To determine the sensing mechanism of n-type semiconductors, here, using ZnO as a model, we investigated the effects of the surface structure and electrode location on the sensing properties of polar orientated films. The top surface of the 001 orientated ZnO film shows a significantly higher response toward n-butylamine, acetone, and ethanol than the bottom surface. The sensitivity is boosted with greater exposure of the Zn-terminated (001) polar plane and higher 3-coordinated Zn (Zn3c) atomic density. Zn3c is demonstrated to act as a sensing active center, with an atomic scale gas sensing mechanism presented. Furthermore, we discovered that no electron depletion layer (EDL) actually exists on the sensing active surface of ZnO, which challenges the widely accepted EDL sensing model for n-type semiconductors. This polar structure sensing model not only contributes to the understanding of crystal facet-dependent sensing properties but also provides an atomic-scale structural model for design of high-performance film sensors.
Li et al. (Fri,) studied this question.