Abrupt Thermal Shock of (NH ₄ ) ₂ Mo ₃ S ₁₃ Leads to Ultrafast Synthesis of Porous Ensembles of MoS ₂ Nanocrystals for High Gain Photodetectors

Ultrafast synthesis of high-quality transition-metal dichalcogenide nanocrystals, such as molybdenum disulfide (MoS ₂ ), is technologically relevant for large-scale production of electronic and optoelectronic devices. Here, we report a rapid solid-state synthesis route for MoS ₂ using the chemically homogeneous molecular precursor, (NH ₄ ) ₂ Mo ₃ S ₁₃ ·H ₂ O, resulting in nanoparticles with estimated size down to 25 nm only in 10 s at 1000 °C. Despite the extreme nonequilibrium conditions, the resulting porous MoS ₂ nanoparticles remain aggregated to preserve the form of the original rod shape bulk morphology of the molecular precursor. This ultrafast synthesis proceeds through the rapid decomposition of the precursor and rearrangement of Mo and S atoms coupled with simultaneous efficient release of massive gaseous species, to create nanoscale porosity in the resulting isomorphic pseudocrystals, which are composed of the MoS ₂ nanoparticles. Despite the very rapid escape of massive amounts of NH ₃ , H ₂ O, H ₂ S, and S gases from the (NH ₄ ) ₂ Mo ₃ S ₁₃ ·H ₂ O mm sized crystals, they retain their original shape as they convert to MoS ₂ rather than undergo explosive destruction from the rapid escape process of the gases. The obtained pseudocrystals are made of aggregated MoS ₂ nanocrystals exhibit a Brunauer-Emmett-Teller surface area of ∼35 m ² /g with an adsorption average pore width of ∼160 Å. The nanoporous MoS ₂ crystals are solution processable by dispersing in ethanol and water and can be cast into large-area uniform composite films. Photodetectors fabricated from these films show more than 2 orders of magnitude higher conductivity (∼6.25 × 10 ^-6 S/cm) and photoconductive gain (20 mA/W) than previous reports of MoS ₂ composite films. The optoelectronic properties of this nanoporous MoS ₂ imply that the shallow defects that originate from the ultrafast synthesis act as sensitizing centers that increase the photocurrent gain via two-level recombination kinetics.