A flexible bimodal sensor array for simultaneous sensing of pressure and temperature

www.advmat.de www.MaterialsViews.com COMMUNICATION A Flexible Bimodal Sensor Array for Simultaneous Sensing of Pressure and Temperature Nguyen Thanh Tien, Sanghun Jeon, Do-Il Kim, Tran Quang Trung, Mi Jang, Byeong-Ung Hwang, Kyung-Eun Byun, Jihyun Bae, Eunha Lee, Jeffrey B.-H Tok, Zhenan Bao, Nae-Eung Lee,* and Jong-Jin Park* An electronic skin (e-skin) is comprised of arrays of pixels that function as sensing devices for various targeted external stimuli.[1–17] It also consists of signal processing circuits embedded in large-area flexible or stretchable substrates Typically, a defined number of pixels are designed to sense specific types of external stimuli In many previously reported works, single modality e-skin in which one sensor in a single pixel measures a unique sensing parameter in either pressure[11–19] or strain[19,20] has been investigated In practical applications of highly functional e-skin, e.g., artificial finger, multimodality in simultaneous sensing of multiple stimuli such as temperature, strain and pressure, is required In previous reports on multimodal sensing,[21–23] multiple sensors that are integrated into a single pixel are able to sense multiple stimuli simultaneously However, large-area integration of the multiple sensors with different sensing principles in a pixel of the multimodal e-skin requires sophisticated fabrication processes Furthermore, quantitative measurements for precise sensing of arbitrary input stimuli, e.g., strain, pressure and temperature, from the sensing devices in single or multimodal e-skin have rarely reported even though their electrical responses to known input stimuli were often measured.[1–17] N T Tien,[+] D.-I Kim, T Q Trung, B.-U Hwang, N.-E Lee School of Advanced Materials Science & Engineering and Sungkyunkwan University Suwon, Kyunggi-do, 440-746, Republic of Korea E-mails: nelee@skku.edu M Jang, N.-E Lee SKKU Advanced Institute of Nano Technology (SAINT) Sungkyunkwan University Suwon, Kyunggi-do, 440-746, Republic of Korea S Jeon,[+] K.-E Byun, J Bae, E Lee, J.-J Park Samsung Advanced Institute of Technology Samsung Electronics Corporation Yongin, Kyunggi-do, 446-712, Republic of Korea E-mails: jongjin00.park@samsung.com S Jeon Department of Applied Physics Korea University Sejong City, 339-700, Korea J B.-H Tok, Z Bao Department of Chemical Engineering Stanford University Stanford, California, 94305, USA [+]These authors contributed equally to this work DOI: 10.1002/adma.201302869 Adv Mater 2013, DOI: 10.1002/adma.201302869 In flexible e-skin, the target signals from sensing elements under multiple stimuli are often influenced by the other stimuli including strain experienced by the e-skin For example, signals from both temperature and pressure sensors in a matrix are influenced by other external stimuli of pressure and temperature, respectively.[21,22,24] To date, signals originating from mechanical interferences are minimized at the level of sensing device by simply calibrating the output signals, which are usually achieved via designing compensating circuits[18,19] and reference devices.[20,21] However, this approach has only been met with limited success to date Another approach is to employ sensing materials that are resistant to strain such that strain induced interference can be avoided.[16,22] This approach requires the usage of rigid materials, which often require special processing steps, complex fabrication designs and integration processes onto flexible substrates.[23] In addition, other limitations such as structural complexity due to the integration of heterogeneous sensing materials and devices, large power consumption, cross-sensitivity to multiple stimuli, and inaccurate signal read-out, have all greatly complicated this approach In most of flexible e-skin investigations, issues relating to signal interference by mechanical deformation or other stimuli have yet been addressed in detail To address the above mentioned challenges, we hypothesized that another viable approach is to separate or decouple a target sensing signal from interferences stemming inherently from stress-related deformation of flexible sensing elements in flexible e-skin In this work, we resolve the subjected interferences by using a field-effect transistor (FET) sensor platform integrated with multi-stimuli responsive materials as subcomponents of gate dielectrics and channel FET is an ideal platform for multi-stimuli e-skin sheets, and various flexible or rigid stimuli-responsive FETs for chemical, biological, and physical sensors have been previously reported.[24–31] As mentioned, signal interference generated via global straining of the flexible FET sensors has yet been addressed Although FET’s device physics and analytical equations have extensively been developed to allow quantitative analysis of the device’s parameters,[32–34] however a detailed analysis of the FET sensors under multiple physical stimuli has rarely been reported In this report, we describe the direct integration of both the piezopyroelectric gate dielectric and piezo-thermoresistive organic semiconductor channel into FET platforms, and the resulting device is able to respond to two stimuli in pressure (or strain) and temperature simultaneously and disproportionally In our previous works, we had shown the possibility of extracting temperature[35] or pressure[36] responsitivities of functional © 2013 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim wileyonlinelibrary.com www.advmat.de COMMUNICATION www.MaterialsViews.com gate dielectrics integrated in FET platforms by using AC gate bias technique Herein, we extend our methods to elaborate the precise and quantitative separation of unknown, simultaneously applied stimuli of pressure-temperature or straintemperature by decoupling the output signal from the single FET platform with stimuli-responsive gate dielectric and semiconductor channel, and further demonstrate the applicability to bimodal sensing from the FET array fabricated on flexible substrate In our first step to realize sensor functions for application to e-skin, we construct a device array that mimics human finger functions, which is comprised of an array of pressure and temperature sensor pixels displayed on top of a flexible platform Our sensor has high sensitivity to external stimuli, while being able to differentiate these exposed stimuli We have chosen to first incorporate a nanocomposite material as the gate dielectric and an organic semiconductor as the channel to the physically responsive FET (physi-FET) platform (Figure 1a) The nanocomposite material serves three primary functions, namely: (i) enhance the electro-physical coupling effects, (ii) improve the stability in their stimuli-responsive properties, and (iii) allow simultaneous analysis of pressure-temperature or straintemperature sensing parameters Since the organic FET structure has functional piezo- and pyroelectric nanocomposite gate dielectrics, as well as piezo- and thermoresistive organic semiconductor, pentacene, channel, it is able to thus simultaneously measure changes in both strain and heat This is because the two chosen materials are able to respond to pressure (or strain) and temperature simultaneously, but in a disproportionate manner Specifically, the nanocomposite material we have used as a gate dielectric is a mixture of poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) and BaTiO3 (BT) nanoparticles (NPs) The transmission electron microscopy (TEM) image of BT NPs in the nanocomposite are depicted as insets in Figure 1a When introducing an AC gate bias to the FET having poled functional gate dielectric with an electrical field (Figure 1b), the amplitude and offset values (IDamp and IDoffset, respectively) of the modulated drain current (ID) include the information on channel transconductance (gm) and the effective remnant polarization (Pr) of the piezo-pyroelectric gate dielectric layer We can hence estimate the gm value and its equivalent voltage in Pr V0 = PCr , via both Equations (1) and (2) below: offset V0 = VG0 ID amp ID (1) amp gm = ID VG0 (2) where C and VG0 are capacitance of gate dielectric and amplitude of applied AC gate bias, respectively Figure Illustration of our approach a) The structure of physically responsive field-effect transistor (physi-FET) with the bottom-gated and top-contact structure, where the gate dielectric is comprised of P(VDF-TrFE) or nanocomposite of P(VDF-TrFE) and BaTiO3 nanoparticles and the channel is organic semiconductor of pentacene b) Changes in ID signals of physi-FET with P(VDF-TrFE) upon applying pressure and temperature c) Responses of channel trans-conductance (gm) in a FET with highly crystalline P(VDF-TrFE) when applying pressure and temperature simultaneously Error bars were drawn with 3X magnification d) Responses of equivalent voltage V0 upon applying pressure and temperature Error bars were drawn with 30X magnification wileyonlinelibrary.com © 2013 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Adv Mater 2013, DOI: 10.1002/adma.201302869 www.advmat.de www.MaterialsViews.com gm M1 M2 = V0 M3 M4 P T (3) where g m and V0 are changes in gm and V0 values under subjection to various stimuli (P, ΔP, T, ΔT); and M1, M2, M3, and M4 are all sub-components of the matrix M By using Equation (3), both values in M3 and M4 were directly correlated to piezoelectric coefficient (d33) and pyroelectric coefficient (p3), which were again associated with the changes in Pr of the nanocomposite material under application of normal stress and temperature, respectively We have previously reported the results for both positive pyro- and piezoelectricity in highly crystalline P(VDF-TrFE).[35,36] Positive pyroelectricity indicated that Pr would increase at elevated temperature, while positive piezoelectricity means Pr would decrease under applied pressure In combining highly crystalline P(VDF-TrFE) and BT NPs with positive piezoelectricity and negative pyroelectricity, d33 and p3 of BT/P(VDF-TrFE) NCs are observed to increase and decrease, respectively In regard to both M1 and M2, corresponding responses in gm with varying P and T are not straightforward However, our experimental data indicated that piezo-resistivity of pentacene channel with a larger P-dependence of effective channel mobility are the main contributions to M1, while gate dielectric capacitance with higher T sensitivity contribute more to M2, respectively (Figure S2) The linear responses of the materials, e.g., T- and P-dependences of measured gm and V0, are directly utilized in sensing applications In such a situation, the proposed matrix approach can then be utilized Adv Mater 2013, DOI: 10.1002/adma.201302869 COMMUNICATION Derivations and fundamental electrical characteristics for extracting gm and V0 values from the modulated ID are detailed in Equation S1 and Figure S1, Supporting Information In our previous works, we validated our model by fitting output (ID–VD) and transfer (ID–VG) characteristic, as well as using AC gate bias technique for extraction of Vo values under the change of individual stimulus (i.e., pressure (P) and temperature (T)).[35,36] The method can be extended to extract gm as well as Vo under simultaneous changes of both T and P Next, we proceed to investigate temperature sensing while applying a pressure to the FET Specifically, we measured the changes in gm and V0 when subjected to varying applied P and T, simultaneously Methods to heat and pressurize the FET was described in the Methods section Figure 1c and 1d showed the responses of gm and V0 with P and T, which were extracted from a FET comprised of highly crystalline P(VDF-TrFE) gate dielectric Sampling T was measured from 35 to 40 °C with an increment interval of °C, and P was from to 0.5 N mm–2 with an interval of 0.1 N mm–2 Each point plotted in Figure 1e and 1d was derived from averaging the values derived from a total of 10 collected data points, as measured under the same condition Our obtained data indicated that responses to P changes were relatively smaller in both gm and V0 values (≈5% in the applied pressure range) when compared to those of T changes Even though the gm and V0 responses by P changes are relatively small, the observed linearity in Figure 1c and 1d suggested that the relationship of gm and V0 vs P and T can be correlated via a characteristic matrix M, as shown in Equation (3) below: However, in most general cases in which functional materials showed non-linear relationships, e.g., Arrhenius behaviorE aof − effective channel mobility, µeff, with T, μeff = μeff e k B T , and reciprocal dependence of C = ε 0ε r d0 (1− EP ) with high pressure (Figure S3), a Jacobian matrix approach is instead needed (for details, see Equation S2) Extending the temperature range toward 70–80 °C also result in reduction of accuracy due to non-linear responses of P(VDF-TrFE).[35] For crystalline P(VDF-TrFE), its pyroelectricity (µC/K) property was found to be more significant than piezoelectricity (pC/N) Upon comparing their responsiveness, which is indicated by µC/K vs pC/N, we realized that the value of M3 is significantly smaller than M4 (Table S1) We attribute such a huge difference in its matrix’s components to a phenomenon known as “ill-conditioning” in linear algebra.[37] Generally, it will lead to inaccuracy when used to extract both parameters in P and T from the data in Figure 1e and 1f To resolve this issue, we instead use the approach in tuning the electro-physical coupling properties of the functional gate dielectric materials For tuning of functional gate dielectrics, our approach described here employs the nanocomposite properties of the highly crystalline materials in both P(VDF-TrFE) and BaTiO3 NPs To study contributions from the NPs’s crystallinity to piezoelectricity, we have utilized the Piezo-response Force Microscopy (PFM) method Figure 2a and 2d illustrated the topographic images of the nanocomposite comprised of 20 wt% NPs Figure 2b and 2c exhibited PFM images of highly crystalline P(VDF-TrFE) with the NPs right after poling and after 40 hrs, respectively; while Figure 2e and 2f depicted PFM images of low crystalline P(VDF-TrFE) with the NPs In this experiment, both negative (V = –30 V, dark line) and positive (V = +30 V, bright line) polarization patterns were written on the surface of P(VDF-TrFE), with and without the NPs together with the crystalline P(VDF-TrFE) The scan speed of the tip was 0.1 Hz for µm/s, and the effective poling time was about 50 s, which allowed the writing of equidistant lines PFM images were obtained by the application of an AC bias of amplitude V and frequency 40 kHz during highly crystalline P(VDF-TrFE) showed pronounced piezo-response images for both initial polarization pattern even after 40 hrs (Figure 2b and 2c), indicating that the crystallinity of P(VDF-TrFE) has significantly enhanced its piezo-response characteristics The effect of initial polarization performance and polarization, with respect to relaxation time, were also quantified in a series of PFM measurements PFM signals of Figure 2g were consistent with piezo-response images of Figure 2b, 2c, 2e, 2f, and Figure 2h, 2i, 2j depicted the time dependence of the maximum piezo-response in both positive and negative polarization patterns and the cross-section of the resulting piezo-response signal, in comparison with the calculated distribution of the electric field created by the tip The signs and the intensities of PFM signals indicated the direction and the amount of sample polarization, respectively As shown from the PFM signal panels in Figure 2h and 2i, highly crystalline P(VDF-TrFE) with BT NPs exhibited enhanced piezoelectric performances, together with increasing signal with respect to relaxation time Simple exponential decay mechanism could not sufficiently account for the relaxation behaviors of the prepared nanocomposites in both positive and negative polarization directions © 2013 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim wileyonlinelibrary.com www.advmat.de COMMUNICATION www.MaterialsViews.com Instead, a stretched exponential dependence equation was used to fit the obtained experimental data in both polarization directions The fitting was done with the KohlrauschWilliams-Watts formula (Equation (4)), which is commonly used to describe relaxation in the system with dipole-dipole interactions, (d33 ) ef f ≈ e − t t0 b (4) where t is the time,[38] b is a measure of the distribution of defects that control the decay process, and to is the measure of the effective relaxation time In our investigation, the b value varies with the crystallinity of P(VDFTrFE) and the NP content in nanocomposites For nanocomposite of highly crystalline P(VDF-TrFE) and NPs, the parameter b was much higher than the control sample, low crystalline (P(VDF-TrFE) High b value for high crystallinity P(VDF-TrFE) incorporated with BaTiO3 NPs exhibit high homogeneity and low defect distribution Also, the effective relation time of high crystalline P(VDF-TrFE) with the NPs, low crystalline P(VDF-TrFE) with the NPs, and low crystalline P(VDF-TrFE) without the NPs were found to be 693, 277 and 35 hrs, respectively This indicates that the crystallinity of the piezoelectric polymer and the employment of the NPs both played crucial roles in improving the effective relaxation time Our PFM study demonstrated the advantages in using the NCs to greatly improve their long term stability as well as their electro-physical coupling properties by adding inorganic BT NPs having higher stability and piezoelectricity than P(VDF-TrFE), thus enabling high reliability when extracting the output sensing parameters The PFM data were also obtained at 70 °C (Figure S4) In comparison with the PFM signal at room temperature, the measurement at an elevated temperature of 70 °C presented a higher PFM signal immediately after applying polling bias on the gate dielectric; however, the reduction in the retention was also faster This condition Figure Piezo-response force microscopy (PFM) images and PFM signals of various nanocomposite materials a) Surface topography image of BaTiO3 NPs in highly crystalline P(VDF-TrFE) matrix PFM images of line polarization patterns obtained with V = 30 V (bright line) and –30 V (dark line) applied to nanocomposite material comprised of highly crystalline P(VDF-TrFE) matrix and BaTiO3 NPs, b) right after poling and c) after 40 hrs d) Surface topography image of BaTiO3 NPs in low crystalline P(VDF-TrFE) matrix PFM images of line polarization patterns obtained with V = 30 V (bright line) and –30 V (dark line) applied to the nanocomposite, e) right after poling, f) after 40 hrs g) Comparison of cross-section of piezoresponse signal across the pattern right after poling and after 40 hrs for both highly crystalline and low crystalline P(VDF-TrFE) matrix h) Comparison of PFM signal evolution with the relaxation time for both samples i) PFM signal window, the difference between the maximum and the minimum PFM signal which were the response of V = +30 V and –30 V, respectively, right after poling (V = ±30 V) and after 40 hrs for various nanocomposite materials j) Relaxation-related time constant, b, of various samples Incorporating BaTiO3 NPs to the highly crystalline P(VDF-TrFE) matrix presents superior characteristics wileyonlinelibrary.com © 2013 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Adv Mater 2013, DOI: 10.1002/adma.201302869 www.advmat.de www.MaterialsViews.com COMMUNICATION might reduce the lifetime of the device at an elevated temperature However, the on-chip polling procedure can be used to reset the sensing accuracy and to extend the device lifetime Another advantage in using crystalline P(VDF-TrFE) and BT NPs is that the different piezo-pyroelectricity of both materials enable us to easily tune their electro-physical coupling properties This in turn can affect the resulting sensitivity and accuracy when we extract a target sensing parameter Supplemental Table S1 shows characteristic matrices in Equation of highly crystalline P(VDF-TrFE) and its nanocomposites with BT NPs of 20 and 40 wt% The values of matrix elements in Table S1 were calculated by using the least-mean-square method, while employing a dataset of more than 5000 measured points from 12 devices According to the data in Table S1, upon adding BT NPs to P(VDF-TrFE) with a concentration of 40 wt% NC, the pressure sensitive coefficient, M3, increases; while the temperature sensitive coefficient, M4, decreases This observation was explained by the enhancement and counterbalance between positive piezoelectricitypositive pyroelectricity of highly crystalline P(VDF-TrFE) and positive piezoelectricitynegative pyroelectricity of BaTiO3 NPs.[35,36,39] In another report,[7] it was also demonstrated that well-alinged crystalline β-phase in nanofibers also contributed to a large enhancement in piezoelectricity, and can enable the tunability of piezo-pyroelectricity Similarity Figure Realization of P-T decoupling and bimodal sensing in a single FET sensor a, Our in both values for M1 and M2 in all devices algorithm to extract two sensing parameters (i.e., P and T), simultaneously b, Standard deviaconfirmed the reliability in our approach to tion in estimated T (σT) value c, Standard deviation in estimated P (σP) value d, Accuracy estimate the components of M matrices for in values as measured by pressure and temperature gauges e, Demonstration of real-time the pentacene channel in FETs for all our bimodal sensing of P-T f, Demonstration of real-time bimodal sensing of both tensile strain (ε)-temperature (T) experiments We also observed that deviation in estimated values of characteristic matrix M was approximately to 5% in all the examined materials, except of gm_ref, V0_ref at certain conditions of Pref and Tref This reffor M4 in 40 wt% NC, which is up to 267% This aberration erence determining step, which can be simultaneously pervalue may be due to the non-linearity in temperature response formed for all devices after the poling process, helps to deterof the material (Figure S5) mine Δgm and ΔV0 and to also estimate P and T from ΔP and Since we know a priori the values of M, both unknown ΔT, respectively If all the devices in an array format can be renvalues in P and T can thus be estimated by using the extracted dered in a uniform manner, the pre-determing step need only gm and V0 values in arbitrary FETs via the inverse matrix, M–1, be performed for only one device The visual schematic of an based on Equation (3) It should be noted that Equation does algorithm that allows us to extract P and T simultaneously from not express any direct relationship between the absolute values measured ID of FETs is shown in Figure 3a This procedure can of gm, V0 and P, T; but rather, it simply highlights their relabe expressed by Equation (5): tive changes The relative changes are expected to be similar in all FETs as they are comprised of the same semiconductor P ref P M1 M2 −1 g m − g m ref = + (5) and gate dielectric, even though they have slightly different gm T Tref V0 − V0 ref M3 M4 and V0 values However, FETs comprised of the same semiconductor, but with different gate dielectrics, should have equal By using Equation (5), we proceeded to determine the accuvalues in both M1 and M2 racy in our collected dataset that were used to calculate the To resolve the issue of different initial gm and V0, each FET data in Supplemental Table S1 Figure 3b shows the relative should have a pre-determining step that gives reference values accuracy percentage of the read-out values from P(VDF-TrFE), Adv Mater 2013, DOI: 10.1002/adma.201302869 © 2013 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim wileyonlinelibrary.com www.advmat.de COMMUNICATION www.MaterialsViews.com 20 wt% NCs and 40 wt% FETs, that matched with the reading values of temperature (T) and pressure (P) using commercial gauges This quantitative comparison excluded the absolute uncertainty of commercial temperature and pressure gauges We observed that 20 wt% NC showed the best accuracy in extracting both the T and P parameters Figure 3c, and 3d showed the different distribution of both applied and estimated values of T and P It was observed that with decreasing M4 from –4.173 to 0.003 V/K, the standard deviation in extracted T (σT) increases from 0.00581 °C to 0.08391 °C (Figure 3d) and corresponds well with the decreased accuracy for T measurements (from 100% to 77.78%, shown in Figure 3d) Similarly, an increase of M3 from 1.094 for highly crystalline P(VDFTrFE) to 2.615 V/MPa for 20 wt% NC was observed, along with a significant increase in accuracy from 64.58% to 99.31% with a tighter standard deviation in extracted P (σP) from 0.02159 of highly crystalline P(VDF-TrFE) to 0.00689 N mm–2 of NC 20 wt% (Figure 3c) However, further increase in M3 to 40 wt% NC led to a decrease in accuracy, as indicated by the increased deviation in σP This was attributed to the non-linear response in temperature of 40 wt% NC (Figure S5), which increases the error values of the M’s components We note that accuracy is estimated as half of the minimum measurement resolution In the case of 20 wt% NC, pressure resolution of the sensor can be considered as 0.07 N mm–2 (≈7 kPa) Pressure sensitivity of the sensor, response slope of Vo vs P, can be estimated as the M3 value, 2.615 mV kPa–1 for 20 wt% NC (Table S1) Higher pressure sensitivity required for e-skin applications requiring detection of 100–500 kPa range can be achieved by enhancing the piezoelectric responsitivity of the NCs through microstructuring Collectively, these results indicate the importance in adjusting the characteristic matrix M through tuning of the electro-physical coupling properties in the nanocomposite material to allow precise quantification of both P and T values Figure 3e showed the real-time responses of both P and T, as extracted from a FET with 20 wt% NCs, upon time-dependent pressurizing and heating of the device A video recording of this experiment is shown in Video 1, Supporting Information Solid red and blue lines indicate the applied T and P, while blank orange circles and green squares are read-out values of T and P Figure 3e also indicated that our obtained accuracy in measuring both sensing parameters is very good The time required for the readout of the pressure or strain at a steadystate signal in our measurement method of “apply and hold” is ≈10 s This sensor response time is sufficient for practical applications that mimic human finger such as holding a hard/ soft-hot/cold object in which target sensing signals are typically constant force, pressure, or strain and temperature In order to further understand the response time of physi-FET under various pressurizing condition, the dynamic responses of a pentacene resistor, metal-20 wt% NC-metal structure and physi-FET to dynamic pressurizing with varying forcing time were measured in a dynamic loading system similar to the system used by Takei et al.[1] The results indicated that the FET responses at a slow forcing condition are attributed to a slow relaxation time for steady-state signal of the pentacene channel due to the long equilibration time of the mechanically stimulated pentacene channel,[36] rather than that of the response of the nanocomposite gate dielectric (Figure S6) As observed, the NCs are not wileyonlinelibrary.com the limiting factor to realize high-speed tactile sensors as the P(VDF-TrFE) was reported to tolerate and respond up to a very high frequency above kHz range.[40] Moreover, we also observed a slight decrease in accuracy after a continuous measurement duration of >4 h We attributed this degradation to the increasing bias stress phenomenon in the FET, which originates from charge trapping in either the pentacene channel or at the interface of the gate dielectrics and pentacene.[41] The de-trapping process by applying positive gate bias at high temperature, re-calibration after a certain time interval, and further optimization of the gate dielectric or passivation layer in the future may help address the reliability issue of our devices We emphasize that the procedure described herein not only decouples the target sensing parameter, T, in which its accuracy is greatly reduced by interference from P; but our approach also provides capability in reading P This capability hence enables the bimodality sensing capability within a single device Upon substituting the sub-components of the characteristic matrices M from a FET with different physical constants (related to other electro-physical couplings), this approach can be further extended to general heterogeneous bimodal sensing applications For instance, if we were to replace the coefficient d33 by e31 (which is often associated with applied planar strain) and Pr (piezoelectric phenomenon), the temperature coupled to tensile strain (under bending) can be extracted in a similar methodology as decoupling P from T Figure 3f showed the read-out values of strain (ε) and T in simultaneous strain-temperature stimulation in real-time Since the device was mounted on a flexible polyimide heater, the low heat capacity of the heater resulted in a large fluctuation in controlling temperature The sensing accuracy of our flexible device was reduced slightly when subjected to repeated bending processes at the bending radii (Figure S7) However, this issue can be easily resolved by performing the pre-determining step of gm_ref and V0_ref values intermittently (Figure 3a) after certain mechanical bending cycles After conducting the predetermining step after 10 000 bending cycles, for example, the readout strain was observed to recover to the value prior to the cyclic bending Our demonstration of bimodal sensing capability also indicated that this approach should readily be extended to simultaneously extract multiple sensing parameters for multimodal sensing To apply our described multimodal sensing concept to a FET array, real-time bimodal sensing of a × device array was investigated by measuring real-time responses of the device array in a custom-built bending system integrated with realtime, multiplexed electrical measurement units The pixel size containing an FET was approximately 0.5 × mm, and our sixteen devices were uniformly distributed over a 1.3 × 1.3 cm2 area Our array system can simultaneously measure the arbitrary T-P values of the contained 16 devices in real-time The measurement details of the array system were described in the Methods section Figure 4a and 4b showed a two-dimensional mapping of measured T and P, respectively, in a device array, when half of the devices, i.e., eight devices, were pressed with a human finger A picture of this experiment was shown in Figure It was clear that the read-out temperature and pressure of pressed devices were higher than the untouched © 2013 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Adv Mater 2013, DOI: 10.1002/adma.201302869 www.advmat.de www.MaterialsViews.com COMMUNICATION Figure Realization of P-T decoupling and bimodal sensing in an array of FET sensors a,b) Read-out pressure and temperature measurements of a sensor array with half of the sensor’s devices are being depressed by a human thumb Pressed devices are observed to show higher pressure and temperature responses, when compared to unaffected devices c,d) Read-out pressure and temperature measurements of a sensor array with all devices being depressed by a human thumb e,f) Read-out pressure and temperature measurements of a sensor array with only one device at (column, row) = (3, 2) being depressed by a blunt object (i.e., cotton swab) The pressure response of the depressed device is measured to be higher than all other unaffected devices, with the temperature of all other unaffected devices being close to room temperature The devices at (2, 3) and (3, 3) are defective devices Moreover, the read-out temperatures of the devices being pressed were also measured to be very close to that of human body temperature Figure 4c and 4d showed the measured values of T and P when all of the devices were pressed by a human thumb As expected, both the temperature and pressure responses in all devices were highly identical with the pressed ones in Figure 4a and 4b In addition, when only one device within the array was individually pressed by using a narrow blunt object, only the read-out pressure of the pressed device was higher than those of the rest; while the read-out temperature of the unaffected devices remained close to room temperature (Figure 4e and 4f) The non-uniformity in performance of the arrayed devices (a critical issue for practical applications) was solved upon applying our pre-determining step of gm_ref and V0_ref values on the initial characteristics of the devices Moreover, such small power consumption of an individual sensing device, which is as low as 600 nW per pixel using the sensor pixel of the one sensor and one switch element (Figure S8), may enable realization of a portable and self-powered e-skin We anticipate that upon optimizing the gate dielectric thickness, we could further reduce operation voltage more and, thus, minimize the devices power consumption Hence, our observed results of spatially resolved responses to both pressure and temperature of flexible Adv Mater 2013, DOI: 10.1002/adma.201302869 FET array indicate that our approach is highly amenable for e-skin application As designed for flexible e-skin, the pressure–temperature bimodal sensing was also demonstrated in a bent state with the bending radii of and cm (Figure S9) Last, this approach can be generalized for other heterogeneous multi-stimuli sensing application For instance, chemicalstrain differentiation can be achieved by utilizing strain (M1) and chemical responsiveness (M2) of semiconducting channel, and strain-responsiveness of functional gate dielectric (M3) In summary, we have successfully demonstrated a general approach to fabricate e-skin that can: (i) extract effects from the target sensing signals, such as P or T, while the flexible sensor is under multimode stimulus; and (ii) enable real-time bimodal sensing using a single FET device by extracting parameters associated with mechanical deformations This concept for real-time bimodal sensing of unknown multi-stimuli was realized in an array format The advantages in integrating FET arrays with multimodal sensing elements in flexible e-skin greatly reduced the complexity in structural integration, eliminated or minimized the signal interferences coupled by strain, significantly reduced the power consumption, and decreased the failure rate in production due to facile integration of FET devices into the circuits Furthermore, it has potential in reducing fabrication costs of large-area flexible e-skins This approach may be extended to © 2013 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim wileyonlinelibrary.com www.advmat.de COMMUNICATION www.MaterialsViews.com realize multimodality in large area flexible e-skin with heterogeneous input stimuli of physical, chemical or biological natures, and also to solve problems associated with strains induced during operative services of flexible electronic systems Experimental Section Preparation of Materials and Devices: The P(VDF-TrFE) (65 mol% of VDF) was purchased from Piezotech S.A The BT NPs with an average diameter of 50 nm and the coupling agent 3-aminopropyltriethoxy silane (APTES) were purchased from Sigma Aldrich The BT NPs were first ballmilled for hrs to disperse the aggregated NPs After milling, the BT NPs were dispersed in an APTES–ethanol solution with a pH of 4–4.5 (adjusted by HCl) for hr The BT NPs and APTES–ethanol mixture was filtered and washed in ethanol to remove the residual APTES The treated BT NPs were then cured at 110 °C for on a hot plate and mixed with N,N-dimethylformamide (DMF) Centrifugation was performed to produce a solvent–particle mixture with small BT NPs The P(VDF-TrFE) and DMF were added to produce solutions of g P(VDFTrFE) in 10 mL DMF with a predefined weight percentage of the BT NPs with respect to P(VDF-TrFE) An inverted-staggered bottom-contact bottom-gate OFET structure (with Ni as the gate electrode, pentacene as the organic semiconductor, and Au as the source/drain electrodes) was used An organic material of tetratetracontane (TTC) was deposited by thermal evaporation at 50 °C as the passivation layer The thickness of the NC gate dielectric layers ranged from 600–700 nm Highly crystalline BT/P(VDF-TrFE) NC gate dielectric layers were obtained by annealing at 140 °C for hrs The channel dimensions of the characterized devices were a length (L) of 40 µm and a width of 800 µm Temperature-Pressure Bimodal Sensing from Single Device: In order to induce piezoelectricity and pyroelectricity of functional gate dielectric that responds to pressure and temperature, an on-chip poling process was conducted by grounded source (S) and drain (D) electrodes while gate electrode is applied –80 V.[36] Applying the pressure to the device positioned on a heating block having a feed-back temperature control was achieved by supplying pressurized N2 gas in a sealed stainless measurement chamber, while the pressure inside the chamber was monitored by a commercial pressure gauge The temperature of the device on the heating block was measured with a thermocouple embedded near the device Sinusoidal AC (alternative current) VG was created by the Tektronix AFG 3102 two-channel Arbitrary/Function Generator The applied VG has the frequency of 0.3125 Hz, V of amplitude and without offset ID was measured with an HP 1415B semiconductor parameter analyzer with time interval of 0.1 s and –5 V of drain bias VD with grounded source Amplitudes and offset values of ID were calculated by fast-Fourier-transform (FTT) method with 64 measured ID values Temperature-Strain Bimodal Sensing from Single Device: Bending of the heated devices was carried out in a custom-built bending system with the polyimide heater attached to the backside the flexible polyimide substrate having the devices by using a heat-conductive epoxy Methodology for estimation of strain in strain-temperature bimodal sensing was shown in Equation S3 For electrical measurements, the same method as the temperature-pressure bimodal sensing was used Piezoelectric-Force-Microscopy Measurement: In order to measure and characterize piezoelectric materials in the nanoscale regime, piezoelectric-force-microscopy (PFM) equipped with the function generator and lock-in amplifier (E-Sweep, SII) was employed Surface topography was obtained in atomic force microscope mode while piezo-response image was obtained in piezo-responsive mode to detect piezoelectric vibrations PFM measures the mechanical response when an electrical voltage is applied to the sample surface with a conductive tip Under the electrical stimulus, the sample locally expands or contracts If the polarization is parallel with the applied electric field, the piezoelectric effect becomes positive, and the sample will locally expand If the polarization is anti-parallel with the electric field, the sample will shrink This sign-dependent behavior indicates that the phase of wileyonlinelibrary.com the cantilever provides the polarization orientation of the sample Cr/ Au coated cantilevers with the resistance of 0.01-0.02 Ω and tip apex radius of less than 50 nm were used (Nanosensors) PFM imaging was acquired under an applied AC voltage with amplitude of V and frequency f = 40 kHz The measurement frequency was chosen far away from the resonant frequencies of the cantilever-sample holder system to avoid the ambiguity of experimental data TEM Measurement: Cross-sectional high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image was taken for detailed analysis of the gate dielectric materials Our obtained HAADF-STEM Z-contrast image clearly distinguishes the BaTiO3 NPs from P(VDF-TrFE) The order of brightness is proportional to mean square atomic number of composed elements Energy dispersive spectroscopy analysis was used for confirming the chemical composition of the NPs We also performed nano-beam diffraction pattern analysis The diffraction pattern clearly confirmed the presence of crystalline BaTiO3 NPs within P(VDF-TrFE) (Figure S10) Supporting Information Supporting Information is available from the Wiley Online Library or from the author Acknowledgements This research was supported by the Basic Science Research Program (Grant No No 2009-0083540 and 2013R1A2A1A01015232) through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (MEST), Republic of Korea Received: June 24, 2013 Revised: August 9, 2013 Published online: [1] K Takei, T Takahashi, J C Ho, H Ko, A G Gillies, P W Leu, R S 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Sharma, M Cölle, P A Bobbert, R A J Janssen, D M de Leeuw, Adv Mater 2008, 20, 975 © 2013 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim wileyonlinelibrary.com ... extract multiple sensing parameters for multimodal sensing To apply our described multimodal sensing concept to a FET array, real-time bimodal sensing of a × device array was investigated by measuring... 10.1002/adma.201302869 FET array indicate that our approach is highly amenable for e-skin application As designed for flexible e-skin, the pressure? ? ?temperature bimodal sensing was also demonstrated... application to e-skin, we construct a device array that mimics human finger functions, which is comprised of an array of pressure and temperature sensor pixels displayed on top of a flexible platform