ARTICLE pubs.acs.org/JPCC
Si(111) Surface Engineered with Ordered Nanostructures by an Atom Transfer Radical Polymerization Placido Mineo,†,‡ Alessandro Motta,† Fabio Lupo,† Lucio Renna,§ and Antonino Gulino*,† †
Dipartimento di Scienze Chimiche, Universita di Catania and I.N.STM UdR of Catania, Viale Andrea Doria 6, 95125 Catania, Italy Istituto per i Processi Chimico Fisici-CNR, Viale Ferdinando Stagno D0 Alcontres, 37-98158 Messina, Italy § STMicroelectronics, Stradale Primosole 50, 95100 Catania, CT, Italy ‡
ABSTRACT: Si(111) substrates were functionalized with a covalent 4-ClCH2C6H4SiCl3 monolayer that binds to the surface using the SiCl3 group and leaves an unreacted CH2Cl group. The remaining benzyl chloride functionality at the top of the Si(111) substrate allowed additional functionalization by an atom transfer radical polymerization process, mediated by copper complexes. Ordered, surface-confined polystyrene assemblies, covalently bound to the silicon, have been obtained. Atomic force microscopy measurements show a long-range order of these nanostructures. X-ray photoelectron spectroscopy provided quantitative results on the surface atomic composition as well as on the nanostructure thickness. The polystyrene structures on the silicon surface were also investigated by attenuated total reflectance infrared spectroscopy.
’ INTRODUCTION Engineering of inorganic surfaces can be the answer to the fabrication of hybrid organic/inorganic nanostructures and represents an advanced method for the production of magnetic, electronic, and photonic devices by a bottom-up approach.13 In the field of silicon surface modification by polymer deposition, covalently assembled polymeric films that grow perpendicular to the silicon surface offer significant advantages.48 In fact, the chain length and the density of the polymer determine the film thickness and the concentration of functional groups, respectively. Recent excellent examples include the functionalization of polymer brush pendant groups with azide-derived fluorescent dyes by Popik and Locklin,9 the formation of layered structures using copper-catalyzed click chemistry by Dinolfo,10 the grafting of acrylic polymers from flat nickel and copper surfaces by surface-initiated atom transfer radical polymerization by Zhu,11 the formation of polymer brushes using a nickel-catalyzed reaction by Senkovskyy and Kiriy,12 and the use of benzyl chloride surfaces for the formation of patterned surfaces by Dressick.1318 However, challenges remain in controlling the reaction rate at the solid state and, as a consequence, in the formation of the resulting nanostructures. For high repeatable processes that can be up-scaled, the control of all synthetic parameters and reaction pathways is fundamental. This, in turn, will allow controlling the growth location and shape of nanostructures. Here, we report an approach for fabricating nanoscale organic structures on silicon surfaces by a covalent-assembly procedure combined with an atom transfer radical polymerization reaction. We demonstrate the growth of molecular polystyrene structures r 2011 American Chemical Society
on the Si(111) surface. The crystalline silicon substrate determines a long-range order of these structures. The reaction parameters of the synthetic process have been optimized.
’ EXPERIMENTAL DETAILS Si(111) substrates, 2 1 cm, obtained from SILTRONIX (France), were first cleaned with “piranha” solution (concd H2SO4/35% H2O2, 70:30 v/v) at room temperature for 10 min, rinsed in double-distilled water for 4 min, etched in 2.5% hydrofluoric acid for 100 s, washed with double-distilled water, and accurately dried with prepurified N2. Both piranha and hydrofluoric acid solutions need to be handled with caution. Subsequently, they were treated for 5 min with ozone using the Ozon-Generator (Fisher 500) system in order to obtain a SiO2 thin (10 Å) layer.19 Freshly cleaned substrates were transferred in a glovebox under a N2 atmosphere and immersed, at room temperature for 20 min, in a 0.5:100 (v/v) n-pentane solution of the trichloro[4-(chloromethyl)phenyl]silane to afford a monolayer of this coupling agent (CA).16,20 The chlorobenzyl-functionalized substrates were then washed and sonicated in npentane for 10 min to remove any physisorbed CA, dried at 120 °C for 3 min, loaded into glass pressure vessels under N2, immersed in freshly prepared solutions containing 6 mL of dry styrene, 0.5 mL of N-methylpyrrolidone, 1.162 mg of CuCl, and 1.79 mg of bipyridyl, and heated for 0.5, 1, 2, 4.5, 17, 24, and 41 h, respectively. We realized that, at reaction temperatures above Received: March 3, 2011 Revised: April 11, 2011 Published: June 07, 2011 12293
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Scheme 1. Proposed Synthesis Pathway for Nanoscale Polystyrene Structures on Si(111), by a Covalent-Assembly Procedure Combined with an Atom Transfer Radical Polymerization Reaction, Mediated by a Copper Complex
100 °C, self-polymerization occurs in pure styrene. Therefore, the reaction temperature was fixed at 90 °C, while excluding light. The bipyridyl chelating ligand maintains the CuCl in solution and also favors the redox process (vide infra). The N-methylpyrrolidone improves the dissolution of the copper salt in styrene. Finally, the functionalized substrates bearing the covalently self-assembled polystyrene structures (SA-PSS) were left to cool to room temperature and repeatedly washed and sonicated with N-methylpyrrolidone, dichloromethane, and toluene to remove any residual physisorbed material. The films strongly adhere to the substrates because they cannot be removed by abrasion with solvent-wetted Kimwipes and were stable for more than 9 months, as evidenced by ATR-FTIR spectroscopy, thus demonstrating that these films are covalently bound to the substrate surfaces. Infrared attenuated reflectance spectra of the monolayers were recorded using a Jasco FT/IR-430 spectrometer, equipped with a Harrick GATR germanium single-reflection ATR (attenuated total reflectance) accessory. One hundred scans (scan range, 4004000 cm1; resolution, 4 cm1) per spectrum were collected. IR (ν/cm) of the SA-PSS: 1450 (s), 1490 (s), 1540 (w), 1596 (w), 2846 (m), 2921 (m), 3024 (m), 3056 (w), 3080 (w).
The surface morphology studies were carried out by atomic force microscopy (AFM), and the images were obtained by an instrument manufactured by the NT-MTD. The noise level before and after each measurement was 0.01 nm. AFM characterizations were performed in a high-amplitude mode (tapping mode) to avoid any possible modification of the grafted organic layer on the surfaces, caused by the interactions with the tip whose nominal curvature radius is 10 nm. X-ray photoelectron spectra (XPS) were measured at 45° relative to the surface plane with a PHI 5600 Multi Technique System, which offers a good control of the photoelectron takeoff angle (base pressure of the main chamber = 2 1010 Torr).2124 Samples were excited with a monochromatized AlKR X-ray radiation using a pass energy of 5.85 eV. No relevant charging effect was observed. The XPS peak intensities were obtained after Shirley background removal.23 The instrumental energy resolution was e0.3 eV. Quantum mechanical DFT calculations were performed at the level of the PBE formalism,25 assuming either isotactic or sindiotactic oligostyrene configurations, to optimize the grafting geometry and to estimate the number of styrene units in a single chain that could reproduce the surface structure mean size, 12294
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Figure 1. ATR-FTIR spectra for representative SA-PSS samples: 1 h (green line), 17 h (red line), 24 h (black line). The spectra of the 1 and 17 h reacted samples are shown in the 35002700 cm1 region (inset). The spectrum of the 24 h reacted sample is in the 35001280 cm1 region and also in the inset. The background has been subtracted from all the spectra. The y axis represents the FTIR-ATR intensity in arbitrary units.
observed with AFM for 1 h reacted samples. The standard allelectron 3-21G basis was used for all atoms.26 Molecular geometry optimization of stationary points used analytical gradient techniques, and calculations were performed using G03 codes.27 Thickness measurements of the SA-PSS were performed using a KLA Tencor P15 profilometer. The experimental uncertainty was about 1 nm. For all samples, scratches of 40 μm were performed on the sample surfaces by application of a controlled force (2.5 newton) over a metal tip. In such conditions, no measurable scratch was detected on a clean silicon surface.
’ RESULTS AND DISCUSSION The SA-PSS were synthesized by an optimized procedure consisting of the covalent grafting of the styrene units to Si(111) substrates that were previously cleaned and silylated (Scheme 1). The silylation reaction was performed under a rigorously inert atmosphere with trichloro[4-(chloromethyl)phenyl]silane, a bifunctional coupling agent that binds both to the substrate and to the styrene. The remaining benzyl chloride functionality at the top of the Si(111) substrates prompted us to induce an atom transfer radical polymerization (ATRP) reaction mediated by a copper complex, as previously reported by Matyjaszewski.4,28 Scheme 1 represents the most probable mechanism.4,28 With this procedure, the rate of chain growth is regulated by the redox reaction between the benzyl chloride group and the Cuþ catalyst.4,28 This produces the copper oxidation to Cu2þ, concomitant reduction of the Cl• radical, due to homolytic fragmentation of the CCl bond, to Cl anion and formation of the surface-bound benzyl radical. Next, the benzyl radical reacts with the styrene monomer to form the surface-bound styrene radical. Afterward, the terminal chloride functionality of the propagating group is restored by reaction of the propagating surface-bound styrene radical with the Cu2þ catalyst that returns the starting Cuþ catalyst and produces the surface-bound styrene chloride. With the cycling of this reaction pathway, polystyrene structures
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grow up covalently bound to the Si(111) surface. The obtained structures strongly adhere to the silicon substrate because it is not possible to remove them with prolonged sonication while heating with organic solvents nor using the Scotch tape decohesion test.29 ATR-FTIR measurements provide information useful to monitor the surface nanostructures. Two spectral regions are diagnostic, namely, that of CH stretching between 2800 and 2900 cm1 and that of the aromatic region around 3050 cm1. Figure 1 shows representative ATR-FTIR spectra for the SA-PSS obtained after 1, 17, and 24 h reaction. For the clarity of the figure, spectra of the 1 and 17 h reacted samples are shown only in the 35002700 cm1 region that contains all the important polystyrene features. In all cases, the main bands at 2921 and 2846 cm1 are assigned to the νa(CH2) and νs(CH2) stretching modes, respectively, of the styrene backbone. Bands at 3024, 3056, and 3080 cm1 are due to the aromatic rings. The whole spectra are coincident with that for polystyrene, thus inferring the presence of polystyrene on the Si(100) substrate after sonication with N-methylpyrrolidone and other solvents. Spectra at different reaction times show interesting increasing polystyrene signal intensities. Surface morphology studies were carried out by atomic force microscopy (AFM). Figure 2 (left) shows representative AFM micrographs for a bare Si(111) and two functionalized substrates reacted 30 min and 1 h with the styrene mixture, respectively. As expected, AFM of the bare Si(111) substrate (Figure 2a) shows a flat surface with a mean roughness (Figure 2a, right) of about 12 Å. Figure 2b,c shows the 30 min and 1 h reacted substrates. Inspection of Figure 2b reveals that some incipient linear nanostructures nucleated and grew up on the substrate surface. The cross-sectional analysis (Figure 2b, right) shows structure heights in the 12 nm range and widths of about 90 nm. The micrograph for a representative 1 h reacted sample (Figure 2c) shows a relevant number of structures having a linear arrangement, superimposed to the surface. The observed nanostructures (Figure 2c, right) have a 28 Å average height. These surface structures evidence a long-range order. Moreover, all the observed structures are almost parallel to each other, thus showing the same direction and resulting in well-defined polymer nanostructures. This observation implies that the nanostructure orientation follows the direction of the rows of the Si atoms in the crystalline Si(111) surface. Because no discrete bonds are expected between the polystyrene single chains, it could be likely that vicinal polystyrene units grafted on the same Si atom row can interact to partially align the phenyl rings and allow ππ styrene interactions. This potential interaction leads to overlap between the electronic states of the individual styrene units. Concerning the molecular spacing of these linear structures as well as the nanostructure growth mechanism, AFM cannot give information at atomic or molecular levels because the curvature radius of the AFM tip is about 10 nm. Nevertheless, the different lengths of these structures should be due to two main reasons: (i) defects on the siloxane monolayer used to functionalize the Si(111) substrate and (ii) the interruption of the radical growth because of coupling reactions of vicinal radicals. Comparison of panels b and c in Figure 2 suggests that the width of these nanostructures depends on the reaction time. In fact, the 1 h reacted samples show nanostructures having an ∼ 150 nm width (Figure 2c), whereas in the 30 min reacted samples, this width is ∼90 nm (Figure 2b). This width increase is mainly due to interchain van der Waals and/or partial ππ styrene interactions that cause, for 12295
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Figure 2. AFM images (left) for the representative (a) bare Si(111) substrate, (b) SA-PSS after 30 min reaction, and (c) SA-PSS after 1 h reaction; their related cross-sectional profiles are on the right.
longer reaction times, these nanostructures to coalesce into a uniform polystyrene layer, thus rendering this order less/or not evident. Taking into account all this and the well-known crosslinking ability of the 4-ClCH2C6H4SiCl3 molecules during the surface grafting,20,32 it seems that the siloxane-functionalized Si(111) template monolayer maintains part of the starting order of the silicon surface and induces this order also to the polystyrene nanostructures. These results as a whole are clear indication of an atom transfer radical polymerization reaction mediated by a copper complex that allows the growth of polystyrene nanostructures on Si(111). Similar ππ styrene interactions have been supposed by an STM study performed on Si(100) that had been exposed to 200 langmuir of styrene, thus producing some styrene lines of about 100 Å.30,31 In that case, the styrene lines were formed by a chain reaction initiated by
Figure 3. Thickness measurements of the SA-PSS samples at different reaction times. The R2 value of the fit is 0.9934. 12296
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Figure 4. Monochromatized AlKR XPS for a representative SA-PSS (1 h) in the C 1s binding energy region. Figure 6. Optimized grafting geometry of the sindiotactic oligostyrene configuration.
Figure 5. Monochromatized AlKR XPS for a representative SA-PSS (1 h) in the Si 2p binding energy region.
dangling bonds on Si(100). The radical was formed at the perfect distance to the neighboring SiH-terminated bond, thus allowing the chain reaction to occur along the Si(100) lines. In our case, similar ππ styrene interactions probably occur along the Si(111) lines and a longer-range order is evident with lines of polystyrene structures in the order of micrometers. Longer reaction times increased the structure density on the surface (2 h reacted samples show a uniform surface coverage) and made the order less evident. The 1 h reacted sample represents the best in order to evidence the structure growth. Thickness measurements were performed using a profilometer. Five different samples have been analyzed for each reaction time, and for each sample, four different area portions were measured. Figure 3 shows the thickness dependence of the SA-PSS samples upon the reaction time. The observed linear dependence is an indication of the atom transfer radical polymerization mechanism. The chemical characterization of the 1 h reacted SA-PSS was carried out with X-ray photoelectron spectroscopy.19,2123 The C 1s band centered at 285.0 eV is due to both aliphatic and aromatic backbones (Figure 4).2124 The presence of weak πfπ* shakeup satellites centered at 293.4 eV, about 8 eV to higher binding energy with respect to the main peak, confirms the presence of aromatic carbon.2123
The band at 286.6 eV, in tune with literature data, is due to the carbon of the unreacted benzyl chloride moiety of the ClCH2C6H4 fragment and to the carbon of the polymer styreneCl endgroups.21 Figure 5 shows the XPS spectrum for a representative SA-PSS (1 h) in the Si 2p binding energy region. The peak at 99.0 eV, with a shoulder at 99.6 eV, accounts for the Si 2p spinorbit components of the silicon. The peak at 103.0 eV is due to the Si 2p signal of the SiO2 thin layer, obtained after 5 min of treatment with ozone (vide supra). The Cl 2p3/2,1/2 spinorbit doublet lies at 199.0 and 200.9 eV.19,2123 No XPS evidence of the presence of copper was found using the adopted reaction stoichiometry. Quantum mechanical DFT calculations2527 (Figure 6) indicate 15 styrene units (containing 127 carbon atoms, including the trichloro[4-(chloromethyl)phenyl]silane moiety) (16 units in the isotactic configuration) per chain needed to reproduce the AFM structures of the 1 h reacted SA-PSS, having a 28 Å average height. From Scheme 1, it emerges that there is always a chlorine atom at the top of the polystyrene surface structures. XPS atomic composition analysis, for 1 h reacted samples, shows a Cl/C ratio of 1/18. The Cl/C ratio for surface areas not covered by polystyrene is 1/7, due to the coupling agent. Therefore, it is possible to estimate a statistical value for the polymer surface coverage (χ) using the simple model: [(1/127)χ] þ (1/7) (1 χ) = 1/18. On the basis of these assumptions, a 65% surface coverage is obtained for the 1 h reacted samples.
’ CONCLUSION In conclusion, the functionalization of the Si(111) surface with a covalent 4-ClCH2C6H4SiCl3 monolayer that leaves a benzyl chloride functionality at the top of the Si(111) substrate allowed additional generation of ordered polystyrene assemblies, covalently bound to the silicon surfaces by means of an atom transfer radical polymerization procedure, mediated by a copper catalyst. It seems that the functionalized Si(111) template layer induces this order because of the well-known cross-linking ability of 4-ClCH2C6H4SiCl3 molecules upon surface grafting.20,32 The whole procedure was optimized by changing the reaction conditions, and those reported (reaction time, temperature, and adopted stoichiometry) in the present study represent the best to 12297
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’ AUTHOR INFORMATION Corresponding Author
*Tel: þ39-095-7385067. Fax: þ39-095-580138. E-mail: agulino@ unict.it.
’ ACKNOWLEDGMENT The authors thank the FIRB project ITALNANONET (RBPR05JH2P) and the CINECA (award No. HP10B0R1E4, 2010) for the availability of high-performance computing resources and support. ’ REFERENCES (1) Mannini, M.; Pineider, F.; Danieli, C.; Totti, F.; Sorace, L.; Sainctavit, P.; Arrio, M. A.; Otero, E.; Joly, L.; Cezar, J. C.; Cornia, A.; Sessoli, R. Nature 2010, 468, 417–421. (2) Yang, W. L.; Fabbri, J. D.; Willey, T. M.; Lee, J. R. I.; Dahl, J. E.; Carlson, R. M. K.; Schreiner, P. R.; Fokin, A. A.; Tkachenko, B. A.; Fokina, N. A.; Meevasana, W.; Mannella, N.; Tanaka, K.; Zhou, X. J.; van Buuren, T.; Kelly, M. A.; Hussain, Z.; Melosh, N. A.; Shen, Z.-X. Science 2007, 316, 1460–1462. (3) Wachowiak, A.; Yamachika, R.; Khoo, K. H.; Wang, Y.; Grobis, M.; Lee, D.-H.; Louie, S. G.; Cromie, M. F. Science 2005, 310, 468–470. (4) Matyjaszewski, K.; Dong, H.; Jakubowski, W.; Pietrasik, J.; Kusumo, A. Langmuir 2007, 23, 4528–4531. (5) LeMieux, M. C.; McConney, M. E.; Lin, Y.-H.; Singamaneni, S.; Jiang, H.; Bunning, T. J.; Tsukruk, V. V. Nano Lett. 2006, 6, 730–734. (6) Xu, F. J.; Xu, D.; Kang, E. T.; Neoh, K. G. J. Mater. Chem. 2004, 14, 2674–2682. (7) Xu, F. J.; Cai, Q. J.; Kang, E. T.; Neoh, K. G. Langmuir 2005, 21, 3221–3225. (8) Juang, A.; Scherman, O. A.; Grubbs, R. H.; Lewis, N. S. Langmuir 2001, 17, 1321–1323. (9) Orski, S. V.; Poloukhtine, A. A.; Arumugam, S.; Mao, L.; Popik, V. V.; Locklin, J. J. Am. Chem. Soc. 2010, 132, 11024–11026. (10) Palomaki, P. K. B.; Dinolfo, P. H. Langmuir 2010, 26, 9677–9685. (11) Chen, R.; Zhu, S.; Maclaughlin, S. Langmuir 2008, 24, 6889–6896. (12) Senkovskyy, V.; Khanduyeva, N.; Komber, H.; Oertel, U.; Stamm, M.; Kuckling, D.; Kiriy, A. J. Am. Chem. Soc. 2007, 129, 6626–6632. (13) Malvadkar, N. A.; Hancock, M. J.; Sekeroglu, K.; Dressick, W. J.; Demirel, M. C. Nat. Mater. 2010, 9, 1023–1028. (14) Malvadkar, N. A.; Demirel, G.; Poss, M.; Javed, A.; Dressick, W. J.; Demirel, M. C. J. Phys. Chem. C 2010, 114, 10730–10738. (15) Malvadkar, N. M.; Sekeroglu, K.; Dressick, W. J.; Demirel, M. C. Langmuir 2010, 26, 4382–4391. (16) Brandow, S. L.; Chen, M.-S.; Dulcey, C. S.; Dressick, W. J. Langmuir 2008, 24, 3888–3896. (17) Chen, M.-S.; Dulcey, C. S.; Chrisey, L. A.; Dressick, W. J. Adv. Funct. Mater. 2006, 16, 774–783. (18) Chen, M.-S.; Brandow, S. L.; Schull, T. L.; Chrisey, D. B.; Dressick, W. J. Adv. Funct. Mater. 2005, 15, 1364–1375.
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