Letter pubs.acs.org/NanoLett
Spectroscopy of Colloidal Semiconductor Core/Shell Nanoplatelets with High Quantum Yield M. D. Tessier, B. Mahler, B. Nadal, H. Heuclin, S. Pedetti, and B. Dubertret* Laboratoire de Physique et d’Etude des Matériaux, CNRS, Université Pierre et Marie Curie, ESPCI, 10 rue Vauquelin, 75005, Paris, France S Supporting Information *
ABSTRACT: Free standing two-dimensional materials appear as a novel class of structures. Recently, the first colloidal twodimensional heterostructures have been synthesized. These core/shell nanoplatelets are the first step toward colloidal quantum wells. Here, we study in detail the spectroscopic properties of this novel generation of colloidal nanoparticles. We show that core/shell CdSe/CdZnS nanoplatelets with 80% quantum yield can be obtained. The emission time trace of single core/shell nanoplatelets exhibits reduced blinking compared to core nanoplatelets with a two level emission time trace. At cryogenic temperatures, these nanoplatelets have a quantum yield close to 100% and a stable emission time trace. A solution of core/shell nanoplatelets has emission spectra with a full width half-maximum close to 20 nm, a value much lower than corresponding spherical or rod-shaped heterostructures. Using single particle spectroscopy, we show that the broadening of the emission spectra upon the shell deposition is not due to dispersity between particles but is related to an intrinsic increased exciton−phonon coupling in the shell. We also demonstrate that optical spectroscopy is a relevant tool to investigate the presence of traps induced by shell deposition. The spectroscopic properties of the core/shell nanoplatelets presented here strongly suggest that this new generation of objects will be an interesting alternative to spherical or rod-shaped nanocrystals. KEYWORDS: Nanoplatelets, nanosheets, nanomembranes, quantum wells, 2D geometry, heterostructure, fluorescence
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design and the synthesis of these novel core/shell structures.13 Here, we study the optical properties of core/shell nanoplatelets with a CdSe core and with different shell thicknesses and compositions. We show that core/shell NPLs with quantum yield up to 80% can routinely be obtained. This increase of the quantum yield is accompanied by a great change in the emission time trace of single core/shell NPLs, and the appearance of quasi-discrete emission states at room temperature. At 20 K, stable emission of nonblinking single NPL is demonstrated with a fluorescence quantum yield close to 100% in the best case. We study the full width half-maximum (fwhm) of the fluorescence emission change with shell thickness and its dependence with temperature. We propose that the increase of the fwhm upon the shell deposition results from the larger phonon coupling of the charge carriers in the CdS shell. The evolution of NPLs fluorescence emission lifetime with the temperature is consistent with the presence of inhomogeneities in the core/shell structure that strongly depend on the quality of the shell deposition. We demonstrate that spectroscopic tools can be used to monitor the quality of a shell growth along with the presence of traps in the colloidal core/shell structure. Our spectroscopic studies of these 2D structures confirm that
emiconductor nanoparticles with two-dimensional geometry, referred in the literature as nanoribbons,1 nanoplatelets (NPLs),2 nanosheets,3 nanomembranes,4 quantumdisks,5 quantum belts,6 or more generally two-dimensional (2D) materials,7 emerge as a novel class of colloidal nanoparticles. Their 2D shape is a nice addition to the collection of shapes already available for semiconductor nanoparticles such as spheres,8 rods,9 tetrapods,10 and nanowires.11 The 2D semiconductor nanoparticles with small thickness compared to the Bohr radius can exhibit 1D confinement.2 These 1D confined particles have electronic properties very close to quantum wells grown by epitaxial techniques on solid substrates.12 Recently, two concomitant studies have described the synthesis of core/ shell NPLs.13,14 Core/shell semiconductor nanoparticles15 and heterostructures16 have been at the center of intense research in the last few decades because these structures have usually better fluorescent quantum yields (QYs),17 more robust optical properties,18 and permit wave function engineering between the core and the shell.19 Moreover, core/shell structures have boosted the development of spherical quantum dots for applications such as biomedical imaging20 and light-emitting diodes.21 The advent of core/shell structures for 2D systems may open the way for applications so far limited to epitaxial quantum wells or multiple quantum wells. Before such developments can be performed, a better characterization and understanding of the core/shell colloidal 2D structures is necessary. The work on core/shell NPLs mainly focused on the © 2013 American Chemical Society
Received: April 27, 2013 Revised: May 21, 2013 Published: June 3, 2013 3321
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Figure 1. (a) Transmission electron microscopy image of core/shell CdSe/CdZnS NPLs. Inset: Scheme of core/shell NPLs. (b) Absorption spectra of solutions of core CdSe NPLs (gray) and CdSe/CdZnS NPLs (black). (c) Photoluminescence spectra of solutions of core CdSe NPLs (gray) and CdSe/CdZnS NPLs (black). (d) Intensity decay of solutions of core CdSe NPLs (gray) and CdSe/CdZnS NPLs (black). All measurements were done at room temperature.
when the void is replaced with a semiconductor, but it suggests that the charge carrier confinement is greater in the thickness direction in the NPL than it is in a quantum dot (QD) with similar emission wavelength. Hence, the emission redshift is stronger upon a shell deposition on a NPL. The emission peak fwhm of NPLs increases from ∼37 to ∼65 meV upon shell deposition (Figure 1c). This broadening is observed for whatever shell thickness is deposited, both for CdS and CdZnS shells regardless of the deposition method. Another characteristic feature of the NPL emission spectrum is its slight asymmetry with a faint red tail emission. These features will be discussed in more details in the next sections. The fluorescence lifetime of core/shell NPLs is significantly longer than core CdSe NPLs but it is shorter than for spherical nanocrystals, even if the fluorescence quantum yield difference calls for care in the comparison. Longer fluorescence lifetime upon shell deposition has already been observed in CdSe/CdS spherical nanocrystals.22 It is attributed to a reduction of the overlap between the charge carriers due to the quasi type II band alignment of the CdSe/CdS structure. In such core/shell structures, at room temperature the electron is delocalized in the whole nanocrystal, while the hole is more localized in the CdSe core. Single NPL Core/Shell Time Trace. In the case of spherical nanocrystals, the shell deposition on CdSe core has been shown to reduce the fluorescence intermittency (known as blinking) at the single particle level.24,25 We show in the following section that a shell deposition on CdSe NPLs has a similar influence. In
core/shell NPLs have optical properties distinct from their spherical- or rod-shaped counterparts. Results and Discussion. We have studied the optical properties of CdSe nanoplatelets capped with either CdS or CdZnS shells. We have synthesized CdSe core NPLs with various lateral dimensions, ranging from 120 nm2 (10 nm × 12 nm) to 400 nm2 (10 nm × 40 nm). On these different NPLs, the shell growth was performed either using a one-pot shell growth,13 or a layer-by-layer process.14 The shell thickness varied from one up to seven monolayers (CdS or CdZnS). TEM images of some studied samples are presented in Supporting Information (Figure S1). We observed that the quantum yield of the core/shell NPLs varied from a few percent to 80% depending on the method chosen for the shell growth. One-pot shell growth gave on average the best quantum yields. With slight modifications to the procedure described in ref 13, we obtained routinely NPLs with QY up to 80% (see experimental part). During the shell deposition, the NPLs conserve their shape and are rather homogeneous (Figure 1a). The shell deposition induces a strong redshift of both the light hole and the heavy hole transitions (Figure 1b). This shift is stronger than the one observed when a CdS shell is grown on a CdSe spherical dot that emits at comparable wavelength than the CdSe core NPL.22 The reason is that in the case of a free electron in vacuum the ground-state energy in a quantum well with a thickness d is identical to the groundstate energy in a spherical quantum dot with a diameter 2d (both cases with infinite barrier).23 This relation does not hold 3322
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Figure 2. (a) Left: Emission time trace of a single core/shell NPL at room temperature (black) and corresponding background noise (red). The time trace is binned at 10 ms. Right: Corresponding normalized intensity distributions. (b) Time dependence of the fluorescence intensity of a single NPL at room temperature (black). Biexponential fit (gray).
Figure 3. (a) Left: Emission time trace of a single core/shell NPL at cryogenic temperature (black) and corresponding background noise (red). The time trace is binned at 20 ms. Right: Corresponding normalized intensity distributions. (b) Time dependence of the fluorescence intensity of a single NPL at room temperature (black). Biexponential fit curve (gray).
∼140 nm2) are rather small compared to other samples. Their quantum yield is close to 80% in solution (n-hexane). The emission time trace of these core/shell NPLs at 300 K under air
Figure 2a, a typical emission time trace of a single core/shell NPL at room temperature is presented. These NPLs have an alloyed Cd0.33Zn0.67S shell and their lateral dimensions (area 3323
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is quite stable. Over minutes, the NPL spends more than 80% of its time in a single emissive state with an average lifetime of 15 ns (Figure 2b). This behavior is in strong contrast with the one observed for CdSe core only NPLs that exhibit a continuous distribution of ON states, associated with a continuous distribution of lifetimes.26 The average observed lifetime of 15 ns is longer than for single CdSe NPLs for which an average lifetime of 4 ns is typically measured at room temperature. It tends to confirm an increase of the exciton lifetime when a shell is added as it has been noticed for ensemble measurements (Figure 1d). We have observed that the single NPL time trace depends greatly on the quality and on the composition of the shell. In CdSe/CdS core/shell NPLs with similar thicknesses than the CdSe/CdZnS core/shell NPLs we just discussed, a more continuous distribution of on-states associated with a more multiexponential intensity decay shape is observed (Supporting Information Figure S2). These NPLs have quantum yields around 40%, lower than the CdSe/CdZnS NPLs, which could result from a lower shell quality deposition and/or from more active surface traps. The CdSe/CdZnS NPLs have also been studied at the single particle level at cryogenic temperature. Emission time traces with long on-time periods are observed at 20 K as illustrated in Figure 3a. The same change of the emission intermittency when the temperature decreases has been observed in CdSe NPLs26 and CdSe/CdS QDs.27 The intensity decay of Figure 3b is well fitted with biexponential decay with lifetimes of 0.8 and 7 ns. We attribute the 7 ns lifetime to the band-edge recombination while we observed that the 0.8 ns lifetime weight is increased with the excitation intensity similarly to multiexcitons in QDs28 (Supporting Information Figure S3). This 0.8 ns lifetime could then be due to several excitons emission. When the temperature decreases, the fluorescence intensity increases. This combined effect of lifetime shortening and QY enhancement when the temperature decreases has recently been observed both on core only NPLs12,26 and on thick shell CdSe/CdS NCs.29 The radiative lifetime of a single core/shell NPL is close to the lifetime of an ensemble of core/shell NPLs measured at the emission maximum at the same temperature (∼7 ns at 25 K for this sample, see Supporting Information Figure S4). On the basis of the intensity comparison at room temperature and at cryogenic temperatures for ensemble measurements, we deduce a quantum yield close to 100% at 20 K for these core/shell NPLs (Supporting Information Figure S5). We conclude that the 7 ns lifetime measured at 20 K is the radiative lifetime of the band edge exciton. This radiative lifetime is longer than the ultrafast lifetime of 0.2−0.3 ns measured for CdSe NPLs. It confirms that the shell tends to increase the radiative lifetime because of the reduction of the overlap between the charge carriers. But this fluorescence decay is still faster than the one observed for spherical NCs at similar temperatures,30 suggesting that the dark/bright energy splitting is much lower in the 2D geometry. Photoluminescence Spectrum. fwhm Increase. As we mentioned earlier, during the shell growth, the fwhm of the photoluminescence spectrum of a solution of core/shell NPLs increases from 37 to 65 meV. The increase of the fwhm could be due to dispersity between nanoplatelets induced by the shell deposition. To test that the fwhm increase is due to heterogeneity between NPLs, we have performed single NPL photoluminescence measurements at room temperature (Figure 4). It appears that the photoluminescence (PL)
Figure 4. Room-temperature photoluminescence spectra of a solution of core/shell NPLs (Dash gray) and of a typical single core/shell NPL (black) of the same sample.
spectrum of a single NPL has a comparable fwhm than for an ensemble of NPLs. Statistics on several core/shell NPLs of different samples confirm the similarity (Supporting Information Figure S6). We conclude that the dispersity between NPLs shell deposition is not the major source of broadening but results from an intrinsic broadening process as it has been suggested in ref 14. The fwhm change upon shell deposition is not limited to the case of NPLs. For spherical core/shell NCs, various fwhm at the single particle level have been observed, depending on the composition and the thickness of the shell. For example, CdSe/ZnS NCs with a thin shell have photoluminescence with a fwhm close to 50 meV,31 whereas CdSe/CdS NCs with a thick shell have much larger fwhm, close to 90 meV.32 In colloidal semiconductor NCs, the intrinsic emission fwhm is dominated at room temperature by thermal effect via exciton−phonon coupling,33 but to the best of our knowledge the increase of the emission fwhm upon shell deposition has not been commented yet. In the case of the NPLs, we hypothesize that the broadening of the emission fwhm results from a stronger exciton−phonon coupling in the shell material than in the core material. To support this hypothesis, we have measured the fwhm in the PL spectrum of CdSe, CdS, and CdTe NPLs (Supporting Information Figure S7). These NPLs have the same crystal structure, they have all pure 1D confinement, and they have identical surface chemistry. For CdSe, CdS, and CdTe NPLs, we respectively measured a fwhm of 37, 94, and 32 meV. It clearly demonstrates that the materials have an influence on the line width. We deduced that the exciton−phonon coupling strength is stronger for CdS nanoplatelets than for CdSe nanoplatelets. For core/shell NPLs, we measured a fwhm that is generally around 60−70 meV, which is between the fwhm of CdSe NPLs and the fwhm of CdS NPLs. In core/shell NPLs, the exciton is coupled with phonons of the core but also with phonons of the shell because of the delocalization of the charge carriers in the shell. Thus we assumed that the linewidth is larger for core/ shell NPLs than for CdSe core only NPLs because of the higher exciton phonon coupling in the shell. Although the fwhm increase for core/shell NPLs can be explained by stronger phonon coupling, other mechanisms may also partake in the broadening. For example, in NPLs the mirror charges have been shown to have a strong effect that results in an increase of the charge carriers confinement.34 This effect is almost exactly compensated by the strong exciton binding energy. The growth 3324
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Figure 5. Evolution with temperature of the PL of an ensemble of core/shell NPLs. (a) Normalized PL spectra for temperatures in the range 10− 300 K. (b) Normalized PL spectrum at 10 K. (c) Evolution versus temperature of the emission maximum. (d) Evolution versus temperature of the fluorescence lifetimes at three different energies of the PL spectrum.
GaInNAs,39 and GaInP2,40 ZnO/CdZnO,37 strained Si,41 InGaN42 layers, and also colloidal CdSe nanowires.43 From this literature, it can be concluded that the inversed S shape and the long lifetime of the low energy side of the PL curve are associated with charge carriers localization in energy traps. All these different materials have in common the presence of some disorder due to local fluctuations.44 The fluctuations can be local changes of composition, strain, surface, or dimension in the material that create local traps for the charge carriers. When the carriers recombine after being trapped, the energy of the emitted photon is smaller than the band edge energy and the carrier lifetime is longer. The inversed S shape behavior is then explained as follows: 300−130 K: In this case, carriers are not trapped in potential fluctuations because they have sufficient thermal energy to pass through the energy barrier of the traps and thus they can recombine at the band-edge. In this case, the maximum of emission follows the band-edge energy and consequently a classical blueshift of the emission is observed when the temperature decreases. 130−50 K: The charge carriers have less thermal energy. In this temperature range, they can be trapped in potential fluctuations, and trap state emission with lower energy than the band-edge appears. The activation of the traps is thus responsible for the low energy emission side of the PL spectrum. Consequently, as more traps are filled with decreasing temperature, a redshift of the emission maximum is observed. The onset of longer lifetime related to traps emission is also consistent with this interpretation.
of a shell on the NPLs reduces the effect of the mirror charges, but the position of mirror charges may fluctuate in a core/shell NPL because its surface is not as smooth and well-defined than the one of core only NPLs. We expect that these fluctuations can broaden homogeneously the NPLs PL. Asymmetry. Another interesting feature of the core/shell NPLs PL spectrum is its asymmetry that we first briefly described in Figure 1c. To understand the origin of this asymmetry, we recorded core/shell NPLs emission spectra at various temperatures between 300 and 10 K (Figure 5a). The PL asymmetry becomes more pronounced when the temperature decreases. At 10 K, a highly asymmetric spectrum with a long tail in the low energy side is obtained (Figure 5b). Two other interesting behaviors can be gleaned from these data. First, when we look at the evolution of the PL emission maximum with temperature, we observe that it exhibits an unusual evolution (Figure 5c). For spherical, rod-shaped, core nanoplatelets, or core/shell nanoparticles, the energy of the emission maximum decreases continuously with temperature.33,35,36 On the contrary, for core−shell NPLs we observed that this evolution exhibits an inversed S shape behavior. The second crucial observation is the comparison of the fluorescence lifetimes for different emission energies versus temperature (Figure 5d). At room temperature, the lifetimes of the low energy side, of the high energy side, and of the emission maximum are similar. But when the temperature decreases, different lifetimes are observed depending on the energy. The higher the energy, the shorter the fluorescence lifetime. Similar behaviors have already been observed in different epitaxially grown quantum wells such as AlGaN,37 AlInAs,38 3325
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the charge carriers. These three effects are also observed when a shell is grown on spherical NCs. The shape of the NPL emission is also affected by the shell growth. First, the fwhm of the core only NPLs increases due to an increase of the phonon coupling with the shell material. Second, the PL peak becomes asymmetric at low temperatures, a signature of defects that appear during the shell growth. Beside its fundamental interest, we believe that our study could help identify potential applications for core/shell NPLs. Materials and Methods. Chemicals. Cadmium nitrate tetrahydrate, zinc nitrate hexahydrate, cadmium acetate dihydrate Cd(OAc)2.2H2O, technical grade 1-octadecene, oleic acid, myristic acid sodium salt, N-methyl formamide, sodium hydrogen sulfide hydrate, sodium sulfide nonahydrate, and selenium in powder were purchased from Sigma Aldrich. Methanol, ethanol, toluene, and n-hexane were purchased from VWR. Synthesis of CdSe/Cd0.33Zn0.67S Core/Shell NPLs by OnePot Shell Growth. These NPLs were obtained according to the procedure described in ref 13. Synthesis of CdSe/CdS Core−Shell Npls by One-Pot Shell Growth. These NPLs were synthesized using a modified procedure described in ref 13 in which chloroform was replaced by N-methylformamide, sodium hydrogen sulfide (NaSH) was used as the sulfur precursor, and cadmium acetate dihydrate (Cd(OAc)2·2H2O) was used as the cadmium precursor. Synthesis of Core/Shell CdSe/CdZnS NPLs by Atomic Layer Deposition. CdSe NPLs with the first excitonic peak at 510 nm are prepared as described in ref 12. Into a 3 mL vial, 200 μL of a solution of CdSe 510 nm in hexane (2 × 10−2 M) is added to 500 μL of N-methylformamide (NMF). Thirty microliters of a freshly prepared 0.3 M solution (30 μL) of Na2S in NMF are added to the mixture. As the exchange of ligands at the surface takes place, the nanoparticles are transferred from the hexane phase to the NMF phase that becomes red. The polar phase is washed twice with hexane and then NPLs are precipitated with 1.5 mL of toluene and 0.5 mL of acetonitrile. The precipitate is dispersed into 0.5 mL of NMF. Then 50 μL of a 0.1 M solution of (Cd(OAc)2·2H2O) in NMF are added and the solution is sonicated for few minutes. NPLs are precipitated with toluene and acetonitrile and dispersed in NMF. For the growth of the successive shells, the addition of the anion was performed as for the first shell. For the growth of the cationic layers, we used 50 μL of a solution composed of (Cd(OAc)2·2H2O) and (Zn(NO3)2·H2O) 0.1 M in NMF. The Cd2+/Zn2+ solution ratio was fixed to 8.5/1.5 for the second shell, 7.5/2.5 for the third shell, and 1/1 for the fourth shell. Nanocrystals are precipitated after each addition step. When the last shell is completed, core/shell NPLs are precipitated with toluene and acetonitrile and dispersed into 1 mL of toluene containing oleic acid (10 μL, 0.03 mmol). The excess of ligand is then eliminated by precipitation with ethanol and core/shell NPLs are finally dispersed in 1 mL of hexane. Nanoparticles were characterized using UV−vis absorption, PL emission, and transmission electron microscopy. Optical Measurements of Ensemble of Core/Shell NPLs at Cryogenic Temperature. NPLs were drop-casted on a sapphire substrate. The NPLs concentration was low with an optical density 350 nm. The sample was mounted in an Oxford optistat CF-V continuous flow cryostat where it was cooled by He vapor exchange gas. To reach a cryogenic temperature, the sample is subjected to a vacuum of less than 10−6 mbar. With this setup, the sample can be cooled to 10 K.
50−10 K: For temperatures below 50 K, we can consider that the carrier can be localized in most of the available traps. As the energy of the traps is connected to the band gap of the materials, it follows a blueshift when the temperature decreases. Then the emission maximum resumes its classical blueshift with decreasing temperatures. It appears that the inversed S shape behavior and the lifetime broadening are strongly related to the sample optical quality. Samples with low QY exhibit a very pronounced inversed S shape and a large lifetime broadening (Supporting Information Figure S8). On the other hand, for samples with high QY the inversed S shape is only weakly visible and the lifetime broadening is reduced (Supporting Information Figure S8). As these energy traps are more present in samples with a low QY, this means that the difference of QY we observed in our first core/shell synthesis was probably related to a difference in the quality of the shell deposition and/or to the presence of a large number of surface traps. Surprisingly, at the single particle level we did not observe the longer lifetimes attributed to the local traps. We performed PL spectra of single core/shell NPLs at cryogenic temperature (Supporting Information Figure S9) to further investigate this difference. The PL emission fwhm of single core/shell NPLs at 20 K is larger (>2 meV) than those obtained for core only CdSe NPLs (
