Thickness-Dependent Room-Temperature Optoelectronic Gas Sensing Performances of 2D Nonlayered Indium Oxide Crystals from a Liquid Metal Printing Process.

Due to excellent gas sensing performances, such as high responsivity, good selectivity, and long-term stability, two-dimensional (2D) nonlayered metal oxide semiconductors have attracted wide attention. However, their thickness-dependent gas sensing behaviors are rarely investigated, which is critical in the development of practical 2D sensors. In this work, 2D InO crystals with a range of thicknesses are realized by extracting the self-limited oxide layer from the liquid indium droplets in a controlled environment. A strong thickness-dependent optoelectronic NO sensing behavior at room temperature is observed. While full reversibility and excellent selectivity toward NO are shown despite the thicknesses of 2D InO, the 1.9 nm thick InO exhibits a maximum response amplitude (Δ/ = 1300) for 10 ppm of NO at room temperature with 365 nm light irradiation, which is about 18, 58, and 810 times larger than those of its 3.1 nm thick, 4.5 nm thick, and 6.2 nm thick counterparts, respectively. The shortest response and recovery times (i.e., 40 s/48 s) are demonstrated for the 1.88 nm thick InO as well. We correlate such a phenomenon with the change in the InO band structure, which is influenced by the thickness of 2D crystals. This work provides in-depth knowledge of the thickness-dependent gas-sensing performances of emerging 2D nonlayered metal oxide crystals, as well as the opportunities to develop next-generation high-performing room-temperature gas sensors.