Universal effect of ammonia pressure on synthesis of colloidal metal nitrides in molten salts
Metal nitrides represent a large class of materials with extensive applications in optoelectronics, energy, and healthcare technologies. For example, GaN and related nitride semiconductors are key materials for solid-state lighting and high-power electronics, TiN and other early transition metal nitrides (TMNs) are widely used in wear-resistant alloys, tool coatings, catalysts and medical implants. Strong metal-nitrogen bonds grant nitrides structural rigidity as well as chemical and thermal stability. However, the covalency of metal-nitrogen bonds necessitates high temperatures to synthesize crystalline metal nitrides. Common synthetic routes include high-temperature solid-state nitridation, crystal growth in supercritical ammonia, molecular-beam epitaxy (MBE), reactive sputtering, and chemical vapor deposition (CVD). The solution synthesis of colloidal nitride nanocrystals (NCs) is rare and particularly challenging because commonly used solvents and surfactants decompose at temperatures far below those required for crystallization of most metal nitrides. Here we report a general approach to solution synthesis of colloidal metal nitride NCs by reacting metal halides and ammonia dissolved in molten inorganic salts at elevated pressures. Successful syntheses of colloidal TiN, VN, GaN, NbN, Mo2N, Ta3N5, W2N, as well as ternary Ti1-xVxN NCs, are demonstrated. These NCs expand the scope of solution-processable technologically important materials.