- Email: [email protected]
Optical properties tailoring by high fluence implantation of Ag ions on sapphire
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 242 (2006) 104–108 www.elsevier.com/locate/nimb
Optical properties tailoring by high fluence implantation of Ag ions on sapphire C. Marques
a,b
, R.C. da Silva a,b, A. Wemans c, M.J.P. Maneira c, A. Kozanecki d, E. Alves a,b,*
a LFI, Dep. Fı´sica, Instituto Tecnolo´gico e Nuclear, Estrada Nacional 10, 2686-953 Sacave´m, Portugal Centro de Fı´sica Nuclear da Universidade de Lisboa, Av. Prof. Gama Pinto 2, 1649-003 Lisboa, Portugal CEFITEC – Departamento de Fı´sica, Faculdade de Cieˆncias e Tecnologia – UNL, 2829-516 Caparica, Portugal d Institute of Physics – Polish Academy of Sciences, 32/46 Lotniko´w Al., 02-668 Warszawa, Poland b
c
Available online 13 September 2005
Abstract Optical and structural properties of single crystalline a-Al2O3 were changed by the implantation of high fluences of Ag ions. Colourless transparent ð1 0 1 0Þ sapphire samples were implanted at room temperature with 160 keV silver ions and fluences up to 1 · 1017 Ag cm2. Surface amorphization is observed at the fluence of 6 · 1016 Ag cm2. Except for the lower fluences (below 6 · 1016 Ag cm2) the optical absorption spectra reveal the presence of a band peaking in the region 450–500 nm, depending on the retained fluence. This band has been attributed to the presence of silver colloids, being thus 1 · 1016 Ag cm2 below the threshold for colloid formation during the implantation. Annealing in oxidizing atmosphere promotes the recrystallization along with segregation of Ag followed by loss through evaporation. Recrystallization is retarded for annealing in reducing atmosphere and the Ag profile displays now a double peak structure after evaporation. Playing with the implantation fluence, temperature and annealing atmosphere controllable shifts of the position and intensity of the optical bands in the visible were achieved. 2005 Elsevier B.V. All rights reserved. PACS: 78.67.n; 61.72.Ww; 61.46.+w Keywords: Ion implantation; Optical properties; Sapphire; Nanoprecipitates
1. Introduction Metallic nanoparticles embedded in dielectric matrixes are of great interest, both from the theoretical and application point of views. Most of the physical properties exhibited by these particles are intriguingly different from those of the respective bulk material and may thus provide new devices. The optical response of clusters is a function of their electronic structure, which strongly depends on their shape, dimension, morphology and, also, on the properties of the * Corresponding author. Address: LFI, Dep. Fı´sica, Instituto Tecnolo´gico e Nuclear, Estrada Nacional 10, 2686-953 Sacave´m, Portugal. Tel.: +351 2199 46086; fax: +351 2199 41525. E-mail address: [email protected] (E. Alves).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.040
host material. The control of the clusters morphology allows designing and tuning the optical absorption characteristics of the system. These linear and also other arising non-linear effects can be used in all sorts of optical devices, from switches to wave guiding or diffraction devices. The non-optical applications include paramagnetic particles for magnetic resonance contrast imaging and metal particles for thermal probing of specific biomolecular interactions [1]. Several methods have been used to produce these nanosystemÕs solid solutions, namely colloidal solution, sol–gel or chemical synthesis, co-sputtering, electrochemical deposition, electron beam lithography, low energy cluster beam deposition or ion implantation [2]. Usually post-annealing is needed to stabilize the system created and/or to promote changes in the morphology and dimensions of the particles
C. Marques et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 104–108
Energy (keV) 1000
1250
1.6
Yield (a.u.)
2.0
Yield (counts/1000)
obtained. Among all these methods the ion implantation technique emerges as the one that allows overcoming the low solid solubility restrictions and to precisely define the confinement zone of the nanostructures. However, the intrinsic defects induced by the ballistic nature of this method and/or extrinsic defects resulting from the presence of the implanted ions may alter the microstructure and its properties thus requiring thermal treatments to allow relaxation to a stable system [3]. The study of all the transformations is essential to understand the behaviour of the implanted system. In this work the optical and structural properties of single crystalline a-Al2O3 were changed by the implantation of high doses of Ag ions. The implantation fluence, the temperature and type of annealing atmosphere produce controllable shifts of the position and intensity of the optical bands in the visible range, thus allowing tailoring the optical properties of the colloid system. The effects of these variables were studied and correlated with the optical behaviour of the samples.
105
1500
1750
displaced Al displaced O Ag x 500
2
200 nm
0
1
Ag
1.2 0 0
0.8
20
40
60
80
100
Depth (nm)
200 nm
0 Al
0.4
15
+
2
1x10 Ag /cm 16 + 2 1x10 Ag /cm 16 + 2 6x10 Ag /cm random
0.0 400
500
600
700
800
Channel Fig. 1. RBS-C spectra of the as implanted samples with fluences up to 6 · 1016 Ag cm2. The random spectra correspond to the sample implanted with 1 · 1015 Ag cm2.
2. Experimental details Colourless transparent synthetic sapphire single crystals, 4 lm thick, with h1 0 1 0i orientation (m-samples) and optically polished surfaces were implanted at room temperature (RT) with 160 keV silver ions. The nominal fluences were in the range of 1 · 1015–1 · 1017 Ag+ cm2. The samples were tilted 8 to avoid channelling effects. The projected range and straggling of the Ag ions were calculated using SRIM2003 [4] to be 47e12 nm, respectively. The SRIM also predicts the production of 2100 vacancies per incident ion. To recover from the implantation damage thermal annealing were carried out at 700 C, 800 C and 1000 C (the later above the melting temperature of silver which is 962 C) for 1 h in oxidizing (air) and reducing (vacuum, 2 · 104 Pa) atmospheres. The samples went directly from RT to the temperature chosen and vice versa. Rutherford backscattering spectrometry (RBS) studies were performed with a 2.0 MeV He+ beam after implantation and after each annealing step to characterise the structural changes. The backscattered particles were detected at 140 and close to 180 using silicon surface barrier detectors with resolutions of 13 keV and 18 keV, respectively. The beam current was measured on the target and kept below 4 nA in order to minimise the effects of charge accumulation at the surface during analysis. The optical absorption (OA) measurements were performed in the 200–900 nm wavelength range at RT, with a Varian Cary 5G UV–Vis–NIR double beam spectrophotometer. 3. Results and discussion 3.1. Structural studies After implantation the distribution of silver is nearly gaussian with the maximum concentration at 40 nm deep
(Fig. 1), in accordance with SRIM prediction (inset of Fig. 1). The implantation of 1 · 1015 Ag+ cm2 produces a thin damage layer of about 65 nm, being the Al minimum yield of 30%. Up to 1 · 1016 Ag cm2 the dechannelling rate reaches 60% with a slight increase in the damage extension, almost 90 nm thick. The implantation damage increases with the fluence and surface amorphization, as seen through RBS-C, is observed at 6 · 1016 Ag cm2 and extends up to 130 nm. The implanted profile with a FWHM of 60 nm is placed inside the disordered surface layer and is responsible for the decrease of the yield in the corresponding Al spectrum. The maximum concentration of Ag in the implanted layer is 6 at%. No channelling was observed on the silver profile for all the implanted fluences. Similar results were found previously in sapphire implanted with gold [5]. The high disorder and the large distribution of the implanted profile and defects in the m-samples indicate that this direction is very radiation sensitive. A study on Er implanted m and c sapphire ((0 0 0 1) orientation) samples reveal that the m-samples are easier to amorphize [6] in agreement with our results. This difference was explained considering the lower displacement energy of oxygen and aluminium along the h1 0 1 0i direction. A bimodal profile appears after implantation of a fluence of 1 · 1017 Ag cm2, peaking at 30 nm and close to the surface (Fig. 2). Similar behaviour was also reported in single crystal SiO2 by Liu et al. [7] and suggest a fast mobility of Ag in a particularly radiation defect rich sample. To recover the implantation damage and stabilize the implanted system, thermal annealing was performed. Since for higher concentrations it is more likely that the implanted ions precipitate in small clusters, the annealing was performed only for the highest dose. The structural evolution of the sample implanted with 1 · 1017 Ag cm2
106
C. Marques et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 104–108
Energy (keV) 1000
1250
1500
1750
2.0 x3
(a) 200 nm
200 nm
0
0 Ag
1.5
Al
1.0
random as implanted <1010> as implanted o <1010> after 700 C o <1010> after 800 C o <1010> after 1000 C o random after 1000 C
Yield (counts/1000)
0.5
0.0 m117
(b)
degrees lower than the bulk material [10] (sapphire melts at 2050 C while the melting point of silver is 962 C) which explains the large losses. Annealing in reducing atmosphere produces similar recrystallization and less Ag loss up to 800 C, as shown in Fig. 2(b). After annealing at 1000 C the Ag profile displays a double peaked structure (at 10 nm and 55 nm) and recrystallization continues. The retained fluence drops to 85% as in air. These results suggest that oxygen could help the recrystallization of the matrix which may contribute to drive the non-soluble silver ion to the surface. However, since major recrystallization is not observed trough RBS it is possible that silver oxides are formed on air, which are known to be unstable at T > 460 C. Major evaporation at 1000 C occurs in similar way irrespective of the atmosphere.
1.5
3.2. Optical studies 1.0
0.5
0.0
400
500
600
700
800
Channel Fig. 2. RBS-C spectra after implantation and after annealing of the sample implanted with 1 · 1017 Ag cm2 on: (a) oxidizing atmosphere and (b) reducing atmosphere. The Al spectra at 700 C was omitted for clarity since it is similar to the as implanted.
is shown in Fig. 2. No channelling effect was observed in the silver profile and the random spectra of the as implanted state is similar up to 800 C treatment and only one is presented for clarity. In all studied samples the annealing in oxidizing atmosphere at 700 C promotes some recrystallization along with the redistribution of Ag (the bimodal as implanted distribution disappears). Annealing at 800 C leads to a 30% loss through the surface by evaporation. At 1000 C evaporation is stronger (Ag loss of 85%) reaching values of the order of 1017 atm m2 s1, in accordance with the values observed in silver beads deposited on sapphire by Erde´lyi et al. [8]. The activation energy estimated at 1000 C, about 9 kJ/ mol, for the Ag diffusion reflect the high mobility of this non-reacting noble metal ion in sapphire (Menning et al. [9] measured D = 2 · 1013 cm2 s1 in sodium silicate glass). The ion can migrate and precipitate into large colloids that eventually coalesce at the surface. Since the surface energy of metals is usually higher than the surface energy of oxides, the decomposition of metal films is energetically favoured. It is also worth notice that the melting point of silver nanostructures is achieved some hundred
The sample implanted with 1 · 1016 Ag cm2 remains nearly transparent, which is evident by the absence of optical absorption in the visible region showed in Fig. 3. The samples implanted with higher doses acquire a goldish yellow coloration, characteristic of the defect and implanted specie rich state. The absorption band at 6.1 eV (203 nm) has long been attributed to F-centres [11] while F+ centres, more unstable, are responsible for the weaker band at 4.8 eV (258 nm) [12]. The surface plasmon resonance (SPR) band at 500 nm (2.48 eV) depends on the retained fluence, the morphology and local environment of the implanted particles. As the dose increases the SPR band becomes stronger and shifts towards longer wavelengths, probably due to the increase of the average size of the precipitates. This band is the signature of the presence of silver colloids and shows that the threshold for colloid formation is in the range of 1 · 1016–6 · 1016 Ag cm2 in these experimental conditions, similar to our previous work on gold implanted sapphire [5]. Assuming that a major fraction of the implanted Ag ions is incorporated into clusters and that the mean free path of the free electrons in Ag metal is larger than the size of the clusters, the average radius of the clusters can be estimated by [13] R¼
VF ; Dx1=2
where VF is the Fermi velocity of the electrons of the metal and Dx1/2 the half width of the absorption peak. This formula, with VF = 1.4 · 106 m/s [14] and the measured Dx1/2 of about 8–11 · 1014 rad/s yields a mean diameter between 1.25 nm and 1.73 nm for the precipitates obtained after implantation. It is worth notice that the SPR band may also be to some extent due to F2þ 2 , absorbing at 443 nm, as observed by Mohanty et al. [15]. However, our previous work with gold implanted sapphire does not show evidence of such centre and only the gold SPR is visible [5]. The larger absorption band presented by the sample implanted
C. Marques et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 104–108
Energy (eV)
Energy (eV) 3
6
5.5
5
4.5
Intensity (a.u.)
3.3 3
4
2.7
2.4
2.1
1.8
1.5
m1x1016 m6x1016 m1x1017 unimplanted sample
F F
107
+
2
1
0 200
225
250
275
300
325
350 400
500
Wavelength (nm)
600
700
800
Wavelength (nm)
Fig. 3. Optical absorption spectra of the virgin sample and after implantation in ultraviolet region (left) and visible region (right).
3.3 3
2.7
2.4
2.1
1.8
1.5
3.0 as implanted o 700 C o 800 C o 1000 C
2.5
Intensity (a.u.)
with 1 · 1017 Ag cm2 may be due to the bimodal distribution observed in Fig. 2(b), resulting from the presence of two different size distributions. This absorption band can be fitted with two gaussian curves with 0.43 eV separations (maxima at 422.6 nm and 496.5 nm, resulting particles sizes of 1.7 nm and 1.4 nm), in agreement with other works [7]. Another possibility for the double peaked SPR is the presence of non-spherical precipitates with multiple resonances [16]. The red shift with increasing dose can also be understood if one considers a possible refraction index gradient after implantation (either from the amorphization of the substrate or due to the presence of Ag ions that can lower the refractive index). Ellipsometric measurements are under way to rule out this possibility. The annealing in air causes the SPR band to vanish (not shown), probably due to the formation of mixed silver oxides in the cubic c-Al2O3 phase present at this temperature and the samples become greyish transparent. The standard silver oxides AgO or Ag2O are not stable above 460 C but may form during the cooling of the samples. However, other compounds or non-stoichiometric oxides may be present. In fact, Menning et al. [9] observed through HRTEM the presence of polycrystalline AgxOy nanoparticles that decoloured their samples. XRD studies are underway to assess this assumption. On the other hand, the samples annealed in vacuum present a strong absorption, mainly the one implanted with the highest dose, Fig. 4. The annealing promoted the loss of silver, as shown in the RBS spectra (Fig. 2) which explains the strong decrease of the optical absorption at 1000 C. Most likely the very small quantity of silver now present is highly dispersed in subnanometer particles. The F centres signal (not shown) also decreases. The SPR red-shift is observed up to 800 C and the samples acquire greenish reflections. The FWHM slightly increases also.
2.0
1.5
1.0
0.5 400
500
60 0
700
800
Wavelength (nm) Fig. 4. Optical absorption spectra for the fluence 1 · 1017 Ag cm2 after annealing at 700 C, 800 C and 1000 C on reducing atmosphere.
4. Conclusions Sapphire implanted with silver show a strong surface plasmon resonance (SPR) band at 450–500 nm after implantation of fluences above 6 · 1016 Ag cm2. The annealing atmosphere plays an important role up to 800 C where the RBS-C spectra show 30% loss of silver after annealing on air. On the contrary, most of the silver is retained when the annealing is performed on reducing atmosphere. The silver almost disappears at 1000 C on both oxidizing and reducing atmospheres. Upon annealing the SPR band red shifts indicating the presence of larger precipitates. Bimodal distributions of the Ag ions were observed after implantation leading to broader distribution of colloid sizes. This distribution is also reflected on larger a SPR band.
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C. Marques et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 104–108
Acknowledgements We wish to acknowledge Jorge Rocha for the Ag implantation and C. Marques acknowledge FCT for its support through the Ph.D. fellowship SFRH/BD/14276/2003. References [1] T.A. Tanton, Trends Biotechnol. 20 (7) (2002) 277. [2] P.D. Townsend, D.E. Hole, Vacuum 63 (2001) 641. [3] M.E. OÕHern, L.J. Romana, C.J. McHargue, J.C. McCallum, C.W. White, in: 15th Int. Symp., 1992, p. 740. [4] J. Ziegler, J. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, 1985, SRIM2003. Available from:. [5] C. Marques, E. Alves, R.C. da Silva, M.R. Silva, A.L. Stepanov, Nucl. Instr. and Meth. B 218 (2004) 139.
[6] E. Alves et al., Nucl. Instr. and Meth. B 166–167 (2000) 183. [7] Z. Liu, H. Wang, H. Li, X. Wang, Appl. Phys. Lett. 72 (15) (1998) 1823. [8] G. Erde´lyi et al., J. Appl. Phys. 80 (3) (1996) 1474. [9] M. Menning, M. Schmitt, H. Schmitt, J. Sol–Gel Sci. Technol. 8 (1997) 1035. [10] P. Buffat, J.P. Borel, Phys. Rev. A 13 (1976) 2287. [11] D.E. Bruce, J.G. Pogatshnik, Y. Chen, Nucl. Instr. and Meth. B 91 (1994) 258. [12] Y. Aoki, T.M. Nguen, Y. Shunya, H. Naramoto, Nucl. Instr. and Meth. B 114 (1996) 276. [13] Y. Saito, D.Y. Shang, S. Suganomata, Ionics 20 (1994) 35. [14] M.A. Omar, Elementary Solid State Physics, Benjamin Cummings, 1993. [15] T. Mohanty, N.C. Mishra, F. Singh, S.V. Baht, D. Kanjilal, Radiat. Measur. 36 (2003) 723. [16] C.F. Bohren, D.R. Huffman, Absorption and Scattering of Light by Small Particles, Wiley Science Paperback Series, John Wiley & Sons, 1998.
Nuclear Instruments and Methods in Physics Research B 242 (2006) 104–108 www.elsevier.com/locate/nimb
Optical properties tailoring by high fluence implantation of Ag ions on sapphire C. Marques
a,b
, R.C. da Silva a,b, A. Wemans c, M.J.P. Maneira c, A. Kozanecki d, E. Alves a,b,*
a LFI, Dep. Fı´sica, Instituto Tecnolo´gico e Nuclear, Estrada Nacional 10, 2686-953 Sacave´m, Portugal Centro de Fı´sica Nuclear da Universidade de Lisboa, Av. Prof. Gama Pinto 2, 1649-003 Lisboa, Portugal CEFITEC – Departamento de Fı´sica, Faculdade de Cieˆncias e Tecnologia – UNL, 2829-516 Caparica, Portugal d Institute of Physics – Polish Academy of Sciences, 32/46 Lotniko´w Al., 02-668 Warszawa, Poland b
c
Available online 13 September 2005
Abstract Optical and structural properties of single crystalline a-Al2O3 were changed by the implantation of high fluences of Ag ions. Colourless transparent ð1 0 1 0Þ sapphire samples were implanted at room temperature with 160 keV silver ions and fluences up to 1 · 1017 Ag cm2. Surface amorphization is observed at the fluence of 6 · 1016 Ag cm2. Except for the lower fluences (below 6 · 1016 Ag cm2) the optical absorption spectra reveal the presence of a band peaking in the region 450–500 nm, depending on the retained fluence. This band has been attributed to the presence of silver colloids, being thus 1 · 1016 Ag cm2 below the threshold for colloid formation during the implantation. Annealing in oxidizing atmosphere promotes the recrystallization along with segregation of Ag followed by loss through evaporation. Recrystallization is retarded for annealing in reducing atmosphere and the Ag profile displays now a double peak structure after evaporation. Playing with the implantation fluence, temperature and annealing atmosphere controllable shifts of the position and intensity of the optical bands in the visible were achieved. 2005 Elsevier B.V. All rights reserved. PACS: 78.67.n; 61.72.Ww; 61.46.+w Keywords: Ion implantation; Optical properties; Sapphire; Nanoprecipitates
1. Introduction Metallic nanoparticles embedded in dielectric matrixes are of great interest, both from the theoretical and application point of views. Most of the physical properties exhibited by these particles are intriguingly different from those of the respective bulk material and may thus provide new devices. The optical response of clusters is a function of their electronic structure, which strongly depends on their shape, dimension, morphology and, also, on the properties of the * Corresponding author. Address: LFI, Dep. Fı´sica, Instituto Tecnolo´gico e Nuclear, Estrada Nacional 10, 2686-953 Sacave´m, Portugal. Tel.: +351 2199 46086; fax: +351 2199 41525. E-mail address: [email protected] (E. Alves).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.040
host material. The control of the clusters morphology allows designing and tuning the optical absorption characteristics of the system. These linear and also other arising non-linear effects can be used in all sorts of optical devices, from switches to wave guiding or diffraction devices. The non-optical applications include paramagnetic particles for magnetic resonance contrast imaging and metal particles for thermal probing of specific biomolecular interactions [1]. Several methods have been used to produce these nanosystemÕs solid solutions, namely colloidal solution, sol–gel or chemical synthesis, co-sputtering, electrochemical deposition, electron beam lithography, low energy cluster beam deposition or ion implantation [2]. Usually post-annealing is needed to stabilize the system created and/or to promote changes in the morphology and dimensions of the particles
C. Marques et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 104–108
Energy (keV) 1000
1250
1.6
Yield (a.u.)
2.0
Yield (counts/1000)
obtained. Among all these methods the ion implantation technique emerges as the one that allows overcoming the low solid solubility restrictions and to precisely define the confinement zone of the nanostructures. However, the intrinsic defects induced by the ballistic nature of this method and/or extrinsic defects resulting from the presence of the implanted ions may alter the microstructure and its properties thus requiring thermal treatments to allow relaxation to a stable system [3]. The study of all the transformations is essential to understand the behaviour of the implanted system. In this work the optical and structural properties of single crystalline a-Al2O3 were changed by the implantation of high doses of Ag ions. The implantation fluence, the temperature and type of annealing atmosphere produce controllable shifts of the position and intensity of the optical bands in the visible range, thus allowing tailoring the optical properties of the colloid system. The effects of these variables were studied and correlated with the optical behaviour of the samples.
105
1500
1750
displaced Al displaced O Ag x 500
2
200 nm
0
1
Ag
1.2 0 0
0.8
20
40
60
80
100
Depth (nm)
200 nm
0 Al
0.4
15
+
2
1x10 Ag /cm 16 + 2 1x10 Ag /cm 16 + 2 6x10 Ag /cm random
0.0 400
500
600
700
800
Channel Fig. 1. RBS-C spectra of the as implanted samples with fluences up to 6 · 1016 Ag cm2. The random spectra correspond to the sample implanted with 1 · 1015 Ag cm2.
2. Experimental details Colourless transparent synthetic sapphire single crystals, 4 lm thick, with h1 0 1 0i orientation (m-samples) and optically polished surfaces were implanted at room temperature (RT) with 160 keV silver ions. The nominal fluences were in the range of 1 · 1015–1 · 1017 Ag+ cm2. The samples were tilted 8 to avoid channelling effects. The projected range and straggling of the Ag ions were calculated using SRIM2003 [4] to be 47e12 nm, respectively. The SRIM also predicts the production of 2100 vacancies per incident ion. To recover from the implantation damage thermal annealing were carried out at 700 C, 800 C and 1000 C (the later above the melting temperature of silver which is 962 C) for 1 h in oxidizing (air) and reducing (vacuum, 2 · 104 Pa) atmospheres. The samples went directly from RT to the temperature chosen and vice versa. Rutherford backscattering spectrometry (RBS) studies were performed with a 2.0 MeV He+ beam after implantation and after each annealing step to characterise the structural changes. The backscattered particles were detected at 140 and close to 180 using silicon surface barrier detectors with resolutions of 13 keV and 18 keV, respectively. The beam current was measured on the target and kept below 4 nA in order to minimise the effects of charge accumulation at the surface during analysis. The optical absorption (OA) measurements were performed in the 200–900 nm wavelength range at RT, with a Varian Cary 5G UV–Vis–NIR double beam spectrophotometer. 3. Results and discussion 3.1. Structural studies After implantation the distribution of silver is nearly gaussian with the maximum concentration at 40 nm deep
(Fig. 1), in accordance with SRIM prediction (inset of Fig. 1). The implantation of 1 · 1015 Ag+ cm2 produces a thin damage layer of about 65 nm, being the Al minimum yield of 30%. Up to 1 · 1016 Ag cm2 the dechannelling rate reaches 60% with a slight increase in the damage extension, almost 90 nm thick. The implantation damage increases with the fluence and surface amorphization, as seen through RBS-C, is observed at 6 · 1016 Ag cm2 and extends up to 130 nm. The implanted profile with a FWHM of 60 nm is placed inside the disordered surface layer and is responsible for the decrease of the yield in the corresponding Al spectrum. The maximum concentration of Ag in the implanted layer is 6 at%. No channelling was observed on the silver profile for all the implanted fluences. Similar results were found previously in sapphire implanted with gold [5]. The high disorder and the large distribution of the implanted profile and defects in the m-samples indicate that this direction is very radiation sensitive. A study on Er implanted m and c sapphire ((0 0 0 1) orientation) samples reveal that the m-samples are easier to amorphize [6] in agreement with our results. This difference was explained considering the lower displacement energy of oxygen and aluminium along the h1 0 1 0i direction. A bimodal profile appears after implantation of a fluence of 1 · 1017 Ag cm2, peaking at 30 nm and close to the surface (Fig. 2). Similar behaviour was also reported in single crystal SiO2 by Liu et al. [7] and suggest a fast mobility of Ag in a particularly radiation defect rich sample. To recover the implantation damage and stabilize the implanted system, thermal annealing was performed. Since for higher concentrations it is more likely that the implanted ions precipitate in small clusters, the annealing was performed only for the highest dose. The structural evolution of the sample implanted with 1 · 1017 Ag cm2
106
C. Marques et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 104–108
Energy (keV) 1000
1250
1500
1750
2.0 x3
(a) 200 nm
200 nm
0
0 Ag
1.5
Al
1.0
random as implanted <1010> as implanted o <1010> after 700 C o <1010> after 800 C o <1010> after 1000 C o random after 1000 C
Yield (counts/1000)
0.5
0.0 m117
(b)
degrees lower than the bulk material [10] (sapphire melts at 2050 C while the melting point of silver is 962 C) which explains the large losses. Annealing in reducing atmosphere produces similar recrystallization and less Ag loss up to 800 C, as shown in Fig. 2(b). After annealing at 1000 C the Ag profile displays a double peaked structure (at 10 nm and 55 nm) and recrystallization continues. The retained fluence drops to 85% as in air. These results suggest that oxygen could help the recrystallization of the matrix which may contribute to drive the non-soluble silver ion to the surface. However, since major recrystallization is not observed trough RBS it is possible that silver oxides are formed on air, which are known to be unstable at T > 460 C. Major evaporation at 1000 C occurs in similar way irrespective of the atmosphere.
1.5
3.2. Optical studies 1.0
0.5
0.0
400
500
600
700
800
Channel Fig. 2. RBS-C spectra after implantation and after annealing of the sample implanted with 1 · 1017 Ag cm2 on: (a) oxidizing atmosphere and (b) reducing atmosphere. The Al spectra at 700 C was omitted for clarity since it is similar to the as implanted.
is shown in Fig. 2. No channelling effect was observed in the silver profile and the random spectra of the as implanted state is similar up to 800 C treatment and only one is presented for clarity. In all studied samples the annealing in oxidizing atmosphere at 700 C promotes some recrystallization along with the redistribution of Ag (the bimodal as implanted distribution disappears). Annealing at 800 C leads to a 30% loss through the surface by evaporation. At 1000 C evaporation is stronger (Ag loss of 85%) reaching values of the order of 1017 atm m2 s1, in accordance with the values observed in silver beads deposited on sapphire by Erde´lyi et al. [8]. The activation energy estimated at 1000 C, about 9 kJ/ mol, for the Ag diffusion reflect the high mobility of this non-reacting noble metal ion in sapphire (Menning et al. [9] measured D = 2 · 1013 cm2 s1 in sodium silicate glass). The ion can migrate and precipitate into large colloids that eventually coalesce at the surface. Since the surface energy of metals is usually higher than the surface energy of oxides, the decomposition of metal films is energetically favoured. It is also worth notice that the melting point of silver nanostructures is achieved some hundred
The sample implanted with 1 · 1016 Ag cm2 remains nearly transparent, which is evident by the absence of optical absorption in the visible region showed in Fig. 3. The samples implanted with higher doses acquire a goldish yellow coloration, characteristic of the defect and implanted specie rich state. The absorption band at 6.1 eV (203 nm) has long been attributed to F-centres [11] while F+ centres, more unstable, are responsible for the weaker band at 4.8 eV (258 nm) [12]. The surface plasmon resonance (SPR) band at 500 nm (2.48 eV) depends on the retained fluence, the morphology and local environment of the implanted particles. As the dose increases the SPR band becomes stronger and shifts towards longer wavelengths, probably due to the increase of the average size of the precipitates. This band is the signature of the presence of silver colloids and shows that the threshold for colloid formation is in the range of 1 · 1016–6 · 1016 Ag cm2 in these experimental conditions, similar to our previous work on gold implanted sapphire [5]. Assuming that a major fraction of the implanted Ag ions is incorporated into clusters and that the mean free path of the free electrons in Ag metal is larger than the size of the clusters, the average radius of the clusters can be estimated by [13] R¼
VF ; Dx1=2
where VF is the Fermi velocity of the electrons of the metal and Dx1/2 the half width of the absorption peak. This formula, with VF = 1.4 · 106 m/s [14] and the measured Dx1/2 of about 8–11 · 1014 rad/s yields a mean diameter between 1.25 nm and 1.73 nm for the precipitates obtained after implantation. It is worth notice that the SPR band may also be to some extent due to F2þ 2 , absorbing at 443 nm, as observed by Mohanty et al. [15]. However, our previous work with gold implanted sapphire does not show evidence of such centre and only the gold SPR is visible [5]. The larger absorption band presented by the sample implanted
C. Marques et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 104–108
Energy (eV)
Energy (eV) 3
6
5.5
5
4.5
Intensity (a.u.)
3.3 3
4
2.7
2.4
2.1
1.8
1.5
m1x1016 m6x1016 m1x1017 unimplanted sample
F F
107
+
2
1
0 200
225
250
275
300
325
350 400
500
Wavelength (nm)
600
700
800
Wavelength (nm)
Fig. 3. Optical absorption spectra of the virgin sample and after implantation in ultraviolet region (left) and visible region (right).
3.3 3
2.7
2.4
2.1
1.8
1.5
3.0 as implanted o 700 C o 800 C o 1000 C
2.5
Intensity (a.u.)
with 1 · 1017 Ag cm2 may be due to the bimodal distribution observed in Fig. 2(b), resulting from the presence of two different size distributions. This absorption band can be fitted with two gaussian curves with 0.43 eV separations (maxima at 422.6 nm and 496.5 nm, resulting particles sizes of 1.7 nm and 1.4 nm), in agreement with other works [7]. Another possibility for the double peaked SPR is the presence of non-spherical precipitates with multiple resonances [16]. The red shift with increasing dose can also be understood if one considers a possible refraction index gradient after implantation (either from the amorphization of the substrate or due to the presence of Ag ions that can lower the refractive index). Ellipsometric measurements are under way to rule out this possibility. The annealing in air causes the SPR band to vanish (not shown), probably due to the formation of mixed silver oxides in the cubic c-Al2O3 phase present at this temperature and the samples become greyish transparent. The standard silver oxides AgO or Ag2O are not stable above 460 C but may form during the cooling of the samples. However, other compounds or non-stoichiometric oxides may be present. In fact, Menning et al. [9] observed through HRTEM the presence of polycrystalline AgxOy nanoparticles that decoloured their samples. XRD studies are underway to assess this assumption. On the other hand, the samples annealed in vacuum present a strong absorption, mainly the one implanted with the highest dose, Fig. 4. The annealing promoted the loss of silver, as shown in the RBS spectra (Fig. 2) which explains the strong decrease of the optical absorption at 1000 C. Most likely the very small quantity of silver now present is highly dispersed in subnanometer particles. The F centres signal (not shown) also decreases. The SPR red-shift is observed up to 800 C and the samples acquire greenish reflections. The FWHM slightly increases also.
2.0
1.5
1.0
0.5 400
500
60 0
700
800
Wavelength (nm) Fig. 4. Optical absorption spectra for the fluence 1 · 1017 Ag cm2 after annealing at 700 C, 800 C and 1000 C on reducing atmosphere.
4. Conclusions Sapphire implanted with silver show a strong surface plasmon resonance (SPR) band at 450–500 nm after implantation of fluences above 6 · 1016 Ag cm2. The annealing atmosphere plays an important role up to 800 C where the RBS-C spectra show 30% loss of silver after annealing on air. On the contrary, most of the silver is retained when the annealing is performed on reducing atmosphere. The silver almost disappears at 1000 C on both oxidizing and reducing atmospheres. Upon annealing the SPR band red shifts indicating the presence of larger precipitates. Bimodal distributions of the Ag ions were observed after implantation leading to broader distribution of colloid sizes. This distribution is also reflected on larger a SPR band.
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Acknowledgements We wish to acknowledge Jorge Rocha for the Ag implantation and C. Marques acknowledge FCT for its support through the Ph.D. fellowship SFRH/BD/14276/2003. References [1] T.A. Tanton, Trends Biotechnol. 20 (7) (2002) 277. [2] P.D. Townsend, D.E. Hole, Vacuum 63 (2001) 641. [3] M.E. OÕHern, L.J. Romana, C.J. McHargue, J.C. McCallum, C.W. White, in: 15th Int. Symp., 1992, p. 740. [4] J. Ziegler, J. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, 1985, SRIM2003. Available from:
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