Excitation of quantum-sized (CuS)n clusters for NIR-II photothermal application: Insights from experiment and DFT calculation

Copper sulfide nanoclusters have an intensified and adaptable near infrared-II (NIR-II) light absorption due to their intriguing electronic structure, which endues them with a promising prospect for photothermal tumor-ablation in nanotherapy. In this study, quantum-sized (CuS)n with cluster-size-dependent distinctiveness of non-shifting, red-shifting, and blue-shifting absorption spectra under photo-excitation were studied experimentally and theoretically; three different stages categorized as molecule, nanocluster and nanoparticle were identified to delineate different mechanisms underlying photo-excitation. Addressing plasmonicity by hole-electron analysis based on time-dependent density functional theory (TD-DFT), we adapted TD-DFT protocol to measure the distribution, collectiveness and coherence of the excited molecular orbital pairs of (CuS)n. The current calculation can also lend weight to plasmonicity evaluation in other semiconducting alloy and compound nanomaterials. We ascertained the correlation of the optical properties of (CuS)n nanoclusters to enhanced plasmonicity arising from an excitonic-dominated excitation along with growing cluster size n, laying down foundations for precise and controllable applications of (CuS)n nanoclusters. In vivo tumor-ablating assessment of (CuS)n nanocluster preliminarily qualified it as an efficient low-dose photothermal transduction agent under a low NIR-II irradiance. This study provides a comprehensive view on (CuS)n nanoclusters preparation, excitation mechanism and application prospects besides uniqueness in methodology, shedding light on further application investigation and engineering for the light-functioning nanoclusters.