Photo-Induced State Shifting in 1D Semiconductor Quantum Wires

Abstract

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.

Intraband Relaxation Dynamics of Charge Carriers within CdTe Quantum Wires

“…3,5,10,18,31−42 These TA data are often complicated and difficult to analyze due to overlapping transient signals. In ref 43, we demonstrated that some of the overlapping inducedabsorption (ΔAbs > 0) and bleach (ΔAbs < 0) signals within the TA data are associated with changes in the electron density distribution induced by photoexcitation; these effects are termed quantum-state renormalization (QSR). 43 In this Letter, we report on the IRD of photoexcited charge carriers within an ensemble of CdTe quantum wires (QWs) characterized using TA spectroscopy.…”

“…In ref 43, we demonstrated that some of the overlapping inducedabsorption (ΔAbs > 0) and bleach (ΔAbs < 0) signals within the TA data are associated with changes in the electron density distribution induced by photoexcitation; these effects are termed quantum-state renormalization (QSR). 43 In this Letter, we report on the IRD of photoexcited charge carriers within an ensemble of CdTe quantum wires (QWs) characterized using TA spectroscopy. 43 The densities of states within the CB and VB of one-dimensional (1D) quantum wires (QWs) contain a continuum associated with the translational motion of the charge carriers along the unconfined dimension.…”

“…43 In this Letter, we report on the IRD of photoexcited charge carriers within an ensemble of CdTe quantum wires (QWs) characterized using TA spectroscopy. 43 The densities of states within the CB and VB of one-dimensional (1D) quantum wires (QWs) contain a continuum associated with the translational motion of the charge carriers along the unconfined dimension. The IRD in these QWs are likely different than those in highly studied QDs, as discrete energetic jumps of the carriers between the quantum-confinement states are not strictly necessary.…”

“…Because total momentum must be conserved, there are likely additional constraints governing the relaxation of the carriers in 1D QWs than in 0D QDs. We employed the QSR model 43 to analyze the TA data and to obtain the time-dependent carrier occupancies within the different spectral features. The temporal profiles of the occupancy features enabled the IRD of the charge carriers through the different sub-bands to be mapped with time.…”

Excitation Energy Dependence of Photoluminescence Quantum Yields in Semiconductor Nanomaterials with Varying Dimensionalities

“…Semiconductor nanoparticles (NPs) have been extensively studied for their abilities to absorb and emit photons across a tunable range of the electromagnetic spectrum due to quantum-confinement effects. − The photoluminescence (PL) quantum yield (Φ PL ) of a sample is a useful metric for assessing the overall quality of the NPs, optimizing synthetic procedures, and improving device efficiencies that incorporate NPs. The Φ PL is the ratio of emission to absorption events.…”