Organic Gas Sensor Research Articles

High selective gas sensors based on surface modified polymer transistor

Gas sensors based on organic field effect transistors have attracted great attentions and achieved much progress because of their inherent merits such as low-cost, room operating temperature, good portability and flexibility. However, high sensitivity and good selectivity are still challenging issues blocking their further development. Herein, we demonstrate a promising strategy for fabricating high selective organic gas sensors through introducing surface modification. Modified substrate with different self-assembling monolayers (SAMs), the sensing properties of polymer OFET gas sensors are varied with terminal group attached on the SAM. The bare device shows significant sensitive to NH 3 , while it becomes much more sensitive to NO 2 if a fluorine substituent SAM adopted. The NH 2 -terminal SAM and CH 3 -terminal SAM bring moderate sensitivity to gases like NH 3 and NO 2 . Those various sensitivity and device parameter evolution are promising to generate a patterning recognition for different gases, which may provide a potential promising approach for improving selectivity of organic gas sensors. • Polymer transistors with varied performance was achieved based on a series of SAM modification. • Gas sensitivities to several gases ranging from oxidative to reductive are critically dependent on the end group of SAM molecules. • Polymer transistor with F-SAM present a high sensitivity up to 780% to NO 2 . • Multiple parameter variations based on the series of devices provide a potential pattern recognition to discriminate gases.

A comparative study of honeycomb-like 2D π-conjugated metal-organic framework chemiresistors: conductivity and channels.

Two-dimensional (2D) π-conjugated conductive metal-organic frameworks (cMOFs, 2DπcMOF) with modulated channel sizes and a broad conductivity range have been reported in the last decade. In contrast, the corresponding comparative studies on their effects on chemiresistive sensing performances, which measure the resistive response toward external chemical stimuli, have not yet been reported. In this work, we sought to explore the structure-performance relationships of honeycomb-like 2D π-conjugated cMOF chemiresistive gas sensors with channel sizes less than 2 nm (the mass transport issue) and broad conductivity in the range from ∼10-8 S cm-1 to 1 S cm-1 (the charge transport issue). As a result, we found that the cMOF with a lower conductivity facilitates the much more sensitive response toward the charge transfer of the adsorbed gases (relative increases in resistance: R = 63.5% toward 100 ppm of NH3 for the as prepared Cu-THQ sensor with the conductivity of ∼10-8 S cm-1). Interestingly, the cMOF with a medium channel size (Cu-THHP-THQ) exhibited the fastest response speed in sensing, although it contains H2en2+ as neutralizing counterions in the channels. From the evaluation of the pore size distribution, it is found that the overall porosity (meso- & micro-pores) of cMOFs, rather than the pore size of the honeycomb structure, would determine their sensing speed. When comparing the performance of two different morphologies of nanorods (NRs) and nanosheets (NSs), NRs showed a slower response and extended recovery time, which can be ascribed to the slower gas diffusion in the more extended 1D channel. Altogether, our results demonstrate the first systematic studies on the effect of various structural parameters on the chemiresistive sensor performance of cMOFs.

High Detectivity Ammonia Gas Sensor of Pentacene Based Organic Material with OXIDE MESH

Ammonia gas sensing properties of pentacene based organic gas sensor fabricated by thermal evaporation method on amorphous oxide semiconductor (a-SiInZnO) mesh. The amorphous oxide semiconductor mesh layer was fabricated by RF sputtering and conventional photolithography. Insertion of a dense mesh layer has been found to significantly improve gas sensing properties. In the case of a conventional pentacene thin film, the SEM image analysis resulted in a uniform surface. This means that ammonia gas cannot be adsorbed well on the surface. The insertion of the mesh layer improves the surface volume ratio of the organic thin film and induces more ammonia gas to be adsorbed on the pentacene surface. As a result, it was confirmed that even when the ammonia gas concentration was 5 ppm or less, the detection rate was higher than that of a conventional pentacene thin film. It is expected to be applied to various high-sensitivity organic-based gas sensors.

Ag-doped WO3 nanostructure films for organic volatile gas sensor application

A novel gas sensor based on Ag-doped WO3 nanostructures was successfully deposited on a glass substrate via electron beam evaporation technique. The microstructure, surface morphology, chemical composition, crystal structure, and optical properties of the pure and Ag-doped WO3 nanostructure thin films were studied by scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray diffractometer, and photoluminescence spectroscopy (PL). Gas-sensing tests revealed that the 15 wt% Ag-doped WO3 nanostructure possess excellent gas-sensing performance for ethanol. The sensitivity of the material toward 100 ppm ethanol was as high as 65 at an optimum operating temperature of 300 °C, which is higher than that of undoped WO3. The response/recovery times 9 s/11 s, great selectivity and outstanding long-term stability was obserevd for the 15 wt% Ag doped WO3 nanostructure sensor. The improvement of ethanol-sensing performance is related to the active space charge regions around the interface of Ag particles and WO3.

UV-enhanced room-temperature ultrasensitive NO gas sensor with vertical channel nano-porous organic diodes

In this work, we successfully proposed an organic semiconductor gas sensor on vertical channel design to exhibit an ultra-sensitive response to nitric oxide gas (10 ppb) at room temperature. The effect of ultra-violet (UV) irradiation (λ =365 nm) on sensing performance of solution processed vertical organic diode (VOD) has been investigated. After implementing a simple and low-cost UV treatment, the sensing response of PBDTTT-C-T based NO sensor is significantly enhanced from 93 % to 233 % at 1 ppm. In addition, we also introduced a new strategy to tune the selectivity of organic gas sensors by using photochromic molecule and organic semiconductor blends. With the 1 Wt% spiropyran dopant, the sensor response to ammonia can be considerably suppressed, hence the response ratio between NO and ammonia can be improved from 2.9 to 16.0, indicating an improved selectivity. The proposed ultrasensitive and selective NO gas sensor have great potential for exhaled breath detection (asthma patients) and environmental monitoring.

Volatile Organic Compounds Gas Sensors Based on Molybdenum Oxides: A Mini Review.

As a typical n-type semiconductor, MoO3 has been widely applied in the gas-detection field due to its competitive physicochemical properties and ecofriendly characteristics. Volatile organic compounds (VOCs) are harmful to the atmospheric environment and human life, so it is necessary to quickly identify the presence of VOCs in the air. This review briefly introduced the application progress of an MoO3-based sensor in VOCs detection. We mainly emphasized the optimization strategies of a high performance MoO3, which consists of morphology-controlled synthesis and electronic properties functional modification. Besides the general synthesis methods, its gas-sensing properties and mechanism were briefly discussed. In conclusion, the application status of MoO3 in gas-sensing and the challenges still to be solved were summarized.

The Room-Temperature Planar Type Gas Sensor Based on DPA-Ph-Dbpzdcn/TPA-Dcpp for NH3 Monitoring Application

Introduction Ammonia (NH3) gas sensor is the indispensable element in safety, air quality monitoring and respiratory analysis fields. The chemoresistor sensors are mainly focused in the investigation of NH3 sensors attributing to the high stability, simple sensing structure, low cost and portable device. However, solving the high working temperature issue and achieving reproducible and stable gas sensing properties by designing novel sensitive material is highly desirable for widespread development of ammonia sensor [1]. Here, the planar room-temperature NH3 sensors are fabricated through spin-coating DPA-Ph-DBPzDCN/TPA-DCPP organic semiconductor materials onto the Al2O3 substrate with Au interdigital electrodes. The two sensors exhibit favorable sensing response as 72.73%/76.4% to 100 ppm NH3 under 98% RH, respectively, also demonstrates the low detection limits, good reproducibility, selectivity and long-term stability. The DFT calculation results further demonstrate that the excellent NH3 sensing characteristics are attributed to the enhanced absorption between NH3 and the N groups of organic molecules, which formed the H2O pre-adsorption under the high RH. This work is expected it can offer some guidance and inspiration for the rational design of sensing materials and further development of room temperature gas sensor. Results and Conclusions The synthesis process of DPA-Ph-DBPzDCN/TPA-DCPP were detailed reported previously [2, 3]. The sensor devices were successful fabricated via spin-coated sensitive materials onto the Al2O3 substrate with Au interdigital electrodes and illustrated in Figure 1. It was illustrated from scanning electron microscopy (SEM) image that the organic sensing films were formed uniformly and tightly. Then, the response transients of sensors in different NH3 concentration under 98% and 25% RH were monitored (Figure 2). The response value improved alone with the increasing of NH3 concentration and performed the highest response value as 72.73%/76.4% to 100 ppm NH3 under the 98% RH, respectively. Furthermore, there had a decreasing trend of the lowest detectable NH3 concentration along with the increasing RH, especially, the TPA-DCPP sensor exhibited the lowest detection limit as 500 ppb under 98% RH.Accordingly, the density functional theory (DFT) calculations were performed to gain further insight into the interaction between gas molecules and the sensitive materials, also the enhanced ammonia sensing properties under the high RH. The three most likely ammonia adsorbing sites (A: trihpenylamine-ortho C on the benzene ring; B: pyrazine-N; C: cyano-N) were selected due to the negative charge was mainly located. The configuration and the charge density difference after NH3 adsorption (Figure 3) demonstrated that the charge transfer occurred through the NH3 given electrons to organic compounds, thereby the conductivity of organic semiconductors increased until to an equilibrium state. Simultaneously, the NH3, H2O, NH3-H2O adsorption energies of the corresponding adsorbing sites were given in Table 1. It was shown that the NH3 adsorbing to organic compounds after H2O pre-adsorption were more capable compared with the NH3 adsorption only. Meanwhile, the NH3 adsorption energy before and after H2O pre-adsorption of the pyrazine-N on TPA-DCPP (-0.1809 eV, -0.4011 eV) was obviously higher than the DPA-Ph-DBPZdCN molecule (-0.14362 eV, -0.15679 eV), resulting in the response value of TPA-DCPP sensor was apparently improved compared to the DPA-Ph-DBPZdCN sensor under the low/high RH, that were attributed to the increasing of the negative charge on pyrazine-N owing to the directly link between and the pyrazine the cyano group, the change of molecular structure leading to a significantly stronger coulombic effect between pyrazine-N with NH3. Taken together, the excellent NH3 sensing characteristics were attributed to the enhanced absorption between NH3 and H2O pre-adsorption organic molecules under the high RH [4]. The DPA-Ph-DBPZdCN/TPA-DCPP could be the promising NH3 organic sensitive materials for the room temperature sensor, meanwhile, the discussion of sensing mechanism bring the further understanding of the organic gas sensors, providing the new concept of organic materials which has potential application value in gas monitoring of production and daily life at room temperature.

Development of Organic Gas Sensor Using Quartz Crystal Microbalance Coated with Plasma-polymerized Films

Development of Organic Gas Sensor Using Quartz Crystal Microbalance Coated with Plasma-polymerized Films

Volatile organic compound gas sensors based on methylammonium lead iodide perovskite operating at room temperature.

Methylammonium lead iodide (MAPbI3) perovskite thin film has been successfully applied to a volatile organic compound (VOC) gas sensor that can operate at room temperature. In this study, ∼100 nm-thick MAPbI3 film shows good reversibility and repeatability as a VOC gas sensor. The resistance of the MAPbI3 film substantially decreases when it is exposed to VOC vapour and recovers back to high resistance when the VOC gas is removed. Adsorption of VOC gas molecules to vacancies in MAPbI3 film might lead to charge trap passivation. The VOC sensor based on perovskite thin film is tested in terms of film thickness, applied bias voltage, and polarity of VOC. We expect that our VOC gas sensor based on solution-processed MAPbI3 operating at room temperature has potential to be developed as a low cost and low power smart gas sensor.

Field-dependent electrical properties of carbon nanotubes from first-principles: negative differential conductance, current oscillations and molecular sensing

In certain materials, and for certain voltage ranges, electrical current flowing through the material decreases when the voltage across the sample is increased. This negative differential conductance (NDC) is important for oscillators, amplifiers, and fast switching devices. In this work, using real time quantum simulations, we show that this phenomenon occurs in isolated finite armchair single wall carbon nanotubes (SWCNT) without end contacts. For metallic SWCNT, like the armchair SWCNT, electron transfer to secondary valleys—the most common cause of NDC—is not expected to be observed, as there are two quantum channels at the Fermi energy available for conduction. The NDC is due to the finite nature of the SWCNT and the existence of excited states that are blocked, similarly to a Coulomb blockade system, thus preventing any further current flow. We also show that the SWCNT conductivity depends on its length and that the current flowing on the SWCNT behaves like a Bloch oscillation that is disrupted in the presence of a molecule, decreasing the conductivity, and thus providing a rationalisation for the behaviour of SWCNT organic gas sensors.

Metal Oxide Nanostructures-Based Gas Sensor for Ethylene Detection: A Review

Increased ethylene content may induce the synthesis of respiratory enzymes and enhance respiration, which accelerated fruit ripening. Monitoring of ethylene concentration is critical in the fruit industry, avoid the fruit from passing through. Research interest has focused on the utilization of nano-sized semiconductor to monitor the ethylene through various processes. As detection for ethylene with excellent sensitivity, superior quantum efficiency, and fast response speed, nano-sized semiconductors are regarded as the most promising directions for nanomaterials. We review the latest achievements with nano-sized material gas sensors as for ethylene detector in the article. Firstly, the classification and structure of semiconductor detectors are introduced (such as resistive transducer; non-resistive transducer; organic semiconductor gas sensor), Then we present several kinds of and their performance, focusing on binary oxides with wide band gap (such as ZnO, SnO2, Ga2O3, Nb2O5, and WO3) and ternary oxides (such as Zn2SnO4, Zn2GeO4, and In2Ge2O7) Eventually, finally, we review and summarize the perspectives in this field and the prospects for future research. Perspectives and outlook on future research directions in this field.

Gas-Sensing Performance and Operation Mechanism of Organic π-Conjugated Materials.

Organic π-conjugated materials, which have advantages of tailorable chemical structures, solution processability, and mechanical flexibility, are promising candidates for applications in low-cost, portable gas sensors that operate at room temperature. Nevertheless, there are still challenging issues in the development of organic semiconductor gas sensors because of issues such as relatively poor sensitivity, slow response, and low recovery. Herein, we summarize the recent progress of gas sensors based on organic conjugated materials and discuss the critical factors related to the sensor performance and the operation mechanism. We highlight six types of typical materials from small molecule to polymer semiconductors with respect to their performance in detecting different gas species through morphology control and molecular engineering. Critical factors from gas kinetics to sensor-analyte interaction are briefly discussed, together with an examination of sensing mechanisms.

A Versatile Method to Enhance the Operational Current of Air-Stable Organic Gas Sensor for Monitoring of Breath Ammonia in Hemodialysis Patients.

Point-of-care (POC) application for monitoring of breath ammonia (BA) in hemodialysis (HD) patients has emerged as a promising noninvasive health monitoring approach. In this context, many organic gas sensors have been reported for BA detection. However, one of the major challenges for its integration with affordable household POC application is to achieve stable performance for accuracy and high operational current at low voltage for low-cost read-out circuitry. Herein, we exploited the stability of the Donor-Acceptor polymer on the cylindrical nanopore structure to realize the sensors with a high sensitivity and stability. Then, we proposed a double active layer (DL) strategy that exploits an ultrathin layer of Poly(3-hexylthiophene-2,5-diyl) (P3HT) to serve as a work function buffer to enhance the operational current. The DL sensor exhibits a sustainable enhanced operational current of microampere level and a stable sensing response even with the presence of P3HT layer. This effect is carefully examined with different aspects, including vertical composition profile of DL configuration, lifetime testing on different sensing layer, morphological analysis, and the versatility of the DL strategy. Finally, we utilize the DL sensor to conduct a tracing of BA concentration in two HD patients before and after HD, and correlate it with the blood urea nitrogen (BUN) levels. A good correlation coefficient of 0.96 is achieved. Moreover, the feasibility of DL sensor integrated into a low-cost circuitry was also verified. The results demonstrate the potential of this DL strategy to be used to integrate organic sensor for affordable household POC devices.

The evaluation of Temperature effect on Peak-to-Peak Line Width (ΔHpp) of conjugating polymer Polypyrrole (Ppy)

The magnetic properties of the neutral and doped polymer polypyrrole (Ppy) nanostructure has been successfully synthesize via chemical method. The nanostructured powder was investigated using electron spin resonance (ESR) measurements. The board ESR signal was attributed to the existence of unpaired electrons. The g-factor of pure Ppy decreased with increasing temperature while that of doped (Ppy) was found to increase linearly from 1.9504 to 1.9925 within a temperature range of 298K - 473 K. This could be due to polaron species formation. The current results demonstrated that polypyrrole (Ppy) could be considered as a good candidate for many applications especially in organic photovoltaic and gas sensors.

Photo-assisted recovery in ammonia sensor based on organic vertical diode

Abstract In this article, a photo-assisted gas sensor was provided to solve the slow recovering issue of the organic gas sensor. The proposed organic gas sensor can detect ammonia in parts-per-billion regime in 30 s and can be multiple uses, but the long recovery time about 5 min places difficulty to realize rapidly multiple-time sensing. Therefore, we used the photo-absorption ability in the organic semiconductor layer to generate the photocurrent and hence to compensate the current drop due to the ammonia absorption. The recovery time was significantly reduced to be only 1 min. The specific wavelength was also found to match the absorption spectrum in different organic sensing materials. As a comparison, the quartz crystal microbalance (QCM) was used to clarify the gas absorption and desorption mechanism. It is believed that this photo-assisted gas detection could be a general method to solve the slow recovery in organic-based gas sensors.

Ultrasensitive and robust organic gas sensors through dual hydrogen bonding

A dual hydrogen bonding-inspired polysquaramide sensor is able to detect ammonia as low as 10 ppt, the lowest among all ammonia sensors to date, and sense nitric oxide as low as 20 ppb. The sensor has outstanding robustness regarding selectivity, time-stability (>240 days), superior to other benchmarked organic electric sensors.

Detection of NO2 Down to One ppb Using Ion-in-Conjugation-Inspired Polymer.

Nitrogen dioxide (NO2 ) emission has severe impact on human health and the ecological environment and effective monitoring of NO2 requires the detection limit (limit of detection) of several parts-per-billion (ppb). All organic semiconductor-based NO2 sensors fail to reach such a level. In this work, using an ion-in-conjugation inspired-polymer (poly(3,3'-diaminobenzidine-squarine, noted as PDBS) as the sensory material, NO2 can be detected as low as 1 ppb, which is the lowest among all reported organic NO2 sensors. In addition, the sensor has high sensitivity, good reversibility, and long-time stability with a period longer than 120 d. Theoretical calculations reveal that PDBS offers unreacted amine and zwitterionic groups, which can offer both the H-bonding and ion-dipole interaction to NO2 . The moderate binding energies (≈0.6 eV) offer high sensitivity, selectivity as well as good reversibility. The results demonstrate that the ion-in-conjugation can be employed to greatly improve sensitivity and selectivity in organic gas sensors by inducing both H-bonding and ion-dipole attraction.

Copper Phthalocyanine and Pentacene Organic Thin-Film Transistor-Structured Ethanol Gas Sensors

In this article, copper phthalocyanine (CuPc) and pentacene double sensing membranes were used in organic thin-film transistor-structured ethanol gas sensors. To study the thickness effect of CuPc layer, various-thick CuPc layers were, respectively, deposited on 50-nm-thick pentacene layer. In addition to the passivation function of the CuPc layer, the performances of the resulting ethanol gas sensors were improved with an increase of the thickness of the CuPc layer until 5 nm and was then degraded with a further increase of its thickness. For the ethanol gas sensors with 50-nm-thick pentacene layer and 5-nm-thick CuPc layer operated at room temperature under 100 ppm ethanol gas concentration, the associated sensitivity was 70%.

Polymorphism in the self-assembled nanostructures of a tris(phthalocyaninato) europium derivative: Phase-dependent semiconducting and NO2 sensing behaviour

A new trifluoroethoxy-substituented tris(phthalocyaninato) europium complex Eu2[Pc(OCH2CF3)4]3 {[Pc(OCH2CF3)4] = 2(3),9(10),16(17),23(24)-tetrakis(2,2,2-trifluoroethoxy)phthalocyaninate}, is designed and synthesized. Introduction of trifluoroethoxy substituents at the phthalocyanine periphery within the triple-decker complex ensures their good solubility in conventional organic solvents. Significantly, depending on the fluorine-participated multiple hydrogen bonding and the π–π interactions in different solvent systems (i.e. n-hexane and water), the self-assembly of the Eu2[Pc(OCH2CF3)4]3 molecules results in well controlled morphologies (nano-bowling bowls, nanobelts and nano-particles) with totally different phases (χ, β and α phases). Furthermore, the sensitivity to NO2 at varied concentrations in the range of 20–1600 ppb, follows the order of nano-bowling bowls in χ phase (SA-1) » nanobelts in β phase (SA-2) > nano-particles in α phase (SA-3), revealing the effect of both phase structure and morphology on sensing performance of the Eu2[Pc(OCH2CF3)4]3. In particular, SA-1 displays unexpected high percent current change of 6.52% to NO2 gas as low as 20 ppb at room temperature, representing thus far the best result of the room-temperature NO2 gas sensing performance in terms of the lowest detection limit with largest response. The present work not only represents the first observation of χ, β and α polymorphs for the sandwich-type triple-decker tetrapyrrole-based rare earth complexes, but more importantly provides an efficient strategy to obtain high performance organic semiconductor-based gas sensors through molecular design and solvent-triggered phase transformations.

Organic Gas Sensor with an Improved Lifetime for Detecting Breath Ammonia in Hemodialysis Patients.

In this work, a TFB (poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-s-butylphenyl)diphenylamine)]) sensor with a cylindrical nanopore structure exhibits a high sensitivity to ammonia in ppb-regime. The lifetime and sensitivity of the TFB sensor were studied and compared to those of P3HT (poly(3-hexylthiophene)), NPB (N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine), and TAPC (4,4'-cyclohexylidenebis[N,N-bis(4-methylphenyl) benzenamine]) sensors with the same cylindrical nanopore structures. The TFB sensor outstands the others in sensitivity and lifetime and it shows a sensing response (current variation ratio) of 13% to 100 ppb ammonia after 64 days of storage in air. A repeated sensing periods testing and a long-term measurement have also been demonstrated for the test of robustness. The performance of the TFB sensor is stable in both tests, which reveals that the TFB sensor can be utilized in our targeting clinical trials. In the last part of this work, we study the change of ammonia concentration in the breath of hemodialysis (HD) patients before and after dialysis. An obvious drop of breath ammonia concentration can be observed after dialysis. The reduction of breath ammonia is also correlated with the reduction of blood urea nitrogen (BUN). A correlation coefficient of 0.82 is achieved. The result implies that TFB sensor may be used as a real-time and low cost breath ammonia sensor for the daily tracking of hemodialysis patients.