Cold Flow as Versatile Approach for Stable and Highly Luminescent

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Cold flow as versatile approach for stable and highly luminescent quantum dot-salt composites Albrecht Benad, Chris Guhrenz, Christoph Bauer, Franziska Eichler, Marcus Adam, Christoph Ziegler, Nikolai Gaponik, and Alexander Eychmueller ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06452 • Publication Date (Web): 02 Aug 2016 Downloaded from http://pubs.acs.org on August 6, 2016

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ACS Applied Materials & Interfaces

Cold flow as versatile approach for stable and highly luminescent quantum dot-salt composites Albrecht Benad,‡ Chris Guhrenz,‡ Christoph Bauer, Franziska Eichler, Marcus Adam, Christoph Ziegler, Nikolai Gaponik*, and Alexander Eychmüller Physical Chemistry, Technische Universität Dresden, Bergstr. 66b, 01062 Dresden, Germany KEYWORDS: Nanocrystals, quantum dots, ionic crystal, LED, luminescent material, embedding

ABSTRACT: Since the beginning of the 80s colloidally synthesized quantum dots (QDs) are in the focus of interest due to their possible implementation for color conversion, luminescent light concentrators and lasing. For all these applications the QDs benefit from being embedded into a host matrix to ensure stability and usability. Many different host materials used for this purpose still have their individual shortcomings. Here, we present a universal, fast and flexible approach for the direct incorporation of a wide range of QDs into inorganic ionic crystals using cold flow. The QD solution is mixed with a finely milled salt, followed by the removal of the solvent under vacuum. Under high pressure (GPa) the salt powder loaded with QDs transforms into transparent pellets. This effect is well known for many inorganic salts (e.g. KCl, KBr, KI, NaCl, CsI, AgCl) from e.g. sample preparation for IR-spectroscopy. With this approach we are able to obtain strongly luminescent QD-salt composites, have precise control over the loading and provide a

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chemically robust matrix ensuring long-term stability of the embedded QDs. Furthermore, we show the photo-, chemical and thermal stability of the composite materials and their use as color conversion layers for a white light-emitting diode (w-LED). The method presented can potentially be used for all kinds of nanoparticles synthesized in organic as well as in aqueous media.


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INTRODUCTION The field of colloidal quantum dots (QDs) has developed increasingly within the last thirty years.1 To date, the syntheses of highly emitting QDs with tunable emission and with photoluminescence quantum yields (PL-QYs) reaching in the best cases unity are well studied.2–8 Current research is focusing on different applications, e.g. in biology, photonics or optoelectronics.9–12 One of the first applications related to everyday life was demonstrated by Samsung researchers.13 They proposed the use of high-quality QDs with narrow emission bandwidth and high QYs for display technologies. We think that the improvement of the processability of as prepared QDs is an important step forward for this field. Commonly, polymers, like poly(methylmethacrylate), are used for QD encapsulation. However, the preservation of the PL-QY during the immobilization of the QDs and the sensitivity to heat, oxygen, light and moisture under illumination in the solid state resulting in a decrease of the QD emission are still challenging problems.14,15 As an alternative to polymers for the protection of the QDs, Otto et al. described a facile approach in which salt crystals (e.g. NaCl, KCl, KBr, etc.) act as host matrix for aqueous-synthesized QDs, resulting in an increase of the long-term stability and temperature stability while preserving the emission behavior.16 Additional studies have shown, that even an increase of the PL-QY of aqueous-synthesized CdTe QDs upon incorporation into a NaCl salt matrix is possible.14,17 However, in spite of the attractiveness of the resulting materials, these methods include a time-consuming crystallization process lasting 1 to 4 weeks. Moreover, QDs synthesized in organic solutions may be encapsulated only after ligand exchange procedures, resulting in a decrease of the initial PL-QYs and a poor reproducibility. To overcome the issue of time Chang et al. developed a co precipitation approach with CdTe QDs using the poor solubility of BaSO4


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in aqueous solution.18 Adam et al.19 and Erdem et al.20 established a liquid-liquid diffusion assisted crystallization (LLDC) process, which makes it possible to incorporate a broad range of oil-based QDs with high PL-QY and narrow emission bandwidths directly into NaCl and LiCl, respectively. Moreover, borax crystals were used as matrix for the incorporation of phase transferred QDs and the fabrication of high-quality LEDs.21 Nevertheless, the resulting low (less than 1 wt%) QD loading in the composite material inherent to all mentioned approaches is still insufficient for most of the applications.19 Besides, the quickly prepared mixed crystals suffer from small crystallite sizes, difficulties in the adjustment of the QD loading and a limited number of matrix materials. Here, we report on a direct incorporation method of oil-based QDs into ionic salt matrices using a material property known from e.g. sample preparation for IR spectroscopy, namely cold flow. This behavior is well-known for “soft” ionic salts, e.g. alkali halides or silver halides, which under high pressure (about 2.2 GPa) feature cold flow. The grain size and moisture of the used salt influence the quality of the resulting pellet. Milled and dried salt powders transfer into transparent pellets under high pressure (see Supporting Information, Figure S1). Using this concept, a variety of oil-based QDs can be incorporated without the need for a phase transfer into the robust salt matrix with a good preservation of the initial PL-QYs. Besides, this method can also be expanded to aqueous-synthesized QD solutions by using e.g. insoluble silver halides as support material. In comparison to other procedures16,17,19,20 this is a fast and versatile technique which enables us to adjust the QD concentration up to very high loadings, to control the size and shape of the resulting crystallite, and the type of matrix material.


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RESULTS AND DISCUSSION There are several suitable salts known from literature, e.g. KCl, KBr, NaCl, CsI, AgI, etc., which show under pressure sintering behavior known as cold flow (see Supporting Information, Table S1).22 Depending on the salt, different ranges of transparency of the resulting pellet are expected. The salts differ mostly in price, color, solubility and hygroscopy, but they also show differences in the pressures required for achieving cold flow. From the broad range of salt materials used in IR-spectroscopy, KBr is most prominent. However, we are focusing in this paper on KCl, which is cheaper and less hygroscopic in comparison to KBr, making it more attractive for future applications. Before the incorporation of the QDs into the ionic salt pellets, the milled salt powder was loaded with the QDs. In general, the washed QD solution is mixed with finely milled KCl powder to form a suspension. Afterwards, the solvent is removed under reduced pressure at a rotatory evaporator resulting in an immobilization of the QDs on the surface of the salt powder grains. Subsequently, the resulting QD-impregnated host matrix undergoes cold flow at a pressure of about 2.2 GPa yielding highly fluorescent pellets of different sizes and thicknesses. For further investigations, three different oil-based QD samples with green, yellow and red emission showing QYs between 26 – 65 % were successfully embedded into the KCl matrix. A successful loading of the pure KCl salt is already indicated by a color change of the white salt under ambient light (see Supporting Information, Figure S2). The resulting impregnated powders as well as the final pellets preserve their pure color and intense emission under UV light (Figure 1a). The sintering process of the salt has no significant influence on the emission color of the pellet. Hence, no prior ligand exchange is needed and the time for the incorporation of the QDs into ionic salt matrices can be drastically reduced.


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Figure 1. (a) True color image under illumination at 365 nm of three different QD systems impregnated on KCl powders and as resulting pellets. (b) True color and microscopic (inset) images of a red-emitting KCl pellet under illumination at 365 nm. Frame (c) shows the transparent red-emitting KBr (left) and KCl (right) pellet under daylight.

From microscopic investigations of the exemplarily chosen red-emitting pellet, it can be seen that the QDs are uniformly distributed within the whole pellet (Figure 1b). As shown in Figure 1c the KBr and KCl pellet loaded with the red-emitting QDs are highly transparent after pressing and preserve an excellent visual quality. This is also seen from the optical transmittance measurements for different QD loadings and KCl salt amounts, respectively (see Supporting Information, Figure S3). Consequently, thin pellets with a lower QD loading show a higher transmittance. Below ~ 500 nm the absorption of the embedded CdSe/CdS QDs starts to dominate. Additionally, scattering effects originated from the high QD loading cannot be suppressed. Here we have to note, that the cold flow technique is also applicable to other matrices, e.g. NaCl, CsI and KBr (see Supporting Information, Figure S4 – S7). Comparative PL measurements of the initial QD solution and the resulting pellet indicate a red shift of the emission while the color purity in terms of full width at half maximum (FWHM) does not change (Figure 2). This is a typical phenomenon, which is known for embedded QDs and may be partly explained by the change of the dielectric constant of the surrounding


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media.17,19,21 Moreover, very similar, moderate red shifts of ~ 15 nm were already observed by Kalytschuk et al. and were explained by reabsorption of the high-energy photons by the incorporated QDs.14 Taking into account the relatively high QD loadings energy transfer processes between closely laying neighbor QDs like shown by Crooker et al. can be also responsible for this behavior.23 This interparticle coupling depends strongly on the particle core and the nature and thickness of the QD shell. In addition, due to a broader size distribution of the yellow-emitting CdSe/ZnS QDs and a resulting higher probability of a direct contact of smaller (donor) and larger (acceptor) QDs, a stronger red shift in comparison to the narrowly sizedistributed green-emitting QDs is expected. Furthermore, scanning electron microscopy (SEM) investigations of QDs loaded on KCl indicate that a phase separation of the QDs and the salt is not observed (see Supporting Information, Figure S8). In opposite, QDs form a sub-monolayer on the salt surface. Although the CdSe/CdS QDs are associated with each other, they are well distributed over the salt surface and do not form bulky aggregates during the impregnation process. However, a non-uniform QD distribution and an agglomeration within the final QD-salt pellet cannot be completely excluded, especially for high QD loadings. This can additionally increase the degree of red shift.


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Figure 2. PL spectra of three different oil-based QD systems in solution (solid line) and as KCl pellet (dashed line).

As a consequence of the observed changes in the luminescence behavior, all salt samples show a moderate decrease in their PL-QYs compared to the stock solution. For the green- and yellow-emitting QDs PL-QY drops from 39 % and 26 % in chloroform to 28 % and 25 % in KCl, respectively. The red sample showed a high PL-QY of 65 % in hexane, which reduces to 55 % in the salt matrix for a loading of 4 mg QDs/g KCl. Comparison of QDs embedded into different salt matrices showed that in KBr the PL-QYs are slightly lower than in KCl (see Supporting Information, Figure S9). We attribute this difference to a higher hygroscopy of KBr (see Supporting Information, Table S1) resulting in the contact of the QDs with the PL quenching water molecules.24 Additionally, this behavior can be due to an influence of the surrounding media (KCl, KBr) itself. It is known, that the stabilization of QDs with chloridecontaining complexes is resulting in a lower degree of hole trapping in comparison to bromidecontaining stabilizers.25–27 More interestingly, further investigations of the QDs in KCl showed that the initial PL-QY of 55 % in the red-emitting pellet is reduced to 34 % after milling to a fine powder. We assume, that the increase of the salt surface by milling results in a higher probability of exposing the QDs to oxygen and atmospheric water. A high level of stability is crucial for the application of the embedded QDs as luminophores. It was already shown in our previous studies that after incorporation of QDs into salt matrices the chemical stability and photostability under illumination is increasing in comparison to commonly used polymers.16,19 For this purpose, we tested the emission stability of the red-emitting sample on a blue LED chip driven at a high current level. The incorporated QDs retained 95 % of their


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initial emission intensity after 96 hours under air (without additional sealing by epoxy coating). As can be seen from Figure S10 (see Supporting Information) the peak position and the shape of the emission band are well preserved. The chemical stability of the resulting pellets against oxidizing agents has been tested with benzoyl peroxide in toluene. The QD solution was compared with the pellet, both placed in a mixture of toluene and oxidizing agent (Figure 3). A pure QD solution was used as reference.

Figure 3. True color image of the chemical stability test from different QD solutions and QDsalt mixed crystals under illumination at 365 nm. I: pure QD solution in toluene, II: QD solution in toluene with benzoyl peroxide, III: KCl pellet in toluene with benzoyl peroxide.

The emission intensity of the pure QD solution under oxidizing conditions is decreased after three days under identical conditions and almost quenched after 7 days (Figure 3). In comparison, the emission of the pellets in the oxidizing solution is nearly unchanged. The incorporation of the QDs into the salt matrix using cold flow provides a tight and robust matrix that protects the QDs against the harsh oxidizing conditions. In addition, the embedded QDs are insoluble in organic solvents.


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From our experience, a commercially available 1 W blue LED heats eventually up to ~ 70 °C during the long time operation. To prove the thermal stability, an exemplarily chosen redemitting pellet was heated up in the high boiling solvent 1-octadecene (ODE). The use of ODE assures a controllable and uniform heating with simultaneous optical characterization of the pellet. This investigations show a thermal stability of the QDs embedded into KCl up to ~ 125 °C without a change of the original strong PL. At higher temperature (~ 175 °C) the PL of the QDs is still observable but starts to decrease. In general, the incorporated QD-salt composites show environmental stability in terms of preserving high PL-QYs for months making them attractive for color conversion and similar applications. Finally, a proof-of-concept white LED was made of green- (80 mg pellet, loading 2 mg/g KCl), yellow- (30 mg pellet, loading 2 mg/g KCl) and red-emitting (20 mg pellet, loading 0.5 mg/g KCl) pellets. The loadings and thicknesses of the pellets were carefully adjusted due to the variation of their PL-QYs (Scheme 1). The pellets are stacked on top of a commercial 1 W blueemitting InGaN LED. This strategy has the advantage of reducing reabsorption within the color conversion layer compared to conventional layers with mixed emitters. Within the layered structure the single emission colors can be arranged from red to green, which reduces reabsorption of the emitted light from the following layers. Under ambient illumination the stacking of the pellets is clearly visible. In operation the proof-of-concept converted LED emits white light.


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Scheme 1. Proof-of-concept preparation of a white LED. The device is fabricated by simply stacking green-, yellow- and red-emitting pellets on a commercially available blue-emitting InGaN LED.

For practical use of the stacked pellets a protective silicone coating has to be used. This combines advantages of both the stability against atmospheric moisture provided by the silicone and the high photo-, chemical and thermal stability of the QD-salt composite materials. CONCLUSION In summary, we have demonstrated a facile and universal “cold flow” approach to the direct incorporation of highly emissive, oil-based QDs into various ionic salt crystals (KCl, KBr, NaCl, CsI,). For the incorporation, neither a time-consuming ligand exchange nor a phase transfer of the QDs into polar media are needed anymore. In addition, the QD loading can be easily adjusted to very high values, as opposed to all previously reported methods of embedding QDs into salt matrices. The resulting ionic surrounding ensures high photo-, chemical and thermal stability, which makes them attractive for applications under rigid conditions. Finally, we demonstrate a simple method for a proof-of-concept preparation of a white LED by stacking green-, yellowand red-emitting pellets on a commercially available blue-emitting InGaN LED. Since this method can be used for any nanoparticle system synthesized in organic solvents and furthermore can utilize many inorganic salt matrices, it opens a path for a variety of new composite materials with new properties and functions. In fact, nanoparticles can be engaged in anion-exchange reactions within the ionic matrix of halide salts which can be used to tune their properties. This will be the focus of a forthcoming paper.


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EXPERIMENTAL SECTION Chemicals. All chemicals used were of analytical grade or of the highest purity available. Cadmiumoxide (CdO, 99.99 %, Aldrich), cesium iodide (CsI, 99.9 %, Alfa Aeser), 1-octadecene (ODE, 90 %, Aldrich), n-octadecylphosphonic acid (ODPA, >97 %, PlasmaChem), 1-octanethiol (>98.5 %, Aldrich), oleic acid (OA, 90 %, Aldrich), oleylamine (OLA, 70 %, Aldrich), potassium bromide (KBr, >99 %, Aldrich), potassium chloride (KCl, >99.5 %, Aldrich), selenium powder (Se, 100 mesh, 99.99 %, Aldrich), sodium chloride (NaCl, 99.7 %, VWR), trioctylphosphine (TOP, 97 %, STREM), trioctylphosphine oxide (TOPO, 99 %, Aldrich), zinc acetate (Zn(OAc)2, 99.99 %, Aldrich). All salts (KCl, KBr, CsI, NaCl) are milled to a fine powder before use and dried at 120 °C in a furnace for at least 24 hours. Synthesis. Alloyed CdSe/ZnS QDs and CdSe/CdS core-shell QDs were prepared according to previous publications.7,27-29 Synthesis of CdSe/ZnS QDs. Green-emitting CdSe/ZnS QDs with an alloyed gradient shell were synthesized according to ref. 28. In a 50 mL three-necked-flask 35.3 mg (0.275 mmol) CdO, 733.9 mg (4 mmol) Zn(OAc)2, 5.5 mL OA, and 20 mL ODE were degassed for 60 min at 100 °C. The resulting suspension was heated under argon atmosphere to 310 °C. Directly afterwards, a solution of 19.7 mg (0.25 mmol) Se and 112.0 mg (3.5 mmol) S in 3 mL TOP was rapidly injected into the flask and the temperature was reduced to 300 °C. After 10 min growth time, the crude solution was cooled to room temperature and the QDs were precipitated two times with 20 mL chloroform and an excess of acetone. Finally, the QDs were redispersed in 4 mL of chloroform.


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Alloyed yellow-emitting CdSe/ZnS QDs were synthesized using the above described strategy with adjustments of the weighed portions using 51.4 mg (0.4 mmol) CdO, 733.9 mg (4 mmol) Zn(OAc)2, 4.5 mL OA, and 20 mL ODE. A solution of 31.6 mg (0.4 mmol) Se and 128.2 mg (4 mmol) S in 3 mL TOP was injected at 310 °C. After a growth time of 6 min at 300 °C the crude solution was cooled to room temperature. Synthesis of red-emitting CdSe/CdS QDs. Wurtzite CdSe core QDs (~ 2.8 nm diameter) were synthesized according to Carbone et al., washed three times with methanol, and finally redispersed in chloroform.30 The concentration of the CdSe core solution was determined according to ref. 31. For the epitaxial growth of 8 monolayers CdS we used a combination of the protocols by Chen et al.7 and Boldt et al.29 Briefly, 3 mL ODE, 3 mL OLA, and 100 nmol of the CdSe core solution were loaded into a 25 mL three-necked-flask. The resulting solution was degassed at 80 °C for 45 min and additionally for 15 min at 120 °C under vacuum. Following, the temperature was raised to 310 °C under nitrogen atmosphere. At 230 °C the injection of the shell precursor solutions of cadmium oleate (diluted in 6 mL ODE) and octanethiol (1.2 equivalent amounts refer to cadmium oleate, diluted in 6 mL ODE) via two separate syringes with a syringe pump was started. The amounts of shell precursors, which were prepared according to ref. 29, were calculated from the core particle size and the required shell thickness. The injection rate was adjusted to add 3 mL per hour. After the precursor addition was finished, 1 mL of OA was injected swiftly into the CdSe/CdS crude solution and the mixture was stirred for additional 60 min at 310 °C. After this annealing step the solution was cooled to room temperature and the resulting QDs were precipitated with acetone/ethanol. Subsequently, the QDs were redispersed in a minimal amount of chloroform and the cleaning procedure was repeated three times. The


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final precipitate was dried under a nitrogen stream, redispersed in n-hexane and passed through a PTFE syringe filter. Preparation of QD-Salt Mixed Crystal Pellets. 500 mg of KCl were mixed under stirring in a 10 mL pointed flask with 2 mL of chloroform or n-hexane (depending on the QDs solvent). A desired amount of the pure QD solution was added (standard loading: 2 mg QDs/g KCl for green- and yellow-emitting samples and 4 mg QDs/g KCl for red-emitting samples). The suspension of KCl and QDs was mixed and the solvent was removed to dryness under reduced pressure with a rotatory evaporator. The QD-impregnated salt was dried additionally under vacuum and milled before further use. According to this strategy also KBr, CsI and NaCl are loaded with QDs. By using a 6 mm pressing die (model P0819 from msscientific GmbH) 100 mg of the QD-impregnated salt undergoes cold flow (at an Atlas 15T manual hydraulic press GS15011 from Specac) which is carried out at 2 tons (equal to 2.2 GPa) for 5 min under vacuum. The resulting pellets are kept under vacuum in a desiccator to prevent loss of transparency. For loadings between 0.5 mg and 20 mg QDs/g salt no significant influence on the resulting pellets concerning transparency have been seen for the red-emitting sample. However, higher loadings typically result in a decrease in transparency and suffer from oily films on the pellet arising from the residual organics from the QDs. Stability Tests. As prepared QD-salt mixed crystal pellets were placed for 7 days in a 0.5 M toluene solution of the strong oxidizing agent benzoyl peroxide. For comparison the same test was performed with the pure QD solutions. The evolution of the PL properties was monitored by a reflex camera.


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Emission Stability Tests. The emission stability of the QD-salt mixed crystal pellets (green, yellow, red) were tested separately with a commercial blue-emitting LED (ASMT-MB00NDF00 model with a dominant wavelength at 460 nm at 350 mA from Avago Technologies), which was driven at 300 mA for 4 days. The LED was placed on an aluminum plate for passive cooling and operated with a 1 kHz on/off rate to avoid a pronounced heat generation. PL spectra were recorded before and after exposure. Temperature Stability Test. A red-emitting pellet (loading: 4 mg QDs/g KCl) was placed in a degassed ODE solution. The temperature was raised for 15 min to 125 °C and 175 °C, respectively, and the PL stability was monitored with a UV lamp. LED Preparation. The green-, yellow- and red-emitting QD-salt composite pellets were used directly for the preparation of a light-emitting diode (LED). Therefore, the pellets were placed on a commercially available blue-emitting LED (ASMT-MB00-NDF00). Absolute Measurement of Photoluminescence Quantum Yields (PL-QYs) and UV/vis Absorption Measurements. Absolute PL-QYs were determined using a Quanta-φ integrating sphere connected to a FluoroLog-3 spectrofluorometer (Horiba Jobin Yvon). QDs in solution were investigated in 10x4 mm quartz cuvettes. QD-salt pellets were characterized within quartzglass slip covered Spectralon®-holders mounted at the bottom of the integrating sphere. For green- and yellow-emitting samples excitation wavelengths of 450 nm and for red-emitting of 500 nm were used, respectively. Pellets of the pure salt were used for blank measurements. Minor differences in the measured values can be explained by the uncertainties of the absolute PL-QY measurements originating mainly from the high sensitivity of the results to the geometry and quality of the pure salt pellets that have been used for blank measurements. To reduce these


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uncertainties, all measurements were repeated at least three times and the average values are reported above. UV/vis absorption spectra of the QD solutions were recorded using a Cary 50 spectrophotometer (Varian). Transmittance Measurements. Total transmittance measurements were performed using a Cary 5000 spectrophotometer (Varian) with an integrating sphere setup. Microscope. Microscopic investigations were carried out on a Zeiss Axiostar plus fluorescence microscope. Scanning Electron Microscopy (SEM) Measurements. SEM images were carried out on a Hitachi SU8020 at 2 kV and 10 µA.

ASSOCIATED CONTENT Supporting Information. True color image of transparent KBr pellet, overview about different host matrices, true color image and PL spectra of QDs embedded into CsI and NaCl pellets, true color images of QD loaded KCl and KBr salts and QDs embedded into KCl and KBr pellets, PL spectra of QDs embedded into KBr pellets, comparison of PL-QYs between initial QD solution and QDs embedded into KBr and KCl pellets, SEM images of the immobilized QDs on the KCl salt surface, optical transmittance measurements, PL spectra of the red-emitting sample before and after operation for 96 hours on a blue LED. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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*E-Mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the M-ERA.NET Network within the project ICENAP, GA 1289/3-1 as well as by the DFG Project EY 16/14-3. We also thank the Department of Inorganic Chemistry of the TU Dresden for sharing their press facilities. REFERENCES (1)

Kovalenko, M. V.; Manna, L.; Cabot, A.; Hens, Z.; Talapin, D. V.; Kagan, C. R.; Klimov, V. I.; Rogach, A. L.; Reiss, P.; Milliron, D. J.; Guyot-Sionnnest, P.; Konstantatos, G.; Parak, W. J.; Hyeon, T.; Korgel, B. A.; Murray, C. B.; Heiss, W. Prospects of Nanoscience with Nanocrystals. ACS Nano 2015, 9 (2), 1012–1057.

(2)

Talapin, D. V.; Lee, J.-S.; Kovalenko, M. V.; Shevchenko, E. V. Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications. Chem. Rev. 2010, 110 (1), 389–458.

(3)

Reiss, P.; Protière, M.; Li, L. Core/Shell Semiconductor Nanocrystals. Small 2009, 5 (2), 154–168.

(4)

Gaponik, N.; Hickey, S. G.; Dorfs, D.; Rogach, A. L.; Eychmüller, A. Progress in the Light Emission of Colloidal Semiconductor Nanocrystals. Small 2010, 6 (13), 1364–1378.


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(5)

Page 18 of 22

Li, J. J.; Wang, Y. A.; Guo, W.; Keay, J. C.; Mishima, T. D.; Johnson, M. B.; Peng, X. Large-Scale Synthesis of Nearly Monodisperse CdSe/CdS Core/shell Nanocrystals Using Air-Stable Reagents via Successive Ion Layer Adsorption and Reaction. J. Am. Chem. Soc. 2003, 125 (41), 12567–12575.

(6)

Greytak, A. B.; Allen, P. M.; Liu, W.; Zhao, J.; Young, E. R.; Popović, Z.; Walker, B. J.; Nocera, D. G.; Bawendi, M. G. Alternating Layer Addition Approach to CdSe/CdS Core/shell Quantum Dots with near-Unity Quantum Yield and High on-Time Fractions. Chem. Sci. 2012, 3 (6), 2028–2034.

(7)

Chen, O.; Zhao, J.; Chauhan, V. P.; Cui, J.; Wong, C.; Harris, D. K.; Wei, H.; Han, H.-S.; Fukumura, D.; Jain, R. K.; Bawendi, M. G. Compact High-Quality CdSe-CdS Core-Shell Nanocrystals with Narrow Emission Linewidths and Suppressed Blinking. Nat. Mater. 2013, 12 (5), 445–451.

(8)

Jun, S.; Jang, E. Bright and Stable Alloy Core/Multishell Quantum Dots. Angew. Chemie Int. Ed. 2013, 52 (2), 679–682.

(9)

Dai, X.; Zhang, Z.; Jin, Y.; Niu, Y.; Cao, H.; Liang, X.; Chen, L.; Wang, J.; Peng, X. Solution-Processed, High-Performance Light-Emitting Diodes Based on Quantum Dots. Nature 2014, 515 (7525), 96–99.

(10)

Gao, X.; Cui, Y.; Levenson, R. M.; Chung, L. W. K.; Nie, S. In Vivo Cancer Targeting and Imaging with Semiconductor Quantum Dots. Nat. Biotechnol. 2004, 22 (8), 969–976.

(11)

Bilan, R.; Fleury, F.; Nabiev, I.; Sukhanova, A. Quantum Dot Surface Chemistry and Functionalization for Cell Targeting and Imaging. Bioconjugate Chem. 2015, 26 (4), 609– 624.

(12)

Wang, Y.; Leck, K. S.; Ta, V. D.; Chen, R.; Nalla, V.; Gao, Y.; He, T.; Demir, H. V.; Sun,


18

Page 19 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


H. Blue Liquid Lasers from Solution of CdZnS/ZnS Ternary Alloy Quantum Dots with Quasi-Continuous Pumping. Adv. Mater. 2015, 27 (1), 169–175. (13)

Jang, E.; Jun, S.; Jang, H.; Lim, J.; Kim, B.; Kim, Y. White-Light-Emitting Diodes with Quantum Dot Color Converters for Display Backlights. Adv. Mater. 2010, 22 (28), 3076– 3080.

(14)

Kalytchuk, S.; Zhovtiuk, O.; Rogach, A. L. Sodium Chloride Protected CdTe Quantum Dot Based Solid-State Luminophores with High Color Quality and Fluorescence Efficiency. Appl. Phys. Lett. 2013, 103 (10), 103105.

(15)

Pechstedt, K.; Whittle, T.; Baumberg, J.; Melvin, T. Photoluminescence of Colloidal CdSe/ZnS Quantum Dots: The Critical Effect of Water Molecules. J. Phys. Chem. C 2010, 114 (28), 12069–12077.

(16)

Otto, T.; Müller, M.; Mundra, P.; Lesnyak, V.; Demir, H. V.; Gaponik, N.; Eychmüller, A. Colloidal Nanocrystals Embedded in Macrocrystals: Robustness, Photostability, and Color Purity. Nano Lett. 2012, 12 (10), 5348–5354.

(17)

Müller, M.; Kaiser, M.; Stachowski, G. M.; Resch-Genger, U.; Gaponik, N.; Eychmüller, A. Photoluminescence Quantum Yield and Matrix-Induced Luminescence Enhancement of Colloidal Quantum Dots Embedded in Ionic Crystals. Chem. Mater. 2014, 26 (10), 3231–3237.

(18)

Chang, Y.; Yao, X.; Zhang, Z.; Jiang, D.; Yu, Y.; Mi, L.; Wang, H.; Li, G.; Yu, D.; Jiang, Y. Preparation of Highly Luminescent BaSO4 Protected CdTe Quantum Dots as Conversion Materials for Excellent Color-Rendering White LEDs. J. Mater. Chem. C 2015, 3 (12), 2831–2836.

(19)

Adam, M.; Wang, Z.; Dubavik, A.; Stachowski, G. M.; Meerbach, C.; Soran-Erdem, Z.;


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 22

Rengers, C.; Demir, H. V.; Gaponik, N.; Eychmüller, A. Liquid-Liquid Diffusion-Assisted Crystallization: A Fast and Versatile Approach Toward High Quality Mixed Quantum Dot-Salt Crystals. Adv. Funct. Mater. 2015, 25 (18), 2638–2645. (20)

Erdem, T.; Soran-Erdem, Z.; Sharma, V. K.; Kelestemur, Y.; Adam, M.; Gaponik, N.; Demir, H. V. Stable and Efficient Colour Enrichment Powders of Nonpolar Nanocrystals in LiCl. Nanoscale 2015, 7 (42), 17611–17616.

(21)

Adam, M.; Erdem, T.; Stachowski, G. M.; Soran-Erdem, Z.; Lox, J. F. L.; Bauer, C.; Poppe, J.; Demir, H. V.; Gaponik, N.; Eychmüller, A. Implementation of High-Quality Warm-White Light-Emitting Diodes by a Model-Experimental Feedback Approach Using Quantum Dot–Salt Mixed Crystals. ACS Appl. Mater. Interfaces 2015, 7 (41), 23364– 23371.

(22) Derrick, M. R.; Stulik, D.; Landry, J. M. Infrared Spectroscopy in Conservation Science; Scientific Tools for Conservation; Getty Conservation Institute: Los Angeles, 2000. (23)

Crooker, S. A.; Hollingsworth, J. A.; Tretiak, S.; Klimov, V. I. Spectrally Resolved Dynamics of Energy Transfer in Quantum-Dot Assemblies: Towards Engineered Energy Flows in Artificial Materials. Phys. Rev. Lett. 2002, 89 (18), 186802.

(24)

Pechstedt, K.; Whittle, T.; Baumberg, J.; Melvin, T. Photoluminescence of Colloidal CdSe/ZnS Quantum Dots: The Critical Effect of Water Molecules. J. Phys. Chem. C 2010, 114 (28), 12069–12077.

(25)

Greaney, M. J.; Couderc, E.; Zhao, J.; Nail, B. A.; Mecklenburg, M.; Thornbury, W.; Osterloh, F. E.; Bradforth, S. E.; Brutchey, R. L. Controlling the Trap State Landscape of Colloidal CdSe Nanocrystals with Cadmium Halide Ligands. Chem. Mater. 2015, 27 (3), 744–756.


20

Page 21 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(26)

Wang, Y.; Zhang, Y.; Zhang, W. First-Principles Study of the Halide-Passivation Effects on the Electronic Structures of CdSe Quantum Dots. RSC Adv. 2014, 4 (37), 19302– 19309.

(27)

Sayevich, V.; Guhrenz, C.; Sin, M.; Dzhagan, V. M.; Weiz, A.; Kasemann, D.; Brunner, E.; Ruck, M.; Zahn, D. R. T.; Leo, K.; Gaponik, N.; Eychmüller, A. Chloride and IndiumChloride-Complex Inorganic Ligands for Efficient Stabilization of Nanocrystals in Solution and Doping of Nanocrystal Solids. Adv. Funct. Mater. 2016, 26 (13), 2163–2175.

(28)

Bae, W. K.; Kwak, J.; Lim, J.; Lee, D.; Nam, M. K.; Char, K.; Lee, C.; Lee, S. Multicolored Light-Emitting Diodes Based on All-Quantum-Dot Multilayer Films Using Layer-by-Layer Assembly Method. Nano Lett. 2010, 10 (7), 2368–2373.

(29)

Boldt, K.; Kirkwood, N.; Beane, G. A.; Mulvaney, P. Synthesis of Highly Luminescent and Photo-Stable, Graded Shell CdSe/CdxZn1–xS Nanoparticles by In Situ Alloying. Chem. Mater. 2013, 25 (23), 4731–4738.

(30)

Carbone, L.; Nobile, C.; De Giorgi, M.; Sala, F. Della; Morello, G.; Pompa, P.; Hytch, M.; Snoeck, E.; Fiore, A.; Franchini, I. R.; Nadasan, M.; Silvestre, A. F.; Chiodo, L.; Kudera, S.; Cingolani, R.; Krahne, R.; Manna, L. Synthesis and Micrometer-Scale Assembly of Colloidal CdSe/CdS Nanorods Prepared by a Seeded Growth Approach. Nano Lett. 2007, 7 (10), 2942–2950.

(31)

Jasieniak, J.; Smith, L.; Embden, J. Van; Mulvaney, P. Re-Examination of the SizeDependent Absorption Properties of CdSe Quantum Dots. J. Phys. Chem. C 2009, 113 (45), 19468–19474.


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Table of Contents


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Cold Flow as Versatile Approach for Stable and Highly Luminescent

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