Sensitive and selective detection of target gases is the ultimate goal for commercialization of graphene gas sensors. Here, ultrasensitive n-channel graphene gas sensors were developed by using n-doped graphene with ethylene amines. The exposure of the n-doped graphene to oxidizing gases such as NO2 leads to a current decrease that depends strongly on the number of amine functional groups in various types of ethylene amines. Graphene doped with diethylenetriamine (DETA) exhibits the highest response, recovery, and long-term sensing stability to NO2, with an average detection limit of 0.83 parts per quadrillion (ppq, 10–15), due to the attractive electrostatic interaction between electron-rich graphene and electron-deficient NO2. Our first-principles calculation supported a preferential adsorption of NO2 on n-doped graphene. In addition, gas molecules on the n-channel graphene provide charged impurities, thereby intensifying the current decrease for an excellent response to oxidizing gases such as NO2 or SO2. On the contrary, absence of such a strong interaction between NH3 and DETA-doped graphene and combined effects of current increase by n-doping and mobility decrease by charged impurities result in a completely no response to NH3. Because the n-channel is easily induced by a top-molecular dopant, a flexible graphene sensor with outstanding NO2 detection capability was successfully fabricated on plastic without vertical stacks of gate-electrode and gate-dielectric. Our gate-free graphene gas sensors enabled by nondestructive molecular n-doping could be used for the selective detection of subppq-level NO2 in a gas mixture with reducing gases.
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 NO 2 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 NH 3 , CO 2 , and relative humidity condition; the source−drain current decreases upon NH 3 exposure, while response to CO 2 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.
Iontronic graphene tactile sensors (i‐GTS) composed of a top floating graphene electrode and an ionic liquid droplet pinned on a bottom graphene grid, which can dramatically enhance the performance of capacitive‐type tactile sensors, are presented. When mechanical stress is applied to the top floating electrode, the i‐GTS operates in one of the following three regimes: air–air, air–electric double layer (EDL) transition, or EDL–EDL. Once the top electrode contacts the ionic liquid in the i‐GTS, the spreading behavior of the ionic liquid causes a capacitance transition (from a few pF to over hundreds of pF). This is because EDLs are formed at the interfaces between the electrodes and the ionic liquid. In this case, the pressure sensitivity increases to ≈31.1 kPa−1 with a gentle touch. Under prolonged application of pressure, the capacitance increases gradually, mainly due to the contact line expansion of the ionic liquid bridge pinned on the graphene grid. The sensors exhibit outstanding properties (response and relaxation times below 80 ms, and stability over 300 cycles) while demonstrating ultimate signal‐to‐noise ratios in the array tests. The contact‐induced spreading behavior of the ionic liquid is the key for boosting the sensor performance.
Research on pressure sensors using elastic materials is actively underway to develop next-generation devices for applications in healthcare monitoring, haptic sensors, and electronic skin. Capacitive-type pressure sensors have several advantages among various types of pressure sensors because they can be manufactured in a small size, and their sensitivity is superior to other types of pressure sensors. Capacitive-type pressure sensors consist of two electrodes and an intermediate active layer, typically made up of elastomer such as polydimethylsiloxane (PDMS). Because a change in thickness of the elastomer upon applied pressure is a key operating mechanism, viscoelastic properties of elastomer should be considered when optimizing sensor performance. In this study, we examined viscoelastic properties of PDMS by controlling its cross-linking density. Effects of cross-linking density on performances of pressure sensors were also analyzed. Dynamic mechanical analysis and swelling method were employed to measure an accurate cross-linking density which significantly affected sensitivity, hysteresis, and recovery rate of pressure sensors. Performances of pressure sensors were also studied according to surface patterns (i.e., dome, square) of PDMS and surface treatment (i.e., UV/ozone, perfluordecyltrichlorosilane). Pressure sensing characteristics were optimized by incorporating dome structure and appropriate surface treatment on PDMS. Results of this study demonstrate that viscoelastic response of elastomer is vital for optimizing properties of capacitive-type pressure sensors.
The growth mechanism of soluble acene is highly dependent on the remaining residual solvent following solution processing. The relationship between the amount of residual solvent and the growth modes of a prototypical soluble acene, 6,13-bis(triisopropylsilylethynyl)pentacene (TIPSpentacene) are examined under spin casting TIPS-pentacene/insulating polymer blends. Changing spin time of the blend solution allows to control the amount of residual solvent, which significantly determines the growth modes of TIPS-pentacene vertically segregated onto the insulating polymer. In situ observation of crystal growth reveals that excess residual solvent in short spin time induces a convective flow in a drying droplet, thereby resulting in 1D growth of TIPS-pentacene crystals. On the other hand, optimal amount of residual solvent in a moderate spin time results in 2D growth of TIPS-pentacene crystals. The well-developed 2D spherulites allow for higher field-effect mobility than that of the 1D crystals because of the higher perfectness and coverage of TIPS-pentacene crystals. The use of other types of soluble acene and insulating polymer only changes the kinetics of crystallization, while the transition of growth mode from 1D to 2D is still observed. This general growth mechanism facilitates the understanding of crystallization behavior of soluble acene for the development of high-performance organic transistors.
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