giant-shell quantum dots

Accepted Manuscript Bright and high-photostable inner-Mn-doped core/giant-shell quantum dots

Ruilin Xu, Bo Huang, Tian Wang, Yufen Yuan, Lei Zhang, Changgui Lu, Yiping Cui, Jiayu Zhang PII:

S0749-6036(17)30927-8

DOI:

10.1016/j.spmi.2017.07.017

Reference:

YSPMI 5127

To appear in:

Superlattices and Microstructures

Received Date:

14 April 2017

Revised Date:

29 June 2017

Accepted Date:

07 July 2017

Please cite this article as: Ruilin Xu, Bo Huang, Tian Wang, Yufen Yuan, Lei Zhang, Changgui Lu, Yiping Cui, Jiayu Zhang, Bright and high-photostable inner-Mn-doped core/giant-shell quantum dots, Superlattices and Microstructures (2017), doi: 10.1016/j.spmi.2017.07.017

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ACCEPTED MANUSCRIPT Bright and high-photostable inner-Mn-doped core/giant-shell quantum dots Ruilin Xua, Bo Huanga, Tian Wangb, Yufen Yuana, Lei Zhanga, Changgui Lua, Yiping Cuia, and Jiayu Zhanga,* aAdvanced

Photonic Center, Southeast University, Nanjing 210096, China

bDepartment

of Physics, Nanjing Normal University, Nanjing 210023, China

*E-mail: [email protected]. Abstract: Compared with quantum-dot (QD) displays, QD lighting possesses higher demand of photostability. Owing to high photostability from the combination of inner independent luminescence center and thick shell (≥ 15 monolayers (MLs)), inner-Mndoped core/giant-shell QDs with bright wide emission are a promising candidate for QD lighting. Aiming at bright and high-photostable giant QDs with low time cost (giant-shell growth time: within 20 min), we put forward the perfect combination strategy of hot-injection nucleation doping and optimized “flash” synthesis, going beyond the combination strategy of one-pot growth doping and typical “flash” synthesis, which led to an increase in photoluminescence (PL) quantum yield (QY) of giant Mn-doped CdS/ZnS QDs (ZnS shell: ~18 MLs) from ≤ 20% to 40%. The PLQY was enhanced to 45% by light annealing. Using traditional LED as the reference, these simply-encapsulated QDs can exhibit the high photostability, throwing light of the application of these inner-Mn-doped core/giant-shell QDs even for QD lighting. Keywords: quantum dots, photostability, nucleation doping, giant shell, “flash” synthesis.

ACCEPTED MANUSCRIPT 1. Introduction Colloidal quantum dots (QDs) possess many merits, such as solution processability, size-tunable emission wavelengths, narrow emission linewidths and so on [1–3], but the photostability is the key factor for their practical applications [3–6]. For QD displays, the most important issue is improving photostability, which will also be tested in the practical application. But for QD lighting, much higher demand of photostability must be met, which puts forward the requirement of high-photostable QDs for further encapsulation. On the one hand, the emission from Mn2+ dopants, capturing the energy of photo-generated excitons rapidly and then functioning as independent luminescence centers, usually exhibits less sensitivity to traps than exciton emission, which means Mn2+ emission possesses the inherent photostability [7–9]. On the other hand, giant shell with wide bandgap can significantly improve QDs’ photostability by decreasing the wave-function overlap of exciton (or luminescence center) and surface traps [3–6]. By the way, the high photostability of YAG:Ce yellow phosphors is partly attributed to the particle size in micron scale [10– 12], which means most of luminescence centers are away from surfaces. Accordingly, inner-Mn-doped core/giant-shell QDs are a promising candidate for QD lighting. High-crystallinity giant shells can be prepared within 20 min by the optimized “flash” synthesis at high temperature of 320 °C [6,13], which also promotes the diffusion of Mn2+ dopants. Usually, besides cation exchange, there are two doping strategies: growth doping [6,14,15] and nucleation doping [15–17]. Inner-Mn-doped cores can be easily prepared by the nucleation-doping strategy, which matches well with the

ACCEPTED MANUSCRIPT optimized “flash” synthesis in favor of further diffusion of Mn2+ dopants. Additionally, multi-injection of precursor solution at slower speed is beneficial to narrow the size distribution, to decrease the defects, and to break through the size limitation of typical “flash” synthesis (diameter: ~17 nm). For the combination strategy

of

one-pot

growth

doping

and

typical

“flash”

synthesis,

the

photoluminescence (PL) quantum yield (QY) of giant CdS/ZnS QDs (ZnS shell: ~18 MLs) were significantly decreased to ≤ 20%. While for the perfect combination strategy of hot-injection nucleation doping and optimized “flash” synthesis, giant CdS/ZnS QDs (ZnS shell: ~18 MLs) exhibit the relatively high PLQY of 40%. And the PLQY was enhanced to 45% by light annealing. These PMMA-coated giant Mndoped QDs exhibit high photostability when encapsulated on a LED for stability testing, which may promote practical applications of inner-Mn-doped core/giant-shell QDs for QD lighting. 2. Experiment section Following the method of Yang et al., Mn-doped CdS QDs were synthesized by the nucleation doping [17]. In order not to introduce additional local strain derived from the lattice mismatch between MnS and CdS, an interface buffer layer (~1 monolayer (ML)) was introduced for the QD growth by using a designed structure of MnS/ZnS/CdS (NZn ≈ 2 NMn ≈ 0.1 NCd; also called CdS:Mn QDs) [17]. Usage of Cd precursor and growth-time were controlled to obtain the nearly same first-exciton absorption (Abs) peak at ~450 nm. The QDs were washed by precipitating ~3 times with acetone and finally were redispersed in small amounts of ODE for the next

ACCEPTED MANUSCRIPT “flash” synthesis. The strategy of optimized “flash” synthesis was based on the typical “flash” synthesis [6,13]. 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, the temperature was then decreased to 320 °C. At 320 °C, a solution containing 80 nmol of seed QDs was injected. Unlike the typical “flash” synthesis, the precursor solution was slowly dropped into the flask after a solution containing only seed cores was injected firstly. When the temperature returned to 320 °C, the S precursor solution was slowly dropped within ~5 min, followed by annealing for ~10 min. Finally the reaction was stopped by natural cooling to room temperature. The QDs were washed by precipitating ~3 times with acetone and finally were redispersed in toluene or hexane. The

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 one Tecnai G2 Transmission Electron Microscope and another JEM-2100 transmission electron microscope. X-ray diffraction (XRD) spectra were recorded on an Ultima IV X-ray diffractometer. The image of time-resolved PL spectra was obtained by a streak camera (Hamamatsu C5680).

ACCEPTED MANUSCRIPT 3. Results and discussion Figure 1a shows the TEM image of Mn-doped CdS cores with the average diameter of ~4 nm, prepared by hot-injection nucleation-doping strategy. The TEM image of Fig. 1b shows giant Mn-doped CdS/ZnS QDs with the average diameter of ~21 nm, obtained by the optimized “flash” synthesis. For the typical “flash” synthesis, a solution containing both seed cores and S precursors is swiftly injected into the solution containing cation precursors in the flask, which leads to a wide size distribution and a size limitation (average diameter: ~17 nm) [6,13]. To narrow the size distribution and to break through the size limitation, the single-injection was changed into the multi-injection. As shown in Fig. 1b, as-prepared QDs possess a large average diameter of ~21 nm, but the size distribution remains relatively narrow. The inset of Fig. 1(b) shows the high-resolution TEM image of one QD, indicating the high-crystallinity of these QDs prepared by the optimized “flash” synthesis. Figure 2 shows the PL spectra of Mn-doped seed cores at different excitation wavelengths. The PL spectra include two components: Mn2+ emission (~580 nm) and surface state emission (~640 nm). At shorter excitation wavelengths (≤ 400 nm), the PL spectra are mainly attributed to Mn2+ emission, while at longer excitation wavelengths (≥ 425 nm), the PL spectra are mainly attributed to surface state emission. This phenomenon suggests spatial heterogeneity, originating from the growth process by using a designed structure of MnS/ZnS/CdS (shown in the inset of Fig. 2). The thin ZnS layer (~1 ML) was introduced as an interlayer to reduce the lattice mismatch between MnS and CdS [17], avoiding the introduction of additional

ACCEPTED MANUSCRIPT strain in QDs prepared by the nucleation doping. Due to the spatial heterogeneity, an excitation light with a longer wavelength excites the outer shell of QDs, which decreases the exciton-Mn2+ energy transfer (ET) rate [18] but increases the exciton trapping rate by surface traps, leading to less Mn2+ emission but more surface state emission. By the way, one-pot growth doping is a facile method to prepare Mn-doped CdS QDs, but Mn2+ dopants are mainly distributed in the outer layer of seed cores prepared by this strategy, resulting from the lattice ejection [14]. These QDs usually exhibit almost no PL, because Mn2+ dopants in the QDs without good crystallinity are close to abundant surface defects (excessive S2+) in the QDs. The crystallinity of seed cores will be improved in the high-temperature “flash” synthesis of giant shells, while the further diffusion of Mn2+ dopants from the outer cores will probably lower the exciton-Mn2+ ET rate [6,18], and then will decrease the PLQY. But for the Mn-doped seed cores prepared by hot-injection nucleation-doping strategy, they usually possess higher crystallinity and narrower size distribution; and the requirement of diffusions of inner Mn2+ ions well matches the high-temperature “flash” synthesis in favor of further Mn2+ diffusion. Accordingly, the perfect combination of nucleation doping and optimized “flash” synthesis usually results in a considerable PLQY. Figure 3(a) shows evolutions of Abs and PL spectra of Mn-doped QDs with ZnSshell thickness. With the growth of giant ZnS shell, the Abs-spectrum exhibits a rapid increase in short wavelengths (≤ 360 nm), and the first exciton Abs-peak of CdS cores was widened and blueshifted at some extent, resulting from the alloying at the core-

ACCEPTED MANUSCRIPT shell interface. Meanwhile, with the increasing strain, the PL peak of Mn2+ emission was redshifted from ~580 nm to ~620 nm. The component of surface state emission from CdS was suppressed by ZnS capping combined with subsequent thermal annealing, leading to more symmetrical PL spectra. The thicker ZnS shell can significantly improve the QDs’ photostability, while it also leads to a greater strain, resulting in the redshift of PL peak. This feature favors the acquisition of highphotostable QDs for warm white LEDs. Additionally, for the Mn-doped CdS cores, the PLQY is sensitive to the Mn2+ distribution and surface ligands, and then the acquisition of high PLQY needs a precise control. While the optimized “flash” synthesis is in favor of obtaining the core/shell QDs with high exciton-Mn2+ ET rate and high-crystallinity, resulting in facile acquisition of high PLQY. For the giant Mn-doped CdS/ZnS QDs with ZnS shell of ~12 MLs, the PLQY can be as high as ~60%. Figure 3(b) shows a streak camera image of time-resolved PL spectra of these giant Mn-doped CdS/ZnS QDs, exhibiting a long PL lifetime in the timescale of milliscond (ms) [6,14,19]. Such long lifetime of Mn2+ emission is generally longer than that of the band-edge or defectrelated emission of host, and that of biological background fluorescence, providing good opportunities to eliminate background fluorescence for biosensing and bioimaging [19,20]. And for the chemo/biosensing and bioimaging, the stability of these bright QDs with long PL lifetime is high enough to make them as an ideal candidate. Compared with QD displays, QD lighting possesses higher demand of

ACCEPTED MANUSCRIPT photostability. Accordingly, in order to meet the higher demand of photostability for QD lighting, a thicker shell is more favorable [3–6]. But the PLQY of core/shell QDs usually began to drop off obviously with the increasing shell thickness of ≥ 15 MLs. First, the electron and hole have a higher probability of leakage into the thicker giant shell. Second, more defects generate in the thicker giant shell. The decrease in the PLQY is attributed to the integrated effect of these two factors. XRD patterns of Fig. 4 show certain phase transition of giant Mn-doped CdS/ZnS QDs with the ZnS-shell thickness increasing from ~12 MLs to ~25 MLs. For the QDs with ZnS shell of ~12 MLs, zinc-blende (ZB) phase dominates in the mixed phase; while for the QDs with ZnS shell of ~25 MLs, wurtzite (WZ) phase dominates in the mixed phase. Because the band gap of WZ ZnS is ~50 meV greater than that of ZB ZnS [21,22], such transition suppresses the leakage of electron and hole into the outer shell at certain extent. Meanwhile, the generation of more defects is unavoidable due to the combination of phase transition and growth of additional thicker shell. For the QDs with giant ZnS shell of ~25 MLs, the PLQY is reduced to ≤ 25% due to the comprehensive effects of above-mentioned factors. While for the QDs with giant ZnS shell of ~18 MLs, the PLQY remains as high as 40%, and it can also be enhanced to 45% by light annealing. The good combination of high photostability and high PLQY makes the giant QDs with ZnS shell of ~18 MLs are the best candidate for QD lighting in this research. Figure 5a shows the photo of giant CdS/ZnS QDs with the giant ZnS shell of ~18 MLs (PL peak: 616 nm) in n-hexane under the UV light. Fortunately, the giant shell

ACCEPTED MANUSCRIPT not only improves the photostability of QDs, but also favors the acquisition of desired warm light [23–25]. The photostability is the most critical factor for practical application in QD lighting. To verify the photostability of these QDs, these PMMAcoated QDs were directly encapsulated on the surface of a violet LED for photostability testing, shown in Fig. 5b. The PL spectra of Fig. 5c show dual emission: violet light emission from the LED chip and Mn2+ emission from these giant QDs. To exclude the effects of some unstable factors, the PL spectra were normalized by the PL peak of violet light from the LED chip with the verified high photostability. Obviously, Figure 5c roughly shows the high photostability of these giant QDs. The detailed evolution of PL of the Mn2+ emission is shown in Fig. 5d. At the beginning (within ~5 hours), the PLQY is enhanced from 40% of the fresh QDs to 45% due to light annealing [26–28], which may improve the crystallinity and surface smoothing of the fresh QDs. Afterwards, the PL slightly decreased and then it became stable during the continuous exposure to violet light for ~10 hours. Fortunately, the PL can recover again after an interruption of exposure, showing a good reversibility. Overall, using traditional LED as the reference, these simplyencapsulated QDs can exhibit the high photostability. Compared to the giant Mndoped QDs with ZnS shell of 14 MLs in our previous study [6], these giant Mn-doped QDs with ZnS shell of ~18 MLs exhibited higher photostability when irradiated at the close light power density (~1.5 W/cm2), throwing light of application of these innerMn-doped core/giant-shell QDs even for QD lighting. Undoubtedly, inner-Mn-doped core/giant-shell QDs are one of the most promising candidates for the remote-type

ACCEPTED MANUSCRIPT QD-LEDs [6, 29–30]. 4. Conclusions

Aiming at bright and high-photostable QDs with low time cost, we put forward the combination strategy of hot-injection nucleation doping and optimized “flash” synthesis. Compared to Mn-doped cores prepared by the one-pot growth-doping strategy, the Mn-doped cores with higher-crystallinity and narrower size distribution, prepared by the hot-injection nucleation-doping strategy, possess the inner Mn2+ distribution, which are suitable for further diffusion and are more compatible with the next optimized “flash” synthesis. Compared to giant QDs prepared by the typical “flash” synthesis, giant QDs prepared by the optimized “flash” synthesis can break through the size limitation and possess the narrower size distribution. Accordingly, the perfect match between hot-injection nucleation doping and optimized “flash” synthesis leads to the high PLQY. Even for the giant Mn-doped CdS/ZnS QDs with ZnS shell of ~18 MLs, the PLQY can be as high as 40%, and can also be enhanced to 45% by light annealing. These PMMA-coated giant Mn-doped QDs exhibit high photostability when encapsulated on a LED for stability testing, which throws light of applications of inner-Mn-doped core/giant-shell QDs even for QD lighting. Looking further ahead, inner-Metal-doped core/giant-shell QDs are one of the most promising candidates for QD lighting.

Acknowledgments This work was supported by the National Basic Research Program of China (973

ACCEPTED MANUSCRIPT Program, 2015CB352002), the Science and Technology Department of JiangSu Province (BE2016021) and the Fundamental Research Funds for the Central Universities.

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ACCEPTED MANUSCRIPT Figure Captions Fig. 1. TEM images of Mn-doped CdS cores with the diameter of ~4 nm (a) and giant CdS/ZnS QDs with the diameter of ~21 nm (b). Inset of Fig. 1(b): High-Resolution TEM image of one giant Mn-doped CdS/ZnS QD. Fig. 2. PL spectra of Mn-doped seed cores at different excitation wavelengths. The inset image shows the seed core with a designed structure of MnS/ZnS/CdS (also called CdS:Mn QDs). The thin ZnS interlayer (~1 ML) was introduced to reduce the lattice mismatch between MnS and CdS, avoiding the introduction of additional strain in QDs prepared by the nucleation doping. Fig. 3. Evolutions of Abs and PL spectra of Mn-doped QDs with ZnS-shell thickness (a) and a streak camera image of time-resolved PL spectra of giant Mn-doped CdS/ZnS QDs (b). Fig. 4. XRD patterns of the giant Mn-doped CdS/ZnS QDs with ZnS shell of ~12 MLs and ~25 MLs respectively. Fig. 5. (a) Giant Mn-doped CdS/ZnS NCs (ZnS shell: ~18 MLs) in n-hexane under the UV light. (b) A lighted LED-lamp fabricated using a commercial violet-LED chip combined with these PMMA-coated NCs under the rated power (0.08 W). (c) Evolution of normalized PL spectrum of the LED-lamp operated under the rated power. (d) Evolution of PL intensity of Mn2+ emission; note: for the time variable, the detailed time of interruption of exposure (~10 hours) is not included.

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ACCEPTED MANUSCRIPT Highlights 1. The structure of inner-Mn-doped core/giant-shell favors the acquisition of high photostability. 2. Aiming at bright and high-photostable giant quantum dots (QDs) with low time cost, we put forward the perfect combination strategy of hot-injection nucleation doping and optimized “flash” synthesis. 3. Inner-Metal-doped core/giant-shell QDs are one of the most promising candidates for QD lighting.