Proceedings of nanoGe September Meeting 2017 (NFM17)
Publication date: 20th June 2016
Copper sulfide (Cu2-xS) nanocrystals have been shown to possess highly tunable localized surface plasmon resonances (LSPRs) in the near-infrared (NIR) spectral region, depending on the amount of Cu+ vacancies.[1,2,3] Here, we present that we can reversibly tune the hole carrier density, and hence, the LSPR in the NIR spectral region, of covellite CuS nanocrystals by spectroelectrochemical methods. We have prepared thin films of degenerately p-doped CuS nanocrystals and by controlling the potential in an electrochemical cell we inject electrons into the NCs, which annihilates the excess holes in the top of the valence band (and hence the LSPR) and shifts the band edge towards the NIR. Furthermore, the injected electrons reduce the covalent disulfide bonds in the covellite CuS unit cell, which results in the reduction of the anionic sublattice from an overall -1 oxidation state to an oxidation state of -2.[3] The electrochemical charge injection is fully reversible and we can cycle several times between a thin film of covellite CuS NCs (Eg = 2.0 eV, strong LSPR) and low-chalcocite Cu2S NCs (Eg = 1.2 eV, no LSPR) by reducing and oxidizing the sulfide sublattice, as evidenced by X-ray Diffractometry and Raman spectroscopy measurements. Interestingly, we find that the fully stoichiometric low-chalcocite Cu2S nanocrystals prepared with our electrochemical approach display efficient air stable radiative recombination near the band edge, centered around 1050 nm. These stoichiometric low-chalcocite Cu2S nanocrystals are obtained by electrochemical reduction of the anionic sublattice and irreversible intercalation of Cu+ ions into the lattice. Our results show that we have dynamic control over the hole doping density and the crystal structure of Cu2-xS nanocrystals, eventually resulting in air stable NIR photoluminescence, which might impact on the successful implementation of copper sulfide nanocrystals into photovoltaic devices or applications such as NIR optical switches and smart windows.
References
[1] J. Luther et al., Nat. Mater., 2011, 10, 36
[2] W. van der Stam, A. C. Berends. C. de Mello Donega, ChemPhysChem, 2016, 17, 559.
[3] Y. Xie et al., J. Am. Chem. Soc., 2013, 135, 17630.
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