NO2-Affinitive Conjugated Polymer for Selective Sub-Parts-Per-Billion NO2 Detection in a Field-Effect Transistor Sensor.

Conjugated polymers (CPs) have provided versatile semiconducting implements for the development of soft electronic devices. When three CPs with the same conjugated framework but different side chains were adopted in the field-effect transistor (FET) sensor for NO2 detection, the response to NO2 showed an opposite tendency to the charge carrier mobility of each CP. Morphological and structural characterizations revealed that the flexible glycol side chain enhances NO2 affinity as well as prevents the formation of lamellar stacking of the CP chains, thereby providing routes for the facile diffusion of NO2. Additionally, theoretical calculations for CP-NO2 complex formation at the molecular level support the relatively low energy barrier for inter-chain transition of NO2 between the glycol-based conjugated frameworks, which implies the spontaneous internal diffusion of NO2 to the semiconductor-dielectric interface in the FET-based sensor. As a result, the CP with a NO2-affinitive morphology exhibited an exceptional sensitivity of 13.8%/ppb upon NO2 (100 ppb) exposure for 50 s and provided excellent selectivity to the FET-based sensor toward other environmentally abundant harmful gases, such as SO2, CO2, and NH3. In particular, the theoretic limit of detection reached down to 0.24 ppb, which is the lowest value ever reported for organic FET-based NO2 gas sensors.

Enhanced Gas Sensing Properties of Graphene Transistor by Reduced Doping with Hydrophobic Polymer Brush as a Surface Modification Layer.

Surface modification layer of a silicon substrate has been used to enhance the performance of graphene field-effect transistors (FETs). In this report, ultrathin and chemically robust polymer brush was used as a surface modification to enhance the gas sensing properties of graphene FETs. The insertion of the polymer brush decreased substrate-induced doping of graphene. This leads to a huge increase in field-effect mobility as well as a minimum shift of the Dirac point voltage. The use of the polymer brush enables fast detection of target gas molecules because graphene sensing modality can be maximized at the undoped state of graphene. The increase of source-drain current, as well as the abrupt decrease of electron mobility upon NO2 exposure, was utilized for the instantaneous detection, and a limit of detection of 4.8 ppb was achieved with graphene FETs on PS brush. We also showed excellent cross-sensitivity of graphene gas sensors to NH3, CO2, and relative humidity condition; the source-drain current decreases upon NH3 exposure, while response to CO2 or relative humidity condition is extremely low. Our results prove that reducing the substrate-induced doping of graphene with a polymer brush is a direct method for boosting the gas sensing properties of graphene FETs.

Effect of Crystallization Modes in TIPS-pentacene/Insulating Polymer Blends on the Gas Sensing Properties of Organic Field-Effect Transistors.

Blending organic semiconductors with insulating polymers has been known to be an effective way to overcome the disadvantages of single-component organic semiconductors for high-performance organic field-effect transistors (OFETs). We show that when a solution processable organic semiconductor (6,13-bis(triisopropylsilylethynyl)pentacene, TIPS-pentacene) is blended with an insulating polymer (PS), morphological and structural characteristics of the blend films could be significantly influenced by the processing conditions like the spin coating time. Although vertical phase-separated structures (TIPS-pentacene-top/PS-bottom) were formed on the substrate regardless of the spin coating time, the spin time governed the growth mode of the TIPS-pentacene molecules that phase-separated and crystallized on the insulating polymer. Excess residual solvent in samples spun for a short duration induces a convective flow in the drying droplet, thereby leading to one-dimensional (1D) growth mode of TIPS-pentacene crystals. In contrast, after an appropriate spin-coating time, an optimum amount of the residual solvent in the film led to two-dimensional (2D) growth mode of TIPS-pentacene crystals. The 2D spherulites of TIPS-pentacene are extremely advantageous for improving the field-effect mobility of FETs compared to needle-like 1D structures, because of the high surface coverage of crystals with a unique continuous film structure. In addition, the porous structure observed in the 2D crystalline film allows gas molecules to easily penetrate into the channel region, thereby improving the gas sensing properties.