Evidence of Spontaneous Multilayer Formation for Disperse Red-1 at

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Evidence of Spontaneous Multilayer Formation for Disperse Red-1 at a Fused-Silica/2-Propanol Interface Jessica A. Ekhoff, Sarah G. Westerbuhr, and Kathy L. Rowlen* Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309 Received June 6, 2001. In Final Form: August 10, 2001 The film structure of a nonlinear optical azo dye (Disperse Red-1, DR-1) was probed by a combination of spectroscopic and microscopic techniques. Second harmonic generation (SHG) and linear dichroism provided evidence for the spontaneous formation of DR-1 multilayers at a 2-propanol/fused-silica interface under equilibrium conditions. Specifically, the adsorption isotherm measured by SHG exhibited two distinct adsorption regimes; the second regime (>10-3 M) being indicative of multilayer formation in which some degree of polar order was retained. The trend in apparent tilt angle measured by linear dichroism was consistent with multilayer formation at solution-phase concentrations above 10-3 M. Multilayer formation was directly observed, on a dip-coated substrate, by atomic force microscopy and further confirmed by ellipsometry. The significance of spontaneous multilayer formation with z-axis polar order in higher layers is the potential for growth of molecular multilayer systems without the need for covalent or sequential deposition.

Introduction Organic surface films are becoming integral elements in many nonlinear optical applications. Poled polymer films containing organic dye molecules have been utilized as optical waveguides, ultra-high-speed optical switches, and waveguide gratings.1,2 Surface films have also been used as biosensors.3 Azo, stilbene, cyanine, and merocyanine dyes are of interest because many possess the large molecular hyperpolarizabilities (β) requisite for nonlinear processes. In particular, azo dyes are relatively inexpensive and readily available, making them attractive for large-scale commercial applications. Azo dyes have been used as liquid crystals, in optical polymers, and in thin films.2 One of the most widely studied azo dyes is Disperse Red-1 (DR-1) due to its large hyperpolarizability. The physical and optical properties of organic thin films depend on molecular organization and structure within the film.3 Molecular orientation, angular distribution in the orientation, and polar order are key parameters with respect to thin film optical properties.4-8 Second harmonic generation (SHG) is a commonly used technique to probe polar order and molecular orientation.9,10 SHG is inherently surface and interface selective and can be used to study solid-liquid, solid-air, air-liquid, and liquidliquid interfaces. With SHG, it is possible to obtain submonolayer, monolayer, and even multilayer structural information for oriented systems in which noncentrosym* To whom correspondence should be addressed. E-mail: rowlen@ spot.colorado.edu. (1) Kaino, T. J. Opt. A: Pure Appl. Opt. 2000, 2, R1-R7. (2) Delaire, J. A.; Nakatani, K. Chem. Rev. 2000, 100, 1817-1845. (3) Ulman, A. An Introduction to Ultrathin Organic Films From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, 1991. (4) Cnossen, G.; Drabe, K. E.; Wiersma, D. A. Langmuir 1993, 9, 1974-1977. (5) Kikteva, T.; Star, D.; Zhao, Z.; Baisley, T. L.; Leach, G. W. J. Phys. Chem. B 1999, 103, 1124-1133. (6) Pinnow, M.; Marowsky, G.; Sieverdes, F.; Mobius, D. Thin Solid Films 1992, 210/211, 231-233. (7) Buscher, C. T.; McBranch, D.; Li, D. J. Am. Chem. Soc. 1996, 118, 2950-2953. (8) Ishibashi, K.-i.; Sato, O.; Baba, R.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Colloid Interface Sci. 2001, 233, 361-363. (9) Simpson, G. J. Appl. Spectrosc. 2001, 55, 16A-32A. (10) Corn, R. M.; Higgins, D. A. Chem. Rev. 1994, 94, 107-125.

metry is retained.10-16 However, SHG alone yields only an apparent mean orientation angle.9 Additional spectroscopic techniques are required in order to determine the angular distribution. In our laboratory, the utility of SHG in combination with linear dichroism, specifically angle-resolved absorbance with photoacoustic detection (ARAPD), has been demonstrated for organic thin films.17 In this work, the coverage dependence of DR-1 physisorbed onto fused-silica was examined using a combination of SHG, ARAPD, ellipsometry, and atomic force microscopy (AFM). Each technique provided unique information regarding film structure. Taken as a whole, the evidence indicated island structure with spontaneous multilayer formation at solution concentrations exceeding 10-3 M. The multilayer structure retains some degree of polar order, with the apparent orientation angle measured by SHG shifting from 44 to 49° as coverage increases. Experimental Section The following is a summary of the instrumentation used in SHG and ARAPD experiments; a detailed description has been previously published.17 SHG experiments were conducted using the fundamental beam (1064 nm) of a Q-switched Nd:YAG laser (Spectra Physics, GCR-11). A total internal reflection (TIR) flow cell geometry was utilized. The flow cell consisted of a 50 µm Teflon spacer (Dupont FEP, 200CLZ) sandwiched between a fused-silica prism (Esco, S1-UV) and a fused-silica microscope slide (Esco, UV grade). The slide was equipped with inlet and outlet ports to allow for flow of the dye solution through the cell. The excitation beam was passed through an aperture, a Glan laser prism, a stepper motor controlled half-wave plate, a focusing lens, and a visible blocking filter before entering the TIR cell. After the TIR cell, the second harmonic (SH) was directed through (11) Hanken, D. G.; Naujok, R. R.; Gray, J. M.; Corn, R. M. Anal. Chem. 1997, 69, 240-248. (12) Schoondorp, M. A.; Schouten, A. J.; Hulshof, J. B. E.; Feringa, B. L. Langmuir 1993, 9, 1323-1329. (13) Wijekoon, W. M. K. P.; Asgharian, B.; Prasad, P. N. Thin Solid Films 1992, 208, 137-144. (14) Ashwell, G. J.; Jefferies, G.; Ranjan, R. Electron. Lett. 1996, 32, 59-61. (15) Ashwell, G. J. J. Mater. Chem. 1999, 9, 1991-2003. (16) Flory, W. C.; Mehrens, S. M.; Blanchard, G. J. J. Am. Chem. Soc. 2000, 122, 7976-7985. (17) Simpson, G. J.; Westerbuhr, S. G.; Rowlen, K. L. Anal. Chem. 2000, 72, 887-898.

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an IR blocking filter, a sheet polarizer, a 532 nm interference filter and finally to a photomultiplier tube (PMT, Hamamatsu R268) for detection. The PMT response was digitized on an oscilloscope (HP 54602B), and the averaged signal (64 pulses, 2 Hz) was transferred to a personal computer. The signal was recorded as a function of incident beam polarization for both the s-polarized and the p-polarized SH. ARAPD experiments were conducted using the SH (532 nm) of the Q-switched Nd:YAG laser. The same TIR cell was utilized, with the addition of an acoustically coupled piezoelectric transducer (PZT). Prior to the TIR cell, the 532 nm beam was passed through an aperture, a Glan laser prism, and a stepper motor controlled double Fresnel rhomb. The PZT was slightly offset from the excitation spot in order to temporally filter out acoustic background signals due to scattered light. The PZT signal was amplified (Panametrics, ultrasonic preamp, 60db) and digitized on an oscilloscope. The averaged signal (64 pulses, 2 Hz) was transferred to a personal computer. As with the SHG experiments, the signal was recorded as a function of polarization of the incident beam. DR-1 (Aldrich, ∼95%) solutions were prepared in 2-propanol (Fisher, optima grade). The fused-silica prism and Teflon spacer were cleaned with sequential sonication in acetone (Fisher, reagent grade), chloroform (Fisher, reagent grade), and methanol (Burdick & Jackson, high purity). The fused-silica microscope slide, syringes, and tubing were sequentially rinsed with the same solvents. The dye solutions were introduced to the flow cell using a syringe pump set at a flow rate of 0.16 mL/min. The solutions were allowed to equilibrate for 10 min prior to each measurement, and flow was kept constant throughout the entire experiment. The flow cell was rinsed with high-purity methanol between each measurement to reestablish a baseline signal. All experiments were conducted at room temperature (typically ∼22 ( 0.5 °C). AFM measurements were conducted on a Digital Instruments Nanoscope III multimode AFM. Small pieces (∼1 cm × 1 cm) of a fused-silica microscope slide (Esco) were solvent cleaned by the method described above. Surface films were prepared by immersing the fused-silica pieces into the dye solution for approximately 15 min with stirring. The pieces were then removed from the solution slowly to allow for uniform solvent evaporation at the solvent/air interface. The surfaces were imaged in air with tapping-mode AFM using etched silicon probes (TESP, Digital Instruments). Images were obtained using a scan rate of 1 Hz, a square scan size of 10 µm, and proportional and integral gains of 0.3 and 0.5, respectively. Ellipsometry measurements were made using an AutoEL ellipsometer (Rudolph Research). Surface films were prepared using the procedure described above, with silicon wafer pieces as the substrate. An index of refraction of 1.5 was assumed for the organic dye layer.3

Results and Discussion The UV-visible absorption spectra of DR-1 in 2-propanol and on a fused-silica surface are shown in Figure 1. There were no significant changes in the absorbance spectrum, which might indicate H or J aggregation,5 with increasing dip-coat solution. The linear dichroism experiments utilized 532 nm excitation to probe the π-π* transition centered at 475 nm.2 The measured extinction coefficient of DR-1 in 2-propanol at 532 nm is 17 000 L/(mol-cm). Semiempirical calculations (ZINDO/S with configuration interaction) indicate that the transition dipole moment vector is 3.5° from the molecular long axis. The change in permanent dipole is also along the long axis of the molecule. As is common with many rodlike molecules, there is a single dominant molecular hyperpolarizability tensor element, βzzz, which is directed along the long axis.17 The subscripts, zzz, refer to the molecular coordinate system. For the trans isomer of DR-1, βzzz is ∼45 × 1030 esu.2 The cis form of DR-1 is highly unstable and rapidly undergoes thermal isomerization to the trans configuration.2

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Figure 1. UV-visible absorbance spectra and molecular structure of DR-1. The solid line is DR-1 in 2-propanol (λmax ) 475 nm), and the dashed line is 8 × 10-4 M DR-1 dip-coated from 2-propanol onto fused-silica (λmax ) 483 nm). The dipcoated spectrum has been multiplied by 15 for comparative purposes.

Second Harmonic Generation. SHG is the nonlinear conversion of 2 photons of frequency (ω) to a single photon of frequency (2ω), which occurs when there is a break in symmetry between two randomly oriented media. The SH intensity (I2ω) generated at an interface is directly related to the surface nonlinear susceptibility, χ(2). For experiments in which the surface is rotationally isotropic about the surface normal, χ(2) can be reduced to three nonzero elements, χzzz, χzxx, and χxxz, where the subscripts refer to the laboratory coordinate system.10 In this case, the relative values of s- and p-polarized components of I2ω can be described by the following two equations, which are specific for a TIR geometry:17

Is2ω ) C|s1 sin(2γ) χxxz|2(Iω)2

(1)

Ip2ω ) C|s5χzxx + cos2(γ) (s2χxxz + s3χzxx + s4χzzz - s5χzxx)|2(Iω)2 (2) where γ is the polarization angle of the incident beam, C is an experimental constant, and si values are fitting coefficients related to the electric fields of the SH and incident light at the surface.17 The molecular βzzz is related to χzzz, χzxx, and χxxz as17

χzzz ) Ns〈cos3 θ〉βzzz χzxx ) χxxz )

1 N 〈cos θ sin2 θ〉βzzz 2 s

(3) (4)

where Ns is the molecular surface density and θ is the molecular long axis orientation with respect to surface normal. An important aspect of eqs 3 and 4 is that I2ω is dependent upon both surface number density and molecular orientation. To develop an accurate picture of surface adsorption, it is necessary to evaluate both Ns and θ independently. Simpson and Rowlen recently reported the theory and an experimental geometry that minimizes SHG sensitivity to molecular orientation, thereby allowing for independent determination of coverage and molecular orientation.18 For molecules with a dominate βzzz, the theoretical orientation-insensitive (18) Simpson, G. J.; Rowlen, K. L. Anal. Chem. 2000, 72, 33993406.

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obtained when the data below 10-3 M are used, with R2 ) 0.94, K1 ) (5 ( 2) × 103 M-1, and ∆Gads ) -21.2 ( 0.1 kJ/mol. The quality of the Langmuir fit at low concentrations supports the idea of two distinct adsorption regimes, with a transition near a solution concentration of 10-3 M. Multilayer formation can be observed by SHG if polar order is retained. Systems of this type are generally synthesized through covalent linkages or a controlled Langmuir-Blodgett depositon.3,11-16,19 For example, Hanken et al. synthesized noncentrosymmetric multilayer films through zirconium phosphonate self-assembly.11 Ashwell recently showed multiple methods for generating noncentrosymmetric multilayers using the LangmuirBlodgett technique.15 In both cases, the SH signal was observed to increase quadratically with layer number. If it is assumed that a monolayer is formed at solution concentrations ∼10-3 M and that a second layer is complete at 4 × 10-3 M (which is the solubility limit), using a zerozero intercept, the data in Figure 2 are consistent with a quadratic relationship between I2ω and the number of layers (R2 ) 1). Multilayer adsorption is commonly described by the Brunauer-Emmett-Teller (BET) isotherm. However, the BET isotherm is best suited for gas-phase applications and does not generate a calculated adsorption energy.20 Wang et al. developed a liquid/surface multilayer adsorption isotherm model that assumes a unique equilibrium constant for molecule/surface interaction, K1, and a subsequent equilibrium constant, K2, for all subsequent layers based on molecule/molecule interactions.20 The functional form is

Figure 2. (A) Adsorption isotherm measured by SHG of DR-1 at the fused-silica/2-propanol interface. Error bars, from three measurements, are smaller than the symbol size. The solid line shows the multilayer model fit (eq 6), R2 ) 0.95, K1 ) (3.6 ( 1.6) × 103 M-1, and K2 ) 81 ( 30 M-1. The dashed line shows a Langmuir fit (I2ω ) bK1M/(1 + K1x), where M is solution concentration in molarity and b is a fitting coefficient) to the data representing the first monolayer, R2 ) 0.94, b ) 1.25, K1 ) (5.2 ( 1.7) × 103 M-1, and ∆Gads ) -21.2 ( 0.1 kJ/mol. (B) Proposed scheme for multilayer organization, where the circles and triangles represent the -OH and -NO2 groups of DR-1, respectively.

polarization rotation angle, γ*, is given by the following relationship, which is specific for a TIR geometry:18

(

γ* ) cos-1

)

s5 3s4 + s5 + s2 + s3

1/2

(5)

The fitting coefficients, si, are all functions of the refractive index of the surface and the solution and the angle of incidence. It should be noted that the TIR geometry results in complex values of the fitting coefficients, with the imaginary contributions typically dominant.18 For the experimental conditions used here, the orientationinsensitive angle was calculated to be 60.1° under the assumption that the refractive index of the surface does not change with increasing surface coverage. SHG Adsorption Isotherm. The SHG adsorption isotherm for DR-1 at the fused-silica/2-propanol interface was measured with an incident beam polarization of 60.1° and p-polarized SH. The data are shown in Figure 2. A Langmuir fit to the entire data set yields R2 ) 0.92, K1 ) (2.0 ( 0.4) × 103 M-1, and ∆Gads ) -18.1 ( 0.1 kJ/mol. However, there appears to be a clear break in the trend at concentrations near 10-3 M. A better Langmuir fit is

φ)

K1C (1 - K2C)[1 + (K1 - K2)C]

(6)

where φ is the total multilayer adsorption (φ ) φ1 + φ2 + φ3 ..., where each is the fractional coverage in a given monolayer), which can exceed a value of 1, and C is the adsorbate concentration (M). Regression of the data in Figure 2 to eq 6 (R2 ) 0.95) yields a K1 of (4 ( 2) × 103 M-1 and a K2 of 81 ( 30 M-1. The respective ∆Gads values are -20.3 ( 0.2 and -10.9 ( 0.2 kJ/mol. Previous investigations of p-nitrophenol serve as useful comparisons, since the molecule is functionally similar to DR-1. Corn and co-workers investigated the interfacial orientation of p-nitrophenol using SHG.21 They reported ∆Gads values of -17 and -19 kJ/mol for p-nitrophenol at fused-silica/chloroform and air/water interfaces, respectively.21 It was noted that these ∆Gads values compare well with the strength of a typical hydrogen bond of approximately 20 kJ/mol.21 Phase SHG experiments were also conducted on p-nitrophenol at the water/air interface in order to determine the absolute orientation.21 It was determined that the -OH group in p-nitrophenol is directed into a water layer with the nitro functional group at the air/water interface, consistent with hydrogen bonding through the -OH.21 For DR-1, either end of the molecule could facilitate physisorption to the fused-silica surface through hydrogen bonding. Fused-silica surfaces exhibit a distribution of pKa values but are generally regarded as somewhat acidic, with a significant number of deprotonated surface silanol (19) Doughty, S. K.; Simpson, G. J.; Rowlen, K. L. J. Am. Chem. Soc. 1998, 120, 7997-7998. (20) Wang, J.; Huang, C. P.; Allen, H. E.; Cha, D. K.; Kim, D.-W. J. Colloid Interface Sci. 1998, 208, 518-528. (21) Higgins, D. A.; Abrams, M. B.; Byerly, S. K.; Corn, R. M. Langmuir 1992, 8, 1994-2000.

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Figure 3. Apparent orientation angles measured by SHG as a function of concentration for DR-1 at the fused-silica/2propanol interface. The error bars represent the standard deviation of five measurements.

groups.22 It is postulated that negatively charged regions of the surface would disfavor hydrogen bonding through the -NO2 group of DR-1, thereby leading to preferential adsorption through hydrogen bonding of the -OH group. The polar order could be retained in subsequent layers through a hydrogen bond donor-acceptor arrangement of the -OH and -NO2 functional groups as well as NO2NO2 repulsive interactions (see Figure 2B). SHG Orientation Measurements. The apparent orientation angle of DR-1 at the fused-silica surface was probed by rotating the linear polarization of a 1064 nm excitation beam through 180° for both s-polarized and p-polarized SH.17 The polarization curves were fit to eqs 1 and 2. χ values were extracted from the fits and used to calculate the SHG orientation parameter, D:

D≡

〈 〉

Figure 4. Apparent orientation angles measured by linear dichroism as a function of concentration for DR-1 at the fusedsilica/2-propanol interface. Errors bars represent the standard error of the mean (N ) 10). The solid line is presented only as a guide to the eye.

concomitant reduction in the angular distribution in that layer, and formation of less well ordered multilayers. Linear Dichroism. ARAPD is a linear dichroism technique that can be used to determine molecular orientation at the sub-monolayer to multilayer level.19,24 Molecular orientation can be determined by varying the linear polarization of the incident beam while monitoring the resulting change in the photoacoustic signal. For the TIR geometry, the relationship between absorbance, A, and polarization angle, γ, is described by17

Atot ) C(ax + (ay - ax) sin2 γ + K[sin2 γ(2az - ax + ay) + ax - 2az]) (8) K ≡ 〈cos2 θ〉 ) cos2 θap

(9)

3

χzzz cos θ ) = cos2 θap cos θ χzzz + 2χzxx

(7)

When a narrow angular distribution is assumed, the apparent orientation angle, θap, may be calculated by θap ) cos-1(D1/2). Apparent orientation angles were determined as a function of DR-1 solution concentration (8 × 10-5 to 4 × 10-3 M), and the results are shown in Figure 3. There is a clear trend of increasing apparent orientation angle (with respect to surface normal) with increasing concentration. Consideration of the SHG results independently leads to several possible explanations for the observed changes in orientation. If the assumed narrow angular distribution is correct, the data imply that at low surface coverage the mean tilt angle is ∼44° and that increased packing, with increased coverage, shifts the tilt angle to ∼49°. If the assumed narrow angular distribution is incorrect, the data imply that the molecules are relatively poorly oriented at low surface coverage, but their net orientation increases with increasing surface coverage. A shift in the apparent tilt angle toward the surface plane is consistent with a reduced angular distribution.23 It is postulated that the change in apparent orientation angle is due to a combination of improved packing in the first layer, with a

where Atot is the sample absorbance, γ is the polarization of the incident beam, ai values are fitting coefficients related to the electric field intensities of each component at the surface, K is the absorbance orientation parameter, and θap is the apparent molecular orientation with respect to surface normal (for a long axis transition) under the assumption of a narrow angular distribution. The constant, C, is equal to the product of the squares of the incident electric field polarization vector and the transition moment vector and is experimentally determined through fits of the data to eq 8. Linear Dichroism Adsorption Isotherm. For linear dichroism, the absorbance magic angle can be used to probe surface coverage independent of changes in molecular orientation. For the TIR geometry, fused-silica and 2-propanol, the calculated magic angle is 55°; therefore, the adsorption isotherm was measured by maintaining an excitation polarization of 55° while the solution concentration was varied. The signal as a function of solution concentration was linear (R2 ) 0.99 with a slope equal to (6 ( 0.3) × 103 arbitrary units/M). In this case, the photoacoustic signal is dominated by contributions from the bulk solution. For example, on the basis of the penetration depth of the evanescent field (∼365 nm),25 a solution concentration of 4 × 10-3 M (highest concentra-

(22) Foissy, A.; Persello, J. In The Surface Properties of Silicas; Legrand, A. P., Ed.; John Wiley & Sons: New York, 1998. (23) Simpson, G. J.; Rowlen, K. L. J. Am. Chem. Soc. 1999, 121, 2635-2636.

(24) Doughty, S. K.; Rowlen, K. L. J. Phys. Chem. 1995, 99, 21432150. (25) Harrick, N. J. Internal Reflection Spectroscopy; John Wiley & Sons: New York, 1967.

Spontaneous Multilayer Formation for Disperse Red-1

Figure 5. Tapping-mode AFM images of DR-1 dip-coated on fused-silica. Lighter shades of gray represent higher features, with the maximum feature height equal to ∼40 nm. (A) 8 × 10-4 M, mean island height ) 16 nm, bearing area ) 0.84%, weighted average height ) 3.3 Å. (B) 2 × 10-3 M, mean island height ) 13.6 nm, bearing area ) 16%, weighted average height ) 23.4 Å. (C) 4 × 10-3 M, mean island height ) 15.7 nm, bearing area ) 20%, weighted average height ) 33.6 Å.

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tion), and a fractional coverage of 0.1 monolayers (∼1013 molecules/cm2), the bulk contribution is an order of magnitude greater than that from the surface. Because linear dichroism is not surface selective, it is not possible under these conditions, even with the TIR geometry, to separate bulk and surface contributions to the overall signal magnitude. However, it is possible to separate the polarization dependence of the adsorbate from the isotropic response of the bulk. Linear Dichroism Orientation Measurements. The apparent molecular orientation of DR-1 at the fused-silica/ 2-propanol interface was determined as a function of surface coverage, as shown in Figure 4. As a control experiment, a similar but nonadsorbing dye was used to ensure that the bulk contribution exhibited no polarization dependence. Azobenzene (4 × 10-3 M) in 2-propanol does not adsorb significantly to fused-silica, as determined by dip-coating with absorbance detection. The polarization response of 4 × 10-3 M azobenzene was flat with θap ) 55.7 ( 0.3°, as expected for an isotropic system (magic angle ) 55° for the TIR geometry). In Figure 4, for DR-1, both the magnitude and the trend in θap as a function of concentration are much different from those observed by SHG. The difference in magnitude indicates that the assumption of a narrow angular distribution is not valid.17 For the linear dichroism determined θap as a function of solution concentration of DR-1, there is an initial increase with increasing coverage up to ∼10-3 M. Beyond ∼10-3 M, the trend reverses and the apparent orientation angle decreases. As with the SHG orientation data, at low coverage, the initial increase in orientation angle is consistent with a decrease, or narrowing of, the angular distribution.26 In this regime, increased coverage may improve packing, which, in turn, narrows the angular distribution. Assuming that the transition point near 10-3 M represents the formation of multilayers, as indicated by the SHG adsorption isotherm (Figure 2), it is not unexpected that additional layers would exhibit increased disorder (leading to a shift of the apparent orientation angle toward smaller tilt angles). The same trend would not necessarily be observed in the SHG apparent orientation angle with coverage, since SHG selectively probes only those molecules in higher layers whose alignment preserves noncentrosymmetry.11-16 Under certain conditions, it is possible to combine SHG and APAPD orientation measurements in order to extract the mean orientation angle and the width of the angular distribution.17 In this work however, such is not the case. With the assumption of a Gaussian distribution in tilt angles, the combination of the data at low coverage does not yield an analytic solution.17,26 It is not clear whether the failure to converge to a solution is due to a breakdown in the distribution assumption or an extraordinarily large width of the distribution. The latter seems less likely since the SHG signal magnitude (S) was significantly larger (S/B ∼ 50) than the background (B).22,26 At higher coverage, i.e., above the transition point of 10-3 M, the combination of data was not expected to (nor did it) yield a solution since the two techniques probe different aspects of the film. Taken as a whole, the spectroscopic techniques provide strong evidence for spontaneous multilayer formation of DR-1 at the fused-silica/2-propanol interface. Noncentrosymmetric multilayers are generally synthesized through covalent linkages or controlled LangmuirBlodgett deposition.3 (26) Simpson, G. J. Molecular Orientation At Surfaces and Interfaces. Doctoral Thesis, University of Colorado, 2001.

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Figure 6. Correlation of weighted average heights calculated from AFM images and ellipsometric thickness measurements. The open triangles represent the AFM results, and the solid circles represent the ellipsometry results (error bars correspond to the standard deviation of five measurements). The solid line is presented only as a guide to the eye.

This work demonstrates the potential for spontaneous formation of nonlinear optical multilayer systems from dilute solutions, unlike liquid crystals, which are generated from concentrated starting materials and interaction with a “rubbed” substrate.27 The work also demonstrates the utility combining surface selective and linear spectroscopic techniques for probing thin film structure. AFM and Ellipsometry. In an attempt to further explore the DR-1 fused-silica system, AFM and ellipsometry were employed to characterize film morphology. However, for these two techniques, it was necessary to dip-coat the film onto the appropriate surface. The surface was extracted slowly (∼1 cm/min) in order to prepare a uniform film under solvent evaporation conditions.28,29 Therefore, direct comparison of the resulting film under these conditions with the film produced under equilibrium conditions is limited. However, many of the forces that result in molecular adsorption to the surface are the same in both cases.30 AFM images of dip-coated DR-1 films on fused-silica are shown in Figure 5. Multilayer islands are readily apparent, with increasing surface coverage occurring with increasing solution-phase concentration. Analysis of the AFM images, using the Nanoscope bearing analysis software, allowed for determination of the fraction of each image above the background. From this value, combined (27) Walba, D. M. Adv. Synth. React. Solids 1991, 1, 173-235. (28) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Nature 1993, 361, 26. (29) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303-1311. (30) Schwartz, D. K. Annu. Rev. Phys. Chem. 2001, 52, 107-137.

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with the mean height of the islands, a weighted average height was calculated for each concentration. The average height is plotted as a function of dip-coat solution concentration in Figure 6. It is important to note that the relatively large height of the islands may have prevented an accurate determination of sub-monolayer or even monolayer coverage by AFM. All ellipsometry measurements were performed on silicon substrates. As silicon contains a surface SiO2 layer, it is reasonable to assume that the surface chemistry is similar to that of fused-silica.26 The ellipsometric thickness as a function of dip-coat solution concentration is shown in Figure 6. The calculated molecular length of DR-1 is ∼15 Å; therefore, the maximum thickness of a complete monolayer should be ∼15 Å. The spectroscopic measurements indicate some molecular tilt; therefore, monolayer coverage would yield an ellipsometric thickness less than 15 Å. The ellipsometric thickness at a dip-coat solution concentration of 1 × 10-3 M, assuming monolayer coverage, corresponds to a molecular tilt angle of 75°. This tilt angle is consistent with the apparent orientation angle measured by linear dichroism (∼72°). Note that the measured film thickness exceeds 15 Å at dip-coat solution concentrations above 2 × 10-3 M. The good correlation between the ellipsomtry and the AFM results indicates that the measured ellipsometric thickness, even at lower coverage, is due to island/multilayers rather than a uniform surface coverage. Conclusions SHG and linear dichroism based spectroscopic studies provided evidence for the spontaneous formation of DR-1 multilayers at a fused-silica/2-propanol interface. Specifically, an adsorption isotherm measured by SHG showed evidence of two distinct adsorption regimes. The second regime appeared to be indicative of multilayer formation in which some degree of polar order was retained. At low coverage, the trend in the apparent tilt angle measured by SHG was consistent with a narrowing of the angular distribution. At higher coverage, the trend in apparent tilt angle was attributed to a combination of contributions from the first layer and subsequent layers. The trend in apparent tilt angle measured by linear dichroism provided supporting evidence of multilayer formation at solutionphase concentrations above 10-3 M, consistent with the two regimes observed in the adsorption isotherm measured by SHG. AFM images of dip-coated substrates show multilayer island formation. Ellipsometry measurements exhibit values greater than the maximum height of one monolayer and correlate well with the AFM results. Acknowledgment. The authors gratefully acknowledge funding from the National Science Foundation. LA010853K