5826
Langmuir 1998, 14, 5826-5833
Control of Structural Order in Self-Assembled Zirconium Alkylphosphonate Films T. Vallant, H. Brunner, U. Mayer, and H. Hoffmann* Institute of Inorganic Chemistry, Vienna University of Technology, Getreidemarkt 9, A-1060 Wien, Austria Received April 23, 1998. In Final Form: July 10, 1998 Monolayer films of zirconium alkylphosphonates Zr(O3P-R) (R ) C14H29, C18H37, C22H45) with different long-chain alkyl groups have been prepared by adsorption from dilute solutions of the corresponding alkylphosphonic acids (molecular self-assembling) onto different primer mono- and multilayers on gold and silicon substrates. The surface orientation of the film molecules and the film structure has been investigated as a function of the hydrocarbon chain length of the adsorbate molecules, the composition and structure of the primer layer, and the adsorption temperature, using ellipsometry and infrared spectroscopy in the external reflection mode. Structural order decreases with decreasing hydrocarbon chain length and increasing temperature, spanning the range from a densely packed, highly ordered structure for the docosyl (C22) compound adsorbed at low temperatures (7 °C), exhibiting close-to-vertical chain orientation and a surface coverage near the theoretical maximum, to a liquidlike isotropic structure for the tetradecyl (C14) compound. Similarly, disorder in short-chain primer layers degrades the overlayer film structure. This effect is particularly noticeable in multilayer films of R,ω-biphosphonic acids used as a template for a terminal docosylphosphonate sample layer, where disorder increases rapidly with the number of layers in the template film. Compared to other self-assembling adsorbates, the structure of alkylphosphonate monolayers is controlled by similar parameters, but more rigorous conditions (long hydrocarbon chains, well-ordered primer layers, low adsorption temperatures) are required to achieve the same densely packed and highly ordered film structures.
Introduction Monolayer and multilayer films of transition metal alkylphosphonates on different solid substrates have first been described by Mallouk and co-workers in 19881,2 as a new class of self-assembling adsorbates and as a promising alternative to the classical Langmuir-Blodgett (LB) films, which suffer from several limitations such as the time-consuming and tedious preparation procedure or the generally low chemical and mechanical stability. In contrast, metal alkylphosphonates can be adsorbed as strongly bound monolayer films on a variety of different substrates by a simple two-step procedure, consisting of dipping an appropriately functionalized substrate alternately into solutions of a suitable metal ion (e.g., Zr4+, Hf4+, Zn2+, Cu2+) and of a long-chain alkylphosphonic acid. Using R,ω-biphosphonic acids in the second step opened up a simple, repetitive procedure for the fabrication of multilayer films with precisely controllable thickness and layer sequence, which has been exploited over the past decade for a number of different applications3,4 including nonlinear optical materials,5-7 electroluminescent devices,8 immobilization of biomolecules,9 or solar energy storage devices based on photoinduced electron-transfer reactions.10-13 (1) Lee, H.; Kepley, L. J.; Hong, H.-G.; Mallouk T. E. J. Am. Chem. Soc. 1988, 110, 618. (2) Lee, H.; Kepley, L. J.; Hong, H.-G.; Akhter, S.; Mallouk, T. E. J. Phys. Chem. 1988, 92, 2597. (3) Katz, H. E. Chem. Mater. 1994, 6, 2227. (4) Thompson, M. E. Chem. Mater. 1994, 6, 1168. (5) Katz, H. E.; Wilson, W. L.; Scheller, G. J. Am. Chem. Soc. 1994, 116, 6636. (6) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991, 254, 1485. (7) Hanken, D. G.; Corn, R. M. Anal. Chem. 1995, 67, 3767. (8) Katz, H. E.; Bent, S. F.; Wilson, W. L.; Schilling, M. L.; Ungashe, S. B. J. Am. Chem. Soc. 1994, 116, 6631. (9) Xu, X.-H.; Yang, H. C.; Mallouk, T. E.; Bard, H. J. J. Am. Chem. Soc. 1994, 116, 8386.
The primary concern for all these applications is the degree of structural order achievable in an adsorbate film of given thickness and composition. Related studies on other self-assembling adsorbate systems14 such as longchain organosulfur and organosilane compounds have shown that structural order is primarily controlled by chain-chain interactions and is therefore directly related to the orientation and packing density of the hydrocarbon chains in these films. Various parameters such as chain length, adsorption temperature, substrate surface composition, adsorption time, etc. have been shown to influence the film structure and, if carefully controlled, allow a highly reproducible fabrication of crystalline-like, well-ordered films. In contrast, alkylphosphonate films have been repeatedly characterized as comparatively disordered,15-20 containing a large number of structural defects. Several studies have been devoted to the identification and optimization of structure-controlling parameters in this particular adsorbate system.15,17,18,20,21 General agreement has emerged from these reports that alkylphosphonate (10) Ungashe, S. B.; Wilson, W. L.; Katz, H. E.; Scheller, G.; Putvinski, T. M. J. Am. Chem. Soc. 1992, 114, 8717. (11) Vermeulen, L. A.; Snover, J. L.; Sapochak, L. S.; Thompson, M. E. J. Am. Chem. Soc. 1993, 115, 11767. (12) Snover, J. L.; Thompson, M. E. J. Am. Chem. Soc. 1994, 116, 765. (13) Vermeulen, L. A.; Thompson, M. E. Chem. Mater. 1994, 6, 77. (14) Ulman, A. Chem. Rev. 1996, 96, 1533. (15) Schilling, M. L.; Katz, H. E.; Stein, S. M.; Shane, S. F.; Willson, W. L.; Buratto, S.; Ungashe, S. B.; Taylor, G. N.; Putvinski, T. M.; Chidsey, C. E. D. Langmuir 1993, 9, 2156. (16) Yang, H. C.; Aoki, K.; Hong, H.-G.; Sackett, D. D.; Arendt, M. F.; Yau, S.-L.; Bell, C. N.; Mallouk, T. E. J. Am. Chem. Soc. 1993, 115, 11855. (17) O’Brien, J. T.; Zeppenfeld, A. C.; Richmond, G. L.; Page, C. J. Langmuir 1994, 10, 4657. (18) Frey, B. L.; Hanken, D. G.; Corn, R. M. Langmuir 1993, 9, 1815. (19) Zeppenfeld, A. C.; Fiddler, S. L.; Ham, W. K.; Klopfenstein, B. J.; Page, C. J. Am. Chem. Soc. 1994, 116, 9158. (20) Bent, S. F.; Schilling, M. L.; Wilson, W. L.; Katz, H. E.; Harris, A. L. Chem. Mater. 1994, 6, 122.
S0743-7463(98)00462-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/09/1998
Control of Structural Order
films are intrinsically more disordered than other classes of self-assembled films, which was ascribed to certain inherent properties such as the bulky metal phosphonate headgroups, the sterically demanding binding geometry of the metal ions, and their relatively easy displacement and dissolution in the intermediate, surface-exposed state. The motivation for this work was therefore the question whether these unfavorable properties can be overcome by a suitable choice of film preparation parameters such as the hydrocarbon chain length of the film molecules, the composition and structure of the primer layer, or the film adsorption temperature. The influence of some of these parameters has been investigated before,16-18,20,21,41 but in only one report41 has an alkylphosphonate monolayer of comparable quality to highly organized, self-assembled films been fabricated, using a rather cumbersome combination of LB and self-assembling preparation techniques. In the present study, we have prepared zirconium alkylphosphonate films with a C14 alkyl group (tetradecyl), a C18 group (octadecyl), and a C22 group (docosyl) by self-assembling from dilute solutions of the corresponding alkylphosphonic acids and adsorbed them onto different primer layers on two types of substrates, a metal (gold) and a dielectric (silicon) substrate, using ellipsometry and infrared spectroscopy in the external reflection mode for film characterization. As we have shown previously,22-24 external reflection IR spectra on silicon are particularly sensitive for the detection of structural transitions in monolayer films. Materials and Methods Compounds, Solvents, and Substrates. The following compounds and solvents were commercially available and were used as received: Octadecyltrichlorosilane (Aldrich, 95%), phosphorus oxychloride (Aldrich, 99%), zirconyl chloride octahydrate (Aldrich, 98%), 2,4,6-collidine (Aldrich, 99%), hydrochloric acid (Aldrich, 37% aqueous solution), lithium aluminum hydride (LiAlH4, Aldrich, 1.0 M solution in THF), n-hexane (Aldrich, 95%), toluene (Aldrich, 99.8%), acetone (Aldrich, 99.9%), acetonitrile (Aldrich, 99.9%), and ethanol (Austria Hefe AG, 99.8%). 11-Hydroxyundecanethiol (HUDT),11-hydroxyhexadecanethiol (HHDT), and methyl 11-(trichlorosilyl)undecanoate (CH3OOC(CH2)10SiCl3, MTSUD) were synthesized as described previously.22,25 1,12-Dodecanediylbis(phosphonic acid) ((OH)2OP(CH2)12-PO(OH)2, di-DDPA) was prepared by the MichaelisArbuzov reaction26 of 1,12-dibromododecane (Aldrich, 98%) with triethyl phosphite (Aldrich, 99%). 1,12-Dibromododecane (0.017 mol) and P(OC2H5)3 (0.042 mol) were refluxed at 140 °C for 3 h with evolution of ethyl bromide. Excess triethyl phosphite and the byproduct C2H5PO(OC2H5)2 were removed under vacuum, and the residual solution was cooled to room temperature. Twenty-five milliliters of concentrated (37%) aqueous HCl solution was added, and heating was resumed at 100 °C for 14 h. The solvent was evaporated, and the white residue was washed several times with acetonitrile and dried in a vacuum to give 4.4 g (88%) of 1,12-dodecanediylbis(phosphonic acid): 1H NMR (D2O) δ(ppm) ) 1.1-1.5 (m, 20 H), 1.65-1.75 (m, 4 H, -CH2-P-); 31P NMR (D2O) δ(ppm) ) 28.2. NMR spectra were measured on a Bruker 250 MHz spectrometer, and the chemical shifts were referenced against external tetramethylsilane (1H NMR) or external phosphoric acid (31P NMR). The monophosphonic acids (21) Byrd, H.; Whipps, S.; Pike, J, K.; Ma, J.; Nagler, S. E.; Talham, D. R. J. Am. Chem. Soc. 1994, 116, 295. (22) Hoffmann, H.; Mayer, U.; Krischanitz, A. Langmuir 1995, 11, 1304. (23) Brunner, H.; Vallant, T.; Mayer, U.; Hoffmann, H. Surf. Sci. 1996, 368, 279. (24) Brunner, H.; Mayer U.; Hoffmann H. Appl. Spectrosc. 1997, 51, 209. (25) Brunner, H.; Vallant, T.; Mayer, U.; Hoffmann, H. Langmuir 1996, 12, 4614. (26) Bhattacharya, A. K.; Thyagarajan, G. Chem. Rev. 1981, 81, 415.
Langmuir, Vol. 14, No. 20, 1998 5827
Figure 1. Preparation scheme for zirconium alkylphosphonate films on silicon and gold substrates primed with HO-terminated monolayers docosylphosphonic acid (DCPA), octadecylphosphonic acid (ODPA), and tetradecylphosphonic acid (TDPA) were synthesized in the same way as di-DDPA using the corresponding monobromides 1-bromodocosane (Aldrich, 96%), 1-bromooctadecane (Aldrich, 96%), and 1-bromotetradecane (Aldrich, 97%). After being washed with acetonitrile, the monophosphonic acids were recrystallized twice from n-hexane and dried under vacuum. DCPA: 31P NMR (EtOD) δ(ppm) ) 31.2, Fp ) 99 °C; ODPA: 31P NMR (EtOD) δ(ppm) ) 28.8, Fp ) 96 °C (98.5 °C27,28); TDPA: 31P NMR (EtOD) δ(ppm) ) 29.7, Fp ) 94 °C (96 °C 27,28). Gold-coated glass slides (25 × 15 mm2) with preferential (111) orientation, which had been prepared by sputter deposition of approximately 200 nm of gold onto glass slides coated with a thin chromium adhesive layer, were obtained from Pharmacia (Uppsala, Sweden) and were cleaned by ultrasonic treatment in toluene, rinsing with acetone and ethanol, and blow-drying in high-purity nitrogen followed by a 15 min exposure to a UV/ ozone atmosphere in a commercial cleaning chamber (Boekel Industries, model UVClean) equipped with a low-pressure mercury quarz lamp (λmax ) 185 and 254 nm). p-doped, (100) oriented and single-sided polished silicon wafers (Wacker Chemitronic, test grade, 14-30 Ω cm resistivity, 0.5 mm thickness) were cut into pieces of appropriate size (25 × 18 mm2) and were cleaned in the same way as the gold slides. This treatment yields a hydrophilic, contamination-free surface with a native oxide layer of 12-14 Å thickness, as routinely checked by ellipsometry. Preparation of the Adsorbate Films. HO-Terminated Primer Layers on Gold and Silicon. Monolayers of 11-hydroxyundecanethiol (HUDT) and 16-hydroxyhexadecanethiol (HHDT) were formed on gold substrates by immersing the freshly cleaned substrates for about 30 min into 1 mmol L-1 solutions of the thiol compounds in n-hexane, followed by a standard cleaning procedure consisting of sonication in toluene, rinsing with acetone and ethanol, and drying in a stream of high-purity nitrogen. A monolayer of 11-hydroxyundecylsiloxane (HUDS, HO(CH2)11SiOx) on native silicon was prepared, as described previously,25 from a precursor monolayer of the corresponding undecanoic acid methyl ester (CH3OOC(CH2)10SiOx) by in-situ reduction of the terminal ester groups with LiAlH4. Zirconium Alkylphosphonate Overlayers. Figure 1 shows schematically the procedure that was used in this study to couple overlayers of different alkylphosphonic acids to HO-terminated primer layers. The terminal hydroxyl groups of the primer layers were first converted in a dry nitrogen atmosphere to phosphoric acid groups (-O-PO(OH)2) by a 2 h immersion in a solution of (27) Moedritzer, K.; Irani, R. R. J. Inorg. Nucl. Chem. 1961, 22, 297. (28) Moedritzer, K.; Maier, L.; Groenweghe, L. C. D. J. Chem. Eng. Data 1962, 7, 307.
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Table 1. Band Assignments and Peak Wavenumbers (in cm-1) in IR Reflection Spectra and Ellipsometric Film Thicknesses of Alkylphosphonate Overlayers Coupled to Different Primer Layers on Gold and Silicon Substrates sample vibrations sample layer
adsorption temp (°C)
νs(CH2)
νas(CH2)
Si/UDSPA-Zr/ Si/UDSPA-Zr/di-DDPA-Zr/
UDTPA-Zr TDPA ODPA DCPA HDTPA-Zr TDPA ODPA DCPA UDSPA-Zr TDPA TDPA TDPA ODPA ODPA ODPA DCPA DCPA DCPA di-DDPA-Zr DCPA
22 22 22 22 22 22 22 22 22 7 22 70 7 22 70 7 22 70 22 22
Si/UDSPA-Zr/di-DDPA-Zr/ Si/UDSPA-Zr/(di-DDPA-Zr)2/
di-DDPA-Zr DCPA
22 22
2856 2858 2858 2850 2851 2857 2850 2849 2859 2858 2858 2858 2850 2851 2858 2850 2850 2850 2858 2847a 2858a 2857 2956
2925 2926 2926 2919 2922 2927 2920 2918 2930 2929 2929 2929 2918 2920 2929 2919 2919 2918 2931 2917a 2930a 2930 2929
primer layer Au/ Au/UDTPA-Zr Au/ Au/HDTPA-Zr Si/ Si/UDSPA-Zr
a
νs(CH3)
νas(CH3)
(2880) 2880 2880
2963 2967 2967
2880 2880 2880
2963 2966 2966
b b b 2880 2880 2882 2880 b b
2968 2967 2967 2966 2967 2967 2968 2967 2967
2880
2966
22.1 16.7 22.1 29.1 29.2 16.7 23.0 29.2 20.2 17.0 17.1 16.9 24.1 23.1 22.2 29.4 28.6 28.0 25.9 28.2
2880
2967
25.7 27.3
sample thickness d [Å]
Band splitting. b Insufficient signal-to-noise ratio in the IR spectra.
Infrared Reflection Spectra. External reflection infrared (ERIR) spectra were measured with a custom-made reflection optical system connected to a Mattson RS FT-IR spectrometer equipped with a narrow-band MCT detector, as described in detail elsewhere.30 p-polarized light at an incidence angle of 80° was chosen for both gold and silicon substrates. A total of 1024 scans at 4 cm-1 resolution were averaged for each spectrum from the film-covered substrate and the clean reference surface. The interferograms were apodized using a triangular function and were zero-filled to yield spectra with one data point per wavenumber. A linear baseline correction was applied to the raw spectra in order to remove baseline offsets and slopes due to spectrometer instabilities and/or irreproducible mounting of sample and reference.
POCl3 (5 mM) and 2,4,6-collidine (5 mM) in acetonitrile, followed by rinsing with acetonitrile, sonication in acetone and water for 1 min each, rinsing with ethanol, and drying with nitrogen. Subsequent exposure of the so-treated substrates to an aqueous solution of ZrOCl2 (5 mM) for 30 min yielded the activated primer layers as zirconium phosphates of ω-hydroxyundecanethiol (UDTPA-Zr) and ω-hydroxyhexadecanethiol (HDTPA-Zr) on gold and of ω-hydroxyundecylsiloxane (UDSPA-Zr) on silicon. These primer layers were subsequently coupled directly to overlayers of different alkylphosphonic acids by overnight immersion in a 1 mM ethanolic solution of TDPA, ODPA, or DCPA. Alternatively, spacer layers of di-DDPA-Zr were adsorbed on top of the primer layers by alternate exposure to aqueous solutions of di-DDPA (1 mM) overnight and ZrOCl2 (5 mM) for 30 min and thorough rinsing with water, acetone, and ethanol after each step, yielding one layer of di-DDPA-Zr per adsorption cycle. These spacer layers were capped with terminal layers of DCPA as described above. Ellipsometric Measurements. Ellipsometric film thickness measurements were carried out on a Plasmos SD 2300 ellipsometer with a rotating analyzer and a He-Ne laser (λ ) 632.8 nm) at 68° incidence as the light source. Slightly different measurement protocols were used for gold and silicon substrates. On gold, the substrate’s optical constants (refractive index n, absorption coefficient k) were first determined using an isotropic model of two semiinfinite phases (air/substrate). Values of n ) 0.165 ( 0.005 and k ) 3.57 ( 0.01 were reproducibly obtained. The thickness of an adsorbate film of the film-covered substrates was then measured using an isotropic three-phase model (air/ adsorbate/substrate) with the previously determined optical parameters n and k for the gold substrate and assumed values of n ) 1.50 and k ) 0 for the adsorbate, the latter representing typical values for alkylphosphonate films7,19 as well as for other transparent, solid-state organic materials.29 On silicon, an isotropic three-phase model (Si/SiO2/air) was used for the substrate and a four-phase model (Si/SiO2/adsorbate/ air) for the sample. A measurement of the clean reference Si/SiO2 yielded the thickness of the native oxide layer using literature values23 for the optical constants of Si (n ) 3.865, k ) 0.020) and SiO2 (n ) 1.465, k ) 0). Subsequent measurement of the film-covered substrate and substitution of the substrate parameters together with assumed values for the adsorbate (n ) 1.50 and k ) 0) into the four-phase algorithm allowed the calculation of the adsorbate layer thickness.
Ellipsometry. Film thicknesses of the alkyl phosphonate overlayers on gold and silicon substrates were determined by measuring the overall thickness of the adsorbate layers (primer layer + sample layer) and subtracting the thickness of the particular primer layer. The results are listed in the last column of Table 1. The experimental accuracy and reproducibility can be estimated from the measured thicknesses of the primer layers UDTPA-Zr (22.1 Å), HDTPA-Zr (29.2 Å), and UDSPAZr (20.2 Å), which were obtained as average values from a large number of samples with a variation of less than ( 0.5 Å. The measured thicknesses of the alkylphosphonate overlayers in Table 1 depend primarily on the chain length of the alkyl substituent. Values between 16.7 and 17.1 Å for TDPA, 22.1-23.1 Å for ODPA, and 28.6-29.2 Å for DCPA were reproducibly obtained for room temperature adsorption (22 °C) on either Au/UDTPAZr, Au/HDTPA-Zr, or Si/UDSPA-Zr without any noticeable effect of the substrate or the particular primer layer underneath the alkylphosphonate film. These measured thicknesses increase linearly with the number of carbon atoms of the alkyl groups with an increment of approximately 1.5 Å per C atom. A small, but significant influence of the adsorption temperature on the thickness
(29) Parikh, A. N.; Allara, D. L.; Azouz, I. B.; Rondelez, F. J. Phys. Chem. 1994, 98, 7575.
(30) Hoffmann, H.; Mayer, U.; Brunner, H.; Krischanitz, A. Vib. Spectrosc. 1995, 8, 151.
Results
Control of Structural Order
Langmuir, Vol. 14, No. 20, 1998 5829
Figure 2. Infrared reflection spectra of tetradecyl-(TDPA), octadecyl-(ODPA) and docosyl-(DCPA) phosphonate overlayers adsorbed on (A) gold substrates primed with -S(CH2)11OPO3-Zr (UDTPA-Zr); (B) gold substrates primed with -S(CH2)16OPO3-Zr (HDTPAZr); (C) silicon substrates primed with -OxSi(CH2)11OPO3-Zr (UDSPA-Zr). The spectra of the primer layers are shown in the bottom part of each figure. Band assignments and peak frequencies are listed in Table 1.
of alkylphosphonate overlayers on Si/UDSPA-Zr primer layers is clearly noticeable. Both DCPA and ODPA films show a thickness decrease with increasing adsorption temperature from 29.4 to 28.0 Å (DCPA) and 24.1 Å to 22.2 Å (ODPA) between 7 and 70 °C. The thickness of TDPA films, on the other hand, remains essentially constant at 17.0 ( 0.1 Å within this temperature range. A subtle trend is finally observable for DCPA overlayers on UDSPA-Zr primed silicon substrates with additional biphosphonic acid (di-DDPA-Zr) spacer layers between the primer layer and the sample layer: With an increase of the overall underlayer thickness from 20.2 Å (UDSPAZr, no spacer layer) to 46.1 Å (UDSPA-Zr/di-DDPA-Zr, one spacer layer) and to 71.8 Å (UDSPA-Zr/di-DDPAZr/di-DDPA-Zr, two spacer layers), the DCPA overlayer thickness decreases from 28.6 Å (no spacer layer) to 28.2 Å (one spacer layer) and to 27.3 Å (two spacer layers). Infrared Spectroscopy. Alkylphosphonate Overlayers on Monolayer-Coated Gold and Silicon Substrates. Figure 2 shows IR reflection spectra of three different alkylphosphonate overlayers (TDPA, ODPA, DCPA) on Au/UDTPA-Zr (Figure 2A), Au/HDTPA-Zr (Figure 2B), and Si/UDSPA-Zr (Figure 2C) primer layers, which were obtained by subtracting the primer layer spectra shown in the bottom parts of Figure 2 from the complete adsorbate film spectra (primer layer + sample layer). Only the CHstretching region is presented here, since the lower frequency absorptions remain essentially unchanged and contain no additional information about the film properties. Four main absorption peaks (ν(CH2) and ν(CH3) stretching vibrations of the hydrocarbon chains) appear in the spectra of Figure 2, whose peak maxima and band assignments are listed in Table 1. On gold substrates primed with either UDTPA-Zr or HDTPA-Zr, the overlayer spectra appear fairly similar despite substantial differences in the absolute band intensities. In both cases, the CH2 intensities νas(CH2) and νs(CH2) decrease with increasing hydrocarbon chain length (TDPA f ODPA f DCPA), opposite to the intrinsic absorption coefficients (k-values), which increase approximately linear with the number of CH2 groups. Since the band intensities in IR reflection spectra on metal substrates depend on both the k-values and the surface orientation of the transition dipole moment vectors, this intensity decrease with increasing k-values indicates some significant structural rearrange-
ments within this series of homologeous film molecules, which will be discussed in detail later. In addition to the intensity changes in parts A and B of Figure 2, also the peak maxima for νs(CH2) and νas(CH2) shift from 2858 and 2927 cm-1 for TDPA films to 2850 and 2919 cm-1 for DCPA on both UDTPA-Zr and HDTPA-Zr primer layers, whereas the corresponding band positions for the intermediate compound ODPA shift from high frequencies on UDTPA to low frequencies on HDTPA. The most significant influence of the primer layer is expressed in the absolute band intensities: The same alkylphosphonate overlayer shows less than half the ν(CH2) band intensities on Au/HDTPA-Zr in comparison to Au/UDTPA-Zr as a primer layer. In contrast, the CH3 intensities of νas(CH3) and νs(CH3) are practically equal between parts A and B of Figure 2, which rules out a significant surface coverage effect on the band intensities. Completely different spectra are obtained for the primer layer UDSPA-Zr and the alkylphosphonate overlayers on a silicon substrate (Figure 2C). Both the primer layer and the TDPA overlayer show inverted absorption spectra with downward-pointing ν(CH2) bands around 2860 and 2930 cm-1. With an increase of the chain length of the alkylphosphonate molecules, these absorptions turn into positive, upward-pointing peaks at 2850 and 2920 cm-1 for ODPA and further grow in the positive direction for DCPA. The ν(CH3) peaks appear as fairly weak, downward pointing absorptions with constant intensities and peak wavenumbers (νas(CH3) at 2967 cm-1 and νs(CH3) at 2880 cm-1) within this series. The main reason for these large spectral differences of alkylphosphonate overlayers on gold and silicon substrates lies in the different optical properties of a metal (gold) and a dielectric (silicon) substrate, which will be discussed in detail later. Figure 3 shows the influence of the adsorption temperature on IR reflection spectra of different alkylphosphonate overlayers on Si/UDSPA-Zr. Each film of the three homologeous compounds TDPA, ODPA, and DCPA was prepared at 7, at 22, and at 70 °C, and the corresponding IR spectra shown in Figure 3 were obtained at room temperature after subtraction of the primer layer spectrum. The band assignments and peak maxima are included in Table 1. A first inspection of the spectral changes in Figure 3 reveals strong similarities to a variation of the hydrocarbon chain length at room tem-
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Figure 4. Infrared reflection spectra of docosylphosphonate overlayers on silicon substrates primed with UDSPA-Zr and different numbers n of O3P-(CH2)12-PO3-Zr (di-DDPA-Zr) spacer layers (n ) 0, Si/UDSPA-Zr/DCPA; n ) 1, Si/UDSPAZr/di-DDPA-Zr/DCPA; n ) 2, Si/UDSPA-Zr/di-DDPA-Zr/diDDPA-Zr/DCPA). The spectra are slightly offset for clarity. Band assignments and peak frequencies are listed in Table 1.
derivative-shaped absorption profiles in the νs(CH2) and νas(CH2) region with fairly weak, positive maxima at low wavenumbers (2917 and 2847 cm-1) and more intense, negative peaks at higher wavenumbers (2858 and 2930 cm-1). Figure 3. Infrared reflection spectra of TDPA, ODPA, and DCPA overlayers adsorbed on UDSPA-Zr primed silicon substrates at three different temperatures, 7 °C (I), 22 °C (II), and 70 °C (III). Band assignments and peak frequencies are listed in Table 1.
perature (Figure 2C). ODPA overlayers, for example, yield a typical “short-chain” spectrum with downward-pointing ν(CH2) absorptions at 2860 cm-1 (νs(CH2)) and 2930 cm-1 (νas(CH2)) at 70 °C, which convert into a “long-chain” spectrum with positive, red-shifted bands at 2850 cm-1 (νs(CH2)) and 2920 cm-1 (νas(CH2)) at lower temperatures (22 and 7 °C). The TDPA spectra, on the other hand, remain short-chain type (downward-pointing ν(CH2) bands at high wavenumbers) throughout the investigated temperature range with slightly increasing band intensities (growth in the negative direction) with increasing temperature, whereas the DCPA spectra remain long-chain type (upward-pointing ν(CH2) bands at low wavenumbers) with a significant intensity increase (growth in the positive direction) with decreasing adsorption temperature. The ν(CH3) absorptions in Figure 3 appear again as fairly weak, downward pointing features around 2967 cm-1 (νas(CH3)) and 2880 cm-1 (νs(CH3)) with little changes in either position or intensity. Docosylphosphonate Overlayers on Multilayer-Coated Silicon Substrates. Figure 4 shows IR reflection spectra of DCPA overlayers on UDSPA-Zr primed silicon substrates with different numbers n of di-DDPA-Zr spacer layers inserted between the primer layer and the DCPA capping layer. The corresponding band assignments and peak frequencies are listed in the bottom part of Table 1. Without a spacer layer (n ) 0), the typical long-chain spectrum with two intense, positive ν(CH2) bands at 2850 cm-1 (νs(CH2)) and 2919 cm-1 (νas(CH2)) is obtained, which converts completely into a short-chain spectrum (downward-pointing ν(CH2) peaks shifted to higher wavenumbers) for two spacer layers (n ) 2). For n ) 1, a mixture between these two types of spectra is obtained, yielding
Discussion Structural Effects in IR Reflection Spectra on Metal and Nonmetal Substrates. To understand and interpret the IR spectroscopic results presented here and their implications with respect to the structure of alkylphosphonate monolayers, we have to review briefly the fundamental relationships between the surface orientation of adsorbate molecules and their band intensities in IR reflection spectra on metal and nonmetal substrates. A detailed account on this subject can be found in several previous publications.22-24,31 We will restrict the discussion here to the ν(CH2) stretching vibrations of adsorbate molecules with long hydrocarbon chains, which provide the most accurate information about the hydrocarbon chain orientation and the overall film structure. Assuming a perfect all-trans geometry of the hydrocarbon chains on the substrate surface, the band intensities for νas(CH2) and νs(CH2) in a hypothetical monolayer film can be calculated as a function of the tilt angle β of the ν(CH2) dipoles with respect to the surface normal orsunder the simplifying assumption of rotational symmetry around the chain axis, for which βas ) βs ) βsas a function of the chain tilt angle R, which is related to β by a simple trigonometric relationship sin2 R ) 2 cos2 β.32 This is shown in Figure 5 for two different substrates, a metal (gold) and a dielectric (silicon) material, using p-polarized radiation at an incidence angle of 80°. Of primary interest for the present study are the spectral changes caused by structural transitions between a highly ordered film (31) Mielczarski, J. A. J. Phys. Chem. 1993, 97, 2649. (32) The hydrocarbon chain axis and the dipole moment vectors for νas(CH2) and νs(CH2) are mutually orthogonal to each other. Their tilt angles with respect to the surface normal (or any other arbitrary reference axis) therefore obey the relationship cos2R + cos2βas + cos2βs ) 1. For βas ) βs ) β, the chain tilt angle R and the dipole moment tilt angle β are related through sin2R ) 2 cos2 β.
Control of Structural Order
Langmuir, Vol. 14, No. 20, 1998 5831 Table 2. Experimental and Calculated Monolayer Thicknesses for Zirconium Alkylphosphonates with Different Alkyl Substituents compound
dell (Å)a
dcalc (Å)b
dcryst (Å)c
dell/dcalc (%)
Zr-TDPA (C14) Zr-ODPA (C18) Zr-DCPA (C22)
16.8 22.7 29.0
21.6 26.6 31.6
21.5 26.0 30.5d
77.8 85.3 91.8
a Average ellipsometric thicknesses for films prepared at room temperature on different primer layers (values from Table 1). b Calculated molecule length with an all-trans extended alkyl group.33 c Crystallographic layer thickness determined from solidstate Zr(HPO4)(RPO4) compounds.34 See text for details. d Extrapolated value from ref 34.
Figure 5. Calculated tilt angle dependence of the ν(CH2) intensities of a hydrocarbon compound adsorbed on a gold and a silicon substrate for p-polarized radiation incident at 80°. The dipole moment tilt angle β is assumed to be equal for the asymmetric and the symmetric mode of ν(CH2) and is related to the chain tilt angle R through sin2 R ) 2 cos2 β (see text). The following parameters have been used for these calculations: peak frequency, 3000 cm-1; adsorbate thickness, 10 Å; optical parameters (n + i*k, n, refractive index, k, absorption coefficient), 1.5 + i*0.1 (adsorbate), 3.0 + i*30 (gold), 3.42 + i*0 (silicon). See ref 23 for details about the calculations.
structure with close-to-perpendicular chain orientation (R ≈ 0°, β ≈ 90°) and a disordered, liquidlike structure, where the tilt angles R and β approach a uniform, isotropic value of 54.7° (“magic angle”) due to a completely random orientational distribution. It can be seen from Figure 5, that such an order-disorder transition on a metal substrate is accompanied by an intensity increase of the ν(CH2) absorptions, because only the perpendicular dipole moment component is probed on a metal surface, which is zero for β ) 90° and increases with decreasing β (increasing R). On silicon, on the other hand, both perpendicular and parallel dipole moment components can be probed, which gives rise to a more complex intensity vs tilt angle relationship, consisting of regions of positive, upward-pointing absorptions (67° < β < 90°) and regions of “negative”, downward-pointing peaks (β < 67°). For the specific case of a structural transition from β ≈ 90° to β ≈ 54.7°, a band inversion is predicted in Figure 5; i.e., the ν(CH2) absorption bands point upward for an ordered film structure (β ) 90°, R ) 0°) and point downward for a disordered structure (R ) β ) 54.7°). We will show in the following, that such structural order-disorder transitions in alkylphosphonate overlayers can be caused by different parameters such as the hydrocarbon chain length and the adsorption temperature as well as the structure of the underlying primer layer. Relationship between the Film Structure and the Hydrocarbon Chain Length in Alkylphosphonate Overlayers. We will first consider the effects of changing the hydrocarbon chain length in alkylphosphonate overlayers prepared at room temperature (22 °C) on different primer layers. The ellipsometric film thickness measurements (Table 1) yield, within experimental uncertainty, constant values for TDPA, ODPA, and DCPA films on three different primer layers (Au/UDTPA-Zr, Au/HDTPA-Zr, Si/UDSPA-Zr). In Table 2, the average of these values dell for each compound is compared to the calculated length dcalc of the molecule with an all-trans extended hydrocarbon chain33 and to the experimental film thickness dcryst derived from crystallographic literature data
for layered zirconium phosphates of the type (HOPO3)Zr(O3POR) (R ) n-alkyl).34 These latter compounds exhibit upon intercalation with the corresponding alcohols R-OH a highly ordered sheet structure, in which inorganic (HOPO3)Zr layers are separated by long-chain organic bilayers (O3POR)2 in a head to head configuration with the chain axes of the alkyl groups R oriented perpendicular to the Zr4+ plane. Half the lattice spacing between two Zr4+ layers therefore equals the thickness of a Zr(O3POR) layer with vertically aligned hydrocarbon chains. The crystallographic data dcryst in Table 2 agree nicely with the calculated lengths dcalc, whereas the ellipsometric thicknesses dell measured here are consistently lower by 2-5 Å. An interesting trend is observed for the ratios dell/dcalc, which are included in the last column of Table 2. Since dcalc represents the maximum thickness for a perfectly packed, densest assembly of vertically aligned film molecules, the ratio dell/dcalc is commonly used as a measure of the surface coverage.29 Thus, the corresponding data in Table 2 indicate that the surface coverage increases from 78% for the tetradecyl compound to 92% for the docosyl compound, which is accompanied by an increase in structural order, as will be discussed below. This trend also explains the unusually high slope of 1.5 Å per carbon atom for the ellipsometric film thicknesses as a function of the hydrocarbon chain length. Since the highest possible increment for vertical chain orientation is 1.26 Å/C-atom,16 the observed slope of 1.5 Å/C-atom contains as a second contribution the rise in surface coverage with increasing chain length. Much stronger changes with the hydrocarbon chain length are observed in the IR reflection spectra (Figure 2). Qualitatively, the spectra for TDPA, ODPA, and DCPA films on gold (parts A and B of Figure 2) are fairly similar. The most remarkable difference are the decreasing ν(CH2) (33) The distance between the zirconium ions and the first C atom of the alkyl group, derived from the layer spacing of (Zr(O3PH)2) and standard P-H and P-C bond lengths, amounts to 3.25 Å (ref 19). Each CH2 group contributes 1.26 Å and the terminal CH3 group an additional 1.92 Å (Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074). The length of a Zr (O3P-(CH2)nCH3) molecule can therefore be calculated as dcalc ) 5.17 + 1.26n for different, all-trans extended alkyl substituents. (34) Yamanaka, S.; Matsunaga, M.; Hattori, M. J. Inorg. Nucl. Chem. 1981, 43, 1343. Although the structure of γ-zirconium phosphate, from which the alkyl-substituted compounds were derived in this study, has later been shown to be incorrect (Alberti, G.; Bernasconi, M. G.; Casciola, M. React. Polym. 1989, 11, 245), the basal spacings d001 determined in this paper still represent the best experimental reference data for the ideal rigid rod structure (the uniform, densest packing of all-trans hydrocarbon chains with perpendicular chain axis orientation) of alkylphosphonate monolayers (see also ref 41). In comparison, R-zirconium phosphates, which directly form layered alkylphosphonates of the type Zr(O3P-R)2 (Alberti, G. In Comprehensive Supramolecular Chemistry; Alberti, G., Bein, T., Eds.; Elsevier: Oxford, 1996; Vol. 7, pp 151-187) and represent the closest monolayer analogues in terms of chemical composition, show larger tilt angles (>30°) of the alkyl groups and therefore yield significantly smaller layer spacings. We appreciate the valuable comments of one reviewer regarding these differences between R- and γ-ZrP structures.
5832 Langmuir, Vol. 14, No. 20, 1998
intensities with increasing chain length (TDPA > ODPA > DCPA). The exact opposite trend would be expected from the increase in surface coverage and absorption strength (k-values) with increasing number of CH2 groups per molecules. The observed spectral differences must therefore be caused by fundamental changes in the film structure within this series of homologeous compounds. DCPA films show only weak ν(CH2) intensities on both Au/UDTPA-Zr (Figure 2A) and Au/HDTPA-Zr (Figure 2B) in close similarity to reported spectra of self-assembled fatty acid monolayers on oxidized aluminum and copper surfaces,35 to self-assembled alkylsiloxane monolayers on gold surfaces,23,36 as well as to spectra of LangmuirBlodgett films of carboxylic acid salts on metal substrates.37 In all these adsorbate systems, the hydrocarbon chains exhibit a densely packed, uniform structure with the chain axes oriented close to perpendicular to the substrate surface (R ≈ 0°), resulting in a close to parallel orientation (β ≈ 90°) of the transition dipole moments for νas(CH2) and νs(CH2), which, according to Figure 5, leads to vanishingly small absorption intensities on a metal substrate. Upon reduction of the hydrocarbon chain length, the band intensities increase due to an increase of the average chain tilt angle accompanied by an increase of the perpendicular dipole moment components of the ν(CH2) vibrations. Concurrently, the peak maxima shift to higher wavenumbers, which signal emerging chain defects (trans-gauche isomerization) caused by a destruction of a crystalline-like, dense packing of the hydrocarbon chains. Thus, the film structure changes from a closeto-vertical molecule orientation toward a randomized, isotropic orientation with decreasing hydrocarbon chain length (DCPA f ODPA f TDPA). The exact same changes take place on Si/UDSPA-Zr primer layers (Figure 2C), although the optical properties of silicon as a nonmetal, dielectric material cause a completely different spectral response (Figure 5): The highly ordered DCPA film with close to vertical hydrocarbon chains shows positive ν(CH2) absorptions. With decreasing chain length (DCPA f ODPA), the average chain tilt angle R increases and the ν(CH2) intensities decrease and eventually invert (ODPA f TDPA) for R approaching the isotropic angle 54.7°. As on gold substrates, this structural transition is accompanied by a high-frequency shift of the ν(CH2) peak maxima (Table 1). Similar spectral changes have been detected in previous studies of alkylsiloxane monolayer films on silicon 22,23,38,39 and have been shown to be highly specific for structural order-disorder transitions as a function of the hydrocarbon chain length and the surface coverage. Influence of the Primer Layer Structure. A comparison of the IR spectra of alkylphosphonate overlayers on two different primer layers with 11 C-atoms (UDTPA-Zr, Figure 2A) and 16 C-atoms (HDTPA-Zr, Figure 2B) shows in both cases the same qualitative changes described above (increasing ν(CH2) intensities with decreasing chain length in the overlayer). However, the absolute band intensities are markedly weaker on the longer-chain primer layer HDTPA-Zr, which indicates a smaller chain tilt angle and a higher degree of structural order for any particular overlayer according to the above argumentation. A surface coverage effect on the IR (35) Tao, Y.-T. J. Am. Chem. Soc. 1993, 115, 4350. (36) Allara, D. L.; Parikh, A. N.; Rondolez, F. Langmuir 1995, 11, 2357. (37) Allara, D. L.; Swalen, J. D. J. Phys. Chem. 1982, 86, 2700. (38) Jeon, N. L.; Finnie, K.; Branshaw, K.; Nuzzo, R. G. Langmuir 1997, 13, 3382. (39) Vallant, T.; Brunner, H.; Mayer, U.; Hoffmann, H.; Leitner, T.; Resch, R.; Friedbacher, G. J. Phys. Chem., in press.
Vallant et al.
intensities must be negligible based on the identical ellipsometric film thicknesses measured on both primer layers (Table 1). The higher degree of structural disorder in the short-chain primer layer UDTPA-Zr, which is indicated by the high ν(CH2) frequencies in the primer layer spectrum (Table 1), is therefore transferred to the overlayers and leads to more disordered alkylphosphonate films in comparison to a HDTPA-Zr primer layer, whose ν(CH2) absorptions appear at significantly lower wavenumbers. This correlation between the primer layer and the overlayer film structure is expressed even more clearly in the DCPA overlayer spectra on a silicon substrate (Figure 4), where the thickness of the primer layer has been intentionally increased by adsorption of biphosphonic acid spacer layers. Without a spacer layer (n ) 0), an ordered overlayer structure with close to vertically oriented hydrocarbon chains is obtained as judged from the strong positive ν(CH2) absorptions. With one spacer layer (n ) 1), these bands turn into derivative-shaped absorptions with small residual peaks in the upward direction and rather strong and broad absorptions shifted to higher wavenumbers and pointing in the negative direction. In analogy to previously reported spectra of tetradecylsiloxane monolayers prepared in a high humidity atmosphere22 or of submonolayer films of long-chain alkylsiloxanes,39 this band splitting into positive and negative peak maxima must originate from different structural domainssregions with undisturbed, vertically oriented chains and regions with isotropic, randomly oriented film molecules. The ordered domains (positive ν(CH2) absorptions) disappear completely with two spacer layers (n ) 2), resulting in a homogeneous, liquidlike DCPA overlayer. Influence of the Film Adsorption Temperature. The ellipsometric film thicknesses for different adsorption temperatures in Table 1 show a weak, but significant, trend toward thinner (or lower-density) films with increasing temperature for the longer-chain overlayers DCPA and ODPA, corresponding to a decrease in surface coverage dell/dcalc from 93.0% to 88.6% for DCPA and 90.6% to 83.5% for ODPA films, respectively, between 7 and 70 °C. TDPA films, on the other hand, show no change in thickness over the investigated temperature range. This behavior correlates nicely with the IR spectroscopic data (Figure 3), which show large spectral changes as a function of temperature for DCPA and ODPA films but only minor differences for the TDPA spectra. Using the model calculations shown in Figure 5, we can interpret this temperature dependence of the IR spectra in terms of a continuous change of the average hydrocarbon chain tilt angle R between a completely random orientation (R ) 54.7°), corresponding to a disordered, isotropic film structure, and a uniform, vertical alignment (R ) 0°) representing a perfectly ordered, crystalline-like phase. TDPA films remain structurally disordered with a nearly constant, average chain tilt angle close to the isotropic value (54.7°) over the investigated temperature range 7-70 °C. In ODPA films, the average chain tilt angle decreases from values >35° at 70 °C (inverted ν(CH2) peaks) to values
