The Effect of the Mid-Shell Thickness on the Charge-Carrier Dynamics of the Green-Light Emitting InP/ZnSe/ZnS Core-Shell Quantum Dots

Ultrafast charge carrier dynamics of green-light emitting InP/ZnSe/ZnS core-shell quantum dots (QDs) with three different ZnSe thickness of 2.0 nm (gQD-20), 2.4 nm (gQD-24), or 3.3 nm (gQD-33) are interrogated by femtosecond transient absorption (TA) and femtosecond stimulated Raman spectroscopy (FSRS). Auger-type cooling of hot electron into the band-edge level of InP is clearly manifested in the TA spectra, giving its time constant of approximate to 222, 300, or 349 fs for gQD-20, gQD-24, or gQD-33, respectively, indicating that the state-filling to the 1S(e) of InP is significantly slowed down with the increase of the mid shell thickness. The global analysis of the whole TA spectra turns out to be extremely useful to explain the dynamic behavior of the heterogeneous ensemble of single and/or bi- (or multi-) excitons in the presence of the Stark-shift. The hole-relaxation dynamics into the band-edge 1S (h) of the core InP is revealed in the bleaching dynamics of the FSR bands corresponding to the phonon mode of InP or ZnSe, giving the estimated lifetime of 1.0-1.4 ps for gQD-20. The hole-relaxation dynamics by the phonon emission seem to be little sensitive to the ZnSe shell thickness. The effect of the ZnSe mid-shell thickness (in the range of 2.0-3.3 nm) on the ultrafast charge carrier dynamics of the green-light emitting InP/ZnSe/ZnS core-shell quantum dots as well as their hole-relaxation dynamics are revealed by the femtosecond time-resolved transient-absorption and femtosecond stimulated Raman spectroscopy.image