Article pubs.acs.org/Langmuir
A Photolithographic Approach to Spatially Resolved Cross-Linked Nanolayers Keita Fuchise,†,‡ Peter Lindemann,§ Stefan Heißler,§ Hartmut Gliemann,§ Vanessa Trouillet,∥ Alexander Welle,†,⊥ Jonathan Berson,#,∇ Stefan Walheim,#,∇ Thomas Schimmel,*,#,∇ Michael A. R. Meier,*,‡ and Christopher Barner-Kowollik*,†,⊥ †
Preparative Macromolecular Chemistry, Institut für Technische Chemie und Polymerchemie (ITPC), Karlsruhe Institute of Technology (KIT), Engesserstr. 18, 76128 Karlsruhe, Germany ‡ Laboratory of Applied Chemistry, Institute of Organic Chemistry (IOC), Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany § Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ∥ Institut für Angewandte Materialien (IAM) and Karlsruhe Nano Micro Facility (KNMF), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ⊥ Institut für Biologische Grenzflächen (IBG), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany # Institute of Nanotechnology (INT), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ∇ Institute of Applied Physics and Center for Functional Nanostructures (CFN), Karlsruhe Institute of Technology (KIT), 76128 Karlsruhe, Germany S Supporting Information *
ABSTRACT: The preparation of cross-linked nanosheets with 1−2 nm thickness and predefined shape was achieved by lithographic immobilization of trimethacryloyl thioalkanoates onto the surface of Si wafers, which were functionalized with 2(phenacylthio)acetamido groups via a photoinduced reaction. Subsequent cross-linking via free radical polymerization as well as a phototriggered Diels−Alder reaction under mild conditions on the surface led to the desired nanosheets. Electrospray ionization mass spectrometry (ESI-MS), X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToFSIMS), as well as infrared reflection-absorption spectroscopy (IRRAS) confirmed the success of individual surface-modification and cross-linking reactions. The thickness and lateral size of the cross-linked structures were determined by atomic force microscopy (AFM) for samples prepared on Si wafers functionalized with a self-assembled monolayer of 1H,1H,2H,2Hperfluorodecyl groups bearing circular pores obtained via a polymer blend lithographic approach, which led to the cross-linking reactions occurring in circular nanoareas (diameter of 50−640 nm) yielding an average thickness of 1.2 nm (radical crosslinking), 1.8 nm (radical cross-linking in the presence of 2,2,2-trifluoroethyl methacrylate as a comonomer), and 1.1 nm (photochemical cross-linking) of the nanosheets.
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INTRODUCTION Received: December 27, 2014 Revised: February 21, 2015 Published: February 23, 2015
Nanosheets and nanomembranes, free-standing ultrathin films with a thickness of 1−100 nm, attracted significant attention in © 2015 American Chemical Society
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Langmuir Scheme 1. Photolithographic Approach to Prepare Nanosheets with a Spatially Resolved Structure
the field of nanotechnology due to their structural features and potential applications. Nanosheets have been fabricated from organic materials, such as spin-coated polymers,1 layer-by-layer films of polyelectrolytes,2−4 self-assembled multiblock copolymers,5−7 cross-linked amphiphilic Langmuir−Blodgett films,8 cross-linked self-assembled monolayers (SAMs),9−13 and densely interpenetrating hybrid network nanomembranes of cross-linked polyacrylate and ZrO2,14 as well as inorganic materials such as graphene15,16 and other thin layers exfoliated from transition metal dichalcogenides, transition metal oxides, as well as other two-dimensional (2D) compounds.17,18 Nanosheets find applications as transparent conductive coatings,19 nanoseparation membranes,20 nanosensors for electrochemical21 or nanoelectromechanical components,22 transmission electron microscope (TEM) supports,23,24 biomedical applications,1 and nanocomposite materials.25,26 The bottomup approach for the preparation of nanosheets including 2D polymers,27,28 which are defined as monolayered or sheet-like covalent polymers with periodic and two-dimensional structures, from small molecules is advantageous in terms of control of the size, shape, and properties of the resulting nanosheets. Cross-linking reactions at a gas/liquid interface or at the surface of solid substrates have advantages compared to reactions in solution, since the preorganization of monomers in a planar structure supports the formation of a periodic 2D structure. For example, Schlüter and co-workers recently reported the synthesis of a 2D polymer from a C3-symmetric monomer with a triangle-pole-like structure at a water−air interface.29 The cross-linking of the monomers was achieved by a phototriggered cycloaddition reaction. The resulting polymer was collected on a SiO2-coated silicon wafer and analyzed by atomic force microscopy (AFM), revealing the size of the 2D polymer to exceed several micrometers.30,31 Similarly, Müllen and colleagues have reported the synthesis of 2D polymers by the polymerization of C3- and C6-symmetric monomers based on direct coupling reactions on a silver surface, although the exfoliation of the 2D polymer from the surface was not demonstrated.32−34 The synthesis of nanosheets exhibiting a chemical or topographical pattern as well as a designed shape is essential for the development of functional nanoscale devices. The preparation of nanosheets with patterns of surface functional groups and those with designed shape have only been achieved by the cross-linking reaction of self-assembled monolayers of aryl thiols on a gold surface. The nanosheets with patterns of surface functional groups were prepared via two-step cross-linking reactions of the SAM consisting of biphenyl thiols using electron flood exposure.10,12 The nanosheets with designed shape have been prepared by the
treatment of the SAM of biphenyl thiols with UV/ozone employing a shadow mask followed by the cross-linking reaction using electron flood exposure11 or by reactive ion etching in an oxygen/argon plasma.13 These procedures require rather harsh conditions for the synthesis of nanosheets with defined geometries. Thus, more straightforward and milder methods for pattern formation and exfoliation of nanosheets still need to be developed. Recently, various phototriggered reactions have been employed to prepare spatially resolved patterns on material surfaces, which include ligations between azido moieties and (oxa)-dibenzocyclooctynes generated from their cyclopropenone derivatives,35,36 olefin/(meth)acryloyl groups and nitrile imines generated from tetrazole,37,38 hydroxylamines and formylphenyl groups generated from o-tetrahydropyranyl-2nitrobenzyl alcohol derivatives,39 o-naphthoquinone methide and vinyl ethers,40 o-naphthoquinone methide and thiols,41 and o-quinodimethane and maleimides.42 We have recently reported that the surface of silicon wafers functionalized with 2-(phenacylthio)acetamido groups can be functionalized with dienes,43 primary amines,44 primary alkoxyamines,44 and thiols44 under irradiation with UV light in a spatially resolved manner. Especially the reaction of 2-(phenacylthio)acetamides and thiols was considered to be useful for the preparation of nanosheets, because it enables both the spatially resolved surface functionalization of solid substrates and the potentially mild exfoliation of the generated nanosheets by cleavage of the disulfide bonds between the immobilized molecule and the substrate surface. Herein, we introduce a photolithographic method to fabricate cross-linked nanosheets with predetermined shape by combining the spatially-resolved immobilization of multifunctional reactive monomers on a solid substrate and their subsequent cross-linking as depicted in Scheme 1, exploiting the above-noted photochemical avenue. The strategy was implemented via the spatially resolved immobilization of a newly synthesized trifunctional methacrylate monomer on Si wafers prefunctionalized with 2-(phenacylthio)acetamido groups. The targeted immobilization of the trimethacrylate thioalkanoate compound (3) was achieved using the phototriggered reaction of the thiol group of 3 and the thioaldehyde groups in situ generated from the 2-(phenacylthio)acetamido moieties on the Si surface. Finally, the desired nanosheets were prepared by subsequent cross-linking of the immobilized methacrylates via variable pathways. The resulting spatially resolved cross-linked structures were analyzed by X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and infrared reflection3243
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The analyzer transmission function, Scofield sensitivity factors,52 and effective attenuation lengths (EALs) for photoelectrons were applied for quantification. EALs were calculated using the standard TPP-2 M formalism.53 All spectra were referenced to the C 1s peak (C−C, C− H) at 285.0 eV binding energy controlled by means of the well-known photoelectron peaks of metallic Cu, Ag, and Au. ToF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry). ToF-SIMS was performed on a ToF-SIMS instrument (ION-TOF GmbH, Münster, Germany) equipped with a Bi cluster liquid metal primary ion source and a nonlinear time-of-flight analyzer. Samples were rinsed and sonicated in HPLC grade water (Carl Roth) prior to analysis. The Bi source was operated in the “bunched” mode providing Bi1+ or Bi3+ ion pulses at 25 keV energy and a lateral resolution of ∼4 μm. The short pulse length allowed for high mass resolution to analyze the complex mass spectra of the immobilized organic layers. Images larger than the maximum deflection range of the primary ion gun of 500 × 500 μm2 were obtained using the manipulator stage scan mode. Negative polarity spectra were calibrated on the C−, CH−, and CH2− peaks. Positive polarity spectra were calibrated on C+, Si+, and small hydrocarbon peaks. Primary ion doses were kept below 1011 ions cm−2 (static SIMS limit). Infrared Reflection−Absorption Spectroscopy (IRRAS). The infrared spectra of the layers were recorded using a VERTEX 80 FT-IR spectrometer (Bruker Optics, Ettlingen, Germany) equipped with a grazing angle (80°) specular reflection spectroscopy accessory. The measurement chamber was continuously purged with dry air. The acquisition time was close to 20 min at a resolution of 2 cm−1. The spectra are reported as absorbance −log(I/I0), where I is the intensity of the radiation after interacting with the sample and I0 is the intensity of the radiation after passing a deuterated hexadecanethiol (HS(CD2)15CD3) self-assembled monolayer (SAM) on gold. Atomic Force Microscope (AFM). The cross-linked layers were characterized by topography and friction force images measured by AFM. All the samples were cleaned by the snow-jet method just before the AFM measurements.54 The wafers were exposed to a jet of CO2 ice crystals, which were produced by expanding CO2 through a nozzle (Snow Jet model K4-05, Tectra Frankfurt/Germany). Using this technique, surface contaminants are removed either by mechanical impact or by dissolution in CO2. The AFM images were recorded with a commercial Bruker Dimension Icon-PT in contact mode in a liquid cell filled with demineralized water using Mikromasch CSC37/PT as an AFM tip (typical force constant of 0.3 N m−1). The AFM tip was scanned at an angle of 90° relative to the longitudinal axis of the cantilever in several scan ranges. The obtained AFM images were evaluated with the NanoScope (version 8.15, BRUKER) software. Preparation of Si Wafers Functionalized with Phenacylthioacetamido Groups (4 and 10). For the preparation of 4, single side polished Si wafers (1.0 cm × 1.0 cm) were cleaned by ultrasonication for 10 min in CH2Cl2. The wafers were dried under a nitrogen stream. The surfaces were preactivated by plasma cleaning of the polished side of the Si wafers using air plasma for 10 min in PDC-002 (Harrick Plasma, Ithaca, USA, 200 W). The Si wafers were subsequently placed in small glass vials containing a solution of N-3(triethoxysilyl)propyl-2-[(2-oxo-2-phenylethyl)thio]acetamide (4.0 mg, 9.6 μmol) in toluene (1.0 mL). The solutions were heated to 50 °C for 2 h and left overnight at ambient temperature. The wafers were subsequently ultrasonicated in CH2Cl2 for 10 min to remove any physisorbed silanes. Functionalization of 9 using N-3-(triethoxysilyl)propyl-2-[(2-oxo-2phenylethyl)thio]acetamide was carried out via a similar procedure without plasma cleaning to obtain Si wafers functionalized with phenacylthioacetamido functions on the circular pattern (10). Preparation of Si Wafers Functionalized with 3 (5, 6a, and 11a). For the preparation of the Si wafers functionalized with phenacylthioacetamido groups without pattern (5) and with the circular pattern (11a), 4 and 10 were directly placed in a headspace vial sealed with a PTFE inner liner. A stock solution of 3 in CH2Cl2 (2.0 mg mL−1) was deoxygenated by flowing nitrogen for 3 min, and 1.0 mL of the solution was injected into the reaction vial. For the preparation of the Si wafers functionalized with phenacylthioacetamido
absorption spectroscopy (IRRAS). The topographical features of the resulting structures were characterized by AFM to confirm the formation of the nanosheets with a thickness of 1− 2 nm and a designed shape.
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EXPERIMENTAL SECTION
Materials. Pentaerythritol (Aldrich, ≥99%), methacrylic acid (Aldrich, 99%), 2,2,2-trifluoroethyl methacrylate (TFEMA, TCI, >98.0%), 3-mercaptopropionic acid (Aldrich, ≥99%), 2,3-dimethylbutadiene (Alfa Aesar, 98%), 2,2′-azobis(isobutyronitrile) (AIBN, ACROS, 98%), triphenylmethyl chloride (Fluka, ≥95%), N,N′diisopropylcarbodiimide (DIC, Acros, 99%), 4-dimethylaminopyridine (DMAP, Aldrich, 99%), n-propylamine (Aldrich, 98%), triethylamine (Aldrich, 99%), triethylsilane (Aldrich, 99%), 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS, Aldrich, 97%), hydroquinone (Fluka, ≥99%), sulfuric acid (Roth), trifluoroacetic acid (TFA, Aldrich, 99%), n-hexane (VWR, Normapur grade), toluene (VWR, 99.5%), tetrahydrofuran (THF, VWR, 99.9%), ethyl acetate (EtOAc, VWR, Normapur grade), dichloromethane (CH2Cl2, VWR, 99.5%, for synthesis), sodium hydrogen carbonate (NaHCO3, Roth, ≥99%), magnesium sulfate (MgSO4, Roth, >99%). TFEMA was purified by passing through a short column of neutral alumina using dichloromethane as the eluent. AIBN was recrystallized from methanol twice. For the surface modification of Si wafers, toluene (Roth, Rotisolv), acetone (Fischer scientific, Electronic (MOS) grade), and CH2Cl2 (Fischer Scientific, anhydrous, synthesis grade) were used without further purification. Single-side polished Si wafers for the preparation of the functionalized Si wafers (4, 5, and 6a−6d) were purchased from Si-Mat silicon Materials (CZ, orientation ⟨100⟩, B-doped, resistivity 1−30 Ω cm, Landsberg, Germany). The gold-coated Si wafers (for 8a−d) were obtained from Georg-Albert-PVD (Silz, Germany). 3(Triphenylmethylthio)propionic acid,45 11-(triphenylmethylthio)undecanoic acid, 4 6 2-(phenacylthio)acetic acid, 4 3,4 7 N-3(triethoxysilyl)propyl-2-[(2-oxo-2-phenylethyl)thio]acetamide,43 4(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMT-MM),48 and O,O′,O″,O‴-tetra(2-formyl-3-methylphenyl)pentaerythritol (7)49 were synthesized as previously reported. The syntheses of O,O′,O″-trimethacryloyl pentaerythritol (1), O,O′,O″trimethacryloyl-O‴-[3-(triphenylmethylthio)propionyl] pentaerythritol (2), O,O′,O″-trimethacryloyl-O‴-[11-(triphenylmethylthio)undecanoyl] pentaerythritol (2′), O,O′,O″-trimethacryloyl-O‴-3-mercaptopropionyl pentaerythritol (3), O,O′,O″-trimethacryloyl-O‴-11mercaptoundecanoyl pentaerythritol (3′), N-n-propyl-2(phenacylthio)acetamide, and Si wafers functionalized with a selfassembled monolayer of 1H,1H,2H,2H-perfluorodecyl group (9)50 are described in the Supporting Information. Characterization. Nuclear Magnetic Resonance (NMR). Spectra were recorded on a Bruker AVANCE DPX spectrometer operating at 300 MHz for 1H NMR measurements and at 75 MHz for 13C NMR measurements. CDCl3 was used as solvent, and the resonance signal at 7.26 ppm (1H) and 77.16 ppm (13C) served as the reference for the chemical shift δ. UV−vis Spectroscopy. UV−vis spectra were recorded on a Varian Cary 300 Bio spectrophotometer. High Resolution Fast-Atom-Bombardment-Mass Spectra (HRMSFAB). HRMS-FAB spectra were measured on a Finnigan MAT 95. Electron Spray Ionization Mass Spectra (ESI-MS). ESI-MS spectra were recorded on an Advion Expression CMS instrument at a methanol flow rate of 300 μL min−1 and a mass range of 100−1200 m/ z. X-ray Photoelectron Spectroscopy (XPS). XPS measurements were performed using a K-Alpha XPS instrument (ThermoFisher Scientific, East Grinstead, U.K.). Data acquisition and processing using the Thermo Avantage software is described elsewhere.51 All samples were analyzed using a microfocused, monochromated Al Kα X-ray source (400 μm spot size). The K-Alpha charge compensation system was employed during analysis, using electrons of 8 eV energy and lowenergy argon ions to prevent any localized charge buildup. The spectra were fitted with one or more Voigt profiles (BE uncertainty: ±0.2 eV). 3244
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Langmuir Scheme 2. Synthetic Strategy toward the Trifunctional Monomers 3 and 3′
groups along the wave pattern (6a), the Si wafer was fixed on the sample holder shown in Figure S3 (Supporting Information) and placed in a headspace vial sealed with a PTFE inner liner. A 4.5 mL portion of the deoxygenated stock solution of 3 in CH2Cl2 (2.0 mg mL−1) was injected into the reaction vial. The vial was irradiated employing a UV light compact low-pressure fluorescent lamp (Philips CLEO Compact PL-L: λmax = 355 nm, refer to Figure S1a, Supporting Information) for 4 h by revolving around at a distance of 40−50 mm in a custom-built photoreactor (see Figure S2, Supporting Information). After the reaction, the Si wafer was taken off from the sample holder. The wafers were rinsed with and ultrasonicated in CH2Cl2 for 10 min to remove physisorbed materials. The wafer was dried under a dry nitrogen stream to afford 5, 6a, and 11a. Cross-Linking Reaction of 6a and 11a. 6a or 11a were placed in a headspace vial sealed with a PTFE inner liner. A stock solution of AIBN (1.0 mL, 5.0 μmol, and 5.0 mmol mL−1 in toluene) was added to the vial. After purging with N2 to remove oxygen from the reaction solution, the vial was heated at 60 °C for 72 h. The resultant Si wafer was washed with and ultrasonicated in CH2Cl2 for 10 min to obtain 6b or 11b. For the preparation of 6c or 11c, the reaction was carried out using AIBN (1.0 mL, 5.0 μmol, and 5.0 mmol mL−1 in toluene) and TFEMA (55.7 mg, 332 μmol) with similar procedures. For the preparation of 6a′, 6a was soaked in a stock solution of 2,3dimethylbutadiene (1.0 mL, 24 μmol, and 2.0 mg mL−1 in CH2Cl2) in a vial sealed with PTFE. After purging with N2 to remove oxygen from the reaction solution, the vial was irradiated by UV light (λmax = 355 nm) from a Philips CLEO Compact PL-L lamp at ambient temperature for 4 h. The purification was carried out with similar procedures to the preparation of 6b and 11b. For the preparation of 6d or 11d, the reaction was carried out using 6a′ or 11a, a stock solution of the tetraphotoenol 7 (1.0 mL, 3.3 μmol, and 2.0 mg mL−1 in MeCN), and a Cosmedico Arimed B6 lamp (λmax = 320 nm) with similar procedures to the preparation of 6a′. Preparation of the Samples for IRRAS. Preparation of GoldCoated Si Wafers Functionalized by 3′ (8a). Gold-coated Si wafers (>1.5 cm (width) × >1.5 cm (length)) were prepared from one-side polished Si wafers (CZ, orientation ⟨100⟩, B-doped, resistivity 5−22 Ω cm) coated with a 5.0 nm Ti adhesion layer and, subsequently, with a 100 nm thick Au layer using physical vapor deposition (PVD). The gold-coated Si wafers were placed in a solution of 3′ (4.0 mL, 0.50 mg mL−1) in ethanol at ambient temperature overnight (>16 h). The wafers were washed with ethanol and dried under a nitrogen stream to obtain 8a. Cross-Linking Reactions of 8a. The cross-linking reactions of 8a were carried out with AIBN to obtain 8b, with AIBN and TFEMA to obtain 8c, and with 7 to obtain 8d. 8b was prepared by the reaction using a stock solution of AIBN (0.560 mL, 11.4 μmol, and 20.3 mmol L−1 in toluene) and toluene (1.69 mL) at 60 °C for 72 h under an argon atmosphere. 8c was prepared by the reaction using a stock solution of AIBN (0.560 mL, 11.4 μmol, and 20.3 mmol L−1 in toluene), TFEMA (170 mg, 1.01 mmol), and toluene (1.69 mL) at 60 °C for 72 h under an argon atmosphere. 8d was prepared by the reaction using a stock solution of 7 (2.25 mL, 7.39 μmol, and 2.00 mg mL−1 in MeCN) under the irradiation of the UV light from a Cosmedico Arimed B6 (λmax = 320 nm, refer to Figure S1b, Supporting Information) lamp at ambient temperature for 4 h. After
the reaction, the wafers were washed with CH2Cl2 to remove physisorbed reactants and dried under a nitrogen stream.
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RESULTS AND DISCUSSION
Molecular Design and Synthesis of Trimethacrylate Thioalkanoates. Two trimethacrylate thioalkanoates (trimethacryloylthiopropionate (3) and the trimethacryloylthioundecanoate (3′)) were employed as monomers with multiple reactive groups to be immobilized on a surface of solid substrates, as depicted in Scheme 2. The methacrylate groups of 3 and 3′ can be cross-linked by radical or ionic polymerization, nucleophilic addition reactions, or cycloaddition reactions. 3 and 3′ can be immobilized onto a solid substrate functionalized with 2-(phenacylthio)acetamido groups via the phototriggered reaction of the thiol group and the thioaldehyde group generated in situ from the 2-(phenacylthio)acetamido moiety under UV irradiation.44,55,56 Besides the immobilization using the phototriggered reaction, 3 and 3′ can be immobilized on a gold surface exploiting the high aurophilicity of the thiol groups. Such an approach does not allow for spatially resolved immobilization of the monomers, but it is a convenient way of covalently covering larger areas of gold surfaces with 3 and 3′ (see below). To obtain 3 and 3′, first pentaerythritol trimethacrylate (1) was synthesized by acid-catalyzed esterification of pentaerythritol using methacrylic acid. 2 and 2′ were subsequently prepared by mild esterification of 1 using 3(triphenylmethylthio)propionic acid or 11(triphenylmethylthio)undecanoic acid and DIC. Finally, 3 and 3′ were synthesized by deprotection of 2 and 2′ using TFA and triethylsilane. It is noteworthy that 3 and 3′ were stable in the absence of bases in spite of the potential reactivity of the thiol group against the methacryloyl groups. The formation of the disulfide bond by phototriggered reaction of a 2-(phenacylthio)acetamido compound and 3 was first evidenced using a model reaction of 3 (2 equiv) and N-npropyl-2-(phenacylthio)acetamide (1 equiv) in CH2Cl2 under UV light irradiation (λmax = 355 nm) at ambient temperature for 1 h (see the Supporting Information). The resulting mixture was analyzed by electrospray ionization mass spectrometry (ESI-MS), in which the observed peaks correspond to cationic sodium adducts of unreacted 3 (m/z = 451.62), cationic potassium adducts of unreacted 3 (m/z = 467.61), and cationic sodium adducts of the asymmetric disulfide generated by the phototriggered reaction of 3 and N-n-propyl-1-(thioformyl)formamide (m/z = 582.89), as shown in Figure 1. It should be noted that the methacryloyl groups were not consumed in the reaction and no oligomeric or polymeric products of the methacrylates were observed. Spatially Resolved Immobilization of 3 onto Si Wafers Functionalized with 2-(Phenacylthio)acetamido Groups. Si wafers functionalized with 2-(phenacylthio)acetamido groups 3245
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to assess the ability of 3 to be immobilized on 4, we immersed a surface of 4 in a solution of 3 (1.0 mL, 2.0 mg mL−1 in CH2Cl2) and irradiated the surface with UV light (λmax = 355 nm) for 4 h at ambient temperature in order to obtain Si wafers functionalized with trimethacrylate moieties (5, Scheme 3). Successful functionalization of 4 by 3 was confirmed by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary-ion mass spectrometry (ToF-SIMS). Figure 2 shows the C 1s spectra of 4 and 5. The main peak in the C 1s spectrum at 285.0 eV was assigned to the CC and CH bonds. The component at 286.7 eV was attributed to CO and CN bonds. The peak intensity of the CO bonds and ester linkages (OCO) observed at 289.3 eV, originating from methacryloyl groups, significantly increased after the reaction.57 In the ToF-SIMS measurement of 5, methacrylate anions (C4H5O2−, m/z = 85.03) and disulfur anions (S2−, m/z = 63.94) were detected as fragment ions generated from the trimethacrylate structure functionalized onto the surface of the Si wafer via disulfide bonds, as shown in Figure S5 (Supporting Information).
Figure 1. ESI-MS spectra of the products generated in the model reaction of 3 and N-n-propyl-2-(phenacylthio)acetamide. The formation of the targeted disulfide compound was detected in the spectrum.
(4) were prepared from single side polished Si wafers (1 cm × 1 cm) and N-3-(triethoxysilyl)propyl-2-[(2-oxo-2-phenylethyl)thio] acetamide by a previously reported procedure.43 In order
Scheme 3. Preparation of Si Wafers Functionalized with Trimethacrylate Moieties with and without the Wave Pattern for XPS and ToF-SIMS Analysis
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Figure 2. Comparison of the C 1s XPS spectra of (a) 4 and (b) 5. The spectra are normalized to maximum intensity. The successful functionalization of 4 by 3 was evidenced from the increase in the signal intensity of CO bonds at 286.7 eV and OCO bonds at 289.3 eV.
After this proof of concept experiment, the phototriggered immobilization of 3 onto 4 was performed in a spatially controlled manner to prepare Si wafers with a wave pattern of trimethacrylate moieties on the surface (6a, Scheme 3). The preparation of 6a was achieved by the reaction of 4 in a solution of 3 in CH2Cl2 (2.0 mg mL−1) under UV light irradiation (λmax = 355 nm) at ambient temperature for 4 h utilizing a shadow mask with a wave pattern (refer to Figure S3, Supporting Information, for an image of the shadow mask and the sample holder). ToF-SIMS measurement of 6a confirmed that 3 successfully functionalized the surface of 4 with the wave pattern featuring a good spatial resolution between the irradiated and nonirradiated parts, as shown in Figure 3a. Methacrylate anions (C4H5O2−, m/z = 85.03) and disulfur anions (S2−, m/z = 63.94) were detected from the irradiated part as fragment ions originated from the methacrylates and the disulfide bonds of the monomer structure on the surface, respectively. The signal intensity of benzoyl cations (C7H5O+, m/z = 105.03) originating from 2-(phenacylthio)acetamido groups on the surface was lower in the irradiated part of 6a than in the nonirradiated part, which confirmed the conversion of the phenacylthioacetamido group on the reaction with 3. Cross-Linking Reactions of the Trimethacrylate Monomers Immobilized on Solid Substrates. The crosslinking reaction of the methacrylate moieties on 6a was carried out with three alternative methods, i.e., (a) a surface radical polymerization using only AIBN to give 6b, (b) a surface radical polymerization using AIBN and TFEMA to give 6c, and (c) a phototriggered Diels−Alder reaction using O,O′,O″,O‴tetra(2-formyl-3-methylphenyl)pentaerythritol (7) to give 6d, as shown in Scheme 3. The preparation of 6b using AIBN was carried out in toluene at 60 °C for 72 h with [AIBN]0 = 5.0 mmol L−1. A small amount of 2-cyano-2-propyl cations (C4H6N+, m/z = 68.05) was detected from the wave pattern of 6b, which indicated that the 2-cyano-2-propyl group originating from AIBN reacted with the methacrylates on the surface of 6a to initiate the cross-linking reaction. The preparation of 6c using AIBN and TFEMA was carried out in toluene at 60 °C for 72 h with [TFEMA]0 = 0.33 mol L−1 and [AIBN]0 = 5.0 mmol L−1. Fluoride anions (F−, m/z = 19.00), trifluoromethyl anions (CF3−, m/z = 69.00), 2,2,2trifluoroethoxy anions (C2H2OF3−, m/z = 99.01), and C4H6N+
Figure 3. ToF-SIMS images of (a) Si wafer functionalized with 3 along wave pattern (6a), (b) 6a cross-linked by AIBN (6b), (c) 6a crosslinked by TFEMA and AIBN (6c), (d) 6a reacted with 2,3dimethylbutadiene (6a′), and (e) 6a′ cross-linked by 7 (6d). The S2− ion observed in all images evidenced that the disulfide bonds between the (cross-linked) trimethacrylate moieties and the surface are intact in each modification process. The C4H6N+ ion observed in the images of 6b and 6d, the C2H2OF3− ion observed in the image of 6c, and the C8H7O2− ion observed in the image of 6d evidenced the incorporation of the structure originating from the employed crosslinking reagents to the surface structure.
were detected along the wave pattern in the ToF-SIMS image of 6c, which indicated that TFEMA and AIBN selectively reacted with the methacrylates on the surface of 6a and initiated the cross-linking reaction (the distribution of F− and CF3− anions in the ToF-SIMS image of 6c is shown in Figure S6, Supporting Information). For the preparation of 6d, 6a was cross-linked with 7 in a Diels−Alder reaction after masking of the 2-(phenacylthio)acetamido groups remaining on the surface using 2,3-dimethylbutadiene, because the thioaldehydes generated in situ from the 2-(phenacylthio)acetamido groups on 6a are also reactive to 7, and thus the cross-linking reaction would occur on the entire surface without the masking. The reaction of 6a and 2,3-dimethylbutadiene was carried out in CH2Cl2 at ambient temperature by UV light irradiation (λmax = 355 nm) for 4 h in the presence of [2,3-dimethylbutadiene]0 = 2.0 mg mL−1 to give 6a′. The wave pattern of the C7H5O+, which was observed in the image of 6a, was not observed in the image of 6a′, which evidenced the successful conversion of the remaining 2-(phenacylthio)acetamido groups on 6a. The C4H5O2− and the S2− were still observed in the image of 6a′, which indicated that the trimethacrylate monomers on the surface remained intact during the reaction. The methacrylate groups on 6a′ were subsequently reacted with 7 in MeCN at ambient temperature by UV light irradiation (λmax = 320 nm) for 4 h in the presence of [7]0 = 2.0 mg mL−1. The 2-formyl-3methylphenoxide anion (C8H7O2−, m/z = 135.05) originated from the partly reacted 7 was detected only within the wave 3247
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Scheme 4. Structures of the 3 and 3′ Immobilized on a Si Wafer and a Gold-Coated Si Wafer and Preparation of the Samples (8a−d) for IRRAS Measurementsa
a
The cross-linking reactions were carried out with the same procedures as for the preparation of 6b−d.
pattern of 6d, which indicated that 7 selectively reacted with the methacrylates on 6a under the employed reaction conditions. The disulfide bonds were stable in all cross-linking reactions, since S2− was detected from the wave pattern in the images of 6b, 6c, and 6d. The C4H5O2− ion was still detected in the images of 6b, 6c, and 6d (even after the cross-linking reaction), as this anion is also generated from the reacted methacryloyl groups.58 The complete consumption of the C C bonds from the trimethacrylate monomers on the surface after the cross-linking reactions was confirmed by infrared reflection-absorption spectroscopy (IRRAS) measurements, as described in the following section. In order to confirm the progress of the cross-linking reactions of the trimethacrylate monomers on solid substrates, IRRAS measurements were carried out (refer to Figure 4). Direct IRRAS measurements of the samples prepared from 5 by cross-linking reactions using AIBN, AIBN and TFEMA, and 7 were inconclusive because of the very low S/N ratio of the spectra caused by characteristics of Si wafers. Thus, we performed the IRRAS measurements of samples prepared on gold-coated Si wafers, as it is well-known that the presence of the surface gold layer drastically increases the S/N ratio of IRRA spectra.59 The 11-thioundecanoate having three methacrylate moieties (3′) was designed and utilized to mimic the structure and surface distance of 3 immobilized on 4, as shown in Scheme 4. The gold-coated Si wafers functionalized with 3′ (8a) were prepared by the direct immobilization of 3′ on goldcoated Si wafers in an ethanol solution at ambient temperature based on the formation of AuS bonds. 8a was cross-linked by AIBN (8b), TFEMA and AIBN (8c), and 7 (8d) by the respective cross-linking reactions in the same manner as the cross-linking reactions for the samples prepared on Si wafers. Figure 4 shows the IRRA spectra of 8a−8d. In the spectrum of 8a, characteristic peaks originating from the methacryloyl groups of the immobilized monomers are observed at 1731 cm−1 (νCO, peak a), 1640 cm−1 (νCC, peak b), 1327 cm−1 (δCCH, peak c), and 1298 cm−1 (νCOC, peak d). The disappearance of peaks b and c, as well as the blue shift of peak a to 1746 cm−1 (νCO, peak a′) in the spectra of 8b− d, evidenced the consumption of CC bonds in all crosslinking reactions. Strong signals of methylene groups at 2927
Figure 4. IRRA spectra of 3′ immobilized on gold-coated Si wafer (8a), 8a cross-linked by AIBN (8b), TFEMA and AIBN (8c), and 7 (8d). The peak b corresponding to CC bonds of the trimethacryloyl groups on 8a disappeared in the spectra of 8b−d, which evidenced complete conversion of the CC bonds and the progress of the crosslinking reactions.
cm−1 (νas, peak e) and 2850 cm−1 (νs, peak f), as well as strong peaks of CF bonds at 1287 cm−1 (νCF, peak h) and 1188 cm−1 (νCF, peak i), were observed in the spectrum of 8c, which evidence the incorporation of TFEMA into the crosslinked structure on the surface. In the spectrum of 8d, peaks at 1700 cm−1 (νCO, peak j), 1169 cm−1 (ArCO, peak k), 1472 cm−1 (δ-OCH2, peak l), and 1266 cm−1 (ArO CH2, peak m) indicate that 7 was incorporated into the crosslinked structure on the surface and some amount of 2-formyl-3methylphenoxy groups remained unreacted in the reaction. These results clearly underpin the successful formation of the cross-linked structure on the surface via the different crosslinking reactions. Thickness of the Cross-Linked Nanosheets. In order to assess the thickness of the generated cross-linked nanosheets in a reliable fashionwhich is not possible on the macroscopically generated wave patterns of 6a−d due to non-well-defined edges due to light refractionSi wafers functionalized with a selfassembled monolayer of 1H,1H,2H,2H-perfluorodecyl groups (FDTS layer) with circular pores (9) were prepared by polymer blend lithography.50 These patterns were employed for the preparation of the samples for AFM measurements to 3248
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Langmuir Scheme 5. Preparation of the Samples for AFM Measurementsa
a
Each surface modification step was carried out with the same procedures as for the preparation of 4 and 6a−d.
layers of 10 and 11a were almost 0.3 nm thinner than that of the FDTS layer, as shown in the cross-section diagrams of Figure 5a and b. Thus, the average thickness of the 2(phenacylthio)acetamido layer on 10 and the trimethacrylate layer on 11a was calculated to be 1.0 nm. The average thickness of the circular layer increased to 1.2 nm for 11b, 1.8 nm for 11c, and 1.1 nm for 11d after the cross-linking reaction of 11a, which is rational with respect to the progress of the crosslinking reaction. The border of the circular layer and the FDTS layer in the topography of 11b and 11d, shown in Figure 5c and e, was distinguished by the comparison with the friction force image for the calculation of the average thickness. The crosslinked layers of 11b and 11d were thus 0.2 and 0.1 nm thicker than the original circular layers of 11a. The increase in the thickness of the circular layers was smaller for 11b and 11d, since only the 2-cyanoprop-2-yl groups generated from AIBN or 7 were newly incorporated to the surface structure. In contrast, the average thickness of the circular layers on 11c was 0.8 nm thicker than those of 11a, thicker than the FDTS layer, and the thickest among 11b−d becausein addition to the trimethacrylate monomers on the surface of 11aa fraction of TFEMA as a comonomer was incorporated in the cross-linking reaction. Importantly, the thickness of the cross-linked structure of 11b−d was extremely small (1−2 nm) and in agreement with typical thicknesses for molecular layers. Thus, from the results of AFM, it was evidenced that nanosheets with 1−2 nm thickness can be prepared by the immobilization of multifunctional monomers onto Si wafers followed by crosslinking reactions using different reagents.
determine the actual thickness of the cross-linked trimethacrylate structures. 9 (FDTS hole pattern) was prepared as described previously and shown in Figure S4 (Supporting Information).50 The bare Si surface within the circular pores/ holes in the FDTS monolayer of 9 was functionalized by the reaction with N-3-(triethoxysilyl)propyl-2-[(2-oxo-2phenylethyl)thio]acetamide to prepare a Si wafer with a circular pattern of 2-(phenacylthio)acetamido groups on the surface (10), as shown in Scheme 5. 10 was reacted with 3 under UV light irradiation (λmax = 355 nm) to prepare Si wafers featuring trimethacrylates on the circular pattern (11a). The cross-linking reactions of the methacrylates on 11a were carried out with (a) AIBN, (b) AIBN and TFEMA, and (c) 7 to prepare 11b, 11c, and 11d, respectively. The obtained surfaces 11a−d were cleaned by sonication in CH2Cl2 for 10 min subsequently using the snow-jet method to remove physisorbed contaminants. 10 and 11a−d were analyzed by AFM in contact mode to obtain topographies and friction force images, as shown in Figure 5. The friction force images showed the position and the size of the circular layers,60 which revealed that the shape of the circular layers was retained after the immobilization of 3 and the subsequent cross-linking reaction. The diameter of the circular layers of 10 and 11a−d was in the range 50−640 nm. The thickness of the functionalized circular layers was determined by measuring the difference in the average height of the FDTS and the circular layer observed in each topography, since the height of the FDTS layer has been reproducibly determined as 1.3 nm.50 The average thickness of the circular layers was calculated by analyzing 15 circular layers observed in each topography, as shown in Figure 6. Circular 3249
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Figure 5. Topographies (left), friction force images (center),60 and representative cross-section diagrams (right) of (a) 10, (b) 11a, (c) 11b, (d) 11c, and (e) 11d measured by AFM in contact mode in deionized water. The white bars in the topographies are indicating the positions of the crosssection. The increase in the thickness of the circular layers on 11b−d in comparison to 11a evidenced the progress of the cross-linking reactions. Nanosheets with different thickness were obtained depending on the utilized cross-linking reaction.
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CONCLUSIONS We present a straightforward strategy for the preparation of sheet-like polymers with a thickness of 1−2 nm on surfaces via the immobilization of trimethacrylate monomers on a Si wafer by a photolithographic process and subsequent cross-linking of the monomer units by three different approaches. The model reaction of 3 and a N-n-propyl-2-(phenacylthio)acetamide evidenced that the phototriggered reaction between 3 and the 2-(phenacylthio)acetamido group generates the targeted disulfide compound. The successful functionalization of the surface of Si wafers by trimethacrylate monomers using phototriggered reactions was confirmed by XPS and ToFSIMS measurements. The results of ToF-SIMS measurement for 6a revealed that the trimethacrylate monomers functionalized the surface of Si wafers through the disulfide bonds in a spatially resolved manner by the utilization of a macroscopic shadow mask. The progress of the cross-linking reaction was
evidenced by ToF-SIMS, IRRAS, and AFM measurements. The ToF-SIMS measurements for 6b−d revealed that the shape of the original wave-shaped pattern developed on 6a and the disulfide bonds between the trimethacrylate moieties and the solid surface was retained after the cross-linking reactions. IRRAS measurements evidenced the consumption of the CC bonds of the immobilized trimethacrylate moieties on the crosslinking reactions. The average thickness of the cross-linked structure was different depending on utilized cross-linkers and determined to be between 1.1 and 1.8 nm by measuring the cross-linked structures grown in the circular pores within a FDTS monolayer prepared on a silicon wafer. The highest layer thickness (1.8 nm) was obtained from the cross-linking reaction using TFEMA and AIBN. 3250
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Nanostructures”. The authors further thank the group of Prof. Wolfgang Wenzel (KIT) for estimating the size and shape of the employed monomer.
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Figure 6. Schematic diagram of the average thickness ⟨d⟩ of the circular layers in 10 and 11a−d. The average thickness increased from 1.0 to 1.1−1.8 nm after the cross-linking reactions.
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ASSOCIATED CONTENT
S Supporting Information *
Synthesis of thioalkanoate with trimethacryloyl groups (3 and 3′); model reaction of 3 and N-n-propyl-2-(phenacylthio)acetamide; preparation of Si wafer functionalized with the FDTS layer and the circular pattern of the bare Si surface; images of the employed shadow mask and the photoreactor; emission spectra of the employed lamps and absorption spectra of the photoreactive compounds; experimental details on the polymer blend lithography procedure; mass spectra observed in the ToF-SIMS measurement of 5; ToF-SIMS images of 6c showing the distribution of F− and CF3− anions. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS C.B.-K. acknowledges financial support from the Karlsruhe Institute of Technology (KIT) in the context of the Helmholtz STN program as well as the German Research Council (DFG). K.F. acknowledges financial support from Japan Society for Promotion of Science (JSPS). S.W., T.S., and J.B. acknowledge support by the DFG-Center for Functional Nanostructures (CFN) at the KIT and by the Baden-Wuerttemberg Foundation within the Network of Excellence “Functional 3251
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DOI: 10.1021/la505011j Langmuir 2015, 31, 3242−3253
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DOI: 10.1021/la505011j Langmuir 2015, 31, 3242−3253
