Construction of Ag-doped Zn–In–S quantum dots toward white LEDs and 3D luminescent patterning

Abstract

In this work, green-luminescent Ag-doped Zn–In–S quantum dots (Ag:Zn–In–S d-QDs) were successfully synthesized in oleylamine media with dodecanethiol via a one-pot noninjection synthetic strategy. The effect of Ag-doping concentration, reaction temperature, and reaction time on the photoluminescence (PL) properties of Ag:Zn–In–S d-QDs were systematically investigated. It is verified that green fluorescence of Ag:Zn–In–S d-QDs originates from the successful doping of Ag into the Zn–In–S host nanocrystals. ZnS shell was further coated on d-QDs to provide effective passivation, significantly improving the PL quantum yield from 7 to 28%. Then we employed 3D printing technology to achieve versatile low toxic fluorescent patterns towards anticounterfeit and optoelectronic applications for the first time. Also, Ag:Zn–In–S/ZnS d-QDs, as green phosphors, were used to fabricate a high-quality white light-emitting diode with a color rendering index of 90.3.

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Introduction

Luminescent colloidal semiconductor quantum dots (QDs) have been extensively explored in the past two decades for their potential applications in numerous fields including photovoltaic devices, light-emitting devices and biological labels, etc.^1–8 Among the diverse kinds of QDs developed up to now, those with high performance are usually II–VI type (e.g. CdTe, CdSe) and IV–VI type (e.g. PbSe, PbS) QDs.^9–14 Unfortunately, these materials suffer from the intrinsic toxicity of cadmium and lead, which limits their application prospects to a great extent.

Recently, transition-metal ions-doped QDs have gained especial interest since they exhibit desired properties and functions like larger Stokes shifts and longer PL lifetimes, as well as enhanced thermal and chemical stabilities.¹⁵ Thus, much effort has been devoted to the incorporation of impurities into semiconductor lattices. To date, Mn and Cu ions are mostly used as the impurities in doped-QDs (d-QDs).^16–25 The Mn ion often restricts the PL emission in a narrow range of 570–610 nm, resulting from the Mn^{2+ 4}T₁–⁶A₁ transition.¹⁹ The emission of Cu d-QDs covers from blue to near-infrared, which can be attributed to the transition from the conduction band of the host nanocrystals (NCs) to the Cu-d states.^26–28 As another group-IB element, silver plays an important role in semiconductors.^29–32 Liu et al. reported the efficient dopant luminescence and tailored p/n conduction of CdS and CdSSe QDs with substitutional heterovalent doping.³³ Wang's group demonstrated that doping Ag impurities into CdTe QDs could efficiently enhance the fluorescence intensity and the radiative rate.³⁴ Even though high-quality d-QDs were obtained, highly toxic cadmium element was still involved in host materials.

As a promising benign alternative to heavy metal-based semiconductor QDs, ternary alloyed QDs like CuInS₂ and AgInS₂ type QDs have attracted increasing attention.^35–41 For Ag–In–S type QDs, AgInS₂–ZnS quaternary system has been developed via different ways to enrich their optical properties.^42–49 Gabka et al. reported the synthesis of quaternary Ag–In–Zn–S NCs with emission wavelength ranging from 556 to 696 nm by a simple hot injection method.⁴⁹ However, most of the preparation of alloyed AgInS₂–ZnS NCs involved sophisticated precursors or multicomponent precursors and ligands [1-dodecanethiol (DDT), oleic acid (OA) and trioctylphosphine (TOP)].^42–48 It is still highly needed to develop a simple method for the construction of new Ag–In–Zn–S quaternary system with green fluorescence below 530 nm. Recent reports show that ternary chalcogenide Zn–In–S is a near-ideal host material for doping owing to its well-developed synthetic approach, composition-tunable band gap, and high chemical stability.²⁵ For instance, Zhang et al. reported a series of Cu doped Zn–In–S d-dots with composition-tunable emission over the entire visible spectrum.²⁷ By doping Ag into Zn–In–S host NCs, a new Ag–In–Zn–S quaternary system with good optical properties might be available.

For the application of QDs, light-emitting diodes (LEDs) with different down-converted materials have been investigated owing to their outstanding merits.⁵⁰ Traditional commercial WLEDs always show low color rendering index (CRI), arising from the lack of the green and red spectral regions. Therefore, further researches to construct more Cd-free high-quality QDs with green fluorescence make a lot of sense. In addition, taking into account of the stable PL of QDs, exploring more applications is also meaningful.

Here, we describe the preparation of a new kind green-emitting Ag-doped Zn–In–S QDs (Scheme 1). The Ag-doped QDs were synthesized via a simple one-pot noninjection synthetic strategy using commercially available chemicals silver acetate, zinc acetate, indium(III) chloride, sulfur as precursors and dodecanethiol as ligand in oleylamine media. Non-fluorescent Zn–In–S host NCs exhibited green fluorescence after doping with Ag ion, and the doping concentration of Ag ion makes significant effect on the optical properties of Zn–In–S NCs. Ag:Zn–In–S/ZnS core–shell d-QDs with ZnS layers coated on Ag:Zn–In–S core d-QDs were further obtained to emit strong green luminescence. For application, we applied the green-luminescent d-QDs combined with yellow and red phosphors as down-converted materials to fabricate high-quality WLEDs, which could improve CRI from 83.2 (yellow and red phosphors based WLED) to 90.3 under applied current of 350 mA. Furthermore, as a promising fabrication technique, three-dimensional (3D) printing technique was employed to construct 3D fluorescent QDs–polymer composites with different patterns for the first time, which provides a new insight into the versatile applications of QDs.

Scheme 1 Schematic preparation of Ag:Zn–In–S/ZnS d-QDs and their applications in PL inkjet printing, 3D printing and WLEDs.

Experimental

Materials

Silver acetate (AgOAc, 99%), indium(III) chloride (InCl₃, 99.9%), zinc acetate [Zn(OAc)₂, 99.9%], oleylamine (OAm, 90%) and octadecene (ODE, 90%) were purchased from Aldrich. Sulfur sublimed (99.5%) and dodecanethiol (DDT, 98%) were acquired from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Photosensitive resin was obtained from QSQM International Corporation. All chemicals were used as received.

Synthesis of Ag:Zn–In–S core d-QDs

Ag:Zn–In–S d-QDs were prepared by one-pot noninjection procedure under N₂ flow. Typically, AgOAc (0.00495 g, 0.03 mmol), Zn(OAc)₂ (0.037 g, 0.2 mmol), InCl₃ (0.0442 g, 0.2 mmol), and sulfur powder (0.051 g, 1.6 mmol) were added into a 50 mL flask along with 5 mL of DDT and 6 mL of OAm at room temperature. The mixture was degassed with N₂ and stirred for 20 min. Then, the solution was heated to 100 °C at a heating rate of 20 °C min⁻¹ for 10 min. After that the temperature was increased to 200 °C and kept for a certain time to accomplish the growth of Ag:Zn–In–S d-QDs. After the achievement of particle growth, the reaction mixture was cooled to 75 °C and then 20 mL toluene was injected into the flask. The obtained d-QDs were precipitated from toluene with acetone, washed twice, and redispersed in toluene or dried in a desiccator for further characterization.

Synthesis of Ag:Zn–In–S/ZnS core/shell d-QDs

Formation of the ZnS shell covering the Ag:Zn–In–S core d-QDs was performed in the original Ag:Zn–In–S reaction mixture after a growth time of 10 min. When the temperature of the reaction mixture was cooled to 100 °C under N₂ atmosphere, 0.4 mmol Zn(OAc)₂ dissolved in 0.1 mL of OAm and 0.9 mL of ODE were injected into the reaction system under magnetic stirring. Then, the system temperature was raised to 240 °C with a heating rate of 20 °C min⁻¹. The reaction mixture was further heated at this temperature for 20 min to complete the growth of ZnS shell. Ag:Zn–In–S/ZnS core/shell d-QDs were purified by the similar process of Ag:Zn–In–S d-QDs as described above.

Inkjet printing of d-QD solution

For inkjet printing, 8 mL toluene solution of d-QDs of about 1 mg mL⁻¹ was used as fluorescent ink and was transferred to the ink cartridge of the Jetlab®II Precision Printing Platform at room temperature. The inkjet prints the patterns on the drop-on-demand (DOD) mode. And some predefined pictures were used in the printing program. A stroboscopic camera was employed to observe printing process. The printing process accomplished in about 90 s and was repeated for 4 times to complete full printing of the d-QDs on the substrate.

3D printing of fluorescent patterning

3D fluorescent structures were printed with UnciaTM 3D Printer by Digital Light Processor (DLP) technology to cure liquid photopolymer layer by layer until the completion of the final model. Wavelength of the light used in printing process is about 410 nm. The printing process can be completed in 3 to 4 hours.

Preparation of WLEDs

Solid powders of Ag:Zn–In–S/ZnS d-QDs were obtained by drying them in a vacuum oven for a period of time and directly used as green nanophosphors. For the fabrication of WLEDs, firstly, blue-light-emitting LED chips (∼460 nm) were fixed on LED bases. The LED chips and the power supply were connected by gold wires. The silicone (Dow Corning Co.) was mixed with Ag:Zn–In–S/ZnS d-QDs, YAG-05 yellow phosphor and R635 red phosphor [Intematix Photonics (SZ) Co., Ltd.; d-QDs/yellow phosphor/red phosphor = 76/12/12 (w/w/w)]. Subsequently, the fluorescent mixture was vacuumed to eliminate the bubbles formed during mixing. Then, the mixture was loaded on the pre-prepared LED chips and heated at 150 °C for 60 min to cure them. Performances of the prepared LEDs was tested in a ZWL 600 optical instrument. For the fabrication of control device, only the converted materials were changed, that is, without as-prepared green QDs, only YAG-05 (yellow) and R635 (red) phosphors [YAG-05/R635 = 1/1 (w/w)] were used as converted materials.

Characterizations

PL spectrometry and ultraviolet-visible (UV-vis) absorption were carried out on a Varian Cary Eclipse spectrophotometer and a PerkinElmer Lambda 850 UV-vis spectrometer, respectively. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) observation were achieved on a Tecnai G2 F30 S-TWIN transmission electron microscope. Fourier transform infrared (FT-IR) spectrometry was obtained on a Nicolet 6700 FT-IR spectrometer. X-ray diffraction (XRD) was recorded on a Rigaku Corp. D/max-rC rotating-anode powder X-ray diffractometer using a copper target. Energy-dispersive X-ray (EDX) spectra were performed on a Hitachi S-4800 field-emission scanning electron microscope, which owns an energy-dispersive X-ray spectrometer. Time-resolved fluorescence decay curves were taken with Edinburgh FL 900 photocounting system. The quantum yield was measured by comparison with quinine sulfate in 0.10 M H₂SO₄ solution as the standard with QY of 54%. Electroluminescence (EL) spectra, color rendering index (CRI) and luminous efficiency (LE) of as-prepared WLEDs were recorded with ZWL-600 spectral analysis system of Hangzhou ZVISION Photoelectric Technology Co., Ltd.

Results and discussion

TEM images and XRD analysis

The low-magnification image of Ag:Zn–In–S d-QDs obtained from TEM is presented in Fig. 1a. The as-prepared d-QDs are near-cubic and well-distributed. Almost no particle aggregations are discovered. The average size of d-QDs is about 9.2 nm with a relatively narrow size distribution. The HRTEM pictures shown in Fig. 1b and c reveal a relatively good crystalline structure of the d-QDs with continuous lattice fringes through the whole particle. The fringe spacing is 0.207 nm, corresponding to (102) planes of zinc blende Zn–In–S crystals. The crystalline structure of as-prepared d-QDs was further characterized by XRD. Fig. 1d shows the characteristic peaks of the zinc blende (cubic) structure of the obtained d-QDs, which is consistent with TEM examination. And the pattern of the cubic lattice structure is maintained in Ag:Zn–In–S/ZnS core/shell QDs. It is noted that the diffraction peaks of d-QDs shift to larger angles after shell coating, which may originate from the smaller lattice constant of ZnS compared with Zn–In–S or the formation of an alloy between the core and the shell.⁴⁹ In addition, we can also deduce that doping Ag into the host alloyed NCs does not cause phase transformation of the crystal structure. The results demonstrate the successful preparation of well-defined Ag:Zn–In–S d-QDs and Ag:Zn–In–S/ZnS d-QDs.

Fig. 1 TEM image of as-synthesized Ag:Zn–In–S d-QDs under a Ag/(Zn + In) precursor mole ratio of 7.5% and a Zn/In molar ratio of 1 : 1 with a reaction temperature of 200 °C (a). HRTEM images of as-synthesized Ag:Zn–In–S d-QDs (b and c). XRD patterns of the as-prepared Ag:Zn–In–S and Ag:Zn–In–S/ZnS d-QDs (d).

Investigation of optimal reaction conditions

To obtain Ag:Zn–In–S d-QDs with good optical properties, the optimal preparative parameters were explored. We investigated the temporal revolution of optical spectra of the resulting Ag:Zn–In–S d-QDs prepared under a Ag/(Zn + In) precursor mole ratio of 7.5% with a growth temperature of 200 °C. Broad absorption peaks instead of well-defined exciton absorption peaks are observed (Fig. 2a), which is the characteristic of ternary and quaternary alloyed NCs.^27,40 The profile of absorption spectra is kept almost unchanged during the whole growth process, which demonstrates that chemical composition of the resulting QDs keeps constant. PL emission spectra are shown in Fig. 2b, the PL emission peak remains almost unchanged during the whole growth process, suggesting a fast nucleation and growth process under the reaction conditions. And the strongest intensity of PL emission could be observed when growth time is 10 min. Afterward, the PL intensity decreases gradually along with the prolonging of reaction time. It is stated that excessive reaction time may give rise to detachment of ligands from the NCs surface, resulting in a mass of surface defects, which is common in the preparation of NCs.

Fig. 2 The effects of the reaction time on UV-vis absorption of Ag:Zn–In–S d-QDs synthesized under a Ag/(Zn + In) precursor mole ratio of 7.5% and a Zn/In molar ratio of 1 : 1 with a reaction temperature of 200 °C (a). The effects of the reaction time on PL of as-prepared Ag:Zn–In–S d-QDs (b). PL spectra of as-prepared Ag:Zn–In–S d-QDs with different Ag doped concentration (c). PL spectra of Ag:Zn–In–S d-QDs under different growth temperatures (d).

Fig. 2c shows PL spectra of the resulting Ag:Zn–In–S d-QDs synthesized with different molar ratios of Ag/(Zn + In) precursor, while the amount of In and Zn precursors and all other conditions remain unchanged. PL intensity and emission peak of the resulting Ag:Zn–In–S d-QDs exhibit a great dependence on the content of Ag precursor. To investigate the origin of the observed PL emission, control experiment was also carried out. Zn–In–S NCs were synthesized with the absence of Ag ion, and no PL emission was detected. However, when the molar ratio of Ag/(Zn + In) precursor increases to 2.5%, PL emission is observed, which indicates that PL emission of the composite QD system results from the doping of Ag component. As Ag concentration increases from 2.5% to 12.5%, the PL emission peak shifts from 480 nm to 530 nm monotonously. And the PL intensity shows a maximum value when the concentration of the Ag precursor is about 7.5%. Further increasing Ag concentration leads to the decrease of PL intensity and PL emission peaks maintain at about 530 nm. The decrease in PL might be attributed to the lattice mismatch accompanied by the generation of defect states in QDs, as well as the formation of the black-colour Ag₂S NCs.^51,52

We further investigated the effect of reaction temperature on the formation of Ag:Zn–In–S d-QDs (Fig. 2d). When the reaction temperature is 190 °C, PL emission has low intensity. When the reaction temperature increases to 200 °C, the PL intensity of Ag:Zn–In–S d-QDs increases sharply and shows PL QY of 7%. This may be attributed to the incorporation of Ag component into the host crystal lattice at the high temperature. Further raise of temperature to more than 220 °C causes a decrease in PL intensity, and the d-QDs become unstable that precipitation occurs. The phenomenon may be due to the gradual decomposition of DDT which acts as the stabilized ligand in the reaction system.

The as-prepared Ag:Zn–In–S d-QDs have relatively low QY owing to the structural defects, which may decrease their photostability and limit their application. It is demonstrated that the PL QY and photostability of NCs can be significantly improved by using suitable materials with a higher band gap to passivate surface of the core NCs. ZnS was chosen as the shell material for surface passivation, owing to its large bulk band gap (E_g = 3.6 eV), chemical stability and nontoxic character.²⁷ Ag:Zn–In–S d-QDs was covered with ZnS shell by means of in situ growth. The Zn precursor was directly injected to the crude Ag:Zn–In–S reaction solution without additional sulfur source, as the existence of an excess sulfur source in the reaction mixture for the first step. The successful coating of the ZnS shell led to a strong enhancement of the PL intensity (QY increased from 7 to 28%).

EDX and FT-IR analysis

We further adopted EDX spectroscopy technique to identify the composition of the Ag:Zn–In–S core d-QDs and Ag:Zn–In–S/ZnS core/shell d-QDs. As shown in Table 1, silver, zinc, indium and sulfur are detected in both core and core/shell d-QDs. In core d-QDs, mole proportion of elements zinc and indium is close to the ratio of raw materials, suggesting the completeness of reaction proceeds. Moreover, the zinc content on the surface of core/shell d-QDs is obviously higher than that of core d-QDs, which is due to the introduction of ZnS shells. In addition, FT-IR spectra analysis was carried out to verify the surface chemistry of the as-prepared Ag:Zn–In–S d-QDs (Fig. S1†). The Ag:Zn–In–S d-QDs have quite similar FT-IR spectrum to that of DDT. The intense peaks at 1460 cm⁻¹ and 721 cm⁻¹ belong to the deformation vibration of S–CH₂ and the stretching vibration of S–C respectively, indicating that the DDT has been successfully capped on the Ag:Zn–In–S d-QDs.

Table 1 EDX results of Ag:Zn–In–S and Ag:Zn–In–S/ZnS d-QDs with Ag/(Zn + In) precursor molar ratio of 7.5% under a Zn/In molar ratio of 1 : 1

	Ag:Zn–In–S	Ag:Zn–In–S/ZnS
Zn atomic%	16.18	29.80
In atomic%	19.67	16.35
S atomic%	58.58	49.72
Ag atomic%	5.62	4.13

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Time-resolved fluorescence decay analysis

Time-resolved fluorescence of the Ag:Zn–In–S core d-QDs and Ag:Zn–In–S/ZnS core–shell d-QDs were performed under an excitation of 405 nm to explore their dynamical features, as shown in Fig. 3. The PL decay curves of as-prepared d-QDs can be well fitted with a biexponential function, using the following eqn (1):

Y(t) = A₁ exp(−τ/τ₁) + A₂ exp(−τ/τ₂) (1)

where τ₁ and τ₂ represent the decay time of the PL emission and A₁, A₂ are their fractional contribution, respectively. The average lifetime (τ_av) is calculated according to eqn (2):⁴¹

τ_av = ∑A_iτ_i²/∑A_iτ_i (2)

Fig. 3 Time-resolved fluorescence decay curves of synthesized Ag:Zn–In–S core with Ag/(Zn + In) precursor molar ratio of 7.5% under a Zn/In molar ratio of 1 : 1 and the corresponding core/shell d-QDs.

The fitting parameters are summarized in Fig. 3. τ₁ of two kinds both is about 100 ns, and τ₂ is 265.3 and 350.1 ns for core and core–shell d-QDs, respectively. In addition, the amplitude A₂ with a longer lifetime accounts for more amount of the total spectrum. According to previous reports, the long decay components can be ascribed to donor–acceptor recombination and conduction band-to-acceptor recombination, which is associated with internal defects that serve as donor or acceptor sites.⁴¹ The average PL decay lifetime for the core d-QDs is determined to be 223.3 ns. Owing to the surface passivation of the ZnS shell, the average lifetime for core/shell d-QDs increases to 309.0 ns. This concluded hundreds of nanoseconds PL lifetime confirms that the emission originated from the Ag dopant but not from surface states of host NCs.²⁷

Applications of d-QDs in 3D printing and inkjet printing patterns

The desirable PL of as-prepared d-QDs renders this nanomaterial promising for various applications. For instance, green PL patterns were realized through 3D printing technology and an inkjet printing platform. Fig. 4a shows the picture of UnciaTM 3D printing device. Fig. 4b and c exhibit the fluorescent pictures of 3D printing patterning under 365 nm ultraviolet light with use of the mixture of Ag:Zn–In–S/ZnS d-QDs and photosensitive resin as printing material. Interestingly, the 3D printing patterns preserve bright green fluorescence after the printing process. The PL throughout the area is homogeneous, indicating the stability of as-prepared d-QDs. Fig. 4d exhibits the principle of inkjet printing. In an electric field, the piezoelectric materials was squeezed which resulted in a pressure pulse and ejected injected ink droplets from the nozzle. Finally, a predefined pattern forms on the substrate. Fig. 4e and f show the confocal microscopy images of as-prepared patterns under 405 nm excitation. The patterns are intact and keep the PL properties of the d-QDs. The successful patterning of the PL d-QDs here indicates the easy manipulation of these d-QDs and their potential application in the optoelectronic devices and anti-counterfeit field.

Fig. 4 Photograph of UnciaTM 3D printer (a). 3D printing structures with green fluorescence under 365 nm ultraviolet light (b and c). Schematic illustration of piezoelectric printing (d). Confocal microscopy images of inkjet printing patterns of Ag:Zn–In–S/ZnS d-QDs (e and f).

Application of d-QDs in WLED

With merits of high PL efficiency and low toxicity, the as-prepared Ag:Zn–In–S/ZnS d-QDs with green fluorescence could be promising green-emitting conversion materials in WLEDs. So far, a variety of high-quality Cd-free QDs have been applied in the fabrication of WLEDs, and the corresponding parameters have been summarized in Table S1.† Those LEDs with high CRIs (>90) may be ascribed to their broader covering of green, yellow and red spectra. While those adopting only yellow or yellow and red color-converting materials always exhibit relatively lower CRIs. Moreover, the CRI is related to the applied current that the higher current the lower CRI.³⁶ In this case, WLEDs were fabricated using green Ag:Zn–In–S/ZnS d-QDs, YAG-05 yellow phosphors and R635 red phosphors as light conversion materials, and were tested under applied current of 350 mA (much higher than 20 mA used in many previous works as shown in Table S1†). A WLED without as-prepared d-QDs was also constructed as the control device to investigate the effect of green Ag:Zn–In–S/ZnS d-QDs on the performance of WLEDs. The EL spectra for both of the devices are shown in Fig. 5a. EL spectrum of the d-QDs-based WLED exhibits obvious emission in the green region (500–550 nm), and a distinct peak at about 526 nm is observed in comparison with that of the control WLED. The addition of green Ag:Zn–In–S/ZnS d-QDs enhances the CRI from 83.2 to 90.3 under applied current of 350 mA, which is comparable with those previously reported high-quality WLEDs tested under current of 20 mA (Table S1†). The LE value for the as-constructed device is 43.7 lm W⁻¹ at 350 mA, which is much higher than that of common incandescent light bulbs (LE < 18 lm W⁻¹). We analysed these emission spectra by CIE (Commission Internationale de l'Eclairage) 1931 chromaticity coordinates. As shown in Fig. 5b, the coordinate of the control WLED is realized at (0.33, 0.29) while that of the d-QDs-based WLED is located at (0.34, 0.34). Fig. 5c and d show the images of the fabricated LED under power-off and -on situations, respectively. The LED lamp could emit bright white light for illuminating an image in the dark (Fig. 5e and f), fully suggesting that the low-toxic d-QDs with excellent optical performances are promising materials for illumination applications.

Fig. 5 EL spectra of the as-prepared white LEDs operated at 350 mA, respectively (a). Placement of the emission spectra of the corresponding WLEDs on the CIE 1931 chromaticity chart (b). Photographs of the LED lamp (employing Ag:Zn–In–S/ZnS d-QDs as green down-conversion materials) under power off and on (c and d). Images of the LED device in the dark (e and f).

Conclusions

In summary, we successfully developed a simple one-pot noninjection synthetic route to prepare highly green-luminescent Ag-doped Ag:Zn–In–S QDs in oleylamine media. Green fluorescence of as-prepared d-QDs arises from the effective doping of Ag into Zn–In–S host NCs. The passivation of the ZnS shell layers efficiently protects the Ag:Zn–In–S core d-QDs, endowing these novel QDs with a high PL QY. Moreover, we fabricated fluorescent patterns with good stability through 3D printing technology for the first time. We also exploited the green-luminescent d-QDs as a novel phosphor to successfully construct a WLED. The QD-WLED can produce bright white light with high color rendering properties (CRI = 90.3) and satisfactory luminous efficiencies, suggesting great potential application of these QDs in the solid-state lighting systems.

Acknowledgements

This work was supported by National Natural Science Foundation of China (21474052), Natural Science Foundation of Jiangsu Province (BK20131408), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Jiangsu Overseas Research & Training Program for University Prominent Young & Middle-aged Teachers and Presidents.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra05689j

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