Rapid and Wide-Range Detection of NOx Gas by N-Hyperdoped Silicon with the Assistance of a Photovoltaic Self-Powered Sensing Mode.

Wide-dynamic-range NOx sensors are vital for the environment and health purposes, but few sensors could achieve wide-range detection with ultralow and ultrahigh concentrations at the same time. In this article, the microstructured and nitrogen-hyperdoped silicon (N-Si) for NOx gas sensing is investigated systematically. Working by the change of surface conductivity, the sensor is ultrasensitive to low concentrations of NOx down to 11 ppb and shows a rapid response/recovery time of 22/33 s for 80 ppb. When the NOx concentration increases and exceeds a threshold value (10-50 ppm), an n-p conduction-type transition is observed due to the inversion of the conduction type of major carriers, which limits the dynamic range of the sensor at high concentration. However, when the sensor works in a photovoltaic self-powered mode under the asymmetric light illumination, the limitation can be successfully overcome. Therefore, with the combination of the two working principles, a wide dynamic range stretching over 6 orders of magnitude (∼0.011-4000 ppm) can be achieved.

"Infinite Sensitivity" of Black Silicon Ammonia Sensor Achieved by Optical and Electric Dual Drives.

The microstructured and hyperdoped silicon as a superior photoelectric and photovoltaic material is first studied as a gas-sensing material. The material is prepared by femtosecond-laser irradiation on selenium-coated silicon and then fabricated as a conductive gas sensor, targeting ammonia. At room temperature, the sensitivity, response time, repeatability, distinguishability, selectivity, and natural aging effect of the sensor have been systematically studied. Results show that such black silicon has good potential for application as an ammonia-sensing material. On the basis of its unique optoelectronic properties, an additional optical drive is proposed for the formation of an optical and electric dual-driven sensor, which is achieved by asymmetric light illumination between the two electrode regions. In a certain range of applied voltage, the sensitivity is enhanced dramatically and even tends to be infinite. For the aged device with degraded sensitivity, a two-order increment is obtained for 500 ppm of NH3 under the extra optical drive. A mechanism based on Dember effect is proposed for explaining such a phenomenon.