Solution-Processed Ge1-x Snx Alloy Nanocrystal Thin Films with High Electrical Conductivity and Tunable Energy Gaps

Ge1-xSnx nanocrystals (NCs) are a class of direct-gap semiconductors that show size- and composition-tunable energy gaps and enhanced absorption and emission properties compared to single-element Ge NCs. With decreasing size and increasing Sn content, optical transition oscillator strength and absorption increases, making these NCs attractive for optoelectronic devices, field effect transistors, and charge storage applications. Herein, we report the synthesis of Ge1-xSnx NCs with varying sizes (ranging from 4.7 +/- 0.6 to 8.6 +/- 1.9 nm) and varying Sn compositions (x = 0.01-0.08), followed by successful exchange of insulating surfactant ligands with molecular metal chalcogenides (MCCs), to produce solution-processed conductive NC thin films. Structural and surface analysis of pre- and post-exchanged NCs indicates a diamond cubic structure and replacement of amine surface ligands with the MCC. Electron micrographs of alloy NCs show a notable decrease in size upon ligand exchange, which is consistent with the etching induced by chalcogenide ligands. The size confinement effects have resulted in energy gaps that are significantly blue-shifted from bulk Ge for the Ge1-xSnx alloy quantum dots with composition-tunable solution-state (1.68-1.26 eV for x = 0.01-0.08) energy gaps and solid-state (1.54-1.20 eV for x = 0.01-0.08) absorption onsets. Electrical characterization of the uniform NC films (thickness = 197 +/- 5 nm) reveals that the films are insulating prior to ligand exchange and show >3 orders of magnitude increase in conductivity (3.5 x 10(-6) S/cm for Ge0.92Sn0.08 NCs) upon functionalization with MCC. The electrical conductivity of the films increases with the increasing Sn composition (1.2 x 10(-6)-3.5 x 10(-6) S/cm for x = 0.01-0.08), which is consistent with the increased spin-orbital coupling and reduction in energy gaps realized through homogeneous alloying of cubic Ge and alpha-Sn.