Facile Ligand Exchange of Ionic Ligand-Capped Amphiphilic Ag2S Nanocrystals for High Conductive Thin Films

A surface ligand modification of colloidal nanocrystals (NCs) is one of the crucial issues for their practical applications because of the highly insulating nature of native long-chain ligands. Herein, we present straightforward methods for phase transfer and ligand exchange of amphiphilic Ag2S NCs and the fabrication of highly conductive films. S-terminated Ag2S (S–Ag2S) NCs are capped with ionic octylammonium (OctAH+) ligands to compensate for surface anionic charge, S2–, of the NC core. An injection of polar solvent, formamide (FA), into S–Ag2S NCs dispersed in toluene leads to an additional envelopment of the charged S–Ag2S NC core by FA due to electrostatic stabilization, which allows its amphiphilic nature and results in a rapid and effective phase transfer without any ligand addition. Because the solvation by FA involves a dissociation equilibrium of the ionic OctAH+ ligands, controlling a concentration of OctAH+ enables this phase transfer to show reversibility. This underlying chemistry allows S–Ag2S NCs in FA to exhibit a complete ligand exchange to Na+ ligands. The S–Ag2S NCs with Na+ ligands show a close interparticle distance and compatibility for uniformly deposited thin films by a simple spin-coating method. In photoelectrochemical measurements with stacked Ag2S NCs on ITO electrodes, a 3-fold enhanced current response was observed for the ligand passivation of Na+ compared to OctAH+, indicating a significantly enhanced charge transport in the Ag2S NC film by a drastically reduced interparticle distance due to the Na+ ligands.