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

Here, nitrogen-doped molybdenum diselenide (MoSe 2 ) films with nanoscaled thicknesses were fabricated by ionizing N 2 carrier gas in a plasma-assisted chemical vapor reaction furnace under different mixtures of N 2 and H 2 ratios and the amount of nitrogen doping was controlled by the relative flow rate of N 2 . Nitrogen radicals diffused deeply in MoSe 2 films once the reaction was activated in a N 2 -rich environment, resulting in a remarkable NH 3 response of 430% at 25 ppm, 6.72 times the response against NO 2 . In contrast, nitrogen doping is limited at the surface under a H 2 -rich environment during the synthesis, resulting in a weaker response against NH 3, which is only 37.5% of that against NO 2 as prior researches. The enhanced NH 3 response instead of NO 2 in N-doped MoSe 2 films with a bulk-like nitrogen doping can be attributed to the valence band being closer to the oxidation potential of NH 3, which is favorable to donate electrons by NH 3 molecules, along with a larger electron affinity than the reduction energy of NO 2, 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 MoSe 2 film is close to 5 nm. The nanoscaled depths of nitrogen doping in MoSe 2 films can easily determine the sensing target through atmospheric modification during the growth process, broadening the application potential of 2D-material-based gas sensors.