# Unlocking Klockmannite: Formation of Colloidal Quasi‐2D CuSe Nanocrystals and Photo‐Physical Properties Arising From Crystal Anisotropy

## TL;DR

Scientists developed a new method to create copper selenide nanocrystals with unique optical properties in the near-infrared range.

## Contribution

A thiol-free colloidal synthesis method for quasi-2D klockmannite CuSe nanocrystals with controlled shape and strong NIR plasmonic absorption.

## Key findings

- Large klockmannite nanosheets and uniform triangular nanoplatelets were produced using a thiol-free hot injection method.
- Crystal anisotropy leads to optical anisotropy and hyperbolic regime in the near-infrared spectral range.
- Ultrafast photophysical behavior includes hot-hole cooling and coherent phonon generation in klockmannite phase.

## 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 micrometers, 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 analyzed 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 behavior 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.

Thiol‐free hot injection approach yields quasi‐2D klockmannite CuSe nanocrystals, specifically nanosheets and uniform triangular nanoplatelets. This study reconsiders the klockmannite from the point of view of crystal anisotropy and demonstrates its effect on plasmonic response in the near‐infrared spectral region with the theoretical emergence of hyperbolicity and mixing of propagating and evanescent fields in triangular nanocrystals.