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, S^2-, 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.