CONTROLLING SOLID-STATE REACTION-MECHANISMS USING DIFFUSION LENGTH IN ULTRATHIN-FILM SUPERLATTICE COMPOSITES

The synthetic parameters used by solid-state chemists have essentially been limited to temperature, pressure, composition, and time. Diffusion length can also be used as a synthetic parameter to alter the pathway of a solid-state reaction. Modulated, binary, ultrathin films of similar stoichiometry have been made whose repeat distance varies from 18 to 128 angstrom. For samples whose modulation length is 38 angstrom or larger, the films evolve upon heating as thin-film diffusion couples, initially growing an amorphous layer and then crystallizing a stable product, MoSe2, at the interface between individual elemental layers. For samples of 27 angstrom or smaller, the films evolve to a homogeneous, amorphous alloy before crystallizing MoSe2. This implies a critical layer thickness, below which it is possible to form a homogeneous, amorphous alloy. There is a difference in nucleation temperature of several hundred degrees between the two length scales reflecting the importance of interfaces in aiding nucleation. The synthetic importance of these results-the ability to control synthetic variables to reach desired reaction intermediates in solid-state reactions-is highlighted.