The binding energy of biexcitons in alloy Zn x Cd 1− x S quantum dots detected by femtosecond laser spectroscopy

Biexcitons localized at Zn x Cd 1− x S quantum dots ( x = 0.37 or 0.45) with a diameter of ~ 45 Å synthesized by two different methods were studied by femtosecond laser spectroscopy. The spectral features of ultrafast transient absorption spectra at the short-time delay of 70 fs are associated with the three lowest energy transitions of quantum dots. The shapes of the transient absorption bands were modeled by fitting to linear absorption. The spectral positions of the absorption components of the excited state in the transient spectra take into account the energy of the biexciton coupling. By fitting the experimental transient absorption spectra of Zn x Cd 1− x S QDs, the binding energies of biexcitons were determined. The biexciton binding energies vary from 16.6 to 37 meV depending on the biexciton transition excited at the Zn x Cd 1− x S quantum dot.