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2007, 111, 8142-8144 Published on Web 05/23/2007
Pulsed-Force-Mode AFM Studies of Polyphenylene Dendrimers on Self-Assembled Monolayers Hua Zhang,*,† Klaus Mu1 llen,‡ and Steven De Feyter§ School of Materials Science and Engineering, Nanyang Technological UniVersity, 50 Nanyang AVenue, Singapore 639798, Singapore, Max-Planck-Institut fu¨r Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany, and DiVision of Molecular and Nanomaterials, Department of Chemistry and INPAC - Institute of Nanoscale Physics and Chemistry, Katholieke UniVersiteit LeuVen (KULeuVen), Celestijnenlaan 200F, B-3001 HeVerlee, Belgium ReceiVed: May 3, 2007
Pulsed-force-mode atomic force microscopy (PFM-AFM) with a chemically modified tip was employed to measure the topography and adhesion force images of homoaggregates of fourth generation polyphenylene and carboxylic-acid-functionalized second generation polyphenylene dendrimers on hydrophilic self-assembled monolayers (SAMs). Although from the AFM topographic image the dendrimers could not be discriminated, from the adhesion image, the respective homoaggregates were easily discriminated. The determination is based on the different adhesive interactions between the dendrimers and the chemically modified tip, which are related to the chemical nature of the outer-surface functional groups, and the adsorbed water layer on hydrophilic surfaces under ambient conditions. It shows that PFM-AFM with chemically modified tips has nanoscale chemical spatial resolution.
Atomic force microscopy (AFM) has the ability to image surface structures on the nanometer scale.1 Dendrimers, functional materials with a natively branched regular structure,2 have been observed with AFM on a variety of substrates, such as mica,3 graphite,3b,c,4 glass,3b a charged solid surface,5 silicon/ silica,6,7 gold,7 and hydrophobically functionalized gold.8 However, a conventional atomic force microscopy has no sensitivity to the chemical properties of a sample surface. With the development of chemical force microscopy (CFM),9 chemically inhomogeneous surfaces have been imaged and the intermolecular forces were investigated.10 The pulsed-force-mode (PFM) AFM technique11 has been successfully applied to measure the local stiffness and adhesion properties of a sample, simultaneously with the topographic image. Although PFM-AFM with chemically functionalized tips has been used to discriminate the chemical functional groups on micropatterned self-assembled monolayers (SAMs) consisting of COOH- and CH3-terminated regions,12 no advantage has been established compared to friction force microscopy (FFM),10a,b,13 a simple method used for characterization of inhomogeneous surfaces. In this letter, PFM-AFM and CFM were combined to study the adhesive properties between chemically modified AFM tips and two homoaggregates of polyphenylene-based dendrimers. These dendrimers, G2Td(COOH)16 (Figure 1A) and G4Td (Figure 1B), are adsorbed on self-assembled monolayers (SAMs). The polyphenylene dendritic scaffold ensures that the carboxylic acid groups are at the rim of the G2Td(COOH)16 * Corresponding author. Phone: +65-6790-5175. Fax: +65-6790-9081. E-mail:
[email protected]. † Nanyang Technological University. ‡ Max-Planck-Institut fu ¨ r Polymerforschung. § Katholieke Universiteit Leuven (KULeuven).
10.1021/jp073388u CCC: $37.00
dendrimer. The adhesion force mapping obtained by PFM-AFM using chemically modified tips was used to distinguish homoaggregates of both dendrimers, achieving nanoscale chemical spatial resolution. This approach is superior to FFM which exerts a large frictional force: this often results in deformation and/or dislocation of structures, especially soft structures (e.g., polymers, biomolecules, etc.), that are weakly physisorbed on a surface. These drawbacks are overcome by the PFM-AFM technique. AFM measurements were carried out on samples freshly prepared by sequentially spin-coating both dendrimers onto selfassembled monolayers of HO-C11H22-SH or HO2C-C10H20SH on a gold-coated Si substrate. The sequential deposition assures the formation of dendrimer homoaggregates.14 By using PFM-AFM with a gold-coated tip modified with carboxy-groupterminated alkyl thiols (COOH tip), the topography and adhesion images of dendrimer aggregates deposited on a hydroxyl-groupterminated SAM-coated gold substrate (OH substrate) were obtained simultaneously. From the topography image (Figure 2A), the formation of aggregates was observed. In the adhesion image (Figure 2B), the dark and bright image contrast reflects a weak and strong adhesion signal, respectively. Therefore, the adhesion signal of the aggregated dendrimers is lower than that of the OH substrate. This can be explained by considering the nature of the OH substrate, the dendrimer molecules, and the COOH tip. First, although there are some carboxylic acids at the rim of G2Td(COOH)16 dendrimers, the molecular surface is still substantially hydrophobic. G4Td and its aggregates are completely hydrophobic. Both the OH substrate and COOH tip are hydrophilic in nature and hydrogen bonding interactions are possible, resulting in a relatively high adhesion force, whereas the adhesion force between the COOH tip and the dendrimer © 2007 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 111, No. 23, 2007 8143
Figure 1. Molecular structure of G2Td(COOH)16 (A) and G4Td (B) dendrimers.
Figure 2. Topography (A, C, and E) and adhesion (B, D, and F) images of the mixed aggregated dendrimers, G2Td(COOH)16 and G4Td, on SAM-coated Au substrates observed by PFM-AFM with a chemically modified Au tip. Square- and circle-marked aggregates represent G2Td(COOH)16 and G4Td aggregates, respectively. The tip and substrate are modified with SAMs terminated with COOH and OH (A and B), OH and OH (C and D), and OH and COOH (E and F) groups, respectively.
molecules is relatively low. Second, when a sample is imaged under ambient conditions, the hydrophilic OH substrate and COOH tip are more susceptible to atmospheric moisture leading to a capillary interaction, thereby increasing the adhesion force. Importantly, the adhesion image (Figure 2B) shows a difference in contrast between the aggregates. There are two different adhesion signals showing up, marked as squares and circles, where the squares exhibit a higher adhesion force than the circles. Compared to the hydrophobic surface of G4Td aggregates (Figure 1B), the G2Td(COOH)16 aggregates (Figure 1A) are somehow more hydrophilic, although both dendrimers are hydrophobic relative to the OH surface as mentioned above. The G2Td(COOH)16 aggregates therefore exhibit a stronger adhesion interaction with the hydrophilic COOH tip and show a higher adhesion signal in the adhesion
image (Figure 2B, squares) as compared to the G4Td aggregates (Figure 2B, circles). Also, hydrogen bonding between G2Td(COOH)16 and the COOH tip is expected to result in an increase of the adhesion force. In addition to carboxyl-modified tips, by using a hydroxylfunctionalized tip, the adhesion properties of the G2Td(COOH)16 and G4Td aggregates on a OH substrate (Figure 2C and D) or COOH substrate (Figure 2E and F) were measured. The obtained results are similar to those measured with the COOH tip (Figure 2A and B). Compared to the topography images (Figure 2C and E), the different adhesion signals (Figure 2D and F) were used to discriminate the different dendrimer aggregates. In line with the experiments with COOH tips, the high (squares) and low (circles) adhesion signals were attributed to aggregates of G2Td(COOH)16 and G4Td, respectively.
8144 J. Phys. Chem. C, Vol. 111, No. 23, 2007 Since the size of the aggregated dendrimers measured in the topographic images (Figure 2A, C, and E) is in the range 50600 nm, significantly larger than the AFM tip radius, the observed differences in adhesion force are not related to the size of the aggregates. Importantly, the current method shows a high chemical sensitivity. As shown in the adhesion image (Figure 2F), two dendrimer aggregates were clearly discriminated, masked as M (G4Td) and N (G2Td(COOH)16), although only one dendrimer aggregate appeared in the topographic image (Figure 2E, diamond area). Although it was previously demonstrated that G2Td(COOH)16 and G4Td aggregates on mica could be discriminated by PFM-AFM using a bare Si tip,14 the current method, using chemically modified tips, is clearly more versatile to characterize and distinguish different functional groups on surfaces. In conclusion, by combining PFM-AFM with a gold-coated AFM tip modified with COOH- or OH-group-terminated SAMs, the adhesion properties of two different types of aggregated polyphenylene dendrimers adsorbed on a COOH- or OH-groupterminated SAM-modified substrate have been visualized. G2Td(COOH)16 and G4Td aggregates are easily discriminated. This study is significant for the following reasons. (1) It illustrates that PFM-AFM with a chemically modified tip shows high chemical sensitivity with nanoscale spatial resolution. (2) Compared to friction force microscopy (FFM), whose high shear force may move, deform, or even destroy surface features, PFMAFM reduces the shear force between the tip and the sample11 and allows the identification of weakly adsorbed dendrimer aggregates. Therefore, the latter technique is more suitable for imaging soft samples.15 Experimental Section Materials. The polyphenylene dendrimers14,16 are denoted as G2Td(COOH)16 and G4Td (as shown in Figure 1). CH2Cl2, tetrahydrofuran (THF) (Spectroscan, Labscan Ltd., Dublin), 11mercapto-1-undecanol (HO-C11H22-SH), and 11-mercaptoundecanoic acid (HO2C-C10H20-SH) (Aldrich Chemical Co., Inc.) were used as received. The concentration of G2Td(COOH)16 in THF and G4Td in CH2Cl2 was 1.5 × 10-7 and 4.0 × 10-6 M, respectively. Chemically Modified Au Substrate and Au-Coated Tip. The chemically modified Au substrate and Au-coated tip were prepared by methods described in the literature.10c,17 Briefly, the piranha solution, V(98% H2SO4)/V(30% H2O2) ) 7:3, cleaned Si wafers and commercial AFM tips were coated with 1 nm of Ti as an adhesion layer and subsequently 10 nm of Au. The Au-coated Si substrates and tips were immersed for 24 h in a 1.0 × 10-3 M solution of HO-C11H22-SH or HO2CC10H20-SH in ethanol in order to form self-assembled monolayers (SAMs). These SAM-modified Au substrates and tips were used immediately upon rinsing with ethanol and Milli-Q water and after drying in a N2 flow. Sample Preparations. A SAM-modified Au substrate was spin-coated under ambient conditions with the G2Td(COOH)16 solution at a speed of 5000 rpm for 90 s, and subsequently with the G4Td solution at a speed of 2000 rpm for 20 s. Atomic Force Microscopy (AFM). The samples were imaged under ambient conditions with a Discoverer TMX 2010 AFM system (Santa Barbara, CA), operated in the pulsed force mode (PFM) (Wissenschaftliche Instrumente und Technologie GmbH (WITec), Germany), to investigate the local adhesion simultaneously with the sample topography. The PFM was added to the AFM system as an external module. The PFM electronics introduces a sinusoidal modulation of the z-piezo
Letters of the AFM with an amplitude between 10 and 500 nm at a user-selectable frequency between 100 Hz and 2 kHz. Due to this rather large amplitude, a full force curve is measured during each period of the oscillation. From the force curve, the adhesion can be directly obtained by setting the appropriate AFM electronic triggers of the PFM module. For the PFM-AFM measurements, a COOH- or OH-group-terminated SAM-modified Au-coated tip (COOH or OH tip) was used. A piezoelectric tube scanner was used with a scan range of 70 µm × 70 µm in the XY-direction and 2.4 µm in the Z-direction. The z-scanner was calibrated using a silicon grating with a step height of 25.5 ( 1.0 nm (Silicon-MDT Ltd., Moscow, Russia). The images have not been processed except for leveling and contrast enhancement. The fast and slow scanning directions are horizontal and vertical, respectively. Acknowledgment. H.Z. is grateful to Prof. Frans C. De Schryver at KULeuven for his kind assistance and support and Prof. Rongchao Jin at Carnegie Mellon University for his helpful suggestion and discussion. K.M. and S.D.F. thank the Belgian Federal Science Policy through IAP-P6/27. References and Notes (1) Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. ReV. Lett. 1986, 56, 930. (2) (a) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic Molecules: Concepts, Synthesis, PerspectiVes; VCH: Weinheim, Germany, 1996. (b) Zeng, F.; Zimmerman, S. C. Chem. ReV. 1997, 97, 1681. (c) Fre´chet, J. M. J. Science 1994, 263, 1710. (3) (a) Hellmann, J.; Hamano, M.; Karthaus, O.; Ijiro, K.; Shimomura, M.; Irie, M. Jpn. J. Appl. Phys. 1998, 37, L816. (b) Sheiko, S. S.; Eckert, G.; Ignat’eva, G.; Muzafarov, A. M.; Spickermann, J.; Ra¨der, H. J.; Mo¨ller, M. Macromol. Rapid Commun. 1996, 17, 283. (c) Huck, W. T. S.; van Veggel, F. C. J. M.; Sheiko, S. S.; Mo¨ller, M.; Reinhoudt, D. N. J. Phys. Org. Chem. 1998, 11, 540. (4) (a) Stocker, W.; Karakaya, B.; Schu¨rmann, B. L.; Rabe, J. P.; Schlu¨ter, A. D. J. Am. Chem. Soc. 1998, 120, 7691. (b) Liu, D. J.; Zhang, H.; Grim, P. C. M.; De Feyter, S.; Wiesler, U. M.; Berresheim, A. J.; Mu¨llen, K.; De Schryver, F. C. Langmuir 2002, 18, 2385. (5) (a) Bliznyuk, V. N.; Rinderspacher, F.; Tsukruk, V. V. Polymer 1998, 39, 5249. (b) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171. (6) McKendry, R.; Huck, W. T. S.; Weeks, B.; Florini, M.; Abell, C.; Rayment, T. Nano Lett. 2002, 2, 713. (7) Zhang, H.; Elghanian, R.; Amro, N. A.; Disawal, S.; Eby, R. Nano Lett. 2004, 4, 1649. (8) Iyer, J.; Hammond, P. T. Langmuir 1999, 15, 1299. (9) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (10) (a) Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943. (b) Vezenov, D. V.; Noy, A.; Rozsnyai, L. F.; Lieber, C. M.; J. Am. Chem. Soc. 1997, 119, 2006. (c) Zhang, H.; He, H. X.; Wang, J.; Liu, Z. F. Langmuir 2000, 16, 4554. (11) Rosa-Zeiser, A.; Weilandt, E.; Weilandt, H.; Marti, O. Meas. Sci. Technol. 1997, 8, 1333. (12) Okabe, Y.; Furugori, M.; Tani, Y.; Akiba, U.; Fujihira, M. Ultramicroscopy 2000, 82, 203. (13) (a) Papastavrou, G.; Akari, S. Nanotechnology 1999, 10, 453. (b) van der Vegte, E. W.; Subbotin, A.; Hadziioannou, G.; Ashton, P. R.; Preece, J. A. Langmuir 2000, 16, 3249. (14) Zhang, H.; Grim, P. C. M.; Vosch, T.; Wiesler, U.-M.; Berresheim, A. J.; Mu¨llen, K.; De Schryver, F. C. Langmuir 2000, 16, 9294. (15) (a) Kwak, K. J.; Kudo, H.; Fujihira, M. Ultramicroscopy 2003, 97, 249. (b) Kwak, K. J.; Sato, F.; Kudo, H.; Yoda, S.; Fujihira, M. Ultramicroscopy 2004, 100, 179. (c) Schneider, M.; Zhu, M.; Papastavrou, G.; Akari, S.; Mo¨hwald, H. Langmuir 2002, 18, 602. (16) (a) Zhang, H.; Grim, P. C. M.; Foubert, P.; Vosch, T.; Vanoppen, P.; Wiesler, U.-M.; Berresheim, A. J.; Mu¨llen, K.; De Schryver, F. C. Langmuir 2000, 16, 9009. (b) Zhang, H.; Grim, P. C. M.; Liu, D. J.; Vosch, T.; De Feyter, S.; Wiesler, U. W.; Berresheim, A. J.; Mullen, K.; Van Haesendonck, C.; Vandamme, N.; De Schryver, F. C. Langmuir 2002, 18, 1801. (17) (a) He, H. X.; Zhang, H.; Li, Q. G.; Zhu, T.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 3846. (b) Zhang, H.; He, H. X.; Wang, J.; Mu, T.; Liu, Z. F. Appl. Phys. A 1998, 66, S269. (c) Zhang, H.; He, H. X.; Mu, T.; Liu, Z. F. Thin Solid Films 1998, 327, 778. (d) He, H. X.; Huang, W.; Zhang, H.; Li, Q. G.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 517.
