Article Cite This: J. Phys. Chem. C 2017, 121, 24816-24823
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Engineering Bicolor Emission in 2D Core/Crown CdSe/CdSe1−xTex Nanoplatelet Heterostructures Using Band-Offset Tuning Marion Dufour,† Violette Steinmetz,‡ Eva Izquierdo,† Thomas Pons,† Nicolas Lequeux,† Emmanuel Lhuillier,‡ Laurent Legrand,‡ Maria Chamarro,‡ Thierry Barisien,‡ and Sandrine Ithurria*,† †
Laboratoire de Physique et d’Etude des Matériaux, PSL Research University, CNRS UMR 8213, UPMC Sorbonne Université, ESPCI Paris, 10 rue Vauquelin, 75005 Paris, France ‡ Sorbonne Universités, UPMC University Paris 06, CNRS-UMR 7588, Institut des Nanosciences de Paris, 4 place Jussieu, 75005 Paris, France S Supporting Information *
ABSTRACT: Colloidal 2D nanoplatelets (NPLs) are a class of nanoparticles that offer the possibility of forming two types of heterostructures, by growing either in the confined direction or perpendicular to the confined direction, called core/crown NPLs. Here, we demonstrate that bicolor emission can be obtained from 2D NPLs with a core/crown geometry. To date, for CdSe/CdTe NPLs with type-II band alignment, only charge transfer emission has been observed due to the very fast (99%), oleic acid (OA) (Aldrich 90%), trioctylphosphine (TOP) (Aldrich, 97%), selenium (Strem Chemicals 99.99%), tellurium (Aldrich, 30 mesh, 99.997%), hexane (Carlo Erba, 95%), ethanol (Carlo Erba, 99.9%), and methanol (Carlo Erba, 99.9%) were obtained from commercial sources. 24817
DOI: 10.1021/acs.jpcc.7b06527 J. Phys. Chem. C 2017, 121, 24816−24823
Article
The Journal of Physical Chemistry C mL three-neck flask, 24 mg of Cd(Ac)2, the NPLs in ODE, and 45 μL of oleic acid are mixed and vacuumed at room temperature for 10 min. Under an argon flow the temperature is set to 215 °C. In parallel, a mix of a variable ratio of TOPSe (1 M) and TOPTe (1 M) in ODE is prepared (total anion concentration of 0.025 M). When the temperature is reached, 2 mL of this second solution is injected at a rate of 2 mL/h into the reaction media. When the desired injection volume is reached (2 mL), the mixture is cooled to room temperature and washed once with ethanol. The core/crown CdSe/CdSexTe1−x NPLs are dispersed in hexane and stored in the dark to prevent them from photodegradation. Aliquots are taken every 0.25 mL of the anionic mixture injected in the three-neck flask.
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RESULTS AND DISCUSSION To grow CdSe/CdSe1−xTex core/crown NPLs, we first synthesized CdSe core NPLs using a procedure from the literature.24 These NPLs have a lateral extension of 9 nm × 22 nm and a thickness of 4 monolayers (MLs) (5 planes of cadmium and 4 planes of selenium), which produce a PL peak at 510 nm. The growth of the crown was ensured by mixing Cd precursors and CdSe core NPLs at high temperature (215 °C), followed by slowly injecting the Se and Te precursors (TOPSe and TOPTe, respectively). The thickness of the NPLs remained unchanged (1.3 nm), while their lateral extension could reach 20 nm × 100 nm (see the electron microscopy images in Figures 1c and S1 and S2). A series of NPLs with an increasing ratio of Te in the crown was synthesized with compositions ranging from pure CdSe to pure CdTe with a 10% increment in the Te content. More details regarding the synthesis and characterization of the materials are given in the Methods and Supporting Information, see Figures S1−S4 and Table S1. The room-temperature (RT) absorption spectra of the 11 core/crown NPLs are presented in Figure 1a and show two types of optical features. First, the peaks relative to the CdSe core (510 nm, which is associated with the heavy hole exciton transition, and 478 nm, which is associated with the light hole exciton transition) can be observed for all NPLs. A second feature resulting from the crown appears at redder wavelengths. The energy of this red peak clearly does not follow a monotonic trend with the Te content. The behavior is the signature of the bowing effect. This effect is observed in ternary alloy and is a deviation to the linearity between the energy gap of the two pure semiconductors.32,33 It consists, in the ternary alloy CdSe1−xTex, of the lowering of the band-gap energy with the ratio x. It will be further discussed later. The RT PL spectra excited at 420 nm and associated with the CdSe/CdSe1−xTex core/crown NPLs are presented in Figures 1b and 2. From sample to sample, the spectra drastically change. Four main behaviors can be observed as a function of the Te content. (i) When 0 < x ≤ 0.2, we observe a single large PL peak with a typical full width at half-maximum (fwhm) of 160 meV (see Figure 2c). Previously,34,35 it has been shown that even minimal Te content, compared to pure CdSe cores, can lead to significant peak broadening and asymmetric peak shapes. This phenomenon is the signature of exciton trapping due to an inhomogeneous alloy composition.34 (ii) For 0.3 < x < 0.5, we observe a single peak, but it is narrower. The peak is also asymmetric and presents a long tail at lower energies. When x = 0.5, the fwhm of the emission peak is equal to 70 meV (19 nm). This result is attributed to direct exciton recombination in the alloys.36−38The peak observed in this range originated from recombination inside the crown as the
Figure 2. (a) RT absorption spectra of 9 aliquots measured during the synthesis of the CdSe/CdSe0.4Te0.6 core/crown NPLs. First gray spectrum represents the NPLs before injection. Between each aliquot, the surface of the NPLs increased by an amount approximately equal to that of the surface of the core NPLs (see Supporting Information). Spectra were normalized at 420 nm. (b) RT PL spectra of the corresponding aliquots measured during the synthesis of the CdSe/ CdSe0.4Te0.6 core/crown NPLs. Excitation wavelength is 420 nm. Absorption and PL background lines were shifted upward for clarity. (Inset) Blue emission contribution versus the injected volume. (c−f) PL spectra and schemes of the four types of emission depending on the crown composition in the core/crown heterostructures.
Stokes shift between the absorption and the emission is below 25 meV (see Figure 5a). Thus, for 0 < x < 0.5, the PL peak is attributed to the crown emission, which was confirmed by the fact that the PL peak was subject to a bowing effect that was similar to the one observed during absorption. (iii) When 0.6 < x < 0.9, the spectra present two emission peaks with very different line widths. The high-energy peak presents a smaller Stokes shift than that of the crown absorption and a fwhm below 60 meV, which corresponds to the crown alloy emission. A contrario, the second peak at lower energy is clearly red shifted compared to the absorption and presents a fwhm of more than 130 meV. This large peak is similar to the one observed for (iv) x = 1, which has been studied and attributed to either the signature of type-II band alignment28 or, at least, recombination through interface states.29 In the third region (iii), for the intermediate Te and Se contents, we observe a bicolor emission made of two overlapping peaks (see Figure 2e). These two peaks were fitted using two Gaussian curves centered at 588 and 635 nm with fwhm values of, respectively, 17 and 57 nm. We estimated that ∼25% of the signal magnitude at 588 nm (the maximum of the blue peak) actually resulted from the overlap with the red peak. To ensure that these two peaks indeed originated from the same NPLs, excitations of the photoluminescence spectra (PLE) centered on the two emissions were performed (see the 24818
DOI: 10.1021/acs.jpcc.7b06527 J. Phys. Chem. C 2017, 121, 24816−24823
Article
The Journal of Physical Chemistry C
Figure 3. (a) RT (red line) and low-temperature (T ≈ 5 K, blue line) absorption spectra of thin solid films prepared from the initial solution of CdSe/CdSe0.4Te0.6 core/crown NPLs. (b) Evolution of the CdSe/CdSe0.4Te0.6 NPL PL spectra at ∼5 K with increasing dispersions on glass slides. Degree of dilution of the used solutions (impregnation) compared to the original solution is indicated above each spectrum. For the 6000 times dilution 2 spectra are plotted corresponding to 2 different positions. (c) Typical PL spectrum, at ∼5 K, of a single CdSe/CdSe0.4Te0.6 core/crown NPL (dark blue). Dark-green curve is a magnification of the emission of the interface state (×10). (d) PL dynamics associated with the Xcrown line (562.7 nm) at ∼5 K and monoexponential fit of the decay (red solid line).
temperature interval.31 A slight narrowing of the absorption line width, approximately 495 nm, was also observed and was sufficient to reveal the crown light−hole and core heavy−hole excitonic transitions.26 During the experiments, the excitation source was tuned to 515 nm (continuous wave (cw) argon laser) so that only higher lying states of the crown exciton progression became populated. In this configuration, the interface states were populated during the diffusion of excitons from the crown to the interface.43 Figure 3b shows the evolution of the PL at 5 K for various NPL coverage rates. As the NPL density decreases, the inhomogeneous Xcrown line splits into sharp peaks while the Xint emission remains identical (see Figure 3b). By diluting the sample we measured the signal from a single NPL, which was characterized by an extremely narrow emission (see Figure 3c). We ensured that the PL signal originated from single NPL by studying emission fluctuations over 170 s (see Figure S6 in the Supporting Information). Several observations evidenced the intrinsic nature of the measured PL. First, the Xcrown line widths were found within the 380−450 μeV range, which compared very well to the emission width values that have been reported for pure CdSe NPLs.31,44 Second, the emission exhibited blinking and spectral diffusion that was responsible for the wellresolved energy shifts of the maximum emission. Third, measurements of the PL intensity decay for the Xcrown line were performed at the single NPL level. The decay measurements conducted using ∼20 NPLs were fitted using monoexponential functions and provided a lifetime range from 70 to 190 ps (a typical trace is shown in Figure 3d). Notably, signatures of longer constants in the PL decay curves were not observed. Our results indicate that such long-time contributions, when present, had a much weaker contribution (at least 1 order of magnitude) at low temperatures compared to the RT experiments (see the discussion below). The measurements of the subnanosecond decay times associated with bright emission provide strong evidence that the Xcrown line corresponds to the radiative recombination of a “direct” exciton generated in the crown (with the electrons and holes confined in the crown). Similar decay times (∼200 ps) have been reported at 20 K for single CdSe NPLs, in which the emission was only determined by the lowest band-edge exciton.31 Fourth, concerning the interface emission band (Xint), it is indeed remarkable that it is always present in the single NPL emission, as shown in Figure 3c. The associated
Supporting Information, Figure S5) and exhibited perfect overlapping. We then investigated the effect of the lateral crown extension on the PL emission (see Figure 2a and 2b and Supporting Information Figures S2 and S3). The PL peak was already strongly red shifted after the injection of 250 μL of the precursors. This first large peak at 630 nm corresponds to the recombination through the interface. The second peak at 580 nm is related to the crown emission and only appeared when the crown became larger (after injecting 500 μL of the precursors). This peak became increasingly predominant as the crown size further increased (inset of Figure 2b). When we synthesized a crown with a spatial extension of only few nanometers, the core/crown interface played an important role and was the only source of recombination. The 2D recombination of the crown exciton became possible when the crown size became sufficiently large. For NPLs, theoretical studies have indeed shown that the exciton oscillator strength is an increasing function of the lateral size before it reaches a plateau.39 This may partly explain the progressive emergence of the crown emission with the increasing extension. The key role of the crown size on the relative magnitude of the two emissions is certainly the reason why Demir’s group did not report the bicolor emission behavior within CdSe/CdSe1−xTex core/crown NPLs. They focused on smaller crown sizes and lower Te contents to finely tune the emission from green to red with an efficient emission quantum yield.35 To gain insight into the intraplatelet nature of the bicolor emission, microPL experiments were conducted using the CdSe/CdSe0.4Te0.6 NPLs with the two PL peaks that had the closest magnitude. The spectroscopy measurements were carried using diluted NPL films to ensure the identification of the individual optical responses. The experiments are performed at low temperature (T ≈ 5 K) to reduce the homogeneous broadening of the emission line that results from exciton−phonon coupling.40,41 Figure 3a compares the absorption of a low optical density film at RT and 5 K. When lowering the temperature to 5 K, the film spectrum presents a rigid blue shift (∼100 meV) (see Figure 3a). Such a shift is expected in bulk semiconductors42and is primarily explained by band shifting resulting from lattice contraction and varying electron−lattice interactions. The same type of behavior appears in NCs. For comparison, a shift of ∼75 meV was measured in the CdSe NPLs within the same 24819
DOI: 10.1021/acs.jpcc.7b06527 J. Phys. Chem. C 2017, 121, 24816−24823
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Figure 4. RT time-resolved spectroscopy. Excitation wavelength is 407 nm. (a) PL signal for a solution of CdSe/CdSe1−xTex with x = 0.6 at early times (t < 1.5 ns) after the pulsed excitation. For comparison, the steady signal with two emission peaks is plotted in dark red. (b) PL decay at early times (t < 1.5 ns) for CdSe/CdSe1−xTex for three different x ratios of Te (x = 0.5, 0.6, 0.8). Narrow bandpass filter is used to collect the photons at approximately 590 nm (mainly crown emission). (c) PL decay at the long time scale for CdSe/CdSe1−xTex for three different x ratios of Te (x = 0.5, 0.6, 0.8). Same filter as for b is used. (d) PL decay at the long time scale for CdSe/CdSe1−xTex for two different x of Te (x = 0.6, 0.8) (mainly associated with the interface emission). For x = 0.5, the core/crown NPLs do not exhibit emission from the interface. Narrow bandpass filter is used to collect the photons at approximately 630 nm.
electrons was expected to occur over a subpicosecond time scale.27,29 The time dependence of the blue peak emission within the short time scale is plotted in Figure 4b for three samples: x = 0.5 when only the crown emits, x = 0.6 when both the crown and the interface emit, and x = 0.8 when the crown barely emits. We measured very distinct behaviors. At a high Te content (x = 0.8), we observe a very fast decay with a constant below the instrumental resolution (∼15 ps), which already corresponds to ∼30% of the integrated time-resolved PL signal. When the Te content is reduced (x = 0.5, 0.6) the dynamics considerably slow down, and the decays can be fitted with two exponentials with characteristic times of 35 and 160 ps, respectively. Moreover, the weighting factors of those fast components in the total integrated time become much weaker, only a few percent. We then investigated the dynamics at a longer time scale (>nanoseconds) for the crown emission (Figure 4c) and interface emission (Figure 4d).36 The decays could, in general, be described by multiexponential dynamics that depend on x. In particular, the crown and interface emission decays are very similar for the CdSe/CdSe0.4Te0.6 samples. However, at least four components are required to achieve a satisfactory fit. The decay times (and respective weighting factors) of 6 ns (0.627), 60 ns (0.290), 250 ns (0.079), and 2.0 μs (0.004) were measured for the crown peaks, whereas the values of 18 ns (0.38), 80 ns (0.44), 350 ns (0.15), and 1.7 μs (0.03) characterized the interface peak dynamics. Notably, the first three constants and their weights were determined with a rather low uncertainty (of a few percent), whereas the uncertainty became higher but still remained moderate (∼10%) for the last constant due to the limited extension of the observed temporal window. In a general manner, the association of a given constant to a particular process is rather spurious, and the presence of a decay time distribution should be considered as the global signature of a specific phenomenon. For example, models of exciton transport via hopping47 and excitons that recombine with randomly distributed traps lead to multiexponential decays. It is thus tempting to ascribe the lengthening of the decays to the introduction of traps in the percolation path of the excitons through alloying. However, the RT observations of multiexponential dynamics with constants in the same range as those during the relaxation of single homo-NPLs of CdSe31,48 precludes only assigning the decay lengthening to the introduction of the alloy. Recently, the excited state dynamics
signal is observable in the PL spectrum but not in the timeresolved emission spectrum (using the synchroscan camera), whereas the intensity would have been expected to be well above the measurable threshold for subnanosecond decays, which confirms the long-lived nature of the state. All single NPL spectra present a broad spectral extension (fwhm of ∼100 meV) and relatively weak amplitude in the interface state emission compared to the crown emission. We also noted an apparent structure in the spectrum that was NPL dependent. In a very simple scheme, when intrinsic responses are measured, the ratio γ between the area under the crown line and the interface state band in a single NPL spectrum should roughly equal the ratio of their counterpart contributions in the inhomogeneous spectrum. We actually found minimal variations in γ within the observed single NPLs and found that γ ≈ 0.6−0.65 was directly comparable to the value extracted from the film PL following spectrum deconvolution. At low temperatures, the single-platelet emission was thus composed of a narrow exciton line that was similar to a quantum-well direct free-exciton line and a broad (∼100 meV) low-energy band with much longer dynamics. In epitaxial double-quantum wells, low-temperature inhomogeneously broadened indirect exciton PL exhibits widths in the meV to tens of meV range,45,46 which results in a difference of more than 1 order of magnitude. One possible explanation is that the emission originates from the recombination of an electron− hole pair with one or both of the carriers in deep traps, as suggested by Cassette et al. 29 The bandwidth of X int characterized the distribution of the energy levels associated with the traps. These traps may have been due to a gradient in the composition all along the interface between the core and the crown.26 In the following section, to further quantify the emission dynamics and characterize the bicolor emission, we describe the time-resolved PL spectroscopy results obtained at RT. Figure 4a presents the PL spectrum at early times ( Eexc, which occurs at high Te content levels (see the left side of Figure 5b), the electron
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CONCLUSION In summary, we observed bicolor emission in core/crown heterostructures by adjusting the CdSe1−xTex crown composition, which enabled fine tuning of the conduction band offset compared to the excitonic binding energy. The short time dynamics showed that when Coulomb attraction was strong and the binding states were energetically favorable, nonradiative quenching of the crown exciton was also drastically reduced, very likely due to the less efficient charge transfer toward the core. In addition, an unambiguous demonstration of bicolor emission from individual nanoplatelets was accomplished. This work unveils that model 2D geometries enable new possibilities at the single object level for designing multicolor emission, as opposed to using QDs. Future works should be dedicated to the design of bicolor emitters with larger energy splitting between the two emissions.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06527. 24821
DOI: 10.1021/acs.jpcc.7b06527 J. Phys. Chem. C 2017, 121, 24816−24823
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The Journal of Physical Chemistry C
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Materials characterization, characterization of the core synthesis, characterization of the core/crown CdSe/ CdSe1−xTex for x = 0.1...1 by absorption, photoluminescence, TEM, XRD, and EDX, and spectroscopic characterization of telluride-rich core/crown NPLs with dual emission (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Emmanuel Lhuillier: 0000-0003-2582-1422 Sandrine Ithurria: 0000-0002-4733-9883 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Agence Nationale de la Recherche for funding through grant Nanodose and H2DH. This work has been supported by the Region Ile-de-France in the framework of DIM Nano-K. This work was supported by French state funds managed by the ANR within the Investissements d’Avenir programme under reference ANR-11-IDEX-0004-02, and more specifically within the framework of the Cluster of Excellence MATISSE.
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DOI: 10.1021/acs.jpcc.7b06527 J. Phys. Chem. C 2017, 121, 24816−24823
Article
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DOI: 10.1021/acs.jpcc.7b06527 J. Phys. Chem. C 2017, 121, 24816−24823
