Applications of Ring-Opening Metathesis Polymerization - American

Chapter 22

Polymer-Inorganic Nanocomposites from Si-Based Substrates: Applications of Ring-Opening Metathesis Polymerization

Downloaded by CORNELL UNIV on August 29, 2016 | http://pubs.acs.org Publication Date: September 19, 2008 | doi: 10.1021/bk-2008-0996.ch022

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Jun-Hyun Kim1, Joshua A. Kellar , Mark C. Hersam , and SonBinh T. Nguyen 1,*

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Departments of Chemistry and Materials Science and Engineering, Northwestern University, 2145 Sheridan Road, Evanston IL 60208

This contribution outlines four different ROMP-based (ROMP = ring-opening metathesis polymerization) approaches to prepare polymer overlayers possessing either hydroxyl or carboxylic acid functionalities on oxidized silicon surface or silica (SiO ) nanoparticles. The morphology, elemental composition, and properties of these polymer overlayers can be characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force micro scopy (AFM), transmission electron microscopy (TEM), ellipsometry, and Fourier-transform infrared spectroscopy (FTIR). 2

© 2008 American Chemical Society

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Nagarajan and Hatton; Nanoparticles: Synthesis, Stabilization, Passivation, and Functionalization ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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Introduction The controlled fabrication of well-defined organic overlayers on solid substrate has long been a key research focus in many technological areas including adhesion, lubrication, coatings, information storage, biomimetic materials, and electronics (l^f). Organic overlayers can endow the underlying substrates with additional mechanical, thermal, optical, and electrical properties that can subsequently be utilized in a synergistic fashion in microelectronics, microelectromechanical systems (MEMS) devices, and biological sensors (5-7) In particular, covalently attached polymer layers (e.g., brushes, films) on surfaces can facilitate the fabrication of complex architecture possessing multiple organic, biomolecular, and inorganic components, allowing for novel combination of properties and functions (8). In theory, such polymer overlayers can readily be prepared in a controlled manner from a wide range of fimctionalized monomers to yield multifunctional films that have tunable density and thickness (9,10). However, to construct these films, reliable synthetic methods that allow for uniform polymer growth as well as being tolerant to a variety of functional groups must be developed. Typically, polymer overlayers can be fabricated by two different approaches: physisorption on solid substrates or covalent attachment to the substrate surface (77). The first strategy is widely practiced in industry and includes methods such as spin-coating, ink-jet printing, screen printing, and selective physisorption of block copolymers (12,13). Covalent attachment of polymers to surfaces is a newer methodology that can, in theory, offer films that are thinner but also thermally and solvolytically more stable (14). As such, it has received considerable attention, especially in the context of surface-initiated (SI) living polymerization where the molecular weight of individual polymer chains can be precisely controlled to give polymers with narrow polydispersity (15,16). Not only should this approach afford polymer films with tunable thickness, the directional growth of the polymer chain may allow for a reliable, controlled placement of many functional groups within the polymer overlayer and with respect to the surface (8,17,18). Thus, many researchers have attempted to develop SI living polymerization strategies, based on well-known solution techniques such as nitroxide-mediated controlled radical polymerization (19-22), atom-transfer radical polymerization (75), anionic (22), cationic (23), and ring-opening metathesis polymerization (ROMP) (10,24,25), to fabricate polymer overlayers (brushes) on solid substrates. Among the aforementioned strategies, SI-ROMP stood out for its low activation energy (i.e. reactions can be carried out under ambient conditions) and ease of application, especially given the recent development of highly active, single-component olefin metathesis catalysts (26). Since 1999, SI-ROMP catalyzed by well-defined metal-alkylidene has been employed by several groups


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(10,27,28) to prepare polymer films and nanopatterns on gold or oxidized silicon. In addition, as ruthenium catalysts for ROMP are known to be functional-group-tolerant and living in diverse solvents (from benzene to water), monomers containing a wide range of functionalities have also been incorporated into a number of surface polymer films (10,22,26,27). Unfortunately, back-biting or chain-transfer between neighboring ROMP polymer chains (Figure 1) still made it difficult to produce polymer overlayers with a wide rage of tunable thicknesses unless surface density of the initiating groups is carefully controlled. (29).

Figure 1. Schematic illustration ofpossible side reactionsfromsurface initiated ring-opening metathesis polymerization (SI-ROMP) on an oxide substrate.

Our groups have had long-standing interests in integrating functional organic materials with silicon-based substrates for material and biological applications. To this end, we have explored four different ROMP-based approaches to prepare functionalized polymer films on oxidized Si(l 11) surface. We aimed to compare polymer overlayers obtained from two different SIROMP-based methods to those derived via in-situ polymerization and grafting processes. Through evaluating the different film morphologies, we have extended the two best strategies to the synthesis of discrete polymer-coated silica nanoparticles. These results are described herein.

Experimental Section General Materials and Methods Grubbs catalyst (PCy ) Cl Ru=CHPh, 5-(bicyclo[2.2. l]hept-2-enyl) trichloro silane (NBE-SiCl ), 5-norbornene-2-carboxylic acid (NBE-COOH), and bicyclo[2.2.1]hept-2-ene (NBE) were purchased from Aldrich and used as 3

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Figure 2. Schematic illustration ofpolymer film growth on oxidized Si(lll) wafers using four different approaches


308 received. 5-(Bicyclo[2.2. l]hept-2-enyl)triethoxysilane (NBE-Si(OEt) ) was purchased from Gelest and used as received. 5-£jco-norbornene-2-ol (NBE-OH) was prepared according to literature procedure (28). All solvents were dried over neutral alumina via the Dow-Grubbs solvent system (29). A l l polymerizations were carried out at room temperature either in a N -filled glovebox or using standard Schlenk techniques, unless otherwise noted. Si(l 11) substrates (~ 1.5 cm in area, Virginia Semiconductor) were oxidized with piranha solution (3:1 cone. H SO4:30%H O2) following reported procedures (30) prior to the attachment of norbornenyl moieties (NBE-SiCl , NBE-Si(OEt) ) (31). Figure 2 illustrates how we construct polymer brushes on oxidized Si(l 11) wafers utilizing ROMP methods. 3

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Method 1 - Conventional SI-ROMP A solution of NBE-SiCl or NBE-Si(OEt) in toluene (1% v/v) was prepared in a N -filled glovebox immediately before use. Oxidized Si(l 11) wafers were immersed in this monomer solution (0.1 mL in 10 mL of toluene) for 1 h (for NBE-SiCl (or overnight for NBE-Si(OEt) )) to form norbornenyl moieties at the interface. The oxidized S i ( l l l ) wafers were then washed with toluene ( 2 x 4 mL) and methylene chloride (CH C1 , 2 x 4 mL) followed by submersion in a solution of Grubbs catalyst (17 mM in CH C1 ) for 30 min to induce ringopening of the surface-attached norbornenyl groups. Next, the oxidized Si(l 11) wafers were thoroughly rinsed with CH C1 ( 3 x 4 mL) to remove any unbound and/or physisorbed catalysts. Surface growth of polymer brushes were then commenced by immersing the initiated wafers in a solution containing norbornene-based monomer (100 mM of NBE-COOH or NBE-OH in CH C1 ) at room temperature for 30 minutes. (The polymerization reaction was believed to have started from the surface of S i ( l l l ) wafer where Grubbs catalysts were initially confined (32).) The samples were then removed from the glovebox and the polymerization was quenched with a few drops of ethyl vinyl ether. The wafers were then rinsed with CH C1 ( 2 x 4 mL), sonicated (Fisher FS6 ultrasonic cleaner) in CH C1 (4 mL) for 10 min, rinsed with CH C1 (10 x 2 mL), and dried in a stream of nitrogen to remove any residual and physisorbed species prior to analyses. 3

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Method 2 - Solventless SI-ROMP In a N -filled glovebox, oxidized S i ( l l l ) wafers were functionalized with either NBE-SiCl or NBE-Si(OEt) monomer and then treated with Grubbs catalysts in the same manner as described in method 1. The wafers were then cleaned with CH C1 ( 3 x 4 mL), dried in a stream of N gas, and placed 2

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309 separately in individual round-bottom flasks. In a separate Schlenk flask was placed a quantity of volatile NBE monomer (-0.032 g) and this flask was connected to the wafer-containing flask using a vacuum transfer manifold. The assembly was removed from the glovebox and connected to a Schlenk line. After three cycles of free-pump-thaw, the NBE monomer was vacuumtransferred over to the round-bottom flask containing the catalyst-immobilized Si(l 11) wafer to form polymer brushes on its surface. The round-bottom flask was kept at -78°C in an acetone-dry ice bath during the transfer, which took about 15 minutes. After the NBE monomer solid has all been transferred, the transfer valve was shut off, the cooling bath was removed, and the assembly was allowed to warm up to room temperature over the next hour. The wafer was then removed from the flask and cleaned using the same purification steps as described for method 1.

Method 3 - In-situ Copolymerization In a N -filled glovebox, oxidized S i ( l l l ) wafer was treated with either NBE-SiCl or NBE-Si(OEt) monomer and rinsed twice each with toluene and CH C1 , as described in method 1. In a separate flask, a CH C1 solution (1 mL) of Grubbs catalyst (0.0034 mmol) was injected into a CH C1 solution (2 mL) of the desired norbornene monomer (0.34 mmol) to initiate polymerization. The NBE-fiinctionalized S i ( l l l ) wafer was then quickly immersed in this polymerizing solution. After 30 min, a few drops of ethyl vinyl ether were then added to terminate the polymerization. Before analyses, the wafer was cleaned using the same purification steps as described for method 1. 2

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Method 4 - Post-polymerization Attachment In a N -filled glovebox, a CH C1 solution (1 mL) of Grubbs catalyst (0.0034 mmol) was injected into a 1:1 mixture of NBE-OH and NBE-Si(OEt) (0.34 mmol total) in CH C1 (2 mL) to prepare random copolymers. After 30 min, a few drops of ethyl vinyl ether were added to the reaction to terminate the polymerization. An oxidized S i ( l l l ) wafer was then placed in this polymer solution overnight. Before analyses, the wafer was cleaned using the same purification steps as described for method 1. 2

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ROMP on Silica Nanoparticles Silica (Si0 ) nanoparticles of -110 nm in diameter were prepared by the conventional Stobber method (33), and functionalized with NBE-Si(OEt) overnight using a modified literature procedure (34,35). In essence, freshly 2

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prepared Si0 nanoparticles in ethanol solution (100 mL, -7 x 10 particles/mL) is treated with NBE-Si(OEt) (-50 |iL, 0.28 mmol) overnight (with vigorous stirring) and then purified by centrifugation (Eppendorf centrifuge 5804R, 15 amp version, 5000 rpm for 30 minutes) to remove unreacted monomers, small Si0 nanoparticles, and any impurities. The resulting solution of NBE-Si(OEt) -fimctionalized Si0 nanoparticles were then treated with sodium sulfate for 15 minutes and then with activated 3A molecular sieves overnight to remove any excess water before diluting in dry CH C1 (1:40 v/v). Subsequent polymerization of norbornene-type monomers (NBE-COOH or NBE-OH) on Si0 nanoparticles was carried out following the same procedures described above for methods 1 and 3. The diluted solution of the nanoparticles was mixed with NBE-COOH (or NBE-OH) monomer (0.34 mmol) and exposed to Grubbs catalyst (3.4 |umol) to initiate polymerization. The final solution was quenched with excess ethyl vinyl ether (-0.5 mL) after 1 h, concentrated to -5 mL on a rotary evaporator, and combined with enough ethanol to make a 40-mL solution. The resulting colloidal solution was subjected to centrifugation (5000 rpm for 30 minutes) and the mother liquor was then decanted. To the settled nanoparticles were added ethanol (- 40 mL) and the resulting mixture was sonicated for 2 minutes in a laboratory sonicator before being centrifiigated one final time. After this final step, the particles were redispersed in ethanol (-10 mL) using sonication (-2 minutes). 2

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Results and Discussions Polymer Brushes on Oxidized Si(lll) Wafers We began by examining a variety of ROMP-based approaches for growing polymers of norbornene-derivatives from oxidized Si(l 11) surfaces. We aimed to find the best ROMP-based method(s) that would allow us to prepare functional polymer-inorganic composites of Si0 nanoparticles. Figures 3 and 4 show representative SEM (scanning electron microscopy, Hitachi S-4500) and A F M (atomic force microscopy, Autoprobe CP Research) images for ROMPbased polymer films grown on oxidized S i ( l l l ) surfaces using four different methods: conventional SI-ROMP, solventless SI-ROMP, in-situ copolymerization, and post-polymerization attachment. In solutions of NBE-OH monomer with similar concentrations (-100 mM), all four of the aforementioned methods lead to polymer films on the surface of the oxidized S i ( l l l ) wafers; however these films have very different roughnesses. While methods 1 and 2 lead to the formation of smooth, thin polymer overlayers, methods 3 and 4 generated thick polymer films with rough surfaces (Table I, cf root-mean-square (rms) roughness). SEM images of the wafers prepared by methods 1 and 2 (Figures 3a-b, both side and top views) 2


311 show thin, relatively flat polymer films covering the entire S i ( l l l ) substrate. This is consistent with the corresponding A F M images (Figures 4a-b) where the top-down view shows surfaces that are uniformly covered with short brushes except for a few sporadic long spikes "sticking up" over the landscape. Presumably, this morphology is a consequence of the inherent high strain in our initiating moieties (NBE-SiCl or NBE-Si(OEt) ), which made these cyclic olefins more susceptible to crosslinking side reactions comparing to terminalolefin initiating groups (Figure 1) (36). In addition, the height of the observed short polymer brushes is inversely affected by the surface concentration of the initiating moieties: the higher this density, the more likely polymer growth will be terminated via crosslinking/back-biting reactions (Figure 1). This latter characteristic is one of the main drawbacks in conventional SI-ROMP procedures when the catalyst is reactive enough to metathesize both strained and non-strained C=C bonds: the growth of ROMP-based polymer brushes on surfaces is usually self-limiting (36). Previous studies from several research groups have suggested that the thickness of the polymer films in SI-ROMP can be controlled by adjusting the surface density and distribution of the initiating olefin moieties and by increasing the concentrations of monomer in solution (32,36,37). However, polymer growth often stops within 1 h due to either catalyst deactivation or backbiting reaction (Figure 1) (32,36,38). Consistent with these limitations, in our hands, methods 1 and 2 were less likely to produce long polymer brushes, presumably due to crosslinking between adjacent polymer chains within a polymer brush. Our attempts to evaluate the likelihood of these crosslinking events through measuring the surface density of the Ru-initiated monomers were unsuccessful due to overlaps of the Ru3d peaks with the CI s signals in the XPS spectra. As a strategy, method 2 is rather limited: it requires a volatile monomer that can be vacuum-transferred onto a surface that has been modified with Grubbs catalyst. Presumably, the polymerization occurs as the monomer is deposited on the surface of the wafer. This method consistently give very thin polymer film (< 10 nm) on the substrate, regardless of the amount of deposited monomers (from 30 mg to 500 mg of NBE for an oxidized Si(l 11) wafer that is - 1.5 cm in area). This data again supports a self-limiting polymer growth mechanism. Methods 3 and 4 generated rough and thick polymer films as expected of a predominantly solution polymerization process where the catalyst is not bound to the surface and can access more monomer molecules in solution. In method 3, the catalyst in solution can incorporate surface-bound monomers into a growing polymer chain, giving polymer films that are thicker than those prepared by methods 1 and 2 but whose thicknesses vary significantly over the coverage area (Figures 3c and 4c). This variation is a consequence of the great discrepancy between solution polymer growth (fast) and surface incorporation (much slower) kinetics: as time progresses, the lengths of the most recently bound polymer chains will be much longer than those that become surface-bound at earlier


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Figure 3. SEM images ofpolymer films on oxidized Si(lll) wafers, (a-c) Side (left) and top (right) views of wafers that were subjected to methods 1-3, respectively, (d) Side view ofa wafer that was subjected to method 4. (e) Side view of a bare oxidized Si(lll) wafer.



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Figure 3. Continued.

Table I. Thicknesses and RMS Roughness of Polymer Brushes on SiWafers, Measured by Ellipsometry and AFM Type offilm

Cone.

Monomer 1 vol%

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2.5 ± 0.3 (NBE-Si(OEt) ) or 3.0 ± 0.3 (NBE-SiCl )

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5.8 ± 0.6 (0.55)

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8.2 ± 0.8

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8.2 ±2.7 (1.1)

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21.5 ±6.7 (9.2)

48.8 (25.5)

20.3 ± 6.9

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9.9 ± 2.5

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rms = root-mean-square roughness, values are indicated in parentheses.



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Figure 4. AFM images ofpolymer films on oxidized Si(lll) wafers, a) Poly(NBE-OH)frommethod 1. b) Poly(NBE)frommethod 2. c) Poly(NBE-OH) from method 3. d) Poly[NBE-OH-r-NBE-Si(OEt) ] from method 4.


316 times. The result is a rougher (Table I) but thicker polymer film comparing to those obtained with methods 1 and 2. As a post-polymerization attachment strategy, method 4 requires ROMPbased polymers containing functional groups that can form covalent bonds with the surface of oxidized Si(l 11) wafers. Overnight treatment of such a wafer with a solution of a random copolymer of NBE-OH and NBE-Si(OEt) (1:1 molar ratio) results in a very rough polymer film (Table I) as observed by A F M (Figure 4d) and SEM (Figure 3d). Presumably, the polymer attachment took place between the surface hydroxyl groups of the oxidized S i ( l l l ) wafer and the triethoxysilyl groups of the polymer. As this "exchange" reaction is fairly slow, crosslinking of the free (i.e., not surface-bound) Si(OEt) groups between adjacent side chains or neighboring polymer chains can also occur at the same time. Both of these processes can result in irregular and thick films possessing clumpy and rough morphologies. The thickness of each organic/polymer overlayer and its uniformity, as measured by root-mean-square (rms) roughness, was determined using ellipsometry and A F M line profiling (Table I). All fiinctionalized wafers were found to have increased thickness compared to a bare wafer (0.6 ± 0 . 6 nm), signifying the presence of an overlayer film. For wafers that were fiinctionalized with NBE-Si(OEt) and NBE-SiCl monolayers, as well as those that were coated with thin polymer overlayers (methods 1 and 2), their film-coated surfaces are smooth and flat with low rms values that were consistent with SEM images. In contrast, wafers that were treated with methods 3 and 4 have thicker and rougher overlayers as indicated by their higher rms values. (To confirm the covalent growth of polymer overlayers in methods 1-3, oxidized but unfonctionalized S i ( l l l ) substrates (i.e., not treated with the initiating NBESi(OEt) and (NBE-SiCl ) monomers) were treated with solutions containing free monomers and Grubbs catalyst under the experimental conditions described for each method. Negligible thickness gains were observed after extensive cleaning to remove physisorbed materials.) Figure 5 displays the attenuated total reflection (ATR) IR spectra (Thermo Nicolet, Nexus 870) of the poly(NBE-OH) films on oxidized S i ( l l l ) wafers prepared by methods 1-4. Both thin (20 nm in thickness, methods 3 and 4) polymer films show very strong v and v C-H stretches at 2927 and 2858 cm" fromthe norbornene polymer backbones, while the unfunctionalized oxidized Si(l 11) wafer does not exhibit any distinctive peaks in the aliphatic C-H region after the background correction. ' All spectra of poly(NBE-OH)-coated Si(l 11) wafers are similar, suggesting formation of chemically analogous polymer overlayers. The representative X-ray photoelectron spectroscopy (XPS, Omicron, ESCA Probe) survey scans of the poly(NBE-OH)-coated wafers are shown in Figure 6. The spectrum of oxidized S i ( l l l ) wafer exhibits core-level peaks characteristic of Si/Si0 : Si2s at 151 eV, Si2p at 99 eV, and Ols at 532 eV, as 3

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well as the CIs adventitious peak at 285 eV. ' As polymer overlayers were grown on the surface using methods 1 and 2, the CIs peak intensity increased relative to those for the Si2s and Si2p peaks, consistent with the presence of carbon-rich films on the Si(l 11) substrate. The core-level peaks in the Si2p and Si2s regions almost completely disappeared for samples prepared by methods 3 and 4 where the polymer films are over 20-nm thick, as would be expected for substrates being covered with a thick insulating organic overlayer. Thus, surface growth of poly(NBE-OH) on oxidized S i ( l l l ) surface was evidenced by the attenuation of the underlying Si signals and the increase in intensity for the Cls signal in the XPS survey scans.

Polymer Overlayers on S i 0 Nanoparticles 2

As methods 1 and 3 allow us to grow polymer overlayers with controlled thickness and relative uniformity on oxidized silicon substrates, we extend them to the construction of discrete polymer-coated Si0 nanoparticles having low polydispersity and good stability. The major advantages of using Si0 nanoparticles as core materials are: their ready availability, low polydispersities (e.g., less than 5%), tunable sizes, and easy surface modification chemistry with a variety of organic molecules. We employed NBE-COOH and NBE-OH as monomers to demonstrate the preparation of polymer overlayers having chemical functionalities that can be easily manipulated. Figure 7 shows the SEM images of [NBE-Si(OEt) ]-fimctionalized Si0 nanoparticles as well as Si0 nanoparticles possessing poly(NBE-OH) and poly(NBE-COOH) overlayers prepared from methods 1 and 3. While method 1 yielded aggregations of particles (Figure 7b), method 3 produced discrete polymer-coated Si0 nanoparticles (Figures 7c and 7d), surprisingly. We suspect that the aggregated particles prepared by method 1 arise from the interand intraparticle cross-linking process during the centrifugation step used after catalyst treatment to separate the free catalyst molecules away form surfacecaptured catalysts. Such centrifugation can decrease the average particleparticle distance and cause interparticle crosslinking via reactions between the surface-capture catalyst groups on one particle and unreacted norbornene groups on the neighboring particles. Interestingly, the particle diameters obtained from transmission electron microcopy (TEM) analysis were -30% smaller than those obtained by SEM, possibly due to the desolvation of nanoparticles under the high vacuum and intense electron beam in TEM. 2

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Conclusion While norbornene-based ROMP polymers can be covalently attached to oxidized S i ( l l l ) surfaces using the four methods described in this manuscript,


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Figure 5. ATR-IR spectra ofpoly(NBE-OH) films on oxidizedSi(lll) wafers as well as a spectrum of the bare wafer. The sample with monolayer coating is obtained with NBE-SiCl . The sample with thin polymer coating is obtained through method 1. The sample with thick polymer coating is obtained through method 3. 3

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Figure 6. XPS survey scans ofpoly(NBE-OH)filmson oxidized Si(lll) wafers as well as a spectrum of the bare wafer. The sample with monolayer coating is obtained with NBE-SiCl . The sample with thin polymer coating is obtained through method 1. The sample with thick polymer coating is obtained through method 3. 3



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Figure 7. SEM images ofpolymer-coated Si0 nanoparticles. (a) NBE-Si(OEt) -functionalized. (b) Poly(NBE-COOH)-coated using method I. (c) poly(NBE-OH)-coated using method 3. (d) Poly(NBE-COOH)-coated using method 3. 2

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the resulting polymer films possessing very different thickness and morphology. If thin polymer films between 5-20 nm are desirable, SI-ROMP techniques are the most appropriate. However, in-situ polymerization seems to be the best strategy for achieving thicker, but still relatively uniform, polymer films on oxidized Si(l 11) surfaces as well as discrete polymer-coated S i 0 nanoparticles. Post-polymerization surface attachment through alkoxysilane sidechains is perhaps the least controlled method in our hands, giving thick and highly irregular coatings. Detail studies of polymer films with tunable thicknesses on oxidized S i ( l l l ) surfaces and Si0 nanoparticles using method 3 are currently underway. 2

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Acknowledgments Financial support by the NSF (DMR-0094347, DMR-0520513, ECS0506802, and EEC-0647560) is appreciated. We acknowledge Mr. Mark Anderson for assistance with the ellipsometric measurements. The SEM, TEM,


320 and XPS work were performed in the EPIC and Keck-II facilities of the NUANCE Center at Northwestern University (supported by NSF-NSEC, NSFMRSEC, Keck Foundation, the state of Illinois, and Northwestern University).

References 1. 2.


3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

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