Quantum Dots

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Full-Spectral Fine-Tuning Visible Emissions from Cation Hybrid Cs1-mFAmPbX3 (X=Cl, Br and I, 0#m#1) Quantum Dots Daqin Chen, Xiao Chen, Zhongyi Wan, and Gaoliang Fang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 01 Jun 2017 Downloaded from http://pubs.acs.org on June 1, 2017

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ACS Applied Materials & Interfaces

Full-Spectral Fine-Tuning Visible Emissions from Cation Hybrid Cs1-mFAmPbX3 (X=Cl, Br and I, 0≤ ≤m≤ ≤1) Quantum Dots

Daqin Chen*, Xiao Chen, Zhongyi Wan, Gaoliang Fang College of Materials & Environmental Engineering, Hangzhou Dianzi University, Hangzhou, 310018, China

Abstract Full color visible emissions are particularly crucial for applications in displays and lightings. In this work, we developed a facile room temperature ligand-assisted supersaturated recrystallization synthesis of monodisperse, cubic structure Cs1-mFAmPbX3 (X=Cl, Br and I or their mixture of Cl/Br and Br/I, 0≤m≤1) hybrid perovskite quantum dots. Impressively, cation substitution of Cs+ by FA+ was beneficial to finely tune band gap and exciton recombination kinetics, improve structural stability and raise absolute quantum yields up to 85%. With further assistant of anion replacement, full-spectral visible emissions in wavelength range of 450-750 nm, narrow full-width at half-maxima and wide color gamut encompassing 130% of NTSC TV color standard were achieved. Finally, Cs1-mFAmPbX3-polymer films retained multi-color luminescence are prepared and a prototype white light-emitting diode device was constructed using green Cs0.1FA0.9PbBr3 and red Cs0.1FA0.9Br1.5I1.5 quantum dots as color converters, certainly suggesting their potential applications in optoelectronics field.

Keywords: perovskite, quantum dots, optical materials, luminescence, LED

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ACS Paragon Plus Environment


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Introduction Currently, metal-halide perovskite semiconductor nanocrystals (NCs), also called them perovskite quantum dots (QDs), have been studied extensively as next generation optoelectronic materials because of unique optical properties, such as wide color gamut, high luminous efficiency as well as narrow spectral lines.1-4 Compared to their bulk counterparts, the perovskite QDs with remarkable quantum confinement effect and enhanced optical performance possess potential applications in many fields including biolable, photocatalysis, solar cell, lasing, lighting and display.5-10 To date, QDs already enter into TV industry and infiltrate into other industrial chain at an alarming rate, and the most common type of QDs is cadmium or zinc based chalcogenides, which have been dominant in the field of QDs for decades.11-14 However, recent reported semiconducting metal-halides with perovskite structure show strong competitiveness. On this hand, colloidal MAPbX3 (MA=CH3NH3, X =Cl, Br and I) QDs, CsPbX3 inorganic perovskite QDs (IPQDs) as well as other one-dimensional perovskite nanowires and two-dimensional perovskite nanosheets with photoluminescence quantum yields (PLQYs) of 50~90%, spectrally narrow and broadly tunable emissions are highly concerning and encouraging.2,15-23 Recently, a new type of hybrid organic−inorganic lead halide perovskite QDs, FAPbBr3 (FA=CH(NH2)2, formamidinium), were successfully fabricated by a hot-inject (HI) route. These FAPbBr3 QDs exhibited bright PL with adjustable spectral wavelength in the green emission range via controlling their sizes,24,25 overcoming the difficulties faced by CsPbBr3 NCs in achieving bright and stable emission at 530-535 2


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nm. In addition, surface self-passivation, high PLQY(~85%), narrow full width at half maximum (FWHM) and high stability make this organic−inorganic perovskite family promising application in both optical and optoelectronic fields. Notably, although PL can be tuned in a range from 470 nm to 535 nm via size effects, the emission range of QDs is not large enough and NH4Pb2Br5 impurity (typically amounting to 5-10%) is usually detected on account of FA+ thermal decomposition during high-temperature reaction. Herein, we reported for the first time the synthesis of cubic structure Cs1-mFAmPbX3 (X=Cl, Br, I, or their mixture, 0≤m≤1) cation hybrid perovskite QDs (HPQDs) and investigated their full-spectral fine-tuning visible emissions via simultaneous cation and anion substitution. These HPQDs were fabricated by a ligand-assisted supersaturated recrystallization (LASR) route at room temperature (RT), accomplishing in a few seconds by transferring Cs+, FA+, Pb2+, and X− precursor ions from soluble solvent to insoluble one without concerning FA+ thermal decomposition. In fact, similar LASR technique has been previously adopted to produce brightly luminescent colloidal CH3NH3PbX3 organometallic perovskite QDs as well as CsPbX3 inorganic perovskite QDs by Zhong and Zeng groups.15,19 This simple strategy is beneficial to get rid of high-temperature heating, inert gas, as well as elaborated injection conditions and achieve QDs with controllable shapes and high PLQYs. 15,19,26

Experimental section Materials and chemicals Lead chloride (PbCl2, aladdin, 99.99%), lead bromide (PbBr2, Macklin, 99.0%), 3



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lead iodide (PbI2, aladdin, 99.9%), cesium bromide (CsBr, Macklin, 99.5%), formamidinium bromide (FABr, Xi'an Polymer Light Technology Corp., 99.5%), oleic acid (OA, Aldrich, 90%), oleylamine (OM, Aldrich, 80–90%), dimethylformamide (DMF, Macklin, 99.5%), dimethyl sulfoxide (DMSO, Macklin, 99.5%) and toluene (C7H8, Macklin, 99.0%) were used as received without further purification. Synthesis of Cs1-mFAmPbX3 QDs In a typical synthesis of Cs0.5FA0.5PbBr3, PbBr2 (0.2 mmol), CsBr (0.1 mmol) and FABr (0.1 mmol) were dissolved in DMF or DMSO solution (5 mL). DMF or DMSO acts as a good solvent to dissolve the inorganic salts and small molecules. After completely dissolving, OA (100 µL) and OM (50 µL) were added to stabilize the precursor solution. Then, 0.4 mL of precursor solution was quickly injected into 5 mL toluene (bad solvent) to induce QD crystallization via vigorous stirring. Bright green emission was observed immediately after the injection. Other samples with different emission colors were fabricated with the mixture of stoichiometric amounts of PbX2 and AX (X = Cl, Br, I, A=Cs, FA) by a similar procedure. All above operations were implemented at room temperature, which varies with seasons from ~ 0 ℃ in the winter of Hangzhou to ~30 ℃ in the summer of Hangzhou. Preparation of QDs embedded PMMA film A slightly modified method reported by Kovalenko et al was used to disperse the as-prepared QDs into a polymethylmethacrylate (PMMA) matrix.24 Firstly, 1g PMMA were completely dissolved into 10 mL toluene at 80 ℃ and cooled down to room temperature as stock solution (100 mg/mL). Then, 1 mL PMMA solution was mixed 4


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with 1mL Cs1-mFAmPbX3 QDs solution in toluene (10 mg/mL) and dropwise casted on the cleaned glass substrate followed by drying in vacuum. Construction of QDs-based white light-emitting diode (WLED) As a proof-of-concept experiment, LED devices were constructed by directly coupling the as-fabricate QD films on the InGaN blue chip. Opaque silica gels were filled around the edges of device to avoid the leakage of blue emitting. Characterization X-ray diffraction (XRD) analysis was carried out to identify phase structure of the as-prepared QDs using a powder diffractometer (MiniFlex600 RIGAKU) with CuKα radiation (λ=0.154 nm) operating at 40 kV. Microstructure observation was performed on a JEOL JEM-2010 transmission electron microscope (TEM) equipped with selected area electron diffraction (SAED) at 200 kV accelerating voltage. TEM specimen was prepared by directly drying a drop of a dilute toluene dispersion solution of QDs on the surface of a copper grid. Absorption and emission spectra were recorded on an Edinburgh Instruments (EI) FS5 spectrofluorometer equipped with a continuous (150 W) and pulsed xenon lamps. Time-resolved spectra of QDs 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 nm picosecond laser. Electroluminescence (EL) spectra of the constructed devices were recorded in a HAAS-2000 sepctroradiometer (Everfine) under the forward bias of 20 mA. Absolute photoluminescence Quantum yield (PLQY), defined as the ratio of 5



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emitted photons to absorbed ones, was measured by a spectrofluoremeter (FS5). An integrating sphere was mounted on the spectrofluoremeter with the entrance and exit ports located in 90o geometry. The QD sample was located in the center of the integrating sphere. All the recorded spectroscopic data were corrected for the spectral responses of both the spectrofluoremeter and the integrating sphere. The responses of the detecting systems (integrating sphere, monochromators and detectors) in photon flux were determined using a calibrated tungsten lamp. Based on this setup, PLQY is calculated based on the following equitation

η=

Lsample number of photons emitted = number of photons absorbed E reference -E sample

(1)

where η represents QY, Lsample the emission intensity, Ereference and Esample the intensities of the excitation light not absorbed by the reference and the sample respectively. The difference in integrated areas between the sample and the reference represents the number of the absorbed photons. The emitted photons were determined by integrating the related emission band. According to the relationship between wavelength (in unit of nm) and band gap (in unit of eV) λ =

hc hc , λ +∆λ = , the emission spectral red-shifting degree Eg E g +∆E g

can be derived as follow

∆λ =∆E g

hc E（ g E g +∆E g）

(2)

where ∆λ represents spectral red-shifting degree, ∆E g is band gap difference before and after the substitution of Cs+ by FA+, E g is band gap of QDs, h is Planck constant and c is light velocity. 6


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Results and Discussion

Figure 1. Schematic illustration of the synthesizing procedure of Cs1-mFAmPbX3(X=Cl, Br, I) HPQDs by a RT LASR strategy: (a) forming stable DMF (or DMSO) precursor solution of FA+, Cs+, Pb2+ and X- ions, (b) introducing precursor solution into toluene to induce QD recrystallization, (c) luminescent photograph of Cs0.1FA0.9PbBr3 QDs in solution under the irradiation of UV (375 nm) lamp.

The detailed LASR procedure is schematically illustrated in Figure 1. Notably, the long chain organic surfactants such as OA and OM were added to control the size and crystallinity of products.27,28 Typically for fabricating Cs0.2FA0.8PbBr3 QDs, PbBr2 (0.2mmol), CsBr (0.04mmol) and FABr (0.16mmol) were dissolved in 5 mL dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) to form the precursor solution of FA+, Cs+, Pb2+ and Br- ions. Meanwhile, appropriate amounts of OA (100 µL) and OM (50 µL) were injected into DMF precursor solution. Herein, DMF/DMSO acts as a good solvent to dissolve inorganic and organic salts. After stabilization, the DMF/DMSO precursor solution is dropwise introduced into toluene acting as a bad solvent under continuous stirring, inducing recrystallization of Cs0.2FA0.8PbBr3 HPQDs. Impressively, a green-yellow colloidal solution was immediately formed, yielding bright green light under the irradiation of UV lamp. 7



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Anion hybrid halide perovskites QDs, including CsPb(Cl/Br)3, CsPb(Br/I)3, FAPb(Cl/Br)3 and FAPb(Br/I)3 can be readily prepared by simply combining appropriate ratios of PbX2 (X=Cl, Br and I) salts (Figure S1). With this method, the as-prepared QDs exhibit tunability of their band-gap energies through the entire visible spectral region. Nonetheless, it is still difficult to realize fine-shifting of emission wavelength. For example, CsPbBr3 QDs usually show green emission at around 520 nm and their PL maxima can be shifted towards long-wavelength by increasing the size of QDs or forming CsPbBr3-xIx solid-solution compounds. However, it is difficult to separate QDs to different sizes with the current technology and an obvious drop of PLQY and stability after the formation of CsPbBr3-xIx is usually unavoidable, probably owing to phase separation into CsPbBr3 and CsPbI3.29 Furthermore, CsPbBr3-xIx QDs exhibit quickly spectral shift with increase of I- content so that it is necessary to accurately regulate the proportion of I- to Br-.30-33 Herein, we demonstrate that cation hybrid halide perovskite QDs, Cs1-mFAmPbBr3, produced by adjusting Cs/FA ratio, could easily achieve fine-tuning of emission wavelength within 1~2 nm and retain high PLQYs (55-85%). Further anion replacement of Br- by Cland I- will eventually result in full-spectral tunable visible emissions from these hybrid Cs1-mFAmPbX3 (X=Cl, Br, I, 0≤m≤1) QDs. The microstructure characterizations of Cs1-mFAmPbBr3 HPQDs are shown in Figure 2. XRD patterns of Cs1-mFAmPbBr3 QDs synthesized via LASR are well coincident with cubic CsPbBr3 phase (JPCDS No. 75-0412) and increasing FA+ content from 0 to 100% induces gradual shift of diffraction peaks towards low-angle 8


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without any detectable impurity phase (Figure 2a), confirming that Cs+ cations in CsPbBr3 host can be completely substituted by FA+ ions and the radius of FA+ is larger than that of Cs+. Similarly, pure phase can also be recognized when Br- anions are partially replaced by Cl- or I- in Cs1-mFAmPbBr3 host (Figure S2). Transmission electron microscope (TEM) observation on the typical Cs0.1FA0.9PbBr3 QDs (Figure 2b) demonstrates that they are cubic-shaped and monodispersed with sizes of 6~10 nm. The corresponding selected area electron diffraction (SAED) pattern (inset of Figure 2b) confirms that these QDs are pure cubic phase. High-resolution TEM (HRTEM) micrograph of several QD particles (Figure 2c) verifies their single-crystalline nature with high-crystallinity. The lattice fringes are clearly resolved, and two typical d-spacings of 0.42 nm and 0.29 nm are observed, corresponding to (110) and (200) planes of cubic perovskite phase, respectively. Notably, the gradual replacement of Cs+ by FA+ will not alter the cubic shape of the initial CsPbBr3 QDs and the increase in FA/Cs ratio will not lead to an obvious change of sizes, as evidenced by TEM observation (Figure S3). (a) (100) (110)

Cs1-mFAmPbBr3

(200) (210)

(220) (221)

(211)

(111)

m=1

m=0.9 m=0.8

Intensity (a.u.)

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m=0.6 m=0.5 m=0.4 m=0.2 m=0.1 m=0

JPCDS No.75-0412 10

15

20

25

30

2(θ) degree

35

40

45

50

20

21

2θ (degree)

22

9



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(c)

(b)

d(110)=0.42nm

50 nm

5 nm

d(200)=0.29nm

Figure 2. Cs1-mFAmPbX3 HPQDs and structural characterization. (a) XRD patterns of typical QDs. Bars represent standard diffraction data of cubic CsPbBr3 (JPCDS No. 75-0412). (b, c) Representative TEM and HRTEM images of Cs0.1FA0.9PbBr3 QDs. Inset of (b) is the corresponding SAED pattern and inset of (c) schematically shows Cs1-mFAmPbX3 crystal structure where orange, green and red spheres represent Cs+, Pb2+ and X- ions, respectively and organic group represents FA+ cation.

We further investigated the optical properties of Cs1-mFAmPbBr3 HPQDs. In PL spectrum of CsPbBr3 product (Figure 3a), a narrow emission band centered at 519 nm with FWHM of 22 nm (~100 meV) was observed. The UV-vis absorption spectrum shows a band edge at 516 nm (Figure 3a) and the relative smaller Stokes shift of 14 meV, implying that the PL luminescence of CsPbBr3 QDs originates from direct exciton recombination.30,33-35 Impressively, the substitution of Cs+ cations by FA+ ones is beneficial for flexible and elaborate controllability on PL color of HPQDs. With increase of FA+ content, both absorption and PL spectra simultaneously move towards long-wavelength without significant alteration of the corresponding FWHMs (Figure 3a, Table S1). All these results are due to the variation of band gap as well as exciton binding energy with modification of FA/Cs ratio in the Cs1-mFAmPbBr3 HPQDs. As schematically illustrated in Figure 3b, the FA+-content dependent energy band structure of QDs was determined according to the position of the emission peak. 10


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As shown in the inset of Figure 2c, each Pb2+ ion coordinates to six Br- ones in the cubic Cs1-mFAmPbBr3 HPQDs and these PbBr64− octahedra are connected to each other by sharing Br- ions to form a three dimensionally linked network. Interstitial Cs+/FA+ cations will stay in the octahedron PbBr64− cages.24,30-32,36-38 It has been previously reported that the valence band and conduction band for CsPbBr3 QDs are predominately originated from Br 4p and Pb 6p orbits and exciton recombination luminescence is dominantly confined within PbBr64− octahedron.39,40 With the replacement of Cs+ by FA+, Pb-Br bond lengths and angles in PbBr64− octahedron are probably altered, which in turn modifies electronic structure (i.e., band gap) of CsPbBr3 QDs.

Absorption/PL intensity (a.u.)

(a)

FAPbBr3 Cs0.2FA0.8PbBr3 Cs0.5FA0.5PbBr3 Cs0.8FA0.2PbBr3 CsPbBr3

400

450

500

550

600

650

700

Wavelength (nm) λ ex=375 nm

0

(d)80

Cs1-mFAmPbBr3 m=0 m=0.1 m=0.2 m=0.4

Absolute PLQY (%)

m=0.5 m=0.6 m=0.8 m=0.9 m=1.0

Lifetime (ns)

(c)

Intensity (a.u.)

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200

400

600

800

Cs1-mFAmPbBr3

60 40 20

80 60 40 20 0.0

0.2

Time (ns)

0.4

0.6

0.8

1.0

FA content

Figure 3. (a) Representative optical absorption/PL spectra (λex=375 nm) and (c) time-resolved PL decays for the Cs1-mFAmPbBr3 HPQDs. (b) Schematically illustration of the variation of band gap with increase of FA+ content in HPQDs. (d) Dependence of lifetime and absolute PLQY on the 11



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FA+ content in HPQDs.

To gain more insight into exciton recombination dynamics, time-resolved PL spectra of Cs1-mFAmPbBr3 QDs were recorded, as shown in Figure 3c. Owing to their non-single-exponential feature, the decay lifetime can be evaluated according to the equation of

τ ave = ∫ I (t )dt / I p , where I(t) is the time-dependent PL intensity, and Ip

the peak intensity in the decay curve. Compared to that of pure CsPbBr3 IPQDs, the average PL lifetime of Cs1-mFAmPbBr3 HPQDs was gradually enhanced with increase of FA/Cs ratio (Table S1, Figure 3d). This indicates that the decrease of band gap with addition of FA+ cation is probably accompanied by an increase in exciton binding energy. In fact, PL lifetimes of CsPbBr3 IPQDs reported by several groups are indeed much smaller than that of FAPbBr3 QDs prepared by a hot injection method at high temperature (140-200 0C).24 To confirm this conclusion, we further examined the lifetime variation of Cs1-mFAmPbBr2Cl HPQDs, as shown in Figure S4. As expected, similar variation trend for PL lifetime is observed, verifying that FA+ doping is the dominant factor for modifying exciton recombination kinetics. As tabulated in Table S1 and shown in Figure 3d and Figure S5, all the Cs1-mFAmPbBr3 QDs show high PLQYs (55-85%) and more interestingly, increasing FA+ content from 0 to 90% results in monotonous enhancement of PLQY. However, PLQY will be obviously lowered when the Cs+ cations were complete replaced by FA+ ones (forming FAPbBr3 QDs). One of the most important factors to improve PLQY is proposed to be due to the enhanced structure stability of HPQDs with the addition of FA+ cation. To maintain a high-symmetry cubic structure of ABX3 halide

12


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perovskite, a geometric parameter called tolerance factor t =

RA + R X is 2( RB + RX )

generally required to be close to 1,41-45 where RA and RB are the ionic radii of the Aand B- site cations, respectively, and RX is the ionic radius of the halogen anion.46,47 As the Pb2+ cation (R=0.119 nm) in B-site is essential for the electronic structures of Cs1-mFAmPbBr3 QDs and its radius is large, it is beneficial to stabilize perovskite structure when Cs+ cation (R=0.167 nm) in A-site is gradually replaced by FA+ cation with larger size (R=0.205 nm) to draw tolerance factor closer to 1 (Table S2). Similarly, taking CsPbBrI2 as a representative example, the replacement of Br(R=0.196 nm) by I- with larger size (R=0.220 nm) reduces the value of tolerance factor and makes QDs unstable, while the substitution of Cs+ by FA+ will restore stability of QDs. As demonstrated in Figure S6, with the extension of resting time, the emission color changes from red to yellow and finally to green in the solution of CsPbBrI2 QDs owing to the gradual release of I- anions from perovskite host, while no obvious color change is found for Cs0.1FA0.9PbBrI2 sample, confirming the vital role of FA+ doping for improving structural stability of HPQDs. Additionally, we further examined the stability of Cs1-mFAmPbBr3 QDs. As shown in Figure S7, no significant alteration of PL positions and intensities for three typical Cs0.9FA0.1PbBr3, Cs0.5FA0.5PbBr3 and Cs0.1FA0.9PbBr3 QDs for the duration of one month confirms that the Cs-FA alloy will not separate into pure Cs and FA domains with the elongation of time.

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Figure 4. Full-spectral fine-tuning visible emissions from hybrid Cs1-mFAmPbX3 (X=Cl, Br, I, 0

Quantum Dots

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