A field-effect transistor (FET) with two-dimensional (2D) few-layer MoS₂ as a sensing-channel material was investigated for label-free electrical detection of the hybridization of deoxyribonucleic acid (DNA) molecules. The high-quality MoS₂-channel pattern was selectively formedthrough the chemical reaction of the Mo layer with H₂S gas. The MoS2 FET was very stable in an electrolyte and inert to pH changes due to the lack of oxygen-containing functionalities on the MoS2 surface. Hybridization of single-stranded target DNA molecules with single-stranded probe DNA molecules physically adsorbed on the MoS₂ channel resulted in a shift of the threshold voltage (V_th) in the negative direction and an increase in the drain current. The negative shift in V_th is attributed to electrostatic gating effects induced by the detachment of negatively charged probe DNA molecules from the channel surface after hybridization. A detection limit of 10 fM, high sensitivity of 17 mV/dec, and high dynamic range of 10⁶ were achieved. The results showed that a bio-FET with an ultrathin 2D MoS₂ channel can be used to detect very small concentrations of target DNA molecules specifically hybridized with the probe DNA molecules.

The graphene/SiO₂ system is a promising building block for next-generation electronic devices, integrating the high electromagnetic performance of graphene with the mature technology of Si-based electronic devices. It is well known that the electromagnetic performance of graphene/SiO₂ is dramatically reduced by structural defects, such as wrinkles and folding, which are suspected to result from water droplets. Therefore, understanding water diffusion between graphene and SiO₂ is required for controlling structural defects and thus improving the electromagnetic performance of this system. Although the behavior of water between graphene and atomically flat mica has been investigated, the characteristics and effects of diffused water between graphene and SiO₂ remain unidentified. We have investigated water diffusion between monolayer graphene and SiO₂ under high humidity conditions using atomic force microscopy. For a relative humidity of over 90%, water diffuses into graphene/SiO₂ and forms an ice-like structure up to two layers thick. Liquid-like water can further diffuse in, stacking over the ice-like layer and evaporating relatively easily in the air causing graphene to wrinkle and fold. By similarly investigating water diffusion between graphene and mica, we argue that water-induced wrinkle formation depends on the hydrophilicity and roughness of the substrate.