1961
J. Phys. Chem. 1993,97, 1961-1969
Profile and In-Plane Structures of Self-Assembled Monolayers on Ge/Si Multilayer Substrates by High-Resolution X-ray Diffraction Employing X-ray Interferometry/Holography S. Xu,* R. F. Fischetti, and J. K. Blasie' Department of Chemistry and the Laboratory for Research on the Structure of Matter, University of Pennsylvania, and Biostructures Institute, University City Science Center, Philadelphia, Pennsylvania 19104
L. J. Peticolas and J. C. Bean AT & T Bell Laboratories, Murray Hill, New Jersey 07974
Received: September 10, 1992; In Final Form: December 10, 1992
X-ray interferometry has been used to btudy the profile structures of chemisorbed alkylsiloxane (hexadecyl-, octadecyl-, and eicosyl-) monolayer films on Ge/Si multilayer substrates of the type 2(Ge&o), Le., two superlattice unit cells, each consisting of 2 Ge monolayers and 30 Si monolayers, as fabricated by molecular beam epitaxy (MBE).Analysis was performed in three steps. First, based on the structural specifications of the cleaned, bare substrates acquired from the MBE fabrication, the actual profile structures of selected multilayer substrates were determined via a model refinement analysis of their meridional X-ray diffraction. The partial profile structures of these substrates (e+, GezSi3oGez) were then employed as the reference profile structure to perform a highly constrained, real-space refinement analysis to derive the profile structures of these multilayer substrates including the particular self-assembled monolayers on their surface, as relative electron density profiles from their meridional X-ray diffraction. Lastly, a real-space refinement was applied to convert the relative electron density profiles to their equivalent profile structures in absolute electron density. These profile structures demonstrate the alkyl chain configurations in these self-assembled monolayers are all-trans and tilted, with respect to the surface normal. The tilt angles are approximately 32 f 3O, 31 3O, and 30 3' for the 16C, 18C, and 20C chains, respectively. The terminal methyl endgroups of each monolayer seem to be well ordered in the profile structure, all lying in the same surface plane to within [qr]critr IF(qz)lspa 0 rapidly and monotonically, and thereby (F(q, > [qr]&(ats2 (F(qr)(bz.Therefore, our meridional X-ray scattering from these multilayer specimensI(q,) for (qz)ciit C ( & , i n I(4,) I (qr)max is dominated by the kinematical X-ray diffraction arising from their electron density contrast profiles
Ils(z).
For our composite inorganic multilayer+rganic overlayer system, the Ge/Si multilayer substrate has very sharp, large amplitude features in its electron density contrast profile, and the Ge/Si multilayer substrate profile structure is essentially known from its MBE fabrication specifications. The organic overlayers represent a small perturbation of the profile structure of the composite system, since their density contrast profiles are anticipated to possess much smaller amplitude features than those of the Ge/Si multilayer. Thus, we were able to use the known multilayer substrate profile structure as the reference structure to determine the profile structure of the organic overlayers by X-ray interferometry. Following the reference,2l the kinematical meridional X-ray diffraction for the composite multilayer structures, as shown in Figure lc,d is given by eq 3, where IF(qz)lkinz
is the total kinematical structure factor modulus squared, lFk(q# and ~ u ( q , ) lare z the kinematical structure factor moduli squared of the known multilayer substrate and unknown SAM overlayer profile structures, respectively. +k(q,) and +u(q,) are the phases of their respective structure factors (each referenced to the center of mass of their respective profile structure), and Akuis the distance along z between the center of mass of the Ge/Si multilayer and the SAM. The effects of the third term in eq 3, the critical interferencebetween the Ge/Si multilayer and the SAM structure factors, are readily apparent from the differencesbetween Figure 1a,c and between Figure 1b,d. The presence of minima in Figure lc,d, which do not exist in Figure la,b, respectively, is a clear indication of thedestructive interferenceeffects. The kinematical X-ray diffraction from these composite Ge/Si multilayer-SAM organic overlayer specimens ranged over only one order of magnitude for (qz)min I (qr) I (qJmaxrdue exclusively to the narrow width of the Ge features in the profile structure of these Ge/Si multilayer substrates fabricated by MBE. This is a very significant improvement in the resulting signal-to-noise over this range of 4,. in comparison with the X-ray specular reflectivity data from SAMs on uniform silicon wafer substrates.I6 Anrlysis For the analysis presented here, it was assumed that for qr 2 0.015 A, IF(qz)lkin2 for both the bare Ge/Si multilayer substrates
The Journal of Physical Chemistry, Vol. 97, No. 9, 1993 1963 and thecompositeGe/Si multilayer-SAM overlayer systems could be obtained from their Lorentz-corrected,meridionalelastic X-ray scattering I(q,) by substraction of (F(q,)(,pa2,as approximated by theLorentzcorrected scattering [I(q,)],,froma bare, uniform silicon wafer substrate.29 A Lorentz factor of q, was applied, as previously described, arising from the o-oscillation of the specimens.30 This procedure resulted in the kinematical X-ray diffraction from these specimens being confined to the q, range between (qz)mln 0.02 A-l and (qz)max 0.08 A-1. All Fourier analyses,both via interferometry and holography, were therefore restricted to this q, window. X-ray interferometric analysis was performed using a highly constrained, real-space refinement algorithm3’to implement the interferometric phasing of the IF(qz)lk,n2, rather than the pointby-point phasing in q, space, as described in reference.21 The advantage of this algorithm is that one can avoid several sources of error to which the q, point-by-point phasing is highly sensitive, especially counting statistics errors over regions of qr for which the structure factor for either the known or unknown profile structure is small, and errors in the relative scaling of the diffraction data sets employed. To implement interferometric phasing, we must first establish the relative electron density profile for the ‘‘knownn Ge/Si multilayer substrate. The initial models for the two unit cell Ge/Si multilayer substrates were developed on an absolute electron density scale, guided by our specifications for their fabrication (see Methods). Electron density levels and lattice constants for Ge and Si were calculated, based on relevant data in reference.32 The initial calculated values for Ge and Si in absolute electron density scale were 1.40 and 0.70 e-/A3, respectively, and the layer thicknesses for Ge and Si were 2.83 and 32.58 A, respectively. The initial models were then relaxed via a model refinement procedure, utilizing comparisons of the calculated meridionaldiffractionand its unique Fourier transform, the Patterson function, for the models with their corresponding experimentalmeridional diffraction and Patterson functions. The same values of (qz)&n and (qz)max truncation as described above for the experimental meridional diffraction data were applied to the model meridional diffraction functions, in order to properly match the corresponding experimental Patterson function, and thus produce (see below) a relative electron density profile for the Ge/Si multilayer substrates fully consistent with such truncation. By successively adjusting these absolute electron density models within their fabrication errors, we were able to refine to the models yielding the best agreement with the experimentalmeridional diffraction and corresponding Patterson functions,as shown in Figures 2a and 3a. Thegermanium features are evidently not as sharp as one would expect ideally from their fabrication specifications. It shows that such substrates are in fact a strained-layer superlattice, with substantial interlayer diffusion resulting in Ge/Si interface br~adening.~’Figures 2b and 3b are the corresponding relative electron density profiles calculated from Figures 2a and 3a, respectively,viadoubleFourier transformationsubjecttothesameq, trunctionas theexperimental diffraction data. Therefore, Figures 2a.b and 3a,b represent our initial knowledge of the profile structures for the two bare Ge/Si multilayer substrates that were later used to form C16 and C20 alkylsiloxane SAM’s on their surface. However, since these Ge/ Si multilayer substrates had been exposed toair and subsequently were subjected to several chemical processes during alkylation (see Methods), we anticipated that their surfaces may be subject to some further relaxation, and/or modification, the internal structure of these multilayer substrates presumably being the most stable. Therefore, instead of utilizing the entire relative electron density profile structures of the two unit cell Ge/Si multilayer substrates as our trial functions to initiate the constrained real-space refinement, we used just the interior portions of Figures 2b and 3b, containing only the twoGe-feature
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Xu et al.
1964 The Journal of Physical Chemisrry, Vol. 97, No. 9, 1993 0
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Figure 2. (a) Refined absolute electron density profile model for the bare, two unit cell Ge/Si multilayer substrate onto which a C16 alkylsiloxane S A M was subsequently chemisorbed; (b) relative electron density profile model of the bare substrate calculated from the profile model in (a) via double Fourier transform, subject to qr truncation; (c) experimental relativeelectrondensity profileof the bare substrate derived via X-ray interferometry; (d) experimental relative electron density profde of the C16 alkylsiloxane S A M on the two unit cell Ge/Si multilayer substrate derived via X-ray interferometry; (e) relative electron density profile model of the C16 alkylsiloxane S A M on the Ge/Si substrate, calculated from the profile model in (f) via double Fourier transform subject toq, truncation; ( f ) refined absoluteelectrondensity profilemodel for the C16 alkylsiloxane S A M on the two unit cell Ge/Si multilayer substrate.
2 (0 Figure 3. (a) Refined absolute electron density profile model for the bare, two unit cell Ge/Si multilayer substrate onto which a C20 alkylsiloxane S A M was subsequently chemisorbed; (b) relative electron density profile model of the bare substrate calculated from the profile model in (a) via double Fourier transform subject to the qr truncation; (c) experimental relative electron density profile of the bare substrate derivedvia X-ray interferometry; (d) experimental relativeelectron density profile of theC2O alkylsiloxaneSAM on the two unit cell Ge/Si multilayer substrate derived via X-ray interferometry; (e) relative electron density profile model of the C20 alkylsiloxane S A M on the Ge/Si substrate, calculated from the profile model in ( f ) via double Fourier transform subject to the qr truncation; (f) refined absolute electron density profile model for theC20alkylsiloxaneSAMonthe twounitcellGe/Simultilayer substrate.
peaks and intervening silicon feature on the left and omitting the 1 e f eature surface Si30 layer features including the derivative l'k on the right correspondingto the Si3o/He interface, Le., the profile structure of only the Ge2Si30Gezportion of the reference Ge/Si multilayer profile structure.31 Figures 2c and 3c contain the experimental relative electron density profile structures A&) for thecompositeGe/Si multilayer-C16andC20 SAM structures, respectively, so-derived from their meridional X-ray diffraction data. By inspection of the features in the neighborhood of z 0 A in thesc relative electron density profiles corresponding to the Silo/He interface for the bare Gt/Si multilayer substrates, namely in Figure 2 b 4 for the C 16 case and Figure 3 b 4 for the C20 case, we can see that the refined model profile structures for bare substrates agree very well with the actual experimental bare substrate profile structures, which indicates that their silicon surface remained essentially unmodified before the alkylation. But once these multilayer substrates are alkylated with the C16 or C20 SAM, additional, more complex features appear about z 0 A, indicating that some modification of the silicon surface occurred during the alkylation. To understand the nature of these new features and theother additional features in the organic overlayer region of these experimental relative electron density profiles, it is necessary to establish real-space, absolute electron density profile models that will account for all the individual features in Figures 2d and 3d. Figures 2f and 3f are the refined absolute electron density profile models for the C16 and C20 SAM cases, respectively. Their corresponding relative electron density profile models are
shown in Figures 2e and 3e, respectively. These relative electron density profile models were produced by frst Fourier transforming theabsoluteelectrondensityprofile model toget themodel profile's kinematical structure factor. We then applied the experimental values for the ni&( and (qJmaxtruncation, and performed an inverse Fourier transform of the model profile's kinematical structure factor to obtain the relative electron density model profile. This procedure was earlier referred to as a double Fourier transform, with the q, truncation window employed. We then refined the model profiles until their relative electron density model profiles were almost identical to the experimental relative electron density profiles in Figures 2d and 3d. In this real-space model refinement, we found that each feature in the absolute electron density profile model has a one-to-one correspondence with its counterpart in the relative electron density profile model, both in position (a0.1 A) and density level (a0.01 e-/A3). We thereby established that, in the relative electron density profile structures, the two peak features at z -74.0 A and z -37.0 A, arise from the two Gez layers; the features within the region -25 A Iz 5 +5 A uniquely arise from the two interfaces Si/SiO, andSiO,/CH2 (see Discussion),andthelast derivative-like feature at z 22.0 and 30.5 A of the two different SAM's, respectively, is due to a sharp interface between the terminal methyl endgroups and He, i.e., the -CH,/He interface. Hence, these refined absolute electron density profile models provide us with a rather precise knowledge of the profile structures of the composite Ge/ Si multilayer+rganic SAM overlayer systems.
(A)
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The Journal of Physical Chemistry, Vol. 97, No. 9, 1993 1965
Self-Assembled Monolayers
Discussion The highly constrained, real-space refinement algorithm provides one solution of a finite number of solutions for the phase of the kinematical structure factor, using the phase dominance of the known reference structure to force the box-refinement algorithm to converge to the local structure solution closest to the trial ~tructure.3~J5To prove the correctness of the so-derived, experimental relative electron density profiles described above, the method of X-ray holography was used in which the profile structure for the overlayer can be made to be contained explicitly in the Patterson function, P ( Z ) . The ~ ~ unique Fourier transform of the kinematical structure factor modulus squared, p(qr)lkln2, without phase information, yields the Patterson function, P(z), namely the autocorrelation function for the relative electron density profile structure A ~ ( Z ) . ~ That * - ~ ~P(z) is the autocorrelation of the relative, and not the absolute electron density profile (i.e., Ap(z) vs pPb(z)), is a simple consequence of the truncation of the meridional diffraction data for qz < (q&” mentioned in the Analysis section. Fourier transformation of eq 3 provides the half-Patterson function for the profile structure of the composite Ge/Si multilayerSAM overlayer system below:
+
P(zL0) = Pk(ZL0) P,(ZLO)
+
AP,(-zWhw”
- 4)(4)
where Ilt denotes the convolution operation. Pk is the autocorrelation function of the known multilayer substrate relative electron density profile A&) occurring about z = 0 A in P(z); P, is the autocorrelation function of unknown organic overlayer relative electron density profile Apu(z) occurring about z = 0 A in P(z);the third term in the sum is the cross-correlation function between the known and the unknown relative electron density profilesshiftedtoz = A&,,(setResults). Ifthemultilayersubstrate reference profile structure is chosen such that Apk(z) 6(z) (Dirac delta-function) and Ak, is larger than the longest z-translation vector in the autocorrelation of the known structure [Apr(z)12and in the auto-correlation of the unknown structure [hpu(z)]2,then the half-Patterson function for the profile structure of the composite system contains explicitly only the relative electron density profile of the unknown overlayer structure itself in the proximity of Ak,, i.e.
-
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P W A k , ) = AP,(Z)*W
= APUW
(5)
This approach is simply off-axis Fourier holography, in which the unknown structure itself is uniquely reconstructed in this particular region of the Patterson function, obtained via a unique Fourier transformation of the hologram IF(qz)lk1n2.” We can use this approach to prove that the relative electron density profile structures of the organic overlayers derived above via X-ray interferometry are indeed correct. Figure 4a,c,e shows the halfPatterson functions of bare, two-bilayer Ge/Si substrates, the two-bilayer Ge/Si substrate plus C16 alkylsiloxane SAM, and the two-bilayer Ge/Si substrate plus C20 alkylsiloxane SAM. Figure 4b,d,f shows their corresponding relative electron density profiles derived via X-ray interferometry. The Patterson functions are aligned with their corresponding relative electron density profile structures, such that the origin of the Patterson function is located at the same z-axis position as the Gez peak feature at the left edge of the corresponding relative electron density profile. By inspection of the features in the Patterson functions and in the corresponding relative electron density profiles over the region -5 A < z < 40 A, it is readily apparent that the Gez peak features at the left edge of the relative electron density profile (which approximates a delta-function) is convoluted with the organic overlayer features as the right edge of the profile in the region -5A 0 A-1) for the 1-bilayer films were difficult to detect due to the complications of weaker counting statistics, substrate scattering and detector nonuniformity. The existenceof this one reflection on the qxvaxis, essentially invariant to rotation about the substrate normal, and the absence of other reflections, weaker by at least an order of magnitude, over a wide range of qxv (for qz = 0 A-1) suggests that in-plane structures of these SAM's contain small, orientationally disordered domains which possess a lattice that is most likely hexagonal (or only very slightly distorted hexagonal) with nearest neighbor chain tilt (to be consistent with their profile structures) resulting in the six degenerate Bragg rods observed 14,J (4.17 A)-l to have a strong component on the %, axis as well as a component off the qxyaxis; while chain tilt angle (and not tilt direction) disorder would significantly weaken the off-q,, component of these Bragg rods at lqxylsi (4.17 &-I, the degenerate Bragg rods for this lattice andchain tiltdirectionoocumngat thenext higher(q,Jr 3IlY4.17 A)-' would have no componenton the qxyaxis and only components well off the qxr axis. This lattice would have a high in-plane density corresponding to 18-20 AZ/chain and an in-plane
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1968 The Journal of Physical Chemistry, Vol. 97, No. 9, I993
resolution limited. The first reflection on the qxyaxis (at qz = 0 A-1) most likely corresponds to the two superimposed f(02) Bragg rods and the second reflection off the qxv axis (at qr = 1 / 5 3 A) most likely corresponds to the four superimposed i(1l), i(11) Bragg rods for a distorted hexagonal lattice (using face-centered rectangular indexing) with b / a 2.08 anda nearest neighborchain tilt direction. The tilt-angle with this b / a distortion corresponds to approximately 36 i 5". The corresponding area/ chain for this distorted hexagonal lattice would be 17-19 A*.
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Figure8. (a) Countour plot of the in-plane/out-of-plane X-ray diffraction from a C16 alkylsiloxane SAM on the SiO, surface of a uniform silicon substrate,I(q,,q,); (b) Stripintegral over0 A- Iqr 50.0489A-I showing the reflection on the qxyaxis at qxy= (1/4.27A); (c) Stripintegral over
0.1998A-1 Iqxy5 0.2497A-l showing the qr depcndenceof the reflection centered a t qxy (1 /4.27A); this monotonically decaying qr dependence effectively vanishes for qr 1 0.06 A-1, the maxima at larger qr arising from the tail of the second reflection shown in (d,e); (d) Stripintegral over 0.1704A-l 5 qr I0.2198 A-1 showing the second reflection off the qxy axis a t qrV z (1/3.70A). (e) Stripintegral over 0.2497 A-l 5 qxV I0.2970 A-l showing the qr dependence of the second reflection at qxv r (1/3.70A) with the maxima occurring a t qr = (1/5.55 A).
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correlation length of -260 A, assuming an exponential decay of interchain in-plane correlations, or a domain size of 130 A, assuming well-ordered in-plane domains of finite size (Figure 7). Subsequently, we have been able to detect one reflection at IqxyI (1/4.27 A) and qz = 0 A-I and a second reflection at Iq,J z (1/3.70 A) and qr = (1/5.55 A) from analogous 16C and 2OC SAM's on the SiO, surface of silicon substrates (Figure 8). These studies also employed a doubly-focused synchrotron radiation source [with a cylindrically-bentNi mirror for vertical focus and a cylindrically-bent,ribbed Si( 111) crystal for horizontal focus] on beamline X9-A at the National Synchrotron Light Source and X-rayfilm as a simple 2dimensional detector with an excellent uniformity response, but poor dynamic range; the resulting Aqxv resolution was considerably inferior to that of Figure 7 with the widths of the reflections along qxy shown in Figure 8b,d being
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cooclusioas We have utilized X-ray interferometryto deriveunambiguously the relative electron density profiles of C16, C18, and C20 siloxylalkane self-assembled monolayers chemisorbed on the surface of two unit cell Ge/Si multilayer substrates. X-ray holography was used to prove the correctness of the so-derived SAM overlayer profile structures. We have found that the C16, C18, and C20 siloxylalkanechain configurationsin these relatively dense SAM's are all-trans and tilted away from substrate surface normal, with the average tilt angle of 32", 31°, and 30°, respectively, at room temperature (-20 "C). Each of these SAMs forms a smooth surface microscopically, with all the terminal methyl endgroups well ordered along the profile z axis, aligned in a single plane parallel to the substrate surface to within < f l A. These results from the profile structures of the C16 and C20 SAMs are in general agreement with their in-plane structures determined by high-resolutionX-ray diffraction,namely distorted hexagonal ( b / a 2.08) with a nearest neighbor chain tilt direction. Our results suggest that a chemisorbed SAM is probably better than a physisorbed Langmuir-Blodgett monolayer (LBM) for subsequent ultrathin LB multilayer deposition.39 We have detected the oxidation of silicon at the substrate surface to form a SO, layer without any prior assumption as to its existence or nature. In the case of the SAM's, this was induced during the prealkylation cleaning procedures. We have also found that the MBE-fabricated multilayer substrate structures employed experienced layer-to-layer interdiffusion, and thereby relaxation at the strained layer-layer interfaces. Acknowledgment. This work was supported by the National Science Foundation (NSF) Materials Research Laboratories (MRL) Project under Grant No. DMR9 1-20668and the National Institutes of Health (NIH) Grants GM-33525 and RROl1633. We would also like to thank Prof. John M. Murray (University of Pennsylvania) for use of his 2-D scanning microdensitometer. References and Notes (1) Bigelow, W. C.; Pickett, D. L.; Zisman, W. A. J. Colloid Interface
Sci. 1946,1 , 513.
(2) Zisman, W. A. Ado. Chem. Ser. 1964,43, 1. (3) Sagiv, J. J . Am. Chem. Soc. 1980,102,92. (4) Pomerantz, M.; Segmtiller,A.; Netzer, L.; Sagiv. J. ThinSolid Films 1985, 132,153. (5) Netzer, L.; Iscovic. R.; Sagiv, J. Thin Solid Films 1983,99,235. (6) Netzer, L.; Iscovic, R.; Sagiv, J. Thin Solid Films 1983, 100, 67. (7) Maoz, R.;Sagiv, J. J . Colloid Interface Sei. 1984, 100, 465. (8) Sabatani, E.;Maoz, R.; Sagiv, J.; Rubinstein, I. J . Electro. Anal. Chem. Interfacial Electrochem. 1987,219, 365. (9) Cohen, S.R.; Nassman, R.;Sagiv, J. Phys. Reo. Ltr.1987,58,1208. (IO) Strong, L. S.;Whitcsides, 0.M. hngmuir 1988, 4, 546. (11) Bain, C. D.; Troughton, E. B.; Tao, Y.T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J . Am. Chem. Soc. 1989,I l l , 321. (12) Nuzzo, R.G.;Dubois, L. H.; Allara, D. L. J. Am. Chrm. Soc. 1990, 112, 558. (1 3) Nuzzo, R. 0.; Korenic, E. M.; Dubois, L. H.J . Chem. Phys. 1990, 93, 767. (14) Li, T. T.;Weaver, M. J. J . Am. Chem. Soc. 1984,106,6107. (IS) Porter, M.D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E.D. J. Am. Chem. Soc. 1987,109,3359. (16) Tidswell, I. M.; Ocko, B. M.; Pershan. P. S.;Wasserman. S. R.; Whitesides, G.M.; Axe, J. D. Phys. Reo. 1990, B 41, 1111. (17) Wasserman, S. R.; Tao, Y.T.; Whitesides, G.M. Lungmuir 1989, 5, 1074. (18) Holmes-Farley, S.R.; Reamey. R. H.; McCarthy, T. J.; Deutch, T.; Whitesidcs, G.M. Lungmuir 1985, I, 725. (19) Chidsey. C. E.D.; Liu, G. Y.;Rowntree, P.; Scoles,G. J . Chrm. Phys. 1989,91,4421.
Self-hembled Monolayers (20) 1. K. J. (21) (22) 1969. (23) (24)
Bean, J. C.; Feldman, L. C.;Fiory, A. T.; Nakahara, S.; Robinson, Vac. Sci. Technol. 1984, A 2, 436. h l a u e r , W.;Blasie, J. K . Acra Crysra/logr. 1971, A 27, 456. Smith, H.M. Principles of Holography; John Wiley: New York,
Bean, J. C.; Sadowski, E. A. J . Yac. Sci. Techno/. 1982, 20, 137. Bean, J. C.; Sheng, T. T.;Feldman, L. C.; Fiory, A. T.;Lynch, R.
T. Appl. Phys. L r r r . 1984,44, 102. (25) Bean. J. C.; Bccker,G. E.; Petroff, P. M.;Scidel, T. E.J. Appl. Phys. 1971, 98, 901. (26) Cowley, J. M. Di/fracfion Physics, 2nd 4.; North-Holland: Amsterdam, 1981. (27) Skita, V.; Richardson, W.; Filipkowski, M.; Garito, A. F.; Blasie, J. K. J. Phys. France 1986,47, 1849. (28) Fischetti, R. F.; Filipkowski, M.; Garito, A. F.; Blasie, J. K. Phys. Rev. 1988, B 37, 4114. (29) Amador, S. M.; Pachcncc, J. M.; Fischetti. R.; McCauley, J. P. Jr.; Smith,A. B. 111; Dutton, P. L.; Blasie, J. K.Mar. Res. Soc.Symp. Proc. 1990, 177, 393. (30) Skita, V.; Filipkowski, M.; Garito, A. F.; Blasie, J. K. Phys. Rev. 1986, B 34, 5826.
The Journal of Physical Chemistry, Vol. 97, No. 9, 1993 1969 (31) Xu, S.;Murphy, M. A.; Amador, S. M.;Blasie, J. K. J. Phys. I France 1991, 1, 1131. (32) Weast, R. C. CRC Handbook of Chemistry and Physics. 65th 4.; CRC Press: Boca Raton, 1984-85. (33) Vandenberg, J. M.; Bean, J. C.; Hamm, R. A,; Hull, R. Appl. Phys. k t r . 1988, 52. 1152. (34) Stroud, R. M.; Agard, D. A. Biophys. J. 1979, 25, 495. (35) Makowski, L. Appl. Cryst. 1981, 14, 160. (36) AI-Bayati,A.H.;Omnan-Rossiter, K.G.;vanden Berg, J.A.;Armour, D. G. Sur/. Sci. 1991, 241, 91. (37) Ulman, A. Aninrroducrion to ulrrarhinorganicfilms from LangmuirBlodgett to Sey-Assembly; Academic Press Inc: San Dicgo, 1991. ( 3 8 ) Hautman, J.; Bareman. J. P.; Mar, W.; Klein, M. L. J. Chem. Soc.. Faraday Trans. 1991,87.2031. (39) Murphy, M. A.; Blasie, J. K.; Peticolas, L. J.; Bean, J. C. Langmuir, in press. (40) Fischetti, R. F.; Xu,S.;Blasie. J. K. Mater. Res. Soc. Proc. 1991, 208, 23 1.
