NANO LETTERS
Catalytic Microcontact Printing without Ink
2003 Vol. 3, No. 10 1449-1453
Xue-Mei Li, Ma´ria Pe´ter, Jurriaan Huskens,* and David N. Reinhoudt* Laboratory of Supramolecular Chemistry and Technology, MESA+ Research Institute, UniVersity of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands Received June 20, 2003; Revised Manuscript Received August 5, 2003
ABSTRACT A novel microcontact printing technique is described that does not require ink. Patterns were created by direct contact of oxidized PDMS stamps with silyl ether-derivatized, acid-labile SAMs on gold. The surface of the stamps was oxidized by oxygen plasma to give a layer of silicon oxide. These stamps when placed on TMS and TBDMS SAMs selectively hydrolyzed these adsorbates in the areas of contact. The hydrolysis and its time dependence were studied by XPS and AFM. Because this process does not involve ink, diffusion processes, which limit the resolution in conventional microcontact printing, are avoided.
Introduction. Patterning surfaces via printing of molecules is of current interest.1 The most extensively explored technology is microcontact printing (µCP) of alkanethiols on gold.2,3 The method employs an elastomeric stamp which is duplicated from a master by curing a prepolymer of, for example, poly(dimethyl siloxane) (PDMS). After soaking in an alkanethiol ink solution, the stamp transfers the ink to a metal substrate (mostly gold) by conformal contact between the stamp and the substrate. The thiol chemisorbs to the metal, thus forming a self-assembled monolayer (SAM) in the contacted area. These layers control the properties of the interface such as wettability, adhesion, friction, molecular recognition, and chemical reactions.4-10 The placement of molecules on surfaces by microcontact printing avoids the most common limitation of optical lithography, the diffraction of light, while retaining one of its advantages, viz. large area patterning (>1 cm2) in a single step. The printed layers have also been used as etch resists owing to their high stability and order. The smallest printable patterns of thiols depend on the pattern definition on a stamp, the amount of ink applied to the stamp, the printing force, the printing duration, the etch system, the shape of the structures, and the fill factor of the pattern.11 It has been found that accurate reproduction of patterns realized in PDMS stamps on gold substrates was problematic on a scale of smaller than 500 nm due to the diffusion of ink molecules from the contacted to the noncontacted areas.12 Ink diffusion is closely related to the vapor pressure of the ink used.1 Thus, ink diffusion processes may be restricted by the use of heavier inks.13,14 Even palladium nanoparticles have been transferred, which initiated metallization of * Corresponding author. Fax: +31 53 4894645. Phone: +31 53 4892980. Email:
[email protected]. 10.1021/nl034423l CCC: $25.00 Published on Web 08/20/2003
© 2003 American Chemical Society
copper.15,16 Patterns were faithfully transferred with high edge resolution, indicating that nanoparticles are of potential use as ink for microcontact printing. However, potential problems with printing nanoparticles reside in the control of the amount and order of material transferred. Recently, we have reported the use of a catalytically active ink for pattern creation in self-assembled monolayers.17 Sulfonic acid-functionalized monolayer-protected gold nanoclusters (MPCs) were used as ink for µCP. The MPCs were transferred onto a trimethylsilyl ether (TMS) SAM by µCP, where they catalyzed locally the hydrolysis of TMS in the contacted areas, leading to a chemically patterned substrate. This method induces a new concept in microcontact printing where new functional groups are created by a chemical reaction during stamping. Although the MPCs are quite heavy and do not diffuse, the granular nature of the MPCs may cause inhomogeneity in inking. A possible solution might be to attach the catalyst to the stamp and create patterns purely by making contact between the stamp and the SAM substrate. In this letter, we describe the use of surface-oxidized stamps in microcontact printing. The stamps were brought into contact with SAMs of acid-labile adsorbates. Patterned substrates are created because of the presence of a layer of silicon oxide on the outer surface of the stamps, which is acidic enough to hydrolyze these adsorbates. Experimental Section. Chemicals. 11-Mercapto-1-undecanol, and tert-butyldimethylsilyl chloride (TBDMS-Cl) were purchased from Aldrich. Preparation of bis(ω-trimethylsiloxyundecyl)disulfide ((TMS-OC11H22S)2) was published elsewhere.17 Water was purified by Millipore membrane units. All solvents used were of reagent grade and were used as received.
Bis(ω-tert-butyldimethylsiloxyundecyl)disulfide (TBDMSOC11H22S)2). Iodine (0.5 g, 3.10 mmol) was added to 11mercapto-1-undecanol (1.0 g, 4.89 mmol) in dichloromethane (50 mL). The solution was stirred for 1 h. The reaction mixture was washed twice with 1 M aqueous Na2S2O3 and twice with water, and subsequently dried over Na2SO4. After removing the solvent, bis(ω-hydroxyundecyl)disulfide was obtained in a quantitative yield as a white solid. 1H NMR (CDCl3) δ (ppm): 3.65 (t, 4H, J ) 7.6 Hz), 2.70 (t, 4H, J ) 4.3 Hz), 1.50-1.75 (m, 8H), 1.20-1.40 (m, 28H). To a solution of bis(ω-hydroxyundecyl)disulfide (200 mg, 0.50 mmol) in dry THF (50 mL), were added TBDMS-Cl (150 mg, 1.2 mmol) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) (164 mg, 1.10 mmol). The reaction mixture was stirred overnight under argon. After filtering off the precipitate from the reaction mixture, the solution was washed twice with water and dried over Na2SO4. After flash column chromatography (silica, hexane/CH2Cl2, 2/1), the TBDMS adsorbate was obtained as an oil (160 mg, 51%). 1H NMR (CDCl3) δ (ppm): 3.57 (t, 4H, J ) 6.5 Hz), 2.65 (t, 4h, J ) 7.3 Hz), 1.59-1.69 (m, 8H), 1.43-1.50 (m, 28H), 0.87 (s, 18H), 0.11 (s, 12H). MS (FAB-MS) m/z: 635.6 ([M+H]+; calcd. for C34H74O2S2Si2: 634.5). Monolayer Preparation. All glassware used for monolayer preparation was immersed in piranha solution (concentrated H2SO4 and 33% aqueous H2O2 in a 3:1 ratio). (Warning! Piranha solution should be handled with caution: it has been reported to detonate unexpectedly.) The glassware was rinsed with a large amount of water (Millipore). Nearly atomically flat gold substrates were obtained from Ssens BV (Enschede, The Netherlands) as a layer of 20 nm gold on titanium (2 nm) on silicon. Before use, the substrates were treated with oxygen plasma (5 min) and ethanol (5 min). Self-assembled monolayers (SAMs) were prepared by immersing the freshly cleaned gold substrates in a 1 mM adsorbate solution at room temperature for 14 h. The SAMs were rinsed by chloroform, ethanol, and water, and dried under a stream of nitrogen. Oxidation of PDMS Stamps. PDMS stamps were prepared according to published procedures.24 Surface modification of the stamps was achieved in two ways. The stamp was treated with O2 plasma for 15 s under the following conditions: P(O2) ∼0.2 mbar, load coil power ∼120 W or UV/ozone etching for 1 h. The stamps after oxidation treatment were immediately put in water and kept there. Oxidation is believed to lead to an oxidation thickness of a few nanometers as discussed before,21 but was not investigated here. Catalytic Pattern Creation. SAMs of TMS and TBDMS on gold were used as substrates for microcontact printing. Oxidized PDMS stamps were taken out from water, blown dry with N2, and brought into contact with the SAMs. No additional pressure was applied to the stamp. The stamp was in contact with substrate at different time to investigate the extent of hydrolysis. Flat stamps were used for preparation of substrates for XPS analysis. NMR. NMR spectra were recorded at 25 °C using a Varian Inova 300 NMR spectrometer. 1H NMR chemical shifts (300 1450
Scheme 1
MHz) are given relative to residual CHCl3 (7.25 ppm) as an internal standard. FAB-MS. Mass spectra were recorded with a Finnigan MAT 90 spectrometer using NBA/NPOE as a matrix. XPS. X-ray spectroscopy was performed on a Quantum 2000 scanning Esca microprobe; the Quantum 2000 uses a Quartz crystal monochromator and a scanning electron source that excites the aluminum anode to produce a focused X-ray beam. The created photoelectrons pass through a spherical capacitor energy analyzer and are detected with a multichannel detector (16 channels). Surface survey data was collected followed by high-resolution scans over C1s (278298 eV), O1s (525-545 eV), S2s (222-242 eV) and Si2s (145-165 eV). Peak areas were calculated using a Gaussian fit program. AFM. The AFM measurements were carried out with a NanoScope III multimode AFM (Digital Instruments, Santa Barbara, CA). Contact mode AFM images were acquired in air with commercial Si3N4 tips. Results and Discussion. Silyl ethers are employed as protective groups in organic synthesis. They are labile to acid or base hydrolysis, and their acid stability is quite dependent on the local steric environment.18 The preparation of the TMS adsorbate (Scheme 1) is published elsewhere.17 Bis(ω-tert-butyldimethylsiloxyundecyl)disulfide ((TBDMS-OC11H22S)2, TBDMS adsorbate) was synthesized in a fashion similar to that shown in Scheme 1. 11-Mercapto-1-undecanol (MUD) was oxidized to the disulfide by iodine in a quantitative yield. The disulfide was then converted into the TBDMS adsorbate by reacting with tert-butyldimethylsilyl chloride (TBDMS-Cl). Self-assembled monolayers of TMS and TBDMS adsorbates were prepared by immersion of freshly cleaned gold substrates in the corresponding adsorbate solutions (1 mM) Nano Lett., Vol. 3, No. 10, 2003
Scheme 2
for 14 h. Water contact angles of TMS (78°/28°, advancing/ receding) and TBDMS (84°/25°) indicate that the SAMs are somewhat hydrophobic. The hysteresis between the advancing and receding angles is an indication of the order of the SAM. For both SAMs, the hysteresis is much larger than 20°, indicating a fairly poor order of these SAMs. The large hystereses observed are not surprising because of the presence of bulky terminal groups. X-ray photoelectron spectroscopy (XPS) was performed on both SAMs and the presence of silicon was confirmed. The molar ratios between C and Si correspond to those in the adsorbates, confirming the SAM formation. PDMS stamps were treated with UV/ozone for 1 h or oxygen plasma for 15 s. The stamps were put in water immediately thereafter. For pattern creation (see Scheme 2), the stamps were taken out, blown dry, and brought into contact with preformed SAMs of TMS or TBDMS with varying contact times. Contact mode AFM was applied to image the surfaces. Both UV/ozone and O2 plasma treatment are known to produce a thin, glassy silicon oxide on the stamp,19,20 which makes the stamp hydrophilic. However, stamps treated with UV/ozone showed shrinkage of line patterns when compared to untreated stamps. Oxygen plasma treatment did not modify the feature sizes of the stamp but induced buckles and cracks.21 After oxidation, the stamps were kept in water in order to maintain the hydrophilicity. The hydrophobicity of the stamps recovered when the stamps are exposed to air in a matter of hours due to the transport of low molecular weight PDMS to the surface.22 Stamps treated by O2 plasma have been applied before in µCP of polar substances, thus broadening the scope of ink types that can be applied. In one case,15,19 oxidized stamps have been used to transfer a Pd(II) complex which initiated the electroless deposition of copper onto the patterns with high resolution. Using such stamps, Whitesides et al.6 have demonstrated µCP of poly(ethylene imine) (PEI) on a reactive SAM, yielding patterns of PEI with submicron resolution. Delamarche et al.20 recently showed the chemical modification of the oxidized stamps with poly(ethylene glycol) (PEG), also with the aim of preservation of the hydrophilicity of the stamps. Silica is known to be acidic, and we found that the TMS adsorbate was hydrolyzed within a few hours after purification by flash column chromatography over silica, most likely by traces of silica remaining in the collected product. This triggered us to investigate the hydrolysis of a TMS SAM by an oxidized PDMS stamp to create patterned surfaces. In Figure 1a and b AFM height and friction images are shown of a TMS SAM after 1 min of contact with a patterned Nano Lett., Vol. 3, No. 10, 2003
Figure 1. Contact mode AFM height (a, c, e, z range 10 nm) and friction (b, d, f, z range 0.2 V) images of a TMS SAM after contact (a, b, e, f: 1 min; c, d: 5 min) with surface-oxidized PDMS stamps treated by O2 plasma (a, b, c, d) and UV/ozone (e, f).
oxidized stamp (treated by O2 plasma). The images corresponded to the feature sizes on the stamp, indicating hydrolysis on the sites of contact. The broader stripes are the areas of contact, which appear brighter in the friction image as observed before for pattern creation with catalytic MPCs.17 No significant height difference is observed in the height profile. Furthermore, these patterns are stable, even with 5 min of contact the features still retain their submicron edge resolution (less than 100 nm, Figures 1c and d), indicating this process is indeed diffusion-free. Patterning on TMS SAMs was also carried out with UV/ ozone treated stamps. This also showed patterns but with modified feature sizes as shown in Figures 1e (height) and f (friction). First, a decrease of line width is observed. Second, due to the prolonged stamp treatment, new grooves result on the stamp features which do not make contact during printing. As a reference, untreated PDMS stamps were brought into contact with TMS SAMs. In this case, no patterns were observed in the AFM images confirming that the pattern creation was indeed caused by the silicon oxide layer created by oxidation. To verify the extent of the surface hydrolysis, oxidized flat stamps (treated by either method) were used for µCP on TMS SAMs with different periods of time. X-ray photo1451
Figure 2. Silicon concentration in a TMS SAM as a function of contact time with an oxidized PDMS stamp as detected by XPS (the line is a guide to the eye).
Figure 3. Contact mode AFM height (left, z range 20 nm) and friction (right, z range 0.2 V) images of a TBDMS SAM after contact with a PDMS stamp treated by UV/ozone for 12 min.
electron spectroscopy (XPS) shows the relative changes of the silicon concentration versus the contact time (Figure 2). After short contact times, the Si concentration remained unchanged within experimental error. After 30 min of contact, the silicon concentration decreased appreciably, indicating that significant hydrolysis had taken (approximately 30%). This behavior seems to contrast the AFM friction behavior shown above, where a clear contrast is observed already after 1 min of contact. However, this is in agreement with results reported previously that no full hydrolysis is needed to obtain contrast in AFM.17 Catalytic patterning was also attempted on TBDMS SAMs. The substrates after µCP with oxidized stamps (treated by UV/ozone) after different printing times were imaged by contact mode AFM. Figure 3 shows the height and friction images of a TBDMS SAM after 12 min of contact with an oxidized stamp. The thinner stripes are the areas of contact, which showed higher friction, which is attributed to the hydrolysis. No height difference was observed.23 Within 2 min of contact, contrast was observed neither in height nor in friction. After 5 min of contact, a slightly discernible contrast was observed in the friction image. With Si XPS, the same decreasing trend was found as for TMS SAM. However, some XPS results showed significantly higher Si concentration on the surface, which was probably due to the deposition of low molecular weight PDMS to the substrates, thus preventing a more quantitative comparison. Nevertheless, the TBDMS SAMs appear to react slower to the 1452
catalytic hydrolysis, as expected from the increased steric bulk compared to TMS. Conclusions. For the first time, direct catalytic µCP by has been performed without ink transfer. Oxidized PDMS stamps were prepared and used for catalytic patterning. As a result of the presence of silicon oxide on the stamp, catalysis took place in the contacted areas on preformed TMS and TBDMS SAMs. Unlike the traditional µCP, which relies on the ink transfer from the stamp to the substrate, this process does not involve an ink transfer, thus the pattern creation process is diffusionless. Although a relatively long contact time is needed for pattern creation, submicron edge resolution is obtained. Stamps need to be reactivated, however, after certain periods of time due to the transport of low molecular weight PDMS to the surface,22 and the catalytic process is rather slow. Currently, we are exploring the covalent linkage of a more active catalyst to the stamp, which would solve both problems. Extension of such an approach to scanning probe lithography would result in a local nanofabrication tool via this novel surface patterning technique. Acknowledgment. This research is supported by the Technology Foundation STW, applied science division of NWO, and the technology program of the Ministry of Economic Affairs (Simon Stevin Nanolithography Award to D.N.R., project number-TST4946). References (1) Delamarche, E.; Schmid, H.; Bietsch, A.; Larsen, N. B.; Rothuizen, H.; Michel, B.; Biebuyck, H. J. Phys. Chem. B 1998, 102, 33243334. (2) Xia, Y. N.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 551-575. (3) 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. (4) Lahiri, J.; Ostuni, E.; Whitesides, G. M. Langmuir 1999, 15, 5, 20552060. (5) Xia, Y. N.; Whitesides, G. M. Annu. ReV. Mater. Sci. 1998, 28, 153184. (6) Yan, L.; Huck, W. T. S.; Zhao, X.-M.; Whitesides, G. M. Langmuir 1999, 15, 1208-1214. (7) Yan, L.; Zhao, X.-M.; Whitesides, G. M. J. Am. Chem. Soc. 1998, 120, 6179-6180. (8) Vaeth, K. M.; Jackman, R. J.; Black, A. J.; Whitesides, G. M.; Jensen, K. F. Langmuir 2000, 16, 8495-8500. (9) Huck, W. T. S.; Stroock, A. D.; Whitesides, G. M. Angew. Chem., Int. Ed. 2000, 39, 1058-1061. (10) Husemann, M.; Mecerreyes, D.; Hawker, C. J.; Hedrick, J. L.; Shah, R.; Abbott, N. L. Angew. Chem., Int. Ed. Engl. 1999, 38, 647-649. (11) Michel, B.; Bernard, A.; Bietsch, A.; Delemarche, 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. (12) Libioulle, L.; Bietsch, A.; Schmid, H.; Michel, B.; Delamarche, E. Langmuir 1999, 15, 300-304. (13) Liebau, M.; Huskens, J.; Reinhoudt, D. N. AdV. Funct. Mater. 2001, 11, 147-150. (14) Bernard, A.; Renault, J. P.; Michel, B.; Bosshard, H. R.; Delamarche, E. AdV. Mater. 2000, 12, 1067-1070. (15) Hidber, P. C.; Helbig, W.; Kim, E.; Whitesides, G. M. Langmuir 1996, 12, 1375-80. (16) Hidber, P. C.; Nealey, P. F.; Helbig, W.; Whitesides, G. M. Langmuir 1996, 12, 5209-5215. (17) Li, X.-M.; Paraschiv, V.; Huskens, J.; Reinhoudt, D. N. J. Am. Chem. Soc. 2003, 125, 4279-4284. Nano Lett., Vol. 3, No. 10, 2003
(18) Greene, T. W.; Wuts, P. G. M. ProtectiVe Groups in Organic Synthesis; John Wiley &Sons: New York, 1991. (19) Kind, H.; Geissler, M.; Schmid, H.; Michel, B.; Kern, K.; Delamarche, E. Langmuir 2000, 16, 6367-6373. (20) Donzel, C.; Geissler, M.; Bernard, A.; Wolf, H.; Michel, B.; Hilborn, J.; Delamarche, E. AdV. Mater. 2001, 13, 1164-1167. (21) Chua, D. B. H.; Ng, H. T.; Li, S. F. Y. Appl. Phys. Lett. 2000, 76, 721-723. (22) (a) Owen, M. J.; Smith, P. J. J. Adhes. Sci. Technol. 1994, 8, 10631075. (b) Fritz, J. L.; Owen, M. J. J. Adhes. 1995, 54, 33-45. (c) Kim, J.; Chaudhury, M. K.; Owen, M. J. J. Coll. Interface Sci. 2000, 226, 231-236. (d) Bausch, G. G.; Stasser, J. L.; Tonge, J. S.; Owen,
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M. J. Plasmas Polym. 1998, 3, 23-34. (e) Hillborg, H.; Gedde, U. W. Polymer 1998, 39, 1991-1998. (f) Hillborg, H.; Ankner, J. F.; Gedde, U. W.; Smith, G. D.; Yasuda, H. K.; Wikstro¨m, K. K. Polymer 2000, 41, 6851-6863. (23) As seen in the height profile, some physisorbed material was present, which was quite difficult to remove. It probably consists of aggregates of TBDMS adsorbates, because there was no friction differences between those spots and the rest of the SAM. (24) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002-2004.
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