Polyurethane

Dec 28, 2017 - Colloidal quantum dots (QDs) are gaining prominence in the lighting and display industry, due to their tunable and saturated emission c...
5 downloads 22 Views 6MB Size
Download PDF
Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Aqueous-Based Cadmium Telluride Quantum Dot/Polyurethane/ Polyhedral Oligomeric Silsesquioxane Composites for Color Enhancement in Display Backlights Julian Schneider,† Tetiana Dudka,† Yuan Xiong,† Zhenguang Wang,† Nikolai Gaponik,‡ and Andrey L. Rogach*,† †

Department of Materials Science and Engineering & Centre for Functional Photonics (CFP), City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong S.A.R. ‡ Physical Chemistry, TU Dresden, Bergstr. 66b, 01062 Dresden, Germany S Supporting Information *

ABSTRACT: Colloidal quantum dots (QDs) are gaining prominence in the lighting and display industry, due to their tunable and saturated emission colors. Their primary application is currently in optical enhancement films for large sized liquid crystal displays, improving the performance in terms of color reproduction and optical efficiency. While most current QD materials in this regard are synthesized and processed in organic media, this work introduces aqueous-based CdTe QDs as a viable alternative for display applications. After discussing relevant aspects of the aqueous synthesis, we demonstrate the fabrication of all-water processed free-standing CdTe QD/polyurethane films, with high flexibility and transparency. Additional introduction of polyhedral oligomeric silsesquioxane results in a better dispersion of the QDs in the polymer matrix and improves the optical properties and especially photo/thermal stabilities of the composites, which is crucial regarding their application in display backlights.

■

in water.17,18 CdTe QDs show tunable excitonic absorption and emission, extending through most of the visible spectral range to the near-infrared, from around 490 to 750 nm. They are commonly synthesized using an aqueous-based synthesis; the introduction of short-chain thiol ligands19,20 and finetuning of the synthetic conditions have enabled the fabrication of these and a range of other high quality II−VI QDs in aqueous media, with photoluminescence quantum yields (PL QYs) well above 50%.21−26 Apart from their role as stabilizers, aqueous phase ligands offer a vast variety of surface functionalities.19 With the thiol group binding to the QD surface, the other functional moiety of the molecule (e.g., alcohol, carboxylic acid, amine) is exposed to the surrounding medium and provides versatile opportunities for postsynthetic modifications.27 In comparison to organic-based QDs, these functionalities add the benefit of covalent links to surrounding matrix materials, which is also helpful to increase the stability of embedded QDs.18,28,29

INTRODUCTION Color performance is currently one of the main parameters of interest in the display industry.1−3 In this regard, displays based on organic light emitting diodes (OLEDs) and/or quantum dot (QD)-based technologies enable access to a wide spectrum of saturated colors.4,5 QDs, which are heavily explored toward new generations of optoelectronic devices,6,7 are currently employed as passive color conversion materials in liquid crystal display (LCD) backlights, due to their tunable emission wavelengths with a narrow emission line width and broad high energy absorption bands.8−12 Furthermore, in comparison to OLEDs, QDs maintain their saturated colors also at high brightness levels, which gives them an edge over organic light emitting materials. While spherical CdSe or InP core/shell QDs are currently the most commonly used materials in this regard, there are a number of other QD materials that show promising properties for use in displays. Examples include perovskite nanocrystals or CdSe nanoplatelets, which have emerged in recent years with extremely narrow emission line widths, giving access to highly saturated emission colors.13−15 Due to their polarized emission, CdSe/ CdS core/shell nanorods can also be employed to enhance display performances.15,16 While the latter materials require rather complex synthesis and processing techniques, CdTe QDs have the advantage that they can be completely processed © XXXX American Chemical Society

Special Issue: Prashant V. Kamat Festschrift Received: November 8, 2017 Revised: December 14, 2017

A

DOI: 10.1021/acs.jpcc.7b11027 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

1:0.2:1.2. Cadmium acetate dihydrate (2 mmol) was dissolved in 50 mL of water in a two-neck flask. After addition of 2.4 mmol of the TGA ligand, the pH of the solution was adjusted to pH 12, using 1 M NaOH. The precursor solution was stirred for 5 min, and 0.4 mmol of NaTeO3 was added. To start the reaction, 0.08 g of NaBH4 was added and the flask was heated up to 100 °C, after mounting a condenser. The solution was refluxed at 100 °C to promote the growth of the CdTe QDs. After a designated amount of refluxing time, the reaction was stopped by cooling the mixture to room temperature. The product was precipitated by addition of an excess of ethanol, dried in air, and stored at 4 °C. Fabrication of CdTe QD/PU/POSS Composite Films. Designated amounts of CdTe QDs were precipitated with ethanol in a centrifuge. After removal of the supernatant, the QDs were dried and redispersed in water, yielding concentrated solutions of QDs. The dispersion of carboxylated PU was added to the QD solution in a volumetric ratio of 5:1 under vigorous stirring, resulting in a homogeneous CdTe QD/PU dispersion. For the fabrication of the films, the dispersions were drop-casted on clean glass substrates in amounts of 60 μL/cm2. After overnight drying under ambient laboratory conditions, the resulting samples were easy to peel off from the substrates, yielding stable and highly flexible freestanding films. Samples with additional OctaTMA POSS were prepared by adding 5 wt % of POSS to water, prior to the redispersion of the QDs. Characterization. UV−vis absorption spectra of both solutions and films were measured on a Varian Cary 50 spectrophotometer, and steady-state PL spectra were recorded on a Varian Cary Eclipse fluorescence spectrophotometer. Time-resolved PL spectra were recorded on a FLS920P fluorescence spectrophotometer (Edinburgh Instruments), using a picosecond pulsed diode laser (EPLED-320 320 nm) as an excitation source. The decay data were deconvoluted with the instrument response function and fitted with a multiexponential fit to derive the average fluorescence lifetimes by taking the weighted average of the exponential components. Absolute PL QYs were measured on the same spectrophotometer, using an integrating sphere. For the microscopic study of the films, a Leica DMI6000B fluorescence microscope with a 12 V/100 W halogen lamp was employed. The microscope was equipped with three band-pass filters, one for the blue (DAPI), green (GFP), and red (Cy3, RFP) spectral region, respectively. Transmission electron microscopy has been performed on a Philips CM-20 microscope.

Potential materials for color conversion applications require high PL QYs in the solid state, with long-term stability under high temperature and high photon flux conditions.30−32 Therefore, identification of suitable matrix materials or encapsulation methods is crucial for the successful employment of colloidal QDs.33 Polyurethanes (PUs) are a class of polymers which have multiple applications, from skateboard wheels to high performance surface coatings and medical devices.34 Several studies have explored PU as a matrix material for light emitting QDs,28,29,35 with a particular focus on the protection provided by the matrix.36 Water-based carboxylated PU offers excellent opportunities in this regard, yielding highly transparent, flexible, and mechanically robust films after polymerization and solvent evaporation, as recently presented for a composite with luminescent Cu-nanoclusters.37 While polymers are soft materials, protection of QDs against environmental influences is often not sufficiently guaranteed.1 Encapsulation of QDs in inorganic matrixes has shown promise in this regard,38−41 but also hybrid materials such as polyhedral oligomeric silsesquioxanes (POSS) have been used to beneficially influence the properties of composite films. POSS consists of a silica cage with different attached functionalities. Depending on the functional groups, POSS is miscible with organic- but also aqueous-based systems and can be used for a variety of applications.42,43 For example, mercaptoisobutyl-POSS has been employed by Wang et al. as a ligand in the organic synthesis of CdSe QDs.44 Furthermore, Wang et al. also demonstrated the incorporation of highly luminescent carbon dots into a POSS matrix, and the use of resulting composites for solid-state lighting.45 Recently, Huang et al. have presented the successful integration of perovskite QDs into a POSS matrix, thus increasing the stability against water and enhancing the performance of perovskite-based LEDs.46,47 For a number of polymer composites, POSS has been used as a filler, improving the thermal stability and mechanical properties of polymers, in particular for aqueous-based PU.48,49 In this work, we demonstrate CdTe QD/PU/POSS composite films for color conversion in display backlights. After introducing the scalable and all-water-based fabrication process, the resulting films are characterized with respect to their optical properties. POSS is shown to play an important role as an additional filler and stabilizer in the composite, by enhancing the optical properties and greatly improving their photo/thermal stabilities. With the improved performance, CdTe QD/POSS/PU composites are used for solid-state color conversion with a blue LED display backlight.

■

■

RESULTS AND DISCUSSION CdTe QDs as Water Processable Color Converters. The growth of CdTe QDs in aqueous medium is controlled by multiple reaction parameters, viz., the pH of the solution, the concentration of precursors, and the nature of the ligand. Figure S1a shows a representative growth trend for CdTe QDs, illustrated by the characteristic shift of the emission peak to longer wavelengths upon reaction time. By translating the spectra into emission maxima, it is easy to demonstrate and compare the emission peak shifts and corresponding growth kinetics for different QD synthetic conditions, as shown in Figure S1b for three cases with different thiol ligands, namely, thioglycerol (TG), mercaptopropionic acid (MPA), and thioglycolic acid (TGA). The synthesis with MPA as a ligand enables the fastest growth, and yields CdTe QDs with the largest size and emission in the red after 4 h of growth. For the

EXPERIMENTAL SECTION Materials. Cadmium acetate dihydrate (≥98%), sodium borohydride (99%), sodium tellurite (99%), and ethanol (analytical grade) were purchased from Sigma-Aldrich. Thioglycolic acid (TGA), thioglycerol (TG), mercaptopropionic acid (MPA), and sodium hydroxide (≥98%) were obtained from AccuChem. Octa(tetramethylammonium) polyhedral oligomeric silsesquioxane (OctaTMA POSS) was purchased from Hybrid Plastics, and carboxylated polyurethane (PU) was purchased from Shenzhen Jinxiu Waterproof Material Co., Ltd. The blue LED backlight panel was purchased from Yicai Lighting Co., Shenzhen. Milli-Q water was used in all syntheses and spectroscopic measurements. Synthesis of CdTe QDs. CdTe QDs were synthesized as previously reported,25 with an optimized Cd:Te:ligand ratio of B

DOI: 10.1021/acs.jpcc.7b11027 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. (a) Photograph of a 1 L batch sample of CdTe QDs. (b) PL spectra of red and green emitting CdTe QDs used in this work, with respective values of fwhm and PL QY. (c, d) TEM images of green (c) and red (d) emitting CdTe QDs. Scale bars are 50 nm.

Figure 2. CdTe QD/PU composite films, fabricated with red and green emitting CdTe QDs. Photographs of (a) free-standing films under ambient light, showing the transparency and uniformity and (b) the films on curved glass under UV irradiation; scale bars are 2 cm. (c) Fluorescence micrographs present the emission uniformity of the red (left) and green (right) films on a micrometer scale. (d, e) Characterization of CdTe-PU composite films by UV−vis absorption and PL spectroscopy for (d) individual green and red emitting and (e) mixed multi-QD/PU films.

same growth times, syntheses with TGA and TG yield CdTe QDs with emission in the yellow and green spectral range, respectively. These differences in the growth kinetics originate mainly in the binding affinities of the thiol ligands to the surface of QDs. Ligands with higher binding affinity, such as TG, more efficiently suppress the growth of the QDs, as they hinder the deposition of monomer species, by blocking the surface of nanoparticles.17 On the other hand, MPA has a lower binding affinity, hence allowing for fabrication of larger QDs with emission maxima reaching 800 nm, after several days of growth. Figure S1b demonstrates that the right choice of stabilizer facilitates precise manipulation of the emission maxima of CdTe QDs by variation of the reaction time. With regard to color gamut in displays, this enables the well-

controllable fabrication of QDs with well-defined emission wavelengths, as growth times in the aqueous syntheses of II− VI semiconductor QDs are typically on a scale of hours, instead of seconds, as in high temperature conditions of syntheses conducted in organic solvents. Another advantage of the aqueous synthesis approach, namely, the ability to scale up batch type synthesis in the flask to a liter scale without compromising the quality of the material, is presented by the photograph of the red emitting CdTe QDs given in Figure 1a. For the studies performed in this work, we have chosen two species of TGA-capped CdTe QDs, whose excitonic emission peaks are located at 540 and 630 nm (Figure 1b), with PL QYs of 65 and 60%, respectively. The spectral line widths of the emission peaks, which define the color purity and correspondC

DOI: 10.1021/acs.jpcc.7b11027 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

micrographs to further highlight QD distribution on a microscopic level. In these images, the QDs show no sign of agglomeration, which results in microscopically uniform fluorescence intensities. Absorption and PL spectra are shown for individual and double-size mixed CdTe QD/PU composite films in parts d and e of Figure 2, respectively. The main contribution to the optical properties of the composite films comes from CdTe QDs, with a negligible influence from the PU polymer (Figure S3). The first absorption peak and the excitonic emission of the individual green emissive film are located at 510 and 545 nm, respectively. For the red emissive composite, the absorption peak is at 601 nm, while the excitonic emission is at 638 nm. In comparison to the values recorded in solution, the first absorption peaks do not shift, while the excitonic emission of CdTe QDs in both composites is shifted by 5 nm (green) and 8 nm (red) to longer wavelengths. Among several possible causes for such a shift are the change in the dielectric surrounding medium, fluorescence resonance energy transfer (FRET) emerging from agglomeration of QDs into clusters, and the inner filter effect (reabsorption).39,40,51,52 Recent work by Koc et al. discussed these effects in composite films of CdSe/CdS QDs and poly(styrene-ethylene-butylene-styrene). They highlighted that optical properties of QDs dispersed in solid polymer matrixes show complex behavior, with effects from concentration dependent photon scattering, reabsorption, and the degree of QD agglomeration.51 The absolute PL QYs of our samples are found to be 19% for the green emitting and 18% for the red emitting CdTe QD film, which corresponds to a drop of around 70% compared to the respective PL QYs in solution. Time-resolved PL spectroscopy reveals average PL lifetimes of 31 ± 0.5 and 26 ± 0.5 ns for the green and red emitting QDs in the individual films, which in combination with the decreased PL QYs indicate the stronger contribution from nonradiative decay channels. Rather than solely originating from effects of reabsorption, these results, especially the changes in radiative and nonradiative PL lifetimes (see the Supporting Information, Tables S1 and S2) point toward changes in the QD’s state and surrounding, which will be discussed in detail below. Figure 2e shows spectra of mixed green and red emitting CdTe QDs in PU, demonstrating the fabrication of composites with multiple species of QDs. Such multi-QD composites provide an alternative to stacked films of individual QDs in the final backlight device structure.16 The absorption spectra in Figure 2e show well-defined excitonic features of green and red emitting CdTe QDs in the mixed film. Excitation at 450 nm leads to the excitonic emission of both species at 542 and 619 nm in the films with lower QD concentration. In the films with higher QD concentration, the PL peaks shift to 544 and 624 nm, respectively. We note that, apart from the peak position, the intensity of each component is also highly concentration dependent in such multi-QD films. Similar results have been previously reported for mixed QDs, dispersed in LiCl macrocrystals, where double-sized distributions of QDs experienced strong nonradiative FRET.53 Optical Characteristics and Improved Stability of CdTe QD/PU/POSS Composites. While the facile fabrication of the aqueous-based CdTe QD/PU composites is an advantage of this kind of system, the affected emission characteristics after QD dispersion in the PU matrix indicate a strong, but undesirable effect of the surrounding matrix material. To understand the origin of this effect and to improve

ing accuracy in the color space, confirm their quality as QD color converters, with full width at half-maximum (fwhm) values of 40 nm for the green emitting and 46 nm for the red emitting CdTe QDs. In the aqueous synthesis, the growth mechanism of QDs is mostly governed by Ostwald ripening, which typically results in size broadening with continuous reaction times.50 Therefore, aqueous CdTe QDs often show rather broad emission line widths, in comparison to their counterparts, synthesized in organic media, where synthesis typically follows a diffusion controlled growth.17 However, optimized synthetic parameters can prevent such size broadening and enable the synthesis of larger CdTe QDs with a relatively narrow spectral line width, suitable for high quality color conversion. The corresponding absorption spectra in Figure S2a also show well-defined excitonic peaks for both green and red emitting CdTe QD species, located at 510 and 601 nm, respectively. After deposition of the CdTe QDs in the solid state, their emission characteristics may be subject to changes, caused by aggregation and/or the inner filter effects at higher QD concentrations (reabsorption), caused by relatively small Stokes shifts of ∼30 nm. PL lifetimes can provide further information on the emission characteristics of QDs, and may help to identify changes after deposition in the solid state. In combination with the PL QY, PL lifetimes can be used to evaluate radiative and nonradiative decay channels. Figure S2b shows PL decay curves, for both green and red emitting CdTe QDs, with average PL lifetimes of 39 ± 0.5 and 33 ± 0.5 ns, respectively. In comparison to green emitting CdTe QDs, the red emitting QDs show a shorter average PL lifetime, in combination with lower PL QYs, which indicates exciton recombination with a larger contribution from fast nonradiative decay channels (see the Supporting Information, Tables S1 and S2). This effect can be related to the formation of QD agglomerates in solution, as discussed in detail in ref 26. The TEM images, presented in Figure 1c,d, further indicate this trend and show particles with sizes of 3.1 ± 0.5 and 4.7 ± 0.9 nm, respectively. Fabrication and Optical Characterization of CdTe QD/PU Composites. To fabricate CdTe QD/PU composite films, designated amounts of dried QD powders are redissolved in a fixed amount of water, and mixed with the aqueous carboxylated PU dispersion. Initial experiments with the system showed negative effects from an excessive amount of water on the quality of the resulting films, which were rather inhomogeneous and experienced cracking. Thus, the amount of water is optimized and fixed to a volumetric mixture of 1:5, additional water to aqueous PU dispersion. The composite films are fabricated by drop-casting the CdTe QD-PU mixtures on glass substrates. After overnight aging under ambient conditions, dried films with an average thickness of 160 μm are obtained, which can easily be detached from the glass substrates. Examples of the fabricated films under ambient and UV light exposure are shown in Figure 2a and b, respectively. The free-standing composite films are highly transparent under ambient light and show good uniformity with bright luminescence under UV irradiation, as demonstrated in Figure 2a,b. Free-standing PU-based films (shown in Figure 2b) are flexible and possess high tensile strength, which was previously highlighted by Wang et al.37 While parts a and b of Figure 2 demonstrate the good macroscopic uniformity of the composite films, Figure 2c presents their fluorescence D

DOI: 10.1021/acs.jpcc.7b11027 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 3. Comparison of the CdTe QD/POSS/PU composite films to the CdTe QD/PU composite with regard to (a) emission characteristics, (b, c) PL decay curves of green (b) and red (c) emitting films with solid lines presenting corresponding multiexponential fits, (d) thermal stability, and (e) photostability under ambient light.

prolonged average PL lifetimes of the green (36 ± 0.5 ns) and red (34 ± 0.5 ns) emitting composites, which are derived from PL decay curves shown in Figure 3b,c. As summarized in Tables S1 and S2 (Supporting Information), both kinds of samples show improved optical properties, due to the smaller contribution from fast nonradiative decay channels in the CdTe QD/POSS/PU composites. Perhaps most importantly, the enhanced optical characteristics are accompanied by strongly improved stabilities of the CdTe QD/POSS/PU composites, in comparison to pure CdTe QD/PU films. Figure 3d shows that heating of films in air to 80 °C for up to 8 h has very little effect on the PL intensity of the CdTe QD/POSS/ PU composites. Further thermal stability measurements reveal that the films retain 63% of their initial PL intensity after 48 h of continuous heating. Similar results are obtained for the photostability tests under ambient light conditions, as presented in Figure 3e. In comparison to the CdTe QD/PU composites, addition of POSS increases the photostability of the films significantly, so that they still retain 60% of their

the performance of the resulting composite materials, we introduce an additional componentpolyhedral oligomeric silsesquioxane (POSS)which has shown a beneficial role in several composite systems.45−47 In this work, water-soluble OctaTMA POSS is chosen; the resulting CdTe QD/PU/POSS films showed similar mechanical characteristics compared to the CdTe QD/PU films, with high transparency and flexibility. In comparison to CdTe QD/PU films, the absorption and PL spectra of the CdTe QD/POSS/PU films show similar features. However, there are some notable differences in terms of several key issues. As shown in Figure 3a, introduction of OctaTMA POSS reduces the emission peak shifts after embedding the CdTe QDs into the solid matrix. Films with the same QD concentrations show emission peak shifts of less than 5 nm from the original peak position in solution. Furthermore, PL QYs of the CdTe QD/POSS/PU composites are 28% (green) and 24% (red), which are both above the PL QY values of 19 and 18% for the green and red emitting CdTe QD/PU composite films. This is further reflected in the E

DOI: 10.1021/acs.jpcc.7b11027 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 4. Color performance of the CdTe QD/POSS/PU composite. (a) Emission profile of the backlight unit, containing one red and one green emitting CdTe QD/POSS/PU film. A corresponding photograph of the white light emitting device is presented in the inset. (b) CIE 1931 color space with a color triangle (in white), based on the green and red emitting CdTe QDs used in this work. The second color triangle (dashed black) shows the possible color space of a backlight unit using another species of CdTe QDs, as discussed in text. (c, d) Full scale backlight prototype with a mixed CdTe QD/POSS/PU film under ambient light (c) and UV excitation (d).

such green and red emitting films offer a white light source, as shown in the inset of Figure 4a, with the corresponding triplepeak spectral emission profile. The combined white emitting backlight panel is driven with a voltage of 2.9 V and a current of 12 mA. The color coordinates of this light source are calculated in the CIE 1931 color space, created by the International Commission on Illumination. Translation of the spectral data results in color coordinates (x, y) of 0.29, 0.68 and 0.66, 0.34 for the green and red QDs, respectively. In combination with a blue LED, which has the coordinates 0.15, 0.03, we obtain the CIE color triangle, as presented in white in Figure 4b. The white color triangle presented in Figure 4b covers 93% of the NTSC standard, and represents the red and green emitting CdTe QD/POSS/PU composites previously discussed in this work. Significant improvement of the color space is possible by using QDs with emission wavelengths located in the shorter green spectral range. While the shift toward shorter green emission wavelengths is difficult to achieve with TGAbased CdTe QDs, CdTe QDs synthesized with TG show promising properties in this respect, as presented in Figure S1b. An example of such TG stabilized CdTe QDs is presented in Figure S4 with the corresponding emission data. While the PL QYs of TG stabilized CdTe QDs are typically lower (32%), the emission peak is located at 525 nm with a spectral line width of only 27 nm. The employment of such QDs would result in a much wider color space, predominantly in the green and blue spectral region, with an improved gamut of 116% NTSC, as indicated by the dark dashed line in Figure 4b. Besides the improvements for the color gamut, the shift in the emission peaks would also result in reduced color cross talk in the color filters, enhancing the transmission efficiency of the display. Figure 4c,d shows a full backlight prototype under ambient light and UV excitation in the double-size mixed QD configuration. While in this case the QD concentrations are not optimized for the white balanced color conversion, the images demonstrate that all-water-based CdTe/POSS/PU composites can easily be integrated onto blue LED panels, creating cheap and large scale white light sources.

initial PL intensity after 7 days of ambient light exposure. These results confirm the beneficial role of POSS and point toward better individual stabilization of the QDs in the PU/ POSS composites. Given the bulky structure of POSS and the negative charge on its oxygen atoms, steric stabilization as well as stabilization by electrostatic repulsion may help to prevent the formation of CdTe QD clusters in the films. Such electrostatic stabilization was also indicated by additional experiments in solution, where aggregated CdTe QDs could be redispersed with a high stability, after addition of POSS. In films, the lower degree of QD agglomeration can also be concluded from comparison of the emission peak shifts in Figure 3a, which show that films without POSS show larger shifts at the same CdTe QD concentrations. Instead of the inner filter effect,51 which should only depend on the CdTe QD concentration in the film, strong peak shifts are a sign for the formation of CdTe QD clusters. A larger drop to a remaining PL QY of 8% for the CdTe QD/PU composite at higher concentrations (in comparison to a PL QY of 19% for the CdTe QD/POSS/PU composite) reinforces this argument. Overall, the addition of OctaTMA POSS helps to physically separate the CdTe QDs in the polymer matrix, resulting in increased stability and improved optical properties of QDs. While the QD stabilization by POSS most probably accounts for the improved photostability, the enhancement of the thermal stability is more likely to originate in the changed structure of the polymer matrix, as was previously reported for physical blends of POSS/PU. In such composites, the bulky POSS modulates the polymer into a more rigid structure,54 which is prone to less damage under heat, thus maintaining better protection for the CdTe QDs against oxygen and moisture. CdTe QD/POSS/PU Composites in Display Backlights. As emphasized before, easy tunability of the emission wavelength is an advantage of the aqueous synthesis of CdTe QDs (see Figure S1). CdTe QDs, which are used in this work, have emission wavelengths of 540 and 630 nm in solution, which provides a good match to the color filters of commercial LCDs.5,9 In the previous section, we demonstrated the beneficial role of POSS in the CdTe QD/PU/POSS composites; in combination with a blue LED backlight panel, F

DOI: 10.1021/acs.jpcc.7b11027 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

■

(4) Erdem, T.; Demir, H. V. Colloidal Nanocrystals for Quality Lighting and Displays: Milestones and Recent Developments. Nanophotonics 2016, 5, 74−95. (5) Talapin, D. V.; Steckel, J. Quantum Dot Light-Emitting Devices. MRS Bull. 2013, 38, 685−691. (6) Kamat, P. V. Quantum Dot Solar Cells. The Next Big Thing in Photovoltaics. J. Phys. Chem. Lett. 2013, 4, 908−918. (7) Dai, X.; Deng, Y.; Peng, X.; Jin, Y. Quantum-Dot Light-Emitting Diodes for Large-Area Displays: Towards the Dawn of Commercialization. Adv. Mater. 2017, 29, 1607022. (8) Kim, T.-H.; Cho, K.-S.; Lee, E. K.; Lee, S. J.; Chae, J.; Kim, J. W.; Kim, D. H.; Kwon, J.-Y.; Amaratunga, G.; Lee, S. Y.; et al. Full-Colour Quantum Dot Displays Fabricated by Transfer Printing. Nat. Photonics 2011, 5, 176−182. (9) Luo, Z.; Xu, D.; Wu, S.-T. Emerging Quantum-Dots-Enhanced LCDs. J. Disp. Technol. 2014, 10, 526−539. (10) Chen, H.; He, J.; Wu, S. T. Recent Advances on Quantum-DotEnhanced Liquid Crystal Displays. IEEE J. Sel. Top. Quantum Electron. 2017, 23, 1900611. (11) Kim, H.-J.; Shin, M.-H.; Lee, J.-Y.; Kim, J.-H.; Kim, Y.-J. Realization of 95% of the Rec. 2020 Color Gamut in a Highly Efficient LCD Using a Patterned Quantum Dot Film. Opt. Express 2017, 25, 10724−10734. (12) Rogach, A. L. Semiconductor Nanocrystal Quantum Dots; Springer: Vienna, Austria, 2008. (13) Huang, H.; Polavarapu, L.; Sichert, J. A.; Susha, A. S.; Urban, A. S.; Rogach, A. L. Colloidal Lead Halide Perovskite Nanocrystals: Synthesis, Optical Properties and Applications. NPG Asia Mater. 2016, 8, e328. (14) Zhou, Q.; Bai, Z.; Lu, W. G.; Wang, Y.; Zou, B.; Zhong, H. In Situ Fabrication of Halide Perovskite Nanocrystal-Embedded Polymer Composite Films with Enhanced Photoluminescence for Display Backlights. Adv. Mater. 2016, 28, 9163−9168. (15) Cunningham, P. D.; Souza, J. B., Jr.; Fedin, I.; She, C.; Lee, B.; Talapin, D. V. Assessment of Anisotropic Semiconductor Nanorod and Nanoplatelet Heterostructures with Polarized Emission for Liquid Crystal Display Technology. ACS Nano 2016, 10, 5769−5781. (16) Srivastava, A. K.; Zhang, W.; Schneider, J.; Rogach, A. L.; Chigrinov, V. G.; Kwok, H.-S. Photoaligned Nanorod Enhancement Films with Polarized Emission for Liquid-Crystal-Display Applications. Adv. Mater. 2017, 29, 1701091. (17) Jing, L. H.; Kershaw, S. V.; Li, Y. L.; Huang, X.; Li, Y. Y.; Rogach, A. L.; Gao, M. Y. Aqueous Based Semiconductor Nanocrystals. Chem. Rev. 2016, 116, 10623−10730. (18) Lesnyak, V.; Gaponik, N.; Eychmüller, A. Colloidal Semiconductor Nanocrystals: The Aqueous Approach. Chem. Soc. Rev. 2013, 42, 2905−2929. (19) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Hoppe, K.; Shevchenko, E. V.; Kornowski, A.; Eychmüller, A.; Weller, H. ThiolCapping of CdTe Nanocrystals: An Alternative to Organometallic Synthetic Routes. J. Phys. Chem. B 2002, 106, 7177−7185. (20) Rogach, A. L.; Katsikas, L.; Kornowski, A.; Su, D. S.; Eychmüller, A.; Weller, H. Synthesis and Characterization of ThiolStabilized CdTe Nanocrystals. Ber. Bunsen. Phys. Chem. 1996, 100, 1772−1778. (21) Rogach, A. L.; Franzl, T.; Klar, T. A.; Feldmann, J.; Gaponik, N.; Lesnyak, V.; Shavel, A.; Eychmüller, A.; Rakovich, Y. P.; Donegan, J. F. Aqueous Synthesis of Thiol-Capped CdTe Nanocrystals: Stateof-the-Art. J. Phys. Chem. C 2007, 111, 14628−14637. (22) Guo, J.; Yang, W. L.; Wang, C. C. Systematic Study of the Photoluminescence Dependence of Thiol-Capped CdTe Nanocrystals on the Reaction Conditions. J. Phys. Chem. B 2005, 109, 17467− 17473. (23) Shavel, A.; Gaponik, N.; Eychmüller, A. Factors Governing the Quality of Aqueous CdTe Nanocrystals: Calculations and Experiment. J. Phys. Chem. B 2006, 110, 19280−19284. (24) Xu, Y. L.; Hao, J. J.; Niu, X. Y.; Qi, S. D.; Chen, H. L.; Wang, K.; Chen, X. G.; Yi, T. Seed-Mediated Growth Approach for Rapid Synthesis of High-Performance Red-Emitting Cdte Quantum Dots in

CONCLUSIONS We explored the use of CdTe QDs as color converters in display backlights, making use of the inherent advantage of their well-controlled and up-scalable synthesis. With narrow spectral emission line widths and PL QYs of beyond 60%, two species of red and green emitting CdTe QDs are assembled in a matrix of carboxylated PU, yielding highly flexible freestanding composite films. To address issues of QD aggregation and corresponding negative effects on their optical characteristics, we included an additional componentOctaTMA POSSas an electrostatic and steric stabilizer. The addition of this bulky compound not only improved the optical properties but also enhanced the photostability and thermal stability of the resulting composites, thus achieving a better stabilization of the QDs in the surrounding matrix. With the CdTe QD/POSS/PU composites, we fabricated a white backlight source with a color space achieving 93% of NTSC standard. Since the optical properties of such composite materials are defined by the characteristics of QDs, further improvements are directly related to future developments in the synthesis. As a fully water-based process, the presented strategy is easily scalable and holds promise for future applications.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b11027.

■

Growth kinetics of CdTe QDs stabilized by different ligands; optical characterization of TGA stabilized CdTe QDs; optical characterization of PU films; and optical characterization of TG capped CdTe QDs (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Julian Schneider: 0000-0002-3052-2622 Nikolai Gaponik: 0000-0002-8827-2881 Andrey L. Rogach: 0000-0002-8263-8141 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We acknowledge financial support from the NSFC/RGC Joint Research Scheme (N_CityU108/17), from the Germany/ Hong Kong Joint Research Scheme sponsored by the Research Grants Council of Hong Kong and the German Academic Exchange Service (G-CityU110/14 and DAAD 57136466), and from the Alexander von Humboldt Foundation.

■

REFERENCES

(1) Chen, J.; Hardev, V.; Yurek, J. Quantum Dot Displays: Giving LCDs a Competitive Edge through Color. Inf. Display 2014, 11, 4− 13. (2) Wang, K.; Sun, X. W. Quantum-Dot and Quantum-Rod Displays - the Next Big Wave. Inf. Display 2016, 32, 6−14. (3) Chen, H.; Zhu, R.; Tan, G.; Li, M.-C.; Lee, S.-L.; Wu, S.-T. Enlarging the Color Gamut of Liquid Crystal Displays with a Functional Reflective Polarizer. Opt. Express 2017, 25, 102−111. G

DOI: 10.1021/acs.jpcc.7b11027 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C Aqueous Phase and Their Application in Detection of Highly Reactive Oxygen Species. Chem. Eng. J. 2016, 299, 201−208. (25) Wu, S. L.; Dou, J.; Zhang, J.; Zhang, S. F. A Simple and Economical One-Pot Method to Synthesize High-Quality Water Soluble Cdte Qds. J. Mater. Chem. 2012, 22, 14573−14578. (26) Kershaw, S. V.; Kalytchuk, S.; Zhovtiuk, O.; Shen, Q.; Oshima, T.; Yindeesuk, W.; Toyoda, T.; Rogach, A. L. Multiple Exciton Generation in Cluster-Free Alloy CdxHg1‑xTe Colloidal Quantum Dots Synthesized in Water. Phys. Chem. Chem. Phys. 2014, 16, 25710−25722. (27) Hines, D. A.; Kamat, P. V. Recent Advances in Quantum Dot Surface Chemistry. ACS Appl. Mater. Interfaces 2014, 6, 3041−3057. (28) Cao, X. D.; Li, C. M.; Bao, H. F.; Bao, Q. L.; Dong, H. Fabrication of Strongly Fluorescent Quantum Dot-Polymer Composite in Aqueous Solution. Chem. Mater. 2007, 19, 3773−3779. (29) Luo, X. T.; Han, J. S.; Ning, Y.; Lin, Z.; Zhang, H.; Yang, B. Polyurethane-Based Bulk Nanocomposites from 1-ThioglycerolStabilized CdTe Quantum Dots with Enhanced Luminescence. J. Mater. Chem. 2011, 21, 6569−6575. (30) Wang, C.-F.; Fan, F.; Sabatini, R. P.; Voznyy, O.; Bicanic, K.; Li, X.; Sellan, D. P.; Saravanapavanantham, M.; Hossain, N.; Chen, K.; et al. Quantum Dot Color-Converting Solids Operating Efficiently in the kw/cm2 Regime. Chem. Mater. 2017, 29, 5104−5112. (31) Abe, S.; Joos, J. J.; Martin, L. I. D. J.; Hens, Z.; Smet, P. F. Hybrid Remote Quantum Dot/Powder Phosphor Designs for Display Backlights. Light: Sci. Appl. 2016, 6, e16271. (32) Kim, H. Y.; Yoon, D. E.; Jang, J.; Lee, D.; Choi, G. M.; Chang, J. H.; Lee, J. Y.; Lee, D. C.; Bae, B. S. Quantum Dot/Siloxane Composite Film Exceptionally Stable against Oxidation under Heat and Moisture. J. Am. Chem. Soc. 2016, 138, 16478−16485. (33) Kershaw, S. V.; Jing, L.; Huang, X.; Gao, M.; Rogach, A. L. Materials Aspects of Semiconductor Nanocrystals for Optoelectronic Applications. Mater. Horiz. 2017, 4, 155−205. (34) Santerre, J. P.; Woodhouse, K.; Laroche, G.; Labow, R. S. Understanding the Biodegradation of Polyurethanes: From Classical Implants to Tissue Engineering Materials. Biomaterials 2005, 26, 7457−7470. (35) Zhu, M.; Peng, X. Q.; Wang, Z. W.; Bai, Z. L.; Chen, B. K.; Wang, Y. T.; Hao, H. Y.; Shao, Z. Q.; Zhong, H. Z. Highly Transparent and Colour-Tunable Composite Films with Increased Quantum Dot Loading. J. Mater. Chem. C 2014, 2, 10031−10036. (36) Lesnyak, V.; Dubavik, A.; Plotnikov, A.; Gaponik, N.; Eychmü ller, A. One-Step Aqueous Synthesis of Blue-Emitting Glutathione-Capped ZnSe1‑xTex Alloyed Nanocrystals. Chem. Commun. 2010, 46, 886−888. (37) Wang, Z.; Chen, B.; Zhu, M.; Kershaw, S. V.; Zhi, C.; Zhong, H.; Rogach, A. L. Stretchable and Thermally Stable Dual Emission Composite Films of On-Purpose Aggregated Copper Nanoclusters in Carboxylated Polyurethane for Remote White Light-Emitting Devices. ACS Appl. Mater. Interfaces 2016, 8, 33993−33998. (38) 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, 5348−5354. (39) Adam, M.; Erdem, T.; Stachowski, G. M.; Soran-Erdem, Z.; Lox, J. F. L.; Bauer, C.; Poppe, J.; Demir, L. V.; Gaponik, N.; Eychmüller, A. Implementation of High-Quality Warm-White LightEmitting Diodes by a Model-Experimental Feedback Approach Using Quantum Dot-Salt Mixed Crystals. ACS Appl. Mater. Interfaces 2015, 7, 23364−23371. (40) 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, 103105. (41) Adam, M.; Gaponik, N.; Eychmüller, A.; Erdem, T.; SoranErdem, Z.; Demir, H. V. Colloidal Nanocrystals Embedded in Macrocrystals: Methods and Applications. J. Phys. Chem. Lett. 2016, 7, 4117−4123.

(42) Rizvi, S. B.; Yildirimer, L.; Ghaderi, S.; Ramesh, B.; Seifalian, A. M.; Keshtgar, M. A Novel POSS-Coated Quantum Dot for Biological Application. Int. J. Nanomed. 2012, 7, 3915−3927. (43) Zhao, X.; Du, J.; Wu, Y. Z.; Liu, H. Z.; Hao, X. P. Synthesis of Highly Luminescent POSS-Coated CdTe Quantum Dots and Their Application in Trace Cu2+ Detection. J. Mater. Chem. A 2013, 1, 11748−11753. (44) Wang, Y.; Vaneski, A.; Yang, H. H.; Gupta, S.; Hetsch, F.; Kershaw, S. V.; Teoh, W. Y.; Li, H. R.; Rogach, A. L. Polyhedral Oligomeric Silsesquioxane as a Ligand for CdSe Quantum Dots. J. Phys. Chem. C 2013, 117, 1857−1862. (45) Wang, Y.; Kalytchuk, S.; Wang, L. Y.; Zhovtiuk, O.; Cepe, K.; Zboril, R.; Rogach, A. L. Carbon Dot Hybrids with Oligomeric Silsesquioxane: Solid-State Luminophores with High Photoluminescence Quantum Yield and Applicability in White Light Emitting Devices. Chem. Commun. 2015, 51, 2950−2953. (46) Huang, H.; Chen, B. K.; Wang, Z. G.; Hung, T. F.; Susha, A. S.; Zhong, H. Z.; Rogach, A. L. Water Resistant CsPbX3 Nanocrystals Coated with Polyhedral Oligomeric Silsesquioxane and Their Use as Solid State Luminophores in All-Perovskite White Light-Emitting Devices. Chem. Sci. 2016, 7, 5699−5703. (47) Huang, H.; Lin, H.; Kershaw, S. V.; Susha, A. S.; Choy, W. C. H.; Rogach, A. L. Polyhedral Oligomeric Silsesquioxane Enhances the Brightness of Perovskite Nanocrystal-Based Green Light-Emitting Devices. J. Phys. Chem. Lett. 2016, 7, 4398−4404. (48) Madbouly, S. A.; Otaigbe, J. U. Recent Advances in Synthesis, Characterization and Rheological Properties of Polyurethanes and POSS/Polyurethane Nanocomposites Dispersions and Films. Prog. Polym. Sci. 2009, 34, 1283−1332. (49) Nanda, A. K.; Wicks, D. A.; Madbouly, S. A.; Otaigbe, J. U. Nanostructured Polyurethane/POSS Hybrid Aqueous Dispersions Prepared by Homogeneous Solution Polymerization. Macromolecules 2006, 39, 7037−7043. (50) Talapin, D. V.; Rogach, A. L.; Shevchenko, E. V.; Kornowski, A.; Haase, M.; Weller, H. Dynamic Distribution of Growth Rates within the Ensembles of Colloidal II−VI and III−V Semiconductor Nanocrystals as a Factor Governing Their Photoluminescence Efficiency. J. Am. Chem. Soc. 2002, 124, 5782−5790. (51) Koc, M. A.; Raja, S. N.; Hanson, L. A.; Nguyen, S. C.; Borys, N. J.; Powers, A. S.; Wu, S.; Takano, K.; Swabeck, J. K.; Olshansky, J. H.; et al. Characterizing Photon Reabsorption in Quantum Dot-Polymer Composites for Use as Displacement Sensors. ACS Nano 2017, 11, 2075−2084. (52) Hoffman, J. B.; Alam, R.; Kamat, P. V. Why Surface Chemistry Matters for QD−QD Resonance Energy Transfer. ACS Energy Lett. 2017, 2, 391−396. (53) Erdem, T.; Soran-Erdem, Z.; Kelestemur, Y.; Gaponik, N.; Demir, H. V. Excitonic Improvement of Colloidal Nanocrystals in Salt Powder Matrix for Quality Lighting and Color Enrichment. Opt. Express 2016, 24, A74. (54) Tanaka, K.; Chujo, Y. Advanced Functional Materials Based on Polyhedral Oligomeric Silsesquioxane (POSS). J. Mater. Chem. 2012, 22, 1733−1746.

H

DOI: 10.1021/acs.jpcc.7b11027 J. Phys. Chem. C XXXX, XXX, XXX−XXX