Spatially deterministic nucleation of 2D semiconductors by etching flux

Nucleation control is fundamental to semiconductor deposition, governing when, where and how crystalline materials form. Conventional strategies can regulate nucleation density and, through area-selective growth, confine deposition to designated regions^1-13. However, they generally do not determine where an individual nucleus forms within the growth regions, leaving even single-nucleation events spatially stochastic and limiting deterministic construction of crystalline materials. Here we report etching-flux-mediated single-centred nucleation of two-dimensional (2D) semiconductors to deterministically localize a single nucleation event. The etching flux released from the barrier suppresses nuclei near the pattern boundary, leaving a single surviving nucleus at the pattern centre, and systematic experiments elucidate the mechanism and establish nucleation design rules for zero, single and multiple nucleation regimes. Etching-flux-mediated single-centred nucleation enables single-crystal molybdenum disulfide growth at the 10-μm scale, field-effect mobilities of up to 117 cm² V^-1 s^-1 and large-area uniformity with process compatibility. This in-plane chemical-flux strategy realizes spatially programmed growth, demonstrating nucleation-growth decoupling for line-shaped single crystals, multiple transistors integrated within a large common crystal, and aligned 2D lateral heterostructures for self-aligned contacts. These capabilities open a path towards advanced 2D electronic integration and expand semiconductor deposition from controlling where materials grow to controlling where and how crystals can begin to form.