Size effects of tin oxide quantum dot gas sensors: from partial depletion to volume depletion

The grain size effect is one of the fundamental characteristics of semiconductor gas sensors. However, it has not been fully understood due to the absence of studies on the volume-depleted grains. In this work, the gas-sensitive SnO2 quantum dots (QDs) from partial depletion to volume depletion are prepared to discuss the size effects. A facile aqueous-based method is used to prepare the size-controllable SnO2 QDs of 2.0–12.6 nm. The resistance shows a monotonically negative size effect while the response reaches the optimization when the grain radius is comparable to the depletion layer width. It is suggested that the design of highly sensitive gas sensors should consider the equal importance of the control of grain size and depletion layer width. The computational results illustrate size-dependent energy level of donors and number of quasi-free electrons, which are responsible for the negative size effect of resistivity in the volume-depleted SnO2 crystallites. This work provides a comprehensive understanding of grain size effects from partial depletion to volume depletion in semiconductor gas sensors.