(color online) (a) Cross-sectional view of the single-layer MoS₂-based photodetector under the focused laser beam (top). Photoresponsivity of the MoS₂ phototransistor (bottom). Spatial map of the photocurrent of the device (inset).^[59] (b) Optical image of a back-gated monolayer MoS₂ photodetector on SiO₂/silicon substrate (top left). The energy band diagram (bottom) of the metal–MoS₂ junction (top) after photoexcitation with an optical pulse (bottom left). The photoresponse time characterization (right). The plots show two distinct timescales: a fast timescale of ∼ 4.3 ps and a slow timescale of ∼ 105 ps.^[60] (c) Schematic and measurement circuit of the few-layer ReS₂ phototransistors.^[64] (d) Photoresponsivity as a function of the incident power of few-layer ReS₂ phototransistors (left). Weak signal detection in a five-layer ReS₂ phototransistor using lighter and limited fluorescent lighting as the weak light sources (right).^[64]