Separation of the ultrafast electron and hole dynamics in the CdSe QDs adsorbed onto TiO2 nanostructured electrodes

Figure 2 shows a typical kinetic trace of the TG signal of CdSe QDs adsorbed onto a TiO2 nanostructured film (prepared by CBD method for 24 h adsorption) measured in air. The vertical axis was plotted on a logarithmic scale. Three decay processes (indicated as A, B, and C in Fig. 2) can be clearly observed. We found that the TG kinetics shown in Fig. 2 could be fitted very well with a double exponential decay plus an offset, as shown in eq. (2):

y = A1e^‘/ч + A2 e~l/ч + y0 (2)

where A1, A2 and yo are constants, and t1 and t2 are the time constants of the two decay processes (A and B in Fig. 2). Here, the constant term y0 corresponds to the slowest decay process (C in Fig. 2), in which the decay time (in the order of ns) is much larger than the time scale of 100 ps measured in this study. The time constants of the fast (t1) and slow (t2) decay processes of photoexcited carriers in air are 6.3 ps and 82 ps, respectively (Table 1). As mentioned above, t1 and t2 are independent of the pump intensity under our experimental conditions, so the three decay processes are mostly due to one-body recombination such as carrier trapping and carrier transfer.

In order to separate the photoexcited electron dynamics and hole dynamics that make up the TG kinetics, the TG kinetics of the same sample was measured both in air and in a Na2S aqueous solution (hole acceptor) (1 M) (Shen et al., 2010a). As shown in Fig. 3, a large difference can be clearly observed between the TG responses measured in air and in the Na2S solution. By normalizing the two TG responses at the signal intensity of 90 ps, we found that they overlapped with each other very well for time periods of longer than 15 ps, but the fast decay process apparently disappeared when the time period was less than 10 ps in the TG kinetics measured in the Na2S solution (hole acceptor). This great difference can be explained as follows. In air, both hole and electron dynamics in the CdSe QDs could be measured in the TG kinetics. In the Na2S solution, however, photoexcited holes in the CdSe QDs will transfer quickly to the electrolyte and only electron dynamics should be measured in the TG kinetics. Therefore, the "apparent disappearance" of the fast decay process in the Na2S solution implies that the hole transfer to S2- ions, which are supposed to be strongly adsorbed onto the CdSe QD surface, can be too fast in these circumstances as indicated by Hodes (Hodes, 2008) and therefore could not be observed under the temporal resolution (about 300 fs) of our TG technique. This observation is particularly important, because the result directly demonstrated that the transfer of holes to sulfur hole acceptors that are strongly adsorbed on the QD surface could approach a few hundreds of fs. An earlier study on the dynamics of photogenerated electron-hole pair separation in surface-space-charge fields at GaAs(100) crystal/oxide interfaces using a reflective electro-optic sampling method

Time (ps)

showed that the hole carrier transit time was faster than 500 fs (Min et al., 1990). We believe an ultrafast hole transfer time from the QDs to hole acceptors that are strongly adsorbed on the QD surface is a more feasible and reasonable explanation, since photoexcited holes can more easily reach the surface of QDs with diameters of a few nm. The TG response measured in the Na2S solution, which is considered to only relate to electron dynamics as mentioned above, can be fitted well with eq. (2). As shown in Table 1, besides the slower decay process, a faster decay process with a decay time of 9 ps was also detected in the TG
response measured in the Na2S solution. Such a faster decay process with a characteristic time of a few picoseconds in the TG response measured in the Na2S solution was considered to correspond to electron transfer from the QDs in direct contact with the TiO2 (first layer of deposited QDs) (Guijarro et al., 2010a, 2010b). It is worth noting that the relative intensity Ai (0.07) measured in the Na2S solution is much smaller compared to the A1 (0.39) measured in air and it could be ignored here. The slower relaxation process in the TG response was not influenced by the presence of the Na2S solution, as shown in Fig. 3. The decay time t2 (85 ps) and the relative intensity A2 (0.31) for the slower decay process in the TG response measured in the Na2S solution are almost the same as those measured in air (Table 1). The slower electron relaxation mostly corresponds to electron transfer from the CdSe QDs to TiO2 and trapping at the QD surface states, in which the decay time depends to a great extent on the size of the QDs and the adsorption method that is used (Guijarro et al., 2010a, 2010b; Shen et al., 2006, 2007; Diguna et al., 2007b). The slowest decay process (with a time scale of ns) may mostly result from the non-radiative recombination of photoexcited electrons with defects that exist at the CdSe QD surfaces and at the CdSe/CdSe interfaces. The difference between the two TG responses measured in air and in Na2S solution (normalized for the longer time), which was termed the "difference response", is believed to correspond to the photoexcited hole dynamics in the CdSe QDs measured in air. As shown in Fig. 3, the difference response decays very fast and disappears around 10 ps and can be fitted very well with a one-exponential decay function with a decay time of 5 ps (Table 1). Thus, we did well in separating the dynamics of photoexcited electrons and holes in the CdSe QDs and found that the hole dynamics were much faster than those of electrons. Some papers have also reported that the hole relaxation time is much faster than the electron relaxation time in CdS and CdSe QDs (Underwood et al., 2001; Braun et al., 2002). In air, the fast hole decay process with a time scale of about 5 ps can be considered as the trapping of holes by the CdSe QD surface states. This result is in good agreement with the experimental results obtained by a femtosecond fluorescence "up-conversion" technique (Underwood et al., 2001).

Thus, by comparing the TG responses measured in air and in a Na2S solution (hole acceptor), we succeeded in separating the dynamic characteristics of photoexcited electrons and holes in the CdSe QDs. We found that charge separation in the CdSe QDs occurred over a very fast time scale from a few hundreds of fs in the Na2S solution via hole transfer to S2- ions to a few ps in air via hole trapping.

TG kinetics______ A1______ t1 (ps)_______ A_______ t2 (ps)________ yo_____

In air 0.39± 0.01 6.3+0.4 0.29 ±0.01 82 ±7 0.27± 0.01

In Na2S 0.07+ 0.01 9 ±1 0.31+ 0.01 85 ±1 0.25+ 0.01

Difference 0.33± 0.01 5.0± 0.3 – – –

Table 1. Fitting results of the TG responses of CdSe QDs adsorbed onto nanostructured TiO2 films measured in air and in Na2S solution (hole acceptor) as well as their "difference response" as shown in Fig. 3 with a double exponential decay equation (eq. (2)). t1 and t2 are time constants; A1, A2 and y0 are constants (Shen et al., 2010a).