Colloidal Mercury-Doped CdSe Nanoplatelets with Dual Fluorescence

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Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

Colloidal Mercury-Doped CdSe Nanoplatelets with Dual Fluorescence Tom Galle,†,⊥ Miri Kazes,*,‡,⊥ Rene ́ Hübner,§ Josephine Lox,† Mahdi Samadi Khoshkhoo,† Luisa Sonntag,† Remo Tietze,† Vladimir Sayevich,† Dan Oron,‡ Andreas Koitzsch,∥ Vladimir Lesnyak,*,† and Alexander Eychmüller† †

Physical Chemistry, TU Dresden, Bergstr. 66b, 01062 Dresden, Germany Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 76100, Israel § Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf e.V., Bautzner Landstr. 400, 01328 Dresden, Germany ∥ Leibniz Institute for Solid State and Materials Research, IFW-Dresden, Helmholtzstr. 20, 01069 Dresden, Germany

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S Supporting Information *

ABSTRACT: Quasi-two-dimensional (2D) CdSe nanoplatelets (NPLs) are distinguished by their unique optical properties in comparison to classical semiconductor nanocrystals, such as extremely narrow emission line widths, reduced Auger recombination, and relatively high absorption cross sections. Inherent to their anisotropic 2D structure, however, is the loss of continuous tunability of their photoluminescence (PL) properties due to stepwise growth. On top of that, limited experimental availability of NPLs of different thicknesses and ultimately the bulk band gap of CdSe constrain the achievable PL wavelengths. Here, we report on the doping of CdSe NPLs with mercury, which gives rise to additional PL in the red region of the visible spectrum and in the near-infrared region. We employ a seeded-growth method with injection solutions containing cadmium, selenium, and mercury. The resulting NPLs retain their anisotropic structure, are uniform in size and shape, and present significantly altered spectroscopic characteristics due to the existence of additional energetic states. We conclude that doping takes place by employing elemental analysis in combination with PL excitation spectroscopy, X-ray photoelectron spectroscopy, and singleparticle fluorescence spectroscopy, confirming single emitters being responsible for multiple distinct emission signals.

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nism.22,23 Inherent to the quantum confinement nature of these 2D structures, however, is the discrete character of thickness variations in steps of monolayers (MLs) and, therefore, the absence of continuous tunability. Very recent synthetic developments opened up the attainable emission wavelengths of CdSe NPLs to about 625 nm for the eight-ML species24,25 and resulted in the continuous tuning of their optical properties by means of an advanced surface design.26,27 However, a further shift into the low-energy domain of the electromagnetic spectrum requires other synthetic strategies such as alloying, doping, or the preparation of type II heterostructures since pure CdSe is ultimately restricted by its relatively large bulk band gap of >1.7 eV.28 As is well known, doping of semiconductor materials gives rise to a multitude of magnetic,29,30 electronic,31,32 or optical33 properties. However, it has been proven difficult to employ

INTRODUCTION The colloidal synthesis of semiconductor nanocrystals (NCs) allows the exploitation of size-dependent quantum confinement effects and the controlled tunability of associated physical properties. Cadmium selenide has served as a model system for wet-chemical synthesis since the pioneering efforts on spherical quantum dots in 1993.1 Continuous efforts have since led to the control of the size, crystal structure,2−4 and shape of these NCs, resulting in the availability of a variety of species, such as nanorods,5,6 tetrapods,7,8 and nanoplatelets (NPLs).9−11 Among them, quasi-two-dimensional (2D) NPLs present a very interesting subset of properties, such as the absence of roughness on the top and bottom facets,11,12 leading to extremely narrow photoluminescence (PL), relatively high absorption cross sections,13−15 and suppressed Auger-recombination rates,16 which make them prime candidates for lasing and light-emitting diode applications.17−19 Due to these unique properties, they have sparked efforts in understanding their electronic structure20,21 and, especially in the case of the zinc-blende particles, the underlying formation mecha© XXXX American Chemical Society

Received: February 26, 2019 Revised: June 21, 2019 Published: June 21, 2019 A

DOI: 10.1021/acs.chemmater.9b00812 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

again. In the next step, Se powder (12 mg, 0.15 mmol) was dispersed in 1.5 mL of ODE and treated by ultrasonication for 30 min. The dispersion formed was added to the flask at 100 °C, followed by an additional degassing step. Hereafter, the flask was filled with nitrogen and the temperature was raised to 250 °C. Meanwhile, Cd(OAc)2· 2H2O (120 mg, 0.45 mmol) was swiftly added at 200 °C to the mixture. Upon reaching 250 °C, the flask was left to react for 10 min. Injection Solutions. Two injection solutions were prepared. As a selenium precursor, we used selenium dioxide (27.0 mg, 0.24 mmol) dispersed in 3 mL of ODE. The cationic precursor consisted of mercury acetate (70.2 mg, 0.22 mmol) and cadmium acetate dihydrate (431.8 mg, 1.62 mmol) in 3.7 mL of ODE and 2.3 mL of oleic acid. The solutions were stirred while raising the temperature up to 100 °C until a clear solution formed. The selenium precursor was heated up to 200 °C until a clear yellow solution was formed and no more solid SeO2 was observed. Upon cooling to room temperature, both precursor solutions were loaded into syringes. Seeded Growth. The injection of the prepared solutions took place at either 200 or 240 °C, typically 2 min after the completion of the CdSe NPLs growth described above. The injection was done by means of a syringe pump at a rate of 4.5 mL/h for 10 min. Upon addition of the precursors, the reaction mixture turned black. As soon as the injection was completed, the flask was cooled in a water bath, and meanwhile, 1 mL of oleic acid was added at 180 °C. The NPLs were precipitated using a hexane/ethanol mixture and dispersed in hexane. Further centrifugation in hexane without the addition of a nonsolvent was carried out to separate spherical nanocrystals formed as a byproduct. Characterization. Optical Absorption, PL, and PL Excitation (PLE) Spectra. Optical absorption, PL, and PL excitation (PLE) spectra of diluted NPL solutions in hexane were recorded using a Cary 60 spectrophotometer (Agilent Technologies) and a FluoroMax4 spectrofluorometer (Horiba Jobin Yvon), respectively. Timeresolved PL measurements were carried out on a FluoroLog-3 spectrofluorometer (Horiba Jobin Yvon) equipped with a pulsed laser diode (λ = 410 nm, 0)(2) denotes an average photon pair number over the pulse difference range between 3 and 30. In this scale, a G(2) factor of 0% is indicative of exclusively single-photon emission, whereas a G(2) factor of 100% means either a multiphoton emitter or an ensemble of particles. Typically, single quantum dots have been shown to exhibit a low probability of simultaneous multiple photon emission due to the rapid nonradiative Auger decay of multiple-excited states, corresponding to a value of G(2) close to zero. In the case of NPLs, previous studies have reported a non-negligible biexciton quantum yield leading to appreciably larger G(2) values.19 However, this was not the case for Te-doped NPLs.41 Figure 4a presents the PL spectrum of a single particle of the four-ML Hg:CdSe240 sample showing a single PL peak at 633

Figure 4. Single-particle emission properties of the sample Hg:CdSe240: (a) PL spectrum and PL lifetime (as the inset), (b) G(2) statistics, and (c) blinking trace.

nm. Almost all measurements performed on the sample Hg:CdSe240 showed a single PL peak (see Figure SI8a for full statistics). The single-photon correlation measurement shown in Figure 4b gives a G(2) factor of 10%, which is a signature of a single-particle emitter. The PL lifetime, displayed in Figure 4a (inset), was measured simultaneously by summing over all photons arriving at the same time bins. Statistics over 16 E


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Figure 5. Three single-particle measurements of the Hg:CdSe200 sample. Particles a−c, left to right. (a) PL spectra with a double peak of two emitting states. The black dashed line marks the spectral separation profile of the 700 nm dichroic beam splitter used in the two-color correlation measurements. (b) Single-color photon correlation measurements. The low G(2) factors are the signature of a single emitter. (c) Two-color photon correlation measurements acquired in sequence. Here, the degree of photon correlation is lower, suggesting multiple emitting states. (d) Two-color blinking statistics. Signals detected from the spectral regions above and below the 700 nm dichroic beam splitter cutoff are marked in red and blue, respectively. The blinking statistics show that the emission from the two spectral regions is partially uncorrelated. (e) Two-color PL lifetime measurements fitted by a biexponential function. The contribution of the long component is higher in the red channel.

shows a larger contribution of the longer lifetime component. It is important to note that the difference with the PL lifetime values determined from the single-particle measurements and lifetimes measured in solution, at the ensemble level, is brought about by an average emission dynamics over all emitting species in the ensemble.

In the context of the observed coupling between the two emission lines, two processes typically can be implicated. One is the Förster resonance energy transfer (FRET),43,44 where an excited donor chromophore may transfer energy to an acceptor chromophore through nonradiative dipole−dipole coupling interaction. The other is the Auger-recombination process,45 in which energy from recombination of an electron−hole pair is F


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mainly the NPL edges. One can conclude that in Hg:CdSe200 mercury atoms are located primarily on the surface of the NPLs (4% Hg content via ICP vs 14% Hg via XPS), whereas in Hg:CdSe240, the distribution of mercury is more even throughout the platelets (8% Hg via ICP-OES vs 5% Hg via XPS). In the 3d3/2 and 3d5/2 spin−orbit coupled doublet of Cd, the spectra can be fitted by three species for both samples (see Figure 6a). The main strong signals at 412.0 and 405.3 eV for Cd 3d3/2 and Cd 3d5/2, respectively, are attributed to Cd2+ ions in the bulk of NPLs.48−50 We assign the weaker component at lower binding energies to cadmium ions near mercury (411.1 and 404.4 eV for Cd 3d3/2 and Cd 3d5/2, respectively) and the third component at higher binding energies to the surface Cd2+ ions bound to oleic acid (i.e., Cdoleate) (412.6 and 405.9 eV for Cd 3d3/2 and Cd 3d5/2, respectively). The binding energy position for surface cadmium ions is in good agreement with the formation of more oxidized Cd species (i.e., bound to a more electronegative atom, such as oxygen) reported in the literature.48−51 The spectra in the Hg region can also be fitted by three species for both samples. In Hg:CdSe200, a minor contribution from the Si substrate is also observable (Figure 6b bottom). We attribute the main strong signals at 104.9 and 100.8 eV for Hg 4f5/2 and Hg 4f7/2, respectively, to Hg2+ ions in the bulk of the NPLs. The position of the binding energy is very close to the values for Hg2+ ions bound to S2− ions reported in the literature.52 We also assign the weaker component at higher binding energies to the surface Hg2+ ions bound to oleic acid (Hg-oleate) (105.8 and 101.7 eV for Hg 4f5/2 and Hg 4f7/2, respectively). A third component is also detected at 103.9 and 99.8 eV for Hg 4f5/2 and Hg 4f7/2, respectively, the binding energy of which is very close to the values reported for Hg0.53,54 In the case of the Se 3d signal, analysis of the XPS spectra indicates that at least three types of selenium are present in the NPLs (Figure 6c). The main signals at 54.8 and 53.9 eV for Se 3d3/2 and Se 3d5/2, respectively, are attributed to Se2− ions in the bulk of NPLs, and the second component at 55.3 and 54.4 eV is related to the surface Se atoms.48,49 We assign the third component at lower binding energies to selenide ions bound to mercury (53.5 and 52.6 eV for Se 3d5/2 and Se 3d3/2, respectively). As can be seen in Figure 6, the surface-to-bulk ratio of Cd as well as Se for Hg:CdSe240 is higher than that detected in Hg:CdSe200, whereas the surface-to-bulk ratio of Hg in Hg:CdSe200 is higher than that in Hg:CdSe240. Based on this analysis, we assume that in Hg:CdSe240 sample mercury is rather firmly incorporated in the interior of the platelets, whereas in Hg:CdSe200 NPLs, a large fraction of mercury atoms is located on the surface. The presented results indicate that the doping of CdSe NPLs with mercury takes place in two different pathways. Certain amounts of mercury atoms are incorporated into the NPLs on or close to the surface at 200 and 240 °C, probably through readily available surface sites during the seeded growth. This process appears thermodynamically favorable owing to very similar crystal lattice parameters of CdSe and HgSe phases. This leads to the appearance of the first red peak in both samples. Differences in the employed injection temperature as well as the amount and distribution of the impurities might account for the variation in the wavelength of the PL maxima of this first red peak. We may assume that the decreasing intensity of the first red peak in the double-emission samples is caused by the doping on interstitial sites when the

transferred to a spectator electron or hole to create a hot carrier, rather than being released as a photon. Typically, when FRET takes place, the lifetime of the donor shortens, whereas the acceptor lifetime becomes longer, which is not the case here. This suggests multiple emitting centers with possible sporadic electronic coupling in between leading to Auger recombination.46 This is consistent with a picture of differently sized doping centers within a single NPL, where excitons residing at nearby centers are close enough to have some degree of electronic coupling between them. It is expected that the separation between impurities influences the PL characteristics of doped nanocrystals considerably.35 We noted that the ratio of intensities between the two additional peaks in the four-ML Hg:CdSe200 NPLs sample was not constant over time. A freshly synthesized sample showed a ratio of I(638)/I(778) = 0.89. Upon storage, the intensity of the first red PL peak increased initially over the first 2 weeks of storage, reaching a ratio of 0.97 (Figure SI11). The trend then inverted, with the first red PL peak decreasing in intensity, reaching a final ratio of 0.27 after 10 weeks of storage. This behavior might be associated with the relocation of mercury atoms inside the crystal lattice.37 At the same time, the residual band-edge emission of CdSe NPLs did not follow a similar trend relative to the second red peak, remaining constant with the ratio of I(514)/I(778) = 0.10−0.13 throughout the observed time frame. The sample Hg:CdSe240 showed a barely noticeable red shift of the additional red peak from 615 to 619 nm over 8 weeks without any changes in the FWHM (Figure SI12). To understand the chemical bonding pattern in the NPLs that might provide further insight into the mechanism of doping, we performed XPS characterization of Hg:CdSe200 and Hg:CdSe240 samples. As seen in Figure 6, we were able to

Figure 6. Core-level XPS spectra of (a) Cd 3d, (b) Hg 4f, and (c) Se 3d regions of Hg:CdSe240 (top) and Hg:CdSe200 (bottom) NPLs.

detect cadmium, mercury, and selenium (the data for carbon and oxygen are not presented here), probing the individual core-level regions of Cd 3d, Hg 4f, and Se 3d, respectively. The Cd/Se/Hg ratios are calculated to be 0.49:0.37:0.14 and 0.64:0.31:0.05 for Hg:CdSe200 and Hg:CdSe240, respectively, by taking the photoionization cross sections of 8.22, 11.91, 8.43, 10.75, 1.00, and 1.4647 for Cd 3d3/2, Cd 3d5/2, Hg 4f5/2, Hg 4f7/2, Se 3d3/2, and Se 3d5/2, respectively, using the ratio of the Cd 3d, Hg 4f, and Se 3d peak areas. A significant difference between the results of the XPS and ICP-OES analyses indicates a different distribution of Hg atoms throughout the NPLs, taking into account that ICP-OES probes the entire macroscopic composition of the material, whereas XPS probes mainly its surface. Here, one should consider the tendency of the NPLs to form stacks via face-to-face self-assembly upon drying concentrated solutions, which can lead to exposure of G


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Chemistry of Materials reaction takes place at 200 °C. In this case, self-purification effects are responsible for the development of the PL characteristics over time. Contrary to this, doping in the 240 °C sample takes place by substituting Cd in the crystal lattice, which leads to a long-term stable emission. The second red peak might originate from a different mechanism, which includes adsorption of mercury onto the CdSe NPL surface and subsequent overgrowth of this impurity by CdSe, which is controlled kinetically.35 At 240 °C, the residence time, in which a mercury ion is trapped onto the surface of the NPL, might be too long in comparison to the growth rate of the crystals, which is why no deep incorporation takes place, i.e., dopant atoms remain on the surface. At milder temperatures, however, this pathway is open and leads to mercury impurities located deeper inside the crystal lattice and energetically deeper inside the band gap of CdSe (Figure 7),

of the dopant-related emission in our samples. The radiative recombination of electrons and holes involving holes trapped on dopant-related acceptor states typically occurs on the time scale of hundreds of nanoseconds,57 i.e., at least 1 order of magnitude slower than our PL lifetime values, since it includes redox reactions of dopant ions (Cu+/Cu2+, Ag+/Ag2+). Nevertheless, based on the data collected, we cannot completely exclude the hole-trapping mechanism that would require additional advanced characterization of the samples, which is out of the scope of this investigation.

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CONCLUSIONS We have developed a simple method to incorporate mercury ions into the cubic crystal lattice of CdSe NPLs, which gives rise to an altered electronic structure and therefore new spectroscopic features. The key strategy is the synthesis of parent CdSe NPLs and subsequent incorporation of small amounts of mercury in a seeded-growth approach. The resulting NPLs are rectangular and uniform in size and shape and show up to two distinct emission maxima. Using singleparticle fluorescence spectroscopy, we demonstrated that single emitters are responsible for these signals. As follows from the results, the doping with mercury takes place in a rather arbitrary manner. This most probably leads to large variations of dopant concentration, location, and distribution inside the nanocrystal ensemble, which results in significant differences in the PL characteristics of single NPLs and peak broadening of the sample ensemble. Our results suggest that the injection temperature plays a crucial role in governing the incorporation of the dopant ions since single emitters showing two red PL signals appear only at milder conditions, whereas the higher injection temperature of 240 °C leads exclusively to one additional red emission. Further exploration of precursor NPLs of different thicknesses, reaction parameters, and doping agents is therefore a promising approach to enable the controlled tuning of PL throughout the NIR region, even though the very character of nanocrystal doping will certainly be a challenging task. While the exact mechanism, in which doping takes place, remains still elusive, an important step toward the controlled manipulation of spectroscopic properties via introduction of impurities into NPLs has been taken.

Figure 7. Energy diagram of pure (center) and mercury-doped CdSe NPLs. The high-temperature synthesis route results in a firm binding of mercury atoms within the platelets responsible for a stable singlepeak emission, whereas the low-temperature approach allows for both loosely (probably via interstitial incorporation) and firmly (substitutional) bound mercury atoms. The first causes stable single-peak emission, whereas the second creates two radiative recombination pathways of lower and higher energies. Since loosely bound Hg atoms change their location/surrounding over time, becoming firmly incorporated into the lattice, the contribution of the higher-energy pathway decreases upon storage.

resulting in the second red peak shifted to the NIR. This assumption is supported by the supposedly low diffusivity of Hg2+ in semiconductor lattices.55,56 On long time scales, the diffusion of mercury deeper inside the crystal takes place, further enhancing the change of intensity ratios between the first and second red peaks, as we have observed experimentally over a course of 10 weeks. In addition to this, distribution of Hg atoms may be strongly affected by the synthesis temperature, where 200 °C is insufficient to achieve a homogeneous distribution of Hg dopants within the crystal structure that eventually leads to a broad distribution of their energy states reflected in a more complex emission pattern. Hence, at a higher temperature of the growth (240 °C), Hg atoms have enough energy input to be evenly distributed in the NPLs reflected in a single-emission process, as implied by the XPS results discussed above. As displayed in Figure 7, we assume that in Hg-doped CdSe NPLs the recombination processes include nonradiative electron transitions from the CdSe conduction band to energy levels introduced by mercury atoms within the band gap, followed by radiative recombination with the hole residing in the valence band. An alternative mechanism could include hole trapping, creating an acceptor level with subsequent radiative electron transition from the conduction band, as proposed for the case of copper-37 and silver-doped39 CdSe NPLs. We rationalize our model based on the relatively short PL lifetimes

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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.chemmater.9b00812. Additional TEM images, PL spectra, and results of single-particle measurements; XRD pattern of Hg-doped CdSe NPLs; HAADF-STEM image and corresponding element maps of Hg:CdSe240 NPLs; TEM image and optical characterization of the quasispherical nanoparticles; fitting of the two PL peaks of the sample Hg:CdSe200 using a nonlinear function; PL spectra of the sample Hg:CdSe200; five-ML CdSe NPLs doped with Hg; and histograms of PL peak positions and G(2) factors (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.K.). *E-mail: [email protected] (V.L.). H


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Miri Kazes: 0000-0003-0796-3945 Dan Oron: 0000-0003-1582-8532 Vladimir Lesnyak: 0000-0002-2480-8755 Alexander Eychmüller: 0000-0001-9926-6279 Author Contributions ⊥

T.G. and M.K. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

Renate Schulze (TU Dresden) is gratefully acknowledged for ICP-OES measurements. T.G. and V.L. acknowledge the support by the DAAD project 57447520. The use of HZDR Ion Beam Center TEM facilities and the funding of TEM Talos by the German Federal Ministry of Education of Research (BMBF), Grant No. 03SF0451 in the framework of HEMCP, are gratefully acknowledged. This work was supported by the EU Horizon 2020 Project MiLEDi (779373).

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