Low-Threshold Stimulated Emission Using Colloidal Quantum Wells

Apr 28, 2014 - Similar results were obtained for NPLs with different thicknesses of the CdS shell, with no clear dependence of the threshold on shell ...
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Low-Threshold Stimulated Emission Using Colloidal Quantum Wells Chunxing She,†,○ Igor Fedin,†,○ Dmitriy S. Dolzhnikov,† Arnaud Demortière,‡,§ Richard D. Schaller,∥,⊥ Matthew Pelton,*,∥,# and Dmitri V. Talapin*,†,∥ †

Department of Chemistry and James Frank Institute, University of Chicago, Chicago, Illinois 60637, United States Electron Microscopy Center, Argonne National Laboratory, Argonne, Illinois 60439, United States § Physics Department, Illinois Institute of Technology, Chicago, Illinois 60616, United States ∥ Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States ⊥ Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States # Department of Physics, University of Maryland, Baltimore County, Baltimore, Maryland 21250, United States ‡

S Supporting Information *

ABSTRACT: The use of colloidal semiconductor nanocrystals for optical amplification and lasing has been limited by the need for high input power densities. Here we show that colloidal nanoplatelets produce amplified spontaneous emission with thresholds as low as 6 μJ/ cm2 and gain as high as 600 cm−1, both a significant improvement over colloidal nanocrystals; in addition, gain saturation occurs at pump fluences 2 orders of magnitude higher than the threshold. We attribute this exceptional performance to large optical cross-sections, slow Auger recombination rates, and narrow ensemble emission line widths. KEYWORDS: Nanoplatelets, core/shell nanocrystals, stimulated emission, colloidal quantum wells, optical gain that can be obtained in a close-packed film of these nanocrystals. In contrast, low-threshold lasing has long been obtained using epitaxially grown quantum wells (QWs).17−20 In these structures, carriers are confined in only one dimension, and Auger recombination rates at ASE thresholds are expected to be lower than those in QDs. However, the techniques generally used to produce QWs, such as molecular beam epitaxy,21 are expensive and have low throughput; moreover, only a relatively small number of QWs can be stacked above one another, limiting the volume of gain medium that can be produced. Recently, methods have been developed for the colloidal synthesis of thin, flat semiconductor nanocrystals, including nanoribbons,22,23 quantum disks,24 and nanoplatelets25−27 (NPLs). Carriers in these flat nanocrystals are confined in only one dimension, making them the colloidal equivalent of QWs.26,28 Time-resolved optical measurements have shown that high-energy carriers in NPLs relax to the band edge on time scales much faster than recombination times,29 as needed for stimulated emission and lasing. In this work, we demonstrate ASE using close-packed films of NPLs and show that ASE thresholds are lower and gain saturation is higher than for other semiconductor nanocrystals. We study both CdSe NPLs and CdS/CdSe/CdS shell/core/

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ver since highly luminescent colloidal semiconductor nanocrystals, or quantum dots (QDs), were first synthesized,1 there has been an abiding interest in using them as laser gain media, due to their tunable emission wavelengths, low cost, and solution processability.2 However, nearly a decade passed before the first demonstrations of optical gain3−5 and lasing6−8 from colloidally synthesized QDs. These were achieved only under pulsed excitation at high energy densities: thresholds on the order of 1 mJ/cm2 or higher for amplified spontaneous emission (ASE) in films of QDs were usually observed,7,9−11 although recent advances12−14 have pushed the thresholds down to less than 100 μJ/cm2. The high thresholds were attributed to Auger processes: optical gain requires excitation of more than one exciton in each QD, but multiple excitons undergo rapid, nonradiative Auger recombination.15 Attempts to reduce thresholds therefore focused on reducing the effects of Auger recombination. For example, charged QDs16 or carefully engineered core/shell QDs9 can have spectrally separated absorption and emission energies, enabling gain without excitation of multiple excitons. Alternatively, nanocrystal heterostructures can be engineered for reduced Auger recombination rates.12 Reduced Auger rates and lower thresholds were also obtained for QDs embedded in thick shells with different shapes, including rods,11 tetrapods,14 and spheres13 (or “giant” QDs). In this case, optical absorption at the excitation energy is also increased,13 leading to thresholds as low as approximately 26 μJ/cm2. The lower threshold, however, comes at the expense of the maximum obtainable gain because the thick shells reduce the maximum exciton density © 2014 American Chemical Society

Received: February 28, 2014 Revised: April 16, 2014 Published: April 28, 2014 2772

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Figure 1. (a) Schematic structures of CdSe and CdS/CdSe/CdS nanoplatelets (NPLs). Cd, Se, and S atoms are in blue, red, and green, respectively. (b) Absorption spectra of CdSe and xCdS/CdSe/xCdS (x = 1, 3, and 8) NPLs in hexane. (c,d) Transmission electron microscope images of (c) CdSe and (d) 3CdS/CdSe/3CdS NPLs, viewed from the top. The scale bars are 10 nm. Insets are photographs of luminescence from solutions of NPLs in hexane.

Figure 2. (a) High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image of an 8CdS/CdSe/8CdS NPL, viewed from the side. The scale bar is 3 nm. (b) Line profile from the HAADF-STEM image. The sharp lines correspond to the positions of Cd planes, and the gaps between the sharp lines correspond to S planes in the shell region and Se planes in the core region. (c) Elemental mapping by energy dispersive X-ray (EDX) spectroscopy along the red line in panel a, showing Se in the core region, S in the shell region, and Cd in both regions.

shell NPL heterostructures, illustrated in Figure 1a. We start with CdSe NPLs with lateral dimensions of 27 ± 3 and 6.8 ± 1.5 nm (see Figure 1c), synthesized according to published procedures25,29 (see the Supporting Information for details). The thickness of the platelets is 1.2 nm, equivalent to 4 complete monolayers (MLs) of CdSe with an additional layer of Cd atoms, so that both sides of the NPL are Cd-terminated (see Figure 2). The NPLs form stable colloidal solutions (see the inset in Figure 1c), which show very narrow excitonic peaks in absorption (see Figure 1b and Supporting Information Figure S2) and emission (see Supporting Information Figure S1), with an emission maximum at 2.42 eV (512 nm).25,26

NPL heterostructures, shown in Figure 1d, are produced by growing CdS shells with thicknesses from 1 to 8 MLs on either side of the CdSe NPLs, using the technique of colloidal atomic layer deposition (c-ALD, see the Supporting Information for details).30 We refer to the resulting structures as xCdS/CdSe/ xCdS, where x corresponds to the thickness of the shell in MLs. As shown in Figure 1b, the absorption and emission spectra of the NPLs shift continually to lower energy as x increases, due to the extension of the electron wave function into the CdS shell (see Supporting Information Figures S1 and S2 for more examples). The c-ALD process enables the growth of an atomically flat CdS shell with ML control over thickness.30 This is illustrated in Figure 2 for a single 8CdS/CdSe/8CdS NPL. 2773

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Figure 3. (a,b) Emission spectra from films of (a) CdSe and (b) 3CdS/CdSe/3CdS NPLs for different pump fluences. (c) Normalized integrated emission intensity from the film of 3CdS/CdSe/3CdS NPLs as a function of pump fluence. The dots are experimental data, and the red lines are two linear fits for different regions of pump fluence. The inset shows a histogram of the amplified spontaneous emission (ASE) thresholds for different films of 3CdS/CdSe/3CdS NPLs; each film showed similar thresholds from multiple spots.

Figure 4. (a) Integrated emission intensity, I, as a function of stripe length, l, from a film of 3CdS/CdSe/3CdS NPLs, for a pump fluence of 200 μJ/ cm2. The dots are experimental data, and the solid line is a fit to I = Io + A(egl − 1)/g, where A is a constant proportional to the spontaneous emission power density and g is modal gain. (b) Integrated emission intensity as a function of pump fluence, showing saturation of ASE at high pump fluences. The dots are experimental data, and the red line is an exponential fit; saturation is defined as the fluence at which the intensity reaches half of its maximum value. The inset shows the same data on a linear scale.

(An NPL with a thick shell was chosen in this case to clearly illustrate the shell/core/shell structure.) In order to observe ASE, we deposited CdSe NPLs on glass substrates by spin coating; the resulting films are optically uniform and transparent, with a thickness of 168 ± 19 nm, a refractive index at 632 nm of 1.7 ± 0.2, and thus a packing density of 50 ± 15%. (See the Supporting Information for details.) We excited the films with frequency-doubled pump pulses from an amplified Ti:sapphire laser system, focused to a stripe along the sample. (See the Supporting Information for details.) When the pump fluence exceeds a certain threshold, a narrow peak due to ASE appears on the lower-energy side of the broader photoluminescence band, and the intensity of this peak increases rapidly with pump fluence. This is shown in Figure 3a for a film of CdSe NPLs that has a threshold fluence of approximately 17 μJ/cm2. This ASE threshold is lower than the lowest reported values for complex QDs specially optimized for low-threshold lasing.12,13

Even lower thresholds are obtained for the NPL heterostructures. Figure 3b,c shows representative data for a film of 3CdS/CdSe/3CdS NPLs with a thickness of 135 ± 13 nm, a refractive index at 650 nm of 1.9 ± 0.1, and thus a packing density of 64 ± 6%. For this film, the ASE threshold is 8.6 μJ/ cm2. Similar results were obtained for NPLs with different thicknesses of the CdS shell, with no clear dependence of the threshold on shell thickness (see Supporting Information Figure S3); we therefore study in the following on 3CdS/ CdSe/3CdS NPLs. The inset of Figure 3c summarizes the results from 25 films. Out of these, two show thresholds below 10 μJ/cm2, and the best film shows a threshold of 6 μJ/cm2, more than 4 times lower than the best reported value for colloidal nanocrystals.13 The modal gain of the NPL films is estimated using the variable-stripe-length (VSL) method,31 as illustrated in Figure 4a. A fit to the data gives a gain of 600 ± 100 cm−1, approximately a factor of 4 larger than the largest values 2774

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Figure 5. (a) Dynamics of the lowest-energy excitons for 3CdS/CdSe/3CdS NPLs in hexane. The samples are excited with different fluences at a photon energy of 3.1 eV, and their transient absorption is probed at a photon energy of 1.98 eV. The signals are normalized to their values at long delay times. The inset shows examples of the dynamics obtained by subtracting the normalized data for a pump fluence of 0.7 μJ/cm2, where Auger recombination is absent. These multiexponential short-time dynamics correspond to Auger recombination, with 1/e decay times of 420 ± 30 and 510 ± 50 ps for pump fluences of 121 and 13 μJ/cm2, respectively. (b) Time-dependent photoluminescence from a film of 3CdS/CdSe/3CdS NPLs, showing fast decay due to Auger recombination at pump fluences below the ASE threshold. The samples are excited with a photon energy of 3.1 eV. The insets show the corresponding spectra, indicating the ASE threshold at approximately 40 μJ/cm2.

reported for QDs.7 Moreover, the gain saturates at very high pump fluences. Figure 4b shows gain saturation at a pump fluence of 800 μJ/cm2, more than 2 orders of magnitude higher than the ASE threshold. By comparison, gain saturation in QDs typically occurs for fluences about twice the ASE threshold.13,32 The lowest ASE thresholds we have observed for NPL films are comparable to or lower than the lowest reported thresholds for any solution-processed materials. For example, the best reported thresholds for semiconducting polymers are approximately 6 μJ/cm2,33 and typical thresholds are an order of magnitude higher. A very recent report showed an ASE threshold of 12 μJ/cm2 in films of organic−inorganic lead halide perovskites.34 The thresholds we have observed in NPL films remain at least an order of magnitude higher than the lowest thresholds obtained for vacuum-deposited organic semiconductors35 or epitaxial multiple-quantum-well structures;36 however, solution processing offers advantages in terms of cost and integrability, and the modal gain provided by the NPL films is at least an order of magnitude larger than those provided by the vacuum-deposited materials.36,37 The performance of QDs as gain media is believed to be limited by Auger recombination. We therefore used transient absorption measurements15 to determine Auger recombination rates in NPL solutions (see the Supporting Information for details). As shown in Figure 5a, increasing the pump fluence results in the emergence of fast decay components, characteristic of Auger recombination.15 By subtracting the dynamics of single excitons,15 we obtained Auger recombination that is nonsingle exponential, similar to the results obtained in transient photoluminescence measurements.38 The 1/e decay time is 510 ± 50 ps for a pump fluence of 13 μJ/cm2, significantly longer than the Auger lifetime for QDs with comparable emission wavelength.13 A recent report also

suggests that Auger lifetimes for CdSe NPLs can be as long as 10 ns.38 To complement the transient absorption measurements of Auger rates, we used a streak camera to measure the dependence of photoluminescence (PL) decay on pump fluence using NPL films (see the Supporting Information for details). To observe competition between Auger processes and ASE, we excited the samples at a series of pump fluences, both below and above ASE thresholds. To obtain a strong signal before reaching threshold, we used a NPL sample with a relatively high threshold of approximately 40 μJ/cm2. Figure 5b shows that there is a fast PL decay component with a time constant of 600 ± 100 ps, again corresponding to Auger recombination. The amplitude of the fast component increases with pump fluence below the ASE threshold, and then decreases at the threshold where ASE dominates (see Supporting Information Table S1). Above threshold, the luminescence is dominated by stimulated emission. The decay time for stimulated emission, corresponding to the gain lifetime, is limited by the instrument response time but is at least an order of magnitude shorter than the Auger recombination time. Auger recombination thus provides little competition with ASE, leading to low thresholds. Additional factors may also be responsible for the superior gain characteristics of the NPL films. One factor is likely the strong optical absorption of the NPLs at the pump-photon energy (3.1 eV): we measure an absorption cross-section of 3.1 × 10−14 cm2 for CdSe NPLs (see the Supporting Information for details). This large absorption cross-section is likely a result of the large physical cross-section of the NPLs. It is increased further by growing the CdS shell (see Supporting Information Figure S4): 3CdS/CdSe/3CdS NPLs have an absorption crosssection of 1.1 × 10−13 cm2, which helps explain their lower ASE 2775

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threshold compared to the CdSe NPLs. The shell also reduces the threshold by increasing the luminescence quantum yield of the NPLs.39 Another important factor in the low-threshold gain and high gain saturation is the high excitonic density of states available for stimulated emission. The large exciton binding energy in the NPLs results in a strong exciton emission peak at room temperature. The two-dimensional joint density of states for electron−hole pairs is concentrated into this narrow peak, providing a high gain coefficient at the exciton energy. Moreover, several excitons with the same emission energy can occupy the NPL,38 allowing the gain to increase well above the threshold before it saturates. This is in contrast to CdSe QDs, which can accommodate at most two band-edge excitons before addition excitation leads to the occupation of higherlying quantum-confined states.40 The high density of states at the exciton peak is further assured by the absence of inhomogeneous spectral broadening in the NPL ensemble. The synthesis of NPLs and the subsequent c-ALD growth of the shells result in an ensemble of atomically flat platelets that all have the same thickness, within a single ML, and thus all have the same emission energy. The absence of inhomogeneous broadening in NPL ensembles has previously been established through single particle measurements: the room-temperature emission line width for a single NPL is nearly identical to the line width for a solution of NPLs.35 It can also be seen by examining the photoluminescence excitation (PLE) spectra of the NPL solutions for different emission energies.30 If the NPL ensemble were inhomogeneously broadened, monitoring emission at a particular wavelength would select a subensemble of the NPLs, and the PLE spectra would vary depending on emission energy.41 As shown in Figure 6 and Supporting Information Figure S5, the PLE spectra are identical for different emission energies, indicating that all of the NPLs in the ensemble have the same excitation spectrum. The xCdS/CdSe/xCdS NPLs show a larger line width than the CdS NPLs, but this is due to increased homogeneous broadening (most likely due to increased electron−phonon coupling34), not inhomogeneous broadening. The homogeneous broadening means that the NPLs can support gain over a reasonably broad spectra bandwidth, as required for practical laser operation. In conclusion, colloidal nanoplatelets are in many respects a superior medium for light amplification and optical gain than conventional colloidal nanocrystals. Films of NPLs show low ASE thresholds and high saturated gain, which can ultimately be traced to the unique synthetic process used to make the NPLs and the one-dimensional confinement of carriers in the NPLs. Our NPL films already outperform the best reported films of colloidal QDs, and a further reduction in ASE threshold is likely possible by reducing nonuniformities and scattering in the NPL films.14,42 The use of NPLs thus has the potential to turn colloidal-nanocrystal lasers into a practical reality, combining the advantages of quantum wells with the advantages of a colloidal system. Key among these latter advantages is solution processability, which opens up the possibility of integrating low-cost lasers into nearly any system, including flexible substrates, optical fibers, microfabricated waveguides, and lab-on-a-chip systems.

Figure 6. Emission spectrum (green) and photoluminescence excitation (PLE) spectra (black, red, and blue) for (a) CdSe and (b) 3CdS/CdSe/3CdS NPLs in hexane. For each sample, PLE spectra were recorded for three different emission energies (λem), indicated by the three arrows; the identical spectra indicate the absence of inhomogeneous broadening.

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ASSOCIATED CONTENT

* Supporting Information S

Methods of material synthesis and experimental details. Additional optical absorption and emission spectra. Tabulated time constants for Auger recombination. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(M.P.) E-mail: [email protected]. *(D.V.T.) E-mail: [email protected]. Author Contributions ○

(C.S. and I.F.) These authors contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357. This work was supported by the Keck Foundation and by the University of Chicago and the Department of Energy under Department of Energy Contract No. DE-AC0206CH11357, awarded to UChicago Argonne, LLC, operator of Argonne National Laboratory. D.V.T. thanks the David and Lucile Packard Fellowship and the Samsung Global Research Outreach Program. This work used facilities supported by the University of Chicago NSF MRSEC Program under Award Number DMR-0213745. A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the 2776

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(32) Chan, Y.; Caruge, J. M.; Snee, P. T.; Bawendi, M. G. Appl. Phys. Lett. 2004, 85, 2460−2462. (33) Xia, R. D.; Heliotis, G.; Bradley, D. D. C. Appl. Phys. Lett. 2003, 82, 3599−3601. (34) Xing, G.; Mathews, N.; Lim, S. S.; Yantara, N.; Liu, X.; Sabba, D.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Nat. Mater. 2014, 13, 476− 480. (35) Aimono, T.; Kawamura, Y.; Goushi, K.; Yamamoto, H.; Sasabe, H.; Adachi, C. Appl. Phys. Lett. 2005, 86, 071110. (36) Thijs, P. J. A.; Tiemeijer, L. F.; Kuindersma, P. I.; Binsma, J. J. M.; Vandongen, T. IEEE J. Quantum Electron. 1991, 27, 1426−1439. (37) Shirakawa, K.; Yamaoka, H.; Toriyama, Y.; Takahashi, N. S.; Adachi, C. Proc. SPIE 2009, 74151H. (38) Kunneman, L. T.; Tessier, M. D.; Heuclin, H.; Dubertret, B.; Aulin, Y. V.; Grozema, F. C.; Schins, J. M.; Siebbeles, L. D. A. J. Phys. Chem. Lett. 2013, 4, 3574−3578. (39) Tessier, M. D.; Mahler, B.; Nadal, B.; Heuclin, H.; Pedetti, S.; Dubertret, B. Nano Lett. 2013, 13, 3321−3328. (40) Efros, A. L.; Rosen, M. Annu. Rev. Mater. Sci. 2000, 30, 475− 521. (41) Talapin, D. V.; Haubold, S.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. J. Phys. Chem. B 2001, 105, 2260−2263. (42) Sundar, V. C.; Eisler, H. J.; Deng, T.; Chan, Y. T.; Thomas, E. L.; Bawendi, M. G. Adv. Mater. 2004, 16, 2137−2141.

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