“Flash” synthesis of “giant” Mn-doped CdS/ZnS nanocrystals for high photostability

Abstract

Colloidal semiconductor nanocrystals (NCs) could exhibit good photostability if a thick shell (usually more than 10 monolayers (MLs)) with wide bandgap is epitaxially grown onto them. But the photoluminescence (PL) quantum yield (QY) of this kind of “giant” NCs is significantly reduced by the accumulation of non-radiative defects during the rapid growth of thick shell. By optimizing the ratio of ligands and the concentration of precursors, we developed a lower-temperature (L-T) “flash” synthesis method to prepare giant Mn-doped CdS/ZnS core/shell NCs. The thick ZnS shell was overcoated at the injection- and growth-temperature of 340 and 315 °C respectively, and ZnS shell with thickness up to 18 MLs could be grown within only 9 min. Because of the suppressed diffusion of Mn²⁺ dopants and the accompanying limited alloying forming, the energy transfer from the photoexcited exciton in NCs' core towards Mn²⁺ dopants remains rapid, resulting in the increase of PL-QY from 25% (typical “flash”) to 36% (L-T “flash”) for the doped NCs with 14 ML ZnS-shell, much higher than the undoped “flash” CdS/ZnS (13 MLs) NCs' PL-QY of less than 1%. Compared with PMMA-coated thin-shell NCs with the continuing PL degradation, PMMA-coated giant ZnS-shell NCs exhibit the high photostability, throwing light on the application of “flash” giant ZnS-shell NCs for displays and lighting. The high-photostable “flash” giant Mn-doped NCs, with broad band emission and big redshifted emission peak position of ∼620 nm, are used as color converters in white light-emitting diodes with a color rendering index of 80, color coordinates of (0.34, 0.38), and a correlated color temperature of 5397 K.

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Introduction

Owing to their excellent fluorescence performance, colloidal semiconductor nanocrystals (NCs) have been attracting considerable attention for their applications in flatpanel displays and flat lighting.^1–3 One of the most important features of NCs is the large specific surface area, resulting in a low coordination number of many surface atoms. Unsaturated dangling bonds can be terminated by proper capping ligands or surface reconstructions, resulting in suppression of non-radiation transitions. But under the action of light, heat and air, the state of their surface will be changed, leading to the degradation of fluorescence. Improving the stability, especially the photostability, of colloidal semiconductor NCs is a crucial success factor for their practical applications in white light-emitting diodes (W-LEDs).

NCs' stability can be improved by surface passivation. The effects of air and water can be reduced by coating NCs with oxides such as SiO₂, Al₂O₃, et al.^4–6 Besides, covering the NCs with a wider-gap material, forming core/shell structures, has been considered as an effective strategy to eliminate surface traps of NCs.^7–9 Furthermore, alloyed or gradient shells have been recently developed for the passivation of NCs.^10,11 The thicker shell, the less wave-function overlap of exciton (or luminescence center) and surface traps, and then the weaker effect thrown to it by external circumstances.^12,13 Hence, giant NCs with a thick shell (usually more than 10 monolayers (MLs)) can be used to effectively improve the stability of NCs.^13,14 For the Mn-doped NCs, the emission is attributed to the Mn²⁺ ion d–d transition, which is excited by the host exciton energy transfer (within a few to tens of picoseconds).^15,16 Such fast energy transfer process will effectively compete with nonradiative decay by trap states, making Mn²⁺ ion emission more robust than exciton luminescence in undoped NCs.^12,16 Furthermore, the d-electrons are almost localized in the Mn²⁺ ion and do not spread over the entire nanocrystal, thus the Mn²⁺ ion emission is less sensitive to nonradiative decay channels on the NC surface.^12,17 Therefore, the Mn-doped NCs exhibit better photostability than the undoped NCs, and Mn²⁺ ion doping can become a way to enhance the photostability of NCs. Additionally, the Mn-doped NCs not only complement the absent red component for YAG:Ce-phosphor-based W-LEDs, but also have the advantages of no self-quenching due to the large Stokes shift.^12,18 Recently, Mn-doped NCs with thermal stability have been designed to be applied in high color rendering W-LEDs with high density emitters.^12,18,19 In conclusion, the synthesis of giant Mn-doped NCs is a good choice for pursuing high-photostable phosphors applied in W-LEDs.

The most widespread core/shell synthesis method is probably the successive ion layer adsorption and reaction (SILAR), a layer-by-layer approach that allows for a precise tuning of the shell thickness, but remains time-consuming. Although “giant” CdSe/CdS NCs could be successfully synthesized with annealing times of 20–30 min per CdS layer,²⁰ Ghosh et al. recently reported that optimal optical properties could only be achieved with annealing times of 3.5–4 h per CdS layer, corresponding to more than two days of continuous synthesis to grow 15 CdS layers.²¹ 2012 Galland et al. described a method to fabricate CdSe/CdS NCs with shell thickness up to 25 MLs by continuous injection of precursors, costing about 20 hours.²² 2014 Christodoulou et al. applied a procedure, similar to the one proposed by Galland et al., to synthesize giant NCs with shells up to ∼20 MLs, which reduced the growth time to less than 4 hours, while led to an excellent photoluminescence (PL) quantum yield (QY) of up to 90%.²³ 2013 another approach was taken by Cirillo et al., where the synthesis of wurtzite CdSe/CdS NCs with a final diameter of up to 17 nm was achieved in merely 3 minutes, but the QY of these high-cadmium giant NCs, though as an excellent undoped system with higher QY than the CdS/ZnS NCs, was also significantly reduced by the accumulation of non-radiative defects in the giant shell during the rapid growth of the thick shell.²⁴

Aiming at cadmium-free or low-cadmium NCs, we developed a “flash” synthesis of giant ZnS-shell NCs for the improvement of NCs' photostability. By optimizing the ratio of ligands and the concentration of precursors, “flash” synthesis of giant Mn-doped CdS/ZnS NCs was conducted at a lower-temperature (L-T) to improve the exciton-Mn energy transfer rate, resulting in the increase of PL-QY from 25% (typical “flash”) to 36% (L-T “flash”) for the doped NCs with 14 ML ZnS-shell, much higher than the undoped “flash” CdS/ZnS (13 MLs) NCs' PL-QY of less than 1%. ZnS shell with thickness up to 18 MLs could be grown within only 9 min at the injection-temperature (IT) and growth-temperature (GT) of 340 and 315 °C respectively. Compared with the corresponding thin-shell NCs, PMMA-coated giant ZnS-shell NCs exhibit the high photostability, throwing light on the application of “flash” giant ZnS-shell NCs for displays and lighting.

Experimental section

Chemicals

Cadmium oxide (CdO, 99.99%), zinc stearate (ZnSt₂, 12.5–14% ZnO), 1-octadecene (ODE, 90%), stearic acid (SA, 99%), and tri-n-octylphosphine oxide (TOPO, 98%) were purchased from Alfa Aesar. Manganese(II) stearate (MnSt₂, > 95%) was purchased from Wako. Zinc oxide (ZnO, 99.9%), oleic acid (OA, 90%), oleylamine (OAm, 70%), sulfur (S, 99.98%), and tri-n-octylphosphine (TOP, 97%) were purchased from Sigma-Aldrich. All chemicals were used directly without any further purification.

Synthesis of the thin-shell Mn-Doped CdS/ZnS NCs

In a typical procedure, 0.39 g of CdO, 3.41 g of SA were loaded in a 250 mL three-neck flask for preparation of Cd precursor. The mixture, firstly flushed with argon (Ar) for 20 min, was heated to 150 °C and kept at that temperature for 15 min. The acquired colorless clear solution, as the Cd precursor, was cooled to room temperature followed by addition of 0.09 g of MnSt₂, 0.48 g of S, and 100 mL of ODE. The mixture was flushed with Ar for 20 min before it was heated to 260 °C. As soon as the temperature reached 260 °C, 10 mL of OAm was injected, followed by the drop wise addition of Zn precursor solution at the rate of 5 mL min⁻¹. The Zn precursor solution was prepared by dissolving ZnSt₂ (3 g) in ODE (22.5 mL) during the heating under Ar flow. The amount of Zn precursor solution added was varied (between 6 and 22.5 mL) in order to tune the thickness (1–3 mL) of the ZnS shell. After 15 min annealing, the reaction was stopped by rapidly cooling to room temperature. The nanocrystals were purified by the addition of acetone and methanol, centrifugation at 4000 rpm for 8 min, and redispersion in normal hexane. The purification was repeated 3 times. The obtained NCs were dispersed in minimal TOP for further “flash” synthesis.

“Flash” synthesis of giant-shell Mn-doped CdS/ZnS NCs

ZnO and oleic acid were mixed with 4 g of TOPO in a three neck flask. The molar ratio of ZnO, OA and S is fixed as 1 : 5 : 3, and the amount of ZnO was varied between 1 and 4 mmol in order to tune the thickness of the ZnS shell. The reaction mixture was heated to 100 °C while flushing with argon for one hour. The temperature was then increased to ∼350 °C. After the solution became colorless, a solution containing 80 nmol of thin-shell Mn-doped CdS/ZnS NCs and sulfur in TOP was injected at the IT of 340–365 °C. After several minutes of ZnS-shell growth (typical “flash”: 6 min at the IT and GT of 355 and 330 °C respectively; L-T “flash”: 9 min at the IT and GT of 340 and 315 °C respectively), the reaction was stopped by natural cooling to room temperature. The nanocrystals were purified by the addition of acetone and methanol, centrifugation at 4000 rpm for 5 min, and redispersion in toluene or normal hexane. The purification was repeated 3 times.

Characterization

Absorption (Abs) and PL spectra were measured with a Shimazu UV3600 spectrophotometer and an Edinburgh F900 fluorescence spectrophotometer, respectively. Transmission electron microscopy (TEM) images were recorded with a Tecnai G2 Transmission Electron Microscope. X-ray diffraction (XRD) spectra were recorded on a D/max 2500VL/PC diffractometer using Cu Kα radiation. Electron paramagnetic resonance (EPR) spectra were obtained with an X-band EMX-10/12 spectrometer. All the characterizations were carried out at room temperature.

Results and discussions

According to the literature about a typical “flash” synthesis of CdSe/CdS NCs,²⁴ the GT is set as 330 °C, the ratio of ZnO, OA and S is set as 1 : 5 : 3, and, corresponding to 4 g of TOPO, the molar quantity of seeded NCs (shown in Fig. 1b) is set as 80 nmol by us. In that literature, the molar ratio of TOP to S is not fixed, while here we turn to a fit molar ratio of TOP to S, in purpose of acquiring giant NCs with both the relatively narrow size distribution and the biggest NC size. As shown in Fig. 1a, the average diameter of NCs increases at the ratio of less than 1.15 and then decreases at the ratio of more than 1.15 with the increasing ratio of TOP to S. TOPS as a low-reactivity S precursor is introduced to avoid the homogeneous nucleation of separate ZnS NCs through the formation of a strong P–S bond.^25–27 Accordingly, when the ratio is less than 1, besides TOPS, there is still unreacted elemental sulfur as the S precursor, whose reactivity is much higher than TOPS, leading to a broadening of size distribution but a decrease in average diameter of synthesized NCs with the homogeneous nucleation of separate ZnS NCs. As shown in Fig. 1c, there formed the NCs with the widest size distribution at the ratio of 0.6. Additionally, at this ratio fluorescence of prepared NCs almost quenched, because too many defects were generated from too much unreacted elemental sulfur with too high reactivity. When the ratio is larger than 1.15, the reactivity of ZnS monomer is restricted by the excessive TOP,^28,29 resulting in a slow reaction rate, so that the average diameter of NCs decreases. In a word, the proper fixed ratio of TOP to S is slightly larger than 1 (such as 1.15) for preparation of as giant NCs as possible with relatively narrow size distribution.

Fig. 1 Evolutions of average diameter of NPs and several typical TEM images. (a) evolutions of average diameter of NPs with the increasing ratio of TOP to S (circle) and the increasing concentration of Zn precursor (square) respectively; (b) the TEM image of the seeded Mn-doped CdS/ZnS NCs (1 ML ZnS); (c) the TEM image of NCs with the widest distribution (TOP:S = 0.6); (d) the TEM image of the typical “flash” NCs corresponding to 0.214 mol L⁻¹ Zn; (e) the TEM image of “flash” NPs corresponding to 0.233 mol L⁻¹ Zn; (f) the TEM image of “flash” NPs corresponding to the half molar quantity of seeded NCs injected. The inset is the corresponding HR-TEM image of one NP. Each white scale bar is 20 nm, while each dark-blue bar is 10 nm.

Fig. 1a also shows the evolution of average diameter of nanoparticles (NPs) with the increasing concentration of precursors (the molar ratio of ZnO, OA, S and TOP is fixed as 1 : 5 : 3 : 3.45, but the molar quantity can change relative to 4 g of TOPO, that is to say, the concentrations of precursors can change proportionally). The average diameter of NPs increases with the increasing concentration, while the increase tendency suddenly increases at the Zn concentration of more than 0.21 mol L⁻¹. Fig. 1d shows the typical TEM image of NCs synthesized at the Zn concentration of 0.20 mol L⁻¹, and the inset high resolution (HR) TEM image in Fig. 1d confirms the single crystalline nature. While, as shown in Fig. 1e, several small NCs coalesce into multi-core NPs, resulting in the rapid increase of average diameter of NPs; and the inset HR-TEM image in Fig. 1e confirms the polycrystalline nature. Additionally, another phenomenon of coalesce (shown in Fig. 1f) is observed also when decreasing the molar quantity of injected NCs from 80 nmol to 40 nmol. The latter is interesting but reasonable, owing to the reason that the activation energy of coalescence decreases with the decreasing particle concentration due to the decreasing molar quantity of injected NCs.^30,31 In the former situation, the fixed molar quantity of injected NCs and the increasing concentration of precursors factually lead to the decreasing particle concentration, resulting in the decreasing activation energy of coalescence; while at the former situation, the NC units in the coalesced NPs are grown greater than that at the latter situation due to the high concentration. Accordingly, the combination of optimized ligands, reasonably high concentration of precursors and reasonably low molar quantity of NCs injected is favor of preparation of as giant NCs as possible, avoiding the formation of multi-core NPs from the coalescence of NCs.

Fig. 2a exhibits spectra of three samples prepared at different ITs and GTs. The optically excited “flash” giant Mn-doped CdS/ZnS NCs exhibit two PL bands. The blue band is assigned to the emission from the recombination of the quantum-confined excitons at the band gap of CdS cores, and the red band is assigned to the emission from Mn²⁺ dopants (⁴T₁ to ⁶A₁).³² Usually for the Mn-doped CdS NCs with thin ZnS-shell, CdS-core's band-edge emitting is suppressed. It is suggested that there must be a certain diffusion of Mn²⁺ dopants from CdS-core towards ZnS-shell during the “flash” synthesis. As shown in the left of Fig. 2a, there is a greater CdS-band-edge emitting at higher IT and GT, which suggests that the higher IT and GT will result in the greater diffusion of Mn²⁺ dopants.^33,34 Additionally, there is a greater blueshift of CdS-core's band-edge emitting (more details in Fig. S1†) at higher IT and GT, which illustrates there forms a thicker alloy buffer layer between the core and the shell at higher IT and GT.^35–37 The alloy buffer layer alleviates the lattice strain,³⁸ which releases the interfacial pressure, resulting in the prevention on the redshift of PL of Mn²⁺ dopants.³⁹ The blue line shows bigger redshifted emission peak position of ∼628 nm, attributed to the thinner alloy layer and a remained sufficient size of NCs, acquired by the multi-injection. In order to suppress the diffusion of Mn²⁺ dopants, we adopted the lower IT and GT (340 and 315 °C respectively). Fig. 2b shows the TEM image of Mn-doped CdS/ZnS NCs acquired by the L-T “flash” synthesis, with the average diameter of 16.3 nm, a little smaller than ∼18 nm corresponding to typical “flash” synthesis mentioned above. Fig. 2c shows evolutions of Abs and PL spectra of the L-T “flash” giant Mn-doped CdS/ZnS NCs. During the L-T “flash” synthesis of giant NCs, the first exciton Abs-peak of CdS-core blueshifts with a rapid increase of Abs-spectrum in the short wavelength (less than 360 nm), while exciton emission from CdS-core does not appear because of the suppressed diffusion of Mn²⁺ dopants. The PL peak of Mn²⁺ dopants shifts from 588 nm to 620 nm (a redshift of 32 nm) at a slower rate, which suggests a slower chemical reaction rate in the process of alloying forming.

Fig. 2 Spectra of giant Mn-doped CdS/ZnS NCs and the TEM image of NCs acquired by the L-T “flash” synthesis. (a) several types of giant Mn-doped CdS/ZnS NCs synthesized at different ITs and GTs; (b) the TEM image of NCs acquired by the L-T “flash” synthesis and the corresponding HR-TEM image of one NC (the main and inset scale bar is 50 nm and 10 nm respectively); (c) evolutions of Abs (blueshift) and PL (redshift) spectra of giant Mn-doped CdS/ZnS prepared by the L-T “flash” synthesis.

Fig. 3 shows the schematic diagram about the “flash” giant Mn-doped CdS/ZnS NCs. When synthesized at such high temperature, there forms an alloyed interface between the core and the shell, resulting in the core–alloy–shell structure. It is concluded that the exciton is highly localized, from the spatial distribution of exciton's wave-functions corresponding to the schematic diagram of band alignments.^40,41 Since ZnS-shell can't be excited by the light with the wavelength of greater than 350 nm, the photo-excitation of these “flash” NCs is always in the CdS-core, when excited by the usually applied blue-violet light. Accordingly, there will be a much lower exciton-Mn energy transfer due to the less wave-function overlap of exciton and Mn²⁺ dopants, if the Mn²⁺ dopants diffuse too much from CdS-core to ZnS-shell.¹⁶ It is worth noting that “flash” synthesis method is a good way to synthesize giant core–shell NCs, in purpose of improving the stability, especially the photostability, of NCs. As shown in Fig. 3, the thicker shell will lead to the less wave-function overlap of exciton (or luminescence center, here such as Mn²⁺ dopants) and surface traps, resulting in the insensitivity to surroundings and the corresponding improvement of photostability.^12–14

Fig. 3 Schematic diagram of core–alloy–shell structure, band alignments, spatial distribution of exciton's wave-functions, and energy-transfer processes of “giant” Mn-doped CdS/ZnS NCs.

Fig. 4a shows the main competition among exciton recombination, exciton trapping and exciton-Mn energy transfer processes, from photoexcitation to Mn²⁺ PL emission. Usually, exciton-Mn energy transfer in Mn-doped CdS/ZnS nanocrystals can readily outcompete the exciton trapping by an order of magnitude.¹⁶ However, with the accumulation of non-radiative defects in the giant shell during the rapid growth of the thick shell, the competition becomes sensitive to exciton-Mn energy transfer. Fig. 4b exhibits the effect of alloying forming to the exciton-Mn energy transfer rate. In the process of alloying forming, there form an alloy buffer layer and a smaller core, resulting in a stronger quantum confinement effect. If all the Mn²⁺ dopants are in the core, alloying forming will promote the exciton-Mn energy transfer due to the greater wave-function overlap of exciton and Mn²⁺ dopants, resulting from the stronger quantum confinement effect. While, in the situation of great diffusion of Mn²⁺ dopants (as shown in the left of Fig. 4b), the exciton-Mn energy transfer rate is reduced by the less wave-function overlap of exciton and Mn²⁺ dopants, resulting from both the stronger quantum confinement effect and the increased distance between the Mn²⁺ dopants and the smaller core. Accordingly, during the “flash” synthesis at too high temperature, both great diffusion of Mn²⁺ dopants and alloying forming significantly decrease the exciton-Mn energy transfer rate, resulting in an evident decrease in QY. In the L-T “flash” synthesis, the diffusion of Mn²⁺ dopants is suppressed and the accompanying alloying forming is limited. Accordingly the energy transfer from the exciton in NCs' core towards Mn²⁺ dopants remains rapid, resulting in the increase of PL-QY from 25% (typical “flash”) to 36% (L-T “flash”) for the doped NCs with 14 ML ZnS-shell, much higher than the undoped “flash” CdS/ZnS (13 MLs) NCs' PL-QY of less than 1%.

Fig. 4 Detailed schematic diagrams of the competition in “flash” Mn-doped CdS/ZnS NCs. (a) the main competition among exciton recombination, exciton trapping and exciton-Mn energy transfer processes. (b) the effect of alloying forming to the exciton-Mn energy transfer rate.

Fig. 5 shows EPR spectra of three types of Mn-doped CdS/ZnS NCs synthesized at different temperatures. All the spectra of Mn²⁺ dopants exhibit a broad resonance at g ≈ 2, and they all exhibit a six line hyperfine structure. The six lines originate from hyperfine interaction with the ⁵⁵Mn nuclear spin (I = 5/2). The hyperfine coupling constant A, from these NCs with ZnS shell thickness of 1 ML, 14 MLs, and 15 MLs, are 69.6, 68.8, and 68.5 G, respectively. The values lie between the values of bulk CdS:Mn (69.9 G) and ZnS:Mn (68.4 G),^42,43 indicating that Mn is substitutionally incorporated in the host.⁴⁴ Obviously, “flash” giant Mn-doped NCs synthesized at higher temperature possess a lower constant A, reasonably resulting from the greater diffusion from CdS-core towards ZnS-shell.

Fig. 5 EPR spectra of three types of Mn-doped CdS/ZnS NCs.

Fig. 6 shows evolutions of PL of Mn-doped thin-shell (3 MLs) and “giant” (14 MLs) NCs under the strong-violet light with a peak wavelength of 400 nm. Compared to thin-shell NCs, giant NCs in solution usually tend to aggregate and then precipitate due to their poor dispersion stability, resulting in a short time observation over PL of giant NCs in solution. Accordingly, both giant and thin-shell NCs, dispersed in PMMA–toluene solution, were spin-coated on the silica substrate to form NC films for the study of photostability. The PL of the PMMA-coated thin-shell NCs (with the QY of 51%) degraded continuously to less than 40% within 20 hours, while the PL of “giant” NCs (with the QY of 36%) decreased slightly at the initial stage and then it remained stable at ∼88% of its initial PL. This suggests that immediately after excitation the exciton has a low probability of being located close to the surface of giant-shell NCs, internally resulting in the insensitivity to the external environment. NCs with thinner shells may possess higher original QY, but, due to their poor photostability, they are hard to be applied for displays and lighting. Giant NCs (shell thickness: not more than 15 MLs) with the considerable QY exhibiting the super photostability, obtained by the “flash” synthesis, may be the optimum selection for NCs' industrial application in displays and lighting.

Fig. 6 Evolutions of PL of Mn-doped “usual” (3 MLs) and “giant” (14 MLs) NCs under the strong violet light with a peak position of 400 nm, from a 60 watts lamp containing 40 LED beads.

To demonstrate the potential application of these photostable giant Mn-doped CdS/ZnS NCs with broad band emission and big redshifted emission peak position of ∼620 nm, a W-LED lamp was fabricated using a commercial blue-LED chip (as shown in Fig. 7a; λ_peak = 465 nm, rated current = 60 mA) combined with yellow phosphors (YAG:Ce) and as-prepared giant Mn-doped CdS/ZnS NCs (∼14 MLs), dispersed in PMMA matrix. Fig. 7b and c shows the photographs of a W-LED lamp before and after an applied forward current of 60 mA. As shown in Fig. 7d of its electroluminescence (EL) spectrum, there locate the emission band at ∼605 nm, mainly attributed to the giant Mn-doped CdS/ZnS NCs, and the emission band at ∼470 nm attributed to the blue-LED chip. The color temperature could be adjusted in a wide range from 7000 K to 3000 K, by increasing the amount of giant Mn-doped NCs, while the color rendering index (CRI) value could remain above 85 at the same time. Under a 60 mA forward-bias current, the fabricated W-LED showed a good performance, with a CRI of 80, color coordinates of (0.34, 0.38), and a correlated color temperature (CT) of 5397 K.

Fig. 7 The application of low-cadmium giant Mn-doped CdS/ZnS NCs to W-LEDs with high CRI. (a) a commercial blue-LED chip. (b and c) photographs of a W-LED under zero current and forward current of 60 mA respectively. (d) EL spectrum of the W-LED operated under forward current of 60 mA.

Conclusions

In conclusion, under the optimization of the ratio of ligands and the concentration of precursors, giant Mn-doped CdS/ZnS core/shell NCs with the thickness of ZnS shell up to 18 MLs were prepared by the L-T “flash” synthesis within only 9 min at the IT and GT of 340 and 315 °C, which can suppress the diffusion of Mn²⁺ dopants towards ZnS-shell and can limit the alloying forming. Accordingly, the energy transfer from the exciton in NCs' core towards Mn²⁺ dopants remains rapid, resulting in the increase of PL-QY from 25% (typical “flash”) to 36% (L-T “flash”) for the doped NCs with 14 ML ZnS-shell, much higher than the undoped “flash” CdS/ZnS (13 MLs) NCs' PL-QY of less than 1%. The high-photostable “flash” giant Mn-doped NCs, with broad band emission and big redshifted emission peak position of ∼620 nm, can be used as color converters in W-LEDs with an optimized CRI of above 80, and an adjustable CT of 3000–7000 K. Compared with PMMA-coated thin-shell NCs with the continuing PL degradation, PMMA-coated giant ZnS-shell NCs exhibit the high photostability we constantly strive to pursue, throwing light on the application of “flash” giant ZnS-shell NCs for lighting and displays.

Acknowledgements

This work is supported by the National Basic Research Program of China (973 Program, 2012CB921801), the Science and Technology Department of JiangSu Province (BE2012163) and the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1443).

Notes and references

1. T. Erdem and H. V. Demir, Nanophotonics, 2013, 2, 57–81.
2. W.-S. Song, J.-H. Kim, J.-H. Lee, H.-S. Lee, Y. R. Do and H. Yang, J. Mater. Chem., 2012, 22, 21901–21908.
3. Q. Sun, G. Subramanyam, L. Dai, M. Check, A. Campbell, R. Naik, J. Grote and Y. Wang, ACS Nano, 2009, 3, 737–743.
4. T. D. Schladt, K. Koll, S. Prüfer, H. Bauer, F. Natalio, O. Dumele, R. Raidoo, S. Weber, U. Wolfrum, L. M. Schreiber, M. P. Radsak, H. Schild and W. Tremel, J. Mater. Chem., 2012, 22, 9253–9262.
5. P. Yang, Z. Yuan, J. Yang, A. Zhang, Y. Cao, Q. Jiang, R. Shi, F. Liu and X. Cheng, CrystEngComm, 2011, 13, 1814–1820.
6. C. Hu, A. Gassenq, Y. Justo, K. Devloo-Casier, H. Chen, C. Detavernier, Z. Hens and G. Roelkens, Appl. Phys. Lett., 2014, 105, 171110.
7. T.-L. Nguyen, M. Michael and P. Mulvaney, Chem. Mater., 2014, 26, 4274–4279.
8. F. Pietra, R. J. A. van Dijk-Moes, X. Ke, S. Bals, G. van Tendeloo, C. D. M. Donega and D. Vanmaekelbergh, Chem. Mater., 2013, 25, 3427–3434.
9. L. Li and P. Reiss, J. Am. Chem. Soc., 2008, 130, 11588–11589.
10. K. Boldt, N. Kirkwood, G. A. Beane and P. Mulvaney, Chem. Mater., 2013, 25, 4731–4738.
11. J. Lim, W. K. Bae, D. Lee, M. K. Nam, J. Jung, C. Lee, K. Char and S. Lee, Chem. Mater., 2011, 23, 4459–4463.
12. X. Yuan, J. Zheng, R. Zeng, P. Jing, W. Ji, J. Zhao, W. Yang and H. Li, Nanoscale, 2014, 6, 300–307.
13. K.-H. Lee, J.-H. Lee, H.-D. Kang, B. Park, Y. Kwon, H. Ko, C. Lee, J. Lee and H. Yang, ACS Nano, 2014, 8, 4893–4901.
14. X. Dai, Z. Zhang, Y. Jin, Y. Niu, H. Cao, X. Liang, L. Chen, J. Wang and X. Peng, Nature, 2014, 515, 96–99.
15. Y. Hefetz, W. C. Goltsos, A. V. Nurmikko, L. A. Kolodziejski and R. L. Gunshor, Appl. Phys. Lett., 1986, 48, 372–374.
16. H.-Y. Chen, S. Maiti and D. H. Son, ACS Nano, 2012, 6, 583–591.
17. K. P. Kadlag, M. J. Rao and A. Nag, J. Phys. Chem. Lett., 2013, 4, 1676–1681.
18. X. Yuan, R. Ma, W. Zhang, J. Hua, X. Meng, X. Zhong, J. Zhang, J. Zhao and H. Li, ACS Appl. Mater. Interfaces, 2015, 7, 8659–8666.
19. P.-C. Shen, M.-S. Lin and C.-F. Lin, Sci. Rep., 2014, 4, 5307.
20. Y. Guo, K. Marchuk, S. Sampat, R. Abraham, N. Fang, A. V. Malko and J. Vela, J. Phys. Chem. C, 2012, 116, 2791–2800.
21. Y. Ghosh, B. D. Mangum, J. L. Casson, D. J. Williams, H. Htoon and J. A. Hollingsworth, J. Am. Chem. Soc., 2012, 134, 9634–9643.
22. C. Galland, S. Brovelli, W. K. Bae, L. A. Padilha, F. Meinardi and V. I. Klimov, Nano Lett., 2013, 13, 321–328.
23. S. Christodoulou, G. Vaccaro, V. Pinchetti, F. de Donato, J. Q. Grim, A. Casu, A. Genovese, G. Vicidomini, A. Diaspro, S. Brovelli, L. Manna and I. Moreels, J. Mater. Chem. C, 2014, 2, 3439–3447.
24. M. Cirillo, T. Aubert, R. Gomes, R. van Deun, P. Emplit, A. Biermann, H. Lange, C. Thomsen, E. Brainis and Z. Hens, Chem. Mater., 2014, 26, 1154–1160.
25. J.-Y. Chang, M.-H. Tsai, K.-L. Ou, C.-H. Yang and J.-C. Fan, CrystEngComm, 2011, 13, 4236–4243.
26. H. Liu, J. S. Owen and A. P. Alivisatos, J. Am. Chem. Soc., 2007, 129, 305–312.
27. A. L. Washington and G. F. Strouse, Chem. Mater., 2009, 21, 3586–3592.
28. K. Yu, X. Liu, Q. Zeng, D. M. Leek, J. Ouyang, K. M. Whitmore, J. A. Ripmeester, Y. Tao and M. Yang, Angew. Chem., Int. Ed., 2013, 52, 4823–4828.
29. R. K. Čapek, D. Yanover and E. Lifshitz, Nanoscale, 2015, 7, 5299–5310.
30. X. Xue, R. L. Penn, E. R. Leite, F. Huang and Z. Lin, CrystEngComm, 2014, 16, 1419–1429.
31. S. Yin, F. Huang, J. Zhang, J. Zheng and Z. Lin, J. Phys. Chem. C, 2011, 115, 10357–10364.
32. Y. Yang, O. Chen, A. Angerhofer and Y. C. Cao, Chem.–Eur. J., 2009, 15, 3186–3197.
33. S. C. Erwin, L. Zu, M. I. Haftel, A. L. Efros, T. A. Kennedy and D. J. Norris, Nature, 2005, 436, 91–94.
34. N. Y. Jamil and D. Shaw, Semicond. Sci. Technol., 1995, 10, 952–958.
35. M. Danek, K. F. Jensen, C. B. Murray and M. G. Bawendi, Chem. Mater., 1996, 8, 173–180.
36. X. Zhong, M. Han, Z. Dong, T. J. White and W. Knoll, J. Am. Chem. Soc., 2003, 125, 8589–8594.
37. X. Zhong, Y. Feng, W. Knoll and M. Han, J. Am. Chem. Soc., 2003, 125, 13559–13563.
38. D. Choi, J.-Y. Pyo, Y. Kim and D.-J. Jang, J. Mater. Chem. C, 2015, 3, 3286–3293.
39. S. Ithurria, P. Guyot-Sionnest, B. Mahler and B. Dubertret, Phys. Rev. Lett., 2007, 99, 265501.
40. L.-D. Zhao, J. He, S. Hao, C.-I. Wu, T. P. Hogan, C. Wolverton, V. P. Dravid and M. G. Kanatzidis, J. Am. Chem. Soc., 2012, 134, 16327–16336.
41. L.-D. Zhao, V. P. Dravid and M. G. Kanatzidis, Energy Environ. Sci., 2014, 7, 251–268.
42. D. M. Hofmann, A. Hofstaetter, U. Leib, B. K. Meyer and G. Counio, J. Cryst. Growth, 1998, 184/185, 383–387.
43. B. J. Park, W. B. Im, W. J. Chung, H. S. Seo, J. T. Ahn and D. Y. Jeon, J. Mater. Res., 2007, 22, 2838–2844.
44. B. Yang, X. Shen, H. Zhang, Y. Cui and J. Zhang, J. Phys. Chem. C, 2013, 117, 15829–15834.

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Footnote

† Electronic supplementary information (ESI) available: Blushifts of spectra of undoped “flash” CdS NCs, and XRD- and SAED-patterns of Mn-doped CdS/ZnS NCs. See DOI: 10.1039/c5ra17200d

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