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with a lock-in amplifier. The relative spontaneous polarization was obtained byintegrating the pyroelectric current over temperature.

Received 5 June; accepted 28 October 1997.

1. Durr, W. et al. Magnetic phase transition in two-dimensional ultrathin Fe films on Au(100). Phys. Rev.Lett. 62, 206–209 (1989).

2. Farle, M. & Baberschke, K. Ferromagnetic order and the critical exponent g for a Gd monolayer: anelectron-spin-resonance study. Phys. Rev. Lett. 58, 511–514 (1987).

3. Mermin, N. D. & Wagner, H. Absence of ferromagnetism or antiferromagnetism in one- or two-dimensional isotropic Heisenberg models. Phys. Rev. Lett. 17, 1133–1136 (1966).

4. Dowben, P. A., McIlroy, D. N. & Li, D. in Handbook on the Physics and Chemistry of Rare Earths (edsGschneidner, K. A. Jr & Eyring, L.) Ch. 159 (Elsevier, Amsterdam, 1997).

5. Ishikawa, K., Yoshikawa, K. & Okada, N. Size effect on the ferroelectric phase transition in PbTiO3

ultrafine particles. Phys. Rev. B 37, 5852–5855 (1988).6. Karasawa, J., Sugiura, M., Wada, M., Hafid, M. & Fukami, T. Ultra-thin lead titanate films prepared by

tripole magnetron sputtering. Integrat. Ferroelectr. Lett. 12, 105–114 (1996).7. Palto, S. et al. Ferroelectric Langmuir-Blodgett films. Ferroelectr. Lett. 19, 65–68 (1995).8. Bune, A. et al. Novel switching phenomena in ferroelectric Langmuir-Blodgett films. Appl. Phys. Lett.

67, 3975–3977 (1995).9. Blinov, L. M., Fridkin, V. M., Palto, S. P., Sorokin, A. V. & Yudin, S. G. Thickness dependence of

switching for ferroelectric Langmuir films. Thin Solid Films 284–285, 474–476 (1996).10. Sorokin, A., Palto, S., Blinov, L., Fridkin, V. & Yudin, S. Ultrathin ferroelectric Langmuir-Blodgett

films. Mol. Mater. 6, 61–67 (1996).11. Scott, J. F. Phase transitions in ferroelectric thin films. Phase Trans. 30, 107–110 (1991).12. Tilley, D. R. in Ferroelectric Thin Films: Synthesis and Basic Properties (eds Paz de Araujo, C., Scott, J. F.

& Taylor, G. F.) 11–45 (Gordon & Breach, New York, 1996).13. Scott, J. F. Properties of ceramic KNO3 thin film memories. Physica B 150, 160–167 (1988).14. Lovinger, A. J. Ferroelectric polymers. Science 220, 1115–1121 (1983).15. Wang, T. T., Herbert, J. M. & Glass, A. M. (eds) The Applications of Ferroelectric Polymers (Chapman &

Hall, New York, 1988).16. Furukawa, T. Ferroelectric properties of vinylidene fluoride copolymers. Phase Transit. 18, 143–211

(1989).17. Legrand, J. F. Structure and ferroelectric properties of P(VDF-TrFE) copolymers. Ferroelectrics 91,

303–317 (1989).18. Kimura, K. & Ohigashi, H. Polarization behavior in vinylidene fluoride-trifluroethylene copolymer

thin films. Jpn J. Appl. Phys. 25, 383–387 (1986).19. Ducharme, S. et al. Critical point in ferroelectric Langmuir-Blodgett polymer films. Phys. Rev. B 57,

25–28 (1998).20. Allenspach, R. & Bishof, A. Magnetization direction switching in Fe/Cu(110) epitaxial films:

temperature and thickness dependence. Phys. Rev. Lett. 69, 3385–3388 (1992).21. Qu, B. D., Zhang, P. L., Wang, Y. G., Wang, C. L. & Zhong, W. L. Dielectric susceptibility of

ferroelectric thin films. Ferroelectrics 152, 219–224 (1994).22. Yamamoto, T. Calculated size dependence of ferroelectric properties in PbZrO3-PbTiO3 system.

Integr. Ferroelectr. 12, 161–166 (1996).23. Wang, C. L., Zhong, W. L. & Zhang, P. L. The Curie temperature of ultra-thin ferroelectric films. J.

Phys.: Cond. Matter 3, 4743–4749 (1992).24. Blinov, L. M. Langmuir films. Sov. Phys. Usp. 31, 623–644 (1988).25. Palto, S. et al. Ferroelectric Langmuir-Blodgett films showing bistable switching. Europhys. Lett. 34,

465–469 (1996).26. Zhang, R. & Taylor, P. L. Theory of ferroelectric-paraelectric transitions in VF2/F4E random

copolymers. J. Appl. Phys. 73, 1395–1402 (1993).

Acknowledgements. We thank P. A. Dowben and J. F. Scott for suggestions for improving the manuscript.Work at the University of Nebraska was supported by the USA NSF Division of Electronic andCommunications Systems and by the Nebraska Research Initiative through the Center for MaterialsResearch and Analysis. Work at the Institute of Crystallography was supported by INTAS.

Correspondence and requests for materials should be addressed to S.D. (e-mail: ducharme@unlinfo.unl.edu).

Surface-inducedstructureformationof polymerblendsonpatternedsubstratesMartin Boltau, Stefan Walheim, Jurgen Mlynek,Georg Krausch* & Ullrich Steiner

Fakultat fur Physik, Universitat Konstanz, 78457 Konstanz, Germany. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Phase separation in bulk mixtures commonly leads to an iso-tropic, disordered morphology of the coexisting phases1. Thepresence of a surface can significantly alter the phase-separationprocess, however2,3. Here we show that the domains of a phase-separating mixture of polymers in a thin film can be guided intoarbitrary structures by a surface with a prepatterned variation ofsurface energies. Such a pattern can be imposed on a surface byusing printing methods for depositing microstructured molecular

* Present address: Institut fur Physikalische Chemie, Ludwig Maximilians Universitat Munchen, There-sienstrasse 39, 80333 Munchen, Germany.

films4, thereby allowing for such patterns to be readily transferredto a two-component polymer film. This approach might provide asimple means for fabricating polymer-based microelectroniccircuits5 or polymer resists for lithographic semiconductorprocessing.

Our approach was quite general and works for a large number ofbinary polymer pairs and for a variety of different substrates. Toidentify an appropriate system, we studied the phase morphology ofa polymer pair on homogeneous substrates of varying surfaceenergies6. The experimental procedure was simple: the two poly-mers were dissolved in a common solvent, a drop of the solution wasplaced on a substrate, and the thin film was prepared by rapidrotation of the substrate (spin-coating). As the polymers arestrongly incompatible, demixing takes place during the spin-coat-ing procedure. The domain morphology that forms during solventevaporation was observed using atomic force microscopy (AFM)(Fig. 1). Thin films of a polystyrene (PS)–polyvinylpyridine (PVP)mixture were spun-cast onto a gold (Au) substrate which waspartially covered by a self-assembled monolayer (SAM). As PSand PVP are strongly incompatible, coexisting phases of thealmost pure polymer components are found after film preparation6.The AFM image shows a homogeneous film surface on the bare Ausubstrate (top, Fig. 1a), whereas a corrugated film topography isfound on the SAM-covered part of the substrate. The friction modeimage (Fig. 1b), which provides a material contrast at the filmsurface7, reveals that these surface undulations coincide with alateral composition variation at the surface of the polymer film.The Au surface (top, Fig. 1b), which shows no contrast in thefriction image, is covered by only one polymer species. For infor-

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Figure 1AFM images (50 3 50 mm2) of a PS/PVPblend (50%:50%w/w) spun-cast

from a tetrahydrofurane (THF) solution (1.5% polymer by weight) onto a Au

surface (50 nm of Au evaporated onto a silicon oxide surface). a, Topography

image of the sample as cast. The friction mode image7 in b exhibits a material

contrast between PS and PVP at the surface. In c, the PS phase was removed by

dissolution in cyclohexane and an AFM topography image of the remaining PVP

was taken at the same location, as in a. The images show a different domain

morphology for the top half (bare Au surface) and the bottom half of the sample

where the Au surface was hydrophobized by depositing a self-assembled

monolayer (SAM) of octadecylmerctaptan before polymer deposition4. In d, e,

the PS (light grey) and PVP (dark grey) phase distribution in the film is visualized

by a superposition of cross-sections at the locations indicated by lines in a and c.

The bilayer in d forms on the more polar gold surface, whereas a laterally

separated phase morphology is found on the SAM-covered substrate.

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878 NATURE | VOL 391 | 26 FEBRUARY 1998

mation on the polymer composition inside the film, the PS phasecan be removed by immersing the sample in cyclohexane, which is asolvent for PS, but does not dissolve PVP. In Fig. 1c, the AFM imageshows the remaining PVP phase after removal of the PS. This imageexhibits the same lateral structure as that in Fig. 1a, with anincreased vertical contrast. The film composition normal to thesubstrate surface is displayed as a superposition of cross-sectionsfrom Fig. 1a and c. For the homogeneous film areas, a PS/PVPbilayer is formed, with the more polar PVP next to the Au substratesurface. On the nonpolar SAM surface, no preferential adsorptiontakes place, and a laterally disordered domain morphology of PSand PVP is seen (Fig. 1e). The strong topographic variation of the

domain morphology on the SAM-covered part of the surface (Fig.1a, e) is due to the different solubility of the two polymers in theircommon solvent6.

The results shown in Fig. 1 suggest a strategy for the transfer of alateral variation in surface energy into a composition pattern of apolymer film. To create a surface with laterally varying surfaceenergies, micro-contact printing technology was used4. After spin-coating a PS/PVP solution onto the prepatterned surface, the imagein Fig. 2a is observed. The surface corrugation from Fig. 1a is nowperfectly aligned in a grid pattern. As in Fig. 1, the topographicundulation originates from a lateral organization of the polymerphases, as confirmed by friction-mode AFM (results not shown).This is corroborated by removing the PVP phase by immersing thesample in ethanol, which is a selective solvent for PVP. Theremaining PS domains show a remarkable contrast in the AFMimage (Fig. 2b): stripes with a rectangular cross-section. Thepolymer domain morphology in Fig. 2 is due to the strong affinityof the PVP to the more polar Au surface, displacing the PS towardsthe SAM-covered parts of the substrate. As in Fig. 1a and e, the PVPstripes protrude from the film surface, giving rise to the topographicimage observed in Fig. 2a. This experiment demonstrates a transferof a difference in surface energies into a composition variation of abinary polymer film. The perfect replication process extends overmuch larger length scales than those shown in the AFM images andis limited only by the sample size. Note that the rapid quench bysolvent evaporation leads to domain structures far from their

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Figure 2 The same PS/PVP blend as in Fig. 1, spun-cast on a patterned Au

substrate. The lateral surface pattern was created by microcontact printing,

whereby a polydimethyl-siloxane stamp is soaked in a octacedyl mercaptan

solution and placed onto a Au surface. A self-assembled monolayer forms only

on surface regions where the stamp touches the substrate surface. The resulting

Au/SAM pattern (2.4 mm periodicity) features stripes of alternating surface

energy with only little topographic variation.The AFM images show a80 3 80 mm2

area: a, as cast; b, after removal of PVP by dissolution in ethanol. The PS and PVP

phases are organized in a surface-induced fashion in which PVP lies on the bare

Au surface regions, displacing the PS onto the SAM stripes. The perspective

representation in c visualizes the rectangular cross-section of the 65-nm-high PS

stripes in b. The dotted areas in c indicate the SAM-covered substrate regions. In

the bottom parts of a and b, where the substrate is completely SAM covered, the

disordered structure is the same as that in Fig.1.

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Figure 3 Films of PS and PSBr (50:50%w/w) spun-cast from toluene solution (2–

4% by weight) onto a patterned silicon wafer which features alternating stripes of

oxidized (SiOx) and hydrogen-terminated silicon (SiH). The substrate pattern was

generated by standard optical lithography combined with an etch in hydrofluoric

acid. AFM images of the film as cast (a, c) were 30 3 30 mm2, and 15 3 15 mm2 (e).

In b, d and f, cross-sections are shown from the locations indicated by lines in a, c

and d, and corresponding images where the PS was removed by dissolution in

cyclohexane: PS, light grey; PSBr, dark grey. The imperfect order in a can be

improved by either changing the film thickness from 65 to 175nm (c) or by

reducing the periodicity of the SiOx/SiH grid from 8 to 1.5mm (e).

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NATURE | VOL 391 | 26 FEBRUARY 1998 879

thermodynamic equilibrium. The rectangular cross-section of thePS phase in Fig. 2c is a direct consequence of this rapid solidificationof the binary polymer blend6. In contrast, wetting experiments of asingle liquid on a patterned substrate show a more roundedtopography8.

These results were achieved only after careful optimization ofseveral experimental parameters. For a given pattern periodicity,perfect lateral organization of the polymer phases is observed onlyfor a given film thickness. To explore the optimum conditions forthe self-organization process and to demonstrate the universality ofour approach, a second binary polymer mixture on a differentprestructured substrate surface was investigated. Mixtures of PS anda partially brominated polystyrene (PSBr) were spun-cast ontopatterned silicon oxide (SiOx)/hydrogen-terminated silicon (SiH)surfaces with varying grating periodicities. AFM cross-sections ofthe as-cast films and of the remaining PSBr phases after removingthe PS (data not shown) are superimposed to yield the cross-sectional domain morphologies. In Fig. 3a, b a partial surfacedirected reorganization of PSBr and PS domains has taken place,but the lateral organization is incomplete. Figure 3a, b reveals aphase behaviour that is very similar to the separation of a PS/PSBrmixture on homogeneous SiOx and SiH surfaces, respectively: PS/PSBr bilayers on the SiH surface regions and a lateral PS/PSBr phasemorphology with ,1-mm-wide PSBr domains surrounded by thePS phase on SiOx. As the film thickness is increased, the quality ofthe pattern replication successively improves (Fig. 3c, d). For a givenfilm thickness (that is, 65 nm in Fig. 3a), on the other hand, areduction of the substrate periodicity leads to a better lateral orderof the domain morphology (Fig. 3e, f). The results shown in Fig. 3suggest an underlying principle that governs the ordering process.We compared the ,1 mm average size of the PSBr domains on theSiOx surface regions in Fig. 3a with the 750-nm stripe width of theSiH/SiOx substrate pattern in Fig. 3e: this indicated that completeordering of the polymer phases can occur only for pattern sizescomparable to the characteristic length of the lateral domainmorphology on the respective homogeneous substrate. Althoughthe details of the surface-induced demixing process are not yetknown, the existence of a well defined lateral length scale suggests abulk- or surface spinodal instability which develops during the spin-coating process. A quantitative study of phase separation duringspin-coating is currently underway.

We have made use of two fundamental thermodynamic princi-ples—phase separation of a binary fluid and selective adsorptionfrom a binary mixture—to transpose a variation in surface energiesof a flat substrate into a concentration variation of a binary polymerfilm. The micrometre size structures perfectly replicate the substratepattern over lateral dimensions which are limited only by the samplesize (1 cm2 in our case). The size of the structures that can beproduced is at present dictated by the availability of substratetemplates, and lateral structure sizes of less than 100 nm shouldbe feasible. We emphasize that the structures reported here arereplicated from a prepatterned substrate, rather than driven by amolecular self-organization process, as for example in microphaseseparated block-copolymer melts9. In these systems, the morphol-ogy of the lateral structure is determined by the architecture of theconstituent molecules, and long-range order is difficult to achieve10.But with our approach, a given experimental system (that is, aparticular polymer blend) can be used to produce structures of

widely varying morphologies and length scales. In contrast to barepatterned SAM substrates, the patterned polymer films are thickenough to allow further semiconductor processing steps such asreactive ion etching. Moreover, the polymer layers can be lifted offthe substrates, and several structured films can be superimposed.Therefore, patterned SAM surfaces can be used as templates formultiple pattern replication. We also note that the manufacture ofthe structured polymer films takes just a few minutes, is inexpen-sive, and, once optimized, is highly reproducible. From a techno-logical viewpoint, various applications can be envisaged, such as theformation of photonic bandgap materials11, patterned polymersurfaces with selective molecular recognition capabilities12, andoptical devices13. In addition to their technological relevance, lateralnanoscale structures in polymer films could provide a new tool tostudy phase equilibria near surfaces on a molecular level. M

Received 19 September; accepted 9 December 1997.

1. Gunton, J. D., San Miguel, M. & Sahni, P. S. in Phase Transitions and Critical Phenomena (eds Domb,C. & Lebovitz, J. L.) Vol. 8, 267–466 (Academic, London, 1983).

2. Jones, R. A. L., Norton, L. J., Kramer, E. J., Bates, F. S. & Wiltzius, P. Surface-directed spinodaldecomposition. Phys. Rev. Lett. 66, 1326–1329 (1991).

3. Krausch, G., Kramer, E. J., Rafailovich, M. H. & Sokolov, J. Self assembly of a homopolymer mixturevia phase separation. Appl. Phys. Lett. 64, 2655–2657 (1994).

4. Xia, Y., Zaho, X.-M. & Whitesides, G. M. Pattern transfer: Self-assembled monolayers as ultrathinresists. Microelectr. Eng. 32, 255–268 (1996).

5. Service, R. F. Patterning electronics on the cheap. Science 278, 383–384 (1997).6. Walheim, S., Boltau, M., Mlynek, J., Krausch, G. & Steiner, U. Structure formation via polymer

demixing in spin-cast films. Macromolecules 30, 4995–5003 (1997).7. Binning, G., Quate, C. F. & Gerber, C. Atomic force microscope. Phys. Rev. Lett. 56, 930–933 (1986).8. Abbot, N. L., Folkers, J. P. & Whitesides, G. M. Manipulation of the wettability of surfaces on the 0.1-

to 1-micrometer scale through micromachining and molecular self assembly. Science 257, 1380–1382(1992).

9. Morkved, T. L., Wiltzius, P., Jaeger, H. M., Grier, D. G. & Witten, T. A. Mesoscopic self-assembly ofgold islands on diblock-copolymer films. Appl. Phys. Lett. 64, 422–424 (1994).

10. Morkved, T. L. et al. Local control of microdomain orientation in diblock copolymer thin films withelectric fields. Science 273, 931–933 (1996).

11. Haus, J. W. in Quantum Optics of Confined Systems (eds Ducloy, M. & Block, D.) 101–141 (KluwerAcademic, Dordrecht, 1996).

12. Prime, K. L. & Whitesides, G. M. Self-assembled organic monolayers: Model systems for studyingadsorption of proteins at surfaces. Science 252, 1164–1167 (1991).

13. Halls, J. et al. Efficient photodiodes from interpenetrating networks. Nature 376, 498–500 (1995).

Acknowledgements. This work was supported by the Deutsche Forschungsgesellschaft (DFG), theVolkswagenstiftung, and NATO. U.S. acknowledges the financial support from a research fellowship(Habilitations-Stipendum) of the DFG. We thank E. J. Kramer and J. Heier for their help preparing thePDMS stamps.

Correspondence and requests for materials should be addressed to U.S. (e-mail address:Ulli.Steiner@uni-konstanz.de).

Table 1 Molecular characteristics of polymers

Polymer Mw Polydispersity index..............................................................................................................................................................................

PS 94.9K 1.06..............................................................................................................................................................................

PVP 115K 1.03..............................................................................................................................................................................

PSBr 215K 1.02..............................................................................................................................................................................Mw, weight average molecular weight. All materials were used as obtained from the PolymerStandardsService (Mainz,Germany). The degree of brominationof the PSBrwas 52.2mol%.

Originof upper-oceanwarmingandElNinochangeondecadal scalesin the tropical PacificOceanRong-Hua Zhang*, Lewis M. Rothstein*& Antonio J. Busalacchi†

* Graduate School of Oceanography, University of Rhode Island, Narragansett,Rhode Island 02882, USA† Laboratory for Hydrospheric Process, NASA Goddard Space Flight Center,Greenbelt, Maryland 20771, USA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

The cause of decadal-scale variability in the tropical PacificOcean—such as that marked by the 1976–77 shift in the ElNino/Southern Oscillation1–7—is poorly understood. Unravellingthe mechanism of the recent decade-long warming in the tropicalupper ocean is a particularly important challenge, given the linkto El Nino variability, but establishing the hypothesized inter-annual/decadal oceanic connections between middle latitudes andtropics has proved elusive8. Here we present observational evi-dence that Pacific upper-ocean warming and decadal changes inthe El Nino/Southern Oscillation after 1976 may originate from