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(Color online) (a) Schematic diagram of TPA and subsequent relaxations. After TPA, the carriers decay through (1) intraband relaxation, (2) intraband absorption, (3) Auger process, (4) radiative recombination, or (5) trapping to surface/defect states. (b) HRTEM image of GSH-capped CdSe QDs; the insets show both the artistic impression and the size distribution. (c) XRD pattern of the QDs. The solid line is the XRD measurement. The dashed line is the fit with Gaussian curves. (d) Spectra of the one-photon absorption (circle) and one-photon-excited fluorescence excited at (solid line) for the QDs in aqueous solution. The dashed curves are the theoretical fits.
(a) Transient absorption in aqueous solution of GSH-capped CdSe QDs measured with laser pulses to a wavelength of . The dashed curve is the autocorrelation between the pump and probe pulses. The inset shows the relaxation processes measured at various pump intensities of 180, 130, 80, and (from the top down). At these pump intensities, two-photon-excited pairs per QD are , 2.6, 1.1, and 0.7, respectively. The solid lines for are two-exponential fitting curves with and . The solid lines for are fitted using the model of quantized decay steps and Poisson distribution for initial states. (b) Effective Auger relaxation time vs pump intensity or . The triangles are the symbols for . The inset illustrates pathways for the relaxation of excited electrons or holes.
Dispersion of the TPA cross section for GSH-capped CdSe QDs. The solid and dashed curves are the theoretical results. The inset shows scans measured at various wavelengths with laser intensities of or less.
(a) Effective TPA coefficient vs laser intensity or . The filled circles are the extracted values from the scans. The inset shows several examples of scans at with laser intensities of 24, 68, 139, and (from up to down). (b) vs laser wavelength. The dotted lines are the guideline for square dependence on wavelength.
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