Photophysical Properties of Water-Soluble CdTe/CdSe/ZnS Core/Shell/Shell Nanocrystals Emitting at 820 nm

The photophysical properties of near-infrared-I (NIR-I)-emissive type-II semiconductor nanocrystals (NCs), such as their excitonic properties and multiphoton absorption (MPA), have been rarely reported. In this work, the photophysical properties of water-soluble CdTe/CdSe/ZnS core/shell/shell NCs emitting at 820 nm were investigated using various spectral tools. Through the measurements of temperature-dependent photoluminescence (PL) spectra, the coefficient of the band gap change of CdTe/CdSe/ZnS core/shell/shell NCs was determined as-1.01 × 10^-5 K^-1, while the coupling strength of exciton-acoustic phonon and exciton longitudinal optical (LO) phonon and the LO phonon energy were determined to be 120 μeV/K and 26 and 77 meV, respectively. Their linear absorption cross section was determined as 1.09 × 10^-15 cm² at 500 nm, using femtosecond transient absorption spectroscopy. More importantly, the MPA cross sections of CdTe/CdSe/ZnS NCs excited in the NIR-II (1000-1340 nm) and III regions (1620-1840 nm) were measured, with the maximum two- A nd three-photon absorption cross sections of NCs being 3.80 ± 0.19 × 10⁶ GM at 1000 nm and 1.28 ± 0.06 × 10^-76 cm⁶ s² photon^-2 at 1700 nm, respectively. In addition, their biocompatibility and NIR-III-excited in vitro bio-imaging have been demonstrated. The good water dispersibility, high exciton stability, and strong multiphoton-excited PL brightness ensure that these NIR-I-emitting CdTe/CdSe/ZnS NCs will be very promising in the applications of various optoelectronic devices and deep-tissue biological imaging.