Mesoscopic selfassembly of gold islands on

Mesoscopic selfassembly of gold islands on diblockcopolymer films T. L. Morkved, P. Wiltzius, H. M. Jaeger, D. G. Grier, and T. A. Witten Citation: Appl. Phys. Lett. 64, 422 (1994); doi: 10.1063/1.111118 View online: http://dx.doi.org/10.1063/1.111118 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v64/i4 Published by the AIP Publishing LLC.

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Mesoscopic

self-assembly

of gold islands on diblock-copolymer

films

T. L. Morkved The James Franck Institute and Department of Physics, The University of Chicago, 5640 South Ellis Ave., Chicago, Illinois 60637

P. Wiltzius AT&T Bell Laboratories, 600 Mountain Avenue, Murray

Hill, New Jersey 07974

H. M. Jaeger, D. G. Grier, and T. A. Witten The James Franck Institute and Department of Physics, The University of Chicago, 5640 South Ellis Ave., Chicago, Illinois 60637

(Received 16 July 1993; accepted for publication 8 November 1993) We describe the fabrication and characterization of self-assembled gold island arrays on diblock-copolymer thin films. The natural tendency of these polymers to form ordered phases is used to induce selective aggregation of evaporated gold metal during an annealing process. We obtain well-defined, nanoscale island arrays aligned with one of the copolymer blocks. Near perfect segregation is achieved between the two domains. Two types of diblock-copolymer systems are discussed, together with the resulting island patterns. Nanometer-size structures are becoming an increasingly important area of investigation. This mesoscopic regime is characterized by unique electronic and optical properties, which have no analogs in macroscopic bulk behavior and may be used to design new classes of electronic devices.’ The standard technique for fabricating mesoscopic structures with small lateral feature sizes is to inscribe the desired patterns into a suitable template material (“resist”) by electronbeam lithography.’ For nanofabrication with feature sizes below 30 nm, however, conventional electron-beam lithography becomes exceedingly difficult. Recent improvements of this technique include ultrahigh resolution inorganic resist materials3 as well as the use of scanning tunneling or atomic force microprobes A fundamentally different alternative that is not limited by the resolution of the pattern generation equipment or the resist material, is to utilize the natural tendency of certain materials to form ordered phases on mesoscopic length scales. Candidates for such self-assembly include arrays of latex spheres; proteins,6 or polymers.7 Typically, the ordered structures formed by these materials have been used as masks for replicating patterns into underlying substrates by evaporation or etching. A refinement of the self-assembly process involves a mixture of assembly species which segregate into ordered domains where one species can be selectively doped or decorated to achieve conducting domains. Here we report on a first step in this direction; selective ordering of gold islands on diblock-copolymer surfaces. Diblock-copolymers spontaneously form ordered configurations such as the lamellar, cylindrical, or spherical phases below an order-disorder transition temperature. These equilibrium configurations are essentially controlled by the volume fraction of the two different polymer blocks that are joined together to form the copolymer.8 Using symmetric polystyrene-polymethylmethacrylate’ (PS-PMMA) diblock-copolymers we have prepared thin films of the lamellar phase; and using asymmetric polystyrene-poly (2-vinylpyridine)” (PS-PVP) we have prepared thin films of the spherical phase. PS-PVP or PS-PMMA were dissolved in toluene at a concentration of 1% by weight. Polymer films of 422

Appl. Phys. Lett. 64 (4), 24 January 1994

about 50 nm thickness were prepared on NaCl crystals by spin coating at 5000 rpm and then annealing at 145 “C for 8 h. Gold metal was then resistively evaporated onto the polymer films at room temperature at a pressure of 5 X 10m6Torr. The nominal gold thickness was 0.5 nm deposited at a rate of 0.01 rnn/s as measured by a quartz crystal monitor. The samples were further annealed for at least 24 h under vacuum at 145 “C and allowed to cool to room temperature. After annealing, the NaCl substrate was dissolved in water, producing a free-standing, gold-decorated copolymer film, which was then picked up on a transmission electron microscope (EM) grid. For comparison, polystyrene” (PS) homopolymer films were prepared in the same manner. The morphology of thin gold films on homopolymers have been extensively studied for the cases of PS and PVP.” Figure l(a) is a TEM micrograph showing the island pattern we obtained on a PS film. Gold does not wet the polymer, resulting in a distribution of discrete gold islands. This resulting morphology is typical for gold island formation on isotropic substrates.13 Figures l(b) and l(c) demonstrate the striking difference in morphology when we switched to diblock-copolymer films. Here we found island patterns that replicate the ordering of the underlying polymer blocks. Gold islands on symmetric PS-PMMA align into locally parallel, wormlike stripes [Fig. l(b)], corresponding to sheets of lamellae formed by this copolymer. On asymmetric PS-PVP, the islands appear to replicate the packing of PVP spheres surrounded by a matrix of PS [Fig. l(c)]. The strong and uniform contrast and high resolution of the TEM micrographs allows for quantitative image analysis of digitized micrographs, such as those in Fig. 1, by thresholding the images, and then segmenting the binary images into features. The discreteness of the digitized images accounts for errors in the centroid locations of at most 0.7 nm. The distribution of nearest-neighbor separations, P(r), calculated from a Delaunay triangulationt4 of the centroid locations appears in Fig. 2 and qualitatively distinguishes decorations on the three polymers. On the homopolymer, the distribution of nearest-neighbor separations peaks at 13 nm reflecting the island growth dynamics under the given an0 1994 American Institute of Physics


FIG. I. ‘T’EM micr~~prr~phsof gold island form&m on PS homopolymer (a), symmetric PS-PMMA diblock-copolymer ih), and asymmetric PS-PVP ~liblo~k=copni~mer icl thin films. AI1 three samples were prepared under identical conditions. The nominal gold coverage was’O.5 nm and the annealing time: 50 h at 145 ‘C The size of each frame is 1 AcrnX 1 pm: the six of the bar in ic) indicates 100 nm. The average gold island tliamctrr is h mn on tltc horwpnlymer end 7 nm on the copolymers with R full width at half-maximum of about :! nm each.

ne3ling conditions. In contrast. the distributions for the spherical and lamellar phases additionally reflect the length scsles provided by the rmderlying copolymers. In the spherical phase, the peak in the nearest-neighbor separations has shifted to 28 nm presumably corresponding to the intrinsic s PL?cinE-. of the PVP spheres. The small peak around 7 nm is due to occasional smaller, secondary islands near the same PVP region, which we find to coalesce into a large main island with longer annealing times. In the Iamellar phase, the pe3k at lower spacings corresponds to islands distributed along a stripe. The broad peak at larger distances corresponds to separations spanning 3dj:~cccntstripes and is broadened both by the random placement of islands within a stripe and by the larger spacing between groups of islands on a single stripe. Using the data of Green et uZ.,~~we can calculate the repeat distance for the lamellar microdomains of our PS-PMMA diblock copolymer to be 35 nm, which is in 3grecmcnt with the position of the broad peak in Fig. 2. With short annenling times, we find that the lamellae still contain several islands across their width, but these eventually coalesce along the ccntcr of the stripes. These observations sugueest that while islands preferentially decorate the stripes or spheres, near the preferred stripe or sphere the coalescence process may be similar to that on isotropic homopolymcrs. With spin co3ting and relatively low temperature annealing, as in this work, the copolymer films do not reach full equilibrium. Ir, Consequently, the gold island decorations in Figs. 1(b) and I IC) exhibit only partially developed, intcrmediate rnnge order. ” In equilibrium, asymmetric PS-PVP forms 3n ordered close packing of PVP spheres surrounded by PS. This translational ordering appears to survive only locally in our spin c;wt films. To gauge the degree of ordering we plot the pair correlation function averaged over angles, g(r)> in Fig. 3. The dashed curve in Fig. 3 is a nonlinear kist-sclunres

fit to g(r)-

1 -[I

-g,,(rjlcf-ri%,

where g,(r)

is

the correlation function for an ideal triangular lattice modified by a Gaussian broadening factor to account for random displncements. The cxponcntial factor, p ...r!C, accounts for topolo@ical defects in the lattice.‘” This three parameter fit App!. Phys. L&t., Vol. 64, No. 4, 24 January 1994

confirms the nearest-neighbor spacing of 28 nm and gives 3 correlation length, 5, of about I.4 nearest-neighbor spacings with random displacements extending to (3.2 lattice spacings. The quality of the fit is consistent with the conjecture that this is a triangular lattice albeit with a high defect density. In contrast, the homopolymcr decoration clearly lacks any longrange structure. This further demonstrates that the PS-PVP film gold island decoration corresponds to the assunted intrinsic placement of the PVP spheres whereas the homopolynter decoration reflects only the growth dynamics of the gold islands. We expect that enhanced control of the polymer casting process will produce gold decoration with longer-ranged order. The specific nature of the sclcctivc gold-polymer interaction which causes the self-assembly is still in question. Two possibilities seem most likely; contact or longer-range van der Waals interaction. In the first case, one block of the copolymer preferentially adsorbs to the gold surface. For example, Russell et uI.” investigated thin PS-PMMA copolymer films on gold substrates and found that at equilibrium 1’S is preferentially located at the substrate. For PS-PVP mol-

15

30

45

60

r m-9 FIG. 2. Distributions, P(r), of island separations, r, for the thrct: siunples shown in Fig. I : PS homopolymer (solid line), symmetric PS-PMMA diblock-copolymer (dotted line), and asymrnrtric PS-PVP diblockcopolymer (dashed lint). Morkved et al.

423


IO 0

50

100

150

r m-0 FIG. 3. Correlation function, g(r), for gold islands on asymmetric PS-PVP (top). The dashed line is a fit to an ideal triangular lattice. The first peak corresponds to smaller, secondary islands over a single PVP sphere and was not included in the fit. For comparison, g(r) for gold islands on PS homopolymer is also shown (bottom).

,,ecules on silver surfaces, Tsai et 01.~’observed that the PVP block preferentially adsorbed to the surface by bonding through the nitrogen atoms while the PS block was positioned away from the surface. Kunz et al.‘2,21 attributed this same mechanism to a strong gold-PVP interaction. These findings make preferential contact interaction a possible explanation for the selective segregation if both blocks are at the surface,‘5 the gold islands are below the surface, or the gold island pull the PVP chains to the surface during annealing. If, on the other hand, only the PS block is at the surface,22 then self-assembly may be driven by van der Waals forces caused by the higher polarizability of the polar blocks (PVP and PMMA). Gold is mobile on P&l2 allowing the gold islands to aggregate and coalesce, but stabilization above the polar blocks might be provided by the van der Waals interactions. Indeed, close inspection of micrographs like Fig. l(b) provides sufficient contrast to allow us to observe the lamellae in TEM micrographs even without prior selective staining. If this contrast between stripes is due to the 14% larger density of PMMA relative to PS,23 then the gold islands appear to aggregate almost exclusively above the PMMA block. A final question concerns the role of the free-surface profile. For thin diblock-copolymer films, steps form at the free surface in order to accommodate the microphaseseparated morphology.22 This may influence the island decoration process. Indeed, Kojima and Magi1124 have prepared similar gold island decorations on crystalline diblock copolymers, where the islands form at the interface between the crystalline and noncrystalline polymer due to a high probability for nucleation at step sites, similar to gold island decoration at the crystal edges of polyethylene.Z In our work, however, it is seen that the gold islands do not position themselves at the interface between the two polymer blocks but instead are centered above the preferred stripes. The self-assembly process using diblock-copolymer films offers several advantages for mesoscopic structure fabrication, namely relative simplicity and size scale attainability below 10 nm. The annealing process leads to near perfect segregation between gold-decorated and nondecorated re424

Appl. Phys. Let,

Vol. 64, No. 4, 24 January 1994

gions (Fig. 1) due to the favorable polymer/gold interaction. In this respect our approach overcomes a limitation observed in the self-assembly of copolymers with internally attached metalorganic groups,26 where annealing appears to lead to a loss of pattern definition due to isotropic metal particle diffusion. A better understanding of the polymer/metal interaction along with copolymer thin film morphology27 should enable us to fabricate specific desired mesoscopic structures, We wish to thank R. Josephs for assistance with the TEM facilities, and G. T. Pickett and Sidney R. Nagel for many stimulating discussions. This work was supported by the NSF Materials Research Laboratory (MRL) at the University of Chicago under Grant No. DMR 8819860. PW thanks the Chicago MRL for hospitality under the Industrial Visitor Program. T.L.M. acknowledges NSF graduate fellowship support. H.M.J. acknowledges fellowship support from the David and Lucile Packard Foundation.

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