gold surfaces for polymer brushes patterning

Electrochimica Acta 54 (2009) 5127–5136

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Local direct and indirect reduction of electrografted aryldiazonium/gold surfaces for polymer brushes patterning Fanny Hauquier, Tarik Matrab, Frédéric Kanoufi, Catherine Combellas ∗ Laboratoire Environnement et Chimie Analytique, CNRS UMR7121, ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France

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Article history: Received 7 November 2008 Received in revised form 16 January 2009 Accepted 22 January 2009 Available online 31 January 2009 Keywords: Aryldiazonium ATRP Polymer brush SECM Patterning

a b s t r a c t The patterning of conductive substrates by polymer brushes may be achieved by using successively scanning electrochemical microscopy (SECM) and atom transfer radical polymerization (ATRP). After the surface functionalization by a brominated aryldiazonium initiator, SECM allows the local reduction at the micrometer scale of the initiator grafted layer. Different channels sizes involved in charge transport within the initiator layers are evidenced by combining SECM, CV and observation of the aryl-grafted layer transformation. ATRP is performed on the SECM patterned substrate. Inside the pattern, the lower density of initiator decreases the polymer thickness. The pattern resolution is enhanced when the direct mode of the SECM is used instead of the mediated indirect mode. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction Much interest is currently directed toward new ways to modify surfaces of solid substrates. In this respect, surface grafting of polymer chains is a much effective method. Grafting from surfaces well organized polymeric systems with controlled length is especially promising. Such polymer brushes are particularly interesting since they can exhibit large and reversible deformations from stretched brush to collapsed mushroom when submitted to a stimulus such as a change of pH or ionic strength. These ordered macromolecular objects can be obtained from radical polymerization. The development of micropatterned polymer structures constitutes an attractive approach for designing novel smart surfaces, actuators, microdevices or chemical/biological sensors. The principal methods for producing such micropatterned surfaces use electron-beam lithography, [1–5] photolithography, [6–13] microcontact printing, [14] nanoimprinting, inkjet printing, [15] Langmuir–Blodgett or dip-pen lithography. In a previous work, we have used the scanning electrochemical microscopy (SECM) to pattern insulating surfaces with polymer brushes of controlled dimension obtained from atom transfer radical polymerization (ATRP). [16] The combination of both techniques, SECM and ATRP, is an innovative lithography to design smart surfaces. This process was used to grow from Si or glass substrates films of various grafting densities and therefore various thicknesses

∗ Corresponding author. E-mail address: [email protected] (C. Combellas). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.01.059

or elongations in the locally etched regions. This lithography procedure can be successfully used to design either empty microdomains surrounded by polymer brushes or isolated microdomains of polymer brushes. Here, the same microelectrochemical strategy is generalized to conducting substrates such as gold surfaces. The objective is to pattern insulating or conductive surfaces with polymer structures of controlled morphologies and to provide an interesting alternative to lithography. Styrene (S) and glycidyl methacrylate (GMA) were chosen for this study. PGMA is a potential surface linker for biomolecules and has promising applications in advanced biotechnology, such as DNA separation, targeted drug delivery, enzyme immobilization, and immunological assay [17–19] because of the ease in the conversion of epoxide groups into a variety of functional groups, such as –OH, –NH2 , and –COOH. On the other hand, PS was chosen because of its hydrophobic character. 2. Experimental 2.1. Surface characterization The thickness of the organic monolayer was estimated using a Sentech SE 400 ellipsometer with a He–Ne laser and an angle of incidence of 70◦ . The refractive index of the initiator layer was set at n = 1.46, and the refractive and optical indexes n and k for the gold substrate were obtained from the clean bare surfaces. Uncertainty ±1 Å. The morphologies of the pattern were determined using a Fogale Nanotech Microsurf 3D optical profilometer or were imaged using

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an INCAx-sight scanning electron microscope (SEM) from Oxford instruments. The amount of polymer grafted on the patterned surface was estimated by interferometric microscopy. The measurement is based on the non-contact acquisition of an interferometric image of a surface topography from the modification of interferometric fringes pattern upon the topography of a reflecting surface. Interference fringes are obtained from the recombination on a CCD camera of two parent half-beams, a first beam is reflected on a reference mirror while the second analyzing beam is reflected on the tested surface. On a reflecting surface covered with thin polymer coatings, the analyzing beam crosses the polymer layer before its reflection on the gold surface. Therefore the path difference introduced by the polymer crossing is 2np t, where np is the polymer optical index (np = 1.46) and t is the polymer film thickness. The interferometric image of a patterned surface can then be transformed into an image of the differential thickness of the polymer coating over the Au surface, as presented in Figure S1. AFM images were recorded with a PicoSPM II microscope from Molecular Imaging in the tapping mode under ambient conditions. We used silicon nitride cantilevers with a cantilever length of 125 ␮m. 2.2. Electrochemical treatment of gold wafers The synthesis of the starting diazonium salt, + N2 –C6 H4 – CH(CH3 )–Br, was previously described [20]. Gold plaques were 1–2 cm2 pieces of a 4 in. diameter × 500 ␮m thickness silicon wafer coated by a 1000 Å gold layer (Aldrich). Gold orientation is nominally highly polycrystalline with 1 1 1 orientation. Electrochemical grafting of this diazonium salt onto the surface was achieved by chronoamperometry for 300 s, at a potential 300 mV more negative than the diazonium salt peak potential (measured on carbon). At this potential, the gold surface should not be covered by an oxide layer. The gold wafers were then thoroughly rinsed under sonication in deaerated acetone. 2.3. Patterning Patterning on the layer was done with a home-made scanning electrochemical microscope using a CHI potentiostat. In the feedback mode, typically, reduction of the initiator layer was achieved in a solution of DMF containing NBu4 BF4 and 2,2 -dipyridyl as the mediator, by applying a reductive potential of −2.1 V/SCE to a platinum working microelectrode (diameter: 25 ␮m). The counter electrode was a platinum wire, and the reference electrode was Ag/AgCl. The whole device was kept under nitrogen in a polyethylene glovebag during the experiment. The microelectrode-substrate positioning was obtained from the experimental approach curve of ferrocyanide onto the aryl-grafted gold surface. In the direct mode in a 2-electrode configuration, the gold surface modified by the initiator layer is used as the working electrode and the microelectrode is used as the counter and the reference electrodes. The reduction of the initiator layer was achieved in a solution of DMF containing NBu4 BF4 (5 mM), by applying a reductive potential of −2.6 V to the gold surface. This potential was determined by cyclic voltammetry in the same 2-electrode configuration as the onset of the gold covered substrate reduction. In this mode, the microelectrode was positioned at a close distance from the substrate using the “negative feedback” obtained from AC-current-SECM [21]. It consists of applying a 10 mV amplitude sinusoidal oscillation (E = 0 V) at the microelectrode and recording the AC tip current or the AC tip impedance. The tip is then withdrawn from a known distance from the substrate. The exact tip–substrate distance is then obtained after the experiment by recording the SECM approach curve in the feedback mode with an

acetonitrile solution of ferrocene. The whole device was kept under nitrogen in a polyethylene glovebag during the experiment. 2.4. Polymerization Glycidyl methacrylate, GMA, and styrene, S, (Aldrich) were distilled prior to polymerization and stored at 4 ◦ C. CuBr, CuBr2 and 2,2 -dipyridyl (Aldrich) were used as received. Surface-initiated ATRP was undertaken on the bromide derivated surface following the procedure described by Yu et al. [22] for GMA and Hikita et al. [23] for styrene. 2.4.1. Surface-initiated ATRP of GMA The detailed procedure for preparing the polymer brushes is the following. First, a 100 mL Schlenk flask equipped with a magnetic stirring bar and sealed with a rubber septum was deoxygenated under vacuum followed by back-filling with nitrogen for three times. The CuBr (50 mg, 0.35 mmol) and CuBr2 (19.5 mg, 0.087 mmol) powders, and the initiator grafted gold wafer were introduced into the flask under a nitrogen flow. A mixture containing GMA (9.5 mL, 70 mmol), and 2,2 -dipyridyl (137.5 mg, 0.87 mmol), previously degassed, was added to the polymerization flask using a double-tipped needle under a nitrogen flow. The flask was placed at room temperature for several hours. The polymerization was stopped by cooling and opening the flask in order to expose the catalyst to air. The gold wafer-PGMA hybrids were thoroughly rinsed in dichloromethane under sonication for five periods of 5 min. 2.4.2. Surface-initiated ATRP of S CuBr (390 mg, 2.7 mmol) and a piece of the initiator grafted gold wafer were placed into a 100 mL Schlenk flask equipped with a magnetic stirring bar and sealed with a rubber septum and deoxygenated by a nitrogen flow. 2,2 -dipyridyl (1.17 g, 7.5 mmol) was placed into the two-neck round-bottom flask, and the flask was evacuated and backfilled with nitrogen. To this flask, 10 mL of toluene was added and this solution was stirred for 20 min under nitrogen. The resulting solution was transferred through a cannula to the Schlenk flask. In a second two-neck round-bottom flask, styrene (24 mL, 210 mmol) was deoxygenated by a nitrogen flow. The monomer solution was transferred through a cannular, and the flask was held at 110 ◦ C. After several hours, the substrate was removed from the flask, washed with dichloromethane, and sonicated in toluene and dichloromethane. 3. Results and discussion 3.1. ATRP polymer growth from Au surfaces First, the electrochemical reduction of a bromoaryl diazonium salt was used to prepare a brominated initiator layer on the gold surface. Immobilization of the initiator by such electrografting was preferred to the use of alkanethiol self-assembled monolayers because it leads to more stable layers when submitted to highly reductive or oxidative potentials; it also resists to sonication and time [24,25]. Indeed, the electrografting of electrodes by diazonium salts provides the covalent attachment of an organic layer onto the electrode surface. The covalent bond between the substrate and the organic layer has been demonstrated both experimentally [26–28] and theoretically [29]. This is a particularly important feature in the preparation of covalently bonded polymer brushes on the surface, as a uniform and dense layer of initiator covalently tethered to the surface is compulsory. Especially, the polymer brush is more stable over a spin-coated polymer layer with regard to solvent treatment and harsh conditions such as high temperature, because of the covalent bond between the polymer and the substrate. In this

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A linear increase in the grafted layer thickness with the polymerization time is observed for both polymers. These results indicate that the process of surface-initiated ATRP is controlled in the two cases. Furthermore, the polymerization rate varies with the nature of the monomer. Surface-initiated polymerization of styrene gives only a 16 nm thick PS film after 6 h, compared to the 28 nm thick film obtained when the graft polymerization was carried out in the presence of GMA. This marked difference can be attributed to the already reported difference between the two polymerization kinetics [22,23]. 3.2. SECM patterning of Au surfaces with polymers Fig. 1. Evolution of the film thickness t with polymerization time deduced from ellipsometric measurements (♦: PGMA, : PS).

respect, aromatic moieties derived from diazonium ions have been proposed to initiate polymer growths from iron [20,30] or carbonaceous surfaces (diamond, nanotubes. . .) [31–33]. Radical polymerization shows many advantages in comparison with other types of polymerization; typical examples are its applicability to various kinds of monomers and the absence of need for strict purification processes. However, radical species are so reactive that care should be taken for the control of the reaction. Atom transfer radical polymerization minimizes the radical concentration, which results in a precise chain growth control [34–40]. Therefore, the method allows to prepare various kinds of block copolymers such as diblock, triblock, and gradient copolymers with well-controlled block lengths. There have been many reports for the preparation of “grafted from” type polymer brushes with the ATRP method, that exploit its living nature and compatibility with various kinds of monomers. They show that it is possible to control the surface properties systematically by controlling the initiator density on the substrates or by selecting the monomer. ATRP is suitable for the polymerization of acrylic and vinylic monomers such as styrene (S) or glycidyl methacrylate (GMA). The presence of a grafted polymer layer on the surface was ascertained by ellipsometry after the substrate has been washed exhaustively and sonicated with several solvents. The thickness of the polymer brushes, grown on the surface, was followed as a function of the polymerization time in the presence of the two monomers as shown in Fig. 1.

Following the procedure described previously, we have patterned surfaces with PS and PGMA polymer brushes. Different strategies have been proposed to pattern surfaces with polymer brushes; they are mainly based on the use of light or beam irradiation of masked surfaces [1–5]. Following the procedure described previously [16] but using a bromoaryl moiety as the initiator for ATRP [20,30–33], we propose an electrochemical alternative based on the use of scanning electrochemical microscopy in the feedback (indirect) mode or in the direct mode. Scheme 1 outlines the synthesis pathway for the preparation of micropatterned brushes of PS and PGMA polymers on a gold surface. As previously reported, the microelectrode of the SECM will be used as an electrochemical eraser of the initiator layer, in order to impede ATRP in the reduced regions. In this way, it will be possible to create structures of polymer brushes. 3.2.1. SECM approach curves of electrografted Au surfaces As the pattern formation depends on the local electrochemical etching of the initiator layer, we first dealt with the SECM characterization of an electrografted 1-bromoethylaryl layer obtained from the reduction of the corresponding diazonium ion. To do so, SECM experiments were achieved above the initiator layer covered gold surface. The approach curves obtained for ferrocyanide, terephthalonitrile, 2,2 -dipyridyl, p-tolunitrile and ferrocene are displayed in Fig. 2. The curves were fitted by the general analytical expressions given for an irreversible charge transfer at the substrate [41,42]. In an aqueous medium, in the presence of potassium ferrocyanide, a decrease of the faradic current with the distance L is observed. Nevertheless, the approach curve deviates from the

Scheme 1. Preparation of micropatterned brushes on gold substrate by scanning electrochemical microscopy (SECM) in the indirect (top left) and the direct (bottom left) modes.

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film. The organic redox probe could then explore both larger and smaller pores, and the rate of ET at the larger pores would correspond to the value observed for the aqueous redox probe (here a value of 5 × 10−4 cm/s was obtained from the SECM approach curve). The electron transfer rates depend on the layer preparation and on the position at the surface, however we detected from SECM approach curves that they generally fall within, in cm/s, 0.002 and 0.004 for terephthalonitrile, 0.006 and 0.01 for 2,2 -dipyridyl and 0.013 and 0.033 for p-tolunitrile. The trend is that the more reducing the anion radical, the faster the ET rate at the layer. Finally, the oxidizing approach curve performed with ferrocene as the redox probe provides, with a rate of 0.012 cm/s, an intermediate situation between the 2,2 -dipyridyl and the p-tolunitrile. Fig. 2. SECM approach curves obtained at a Pt disk UME tip (radius a = 12.5 ␮m) on an initiator layer covered Au substrate in the presence of: ( ) potassium ferrocyanide (5 × 10−3 mol L−1 in H2 O + 0.1 mol L−1 KCl); ( ) terephthalonitrile (5 × 10−2 mol L−1 in DMF + 0.1 mol L−1 NBu4 BF4 ); ( ) 2,2 -dipyridyl (5 × 10−2 mol L−1 in DMF + 0.1 mol L−1 NBu4 BF4 ); ( ) ferrocene (5 × 10−3 mol L−1 in DMF + 0.1 mol L−1 NBu4 BF4 ); ( ) p-tolunitrile (5 × 10−2 mol L−1 in DMF + 0.1 mol L−1 NBu4 BF4 ). The solid lines are the theoretical curves for finite electron-transfer kinetics. The dashed and dotted lines are the theoretical curves for a conducting substrate and for a totally insulating substrate respectively.

expected curve for a totally insulating substrate. The electroreduction of the diazonium salt on gold surfaces gives a polyaryl multilayer (2–3 nm from ellipsometry) that is likely more complex than the classical self-assembled monolayers [43,44]. It is also less densely packed than classical SAMs, as it does not block the electron transfer (ET) to potassium ferricyanide as kapp , obtained from the fit of SECM approach curves, reaches a value of 5 × 10−4 cm/s. The blocking of the electrode by the electrografted layer was confirmed by a cyclic voltammetry (CV) study. Indeed, the CVs for the oxidation of ferrocyanide at naked and electrografted Au surfaces are given in Figure S2. The potential dependency of the apparent heterogeneous ET, khetCV , at the naked and grafted electrodes can be obtained from mathematical transformation of the CV curves into Tafel plots (given in Figure S3). We followed the procedure detailed in Refs. [45–47]. The values of the apparent standard heteroge0 , are obtained by extrapolation of such Tafel plots neous ET, khetCV 0 at E = E0 . Once grafted by the polyaryl multilayer, the khetCV value for ferrocyanide oxidation has decreased by a factor of 3–5 × 103 . This decrease is not high enough to account for tunnelling electron transfer along a distance as long as 2 nm. The low electron transfer rate would rather indicate ET at defects sites, as was already proposed for other similarly electrografted systems [45–49]. As already observed too, for the organic redox probes, the approach curve was always higher, indicating a higher regeneration of the redox probe at the electrografted layer. The values of the electron transfer rates at the substrate depend on the redox probe and also on the substrate surface, but they are always 5–10 times higher than the value obtained with the aqueous ferrocyanide. The difference between aqueous and organic probes can be interpreted by either (i) permeation of the organic probes within the hydrophobic layer followed by an ET at the surface or (ii) ET at smaller pinholes. We make here a distinction between large defect sites and pinholes, and define the former ones as pores or defects of large, ␮m or sub-␮m, dimension (macropores) than the latter ones that would rather characterize smaller meso- or nano-pores. This distinction is important as different contributions from those defects could be expected according to the solvation properties of the electrochemical solvent used. Indeed, transport of aqueous redox probes into macropores or ␮m scale defects in the organic layer could be expected, while such species would not, owing to capillarity restrictions (related to the hydrophobicity of the organic layer), explore within nanopores or pinholes in a 2 nm thick organic

3.2.2. Patterning in the feedback mode of the SECM In the feedback mode of the SECM, the initiator layer is scanned with a microelectrode electrogenerating the reduced form of a redox mediator M. This step leads to the local reduction of the C–Br bond and its conversion into a C–H bond. In that way, the initiation step is deactivated in the reduced area. Following that, the whole surface is submitted to the polymerization; this process provides a local etching of the surface. In previous works, water condensation was used to estimate pattern dimensions on silicon or glass substrates [16]. Here, the hydrophobicity contrast is not so high and several techniques, such as SEM or interferometric microscopy, were used to estimate the pattern width, w. The SEM image (Fig. 3) displays the gold surface covered by a PGMA film that exhibits several clear lines, which are consistent with the patterns. The larger white protrusions observed at one end of each etched line are due to a longer reduction time at this place; indeed, the SECM tip positioning was deduced from the approach curve performed there. The morphology of the patterns is governed by the tip velocities from 200 to 50 ␮m/s. For both polymers, the evolution of the pattern width as a function of the tip velocity v and the microelectrode/substrate separation was investigated (Fig. 4). For all distances between the substrate and the microelectrode, the pattern width decreases as the tip velocity increases. As a comparison, data obtained for an insulating substrate are also reported in Fig. 4 (×). For a similar distance between the microelectrode and the substrate, the pattern dimensions are smaller for the conducting substrate. In previous works, we have attributed the patterns width evolution to the diffusive-convective transport of the tip-

Fig. 3. SEM image of the patterned PGMA brush.

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Fig. 4. Evolution of the pattern width w as a function of the tip velocity v and the microelectrode/substrate distance d (with L = d/a). Local reduction in the indirect mode in the presence of 2,2 -dipyridyl (5 × 10−2 mol L−1 in DMF + 0.1 mol L−1 NBu4 BF4 ): L = 1; ( ), ( ) and ( ) L = 0.7; ( ) L = 0.5; ( ) L = 0.45; ( ) insulating substrate at L = 0.5; Local reduction in the direct mode (DMF + 5 mmol L−1 NBu4 BF4 ) ( ) L = 1.6. L values are given with a ±0.08 (±1 ␮m) confidence.

electrogenerated reagent (here the reducer M•− ) at the substrate surface. The difference of the pattern width on Au and insulating surfaces may be due to the evolution of the diffusion cone with the conductive nature of the substrate. Indeed, partial regeneration of the redox mediator, M, is expected at the surface of a partially conducting substrate, such as the gold surface covered by an organic layer. This feedback impedes the lateral expansion of the electrogenerated reagent over the conductive substrate and the higher the conductivity of the substrate, the lower the lateral expansion of the electrogenerated reagent. For a given tip scan rate, when the microelectrode/substrate distance decreases, the pattern width decreases. Owing to the possible regeneration of M at the substrate, it seems surprising that the expansion of the tip-electrogenerated reagent varies so significantly within the explored tip/substrate separation distance, L. The variations observed could indicate that at shorter L the substrate behaves as a partially conducting substrate (focusing the reagent around the microelectrode disk) while at higher L it presents a more pronounced insulating character (allowing the reagent expansion over a wider region). This situation could be due to the influence of the solution ohmic drop onto the (unbiased) substrate kinetics [50] or the intervention of kinetic limitations related to lateral charge propagation within the layer [51–53]. The chemical content of the pattern was investigated by interferometric microscopy that allows to follow the pattern depth and then to check whether it contains polymeric material or not. Fig. 5 shows the evolution of the ratio e/t, where e and t are the pattern depth and the film thickness, respectively, as a function of the tip velocity. We can see that there is not much effect of the velocity on this ratio and that the mean e/t value is around 0.4. If all the C–Br bonds present at the surface were totally reduced into C–H bonds, the value would aim toward 1, as previously observed for the local patterning of insulating silanized glass or Si surfaces [16]. However, in our case, the pattern depth is lower than the polymer thickness, which suggests that there is some polymer inside the pattern. In order to ascertain this suggestion, AFM analysis of the polymer brushes modified surfaces was achieved. Fig. 6A shows the initial morphology of a gold surface modified by bromoaryl diazonium. The surface is smooth and its average roughness is of the order of 15 Å. The enlarged image of this surface, at 500 nm × 500 nm scale, shows the presence of gold terraces as shown in Fig. 6A inset. However, the image presents a slight difference by comparison with the virgin substrate at the same scale since small ∼20–30 nm diameter

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Fig. 5. Evolution of the ratio e/t as a function of the tip velocity. Local reduction in indirect mode in the presence of 2,2 -dipyridyl (5 × 10−2 mol L−1 in DMF + 0.1 mol L−1 NBu4 BF4 ): ( ) L = 0.65, ( ) and ( ) L = 0.7, ( ) L = 0.5; terephthalonitrile (5 × 10−2 mol L−1 in DMF + 0.1 mol L−1 NBu4 BF4 ) ( ) L = 0.5; Local reduction in the direct mode (DMF + 5 mmol L−1 NBu4 BF4 ) ( ) L = 1.6.

nodules are observed on the grafted gold surface. The presence of such topography has been attributed to the grafting of aryl groups onto the surface [48,54–56]. After local reduction and ATR polymerization, the AFM tip was placed outside the patterned area (Fig. 6B). The morphology of the initiator layer has completely disappeared and has been replaced by large spheroids with diameters around 150 nm. Consequently, the average roughness increased to 14 nm. These dramatic changes in the morphology reflect the growth of polymer brushes onto the initiator layer. Next to that, the AFM tip was removed and placed inside the patterned area (Fig. 6C). In this area, the initial morphology of the gold surface modified by bromoaryl groups is not totally recovered and the same large spheroids, which are consistent with polymer brushes, are persistent. These results confirm the presence of some polymer inside the pattern, which means that the reduction of the initiator is quite inefficient. However, areas without trace of polymer inside the pattern may also be observed. The comparison of the different cross-sections (Fig. 6D) reveals that the roughness inside this area is the roughness of a surface only covered by aryl groups derived from diazonium salts. In this area whose width varies from 1 to 3 ␮m diameter, the C–Br bonds are totally reduced. Even if there is still some polymer inside the reduced area, local modification is still visible. This phenomenon can be explained by the behavior of the polymer brushes. Indeed, this ordered macromolecular structure exhibits large and reversible deformations from a stretched brush to a collapsed mushroom. Outside the pattern, there is a uniform and dense layer of initiator covalently tethered to the surface. This dense initiator layer leads to the formation of a stretched brush layer. On the contrary, inside the local modification, the initiator layer is partially reduced and then partially debrominated. The initiator density decreases but is still sufficient to initiate an ATR polymerization. In that way, a lower density of polymer brushes facilitates its spreading over the surface and therefore yields an apparent thinner film. The difference between the more or less elongated states explains this local patterning. The presence of some polymer inside the patterns then indicates that the C–Br reduction of the initiator layer is not complete. Two hypotheses may explain this behavior: (i) the anion radical of the redox mediator is not enough reducing to reduce completely the initiator layer and to prevent totally the polymerization in the reduced area, or (ii) the reduction is incomplete because of transport limitations of the reducer within the initiator layer. Several experiments were achieved to test these assumptions.

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Fig. 6. Tapping-mode (5 ␮m × 5 ␮m) AFM images for gold modified surface (A) after grafting of the initiator layer (local zoom inset), (B) after ATR polymerization outside pattern (C) after ATR polymerization inside pattern and (D) cross-sections of the surfaces.

3.2.3. Electrochemical reduction of bromoethylbenzene— influence of the reductive strength? As a preliminary test, we have investigated the reductive behavior of a solution of 1-bromoethylbenzene, 1, in order to mimic, in solution, the reductive behavior of the ATRP initiator immobilized on the Au surface. The cyclic voltammograms of the reduction of a DMF solution of 1 present a large and irreversible peak, Ep − Ep/2 = 145 mV, located at −1.67 V vs SCE for an electrode potential scan rate of 0.1 V/s. The shape, location and peak potential variations with the potential scan rate clearly indicate that the reaction is under the kinetic control of a slow electron transfer step characterized by a transfer coefficient of ˛ ∼0.32. As observed for analogous alkylbromides and arylalkylbromides, the reduction is kinetically controlled by the first dissociative electron transfer that corresponds to the debromination of 1 [57,58]. The homogeneous reduction of 1 by 2,2 -dipyridyl could not be studied by cyclic voltammetry as 2,2 -dipyridyl is more difficult to reduce than 1. We have then performed the redox catalysis of 1 by a more oxidizing redox mediator such as phthalonitrile. The cyclic voltammograms of these redox catalysis experiments (for 2 concentrations of redox mediator and 2 concentrations of 1) coincide with the general scheme of homogeneous reduction of alkylaryl halides (RX): [59,60] M + e = M•−

(1)

k2

M•− + RX−→M + R• + X− k3

M•− + R• −→MR− k4

M•− + R• −→M + R−

(2) (3) (4)

where the radical R• can either couple with M•− via a radical–radical coupling reaction (3) or get reduced by ET with M•− (4). In a typical redox catalysis experiment, the reduction peak currents of the redox mediator in the absence and in the presence of the RX moieties are compared (see the example in Figure S4). A current increase is expected upon RX addition as reaction (2) regenerates M at the electrode surface. The current increase depends on the electrode potential scan rate, the excess factor [RX]/[M] and on the kinetic parameters k2 and k3 /k4 . The simulation of the redox catalysis experiments allows the estimation of the first homogeneous dissociative electron transfer rate, k2 , and of the competition between the two ensuing reactions for the fate of the radical. This is obtained from the comparison of experimental data with the appropriate theoretical working curves obtained for example from Digisim® , an example is given in Figure S5. Assuming the radical–radical coupling reaction at a rate of, k3 = 107 M−1 s−1 , k2 = 8 × 105 M−1 s−1 and k4 = 6 × 108 M−1 s−1 (average of 10 values) are obtained. These values fall in the range of what could be expected from ET theories. Indeed, the value of k2 for the redox

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catalysis of 1 by the anion radical of phthalonitrile is in agreement with the heterogeneous dissociative reduction of 1 and Savéant description of homogeneous dissociative ET [61]. Briefly, from the peak reduction potential of 1 at 0.1 V/s, one can compare the C–Br bond dissociation energy of 1 to that of C–Br in PhCH2 Br (D = −2/3 Ep = −20 mV) and obtain the E0 of the 1st dissociative ET of 1, E 0 = EPhCH2 Br − D = 0.61 V vs SCE [62]. From the knowledge of the E0 , the value of the homogeneous rate constant k2 would fit the Savéant homogeneous dissociative ET theory, if the homogeneous ET has an intrinsic activation barrier of 0.73 eV in agreement with values obtained for alkyl halides and from the predicted value of 0.74 eV. The value of the 2nd homogeneous ET rate constant, k3 , is also in good agreement with data from the reduction dynamic of the ethylphenyl radical [63]. From this homogeneous investigation and from Savéant dissociative homogeneous ET, one can then predict that the homogeneous rate constants for 1 reduction by the anion radicals of 2,2 -dipyridyl and p-tolunitrile are respectively 2 × 108 and 3 × 109 M−1 s−1 , meaning that the reduction of 1 is close to diffusion control. It is then expected that the surface bound ATRP initiator will get reduced by these redox catalysts at rates close to diffusion limit. The incomplete reduction of the initiator layer is then likely not due to the reducing strength of the redox catalyst. Furthermore, local reduction was also achieved using two other redox couples, namely p-tolunitrile (E◦ = −2.36 V vs SCE) that displays a more negative standard potential than 2,2 -dipyridyl, and terephthalonitrile (E◦ = −1.50 V vs SCE) with a more positive standard potential than 2,2 -dipyridyl. In both cases, the local reduction leads to the formation of patterns with similar dimensions with incomplete reduction. There is, however, a little influence of the mediator reducing power on the pattern width. It seems that, for a given L value, the less reducing anion radical of terephthalonitrile generates slightly wider patterns while the more reducing anion radical of p-tolunitrile generates patterns slightly thinner. These experiments were performed only once and should be taken with care but they seem to correlate the trends in the approach curves depicted in Fig. 2. As the pattern depth is concerned, even the use of a mediator with a higher reducing power does not allow the total reduction of the C–Br bonds since some polymer still grows inside the pattern. It then appears that the mediator reducing power has not much influence on the pattern depth, meaning that with the two most reducing anion radicals used, the reduction of the initiator layer is achieved at its maximum yield. This is in agreement with what was observed on Si surfaces where the initiator layer was completely reduced with 2,2 -dipyridyl. 3.2.4. Patterning in the direct mode of the SECM In order to improve the microelectrochemical patterning procedure we have performed the direct local reduction of the initiator layer electrografted on Au surfaces, in opposition with its indirect reduction in the presence of a mediator. This study will also allow to compare the extent of patterning when the reduction is achieved, in the former situation, from the lower (in contact with the Au substrate) or from the upper (in contact with the solution) side of the initiator organic layer. The same setup is used, namely the microelectrode is placed near the initiator grafted gold surface. For a better control of the electrochemical processes, a 2-electrode configuration was preferred [49]. The gold surface modified by the initiator layer is used as the working electrode in a resistive dilute electrolytic solution of DMF (5 mM of NBu4 BF4 ) and the microelectrode is used as counter electrode in order to focus the reduction. The reduction of the initiator layer was achieved in a solution of DMF containing only 5 mM of NBu4 BF4 , by applying a reductive potential of −2.6 V to the gold surface vs the Pt microelectrode tip. This potential has been chosen as the onset of the reduction wave observed by cyclic voltammetry

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in the same 2-electrode configuration, for the reduction of the Au electrografted initiator layer. The preliminary experiments show that it is possible to decrease the pattern width by using the direct reduction instead of the indirect one. For the same velocity and a higher microelectrode/substrate distance than in the indirect mode, the pattern width, estimated by SEM, is clearly lower as shown in Fig. 4 (+). Moreover, interferometric measurements show that this configuration has an effect on the evolution of the ratio e/t when the tip is moved near the substrate. Increasing the tip velocity decreases the pattern depth. Furthermore, this ratio reaches a value of 0.84 when the tip is moved at 5 ␮m/s, meaning that the major part of the initiator layer is reduced. When using a lower velocity, a total reduction of the C–Br bonds should be expected. In the direct mode, the local surface reduction is then more efficient than in the indirect mode, even though the direct reduction uses less reducing potentials (Esubs ∼−1.6 V vs SCE compared with the 2,2 -dipyridyl radical anion, E0 = −2.1 V vs SCE). This suggests that, at least for slow tip speeds, the direct charge transfer from the Au surface to the initiator layer is more favoured than the charge injection from the upper side of the layer that is connected to the solution. This could be related to the ease of transport of charges and species within the organic layer. 3.3. Toward a picture of charge transport at diazonium electrografted multilayers Contrary to what is observed for the homogeneous reduction of 1 and previous patterning of insulating surfaces, the local mediated reduction of the initiator layer electrografted on Au is not complete and reaches, at best, 60%. This means that some Br terminated functions are still available within the locally reduced initiator layer that cannot be accessed by the microelectrogenerated reagent. It agrees well with the ellipsometric characterization that indicates that the electrografting procedure leads to the growth of a multilayer structure, most likely consistent with the large nodules observed by AFM (inset of Fig. 6A). The incomplete reduction of C–Br functions of the initiator multilayer means that the Br functions buried in the core of these nodules do not get reduced at the timescale of the patterning (<10 s). The progressive controlled growth of polymer structures demonstrates that these hindered C–Br functions are still available for the reduction step that occurs during the ATRP, however the timescale of the ATRP polymerization is much longer (>1 h). It is not surprising that the more organized self-assemblies of silanes (or thiols) are reduced more steadily than the more disordered electrografted aryl multilayers obtained by the reduction of diazonium ions. The partial accessibility of the C–Br functions of the electrografted layer may be correlated to the difficulty to quantify electroactive groups present in a compact organic layer obtained from the electrografting of diazonium ions [28,64]. For the reductive local transformation of the material during the patterning, the electron transfer across the electrografted multilayer may be decomposed into different parallel processes when permeation is neglected, as presented in Scheme 2. They correspond to: (1) the ET at macropores or (2) at nanopores and, (3) the irreversible reduction of the layer by the reducer. The global ET process, described by the total substrate current, iET , is the sum of all these different processes whose characteristic currents are, respectively, imp , inp , ired : iET = imp + inp + ired

(5)

The current required for the local material transformation during the SECM patterning of the electrografted layer may be roughly estimated from [65]

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Scheme 2. Possible paths for transport of a redox probe through an electrografted multilayer. (1) Transport of organic and aqueous probes in macropores of radius Rd , (2) transport of organic probes in meso- or nano-pores of radius Rd , (3) partial mediated reductive local transformation (into a gray material) of the multilayer only when A = Ox and B = Red. In (1) and (2) discharge of B at the conductive surface occurs whatever the redox nature of B (reducer or oxidant).

ired = 2F0.6Br wv

(6)

where F is the faraday,  Br represents the surface concentration of the initiator molecule (<5 × 10−9 mol/cm2 for a 2–3 nm thick layer), w is the pattern width and v is the tip writing speed. Its maximal value is 2 × 10−8 A, which is considerably small compared to the 5 × 10−7 A flowing through the substrate. The major path for the electron flow into the system during the patterning and during the approach curves is then related to the mass transport within the electrografted layer by transport through pinholes and defects toward the Au electrode. As the layer is partly reduced by the different reducers, permeation through the whole layer is likely not favoured and transport through defects is preferred. Actually, permeation through the film involves very small channels; it is likely prevented in mediated reduction owing to the difficulty to transport both the reducing species and, for the respect of local electroneutrality, the electrolyte cation within the dense multilayer. This is indeed consistent with the heterogeneity in the approach curves recorded from sample-to-sample even though the same thickness of initiator layer was immobilized. Following Mirkin’s formalism [41,66] the effective ET rate constant obtained by fitting the experimental approach curves to theory, as made in Fig. 2, then leads to the estimate of the ET for the transport through defects (macropores, kmp and nanopores, knp ): keff = kmp + knp

(7)

Generally the mass transfer limited ET at pinholes of average surface density Np and average radius Rp can be measured by SECM as it is characterized by an apparent ET rate constant given by: [45,46,67,68] kp = 4Np DRp =

4p D Rp

(8)

where D is the solution diffusion coefficient of the redox probe (the pinhole is assumed to be filled by solution),  p is the surface fraction of the pinholes (p = Np Rp2 ), with the index p = mp or np. The lower rate observed for aqueous redox mediators would depict transport through macropores (and/or partial conductivity if possible) of the organic layer. The transport of hydrophilic ions through meso- or nano-pores is prevented owing to capil-

larity restrictions (hydrophobic pores inaccessible to the aqueous solution) and/or the organic layer shrinking. From the inspection of the Au grafted layer by cyclic voltammetry with ferrocyanide as the aqueous redox probe, the surface coverage of the larger defects (macropores) may be estimated. It is obtained from the ET rate constant extracted from cyclic voltammetry (Figures S2 and S3) at the naked, 0 0 , and electrografted, khetCV,Au-R , Au electrodes as mp = khetCV,Au

0 0 khetCV,Au-R /khetCV,Au ∼ 2 − 3 × 10−4 . Combined with (8) and the SECM inspection of the surfaces, one obtains the average defect radius, Rmp = 4 mp D/kSECM,FeCN ∼50 nm, and their density Nmp ∼3 × 106 cm−2 . The aqueous redox probe transfers through very large pores in the grafted film that are largely distant from each other, as the average distance between two neighbouring pores can 1/2 be roughly estimated as R0 ∼ Rmp /mp ∼ 3 ␮m [67]. It is consistent with the AFM image since such large pores could be the black region observed in the inset of Fig. 6A. This picture also confirms the recent question arisen by Downard and co-workers [69,70] or McDermott and Kariuki [48,55] concerning the use of redox probes for the inspection of electrografted multilayers. We believe that the combination of SECM and CV gives complementary characterization and interesting insight into such complex film structures. Again, the SECM demonstrated that this description is reasonable; indeed, the proper patterning of the surface required the perfect parallelism of the tip travel plane and the Au surface plane. If this alignment is easy to perform with perfectly insulating surfaces, it is more delicate with the electrografted Au surfaces because linescan of the surface shows heterogeneities in the measured feedback current along the linescan. Such heterogeneities are commonly observed when inspecting material surfaces [65,71]; here, they could be accounted for the peculiar large pores size and distribution. When considering the organic redox probes, higher feedbacks are observed. It is then expected that the defects contribution will be the sum of the transport through the same large defects and the transport through smaller pores because of their better wetting and swelling by the organic solvent [70]. The effective heterogeneous rate constant measured by the SECM reads as the sum of both contributions (7), among which that of the macropores may

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be neglected, and: keff = 4Nmp DRmp + 4Nnp DRnp ∼

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4. Conclusion 4np D Rnp

(9)

with Np ,  p and Rp , respectively the surface density, surface coverage, radius and heterogeneous rate constant for the transport through the two different defects; p = mp, large defects or macropores, p = np smaller mesopores. As the less reducing redox probe (anion radical of terephthalonitrile) presents a keff very close to that of the 2,2 -dipyridyl anion radical, despite a 600 mV E0 difference, the value for the former probe could represent a rough estimate of the contribution of the transport within both large and smaller pores, and one could then estimate the product Nnp Rnp ∼ kSECM /4DPht ∼75 cm−1 . If one considers that those pores have an average radius of Rnp ∼10 nm, it is expected that they have a surface density of Nnp ∼7.5 × 107 pores per cm2 or a surface coverage of  np ∼2.5 × 10−4 . From the cyclic voltammetry of ferrocene oxidation at the Au electrografted electrode, a value of the surface coverage of all the defects of  d ∼4 × 10−4 , may be obtained (assum0 ∼ 4 cm/s). This value is close to ing that on naked Au electrode khet that observed with the aqueous redox probe for the macropores. Owing to the film reconstruction upon the solvent change (from H2 O to DMF), it is difficult to know whether the pores structure observed in the aqueous environment is maintained and therefore to conclude if  d =  mp +  np . Anyway, the nanopores implied in the organic redox probes are likely smaller than 16 nm. The higher ET rate observed for the more reducing redox probes and for the ferricinium cation could depict some diffusion limitations related to the dimension of the redox probe compared to the pores dimension. Indeed, even though all species investigated have similar diffusion coefficients, the ion pair they will form to transport charges within the pores could be very different. For example, the Fc+ ,BF4 − pair is much smaller than the M•– ,NBu4 + one. Moreover, the negative charge in the different radical anions is mainly delocalized around the N atoms giving rise to dipoles that could generate ion pairs with different sizes. Finally, the apparent increase of keff when the E0 of M decreases could be a manifestation of the potential drop within the pores between the film–solution and film–electrode interfaces. The more negative the E0 , the larger the potential drop and the more efficient the migration transport of the ion within the pores. The direct mode reduction only requires transport of the electrons through the (partly conductive [72]) aryl layer in conjunction with the injection and transport/permeation of the cation of the electrolyte of this layer. In opposition, the indirect reduction by the electrogenerated reagent requires the transport of a more voluminous molecular assembly and therefore requires larger channels. Finally, the great dependence of the pattern depths on the tip rate (or the reduction time) reveals the kinetics of the charge transport into the electrografted layer structure. For a tip scan rate of the order of 10 ␮m/s, the initiator layer is half-reduced, the corresponding time of flight of the substrate by the tip,  = 2 a/v ∼ 2 s, gives an estimate of the characteristic time of this phenomenon. This transport is likely limited by the diffusion of the electrolyte cations within the 3 nm thick initiator layer with an apparent diffusion coefficient of the order of 5 × 10−14 cm2 /s and suggests constrained diffusion into very thin channels or very slow permeation rates. In the indirect mode, the pattern depth does not depend on the tip rate (or the reduction time); owing to the fastest tip rate used, this shows that the limiting process that characterizes the transport of the mediated reduction of the upper initiator layer occurs at a characteristic time shorter than 0.05 s, two orders of magnitude faster than for the direct reduction. This also supports that the mediated reduction engages only the superficial layer of the initiator and transport into larger channels.

The local etching of polymer brushes onto conducting substrates may be achieved by a method previously described for insulating substrates. It is based on the overall on atom transfer radical polymerization and scanning electrochemical microscopy. After the surface functionalization by a brominated initiator, SECM allows the local reduction at the micrometer scale of the initiator layer previously grafted on the gold surface. As already described for an initiator grafted layer onto an insulating substrate, the initiator decomposition consists of the reductive debromination of the initiator by a radical anion electrogenerated at the microelectrode tip of the SECM. ATR polymerization on these surfaces leads to the formation of a local patterning whose morphologies depend on the tip velocity. The pattern width decreases as the tip velocity increases. Moreover, the closer the microelectrode to the surface, the lower the width. Nevertheless, contrary to an insulating substrate, such as glass or silicon oxide, the local reduction of the initiator layer is incomplete. Furthermore, the use of a more reducing mediator has no influence on the efficiency of this local reduction. The persistence of some C–Br bonds into the reduced area enables the growth of some polymer chains. Inside the pattern, the lower density of initiator decreases the polymer thickness. An alternative method, using the gold substrate as the working electrode and the microelectrode as the counter electrode, gave a better pattern resolution. This local electrochemical etching, that does not require a mediator, could be a promising alternative for the patterning of conducting substrates. On the contrary to what was observed on insulating substrates, the incomplete reduction of the initiator layer is quite frustrating. Indeed, it means that a complete debromination of the initiator layer is hardly obtained by an electrochemical means owing to the complex structure of the initiator multilayer. In the present case, this would make the masking technique superior to the SECM alternative proposed here. Finally, the combination of SECM, CV and observation of the aryl-grafted layer transformation allows to evidence the different channels involved in charge transport within these multilayers. It seems that aqueous redox probes are vehicled within large defects of about 50 nm while the transport of organic electroactive species exploit 5–10 times smaller meso- or nano-pores. The slow permeation of smaller species is likely possible during the direct injection of charges from the electrode surface. Acknowledgments F. Hauquier and T. Matrab acknowledge the ANR for financial support via the ANR-06-BLAN-0368 project. J. Ghilane from ITODYS is thanked for help for the acquisition of the AFM images. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2009.01.059. References [1] Y. Tsujii, M. Ejaz, S. Yamamoto, T. Fukuda, K. Shigeto, K. Mibu, T. Shinjo, Polymer 43 (2002) 3837. [2] U. Schmelmer, R. Jordan, W. Geyer, W. Eck, A. Golzhaüser, M. Grunze, A. Ulman, Angew. Chem., Int. Ed. 42 (2003) 559. [3] I.S. Maeng, J.W. Park, Langmuir 19 (2003) 4519. [4] S.J. Ahn, M. Kaholek, W.-K. Lee, B. LaMattina, T.H. LaBean, S. Zauscher, Adv. Mater. 16 (2004) 2141. [5] Q. He, A. Küller, M. Grunze, J. Li, Langmuir 23 (2007) 3981. [6] M. Husemann, M. Morrison, D. Benoit, J. Frommer, C.M. Mate, W.D. Hinsberg, J.L. Hedrick, C.J. Hawker, J. Am. Chem. Soc. 122 (2000) 1844. [7] P. Iwata, P. Suk-In, V.P. Hoven, A. Takahara, K. Akiyoshi, Y. Iwasaki, Biomacromolecules 5 (2004) 2308. [8] F. Zhou, L. Jiang, W. Liu, Q. Xue, Macromol. Rapid Commun. 25 (2004) 1979.

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