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Article Cite This: ACS Omega 2018, 3, 2706−2714
Concurrent Ultrafast Electron- and Hole-Transfer Dynamics in CsPbBr3 Perovskite and Quantum Dots Jayanta Dana,†,‡ Partha Maity,† Biswajit Jana,† Sourav Maiti,†,∥ and Hirendra N. Ghosh*,†,‡,§ †
Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India Homi Bhabha National Institute, Anushakti Nagar, Mumbai 400094, India § Institute of Nano Science and Technology, Mohali, Punjab 160062, India ∥ Department of Chemistry, Savitribai Phule Pune University, Ganeshkhind, Pune 411007, India ‡
S Supporting Information *
ABSTRACT: Ultrafast charge-transfer (i.e., electron and hole) dynamics has been investigated between the cesium lead bromide (CsPbBr3, CPB) perovskite nanocrystals (NCs) and cadmium selenide (CdSe) quantum dots (QDs) as a new composite material for photocatalytic and photovoltaic applications. The CPB NCs have been synthesized and characterized by high-resolution transmission electron microscopy (HR-TEM) and X-ray diffraction (XRD) pattern. The redox levels (i.e., conduction band (CB) and valence band (VB)) of the CPB NCs and CdSe QDs suggest the feasibility of photoexcited electron transfer from CPB NCs to CdSe QDs and photoexcited hole transfer from CdSe QDs to CPB NCs, and it has been confirmed by both steady-state and time-resolved spectroscopy. To investigate the electron- and hole-transfer dynamics in ultrafast time scale, we have performed femtosecond upconversion and femtosecond transient absorption studies. The measured electron-transfer time from CPB NCs to CdSe QDs and hole-transfer time from CdSe QDs to CPB NCs were found to be 550 and 750 fs, respectively. Interestingly, the charge-transfer process found to be restricted in CPB/CdSe@CdS core-shell system where electron transfer from CPB NCs to core shell takes place, but the hole transfer from core shell to CPB NCs found to be restricted due to CdS shell making the process thermodynamically nonviable. Our observation has suggested that after the photoexcitation of CPB NCs/CdSe QDs composite system, a charge-separated state is formed where the electrons are localized in CB of CdSe QDs and holes are localized in VB of CPB NCs. This makes the composite system a better material for efficient light harvesting and photocatalytic material as compared to the individual ones.
1. INTRODUCTION Since last decades, one of the broad research interests on nanocrystal (NC) semiconductor quantum dots (QDs) are due to their smart optical properties including size tunable band gap, high exciton lifetime, high quantum yield (QY), multiexciton generation, high photostability, easy synthesis protocol, etc.1−7 These QDs are promising materials for the thirdgeneration solar cell, bioimaging, various photocatalytic and water-splitting devices, etc.8−12 Since the last decade, among all of the QDs materials, metal chalcogen QDs have been well studied in literature with regards to both fundamental and application point of view.2−4,7 However, very recently, one of the most important growing subject of quantum dot research is organic−inorganic metal halide (MPbX3: M = CH3NH3, HC(NH3)2Cs; X = Cl, Br, I) perovskites.13−18 The perovskite materials have achieved maximum attention as solar cell efficiency of CH3NH3PBI3 exceeded 20%.19,20 However, the stability of CH3NH3PbI3 is found to be a big issue due to its sensitivity to moisture, which leads to hydrolysis and degradation of the perovskite materials.21−26 So, it was utmost © 2018 American Chemical Society
important to have a highly stable perovskite material for all kinds of basic applications. Recently, Kovalenko and his team have synthesized all inorganic lead halide (CsPbBr3, CPB) quantum dots, which have a long-term stability.14 Yang et al. reported the different crystalline phase of CsPbBr3 after varying the ratio of Cs and Pb.27 Interestingly, it has been observed that the band gap of all of the inorganic perovskite materials can be tuned from UV to visible by changing either the size or the composition.13,18,28,29 The size- and composition-dependent band gap tunability and carrier dynamics for different kinds of all-inorganic perovskite materials have been explored in literature.30−33 The tunable band gap in the UV to visible region, narrow full width at half-maximum of the photoluminescence (PL) spectra, very high photoluminescence quantum yield (PLQY), reduced luminescence blinking, and high carrier mobility (∼4500 cm2 V−1 s−1) make these Received: February 15, 2018 Accepted: February 23, 2018 Published: March 7, 2018 2706
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Figure 1. (A) HR-TEM image, (B) XRD pattern and (C) UV−vis absorption (olive solid line) and photoluminescence spectra (red dotted line) of CPB NCs.
Figure 2. Steady-state, (A) UV−vis absorption, and (B) photoluminescence spectra of (a) CPB, (b) CdSe QDs, and (c) colloidal mixture of CPB and CdSe QDs. Spectrum (d) shows the simple addition of spectra (a) and (b). (C, D) Upconversion luminescence decay traces of (a′) CPB at 487 nm, (b′) CdSe at 570 nm, (c′, c″) mixture of CPB and CdSe at 487 and 570 nm, respectively. For Upconversion photoluminescence measurements, the samples were excited at 400 nm.
perovskites better alternative materials as compared to binary II−VI semiconductor QDs for light-harvesting material.16,34−37 Therefore, the perovskite NCs can be efficiently used as gain medium of the laser, light-emitting diodes, photon emitter, photodetector, etc.35,37−42 Although it has been realized that all-inorganic perovskites are much superior materials in terms of stability, their powerconversion efficiency is way poorer as compared to organic− inorganic metal halide perovskite materials. Kulbak et al. reported ∼6.6% solar efficiency inorganic-based cesium lead trihalide perovskite material.43 Recently, Swarnkar et al.44 reported a power-conversion efficiency of maximum 10.77% for the quantum dot-induced phase stabilization of α-CsPbI3, which is the highest efficiency for all of the inorganic perovskite materials so far. It has been realized that to get a higher photovoltaic performance, the photogenerated hole has to be removed very fast before it corrodes the photoanode. It is always a challenging task to find suitable hole-transporting materials to stop the carrier recombination. In addition, another important factor for a lower efficiency of the all of the inorganic perovskites is limited absorption of solar radiation (below 600 nm). Now to tackle the dual problem concept of cosensitization can be incorporated where with the perovskite material another II−VI quantum dot material can be used. Due to suitable band energy level alignment, a better charge separation can take place through interfacial electron- and holetransfer reactions. Synergistically, both perovskite and II−VI QDs can behave as supersensitizer and absorb more solar light
as compared to individual material. Earlier, we have demonstrated ultrafast charge-transfer dynamics between CPB NCs and dibromo fluorescence (DBF) molecule where the photoexcited hole from CPB was extracted by DBF and the photoexcited DBF injected an electron into the conduction band (CB) of CPB, resulting grand charge separation.45 Earlier, we have also demonstrated the ultrafast charge-transfer dynamics of II−VI quantum dots after sensitization with different molecular adsorbates.46−48 However, till date, chargetransfer dynamics between the perovskite NCs and II−VI QDs have never been discussed in literature. Herein, we are reporting the electron- and hole-transfer dynamics between photoexcited CsPbBr3 (CPB) perovskite nanocrystals (NCs) and CdSe QDs by ultrafast time-resolved absorption and luminescence spectroscopy. The synthesized CsPbBr3 (CPB) has been characterized by high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), and steady-state optical studies. The redox band alignment of CPB NCs49 and CdSe QDs50 suggests that the electron transfer from CB of CPB NCs to CB of CdSe QD and hole transfer from valence band (VB) of CdSe QDs to VB of CPB NCs can concomitantly take place. Selective excitation through steady-state and time-resolved luminescence spectroscopic studies confirm the above processes. To determine the electron- and hole-transfer times in the above system, we carried out the femtosecond upconversion with femtosecond transient absorption (TA) studies and correlated the data. To reconfirm the above processes further, we carried out ultrafast 2707
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valence band (VB) maxima of the CPB NCs lie energetically higher and lower than the CB and VB of CdSe QDs, respectively.49,50 Therefore, upon photoexcitation at 400 nm light, both CdSe QDs and CPB can be excited, where photoexcited electron can be transferred from CPB NCs to CdSe QDs and photoexcited hole from CdSe QDs can be transferred to CPB, as both the processes are thermodynamically feasible. Now to determine the time scale of both electronand hole-transfer processes, time-resolved upconversion measurements have been carried out for both CdSe QDs and CPB NCs at their PL maxima position both in absence and presence of each other after exciting them at 400 nm laser light, as shown in Figure 2C,D. Figure 2C-a′ shows the upconversion decay trace of CPB measured at the wavelength of 487 nm and can be fitted multiexponentially with time components of τ1 = 5 (±0.2) ps (29%), τ2 = 60 (±2) ps (21%), and τ3 = >100 ps (50%) (Table 1, SI). Interestingly, the decay components become faster in the presence of CdSe QDs (Figure 2C-c′), which can be fitted with time components of τ1 = 0.55 (±0.1) ps (47%), τ2 = 2 (±0.25) ps (27%), τ3 = 30 (±1) ps (17%), and τ4 = >100 ps (9%) (Table 1, SI). The extremely fast component (550 fs) in the decay trace can be attributed to the electron transfer from CPB NCs to CdSe QDs as the process is energetically viable (Scheme 1).51,52 The PL decay trace of CdSe QDs measured at 570 nm is shown in Figure 2C-b′ and has been fitted multiexponentially with time constants of τ1 = 4 (±0.2) ps (24%), τ2 = 60 (±2) ps (23%), and τ3 = >100 ps (53%) (Table 1, SI). Interestingly, this decay kinetics becomes faster when CdSe QDs interacts with CPB NCs, as illustrated in Figure 2D-c″, which can be fitted multiexponentially with time constants of τ1 = 0.6 (±0.1) ps (29%), τ2 = 1.5 (±0.20) ps (30%), τ3 = 20 (±1) ps (23%), and τ4 = >100 ps (18%) (Table 1, SI). Herein, the faster decay component (600 fs) can be attributed to the hole-transfer time from CdSe QDs to CPB NCs.48 For a better understanding of this charge-transfer process in two different nanocrystals interface, we impose the band edge engineering concept. Therefore, we have deliberately carried out the charge-transfer interaction between CPB NCs and CdSe@CdS core shell, where core is CdSe QD. The steadystate optical absorption spectra of CPB NCs, CdSe@CdS core shell, and the mixture of these two are shown in Figure 3A-a−c, respectively. The absorption spectrum of the mixture (c) is not changed from pure CPB NCs or CdSe@CdS core shell, which signifies that there is no complexation between these NCs like CdSe QDs. Unlike CdSe, the steady-state PL spectrum of the mixture of CPB NCs and CdSe@CdS core shell is different. Figure 3B-a,b depicts the PL spectra of CPB NCs and CdSe/ CdS core shell, respectively, after 400 nm excitation. Figure 3Bc shows the PL spectrum of colloidal mixture of CPB and CdSe@CdS core shell. Similar to CdSe QDs (Figure 2B-c′), the PL spectrum of the mixture of CPB-CdSe@CdS core shell shows that the PL of CPB NCs is drastically quenched. However, the PL of CdSe@CdS core shell has not been quenched in the presence of CPB NCs, which is different from CdSe QDs. To realize the distinct charge (electron and hole) transfer processes, we have drawn the energetics of CPB and CdSe@CdS core shell in Scheme 2.49,53−55 The band energy alignment of the CdSe@CdS core shell suggests a quasi−type II structure in which valence bands of CdSe QD and CdS QD have a higher energy offset.56,57 Again, Scheme 2 suggests that the CB and VB of the CPB NCs lie energetically higher than the CB and VB of the CdSe@CdS core shell. Thus, upon
spectroscopic studies between CPB NCs and CdSe@CdS core shell and the results have been discussed.
2. RESULTS AND DISCUSSION To analyze the morphology of the synthesized CsPbBr3 nanocrystals, we carried out the HR-TEM and XRD studies. Figure 1A shows the HR-TEM images of the synthesized CPB NCs. The size (edge length) of the cubic NCs was measured to be 10 nm. The XRD pattern is shown in Figure 1B. Three major peaks (2θ) at ∼15, 21.69, and 30.5° for [100], [110], and [200] planes, respectively, reveal the cubic phase of CsPbBr3.14 To characterize the CPB NCs optically, we have performed the steady-state absorption and luminescence spectroscopic measurements. The first excitonic absorption of CPB NCs appears at 472 nm, as seen in Figure 1C, and the corresponding band edge PL peak appears at 487 nm, with a very high PLQY (∼85%, calculation shown in the Supporting Information (SI)). The main endeavor of the present study is to investigate the interfacial electron- and hole-transfer dynamics of CPB NCs in the presence of CdSe QDs and CdSe/CdS core-shell nanocrystals. A study of the ground-state interaction is one of the key measurements for such charge-transfer reactions. To examine the interaction between the CPB NCs and CdSe QDs, we have carried out the steady-state optical absorption and PL studies. Figure 2A-a−c shows the steady-state optical absorption spectra of CPB NCs, CdSe QDs, and colloidal mixture of CPB NCs and CdSe QDs, respectively, in toluene. We maintained the concentration ratio of CPB and CdSe QDs ∼1:1 (calculation is shown in the SI) to perform all of the optical studies. The absorption peaks at 472 nm for CPB NCs and at 555 nm for CdSe QDs are almost unchanged in the CPB/CdSe QDs mixture. The spectrum (d) shows the simple addition of (a) and (b), which is not very different from the spectrum (c) except in the blue region of the spectrum. A higher absorption in the blue region of the spectrum suggests that there might be interaction in the high-energy states of both CdSe QD and CPB NCs. Figure 2B-a shows the steady-state PL spectrum of CPB NCs with peak maxima at 487 nm having ∼85% PLQY in toluene. On the other hand, the band edge emission peak maxima of the CdSe QDs appear at 570 nm (Figure 2B-trace b), with PLQY ∼23% in toluene (calculation shown in SI). Figure 2B-trace c shows that on the addition to CPB and CdSe QDs, the emission intensity of CPB at 487 nm gets drastically reduced and disappeared at the CdSe QDs position (570 nm). The energetics of CPB and CdSe QDs drawn in Scheme 1 suggest that the conduction band (CB) and Scheme 1. Schematic49,50 Representation of Electron Transfer from CB of CPB NC to CdSe QD and Hole Transfer from VB of CdSe QD to CPB NC
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Figure 3. Steady-state, (A) UV−vis absorption, and (B) PL spectra of (a) CPB NCs, (b) CdSe/CdS core shell, and (c) colloidal mixture of CPB NCs and CdSe/CdS core shell. Spectrum (d) shows the simple addition of spectra (a) and (b). (C, D) PL upconversion decay traces of (a′) CPB NCs at 487 nm, (b′) CdSe/CdS core shell at 600 nm, and (c′, c″) mixture of CPB NCs and CdSe/CdS core shell at 487 and 600 nm, respectively.
To reconfirm the electron transfer from CPB to CdSe@CdS core shell and the restriction of hole transfer from core shell to CPB and to determine the time constants of charge-transfer processes, we have carried out femtosecond upconversion luminescence studies after exciting the samples at 400 nm. Figure 3C-a′ is the luminescence decay trace of CPB NCs at 487 nm, which we have already mentioned earlier (Figure 2Ca′) and has been fitted biexponentially (Table 1, SI). Interestingly, in the presence of CdSe@CdS core shell, the CPB PL decay is much faster, as shown in Figure 3C-c′, and can be fitted biexponentially with time constants of τ1 = 0.65 (±0.1) ps (64%) and τ2 = 15 (±0.75) ps (33%) (Table 2, SI). This faster component of the decay can be attributed to the electron transfer from CPB NC to CdSe@CdS core shell. On the other hand, the PL decay kinetics of CdSe@CdS core shell in the absence and presence of CPB NCs at 600 nm have been shown in Figure 3D-b′,c″, respectively. It is seen that the emission decay kinetics of the CdSe@CdS core shell are not changed significantly in the absence and presence of CPB NCs. The PL decay trace of CdSe@CdS core shell can be fitted multiexponentially with time constants of τ1 = 10 (±0.5) ps (21%), τ2 = 50 (±2) ps (31%), τ3 = 150 (±5) ps (3%), and τ4 = >1 ns (45%) in the absence of CPB NCs (Table 2, SI) and τ1 = 7 (±0.5) ps (33%), τ2 = 25 (±2) ps (25%), τ3 = 130 (±5) ps (7%), and τ4 = >1 ns (35%) in the presence of CPB NCs (Table 2, SI). The faster and longer components represent the trap state and the exciton recombination, respectively.58,59 However, little change in decay dynamics might be due to leaking of hole from the core of the CdSe QDs to the CPB NCs through the CdS shell. Therefore, in the present investigation, it has been observed that both electron and hole transfer take place simultaneously in the CPB and CdSe QDs system; however, in the case of CPB and CdSe@CdS core-shell system, electron transfer from photoexcited CPB NCs to core shell is facile, but the hole transfer from photoexcited CdSe@CdS core shell to CPB NCs is restricted. From a steady-state and time-resolved PL studies, it is confirmed that the electron transfer takes place from photoexcited CPB NCs to CdSe QDs and the hole is
Scheme 2. Allowed Electron-Transfer and Restricted HoleTransfer Processes from CPB NC to CdSe/CdS Core Shell and CdSe/CdS Core Shell to CPB NC, respectively49,53−55
photoexcitation, the photoexcitation of electron from CPB NC to CdSe@CdS core shell can take place, as the process is energetically favorable. However, photoexcited hole transfer from CdSe@CdS core shell to CPB NC is restricted due to the presence of CdS shell, which makes the process energetically unfavorable. In the case of CPB and CdSe QDs mixture, we have seen that the PL quenching of both CPB and CdSe QDs is due to electron transfer from CPB NCs to CdSe QDs and hole transfer from CdSe QDs to CPB NCs, respectively. Herein, the PL quenching of CPB NCs in the presence of CdSe@CdS core shell can be attributed as electron transfer from CPB NC to core shell. However, the PL intensity of the CdSe@CdS core shell decreases marginally in the presence of CPB, suggesting that the hole transfer is restricted from core shell to CPB NCs. The hole is confined in the core CdSe due to large VB offset energy core CdSe QD and shell CdS QD (Scheme 2). Although the hole-transfer process is not thermodynamically viable, we have observed a marginal PL quenching of the CdSe@CdS core shell, which can be attributed to the leaking of hole from the CdSe core of the core shell to CPB NC through CdS shell QD. 2709
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ACS Omega transferred from photoexcited CdSe QDs to CPB NCs. At this point, we would like to mention that it is important to find out the viability of energy transfer between CPB NCs and CdSe QDs and CdSe@CdS core shell as the PL spectra of CPB NCs nicely overlap with the absorption spectra of the QD materials. Therefore, the energy transfer from photoexcited CPB NCs to the QD materials is expected. However, in the steady-state PL spectra in both systems, we did not observe any increment in QD PL. So energy-transfer process in the above-mentioned systems can be ruled out. To reconfirm the electron- and hole-transfer dynamics between CPB NCs and CdSe QDs more precisely, femtosecond transient absorption spectroscopic studies have been carried out and correlated the electron- and hole-transfer dynamics with fluorescence upconversion data. Figure 4a shows
To monitor the electron- and hole-transfer dynamics between CPB NCs and CdSe QDs, we probed the bleach kinetics at the key wavelengths in different systems. Figure 5A-a
Figure 5. TA bleach dynamics of (a, b) CPB NCs and CPB/CdSe QD system at 477 nm and (c, d) CdSe QDs and CPB/CdSe QD system at 555 nm after 400 nm excitation.
represents the bleach kinetics of pure CPB NCs at 477 nm. The growth of the bleach kinetics can be fitted biexponentially with time constants of τ1 = 150 (±7.5) fs (70%) and τ2 = 1 (±0.1) ps (30%) (Table 3, SI), which can be attributed to the carrier cooling time from upper excitonic states to the band edge states of CPB NCs.51,61 Now, the bleach recovery kinetics can be fitted with time components of 5 (±0.25) ps (−26%), 70 (±3) ps (−35%), and >1 ns (−37%) (Table 3, SI), which can be attributed to the charge recombination dynamics between photoexcited electron and hole in CPB NCs. The bleach recovery kinetics matches nicely with the upconversion PL decay (Figure 7, SI). This observation suggests that majority of the charge carriers recombine through radiative process, which makes them a high-QY material.62 Now, it has been observed that bleach recovery kinetics at 477 nm drastically change in the CPB/CdSe QD system, as shown in Figure 5A-b, where the bleach growth time is found to be pulse-width limited (1 ns (−18%) (Table 3, SI). The bleach recovery found to be much faster in the CPB/CdSe QD system as compared to pure CPB NCs is attributed to the fast electron transfer from CPB NCs to CdSe QDs.51,62 Herein, the 0.65 ps component represents the electron-transfer time from CPB NCs to CdSe QDs, which closely matches with the electron-transfer time constants determined from the fluorescence upconversion data. Now, the transient bleach recovery kinetics for CdSe QDs and CPB/ CdSe QDs system were monitored at 555 nm, which is the first excitonic bleach peak for CdSe QDs, and are shown in Figure 5B. Figure 5B-c indicates the first excitonic bleach recovery kinetics of CdSe QDs that can be fitted with biexponential growth constants of 100 (±10) fs (75%) and 0.5 (±0.05) ps (25%) (Table 3, SI), where the longer growth component can be attributed to the electron cooling time from upper electronic states to the band edge states.58,59 Now, the 1S bleach recovery can be fitted multiexponentially with time constants of τ1 = 6 (±0.25) ps (−35%), τ2 = 100 (±5) ps (−21%), and τ3 = >1 ns (−44%) (Table 3, SI). The faster (6 ps) and longer (100 ps and > 1 ns) time components represent the carrier recombination
Figure 4. TA spectra of (a) CPB NCs, (b) CdSe QDs, and (c) mixture of CPB NCs and CdSe QDs at different time delays after 400 nm pulse excitation in toluene.
the TA spectra of CPB after 400 nm laser excitation at different time delays. We used very low pump intensity to minimize the multiexciton generation and pump fluency is calculated in the SI. The spectra at all of the time delays show a sharp negative absorption change (bleach) peaking at 477 nm, which can be attributed to the ground state excitonic absorption, which nicely matches with the steady state absorption spectrum (Figure 1A-a). In addition, the bleach photoinduced absorption (PA) was observed at 485−530 nm, which can be attributed to hot exciton induced absorption.30 The TA spectra of CdSe QDs consisting of two negative absorption bands (bleach) at 555 and 480 nm due to 1S and 1P electronic transition, respectively, are shown in Figure 4b. A weak photoinduced absorption band in the red region of the spectra suggests the presence of minimum-defect states in CdSe QDs. Similar to the steady-state optical absorption spectrum, the TA spectra of the CPB NCs and CdSe QDs mixture show individual spectral feature, which has been illustrated in Figure 4c. Herein, one negative absorption band appearing at 555 nm can be attributed to 1S excitonic bleach of CdSe QDs, whereas the second one, which is quite broad, can be attributed to the combination of excitonic bleach of CPB NCs and 1P excitonic bleach of CdSe QDs, where primarily the bleach is dominated by CPB NCs. It is interesting to see that the photoinduced absorption beyond 600 nm is very low for CPB NCs and CdSe QDs, attributed to the minimum-defect states in our materials. But this photoinduced absorption beyond 600 nm is increased in the composite system (shown in Figure 2, SI) compared to the isolated CPB NCs and CdSe QDs, which reveal an increase in the carrier density at the CdSe QDs transferred from CPB NCs.46,60 2710
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ACS Omega due to trapping and exciton recombination, respectively.58,59 Now, Figure 5B-d indicates the first excitonic bleach recovery kinetics of CdSe QDs in the presence of CPB NCs that can be fitted with biexponential growth constants of 150 (±10) fs (75%) and 0.4 (±0.05) ps (25%) and multiexponential recovery components with time constants of τ1 = 5 (±0.2) ps (−23%), τ2 = 22 (±1.5) ps (−34%), τ3 = 130 (±5) ps (−12%), and τ3 = >1 ns (−31%) (Table 3, SI). It is interesting to see that the growth component of CdSe QDs at the excitonic wavelength in the presence of CPB NCs does not change (Figure 5B-d), revealing that electron in CdSe QDs is not affected in the presence of CPB NCs. However, the bleach recovery found to be little faster as compared to pure CdSe QDs might be due to the hole transfer from CdSe QDs to CPB NCs. To investigate the charge-transfer dynamics in the CdSe@ CdS core shell in the presence of CPB NCs in ultrafast time scale, femtosecond transient absorption studies have been carried out for pure CdSe@CdS core shell and CdSe@CdS/ CPB NCs systems and are shown in the Supporting Information (Figure 6, SI). To follow the charge-transfer dynamics in the CdSe@CdS/CPB NCs composite system, we have monitored the bleach recovery kinetics at different excitonic wavelengths (477 and 590 nm) and compared them with the bleach recovery kinetics in the pure CdSe@CdS QDs and CPB NCs, respectively, and shown in Figure 6. Figure 6a,b
3. CONCLUSIONS In summary, the interfacial charge (both electron and hole) transfer between photoexcited CsPbBr3 (CPB) NCs and CdSe QDs and CdSe@CdS core shell has been demonstrated through steady-state and ultrafast time-resolved PL and absorption techniques. To start with, both CPB NCs and QD core shells were synthesized and characterized by HR-TEM and XRD measurement techniques. The energy-level diagram suggests that the electron transfer from the photoexcited CPB NCs to CdSe QDs and the hole transfer from CdSe QDs to CPB NCs are thermodynamically viable processes. Both the steady-state and time-resolved PL studies suggest the concurrent electron and hole transfers from CPB NCs to CdSe QDs and from CdSe QDs to CPB NCs, respectively. On the other hand, in case of CPB NCs and CdSe@CdS core-shell system, the photoexcited electron transfer from CPB NC to the core shell is found to be facile, whereas the hole transfer from core shell to CPB NCs is found to be restricted due to the presence of CdS shell. To monitor the charge-transfer dynamics in an ultrafast time scale, two complimentary techniques, femtosecond transient absorption and fluorescence upconversion techniques, were employed. The photoexcited electrontransfer times from CPB NCs to CdSe QDs and CdSe@CdS core shell were measured to be 550 and 650 fs, respectively. On the other hand, the hole-transfer time from the photoexcited CdSe QDs to CPB NCs is found to be 750 fs. The present investigation suggests that in CPB NCs and CdSe QDs composite system, a grand charge separation, where all of the electrons are localized to CdSe QDs and the holes are localized in CPB NCs, takes place on photoexcitation. These chargeseparated composites can be used as efficient materials for photocatalysis and photovoltaic applications. 4. EXPERIMENTAL SECTION 4.1. Materials. Cesium carbonate (Cs2CO3), lead bromide (PbBr2), cadmium oxide (CdO, 99.5%), selenium powder (Se, 99.99%), sulfur powder (S, 99.99%), octadecene (ODE, 90%), trioctyl phosphine (TOP, 90%), oleic acid (OA, 90%), and oleylamine were purchased from Sigma-Aldrich and used without further purification. tert-Butanol (tBuOH) (AR grade) was used to purify the crude product and toluene was used to disperse the purified samples. 4.2. Synthesis. 4.2.1. Synthesis of CsPbBr3 QDs. The colloidal CPB QDs has been synthesized after following the method by Kovalenko and his co-workers.14 To synthesize the CPB QDs, first we made the Cs-oleate stock solution. In the three-neck round-bottom (RB) flask, 1.25 mmol (∼0.41 g) Cs2CO3 was taken along with 20 mL ODE and 4 mmol OA (∼1 mL). To dry this precursor, it was heated at 130 °C for 1 h then further heated to 150 °C for the complete dissolution of Cs2CO3. The resulting solution formed Cs-oleate and was used as Cs precursor. In another three-neck RB flask, 0.188 mmol PbBr2 (∼0.069 g) and 5 mL ODE were taken and heated to 120 °C in vacuum for 1 h to dry the solution completely. After complete drying, the mixture was heated to 180 °C and 0.4 mL Cs-oleate was injected at that temperature. Finally, the reaction was quenched in ice bath within 1 min after the injection of Cs precursor. The synthesized CPB was cleaned by precipitation in tertiary butanol and dissolved in toluene for further use. 4.3. Synthesis of CdSe QDs and CdSe@CdS Core Shell. We have followed the high-temperature synthesis method reported by Peng and his co-workers to synthesize the
Figure 6. TA bleach dynamics of (a) CPB NCs at 477 nm, (b) CdSe@ CdS/CPB NCs at 477 nm, (c) CdSe@CdS core shell at 590 nm, and (d) CdSe@CdS/CPB at 590 nm after 400 nm excitation.
shows the bleach kinetics of CPB NCs at 477 nm in the absence and presence of CdSe@CdS core shell. The bleach dynamics of pure CPB NCs has already been discussed in the earlier part of this article. It is interesting to see that the bleach kinetics of CPB NCs at 477 nm drastically changes in the presence of CdSe@CdS core shell, which can be fitted with pulse-width-limited growth (1 ns (−7%) (Table 4, SI). Pulse-width-limited bleach growth kinetics clearly suggest the transfer of hot electrons from CPB NCs to the core shell. However, it is quite interesting to see that bleach recovery kinetics at 590 nm looks marginally different in the case of pure CdSe@CdS core shell and CPB/ CdSe@CdS core shell system. This observation clearly indicates that hole transfer from the core shell to CPB NCs is restricted and quite matches with the fluorescence upconversion data. 2711
DOI: 10.1021/acsomega.8b00276 ACS Omega 2018, 3, 2706−2714
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ACS Omega monodisperse CdSe QDs.63 In brief, first Se stock solution was prepared by dissolving 0.5 mmol Se in 1 mL TOP by ultrasonication. CdO (1 mmol) and OA (4 mmol) were taken in a three-neck RB flask along with 15 mL ODE and heated to 250 °C in an inert atmosphere for the complete dissolution, suggesting the formation of Cd-oleate. Then, the reaction mixture was heated to 280 °C and the Se stock solution injected in a single shot at that temperature. To get the desired size of CdSe QDs, the reacting solution was left at 280 °C for few minutes and then cooled down to room temperature. The synthesized CdSe QDs were cleaned by precipitation in methanol two to three times and dissolved in toluene for further studies. The CdSe/CdS core shell was synthesized after following the method reported by Peng et al.64 To synthesize the core shell, the newly synthesized CdSe core was used as the seed. First, Cd-oleate was prepared by refluxing the mixture of 54 mg of CdO, 800 μL of OA, and 200 μL TOP in 10 mL ODE at 180 °C in inert Ar gas atmosphere into a three-neck round-bottom flask. Then, the mixture was cooled down to room temperature. 12.5 mL of synthesized CdSe QDs was added into the Cdoleate and heated to 120 °C for 25 min to completely remove toluene. After that, this mixture was heated to 200 °C. Meanwhile, the S solution was made by dissolving 13.4 mg of S powder in 200 μL of TOP in 5 mL of ODE. This S solution was added dropwise into the reaction mixture at 200 °C for 1 h and left for 30 min for complete growth of the CdS shell. Then, the final mixture was cooled down to room temperature and precipitated by methanol and dissolved in toluene for further use.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b00276. Details about the instrumentation, HR-TEM image of CdSe and CdSe@CdS, transient absorption spectra of CdSe@CdS core shell, and all of the fitted tables (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected],
[email protected]. ORCID
Partha Maity: 0000-0002-0293-7118 Sourav Maiti: 0000-0003-1983-9159 Hirendra N. Ghosh: 0000-0002-2227-5422 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by “DAE-SRC Outstanding Research Investigator Award” (Project/Scheme No. DAE-SRC/2012/ 21/13-BRNS) granted to H.N.G. We acknowledge Prof. Anindya Dutta, Department of Chemistry, IIT Mumbai for providing fluorescence up-conversion. J.D. and S.M acknowledge CSIR and P.M. acknowledges DAE for research fellowship, respectively.
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