One-Step Dipping Method for Covalently Grafting ... - ACS Publications

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One-Step Dipping Method for Covalently Grafting Polymer Films onto a Si Surface from Aqueous Media Junhong Zhang,† Ming Li,*,† Wenqi Zhang,‡ and Liqiang Cao‡ †

State Key Laboratory of Metal Matrix Composites, School of Material Science and Engineering, Shanghai Jiao Tong University, No. 800 Dongchuan Road, Shanghai 200240, China ‡ National Center for Advanced Packaging Co., LTD., 200 Linghu Boulevard, Wuxi 214000, China S Supporting Information *

ABSTRACT: A facile and one-pot dipping method was proposed in this article for the first time to prepare vinylic polymer films on a silicon (Si) surface. This novel process was conducted in acidic aqueous media containing 4-nitrobenzene diazonium (NBD) tetrafluoroborate, hydrofluoric acid (HF), and vinylic monomers at room temperature in the open air and without any apparatus requirement. The formation of the polyvinyl film was confirmed by corroborating evidence from ellipsometry, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and atomic force microscope (AFM) analysis. The results revealed that both polymers of poorly water soluble methyl methacrylate (MMA) and water-soluble acrylic acid (AA) monomers were covalently grafted onto the Si surface via this simple process. The polyvinyl film was composed of polynitrophenyl (PNP) and polyvinyl, where PNP was doped into polyvinyl chains throughout the entire film. From a mechanistic point of view, the simple dipping method took advantage of the ability of the NBD cation to be spontaneously reduced at the Si surface at open circuit potential, providing aryl radicals. These radicals can be covalently bonded to the Si surface to form the PNP primer layer. Although the PNP sublayer was thinner and difficult to detect, it was necessary to graft polyvinyl chains. Furthermore, the aryl radicals were used to initiate the polymerization of vinylic monomers. The radical-terminated polyvinyl chains formed in the solution were then added to the aromatic rings of the primer layer to form the expected polyvinyl film. approaches. The surface is first modified with diazonium salts and then used as a macroinitiator to grow polymer chains through surface-initiated polymerization methods, such as atom-transfer radical polymerization (ATRP),14−18 ring opening,19 and click chemistry.20,21 In the resulting polymer coating, the aryl layers provide a covalent bond with the substrate and a covalent link with macromolecular species. Diazonium-based surface-initiated polymerization methods can be used with many monomers from several substrates, leading to a thicker polymer film. However, it is conducted in two steps, which makes industrial applications difficult. In recent years, one pot diazonium-based surface electroinitiated emulsion polymerization has been proposed to covalently bond compact and stable polymer films to electrodes.2,23,24 It relies on a radical initiation step by aryl radicals with the formation of a primary sublayer, followed by propagation and chain transfer on the aromatic rings of the primary sublayer. In most of the literature, electrochemistry is used to induce the formation of aryl radicals.25 Recently, some results concerning the formation of aryl radicals at some reducing

1. INTRODUCTION The organic modification of the silicon (Si) surface has become an attractive topic because of a wide range of potential application in the areas of microelectronics, biosensors, and molecular electronics.1−4 Therefore, it is of great importance to prepare organic polymer on a Si substrate. In recent years, the modification of the Si surface with the electrochemical reduction of diazonium salts has become attractive because it can be performed in aqueous media, which is more acceptable for industrial applications.5−8 In the best understood reaction, a diazonium salt with an electrolyte such as tetrabutylammonium cations is reduced by an externally applied potential, typically at around −1 V versus the saturated calomel electrode (SCE). The mechanism of the electrografting of diazonium salts has been extensively investigated in the literature. It involves a reductive electron transfer to the diazonium salt concerted with the cleavage of dinitrogen.9,10 The aryl radicals thus formed are covalently bonded to the electrode surface.11 Furthermore, aryl radicals formed in excess can react with already-grafted aryl groups to form a final polynitrophenyl (PNP) coating.12,13 It is a simple one-step, versatile way to graft organic coatings onto electrodes, but it delivers only a very thin film. Diazonium salts are also employed to bond macromolecules to the surface of a substrate by grafting from and grafting to © XXXX American Chemical Society

Received: May 20, 2016 Revised: July 20, 2016

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2.3. Grafting of a Polyvinyl Film. The solution for grafting a polyvinyl film onto the Si surface was prepared as follows. First, a small quantity of mineral acid (HCl) was added to a certain volume of ultrapure Milli-Q water to lower the pH to 2 (because NBD is stable in water only for pH 99%) and AA (>99%). The surfactant was sodium dodecyl sulfate (SDS, 98.5%), which was used to solubilize the non-watersoluble vinlic monomers and to improve the wettability of the Si substrate. Concentrated hydrochloric acid (HCl) and 40% HF were purchased as reagent grade. All reactants were used as received, and in particular, vinylic monomers were not distilled to remove commercial inhibitors. To avoid potential pollution, the fluorion was recycled by adding calcium hydroxide (Ca(OH)2, 80%) to the solution after grafting. Aqueous solutions were prepared with ultrapure Milli-Q water (Millipore Milli-Q water purification system). 2.2. Substrate Treatment. The Si substrate was cut into 1 × 2 cm2 pieces from 10 Ω·cm p-type Si(100) wafers. The substrate was then ultrasonically washed for 5 min in succession with acetone, ethanol, and water and dried in air. Hence, the Si surface was originally covered by oxide. B

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Figure 1. ATR-IR spectra of the original Si surface and the polyvinyl/ Si surface dipped in the solution containing corresponding vinylic monomers NBD and HF.

the PMMA film. For the PAA film, the spectrum exhibited major adsorption bands: 2980 and 2940 cm−1 attributed to the stretching vibration of CH3 and CH2 groups, respectively, 1450 cm−1 for the bending vibration of the CH2 group, 1726 cm−1 for the stretching vibration of the ester carbonyl group (C O), and a broad band at 3250 cm−1 attributed to the stretching vibration of the OH group. The PNP-like composition in the two polymer film can be observed through the NO2 adsorption bands near 1530 and 1350 cm−1 and the phenyl adsorption band near 1600 cm−1. Therefore, the ATR-IR results clearly revealed that the grafted polymer film was composed of PNP and polyvinyl. In addition, the notable absence of vibrational features related to the diazonium tetrafluoroborate moiety, such as the ν-(N ≡ N+) peak near 2280 cm−1 and the strong and broad ν(BF4) mode expected at around 1050 cm−1, suggested that diazonium salts was not merely adsorbed on the surface. The absence of the in-plane ring breathing ν(CC) modes at around 1525 and 1503 cm−1 also demonstrated that the organic film grafted onto the Si substrate was a polymer other than a vinylic monomer. XPS analysis was carried out for a PMMA film with 19.5 nm thickness to confirm the nature of the grafted film, as shown in Figure 2. The carbon, oxygen, and nitrogen peaks of the PMMA film were clearly observed on the survey spectrum. Because of the thinner film, the Si signal of the surface was still visible. The Si 2p core level centered at 100 eV was further divided into two peaks: the larger one at a lower binding energy was attributed to the Si−Si bond, and the smaller one at a higher binding energy may correspond to the Si−C bond, which can support the existence of a covalent bond between Si and the PMMA film. The C 1s core-level spectrum presented four peaks. The larger one at a lower binding energy (284.9 eV) corresponded to the alkyl and phenyl groups (CH3−CH2−, C6H5−). The carbonyl ester group (COO) appeared in the peak at 289 eV, and the small component at 287 eV was assigned to the ester −C−O− simple bond. This strongly confirmed the PMMA film. The binding energy component of the C 1s core-level spectrum at 286 eV indicated carbon carrying nitro groups (NO2). The N 1s core level gave two perfectly separated peaks centered at 399.7 and 406 eV. The peak at high binding energy corresponded to the nitro group (N−O) arising from PNP, and the peak centered at 399.7 eV was characteristic of an amino group (N−H) that may be due to the reduction of nitro groups. This suggested the presence of PNP moieties in the PMMA film. Therefore, these XPS results

Figure 2. Survey, C 1s core-level, and N 1s core-level XPS spectra of the PMMA film grafted on the Si surface.

revealed that the PMMA film was composed of PNP and PMMA, which was consistent with the ATR-IR results. Clearly, the absence of the BF4− counterion that composed part of the diazonium salt (typically appearing at about 686.5 eV) was evidenced in the survey XPS spectra. In addition, no N 1s signal for −N2+ (at approximately 403.8 eV) was observed, indicating that the diazonium moiety had been lost. This reflected the spontaneous reducing of diaznonium salt after dipping the Si substrate into the solution. The PAA film with 21 nm thickness was also characterized by XPS. Figure 3 displays the characteristic carbon and nitrogen peaks of the PAA film. The C 1s core-level spectrum presented three peaks. The smaller one at higher binding energy (289 eV) corresponds to the carboxyl group (COOH), alkyl groups (−CH2−, −CH3−) appeared in the peak centered at 284.9 eV, and the small component at 286 eV indicated carbon carrying nitro groups (NO2). The former two peaks reflected the grafting of PAA. The last small peak suggested the presence of PNP moieties in the PAA film. Regarding the nitrogen signal, the highest-energy peak at 406 eV was obviously attributed to the N 1s in nitro groups (NO2), and the component at 399.7 eV can be attributed to the amino group (NH2). This also confirmed the presence of PNP in the PAA film, which was similar to that of the PMMA film. In addition, the peak at 403.8 eV attributed to azo groups (−NN−) was not seen, which reflected the reduction of the diazonium salt. The surface morphology of the PMMA and PAA films was investigated by AFM. Figure 4 shows the AFM height images of PMMA and PAA films with the same thickness on different scales (1 × 1 μm2 and 500 × 500 nm2). The surface roughness of the PMMA film was 2.496 nm over 1 μm2 areas, but that of the PAA film was only 1.67 nm over 1 μm2 areas. The greater surface roughness of the PMMA film was probably due to the poor affinity between the solvent (water) and the PMMA chains. Because water was a bad solvent for PMMA, chains tended to aggregate, giving rise to “mushroom” structure, as shown in the AFM phase images of the PMMA film (Figure S1a,b). The AFM phase images of the PMMA film evidenced globular morphology. On the contrary, the images of PAA film C

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Figure 3. C 1s core-level and N 1s core-level XPS spectra of a PAA film grafted on a Si surface.

Figure 4. AFM height images in tapping mode for (a, b) the PMMA film and (c, d) the PAA film grafted onto the Si surface by the one-step dipping method.

and water-soluble monomers (AA) can be polymerized and grafted onto the Si surface to form a corresponding polyvinyl film by a one-step dipping method. The two polymer films were composed of PNP and polyvinyl, but the location of the nitrophenyl moieties was still unclear. To investigate the structure of the film, the PMMA film with different thicknesses taken as a reference film was further analyzed by XPS (Figure S2). Table 1 shows the chemical composition of the PMMA film with different thicknesses measured by XPS. Because XPS is sensitive only to the outer ∼10 nm of a coating,32 XPS analysis of the PMMA film with different thicknesses allowed a detailed study of the whole thickness of the film, from the upper part (PMMA superficial) to the inner part (interface area

looked smoother and more homogeneous (Figure S1c,d) because the AA monomers were totally water-soluble. In addition, the degree of polymerization of water-soluble vinylic monomer was higher than that of the non-water-soluble one, 23 and the grafted film with a higher degree of polymerization tended to grow quickly. Therefore, the thickness of the PAA film was larger than that of the PMMA film that was grafted at the same solution concentration (13.5 × 10−3 M SDS, 0.63 M vinylic monomer, 4.2 × 10−3 M NBD, and 3% HF) and reaction time (15 min). The thickness of the PAA film was 369 nm whereas that of the PMMA film was 284 nm. 3.2. Structure of a Polymer Film. We already demonstrated that poorly water soluble monomers (MMA) D

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film through the one-step dipping method was split into three steps: (i) the formation of nitrophenyl radicals; (ii) the initiation of the vinylic monomers radical polymerization in solution by nitrophenyl radicals (Scheme 1) and the formation

Table 1. Chemical Composition of the Si Surface after Grafting PMMA with Different Thicknesses atom % concentration PMMA thickness (nm)

CO/C

N

19.5 213 420

7.43 7.14 7.27

6.21 6.07 6.47

Scheme 1. Polymerization of Monomers Initiated by Nitrophenyl Radicals

between Si and the PMMA film). The 420 nm PMMA film corresponds to the top of the film, the 213 nm PMMA film corresponds to the middle part, and the 19.5 nm PMMA film corresponds to the inner part. It was clearly seen that the Celement component of CO changed little as the film thickness increased. In addition, the N atom % concentration also varied little with the increasing film thickness. These results revealed that a nitrophenyl-rich sublayer was not detected, and the chemical composition was homogeneous throughout the whole film. Therefore, the nitrophenyl moieties were incorporated into the PMMA chains throughout the whole film. To further investigate the film structure, three samples were compared by IR with a focus on the intensity of the phenyl group absorption bands, the signature of nitrophenyl moieties. Sample I was a PNP layer made from the solution of NBD and HF without MMA monomer for 15 min, and its thickness was 58 nm. Samples II and III were PMMA films made from a solution containing NBD, HF, and MMA monomers for 5 and 15 min, and their thicknesses were 61.8 and 284 nm, respectively. Therefore, sample II corresponds to the inner part of sample III. The FTIR spectra of the three surfaces are displayed in Figures 1, S3, and S4. The intensity of phenyl group IR adsorption bands in the FTIR spectrum is shown in Table 2. The intensity value of the phenyl group adsorption

Scheme 2. Grafting of Nitrophenyl Radicals and Polyvinyl Chains on the Si Surface

of the aryl primer layer (step I in Scheme 2); and (iii) interaction of the macroradical chains with the primer layer to form a grafted copolymer layer and the growth of the polymer film by the successive grafting of polyvinyl chains and nitrophenyl radicals (step II of Scheme 2). This mechanism was supported by the well-known mechanism for the thickening of PNP layer33,34 and the electrochemical grafting of a vinylic polymer.22−24 The aryl primer layer was necessary for the grafting of the polyvinyl chains, but it may be extremely thin, which caused it to be difficult to detect in our results. Therefore, the final polymer film was a homogeneous copolymer composed of PNP and polyvinyl, and PNP acted as a node or cross-linker between polyvinyl chains. This mechanism suggested that the grafting process can be applied to various vinylic monomers. In our experiment, MMA (very poorly soluble in water) and AA (totally miscible in water) monomers were selected to demonstrate that the water monomer solubility was not a restricting parameter. The second and third steps of the grafting process were purely chemical processes, so they were explainable by the electroless dipping method. However, the formation of the nitrophenyl radicals in the first step was an electrochemical process. The nitrophenyl radicals resulted from the reduction of NBD, so we must propose a mechanism for the spontaneous reduction of NBD in our experiment. It was tempting, especially when considering the Si surface dipped into HF solutions, to propose a cation exchange or charge injection mechanism for the nitrophenyl radicals’ formation. This might be qualitatively similar to the redox mechanism between the more reducing metal and the diazonium salt.27−29 The grafting is possible only if the surface can provide electrons for the reduction of the diazonium salt into an aryl radical. We thus proposed a scheme (Scheme 3) in which the diazonium salt was activated at the Si surface in the absence of an externally applied potential, quickly forming nitrophenyl radicals, followed

Table 2. Intensity of Phenyl Group IR Adsorption Bands in the PNP Film and PMMA Film on the Si Surface films

thickness (nm)

solution composition

νphenyl (1600 cm−1) (%)

15 min PNP 5 min PMMA 15 min PMMA

58 61.8 284

HF, NBD HF, NBD, MMA HF, NBD, MMA

0.858 0.081 0.911

band of sample III (0.911%) was similar to that of sample I (0.858%), which revealed that the two samples had the same content of PNP. However, the intensity value of the phenyl group adsorption band of sample II (0.081%) was 10 times lower than that of sample I (0.858%), which reflected that the nitrophenyl moieties were not localized in the inner part of the PMMA film. The IR results of three samples suggested that the nitrophenyl groups may be doped into PMMA chains throughout the entire film, which was consistent with the XPS results. 3.3. Proposed Mechanism. FTIR spectroscopy, XPS, AFM microscopy, and ellipometric thickness measurements all provided evidence that the PMMA film and PAA film can be successfully grafted onto the Si surface through a simple dipping method. In the polyvinyl film, the PNP moieties were doped into the polyvinyl chains. In this section, we aimed at describing one tentative mechanism for this spontaneous grafting process at the Si surface, which can fully explain our experimental results. Similar to the surface electroinitiated emulsion polymerization method, the grafting of the polyvinyl E

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Langmuir Scheme 3. Spontaneous Reduction of NBD at the Si Surface in HF Aqueous Solution

by surface attachment. First, the oxide originally covering the Si surface was removed in the solution with HF, and the hydrogenated Si surface was thus prepared. This was the most widely used cleaning technology for the Si surface.35−37 Then, HF would further attack the hydrogen-terminated Si surface. It involved the replacement of a surface hydrogen atom with F− in the HF solution. The replacement of H by F− required a hole, which resulted in a neutralized Si−F bond. Therefore, during the replacement process, one electron may transfer from the Si−Si bond to the NBD cation in the vicinity of the Si surface. Trucks and coauthors had proposed the transfer of one electron from the Si−Si bond to the hydrogen of the Si−H bond during HF etching of the Si surface.38 This can lead to the local generation of nitrophenyl radicals by the loss of N2. These radicals then attack the Si−H to attach to the surface of the Si substrate through covalent bonds, which would explain the appearance of the Si−C peak seen in the XPS spectra. The nitrophenyl radical can also initiate the polymerization of monomers in thickening the film (Schemes 1 and 2). Furthermore, the Si−SiF bond was broken by reacting with HF, and a silicon atom was dissolved in the solution with the removal of the fluorine-terminated surface silicon as SiF4, leaving a new Si−H surface. The electronegative Si−H bond would reduce more NBD cations in the HF solution. This redox reaction among Si, HF, and diaznoum salt and the formation of the Si−H surface was steadily recycled, so the thicker polymer film could be formed. Stewart and coauthors proposed an OCP-based mechanism for the spontaneous reduction of diazonium salts on the Si−H surface in contact with 0.5 mM diazonium salt in CH3CN.30 The spontaneous oxidization of Si−H may provide a complementary means of reduction of the NBD cation in our experiment. However, in our experiment, no PMMA or PNP signal was observed in the FTIR spectra (Figure S5) after the Si−H surface was dipped in the aqueous solution with NBD and MMA monomers but without HF. That may be attributed to the metastable Si−H surface. It likely reacted with oxygen or water in solution to produce oxides or hydroxides. When HF was added to the solution, a thicker polymer film could be prepared on the Si surface.31 It should be noted that the thickness of the film increased as the HF concentration in the solution increased, as shown in Figure 5. These results tended to indicate that the anodic HF etching of Si was of prime importance for the spontaneous reduction of the diazonium salt. In addition, the experiment was also carried out in the solution containing MMA monomers and HF but without NBD. No polymer was observed on the Si surface, as shown in

Figure 5. Thickness of the PMMA film on the Si substrate as a function of the volume percent of HF.

the ATR-IR spectrum (Figure S5). This result strongly suggested that NBD was also important for the one-step dipping method. If the OCP of the Si surface in contact with the solution with HF, NBD, and monomers was more negative than the reduction potential of the diazonium itself (+0.3 V versus SCE), then it was possible to have a spontaneous reduction. The expected etching morphology was observed in the crosssectional images of the PMMA/Si sample (Figure 6). A rough

Figure 6. SEM cross-sectional images of the PMMA/Si sample grafted for 15 min.

and porous silicon (PS)-like structure was observed at the interface between Si and the PMMA film. However, when the Si surface was dipped into only the HF solution, the PS-like structure was not seen in the cross-sectional image (Figure S6). That was because the H-terminated surface was essentially stable considering the relatively slow etching rate of HF solution. Nevertheless, PS was chemically formed without the need for an external current or voltage source by adding nitric acid to the HF solution.39,40 In this stain etching technology, nitric acid was reduced at the Si surface by injecting holes into F

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electrografting from diazonium salt solutions. ACS Appl. Mater. Interfaces 2010, 2, 1184−1190. (6) Allongue, P.; de Villeneuve, C. H.; Pinson, J.; Ozanam, F.; Chazalviel, J. N.; Wallart, X. Organic monolayers on Si(111) by electrochemical method. Electrochim. Acta 1998, 43, 2791−2798. (7) Allongue, P.; Henry de Villeneuve, C. H.; Cherouvrier, G.; Cortes, R.; Bernard, M. C. Phenyl layers on H-Si(111) by electrochemical reduction of diazonium salts: monolayer versus multilayer formation. J. Electroanal. Chem. 2003, 550-551, 161−174. (8) Rappich, J.; Merson, A.; Roodenko, K.; Dittrich, T.; Gensch, M.; Hinrichs, K.; Shapira, Y. Electronic properties of Si surfaces and side reactions during electrochemical grafting of phenyl layers. J. Phys. Chem. B 2006, 110, 1332−1337. (9) Mahouche-Chergui, S.; Gam-Derouich, S.; Mangeney, C.; Chehimi, M. M. Aryl diazonium salts: a new class of coupling agents for bonding polymers, biomacromolecules and nanoparticles to surface. Chem. Soc. Rev. 2011, 40, 4143−4166. (10) Mooste, M.; Kibena, E.; Kozlova, J.; Marandi, M.; Matisen, L.; Niilisk, A.; Sammelselg, V.; Tammeveski, K. Electrografting and morphological studies of chemical vapour deposition grown grapheme sheets modified by electroreduction of aryldiazonium salts. Electrochim. Acta 2015, 161, 195−204. (11) Boukerma, K.; Chehimi, M. M.; Pinson, J.; Blomfield, C. X-ray photoelectron spectroscopy evidence for the covalent bond between an iron surface and aryl groups attached by the electrochemical reduction of diazonium salts. Langmuir 2003, 19, 6333−6335. (12) Anariba, F.; DuVall, S. H.; MeCreery, R. L. Mono- and multilayer formation by diazonium reduction on carbon surfaces monitored with atomic force microscopy “scratching. Anal. Chem. 2003, 75, 3837−3844. (13) Bernard, M.−C.; Chausse, A.; Cabet-Deliry, E.; Chehimi, M. M.; Pinson, J.; Podvorica, F.; Vautrin-UI, C. Organic layers bonded to industrial, coinage, and noble metals through electrochemical reduction of aryldiazonium salts. Chem. Mater. 2003, 15, 3450−3462. (14) Matrab, T.; Chehimi, M. M.; Perruchot, C.; Adenier, A.; Guillez, A.; Save, M.; Charleux, B.; Cabet-Deliry, E.; Pinson, J. Novel approach for metallic surface-initiated atom transfer radical polymerization using electrografted initiators based on aryl diazonium salts. Langmuir 2005, 21, 4686−4694. (15) Hauquier, F.; Matrab, T.; Kanoufi, F.; Combellas, C. Local direct and indirect reduction of electrografted aryldiazonium/gold surfaces for polymer brushes patterning. Electrochim. Acta 2009, 54, 5127−5136. (16) He, W.; Jiang, H.; Zhang, L.; Cheng, Z.; Zhu, X. Atom transfer radical polymerization of hydrophilic monomers and its application. Polym. Chem. 2013, 4, 2919−2938. (17) Iruthayaraj, J.; Chernyy, S.; Lillethorup, M.; Ceccato, M.; Ron, T.; Hinge, M.; Kingshott, P.; Besenbacher, F.; Pedersen, S. U.; Daasbjerg, K. On surface-initiated atom transfer radical polymerization using diazonium chemistry to introduce the initiator layer. Langmuir 2011, 27, 1070−1078. (18) Mineo, P.; Motta, A.; Lupo, F.; Renna, L.; Gulino, A. Si(111) surface engineered with ordered nanostructures by an atom transfer radical polymerization. J. Phys. Chem. C 2011, 115, 12293−12298. (19) Delamar, M.; Dbsarmot, G.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Savfant, J.-M. Modification of carbon fiber surfaces by electrochemical reduction of aryl diazonium salts: application to carbon epoxy composites. Carbon 1997, 35, 801−807. (20) Mahouchea, S.; Mekni, N.; Abbassi, L.; Langa, P.; Perruchot, C.; Jouini, M.; Mammeri, F.; Turminec, M.; Romdhane, H. B.; Chehimi, M. M. Tandem diazonium salt electroreduction and click chemistry as a novel, efficient route for grafting macromolecules to gold surface. Surf. Sci. 2009, 603, 3205−3211. (21) Li, H.; Cheng, F.; Duft, A. M.; Adronov, A. Functionalization of single-walled carbon nanotubes with well-defined polystyrene by “Click” coupling. J. Am. Chem. Soc. 2005, 127, 14518−14524. (22) Tessier, L.; Deniau, G.; Charleux, B.; Palacin, S. Surface electroinitiated emulsion polymerization (SEEP): a mechanistic approach. Chem. Mater. 2009, 21, 4261−4274.

the valence band of the semiconductor, thereby enabling Si to etch under OCP. Such a mechanism may be plausible for the other oxidizing agent such as the NBD cation. Our results agree well with this theory. This spontaneous reduction of diazonium salt provided an attractive venue to covalently bond various vinylic polymers to the Si surface to prepare a highly passivating insulator. It may find many applications in microelectronics and other areas.

4. CONCLUSIONS The synthesis of a polymer film on a Si surface was studied using a new spontaneous redox of diazonium salts in the presence of vinylic monomers. This method relied on the ability of diazonium salts to be spontaneously reduced on the Si surface in the presence of HF in the solution, and the reduction acted both as an initiator for the conventional radical polymerization of vinylic monomers and as a primer sublayer. Various vinylic polymers were prepared on the Si surface through this simple method. The polymer film was composed of PNP and polyvinyl, and PNP acted as a cross-linker between PMMA chains. This was the first technique that has been used to graft a stable organic polymer layer with a large range of thickness on the Si surface in a fast one-step process working in water solution at room temperature in the open air and without any apparatus requirements.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01931. AFM phase images in tapping mode, XPS spectrum of a PMMA film, ATR-IR spectra of Si surfaces, and SEM cross-sectional images of etched Si in solution (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-21-3420-2748. Fax: +86-21-3420-2748. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Basic Research Program of China (973 Program no. 2015CB057200) and the National Center for Advanced Packaging Co., Ltd.

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REFERENCES

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DOI: 10.1021/acs.langmuir.6b01931 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.6b01931 Langmuir XXXX, XXX, XXX−XXX