Unlocking klockmannite: formation of colloidal quasi-2D CuSe nanocrystals and photo-physical properties arising from crystal anisotropy

TL;DR

This study presents a ligand-free colloidal synthesis of quasi-2D klockmannite copper selenide nanocrystals, revealing their anisotropic optical properties, hyperbolic behavior, and ultrafast photophysical dynamics driven by crystal anisotropy.

Contribution

It introduces a novel ligand-free synthesis method for shape-controlled klockmannite nanocrystals and analyzes their unique optical anisotropy and hyperbolic properties using advanced modeling.

Findings

Large nanosheets and triangular nanoplatelets synthesized with controlled shape.

Pronounced optical anisotropy and hyperbolic regime in NIR spectral range.

Ultrafast photophysical processes including hot-hole cooling and phonon generation observed.

Abstract

Copper selenide is an exceptional quasi-layered monolithic material that exhibits both semiconducting and metallic properties in adjacent visible and near-infrared (NIR) spectral ranges. Here we introduce a thiol-free colloidal synthesis for generating quasi-2D klockmannite copper selenide nanocrystals via hot injection method, achieving shape control by tuning the injection temperature and precursor concentrations without any additional ligands. This approach produces large klockmannite nanosheets with lateral sizes from 200 nm to several micrometres, as well as uniform triangular nanoplatelets with sizes of 12-25 nm that are monocrystalline and display strong NIR plasmonic absorption. The spectral features of the anisotropic klockmannite phase in the NIR have been analysed using complex-scaled discrete dipole approximation (CSDDA) calculations, which reveal pronounced optical anisotropy and the emergence of hyperbolic regime. The combined effect of propagating and evanescent fields is regarded as the underlying reason of such modes in the hyperbolic domain. Finally, the ultrafast photophysical behaviour of the material in klockmannite phase is examined, including hot-hole cooling, trapping, and coherent phonons generation. Our findings emphasize the important role of the intrinsic crystal anisotropy in governing the physical properties of nanoscale klockmannite.