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Chemical Modification of Polypropylene and Polystyrene through Fluorocarbon Ion Beam Deposition Wen-Dung Hsu,† Christopher Fell, Sharon Pregler, and Susan B. Sinnott* Department of Materials Science and Engineering, UniVersity of Florida, GainesVille, Florida 32611-6400 ReceiVed: May 23, 2009; ReVised Manuscript ReceiVed: July 23, 2009
Classical molecular dynamics (MD) simulations are used to examine the deposition of polyatomic fluorocarbon (FC) beams on polypropylene (PP) and polystyrene (PS) surfaces. The goal is to investigate the ways in which different FC ions, in this case CF3+ and C3F5+, chemically modify the two different polymer surfaces. The simulations predict that the chemical reactions that occur upon impact are highly localized. As a result, with the same incident energy, CF3+ ions generate more PP chain fragments and facilitate more etching of the surface than C3F5+ ions. In contrast, C3F5+ ions promote more cross-linking between PP chains and the growth of FC films on the PP surface. In PS, there is more penetration of the ions than in PP as well as increased formation of CF2 particles, which indicates that deposition on PS yields fluorocarbon films more easily than deposition on PP. The simulations thus provide important insights into the complex mechanisms associated with the processes used to engineer polymer thin films in FC ion beams and plasmas and illustrate how differences in polymer structure ultimately influence such properties as sputtering, chemical modification, and thin film growth. Introduction Polypropylene (PP) and polystyrene (PS) are used in a variety of applications including packaging, containers, and laboratory equipment. They have high stiffness, comparable density, and good fatigue resistance relative to other polymers.1-3 Plasma treatment is widely used to chemically modify polymer surfaces and/or to deposit polymer thin films. For example, the growth of fluorocarbon (FC) thin films on polymer surfaces can yield unique physical and chemical characteristics that play essential roles in microelectronics, antifouling, and medical applications and, in particular, can engineer the chemical and thermal resistances, dielectric constants, and tribological properties of treated surfaces.4-7 While plasma treatment of polymer surfaces can tailor the surface properties for various applications, there is still much that is not known about the mechanisms associated with the process itself due to the complex environment within the plasma. Mass-selected ion beam deposition processes8-11 are therefore used to isolate the contributions of particular ions that are thought to be present in plasmas, and computer simulations of this process provide important insights into the chemical reactions that occur which are complementary to insights from experimental data. Here, classical molecular dynamics (MD) simulations12 are used to examine the continuous deposition of FC ions onto an R-isotactic polypropylene (PP) and a syndiotactic polystyrene surface (PS) at experimental fluences.9 The effect of a methyl group versus a phenyl group on the PP and PS backbones on deposition chemistry is compared. The two FC ions that are examined, C3F5+ and CF3+, are chosen because they are thought to exist in low-energy FC plasmas11,13 and were previously used to examine the chemical modification of polymer surfaces.14-17 * To whom correspondence should be addressed: e-mail
[email protected]. † Present address: Department of Materials Science and Engineering, National Cheng Kung University, No. 1 Ta-Hsueh Road, Tainan, Taiwan, ROC.
The deposition process is expected to modify the chemical and electrochemical properties of the polymer surface. The simulations elucidate the details of this modification by allowing us to predict the depth profiles of the incident atoms, the chemical products that are produced, the nature of the etched fragments, and additional details regarding the chemical modification of the surface. Computational Details The MD simulations numerically integrate Newton’s equations of motion with a third-order Nordsieck predictor-corrector algorithm.12 They predict the position, velocity, and acceleration of the atoms in the system as a function of time. Here, the shortrange interatomic interactions in the system are calculated by use of the second-generation reactive bond order (REBO) potentials,17,18 while the long-range interatomic interactions are calculated by use of a standard Lennard-Jones (LJ) potential.19 These potentials are connected smoothly to one another by spline interpolation.20 The REBO potentials utilized are capable of predicting new bond formation and bond breaking,21-24 which is crucial to accurately model the outcome of polyatomic ion beam deposition. However, the REBO potentials are presently unable to explicitly model true charging of the atoms or electronic excitations; therefore, ions with positive charges are treated as reactive radicals. The simulations thus assume that the cumulative effect of the deposition processes does not depend too greatly on the inclusion of explicit charging; to the degree that this is true, these empirical potentials provide qualitatively correct predictions and thus important insights into the modification of polypropylene in comparison to polystyrene results by mass-selected polyatomic atoms. The initial surface of PP used in the simulations can be seen in Figure 1a. It is composed of 12 layers that contain six chains per layer. Each chain contains nine monomers of the polypropylene repeat unit (-CH2CHCH3-) along the long side of the surface at 41.58 Å. The short side of the surface slab is composed
10.1021/jp904833w CCC: $40.75 2009 American Chemical Society Published on Web 09/17/2009
Fluorocarbons on Polypropylene and Polystyrene
J. Phys. Chem. C, Vol. 113, No. 41, 2009 17861 13 600 atoms. The initial surface of PS is shown in Figure 1b. It consists of eight layers of polystyrene for a total depth of 56 Å. The PS chains are aligned along the shorter, 30 Å, side of the slab and contain 12 repeat units of (-CH2CHC6H5-), with about 10 000 atoms total in the surface slab. For each system, periodic boundary conditions12 are applied within the surface plane to mimic an infinite surface. Each polymer chain ends at the boundary and then effectively wraps around on itself such that there are no surface slab edge effects. A thermostat is applied to approximately 50% of the atoms in the surface to maintain the system temperature at 300 K during deposition. The thermostat region consists of one and a half bottom layers of the surface and its edges such that the “active” area of the surface slab is about 1.32 × 27 Å2 for the PP surface and 32 × 20 Å2 for the PS surface. The active area of the slab can be seen in Figure 1a for PP and Figure 1b for PS. The atoms that are “active” evolve freely in response to forces from the neighboring atoms according to Newton’s equation of motion without any additional constraints. In contrast, the thermostat atoms have Langevin friction and stochastic forces25,26 applied to them to imitate the heat dissipation process of much larger surfaces that dissipate excess energy through thermal atomic fluctuations. The thermostat atoms prevent the surface from translating in response to polyatomic ion beam deposition and from heating up as a whole, although localized heating can occur, especially at impact sites. Prior to deposition, each polymer substrate is relaxed at 300 K for 20 ps, at which point the system potential energy fluctuates by 0.0033 eV/atom around a constant value as a function of time. The fluorocarbon beams, C3F5+ and CF3+, are individually deposited onto the active area of the PP and PS surfaces. The beam consists of 320 C3F5+ ions and 535 CF3+ ions such that the total F fluence is 1.8-2.5 × 1016 atoms/cm2; these values are comparable to experimental values that correspond to ion currents of about 80 nA.9 The total kinetic energy is 50 eV/ion and the incident angle is normal to the surface. Previous studies indicate that angled deposition lowers the extent of incident energy that is directed into the surface but does not lead to any new phenomena;27 for this reason, deposition at angles other than normal to the surface is not considered here. Both ions are continuously deposited onto the surface at randomly selected locations within the active areas and are randomly oriented relative to the surface. The time interval between ion collisions with the surface is around 1.5 ps, and after every five ions are deposited, the entire system is equilibrated for 12.5 ps; these times were arrived at through empirical means when they were found to be sufficient to maintain the surface temperature at around 300 K. After the ion beam deposition process is complete, the system is further equilibrated for 220 ps. The time step used in the simulations is 0.20 fs. Results and Discussion
Figure 1. Snapshots of the initial polymer surface displaying thermostat and active atoms before simulation: (a) polypropylene; (b) polystyrene.
of six chains and is 35.7 Å long. The depth normal to the surface is 55.08 Å, and the PP system as a whole contains approximately
Figure 2 shows snapshots of the PP and PS surfaces following the deposition of C3F5+ and CF3+ ion beams. The C3F5+ ion beam modifies the first six layers of the PP surface, while the CF3+ ion beam modifies approximately 25 Å from the surface. The ions penetrated the PS surface 10 Å deeper than PP. Figure 2 also indicates the original surface level with a solid line. On examining the figure it is clear that following deposition the PP surface swells due to the injection of C3F5+ ions, but the surface level decreases following the deposition of CF3+ ions as a result of extensive surface etching. The results thus suggest that the C3F5+ ion beam deposition facilitates the growth of a FC thin film but the CF3+ ion beam deposition modifies the PP
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Figure 2. Postirradiation snapshots of (a) C3F5+ on PP, (b) CF3+ on PP, (c) C3F5+ on PS, and (d) CF3+ on PS. The horizontal bar indicates the initial surface level.
substrate mainly by etching it. This swelling of polymer surfaces as a result of FC ion beam deposition was also predicted to occur in previous MD simulations.17 In particular, the deposition
Hsu et al. of the same FC ions onto PS resulted in swelling in the case of both C3F5+ and CF3+ (see Figure 2). Thus, the difference in results for PP and PS under comparable conditions suggests that the structure of the polymer chain and the side groups may control the FC-polymer interactions. In other words, determining whether a FC deposition process will produce greater uptake than etching or vice versa cannot be simply predicted by the size and incident kinetic energy of the incident ions. Rather, the outcome of the deposition depends on the details of the FC-polymer interaction. A quantitative analysis of the depth profile of the incident carbon and fluorine atoms is given in Figure 3. The shape of the depth profile indicates a broadly shaped curve in both cases. The curve peak occurs at a depth of 0.0-6 Å in PP and at about 15 Å in PS for the C3F5+ ion beam deposition case. For the CF3+ particle, the peak occurs at 6-12 Å in PP and at about 10-18 Å in PS. Figure 3 also indicates that the C3F5+ ion beam has higher deposition yield overall than the CF3+ ion beam. The C3F5+ particles penetrate slightly deeper in the PS substrate by about 40 Å, while the CF3+ ions penetrate about 20 Å deeper than in PP in Figure 3. The penetration depth between the two different ions follows the same trend for PS as with PP. Specifically, CF3+ travels slightly deeper than C3F5+ by about 20 Å. There are several factors that influence the degree to which the C3F5+ ion and its fragments modify a polymeric surface. For example, since both ions have the same incident kinetic energy on impact, the CF3+ ion possesses a higher incident velocity than C3F5+. In addition, the CF3+ ion is the smaller of the two and thus would be expected to propagate further into the polymer substrate. In particular, the particles penetrate into the PS surface by an additional 20-30 Å over PP. This is because the bulkier phenyl group in PS spaces out the backbone chains about 2 Å more than the methyl group in PP, which ultimately allows deeper penetration of ions in the PS. In Figure 3, the difference in ion depth in PP is about twice as much as in PS. One can thus conclude that, in general, PP does not easily allow larger particles to penetrate to as great a depth as PS. These results are consistent with previous findings for these two polyatomic FC ions deposited on PS14,16,17 and poly(vinylidene fluoride-trifluoroethylene) P(VDF-trFE) copolymer.14 Various chemical products are formed during deposition, as indicated in Figure 4. In this respect, there are many similarities in the results for the PP and PS substrates. Although PP and PS have almost the same chemical ratio of atoms, the extra carbon and hydrogen atoms in the bulky phenyl groups in PS are responsible for slight differences in deposition outcomes. The products formed as a result of C3F5+ ion beam deposition for both substrates include C3F5, CnFm (where n > 3 and m > 5), C2Fn, C3Fn (where n > 0 and n * 5), CF2, CF, and F. In PP, the first two species account for 78% of the products formed, where C3F5 accounts for 59% and CnFm accounts for 19%. This indicates that the majority of the products are the size of the original ion or slightly larger. In PS, 39% of the product formed is C3F5 and 3.9% is CnFm. For both polymer substrates, a substantial amount of CF2 is formed. In PP it accounts for 5% of the products formed in C3F5+ deposition, and in PS it accounts for 15.6%. This is significant because the CF2 radical is a major building block of FC films.28 Overall, about 28% of all products form chemical bonds to the PP surface and 26% of products bond to the PS surface on the time scales of the simulations. These results are qualitatively consistent with our previous findings for FC deposition on P(VDF-trFE).14
Fluorocarbons on Polypropylene and Polystyrene
Figure 3. Depth profiles of carbon and fluorine atoms after deposition of (a) C3F5+ on PP, (b) C3F5+ on PS, (c) CF3+ on PP, and (d) CF3+ on PS.
The products formed as a result of CF3+ ion beam deposition on both PP and PS substrates include CF3, F, CF2, C2Fn (where n > 0), CF4, CF, C3Fn (where n > 0 and n * 5), and F2. In PP, CF3 accounts for 37%, F accounts for 31%, and CF2 accounts for 12% of the products. As was the case for C3F5+ ion beam deposition on PP, the most abundant chemical product is the incident ion itself; however, the total fraction is much smaller than was the case for C3F5+. This result indicates that the CF3+ ions have a higher probability of dissociating on impact. The
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Figure 4. Molecular weight distributions of chemical products formed after deposition of (a) C3F5+ on PP, (b) C3F5+ on PS, (c) CF3+ on PP, and (d) CF3+ on PS.
second and third most abundant products are F and CF2, respectively, and 96% of them covalently bond with the PP surface. In PS, mostly F is formed in the CF3+ deposition which takes 49%, 6% of which attached themselves to the PS, either replacing a H or capping a chain. The next most dominant species are CF2 and CF3, which account for 24% and 23% of the products, respectively. Here, the two species have roughly the same occurrence, but the ratio is different in PP, which has
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TABLE 1: Scattering Yields for CF3+ Deposition on PP and PS CF3+ on PP CF4 CF3+ CF2 CF F2 F CnFm
3.0 44.8 22.4 3.7 3.7 20.9
CF3+ on PS 33.0 23.5 2.0 2.0 10.8 4.8
TABLE 2: Scattering Yields for C3F5+ Deposition on PP and PS C3F5+ C 3 F4 C 3 F3 C 2 Fn CF2 CF F
C3F5+ on PP
C3F5+ on PS
36.1 7.2 7.2 18.1 19.3 4.8 7.2
32.5 3.8 22.1 20.8 5.8 8.8
more CF3 particles than CF2. Since PP and PS are both composed of initially saturated polymer chains, the results suggest that F and H exchange is energetically favorable when the incident CF3+ ion dissociates on impact. Thus the total percentage of chemical products that bond to the surface is 65%, a much higher overall percentage than is the case for C3F5+ ion beam deposition. Tables 1 and 2 show the yields for CF3+ and C3F5+ deposition on PP and PS. In the case of CF3+ deposition, CF3, CF2, and F are the major products that are scattered away for both the PP and PS surfaces. There is about 25% more CF3 scattering and 50% more F scattering in PP than in PS. In the case of C3F5+ deposition, C3F5, C2F3, and CF2 are the major products that are scattered. Comparison of the results for PP and PS also follows a similar trend with generally more particles scattered in PP than in PS for both CF3+ and C3F5+ deposition cases. These tables illustrate that the incident particle, CF3+ or C3F5+, does not easily dissociate; however, when it does, CF2 and F are the most common products. Chemical products that do not bond with the incident ions or their fragments are also etched from the PP and PS surfaces. In the case of C3F5+ ion beam deposition, the most common products formed are H, CH3, and CH2. There are also a few large products that are composed of more than three carbon atoms that scatter away from the surface, but most products are small. This is supported by Table 2, which indicates that C3F5+ does not easily dissociate when deposited on PP or PS. In the case of CF3+ ion beam deposition, the majority of the chemical products are H, CH3, and CnHm (where n > 3 and m > 5). The significant fraction of larger chemical products that are etched away from the surfaces indicates that CF3+ is more efficient at fragmenting the PP and PS polymer chains and facilitating etching. The differences between the interactions of the C3F5+ and CF3+ ions on these polymer surfaces can be attributed to the differences in their size and incident velocity. Since the impactinduced chemical reactions occur in a localized region of the surface, the kinetic energy transferred to the surface during the impact of CF3+ ions is higher than in the case of the C3F5+ ions even though the overall incident energies are the same in both cases. Because the C3F5+ ions are larger, they have more degrees of freedom to vibrate, rotate, and twist, all of which can ultimately dissipate energy and ultimately reduce the amount
Figure 5. Degree of etching after deposition of (a) C3F5+ on PP, (b) C3F5+ on PS, (c) CF3+ on PP, and (d) CF3+ on PS.
of energy transferred to the surface. In contrast, CF3+ ions are smaller and are more efficient at chemically modifying the PP. The extent of PP and PS surface etching is indicated in Figure 5. In Figure 5a, the amount of etching increases slowly throughout the deposition until late in the process, when it increases sharply and flattens out for the PP. This increase is due to the fact that the C3F5+ ions are not efficient at breaking bonds within the PP, but when enough of them have impacted the surface, relatively large fragments are etched away. In Figure 5b, the ratios between etched C and H atoms are nearly identical.
Fluorocarbons on Polypropylene and Polystyrene The curve plateaus at 30% less fluence than the PP case. The amount of C etched in PS from C3F5+ deposition is less than in PP, so PS loses less material during radiation. The CF3+ ion beam has etched a much higher density of atoms from the surface than the C3F5+ ion beam for the PP, as shown in Figure 5c. In Figure 5d, for PS, the CF3+ follows a similar pattern to the etching profile in PP. The etching concentration is greater in PP under CF3+ with the carbon etching significantly higher than the PS, similar to the C3F5+ ion. This ratio fluctuates dramatically in the initial stages of deposition and converges to a steady value as the deposition proceeds. It converges to around 3 in the case of C3F5+ ion beam deposition and to around 2 in the case of CF3+ ion beam deposition. The ratio of C and H for both ion particles on PS is nearly identical. It should be noted that the H-to-C ratio in an intact PP chain is 2, which indicates that the hydrogen and carbon atoms are equally likely to be etched. It is also indicative of the fact that the ions are breaking the PP chains. In contrast, the higher H/C ratio in the case of C3F5+ ion beam deposition on PP indicates that hydrogen atoms have a higher probability of being etched away than carbon atoms. It also indicates that the C3F5+ ions are only able to break the C-H bonds on the PP backbones or the backbone-CH3 bonds on impact. In both C3F5+ and CF3+ ion depositions on PS, the C-to-H ratio is nearly identical, and considerably less mass is lost than in deposition on PP. This implies that deposition on PS results in the growth of thin films more readily than does deposition on PP. The majority of the etched particles are C3F5 and the second most populous etched products are C2Fn and CF2 in the case of C3F5+ ion beam deposition. Combining this information with the chemical products that remain embedded in the polymer (illustrated in Figure 4) indicates that C3F5 is the most common chemical product formed during the deposition of this polyatomic FC ion. In other words, the C3F5+ ion does not easily dissociate; however, when it does dissociate, CF2 and C2F3 are the most common chemical products. In the case of CF3+ ion beam deposition, the most common etched product is also CF3 and the second most common etch particles are CF2 and F. Combined with the results shown in Figure 4, it is clear that CF3+ ions are most likely to dissociate to CF2 and F when they impact the surface. Similar FC decomposition mechanisms were predicted previously for these same FC ions on PS and P(VDF-trFE).14,16,17 The efficiency of the dissociation predicted to occur for PP is, however, different from the other polymer surfaces considered. Specifically, the deposition of CF3+ ions on PP and P(VDF-trFE) had higher dissociation probabilities than on PS. As discussed earlier, the PS polymer is more open due to the presence of the phenyl ring, which allows more incoming particles to penetrate the surface and ultimately become trapped. In contrast, both PP and P(VDF-trFE) have fewer open spaces between their backbones, so the probability of particles impacting and etching the polymer is greater than in PS. The etching data indicate that the C3F5+ ion beam is steadily adding FC character to the PP surface or, in other words, facilitates the growth of FC thin films. This process is expected to increase the wear resistance of the PP relative to the untreated surface. In contrast, in the case of CF3+ ion beam deposition, the PP is steadily etched and accommodates FC fragments that penetrate into the surface. The deposition of this ion might thus be used to clean the PP surface with minimal chemical modification of the surface structure relative to the untreated surface. Conclusions The deposition of a fluorocarbon ion beam onto R-isotactic polypropylene and syndiotactic polystyrene surfaces is consid-
J. Phys. Chem. C, Vol. 113, No. 41, 2009 17865 ered here. In particular, the relative effects of C3F5+ and CF3+ ions are examined in classical molecular dynamics simulations. PS allows the incident ions to penetrate the surface more than is the case for PP, due to a slightly wider spacing between the backbone chains. The C3F5+ ion beam is predicted to facilitate the growth of a fluorocarbon thin film on the PP and PS surfaces, while the CF3+ ion beam is predicted to primarily etch each surface. There are some differences in the details of the resulting chemistries on the PS and PP surfaces, but the overall behavior for both polymers is similar. In the case of C3F5+ deposition, there are considerably more F and CF2 products formed than was the case for CF3+ deposition, and more of these products form during deposition on PS than on PP. These species are crucial in film growth, so this analysis implies that fluorocarbon films should form more easily on PS than on PP. These findings may be used to enable engineering of the surface properties of the polymer surface for particular applications where thin film deposition or etching is required. Acknowledgment. We gratefully acknowledge the support of the National Science Foundation through Grants CHE0200838 and CHE-0809376. References and Notes (1) Paukkeri, R.; Lehtinen, A. Polymer 1993, 34, 4075. (2) Paukkeri, R.; Vaananen, T.; Lehtinen, A. Polymer 1993, 34, 2488. (3) Randall, J. C. Macromolecules 1997, 30, 803. (4) Tanaka, K.; Inomata, T.; Kogoma, M. Thin Solid Films 2001, 386, 217. (5) Takahashi, K.; Itoh, A.; Nakamura, T.; Tachibana, K. Thin Solid Films 2000, 374, 303. (6) da Costa, M.; Freire, F. L.; Jacobsohn, L. G.; Franceschini, D.; Mariotto, G.; Baumvol, I. R. J. Diamond Relat. Mater. 2001, 10, 910. (7) Wang, J. H.; Chen, J. J.; Timmons, R. B. Chem. Mater. 1996, 8, 2212. (8) Akin, F. A.; Jang, I.; Schlossman, M. L.; Sinnott, S. B.; Zajac, G.; Fuoco, E. R.; Wijesundara, M. B. J.; Li, M.; Tikhonov, A.; Pingali, S. V.; Wroble, A. T.; Hanley, L. J. Phys. Chem. B 2004, 108, 9656. (9) Wijesundara, M. B. J.; Ji, Y.; Ni, B.; Sinnott, S. B.; Hanley, L. J. Appl. Phys. 2000, 88, 5004. (10) Wijesundara, M. B. J.; Zajac, G.; Fuoco, E.; Hanley, L. J. Adhes. Sci. Technol. 2001, 15, 599. (11) Yanai, K.; Karahashi, K.; Ishikawa, K.; Nakamura, M. J. Appl. Phys. 2005, 97, 053302. (12) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Oxford University Press: New York, 1986. (13) Gaboriau, F.; Cartry, G.; Peignon, M. C.; Cardinaud, C. J. Phys. D: Appl. Phys. 2006, 39, 1830. (14) Hsu, W. D.; Jang, I.; Sinnott, S. B. J. Vac. Sci. Technol., A 2007, 25, 1084. (15) Hsu, W. D.; Jang, I.; Sinnott, S. B. Chem. Mater. 2006, 18, 914. (16) Jang, I.; Sinnott, S. B. Appl. Phys. Lett. 2004, 84, 5118. (17) Jang, I. K.; Sinnott, S. B. J. Phys. Chem. B 2004, 108, 18993. (18) Brenner, D. W.; Shenderova, O. A.; Harrison, J. A.; Stuart, S. J.; Ni, B.; Sinnott, S. B. J. Phys.: Condens. Matter 2002, 14, 783. (19) Frankland, S. J. V.; Brenner, D. W. Chem. Phys. Lett. 2001, 334, 18. (20) Sinnott, S. B.; Shenderova, O. A.; White, C. T.; Brenner, D. W. Carbon 1998, 36, 1. (21) Neyts, E.; Bogaerts, A.; Gijbels, R.; Benedikt, J.; van de Sanden, M. C. M. Diamond Relat. Mater. 2004, 13, 1873. (22) Neyts, E.; Bogaerts, A.; Gijbels, R.; Benedikta, J.; van de Sanden, M. C. M. Nucl. Instrum. Methods Phys. Res., Sect. B 2005, 228, 315. (23) Ni, B.; Andrews, R.; Jacques, D.; Qian, D.; Wijesundara, M. B. J.; Choi, Y. S.; Hanley, L.; Sinnott, S. B. J. Phys. Chem. B 2001, 105, 12719. (24) Ni, B.; Sinnott, S. B. Phys. ReV. B 2000, 61, R16343. (25) Pastorino, C.; Kreer, T.; Muller, M.; Binder, K. Phys. ReV. E 2007, 76. (26) Tobias, D. J.; Martyna, G. J.; Klein, M. L. J. Phys. Chem. 1993, 97, 12959. (27) Jang, I.; Ni, B.; Sinnott, S. B. J. Vac. Sci. Technol., A 2002, 20, 564. (28) Abrams, C. F.; Graves, D. B. J. Appl. Phys. 1999, 86, 5938.
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