Catalytic Scanning Probe Nanolithography (cSPL): Control of the AFM

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Catalytic Scanning Probe Nanolithography (cSPL): Control of the AFM Parameters in Order to Achieve Sub-100-nm Spatially Resolved Epoxidation of Alkenes Grafted onto a Surface Vincent Mesquita,† Julien Botton,‡ Dmitry A. Valyaev,‡ Cyril François,‡ Lionel Patrone,†,§ Teodor Silviu Balaban,‡ Mathieu Abel,† Jean-Luc Parrain,*,‡ Olivier Chuzel,*,‡ and Sylvain Clair*,† †

Aix Marseille Université, CNRS, Université de Toulon, IM2NP UMR 7334, 13397 Marseille, France Aix Marseille Université, Centrale Marseille, CNRS, iSm2 UMR 7313, 13397 Marseille, France § Institut Supérieur de l’Electronique et du Numérique, CNRS, IM2NP UMR 7334, 83000 Toulon, France ‡

S Supporting Information *

ABSTRACT: Scanning probe lithography (SPL) appears to be a reliable alternative to the use of masks in traditional lithography techniques as it offers the possibility of directly producing specific chemical functionalities with nanoscale spatial control. We have recently extend the range of applications of catalytic SPL (cSPL) by introducing a homogeneous catalyst immobilized on the apex of a scanning probe. Here we investigate the importance of atomic force microscopy (AFM) physical parameters (applied force, writing speed, and interline distance) on the resultant chemical activity in this cSPL methodology through the direct topographic observation of nanostructured surfaces. Indeed, an alkene-terminated self-assembled monolayer (alkene-SAM) on a silicon wafer was locally epoxidized using a scanning probe tip with a covalently grafted manganese complex bearing the 1,4,7-triazacyclononane macrocycle as the ligand. In a post-transformation process, N-octylpiperazine was covalently grafted to the surface via a selective nucleophilic ring-opening reaction. With this procedure, we could write various patterns on the surface with high spatial control. The catalytic AFM probe thus appears to be very robust because a total area close to 500 μm2 was patterned without any noticeable loss of catalytic activity. Finally, this methodology allowed us to reach a lower lateral line resolution down to 40 nm, thus being competitive and complementary to the other nanolithographical techniques for the nanostructuration of surfaces.

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INTRODUCTION The quest for developing functional materials on the nanometer scale has stirred the scientific community over the past two decades, resulting in the emergence of scanning probe lithography (SPL) methods. By the use of a scanning probe microscope to alter surfaces, either by selective degradation, chemical modification, or the feeding of additional material, SPL techniques have ensured the creation of functionalized surfaces with high control of the spatial resolution.1−7 Among such techniques, dip-pen nanolithography (DPN) has arisen as one of the most flexible ways to access such structures.8−15 Even if dip-pen nanolithography has already shown tremendous industrial potential, it still suffers from some drawbacks such as the limited choice of molecular inks and their compatibility © XXXX American Chemical Society

with the substrates. To overcome these constraints, complementary approaches have recently been developed, including constructive lithography16,17 and catalytic scanning probe lithography (cSPL).18,19 This technique constitutes a unique tool for controlling the covalent grafting of molecular objects on a surface with nanoscale resolution. Therefore, the chemical modification process can be initiated locally by limiting the reacting system to a confined volume at the interface between the substrate and a catalytically active nanoscale probe, as illustrated in Figure 1. Received: February 12, 2016 Revised: March 21, 2016

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DOI: 10.1021/acs.langmuir.6b00543 Langmuir XXXX, XXX, XXX−XXX

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important to note here that we have shown that the use of a homogeneous supported catalytic species in a heterogeneous reaction system allows us to achieve robust covalent grafting using mild reaction conditions and to open new opportunities for a wide variety of chemically selective surface reactions. Alkene-terminated self-assembled monolayers (alkene-SAMs) on silicon have been locally epoxidized using a Mn complex with a 1,4,7-triazacyclononane (TACN) ligand grafted onto an AFM tip as a catalytic system as depicted in Figure 2 with the subsequent grafting of N-octylpiperazine onto the modified area. To ensure the expansion of this research field toward mass production through parallelization methods or alternative techniques,31−33 it appears crucial to have a robust lithographic system with nanoscale resolution at our disposal. On the basis of our preliminary results,30 a detailed study of the relationship between physical AFM parameters and the resultant chemical activity in this cSPL methodology is being described herein through the direct topographic observation of nanostructured surfaces. Fundamental processes, involved in such a challenging double-heterogeneous nanolithographic approach, have been investigated in this study to gain insight into such a confined metal-catalyzed reaction system. Our results have allowed us to spatially control the lithography of both extended and nanosized arrays of patterns.

Figure 1. Schematic representation of the interface between the SAMcoated substrate and the catalytic probe.

Despite recent contributions20−27 and because of modest reproducibility related to the inherent complexity of such systems and the limited scope of the catalytic processes described, cSPL is still in its infancy. Recently, Davis and Hanyu established the detailed reaction mechanisms of surfaceconfined Suzuki−Miyaura and Mizoroki−Heck reactions using cSPL methods through proper control of the various reaction parameters.28 Interestingly, Braunschweig and coworkers were able to elucidate the kinetics of a surface-confined Huisgen 1,3-dipolar cycloaddition reaction and measure the corresponding activation volume.29 Although most cSPL approaches were performed using catalytic probes coated with transition metals or their oxides (thin films or nanoparticles),20−25 we proposed recently, in preliminary studies, to extend the cSPL technique by the immobilization of a homogeneous transition-metal catalyst on the tip of an atomic force microscope (AFM) probe.30 It is

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MATERIALS AND METHODS

The AFM experiments were performed at room temperature (25 °C) with an Agilent 5500 microscope on a 10-undecenyltrichlorosilane SAM on a silicon wafer. The wafer had laser marks that are recognizable with optical microscopy so that the exact location of the nanolithography patterns could be easily regained after ex situ treatments. The chosen catalytic system, a 1,4,7-triazacyclononane (TACN) ligated manganese complex, was shown to be highly active and selective for the epoxidation of terminal aliphatic alkenes using aqueous H2O2 as an oxygen atom source at 0−25 °C in acetonitrile in both homogeneous34,35 and immobilized-on-silica versions.36 Catalytic

Figure 2. Schematic representation of the local alkene epoxidation by a catalytic AFM probe covalently functionalized with the Mn-TACN complex and H2O2 as a sacrificial oxidant in acetonitrile, followed by an AFM-free nucleophilic ring-opening of the epoxide sites reacting with Noctylpiperazine. B


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Langmuir AFM probes were obtained by coating commercial silicon tips with the appropriate TACN-type ligand bearing an alkyl chain ending with a Si(OMe)3 anchoring group. More details, including the synthesis and grafting of the ligand, about our standard experimental procedure can be found in previous work30 and in the Supporting Information (SI). The working area of the 10-undecenylsilanyl SAM on SiO2/Si was first imaged in air in tapping mode with a catalytic tip. Then, a local AFM-catalyzed epoxidation process was conducted by introducing a solution of 50 mM H2O2 in acetonitrile (MeCN) and scanning a defined area in contact mode (writing mode). The modified area could be immediately observed in the phase image of AFM in tapping mode (reading mode).37 The epoxidized pattern was then rinsed thoroughly with acetonitrile and revealed by the selective grafting of Noctylpiperazine from a 2 mM solution in MeCN under soft Lewis acid activation, which was previously performed with 4 mM LiOTf overnight at 25 °C.38,39 The wafer surface was then rinsed consecutively with MeCN (15 min) and water (15 min) and sonicated in a MeCN/H2O (1:1) mixture (20 min) before final characterization by AFM in air in tapping mode, this time with a noncatalytic tip. AFM images were treated and analyzed using the WsXM software package.40 Quantification of the grafting process was obtained by applying a first-order flattening filter to the substrate and measuring the 3D volume of the grafted structure. An average grafting height was then calculated by dividing this volume by the grafted surface area. Error bars were extracted by considering the imperfect flatness of the surface and the intrinsic roughness of the alkene-derivatized SAM surface. The latter amounted to about 0.2 nm, a value that consequently limited the sensitivity of the measurements. For all of the experiments, AppNano ACT probes were used (cantilever length 125 μm, nominal frequency 300 kHz). The spring constant was calibrated by a thermal tuning method on a Bruker Multimode 8 apparatus using a Nanoscope V controller and was evaluated to be 21 ± 1 N·m−1. The pristine silicon tip has a nominal radius of curvature of less than 10 nm. The tips with grafted catalyst were characterized using a test grating TGT1 sample (NT-MDT), and we found a tip radius of curvature of 35 ± 15 nm as detailed in the SI. Scanning electron microscope (SEM) images of some tips were additionally obtained, and no particular degradation of the tip shape due to the catalyst coating process could be observed as detailed in the SI. The lithography process was very robust and could deliver highly efficient N-octylpiperazine grafting in most cases. All results, trends, and conclusions presented here could be qualitatively well reproduced. Because the grafting of the catalyst and the precise configuration of the tip end were not perfectly controlled, the threshold values of the different effects showed a variable distribution. Each individual experiment was successfully reproduced at least three times, and average values are presented in Figure 6. The error bars reported reflect the corresponding standard deviation.

characterization by AFM in air in tapping mode, this time with a noncatalytic tip. To ensure the production of well-defined patterns, it appeared that each step (i.e., the immobilization of the homogeneous ligand and subsequent Mn complexation on an AFM tip; the presence of hydrogen peroxide as an oxidant during lithographic processes) should be adequately performed and is crucial to the procedure as a whole. Quality control of our catalytic tips has been conducted through the optimization of the grafting time of the TACN ligand and subsequent visualization by SEM imaging. This study, along with the control experiments, is discussed in detail in the SI. Methodology. Prior to evaluating the limits of this catalytic system and therefore the potential of an immobilized homogeneous catalyst in terms of lateral resolution, it was necessary to elucidate the experimental conditions that control this particular catalytic environment. To gain knowledge of such a complex double-heterogeneous process, the influence of several lithographic parameters on the localized epoxidation reaction was systematically investigated. A standard experiment consists of zigzag scanning of several squared areas of the same size, with well-defined AFM writing parameters (applied force, scanning speed, and interline distance (DIL)). The lithography of arrays of squared patterns, whereby two writing parameters are being incrementally modified, one between consecutive rows and the other one between consecutive columns, allowed us to quantify the influence of these two independent parameters in a straightforward way as pictorially represented in Figure 3. It is noteworthy that the first and the last rows were

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RESULTS AND DISCUSSION The different steps in the experimental procedure of our cSPL technique have been fully detailed recently.30 A brief summary of these procedures described below is presented schematically in Figure 2. More details can be found in the Materials and Methods section and in the SI. The working area of a 10-undecenylsilanyl SAM on SiO2/Si was first imaged in air in tapping mode with a catalytic tip. Then, a local AFM-catalyzed epoxidation process was conducted by introducing a 50 mM solution of H2O2 in acetonitrile (MeCN) and scanning a defined area in contact mode (writing mode). The modified area could be immediately observed in the phase image of AFM in tapping mode (reading mode). The epoxidized pattern was then rinsed thoroughly with MeCN and revealed ex situ by the selective grafting of Noctylpiperazine under soft Lewis acid activation with lithium triflate salts (LiOTf). The wafer surface was then rinsed with MeCN and deionized water and sonicated briefly before a final

Figure 3. Design of a typical experiment for scanning probe lithography. Squared patterns with a zigzag scanning motion were created with varying control parameters P1 and P2, corresponding to the applied force, the rate of speed, and the interline distance DIL. The first and last rows were performed with identical parameters to check for eventual modification of the tip state.

performed under identical conditions in order to assess the possible modification of the tip during the experiment. Table 1 presents some selected experiments with their corresponding experimental parameters. Because the AFM topographic height may be very sensitive to imaging conditions and to the SAM quality, the influence of the writing parameters was evaluated from an array of patterns, realized with a unique catalytic AFM probe, by extracting the average height of each nanostructure on a single AFM image, as illustrated in Figure 3. Assuming that the grafting yield of the N-octylpiperazine chains was directly C


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Langmuir Table 1. Summary of the Nanolithography Experiments Performed and Their Corresponding Reaction Parameters experiment type mechanical degradation influence of applied force resolution limit scanning speed robustness

interline distance (nm)

writing speed (μm·s−1)

applied force (nN)

4

2

4, 8, 16, 32

2

200 to 11 500 200 to 9600

64 8 8

0.2 2 to 20 5

6100 3800 3800

related to the surface density of the epoxide moieties,41 we could then evaluate the conversion rate of the epoxidation reaction by extracting the average height h of the postgrafted Noctylpiperazine chains from AFM images. The higher grafting density is related to an alignment of the N-octylpiperazine chains and to the formation of a well-packed structure. A lower grafting density enables high flexibility of the chain conformations and a lower apparent height of the structure.42 Besides, the optimum packing of upright standing alkyl chains on a flat substrate corresponds to a surface density of about 5 molecules per nm2.43,44 The length of the N-octylpiperazine molecule is equal to 1.51 nm, according to DFT calculations,30 providing a film height of 1.49 nm considering a tilt angle of ∼10°.45 We could measure under optimum conditions a maximum height of 1.3 nm, corresponding therefore to 87% of the theoretical height, leading to an estimated density of 4 molecules·nm−2.42 This value is close to or higher than the values reported for functionalized SAM films.28,29 It is noteworthy that the minimum detectable height was 0.2 nm as a result of the intrinsic roughness of the SAM layer. Mechanical Degradation. The mechanical impact of the writing scanning mode on the SAM has been studied as a function of the applied load. Square areas were scanned in contact mode using a catalytic probe with a constant speed of 2 μm·s−1 and gradually increasing the force from 200 to 11 500 nN in acetonitrile and then imaging (reading) in tapping mode in air. The resulting topographic and phase images are shown in Figure 4. The squared areas were always distinguishable in the phase image as a result of the reordering of the molecules in the SAM.46,47 We found that the interline distance (i.e., the density of scanning lines) is an important parameter influencing the mechanical degradation. For repeated scanning over areas in close proximity to one another (small interline distance), degradation of the SAM was observed for forces between 3000 and 5700 nN, consisting of the formation of a topographic depression of 0.5 to 0.7 nm and associated with higher phase contrast resulting from partial nanoshaving or important reordering of the SAM structure.1 For applied forces larger than 7600 nN, severe degradation of the SAM and the creation of cavities with a depth of ∼1 nm were observed. However, for low applied forces (5700 nN), as shown previously, a partial degradation of the SAM surface can be hypothesized from Figure 4, eventually leading to a slight decrease in the average height of the grafted pattern. The nanolithographic process was also found to affect the tip efficiency, as shown by the slight shift between first and last rows for the onset force required to observe a nonzero pattern. Nevertheless, this tip degradation was probably limited to the very first moments of nanolithography with a fresh tip because the tip catalytic activity was preserved over a long working period, as will be shown in Figure 8. When varying the interline distance inside the writing scan, similar trends were observed qualitatively (Figure 5c−e), but the onset of the saturation regime was obtained at different forces: the larger the interline distance, the higher the saturation onset force. Also, the onset force required to observe a minimal grafting height (nonzero) increased along with interline distances. In a first approximation, the results presented in Figure 5 could be interpreted by defining an effective action radius that characterizes the area under the tip where the catalytic activity produces maximal epoxidation reactions at defined applied load

The data from Figure 5 can be analyzed in further detail using an elastic Hertz model. To obtain an estimation of the amount of reaction produced by the homogeneous catalyst supported by the AFM tip, different parameters have to be considered: the catalytic activity Acat per surface area of the tip, the geometry of the tip (radius of curvature, Rtip; surface contact area, Scontact; and related contacting width Ltip), and the AFM scanning parameters (scanning speed Vscan and interline distance DIL). The estimation of the reaction yield [ΔN] (or the oxygen transfer efficiency per surface area) as a function of the AFM parameters can then be written as [ΔN ] = Acat × Scontact × tcontact

with tcontact =

tcontact = E

L tip Vscan

L tip Vscan L tip

×

DIL

(1)

for the tip scanning a single line and for the tip scanning a squared surface of DOI: 10.1021/acs.langmuir.6b00543 Langmuir XXXX, XXX, XXX−XXX

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Langmuir interline distance DIL. Within the framework of elastic deformation for a mechanical contact between a sphere and a planar surface, a Hertz method can be used to obtain the tip− sample contact area Scontact and contact radius a using the geometry schematized in Figure 6a52,53 ⎛ R tip × F ⎞1/3 a=⎜ ⎟ ⎝ K ⎠

(2)

with K given by eq 3: 2 1 − ν2 2 ⎞ 4 ⎛ 1 − ν1 ⎟ ⎜ + 3 ⎝ E1 E2 ⎠

−1

K=

(3)

Figure 7. Average pattern height as a function of the total reaction time per scanned μm2.

The range of forces in our study was 0 to 9600 nN. The tip radius Rtip was estimated to be 35 nm using a calibration sample as described in the SI. The Hertz model takes into account the elastic behavior between the tip and the silicon substrate. The Young modulus and Poisson ratio are E1 = E2 = 166 × 109 N· m−2 and ν1 = ν2 = 0.22 for the Si tip and Si substrate.54 The resulting K coefficient is 1.16 × 1011 N·m−2. Finally, by combining an elastic model of the tip with a constant catalytic activity, [ΔN] can be written as a function of the AFM parameters scan speed Vscan, interline distance DIL, and applied force F as follows: [ΔN ] = Acat × = Acat

pattern height as a function of the total time spent on writing a 2 × 2 μm2 for an applied force of 3800 nN (interline distance 8 nm). According to the experiments in Figure 5, this value corresponds to an intermediate average height of the pattern, between the detection onset and the saturation threshold. Therefore, it would be possible to evaluate more precisely the fluctuations in height of the scanning speed. The total reaction time (total lithography time) is inversely proportional to the scanning speed. Remarkably, we could not observe any significant loss of activity up to the maximum speed used, which corresponded, for an area of 1 μm2, to a total reaction time of 7 s. To reflect the effective contact time of the catalyst with a single point on the surface, this reaction time can then be rescaled by the ratio of the effective tip contact area by the total pattern area (1 μm2). By considering a tip action area extracted from Figure 6c (tip action radius of 5 nm), the effective tip contact area amounts 79 nm2. The minimum reaction time observed of 7 s corresponds to an effective reaction time of ∼0.6 ms. We can thus conclude that the reaction kinetics takes place at a characteristic time smaller than 10−4 s. To determine if the catalytic transformation was not controlled by the diffusion of H2O2 (i.e., the co-oxidant), we estimated the number of molecules in close proximity to the substrate. The concentration of H2O2 was 50 mM, which represents within the volume of 1 nm high above the SAM whose density was estimated to be 4.0 epoxide functions nm−2 a surface concentration of 3 × 10−2 molecules nm−2. The diffusion rate of H2O2 is estimated to be much faster than the tip displacement. By taking into account that the co-oxidant is present in large excess, we can then assume that our lithographic process is not limited by diffusion. Robustness of the Catalytic System. To evaluate the robustness of our catalytic system, a broad area of a SAM has been patterned by an extensive array of nanostructures with a single catalytic tip. A surface equivalent to 480 μm2 consisting of 32 1 × 1 μm2 and 112 2 × 2 μm2 squares was treated by local epoxidation, followed by a nucleophilic ring-opening epoxide with N-octylpiperazine as a revealing step. The written squares appeared to be homogeneous over the entire process as presented in Figure 8, suggesting no alteration of the catalytic activity. The total number of epoxidation reactions is estimated to be >2 × 109. If we consider a tip action radius of ∼5 nm for the tip and a catalyst density of ∼4 molecules nm−2,43 then we can estimate that the tip is decorated by about 300 active Mn centers. Each Mn atom could thus effectively catalyze at least 6 × 106 epoxidation reactions, which is considerably higher than typical turnover numbers (TON) for the epoxidation of

4πa 4 Vscan × DIL

⎛ R tip × F ⎞4/3 4π × ×⎜ ⎟ Vscan × DIL ⎝ K ⎠

(4)

Because the density of active sites is different for each experimental preparation of the tip and the catalytic activity per site is not known, only a qualitative description can be proposed. The influence of the applied load on the reaction for different line spacing distances can be estimated at a given scan speed as shown in Figure 6b. When a constant catalytic activity is used in eq 4, the reaction yield increases with the force and decreases with the interline spacing distance. The present model could be fitted to the experimental data by adjusting parameter Acat. To this aim, we defined the minimal yield for which topographic contrast can be observed by AFM (providing a nonzero pattern height). Yang et al.55 have previously estimated this value through a combined AFM topographic image and ellipsometry study on SAM growth. A minimum coverage of 40% of the maximum adsorption layer was required to achieve height detection in AFM. This threshold value of 40% can be used to fit our experimental data. We extracted from three independent measurements the average minimum force required to detect effective grafting by AFM (nonzero height) and compared it to the calculated 40% threshold value of the Hertz model (dashed line in Figure 6b). The best fit was obtained for Acat = 0.21 reaction·nm−2·s−1 and is reported along with experimental data in Figure 6d. Finally, within the framework of this model, a theoretical action radius, as defined above, can be derived by considering the forces at which the reaction yield reaches 100% in Figure 6b. The corresponding values are presented together with experimental data in Figure 6c, providing good agreement. Influence of the Scanning Speed. The influence of the scanning speed (Vscan) has been investigated by varying the duration of the contact between the catalyst and the surface during the writing process. Figure 7 shows the evolution of the F


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Figure 9. (a) AFM topographic image of the nanolithography of individual parallel lines spaced by 62 nm. (b) Line profile corresponding to the dashed line in (a) displaying a lateral resolution down to 40 nm.

perform the epoxidation of an alkene-terminated SAM. The reaction was revealed by AFM after the nucleophilic ring opening of the epoxide function by an alkyl-piperazine derivative. The spatial extension of the catalytic activity of the tip as a function of the applied load was analyzed within the framework of an elastic model. By varying the writing speed, we could show that the reaction proceeds faster than a characteristic time of 10−4 s. The catalytic process was able to provide a line resolution of 40 nm and could be used to pattern an area close to 500 μm2. The strategy developed here represents a nanolithography process with high robustness and sharp lateral resolution. Because it is based on a homogeneous catalyst immobilized on the tip, it also has good potential for exploring various chemical reactions that are complementary to heterogeneous catalytic methodologies. In addition, the control of crucial parameters, such as the applied force and the writing speed, will give the opportunity to create chemical and functional gradient surfaces in a similar way to what can be obtained with other techniques using electric field,57 electrochemical,58 diffusion,59 and microfluidic60 methods. Besides its high potential for nanometer scale modifications of surfaces in a well-controlled manner, our system could finally represent an insightful platform for fundamental studies of such a catalytic reaction.

Figure 8. Robustness of the catalytic tip evaluated by writing patterns repeatedly (F = 3800 nN, Vscan = 5 μm·s−1, DIL = 8 nm). A total area equivalent to 480 μm2 could be successfully grafted. This corresponds to the formation of >109 epoxide groups.

terminal alkenes with Mn-TACN complexes under homogeneous conditions (TON up to 1100).34 The observed higher catalytic activity may be explained by the better stabilization of the grafted catalytic species, preventing catalyst deactivation by decomposition. Lateral Resolution Limit. To estimate the resolution limit (narrowest line width) of our cSPL technique, individual parallel lines spaced 62 nm apart were scanned under writing conditions. The applied load was 6100 nN, and the scan speed was 0.2 μm·s−1. These parameters with high load appeared to be optimum because reducing the applied force resulted in poorly defined structures or the absence of measurable grafting. The AFM image of the resulting pattern is presented in Figure 9a, and a topographic profile across the patterns is shown in Figure 9b. A line width of 40 nm could be achieved. This value is of the order of magnitude of a typical tip size and comparable to resolutions in the range of 20 to 50 nm achieved among the other cSPL techniques.23−25 Similarly, dip-pen nanolithography techniques can reach a lateral resolution of 30−50 nm,9,10 or 10 nm in the case of oxidative SPL methodology.56 Our cSPL methodology using a homogeneous immobilized catalyst could be classified with other nanolithographical techniques for the nanostructuration of surfaces.

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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.6b00543. Control experiments on mechanical degradation, optimization of the grafting procedure of the ligand, AFM characterization data and calculation details (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected].

CONCLUSIONS We have performed a comprehensive study of the influence of the control parameters in a local epoxidation reaction using scanning probe lithography in an AFM apparatus. A commercial silicon tip with a supported Mn-TACN complex in the presence of H2O2 was used as a catalyst in order to

Author Contributions

V.M. and J.B. contributed equally. Notes

The authors declare no competing financial interest. G


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ACKNOWLEDGMENTS We thank ANR (grant no. ANR 12 BS10 015 02 “CASPARES”), la région Provence-Alpes-Côte d′Azur (PACA ̈ region council, project “Nanomosaique”), Aix-Marseille University, and the Centre National de la Recherche Scientifique (CNRS) for financial support. Alain Combes and Mathieu Koudia are gratefully acknowledged for technical support.

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Catalytic Scanning Probe Nanolithography (cSPL): Control of the AFM

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