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.