Nanostructured Diblock Copolymer Films: A Grazing Incidence Small

From grazing incidence small-angle neutron scattering (GISANS), the microphase separation structure is determined in addition. Although these ultrathi...
0 downloads 0 Views 265KB Size
Download PDF
7778

Langmuir 2003, 19, 7778-7782

Nanostructured Diblock Copolymer Films: A Grazing Incidence Small-Angle Neutron Scattering Study† P. Mu¨ller-Buschbaum,*,‡ R. Cubitt,§ and W. Petry‡ TU Mu¨ nchen, Physik-Department, LS E13, James-Franck-Strasse 1, 85747 Garching, Germany, and Institut Laue-Langevin, 6 rue Jules Horowitz, b.p. 156, 38042 Grenoble, France Received October 21, 2002. In Final Form: February 6, 2003

The destabilization of confined ultrathin poly(styrene-block-paramethylstyrene) films with a fully deuterated polystyrene block in the strong segregation limit is investigated. Using the spin-coating technique, polymer films with thicknesses smaller than the radius of gyration of the unperturbed polymer molecule were prepared on top of solid substrates. Successively these films have been annealed by storage under a toluene atmosphere. The evolving surface topographies are investigated with atomic force microscopy. From grazing incidence small-angle neutron scattering (GISANS), the microphase separation structure is determined in addition. Although these ultrathin films are hardly detectable with reflectivity measurements, in GISANS a good signal was obtained. The time dependence of the characteristic lateral length of the nanostructures follows an Avrami growth law.

Introduction Engineered nanostructures such as quantum dots, nanowires, nanotubes, or nanolayers are of strong interest for basic research as well as with respect to future applications.1-3 This includes the option of new properties due to surface effects and size reduction down to a regime in which a characteristic length scale of a physical phenomenon becomes comparable with the typical length scale of the nanostructure.4 As a consequence, the control of nanostructured surfaces marks the first basic step. Complementary to lithographic patterning techniques, the creation of nanostructures due to self-organization has become increasingly important. Following the approach of soft-lithography, structures down to several tens of nanometers are accessible5 and most likely these techniques are utilized for the creation on anisotropic surface pattern comparable to the common resist technique in the semiconductor industry. Isotropic structures are easily created by the concept of self-assembly.6-10 While phaseseparated surface structures in polymer blend films are typically of micrometer size, microphase separation in diblock copolymer systems is well-known to offer nano* Corresponding author. † Part of the Langmuir special issue dedicated to neutron reflectometry. ‡ TU Mu ¨ nchen. § Institut Laue-Langevin. (1) Thurn-Albrecht, T.; Schotter, J.; Ka¨stle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126. (2) Clusters and Nanomaterials; Kawazoe, Y., Kondon, T., Ohno, K., Eds.; Springer-Verlag: Berlin, 2002. (3) Bauer, G.; Pittner, F.; Schalkhammer, T. Mikrochim. Acta 1999, 131, 107. (4) Russell, T. P Science 2002, 297, 964. (5) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Science 1996, 272, 85. (6) Whitesides, G. M.; Mahias, J. P.; Seto, C. T. Science 1990, 254, 1312. (7) Aksay, A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273, 892. (8) Thurn-Albrecht, T.; DeRouchey, J.; Jaeger, H. M.; Russell, T. P. Macromolecules 2000, 33, 3250. (9) Spatz, J. P.; Mo¨ller, M.; Noeske, M.; Behm, R. J.; Pietralla, M. Macromolecules 1997, 30, 3874. (10) Mu¨ller-Buschbaum, P.; Wolkenhauer, M.; Wunnicke, O.; Stamm, M.; Cubitt, R.; Petry, W. Langmuir 2001, 17, 5567.

structures in the bulk as well as in thin films.11-14 Recently, it was shown that from a destabilization of confined ultrathin films nanostructured polymeric surfaces result as well.15-17 Following the scaling laws of dewetting due to the reduced amount of polymeric material available, the size of the resulting structures decreases with decreasing film thickness. Destabilizing confined ultrathin diblock copolymer films thus offers the opportunity to introduce two intrinsic lateral length scales, the characteristic periodicity of the microphase separation structure L0 and the most prominent in-plane length of the dewetting structure Λ. For a large range of molecular weights of the diblock copolymers, L0 < Λ is fulfilled and the primary structure is given by the dewetting process. The resulting structures are pancake-shaped droplets with an internally periodically arranged microphase separation structure.10 As usual for self-assembled structures, it is isotropic, which is helpful for many applications such as sensors or templates (function does not depend on the orientation of the structure). In this case, not only does the characteristic lateral length have to be controlled but in addition the type of structure installed at the surface is of importance; final dewetting states are no longer sufficient. Due to the minimization of the contact line area, the final dewetting state is given by a drop which degenerates in the case of ultrathin films into a pancake-shaped droplet. However, since there is no need to restrict ourselves only to final states, with the presented investigation we address the intermediate states which are accessible during the destabilization process. (11) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: Oxford, 1998. (12) Mansky, P.; Russell, T. P.; Hawker, C. J.; Mays, J.; Cook, D. C.; Satija, S. K. Phys. Rev. Lett. 1997, 79, 237. (13) Torikai, N.; Noda, I.; Karim, A.; Satija, S. K.; Han, C. C.; Matsushita, Y.; Kawakatsu, T. Macromolecules 1997, 30, 2907. (14) Vignaud, G.; Gibaud, A.; Gru¨bel, G.; Joly, S.; Ausserre, D.; Legrand, J. F.; Gallot, Y. Physica B 1998, 248, 250. (15) Mu¨ller-Buschbaum, P.; Gutmann, J. S.; Stamm, M.; Cubitt, R.; Cunis, S.; von Krosigk, G.; Gehrke, R.; Petry, W. Physica B 2000, 283, 53. (16) Mu¨ller-Buschbaum, P.; Gutmann, J. S.; Stamm, M. Phys. Chem. Chem. Phys. 1999, 1, 3857. (17) Mu¨ller-Buschbaum, P.; Gutmann, J. S.; Cubitt, R.; Stamm, M. Colloid Polym. Sci. 1999, 277, 1193.

10.1021/la0267241 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/02/2003

Nanostructured Diblock Copolymer Films

While on a local scale the surface structure is nicely pictured by atomic force microscopy (AFM), to obtain statistically significant information scattering is helpful. With grazing incidence small-angle scattering, a length regime from molecular to micrometer-sized and thus comparable to that of AFM is addressed.18-20 The combination of partial deuteration and the use of neutrons to probe the surface structures offers access to the internal structures as well.15 The article is structured as follows: The introduction is followed by an Experimental Section describing the sample preparation and the techniques used. The section Results and Discussion is followed by Summary and Outlook. Experimental Section Sample Preparation. To ensure reproducible preparation conditions, the native oxide covered Si(100) surfaces (MEMC Electronic Materials Inc., Spartanburg) were cleaned prior to the spin coating. The cleaning includes 15 min at 80 °C in an acid bath consisting of 100 mL of 80% H2SO4, 35 mL of H2O2, and 15 mL of deionized water, rinsing in deionized water, and drying with compressed oil-free nitrogen. To obtain confined ultrathin films, a highly diluted diblock copolymer-toluene solution was spin coated utilizing typical spinning parameters (1950 rpm for 30 s). The diblock copolymer poly(styrene-block-paramethylstyrene), denoted P(Sd-b-pMS) was prepared anionically (Polymer Standard Service, Mainz, Germany). The nearly symmetric P(Sdb-pMS) has a fully deuterated polystyrene (PSd) block and a protonated poly(paramethylstyrene) (PpMS) block. The degree of polymerization of the PSd block compared to the total chain is fPSd ) NPSd/N ) 0.47. From the molecular weight Mw ) 230 000 g/mol (narrow molecular weight distribution Mw/Mn ) 1.08) and the polymer-polymer interaction parameter of PSd and PpMS χ ) A + B/T with A ) -0.011 ( 0.002 and B ) 6.8 ( 1 K,21 a value of χN ∼ 24.0 is calculated. Thus the investigated system belongs to the strong segregation regime.22 After the spin coating, homogeneous P(Sd-b-pMS) films result. Their thicknesses as measured with X-ray reflectivity were 1.5 ( 0.2 nm, which is very small as compared to the radius of gyration in the bulk Rg ) 13.6 nm.23,24 The homogeneity of the film was checked with atomic force microscopy and from the small value of the surface roughness (on the order of 0.5 nm) obtained from the X-ray data. To destabilize the initially homogeneous films, the samples were stored under toluene vapor (pressure p ) 0.8p0, temperature ) 296 K) for a fixed time interval. Storage times of 1, 2, 3, 4, 5, 6, 8, and 12 h were taken. After this exposure time, the samples were quenched to ambient air and examined. Several identical samples were prepared and investigated, exhibiting the presented results. Atomic Force Microscopy. Using a PARK Autoprobe CP atomic force microscope, the sample surface was probed. Micrographs were recorded at different sample positions. In the investigated confined ultrathin film thickness regime, the polymeric structures are highly sensitive and only in the applied noncontact mode is a tip-induced sample degradation minimized. The silicon gold-coated conical cantilevers have resonant frequencies of about f ) 111 kHz and a spring constant of =2.1 N m-1. All measurements were performed in air at room temperature. At each individual sample position, scans with different ranges from 0.2 µm × 0.2 µm up to 10 µm × 10 µm were performed. From the raw data, the background due to the scanner tube (18) Salditt, T.; Metzger, T. H.; Peisl, J.; Reinecker, B.; Moske, M.; Samer, K. Europhys. Lett. 1995, 32, 331. (19) Naudon, A.; Babonneau, D.; Thiaudiere, D.; Lequien, S. Physica B 2000, 283, 69. (20) Mu¨ller-Buschbaum, P.; Casagrande, M.; Gutmann, J. S.; Kuhlmann, T.; Stamm, M.; Cunis, S.; von Krosigk, G.; Lode, U.; Gehrke, R. Europhys. Lett. 1998, 42, 517. (21) Schnell, R.; Stamm, M. Physica B 1997, 234, 247. (22) Helfhand, E.; Wassermann, Z. R. Macromolecules 1980, 13, 994. (23) Jung, W. G.; Fischer, E. W. Macromol. Chem. Macromol. Symp. 1988, 16, 281. (24) Bartels, V. T.; Abetz, V.; Mortensen, K.; Stamm, M. Europhys. Lett. 1994, 27, 371.

Langmuir, Vol. 19, No. 19, 2003 7779 movement is fully subtracted to determine the values of the rootmean-square (rms) roughness over the complete scan area. In addition to the rms roughness, which displays statistical information perpendicular to the sample surface, statistical information parallel to the surface is obtained from the power spectral density function (PSD).25,26 The PSD is calculated from the scanning force microscopy height data by a 2D Fourier transformation and radial average of the isotropic Fourier space data. Due to the different scan ranges in real space, the PSDs cover different intervals in the reciprocal space. Thus a combination of PSDs related to different scan ranges enlarges the covered interval in reciprocal space as compared to one individual PSD. In the following, the combined PSD data are called the master curve. The master curve is equivalent to a scattering signal and thus shows the existence of a most prominent inplane length scale which might be present within the resolvable range. If a distinct peak is present in the master curve, the most prominent in-plane length Λ is extracted from its position. With the rms roughness and the master curve, the sample surface is statistically described. Grazing Incidence Small-Angle Neutron Scattering (GISANS). GISANS measurements were performed at the D22 beamline at the neutron reactor ILL (Grenoble). In contrast to the common transmission geometry, we employed a reflection geometry. The sample was placed horizontally on a two-circle goniometer with a z-translation table.27 Extremely narrow crossslits with typical openings of millimeters and a large collimation distance were used. The background was optimized due to the completely evacuated pathway except a small region of (10 mm in front and behind the sample. Experiments were carried out at wavelengths of 0.6 and 1.0 nm (wavelength selector, ∆λ/λ ) 10%). Details concerning the beamline are reported elsewhere.27 The scattered intensity at one fixed angle of incidence Ri is detected with a two-dimensional detector (128 × 128 pixel array). Due to the larger sample-detector distance of 17.66 m, a resolution of 4.45 × 10-3 nm-1 (2.69 × 10-3 nm-1) was achieved at a wavelength of λ ) 0.6 nm (1.0 nm). The small-angle scattering information is extracted from horizontal slices (with respect to the sample surface) taken at the critical angle of PpMS. Statistics of these slices are improved by integrating the intensity (∆qz ) (1.91 × 10-2 nm-1). Typical counting times were on the order of 8 h to obtain reasonable statistics. Thus it is not possible to perform in situ neutron scattering having the sample stored under toluene vapor.

Results and Discussion A. GISANS Data. A typical GISANS scattering pattern obtained at a fixed incident angle Ri is shown in Figure 1. To achieve surface sensitivity, the critical angle of deuterated polystyrene PSd was chosen as the angle of incidence. The two-dimensional scattering pattern can be understood to be built up from several horizontal lines at different exit angles Rf or from several vertical lines at different out-of-plane angles ψ.28 The denotations horizontal and vertical refer to the sample surface. As typical features, the specular peak at Rf ) Ri and ψ ) 0 and the Yoneda peak at Rf ) Rc (critical angle of surface structure29,30) and ψ * 0 are visible. The grazing incidence smallangle scattering information is obtained from horizontal cuts at a fixed angle Rf and thus depends on ψ only. The Yoneda peak is split from a central peak into two peaks which are located symmetrically to the center position. (25) Gutmann, J. S.; Mu¨ller-Buschbaum, P.; Stamm, M. Faraday Discuss. 1999, 112, 285. (26) Mu¨ller-Buschbaum, P.; Gutmann, J. S.; Stamm, M. Macromolecules 2000, 33, 4886. (27) Guide to Neutron Research Facilities at the ILL; Bu¨ttner, H. G., Lelievre-Berna, E., Pinet, F. Eds.; Institut Laue-Langevin: Grenoble, France, 1997; p 32. (28) Salditt, T.; Metzger, T. H.; Peisl, J.; Goerigk, G. J. Phys. D: Appl. Phys. 1995, 28, A236. (29) James, R. W. In The Optical Principles of the Diffraction of X-rays; OxBow Press: Woodbridge, CT, 1962. (30) Yoneda, Y. Phys. Rev. 1963, 131, 2010.

7780

Langmuir, Vol. 19, No. 19, 2003

Mu¨ ller-Buschbaum et al.

Figure 2. Double logarithmic plot of GISANS data (measured at a wavelength of 0.6 nm) from samples stored under a toluene atmosphere. From the bottom to the top, the storage time increases (t ) 2, 3, 4, and 12 h). The arrow marks a position related to a peak which is expected from an internal structure ξL as explained in the text. The dashed line indicates the resolution limit. All curves are shifted along the y-axis for clarity. Figure 1. Typical GISANS data exhibiting the most prominent features such as the specular and the Yoneda peaks. The exit angle is denoted Rf, and the angle perpendicular to the scattering plane (defined by the incident beam) is denoted ψ. High intensity is shown in white and low intensity in black. Due to the presence of a well-defined surface structure, the Yoneda peak is split into two peaks.

This results from the presence of a strong structure factor peak in the GISANS signal.17 With the sample surface defining the (x,y)-plane and the incidence beam being directed along the x-axis, a scattering vector b q ) (qx, qy, qz) is calculated using qx ) 2π/λ[cos(ψ) cos(Rf) - cos(Ri)], qy ) 2π/λ[sin(ψ)cos(Rf)], and qz ) 2π/λ[sin(Rf) + sin(Ri)]. As a consequence, the GISANS signal depends basically on the in-plane wave vector component qy. Only at the specular position is no in-plane wave vector component present. Consequently the 2D detector mainly collects diffusely scattered information. Since the rms surface roughness is small, satisfying σrmsqy < 1, the diffusely scattered intensity is calculated in the framework of the distorted-wave Born approximation (DWBA). For a given illuminated surface area A, a refractive index n, and wavelength λ, the differential cross section is given by19,31

dσ Aπ2 ) 4 (1 - n2)2|Ti|2|Tf|2F(q b) dΩ λ

(1)

Thus the Fresnel transmission functions Ti,f act only as overall scaling factors in the GISANS geometry, since Ri and Rf are fixed and the diffuse scattering factor F(q b) is directly detected. For N identical and centrosymmetrical objects with a random orientation, the diffuse scattering factor can be approximated19

F(q b) ∼ NS(q b)P(q b)

(2)

to depend on the form factor of the individual objects P(q b) and to depend on the structure factor S(q b). A mathematical description of the form factor depends on the type of object such as sphere, cylinder, or slab, while the structure factor directly yields the most prominent in-plane length ξ. (31) Salditt, T.; Metzger, T. H.; Brandt, Ch.; Klemradt, U.; Peisl, J. Phys. Rev. B 1995, 51, 5617.

If the incident or exit angle Ri,f is equal to the critical angle Rc(A) of material A, the transmission functions have a maximum, which is called the Yoneda peak.30 Therefore, out-of-plane cuts at the positions Rf ) Rc(A) increase the scattering contribution of material A.32 Figure 2 shows GISANS data taken at the critical angle of PpMS (below the critical angle of PSd), which are thus mostly sensitive to structures related to the PpMS part of the diblock copolymer. Data measured at different destabilization times are compared. From the bottom to the top, the storage time under toluene vapor increases. Irrespective of the annealing time, all GISANS data show a peak, which shifts toward smaller qy values with increasing toluene atmosphere storage time. Thus with ongoing destabilization the evolving structures are increasing in their lateral size. In addition, the peak height increases which results from an increased scattering contrast. During the destabilization, the height of the structures increases as well. B. AFM Data. In real space, this is nicely pictured with AFM. In Figure 3, AFM data at a fixed scan range of 1 × 1 µm2 are shown. Already after 1 h of storage time (Figure 3A), a nanostructured surface has evolved. A highly interconnected network of ribbonlike structures covers the substrate surface. The height of the surface features is 2 nm, and a most prominent in-plane length of 63 nm is calculated. This length depicts the distance between the nanostructures, whereas the width of the nanostructures is 30 nm only. Compared to the bulk key parameters such as the radius of gyration Rg ) 13.6 nm23,24 of the unperturbed chain and the bulk lamellar spacing L0 ) 45.0 nm,33,34 the surface structures are similar in their size. As a consequence, this nanostructure observed in the early state of structure creation is basically determined by the dimensions of the diblock copolymer molecules itself. It marks a sort of lower limit with respect to the size unless only single chains are addressed.35 After 3 h of storage time (Figure 3B), the surface structure has coarsened parallel and perpendicular to the substrate surface. A surface feature height of 4.5 nm, a (32) Mu¨ller-Buschbaum, P.; Stamm, M. Physica B 1998, 248, 229. (33) Giessler, K. H.; Rauch, F.; Stamm, M. Europhys. Lett. 1994, 27, 605. (34) Giessler, K.-H.; Endisch, D.; Rauch, F.; Stamm, M. Fresenius’ J. Anal. Chem. 1993, 346, 151. (35) Mu¨ller-Buschbaum, P.; Hermsdorf, N.; Gutmann, J. S.; Stamm, M.; Cunis, S.; Gehrke, R.; Petry, W. To be published.

Nanostructured Diblock Copolymer Films

Langmuir, Vol. 19, No. 19, 2003 7781

Figure 3. AFM pictures (scan range of 1 × 1 µm2) of the topology signal as observed after storage under toluene vapor. The samples shown differ in the storage time: (A) 1, (B) 3, (C) 4 h. The height scaling is chosen individually for each pattern to depict the in-plane structure more clearly.

Figure 4. Comparison between the master curve calculated from the AFM data (dots) and the GISANS data measured at different wavelengths (0.6 nm, circles; 1.0 nm, triangles). The corresponding toluene storage time is 4 h. The dashed lines depict the resolution limit which is different for the experimental techniques. In this double logarithmic presentation, all curves are shifted along the y-axis for clarity.

width of 110 nm, and an in-plane distance of 200 nm result. Therefore several diblock copolymers have organized into the nanostructured regions covered with polymeric material. Still the rest of the ribbonlike structure remained. After 4 h of storage time (Figure 3C), this ribbon structure is broken into isolated droplets. Connected with a further coarsening, the droplet height increased to 6.5 nm and a most prominent in-plane length of 314 nm is calculated. The characteristic width of the surface structures is difficult to estimate because rather irregularly shaped droplets are built up. Only in the final dewetting state, as shown recently,10 are droplets with rather circular contact lines toward the substrate installed. C. Comparison of GISANS with AFM. The statistical relevance of AFM data is small as compared to that of GISANS data because only a very small surface area is probed. On the other hand, the master curves calculated from the AFM data enable a direct assignment of the peaks observed in the GISANS data.16 With AFM, real-space information is probed, which includes an easy interpretation. Figure 4 demonstrates this for the storage time of 4 h. The GISANS data measured at 0.6 and 1.0 nm and the master curve (dots) exhibit a well-pronounced peak at a position marked with the arrow. The corresponding in-plane distance is 314 nm, and thus structure factor information, the distance between the polymeric droplets, is probed. As a consequence, the AFM data shown are statistically significant for the type of surface topography being present.

Anyhow, AFM probes the surface topography only. GISANS enables an additional view inside the surface structures.17 The chemical composition is addressed due to the contrast introduced by the deuteration of one block (namely, the PSd block). Because both blocks dPS and PpMS differ only by one methyl group, despite the deuteration, their chemical and mechanical properties are very similar. Therefore surface characterization methods such as friction and stiffness measurements36 yield not enough contrast to distinguish between the components and a selective dissolution37 of one component is not possible. However, due to the contrast generated by the deuteration, possible substructures are detectable with GISANS. Differences in the slope between the master curve and the GISANS data in the region at large qy can be attributed to the influence of the background level in the GISANS experiments. A change in the neutron wavelength changes the slope in this region, although the same region in reciprocal space is measured. A second peak is present neither in the master curve nor in the GISANS data. As a consequence, none of the surface structures is highly monodisperse, which is easily understood from the AFM data shown in Figure 3C, and no internal structure is present. A monodisperse structure would give rise to a form factor maximum (yielding an additional peak), and an internal structure would add a further periodicity resulting in a second peak. An internal structure might be expected since diblock copolymers offer substructures resulting from a microphase separation process. In the final dewetting states, a microphase structure was observed for the same sample system.10 It was directly probed from the presence of a second peak in the GISANS data. However, with decreasing film thickness the visibility of this microphase separation structure peak decreases as well and in the case of the film thickness addressed in this investigation it was only hardly observable. Again we observe this effect, after 12 h of toluene atmosphere storage; the GISANS intensity at a qy position related to this microphase separation structure is slightly increased (see arrow and rhombs in Figure 2). Nevertheless, a strong peak is not detectable due to the small number of repetition units which can be put inside the droplets (roughly 7) and due to the low statistics related to the available neutron flux. An improvement of the statistics by an increase of the counting times is hardly possible within the common neutron beamtimes. The data presented were counted for 8 h each. (36) Krausch, G.; Hipp, M.; Bo¨ltau, M.; Mlynek, J. Macromolecules 1995, 28, 260. (37) Walheim, S.; Bo¨ltau, M.; Mlynek, J.; Krausch, G.; Steiner, U. Macromolecules 1997, 30, 4995.

7782

Langmuir, Vol. 19, No. 19, 2003

Mu¨ ller-Buschbaum et al.

following an Avrami law45,46

Λ(t) ) Λ0 + a(1 - exp[-(t/τ)N])

Figure 5. Lateral lengths Λ as a function of the toluene atmosphere storage time. Data from GISANS (circles) are compared to data measured with AFM (crosses). The solid line is a fit following an Avrami law.

Anyhow, the GISANS data of the intermediate destabilization steps do not show at least a sign of a peak in this qy range (circles, triangles, and crosses in Figure 2). Thus in these early stages of dewetting no internal periodically arranged structure is present. D. Model. It is quite difficult to estimate the glass transition temperature of an ultrathin polymer film stored under solvent vapor. Even without solvent vapor, the influence of the film thickness on the glass transition temperature is an actually investigated scientific area.38-41 While commonly the dewetting of an initially homogeneous film results from annealing above the glass transition temperature, in a solvent-driven glass transition the polymer film is plasticized by the incorporation of solvent molecules.42-44 The original homogeneous polymer film is replaced by a highly concentrated polymer-toluene solution layer which introduces a mobility to the diblock copolymer molecules. In addition, the surface tension, viscosity, and van der Waals interaction with the substrate are markedly changed by the incorporated solvent molecules as compared to the parameters for the polymer melt. Figure 5 shows the increase of the lateral lengths Λ detected with GISANS (circles) and with AFM (crosses) as a function of the toluene atmosphere storage time. Within the range of error, both techniques determine the structure factor information, namely, the distance between the surface structures. After a nanostructured surface pattern is installed, it coarsens further and the characteristic lateral length increases. The solid line is a fit (38) Forrest, J. A.; Dalnoki-Veress, K.; Dutcher, J. R. Phys. Rev. E 1997, 56, 5705. (39) Kim, J. H.; Jang, J.; Zin, W. C. Langmuir 2000, 16, 4064. (40) Forrest, J. A.; Mattsson, J. Phys. Rev. E 2000, 61, R53. (41) Tsui, O. K. C.; Russell, T. P.; Hawker, C. J. Macromolecules 2001, 34, 5535. (42) Laschitsch, A.; Bouchard, C.; Habicht, J.; Schimmel, M.; Ru¨he, J.; Johannsmann, D. Macromolecules 1999, 32, 1244. (43) Chen, W. L.; Shull, K. R.; Papatheodorou, T.; Styrkas, D. A.; Keddie, J. L. Macromolecules 1999, 32, 136. (44) Yim, H.; Kent, M. S.; Hall, J. S.; Benkoski, J. J.; Kramer, E. J. J. Phys. Chem. B 2002, 106, 2474.

(3)

with a growth exponent N ) 3 and a time constant τ ) 4 h. Because this model makes no assumptions on the physics of the underlying growth mechanism, despite the transformation of a phase I into a phase II, the determined growth exponent only shows that the growth is twodimensional (d ) N - 1). While the intermediate stages are well described by this coarsening process, in the late stages the surface structure breaks up into isolated islands. This slows down the further increase in the characteristic lateral lengths Λ. A further minimization of the surface energies mainly affects the shape of the installed surface structures only. In the final state, a nearly circular contact line results. In addition to this primary structure given by the dewetting, a microphase separation structure is built up in the late stages fulfilling L0 < Λ. Thus a host structure (polymeric islands) is installed via a dewetting process first, whereas the internal order inside this host structure (microphase separation structure) evolves within later stages of the process. This behavior is markedly different as compared to that of bulk films. Summary and Outlook Confined ultrathin polymer films with thicknesses of 1.5 nm are already quite difficult to detect with neutron reflectivity. Due to the destabilization, the surface roughnesses increase, which increases the difficulties with respect to reflectivity experiments. On the other hand, due to the installation of a nanostructured surface, with GISANS a signal becomes accessible. From the comparison with AFM, it is proved that a structure factor of the evolving structures is measured with GISANS. In addition, due to the chosen contrast between both blocks, resulting from deuteration of one block, the microphase separation structure is accessible in the GISANS experiment. With the currently available neutron flux, however, at least in the reported film thickness regime, this is still challenging. With respect to the creation of nanostructured polymeric surfaces, the presented option using even intermediate dewetting states of confined ultrathin diblock copolymer films offers two advantages. First, even smaller structures can be created which tackle the limit of single molecules and whose size is basically given by the molecular dimensions itself. Second, the restriction of obtaining only a dropletlike surface pattern is overcome and ribbon structures are accessible as well. This might open up new possibilities with respect to applications. Acknowledgment. We thank J. Kraus for his help during the GISANS measurements at the D22 beamline. This work was supported by the BMBF (Fo¨rderkennzeichen 03DUOTU1/4). LA0267241 (45) Kolmogorov, A. N. Izv. Akad. Nauk SSSR Otd. Mat. (Russian) 1937, 3, 335. (46) Avrami, M. J. Chem. Phys. 1939, 7, 1103.