Metallic-like Stoichiometric Copper Sulfide Nanocrystals: Phase- and Shape-Selective Synthesis, Near-Infrared Surface Plasmon Resonance Properties, and Their Modeling

Abstract

In the realm of semiconductor nanomaterials, a crystal lattice heavily doped with cation/anion vacancies or ionized atomic impurities is considered to be a general prerequisite to accommodating excess free carriers that can support localized surface plasmon resonance (LSPR). Here, we demonstrate a surfactant-assisted nonaqueous route to anisotropic copper sulfide nanocrystals, selectively trapped in the covellite phase, which can exhibit intense, size-tunable LSPR at near-infrared wavelengths despite their stoichiometric, undoped structure. Experimental extinction spectra are satisfactorily reproduced by theoretical calculations performed by the discrete dipole approximation method within the framework of the Drude-Sommerfeld model. The LSPR response of the nanocrystals and its geometry dependence are interpreted as arising from the inherent metallic-like character of covellite, allowed by a significant density of lattice-constitutional valence-band free holes. As a consequence of the unique electronic properties of the nanocrystals and of their monodispersity, coherent excitation of symmetric radial breathing modes is observed for the first time in transient absorption experiments at LSPR wavelengths.

New materials for tunable plasmonic colloidal nanocrystals

“…In another work, Xie et al synthesized disk-shaped covellite (CuS) NCs via a hot-injection route and studied in detail their optical behaviour. 71 In these NCs, due to the peculiar 2D metallic character of covellite (as discussed earlier), the plasmonic behaviour can be considered as a set of in-plane collective hole oscillations, one for each metallic sheet that are stacked on top of each other in a disk. Xie et al did not find compelling evidence of an additional optical absorption besides the broad band peaked at 1300 nm.…”

Chemical Control of Plasmons in Metal Chalcogenide and Metal Oxide Nanostructures

“…[ 26 ] As a result, CuS behavior is more metallic in nature and is an unconventional vacancy-undoped chalcogenide. The diffi culty in synthesizing CuS is the reason why their optical properties have not been well studied.…”

“…The diffi culty in synthesizing CuS is the reason why their optical properties have not been well studied. However, hexagonal disks have been synthesized colloidally [ 26 ] with lengths tunable from 10-28 nm with a constant ca. 5 nm height.…”

Ultrafast Hole Trapping and Relaxation Dynamics in p-Type CuS Nanodisks

“…Among them, the hexagonal covellite (CuS) is of particular interest due to its significant density of free carriers (holes) in the valence band accounting for its unique metallic conductivity and the strong localized surface plasmon resonance (LSPR) in NIR. [5][6][7][8] These characteristics, together with its suitable band gap (2.2 eV), make CuS potentially ideal as low-cost light-harvesting and charge-transport materials in photovoltaics and photocatalysis. 7,[9][10][11] The successful utilization of CuS nanocrystals in photocatalysis and photovoltaic devices largely depends on the trapping and relaxation dynamics of charge carriers in CuS nanocrystals.…”