Organic transistors based on airbrushed small molecule-insulating polymer blends with mobilities exceeding 1 cm² V⁻¹ s⁻¹

Abstract

Spray-coating, has recently fueled scientific interest as a versatile solution-processing technique for the realization of organic electronic devices, such as organic field-effect transistors (OFETs). In the present work, air-brush method was used for the deposition of semiconducting blends of triisopropylsilylethynyl-pentacene (TIPS-PEN) and common insulating polymers of polystyrene or polymethylmethacrylate. The use of such blend systems not only resulted in an improved wet film formation but also enabled efficient control over the crystallization process. A systematic study on the effect of different composition ratio on the morphology and crystallinity of the sprayed films as well as their macroscopic uniformity, was carried out. Both blend systems revealed well-ordered TIPS-PEN crystalline domains on the top surface, indicative of the pronounced phase separation phenomena. The optimized airbrushed OFETs exhibited excellent electrical characteristics with a maximum hole mobility value of 1.3 cm² V⁻¹ s⁻¹, negligible hysteresis, near-zero turn-on voltages and on/off current ratio greater than 10⁵. Additionally, the transistors revealed good long-term environmental stability, with no significant degradation after a period of 13 months. These results represent an important step for present and future applications of spaying techniques toward the controlled growth of high performance and environmentally stable OFETs.

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1. Introduction

The significant progress in the design and synthesis of solution-processed organic semiconductors has led to the development of well performing Organic Field-Effect Transistors (OFETs) with high mobility values,^1–3 clearly exceeding those of amorphous silicon-based devices (μ > 1 cm² V⁻¹ s⁻¹).⁴ Moreover, solution-processed OFETs possess advantageous features over their vacuum processed counterparts, such as low-temperature processing and compatibility with low-cost, large-area fabrication processes. Typical solution-processing techniques for industrial manufacturing, include blade coating,^5,6 ink-jet printing,^7,8 gravure printing^9,10 as well as the recently emerged spraying technologies^11–13 which have undoubtedly attracted significant interest. Their ease of use, fast deposition and adaptability with roll-to-roll (R2R) processes, renders them ideal techniques from an industrial point of view. To date, several variants of spray-coating approaches have been proposed for the realization of organic electronic devices, with air-brush,¹⁴ ultrasonic¹⁵ and electrostatic spraying¹⁶ being the most widely investigated. In the case of OFETs, several studies have focused on the spray-processing of organic small molecules such as soluble acenes due to their high charge carrier mobility and enhanced stability characteristics. In a study by Azarova et al., spray deposited OFETs based on the soluble acene 2,8-difluoro-5,11-triethylsilylethynyl anthradithiophene (dif-TES-ADT) were reported.¹¹ Due to the optimized spraying conditions with respect to the device characteristics, good performing devices with a maximum mobility of 0.2 cm² V⁻¹ s⁻¹ and a good inter-device uniformity were successfully obtained. A similar study was performed by Owen et al., resulting in a mobility value as high as 0.3 cm² V⁻¹ s⁻¹.¹⁷ A more controllable spraying system was introduced by Shao et al., who reported on ultrasonically sprayed OFETs based on triisopropylsilylethynyl-pentacene (TIPS-PEN).¹⁵ By optimizing the spraying parameters, a maximum mobility of 0.36 cm² V⁻¹ s⁻¹ was recorded. In an attempt to further increase control over the spraying process and thus achieve higher performance devices, we reported on the process optimization of electrostatic - sprayed soluble acenes.¹⁶ The fabricated TIPS-PEN OFETs showed a high mobility value up to 0.78 cm² V⁻¹ s⁻¹. Despite the promising transistor performance in the aforementioned studies, improving the film-forming properties and taking control over the complex crystallization process, remains challenging. The approach of blending small molecules with semiconducting or insulating polymers has been widely used in various solution processes due the efficient control of crystallization behavior, as well as the enhancement of the transistor characteristics (i.e., mobility, subthreshold slope).^18–20 In a recent work, we introduced the concept of electrospray-printing acene:semiconducting polymer blends.¹² Blending the polymer with the active material resulted indeed in favorable morphological characteristics which in combination with the high control over the process given by the electrospraying technique, yielded high performing OFETs with mobility as high as 1.7 cm² V⁻¹ s⁻¹ and improved device performance consistency.

Based on the latter study, we herein examine the concept of TIPS-PEN:insulating polymer (PMMA or PS) films for the realization of high performance top-contact OFETs based on air-brush spraying technique. A detailed morphological examination of the produced films under the optimized process parameters is conducted. The macroscopic uniformity of the films along with the effect of the composition ratio on the microstructural morphology and device performance is thus qualitatively investigated. Both blend systems (PS or PMMA blended) showed excellent electrical behavior exhibiting among the highest mobility values reported for airbrushed OFETs to date. Last but not least, the fabricated transistors exhibited enhanced stability characteristics after exposing them in atmospheric conditions for long time period, a critical aspect for real-world applications.

2. Experimental section

2.1 Materials

Triisopropylsilylethynyl (TIPS)-pentacene (-pentacene, ≥99.0%) was purchased from Ossila and used without further purification. Polystyrene (PS, M_w = 280 kDa), polymethylmethacrylate (PMMA, M_w = 996 kDa), as well as anisole (99.7%, anhydrous) were purchased from Sigma-Aldrich and used without further purification (Fig. 1a).

Fig. 1 (a) Molecular structures of TIPS-PEN and the insulating polymers PS and PMMA, employed in this study. (b) Layout of the bottom gate-top contact OFET device studied in this work. (c) Schematic drawing of the spray-coating method. (d) Schematic illustration showing the drying mechanism and the crystalline formation in each different region of the sample. The POM images show the morphology of a representative TIPS-PEN:PMMA sprayed films. The scale bar is 50 μm.

2.2 Sample preparation

A bottom-gate, top-contact (Fig. 1b) configuration was used for the fabrication of the OFET devices. Highly doped p-type silicon substrate with 300 nm thermal oxide (SiO₂) layer was used as a common gate electrode and dielectric, respectively. The substrates were cleaned by ultrasonication in acetone for 10 minutes and isopropylalcohol (IPA) for 10 minutes followed by oxygen plasma cleaning (80 watt for 2 min). For the neat TIPS-PEN samples hexamethyldisilazane (HMDS) was spin-coated (3500 rpm for 1 min, 120 °C baking) onto SiO₂ substrates prior the deposition of semiconductor. The semiconducting solutions were airbrushed directly onto the substrates under atmospheric conditions. After semiconductor's deposition, source and drain Au electrodes (50 nm) were deposited by thermal evaporation through a shadow mask. The channel length of the transistor was varied from 30 to 80 μm while the width was 1000 μm.

2.3 Air-brush spraying method

In this study, TIPS-PEN:insulating polymer blends were sprayed with a commercially available Iwata airbrush (HP-CR model). The aerosolized droplets were collected on the substrates using low pressure nitrogen (N₂) as the transporting gas (see Fig. 1c). TIPS-PEN:PS or PMMA blends were prepared in anisole with the TIPS-PEN weight fraction being varied from 0.2 to 0.8. The optimization of the spraying process was carried out by tuning the spraying parameters such as the nozzle to substrate distance (∼80 mm), the substrate temperature (80 °C) and the nitrogen pressure (∼7 psi). Before the deposition of the blended solutions, the substrates were kept at 80 °C. Following, the substrates were placed in a fixed and planar position, while the solution was loaded in the reservoir and sprayed. During the spraying, the airbrush was positioned perpendicular to the substrate. The radius of the spraying cone was found to be approximately 20 mm. The ejected droplets were collected and merged over the entire substrate (15 mm × 20 mm) until a continuous wet film was formed. The resulted films were left to evaporate freely in air without any further thermal annealing. Finally, the samples were placed in a vacuum chamber (0.1 mbar, 60 min) in order to remove the residual solvent.

2.4 Characterization

For the investigation of the surface morphology, atomic force microscopy (AFM) and polarized optical microscopy (POM) measurements were performed via a NTEGRA Scanning Probe Microscope (NT-MDT) and a Zeiss aus Jena polarized optical microscope, respectively. AFM measurements were conducted in the tapping mode for better image acquisition using rectangular silicon cantilevers with 10 nm nominal tip curvature and at a resolution of 512 points per line. In order to determine the crystalline quality of the films, X-ray diffraction (XRD) measurements were performed in the angular range (2θ) of 4° to 30° with a step size of 0.02° which is the range where the main reflections of the semiconductors are apparent, via a D-5000 (Brucker) diffractometer with CuKα₁ (2.2 kW X-ray tube) monochromatic radiation source, operating at 40 kV and 40 mA.

The electrical properties of the airbrushed OFETs were investigated using a Keithley 4200SCS semiconductor parameter analyzer under dark and ambient conditions, at room temperature. The field-effect mobility was calculated in the saturation regime according to the following equation:

where W (1000 μm) is the width of the channel, L (30–80 μm) the length of the channel, C_i is the total measured capacitance per unit area, V_GS is the applied gate voltage and I_DS is the drain current.

3. Results and discussion

3.1 Morphology and structure of the air-brushed blends

One of the most important aspects in all variants of spraying coating, is the uniformity of the produced liquid films, as it greatly affects the resulted morphology during the drying process. Especially in the case of crystalline semiconductors, extensive fluctuations in the volume of the film or uncovered regions may result in incomplete and/or anisotropic crystallization. Another important factor which has to be considered, is the drying rate of the ejected droplets. Generally, if the drying is rapid, each individual droplet tends to maintain its shape, due to the pinning of its contact line onto the substrate. In this case a hydrodynamic flow directed from the center of the pinned droplet toward the contact line, is established, in order to compensate the solvent loss at the periphery.²¹ The resulting shape of the dried droplet is often referred as “coffee-stain”.²² Such formations have detrimental effects on the morphology of crystalline semiconductors, as they induce extensive crystalline anisotropy, discontinuities and poor film coverage. Furthermore, the physical properties of the solvent can alter the size of the ejected droplets during the atomization process. Particularly, high viscosity solutions prevent the disintegration of the medium upon atomization, leading to larger droplet size. Similar effects are observed for high surface tension liquids. In this work, anisole was used as the solvent, as it exhibits balanced intrinsic characteristics (n = 1.05 cP, σ = 35.0 dyne cm⁻¹) for efficient atomization, while its average boiling point (T_b = 154 °C) provides sufficient time to the incident droplets to coalesce onto the substrate and form a complete layer, before drying. Moreover, the incorporation of the polymeric binder in the TIPS-PEN solution was found to significantly improve the rheological and wetting properties of the sprayed films on the SiO₂/Si substrates, without affecting the spraying scheme.

Despite the uniform film obtained, the lack of solvent evaporation control during the spraying process, induces a macroscopic crystalline anisotropy. Fig. 1d illustrates the proposed drying mechanism and the crystallization scheme in the different regimes of the sample, upon evaporation. Specifically, when the sprayed layer is left to dry freely, the evaporation rate at the contact edges is significantly higher compared to the central regions, due to the temperature gradient between the top (film/air ∼ 23 °C) and the bottom (film/substrate ∼ 80 °C) interface of the wet layer. In this case, the resulting crystalline film can be divided in three consecutive regimes (edges, intermediate domains and center) according to the observed morphological characteristics, as shown in the POM images (Fig. 1d) of a TIPS-PEN:PMMA film. Particularly, at the initial drying stages, the phase-separated TIPS-PEN molecules are transferred through convective flow at the contact periphery of the liquid film and initiate nucleation centers, resulting in a radial crystal growth towards the inner regions of the sample. The nucleation zone is mainly dominated by irregular-shaped crystals growing from the localized grain aggregations. After the nucleation step, the crystal growth (at the intermediate region) occurs in a more isotropic manner, giving rise to the formation of oriented and relatively uniform needle-like crystals. The lack of spherulitic growth and the presence of large sized crystals reveal an efficient vertical phase separation between the components of the blend, basically driven by the slow drying of the samples. The observed textured crystallization on the top surface of the sprayed blends can be attributed to the relatively low viscosity solutions used in this work. Indeed, it has been previously shown that low viscosity inks favor the vertical phase separation, thus assisting TIPS-PEN molecules to develop into large crystalline domains.^2,23 Finally, the morphological characteristics of the sprayed films significantly change in the center region of the substrate, in which bulky and irregularly shaped crystal grains can be observed. Nevertheless, the largest part of the film surface is characterized by relatively uniform and homogeneous crystal distribution.

It has been previously shown that the molecular weight (M_w) of the polymeric binder may cause significant changes in the segregation characteristics and the subsequent crystallization of the semiconductor.²⁴ Specifically, when using low M_w amorphous polymers, the segregation effects are weak resulting in mixed phase surfaces. In such a case the polymer phase may disrupt the crystalline formation, leading to a noticeable performance degradation. On the other hand, high M_w polymers may induce stronger vertical phase segregation, resulting in high quality phase separated crystalline structures which are beneficial for high charge carrier transport.^2,25,26 Taking into account the abovementioned aspects, high M_w polymer binders (PS and PMMA) were employed in the present study.

The effects of the different binding polymer and of the composition ratio on the crystalline morphology of the sprayed blends were investigated by means of AFM and POM measurements, as shown in Fig. 2. Both types of blends exhibited strong optical birefringence and distinct microcrystalline features on the top surface, indicative of the presence of phase separation phenomena, even though their morphological characteristics vary in terms of crystal size and shape. For the lower TIPS-PEN weight fraction (0.2), both blends showed a dendritic and branched morphology indicating that the crystal growth is likely hindered by the binding polymer. Such features are related with molecular disorder within the crystal domains and as a consequence with degraded electrical performance characteristics.²⁷ This is in accordance with the results reported by Madec et al. for drop-casted TIPS-PEN:polymer blends.²⁸ Interestingly, TIPS-PEN:PMMA blends revealed more uniform crystalline structures and homogeneous distribution, while in TIPS-PEN:PS films the crystals appeared to have an indefinite shape and size. The abovementioned observations indirectly implies a difference in the phase segregation strength between the two systems.⁸ The addition of more TIPS-PEN content (50%) in the blends resulted in the formation of elongated polycrystalline domains with enhanced continuity and orientation. The average crystal size was found ranging between 7–15 μm, while the apparent color variations within the crystals are attributed to thickness fluctuations. Further increase of TIPS-PEN fraction (0.8) did not show any pronounced effect on the crystallization scheme, even though larger crystallites were observed for both blends. It is assumed in this case that the crystal growth is mainly determined by the solvent evaporation and is not limited by the presence of the polymeric binder. Nevertheless, this qualitative assumption could not fully elucidate the stratification profile and the interface characteristics of the films. It should be noted, that under such conditions, TIPS-PEN:PS sprayed blends displayed a decent degree of uniformity and orientation. The considerable improvement in our approach can be perceived by comparing the morphology of the sprayed blends with that of neat TIPS-PEN films (see Fig. S1†). Specifically, neat films showed irregular shaped crystal domains and limited crystal coverage. This fact underlines the significant importance of using a polymeric binder when spraying a crystalline semiconductor.

Fig. 2 AFM topography of the (a–c) TIPS-PEN:PS and (d–f) TIPS-PEN:PMMA sprayed blends on SiO₂/Si substrates for the different ratios of 0.2 : 0.8, 0.5 : 0.5 and 0.8 : 0.2, respectively. The insets show the corresponding POM image. The scale bar is 20 μm.

We further investigated the structural properties of the airbrushed blends using out-of-plane XRD measurements (see Fig. S2†). The XRD spectra of the optimized blends showed sharp (00l) Bragg peaks indicating the presence of highly-ordered crystalline structures in the blended films. According to Bragg's equation (2d sin θ = nλ, λ = 1.54 Å), the primary (001) diffraction peak at around 2θ ≈ 5.36° corresponds to an interlayer spacing of about 16.5 Å, which is consistent with the c-axis unit cell of TIPS-PEN.²⁹ The comparison of the blends with the neat airbrushed TIPS-PEN films (inset of Fig. S2†), suggests that the blended films maintain a high degree of crystalline order, capable of generating high charge carrier mobilities.

3.2 Electrical properties of the airbrushed blends

The evidence of vertical stratification was further confirmed by measuring the capacitance–voltage (C–V) characteristics (at 1 MHz) of metal–insulator–semiconductor (MIS) capacitors based on the present blends (see Fig. S3†). The data collected for each different ratio, are presented in Fig. 3. As can be observed, the overall device capacitance is lower than that of 300 nm SiO₂ (∼10.9 nF cm⁻²), indicating the presence of a dielectric interlayer within the films. Furthermore, there is a clear trend of decrease in the total capacitance as the polymeric binder content increases, for both blend types. Assuming two parallel-plate capacitors in series, we estimated the contribution of the polymeric layer in the total capacitance and its corresponding thickness (as can be seen in Fig. 3a), using the following equation:
where C is the capacitance per unit area (F m⁻²), ε is the relative permittivity of the material, (ε_SiO2 = 3.9, ε_PS = 2.4, ε_PMMA = 2.6), ε_o = 8.854 × 10⁻¹² (F m⁻¹) is the permittivity of free space and d is the thickness (m). To verify the aforementioned assumption, AFM (Fig. 3b) measurements were carried out on the remaining polymeric layer after selective etching of TIPS-PEN from the TIPS-PEN : PS (0.8 : 0.2) films. The value of 63 nm, as measured in the corresponding section profile, is consistent with the mean thickness value estimated (∼66 nm) using the capacitance calculation. It should be noted that the apparent variations in the capacitance values is due to the thickness fluctuations of the wet films formed during airbrush spraying. For the precise calculation of the field-effect mobility, the capacitance was measured individually for each transistor.

Fig. 3 (a) Measured total capacitance (C_i) values of the airbrushed blends and the estimated thickness (d_pol) of the polymeric interlayer for each different ratio. The mean values were extracted from at least 8 MIS capacitors (b) AFM topography and the corresponding section profile (white line) of cyclohexane-etched TIPS-PEN : PS (0.8 : 0.2) airbrushed film.

Fig. 4(a and b) shows the I–V transfer and output (insets) characteristics of the best performing sprayed TIPS-PEN:PS and TIPS-PEN:PMMA OFETs, respectively. Both blends exhibited unipolar p-type behavior with a clear turn-on point and distinct linear and saturation regimes, while negligible current hysteresis for forward and backward scans of the gate voltage was recorded, indicating the efficient self-passivation of the SiO₂ surface by the polymeric binder. Overall the optimized transistors exhibited high hole mobility with a maximum value of 1.3 cm² V⁻¹ s⁻¹ for the TIPS-PEN : PS blends (@0.8 : 0.2 ratio) and around 0.5 cm² V⁻¹ s⁻¹ (@0.8 : 0.2 ratio) for TIPS-PEN:PMMA blends. These values are comparable to that of drop-cast or inkjet-printed transistors based on blends and among the highest obtained for sprayed OFETs. The performance of the airbrushed blends was further evaluated by comparing their transistor characteristics with that of neat TIPS-PEN (see Fig. S4†). Particularly, the maximum obtained mobility for neat TIPS-PEN OFETs was found to be around 0.15 cm² V⁻¹ s⁻¹, while the characteristic curves consistently exhibited high hysteresis and unstable behaviour under environmental conditions. The significantly higher mobility of blend films can be ascribed to the “zone-refinement effect” which occurs during phase separation and excludes impurities from the crystalline layer.³⁰ Additionally, the presence of a polymeric binder reduces the rate of solvent evaporation which leads to better crystalline morphology.³¹

Fig. 4 I–V transfer and output (inset) electrical characteristics of the best performing airbrush sprayed (a) TIPS-PEN:PS and (b) TIPS-PEN:PMMA OFETs. Statistical diagrams showing the average (c) mobility, (d) threshold voltage and (e) on/off current ratio values of both sprayed blends.

The device-to-device uniformity was examined by calculating the basic transistor parameters (μ, V_T, I_on/I_off) for each ratio of the tested blends as shown in the Fig. 4c–e. For the accurate comparison of the different samples, transistors were picked from the same regions. The TIPS-PEN:PS blends exhibited a strong tendency to increase the field effect mobility upon increasing the TIPS-PEN weight fraction. Particularly, the average hole mobility value was found to increase as follows: 0.16 ± 0.12 cm² V⁻¹ (@0.2 : 0.8), 0.30 ± 0.23 cm² V⁻¹ (@0.5 : 0.5) and 0.75 ± 0.38 cm² V⁻¹ (@0.8 : 0.2). In contrast, only slight changes were observed for TIPS-PEN:PMMA blends, with the average mobility ranging between 0.15–0.27 cm² V⁻¹ s⁻¹.

The variations in the mobility values for the different ratios, especially in the case of TIPS-PEN:PS, can be ascribed to the different topographical features and the quality of the critical interfaces, induced by the phase separation phenomena. As evidenced by the POM measurements, the airbrushed blends consist of irregular crystals and a highly branched morphology for low TIPS-PEN weight fraction, which is directly related with the decreased performance of the transistors. Increasing the TIPS-PEN content resulted in distinct ribbon-like separated crystals, which are consistent with the high charge carrier mobility. As in this case both blend systems exhibit high quality phase separated domains at the air–film interface, we hypothesize that the performance discrepancy is most likely attributed to the different inherent properties of the polymeric binders (i.e., surface energy or polarity), as well as to the disparate interfacial characteristics. To further confirm the latter assumption, AFM measurements were performed in the cyclohexane-etched blends as shown in Fig. S5.† In the case of TIPS-PEN:PS the relatively smooth and neat layer of the remaining PS film indicates a well-formed interface, contrary to the rougher and inhomogeneous surface of TIPS-PEN:PMMA films. This is also evidenced by the phase images of TIPS-PEN:PMMA, which show crystalline fractions decorating the polymeric surface. In addition, TIPS-PEN:PS blends showed higher threshold voltage values, probably associated with the larger crystalline domains and the consequent limited charge carrier injection. Further, both blends exhibited high values of I_on/I_off, ranging from 10⁴ to 10⁶, without any pronounced trend. It is well established that among the two commonly used blend systems, the airbrushed TIPS-PEN:PS system provides better performing OFETs.

The long-term environmental stability of the fabricated OFETs was investigated by exposing a batch of devices over a period of 13 months under atmospheric conditions (T = 22 °C, relative humidity R_H = 55%). As shown in Fig. 5, both blends demonstrated excellent air stability under such prolonged period of time, with only a slight decrease in the on-current. The increase in the off-current levels which results in a decrease of about two orders of magnitude in the I_on/I_off ratio, can be attributed to the gradual degradation of the crystalline semiconductor. The slight hysteresis in the freshly prepared OFETs is probably related to residual solvents in the films. Surprisingly, no significant shifts in the threshold voltage of the exposed OFETs were observed, confirming the good self-encapsulation capability of such blend systems. Such enhanced stability characteristics have been observed in similar small molecule/insulating polymer blends.^5,32 We assume that the use of blend films not only effectively passivates the hydroxyl groups of SiO₂ surface limiting the formation of interfacial traps, but also maintains the charge transport capability of semiconductor by repelling the accumulation of impurities.³⁰

Fig. 5 I–V transfer electrical characteristics measured in airbrush sprayed OFETs based on TIPS-PEN/PS (a) as well as TIPS-PEN/PMMA (b) blends right after fabrication and after 13 months in air and moisture.

4. Conclusions

In summary, we have herein demonstrated high performance airbrushed OFETs based on semiconductor:insulating polymer blends. Specifically, TIPS-PEN was blended with high M_w PS or PMMA in different composition ratios and sprayed on top of SiO₂/Si substrates. Using such blends via airbrush technique, significantly improved the film forming behavior of the semiconducting solution. Further, by optimizing the blend parameters, continuous and well-ordered TIPS-PEN crystal structures were formed on the top surface, indicating the efficient phase separation between the two components. The investigation of the effect of the composition ratio on the film morphology revealed that higher TIPS-PEN weight fractions (≥50%) bring about better crystalline characteristics yielding enhanced charge carrier properties. Both blend systems exhibited excellent I–V transistor behavior with negligible current hysteresis, on/off > 10⁵ and near-zero turn-on voltages. The best performing transistors (TIPS-PEN:PS) showed mobilities exceeding the benchmark values of amorphous silicon (0.5–1 cm² V⁻¹ s⁻¹). Finally, the air-brushed devices exhibited exceptional environmental stability with minimum performance degradation after exposing them for more than a year in atmospheric conditions. These results highlight the great versatility of semiconducting blends and underline the great potential of spraying methods toward the realization of cost-effective electronic devices.

Acknowledgements

This work was supported by: EC Project REGPOT-286022 ROleMak. The authors gratefully acknowledge Mr Nikolaos Kipouros for the POM measurements.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22342g

‡ Contributed equally.

§ Present address: Department of Bioelectronics, Ecole Nationale Superieure des Mines, CMP-EMSE, MOC, 13541 Gardanne, France.

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