Controllable Vertical Nitrogen Doping in Nanoscaled Molybdenum Diselenide Films for Selective Sensing of NH₃ and NO₂ Gases

Here, nitrogen-doped molybdenum diselenide (MoSe2) films with nanoscaled thicknesses were fabricated by ionizing N2 carrier gas in a plasma-assisted chemical vapor reaction furnace under different mixtures of N2 and H2 ratios and the amount of nitrogen doping was controlled by the relative flow rate of N2. Nitrogen radicals diffused deeply in MoSe2 films once the reaction was activated in a N2-rich environment, resulting in a remarkable NH3 response of 430% at 25 ppm, 6.72 times the response against NO2. In contrast, nitrogen doping is limited at the surface under a H2-rich environment during the synthesis, resulting in a weaker response against NH3, which is only 37.5% of that against NO2 as prior researches. The enhanced NH3 response instead of NO2 in N-doped MoSe2 films with a bulk-like nitrogen doping can be attributed to the valence band being closer to the oxidation potential of NH3, which is favorable to donate electrons by NH3 molecules, along with a larger electron affinity than the reduction energy of NO2, hindering electron extraction toward gas molecules. The doping depths of both surface-limited doping and bulk-like doping can be precisely controlled at the nanoscale since the whole thickness of a MoSe2 film is close to 5 nm. The nanoscaled depths of nitrogen doping in MoSe2 films can easily determine the sensing target through atmospheric modification during the growth process, broadening the application potential of 2D-material-based gas sensors.