Article Cite This: Chem. Mater. 2018, 30, 5714−5725
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Overcoming the Complex Excited-State Dynamics of Colloidal Cadmium Selenide Nanocrystals Involved in Energy Transfer Processes Chenjia Mi, Mersedeh Saniepay, and Rémi Beaulac* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, United States
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S Supporting Information *
ABSTRACT: Semiconductor nanocrystals are often characterized by complex excited-state dynamics which reflect the inhomogeneous character of ensemble of nanocrystals. A new hybrid inorganic−organic donor−acceptor system involving CdSe nanocrystals and paramagnetic nitronyl nitroxide free radicals is shown to lead to efficient Förster (dipolar) resonance energy transfer. This transfer process, which is monitored by steady-state and time-dependent photoluminescence quenching experiments, occurs on a time scale similar to that of the intrinsic recombination in CdSe nanocrystals, allowing to unravel some of the complexity associated with the excited-state photophysics of semiconductor nanocrystals. A Stern−Volmer formalism that can handle the multiexponential nature of the time-dependent excited-state kinetics of CdSe nanocrystals is developed, leading to excellent agreement between steady-state and time-dependent photoluminescence data when a log-normal distribution model is used for the intrinsinc recombination rate constant and a Poisson distribution for the number of bound quenchers per emitter.
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INTRODUCTION Colloidal semiconductor nanocrystals (NCs), with CdSe as one of the most studied representative,1−3 are fascinating chemical objects with size-extensive electronic properties and are often proposed as ideal, or at least desirable, candidates for a plethora of applications involving the transfer or conversion of energy.4−8 Such commonplace statements prelude many studies dedicated to NCs, concealing the fact that these materials are also, perhaps irremediably, plagued by extremely complex excited-state dynamics that remain mostlyin some cases, completelymisunderstood in spite of decades of intense efforts aimed at controlling and mitigating the undesirable outcomes linked to these effects.9−14 Photoluminescence (PL) intermittency (so-called “blinking”) is perhaps the most readily identifiable one of these effects,15−18 which are generally attributed to photoinduced surface trapping processes that subsequently lead to various observable NC aging outcomes.15,18 Another direct manifestation of the complexity of CdSe NCs excited-state dynamics is the ubiquitous “multiexponential” character of their PL decay signals,19−22 which betokens the highly inhomogeneous character of NC ensembles where structural, environmental, and hysteretic variations lead to considerable variations on the energetics of charge-carrier recombination and trapping processes.11,12,23−25 A direct characterization of the underlying distributed chemical nature of NCs is a challenging task to undertake, and a general approach which is often adopted is to treat NC PL decays phenomenologically, with models that offer little to no physical © 2018 American Chemical Society
insight other than reproducing well the observed decay dynamics.19,21 Photoinduced donor−acceptor (DA) processes provide an excellent platform to investigate complex excited-state dynamics, and conveniently chosen chemical entities (acceptors) can be used to systematically perturb, and thus probe, the excited-state relaxation of NCs. Many DA studies involving NCs have now been reported, with either electron transfer26−33 or energy transfer34−45 as the dominant mechanism of interaction. Nevertheless, quantitative characterizations of DA processes involving NCs are generally challenging endeavors due to the complexity of the intrinsic photophysical behavior of NCs, with the consequence that studies based on steadystate (time-independent) quantities often do not lead to physical pictures that are consistent with those obtained from analyses of time-dependent data. Here, nitronyl nitroxide (NN) free radicals, which are wellknown molecular species,46−48 are used for the first time as molecular acceptors to probe the complex dynamics of CdSe NCs. As we demonstrate, an important characteristic exhibited by a specific NN radical (4-carboxyphenyl-NN, or CPNN) is the interesting fact that it binds quite strongly to the NC surface while interacting with CdSe NCs neither too strongly nor too weakly, as measured from simple PL quenching experiments. This intermediate regime of DA interaction Received: May 30, 2018 Revised: July 22, 2018 Published: July 23, 2018 5714
DOI: 10.1021/acs.chemmater.8b02271 Chem. Mater. 2018, 30, 5714−5725
Article
Chemistry of Materials
The dilution effect and inner-filter effect were later corrected with a mathematical method. NMR Measurements. 1H NMR spectra were collected on an Agilent DDR2 500 MHz NMR spectrometer equipped with a 7600AS 96 sample autosampler running VnmrJ 3.2A, using a 45° pulse angle, 10 s relaxation time sequence, and 32 scans. To a known concentration of NCs suspended in deuterated benzene was added ferrocene as an internal standard. The concentration of ligands was determined relative to ferrocene by integration of the terminal methyl (chemical shift of 1.2 ppm) and ferrocene (chemical shift of 4.0 ppm) peaks.50 EPR Measurements. EPR spectra were recorded on a Bruker ESP-300E X-band EPR spectrometer. The microwave frequency was set at 9.86 × 109 Hz, and the power was 12.63 mW with 12 dB attenuation. The modulation frequency was 100 kHz, and the time constant was 20.48 ms. Acquired data were averaged over 10 scans. For sample preparation, CPNN radicals (with/without NCs) were dissolved in toluene in quartz EPR tubes and were degassed with freeze−pump−thaw technique for five cycles of 5 min prior to the experiment.
allows to capture precisely the nature of the mechanism responsible for the PL quenching, attributed here to efficient dipolar energy transfer and described using a fully coherent analysis of both steady-state and time-dependent measurements based on Poissonian DA interactions and a log-normal distribution of intrinsic recombination rate constants for CdSe NCs.
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EXPERIMENTAL SECTION
Chemicals. All chemicals were used directly as purchased unless otherwise specified. Cadmium nitrate tetrahydrate, myristic acid, and oleic acid were purchased from Sigma-Aldrich. Selenium dioxide (SeO2), anhydrous methanol, and 1-octadecene (ODE) were purchased from Acros. Pentane was purchased from Macron and dried through alumina column. Ethyl acetate and dichloromethane (DCM) were purchased from Macron and dried with molecular sieves prior to use. All NMR solvents were purchased from Cambridge Isotope Laboratories. Syntheses. (i) CdSe NCs: the synthesis procedure was adopted from Cao et al.49 with slight modifications.50 In short, cadmium myristate (CdMyr2) and SeO2 were mixed with ODE, degassed with Schlenk technique, and heated under nitrogen while stirring to a temperature such that the desired size was reached. Oleic acid was added, and then the reaction was cooled to room temperature. Average diameters were obtained from comparison with empirical sizing curves.51 Detailed procedures and resulting product NMR and absorption profiles are described in the Supporting Information (text and Figure S1). (ii) CPNN radical: 2-(4-carboxyphenyl)-4,4,5,5tetramethylimidazoline-1-oxyl-3-oxide was prepared by adapting the procedure from Ullman et al.46 with modification. In short, bishydroxylamine species was coupled with aldehyde species to yield the imidazoline ring, followed by oxidation with NaIO4 to obtain the nitronyl nitroxide free radical. The detailed procedure is described in the Supporting Information (text and Figure S2). Optical Spectroscopy. UV−vis spectra were measured with an Olis 17 UV/vis/NIR spectrometer. Steady-state (continuous-wave) and time-dependent PL spectra were measured with a home-built spectrometer, consisting of Horiba iHR monochromators and a Horiba Symphony II CCD detector. The PL quantum yield was measured with an integration sphere (Hamamatsu Quantaurus, C11347). (i) Steady-state PL instruments and parameters: the excitation light source was a tungsten lamp followed by an iHR 320 monochromator. The beam was focused at the center of the 1 cm sample cuvette. The emission was collected at 90° angle to the incident beam with a CCD camera (Horiba Jobin−Yvon Symphony II, liquid N2-cooled) after an iHR 550 monochromator. (ii) Timedependent PL instruments and parameters: the time-dependent PL decays were measured with a T900 (Edinburgh) time-correlated single photon counting (TCSPC) card. The excitation light source was a 405 nm diode laser from Picoquant (LDH-D-C-405M). The laser beam coincided with the steady-state excitation beam, although during the time-dependent measurements the CW beam was blocked. The laser pulse frequency was 125 kHz for samples i and ii and 1 MHz for samples iii and iv. The laser intensity was adjusted so that the photon emission rate (stop rate) was smaller than 3% of the pulse frequency to ensure single photon counting within each time window and to minimize multiple excitation. The emission was collected by a PMT detector (Hamamatsu H7422-40) through the same iHR 550 monochromator. PL Quenching Experiment. For a typical quenching study, 2.000 mL of CdSe NCs suspended in dichloromethane (DCM) was added into a 1 cm cuvette with a small magnetic stir bar. The UV−vis absorption, steady-state, and time-dependent PL emission spectra were measured. During the steady-state and time-dependent PL measurements the suspension was under stirring to minimize the impact from photoannealing and photodeposition of NCs. Then each time 20.0 μL of CPNN DCM solution was titrated directly into the cuvette with a micropipet. UV−vis absorption, steady-state, and timedependent PL emission were measured again and after each titration.
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RESULTS Figure 1a shows the absorption and PL spectra of 3 nm CdSe NCs, and Figure 1b shows the absorption spectrum of the
Figure 1. (a) UV−vis absorption (blue) and PL emission (red) spectra of CdSe NCs (3.0 nm diameter, 1.57 μM in DCM). (b) Spectral overlap between the PL emission (red dashed line, areanormalized intensity units) of the CdSe NCs and the absorption profile of CPNN (blue solid line, molar absorptivity units), whose structure is shown here.
CPNN free radical studied here (structure shown in the inset). This free radical is characterized by a single band in the visible, characteristic of NN radicals with aromatic substituents. Assuming that the whole band corresponds to a single electronic transition (there are in fact two in there),52 a total oscillator strength of ∼5 × 10−3 is estimated, consistent with the weakly allowed n → π* character of this transition.52,53 We note that this oscillator strength is almost 2 orders of magnitude larger than for the analogous transition of 5715
DOI: 10.1021/acs.chemmater.8b02271 Chem. Mater. 2018, 30, 5714−5725
Article
Chemistry of Materials
Figure 2. (a) PL spectrum of d = 3.0 nm CdSe NCs ([NC] = 1.57 μM in DCM), with different amounts of added CPNN, final [CPNN] between 0 and 12.8 μM in DCM, corresponding to [CPNN]/[NC] ratios of 0 to ∼9, after correction for dilution effects. (b) Decrease of the integrated PL intensity with increasing concentration of CPNN. Each point gives the area-integrated PL intensity relative to that of NCs without quencher (PL intensities corrected for dilution effect; see text for details). The dashed curve is a guide to the eyes. (c) PL decay dynamics (red curves) of the spectra shown in (a), underscoring the highly multiexponential nature of the excitonic recombination in these samples. (d) Average time of emission (purple diamonds) and time-integrated PL decay curves (red circles) extracted from the data in (c) (each decay curve was also normalized such that I(t=0) = 1, prior to the time-integration). Dashed curves are guides to the eyes.
Table 1. CdSe NCs Photophysical Data NC sample a
i ii iii iv
NC diam (nm)
λ1Sb (nm)
± ± ± ±
539 573 571 587
2.99 3.74 3.68 4.16
0.30 0.37 0.37 0.42
[NC] (μM) 1.6 0.9 0.9 1.5
± ± ± ±
0.2 0.1 0.1 0.2
Φ0c (%) 16.0 16.7 6.7 11.0
± ± ± ±
0.9 0.9 2.2 1.4
⟨t⟩0d (ns) 61.2 40.4 44.3 34.4
± ± ± ±
2.2 1.9 2.3 1.4
KSVe (mM−1) 223 283 127 90
± ± ± ±
2 3 2 2
a
Sample shown in the main part of the text; the other samples are shown in the Supporting Information. bWavelength of the 1Se1S3/2 peak of the NC sample absorption profile, the uncertainty is ±1 nm for all samples. cPhotoluminescence quantum yield in the absence of quencher. dAverage time of emission in the absence of quencher, obtained from eq 1. eStern−Volmer constant, obtained from eq 4 and the data in Figure 3 (for sample i, other samples shown in the Supporting Information).
TEMPO.54 As shown in Figure 1b and as will be discussed in further detail in the last section of the Discussion, the spectral overlap between this absorption feature and the PL transition of the CdSe NCs is conducive to efficient energy transfer from photoexcited CdSe NCs to CPNN. Upon addition of CPNN, the overall PL intensity of the CdSe NCs decreases, as shown in Figure 2a. The effect is relatively strong, and micromolar concentrations are sufficient to lead to quantitative PL quenching: with about three radicals added per NC, the PL can be reduced to about half the starting intensity. This PL quenching efficiency, as measured in a radical-per-NC basis, is at least 2 orders of magnitude larger than that observed for 4-amino-TEMPO, another nitroxide free radical that can interact with CdSe NCs.54 As shown below, the striking difference between the two nitroxide radicals is due not only to the large difference in the binding affinity of amines vs carboxylates but also to a fundamental change in the mechanism of quenching between TEMPO and NN radicals. It is important to note that although the spectral overlap between the CdSe NCs and CPNN radicals could in
principle lead to direct absorption of both the excitation beam and the CdSe NC PL to yield an apparent quenching of the PL,55,56 at the low concentrations used here this “inner-filter” effect is practically negligible, accounting for less than 0.3% of the observed quenching. Nonetheless, both inner-filter and dilution effects have been corrected for based on the CPNN absorption profile and the experimental titration method. The PL quenching effect can also be directly quantitatively characterized in the time domain, as shown in Figure 2c. As typically observed for CdSe NCs,20,21,57 the excitonic recombination is characterized by strong deviations from single-exponential decay dynamics associated with simple firstorder relaxation processes, which are signatures of complex underlying inhomogeneities from NC to NC, leading to highly distributed excitonic recombination rate constants.19,21,23,25,58 As described in the Discussion section, such decays can be fitted empirically with phenomenological models such as multiexponential decays. Alternatively, the overall dynamics of complicated decays can be characterized without any specific a priori model by using the average time of emission values, 5716
DOI: 10.1021/acs.chemmater.8b02271 Chem. Mater. 2018, 30, 5714−5725
Chemistry of Materials
∫ I0(t ) dt I(t = 0) jij I0 zyz = j z ∫ I(t ) dt I0(t = 0) k I {TD
which corresponds to the first moment of the experimental decay curve:19 ∞
⟨t ⟩ =
∫0 I(t )t dt
(3)
indicating the negligible role that ultrafast (shorter than 1 ns here) or very long (longer than 1 μs here) processes appear to play in the overall PL quenching observed here. In other words, it is clear that the same overall physics is adequately captured by the steady-state and time-dependent data reported in Figures 2a and 2c, respectively. Performing a linear fit on the low concentration range (
