Extremely Suppressed Energetic Disorder in a Chemically Doped Conjugated Polymer.

Chemical doping can be used to tune the optoelectronic properties of conjugated polymers (CPs), extending their applications as conducting materials. Unfortunately, chemically doped CP films containing excess dopants exhibit an increase in energetic disorder upon structural alteration, and Coulomb interactions between charge carriers and dopants also affect such disorder. The increase in energetic disorder leads to a broadening of the density of states, which consequently impedes efficient charge transport in chemically doped CPs. However, the molecular origins that are inherently resistant to such incidental increase of energetic disorder in chemically doped CPs have not been sufficiently explored. Here, it is discovered that energetic disorder in chemically doped CPs can be suppressed to a level close to the theoretical limit. Indacenodithiophene-co-benzothiadiazole (IDTBT) doped with triethyloxonium hexachloroantimonate (OA) exhibits disorder-free charge-transport characteristics and band-like transport behavior with astonishing carrier mobility as a result of reinforced 1D intramolecular transport. Molecular structure of IDTBT provides a capability to lower the energetic disorder that generally arises from the inclusion of heterogeneous dopants. The results suggest the possibilities of implementing disorder-free CPs that exhibit excellent charge transport characteristics in the chemically doped state and satisfy a prerequisite for their availability in the industry.

Ultrasensitive N-Channel Graphene Gas Sensors by Nondestructive Molecular Doping.

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.

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.