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Out‐of‐Plane Homojunction Enabled High Performance SnS2 Lateral Phototransistor

2D materials have been extensively applied in electronic and optoelectronic devices due to their unique and extraordinary electrical and optical properties. Among them, tin disulfide (SnS2) has attracted enormous research interests due to its low‐cost, earth‐abundant, environment friendly properties as well as superior performance in photodetectors. Here, an outstanding in‐plane SnS2 phototransistor with an out‐of‐plane built‐in electric field enabled by molybdenum oxide (MoO3) surface functionalization is demonstrated. The photocarriers are generated and effectively separated by the vertical electric field, leading to a high photoresponsivity of 2.3 × 103 A⋅W−1, photoconductive gain of >105, external quantum efficiency exceeding 8%. It also demonstrates a low noise power density, revealing an ultrasensitive photodetection with specific detectivity up to 3.2 × 1012 Jones. The formation of the out‐of‐plane built‐in electric field is validated by the output electrical transport measurement of SnS2 vertical transistor, and further corroborated by in situ ultraviolet photoelectron spectroscopy and X‐ray photoelectron spectroscopy characterizations. The findings promise surface functionalization as an effective approach to induce an internal electric field in 2D multilayer SnS2 towards its application in high performance photodetection.