Decoding the entire (fs-μs) excited-state dynamics and energetics in toxic-metal-free CuInS₂ QDs

Among the red-NIR emitting toxic-metal-free quantum dots (QDs), CuInS₂ QDs are emerging as the forerunner. However, optical spectroscopy, excited-state energetics and dynamics of CuInS₂ QDs, especially the exact reasons behind the large Stokes shift (5051 cm^-1/630 meV) and the large FWHM of PL emission (2444 cm^-1/300 meV), remain poorly understood. Steady-state PL measurements from room temperature (RT, 300 K) to liquid-helium-cryogenic temperature (LHCT, 22 K) revealed two emission bands with PL_max at 660 and 535 nm, respectively. (a) Activation energies of the 660 and 535 nm bands are measured to be 93.1 and 8.5 meV, respectively, indicating tightly bound and loosely bound electron-hole pairs, respectively. (b) PL decay patterns of these two bands at LHCT are quite different. These results clearly demonstrate that the nature of the 660 and 535 nm states is quite different. Although this QD has an excited-state lifetime of ∼4.5 microseconds (μs), dynamical analysis has so far been reported only up to 5 nanoseconds (ns). Exploring ultrafast femtosecond (fs) to slow (µs) dynamics, in addition to the 495 nm band (absorption band-edge), three additional excited-states at 535, 635 and 660 nm could be identified. Moreover, dynamical time constants for transitions from (a) 495 to 535 nm state, (b) 535 to 635 nm state, and (c) 635 to 660 nm state, have been estimated to be 560 fs, 810 fs and 0.1 µs, respectively. Exploring very detailed spectral and dynamical analyses, the charge-carrier hopping through the manifolds of the excited-states is shown to be responsible for the broad and large Stokes shifted PL emission for CuInS₂ core QDs.