Incorporating Ytterbium (III) and Bismuth (III) ions into NaInS2 nanocrystals toward enhanced visible absorption and near-infrared emission

Constructing lanthanide doped semiconductor nanocrystals (Ln-SNCs) is a promising strategy for combining of bright, long-lived, and spectrally narrow Ln 3+ emission, along with strong broadband absorption and low-phonon-energy crystalline environment of semiconductor to make new spectral-conversion nanophosphors. NaInS 2 SNC is one of the most important host series for its high stability and non-toxicity as compared to those widely explored Ln doped perovskite SNCs. Such a family SNCs provide high cation coordination numbers (CN) of 6, which fulfills the cation the fundamental prerequisite for Ln 3+ doping. However, the photoluminescence quantum yields (PL QYs) of Yb 3+ doped NaInS 2 are still below 8% in most SNCs thus far, restricting their further application in near-infrared NIR bio-probe. Here, we demonstrate a strategy for accessing Yb 3+ /Bi 3+ -codoped NaInS 2 SNCs for strong NIR emission. In order to improve the NIR emission, we introduce Bi 3+ ions to create new excitation channels, aiming at enhancing the ultra-violet (UV) and visible light absorption. Furthermore, an ZnS outer-layer growth on Ln-doped NaInS 2 :Bi passivates the surface of SNCs and suppresses the surface defect related non-radiative recombination losses. Our NaInS 2 :Yb/Bi@ZnS SNCs exhibit efficient NIR emission around 1000 nm with a notable PL QY of 9.5%, suggesting their potential applications in NIR bio-imaging. • Herein, we demonstrate the successful synthesis of colloidal NaInS 2 :Yb/Bi@ZnS nanocrystals (∼15 nm). • Bi 3+ was introduced into the NaInS 2 lattice to give rise to a new optical absorption band at 460 nm, which is more conducive to excitation by commercial 450 nm LEDs. • ZnS shell was constructed to eliminate the quenching sites on the SCNs surface as well as spatial isolate of SCNs from surrounding deactivators. • As a results, NaInS 2 :Yb/Bi@ZnS sample show circa 61 and 77 times more intense NIR emission under excitation at 390 and 460 nm as that of NaInS 2 :Yb/Bi SCNs and thus the relative Yb 3+ emission quantum yield increases to about 10%. • The first principle density functional theory (DFT) calculations shew that the doping of Bi 3+ reduced the state energy near CBM to 1.36 eV, and thus enhance the energy transfer to Yb 3+ f -electrons. • Such strategies, combining with the superior luminescence performance of Ln 3+ ions and the large absorption cross-section of non-toxic, heavy metal-free SCNs, obtain high-efficiency NIR SCNs, which might lead future applications such as NIR LEDs, and NIR bio-probes.