DOI: 10.1021/la903465f 1449Langmuir 2010, 26(3), 1449–1451 Published on Web 12/01/2009

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© 2009 American Chemical Society

Catalytic Microcontact Printing on Chemically FunctionalizedH-Terminated Silicon

Alexander A. Shestopalov,† Robert L. Clark,*,‡ and Eric J. Toone*,†

†Department of Chemistry, Duke University, Durham, North Carolina 27708 and ‡Hajim School of Engineeringand Applied Sciences, University of Rochester, Rochester, New York 14627

Received September 14, 2009. Revised Manuscript Received October 23, 2009

We report a novel inkless soft lithographic fabrication protocol that permits uniform parallel patterning of hydrogen-terminated silicon surfaces using catalytic elastomeric stamps. Pattern transfer is achieved catalytically via reactionbetween sulfonic acid moieties covalently bound to an elastomeric stamp and a Boc-functionalized SAM grafted topassivated silicon. The approach represents the first example of a soft lithographic printing technique that createspatterns of chemically distinctive SAMs on oxide-free silicon substrates.

Microcontact printing (μCP) is a micro- and nanopatterningtechnique that uses elastomeric stamps to pattern various solidsurfaces with inorganic, organic, and biologicalmolecules.1-3 Firstreported by Whitesides and co-workers in the early 1990s,4 thetechnique has become amainstay of surface science and is routinelyused to createmicropatterns of self-assembledmonolayers (SAMs)of thiols and silanes on metal and oxide surfaces.1-3,5-11 Tradi-tional μCP is restricted to surfaces that undergo rapid irreversiblereaction with molecular inks and suffers from several limitationsthat preclude the accurate replication of submicrometer features.Processes that include diffusive spreading of molecular inks1,6,12

and the deformation of elastomeric stamps13,14 permit only alimited number of organic SAMs to be successfully patterned atresolutions below 500 nm.Moreover, traditional μCPmethods arelargely restricted to thiol/metal and silane/oxide systems andcannot pattern many important hard substrates such as poly-crystalline silicon and germanium. The formation of monolayerson these surfaces usually requires prolonged reaction times andharsh conditions (high temperature, inert atmosphere, and UVirradiation), which are incompatible with the traditional stampmaterials and μCP conditions.

Recently, we have developed several inkless μCP techniquesthat transfer pattern from an elastomeric stamp bearing an

immobilized catalyst to a preformed functionalized SAM.15-17

By employing a biochemical or chemical reaction between acatalyst immobilized on the stamp and the corresponding sub-strate immobilized as a SAM on gold, the diffusive limitations oftraditional μCP are obviated and the accurate replication ofpattern features with sub-50-nm edge resolution (limited by thesize of gold grains) is possible. Moreover, the use of readilyfunctionalized, rigid polyurethane-acrylate (PU) polymers asstamp materials18,19 circumvents the deformation behavior ofPDMS-based stamps, and features with extremely low aspectratios (<0.042) can be replicated.16

Despite the advantages of inkless soft lithography,20 the techni-que remains limited by the use of gold as an underlying SAMsubstrate, which is amaterial that supports only relatively unstableSAMs and limits the resolution of printed features to the grain sizeof gold. To expand the utility of inkless μCP, we sought flatpolycrystalline surfaces that support chemically robust, stableSAMs. To enhance the resolution of catalytic μCP further and toextend its applicability to a broader group of underlying substrates,including silicon, we sought to immobilize Boc-functionalized

Figure 1. Catalytic μCP on Boc-modified SAMs on silicon.

*To whom correspondence should be addressed. E-mail: eric.toone@duke.edu; rclark@rochester.edu.(1) Perl, A.; Reinhoudt, D. N.; Huskens, J. Adv. Mater. 2009, 21, 2257–2268.(2) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1–68.(3) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550–575.(4) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002–4.(5) Biebuyck, H. A.; Larsen, N. B.; Delamarche, E.;Michel, B. IBM J. Res. Dev.

1997, 41, 159–170.(6) Delamarche, E.; Schmid, H.; Bietsch, A.; Larsen, N. B.; Rothuizen, H.;

Michel, B.; Biebuyck, H. J. Phys. Chem. B 1998, 102, 3324–3334.(7) Gates, B. D.; Xu, Q.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides,

G. M. Chem. Rev. 2005, 105, 1171–1196.(8) Michel, B.; Bernard, A.; Bietsch, A.; Delamarche, E.; Geissler, M.; Juncker,

D.; Kind, H.; Renault, J. P.; Rothuizen, H.; Schmid, H.; Schmidt-Winkel, P.;Stutz, R.; Wolf, H. IBM J. Res. Dev. 2001, 45, 697–719.(9) Rogers, J. A.; Nuzzo, R. G. Mater. Today 2005, 8, 50–56.(10) Weibel, D. B.; DiLuzio, W. R.; Whitesides, G. M. Nat. Rev. Microbiol.

2007, 5, 209–218.(11) Zhao, X.-M.; Xia, Y.; Whitesides, G. M. J. Mater. Chem. 1997, 7, 1069–

1074.(12) Larsen, N. B.; Biebuyck, H.; Delamarche, E.; Michel, B. J. Am. Chem. Soc.

1997, 119, 3017–3026.(13) Bietsch, A.; Michel, B. J. Appl. Phys. 2000, 88, 4310–4318.(14) Delamarche, E.; Schmid, H.; Michel, B.; Biebuyck, H. Adv. Mater. 1997, 9,

741–746.

(15) Shestopalov, A. A.; Clark, R. L.; Toone, E. J. J. Am. Chem. Soc. 2007, 129,13818–13819.

(16) Shestopalov, A. A.; Clark, R. L.; Toone, E. J. Nano Lett., submitted forpublication, 2009.

(17) Snyder, P. W.; Johannes, M. S.; Vogen, B. N.; Clark, R. L.; Toone, E. J.J. Org. Chem. 2007, 72, 7459–7461.

(18) Choi, S.; Yoo, P. J.; Baek, S. J.; Kim, T. W.; Lee, H. H. J. Am. Chem. Soc.2004, 126, 7744–7745.

(19) Yoo, P. J.; Choi, S.-J.; Kim, J. H.; Suh, D.; Baek, S. J.; Kim, T. W.; Lee,H. H. Chem. Mater. 2004, 16, 5000–5005.

(20) Li, X.-M.; Peter, M.; Huskens, J.; Reinhoudt, D. N. Nano Lett. 2003, 3,1449–1453.

1450 DOI: 10.1021/la903465f Langmuir 2010, 26(3), 1449–1451

Letter Shestopalov et al.

molecules on polycrystalline silicon and transfer pattern to thissurface using an acidic PU stamp (Figure 1).

SAMs covalently bound to silicon have gained significantattention for (bio)chemical sensing and electronic applicationsbecause of their exceptional stability and their ability to passivatesilicon surfaces toward chemical oxidation.21 Most chemicalapproaches to silicon functionalization are based on the hydro-silylation of Si-H groups on the surface with alkenes, a processcatalyzed by heat, peroxides, metalloorganic catalysts, or UVlight.22-26 To form Boc-modified SAMs on silicon, we preparedBoc-protected aminoalkene 5 from 11-bromo-1-undecene in foursteps (Figure 2). Subsequently, a freshly prepared H-terminatedsilicon surface was reacted with a mixture of Boc-modified andblank alkenes 5 and 6 (1/4 v/v) for 2 h underUV light in a nitrogenatmosphere (Figure 3). The terminal Boc group in 5 does notallow the formation of highly ordered SAMs because it hindersnoncovalent chain-chain interactions. We decided to use mixedmonolayers to separate Boc-protected molecules further apartand achieve higher density and better association between theSAM components to help protect underlying passivated siliconfrom air or water oxidation.

XPS spectra of H-terminated and Boc-modified substratessupport the formationof SAMsdirectly grafted to silicon throughSi-C bonds. The Si 2p signals in both substrates completely lackthe silicon oxide peak at 103 eV indicative of the native oxidelayer. The XPS C 1s signal of the Boc-modified surfaces indicatesthe presence of both PEG and alkyl carbons, but the H-termi-nated surface shows only a small residual C 1s peak (Figure 3).

In our previous work, we demonstrated that a sulfonic acid-modified polyurethane-acrylate stamp effectively transfers bothmicro- and nanoscale patterns to Boc-functionalized SAMs ongold by selectively deprotecting Boc moieties within the contactarea.16 We have also shown that the use of the polyurethanestamp material eliminates the deformation behavior of PDMS-based stamps, permitting the defect-free replication of featureswith a very low aspect ratio (0.042). Here we utilized an identicalacidic stamp to selectively deprotect Boc-modified SAMs onpassivated silicon and to prepare patterns of protected and freeamino regions. The stamp was prepared by reacting a mixturecontaining monomers 7 and 8 with 2-mercaptoethanesulfonicacid at 50 �C followed by deoxygenation and UV-inducedpolymerization of the resulting solution either between two glassplates or between a glass plate and a patterned master to produceflat or patterned acidic stamps, respectively (Figure 4). Anunfunctionalized flat polyurethane-acrylate stamp comprisingonly monomers 7 and 8 was also prepared as an inactive control.

To establish the ability of a sulfonic acid-modified stamp todeprotect Boc-modified monolayers on silicon and to determinethe efficiency of deprotection, a single H-terminated silicon chipwas functionalized with amixture of alkenes 5 and 6 (1/4 v/v) andreacted in several areas with both flat acidic and blank(unfunctionalizezd) stamps (Figure 5).

The resulting surfaces were analyzed by XPS at six differentpoints to determine both the uniformity of the monolayer acrossthe sample and the efficiency of Boc ablation by the acidic PU

Figure 2. Synthesis of Boc-protected aminoalkene 5.

Figure 3. Preparation of Boc-protected SAMs on silicon andXPSspectra of Boc-modified and H-terminated surfaces.

Figure 4. Chemical composition and SEM images of the patternedsulfonic acid-modified stamp.

(21) Puniredd, S. R.; Assad, O.; Haick, H. J. Am. Chem. Soc. 2008, 130, 13727–13734.(22) Strother, T.; Hamers, R. J.; Smith, L.M.Nucleic Acids Res. 2000, 28, 3535–

3541.(23) Wagner, P.; Nock, S.; Spudich, J. A.; Volkmuth, W. D.; Chu, S.; Cicero,

R. L.; Wade, C. P.; Linford, M. R.; Chidsey, C. E. D. J. Struct. Biol. 1997, 119,189–201.(24) Clare, T. L.; Clare, B. H.; Nichols, B. M.; Abbott, N. L.; Hamers, R. J.

Langmuir 2005, 21, 6344–6355.(25) Strother, T.; Cai, W.; Zhao, X.; Hamers, R. J.; Smith, L. M. J. Am. Chem.

Soc. 2000, 122, 1205–1209.(26) Lasseter, T. L.; Clare, B. H.; Abbott, N. L.; Hamers, R. J. J. Am. Chem.

Soc. 2004, 126, 10220–10221.

DOI: 10.1021/la903465f 1451Langmuir 2010, 26(3), 1449–1451

Shestopalov et al. Letter

stamp. Following XPS analysis, the substrate was treated with aTFA/CH2Cl2 solution (1/3 v/v) at roomtemperature for 30min tocompletely remove Boc groups from all locations, and theresulting surface was reanalyzed by XPS to determine the carbonconcentration in a fully deprotected substrate.

Figure 6 shows that the carbon concentration in the Boc-protected areas decreases slightly from point 1 (C1s/Si2p= 2.046)to 6 (C1s/Si2p = 1.908), which is a result of the slight inhomo-geneities in light intensity across the substrate surface from thesingleUV light source. XPS analysis also revealed that an inactivestampproduces no change in theBoc substrate (point 5, C1s/Si2p=1.909). However, the acidic stamps decreased the carbon con-centration in areas 2 and 4 (5 and 40 min reaction times, respecti-vely) by approximately the same amount as treatment with TFA/CH2Cl2 solution (Figure 6), demonstrating that an acidic PUstamp completely deprotects Boc-functionalized monolayers onpassivated Si in as little as 5 min at room temperature.

To evaluate the effectiveness of the sulfonic acid-modified PUstamp for catalytic pattern transfer, we reacted both acid-func-tionalized and control PU stamps containing 6.5 μm features withBoc-functionalized SAMs for 5 min at room temperature. Theresulting substrates were analyzed by SEM to evaluate the chemi-cal differences between protected and deprotected (patterned)regions.WhereasBoc-modifiedmonolayer surfaces exposed to aninactive PU stamp showed no discernible features, those exposedto acidic stamps showed feature patterns cognate to the stamp(Figure 7). The acidic stamp selectively deprotected SAMs inregions of conformal contact, creating distinctive areas of Boc-protected and free amine groups. The pattern was uniform acrossthe entire stamped area. The contrast between the protected anddeprotected regions was relatively low, presumably because of thelow surface population of Boc groups.

In conclusion, we have demonstrated that the sulfonic acid-modified PU stamp effectively deprotects Boc-modified SAMsonpassivated silicon, creating uniform patterns of protected and freeamine functionalities. The protocol makes use of a very stableSAM directly grafted to silicon through Si-C bonds and allowsfor the direct attachment of desirable chemical functionalities tothe patterned substrate. This protocol is, to the best of ourknowledge, the first example of a soft lithographic printingtechnique that creates regions of chemically distinctive SAMson oxide-free silicon substrates. We continue to explore theabilities of catalytic μCP on technologically important substratessuch as silicon and will report our results in due course.

Acknowledgment. The financial support of the NSF (DMI0600513) is gratefully acknowledged.

Supporting Information Available: Experimental details.This material is available free of charge via the Internet athttp://pubs.acs.org.

Figure 5. Reactions of the Boc-modified silicon surface with cat-alytic and inactive stamps.

Figure 6. XPSanalysis of the reactions of theBoc-modified siliconsurface with catalytic and inactive stamps.

Figure 7. SEM images of the patterned Boc-protected substrates.