Facile Ligand Exchange of Ionic Ligand-Capped Amphiphilic AgS 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 AgS NCs and the fabrication of highly conductive films. S-terminated AgS (S-AgS) NCs are capped with ionic octylammonium (OctAH) ligands to compensate for surface anionic charge, S, of the NC core. An injection of polar solvent, formamide (FA), into S-AgS NCs dispersed in toluene leads to an additional envelopment of the charged S-AgS 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-AgS NCs in FA to exhibit a complete ligand exchange to Na ligands. The S-AgS 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 AgS 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 AgS NC film by a drastically reduced interparticle distance due to the Na ligands.