Ammonia pressure controls colloidal metal nitride synthesis in molten salts

Abstract

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 electronics1,2. TiN and other early transition metal nitrides (TMNs) are widely used in wear-resistant alloys, tool coatings, catalysts and medical implants3. Strong metal–nitrogen bonds grant nitrides structural rigidity as well as chemical and thermal stability4. However, the covalency of metal–nitrogen bonds necessitates high temperatures to synthesize crystalline metal nitrides. Common synthetic routes include high-temperature solid-state nitridation5, crystal growth in supercritical ammonia6, molecular-beam epitaxy (MBE)7, reactive sputtering8,9 and chemical vapour deposition1,10–12. The solution synthesis of colloidal nanocrystals (NCs) has been demonstrated for late TMNs with relatively weak chemical bonds13–17, whereas the synthesis of early TMN NCs is challenging because it requires temperatures far above the stability range of commonly used solvents. Here we report a general approach to solution synthesis of refractory 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, TaN, W2N and ternary Ti1−xVxN NCs are demonstrated. These NCs expand the scope of solution-processable technologically important materials. A molten salt, high-pressure synthesis method enables solution-based production of diverse refractory metal nitride nanocrystals, expanding access to technologically important nitride materials for advanced applications.