Macromonomer-induced CdTe quantum dots toward multicolor fluorescent patterns and white LEDs

Abstract

We report an available route for the synthesis of poly(methacrylic acid) (PMAA) macromonomers/N-acetyl-L-cysteine (NAC)-co-capped CdTe quantum dots (QDs) toward multicolor fluorescent patterns and warm white LED devices. The properties of the as-prepared CdTe QDs herein were thoroughly investigated by Fourier transform infrared spectra (FT-IR), ultraviolet-visible (UV-vis), photoluminescence (PL), X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. We have found that the PMAA/NAC-capped CdTe QDs exhibited stable and strong photoluminescence (PL) with a narrow size distribution and a high quantum yield (QY) of ∼45%. Moreover, the as-prepared CdTe QDs demonstrated a quick growth rate and favorable fluorescent stability without any aggregation and phase separation when they were blended with a polymer matrix. Subsequently, by virtue of their excellent polymer-compatibility, we extended the water-soluble PMAA/NAC-capped CdTe QDs as “fluorescent ink” for multicolor fluorescent patterns by silk screen printing, which may find application in the anti-counterfeit field. In addition, we explored the robust CdTe QDs as “red phosphor” to fabricate a white LED device, the resulted LED emitted a higher rendering color index (CRI) warm white light compared to the conventional phosphor-only LEDs. It is believed that the route reported here could offer an available pathway to achieve robust QDs for diverse applications.

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1. Introduction

Quantum dots (QDs) with unique physical characteristics have attracted great fundamental and practical interests due to their appealing applications in the fields including solar cells,¹ photodetectors,² biolabeling,³ and light-emitting diodes (LED).⁴ Typically, the II-VI semiconductor QDs, such as CdSe, CdTe and CdS, were the hot research topic in the past decades.^5–8 For practical applications, QDs are usually required to combine with polymers; however, luminescence diminishing or quenching of QDs often occurs owing to the aggregation and phase separation during the incorporation of QDs into polymer matrices, thus limiting their applications.^9–13 Up to now, tremendous efforts have been devoted to improve the polymer-compatibility of QDs including the introduction of polymerizable ligands^14–19 and the modification of QDs using polymerizable surfactants.^20–22 For instance, Dubertret et al. first reported the modification of QDs with polymers composed of the mixture of 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethyleneglycol)] and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine.²³ Peng's group improved the stability of the CdSe/CdS core/shell QDs through the ring-closing metathesis of vinyl end-functionalized dendrons.²⁴ Zhang et al. employed octadecyl-p-vinyl-benzyldimethylammonium chloride as a polymerizable surfactant to prepare bulk CdTe/polystyrene nanocomposites, which were transparent and showed no signs of phase separation or agglomeration.²⁵ Gao et al. used a high molecular mass amphiphilic tri-block copolymer consisting of a poly(butylacrylate) part, a poly(ethylacrylate) part and a poly(methacrylic acid) part to directly encapsulate the QDs.²⁶ However, the introduction of polymeric layer usually decreases the performance of the QDs and affects the size-tunable optical properties of the QDs in many cases. Therefore, further study is still needed to effectively synthesize high performance QDs with desirable polymer-compatibility and size-tunable optical properties.

Herein, we describe the synthesis of water-soluble CdTe QDs co-stabilized by N-acetyl-L-cysteine (NAC) and poly(methacrylic acid) (PMAA) macromonomers toward multicolor fluorescent patterns and warm white LED devices (Scheme 1). First, PMAA macromonomers were fabricated as polymeric ligands via catalytic chain transfer polymerization (CCTP) technique according to our previous work.^27–29 CCTP is a powerful synthetic route to produce a relatively low-molecular weight polymer chain. Subsequently, water-soluble CdTe QDs were prepared using NAC and the as-prepared PMAA macromonomers as stabilizing agents under microwave irradiation. Not only do the resulted CdTe QDs retain the performance originated from the short-straight NAC ligand, such as highly efficient fluorescence, good photostability and quantum yield (QY) of ∼45%, but they also exhibit favourable polymer-compatibility and size-tunable optical properties with a quick growth rate. Finally, taking advantage of the as-prepared QDs with good polymer-compatibility, we extended the robust CdTe QDs as “fluorescent ink” to print elegant photoluminescent patterns and as color convert materials to fabricate warm white LEDs. It is believed that the route reported here could offer an available pathway to achieve robust QDs for diverse applications.

Scheme 1 Illustration of the route for the synthesis of PMAA/NAC-capped CdTe QDs and their applications on white LED and photoluminescent patterns.

2. Experimental

2.1 Materials

Cadmium chloride (CdCl₂·2.5H₂O), N-acetyl-L-cysteine (NAC), tellurium powder, sodium borohydride (NaBH₄), sodium hydroxide (NaOH) and poly(vinyl pyrrolidone) (PVP, M_n = 58000) were purchased from standard sources. All the materials were used as received without further purification. The poly(methacrylic acid) (PMAA) macromonomer was prepared via catalytic chain-transfer polymerization (CCTP) as our previous case.²⁷ The evidence for the successful preparation of low-molecular-weight PMAA macromonomer via CCTP can be demonstrated by gel permeation chromatography (GPC) characterization in Fig. S1, ESI.†

2.2 Synthesis of fresh oxygen-free NaHTe

The intermediator sodium hydrogen telluride (NaHTe) was prepared as reported.³⁰ In a typical procedure, NaBH₄ (22.7 mg, 0.6 mmol) was initially transferred into a 5 mL vial, and then 1 mL of DI water was added, followed by adding tellurium powder (31.9 mg, 0.25 mmol). The vial was kept in an ice-water bath for several hours. Note: during the reaction, a small outlet connected to the vial was kept open to discharge the pressure from the resulting hydrogen.

2.3 Synthesis of PMAA/NAC-co-capped CdTe QDs

The high-quality CdTe QDs were synthesized according to our previous work.³¹ CdTe precursor solution was prepared by adding fresh oxygen-free NaHTe solution to a N₂-deaerated CdCl₂ solution at pH = 10 in the presence of NAC and PMAA macromonomers. The typical concentrations were [Cd²⁺] = 0.5 mmol L⁻¹, [NAC] = 0.5 mmol L⁻¹, [PMAA] = 0.5 mmol L⁻¹, [Te] = 0.25 mol L⁻¹, respectively. The reaction was carried out under microwave irradiation (microwave digestion system XH-MC-1) (900W) at the temperature of 95 °C. A series of high-quality CdTe QDs were prepared by controlling the reaction time. Solid CdTe powders were obtained by adding absolute ethanol to precipitate the solution sample, repeated washing and drying in desiccators. Finally, the CdTe QDs powders were redissolved in DI water for further use.

2.4 Multicolor fluorescent patterns from CdTe QDs

2 g CdTe QDs aqueous solution was initially magnetically blended with polyvinyl pyrrolidone (PVP) aqueous solution (8 g, 2 wt %) to form a homogeneous solution. The solution was cast on the printing mask of a silk screen printing device, and was penetrated through the pattern screen (140 mesh) onto the PVP film substrate via spin-coating.

2.5 Preparation of white LEDs

The GaN LED chip with the peak wavelength centered at 460 nm was purchased from Bridgelux Co. The blue-light-emitting microchip was attached on the bottom of the LED base without overcoat material above the chip. The two leads on the LED were prepared to connect to the power supply. Afterwards, the thermocurable resin (silicone, Dow-Corning Co.) was mixed with CdTe QDs and phosphor (QDs/phosphor = 1/6 wt/wt) and put in a vacuum chamber to remove the bubbles. About 30 μL of the QD-phosphor mixture was dispensed on the LED chip and thermally cured at 150 °C for 1 h. Finally, the optical lenses were placed on the top of the LED chip and the voids were filled with the silicone. The ultimate LED was then further cured at 150 °C for 1 h. All the optical performances were measured using a ZWL-600 instrument with integral sphere.

2.6 Characterizations

Transmission electron microscopy (TEM). The particle diameter and lattice fringes were examined with a JEOL JEM-2010 transmission electron microscope. A drop of CdTe QDs aqueous solution was placed on a copper grid that was left to dry before transferred into the TEM sample chamber. The particle diameter was estimated by using ImageJ software analysis of the TEM micrographs.

Spectroscopy. Fourier-transform infrared (FT-IR) spectra were recorded on a Nicolet 6700 FT-IR spectrometer. The X-ray diffraction (XRD) pattern was conducted on a Bruker-AXS D8 ADVANCE X-ray diffractometer with Cu-Kα radiation (λ = 0.1542 nm) at a scanning speed of 6°/min over 2θ range of 10°–70°. Ultraviolet-visible (UV-vis) absorption spectra were recorded by a UV-vis spectrometer with the scan range of 290–800 nm (Lambda 950, Perkin-Elmer). Photoluminescence (PL) spectra were measured on a Cary Eclipse fluorescence spectrophotometer at room temperature operating with a 350 nm laser beam as a light source and Xe lamp as excited source; the excitation and emission slits were 5 nm. Time-correlated single-photon counting (TCSPC) data were performed on an SLM 48000 spectrofluorometer using a He:Cd laser as the excitation source.

Quantum yield (QY) calculation. QY was measured according to established procedure (J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Kluwer Academic/Plenum Publishers, New York, 2nd edn, 1999) by using rhodamine 6G in ethanol as a reference standard (Q = 0.95). The quantum yield was calculated using eqn 1:where Q is QY, S is the measured integrated emission intensity, A is the optical density and N is the refractive index. The subscripts s and x refer to the standard rhodamine 6G and test samples respectively. The UV-vis absorption spectrometer was used to determine the absorbance of the samples at 550 nm. The concentration of the samples for QY estimation should allow the first excitonic absorption peak to be below 0.05 in order to avoid any significant reabsorption. A Varian Cary Eclipse spectrophotometer was used to excite the samples at 550 nm and to record their photoluminescence spectra.

3. Results and discussion

The synthesis of water-soluble CdTe QDs was done based on our previous precedure³¹ in the presence of PMAA macromonomers prepared by the CCTP technique and NAC short-straight ligands. The PMAA macromonomers with M_n = 1576 contain abundant carboxylic anchors, which can easily coordinate with metal ions (Fig. S1, ESI†). To demonstrate the successful synthesis of dual ligand-capped CdTe QDs, we initially investigated the FT-IR spectra of pure PMAA macromonomers, PMAA/NAC-capped CdTe QDs and NAC-capped CdTe QDs for comparison, as shown in Fig. 1. The FT-IR spectrum of the PMAA macromonomer (curve 1) shows a strong peak at 1700 cm⁻¹, which is ascribed to the C=O stretching vibration. When the PMAA macromonemer acts as a ligand in the construction of CdTe QDs (curve 2), this C=O stretching vibration peak shifts to the position observed at 1635 cm⁻¹ during the coordinating interaction between the PMAA macromonomers and Cd²⁺.²⁸ However, the C=O stretching vibration peak can not be seen in the control sample (curve 3), which proves that the PMAA macromonomers successfully coordinate with Cd²⁺.

Fig. 1 FT-IR spectra of (1) the PMAA macromonomer via CCTP, (2) PMAA/NAC-capped CdTe QDs, and (3) NAC-capped CdTe QDs.

We then investigated the effect of NAC/PMAA molar ratios on the luminescence QYs of CdTe QDs, as shown in Fig. 2a. Several runs were performed at different NAC/PMAA molar ratios, ranging from 0 to 4/1 (mol/mol). We found increasing the NAC molar concentration resulted in the increase of luminescence QY, which gradually levelled off when the NAC/PMAA molar ratios reached 1/1, so we fixed the NAC/PMAA ratio at 1/1. An indication for comparing the refluxing time of the CdTe QDs capped by both PMAA and NAC with that by only NAC comes from the PL peak position vs. time curves, as displayed in Fig. 2b. After refluxing for 90 min, the characteristic emission peak position of PMAA/NAC-capped CdTe QDs is 635 nm (curve 1), however, in the case of NAC-capped CdTe QDs, it takes more than 180 min to reach 635 nm of PL emission (curve 2). The different growth rates in both cases confirmed the effect of the PMAA macromonomers on the growth of QDs, which can be explained by the PMAA macromonomers having a higher molecular weight and stronger steric hindrance compared to the short-straight NAC, increases the size of the CdTe QDs.^32,33 From this point of view, our strategy can accelerate the growth rate of QDs, which may provide a reference method to fabricate high-quality near infrared red (NIR) QDs.

Fig. 2 (a) Effect of different molar ratios between NAC and PMAA on QYs of CdTe QDs prepared at the pH = 10, and [Cd²⁺]/[HTe⁻]/[stabilizer] = 2/1/4 (mol/mol/mol). (b) Peak position of PL emission as a function of reaction time with (1) PMAA/NAC-capped CdTe QDs and (2) NAC-capped CdTe QDs obtained strictly under the same conditions. (c) Normalized UV-vis absorption spectra of PMAA/NAC-capped CdTe QDs during their growth in refluxing synthesis, and (d) the corresponding PL spectra. Inset of (c): the digital images under an ultraviolet lamp of five typical as-prepared CdTe QDs.

The UV-vis absorption spectra and PL spectra of five typical as-prepared CdTe QDs as a function of reaction time can be observed in Fig. 2c and 2d. On prolonging the microwave heating time, the growth of the CdTe QDs was clearly indicated by the red-shift of both absorption and emission spectra, and their bright fluorescence of color appears to vary from green to red, exhibiting “quantum size effects”. Their corresponding digital images under an ultraviolet lamp are also displayed in the inset of Fig. 2c. According to the method reported by Peng's group,³⁴ the estimated mean particle sizes with the progress of growth were 3.1 nm, 3.4 nm, 3.7 nm, 4.1 nm, 5.0 nm, respectively. Simultaneously, the sharp emission with full width at half-maximum (fwhm) of ca. 50 nm suggests the narrow size distribution of the as-prepared CdTe QDs and few electronic defect sites. Compared with the fluorescence emission of rhodamine 6G, the calculated QY of the CdTe QDs was ∼45%. The PMAA macromonomers via CCTP do not influence the “quantum size effects” of the CdTe QDs due to their low molecular weight compared with conventional polymers.

The as-prepared CdTe QDs were further characterized by TEM and XRD (Fig. 3). In the TEM image (Fig. 3a), the QDs appear as spherical particles without any aggregation. The HRTEM image of an individual nanocrystal is shown in Fig. 3b, from which the atomic lattice fringes of the CdTe nanocrystal are clearly visible. The selected area electron diffraction (SAED) pattern in Fig. 3c appears as broad diffuse rings due to the small particle size and good dispersity in solution. The powder XRD profile of the QDs shows broad peaks typical for nanocrystals, which are assigned to the (111), (220) and (311) planes of the cubic zinc blended structure of the bulk CdTe crystal. The addition of PMAA macromonomers as stabilizing agent does not change the structure of the CdTe QDs.

Fig. 3 (a) TEM image, (b) HRTEM image, (c) SAED pattern and (d) XRD pattern of PMAA/NAC-capped CdTe QDs with the PL emission at 635 nm.

Fig. 4 (a) A typical time-resolved fluorescence decay curve of PMAA/NAC-capped CdTe QDs (λ_ex = 405 nm) measured at the PL peak of 605 nm. (b) PL spectra of CdTe QDs stabilized together by PMAA and NAC with (1) fresh sample; (2) 6 months.

It is worth noting that a key factor in the applications of QDs is their stability, so we assessed the stability of the resulted CdTe QDs. Time-resolved photoluminescence measurements were performed using time-correlated single-photon counting methodology. The decay curve at the maximum of the exciton peak is depicted in Fig. 4a. Decay traces for the sample was well fitted with biexponential function based on nonlinear least-square, using the following expression (eqn 2).³⁵

Y(t) = α₁exp(−t/τ₁) + α₂exp(−t/τ₂) (2)

Where α₁, α₂ are fractional contributions of time-resolved decay lifetimes τ₁, τ₂ and average lifetime [small tau, Greek, macron] could be concluded from eqn 3:

For PMAA/NAC-capped CdTe QDs, [small tau, Greek, macron] is 67.36 ns, which is ∼2 times longer than conventional thiol-capped CdTe QDs.³⁶ A distribution in decay times is expected as a result of a variation in the non-radiative decay rates.³⁷ The longer fluorescence-decay time expresses that the polymer ligand in this system could prevent the aggregation of QDs, decrease the defects of QDs and further avoid non-radiative recombination. We also investigated their long-term stability with respect to the photoluminescence properties. The PL spectra of the as-prepared CdTe QDs under ambient conditions, which were stored in air and room light with different times of (1) a fresh sample and (2) 6 month old sample, are displayed in Fig. 4b. We can observe a slight red-shift of the PL peak about 3 nm after 6 months, along with about 5% decrease of the apparent fluorescence intensity, which demonstrates the resulted CdTe QDs are stable under ambient conditions. The good fluorescent stability of the QDs might be correlated with the polymer ligands on their surface, which heal defects of the QDs.

In addition, we compared the photostability of the QD/polymer nanocomposites derived from CdTe QDs capped by PMAA/NAC dual ligands and NAC as the solo ligand, respectively. As seen in Fig. 5a, the initial PL intensity of both QDs/polymer nanocomposites is almost the same (curve 1 and 2). Whereas, the PL intensity of NAC-capped CdTe/PVP composites declined by ∼45% after being stored under ambient conditions for one week (curve 4), while the PL intensity of PMAA/NAC-capped CdTe/PVP composites only showed a slight decrease (<8%, curve 3), manifesting the good polymer-compatibility of the dual ligand-capped QDs. Coupled with their favourable fluorescence, the as-prepared QDs might be utilized as “fluorescent ink” for versatile photoluminescent patterns on polymer film. We used silk screen printing, an easily-handled method, for fluorescent patterns in our case. The “inks” with different fluorescence obtained from CdTe QDs with the PL emission peaks centring at 540 nm, 560 nm and 625 nm, respectively, were screened through woven meshes onto a PVP substrate by extruding the scraper blade. As seen in Fig. 5b and 5c, the flower patterns are invisible on the PVP film under daylight (Fig. 5b), but exhibit bright fluorescent multi-colors under the UV lamp (Fig. 5c), which may offer a further development for optoelectric devices and in the anti-counterfeit field.

Fig. 5 (a) PL spectra of QD/PVP nanocomposites derived from (1) PMAA/NAC-capped CdTe QDs, (2) NAC-capped CdTe QDs and their corresponding PL spectra stored after one week, respectively (3,4). Photographs of the patterned films from QD/PVP nanocomposites via screen printing under (b) daylight and (c) UV light.

The semiconductor QDs turn out to be promising candidates for the fabrication of white LEDs.^38–42 Typically, the addition of conventional QDs always results in a decrease in PL intensity due to the phase separation of QDs in the polymer, which limits their applications.⁴³ Taking advantage of the favorable polymer-compatibility of the resulted QDs, we directly employed the as-prepared solid red QDs to construct LED devices. The white LED was fabricated from the color convert materials using a mixture of yellow phosphors, red CdTe QDs and silicone, with their diagram illustrated in Fig. 6b. The as-prepared LED lamp could produce bright white light for illuminating an image in the dark, as shown in Fig. 6a. From this point of view, we believed the emitted blue light was transformed into white light as it passed through the color convert materials. The color of the emission can be better quantified with the CIE (Commission Internationale d'Éclairage) 1931 chromaticity coordinates, which serves to specify how the human eye perceives light with a given spectrum. As seen in Fig. 5c, the coordinates of the LEDs are located at (0.3485, 0.3634) belonging to the white gamut. The luminescence spectrum of the device takes on two emissions (Fig. 5d), one from the LED chip centered at 460 nm and the other from the corresponding visible component in the range of 480∼770 nm emanated from the QD-phosphor nanocomposite. This reveals the pronounced contribution of red QDs for high-color rendering index (CRI) lighting owing to their extraordinarily broad emission.^44,45

Fig. 6 (a) The photograph of the as-prepared LED device in the dark. (b) Schematic view of the fabrication of the white LED device with QD-phosphor nanocomposites. (c) Placement of the emission spectrum of the white LED on the CIE 1931 chromaticity chart. (d) Emission spectrum of the corresponding white LED lamp.

We also measured the performance of white LEDs before and after blending with red QDs, as summarized in Table 1. We found the CRI increased from 76.1 to 84.0 and the correlated color temperature changed to warm white, which can be attributed to the red QDs compensating for the deficient emission of the phosphor in the red region. Therefore, the LED fabricated with the QD-phosphor nanocomposite represents a more warm color than the phosphor-only used LEDs. However, the luminous efficacy of the white LEDs fabricated with QDs hardly improved compared to the phosphor-only used LED, which can be explained by the addition of red CdTe QDs influencing the luminous flux of the LED, leading to slight decrease of the luminous efficacy.

Table 1 Performance of white LED before and after blending red PMAA/NAC-capped CdTe QDs

	CRI	Color Temp.	CIE (x, y)	Luminous Efficacy
With QDs	84.0	4913 K	0.3485, 0.3634	56.4 lm W^−1
Without QDs	76.1	6125 K	0.3190, 0.3499	68.1 lm W^−1

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4. Conclusions

In summary, we have demonstrated the synthesis of water-soluble CdTe QDs co-stabilized by low-molecular weight PMAA macromonomers and thiol-containing NAC. The as-prepared CdTe QDs are of narrow size distribution, highly efficient fluorescence, good fluorescence lifetime, fine photostability and exhibit the obvious advantage of polymer compatibility. More interestingly, the addition of polymeric PMAA macromonomers accelerates the growth rate of QDs, which may provide a reference method to fabricate high-quality NIR QDs. By virtue of their favorable properties, we also exploited the robust CdTe QDs as “fluorescent inks” to sketch multicolor elegant patterns via silk screen printing and as “red phosphor” for warm white LED fabrication. In this way, this route might constitute a promising way to achieve robust QDs for diverse applications.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21076103, 21176122), National Natural Science Foundation of China-NSAF (10976012), Natural Science Foundations for Jiangsu Higher Education Institutions of China (09KJB530005) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Footnote

† Electronic supplementary information (ESI) available: GPC profile of the PMAA macromonomer via CCTP. See DOI: 10.1039/c2ra20950k

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