The First Organosiloxane Thin Films Derived from SiCl3-Terminated

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The First Organosiloxane Thin Films Derived from SiCl3-Terminated Dendrons. Thickness-Dependent Nanoand Mesoscopic Structures of the Films Deposited on Mica by Spin-Coating Zhongdang Xiao,† Chengzhi Cai,* Aurelie Mayeux, and Alexandra Milenkovic Department of Chemistry, University of Houston, Houston, Texas 77204-5003 Received May 28, 2002. In Final Form: July 11, 2002

This article describes a new type of organosiloxane thin films derived from SiCl3-terminated carbosilane dendrons of the second, third, and fourth generations, containing 9, 27, and 81 SiCl3 terminal groups. The films were deposited on mica surfaces by spin-coating from solutions of the dendrons with a series of concentrations. The resulting submonolayer to multilayer films were investigated with atomic force microscopy (AFM). AFM studies showed that the morphology of the films was highly dependent upon the generation of the dendrons and the film thickness. Mesoscopic ring, disk, or hole structures were observed. These structures were composed of nanoparticles with sizes corresponding to one dendron molecule or the cluster of a few laterally bound molecules. At submonolayer coverage, the molecules tended to flatten and spread out on mica surfaces. Significantly, this study showed that molecularly flat monolayers could be obtained with the dendrons containing up to 81 SiCl3 terminal groups. Prior to curing, the dendron films could be shaved by an AFM tip, while the films became robust after curing at elevated temperatures, indicating the formation of a strong siloxane network among the dendron molecules. Direct AFM observations indicated that the ring structures were formed from isolated droplets during the evaporation of the liquid thin films. The mechanisms for the formation of the observed film morphology are discussed, based on the structure and properties of the dendrons and the effect of the evaporation process.

Introduction Dendrimers are monodisperse molecules with a regular and highly branched three-dimensional structure.1-5 The size and structure of dendrimers can be tailored by synthesis, and a wide variety of functional groups can be incorporated in the internal and periphery of the dendrimers. The unique structure and properties of this new class of materials have attracted widespread interest. While most of the research focuses on the chemical and physical properties of dendrimers in solution, growing interest has recently also been directed to the interfacial properties that are relevant to a wide range of potential applications of dendrimers, for example, in chemical and biological sensors, liquid crystal displays, organic light emitting diodes, electro-optic films, coating of microelectronic and MEMS devices, and nanolithography resists.3,6,7 Dendrimer thin films have been prepared by LangmuirBlodgett techniques, self-assembly on solid substrates from solution, and spin-casting.3,6,7 The majority of the dendrimer films reported so far are based on poly(amido * To whom correspondence should be addressed. Tel: 713-7432710. Fax: 713-743-2709. E-mail: [email protected]. † Future address: National Laboratory of Molecular & Biomolecular Electronics, Southeast University, Nanjing, 210096, P. R. China. (1) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A. G., III Angew. Chem., Int. Ed. Engl. 1990, 29, 138-175. (2) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic Molecules; VCH: Weinheim, 1996. (3) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. Rev. 1999, 99, 1665-1688. (4) Grayson, S. K.; Frechet, J. M. J. Chem. Rev. 2001, 101, 38193867. (5) Matthews, O. A.; Shipway, A. N.; Stoddart, J. F. Prog. Polym. Sci. 1998, 23, 1-56. (6) Tully, D. C.; Frechet, J. M. J. Chem. Commun. 2001, 1229-1239. (7) Tsukruk, V. V. Adv. Mater. 1998, 10, 253-257.

amine) (PMMA), poly(benzyl ether), poly(propyleneimine), and carbosilane dendrimers. The terminal groups at the periphery of these dendrimers include NH2, OH, COOH, ether, thiol, and alkyl groups. The dendrimers bind on a variety of substrate surfaces through physisorption or formation of ionic, amide, or Au-S bonds, usually through the terminal groups. Both theoretical and experimental studies of dendrimer films have shown that isolated dendrimer molecules tend to flatten and spread out on substrate surfaces when the interfacial interaction is strong.3,6,7,8-10 The preferred conformation of dendrimers balances the entropy factor that favors a spherical shape and the enthalpy factor that tends to maximize the interactions with the substrate surface and thus compresses the molecule. We have recently been interested in a new type of organosiloxane films derived from focally functionalized dendrons whose periphery consists of multiple SiX3 groups (X ) Cl, Ome; Figure 1A). Although self-assembled monolayers (SAMs) of single-chain alkylsiloxanes on SiO2/ Si, glass, and mica have been intensively studied11-19 and recently Frechet and co-workers have reported the self(8) Mansfield, M. L. Polymer 1996, 37, 3835-3841. (9) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171-2176. (10) Sidorenko, A.; Zhai, X. W.; Peleshanko, S.; Greco, A.; Shevchenko, V. V.; Tsukruk, V. V. Langmuir 2001, 17, 5924-5931. (11) Schwartz, D. K. Annu. Rev. Phys. Chem. 2001, 52, 107-137. (12) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (13) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (14) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151-256. (15) Stevens, M. J. Langmuir 1999, 15, 2773-2778. (16) Resch, R.; Grasserbauer, M.; Friedbacher, G.; Vallant, T.; Brunner, H.; Mayer, U.; Hoffmann, H. Appl. Surf. Sci. 1999, 140, 168175. (17) Lambert, A. G.; Neivandt, D. J.; McAloney, R. A.; Davies, P. B. Langmuir 2000, 16, 8377-8382.

10.1021/la026001h CCC: $22.00 © 2002 American Chemical Society Published on Web 08/15/2002

Organosilane Films from SiCl3-Terminated Dendrons

Figure 1. Cartoon illustration of the concept of using dendrons terminated with multiple surface-active groups (R) to control the spacing between the functional moieties located at the focal point of the dendrons (A). As a comparison, in a conventional mixed monolayer without phase separation, the functional moieties are randomly distributed (B), and a significant portion of them do not have the optimal spacing between each other.

assembled monolayers of dendrons containing one SiCl3 group at the focal point,20 thin films derived from the wellknown SiCl3-terminated dendrons/dendrimers21-25 remained unexplored prior to our preliminary communication.26 Our interest in such dendron films is related to our ongoing research aiming to precisely control the spacing between functional groups in organic thin films and the location and orientation of individual functional molecules in organic nanostructures and single-molecule devices. The focally functionalized and SiX3-terminated dendrons (Figure 1A) appear to be ideal building blocks for our purposes. Upon deposition on a polar surface under ambient conditions, the SiCl3 groups in such dendrons should be readily hydrolyzed to Si(OH)3 groups by the surface-bound water.27 The dendrons are expected to flatten out and spread on the polar surfaces to maximize the interaction between the polar periphery and the polar substrate surface.3,6,7,8 In this ideal bonding geometry, the functional group located at the focal point of the dendrons should appoint normal to the film surface. As illustrated in Figure 1A, the spacing between the functional groups (18) Vallant, T.; Brunner, H.; Mayer, U.; Hoffmann, H.; Leitner, T.; Resch, R.; Friedbacher, G. J. Phys. Chem. B 1998, 102, 7190-7197. (19) Britt, D. W.; Hlady, V. J. Colloid Interface Sci. 1996, 178, 775784. (20) Tully, D. C.; Wilder, K.; Frechet, J. M. J.; Trimble, A. R.; Quate, C. F. Adv. Mater. 1999, 11, 314-318. (21) Zhou, L. L.; Roovers, J. Macromolecules 1993, 26, 963-968. (22) Vandermade, A. W.; Vanleeuwen, P. J. Chem. Soc., Chem. Commun. 1992, 1400-1401. (23) Frey, H.; Schlenk, C. Top. Curr. Chem. 2000, 210, 69-129. (24) Terunuma, D.; Kato, T.; Nishio, R.; Matsuoka, K.; Kuzuhara, H.; Aoki, Y.; Nohira, H. Chem. Lett. 1998, 59-60. (25) Kim, C.; Park, E.; Kang, E. Bull. Korean Chem. Soc. 1996, 17, 419-424. (26) Xiao, Z. D.; Cai, C. Z.; Deng, X. B. Chem. Commun. 2001, 14421443. (27) Tripp, C. P.; Hair, M. L. Langmuir 1995, 11, 149-155.

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of the adjacent dendrons in the film can then be controlled by the size (generation) of the dendron and the steric hindrance of the branches in the dendron. The ability to precisely control the spacing between functional groups in thin films is highly desirable in many applications, such as sensors and model systems based on the interactions of surface functional groups with target molecules of various sizes and stereoelectronic properties.28-30 The average density of functional groups in thin films is commonly adjusted by co-deposition of a mixture of inert and functionalized single-chain absorbates as illustrated in Figure 1B. However, considering the high mobility of such absorbates on the surface in the physisorption state (prior to chemisorption),11,31 it is conceivable that phase separation into microscopic or mesoscopic domains with a distinct density of the functional groups is difficult to prevent, especially when these groups can strongly interact with each other, for example, via hydrogen bonding. Although phase separation in mixed SAMs has indeed been observed in several systems32-37 and it precludes the molecular level control of spacing between the functional groups, this potential problem has often been ignored in many studies. Even in the ideal mixed SAMs where the functional groups are randomly distributed on the film surface (Figure 1B), the spacing between these groups follows the Gaussian distribution, and hence a significant portion of the spacings are not optimal. The unique feature of focally functionalized dendrons (Figure 1A) may also be used to bridge the gap between the top-down and bottom-up approaches for fabrication of nanoscale molecular structures. Oxide nanostructures can be fabricated on passivated silicon surfaces by electron beam lithography or scanning probe microscopy.38-44 The dendron molecules can selectively self-assemble on the oxide nanostructures, and serve as “molecular glue” to fix the functional moiety on the nanostructure. Since the base of the dendrons can be a few nanometers across upon spreading on the polar surface, this allows the attachment of a few and eventually only one functional moiety at selected locations of the surface. (28) Southern, E.; Mir, K.; Shchepinov, M. Nat. Genet. 1999, 21, 5-9. (29) Niemeyer, C. M.; Blohm, D. Angew. Chem., Int. Ed. 1999, 38, 2865-2869. (30) Houseman, B. T.; Mrksich, M. Angew. Chem., Int. Ed. 1999, 38, 782-785. (31) Sung, M. M.; Carraro, C.; Yauw, O. W.; Kim, Y.; Maboudian, R. J. Phys. Chem. B 2000, 104, 1556-1559. (32) Heise, A.; Stamm, M.; Rauscher, M.; Duschner, H.; Menzel, H. Thin Solid Films 1998, 329, 199-203. (33) Lewis, P. A.; Donhauser, Z. J.; Mantooth, B. A.; Smith, R. K.; Bumm, L. A.; Kelly, K. F.; Weiss, P. S. Nanotechnology 2001, 12, 231237. (34) Lewis, P. A.; Smith, R. K.; Kelly, K. F.; Bumm, L. A.; Reed, S. M.; Clegg, R. S.; Gunderson, J. D.; Hutchison, J. E.; Weiss, P. S. J. Phys. Chem. B 2001, 105, 10630-10636. (35) Sawaguchi, T.; Sato, Y.; Mizutani, F. J. Electroanal. Chem. 2001, 496, 50-60. (36) Smith, R. K.; Reed, S. M.; Lewis, P. A.; Monnell, J. D.; Clegg, R. S.; Kelly, K. F.; Bumm, L. A.; Hutchison, J. E.; Weiss, P. S. J. Phys. Chem. B 2001, 105, 1119-1122. (37) Kang, J. F.; Liao, S.; Jordan, R.; Ulman, A. J. Am. Chem. Soc. 1998, 120, 9662-9667. (38) Fuierer, R. R.; Carroll, R. L.; Feldheim, D. L.; Gorman, C. B. Adv. Mater. 2002, 14, 154. (39) Gorman, C. B.; Carroll, R. L.; He, Y. F.; Tian, F.; Fuierer, R. Langmuir 2000, 16, 6312-6316. (40) Yasin, S.; Hasko, D. G.; Ahmed, H. Appl. Phys. Lett. 2001, 78, 2760-2762. (41) Shklyaev, A. A.; Shibata, M.; Ichikawa, M. Surf. Sci. 2000, 447, 149-155. (42) Amro, N. A.; Xu, S.; Liu, G. Y. Langmuir 2000, 16, 3006-3009. (43) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S. H.; Mirkin, C. A. Science 1999, 283, 661-663. (44) Xu, S.; Liu, G. Y. Langmuir 1997, 13, 127-129.

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Figure 2. Idealized formulas of the dendrons 1-3 (left) and their gas-phase structures (right) geometrically optimized by MM2, MOPAC in CS ChemBats3D Pro, CambridgeSoft.

As compared to the single-chain alkyltrichlorosilanes (Figure 1B), the presence of multiple SiCl3 groups on the large dendron molecule should greatly reduce the mobility of the molecules upon adsorption on the surface and improve the stability of thin films and nanostructures. Nuzzo et al. proposed that the low edge resolution of patterns formed by microcontact printing (µCP) of octadecyltrichlorosilane (OTS) on the SiO2/Si surface might be due to diffusion and migration31 of OTS molecules at the edge of the patterns.45 Indeed, replacing OTS with docosyltrichlorosilane possessing a higher molecular weight led to a significant improvement in the edge resolution.45 Accordingly, using SiCl3-terminated dendrons may further improve the resolution of µCP of organosiloxanes. Despite the above promises, SiCl3-terminated dendrons are believed to be highly sensitive to moisture and difficult to handle. This is probably the reason thin films of such dendrimers have not been reported. During deposition on a polar surface, besides binding laterally on the substrate surface, the hydrolyzed dendrons may also aggregate in the vertical direction, resulting in a rough film surface.46 Thus arises a key question: Are the SiCl3-terminated (45) Finnie, K. R.; Haasch, R.; Nuzzo, R. G. Langmuir 2000, 16, 69686976. (46) Fadeev, A. Y.; McCarthy, T. J. Langmuir 2000, 16, 7268-7274.

dendrons able to form homogeneous and flat monolayers? To address this question and to learn about the intriguing behavior of SiCl3-terminated dendrons on polar surfaces, we decided to investigate the morphology of thin films derived from the model compounds 1-3.24 As shown in Figure 2, 1-3 are second to fourth generation carbosilane dendrons containing 9, 27, and 81 SiCl3 terminal groups, with molecular weights of 1904, 5713, and 17 140, respectively. Figure 2 also shows a gas-phase structure with the dimension for each compound, obtained by geometric optimization with the MM2 program. The films were deposited on a mica surface by spin-casting with solutions of 1-3 in a series of concentrations. The film morphology was studied by atomic force microscopy (AFM). Experimental Section According to the reported procedure, SiCl3-terminated carbosilane dendrons 1-324 were synthesized from phenyltrichlorosilane by repeating allylation of the SiCl3 terminal groups with allylmagnesium bromide and hydrosilylation of the resulting terminal allyl groups with HSiCl3 catalyzed by H2PtCl6/i-PrOH in tetrahydrofuran (THF). Caution: HSiCl3 is a highly toxic, highly reactive, and volatile compound. Each of the allylterminated dendrons was purified by flash chromatography and characterized by 1H and 13C NMR. The hydrosilylation was monitored by 1H NMR until the allylic signals disappeared. This divergent approach has been widely used for the synthesis of


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Figure 3. Typical tapping mode AFM images (each 5 × 5 µm2) of films of 1 deposited on mica surfaces by spin-casting of a THF solution of 1 with the concentration indicated in the images. The z-scale (gray scale) for images a-h is 8 nm and for images i-l is 5 nm. carbosilane dendrimers and is known to introduce defects to highgeneration dendrimers.23,47 After the completion of the last hydrosilylation, the solvent and unreacted HSiCl3 were removed under a vacuum at 40 °C, and anhydrous THF was added to dissolve most of the residue. The solution containing a trace amount of insoluble materials was filtered and diluted with anhydrous THF to a series of concentrations ranging from 10-8 to 10-3 M. These solutions were used immediately for spin-casting films. Spin-casting was carried out under ambient conditions with a relative humidity of 40% using a model WS-400A-6NPP spin coater from Laurell Tech. Co. A drop of the dendron solution was placed on a freshly cleaved muscovite mica (Structure Probe), and the substrate was immediately spun. The spin rate reached 2000 rpm within 5 s and was maintained at 2000 rpm for 115 s. Curing of the films was performed in an oven at 115 °C overnight, and the samples were then slowly cooled to room temperature. AFM studies were performed in ambient conditions with a Multimode Nanoscope IIIa (Digital Instruments). Tapping mode AFM images were obtained using an Ultrasharp NSC12 Si cantilever (Silicon-MDT Ltd.) with a typical spring constant of ∼14 N/m and a resonant frequency of ∼310 kHz. Height and phase images were recorded simultaneously. For each sample, several images were acquired at different spots in the center area (ca. 3 × 3 mm2) of the sample to ensure the homogeneity. To measure the thickness of some films, part of the film was shaved by AFM in contact mode with a loading force of ∼150 nN using a silicon nitride cantilever tip (typical spring constant, 0.6 N/m). After shaving, the area including the shaved region was imaged in contact mode with a low loading force. The films were intact under this force as verified by “zooming out” imaging of the previously scanned area.

Results Mesoscopic Morphologies. Figures 3-5 are the typical AFM images of a series of films deposited with decreasing concentrations of 1-3. All image sizes in (47) Lorenz, K.; Mulhaupt, R.; Frey, H.; Rapp, U.; Mayerposner, F. J. Macromolecules 1995, 28, 6657-6661.

Figures 3 and 5 are 5 × 5 µm2, and those in Figure 4 are 4 × 4 µm2. Note that the z-scales (contrast) for the images are different; the roughness data of the images in Figures 3-5 are summarized in Table 1. It is clear from these images that the mesoscopic morphology of the films is highly dependent upon the generation of the dendrons and the film thickness. In general, holes, rings, or disks, and dots are the main features in these films. Films deposited from a high concentration (all concentrations are specified in the corresponding images in Figures 3-5) solution of 1-3 consisted of cellular holes as shown in Figures 3a, 4a, and 5a, with a mean depth of 4.2, 6.5, and 11.0 nm, respectively. Relatively flat multilayers of 1 and 2 (Figures 3b and 4b) could be prepared using a suitable concentration. Lowering this concentration led to the formation of irregular holes (Figures 3c and 4c). In contrast, even slightly lowering the solution concentration of 3 switched the cellular hole structures in Figure 5a to large irregular hole structures in Figure 5b. Below a certain concentration, the above hole structures in the thicker films were replaced by ring structures as shown in Figures 3d, 4d-f, and 5c-g. The ring structures appeared in several concentration ranges for each series of the films. In general, for higher generation dendrons, ring structures were present in films deposited with a wider range of concentrations, for example, Figure 3d,f,g for 1, Figure 4d-f,j,k for 2, and Figure 5c-g,i,j,l for 3. The inner diameters of most of the rings were in the range of 50-700 nm and quite homogeneous for each of the films. The rims of the rings were about tens of nanometers wide and less than 10 nm high and became thinner and lower with decreasing concentration. Besides rings, large aggregates or relatively small aggregates (nanoparticles or “dots”) were also present in these films. Between the concentration ranges for ring structures, grainy structures (Figures 3e,l, 4l, and 5h) and flat monolayers (Figures

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Figure 4. Typical tapping mode AFM images (each 4 × 4 µm2) of films of 2 deposited on mica surfaces by spin-casting of a THF solution of the concentration indicated in the images. The z-scale for images a-h is 15 nm and for images i-l is 5 nm.

Figure 5. Typical tapping mode AFM images (each 5 × 5 µm2) of films of 3 deposited on mica surfaces by spin-casting of a THF solution of the concentration indicated in the images. The z-scale for all images is 15 nm.

3h-j, 4h,i, and 5k, see below) were formed, as well as disk structures (Figures 3h,i,k and 4g). The disk structures were found only in films deposited from 1 and 2. The diameters of the disks were typically 150-500 nm, and the heights of the disks were about 0.1-0.2 nm in Figure 3h,i, 0.3 nm in Figure 3k, and 0.5 nm in Figure 4g. Significantly, the films corresponding to Figure 3h-j (prepared from 1) are very flat. With a roughness of 0.11-

0.17 nm root mean square (rms) over 5 × 5 µm2 (Table 1), they are nearly as flat as the widely studied SAMs derived from OTS, for which the roughness was 0.10 nm over 1 × 1 µm2.16 These films are monolayers as shown by the highresolution AFM images (see below). Flat monolayers with a roughness of 0.20-0.33 nm were also obtained from the third-generation dendron 2 over a concentration range of


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Table 1. Root Mean Square (rms) Roughness (nm) of Films of 1-3, Obtained from the AFM Data Corresponding to Figures 3-5 image figure

a

b

c

d

e

f

g

h

i

j

k

l

3 4 5

0.66 2.06 4.60

0.95 1.13 2.98

2.77 2.33 0.70

0.68 1.26 1.33

0.37 0.50 0.97

0.32 0.57 0.74

1.43 0.42 0.51

0.17 0.33 0.43

0.14 0.20 0.69

0.11 1.30 0.38

0.14 0.30 0.35

0.23 0.31 0.24

1 × 10-6 to 2 × 10-6 M (Figure 4h,i). Furthermore, molecularly flat (0.35 nm rms) monolayers of the fourthgeneration dendron 3 containing about 81 SiCl3 groups could be deposited from a 4 × 10-7 M solution, although some large aggregates were also present in the films (Figure 4k). Microscopic Morphologies. Tapping mode AFM images zooming in on a region in Figures 3-5 reveal that most of the films consist of nanoparticles; the disks, rings, and dots are the clusters of the nanoparticles. Both the height and phase images were recorded simultaneously, and selected images are presented in Figure 6. In most cases, the phase images provide better resolution than the corresponding height images, in agreement with previous observations on other carbosilane dendrimer films.48 Figure 6a focusing on a disk in Figure 3k shows that the disk consists of nanoparticles. According to the cross-section plot (not shown), the disk is ∼0.3 nm above the bare mica surface. Figure 6b reveals more details of a ring in Figure 5l. The ring diameter is ∼400 nm, the rim is ∼35 nm wide and ∼1.5 nm high, and it is made of clusters of nanoparticles. Isolated nanoparticles are also “pinned” on the flat mica surface both inside and outside the ring. From the AFM images (not shown), the surface coverage by the films derived from 1-3 appears to be increasing with higher concentrations of the deposition solutions, as expected. However, the exact coverage was difficult to measure, especially for the homogeneous, high-coverage films where the distance between the adjacent molecules becomes comparable to the AFM tip size. Here, we use the term “monolayer” for a layer of dendron molecules that are laterally interconnected into a 2-D network on the substrate surface. For dendron molecules that can spread to a large extent on the surface, their surface density or coverage may vary over a wide range. Indeed, for films deposited from 1, the monolayers were formed when the solution concentrations were higher than 4 × 10-6 M, and the monolayers remained for concentrations ranging from 4 × 10-6 to 8 × 10-6 M (Figure 3j-h). The high-resolution image in Figure 6c focusing on a region in Figure 3j shows that the film is amorphous and consists of interconnected nanoparticles. Combining the high-resolution images with the thickness measurement described later, we believe that the films corresponding to Figures 3h-j, 4h,i, and 5k are monolayers. Careful inspection of the images of Figure 3h,i reveals the presence of disk structures. As will be discussed later, these disks with a height of 0.1-0.2 nm represent domains with a slightly higher density of the molecules than the surrounding monolayer. Figure 6d is a high-resolution AFM image zooming in on a region in Figure 5k, corresponding to a monolayer film derived from 3. The nanoparticles shown in Figure 6d,d′ are higher than those in Figure 6c,c′, consistent with the size of the dendron. Deposition of more molecules on the monolayers increases the roughness of the films and produces more aggregates, disks, or ring structures on top of the monolayer. For example, Figure 6e,e′ zooming in on a region in Figure 3f (48) Ponomarenko, S. A.; Boiko, N. I.; Shibaev, V. P.; Magonov, S. N. Langmuir 2000, 16, 5487-5493.

shows part of the ring structures and aggregates formed on top of a monolayer derived from 1. Isolated nanoparticles with lateral dimensions in the range of 8-40 nm were observed in the low-coverage films of 1-3 deposited from low-concentration solutions. Figure 7a is a high-resolution image of a region outside the disks in Figure 3k, showing the isolated nanoparticles of 1 on the mica surface. The average height of these nanoparticles was 0.25-0.55 nm, as measured by the cross-section plot shown in Figure 7a. Parts b and c of Figure 7 are the high-resolution AFM images of flat areas of Figure 4k (from 2) and Figure 5l (from 3), respectively. The small nanoparticles in these images are in general slightly larger that those in Figure 7a. Their heights were in the range of 0.35-0.75 nm for 2 and 0.72-1.6 nm for 3. To measure the thickness of the high-coverage films, part of the film was shaved by contact mode AFM with a high loading force. After the shaving, the area including the shaved region was imaged with a low loading force. The cross section is then used to estimate the height of the films. As an example, Figure 7d shows an AFM image and a cross-section plot of the region containing the shaved area in a film of 2 (Figure 4i). In this way, the average thickness of the monolayers derived from 1 (Figure 3hj), 2 (Figure 4h,i), and 3 (Figure 5k) was measured, and the results are summarized in Table 2. These values are on average higher than that of the corresponding isolated nanoparticles (see above). Influence of Curing. Upon curing at 115 °C, the films became so robust that they could no longer be shaved by the AFM tip even with loading forces as high as 250 nN; before curing, the films can be easily shaved with a loading force of 150 nN. Slight changes of morphology occurred in multilayer films upon curing. For example, the pinhole in Figure 4c expanded after curing (Figure 8a). For the low-coverage films, the main feature of the films remains the same after curing, but the nanoparticles/aggregates in the films became significantly higher, and their surface density decreased. For example, both the images of a submonolayer film of 2 obtained before curing (Figure 4k) and after curing (Figure 8b) show the presence of ring structures. However, the height of the nanoparticles, obtained by averaging the height values of over 2000 nanoparticles and aggregates in the images, in Figure 8b is 0.76 nm, while in Figure 4k it is 0.36 nm. Also, there are fewer nanoparticles in Figure 8b than in Figure 4k. Significantly, no apparent change of morphology for monolayer films was found upon curing, for example, Figure 8c,d versus Figures 3j and 6d. Discussion Molecular Models. Recent studies of the interfacial properties of dendrimers have shown that the shape of dendrimer molecules in solution phase can be deformed upon absorption on a solid surface.3,6-10,49-54 The degree of deformation is dependent upon the substrate surface (49) Wells, M.; Crooks, R. M. J. Am. Chem. Soc. 1996, 118, 39883989. (50) Lackowski, W. M.; Campbell, J. K.; Edwards, G.; Chechik, V.; Crooks, R. M. Langmuir 1999, 15, 7632-7638.

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Figure 7. (a-c) High-resolution tapping mode AFM images and cross-section plots (along the solid line in the images) of low-coverage films derived from 1 (a), 2 (b), and 3 (c). (d) Contact mode AFM image and cross-section plot of a monolayer film from 2 containing a shaved area. Table 2. Average Thickness (nm) of Monolayers Prepared from 1-3, Measured by the Cross Section of the Shaved Monolayers Corresponding to the Figures Indicated in the Table figure thickness

Figure 6. Selected height (left) and phase (right) images zooming in on a region in the images in Figures 3k (a), 5l (b), 3j (c), 5k (d), and 3f (e). The scale bars correspond to 100 nm. The z-scale for height images is 4 nm and for phase images is 40°.

and the chemical and structural nature of the dendrimers, especially the periphery groups and the flexibility of the dendrimers. The structure of low-generation carbosilane (51) Hierlemann, A.; Campbell, J. K.; Baker, L. A.; Crooks, R. M.; Ricco, A. J. J. Am. Chem. Soc. 1998, 120, 5323-5324. (52) Chechik, V.; Crooks, R. M. Langmuir 1999, 15, 6364-6369.

3h

3i

3j

4h

4i

5h

0.70

0.50

0.35

0.80

0.55

0.85

dendrimers is highly flexible, as indicated by the low glass transition temperatures (-30 to -40 °C) for the hydroxyterminated dendrimers of up to the fourth generation with a branching degree of 4.47 The high flexibility allows these dendrimers to flatten on a polar surface. During the deposition of the carbosilane dendrons 1-3 in THF on mica under ambient conditions, the SiCl3 terminal groups of the dendrons should be rapidly hydrolyzed to Si(OH)3 groups by surface-bound and in situ absorbed H2O.27,55 The resulting silanol-terminated dendron molecules possess both a polar periphery and a hydrophobic wedge. Upon absorption on the mica surface, these molecules may adopt several possible binding geometries, as depicted in Figure 9. For isolated small dendrons, the high flexibility of the branches allows the molecules to flatten on the surface to maximize the interaction between the polar terminal groups and the mica surface (Figure 9a). This orientation exposes the hydrophobic part of the molecule (53) Sheiko, S. S.; Buzin, A. I.; Muzafarov, A. M.; Rebrov, E. A.; Getmanova, E. V. Langmuir 1998, 14, 7468-7474. (54) Li, J.; Swanson, D. R.; Qin, D.; Brothers, H. M.; Piehler, L. T.; Tomalia, D.; Meier, D. J. Langmuir 1999, 15, 7347-7350. (55) Miranda, P. B.; Xu, L.; Shen, Y. R.; Salmeron, M. Phys. Rev. Lett. 1998, 81, 5876-5879.


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Figure 8. AFM images of films of 2 (a,b) and 1 (c,d) obtained after curing at 115 °C overnight. The high-coverage film (a) is corresponds to Figure 4c, the low-coverage film (b) to Figure 4k, and the monolayer (c,d) to Figures 3j and 6d, respectively. The scan size for (a-c) is 4 × 4 µm2 and for (d) is 0.5 × 0.5 µm2. The z-scale for (a) is 15 nm and for (b-d) is 5 nm.

on the surface. However, for high-generation dendrons, the rapidly increased steric and bonding constraint as shown in Figure 1 may not permit all polar periphery groups to reach the mica surface (Figure 9b), and it is expected that such dendrons form aggregates more easily than the smaller dendrons. In addition to the molecule-substrate interaction, the intermolecular interaction is critical in determining the structure of the films. Intermolecular interaction leads to the growth of aggregates through binding of mobile molecules in solution and on the surface. The flexible dendron molecules in the aggregates absorbed on surface may be able to orient most of the polar groups including those cross-linked siloxyl groups to interact with the polar surface as illustrated in Figure 9c,d. The difference between models c and d is the density of the dendron molecules in the layer. If the density is low (Figure 9d), the molecules are likely to spread out more than if it is high as in Figure 9c. However, for high-generation and less flexible dendrons, substantial amounts of the polar end groups may not be able to reach the substrate surface and become active sites for the binding of other molecules not only horizontally but also vertically, resulting in the undesired aggregates containing stacks of molecules (Figure 9e). The above models are in good agreement with most of the AFM data. In the AFM images, the films derived from 1-3 are made of nanoparticles whose dimension is in the range of 8-40 nm. Recently, several groups have reported single molecule resolution images for several types of dendrimer films, including carbosilane, PMMA, and polyester, using tapping mode AFM with phase imaging.48,51,54 Compared to the reported images, the nanoparticles in our images are less uniform in size. Combining this with the fact that the dendrons 1-3 are smaller and have a higher tendency to associate, we believe that the observed nanoparticles are a mixture of single dendron molecules and clusters of a few of the molecules. The height values of the isolated nanoparticles on mica measured by

Figure 9. Illustration of a possible binding geometry of the dendron molecules in the films, including isolated molecules with the whole (a) or the partial (b) polar periphery interacting with the mica surface, monolayers of a high (c) and low (d) density of the molecules whose polar groups bind laterally with the adjacent molecules and vertically with the substrate surface, and high aggregates or a multilayer (e) where molecules in the adlayer bind to the silanol groups of the underlying molecules.

AFM are significantly lower than the calculated hydrodynamic diameter of these molecules (Figure 2). The ratio (H/rc) of the measured height (H) of the isolated nanoparticles over the calculated diameter (rc) of the dendron (Figure 2) is 1:7-1:3 for 1, 1:9-1:4 for 2, and 1:5-1:2 for 3. These H/rc values indicate a substantial flattening of the dendrons upon adsorption on mica surface (Figure 9a,b), in agreement with previous observations on other low-coverage dendrimer films where the dendrimers are flexible and the dendrimer-substrate interaction is strong.6,7,9,10,49-54 However, the AFM tip can also deform the dendrons to some extent. Compared to the isolated nanoparticles, the H/rc values for molecules in monolayer films derived from 1, 2, and 3, corresponding to Figures 3h-j, 4h,i, and 5k, increases to 1:5-1:2 for 1, and 1:8-1:3 for 2, and 1:4 for 3. We observed that the thickness of the monolayer prepared from 1 varied in a wide range (0.350.70 nm) and increased with a higher concentration of the deposition solution, that is, a higher density of the molecules on the substrate surface. This can be rationalized by the models in Figure 9c,d. Similarly, Sheiko et al. have shown that hydroxy-terminated carbosilane dendrimers in a Langmuir monolayer can be compressed laterally and remain as a stable monolayer over a wide range of molecular areas.53 Accordingly, the faint disks in Figures 3h,i and 4g that are about 0.1-0.2 and 0.5 nm above the monolayer are likely composed of a higher density of molecules than their surrounding monolayer, corresponding to model c versus model d in Figure 9. The origin of the disk structures will be discussed later. The higher tendency of the dendron 3 that possesses about 81

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SiCl3 terminal groups to form large aggregates in most of the films can be rationalized by the model in Figure 9e in which the dendron molecules cannot facilely arrange all the polar periphery groups to interact with the substrate surface and the adjacent molecules, and hence the vertical binding can compete with the horizontal binding. The Effect of Curing. Intermolecular condensation between the Si(OH)3 end groups in the films deposited from 1-3 may occur at room temperature, but apparently only to a limited extent. In fact, a multilayer film of 2 which was kept at room temperature in air for 2 weeks can still be shaved by an AFM tip. Also, the water contact angles of all films before curing were lower than 16° according to our studies on the wettability of the films (to be reported separately). Both observations indicate that the cross-linking of the silanol groups to form a siloxane network was not completed at room temperature, and the films contained a significant amount of silanol groups. Upon curing at 115 °C, the high-coverage films displayed a marked increase in advancing contact angles of water, approaching 100° for full-coverage films, and the films were so robust that shaving with an AFM tip was not possible even with a very high loading force, in accord with a film structure consisting of an intensively crosslinked siloxane network. Upon curing at 115 °C, the pinholes in the multilayer films expanded (Figure 4c vs Figure 8a). This is likely due to the inter- and intramolecular condensation of a large number of silanol groups in the films, which shrinks the molecules and hence the film. The same explanation may apply to the low-coverage films where the average height of the aggregates increased markedly from 0.36 to 0.76 nm upon curing (Figure 4k vs Figure 8b). The density of isolated nanoparticles in these films reduced upon curing, indicating that the nanoparticles aggregated at elevated temperatures. Since mica substrate surfaces possess a low density of hydroxy groups,56 the condensation of the silanol groups in the dendrons should occur mainly intraand intermolecularly. The lack of covalent bonding between the dendron molecules and the substrate surface facilitates the migration and aggregation of the molecules. For the monolayer films, both the mesoscopic and microscopic morphologies remained unchanged, according to the (high-resolution) AFM images, for example, Figure 8c,d versus Figures 3j and 6d. This highly desirable result can be attributed to the unique structure of the dendrons. As illustrated in Figure 9c, the molecules in the monolayer initially interact with the adjacent molecules via multiple Si(OH)3 groups, and the branches of the molecule are flexible, that is, they can extend upon intermolecular condensation. Both the presence of multiple silanol groups and the flexibility of the molecule greatly facilitate the molecule to form at least one OSi-O bond with each of the adjacent molecules, which preserves the extended molecular network structures in the monolayer films. Mesoscopic Morphologies. Interestingly, rings or disks are present in many films shown in Figures 3-5. Several groups have reported the observation of mesoscopic ring structures in thin films deposited on solid substrates by evaporation of a solution or suspension of a variety of materials, such as porphyrin derivatives,57-60 (56) Carson, G. A.; Granick, S. J. J. Mater. Res. 1990, 5, 1745. (57) Schenning, A.; Benneker, F. B. G.; Geurts, H. P. M.; Liu, X. Y.; Nolte, R. J. M. J. Am. Chem. Soc. 1996, 118, 8549-8552. (58) Hofkens, J.; Latterini, L.; Vanoppen, P.; Faes, H.; Jeuris, K.; DeFeyter, S.; Kerimo, J.; Barbara, P. F.; De Schryver, F. C.; Rowan, A. E.; Nolte, R. J. M. J. Phys. Chem. B 1997, 101, 10588-10598. (59) Latterini, L.; Blossey, R.; Hofkens, J.; Vanoppen, P.; De Schryver, F. C.; Rowan, A. E.; Nolte, R. J. M. Langmuir 1999, 15, 3582-3588.

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nanoparticles,61-65 and polymers.66 Compared to these ring structures, those in Figures 3-5 are generally smaller, most of the narrowly distributed ring diameters being in the range of 150-800 nm versus generally more than 1 µm. In addition, they consist of cross-linked dendron molecules and hence are far more robust than the reported ones in which the molecules/particles are held by relatively weak intermolecular forces. Several mechanisms have been proposed to account for the ring formation. Ohara and Gelbart proposed that the rings in their system (a hexane solution of silver or gold nanoparticles covered with a SAM of alkanethiolate, deposited on a carbon TEM grid) originated from the dry holes generated during the evaporation of the liquid film. During the expansion of the dry holes, which is driven by evaporation, the fluid flow accumulates the nanoparticles at the contact line of the hole, forming a rim. The rim pins after accumulation of a sufficient number of the nanoparticles that increase the friction of the flow due to the interaction between the nanoparticles and the substrate surface.67 This mechanism is in accord with the observation that the density of nanoparticles inside the ring is less than outside.61 Other researchers have applied similar mechanisms to rationalize the ring formation in their systems.65 Maillard et al. argued that the hole formation is driven by the Marangoni effect,63 that is, rapid evaporation of a liquid film generates a temperature variation in the field, leading to convective flow and a solute concentration gradient, as well as the surface tension gradient which drives instability and pattern formation. Film rupturing into holes occurs in the low particle density (thinner) areas. The convective flow near the contact line of the hole further increases the local density of the nanoparticles. Nolte et al. studied the formation of ring structures after evaporation of a drop of porphyrin solution on glass.57-60 They concluded that the rings were developed from droplets formed by the rupture of the liquid film due to evaporation.59 In situ confocal fluorescence imaging of the evaporation process revealed the formation of micrometer-sized droplets that rapidly coalesce into larger ones from which rings were generated around the pinned contact line. The researchers believed that the conversion of droplets into rings proceeded via the same mechanism as the one formulated by Deegan et al. for the formation of macroscopic rings such as coffee stains on solid surfaces.68-70 Deegan et al. showed that a radial flow outward from the center of a drying drop drags the solute to the contact line of the drop, and the enrichment of the solute at the contact line forms the rim. The origin of the radial flow was proposed to be the combination of the evaporation, the pinning of the contact line by the solute, (60) Biemans, H. A. M.; Rowan, A. E.; Verhoeven, A.; Vanoppen, P.; Latterini, L.; Foekema, J.; Schenning, A.; Meijer, E. W.; De Schryver, F. C.; Nolte, R. J. M. J. Am. Chem. Soc. 1998, 120, 11054-11060. (61) Ohara, P. C.; Heath, J. R.; Gelbart, W. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 1078. (62) Maenosono, S.; Dushkin, C. D.; Saita, S.; Yamaguchi, Y. Langmuir 1999, 15, 957-965. (63) Maillard, M.; Motte, L.; Ngo, A. T.; Pileni, M. P. J. Phys. Chem. B 2000, 104, 11871-11877. (64) Shafi, K.; Felner, I.; Mastai, Y.; Gedanken, A. J. Phys. Chem. B 1999, 103, 3358-3360. (65) Huang, S. M.; Dai, L. M.; Mau, A. W. H. J. Mater. Chem. 1999, 9, 1221-1222. (66) Hahm, J.; Sibener, S. J. Langmuir 2000, 16, 4766-4769. (67) Ohara, P. C.; Gelbart, W. M. Langmuir 1998, 14, 3418-3424. (68) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Phys. Rev. E 2000, 62, 756-765. (69) Deegan, R. D. Phys. Rev. E 2000, 61, 475-485. (70) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827-829.


and the tendency of the drop to maintain its shape.69 Other possibilities to induce an influx of solute to the rims have also been proposed.62,71 All of the above mechanisms share a common feature, that is, the liquid flow drags solute toward the rim of a hole or a droplet and pins the contact line which defines the ring after the solvent dries out. Both theoretical and experimental studies have concluded that the formation of droplets or holes in an evaporating film is primarily driven by the instability of the film when it thins to a critical thickness.72,73 Beyond this thickness, nucleation at defects or spinodal decomposition can rupture the film to form holes usually with a circular shape to minimize edge energy. Continuously thinning the film by evaporation expands the holes. If the rim of the holes cannot be pinned by the solute, they will “percolate” to form droplets at their interstices.67 This applies not only to dewetting substrates but also to wetting substrates that are related to our system.74 Lipson showed that despite the fact that under equilibrium conditions a water film wets a mica surface, when the film reaches a thickness of 100-1000 Å, “dry holes” nucleate in the film and the water recedes to the wet parts.75-77 During the recession, a thick rim of water is created, which becomes hydrodynamically unstable and breaks up into water droplets on mica. The AFM studies by Bluhm and Salmeron also revealed that supercooled (-36°) water droplets that are condensed on an ice plate do not completely spread on the ice surface but remain at a contact angle of about 3°.78 On the basis of the previous studies outlined above, combined with the chemical nature of the dendrons 1-3, we can provide a tentative account for the origin of the morphology shown in Figures 3-5. We assume that at the early stage of the spin-coating process, the film thins mainly due to the spin inertia and the spreading of the liquid drop on the wetting substrate surface. At the latter stage, evaporation dominates the thinning of the films and generates instability within the film. When the film thins below a critical thickness, the instability causes film rupture through the formation of dry holes leading to droplets.74-78 Most of our attempts to monitor the evolution of the film structures failed, because the films dried out rapidly and the film structures fixed before the first AFM image was obtained, no mater how fast we loaded the sample and carried out the AFM measurement. However, on several occasions we were able to observe that the rings in some films were developed from droplets,59 although these observations cannot exclude the possibility of forming rings through dry holes for the other films.67 For example, Figure 10a is an AFM image obtained after the first scan of a film prepared with 2, showing the presence of liquid droplets on the film. The subsequent scan of the same area provided in Figure 10b revealed that the droplets developed into rings with the same diameter. The proposed process of ring formation is outlined in Figure 11a-d. On the basis of the published data,55,59,79-81 it is reasonable to assume that most of the SiCl3 groups in the dendron molecules have been hydrolyzed to Si(OH)3 groups before the film rupture starts. The Si(OH)3(71) Parisse, F.; Allain, C. Langmuir 1997, 13, 3598-3602. (72) Sharma, A.; Khanna, R. J. Chem. Phys. 1999, 110, 4929-4936. (73) Oron, A.; Davis, S. H.; Bankoff, S. G. Rev. Mod. Phys. 1997, 69, 931-980. (74) Padmakar, A. S.; Kargupta, K.; Sharma, A. J. Chem. Phys. 1999, 110, 1735-1744. (75) Elbaum, M.; Lipson, S. G. Phys. Rev. Lett. 1994, 72, 3562-3565. (76) Samid-Merzel, N.; Lipson, S. G.; Tannhauser, D. S. Physica A 1998, 257, 413-418. (77) Lipson, S. G. Phys. Scr. 1996, T67, 63-66. (78) Bluhm, H.; Salmeron, M. J. Chem. Phys. 1999, 111, 6947-6954.

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Figure 10. Direct AFM observation of the formation of ring structures in a film prepared by spin-coating a 10-5 M THF solution of 2 on mica. After spin-coating, the sample was rapidly loaded to AFM for imaging in tapping mode. The first scan (which took 100 s) gave image a, and the second scan over the same area resulted in image b. The image sizes are 10 × 10 µm2. The z-scale for (a) is 80 nm and for (b) is 15 nm.

terminated dendron molecules should prefer to bind to the polar mica surface and to aggregate (Figure 11a). During the film rupture and the subsequent formation of droplets, the fluid flow brings the mobile dendron molecules to the droplets and thus increases the density of the molecules over the area covered by the droplet (Figure 11b). The rings are formed due to the pinning of the dendron molecules along the contact line of the evaporating droplets. These molecules are dragged to the contact line by the outward flow generated within the droplet by evaporation (Figure 11b), forming the rim (Figure 11c,d).68-70 Therefore, to form a ring, a sufficient amount of dendron molecules must be mobile in the droplet, that is, they should either dissolve in the droplet or absorb on the substrate surface but have a sufficient mobility to be dragged by the outward flow. The large dendron molecules 3 should interact more strongly with the mica surface and thus have a higher adsorption rate and a lower mobility than the smaller dendrons 1 and 2. The presence of ring structures in the low-coverage films of 3 (Figures 5l and 6b), however, indicates that single molecules or small aggregates derived from 3 are actually quite mobile in the system. Formation of larger aggregates of molecules should increase the substrate-molecule interaction and eventually immobilize the molecules. If this process happens before the droplets are generated, the formation of ring structures should be suppressed. It is conceivable that the rate of aggregation and hence immobilization of the dendron molecules on the surface is determined by the surface density of the molecules. Indeed, upon increasing the concentration, the films of 3 changed from ring structures (Figure 5l) to a monolayer (Figure 5k). Also, the formation of the flat monolayers from 1 (Figure 3h-j) and 2 (Figure 4h,i) indicates that at a suitable concentration range, most of the molecules rapidly absorb and bind laterally to form a monolayer on the surface (models in Figure 9c,d) prior to the rupture of the liquid films into droplets. Therefore, only a small amount of dendron molecules are brought to the droplets in which (79) The rupture of a film of chloroform with vapor pressure (0.235 bar) higher than that of THF (0.215 bar, the solvent we used, ref 80) takes 89 s in ambient conditions after deposition (ref 59). The time for hydrolysis of Ph2SiHCl, which is far less reactive than RSiCl3, by water absorbed on a sol-gel film is less than 20 s (the time needed to complete the measurement), and for 1 mM water in heptane it is less than 180 s (ref 81). For a 1 µm thick liquid film (thicker than the critical thickness for film rupture), even ignoring the rapid adsorption of water from air by the solvent (THF), the concentration of water bound on the mica surface is more than 5 mM (ref 55). (80) Strawhecker, K. E.; Kumar, S. K.; Douglas, J. F.; Karim, A. Macromolecules 2001, 34, 4669-4672. (81) Rivera, D.; Harris, J. M. Anal. Chem. 2001, 73, 411-423.

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presses aggregation, and disk or ring structures are expected to form on top of a layer of dendron molecules and are indeed found in Figures 3f,g, 4f,g, and 5d-g,i,j (for a high-resolution image, see Figure 6e). Further increasing the density favors aggregation and immobilization, explaining the grainy film shown in Figures 3e and 5h,j. In some of the low- or high-coverage films, for example, Figures 3i and 4g,j,k, the layer inside the rings is thicker than outside. This may be due to the ring formation (model in Figure 11a-d) concurrent with insertion of the mobile dendron molecules in the droplet into the underlying layer (model in Figure 11e,f). The height and width of the rims in the ring structures should be determined by the concentration of the mobile molecules within the droplet and the ability of the molecules to immobilize (pin) along the contact line of the droplet on the mica or film surface. If the molecules cannot pin the droplet, the droplet will shrink until a sufficient amount of the molecules are accumulated in the rim to stop the shrinking or dry out to form a dot (left side of Figure 11d). This is probably the reason the rim widths are quite similar in each ring structure. Other possible reasons for the presence of large aggregates and/or dots in most of the films is the aggregation of the dendron molecules on the substrate/film surface and in solution during and prior to deposition in the presence of a trace amount of water in the system. The morphology in the multilayer films (Figures 3a-c, 4a-c, and 5a,b) is similar to those reported for a wide variety of polymer films, such as mesogen-terminated carbosilane dendrimers,82 collagen,83 and nanoparticlefilled polymers,84 and is probably due to the dewetting of the films. High concentrations of dendrons increase the viscosity of the solution that may impede the rupture and droplet formation process. Figure 11. Proposed process for the formation of rings, dots, and disks during the evaporation of a liquid film (a) containing the dendron molecules (round circles for those dissolved in solution and oblate circles for those absorbed on the mica surface). The cross-section sketch in (b) represents the rupture of a part of the liquid film into two droplets. The continuous evaporation generates an outward flow within the droplets as indicated by the empty arrows in (b), which drags the mobile dendron molecules to the contact line of the droplet. A sufficient number of these molecules within the larger droplet pin the contact line of the droplet and form the rim in (c) and further the ring in (d) when the droplet dries out, while the small droplet in (b) shrinks and dries out into a dot in (d). The dendron molecules may also aggregate into dots, e.g., the one inside the ring in (c,d). The two faint disks in (f) are formed when the mobile molecules in the droplets in (e) rapidly insert into the surface underneath and are immobilized thereafter.

they rapidly insert into the underlying monolayer and are immobilized thereafter as illustrated in Figure 11e,f, leading to the faint disks shown in Figures 3h,i and 4h. On the basis of the above discussion, for the small dendron 1, at a low surface coverage, the molecules should be mobile and easily brought to the droplets. The accumulation of these molecules in the droplet facilitates them to aggregate and immobilize on the surface underneath. If the immobilization is faster than ring formation, disk structures such as those in Figure 3k are generated. The formation of structures on top of a monolayer, for example, Figure 6e, is probably due to the relatively high mobility or low adsorption rate for molecules on top of the monolayer where the polar groups preferentially interact with the mica surface (Figure 9c,d). The rate of aggregation of these molecules is probably again dependent upon their density. Low density sup-

Conclusion A new type of siloxane thin films based on SiCl3terminated carbosilane dendrons containing 9-81 SiCl3 terminal groups was explored in this study. We demonstrated that very flat (down to 0.11 nm rms over 5 × 5 µm2) monolayers on mica can be obtained with the dendron 1 containing 9 SiCl3 groups; once the concentration is properly adjusted, the monolayer films can be prepared conveniently by spin-coating. Both the third- and fourthgeneration dendrons 2 and 3 can also form flat monolayers, although in general more aggregates and ring structures are present in the films derived from these higher generation dendrons. These results invite further exploration of using focally functionalized dendrons with a surface-active periphery as a new type of thin film platforms for potential applications such as sensors and nanostructures. Their potential advantages include a high stability and an adjustable, well-defined, and large spacing between functional groups in the thin films. Equally interesting is the behavior of the dendrons 1-3 reminiscent of small clusters of single-chain alkylsiloxanes during deposition, which may provide new insights into the self-assembly growth of alkylsiloxane monolayers.11-19,31 The AFM study showed that the film morphology featuring various patterns, such as grain, hole, (82) Coen, M. C.; Lorenz, K.; Kressler, J.; Frey, H.; Mulhaupt, R. Macromolecules 1996, 29, 8069-8076. (83) Thiele, U.; Mertig, M.; Pompe, W. Phys. Rev. Lett. 1998, 80, 2869-2872. (84) Barnes, K. A.; Karim, A.; Douglas, J. F.; Nakatani, A. I.; Gruell, H.; Amis, E. J. Macromolecules 2000, 33, 4177-4185. (85) Vossmeyer, T.; Chung, S. W.; Gelbart, W. M.; Heath, J. R. Adv. Mater. 1998, 10, 351-353.


disk, or ring structures, was highly dependent on the thickness of the films and the generation of the dendrons. The evolution of isolated droplets into rings in a freshly deposited film was captured by AFM imaging. On the basis of these observations and the previous studies by other groups, we propose that during spin-coating, the instability of the liquid film driven by evaporation causes the film rupture into droplets that may develop into rings, disks, or dots when the film dries out. The rate of immobilization of the dendron molecules versus the rate of pattern (droplets, rings, disks) formation determines the amount of the molecules accumulated within the patterns that dictate the final film morphology. Flat films result from rapid immobilization of the molecules, while ring or disk structures signal the presence of significant amounts of mobile molecules during the film rupture. Accordingly, our results suggest that a high surface coverage is required to rapidly immobilize the dendrons 1-3, and molecules in the low-coverage films have a high desorption rate, as indicated by the presence of ring or disk structures in these films. Therefore, formation of the monolayers appears to be a highly cooperative process.

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This study also confirms that at submonolayer coverage the dendron molecules tend to flatten and spread out on mica surfaces.8-10 Another interesting observation is that the siloxane network between the dendron molecules cannot be completely established without curing the films at elevated temperatures. Currently, we are investigating the films prepared with other dendrons possessing different degrees of branching and lengths of branching units, as well as focally functionalized ones. The growth from solution and the use of other substrates such as SiO2/Si allowing more convenient study of the films by ellipsometry, X-ray photoelectron spectroscopy, and attenuated total reflection IR are also under way. Acknowledgment. This work was supported by the Texas Advanced Research Program under Project No. 003652-0365-1999, the Robert A. Welch Foundation, and the Petroleum Research Fund. LA026001H

The First Organosiloxane Thin Films Derived from SiCl3-Terminated

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