Electron and Hole Trapping Dynamics in Semiconductor Nanocrystals: Femtosecond Nonlinear Transmission and Photoluminesce
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CARRIER TRAPPING DYNAMICS AT LOW PUMP INTENSITIES First we examine the time-resolved DT at low pump fluences wp < 0.5 mJ cm -2 where the PL and DT dynamics are independent of the pump intensity. This corresponds to excitation of less than one electron-hole (e-h) pair per NC on average and allows us to avoid the saturation of trap states and fast Auger recombination.
0.4
_ 0.45 ps ....... 3 ps
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....
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DT (0.45 ps)
0.4
..... cw-PL
, • 6dominated by the electron population a 0 PL-(2.6 eV) [10]. Therefore, the initial DT 0 DT (2.6 eV) OW dynamics can be attributed to the _j Idepopulation of the lowest quantized state in the conduction band due to 2 electron trapping. The trapping states 4 3 2 1 0 -1 o responsible for this process are likely the same as those that give rise Delay Time (ps) wele h =to the so-called deep-trap emission FIG. 2. Single-wavelength (/1([= 2.6 eV) PL (solid [11] located in our samples at -1.9 circles) and DT (open squares) dynamics in CdS eV [2]. This explanation is NC's. Solid lines show exponential fits. consistent with direct measurements of buildup dynamics of the deep-trap PL which yield a time-constant of -30 ps [12]. As nonlinear transmission is dominated by the electron population, the initial hole dynamics are almost "invisible" in DT. To resolve the hole dynamics, we make use of a complementary femtosecond technique-PL up-conversion. At early times after excitation, the PL spectra are dominated by the band located at 2.63 eV (the lowest optical interband transition between quantized states) [1]. In contrast to the relatively slow DT decay, the PL relaxation is extremely fast and S10
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occurs on the ps time scale. The drastic difference in early-time PL and DT dynamics is pronounced in single wavelength time-transients, as shown in Fig. 2. The initial decay of the PL occurs on a much faster time scale than that of the DT. Exponential fits to the experimental data yields I and 30 ps relaxation times for the first stages of PL and DT dynamics, respectively. The strong difference in the early-time DT and PL dynamics suggests that the carrier decay is nongeminate and that the electrons and holes have different relaxation paths. In contrast to DT, the PL intensity is determined by the product of the electron and hole occupation numbers and therefore is significantly affected by the dynamics of both carriers. Since DT at 2.6 eV [the IS(e)1S3n(h) transition] decays relatively slowly, the fast I-ps relaxation of the PL intensity can only be attributed to a change in the hole occupation numbers due to trapping at localized states. The magnitude of the low-energy shift of the PL band measured in Ref. I indicates that this state has a depth of about 70 meV. The transition coupling this state to the lowest quantized electon state dominates the cw-emission, as well as DT measured at long times after excitation [see Fig. 1(b)]. CARRIER DYNAMICS AT HIGH PUMP-POWERS: AUGER-PROCESS-INDUCED CHARGE SEPARATION At pump levels above -0.5 mJ cm 2 , the DT dynamics begin to
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