Ammonia pressure controls colloidal metal nitride synthesis 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^1,2. 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 sputtering^8,9 and chemical vapour deposition^1,10-12. The solution synthesis of colloidal nanocrystals (NCs) has been demonstrated for late TMNs with relatively weak chemical bonds^13-17, 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 Ti_1-xV_xN NCs are demonstrated. These NCs expand the scope of solution-processable technologically important materials.