Tuesday, 28 May, 2013
Shedding Light on Vacancy-Doped Copper Chalcogenides: Shape-Controlled Synthesis, Optical Properties, and Modeling of Copper Telluride Nanocrystals with Near-Infrared Plasmon Resonances
I. Kriegel, J. Rodríguez-Fernández, A. Wisnet, H. Zhang, C. Waurisch, A. Eychmüller, A. Dubavik, A. Govorov, and J. Feldmann -
ACS Nano, Vol. 7 (5), pp. 4367-4377 (2013)
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 Cu2xTe) 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 (Cu2xS, Cu2xSe). 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 Cu2xTe. Additionally we show that the Drude model does not appropriately describe the complete set of Cu2xTe 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 Cu2xTe cannot be described as fully free particles and that the effects of localization of holes are important. A similar behavior for Cu2xS and Cu2xSe 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.