Publication date: 15th December 2025
Copper selenide (CuSe) is a quasi-layered monolithic material that uniquely exhibits both semiconducting and metallic properties across the visible and near-infrared (NIR) spectral ranges. This dual behaviour enables excitonic absorption in the visible alongside strong, highly tunable plasmonic resonances in the NIR, providing a versatile platform to explore plasmonic–excitonic interactions, nonlinear optical effects, and anisotropic light–matter coupling.
Despite its potential, synthesising single-phase klockmannite CuSe remains challenging due to its complex layered crystal structure and the variable valence states of copper and selenium. In this work, a simple phosphine- and thiol-free colloidal synthesis is developed using a hot-injection method to produce quasi-2D klockmannite CuSe nanocrystals. Precise morphological control is achieved by adjusting the injection temperature and precursor ratios, without introducing additional ligands. This approach yields large nanosheets with lateral dimensions of ~200 nm to several micrometres, as well as monocrystalline triangular nanoprisms with diagonal sizes of 15–40 nm. Both morphologies feature smooth surfaces and strong NIR plasmonic absorption, which can be tuned by size and shape.
The anisotropic optical response of klockmannite CuSe is further examined through comparison of experimental spectra with theoretical calculations using the complex-scaled discrete dipole approximation (CSDDA). The modelling reveals hyperbolic optical behaviour within the NIR regime, arising from the interplay of propagating and evanescent electromagnetic fields, and leading to characteristic surface modes indicative of strong optical anisotropy.
Finally, ultrafast spectroscopic studies provide further insight into the photophysical behaviour of the material, revealing fast hot-hole cooling, carrier trapping pathways, and the generation of coherent phonons. Together, these results establish a scalable and ligand-minimal route to anisotropic and hyperbolic CuSe nanostructures, offering new opportunities for tunable plasmonic–excitonic coupling and advanced optical functionality in solution-processed nanomaterials.
Authors thank Jivesh Kaushal, Stefan Lochbrunner, Tobias Korn and Stefan Scheel for their contribuitons and DFG for funding of SFB 1477 “Light-Matter Interactions at Interfaces”
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