Langmuir 2003, 19, 3345-3349
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Structural Investigations of Octadecylphosphonic Acid Multilayers G. N. Fontes,† A. Malachias,† R. Magalha˜es-Paniago,†,‡ and B. R. A. Neves*,†,§ Depto. de Fı´sica, ICEx, Universidade Federal de Minas Gerais, Ave. Antonio Carlos, 6627, Belo Horizonte, CEP:30123-970, Brazil, Laborato´ rio Nacional de Luz Sı´ncrotron, Caixa Postal 6192, Campinas, CEP:13084-971, Brazil, and Laborato´ rio de Nanoscopia, Fundac¸ a˜ o Centro Tecnolo´ gico de Minas Gerais, Ave. Jose Candido da Silveira, 2000, Belo Horizonte, CEP:31170-000, Brazil Received November 1, 2002. In Final Form: January 30, 2003 In this work, different multilayer conformations of a linear amphiphilic molecule, octadecylphosphonic acid (OPA), were investigated by atomic force microscopy (AFM), X-ray reflectivity, and X-ray diffraction. It was found that these molecules spontaneously pack into well-organized self-assembled bilayers (SABs) when the structures are formed inside concentrated solutions and/or after slow solvent evaporation. The molecular structure of an OPA SAB was observed to be dependent on its position along a stack of bilayers: when the SAB is at, or close to, the stack surface, OPA molecules are vertically aligned and form 5 nm thick bilayers; when it is in the middle of a stack of hundreds, or thousands, of bilayers, OPA molecules are tilted and form 3.4 nm thick bilayers. The van der Waals interactions among the long OPA alkyl chains were used to explain and also predict some features of the molecular arrangement inside both bilayer types. Using the AFM technique, which also enabled the application of pressure onto a bilayer surface, oblique and hexagonal molecular packings were observed, probably corresponding to both 5 and 3.4 nm thick bilayers, respectively.
Introduction Self-assembled monolayers (SAMs) have been the subject of extensive research for more than a decade due to their technological applications as, for example, lubricants, adhesion promoters, or, more recently, nanolithography resists.1-3 Several different amphiphilic molecules, such as alkanethiols and carboxylic and phosphonic acids have been shown to form SAMs on a variety of substrates with interesting properties.4-8 Besides monolayers, selfassembled bilayers (SABs) and multilayers of various organic molecules have also been widely investigated, especially phospholipid SABs, which are useful models of biological membranes.9-10 Studies on the transition from SAMs, or SABs, to multilayered objects and vice versa have shed some light on the kinetic and/or dynamic processes which control organization and packing at the molecular level.11-12 In a recent paper, Neves et al. have shown that 5 nm thick layers of octadecylphosphonic acid * Corresponding author. E-mail:
[email protected]. † Universidade Federal de Minas Gerais. ‡ Laborato ´ rio Nacional de Luz Sı´ncrotron. § Fundac ¸ a˜o Centro Tecnolo´gico de Minas Gerais. (1) Laibinis, P. E.; Hickman, J. J.; Wrighton, M. S.; Whitesides, G. M. Science 1989, 245, 845. (2) Ulman, A. Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (3) Poirier, G. E. Chem. Rev. 1997, 97, 1117. (4) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (5) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. (6) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (7) Woodward, J. T.; Ulman, A.; Schwartz, D. K. Langmuir 1996, 12, 3626. (8) Woodward, J. T.; Schwartz, D. K. J. Am. Chem. Soc. 1996, 118, 7861. (9) Powers, L.; Pershan, P. S. Biophys. J. 1977, 20, 137. (10) Leikin, S.; Parsegian, V. A.; Rau, D. C. Annu. Rev. Phys. Chem. 1993, 44, 369. (11) Li, D.; Ratner, M. A.; Marks, T. J.; Zhang, C.; Yang, J.; Wong, G. K. J. Am. Chem. Soc. 1990, 112, 7389. (12) Netzer, L.; Sagiv, J. J. Am. Chem. Soc. 1989, 105, 674.
(OPA) evolve to form well-organized SAMs on a mica surface.13 Moreover, such 5 nm thick layers were found to be spontaneously formed inside concentrated solutions and were tentatively described as conventional bilayers, although no experimental evidence of the internal structure of such layers was presented.13 In this work, X-ray reflectivity and atomic force microscopy (AFM) were employed to assess the structure of these OPA layers and, as a consequence, to unambiguously identify them as selfassembled bilayers. Furthermore, OPA multilayers formed by slow solvent evaporation were also investigated by X-ray diffraction and AFM. Although OPA is mostly known for its ability to form SAMs on a large variety of substrates,7,8,13-15 the present results show that bilayers are also natural packing structures for OPA molecules. Additionally, there are two different bilayer structures which are observed in distinct positions of a stack of hundreds, or thousands, of bilayers: 5 nm thick SABs at, or close to, the stack surface; and 3.4 nm thick SABs in the inner region of the stack. Experimental Section n-Octadecylphosphonic acid, 94+%, was used as received from Alfa Aesar to produce 0.1 wt % solutions in absolute ethanol (which corresponds to a 2.1 mM concentration). OPA, CH3(CH)17PO3H2, is a 2.5 nm long linear amphiphilic molecule, with a lengthy alkyl chain (∼2.2 nm) and a polar and hydrogen-bondforming headgroup. Samples with a few 5 nm thick OPA layers were prepared by spread-coating of GaAs and Si substrates as described in refs 13 and 15. Samples comprising hundreds of OPA layers, or more, were produced by slow evaporation of ethanol in the OPA solution. The evaporation time varied from 72 to 240 h depending on solution volume, after which macroscopic OPA agglomerates could be observed at the bottom of the beaker. (13) Neves, B. R. A.; Salmon, M. E.; Russell, P. E.; Troughton, E. B., Jr. Langmuir 2001, 17, 8193. (14) Badia, A.; Lennox, R. B.; Reven, L. Acc. Chem. Res. 2000, 33, 475. (15) Neves, B. R. A.; Salmon, M. E.; Russell, P. E.; Troughton, E. B., Jr. Langmuir 2000, 16, 2409.
10.1021/la0267847 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/04/2003
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Figure 1. AFM images of both sample types investigated in this work: (a) a sample with a few OPA layers prepared by spread-coating; (b) a sample with hundreds of OPA layers prepared by solvent evaporation. The scale bar between the two images indicates their lateral dimensions. Using delicate tweezers (to avoid mechanical stress/strain on the agglomerates), single OPA agglomerates were then carefully placed on either a GaAs or a Si surface, which served as a substrate. All samples were morphologically characterized by a MultiMode atomic force microscope, from Digital Instruments, operating in intermittent contact mode. The structural organization of OPA molecules in the xy plane of a SAB was also determined by AFM using the contact mode of operation. Silicon and silicon nitride cantilevers were employed as AFM probes during intermittent contact and contact operation of the microscope, respectively. The variation of applied load on the SAB surface by the AFM cantilever enabled imaging and structural determination of the SAB under a wide range of applied pressures (from a few to thousands of atmospheres).16 The SAB structural organization in the z direction (parallel to the surface normal) was determined by X-ray reflectivity and X-ray diffraction measurements on samples with either only a couple or a large number of stacked bilayers, respectively. All X-ray data were acquired with a 200 µm × 2 mm collimated beam of wavelength λ ) 1.571 Å. The scattering was collected by a scintillation detector with a standard four-circle diffractometer at the XRD2 beamline of the National Synchrotron Light Laboratory (LNLS) in Campinas, Brazil.
Results and Discussion Figure 1 shows AFM images of both sample morphologies investigated in the present work. In Figure 1a, one or two 5 nm thick layers can be seen (in gray and light gray shades, respectively) partially covering the GaAs substrate surface (dark gray). This is a characteristic image of a sample produced by the spread-coating method, which does not produce evenly covered surfaces.13,15 Typically, in this work, the surface coverage was ∼70% in regions nearby the position where the solution droplet was deposited, as shown in Figure 1a. Within the covered area, ∼60% corresponds to two-layer coverage and ∼40% corresponds to coverage by a single 5 nm thick layer. Due to the nonhomogeneous nature of the deposition process, variations in surface coverage and in the one/two-layer ratio were observed along the surface of all samples investigated in this work.13 Figure 1b portrays a three(16) Barrena, E.; Kopta, S.; Ogletree, D. F.; Charych, D. H.; Salmeron, M. Phys. Rev. Lett. 1999, 82, 2880.
Fontes et al.
Figure 2. Graphs showing X-ray reflectivity data for a sample with a few OPA layers (a) and X-ray diffraction data for a sample with hundreds of OPA layers (b). In both graphs, the solid line indicates the theoretical fitting of the experimental data by the models presented in the text. In (a), the OPA bilayer molecular configuration and the z direction are shown at the top right of the figure.
dimensional (3D) image of a sample formed by thousands of OPA layers produced by slow solvent evaporation. Several OPA layers can be seen in Figure 1b in different shades of gray, and it is important to note that they all are 5 nm thick. It is also important to note that this preparation method produces a vertical stacking of OPA layers, facilitating X-ray investigation of such samples. Finally, the size distribution of OPA layer agglomerates produced by solvent evaporation ranges from a few microns to a few millimeters in lateral dimensions and from hundreds of nanometers to a few microns in thickness. To verify the hypothesis presented in ref 13 that 5 nm thick OPA layers are actually self-assembled bilayers, X-ray reflectivity measurements were carried out on the sample shown in Figure 1a, and the results are plotted in Figure 2a. In this figure, the measured scattered intensity (open squares) is normalized by Fresnel reflectivity RF and is plotted as a function of q, the momentum transfer after scattering.17 With the aim of understanding and modeling such data, it must be remembered that X-ray reflectivity is a restricted case of X-ray diffraction. Since X-rays are primarily scattered by electrons, the diffracted intensity I(q b) is related to the Fourier transform of the electron density F(r b) by I(q b) ∝ |F(r b) exp(iq b‚r b) dr b|2, where b q is the momentum transfer after scattering and |q| ) 4π/λ sin θ, with θ being the measured X-ray scattered angle.17 Therefore, using this relation, it is possible to determine the molecular structure of a 5 nm thick OPA layer since the electron density varies along the OPA molecule. When such layers are deposited on a substrate of known electron density F0, the scattered intensity R(q) can be written, within the Gaussian step-model approximation, as18,19
R(q) RF(q)
N
)|
∑ i)0
(Fi - Fi+1) F0
exp(-iqDi) exp(-q2σi+12/2)|2 (1)
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In this model, each 5 nm thick OPA layer is divided into N layers with electron density Fi and interface roughness i σi; Di ) ∑j)1 Tj is the distance from the substrate to the ith interface, with Tj being the thickness of the jth layer.18 The above equation is a good approximation for momentum values q > 2qc only, where qc is the critical momentum transfer, which depends only on the substrate, and is equal to 0.0446 Å-1 for GaAs.17,18 Considering the experimental observation that there are regions of the sample covered by different numbers of OPA layers (one or two, see Figure 1a), the measured X-ray intensity should be modeled as
βR1(q) + (1 - β)R2(q) R(q) ) RF(q) RF(q)
(2)
where R1(q) and R2(q) are the scattered intensities calculated by eq 1 when there is only one (R1) or there are two (R2) 5 nm thick OPA layers covering the GaAs substrate. The parameter β indicates the fraction of the total scattered intensity that arises from regions covered by one 5 nm thick OPA layer only. Hence, eq 2 was used to model the experimental data, and the best fitting is indicated by a solid line in Figure 2a. The values of the parameters used in such fitting unambiguously identify the observed 5 nm thick layers as self-assembled bilayers, and the molecular arrangement along the z direction is shown in the space-filling model at the top right of Figure 2a. It can be seen that OPA molecules pack themselves in a vertical alignment with the phosphonic headgroups pointing outward, in a structure common to most phospholipid bilayers.9,10 Moreover, electron density values used in the fitting indicate a high-density packing of OPA molecules in the xy plane of the SAB, with each OPA molecule occupying an average area of ∼17 Å2. Finally, the best value for the β parameter was found to be 0.25, indicating that only 25% of the measured intensity came from X-ray scattering in regions covered by a single OPA bilayer and the remaining 75% of the intensity came from regions covered by two bilayers. It is worth comparing these values with the morphological analysis of the sample shown in Figure 1a. Considering covered regions of the substrate only, 40% is covered by a single bilayer and 60% by two bilayers, in poor agreement with the X-ray fitting. However, AFM images of larger areas (not shown), which were used to make the coverage analysis, show a large number of small (
