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Silica-Coated Mn-Doped CsPb(Cl/Br)3 Inorganic Perovskite Quantum Dots: Exciton-to-Mn Energy Transfer and Blue-Excitable Solid-State Lighting Daqin Chen, Gaoliang Fang, and Xiao Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14471 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 1, 2017

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ACS Applied Materials & Interfaces

Silica-Coated Mn-Doped CsPb(Cl/Br)3 Inorganic Perovskite Quantum Dots: Exciton-to-Mn Energy Transfer and Blue-Excitable Solid-State Lighting Daqin Chen*, Gaoliang Fang and Xiao Chen College of Materials & Environmental Engineering, Hangzhou Dianzi University, Hangzhou, 310018, P. R. China ABSTRACT Tunability of emitting colors of perovskite quantum dots (PQDs) was generally realized via composition/size modulation. Due to their bandgap absorption and ionic crystal features, the mixing of multiple PQDs inevitably suffers from reabsorption and anion-exchange effects. Herein, we address these issues with high-content Mn2+-doped CsPbCl3 PQDs that can yield blue-excitable orange Mn2+ emission benefited from exciton-to-Mn energy transfer and Cl-to-Br anion exchange. Silica-coating was applied to improve air stability of PQDs, suppress the loss of Mn2+ and avoid anion-exchange between different PQDs. As a direct benefit of intense multi-color emissions from Mn2+-doped PQD@SiO2 solid phosphors, a prototype white light-emitting diode with excellent optical performance and superior light stability was constructed using green CsPbBr3@SiO2 and orange Mn: CsPb(Cl/Br)3@SiO2 composites as color converters, verifying their potential applications in optoelectronics field.

Keywords: quantum dots; energy transfer; luminescence; perovskite; LED

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ACS Paragon Plus Environment


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Introduction Recently, perovskite quantum dots (PQDs), showing broad chemical tunability, excellent charge transport properties and high photoluminescence quantum yields (PLQYs), have attracted great attention for their promising applications in optoelectronic field including photovoltaic, light emitting diode (LED) and laser.1-10 As for optical materials, tunable multi-color emission is one of the most important issues. Generally, the bandgaps of PQDs and the corresponding luminescence colors can be readily tuned over the entire visible spectral range of 400-780 nm via composition modulation and size modification (known as quantum-size effect).11-19 Additionally, halogen anion exchange was recently demonstrated to be an effective strategy to tune bandgap of PQDs without destruction of shapes and crystal structures of initial particles.20-23 For the construction of luminescent devices, it is common to integrate optical materials with diverse emissions to realize tunability of emitting colors. Considering their bandgap absorption/excitation characteristics, the mixing of multiple PQDs will inevitably suffer from detrimental reabsorption effect. Taking CsPbBrI2 red-emitting PQD as a typical example, its absorption/excitation region extending from ultraviolet to red overlaps with the emitting ranges of other blue CsPbClBr2, green CsPbBr3 and yellow CsPbBr2I PQDs (Figure S1). In addition, strong anion exchange effect was inevitable when mixing these PQDs, certainly limiting their practical applications.24, 25 As an alternative, energy transfer from semiconductor QDs to doped activators has been well exploited to expand the emitting colors of QDs.26-29 Importantly, the excitation region of dopants will be confined within bandgap of QDs and the reabsorption effect may be completely avoided through bandgap modification of QD sensitizers. Generally, efficient energy transfer from QDs to dopants can be expected when energy matching condition between them 2


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was satisfied. Recently, Mn2+ doped PQDs was successfully fabricated and effective Mn2+ doping was achieved by using appropriate manganese precursors.30-36 Bright Mn2+ orange emission was observed due to energy transfer from CsPbCl3 PQDs to Mn2+ activators; however, it was concluded that the substitution of Cl by Br/I induced energy mismatch between PQDs and Mn2+ and remarkably reduced Mn2+ luminescence, i.e., the effective excitation range was restricted to UV-violet short-wavelength region. As far as we know, current commercial phosphor-converted white LEDs are mainly driven by the InGaN blue-emitting chip.37-40 Therefore, it is highly desirable to design and synthesize blue-light excitable orange-emitting Mn2+-doped PQDs with the assistance of energy transfer from PQDs to Mn2+ dopants. In the present work, we re-examined the energy levels of Mn2+ in the CsPb(Cl/Br)3 PQDs and demonstrated that Mn2+: 4T1 emitting-state was actually within bandgap between valence band (VB) and conduction band (CB) of CsPb(Cl/Br)3 PQDs. Importantly, blue-excitable Mn2+ orange emission via energy transfer from CsPb(Cl/Br)3 PQDs to Mn2+ dopants was realized for the first time. It was found that efficient energy transfer from QPDs to Mn2+ was highly dependent on the Mn2+ doping content in the host and intense Mn2+ orange emission was observed under the excitation of blue-light only when Mn2+ content was high enough. Unfortunately, Mn2+ dopants will be expelled from PQD host with the elongation of storing time, especially for high-content Mn2+ doped sample, probably owing to the large difference of ionic radii between Pb2+ and Mn2+.Herein, silica-coating on the Mn2+-doped PQDs was adopted to suppress the loss of Mn2+ and improve air stability of PQDs. As for as we know, this is the first report demonstrating the protecting role of silica-coating to avoid the loss of Mn2+ activators from PQD host. Moreover, the anion exchange reaction between different PQD@SiO2 composites was greatly inhibited, being 3



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beneficial to construct PQDs-based white LEDs (WLEDs). Indeed, by combining green CsPbBr3@SiO2 and orange Mn2+-doped CsPb(Cl/Br)3@SiO2 color converters with a InGaN blue chip, LED devices with tunable color coordinates, correlated color temperature (CCT), color rendering index (CRI) and luminous efficiency (LE) of 40~60 lm W-1 were achieved, verifying their promising application in solid-state lighting.

Experimental Section Materials Cs2CO3 (Aladdin, 99.9%), octadecene (ODE, Aladdin, 90%), oleic acid (OA, Aldrich, 90%), oleylamine (OLA, Aladdin, 80-90%), PbCl2 (Macklin, 99.9%), PbBr2 (Macklin, 99%), MnCl2 (Aladdin, 99%), hexane (Aladdin, 99%), toluene (Aladdin, 99%), poly methyl methacrylate (PMMA, Macklin), tetramethyl orthosilicate (TMOS, Aladdin, 98%). All chemicals were directly used without further purification. Preparation of Cs-oleate Cs2CO3 (0.407 g, 1.25 mmol) was loaded into a 50 mL 3-neck flask along with ODE (18 mL) and OA (1.74 mL), dried for 1 h at 120 oC, and then heated under N2 to 150 oC until all Cs2CO3 were reacted. Notably, since Cs-oleate can precipitate out of ODE at room temperature, it has to be pre-heated to 100 oC before injection. Preparation of Mn2+-doped CsPbCl3 PQDs Taking Mn-to-Pb molar feeding ratio of 6/1 as a typical example, the synthesizing process was described as follows. PbCl2 (0.028 g, 0.1 mmol), MnCl2 (0.119 g, 0.6 mmol), ODE (5.0 mL), OLA (2 mL), and OA (2 mL) were loaded into a 50 ml 3-neck flask and heated to 120 oC under vacuum for 30 min. The temperature was increased to 170 oC and Cs-oleate (1 mL) was quickly injected and 4


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after 5s the solution was quickly cooled down to room temperature by immersion in a cold-water bath. Purification The Mn:CsPbCl3 QDs were extracted from the crude solution by centrifuging at 10000 rpm for 5 min to discard the supernatant containing unreacted precursor and by-products. Finally the precipitate were redispersed in toluene forming stable colloidal solutions for further characterization. Preparation of PbBr2 stock solution PbBr2 (0.294 g, 0.8 mmol), OA (1.6 ml), OLA (1.6 ml), and ODE (10 ml) were loaded into a 50 ml 3-neck flask and heated to 120 oC under vacuum for 30 min. The PbBr2 solution was stored at room temperature. Preparation of Mn: CsPb(Cl/Br)3 PQDs via anion exchange Mn: CsPbCl3 PQDs were dispersed in 24 mL hexane at room temperature. Next, We divide hexane solution equally into 24 portions and PbBr2 stock solution was added to each portion in 0.1 mL increments. After stirring for 30 min, no further shifting of spectral position was detected, indicating the finish of anion exchange. The products were separated from the solution via centrifugation, and redispersed in hexane solution for further XRD, TEM, absorption, PLE/PL and PLQY measurements. Preparation of PQD@SiO2 powders 100 μL TMOS was introduced into a 25 mL open flask containing 10 mL of the colloidal Mn: CsPb(Cl/Br)3 PQDs toluene solution. The open flask was placed at room temperature and in a relative humidity (RH) of 70% (Hangzhou, P. R. China) with continuous stirring. After stirring for 8 5



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h, the precipitates were collected through centrifugation at 10000 rpm for 10 min, and then were dried in vacuum to form PQD@SiO2 composite phosphors. Notably, the powders can be re-dispersed in non-polar organic solvents such as hexane and toluene, however, the solution is translucent owing to the easy aggregation of PQD@SiO2 composites. Preparation of PQD@SiO2-PMMA film 1.5 g PMMA solid particles were added into 25 mL flask containing 10 ml toluene solution and heated to 60 oC with vigorous stirring. After PMMA completely dissolves, PQD@SiO2 phosphors were added into the PMMA-toluene solution. With the vigorous stirring process, all the PQD@SiO2 were dispersed homogeneously in the PMMA-toluene solution. Then, the resultant mixture was dropped on a clear glass through a vacuuming procedure to form the PQD@SiO2-PMMA film. Construction of PQD@SiO2-based WLEDs WLED devices were constructed by directly coupling the as-fabricate green CsPbBr3@SiO2 and orange Mn: CsPb(Cl/Br)3@SiO2 phosphors on the InGaN blue chip. The green and orange phosphors with different ratios were homogenously dispersed in the mixture of A/B gels (1:1) and then dropped on the chip. Additionally, opaque silica gels were filled around the edges of device in order to avoid the leakage of blue light. Characterizations XRD patterns was recorded to identify crystal phase structure for the as-prepared Mn2+-doped CsPb(Cl/Br)3 PQDs using a powder diffractometer (MiniFlex600 RIGAKU) with CuKα radiation (λ=0.154 nm) operating at 40 kV. TEM observation was carried out on a JEOL JEM-2010 transmission electron microscope equipped with an energy dispersive X-ray (EDX) spectroscope 6


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at 200 kV accelerating voltage. Scanning TEM (STEM) images were performed on a FEI aberration-corrected Titan Cubed S-Twin transmission electron microscope operated on the high-angle annular dark-field (HAADF) mode. TEM specimen was prepared by directly drying a drop of a dilute toluene solution of PQDs on the surface of a copper grid. The actual chemical compositions of Mn2+-doped PQDs were determined by Inductively coupled plasma atomic emission spectroscopy (ICP-AES) technique using a Perkin-Elmer Optima 3300DV spectrometer. Fourier transform infrared spectra were recorded on a Nicolet 6700 FTIR spectrometer in the range of 4000~400 cm−1 using the KBr pellet technique. Absorption, PLE, PL and Mn2+ decay curves for the Mn2+-doped CsPb(Cl/Br)3 PQDs were recorded on an Edinburgh Instruments (EI) FS5 spectrofluorometer equipped with a continuous (150 W) and pulsed xenon lamps. Time-resolved spectra for exciton emission of PQDs were detected on a fluorescent lifetime spectrometer (Edinburgh Instruments, LifeSpec-II) based on a time correlated single photon counting technique under the excitation of 375 nm picosecond laser. Photoluminescence quantum yield (PLQY), defined as the ratio of emitted photons to absorbed ones, was determined by a spectrofluorometer (FS5) equipped with the xenon lamp as the excitation source (λex=365 nm) and an 15 cm integrating sphere. Electroluminescence (EL) spectra, Commission Internationale de L’Eclairage (CIE) chromaticity coordinates, color rendering index (CRI), correlated color temperature (CCT) and luminous efficiency (LE) of the designed PQD@SiO2-based

WLED

devices

were

recorded

and

determined

in

a

HAAS-2000

spectroradiometer (Everfine, HAAS-2000) under the operating current of 5~150 mA. All the experiments were carried out at room temperature.

Results and Discussion 7



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Similar to the cases reported previously,30, 31 we firstly prepared Mn2+-doped CsPbCl3 PQDs via a co-precipitation method by introducing MnCl2 precursor into PbCl2 solution followed by adding Cs-oleate to induce crystallization of product. As tabulated in Table S1, the optical properties of the as-prepared Mn2+-doped CsPbCl3 PQDs are highly dependent on the Mn-to-Pb molar feeding ratio. With increase of feeding ratio from 0 to 15, the emission color changes from weak violet to intense orange and finally to bright red, the exciton narrowband emission of CsPbCl3 PQDs shifts towards short-wavelength from 416 nm to 388 nm while the Mn2+ broadband emission gradually moves to long-wavelength from 591 nm to 621 nm (Table S1, Figure S2). Absorption spectra of Mn2+-doped CsPbCl3 PQDs exhibit similar blue-shift with increase of Mn-to-Pb feeding ratio (Figure S2). Notably, compared to strong band-to-band absorption transition, no obvious Mn2+ absorption bands are detected owing to its parity-forbidden d→d transition. Impressively, as evidenced in Figure S3, PLE spectra recorded by monitoring Mn2+ orange emission (600 nm) resemble the corresponding absorption spectra, suggesting the presence of energy transfer from PQDs to Mn2+ dopants.[31, 32] All these results indicate that Mn2+ dopants are successfully incorporated into CsPbCl3 PQDs and the actual Mn2+ content in PQDs increases as the Mn-to-Pb feeding ratio increases. Indeed, ICP-AES measurements determine the actual

compositions of

CsPb0.982Mn0.018Cl3, CsPb0.953Mn0.047Cl3, CsPb0.835Mn0.165Cl3, and

CsPb0.769Mn0.231Cl3 PQDs for the corresponding Mn-to-Pb feeding ratios of 2/1, 6/1, 10/1 and 15/1 (Table S1). As a consequence, the blue-shift of exciton emission is probably ascribed to the modification of PQD bandgap via the gradual substitution of Pb2+ by Mn2+ and the red-shift of Mn2+ emission is possibly attributed to both the adjustment of Mn2+ ligand-field due to lattice contraction and the Mn2+ reabsorption/emission effect. Notably, the PLQY increases from ~0.1% 8


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for pure CsPbCl3 PQDs to 34% for CsPb0.835Mn0.165Cl3 and then quickly decreases to 8% (CsPb0.769Mn0.231Cl3) with further increase of Mn2+ content in PQDs (Table S1), possibly ascribing to high-content Mn2+ concentration quenching. Evidently, an gradual decrease in decay lifetime of Mn2+ in millisecond scale was detected with increase of Mn-to-Pb feeding ratio (Figure S4), confirming the increase of Mn2+ dopants in PQDs and the presence of associated concentration quenching effect. On the other hand, it was reported that MnCl2 precursor was beneficial to incorporate Mn2+ emitting centers into PQDs.31 Therefore, it is possible to produce the Cs-Mn-Cl complex for the high Mn2+ content doped samples, which may also contribute to the quenching of Mn2+ emission. Microstructure characterizations of Mn2+-doped CsPbCl3 PQDs are shown in Figure S5 and Figure 1. X-ray diffraction (XRD) patterns of the as-synthesized PQDs are well coincident with cubic CsPbCl3 phase (JPCDS No. 75-0411) and increasing Mn2+ doping content induces slightly shift of diffraction peaks toward high-angle without any detectable impurity phase (Figure S5), ascribing to the gradual replacement of Pb2+ with larger ionic radii (r=0.133 nm, CN=6) by Mn2+ with smaller one (r=0.097nm, CN=6).41 Transmission electron microscope (TEM) observations on three representative CsPbCl3, CsPb0.953Mn0.047Cl3 and CsPb0.835Mn0.165Cl3 PQDs (Figure 1a-1c) verify that they are all cubic-shaped and monodisersed with sizes of 10-12 nm and the shape and size of PQDs are not significantly altered with increase of Mn2+ doping content. Selected area electron diffraction (SAED) pattern of PQDs (inset of Figure 1c) confirms their pure cubic structure and High-resolution TEM (HRTEM) micrograph of an individual PQD particle (Figure 1d) verifies its single-crystalline nature with high-crystallinity. The lattice fringes are distinctly resolved, and two typical d-spacings of 0.561 nm and 0.396 nm relevant to (100) and (110) planes 9



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of cubic perovskite phase are observed. Additionally, Cs, Pb, Cl and Mn signals are detected in the energy dispersive X-ray (EDX) spectrum (Figure 1e), further supporting the presence of Mn2+ in CsPbCl3 host. (a)

(b)

(c)

200 nm

200 nm

200 nm

(d)

(e) d(100)=0.561nm

d(110)=0.396nm

100

110

Figure 1 TEM images of (a) CsPbCl3, (b) CsPb0.953Mn0.047Cl3 and (c) CsPb0.835Mn0.165Cl3 PQDs. Insets are the corresponding PL photographs under the irradiation of UV lamp and SAED pattern. (d) HRTEM micrograph of an individual CsPb0.835Mn0.165Cl3 particle and its FFT pattern. (e) EDX spectrum of CsPb0.835Mn0.165Cl3 PQDs. C and Cu signals are originated from copper-supported carbon grid. So far, we have demonstrated the possibility of exciton-to-Mn energy transfer assisted Mn2+ orange-red luminescence. Unfortunately, the excitation wavelength is confined within the range from UV to violet for all the Mn2+-doped CsPbCl3 PQDs. To extend the excitation region for their practical application in blue-excitable solid-state lighting, Cl-to-Br anion exchange strategy is adopted to finely tune the bandgap of CsPbCl3 PQDs and the Mn2+ doping-content dependent energy transfer processes from hybrid CsPb(Cl/Br)3 PQDs to Mn2+ activators are systematically investigated. XRD patterns for a typical CsPb0.953Mn0.047Cl3 sample after anion exchange were recorded (Figure S6). Apparently, the crystal structure is retained and XRD peaks monotonically 10


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(a)

Mn: Pb=2:1

Br/Cl=0:1

(b) Mn: Pb=6:1

Br/Cl=0:1

Br/Cl=0.1:0.9

(c)

Mn: Pb=10:1

Br/Cl=0:1

Br/Cl=0.2:0.8

Br/Cl=0.2:0.8

Br/Cl=0.5:0.5

Br/Cl=0.5:0.5

Br/Cl=0.4:0.6 Br/Cl=0.45:0.55 Br/Cl=0.5:0.5 Br/Cl=0.6:0.4 Br/Cl=0.65:0.35

Br/Cl=0.6:0.4 Br/Cl=0.7:0.3

Br/Cl=0.8:0.2

Br/Cl=0.7:0.3

PL intensity (a.u.)

PL intensity (a.u.)

Br/Cl=0.35:0.7

PL intensity (a.u.)

Br/Cl=0.2:0.8

Br/Cl=0.6:0.4 Br/Cl=0.7:0.3

Br/Cl=0.8:0.2 Br/Cl=0.85:0.15

Br/Cl=0.85:0.15

Br/Cl=0.75:0.25 Br/Cl=0.85:0.15

400

500

600

Br/Cl=0.9:0.1

Br/Cl=0.9:0.1

400

700

500

600

700

400

500

600

700

Wavelength (nm)

Wavelength (nm)

Wavelength (nm)

(e)

(d)

Mn:Pb=2:1 λem=600nm

10

PLE intensity (a.u.)

Br/Cl increasing

Log[Mn/Exciton]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

0.1

0.01

Mn-to-Pb feeding ratio 2:1 6:1 10:1

Mn:Pb=6:1 λem=600nm

Br/Cl increasing Mn:Pb=10:1 λem=600nm Br/Cl increasing

400

410

420

430

440

450

460

470

480

490

300

350

400

(f)

450

500

550

Wavelength (nm)

Exciton emission wavelength (nm)

(h)

(g)

Figure 2 Evolution of PL spectra (λex=365 nm) for Mn2+ doped CsPbCl3 PQDs during progressive Branion exchange with different Mn-to-Pb feeding ratios: (a) 2:1, (b) 6:1 and (c) 10:1. (d) Mn-to-exciton emission peak intensity ratio versus exciton emission wavelength for three typical Mn2+-doped CsPb(Cl/Br)3 PQDs. (e) PLE spectra (λem=600 nm) for the CsPb0.982Mn0.018Cl3, CsPb0.953Mn0.047Cl3 and CsPb0.835Mn0.165Cl3 PQDs during progressive Br- anion exchange (Br/Cl ratio gradually increases from left to right). (f-h) Luminescent photographs of the corresponding PQDs under the irradiation of UV lamp. shift toward low-angle with increase of Br- content, as verified by the (200) peak in Figure S6, being in agreement with lattice expansion due to the replacement of Cl- ions (r=0.181 nm ) by larger Br- ones (r=0.196 nm).42 Similar result can also be found for the CsPb0.982Mn0.018Cl3 sample (Figure S7). Importantly, Cl-to-Br anion exchange will not significantly modify the size and shape

11


0Br 0.5Br 1.5Br 2.5Br 4.5Br 6.5Br 8.5Br 12.5Br


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of the final product (Figure S8). As shown in Figure 2a, red-shift of exciton emission from violet to cyan is observed as anion exchange is performed for the CsPb0.982Mn0.018Cl3 sample; however, the Mn2+ orange emission weakens and the Mn-to-exciton emission peak intensity ratio (EPIR) is remarkably reduced from 1.4 to 0.1 when the excition emission is changed from 410 nm to 460 nm (Figure 2d). Fortunately, with increase of Mn2+ dopants in PQDs, intense Mn2+ luminescence is remained (Figure 2b, 2c) and the corresponding Mn-to-exciton EPIRs can reach as high as 50% and 360% for the CsPb0.953Mn0.047Cl3 and CsPb0.835Mn0.165Cl3 PQDs with exciton emissions locating around 460 nm (Figure 2d), respectively. More importantly, anion exchange of Cl- by Br- indeed contributes to red-shift of Mn2+ excitation wavelength region from violet (~400 nm) to blue (~480 nm), as evidenced in Figure 2e, confirming the presence of energy transfer from CsPb(Cl/Br)3 QDs to Mn2+. As demonstrated in Figure S9, the excitation wavelength independent emissions of both quantum dots and Mn2+ dopants are evidenced in the excitation-emission two-dimensional mapping, certainly verifying that the realization of Mn2+ emission is originated from exciton-to-Mn energy transfer. Moreover, PL decay curves of Mn2+ dopants by monitoring 600 nm emission during progressive Br- anion exchange were recorded, as shown in Figure S10-S12. Evidently, a common feature, i.e., a gradual decrease in Mn2+ decay lifetime accompanied by the replacement of Cl- by Br-, was observed for all these three PQD samples, as tabulated in Table S2. For instance, the average lifetime monotonously decreases from 1451 µs to 327 µs with increase of Br/Cl ratio from 0:1 to 0.9: 0.1 for the PQD sample with Mn-to-Pb feeding ratio of 2:1. As above mentioned (Figure S4), Mn2+ lifetime will be prolonged when Mn2+ doping content in PQDs is reduced due to the suppression of Mn2+ concentration quenching. The Mn2+ contents before/after anion exchange were determined with the ICP-AES technique and no obvious 12


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change in Mn2+ content is found, confirming that the shorten lifetime of Mn2+ after Cl-to-Br anion exchange is not due to the loss/change of Mn2+ content but ascribed to the alteration of Mn2+ ligand-field from Cl- dominant octahedron to Br- dominant one. As demonstrated in Figure 2f-2h, the variation of emitting color for the Mn2+-doped PQDs by anion exchange is clearly discernible and the Mn2+ orange/red emission is retained for high-content Mn2+ doped samples. To better enable the comparison of color variation, the influence of Cl-to-Br anion exchange on color coordinates of CsPb0.982Mn0.018Cl3, CsPb0.953Mn0.047Cl3 and CsPb0.835Mn0.165Cl3 PQDs are labeled as black circles in the Commission International de L'Eclairage (CIE) chromaticity diagrams (Figure S13-S15). With the gradual substitution of Cl- by Br-, the emission color quickly changes from orange/red to cyan for the CsPb0.982Mn0.018Cl3 and CsPb0.953Mn0.047Cl3 PQDs (Figure S13, S14) while the emission color almost remains in the orange-magenta region for the CsPb0.835Mn0.165Cl3 PQDs (Figure S15) due to efficient energy transfer from QDs to Mn2+ dopants in the high-content Mn2+-doped sample.

Figure 3 Proposed energy level diagram for Mn2+ emitting center in a free-ion state and in a crystal field of cubic symmetry, energy band structure of CsPb(Cl/Br)3 PQDs (Br/Cl ratio gradual increases from left to right), and energy transfer mechanisms from PQDs to Mn2+ dopants. In generally, the energy levels of Mn2+ can be characterized by Tanabe-Sugano diagram 13



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(Figure S16) and are highly dependent on the crystal-field environment since the radiative transition of Mn2+ is originated from outmost electrons in the 3d5 state.43-45 As shown in Figure 3, the spin-orbit coupled 4G and 4D degenerate energy levels of Mn2+ free-ion state will be split into 4

T1(4G), 4T2(4G),, 4E, 4A1(4G), and 4T2(4D), 4E(4D) multiplet states in a crystal field of cubic symmetry,

respectively. PL and PLE spectra (Figure 2) evidenced that the orange emission assigned to Mn2+: 4

T1→6A1 transition was always detectable during anion exchange, indicating that both 6A1 ground

state and 4T1 excited state of Mn2+ are within bandgap between valence band (VB) and conduction band (CB) of CsPb(Cl/Br)3 PQDs. Therefore, as schematically illustrated in Figure 3, energy transfer from PQD to Mn2+ will definitely occur although some excited states of Mn2+ may be overlapped by CB with increase of Br/Cl ratio via anion exchange. Notably, the exciton emission of PQDs and Mn2+ d→d emitting emerged as competition. Increasing Mn2+ content will significantly enhance energy transfer probability from PQDs to Mn2+ dopants and Mn2+ luminescence will be dominant, especially in pure CsPbCl3 host. Cl-to-Br anion exchange will change Mn2+ doping host from pure CsPbCl3 to CsPb(Cl/Br)3 and remarkably weaken Mn2+ luminescence relative to exciton one. As we all know, the PLQY of CsPbBr3 is far higher than that of CsPbCl3,4 i.e., the radiative transition probability of CsPbBr3 is much higher than that of CsPbCl3. In this case, the excitons may prefer to recombine rather than transfer energy to Mn2+ emitting centers for the Mn2+-doped PQDs with high Br/Cl ratio. Moreover, the decrease of bandgap via anion exchange will make CB approach to Mn2+: 4T1(4G) emitting state, and some electrons in 4

T1(4G) emitting state can return to CB via thermal activation, which also has a detrimental

influence on Mn2+ luminescence. Therefore, it is necessary to keep a delicate balance between Mn2+ emitting and excitation wavelength tuning through appropriate substitution of Cl- by Br- in 14


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the Mn2+-doped CsPb(Cl/Br)3 PQDs.

(a)

0h

(b)

Light on

4h

8h

(c)

Figure 4 (a) Change of the appearance of toluene solutions containing TMOS (left), Mn:CsPbCl3 PQDs (middle) and both TMOS and Mn:CsPbCl3 PQDs (right) during the time period up to 8 h. (b) The corresponding luminescent photographs for these solutions under the irradiation of UV lamp. (c) Multi-color emissions of a series of silica-coated PQD phosphors under the excitation of UV lamp (from left to right: CsPb(Cl0.4Br0.6)3, CsPbBr3, CsPb0.835Mn0.165Cl3, CsPb0.835Mn0.165(Cl0.6Br0.4)3, CsPb0.835Mn0.165(Cl0.5Br0.5)3, CsPb0.835Mn0.165(Cl0.4Br0.6)3, CsPb0.835Mn0.165(Cl0.3Br0.7)3, and CsPb0.835Mn0.165(Cl0.05Br0.95)3). For practical application, the stability of Mn2+-doped PQDs is one of most important factors needing consideration.46-54 Unfortunately, Mn2+ dopants will be expelled from CsPb(Cl/Br)3 host with the elongation of storing time, especially for high-content Mn2+ doped sample, probably owing to the large difference of ionic radii between Pb2+ (r=0.133 nm) and Mn2+ (r=0.097nm). As demonstrated in Figure S17, Mn2+ orange/red emitting color remarkably diminishes after storing for 24 h. Additionally, PQDs will quickly decompose when exposing to ambient environment due 15



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to their strong ionic crystal feature.55, 56 Therefore, it is highly desirable to find a way to improve structural stability and optical performance of Mn2+-doped PQDs. Herein, we design a SiO2-coating strategy to protect Mn2+-doped PQDs against degradation and suppress the loss of Mn2+ from PQD host. Traditionally, tetraethyl orthosilicate (TEOS) has been widely applied as a precursor to grow SiO2 with the assistance of water, ethanol and ammonia,57, 58 which is actually not suitable for coating PQDs as they will be decomposed by water before the formation of SiO2. As an alternative, tetramethyl orthosilicate (TMOS) was demonstrated to enable the growth of SiO2 on the surface of PQDs without significant degradation of PQDs benefited from its low water consuming content and fast hydrolysis rate.52 As shown in Figure 4a, three groups of experiments were carried out, i.e., 10 mL of toluene solution containing 100 μL TMOS (left), 10 mL of colloidal Mn:CsPbCl3 PQDs toluene solution (middle) and 10 mL of colloidal Mn:CsPbCl3 PQDs toluene solution containing 100 μL TMOS (right) in the unsealed beakers were continuously stirred for 8 h at room temperature with the relative humidity (RH) of 70% (Hangzhou, P. R. China). Without the addition of Mn2+-doped PQDs, the solution is transparent and SiO2 cannot be quickly produced (Figure 4a, left). Additionally, the colloidal Mn:CsPbCl3 PQDs toluene solution shows no obvious change after stirring for 8 h (Figure 4a, middle). Impressively, simultaneous introduce of TMOS and Mn2+-doped PQDs into toluene solution will make the solution translucent after stirring for 4 h (Figure 4a, right), indicating that the addition of PQDs will promote the hydrolysis of TMOS and the formation of PQDs@SiO2 composites. As shown in Figure 4b, the homogeneous emitting color is retained after the growth of SiO2. After stirring for 8 h, the Mn2+-doped PQDs@SiO2 powders can be easily obtained via centrifugation and further dry at 60 oC for 12 h. Adopting a similar procedure, a series of SiO2-coated PQDs phosphors, including CsPb(Cl/Br)3 and 16


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Mn2+-doped CsPb(Cl/Br)3 with varied Br/Cl ratios, were successfully obtained and exhibited bright tunable emitting colors (Figure 4c). The corresponding PL and PLE spectra of PQD@SiO2, as presented in Figure S18, are similar to those of pure CsPb(Cl/Br)3 and Mn2+-doped CsPb(Cl/Br)3 colloidal PQDs, confirming that silica-coating will not induce significant alteration on their excitation and emission. The PLQYs for CsPbBr3@SiO2 and Mn2+-doped PQD@SiO2 samples reach as high as 85% and 30%, respectively. (a)

(b)

Mn: CsPbCl3@SiO2 Mn: CsPbCl3 CsPbCl3

SiO2

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

20

30

40

50

2θ (degree)

60

70

80

200 nm

(c)

(d)

200 nm

Figure 5 (a) XRD patterns of CsPbCl3, CsPb0.835Mn0.165Cl3, and CsPb0.835Mn0.165Cl3@SiO2 samples. (b) Bright-field and (c) dark-field TEM images of PQD@SiO2 composite. Inset of (b) is the corresponding SAED pattern. (d) EDX spectrum of PQD@SiO2 product, showing the presence of Si and O signals apart from Cs, Pb, Cl and Mn elements. XRD patterns of CsPbCl3, CsPb0.835Mn0.165Cl3 PQDs and the corresponding Mn2+-doped PQD@SiO2 composite are shown in Figure 5a. As expected, all the samples can be well indexed to a cubic CsPbCl3 phase (JPCDS No. 75-0411). Importantly, an extra broad peak at 2θ=15o-30o assigned to the amorphous structure of silica is observed for the composite product, verifying the successful hydrolysis of TMOS into SiO2. FTIR spectra for both PQD and PQD@SiO2 samples have been recorded (Figure S19). For the PQD@SiO2 sample, vibration of Si–O–Si at 1150 cm−1 and 17



symmetric stretching vibration of Si–O–Si at 800 cm-1, together with a weak band of Si–OH at 960 cm−1, appear, which are originated from the hydrolysis condensation of TMOS and indicate the formation of a cross-linked silica network. Importantly, the strong characteristic C–H stretching vibrations at 2800–3000 cm-1 in PQDs, derived from long hydrocarbon chain portion of OA and OLA on the surface of PQDs, become remarkably weakened in PQD@SiO2, verifying the successful coating of SiO2. As demonstrated by bright-field and dark-field TEM images (Figure 5b, 5c, Figure S20), PQDs were indeed wrapped inside SiO2. Especially, HAADF-STEM image (Figure S21), being sensitive to the atomic number (Z) difference in the sample, exhibits obvious contrast for the PQDs (bright) embedded in the silica matrix (dark), owing to the large difference of atomic number between Cs/Pb (Z=55/82) and Si (Z=14). The corresponding SAED pattern (inset of Figure 5b) shows typical diffraction rings of cubic phase and evidences the existence of Mn2+-doped PQDs among silica matrix. The Cs, Pb, Cl, Mn, Si and O elements are detected in the EDX spectrum of PQDs@SiO2 composite (Figure 5d). All the results confirm the formation of silica-coated Mn2+-doped PQDs. Mn: CsPbCl3

(a)

Mn: CsPbCl3@SiO2

Mn/Exciton

6.0

(d)

3.0 1.5 0.0

Mn/Exciton

0 day

4.5

2.5

0 (b)

2

4

6

8

10

12

2 days

Mn: CsPbCl2 Br Storage time (day) Mn: CsPbCl2 Br@ SiO 2

2.0 1.5 1.0

4 days

0.5 0.0 0

0.4

Mn/Exciton

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(c)

2

4

6

8

10

Storage time (day) Mn: CsPbClBr2

12

Mn: CsPbClBr2@SiO2

0.3

12 days

0.2 0.1 0.0 0

2

4

6

8

10

12

Storage time (day)

Figure 6 Dependence of Mn-to-exciton emission peak intensity ratio on storage time up to 12 18


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days for (a) Mn: CsPbCl3 and Mn: CsPbCl3@SiO2, (b) Mn: CsPbCl2Br and Mn: CsPbCl2Br@SiO2, (c) Mn: CsPbClBr2 and Mn: CsPbClBr2@SiO2 samples. (d) Photographs of orange Mn: CsPbCl3 PQD (left), Mn: CsPbCl3@SiO2 phosphor (middle) and Mn: CsPbCl3@PMMA film (right) under the excitation of UV lamp with elongation of storing time in air. In a further experiment, the stability of silica-coated Mn2+-doped PQDs was investigated. PL spectra of Mn2+-doped CsPb(Cl/Br)3 and the corresponding SiO2-coated PQD composites were recorded when these samples were exposed to air for different times. The normalized PL spectra, shown in Figure S22-S24, demonstrate that Mn2+ emissions relative to exciton ones for all the Mn2+-doped colloidal PQDs weakens quickly with increase of storing time (Figure S22a, S23a, S24a). Impressively, after the formation of the PQD@SiO2 composites, the Mn2+ luminescence is largely retained (Figure S22b, S23b, S24b). Specifically, after storing for 12 days, the orange emission is obviously quenched for CsPb0.953Mn0.047Cl3 PQDs while bright orange luminescence is clearly visible for CsPb0.953Mn0.047Cl3@SiO2 sample (insets of Figure S22); the pink luminescence is converted into blue one for CsPb0.953Mn0.047ClBr2 PQDs owing to the total vanishing of Mn2+ emission while it is evidently remained for CsPb0.953Mn0.047ClBr2@SiO2 sample (insets of Figure S24). The storage-time dependent Mn-to-exciton EPIR for the corresponding PQDs and PQD@SiO2 composites are determined and compared in Figure 6a-6c. Obviously, the EPIRs of colloidal PQD samples considerably drop with increase of storage time and even decrease to zero for the Br- anion exchange samples. As a comparison, Mn2+ luminescence for PQD@SiO2 composites is quite stable over time, showing only ~20% drop over 12 days. As seen in Figure 6d, the CsPb0.953Mn0.047Cl3 PQDs (left) and the corresponding PQDs embedded PMMA film (right) lost most brightness after exposing to air for 12 days, while the PQD@SiO2 composite remains intense orange emitting over a period of 12 days. As evidenced in Figure S25, intense luminescence for the Mn:CsPbCl3@SiO2 sample can be retained even after storing for 72 days in air. Therefore, it can be concluded that silica-coating is indeed beneficial to greatly improve air stability of PQDs 19



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and significantly suppress Mn2+ loss from PQD host. Moreover, another advantage for the silica-coated PQDs lies in the elimination of anion exchange between them. As a typical example, CsPbBr3@SiO2 and CsPb0.953Mn0.047Cl2Br@SiO2 dual-phase embedded transparent PMMA (Figure S26) was prepared. As evidenced in the corresponding PL spectrum (Figure S27), the green exciton emission from CsPbBr3 PQD and the blue exciton emission and orange Mn2+ one from CsPb0.953Mn0.047Cl2Br PQD are distinctively observed. Notably, we have further expanded the experiment for the I-to-Br anion exchange. With the substitution of Br- by I- in PQDs, the conduction band of PQD gradually shifts towards low-energy region and overlaps with Mn2+: 4T1 emitting-state. As a consequence, the exciton-to-Mn energy transfer and Mn2+ orange emission cannot be observed in the iodine-containing samples (Figure S28). Finally, as a proof-of-concept experiment, light-emitting devices were constructed by coupling green CsPbBr3@SiO2 and orange CsPb0.835Mn0.165(Cl0.5Br0.5)3@SiO2 phosphors with an InGaN chip to demonstrate their promising application in blue-light-excitable solid-state-lighting. Electroluminescence

(EL)

spectra

of

LED

chip,

CsPbBr3@SiO2-LED

and

CsPb0.835Mn0.165(Cl0.5Br0.5)3@SiO2-LED driven by 20 mA operation current show a blue emission band of chip (Figure 7a), a narrow green emission band of CsPbBr3@SiO2 (Figure 7b) and a broad orange emission band of CsPb0.835Mn0.165(Cl0.5Br0.5)3@SiO2 (Figure 7c), certainly confirming that the as-prepared Mn2+-doped PQD@SiO2 composites are blue-light-excitable and suitable as red

color-converter

for

phosphor-converted

WLED.

EL

spectra

of

the

designed

[email protected](Cl0.5Br0.5)3@SiO2-LEDs, presented in Figure 7d-7f, consist of blue, green and orange emissions, yielding pure white-light. As tabulated in Table S3, the modification 20


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of CsPbBr3@SiO2 to CsPb0.835Mn0.165(Cl0.5Br0.5)3@SiO2 ratio can finely tune color coordinates, CCT (a)

(d)

(b)

(c)

(e)

(f)

(h)

EL intensity (a.u.)

(g)

150 mA

5 mA

400

450

500

550

600

650

700

(i)

400

750

0h 6h 12h 18h 24h

EL intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


450

500

550

600

650

700

750

Wavelength (nm)

Figure 7 EL spectra of (a) commercial InGaN blue chip and the designed light-emitting devices by coupling blue chip with (b) green CsPbBr3@SiO2, (c) orange Mn2+-doped CsPb(Cl0.5Br0.5)3@SiO2 phosphors, and (d-f) their mixtures with different green-to-orange ratios (from left to right: 1:5, 1.5: 5, 2:5). Inset of (e) is the corresponding white-light-emitting photograph for the constructed PQD@SiO2-based device driven by 20 mA forward current. (g) Color triangle of blue LED, green and red PQD@SiO2 phosphors and CIE chromaticity coordinates of WLED devices. EL spectra of WLED as a function of (h) operating current and (i) different working time intervals. (5942 K~6636 K) and CRI (82.7~84.6) for the designed PQDs-based WLEDs. The LEs of devices reach as high as 20~40 lm W-1 and can be further improved by optimizing device structure, changing phosphor ratio, reducing light scattering and increasing chip efficiency. The color coordinates of blue-chip, silica-coated CsPbBr3 and Mn: CsPb(Cl/Br)3 PQDs are labeled in the

21



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Commission International de L'Eclairage (CIE) chromaticity diagram, constituting the fundamental tri-colors to enable convenient control of optoelectronic performance of LED devices (Figure 7g). Notably, the selectivity of blue-light-excitable orange Mn2+-doped PQD@SiO2 color converter is variable benefited from the easy composition tunability of PQD host via fine Cl-to-Br anion exchange, as shown in the ellipse region of Figure 7g. Furthermore, driving current dependent EL spectra (Figure 7h) show that both green and orange emissions of PQD@SiO2 gradually enhance as the forward current increases from 5 to 150 mA, confirming that these composite phosphors exhibit no obvious saturation toward the incident blue light. The light stability of WLED driven by 20 mA operating current was also examined under different working time intervals. Importantly, no remarkable degradation/change of EL spectra (Figure 7i) was found, and the CRI, CCT as well as color coordinates were also stable with prolonging working time to 24 h, verifying the superior color stability for the constructed PQD@SiO2-based WLED.

Conclusions In summary, efficient exciton-to-Mn energy transfer was realized in the Mn2+-doped CsPbCl3 PQDs with high Mn2+ doping content and Mn2+ orange emission can be effectively excited by incident blue light with the assistance of bandgap modification via Cl-to-Br anion exchange. Furthermore, silica layer was formed on the PQDs using TMOS as the hydrolysis precursor, being beneficial to greatly improve air stability of Mn2+-doped PQDs, significantly inhibit the loss of Mn2+ dopants from PQD host, and effectively eliminate of detrimental anion exchange among different types of PQDs. Finally, blue-light-excitable WLEDs, constructed by combining green CsPbBr3@SiO2 and orange Mn: CsPb(Cl/Br)3@SiO2 phosphors with an InGaN blue chip, exhibited tunable color coordinates, CCT and CRI and LE of 40~60 lm W-1 and superior light stability without 22


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remarkable degradation after working up to 24 h. There results indicate that the silica-coated Mn2+-doped CsPb(Cl/Br)3 PQDs may act as the alternative to traditional red phosphors applied in blue -light-excitable solid-state lighting without concerning adverse reabsorption effect.

ASSOCIATED CONTENT Supporting Information Table S1-S3, and Figure S1-S28. Extra experimental conditions, PLE, PL spectra, decay curves, XRD patterns, TEM images, FTIR spectra, CIE color coordinates, Tanabe–Sugano diagram (TSD) of Mn2+ as well as luminescent photographs mentioned in the text. This information is available free of charge via the Internet at http://pubs.acs.org/. Author Information Corresponding author *E-Mail: [email protected] (D. Q. Chen) Acknowledgements This research was supported by Zhejiang Provincial Natural Science Foundation of China (LR15E020001), National Natural Science Foundation of China (51572065, 61372025, 51372172), and 151 Talent's Projects in the Second Level of Zhejiang Province.

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