Direct Synthesis of Novel Cu_2–xSe Wurtzite Phase

Abstract

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.

Polymorphic Metastability in Colloidal Semiconductor Nanocrystals

“…Reductions in free energy differences between polymorphs and the fast reaction kinetics of nanocrystal nucleation and growth make it possible to synthesize colloidal nanocrystals with metastable crystal structures that in bulk are only accessible at much higher temperatures than those required for the analogous nanocrystal syntheses [12] . In addition to nanocrystalline analogues of bulk materials, entirely new crystal phases can also arise on the nanoscale that have no bulk counterparts [13–23] . Therefore, colloidal nanocrystal chemistry presents both thermodynamic and kinetic advantages in the preparation of metastable materials, and promises to be fruitful for the discovery of new metastable materials with unique properties.…”

“…One approach that has proven successful in controlling reaction kinetics is leveraging predictable trends in the reactivities of molecular precursors to affect the rates and/or conditions under which nanocrystal nucleation and growth occur [60–62] . In this regard, diorganyl dichalcogenides lend themselves to kinetically controlled syntheses, as their reactivities can be modulated by changing the identity of the organic substituent [17,63–65] . For example, in 2013, Vela et al.…”

Diorganyl Dichalcogenides as Surface Capping Ligands for Germanium Nanocrystals

“…Diorganyl dichalcogenide compounds have been used as molecular precursors in the synthesis of a variety of colloidal semiconductor NCs with controlled composition and morphology. − The relative bond dissociation energies of carbon–chalcogen (C–Q) and chalcogen–chalcogen (Q–Q) bonds in the diorganyl dichalcogenide reactants have been used to explain the observed compositions and morphologies. A low bond dissociation energy for C–S bonds gives S-rich phases in the Fe x S y system .…”

“…A low bond dissociation energy for C–S bonds gives S-rich phases in the Fe x S y system . The metastable wurtzite phase Cu 2– x Se can be achieved by the reaction of precursors with high bond dissociation energies, while low bond dissociation energies give thermodynamic products . In both cases, alkylamines or other coordinating solvents are important to the formation and reactivity of chalcogenide intermediates.…”

Tolman’s Electronic Parameter of the Ligand Predicts Phase in the Cation Exchange to CuFeS2Nanoparticles

“…Further attempts at postsynthetic phase conversion were performed using a high-temperature pXRD furnace to further rule out thermal interconversion between the phases. Metastable WZ-like CuInS 2 will convert to the thermodynamically stable CP phase between 400 and 500 °C, whereas the metastable WZ Cu 2 Se phase transforms to the cubic at only 150 °C. , WZ-like CuFeS 2 nanocrystals that were synthesized through cation exchange in the presence of amine ligands were heated to 500 °C. pXRD patterns were taken at room temperature and every 100° starting at 100 °C (Figure ).…”