Ultra Stable Quantum Dot Composite Film in Severe Environment

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Functional Inorganic Materials and Devices

Ultra Stable Quantum Dot Composite Film in Severe Environment Zunxian Yang, Yuxiang Zhang, Jiahui Liu, Jingwei Ai, Shouqiang Lai, Zhiwei Zhao, Bingqing Ye, Yushuai Ruan, Tailiang Guo, Xuebin Yu, Gengxu Chen, Yuanyuan Lin, and Sheng Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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ACS Applied Materials & Interfaces

Ultra

Stable Quantum Dot Composite Film in Severe

Environment

Zunxian Yang1*, Yuxiang Zhang1, Jiahui Liu1, Jingwei Ai1, Shouqiang Lai1, Zhiwei Zhao1, Bingqing Ye1, Yushuai Ruan1, Tailiang Guo1*, Xuebin Yu2, Gengxu Chen1,Yuanyuan Lin1, Sheng Xu1

1

National & Local United Engineering Laboratory of Flat Panel Display Technology,

Fuzhou University, Fuzhou 350116, P. R. China 2

Department of Materials Science, Fudan University, Shanghai 200433, P. R. China

Abstract Semiconductor quantum dots (QDs) have attracted extensive attention due to their remarkable optical and electrical characteristics. While the practical application of quantum dots and further the quantum dot composite films have greatly been hindered mainly owing to their essential drawbacks of extreme unstability under the oxygen and water environments. Herein, one simple method has been employed to enhance enormously the stability of CdxZn1-xSeyS1-y quantum dot composite film by a combination of CdxZn1-xSeyS1-y quantum dots and particular polyvinylidene Fluoride (PVDF), which is characteristic of closely arranged molecular chains and strong hydrogen bonds. There are many particular advantages in the quantum dot/PVDF composite films such as easy process, low cost, large area fabrication, and especially

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extreme stability even in the boiling water for more than 240 minutes. By employing the K2SiF6:Mn4+ as red phosphor, a prototype white LED with a color coordinates of (0.3307, 0.3387), Tc of 5568K, color gamut 112.1NTSC(1931)% at 20mA has been fabricated, and there is little variation under different excitation currents, indicating that the quantum dot/PVDF composite films fabricated by this simple blade-coating process make them ideal candidates for LCD backlight utilization via assembling white LED on a large scale owing to its ultra-high stability under the severe environment.

Key Words：Quantum dot composite film; water/oxygen stability; thermal stability; PVDF microencapsulation; photoluminescent emission. Introduction Semiconductor quantum dots (QDs), as one of solution-processed colloidal nanomaterials, have attracted more and more attentions for three decades

1-4

due to

the unique size-dependent optical properties, high photoluminescence quantum efficiency, tunable wavelengths and so on. These outstanding properties make quantum dots a suitable candidates in many fields including light-emitting devices (LEDs)

5, 6

, solar cells7-9, photoconductors10-12 and field-effect transistors13 and so on.

Recently, novel QD/polymer composites have been fabricated to be used as the light down-conversion component of solid-state light-emitting diodes (LEDs) for display application.14, 15 As the quantum dots enhanced liquid crystal display by virtue of their ultra-excellent characteristics 16-18, which attracts more and more interests from


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the researchers in quantum dot display fields. However, there are still many challenges for the application of quantum dot composite film in many fields including display, probably owing to the intrinsic unstable properties of quantum dots especially when inevitably contacted with the moisture and oxygen in the air. Usually, there are probably two strategies to overcome the inherent shortcomings of quantum dots in the composite film via cutting off the contact with water and oxygen in the air. One strategy is to fabricate and further encapsulate the quantum dot composite film device completely in an environment without air, i.e. in the glove boxes. This will greatly increase the cost of devices mainly owing to the expensive cost of the complicated fabrication and encapsulating processes, but possibly weaken their optical performance and restrict subsequent application because of the inevitable optical blocking and absorption of the encapsulating layer. Another effective strategy is to confine the quantum dots into inorganic shells or into polymer matrix including coating a multiple shells

19

, aluminum doping20 or

dispersing the quantum dots into polymer matrix 14, 21. Presently, there are still many difficulties in forming a uniform dense and stable inorganic shell outside. Therefore, once suitable polymers and feasible processes employed, dispersing the quantum dots into polymer matrix may be an effective solution. Poly(vinylidene) fluoride (PVDF) is characteristic of simple film-formation via solvent evaporation, excellent thermal, hydrophobic stability, high optical permeability and selectivity, as well as strong mechanical properties mainly owing to its closely arranged molecular chains and strong hydrogen bonds 22. Because of its



excellent characteristics, PVDF is widely studied

23-25

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and possibly an ideal

candidate for forming semiconductor quantum dots/polymer film with the characteristics of simple fabrication, excellent optical and mechanical stability. Generally, polymer powder easily dissolves into some strong polar solvents such as N, N-dimethylformamide (DMF)26, while the quantum dots with the organic ligands, such as hydrophobic oleic acid (OA) can only easily dissolve into some non-polar or weak polar solvents , such as hexane, octane, chloroform27, 28. Thus, there are still many difficulties in choosing a suitable solvent to dissolve the polymer powder (PVDF) and quantum dots simultaneously. In this paper, semiconductor quantum dots/PVDF composite films have been fabricated just by a combination of quantum dot-chloroform solution and PVDF DMF solution, followed by a facile blade-coating approach. The as-fabricated composite films exhibit homogeneous fluorescence and morphology, ultra stability against water and heat even in the boiling water for 240 minutes. Furthermore, a prototype white LED with a color coordinate of (0.3307, 0.3387), Tc of 5568K, color gamut 112.1NTSC% (1931) at 20mA had been fabricated by integrating CdxZn1-xSeyS1-y green emissive composite films and red emissive K2SiF6:Mn4+ phosphor with blue chips and excellent color stability under the different currents had been obtained.

Results and discussion As illustrated in Fig. 1, the stability tests against the sever environment for the CdxZn1-xSeyS1-y /PVDF composite films and their typical fabrication processes were


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demonstrated briefly. As shown in Fig. 1(a), a piece of as-fabricated composite film dipped into hot water at different temperatures to effectively investigate the stability of composite film against the water, oxygen and heat in severe environment. The photograph of the composite film was taken once in a certain time, when exposed to UV

light

shining

instrument.

And

the

measurement

of

corresponding

photoluminescence with fluorescence spectrometer was also obtained at the same position of the film. As described in Fig. 1 (b), the fabrication processes for the composite films are mainly included as follows: ⅰ) formation of quantum dot solution and PVDF solution, respectively; ⅰ) mixing the quantum dot chloroform solution and PVDF solution together; ⅰ) pouring out and blade-coating the mixture solution onto glass substrate; ⅰ) keeping the precursor composite film in 30 ⅰ vacuum environment for crystallizing PVDF, evaporating the solvent off the composite film and then confining the quantum dots in the PVDF matrix. Actually, because PVDF polymers are apt to dissolve in some strong polar solvent while hydrophobic oleic acid (OA) decorated quantum dots can easily dissolve in non-polar or weak polar solvents, there are still many challenges in exploring the compatible solvents for both, forming a uniform mixture solution and finally uniform quantum dots/PVDF composite film. As shown in Fig. 2(a) and (b), quantum dots could disperse easily in a mixture solvent of chloroform and DMF and finally to form uniform solution (S3 in Fig. 2(a) and S6 in Fig. 2(b)). However, there was a clear multi-layer in quantum dot solution with hexane/DMF solvent (S1 in Fig. 2(a) and S4 in Fig. 2(b)) or octane/DMF solvent (S2 in Fig. 2(a) and S5 in Fig. 2(b)). Such significant differences



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possibly originated from the nature characteristics of the solvent among chloroform, hexane and octane. According to the Similar Phase Solution Principle, the non-polar hexane and octane could not dissolve in the strong polar solvent, while the weak polar solvent such as chloroform could easily dissolve in the strong polar solvent and finally to form uniform solution. Therefore, the chloroform and DMF compatible solvents could be suitable for the formation of uniform solution for quantum dots/PVDF polymer mixture and we finally obtain the uniform ultra-compact quantum dot/PVDF composite film by encapsulating the quantum dots into the PVDF polymer matrix when solvent evaporated29,

30

. Besides, this was also possibly due to PVDF’s

particular characteristic of strong hydrogen bonds and Van der Waals forces. In fact, the volume ratio of chloroform to DMF in the mixture solutions would exert great effects on the morphology of the as-fabricated composite film (see Supporting Information Fig.S1). The as-fabricated composite films with different ratio of precursor solution were shown in Supporting Information Table S1, respectively. When fixing the volume of total mixture solution, with the increase of chloroform volume in the mixture solution, there was a remarkable change in the uniformity, the color of the composite film as well as the adhesion with the glass substrate. The sample S0.25ml chloroform was smooth, uniform, nearly white and evenly adhered to the glass substrate, while the sample S0.5ml

chloroform

was uniform, light green and

completely separated from the glass substrate, the S0.75ml

chloroform

exhibited the

characteristics of some non-uniformity, green and complete separation from the glass substrate, finally, the S1ml chloroform shrunk greatly into a huddle. This phenomenon was


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possibly due to the different solubility of quantum dots and PVDF in the mixed solvent. Too much chloroform would impede the solubility of PVDF, while excessive DMF would similarly hinder the solubility of the QDs. In others words, boiling point and hydrophily of chloroform and DMF, which would arouse the difference in the internal stress, finally result in the difference in the uniformity, color of the composite film and the adhesion with the glass substrate. And the ratio impact of chloroform to DMF were farther confirmed by the PL decay lifetime investigation. PL decay lifetime was usually a hallmark for quantum dots and quantum dot composite films mainly because of the direct association between the PL lifetime and the optical performance. The PL decay dynamics characteristics and corresponding optical performances of CdxZn1-xSeyS1-y /PVDF composite film fabricated by different processes including different solvent ratios and solvent evaporation rate as well as the dilute quantum dots chloroform solution were explored and analyzed in detail (see Fig. 2(c), Supporting Information Table S2, Fig.S2, Fig.S3, Fig.S4 and Fig.S5). The PL decay lifetime (τ) curves of all the samples (S0.25ml chloroform, S0.5ml chloroform, S0.75ml chloroform,

S1ml chloroform and SQDs solution) were fitted with bi-exponential decay function

(see Fig. 2(c) and Supporting Information Table S2), which referred to two recombination components: radiative recombination (τ1) and delayed recombination (τ2). Generally, the faster decay component (10-40ns) was attributed to the radiative recombination while the other with longer decay time (60-120ns) belonged to the delayed recombination indicating that the existence of long-lived trap states on the surface of QDs31, 32. As for chloroform diluted quantum dot solution sample SQDs



solution,

its radiative recombination and prefactor was 21.6ns (τ1) and 96.03% (f1) while

the delayed recombination and prefactor was 83.1ns (τ2) and 3.97% (f2)，where the prefactors denoted population of the components, respectively. The average recombination lifetime (τavg) of SQDs solution, which was estimated by the equation: τavg = f1τ1+ f2τ2, was 24.03 ns. And the QY of quantum dots was 86%, which was estimated by slope of the standard fluorescent dye and quantum dots (see Supporting Information Fig.S2). The S0.5ml

chloroform

with PL decay lifetime (τ1= 21.3ns,

f1=97.29%, τ2= 98.7ns, f2=2.71% and τavg=23.40ns) exhibited a similar decay time characteristics as compared with those of SQDs solution. As for S0.25ml chloroform and S1ml chloroform,

their τavg were 18.41ns and 20.21ns, respectively. When compared with those

of SQDs solution, the decay lifetimes of these samples declined by more than 23.3% and 15.8% respectively. Generally, the longer decay lifetime means the more effective electrons, holes and excitons under same excitation environments that would finally lead to better optical performance. Therefore, the ratio of chloroform to DMF would directly affect the viscidity, uniformity, solvent volatility of precursor solutions as well as the adhesion of mixture solution to the glass substrate, ultimately the color and morphology and the PL decay lifetime of composite films. High ratio of chloroform to DMF greatly reduced the viscidity and the adhesion to the glass substrate, increased the solvent volatility and changed the solubility or dispersion of quantum dots and PVDF polymer matrix in solution, respectively, and finally the dispersion of quantum dots in the PVDF matrix. High ratio of chloroform to DMF would facilitate the QDs solubility but impede dissolving PVDF. While too low ratio of chloroform to DMF


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would also reduce the solubility or dispersion of quantum dots and PVDF polymer matrix in solution and finally the dispersion of quantum dots in the PVDF matrix. Too low ratio of chloroform to DMF would facilitate the PVDF solubility but impede dissolving QDs. Only when the ratio of chloroform to DMF of 1:3 (just like that in the sample S0.5ml chloroform), the uniform CdxZn1-xSeyS1-y /PVDF composite film with good optical performance were obtained. In addition, hydrophobic oleic acid (OA) decorated quantum dots could only dissolve or disperse into non-polar or weak polarity solvent. When chloroform (weak polarity solvent) dissolved in DMF (strong polar solvent) as chloroform micelles, the quantum dots and PVDF were able to disperse in chloroform micelles and the solvent DMF respectively and ultimately a stable colloidal solution formed. Besides, ambient temperature and pressure were another two important factors for the actual ratio of chloroform to DMF in the composite film, the numbers of chloroform micelles and finally the stability of the colloidal solution and the dispersion of quantum dots and PVDF matrix in the composite film. This influence was also confirmed by their PL decay lifetime investigation mentioned above. Supporting Information Fig.S3 further indicated the PL decay lifetime characteristics of the quantum dot composite films which derived from the quantum dot solution with the different optical density (OD) value under relatively low-temperature and high-pressure environment (Note: this meant moderate solvent evaporation rate of the composite film) and the PL decay lifetime characteristics of the original quantum dot solution. The results showed PL decay lifetimes of quantum dot composite films decreased sharply as compared with that of



original quantum dot solution and the concentration of quantum dot in precursor influenced the PL decay lifetime of composite films to some extent. Thus, this further confirmed the effect of chloroform ratio to DMF in precursor solution on the PL decay lifetime characteristics of composite films in Fig. 2(c). Under a relative low-temperature and high-pressure environment, the solvents would evaporate slowly and chloroform in the film evaporated more quickly than DMF, and the quantum dots would separate out from the PVDF matrix mainly owing to the transient reduced viscidity of composite film and the reduced ratio of chloroform to DMF in the composite film. This would greatly influence the PL performance of the composite films. As shown in Supporting Information Fig.S4 and Supporting Information Fig.S5, the UV radiation obviously influenced the photoluminescence of the composite film by sheltering part of the composite film and varying the radiation time possibly owing to the variation in the solvent evaporation rate of the composite film, similar to these phenomena in Fig. 2(c) and Supporting Information Fig.S3. The morphology and phase of the CdxZn1-xSeyS1-y /PVDF composite films was systematically investigated by the X-ray diffraction (XRD), Scanning Electron Microscopy(SEM) and fluorescence optical microscopy, respectively (see Fig. 2 (d) and Supporting Information Fig.S6). For comparison, the phase of pure PVDF and green QDs powder were also provided (Fig. 2(d)). The X-ray diffraction patterns of as-fabricated composite film revealed that the composite film mainly consisted of the PVDF mixed phases with the sharp and strong peaks at the 20.2° and 18.6°, which were probably attributable to the (110) of β phase and (120) of α phase22, respectively.


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The XRD pattern of the composite film showed minor broad peak at 27.1°, probably corresponding to the (101) plane of CdSe, which was in well agreement with that of the pure QDs powder. As shown in Fig. 2(d), the XRD patterns of pure PVDF only included one sharp and strong peak at the 20.2°, which was possibly attributable only to the (110) of pure β phase and different from those of the composite film. Supporting Information Fig.S6 (a-b) showed the cross-section SEM image and the surface SEM image of composite film. The composite film consisting of compact PVDF matrix without obvious holes or voids in the film would effectively protect the confined quantum dots from the water, oxygen and heat of surrounding environment, which resulted in the highly improved reliability of the composite film in severe environment. As shown in Supporting Information Fig.S6 (c-d), the uniform fluorescence optical photographs of the quantum dot composite film further demonstrated the uniformly dispersed quantum dots in the PVDF matrix. This was mainly attributable to the uniform precursor solution with optimized volume ratio solvents and the following suitable process temperature and pressure in fabricating the composite film. By simply adjusting the composition of precursor solution, a series of CdxZn1-xSeyS1-y quantum dots with different fluorescence wavelengths were successfully synthesized for further use in CdxZn1-xSeyS1-y based composite films (See Fig. 3 and Supporting Information Fig.S7). Rather than the traditional synthesis processes for shell-core quantum dots that include many synthesis steps, we produce the CdxZn1-xSeyS1-y quantum dots with high performance and different fluorescence wavelengths via



simple one-pot method that also suitable for mass production (see Fig. 3(a) and (d), Supporting Information Fig.S7)33, 34. As shown in Fig. 3(b) and (e), those quantum dot/PVDF polymer composite films were characteristic of excellent fluorescence and similar to those of their own quantum dot solutions with different fluorescence wavelengths, respectively, mainly owing to their particular particle sizes35. Additionally, the quantum dot composite film with dimension of 8 cm×15cm (see Fig. 3(c)) and thickness of 45±5µm (see Supporting Information Fig.S8, Fig.S6 (a)) was simply fabricated by knife coating fabrication method and exhibited uniform photoluminescence characteristic when exposed to UV radiation, which presented the possibility for mass production of LCD backlight. As results of uniform nanoparticle dispersion and polymer encapsulation, the CdxZn1-xSeyS1-y quantum dots/PVDF composite films exhibited excellent stability against water, oxygen and heat exposure (Fig. 4). No obvious PL degradation has happened for the quantum dot composite films when immersed into water even for more than 54 days (see Fig. 4(a)). To accelerate the water/thermal stability test, the samples immersed directly into hot water at different temperatures for different time with their PL monitored periodically under UV illumination (Fig. 4(b)). The photographs of water/thermal stability test represents no distinguishable PL decay in their photoluminescence, indicating the excellent water and thermal resistance for composite films. As shown in Fig. 4(c) and (d), as the change of of the water temperature and heating time, there were still some slight fluctuations in the PL intensities for composite films according to the PL intensity three-dimensional test


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data, presumably because few quantum dots unconfined by PVDF matrix or some holes/pores appeared when some residual solvent in PVDF matrix evaporated during heating process. Particularly, when the temperature of the hot water rose from 50ⅰ to 100ⅰ, the PL intensity kept stable even for more than 60 minutes. Therefore, the high water/oxygen/thermal stability of the CdxZn1-xSeyS1-y /PVDF composite film in the water-boiling tests was possibly attributable to the particular architecture of the quantum dots confined into the PVDF matrix in micro/nano-scale. For comparison, the CdxZn1-xSeyS1-y /PMMA composite film was also fabricated by the processes as same as those of the CdxZn1-xSeyS1-y /PVDF composite film and then immersed into the boiling water. An apparent deformation happened when the CdxZn1-xSeyS1-y /PMMA composite film immersed in hot water just for only a few seconds while no obvious deformation for CdxZn1-xSeyS1-y /PVDF composite film that boiled in hot water even for more than four hours (see Supporting Information Fig.S9 and Video S1). This was mainly attributable to their significant difference in thermal properties and hygroscopicity. The PVDF matrix was characteristics of thermal stability and hydrophobicity while the PMMA matrix exhibited poor thermal stability and hygroscopicity, which finally resulted in their significant difference in the water/oxygen/thermal stability. Very interestingly, it was found that the fluorescence intensity of the quantum dots/PVDF composite film irradiated under 6W UV light increased at first with the irradiating time and then became stable (see Supporting Information Fig.S10), which was similar to the previous report36. This phenomenon was possibly attributable to the defect reduction of quantum dots in the composite



film when exposed to enough UV irradiation

37, 38

and the solvent reduction in the

composite film during the UV irradiation. Moreover, the ascent rate in fluorescence intensity of the quantum dots/PVDF composite film when exposed to UV irradiation decreased with the evaporation rate of the solvent in fabricating composite film. Especially, the fluorescence intensity of the quantum dots/PVDF composite film with slow evaporation rate increased obviously only in several minutes when exposed to the 365nm UV lamp (6W) radiation. This was possibly attributable to the more defects of quantum dots resulting from higher solvent content in the composite film with slow evaporation rate, agreeing well with the slight fluctuations in the PL intensities of composite films versus the boiling temperature and boiling time in hot water in Fig. 4(c) and (d). Thanks to their process simplicity, low cost, high stability in water/oxygen/thermal environments, suitability for fabricating composite film with large area, uniform quantum dots distribution in film and high color purity, these composite films offered tremendous potential for different applications. Especially, for the down converters in the backlight of LCDs, quantum dots/PVDF composite film would absorb many excitation light photons with energy greater than their own band gap and gave off photoluminescence with energy equal to their own band gap. A prototype phosphor-converted LED was fabricated by encompassing the composite film into UV cure adhesive with red emissive phosphor, K2SiF6:Mn4+. This red phosphor, K2SiF6:Mn4+, exhibited different photoluminescence performance when excited with different excitation wavelengths (see Supporting Information Fig.S11 ), while the


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maximum photoluminescence of the K2SiF6:Mn4+ phosphor was obtained when excited by the 465nm light. We ultimately fabricated a prototype device of white LED by combination of the red K2SiF6:Mn4+ phosphor, green quantum dots composite film with high quantum yield and stability (see Fig. 5(a)), and the blue LED chip that used as a pump to activate the phosphors and quantum dots to produce the white color (see Fig. 5(b)). Furthermore, the good electroluminescence spectrum for this white LED device was obtained (see Fig. 5(c)), indicating highly uniformity of color distribution. It was more convenient to adjust its CIE coordinates and realize the white photoluminescence just by changing the content of red K2SiF6:Mn4+ phosphor in the phosphor precursor layer. As indicated in Fig. 5(d), the color coordinates of the as-fabricated model marked as a hollow pentagonal star in the CIE diagram was (0.3307, 0.3387), the color temperature Tc was 5568K, luminous efficiency was 41.6lm/W and further a color gamut 112.1NTSC(1931)% were obtained at 20mA. As show in Supporting Information Table S3, when the currents of the device changed from 20mA to 50mA, the CIE and CCT changed slightly, showing good stability with the drive currents. Furthermore, the EL stability of the white LED at the high initial brightness of 1.9387×104cd/m2 and the constant current of 50 mA was tested. By monitoring the brightness and EL spectrum, the WLED obtained 91% initial brightness after the 3 hours’ radiation at the initial brightness of 1.9387×104cd/m2 and there was little variation in EL spectrum.(see Supporting Information Fig.S12). Theses results demonstrated that the quantum dots/PVDF composite film was characteristic of particular water and oxygen stability and the composite film



exhibited further tremendous potential in many applications including the backlight for the LCDs. Of course, it is our future work to replace the red K2SiF6:Mn4+ phosphor by the red quantum dots composite film and produce the white LED with pure quantum dots composite film as the down converters.

Conclusions In summary, a novel CdxZn1-xSeyS1-y /PVDF composite film was fabricated by an extremely stable, facile, low-cost, large-scale production solution, followed by blade-coating the mixture of quantum dot/chloroform solution and PVDF/DMF solution. This composite film exhibited extremely stable photoluminescence performance against the water, oxygen and heat even in severe environment mainly owing to the particular architecture consisting of quantum dot photoluminescence emitters and the PVDF structure-supporter and micro-encapsulator, which was characterized by closely arranged molecular chains and strong hydrogen bonds. The as-fabricated quantum dot composite film exhibited a uniform distribution, ultra-stable photoluminescence performance against the water, oxygen and heat even in the boiling water for more than 240minutes when exposed to the UV radiation. Furthermore, a white LED was fabricated simply just by combining the green emissive films with the red emissive K2SiF6:Mn4+ phosphor via the activation of the blue LED chip mainly owing to the ultra-stability in the photoluminescence performance of the quantum dot composite film against the water, oxygen and heat even in severe environments. The prototype phosphor-converted LED exhibited


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excellent photoluminescence performance with a CIE-1931 coordinates of (0.3307,0.3387), Tc of 5568K, color gamut 112.1NTSC(1931)% at 20mA, indicating that the quantum dot composite film fabricated by this simple blade-coating process made them ideal candidates for LCD backlight utilization owing to its ultra-high stability under the severe environment.

Experimental Synthesis of Quantum Dots. The quantum dots were prepared in a similar way to that reported in previous works

39, 40

and with some modifications. In a typical

synthetic procedure of green quantum dots with PL=527nm, 0.4mmol of CdO, 4mmol of zinc acetate dihydrate, 6.5ml oleic acid, and 20 mL of 1-octadecne were added into a 100 mL round flask. The mixture was heated to 150 °C, stirred quickly, filled with Ar gas and keep for 10 minutes, and further heated to 300°C. At this temperature, 0.4 mmol of Se powder and 4 mmol of S powder were completely dissolved into 3 mL of TOP and subsequently, the solution were quickly injected into the reaction flask. After the injection, the temperature of the reaction flask was keep at 300 °C for promoting the growth of QDs, After 10 min of reaction, the reactor cooled down to room temperature and the resulting quantum dot solutions were obtained. Of course, changing the cation ratio of Cd and Zn or the order of anion of Se and S could acquire the quantum dots with different photoluminescence wavelength. The typical processes for synthesizing the red/green/blue fluorescence quantum dots have been shown in Supporting Information Table S4, respectively.



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Fabrication of Quantum Dot Composite Film. Typically, quantum dots were dissolved into 0.5ml chloroform to form solution 1 and simultaneously, 0.336g PVDF power were added into 1.5 ml DMF to obtain solution 2, and then the two solutions obtained were mixed together with vigorously stirring. Then, the sticky mixture was poured onto glass substrate, and the precursor films with different thicknesses were obtained just by adjusting the blade of the knife coater. Then the precursor film, along with their substrates, was put into a vacuum oven to remove the solvent and the PVDF with semi-crystalline morphology were formed. Uniform freestanding CdxZn1-xSeyS1-y /PVDF composite film was obtained by peeling it off from the substrate. Fabrication of the White LED devices. Firstly, K2SiF6:Mn4+ phosphor was dispersed into UV cure adhesive and the mixture was spin-coated onto the glass substrate with speed of 3000rpm. Secondly, the red luminescent film was obtained by a 5-min UV curing process. Thirdly, the QDs/PVDF composite film was put onto the red emissive layer. Finally, the two luminescent layers put onto a blue LED chip. Materials Characterization. The phase of the products was examined by using X'Pert Pro MPD X-ray diffraction meter with Cu Kα radiation (λ=1.5418 Å, Philips, Holland). The morphology of these films was evaluated by using a NanoSEM 230 field emission scanning electron microscope (FE-SEM, Nova NanoSEM 230, FEI, US). Room temperature UV–vis absorption spectra were measured with a Shimazu UV3600

UV–vis diode

array spectrometer.

Photoluminescence

(PL) and

photoluminescence excitation (PLE) spectra were collected by a Shimazu F4600 spectrometer. Fluorescent quantum yields (QY) of the quantum dots were measured


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and estimated by comparing their fluorescence intensities with primary standard dye rhodamine-B solutions under the same optical density and excitation wavelength. The microscopic images of films were characterized by fluorescent microscope （Olympus BX51M）. PL decay characteristics were obtained from a fluorescence lifetime spectrometer (Horiba IBH-TemProm) by using 405 nm laser as an excitation source. The time-resolved PL decay curves were fitted with a bi-exponential function of time (t) where f1 and f2 are prefactors, τ1 and τ2 are the time constants. The average recombination lifetime (τavg) was estimated with the f and τ values from the fitted curve data according to the following equation: τavg=f1τ1+ f2τ2. In water resistance test, the composite films without further protection were immersed into water for different time and then the PL integrated intensity of composite films was measured and obtained. In the UV radiation test, the films were radiated by the UV 365-nm lamp (6W) for different time at the room temperature and the PL intensity of composite films was measured at a certain time. In water and temperature resistance test, the film without further protection was immersed into water at different temperature, the PL intensity of composite films at the same location were measured at different temperature and time. In order to maintain each test in the same condition, the quantum dot films were grown on rectangular substrate. And the as fabricated film was marked at their four corners and sandwiched between two same glasses, then put it in the holder of Hitachi F4600 fluorescence spectrometer for test. The testing parameters including the excitation wavelength, the width of the slit, and



the scanning speed, PMT voltage and so on were kept constant during the whole fluorescence testing course.

ASSOCIATED CONTENT Supporting Information Fig. S1: the photographs of quantum dot composite films fabricated with different solvents; Table S1: The composition of precursor solution for fabricating composite film samples; Table S2: the PL lifetime τ and the quantum yields of the quantum dot composite films fabricated with different solvent under the same batch with the middle evaporated speed; Fig. S2: The QY of quantum dots obtained by the slope method; Fig. S3: the PL decay lifetime of the quantum dot composite films fabricated under relatively low temperature and high pressure environment by the quantum dot solution with the different optical density (OD) value, and that of the original quantum dot solution; Fig. S4: the photo image of the film with and without the shelter under UV radiation; Fig. S5: the time-dependent PL spectra of sample under the constant 465nm excitation for different time; Fig. S6: The SEM images and the fluorescence optical microscopy images of the composite film; Fig. S7: the PL spectra of the precursor CdxZn1-xSeyS1-y quantum dots for fabricating the quantum dot composite film; Fig. S8: the step profile of the film, which cut by blade for test the thickness; Fig. S9: the photos of PMMA/QDs film and PVDF/QDs film before/after being immersed in boiling wate; Video S1: vivid reliability test of quantum dots-PVDF composites film in boiling water; Fig. S10: The PL stability of composite


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film frabricated with middle evaporation speed under continue UV lamp(6w) irradiated versus irradiation time; Fig. S11: the excitation photo luminescence spectra of red emissive phosphor: K2SiF6:Mn4+ under different excitation wavelength. Table S3: the luminance, CCT, and CIE color coordinates for a prototype of white lighting emitting diode under different drive currents; Table S4: the typical processes for synthesizing the red fluorescence quantum dots, the green fluorescence quantum dots and the blue fluorescence quantum dots, respectively.

Author information Corresponding Authors *E-mail: [email protected]

(Z. Yang)

*E-mail:[email protected]

(T. Guo)

Notes The authors declare no competing financial interest.

Acknowledgements Part of the work was funded by the Natural Science Foundation Program of China (61574039), the Natural Science Foundation Program of Fujian Province (2015J01252) and Key Research and Development Plan of Ministry of Science and Technology of China (2016YFB0401503, 2016YFB0401305, 2016YFB0401103).



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Fig. 1 Schematic illustration of (a) stability test and (b) the fabrication processes for CdxZn1-xSeyS1-y quantum dot /PVDF composite films indicating that the CdxZn1-xSeyS1-y quantum dots were confined into the PVDF polymer matrix. (The fabrication processes: formation of quantum dot solution and PVDF solution, respectively; mixing the quantum dot solution and PVDF solution together; pouring out and blade-coating the mixture solution onto substrate; keeping the precursor composite film into 30ⅰvacuum environment for crystalizing PVDF, evaporating the solvent off the composite film and confining the quantum dots in the PVDF matrix)


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Fig. 2 (a) the photograph of the quantum dot solution in different solvents. (b) the photograph of the quantum dot solution in different solvents when exposed to UV radiation. (c) The PL decay characteristics of different quantum dot composite films fabricated under relatively high temperature and low pressure and chloroform diluted quantum dot solution, respectively. (d) XRD patterns of pure PVDF films, pure quantum dots and quantum dots/PVDF composite films fabricated with 0.336g PVDF and 0.5ml quantum dots solutions. (Note: S1 refers to the photograph of the quantum dot hexane/DMF solution; S2 refers to the photograph of the quantum dot octane/DMF solution; S3 refers to the photograph of the quantum dot chloroform/DMF solution. S4, S5 and S6 are the photographs of S1, S2 and S3 when exposed to UV radiation, respectively.).



Fig. 3 (a) the photographs of the CdxZn1-xSeyS1-y quantum dot solutions under UV radiation. (b) the photographs of CdxZn1-xSeyS1-y quantum dots/PVDF composite films under UV radiation. (c) The photograph of large-area quantum dot composite film under UV radiation. (d) UV–vis absorption of kinds of the quantum dots. (e) PL spectra of the quantum dot composite films.


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Fig. 4 Water and thermal resistance characterization of quantum dot composite film. (a) The photographs of composite film in water under UV irradiation taken for different time(0h,408h,1056h,1272h,1296h). (b) the photographs of composite film in hot water under irradiation taken at different temperatures and for different time(25ⅰ,60min; 40ⅰ,60min;

55ⅰ,60min;

70ⅰ,60min;

80ⅰ,60min;

90ⅰ,30min;

90ⅰ,60min;

100ⅰ,30min; 100ⅰ,60min.). (c) The PL intensity three-dimensional surface of composite films versus the temperature of the hot water and the boiling time. (d) The PL intensity of composite films boiled for different time in different temperature water.



Fig. 5 (a) Schematic diagram of the configuration for the prototype LED device. (b) photographs of white LED when drived by 20 mA operation current. (c) Emission spectrum of the white LED fabricated by green emissive CdxZn1-xSeyS1-y /PVDF composite films, red emissive phosphor and blue LED. (d) The color coordinate (black star) and the color triangle of obtained white LED is exhibited in CIE 1931 diagram.


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Table of Contents


Ultra Stable Quantum Dot Composite Film in Severe Environment

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