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  • 1498

    Introduction

    Self-assembled monolayers (SAMs) form when organicmolecules spontaneously chemisorb on the surfaces ofsolids (e.g. organic thiols and disulfides on gold, silver,and copper or carboxylic acids on the surface of alumina).1'2The most robust and best characterized SAMs are thosecomprising alkanethiolates on gold.l By variation of thelength of the alkane chain and the identityof the functionalgroup at its terminus, the thickness of the organic layerand the chemical properties of the exposed interface canbe controlled with great precision. We and others haveused these SAMs for studies in tribology,t adhesion,awetting,s and other fields.6 In this paper, we describe atechnique for patterning the formation of SAMs, using anelastomeric stamp, that can routinely produce patternswith dimensions from 1 to 100 lrm;7 features as small as0.2 mm (200 nm) have been generated using this procedure,although these very small features are not always easilyreproduced. These patterned surfaces have geometricallywell-defined regions with different chemical and physicalproperties. We demonstrate a number of uses for them.

    * To whom correspondence should be addressed.o Abstract published in Aduance ACS Abstrocts, April 1, 1994.(1) Nuzzo, R. G.; AIIara, D. L. J. Am. Chem. Soc. 1983, 105, 448\.

    Troughton, E. B.; Bain, C. D.; Whitesides, G.M.; Nuzzo, R. G.; Allara,D. L.; Porter, M. D. Langmuir 1988, 4, 365.

    (2) Laibinis, P. E.; Hickman, J.; Wrighton, M. S.; Whitesides, G. M.Science (Washington, DC) 1989, 245, 845.

    (3) Salmeron, M. B. MRS 8u11.1993, 18, 20-25.(4) Al lara, D. L. ;Hebard, A. F. ;Padden, F. J. ;Nuzzo, R. G.;Falcone,

    D. R. J. Vac. Sci.Technol. A 1983,376. Haussling, L.; Ringsdorf, H.;Schmit t , F. J. ; Knol l , W. Langmuir 1991, 7,1837.

    (5) Bain, C. D. ;Whitesides, G. M. J. Am. Chem. Soc. 1988, 110,6560.Bain, C. D. ; Whi tesides, G. M. J. Am. Chem. Soc. 1989,111,7155. Bain,C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110,3665. Bain, C. D.;Whitesides, G. M. Science (Washington, DC) 1988,240,62. Bain, C. D.;Whitesides, G. M. J. Am. Chem. Soc. 1989, 111,7164.

    (6) See for example Black, Andrew J.; Wooster, Timothy T.; Geiger,Will iam E. J. Am. Chem. Soc. 1993, 115,7924-5. Duerig, U.; Zueger, O.;Michel, B.; Haeussling, L.i Ringsdorf, H. Phys. Reu. B 1993, 8, 1711-17.Willner,Itamar;Rubin, Shai; Cohen, Yael J. Am. Chem. Soc. 1993, 115,4937-8. Ong, T. Hui ;Davies, Paul B. ;Bain, Col in D. Langmuir 1993,9,1836-45. Huang, Jingyu; Hemminger, John C. J. Am. Chem. Soc. 1993,115,3342-3. Stenger, David A.;Georger, Jacque H.;Dulcey, Charles S.;Hickman, James J.;Rudolph, Alan S.; Nielsen, Thor B.;McCort, StephenM.; Calvert, Jeffrey M. J. Am. Chem. Soc. 1992, 114, 8435-42.

    (?) Kumar, A.; Whitesides, G. M. Appl. Phys. Lef f ., in press.

    copyright o 1ee4 rry the A-""i"""TfJilit""$ !T#",sxTYJ*;13i',;J?""*i..ion of the copyright owner.

    Patterning Self-Assembled Monolayers: Applications inMaterials Science

    Amit Kumar, Hans A. Biebuyck, and George M. Whitesides'

    Department of Chemistry, Haruard Uniuersity, 12 Oxford Street,C ambridge, Massachusetts 02 1 38

    Receiued February 18, 19948

    This paper describes an experimentally simple technique, based on stamping or contact printing, topattern the adsorption of alkanethiolates on surfaces of gold, on scales from 0.2 to 100 irm, and illustrateslhe use of these paiterned surfaces. An elastomeric stamp, fabricated from poly(dimethylsiloxane) (PDIVIS)'was used to deliver alkanethiol to predetermined regions of the surface of an evaporated gold film. Withthis technique, organic surfaces patterned with well-defined regions exhibiting different chemical andphysical propertiei have been produced. This technique was used to pattern the adsorption of single andmultiple SAMs on a single substrate. This method of preparing patterned surfaces has been used in avariety of applications, including the preparation of well-defined, heterogeneous substrates for scanningprobe microscopies, the formation of microelectrodes, the formation of microstructures of silicon, thepreparation of substrates for the study of condensation figures, and the preparation of substrates forpatterned formation of microcrystals.

    Results and Discussion

    Preparation of the Elastomeric Stamps and For-mation of Patterned Surfaces. Figure 1 diagrams theprocedure for the formation of the stamp and its use toprepare patterned surfaces.? (More description is pre-sented in the Experimental Section.) The stamp was usedin conjunction with an "ink" consisting of a solution of analkanethiol in ethanol, to print with SAMs on surfaces ofgold. The stamp was fabricated by casting poly(dimeth-ylsiloxane) (PDMS) on a master having the desiredfeatures. We used several types of masters for thefabrication of stamps: commercially available transmissionelectron microscopy grids, structures prepared photo-lithographically by exposing and developing a photoresistpattern on a solid support (typically a silicon wafer), andsemiconductor wafers that have been processed photo-lithographically and metalized. After curing, the PDMSstamp was peeled away from the master. The master couldbe reused a number of times to produce multiple copiesof the stamp. Figure lb shows a photograph of arepresentative stamp. The square, opaque area is theregion that contains the desired microscopic features.Figure lc shows a magnified view of the features on arepresentative stamp. This stamp was exposed to anethanolic solution (1-10 mM) of the alkanethiol that wasto be patterned on the surface. We believe that the PDMSswelled slightly upon exposure to the solution.s The thiolwas transferred to the surface by bringing the stamp andthe gold into contact. Although the surface of the goldwas not smooth on a molecular scale (these evaporatedfilms typically have corrugations of 50-100 A), theelastomeric nature of PDMS allowed for conformal contactbetween the stamp and the surface. After removal of thestamp, the gold surface could be used in studies wheresingle, patterned SAMs were desired. Alternatively, thegold could be exposed to a solution of another alkanethiolor dialkyl dilsulfide to derivatize the remainingbare regions

    (8) We believe the swelling of the polymer did not distort the features.

    @ 1994 American Chemical Society07 43-7 463 I I 4 I 2410-1498$04. 50/ 0

  • Patterning Self - Assembled Monolayers

    PD\TS

    Langmuir, Vol. 10, No. 5, 1994 1499

    Photoresist pattern(1 -2 um th i ckness t

    I oo.tt is peeted

    I au'av from the

    Y master

    I eof f S is exposed ro aI solut ion containing

    I H S ( C I I 2 r 1 5 C I t . 1

    Ai*ftur*;!'oiL-"{.ff{;o*t}, t r' # a I k a n et h i ol

    Stamping onto goldsubstrate transfersthiol -J S A M s ( - l - 2 n m r

    A u ( 2 0 0 n m l + T i ( 5 n m r

    h rd rophob i . - - - l

    h r d roph i l i c - t I

    i f

    Surface is exposed toso lu t ion conta in ingHS(CH2) l tOH

    SE]VI

    Figure l. Procedure for fabrication of the pattrned elastomeric stamp, and for preparation of patterned surface. (a) An erposedand developed photoresist pattern on a silicon wafer was ueed aB the master. Poly(dimethylsilorane) (PDMS) was pol5'merizld onthe master and carefully peeled away. The stamp was inked by erposure to an ethanolic solution (1-10 mM) of the appropriatealkanethiol, brought into contact with the gold substrate, and removed. The substrat was then washed for 2-4 s with a solution ofanother alkanethiol (1-10 mM in ethanol). The substrate was finatly washed for 10 s with ethanol and dried in a stream of nitrogen.tb) Photograph of a stamp. The region with the featutes are in the square section. (c) A magnified view of the featurea in the aquaresection. (d) This electron micrograph is lepresntative of a patterned surface. The etamp having square features, 10 tm on 8 side,was used to pattem the adsorption of HS(CHtroCHg on the gold (light regions); the gold was subsequently erposed to HS(CH2)D-CH,OH (dark regions).

    with a second SAM. Multiple printing steps with the samestamp (we have used the same stamp up to 100 times overa period of several months with no evidence of loss ofperformance), with different stamps, and with differentalkanethiols could be carried out on a single substrate.This flexibility made it straightforward to prepare a rangeof surfaces having both simple and complex patternsconsisting of one or several SAMs, using simple stamps.

    Figure ld shows a scanning electron micrograph (SEM)of a representative surface. A stamp having squares 10sm on a side (shown in Figure 1b,c) was used to stamp apattern of HS(CHz)rsCHs (light) on gold, and the remain-ing regions were exposed to HS(CHz)roCHzOH (dark). Themicrograph shows that the edge resolution is on thesubmicrometer scale, but it is not clear whether theboundary between SAMs is abrupt on molecular dimen-sions. We believe that the contrast observed in this imageis primarily due to attenuation of secondary electrons byimpurities adsorbed on the SAMs. More contaminantsadsorb onto the high free energy surfaces that form whenSAMs are made from alkanethiols with terminal hydroxyls(and other hydrophilic functionalities) than on SAMs

    prepared from alkanethiols with terminal methyls; con-sequently, the hydrophilic regions appear darker.e

    Parts a(M), b(M), and c(M) of Figure 2 show severalSEMs of complex microelectronic patterns that wereproduced using conventional lithography. These struc-tures were used in turn as masters to produce stamps,which reproduced corresponding patterns in SAMs of HS-(CHz)rsCH3 on gold surfaces. The SAMs formed fromHS(CH2) lsCHs were hydrophobic, low free-energy surfaces.The patterned gold was washed with an ethanolic solution(1-10 mM) of HS(CH2)10CH2OH to functionalize the bareregions. Parts a(P),

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