Synthesizing Amorphous Precursors Through Control of Local Composition

Synthesizing compounds predicted to have exceptional properties that are close to or below the ground state convex hull has proven to be very challenging, as avoiding the formation of more thermodynamically or kinetically stable mixtures of known compounds is often required. A homogenous amorphous phase has been suggested as a very general reaction intermediate. However, the preparation of amorphous phases of controlled composition is also very challenging. We postulated that amorphous intermediates with controlled composition can be made by avoiding the formation of regions with compositions close to that of the known compounds. Specifically, we demonstrated that we could avoid the formation of PbSe and MoSe2 by sequentially depositing nonstoichiometric ultrathin sub-monolayer thickness layers on a nominally room temperature substrate. The substrate temperature needs to be low enough to minimize surface diffusion, preventing agglomeration of elements and resultant concentration gradients. The amount of diffusion required to form nucleation embryos can be controlled by changing the difference between the composition of the film and the stoichiometry of the compound in question. Large enough differences should result in amorphous intermediates in most systems. The presence of more than two elements will further suppress the nucleation of binary compounds, making this approach particularly useful to prepare amorphous precursors for the synthesis of metastable ternary and quaternary compounds.