Photo-Induced State Shifting in 1D Semiconductor Quantum Wires

We report on the time-dependent, energetic shifting of quantum-confinement states within photoexcited CdTe quantum wires that were identified by using transient absorption (TA) spectroscopy. The initial photoexcitation promotes an electron to states in the conduction band (CB) and generates a hole in the valence band (VB), and the changes in carrier densities give rise to quantum-state renormalization (QSR). This mechanism is akin to the generation of electron and hole polarons that alter the effective masses of the states in the CB and VB that are probed in the TA experiments. Since the energies of the quantum-confinement states are inversely proportional to the effective mass of the carrier, the states undergo QSR that evolves as the carriers relax to the band edge. A simple model is presented which separates the bleach and induced-absorption signals associated with QSR in the TA data from additional bleach signals attributed to the occupancy of carriers in the shifted states. The states are observed to shift independently with time as the carriers relax. The lowest-energy occupancy feature for the CdTe quantum wires shifts from-25 to +3 meV within 600 fs and then to-17 meV on longer time scales after excitation. The Stokes shift of the photoluminescence of the CdTe quantum wires is dominated by the long-time scale QSR of the band-edge states. QSR is likely prominent in other direct-gap semiconductor nanoparticles, especially those with a significant Stokes shift of the photoluminescence from the absorption band-gap transitions.