Nucleation-Limited Phase Formation in the Nb–Se Binary System

Predicting the appropriate conditions for the synthesis of inorganic compounds anticipated by theory to be stable is not possible because not enough is known about the interplay between experimental parameters and solid-state reaction pathways. To systematically probe how experimental parameters impact the early stages of a solid–solid reaction, the reaction between ultrathin Nb and Se layers deposited onto a nominally ambient temperature substrate was examined as a function of their thickness, the ratio of their thicknesses, and the annealing temperature and time. For elemental layer thicknesses less than a monolayer, the elements react during the deposition to form an amorphous intermediate, except for films with compositions near a 1 to 2 ratio of Nb to Se, which nucleate and grow of NbSe 2 during the deposition. Surprisingly, annealing homogeneous amorphous intermediates with compositions close to the stoichiometries of the other 5 thermodynamically stable niobium selenides above the temperature where diffusion occurs did not result in their formation in the temperature ranges explored. NbSe 2 formed from an amorphous intermediate with a 3 to 4 ratio of Nb to Se rather than Nb 3 Se 4 . NbSe 2 also nucleated when precursors containing concentration gradients containing interfacial regions close to a 1:2 ratio of Nb to Se were annealed. Elementally modulated precursors with more Nb- or Se-rich interfaces interdiffused, forming amorphous intermediates. Nucleation rather than diffusion is the rate-limiting step in the formation of a crystalline phase in this system. NbSe 2 is the easiest compound to nucleate in this system because it has the simplest crystal structure with the smallest unit cell of all the niobium selenides, and its structure has a low-energy van der Waals surface without dangling bonds, which reduces the surface energy of forming embryos. The data presented suggests that the inability to synthesize many of the compounds computationally predicted to be stable results from the difficulty of nucleating their often-complex predicted structures relative to other compounds with simple structures.