- Email: [email protected]
polymethylmethacrylate composite films
Journal of Alloys and Compounds 647 (2015) 578e584
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Preparation and multicolored fluorescent properties of CdTe quantum dots/polymethylmethacrylate composite films Yanni Huang, Jianjun Liu*, Yingchun Yu, Shengli Zuo State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 December 2014 Received in revised form 21 April 2015 Accepted 20 May 2015 Available online 22 June 2015
A new simple route was presented for the preparation of stable fluorescent CdTe/polymethylmethacrylate (CdTe/PMMA) composite films by using hydrophilic thioglycolic acid capped CdTe quantum dots (TGA-CdTe QDs) and polymethylmethacrylate (PMMA) as raw materials. The TGA-CdTe QDs were firstly exchanged with n-dodecanethiol (DDT) to become hydrophobic DDT-CdTe QDs via a ligand exchange strategy, and then incorporated into PMMA matrix to obtain fluorescent CdTe/PMMA composite films. The structure and optical properties of DDT-CdTe QDs and CdTe/PMMA composite films were investigated by XRD, IR, UV and PL techniques. The results indicated that the obtained DDT-CdTe QDs well preserved the intrinsic structure and the maximum emission wavelength of the initial water-soluble QDs and the resulting 6.10 wt% CdTe/PMMA composite film exhibited significantly enhanced PL intensity. Furthermore, the multicolored composite films with green, yellow-green, yellow and orange light emissions were well tuned by incorporating the CdTe QDs of various maximum emission wavelengths. The TEM image demonstrated that the CdTe QDs were well-dispersed in the PMMA matrix without aggregation. Superior photostability of QDs in the composite film was confirmed by fluorescence lifetime measurement. Thermo-gravimetric analysis of CdTe/PMMA composite films showed no obvious enhancement of thermal stability compared with pure PMMA. © 2015 Elsevier B.V. All rights reserved.
Keywords: CdTe quantum dot Ligand-exchange Polymer Fluorescence Composite film Thermal evaporation
1. Introduction Semiconductor quantum dots (QDs)-polymer composites have attracted great attention due to their excellent optical properties and potential applications in the fields including light-emitting devices (LEDs) [1,2] nonlinear optical devices [3,4] biological labels [5,6] etc. Significant efforts have been devoted to fabricate the QD-polymer composites with tunable fluorescence property and with improved film-formed performance using various polymers such as poly (vinyl alcohol) (PVA) [7], polystyrene [8], nylon [9], fluoropoly (ether ether ketone) (FPEEK) [10], styrene/ maleic anhydride copolymer [11] etc. Polymethylmethacrylate (PMMA) with high transparency, high mechanical stability and a relative low price can be used in a wide range of applications which can be even more expanded by the incorporation of QDs into its matrix. CdSe/PMMA composite films with enhanced electrical conductivity were reported by Al-Hosiny's group [12].
* Corresponding author. E-mail address: [email protected] (J. Liu). http://dx.doi.org/10.1016/j.jallcom.2015.05.230 0925-8388/© 2015 Elsevier B.V. All rights reserved.
Khanna et al. [13] fabricated transparent CdS/PMMA composite films with tunable optical properties by a thermal evaporation method. Among the various QDs, CdTe QDs of different sizes have tunable emission from green to red due to quantum confinement are favorable for optoelectronic devices [14,15]. To date, CdTe QDs with desired optical properties can be synthesized via two synthetic routes including organic-phase and aqueous-phase approaches. The oil-soluble QDs can be obtained by an organometallic synthetic route with trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) as stabilizing reagents [16,17], which is toxic, expensive and unfavorable for large-scale production. While the aqueous synthesis is reagent-effective, less toxic [18,19] and the obtained hydrophilic QDs can be transferred into organic phase under relatively mild conditions [20]. Incorporating CdTe QDs into polymers to fabricate light-emitting films may maintain or enhance the PL stability of QDs and further expand their applications in photovoltaic field. Synthesis of PMMA/CdTe composite fibers via an electrospinning route was demonstrated by Zhu et al. [21]. Herein, we provide a facile and effective route to prepare flexible CdTe/PMMA composite films by using the aqueous phase
Y. Huang et al. / Journal of Alloys and Compounds 647 (2015) 578e584
synthesized CdTe QDs and PMMA as raw materials. Firstly, to obtain homogeneous solution of QDs and PMMA, a ligand-exchange procedure was used to render QDs to be hydrophobic, and then flexible transparent CdTe/PMMA composite films with enhanced PL intensity were fabricated by a thermal evaporation process. The thermal stability, photostability, and the distribution of the CdTe QDs in the polymer matrix were well studied. 2. Materials and methods 2.1. Chemicals CdCl2$2.5H2O (99.9%), thioglycolic acid (TGA; 98%), tellurium powder (99.9%), sodium borohydride (NaBH4), sodium hydroxide (NaOH), n-dodecanethiol (DDT), methyl methacrylate (MMA), benzoyl peroxide (BPO), acetone, toluene, cyclohexane and ethanol were all analytical grade and purchased from the Beijing Chemical Reagents Company. All chemicals were used without additional purification. All aqueous solutions were prepared with Milli-Q water (Millipore). 2.2. Synthesis of water-soluble CdTe QDs Water-soluble TGA capped CdTe QDs were synthesized according to our previous work [22]. Typically, the CdTe precursor solution was prepared by the addition of fresh oxygen-free NaHTe solution to a N2-saturated CdCl2 solution at pH ¼ 10 in the presence of TGA stabilizer. The molar ratio of Cd2þ/TGA/HTe was set as 1:3:0.1 ([Cd2þ] ¼ 4 mmol/L). Under vigorous stirring in N2 atmosphere, a series of water-soluble CdTe QDs were prepared by controlling refluxing time: 0.5 h, 4 h, 10 h and 20 h. The TGA-CdTe QD powders were precipitated from the water phase with acetone for XRD and FT-IR characterization. 2.3. Preparation of oil-soluble DDT-CdTe QDs via a ligand-exchange procedure The oil-soluble DDT-CdTe QDs were prepared as follows. First, 30 mL as-prepared TGA capped CdTe QDs solution, 10 mL ndodecanethiol and 30 mL acetone were added into a 200 mL flask and the mixture was vigorously stirred at 65 C for 1 h. The CdTe QDs were slowly transferred into n-dodecanethiol phase from water phase. Then, the upper organic phase was separated and centrifuged at 5000 rpm for 10 min. The precipitate was washed with toluene twice and dried at 70 C for 10 min. Finally, 0.0710 g of the obtained solid powder, 10 mL toluene and 10 mL cyclohexane were added into a 50 mL conical flask with stopper and stirred at 65 C until all solid dissolved to obtain a homogeneous solution for further experiments. The oil-soluble CdTe QDs with different maximum emission wavelength lem were prepared by using CdTe QDs aqueous solution with refluxing time of 0.5 h, 4 h, 10 h and 20 h as raw material. For XRD and FT-IR characterization, the DDT-CdTe QD powders were employed. 2.4. Preparation of fluorescent CdTe/PMMA composite films via a thermal evaporation method Firstly, the PMMA was prepared by polymerization of the MMA monomers initiated by BPO as described previously [13], with a number average molecular weight Mn ¼ 193535 Da. Then, a certain amount of as-prepared oil-soluble CdTe QDs solution was added to 4.06 g of PMMA toluene solution (7.4 wt%) to form a homogeneous solution under stirring at 65 C. Finally transparent films of CdTe/ PMMA composite were prepared by further heating the mixed solutions at 100 C for 4 h in an oven.
579
2.5. Characterization of samples The molecular weight of the synthesized PMMA was determined on a Waters 515-717-2410 gel permeation chromatography (GPC) system with THF as the eluting solvent and a polystyrene standard. X-ray diffraction (XRD) patterns were measured on a Bruker D8ADVANCE X-Ray diffractometer with Cu Ka radiation. Fouriertransform infrared (FT-IR) spectra were obtained on a Thermo Nicolet 6700 IR spectrophotometer. Photoluminescence (PL) spectra were recorded on a Hitachi F-7000 fluorescence spectrophotometer equipped with a Xe lamp. The excitation wavelength was set at 365 nm, tube voltage was 400 V, scanning speed was 1200 nm/min and the excitation and emission slits were both 5 nm. UVevisible absorption spectra were collected on a Shimadzu UV 3600 UV-VISNIR spectrophotometer. The Thermo-gravimetric analysis of the sample were carried out by simultaneous thermogravimetrydifferential scanning calorimetry (TG-DSC), performed with a NETZSCH STA 409 PG/PC thermal analysis system at 10 K/min under an atmosphere of N2. The morphology of the obtained samples was determined by transmission electron microscopy (TEM, Hitachi 800). The CdTe/PMMA sample was ultramicrotomed with a Leica EM-FC6 Instruments at 90 C using glass knife and ready to be imaged in the TEM. Photographic images of the samples were captured by digital camera (100 IS, Canon). Fluorescent images were recorded on Laser scanning confocal microscope (FV1000) and a diode laser (Spectra-Physics, Cyan Scientific, l ¼ 405 nm) was used as the excitation source. Time-correlated single-photon counting (TCSPC) data were performed on FL 900 spuoectroflrometer using a hydrogen lamp as the excitation source, lex ¼ 375 nm. 3. Results and discussions 3.1. The preparation of CdTe/PMMA composite film The process of the preparation of CdTe/PMMA composite film is demonstrated in Scheme 1. The TGA-capped CdTe QDs were firstly prepared via an aqueous synthetic strategy, and then became oilsoluble by exchanging the initial ligand with DDT through a ligand-exchange strategy [23], as shown in Scheme 1a. To improve the ligand-exchange efficiency, the addition of a polar organic solvent is necessary [24]. The PL spectra of CdTe QDs with different refluxing time before and after the ligand-exchange procedure are displayed in Fig. 1. The water-soluble CdTe QDs with refluxing time of 0.5 h, 4 h, 10 h and 20 h show the maximum emission wavelength lem at 541 nm, 556 nm, 567 nm and 592 nm, respectively, and the mean sizes of QDs can be estimated to be 2.21 nm, 2.68 nm, 3.16 nm and 3.42 nm, shown in Surpporting Materials. After the ligandexchange procedure, the obtained oil-soluble CdTe QDs show the maximum emission wavelength lem at 539 nm, 555 nm, 566 nm and 588 nm. The emission peak of the oil-soluble DDT-CdTe QDs solution shows no obvious change compared with the corresponding aqueous CdTe QDs, indicating that the CdTe QDs were well protected by DDT without aggregation after the ligand-exchange procedure. The best quantum yield (QY) with 75% for the watersoluble CdTe QDs can be obtained at the refluxing time of 4 h, and the QY for the corresponding oil-soluble CdTe QDs is 13.5%, using rhodamine 6G (QY ¼ 95%) as the reference standard. The declining of QY is probably due to the formation of new surface defects during water-to-oil processes [25]. When illuminated under ultra violet lamp of 365 nm, the oil-soluble CdTe QDs show green, yellow-green, yellow and orange light emissions (shown in Fig. S2) corresponding to the lem of 539 nm, 555 nm, 566 nm and 588 nm, respectively. The CdTe/PMMA composite films were obtained by evaporating the solutions of oil-soluble CdTe QDs and PMMA in mixed solvent of toluene and cyclohexane, as shown in Scheme 1(bed).
580
Y. Huang et al. / Journal of Alloys and Compounds 647 (2015) 578e584
Fig. 2. XRD patterns of TGA-CdTe QDs powder (a) DDT-CdTe QDs powder (b) PMMA (c) 6.10 wt% CdTe/PMMA composite film (d). Inset shows magnified (a) and (b) patterns.
Scheme 1. Schematic illustration of the preparation of CdTe/PMMA composite film. (a) Ligand-exchange of TGA-capped CdTe QDs by DDT during phase transfer procedure. (b) Preparation of PMMA by polymerization of the monomers. (c) Encapsulation of DDTcapped CdTe by PMMA in mixed solvent of toluene and cyclohexane. (d) Thermal evaporation of solvents to obtain CdTe/PMMA film.
Fig. 2 shows the XRD patterns of TGA-CdTe QDs powder, DDTCdTe QDs powder, pure PMMA and 6.10 wt% CdTe/PMMA composite film. Same diffraction peaks in the TGA-CdTe QDs powder
(curve a) and DDT-CdTe QDs powder (curve b) with 2q at 24.22 ,40.58 ,47.17 indexed to (111), (220) and (311) plane of the cubic zinc blende structure like bulk CdTe (JCPDF No.19-0191) are clearly observed, indicating the intrinsic structure of CdTe QDs is preserved during the ligand-exchange procedure. The XRD pattern of PMMA (curve c) shows a broad diffraction peak at 2q ¼ 16.33 , typical of an amorphous material, together with two peaks of lower intensities centered at 30.69 and 42.51 [26,27]. Besides, similar XRD patterns can be found in Fig. 2(c) and (d) corresponding to the pure PMMA and the 6.10 wt% CdTe/PMMA composite film but there are little XRD peak shifts to lower 2q after incorporating QDs into PMMA, indicating the slightly change of the orientation of the
Fig. 1. PL spectra of CdTe QDs with refluxing time of (A) 0.5 h (B)4 h (C)10 h and (D)20 h before and after the ligand-exchange procedure. (a) Organic solvents; (b) CdTe QDs in water; (c) CdTe QDs in oil.
Y. Huang et al. / Journal of Alloys and Compounds 647 (2015) 578e584
581
3.2. The fluorescent property, photostability and thermal stability of CdTe/PMMA composite film
Fig. 3. FT-IR spectra of TGA-CdTe QDs powder (a) DDT-CdTe QDs powder (b) PMMA (c) 6.10 wt% CdTe/PMMA composite film (d).
PMMA chains and the interaction between CdTe QDs and polymer chains. No characteristic diffraction peak of CdTe QDs is detected for CdTe/PMMA composite film, which is explained by the small amounts, high dispersion and low crystallinity of CdTe QDs in CdTe/ PMMA composite film. The FT-IR spectra of CdTe QDs, PMMA and the 6.10 wt% CdTe/ PMMA composite film are shown in Fig. 3. The characteristic peaks at 1580 cm1 and 1400 cm1 (curve a) correspond to the carboxyl group attributing to TGA molecules on the surface of CdTe QDs. Compared with the TGA-CdTe QDs, the characteristic peaks of the carboxyl group are not obvious in the FT-IR spectra of the DDT-CdTe QDs (curve b). In addition, the sharp IR absorption peaks at 2920 cm1 and 2848 cm1 of CeH stretching vibration, together with the peak at 720 cm1 of the ethyl groups (n > 4) swinging in the plane, strongly prove the existence of DDT molecule, indicating that the original hydrophilic ligands, TGA capped on the CdTe QDs have been exchanged by the hydrophobic DDT molecule during the ligand-exchange process. The FT-IR spectrum of 6.10 wt% CdTe/PMMA composite film (curve d) almost perfectly matches that of pure PMMA (curve c), indicating that the CdTe QDs are well-embedded in composite film instead of adsorbing only on the PMMA surface. The peak at 2961 cm1 is assigned to the stretching vibrations of CeH of both methyl and ethyl group, while the peaks at 1451 cm1 and 1380 cm1 are due to their bending vibrations. Moreover, the C]O absorption vibration appears at 1728 cm1 and the stretching vibration of CeO is at 1148 cm1.
Fig. 4A shows that the loading amount from 2.12 wt% to 6.10 wt% of CdTe QDs has significant effect on the PL property of CdTe/PMMA composite film. There is an increasing PL intensity and red-shift (from 542 nm to 553 nm) of the emission peak, which may be ascribed to the growth and ripening of more CdTe QD grains leading to the decrease of surface defects while increasing CdTe QDs loading amount [28]. Fig. 4B shows that there is an increasing absorption intensity and a slightly red-shift of the absorption peak along with the increasing CdTe QDs loading amount from 2.12 wt% to 6.10 wt%, which indicates the slightly growth of QDs in PMMA matrix with the increase of CdTe QDs loading amount. This result is well consistent with that obtained by PL measurement. However, CdTe QDs are expelled from the surface of the composite film when the CdTe QDs loading amount is increased to 9.78 wt% (Fig. S3), which implies that there is an existence of best loading amount of CdTe QDs in polymer. Fig. 5A shows the PL spectra of CdTe/PMMA composite films incorporated by CdTe QDs of different lem. By incorporating the CdTe QDs with lem of 539 nm, 555 nm, 566 nm and 588 nm into PMMA matrix, the obtained composite films display sizedependent fluorescence emissions at 538, 553, 565 and 584 nm, respectively, corresponding to green, green-yellow, yellow, and orange light emissions as shown in Fig. 5B. This result demonstrates that the multicolored fluorescence of the CdTe/PMMA composite film could be adjusted by varying the lem of incorporated CdTe QDs. The microstructures and fluorescence properties of CdTe/PMMA composite films incorporated by QDs of different lem were further characterized by using laser confocal fluorescence microscopy (LCFM). As shown in Fig. 6(aec), the CdTe/PMMA composite films are composed of many spherical particles with 2e4 mm in size. The microsphere is attributed to the CdTe/PMMA composite spherulite, which is one of the common morphology of PMMA or composites with PMMA [29]. The observation of uniform fluorescence emission throughout the microspheres suggests that the CdTe QDs are well compatible with the PMMA matrix in our system. Furthermore, it is found that varying the lem of incorporated CdTe QDs has no obvious influence on the microstructure of CdTe/PMMA composite and the size-dependent fluorescence property of CdTe QDs in PMMA matrix remains well. Fig. 7A shows that the variation in excitation wavelength from 300 nm to 460 nm does not lead to any change in the position of emission peak for CdTe/PMMA composite film, and the PL intensity becomes more higher with the decreasing excitation wavelength, thus proving an excellent quantum confinement with narrow size distribution for CdTe QDs in PMMA matrix [13,30]. The TEM image
Fig. 4. PL spectra (A) and UVevisible absorption spectra (B) of CdTe/PMMA composite films. Incorporated QDs: lem ¼ 555 nm, yellow-green.
582
Y. Huang et al. / Journal of Alloys and Compounds 647 (2015) 578e584
Fig. 5. Normalized PL spectra of 6.10 wt% CdTe/PMMA composite films incorporated by CdTe QDs of different lem (A) and their images(B) under room light (top) and 365 nm UV irradiation (bottom). From left to right: PMMA; a. QDs: lem ¼ 539 nm, green; b. QDs: lem ¼ 555 nm, yellowegreen; c. QDs: lem ¼ 566 nm, yellow; d. QDs: lem ¼ 588 nm, orange.
Fig. 6. Fluorescence confocal microscopy images of 6.10 wt% CdTe/PMMA composite films incorporated by CdTe QDs of different lem. From left to right: a. QDs: lem ¼ 539 nm, green; b. QDs: lem ¼ 566 nm, yellow; c. QDs: lem ¼ 588 nm, orange.
Fig. 7. PL spectra of 6.10 wt% CdTe/PMMA composite film at different excitation wavelengths (A) and its TEM image (B). Incorporated QDs: lem ¼ 555 nm, yellowegreen.
of 6.10 wt% CdTe/PMMA composite slice (Fig. 7B) shows that CdTe QDs are well-dispersed in the PMMA matrix, revealing the approach provided in our work effectively avoids QDs aggregation.
Fig. 8 shows the PL spectra and time-resolved fluorescence decay curves of three samples including TGA-CdTe QDs powders, DDT-CdTe QDs powders and 6.10 wt% CdTe/PMMA composite film,
Y. Huang et al. / Journal of Alloys and Compounds 647 (2015) 578e584
583
Fig. 8. PL spectra (A) and Time-resolved fluorescence decay curves (B) of CdTe QDs powders and 6.10 wt% CdTe/PMMA composite film. Initial CdTe QDs aqueous solution: 4 h, lem¼556 nm.
Table 1 The decay time parameters for samples. Items
Samples
A
Lifetime components ti/ns
Bi ( 102)
a
TGA-CdTe QDs Powder DDT-CdTe QDs Powder 6.10 wt% CdTe/PMMA Film
40.24
t1¼3.720 t2¼8.097 t3¼34.05 t1 ¼ 1.938 t2 ¼ 9.648 t3 ¼ 34.16 t1 ¼ 2.113 t2 ¼ 9.294 t3 ¼ 35.57
B1 B2 B3 B1 B2 B3 B1 B2 B3
b
c
50.13
66.76
¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼
4.921 2.627 0.4756 4.310 3.094 1.254 4.460 2.797 1.266
Relative contribution of ti (%)
Average lifetime tAV/ns
Chi-square c2
32.82 38.14 29.04 10.31 36.84 52.85 11.71 32.32 55.97
14.20
1.199
21.80
1.061
23.16
1.186
all are prepared by using the same initial CdTe QDs aqueous solution (4 h, lem ¼ 556 nm). As shown in Fig. 8A, there is an enhancement of the PL intensity and an obvious blue shift of the emission peak for CdTe QDs solid after being exchanged by DDT and capsulated by PMMA, indicating that PMMA plays an important role to inhibit the aggregation of CdTe QDs as well as to modify the surface of the QDs. To study the photostability of CdTe QDs in CdTe/ PMMA composite film, the time-correlated single-photon counting (TCSPC) methodology was performed. Comparative TCSPC studies are presented in Fig. 8B. The decay curves of CdTe QDs powders and 6.10 wt% CdTe/PMMA composite film are well fitted with triexponential function Y(t) based on nonlinear least squares, using the following expression:
YðtÞ ¼ A þ
3 X
Bi expðt=ti Þ
(1)
i¼1
Where Bi are the amplitude of time-resolved decay lifetimes ti and the average lifetime tAV could be concluded from a typical formula:
tAV ¼
B1 t1 2 þ B2 t2 2 þ B3 t3 2 B1 t1 þ B2 t2 þ B3 t3
(2)
The decay time parameters for samples are given in Table 1. There are two shorter lifetimes (<10 ns) and a longer lifetime (>30 ns) for all the three samples given by data fitting. The shorter lifetime components can be mainly attributed to the intrinsic recombination of initially populated core states [31], while the longer lifetime component in the fluorescence decay is caused by the radiative recombination of electrons and holes on the surface involving surface-localized states [32]. The relative contribution of the longer lifetime (>30 ns, t3) is 29.04%, 52.85% and 55.97% for TGA-CdTe QDs powders, DDT-CdTe QDs powders and 6.10 wt% CdTe/PMMA composite film, respectively, indicating that the
Fig. 9. Thermo-gravimetric analysis of PMMA (a) 4.15 wt% CdTe/PMMA (b) 6.10 wt% CdTe/PMMA (c).
surface-related emission for CdTe QDs is obviously increased after ligand-exchange procedure and further increased by being capsulated into PMMA matrix. By comparing the average lifetimes of the samples, we can see that the average fluorescence lifetime tAV of CdTe QDs is prolonged upon their incorporation into polymeric matrix to form CdTe/PMMA composite film. For 6.10 wt% CdTe/ PMMA composite film, the average lifetime is 23.16 ns, which is 1.36 ns longer than that of isolated CdTe QDs (21.80 ns) precipitated from oil phase, and 8.96 ns longer than that of the one (14.20 ns) obtained from water phase. The polymer matrix in the system could restrain the aggregation of CdTe QDs avoiding selfquenching, and delay the fluorescence decay process [33]. As shown in Fig. 9, TGA analysis displays two stages decomposition behavior for CdTe/PMMA composite films, the initial weight
584
Y. Huang et al. / Journal of Alloys and Compounds 647 (2015) 578e584
loss between 150 C and 270 C is due to the vaporization of trapped solvent from PMMA matrix and the second-stage weight loss between 270 C and 430 C is assigned to the decomposition of PMMA itself. The observations indicate that the incorporation of CdTe QDs with different amounts into PMMA matrix only slow the decomposition before about 350 C but next rapid decomposition up to 430 C is almost the same for CdTe/PMMA and pure PMMA. It is concluded that there is no great difference between decomposition of CdTe/PMMA composite and pure PMMA and no obvious dependence of thermal stability for PMMA on QDs loading amount. The remained residues at 900 C for CdTe/PMMA composite films can be considered as the mixture of CdTe and the carbon residues that may form during the decomposition [13]. 4. Conclusions In summary, transparent CdTe/PMMA composite films with enhanced fluorescence were fabricated by using water phase CdTe QDs and PMMA as raw materials. The phase transfer of CdTe QDs from aqueous phase into the oil phase was performed using hydrophobic DDT, followed by the addition of PMMA and then thermal evaporation, resulting in uniform transparent composite films with stable and excellent fluorescence property. The multicolored composite films with green, yellow-green, yellow and orange light emissions were achieved by incorporating different lem of CdTe QDs into PMMA matrix. We believe this facile approach can be easily applied to other semiconductor QDs to develop highly functional QD/polymer composite films that could be potentially used in photonic devices and optical materials. Acknowledgments This work was financially supported by the National Natural Science Foundation of China under Grant No. 11172043. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2015.05.230. References [1] B. Dubertret, P. Skourides, D.J. Norris, V. Noireaux, A.H. Brivanlou, Science 298 (2002) 1759e1762.
[2] N. Tessler, V. Medvedev, M. Kazes, S. Kan, U. Banin, Science 295 (2002) 1506e1508. [3] A.A. Mamedov, A. Belov, M. Giersig, N.N. Mamedova, N.A. Kotov, J. Am. Chem. Soc. 123 (2001) 7738e7739. [4] F. Qian, Y. Li, S. Gradecak, H.G. Park, Y. Dong, Y. Ding, Z.L. Wang, C.M. Lieber, Nat. Mater. 7 (2008) 701e706. [5] N. Pradhan, D.M. Battaglia, Y. Liu, X. Peng, Nano Lett. 7 (2007) 312e317. [6] Y. Zhang, Y. Li, X.P. Yan, Small 5 (2009) 185e189. [7] B.T. Suo, X. Su, J. Wu, D. Chen, A. Wang, Z.H. Guo, Mater. Chem. Phys. 119 (2010) 237e242. [8] M. Mao, C.H. Zhou, H.B. Shen, H.Z. Wang, S.J. Wang, L.S. Li, Mater. Lett. 111 (2013) 97e100. [9] Y. Yuan, F.-S. Riehle, R. Nitschke, M. Krüger, Mater. Sci. Eng. B 177 (2012) 245e250. [10] L.J. Zhu, P.F. Huo, Q. Wang, F.X. Liang, C.L. Zhang, J.L. Li, X.Z. Qu, G.B. Wang, Z.Z. Yang, Chem. Commun. 49 (2013) 5283e5285. [11] B. Martín-García, Pedro M.R. Paulo, Sílvia M.B. Costa, M.M. Vela-zquez, J. Phys. Chem. C 117 (2013) 14787e14795. [12] N.M. Al-Hosiny, S. Abdallah, M.A.A. Moussa, A. Badawi, J. Polym. Res. 20 (2013) 1e8. [13] P.K. Khanna, N. Singh, J. Lumin 127 (2007) 474e482. [14] S.D. Hodgsonn, W.S.M. Brooks, A.J. Clayton, G. Kartopu, V. Barrioz, S.J.C. Irvine, Nano Energy 4 (2014) 1e6. [15] I. Su0 arez, H. Gordillo, R. Abargues, S. Albert, J. Mart0 -nez-Pastor, Nanotechnology 22 (2011) 435202e435209. [16] C.B. Murray, D.J. Norris, M.G. Bawend, J. Am. Chem. Soc. 115 (1993) 8706e8715. [17] S.F. Wuister, F. Driel, A. Meijerink, J. Lumin 102e103 (2003) 327e332. [18] A.L. Rogach, T. Franzl, T.A. Klar, J. Feldmann, N. Gaponik, V. Lesnyak, A. Shavel, A. Eychmüller, Y.P. Rakovich, J.F. Donegan, J. Phys. Chem. C 111 (2007) 14628e14637. [19] C.Q. Dong, J.C. Ren, Luminescence 27 (2012) 199e203. [20] N. Gaponik, D.V. Talapin, A.L. Rogach, K. Hoppe, E.V. Shevchenko, A. Kornowski, A. Eychmüller, H. Weller, J. Phys. Chem. B 106 (2002) 7177e7185. [21] L. Zhu, S.Y. Yang, J. Wang, C.F. Wang, L. Chen, S. Chen, Mater. Lett. 99 (2013) 54e56. [22] J.J. Liu, Z.X. Shi, Y.C. Yu, S.L. Zuo, J. Colloid Interface Sci. 342 (2010) 278e282. [23] L.H. Yuwen, B.Q. Bao, G. Liu, J. Tian, H.T. Lu, Z.M. Luo, X.R. Zhu, F. Boey, H. Zhang, L.H. Wang, Small 7 (2011) 1456e1463. [24] N. Gaponik, D.V. Talapin, A.L. Rogach, A. Eychmüller, H. Weller, Nano. Lett. 2 (2002) 803e806. [25] J. Ding, X. Wang, L.H. Zhuo, B. Tang, J. Mater. Chem. 19 (2009) 3027e3032. [26] M.L. Saladino, T.E. Motaung, A.S. Luyt, A. Spinella, G. Nasillo, E. Caponetti, Polym. Degrad. Stab. 97 (2012) 452e459. [27] F.J. Balta-Calleja, C.G. Vonk, Elsevier, Amsterdam, 1989, pp. 191e193. [28] O.O. Matvienko, Y.N. Savin, A.S. Kryzhanovska, O.M. Vovk, M.V. Dobrotvorska, N.V. Pogorelova, V.V. Vashchenko, Thin Solid Films 537 (2013) 226e230. [29] C.A. Daniels, Technomic Publishing Co. Inc., Lancaster, Basel, 1989, pp. 11e12. [30] P.K. Khanna, P. More, B.G. Bharate, A.K. Vishwanath, J. Lumin 130 (2010) 18e23. [31] X.Y. Wang, L.H. Qu, J.Y. Zhang, X.G. Peng, M. Xiao, Nano Lett. 3( (2003) 1103e1106. [32] L.H. Qu, X.G. Peng, J. Am. Chem. Soc. 124 (2002) 2049e2055. [33] C.R. Kagan, C.B. Murray, M. Nirmal, M.G. Bawendi, Phys. Rev. Lett. 76 (1996) 1517e1520.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Preparation and multicolored fluorescent properties of CdTe quantum dots/polymethylmethacrylate composite films Yanni Huang, Jianjun Liu*, Yingchun Yu, Shengli Zuo State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 December 2014 Received in revised form 21 April 2015 Accepted 20 May 2015 Available online 22 June 2015
A new simple route was presented for the preparation of stable fluorescent CdTe/polymethylmethacrylate (CdTe/PMMA) composite films by using hydrophilic thioglycolic acid capped CdTe quantum dots (TGA-CdTe QDs) and polymethylmethacrylate (PMMA) as raw materials. The TGA-CdTe QDs were firstly exchanged with n-dodecanethiol (DDT) to become hydrophobic DDT-CdTe QDs via a ligand exchange strategy, and then incorporated into PMMA matrix to obtain fluorescent CdTe/PMMA composite films. The structure and optical properties of DDT-CdTe QDs and CdTe/PMMA composite films were investigated by XRD, IR, UV and PL techniques. The results indicated that the obtained DDT-CdTe QDs well preserved the intrinsic structure and the maximum emission wavelength of the initial water-soluble QDs and the resulting 6.10 wt% CdTe/PMMA composite film exhibited significantly enhanced PL intensity. Furthermore, the multicolored composite films with green, yellow-green, yellow and orange light emissions were well tuned by incorporating the CdTe QDs of various maximum emission wavelengths. The TEM image demonstrated that the CdTe QDs were well-dispersed in the PMMA matrix without aggregation. Superior photostability of QDs in the composite film was confirmed by fluorescence lifetime measurement. Thermo-gravimetric analysis of CdTe/PMMA composite films showed no obvious enhancement of thermal stability compared with pure PMMA. © 2015 Elsevier B.V. All rights reserved.
Keywords: CdTe quantum dot Ligand-exchange Polymer Fluorescence Composite film Thermal evaporation
1. Introduction Semiconductor quantum dots (QDs)-polymer composites have attracted great attention due to their excellent optical properties and potential applications in the fields including light-emitting devices (LEDs) [1,2] nonlinear optical devices [3,4] biological labels [5,6] etc. Significant efforts have been devoted to fabricate the QD-polymer composites with tunable fluorescence property and with improved film-formed performance using various polymers such as poly (vinyl alcohol) (PVA) [7], polystyrene [8], nylon [9], fluoropoly (ether ether ketone) (FPEEK) [10], styrene/ maleic anhydride copolymer [11] etc. Polymethylmethacrylate (PMMA) with high transparency, high mechanical stability and a relative low price can be used in a wide range of applications which can be even more expanded by the incorporation of QDs into its matrix. CdSe/PMMA composite films with enhanced electrical conductivity were reported by Al-Hosiny's group [12].
* Corresponding author. E-mail address: [email protected] (J. Liu). http://dx.doi.org/10.1016/j.jallcom.2015.05.230 0925-8388/© 2015 Elsevier B.V. All rights reserved.
Khanna et al. [13] fabricated transparent CdS/PMMA composite films with tunable optical properties by a thermal evaporation method. Among the various QDs, CdTe QDs of different sizes have tunable emission from green to red due to quantum confinement are favorable for optoelectronic devices [14,15]. To date, CdTe QDs with desired optical properties can be synthesized via two synthetic routes including organic-phase and aqueous-phase approaches. The oil-soluble QDs can be obtained by an organometallic synthetic route with trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) as stabilizing reagents [16,17], which is toxic, expensive and unfavorable for large-scale production. While the aqueous synthesis is reagent-effective, less toxic [18,19] and the obtained hydrophilic QDs can be transferred into organic phase under relatively mild conditions [20]. Incorporating CdTe QDs into polymers to fabricate light-emitting films may maintain or enhance the PL stability of QDs and further expand their applications in photovoltaic field. Synthesis of PMMA/CdTe composite fibers via an electrospinning route was demonstrated by Zhu et al. [21]. Herein, we provide a facile and effective route to prepare flexible CdTe/PMMA composite films by using the aqueous phase
Y. Huang et al. / Journal of Alloys and Compounds 647 (2015) 578e584
synthesized CdTe QDs and PMMA as raw materials. Firstly, to obtain homogeneous solution of QDs and PMMA, a ligand-exchange procedure was used to render QDs to be hydrophobic, and then flexible transparent CdTe/PMMA composite films with enhanced PL intensity were fabricated by a thermal evaporation process. The thermal stability, photostability, and the distribution of the CdTe QDs in the polymer matrix were well studied. 2. Materials and methods 2.1. Chemicals CdCl2$2.5H2O (99.9%), thioglycolic acid (TGA; 98%), tellurium powder (99.9%), sodium borohydride (NaBH4), sodium hydroxide (NaOH), n-dodecanethiol (DDT), methyl methacrylate (MMA), benzoyl peroxide (BPO), acetone, toluene, cyclohexane and ethanol were all analytical grade and purchased from the Beijing Chemical Reagents Company. All chemicals were used without additional purification. All aqueous solutions were prepared with Milli-Q water (Millipore). 2.2. Synthesis of water-soluble CdTe QDs Water-soluble TGA capped CdTe QDs were synthesized according to our previous work [22]. Typically, the CdTe precursor solution was prepared by the addition of fresh oxygen-free NaHTe solution to a N2-saturated CdCl2 solution at pH ¼ 10 in the presence of TGA stabilizer. The molar ratio of Cd2þ/TGA/HTe was set as 1:3:0.1 ([Cd2þ] ¼ 4 mmol/L). Under vigorous stirring in N2 atmosphere, a series of water-soluble CdTe QDs were prepared by controlling refluxing time: 0.5 h, 4 h, 10 h and 20 h. The TGA-CdTe QD powders were precipitated from the water phase with acetone for XRD and FT-IR characterization. 2.3. Preparation of oil-soluble DDT-CdTe QDs via a ligand-exchange procedure The oil-soluble DDT-CdTe QDs were prepared as follows. First, 30 mL as-prepared TGA capped CdTe QDs solution, 10 mL ndodecanethiol and 30 mL acetone were added into a 200 mL flask and the mixture was vigorously stirred at 65 C for 1 h. The CdTe QDs were slowly transferred into n-dodecanethiol phase from water phase. Then, the upper organic phase was separated and centrifuged at 5000 rpm for 10 min. The precipitate was washed with toluene twice and dried at 70 C for 10 min. Finally, 0.0710 g of the obtained solid powder, 10 mL toluene and 10 mL cyclohexane were added into a 50 mL conical flask with stopper and stirred at 65 C until all solid dissolved to obtain a homogeneous solution for further experiments. The oil-soluble CdTe QDs with different maximum emission wavelength lem were prepared by using CdTe QDs aqueous solution with refluxing time of 0.5 h, 4 h, 10 h and 20 h as raw material. For XRD and FT-IR characterization, the DDT-CdTe QD powders were employed. 2.4. Preparation of fluorescent CdTe/PMMA composite films via a thermal evaporation method Firstly, the PMMA was prepared by polymerization of the MMA monomers initiated by BPO as described previously [13], with a number average molecular weight Mn ¼ 193535 Da. Then, a certain amount of as-prepared oil-soluble CdTe QDs solution was added to 4.06 g of PMMA toluene solution (7.4 wt%) to form a homogeneous solution under stirring at 65 C. Finally transparent films of CdTe/ PMMA composite were prepared by further heating the mixed solutions at 100 C for 4 h in an oven.
579
2.5. Characterization of samples The molecular weight of the synthesized PMMA was determined on a Waters 515-717-2410 gel permeation chromatography (GPC) system with THF as the eluting solvent and a polystyrene standard. X-ray diffraction (XRD) patterns were measured on a Bruker D8ADVANCE X-Ray diffractometer with Cu Ka radiation. Fouriertransform infrared (FT-IR) spectra were obtained on a Thermo Nicolet 6700 IR spectrophotometer. Photoluminescence (PL) spectra were recorded on a Hitachi F-7000 fluorescence spectrophotometer equipped with a Xe lamp. The excitation wavelength was set at 365 nm, tube voltage was 400 V, scanning speed was 1200 nm/min and the excitation and emission slits were both 5 nm. UVevisible absorption spectra were collected on a Shimadzu UV 3600 UV-VISNIR spectrophotometer. The Thermo-gravimetric analysis of the sample were carried out by simultaneous thermogravimetrydifferential scanning calorimetry (TG-DSC), performed with a NETZSCH STA 409 PG/PC thermal analysis system at 10 K/min under an atmosphere of N2. The morphology of the obtained samples was determined by transmission electron microscopy (TEM, Hitachi 800). The CdTe/PMMA sample was ultramicrotomed with a Leica EM-FC6 Instruments at 90 C using glass knife and ready to be imaged in the TEM. Photographic images of the samples were captured by digital camera (100 IS, Canon). Fluorescent images were recorded on Laser scanning confocal microscope (FV1000) and a diode laser (Spectra-Physics, Cyan Scientific, l ¼ 405 nm) was used as the excitation source. Time-correlated single-photon counting (TCSPC) data were performed on FL 900 spuoectroflrometer using a hydrogen lamp as the excitation source, lex ¼ 375 nm. 3. Results and discussions 3.1. The preparation of CdTe/PMMA composite film The process of the preparation of CdTe/PMMA composite film is demonstrated in Scheme 1. The TGA-capped CdTe QDs were firstly prepared via an aqueous synthetic strategy, and then became oilsoluble by exchanging the initial ligand with DDT through a ligand-exchange strategy [23], as shown in Scheme 1a. To improve the ligand-exchange efficiency, the addition of a polar organic solvent is necessary [24]. The PL spectra of CdTe QDs with different refluxing time before and after the ligand-exchange procedure are displayed in Fig. 1. The water-soluble CdTe QDs with refluxing time of 0.5 h, 4 h, 10 h and 20 h show the maximum emission wavelength lem at 541 nm, 556 nm, 567 nm and 592 nm, respectively, and the mean sizes of QDs can be estimated to be 2.21 nm, 2.68 nm, 3.16 nm and 3.42 nm, shown in Surpporting Materials. After the ligandexchange procedure, the obtained oil-soluble CdTe QDs show the maximum emission wavelength lem at 539 nm, 555 nm, 566 nm and 588 nm. The emission peak of the oil-soluble DDT-CdTe QDs solution shows no obvious change compared with the corresponding aqueous CdTe QDs, indicating that the CdTe QDs were well protected by DDT without aggregation after the ligand-exchange procedure. The best quantum yield (QY) with 75% for the watersoluble CdTe QDs can be obtained at the refluxing time of 4 h, and the QY for the corresponding oil-soluble CdTe QDs is 13.5%, using rhodamine 6G (QY ¼ 95%) as the reference standard. The declining of QY is probably due to the formation of new surface defects during water-to-oil processes [25]. When illuminated under ultra violet lamp of 365 nm, the oil-soluble CdTe QDs show green, yellow-green, yellow and orange light emissions (shown in Fig. S2) corresponding to the lem of 539 nm, 555 nm, 566 nm and 588 nm, respectively. The CdTe/PMMA composite films were obtained by evaporating the solutions of oil-soluble CdTe QDs and PMMA in mixed solvent of toluene and cyclohexane, as shown in Scheme 1(bed).
580
Y. Huang et al. / Journal of Alloys and Compounds 647 (2015) 578e584
Fig. 2. XRD patterns of TGA-CdTe QDs powder (a) DDT-CdTe QDs powder (b) PMMA (c) 6.10 wt% CdTe/PMMA composite film (d). Inset shows magnified (a) and (b) patterns.
Scheme 1. Schematic illustration of the preparation of CdTe/PMMA composite film. (a) Ligand-exchange of TGA-capped CdTe QDs by DDT during phase transfer procedure. (b) Preparation of PMMA by polymerization of the monomers. (c) Encapsulation of DDTcapped CdTe by PMMA in mixed solvent of toluene and cyclohexane. (d) Thermal evaporation of solvents to obtain CdTe/PMMA film.
Fig. 2 shows the XRD patterns of TGA-CdTe QDs powder, DDTCdTe QDs powder, pure PMMA and 6.10 wt% CdTe/PMMA composite film. Same diffraction peaks in the TGA-CdTe QDs powder
(curve a) and DDT-CdTe QDs powder (curve b) with 2q at 24.22 ,40.58 ,47.17 indexed to (111), (220) and (311) plane of the cubic zinc blende structure like bulk CdTe (JCPDF No.19-0191) are clearly observed, indicating the intrinsic structure of CdTe QDs is preserved during the ligand-exchange procedure. The XRD pattern of PMMA (curve c) shows a broad diffraction peak at 2q ¼ 16.33 , typical of an amorphous material, together with two peaks of lower intensities centered at 30.69 and 42.51 [26,27]. Besides, similar XRD patterns can be found in Fig. 2(c) and (d) corresponding to the pure PMMA and the 6.10 wt% CdTe/PMMA composite film but there are little XRD peak shifts to lower 2q after incorporating QDs into PMMA, indicating the slightly change of the orientation of the
Fig. 1. PL spectra of CdTe QDs with refluxing time of (A) 0.5 h (B)4 h (C)10 h and (D)20 h before and after the ligand-exchange procedure. (a) Organic solvents; (b) CdTe QDs in water; (c) CdTe QDs in oil.
Y. Huang et al. / Journal of Alloys and Compounds 647 (2015) 578e584
581
3.2. The fluorescent property, photostability and thermal stability of CdTe/PMMA composite film
Fig. 3. FT-IR spectra of TGA-CdTe QDs powder (a) DDT-CdTe QDs powder (b) PMMA (c) 6.10 wt% CdTe/PMMA composite film (d).
PMMA chains and the interaction between CdTe QDs and polymer chains. No characteristic diffraction peak of CdTe QDs is detected for CdTe/PMMA composite film, which is explained by the small amounts, high dispersion and low crystallinity of CdTe QDs in CdTe/ PMMA composite film. The FT-IR spectra of CdTe QDs, PMMA and the 6.10 wt% CdTe/ PMMA composite film are shown in Fig. 3. The characteristic peaks at 1580 cm1 and 1400 cm1 (curve a) correspond to the carboxyl group attributing to TGA molecules on the surface of CdTe QDs. Compared with the TGA-CdTe QDs, the characteristic peaks of the carboxyl group are not obvious in the FT-IR spectra of the DDT-CdTe QDs (curve b). In addition, the sharp IR absorption peaks at 2920 cm1 and 2848 cm1 of CeH stretching vibration, together with the peak at 720 cm1 of the ethyl groups (n > 4) swinging in the plane, strongly prove the existence of DDT molecule, indicating that the original hydrophilic ligands, TGA capped on the CdTe QDs have been exchanged by the hydrophobic DDT molecule during the ligand-exchange process. The FT-IR spectrum of 6.10 wt% CdTe/PMMA composite film (curve d) almost perfectly matches that of pure PMMA (curve c), indicating that the CdTe QDs are well-embedded in composite film instead of adsorbing only on the PMMA surface. The peak at 2961 cm1 is assigned to the stretching vibrations of CeH of both methyl and ethyl group, while the peaks at 1451 cm1 and 1380 cm1 are due to their bending vibrations. Moreover, the C]O absorption vibration appears at 1728 cm1 and the stretching vibration of CeO is at 1148 cm1.
Fig. 4A shows that the loading amount from 2.12 wt% to 6.10 wt% of CdTe QDs has significant effect on the PL property of CdTe/PMMA composite film. There is an increasing PL intensity and red-shift (from 542 nm to 553 nm) of the emission peak, which may be ascribed to the growth and ripening of more CdTe QD grains leading to the decrease of surface defects while increasing CdTe QDs loading amount [28]. Fig. 4B shows that there is an increasing absorption intensity and a slightly red-shift of the absorption peak along with the increasing CdTe QDs loading amount from 2.12 wt% to 6.10 wt%, which indicates the slightly growth of QDs in PMMA matrix with the increase of CdTe QDs loading amount. This result is well consistent with that obtained by PL measurement. However, CdTe QDs are expelled from the surface of the composite film when the CdTe QDs loading amount is increased to 9.78 wt% (Fig. S3), which implies that there is an existence of best loading amount of CdTe QDs in polymer. Fig. 5A shows the PL spectra of CdTe/PMMA composite films incorporated by CdTe QDs of different lem. By incorporating the CdTe QDs with lem of 539 nm, 555 nm, 566 nm and 588 nm into PMMA matrix, the obtained composite films display sizedependent fluorescence emissions at 538, 553, 565 and 584 nm, respectively, corresponding to green, green-yellow, yellow, and orange light emissions as shown in Fig. 5B. This result demonstrates that the multicolored fluorescence of the CdTe/PMMA composite film could be adjusted by varying the lem of incorporated CdTe QDs. The microstructures and fluorescence properties of CdTe/PMMA composite films incorporated by QDs of different lem were further characterized by using laser confocal fluorescence microscopy (LCFM). As shown in Fig. 6(aec), the CdTe/PMMA composite films are composed of many spherical particles with 2e4 mm in size. The microsphere is attributed to the CdTe/PMMA composite spherulite, which is one of the common morphology of PMMA or composites with PMMA [29]. The observation of uniform fluorescence emission throughout the microspheres suggests that the CdTe QDs are well compatible with the PMMA matrix in our system. Furthermore, it is found that varying the lem of incorporated CdTe QDs has no obvious influence on the microstructure of CdTe/PMMA composite and the size-dependent fluorescence property of CdTe QDs in PMMA matrix remains well. Fig. 7A shows that the variation in excitation wavelength from 300 nm to 460 nm does not lead to any change in the position of emission peak for CdTe/PMMA composite film, and the PL intensity becomes more higher with the decreasing excitation wavelength, thus proving an excellent quantum confinement with narrow size distribution for CdTe QDs in PMMA matrix [13,30]. The TEM image
Fig. 4. PL spectra (A) and UVevisible absorption spectra (B) of CdTe/PMMA composite films. Incorporated QDs: lem ¼ 555 nm, yellow-green.
582
Y. Huang et al. / Journal of Alloys and Compounds 647 (2015) 578e584
Fig. 5. Normalized PL spectra of 6.10 wt% CdTe/PMMA composite films incorporated by CdTe QDs of different lem (A) and their images(B) under room light (top) and 365 nm UV irradiation (bottom). From left to right: PMMA; a. QDs: lem ¼ 539 nm, green; b. QDs: lem ¼ 555 nm, yellowegreen; c. QDs: lem ¼ 566 nm, yellow; d. QDs: lem ¼ 588 nm, orange.
Fig. 6. Fluorescence confocal microscopy images of 6.10 wt% CdTe/PMMA composite films incorporated by CdTe QDs of different lem. From left to right: a. QDs: lem ¼ 539 nm, green; b. QDs: lem ¼ 566 nm, yellow; c. QDs: lem ¼ 588 nm, orange.
Fig. 7. PL spectra of 6.10 wt% CdTe/PMMA composite film at different excitation wavelengths (A) and its TEM image (B). Incorporated QDs: lem ¼ 555 nm, yellowegreen.
of 6.10 wt% CdTe/PMMA composite slice (Fig. 7B) shows that CdTe QDs are well-dispersed in the PMMA matrix, revealing the approach provided in our work effectively avoids QDs aggregation.
Fig. 8 shows the PL spectra and time-resolved fluorescence decay curves of three samples including TGA-CdTe QDs powders, DDT-CdTe QDs powders and 6.10 wt% CdTe/PMMA composite film,
Y. Huang et al. / Journal of Alloys and Compounds 647 (2015) 578e584
583
Fig. 8. PL spectra (A) and Time-resolved fluorescence decay curves (B) of CdTe QDs powders and 6.10 wt% CdTe/PMMA composite film. Initial CdTe QDs aqueous solution: 4 h, lem¼556 nm.
Table 1 The decay time parameters for samples. Items
Samples
A
Lifetime components ti/ns
Bi ( 102)
a
TGA-CdTe QDs Powder DDT-CdTe QDs Powder 6.10 wt% CdTe/PMMA Film
40.24
t1¼3.720 t2¼8.097 t3¼34.05 t1 ¼ 1.938 t2 ¼ 9.648 t3 ¼ 34.16 t1 ¼ 2.113 t2 ¼ 9.294 t3 ¼ 35.57
B1 B2 B3 B1 B2 B3 B1 B2 B3
b
c
50.13
66.76
¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼
4.921 2.627 0.4756 4.310 3.094 1.254 4.460 2.797 1.266
Relative contribution of ti (%)
Average lifetime tAV/ns
Chi-square c2
32.82 38.14 29.04 10.31 36.84 52.85 11.71 32.32 55.97
14.20
1.199
21.80
1.061
23.16
1.186
all are prepared by using the same initial CdTe QDs aqueous solution (4 h, lem ¼ 556 nm). As shown in Fig. 8A, there is an enhancement of the PL intensity and an obvious blue shift of the emission peak for CdTe QDs solid after being exchanged by DDT and capsulated by PMMA, indicating that PMMA plays an important role to inhibit the aggregation of CdTe QDs as well as to modify the surface of the QDs. To study the photostability of CdTe QDs in CdTe/ PMMA composite film, the time-correlated single-photon counting (TCSPC) methodology was performed. Comparative TCSPC studies are presented in Fig. 8B. The decay curves of CdTe QDs powders and 6.10 wt% CdTe/PMMA composite film are well fitted with triexponential function Y(t) based on nonlinear least squares, using the following expression:
YðtÞ ¼ A þ
3 X
Bi expðt=ti Þ
(1)
i¼1
Where Bi are the amplitude of time-resolved decay lifetimes ti and the average lifetime tAV could be concluded from a typical formula:
tAV ¼
B1 t1 2 þ B2 t2 2 þ B3 t3 2 B1 t1 þ B2 t2 þ B3 t3
(2)
The decay time parameters for samples are given in Table 1. There are two shorter lifetimes (<10 ns) and a longer lifetime (>30 ns) for all the three samples given by data fitting. The shorter lifetime components can be mainly attributed to the intrinsic recombination of initially populated core states [31], while the longer lifetime component in the fluorescence decay is caused by the radiative recombination of electrons and holes on the surface involving surface-localized states [32]. The relative contribution of the longer lifetime (>30 ns, t3) is 29.04%, 52.85% and 55.97% for TGA-CdTe QDs powders, DDT-CdTe QDs powders and 6.10 wt% CdTe/PMMA composite film, respectively, indicating that the
Fig. 9. Thermo-gravimetric analysis of PMMA (a) 4.15 wt% CdTe/PMMA (b) 6.10 wt% CdTe/PMMA (c).
surface-related emission for CdTe QDs is obviously increased after ligand-exchange procedure and further increased by being capsulated into PMMA matrix. By comparing the average lifetimes of the samples, we can see that the average fluorescence lifetime tAV of CdTe QDs is prolonged upon their incorporation into polymeric matrix to form CdTe/PMMA composite film. For 6.10 wt% CdTe/ PMMA composite film, the average lifetime is 23.16 ns, which is 1.36 ns longer than that of isolated CdTe QDs (21.80 ns) precipitated from oil phase, and 8.96 ns longer than that of the one (14.20 ns) obtained from water phase. The polymer matrix in the system could restrain the aggregation of CdTe QDs avoiding selfquenching, and delay the fluorescence decay process [33]. As shown in Fig. 9, TGA analysis displays two stages decomposition behavior for CdTe/PMMA composite films, the initial weight
584
Y. Huang et al. / Journal of Alloys and Compounds 647 (2015) 578e584
loss between 150 C and 270 C is due to the vaporization of trapped solvent from PMMA matrix and the second-stage weight loss between 270 C and 430 C is assigned to the decomposition of PMMA itself. The observations indicate that the incorporation of CdTe QDs with different amounts into PMMA matrix only slow the decomposition before about 350 C but next rapid decomposition up to 430 C is almost the same for CdTe/PMMA and pure PMMA. It is concluded that there is no great difference between decomposition of CdTe/PMMA composite and pure PMMA and no obvious dependence of thermal stability for PMMA on QDs loading amount. The remained residues at 900 C for CdTe/PMMA composite films can be considered as the mixture of CdTe and the carbon residues that may form during the decomposition [13]. 4. Conclusions In summary, transparent CdTe/PMMA composite films with enhanced fluorescence were fabricated by using water phase CdTe QDs and PMMA as raw materials. The phase transfer of CdTe QDs from aqueous phase into the oil phase was performed using hydrophobic DDT, followed by the addition of PMMA and then thermal evaporation, resulting in uniform transparent composite films with stable and excellent fluorescence property. The multicolored composite films with green, yellow-green, yellow and orange light emissions were achieved by incorporating different lem of CdTe QDs into PMMA matrix. We believe this facile approach can be easily applied to other semiconductor QDs to develop highly functional QD/polymer composite films that could be potentially used in photonic devices and optical materials. Acknowledgments This work was financially supported by the National Natural Science Foundation of China under Grant No. 11172043. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2015.05.230. References [1] B. Dubertret, P. Skourides, D.J. Norris, V. Noireaux, A.H. Brivanlou, Science 298 (2002) 1759e1762.
[2] N. Tessler, V. Medvedev, M. Kazes, S. Kan, U. Banin, Science 295 (2002) 1506e1508. [3] A.A. Mamedov, A. Belov, M. Giersig, N.N. Mamedova, N.A. Kotov, J. Am. Chem. Soc. 123 (2001) 7738e7739. [4] F. Qian, Y. Li, S. Gradecak, H.G. Park, Y. Dong, Y. Ding, Z.L. Wang, C.M. Lieber, Nat. Mater. 7 (2008) 701e706. [5] N. Pradhan, D.M. Battaglia, Y. Liu, X. Peng, Nano Lett. 7 (2007) 312e317. [6] Y. Zhang, Y. Li, X.P. Yan, Small 5 (2009) 185e189. [7] B.T. Suo, X. Su, J. Wu, D. Chen, A. Wang, Z.H. Guo, Mater. Chem. Phys. 119 (2010) 237e242. [8] M. Mao, C.H. Zhou, H.B. Shen, H.Z. Wang, S.J. Wang, L.S. Li, Mater. Lett. 111 (2013) 97e100. [9] Y. Yuan, F.-S. Riehle, R. Nitschke, M. Krüger, Mater. Sci. Eng. B 177 (2012) 245e250. [10] L.J. Zhu, P.F. Huo, Q. Wang, F.X. Liang, C.L. Zhang, J.L. Li, X.Z. Qu, G.B. Wang, Z.Z. Yang, Chem. Commun. 49 (2013) 5283e5285. [11] B. Martín-García, Pedro M.R. Paulo, Sílvia M.B. Costa, M.M. Vela-zquez, J. Phys. Chem. C 117 (2013) 14787e14795. [12] N.M. Al-Hosiny, S. Abdallah, M.A.A. Moussa, A. Badawi, J. Polym. Res. 20 (2013) 1e8. [13] P.K. Khanna, N. Singh, J. Lumin 127 (2007) 474e482. [14] S.D. Hodgsonn, W.S.M. Brooks, A.J. Clayton, G. Kartopu, V. Barrioz, S.J.C. Irvine, Nano Energy 4 (2014) 1e6. [15] I. Su0 arez, H. Gordillo, R. Abargues, S. Albert, J. Mart0 -nez-Pastor, Nanotechnology 22 (2011) 435202e435209. [16] C.B. Murray, D.J. Norris, M.G. Bawend, J. Am. Chem. Soc. 115 (1993) 8706e8715. [17] S.F. Wuister, F. Driel, A. Meijerink, J. Lumin 102e103 (2003) 327e332. [18] A.L. Rogach, T. Franzl, T.A. Klar, J. Feldmann, N. Gaponik, V. Lesnyak, A. Shavel, A. Eychmüller, Y.P. Rakovich, J.F. Donegan, J. Phys. Chem. C 111 (2007) 14628e14637. [19] C.Q. Dong, J.C. Ren, Luminescence 27 (2012) 199e203. [20] N. Gaponik, D.V. Talapin, A.L. Rogach, K. Hoppe, E.V. Shevchenko, A. Kornowski, A. Eychmüller, H. Weller, J. Phys. Chem. B 106 (2002) 7177e7185. [21] L. Zhu, S.Y. Yang, J. Wang, C.F. Wang, L. Chen, S. Chen, Mater. Lett. 99 (2013) 54e56. [22] J.J. Liu, Z.X. Shi, Y.C. Yu, S.L. Zuo, J. Colloid Interface Sci. 342 (2010) 278e282. [23] L.H. Yuwen, B.Q. Bao, G. Liu, J. Tian, H.T. Lu, Z.M. Luo, X.R. Zhu, F. Boey, H. Zhang, L.H. Wang, Small 7 (2011) 1456e1463. [24] N. Gaponik, D.V. Talapin, A.L. Rogach, A. Eychmüller, H. Weller, Nano. Lett. 2 (2002) 803e806. [25] J. Ding, X. Wang, L.H. Zhuo, B. Tang, J. Mater. Chem. 19 (2009) 3027e3032. [26] M.L. Saladino, T.E. Motaung, A.S. Luyt, A. Spinella, G. Nasillo, E. Caponetti, Polym. Degrad. Stab. 97 (2012) 452e459. [27] F.J. Balta-Calleja, C.G. Vonk, Elsevier, Amsterdam, 1989, pp. 191e193. [28] O.O. Matvienko, Y.N. Savin, A.S. Kryzhanovska, O.M. Vovk, M.V. Dobrotvorska, N.V. Pogorelova, V.V. Vashchenko, Thin Solid Films 537 (2013) 226e230. [29] C.A. Daniels, Technomic Publishing Co. Inc., Lancaster, Basel, 1989, pp. 11e12. [30] P.K. Khanna, P. More, B.G. Bharate, A.K. Vishwanath, J. Lumin 130 (2010) 18e23. [31] X.Y. Wang, L.H. Qu, J.Y. Zhang, X.G. Peng, M. Xiao, Nano Lett. 3( (2003) 1103e1106. [32] L.H. Qu, X.G. Peng, J. Am. Chem. Soc. 124 (2002) 2049e2055. [33] C.R. Kagan, C.B. Murray, M. Nirmal, M.G. Bawendi, Phys. Rev. Lett. 76 (1996) 1517e1520.