Optical Fiber HMS Ammonia Sensor with Cellular-like\nN‑CQDs/Mg<sub><i>x</i></sub>Zn<sub>1–<i>x</i></sub>O at Room Temperature

This\npaper presents an optical fiber hollow malposition structure\n(HMS) elaborated with cellular-like N-CQDs/Mg<sub><i>x</i></sub>Zn<sub>1–<i>x</i></sub>O for detecting ammonia\n(NH<sub>3</sub>) at room temperature. The gas detection was realized\nby combining the HMS to excite the surface evanescent field on the\nouter surface of the hollow core fiber (HCF) and the surface-coated\ncellular-like N-CQDs/Mg<sub><i>x</i></sub>Zn<sub>1–<i>x</i></sub>O material for improving sensitivity. Experimental\nresults demonstrate that the sensor coated with N-CQDs/Mg<sub>0.2</sub>Zn<sub>0.8</sub>O exhibits the highest sensitivity, reaching 8.9\npm/ppm, which is 11.6 times higher than that with Mg<sub>0.2</sub>Zn<sub>0.8</sub>O. Additionally, the NH<sub>3</sub> sensor can determine\na minimum concentration of 1 ppm and has response and recovery times\nof 6.2 and 7.1 s, respectively. Density functional theory (DFT) calculations\nconfirm that N-CQDs reduces the bandgap of the composite material\nand increase the carrier concentration on its surface, thereby enhancing\ngas adsorption capacity. The intrinsic characteristics of the proposed\nsensor including small size, high sensitivity, and good stability\nreveal its promising prospects in detecting low concentrations of\nNH<sub>3</sub>.