Synthesis and Micropatterning of Photocatalytically Reactive Self

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Synthesis and Micropatterning of Photocatalytically Reactive SelfAssembled Monolayers Covalently Linked to Si(100) Surfaces via a Si−C Bond Michael K. F. Lo,† Matthew N. Gard,‡ Bryan R. Goldsmith,§ Miguel A. Garcia-Garibay,‡,§ and Harold G. Monbouquette*,†,§ †

Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, 5531 Boelter Hall, Los Angeles, California 90095, United States ‡ Department of Chemistry and Biochemistry, University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, California 90095, United States § California NanoSystems Institute, University of California, Los Angeles, Los Angeles, California 90095, United States ABSTRACT: Selective generation of an amine-terminated selfassembled monolayer bound to silicon wafers via a silicon−carbon linkage was realized by photocatalytically reducing the corresponding azide-terminated, self-assembled monolayers (AzSAMs). The Az-SAM was obtained by thermal deposition of 11-chloroundecene onto a hydrogen-terminated silicon wafer followed by nucleophilic substitution of the chloride with the azide ion in warm N,N′-dimethylformamide (DMF). The presence of the terminal azide group on the SAM was confirmed by reflection absorption infrared spectroscopy (RAIRS), by X-ray photoelectron spectroscopy (XPS), and by detecting the formation of a triazole upon reaction of the azide with an activated alkyne. The desired terminal amine groups were generated by photocatalytic reduction of the Az-SAM with cadmium selenide quantum dots (CdSe Qdots) using λ > 400 nm. Analysis of the reduced SAM by XPS gave results that were consistent with those obtained with an amine-terminated surface obtained by reducing the Az-SAM with triphenylphosphine. To demonstrate the feasibility of using the Az-SAM for surface patterning, a sample was coated with adsorbed CdSe Qdots and exposed to the output of a diode laser at λ = 407 nm through a micropatterned mask. Using a SEM, the pattern formed in this manner was revealed after removing the CdSe Qdots and subsequently adsorbing 10 nm gold nanoparticles (AuNPs) to the positively charged terminal-amine groups. The formation of the pattern by CdSe-photocatalyzed reduction of the azide demonstrates a novel route to create features by selective modification of organic monolayers on silicon wafers.

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temperature,4,11,14,15 in oxidative11,12 and harsh chemical environments.16−18 With proper functionalization and spatial resolution, these SAMs may provide a robust platform for the bottom-up fabrication of metal−insulator−semiconductor structures that exhibit minimal current leakage. The concept explored in this Article involves the preparation of a photoactive SAM that can be used for the development of surface patterns. The specific chemistry selected for this study involves the use of an azideterminated monolayer that can be photochemically modified into one that has amine-terminated chains. To develop a strategy for the synthesis of the desired azideterminated SAM, we considered various options. Since the early studies by Lindford and Chidsey,19,20 multiple synthetic routes to forming Si−C SAMs have been demonstrated. In general, such surfaces can be prepared using a variety of methods. These

INTRODUCTION Modified silicon surfaces have received significant research attention in recent years as feature sizes of commercial semiconductor products shrink toward the molecular scale.1−4 The electronic properties of these exceedingly small devices become increasingly dependent on the native properties of increasingly small clusters of molecules, causing issues in current leakage, power consumption, and heat dissipation.3,5 Organic insulator-on-silicon represents a promising material that can replace the silicon oxides in curbing current leakage.4,6,7 In fact, Faber et al. showed that octadecyl-terminated monolayers on silicon have better insulating properties than those on silicon oxide.6 By modifying the terminating groups of these aliphatic monolayers with designer receptors, the conductance of silicon surfaces can be modified and tuned for applications in molecular sensing.8 Preparing carbon-on-silicon (Si−C) self-assembled monolayer (SAM) is straightforward.4,9,10 The covalent,11 low bond polarity,12 and high bond strength (82 kcal/mol)13 nature of the Si−C bond make these SAM surfaces stable at high © 2012 American Chemical Society

Received: July 17, 2012 Revised: October 17, 2012 Published: October 21, 2012 16156

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Scheme 1. Derivatization Pathway from Cl-SAM to Colloidal Gold Decorated Self-Assembled Monolayer on N-type Silicon Wafer

include thermal activation,15,16,21−27 the use of UV,14,23,28−30 visible light31−33 or radical initiators;17,34−37 reactions with lithium,38−40 Grignard reagents,28,38 and Lewis acid catalysts;14,28 and scribed silicon methods.41−43 Among these, the thermal and visible light methods for reacting α-olefins with Hterminated Si surfaces (Si−H) are popular for several reasons. Thermal deposition yields SAMs of notably consistent quality, with no catalysts required, as long as the preparation is carried out under inert conditions. In most cases, a dilute solution of the olefin at as low as 2.5% by volume is needed, which is advantageous for custom-made olefins.9,22 With the visible lightactivated method, SAMs are prepared by spreading a thin layer of the neat olefin over the Si−H surface and then exposing the system to light. Mild room temperature conditions are sufficient to activate the self-assembly process without additional radical initiators or catalysts.31 However, this technique requires compounds that are liquid at moderate temperature, and the density of the SAM prepared by the light-activated process is less than that generated by thermal grafting.44 The density of the monolayer is influenced also by the size of the olefin relative to the reaction site on the silicon surface. For a Si(100) surface, the minimum end-on area required for a straight aliphatic chain (18.5 Å2 for crystalline n-C33H68)19 is larger than the area per silicon surface site, which is about 14.8 Å2.24 Molecular simulations reveal the maximum surface coverage of olefin on Si(100) is about 0.61 per silicon site.24 Therefore, SAMs assembled with olefins of larger terminal groups than methyl moieties are expected to be less densely packed.25,44 The unreacted hydrogen-terminated silicon surface sites slowly oxidize in air, forming silicon oxides.24 Therefore, a conscious effort should be made to generate SAMs with the smallest headgroup possible to obtain the highest coverage, while maintaining the desired level of flexibility for subsequent chemical modifications. Currently, the versatility of these chemically and physically robust surfaces is limited by the compatibility of desired ωfunctional groups with the highly reactive Si−H-terminated surfaces.11,12 Reactive functional groups, such as bromo,31,37,45 iodo,46 aldehyde,47,48 hydroxyl,37,49 nitro,37 amino,50,51 and azido52 groups, react in undesired ways with the Si−H surfaces. Often, the silicon surface abstracts electrons from these species, leading to the incorporation of the undesired ω-functional group of the olefin.45 For instance, the oxygen atom of the ω-aldehyde group can compete with the olefin for reacting with the silicon

site, forming a Si−O−C linkage and the CC double bond at the free terminus.47,48 Electron-rich primary amines also are known to react with Si−H-terminated surfaces, yielding SAMs with Si−N linkages.50,51 Both Sieval25 and Ara53 showed that acyl protected amines, such as phthalimides and amides, are compatible with Si−H-terminated surfaces, but the resulting monolayer is not close-packed due to the large size of the terminal imide and acyl groups.25,53 Finally, the reductive nature of the Si−H surface prohibits the direct assembly of azidefunctionalized olefins.52,54 Therefore, an alternate preparation of azide-terminated SAMs (Az-SAM) is proposed here via the thermal deposition of an ω-halogen-terminated SAM, which is subsequently reacted with sodium azide in dimethylformamide to yield the desired alkyl-azide groups.31,55,56 The choice of the appropriate halogenated olefin involves a balance between reactivity and thermal stability. Theoretically, 11-bromoundecene would be ideal, because once the alkene is bound to the silicon wafer via a silicon−carbon (Si−C) linkage, the bromine can be replaced with azide, a small but strong nucleophile, to generate the desired Az-SAM.31,55,56 However, when Marrani et al. attempted to create a bromo-terminated SAM via the light-activated deposition of 11-bromoundecene onto a Si−H surface,31 a significant fraction of the silicon sites were modified with bromine atoms (Si−Br) instead of aliphatic chains. It was suggested that photochemical activation at ambient temperature leads to Br radicals, which result in the formation of a heterogeneous surface with Si−alkyl and Si−Br sites. This was probably the result of an electron transfer reaction to the C−Br bond from the Si−H surface at ambient temperature,31 leading to the formation of a heterogeneous surface with alkyl and brominated sites.19,37,45 Because the Si−Br bonds spontaneously dissociate in solvents or react with oxygen, the resulting SAMs have less than maximal coverage.37 In another study by Cohen et al., the bromo-terminated SAM was formed by immersing a Si(111)−H surface directly into neat 11-bromoundecene while heating at 200 °C for 16−20 h.57 The authors observed the C−H stretch from methyl groups at 2960 cm−1, suggesting that homolytic cleavage of the C−Br bond and hydrogen abstraction occur under the reaction conditions.45 An alternative approach suggested by Linford et al. involves the direct preparation of a mixed SAM by depositing a 50/50 mixture of 11-chloroundecene and didodecanoyl peroxide onto a silicon wafer.19 Although the aliphatic chloride is a poorer leaving group than are bromides or iodides, azide ions are known to replace chlorides nucleophili16157

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cally in DMF.58,59 Thus, the unsaturated chloride represents a promising candidate for the preparation of an Az-SAM on Siwafers, although little is known about the preparation of a homogeneous Cl-terminated SAM on silicon surfaces. Az-SAM surfaces are expected to display high reactivity toward a wide range of species, including dipolar cycloadditions (“Click” reactions) and reduction reactions. Numerous research groups have explored the 1,3-dipolar cycloaddition of alkynes and azides to form 1,2,3-triazoles.31,56,60−63 Dyes,56,63 biomolecules,60 and ferrocenes31,61 modified with an alkyne can be “clicked” covalently to azide-containing surfaces, with negligible sidereactions.60 Organic azides are also photolytically active, giving rise to highly reactive nitrenes, which are capable of inserting into the C−H bonds of nearby molecules.64,65 Finally, organic azides can be reduced chemically with phosphines66,67 or photocatalytically with CdS or CdSe quantum dots (Qdots).67,68 Phosphines can either ligate to the azide group with a molecule of interest via Staudinger ligation66 or yield the corresponding primary amines.67 In this study, we create high density Az-SAMs by thermal deposition of Cl-terminated SAMs and demonstrate the formation of patterns with terminal amines in the Az-SAM background by spatially and selectively reducing terminal azide groups (Scheme 1).

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Generation of Az-SAM from Cl-SAM. A sample of Cl-SAM on Si prepared as described above and 20 mg of NaN3 were placed under vacuum for 1 h prior to adding 2 mL of anhydrous DMF under argon. The submerged SAM was maintained at 65 °C for 4 days in the dark. The Si-supported SAM was removed from the DMF solution, incubated in water for 3 h, and cleaned by sonication in water, THF, and methanol for 10 min each. Surfaces were dried with argon, and samples were stored in the dark. Formation of a 1,2,3-Triazole-Terminated SAM from the AzSAM and Trifluoroethyl Propiolate (TFEP) via a 1,3-Dipolar Cycloaddition Reaction. A dried Az-SAM on Si was placed into a scintillation vial, and 20 μL of neat TFEP was spread on the surface evenly via natural wetting. The reaction was allowed to occur for 48 h in the dark at room temperature. The surface was ultrasonic cleaned in THF, methanol, and water for 10 min each. Surfaces were dried with argon, and the samples were stored in the dark. Photocatalytic Reduction of Az-SAMs in Bulk Solutions of DEAET-Capped CdSe Qdots. An Az-SAM preparation was placed into a 100 mM solution of sodium formate containing ∼10−6 M CdSe Qdots in water and was briefly sparged with argon for 30 min. The vial was exposed to a 450 W Hanovia arc lamp (Ace Glass Inc., Vineland, NJ) through a 400 nm cutoff filter from Schott North America, Inc. (Duryea, NY) for 3 h and was cooled in an ice bath. The SAM was then agitated in filtered 6 M aqueous guanidine hydrochloride (GHCl) solution for 24 h to remove any absorbed Qdots.67 SAMs were ultrasonic cleaned in water, ethanol, and water again for 10 min each before drying with argon. Photocatalytic Reduction of Az-SAM with Preadsorbed DEAET-Capped CdSe Qdots. Inside a dark room, CdSe Qdots were allowed to adsorb onto an Az-SAM surface (0.5 cm × 0.5 cm) overnight, and excess Qdots were removed by dipping gently into a buffer solution containing 1.0 mM DEAET, 1.5 mM MES, and 10 mM sodium formate at pH 5. The surface appeared hydrophilic. The wafer was sandwiched immediately between two coverslips, and the SAM side was irradiated with a Coherent Cube (Santa Clara, CA) 407 nm laser with a power output of 10 mW for 15 min or for 4 h. The SAM was placed immediately into a 6 M aqueous GHCl solution overnight (12 h minimum) to desorb Qdots from the surface. The SAM was ultrasonic cleaned in water, THF, and MeOH for 10 min each. Water dewetted from the surface readily. The sample was dried with a stream of argon and stored in the dark for analysis. Additionally, two separate control experiments were conducted. An Az-SAM with preadsorbed CdSe Qdots was kept away from the laser excitation for 15 min. Separately, an Az-SAM without CdSe Qdots was exposed to laser excitation. For the micropatterning experiment, an Az-SAM preadsorbed with CdSe was exposed to the laser beam behind a 110 μm pitched grid mesh on top of the coverslip for 15 min. The same cleaning procedure was performed. Chemical Reduction of Az-SAMs by Reduction with Organic Phosphines. Dried Az-SAMs and 13.8 mg of PPh3 were vacuum dried for 1 h in a vial with septum and backfilled with dried argon. Five milliliters of freshly distilled THF was injected into the vial under a positive atmosphere of argon and was shaken for 2 days at room temperature. Alternately, amine-terminated SAMs (NH2-SAM) were produced by reducing the corresponding Az-SAMs (NH2-SAM) by dipping them into a THF solution containing 3% PBu3 and 3% H2O. The surfaces were left in water for 1 day and were cleaned by sonication in THF, water, and MeOH for 15 min each. Surfaces were dried with argon and stored in the dark. Gold Nanoparticle (AuNP) Tagging of Terminal Amines Formed by Partial Photocatalytic Reduction of Az-SAMs. Five hundred microliters of a stock solution of 10 nm AuNPs was concentrated by centrifuging at 14 000 rotations per minute (18 G) for 30 min. The supernatant (460 μL) was removed, and a pH 7.0 buffered solution (Fisher Chemicals, Pittsburgh, PA) of 10 mM NaCl/ 0.1 mM sodium citrate was added to give a final volume of ca. 240 μL. The pink-colored solution appeared homogeneous. Inside a plastic centrifuge tube, each SAM was further cleaned with MeOH and then with water for 10 min each prior to incubating in an aliquot of the buffer. Another aliquot of AuNP solution of the same volume was added using a micropipet. SAMs were agitated on a lateral shaker in the dark for 6 h. SAMs were removed carefully from the solution and were rinsed

EXPERIMENTAL SECTION

Chemicals. Semiconductor-grade hydrofluoric acid (HF) 48 wt % and hydrochloric acid (HCl) 25 wt % were obtained from Riedel-de Haën (Germany); sodium azide 99.5%, mesitylene 99%, and 1octadecene 99.8% were obtained from Fluka (Switzerland); sodium citrate (NaCit) 99.8%, hydrogen peroxide (H2O2) 30%, and common solvents were obtained from Fisher Scientific (Pittsburgh, PA); samples of 2-(N,N-diethylamino)ethanethiol (DEAET) 96%, sodium formate 98%, triphenylphosphine (PPh3) 99%, tributylphosphine (PBu3) 99%, 2-(N-morpholino)ethylenesulfonic acid (MES) 99%, 1-chlorodecane 97%, and 11-chloroundecene 97% were obtained from Aldrich (Milwaukee, WI); and anhydrous N,N-dimethylformamide (DMF) 99.5% was obtained from Acros Organics (Morris Plains, NJ). Mesitylene, dichloromethane (DCM), tetrahydrofuran (THF), and 11-chloroundecene were distilled and stored in Teflon-coated septum vials; all other chemicals were used as received. Deionized (DI) water (18 MΩ cm) was obtained from a Millipore Milli-Q system with the organic-free kit installed (Billerica, MA). Ten nanometer gold colloids (AuNPs) in water were obtained from Ted Pella Inc. (Redlands, CA). 2,2,2-Trifluoroethyl propiolate (TFEP) was synthesized following a procedure reported by Jung et al.69 Fresh DEAET-capped CdSe Qdots were prepared by a method described in detail previously.68 N-type 10− 20 Ω cm test-grade single-sided Si(100) wafers were obtained from Silicon Quest International (San Jose, CA). Formation of Cl-SAMs on Si(100) Surfaces by Thermal Deposition. Si(100) wafers were diced into 0.5−1 cm square shards and were degreased by ultrasonic cleaning in acetone, methanol, and isopropanol for 2 min each. After ultrasonic cleaning for 10 min in water three times, they were submerged in a HCl:H2O2:H2O (1:2:7) mixture for 5 min, followed by ultrasonic cleaning three times in water for 10 min each. Inside a glovebag purged with dry argon for 1 h, the shards were dipped into an argon-saturated 4.8 wt % HF solution for 5 min. The aqueous HF solution spontaneously dewetted from the hydrophobic hydrogen-terminated silicon surfaces. The shards were dried with argon and placed immediately into a flask containing 5 mL of argon-purged 10% v/v 11-chloroundecene in mesitylene. The flask was subsequently heated in an oil bath at 165 °C for 12 h under a positive pressure of dried argon. After the reaction, the flask was cooled to room temperature. Shards were rinsed with DCM, followed by ultrasonic cleaning in ethanol for 5 min and in water for 10 min. Surfaces were dried with argon and stored in the dark. For control studies, the same procedure was repeated with 1-octadecene or 1-chloroundecane in place of 11chloroundecene. 16158

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immediately with a 100 μM citrate solution, water, and MeOH before drying with argon. SAMs were stored in the dark before analysis.

Article

RESULTS AND DISCUSSION Preparation of Chloro-Terminated SAMs (Cl-SAMs) on Silicon. The goal of this phase of the study was to establish a reproducible process to yield a dense surface monolayer terminated with a reactive chloride group. The target surfaces prepared only with 11-chloroundecene in distilled mesitylene were expected to be less hydrophobic and hence should display a smaller contact angle than the 99° measured by Linford for SAMs on Si(111) composed of a 50:50 mix of Cl- and methylterminated species.19 Indeed, a 10 vol % solution of 11chloroundecene in mesitylene yielded Cl-SAMs with an average contact angle of 88° (Table 1). As expected, this contact angle is

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INSTRUMENTATION Reflection Absorption Infrared Spectroscopy (RAIRS). All IR spectra of SAMs were obtained with a DigiLab (Canton, MA) FTS-175C single IR beam spectrometer fitted with a mercury−cadmium−telluride (MCT) detector, which was cooled by liquid nitrogen. Beam incidence and takeoff angles of 68° for Si(100) surfaces were used for the highest signal-tonoise ratio.32 The sample stage chamber was purged constantly with dry nitrogen to reduce artifacts from water vapor. All spectra, including the blank spectra, were recorded at 4 cm−1 resolution, averaged over 1024 scans, and flattened to the baseline. The sensitivity ratio was kept at 1 throughout the experiment. Contact Angle. Contact angle data were obtained using a First Ten Ångstroms FTA4000 goniometer (Portsmouth, VA). Contact angles of 5 μL drops of water were measured on at least three different areas of the surface. Measurements were obtained immediately after water droplets were placed on the surface using a micropipet. All droplets were allowed to spread onto the surface freely upon release from the tip of the micropipet. Reported contact angles are mean values with corresponding standard deviations. Tapping Mode Atomic Force Microscopy (TM-AFM). All AFM images were obtained using a Digital Instruments NanoScope IIIa scanning probe microscope equipped with a Multimode AFM scanning head (Santa Barbara, CA). Tapping mode AFM cantilevers with aluminum reflex coating (AppNano or Vista Probes) were obtained from NanoScience Instruments (Phoenix, AZ). AFM cantilevers were tuned and operated around ∼300 kHz. Samples were mounted onto the sample stage with double-sided tape, and images were recorded on the forward trace with scan rates between 0.50 and 0.75 Hz at 512 samples/ line and 512 lines per image. The image was flattened vertically and horizontally (second degree flattening). X-ray Photoelectron Spectroscopy (XPS). All XPS data were obtained by exposing samples to monochromatic Al Kα radiation (1486.6 eV, 300 W) in an Electron Spectroscopy Chemical Analysis (ESCA) instrument manufactured by Omnicon NanoTechnology USA (Eden Prairie, MN). The takeoff angle was 90°, the pass energy was set at 20 eV, and samples were scanned at least twice. All data were acquired below 10−9 Torr. Atomic sensitivity factors of 0.42 for N(1s), 0.37 for Cl(2s), and 0.25 for Si(2p) were used for yield calculations.70 All spectra were referenced to the Si(2p3/2) peak at 99.0 eV B.E.71 XPSPEAK 4.1 peak fitting software was used to first subtract the peaks with the Shirley-type background spectrum followed by deconvolution and integration of the peak areas.72 Scanning Electron Microscopy (SEM). All SEM images were recorded with a JEOL JSM-6700F field emission scanning electron microscope (Peabody, MA) on an aluminum stage. The probe current was set to 9. The brightness and contrast for some images were readjusted using ImageJ 1.41o (NIH) software. Optical Microscopy. The color image of the mask with the 110 μm dot pattern array was recorded with the Nikon Eclipse ME600 optical microscope (Melville, NY) at 50× zoom. The image was captured and calibrated with a QImage Micropublisher 5.0 RTV camera (Surrey, British Columbia) connected to a PC via a firewire (IEEE-1394) cable. Finally, it was processed with ImageJ 1.41o software.

Table 1. Comparison of Contact Angles and IR Data for Depositing Cl-SAM on Si(100) RAIRS wavenumbers (cm−1) adsorbate

duration (h)

average contact angle (deg)

−CH2− (νas)

−CH2− (νs)

11-chloroundecene 1-octadecene 1-chlorodecane

12 6 12

88 ± 1 107 ± 1 35 ± 4

2925 2923 N/A

2854 2853 N/A

slightly less than the 92° reported for an 11-fluoroundecenemodified SAM on Si(100).32 RAIRS data showing the methylene symmetric and antisymmetric signals of the Cl-SAM were at 2854 and 2925 cm−1, respectively (Figure 1). A SAM formed by

Figure 1. RAIRS spectra of, from top to bottom, control, Cl-SAM, AzSAM, and NH2-SAM (from phosphine reduction); signals near 2300 cm−1 correspond to carbon dioxide.

deposition of 1-octadecene on hydrogenated Si(100) by the same procedure described above was created for comparison purposes (Table 1). Methylene stretches were detected easily at 2853 and 2923 cm−1, as expected, and the SAMs had an average contact angle of 107°. A well-ordered hexadecyl-terminated SAM on Si(100) described in the literature yielded a similar contact angle of approximately 110° and similar symmetric and antisymmetric methylene stretches recorded at 2854 and 2923 cm−1, respectively.32 These literature data provide a benchmark for well-ordered, tightly packed hydrophobic SAMs on Si(100). Disordered liquid-like monolayers would be expected to display smaller contact angles (more hydrophilic), and methylene stretches would occur at longer wavenumbers.32 The data for our Cl-SAMs and methyl-terminated SAMs are comparable with the available literature data, suggesting that they are well-ordered and reasonably packed. 16159

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Figure 2. High-resolution XPS spectra of a Cl-SAM in the Si(2p) (left) and Cl(2s) (right) regions.

Figure 3. High-resolution XPS spectra in the N(1s) region (left) and F(1s) region (right) of an Az-SAM sample.

401.0 BeV were similar to those described in previous studies.67 The total area under the 404.7 BeV signal was about one-half of the 401.0 BeV signal. The overall yield for the conversion from Cl-SAM to Az-SAM (N/Cl) was calculated by dividing the integrated area ratios of N/Si over Cl/Si from respective SAMs made in the same batch, thereby giving an estimated conversion of ∼70%. When an octadecyl-terminated SAM was used in place of a Cl-SAM as a control, no azide signal was detected at 2100 cm−1. The average contact angle of the Az-SAMs was 90 ± 2°, which is higher than that reported for SAMs on gold surfaces terminated with aromatic azides (78°)67 and aliphatic azide (77°).55,61 A typical TM-AFM image of the Az-SAM surface sample revealed an atomically flat surface with an RMSroughness of ca. 0.3 nm (Figure 4A). The Az-SAMs were stable for months when stored in the dark. To confirm the reactivity of the Az-SAM, an activated alkyne, 2,2,2-trifluoroethyl propiolate (TFEP), was synthesized and reacted with the Az-SAM via a 1,3-dipolar cycloaddition at room

To ensure that the Si−H surface was hydrosilylated by 11chloroundecene via its alkenyl terminus, an alternate SAM deposition process was carried out at 165 °C for 12 h with 1chloroundecane in lieu of 11-chloroundecene. Should the chlorine atom cleave homolytically at the hydrogen-terminated surface, methylene signals would be observable on the RAIRS spectrum. However, the resulting surface lacked any methylene signals and appeared hydrophilic (Figure 1), suggesting minimal (if any) alkylation as a result of cleavage of the C−Cl bond.19 Analysis of the resulting Cl-SAMs by XPS showed the expected Cl(2s) signal at 271.5 eV (binding electron volts, BeV) (Figure 2).73 A single signal indicated that Cl atoms are present in only one state. Because only ca. 61% of the hydrogenterminated Si(100) surface sites can be grafted with aliphatic chains, the remaining silicon sites were expected to be oxidized upon exposure to air and water. The expected signal for oxidized silicon surface atoms was observed at 102.8 eV in the XPS spectrum (Figure 2).24 The combined RAIRS and XPS data suggest the Cl-SAM was grafted onto the silicon surface at high density and with chlorine being the surface terminal atom. Preparation of Aliphatic Az-SAMs. The Az-SAMs on silicon were obtained successfully from the corresponding chloride (Scheme 1). A strong signal near 2100 cm−1 appeared on the RAIRS spectrum after exposing the Cl-SAM to a DMF solution of NaN3 at 65 °C for 4 days while the methylene stretches remain (Figure 1c). The new peak corresponds to the azide moiety attached to an aliphatic carbon.31,61,67 Repeated ultrasonic cleaning of the azide-terminated surfaces in THF and water did not diminish the azide signal, indicating that the azide group was covalently bound to the aliphatic chains. Analysis of the Az-SAM by XPS confirmed the presence of azides (Figure 3). Two characteristic N(1s) signals at 404.7 and

Figure 4. 1 μm2 TM-AFM images of Az-SAM (A) and NH2-SAM (B). 16160

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Figure 5. High-resolution XPS analysis of the TFEP modified azide SAM, showing the N(1s) region (left) and the F(1s) region (right).

Figure 6. RAIRS spectrum of an Az-SAM before (solid line) and after (dotted line) being irradiated with a Hanovia arc lamp in an aqueous solution of CdSe Qdots (left) or with a 10 mW 407 nm laser in an aqueous solution of CdSe Qdots (right).

Figure 7. XPS analysis of Az-SAM chemically reduced by PPh3 in the N(1s) region (left) or photocatalytically reduced with DEAET-capped CdSe Qdots (right).

temperature without copper catalyst.74 Even though this procedure required a longer reaction time, it is still advantageous as it eliminates the use of copper. After reaction, the methylene stretches remained roughly constant, while the IR stretch at 2100 cm −1 disappeared. A new peak emerged at 1756 cm −1 corresponding to the carbonyl stretch of the ester and was accompanied by the combination band of silica stretches around 1720 cm−1.75 XPS data showed that the N(1s) signal at 404.7 BeV disappeared and the one at 401.0 BeV was broadened. The latter signal could be deconvoluted into two peaks with a pattern similar to those reported for triazole species bound to a Si(100) surface that were prepared in a similar manner.62,67 Furthermore, a strong F(1s) signal appeared at 689.3 BeV (Figure 5), while the original Az-SAM only showed a very weak F(1s) signal (Figure 3 inset), most likely arising from the HF etch.46

Partial Reduction of Az-SAMs To Give Terminal Amine Groups. Aromatic azides were shown previously to reduce photocatalytically to aromatic amines at the surface water interface using CdSe Qdots suspended in water in the presence of a sacrificial electron donor at wavelengths greater than 400 nm.67,68 After applying the same photocatalytic reduction technique, the IR intensity of the Az-SAM was attenuated at 2100 cm−1, while the methylene C−H stretch absorption signals were not affected significantly (Figure 6, left). The N−H stretch is not observed on the RAIRS spectrum, similar to previous reports of amine-terminated self-assembled monolayers.67,76−78 The infrared absorption bands of surface-bound species are governed by the surface-selection rule, where the absorption is permitted for only the perpendicular component of a bond relative to the surface normal of a metal surface.79 Therefore, the 16161

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Figure 8. 1 μm2 SEM images of AuNPs adsorption on photocatalytically reduced Az-SAM: preadsorbed with Qdots and exposed to (A) no laser, (B) for 15 min; or in bulk solutions of Qdots and exposed to laser for (C) 5 min, and (D) 4 h; AuNP adsorption on phosphine reduced Az-SAM (E).

more protonated NH2 groups in the photocatalytically reduced SAM than in the phosphine reduced SAM. Intuitively, this should have resulted in a lower contact angle for the latter case due to the presence of ionic species. However, in low ionic strength conditions, such as in the deionized water used for these contact angle measurements, the protons may be lost from the surface and the in-plane amine groups may reorganize and increase the overall chain ordering. This may effectively increase the contact angle of the overall SAM. Furthermore, the azide absorption in the photocatalytically reduced azide SAM as shown in Figure 6 is still detectable by RAIRS. Therefore, the mixed azide and amine SAM in the latter case may have a slightly higher contact angle than those obtained by phosphine reduction. Titrating the surface by measuring the adhesion forces using chemical force measurement in buffers of known pH and ionic strength may shed light on the actual state of the NH2 surface.78 The NH2 surface is expected to be more ionizable than the azideterminated SAM. The results above suggest the chemically and photocatalytically reactive azide group was positioned at the SAM terminus. Photocatalytic Reduction by Adsorbed CdSe Qdots Using a 407 nm Laser. Adsorbed Qdots are potentially advantageous in comparison to phosphine reagents for Az-SAM reduction because the extent of the reduction can be controlled precisely by the density of adsorbed Qdots on the azide surface. The surface density of the amine species is important as it contributes significantly to the observed pK1/2, which is defined as the pH value at which one-half of the NH2-SAM surface moieties are protonated.76 Previously, Schweiss et al. showed that a SAM consisting of carboxylic acid-terminated groups diluted with methyl groups is more likely to be deprotonated near the bulk carboxylic acid pKa (∼3) than those at high surface concentration (pK1/2 ≈ 5.5−8.5).81 Analogously, NH2-SAMs diluted by azide-terminated species may be expected to elevate the pK1/2 of a pure amine SAM closer to the one in the bulk pKa (∼9). Pure amine-terminated surfaces are more acidic (pK1/2 ≈ 4−5) due to local charge−charge repulsion of the protonated

lack of N−H absorption suggests the N−H bonds of the amine groups may be positioned parallel to the plane of the surface.78 Analysis of the Az-SAM by XPS after photocatalytic reducing with bulk CdSe Qdots revealed a small azide N(1s) signal at 404.9 BeV in addition to the amine N(1s) peaks at 400.3 and 402.0 BeV (Figure 7). One of the amine N(1s) peaks was shifted downfield possibly due to protonation, which renders the quaternary ammonium nitrogen more electron poor than that of a free amine.80 A control experiment was performed where the Az-SAM was exposed to light greater than 400 nm for 2 h in an aqueous EtOH solution of 10 mM 2-(N,N-diethylamino)ethanethiol (DEAET) without CdSe Qdots. The N(1s) peaks corresponding to the azide moiety were unchanged after this experiment, as expected by lack of direct azide absorption at the chosen wavelength. To verify the presence of primary amines after the photocatalytic reduction of the Az-SAM with CdSe Qdots, an NH2SAM was prepared by reducing chemically the Az-SAM with PPh3 for 48 h followed by hydrolysis in water for 2 days. The TMAFM micrograph revealed a surface topology similar to the AzSAM, which was also atomically flat (Figure 4B). XPS analysis of this NH2-SAM also revealed a signal near 401 BeV (Figure 7) and a small signal at 404.5 BeV. The latter signal is likely due to the presence of unreacted azide groups, which should also be accompanied by a small 401.1 BeV band. Support for this interpretation was obtained by deconvoluting the large 401 BeV signal into an azide peak at 401.1 BeV and an amine peak at 400.2 BeV. By integrating the N(1s) peak areas, it could be inferred that approximately 90% of the azide groups had been converted to amines. NH2-SAMs obtained by reducing the Az-SAMs with PBu3 (Figure 1d) yielded results similar to those obtained with PPh3. The N−H stretch is again not present despite the C−H stretches remaining in the RAIRS spectrum for the azideterminated SAM reduced by phosphine. The NH2-SAMs derived from PPh3 or PBu3 showed an average contact angle of 59°, which is slightly smaller than the photocatalytically reduced AzSAM (64°). This is contrary to the XPS data, where there are 16162

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amine groups.82,83 Therefore, we could expect that fewer negatively charged AuNPs should be absorbed onto the AzSAM or NH2-SAM at neutral pH (Figure 8A and E). By controlling the extent of photoreduction, it should be possible to adjust the surface density of amine groups and manipulate the extent of binding of negatively charged AuNPs at neutral pH. After laser irradiation of the Az-SAM with preadsorbed CdSe Qdots for 4 h, the intensity of the azide stretch was attenuated (Figure 6, right). Although no significant change to the IR intensity of the azide stretch was observed after exposure to the laser for only 15 min, this surface adsorbed AuNPs at relatively high density (Figure 8B). The distribution of AuNPs appeared to be homogeneous, and few aggregates were observed. A control experiment where the CdSe Qdot-coated Az-SAM was not irradiated with the laser led to adsorption of AuNPs with a very low density (Figure 8A). Additionally, a pristine Az-SAM without any adsorbed CdSe Qdots that was exposed to the laser for 2 h showed little to no AuNP binding. These results demonstrate that both CdSe Qdots and the laser irradiation were required for the reduction of azides to amines and for the subsequent adsorption of AuNPs to occur. The same sample presented in Figure 6 (right) was also incubated in a solution of AuNPs in 10 mM NaCl/0.1 mM NaCit at neutral pH for 7 h, but few adsorbed AuNPs were seen, as shown in Figure 8D. When the Az-SAM was reduced photocatalytically with a bulk solution of CdSe Qdots under laser illumination for 5 min, some AuNPs were adsorbed afterward (Figure 8C). These observations suggest that the extensively reduced sample in Figure 6 (right) harbored surface amines with a pK1/2 depressed to the extent that few protonated amine groups were available for AuNP binding at a neutral pH. This result is similar to that observed with the NH2-SAM obtained from reducing Az-SAM with PBu3 (Figure 8E), where few AuNPs were adsorbed. Judging by the small change in the IR intensity of the azide peak associated with activation of the surface for AuNP binding, the pK1/2 of the surface amines appears to be highly sensitive to their surface concentration. Using this photocatalytic Qdot technique, the extent of reduction of surface azides to amines can be controlled deliberately to give a readily observable result, AuNP adsorption. Micropatterning by Controlled Photocatalytic Surface Reduction. Selected areas of an Az-SAM with preadsorbed CdSe Qdots were reduced photocatalytically upon irradiating with the 407 nm laser behind a mask consisting of a 110 μmpitched dot array. The Az-SAM was sandwiched between two coverslips. As expected, areas within the dots receiving laser irradiation led to photocatalytic reduction of the surface azide to the corresponding amine, while areas covered by the mask did not react. Upon removing the CdSe Qdots with 6 M GHCl and incubating with AuNPs in a pH 7 buffer solution containing 10 mM NaCl and 0.1 mM NaCit, an AuNP composed pattern that resembled the mesh mask was seen readily under the SEM (Figure 9). The brighter regions highlight the photocatalyzed area where AuNPs associate (amines), while the darker regions represent the unreduced azides. This study demonstrates that photocatalytic reduction of surface groups can be used to pattern areas of amines in an azide background over a large area. The size of the features on the Az-SAM was slightly bigger than the mask, which is composed of patterns printed onto a thin plastic film, and rings and grids were interconnecting the dots. This pattern was caused by the diffraction and constructive/destructive interferences of the laser beam going through the dot openings on the mask.84 These pattern characteristics reflect the photocatalytic sensitivity of the CdSe Qdots and also the

Figure 9. SEM image of partially reduced Az-SAM behind a mesh mask (left); and optical microscope image of mask (right); scale bar = 100 μm.

selective adsorption of AuNP to the photocatalytically reduced regions, including where photocatalytic reduction has occurred due to the diffracted light. Efforts to reduce such diffracted light effects could include applying a thin layer of aqueous solution containing necessary sacrificial electron donors to eliminate air gaps,85 and arranging the Az-SAM and the side of the printed micropattern in direct contact without the SAM-side coverslip.86 Control experiments where no CdSe Qdots were preadsorbed to the Az-SAM prior to laser illumination through the mask showed no discernible patterns after incubating the surface in the AuNP solution.

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CONCLUSION In this work, well-ordered azide-terminated and amineterminated self-assembled monolayers (SAMs) covalently bound to the (100) surface of silicon wafers were prepared. Densely packed SAMs were obtained by thermal deposition of 11-chloroundecene onto hydrogen-terminated Si(100) at 165 °C. Terminal azide moieties were installed in a NaN3-saturated DMF solution heated to 65 °C for 4 days. RAIRS and XPS data provided evidence for the presence of azides on the SAM. A dramatic difference in the N(1s) signals from XPS was observed before and after reacting the azide SAM with 2,2,2-trifluoroethyl propiolate. The appearance of an intense F(1s) XPS signal suggested the azido group was readily available for the formation of triazole via 1,3-dipolar cycloaddition. The Az-SAM also could be reduced with phosphine reagents, or with bulk CdSe Qdot photocatalysts and light at >400 nm to yield the corresponding amines. In the former case, RAIR spectra showed little to no azide peak at 2100 cm−1, suggesting high conversion of the azide to the amine, while the azide signal was still readily observable in the latter case. However, the N(1s) region of the respective XPS spectra reflected incomplete reduction of the surface azide to surface amine in both cases. Negatively charged AuNPs at neutral pH had little affinity for the phosphine-reduced NH2-SAM, due to the depressed pK1/2 resulting from the high density of surface amines. Because the observed pK1/2 of the surface was controlled by the density of amines relative to azides, the pK1/2 of the amines could be tuned by reducing a smaller proportion of the azide on the SAM. A partial amine SAM with the remaining azide groups serving as a diluent could be generated photocatalytically using CdSe Qdots and a 407 nm laser. The extent of photocatalytic reduction could be controlled by adjusting the exposure time to the laser. AuNP adsorption was observed after short laser exposure (∼5 min), but little AuNP adsorption was seen after longer laser exposure (∼4 h) presumably due to the greater extent of azide reduction at longer exposure times resulting in lower surface pK1/2 values of the surface amines. Using Qdot photocatalysts, preadsorbed on an Az-SAM, a micropattern of terminal amines in an Az-SAM background was generated by illuminating the surface through a patterned mask. The creation 16163

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of a reactive surface amine micropattern in a differentially reactive azide SAM background offers the possibility to build functionality upon the pattern using the full toolbox of synthetic organic chemistry.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the Focus Center Research Program (FCRP) through the Functional Engineered Nano Architectonics (FENA) Center at UCLA. We also acknowledge Dr. Zheng Xu of the Department of Material Science (Professor Yang’s Group, UCLA) for collecting the XPS data.

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