Electrochemical lithium doping of Cu2−xS nanocrystal assemblies for tuning their near infrared absorbance

The charge carrier density of copper sulfide nanocrystals (Cu2−xS NCs) is sensitive to variations in the atomic composition, which determines the nature of sulfur bonding (sulfur-to-sulfur bonding or copper-to-sulfur bonding) in the lattice. Therefore, the fine control of the composition of Cu2−xS NCs, particularly in thin-film assemblies, provides a versatile strategy for tuning the electronic properties of materials that can be directly applied in electronic devices. Herein, we report that the atomic composition of the Cu2−xS NC assemblies (x = 0.9; cation/anion ratio = 1.1/1) can be controlled by introducing monovalent lithium ions into the assemblies (yielding Li0.7Cu2−xS NCs; x = 0.9; cation/anion ratio = 1.8/1) and reversibly extracting these cations from the assemblies through electrochemical methods. The electrochemically controlled uptake and release of lithium ions in Cu2−xS NC assemblies enabled the systematic tuning of the characteristic near-infrared absorbance (NIR) of the thin-film assemblies based on the localized surface plasmon resonance; NIR absorbance at 1300 nm wavelength, for example, could be controlled by more than 75% by exploiting the reversible doping process.