Shedding Light on Vacancy-Doped Copper Chalcogenides: Shape-Controlled Synthesis, Optical Properties, and Modeling of Copper Telluride Nanocrystals with Near-Infrared Plasmon Resonances

Abstract

Size- and shape-controlled synthesis of copper chalcogenide nanocrystals (NCs) is of paramount importance for a careful engineering and understanding of their optoelectronic properties and, thus, for their exploitation in energy- and plasmonic-related applications. From the copper chalcogenide family copper telluride NCs have remained fairly unexplored as a result of a poor size-, shape-, and monodispersity control that is achieved via one-step syntheses approaches. Here we show that copper telluride (namely Cu(2-x)Te) NCs with well-defined morphologies (spheres, rods, tetrapods) can be prepared via cation exchange of preformed CdTe NCs while retaining their original shape. The resulting copper telluride NCs are characterized by pronounced plasmon bands in the near-infrared (NIR), in analogy to other copper-deficient chalcogenides (Cu(2-x)S, Cu(2-x)Se). We demonstrate that the extinction spectra of the as-prepared NCs are in agreement with theoretical calculations based on the discrete dipole approximation and an empirical dielectric function for Cu(2-x)Te. Additionally we show that the Drude model does not appropriately describe the complete set of Cu(2-x)Te NCs with different shapes. In particular, the low-intensity longitudinal plasmon bands for nanorods and tetrapods are better described by a modified Drude model with an increased damping in the long-wavelength interval. Importantly, a Lorentz model of localized quantum oscillators describes reasonably well all three morphologies, suggesting that holes in the valence band of Cu(2-x)Te cannot be described as fully free particles and that the effects of localization of holes are important. A similar behavior for Cu2-xS and Cu(2-x)Se NCs suggests that the effect of localization of holes can be a common property for the whole class of copper chalcogenide NCs. Taken altogether, our results represent a simple route toward copper telluride nanocrystals with well-defined shapes and optical properties and extend the understanding on vacancy-doped copper chalcogenide NCs with NIR optical resonances.

New materials for tunable plasmonic colloidal nanocrystals

“…5d). 29 On the other hand, the work of Li et al More in general, they were able to show that oxidizing conditions, which would increase the number of free carriers, are indeed translated into an increased conductivity of a film of Cu 2Àx S NCs. The conductivity could be then reversibly decreased under reducing conditions.…”

“…63 Imaging, sensing and photothermal therapy do not conclude the range of potential application for copper chalcogenide NCs. NCs of these materials, because of their intrinsic ability to undergo cation exchange, 64 29 This suggests that a large fraction of the holes in these copper-deficient chalcogenides is actually localized. It was therefore concluded that the shape of the NCs has little influence on their plasmonic response.…”

Doped Nanocrystals as Plasmonic Probes of Redox Chemistry

“…We exploit the well-known chemical sensitivity of copper(I) sulfide, [17] which can be summarized by the chemical reaction: [18] Cu 32 S 16 Ð Cu 32-n S 16 …”

“…We hypothesized that the addition of amines, which bind strongly to copper, should assist in extracting copper from the copper sulfide lattice, thus pushing the equilibrium indicated in Equation [2] to the right. More copper-deficient stoichiometries (Cu 30 S 16 , Cu 29 S 16 , and so on) are also possible. [21] Upon addition of amine to stoichiometric copper sulfide nanocrystals, we observed that a strong LSPR mode appeared.…”

“…The ca. 1 eV peak position of the LSPR mode achieved by the addition of 200 mm iodine corresponds to a stoichiometry of Cu 29 S 16 or Cu 1.8 S (see the Supporting Information), known to be the digenite phase. Concomitant with copper extraction, copper iodide was formed, as verified by the appearance of its characteristic absorption band at 410 nm (Supporting Information, Figure S3 a).…”