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Multiple Transmission-Reflection Infrared (MTR-IR) Spectroscopy of Arachidic Acid LB Films on Hydrophilic and Hydrophobic Silicon Surfaces Peng-Feng Guo, Hong-Bo Liu, Xiang Liu, Hong-Fang Li, Wen-Yi Huang, and Shou-Jun Xiao* State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures, Nanjing UniVersity, Nanjing 210093, China ReceiVed: July 10, 2009; ReVised Manuscript ReceiVed: NoVember 24, 2009
Our recently developed multiple transmission-reflection infrared (MTR-IR) spectroscopy method was applied to measure arachidic acid Langmuir-Blodgett films on hydrophilic and hydrophobic silicon substrates. Highquality MTR-IR spectra at both s and p polarizations were obtained. The MTR-IR absorption intensity at different incident angles was found to be linearly correlated with the transmission times (N) for arachidic acid monolayers on a hydrophilic silicon substrate. Significant differences in polarization-dependent intensities were observed for some specific vibration modes, for example, CH3 and CH2 stretching, carboxylic acid stretching, and CH2 bending, on different types of monolayer and multilayer films. By means of polarizationdependent spectra, the molecular orientation was numerically calculated, and the molecular structures such as the crystalline packing of hydrocarbon chains and the trans and cis configurations of carboxylic acid species were deduced. Obvious and repeatable intensity differences for the same vibration mode from s to p polarization strongly confirmed the stability and credibility of our MTR-IR method. 1. Introduction Because self-assembled molecular monolayers on infraredtransparent semiconductors such as silicon and germanium are gaining more attention in molecular and nanoelectronics and biosensing, we recently developed the technique of multiple transmission-reflection infrared (MTR-IR) spectroscopy to analyze silicon-based molecular monolayers qualitatively and quantitatively regarding their chemical and structural information.1-3 Infrared spectroscopy is always one of the most powerful tools for measuring substrate-surface-based ultrathin films of soft materials. For example, infrared reflection absorption spectroscopy4 is well-known as a routine laboratory tool for nondestructive analyses of thin organic films and adsorbates on metallic surfaces, including Langmuir-Blodgett (LB) films,5-14 self-assembled monolayers,15,16 and covalently attached monolayers.17,18 Although there have been a few examples of this singlereflection approach on silicon-based monolayers,19 the penetration of infrared light through the silicon crystal causes the loss of signal in the detector and results in a low signal-to-noise ratio. Especially for p-polarized light incident at grazing angles (close to the Brewster angle of the air/silicon interface, 73.6°), nearly all light passes through the silicon crystal and escapes from the detector. Over the years, high-quality infrared spectra for silicon-based monolayers have been obtained only with the multiple-internal-reflection (MIR) method in an attenuated-totalreflection (ATR) silicon crystal prism.20-24 The MIR technique works both for in situ and ex situ measurements. However, the MIR prism is specifically fabricated, and so, it is costly and often needs to be recycled after measurements. With the increasing interest in silicon-based organic and biological functionalizations, more convenient IR methods are needed. Actually, several infrared spectroscopies have been developed recently. The simplest approach is Brewster-angle transmission infrared (BAT-IR) spectroscopy.25,26 By changing the incidence * Corresponding author. Tel.: 86-25-83621001. Fax: 86-25-83314502. E-mail:
[email protected].
angles during spectral measurements, multiple-angle incidence resolution IR spectroscopy was developed by applying an arithmetic regression model to obtain in-plane and out-of-plane mode spectroscopies.27,28 However, single-transmission approaches for monolayer and submonolayer molecules with weak absorptions face a tough challenge both for good-resolution spectra and for quantitative analyses. A sandwich configuration (the GATR, or grazing-angle ATR accessory, from Harrick Scientific Products, Inc.) with a monolayer between two highrefractive-index solid materials (e.g., Ge/monolayer/Si) has also been developed.29-31 High-quality spectra from the GATR can be obtained only at p polarization, and no signal can be obtained at s polarization. This limits the accessible molecular structural information in the parallel direction to the substrate surface. Moreover, GATR operates according to a loading-forcedependent method, and therefore, it is a contact-mode configuration and not a true nondestructive methodology. The air gap between the sample and germanium surfaces often renders the spectral intensity nonrepeatable. Our MTR-IR method, originated from Greenler’s parallelmirror configuration, can theoretically enhance the infrared signal by up to 20 times.32 Its core design is that the infrared light is forced to bounce back and forth between two metal mirrors and thus penetrates the infrared-transparent silicon chip many times (hence the name multiple transmission-reflection). Such multiple interactions between the infrared light and the monolayer molecules can be realized only with the MIR-ATR configuration. However, with the help of two parallel gold mirrors, we reached the same sensitivity as can be obtained with the MIR method. Moreover, the MTR-IR method allows for the use of standard silicon wafers with variable thicknesses from 0.3 to 1.0 mm without further mechanical processing, which is a distinctive advantage of the MTR-IR method. We envision that the MTR-IR approach will render silicon-based monolayer research work much easier and less expensive. In our previous reports, we investigated covalently bonded monolayers of undecylenic acid and its derived N-succinimidyl
10.1021/jp909027e 2010 American Chemical Society Published on Web 12/16/2009
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Figure 1. Schematic of MTR setup showing multiple transmissions and reflections with simplified transmission times N ) 4, where N ) 4 is indicated by passing through the silicon chip four times with the solid line beam. Iin and Iout represent the incidence and output light intensities, respectively. The broken lines represent other reflections and transmissions occurring on the silicon chip, which are not plotted completely for simplicity. All multiple transmissions and reflections are considered in our calculations.1,3
ester on silicon surfaces1 and the dynamic process of alkanephosphate self-assembly on ultrathin TiO2-coated silicon crystals.2 More experimental details are needed to demonstrate the advantage of our MTR-IR approach. One obvious strategy is to investigate a well-characterized ultrathin film. The LB films of long-chain alkyl acids such as arachidic acid (AA) have been extensively studied by all types of infrared spectroscopies, such as transmission infrared spectroscopy, infrared reflection absorption spectroscopy, ATR-IR spectroscopy, among others.5-9,24 Therefore, we examined pure AA Langmuir monolayers and LB multilayer films rather than AA salt films with MTR-IR spectroscopy, because AA salts have fewer infrared features in the carbonyl stretching region. In this work, we extended our MTR-IR measurement range from 4000 to 1150 cm-1, whereas we limited the range to 4000-1500 cm-1 in our previous report.2 To the best of our knowledge, these spectra are the most sensitive polarization-dependent spectra of AA monolayers and multilayers at both p and s polarizations obtained to date. Then, the dichroic ratios (DRs), As/Ap (where As is the absorbance at s polarization and Ap is the absorbance at p polarization), of some specific groups could be obtained, and thus, by simulation, the orientations of these groups could be calculated. When LB films were deposited in a monolayer and in several multilayers, the crystalline packing of the alkyl chains and the configuration of the fatty acid dimer were different, which was exhibited clearly in the MTR-IR spectra. With this example, we demonstrate that MTR-IR spectroscopy is a powerful and convenient tool for sensitive measurements of molecular structures and orientations of monolayer and multilayer films on infraredtransparent substrates. The structure of this article is as follows: First, we briefly introduce the MTR-IR method. Then, we validate the linear relationship between the absorbance and the transmission times with different incidence angles. Finally, we measure the spectra at both s and p polarizations and analyze them in detail, both qualitatively and quantitatively, for the molecular and functionalgroup structures and orientations by means of the structure-infrared effect relationship and dichroic ratios. 2. Methods and Materials 2.1. Deposition of a Langmuir Monolayer and LB Multilayers. First, a double-side-polished silicon wafer (〈111〉oriented, p-type, B-doped, 15 Ω cm resistivity, and 0.5 mm thickness) was cut into rectangular shapes (20 mm × 30 mm) and then cleaned with heated piranha solution [1:3 (v/v) mixture of 30% H2O2/95% H2SO4] for 30 min to remove impurities and obtain hydrophilic surfaces (SiO2/Si). To get a hydrophobic surface (Si-H/Si), the silicon chip was soaked in 1% HF for 30 s.
The Langmuir monolayer was prepared on a KSV500 Langmuir trough (KSV Instruments Ltd., Helsinki, Finland). The molecular monolayer on the surface of a subphase (deionized water, resistivity g 18.0 MΩ cm) was formed by spreading arachidic acid (AA) solution (1 mg/mL) in chloroform (AR grade). A compression rate of 5 mm/min was used to reach a surface pressure of 25 mN/m, and at this constant surface pressure (25 mN/m), the monolayer was transferred onto the silicon substrate by vertical dipping with a dipping speed of 2 mm/min. The temperature was maintained at 20 °C. Both sides of the silicon chip were covered with AA molecules. The π-A isotherm and transfer ratios are shown in Figures S1 and S2, respectively, in the Supporting Information. 2.2. Preparation of Au Mirrors. The Au mirror was prepared by sputtering of 2000 Å of Au onto a single-sidepolished silicon surface with 100 Å of sputtered Cr as the seed layer. 2.3. Multiple Transmission-Reflection Infrared (MTRIR) Spectroscopy. Infrared spectra were recorded on a Bruker V80 instrument at 4 cm-1 resolution with a deuterated triglycine sulfate (DTGS) detector. To accomplish the multiple transmission -reflection measurements, two Au mirrors were used (mirrors I and II) to control the transmission-reflection times (Figure 1). The measured spectrum was referenced to a native clean silicon chip. No smoothing was done for our experimental spectra. 2.4. Description of MTR-IR Method and Its Theoretical Consideration for Orientation. Details of the MTR-IR setup and theoretical considerations were described in our previous reports.1,3 To clarify the data treatment in the following sections, a core schematic is shown in Figure 1, and a brief theoretical summary is provided. A double-side-polished silicon sample was placed between the two Au mirrors I and II, which were used to compel the incoming infrared light (Iin) to reflect back toward the silicon and to enable multiple transmissions and reflections through the silicon chip to occur. Finally, all output infrared light (Iout) with molecular information was directed to the detector. High-quality spectra at both s and p polarizations were recorded, and thus, the dichroic ratios As/Ap of some specific groups could be obtained and applied to calculate the molecular orientations. The number of simplified transmission times (N) was defined as the number of times the beam passed through the silicon chip until it left for the detector, assuming no reflection on the silicon surface. In Figure 1, N is equal to 4. N can be adjusted to different values by (a) increasing or decreasing the distance between the two Au mirrors, (b) choosing different infrared incidence angles, and/or (c) using the silicon chips with different lengths. In our work, we fixed
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the silicon chip length and preferred the first two approaches to obtain our anticipated transmission times. Using the above terms, the absorbance A can be written as
A ) -log10
Iout Iout(0)
(1)
where Iout(0) is the output light intensity of a reference silicon chip without organic monolayers and Iout is the output light intensity of a silicon chip coated with organic monolayers. During simulation, all transmissions through the silicon chip and all reflections occurring on the gold mirrors and on the air/ silicon interfaces were included.1,3 Both Iout(0) and Iout were calculated through iterative calculations, in accordance with our previous publication.1 The basic theory for the calculation of molecular orientations from IR spectra, particularly in thin films, is the so-called Berreman effect.33,34 For an organic monolayer/Si system, the adsorbed monolayer is usually regarded as a uniaxial symmetric system. In this case, the molecular orientation is associated with the absorption index k, which is the imaginary part of the complex refractive index (nˆ ) n + ik). The maximum absorption index, kmax, of one species is the absorption index when all of its transition dipole moments are oriented along the same direction. kmax can be obtained from the equation
kmax ) kx + ky + kz ) 3kiso
(2)
where kz is the component of absorption index perpendicular to the surface and kx and ky are parallel components. When the molecules are in an isotropic state, kz, kx, and ky are equal to each other and are also defined as the isotropic absorption index, kiso. In an ordered film, the tilt angle, γ, of the alkyl chain from the surface normal (z) is related to k as35
kx ) ky ) kmax[s(sin2 R)/2 + (1 - s)/3]
(3)
kz ) kmax[s cos2 R + (1 - s)/3]
(4)
1 s ≡ 〈P2 cos γ〉 ) 〈3 cos2 γ - 1〉 2
(5)
where R is the angle between the transition dipole moment and the alkyl chain axis (for the all-trans conformation, R ) 90°) and P2 cos γ is the second-degree Legendre polynomial. Because of the ordering of organic films, the respective absorbances As at s polarization and Ap at p polarization are different. Their DR (As/Ap) can be related to the tilt angle γ from the surface normal according to eqs 1-5, and thus, a simulated curve of DR against γ can be obtained. The experimental DR value will map onto one tilt angle in the curve, which is taken as the orientation of this functional group. 3. Results and Discussion 3.1. Brewster-Angle Transmission (BAT) and Multiple Transmission-Reflection (MTR) Infrared Spectra of an AA Monolayer on a Hydrophilic Silicon Surface. The primary goal of the MTR setup is to achieve signal enhancement. Theoretically, the absorbance has a linear relationship with N. Because a BAT-IR signal corresponds to an MTR-IR signal at N ) 1, the absorbance for p-polarized light at N from MTR-IR spectroscopy should be N times that from BAT-IR spectroscopy for the same vibration mode. To demonstrate this statement, we performed an MTR-IR measurement at N ) 10 and a BATIR measurement for an AA monolayer on a hydrophilic silicon chip at an incidence angle of 74° (the Brewster angle between silicon and air). We used nonpolarized light for both measure-
Figure 2. Experimental MTR (N ) 10 at an incidence angle of 74°, upper trace) and BAT (N ) 1 at an incidence angle of 74°, lower trace) infrared spectra of an AA monolayer on a double-sided polished hydrophilic silicon chip.
ments. According to Beer’s law, the absorbance should be the same regardless of whether we used polarized or nonpolarized light, assuming identical absorption coefficients for both s and p polarizations. Two traces in the range 2600-3200 cm-1 are compared in Figure 2: the bottom trace is from BAT (corresponding to N ) 1) and the upper trace from MTR at N ) 10. Four characteristic absorption bands of AA are obvious: the two bands at 2916 and 2850 cm-1 are the asymmetric and symmetric stretching modes of CH2, respectively, and those at 2960 and 2879 cm-1 are the corresponding asymmetric and symmetric stretching modes of CH3. Because of the abundance of CH2 groups and their strong absorbance, the CH3 band at 2879 cm-1 between two CH2 bands is overlapped and cannot be observed clearly. The frequencies of the two CH2 bands are sensitive to the conformation of the hydrocarbon chains. The relatively low values for the CH2 stretching bands at 2916 and 2850 cm-1, compared to values for the gauche conformer in the liquid state at 2928 and 2856 cm-1, indicate a highly ordered all-trans conformations of the alkyl chains.36-38 The experimental absorbance at 2916 cm-1 obtained by MTR-IR spectroscopy is about 9 times that obtained by BAT-IR, corresponding very well to the theoretical linear relationship (10 times). 3.2. Quantitative Analysis of the Linear Relationship between Absorbance (A) and Transmission Times (N). Numerical simulations in our previous articles1,3 gave a linear relationship between the absorbance and N up to N ) 14. In fact, the linear relationship can be kept for higher values of N up to 20 at around 70° incidence angle according to Greenler’s work.32 However, the signal-to-noise ratio reaches its maximum at around N ) 14 and then tends to decrease because of light loss and interference.1 Practically, if a good-quality spectrum can be obtained at lower N (we often use N ) 6-8 to obtain adequate spectra), higher N values are not considered. In this work, we experimentally demonstrate the linear relationship by recording and analyzing a series of spectra for N ) 4, 6, 8, and 10 at incidence angles of 50°, 60°, and 70°. These spectral absorbances fall within the linear range derived by both Greenler and us.1,32 Two parameters in the experimental setup were adjusted to achieve the required transmission times (N): the distance between the two gold mirrors and the incidence angle. At each incidence angle of 50°, 60°, or 70°, four values of N (4, 6, 8, and 10) were achieved by remodeling the distance between the two parallel Au mirrors. The incidence angle affects
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Figure 3. Experimental MTR-IR spectra at incidence angles of (a) 50°, (b) 60°, and (c) 70°. Four N values, 4, 6, 8, and 10, were achieved at each incidence angle by remodeling the distance between Au mirrors I and II.
the reflectance and transmittance at the air/silicon interface for both s- and p-polarized light. However, two parallel gold mirrors force the incoming infrared light, regardless of the type of propagation mode (s or p polarization), to the detector. Considering the flexible use of the MTR setup, nonpolarized light was applied in this measurement. The evolutions of the signal enhancement at 50°, 60°, and 70° are shown in Figure 3a-c, respectively, for N ) 4, 6, 8, and 10. For the same value of N, the MTR-IR spectra at 50°, 60°, and 70° show a few discrepancies in the absorption intensities. As N increases from 4 to 10, the intensities of the IR bands at 2850 and 2916 cm-1 linearly increase, as demonstrated in Figure 4. The baseline distortion of the IR spectra becomes more severe from 70° to 50°, which is disadvantageous for quantitative analyses. The baseline deformation also becomes obvious when N is increased to a larger value such as 14 (not shown here), because of light loss and interference. From the above results, the conclusion can be made that an incidence angle of around 70° presents a better maneuverability for goodquality spectra than lower incidence angles. Numerical simulations were carried out to analyze the relationship between the CH2 stretching absorbance at 2916 cm-1 [υas(CH2)] and N. For the υas(CH2) band, the refractive index nLB was adopted from previous reports as 1.5.6,39,40 The refractive indices of silicon (nSi), air (nair), and gold (nAu) are 3.42, 1.0, and 2.018 + 21.087i,41 respectively. An AA monolayer thickness of 2.8 nm was also adopted in our simulations.6 An uncertain parameter for simulation is the value of the maximum absorption index kmax. kmax is a function of the molecular packing density and orientation. Diversified values of kmax ranging from 0.4 to 1.0 have been used for alkyl chains in previous works to fit experimental data.6,7,15,22,35 The tilt angle of the AA alkyl chain in LB films was determined to be less than 30°.6,7 In Figure 4, we present the simulation relationship between absorbance and N with three kmax values, 0.55, 0.65, and 0.75, which are close to our experimental data. In addition, three tilt angles of 10°, 20°, and 30° were used for the simulations. To obtain the total absorbance, both p- and s-polarized spectra were simulated, and then they were counterpoised to obtain the nonpolarized absorbance.42 The simulated absorbances at different tilt angles for the same kmax value exhibit little discrepancy. By comparing the experimental curves (black lines) with the simulated curves, the kmax value of the AA monolayer was determined to be ∼0.65. 3.3. Polarized IR Spectra of an AA Monolayer on a Hydrophilic Silicon Surface. From the results of the preceding sections, we fixed the incidence angle at 70° for the investigation of molecular structures and orientations. In our setup, the silicon chip was placed vertically. Therefore, the p and s polarizations corresponded to 0° and 90° polarizations, respectively. To view clearly the evolution of the molecular structure, the primary IR
beam was polarized to 0° (p-polarized), 30°, 60° and 90° (s-polarized) using a KRS-5 polarizer, and the measured spectra are collected in Figure 5. The C-H stretching vibrations are shown in Figure 5a. The CH2 stretching shows a blue shift compared to that of disordered bulk AA at 2928 and 2856 cm-1.36-38 The CH3 asymmetric stretching is split into two peaks, in-plane stretching [υin as(CH3)] at 2965 cm-1 and out-of-plane stretching [υout as (CH3)] at 2957 cm-1.35,43 By assuming a perfectly ordered monolayer on the silicon surface, the normal (z) and parallel (x) components of transition dipole moments contribute to the absorbance at 0° polarization, whereas the parallel (y) component contributes to that at 90° polarization. At 0° polarization, both methyl in-plane (2965 cm-1) and out-of-plane (2958 cm-1) asymmetric stretching bands can be observed. With the evolution from 0° to 90°, gradually, the in-plane stretching becomes weaker, and the outof-plane stretching appears clearer at 90° polarization. This evolution illuminates a parallel vector of transition dipole moment of υinas(CH3) along the x direction of the silicon surface; that is, the plane consisting of the three H atoms of CH3 is parallel to the silicon surface. Similarly, because of the z component of the transition dipole moment of the symmetric stretching, a peak at 2878 cm-1 emerges at 0° polarization and disappears at 90° polarization. Because the AA Langmuir monolayer is a highly ordered film with a small tilt angle of the alkyl chain from the surface normal, according to the surface selection rule, much less CH2 asymmetric stretching υas(CH2) at 2916 cm-1 and symmetric stretching υs(CH2) at 2850 cm-1 were obtained at 0° polarization than at 90° polarization. For the orientation analysis of thin films by IR spectroscopy, a large number of works have focused on obtaining the absorption index k of the molecule and the tilt angle of the molecular longitudinal axis. Generally, the k value of a system can be obtained by the following two methods: Parikh and Allara6 and Allara and Nuzzo15 assumed that the oscillator strengths and the molecular density do not change appreciably upon molecular chemisorption and, therefore, that the isotropic absorption index kiso can be determined from Kramers-Kronig transformation of the experimental transmission spectra of the film compound in a KBr pellet. Then, the kmax value can be obtained by the relationship kmax ) 3kiso. Another commonly used method is fitting the k value with the tilt angle of the uniaxial Langmuir monolayer.35,44 These two methods can be called “spectrum fitting”. In our case, we used the 2916 cm-1 band collected by MTRIR spectroscopy at both s and p polarizations for the simulation of kmax and the tilt angle of AA molecules. According to Figure 4, kmax was estimated to be around 0.65. From our theoretical treatment, the fitting curves of As and Ap versus tilt angle can be independently obtained by varying kmax. For example, two groups (As and Ap) of five fitting curves with kmax ) 0.62, 0.63,
MTR-IR Spectroscopy of Arachidic Acid LB Films on Si
Figure 4. Simulated and experimental data of the band at 2916 cm-1 presenting a linear relationship between absorbance and N at (a) 50°, (b) 60°, and (c) 70° incidence. Two key parameters, tilt angle and kmax, were predicted according to previous works. Three kmax values, 0.55, 0.65, and 0.75, were chosen for fitting. By comparison of the experimental data (black lines) with the simulated data, a kmax value of ∼0.65 was determined for the AA monolayer on a hydrophilic silicon surface. At each kmax value, three possible tilt angles (10°, blue; 20°, green; 30°, red) were used for fitting. Among the results, the absorbances simulated with kmax ) 0.65 and γ ) 10° are the closest ones to the experimental values. However, kmax ) 0.65 and γ ) 10° are roughly evaluated here, and a much more delicate simulation for kmax and γ is described in Figure 6 in the next section. The parameters used for simulation are as follows: film thickness ) 2.8 nm, nLB ) 1.5, nsi ) 3.42, nair ) 1.0, and nAu ) 2.018 + 21.087i.
0.638, 0.64, and 0.65 are presented in Figure 6a. For the same sample, our experimental absorbance results were As ) 0.02000
J. Phys. Chem. C, Vol. 114, No. 1, 2010 337 and Ap ) 0.01446. Using the experimental values of As and Ap in the simulation equations, we have a system of two independent nonlinear equations [As (or Ap) relates to kmax and γ]. Thus, two unknowns (kmax and γ) can be solved from the two equations. In Figure 6a, we applied the fitting curves to obtain the approximate solutions of the two unknowns (kmax and γ). Only at kmax ) 0.638, could the same tilt angle of 16.5° be obtained both from s- and p-polarized spectra simultaneously. When other kmax values were used, we obtained two different tilt angles for the same sample at s and p polarizations. For example, for kmax ) 0.62, the tilt angle at s polarization is 9°, whereas that at p polarization is 19°. A discrepancy of 10° exists for the same sample, which is not valid. We also applied the “DR fitting” method described in our previous report to determine the tilt angle, where DR was defined as DR ) As/Ap.1 Our experimental result of DR ) As/ Ap ) 1.383 corresponds to a tilt angle of 16.50° ( 0.03°, within a kmax range from 0.62 to 0.65 in the inset of Figure 6b. This value is in accordance with other people’s reports.7,12 Infrared spectroscopy is able to distinguish the crystalline arrangement of hydrocarbon chains due to the fact that the crystal field splitting of the methylene scissoring δ(CH2) at ∼1470 cm-1 is very sensitive to the crystallographic subcell packing.9,36,45 For example, an orthorhombic subcell packing in orthorhombic or monoclinic crystals is associated with splitting of the δ(CH2) scissoring band, whereas a hexagonal subcell packing in hexagonal or pseudohexagonal crystals is associated with a δ(CH2) singlet. For the monolayer film on the hydrophilic silicon surface in Figure 5b, only a singlet at 1468 cm-1 was observed at all polarizations [the small peak at 1437 cm-1 for 0° polarization is due to carboxylic symmetric stretching15,46 and not to the splitting of δ(CH2) scissoring]. The singlet band indicates a hexagonal or pseudohexagonal packing of hydrocarbon chains. In addition, the scissoring δ(CH2) is in the plane parallel to the substrate surface; therefore, it evolves to larger strengths from 0° to 90° polarization. Because we deposited pure fatty acid AA molecules on silicon, the COOH peak was expected to appear near 1700 cm-1, rather than an arachidate anion to appear near 1545 cm-1.15,37,47,48 In Figure 5b, a broad band between 1600 and 1800 cm-1 is assigned to the carboxylic CdO stretching vibration. This peak can be deconvoluted into three peaks at 1732, 1714, and 1700 cm-1. The peak at 1732 cm-1 indicates that there are some interactions between Si-OH from the hydrophilic silicon surface and COOH from the AA molecules. The appearance of peaks at 1714 and 1700 cm-1 suggests that large numbers of carboxylic acid species are present on the hydrophilic silicon surface. The gradual decrease of the 1714 cm-1 peak from 0° to 90° polarization indicates that the transition dipole moment of carboxylic asymmetric stretching is tilted more toward the surface normal. The carboxylic symmetric stretching at 1437 cm-1 can be observed only as a small trace in the 0° polarized spectrum, and it decreases dramatically as the polarization changes from 0° to 90°, which also supports the conclusion that the carboxylic stretching transition dipole moment is in the surface normal direction of the silicon chip. 3.4. AA Multilayers on Silicon Substrates. To obtain more information about the molecular structure and orientation, we deposited two kinds of multilayers on silicon. One was a Z-type multilayer on the hydrophilic (SiO2/Si) substrate, which was determined by transfer ratios of less than 0.2 during the downstroke and near 1.0 during the upstroke in the deposition process. The other was the Y-type multilayer on the hydrophobic (Si-H/Si) substrate, which was determined by transfer ratios
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Figure 5. Polarized MTR-IR spectra of the AA monolayer on a hydrophilic silicon surface at 70° incidence and N ) 10: (a) 2800-3000 and (b) 1150-1850 cm-1.
Figure 6. Calculated (a) As (upper part) and Ap (lower part) and (b) DR against the tilt angle of AA molecules on silicon surfaces using five kmax values, 0.62, 0.63, 0.638, 0.64, and 0.65, for the peak at 2916 cm-1. The parameters for calculation are as follows: N ) 10, angle of incidence ) 70°, thickness of monolayer ) 2.8 nm, nLB ) 1.5, nsi ) 3.42, nair ) 1.0, and nAu ) 2.018 + 21.087i. The experimental As () 0.02000) and Ap () 0.01446) values in a are indicated as two horizontal black lines, and their intersection points with the simulated lines indicate the tilt angles. The experimental DR value (1.384) in the DR fitting curves in b indicates a tilt angle of 16.5°. The inset in b magnifies the intersection region of the small rectangle containing five simulation lines.
near 1.0 throughout the downstroke and upstroke processes (Figure S2 in the Supporting Information). The stronger absorption intensities of multilayers can clarify weak bands below 1800 cm-1, which are ambiguous in the spectra of the monolayer. Furthermore, we expect the fine differences between intermolecular interactions in Z-type films and those in Y-type films to be represented in their MTR-IR spectra. 3.4.1. Orientation and Crystal Structure of Hydrocarbon Chains in Multilayer Films. The hydrocarbon stretching bands of LB multilayer films are shown in Figures 7a and 8a. Obviously, larger numbers of monolayers give stronger absorbances. We list the DR values and tilt angles of the alkyl chain from different kinds of films in Table 1, using data for the CH2 asymmetric stretching band at 2916 cm-1. With increasing number of monolayers, the alkyl chain tilts more from the surface normal, as reported previously for LB films.49 The Z-type monolayer film has a tilt angle of 16.5°, 10° less than those of its corresponding multilayer films: 28°, 24°, and 26.5° for films of two, three, and four monolayers, respectively. In addition, the tilt angle of the Y-type film for the first two monolayers is less than 1°, which means that the alkyl chain is nearly perpendicular to the silicon surface. This is due to the formation of hydrogen bonds in the head-to-head COOH configuration between the first and second monolayers. In contrast, for the
Y-type film of four monolayers, a hydrophobic repulsion exists between the second and third monolayers, and thus, it results in a relatively larger tilt angle for the additional bilayer film. The δ(CH2) scissoring band in the film of Z-type multilayers, shown in Figure 7b, is different from that of the corresponding one-monolayer film. It splits the one-monolayer singlet of 1468 cm-1 into two bands at 1471 and 1464 cm-1. This is caused by the crystal field from the crystalline packing of hydrocarbon chains. The splitting gradually becomes obvious with increasing number of monolayers. For the film of two monolayers in Figure 7b, the band exhibits a broader shape than in the monolayer film. The δ(CH2) scissoring band of the second monolayer can be obtained by subtraction of the one-monolayer spectrum from the integrated two-monolayer spectrum. This is illustrated in the inset, where the two split peaks can be clearly seen. This is due to the orthorhombic or monoclinic crystal structure of the second monolayer, which differs from the pseudohexagonal structure of the first monolayer. As the number of monolayers is increased to 3 and 4, this splitting can be easily distinguished without the subtraction treatment. From the tilt angle of 26° ( 2° for the films of two to four monolayers in Table 1, we conclude that the second, third, and fourth monolayers are arranged with nearly the same structure: a
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Figure 7. Polarized MTR-IR spectra of Z-type AA films at 70° incidence and N ) 10: (a) 2700-3050 and (b) 1150-1850 cm-1.
Figure 8. Polarized MTR-IR spectra of Y-type AA films at 70° incidence, N ) 10: (a) 2700-3050 and (b) 1150-1850 cm-1.
TABLE 1: DR Values and Tilt Angle of Alkyl Chains Obtained from the CH2 Asymmetric Stretching Band at 2916 cm-1 film type (substrate)
AA film (no. of layers)
DR (As /Ap)
tilt angle (deg)
Z-type (SiO2/Si) Z-type (SiO2/Si) Z-type (SiO2/Si) Z-type (SiO2/Si) Y-type (Si-H/Si) Y-type (Si-H/Si)
1 2 3 4 2 4
1.383 1.144 1.255 1.158 1.612 1.309
16.5 28 24 26.5
