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Application of rubrene air-gap transistors as sensitive MEMS physical sensors Marco J Pereira, Micaela Matta, Lionel Hirsch, Isabelle Dufour, Alejandro L. Briseno, Sai Manoj Gali, Yoann Olivier, Luca Muccioli, Alfred Crosby, Cedric Ayela, and Guillaume Wantz ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15319 • Publication Date (Web): 06 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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ACS Applied Materials & Interfaces

Application of rubrene air-gap transistors as sensitive MEMS physical sensors Marco J. Pereira1, Micaela Matta2, Lionel Hirsch1, Isabelle Dufour1, Alejandro Briseno3, Sai Manoj Gali2, Yoann Olivier4, Luca Muccioli2,5, Alfred Crosby3, Cédric Ayela1* and Guillaume Wantz1* Univ. Bordeaux, IMS, CNRS, UMR 5218, Bordeaux INP, ENSCBP, F-33405 Talence, France Univ. Bordeaux, ISM, CNRS, UMR 5255, F-33405 Talence, France 3 Polymer Science & Engineering University of Massachusetts Amherst 120 Governor’s drive, Amherst, MA 01003, USA 4 Laboratory for Chemistry of Novel Materials University of Mons Place du Parc 20, B-7000 Mons, Belgium 5 Department of Industrial Chemistry “Toso Montanari”, University of Bologna, I-40136 Bologna, Italy 1 2

*Authors

to whom correspondence should be addressed: [email protected], [email protected]

KEYWORDS: organic MEMS, OFET, air-gap transistor, crystal, rubrene, pressure sensor, charge injection ABSTRACT: Micro electromechanical systems (MEMS) made of organic materials have attracted efforts on development a new generation of physical, chemical and biological sensors, for which the electromechanical sensitivity is the current major concern. Here, we present an organic MEMS made of a rubrene single crystal air-gap transistor. Applying mechanical pressure on the semiconductor results in high variations of drain current: an unparalleled Gauge factor above 4000 has been measured experimentally. Such a high sensitivity is induced by the modulation of charge injection at the interface between the gold electrode and the rubrene semiconductor as an unusual transducing effect. Applying these devices to the detection of acoustic pressure shows that force down to 230 nN can be measured with a resolution of 40 nN. This study demonstrates that MEMS based on rubrene air-gap transistors constitute a step forward to the development of high performance flexible sensors.

INTRODUCTION Since the first commercialization of silicon strain gauges in the 60’s1, micro electromechanical systems (MEMS) have been at the heart of intense research. However, the majority of these systems are based on silicon material and associated micro and nano machining techniques that suffer from inherent high cost and environment unfriendly fabrication processes. In order to overcome these major concerns, MEMS based on organic materials are currently explored. Organic semiconductors are viewed as a promising replacement for silicon for many applications in electronics and photovoltaic. In addition, they enable the development of flexible MEMS which represent a good prospect for several applications such as artificial skin and wearable electronics2–6. To achieve competitive MEMS, high electromechanical sensitivity (the ratio of the relative variation of the electrical sensor output to the mechanical strain, also called gauge factor – GF) and ease of fabrication are required. These two parameters are dependent on the transducing effect implemented into the devices. In organic MEMS, piezoresistivity, capacitance and piezoelectricity are the three main transducing mechanisms explored so far7. Piezoresistive materials present the sufficient mechanical stability required for industrial applications, however, they suffer from drawbacks such as temperature dependence, nonlinear response, low scale factor and relatively low sensitivity. In some studies different

proportions of highly conductive graphene8 and multiwalled carbon nanotubes9 were dispersed into polydimethylsiloxane (PDMS) to obtain the maximum sensitivity, response time and detection limit. In a different approach, Thuau et al. improved by a factor of 3.7 the electromechanical sensitivity by combining a low-cost piezoresistive organic MEMS with an organic field effect transistor (OFET)10. The variation of strain into the piezoresistive MEMS cantilevers triggered the gate voltage in the OFET, resulting in increase of the drain current of the transistor. Regarding capacitive transduced MEMS, their sensitivity is typically higher than piezoresistive MEMS. In 2014, Zang et al. showed for the first time the possibility to measure sound pressure by means of an organic transistor integrating an aluminum floating gate11. By measuring changes in capacitance due to gate vibrations, they were able to reach fast time response and low-pressure detection under 0.5 Pa. Also, the sensitivity of organic MEMS has been improved by structuring the gate dielectric made of elastomer PDMS12,13. Rectangular pillars or pyramidal shapes were fabricated and tested as thin-film capacitors and gate dielectric into OFETs based on rubrene single crystals or as flexible transparent films in triboelectric nanogenerators13. Sensitivity increases were measured with the patterned films. These variations were attributed to the presence of air voids inside the patterned dielectric, which decrease the elastic resistance and dielectric constant of the film. Mannsfeld et al. were able to measure the pressure induced by objects as small as a fly12. This approach

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has also been used by Fan et al. in triboelectric nanogenerators to detect the pressure applied by a bird feather13. With their respective transducing mechanism these state-of-the-art sensors have been able to detect pressures equal to 3 Pa12 and 0.4 Pa13 with a surface of detection respectively equal to 64 mm2 and 3 cm2. These pressures correspond to an applied force on the sensor of approximately 150 µN. Although piezoelectric materials can be efficient in transducing mechanical strains into electrical signals, OFETs were used again to further enhance the organic MEMS sensitivity. In one of these examples, a polarized piezoelectric polymer (PVDF-TrFE) can be used as the gate dielectric material into OFETs14. A strain applied to the device induces a change of polarization in the piezoelectric gate, which therefore affects the charge density in the semiconductor. By this mean, the electromechanical sensitivity (also called gauge factor in the case of piezoresistivity) was demonstrated to reach 600 for low strain levels. All these examples demonstrate the use of OFET as an efficient strategy to enhance the electromechanical sensitivity of integrated organic MEMS. Nevertheless, in most of cases, the transducer element is integrated into a supporting material. Combining both elements into one single device will enable to further mature the field of organic MEMS by fabricating competitive devices with high sensitivity. This study presents an innovative solution to transduce mechanical stimuli into electrical signals, in order to reach high sensitivities. The MEMS are designed using suspended rubrene single crystals on top of air-gap transistors as electromechanical transducers. Rubrene single crystals ensure the mechanical stiffness of the suspended clamped-clamped bridge, as well as being the core sensitive element of the integrated transducer. Indeed, a mechanical pressure applied on top of the crystal induces significant increases of drain current. This MEMS configuration exhibits an unprecedented high electromechanical sensitivity, with gauge factors above 4000.

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In a previous study we demonstrated that the intermolecular modifications induced by such mechanical strain cannot be responsible of such a sensitive transduction effect15, while here we evidence a mechanical dependence of hole injection in the semiconductor as the main transducing mechanism leading to those very high gauge factors. As a model application, the MEMS were used as acoustic pressure sensors, to detect sound waves pressure down to 23 Pa with a resolution around 4 Pa. With a surface of detection around 0.01 mm2, this pressure corresponds to force of 230 nN applied to the sensor with a resolution of 40 nN, making such devices particularly sensitive. Furthermore, the devices have been fabricated in just a few simple steps, showing the possibility of obtaining highly sensitive organic MEMS sensors while keeping the fabrication process fast and cheap.

MATERIALS AND METHODS Architecture of the sensor The original MEMS presented here consist in rubrene bottom-gate bottom-contacts air-gap transistors. Their fabrication is inspired by the work of Menard et al.16. The airgap is 7 µm-deep and the channel length is 100 µm, while its width is the one of the laminated rubrene crystal itself (under 100 µm). As shown in Figure 1a, the device is composed of three layers: a microstructured air-gap substrate is made of polydimethylsiloxane (PDMS), on top of which gold (Au) electrodes are evaporated and where a single crystal of rubrene is laminated. Rubrene is selected as organic semiconductor because of its high mobilities (up to 40 cm2.V-1.s-1)17, and interesting mechanical flexibility as thin crystals18,19.

Figure 1. Illustrative schematics, microscopic image and optical profilometer images of the OFET MEMS (a) Representation of the device under mechanical stress, (b) microscopic image a single crystal of rubrene on an air-gap structure, (c) and (d) 3D images of a single crystal of rubrene obtained by optical profilometry (one can spot the shadowing effect on the pictures of the cantilever used to strain the crystal), (c) at rest, and (d) under stress.


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ACS Applied Materials & Interfaces First, to fabricate the PDMS substrates containing the microstructured drain and source patterns, a moulding process is used, starting with the fabrication of a silicon master. For this, a Si-wafer was cleaned in acetone, ethanol, isopropanol baths during 15 min. Then a thin layer (7 µm) of negative photoresist (AZnLOF2070 from MicroChemicals GmbH Ulm) was spin coated on the wafer, 10 s at 500 rpm with an acceleration of 100 rpm·s-1 and then 30 s at 4000 rpm with an acceleration of 500 rpm.s-1. A soft bake was processed at 110°C on a hot plate during 6 min before UV exposition (18 s with 19.5 mW/cm2). Then a post exposure bake was performed at 110°C during 5 min. Finally, the non-polymerized resin was removed in a development bath (AZ326MIF from MicroChemicals GmbH Ulm) with agitation during 2 min at room temperature. Special attention was paid to obtain well-defined undercut features during the etching process, preventing any short circuits between the final gate and source/drain electrodes. To reach this goal, the development time of the resin was optimised to obtain visible undercuts. Moreover, to decrease the adhesion between the Si mould and the PDMS, a self-assembled monolayer of trichloro(1H,1H,2H,2H-perfluorooctyl)silane (from SigmaAldrich) was deposited under primary vacuum on the mould. Afterward, a mixture of PDMS, Sylgard 184 (from Neyco), was prepared with a weight ratio precursor/monomer of 1:5. After mixing, the PDMS solution was degassed in a vacuum chamber for approximately 1 hour. Finally, the mixture was poured onto the previous Si mould and cured overnight at 50°C under vacuum. Then, the patterned chips were cut from the PDMS bulk and glued on clean glass substrates. A thin gold layer of 30 nm-thick was then deposited on the overall sample surface using electron-beam evaporation under vacuum (10- 7 mbar) with a deposition rate between 0.06 and 0.1 nm·s-1. Subsequently, rubrene crystals were manually laminated between the source and drain electrodes. As the charge carrier mobility is anisotropic in rubrene crystals, the long crystal axis20, along which the mobility is the highest, was aligned along drain source electrodes (see in the Figure 1b). Rubrene crystals were grown using the PVT (Physical Vapor Transport) process, described by Laudise et al.21, starting from commercial rubrene powder (Sigma-Aldrich) sublimated at 300°C under an Argon flow of 15 mL.min- 1, after three successive purifications. The growth time was 40 min in total, starting from room temperature. This method enables to grow crystals with various thicknesses from 100 nm to 4 µm. To demonstrate the influence of the interface between the Au electrodes and the semiconductor on the transducing effect, a second batch of MEMS was fabricated by adding a 5 nm-thick layer of molybdenum oxide (MoO3) on top of the Au electrodes. This layer was evaporated by electron-beam evaporation under vacuum (10- 7 mbar) with a deposition rate of 0.1 nm·s-1 prior to the lamination of the rubrene crystal. Characterization of the electromechanical sensitivity To quantify the electromechanical sensitivity of the present organic MEMS, we used the Gauge Factor (GF). In the present case, GF in Eq.1 represents the relative variation of electrical drain current ID of the air-gap transistor for a mechanical strain 𝜺 applied to the beam, here, the rubrene crystal: Δ𝐼𝐷 𝐼𝐷 Eq 1 𝐺𝐹 = 𝜀

To induce different strain values into the rubrene crystal, a small piezo-controlled tip from Imina Technology was used to apply a force in the middle of the beam, as shown in Figure 1a. For each applied force, the deflection profile of the beam is recorded in situ by optical profilometry (Veeco NT9080). This profile corresponds to the one of a clamped-clamped cantilever with a force applied in the middle of the structure. We assume that the cantilever profile is constant along both the width and the thickness of the crystal, and that this profile is well reproduced by equation 2: 𝐿 For 𝑥 ∈ 0, , 2 Eq 2 𝑥2 𝑥3 𝛿(𝑥) = 4𝛿𝑚𝑎𝑥 3 2 ― 4 3 𝐿 𝐿

[ ]

(

)

Equation 3 is used to calculate the strain experienced by the crystal (see Supplementary information –S7 for further details): 𝜀=

1

𝐿2

𝐿

0

∫

[

2

12𝛿𝑚𝑎𝑥 𝑑 𝛿(𝑥) 2 𝑑𝑥 = 𝑑𝑥 5 𝐿2

]

Eq 3

where, 𝛿 is the deflection along the x-axis (x = 0 corresponds to one clamped-end of the beam), 𝜀 is the total strain along the cantilever, L is the length of the beam (channel length of the organic transistor) and 𝛿𝑚𝑎𝑥 the maximum deflection in the middle of the rubrene beam (x = L/2). Figures 1c and 1d show optical profilometry 3-dimensional profiles of a rubrene crystal beam at rest (without any applied force to the crystal) and under stress (with an applied force), respectively. On these images, the shadow of the tip applying the force is visible. The deflection profile is extracted along the source/drain direction. In addition to this optical characterization, three other piezocontrolled tips were used as electrical contacts for the source, drain and gate. For each applied force to the device, output and transfer curves of the air-gap transistor were recorded. Electrical measurements were performed in air and in the dark after a rest of at least one hour in dark. This rest allows a completely release the photo-induced charges in rubrene allowing reproducible data acquisition. Drain currents were measured in the saturation regime (VDS = – 50 V). One should note that the deformation applied to the crystal is fully reversible. Removing the tip enabled the crystal to get back to his initial position and to restore the initial electrical characteristics. Furthermore, care has been taken not to overload the crystal, to avoid its breakage. Pressure sensor characterization Sound pressure was used to evaluate the performance of the MEMS as pressure sensor. For this, an audio-speaker was positioned 10 cm above the MEMS, and connected to a network analyzer (Agilent E5061B) delivering a signal with a controlled frequency and power (POUT) and measuring the ratio between injected and measured signal. A Keithley 2614B sourcemeter was connected to the source, gate and drain, in order to power up the air-gap transistor. To measure the output signal from the air-gap MEMS sensor, the drain was connected to the network analyzer (PIN) using a 10 MΩ resistor, as shown in the scheme presented in Figure 2.


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Figure 2. Scheme of the set up used to characterize the air-gap MEMS as a pressure sensor.

To determine the limit of detection of the MEMS sensor, power sweeps were performed, from – 45 dBm to – 8 dBm (electrical power delivered by the network analyzer) for each of the following frequencies: 113, 213, 313, 413, 513, 613, 713, 813, 913 and 1013 Hz. The dBm corresponds to the power ratio in decibels referenced to one milliwatt. Although all the measurements were done inside a Faraday cage, frequency values were chosen in order to avoid electromagnetic noise coming from the harmonics of the power supply (50 Hz, European grid). Also, it was considered that for a given electrical power applied to the speaker, the corresponding acoustic pressure applied to the sensors is frequency dependent. To take into account this dependence, a calibrated microphone was placed a posteriori to record at the same position the power sweep, enabling a “conversion” from electrical power delivered by the network analyzer (dBm) to the sound power measured by the microphone (dB) with a homemade Matlab script. The conversion of the sound power (dB) into an applied pressure is obtained with 0 dB acoustic corresponding to 20 µPa (standard reference of hearing threshold).Finally, the limit of detection of the air-gap MEMS pressure sensor was determined by using three times the standard deviation of the baseline of the output signal. In addition, the cut-off frequency was determined by simply applying a frequency sweep with a power of – 10 dBm.

RESULTS AND DISCUSSION Electromechanical properties of rubrene air-gap transistors First, the fabricated devices were tested as strain sensors to determine their electromechanical sensitivity. Increasing forces were applied to the top surface of the rubrene crystals and for each of them the strains were calculated, and the drain currents recorded. One should note that the air-gap configuration is the best one to transmit directly the force to the crystal. Moreover, using air as dielectric allows the deformation of the crystal without changing the mechanical/electrical interface between the semiconductor and the dielectric. Figure 3a shows the profiles of the crystal for different applied forces. The top line is the crystal at rest and each consecutive line corresponds to an increasing force. The higher the force is, the more deflected the crystal is and the larger the corresponding strain is. Due the manual lamination process, the rubrene crystals are never perfectly flat between the two electrodes. This induces an initial curvature, either positive or

negative, as evident from Figure 3a. Figure 3b shows the resulting transfer curves for the corresponding strain values applied to the rubrene crystal and presented in Figure 3a. As the rubrene is a p-type organic semiconductor, the transistors were tested in the range of 0 V to –50 V for the gate voltage (VGS) and a constant drain voltage (VDS) of –50 V. At rest, the ON/OFF current ratio is higher than 100 and the mobility is calculated to 2.68 cm2.V-1.s-1 at VGS = –50 V using the Equation S1. When applying a positive strain into the rubrene crystal, the resulting transfer curves show that a higher strain induces higher drain currents. At the maximum strain of 0.06%, the ON/OFF current ratio increases, compared to the curves at rest, as the mobility is multiplied by more than a factor two (from 2.68 to 6.65 cm2.V-1.s-1). The resulting gauge factor (GF, see Eq. 1) of the air-gap MEMS was calculated to quantify the electromechanical sensitivity. Figure 3c shows the results of one representative device. Tremendously high GF were obtained, above 4000, which is one of the largest sensitivity reported in the literature14,22. This result has been obtained by probing the electrical response along the b-axis of the crystal. It is well known that this axis shows the best mobilities compared with the two other ones. The electromechanical response of the axis a and c have not been probed, respectively due to a smaller electromechanical response15 and the impossibility to laminate the crystal on the edge. To investigate the transducing mechanism inducing these high variations of drain current, the capacitive GF was first calculated. Indeed, as the gap between the rubrene semiconductor and the gate is decreasing when the force applied to the crystal is increasing, the dielectric capacitance is intrinsically changing. By the way, capacitive changes have been already mentioned as one widely used transducing mechanism in organic MEMS integrating transistors. In the present case, it can be seen from Figure 3c that the GF associated to the capacitive changes is relatively high (around 250) but remains lower than the drain current GF at 4200. The capacitance is calculated by using the measured deflection profile. Thus, variation of capacitance cannot by itself explain these high GFs. Other possibilities could include piezoresistive effects, however we showed in a previous investigation15, and confirmed by further calculations (Supplementary information: Molecular dynamics simulations), that they are responsible for GFs around 25 for rubrene single crystals under uniaxial strain along the main


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ACS Applied Materials & Interfaces crystal axis. Similar values were also reported by ReyesMartinez et al.23 In fact, in the hypothesis of clamped-clamped beam the application of a vertical force should induce expansion of all the layers of the crystal, where injected charges

are flowing. The mechanical setup employed here is indeed not very different from uniaxial horizontal strain, and accordingly piezoresistivity variations are expected to be in the same order of magnitude than the ones found in references15,23.

Figure 3. Mechanical and electrical responses of the OFET MEMS under stress (a) Deflection profiles of a crystal for different forces applied (b) Resulting transfer curves for a device with uncoated gold electrodes, (c) and (d) Variation of capacitance and drain current and corresponding Gauge factors (GF) at VDS = VGS = – 50 V, for devices without and with MoO3 coating, labelled “Au” and “Au+MoO3”, respectively.

To elucidate the transduction mechanism in the air-gap MEMS, the variation of the charge injection at the semiconductor / electrode interface has been then considered as the primary source. In order to assess the validity of this assumption, we improved the charge injection by adding, in a second device, a 5 nm layer of MoO3 evaporated between the gold electrodes and the crystal. MoO3 has previously been used with a thin film of pentacene to improve contact properties24,25. Indeed, by reducing the roughness of the electrode surface, MoO3 enables a better contact with the crystal, as well as a reduction of the injection barrier height at the interface between the organic semiconductor and the gold electrode. Figure S4 shows the drain current for the two electrodes configurations, with and without MoO3. With MoO3, the ON/OFF ratio is higher by almost four orders of magnitude in the transfer curves. This improvement can be attributed to a decrease of the electrodes roughness and an interfacial doping26. AFM images of Au and Au/MoO3 electrodes can be found in the Supporting Information (Figure S5). The surface of Au/MoO3 electrodes exhibit indeed a smaller roughness, since Zrms = 2.8 nm while Zrms = 4.6 nm is measured for Au electrodes. Moreover, Wu et al.27 shown recently that compressive strains on rubrene lead to a decrease of the work function and a consequent enhancement of hole injection. Since in our air gap MEMS, compressive forces are present at the contacts as soon as a force is applied to the crystal, and on the bottom layers of the crystal where the charge injection occurs, we can assume that effects described

in27 play a major role in increasing the GF. However, one cannot disentangle here if the decrease of the injection barrier or the increase of the contact area upon strain is dominating the enhanced charge injection inside the crystal. To evidence the impact of charge injection in the electromechanical response of the air-gap MEMS, Figures 3c and 3d show both the relative variation of drain current and capacitance for different applied strains on devices without MoO3 and with MoO3, respectively. The relative variation of drain current is in the same order of magnitude as the capacitive variation for the device with MoO3. The difference between capacitive GF and drain current GF in Figure 3d can be due to: (i) an approximation in the calculation of the capacitance as the entire capacitance is extrapolated from a crystal profile, and (ii) the contact resistance is reduced thanks to MoO3, but not totally inhibited. This result evidences however that there is no more amplification of the drain current when the charge injection at the interface between the rubrene and drain/source electrodes is improved. Thus, the tremendously high drain current variations of the rubrene air-gap MEMS can be unambiguously attributed to the modulation of the charge injection at the interface between the crystal and the electrodes when a force is applied to the crystal. Detection of sound pressure As a model application, the presented air-gap MEMS devices are tested as acoustic pressure sensors. For this, a sound wave


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was applied above the MEMS and the electrical response of the sensors was recorded (see materials and methods section for experimental details). Indeed, applying an acoustic wave above the suspended rubrene crystal induces a vibration of the latter due to the acoustic pressure, resulting in changes in the electrical response of the air-gap transistor. Figure 4a shows the response of an air-gap MEMS under a power sweep between – 50 dBm and – 8 dBm (the maximum of the power range was limited by the saturation threshold of the network analyzer). The plots in Figure 4a depict the electrical response of the sensor averaged over the ten frequencies tested (described in the material and method section). From this figure, an increase of the output signal is observed for electrical power above 20 dBm when the sensor is ON (VDS = – 50 V). A much smaller increase of the output signal can be seen in the OFF state (VDS = – 20 V), and this increase is observable at higher electrical power (above – 16 dBm). The response in the OFF state can be due to the softness of PDMS supports inducing small deformations as all the MEMS is subjected to the acoustic pressure. Figure 4b shows the frequency dependent limit of detection of acoustic pressure, expressed in Pa, of the air-gap MEMS sensors. The results show that the devices can detect low acoustic levels, 89 dB at 713 Hz, which corresponds to a force of 6 nN (0.6 Pa). However, we considered that our limit of detection corresponds to the highest obtained value so far,

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around 230 nN (23 Pa) in the range of frequencies tested. Such a low limit of detection has never been reported in the literature on organic MEMS. The smallest change of force that the sensor can detect, shown in Figure 4c, was calculated for each sensor by dividing the three times standard deviation of the response obtained between – 50 dBm and – 25 dBm by the linear regression coefficient obtained, which corresponds to the sensitivity of 8.10- 6 dBm-1, between the limits of detection and – 8 dBm. The sensitivity is not expressed in Newton because the linear regression is done on dBm values which is a logarithm scale. The best resolution achieved was 6 nN (0.6 Pa). However, here again, we assume that our resolution corresponds to the maximum obtained, around 40 nN (4 Pa) in the range of frequencies tested. Finally, the cut-off frequency was determined by simply applying a frequency sweep with an electrical power of – 10 dBm. As shown in Figure 4d, the sensor response decreases drastically above 2 kHz. The cut-off frequency of 2 kHz is considered high for such organic devices, as the response time is limited by the finite mobility of the charges inside the semiconductor. Overall these results demonstrate the possibility to exploit the variation of charge injection as a mechanism of transduction and thus highlight the opportunity to use organic MEMS has a simple transducing block to reach high sensitivities in pressure sensing.

Figure 4. Sensor characteristics, (a) Average output for a power sweep when the sensor is OFF and ON, (b) Limits of detection for the frequencies tested and (c) Resolution for the frequencies tested, (d) cut-off frequency for a sensor at – 10 dBm.

CONCLUSION To conclude, in this study, the use of rubrene air-gap transistors as organic MEMS with integrated transduction for physical sensing applications has been reported for the first time. Modulation of the charge injection at the interface between the rubrene semiconductor and the drain/source electrodes has been evidenced as the transduction mechanism enabling such experimental gauge factors. The air-gap

architecture as a simple block of transduction exhibits tremendously high strain sensitivity, with Gauge Factor up to 4200, showing that this simple transducing mechanism is particularly efficient compared to the three commonly used: piezoresistivity, capacitance and piezoelectricity. Moreover, this original MEMS has been used as a sensor able to detect low acoustic pressures. The limit of detection has been measured down to 23 Pa, with a resolution of 4 Pa and a sensitivity of


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ACS Applied Materials & Interfaces 1.10- 5 Pa- 1. The presented electromechanical transduction mechanism proves the possibility to use low-cost organic materials to develop highly sensitive MEMS physical sensors. Furthermore, the simple architecture of the air-gap transistor, where the semiconductor is used also as mechanical support, opens the way towards innovative and highly sensitive transduction schemes.

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ASSOCIATED CONTENT Supporting Information

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Figure S5. AFM measurements of the electrodes surfaces of the air-gap MEMS. Equation S2. Relation between deflection and length of a curve.

AUTHOR INFORMATION Corresponding Authors Dr. Cédric Ayela, Dr. Guillaume Wantz Université de Bordeaux, ENSCBP Laboratoire IMS, UMR 5218 16, Av Pey Berland, 33607 Pessac, FRANCE tél: +33 6.58.73.26.00 [email protected] [email protected]

Author Contributions C.A., G.W. and M.P. conceived and designed the experiments. C.A., G.W. and M.P. carried out the fabrication, optimization and characterization of the sensors. M.M., L.M., S.M. and Y.O. carried out additional simulations on the rubrene single crystals. All authors contributed to the scientific discussion and redaction of the manuscript.

Conflict of interests The authors declare that there is no conflict of interest regarding the publication of this article.

ACKNOWLEDGMENT The collaboration between the University of Bordeaux and the University of Massachussetts, and the research activity of M.P., M.M., S. M. G., and L. M. were supported by the LabEx AMADEus (ANR-10-LABX-42) in the framework of IdEx Bordeaux (ANR-10-IDEX-03-02) i.e. the Investissements d’Avenir programme of the French government managed by the Agence Nationale de la Recherche.

ABBREVIATIONS AFM, Atomic Force Microscopy; GF, Gauge Factor; MEMS, Micro Electro Mechanical Systems; OFET, Organic Field Effect Transistor; PDMS, PolyDiMethylSiloxane.


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