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Quantum dots (QDs) based composite thin films show great promise in the application areas ranging from light-emitting diodes (LEDs), nonlinear optical devices to luminescent solar concentrators. We propose and demonstrate a facile method to fabricate ZnSe:Mn QDs/Poly (lauryl methacrylate-co- ethylene glycol dimethacrylate) (QDs/ Poly(LMA-co-EGDMA) composite thin films which show high transmittance up to 90%. Moreover, the as prepared QDs/P composite thin films exhibits high quantum yields (QYs) which could be well tuned from 35% to 67%.The enhanced transparency and QYs of these QDs/P composite thin films are attributed to the triple function of surface decorated 1-dodecanethiol (DDT) including the reducing of surface defects, oxidation resisting and enhancement of dispersity. These prominent performances make them very attractive for applications in various future light-emitting technologies and advanced optoelectronic devices.
© 2015 Optical Society of America
1. Introduction
Composite thin films based on semiconductor colloidal nanocrystals (quantum dots, QDs) with stable and strong luminescence have attracted significant research interest over the past few decades owing to their application prospects in the fields from light-emitting diodes (LEDs), nonlinear optical devices to luminescent solar concentrators which is due to their large Stokes shift, suitable absorption cross-section and size-dependent tunable emission [1–5]. QDs were usually synthesized via colloidal chemistry approaches and then embedded into polymer matrix to form flexible luminescence composite thin film which possess unique mechanical properties such as flexiblity, foldability and crack resistance [6]. The main optical characteristics of composite thin film based on luminescence QDs are high transmittance, low-reabsorption and high quantum yield (QYs). In order to achieve high optical performance, the materials used for light absorption and conversion devices must usually exhibit low reabsorption and high fluorescence quantum efficiency [7,8]. Even though CdSe QDs exhibit high luminescence QYs up to 52.2%, composite thin film based on the CdSe QDs is still hindered by the reabsorption resulting from the small stokes shift in undoped CdSe QDs [9].
Until today, many strategies have been tried to reduce the reabsorption in the luminescent composite thin film. Mutlugun et al. and Meinardi et al. used QDs with core/shell structure to decrease the reabsorption [10,11]. These core/shell QDs exhibit large Stokes shift resulting from the increased energy separation between light absorption and emission in two distinct parts of the materials. The core and shell are usually composed of materials with similar lattice spacing, but different energy gap to overcome the lattice mismatch and reabsorption [12,13]. the lattice mismatch in many core and shell materials along the complicated synthetic procedures are still the big challenges of core/shell QDs based composite thin films.. Besides this, light transmittance is another critical factor in the composite thin films. Higher transmittance means the lower loss of energy, which is beneficial to the working efficiency of optoelectronic devices. However, the transmittance is still hindered by the low transparency of the matrix caused by the massive aggregation of QDs during the process of incorporation into polymers.
The recently developed doped quantum dots (D-QDs) show great potential as an alternative to reduce the reabsorption. In such D-QDs, light absorption is dominated by the host, while emission is dominated by the doped impurities, which resulted in a large Stokes shift up to hundreds of meV [14,15]. Furthermore, compared to traditional organic dyes, D-QDs exhibit lower concentration quenching threshold, higher thermal and chemical stability [16,17]. Although there are so many advantages, the optical properties of the D-QDs are influenced by many factors, especially the surface modification [18,19]. Improper surface modification would induce the agglomeration of D-QDs in matrix and reduce the transparency of composite thin films resulting in a decrease of transmittance. Besides, free radical species generated from the decomposition of the polymerization initiator usually show strong oxidative property, which not only oxidize the surfactant and reduce the dispersibility of QDs in polymer, but also result in additional defects degrading the optical properties of QDs [20,21].
Monodisperse QDs exhibit high QYs were usually synthesized in high boiling point organic solvent and modified with hydrophobic molecules. The surface hydrophobic molecules modification requires that the monomers should present similar polarity to the QDs. Among several polymer system, LMA and EGDMA exhibit similar polarity to hydrophobic surfactant, which turn to avoid the aggregation of QDs and enhance the transparency of composite thin films [22]. Moreover, the mechanical performance of the polymer could be regulated by the chemical dosage ratio between LMA and EGDMA. In this article, we demonstrated a facile strategy to fabricate ZnSe:Mn QDs/Poly(LMA-co-EGDMA) composite thin films with low-reabsorption, high transparency and high QYs. ZnSe:Mn QDs were synthesized by a modified high boiling organic solvent method using oleylamine (OLA) and DDT as the surfactant. The as synthesized QDs show decent dispersibility in polymer due to the similar polarity between the surfactants and monomer. It is worth noting that the DDT surfactant molecules adsorbed on the surfaces of ZnSe:Mn QDs could reduce the surface defects and aggregation of QDs resulting into higher QYs.This could further increase the transparency and QYs of composite thin films. Moreover, the as synthesized ZnSe:Mn QDs/Poly(LMA-co-EGDMA) composite thin films show prominent optical properties and could be a potential candidate for the applications ranging from light-emitting diodes (LEDs), optoelectronic detector to luminescent solar concentrators.
2. Experimental section
2.1 Chemical and reagents
1-octadecene (ODE, 90%), 1-dodecanethiol (DDT, 98%), zinc stearate (ZnSt2, 12.5-14% ZnO), selenium power (200 mesh, 99.999%), tetramethylammonium hydroxide pentahydrate (TMAH, 98%), manganese (II) chloride, anhydrous (MnCl2, 97%), stearic acid (SA, 90%) and ethylene glycol dimethacrylate (EGDMA, 98%) were purchased from Alfa Aesar Chemical Company. Oleylamine (OLA, 80-90%), lauryl methacrylate (LMA, 96%) and 2, 2-Dimethoxy-2-phenylacetophenone (DMPA, 99%) were purchased from Aladdin Chemical Company. Chloroform (98.5%), acetone (99.5%) and methanol (99.5%) were purchased from Sinopharm Chemical Reagent Company. All chemicals were used without further purification.
2.2 Synthesis of ZnSe:Mn QDs
In a typical synthesis procedure, 10 mL of ODE, 0.1 mmol of ZnSt2, 2 μmol of MnSt2 and 1 mL of OLA were mixed in a 100 mL three-neck round-bottom flask which was degassed with argon and then heated up to 250 °C. Then, 0.4 mmol of Se and 60 μL of DDT were dispersed into 2 mL of OLA as Se precursor and injected into the flask. The obtained mixed solution was heated up to 280 °C and maintained at this temperature for 10 minutes. The solution was cooled down to 230 °Cfollowed by an injection of 2 mL of Zn precursors which was prepared by dissolving 2 mmol of ZnSt2 and 2 mmol of SA in 10 mL of ODE. The reaction mixture was maintained at 230 °C for 30 minutes and then cooled down to 160 °C. A certain amount of DDT was further injected into the flask and maintained at this temperature for 10 minutes to modify ZnSe:Mn QDs surface with DDT. Then, the flask was rapidly cooled down to room temperature, and the as-synthesized ZnSe:Mn QDs were washed three times by acetone and chloroform. The ZnSe:Mn QDs were then dispersed in 2mL of chloroform for further characterizations and applications.
2.3 Fabrication of ZnSe:Mn QDs/polymer composite thin films
In a typical synthesis, a designed amount of ZnSe:Mn/chloroform solution was added to the polymer precursor solution mixed with 350 μL of LMA, 70 μL of EGDMA and 1 wt% of 2-Dimethoxy-2-phenylacetophenone (DMPA) as an initiator. The mixture solution was treated with ultrasonic for 5 minutes to acquire a clear solution. In order to obtain different parameters, we need to make a mold with different parameters of silica gel sheeting tailored. For example, we tailored 0.5 mm thick silica gel sheeting to a spacer with a size of 45 mm × 12 mm and placed it between two pieces of glass slide obtaining a relative sealed environment. Then the solution was injected into the space and another injector was used to extract the air inside simultaneously. The samples were photo-polymerized with UV light. We can also synthesize a film as thin as 20 μm without the silica gel sheeting.
2.4 Instruments and characterization
The UV-Vis absorption spectra and transmission spectrum were performed on a Perkin Elmer Lambda 25 UV-visible spectrophotometer. Fluorescence spectra were obtained with a fluorescence spectrophotometer (Jasco FP6500, Japan) at room temperature with a 450 W xenon lamp as an excitation source. Fourier Transform Infrared Spectra were collected with a Bruker VERTEX 70 Fourier Transform Infrared Spectrometer. X-ray Photoelectron Spectra (XPS) were recorded by Kratos AXIS-ULTRA DLD-600W Spectrometer. The crystallographic information of ZnSe:Mn QDs was characterized by X-ray diffraction (X'Pert PRO from PANalytical B.V with Cu Kα radiation). TEM images were recorded by FEI Tecnai G2 20 U-Twin. Thermo Gravimetric Analysis was collected with a Perkin Elmer Pyris1 TGA analysis meter. The QYs was measured and derived according to a previous reported comparative method [23].
3. Results and discussion
In this article, a facile method was demonstrated to fabricate ZnSe:Mn QDs/polymer (QDs/P) composite thin films with low reabsorption, high transmittance and high QYs. Large stokes shift in QDs is a key factor to achieve low-reabsorption and improve the luminescence properties of QDs/polymer composite thin film. As shown in Fig. 1(a), the light absorption of ZnSe:Mn QDs is dominated by the semiconductor host ZnSe with a peak at about 400 nm, while emission is mediated by the metal impurities Mn with an emission peak at about 590 nm. Hence, the ZnSe:Mn QDs synthesized by high boiling organic solvent method exhibit a large stoke shift up to 190 nm, which would significantly reduce the reabsorption and enhance the luminescence efficiency of QDs/P composite thin films.
Fig. 1 Morphology, crystal structure and optical properties of ZnSe:Mn QDs. Absorption and emission spectra of ZnSe:Mn QDs (A); PL spectra of ZnSe:Mn QDs treated with different amounts of DDT (B); XRD spectra of treated and untreated ZnSe:Mn QDs (C); TEM images of QDs untreated (D) and treated (E) with DDT. Insets on the bottom right corner of the TEM images are corresponding HRTEM images with identical scale bars of 5 nm.”
DDT is used as efficient surfactant species and could enhance the emission efficiency of D-QDs. Figure 1(b) shows the photoluminescence (PL) spectra of DDT treated ZnSe:Mn dispersed in chloroform, which normalized by the absorption at 400 nm. The PL intensity increases with the addition of DDT, and then maintains at a saturation value. The derived QYs of ZnSe:Mn QDs show a gradual increase from 61.5% to 75.6% with a maximum enhancement of 14.1%.The resulted DDT modification could effectively reduce the abundant surface defects and non-radiative recombination. Moreover, DDT molecules could withdraw the surface holes quenching band-edge PL, which resulted in the enhancement of Mn related PL [24,25]. Although the DDT molecules show high affinity to the surface of QDs and could substitute OLA, there is a saturated adsorption ability due to the limited surface area of QDs. Hence, there is an emission enhancement and saturation of ZnSe:Mn QDs, and 1 mL DDT is the optimized dosage without noted every time in this research.
Figures 1(c)-1(e) reveal the crystal phase and morphology of ZnSe:Mn QDs before and after the DDT. As shown in Fig. 1(c), both DDT treated and untreated ZnSe:Mn QDs exhibit high crystallinity and could be clearly indexed into cubic phase with zinc-blende structure. TEM images (Figs. 1(d)-1(e)) shows ZnSe:Mn QDs with similar size and monodispersity before and after the treatment. These aforementioned results manifest that the addition of DDT exhibits fewer effects on the morphology and crystal phase. Figure 2(a) presents the FTIR spectra of QDs before and after the treatment with 1 mL of DDT. The peaks at 2921 cm−1, 2852 cm−1 can be attributed to C-H stretching vibration, while peaks at 1465 cm−1, 1417 cm−1 could be ascribed to C-H bending vibration. The peak appears at 721 cm−1 resulted from CH2 rocking vibration. The peaks at 3430 cm−1, 1562 cm−1 and 1052 cm−1 respectively belong to N-H stretching vibration, deformation vibration and C-N stretching vibration of OLA molecules. After the treatment with DDT, the characteristic peaks of OLA became weaker, while the emerging obvious peak at 605 cm−1 could be attributed to the C-S stretching vibration from the modified DDT molecules. The further element analysis manifests (Fig. 2(b)) that the nitrogen element decreased from 9.61% to 6.01%, while sulphur increased from 7.01% to 14.87% after the treatment with 1 ml DDT. The 7.01% of sulphur element primarily comes from the precursor. Further 7.86% increase implies that abundant DDT molecules have been adsorbed to the QDs surface and substitute part of OLA owing to the different adsorption capacity between OLA and DDT [26,27]. The morphology and crystal phase analysis (Figs. 1(d)-1(e)) above prove that the surface modification with DDT would not have any considerable effects on the shape and structure of QDs.
Fig. 2 Surface molecules and element composition of ZnSe:Mn QDs. FTIR spectra (A) and element composition histogram derived from XPS of QDs (B) before and after treatment with DDT.
QDs/P composite thin films were fabricated according to the route illustrated in Fig. 3, where untreated and treated QDs are noted as QDs/P1 and QDs/P2, respectively. All of these QDs/P were synthesized by adding 1 wt% of DMPA as polymerization initiator. Figures 4(a)-4(b) presents the absorption and emission spectra of QDs, QDs/P1and QDs/P2, where the emission spectra were normalized by the absorption at 400 nm. There is no obvious shift in emission of QDs before and after embedding into the polymer matrix. The further analysis reveals that the corresponding QYs of QDs/P1 decreased from 61.5% to 35.4%, while that of QDs/P2 decreased from 75.6% to 67.1%, compared to that of as synthesized QDs solution. The net QYs decreasing of QDs/P1 and QDs/P2 are 26.1% and 8.5%, respectively. The higher QYs of QDs/P2 compared to that of QDs/P1 thin film, could be attributed to the decreased surface defect and enhanced dispersibility in polymer.. This improved dispersibility is due to the surface decoration of QDs with DDT. Figure 4(c) presents the PL spectra of QDs/P thin film which are embedded with DDT treated QDs. The emission intensity shows similar tendency to that of DDT treated QDs dispersed in chloroform (Fig. 1(e)). However, the QYs of QDs/P thin film increases from 35.4% to 67.1% with a net increase of 31.7%. This net QYs enhancement of 31.7% is higher than that of 14.1% in case of DDT treated QDs in chloroform. It is notable that the QYs enhancement of QDs/P is much higher than that of QDs dispersed in chloroform. This enhancement could be due to the superficial DDT molecules which generate sulfide exposing to radical species and exhibit excellent oxidation resisting property [28].
Fig. 4 Optical properties of QDs, QDs/P1 and QDs/P2. Absorption and PL spectra of QDs/P1 (A) and QDs/P2 (B); PL spectra of QD/P composite thin films treated with different amount of DDT (C).
To test the hypothesis, different amounts of initiator (DMPA)were added to synthesize the QDs/P composite thin films. As revealed in Fig. 5(a), the QYs of QDs/P first increased and then decreased with the increase of DMPA. The UV light is usually utilized to drive the polymerization and cross-linking reaction, while it could also induce redox reaction to generate surface defect and reduce the QYs. The small increase of DMPA from 0.2 to 1% would shorten reaction process to reduce the defects induced by UV radiation and enhance the QYs of QDs/P composite thin films. When the concentration of DMPA exceeds 1%, the UV light would induce abundant oxidative radical species affecting the luminescence property. Hence, the UV radiation is prominent for the QYs deterioration with small amounts of initiators, while destructive effects of initiator-induced radical species became remarkable with high concentration of initiator. Besides, nanoparticles concentration is another critical factor on the luminescence efficiency of QDs/P composite thin films. Figure 5(b) shows the QYs evolution of QDs/P composite thin films with different amount of QDs inferred from the thermogravimetric analysis (Fig. 7). In general, higher concentration of QDs leads to the higher QYs.. Also, QDs/P2 thin films show obviously higher QYs than that of QDs/P1 which is mainly due to the surface modification of QDs with DDT. Nevertheless, QYs evolution of QDs/P2 shows a significant difference to QDs/P1 at low concentration of DMPA. The increasing of QDs could reduce the average free radical species around each particle, and resulted in weakening of oxidation ability.
Fig. 5 QYs evolution of QDs/P composite thin films. QYs of QDs/P composite thin films synthesized with different amounts of DMPA (A); QYs of QDs/P composite thin films synthesized with different amounts of QDs (B).
DDT modification not only reduce the surface defect but also improve the dispersibility of QDs resulting in improved transparency of composite thin films. Figure 6(a) shows the photograph of polymer only, QDs/P1 and QDs/P2 under the irradiation of natural light, which manifests that the blank polymer, QDs/P1 and QDs/P2 thin films are transparent with high transmittance. Figure 6(b) shows the photographs of polymer only, QDs/P1 and QDs/P2 under 365nm UV light. QDs/P2 show obviously enhanced yellowish emission than that of QDs/P1. Figure 6(c) shows the transmission spectra of blank polymer and QDs/P with a thickness of 0.5 mm. The blank polymer thin film shows average transmittance as high as 91.23% in the visible region. QDs/P1 with 0.3 wt% of ZnSe:Mn QDs calibrated from TGA analysis as shown in Fig. 7.The as prepared QDs/P1 maintained the average transmittance higher than 88% in the range from 500 nm to 800 nm, which is higher than that of previous reported QDs/cellulose nanofibers composite thin film (~80%) [28]. The high transmittance is mainly due to the dispersibility of QDs inmonomer based on the similar polarity. The transmittance of both QDs/P1 and QDs/P2 exhibit a rapid decrease from 450 to 400 nm, which is attributed to the intense absorption of ZnSe host. It is worth noting that the QDs/P2 shows 2% higher transmittance than that of QDs/P1 in the range from 500 nm to 800 nm due to the presence of DDT molecules which have stronger affinity than OLA molecules. Hence DDT treated QDs would lose less surface surfactant in the polymerization process, and show higher dispersity to maintain the high transmittance of the as prepared composite thin films.
Fig. 6 Optical properties of QDs/P composite thin films. Photograph of QDs/P composite thin films under daylight (A), UV irradiation (B) and Transmission spectra (C) of QDs/P composite thin films. The samples from left to right are blank polymer, QDs/P1 and QDs/P2 respectively.
Fig. 7 Thermogravimetric analysis (TGA) of the blank polymer and QDs/P thin films. The enlarged images in the inset reveals the mass concentration of inorganic QDs is about 0.3%. The obvious peak shift resulted from the surface decorated surfactant.
4. Conclusions
To summarize, ZnSe:Mn QDs based QDs/P composite thin films were fabricated by a facile strategy. Alkylthiol were selected as the surfactant molecules, which could be feasibly decorated on the surface of ZnSe:Mn QDs increasing the QYs of QDs by both reducing the surface defects and acting as hole-withdrawing species. Moreover, the DDT molecules exhibits oxidation resisting property to initiator induced oxidative free radicals and could well maintain the QYs of QDs which were embedded into Poly(LMA-co-EGDMA). Meanwhile, DDT molecules enhanced dispersibility, which leads to the improved transparency. This kind of QDs/P composite thin films show high transmittance and high QYs up to 90% and 67.1%, respectively. These QDs/P composite thin films are promising candidates for applications in various future light-emitting technologies and advanced optoelectronic devices.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 11274127, 51303046, 51173038) and Ph.D. Programs Foundation of Ministry of Education of China (No. 20114208130001, No. 20134208120001). We would also like to acknowledge the Analytical and Testing Center of HUST for assistance in material characterizations.
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