In Situ Crystallization Synthesis of CsPbBr3 Perovskite Quantum Dots

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Functional Inorganic Materials and Devices

In Situ Crystallization Synthesis of CsPbBr3 Perovskite Quantum Dots Embedded Glasses with Improved Stability for Solid-State-Lighting and Random Upconverted Lasing Shuo Yuan, Daqin Chen, Xinyue Li, Jiasong Zhong, and Xuhui Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05155 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018

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ACS Applied Materials & Interfaces

In Situ Crystallization Synthesis of CsPbBr3 Perovskite Quantum Dots Embedded Glasses with Improved Stability for Solid-State-Lighting and Random Upconverted Lasing Shuo Yuan1, Daqin Chen1,2*, Xinyue Li1, Jiasong Zhong1, Xuhui Xu3,* 1

College of Materials & Environmental Engineering, Hangzhou Dianzi University, Hangzhou, Zhejiang, 310018, P. R. China

2

College of Physics and Energy, Fujian Normal University, Fuzhou, 350117, P. R. China

3

College of Materials Science and Engineering, Kunming University of Science and Technology, Kunming, 650093, P. R. China Corresponding authors E-Mail: [email protected]; [email protected]

1

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Abstract All-inorganic cesium lead bromide CsPbBr3 perovskite quantum dots (QDs) are emerging as potential candidates for the applications in optoelectronic devices but suffer from poor long-term stability due to their high sensitivity to UV irradiation, heat and especially to moisture. Although great advances in improving stability of perovskite QDs have been achieved by surface modification or encapsulation in polymer and silica, they are not sufficiently refrained from external environment due to non-dense structures of these protective layers. In this work, in situ nanocrystallization strategy is developed to directly grow CsPbBr3 QDs among a specially designed TeO2-based glass matrix. As a result, QDs embedded glass shows typical bright green emission assigned to exciton recombination radiation and significant improvement of photon-/thermal stability and water resistance due to the effective protecting role of dense structural glass. Particularly, ~ 90% of emission intensity is even remained after immersing QDs embedded glass in water up to 120 h, enabling them to find promising applications in white-light-emitting device with superior color stability and low-threshold random upconverted laser under ambient air condition. Keywords perovskites; quantum dots; glass; white-light-emitting diode; random laser

2


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1. Introduction Recently, all-inorganic cesium lead bromide CsPbBr3 perovskite quantum dots (QDs) have aroused wide attentions owing to high photoluminescence quantum yield (PLQY, up to 90%) with surface self-passivation effect and narrow full width at half maximum (FWHM, ~20 nm).1-5 These emissive features enable them to find promising applications in optical and optoelectronic fields.6-12 To date, a variety number of wet-chemical methods,

including hot injection, ligand-assisted

supersaturated recrystallization, solvothermal reaction and even ultrasonication, have been developed to prepare perovskite QDs.1, 13-17 Nevertheless, one of major issues to obstruct the application of CsPbBr3 QDs is the long-term stability, especially moisture resistance, which has not been well addressed so far. Generally, CsPbBr3 QDs show strong ionic nature and high surface energy and will quickly degrade to their components when directly contacting with polar solvents such as water.18 To resolve this problem, several strategies have been adopted by researchers such as surface modification or encapsulation in polymers and inorganic matrix.19-28 For example, Manna et al. demonstrated that X-ray irradiation on the CsPbBr3 films can induce intermolecular C=C bonding of the organic ligands coated on the surface of QDs to enhance the stability;19 Fu et al prepared PVP-modified CsPbBr3 nanocrystals (NCs) and embedded them into microhemispheres (MHSs) of polystyrene matrix to prepare “water-resistant” NCs@MHSs hybrids as multicolor multiplexed optical coding agents;20 Wang et al and others successfully embedded CsPbBr3 QDs into a wider band gap Cs4PbBr6 matrix, enabling to improve thermal stability and operate at 3



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relatively high temperature.26, 27 Li et al prepared perovskite QD/silica composites by embedding QDs in silica spheres derived from tetramethyl orthosilicate (TMOS) in “waterless” toluene21 and similarly Zhang et al realized the coating of silica on the surface of QDs via slowly hydrolyzing organosilicon capping agent.22 Despite numerous advances in improving water resistance of perovskite QDs, they were not sufficiently protected from external environment probably due to non-dense structures of surface protective layer and instability of silica prepared by wet-chemical route.

Scheme 1 A glass crystallization strategy to fabricate QDs embedded glass; CsPbX3 crystal structure and luminescent photograph of QDs@glass powder under irradiation of UV lamp (365 nm) are also provided.

Herein, we develop a strategy to incorporate CsPbBr3 perovskite QDs into a more stabilized and tight medium, i.e. inorganic TeO2-based glass. The core technologies involve the design of appropriate glass composition and the in situ nanocrystallization of CsPbBr3 QDs from glass matrix (Scheme 1). In fact, Zn/Cd/Pb-based sulfide QDs embedded glasses have been previously fabricated to disperse QDs as the bulky materials.29-32 Recently, Wang and Xiang et al have tried to grow CsPbBr3 particles in phosphorosilicate and borosilicate glasses to find possible application in white-light-emitting diodes (wLEDs) but no detailed water resistance performance was studied.33-36 Additionally, the requirement of high-melting temperature (up to 1100 oC) for preparing precursor glass (PG) will result in remarkable volatilization of 4


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Br sources and reduce the amount of crystallized CsPbBr3 phase in glass, leading to weak luminescence and low absolute PLQY. In the present work, we report on the successful achievement of controllable CsPbBr3 QD crystallization in the specially designed low-melting TeO2-based glasses containing appropriate Cs/Pb/Br precursor sources for the first time. The as-prepared CsPbBr3 QDs embedded glasses (QDs@glass) exhibit high PLQY of ~70%, superior photo-/thermal-stability and excellent water resistance, enabling them to find practical applications in optoelectronic fields. As a proof-of-concept experiment, the present QDs@glass nanocomposites are demonstrated to be suitable to apply in warm wLEDs and random upconverted (UC) lasing. 2. Experimental Section Preparation of CsPbBr3 QDs@glass: The glass matrix was designed with the compositions of (55-85)TeO2-2Al2O3-14H3BO3-16ZnO-13Na2CO3 (mol%), and the added perovskite-related components were Cs2CO3-PbBr2-KBr and CsBr-PbBr2. The raw materials were well mixed and ground into powders, and melted in muffle at 550~950 ℃ for 30 min under an ambient atmosphere to achieve precursor glass. Then the QDs@glass was achieved through controlled in situ glass crystallization via heating at 200~400 ℃ for 2~12 h. The obtained specimens were optical polished or ground into powders for subsequent characterization and usage. Characterizations: X-ray diffraction (XRD) analysis was carried out to identify the phase structure of the as-prepared samples using a powder diffractometer (MiniFlex600 RIGAKU) with Cu Kα radiation (λ= 0.154 nm) operating at 40 kV. The 5



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actual composition of the glass was detected by X-ray photoelectron spectroscopy (XPS) using a VG Scientific ESCA Lab Mark II spectrometer equipped with two ultra-high vacuum 6 (UHV) chambers. All the binding energies were referenced to the C1s peak of the surface adventitious carbon at 284.8 eV. Fourier transform infrared (FTIR) spectra were measured via a Perkin-Elmer IR spectrometer using the KBr pellet technique. The Raman spectra in the range of 100-1000 cm-1 were determined by a LabRam HR Raman spectrometer operated with 633 nm as the excitation source. Microstructure observations of QDs@glass were carried out on a JEOL JEM-2010 transmission electron microscope (TEM) operated at 200 kV accelerating voltage. Scanning TEM (STEM) images and the related elemental mappings on the sample were taken on a FEI aberration-corrected Titan Cubed S-Twin transmission electron microscope operated on a high-angle annular dark-field (HAADF) mode. TEM and STEM specimens were prepared by directly drying a drop of a dilute ethanol solution dispersed with QD@glass pieces on the surface of a copper grid. Photoluminescence (PL) and PL excitation (PLE) spectra were recorded using an Edinburgh Instruments (Edinburgh, UK) FS5 spectrofluorometer equipped with a continuous (150 W) lamp. Two-dimensional excitation-emission mapping of QDs@glass was measured by continuously changing excitation wavelength with a fixed step of 1 nm, and the offset between excitation wavelength and emission one was set to be 30 nm to reduce scattering light. Time-resolved spectra of CsPbBr3 QDs@glass were detected on a fluorescent lifetime spectrometer (Edinburgh Instruments, LifeSpec-II) based on a time correlated single photon counting (TCSPC) technique under the excitation of 375 6


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nm picosecond laser. Internal PL quantum yield (PLQY), defined as the ratio of emitted photons to absorbed ones, was determined by a spectrofluorometer (FS5) equipped with a 15 cm integrating sphere and the xenon lamp as excitation source. Construction and measurements of QDs@glass-based wLEDs: The wLED devices were constructed by coupling the as-fabricated QDs@glass on the InGaN blue chip. Opaque silica gels were filled around the edges of the device to avoid the leakage of blue light. Green-emitting CsPbBr3 QDs@glass powder and red-emitting Eu2+: CaAlSiN3 phosphor were blended with silicone gel A/B in order to directly paint on the surface of InGaN blue chip. Photoelectric parameters, including spectral power and photon distribution of the total radiant flux of the emission spectra, color rendering index (CRI), correlated color temperature (CCT), luminous efficiency (LE), and Commission Internationale de L'Eclairage (CIE) chromaticity coordinates, were measured using a HAAS-2000 spectroradiometer (Everfine, P. R. China) equipped with an integrating sphere of 50 cm diameter under the forward bias of 10~100 mA. Design and measurements of QDs@glass-based random laser: For the random lasing measurement, a homemade micro-PL system (OlympusBX-52 microscope and a 20×0.8 NA objective lens) was used to focus 800 nm femtosecond laser beam to different area spots of QDs@glass. A Ti: sapphire femtosecond laser (Coherent Libra) integrated with an optical parametric amplifier (Coherent OPerA Solo), which generates femtosecond pulses (50 fs,1 kHz) was used as the excitation source. The QDs@glass located inside the low-temperature (77 K) chamber was excited by the laser spot. Upconversion emission observed from the QDs@glass was then collected 7



by the same objective lens when the angle of detection is 0°. The detection of other angles (30° and 60°) chooses another objective lens. The received optical signal emitted from the sample was either coupled to a conventional CCD camera for the recording of the near-field image or attached to a monochromator (Princeton SpectraPro 2750 integrated with a ProEM EMCCD camera with spectral resolution less than 0.1 nm) for spectroscopic analysis. 3. Results and Discussion (a)

(b)

JCPDS No. 18-0364

Precursor glass QDs@glass

122

PG 753

o

280 C/2h o

300 C/2h ♦ ♦ ♦♦

Intensity (a.u.)

o

250 C/2h

Intensity (a.u.)

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670 308

472

o

300 C/10h JCPDS No. 54-0752

10

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(c)

30

40 2θ (degree)

50

QDs

60

70 100

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-1

Raman Shift (cm )

(d)

d(200)=2.91Å

100 nm

(e)

glass matrix

(f)

10 nm Cs (g)

Pb (h)

Br

Figure 1 (a) XRD patterns of PG and QDs@glass obtained by glass crystallization at different temperatures and times. Bars represent standard diffraction data of cubic CsPbBr3 (JCPDS No. 54-0752) and monoclinic one (JCPDS No. 18-0364). (b) Raman spectra of PG and QDs@glass samples (λex=633 nm). (c) TEM and (d) HRTEM images of QDs@glass. (e) HAADF-STEM image with associated (f) Cs, (g) Pb and (h) Br elemental mapping for several precipitated particles distributed in glass (red rectangle region in (e)). 8


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After

exploring

tremendous

fundamental

glass

compositions

and

perovskite-related components, TeO2-based inorganic glasses are adopted as hosts, Cs2CO3-PbBr2-KBr (CPK) and CsBr-PbBr2 (CP) serve as the source of perovskites, as tabulated in Table S1 and S2, respectively. The melting temperature can be lowered down to 650 ℃ for 30 min to obtain transparent precursor glasses. Introducing CPK into low-content TeO2-based glass can induce intense green emission after melt-quenching (Table S1), indicating the formation of CsPbBr3 QDs inside glass via self-crystallization. Unfortunately, the main crystalline phases are NaBr and CsBr and no any diffraction peaks assigned to CsPbBr3 QDs are observed in XRD pattern (Figure S1), suggesting that the content of CsPbBr3 in glass is quite low. Further modifying TeO2 content can reduce the unwanted impurity phases but only produce hexagonal Cs4PbBr6 (JCPDS No. 73-2478) product after glass crystallization (Figure S2). As an alternative, CP components are used to substitute CPK in the following section. As tabulated in Table S2, the added content of CsBr-PbBr2 significantly affected the precipitated phases among glass matrix and with increase of CP content, the crystallized phase gradually changed from pure hexagonal Cs4PbBr6 to pure cubic CsPbBr3. Figure 1a shows XRD patterns of PG and QDs@glass samples obtained via glass crystallization at different temperatures/times using 10CsBr-20PbBr2 (mol%) as perovskite-related component. Evidently, the as-prepared PG is indeed amorphous structure and pure cubic CsPbBr3 phase (JCPDS No. 54-0752) is precipitated after glass crystallization (250-300oC/2h). In addition, further increasing crystallization 9



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time to 10 h will induce phase transition from cubic CsPbBr3 to monoclinic one (JCPDS No. 18-0364). X-ray photoelectron spectroscopy (XPS) measurements evidence the existence of Te, B, Al, Zn, O, Na, Cs, Pb and Br signals and the slight shift of Cs, Pb and Br peaks toward larger binding energy after glass crystallization probably attributes to the modification of ligand-fields surrounding these elements (Figure S3). Fourier transform infrared (FTIR) spectra (Figure S4) confirm that glass network structure mainly consists of [TeO4], [BO4] and [BO3] units, where the related stretching vibrations of Te-O and B-O bonds are clearly observed.37,38 As a supplement, Raman spectra of PG and QDs@glass samples were recorded to trace the structural variation after glass crystallization. As shown in Figure 1b, both PG and GC exhibit similar strong peaks at 753 cm-1, 670 cm-1 and 472 cm-1 assigned to stretching vibration of TeO4 trigonal bipyramid, stretching and bending vibrations of Te-O-Te group. Notably, two extra peaks at 308 cm-1 and 122 cm-1, attributing to the motion of Cs+ cations and the second-order phonon mode of the [PbBr6]4octahedron,39,40 are clearly discerned for QDs@glass sample, confirming the formation of CsPbBr3 particles among glass matrix. Transmission electron microscope (TEM) image of QDs@glass (Figure 1c) clearly demonstrates the homogeneous distribution of CsPbBr3 QDs with particle size of 6-10 nm among glass matrix. High-resolution

TEM

(HRTEM)

micrograph

(Figure

1d)

verifies

their

single-crystalline nature with high-crystallinity and distinctly resolved lattice fringes. High-angle annular dark-field (HAADF) scanning TEM (STEM) observation, which is sensitive to the atomic number (Z) difference in the product,41,42 is used to 10


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characterize the prepared QDs embedded glass. As exhibited in Figure 1e, owing to the large difference of atomic number between Cs/Pb/Br (Z=55/82/35) and Te/B/O (Z=52/5/8), the obvious contrast for the CsPbBr3 QDs (bright) and the glass matrix (dark) is distinctly discernable, evidencing the successful precipitation of QDs from the TeO2-based inorganic glass. Apparently, EDS elemental mappings verify the distribution of Cs, Pb and Br in the same particle regions, i.e., segregation of Cs/Pb/Br in the QDs (Figure 1f-1h). PL

Abs/PLE/PL intensity (a.u.)

(a)

(b)

PG o 250 C/2h o 280 C/2h o 300 C/2h o 300 C/10h

PLE

Absorption

QDs@glass 350

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(c)

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τave=5.5 ns

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Experimental data Fitted curve

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


300

400

500 280

R =0.99 Eb=42 meV

300

320

340

360

380

400

420

440

460

T (K)

Time (ns)

Figure 2 (a) Absorption, PL (λex=370 nm) and PLE (λem=522 nm) spectra of PG and QDs@glass samples. (b) Two-dimensional excitation-emission mapping of QDs@glass. (c) A typical time-resolved PL trace for QDs@glass and the corresponding fitted curve. (d) Integrated emission intensity as a function of temperature from RT to 453 K.

We further studied the optical properties of CsPbBr3 QDs embedded glass. The precursor glass is transparent and colorless and gradually turn to be translucent and light yellow after glass crystallization (Figure S5), indicating the successful growth of QDs among glass matrix. As evidenced in Figure 2a, no obvious emission signal was 11



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detected in photoluminescence (PL) spectrum for PG sample, while a narrow emission band located at ~522 nm with calculated FWHM of 22 nm (~110 meV) was observed for all the QDs@glass samples. Notably, no significant quantum confinement effect is found since the sizes of QDs (6~10 nm) in glass is close to theoretical Bohr diameter (7 nm).43 The optimal crystallization temperature/time is 280 oC/2h and the absolute PL quantum yields (PLQYs) of 60-70% were determined for the QDs@glass samples from different batches (Figure S6). The UV-vis absorption spectrum shows a band edge at 518 nm with a relative smaller Stokes shift of 18 meV and PL excitation (PLE) spectrum exhibits similar band-to-band absorption transition (Figure 2a), implying that the luminescence of CsPbBr3 QDs originates from direct exciton recombination.14 Two-dimensional excitation-emission mapping evidences that the position and shape of the emission band are independent on the excitation wavelengths (260~500 nm) (Figure 2b). These results confirm that the green emission is not from the defect-related emitting states but originated from exciton recombination within [PbBr6]4- octahedron (Scheme 1) for the present QDs@glass. Impressively, the synthesis of QDs@glass samples by this facile glass-preparing route can be easily scaled up, and about 10 g of products with a high synthetic productivity of 80% and highly efficient luminescence (Scheme 1, Figure S5) can be obtained for each batch. Figure 2c exhibits time-resolved decay curve of QDs@glass and the corresponding fitted curve. Owing to the non-single-exponential feature, the average decay lifetime can be evaluated according to the following expression44, 45 12


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τ ave = ∫ I ( t ) dt / I 0

(1)

where I(t) is the time-dependent emission intensity and I0 is the peak intensity. The determined lifetime value of 5.5 ns is in nanosecond scale, further confirming the intrinsic excitation recombination nature of QDs embedded in glass.1 Different to the case of colloidal CsPbBr3 QDs (8~9 ns) prepared by wet-chemical route,1,14 the present CsPbBr3 QDs are precipitated from glass through melt-quenching and subsequent heat-treatment. Therefore, some defects may be produced in QDs or on the interfaces between QDs and glass, leading to short lifetime of QD@glass. Additionally, temperature-dependent PL spectra of QDs@glass were recorded, as shown in Figure S7. The integrated emission intensity gradually decreases with elevation of temperature (Figure 2d). Accordingly, the important physical parameter of exciton binding energy can be estimated with the help of the following equation14,15

I (T ) =

I0 1 + Ae − Eb /( kBT )

(2) where I(T) and I0 are emission intensities at temperature T and 0 K, respectively, Eb is exciton binding energy, and kB is Boltzmann constant. The fitted curve is also plotted in Figure 2d and the evaluated exciton binding energy is 42 meV. Such experimental determined Eb value for the present QDs@glass is consistent with previously reported value of colloidal CsPbBr3 QDs (40-44 meV)14 but slightly larger than that of bulk CsPbBr3 (35 meV) probably owing to the effect quantum confinement.46, 47 Notably, the exciton binding energy is far higher than the thermal disturbance energy (26 meV) at room temperature (RT), enabling exciton formation and radiative recombination at 13



RT and ensuring highly efficient luminescence for the investigated QDs@glass. In a further experiment, the stability of the as-prepared QDs@glass, including photostability, water resistance and thermal stability, is systematically investigated. Firstly, QDs@glass was illuminated by UV lamp for different durations and the related PL spectra were recorded to evaluate photostability of CsPbBr3 QDs embedded in inorganic TeO2-based glass. As exhibited in Figure 3a, no significant change for both PL intensity and FWHM is detected owing to the well protecting role of dense structural glass on CsPbBr3 QDs. Secondly, QDs@glass was immersed into aqueous solution for different times (up to 45 days) and the corresponding PL spectra were detected (Figure S8). As shown in Figure 3b, remnant PL intensity only slightly decreases with elongation of immersing time in water, ~ 90% of initial emission intensity is remained after 120 h and even about 60% of PL intensity is still obtained for storing time up to 45 days. As seen in Figure 3c, QDs@glass in water retains intense green luminescence 28

(a)

Remnant PL

26

Light off

0.8

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24 23 22

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1.2

FWHM (nm)

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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21 0

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1.0

(d)

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QDs@glass

(e)

Colloidal QDs o 100 C cycle heating o 100 C cycle cooling o 150 C cycle heating o 150 C cycle cooling o 200 C cycle heating o 200 C cycle cooling

o

0.6 0.4

0.8

Intensity (a.u.)

100 C cycle heating o 100 C cycle cooling o 150 C cycle heating o 150 C cycle cooling o 200 C cycle heating o 200 C cycle cooling

0.8

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


0.6 0.4 0.2

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Figure 3 (a) Photostability test of QDs@glass illuminated with 365 nm UV lamp (20 W): remnant PL intensity and the variation of FWHM as a function of the irradiation time. (b) Water resistance test by directly immersing QDs@glass in aqueous solution: remnant PL intensity versus store time. As a comparison, the change of PL intensity of colloidal QDs in water is also provided in (b). (c) Luminescent photographs of QDs@glass in water with elongation of storing time (from left to right: 0, 1, 3, 7, 10, 15, 30, 45 days). Temperature-dependent PL intensities for (d) QDs@glass and (e) colloidal QDs via three heating/cooling cycles at 100, 150 and 200 oC, respectively.

over a period of 45 days. As a comparison, it is found that colloidal CsPbBr3 QDs are quickly decomposed and their emission intensity is only ~5% left after immersing in water for 2 hours (Figure S9, inset of Figure 3b). Therefore, it can be concluded that inorganic glass host is indeed beneficial to efficiently protect CsPbBr3 QDs from erosion of exterior environment and greatly improve their moisture resistance. To evaluate the thermal stability of CsPbBr3 QDs induced by incorporating them into glass host, we compared temperature-dependent PL intensity for QDs@glass and colloidal QDs via a heating/cooling (100, 150 and 200 oC) cycle procedure, as shown in Figure 3d and 3e. Notably, the integrated PL intensity for excitonic luminescence of CsPbBr3 QDs in glass was found to be slightly weakened after being heated up to 200 oC and then cooled down to RT (Figure 3d), which is totally different from the case of colloidal QDs whose PL was hardly detected after undergoing the same heating/cooling cycles (Figure 3e). PL intensity of QDs@glass can be retained about 60-70% of the original one after experiencing three heating/cooling cycles at 100, 150 15



and 200 °C, far better than that of colloidal counterparts (

In Situ Crystallization Synthesis of CsPbBr3 Perovskite Quantum Dots

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