Impact of Shell Growth on Recombination Dynamics and Exciton

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Impact of Shell Growth on Recombination Dynamics and Exciton−Phonon Interaction in CdSe−CdS Core−Shell Nanoplatelets ACS Nano Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/07/18. For personal use only.

Alexander W. Achtstein,*,† Oliver Marquardt,‡ Riccardo Scott,† Mohamed Ibrahim,† Thomas Riedl,§ Anatol V. Prudnikau,∥,⊥ Artsiom Antanovich,∥ Nina Owschimikow,† Jörg K. N. Lindner,§ Mikhail Artemyev,∥ and Ulrike Woggon† †

Institute of Optics and Atomic Physics, Technical University of Berlin, Strasse des 17. Juni 135, 10623 Berlin, Germany Weierstraß Institute for Applied Analysis and Stochastics, Mohrenstraße 39, 10117 Berlin, Germany § Department of Physics, Paderborn University, Warburger Strasse 100, 33098 Paderborn, Germany ∥ Research Institute for Physical Chemical Problems of Belarusian State University, 220006 Minsk, Belarus ⊥ Physical Chemistry, TU Dresden, Bergstrasse 66b, 01062 Dresden, Germany ‡

S Supporting Information *

ABSTRACT: We investigate the impact of shell growth on the carrier dynamics and exciton−phonon coupling in CdSe−CdS core−shell nanoplatelets with varying shell thickness. We observe that the recombination dynamics can be prolonged by more than one order of magnitude, and analyze the results in a global rate model as well as with simulations including strain and excitonic effects. We reveal that type I band alignment in the hetero platelets is maintained at least up to three monolayers of CdS, resulting in approximately constant radiative rates. Hence, observed changes of decay dynamics are not the result of an increasingly different electron and hole exciton wave function delocalization as often assumed, but an increasingly better passivation of nonradiative surface defects by the shell. Based on a global analysis of time-resolved and time-integrated data, we recover and model the temperature dependent quantum yield of these nanostructures and show that CdS shell growth leads to a strong enhancement of the photoluminescence quantum yield. Our results explain, for example, the very high lasing gain observed in CdSe−CdS nanoplatelets due to the type I band alignment that also makes them interesting as solar energy concentrators. Further, we reveal that the exciton-LO-phonon coupling is strongly tunable by the CdS shell thickness, enabling emission line width and coherence length control. KEYWORDS: nanoplatelets, CdSe, strain, recombination dynamics, CdS platelets and their heterostructures.25,26 Also high exciton binding energies (of >100 meV) have been predicted and measured,3,14,22,27 confirming the presence of robust excitons even at room temperature. Recently core−shell nanoplatelets and similar heterostructures have received increasing attention due to a broad range of properties, which can be engineered in these heterostructures.28−33 In this paper, the impact of shell growth on carrier dynamics and exciton−phonon coupling in CdSe-CdS core−shell nanoplatelets is investigated in detail. The recombination dynamics of CdSe core-only nanoplatelets can be slowed down

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emiconductor nanoparticles receive growing attention due to their promising optoelectronic properties. Among them, semiconductor nanoplatelets and sheets1−6 are interesting due to their directed emission7 and polarization,8 fast radiative lifetimes9,10 related to the Giant Oscillator Strength effect3,11,12 allowing high quantum yields,13 promising lasing properties,14−16 and high two photon absorption.16−18 Further, a well width dependent high dark−bright splitting19,20 of 3−6 meV and first applications such as strong electroabsorption response21−23 as well as efficient field effect devices24 have been demonstrated. The unusually small coupling to phonons3 makes especially II−VI nanoplatelets potential candidates for materials with high lateral conductivity or low dephasing rates. Further, strong alterations of the emission spectra and decay times have been predicted for © XXXX American Chemical Society

Received: June 25, 2018 Accepted: September 4, 2018

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DOI: 10.1021/acsnano.8b04803 ACS Nano XXXX, XXX, XXX−XXX

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Cite This: ACS Nano XXXX, XXX, XXX−XXX

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ACS Nano by an order of magnitude depending on the CdS shell thickness. Using a global rate model, the observed changes are shown not to be caused by an increasingly differing electron and hole exciton wave function localization, e.g., as often expected, by a type II or I 1/2 junction. Instead, we find all the studied CdSe−CdS platelets to exhibit a type I band alignment, irrespective of the shell thickness, and the prolongation of the photoluminescence (PL) lifetime is caused by better passivation of nonradiative surface defects with increasing shell thickness. The intrinsic radiative lifetimes remain constant. Our results directly explain the low lasing thresholds of CdSe−CdS nanoplatelets. Apart from the timeresolved dynamics, the exciton-LO-phonon coupling is shown to be tunable by shell thickness. Here the overlap of the exciton wave function with the core phonon mode decreases with increasing shell thickness. Potential implications of the findings are discussed and results compared with theoretical modeling of the electronic states in the core−shell nanoplatelets.

RESULTS AND DISCUSSION Here, 4.5 monolayer (ML) CdSe nanoplatelets with 8−9 × 10 nm2 size were synthesized according to refs 2 and 34. Coreonly platelets were overgrown with 1−3 ML CdS by a colloidal layer-by-layer deposition technique.35 Representative absorption spectra are shown in Figure S1 of the Supporting Information. The samples were then embedded in poly(lauryl methacrylate-co-methyl methacrylate)-polymer on fused silica substrates and mounted in a cryostat. A pulsed Ti:Sa laser at 430 nm was used for excitation. Time resolved and integrated PL detection was realized by using a Streak camera and spectrometer. For details, see Methods. Figure 1 shows temperature dependent PL spectra of CdSen ML CdS (n = 0,1,2,3) nanoplatelets for two exemplary temperatures (4 and 300 K) together with representative TEM images of core only and CdSe-1 ML CdS nanoplatelets. The red-shift occurring with increasing shell thickness can be attributed to a reduction of the confinement related valence and conduction band offsets. The HOMO−LUMO gap of the CdS shell is smaller than that of the oleic acid ligands (≈8 eV9) and leads to a stronger delocalization of the CdSe core exciton into the CdS shell, resulting in an increasing red-shift. Additionally, as shown later by simulations (Figure 5), dielectric contrast effects to the ligand shell lead to a further red-shift of the emission. In line with recent reports, the coreonly nanoplatelets exhibit a double emission,9,33,36 with an origin that is still under debate. A temperature dependent red-shift of the PL emission can be observed. Figure 2a shows the temperature dependence of the emission maximum obtained from log−normal fits to the temperature dependent emission in the range of 4−300 K. A first observation is that the amount of the red-shift and with it the high temperature slope lowers with increasing CdS layer thickness. Fits to a semiempirical model for the band gap shift37,38 i 2 Emax = E0 − aepjjjj1 + θ / T e − k

yz zz 1 z{

Figure 1. PL emission of 4.5 ML core-only and 1−3 ML (CdS) CdSe−CdS core−shell nanoplatelets at 4 K (a) and 300 K (b). Clearly, a splitting can be observed for the core-only nanoplatelet at low temperatures in line with ref 9. (c) Transmission electron micrographs of 4.5 ML CdSe core-only (viewed onto the basal plane) and 4.5 ML CdSe 1 ML CdS nanoplatelets (viewed edgeon). The platelet thickness corresponds to 4.5 ML CdSe and 1 ML CdS on each basal plane. For TEM characterization of the other samples, see the Supporting Information.

Figure 2. (a) Extracted temperature dependence of the excitonic band gap deduced from PL for 4.5 ML CdSe platelets with 0 (low energy peak) and 1, 2, and 3 ML CdS shell as well as fits to eq 1. (b) Deduced exciton−phonon coupling strength aep and (c) average phonon temperature from the fits to eq 1 in (a).

(1)

with aep, the exciton−phonon coupling strength, and θ = ⟨ℏω⟩/kB, the average phonon temperature, are indicated as straight lines. The results are displayed in Figure 2b and c.

For the core-only platelets aep is not far from the 21 meV phonon coupling measured in MBE grown ZB CdSe B


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ACS Nano epilayers;39 however, the average phonon temperature is lower than in these epilayers (203 K). A lower θ can be related to a larger contribution of low energy acoustic phonons, since the coupling to acoustic phonons by deformation potential scattering scales with 1/Lz in 2D systems, with the well width Lz.40 aep decreases with the number of monolayers in Figure 2b, and with it the high temperature slope in (a). Conversely, a lower contribution of high energy optical phonons reduces the average coupling aep, most likely related to the reduced overlap of the CdSe core exciton wave function with the core optical phonon modes, defining the interaction strength.41,42 Hence, the exciton-LO-phonon coupling is reduced with the introduction of a shell due to the different localization volume of the exciton, analogous to results on CdSe−CdS quantum dots.43 Once the band offset is lowered for the CdS coating, the exciton spreads more into the CdS barrier so that the overlap and interaction with the core phonon modes is reduced with increasing CdS layer thickness. In the CdS layer, only CdS-like phonons exist, which, in the absence of nonlinear interaction, cannot contribute to the CdSe core’s phonon coupling and thus not influence the band gap renormalization. Further, we observe a decrease of the average phonon temperature (and with it the average phonon energy) with the number of CdS layers in Figure 2c. It can be interpreted in terms of the contributions of low energy acoustic and high energy optical phonons. The vibronic levels in the CdS are less populated as compared to CdSe at the same temperature, as, e.g., the LO phonon energy in CdS is considerably higher (∼38 meV44). Hence, once the platelet is overgrown, a reduction of coupling to polar LO-phonon modes (of energy 25 meV for ZB CdSe45) takes place for the reason discussed above. Acoustic modes, however, have low energies (in the low meV range) in both materials, so that CdS acoustic phonons still contribute to the band gap renormalization. Due to both described effects, acoustic phonons dominate the average phonon energy and temperature dependence for thick CdS shells, while in the thin shell and core-only systems a considerable part of the phonon interaction is contributed by high energy (e.g., LO and TO) phonons. Essentially, this behavior enables control of the dephasing and with it the emitter line width and coherence length by the CdS shell thickness. Figure 3 shows the normalized time-resolved PL emission of our samples at selected temperatures. The strong alteration of the PL decay with temperature is reduced with increasing CdS shell thickness. This is a first indication that temperature activated nonradiative channels may be strongly reduced by a shell overgrowth. A second observation is the nearly unaltered PL decay at low temperatures for the 0, 2, and 3 ML samples. The 1 ML sample shows slightly faster recombination dynamics at low temperature as compared to the 0 ML sample. The high temperature (300 K) dynamics is comparable to the 2 ML sample and prolonged with respect to the core only sample, due to better suppression of (thermally activated) nonradiative recombination through the shell. The faster decay of the 1 ML sample at 4 K may be indicative for (structural) defects in the one monolayer shell, which act as trapping centers at low temperature. If the shell growth is continued layer-by-layer, these centers may disappear prolonging the low temperature decay dynamics. Comparing the core only, 2 and 3 ML samples (and not taking into account the 1 ML sample for the reasons above), the nearly

Figure 3. Time-resolved PL dynamics of 0, 1, 2, and 3 ML shell platelet samples at different temperatures along with biexponential decay fits (lines) taking the convolution with the instrument response function into account. Prolongation of the decay dynamics with increasing CdS shell thickness is observed at elevated temperatures.

constant 4 K decay dynamics points to the hypothesis, that at very low temperature, when thermally activated nonradiative processes are less relevant, the radiative decay rates are nearly identical for the samples and independent of the CdS layer thickness. To investigate these findings, we analyze the results with a rate equation model, successfully applied to CdSe nanoplatelets.9 We model a three level system that consists of a crystal ground state (no exciton state), a ground state exciton state (GS), and an excited exciton state (ES); see the Supporting Information for a sketch of the level scheme. As a result, biexponential PL decays I(t) = A e−t/τF + B e−t/τS with a fast (index F) and a slow (index S) decay constant are obtained as solution to the rate equation system for the two emissive states ES and GS nES ̇ = −nES(Γ rES + Γ nr ES + γ0(nΔ + 1)) + nGSγ0nΔ

(2)

r nr nGS + Γ GS + γ0nΔ) + nESγ0(nΔ + 1) ̇ = −nGS(Γ GS

(3)

with nES,GS being the state populations, ΓrES,GS being the t radiative decay rate, and Γ inr = Γ inr,0 + Γ inr,00e−Δ Ei / kbT (with i = ES,GS) being the nonradiative decay rate (constants). nΔ = (eΔE/kbT − 1)−1 is a Bose occupation statistics factor reflecting the thermally activated LO-phonon scattering processes between GS and ES (and vice versa) with ΔE = 25.6 meV being the energy difference between both emissive states taken from the core sample’s PL. γ0 is the zero temperature ES → GS scattering rate. (See the Supporting Information for details.) Instead of using the biexponential decay lifetimes obtained from the well matching biexponential fits in Figure 3, we use C


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ACS Nano the decay constants λF,S = 1/τF,S, which are plotted in Figure 4. Further, we plot the time integrated PL emission (TI) versus

considerably altered by adding a CdS shell. As the core-only platelets form a type I band alignment to the oleic acid ligands with much higher HOMO−LUMO gap, the only weak alteration of the radiative rates for the core−shell platelets points also to a type I band alignment for all CdSe−CdS samples. Otherwise a strong alteration of the transition rate constant is expected due to the formation of a spatially indirect exciton with reduced e−h overlap. Rajadell et al.25 have predicted that, e.g., a variation of the conduction band offset results in only slight variations of the radiative rates for a type I alignment, while considerable effects are expected for type II alignments. In line with only slight alterations of the radiative rates we find that the ES→ GS exciton scattering rate stays constant as well, also pointing to no significant changes in the band alignment. A permanent dipole moment of a spatially indirect exciton would result in an alteration of the inter level scattering rates. This is in contrast to the observations, and can be excluded. The strongest effect of the CdS shell growth is an efficient passivation and suppression of nonradiative defect sites, which leads to a strong reduction of the nonradiative rates Γnr i (see Table 1). To model the overlaps and transition energies, we have computed the ground state electronic properties of the systems under consideration using an eight-band k·p model, taking strain, piezoelectricity, and excitonic effects into account.46 The elastic properties of the system were computed using linear elasticity theory47 and both formalisms were implemented within the Sphinx-software library.48,49 In the first step, simulations have been performed taking only bulk electronic properties into account. Second, strain and piezoelectric potentials were taken into account. Finally, we have incorporated ground state electron−hole interaction selfconsistently within the Hartree approximation.50 The respective transition (band gap) energies between the electron and hole ground states, Ψe and Ψh, are shown as a function of the thickness of the CdS shell in Figure 5a. It is seen that the CdS shell has a significant influence on the transition energies and that strain and excitonic effects have a significant influence on the electronic properties of the system under consideration and thus need to be taken into account. The exciton binding energies of ∼200 meV obtained from our modeling are in line with recent experimental and theoretical findings for CdSe core-only platelets.3,14,22,27 To quantify the contribution of electron−hole recombination inside and outside the CdSe core, we have computed the 2 ground state electron−hole overlap as 6 = |⟨Ψ|Ψ e h⟩| for the core, shell, and ligand region as a fraction of the total overlap. Table 1 shows a comparison of the obtained radiative and nonradiative rates and the e−h overlap integrals in the CdSe core (6C), the CdS shell (6S ), and the ligand layer 6L for the GS. In line with experiments, the overlap integral in the core,

Figure 4. Fast (λF) and slow (λS) decay rates for 0 ML (a), 1 ML (b), 2 ML (c), and 3 ML (d) shell samples obtained from biexponential fits to the PL decay transients. Time-integrated PL intensity (TI) and (for CdSe core-only platelets) the ES to GS time integrated emission intensity ratio (R) are also added. Global fits to our rate model are indicated as corresponding lines.

temperature to gain more insight into the nonradiative processes. In the core-only sample, the higher and lower emissive state can be resolved clearly. Because of higher inhomogeneous broadening, this is not possible in the core− shell samples. Hence, we plot the experimentally accessible time-integrated PL intensity ratio (R) of ES and GS additionally as a function of temperature only for the coreonly sample. As all the mentioned experimental quantities are fully described with our rate equation model (see the Supporting Information), we use a global model fit with shared parameters to fit the experimental data in Figure 4 for each sample. Table 1 shows the results of our global fits. A first unexpected observation is that the radiative rates of both states stay approximately constant among the four samples. Hence, apart from a slight deviation for the 0 ML core-only sample, the electron−hole wave function overlap is not Table 1. Results of Global Fits to Each Sample in Figure 4a CdS layers 0 1 2 3

ML ML ML ML

ΓrGS (ns−1) 25 23 23 26

± ± ± ±

2 2 2 2

ΓrES (ns−1) 66 83 81 76

± ± ± ±

6 8 7 7

−1 Γnr,00 GS (ns )

66 21 16 11

± ± ± ±

5 2 1 1

−1 Γnr,00 ES (ns )

245 149 64 23

± ± ± ±

20 13 4 2

γ0 (ns−1) 103 99 99 93

± ± ± ±

7 6 7 6

6C (%)

6S (%)

6L (%)

99.829 97.509 97.183 96.752

0.000 2.477 2.816 3.248

0.171 0.014 0.001 0.000

a Additionally, the electron-hole ground state overlaps inside the core, shell, and ligand, 6C , 6S, and 6L obtained from theoretical modeling are given for the respective shell thicknesses.

D


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Figure 6. Temperature dependent quantum yields (TI and TR) obtained from rate equation model for core-only and core-3 ML CdS nanoplatelets. Good agreement with reference data points from literature can be seen.

order of a few mV with their extrema outside of the CdSe core such that their influence on the electron and hole ground states is negligible. The ground state charge densities of electron and hole are shown for the core-only system and for a CdSe platelet with a 3 ML thick CdS shell in Figure 5b. While most of the charge density is inside the CdSe core, a visible nonzero charge density in the ligand or CdS shell remains. Focusing again on the results of Table 1, it shows that overcoating nanoplatelets with CdS improves their quantum yield (QY). To get a deeper insight in the temperature dependent QY, we compare in Figure 6 the temperature dependent quantum yield for the core-only and CdSe-3 ML CdS samples from calculations based on the obtained radiative and nonradiative rates in Table 1 (see the Supporting Information). We see that for the core-only nanoplatelets the QY strongly drops with temperature to slightly above 30% at room temperature. The value is in good agreement with measured quantum yields for heavy hole excitons in CdSe platelets at room temperature reported by Scott et al.17 (29%) and Ithurria et al.11 (30−40%). Also at low temperature, values above 85% have been reported.28 An increasing shell thickness can improve the QY to over 80% at room temperature for the 3 ML samples in quantitative agreement with reports from synthesis by Tessier et al.53 (80%) and She et al.15 These reference values plotted in Figure 6 clearly substantiate the validity of our approach. We further display in the figure a comparison of the quantum yields obtained by CW (TI) and fs (TR) (sub radiative lifetime pulse width excitation), where the QY is defined as the time integrated average number of photons emitted from both states per absorbed photon. As the ES and GS populations are time dependent for pulsed excitation, a slight alteration of the QY is observed. However, the simple CW approximation (TI), where the QY is defined as the sum of the radiative rate constants divided by the sum of all population decay related rate constants is used, turns out to be a very good approximation. The good agreement of the QYs and decay constants obtained from modeling the time-resolved and integrated data with literature and our calculations further substantiates the validity of our global fit model.

Figure 5. (a) Ground state transition energy as a function of CdS shell thickness taking an increasing number of influences, from only confinement to the excitonic effects, into account. (b) Electron (red) and hole (blue) ground state charge densities at 2% (transparent) and 20% (solid isosurface) of their respective maximum for a 4.5 ML thick CdSe platelet (dark gray) without and with a 3 ML thick CdS shell (light gray).

which is proportional to the radiative rate, is practically unaltered within the error margin. Hence, in the CdSe−CdS platelets there is, against expectations, no formation of an indirect band gap, so that the band alignment is still type I. Further, we see that the trend of a reduction of the nonradiative rates upon CdS coating can be explained mainly by the reduction of the wave function spread into the ligand layer, where nonradiative processes, e.g., by quenchers intercalated between the ligands can take place. The energy transport to those defects is most probably a more tunnelinglike phenomenon to a spatial distribution of trapping sites, so we do not expect a simple linear correlation between the nonradiative rates and the overlap integral. Nevertheless, it explains the observed trends in the nonradiative rates. We remark that the nearly unaltered wave function overlap and better passivation of nonradiative decay channels directly explains the improved, much lower lasing threshold of CdSe− CdS nanoplatelets reported, e.g., in refs 14, 15, 51, and 52. The spontaneous and stimulated emission rates are directly proportional via the Einstein relations.40 The high stimulated emission is a direct result of the type I character of the CdSe− CdS band alignment and the high electron−hole overlap in nanoplatelets. Further, the type I band alignment makes the CdSe−CdS platelets suitable for solar energy concentrators.30 Big shells can be grown, act as sensitizer, and transfer the photogenerated excitons to the core. The computed lowest exciton transition energies (at zero temperature) shown in Figure 2 are in good agreement with our measured PL data at T = 4 K. While strain leads to a blue shift of the transition energy of ∼ 75 meV for 3 ML CdS nanoplatelets, we note that piezoelectric potentials are of the E


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CONCLUSION In summary, we have investigated the impact of shell growth on the carrier dynamics and exciton−phonon coupling in CdSe−CdS core−shell nanoplatelets with varying shell thickness. We showed that the recombination dynamics can be prolonged by more than 1 order of magnitude. A global rate equation model and simulations including strain and excitonic effects have demonstrated a type I band alignment. This results in approximately constant electron−hole wave function overlap and radiative rates. Therefore, the alterations in the decay dynamics are not the result of increasingly spatially indirect excitons like often assumed, but an increasingly better passivation of nonradiative surface defects by the shell. This is substantiated by investigating the temperature dependent quantum yield based on a global analysis of time-resolved and time-integrated data. It shows that CdS shell growth leads to a strong increase of the PL quantum yield. Our results explain, for example, the very high lasing gain observed in CdSe−CdS nanoplatelets due to the type I band alignment and makes them interesting model systems as solar energy concentrators. Further, we reveal that the exciton−phonon interaction in CdSe−CdS nanoplatelets is strongly tunable by the CdS shell thickness and reduced for thicker shells. This will allow for exciton emission line width and coherence length control by the CdS shell thickness.

Artsiom Antanovich: 0000-0001-8533-7347 Mikhail Artemyev: 0000-0002-6608-0002 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS R.S., U.W., and A.W.A. acknowledge DFG Grants WO477-1/ 32 and AC290-2/1, M.A. the CHEMREAGENTS program, and A.A. BRFFI Grant No. X17KIG-004. Figure 5 was prepared using VMD.54 O.M. acknowledges support by Deutsche Forschungsgemeinschaft (DFG) via SFB 787. REFERENCES (1) Joo, J.; Son, J. S.; Kwon, S. G.; Yu, J. H.; Hyeon, T. LowTemperature Solution-Phase Synthesis of Quantum Well Structured CdSe Nanoribbons. J. Am. Chem. Soc. 2006, 128, 5632−5633. (2) Ithurria, S.; Dubertret, B. Quasi 2D Colloidal CdSe Platelets with Thicknesses Controlled at the Atomic Level. J. Am. Chem. Soc. 2008, 130, 16504−16505. (3) Achtstein, A. W.; Schliwa, A.; Prudnikau, A.; Hardzei, M.; Artemyev, M. V.; Thomsen, C.; Woggon, U. Electronic Structure and Exciton−Phonon Interaction in Two-Dimensional Colloidal CdSe Nanosheets. Nano Lett. 2012, 12, 3151−3157. (4) Wang, Q.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J.; Strano, M. Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699. (5) Gerdes, F.; Navío, C.; Juárez, B. H.; Klinke, C. Size, Shape, and Phase Control in Ultrathin CdSe Nanosheets. Nano Lett. 2017, 17, 4165−4171. (6) Nasilowski, M.; Mahler, B.; Lhuillier, E.; Ithurria, S.; Dubertret, B. Two-Dimensional Colloidal Nanocrystals. Chem. Rev. 2016, 116, 10934−10982. (7) Scott, R.; Heckmann, J.; Prudnikau, A. V.; Antanovich, A.; Mikhailov, A.; Owschimikow, N.; Artemyev, M.; Climente, J. I.; Woggon, U.; Grosse, N. B.; Achtstein, A. W. Directed Emission of CdSe Nanoplatelets Originating from Strongly Anisotropic 2D Electronic Structure. Nat. Nanotechnol. 2017, 12, 1155. (8) Yoon, D.-E.; Kim, W. D.; Kim, D.; Lee, D.; Koh, S.; Bae, W. K.; Lee, D. C. Origin of Shape-Dependent Fluorescence Polarization from CdSe Nanoplatelets. J. Phys. Chem. C 2017, 121, 24837−24844. (9) Achtstein, A. W.; Scott, R.; Kickhöfel, S.; Jagsch, S. T.; Christodoulou, S.; Bertrand, G. H.; Prudnikau, A. V.; Antanovich, A.; Artemyev, M.; Moreels, I.; Schliwa, A.; Woggon, U. p-State Luminescence in CdSe Nanoplatelets: Role of Lateral Confinement and a Longitudinal Optical Phonon Bottleneck. Phys. Rev. Lett. 2016, 116, 116802. (10) Kunneman, L. T.; Schins, J. M.; Pedetti, S.; Heuclin, H.; Grozema, F. C.; Houtepen, A. J.; Dubertret, B.; Siebbeles, L. D. A. Nature and Decay Pathways of Photoexcited States in CdSe and CdSe/CdS Nanoplatelets. Nano Lett. 2014, 14, 7039−7045. (11) Ithurria, S.; Tessier, M. D.; Mahler, B.; Lobo, R. P. S. M.; Dubertret, B.; Efros, A. L. Colloidal Nanoplatelets with TwoDimensional Electronic Structure. Nat. Mater. 2011, 10, 936−941. (12) Naeem, A.; Masia, F.; Christodoulou, S.; Moreels, I.; Borri, P.; Langbein, W. Giant Exciton Oscillator Strength and Radiatively Limited Dephasing in Two-Dimensional Platelets. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 121302. (13) Polovitsyn, A.; Dang, Z.; Movilla, J. L.; Martín-García, B.; Khan, A. H.; Bertrand, G. H. V.; Brescia, R.; Moreels, I. Synthesis of AirStable CdSe/ZnS Core−Shell Nanoplatelets with Tunable Emission Wavelength. Chem. Mater. 2017, 29, 5671−5680. (14) Grim, J. Q.; Christodoulou, S.; Di Stasio, F.; Krahne, R.; Cingolani, R.; Manna, L.; Moreels, I. Continuous-Wave Biexciton Lasing at Room Temperature using Solution-Processed Quantum Wells. Nat. Nanotechnol. 2014, 9, 891−895.

METHODS Here 4.5 monolayer (ML) CdSe nanoplatelets with 8−9 × 10 nm2 size were synthesized according to refs 2 and 34 and characterized by transmission electron microscopy (TEM) (JEOL ARM200F operated at 200 kV and Zeiss Leo 906E at 120 kV). In a second step, the coreonly platelets were overgrown with 1−3 ML CdS by a colloidal layerby-layer deposition technique.35 The final lateral size of the core− shell platelets increases from the CdSe-1 ML CdS core−shell platelets to the CdSe-3 ML CdS core−shell platelets from 10 × 11 to 18 × 22 nm2. (See also details of TEM characterization in Figure 1c and the Supporting Information, e.g., for the confirmation of layer-by-layer CdS shell growth.) The dispersions where precipitated with acetonitrile and redispersed in toluene. The procedure was iterated three times, and the samples were then embedded in poly(lauryl methacrylate-co-methyl methacrylate) polymer on fused silica substrates and subsequently mounted in a CryoVac Micro cryostat. A 150 fs, 75.4 MHz repetition rate pulsed Ti:Sa laser at 430 nm was used for confocal excitation with a 0.4 NA objective (≈2 W/cm2 CW equivalent excitation density). Time resolved and integrated photoluminescence (PL) detection was realized by using a Hamamatsu C5680 Streak camera and a Horiba IHR550 spectrometer with an attached LN2 cooled CCD.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b04803. Details of TEM characterization, absorption sprectra, rate equation model, simulations using eight-band k·p model and quantum yield derived from dynamics (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Alexander W. Achtstein: 0000-0001-8343-408X Anatol V. Prudnikau: 0000-0002-4088-4942 F


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