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Effects of Molecular Orientation and Size in Sputtering of Model Organic Crystals† Karim Hamraoui and Arnaud Delcorte* Unite´ PCPM, UniVersite´ catholique de LouVain, 1 Croix du Sud, B-1348, LouVain-la-NeuVe, Belgium ReceiVed: June 26, 2009; ReVised Manuscript ReceiVed: October 29, 2009
Using coarse-grained molecular dynamics simulations, we investigate the interaction of kiloelectronvolt C60, Au3, and Au projectiles with model polyethylene-like crystalline targets (hexacontane: C60H122). Two different orientations of hexacontane molecules were examined: horizontal and vertical. We observe strong structural effects related to the molecular orientation of the target on the sputtering yields. The simulations show that the sputtered mass increases dramatically when going from vertical to horizontal hydrocarbon chains (∼4-8 times, depending on the projectile). With the latter system, large chunks of organic material including many molecules are emitted (beyond 14 kDa). The observed differences are rationalized in terms of energy deposition, pressure wave development, and crater formation, in relation to the specific structure of the samples. This pronounced influence of the target structure might have implications for the mass spectrometric analysis of liquid-crystal-type materials and biological samples, including cells and tissues, where bilayered phospholipid membranes are a major constituent of the sample. 1. Introduction Bombardment of organic materials and polymers by energetic ions has a number of applications in surface treatment and analysis, such as ion-beam lithography1,2 and secondary ion mass spectrometry (SIMS).3,4 In SIMS, the nature of the fragments sputtered during ion bombardment is analyzed to identify the molecular composition of the target. Polyatomic ion sources (SF5+,5 Aun+,6 Bin+,7 and C60+8) have demonstrated their usefulness for this type of experiment, since it could be shown that surface molecules were desorbed with greater efficiency under cluster bombardment than with atomic projectiles.5,8 They also opened the door to molecular depth-profiling and, with wellfocused beams, to 3D molecular imaging.9-11 From the fundamental viewpoint, several questions remain and a more complete understanding of the sputtering and ionization processes of organic materials appears to be a requirement for the correct interpretation of the experiments. Many theoretical studies have already investigated the difference between processes initiated by polyatomic and atomic projectiles in various targets including metals, semiconductors, organic layers, or bulk polymers, and how the mechanism of molecular desorption changes as the size of the projectile increases.12-21 For cluster projectiles such as C60, the primary kinetic energy is partitioned between many constituent atoms as the projectile breaks up upon impact. The energy of individual atoms is, therefore, low, which results in a comparatively small penetration depth and damage only in the topmost layers of the solid. The collective action of the cluster constituents creates a localized region of high energy density at the surface that results in crater formation and the ejection of large quantities of material in the gas phase. A limited penetration depth and a large sputtering yield are believed to be responsible for the improved depth profiling capabilities of cluster beams, as the damage induced by projectile impact is immediately removed. Unlike polyatomic projectiles, most of the primary energy of atomic
projectiles is deposited deep inside the material and cannot contribute to ejection.13,22 With the help of molecular dynamics (MD) simulations, much of the physics of atomic and polyatomic projectile induced sputtering could thus be explained. However, the importance of structural factors in organic materials, such as molecular orientation and crystallinity, has not been fully assessed yet. Crystalline organic materials are important in materials science and nanotechnology (self-assembled layers, Langmuir-Blodgett (LB) films, liquid crystals), but also in biology (cell membranes),23 and so is their analysis with ion beam techniques (LB layers,24-26 phospholipids,27,28 cells,10,29,30 tissues31-33). A recent theoretical study involving Langmuir-Blodgett layers showed that crystallinity and molecular orientation play indeed a role in the energy deposition and sputtering processes upon C60 bombardment.34 In this contribution, we focus on structural effects observed in long, aligned alkanes bombarded by atomic and poyatomic projectiles. The chosen targets are polyethylenelike crystals (hexacontane: C60H122) with different orientations. They were chosen because they are among the simplest models of organic crystals, and they are observed experimentally.35 The purpose of this work is to study the effect of the crystalline orientation of the hexacontane molecules on the sputtering yields and crater formation, upon impact of polyatomic (C60 and Au3) and atomic (Au) projectiles with 5-25 keV of kinetic energy. The dynamics of energy transfer in a layered material, the projectile penetration, the damage created in the solid, and the effect of the projectile energy are also analyzed. In several ways, this investigation is complementary to the studies of involving arachidic acid LB layers.34,36 For instance, it shows new effects that could not be observed with smaller LB molecules and generalizes the observations to a series of projectiles. Among the crystalline orientation-related effects, we observe large yield differences between vertical and horizontal chains, peculiar crater shapes, and even layer delamination. 2. Computational Details
†
Part of the “Barbara J. Garrison Festschrift”. * To whom correspondence should be addressed. E-mail: arnaud.delcorte@ uclouvain.be.
The classical method of molecular dynamics simulations37 is used to study the sputtering of the systems of interest (SPUT
10.1021/jp906004v 2010 American Chemical Society Published on Web 12/28/2009
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TABLE 1: Morsea and Lennard-Jonesb Interaction Parameters Morse
De (eV)
1-2 1-3
3.6 0.03
1-2 1-3
3.6 0.03
R (Å-1)
rcutoff (Å)
CH2-CH2 1.53 2.55
2 2
5.0 5.0
CH2-CH3 1.53 2.55
2 2
5.0 5.0
re (Å)
Lennard-Jones
ε (eV)
σ (Å)
rcutoff (Å)
C-CH2, C-CH3 Au-CH2, Au-CH3 CH2-CH2 CH2-CH3 CH3-CH3
0.0120 0.0040 0.0052 0.0062 0.0076
3.40 3.04 3.85 3.90 3.90
7.65 7.65 7.65 7.65 7.65
a
The 1-2 interactions are defined as the nearest neighbor interactions, and the 1-3 are the next nearest neighbor interactions. b A 12-6 Lennard-Jones potential is used.
code13). Briefly, the position and velocity of each particle as a function of time is determined by numerically integrating Hamilton’s equations of motion. The energy and forces in the system are described by empirical interaction potentials (Table 1). In order to reduce the computational expense, certain atoms of the target are grouped to form united atoms or particles. The advantages of such a coarse-grained approach are that there are fewer particles, the potentials are simpler and thus quicker to calculate, and the fast H-vibration is eliminated which allows for a larger time step to be used in the integration.38,39 In biology, coarse-graining is routinely used for the MD simulations of lipid membranes.40 The hydrocarbon samples used for this study are hexacontane (HC: C60H122) and larger linear polyethylene molecules (PE: C300H602). Information on the crystalline packing of the HC chains was taken from published experimental data.35,41 The modeled HC and PE crystals have an orthorhombic structure with Pca21 symmetry and unit cell dimensions of a ) 7.121 Å, b ) 4.851 Å, and c ) 2.548 Å. This primitive cell contains 12 atoms (4 carbon and 8 hydrogen) with C-C and C-H bond lengths of 1.53 and 1.07 Å, respectively. The basic cell was coarse-grained by using the corresponding carbon atoms for the coordinates of the CH2 and CH3 united atoms. The HC molecules, shown in Figure 1, contain thus 58 CH2 particles of 14 amu and 2 CH3 particles of 15 amu. Because of the crystalline nature of the HC sample investigated in our simulations, two chain orientations with respect to the surface normal were studied: horizontal (HCh) and vertical (HCv). Side views of the HC systems with horizontal and vertical hydrocarbon chains and the different aiming points are also shown in Figure 1. The specifics of the computational cells used for the simulations of HC and PE molecular samples are summarized in Table 2. The obtained solids, with the united atom positions defined as described above, underwent several stages of relaxation in order to reach the equilibrium configuration. In the coarse-grained model, a Lennard-Jones potential was used to describe the interactions of the particles located on different HC and PE molecules (CH2-CH2, CH2-CH3, and CH3-CH3) as well as the C-CHx and Au-CHx potentials between atoms from the projectiles (C60, Au3, and Au) and particles in the sample. The values of ε and σ were chosen from previous studies describing linear hydrocarbons.42,43 With these values, the intermolecular binding energy of a bulk HC molecule is ∼5.2 eV. For the intramolecular interactions, the model must
Figure 1. Side view of the hexacontane crystals with (a) horizontal and (b) vertical carbon chains. The polymer chains are represented by green dots, and the end groups, by tan dots. (c) Top view of the unit cell with the positions of the impact points for the vertical chain solid.
TABLE 2: Characteristics of the Polyethylene Samples sample
HCv
HCh
PEh
formula molecular weight number of molecules total number of CH2/CH3 particles atom-equivalent number cell size -x (Å) -y (Å) -z (Å) number of trajectories at 5 keV number of trajectories at 15 keV number of trajectories at 25 keV
C60H122 842 9048 542880 1646736 279 275 155 5 5 1
C60H122 842 11600 696000 2111200 391 279 140 5 5 1
C300H602 4202 2400 720000 2164800 401 301 140 1
allow molecules to store internal energy up to the point when they dissociate. A Morse potential44 between adjacent CH2 and CH3 particles was chosen to account for the dissociating bond
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stretch term, with parameters that reflect the bond strength and equilibrium distance in linear hydrocarbons.42,43 The other necessary interactions are between CHx particles separated by one CHx particle (the 1-3 interaction), which are modeled by a Morse potential with a small well depth.38 This pair potential allows the particles to interact if the molecule is dissociated and also provides an equilibrium configuration of the sample in which the molecules adopt the appropriate zigzag shape. This type of interaction is preferred to an angle bend term that does not allow for dissociation and is therefore limiting for sputtering simulations. For CHx particles of a molecule that are separated by two or more particles, a Lennard-Jones potential with the same parameters as the one used for intermolecular interactions is used. The parameters for the Lennard-Jones and Morse interactions are given in Table 1. In all of the simulations, the C-C interactions of the C60 projectile are described by the AIREBO potential,45 an extension of the REBO potential46 including long-range interactions. To model the Au-Au interaction of the Au3 projectile, we used the molecular dynamics/ Monte Carlo corrected effective medium (MD/MC-CEM).47 In this study, we model the bombardment of HC and PE by C60, Au3, and Au projectiles. The projectiles were given 5, 15, and 25 keV of initial kinetic energy and were aimed normal to the surface. The atoms within the incident clusters were given initial velocities of zero with respect to the cluster center of mass, and the cluster orientation was determined randomly. Several trajectories were performed for each target-projectile combination, and they were computed up to a time of 25 ps. In all of the sputtering calculations, a rigid and a stochastic region (at 0 K) were employed along the crystal sides and bottom, in order to absorb the pressure waves induced by the projectiles.18,48 3. Results and Discussion The discussion begins with the description of the sputtering yields and crater characteristics (section 3.1). The dynamics of energy transfer in a layered material is discussed next (section 3.2), and the effects of the projectile energy are evaluated (section 3.3). Finally, our results are compared to other theoretical studies of organic and crystalline material sputtering (section 3.4). 3.1. Sputtering Yields and Crater Formation. Crosssectional views of the temporal evolution of the simulations of 5 keV C60, Au3, and Au impinging on the samples with horizontal (HCh) and vertical hexacontane molecules (HCv) are illustrated in Figure 2, and the statistics of the sputtered species at 25 ps are summarized in Table 3. Side views along the x and y axis of the situation at 20 ps are also shown in Figure 3 for sample HCh bombarded by 5 keV C60. The statistics of sputtering indicate that the total sputtered mass is reduced by a factor between 4 and 8 when going from the system with horizontal hexacontane molecules to the one with vertical molecules, depending on the projectile. The flux sputtered from the sample with horizontal chains consists of a mix of fragmented and intact hexacontane molecules. Table 3 shows that, for each projectile, the total number of sputtered fragments is also larger for the solid with horizontal molecules than for the solid with vertical molecules. An enhancement between 1.4 and 1.7 is observed, depending on the projectile. However, the difference of yield is mostly due to the ejection of intact C60H122 molecules and large clusters from the sample with horizontal chains. For cluster projectiles, an average of 29 C60H122 molecules is emitted with this arrangement. In contrast, polyatomic and atomic projectiles with 5 keV of energy are not able to desorb intact molecules from the hexacontane crystal
Hamraoui and Delcorte with vertical chains. The orientation of the chains also affects the nature of the sputtered flux in a more subtle way. As shown in Table 3, the ratio of the numbers of ejected CH3 (end group) versus CH2 (main chain), either as standalone particles or in larger fragments, is strongly influenced by the molecular arrangement. In sample HCh, this ratio remains quite close to the stoichiometric value (3.4%), except for Au bombardment. For all of the projectiles, that ratio is 2 times larger or more with sample HCv (11% upon C60 bombardment). Therefore, the emission of chain end fragments is much more favored with sample HCv. This orientational effect might be used by experimentalists as a diagnostic tool to investigate molecular orientations at surfaces. Finally, the results of Table 3 show that more carbon atoms of the C60 projectile are backscattered or desorbed with sample HCh (35) than with sample HCv (4). Pronounced differences between the sample with horizontal and vertical hexacontane molecules can also be observed in the snapshots of the trajectories shown in Figure 2. The side views in Figure 2 only show a 40 Å thick section of the target cut around the impact point. The snapshots at 25 ps show that the projectile impacts create craters in both systems but with very different features. First, the crater volume is much larger with sample HCh, in agreement with the larger numbers of ejected C60H122 molecules (Figure 2a). For that system, the formation of the crater around the impact point is accompanied by two breaches opening at the interface with the nearest hexacontane layers. The full side views of sample HCh upon C60 bombardment, Figure 3, show the extent of the material flow into the gas phase. They also clearly indicate directionality in the flow of ejecting molecules. As signified by the two white arrows (Figure 3b), the departing molecules and clusters follow two of the main crystalline directions, as if the molecular planes were gliding on top of each other. In this case, it is clear that the emission of molecules and clusters extends beyond 20 ps. Even though the final volume of the crater is slightly different for the three projectiles, the characteristic shape is conserved, indicating that it is mainly driven by the structure of the material and not by the projectile type. For sample HCv, the crater is generally smaller but the behavior is projectile-dependent. The crater formed upon fullerene bombardment has a spherical cap shape and is less deep than the one formed in sample HCh. In contrast, the craters formed upon gold bombardment (Au3 and Au) can be described more adequately as tracks, with a cylindrical shape and a larger depth than those formed in sample HCh. The statistics of damage are reported in Table 4. The level of damage is quantified by the numbers of atoms with at least one bond severed during the penetration of the projectile in the solid (defined as radicals in a previous study39). The total numbers and the numbers remaining in the solid at the end of the trajectory are listed for each projectile. The data in Table 4 indicate that the number of radicals induced in the solid is independent of the molecular orientation and slightly influenced by the projectile type. However, at the end of the trajectories (25 ps), more radicals remain in the solid with HCv than HCh; i.e., a smaller number of them are entrained with the sputtered flux in the vertical chain configuration. For instance, upon fullerene impact, only 23% of the damage created in sample HCh remains in the solid, vs 48% for sample HCv. This observation is consistent with the much smaller sputtering yields computed for sample HCv (Table 3). In order to clearly differentiate effects related to the chain length and to the molecular orientation, we designed a sample with horizontal chains of polyethylene (PEh), about 5 times
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Figure 2. Cross-sectional view of the temporal evolution of the interaction of 5 keV C60, Au3, and Au projectiles (normal incidence) with the samples with horizontal (a: HCh) and vertical hexacontane molecules (b: HCv). The cross section is centered at the impact point and is 40 Å thick. The projectile atoms are white.
larger than hexacontane (Figure 4). The PE chains in the top layers were kept free to move (no rigid boundaries in the direction of the chains). The numbers of particles ejected from sample PEh are summarized in Table 3. With that sample, the emission of intact C300H602 molecules does not occur, which clearly shows that increasing the molecular length makes desorption of intact molecules more difficult, even in the “horizontal” case. The result concerning entire molecules was somewhat expected, because longer chains have a larger binding
energy to the surface and their complete unzipping prior to desorption is more energy costly. Table 3 shows, in addition, that the number of fragments ejected from PEh is also lower than those calculated from the hexacontane solid with horizontal chains (HCh) except in the case of atomic Au impact. The sputtering results of Table 3 show that the yield calculated with horizontal hexacontane is quenched (i) for a given orientation, when the molecular size increases and (ii) when the molecular orientation with respect to the surface plane
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TABLE 3: Statistics of the Sputtered Species in the Target with Horizontal (HCh and PEh) and Vertical (HCv) Carbon Chains (5 keV Impacts)a projectile
C60
Au3
Au
sample
HCh
HCv
PEh
HCh
HCv
PEh
HCh
HCv
PEh
intact molecules intact molecules in larger clusters fragments ratio CH3/CH2 (%) atoms from the projectile total sputtered mass (kDa)
7 21 162 3.6 35 38.3
0 0 120 10.9 4 4.8
0 0 130
6 14 141 4.1 0 35.6
0 0 81 7.4 0 4.4
0 0 42
7 3 127 7.3 0 22.5
0 0 80 24.0 0 5.3
0 0 94
a
17 3.3
0 0.8
0 2.2
These values are an average over 5 (HCv), 5 (HCh), and 1 (PEh) trajectories.
Figure 3. Side views along the y (a) and x (b) directions of sample HCh, 20 ps after the impact of a 5 keV C60 projectile (impact point number 1). The white arrows indicate the crystallographic directions along which the sputtered material flux is aligned.
TABLE 4: Total Number of Radicals in the Simulation for 5 keV Impacts (These Values Are an Average over Five Trajectories) projectile
C60
Au3
Au
sample
HCh
HCv
HCh
HCv
HCh
HCv
number of radicals created in the solid number of radicals in the solid at 25 ps
241
235
254
263
225
257
56
112
100
172
108
144
and the beam incidence is modified. In parallel, recent calculations with Langmuir-Blodgett films of arachidic acid34 and with octatetraene crystals49 have shown that such smaller molecules could be desorbed intact even with an orientation that is closer to the surface normal (14 and 30-40°, respectively). It is therefore the conjunction of these two physical factors that
Figure 4. Cross-sectional view of the interaction between a 5 keV C60 and a polyethylene molecular sample (PEh: C300H602): (a) 0 ps; (b) 25 ps. The thickness of the cross section is 40 Å. The C atoms of the C60 projectile are white.
governs the yield, for the same bombardment conditions and, to some extent, even for different projectile types. The extremely large yields obtained with sample HCh, and the shape of the observed crater, can be understood considering the orientation of the molecules and the specifics of energy dissipation and pressure wave development in a layered material. These effects are investigated in the next section. 3.2. Dynamics of Energy Transfer in a Layered Material. Recent studies have demonstrated the importance of the first few hundreds of femtoseconds of the projectile-surface interaction for the determination of the sequence of events leading to crater formation and sputtering. In particular, the mesoscale energy deposition footprint (MEDF) model uses the distribution of energy in the surface of the sample at 90% projectile energy transfer as an input for a hydrodynamical model producing the volume of the sputtered material cone as an output.50,51 From
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TABLE 5: Energy Depositiona (keV) Calculated in the Topmost Layers of Samples HCh and HCv (30 Å of Thickness) and Penetration Deptha (Å) of the Projectile Center Mass (5 keV Impacts) (The Values after ( Indicate Standard Deviation) projectile
C60
Au3
Au
sample
HCh
HCv
HCh
HCv
HCh
HCv
energy in the surface at 90% energy transfer energy in the surface at 1 ps penetration depth at 90% energy transfer penetration depth at 400 fs
4.14 ( 0.06
4.20 ( 0.07
2.09 ( 0.06
2.16 ( 0.35
2.36 ( 0.42
2.22 ( 1.01
2.13 ( 0.07
2.69 ( 0.03
1.74 ( 0.08
2.27 ( 0.40
1.70 ( 0.16
1.75 ( 0.92
12.32 ( 1.15
14.70 ( 0.95
48.93 ( 3.20
59.17 ( 10.17
57.61 ( 9.40
67.15 ( 22.27
12.89 ( 4.73
24.30 ( 4.13
63.97 ( 0.90
70.89 ( 7.36
85.54 ( 8.91
94.62 ( 20.93
a
These values are an average over five trajectories.
this viewpoint, it appears that large sputtering yield differences such as those revealed upon bombardment of crystalline hexacontane layers might also relate to differences in the way energy is initially imparted to the surface atoms and the genesis of the interaction. To verify this hypothesis, the energy deposition by the projectile in the topmost layers of samples HCh and HCv and the penetration depth of the projectile center mass have been analyzed. For this analysis, we defined the surface region as the top 30 Å of the sample. Energy in the surface was computed as the total kinetic energy plus the total potential energy of the particles. Table 5 indicates the energy deposited in the surface layer at 90% projectile energy transfer and at 1 ps. In addition, we list the penetration depths of the projectile centers of mass at 90% energy transfer and at 400 fs (corresponding to the middle vignettes of Figure 2). In agreement with previous studies, our calculations indicate that the quantity of energy deposited in the surface between 0 and 1 ps depends on the projectile.20,49-52,60 At 90% energy transfer, it is about twice larger with fullerenes than with Au3 and Au. The differences of energy in the surface correlate well with the respective penetration depths of the different projectiles. However, the data in Table 5 also show that, at the same time, for a given projectile, the energy in the surface layer at 90% energy deposition is almost similar for both molecular arrangements (HCv and HCh) and the projectile penetration depths are only slightly larger with sample HCv. For sample HCv, the actual values of the penetration depths at 90% energy transfer indicate that the different projectiles deposit almost all of their energy inside the first molecular layer. Therefore, they tend to impart their energy in internal modes and break up the molecules, instead of transferring them the translational energy needed for emission. This effect is illustrated in Figure 5 with the positions of the particles that have moved vertically by more than 4 Å after 1 ps. White dots indicate particles moving downward, and red dots represent particles moving toward the vacuum. The volume of atoms which have moved toward the vacuum is significantly higher in the sample with horizontal hexacontane molecules than in the one with vertical molecules. The splitting plane between upward and downward motion is observed around 15-25 Å below the surface for sample HCh. Molecules above that plane receive mostly upward momentum, which allows them to be eventually ejected. In sample HCv, that up-down splitting plane cuts through the molecules, irrespective of the projectile. Even though Figure 5 shows slight differences related to the choice of projectile, the observed patterns and the numbers of upward moving atoms are mainly induced by the structure of the samples. Table 5 also provides information about the dynamics of energy dissipation in the different samples. After having deposited most of their energy in the surface, the projectiles continue to sink in sample HCv and they eventually bury deeper
than in sample HCh. The vertical orientation of the hexacontane molecules apparently helps the projectile to penetrate deeper into the target. In particular, the average depth of the fullerene center of mass at 400 fs is twice larger in HCv than HCh (see also Figure 2). However, after 90% energy deposition, the role of the projectile in the energy transfer and dissipation processes is minor. Despite the continued penetration of the projectiles in HCv, the quantity of energy remaining in the surface layer at 1 ps indicates that dissipation of the energy toward the bulk is faster in sample HCh. This mechanism of low-energy projectile atoms slowly “sinking” along the molecular chain direction should not be confused with the channeling of high energy atomic ions observed in metals and other crystalline materials. The energy dissipation mechanism and the generated pressure waves are strongly influenced by the orientation of the sample molecules and layers. In both targets, the energy dissipates faster in the direction perpendicular to the hexacontane molecules than in the parallel direction. In particular, quantitative measurements show that the pressure wave travels faster in the z direction in sample HCh and in the x and y directions in sample HCv. This effect can be visualized, for instance, in Figure 2. At 400 fs, the shape of the disturbed zone is quite deep and narrow in target HCh, while it is wider and more confined in the surface with sample HCv. For a better understanding of the effect of the layered structure on the pressure wave development and the energy dissipation in general, we studied the atomic displacements at the interface between two layers of sample HCh. In Figure 6, the average displacement of the CH3 end groups perpendicular to the interface, in the topmost layers of sample HCh, is plotted as a function of time (5 keV C60 bombardment). The chosen region from which the interface motion was averaged is illustrated in Figure 6b. As illustrated in Figure 6a, immediately after the pressure wave reaches the interface at ∼400 fs, the positions of the CH3 particles forming the two sides of the interface take opposite directions and the gap between the two hexacontane layers at the surface starts increasing with time. It is therefore the motion and partial reflection of the pressure wave at the interface that triggers the opening of wide interlayer breaches, accompanying the crater formation, in the system with horizontal carbon chains. Although the pressure waves induced by the different projectiles exhibit slightly different features, the effects are qualitatively similar and so are the final crater geometries. 3.3. Effect of the Projectile Energy. Because the size of the volume energized in the beginning of the interaction depends on the projectile energy,39,50 we expect to measure the effect of that parameter on the indicators of cratering and sputtering of hexacontane crystals. To investigate the behavior of our samples for higher projectile energies, a series of impacts at 15 and 25 keV were computed. Figure 7 shows the dependence of the
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Figure 5. Position of particles which have moved by more than 4 Å after 1 ps in the systems with (a) horizontal and (b) vertical carbon chains (5 keV impacts). Red and white dots represent species which have moved toward the vacuum and the solid, respectively.
sputtering yields (number of intact molecules, Figure 7a; number of fragments, Figure 7b; total sputtered mass, Figure 7c) on the projectile energy for 5, 15, and 25 keV C60, Au3, and Au impact at normal incidence. The results at 5 and 15 keV are averages over five trajectories with the impact points as defined in Figure 1. In contrast, only one trajectory was computed for each projectile at 25 keV. Therefore, the observation at 25 keV should be considered only as indicative of a trend, rather than a quantitative result. A cross-sectional view at 25 ps of the HCv sample bombarded by 25 keV C60 is displayed in Figure 8. In contrast with the 5 keV calculations, ejection of intact C60H122 molecules from sample HCv is observed with gold projectiles as the energy reaches 15 keV. At 25 keV, the ejection of intact molecules is also observed upon fullerene impact. Over the considered range of energies and projectiles, the number of intact molecules emitted from sample HCv remains much lower than that observed with sample HCh. Figure 7 shows that the total sputtered mass and the number of fragments sputtered from targets HCh and HCv also increase monotonically with the projectile energy. Figure 8 explains the mechanism by which
intact molecules can be ejected from sample HCv. Upon 25 keV C60 impacts, the energy carried by the pressure wave is such that decohesion occurs at the interface between the two molecular layers, locally lifting up the top layer up to the point that it breaks open and liberates intact molecules. This effect starts occurring at a lower projectile energy with Au and Au3 projectiles because they inherently deposit their energy deeper than C60, i.e., closer to the interface of the hexacontane layers. In spite of this delamination mechanism, the difference in terms of sputtered material observed between the two hexacontane crystals at 5 keV remains very important at high energy. 3.4. Comparison with Recent Studies and Experimental Perspectives. Our simulations indicate that the sputtering yield of hexacontane samples is largely influenced by the crystalline orientation. Upon 15 keV fullerene impact, the total sputtered mass increases from ∼20 to ∼160 kDa when going from the target with vertical hexacontane molecules to the one with horizontal molecules. The interaction between C60 and another type of crystalline organic sample, a Langmuir-Blodgett film of arachidic acid, AA (449 Da), was recently studied using
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Figure 6. (a) Average x-coordinates of the CH3 particles in the top two surface layers (0 and 4 Å under the surface plane) as a function of time for a 5 keV C60 impact. (b) Area over which the positions of the moving CH3 particles were averaged (red rectangle).
coarse-grained MD simulations. Under 15 keV C60 bombardment with an incidence angle of 0° (normal to the surface), i.e., almost aligned with the molecules such as sample HCv, the authors report a total sputtered mass of ∼72 kDa.34 The sputtered mass does not vary strongly when the incidence angle is modified from -25 to +45° (values in the range ∼70 f ∼84 kDa). These values are much larger than those obtained for HCv, which can be explained by the smaller size of the molecules. At 15 keV, C60 projectiles primarily deposit their energy in the top two molecular layers (2.6 nm thickness per layer) and molecules in the top layer should primarily receive upward momentum. Because of the small binding energy between layers, intact AA molecules from the first and even the second layer are easily desorbed. For the same reason, the influence of the incidence angle with respect to chain orientation is expected to be much less than that for hexacontane layers. With both arachidic acid and hexacontane layers, a strong dependence of the final projectile penetration depth on the angle between the incident beam and the molecule orientation is observed. As was reported in Table 5 for our system, the penetration depth increases by a factor of ∼2 when going from incidence perpendicular to the chains to incidence aligned with the chains. The main argument invoked in ref 34, the opening of the lattice along the chain direction, is also valid in our case. However, our results also demonstrate that this difference of penetration does not influence the quantity of energy deposited in the surface layer in our system. On the contrary, at 1 ps, less energy is transferred in depth with HCv than with HCh, which was explained by the observation that, after the first hundred femtoseconds of the interaction, the energy is transferred by the pressure wave and no longer by the projectile (section 3.2).
Figure 7. Energy dependence of the sputtering yield. (a) Number of intact molecules ejected from the solid. (b) Number of sputtered fragments. (c) Total sputtered mass. The data at 5 and 15 keV are averages over five trajectories, and the data at 25 keV are obtained from a single trajectory. The vertical error bars indicate the standard deviation in the case of Au3 projectiles.
In our system, that mechanism is more efficient perpendicular to the chains. The delamination process, observed in sample HCh with the formation of large interlayer breaches and in sample HCv at 15 and 25 keV with the separation of the two molecular layers (Figure 8), does not seem to occur in the LB layer system.34 This might be due to the larger interlayer binding energies of the LB system (0.14 eV for COOBa-COOBa interactions in the LB system against 0.0076 eV for CH3-CH3 interactions in our system), combined with the lower binding energy within the layer. This delamination process is largely responsible for the specific crater shapes observed in our simulations. In another study, we investigated the sputtering of amorphous molecular samples of polyethylene with molecular weights ranging from 0.3 up to 14 kDa.52,53 The calculated values of
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Figure 8. Cross-sectional view of the interaction of a 25 keV C60 projectile with sample HCv, leading to the ejection of intact C60H122 molecules at 25 ps (impact point number 1 - see Figure 1). The thickness of the cross section is 40 Å. The carbon atoms of the projectile are white.
Hamraoui and Delcorte for C60, 60-70 Å for Au3, and 85-95 Å for Au projectiles. These results are similar to those obtained with other lightelement materials such as molecular benzene,58,60,61 water ice,51 graphite,55 and fullerite.55 A direct experimental verification of our predictions with SIMS experiments is outside the scope of this article. Nevertheless, the phase diagrams and crystalline structures of alkane molecular samples,62 including hexacontane,35 are largely studied and ion beam analyses of such molecular crystals, with different orientations, have been reported in the literature (ion scattering on hexatriacontane, C36H74).63 Therefore, we are confident that our model can be experimentally tested. In terms of applications, the structural effect observed in our calculations might be important for the analysis of liquid-crystal-type materials and biological samples, including cells and tissues, where phospholipid bilayered membranes are a major constituent of the sample.23 Our simulations predict that, for such samples, the yield of sputtered molecules should strongly depend on the orientation and length of the molecular layers present at the surface. In addition, they confirm that the fraction of sputtered species including molecular end groups is also influenced by the molecular orientation and/or conformation. 4. Conclusion
the yields compared well with the experimental values obtained for polylactic acid and Irganox 1010 (1177 Da).54 For 1.4 kDa oligomers bombarded by 15 keV C60, a total sputtered mass of ∼80 kDa was calculated,53 which is close to the values obtained in ref 34 for crystalline samples of arachidic acid. Under 5 keV C60 bombardment, it could be shown that the sputtered mass was varying between ∼15 and ∼32 kDa depending on the chain length and the level of entanglement of the polymer.52 The largest yields (∼32 kDa) were obtained for the smallest molecules, PE oligomers with only 10 repeat units or icosane (C20H42). Because of the size of icosane, entanglement did not play any role in this sample. In the HCh system, the average sputtered mass upon 5 keV C60 impact is 38 kDa, i.e., larger than that calculated for amorphous icosane even though the chains are 3 times longer. As was indicated above, less material was also ejected from arachidic acid layers than from horizontal hexacontane despite the smaller molecular size of AA. In sample HCh, the reflection of energy at the interface between the layers and the easiness to unzip and desorb long, nonentangled, horizontal molecules obviously contribute to generate extremely high sputtering yields (Figure 3). In addition, the structure of the material critically influences the crater formation. For elemental targets (Ar,55 Ag,18 Au,55 Si,56,57 C55), amorphous organic materials52 and crystalline solids made of molecules such as benzene38,58,59 and fullerite,55 the crater geometry was consistently found to be hemispherical. In contrast, with arachidic acid34 and hexacontane layers, the crater geometry is generally not hemispherical. The craters formed in sample HCh upon atomic and polyatomic impacts have similar irregular shapes with breaches at the layer interfaces. The shape of the crater formed in sample HCv by C60 is a spherical cap, with a height that is much smaller than the radius, and that formed upon gold bombardment (Au3 and Au) is essentially cylindrical (track). The directionality of the sputtered flux evidenced in Figure 3b is reminiscent of similar effects observed with other crystalline materials.58,59 Concerning the penetration depth of different projectiles, our results involving C60, Au3, and Au are in broad agreement with the literature. Considering both molecular orientations, our calculations at 5 keV provide a penetration depth of 10-25 Å
The simulations of the interaction of atomic and cluster projectiles with hexacontane targets show that the sputtered mass (sputtering yield) increases dramatically when going from vertical to horizontal hexacontane molecules (between 3 and 8 times depending on the projectile). The directionality of the sputtered flux observed for the sample with horizontal chains, irrespective of the projectile nature, is reminiscent of structural effects also observed upon sputtering of inorganic monocrystalline targets. In essence, the mechanistic explanation of the yield enhancement is the following: For vertical HC molecules, the energized nanovolume is entirely included in the top molecular layer and the hydrocarbon chains in that volume undergo extensive bond breakings and important stresses. At 5 keV, no intact molecule can be sputtered, only small fragments. In the case of horizontal HC molecules, two factors are at play. First, the energy deposition by the projectile in the sample surface creates a region with intense upward momentum in the top 20-30 Å of the sample, which is able to lift up and eventually unzip large numbers of molecules, because they lie parallel to the surface. Second, the reflection of the pressure wave at the interface between two molecular layers tends to confine the energy in the impact region and to reinforce the desorption mechanism. Layer separation (delamination) is induced by the motion and partial reflection of the pressure wave at the interfaces in the HCh system, for all energies, and in the HCv system, beyond 15 keV. The numbers of fragmented and intact HC molecules and the total sputtered mass increase monotonically with increasing projectile kinetic energy (5 f 25 keV), but large differences of sputtering yields remain between sample HCh and sample HCv at all energies. The emission of intact molecules from the target with vertical hexacontane is initiated at 15 keV upon impact of gold projectiles (Au3 and Au) and at 25 keV under C60 bombardment. It is caused by the delamination and rupture of the top molecular layer. Acknowledgment. The authors wish to thank Prof. Barbara Garrison for fruitful discussions about these results and for the access to her simulation code and to the resources of her group during the summer of 2008. K.H. is grateful to UCL for financial
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