LETTER pubs.acs.org/NanoLett
Efficient Charge Separation in Multidimensional Nanohybrids Xiaohui Peng,† James A. Misewich,‡ Stanislaus S. Wong,†,‡ and Matthew Y. Sfeir*,§ †
Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794, United States Condensed Matter Physics and Materials Sciences Department and §Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States
‡
bS Supporting Information ABSTRACT: We report unidirectional charge transfer in multidimensional nanohybrids, consisting of a quantum dot, an electronically active molecular linker, and a carbon nanotube. After covalent attachment to the nanotube, only emission consistent with the negatively charged quantum dot exciton ion rather than the neutral exciton is observed, showing nearly monoexponential recombination kinetics and an average lifetime of 3.5 ns. Using kinetic models, we explain how charge transfer is biased at the expense of other decay pathways. KEYWORDS: DWNTs, CdSe nanocrystals, charged quantum dots, trion, charge transfer, resonance energy transfer
D
espite the promise of quantum dot (QD) based solar devices, their effective implementation has been limited. In particular, the charge extraction process, a crucial step for functional device operation, is often inefficient due to the spatial confinement of the electron and hole wave functions to the interior of the QD and their corresponding Coulomb interaction.1 Furthermore, this effect is exacerbated by the bulky insulating organic ligands used to control particle growth.2 Much of the literature on charge transfer interactions in these materials has been concerned with QD films, where surface modification of quantum dots with short ligand species has been shown to dramatically improve interdot tunneling rates3 with electron mobilities reaching as high as 16 cm2 V1 s1.4 The majority of these experimental results suggests that conduction in QD films is mediated by surface (defect) states, with only recent results using inorganic ligands showing evidence for strong electronic coupling.4 For example, transport measurements imply that the tunneling barriers between adjacent dots are smaller than would be expected for an insulating organic layer suggesting some electronic interaction (hybridization) between the organic ligand and the QD.2c In addition, the absorption red shift associated with QD films has been found to originate from polarization effects rather than from strong electronic coupling, most likely caused by a redistribution of carrier density to the surface of the QDs.5 On the other hand, in the context of a light harvesting application, surface modification has also been shown to have potentially detrimental effects upon the photoexcited exciton lifetime.6 This tension between the beneficial and detrimental effects of surface states is most evident in QD-sensitized solar devices.7 In this architecture, poor passivation of the QD surface can increase the amount of surface states that act as recombination centers r 2011 American Chemical Society
and decrease the chemical stability of the cell. As a result, the design of any QD based solar device based upon interfacial charge transfer, must simultaneously control (1) charge separation of the photogenerated exciton, (2) forward charge transfer, and (3) back transfer rates for maximum efficiency.8 Recent demonstrations of ligand-exchanged QD film photovoltaic devices based either on Schottky barriers9 or on heterojunction based approaches10 have suggested that such control is attainable and can be applied to more complex interfaces. Although the photophysics of various heterostructure-type architectures have been examined in the literature, a fully chargeseparated product has not been reported. QD-based charge transfer structures have demonstrated effective photoluminescence quenching, particularly in QDpyreneNT hybrid systems11 and QDNTpolymer hybrids,12 suggesting that efficient exciton dissociation and carrier transport can be achieved by properly tuning the electronic coupling of the donor and acceptor. However, apart from charge transfer, resonant energy transfer has also been found to occur between QDs and CNTs.13 While devices which exploit this process, caused by near-field dipolar coupling between QDs and corresponding acceptors, have been shown to be useful for solar harvesting,14 it is detrimental to a device based primarily upon charge transfer interactions. As charge and resonance energy transfer are competing donoracceptor interactions, it is essential to be able understand the design rules which can favor one process over the other.
Received: May 17, 2011 Revised: August 17, 2011 Published: October 07, 2011 4562
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Nano Letters Herein we apply surface modification concepts to a heterostructure of QDs and double-walled carbon nanotubes (DWNTs) and show that optical probes can be used to track the charge carrier interactions. We conclude that fast and efficient unidirectional charge transfer can be achieved under favorable electronic and geometric conditions. In effect, by using short, electronically active ligands between the QD and NT species, we are able to create intermediate “surface traps” whose formation is found to be crucial for attaining long-lived charge separation in the fully assembled heterostructure; i.e., kinetic control is achieved by preventing radiative recombination (within the QD) and resonance energy transfer (to the DWNT). We observe that small back transfer rates result in a nearly permanent charge separated state, such that only light emission corresponding to a charged QD exciton is observed in photoluminescence measurements. As such, our QD-CNT hybrids provide an interesting model for achieving efficient charge separation, thereby shedding light upon requirements for the design and fabrication of QDbased solar devices. CdSe QDs (d = 3.4 nm) capped with oleic acid have been prepared according to previously reported methods.15 For CdSeAET-DWNT composites, ligands of as-prepared QDs in solution were replaced by 2-aminoethanethiol (AET). Afterward, nanotubenanocrystal composites were synthesized by reacting CdSeAET QDs with oxidized DWNTs dispersed in dimethylformamide (DMF) in the presence of carbodiimide coupling reagents. A similar procedure was used to prepare CdSe/ ZnSAETDWNT composites. The composite samples used for optical measurements were freshly prepared and dispersed in ethanol by sonication. For the QDAET samples, the concentration in ethanol solution was adjusted to be similar to our best estimates of the QD concentration in the composite samples. Solutions were continuously stirred during data acquisition. Photoluminescence (PL) measurements were taken using a tunable high-repetition rate (250 kHz) Ti:sapphire-based amplified laser system for photoexcitation. Solutions were excited at grazing incidence with a 100 mm lens and PL was collected with a home-built optical microscope coupled to a spectrometer equipped with a liquid-nitrogen-cooled CCD camera (for spectra collection) and an avalanche photodiode (for time-correlated single photon counting measurements). All data were collected in the low-fluence regime where the decay kinetics were verified to be independent of laser power. More experimental details can be found in the Supporting Information section. In previous work, we have demonstrated the synthesis of CdSe quantum dots and DWNT heterostructures using a short aminothiol linker, namely, AET. We found that the replacement of the original oleic acid ligands on the surface of CdSe QDs with AET gave rise to the appearance of a strong trap emission in the near-infrared (NIR) region of the spectrum and that this emission subsequently disappeared upon conjugation with the DWNTs.16 We speculated that this quenching could arise from either a charge or resonance energy transfer interaction between the QD and DWNT. Here, we use time-resolved photoluminescence to unambiguously demonstrate that the charge transfer process dominates. The presence of the AET-induced NIR emission and associated exciton kinetics provides for a direct optical signature for the presence of deeply trapped carriers in the vicinity of the QD surface and allows us to monitor what happens to these trapped carriers upon functionalization. To allow for comparison with the results obtained on the QDAETDWNT heterostructures,
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Figure 1. (A) Steady-state photoluminescence spectra of CdSeAET (black) and of CdSeAETDWNT composites (blue). The exciton emission in the CdSeAETDWNT complex is approximately 7 weaker than the CdSeAET exciton emission. (B) Normalized exciton (orange at 599 nm) and trap (red at 795 nm) emission decay of CdSeAET on a log scale. The instrument response function is shown for reference (black).
we first discuss the photoluminescence features of the isolated, functionalized quantum dots in detail. Figure 1A shows the steady-state luminescence spectrum of oleic acid-capped CdSe QDs upon ligand exchange with AET. The spectrum consists of a relatively narrow feature at 599 nm, corresponding to the radiative recombination of band gap excitons and a broader emission band in the near-infrared range, peaking around 795 nm. In Figure 1B, we show time-resolved emission measurements for each of these bands on a loglog scale in order to highlight the different decay components and overall time window difference in the visible and NIR regions. The exciton luminescence (orange curve) is multiexponential with a relatively short total lifetime, with early time decay dynamics occurring faster than the instrument responsive function (black curve), which is indicative of a very efficient nonradiative contribution. On the other hand, the NIR trap emission (red curve) is found to exhibit a long lifetime relative to intrinsic radiative processes in CdSe QDs, with significant recombination still occurring on microsecond time scales. The recombination kinetics in the NIR 4563
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Nano Letters can be compared directly with quantitative models that consider the effect on the QD PL of the quantity and energy distributions of surface traps.17 Both the energy shift in the PL (>0.4 eV) and the decay kinetics are consistent with the presence of deep carrier traps, of the type that have been shown to dramatically affect the PL efficiency. This is consistent with reports of thiol ligands as hole acceptors since their redox energy level is situated at a noticeably higher energy level than that of the valence band of CdSe18 and due to the low electronegativity of sulfur as compared with oxygen in either oleic acid or trioctylphosphine oxide (TOPO).19 In the case of AET binding, the presence of weak NIR emission was also reported, accompanied by a dramatic decrease in the total PL rate. When the CdSeAET complex was covalently linked to DWNTs, the optical signature of trapped charges disappeared, affecting both the steady-state and time-resolved PL. As is noted above, the NIR emission band is no longer observable, although the visible exciton emission can still be clearly seen (Figure 1A). The emission of the excitonic state is considerably weaker than before covalent attachment to the DWNT. Our best estimates put the normalization factor at ∼7 (accounting for factors such as differences in laser power, count rate, and concentration). There is considerable uncertainty in this estimate; this is discussed further in the Supporting Information section. Furthermore, the decay kinetics of the exciton band now exhibits the signature of charged exciton (trion) photoluminescence (Figure 2, blue) rather than a species with a nearby trapped charge. These types of exciton ion (trion) complexes form upon photoexcitation of a charged QD and have been shown to be weakly emissive. In our system, the observed TRPL kinetics is consistent with previous reports of trion emission in both films20 and isolated21 CdSe-based QDs. In general, we observe similar biexponential kinetic behavior to what has been reported, with average lifetimes that are shorter than the neutral radiative lifetime (typically 59 ns). A fit to the luminescence decay curve of our covalent complex reveals that more than 90% of the emission is contained in a single exponential with a lifetime of ∼2.7 ns (dotted line in Figure 2). In addition, a small amount of carrier recombination is found to occur with a lifetime of ∼11 ns (leading to an average total lifetime of 3.5 ns). In all cases, the results were found to be independent of incident laser power. It is important to note that while previous reports have relied upon statistical fluctuations of the QD charge state for the observation of the trion luminescence, this species is isolated in our assembled heterostructure. The ability to directly compare solutions of the unbound individual components (neutral) and the covalently bound hybrid structure (charged) allows us to have high confidence in our assignment, as the emission recombination kinetics change from a complex multiexponential curve (Figure 2, gray) to a nearly monoexponential one (Figure 2, blue). This implies that the rate of the nonradiative contribution, usually attributed to Auger recombination involving the extra charge carrier, is no longer fluctuating in time. Instead, the emission rate appears to be time-independent, suggesting a long-lived (nearly permanent) charge separated state between the QD and the DWNTs. In fact, the exciton in the CdSeAETDWNT complex has a significantly longer average lifetime as compared with that associated with the unbound particles. Without accounting for a long-lived charged exciton, the presence of the nanotube would be expected to provide only additional nonradiative recombination channels, with the effect of shortening the overall total PL lifetime. These results imply
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Figure 2. Normalized time-resolved photoluminescence measurements of the exciton decay in CdSe-AET (gray) and CdSe-AET-DWNT composites (blue).
that the electronic interaction between the QD and the DWNT is dominated by a surface-mediated charge transfer process rather than by resonance energy transfer losses. To support our hypothesis above, we have analyzed the rates of different possible decay channels of the quantum dot excitation into the nanotube. There are two external nonradiative relaxation pathways of photogenerated excitons, namely, (i) resonance energy transfer (RET) of both charge carriers via a near-field electromagnetic interaction and (ii) charge transfer (CT) of one charge carrier. Both of these processes are highly dependent upon the physical separation between donor and acceptor. In the dipole limit, the RET rate is inversely proportional to the sixth order of magnitude of distance,13a whereas the interfacial charge transfer rate decays much more rapidly, falling off exponentially with the tunneling distance.22 Similarly, the disappearance of both direct (NIR emission) and indirect (fast, multiexponential PL dynamics) evidence for efficient surface trapping is unexpected in the absence of a faster de-excitation channel. We note that the rate of the initial surface trapping step will depend highly upon the number of traps and their energy distribution. In our material, we have tried to engineer an ultrafast trapping process, by maximizing the coverage of AET on the surface of the particle. As such, we would not expect this surface trapping process, thought to be localized at the sulfurQD interface, to be affected by the presence of the DWNT, which is bonded through the nitrogen atom of the AET. In order to explain the observed competition between surfacemediated charge transfer and resonance energy transfer, we calculate the distance-dependent rates and compare them with our experimental conditions. We start first with a calculation of the resonance energy transfer rates, shown as the black line in Figure 3. Since the physical size of the nanotube is larger than the surfacesurface distance (r) to the QD, the typical dipole approximation (and corresponding r6 rate dependence) is no longer valid. As such, we have followed the methodology of Swathi and Sebastian23 to more precisely treat the transition density in the nanotube. In this calculation, the QD is approximated as an ideal dipole, with a transition moment taken from calculations in Govorov et al.24 We have modeled the DWNT as two noninteracting semiconducting SWNTs, (14, 13) and (9, 8), with the appropriate chiral vectors to give a NTNT spacing of 4564
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Nano Letters
Figure 3. Calculations of the distance-dependent charge and energy transfer rates for an ideal dipole (p = 13 D) and a DWNT composed of (14, 13) and (9, 8) chiral vectors. The dielectric medium was set to be slightly higher than water (ε = 2).
∼0.34 nm. We have simplified the calculation by using the singleelectron picture to calculate the electronic structure of the nanotube (ignoring many-body effects). We argue that the error introduced by this treatment is compensated for by the variation in the type (metallic, semiconducting) and size (14 nm) of nanotubes present in our sample. If we assume that the exciton is located in the center of the QD, then we can determine the expected RET rate for the CdSeAETDWNT. The surfacesurface distance is 2.17 nm and the calculated rate is 1.3 ns1; this is ∼5 faster than the observed total lifetime (3.5 ns). Regardless of the exact numbers given by the calculations, it is abundantly clear that a faster overall rate should be observed upon binding to the DWNT, in contradiction with the experimental results. Next we turn our attention to the expected charge transfer rates. According to Marcus theory,25 the rate of CT between CdSe and CNTs can be estimated based on the free energy change of the reaction, the reorganization energy, and the electronic coupling strength. Importantly, the electronic coupling strength decays exponentially with distance. Therefore, the rate of charge transfer is expected to decrease much more rapidly with distance than that of energy transfer and can be expressed as kET = k0 exp(βr), where k0 is the rate at the contact distance for the donor and acceptor, r is the separation between donor and acceptor or the length of the bridge which is about 4.7 Å in our study, and β is a structure-dependent attenuation factor, which correlates the rate of electron transfer with the chemical structure of the bridge. In the case where the saturated hydrocarbon molecules are used as the bridge (e.g., alkanethiol), β has been reported in the range of 0.81.0 Å.26 Given the potentials of conduction band and valence band of CdSe and NTs as well as the dielectric properties of solvent, we obtain the distance scaling of CT shown in Figure 3 (red curve). The comparisons of the RET and CT scaling behavior are supportive of the hypothesis that we are observing predominantly trion emission as the observed changes in the PL kinetics cannot be explained by invoking a quenching process (CT or RET) alone. For example, if we assume that the carriers are located near the center of the QD, then the expected charge transfer rate is too low (104.28 As an additional check on our work, an analogous heterostructure in which RET is expected to dominate was prepared using coreshell CdSe/ZnS QDs after ligand exchange with AET. Given that the band gap of ZnS is wider than that of CdSe, forming a type-I coreshell structure, the electron and hole are expected to be more confined within the CdSe core by the ZnS shell. In particular, the hole wave function has a negligible probability of spreading into the ZnS layer,1a thereby lowering the probability of hole trapping onto the surface of QDs. For a material without an effective charge trapping step, we expect that RET will dominate the electronic interaction and effectively increase the total nonradiative contribution to the total lifetime. Similar experiments in this regime have recently been reported for CdSe/ZnS QDs on graphene.29 Experimentally, the asprepared CdSe/ZnS QDs exhibited highly efficient emission with nearly monoexponential decay due to the improved surface passivation provided by the ZnS shell. Upon AET ligand exchange, a multiexponential decay of the exciton of AET CdSe/ZnS suggested that the relaxed, lowest-lying core exciton 4565
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Nano Letters
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relaxation process affects each component identically ̅Γtot ½QD NT ¼ ̅Γtot ½QD þ ΓRET ;
¼
Figure 4. (A) The steady-state photoluminescence spectra of CdSe/ ZnSAET (pink) and CdSe/ZnSAETDWNT composites (blue). (B) Normalized time-resolved photoluminescence measurements of the exciton decay in CdSe/ZnSAET (pink) and CdSe/ ZnSAETDWNT composites (blue).
remains weakly coupled to the surface states. However, no obvious trap emission was observed in CdSe/ZnSAET, even upon an extensive ligand exchange process (Figure 4A). Similar to the uncapped QDs, covalent binding of the CdSe/ ZnSAET material to DWNTs resulted in a noticeable decrease in emission intensity. However, a significant difference is observed in the time-resolved photoluminescence data (Figure 4B), where the kinetics reflect the appearance of an additional quenching channel resulting in an overall smaller average total lifetime. Also by contrast with CdSeAET DWNT, the exciton decay of CdSe/ZnSAETDWNT remained a multiexponential process. On the basis of these overall observations, it is conceivable that the charge-separated state could not be formed within the CdSe/ZnS AETDWNT heterostructures due to the substantial impairment of the surface trapping process. As a consequence, RET became a more probable relaxation pathway and can account for the significant decrease of exciton emission. From the observed TRPL decays, it is possible to estimate the RET rate from the CdSe/ZnS QD to the DWNT and the overall decrease in quantum yield. Although the decays are multiexponential, we can assume that the additional nonradiative
∑i Ai τi ∑i Ai τi 2
̅Γtot ¼
1 τ̅
In the above equation, the amplitudes (A) and lifetimes (τ) are extracted from fitting each of the data sets (with and without DWNTs). From this analysis, we are able to satisfactorily fit the PL decay curves with three components. The lifetimes and amplitudes obtained by fitting are used to compute the RET rate; it is determined to be 0.22 ns1. This should give a corresponding reduction in the quantum yield of ∼2.3 before accounting for the absorption of the incident and emitted light by the presence of the broadly absorbing nanotube species. In practice, it was difficult to accurately resolve the absolute absorbance of the QDs from the DWNT baseline. However, our best estimate gives a change in PL intensity of ∼3.5. Using our kinetic models, the CdSe/ZnSAETDWNT complexes, with a surfacesurface distance of 3.17 nm, has a calculated RET rate of 0.3 ns1. This value matches very well with the experimental value (0.2 ns1) and is well within the expected error. Since we have already established that charge transfer from the interior of the dot is inefficient, these results explain why the charged-separated state can be formed in CdSe but not in CdSe/ ZnS complexes. Taken all together, these results provide strong evidence that an ultrafast trapping process (perhaps into a discrete molecular energy level) is a crucial intermediate in the formation of the observed long-lived charge-separated state, and an efficient solar device based on these inorganic nanostructures. We have demonstrated the formation of a long-lived chargeseparated state within DWNTCdSe heterostructures. This process is induced by hole-trapping ligands at the surface of CdSe together with fast interfacial charge transfer to a DWNT. A high coverage of AET on the surface of CdSe ensured not only the high efficiency of trapping process but also the high statistical probability that one particular type of charge carriers localizes near the interface. This is accompanied by the recovery of nearly monoexponential charged exciton photoluminescence in CdSe. Moreover, it cannot be overemphasized that precise control of both the proximity of QDs to the DWNTs and the surface chemistry is crucial for controlling the different kinetic pathways.
’ ASSOCIATED CONTENT
bS
Supporting Information. The experimental methods and materials. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT Research carried out in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy 4566
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Nano Letters Sciences, under Contract No. DE-AC02-98CH10886. SSW specifically acknowledges the U.S. Department of Energy Office of Basic Energy Sciences, Division of Materials Sciences and Engineering for support of additional spectroscopy work and for personnel support (XP and SSW) as well. We thank C. T. Black for useful discussions and feedback on the manuscript.
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