Lattice Disorder-Engineered Energy Splitting between Bright and Dark

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

Lattice Disorder-Engineered Energy Splitting between Bright and Dark Excitons in CsPbBr Quantum Wires 3

Huaiyi Ding, Mei Liu, Nan Pan, Yiyun Dong, Yue Lin, Taishen Li, Jiangtao Zhao, Zhenlin Luo, Yi Luo, and Xiaoping Wang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00551 • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 7, 2019

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The Journal of Physical Chemistry Letters

Lattice Disorder-Engineered Energy Splitting between Bright and Dark Excitons in CsPbBr3 Quantum Wires Huaiyi Ding†,‡, Mei Liu†, Nan Pan*,†,‡,§, Yiyun Dong†, Yue Lin†, Taishen Li†,‡, Jiangtao Zhao∥, Zhenlin Luo∥, Yi Luo†,‡,§ and Xiaoping Wang*,†,‡,§ † Hefei

National Laboratory for Physical Sciences at the Microscale, University of Science and

Technology of China, Hefei, Anhui 230026, P. R. China ‡Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science

and Technology of China, Hefei, Anhui 230026, P. R. China § Key

Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences,

School of Physical Sciences, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China ∥ National

Synchrotron Radiation Laboratory and CAS Key Laboratory of Materials for Energy

Conversion, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China KEYWORDS: strain, disorder, electron-hole exchange interaction, Rahsba effect, bright-dark exciton splitting.

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ABSTRACT: Excitons in nanostructured semiconductors often undergo strong electron-hole exchange interaction, resulting in bright-dark exciton splitting with the dark exciton usually being the lower energy state. This unfavorable state arrangement has become the major bottleneck for achieving high photoluminescence quantum yield (PLQY). However, the arrangement of dark and bright exciton states in lead halide perovskites is under intense debate due to the involvement of many complicated factors. We present here the first experimental evidence to demonstrate that the strain is a crucial factor in tuning the energy splitting of the bright and dark excitons, resulting in different PL properties.

TOC Graphic

In quantum-confined nanocrystals, such as quantum dots, the enhanced wave-function overlap of electron and hole induces larger binding energy1 and higher radiative transition rate of exciton than that in the bulks.2 This merit, along with the others such as full-color tunability through size and/or composition, high color purity and durability, flexibility and low processing cost, makes quantum dots one of the most excellent candidates for the next-generation lighting and display devices.3-5 However, the quantum confinement can also greatly enhance electron-hole exchange interaction,


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resulting in an energy splitting (ΔE) between optically-allowed (bright) and optically-forbidden (dark) states.6-12 For most materials, the former has a higher energy and ΔE is on the order of 2100 meV.2 As a result, if there are significant nonradiative defects to annihilate the dark excitons, the photoluminescence quantum yield (PLQY), which is in principle negatively correlated with the factor, exp(ΔE/kT), is seriously suppressed at cryogenic temperatures and sometimes even at room temperature (when ΔE>26 meV). Considerable efforts have been made to solve the problem related to bright-dark exciton splitting. For example, an electron-hole overlap tuning technology was used to suppress the electron-hole exchange interaction in II-VI group hetero-structured quantum dots.13-15 Very recently, Becker el al. proposed a new scenario that the order of bright and dark excitons could be reversed by strong spin-orbit coupling and Rashba effect16 in perovskite cesium lead halide nanocrystals,17 which was used to explain the unique properties of the cesium lead halide quantum dots, namely the ultrahigh PLQY near unity and fast PL decay less than 1 ns. However, such a hypothesis has been challenged by other studies,18,

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when the diversity of

multiphase structure and disordering of different groups in different samples were considered.19 In this case, the lattice disorder can often be unintentionally introduced into the perovskite quantum dots during their preparation processes;20, 21 even with the same preparation method, it cannot be sufficiently controlled, as reflected by their different PL fine structures at 4 K.22, 23 Since the crystal field can be partially cancelled out by the lattice disorder and/or the multiphase formation, the suggested Rashba effect could be largely reduced, leading to the resulted different PL properties.1719

However, to the best of our knowledge, there is no direct evidence to confirm that the lattice

disorder has the ability to tune the splitting of the bright and dark excitons. In this work, we designed a series of experiments to directly demonstrate the lattice disorder modulated bright-dark exciton splitting in CsPbBr3 quantum wires (QWs). The disorder is induced


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by lattice strain, which can be controllably generated (relaxed) as the temperature rapidly cools down (heats up) and stabilizes. In-situ X-ray diffraction (XRD) measurement evidences that, after each rapid fall/rise of temperature, the lattice constant (strain) of the QWs gradually changes at the target temperature for about one hour to get stabilized. This unique feature allows us to probe the strain/disorder-dependent PL evolution of the QWs. By variable-temperature time-resolved photoluminescence (TRPL) and steady-state photoluminescence (SPL), it is clearly shown that the disorder enhances the bright-dark exciton splitting energy while reduces the PLQY. Ultrafine CsPbBr3 QWs were synthesized through a solution-based method described in our previous work,24 which is also detailed in the Supplementary section 1. Figure 1a shows the representative transmission electron microscopy (TEM) image of the samples. Due to the instability of the ultrafine QWs under high-energy electron irradiation, it is difficult to directly measure the diameters from high-magnification TEM images. However, due to the strong quantum confinement effect, the SPL spectra of the QWs with different diameters give rise to distinct peak positions at cryogenic temperatures, which is very useful to estimate their diameters. As shown in Figure 1b, a series of separate emission peaks at 450, 460, 467, 474 and 483 nm are clearly observed in the SPL spectra of the QWs bundle at 80K, corresponding to the CsPbBr3 QWs with the diameters of 4, 5, 6, 7 and 8 unit-cell, respectively. More details about the diameter estimation are given in Supplementary Figure. S1 and S2. As can be seen, all of the diameters are much smaller than the Bohr diameter of exciton (about 7 nm) in bulk CsPbBr3, 25,26 a strong electronhole exchange interaction induced bright-dark exciton splitting is therefore expected. To clearly demonstrate this bright-dark exciton splitting, the temperature, magnetic-field and diameter dependences of the TRPL are systematically investigated.


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TRPL of CsPbBr3 QWs with a 5 unit-cell diameter (with 4nm band width) at various temperatures from 10 to 120 K are shown in Figure 1c, clearly exhibiting two decay channels with significantly different decay rates. The lifetime of the fast decay channel is less than 100 ps, even after convoluting with the instrument response, whereas the slow decay path, of which the lifetime is longer than 1 μs, almost goes out of the measurement range at 10 K. As the temperature increases, the slow decay component gradually becomes faster and eventually dominates the PL. At 120 K, the fast decay component cannot be resolved any more, whereas the lifetime of the slow component significantly shortens to a few nanoseconds. All of these phenomena are the typical features of the bright-dark exciton splitting, which can be explained by the energy diagram in Figure 1d. The fast decay comes from the direct recombination of the bright exciton and its lifetime is affected by the relaxation of the bright state to the dark state. The slow component should be also from the bright state, but is generated from the back-donation of thermal activated dark state, i.e. a delayed PL process. The quantitative description of the processes can be obtained from the exciton decay equation,19

 0 N B   pB  d  pB    B   0  N B  1      0 N B   D   pD  dt  pD    0  N B  1

(1)

where pB and pD are the populations of bright and dark excitons, ΓB and ΓD are decay rates of the bright and the dark excitons, respectively, γ0 is the bright-dark relaxation rate at zero temperature and NB=1/(exp(ΔE/kT)-1) is the phonon number at a temperature T. By solving these equations, the exciton populations can be derived as,  pB  Bs exp   s t   B f exp   f t      pD  Ds exp   s t   D f exp   f t 

(2)


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As the assumption γ0>>ΓB>>ΓD, at low temperature holds, kT

Lattice Disorder-Engineered Energy Splitting between Bright and Dark

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