Computer Simulations of the Sputtering of Metallic, Organic, and Metal

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Computer Simulations of the Sputtering of Metallic, Organic, and Metal−Organic Surfaces with Bin and C60 Projectiles A. Delcorte,* Ch. Leblanc, C. Poleunis, and K. Hamraoui Institute of Condensed Matter and Nanosciences - Bio & Soft Matter, Université catholique de Louvain, Croix du Sud, 1 bte L7.04.01; B-1348 Louvain-la-Neuve, Belgium ABSTRACT: This study focuses on the microscopic modeling of 0− 25 keV Bi1−3−5 and C60 cluster impacts on three different targets (Au crystal, adsorbed Au nanoparticle, and organic solid), using molecular dynamics simulations, and on the comparison of the calculated quantities with recent experimental results, reported in the literature or obtained in our laboratory. The sputtering statistics are reported, showing nonlinearity of the sputtering yields with the number of cluster atoms at the same incident velocity for Bi1−5 bombardment. They are compared to experiments (especially for the organic target), and the microscopic explanation of the observations is analyzed. The results show that the respective behaviors and performances of the different projectiles are strongly dependent on the target, with clusters of heavy Bi atoms being more efficient at sputtering gold and, conversely, fullerene clusters inducing the largest sputtering yields of the organic material (mass matching). For organic targets, some important and novel conclusions of this work are the following: (i) The increase of the sputtering yield when going from Bi atoms to Bi clusters is insufficient to explain the much larger increase of characteristic ion yields, suggesting a projectile-dependent ionization probability. (ii) The extent of molecular fragmentation follows the order of Bi > Bi3 > Bi5 > C60, that is, softer emission with larger clusters. (iii) Even 5−10 keV Bi atoms create collective molecular motions and craters in the polymeric solid, though the collision cascades are rather dilute. Finally, a second series of simulations performed at low energies predict that 0.1−1 keV Bin clusters should not provide better results for sputtering and depth profiling than isoenergetic single atoms. as a function of energy for the latter clusters.12 Earlier results obtained on organic dyes also indicated that the molecular ion yields measured with Bi3−5 ions were comparable to those generated by SF5 and C60.13 However, a detailed analysis of protein mass spectra using multivariate analysis showed that the fragmentation patterns were affected by the type of cluster, an issue for SIMS analysts.14 The interaction of Bin clusters with surfaces has not been studied by theoretical methods yet. Nevertheless, a large number of molecular dynamics (MD) simulations have been performed with Aun clusters, which have a very close atomic mass, mostly for metal targets,15−18 but also for other materials.19,20 Of interest regarding the analytical instrumentation and applications, MD simulations comparing Au3 and C60 impacts in light element targets also exist (bulk water ice,21,22 water ice overlayers,23 bulk hydrocarbons24). Upon 5 keV bombardment conditions, C 60 and Au 3 , respectively, eject ∼1600 and ∼1000 water molecules from water ice, creating huge craters in the target.21 The mass of hydrocarbons sputtered by C60 and Au3 are also comparable, though slightly larger with C60.24 In both cases, Au3 penetrates significantly deeper in the solid than C60 and the shape of the crater is more elongated with Au3 impacts. The emission

1. INTRODUCTION Small heavy metal clusters (Aun+,1 Bin+2) have become reference projectiles for sample analysis and imaging by secondary ion mass spectrometry (SIMS).3 In particular, Bin+ beams became very popular after their introduction by a major instrument manufacturer in 2004.2 First, the molecular secondary ion yields obtained upon cluster bombardment of organic samples are much larger than those obtained with atomic projectiles, including Au+ and Bi+.4,5 Second, because of the design of the liquid metal ion guns by which they are produced, much better focusing of the beam can be obtained with small metal clusters than with light-element clusters, such as SF5+6 and C60+7 ( Bi5 > C60. Figure 5 shows the yield distribution of Au clusters sputtered from sample N upon 25 keV bombardment with the Bi1−5 and

selection of Bin clusters and impact energies allows us to check the nonlinearity of the yield for projectiles with the energy of 5 keV per atom. Figure 6 shows the sputtering yield per 5 keV

Figure 6. Sputter yield per 5 keV atom for samples M (gray) and N (black) bombarded by Bi atoms and clusters.

atom for samples M and N bombarded by Bi atoms and clusters. In that representation, a nonlinear yield is indicated by an increase of Y/n for the different projectiles. For the gold crystal (sample M), the yield is strongly nonlinear, with Y/n being 9 and 8 times larger for Bi3 and Bi5 impacts than for Bi impacts. Nonlinearity is also observed for sample N, but with a smaller enhancement factor (between 2 and 3). Nonlinearity of the sputtered mass can also be checked for the organic sample, even though the selected impact energies do not allow a direct comparison, such as the one in Figure 6. The calculated and experimental sputtering data of Figure 3a are plotted in Figure 3b in terms of sputtered mass per projectile atom Ym/n as a function of energy per atom. The different simulation data sets were fitted by a straight line with a (0,0) intercept. The fits to the data sets corresponding to the three considered clusters, Bi3, Bi5, and C60, form almost a single line, whereas the line corresponding to single Bi is clearly separated. In that representation, the data points sitting on the same line (i.e., same proportionality factor between Ym/n and E/n) can be considered as having the same “nonlinearity factor”. Figure 3b confirms, therefore, the existence of a nonlinear enhancement of the sputtering when going from the atomic projectile to the clusters. For the three clusters, the data points corresponding to 1 keV bombardment fall significantly under the fit line, suggesting the presence of a sputtering threshold around that energy. The experimental values reported by Muramoto et al.12 for trehalose samples are also slightly under the calculated lines for Bi, Bi3, and C60, whereas the value measured for 50 keV Bi5 is above. Given the aforementioned differences between the experiment and the simulation, these slight discrepancies are not surprising. However, the positions of the experimental data points indicate that the nonlinearity factors follow the order of Bi5 > Bi3 > Bi, while the simulations suggest the order of Bi5 ≈ Bi3 > Bi. 3.2. Microscopic View of the Impacts. Microscopic views of the impact regions at the end of the trajectories are shown in Figure 7 for sample M upon 15 keV impacts by Bin and C60 projectiles. The cross sections of the sample around the impact point show that the three clusters create hemispherical craters

Figure 5. Calculated yield distribution of Aun clusters sputtered from sample N upon 25 keV bombardment by Bin and C60 projectiles.

C60. The lines represent the best exponential fit of the results (Y(m) ∼ m−X), with m being the fragment mass and Y(m) the corresponding fragment abundance. For all of these fits, the exponent X is comprised between 1.9 and 2.1. Therefore, despite differences of absolute yields, one cannot state that there is a significant difference of sputtered Au cluster distributions for the considered series of projectiles impinging on sample N. Nonlinearity of the Sputtering. The sputtered masses and the number of ejected molecules reported in Figures 2 and 3 clearly indicate that Bi3−5 clusters induce more emission than isoenergetic Bi atoms. It is, therefore, useful to check the nonlinearity of the enhancement, a feature that is commonly reported for cluster projectiles. Strictly speaking, there is nonlinearity of the sputtering yield Y (or the sputtered mass Ym) when the yield generated by a cluster of An (with n constituents) is larger than the yield produced by n independent atoms of the element A, with the same energy per atom (or velocity). In the cases of samples M and N, our 2745

dx.doi.org/10.1021/jp308411r | J. Phys. Chem. C 2013, 117, 2740−2752

The Journal of Physical Chemistry C

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Figure 7. Cross-sectional side views (2 nm thickness) of the craters formed in sample M upon 15 keV bombardment by Bin and C60 projectiles. Time: 15 ps.

Figure 8. Cross-sectional side views (6 nm thickness) of the craters formed by Bi5 and C60 clusters in sample N. Top panel: 15 keV. Bottom panel: 25 keV. Time: 25 ps.

microstructure over larger time scales. In contrast, atomic Bi does not create a real crater, but rather a hemispherical damaged region where the solid is amorphized, even after 30 ps. A similar difference between monatomic (Ga) and cluster (C60) bombardment was also observed by Postawa et al. for impacts on a silver crystal.60 While atomic projectiles generate

of approximately 4 nm in width and 2 nm in depth, with slight variations depending on the considered clusters but also on the specific impact point. The rims of the craters have an elevation of three to four atomic layers. The walls of the craters and the rims are essentially recrystallized at the end of the trajectories. Therefore, one does not expect further changes of the 2746



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Figure 9. 10 keV impacts of Bi, Bi5, and C60 projectiles in sample P. Top panel: Cross-sectional side views (4 nm thickness) of the craters. Time: 25 ps. Bottom panel: Successive positions of the atoms in motion (>20 eV of kinetic energy) until the projectile has transferred 90% of its energy to the target. The legend on the side corresponds to the time in femtoseconds. The red dots in (b, c) represent the position of the center of mass of the projectiles when their kinetic energy falls below 10 eV (99.9% energy transfer).

rather open collision cascades for most impact points,61 the energy density deposited by the clusters in a very small volume of the sample surface induces the development of spherical pressure waves in the solid and collective motion of atoms, leading to the observed craters and to large sputtering yields. This difference of behavior explains the comparatively low sputtering yields obtained with atomic Bi (Figure 2a). However, all the clusters are not equivalent. C60 sputters significantly less gold than isoenergetic Bi3 and Bi5, especially beyond 5 keV, and the craters are consistently smaller. The explanation lies in the comparatively low mass ratio between the projectile constituents and the target atoms. Because of the low mass of carbon, C60 is essentially backscattered from the surface and the energy reflected and lost upon backscattering amounts to ∼40% of the initial kinetic energy of the projectile for samples M and N. In contrast, because of the similar masses of Bi and Au, and the high energy per atom in the Bin projectiles, their energy is efficiently transferred to the solid. The sputtering of sample N is illustrated in Figure 8 for Bi5 and C60 impacts at 15 and 25 keV. Visually, the effects of Bi and Bi3 bombardment of the nanoparticle at those energies are similar to those of C60 and Bi5, respectively. For both energies, the impacts of Bi3−5 induce much more fragmentation. At 25 keV, Bi3−5 is able to disintegrate the nanoparticle, while Bi and C60 still leave a large portion intact. The explosion of the nanoparticle upon Bi3−5 bombardment is accompanied by the creation of a wide crater in the organic material. Note that a significant portion of the nanoparticle fragments receive lateral momentum and redistribute on the surface around the impact

point, especially in the case of C60. The microscopic views also help us understand why more material is sputtered from sample N than from sample M. The reason is twofold. First, the average binding energy is less for the nanoparticle than the flat crystal surface, and second, the energized gold nanovolume (i.e., the entire nanoparticle at 25 keV) is much less constrained by the surrounding material in the case of the nanoparticle. For these reasons, the fragments can easily expand sideways and upward and even, to some extent, downward, into the organic substrate. The movies of the trajectories indicate that the vertical dispersion of the Au-NP fragments in the softer matrix is more pronounced with Bi3 and Bi5 than with Bi and C60. In the case of the flat crystal surface, the material of the expanding crater remains strongly bonded to the gold surface and a large fraction of it does not receive enough energy to detach, eventually forming the rim of the crater. A large part of the energy is also dissipated in the rest of the solid via pressure waves. For sample N, the influence of the impact point with respect to the position of the nanoparticle was checked upon C60 bombardment. For central and peripheral impacts, 25 keV C60 induces the disintegration of the top part of the nanoparticle and the formation of many smaller clusters moving sideways (Figure 8). The lower Au yields calculated for the peripheral impacts in Table 3 can be explained by the larger number of Au atoms and clusters moving sideways (and sometimes downward) rather than upward, which cancels a fraction of the sputtered flux. For interfacial impacts (not shown), a large hemispherical crater develops in the polymer (5.5, 6.5, and 8.0 nm depth at 5, 15, and 25 keV, respectively). 2747



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cell, and therefore, their penetration depth cannot be estimated from these simulations (>15.5 nm). One point to emphasize here is that, despite the dilute nature of the collision cascade of 10 keV Bi in sample P, a relatively large crater is created in the sample, indicating collective atomic motion in the sample. Therefore, deriving the sputter yield from the linear cascade theory for such a case scenario would be inappropriate. This observation is in contrast to the case of the Bi bombardment of the gold crystal (sample M). The picture of the energetic collisions in the first hundreds of femtoseconds of the interaction allows us to speculate about the reasons of the observed discrepancy between the sputtering yield and the ion yield enhancements when going from atomic Bi projectiles to clusters. In addition to the possibility of preformed ions,66,67 it has long been thought,68,69 and also shown in previous simulation works,70 that significant ionization occurs in the energetic collision cascade, at the beginning of the interaction. It seems very difficult to explain the aforementioned discrepancy with the concept of preformed ions, because in that case, the sputtering and ion yields should, in principle, be proportional to each other. Instead, free protons and electrons, produced with a non-negligible kinetic energy in the tracks of the projectile and the energetic recoil atoms shown in the bottom panel of Figure 9, should expand in every direction with the forming crater, like those recoil atoms. In the case of clusters (Figure 9b,c), the dense and surface-localized “cascade” should favor the recombination of these protons and electrons with the departing fragments and molecules. In contrast, for atomic Bi (Figure 9a), the dilute nature of the cascade and its development in the depth of the target suggest low recombination rates despite the formation of a rather large crater and the emission of a significant number of fragments and molecules. The parallel increase of the number of departing species and available ionizing agents in the top surface layer, occurring upon cluster bombardment, should naturally lead to nonlinear effects (e.g., a square dependence of the ion yields versus the sputter yields5,71). The argument of a nonlinear increase of ionization with respect to the sputtering yields should be especially valid for secondary ionization events resulting from the interaction of two partners, such as molecular protonation, electron capture, and electron stripping from a molecular fragment by an energetic electron produced in the cascade. The energy dependence of the implantation depth of the Bi, Bi3, and Bi5 projectiles in sample P at the end of the simulations is reported in Figure 10. The data obtained by Muramoto et al. for trehalose targets in the aforementioned experimental study are also displayed on the same graph, together with the SRIM calculation results they discussed in their article.12 The experimental values reported in Figure 10 have not been corrected for the incidence angle of 45°, therefore, they should underestimate the penetration depth by a factor of cos(45°). First, our simulation results indicate a quite different behavior for Bi atoms and Bi3−5 clusters. The evolution of the calculated implantation depth of Bi3 and Bi5 between 0.1 and 10 keV can be reasonably described by a logarithmic function (red and blue dashed lines). In contrast, the implantation depth of single Bi atoms, very close to the cluster data below 1 keV, increases much faster above this energy. At 10 keV and beyond, the implantation depth of single Bi exceeds the thickness of sample P and can, therefore, no longer be obtained from our simulations. A second observation is that, for all energies, the penetration depth of Bi5 clusters is slightly larger than that of

More Au fragments mix with the polymer, in the crater and in the departing plume, and the rest of the nanoparticle moves sideways, pushing a rim of organic material. In comparison with the experimental results of Yang et al. for Au-NPs adsorbed on SiO2,53 our results for a semiembedded nanoparticle suggest that the outcome of the sputtering event will also be influenced by the Au-NP substrate in several ways. The energy dissipation and reflection will be different, with a possible influence on the sputter yields, and the Au-NP fragments will penetrate the soft and not the hard substrate, possibly leading to a deterioration of the depth profiling resolution for small Au-NPs embedded in soft matrixes. In addition, the desorption of intact nanoparticles and of Au clusters forming a substantial portion of the original NPs, observed upon metal cluster bombardment of Au-NPs adsorbed on harder substrates,62 should be less favored with a soft matrix that absorbs a large amount of the initial energy in plastic deformation. The bombardment of sample P is illustrated in Figure 9, for 10 keV Bi, Bi5, and C60 impacts. Considering the lower energies, the effect of the bombardment is dramatic with respect to samples M and N. Large hemispherical craters are formed with all the cluster projectiles. Roughly, the volumes of the craters at the end of the trajectories are 30−50 times bigger, and the displaced mass is 1.5−2.5 times larger upon 10 keV bombardment of the polymer, in comparison with 15 keV bombardment of the gold crystal. Atomic Bi projectiles are also able to induce craters in the polymeric material, but they are rather ill-defined, more narrow, and with a conical shape. The validity of those results is difficult to assess experimentally because, to our knowledge, there exists no report concerning the measurement of craters induced by keV projectiles in organic solids. In addition, because of the much larger computational expense, there are very few simulations of organic material bombardment by keV projectiles using a more sophisticated representation of the target. Nevertheless, full atomistic simulations have shown that 5 keV atomic projectiles induce the disintegration of a ∼8 nm polystyrene sample63 and even sub-keV atomic projectiles destroy an ∼4 nm benzene crystal.64 Recently, mixed atomistic/coarse-grained simulations of benzene ice bombardment by 15 keV clusters (C60, Ar18, and Ar60) showed the formation of ∼15 nm wide hemispherical craters.65 In coarse-grained simulations, the bombardment of a benzene crystal by a 5 keV atomic Au projectile induces the development of a more conical crater,22 as observed for Bi impacts in Figure 9a, but it is shown here for the first time with an amorphous polymer target, comparable to real-world samples. The bottom panel of Figure 9 shows the successive positions of the cascade atoms in the beginning of the interaction. The atomic cascade becomes denser and more condensed in the surface region when going from Bi to Bi5 and finally C60, in correlation with the shape of the craters observed in the top panel. For Bi5 and C60, the plots also indicate the point reached by the center of mass of the projectile when almost all of its energy is transferred to the target (9.99 keV). Whereas the light C60 atoms, with an energy of 166.7 eV each are quickly stopped and often backscattered toward the vacuum, the heavier and more energetic Bi atoms implant at a depth that roughly corresponds to the bottom of the crater, when it is fully developed at a later time. Motions of the projectile atoms after the time indicated for the red point still occur, but they are caused by the slower dynamics of the surrounding material rather than by the projectile itself. At 10 keV, the single Bi atoms reach the bottom of the simulation 2748



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yields for those projectiles might have consequences for analytical applications. The energy dependence of the sputtered quantities below 1 keV is represented in Figure 11 for atomic Bi projectiles and

Figure 10. Energy dependence of the implantation depth of Bi3−5 projectiles in organic targets. Comparison between the implantation depth of the center of mass of the projectile in the case of sample P (colored symbols) and the experimentally determined implantation depth for a trehalose film (black symbols).12 The measured implantation depths correspond to a 45° incidence angle. The vertical bars are the experimental errors reported for the measurements. The calculated SRIM values, also reported in ref 12 for the implantation of Bi in trehalose, are signified by the dashed black line. The variations of implantation depth in the MD simulations (not shown) do not exceed 15% of the reported averages.

Figure 11. Polymer sputtering by low-energy Bi projectiles. Energy dependence of the sputtering for samples with different chain lengths (see text for details). The black dots at 1000 eV correspond to the sputtered masses (not the volumes) measured for the bombardment of trehalose.73

three polymeric samples, including sample P and two others, with, respectively, shorter (icosane: ICO; 20 units) and longer chain lengths (LP; 250 units). The evolution is quasi-linear beyond 200 eV. The absolute values of the sputtered masses depend significantly on the chain length, with shorter chains producing higher emission yields. Indeed, the results show that the shorter chains can be sputtered intact even at these low energies, whereas longer chains cannot. For ICO upon Bi bombardment, the mass ejected as entire molecules amounts to ∼25% of the total sputtered mass. For longer molecules, it is negligible at these energies. A similar difference as a function of the chain length was also observed upon C60 bombardment at higher energies.30 The evolution of the sputtering yields upon 0−1.5 keV bombardment of trehalose samples by Cs+ and Xe+ primary ions was reported by Wehbe and Houssiau.73 In agreement with our simulations of Bi bombardment, the energy dependence they observe is linear beyond 100 eV. For direct comparison, their measured sputter yield numbers have been transformed in mass equivalents and are plotted in Figure 11 for the case of 1 keV bombardment. They fall in the range of sputtered masses measured for 1 keV bombardment of samples P and ICO. The sputtered masses are summarized for 1 keV Bi, Bi3, and Bi5 in Figure 12a. The results obtained with C60 projectiles are also plotted for comparison. In contrast with higher energies (5−10 keV), the sputtered mass decreases with increasing projectile nuclearity for the three considered PE samples. For light element samples and larger energies, it was shown in previous studies that the sputtered mass was inversely proportional to projectile range and to the average depth of the energy deposition in the sample.21,23 In our case, the penetration depth D of the projectiles at all energies between 0 and 1 keV follow the order of DBi5 > DBi3 > DBi (Figure 10). In the same line as in refs 24 and 74, Figure 12b shows the energy distributions in the surface of sample P at a time when the projectiles have transferred 90% of their energy (900 eV) to the target. In agreement with the ranges, Figure 12b shows that the energy distribution broadens toward the depth of the target with increasing nuclearity for Bin projectiles. A simple

Bi3 clusters. The experimental data for Bi3 and Bi5 seem to indicate the opposite trend between the two clusters. However, they are scarce and correspond to significantly higher energies, and the error bars reported by the authors are large, making any conclusion difficult. The larger penetration depth of 25 keV Bi ions measured in trehalose (17.9/cos(45°) = 25.3 nm12) is consistent with our results for sample P. Though the materials and the computational methods differ, the trend of the SRIM calculations for single Bi impacts is also similar to the one observed in our MD simulations. Nevertheless, the strong difference of behavior indicated by the MD data between Bi atoms and clusters suggest that, in contrast with what is proposed in ref 12, the penetration depth of a Bin cluster with a kinetic energy E cannot simply be approximated by the penetration depth of a single Bi atom with a kinetic energy E/n. This hypothesis, consistent with an independent motion of the constituents of the clusters, is not verified in the MD simulations. As suggested by the time-evolution of the dynamics, the Bi atoms rather tend to remain together over a large portion of their trajectory in the solid (see Figure 9b), as was also observed for Au3 impinging on water ice.21 The more complex behavior of small Bi clusters is also indicated by the fact that Bi5 penetrates deeper than isoenergetic Bi3 according to our calculations. 3.3. Low-Energy Bombardment of Polymers. The use of sub-keV energy beams of atomic ions (Cs, Xe) has proved to be a very interesting alternative to more energetic cluster projectiles (C60, SF5) for the molecular depth-profiling of polymers72 and amino acids.73 The explanation is related to the low penetration depth of low-energy projectiles and, in the case of Cs, to chemical effects in the modified surface created after the initial transient. In the course of this study, it was, therefore, judged useful to carefully compare the behavior of Bin projectiles in the energy range of 0−1000 eV. In particular, the observation of different penetration depths and sputtering 2749



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Figure 12. Polymer sputtering by low-energy Bin and C60 clusters. (a) Mass (volumes) sputtered by 1 keV clusters. (b) Energy distributions in the surface when the projectiles have transferred 90% of their energy (900 eV) to the target. (c, d) Time-evolution of the energy dissipation in the polymer surface for (c) Bi and (d) Bi5 projectiles.

yields is observed when going from Bi to Bi3−5 projectiles. For organic samples, the calculated sputtered masses are in agreement with recently reported experiments. The comparison with experimental results also indicates that the increase of the sputtering yield when going from Bi atoms to Bi clusters (a factor of ∼2) is largely insufficient to explain the experimentally observed increase in molecular ion yields (1−2 orders of magnitude). Therefore, a projectile-dependent ionization probability must be assumed. The extent of molecular fragmentation follows the order of Bi > Bi3 > Bi5 > C60, that is, softer emission with larger clusters. In the simulations, the different sputtering yields are explained by the different projectile penetration, energy transfer, and crater formation. One important observation is that even 5−10 keV Bi atoms create conical craters in the polymeric solid, though the collision cascades are rather dilute, questioning the use of the linear collision cascade theory to explain sputtering in such a situation. The energy dependence of the penetration depth suggests a significantly different behavior for single Bi atoms and Bi3−5 clusters. Finally, simulations performed at low energies predict that sub-keV Bin clusters should not provide better results for sputtering and depth profiling than isoenergetic single atoms.

interpretation of the unexpected result of Figure 12a might then be that Bin clusters waste a larger part of their energy in the depth of the sample, thereby causing less sputtering. It should be noted, however, that, in this low-energy range, the largest part of the Bin projectile energy is still placed in the topmost layers of the sample, which is rather favorable for sputtering. For more details, Figure 12c,d gives the time evolution of the energy dissipation in the polymer surface over the first 1500 fs of the interaction for 1 keV Bi and Bi5 bombardment of sample P. For Bi impacts, the main energy peak is between 1 and 2 nm under the surface and then the energy dissipates slowly in the organic medium. For Bi5 impacts, the peak is slightly less intense but it spreads from the topmost layer to the region between 2 and 3 nm over the first 500 fs. Therefore, the energy dissipates in deeper layers and there is comparatively less energy reflection through sputtering. In terms of the aforementioned experimental application for organic depth profiling, dissipation of the energy in deeper layers and reduction of the sputtering are not desired. For these reasons, our predictions suggest that Bin clusters should not provide better results than atomic Bi in that energy range.

4. CONCLUSION The theoretical study of the bombardment of polymeric, metallic, and hybrid targets by keV Bi1−3−5 and C60 clusters shows that the respective behaviors and performances of the different projectiles are strongly dependent on the target. Bi clusters are more efficient at sputtering gold, whereas C60 shows the largest sputtering yields for the polymeric sample, confirming the importance of mass matching in cluster-induced sputtering. For all the targets, nonlinearity of the sputtering

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*Phone: +3210473596. Fax: +3210472005. E-mail: arnaud. [email protected]. Notes

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ACKNOWLEDGMENTS The authors wish to thank Professor Barbara J. Garrison from Penn State University for the access to her simulation code. Dr. Martin Seah is gratefully acknowledged for his very useful insights in the discussion of the sputtering yields of gold surfaces and nanoparticles. K.H. and part of this work were supported by the French Community of Belgium via the Concerted Research Action Programme (ARC NANHYMO: convention 07/12-003). The complementary support of the European Community via the FP7 project 3D-Nanochemiscope (Grant agreement: CP-TP 200613-2) is also acknowledged. A.D. is a Senior Research Associate of the Belgian Fonds National pour la Recherche Scientifique (FNRS).

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