Highly Sensitive Air-Stable Easily Processable Gas Sensors Based on

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 43831−43841

Highly Sensitive Air-Stable Easily Processable Gas Sensors Based on Langmuir−Schaefer Monolayer Organic Field-Effect Transistors for Multiparametric H2S and NH3 Real-Time Detection Alexey S. Sizov,†,‡ Askold A. Trul,†,‡ Viktoriya Chekusova,†,‡ Oleg V. Borshchev,†,‡ Alexey A. Vasiliev,‡,§ Elena V. Agina,†,‡ and Sergei A. Ponomarenko*,†,∥

ACS Appl. Mater. Interfaces 2018.10:43831-43841. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 12/20/18. For personal use only.

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Enikolopov Institute of Synthetic Polymeric Materials of the Russian Academy of Sciences, Profsoyuznaya st. 70, Moscow 117393, Russia ‡ Printed Electronics Technologies LLC, Profsoyuznaya st. 70, Office 410, Moscow 117393, Russia § NRC Kurchatov Institute, Kurchatov Complex of Physical and Chemical Technologies, Akademika Kurchatova pl. 1, Moscow 123182, Russia ∥ Chemistry Department, Lomonosov Moscow State University, Leninskie Gory 1-3, Moscow 119991, Russia S Supporting Information *

ABSTRACT: A combination of low limit of detection, low power consumption, and portability makes organic field-effect transistor (OFET) chemical sensors promising for various applications in the areas of industrial safety control, food spoilage detection, and medical diagnostics. However, the OFET sensors typically lack air stability and restoration capability at room temperature. Here, we report on a new design of highly sensitive gas sensors based on Langmuir−Schaefer monolayer organic field-effect transistors (LS OFETs) prepared from organosilicon derivative of [1]benzothieno[3,2-b][1]-benzothiophene. The devices fabricated are able to operate in air and allow an ultrafast detection of different analytes at low concentrations down to tens of parts per billion. The sensors are reusable and can be utilized in real-time air-quality monitoring systems. We show that a direct current response of the LS OFET can be split into the alteration of various transistor parameters, responsible for the interactions with different toxic gases. The sensor response acquiring approach developed allows distinguishing two different gases, H2S and NH3, with a single sensing device. The results reported open new perspectives for the OFET-based gas-sensing technology and pave the way for easy detection of the other types of gases, enabling the development of complex air analysis systems based on a single sensor. KEYWORDS: monolayer organic field-effect transistors, chemical sensors, Langmuir−Schaefer monolayers, sensing mechanism, multiparametric detection

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INTRODUCTION Organic field-effect transistors (OFETs) have garnered considerable attention because of their application in largearea, low-cost, and low-power electronics.1−5 Compared to their inorganic counterparts, such devices provide a broad selection of organic semiconductors (OSCs),6 solution processing,7,8 device mechanical flexibility,9 transparency,10 and large-scale manufacturing via various printing techniques.3,11 The reliability of organic electronic devices still remains a crucial issue due to a strong sensitivity of OSCs to chemical, optical, and mechanical impacts,12,13 thus limiting the OFET technology’s commercialization as a competitor to traditional silicon electronics. Although such sensitivity is traditionally considered a drawback, it simultaneously enables utilization of OFETs in various sensing applications,14−19 including detection of chemical, physical, or biological agents. Indeed, at the moment, light,20 pressure,21 and various chemical22 and biochemical23 sensors based on OFETs have © 2018 American Chemical Society

been reported. In particular, the OFET chemical sensors operating in liquid phase demonstrate an excellent biocompatibility, which creates a perfect platform for the point-of-care testing.14 Such novel applications imply clinical tests being performed directly or near the site of a patient care (even at the patient’s house), allowing a timely initiation of appropriate therapy. At the same time, gas chemical sensors based on organic transistors enable detection of target volatiles at very low concentrations down to the parts per billion (ppb) level.24 A combination of such low limit of detection (LOD), low power consumption, and portability makes OFETs perfect candidates for various applications in the areas of industrial safety control, food spoilage detection at early stages, and medical diagnostics via the exhaled breath analysis.14,25,26 Received: September 5, 2018 Accepted: November 22, 2018 Published: November 22, 2018 43831

DOI: 10.1021/acsami.8b15427 ACS Appl. Mater. Interfaces 2018, 10, 43831−43841

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ACS Applied Materials & Interfaces

Figure 1. (a) LS OFET device architecture, (b) chemical structure of organosilicon dimer O(Si-Und-BTBT-Hex)2 in the closed conformation, (c) atomic force microscopy (AFM) morphology of the LS film of the dimer, (d) output, and (e) transfer curves of the LS OFET in dry air. The transfer curve is measured at Vd = −40 V.

high charge carrier mobility (∼10 cm2/(V s)), the device can be operated only under nitrogen atmosphere or being encapsulated, whereas no electrical response was observed under ambient conditions, thus limiting any practical application of the sensor. Therefore, a solution-processable fast and scalable approach for the fabrication of air-stable highly sensitive and selective gas sensors is still in great demand. Although a monolayer OFET device is strongly sensitive to the environment, a problem of the adequate response acquisition procedure from the OFET sensors requires special attention. Generally, all OFET key parameters may change independently upon exposure to the analyte, making such device a multiparametric chemical sensor,14 but complicating an unambiguous sensor readout. In particular, on-state current,34,35 field-effect mobility,36,37 threshold voltage,32,36,37 subthreshold slope (SS),29 and off-state current38 of the OFETs have been reported to be influenced by the analyte− sensor interactions. Therefore, a careful analysis of the monolayer OFETs’ response should be implemented in order to optimize the sensor’s device performance. Although low response time of nanoporous [1]benzothieno[3,2-b][1]benzothiophene (BTBT)-based OFET sensors to formaldehyde has already been demonstrated,39 the application of the multiparametric detection principle would allow distinguishing several different gases with a single OFET sensing device. The other important issue for organic sensors is a baseline drift problem,24 which should be especially addressed in the case of 1−2 monolayer OSC films. To compensate the baseline drift, the charge trapping dynamics analysis that has been developed by the group of de Leeuw for NO2 sensors based on ZnO field-effect transistors40,41 can be applied. The algorithm

Nowadays, various OFETs have been reported to provide sensing capabilities for detection of a toxic gas. For example, organic field-effect transistors based on diketopyrrolopyrrolebithiophene- or 9,9-dialkyl-9H-fluorene-based polymers as OSC are able to detect ppb concentrations of ammonia27 or hydrogen sulfide,28 respectively. However, such OFETs typically lack the air stability and sensor restoration capability at room temperature. It has been also demonstrated that conjugated oligomers, for example, polycrystalline films of 6,13-bis(triisopropylsilylethynyl)-pentacene29 or single crystals of dinaphtho[3,4-d:3′,4′-d′]benzo[1,2-b:4,5-b′]dithiophene,30 can be used for chemical sensing. In this context, special attention has to be paid to monolayer OFETs,31 as the chargetransporting layer in such devices is directly exposed to the atmosphere containing a measured gas (analyte), thus providing both high sensitivity and low response time of the sensor. Indeed, self-assembling monolayer field-effect transistors (SAMFETs) prepared from chloro[11-(5⁗-ethyl2,2′:5′,2″:5″,2‴:5‴,2⁗-quinquethien-5-yl)undecyl]dimethylsilane, Cl-Si-Und-5T-Et, demonstrated a low detection limit, which was further improved by introduction of a receptor layer, capable of specific interactions with a target analyte, on top of the semiconductor.32 A disadvantage of this approach is relatively slow OSC self-assembling monolayer fabrication that involves formation of the covalent bindings between the OSC molecules and the dielectric layer, limiting practical implementations of such devices. In a recent work, excellent sensitivity of 2,9-didecyldinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]thiophene monolayer OFETs to ammonia has been demonstrated, where the monolayer was formed by fast and scalable dual solution shearing technique.33 Despite the strong sensing properties to ammonia (LOD ∼ 10 ppb) and 43832


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The Langmuir−Schaeffer (LS) technique has been successfully applied to the fabrication of efficient SAMFETs based on a number of organosilicon derivatives of conjugated oligomers.50 In the case of O(Si-Und-BTBT-Hex)2, the monolayer semiconducting film has a uniform morphology (Figure 1c) with the layer thickness corresponding approximately to a half of the theoretically calculated length of the molecule in its extended conformation (66 Å), which corresponds to a vertical orientation of the BTBT units within the monolayer with a kink on the disiloxane unit of the molecule in the closed conformation. The uniform film has a high substrate coverage (>98%), which results in excellent device reproducibility. The detailed investigation of the monolayer structure has been reported previously.49 Electrical properties of the LS OFETs were typical for p-type organic semiconductors. The output characteristics (Figure 1d) show a clear dependence of the saturation current on the gate bias applied, which confirms the presence of the pronounced field effect. It should be mentioned that the output curves are linear in the low drain voltage region, which is evidence of a small impact of contact effects on the electrical performance. A typical transfer curve is shown in Figure 1e, allowing extraction of the LS OFET key electrical parameters: field-effect mobility in the saturation regime μ = 10−2 cm2/ (V s), threshold voltage VT = −5 V, on-state current Ion ∼ 10−6 A, off-state current Ioff ∼ 10−10 A, and subthreshold slope SS = 5 V/decade. Device-to-device variation of the threshold voltage was as small as ±5 V due to high uniformity of the LS film. Electrical performance of the LS OFETs based on O(Si-Und-BTBT-Hex)2 was among the average values for this type of organic transistors.49 Figure 2a shows dynamic response of the LS OFET chemical sensor to the exposure to 200 ppb of ammonia in dry air. During the experiment, both gate and drain voltages were kept at constant biases of −40 V, corresponding to a saturated regime of the transistor. Current in the device permanently decayed, and the decay rate was continuously changed upon appearance of ammonia. The decay rate in dry air (“zero” decay) should be attributed to bias stress effects in the OFET, discussed below. When NH3 is introduced into the cell, zero decay is combined with an extra effect of the toxic gas presence, resulting in a current decay rate variation. As the toxic gas flow is turned off, the current slowly restores toward the initial value. However, being affected by the gate bias stress, it continues to decrease. Such dynamic response is typical for OFET-based chemical sensors,27,38 but the amplitude of zero decay in the case of monolayer devices is significantly higher than in organic single crystals or bulk films. The data described clearly indicate that in order to be implemented as a low-cost, low power consumption gassensing device, the LS OFET requires a specific response acquiring algorithm. Because the device has rather weak operational stability due to the big hysteresis loop (Figure S1) and a high drain current decreasing rate under continuous bias stress, such devices can hardly be used in electrical circuits, especially when continuous voltage stress is intended. At the same time, they can be applied in sensing applications since they are able to rapidly recover their properties when the bias stress is off (Figure S2). According to the literature data, the bias stress effect in organic semiconductors (a zero decay) originates from both intrinsic and extrinsic factors. In p-type organic transistors with SiO2 dielectric layer upon the continuous test, formation of the

relies on a threshold voltage drift kinetics analysis during the formation of localized charges on the semiconductor− dielectric interface in the presence of the analyte. Application of this technique to the monolayer OFET-based gas sensors should allow at least partial overcoming of the baseline drift problem and moving this technology toward the industrial stage. In this work, we propose a self-assembled Langmuir− Schaefer monolayer field-effect transistor based on organosilicon derivative of [1]benzothieno[3,2-b][1]-benzothiophene (BTBT) for detection of toxic chemical gases such as H2S and NH3. The OSC monolayer has been deposited using a Langmuir−Schaefer technique, which enables formation of a two-dimensional (2D)-crystalline monolayer film with high device yield and excellent reproducibility and could be easy scaled up to industrial applications without losing device performance.42 Chemically inert nature of the organosilicon semiconducting material enables OFET manufacturing under ambient conditions including air and water vapors. The OFETs fabricated demonstrate strong sensitivity to target gases (NH3 and H2S), which is compatible with environmental stability and reusability of a sensor. We show that selectivity can be achieved using the multiparametric detection principle, where specific variations of the device parameters can be correlated to the target gas. Possible mechanisms of the analytes’ interactions with the OFET semiconducting monolayer are discussed.

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RESULTS AND DISCUSSION The proposed gas sensor is based on a Langmuir−Schaefer monolayer organic field-effect transistor (LS OFET) in a bottom contact bottom gate configuration (see Figure 1a). This architecture implies direct exposure of a chargetransporting layer of the organic semiconductor to the environment and enables high sensitivity and fast response of the chemical sensor. As the organic semiconductor, a monolayer Langmuir−Schaeffer film of 1,3-bis[11-(7-hexyl[1]benzothieno-[3,2-b][1]benzothien-2-yl)undecyl]-1,1,3,3-tetramethyldisiloxane, O(Si-Und-BTBT-Hex)2, (Figure 1b) was used. This organosilicon dimer of dialkyl-BTBT was designed and synthesized as described earlier.43 Briefly, the BTBT semiconducting fragment has been chosen because of the high electrical performance reported for OFETs based on this conjugated organic core4,44 and high solubility of dialkyl-BTBT compounds in widely used nonchlorinated organic solvents, in particular, in toluene, which makes them prospective for industrial applications. The presence of the chemically inert tetramethyldisiloxane (−Si(CH3)2−O−Si(CH3)2−) group capable of hydrogen bond formation with water molecules enables processing of this dimer by Langmuir−Blodgett or Langmuir−Schaeffer self-assembling techniques under ambient conditions.45,46 O(Si-Und-BTBT-Hex)2 is stable under normal conditions and does not hydrolyze in the presence of water as opposed to the chlorosilane derivatives of α,ω-dialkyloligothiophenes investigated before.47 SAMFETs based on Langmuir films of organosilicon derivatives of conjugated oligomers demonstrate good reproducibility and air stability, which make them suitable as a perfect platform for gas-sensing applications.45,48 The highlighted environmental stability allows a device to be stored at room temperature and humid air without losing its electrical properties49 and sensitivity to target gases. 43833


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charge trapping in deep states at the semiconductor−dielectric interface was observed.13 Such a process causes a threshold voltage shift together with a current relaxation. As an extrinsic factor, the most important is the presence of moisture at the OSC−gate dielectric interface. A mechanism of the relaxation was described by the local polarization of H2O molecules, which present on the SiO2 surface.51 It should be noted that the role of oxygen present in the air on the current decay is negligible as the relaxation kinetics was found to be the same in dry air and under inert atmosphere of argon (Figure S3). To investigate the origin of the LS OFET drain current relaxation during analyte exposure, a series of transfer curves has been measured in dry air with different concentrations of ammonia. Figure 2b shows dynamics of the transfer curves upon exposure in the dry air containing 200 ppb of NH3. The apparent trend is a shift of the threshold voltage toward negative values during the continuous test. Such kinetics of the OFET characteristics is known to be caused by a charge trapping at the semiconductor−dielectric interface,52 which leads to the threshold voltage shift up to the maximum value, which corresponds to the gate bias applied to the OFET. This phenomenon has been used earlier in OFET chemical sensors utilizing different materials in both liquid53 and gas phases32 and usually can be attributed to adsorption of polar molecules of the analyte onto the surface of the semiconductor layer.29,32 The voltage shift sign depends on the type of organic semiconductor and, in the case of p-type OSC, is expected to be negative, like it was observed in our experiments. To investigate more thoroughly the interactions between the OSC and the analyte, dynamics of the LS OFET key

Figure 2. NH3-sensing properties of the LS OFET. (a) Dynamic response to 200 ppb of ammonia. During the experiment, the LS OFET is kept under direct current biases. The dashed line shows reference measurement without ammonia. (b) Shift of the transfer curves measured continuously in dry air containing 200 ppb of ammonia.

Figure 3. Dependence of the LS OFET key parameters extracted from transfer curves as a function of time upon exposure to different concentrations of ammonia: (a) relative mobility charge μ/μ0, (b) threshold voltage shift ΔVT, (c) relative on-state current change I/I0 (at Vg = −40 V, Vd = −40 V), and (d) threshold voltage relaxation time τ1 as a function of ammonia concentration. The τ1 error bars are less than graph point size. 43834


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OSC film injected from the source electrode into the transistor channel during device operation. In other words, polar NH3 molecule induces a trap state on the surface of the electrically doped semiconductor. Trapped holes screen the gate bias and shift the threshold voltage, but do not affect the mobility. The other important parameter responsible for the OFET− analyte interactions is a subthreshold slope (SS), previously reported to change upon exposure to both ammonia and nitrogen dioxide.29,36 The SS value can be correlated with the effective trap density,55 including both the deep bulk traps and the interface traps, which are present in the pristine organic semiconductor. Such deep traps are filled with holes injected from the source electrodes and do not contribute to the drain current. Additionally, the analyte molecules adsorbed on the semiconducting monolayer under the applied gate bias can also vary the density of deep traps in the undoped semiconductor and change the SS. The constant SS values observed in our experiments (Figure S5) most likely indicate a negligible amount of deep trap states formed in the OSC in the presence of ammonia. The last OFET parameter known to be responsible for transistor−analyte interactions is an off-state current.38,56 The variations of this parameter may originate from the oxidation process as described in ref 38. This process was responsible for a gradual increase of the OFET closed-state current on increasing the analyte (diethyl chlorophosphate) concentration from 10 ppb to 100 ppm. However, in our work, we have used siloxane dimer O(Si-Und-BTBT-Hex)2 with a high thermal oxidation stability43 as a material for the semiconducting monolayer. Owing to a very high chemical stability under ambient conditions, such monolayers do not undergo any chemical reactions with analytes or secondary components during device operation, which should lead to the constant offcurrent. In the bulk OSC films, variations of the Ioff may also arise from a difference in the bulk conductivity of the OFET semiconductor.56 Such effect may occur in the bulk films because the charge transport is considered to be threedimensional only if the gate bias is absent, whereas for the onstate it is limited to 2D and localized in 1−2 molecular layers close to the semiconductor−dielectric interface.57 In the case of LS OFETs, both the on-state and off-state conductivities are localized inside the monolayer due to a very small thickness of the OSC. The specific feature of a chemical sensor based on the LS OFET is the absence of bulk conductivity that is reflected as a negligible Ioff variation. To sum up, careful analysis of the OFET key parameters shows that the charge carrier mobility remains constant during the exposure to ammonia, whereas the threshold voltage shifts. This allows one to suggest that the LS OFET sensing properties originate from the gate-bias-controlled charge trapping mechanism. Briefly, the charge trapping appears only under the gate bias applied, whereas when the source and gate electrodes are grounded, one could observe the restoration of the electrical properties of the LS OFET. The most likely explanation of this observation is that the interactions between the semiconducting monolayer and the analyte have an electrostatic charge−dipole nature, where the charges are injected holes in the semiconducting monolayer, whereas the dipoles are polar ammonia molecules. In other words, when gate bias is applied, ammonia molecules adsorb to the surface of the organic semiconductor and trap the injected holes. The localized holes do not contribute any more to the drain current but screen the gate bias shifting the threshold

parameters was determined in dry air containing ammonia traces. Since an OFET is generally considered as a multiparametric gas sensor,14 its key parameters can change independently under the presence of an analyte. Figure 3 shows dynamics of the OFET key parameters under exposure to different ammonia concentrations in dry air. It was found that variations of the charge carrier mobility (Figure 3a), the off-state current Ioff (Figure S4), and the subthreshold slope S (Figure S5) in the LS OFET upon the introduction of ammonia are relatively weak and cannot be attributed to the presence of the analyte. At the same time, the threshold voltage gradually shifts upon the exposure to ammonia (Figure 3b) and follows a stretched-exponent relaxation41 ÄÅ É l ÅÅ i y β ÑÑÑ| o o o Å t o o ÅÅ−jjj zzz ÑÑÑo ΔVT(t ) = V0m − 1 exp } Å Ñ j z o Å Ñ j z o o ÅÅ k τ1 { ÑÑo o o Å Ñ Å Ñ Ç Ö~ n

(1)

where τ1 is a relaxation time dependent on the analyte concentration, β is a dispersion parameter, and V0 is a maximum threshold voltage shift, equal to VG − VT0, where VT0 is an initial threshold voltage and VG is an applied gate bias. The dispersion parameter β was found to be 0.5−0.6 for all the experiments and independent both of the gate voltage and of the analyte concentration. It was found that the threshold voltage shift follows the stretched-exponent kinetics in a wide range of ammonia concentrations (Figure 3d). This observation indicates that the charge trapping mechanism is responsible for the sensor behavior in the case of interactions with ammonia analyte. Using eq 1, measurements of VT kinetics of the LS OFET sensor allow quantitative calculation of the ammonia concentration in the environment. It should be noted that the LS OFET maximum current, Ion, changes as a consequence of the VT shift, and since the charge carrier mobility is constant (Figure 3a), the Ion current follows the same kinetics as the threshold voltage (Figure 3c). A number of experimental works reported on the charge carrier mobility changes during exposure of the OFET sensors to different analytes.36,37 For example, in ref 36 monolayer organic transistors based on pentacene demonstrated charge carrier mobility increasing during the device’s exposure to NO2 due to donor−acceptor interactions between nitrogen dioxide and the OSC. As nitrogen dioxide is a strong electron acceptor, it is able to withdraw electrons, thus generating holes while interacting with the organic semiconductor film. The mobility in organic semiconductors is known to have a power law dependence on the charge carrier concentration.54 Therefore, in the case of NO2 detection, the mobility increases in the presence of the analyte.36 On the contrary, ammonia is an electron donor; therefore, a decrease of mobility in p-type organic semiconductors is expected. Indeed, some experimental works reported on decrease of the field-effect mobility of the OFETs upon exposure to NH3 due to reduction of positive charges (holes) on a polymer chain of the OSC by a lone electron pair of ammonia.37 However, in the case of ammonia−LS OFET interactions described in this work, the mobility was found to be constant in a wide range of analyte concentrations investigated (Figure 3a), indicating a different sensing mechanism responsible for NH3 detection, rather than those described above for NO2. Although NH3 molecule is still a strong electron donor, its lone pair of electrons can electrostatically interact with induced charges in a monolayer 43835


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Figure 4. Dependence of the LS OFET key parameters extracted from transfer curves as a function of time upon exposure to different concentrations of hydrogen sulfide: (a) relative on-state current change I/I0 (at constant Vg = −40 V, Vd = −40 V), (b) relative mobility change μ/μ0, (c) threshold voltage shift ΔVT, and (d) mobility relaxation time τ2 as a function of hydrogen sulfide concentration (red solid line is an eye guide).

H2S sensors. However, the drain current decay is a result of not only the OFET threshold voltage shift, but also a consistent decrease of the charge carrier mobility with time (see Figure 4b). Whereas the charge carrier mobility remains constant in dry air, it decreases even with insertion of 10 ppb of hydrogen sulfide. At the same time, the threshold voltage shift rate demonstrates only a weak correlation with H2S concentration (see Figure 4c). Therefore, the relative mobility change appears as a measure for hydrogen sulfide concentration estimation. The estimated LOD of H2S is 10 ppb (see SI for details). To use the LS OFET as a hydrogen sulfide sensor, an adequate readout procedure should also be developed. As described above, in the case of ammonia detection, the threshold voltage drift follows a stretched-exponent kinetics (see eq 1), the decay rate τ1 of which can be used to determine the NH3 concentration quantitatively. Similar to eq 1, the charge carrier mobility kinetics can be described by an empirical stretched exponential behavior ÄÅ É ÅÅ i y β ÑÑÑ ÅÅ jj t zz ÑÑ μ(t ) = μ0 expÅÅÅ−jj zz ÑÑÑ ÅÅ jk τ2 z{ ÑÑ (2) ÇÅÅ ÖÑÑ

voltage. When the gate bias is absent, these traps disappear due to thermal relaxation and release of the ammonia molecules. As a result, the LS OFET devices are able to respond to ammonia exposure even at very low ppb-range concentrations. The estimated LOD for ammonia is 50 ppb (see Supporting Information (SI) for details). Although response of the OFET-based chemical sensors to ammonia has been investigated in a significant number of experimental works, the response of such devices to hydrogen sulfide is not fully understood yet.28,30,58−61 Typically, OFETs demonstrate a detection limit of ∼1 ppm in the case of hydrogen sulfide,30 which is significantly higher than in the case of ammonia. The most likely explanation of this fact is the different molecular properties of hydrogen sulfide that is a weak electron donor and has a lower dipole moment as compared to ammonia that leads to weaker interactions with ptype OSC. Nevertheless, OFET-based sensors demonstrate a rapid response to H2S with the drain current variation upon exposure to hydrogen sulfide.61 To understand better the sensing mechanism, detailed investigations of the LS OFET key parameters’ variation upon exposure to H2S have been made. The LS OFETs based on siloxane dimer O(Si-Und-BTBTHex)2 for the detection of hydrogen sulfide were fabricated and evaluated using the same procedure of ammonia sensors described above. Figure 4 shows the LS OFETs’ key parameters as a function of time for various hydrogen sulfide concentrations in dry air. Similar to NH3, the drain current of LS OFET at fixed gate and drain voltages decays both in pure dry air and upon exposure to H2S (see Figure 4a). After the test, the device characteristics restore to their initial values, but the typical restoration time is higher than those in the case of ammonia (i.e. ∼20 and ∼10 min correspondingly after 40 seconds stress in 200 ppb of the target gas; see Figure S6). The drain current decay rate increases with H2S concentration increasing, indicating the possibility of using the LS OFETs as

where τ2 is a mobility relaxation time depending on the H2S concentration, β is a dispersion parameter, and μ0 is the initial charge carrier mobility. The experimentally determined values of β were the same as in the case of ammonia sensing. Similar to NH3, the dispersion parameter was independent of both the gate voltage and the analyte concentration. Figure 4d shows the mobility relaxation time τ2 as a function of H2S concentration. The monotonous decrease of τ2 allows a quantitative calculation of hydrogen sulfide concentration from the mobility relaxation time measurements. Differences observed in the LS OFET key parameter dynamics clearly indicate dissimilar mechanisms responsible 43836


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case of LS OFETs, the charge carrier mobility decays with time and restores to 100% of the initial values under dry air without any applied voltage, which indicates that H2S desorption occurs. The absence of a chemical reaction between H2S and the gold electrodes could be explained by a uniform coverage of the gold electrodes by the OSC monolayer as a result of Langmuir−Schaefer deposition that protects the electrodes from the reaction with hydrogen sulfide. Moreover, chemical modification of the electrode surface normally increases the metal−semiconductor injection barrier that typically results in a contact resistance effect. In the case of LS OFETs, after their exposure to hydrogen sulfide, no contact resistance effect has been observed (Figure S7). It means that the injection process is not affected, and the OSC film itself interacts with the analyte. Therefore, the sensing mechanism of H2S should be further explored. On the basis of the experimental data, we suggest the influence of grain boundaries’ resistance as the main mechanism of H2S detection with the LS OFETs. In polycrystalline sensing materials, electrical conductivity occurs along the percolation paths via grain-to-grain contacts and, therefore, depends on the value of Schottky barrier on the adjacent grains.67 The height of the barrier is modulated by the sorption process of hydrogen sulfide molecules onto the grain boundaries of the OSC film during the LS OFET operation. As an example, reducing gases lead to increase in the conductivity for p-type semiconductor materials, whereas the effect of oxidizing gases is the opposite.67 In the case of hydrogen sulfide, the adsorbed species act as electron donors, generating surface potential barriers on the grains and decreasing both the conductivity of the LS film and the extracted charge carrier mobility. Particularly, the latter was observed in our experiments. It is important to highlight that the sensing mechanisms for ammonia and hydrogen sulfide molecules by the LS OFETs are significantly different. In the former case, the strong electron properties of ammonia allow the molecules to interact with the surface of the OSC film (Figure 5a). Thus, the

for ammonia and hydrogen sulfide detection. Recognition of ammonia by the LS OFETs is based on gate-bias-controlled charge trapping, which results in the constant mobility value and consistent threshold voltage shift with increasing NH3 concentration. Detection of hydrogen sulfide by the LS OFETs, conversely, is followed by decrease of the charge carrier mobility, whereas the threshold voltage shift only slightly depends on the H2S concentration. Thus, the hydrogen sulfide detection mechanism remains unclear and requires additional analysis. First of all, the effects of gate-bias-controlled charge trapping in the OSC monolayer should not be considered as a primary sensing mechanism of H2S. According to a simple electrostatic approach, trapped charge carriers do not contribute to the drain current but screen the gate bias, therefore shifting the threshold voltage similar to the case of ammonia. In principle, increasing the concentration of the analyte should lead to a faster threshold voltage shift kinetics in the case of such charge trapping. However, only a small variation of the threshold voltage shift rate was observed in the case of H2S, which allows one to conclude a small impact of the gate bias screening effect. This result could be explained by the weak electrondonor properties of hydrogen sulfide molecules as opposed to the strong electron-donor properties of NH3. As discussed above, the principal interactions between ammonia molecules and the OSC are based on the electron-donor properties of ammonia, which enables charge trapping. Such type of interactions appears to be relatively weak in the case of H2S due to the poor electron-donating properties of the latter. Furthermore, the correlation between the donor−acceptor properties of the analyte and the threshold voltage shift kinetics is supported by the fact that the sensing mechanism known for ZnO n-type FETs able to detect NO240,62,63 is generally the same as those for NH3 detection by LS OFETs investigated in this work. The other mechanism should be considered since the origin of the LS OFET sensing properties to H2S is the formation of shallow traps in the OSC film while interacting with the H2S molecules. As opposed to fully localized trap states formed by ammonia in the electrically doped semiconductor, the shallow trap sites represent the trap states of the band gap close to a highest occupied molecular orbital level of the pristine OSC.55 When additional shallow trap states are formed in the presence of H2S, a part of the holes is injected below the so-called mobility edge, and the charge transport becomes traplimited.64 This might lead to the experimental mobility decrease as observed in the LS OFETs during their exposure to H2S. However, the mechanism described above is unlikely responsible for hydrogen sulfide detection, mainly because the sensor restoration related to H2S desorption in pure air was observed within ∼20 min, which is longer than in the case of NH3 (10 min; see Figure S6). The shallow trap states are expected to be characterized by faster desorption processes than the localized trap states.65 It means that shallow trap states do not mainly contribute to H2S detection. The third possible mechanism of the LS OFET−H2S interactions is the formation of a chemical bond between the material of electrodes (gold) and the thiol group of H2S. This detection mechanism has been proposed earlier, for example, in the case of hydrogen sulfide detection by gold nanoparticles electrodeposited on single-walled carbon nanotubes.66 However, Au−S chemical bond formation means irreversible analyte sorption and, therefore, the disposable sensor. In the

Figure 5. Suggested mechanisms of ammonia (a) and hydrogen sulfide (b) detection by the LS OFETs. 43837



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CONCLUSIONS AND OUTLOOK To sum up, in this work we designed and realized the first highly sensitive gas sensor based on Langmuir−Schaefer BTBT monolayer organic field-effect transistors. The devices fabricated from organosilicon dimer of dialkyl-BTBT O(SiUnd-BTBT-Hex)2 are able to operate in dry air and allow ultrafast detection and distinguishing of two different analytesammonia and hydrogen sulfideat low concentrations down to tens of ppb. The LS OFET sensors are reusable and can be utilized in real-time air-monitoring systems. We showed that the instantaneous sensor response can be split into variations of two OFET parameters, namely, charge carrier mobility and threshold voltage, as a result of diverse mechanisms of interactions with different toxic gases. The response to ammonia originates from the gate-biascontrolled charge trapping mechanism, whereas the sensitivity to hydrogen sulfide is based on the modulation of grain boundaries’ resistance in the LS film. Finally, we have demonstrated a multiparametric time-resolved gas detection technique, allowing one to distinguish two chemically different gases (H2S and NH3) by a single sensing device. We expect that the approach proposed can be further extended to other types of gases enabling the development of a complex air analysis system based on a single sensor. Preliminary tests of LS OFET sensitivity to NO2 and ethanethiol show even different transistor parameters’ behavior in the presence of these gases at sub-parts per million concentrations, which will be a subject of our further publications. The results obtained pave the way for OFET-based gas-sensing technology toward the real-time electronic nose.

electron pair of nitrogen easily interacts with positively charged holes, being ejected by the field effect into the LS film. In the latter case, hydrogen sulfide molecules have only weak electron-donor properties and do not significantly trap holes on the OSC surface. However, they are capable of diffusing into the monolayer OSC and modulating its conductivity via variation of the potential barrier between the grains (Figure 5b). The differences in the sensing mechanisms for these two gases lead to different LS OFET characteristics’ response, enabling a multiparametric toxic gas detection. As a result, extraction of the threshold voltage shift kinetics allows a quantitative determination of the ammonia concentration in the surrounding air. At the same time, to determine the hydrogen sulfide concentration in air, both threshold voltage shift and mobility variation should be analyzed. As described in the Introduction, the most important issues for OFET-based chemical sensors are the baseline drift problem, the capability of sensor restoration at room temperature, and reusability. In our approach, characteristic relaxation time used for the LS OFET sensor readout is highly reproducible with virtually zero drift (Figure S8). Also, despite the fact that the OFET key parameters degrade during the measurements and under analyte exposure, they restore to the initial values at normal conditions without gate bias applied. It means that both ammonia and hydrogen sulfide desorb from the semiconducting layer, and the LS OFET sensors are reusable and can be utilized in real-time applications (Figures 6 and S9).

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METHODS

Device Fabrication. LS OFETs were prepared on heavily doped silicon substrates with thermally grown oxide SiO2. The dielectric thickness was 290 nm, and its measured capacitance was 13 nF/cm2. Gold source and drain electrodes were thermally evaporated through a shadow mask. The resulting bottom gate bottom contact configuration had a fixed channel size of W = 1000 μm and L = 30 μm. The LS films of BTBT dimer O(Si-Und-BTBT-Hex)2 have been deposited as follows. The spreading solution was prepared by dissolving the BTBT dimer in toluene (Acros, used as received) at the concentration of 0.33 g/L. The solution was spread on the water surface, and the Langmuir film was left for 5 min to evaporate toluene and equilibrate before the compression has been started. Data were collected with a Nima 712BAM system equipped by Brewster angle microscope MicroBAM2 using a Teflon trough and barriers at room temperature. The monolayers were compressed at the speed of 200 mm/min. The LS films were obtained by horizontal transfer on octadecyltrimethoxysilane-modified silicon substrates with a dipping speed of 12 mm/min at constant surface pressure equal to 28 mN/m. Prior to electrical measurements, the unwanted part of the OSC monolayer film was manually removed to prevent parasitic current. The LS OFETs fabricated were annealed at normal conditions during 72 h to remove water traces. Electrical Characterization. Electrical measurements of the LS OFETs were performed with a Keithley 2634B source-meter under controlled environment at room temperature. Saturated field-effect mobility and threshold voltage were extracted by fitting transfer characteristics in the saturated regime with eq 3 iW y Id1/2 = jjj μC zzz k 2L {

Figure 6. Alteration of the charge carrier mobility (a) and threshold voltage (b) of the same LS OFET sensor after a consecutive exposure to 1 ppm H2S, 50 ppb NH3, 10 ppb H2S, and 200 ppb NH3, demonstrating reusability of the device. Alterations at each individual experiment are in good agreement with the data presented in Figures 3 and 4.

1/2

(Vg − VT)

(3)

where L and W are channel length and width, respectively; and C is the dielectric capacitance per unit area. The measurement time for a 43838


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ACS Applied Materials & Interfaces single curve was about 10 s. A delay of 300 ms between the drain/gate voltage sourcing and current measurement was set in order to minimize parasitic effects. Morphological Characterization. AFM measurements were performed with an NT-MDT Solver Next scanning probe microscope under ambient environment using Bruker FESPA silicon probes with resonance frequency of 70 kHz. Sensor Property Evaluation. The sensor properties were tested in a specially designed Teflon chamber with a free volume of ∼2.3 cm3. The LS OFETs’ response to toxic gases have been evaluated in dry airflow containing known amounts of NH3 or H2S diluted with the carrier gas generated by zero air generator (Khimelectronica, Russia). Mixtures of 200−5000 ppb of ammonia and hydrogen sulfide in carrier gas were prepared using a computerdriven GGS-K generator (Monitoring, Russia). The calibrated permeation tubes (Monitoring, Russia) of NH3 and H2S were used. Mixtures of 10−200 ppb of toxic gases in dry air were prepared by additional diluting of the GGS-K generator output with a synthetic airflow from a cylinder controlled by the mass-flow controller (Bronkhorst, The Netherlands). The electrical measurements were performed in a stabilized concentration that was reached after a 1 min flow.

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(3) Kang, B.; Lee, W. H.; Cho, K. Recent Advances in Organic Transistor Printing Processes. ACS Appl. Mater. Interfaces 2013, 5, 2302−2315. (4) Yuan, Y.; Giri, G.; Ayzner, A. L.; Zoombelt, A. P.; Mannsfeld, S. C. B.; Chen, J. H.; Nordlund, D.; Toney, M. F.; Huang, J. S.; Bao, Z. N. Ultra-High Mobility Transparent Organic Thin Film Transistors Grown by an Off-Centre Spin-Coating Method. Nat. Commun. 2014, 5, No. 3005. (5) Baeg, K. J.; Caironi, M.; Noh, Y. Y. Toward Printed Integrated Circuits Based on Unipolar or Ambipolar Polymer Semiconductors. Adv. Mater. 2013, 25, 4210−4244. (6) Usta, H.; Facchetti, A. Large Area and Flexible Electronics; Caironi, M.; Noh, Y.-Y., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA, 2015; Chapter 1, pp 1−100. (7) Reddy, M. R.; Kim, H.; Kim, C.; Seo, S. 2-Thiopene[1]Benzothieno[3,2-B]Benzothiophene Derivatives as Solution-Processable Organic Semiconductors for Organic Thin-Film Transistors. Synth. Met. 2018, 235, 153−159. (8) Mamillapalli, N. C.; Vegiraju, S.; Priyanka, P.; Lin, C. Y.; Luo, X. L.; Tsai, H. C.; Hong, S. H.; Ni, J. S.; Lien, W. C.; Kwon, G.; Yau, S. L.; Kim, C.; Liu, C. L.; Chen, M. C. Solution-Processable EndFunctionalized Tetrathienoacene Semiconductors: Synthesis, Characterization and Organic Field Effect Transistors Applications. Dyes Pigm. 2017, 145, 584−590. (9) Kazantsev, M. S.; Konstantinov, V. G.; Dominskiy, D. I.; Bruevich, V. V.; Postnikov, V. A.; Luponosov, Y. N.; Tafeenko, V. A.; Surin, N. M.; Ponomarenko, S. A.; Paraschuk, D. Y. Highly Bendable Luminescent Semiconducting Organic Single Crystal. Synth. Met. 2017, 232, 60−65. (10) Caranzi, L.; Pace, G.; Sassi, M.; Beverina, L.; Caironi, M. Transparent and Highly Responsive Phototransistors Based on a Solution-Processed, Nanometers-Thick Active Layer, Embedding a High-Mobility Electron-Transporting Polymer and a Hole-Trapping Molecule. ACS Appl. Mater. Interfaces 2017, 9, 28785−28794. (11) Bonacchini, G. E.; Bossio, C.; Greco, F.; Mattoli, V.; Kim, Y. H.; Lanzani, G.; Caironi, M. Tattoo-Paper Transfer as a Versatile Platform for All-Printed Organic Edible Electronics. Adv. Mater. 2018, 30, No. e1706091. (12) Bobbert, P. A.; Sharma, A.; Mathijssen, S. G. J.; Kemerink, M.; de Leeuw, D. M. Operational Stability of Organic Field-Effect Transistors. Adv. Mater. 2012, 24, 1146−1158. (13) Sirringhaus, H. Reliability of Organic Field-Effect Transistors. Adv. Mater. 2009, 21, 3859−3873. (14) Torsi, L.; Magliulo, M.; Manoli, K.; Palazzo, G. Organic FieldEffect Transistor Sensors: A Tutorial Review. Chem. Soc. Rev. 2013, 42, 8612−8628. (15) Someya, T.; Dodabalapur, A.; Huang, J.; See, K. C.; Katz, H. E. Chemical and Physical Sensing by Organic Field-Effect Transistors and Related Devices. Adv. Mater. 2010, 22, 3799−3811. (16) Sportelli, M. C.; Picca, R. A.; Manoli, K.; Re, M.; Pesce, E.; Tapfer, L.; Di Franco, C.; Cioffi, N.; Torsi, L. Surface Analytical Characterization of Streptavidin/Poly(3-Hexylthiophene) Bilayers for Bio-Electronic Applications. Appl. Surf. Sci. 2017, 420, 313−322. (17) Seshadri, P.; Manoli, K.; Schneiderhan-Marra, N.; Anthes, U.; Wierzchowiec, P.; Bonrad, K.; Di Franco, C.; Torsi, L. LowPicomolar, Label-Free Procalcitonin Analytical Detection with an Electrolyte-Gated Organic Field-Effect Transistor Based Electronic Immunosensor. Biosens. Bioelectron. 2018, 104, 113−119. (18) Diacci, C.; Berto, M.; Di Lauro, M.; Bianchini, E.; Pinti, M.; Simon, D. T.; Biscarini, F.; Bortolotti, C. A. Label-Free Detection of Interleukin-6 Using Electrolyte Gated Organic Field Effect Transistors. Biointerphases 2017, 12, No. 05F401. (19) Di Lauro, M.; Berto, M.; Giordani, M.; Benaglia, S.; Schweicher, G.; Vuillaume, D.; Bortolotti, C. A.; Geerts, Y. H.; Biscarini, F. Liquid-Gated Organic Electronic Devices Based on HighPerformance Solution-Processed Molecular Semiconductor. Adv. Electron. Mater. 2017, 3, No. 1700159. (20) Gu, P.; Yao, Y.; Feng, L.; Niu, S.; Dong, H. Recent Advances in Polymer Phototransistors. Polym. Chem. 2015, 6, 7933−7944.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b15427.

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LS OFET electrical properties, reproducibility, environmental stability, and LOD estimation (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sergei A. Ponomarenko: 0000-0003-0930-7722 Author Contributions

E.V.A. and A.S.S. conceived and designed the experiments; O.V.B. synthesized the BTBT derivative; V.C. prepared sensor samples; A.A.T. performed the experiments and processed the data; S.A.P. and A.A.V. supervised the research; A.S.S. wrote the paper. All authors have given approval to the final version of the manuscript. Funding

This work was performed in the framework of Leading science school NSh-5698.2018.3, supported by the Ministry of Science and Higher Education of the Russian Federation. E.V.A., A.S.S., A.T.T., and V.C. are grateful to RFBR (grant 17-0300222) for financial support. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to Aleksander Permyakov (ISPM RAS) and Mikhail Yablokov (ISPM RAS) for technical support of toxic gas measurements.

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REFERENCES

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