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Specific and Reproducible Gas Sensors Utilizing Gas-Phase Chemical Reaction on Organic Transistors Yaping Zang, Fengjiao Zhang, Dazhen Huang, Chong-an Di,* Qing Meng, Xike Gao,* and Daoben Zhu* By virtue of their unique feature of precise chemical detection, electrical sensors have attracted tremendous interest in environmental monitoring, industrial production, and food safety detection.[1–6] Organic field-effect transistor (OFET)-based gas sensors, the promising candidates of electrical sensing, have received particular attention owing to their intrinsic advantages in ultra-low-cost and portage applications.[7–10] So far, ppm and sub-ppm detection of humidity, nitrogen dioxide (NO2), sulfur dioxide (SO2), ammonia (NH3), hydrogen sulfide (H2S) and phosphate vapors, etc.,[11–19] have been successfully realized utilizing various OFETs. In spite of these encouraging achievements of OFET based gas sensors, some crucial sensing performances such as specificity and reproducibility are still far from satisfactory toward widely detecting applications. Therefore, the construction of fast response rate OFET based sensors with excellent sensitivity, specificity, reproducibility, and stability remains challenging and is highly desired. A typical OFET based gas sensor relies on the noncovalent bonds that induce interaction between the analytes and organic semiconductors.[9] Such kind of interaction results in a change in carrier density and/or carrier transport properties in the conductive channel, leading to the alteration of device characteristics. Application of air dielectric layer, introduction of ultrathin heterojunction layer, deposition of ultra-thin organic active layer, and construction of self-assembled monolayer field-effect transistor have been developed to strengthen and/or accelerate the noncovalent interaction between the analytes and the active layer.[17,20–23] Although these approaches contribute greatly to the enhancement of the sensitivity, the improvement of sensing specificity still relies on the development of multi-functional semiconductors and/or receptor molecules from the standpoint Y. P. Zang, F. J. Zhang, D. Z. Huang, Dr. C.-A. Di, Dr. Q. Meng, Prof. D. B. Zhu Beijing National Laboratory for Molecular Sciences Key Laboratory of Organic Solids Institute of Chemistry Chinese Academy of Sciences (CAS) Beijing 100190, P. R. China E-mail: [email protected]; [email protected] Y. P. Zang, F. J. Zhang, D. Z. Huang University of Chinese Academy of Sciences Beijing 100049, P. R. China Dr. X. K. Gao Laboratory of Materials Science Shanghai Institute of Organic Chemistry CAS, Shanghai 200032, P. R. China E-mail: [email protected]

DOI: 10.1002/adma.201305011

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of chemical reactivity. As a representative example, Katz et al. recently put forward a series of OFETs based sensors with both high sensitivity and good specificity by the introduction of different receptor molecules.[12,15,24] These results indicated the important role of chemical reactivity played on the specific and sensitive gas sensors based on OFETs. A chemical reaction usually leads to the transformation of one set of chemical substances to another and involves various electron and/or charge transfer processes. Once appropriate chemical reaction occurred on the surface of active channel of OFETs, the electron and/or charge transfer might be transduced into electrical signal by the devices, allowing detection of particular analytes. Herein, we report a simple chemical approach, which we called “gas-phase reaction assisted detection (GRAD)”, to enabling sensitive, specific and reproducible multi-gas sensors using well-developed n- and p-channel OFETs. Benefiting from introduction of a simple textbook reaction, additional molecular design of multi-functional semiconductors can be avoided. The combination of chemical adsorption, gas-phase reaction and carrier transport is thus a novel strategy toward precise and predictable gas sensors. The molecular structure of organic semiconductors and the device structure are shown in Figure 1. NDI(2OD)(4tBuPh)DTYM2 and pentacene were deposited as the n- and p-channel active layers due to their excellent electrical properties and good stability.[22,25,26] Bottom-gate top-contact OFETs with active layer thickness of 10–20 nm were fabricated and characterized using standard methods (Supporting Information). The typical output and transfer characteristics are given in Figure S1. All the devices exhibit mobility ranging from 0.2–0.7 cm2 V–1 s–1 and show clear FET characteristics with well-defined linear and saturation regimes. The responses of the fabricated devices to NH3, NO2 and HCl were investigated by plotting the source-drain current (IDS) versus exposure time to analytes (Figure 2). For NDI(2OD) (4tBuPh)-DTYM2 based n-channel OFETs, the exposure to 1 ppm NH3 results in obvious increase of IDS while the exposure to 10 ppm NO2 leads to an opposite phenomenon. Since no obvious changes in spectrum before and after the chemical gas exposure were observed during the X-ray photoelectron spectroscopy (XPS) and Ultraviolet-visible (UV-Vis) adsorption spectroscopy measurements (Figure S3 and S4), the exposure to NH3, NO2, HCl does not lead to irreversible chemical changes of organic semiconductors. As an electron donor, NH3 interacts with electron-deficient semiconductor of NDI(2OD)(4tBuPh)DTYM2. It therefore induce more carriers and even fill some electron traps in the conductive channel.[14] The increased IDS is thus attributed to the doping and de-trapping of NH3, which

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COMMUNICATION Figure 1. (a) Molecular structures of NDI(2OD)(4tBuPh)-DTYM2 and pentacene. (b) Schematic diagram of device structure and sensing mechanism.

can be confirmed by UV-Vis adsorption spectroscopy of NH3 treated solution of NDI(2OD)(4tBuPh)-DTYM2 (Figure S5). In addition, the strong oxidizing NO2 gas can capture electrons from the semiconductors easily and leads to a decrease of IDS.[20] The deduction is consistent with negative and positive shifts of threshold voltage upon exposure to NH3 and NO2 (Figure S2), respectively. Meanwhile there is no obvious change in device mobility, indicating relative weak influences on carrier transport. In contrast, p-channel pentacene based OFETs show an opposite phenomenon when exposed to NH3 (1 ppm) and NO2 (1000 ppm). Owing to the hole-accumulation mechanism of p-channel OFETs, the exposure to NH3 (1 ppm) leads to the decrease of IDS and the introduction of NO2 (1000 ppm) increases the IDS obviously (Figure 2d, e). However, both n-channel and p-channel devices are insensitive to HCl. Exposure to 1000 ppm HCl results in negligible IDS changes for NDI(2OD)(4tBuPh)-DTYM2 based devices and slightly increases of IDS for pentacene OFETs. The phenomenon is probably because of relative weak doping and trapping ability of HCl for both electrons and holes. For NDI(2OD)(4tBuPh)-DTYM2 based OFETs after exposure to NH3, the increased IDS is not reversible in four thousand

seconds (Figure S6), suggesting effective chemical adsorption on the semiconducting layer. Interestingly, NH3 adsorbed NDI(2OD)(4tBuPh)-DTYM2 devices show unexpected sensitivity to HCl. When the device was exposed to trace amount of NH3 (1 mL, 1000 ppm) for only few seconds, the IDS increased dramatically (Figure 3a). The introduction of air flow to the polydimethylsiloxane (PDMS) microchannel cell, which aims at removement of free NH3, leads to slightly decreased IDS. However, the exposure of HCl (10 ppm) to NH3 exposed NDI(2OD) (4tBuPh)-DTYM2 devices results in an obvious decrease of IDS. The IDS can be recovered nearly to its initial degree without any additional treatments. This unprecedented HCl detection shows good reproducibility as indicated by the similar responses upon HCl exposure during the cycling detection test (Figure 3a). The total response and recovery time for the detection of 10 ppm HCl is about 10 and 20 seconds, respectively, making the devices serving as one of the fastest response rate OFETs based gas sensors. It is worth noting that this response rate is comparable with that of commercial HCl sensors (20∼300 seconds).[27–31] Therefore, the adsorption of NH3 on active layers enables the precise detection of HCl. To investigate the sensing property of NH3 exposed NDI(2OD)(4tBuPh)-DTYM2 OFETs under different analyte

Figure 2. Drain current for NDI(2OD)(4tBuPh)-DTYM2 OFETs vs time of exposure to (a) 1 ppm NH3, (b) 10 ppm NO2 and (c) 1000 ppm HCl, respectively. Drain current for pentacene OFETs vs time of exposure to (d) 1 ppm NH3, (e) 1000 ppm NO2 and (f) 1000 ppm HCl, respectively. The VGS and VDS are fixed at 10 V and –10 V for n- and p-channel devices, respectively.

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of times when the devices are exposed to a low concentration of analytes. In addition to good reproducibility, our n-channel devices show excellent stability before and after exposure to NH3 (Figure S6), which is comparable with that of many p-channel devices.[15] Benefiting from good stability, air is applied as the carrier gas in our studies to provide a realistic simulation of sensing process. The remarkable reproducibility and stability imply NDI(2OD)(4tBuPh)-DTYM2 based OFETs could serve as an outstanding candidate for sensing applications. In order to study the sensing mechanism, atomic force microscopy (AFM) measurements were performed to investigate the morphology of NDI(2OD)(4tBuPh)-DTYM2 films upon different gases (Figure 4, S7 and S8). Surprisingly, nanoscale particles were formed after exposure of NH3 adsorbed NDI(2OD)(4tBuPh)-DTYM2 to HCl. The Figure 3. Drain current for NH3 exposed NDI(2OD)(4tBuPh)-DTYM2 OFETs vs time of expo- particle size increases from 70 nm to sure to (a) HCl (10 ppm), (b) HCl with vary concentration (10, 50, 80, 100 and 300 ppm). 110 nm along with enhanced HCl concen(c) The relative response of NH3 exposed NDI(2OD)(4tBuPh)-DTYM2 OFETs as a function of HCl concentration (linear and log scale). The red line is the fitting result. (d) Source-drain tration from 10 ppm to 100 ppm. Since the current of sensing OFETs during 10 successive cycles of exposure to NH3 (1/10 v/v) and HCl particles cannot be formed by the exposure of NDI(2OD)(4tBuPh)-DTYM2 film to NH3 and (1/10 v/v) alternately. The VGS and VDS are fixed at 10 V. HCl separately, it can be concluded that the particle should be NH4Cl which comes from chemical reaction between adsorbed NH3 and exposed HCl. The concentration, the HCl concentration was tuned from 10 ppm to 300 ppm. The sensing measurements were performed with existence of Cl− and NH4+ was further testified by the fact that the same method mentioned above and the sensing characteristhe water solution of these particles can react with AgNO3 solutics were recorded in Figure 3b. When the devices were exposed tion (0.1 mol/ml) and the produced gases with NaOH enable to HCl with increasing concentration, the IDS responses color change of the PH test strips, respectively (Figure S9). Moreover, these particles can be removed by a thermal treatincrease dramatically. For example, 10 ppm HCl resulted in ment (80 °C for 30 s), vacuum storage (10–4 Pa for 1 h) and 34% decrease of IDS while the IDS decreased one order of magnitude in 20 seconds when the NDI(2OD)(4tBuPh)-DTYM2 water washing, respectively (Figure 4, S8). The lower thermal device was exposed to 300 ppm HCl. Even exposed to 300 ppm treatment temperature (80 °C) than the sublimation temperaHCl, the device can recover most of its initial degree within ture (100 °C) of NH4Cl might because of nano-scale effect. 150 seconds. More importantly, the ΔI/I0, defined as the ratio The proposed mechanism is further described as folof maximum change in current after exposure to HCl to the lows (Figure 4e). As mentioned above, adsorption of NH3 on initial IDS before exposure to different HCl, increases with NDI(2OD)(4tBuPh)-DTYM2 film leads to effective electron doping. However, the interaction between NDI(2OD)(4tBuPh)the increased HCl concentration. As shown in Figure 3c, the DTYM2 and NH3 is relatively weak which is indicated by the linear ΔI/I0 increase can be observed when the HCl concentration changed from 10 ppm to 300 ppm. Interestingly, a good fluctuant current when air flow is introduced (Figure 4e, 6c). linearity of ΔI/I0 as a function of HCl concentration at the log The airflow blowing on the surface of the semiconductor loosens the fixed NH3 and causes a slight decrease of IDS, scale was observed within the full range of HCl concentration (10–300 ppm) and the determination coefficient R2 is 0.86. The which recovers immediately once the airflow is turned off. good sensing performance under varying HCl concentration Additionally, when the HCl flow is turned on, many adsorbed makes these OFETs sensors suitable for quantitative detection. NH3 molecular desorbed from the semiconducting layer owing The sensing reproducibility and stability are important to dipole-dipole interaction between NH3 and HCl, thus lead parameters for electronic sensors. To test the reproducibility to reduced NH3 doping to the device. We deduce that the of NDI(2OD)(4tBuPh)-DTYM2 based sensing devices, we decreased IDS results from the combined effect of airflow and exposed OFETs to high concentration of NH3 (1/10 v/v) and electrostatic attractions between NH3 and HCl. Owing to the HCl (1/10 v/v) alternately (Figure 3d). Exposure to NH3 results low HCl concentration, only small amount of desorbed NH3 in dramatic increase of IDS, while the device characteristics can react with HCl. Adsorption of the residual NH3 on active layer be recovered by the introduction of HCl. Considering more occurred once again once HCl flow was turned off and enables than 10 successive cycles of the exposure to high concentranearly recovered IDS. Meanwhile, the generated NH4Cl particles tion of NH3 and HCl was achieved (Figure 3d), we deduce deposited on the surface of the film and have no influence on IDS. However, high concentration of HCl (1/10 v/v) creates a the devices might be used for several hundreds to thousands

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COMMUNICATION Figure 4. AFM images of NDI(2OD)(4tBuPh)-DTYM2 films upon exposure to (a) NH3 (1/10 v/v), (b) NH3 (1/10 v/v) and HCl (10 ppm), (c) NH3 (1/10 v/v) and HCl (100 ppm), (d) NH3 (1/10 v/v) and HCl (100 ppm) followed by 80 °C thermal treatment for 30 seconds in air, (e) Cartoon diagram of sensing process (with fixed VDS and VGS) of chemical reaction assisted gas sensor based on n-channel OFETs.

OFETs show expected reproducible response to both low contremendous IDS decrease of nearly three orders and impedes centration and high concentration of NH3 (1 ppm and 1/10 v/v), spontaneous recovery of IDS before the re-introduction of NH3 (Figure 3d and S10d). The ΔI/I0 brought by high concentration even 1 ppm NH3 can decrease the IDS of HCl treated device of HCl (1/10 v/v) is far bigger than which created by HCl with by 36% (Figure 5a, b). The phenomenon is similar to the HCl low concentrations (10−300 ppm) (Figure 3c, 3d) since a large sensing response of NH3 exposed NDI(2OD)(4tBuPh)-DTYM2 amount of HCl can react with most of adsorbed NH3. Theredevices. Similarly, the produced particles can be removed by fore, the interaction and chemical reaction between NH3 and HCl is responsible for reproducible and high sensitive detection of HCl. For most OFET-based gas sensors, the response of output signal is proportional to the analyte concentrations.[1,20] Figure S10a shows the responses of NDI(2OD)(4tBuPh)DTYM2 OFETs upon exposure to NH3 with different concentrations. The ΔI/I0 displays a linear increase with the increment of NH3 concentrations in the range of 1–1000 ppm. However, the plot of ΔI/I0 versus HCl concentrations just show a linearity in the range of low concentrations in our HCl detection process (Figure 3c). Considering the mechanism analysis suggested above, we deduce that the deviation of linearity at the whole range of HCl cencentrations probably created by the multiple effect of airflow, desorption ability of NH3 from semiconductor and interactions between NH3 and HCl. Figure 5. Drain current for pentacene OTFTs vs time of exposure to (a) HCl (1/10 v/v) and NH Pentacene OFETs were constructed to (1 ppm), and (b) HCl (1/10 v/v) and NH (1/10 v/v) alternately for 10 successive cycles. Drain3 3 verify our proposed mechanism. In contrast current for NDI(2OD)(4tBuPh)-DTYM2 OTFTs vs time of exposure to (c) various concentration to unrecoverable decrease of IDS upon expo- (10, 80, 100 and 300 ppm) of NO2, (d) NH3 (1000 ppm) and NO2 (10 ppm). VGS and VDS are sure to NH3, the HCl exposed pentacene fixed at -10 V and 10 V for p- and n-channel devices, respectively.

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(10–1000 ppm) (Figure 6b). No obvious changes were observed when the device was exposed to 10 ppm HBr, perhaps because of weaker chemical interaction between the chemical adsorbed NH3 and low concentration of HBr. The deduction was confirmed by AFM measurement since no NH4Br was formed when 10 ppm HBr was introduced on the surface of NH3 exposed NDI(2OD) (4tBuPh)-DTYM2 film. Moreover, the devices possess pronounced selectivity for NO2 (10 ppm) and SO2 (10 ppm), which are representative coexisting air pollutions. In comparison with a 37% decrease of IDS when the devices was exposed to 10 ppm NO2, no obvious responses were observed after introduction of 10 ppm SO2 (Figure 6c). Thereafter, the exposure to a mixture gas of NO2 and SO2 leads to a same IDS response compared with IDS changes upon exposure to NO2. The possible reason might be that NO2 is an oxidaFigure 6. (a) Drain current changes for NH3 exposed NDI(2OD)(4tBuPh)-DTYM2 OFETs tive gas with stronger withdrawing ability upon exposure to different vapors. Drain current for NH3 exposed NDI(2OD)(4tBuPh)-DTYM2 OFETs vs time upon exposure to (b) HBr (10 ppm) and HCl (10 ppm), and (c) NO2 (10 ppm), than SO2. Although the devices showed comparable response to HCl and NO2, it could SO2 (10 ppm), and mixture of NO2 (10 ppm) and SO2 (10 ppm), respectively. be easier to filter one of these gases by other chemical approaches. These studies thus demonstrate the remarkable selectivity of NDI(2OD)(4tBuPh)the treatments of vacuum storage (10–4 Pa for 1 h) and water DTYM2 based sensors. washing (Figure S8). These results combined together suggest In conclusion, specific and reproducible HCl, NO2, and reasonable mechanism of GRAD process and it's generality to both n-channel and p-channel OFETs. NH3 detections have been realized utilizing gas-phase reaction assisted detection approach. A combination of effective chemIn addition to direct chemical reaction between the adsorbed ical adsorption and chemical reaction occurred on organic tranNH3 and the analytes, reversible and effective detection of sistors enables OFETs based sensors exhibiting outstanding NO2 can be achieved when assisted by the chemical adsorbed sensing performances including good selectivity, excellent NH3. As mentioned above, NO2 can trap electron transport reproducibility, high sensitivity, fast response and recovery rate. in NDI(2OD)(4tBuPh)-DTYM2 based devices and induces This study suggests that the fine-tuning of semiconductor-anaunrecoverable decrease of IDS (Figure 5c). Interestingly, lyte interaction via chemical approach can serve as a shortcut to reversible detection of 10 ppm NO2 was achieved by using achieve predictable sensors using well reported OFETs without NH3 (1000 ppm) exposed devices, with the sensitivity of 50% additional molecular design. (Figure 5d). The phenomenon is probably due to impeded chemical adsorption of NO2 by pre-adsorbed NH3. Therefore, the interaction between the active layer and analytes can be fine-tuned by adsorbed gases to realize sensitive and reproducSupporting Information ible chemical detection. Supporting Information is available from the Wiley Online Library or Selectivity is a crucial parameter and an open issue for pracfrom the author. tical sensing applications, which usually relies on the specific interaction between the organic semiconductors and the analytes. To probe the selectivity of our devices, the sensing property of NH3 exposed NDI(2OD)(4tBuPh)-DTYM2 devices to Acknowledgements several different solvent vapors and chemical gases including The authors acknowledge the kind help from Prof. Yunqi Liu and Prof. isopropanol, chloroform, acetone, o-Dichlorobenzene, hexane, Jizheng Wang for AFM measurements, and partial sensitive detection H2S, HBr, SO2 were investigated (Figure 6). As shown in measurements. We acknowledge financial support from the National Figure 6a, all these solvents except HCl and NO2 gave small Natural Science Foundation (61171055 and 51173200), MOST (2011CB932300), the Chinese Academy of Sciences. responses (ΔI/I < 0.1) even at very high concentration of 1000 ppm. It should be noted that NDI(2OD)(4tBuPh)-DTYM2 based sensor even exhibit outstanding selectivity to the gases Received: October 8, 2013 with similar properties. As a clear example, 10 ppm HCl can Revised: November 24, 2013 be easily detected while the devices are insensitive to HBr Published online: February 8, 2014

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Specific and reproducible gas sensors utilizing gas-phase chemical reaction on organic transistors.

Utilizing a textbook reaction on the surface of an organic active channel, achieves sensitive detection of HCl, NH3 and NO2, with good selectivity, ex...
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