Alkylnitrile Ligands Enable Stable Phase and Electronic Structures of Monodisperse Ag ₂ Se Nanocrystals

Alkylnitriles, N≡CR with R as a long alkyl chain with 11–17 carbons, are introduced for synthesizing and stabilizing Ag2Se nanocrystals (NCs). Their intermediate bonding strength─stronger than alkylamines yet weaker than thiols─enables synthesis of monodisperse monoclinic Ag2Se NCs in a large size range (3–20 nm) at 120–140 °C. AgNO3 dissolved in N≡CR and Se-trioctylphosphine dissolved in NH2Ol are respectively introduced as the silver and selenium precursors, which effectively avoid formation of the Ag0 phase. Given the large excess of alkylamines in the synthesis, the native surface ligands on the as-synthesized NCs are dominated by dynamic and weak NH2Ol ligands, which can be readily replaced by relatively strong and stable N≡CR ligands at ambient temperatures. Unlike NH2Ol-coated Ag2Se NCs, N≡CR-coated ones have high phase (monoclinic) and electronic (either doped or undoped) stabilities, offering two series of Ag2Se NCs for either the short-wavelength or middle infrared window. The findings here indicate that specifically designed surface ligands not only ensure colloidal stability during synthesis and processing but also confer the desired phase and electronic stability to NCs, which may otherwise remain metastable when inappropriate ligands are used.