Roles of Sulfur Sources in the Formation of Alloyed Cu_2–<i>x</i>S_<i>y</i>Se_1–<i>y</i> Nanocrystals: Controllable Synthesis and Tuning of Plasmonic Resonance Absorption

Ternary alloyed Cu_2–<i>x</i>S_<i>y</i>Se_1–<i>y</i> nanocrystals (NCs) were synthesized by using a simple and phosphine-free colloidal approach, in which sulfur powder and 1-dodecanethiol (DDT) were used as sulfur sources. In both cases, the crystal phase transformed from cubic berzelianite to monoclinic djurleite structure together with the morphology evolution from quasi-triangular to spherical or discal with an increase of sulfur content. Accordingly, the near-infrared (NIR) localized surface plasmon resonance (LSPR) absorption of the as-obtained sulfur-rich NCs exhibited obvious red-shift of wavelength and widening of absorption width. When the sulfur powder was chosen as sulfur sources, the LSPR wavelength of the as-obtained alloyed Cu_2–<i>x</i>S_<i>y</i>Se_1–<i>y</i> NCs could be tuned from 975 to 1230 nm with a decrease of selenium content in the NCs. In contrast, the region of the red-shift could be up to 1250 nm for the alloyed NCs synthesized by incorporation of different DDT dosage into the reaction system. The different sulfur sources and the electron donating effects of the DDT as a ligand played an important role in the LSPR absorption tuning. This deduction could be testified by the post-treating the quasi-triangular Cu_2–<i>x</i>Se NCs with DDT under different temperatures and over different reaction time, which exhibited a red-shift of LSPR wavelength up to 450 nm due to coordination of DDT to Cu atoms on the NC surface while incorporating some sulfur anions into the lattice. This study offers a convenient tool for tuning the LSPR absorption of copper chalcogenide NCs and makes them for application in biological and optoelectronic fields