Mechanism and Origin of Exciton Spin Relaxation in CdSe Nanorods

Abstract

The dynamics of exciton spin relaxation in CdSe nanorods of various sizes and shapes are measured by an ultrafast transient polarization grating technique. The measurement of the third-order transient grating (3-TG) signal utilizing linear cross-polarized pump pulses enables us to monitor the history of spin relaxation among the bright exciton states with a total angular momentum of F = +/-1. From the measured exciton spin relaxation dynamics, it is found that the effective mechanism of exciton spin relaxation is sensitive to the size of the nanorod. Most of the measured cross-polarized 3-TG signals show single-exponential spin relaxation dynamics, while biexponential spin relaxation dynamics are observed in the nanorod of the largest diameter. This analysis suggests that a direct exciton spin flip process between the bright exciton states with F = +/-1 is the dominant spin relaxation mechanism in small nanocrystals, and an indirect spin flip via the dark states with F = +/-2 contributes as the size of the nanocrystal increases. This idea is examined by simulations of 3-TG signals with a kinetic model for exciton spin relaxation considering the states in the exciton fine structure. Also, it is revealed that the rate of exciton spin relaxation has a strong correlation with the diameter, d, of the nanorod, scaled by the power law of 1/d4, rather than other shape parameters such as length, volume, or aspect ratio.

Spin relaxation in zinc blende and wurtzite CdSe quantum dots

“…One exponential, associated with exciton recombination had a long nanosecond timescale that could not be fully resolved in our experiments, which had a maximum pump-probe time delay, t p , of 850 ps. The other two exponentials likely correspond to surface trapping dynamics and were on the order of picoseconds to tens of picoseconds, consistent with previous results [10,31,32,11,12]. The timescales and relative amplitudes obtained from the VVVV data were then used to analyze the VHVH and VHHV traces, allowing us to account for the exciton population relaxation dynamics in the CPH-3TG signals and to obtain the exciton spin flip rate.…”

“…However, population in the F ¼ 0 state does not directly contribute to the exciton spin flip rate. Instead, it contributes to the overall positive long delay time offset of the CPH-3TG signals [12].…”

“…Inhomogeneous broadening due to the inherent size-distribution associated with colloidal QD samples can easily obscure this small energy separation. Despite the small energy differences, relaxation dynamics in the fine structure of colloidal QD samples can be measured using a time-resolved spectroscopic technique, cross-polarized heterodyned third-order transient grating (CPH-3TG), recently developed by our group [10][11][12][13][14][15][16].…”

Controlling the Optical Properties of Inorganic Nanoparticles

“…Figure 6b shows a plot of another distinct kind of radiationless relaxation rate in NCs that has been largely overlooked until recently. [111,112] These are the various relaxation pathways within the fine structure states of the lowest exciton. For example, such relaxation processes decide the pathways taken from optical excitation to photoluminescence: say, F ¼ þ1 !…”

“…We have found that the time scales for such relaxation processes are strongly pathway dependent and time constants range from 100 s of femtoseconds to 10 s of picoseconds. [112] The specific rate constant plotted in Figure 6b turns out to be strongly dependent only on the diameter of nanocrystals (and not on length at all), [93] which has enabled us to employ this trend as a 'calibration curve' to estimate how exciton size depends on shape in systems like those NCs shown in Figure 1c. [38] Note that the sign of the total angular momentum flips, and it is this phenomenon that enables us to record these dynamics, even though the states involved are completely obscured by inhomogeneous line broadening.…”