Herein, we report the unprecedented direct synthesis of a recently discovered metastable wurtzite phase of Cu2–x Se. Nanocrystals of Cu2–x Se were synthesized employing dodecyl diselenide as the selenium source and ligand. Optical characterization performed with UV–vis–NIR spectroscopy in solution showed a broad plasmonic band in the NIR. Structural characterization was performed with X-ray diffraction (XRD) and transmission electron microscopy. Variable-temperature XRD analysis revealed that the wurtzite nanocrystals irreversibly transform into the thermodynamic cubic phase at 151 °C. Replacement of dodecyl diselenide with dodecyl selenol yielded cubic phase Cu2–x Se, allowing for polymorphic phase control. An aliquot study was performed to gain insight into the mechanism of phase selectivity. The direct synthesis of this novel wurtzite phase could enable the discovery of new phenomena and expand the vast application space of Cu x Se y compounds.

S Supporting Information E ver-growing demand on our traditional energy resources makes harnessing underutilized alternative energy sources all the more necessary and desirable to the scientific community. 1 Colloidal copper(I) sulfide nanocrystals (NCs) are an attractive photovoltaic device component due to their solar absorption characteristics; 2 Cu 2 S is a p-type semiconductor with a bandgap reported from 1.1 to 1.4 eV, and an absorption coefficient of 10 4 cm −1 . 3 Cu 2 S is also a parent material to ternary and quaternary copper sulfides such as CuInS 2 , which has emissive defects that can be used for LED lighting, biomedical applications and solar concentrating. 4 The light-absorbing characteristics of copper(I) sulfide can be further enhanced by morphologies that increase the absorption cross section of the nanocrystals. Manipulating reaction mixture additives can generate highly faceted Cu 2 S nanocrystals, 5 and careful stabilizer control has been used previously to make Cu 2 S nanoribbons from nanocrystal assemblies in aqueous media. 6 Sadtler et al. were first to synthesize Cu 2 S nanorods through cation exchange with CdS nanorods, a process that lacks atom economy and generates a cadmium byproduct. 7 Additionally, the cation exchange process likely leaves the Cu 2 S doped with Cd ions. Kruszynska et al. discovered that substoichiometric Cu 2−x S nanorods could be grown from nucleations when using tert-DDT as a ligand and sulfur source, 8 but the fully stoichiometric Cu 2 S was not achieved. Further developments to prepare rods structures without cadmium and of controlled stoichiometry are needed.The oriented attachment of Cu 2 S seed particles into larger single crystalline structures offers a means to obtain nanostructures with morphologies, sizes and compositions that may not be possible with traditional monomer-based growth. Oriented attachment of PbSe performed by Cho 9 and later by Koh 10 produced nanowires, nanorings, and nanorods from single-particle building blocks. Oriented attachment has also been used to create single-crystalline chains of TiO 2 crystals, 11 CdTe nanowires, 12 and nanorods of ZnO, 13 CdS and Ag 2 S. 14 The process occurs due to dipole interactions between particles. Hexagonal-like crystal structures (as shown in ZnO, CdS and Ag 2 S) inherently have a dipole due to crystal structure anisotropy, yet control over the surface chemistry was essential to orchestrate the directed attachment in many of these cases. The room temperature crystal structure of stoichiometric Cu 2 S (low chalcocite) and substoichiometric crystal structure djurleite (Cu 1.96 S) consist of copper atoms arranged around a distorted hexagonally close-packed sulfur sublattice 15 (approximated here for simplicity by the hexagonal high chalcocite structure). Provided the surface chemistry can

This study demonstrates that a dialkyl ditelluride reagent can produce metastable and difficult-to-achieve metal telluride phases in nanocrystal syntheses. Using didodecyl ditelluride and without the need for phosphine precursors, nanocubes...