Decoupling amine evolution from sulfur delivery in high-temperature metal sulfide nanocrystal synthesis

Thiourea and its N,N'-substituted derivatives are widely employed as sulfur precursors in the colloidal synthesis of metal sulfide nanomaterials. Here, we demonstrate that these precursors exhibit an intrinsic limitation at elevated temperatures: prior to H₂S evolution, they release reactive amines that perturb nucleation and growth. In situ mass spectrometry reveals that above 160 °C, amine evolution precedes sulfur delivery and coincides with dissolution or suppression of crystalline SrS formation in a model reaction. Mechanistic analysis further shows that all investigated thioureas converge to N,N'-dioleylthiourea (DOlTU) as a common intermediate before H₂S release. By directly employing DOlTU as a sulfur precursor, we decouple amine evolution from sulfur supply and establish a continuous, temperature-dependent nanocrystal growth profile up to 250 °C. These findings identify precursor decomposition chemistry as a decisive parameter in high-temperature metal sulfide synthesis and provide a rational strategy for designing sulfur sources that enable predictable and reproducible nanocrystal formation for optoelectronic materials.