Research article Received: 10 October 2013,

Revised: 9 January 2014,

Accepted: 16 February 2014

Published online in Wiley Online Library

(wileyonlinelibrary.com) DOI 10.1002/bio.2664

Sensitization enhancement of europium in ZnSe/ZnS core/shell quantum dots induced by efficient energy transfer Ni Liu,a Ling Xu,a* Hongyu Wang,a Jun Xu,a Weining Su,b Zhongyuan Maa and Kunji Chena ABSTRACT: Eu-doped ZnSe:/ZnS quantum dots (formed as ZnSe:Eu/ZnS QDs) were successfully synthesized by a two-step wet chemical method: nucleation doping and epitaxial shell growing. The sensitization characteristics of Eu-doped ZnSe and ZnSe/ ZnS core/shell QD are studied in detail using photoluminescence (PL), PL excitation spectra (PLE) and time-resolved PL spectroscopy. The emission intensity of Eu ions is enhanced and that of ZnSe QDs is decreased, implying that energy was transferred from the excited ZnSe host materials (the donor) to the doped Eu ions (the acceptor). PLE reveals that the ZnSe QDs act as an antenna for the sensitization of Eu ions through an energy transfer process. The dynamics of ZnSe:Eu/ZnS core/shell quantum dots with different shell thicknesses and doping concentrations are studied via PL spectra and fluorescence lifetime spectra. The maximum phosphorescence efficiency is obtained when the doping concentration of Eu is approximately 6% and the sample showed strong white light under ultraviolet lamp illumination. By surface modification with ZnS shell layer, the intensity of Eu-related PL emission is increased approximately three times compared with that of pure ZnSe:Eu QDs. The emission intensity and wavelength of ZnSe:Eu/ZnS core/shell quantum dots can be modulated by different shell thickness and doping concentration. The results provide a valuable insight into the doping control for practical applications in laser, light-emitting diodes and in the field of biotechnology. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: Core-shell quantum dots; Energy transfer; Enhanced sensitization luminescence

Introduction II–VI semiconductor nanocrystals (also named quantum dots, QDs) have been studied extensively in recent years due to excellent luminescent properties, such as easily modulated emission wavelength, high resistance to photobleaching, excellent monodispersity and high luminescent yield (1–3). Semiconductor QDs with rare earth (RE) impurities are promising candidates in the fabrication of a new class of light-conversion materials (4,5). However, RE ions have extremely sharp optical absorption cross-sections of the order of 10 21/cm2, which lead to low emission efficiencies (6). Meanwhile, the direct excitation of RE ions is difficult because most of the f–f transitions are Laporte forbidden (7). To overcome this limitation, the concept of sensitization through an antenna effect has been established. The luminescence intensity of RE can be significantly enhanced when doped in a host material with an enlarged absorption cross-section by efficient energy pumping and transferring. Sadhu et al. reported the influence of particle size on photophysical properties of Eu-doped CdS nanocrystals, and analyzed the energy transfer mechanism by time-resolved photoluminescence (PL) spectroscopy (8). Dethlefsen et al. studied Ln3+-modified CdSe and CdSe/CdS QDs. The results showed that the sensitization of Ln3+ in CdSe could be achieved through high-energy excitation of the CdSe host (9). Compared with the extensive research of II–VI materials with transition metal ion doping, the study of ZnSe QDs with RE doping is rare. Furthermore, the QD doping with RE is usually prepared through the organometallic route (10–12). In this paper, we adopted the wet chemical method

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to prepare ZnSe QDs and doped QDs. In comparison to the organometallic route, the wet chemical method is low cost, simple and environmentally friendly. Moreover, the ZnSe semiconductor material is an excellent blue light emitting material and non- or low toxic, which is superb for biological labeling and blue light-emitting diode applications (13,14). It has been reported that the absorption cross-section of ZnSe was 1.1 × 10 18 cm2 (15) and the Eu ion with a relatively simple energy level is perfect luminescence material. Thus, ZnSe QDs doped with Eu acting as sensitizers to favor the phonon absorption will appear to have excellent properties. Energy transfer is a distance-dependent interaction between the host material (the donor) and the dopant (the acceptor) in which excitation is transferred from the former to the latter (16,17). In general, energy transfer is affected by the overlap between the donor emission cross-section and acceptor

* Correspondence to: Ling Xu, National Laboratory of Solid State Microstructures and Jiangsu Provincial Key Laboratory of Photonic and Electronic Materials Sciences and Technology, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, People’s Republic of China. E-mail: [email protected] and [email protected] a

National Laboratory of Solid State Microstructures and Jiangsu Provincial Key Laboratory of Photonic and Electronic Materials Sciences and Technology, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, People’s Republic of China

b

National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China

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N. Liu et al. absorption cross-section, the relative orientation of the interacting dipoles and distance between the donor and the acceptor (< 10 nm) (18). Most studies of energy transfer dynamics are focused on transient metals or two or more RE ions codoped in II–VI semiconductors used for near-infrared light emitting diodes, upconversion materials and solar cells (19,20). However, the research for single Eu ions doping in ZnSe QDs and ZnSe/ZnS QDs with a white emission is relatively scarce. Therefore, in this paper, we investigated the PL spectra, absorption spectra and fluorescent lifetime spectra of ZnSe:Eu QDs and ZnSe:Eu/ZnS systematically. The sensitization mechanism of ZnSe:Eu QDs via the energy transfer process is interpreted in detail. The emission intensity of the Eu-doped ZnSe QDs is increased three times after ZnS shell epitaxial overgrowth. The efficient energy transfer process between ZnSe (host material) and Eu ions (dopant) with different doping concentrations and shell thicknesses is discussed based on PL spectra and fluorescent lifetime spectra.

Experimental Reagents and materials Pure ZnSe QDs, ZnSe:Eu QDs and core/shell ZnSe:Eu/ZnS QDs were prepared using a wet chemical colloidal technique as previously reported (21). The reagents and materials are: zinc nitrate (Zn(NO)3, 99.9%), zinc acetate dehydrate (Zn(OAc)2, 99.9%), samarium acetate (Eu(NO)3, 99.9%), 3-mercaptopropionic acid (MPA, 99%), selenium powder (99.5%), sodium borohydride (NaBH4, 98%), ethanol and deionized water. All reagents were used as received without further purification. Synthesis of 3-mercaptopropionic acid-capped ZnSe:Eu quantum dots NaBH4 (0.38 g) was dissolved in 5 mL deionized water and both were added to a 250 mL flask. The air was pumped off and replaced with N2. When the NaBH4 was fully dissolved, 0.40 g selenium powder was added to the flask. The solution was ultrasonic, dispersed in a nitrogen atmosphere. When the selenium powder was totally dissolved, the clear NaHSe solution was diluted with 100 mL of nitrogen-saturated water and prepared for the next synthesis. Zn(NO)3 (25 mL, 0.1 M), Eu(NO)3 (0.01 M) and 1.2 M LMPA were mixed and the pH was adjusted to 10.3 by dropwise addition of NaOH (2.0 M) solution. The mixed solution was put in a threenecked flask and stirred for 1 h in nitrogen. One hour later, about 45 mL fresh NaHSe solution was injected in to the flask. The mixture was heated with refluxing at 100°C in a nitrogen atmosphere. Aliquots were removed at different reaction time intervals for the next preparation. When the heating was stopped, the solution was precipitated by ethanol. The precipitation was centrifuged. Then the upper layer was removed and the precipitation was washed with ethanol three times and was dried in a vacuum. The samples used for characterization were all redissolved in deionized water.

Then the solution was stirred for 30 min in nitrogen atmosphere. ZnSe:Eu (1 mL, 0.48 mmol) solution was ultrasonic, dispersed in nitrogen until the solution appeared a clear pale yellow. Then the mixture of Zn(NO)3 and MPA were slowly dropped into the ZnSe:Eu solution and the mixture rapidly stirred. The solution was heated at 90°C for 1.5 h. The samples were precipitated by ethanol then centrifuged. The precipitation was washed with ethanol three times and the excess ligand and unreacted precursors were removed. Characterizations The pH value of solution was tested by a PHDZ-01 type meter (Shanghai REX Instrument Factory). The meter was corrected before measurement. The Roman spectra were performed by using a T64000 triple Raman system with the 514.5 nm line of Ar+ laser as the excitation. Room temperature PL spectra were measured using Jobin Yvon Fluorolog-3 system (Jobin Yvon Division company, France), and the excitation source was 325 nm He–Cd laser (Kimmon company, Japan) (30 mW). The room temperature PL excitation (PLE) measurements were carried out using a 450 W xenon lamp as the excitation light resource. The absorption spectra of samples were taken on a 3100 ultraviolet–visible spectrometer (Shimadzu, Japan). The luminescence lifetime spectra of samples were performed by a FLS920 fluorescence spectrophotometer (Edinburgh Instruments Ltd., Livingston, West Lothian, UK) equipped with a 450 W xenon lamp as the excitation source and the pulse frequency is 100 ns.

Results and discussion The ZnSe:Eu QD and ZnSe:Eu/ZnS QD samples we used for measurement were purified and re-dissolved in aqueous solution after being centrifuged and washed. To confirm the microstructure of as-obtained samples, Raman spectra was performed at room temperature, as shown in Fig. 1. The doping concentration of Eu ions is 4% and shell thickness of ZnS is 1.5 monolayer (ML). The Raman peaks at 250/cm and 505/cm are attributed to the longitudinal optical (1LO) phonon mode and 2LO photon mode of ZnSe (22), as shown in Fig. 1(a). For the reported ZnSe polycrystalline nanoparticles (23,24), the 1LO phonon frequencies are 255/cm and 510/cm. A small shift of the two peaks of the ZnSe QDs towards a lower frequency may be associated with the small size and large surface effects. When Eu ions were doped in ZnSe QDs, the main two Raman

Synthesis of synthesis of 3-mercaptopropionic acid-capped ZnSe:Eu/ZnS core/shell quantum dots Zn(NO)3 (4.8 mL, 0.1 M) and LMPA (8.35 M) were mixed together and put in a three-neck flask. The pH of the mixture was adjusted to 10.3 by dropwise addition of NaOH 2.0 M solution.

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Figure 1. The room temperature Raman spectra of: (a) pure ZnSe quantum dots (QDs), (b) ZnSe:Eu QDs and (c) ZnSe:Eu/ZnS QDs. LO, longitudinal optical.

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Luminescence intensity enhanced sensitization energy transfer peaks (1LO photon mode and 2LO photon mode) seem similar and no detectable signal corresponding to any other Eu specie appears in the Raman spectra. It indicates that the dopant of Eu ion do not change the structure of ZnSe host material. The results are consistent with the X-ray diffraction results of them as reported by Wang et al. (17). When the shell of ZnS was deposited on the surface of ZnSe:Eu QDs, a peak of the LO phonon mode of ZnS shell at about 350/cm was observed in Fig. 1(c) (25). This result also confirmed that the ZnS shell has been successfully overgrown on the core ZnSe:Eu QDs. Figure 2(a,b) shows the wide field transmission electron resolution images (TEM) of the core ZnSe:4%Eu and corresponding core/ shell ZnSe:4%Eu/ZnS QDs with a 1.5 ML shell thickness. Homogeneously distributed ZnSe:Eu and ZnSe:Eu/ZnS QDs spherical crystallites are obtained from the graph. The inset of Fig. 2(a) shows an high-resolution TEM image of as-obtained ZnSe:Eu QDs. The average diameter of ZnSe:Eu QDs are about 2.9 nm and the inset images show the periodic lattice. When ZnSe:Eu QDs are overcoated with 1.5 ML shell thickness of ZnS, the sizes of ZnSe:Eu/ZnS QDs are enlarged to 4.5 nm. This increasing size for core/shell QDs compared to that of the core ZnSe:Eu is mainly attributed to the shell deposition of ZnS, because in the control experiment, the core ZnSe:Eu QDs reaction solution was heated at the identical experimental conditions except for the introduction of the S precursor. Moreover, the clear-cut lattice fringes shown in the high-resolution TEM images confirm that the as-obtained ZnSe:Eu and ZnSe:Eu/ZnS QDs are of high crystallinity. Figure 3(a) shows the PL spectra of the pure ZnSe QDs, Eudoped ZnSe and Eu-doped ZnSe/ZnS QDs under 370 nm excitation. Equally, the doping concentration of Eu ions is 4% and shell thickness of ZnS is 1.5 ML. The PL spectrum of the pure ZnSe nanocrystals displays an intense blue emission band centered at 406 nm and the other broad green band centered at around 510 nm (curve a). The former is attributed to the excitonic recombination from the valence band to the conduction band of ZnSe nanocrystals, and the latter results from the electron–hole recombination through shallow surface trap states of ZnSe nanocrystals (26). The band edge emission energy is higher than that of the bulk ZnSe (459 nm, 2.7 eV). This blue shift toward higher energy is attributed to the quantum confinement effect. When Eu ions are doped in pure ZnSe host materials, the characteristic emission peaks of Eu ions are definitely visible in the PL spectrum, which correspond to 5D0 → 7F1 (590 nm), 5D0 → 7F2 (613 nm), 5D0 → 7F3 (652 nm) and 5D0 → 7F4 (695 nm), respectively (27). Simultaneously, the emission bands of ZnSe

Figure 3. (A) PL spectra of the pure ZnSe quantum dots, Eu-doped ZnSe and Eu-doped ZnSe/ZnS quantum dots using 370 nm excitation. (B) Corresponding photos of as-obtained three samples under ultraviolet lamp. PL, photoluminescence.

nanocrystals also exist (curve b), indicating that Eu ions had entered the crystal lattice while not destroying the lattice structures of ZnSe QDs. However, the emission intensity of surface states and that of the ZnSe QD band edge are both decreased. Notably, the decreased PL emission intensities of ZnSe QDs and increased emission intensities of Eu ions indicate that the Eu ions are sensitized through energy transfer process between the ZnSe host and Eu ions. After the epitaxial shell of ZnS is overcoated on to ZnSe:Eu QDs, the two broad emission bands derived from the ZnSe core bathochromically shift by 10–20 nm, and the emission intensity of Eu ions is evidently enhanced threefold compared to that of ZnSe:Eu QDs (curve c). Meanwhile, the emission intensity of ZnSe core QDs in curve c is lower than that of ZnSe:Eu QDs. The luminescence quenching of ZnSe QDs

Figure 2. The wide field transmission electron resolution images (TEM) of the core ZnSe:4%Eu and corresponding core/shell ZnSe:4%Eu/ZnS QDs with a 1.5 ML shell thickness.

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N. Liu et al.

Figure 4. The absorption spectra of ZnSe:Eu and ZnSe:Eu/ZnS and the PL spectrum of ZnSe quantum dots. PL, photoluminescence.

and the PL intensity enhancement of Eu ions suggest the energy transfer originating from the dipole–dipole interaction between them. Similar observations have been made by Sadhu et al., who reported that upon excitation of the CdS host, the energy from non-radiative recombination of electron–hole pairs could be transferred to the high-lying energy levels of Eu ions (8). Fig. 3 (b) show the camera images of pure ZnSe QDs, ZnSe:Eu QDs and ZnSe:Eu/ZnS QDs (from left to right), which emit blue, red and white light under ultraviolet lamp. Compared to the organic ligand emitting materials prepared in the He et al. study (28), this inorganic luminescence material with multicolor emission could potentially be used in laser and light-emitting diode applications due to the simple and economic manufacturing process. To validate the sensitization enhancement for Eu ions by energy transfer between ZnSe QDs and the doped Eu ions, we measured the absorption spectra of ZnSe:Eu and ZnSe:Eu/ZnS and the PL spectrum of ZnSe QDs, as shown in Fig. 4. The PL spectrum of the pure ZnSe QDs exhibits two emission bands at 406 nm and 510 nm, which corresponds to band edge emission and surface defects emission respectively, as discussed above. The characteristic absorption peaks of Eu ions at 390 nm and 450 nm correspond to 7D0 → 5F6 and 7D0 → 5F2 respectively (29). No characteristic absorption peaks of Eu ions are discerned in the absorption spectra of ZnSe:Eu and ZnSe:Eu/ZnS nanocrystals. The absorption cross-section of ZnSe is much larger than that of Eu, as we described in our previous paper. As a result, the absorption peak of Eu ion is overlapped by that of the more intensely absorbed ZnSe, which is consistent with some previous literature (30). Herein we also observe the band overlapping between the PL emission band of ZnSe QDs (the donor) and the absorption bands of ZnSe:Eu and ZnSe:Eu/ZnS QDs (the acceptor). The area of the overlapping between the donor and the acceptor is enlarged after shell growing. Thus, the efficiency of energy transfer is augmented after the ZnS shell is overcoated on to ZnSe:Eu QDs. Figure 5(a) shows the photophysical processes of Eu-doped ZnSe QDs. The characteristic absorption bands of Eu ions are located within the emission band range of ZnSe QDs, allowing the energy transfer between the donor and the acceptor. There are usually two emission bands of semiconductor nanocrystals, corresponding to band edge luminescence band and impurities/ defects luminescence band. The luminescence emission spectra of the core ZnSe QDs show an edge emission band (406 nm, 3.05 eV) and a surface defects emission band (510 nm, 2.45 eV). The band gap edge emission results from the radiative recombination of conductive band-derived electrons and valence band-

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Figure 5. (A) The photophysical processes of Eu-doped ZnSe QDs. (B) White light emission through efficient ET. (ET, energy transfer).

derived holes (Fig. 3). These sharp spectra are commonly located at the absorption edge. Meanwhile, the electrons may be trapped by shallow states of the defects. In general, RE ions doped in II–VI semiconductor nanocrystals function as energy acceptors rather than charge carriers because the charge transfer between the dopant and the host materials is energetically more expensive than the bandgap transition (31). When Eu ions are doped in the host material, energy can be transferred by two pathways, including the energy transferred from the excitonic recombination of ZnSe nanocrystals to Eu ions that leads to the absorption transition 7 D0 → 5F6 of Eu ions, and that transferred in the recombination between the electron in the surface defect state and the hole in the valence band. The electron in the conduction band, which is relaxed to the surface defect state, is subsequently recombined with the hole. The relaxing energy is thus transferred to the nearby Eu ion, which is doped in ZnSe nanocrystals. To confirm the energy transfer between the QDs in our samples, the PLE spectra of the 4% Eu-doped ZnSe nanocrystals were investigated. The PLE spectrum of ZnSe QDs and ZnSe:Eu QDs was shown in Fig. 6. For ZnSe:Eu QDs, the emission wavelength at 613 nm corresponds to the characteristic luminescence of Eu ions and the excitation wavelength varies from 300 nm to 550 nm. The broad band at 390 nm is ascribed to the electronic transition of ZnSe semiconductor, which is consistent with pure ZnSe QDs (curve a). The evident excitation peaks at 397 nm is associated with the resonant excitations of Eu 4f–4f transitions (32). The comparison between the PLE spectrum of the ZnSe:Eu QDs with the absorption spectra of ZnSe:Eu and ZnSe:Eu/ZnS QDs (Fig. 4) reveals that the luminescence from Eu ion originates from the photoexcitations of ZnSe QDs, which distinctly demonstrates that the highly efficient energy transfer from ZnSe host materials to Eu ions accounts for the enhanced light emission of the Eu ions.

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Luminescence intensity enhanced sensitization energy transfer

Figure 6. Photoluminescence excitation spectrum of ZnSe:Eu quantum dots monitoring the photoluminescence peak of Eu ions (613 nm).

It has been reported that when the surface of ZnSe is passivated with ZnS, a ZnSe/ZnS core–shell semiconductor is formed and the quantum yield is greatly improved compared with that for bare ZnSe. Therefore, to enhance the luminescence intensity of Eu, we prepared ZnSe:Eu/ZnS QDs with a two-step wet chemical method: nucleation doping and epitaxial shell growing. The PL spectra of ZnSe:Eu/ZnS QDs with different shell thicknesses are shown in Fig. 7. The intensity of the Eu-related emission (613 nm) is enhanced up to the maximum (1.5 ML thickness) with increasing deposited shell thickness. The inset of Fig. 7 shows the ratio of the surface defect-related emission intensity (ID), band edge emission intensity (IB) and the 5D0 → 7F2 (613 nm) emission intensity (I613) of Eu ions, indicating the effective quantum yield of the RE emission. The ratio of ID/I613 decreases and that of IB/I613 increases when the shell thickness is 0.5 ML. With the shell thickness increasing, the non-radiative recombination probability of defect states is reduced and band edge emission intensity is increased, the increasing ratio of IB/ I613 appears. When the shell thickness is 1.5 ML, the energy transfer efficiency from band edge recombination to Eu ions is higher, so the ratio of IB/I613 is decreased. Moreover, the relative intensity of ID/I613 is minimal suggesting that the epitaxial layer efficiently passivates the surface of the core, which is optimized by the ZnS shell. The Eu ions can be efficiently sensitized through the energy transfer process from ZnSe QDs. With increasing shell thickness (over 1.5 ML), the relative intensity ratio

Figure 7. PL spectra of ZnSe:Eu/ZnS QDs with different shell thicknesses. The inset are the ratio of the surface defect-related emission intensity (ID), band edge emission intensity (IB) and the 5D0→7F2 (613 nm) emission intensity (I613) of Eu ions.

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of defect emission to 613 nm is gradually elevated. When thicker shell material coherently grows on a tiny core, significantly more lattice mismatches are produced following more defect states (33). Hence, the relative intensity ratio of the surface defect-related emission to the 5D0 → 7F2 one (613 nm) is increased. In the meantime, more electrons are trapped by surface defect states and less recombination energy is transferred to Eu ions, thereby reducing the luminescence intensity of Eu. The results indicate that the shell can sufficient passivate the surface of ZnSe:Eu QDs and enhance the energy transfer efficiency between the ZnSe host and Eu ions. However, the shell thickness has an optimum value and 1.5 ML thickness of ZnS shell is excellent for ZnSe:Eu/ZnS QDs. To verify sensitization enhancement of Eu induced by efficient energy transfer between ZnSe host materials and Eu ions, the fluorescence lifetime spectra of ZnSe QDs, ZnSe:Eu QDs and ZnSe:Eu/ZnS QDs with different shell thicknesses were recorded by monitoring the fluorescence peaks of Eu at 613 nm and that of the ZnSe core at 510 nm (Fig. 8). Figure 8(a) exhibits that the decay profile of defect state emission (510 nm) in ZnSe:Eu QDs and ZnSe:Eu/ZnS QDs. The decay time of defect emission of ZnSe:Eu/ZnS QDs is much faster than that of pure ZnS QDs. This shortened decay time originates from the effective energy transfer process from the host material to Eu ions. Meanwhile, the lifetime of defect state PL of ZnSe:Eu/ZnS QDs is decreased slightly after ZnS shell modification compared to that of core ZnSe:Eu QDs. From Fig. 8(b) we can see that the characteristic emission of Eu ions doped in ZnSe exhibits a slow growth with shell thickness increased. The decay profile of ZnSe:Eu/ZnS with

Figure 8. (A) Time-resolved photoluminescence decay curves of pure ZnSe quantum dots (QDs), ZnSe:Eu QDs and ZnSe:Eu/ZnS QDs by monitoring the fluorescence peaks of ZnSe at 520 nm. (B) Time-resolved photoluminescence decay curves monitoring the fluorescence peaks of (613 nm) of Eu in ZnSe:Eu QDs (the same concentration) capped with different shell thicknesses.

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N. Liu et al. investigated. The shell thickness of 1.5 ML is optimal and the relative emission intensity of Eu ions is enhanced evidently along with distinct quenching of the broad defect emission of ZnSe host QDs. The increased fluorescent lifetime values of Eu ions and ZnSe nanocrystals indicate that ZnSe host materials act as an antenna and the highly efficient energy transfer realized from ZnSe QDs to Eu ions. Meanwhile, the energy transfer efficiency in ZnSe QDs is optimized when the doping concentration of Eu ions is 6%. The QDs with multi-color emission are feasible for applications in the fields of laser, light-emitting diodes and biological labels. Acknowledgments

Figure 9. Photoluminescence spectra of ZnSe quantum dots with different Eu dopant concentrations. Samples of a–c have been normalized at 510 nm.

1.5ML is the slowest. When the thickness of the ZnS shell is over 1.5 ML, the strain between ZnSe and ZnS is strengthened, which leads to formation of dislocations for low angle grain boundaries, thereby more defects appear, which are the radiative recombination sites within which the ZnS shell originates. The electrons are trapped due to the surface defects, which prevents the energy to be transferred to Eu ions. This phenomenon concerning ultrathin CdSe/ZnSe quantum wells has already been reported (34). Furthermore, the lifetime of Eu emission decreases with the shell thicker than 1.5 ML, but also longer than that of the 0.5 ML shell thickness. From the lifetime spectra of ZnSe:Eu/ZnS QDs we can see that the sensitization of Eu ions is enhanced when the ZnS shell is overgrown on the surface of ZnSe:Eu QDs. We also observed the PL variations of ZnSe:Eu/ZnS QDs with different doping concentration of Eu ions. Figure 9 shows the PL spectra of ZnSe QDs with identical concentrations of ZnSe (0.005 M) and different doping concentrations of Eu (2%, 4%, 6%, 8%). In the analysis of the sensitization mechanism of Eu ions, the intensity of the defect state emission (510 nm) of as-obtained samples (2%, 4%, 6% doping concentration) are normalized. The emission intensity of Eu (sharp band at 616 nm) is gradually elevated to the maximum with increasing doping concentrations of Eu (from 2% to 6%). In contrast, the emission intensity of ZnSe host material is decreased, revealing that more energy is efficiently transferred from the band edge of ZnSe nanocrystals to Eu ions in their vicinity. The energy transfers of ZnSe QDs doped with 2% and 4% are less efficient than that of ZnSe:6%Eu QDs, because the QDs with lower doping concentrations have fewer energy storage centers than that of the highly doped QDs (35). Increasing the concentration of Eu from 2% to 6% relative to Zn augments the PL emission intensity of 5D0 level transition, which also demonstrates the successful incorporation of Eu ions into ZnSe host materials (the unreacted precursors have been removed by washing). The PL intensity of Eu ions is decreased when the doping concentration of Eu is over 8%. The reason probably stems from the increased irradiative luminescent centers due to the segregation of more defects on the interface at the ZnSe core and ZnS shell (36) and concentration quenching effect of active Eu ions (37).

Conclusions The influences of different shell thicknesses and Eu doping concentrations in ZnSe/ZnS core–shell QDs on Eu sensitization enhancement by the energy transfer process were systematically

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This work was supported by NSF of China (nos. 61376004 and 61071008), and the State Key Program for Basic Research of China (no. 2013CB632101).

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Luminescence intensity enhanced sensitization energy transfer 2+

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shell quantum dots induced by efficient energy transfer.

Eu-doped ZnSe:/ZnS quantum dots (formed as ZnSe:Eu/ZnS QDs) were successfully synthesized by a two-step wet chemical method: nucleation doping and epi...
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