Properties of Single Dendrimer Molecules Studied by Atomic Force

Langmuir 2000, 16, 9009-9014

9009

Properties of Single Dendrimer Molecules Studied by Atomic Force Microscopy† Hua Zhang,‡ P. C. M. Grim,‡ P. Foubert,‡ T. Vosch,‡ P. Vanoppen,‡ U.-M. Wiesler,§ A. J. Berresheim,§ K. Mu¨llen,§ and F. C. De Schryver*,‡ Laboratory for Molecular Dynamics and Spectroscopy, Department of Chemistry, Katholieke Universiteit Leuven (KULeuven), Celestijnenlaan 200F, B-3001 Heverlee, Belgium, and Max-Planck-Institut fu¨ r Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany Received February 14, 2000. In Final Form: May 8, 2000 Well-separated, individual polyphenylene dendrimer molecules have been prepared by spin coating on a mica surface, and subsequently imaged by noncontact atomic force microscopy (NCAFM). The observed height is in good agreement with the size of a single dendrimer molecule, as calculated by molecular dynamics simulation. By using pulsed force mode (PFM) AFM, stiffness and adhesion properties of individual polyphenylene dendrimers have been studied. They could be related to the molecular structure and the chemical nature of the outer surface of the dendrimers and the thin film of water adsorbed on mica when imaged under ambient conditions. Finally, by changing the concentration of the spin-coating solution, two different kinds of aggregates have been characterized.

Introduction Dendrimers, a new kind of functional materials, are attracting increasing attention.1 Because of their unique highly branched regular structures, dendrimers can be used as catalysts when functionalized,2 as energy or charge-transfer systems,3 as a charge-transport or lightemitting layer in organic light-emitting diodes (LEDs),4 and as a template for the preparation of monodisperse nanoparticles, such as Cu,5 Pd, and Pt.6 On the basis of dendrimer structures, self-assembled monolayer (SAM) † Part of the Special Issue “Colloid Science Matured, Four Colloid Scientists Turn 60 at the Millennium”. * To whom correspondence should be addressed. Telephone: +3216-327405. Fax: +32-16-327989. E-mail: Frans.DeSchryver@ chem.kuleuven.ac.be. ‡ Katholieke Universiteit Leuven. § Max-Planck-Institut fur Polymerforschung.

(1) (a) Fischer, M.; Vo¨gtle, F. Angew. Chem., Int. Ed. Engl. 1999, 38, 884. (b) Majoral, J.-P.; Caminade, A.-M. Chem. Rev. 1999, 99, 845. (c) Hearshaw, M. A.; Moss, J. R. Chem. Commun. 1999, 1. (d) Archut, A.; Vo¨gtle, F. Chem. Soc. Rev. 1998, 27, 233. (e) Zeng, F.; Zimmerman, S. C. Chem. Rev. 1997, 97, 1681. (f) Smith, D. K.; Diederich, F. Chem. Eur. J. 1998, 4, 1353. (g) Fre´chet, J. M. J. Science 1994, 263, 1710. (h) Kawa, M.; Fre´chet, J. M. J. Chem. Mater. 1998, 10, 286. (i) Balogh, L.; deLeuze-Jallouli, A.; Dvornic, P.; Kunugi, Y. T.; Blumstein, A.; Tomalia, D. A. Macromolecules 1999, 32, 1036. (2) (a) Reetz, M. T.; Lohmer, G.; Schwickardi, R. Angew. Chem., Int. Ed. Engl. 1997, 36, 1526. (b) Bhyrappa, P.; Young, J. K.; Moore, J. S.; Suslick, K. S. J. Am. Chem. Soc. 1996, 118, 5708. (c) Chow, H.-F.; Mak, C. C. J. Org. Chem. 1997, 62, 5116. (3) (a) Jockusch, S.; Ramirez, J.; Sanghvi, K.; Nociti, R.; Turro, N. J.; Tomalia, D. A. Macromolecules 1999, 32, 4419. (b) Gilat, S. L.; Adronov, A.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. Engl. 1999, 38, 1422. (c) Hofkens, J.; Latterini, L.; De Belder, G.; Gensch, T.; Maus, M.; Vosch, T.; Karni, Y.; Schweitzer, G.; De Schryver, F. C.; Hermann, A.; Mu¨llen, K. Chem. Phys. Lett. 1999, 304, 1. (d) Devadoss, C.; Bharati, P.; Moore, J. S. J. Am. Chem. Soc. 1996, 118, 9635. (e) Balzani, V.; Campagna, S.; Denti, G.; Juris, A.; Serroni, S.; Venturi, M. Acc. Chem. Res. 1998, 31, 26. (f) Liang, D.-L.; Aida, T. Nature 1997, 388, 545. (4) (a) Halim, M.; Pillow, J. N. G.; Samuel, I. D. W.; Burn, P. L. Adv. Mater. 1999, 11, 371. (b) Strohriegl, P. Adv. Mater. 1996, 8, 507. (c) Wang, P. W.; Liu, Y. J.; Devadoss, C.; Bharathi, P.; Moore, J. S. Adv. Mater. 1996, 8, 237. (d) Kraft, A. J. Chem. Soc., Chem. Commun. 1996, 77. (e) Shirota, Y.; Kuwabara, Y.; Inada, H.; Wakimoto, T.; Nakada, H.; Yonemoto, Y.; Kawami, S.; Imai, K. Appl. Phys. Lett. 1994, 65, 807. (5) (a) Zhao, M.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1998, 120, 4877. (b) Balogh, L.; Tomalia, D. A. J. Am. Chem. Soc. 1998, 120, 7355. (6) Zhao, M.; Crooks, R. M. Angew. Chem., Int. Ed. Engl. 1999, 38, 364.

or multilayer films7 onto which noble metal colloids can be deposited,8 as well as resists for scanning probe lithography,9 have been developed. Atomic force microscopy (AFM)10 is a powerful tool to observe the topography of a surface. Using AFM, dendritic structures have been studied on a variety of surfaces, such as mica,11 graphite,11b,c,12 glass,11b and charged solid surfaces.7b,c For example, Hellmann et al. observed the aggregation of a polyether third generation dendrimer on a mica surface when the surface is rinsed with a benzene solution of the dendrimer.11a Sheiko et al. studied the adsorption and aggregation of carbosiloxane dendrimers on mica, glass, and graphite surfaces by casting a dendrimer solution in hexane. From the histogram of the height distribution of dendrimer aggregates on glass, the height of a single dendrimer molecule was obtained.11b By tapping mode AFM, Huck et al. studied films of aggregated generation five (G5) metallodendrimers on mica and graphite, prepared by spin coating a dendrimer solution in nitroethane. The value of the height observed by AFM was half of the calculated diameter, which was attributed to the flattening of the spherical metallodendrimers on the surface.11c AFM results obtained by Stocker et al. on poly(p-phenylene), decorated with third-generation dendrons, indicate the formation of multilayer films on graphite, made up of densely packed nanorods.12 By using electrostatic layer-by-layer deposition, Bliznyuk et al. (7) (a) Wells, M.; Crooks, R. M. J. Am. Chem. Soc. 1996, 118, 3988. (b) Bliznyuk, V. N.; Rinderspacher, F.; Tsukruk, V. V. Polymer 1998, 39, 5249. (c) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171. (8) (a) Rubin, S.; Bar, G.; Taylor, T. N.; Cutts, R. W.; Zawodzinski, T. A., Jr. J. Vac. Sci. Technol. 1996, 14, 1870. (b) Bar, G.; Rubin, S.; Cutts, R. W.; Taylor, T. N.; Zawodzinski, T. A., Jr. Langmuir 1996, 12, 1172. (9) Tully, D. C.; Wilder, K.; Fre´chet, J. M. J.; Trimble, A. R.; Quate, C. F. Adv. Mater. 1999, 11, 314. (10) (a) Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. Rev. Lett. 1986, 56, 930. (b) Takano, H.; Kenseth, J. R.; Wong, S.-S.; O’Brien, J. C.; Porter, M. D. Chem. Rev. 1999, 99, 2845. (11) (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.

10.1021/la000201g CCC: $19.00 © 2000 American Chemical Society Published on Web 06/16/2000

9010

Langmuir, Vol. 16, No. 23, 2000

fabricated self-assembled mono- and multilayer films of polyamidoamine dendrimers on a charged surface, which were observed by AFM.7b,c Very recently, Li et al. observed individual molecules of four different types of core-shell tecto-(dendrimers) on mica by tapping mode AFM.13 In this contribution, we report the imaging of wellseparated, individual polyphenylene dendrimer molecules as well as their aggregates on mica surfaces, obtained by spin coating different concentrations of a dendrimer solution in CH2Cl2. Noncontact AFM (NCAFM) and the recently developed pulsed force mode (PFM) AFM technique14 have been employed to obtain a topographic image and information on the local stiffness and adhesion properties of the individual and aggregated dendrimers. The PFM-AFM is especially useful for imaging soft samples such as polymers, biological molecules, or, in this case, dendrimer molecules, since there is virtually no lateral force damage. Recently, PFM-AFM has been successfully applied to measure local variations in adhesion, stiffness, and electrostatic surface forces in polymer films and self-assembled monolayers, as well as Al on a quartz plate.15 Experimental Section The polyphenylene dendrimer molecule16 used in this study is denoted as G4-Td (as shown in Figure 1A). CH2Cl2 (Spectroscan, Labscan Ltd., Dublin, Ireland) was used as received. Immediately before spin coating, the mica surface was cleaved with Scotch tape. Different concentrations of the G4-Td dendrimer in CH2Cl2 (1.47 × 10-8 and 7.4 × 10-7 M) were spin-coated for 20 s under ambient conditions at a speed of 2000 rpm. After spin coating, the samples were immediately imaged under ambient conditions with a Discoverer TMX 2010 AFM system (ThermoMicroscopes, San Francisco, CA), operated in the noncontact mode. Noncontact AFM (NCAFM) is one of several vibrating cantilever techniques in which an AFM cantilever is vibrated near the surface of a sample. The spacing between the tip and the sample for NCAFM is on the order of tens to hundreds of angstroms. The probe is in the attractive force region, and the cantilever is pulled toward the sample. NCAFM provides a means for measuring sample topography with little or no contact between the tip and the sample. The total force between the tip and the sample in the noncontact regime is very low, generally about 10-12 N. This low force is advantageous for studying soft or elastic samples. For the NCAFM measurements, Si probes (ThermoMicroscopes, San Francisco, CA) were used, with a spring constant of 36-49 N/m and a resonance frequency of 182-193 kHz. Local stiffness and adhesion, simultaneously imaged with the sample topography, were investigated with pulsed force mode (PFM) AFM. The PFM (Wissenschaftliche Instrumente und Technologie GmbH (WITec), Germany) was added to the AFM system as an external module. The PFM electronics introduces (12) Stocker, W.; Karakaya, B.; Schu¨rmann, B. L.; Rabe, J. P.; Schlu¨ter, A. D. J. Am. Chem. Soc. 1998, 120, 7691. (13) Li, J.; Swanson, D. R.; Qin, D.; Brothers, H. M.; Piehler, L. T.; Tomalia, D.; Meier, D. J. Langmuir 1999, 15, 7347. (14) Rosa-Zeiser, A.; Weilandt, E.; Weilandt, H.; Marti, O. Meas. Sci. Technol. 1997, 8, 1333. (15) (a) Marti, O.; Stifter, T.; Waschipky, H.; Quintus, M.; Hild, S. Colloids Surf., A 1999, 154, 65. (b) Krotil, H.-U.; Stifter, T.; Waschipky, H.; Weishaupt, K.; Hild, S.; Marti, O. Surf. Interface Anal. 1999, 27, 336. (c) Miyatani, T.; Okamoto, S.; Rosa, A.; Marti, O.; Fujihira, M. Appl. Phys. A 1998, 66, S349. (d) Leijala, A.; Hautoja¨rvi, J. Textile Res. J. 1998, 68, 193. (e) Miyatani, T.; Horii, M.; Rosa, A.; Fujihira, M.; Marti, O. Appl. Phys. Lett. 1997, 71, 2632. (f) Luzinov, I.; Minko, S.; Senkovsky, V.; Voronov, A.; Hild, S.; Marti, O.; Wilke, W. Macromolecules 1998, 31, 3945. (16) (a) Morgenroth, F.; Reuther, E.; Mu¨llen, K. Angew. Chem., Int. Ed. Engl. 1997, 36, 631; Angew. Chem. 1997, 109 (6), 647. (b) Morgenroth, F.; Ku¨bel, C.; Mu¨llen, K. J. Mater. Chem. 1997, 7, 1207. (c) Morgenroth, F.; Berresheim, A. J.; Wagner, M.; Mu¨llen, K. J. Chem. Soc., Chem. Commun. 1998, 1139. (d) Wiesler, U.-M.; Mu¨llen, K. J. Chem. Soc., Chem. Commun. 1999, 2293. (e) Wiesler, U.-M.; Berresheim, A. J.; Morgenroth, F.; Mu¨llen, K. Macromolecules, submitted.

Zhang et al. a sinusoidal modulation of the z-piezo of the atomic force microscope with an amplitude between 10 and 500 nm at a userselectable 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 local stiffness and adhesion can be directly obtained by setting the appropriate electronic triggers of the PFM module. For the PFMAFM measurements, Si probes (ThermoMicroscopes, San Francisco, CA) with a spring constant of 0.08-0.20 N/m and a resonance frequency of 8-14 kHz were used. A piezoelectric tube scanner was used with a scan range of 7 µm × 7 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, Moscow, Russia). All images presented in this paper have not been processed other than leveling and contrast enhancement. The fast and slow scanning directions are horizontal and vertical, respectively. The molecular model of G4-Td was built and energy minimized in a vacuum by molecular dynamics simulation, implemented in a Merck force field (MMFF)17 minimization method in Spartan (Wavefunction Inc., Irvine, CA).

Results and Discussion Figure 1B is a NCAFM image of the G4-Td dendrimer (as shown in Figure 1A) adsorbed on freshly cleaved mica by spin coating a 1.47 × 10-8 M CH2Cl2 solution. Several white spherical spots are observed on the mica surface. The size of the dendrimers was determined by measuring the height of the individual polyphenylene dendrimers, which is 4.9 ( 0.3 nm (Figure 1C). The uniformity of the white spots and the narrow height distribution leads us to believe that only individual polyphenylene dendrimers are present on the mica surface and that aggregation has not occurred. The observed full width at the baseline level, which is 75 ( 4 nm (Figure 1C), is evidently larger than the observed height because of the geometrical tip/sample convolution effect.18 As a means to quantify this convolution effect, spherical gold particles were measured with NCAFM.19 By using other tips, originating from a different batch, individual dendrimer molecules could be imaged at high resolution. The NCAFM result presented in Figure 1D shows two dendrimer molecules, which appear to be somewhat elongated in the bottom-left direction, probably due to the scanner drift. The full width at the baseline level was determined to be ∼30 nm.20 (17) Halgren, T. A. J. Comput. Chem. 1996, 17, 490. (18) (a) Yang, J.; Laurion, T.; Jao, T.-C.; Fendler, J. H. J. Phys. Chem. 1994, 98, 9391. (b) Markiewicz, P.; Goh, M. C. Langmuir 1994, 10, 5. (19) To elucidate the tip/sample geometrical convolution effect and explain the observed width of the single dendrimers, spherical gold particles (with an average diameter of 16 nm, as measured by TEM) on mica were imaged as a reference by NCAFM. The sample was prepared by dropping the Au aqueous solution on the mica surface and drying under ambient conditions. The observed height and width of the Au particles was 15.6-18.9 and 114-134 nm, respectively. Using these values and the value for the observed width of the G4-Td dendrimer, the radius of curvature of the tip and the dimension of the G4-Td dendrimer molecules can be calculated, using the standard formula L ) 4(Rr)1/2 (where L is the observed full width of the molecule, R is the tip radius, and r is the radius of the (spherical) molecule), for the tip/ sample convolution effect.11b This leads to an average tip curvature radius of 114 ( 43 nm. This large value for the tip radius was confirmed by SEM images of the tips that are used in the experiments. SEM images have been taken of the tip used in the experiments and of several other tips (used and unused ones). They all show a tip radius of around 100 nm, in contrast to manufacturer values of 10-20 nm. From the values for the tip radius and the observed width of the G4-Td molecules, the height of the dendrimer molecules was determined as 6.2 ( 1.7 nm. This demonstrates that the relatively large value of the tip radius accounts for the observed width of the individual dendrimer molecules. (20) Using the observed width of the molecules and the dimension of the dendrimer molecule as input for the equation in ref 19, a tip radius of approximate 23 nm results. This is in accordance with manufacturer values of 10-20 nm when the effect of drift, which leads to overestimation of the observed width of the molecules, is taken into account.

Properties of Single Dendrimer Molecules

Langmuir, Vol. 16, No. 23, 2000 9011

Figure 1. (A) Molecular structure of G4-Td. (B) NCAFM image of well-separated, individual G4-Td dendrimer molecules on mica, spin coated at a concentration of 1.47 × 10-8 M in CH2Cl2. The black to white height difference corresponds to 5.2 nm. (C) Topography profile across the center of a single G4-Td dendrimer molecule presented in part B. (D) High-resolution NCAFM image of individual G4-Td dendrimer molecules on mica. The black to white height difference corresponds to 2 nm. The inset is the topography profile across the center of a single G4-Td dendrimer molecule presented in part D.

To make a comparison of the observed molecular height with the molecular structure of the dendrimer molecule, the molecular model of G4-Td was minimized in energy by molecular dynamics simulation, a force field minimization method in Spartan.17 The lowest-energy molecular conformation of G4-Td is shown in Figure 2A. The structure of G4-Td is like a tetrahedron (Figure 2B), characterized by the four branches A, B, C, and D. The typical dimension of the molecule, that is, the length of one ribbon of the tetrahedron, equals 6.0 nm. However, it is not this dimension that determines the height of the molecule but rather the distance between the apex of one of the branches, for example, branch A, and the plane of the mica surface in contact with, for example, branches B, C, and D, as schematically drawn in Figure 2B. The most probable conformation of a single dendrimer is the conformation in which three of the four branches are in contact with the mica surface. The fourth branch is pointing away from the mica surface. The height of the molecule in this case equals 4.9 nm, which is in perfect

agreement with the experimentally determined value of 4.9 ( 0.3 nm. This provides further evidence that indeed truly individual dendrimer molecules are imaged. By PFM-AFM,14 the stiffness and adhesion properties of the individual dendrimer molecules have been studied. Figure 3 shows the topography, stiffness, and adhesion images of single G4-Td dendrimers on mica. From the topography image (Figure 3A), the height of the single dendrimer molecules was assessed to be 5.0 ( 0.5 nm, consistent with the value obtained from the NCAFM image (Figure 1B and D). Parts B and C of Figure 3 display the stiffness and adhesion images respectively, simultaneously acquired by the PFM module together with the topographic information. For both images, dark image contrast means a low stiffness or adhesion signal while bright image contrast means a high stiffness or adhesion signal. Figure 3B shows that the G4-Td molecules have a high stiffness relative to that of the mica surface. This observation can be explained as follows. First, when imaged under ambient conditions, there is a thin film of

9012

Langmuir, Vol. 16, No. 23, 2000

Zhang et al.

Figure 2. (A) Space-filling view of a G4-Td dendrimer molecule which is minimized in energy by molecular dynamics simulation implemented in a force field minimization method in Spartan. (B) Tetrahedron structure of G4-Td. A, B, C, and D are the four branches of the G4-Td dendrimer.

Figure 3. Topography (A), stiffness (B), and adhesion (C) images of individual G4-Td dendrimer molecules on mica observed by PFM-AFM. The height scales are 4.8 nm, from -0.057 V (dark) to 0.103 V (bright), and from 1.270 V (dark) to 1.756 V (bright), respectively.

water on the mica surface (about 0.3 nm).21 Since mica is hydrophilic and G4-Td is hydrophobic in nature, mica is more susceptible to moisture in air.21 This means that the stiffness of the “water/mica” assembly is measured, which is lower than the stiffness of the bare mica because of the capillary interaction between the tip and the thin film of water on the mica surface.22 Second, as the G4-Td molecules are composed of connected rigid benzene moieties, one would expect a high degree of shape persistence and stiffness. This can be directly derived from the comparison between the observed height of the individual G4-Td dendrimers (4.9 nm for NCAFM and 5.0 nm for PFM-AFM) and the dimension obtained by molecular dynamics simulation (4.9 nm). Very often, when imaging soft organic materials and biological materials, the AFM tip deforms the objects under investigation and (21) Homola, A. M.; Israelachvili, J. N.; McGuiggan, P. M.; Gee, M. L. Wear 1990, 136, 65. (22) Scandella, L.; Schumacher, A.; Kruse, N.; Prins, R.; Meyer, E.; Lu¨thi, R.; Howald, L.; Gu¨ntherodt, H.-J. Thin Solid Films 1994, 240, 101.

reduced height values are observed. In our case, however, the theoretical and experimental height values are identical, which strongly indicates that the G4-Td molecules are rigid. This is confirmed by the experimental result in Figure 3B, in which the G4-Td dendrimers appear stiffer than the mica surface. Similar observations have been reported by Liu et al., who also observed that “mica” appears as a softer surface than the organic monolayer (octadecyltriethoxysilane/mica).23 From Figure 3C it can be clearly seen that the adhesion signal of the G4-Td molecules is much lower than that of the mica substrate. This is not at all surprising when taking into account the nature of the surface groups of the mica, the dendrimer molecules, and, of course, the tip. The mica surface and the silicon tip are both hydrophilic in nature, leading to a relatively high adhesion force, whereas the adhesion force between the tip and the dendrimer molecules, with respectively hydrophilic and hydrophobic surface groups, is relatively low. At the same (23) Kiridena, W.; Jain, V.; Kuo, P. K.; Liu, G. Y. Surf. Interface Anal. 1997, 25, 383.

Properties of Single Dendrimer Molecules

Langmuir, Vol. 16, No. 23, 2000 9013

Figure 4. Topography (A), stiffness (B), and adhesion (C) images of round aggregates of G4-Td dendrimers on mica observed by PFM-AFM. The height scales are 15.4 nm, from 0.017 V (dark) to 0.145 V (bright), and from 1.485 V (dark) to 1.730 V (bright), respectively. Topography (D), stiffness (E), and adhesion (F) images of the chainlike aggregates of G4-Td dendrimers on mica, as observed by PFM-AFM. The height scales are 15.2 nm, from -0.065 V (dark) to 0.271 V (bright), and from 1.453 V (dark) to 1.852 V (bright), respectively.

time, there is a capillary interaction between the tip and the thin film of water on the mica surface,22 which will also increase the adhesion force. This explains why the dendrimer molecules have a dark image contrast in the adhesion image in Figure 3C. When the concentration of the G4-Td dendrimer solution increases, the dendrimers will aggregate. Figure 4 shows the topography, stiffness, and adhesion images, as observed by PFM-AFM, prepared by spin coating a 7.4 × 10-7 M CH2Cl2 solution on mica. Parts A and D of Figure 4 show the topography images of two aggregated states of the dendrimers, as acquired on different areas of the same sample. The aggregates as observed in Figure 4A are round, the height of them being in the range 7.6-13 nm. The aggregates as observed in Figure 4D are differently shaped. The polyphenylene dendrimers are laterally connected and form small dendrimer clusters or sometimes a dendrimer chain. The height of these dendrimer clusters lies within the range 5.5-16 nm. Mostly, round aggregates are observed. When the spincoating speed is decreased, chainlike aggregates can be found in the center of the rotation, where the speed of the spin coating is approximately zero. When the solution is dropped on a mica surface and the solvent removed under ambient conditions, a large area consisting of chainlike dendrimer aggregates appeared, based on self-organiza-

tion. A possible explanation for the variation in the aggregated states of the dendrimer might be the difference in velocity during the spin-coating process. For a small region around the center of rotation, the velocity is approximately zero, resulting in a longer assembly time for the dendrimer aggregates. Also, the evaporation speed of the solvent and the interactions between the dendrimers, the dendrimer molecules, and mica are probably factors which also influence the aggregated state of the dendrimers. The stiffness (Figure 4B and E) and adhesion images (Figure 4C and F) of the aggregated G4-Td dendrimers show a similar contrast to that observed for the individual polyphenylene dendrimers. The aggregates are also stiffer than “mica”, and the adhesion force between the Si tip and the aggregates is lower than that between the tip and mica. These two properties are determined by the thin film of water on the mica surface, the surface of the hydrophobic groups, and the rigid structure of the aggregates. From Figure 4B, a small dark area can be observed in some of the aggregates, which suggests that there is a soft part in the G4-Td rigid aggregates. This could be related to the internal structure of the aggregates, which could vary between the various aggregates present on the mica surface. Also, at these particular positions, the aggregate exhibits a higher adhesion force

9014

Langmuir, Vol. 16, No. 23, 2000

compared to that exhibited by the rest of the aggregate (Figure 4C). Conclusions Well-separated, completely individual G4-Td dendrimer molecules adsorbed on a mica surface were obtained by spin coating a very dilute solution in CH2Cl2 at a concentration of 1.47 × 10-8 M and subsequently imaged by NCAFM. The dendrimer height, obtained from the AFM topography, is in agreement with the size calculated with molecular dynamics simulation. When the concentration of the dendrimers is increased, two different aggregated states are observed by NCAFM, round and chainlike aggregates. By using PFM-AFM, the stiffness and adhesion properties of individual and aggregated polyphenylene dendrimer molecules are measured. Compared to the “mica” substrate, individual and aggregated polyphenylene dendrimer molecules show a high stiffness and a low adhesion, which are determined by their rigid structures, their hydrophobic surface groups, and the

Zhang et al.

adsorbed thin film of water on the mica surface when imaged under ambient conditions. Compared to AFM, PFM-AFM is more sensitive to the physical and chemical properties of materials, and therefore, it can be used to discriminate between different materials and/or different surface properties of samples.24 This is the first time that stiffness and adhesion properties of single dendrimer molecules have been reported. Acknowledgment. The authors thank the DWTC, through IUAP-IV-11, the FWO (Flemish Ministry of Education), and ESF SMARTON for financial support. P.F. and T.V. thank the IWT for a predoctoral scholarship. The collaboration was made possible thanks to TMR project SISITOMAS. Furthermore, we thank Declan Ryan (Department of Chemistry, University College Dublin, Ireland) for his kind donation of the aqueous Au dispersion. LA000201G (24) Zhang, H.; Grim, P. C. M.; Vosch, T.; Wiesler, U.-M.; Berresheim, A. J.; Mu¨llen, K.; De Schryver, F. C. Langmuir, submitted.