family with initial Au film thickness of 2.9 nm (G1–G4).
result confirms the formation of Au NPs by diffusion into ZnO layer showed in OAS. The Si3+ ion irradiation increases the width of Au peak (solidcircles-dashed in Fig. 3b – G3) along with a slight movement to left, therefore; Au migration to glass substrate is confirmed. The second TA treatment (continuous lines in Fig. 3b – G4) moves the Au peak to the initial position. In the last two cases the Au could be in both sides of the interface. From the spectra simulations the slope wide at the left side of the Zn signal is associated to Au diffusion. Similar situation occurs at the slopes corresponding to silicon in glass where Au mixes in the substrate without the presence of Zn. These simulations indicate the formation of well defined interface between glass and ZnO film with low roughness. 3.3. Transmission Electron Microscopy For Transmission Electron Microscopy technique was possible confirm result discussed above. Fig. 4 shows a TEM micrograph of Au NPs embedded in ZnO matrix for sample with thermal annealing treatment (G2) with initial Au layer thickness of 1.6 nm. In this figure black spots represent the Au particles suspended in the ZnO matrix and a corresponding histogram. In the case of the histogram most of Au NPs were in the range of 4– 9 nm in radio and the average radio was 6.61 ± 1.26 nm.
3.2. Rutherford Backscattering Spectrometry 3.4. X-Ray Diffraction Fig. 3a shows a typical RBS spectrum obtained for multilayer system SiO2/Au/ZnO. In addition, energy position for backscattered particles on surface by O, Zn and Au was added. Fig. 3b shows energy range for which the zinc and gold have signal. This case is about a family of samples with initial 2.9 nm Au film and all treatments from G1 to G4. Narrow right peaks correspond to particles backscattered by Au and with tooth shapes by Zn. Spectra analyses indicated that certain amount of Zn and Au remain constant under all treatments. This is an indication that under applied irradiation conditions (ion, energy and fluence) the sputtering effect is negligible. As-growth samples (dash-dot line in Fig. 3b – G1) are indicating well-defined layers. After first TA (open-squares-dashed in Fig. 3b – G2) the Au peak reduces height, becomes wider (larger FWHM) and moves slightly to the right while the fall amplitude at left side of Zn signal becomes wider. This fact showed Au diffusion into the ZnO matrix and thus, the well-defined Au film disappearance. This
Structural evolutions of samples are reflected in XRD patterns as shown in Fig. 5. Preferential orientation of peaks associated with ZnO was found for 2h region around 30°–40°. Here the most intense peak has (0 0 2) orientation in tabulated position (JCPD No. 36-1451) with characteristic of hexagonal lattice with a = 3.24 Å and c = 5.20661 Å. A reduced diffraction of Au is observed in as-growth samples (continuous lines in Fig. 4 – G1). Nevertheless, after first TA (dotdashed in Fig. 4 – G2) the Au diffusion into the ZnO matrix causes NPs formation. Furthermore, the crystallization of Au generates a preferential orientation (1 1 1). This peak is associate with FCC crystalline structure in tabulated position (JCPD No. 04-0784), where a = 4.078 Å. Same structure was reported for Van Huis et al. for Au NPs in MgO matrix [2]. After irradiation (solid-star-dashed line in Fig. 4 – G3) appears new peaks for ZnO associated with plane (1 0 2) and (1 1 0). This
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is a consequence of the privileged orientation induced in the ZnO grain due to structural damage by the ion movement along the film, which permits the emigration of Au from the ZnO layer to SiO2 substrate. The crystallinity of ZnO was kept well under TA or irradiation treatment, which is considered large in comparison with other semiconductors. Also, for the irradiation treatment is more evident the formation of metal Au at preferential orientation (1 1 1). The last TA (dash-dot line in Fig. 4 – G4) generates recovery of irradiation damage in ZnO according to disappearance of peaks oriented (1 0 2) and (1 1 0), while the peak associated to Au remains. This fact confirms the formation of NPs in the samples. 4. Conclusions Post-treatments Au thin film between a semiconductor ZnO film and SiO2 glass substrate is a good quality method to obtain Au NPs in ZnO/SiO2 interface. First TA only facilitates formation of Au NPs in ZnO, which does not take place for glass substrate. The Si3+ ion irradiation produces an enhancement of Au diffusion in the glass allowing the formation of Au NPs in both sides of the interface. ZnO films recover from irradiation damage with the last TA but maintain Au NPs. All the characterizations techniques suggested the formation of nanoparticles in the matrices in study. Small discrepancy in the size of the nanoparticles was found dependent on the method used but in all cases were reported. Further studies are required in order to introduce this procedure as an accurate method to synthesize interfaces metallic NPs. These researches should focus on the relation between metal thin film thickness in multilayer system and Si ion energy. Another important aspect to take into account should be the subsequent TA treatment conditions.
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Particle size (nm) Fig. 4. Transmission electron micrograph of Au NPs embedded in ZnO matrix for sample G2 and corresponding histogram.
The authors thank to F.J. Jaimes and K. López for their collaboration and operation of the Pelletron accelerator at Physic Institute, UNAM. We also want to thank J.M. Hernandez, E. Camarillo, M. Aguilar, H. Cruz-Majarrez, and L. Flores for they support during the grown and characterization process. This work was supported by DGAPA-UNAM under Contract PAPIIT-IN109910 and ICYT-DF under Contract PICCO 10-75. References
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[1] I.P. Jain, G. Agarwal, Surf. Sci. Rep. 66 (2011) 77–172. [2] M.A. van Huis, A.V. Fedorov, A. van Veen, C.V. Falub, S.W.H. Eijt, B.J. Kooi, J.Th.M. De Hosson, T. Hibma, R.L. Zimmerman, Nucl. Instrum. Methods B 191 (2002) 442. [3] T. Karali, N. Can, L. Valberg, A.L. Stepanov, P.D. Townsend, Ch. Buchal, R.A. Ganeev, A.I. Ryasnyansky, H.G. Belik, M.L. Jessett, C. On, Physica B 363 (2005) 88. [4] A.I. Ryasnyansky, B. Palpant, S. Debrus, U. Pal, A.L. Stepanov, Opt. Commun. 273 (2007) 538–543. [5] A.L. Stepanov, R.I. Khaibullin, N. Can, R.A. Ganeev, A.I. Ryasnyansky, C. Buchal, S. Uysal, Tech. Phys. Lett. 30 (2004) 846. [6] T. Pustelny, E. Maciak, Z. Opilski, A. Piotrowska, E. Papis, K. Go laszewska, Eur. Phys. J. Special Topics 154 (2008) 165–170. [7] Y.S. Kivshar, W. Kro´ likowski, O.A. Chubukalo, Phys. Rev. E 50 (1994) 5020. [8] H.S. Eisenberg, Y. Silberberg, R. Morandotti, A.R. Boyd, J.S. Aitchison, Phys. Rev. Lett. 81 (1998) 3383. [9] B.L. Justus, A.L. Huston, A.J. Campillo, Appl. Phys. Lett. 63 (1993) 1483. [10] C. Xu, W.S. Oh, G. Liu, D.Y. Kim, D.W. Goodman, J. Vac. Sci. Technol. A 15 (3) (1997). [11] Y. Takeda, V.T. Gritsyna, N. Umeda, C.G. Lee, N. Kishimoto, Nucl. Instrum. Methods B 148 (1999) 1029. [12] Y. Takeda, J.P. Zhao, C.G. Lee, V.T. Gritsyna, N. Kishimoto, Nucl. Instrum. Methods B 166–167 (2000) 877. [13] H. Amekura, N. Umeda, Y. Takeda, N. Kishimoto, H. Nejo, in: K. Yoshihara (Ed.), Proceedings of the 5th International Symposium on Advanced Physical Fields (APF-5), NRIM, Tsukuba, 2000, p. 203. [14] D.L. Windt, R. Hull, W.K. Waskiewicz, J. Appl. Phys. 71 (1992) 6.
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[23] C. Torres-Torres, A.V. Khomenko, J.C. Cheang-Wong, L. Rodríguez-Fernández, A. Crespo-Sosa, A. Oliver, Opt. Express 15 (2007) 9248. [24] O. Fukuoka, N. Matsunami, M. Tazawa, T. Shimura, M. Sataka, H. Sugai, S. Okayasu, Nucl. Instrum. Methods B 250 (2006) 295. [25] J.R. Kaschny, A. Morrone, A. de Mattos, J.E. Schmidt, M. Behar, Nucl. Instrum. Methods B 218 (2004) 308. [26] D. Ila, Z. Wu, C.C. Smith, D.B. Poker, D.K. Hensley, C. Klatt, S. Kalbitzer, Nucl. Instrum. Methods B 127 (128) (1997) 570. [27] O. Peña, L. Rodríguez-Fernández, J. Roiz, J.C. Cheang-Wong, J. Arenas-Alatorre, A. Crespo-Sosa, A. Oliver, Rev. Mex. Fis. S 53 (2007) 62. [28] R.L. Zimmerman, D. Ila, E.K. Williams, D.B. Poker, D.K. Hensley, C. Klatt, S. Kalbitzer, Nucl. Instrum. Methods B 148 (1999) 1064. [29] G. Mie, Ann. Physik 25 (1908) 216.
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Au nanoparticles formation in ZnO/SiO2 interface by 10 MeV Si3+ irradiation D.R. Hernández-Socorro ⇑, L. Rodríguez-Fernández, H.G. Silva Pereyra Instituto de Física, Universidad Nacional Autónoma de México, Ciudad Universitaria, México, Coyoacan, D.F. 04510, Mexico
a r t i c l e
i n f o
Article history: Received 6 May 2012 Received in revised form 31 December 2012 Accepted 5 January 2013 Available online 25 March 2013 Keywords: ZnO SiO2 SPR Optical properties Au nanoparticles
a b s t r a c t This research proposes a method to obtain Au nanoparticles (NPs) at a ZnO/SiO2 interface. Initially a well defined Au thin film was grew between of ZnO film and SiO2 glass layers. Post-treatment of 10 MeV Si3+ irradiation or/and thermal annealing treatments were used. Samples were analyzed by Optical Absorption Spectroscopy, Rutherford Backscattering Spectrometry, X-Ray Diffraction and Transmission Electron Microscopy. Results showed the pulverization of initial Au thin film and the formation of NPs in both sides of ZnO/SiO2 interface and in between. In general the formations of Au NPs sizes inside both matrices were between 3 and 8 nm. Last thermal treatment was important because caused the ZnO film recovery from irradiation damage. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The starting point of ion induced radiation damage studies was aged 50 years ago with the advent of nuclear energy. Most of interest in this early period had been devoted to the modification of metallic compounds under neutron irradiation. Later the use of implantation for doping semiconductors extended massively. Finally the formation of new phases by ion irradiation, ion implantation, atomic mixing of multilayers and by dynamic mixing during implantation or deposition, constituted an active research area [1]. The inclusion of metal nanoparticles (NPs) embedded in transparent semiconductor and dielectric matrices have many different important applications due to the new optical, electrical and magnetically properties that nanostructure materials can offer in these matrices. The applications of these properties have special interest in opto-electronics field [2–5]. They have been meaningful as very promising materials for sensor application, for example as NH3 sensor [6]. Another sample is the optical nonlinearities providing for Au nanoparticles embedded in these kinds’ matrices [7]. For example, a material with positive sign of c can be used for soliton formation in optical waveguides [7,8] and a material characterized by a negative sign of c and a low efficiency of nonlinear absorption can be applied as an optical limiter of laser radiation due to the divergence produced by self-defocusing [9]. This paper presents the mixing of interfaces by two treatments: annealing and ion beam. As resulted of these treatments, we obtained metallic nanoparticles formation inside semiconductor and insulator matrix. The use of annealing treatment for this ⇑ Corresponding author. Tel.: +52 55 56225160; fax: +52 55 56225009. E-mail address: [email protected] (D.R. Hernández-Socorro). 0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.01.004
purpose is very well known [10], however; the use of ion beam or the combination of both method for modification in solids, thin films, surface and interfaces is novel and actual in the nanoscience field. In recent years, the injection of material into a target specimen in the form of an accelerated ion beam offers a most valuable tool for altering its physical properties in a controlled manner [1,2]. This is particularly important for substances as semiconductors and insulator. In the semiconductor the electrical behavior is determined by extremely small concentrations of certain impurities and in the insulator are possible observing large optical nonlinearity [11], ultrafast response [12], and single electron transport [13]. Most of the research conducted today in this topic has been focus on the metal/Si, metal/metal, metal–oxide/Si, metal/ceramic systems, while a lacks of investigation in insulator/metal/semiconductor systems is present [14–16]. In this investigation a multilayer systems with form SiO2/Au/ ZnO was irradiated with 10 MeV Si3+ ions. This kind of ion and energy were chosen because they do not produce a large ballistic damage in the films and a small surface sputtering effect is expected. Also, Si ions are finally implanted far away from the ZnO/ SiO2 interface. Previous researches with low dose of Si ions implanted in SiO2-glass do not show evidence of effects in light absorption properties of glass [17]. Post-thermal treatments are applied in irradiated multi-layers systems in order to generate Au NPs nucleation in the interface region. 2. Experimental detail Fig. 1 show samples preparation process to generate Au NPs at interface of ZnO film and SiO2-glass substrate. Au thin films were deposited on clean SiO2 substrates. The technique used was direct
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3. Results and discussions 3.1. Optical Absorption Spectroscopy Fig. 2 shows OAS spectrum obtained for three families of samples with initial Au layer thickness (a-1.6 nm, b-2.9 nm, and c3.8 nm) and each family with their four treatments G1–G4. All samples showed declines at around 400 nm corresponding with a
G1 G2 G3 G4
0.3
Intensity (a.u.)
current (DC) magnetron sputtering with a 99.9% purity Au target. Three different Au film thicknesses: 1.6 nm, 2.9 nm and 3.8 nm were grown under a constant deposition rate of 3 ± 0.4 Å/s. In addition, a 160 nm of ZnO thin film was deposited over an Au layer. The technique used was radio frequency (RF) magnetron sputtering with a 117 W of power and a constant deposition rate of 1.34 ± 0.07 Å/s [18]. The target was ZnO with 99.99% purity located at a distance of 15 cm to samples. After deposition, samples were thermally annealed (TA) at 773.15 K in air for 1 h and irradiated by 10 MeV Si3+ ions. After irradiation, selected samples received a second TA treatment under same conditions. TA was carried out in a Thermolyne furnace model 1400. Irradiations were conducted at a fluence of 1.0 1016 Si3+ ions/cm2 using the 3 MV Pelletron tandem accelerator at Physics Institute, UNAM. According to SRIM (Stopping and Range of Ions in Matter) the Si ions implanted in the glass were located at 4 lm beneath Au thin film [19]. Four groups of samples were obtained for each thickness of Au. The treatments performed were as-growth (G1), as-growth plus TA (G2), as-growth plus TA plus irradiation (G3), and asgrowth plus TA plus irradiation plus TA (G4). Samples were characterized using Optical Absorption Spectroscopy (OAS), Rutherford Backscattering Spectrometry (RBS), X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). UV–Vis optical absorption spectra (range 350–800 nm) were obtained with a Varian Cary 5000 UV–Vis–IR spectrophotometer with double beam. Structural properties of films were analyzed using XRD in Bruker Advance D-8 equipment using Cu Ka radiation. Data were obtained through 2h configuration (Bragg–Brentano geometry) between 30° and 50° with a step of 0.02°. RBS measurements were performed at Pelletron accelerator described above and using 2 MeV 4He2+ ion beams. Backscattered ions were registered by a detector located at angle of 165°. RBS spectra were analyzed using RUMP (Rutherford Universal Manipulation Program) simulation software [20,21]. The TEM characterization were carried out using JEOL 2010FasTem type FEG transmission electron microscope operating at 200 kV (point-to-point resolution 1.9 Å).
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Fig. 1. Schematic diagram illustrating of Au NPs formation process at interface between ZnO film and SiO2 substrate.
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800
Wavelength (nm) Fig. 2. OAS results of each family of samples with initial Au layer thickness: (a) 1.6 nm, (b) 2.9 nm, and (c) 3.8 nm.
light absorption of SiO2 substrate and ZnO thin film. As-growth samples (circles-dashed in Fig. 2G1) presented an absorption increment like a hump in range 550–800 nm, which is due to absorption in a simple Au layer. These humps increases directly proportional to initial thickness of Au film as expected. After first thermal annealing (continuous-line in Fig. 2G2) this hump disappears and new peaks at around 600 nm are shown. These peaks are related with formation of Au NPs in ZnO thin films. Indeed, Mishra et al. reported SPR band at red-shift; which was associated with Au
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r¼
mf Dw1=2
ð1Þ
where mf is electron velocity corresponding to Fermi energy of metal, (for Au mf = 1.39 108 cm s1), and Dw1=2 ¼ 2pcðDk=k2p Þ. Where Dk is the full width at half maximum (FWHM) wavelength, and kp is the peak wavelength for the SPR. From optical absorption spectrum is possible to determinate both. The kp depends of metal in NPs formation and Dk is related with the size of NPs. As a result of this approximation, Au NPs sizes into ZnO matrix were between 4 to 8 nm and into SiO2 glass ranged from 3 to 6 nm.
5000 2500
4000 3500
(a)
Zn
2000
Counts
G1 G2 G3 G4
4500
O
1500 1000
Au
500
Counts
NPs in ZnO matrix for annealing treatment in range of 473.15– 873.15 K [22]. This fact confirms that TA treatments allow diffusion of initial Au thin films into a ZnO film, (Fig. 1G2). Also, TA treatments by themselves are unable to induce Au diffusion into a SiO2 substrate. The Si3+ ion irradiation processes resulted in reduction of peak at around 600 nm and a new band showed around 522 nm. This new band is associated with SPR of Au NPs in SiO2 (dashed-lines in Fig. 2G3c) [23]. Ion irradiation leads to diffusion of Au into SiO2 substrate with subsequent formation of NPs. It is well known that ion-irradiation damage creates defects and causes changes in its optical response during irradiation process. Fukuoka et al. and Kaschny et al. reported evidence that non-equilibrium dynamics along the ion track creates conditions that promote diffusion [24,25]. These changes could be from different type and can be treated like a statistical process along the Si ion track. SPR located at 520 nm with previous treatment was reported by Ila et. al. [26]. Post-irradiation with few MeV of He and B ions of SiO2 matrix implanted with Au induced formation of Au NPs in SiO2 matrix. The second TA treatment (short-dashed-lines in Fig. 2G4) caused NPs nucleation in both sides of the interface. This effect is more notorious for the thicker layer of initial Au film (Fig. 2G4c). The overlap of two peaks suggests the presence of Au NPs in both matrices. Previous publications demonstrated that optical properties have strong dependency on the size and geometry of NPs formation [22,24,26]. Early in the last century, Maxwell’s equations were used to forecast absorption of an electromagnetic wave which travels through a solid matrix with suspending metal spheres inside. Therefore, different authors estimate precipitate size from broadening of SPR according with Mie theory [2,27,28]. This theory considers spherical shape for metallic NPs inside the matrix. The average radius r could be calculated according to Doyle’s formula [29]:
3000
0
600
800
1000 1200
1400
1600
1800
Energy (keV)
2500 2000 1500 1000
(b)
500 0 1300
1400
1500
1600
1700
1800
Energy (keV) Fig. 3. (a) Typical RBS spectrum obtained. The arrows indicate location corresponding to O and Au on surface. (b) RBS spectra for a sample family with initial Au film thickness of 2.9 nm (G1–G4).
result confirms the formation of Au NPs by diffusion into ZnO layer showed in OAS. The Si3+ ion irradiation increases the width of Au peak (solidcircles-dashed in Fig. 3b – G3) along with a slight movement to left, therefore; Au migration to glass substrate is confirmed. The second TA treatment (continuous lines in Fig. 3b – G4) moves the Au peak to the initial position. In the last two cases the Au could be in both sides of the interface. From the spectra simulations the slope wide at the left side of the Zn signal is associated to Au diffusion. Similar situation occurs at the slopes corresponding to silicon in glass where Au mixes in the substrate without the presence of Zn. These simulations indicate the formation of well defined interface between glass and ZnO film with low roughness. 3.3. Transmission Electron Microscopy For Transmission Electron Microscopy technique was possible confirm result discussed above. Fig. 4 shows a TEM micrograph of Au NPs embedded in ZnO matrix for sample with thermal annealing treatment (G2) with initial Au layer thickness of 1.6 nm. In this figure black spots represent the Au particles suspended in the ZnO matrix and a corresponding histogram. In the case of the histogram most of Au NPs were in the range of 4– 9 nm in radio and the average radio was 6.61 ± 1.26 nm.
3.2. Rutherford Backscattering Spectrometry 3.4. X-Ray Diffraction Fig. 3a shows a typical RBS spectrum obtained for multilayer system SiO2/Au/ZnO. In addition, energy position for backscattered particles on surface by O, Zn and Au was added. Fig. 3b shows energy range for which the zinc and gold have signal. This case is about a family of samples with initial 2.9 nm Au film and all treatments from G1 to G4. Narrow right peaks correspond to particles backscattered by Au and with tooth shapes by Zn. Spectra analyses indicated that certain amount of Zn and Au remain constant under all treatments. This is an indication that under applied irradiation conditions (ion, energy and fluence) the sputtering effect is negligible. As-growth samples (dash-dot line in Fig. 3b – G1) are indicating well-defined layers. After first TA (open-squares-dashed in Fig. 3b – G2) the Au peak reduces height, becomes wider (larger FWHM) and moves slightly to the right while the fall amplitude at left side of Zn signal becomes wider. This fact showed Au diffusion into the ZnO matrix and thus, the well-defined Au film disappearance. This
Structural evolutions of samples are reflected in XRD patterns as shown in Fig. 5. Preferential orientation of peaks associated with ZnO was found for 2h region around 30°–40°. Here the most intense peak has (0 0 2) orientation in tabulated position (JCPD No. 36-1451) with characteristic of hexagonal lattice with a = 3.24 Å and c = 5.20661 Å. A reduced diffraction of Au is observed in as-growth samples (continuous lines in Fig. 4 – G1). Nevertheless, after first TA (dotdashed in Fig. 4 – G2) the Au diffusion into the ZnO matrix causes NPs formation. Furthermore, the crystallization of Au generates a preferential orientation (1 1 1). This peak is associate with FCC crystalline structure in tabulated position (JCPD No. 04-0784), where a = 4.078 Å. Same structure was reported for Van Huis et al. for Au NPs in MgO matrix [2]. After irradiation (solid-star-dashed line in Fig. 4 – G3) appears new peaks for ZnO associated with plane (1 0 2) and (1 1 0). This
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is a consequence of the privileged orientation induced in the ZnO grain due to structural damage by the ion movement along the film, which permits the emigration of Au from the ZnO layer to SiO2 substrate. The crystallinity of ZnO was kept well under TA or irradiation treatment, which is considered large in comparison with other semiconductors. Also, for the irradiation treatment is more evident the formation of metal Au at preferential orientation (1 1 1). The last TA (dash-dot line in Fig. 4 – G4) generates recovery of irradiation damage in ZnO according to disappearance of peaks oriented (1 0 2) and (1 1 0), while the peak associated to Au remains. This fact confirms the formation of NPs in the samples. 4. Conclusions Post-treatments Au thin film between a semiconductor ZnO film and SiO2 glass substrate is a good quality method to obtain Au NPs in ZnO/SiO2 interface. First TA only facilitates formation of Au NPs in ZnO, which does not take place for glass substrate. The Si3+ ion irradiation produces an enhancement of Au diffusion in the glass allowing the formation of Au NPs in both sides of the interface. ZnO films recover from irradiation damage with the last TA but maintain Au NPs. All the characterizations techniques suggested the formation of nanoparticles in the matrices in study. Small discrepancy in the size of the nanoparticles was found dependent on the method used but in all cases were reported. Further studies are required in order to introduce this procedure as an accurate method to synthesize interfaces metallic NPs. These researches should focus on the relation between metal thin film thickness in multilayer system and Si ion energy. Another important aspect to take into account should be the subsequent TA treatment conditions.
Relative particle number
10
8
6
4
Acknowledgments 2
0 3
4
5
6
7
8
9
10
11
Particle size (nm) Fig. 4. Transmission electron micrograph of Au NPs embedded in ZnO matrix for sample G2 and corresponding histogram.
The authors thank to F.J. Jaimes and K. López for their collaboration and operation of the Pelletron accelerator at Physic Institute, UNAM. We also want to thank J.M. Hernandez, E. Camarillo, M. Aguilar, H. Cruz-Majarrez, and L. Flores for they support during the grown and characterization process. This work was supported by DGAPA-UNAM under Contract PAPIIT-IN109910 and ICYT-DF under Contract PICCO 10-75. References
1200
800
400
ZnO (102)
Au (111)
600
ZnO (110)
ZnO (002)
Intensity (a.u.)
1000
G1 G2 G3 G4 Au (JCPD No04-0784) ZnO(JCPD No36-1451)
200
0 30
33
36
39
42
45
48
51
54
57
60
2θ (Deg) Fig. 5. Typical X-Ray Diffraction profiles (G1–G4). The corresponding peaks of ZnO and Au (1 1 1) metal are indicated.
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