Article pubs.acs.org/JPCC
Charge Delocalization in the Cascade Band Structure CdS/CdSe and CdS/CdTe Core−Shell Sensitized with Re(I)−Polypyridyl Complex Partha Maity,† Tushar Debnath,† Tanmay Banerjee,‡ Amitava Das,*,‡,§ and Hirendra N. Ghosh*,† †
Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India Organic Chemistry Division, National Chemical Laboratotry, Pune 411008, India § CSIR-Central Salt & Marine Chemical Research Institute, 364021 Bhavnagar, Gujarat, India ‡
S Supporting Information *
ABSTRACT: Charge-carrier dynamics of CdS quantum dot (QD) and CdS/ CdSe type-I and CdS/CdTe type-II core−shell nanocrystals (NCs) sensitized with a Re(I)−polypyridyl complex have been carried with special emphasis on studies on carrier delocalization and the role of Re-complex as a hole acceptor and sensitizer molecule. Our investigation confirmed photoexcited hole transfer from CdS and CdS/CdSe to the Re-complex, while no hole transfer was observed in the CdS/CdTe−Re-complex system. This was rationalized by the evaluation of the relative energy levels, which revealed that such hole migration was not energetically favorable due to low-lying highest occupied molecular orbital (HOMO) of the Re-complex as compared with the valence band (VB) of CdTe shell; however, luminescence quenching from upper excited states of Re-complex was observed in the presence of all three QD and core−shell systems, which has been attributed to electron injection from hot state (energetically higher than the LUMO state) of the Re-complex to the conduction band (CB) of the QDs. Transient absorption (λpump = 400 nm, λprobe = 450−750 nm) spectra recorded for Recomplex-sensitized CdS and CdS/CdSe composite in the femtosecond time domain revealed a broad transient absorption band in the 580−750 nm region with a peak around 595 nm, and this was attributed to the cation radical formation for Re-complex, either by capturing photoexcited hole from the NCs or by injecting electron to the CB of the NCs. As anticipated, no such spectrum was observed for the CdS/CdTe−Re-complex composite system after 400 nm excitation. Electron injection from photoexcited Re-complex to CdS QD and CdS/CdSe core−shell was found to be 1.8 ns (27%) (Table 1). Slower growth component (2 ps) of the bleach kinetics can be attributed to electron cooling time from the upper excitonic state to the 1S state.56,58,59 The bleach recovery kinetics at 570 nm for the CdS/CdSe core−shell in the presence of Recomplex can be fitted with biexponential growth with time constants τg1 = 100 fs (−75%) and τg2 = 2.5 ps (−15%) and multiexponential recovery with time constants τ1 = 15 ps (37%), τ2 = 80 ps (25%), τ3 = 400 ps (24%), and τ3 = >1.8 ns (14%) (Table 1). The second slow bleach growth time constant (2.5 ps) can be attributed to electron cooling time from upper excitonic state to the first excitonic state of CdSe QD. Interestingly, the bleach recovery dynamics of CdS/CdSe−Recomplex composite shows a different pattern as compared with pure CdS/CdSe. The bleach dynamic at 570 nm of the composite (Figure 9b) has slower bleach growth that can be attributed to slower electron cooling. This slow bleach growth can be attributed as electron−hole decoupling due to hole transfer from CdSe shell to Re-molecule (Scheme 2A). On the contrary, TA kinetics at 600 nm due to Re-cation radical can be fitted single exponentially with pulse-width-limited (1.8 ns (−68%) (Table 1). The kinetics for the Re-cation radical at 600 nm shows a faster decay component (1 ps),
Figure 6. Transient absorption spectra of (A) CdS QD and (B) CdS− Re-complex composite system in chloroform solution at different time delay following 400 nm laser excitation. In both cases the concentration of CdS QD was kept at ∼1 μM.
intensity at 590 nm and a broad transient signal beyond 750 nm. The bleach signal can be attributed to 1S excitonic transition and the photoinduced absorption (PA) signal can be attributed to a combination of the excited-state absorption of the Re-complex cation radical and the CB electron of the CdS QD, respectively. It is interesting to see that the PA signal in the 550−650 nm region of the CdS−Re-composite exactly matches the cation radical of the Re-complex.51 Scheme 1 and Figure 1 suggest that photoexcitation of CdS−Re-complex composite materials by 400 nm both hole transfer from photoexcited CdS QD to Re-complex, and electron injection from photoexcited Re-complex to CB of CdS QD is a thermodynamically viable process. So in the CdS−Re-complex composite system charge separation takes place by both hole transfer and electron injection. Dynamics at different key wavelengths have been monitored for both CdS QD and CdS− Re-complex composite systems to monitor both charge separation and charge recombination and are discussed as follows. Figure 7 shows the kinetic traces of CdS and CdS−Recomplex at different wavelengths after 400 nm laser excitation. The 1S bleach dynamics of CdS (Figure 7a) at 452 nm, which has pulse-width-limited (1.8 ns (41%) (Table 1); however, the bleach dynamic of CdS−Re-complex at 452 nm
Figure 7. Kinetic decay traces of CdS QD (a), CdS−Re-complex (b) at 452 nm, and (c) CdS−Re-complex at 590 nm in chloroform after 400 nm laser excitation. G
DOI: 10.1021/acs.jpcc.6b01654 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Table 1. Ultrafast Multi-Exponential Fitting Parameters of CdS, CdS/CdSe, and CdS/CdTe Sensitized by Re-Complex Monitored at Key Wavelengths (λ, nm) after 400 nm Laser Excitationa sample CdS CdS−Re-1,2 CdS/CdSe CdS/CdSe−Re-1,2 CdS/CdTe a
λ (nm)
τg1
452 452 590 570 570 600 464
1.8 >1.8
ns ns ns ns ns ns
(41%) (11%) (−67%) (27%) (14%) (−68%)
Percentages in the parentheses represent amplitude of the corresponding exponential functions.
CdTe type-II core−shell at different time delay after 400 nm laser excitation in chloroform solution. The spectra show a narrow negative absorption band at 460−490 nm (peaking at 480 nm) and another broad negative absorption band at the 550−660 nm region (peaking at 620 nm) that can be attributed to excitonic bleach due to 1S excitonic bleach due to CdS core QD and CdTe shell QD, respectively. Apart from the excitonic bleaches, a little positive absorption band has been observed beyond 660 nm, which can be attributed to the absorption due to the trapped electrons in the core−shell structure that arises due to higher lattice mismatch between CdS core and CdTe shell. To monitor the charge-carrier dynamics in the CdS/ CdTe core−shell, we have compared the bleach recovery kinetics at 1S excitonic position of CdS QD for both pure CdS QD and CdS/CdTe core−shell, shown in the Figure 10 inset (left panel). The bleach recovery dynamics of the CdS/CdTe at 464 nm (1S excitonic position of CdS) can be fitted with biexponential growth with time constant τg1 = 100 fs (−85%) and τg2 = 0.8 ps (−15%) and multiexponential recovery with time constants τ1 = 25 ps (15.6%), τ2 = 500 ps (26.4%), and τ3 = >1.8 ns (58%) (Table 1). Interestingly, bleach growth time of pure CdS (Figure 7a), which is
