Visible-NIR transient absorption measurements Sample Clauses

Visible-NIR transient absorption measurements. The spectral width of the exciton bands and the dynamics of electrons and holes can be directly probed by TA spectroscopy. The experimental setups for the TA measurement in the visible-NIR and mid-IR regions are described in Chapter 2. The visible-NIR TA spectra of isolated PbS QDs in heptane measured after 800 nm excitation are shown in Figure 5.2 A. A clear bleach of 1S exciton band near 920 nm and an induced absorption feature from 450 to 800 nm are observed. The 1S exciton bleach is attributed to the state filling of 1S electron and hole levels,30, 50 and the positive feature in the visible region is caused by the red-shift of higher energy bands induced by the presence of the 1S exciton.51-53 The recovery of these two TA features is less than 10% within 600 ps, indicating that most excited QDs are in the long-lived single exciton state. The TA spectra of QDs deposited on sapphire windows (Figure 5.2 B) show similar TA spectral and dynamic features, suggesting negligible interaction of the 1S electrons and holes between the QDs and with the insulating substrate (sapphire window). However, the TA spectra for PbS on TiO2 measured under the same excitation conditions (Figure 5.2 E) show a much more broadened 1S exciton bleach and a faster bleach recovery ( > 90% within 600 ps). As shown in Figure 5.2 C and 5.2 D, both the width of the 1S exciton bleach and its rate of recovery decrease with the Al2O3 insulating layer thickness. It is interesting to note that a clear although broad 1S exciton bleach was observed in the TA spectra, which is much more pronounced than the 1S exciton band in the static absorption spectrum (Figure 5.1 B) of PbS/TiO2 samples. The reason for this discrepancy will be provided below. (A) PbS-Sapphire 24 16 8 24 18 12 12 9 3 0 (B) PbS-3Al2O3/TiO2 (C) PbS-1Al2O3/TiO2 (D) PbS-TiO2 Figure 5.3. Comparisons of mid-IR kinetics of PbS QDs in heptane (black dotted lines) and on sapphire window (A), 3Al2O3/TiO2(B), 1Al2O3/TiO2(C), and TiO2 (D) at 3 (top panels, red lines), 4 (middle panels, blue lines) and 5 (bottom panels, green lines) µm after 800nm excitation. The signals from each substrate have been scaled by a normalization factor such that it has the same initial amplitude at 5 µm as QDs in heptane.