Pervasive Cation Vacancies and Antisite Defects in Copper Indium Diselenide (CuInSe₂) Nanocrystals

Abstract

Copper indium diselenide (CuInSe2) is a prototype ternary compound and group I–III–VI semiconductor with useful optoelectronic properties. CuInSe2 nanocrystals have been of significant interest because of their size-tunable optical properties and lack of toxic heavy metals. Because of the particular vacancy and antisite substitutional point defects in CuInSe2, large stoichiometric deviations can be tolerated, sometimes leading to the so-called ordered vacancy compounds (OVCs). Here, we use Raman spectroscopy of oleylamine-capped CuInSe2 nanocrystals and ab initio lattice dynamics modeling to study the concentration and arrangements of (2vCu– + InCu2+) defect pairs in the nanocrystals. The nanocrystals have randomly distributed defect pairs that become mobile under light excitation and accumulate, as in OVCs, along the [100] direction. Because the high concentration of vacancies in CuInSe2 nanocrystals is compensated by InCu2+ antisite defects, these nanocrystals do not exhibit an optical plasmon resonance like many other copper chalcogenide nanocrystals. Annealing the nanocrystals at a high temperature (600 °C) was found to significantly reduce the defect concentration.