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Abstract
Quantum-dot (QD) ligands were modified and hydrosilylated with a siloxane matrix to improve the quantum efficiency and stability of the QDs. Conventional oleic acid (OA) ligands were exchanged with vinyl ligands without any reduction in the quantum yield. After ligand modification, hydrosilylation was induced between the vinyl ligands on the QDs (vinyl QDs) and a siloxane matrix, resulting in a uniform QD dispersion in the matrix. The hydrosilylated QDs in siloxane showed 23% higher photoluminescence intensity than OA QDs blended in siloxane after storage for 30 days at 85 °C under 85% relative humidity. The QDs also showed 22.3% higher UV/thermal stability than OA QDs in siloxane after 29 h under a high LED photon flux. This study demonstrates that the chemical reaction of QD ligands with polymer matrices can improve the QDs’ dispersion and stability.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Colloidal quantum dots (QDs) have recently received tremendous attention because of their high color purity, high photoluminescence quantum yield (PL QY), and tunable emission wavelength [1–4]. QDs have been commercialized for use in liquid-crystal displays in the form of color-conversion films, which typically contain QDs dispersed in large-area acrylate polymer films sandwiched between two gas-barrier layers to suppress QD degradation by exposure to oxygen and moisture [5–8]. However, it is desirable to eliminate or reduce the use of barrier films to significantly reduce the cost and increase the flexibility of QD films [9]. Direct on-chip packaging would be an ideal low-cost configuration for many applications of light-emitting diodes (LEDs), micro-LEDs, and organic LEDs [10–15], however, the optical properties of QDs are easily deteriorated under high photon flux from light sources and high heat flux caused by Joule heating during device operation [16–20]. For direct on-chip applications, high stability is required under high moisture and oxygen vapor pressure, high temperature, and high photon flux. Researchers have adopted relatively stable polymers to protect QDs [21,22] with poly(methylmethacrylate) (PMMA) being widely investigated due to its high transparency and low permeability to moisture and oxygen. However, poor dispersion and aggregation of QDs in PMMA are known issues [23] resulting in a reduction of the PL QY and a red-shift of the PL emission peak. The PL QY decreases because photons emitted from the QDs are reabsorbed by adjacent non-emitting QDs while the PL emission peak is red-shifted because of energy transfer from small to large QDs [24–26].
Sol-gel condensation has been proposed as a method to improve the stability and dispersion of QDs in polymers [27,28]. Researchers have demonstrated the binding of QDs with hydroxyl-group ligands to a tetraethyl orthosilicate matrix [27] and the binding of QDs with alkoxysilane ligands to a diphenylsilanediol matrix through cross-linking methoxysilane groups by sol-gel condensation with an acid or base catalyst [28]. However, sol-gel condensation reactions generate by-products such as alcohol and water, meaning that extra steps are required to remove the by-products and the acid/base catalysts to prevent them from damaging the QDs and reducing the PL QY [29].
In this work, QDs with vinyl ligands (vinyl QDs) were prepared by ligand exchange from oleic-acid (OA)-ligand QDs. The vinyl QDs formed chemical bonds with a siloxane matrix through hydrosilylation, which required a small amount of metallic Pt catalyst that does not degrade QDs. The degrees of dispersion of the hydrosilylated vinyl QDs and non-reactive QDs in polymer matrices were compared, and the thermal and moisture stability of the hydrosilylated vinyl QDs in siloxane were evaluated with storage at 85 °C/85% RH and their UV/thermal stability was also studied with a high-photon-flux blue LED.
2. Experimental section
2.1 Synthesis of green-emitting CdSe@ZnS/ZnS QDs
CdSe@ZnS/ZnS QDs with oleic acid (OA) ligands were synthesized following a previously reported method to produce alloyed core/shell structured QDs with a thick shell for stability [30]. The collected QDs with OA ligands were mixed with 6-mercapto-1-hexanol (MCH) in a three-necked flask under nitrogen at 120 °C for 4 h, after which the excess MCH was removed by three consecutive precipitations with diethyl ether. Allyl isocyanate was added to the QDs with MCH ligands in chloroform and the mixture was left to react at 40 °C for 14 h to convert the MCH ligands into vinyl groups connected to the QDs by urethane bonds. The solution of QDs with vinyl ligands was diluted with chloroform and methanol, and then excess hexane was added to precipitate the QDs for collection by centrifugation three times. 50 mg of vinyl QDs were dispersed in 1 g of chloroform to produce the QD solution for film preparation.
2.2 Fabrication of color-conversion films
The concentration of QDs was 3wt% in the matrix (97 wt%). Hydrosilylated vinyl QD-siloxane films and OA QD-siloxane films were fabricated by mixing the QD solutions (50 mg/g) with polymethylhydrosiloxane (PMHS, Gelest), tris(vinyl dimethylsiloxy)phenylsilane (TVDSP, Gelest), and vinyl-terminated polydimethylsiloxane (PDMS, Sylgard 184 Silicone Elastomer). After mixing, a platinum catalyst was added to promote the hydrosilylation reaction, which was conducted at 100 °C for 5 min. The vinyl QD-siloxane films were formed by the hydrosilylation reaction between the vinyl groups on the vinyl QDs and the silicon hydride groups (Si-H) on PMHS, while the OA QD-siloxane films were fabricated by blending OA QDs and the siloxane matrix without any chemical bonding. Films of QDs in poly(methylmethacrylate) (PMMA) were prepared as follows: 2.6 g of PMMA was dissolved in 5.6 g of ethyl acetate. Vinyl QD solution (50 mg/g) and the PMMA (30 wt%) solution were blended and the solvent was then removed under air for 1 h. Vinyl QD-PMMA films were subsequently fabricated at 100 °C for 5 min.
2.3 Analytical tools
The PL QYs of QDs in solutions and films were determined using an absolute PL quantum yield spectrometer (Otsuka Electronics, QE-2100) and the PL emission spectra were obtained using a Cary Eclipse fluorescence spectrophotometer (Agilent Technologies, G9803A). The diameter of the QDs was determined using a Cs-corrected high-resolution transmission electron microscope (HRTEM) with an accelerating voltage of 200 kV (JEOL, JEM-ARM200F). The modified functional groups were characterized by Fourier-transform infrared (FT-IR) spectroscopy (JASCO, FT-IR 4600) and the new structure of the QDs was determined by 1H-NMR spectroscopy (Varian Technologies, Unity Inova). Images of the QDs in polymer matrices were obtained by scanning electron microscopy-energy-dispersive spectroscopy (SEM-EDS) with an accelerating voltage of 30 kV (JEOL, JSM-7600F). The electroluminescence characteristics of the QDs in polymer matrices after placing the films on the 3.72 × 3.72 mm2 GaN blue LED chip (λem = 450 nm, DSLAB) were determined using a spectroradiometer (Konica-Minolta, CS-2000) coupled with a voltage-current source unit (Tektronix, Keithley 2400). The pump intensity of blue LED is 0.434 W/cm2. The thicknesses of the films in the backlight unit were almost same to 0.7 mm.
3. Results and discussion
CdSe@ZnS/ZnS QDs with vinyl ligands were synthesized by ligand exchange from QDs with OA. The conventional OA ligands were exchanged with reactive alcohol ligands as the first step. The alcohol groups were then reacted with the isocyanate groups in allyl isocyanate to form vinyl ligands by the formation of urethane bonds. The PL emission spectra and PL QY of the QDs were measured, and the results are shown in Fig. 1. No degradation was observed in the emission peak, full width at half maximum (FWHM), or PL QY after ligand exchange with the QDs maintaining an emission peak at 528 nm with a FWHM of 19 nm. No reduction of the PL QY was observed after ligand exchange from OA to vinyl ligands, being maintained at about 84%, unlike previous reports of reductions in PL QY following ligand exchange [31,32].
Fig. 1 (a) The PL emission spectra of green QDs. Inset shows a TEM image of the QDs (b) PL QY of green QDs in solution with different ligands.
The chemical structures of the ligands were characterized by FT-IR and 1H-NMR spectroscopy, as shown in Fig. 2. The initial OA QDs showed a sp3 C-H stretching peak around 2847-2910 cm−1 in the FT-IR spectrum. The QDs ligand exchanged with 6-mercapto-1-hexanol (MCH) showed an O-H stretching peak around 3000-3600 cm−1. The MCH ligands on the QDs were converted to vinyl groups by reaction with allyl isocyanate to form urethane bonds; this was proved by the sp2 C-H stretching vibration at 3018 cm−1 from the vinyl ligands and the disappearance of the O-H stretching peak. The chemical bonding was also confirmed by 1H-NMR spectroscopy. The presence of OA ligands on the QDs was confirmed by the terminal methyl hydrogen (-CH3) peak at 0.87 ppm in the 1H-NMR spectrum, which disappeared after ligand exchange with MCH. The presence of MCH was confirmed by the terminal methylene hydrogens (-CH2-OH) peak at 3.54 ppm. The vinyl ligands on the QDs were formed by reaction of the MCH groups with allyl isocyanate, which was confirmed by the disappearance of the terminal methylene group (-CH2-OH) hydrogen peak and the appearance of the terminal vinyl group (-CH2) hydrogen peak at 5.26-5.50 ppm. These FT-IR and 1H-NMR results indicate that all the OA ligands on the QDs were exchanged and converted into vinyl ligands.
The synthesized vinyl QDs were mixed with a siloxane matrix and thermally hydrosilylated to form chemical bonds between the QDs and the matrix. The pathway for the hydrosilylation reaction between the vinyl QDs and the siloxane matrix is illustrated in Fig. 3. TVDSP was added as a cross-linker and its phenyl functional groups provided a high refractive index (1.442) and high thermal stability [33]. Vinyl-terminated PDMS was added to prevent cracks forming during thermal curing processes. The silicon hydride groups (Si-H) on PMHS react with the vinyl QDs by hydrosilylation, which was confirmed by the FT-IR spectroscopy results shown in Fig. 4. The vinyl QDs showed a sp2 C-H stretching peak at 3018 cm-1 and PMHS showed a Si-H stretching peak at 2100 cm-1. During curing, the vinyl QDs reacted with PMHS by hydrosilylation, demonstrated by the disappearance of the vinyl ligands’ sp2 C-H stretching peak and the reduction of the PMHS Si-H stretching peak.
Vinyl QDs were connected to the three-siloxane matrix by a thermal curing hydrosilylation reaction. The hydrosilylated vinyl QDs in siloxane were compared with conventional OA QDs dispersed in the same siloxane matrix. The vinyl ligands on the QDs were chemically bonded to the siloxane matrix while OA QDs were simply blended with the siloxane without any chemical bonding between the QDs and the matrix. The PL QY, emission peak, and FWHM of the hydrosilylated vinyl QDs in siloxane were measured and are summarized in Table 1. The PL QY of hydrosilylated vinyl QDs in siloxane was 49.5%, higher than that of OA QDs in siloxane (39.9%). The emission peak of hydrosilylated vinyl QDs in siloxane was at 528.9 nm while the OA QDs in siloxane was at 531.4 nm. A smaller red shift of the PL emission peak was observed for hydrosilylated vinyl QDs compared to the OA QDs in siloxane. The hydrosilylated vinyl QDs in siloxane were also compared to vinyl QDs dispersed in PMMA without any chemical reaction. PMMA was chosen as a control because it has a relatively high thermal stability and excellent optical transparency [21,22]. Hydrosilylated vinyl QDs in siloxane showed a PL QY of 49.5% while vinyl QDs in PMMA was 40.2%. The emission peak of the hydrosilylated vinyl QDs in siloxane was at 528.9 nm, while vinyl QDs in PMMA was at 530.0 nm. A smaller red shift of the PL emission peak was observed for hydrosilylated vinyl QDs in siloxane compared to the vinyl QDs in PMMA. The high PL QY and reduced red shift of the emission peak of the hydrosilylated vinyl QDs in siloxane are attributed to reduced energy transfer caused by improved dispersion of the QDs in the matrix. The degree of dispersion of the QDs in the matrices was determined by SEM-EDS analysis, as shown Fig. 5. The hydrosilylated vinyl QDs were more uniformly dispersed in the siloxane matrix than OA QDs in siloxane and vinyl QDs in PMMA. The excellent dispersion of the vinyl QDs in siloxane contributed to the high QY and minimized red shift of the PL emission peak by reducing the energy transfer from radiative to non-radiative QDs.
Table 1. PL properties of OA QDs in the siloxane matrix, and vinyl QDs in the siloxane matrix and in PMMA
Fig. 5 SEM-EDS mapping images of Zn and S in OA QDs in siloxane, vinyl QDs in PMMA, and vinyl QDs in siloxane.
The thermal and humidity stability of the QDs in the polymer matrices was determined by measuring the PL emission intensity after they had been stored for 30 days at 85 °C and 85% RH. As shown in Fig. 6(a), the hydrosilylated vinyl QDs in siloxane retained 92% of their initial PL intensity, while the OA QDs in siloxane and vinyl QDs in PMMA retained 69% and 80% of their initial PL intensity, respectively. The UV/thermal stability of the QD films was also evaluated using a high-photon-flux blue LED with luminous efficacy of 39.4 lm/W operated at 20 mA and 3 V, with the relative PL intensity plotted in Fig. 6(b). The QDs in the polymer matrices converted 450 nm blue emission into 530 nm green light. Hydrosilylated vinyl QDs in siloxane retained 43.5% of the initial PL intensity while the OA QDs in siloxane and vinyl QDs in PMMA retained 19.7% and 34.1% of the initial PL intensity, respectively, after 29 h. The hydrosilylated vinyl QD-siloxane film is believed to have the strongest barrier properties because of its highly crosslinked polymer network. The high crosslinking density of the matrix leads to low penetration rates of moisture and oxygen due to the reduced free volume of the polymer matrix [34]. These results suggest that the hydrosilylated vinyl QDs in siloxane are highly stable because of the high crosslinking density caused by the hydrosilylation between the vinyl QDs and Si-H groups of the PMHS.
Fig. 6 The time-dependent relative PL intensity of (a) vinyl QDs in siloxane, vinyl QDs in PMMA, and OA QDs in siloxane under 85 °C/85% RH conditions. (b) The time-dependent relative PL intensity of a blue LED on-chip packaged with vinyl QDs in siloxane.
The color-conversion performance of the QDs in polymer matrices was determined on a blue LED as shown in Fig. 7. QDs in the polymer matrices were applied as a color-converting layer on a blue LED with luminous efficacy of 39.4 lm/W operated at 20 mA and 3 V. In this system, the thicknesses of the QDs in the polymer matrices in the backlight unit were 0.7 mm. The QDs absorb the blue emission from the LED and convert it into green light. Commission Internationale de l’Eclairage (CIE) color coordinates were (0.229, 0.712) for hydrosilylated vinyl QDs in siloxane, (0.232, 0.689) for OA QDs in siloxane, and (0.222, 0.717) for vinyl QDs in PMMA. A luminous efficacy of 116.1 lm/W was achieved for the hydrosilylated vinyl QD-siloxane film while 69.1 lm/W and 71.7 lm/W were obtained with OA QDs in siloxane and vinyl QDs in PMMA, respectively. This enhanced efficiency is attributed to the increased PL QY caused by the improved QD dispersion and reduced energy transfer between QDs.
Fig. 7 EL spectrum and images (inset) and (b) CIE color coordinates of an LED on-chip packaged with QDs in polymer matrices.
4. Conclusion
Hydrosilylated vinyl QDs in a siloxane matrix were fabricated by a hydrosilylation reaction between vinyl ligands on QDs and the Si-H groups of PMHS to improve the dispersion and stability of the QDs. Vinyl QDs were prepared by complete exchange of the conventional OA ligands on the QDs with reactive alcohol ligands, which were subsequently reacted with isocyanate groups. No degradation in the PL QY was observed after the ligand exchange and vinyl functionalization. The synthesized vinyl QDs were chemically bonded to the siloxane matrix by hydrosilylation reaction. The hydrosilylated vinyl QDs in siloxane showed excellent, uniform dispersion, as well as higher PL QY and smaller red shift than QDs blended in a polymer matrix. They also showed high stability under 85 °C/85% RH conditions; approximately 92% of the initial PL intensity of hydrosilylated vinyl QDs in siloxane was retained after storage for 30 days, while the PL intensity of the OA QDs in siloxane decreased to 80% and that of vinyl QDs in PMMA decreased to 69%. In the UV/thermal stability test, about 43.5% of the initial PL intensity of hydrosilylated vinyl QDs in siloxane was retained after 29 h under a high photon flux generated by an LED, while OA QDs in siloxane retained 19.7% and vinyl QDs in PMMA retained 34.1%. The excellent stability of the hydrosilylated vinyl QDs in siloxane is attributed to the high crosslinking density suppressing the diffusion of oxygen and moisture. A higher luminous efficacy of 116.1 lm/W was achieved with hydrosilylated vinyl QDs in siloxane, while 69.1 lm/W was obtained with OA QDs blended in PMMA. We successfully demonstrated improved QD dispersion and stability in the polymer matrix with hydrosilylation between the QDs and the siloxane matrix, and believe this chemical reaction of QD ligands with polymer matrices can provide solutions for the problems of QD dispersion and stability in on-chip applications.
Funding
National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea (2012M3A6A7054855).
Acknowledgments
This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea (2012M3A6A7054855).
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