CdS Quantum Dots with Room Temperature Biexciton

May 20, 2015 - Purification of Single Photons from Room-Temperature Quantum Dots. Shih-Wen Feng , Chun-Yuan Cheng , Chen-Yeh Wei , Jen-Hung Yang , Yen...
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Gradient CdSe/CdS Quantum Dots with Room Temperature Biexciton Unity Quantum Yield Michel Nasilowski,†,‡,§ Piernicola Spinicelli,†,‡,§ Gilles Patriarche,⊥ and Benoît Dubertret*,†,‡,§ †

Laboratoire de Physique et d’Etude des Matériaux (LPEM), PSL Research University, ESPCI-ParisTech, 10 rue Vauquelin, F-75231 Paris Cedex 5, France ‡ UMR 8213, CNRS, F-75005 Paris, France § UPMC Univ Paris 06, Sorbonne Universités, F-75005 Paris, France ⊥ Laboratoire Photonique et Nanostructures, CNRS, 91460 Marcoussis, France S Supporting Information *

ABSTRACT: Auger recombination is a major limitation for the fluorescent emission of quantum dots (QDs). It is the main source of QDs fluorescence blinking at the single-particle level. At high-power excitation, when several charge carriers are formed inside a QD, Auger becomes more efficient and severely decreases the quantum yield (QY) of multiexcitons. This limits the efficiency and the use of colloidal QDs in applications where intense light output is required. Here, we present a new generation of thick-shell CdSe/CdS QDs with dimensions >40 nm and a composition gradient between the core and the shell that exhibits 100% QY for the emission of both the monoexciton and the biexciton in air and at room temperature for all the QDs we have observed. The fluorescence emission of these QDs is perfectly Poissonian at the single-particle level at different excitation levels and temperatures, from 30 to 300 K. In these QDs, the emission of high-order (>2) multiexcitons is quite efficient, and we observe white light emission at the single-QD level when high excitation power is used. These gradient thick shell QDs confirm the suppression of Auger recombination in gradient core/shell structures and help further establish the colloidal QDs with a gradient shell as a very stable source of light even under high excitation. KEYWORDS: Quantum dots, quantum yield, blinking, biexciton, multiexcitons, gradient core/shell

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recombination increases with the number of charge carriers present in the NC, and higher-order recombinations are usually very efficiently quenched by the nonradiative Auger mechanism. However, for several applications where optical pumping (lasing) or high brilliance (light-emitting diodes, LEDs) is needed, a high QY of the biexciton or higher-order multiexcitons is required.20 So far, the best result reported for the biexciton QY in colloidal NCs is 40%, averaged on all the studied QDs.17,21 In the present study, we report a new generation of thick shell CdSe/CdS QDs that show, in air and at room temperature 100% QY for the biexciton for all the dots we observed. In addition, they present a completely suppressed blinking, no Auger recombination at least for the charged monoexciton and the biexciton, and a QY of 100% for the monoexciton as well as for the biexciton, strongly reduced Auger recombinations for higher-order multiexcitons and an emission color at the single-particle level that strongly depends

emiconductor nanocrystal quantum dots (QDs) present unique electronic and optical properties.1 Once the synthesis of colloidal QDs has been developed with a fine control of the dimensions of the nanoparticles,2 many studies have focused on improving the brightness of these colloidal nanocrystals (NCs), that is, on increasing their quantum yield (QY). The limitation for high QY is mainly due to Auger recombinations3,4 that lead to QD emission intensity fluctuations at the single-particle level, which can result, for example, in switching between bright, radiative states and dark, nonradiative states. Several strategies have been developed to reduce the Auger recombination to increase the QY of the exciton and suppress blinking. These include surface modifications,5 thick-shell QDs,6,7 gradient structures,8−10 and plasmonic heterostructures.11 Some QDs have been synthesized with strongly diminished blinking and high QYs, even close to unity for the monoexciton,12 but their emission still shows characteristic drops in intensity.13−15 All these works show that a lot of effort has been put into improving the QY, but most of the studies have so far focused on the QY of the monoexciton. Only few recent studies have addressed the issue of biexciton QY.16−19 Indeed, the efficiency of Auger © XXXX American Chemical Society

Received: March 2, 2015 Revised: May 6, 2015

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Figure 1. Optical and structural characterization of QDs. Top panel: b-QDs. Bottom panel: thick-shell QDs. (a, c) PL and absorbance spectra. (b, d) (left) High annular angular dark-field STEM images and (right) EDX profiles along the red lines shown in panels b and d (left).

Figure 2. Time traces of single QDs at different excitation powers. Left panel: b-QDs. Right panel: thick-shell QDs. (a, b) Low excitation power. (c, d) Higher excitation power. (a, left) Intensity trace of a typical individual b-QD. Excitation wavelength: 405 nm. Repetition rate: 1 MHz. (a, right) Normalized occurrences and Poissonian fit of trace in panel a, left. (b, left) Intensity trace of a typical individual thick-shell QD. Excitation wavelength: 405 nm. Repetition rate: 1 MHz. (b, right) Normalized occurrences and Poissonian fit of trace in panel b, left. Both traces were measured at ∼0.05/μs, which represents the average number of excitons per QD per microsecond. (c, d) Higher excitation power. (c, left) Intensity trace of a typical individual b-QD at < N > ∼0.2/μs. Excitation wavelength: 405 nm. Repetition rate: 0.5 MHz. (c, right) Normalized occurrences and Poissonian fit of trace in panel c, left. (d, left) Intensity trace of a typical individual thick-shell QD at ∼0.25/μs. Excitation wavelength: 405 nm. Repetition rate: 1 MHz. (d, right) Normalized occurrences and Gaussian fit of trace in panel d, left. Excitation wavelength: 405 nm. Repetition rate: 1 MHz. For panels a−d, different QDs were analyzed on different configurations of the optical setup. Bin time is 20 ms.

strongly reduced Auger process at cryogenic temperature (below 30K), yielding 100% quantum-yield-QDs at cryogenic temperature.22 The PL of those thick-shell QDs is presented in Figure 1, panel c, and exhibits an emission peak at 667 nm with a full-width at half-maximum (FWHM) of 36 nm. Figure 1, panel d shows scanning transmission electron microscopy (STEM) images of the thick-shell QDs. Their size in the longest dimension is around 30 nm. Figure 1, panel d, right, presents an energy-dispersive X-ray spectrometry (EDX) profile of the sulfur, selenium, and cadmium contents along

on the excitation, yielding white-light emitting QDs at high excitation power. The NCs studied in this article (hereinafter referred to as bulky-shell gradient QDs, or b-QDs) are presented in Figure 1, panels a and b. They are compared to a previous generation of thick-shell CdSe/CdS NCs, which are a benchmark in terms of stability and optical properties, as described in Javaux et al.22 (hereinafter referred to as thick-shell QD) (Figure 1c,d). The intensity trace of those QDs shows no blinking at the singleparticle level, but shows some flickering and the QDs exhibit a B

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Figure 3. Time traces of single QDs in air and under vacuum and autocorrelation functions of PL intensity. (a, left) Intensity traces of typical individual b-QDs in air and under vacuum measured at ∼0.5/μs. Excitation wavelength: 405 nm. Repetition rate: 1 MHz. Bin time is 50 ms. (a, right) Normalized occurrences and Poissonian fit in air and under vacuum of traces in panel a, left. (b, left) Intensity traces of typical individual thickshell QDs in air and under vacuum measured at ∼0.5/μs. Excitation wavelength: 405 nm. Repetition rate: 1 MHz. Bin time is 50 ms. (b, right) Normalized occurrences in air and under vacuum and Poissonian fit under vacuum of traces in panel b, left. (c) g(2) measurements for typical b-QDs. < N > ∼0.025/μs. Excitation wavelength: 405 nm. Repetition rate: 125 kHz. (d) g(2) measurements for typical thick-shell QDs. < N > ∼0.2/μs. Excitation wavelength: 405 nm. Repetition rate: 1 MHz. Bin time is 2 ns for panel c and 3 ns for panel d.

When observed with the eye under UV light on a microscope, the emission of the bulky-shell gradient QDs is completely stable. The trace does not show any fluctuations and seems to be brighter that the best QDs we studied earlier in the group.22 For the thick-shell QDs, at low excitation power, two states can be seen on the trace (Figure 2b). Both states are emissive, but their intensity is different. The brightest state has been attributed to the neutral monoexciton, the dimmest state to the negatively charged exciton.24 The QY of the so-called gray state was measured to be around 40%; for the bright state, the QY has been shown to be at 100%.22 Although these dots are nonblinking, as those two states are radiative, the flickering of the emission between those two states decreases the overall QY of the QD. In contrast, for the b-QDs, the emission trace is perfectly stable and shows only one state without any blinking or flickering (Figure 2a). The intensity distribution can be fitted by Poisson statistics, confirming there is no blinking even at short time scales, in air and at room temperature.25 The complete absence of flickering suggests that Auger processes are bypassed and that the overall QY of those QDs is close to unity for the monoexciton emission. To determine the QY of the b-QDs, we compared the emission intensities of several (∼15) thick-shell and b-QDs in the same conditions of excitation (i.e., ∼0.05/μs, which is the mean number of excitons per QD per microsecond19) (Figure 2a,b). The emission intensity of the only state in the b-QDs perfectly corresponds to the 100%-QY state of the thick-shell QDs. We conclude that the b-QDs have a single emissive state with a 100% QY for the monoexciton at room temperature. At higher excitation power (Figure 2d), more excitons are produced in the NCs, and thus the probability for the QD to get charged increases. In this excitation regime, the thick-shell QDs emission trace oscillates between two states. This results in intense flickering. On the opposite, the trace of the b-QDs

the red line in the STEM image in Figure 1, panel d. The selenium is detected in a region that corresponds to the CdSe core only. No selenium is detected in the CdS shell. On the basis of the STEM images and the EDX profiles, we can infer that the typical thickness of the CdS shell is approximately 12 nm, that is, 40 MLs of CdS deposited on the CdSe core. Figure 1, panel a shows the photoluminescence (PL) and absorbance spectra of the bulky-shell gradient QDs. Their absorbance spectrum exhibits similar features to the thick-shell QD spectrum due to the presence of a large CdS shell. Their emission peak is around 685 nm, and their FWHM is around 55 nm. The emission at lower energy suggests that the quantum confinement effect in these NCs is weaker or that delocalization of charges is more effective.23 Concerning the bQDs’ FWHM, it is larger than that of thick-shell NCs and also larger than the FWHM of other core−shell CdSe/CdS QDs reported in the literature,7,22 suggesting the synthesis might yield QDs with some composition variations. Figure 1, panel b, left, shows STEM images of the b-QDs. The final diameter of those structures is approximately 40 nm, exceeding the size of the thick-shell CdSe/CdS synthesized so far. On the basis of the size of the core after its synthesis (3.2 nm in diameter), we can estimate the thickness of the shell to be around 18 nm on average, that is, approximately 60 monolayers (MLs) of CdS deposited on the CdSe cores. Figure 1, panel b, right, presents the EDX profile along the red line shown in Figure 1, panel b, left. Contrary to what is shown in the profile in Figure 1, panel d, the Se is not only present in a small region like in the thickshell QDs, but also is distributed along approximately 15 nm, while the size of the CdSe cores after synthesis is only 3.2 nm in diameter. In addition, the percentage of selenium decreases slowly on each side of the core, suggesting that during the synthesis of the CdS shell on the CdSe cores, a gradient between the core and the shell materials formed. C

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Figure 4. PL spectra of individual QDs at increasing excitation powers. (a) b-QDs. (b) Thick-shell generation QDs. /μs represents the number of excitons per microsecond per QD. Excitation wavelength: 405 nm. Repetition rate: 40 MHz.

Figure 5. Normalized integrated PL intensity of b-QDs at different temperatures. Top panel: ensemble measurements. Bottom panel: single-QD measurements. (a) Integrated PL intensity from 300 K to 16 K. (b) Evolution of PL decay from 300 K to 16 K. Excitation wavelength: 377 nm. Repetition rate: 100 kHz. (c) Mean of PL intensity of ∼70 single QDs at 300 K in air, in vacuum, and at 30 K. (d) PL decay on a single QD at 300 K in air, in vacuum, and at 30 K. Excitation wavelength: 405 nm. Repetition rate: 250 kHz. (e) g(2) measurements for one b-QD at 30K. Excitation wavelength: 405 nm. Repetition rate: 250 kHz. Bin size is 20 ns.

confirm the presence of a single emissive state, which is the same in air or under vacuum. Here, we focus on the relative QY of the biexciton (XX) versus the monoexciton (X). Figure 3, panels c and d present measurements of the autocorrelation function of the PL intensity, or g(2). For the thick-shell QDs (Figure 3d), under weak excitation, the g(2) measurements yield a ratio QYXX/ QYX28 of around 30%. The QYXX is thus 1/3 of the QYX at room temperature, which confirms that Auger processes are still present in these QDs and are more efficient for the biexciton than for the monoexciton, as already shown by other studies.18,29 On the other hand, in the bulky-shell gradient QDs (Figure 3c), the g(2) measurements yield a ratio QYXX/ QYX close to 100% for all the dots observed (∼30) (we checked that the QDs we observed were individual particles by

remains perfectly stable with a shot-noise limited intensity profile (Figure 2c), suggesting that the emission intensity of the b-QDs does not differ whether the QD is charged or neutral. This observation was confirmed with the study of the QDs’ emission under vacuum. It has previously been shown22 that under vacuum, the thick-shell QDs remain charged,26,27 and their emission stays in a gray state (Figure 3b) with a QY of only 40% for the monoexciton. In the case of b-QDs, even under vacuum, the emission level remains unchanged compared to the emission in air (Figure 3a). This confirms that the emission of b-QDs is insensitive to the presence of O2 and or H2O on their surface. In the right panel of Figure 3, panel a, the normalized occurrences are presented for both the traces in air and in vacuum. Both of them have Poisson statistics that D

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Nano Letters a correlative fluorescence imaging, TEM study, see Supporting Information). This means that the monoexciton and the biexciton have identical QYs. As we showed earlier, QYX ≈ 100%, which leads to QYXX ≈ 100%. To study the QY of higher-order multiexcitons, we deposited a diluted film of QDs on a glass slide and observed the PL of single dots under increasing laser power excitation at room temperature in air. The resulting spectra are shown in Figure 4, panel a and compared to the thick-shell QDs (Figure 4b). In the thick-shell QDs, the emission maximum of the main peak slightly shifts to higher energies when excited at higher power (Figure 4b). This is due to multiexcitons recombining from higher energy levels.30 Besides, higher-order multiexcitonic recombinations occur at higher energies involving even higher energy levels. Nevertheless, the high-energy emissions remain quite low in intensity, especially compared to the main peak intensity. Figure 4, panel a presents the same study on the bulky-shell gradient QDs. The same behavior of the main emission peak with increasing excitation power can be seen, but the behavior at high energies is very different from that of the thick-shell QDs. We can see an emission feature that appears at high energies and at a lower excitation power than for the thick-shell QDs. When the excitation is further increased, the emission peaks become even more visible and reach an intensity similar to that of the main peak. Thus, at high excitation powers, those QDs emit white light. A similar effect has already been observed by Klimov’s group.21 In contrast, in their work, the QY of the biexciton was on average much lower (∼40%) than that for our b-QDs, and the appearance of high-energy emission bands (less intense than in the case of our QDs) occurred at higher excitation powers. The ability to see on the emission spectra high-energy features much more intense and at lower excitation powers compared to the thick-shell QDs is further proof of a strongly reduced Auger process for high-order multiexcitons. The more charges in the QDs, the higher the efficiency of the Auger process. This is the main reason why even at high excitation powers, the emission peaks due to multiexcitonic recombinations are only slightly visible. For these NCs, even if the Auger processes are strongly reduced at low excitation (thus for the trion) at cryogenic temperature, they are still very efficient at room temperature, especially for multiexcitons. On the other hand, for bulky-shell gradient QDs, the emission peaks of the multiexcitons can be as intense as the emission of the main peak. This is another suggestion that the Auger processes in those QDs are efficiently suppressed for high excitonic orders at room temperature (see Figure S4, Supporting Information). Figure 5, panel b shows the PL decay in an ensemble measurement of b-QDs at different temperatures, and Figure 5, panel d shows the same measurement on a single QD. The decay is multiexponential, consistent with the fact that even at low excitation powers, multiexcitons are created in the b-QD. Compared to QDs presented in the literature so far, the lifetimes are much longer: 4 μs, 800 ns, and 150 ns at 300 K in air. These long lifetimes, especially the longest one, suggest that there are charges trapped in the NC.31 When studied under vacuum, the lifetime does not change, which is in agreement with the fact that the Auger processes are suppressed for the trion and the biexciton; thus, no nonradiative channels are opened under vacuum when the QD is forced to stay charged. When studied at low temperature, the lifetimes decrease to 1.5 μs, 350 ns, and 100 ns. This is consistent with the fact that the

band gap alignment of the CdSe and the CdS changes at low temperature, confining the charges in the core.22 Besides, for thick-shell QDs, reducing the temperature increases the PL intensity level, which corresponds to an increase in QY (up to 100% at 30K). This is due to the fact that changing the band gap alignment confines the charges, preventing them from reaching the surface, where Auger recombinations can occur. On the contrary, for b-QDs, we do not observe any PL intensity increase with decreasing temperature (Figure 5a,c) despite the modification of the band alignment (as seen by the decrease of the lifetime). This is indeed the expected behavior for QDs in which Auger processes are completely absent already at room temperature. In conclusion, we have presented a new generation of bulkyshell gradient CdSe/CdS colloidal QDs with 100% QY for the monoexciton as well as for the biexciton. They do not present any flickering and have only one emissive state, whose intensity does not change under vacuum and at cryogenic temperature. The g(2) measurements confirm that the QY of the biexciton is the same as the QY of the monoexciton, which has been shown to be at 100%. Multiexcitons of higher order also have high QYs, yielding white-light-emitting QDs at higher excitation power and making these QDs excellent candidates for several optical applications such as lasing or light-emitting devices.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed synthesis, supplementary EDX profiles and mapping, evolution of PL vs power, and correlative light and electron microscopy. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.nanolett.5b00838.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Al. Efros, J.-P. Hermier, L. Biadala, and M. Bayer for fruitful discussions and thank Agence National de la Recherche for funding through Grant SNAP.

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REFERENCES

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