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Au400 Sputtering of a Polymer with Adsorbed Metal Nanoparticles: A Molecular Dynamics Study Oscar A. Restrepo* and Arnaud Delcorte Institute of Condensed Matter and Nanosciences - Bio & Soft Matter, Universite catholique de Louvain, Croix du Sud, 1 bte 3; B-1348 Louvain-la-Neuve, Belgium ABSTRACT: Using molecular dynamics simulations, this article investigates the interaction of 540 keV Au400 projectiles with polyethylene and with gold-adsorbed nanoparticles of ∼25 Å in diameter. Impacts over the Au nanoparticles and on the polymer are compared in terms of sputtered quantities, projectile penetration, and energy deposition in the substrate, and connections with experiments are made. In the considered energy range, impacts over a nanoparticle emit 1 order of magnitude more organic material than impacts on the polymeric substrate. This is explained by the large range of the heavy Au400 projectile in the polymer and the increased stopping when it first encounters an adsorbed nanoparticle. As a result, we predict that organic material sputtering and analysis with Au400 should be favored by the presence of gold nanoparticles (metal-assisted secondary ion mass spectrometry). The simulations also indicate that the average depth of emission of organic material does not exceed 20 Å, and it is much less for impacts on the nanoparticles. Finally, Au400 projectiles with >10 keV of energy are able to sputter entire nanoparticles via pressure wave development and collective molecular motions in the organic substrate.
1. INTRODUCTION This theoretical study was inspired by two concomitant experimental facts, recently established in the field of organic secondary ion mass spectrometry (SIMS). First, after the discovery that ultrathin metal coatings, as obtained by sample metallization (metal-assisted SIMS), help to increase the detected secondary ion yields of organic samples by up to 2 orders of magnitude,14 it was found that such an enhancement was generally not observed with cluster projectiles, like C60 and even Bin. Instead, a reduction or a stagnation of the yield appeared to be the rule upon such cluster bombardment. Second, a Au400 cluster ion source was used successfully to produce molecular ions from organic materials, with seemingly excellent surface sensitivity and large molecular ion yields.5,6 In this study, we chose to investigate the interaction of Au400 projectiles with a polyethylene crystal (PE) and with Au nanoparticles adsorbed on such a substrate, to explore both the clusterpolymer interaction physics and the effect of adsorbed nanoparticles on that interaction. In a recent article, the comparatively mediocre performance of C60 for sputtering metalorganic layers7 was explained using mechanistic arguments.8,9 It was shown that, in comparison with a 10 keV Ga projectile, an isoenergetic C60 fullerene, with the low atomic mass of its constituents and its relatively low energy per atom, was inefficient to retransfer momentum to the organic material when impacting adsorbed metal nanoparticles. With the same kinetic energy, Au400 projectiles were also unable to sputter r 2011 American Chemical Society
organic material from metal organic surfaces.10 Indeed, once again, upon impact on the nanoparticle, the generated recoil atoms have an energy that is too low to cause further sputtering of the organic material. In addition, the same study demonstrated that, for impacts on the organic substrate, much less sputtering could be observed with Au400 than with C60 or Au3 and that the projectile was implanting quite deeply in the solid.10 These two observations appear to be in disagreement with the aforementioned claims of high sputtering yields and surface sensitivity. However, many of the experiments have been carried out with a projectile energy of 102 keV, i.e., 1 order of magnitude larger than the preliminary theoretical study of ref 10. Therefore, the first goal of this new study is to explore the energy dependence of the projectile penetration, energy deposition, and sputtering of Au400 projectiles, for impacts on both the organic material and the supported metal nanoparticles. The physics of Aun clusters impinging on light materials and nanoparticles of different sizes has been previously studied by Urbassek and collaborators.1113 Thanks to these and other works,14,15 the ranges and interaction of heavy clusters in different types of materials are better understood. It is accepted that the range of a heavy cluster stopped in a light and flat material is increased beyond that of an equi-velocity atom.16 The Received: February 4, 2011 Revised: April 18, 2011 Published: May 24, 2011 12751
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different case scenarios help understand this large difference. In the final section of this article, our new results are critically discussed in comparison with existing experiments.
Figure 1. (a) 3D perspective view of the simulation cell after sample relaxation and before the bombardment. Configuration with the central Au555 nanoparticle (gray) and the Au400 projectile on top (red). The other Au atoms are in yellow, and the PE coarse-grained units are in orange. (b) Top view of the same cell indicating the impact point. For impacts on PE, the gray nanoparticle is removed.
explanation is that the front atoms in the cluster collide with target atoms and transfer sufficient momentum to them to clear the way for the following cluster atoms. As a result, the stopping power of the cluster is reduced, and therefore, its range is increased. This concept was called the “clearing-the-way” effect, and it was introduced by Shulga and Sigmund two decades ago.17,18 Of course, this effect can be strongly modified by the presence of nanoparticles on the surface. In the aforementioned theoretical studies, the emphasis was placed on the cluster ranges and stopping powers in comparison with atomic projectiles. Little attention was paid to the sputtering yields of organic materials upon Au400 bombardment. Therefore, this aspect of the physics, important for mass spectrometric applications, is at the heart of the present study. From the point of view of applications, the characterization of nanoparticles and nanoobjects and their environment gained importance with the advent of nanotechnologies. Nanostructures significantly change the surface properties compared to their flat or bulk counterparts. While their morphology can be studied by various techniques such as SEM, TEM, or AFM, their full physical and chemical characterization remains difficult. Recently, SIMS studies in the event-by-event bombardment mode, using Au400 as a projectile, have provided morphological and chemical information with a resolution of ∼10 nm.1921 These measurements also lead to the determination of the surface coverage. For these specific studies, our MD simulations can also provide a more physical background for the interpretation of the experimental results. To explore the physics of energetic Au400 cluster impacts on organic materials and supported nanoparticles (MetA-SIMS situation), we extend here our previous studies10 using a larger target made of a polyethylene crystal covered by gold nanoparticles. Au400 clusters with energies ranging from 5 to 40 keV are aimed at the same impact points on the surface (Figure 1), and the projectile energy deposition and sputtering characteristics are investigated in each case. Surprisingly, our results show that impacts on the gold nanoparticles induce the emission of ten times more organic material than direct hits on the polymer, suggesting that MetA-SIMS might be a promising approach for the analysis of organic materials using Au400 projectiles. The physics of the projectile penetration and energy deposition in the
2. COMPUTATIONAL METHOD Classical molecular dynamics simulations are used to study the system of interest. Details about the method and the code used for sputtering, developed at Penn State University, can be found in refs 22 and 23. The trajectories were visualized using VMD program.24 The sample used to model the impact of a Au400 cluster with a polyethylene (PE) crystal covered by Au nanoparticles (see Figure 1) was designed via several stages of relaxation. For the PE substrate, a coarse-grained approximation was implemented to reduce the computational expense.25 We first defined an orthorhombic unit cell with lattice parameters given by a(T) = (7.74053 þ 0.000 471 (T 400)) Å, b(T) = (4.45817 þ 0.000 261 (T 400)) Å, and c(T) = (2.52748 þ 0.000 014 (T 400)) Å.26 On the basis of this unit cell, a box with 5200 chains formed of individual elements (198 CH2 and 2 CH3 localized at the ends) and dimensions 300 300 250 Å3 was set up. The initial velocities of all atoms were set using a Gaussian distribution with temperature of 4 K. This value of temperature was chosen to obtain a good agreement with experimental values of the polyethylene structure obtained at the same temperature.27 The nanoparticles on the surface were obtained in two steps. First, before starting relaxation, a random deposition algorithm was used to place the atoms on the top PE surface, creating a fractal structure that mimics how gold deposits when samples are inside of a metallization chamber.28 Second, successive relaxations were achieved until the relaxed nanoparticle structure was obtained. 13 386 Au atoms were used for the gold overlayer. Therefore, the total system consisted of 1 053 386 elements. At the end of the relaxation, the PE surface was covered by a nanostructure of gold islets of various sizes (average diameter of ∼25 Å). The equivalent thickness of the obtained Au layer, if it were uniform, would be 2.6 Å, which corresponds to a quantity of deposited gold of 2.6 nmol/cm2. The obtained surface nanostructure is in good agreement with experiments for that quantity of deposited gold (scanning electron microscopy).29 The Au555 cluster formed in the center of the sample has a particular importance. Indeed, in this study, we compare the effects of impacts of Au400 on that central nanoparticle and impacts on the PE substrate. For the impacts over PE, we used a second simulation cell from which the central cluster was removed before undergoing a last stage of relaxation. Both the Au555 surface nanoparticle and the Au400 projectile were quasispherical, with diameters of ∼25 and ∼20 Å, respectively. Over the nonbombarded five sides, rigid boundaries of 5 Å and stochastic forces at 4 K over a region of 15 Å were implemented using the Langevin equation to provide a heat bath during relaxation. This device permits sample equilibration, and during the bombardment, it absorbs the pressure waves generated by the projectile impact. The interaction potentials used were the MD/MC-CEM potential for AuAu,30 Lennard-Jones potential functions for AuPE and PEPE intermolecular interactions, and Morse potential functions for the intramolecular interactions in PE. More details about the potential parameters can be found in ref 31. 12752
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Figure 3. Yield distribution of PE fragments (number of fragments per impact as a function of mass). Impact on PE at 40 keV is compared to impacts on the central Au555 nanoparticle at 20, 30, and 40 keV.
Figure 2. (a) Sputtered masses of PE fragments for impacts on PE and direct hits on the Au555 nanoparticle. (b) Sputtered masses of gold for the same conditions. Full symbols are for Aun clusters with n < 10, and open symbols represent the total sputtered gold mass, including entire nanoparticles desorbed at much later times.
The impact energies were set to 5, 10, 15, 20, 30, and 40 keV, which is equivalent to 12.5, 25, 37.5, 50, 75, and 100 eV/atom. The total integration time was set to 30 ps. After this time the total ejected mass was gathered and analyzed. It is important to note here that, for the considered metalorganic system and beyond a projectile energy of 10 keV, some late events might still happen after 30 ps, such as late desorption of massive intact nanoparticles. Therefore, caution is used in the discussion of the sputtered mass values. In addition, because of the restricted choice of aiming points (one impact per energy value), the results do not intend to provide a statistically significant description of the sputtering of this system that could, for instance, be quantitatively compared to experiments. Rather, this article focuses on the pronounced differences of projectile penetration, energy deposition, and surface sputtering observed between the two extreme cases of impacts on the polymer and direct hits on a nanoparticle and their energy dependence.
3. RESULTS AND DISCUSSION The first part of the article (Section 3.1) analyzes the distributions of sputtered species. In Section 3.2, the different case scenarios of the interaction of the projectile with the surface are investigated, and the reasons of the observed behaviors are explained. A particular emphasis is placed on the clustercluster
interaction in Section 3.3. Finally, Section 3.4 compares our computational predictions to the available experimental results. 3.1. Energy Dependence of the Sputtering. The sputtered masses of PE fragments and of gold atoms and clusters are shown in Figure 2 as a function of energy, from 5 to 40 keV. The emission of organic fragments, Figure 2a, increases as a function of energy for both impacts on the central Au555 nanoparticle and impacts on the bare PE substrate. However, the sputtered mass difference between these two situations is about 1 order of magnitude. This effect is particularly true for energies equal to or larger than 15 keV. For 10 keV of energy and below, almost no organic fragment emission is observed. A second observation is that the sputtered mass evolution as a function of energy is nonlinear in this energy range. It is best fitted by a square function (full lines in Figure 2a). This is in contrast with some previous results obtained upon bombardment of amorphous polyethylene samples with C60 projectiles32 and with Arn cluster bombardment of van der Waals bonded systems.13 In the case of Au400 projectiles, the region ∼1015 keV can be considered as the threshold of sputtering. The organic fragment emission yield distribution (mass spectrum) is reported in Figure 3 for different energies and impact points (20, 30, 40 keV for direct impacts on the cluster, 40 keV for impact on PE). Given the specifics of our simulations, the statistics are scarce but, nonetheless, indicative of some trends. First, in the low mass range (below 200 Da), the yield distributions are reasonably well fit by exponential functions [Y = A exp(Bx), with R2 values above 0.90.95]. For impacts on the central Au555 nanoparticle, the exponential decay parameter B for the best fit is in the range 0.010.02 depending on the projectile energy. In contrast, it is close to 0.04 for the 40 keV impact on PE. At lower energy, the impacts on PE provide so little yield that such an analysis would be meaningless. The results of Figure 3 show that, in addition to generating higher yields of organic fragments, impacts of Au400 on the nanoparticle tend to sputter larger chain segments. Figure 4 shows the depth of origin of the sputtered PE fragments. The vertical coordinate Z is calculated with respect to the AuPE interface. The average depth and the vertical coordinate of the deepest ejected fragments (Zmax) are reported as a function of the projectile kinetic energy (E0). Because the interface between Au and PE is not flat, these quantities may exhibit an error of a few Angstrom, depending on the exact 12753
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Figure 4. Average depth of origin of the sputtered PE fragments (Zav) and z coordinate of the deepest sputtered fragment (Zmax) as a function of the projectile energy for (a) impacts on PE and (b) impacts on the Au555 nanoparticle. The depths are calculated with respect to the AuPE interface.
emission point on the surface. The depth of emission tends to increase with the projectile energy, but in all cases, most of the sputtered fragments and/or molecules originate in the top 20 Å of the polymer surface. However, there is again a clear distinction between impacts on the PE surface and on the Au555 nanoparticle. At all energies, impacts on the nanoparticle lead to average emission depths that are more than 2 times smaller with respect to impacts on the polymer. This result suggests that ion beam analysis of organic materials with Au400 in the MetA-SIMS configuration, that is, with Au nanoparticles on the surface, should be more surface sensitive than the regular analysis conditions. The sputtering of gold clusters is also analyzed in Figure 2b. A distinction is made between small Aun clusters with n < 10 (full symbols) and the total sputtered gold mass, including entire nanoparticles desorbed intact from the surface (open symbols). For impacts on the central nanoparticle, the mass of small gold clusters (nanoparticle fragments) increases almost linearly with the impact energy (R2 value >0.9). The branching at 10 keV with the curve corresponding to the total mass indicates desorption of some of the surrounding nanoparticles beyond that energy threshold. In contrast, impacts on PE induce very little fragmentation of the surrounding gold nanoparticle or backscattering of the projectile constituents. In that case, the intact nanoparticles start desorbing at the energy threshold of 30 keV. The following sections investigate the physics of the impacts and, in particular, the differences between impacts on the gold nanoparticle and the PE surface, giving rise to these sputtering characteristics. 3.2. Ranges and Energy Deposition in the Surface. To understand the large sputtering yield differences between the impacts on the Au nanoparticle and over PE, additional information has been gathered from the analysis of the projectile penetration and energy transfer to the surface. Figures 5a and 5b show cross-sectional views of the samples upon 40 keV Au400 impacts, when the projectiles reach their maximum penetration depth (8 ps for impact on PE and 4.5 ps for impact on Au nanoparticle). In Figure 5a, corresponding to the impact on PE, the projectile atoms are colored as a function of their position in the cluster, in red for atoms belonging to the cluster outer shell and in yellow for the cluster core. The projectile implants deep in the organic sample (>200 Å), creating a narrow track of broken bonds and damaged material. The predominance of red projectile atoms in the damage track indicates that the projectile is “peeled off” upon interaction with the organic medium (inset of Figure 5a). The picture also evokes
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Figure 5. Cross sections of the target at the time of maximum penetration for impacts at 40 keV, (a) on PE and (b) on the Au555 nanoparticle. The central insets show the origin of the PE fragments (purple) for each case. The left inset shows the effect of “peeling off ” of the projectile at 1.5 ps. The vertical distances are calculated with respect to the AuPE interface.
the “clearing the way” effect,17,18 i.e., the front atoms of the projectile outer shell (red) clear the way for the core atoms (yellow). In contrast, Figure 5b, corresponding to the impact on the gold nanoparticle, shows the formation of a large hemispherical crater with a diameter >100 Å (here, yellow represents the target cluster Au555). The maximum penetration depth (range) is attained when the PE material surrounding the crater reaches its maximum compression. At the end, the polymer crystal relaxes with a crater of only 30 Å of depth. Figure 6a and 6b displays the evolution of the penetration of the center of mass of Au400 as a function of time for different values of the initial kinetic energy. The first observation is that impacts on PE transfer all the projectile energy to the sample, while direct hits on the Au555 nanoparticle induce the backscattering of the center of mass. As the projectile energy increases, the reflection becomes faster (Figure 6b). Second, the different scales on Figure 6a and 6b confirm that the range of Au400 is significantly larger upon impact on PE than on the nanoparticle for all the considered energies. In Figure 6c and 6d, two different estimates of the maximum penetration depth are reported as a function of the impact energy. They are the maximum penetration depth of center of mass, RCM, and the position of the deepest atom, RMax. The divergence between the values of RCM and RMax with increasing energy is an indication of the pulverization of the projectile. This effect is more pronounced for impacts on the Au555 nanoparticle. This phenomenon was also observed for impact on graphite and gold crystals, as was noted by Anders and Urbassek.33 To have a more detailed view of the energy transfer from the projectile to the sample, the simulation cell has been sliced in horizontal slabs of 3 Å each. The evolution of the kinetic energy in each slab was monitored as a function of time for the different trajectories. The kinetic energy was divided by the volume of the slab (3 300 250 Å3) to compare the pressure that one slab exerts over its other two neighbors. However, it is important to point out that the internal pressure (the derivative of the kinetic energy with respect to the volume as it can be deduced from definition of work) is not in phase with the kinetic energy; i.e., when one is maximum the other must be a minimum. Then, we interpret it as the total kinetic energy of the slab normalized to the layer volume at a fixed time t, and that gives us information 12754
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Figure 6. Evolution of the vertical coordinates of the center of mass (CM) as a function of time for impacts (a) on PE and (b) on the Au555 nanoparticle. The pictures (c,d) show the maximum range reached by the projectile center of mass (represented by RCM) and vertical positions of the deepest Au atoms of the projectiles (represented by RMax), for these two types of impacts. The vertical distances are calculated with respect to the AuPE interface.
Figure 8. Fraction of the projectile energy (E3L/E0) transferred to the top 30 Å of the target as a function of the projectile energy.
Figure 7. Evolution of the kinetic energy per volume of layer KE/Vol in the sample for impacts at 5 keV (a,b) and 40 keV (c,d). The left column corresponds to impacts on PE and the right column to impacts on the Au555 nanoparticle. The KE/Vol in each horizontal slab of 3 Å is colorcoded from blue to white (color scales on the right). The time resolution is 50 fs for the first 2000 and 500 fs afterward. The vertical distances are calculated with respect to the AuPE interface.
about how energy transfers between layers and from one time step to the next. The results are displayed in Figure 7 for two energies (5 and 40 keV) and for two different impact points
(on PE and on the Au555 nanoparticle). First, the energy plots of Figure 7 confirm that the energy is transferred in the entire depth of the sample upon impact on PE, while the energy peak is confined in the top 100 Å for impacts on Au555. Second, the energy plots show that, for all the investigated cluster energies, the Au400 cluster impact generates an energy wave in the substrate, with an intensity that is proportional to the impact energy. The decoupling between the projectile implantation and the wave penetration is obvious, for instance, at 5 keV, where the projectile slows down rapidly while the wave continues to progress in the solid with an almost unchanged speed. The comparison between the two impact points indicates that the velocity of the wave is slightly larger when impacts are on PE. At 40 keV, it reaches the bottom of the substrate at ∼4 ps for impacts on PE and at ∼6 ps for impacts on the nanoparticle. Reflection of the energy wave on the bottom of the sample appears to be minor, indicating that the friction region on top of the last rigid layer plays its role. However, there is some reflection 12755
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Figure 9. Side view of a low-energy impact of 5 keV on PE at 4 ps. Each colored slice of PE is 10 Å thick. The surrounding gold nanoparticles have been removed for clarity.
of the energy within the bulk of the substrate, after the initial compression induced by the projectile itself and the developing wave. It is shown by the green shadows below 200 Å in Figure 7c, d, which represents less than 0.5% of the maximum energy per volume in the color bar. Finally, lighter rays developing above the sample surface (negative Z values) indicate the sputtering of surface species, for the impact on the Au555 nanoparticle. Of particular interest is the energy deposition in the top layers of the polymer because the sputtered material is mostly originating from there (Figure 4). The fraction of the projectile energy (E3L/E0) transferred to the top 30 Å of the target, as a function of the initial energy E0, is shown in Figure 8. Irrespective of the projectile energy, impacts on the central cluster always transfer ∼2530% of the total energy to the top 30 Å of the sample; i.e., E3L is proportional to E0. This is probably a consequence of the formation of a hemispherical crater in the top surface region. In contrast, for impacts on the organic material, the transferred energy fraction E3L decreases with increasing total energy E0. This decrease is best fit by a power function of the form E3L/E0 = aE0n with n = 0.475, that is very close to 0.5. The exponent 0.5 means that E3L/E0 is proportional to 1/v0. So, for a massive Au cluster impinging on a soft polymeric material, with the formation of a cylindrical track, our results suggest that the fraction of the projectile energy deposited in the surface layer is roughly proportional to the inverse velocity, i.e., to the time spent in that surface region. In terms of induced sputtering, Figure 8 also helps to rationalize the comparatively mediocre performance of Au400 projectiles when impacts are on the polymer. Some of the characteristics observed in Figure 5 and Figure 7, such as the track formation and the energy wave development, are due to the cluster projectile effect, i.e., the fact that the constituent atoms of the projectile act as a whole rather than independently, but some aspects can be explained with basic atomistic concepts. In particular, the differences between the impact on the PE and the impact on the nanoparticle can be rationalized on the basis of the different atomic mass ratios between the projectile and the target. More precisely, considering the hard sphere approximation and that the atoms of the target are at rest before the collision, when the projectile and the target have the same atomic mass, the sum of the deflection
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Figure 10. Time evolution of the energy per atom for the projectile Au400 (KP), the surface nanoparticle Au555 (KT), and the combination of both Au555 þ Au400 (KPþT). The color coding indicates the projectile energy. Inset: microscopic view of the situation at the maximum of KT for 40 keV bombardment. Atoms of the Au400 projectile are in red, and atoms of the surface Au555 nanoparticle are in yellow.
angles of each pair of colliding atoms is 90°. In this particular situation, the energy transfer is a maximum. Now, as the time evolves, the deflection angles change; the kinetic energy per particle reduces; and interatomic potentials become important. The global effect is the maximum angle of deflection of all the colliding atoms. This explains why, in the Au400Au555 collision, the damage is confined in the surface, with a semispherical crater, much like C60 impinging on PE32 or Au402 impinging on bulk Au.33 In contrast, because of the mass ratio between Au and C, Au is only slightly deflected by collisions with C atoms, and the energy transfer per collision is limited. Therefore, the mass ratio between the projectile and the target atoms certainly plays a major role in the explanation of the different behaviors shown in Figures 57. Note that, in comparison with a bulk Au target, the spherical Au555 nanoparticle has a significant fraction of its atoms at the surface, and therefore, the total cohesive energy is reduced. Consequently, atoms can move with less constraint, increasing the Au emission yield by more than a factor of 2 in comparison with a planar target. This effect is also observed by Zimmermann and Urbassek for a spherical Au cluster of 200 Å of diameter.34 In our case, as a direct result of the different penetration depth and energy transfer scenarios, many more CC bonds are broken when the impact is on the Au555 nanoparticle. Indeed, a large fraction of the Au atoms propelled from the nanoparticle upon impact redistribute their energy in the organic material, inducing numerous CC bond scissions and larger emission yields of organic material than direct impacts on PE. This study of the projectile penetration and energy deposition in the surface explains the evolution of the sputtered masses of Figure 2. At low energy (10 keV), EAu , E0. In that case, the collisions can be considered as elastic. As mentioned before, the transfer of energy at each collision is a maximum, explaining the high negative slope of the kinetic energy KP of atoms of the projectile as well as the high positive slope of KT. Irrespective of the impact energy, more than 60% of E0 is transferred to the surface cluster within that time range. In that first interval, the two clusters go through a
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stage of compression followed by a stage of decompression, as shown by the evolution of their potential energies in Figure 11 (40 keV case). The maximum potential energy around 250 fs corresponds to the local minimum of KPþT in Figure 10. Figure 11 also indicates that the interactions with PE chains only start after the maximum compression of the gold clusters. Additional simulations with the same two clusters in the vacuum, i.e., without a PE substrate, and over a larger range of energies were performed to corroborate the results of Figures 10 and 11. They showed that the behavior of the two clusters is equivalent with or without PE before reaching the maximum compression. In the second time interval, beyond the maximum of KT, the value of KPþT decreases as the energy gets transferred to the PE substrate. The transfer of energy to the substrate is accompanied by a change of slope of the projectile energy KP, which can be explained by the less efficient transfer of energy in collisions between Au and C atoms. A remarkable observation from Figure 10 is that KP and KT behave similarly after the maximum of KT, for example, beyond 400 fs for the 40 keV case. This suggests that the atoms of the projectile and target clusters become undistinguishable after the collision. This effect is even more obvious in Figure 11, where the potential energy curves merge exactly beyond 300 fs, before the beginning of the energy transfer to the substrate. In other words, the organic material does not see the impact of the projectile but rather the combination of both projectile and target cluster atoms, with a broad angular distribution of velocities, resulting from the initial projectiletarget cluster collision. This velocity distribution resulting from the impact is responsible for the energy deposition into a wide semispherical volume just below the two fused clusters. This behavior clearly differs from the impact of a projectile with 955 atoms. In that case, all atoms would move in the same direction until they start the interaction with the PE surface. 3.4. Comparison with Experiments. To our knowledge, there exists no experimental measurement of the sputtering yields of organic materials upon Au400 bombardment that could be compared with our simulations and no published comparison of the specific performance of Au400 with respect to other projectiles in terms of sputtering. In several cases, however, secondary ion sampling depths and secondary ion yields have been reported. In the case of multilayered films bombarded by 136 keV Au400þ, a secondary ion emission depth of 6090 Å was measured for polyatomic secondary ions.35 Our simulations indicate that, even though the range of 40 keV in the PE substrate is ∼200 Å, the sputtered fragments come from a maximum depth of ∼50 Å and from an average depth that does not exceed 20 Å (Figure 4). If an extrapolation of the curves of Figure 4a suggests a larger emission depth at 136 keV, a value of 6090 Å is not unrealistic. Direct impacts on the Au555 nanoparticle even lead to a shallower emission depth of the organic fragments. Therefore, the emission generated by energetic Au400 projectiles remains surface sensitive, and our simulations generally agree with the experimental observations. For biomolecules such as dynorphin, bradykinin, and gramicidin S, a molecular ion yield enhancement of 12 orders of magnitude was reported between 40 keV Au4004þ and 10 keV Au3þ.6 In contrast, with glycine samples, Guillermier et al. showed that, for an acceleration voltage of 24 keV, Au5þ (24 keV) and Au4004þ (96 keV) emit similar yields of molecular ions.36 A difference between these two projectiles only appears 12757
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The Journal of Physical Chemistry C for higher acceleration voltages. In a complementary study, the same group finds that, upon 136 keV impacts, the yield of CN fragments sputtered from glycine is more than 1 order of magnitude larger for Au400 than for Au3.37 The emission yields of intact molecules and CN fragments upon Au400 bombardment are comparable for pure histidine, guanine, and glycine samples.38 Finally, for the system of 50 Å Ag nanoparticles adsorbed on glycine, larger yields of negative organic ions were reported with 136 keV Au4004þ than with 34 keV Au3þ and 26 keV C60þ.19 Nevertheless, even with a 4 times larger kinetic energy, Au400 generates only two times more glycine molecular ions (m/z = 74) than Au3 and two times less glycine dimers (m/z = 149). Given the available experimental evidence, it is therefore very difficult to assess whether Au400 really induces higher molecular ion yields than smaller clusters such as Au3 or C60 with the same total energy, at least in the energy range below 100 keV. In a preliminary theoretical article,10 we showed that, upon 10 keV bombardment, the sputtering of organic material was much less with Au400 than with C60 or even Au3. The present study shows that this energy of 10 keV still corresponds to the threshold of sputtering for Au400 and that a quasi-linear evolution of the yield as a function of energy only appears beyond 20 keV. Therefore, the comparison at 10 keV cannot be simply extrapolated to higher energies. However, the sputtering yields computed so far with C60 and Au400 and the significantly larger penetration depths of energetic Au400 clusters in organic materials at all energies do not plead in favor of larger sputtering yields for the latter projectile. Of course, electronic effects, not taken into account in the simulations, might explain the difference of molecular ion yields sometimes observed in the experiments. In several studies, products of recombination between glycine fragments and metal atoms from the projectile (Au),38 as well as from adsorbed nanoparticles (Ag),19,39 respectively, were also observed in the secondary ion mass spectra. Such rearrangements are not very frequent, but they occur in our simulations. In particular, direct hits at the nanoparticle generate recoil metal atoms that sometimes recombine with fragments of the organic substrate, as well as projectile atoms that hit neighboring nanoparticles and create adducts. Another intriguing experimental observation is that, upon continuous bombardment of biomolecular samples with Au4004þ, an intensity increase followed by saturation was observed, whereas a steady decrease occurred upon Au5þ bombardment.5 It is not clear that this observation corresponds to the case of molecular depth profiling observed with light element clusters. Indeed, in light of our simulations (Figure 5), impacts of 40 keV Au400 clusters lead to the implantation of the gold projectiles up to a depth of ∼200 Å and relatively little sputtering. Upon repeated bombardment, the surface should therefore gradually change from a pure organic material to a mixed metalorganic layer with Au clusters embedded in the organic matrix. In that case, the interpretation of the observed yield enhancement upon continuous bombardment could be given by the results of Figure 2a, i.e., the predicted organic emission yield increase when going from impacts on the organic material to impacts on the adsorbed gold nanoparticles. Finally, the ability of Aun clusters to desorb nanoparticles condensed on lightelement substrates (carbon) was investigated in recent studies, via the use of a collector, and the size distribution of the collected nanoparticles could be determined by TEM.12,40 In particular, it was found that 72 keV Au400 projectiles were able to desorb
