- Email: [email protected]
Ion-induced metal nanoparticles in insulators for nonlinear optical property
Nuclear Instruments and Methods in Physics Research B 206 (2003) 634–638 www.elsevier.com/locate/nimb
Ion-induced metal nanoparticles in insulators for nonlinear optical property N. Kishimoto a
a,*
, Y. Takeda a, N. Umeda b, N. Okubo b, R.G. Faulkner
c
Nanomaterials Laboratory, National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan b University of Tsukuba, Institute of Materials Science, 1-1-1 Ten-nodai, Tsukuba, Ibaraki 305-8573, Japan c IPTME, Loughborough University, Loughborough, Leics LE11 3TA, UK
Abstract Metal nanoparticles embedded in insulators exhibit a strong surface resonance and have been studied for optical switching. In this study, negative Cu ions of 60 keV were implanted into a-SiO2 , MgO 2.4(Al2 O3 ) and LiNbO3 at 10–50 lA/cm2 to 1 1017 ions/cm2 . The resultant nanoparticle morphology was studied by cross-sectional TEM and shown to depend on the substrate species. The a-SiO2 showed the formation of spherical Cu nanocrystals of 10 nm. The MgO 2.4(Al2 O3 ) suppressed particle coarsening even at high dose rates, sustaining crystallinity of the lattice. On the other hand, the LiNbO3 exhibited non-spherical Cu nanocrystals of 10 nm. Ion-induced photon spectroscopy was applied to monitor the ion–substrate interactions from outside of the substrates. The non-linear optical properties were evaluated by a pump–probe method around the plasmon energy of about 2 eV. Although LiNbO3 exhibited a subpicosec non-linear response, ion-induced photon spectroscopy revealed Li-atom release to the vacuum under ion implantation, influencing the Cu nanoparticle formation. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 42.65.An; 78.66.Vs; 78.60.Hk; 79.20.Rf; 78.66.w Keywords: Negative ion implantation; Lithium niobate; Magnesium aluminate spinel; Cu nanoparticle; Nonlinear optical property
1. Introduction Metal nanoparticles embedded in insulators have attracted our attention [1,2], since electrons confined in the metal particles show surface-plasmon resonance and provide ultrafast non-linear response [3,4]. In fabricating metal nanoparticles, employment of ion implantation widens material * Corresponding author. Tel.: +81-29-859-5009; fax: +81-29859-5010. E-mail address: [email protected] (N. Kishimoto).
selection for both metal and substrate species, because of the non-equilibrium atomic injection of immiscible elements and subsequent precipitation of nanoparticles. Particularly, usage of negative ions is effective to alleviate surface charging on insulating substrates [5]. High-flux Cu implantation has enabled us to form self-assembled metal nanoparticles in insulating substrates without applying post-irradiation annealing [6–9]. However, precipitation processes during ion implantation are strongly influenced by ion–substrate interactions including post-collision events, such as radiation-induced diffusion, nucleation and growth
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00804-8
N. Kishimoto et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 634–638
[6,10]. Also, heavy-ion implantation up to high doses is subjected to surface sputtering/sublimation [11,12] and composition change of substrates, as described by the TRIDYN code [13]. Consequently, understanding the in-beam response of substrates is vital to control the nanoparticle morphology. Ion-induced photon spectroscopy is one of the unique methods to probe dynamical ion–substrate interactions [11]. In this paper, we present TEM observation of LiNbO3 implanted with 60 keV Cu ions, in comparison with SiO2 and MgO 2.4(Al2 O3 ). The nanoparticle morphology is compared with the ion-induced photon spectroscopy and the mechanism of nanoparticle formation is discussed with respect to mass transport of the implanted species.
2. Experimental procedures Specimens consist of x-cut LiNbO3 single crystal, a-SiO2 (KU-1â , OH :820 ppm) and singlecrystal spinel MgO 2.4(Al2 O3 ) of (1 0 0)-plane. The disk substrates, polished to optical grade, are 15 mm in diameter and 0.5 mm in thickness. Negative Cu ions of 60 keV were implanted into substrates at various dose rates up to about 50 lA/ cm2 , to a total dose of 1.0 1017 ions/cm2 . The negative ion techniques and the apparatus were described elsewhere [14]. Depth profiles of 60 keV Cu, without considering post-collision processes, were estimated with the TRIDYN code [13]. The projected ranges of 60 keV Cu for LiNbO3 , SiO2 and MgO 2.4(Al2 O3 ) are 30, 45 and 30 nm, respectively, in the case where surface sputtering is not dominant. Specimen temperature was monitored by a thermocouple attached to periphery of the substrate and did not exceed 500 K below 50 lA/cm2 . Time-resolved optical devices, with fast-response CCD cameras (Princeton Instruments: IMAX-512), were used for ion-induced photon spectroscopy [11]. The full detection range was from 900 to 200 nm. The monochromators were placed at the two quartz windows at angles of 145° from the ion beam direction. To eliminate ion-induced emission from the surrounding of a specimen, the beam cross-section was shaped into
635
a 12 mm-diam. circle through a pre-aperture, which was aligned onto the same area of the substrate surface with a Cu contact aperture. Nonlinear transient absorption was measured using the pump–probe method with a femtosecond-laser system [8]. The photon energy of the pumping pulse was tuned near the surface-plasmon resonance. Finally, cross-sectional electron microscopy was conducted to evaluate microstructures in the Cu-implanted region.
3. Results and discussion Metal precipitation behavior significantly depended on the substrate species, although spontaneous precipitation occurred more or less in all materials studied. At high dose rates of 6 50 lA/ cm2 , a-SiO2 showed spontaneous formation of Cu nanocrystals of 10 nm. The in-beam metastable nature of a-SiO2 favored pronounced precipitation and annihilation of defect aggregates in the matrix. A self-assembled 2D arrangement of nanoparticles is explained in terms of steep gradient of radiationinduced diffusion [6]. The MgO 2.4(Al2 O3 ) also showed spontaneous precipitation but the size was 2–5 nm in diameter, much smaller than that of a-SiO2 . The small precipitates with sustained crystallinity may be associated with the superb radiation resistance of the MgO n(Al2 O3 ) [15]. On the other hand, LiNbO3 showed unstable precipitation behavior, different from a-SiO2 or MgO 2.4(Al2 O3 ). Fig. 1 shows a cross-sectional TEM image of LiNbO3 implanted with 60 keV Cu ions at a dose rate of 10 lA/cm2 to a dose of 3 1016 ions/cm2 . The high-resolution image was taken around the projected range of 60 keV Cu in LiNbO3 . The metal particles formed are single Cu crystals of 10 nm or smaller in diameter, and occasionally show Moire patterns with lattice fringes of the matrix LiNbO3 , as seen in Fig. 1. The shape of Cu particles is mostly non-spherical, being either elongated or partially facetted. The Cu nanoparticles are widely distributed from the vicinity of the surface to a depth more than 100 nm, which is much deeper than the projected range of 30 nm. The wide distribution of nanoparticles can be attributed to pronounced radiation-enhanced
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N. Kishimoto et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 634–638
8
Li I
Intensity ( a.u. )
–
6
4
0 200
diffusion of Cu atoms on the lattice of LiNbO3 . It is thus manifested that Cu nanoparticles spontaneously form in LiNbO3 implanted with 60 keV Cu without thermal annealing. Although the spontaneous precipitation is common with a-SiO2 and MgO 2.4(Al2 O3 ), the non-spherical shape and wide distribution are characteristic of nanoparticles formed in crystalline LiNbO3 . As for the LiNbO3 matrix, the lattice crystallinity is well maintained to 3 1016 ions/cm2 , as seen in Fig. 1. The ion dose corresponds to about 50 dpa in the peak region. The good crystallinity after implantation appears that the LiNbO3 may be radiationresistant against heavy-ion irradiation. However, it is not the case, since the LiNbO3 lattice allows Cu atoms to widely migrate and yields plenty of vacancies to accommodate the metal particles. Fig. 2 shows ion-induced photon spectra of LiNbO3 implanted with 60 keV Cu ions at a dose rate of 10 lA/cm2 . Strong and sharp line spectra emerge during Cu ion irradiation on the broad background. The ion-induced photon emission as line spectra implies that isolated atoms in the excited states are released from the substrate to the vacuum, and that the internal optical transitions emit specific photons. The atomic line emission of Li I (3d 2 D3=2 ! 2p 2 P1=2 , etc.: 610.4 nm, 2p 2 P3=2 !
Nb I Cu I
2
Fig. 1. Cross-sectional TEM image of LiNbO3 implanted with 60 keV Cu ions at a dose rate of 10 lA/cm2 to a dose of 3 1016 ions/cm2 .
60 keV Cu => LiNbO3
Li I
Li I
x1/10 x1
400 600 800 Wavelength ( nm )
1000
Fig. 2. Ion-induced photon spectra of LiNbO3 under implantation of 60 keV Cu ions at a dose rate of 10 lA/cm2 .
2s 2 S1=2 , etc.: 670.8 nm, 3s 2 S1=2 ! 2p 2 P1=2 , etc.: 812.6 nm), Cu I (4p 2 P3=2 ! 4s 2 S1=2 : 324.8 nm and 4 p 2 P1=2 ! 4s 2 S1=2 : 327.4 nm) and Nb I (405.9, 408.0, 410.1, 412.4 nm) are detected in accordance with the NIST Atomic Spectra Database [16]. Line emission of O atoms was not detected in the present energy region. The dose dependence of the line spectra of Li, Nb and Cu is shown in Fig. 3. The Li line of 671 nm keeps the strongest peak all the way and begins to decrease towards 1 1017 ions/cm2 . The Nb line of 412 nm gradually increases from the onset and then saturates. On the other hand, the Cu line of 325 nm is initially negligible and gradually increases above about 5 1016 ions/cm2 . Fig. 4 shows calculated dose dependence of surface recession and sputtering yields of Li, Nb, O and Cu from LiNbO3 during implantation of 60 keV Cu ions. The calculation was done by the TRIDYN code [13]. The surface recession due to sputtering at 3 1016 ions/cm2 (corresponding to Fig. 1) is about 10 nm. The dose-dependent tendencies of Li, Nb and Cu calculated are qualitatively in good agreement with the experimental ones in Fig. 3. Namely, the Li atomic release gradually decays with increasing dose and, complementarily, the Nb atomic release increases. These variations of Li and Nb lines indicate that
N. Kishimoto et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 634–638 –
60 keV Cu => LiNbO3
Intensity ( a.u. )
10
Li (671 nm) x1 Nb (412 nm)
5
x1/2
x1/2 Cu (325 nm)
0 0
5 Dose ( 10
16
10 2
ions/cm )
60 keV Cu => LiNbO 3
4
50 O
2
Cu
Li
Nb
0
Sputtering Yield (atoms/ion)
Fig. 3. Dose dependence of ion-induced line spectra of LiNbO3 under implantation of 60 keV Cu ions at a dose rate of 10 lA/ cm2 .
Surface Recession ( nm )
637
0 0
10 20 16 2 Dose ( 10 ions/cm )
30
Fig. 4. Calculated dose dependence of surface recession of LiNbO3 and sputtering yields of Li, Nb, O and Cu from LiNbO3 under implantation of 60 keV Cu ions. The calculation was done by the TRIDYN code.
Li-depleted and Nb-enriched zones are created in the vicinity of the surface. A quantitative difference of Li in Fig. 3 from that in Fig. 4 is a slower decay of Li (experiment), which implies that Li
Fig. 5. Non-linear transient absorption of LiNbO3 implanted with 60 keV Cu at 10 lA/cm2 to 3 1016 ions/cm2 . The pumping and probing energies are 2.16 and 2.05 eV, respectively.
atoms are diffusively supplied from the deeper region, that is, radiation-enhanced diffusion of Li is manifested. As for the Cu atomic evolution, the initially-implanted Cu profile begins to be sputtered by the successive surface recession, and the Cu atomic release reaches a quasi-steady-state between the inward and outward atomic fluxes, where the sputtering yield is unity. In this context, the upper limit of Cu concentration is determined by the balance between the implantation rate and surface-recession rate, irrespective of the nominal dose. The sputtering process does not cause a significant loss of implants below about 5 1016 ions/cm2 and the peak Cu concentration is roughly proportional the nominal dose. It should be pointed out that that radiation-enhanced diffusion of Cu and Li promotes broadening nanoparticle distribution and releasing Li atoms more than the TRIDYN regime. It is speculated from the observed tendencies that the Cu diffusion may be enhanced by the Li diffusion via the Cu–Li ion exchange. The radiation-enhanced diffusion of Cu and formation of the Li-depleted zone near the surface may cause significant nanoparticle rearrangements in the composite but, as seen in Fig. 1, the appropriate implantation condition successfully
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N. Kishimoto et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 634–638
fabricates nanoparticles of 10 nm in diameter. In fact, the linear optical absorption of the nanoparticle composite showed a surface-plasmon resonance at 1.8 eV, as predicted by the Maxwell-Garnett theory [8,17]. Fig. 5 shows the non-linear transient absorption of LiNbO3 implanted with 60 keV Cu at 10 lA/cm2 to 3 1016 ions/cm2 . The ordinate designates difference from the steady-state absorption. The pumping (at t ¼ 0) is conducted with a single-energy laser of 2.16 eV at the surfaceplasmon peak. The absorption spectrum around the peak is measured by probing white light, and the differential signal at 2.05 eV is plotted against the delay time. The pumping light induces a decrease in absorption (bleaching) as compared to the steady-state, and the bleaching quickly recovers. The nanoparticle composite of Cu:LiNbO3 exhibits an ultrafast nonlinear response of subpicoseconds, which is suitable for all-optical switching. Although the unstable behavior of Li and Cu makes the fabrication window narrow, the LiNbO3 substrate has a good potential for the ultrafast response and the wavelength tuning.
4. Summary Negative Cu ions of 60 keV have been implanted into LiNbO3 , a-SiO2 and MgO 2.4(Al2 O3 ) and to 1 1017 ions/cm2 . The nanoparticle morphology significantly depended on the substrate species, though spontaneous precipitation occurred in all cases. In comparison with a-SiO2 and MgO (Al2 O3 ), the LiNbO3 exhibited non-spherical Cu nanocrystals of 10 nm dispersing over a wide depth. Ion-induced photon spectroscopy revealed the strong atomic release of Li and dose-dependent release of Cu implants. The wide distribution of nanoparticles and the strong atomic release indicate that diffusion of Cu and Li is enhanced by ion irradiation and causes the dominant atomic migration. The nanoparticle composite of Cu:LiNbO3 exhibited an ultrafast nonlinear response of sub-picoseconds. Although the LiNbO3 under implantation showed significant Cu migra-
tion, the LiNbO3 substrate is a good option for the ultrafast response and the wavelength tuning. Acknowledgements A part of this study was financially supported by the Budget for Nuclear Research of the MEXT, based on the screening and counseling by the Atomic Energy Commission. The authors are grateful to Dr. H. Amekura, Dr. T. Suga and Ms. J. Lu of NIMS for their assistance in the experiments. References [1] D.J. Rej, R.R. Bartsch, H.A. Davis, R.J. Faehl, J.B. Greenly, W.J. Waganaar, Rev. Sci. Instr. 64 (1993) 2753. [2] R.F. Haglund Jr., L. Yang, R.H. Magruder III, C.W. White, R.A. Zuhr, L. Yang, R. Dorsinville, R.R. Alfano, Nucl. Instr. and Meth. B 91 (1994) 493. [3] J.-Y. Bigot, J.C. Merle, O. Cregut, A. Daunois, Phys. Rev. Lett. 75 (1995) 4702. [4] R.F. Haglund Jr., Mater. Sci. Eng. A 253 (1998) 275. [5] J. Ishikawa, H. Tsuji, Y. Toyota, Y. Gotoh, K. Matsuda, M. Tanjyo, S. Sakaki, Nucl. Instr. and Meth. B 96 (1995) 7. [6] N. Kishimoto, N. Umeda, Y. Takeda, C.G. Lee, V.T. Gritsyna, Nucl. Instr. and Meth. B 148 (1999) 1017. [7] N. Kishimoto, Y. Takeda, N. Umeda, V.T. Gritsyna, C.G. Lee, T. Saito, Nucl. Instr. and Meth. B 166–167 (2000) 840. [8] Y. Takeda, N. Umeda, V.T. Gritsyna, N. Kishimoto, Nucl. Instr. and Meth. B 175–177 (2001) 463. [9] Y. Takeda, C.G. Lee, N. Kishimoto, Nucl. Instr. and Meth. B 190 (2002) 797. [10] H. Hosono, H. Fukushima, Y. Abe, R.A. Weeks, R.A. Zuhr, J. Non-Cryst. Solids 143 (1992) 157. [11] V. Bandourko, T.T. Lay, Y. Takeda, C.G. Lee, N. Kishimoto, Nucl. Instr. and Meth. B 175–177 (2001) 68. [12] V.V. Bandourko, N. Umeda, N. Kishimoto, Nucl. Instr. and Meth. B 193 (2002) 690. [13] W. Moeller, W. Eckstein, Nucl. Instr. and Meth. B 2 (1984) 814. [14] N. Kishimoto, Y. Takeda, V.T. Gritsyna, E. Iwamoto, T. Saito, Ion Implant. Technol. 12 (1999) 342. [15] L.W. Hobbs, F.W. Clinard Jr., J. Phys. 41 (1980) C6. [16] W.C. Martin, J.R. Fuhr, D.E. Kelleher, A. Musgrove, L. Podobedova, J. Reader, E.B. Saloman, C.J. Sansonetti, W.L. Wiese, P.J. Mohr, K. Olsen, NIST Atomic Spectra Database (version 2.0), 2002. Available from
Ion-induced metal nanoparticles in insulators for nonlinear optical property N. Kishimoto a
a,*
, Y. Takeda a, N. Umeda b, N. Okubo b, R.G. Faulkner
c
Nanomaterials Laboratory, National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan b University of Tsukuba, Institute of Materials Science, 1-1-1 Ten-nodai, Tsukuba, Ibaraki 305-8573, Japan c IPTME, Loughborough University, Loughborough, Leics LE11 3TA, UK
Abstract Metal nanoparticles embedded in insulators exhibit a strong surface resonance and have been studied for optical switching. In this study, negative Cu ions of 60 keV were implanted into a-SiO2 , MgO 2.4(Al2 O3 ) and LiNbO3 at 10–50 lA/cm2 to 1 1017 ions/cm2 . The resultant nanoparticle morphology was studied by cross-sectional TEM and shown to depend on the substrate species. The a-SiO2 showed the formation of spherical Cu nanocrystals of 10 nm. The MgO 2.4(Al2 O3 ) suppressed particle coarsening even at high dose rates, sustaining crystallinity of the lattice. On the other hand, the LiNbO3 exhibited non-spherical Cu nanocrystals of 10 nm. Ion-induced photon spectroscopy was applied to monitor the ion–substrate interactions from outside of the substrates. The non-linear optical properties were evaluated by a pump–probe method around the plasmon energy of about 2 eV. Although LiNbO3 exhibited a subpicosec non-linear response, ion-induced photon spectroscopy revealed Li-atom release to the vacuum under ion implantation, influencing the Cu nanoparticle formation. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 42.65.An; 78.66.Vs; 78.60.Hk; 79.20.Rf; 78.66.w Keywords: Negative ion implantation; Lithium niobate; Magnesium aluminate spinel; Cu nanoparticle; Nonlinear optical property
1. Introduction Metal nanoparticles embedded in insulators have attracted our attention [1,2], since electrons confined in the metal particles show surface-plasmon resonance and provide ultrafast non-linear response [3,4]. In fabricating metal nanoparticles, employment of ion implantation widens material * Corresponding author. Tel.: +81-29-859-5009; fax: +81-29859-5010. E-mail address: [email protected] (N. Kishimoto).
selection for both metal and substrate species, because of the non-equilibrium atomic injection of immiscible elements and subsequent precipitation of nanoparticles. Particularly, usage of negative ions is effective to alleviate surface charging on insulating substrates [5]. High-flux Cu implantation has enabled us to form self-assembled metal nanoparticles in insulating substrates without applying post-irradiation annealing [6–9]. However, precipitation processes during ion implantation are strongly influenced by ion–substrate interactions including post-collision events, such as radiation-induced diffusion, nucleation and growth
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00804-8
N. Kishimoto et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 634–638
[6,10]. Also, heavy-ion implantation up to high doses is subjected to surface sputtering/sublimation [11,12] and composition change of substrates, as described by the TRIDYN code [13]. Consequently, understanding the in-beam response of substrates is vital to control the nanoparticle morphology. Ion-induced photon spectroscopy is one of the unique methods to probe dynamical ion–substrate interactions [11]. In this paper, we present TEM observation of LiNbO3 implanted with 60 keV Cu ions, in comparison with SiO2 and MgO 2.4(Al2 O3 ). The nanoparticle morphology is compared with the ion-induced photon spectroscopy and the mechanism of nanoparticle formation is discussed with respect to mass transport of the implanted species.
2. Experimental procedures Specimens consist of x-cut LiNbO3 single crystal, a-SiO2 (KU-1â , OH :820 ppm) and singlecrystal spinel MgO 2.4(Al2 O3 ) of (1 0 0)-plane. The disk substrates, polished to optical grade, are 15 mm in diameter and 0.5 mm in thickness. Negative Cu ions of 60 keV were implanted into substrates at various dose rates up to about 50 lA/ cm2 , to a total dose of 1.0 1017 ions/cm2 . The negative ion techniques and the apparatus were described elsewhere [14]. Depth profiles of 60 keV Cu, without considering post-collision processes, were estimated with the TRIDYN code [13]. The projected ranges of 60 keV Cu for LiNbO3 , SiO2 and MgO 2.4(Al2 O3 ) are 30, 45 and 30 nm, respectively, in the case where surface sputtering is not dominant. Specimen temperature was monitored by a thermocouple attached to periphery of the substrate and did not exceed 500 K below 50 lA/cm2 . Time-resolved optical devices, with fast-response CCD cameras (Princeton Instruments: IMAX-512), were used for ion-induced photon spectroscopy [11]. The full detection range was from 900 to 200 nm. The monochromators were placed at the two quartz windows at angles of 145° from the ion beam direction. To eliminate ion-induced emission from the surrounding of a specimen, the beam cross-section was shaped into
635
a 12 mm-diam. circle through a pre-aperture, which was aligned onto the same area of the substrate surface with a Cu contact aperture. Nonlinear transient absorption was measured using the pump–probe method with a femtosecond-laser system [8]. The photon energy of the pumping pulse was tuned near the surface-plasmon resonance. Finally, cross-sectional electron microscopy was conducted to evaluate microstructures in the Cu-implanted region.
3. Results and discussion Metal precipitation behavior significantly depended on the substrate species, although spontaneous precipitation occurred more or less in all materials studied. At high dose rates of 6 50 lA/ cm2 , a-SiO2 showed spontaneous formation of Cu nanocrystals of 10 nm. The in-beam metastable nature of a-SiO2 favored pronounced precipitation and annihilation of defect aggregates in the matrix. A self-assembled 2D arrangement of nanoparticles is explained in terms of steep gradient of radiationinduced diffusion [6]. The MgO 2.4(Al2 O3 ) also showed spontaneous precipitation but the size was 2–5 nm in diameter, much smaller than that of a-SiO2 . The small precipitates with sustained crystallinity may be associated with the superb radiation resistance of the MgO n(Al2 O3 ) [15]. On the other hand, LiNbO3 showed unstable precipitation behavior, different from a-SiO2 or MgO 2.4(Al2 O3 ). Fig. 1 shows a cross-sectional TEM image of LiNbO3 implanted with 60 keV Cu ions at a dose rate of 10 lA/cm2 to a dose of 3 1016 ions/cm2 . The high-resolution image was taken around the projected range of 60 keV Cu in LiNbO3 . The metal particles formed are single Cu crystals of 10 nm or smaller in diameter, and occasionally show Moire patterns with lattice fringes of the matrix LiNbO3 , as seen in Fig. 1. The shape of Cu particles is mostly non-spherical, being either elongated or partially facetted. The Cu nanoparticles are widely distributed from the vicinity of the surface to a depth more than 100 nm, which is much deeper than the projected range of 30 nm. The wide distribution of nanoparticles can be attributed to pronounced radiation-enhanced
636
N. Kishimoto et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 634–638
8
Li I
Intensity ( a.u. )
–
6
4
0 200
diffusion of Cu atoms on the lattice of LiNbO3 . It is thus manifested that Cu nanoparticles spontaneously form in LiNbO3 implanted with 60 keV Cu without thermal annealing. Although the spontaneous precipitation is common with a-SiO2 and MgO 2.4(Al2 O3 ), the non-spherical shape and wide distribution are characteristic of nanoparticles formed in crystalline LiNbO3 . As for the LiNbO3 matrix, the lattice crystallinity is well maintained to 3 1016 ions/cm2 , as seen in Fig. 1. The ion dose corresponds to about 50 dpa in the peak region. The good crystallinity after implantation appears that the LiNbO3 may be radiationresistant against heavy-ion irradiation. However, it is not the case, since the LiNbO3 lattice allows Cu atoms to widely migrate and yields plenty of vacancies to accommodate the metal particles. Fig. 2 shows ion-induced photon spectra of LiNbO3 implanted with 60 keV Cu ions at a dose rate of 10 lA/cm2 . Strong and sharp line spectra emerge during Cu ion irradiation on the broad background. The ion-induced photon emission as line spectra implies that isolated atoms in the excited states are released from the substrate to the vacuum, and that the internal optical transitions emit specific photons. The atomic line emission of Li I (3d 2 D3=2 ! 2p 2 P1=2 , etc.: 610.4 nm, 2p 2 P3=2 !
Nb I Cu I
2
Fig. 1. Cross-sectional TEM image of LiNbO3 implanted with 60 keV Cu ions at a dose rate of 10 lA/cm2 to a dose of 3 1016 ions/cm2 .
60 keV Cu => LiNbO3
Li I
Li I
x1/10 x1
400 600 800 Wavelength ( nm )
1000
Fig. 2. Ion-induced photon spectra of LiNbO3 under implantation of 60 keV Cu ions at a dose rate of 10 lA/cm2 .
2s 2 S1=2 , etc.: 670.8 nm, 3s 2 S1=2 ! 2p 2 P1=2 , etc.: 812.6 nm), Cu I (4p 2 P3=2 ! 4s 2 S1=2 : 324.8 nm and 4 p 2 P1=2 ! 4s 2 S1=2 : 327.4 nm) and Nb I (405.9, 408.0, 410.1, 412.4 nm) are detected in accordance with the NIST Atomic Spectra Database [16]. Line emission of O atoms was not detected in the present energy region. The dose dependence of the line spectra of Li, Nb and Cu is shown in Fig. 3. The Li line of 671 nm keeps the strongest peak all the way and begins to decrease towards 1 1017 ions/cm2 . The Nb line of 412 nm gradually increases from the onset and then saturates. On the other hand, the Cu line of 325 nm is initially negligible and gradually increases above about 5 1016 ions/cm2 . Fig. 4 shows calculated dose dependence of surface recession and sputtering yields of Li, Nb, O and Cu from LiNbO3 during implantation of 60 keV Cu ions. The calculation was done by the TRIDYN code [13]. The surface recession due to sputtering at 3 1016 ions/cm2 (corresponding to Fig. 1) is about 10 nm. The dose-dependent tendencies of Li, Nb and Cu calculated are qualitatively in good agreement with the experimental ones in Fig. 3. Namely, the Li atomic release gradually decays with increasing dose and, complementarily, the Nb atomic release increases. These variations of Li and Nb lines indicate that
N. Kishimoto et al. / Nucl. Instr. and Meth. in Phys. Res. B 206 (2003) 634–638 –
60 keV Cu => LiNbO3
Intensity ( a.u. )
10
Li (671 nm) x1 Nb (412 nm)
5
x1/2
x1/2 Cu (325 nm)
0 0
5 Dose ( 10
16
10 2
ions/cm )
60 keV Cu => LiNbO 3
4
50 O
2
Cu
Li
Nb
0
Sputtering Yield (atoms/ion)
Fig. 3. Dose dependence of ion-induced line spectra of LiNbO3 under implantation of 60 keV Cu ions at a dose rate of 10 lA/ cm2 .
Surface Recession ( nm )
637
0 0
10 20 16 2 Dose ( 10 ions/cm )
30
Fig. 4. Calculated dose dependence of surface recession of LiNbO3 and sputtering yields of Li, Nb, O and Cu from LiNbO3 under implantation of 60 keV Cu ions. The calculation was done by the TRIDYN code.
Li-depleted and Nb-enriched zones are created in the vicinity of the surface. A quantitative difference of Li in Fig. 3 from that in Fig. 4 is a slower decay of Li (experiment), which implies that Li
Fig. 5. Non-linear transient absorption of LiNbO3 implanted with 60 keV Cu at 10 lA/cm2 to 3 1016 ions/cm2 . The pumping and probing energies are 2.16 and 2.05 eV, respectively.
atoms are diffusively supplied from the deeper region, that is, radiation-enhanced diffusion of Li is manifested. As for the Cu atomic evolution, the initially-implanted Cu profile begins to be sputtered by the successive surface recession, and the Cu atomic release reaches a quasi-steady-state between the inward and outward atomic fluxes, where the sputtering yield is unity. In this context, the upper limit of Cu concentration is determined by the balance between the implantation rate and surface-recession rate, irrespective of the nominal dose. The sputtering process does not cause a significant loss of implants below about 5 1016 ions/cm2 and the peak Cu concentration is roughly proportional the nominal dose. It should be pointed out that that radiation-enhanced diffusion of Cu and Li promotes broadening nanoparticle distribution and releasing Li atoms more than the TRIDYN regime. It is speculated from the observed tendencies that the Cu diffusion may be enhanced by the Li diffusion via the Cu–Li ion exchange. The radiation-enhanced diffusion of Cu and formation of the Li-depleted zone near the surface may cause significant nanoparticle rearrangements in the composite but, as seen in Fig. 1, the appropriate implantation condition successfully
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fabricates nanoparticles of 10 nm in diameter. In fact, the linear optical absorption of the nanoparticle composite showed a surface-plasmon resonance at 1.8 eV, as predicted by the Maxwell-Garnett theory [8,17]. Fig. 5 shows the non-linear transient absorption of LiNbO3 implanted with 60 keV Cu at 10 lA/cm2 to 3 1016 ions/cm2 . The ordinate designates difference from the steady-state absorption. The pumping (at t ¼ 0) is conducted with a single-energy laser of 2.16 eV at the surfaceplasmon peak. The absorption spectrum around the peak is measured by probing white light, and the differential signal at 2.05 eV is plotted against the delay time. The pumping light induces a decrease in absorption (bleaching) as compared to the steady-state, and the bleaching quickly recovers. The nanoparticle composite of Cu:LiNbO3 exhibits an ultrafast nonlinear response of subpicoseconds, which is suitable for all-optical switching. Although the unstable behavior of Li and Cu makes the fabrication window narrow, the LiNbO3 substrate has a good potential for the ultrafast response and the wavelength tuning.
4. Summary Negative Cu ions of 60 keV have been implanted into LiNbO3 , a-SiO2 and MgO 2.4(Al2 O3 ) and to 1 1017 ions/cm2 . The nanoparticle morphology significantly depended on the substrate species, though spontaneous precipitation occurred in all cases. In comparison with a-SiO2 and MgO (Al2 O3 ), the LiNbO3 exhibited non-spherical Cu nanocrystals of 10 nm dispersing over a wide depth. Ion-induced photon spectroscopy revealed the strong atomic release of Li and dose-dependent release of Cu implants. The wide distribution of nanoparticles and the strong atomic release indicate that diffusion of Cu and Li is enhanced by ion irradiation and causes the dominant atomic migration. The nanoparticle composite of Cu:LiNbO3 exhibited an ultrafast nonlinear response of sub-picoseconds. Although the LiNbO3 under implantation showed significant Cu migra-
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