Crystal-Defect-Dependent Gas-Sensing Mechanism of the Single ZnO Nanowire Sensors

Though the chemical origin of a metal oxide gas sensor is widely accepted to be the surface reaction of detectants with ionsorbed oxygen, how the sensing material transduces the chemical reaction into an electrical signal (i.e., resistance change) is still not well-recognized. Herein, the single ZnO NW is used as a model to investigate the relationship between the microstructure and sensing performance. It is found that the acetone responses arrive at the maximum at the NW diameter (D) of similar to 110 nm at the D range of 80 to 400 nm, which is temperature independent in the temperature region of 200 degrees C-375 degrees C. The electrical properties of the single NW field effect transistors illustrate that the electron mobility decreases but electron concentration increases with the D ranging from similar to 60 nm to similar to 150 nm, inferring the good crystal quality of thinner ZnO NWs and the abundant crystal defects in thicker NWs. Subsequently, the surface charge layer (L) is calculated to be a constant of 43.6 +/- 3.7 nm at this D range, which cannot be explained by the conventional D-L model in which the gas-sensing maximum appears when D approximates 2L. Furthermore, the crystal defects in the single ZnO NW are probed by employing the microphotoluminescence technique. The mechanism is proposed to be the compromise of the two kinds of crystal defects in ZnO (i.e., more donors and fewer acceptors favor the gas-sensing performance), which is again verified by the gas sensors based on the NW contacts.