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Synthesis of silica: Metals nanocomposites and modification of their structure by swift heavy ion irradiation
Surface & Coatings Technology 203 (2009) 2432–2435
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Synthesis of silica: Metals nanocomposites and modification of their structure by swift heavy ion irradiation J.C. Pivin a,⁎, F. Singh b, Y. Mishra b, D.K. Avasthi b, J.P. Stoquert c a b c
CSNSM-IN2P3, Bat. 108, 91405 Orsay Campus, France Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India Laboratoire INESS, BP28, 67037 Strasbourg Cedex 2, France
a r t i c l e
i n f o
Available online 27 February 2009 Keywords: Metal nanoparticles Swift heavy ion irradiation Plasmon resonances Magnetic films
a b s t r a c t Ions with energies larger than 100 keV/amu produce high densities of electronic excitations in narrow cylindrical volumes around their path. The perturbation of the electronic system is transferred to the atomic network, leading to structural transformations such as the nucleation of metal particles in glasses and to changes in the particle size, shape or spatial distribution in nanocomposites. Results of our group concerning irradiation effects in silica-metal composites are discussed in terms of thermal spikes and hammering mechanisms. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The interest of nanomaterials lies in the strong variation of most properties of solid particles as a function of their size when it is in the range of 1 to 100 nm (below 1 nm the electronic structure becomes molecular). Ion beam irradiation constitutes an efficient means to modify the nanostructure of solids because of the nanometric size of the volume perturbed by each ion, whatever the stopping regime [1]. The nucleation of particles in metastable solid-solutions and the modifications of the structure of nanocomposites under the effect of electronic excitations and ionizations produced by swift ions have been much less investigated than those occurring in the ballistic regime of slowing down. One knows that the high density of positive charges remaining in the core of swift ion tracks after the ejection of secondary electrons, is liable to provoke Coulomb explosions [2], especially when the target is an insulator. Beside that, the energy deposited in electronic excitations in the core is transmitted to the lattice by electron phonon coupling, which leads to a strong agitation of atoms during some fraction of nanosecond, comparable to a thermal spike [3]. Consequently, the track halo undergoes a pressure spike whenever the density of the molten core is lower than that of the solid phase. A third effect, which has been observed exclusively in amorphous targets, consists of a dilatation perpendicular to the beam direction and a compression parallel to the beam, known under the name of hammering [4]. When this target contains crystalline inclusions, the hammering of the amorphous matrix is liable to provoke an elongation of the inclusions parallel to the beam direction by creep, because of their compression perpendicular to the beam
⁎ Corresponding author. Tel.: +33 169155212; fax: +33 169155268. E-mail address: [email protected] (J.C. Pivin). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.02.033
direction and heating [5]. Thermal spikes and Coulomb explosions may as well lead to a dissolution of particles contained in a matrix or to a rearrangement of these particles in strings along the ion path [6], or also to an anisotropic ripening of the particles with a long dimension parallel to the ion path. These structural transformations have interesting consequences for the optical or magnetic properties of particles embedded in insulating matrices, such as a shift or splitting of their surface plasmon resonance [7], useful for adjusting the properties of wave-guides or optical filters, or a tilt of their easy magnetization axis parallel to the ion beam [8], permitting to record information with high density perpendicular to the surface. Swift Heavy Ions (SHI) irradiation of nanoparticles embedded in a matrix should permit to control their size, shape and interface state (which is of high importance when the proportion of atoms present at the surface becomes noticeable). However, the most relevant effect of SHI irradiation in composite targets, among spikes, explosions, hammering, is not well identified. The size and volume fraction of the particles play probably a major role in all these mechanisms, especially in thermal spikes as will be discussed on the basis of a few simulations. The intention of the present paper is not to propose universal rules but to show some trends for metal particles in a silica matrix. The studied materials were synthesized by ion exchange, sol–gel chemistry or sputtering techniques and details of the investigations can be found in Refs. [8–11]. 2. Nucleation of metal particles When metastable systems such as silicates or substoichiometric SiOx films containing metal ions in solid solution, are irradiated by an ion beam of a few MeV, a reduction of the metal often occurs under the effect of electronic excitations and collision cascades play little role in the nucleation process. Even He ions of 1 MeV, with an electronic
J.C. Pivin et al. / Surface & Coatings Technology 203 (2009) 2432–2435
stopping power Se as low as 300 eV/nm/ion (and a negligible nuclear stopping power), are able to reduce Ag ions inserted by ion exchange in a soda lime glass [9]. In this case, the precipitation kinetics is simply a linear function of the transferred energy Se × ϕ per unit length, ϕ being the ion fluence. Irradiation experiments have also been performed with He, Si or Au ions of a few MeV on gel films containing Fe, Ni, Co or Cu in solid solution. The used gel, with a formula of SiH(OH)3 ,formed from triethoxysilane, is transformed by thermal treatment or irradiation (when free of metal ions Mn+ in solution) into a suboxide SiOx, then decomposed into silica and Si nanoparticles [10]. When the gel contains metal ions, their reduction by Si–H radicals occurs instead of the precipitation of Si. The precipitation kinetics of the transition metals is not a linear function of the energy deposited in electronic excitations, because of their higher solubility in silica. A critical amount of energy is required to reduce enough M atoms and form nuclei with a stable size, so that an overlap of the tracks is needed and the precipitation kinetics obeys a Poisson law [10]. Now, more energetic ions deposit enough energy for reducing Fe ions, even in stoichiometric silica [11]. The solubility of Fe in silica films grown by low temperature processes such as magnetron sputtering (involving the deposition of atoms with low kinetic energies), is so high that less than 2 at.%. Fe is precipitated in films containing an overall concentration of 14 at.% Fe. Irradiation with swift heavy ions, such as for instance 100 MeV Au, proves more efficient than a thermal treatment at high temperature under Ar:H2 atmosphere for reducing these Fe ions, since a reduction yield of 12 at.% in a film, containing 14 at.% Fe is achieved as against 4.5 at.% by annealing at 900 °C. Two other features of nanocomposites formed by ion irradiation are worth to note. First, the formed particles exhibit a much narrower size distribution than those obtained by thermal treatment. Next, they are aligned along the ion beam direction, as shown in Fig. 1. Such a structure is attracting for the perpendicular magnetic recording. However magnetometry and ESR characterizations of SiO2:14% Fe films show that their easy magnetization axis remains parallel to the surface, because of the strong dipolar interaction between the strings of nanoparticles. 3. Dissolution or growth of particles as a function of their size, volume fraction and of the deposited energy density The local increase of temperature after an ion impact, as a function of the distance r to the ion path and time, can be calculated by solving the system of two equations of thermal exchange in the electronic and atomic subsystems. Even for a single phased target, the calculations must be performed step by step (dr, dt) and, when considering a system made of a single spherical particle at the origin of the ordinates (r, z, θ, in
Fig. 1. TEM image of the cross section of a SiO2: Fe film containing 14 at.% Fe, deposited by magnetron sputtering (Fe was in solid solution before irradiation), irradiated with 1013 Au ions/cm2 of 100 MeV.
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cylindrical frame) embedded in a matrix, one has to take into account changes in the specific heat C, thermal conductivity K and the electron– phonon coupling constant g as a function of the position (Fig. 2).
Ce ρ
Ca ρ
→ ATe r ; t At → ATa r ; t At
! → → → → = jd Ke jTe r ; t − g ðTe − Ta Þ + A r ; t ! → → → = jd Ka jTa r ; t + g ðTe − Ta Þ
Where, Ce,a, Ke,a, and Te,a are relative to the electronic (e) and atomic (a) systems respectively, ρ is the mass density of the lattice and A(r,t) is the energy density provided by the incident ion to the electronic subsystem at radius r and time t. Calculations were performed for Ag and Au ions of 100 MeV (with very comparable stopping powers) and Fe, Ag, Au particles of increasing sizes. One important output of these calculations is that Fe particles are melted only when their diameter is smaller than that of melted silica (7.5 nm), while Ag particles with sizes up to 15 nm or Au ones with sizes up to 30 nm are melted in the same conditions. The effect must be ascribed to the difference of thermal conductivity and not to that of melting temperature, because the temperature reached in the smallest particles is in all cases in the range of 2500–3000 K (Fig. 2). A second important result is that the temperature in the particles increases all the more as they are small. When Ag or Au particles are about 2 times larger than the molten cylinder of silica, their temperature exceeds significantly the melting point of the metal only at the particle periphery. Therefore, one can expect that the part of the particle which is outside the cylinder flow more easily in the track and that the particle become elongated during the freezing stage. But the Fe particles which are not melted only when smaller than this cylinder should remain spherical. Irradiation of silica films containing Ag particles with a low volume fraction (1–5%) and small sizes (1–3 nm) compared to that of ion tracks may lead to two opposite results, depending on Se [9]. He or Au ions of 1–3 MeV are not able to produce thermal spikes but enough ionizations at the surface of the particles for promoting the desorption of Ag ions and their diffusion in silica. The result of this is a growth of some particles at the expense of others, in proportion to the amount of transferred energy Se × ϕ (kinetic law which is characteristic of the desorption process). When the same films are irradiated with 100 MeV Au or Ag ions (Se of the order of 10 keV/nm/ion), the particles are dissolved. This dissolution was ascribed to an excessive increase of the desorption rate with respect to that of the diffusion in the matrix [9] (in some respect a Coulombian explosion of the particles). But the above mentioned calculations of the thermal dissipation of the energy give a more simple explanation of this dissolution, which is their melting. For slightly larger volume fractions (6%) and sizes (varying from 2 to 15 nm) of Ag particles in silica, a rearrangement of the particles, in arrays aligned along the ion beam direction was observed by TEM after irradiation with 1015 Si ions of 30 MeV [6]. This rearrangement induces an increase in the dipolar interaction between the particles, with interesting effect a splitting of their surface plasmon resonance peak under polarized light (also observed when the particles become elongated and for same reason which is the anisotropy of electric field). But the particles alignment is not easy to interpret in terms of thermal spikes since the fluences used in this experiment were so high that melted particles should have moved at random from one track to another during the irradiation. Astonishingly the behaviors of 2 metals having very comparable thermal properties (melting point, fusion enthalpy, conductivity…) like Ag and Au are quite different [5,12]. In-situ X-ray diffraction at grazing incidence was used to assess accurately SHI effects on the size of these particles in silica films for comparable initial sizes (4 nm)
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J.C. Pivin et al. / Surface & Coatings Technology 203 (2009) 2432–2435
largest ones, and the diffusion of Au atoms in the molten cylinders of silica induced a growth of the largest particles along the track axis. The interpretation of other TEM experiments on a system, containing only 5% Ag particles with an initial diameter of (9.6 ± 1.5) nm, irradiated with 3 × 1013 Au ions of 120 MeV is less easy, since the smallest particles remained spherical and were dissolved, while coarser particles got ellipsoidal shapes which might be ascribed for a part to the hammering of the matrix [15] The different behaviors of the 2 metals may result from (i) the larger solubility of Ag in silica (up to 6% out of equilibrium [6]), (ii) the highest temperatures reached in small Au particles. Another important factor which has been evidenced by RBS analyses of the films irradiated in same conditions is that Ag atoms evolve from particles with sizes up to 15 nm (their temperature does not reach the boiling point whatever their size), but no electronic sputtering is observed for Au particles of same range of sizes. 4. Effects of stress on the magnetic anisotropy of Fe and FePt particles Fig. 2. Radial distribution of temperature for Ag particles of different diameters (indicated in the frame) 0.1 ns after the ion go past (time at which the temperature in the lattice reaches a maximum). The inset indicates the cylindrical coordinates (y,r) used in calculations.
and a large volume fraction of 15%. In the case of Au particles, a fast increase of the mean particle size was observed up to 7 nm, under irradiation with 90 MeV Ni ions, then a slower growth at high ion fluences. The fast initial growth rate is ascribed to the diffusion of Au atoms in the molten core of tracks (with a diameter estimated to 5 nm) and the second stage to the diffusion in the track halo, because of the larger thermal conductivity in the particles than in the matrix [13]. In the case of Ag particles, all other parameters being similar, the growth rate during the initial stage was found much smaller [13]. An elongation of Au and Ag particles parallel to the ion beam direction was observed by TEM in silica films containing the same concentrations of 15% Ag or Au particles with larger sizes obtained by annealing at 800 °C, after irradiation with Au ions of 120 MeV at a fluence of 3 × 1013/cm2, but it was more significant in the case of Au [13,14] (Fig. 3). It is worth to emphasize that the elongation and complex shape of the Au particles observed in Fig. 3 cannot be ascribed to the hammering of the matrix, because the smallest particles should undergo the largest elongation. In fact, they remain spherical and their proportion decreases upon irradiation and large particles get irregular shape and seem to percolate. Obviously, the small particles were dissolved during spikes, together with the peripheral part of the
Fig. 3. TEM image of the cross section of a SiO2: Au film containing 15 at.% Au, deposited by magnetron sputtering and annealed at 800 °C, then irradiated with 3 × 1013 Au of 120 MeV. After the annealing the Au particles were spherical with a mean diameter of 12.5 nm and the straggling of the Gaussian distribution of sizes was 5.5 nm.
Normally the magnetization of a thin ferromagnetic film is easier parallel to its surface because of the shape anisotropy. An easy magnetization perpendicular to the surface is preferable for a magnetic recording at high density, and this goal can be achieved by irradiation with swift heavy ions in the case of films made of silica embedding small Fe or FePt particles with a low volume fraction (the L10 quadratic FePt phase is interesting for its strong coercivity). Electron spin resonance is the most suitable technique to investigate changes in the magnetization anisotropy of thin insulating films, at least when the magnetization is homogeneous, since in such experiments the resonance field is minimum parallel to the easy magnetization axis and its variation with the tilt angle is given by well known equations [16]. The resonance field of films containing 3–7 vol.% of Fe particles (superparamagnetic at room temperature) [11] or 3 vol.% of FePt compound particles (ferromagnetic at RT) is minimum parallel to the surface in pristine state and becomes perpendicular to the surface after irradiation with 1013 Au ions of 100 MeV as shown in Fig. 4. The tilt of easy magnetization axis is ascribed in both cases to the stress exerted on the particles by the matrix, because of the hammering
Fig. 4. Angular variation of the electron spin resonance (ESR) field as a function of the angle φ between the applied field H and the surface, in a silica film containing 3 vol.% FePt particles, irradiated with 100 MeV Au ions at fluences indicated in the frame (n.i. is for non irradiated). The film was deposited by magnetron cosputtering and particles of the L10 tetragonal phase, with a mean diameter of 11 nm, were formed by annealing at 700 °C in Ar:5% H2 atmosphere for 1 h.
J.C. Pivin et al. / Surface & Coatings Technology 203 (2009) 2432–2435
effect of SHI on amorphous targets [4,5]. Indeed no alignment of the Fe (FePt) particles nor any elongation or formation of nanowires, modifying the shape anisotropy was observed in TEM. A more systematic investigation of the effect of swift heavy ion irradiation on the magnetic properties of Fe particles in silica was performed for different particle diameters in the range of 5 to 20 nm and volume fractions up to 25%. For high volume fractions, only a slight tilt (by 10– 20°) of the easy axis was observed with irradiation, on account of the angular dependence of the resonance field in ESR. But the magnetostriction of the particles induces systematically an increase of coercive field, by a factor of 2 to 3, and of remnant magnetization of the films perpendicular to their surface, after irradiation at enough high ion fluence for producing an hammering effect in the matrix (with 100 MeV Au ions at fluences of 1013 to 1014) [13]. On the contrary, little change of coercive field and a slight decrease of remnant magnetization are measured when the field is applied parallel to the surface. Very similar changes in the angular dependence of the coercivity have been reported for Co particles grown by ion implantation in silica (with a mean size of 8 nm and volume fraction of about 10%) then irradiated with 200 MeV I ions. In this case, an elongation of the Co particles parallel to the ions path was observed in TEM [17], but none for Fe particles of any size in films grown by magnetron sputtering studied by our group. This different behavior of Co and Fe particles is probably due to the twice smaller thermal conductivity of Co (other thermal parameters being comparable). However that may be, the main reason of the anisotropy modification is probably the same in both cases, which is the magnetostriction imposed by the matrix. 5. Conclusions Swift heavy ions induce changes in the size, shape or spatial distribution of metal particles embedded in silica and most probably in other metal-insulator systems which have not yet been investigated. The observed effects are complex functions of the thermodynamic properties of the particles and matrix and geometrical factors affecting their coalescence during the transient thermal spikes induced by irradiation with swift heavy ions. If that is still difficult to propose general rules, applications are foreseeable in optics and magnetism. The trend of particles to rearrange in strings or to become elongated parallel to the ion paths, depending
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on their nature, size and volume fraction, can be used for modifying the absorption wavelength of noble metal particles. The tilt of the easy magnetization axis of Fe or FePt particles from parallel to perpendicular to the surface under the effect of stress, when their volume fraction is limited, is also of high interest for increasing the density of magnetic recording. Ions of lower energies, if inducing no transient melting of the target, are however able to produce a precipitation of metals in oversaturated solid solutions and the obtained systems are interesting for the narrow range of sizes of the particles. Ions with energies closer to the Bragg peak reduce more easily metal ions in solid solutions (for instance Fe in stoichiometric silica) but dissolve these particles when tracks overlap [11]. References [1] J.C. Pivin, in: A. Vaseashta, D. Dimova-Malinovska, J.M. Marshall (Eds.), Nanostructured and Advanced Materials, NATO Science Series, 204, 2005, p. 155. [2] D. Lesueur, A. Dunlop, Rad. Effects 126 (1993) 163. [3] M. Toulemonde, C. Trautmann, E. Balanzat, K. Hjoirt, A. Weidinger, Nucl. Instr. Meth. B216 (2004) 1. [4] S. Klaumünzer, C.L. Li, S. Löffler, M. Rammensee, G. Schumacher, H.C. Neitzer, Rad. Eff. Def. Sol. 108 (1989) 131. [5] J.J. Penninkhof, T. van Dillen, S. Roorda, C. Graf, A. van Blaaderen, A.M. Vredenberg, A. Polman, Nucl. Instr. Met. Phys. Res. B242 (2006) 523. [6] J.J. Penninkhof, A. Polman, L.A. Sweatlock, S.A. Maier, H. Atwater, A.M. Vredenberg, B.J. Kool, Appl. Phys. Lett. 83 (2003) 4137. [7] S. Link, M.A. El-Sayed, J. Phys. Chem. B103 (1999) 8410. [8] J.C. Pivin, S. Esnouf, F. Singh, D.K. Avasthi, J. Appl. Phys. 98 (2005) 1. [9] J.C. Pivin, G. Roger, M.A. Garcia, F. Singh, D.K. Avasthi, Nucl. Instr. Meth. B215 (2004) 373. [10] J.C. Pivin, D.K. Avasthi, F. Singh, A. Kumar, E. Pippel, G. Sagon, Nucl. Instr. Meth. Phys. Res. B 236 (2005) 73. [11] F. Singh, D.K. Avasthi, O. Angelov, P. Berthet, J.C. Pivin, Nucl. Instr. Meth. Phys. Res. B245 (2006) 214. [12] Y.K. Mishra, D.K. Avasthi, P.K. Kulriya, F. Singh, D. Kabiraj, A. Tripathi, J.C. Pivin, I.S. Bayer, Abhijit Biswas, Appl. Phys. Lett. 90 (2007) 73110. [13] F. Singh, PhD thesis, Orsay University (2007). [14] Y.K. Mishra, F. Singh, D.K. Avasthi, J.C. Pivin, D. Malinovska, E. Pippel, Appl. Phys. Lett. 91 (2007) 1. [15] F. Singh, S Mohapatra, J.C. Pivin, D.K. Avasthi, Nucl. Instr. and Meth. in Phys. Research B (in press). [16] C. Chappert, K. Le Dang, P. Beauvillain, H. HUrdequint, D. Renard, Phys Rev B 34 (1986) 3192. [17] C. D'orleans, J.P. Stoquert, C. Estournes, C. Cerruti, J.J. Grob, J.L. Guille, F. Haas, D. Muller, M. Richard-Plouet, Phys. Rev. B67 (2003) 220101.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Synthesis of silica: Metals nanocomposites and modification of their structure by swift heavy ion irradiation J.C. Pivin a,⁎, F. Singh b, Y. Mishra b, D.K. Avasthi b, J.P. Stoquert c a b c
CSNSM-IN2P3, Bat. 108, 91405 Orsay Campus, France Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India Laboratoire INESS, BP28, 67037 Strasbourg Cedex 2, France
a r t i c l e
i n f o
Available online 27 February 2009 Keywords: Metal nanoparticles Swift heavy ion irradiation Plasmon resonances Magnetic films
a b s t r a c t Ions with energies larger than 100 keV/amu produce high densities of electronic excitations in narrow cylindrical volumes around their path. The perturbation of the electronic system is transferred to the atomic network, leading to structural transformations such as the nucleation of metal particles in glasses and to changes in the particle size, shape or spatial distribution in nanocomposites. Results of our group concerning irradiation effects in silica-metal composites are discussed in terms of thermal spikes and hammering mechanisms. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The interest of nanomaterials lies in the strong variation of most properties of solid particles as a function of their size when it is in the range of 1 to 100 nm (below 1 nm the electronic structure becomes molecular). Ion beam irradiation constitutes an efficient means to modify the nanostructure of solids because of the nanometric size of the volume perturbed by each ion, whatever the stopping regime [1]. The nucleation of particles in metastable solid-solutions and the modifications of the structure of nanocomposites under the effect of electronic excitations and ionizations produced by swift ions have been much less investigated than those occurring in the ballistic regime of slowing down. One knows that the high density of positive charges remaining in the core of swift ion tracks after the ejection of secondary electrons, is liable to provoke Coulomb explosions [2], especially when the target is an insulator. Beside that, the energy deposited in electronic excitations in the core is transmitted to the lattice by electron phonon coupling, which leads to a strong agitation of atoms during some fraction of nanosecond, comparable to a thermal spike [3]. Consequently, the track halo undergoes a pressure spike whenever the density of the molten core is lower than that of the solid phase. A third effect, which has been observed exclusively in amorphous targets, consists of a dilatation perpendicular to the beam direction and a compression parallel to the beam, known under the name of hammering [4]. When this target contains crystalline inclusions, the hammering of the amorphous matrix is liable to provoke an elongation of the inclusions parallel to the beam direction by creep, because of their compression perpendicular to the beam
⁎ Corresponding author. Tel.: +33 169155212; fax: +33 169155268. E-mail address: [email protected] (J.C. Pivin). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.02.033
direction and heating [5]. Thermal spikes and Coulomb explosions may as well lead to a dissolution of particles contained in a matrix or to a rearrangement of these particles in strings along the ion path [6], or also to an anisotropic ripening of the particles with a long dimension parallel to the ion path. These structural transformations have interesting consequences for the optical or magnetic properties of particles embedded in insulating matrices, such as a shift or splitting of their surface plasmon resonance [7], useful for adjusting the properties of wave-guides or optical filters, or a tilt of their easy magnetization axis parallel to the ion beam [8], permitting to record information with high density perpendicular to the surface. Swift Heavy Ions (SHI) irradiation of nanoparticles embedded in a matrix should permit to control their size, shape and interface state (which is of high importance when the proportion of atoms present at the surface becomes noticeable). However, the most relevant effect of SHI irradiation in composite targets, among spikes, explosions, hammering, is not well identified. The size and volume fraction of the particles play probably a major role in all these mechanisms, especially in thermal spikes as will be discussed on the basis of a few simulations. The intention of the present paper is not to propose universal rules but to show some trends for metal particles in a silica matrix. The studied materials were synthesized by ion exchange, sol–gel chemistry or sputtering techniques and details of the investigations can be found in Refs. [8–11]. 2. Nucleation of metal particles When metastable systems such as silicates or substoichiometric SiOx films containing metal ions in solid solution, are irradiated by an ion beam of a few MeV, a reduction of the metal often occurs under the effect of electronic excitations and collision cascades play little role in the nucleation process. Even He ions of 1 MeV, with an electronic
J.C. Pivin et al. / Surface & Coatings Technology 203 (2009) 2432–2435
stopping power Se as low as 300 eV/nm/ion (and a negligible nuclear stopping power), are able to reduce Ag ions inserted by ion exchange in a soda lime glass [9]. In this case, the precipitation kinetics is simply a linear function of the transferred energy Se × ϕ per unit length, ϕ being the ion fluence. Irradiation experiments have also been performed with He, Si or Au ions of a few MeV on gel films containing Fe, Ni, Co or Cu in solid solution. The used gel, with a formula of SiH(OH)3 ,formed from triethoxysilane, is transformed by thermal treatment or irradiation (when free of metal ions Mn+ in solution) into a suboxide SiOx, then decomposed into silica and Si nanoparticles [10]. When the gel contains metal ions, their reduction by Si–H radicals occurs instead of the precipitation of Si. The precipitation kinetics of the transition metals is not a linear function of the energy deposited in electronic excitations, because of their higher solubility in silica. A critical amount of energy is required to reduce enough M atoms and form nuclei with a stable size, so that an overlap of the tracks is needed and the precipitation kinetics obeys a Poisson law [10]. Now, more energetic ions deposit enough energy for reducing Fe ions, even in stoichiometric silica [11]. The solubility of Fe in silica films grown by low temperature processes such as magnetron sputtering (involving the deposition of atoms with low kinetic energies), is so high that less than 2 at.%. Fe is precipitated in films containing an overall concentration of 14 at.% Fe. Irradiation with swift heavy ions, such as for instance 100 MeV Au, proves more efficient than a thermal treatment at high temperature under Ar:H2 atmosphere for reducing these Fe ions, since a reduction yield of 12 at.% in a film, containing 14 at.% Fe is achieved as against 4.5 at.% by annealing at 900 °C. Two other features of nanocomposites formed by ion irradiation are worth to note. First, the formed particles exhibit a much narrower size distribution than those obtained by thermal treatment. Next, they are aligned along the ion beam direction, as shown in Fig. 1. Such a structure is attracting for the perpendicular magnetic recording. However magnetometry and ESR characterizations of SiO2:14% Fe films show that their easy magnetization axis remains parallel to the surface, because of the strong dipolar interaction between the strings of nanoparticles. 3. Dissolution or growth of particles as a function of their size, volume fraction and of the deposited energy density The local increase of temperature after an ion impact, as a function of the distance r to the ion path and time, can be calculated by solving the system of two equations of thermal exchange in the electronic and atomic subsystems. Even for a single phased target, the calculations must be performed step by step (dr, dt) and, when considering a system made of a single spherical particle at the origin of the ordinates (r, z, θ, in
Fig. 1. TEM image of the cross section of a SiO2: Fe film containing 14 at.% Fe, deposited by magnetron sputtering (Fe was in solid solution before irradiation), irradiated with 1013 Au ions/cm2 of 100 MeV.
2433
cylindrical frame) embedded in a matrix, one has to take into account changes in the specific heat C, thermal conductivity K and the electron– phonon coupling constant g as a function of the position (Fig. 2).
Ce ρ
Ca ρ
→ ATe r ; t At → ATa r ; t At
! → → → → = jd Ke jTe r ; t − g ðTe − Ta Þ + A r ; t ! → → → = jd Ka jTa r ; t + g ðTe − Ta Þ
Where, Ce,a, Ke,a, and Te,a are relative to the electronic (e) and atomic (a) systems respectively, ρ is the mass density of the lattice and A(r,t) is the energy density provided by the incident ion to the electronic subsystem at radius r and time t. Calculations were performed for Ag and Au ions of 100 MeV (with very comparable stopping powers) and Fe, Ag, Au particles of increasing sizes. One important output of these calculations is that Fe particles are melted only when their diameter is smaller than that of melted silica (7.5 nm), while Ag particles with sizes up to 15 nm or Au ones with sizes up to 30 nm are melted in the same conditions. The effect must be ascribed to the difference of thermal conductivity and not to that of melting temperature, because the temperature reached in the smallest particles is in all cases in the range of 2500–3000 K (Fig. 2). A second important result is that the temperature in the particles increases all the more as they are small. When Ag or Au particles are about 2 times larger than the molten cylinder of silica, their temperature exceeds significantly the melting point of the metal only at the particle periphery. Therefore, one can expect that the part of the particle which is outside the cylinder flow more easily in the track and that the particle become elongated during the freezing stage. But the Fe particles which are not melted only when smaller than this cylinder should remain spherical. Irradiation of silica films containing Ag particles with a low volume fraction (1–5%) and small sizes (1–3 nm) compared to that of ion tracks may lead to two opposite results, depending on Se [9]. He or Au ions of 1–3 MeV are not able to produce thermal spikes but enough ionizations at the surface of the particles for promoting the desorption of Ag ions and their diffusion in silica. The result of this is a growth of some particles at the expense of others, in proportion to the amount of transferred energy Se × ϕ (kinetic law which is characteristic of the desorption process). When the same films are irradiated with 100 MeV Au or Ag ions (Se of the order of 10 keV/nm/ion), the particles are dissolved. This dissolution was ascribed to an excessive increase of the desorption rate with respect to that of the diffusion in the matrix [9] (in some respect a Coulombian explosion of the particles). But the above mentioned calculations of the thermal dissipation of the energy give a more simple explanation of this dissolution, which is their melting. For slightly larger volume fractions (6%) and sizes (varying from 2 to 15 nm) of Ag particles in silica, a rearrangement of the particles, in arrays aligned along the ion beam direction was observed by TEM after irradiation with 1015 Si ions of 30 MeV [6]. This rearrangement induces an increase in the dipolar interaction between the particles, with interesting effect a splitting of their surface plasmon resonance peak under polarized light (also observed when the particles become elongated and for same reason which is the anisotropy of electric field). But the particles alignment is not easy to interpret in terms of thermal spikes since the fluences used in this experiment were so high that melted particles should have moved at random from one track to another during the irradiation. Astonishingly the behaviors of 2 metals having very comparable thermal properties (melting point, fusion enthalpy, conductivity…) like Ag and Au are quite different [5,12]. In-situ X-ray diffraction at grazing incidence was used to assess accurately SHI effects on the size of these particles in silica films for comparable initial sizes (4 nm)
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largest ones, and the diffusion of Au atoms in the molten cylinders of silica induced a growth of the largest particles along the track axis. The interpretation of other TEM experiments on a system, containing only 5% Ag particles with an initial diameter of (9.6 ± 1.5) nm, irradiated with 3 × 1013 Au ions of 120 MeV is less easy, since the smallest particles remained spherical and were dissolved, while coarser particles got ellipsoidal shapes which might be ascribed for a part to the hammering of the matrix [15] The different behaviors of the 2 metals may result from (i) the larger solubility of Ag in silica (up to 6% out of equilibrium [6]), (ii) the highest temperatures reached in small Au particles. Another important factor which has been evidenced by RBS analyses of the films irradiated in same conditions is that Ag atoms evolve from particles with sizes up to 15 nm (their temperature does not reach the boiling point whatever their size), but no electronic sputtering is observed for Au particles of same range of sizes. 4. Effects of stress on the magnetic anisotropy of Fe and FePt particles Fig. 2. Radial distribution of temperature for Ag particles of different diameters (indicated in the frame) 0.1 ns after the ion go past (time at which the temperature in the lattice reaches a maximum). The inset indicates the cylindrical coordinates (y,r) used in calculations.
and a large volume fraction of 15%. In the case of Au particles, a fast increase of the mean particle size was observed up to 7 nm, under irradiation with 90 MeV Ni ions, then a slower growth at high ion fluences. The fast initial growth rate is ascribed to the diffusion of Au atoms in the molten core of tracks (with a diameter estimated to 5 nm) and the second stage to the diffusion in the track halo, because of the larger thermal conductivity in the particles than in the matrix [13]. In the case of Ag particles, all other parameters being similar, the growth rate during the initial stage was found much smaller [13]. An elongation of Au and Ag particles parallel to the ion beam direction was observed by TEM in silica films containing the same concentrations of 15% Ag or Au particles with larger sizes obtained by annealing at 800 °C, after irradiation with Au ions of 120 MeV at a fluence of 3 × 1013/cm2, but it was more significant in the case of Au [13,14] (Fig. 3). It is worth to emphasize that the elongation and complex shape of the Au particles observed in Fig. 3 cannot be ascribed to the hammering of the matrix, because the smallest particles should undergo the largest elongation. In fact, they remain spherical and their proportion decreases upon irradiation and large particles get irregular shape and seem to percolate. Obviously, the small particles were dissolved during spikes, together with the peripheral part of the
Fig. 3. TEM image of the cross section of a SiO2: Au film containing 15 at.% Au, deposited by magnetron sputtering and annealed at 800 °C, then irradiated with 3 × 1013 Au of 120 MeV. After the annealing the Au particles were spherical with a mean diameter of 12.5 nm and the straggling of the Gaussian distribution of sizes was 5.5 nm.
Normally the magnetization of a thin ferromagnetic film is easier parallel to its surface because of the shape anisotropy. An easy magnetization perpendicular to the surface is preferable for a magnetic recording at high density, and this goal can be achieved by irradiation with swift heavy ions in the case of films made of silica embedding small Fe or FePt particles with a low volume fraction (the L10 quadratic FePt phase is interesting for its strong coercivity). Electron spin resonance is the most suitable technique to investigate changes in the magnetization anisotropy of thin insulating films, at least when the magnetization is homogeneous, since in such experiments the resonance field is minimum parallel to the easy magnetization axis and its variation with the tilt angle is given by well known equations [16]. The resonance field of films containing 3–7 vol.% of Fe particles (superparamagnetic at room temperature) [11] or 3 vol.% of FePt compound particles (ferromagnetic at RT) is minimum parallel to the surface in pristine state and becomes perpendicular to the surface after irradiation with 1013 Au ions of 100 MeV as shown in Fig. 4. The tilt of easy magnetization axis is ascribed in both cases to the stress exerted on the particles by the matrix, because of the hammering
Fig. 4. Angular variation of the electron spin resonance (ESR) field as a function of the angle φ between the applied field H and the surface, in a silica film containing 3 vol.% FePt particles, irradiated with 100 MeV Au ions at fluences indicated in the frame (n.i. is for non irradiated). The film was deposited by magnetron cosputtering and particles of the L10 tetragonal phase, with a mean diameter of 11 nm, were formed by annealing at 700 °C in Ar:5% H2 atmosphere for 1 h.
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effect of SHI on amorphous targets [4,5]. Indeed no alignment of the Fe (FePt) particles nor any elongation or formation of nanowires, modifying the shape anisotropy was observed in TEM. A more systematic investigation of the effect of swift heavy ion irradiation on the magnetic properties of Fe particles in silica was performed for different particle diameters in the range of 5 to 20 nm and volume fractions up to 25%. For high volume fractions, only a slight tilt (by 10– 20°) of the easy axis was observed with irradiation, on account of the angular dependence of the resonance field in ESR. But the magnetostriction of the particles induces systematically an increase of coercive field, by a factor of 2 to 3, and of remnant magnetization of the films perpendicular to their surface, after irradiation at enough high ion fluence for producing an hammering effect in the matrix (with 100 MeV Au ions at fluences of 1013 to 1014) [13]. On the contrary, little change of coercive field and a slight decrease of remnant magnetization are measured when the field is applied parallel to the surface. Very similar changes in the angular dependence of the coercivity have been reported for Co particles grown by ion implantation in silica (with a mean size of 8 nm and volume fraction of about 10%) then irradiated with 200 MeV I ions. In this case, an elongation of the Co particles parallel to the ions path was observed in TEM [17], but none for Fe particles of any size in films grown by magnetron sputtering studied by our group. This different behavior of Co and Fe particles is probably due to the twice smaller thermal conductivity of Co (other thermal parameters being comparable). However that may be, the main reason of the anisotropy modification is probably the same in both cases, which is the magnetostriction imposed by the matrix. 5. Conclusions Swift heavy ions induce changes in the size, shape or spatial distribution of metal particles embedded in silica and most probably in other metal-insulator systems which have not yet been investigated. The observed effects are complex functions of the thermodynamic properties of the particles and matrix and geometrical factors affecting their coalescence during the transient thermal spikes induced by irradiation with swift heavy ions. If that is still difficult to propose general rules, applications are foreseeable in optics and magnetism. The trend of particles to rearrange in strings or to become elongated parallel to the ion paths, depending
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on their nature, size and volume fraction, can be used for modifying the absorption wavelength of noble metal particles. The tilt of the easy magnetization axis of Fe or FePt particles from parallel to perpendicular to the surface under the effect of stress, when their volume fraction is limited, is also of high interest for increasing the density of magnetic recording. Ions of lower energies, if inducing no transient melting of the target, are however able to produce a precipitation of metals in oversaturated solid solutions and the obtained systems are interesting for the narrow range of sizes of the particles. Ions with energies closer to the Bragg peak reduce more easily metal ions in solid solutions (for instance Fe in stoichiometric silica) but dissolve these particles when tracks overlap [11]. References [1] J.C. Pivin, in: A. Vaseashta, D. Dimova-Malinovska, J.M. Marshall (Eds.), Nanostructured and Advanced Materials, NATO Science Series, 204, 2005, p. 155. [2] D. Lesueur, A. Dunlop, Rad. Effects 126 (1993) 163. [3] M. Toulemonde, C. Trautmann, E. Balanzat, K. Hjoirt, A. Weidinger, Nucl. Instr. Meth. B216 (2004) 1. [4] S. Klaumünzer, C.L. Li, S. Löffler, M. Rammensee, G. Schumacher, H.C. Neitzer, Rad. Eff. Def. Sol. 108 (1989) 131. [5] J.J. Penninkhof, T. van Dillen, S. Roorda, C. Graf, A. van Blaaderen, A.M. Vredenberg, A. Polman, Nucl. Instr. Met. Phys. Res. B242 (2006) 523. [6] J.J. Penninkhof, A. Polman, L.A. Sweatlock, S.A. Maier, H. Atwater, A.M. Vredenberg, B.J. Kool, Appl. Phys. Lett. 83 (2003) 4137. [7] S. Link, M.A. El-Sayed, J. Phys. Chem. B103 (1999) 8410. [8] J.C. Pivin, S. Esnouf, F. Singh, D.K. Avasthi, J. Appl. Phys. 98 (2005) 1. [9] J.C. Pivin, G. Roger, M.A. Garcia, F. Singh, D.K. Avasthi, Nucl. Instr. Meth. B215 (2004) 373. [10] J.C. Pivin, D.K. Avasthi, F. Singh, A. Kumar, E. Pippel, G. Sagon, Nucl. Instr. Meth. Phys. Res. B 236 (2005) 73. [11] F. Singh, D.K. Avasthi, O. Angelov, P. Berthet, J.C. Pivin, Nucl. Instr. Meth. Phys. Res. B245 (2006) 214. [12] Y.K. Mishra, D.K. Avasthi, P.K. Kulriya, F. Singh, D. Kabiraj, A. Tripathi, J.C. Pivin, I.S. Bayer, Abhijit Biswas, Appl. Phys. Lett. 90 (2007) 73110. [13] F. Singh, PhD thesis, Orsay University (2007). [14] Y.K. Mishra, F. Singh, D.K. Avasthi, J.C. Pivin, D. Malinovska, E. Pippel, Appl. Phys. Lett. 91 (2007) 1. [15] F. Singh, S Mohapatra, J.C. Pivin, D.K. Avasthi, Nucl. Instr. and Meth. in Phys. Research B (in press). [16] C. Chappert, K. Le Dang, P. Beauvillain, H. HUrdequint, D. Renard, Phys Rev B 34 (1986) 3192. [17] C. D'orleans, J.P. Stoquert, C. Estournes, C. Cerruti, J.J. Grob, J.L. Guille, F. Haas, D. Muller, M. Richard-Plouet, Phys. Rev. B67 (2003) 220101.