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.