Atomically thin 2D materials are good templates to grow organic semiconductor thin films with desirable features. However, the 2D materials typically exhibit surface roughness and spatial charge inhomogeneity due to nonuniform doping, which can affect the uniform assembly of organic thin films on the 2D materials. A hybrid template is presented for preparation of highly crystalline small‐molecule organic semiconductor thin film that is fabricated by transferring graphene onto a highly ordered self‐assembled monolayer. This hybrid graphene template has low surface roughness and spatially uniform doping, and it yields highly crystalline fullerene thin films with grain sizes >300 nm, which is the largest reported grain size for C60 thin films on 2D materials. A graphene/fullerene/pentacene phototransistor fabricated directly on the hybrid template has five times higher photoresponsivity than a phototransistor fabricated on a conventional graphene template supported by a SiO2 wafer.

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.

Numerous wearable devices were developed to measure bioelectric signals for continuous healthcare monitoring. The electrode, which interconnects electronics and the human body, significantly affects the signal quality. Although Ag/AgCl electrodes have been commonly used, noble-metal electrodes are more promising in terms of long-term reusability and flexibility. However, the signal-to-noise ratio (SNR) of noble metals is still insufficient for highly accurate biosignal acquisition. In this study, we propose an approach to enhance the electrical characteristics of a noble-metal skin electrode by surface modification using gold nanoparticles. The process parameters for nanoparticle deposition were optimized to maximize the surface area, thereby significantly improving the SNR of the electrode. The SNR value was increased by 51% in electrocardiogram and by 63% in electromyogram (EMG). We also propose an approach to quantify the motion artifact by spectral analysis, and the high flexibility of our electrode reduced the motion noise by 95% compared to the conventional Ag/AgCl electrode. The enhanced electrode interface paves the way for analyzing complex biosignals such as EMG and electroencephalogram in wearable applications.

Electrical doping is essentially required for high‐performance organic thermoelectric (TE) materials; however, the doping efficiency ηd has not been extensively investigated in highly doped organic semiconductors (OSCs). Here, it is demonstrated that the distribution of dopant molecules in a specific position in highly doped OSCs affects the ηd, which is critically related to the Seebeck coefficient S and the electrical conductivity σ. Poly(2,5‐bis(3‐hexadecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) (PBTTT) films are p‐doped with 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) by either solution‐sequential (SSq) doping or vapor doping. SSq doping deposited F4TCNQ only in the amorphous domains of PBTTT films, whereas vapor doping deposited it in both the amorphous and crystalline domains. F4TCNQ molecules in the crystalline domains exhibited a high ηd and led to a rapid increase of the power factor with increasing σ: S2σ ∝ σ0.76. These results provide guidance for the efficient doping of highly doped OSCs and emphasize the importance of doping efficiency in obtaining high‐performance organic TE materials.