Langmuir 1986,2, 805-808 tail group entanglement along with head groups that interact weakly with the subphase as, for example, the long-chain aliphatic alcohols.’* The experimental results obtained with DMPS and with BBPS are in accord with the foregoing interpretations. DMPS forms a weak network on the surface and there is considerable tail entanglement. BBPS is more expanded; consequently, the contribution of tail group entanglement to the viscosity is diminished. The surface network of itself seems to be too weak to have a detectable affect. The only other compound that we have found to be as inviscid as BBPS is va1in0mycin.l~ Valinomycin is doughnut shaped. It does not have long chains which can intertwine nor does it form a network on the surface.
Conclusion Dimyristoylphosphatidylserine (DMPS) monolayers are more expanded at pH 4 than at pH 7. Further, the monolayers at pH 4 are considerably more viscoelastic. On a substrate buffered to pH 4,the viscosity of DMPS films increase from 0.01 (dyn s)/cm a t surface pressure of 10.8 dyn/cm to 594 (dyn s)/cm a t surface pressure 40.6 dyn/ cm. The very large viscosity at pH 4 is attributed to hydrogen bonding through the water between ionized and unionized serine moieties which results in an imperfect network on the surface. Bovine brain phosphatidylserine (BBPS) monolayers show no sensitivity to pH, in contrast to those of DMPS. Further, the viscosity is essentially constant a t 1 (mdyn (18) Abraham, B. M.; Miyano, K.; Xu, S. Q. Ketterson, J. B. Phys. Reu. Lett. 1982, 49, 1643; Phys. Rev. Lett. 1983, 51, 1975. (19) Karle, J. Science (Washington,D.C.)1986, 232, 837. (20) Abraham, B. M.; Ketterson, J. B. Langmuir, 1985, I , 461.
805
s)/cm for all surface densities. These observations can be partially explained by the large number of double bonds which expand the monolayer. The double bonds also reduce entanglement of tail groups, which represses the viscoelasticity. Karlelg has shown that saturating the double bonds in the cyclic decapeptide antamanide produces a subtle change in conformation but does not affect the ability of the resulting compound, perhydroantamanide, to encapsulate Li+. However, the biological activity is lost. In a similar manner, there may be subtle affects, as yet to be discovered, which give rise to the viscosity difference between DMPS and BBPS. The fact that some investigators believe that a monolayer at 50 dyn/cm surface pressure may be compared with a b i l a ~ e r ~whereas l - ~ ~ others believe that 12-13 dyn/cm is the proper comparison pressurez4 would not lead to drastically different concepts for function. However, the contrasting behavior of DMPS and BBPS to a shearing stress would most certainly lead to different concepts for the biological function.
Acknowledgment. We thank Professor Robert MacDonald of the Northwestern University Department of Biochemistry for supplying the first sample of BBPS and are especially grateful for his patience during many helpful discussions. Registry No. DMPS, 2954-45-2;L-serine, 56-45-1. (21) Phillips, M. C.; Williams, R. M.; Chapman, D. Chem. Phys. Lipids 1969. 3. 234. (22!’Hui, S. W.; Cowden, M.; Papahadjopoulos, D.; Parson, D. F. Biochzm. Biophys. Acta 1975, 382, 265. (23) Nagle, J. F. J. Membr. Biol. 1976, 27, 233. (24) Albrecht, 0.;Gruler, H.; Sackmann, E. J . Phys. (Les Ulis, Fr.) 1978, 39, 301.
Effects of Argon Ion Bombardment on the Basal Plane Surface of MoS2 Jeffrey R. Lince,* David J. Carre, and Paul D. Fleischauer Chemistry and Physics Laboratory, T h e Aerospace Corporation, Los Angeles, California 90009 Received J u n e 23, 1986. I n Final Form: August 12, 1986 The surface composition and structure of the basal plane surface of MoSz was modified by 10-keVargon ion bombardment and then studied with X-ray photoelectron spectroscopy. The rigid shift of both sulfur and molybdenum core levels is demonstrated to result from the formation of a small amount of Mo islands on the surface, although there is no a priori evidence for changes in oxidation state at the surface after bombardment. This conclusion is suggested by recent data on the deposition of metals on the MoS2 basal plane that indicate a strong correlation between the Mo or S binding energy and the electronegativity of the metal. Our results are compared with those of previous studies and are explained in terms of recent theories on the effects of surface ion bombardment that suggest that chemicallthermal effects dominate over substrate mass effects for higher projectile ion mass and energy.
Introduction The anisotropy of the MoSz crystal lattice is due to its layered, two-dimensional structure. Cleavage between layers produces a highly inert surface, while cutting along an edge plane causes the formation of dangling bonds that are considerably more reactive. Thus, the catalytic activity of MoSz has been ascribed mainly to adsorption on edge planes,lS2whereas the excellent lubricating properties of
* Author
t o whom correspondence should be addressed.
0743-7463/86/2402-0805$01.50/0
MoSz have been attributed to the basal plane ~ t a b i l i t y . ~ The effects of noble gas ion bombardment on the basal plane of MoSz single crystals have been studied for over 15 with the goal of determining whether defects (1) Matsunaga, M.; Homma, T.; Tanaka, A. ASLE Trans. 1982, 25, 323. (2) Suzuki, K.; Soma, M.; Onishi, T.; Tamaru, K. J . Electron Spectrosc. Relat. Phenom. 1981, 24, 283. (3) Fleischauer, P. D. ASLE Trans. 1983, 27, 82. (4) Williams, R. H.; Higginbotham, I. G.; Whitaker, M. A. B. J. Phys. C 1972, 5, L191.
0 1986 American Chemical Society
Lince e t al.
806 L a n g m u i r , VoE. 2, No. 6, 1986
M o 36 could be produced in the highly unreactive basal plane surface so that its catalytic activity might be i n ~ r e a s e d . ~ ~ ~ Such studies also have implications for improved solid lubricant performance, since ion bombardment (IB) might also be used to determine whether the reactivity of MoS, toward various metals could be increased with a corresponding increase in adhesion. Despite the numerous studies, there is still extensive controversy about the effect of IB on the MoSz basal plane surface. Feng and Chen used Auger electron spectroscopy (AES)to characterize the effect of 300-eV Ar+ ions on the basal plane ~ u r f a c e .After ~ IB, the ratio of the S to Mo Auger signal decreased by a factor of 5. They used this result, in addition to Auger peak shape analysis, to show that sulfur had been preferentially sputtered and that islands of molybdenum metal had formed on the surface. They attributed the increase in chemisorption after IB, observed by Williams et al.,4to the high reactivity of Mo metal. BINDING ENERGY lev) In contrast, Davis and Carver reported that bombardment with 1-keV Ne+ ions produced no detectable metallic s 2P Mo. Using X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy, they argued that the reduction in the S/Mo ratio was due mostly to the creation of sulfur surface vacancies, even for ion doses of 1 X 10ls cm-,. The only effect of ion bombardment on the XPS spectra was to broaden the Mo peak slightly. There were no apparent Mo or S peak shifts.6 Matsunaga et al. have investigated the vapor adsorption AFTER SPUTTERING and IB of MoS, edge and basal surfaces.’ Their AES peak ratios implied that the basal plane surface was initially “enriched” (as compared to the bulk) with sulfur. After IB with 5-keV Ar’ ions, they observed a drop in the AES S/Mo ratio (by a factor of 2), which they interpreted to BEFORE SPUTTERING SPUTTERING -ORE result from removal of an excess surface sulfur layer so that the new surface represented the bulk stoichiometry. 1 6 8 0 1 6 6 0 1640 1 6 2 0 1 6 0 0 1 5 8 0 Clearly, the above studies lack consensus on the effect BINDING ENERGY (eV) of IB on the basal plane surface of MoS,. This paper Figure 1. Examples of X-ray photoelectron spectra of the MoS, presents new ion bombardment results, which will be basal plane surface: (a) Mo 3d doublet before and after argon ion bombardment; (b) S 2p doublet before and after argon-ion compared with those of the previous studies and discussed bombardment. All peaks were referenced to carbon at 285.0 eV. in light of recent theories of IB effects and sputtering These spectra are representative of the many spectra taken in mechanisms. N
_-“I-
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+
this study.
Experimental Section Clean basal plane surfaces were produced by cleaving natural molybdenite crystals in a N2-filled glovebag and then transferring them to the XPS sample chamber without exposure to atmosphere. The sample chamber was pumped to a base pressure of 3 x IO4 torr by both turbomolecular pumping and cryopumping. The samples were analyzed either immediately or after being subjected to IB from a GCA/McPherson Model 36-K ion gun. Prior to IB, the highly unreactive basal plane surface remained clean a t this pressure. Even in preliminary experiments, when the sample was cleaved in air, little oxygen (
