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Ion beam induced sintering of colloidal polystyrene nano-masks
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 257 (2007) 455–458 www.elsevier.com/locate/nimb
Ion beam induced sintering of colloidal polystyrene nano-masks D. Kraus, J.K.N. Lindner *, B. Stritzker Universita¨t Augsburg, Institut fu¨r Physik, Universita¨tsstr. 1, D-86135 Augsburg, Germany Available online 10 January 2007
Abstract Nano-masks of polystyrene formed by self-organized arrangement of colloidal nano-spheres in a hexagonally close-packed monolayer on Si(1 0 0) surfaces were irradiated with 15 and 75 keV C+ ions at room temperature. At fluences above 1015 C/cm2 the polystyrene nano-spheres are observed to form necks at the contact points. This neck formation is thought to result from the near surface reduction of polymer viscosity due to molecular chain scission, which in turn allows to minimize the enormous surface tensions caused by the small radii of surface curvatures at the sphere contact points. This nano-scale effect was observed earlier in colloidal nano-masks of silica and called Ion Beam induced SIntering (IBSI). In polystyrene it is superimposed by the strong volume shrinkage of spheres resulting from polystyrene damage. Both effects cooperate to form novel nano-net structures on the silicon surface. 2007 Published by Elsevier B.V. PACS: 81.16. c; 61.80.Jh; 82.70.Dd; 81.16.Nd; 81.65.Cf Keywords: Nano-masks; Nanostructures; Polystyrene; Ion beam induced viscous flow; Colloids; SEM; TEM
1. Introduction Regular surface patterns with lateral dimensions of a few tens of nanometers can easily and affordably be formed on a large variety of surfaces using nano-sphere lithography [1–3]. This colloidal mask technique employs the selforganization of spherical nanoparticles in a colloidal suspension upon the controlled drying on a solid surface, which typically results in a close-packed arrangement of spheres. Using appropriate drying conditions the spheres can arrange themselves in monolayers, which can be used as a stencil mask for e.g. the deposition of arrays of monomodal nanodots or for the erosion of regularly spaced surface areas. For this, the free interstices between three adjacent spheres serve as a mask opening, the size of which is roughly one fourth of the colloid particle diameter. Typ-
*
Corresponding author. Tel.: +49 821 598 3445; fax: +49 821 598 3425. E-mail address: [email protected] (J.K.N. Lindner).
0168-583X/$ - see front matter 2007 Published by Elsevier B.V. doi:10.1016/j.nimb.2007.01.031
ically, the colloid bead material is polystyrene or silica, with or without functionalized surface groups, and the liquid phase is water, again either in a pure state or with surfactants to modify the surface tensions and inter-sphere forces. The question arises as to whether these nano-masks can be applied for ion implantation processes, which would allow for laterally nanostructured subsurface modifications. Recently we demonstrated [4,5] that upon irradiation of nano-masks made from 100 nm diameter SiO2 beads with light keV ions the masks start to deform at high ion fluences. This deformation is the result of the enormous surface tensions caused by the strong surface curvature at the contact points of nano-spheres. The surface tensions can be released by atom migration to the contact points of the spheres once surface atoms gain enough mobility owing to the ion bombardment. As the mask deformations were observed to be of non-thermal nature and lead to necks between spheres reminiscent of a sintering process, the effect was called Ion Beam induced SIntering (IBSI). It is anticipated that this nano-scale IBSI effect can be
456
D. Kraus et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 455–458
exploited to modify the shape of mask openings of colloidal nano-masks in a controlled way, thus allowing for more flexibility in nano-mask design. In the present paper, it is investigated whether the IBSI effect occurs in nano-masks of polystyrene (PS) beads as well. PS has the advantage that colloidal suspensions with a narrow particle size distribution can be easily obtained for a wide range of bead diameters. However, the polymeric nature of PS, the lower melting point and the smaller stopping power of ions must be taken into account in ion irradiation experiments. 2. Experimental details Monolayers of colloidal polystyrene particles were fabricated by nano-sphere lithography [2,3]. To this end Czochralski Si(1 0 0) wafers were used as substrates, which were cleaned from organic contaminants. In order to obtain homogeneous and hydrophilic surfaces, the substrates were immersed in a modified RCA cleaning solution and ultrasonicated in deionized water. Surfactant-free colloidal polystyrene spheres from Polysciences Inc. with a diameter of 200 nm and a narrow size distribution (coefficient of variance < 8%) were used to create the masks. The PS beads are dispersed in water at a concentration which was adjusted by the addition of DI water and subsequent homogenization by ultrasonication before pipetting a small droplet onto the prepared Si surface. Large-scale monolayer colloidal crystals (up to 50 mm2) were obtained on the silicon substrates by controlled drying of droplets of the PS suspension using a custom-built inclination dryer. The geometry and structure of PS layers was monitored by light and scanning electron microscopy (SEM) with a FEI XL30 ESEM. Monolayers of PS are large enough to localize them with the naked eye by means of their optical interference contrast. This facilitates to cover parts of the monolayers with an Al beam stop foil prior to ion irradiation in order to allow for a direct comparison between unimplanted and implanted areas. Implantations were performed at room temperature with 15 and 75 keV C+ ions in the fluence range of 1–50 · 1015 C/cm2. Even though no charge compensation system was used and the isolating PS spheres were not coated with a conductive film, the current flowing from the sample was stable. Average ion current densities during implantation varied from 1 to 10 lA/cm2, the chamber pressure was in the range of 1– 3 · 10 6 mbar. The morphologies of the implanted samples were investigated by SEM and cross section transmission electron microscopy (XTEM) with a Philips CM30 at 300 keV and a Jeol JEM2100F high-resolution field-emission TEM at 200 keV. Prior to cross section specimen preparation a 100 nm thick Co layer was evaporated onto the samples in order to protect the sensitive polystyrene from attack by organic solvents which are used for cleaning purposes at various stages of the preparation sequence. Final
XTEM sample thinning was accomplished by keV Ar sputtering. 3. Results and discussion SRIM2003 [6] Monte Carlo simulations of the ion ranges predict for 15 keV C+ ions in continuous films of PS (50 at.% C, 50 at.% H, density 1.06 g/cm3) a concentration maximum at 77 nm, i.e. closely above the sphere centre of 200 nm beads, and a broad nuclear stopping power max˚ at 46 nm. Any ion beam induced modifiimum of 11 eV/A cations are predicted to be restricted to the first 130 nm beneath the PS surface. In contrast, 75 keV C+ ions are expected to penetrate the PS beads completely even if the centre of a sphere is hit. In this case, a carbon concentration maximum with a full width at half maximum (FWHM) of 230 nm is simulated to occur at a depth of 100 nm beneath the PS/Si interface. The nuclear stopping ˚ power increases within the PS spheres from 4 to 8 eV/A between the surface and the rear interface with the Si substrate. Thus, the situation is very similar to our previous study [4], where the same ion species and energies were used to irradiate monolayers of 100 nm diameter silica spheres, and either implantation or irradiation was performed at the lower or higher energy. Fig. 1 shows SEM images of a colloidal PS monolayer which in the upper half was irradiated with 1 · 1015 cm 2 75 keV C+ ions. In the lower, non-irradiated half of the monolayer the hexagonal close-packed arrangement of spheres is clearly visible. The layer contains zero- and onedimensional defects originating from variations in the PS bead diameters. In SEM images these defects show up with a bright contrast due to charging effects in the SEM. In the ion irradiated part these defects broaden significantly, an effect which is most pronounced along grain boundaries (marked GB in Fig. 1). The same gap formation was already observed in colloidal silica masks [4]. Strong defect broadening by a collective motion of spheres is visible in real time during SEM imaging of both silica and PS masks when the electron beam hits the sample for a too long time (which is avoided in all images shown here), indicating that the effect is due to surface charging of spheres. As we speculate that this effect can be suppressed by making the spheres conductive prior to ion irradiation or by using an electron flood system, we shall not go into further details with this point. The inset of Fig. 1 shows a limited mask area at a larger magnification. Obviously subtle necks have been formed at the contact points between adjacent spheres, reducing the local surface energies. The same two effects, gap and neck formation, are also observed for 15 keV implantations with no significant differences at this ion fluence. Fig. 2 exhibits SEM images of a PS nano-mask irradiated with 5 · 1016 cm 2 75 keV C+ ions. As is clearly visible, both effects prevail and strong necks are found at this high fluence, leading to a network of spheres interconnected by delicate nano-fibres. The coalescence of spheres by the formation of necks is even more pronounced when
D. Kraus et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 455–458
Fig. 1. SEM survey and detail images of a colloidal PS nano-mask after irradiation with 1 · 1015 cm of the sample. A grain boundary is marked as GB in the survey.
2
457
75 keV C+ ions in the upper half (above the arrow)
Fig. 2. SEM survey and detail images of PS spheres forming a nano-net structure on a Si(1 0 0) surface after irradiation with 75 keV C+ ions at a fluence of 5 · 1016 cm 2.
irradiations are performed with 15 keV instead of 75 keV C+ ions at the same doses (not shown here), resulting in groups of strongly sintered PS particles. This is a strong contrast to our earlier IBSI studies [4] of silica particle
masks, where no significant differences were observed for the same two energies. The energy and fluence dependence of the mean width of necks DN as determined from SEM images is displayed in
458
D. Kraus et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 455–458
Fig. 4. XTEM micrograph of two PS spheres connected by ion beam induced sintering due to implantation with 1 · 1016 cm 2 15 keV C+ ions. The spheres were covered with 100 nm Co as a protection layer. A pyramid structure is formed between the spheres underneath the neck. The position of the original equatorial line of beads is indicated as a dashed line.
starts at the contact points and then shifts to the depth of strongest atomic mobility. 4. Summary Fig. 3. Mean neck diameter (a) and mean sphere diameter (b) in 200 nm PS nano-masks as a function of fluence of 15 and 75 keV C+ ions at room temperature.
Fig. 3(a). A similar trend is visible for both energies, with minimum of neck diameters at 1016 C/cm2. The origin of this behaviour is presently not yet clear. Sputtering certainly has to be taken into account to interpret the decrease of DN between 1015 and 1016 C/cm2, but this would not explain the rise at higher doses and the similarity of curves for the two energies. The situation is complicated by the fact that PS particles themselves shrink with increasing fluence (Fig. 3(b)) owing to the scission of polymer chains and release of volatile CHx groups [7]; this shrinkage seems to saturate at fluences above 1016 C/cm2 (Fig. 3(b)). Since C–H molecular chains within the necks are subjected to such shrinkage effects, too, a complicated fluence dependence may result. This is corroborated by the observation [8] that in thermal sintering of polymers a lower molecular weight and a lower viscosity lead to faster coalescence of particles. Finally, in Fig. 4 it is shown that necks are free standing and not attached to the substrate. Owing to the Co evaporated onto the mask and into the mask openings, the necks can be clearly distinguished from the glue used in the XTEM preparation. It is obvious that the necks form in the 15 keV implanted masks above the equatorial line of the PS beads, roughly at the depth of maximum implanted carbon concentration, indicating that the neck formation
Monolayers of polystyrene nano-beads were formed by self-organization to fabricate nano-masks on Si(1 0 0) for nano-sphere lithography. Ion irradiation with either 15 or 75 keV C+ ions at room temperature deforms the masks at fluences above 1015 C/cm2 by several mechanisms: (a) gap formation preferentially along pre-existing mask defects owing to beam charging, (b) sputtering, (c) irradiation induced PS volume shrinkage, and (d) ion beam induced sintering (IBSI) of adjacent nano-spheres. The latter effect is observed to be energy dependent in PS and leads to the formation of novel nano-net surface structures. References [1] L.M. Liz-Marza´n, P.V. Kamat (Eds.), Nanoscale Materials, Kluwer Academic Publishers, Boston, Dordrecht, London, 2003. [2] H.W. Deckmann, J.H. Dunsmuir, Appl. Phys. Lett. 41 (1982) 377. [3] F. Burmeister, W. Badowsky, T. Braun, S. Wieprich, J. Boneberg, P. Leiderer, Appl. Surf. Sci. 144–145 (1999) 461. [4] J.K.N. Lindner, B. Gehl, B. Stritzker, Nucl. Instr. and Meth. B 242 (2006) 167. [5] J.K.N. Lindner, D. Kraus, B. Stritzker, Materials Research Society Fall Meeting, Boston, Symposium OO, 2005. [6] J.F. Ziegler, J.B. Biersack, U. Littmark (Eds.), The Stopping and Range of Ions in Solids, Pergamon, New York, 1985., version SRIM2003.26. [7] See e.g. M. Iwaki, Nucl. Instr. and Meth. B 175–177 (2001) 368; or F.F. Komarov, A.V. Leontyev, V.V. Grigoryev, M.A. Kamishan, Nucl. Instr. and Meth. B 191 (2002) 728. [8] C.T. Bellehumeur, M.K. Bisaria, J. Vlachopoulos, Polym. Eng. Sci. 36 (1995) 2201.
Nuclear Instruments and Methods in Physics Research B 257 (2007) 455–458 www.elsevier.com/locate/nimb
Ion beam induced sintering of colloidal polystyrene nano-masks D. Kraus, J.K.N. Lindner *, B. Stritzker Universita¨t Augsburg, Institut fu¨r Physik, Universita¨tsstr. 1, D-86135 Augsburg, Germany Available online 10 January 2007
Abstract Nano-masks of polystyrene formed by self-organized arrangement of colloidal nano-spheres in a hexagonally close-packed monolayer on Si(1 0 0) surfaces were irradiated with 15 and 75 keV C+ ions at room temperature. At fluences above 1015 C/cm2 the polystyrene nano-spheres are observed to form necks at the contact points. This neck formation is thought to result from the near surface reduction of polymer viscosity due to molecular chain scission, which in turn allows to minimize the enormous surface tensions caused by the small radii of surface curvatures at the sphere contact points. This nano-scale effect was observed earlier in colloidal nano-masks of silica and called Ion Beam induced SIntering (IBSI). In polystyrene it is superimposed by the strong volume shrinkage of spheres resulting from polystyrene damage. Both effects cooperate to form novel nano-net structures on the silicon surface. 2007 Published by Elsevier B.V. PACS: 81.16. c; 61.80.Jh; 82.70.Dd; 81.16.Nd; 81.65.Cf Keywords: Nano-masks; Nanostructures; Polystyrene; Ion beam induced viscous flow; Colloids; SEM; TEM
1. Introduction Regular surface patterns with lateral dimensions of a few tens of nanometers can easily and affordably be formed on a large variety of surfaces using nano-sphere lithography [1–3]. This colloidal mask technique employs the selforganization of spherical nanoparticles in a colloidal suspension upon the controlled drying on a solid surface, which typically results in a close-packed arrangement of spheres. Using appropriate drying conditions the spheres can arrange themselves in monolayers, which can be used as a stencil mask for e.g. the deposition of arrays of monomodal nanodots or for the erosion of regularly spaced surface areas. For this, the free interstices between three adjacent spheres serve as a mask opening, the size of which is roughly one fourth of the colloid particle diameter. Typ-
*
Corresponding author. Tel.: +49 821 598 3445; fax: +49 821 598 3425. E-mail address: [email protected] (J.K.N. Lindner).
0168-583X/$ - see front matter 2007 Published by Elsevier B.V. doi:10.1016/j.nimb.2007.01.031
ically, the colloid bead material is polystyrene or silica, with or without functionalized surface groups, and the liquid phase is water, again either in a pure state or with surfactants to modify the surface tensions and inter-sphere forces. The question arises as to whether these nano-masks can be applied for ion implantation processes, which would allow for laterally nanostructured subsurface modifications. Recently we demonstrated [4,5] that upon irradiation of nano-masks made from 100 nm diameter SiO2 beads with light keV ions the masks start to deform at high ion fluences. This deformation is the result of the enormous surface tensions caused by the strong surface curvature at the contact points of nano-spheres. The surface tensions can be released by atom migration to the contact points of the spheres once surface atoms gain enough mobility owing to the ion bombardment. As the mask deformations were observed to be of non-thermal nature and lead to necks between spheres reminiscent of a sintering process, the effect was called Ion Beam induced SIntering (IBSI). It is anticipated that this nano-scale IBSI effect can be
456
D. Kraus et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 455–458
exploited to modify the shape of mask openings of colloidal nano-masks in a controlled way, thus allowing for more flexibility in nano-mask design. In the present paper, it is investigated whether the IBSI effect occurs in nano-masks of polystyrene (PS) beads as well. PS has the advantage that colloidal suspensions with a narrow particle size distribution can be easily obtained for a wide range of bead diameters. However, the polymeric nature of PS, the lower melting point and the smaller stopping power of ions must be taken into account in ion irradiation experiments. 2. Experimental details Monolayers of colloidal polystyrene particles were fabricated by nano-sphere lithography [2,3]. To this end Czochralski Si(1 0 0) wafers were used as substrates, which were cleaned from organic contaminants. In order to obtain homogeneous and hydrophilic surfaces, the substrates were immersed in a modified RCA cleaning solution and ultrasonicated in deionized water. Surfactant-free colloidal polystyrene spheres from Polysciences Inc. with a diameter of 200 nm and a narrow size distribution (coefficient of variance < 8%) were used to create the masks. The PS beads are dispersed in water at a concentration which was adjusted by the addition of DI water and subsequent homogenization by ultrasonication before pipetting a small droplet onto the prepared Si surface. Large-scale monolayer colloidal crystals (up to 50 mm2) were obtained on the silicon substrates by controlled drying of droplets of the PS suspension using a custom-built inclination dryer. The geometry and structure of PS layers was monitored by light and scanning electron microscopy (SEM) with a FEI XL30 ESEM. Monolayers of PS are large enough to localize them with the naked eye by means of their optical interference contrast. This facilitates to cover parts of the monolayers with an Al beam stop foil prior to ion irradiation in order to allow for a direct comparison between unimplanted and implanted areas. Implantations were performed at room temperature with 15 and 75 keV C+ ions in the fluence range of 1–50 · 1015 C/cm2. Even though no charge compensation system was used and the isolating PS spheres were not coated with a conductive film, the current flowing from the sample was stable. Average ion current densities during implantation varied from 1 to 10 lA/cm2, the chamber pressure was in the range of 1– 3 · 10 6 mbar. The morphologies of the implanted samples were investigated by SEM and cross section transmission electron microscopy (XTEM) with a Philips CM30 at 300 keV and a Jeol JEM2100F high-resolution field-emission TEM at 200 keV. Prior to cross section specimen preparation a 100 nm thick Co layer was evaporated onto the samples in order to protect the sensitive polystyrene from attack by organic solvents which are used for cleaning purposes at various stages of the preparation sequence. Final
XTEM sample thinning was accomplished by keV Ar sputtering. 3. Results and discussion SRIM2003 [6] Monte Carlo simulations of the ion ranges predict for 15 keV C+ ions in continuous films of PS (50 at.% C, 50 at.% H, density 1.06 g/cm3) a concentration maximum at 77 nm, i.e. closely above the sphere centre of 200 nm beads, and a broad nuclear stopping power max˚ at 46 nm. Any ion beam induced modifiimum of 11 eV/A cations are predicted to be restricted to the first 130 nm beneath the PS surface. In contrast, 75 keV C+ ions are expected to penetrate the PS beads completely even if the centre of a sphere is hit. In this case, a carbon concentration maximum with a full width at half maximum (FWHM) of 230 nm is simulated to occur at a depth of 100 nm beneath the PS/Si interface. The nuclear stopping ˚ power increases within the PS spheres from 4 to 8 eV/A between the surface and the rear interface with the Si substrate. Thus, the situation is very similar to our previous study [4], where the same ion species and energies were used to irradiate monolayers of 100 nm diameter silica spheres, and either implantation or irradiation was performed at the lower or higher energy. Fig. 1 shows SEM images of a colloidal PS monolayer which in the upper half was irradiated with 1 · 1015 cm 2 75 keV C+ ions. In the lower, non-irradiated half of the monolayer the hexagonal close-packed arrangement of spheres is clearly visible. The layer contains zero- and onedimensional defects originating from variations in the PS bead diameters. In SEM images these defects show up with a bright contrast due to charging effects in the SEM. In the ion irradiated part these defects broaden significantly, an effect which is most pronounced along grain boundaries (marked GB in Fig. 1). The same gap formation was already observed in colloidal silica masks [4]. Strong defect broadening by a collective motion of spheres is visible in real time during SEM imaging of both silica and PS masks when the electron beam hits the sample for a too long time (which is avoided in all images shown here), indicating that the effect is due to surface charging of spheres. As we speculate that this effect can be suppressed by making the spheres conductive prior to ion irradiation or by using an electron flood system, we shall not go into further details with this point. The inset of Fig. 1 shows a limited mask area at a larger magnification. Obviously subtle necks have been formed at the contact points between adjacent spheres, reducing the local surface energies. The same two effects, gap and neck formation, are also observed for 15 keV implantations with no significant differences at this ion fluence. Fig. 2 exhibits SEM images of a PS nano-mask irradiated with 5 · 1016 cm 2 75 keV C+ ions. As is clearly visible, both effects prevail and strong necks are found at this high fluence, leading to a network of spheres interconnected by delicate nano-fibres. The coalescence of spheres by the formation of necks is even more pronounced when
D. Kraus et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 455–458
Fig. 1. SEM survey and detail images of a colloidal PS nano-mask after irradiation with 1 · 1015 cm of the sample. A grain boundary is marked as GB in the survey.
2
457
75 keV C+ ions in the upper half (above the arrow)
Fig. 2. SEM survey and detail images of PS spheres forming a nano-net structure on a Si(1 0 0) surface after irradiation with 75 keV C+ ions at a fluence of 5 · 1016 cm 2.
irradiations are performed with 15 keV instead of 75 keV C+ ions at the same doses (not shown here), resulting in groups of strongly sintered PS particles. This is a strong contrast to our earlier IBSI studies [4] of silica particle
masks, where no significant differences were observed for the same two energies. The energy and fluence dependence of the mean width of necks DN as determined from SEM images is displayed in
458
D. Kraus et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 455–458
Fig. 4. XTEM micrograph of two PS spheres connected by ion beam induced sintering due to implantation with 1 · 1016 cm 2 15 keV C+ ions. The spheres were covered with 100 nm Co as a protection layer. A pyramid structure is formed between the spheres underneath the neck. The position of the original equatorial line of beads is indicated as a dashed line.
starts at the contact points and then shifts to the depth of strongest atomic mobility. 4. Summary Fig. 3. Mean neck diameter (a) and mean sphere diameter (b) in 200 nm PS nano-masks as a function of fluence of 15 and 75 keV C+ ions at room temperature.
Fig. 3(a). A similar trend is visible for both energies, with minimum of neck diameters at 1016 C/cm2. The origin of this behaviour is presently not yet clear. Sputtering certainly has to be taken into account to interpret the decrease of DN between 1015 and 1016 C/cm2, but this would not explain the rise at higher doses and the similarity of curves for the two energies. The situation is complicated by the fact that PS particles themselves shrink with increasing fluence (Fig. 3(b)) owing to the scission of polymer chains and release of volatile CHx groups [7]; this shrinkage seems to saturate at fluences above 1016 C/cm2 (Fig. 3(b)). Since C–H molecular chains within the necks are subjected to such shrinkage effects, too, a complicated fluence dependence may result. This is corroborated by the observation [8] that in thermal sintering of polymers a lower molecular weight and a lower viscosity lead to faster coalescence of particles. Finally, in Fig. 4 it is shown that necks are free standing and not attached to the substrate. Owing to the Co evaporated onto the mask and into the mask openings, the necks can be clearly distinguished from the glue used in the XTEM preparation. It is obvious that the necks form in the 15 keV implanted masks above the equatorial line of the PS beads, roughly at the depth of maximum implanted carbon concentration, indicating that the neck formation
Monolayers of polystyrene nano-beads were formed by self-organization to fabricate nano-masks on Si(1 0 0) for nano-sphere lithography. Ion irradiation with either 15 or 75 keV C+ ions at room temperature deforms the masks at fluences above 1015 C/cm2 by several mechanisms: (a) gap formation preferentially along pre-existing mask defects owing to beam charging, (b) sputtering, (c) irradiation induced PS volume shrinkage, and (d) ion beam induced sintering (IBSI) of adjacent nano-spheres. The latter effect is observed to be energy dependent in PS and leads to the formation of novel nano-net surface structures. References [1] L.M. Liz-Marza´n, P.V. Kamat (Eds.), Nanoscale Materials, Kluwer Academic Publishers, Boston, Dordrecht, London, 2003. [2] H.W. Deckmann, J.H. Dunsmuir, Appl. Phys. Lett. 41 (1982) 377. [3] F. Burmeister, W. Badowsky, T. Braun, S. Wieprich, J. Boneberg, P. Leiderer, Appl. Surf. Sci. 144–145 (1999) 461. [4] J.K.N. Lindner, B. Gehl, B. Stritzker, Nucl. Instr. and Meth. B 242 (2006) 167. [5] J.K.N. Lindner, D. Kraus, B. Stritzker, Materials Research Society Fall Meeting, Boston, Symposium OO, 2005. [6] J.F. Ziegler, J.B. Biersack, U. Littmark (Eds.), The Stopping and Range of Ions in Solids, Pergamon, New York, 1985.