Scanning probe block copolymer lithographyJinan Chaia,b,1, Fengwei Huoa,b,1,2, Zijian Zhenga,b,3, Louise R. Giamb,c, Wooyoung Shimb,c, and Chad A. Mirkina,b,c,4

aDepartment of Chemistry, bInternational Institute for Nanotechnology, and cDepartment of Materials Science and Engineering, Northwestern University,2145 Sheridan Road, Evanston, IL 60208

Contributed by Chad A. Mirkin, October 5, 2010 (sent for review September 3, 2010)

Integration of individual nanoparticles into desired spatial arrange-ments over large areas is a prerequisite for exploiting their uniqueelectrical, optical, and chemical properties. However, positioningsingle sub-10-nm nanoparticles in a specific location individuallyon a substrate remains challenging. Herein we have developed aunique approach, termed scanning probe block copolymer lithogra-phy, which enables one to control the growth and position ofindividual nanoparticles in situ. This technique relies on eitherdip-pen nanolithography (DPN) or polymer pen lithography (PPL)to transfer phase-separating block copolymer inks in the formof 100 or more nanometer features on an underlying substrate.Reduction of the metal ions via plasma results in the high-yieldformation of single crystal nanoparticles per block copolymer fea-ture. Because the size of each feature controls the number of metalatomswithin it, the DPN or PPL step can be used to control preciselythe size of each nanocrystal down to 4.8� 0.2 nm.

scanning probe lithography ∣ block copolymer micelles ∣single particle synthesis ∣ nanopatterning

Nanoparticles exhibit size-dependent photonic, electronic, andchemical properties that could lead to a new generation of

catalysts and nanodevices, including single electron transistors,photonics, and biomedical sensors (1–3). In order to realize manyof these targeted applications, researchers need ways of synthe-sizing monodisperse particles while controlling individual particleposition on technologically relevant surfaces. The challenge ofpositioning or synthesizing single sub-10-nm nanoparticles indesired locations is difficult, if not impossible, via current tech-niques including conventional photolithography (4–7). Scanningprobe-based methods such as dip-pen nanolithography (DPN) (8)and polymer pen lithography (PPL) (9) are particularly attractivebecause inked nanoscale tips can deliver material directly todesired locations on various substrates with high registrationand sub-50-nm feature resolution (10). Here we report a uniqueapproach, termed scanning probe block copolymer lithography(SPBCL), which enables one to control individual nanoparticlegrowth and position in situ by using DPN or PPL to patternattoliter volumes of metal ions associated with block copolymersin a massively parallel manner over large areas. Reduction ofthe metal ions via plasma results in the high-yield formationof single crystal nanoparticles per block copolymer feature.Specifically, we demonstrate that pattern dimensions and metalion concentration dictate the size of each nanoparticle, whosediameter can be controlled with remarkable precision downto 4.8� 0.2 nm.

To begin, we identified a polymer with two essential properties.The material must transfer from a scanning probe tip to a surfaceof interest in a controllable way, and it must sequester metalions which can be used subsequently to make nanoparticles.We evaluated the properties of poly(ethylene oxide)-b-poly(2-vi-nylpyridine) (PEO-b-P2VP) in this context (Fig. 1 A and B).Researchers have shown that block copolymers can be used togenerate nanostructures in the 5–100 nm range (11–14). Thewell-defined domain structures of the block copolymer systemcan be used as templates to achieve patterns of functional mate-rials including metals, semiconductors, and dielectrics (15–18).Past block copolymer work described the use of block copolymers

as thin film templates for the synthesis of nanoparticle arrays inmass without control over individual particle position or dimen-sions. In this work, however, we demonstrate addressable andsize-controllable single nanoparticle synthesis using a tip-basedapproach where the block copolymer acts as a delivery matrixfor facile ink transfer and as a synthetic nanoreactor for formingsingle nanoparticles. With this PEO-b-P2VP block copolymer,the P2VP is responsible for concentrating nanomaterial precur-sors through metal ion association for subsequent in situ chemicalsynthesis (19, 20), whereas PEO acts as a delivery block to facil-itate ink transport when used in a scanning probe experiment.Pure PEO is known to be a good ink matrix material for DPN(21), whereas P2VP alone is not a good transport matrix becauseof its low solubility in water at neutral pH. The block copolymerseparates into nanoscale micelles, which not only localizes themetal ions, but also causes the amount of metal ion in each fea-ture to be substantially lower than if the feature was made frompure metal ion ink. Moreover, the time-dependent ink transportcharacteristics of DPN and PPL determine the volume of trans-ferred composite ink, which effectively controls the final featuresize of the nanomaterials formed inside the polymer micelles. In-deed, the final dimensions of the metal nanoparticles that resultfrom plasma reduction of the metal ions in the block copolymerfeatures are smaller than those which define the original features.Importantly, feature size reduction beyond physical tip geometryconstraints is achieved via this approach. It is worth noting thatfeature resolution for conventional DPN is limited by the tipradius of curvature and the water meniscus formed betweentip and substrate; the smallest DPN feature reported to date is15 nm in diameter for an alkanethiol self-assembled monolayerformed on a crystalline Au (111) substrate (22).

Results and DiscussionIn a typical SPBCL experiment, PEO-b-P2VP (PEO MW ¼2;800 g∕mol and P2VP MW ¼ 1;500 g∕mol) was dissolved inan aqueous solution at a concentration of 0.5% (wt∕wt). TheHAuCl4 was added to the solution at a 2∶1 molar ratio ofP2VP:Au. The copolymer-gold salt solution was stirred for24 h, and Si tips (Nanoink) were dipped into the ink solution,followed by drying with nitrogen. The DPN experiment was per-formed on an NScriptor system (Nanoink). The inked tips werebrought in contact with a hexamethyldisilazane-coated Si∕SiOxsurface. Due to the facile transport of PEO under environmentsof high humidity, the deposition of PEO-b-P2VP is rapid, anddots of uniform size were produced with a tip dwell time of0.01 s at 70% relative humidity. This process was repeated

Author contributions: : J.C., F.H., Z.Z., and C.A.M. designed research; J.C., F.H., Z.Z., L.R.G.,and W.S. performed research; J.C., F.H., Z.Z., L.R.G., W.S., and C.A.M. analyzed data;and J.C., F.H., L.R.G., and C.A.M. wrote the paper.

The authors declare no conflict of interest.1J.C. and F.H. contributed equally to this work.2Present address: School of Materials Science and Engineering, Nanyang TechnologicalUniversity, 50 Nanyang Avenue, Singapore 639798.

3Present address: Institute of Textiles and Clothing, Hong Kong Polytechnic University,Hung Hom, Kowloon, Hong Kong, China.

4To whom correspondence should be addressed. E-mail: chadnano@northwestern.edu.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1014892107/-/DCSupplemental.

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1,600 times for a total patterning time below 2 min to generate a40 × 40 array where the distance between features was 500 nm(Fig. 1C). In a representative 20-dot line generated by a singlepen, each feature diameter was approximately 90 nm with a sizedeviation of 8%, as measured by atomic force microscopy(AFM) (Fig. 1D).

When the PEO-b-P2VP∕AuCl4− inked arrays were brought

in contact with a sample surface, micelles were transported to thesubstrates through the meniscus formed at the tip point of con-tact. Subsequent reduction by plasma treatment leads to the for-mation of Au nanoparticles within the aggregated micelles, asdetermined by X-ray photoelectron spectroscopy (Fig. S1). In thisstep, the surrounding polymer matrix was removed by the plasma,leaving square arrays of sub-10-nm Au nanoparticles on the Sisubstrate, as evidenced by SEM (Fig. 1E). The Fourier transformof the SEM image shows a highly ordered pattern characteristicof aligned square arrays (Fig. 1E, Inset). An SEM image at lowermagnification (Fig. S2) remarkably indicates that every spot isoccupied by a single Au nanoparticle in an 9 × 7 array. ThePEO-b-P2VP∕AuCl4

− ink was also patterned on a 50-nm-thickSi3N4 membrane. SEM and transmission electron microscopy(TEM) images reveal that the mean diameter of the Au nanopar-ticles in the array is 8.2� 0.6 nm. One striking observation is thatall of the atoms in a patterned feature go into making a singlenanoparticle. The clear lattice fringes in the high-resolutionTEM image of one of these Au particles are indicative of singlecrystal, face-centered-cubic Au with an interplanar (111) spacingof 0.24 nm (Fig. 1F). Moreover, the characteristic electron dif-fraction pattern (Fig. 1F, Inset) also confirms the single crystalnature of the gold nanoparticles.

Compared to conventional photolithography or contact print-ing, which allows one to create and duplicate preformed masks,and block copolymer lithography, which generates hexagonal ar-rays with spherical domains in most cases (23), we demonstratedflexibility in creating arbitrary patterns with SPBCL throughpiezo-controlled movement of the probe array over a substrate.

This advantage of SPBCL bypasses the need for photomask fab-rication and enables arbitrary pattern formation compatible withthe semiconductor industry integrated circuit design standards.As an example of arbitrary pattern control, we generated theNorthwestern University Wildcat logo consisting of individualsub-10-nm Au nanoparticles (Fig. 2 A and B).

The size of the nanoparticles synthesized in the block copoly-mer micelles depends on a number of parameters, such as thechain length of the copolymer block, the loading concentrationof the metal precursor, and the type of reducing agent (24–26).The time-dependent ink transport characteristics of DPN providean additional route for controlling the size of the nanomaterialssynthesized within the deposited block copolymer nanoreactors.Because the diffusive characteristics of the block copolymer inkare similar to previous reports of feature size dependence on tip-substrate contact time, the nanoparticles synthesized using thisDPN-based approach have dimensions that are linearly depen-dent on the square root of the tip-substrate contact time (27).In a typical DPN experiment under an environment of saturatedhumidity, Au nanoparticles of different diameters were producedby adjusting tip dwell time from 0.01, 0.09, 0.25, 0.49, to 0.81 s(Fig. 3). The Au nanoparticles, prior to the complete removalof block copolymer matrix, were characterized by AFM andTEM (Fig. S3). These data allow one to determine the absoluteand relative sizes of the original polymer spots and resultinggold nanoparticles in a single experiment. Because feature sizeis dependent upon dwell time in a DPN or PPL experiment,one can control the size of the gold nanoparticles based uponthe size of the block copolymer spots generated in the scanningprobe experiment. Therefore, one can create an indirect, but lin-ear relationship between dwell time and nanoparticle size (Fig. 3).In general, the nanoparticles have diameters about 10 timessmaller than that of the original patterned feature. Moreover,when the block copolymer features are large enough (e.g., 450 nmin diameter), more than one Au nanoparticle can form within theoriginally patterned area (Fig. S4). This result demonstrates that

Fig. 1. Tip-based synthesis of single Au nanoparti-cles. (A) Structure and molecular weight of thePEO-b-P2VP used in this study. (B) The block copoly-mer phase separated in aqueous solution to formP2VP cores (blue) surrounded by PEO coronas (red).When HAuCl4 is added to the solution, the proto-nated P2VP cores become associated with AuCl4

−

ions. The hybrid ink is dip coated onto an AFM tipand patterned on the Si substrate. The Au precursorwithin the block copolymer micelles is then reduced,and polymer removed, with plasma treatment.(C) AFM topographical image of a square dot arrayof PEO-b-P2VP∕AuCl4

− ink on an Si∕SiOx surface pat-terned by DPN. (D) Height profile of one line ofPEO-b-P2VP∕AuCl4

− dots demonstrating uniformityof feature size. (E) SEM image of sub-10-nm Aunanoparticles produced by plasma treatment. (Inset)Fourier transform of the SEM image. (F) High-resolu-tion TEM image showing a crystallineAunanoparticlewith a diameter of 8 nm. The measured interplanarspacing of the crystal is 0.24 nm. (Inset) Typical elec-tron diffraction pattern of the synthesized Au (111)nanoparticle.

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it is possible to control the number of particles per depositedfeature.

Importantly, this approach allows for sub-5-nm Au nanoparti-cles to be synthesized and assembled by decreasing the goldprecursor concentration with all other experimental parametersremaining constant. The gold salt loading factor determines thelocal concentration of ions within the polymer micelle: The lowerthe concentration, the smaller the Au nanoparticle. Herein,HAuCl4 was added to the PEO-b-P2VPmicelle solution to obtaina 4∶1 molar ratio of 2-vinylpyridine to gold instead of thepreviously described 2∶1. After stirring for 1 d, the block copo-lymer-gold salt ink was transported from the pen array to thesubstrate followed by plasma reduction of the Au. SEM data showthat sub-5-nm single crystal Au particles form on each spot(Fig. 4A). The size of each Au nanoparticle was measured byscanning TEM (Fig. 4B), and a histogram shows the average dia-meter of the Au nanoparticles is 4.8 nm with a standard deviationof 0.2 nm (4%) (Fig. 4C). Similar results were obtained for arraysof platinum nanoparticles by using the Pt precursor, Na2PtCl4,mixed with PEO-b-P2VP solution as the ink, followed by DPNdeposition and plasma treatment (Fig. S5).

SPBCL by DPN is also amenable to PPL, a recently developedhigh-throughput scanning-probe-based printing method thatcombines the advantages of DPN and microcontact printing,and eliminates many of their shortcomings. For proof-of-conceptpurposes, a 1-cm2 polymer pen array [∼15;000 polydimethylsilox-ane (PDMS) pens] with 80-μm separation between tips was inkedwith the PEO-b-P2VP∕AuCl4

− ink by spin coating. Using anAFM (XEP software, Park Systems Co.) at 80% humidity, eachpen in the PPL array was used to make a 20 × 20 dot array with2-μm spacing between the dots (Fig. 5A). The deposition time for

each dot was only 0.5 s, and thus an array of approximately 25 mil-lion features (400 features per pen) was prepared in less than5 min. The SEM image in Fig. 5B shows the formation of an arrayof single Au nanoparticles where the original block copolymermatrix has been removed by oxygen plasma and demonstratesthat this approach is extremely high yield.

In conclusion, we report SPBCL, a unique approach to synthe-size and position single sub-10-nm nanoparticles within phase-separating block copolymer inks transported to a surface by DPNand PPL. SPBCL is general and can be applied to arbitrary pat-terning and synthesis of sub-10-nm structures on many flat sur-faces over large areas. The technique is not restricted solely toAu and Pt, and in principle can be applied to the synthesis andarbitrary patterning of sub-10-nm structures consisting of othermetals, semiconductors, and potentially any material that can besynthesized within the domains of a block copolymer (28–31). Thisset of capabilities will allow researchers to create nanostructureson surfaces that are small enough to behave as single particledevices (e.g., sensors) that can be integrated with prefabricatedcircuitry and the isolation of individual biological molecules suchas proteins.

Fig. 2. Arbitrary pattern control of single Au nanoparti-cles. (A) A dark field optical microscopy image of the North-western University Wildcat logo pattern made of individualPEO-b-P2VP∕AuCl4

− dot features. (B) SEM image of a mag-nified portion of A showing the formation of Au nanopar-ticle arrays embedded in the polymer matrix upon plasmaexposure. (Inset) A magnified SEM image of a single goldnanoparticle after polymer removal.

Fig. 3. Size distribution of PEO-b-P2VP∕AuCl4− dots and the corresponding

Au nanoparticles synthesized within them as a function of increasingtip-substrate contact time. AFM topographical images of the patternedPEO-b-P2VP∕AuCl4

− features and SEM images of the Au nanoparticlesformed after brief exposure to plasma show a time-dependent size increase(Insets).

Fig. 4. Synthesis of sub-5-nm Au nanoparticles. (A) SEM image of a 3 × 3

array of Au nanoparticles with sub-5-nm diameters. (B) The size of the Aunanoparticles of the array corresponding to Fig. 3A was measured byscanning TEM. (C) Histogram showing the size distribution of sub-5-nm Aunanoparticles.

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Materials and MethodsInk Preparation. The diblock copolymer, PEO-b-P2VP, was purchased fromPolymer Source, Inc., with polydispersity Mw∕Mn ¼ 1.11 and molecularweights of 2,800 and 1;500 g∕mol for the PEO and P2VP, respectively.PEO-b-P2VP was used without purification. HAuCl4 (99.9995%) was pur-chased from Sigma-Aldrich and Na2PtCl4 (99%) was acquired from StremChemicals. The ink was prepared by dissolving the PEO-b-P2VP in 18 MΩ-cmdeionized water at room temperature to make a 0.5% wt∕wt solution andstirred for 24 h. In order to preload the polymers with metal precursors, solid

HAuCl4 or Na2PtCl4 was dissolved in an aqueous solution of PEO-b-P2VP byvigorous stirring for at least 24 h before use (19). The molar ratio of ½AuCl4�−or ½PtCl4�2− to pyridyl groups ranges from 0.1 to 0.5.

DPN Patterning Process. Either single A-type tips or 12 pen M-type 1D tiparrays (Nanoink, Inc.) can be employed for DPN. Before patterning, thecantilever tips were exposed to oxygen plasma for 30 s to render themhydrophilic, which ensured that the ink wetted the cantilever tips completely.Then the tips were dipped in the ink solution and blown dry with nitrogen.DPN patterning was performed in a chamber with environmental control(NScriptor) manufactured by Nanoink, Inc., where the relative humidityranged from 70% to saturation in a temperature range of 25–29 °C. Theinstrument has a motorized stage with a registration of ∼15 nm, and 1D can-tilever tip arrays can be aligned parallel to the substrate by optical leveling.The patterns were generated using InkCAD software (Nanoink, Inc.), whichallows one to control feature position and tip-substrate contact time.

PPL Patterning Process. Polymer pen tip arrays with 80-μm spacing betweentips were prepared as previously reported (10). The pen arrays were madefrom hard PDMS (h-PDMS), which was composed of 3.4 g of vinyl-com-pound-rich prepolymer (vinylmethylsiloxane-dimethylsiloxane -731, Gelest)and 1.0 g of hydrosilane-rich crosslinker (methylhydrosiloxane-301, Sigma).The mixture was stirred, degassed, and poured on top of the soft pen arraymaster. A precleaned glass slide (VWR, Inc.) was then placed on top of theelastomer array and the whole assembly was cured at 70 °C overnight. Finally,the polymer pen array was carefully separated from the pyramid master andtreated with oxygen plasma for 2 min to improve inking. The ink solution ofblock copolymer/metal precursor was spun cast onto the PDMS tip arrays(1 mL ink, 2,000 rpm, 2 min) (Ocean Nanotech). PPL was carried out on anAFM work station with a PPL head manufactured by Park Systems Co., usingXEP software (Park, Inc.) at a relative humidity of ∼80% and temperature of25–29 °C. The leveling between the pen arrays and the sample was adjustedwith an XY scanning tilting stage with a resolution of 4∕1;000°.

Nanoparticle Synthesis. After DPN or PPL, the samples were exposed tooxygen or argon plasma for about 5min in a Harrick Plasma Cleaner operatedat 60 W at a pressure of 100 mtorr. Plasma treatment was used to removethe block copolymer and reduce the metal ions as facilitated by hydrocarbonoxidation (32). Any metal oxide on the nanoparticle surface can be removedby subsequent argon or hydrogen plasma treatment (26).

ACKNOWLEDGMENTS. C.A.M. acknowledges the US Air Force Office ofScientific Research, the Defense Advanced Research Projects Agency, andNational Science Foundation (NSF) (Nanoscale Science and EngineeringCenter Program) for support of this research. C.A.M. is grateful for a NationalInstitutes of Health Director’s Pioneer Award and a National Security Scienceand Engineering Faculty Fellowship from the Department of Defense. J.C.acknowledges the Natural Sciences and Engineering Research Council ofCanada for a Postdoctoral Fellowship. L.R.G. acknowledges the NSF for aGraduate Research Fellowship.

1. Klein DL, Roth R, Lim AKL, Alivisatos AP, McEuen PLA (1997) Single-electron transistormade from a cadmium selenide nanocrystal. Nature 289:699–701.

2. Bakr OM, et al. (2009) Silver nanoparticles with broad multiband linear optical absorp-tion. Angew Chem Int Ed Engl 48:5921–5926.

3. GrahamD, Faulds K, SmithWE (2006) Biosensing using silver nanoparticles and surfaceenhanced resonance Raman scattering. Chem Commun 42:4363–4371.

4. Gates BD, et al. (2005) New approaches to nanofabrication: Molding, printing, andother techniques. Chem Rev 105:1171–1196.

5. Jiang L, Wang W, Fuchs H, Chi L (2009) One-dimensional arrangement of goldnanoparticles with tunable interpartcle distance. Small 5:2819–2822.

6. Sioss JA, Keating CD (2005) Batch preparation of linear Au and Ag nanoparticle chainsvia wet chemistry. Nano Lett 5:1779–1783.

7. Kraus T, et al. (2007) Nanoparticle printing with single-particle resolution. Nat Nano-technol 2:570–576.

8. Piner RD, Zhu J, Xu F, Hong SH, Mirkin CA (1999) “Dip-pen” nanolithography. Science283:661–663.

9. Huo F, et al. (2008) Polymer pen lithography. Science 321:1658–1660.10. Salaita K, Wang YH, Mirkin CA (2007) Applications of dip-pen nanolithography. Nat

Nanotechnol 2:145–150.11. Bates FS, Fredrickson GH (1999) Block copolymers—designer soft materials. Phys Today

52:32–38.12. Ruiz R, et al. (2008) Density multiplication and improved lithography by directed block

copolymer assembly. Science 321:936–939.13. Bita I, et al. (2008) Graphoepitaxy of self-assembled block copolymers on two-dimen-

sional periodic patterned templates. Science 321:939–934.

14. Park S, et al. (2009) Macroscopic 10-terabit-per-square-inch arrays from block copoly-mers with lateral order. Science 323:1030–1033.

15. Spatz JP, Roescher A, Moller M (1996) Gold nanoparticles in micellar poly(styrene)-b-poly(ethylene oxide) films-size and interparticle distance control in monoparticulatefilms. Adv Mater 8:337–340.

16. Park M, Harrison C, Chaikin PM, Gegister RA, Adamson DH (1997) Block copolymerlithography: Periodic arrays of similar to 10(11) holes in 1 square centimeter. Science276:1401–1404.

17. Black CT (2007) Polymer self-assembly as a novel extension to optical lithography. ACSNano 1:147–150.

18. Chai J, Wang D, Fan X, Buriak JM (2007) Assembly of aligned linear metallic patternson silicon. Nat Nanotechnol 2:500–506.

19. Bronstein LH, et al. (1999) Induced micellization by interaction of poly(2-vinylpyri-dine)-block-poly(ethylene oxide) with metal compounds. Micelle characteristics andmetal nanoparticle formation. Langmuir 15:6256–6262.

20. Sidorov SN, et al. (2004) Influence of metalation on themorphologies of poly(ethyleneoxide)-block-poly (4-vinylpyridine) block copolymer micelles. Langmuir 20:3543–3550.

21. Huang L, et al. (2010) Matrix-assisted dip-pen nanolithography and polymer penlithography. Small 6:1077–1081.

22. Hong SH, Zhu J, Mirkin CA (1999) Multiple ink nanolithography: Toward a multiple-pen nano-plotter. Science 286:523–525.

23. Tang CB, Lennon EM, Fredrickson GH, Kramer EJ, Hawker CJ (2008) Evolution of blockcopolymer lithography to highly ordered square array. Science 332:429–432.

24. Spatz JP, et al. (2000) Ordered deposition of inorganic clusters from micellar blockcopolymer films. Langmuir 16:407–415.

Fig. 5. PPL patterning of nanoparticle arrays over large areas. (A) A darkfield optical microscopy image of a large scale pattern of PEO-b-P2VP∕AuCl4

− dots. (The inset shows 20 × 20 dots with 2-μm spacing for each patternwith an individual pen created on a Si∕SiOx surface by a 15,000 pen array.)(B) SEM image of Au particles (bright dot) formed within the PPL-patternedblock copolymer arrays which have been removed by oxygen plasma. Theaverage diameter of Au particles in the array is 9.5 nm.

Chai et al. PNAS ∣ November 23, 2010 ∣ vol. 107 ∣ no. 47 ∣ 20205

ENGINEE

RING

Dow

nloa

ded

by g

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ober

23,

202

0

25. Bhaviripudi S, et al. (2006) Block-copolymer assisted synthesis of arrays of metal na-noparticles and their catalytic activities for the growth of SWNTs. Nanotechnology17:5080–5086.

26. Chai J, Buriak JM (2008) Using cylindrical domains of block copolymers to self-assembleand align metallic nanowires. ACS Nano 2:489–501.

27. Ginger DS, Zhang H, Mirkin CA (2004) The evolution of dip-pen nanolithography.Angew Chem, Int Ed 43:30–45.

28. Abes JI, Cohen RE, Ross CA (2003) Selective growth of cobalt nanoclusters in domainsof block copolymer films. Chem Mater 15:1125–1131.

29. Kang C, et al. (2009) Full color stop bands in hybrid organic/inorganic block copolymerphotonic gels by swelling-freezing. J Am Chem Soc 131:7538–7539.

30. Li X, Lau KHA, Kim DH, Knoll W (2005) High-density arrays of titania nanoparticles

using monolayer micellar films of diblock copolymers as templates. Langmuir

21:5212–5217.

31. Bang J, Jeong U, Ryu DY, Russell TP, Hawker CJ (2009) Block copolymer nanolithogra-

phy: Translation of molecular level control to nanoscale patterns. Adv Mater

21:4769–4792.

32. Jaramillo TF, Baeck S-H, Cuenya BR, McFarland EW (2003) Catalytic activity of

supported au nanoparticles deposited from block copolymer micelles. J Am Chem

Soc 125:7148–7149.

20206 ∣ www.pnas.org/cgi/doi/10.1073/pnas.1014892107 Chai et al.

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