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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Cesium Lead Chloride/bromide Perovskite Quantum Dots with Strong Blue Emission Realized via a NitrateInduced Selective Surface Defect Elimination Process Shixun Wang, Yu Wang, Yu Zhang, Xiangtong Zhang, Xinyu Shen, Xingwei Zhuang, Po Lu, William W. Yu, Stephen V Kershaw, and Andrey L. Rogach J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03750 • Publication Date (Web): 19 Dec 2018 Downloaded from http://pubs.acs.org on December 20, 2018

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The Journal of Physical Chemistry Letters

Cesium Lead Chloride/Bromide Perovskite Quantum Dots with Strong Blue Emission Realized via a Nitrate-Induced Selective Surface Defect Elimination Process Shixun Wang,1# Yu Wang,1# Yu Zhang,1* Xiangtong Zhang,1 Xinyu Shen,1 Xingwei Zhuang,1 Po Lu,1 William W. Yu,1 Stephen V. Kershaw,2 and Andrey L. Rogach2,3*

#

S.W and Y.W contributed equally to this work.

1. State Key Laboratory of Integrated Optoelectronics and College of Electronic Science and Engineering, Jilin University, Changchun 130012, China 2. Department of Materials Science and Engineering, and Centre for Functional Photonics (CFP), City University of Hong Kong, Kowloon, Hong Kong S.A.R. 3. Beijing Institute of Technology, School of Materials Science and Engineering, Beijing 100081, China

1

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The Journal of Physical Chemistry Letters 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

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; *E-mail: [email protected].

ORCID Yu Zhang: 0000-0003-2100-621X; Andrey L. Rogach: 0000-0002-8263-8141

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Abstract

Cesium lead halide perovskites emitting blue light in the range of wavelengths 460 to 470 nm have so far been plagued with rather poor luminescent performance, placing inevitable limitations on the development of perovskite nanocrystal-based blue light-emitting devices. Herein, a selective surface defect elimination process with the help of hydrated nitrates was introduced into the perovskite/toluene solution to strip the undesired surface defects and vacancies, and to boost the photoluminescence quantum yield of true-blue-light emitting (at 466 nm) CsPb(Cl/Br)3 perovskite nanocrystals to the impressive value of 85%. Unlike the conventional passivation strategy, the anionic nitrate ions are able to desorb the undesired surface metallic lead and combine with excess surface metal ions leaving perovskite quantum dots with better crystallinity and fewer surface defects.

TOC GRAPHICS

3



KEYWORDS

lead

halide

perovskite

nanocrystals,

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blue-emitting

materials,

selective surface defect treatment

Lead halide perovskites have emerged as promising candidate materials for several

optoelectronic

applications

including

photovoltaics,1-3

low-threshold

lasers,4,5 photodetectors6-8 and light-emitting diodes (LEDs)9-11 due to their superior charge-transport properties, narrow emission linewidth, and tunable optical bandgaps. In particular, all-inorganic lead halide perovskite nanocrystals, namely cesium lead halide perovskite (CsPbX3, X = Cl, Br, and/or I) quantum dots (QDs) have been found to offer attractive optical properties implying their potential use in optoelectronic devices.12-14 Though both green- and red-emitting CsPbX3 QDs achieved photoluminescence quantum yields (PL QY) of near to 4


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100%,15-16 blue-emitting cesium lead halide perovskites with emission peaked at 460-470 nm still lag behind,9,18,19 hindering possible developments of allperovskite

red-green-blue

(RGB)

displays.

However,

the

exact

underlying

reasons for relatively low PLQYs in chlorinated blue-emitting cesium lead halide QDs have not been fully understood so far. Typically, decreasing the size of CsPbBr3 NCs and varying the mixed Cl/Br halide composition are the main methods to obtain true blue-emitting perovskite QDs, while the possibility of phase separation and the large proportion of surface defects of small-size nanoparticles (due to their high surface-to-volume ratio) are the issues impeding their PL QYs.20,21 Attempts at coating perovskite QDs with additional silica passivating shells showed a limited success in improving their PL QYs.11,22-25 Though

the

recently

reported

blue-emitting

core/shell

CsPbBr3/amorphous

CsPbBrx QDs achieved high PL QYs of over 80%, they suffered from poor thermal stability due to the metastable nature of the amorphous shell.21 Other strategies to improve the PL QY of violet or blue emitting perovskite QDs focused on post-synthetic surface passivation using tetrafluoroborate salts,

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yttrium chloride salts and additional bromide ions.20,26,27 Surface treatment, ideally, should be able to strip or remove surface defects, while leaving behind intact, surface-defect-free perovskite QDs with an improved PL QY.

Herein, we propose a novel strategy to selectively peel away the surface defects

of

CsPb(Cl/Br)3

QDs

using

a

treatment

with

nitrate

ions,

while

maintaining their cubic shape, surface ligands and crystal structure; this results in samples with strong true blue emission in the range of 460-470 nm, and extremely high PL QY of 85%. Lead halide perovskite QDs are ionic and thus very sensitive to polar solvents; their surface ligands have been demonstrated to be highly dynamic and labile.28,29 We have found that nitrate ions dissolved in toluene interact with the uncoordinated Pb ions and excess Cs ions at the surface of perovskite QDs and are able to strip the outer layer of the nanocrystals having high defect densities and poor crystallinity, while at the same time having minimal impact on the rest of the QDs.

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To study the role of nitrate additives, we first conducted the treatment of the mixed-anion CsPb(Cl/Br)3 QDs by a number of different (hydrated and nonhydrated) nitrate salts, including an anhydrous lead nitrate, hexahydrated zinc nitrate,

tetrahydrated

cadmium

nitrate,

hexahydrated

yttrium

nitrate

and

hexahydrated nickel nitrate. Normally, 0.01 M nitrates were separately added into 3 ml QD/toluene solutions (30 mg·ml-1), and 9 min later, the nitrates were easily removed through centrifuging for 1 min at 3000 rpm. Figure 1a demonstrates an overall reduced optical absorption of the treated QDs; we hypothesize here and will discuss in detail later on, that nitrates may peel the undesired crystal surface and even decompose some imperfect crystalline perovskites, thereby reducing the optical absorption. Unlike the samples treated by hydrated nitrates, the one treated by the anhydrous lead nitrate shows a similar absorbance with the pristine sample in the absorption range between 350 to 470 nm. This may be attributed to constraining influence of the crystal water, as will be discussed later on. As for the red-shifts observed for the absorption spectra of all treated samples, we initially inferred that it is a

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combined effect of the changed Cl/Br ratio during the selective surface defect elimination process and some proportion of enlarged perovskite cores due to the recrystallization of some stripped perovskite materials on other particle surfaces, as will be presented later. The PL spectra in Figure 1b illustrate significantly improved emission intensities (PL QYs changing from 12% to 85% for the best performing QDs) of samples treated by different nitrates in toluene. The red shift of the PL maxima mixed-anion CsPb(Cl/Br)3 QDs (Figure 1b) coincides well with the similar red shifts observed in the absorption spectra. We note that for the CsPbCl3 and CsPbBr3 QDs, the PL spectra shown in Figure S1 (Supporting Information, SI), show a minor (0.5 nm) blue-shift upon treatment

with

nickel

nitrate,

rather

than

a

red-shift

observed

for

the

CsPb(Cl\Br)3 QDs in Figure 1b due to the selective surface defect elimination process which leads to the change of the Cl\Br ratio. From Figure S1 we can also see, that only the CsPbCl3 QDs (similar to the mixed-anion CsPb(Cl/Br)3 QDs) show an improvement of the PL intensity, while the CsPbBr3 QDs experienced a slightly weakened PL intensity after the treatment with nickel

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nitrate. This may indicate that the surface defects are the principal issue restricting the PL of the chlorinated cesium lead halide perovskite QDs.

Figure 1. (a) Optical absorption spectra, (b) PL spectra, (c) PL decays fitted by two-exponential functions (see Table 1), and (d) XRD patterns (the panel on the right shows an enlarged part of the (200) plane reflex) of the pristine CsPb(Cl/Br)3 QDs, and CsPb(Cl/Br)3 QDs treated with anhydrous lead nitrate,

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hexahydrated zinc nitrate, tetrahydrated cadmium nitrate, hexahydrated yttrium nitrate, and hexahydrated nickel nitrate, as indicated on the frames. The inset in (b) shows a photograph of the blue-emitting solution of the nickel nitrate treated sample taken under illumination with a 365 nm UV lamp in an ambient environment; the inset in (c) shows an enlarged PL decay figure at PL intensity of 104/e.

Table 1. PL characteristics, including PL peak positions, PL QYs, and the data (average lifetimes, radiative and non-radiative decay rates) related to twoexponential fittings of PL decays for the pristine CsPb(Cl/Br)3 QDs (“no additive”), and the same QDs treated with different nitrates. The deviation of 𝐾𝑟 × 106𝑠 ―1 is calculated when the PL QY uncertainty is +/-0.5% and that of the lifetimes is +/-0.1ns. Additives No Pb(NO3)2 Zn(NO3)2·6H2O Cd(NO3)2·4H2O Y(NO3)3·6H2O

PL peak/nm 462 463 463 464 464

PL QY (%)

𝜏𝑎𝑣𝑔/𝑛𝑠

12 29 50 57 75

𝐾𝑟( × 106𝑠 ―1)

3.4 3.5 3.6 3.6 3.7 10


35±3 82±4 138±6 158±6 202±7

𝐾𝑛𝑟( × 106𝑠 ―1) 259±15 203±15 139±14 119±13 68±14

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Ni(NO3)2·6H2O

466

85

4.2

203±6

38±14

Subsequently, time-resolved PL decay measurements were conducted to study the radiative mechanism of the pristine and nitrate-treated samples. The obtained PL decay curves have been fitted with double-exponential functions, as presented in Figure 1c. The average (effective) PL lifetimes (τavg) were evaluated from the experimental PL kinetics [I(t)] by using the equation28,29

𝜏𝑎𝑣𝑔 =

1 ∞ 𝐼(0)∫0 𝐼(𝑡)

𝑑𝑡

(1)

where the integration was performed numerically. The radiative and nonradiative components in the total PL decay for an ideal system of identical perovskite QDs could be determined via

1 𝐾𝑟

(2)

11


= 𝜏 = 𝜏𝑎𝑣𝑔/QY 𝑟


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1 𝐾𝑛𝑟

𝜏𝑎𝑣𝑔

= 𝜏 = 1 ― 𝑄𝑌 𝑛𝑟

(3)

where τr is the radiative lifetime, τnr is the non-radiative lifetime, Kr is the radiative decay rate, Knr is the non-radiative decay rate and QY is the measured value of PL QYs (expressed as a fraction between 0 and 1) conducted under the same excitation wavelength (365 nm) as with the PL decay measurement. Table 1 lists the measured PL QYs, the average PL lifetime, radiative decay rate, and non-radiative decay rate obtained from equations 1, 2 and 3, respectively. We note that the equation 3 ignores the presence of dark (non-emissive for a period of time, which is related to blinking phenomenon) dots in the ensemble,30,31 which may overestimate the nonradiative decay rates. If the fraction of bright dots is b (b < 1) and the remaining particles are dark on the measurement timescale (the time between pulses in the TCSPC decay time experiment), equation 3 should be corrected as:

12


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𝜏𝑎𝑣𝑔

𝜏𝑛𝑟 = 1 ― 𝑄𝑌𝑓𝑐𝑜𝑟(𝑏,𝑄𝑌) (4)

The corrected function in equation 4 (𝑓𝑐𝑜𝑟(𝑏,𝑄𝑌) ≥ 1) becomes increasingly important as the fraction of “dark” perovskites decrease.32 This fraction has little impact on the non-radiative decay rate when the PL QY value is low enough while it will be overestimated by a factor of between 0 and 1 when the PL QY is high. As can be seen from the Table 1, the treated samples all have increased average lifetime (from 3.5 ns to 4.2 ns, corresponding to the growth trend of their lifetime at PL intensity of 104/e, as shown in the inset in Figure 1c), improved radiative decay rates (from 2.3-fold to 5.8-fold of the pristine value), decreased non-radiative decay rates (from 0.78-fold to 0.15-fold of the pristine value) and high PL QY (from 29% to 85%), while the pristine sample only shows a low average lifetime of 3.4 ns and a weak PL QY of 12%. The enhancement in fluorescent performance may originate from the improved surface state with fewer defects, and better channels of radiative recombination of the treated perovskite QDs, due to the selective elimination of undesired 13



surface defects and vacancies. Notably, as it can be seen from the Table 1, the samples treated by the hydrated nitrates show higher PL QY than that of the sample treated by non-hydrated Pb(NO)3. We infer that the crystal water in nitrates doesn’t directly participate in the surface treatment, but rather provides an appropriate Me-OH2 distance to facilitate good bonding with metal (Me) ions while it has a labile connection with nitrate ions.33 Because of that, the crystal water can constrain metal ions to diffuse into the water-insoluble toluene and/or to realize doping process, and meanwhile will contribute to the mobility of nitrate ions, improving the defect elimination process by the latter. As shown in Figure S2, hydrated acetates are also able to strip away defects and improve PLQY of perovskite QDs to some extent, according to the induced optical absorbance and increased PL intensity. Hydrated sulphates did not cause any enhancement of the PLQYs, as these are stable salts hardly soluble in toluene. Hydrated carbonates damaged the perovskites, when contacting with the uncoordinated lead ions at their surface. As such, the hydrated nitrates appear

14


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to be the ideal choice among several other salts to modestly strip surface defects of perovskites.

Figure 2. (a, b) TEM and (c, d) high-resolution TEM images of pristine CsPb(Cl/Br)3 QDs (a, c) and the same QDs treated by hexahydrated nickel nitrate (b, d). Scale bars on (a, c) are 20 nm.

Insets in (a, b) show the

histograms of the QDs’ average sizes (edge lengths); insets in (c, d) show the corresponding FFT figures. 15



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To further explore this hypothesis, we conducted XRD, TEM and XPS measurements on nitrate-treated and untreated samples. Figure 1d shows XRD patterns of the samples; for all treated QDs, the diffraction from the (200) planes of cubic CsPb(Cl/Br)3 experiences a shift towards higher angles, wherein the hexahydrated nickel nitrate-treated sample has the largest shift of 0.3 degree and the anhydrous lead nitrate-treated sample also shows a 0.2-degree shift, suggesting that such a shift cannot be attributed to possible cationic doping. The shift should be rather considered as the result of the removal of larger lead surface and/or interstitial atoms (154 pm; Pb2+ 120 pm) and the fraction

of

poor-crystalline

surface

perovskite

particles,

according

to

the

unchanged interplanar distance in TEM figures of the nitrate-treated perovskite QDs, as will be presented later.

In the following, we present the data of the selective surface defect elimination process of the CsPb(Cl/Br)3 QDs treated with the hexahydrated

16


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nickel nitrate, as this chemical had the strongest influence in terms of improving the PL QY of the samples. Among all the tested salts, it may have a more suitable binding mode with the crystal water as well as a more appropriate solubility in toluene solution, thereby offering the best condition for the surface defect elimination process. Transmission electron microscopy (TEM) images provided in Figure 2a, c show that the pristine CsPb(Cl/Br)3 QDs are crystalline particles with cubic-shape morphologies, and rather narrow size distributions, with an average edge length of around 10.0 nm. The nitratetreated perovskite nanocrystals, however, show somewhat larger size distribution with an average edge length of about 10.4 nm (Figure 2b). We notice, that unlike the pristine QDs, the nitrate-treated samples display several connected nanocrystals

(see

Figure

S3

in

SI)

probably

originating

from

fusing

of

perovskite QDs together during the defect elimination process, when some of the surface ligands were detached. The presence of such fused (and thus enlarged) QDs in the nitrate-treated samples may also be an additional reason for, and contribution to, the red-shift in their optical absorption and PL spectra

17



to some extent, as observed in Figure 1a, b. According to the fast Fourier transform (FFT) of the high-resolution TEM images (Figure 2c, d), a similar interplanar spacing of 0.32 nm for both the pristine and treated samples was obtained, corresponding to the interplanar spacing of the (111) crystal plane of the cubic phase of CsPb(Cl/Br)3. We note that interplanar distances of both cubic CsPbBr3 and CsPbCl3 for the (111) crystal planes are 0.32 nm, according to the PDF#84-464 and PDF#84-438, respectively. The Energy Dispersive Spectroscopy (EDS) data given in Figure S4 show that the nitrate-treated mixed-anion CsPb(Cl/Br)3 QDs have decreased Cs and Pb atomic percentage, and a lower Cl/Br ratio (1.04 as compared to 1.45 for the pristine sample), supporting the assertion of the removal of surface Cs/Pb atoms from the nonperfect octahedrons with halide vacancies (Figure S3).

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Figure 3. (a) Wide-range XPS spectra, (b) Cs-3d core-level XPS spectra, (c) Pb-4f core-level XPS spectra and (d) FTIR spectra of the pristine (black) and hexahydrated nickel nitrate-treated (red) CsPb(Cl/Br)3 QDs. No XPS peak for Ni-2p could be observed.

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X-ray photoelectron spectroscopy (XPS) was conducted on the pristine and on the hexahydrated nickel nitrate treated CsPb(Cl/Br)3 QDs. As shown in Figure 3a and Figure S5, no peaks corresponding to Ni-2p core level structure were observed in the XPS spectra, in spite of the stronger N-1s signal from the nitrate-treated samples. This reflects the fact that Ni ions were not doped into the CsPb(Cl/Br)3 QDs. For the pristine QDs, the values of the (Cl+Br):Pb ratio derived from the XPS measurements was 2.31, the Cl:Br ratio was 1.61, and the Cs:Pb ratio was 1.11, which indicates the existence of excess Cs ions and/or halide vacancies at certain surfaces offering positive sites to adsorb negative ions.34 For the nitrate treated QDs, the (Cl+Br):Pb ratio increased to 2.71, the Cl:Br ratio reduced to 1.01, and the Cs:Pb ratio decreased to 1.02, revealing removal of some (eventually uncoordinated) Pb and Cs ions from the surface. Figure 3b shows the XPS spectra for the Cs-3d core level, wherein the reduced peak intensity of both Cs-3d5/2 (737.8 eV) and Cs-3d7/2 (724.0 eV) for the nitrate treated sample reveals lower content of Cs as compared to the pristine one. In the Pb-4f core level spectra (Figure 3c), two peaks of Pb-4f5/2

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and Pb 4f7/2 are resolved. For the pristine sample, each of these peaks can be fitted with two peaks, where signals with low and high binding energy are attributed to Pb-Cl/Pb-Br (138.07 eV and 142.90 eV) and Pb in metallic state (141.1 and 136.3 eV), respectively.35 For the nitrate treated samples, on the contrary, the Pb-4f peaks can be fitted with just two peaks which locate at higher binding energies of 143.1 eV (Pb-4f5/2) and 138.2 eV (Pb 4f7/2) with a spin-orbit splitting of 4.9 eV, indicating the presence of lead in the oxidation state II (Pb2+). It seems that the introduced nitrate ions would strip off the labile lead atoms, uncoordinated Pb2+, excess Cs2+, and a part of the halides to leave behind perovskite QDs with fewer surface defects.

To compare the state of the surface ligands in the pristine and nitrate-treated samples, Fourier transform infrared spectroscopy (FTIR) was performed. As shown in Figure 3d, there is no appreciable change between the FTIR spectra except the slightly enhanced peak at 3356 cm-1 corresponding to the N-H stretching of the oleylamine ligand in the nitrate-treated QDs. This indicates better capping of the QD surface by oleylamine ligands, which additionally 21



helps to eliminate surface traps. Since organic surface ligands on perovskite QDs have been shown to be highly dynamic and labile,36,37 they can easily be detached and then re-attached to the nitrate-treated surface, thus perfecting the capping of the peeled QDs and leading to the increased peak intensity as well as improved luminescent stability under ambient conditions (Figure S6). The removal of ligands from the surface would only slightly affect the electronic structure of these materials,38 thereby providing the ideal precondition to conduct our defect elimination process. Besides, the peak in the FTIR spectrum representing the O-H stretching at 3184 cm-1 remains unchanged, which means that the crystal water component in the hexahydrated nickel nitrate was not involved in the defect peeling or at least changing the surface state of the perovskite QDs, as has been previously reported for other kinds of perovskite treatment with traces of water.39,40 We note that the treatment of CsPb(Cl/Br)3 QDs with nitric acid resulted in a prompt increase of the luminescence intensity, but the emission color gradually changed from blue to bluish green within a few minutes (Figure S7), probably due to a combined effect of both

22


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the changed Cl\Br ratio after the nitrate-induced selective surface defect elimination process and an increased size of the QDs.

In conclusion, we showed the ability of different nitrate salts to perform a selective

surface

defect

elimination

process

on

perovskite

QDs,

which

nonetheless keeps the perovskite core capped by labile surface ligands, while removing the undesired surface defects and vacancies. Owing to the nitrate treatment, the PL QY of the truly blue-emitting CsPb(Cl/Br)3 perovskite QDs increased to an impressive value of 85% at an emission wavelength of 466 nm. The facile treatment strategy introduced here reveals the importance of the surface design as related to the optical performance of the true blue-emitting perovskite QDs, and paves the way towards the development of highperformance blue perovskite LEDs.

Supporting Information

Experimental Details and some characterizations are provided in the supporting information.

23



AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]; *E-mail: [email protected].

Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT We acknowledge financial support from the Natural Science Foundation of China (NSFC) (61675086, 61475062, 61722504); National Key Research and Development Program of China (2017YFB0403601); the Research Grant Council of Hong Kong S.A.R. (GRF project CityU11337616); the Special Project of the Province-University Co-constructing Program of Jilin University (SXGJXX2017-3); the Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of

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Sciences; and the Talent Introduction Plan of Overseas Top Ranking Professors by the State Administration of Foreign Expert Affairs (MSBJLG040).

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