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Preparation and Nanoscale Mechanical Properties of Self-Assembled Carboxylic Acid Functionalized Pentathiophene on Mica Jinyu Chen,† Amanda R. Murphy,‡ Joan Esteve,§ D. F. Ogletree,† Miquel Salmeron,*,† and Jean M. J. Fre´chet† Lawrence Berkeley National Laboratory, Berkeley, California 94720, and Department of Chemistry, University of California, Berkeley, California 94720 Received October 24, 2003. In Final Form: April 13, 2004 The oligothiophene derivative 4-(5′′′′-decyl-[2,2′;5′,2′′;5′′,2′′′;5′′′,2′′′′] pentathiophen-5-yl)-butyric acid (D5TBA) was synthesized by Stille cross-coupling methods using functionalized thiophene monomers. The structural and mechanical properties of D5TBA self-assembled monolayers on mica have been studied by atomic force microscopy (AFM). The self-assembled films were prepared by immersing the mica in dilute chloroform or tetrahydrofuran (THF) solutions. The films were predominantly of monolayer thickness with molecules packed in nearly upright orientations. In regions covered with multilayers, the molecules in each monolayer were oriented opposite to those in the neighboring ones, that is, with COOH-COOH and CH3-CH3 contact. The nature of the end group in contact with the substrate depended on the solvent used and the degree of hydration of the substrate, with hydrophobic chloroform solvent favoring the methyl end down and hydrophilic THF favoring the acid group end down. The orientation could also be controlled by dipping using the Langmuir-Blodgett technique.
Introduction The use of atomic force microscopy (AFM) to study selfassembled monolayers (SAMs) has gained increasing importance in recent years. Previous work in our laboratory focused on films made of simple alkane chain molecules with either thiol or silane headgroups to provide strong covalent binding to the substrate1 or amino groups, which bind through ionic interactions.2,3 In the latter case, we have shown that the stability and thickness of the weakly bound films are strongly dependent on the solvent used and on the presence of water on the surface. Films of molecules with chains other than saturated alkanes attract considerable attention because of their potential applications in optical and electronic devices. This is the case of oligothiophenes,4 which are semiconducting and related to electrically conducting polythiophenes.5 The molecular organization of vacuum evaporated oligothiophene thin films2,6,7 as well as the self-assembly of oligothiophenes to form supramolecular ordered monolayers on highly oriented pyrolytic graphite * Corresponding author. † Lawrence Berkeley National Laboratory. ‡ University of California, Berkeley. § Departament de Fı´sica Aplicada i O Å ptica, Universitat de Barcelona, Catalunya. (1) (a) Swalen, J.; Allara, D.; Andrade, J.; Chandross, E.; Garoff, S.; Israelachvili, J.; McCarthy, T.; Murray, R.; Pease, R.; Rabolt, J.; Wynne, K.; Yu, H. Langmuir 1987, 3, 932-950. (b) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (2) Benı´tez, J.; Kopta, S.; Ogletree, D.; Salmeron, M. Langmuir 2002, 18, 6096-6100. (3) Benı´tez, J.; Kopta, S.; Dı´ez-Pe´rez, I.; Sanz, F.; Ogletree, D.; Salmeron, M. Langmuir 2003, 19, 762-765. (4) See references contained in Mullen, K.; Wegner, G. Electronic Materials: The Oligomer Approach; Wiley-VCH: New York, 1998. (5) Fichou, D. Handbook of Oligo- and Polythiophenes; Wiley-VCH: New York, 1998. (6) Servet, B.; Horowitz, G.; Ries, S.; Lagorsse, O.; Alnot, P.; Yassar, A.; Deloffre, F.; Srivastava, P.; Hajlaoui, R.; Lang, P.; Garnier, F. Chem. Mater. 1994, 6, 1809-1815. (7) Fichou, D. J. Mater. Chem. 2000, 10, 571-588.
(HOPG) has been studied extensively.8-11 However, few studies have been done by AFM on oligothiophene films formed from solutions.12,13 Specifically, the structural and mechanical properties of these thin films have yet to be investigated. Information about molecular packing and ordering within SAMs of functionalized thiophene oligomers is useful to increase performance in electronic devices in which these materials are used. In this paper, we present AFM studies of the structure and mechanical properties of SAMs formed by the oligothiophene derivative 4-(5′′′′-decyl-[2,2′;5′,2′′;5′′,2′′′;5′′′,2′′′′] pentathiophen-5-yl)-butyric acid (D5TBA) on mica. A schematic representation of the D5TBA molecule is shown in Figure 1. The molecule contains a central segment made of 5 thiophene units, with a 10-carbon-atom alkyl chain on one side and a shorter butyric acid group on the other. Both chain ends are tilted with respect to the central pentathiophene and exhibit significant conformational mobility. The size of the molecule was calculated using the molecular modeling program Spartan.14 The 5-thiophene segment is made of coplanar thiophene units with alternating orientation of the sulfur moiety forming a chain with a length of 1.70 nm. The butyric acid group contributes a length of 0.62 nm. These dimensions, together with the van der Waals radius of the methyl end group (∼0.2 nm) and the OH group at the acid end (∼0.1 nm), give a length of the entire molecule close to 3.6 nm for a fully extended conformation. (8) Azumi, R.; Go¨tz, G.; Ba¨uerle, P. Synth. Met. 1999, 101, 569-572. (9) Mena-Osteritz, E. Adv. Mater. 2002, 14, 609-616. (10) Stecher, R.; Gompf, B.; Mu¨nter, J.; Effenberger, F. Adv. Mater. 1999, 11, 927-931. (11) Noh, J.; Ito, E.; Nakajima, K.; Kim, J.; Lee, H.; Hara, M. J. Phys. Chem. B 2002, 106, 7139-7141. (12) Sanberg, H.; Henze, O.; Sirringhaus, H.; Kilbinger, A.; Feast, W.; Friend, R. Proc. SPIE-Int. Soc. Opt. Eng. 2001, 4466, 35-43. (13) Schenning, A.; Kilbinger, A.; Biscarini, F.; Cavallini, M.; Cooper, H.; Derrick, P.; Feast, W.; Lazzaroni, R.; Leclere, Ph.; McDonell, L.; Meijer, E.; Meskers, S. J. Am. Chem. Soc. 2002, 124, 1269-1275. (14) Spartan ’02 for Windows; Wavefunction, Inc.: Irvine, CA, 2002.
10.1021/la030395a CCC: $27.50 © 2004 American Chemical Society Published on Web 07/28/2004
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Figure 1. Schematic representation of the structure of 4-(5′′′′decyl-[2,2′;5′,2′′;5′′,2′′′;5′′′,2′′′′] pentathiophen-5-yl)-butyric acid (D5TBA). The dimensions were calculated using bond distances and angles. The 3.65 nm length of the molecule was calculated by adding the van der Waals radii of the terminal methyl and OH groups.
The molecules bind to each other through a variety of interactions, with a strong π-stacking contribution of the thiophene units as well as additional interactions from both the alkyl chains and the polar carboxyl acid moieties. Experimental Section Materials. All chemicals were purchased from Aldrich and used without further purification unless otherwise noted. 2,2bithiophene was purified by filtering it through silica gel using hexane as the eluent. N-bromosuccinimide (NBS) was recrystallized from 1:1 acetic acid/water prior to use. Tetrahydrofuran (THF) was distilled over Na/benzophenone and dichloromethane from calcium hydride just prior to use. The N,N′-dimethylformamide (DMF) used was anhydrous packed under N2. All reactions were performed under dry N2 unless otherwise noted. All extracts were dried over MgSO4, and solvents were removed by rotary evaporation with aspirator pressure. Flash chromatography was performed using Merck Kieselgel 60 (230-400 mesh) silica. Monolayers were prepared on muscovite mica (KAl2(Si3AlO10)(OH)2) from Mica New York Corp. Characterization. Infrared spectra were measured on neat samples unless otherwise indicated with a Mattson Genesis II Fourier transform infrared (FT-IR) instrument with a diffuse reflectance accessory (Pike). Ultraviolet-visible (UV-vis) data were measured with a Varian Cary 50 spectrophotometer. 1H NMR and 13C NMR spectra were recorded with Bruker AMX300, AM-400, or DRX-500 instruments using CDCl3 as the solvent. Matrix assisted laser desorption ionization time-of-flight (MALDITOF) mass spectrometry was performed on a Perseptive Biosystems Voyager-DE instrument in positive ion mode using R-cyano-4-hydroxycinnamic acid or 9-nitroanthracene as the matrix. High resolution mass spectometry (HRMS) using fast atom bombardment (FAB) was done with a Micromass ZAB2-EZ double focusing mass spectrometer (BE geometry). Elemental analyses were performed at the UC Berkeley Microanalysis Laboratory. Atomic force microscopy (AFM) was performed using a homebuilt instrument controlled with commercial RHK electronics. The instrument was operated inside a chamber providing sound isolation and humidity control. Si3N4 cantilevers from NanoProbe (Digital Instruments, Santa Barbara, CA) with a nominal spring constant of 0.12 N/m were used to obtain the images. Forces were determined by multiplying the lever deflection by the spring constant. The x, y, and z distance measurements were calibrated using standard samples. The most important dimensions in this paper are the distances in the z-direction (perpendicular to the surface). All the reported distances have an associated absolute error bar of (0.3 nm and a relative error (within one experiment) of (0.1 nm. While most of the results presented in this work were obtained by using the atomic force microscope in the contact mode, we will also present some experiments using the atomic force microscope in the so-called scanning polarization force mode (SPFM). In
Chen et al. this mode, a conductive cantilever and tip is scanned out of contact with the sample, typically at a height of 10-20 nm over the surface. The cantilever oscillation due to the electrostatic response to an applied voltage of 10 V amplitude and 4 kHz frequency was detected with lock-in amplifiers. The second harmonic amplitude, which depends on the electric polarizability of the surface, is used as the z-feedback control and produces a topographic image. The first harmonic signal, which depends on the contact potential difference between the tip and the sample, is used as the bias feedback and produces a contact potential image.15 The use of the first harmonic bias feedback is known as the Kelvin probe method. Both images are acquired simultaneously. Synthesis. Synthesis and characterization of the D5TBA molecule is given in the Supporting Information. Monolayer Preparation. To deposit films of D5TBA, we used the method of immersion of the mica substrate into solutions of the molecule. Two different solvents, chloroform and tetrahydrofuran (THF), were used. In both cases, solutions were prepared with a 1.86 × 10-5 M (0.1% m/m) concentration. Samples of muscovite mica were cleaved on both sides at ambient conditions and within seconds immersed in the solution for periods ranging from 30 s to hours. The samples were subsequently dried under a stream of dry N2 for several minutes. Another method to produce films is the Langmuir-Blodgett technique. Although the immersion method was used in most of the studies reported here, we will also present a few results using the latter technique, to compare dielectric properties that depend on molecular orientation.
Results and Discussion 1. Films Formed from THF Solutions. 1.1. Island Structure. After a 30 s immersion of mica into THF solutions of D5TBA, the surface was covered with numerous monolayer islands with lateral dimensions of several micrometers, like those shown in the contact mode AFM images of Figure 2. These images were acquired at a negative external load of -18 nN, by pulling the lever away from the sample. Since the pull-off force necessary to break the adhesive contact was close to -30 nN, the total net compressive load was ∼12 nN. This was done in order to minimize mechanical perturbations of the film during imaging. The images in Figure 2 show monolayer islands with a uniform height of 3.5 nm, close to the estimated 3.6 nm length of the molecule (see Figure 1). Since the friction force on the film is lower than that on the exposed mica regions, and lower also than that on films exposing COOH groups (see below), we conclude that the molecules are oriented nearly vertically with the methyl group exposed and the COOH group in contact with the mica substrate. The islands consist of several domains that are distinguished by their different friction forces. Figure 2b shows a friction force map acquired simultaneously with the topography. The gray levels indicate different values of the friction force, light corresponding to high and dark to low friction. Most islands are composed of several domains with different values of the friction force. In Figure 2c, several domain boundaries in each island have been marked with lines. Since the height of the islands is uniform, we conclude that these domains differ in the molecules being oriented differently relative to the scanning direction, as shown in the schematic drawing in Figure 3. In this figure, we show the topographic profile of a particular island with two large and several smaller domains. A topographic profile across the two large domains and the corresponding friction profile are shown in the lower part of the figure. Although the friction force is different in each domain in one scan direction, the width of the friction loop is the same, as can be seen in the friction (15) Hu J.; Xiao X.-d.; Ogletree, D. F.; Salmeron, M. Science 1995, 268, 267.
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Figure 4. Topography (left) and friction (right) images of D5TBA layers formed from THF solution as a function of load. The images show a large island of monolayer height with a small multilayer region (brighter area) containing up to four layers, marked with numbers in part a. The upper layers are removed by the tip as the load increases. The imaging load was (a) -27, (b) 0, (c) 8, (d) 16, and (e) 34 nN. The letters L and R in part a mark the positions analyzed in Figure 5. Figure 2. Images of self-assembled films of D5TBA on mica formed after a 30 s immersion in THF×: (a) topographic image; (b) friction image; (c) the same image as that in part b except with contour lines drawn in to show friction domains. Image size, 5 µm × 5 µm.
Figure 3. Cross sections of the topography and friction force over an island with two distinct friction domains (inset). The friction traces in each scanning direction show that, while the friction force is different in each direction, the amplitude of the friction loop is the same in each domain. This suggests that the molecules in each domain have different orientations relative to the scanning direction, as illustrated in the drawings under the topographic profile.
force profiles corresponding to left-to-right and right-toleft scanning directions in the figure. Unfortunately, we were unable to obtain lattice-resolved images of the D5TBA
molecules in the film, which prevented determination of the molecular orientation in each domain and consequently a quantitative study of the frictional anisotropy. Although the film is composed mostly of single monolayer islands for a coverage of ∼50%, a small number of multilayer aggregates (30%). The contact potential is lower on the D5TBA islands than on mica.
the methyl group to the hydrophobic solvent. At the same time, the immersion of mica in the chloroform displaces the water layer that is present on this hydrophilic substrate in typical humid environments encountered in the laboratory (40-60% in our case). We thus propose that a similar result can be expected in the case of D5TBA SAMs prepared from chloroform solutions. As the micelles adsorb on the mica, they can give rise to stacking sequences where the first layer contacts mica through the methyl ends. We propose that this is the case in regions 2 and 3 and that these regions contain two and three layers, respectively. This is in contrast to the case of the hydrophilic THF solvent that favors micelles exposing the COOH groups. Patches of residual water on the mica substrate might also favor the adsorption of molecules oriented with the COOH groups down, as in regions 1 and 5. Because of the hydrogen and ionic bonding between carboxyl groups and the mica surface, the COOH-down structure proposed for the layers in region 5 should be more stable than the CH3-down structure of regions 2 and 3. To verify this idea, we acquired images of the same area with increasing load. The result is shown in Figure 8, where pairs of images (topography and friction) are shown as a function of increasing load. The first image obtained at -3 nN, Figure 8a, was acquired in the same area as that in Figure 7. Increasing the normal force to -1 nN causes the disappearance of all materials from regions 2 and 3, while SAM film remains in regions 1 and 5. The height of the film in region 5 however is reduced
to 9 nm from its original 15 nm, while the friction remains the same, indicating no change in film termination. This therefore corresponds to the loss of one D5TBA bilayer. The persistence of the films in regions 1 and 5 provides evidence of strong binding to mica, while the disappearance of the film in regions 2 and 3 indicates weak binding. Therefore, we propose that the binding to mica in regions 1 and 5 is by carboxyl groups, while in regions 2 and 3 the film contacts mica through the methyl groups. Notice also that the area occupied by that region has expanded and covers region 3. After the load-induced modifications, the film is stable and no further changes are observed until the load reaches 9 nN, at which point the tip spreads and drags material from the multilayer regions. Finally, at a load of 19 nN, all the film material is swept away by the tip. Large images acquired afterward at -3 nN revealed the presence of material piled up at the edges of the scanned area. In another experiment, we prepared films by immersing mica for 2 or 3 days in the chloroform solution. The long exposure produced a uniform film with the methyl groups contacting the mica surface. This result suggests that chloroform is able to displace all the residual water on the mica surface, while in the short exposures of the previous experiment some residual water was retained by the mica. 3. Dielectric and Electrical Dipole Properties of D5TBA Films. To study the dielectric properties of the D5TBA films, we used the scanning polarization force microscopy (SPFM) mode described in the Experimental Section. Figure 9 shows three pairs of images from a film
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that observed on alkyl silanes and amine SAMs. It is due to the large increase of the polarizability of mica when its surface ions become mobile due to hydration. In the areas covered by the SAM however, no such enhancement occurs and the surface ionic mobility remains low, as in the dry condition. Since this is described in detail in a recent publication, we will omit discussion of this interesting phenomenon here.22 To better determine the relation between the molecular orientation and the electric dipole, we performed experiments with D5TBA monolayers produced using the Langmuir-Blodgett (LB) technique. To that effect, the mica was dipped once through water covered with a monolayer of D5TBA, as shown in the schematic of Figure 10c. Since the carboxylic acid end of the molecule is attracted to the water, the film is forced into contact with the mica with the methyl end group down. Images of such films acquired in contact and SPFM modes are shown in Figure 10a. The height of the islands in the topographic image of Figure 10a is 3.5 nm, similar to that of films formed by the immersion method. The corresponding friction image shows high friction values over the islands, indicating that COOH groups are exposed. SPFM images of the same region (Figure 10b) show that the contact potential is higher over the film than over the mica and of opposite sign to that in Figure 9, where methyl groups were exposed. This result confirms that the positive end of the molecular electric dipole is at the acid group and indicates that when exposed to air it is not ionized to COO-. Conclusions
Figure 10. AFM images of D5TBA films formed by the Langmuir-Blodgett technique, shown schematically in part c, that force the film in a methyl-down orientation: (a) contact mode topography (top) and friction (bottom); (b) noncontact SPFM images of the same area showing topography (top) and contact potential (bottom). The contact potential is higher on the D5TBA.
self-assembled from THF solution. The left images in Figure 9a were acquired in the standard AFM contact mode and correspond to topography (top) and friction (below). Three roughly triangular monolayer islands (∼3.5 nm in height) can be seen. The center and right-hand side images (Figure 9b and c) were acquired in SPFM mode. Because the tip is several tens of nanometers above the surface in this mode, the image resolution is limited. The top image in Figure 9b corresponds to the SPFM topography (second harmonic z-feedback) at a low humidity of 8% at room temperature. The apparent height of the islands is 1.4 nm. This lower value compared to the values obtained in contact mode reflects differences in the polarizability (or dielectric constant) of the D5TBA film relative to the mica. Notice also the modified topography of the upper island, due to damage caused during contact imaging that displaced some of the material. The bottom image corresponds to the surface potential (first harmonic tip bias feedback) and shows that the two lower islands are more negative than the mica (by ∼0.25 V). The negative potential of the two bottom islands indicates that the molecular electric dipole moment points down toward the carboxyl group. At higher humidity, there is no change in the dipolar orientation of the molecule but the topography undergoes a contrast reversal at around 30% (and higher values) relative humidity, where the islands are imaged as depressions. This reversal phenomenon is similar to
The new oligothiophene derivative 4-(5′′′′-decyl[2,2′;5′,2′′;5′′,2′′′;5′′′,2′′′′] pentathiophen-5-yl)-butyric acid (D5TBA) prepared by Stille cross-coupling methods using functionalized thiophene monomers has been used to prepare self-assembled films on a mica surface. The self-assembled films were prepared by immersing mica substrates into THF or chloroform solutions of D5TBA. Monolayers were formed with the molecules oriented in a nearly upright position and bound to each other by additive interaction forces. In the THF solution, mechanically stable monolayers were formed, with the COOH groups binding to the mica substrate and the methyl group exposed. Exposed methyl groups substantially lowered the friction force on the film, as compared to the bare mica. Multilayers of D5TBA were also formed. These were found to be less stable than the monolayer. In the multilayer covered regions, individual layers were stacked with alternating orientations, due to formation of COOHCOOH and CH3-CH3 interfaces. Layers exposing the acid group exhibited high friction forces, while those exposing methyl groups exhibited low friction forces. Upon pressure exerted by the tip, the layers above the first could be removed. An interesting observation was the insertion of molecules from one layer into the next as a result of tip pressure. This insertion did not occur in layers prepared by a long immersion time, presumably due to the formation of more densely packed layers. When using chloroform solutions, the layers were formed with the methyl groups in contact with the mica, except in some patches that had probably retained some of the water present in the mica surface before immersion. The methyl-down films could easily be removed by (22) Diez, I.; Luna, M.; Teheran, F.; Ogletree, D. F.; Sauz, F.; Salmeron, M. Langmuir 2004, 20, 1284-1290.
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scanning with the atomic force microscope tip indicative of weak binding. Films bound by the COOH group adhered much more strongly to mica. Prolonged immersion in the chloroform solution produced films uniformly oriented with the methyl end groups down, due to the effective removal of the mica water film by the hydrophobic solvent. Dipping the mica once into a water trough covered with D5TBA produced monolayer films with the molecules oriented with the methyl groups in contact with the substrate. This forced orientation helped determine the dipolar moment orientation of the D5BTA SAM using the noncontact SPFM mode. The positive end of the molecule
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was determined to be the COOH group, both when pointing up or down toward the mica. Acknowledgment. This work is supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials and Engineering, of the U.S. Department of Energy under Contract No. De-AC0376SF00098. Supporting Information Available: The synthetic procedure and characterization of D5TBA. This material is available free of charge via the Internet at http://pubs.acs.org. LA030395A
