From Monolayer MoS2 FETs to MoS2−MoO2 Nanoheterostructures: Enhanced Self-Powered Visible-Light and NO2 Sensing

Self-powered multifunctional sensors require internal fields that can separate photocarriers and transduce surface adsorption events without continuous external bias. Here, we show that chemical vapor deposition (CVD) growth conditions can tune the morphology of nanostructured molybdenum disulfide (MoS2), forming sulfur-deficient quadrilateral flakes with coexisting molybdenum-sulfur (Mo−S) and molybdenum-oxygen (Mo−O) chemical environments, consistent with MoS2−MoO2 (MSMO) phase coexistence. Comprehensive structural, optical, and chemical characterization confirms the distinct growth regimes of monolayer, few-layer, dendritic, and quadrilateral flake and verifies the morphology and composition of the resulting phases. Devices fabricated from pristine MoS2, gate-modulated MoS2, and the MSMO quadrilateral-flake film were compared under visible illumination and NO2 exposure. The monolayer MoS2 FET exhibits n-type gate modulation with an on/off ratio of ∼ 4 − 7, field-effect mobility of ∼0.2−0.7 cm2V−1s−1, threshold voltage of ∼1.5 − 3.1 V, and subthreshold swing of ∼5 − 8 V.dec−1, indicating conventional but weakly gate-controlled CVD MoS2 transistor behavior. The heterostructure device outperforms the monolayer MoS2 FET device, delivering responsivity of 100 A/W, detectivity of 1012 Jones (at 630 nm, 30 mW optical power), response/recovery times of 200 ms, and operation over the visible spectrum at zero bias. The same junction architecture also enables room-temperature NO2 sensing with a sub-ppm (∼0.1 PPM) limit of detection (LOD) at a low bias of 0.5 V, and this decreases further with illumination. The device exhibits a zero-bias sensing response with a few ppm LOD (∼5 ppm), whereas pristine MoS2 control devices require a higher applied bias and exhibit weaker modulation. Thus, internal interface field enables a faster and measurable self-powered photodetection and NO2 sensing than external electrostatic gating.