2D Behaviors of Excitons in Cesium Lead Halide Perovskite

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2D Behaviors of Excitons in Cesium Lead Halide Perovskite Nanoplatelets Jing Li, Laihao Luo, Hongwen Huang, Chao Ma, Zhizhen Ye, Jie Zeng, and Haiping He J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b00017 • Publication Date (Web): 23 Feb 2017 Downloaded from http://pubs.acs.org on February 26, 2017

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The Journal of Physical Chemistry Letters

2D Behaviors of Excitons in Cesium Lead Halide Perovskite Nanoplatelets

Jing Li,†,§ Laihao Luo,†,‡ Hongwen Huang,*,‡ Chao Ma,‡ Zhizhen Ye,§ Jie Zeng*,‡ and Haiping He*,§ §

State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, P. R. China ‡

Hefei National Laboratory for Physical Sciences at the Microscale, Key Laboratory of

Strongly-Coupled Quantum Matter Physics of Chinese Academy of Sciences, Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China

*To whom correspondence should be addressed. E-mails: [email protected] (HH), [email protected] (HongwenH), [email protected] (JZ)

†These authors contributed equally.

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ABSTRACT: Fundamental to understanding and predicting the optoelectronic properties of semiconductors is the basic parameters of excitons such as oscillator strength and exciton binding energy. However, such knowledge of CsPbBr3 perovskite, a promising optoelectronic material, is still unexplored. Here we demonstrate that quasi-two-dimensional (quasi-2D) CsPbBr3 nanoplatelets (NPLs) with 2D exciton behaviors serve as an ideal system for the determination of these parameters. It is found that the oscillator strength of CsPbBr3 NPLs is up to 1.18 × 104, higher than that of colloidal II-VI NPLs and epitaxial quantum wells. Furthermore, the exciton binding energy is determined to be of ~120 meV from either the optical absorption or the photoluminescence analysis, comparable to that reported in colloidal II-VI quantum wells. Our work provides physical understanding of the observed excellent optical properties of CsPbBr3 nanocrystals, and would benefit the predicting to high-performance excitonic devices based on such materials.

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Very recently, colloidal cesium lead halide perovskite (CsPbX3, X = Cl, Br, and I) nanocrystals have emerged as a class of promising candidates in a variety of optoelectronic devices, such as lasers, light-emitting diodes (LED), and single photon emitters, due to the exceptional optical and electronic properties.1-4 As the properties of CsPbX3 nanocrystals are highly dependent on their shapes, CsPbX3 nanocrystals with tunable shapes of nanocubes, nanowires, and nanoplates have

been

rapidly

developed.1,

5-9

Among

these

distinct

structures,

the

perfect

quasi-two-dimensional (quasi-2D) CsPbBr3 nanoplatelets (NPLs) are of particular interest due to their naturally analogous excitonic behaviors to that of epitaxial quantum well,10 a most common building block in practical optoelectronic devices,11 considering the conclusive role of excitonic behaviors in optoelectronic properties.12-13 Furthermore, the developed low-cost and large-scale colloidal synthesis as well as the exquisitely monolayer-leveled control over the thickness of NPLs enables such quasi-2D CsPbBr3 NPLs to be an ideal platform for both fundamental photophysics studies and high-performance excitonic devices.7-9,

14-15

As a good example,

Alivisatos and co-workers demonstrated the photoluminescence quantum yield (PLQY) value of 84.4 ± 1.8%, 44.7 ± 2.6%, and 10 ± 0.5% for five, four, and three monolayer thick CsPbBr3 NPLs, respectively.7 In another case, Manna and co-workers surprisingly found that the exciton dynamics of CsPbBr3 NPLs were insensitive to the extent of 2D confinement.8 The results implied the unique excitonic properties of quasi-2D CsPbBr3 NPLs. However, the studies of excitonic properties for quasi-2D CsPbX3 NPLs at present are still in a rudimentary stage with respect to the progresses in II-V quasi-2D cadmium chalchogenide NPLs.16-18 In particular, the basic parameters of excitons for quasi-2D CsPbX3 NPLs, such as the oscillator strength and exciton binding energy, are yet to be determined. In principle, the oscillator strength is the figure of merit to quantify the coupling of an emitter to light, which is highly associated with the cross-section of both absorption and spontaneous/stimulated emission.16, 19 While the exciton binding energy, namely the strength of electron-hole interaction, largely dominates the transport and recombination mechanisms of photocarriers, which is critical to the performance of both solar cells and light-emitting devices.20-23 Therefore, knowledge of the oscillator strength and exciton binding energy for quasi-2D CsPbX3 NPLs is essential to understand and predict their many of optoelectronic properties. In this work, we clearly demonstrate the 2D excitonic behaviors for the produced quasi-2D CsPbBr3 NPLs with lateral size of 14.8 and 20.2 nm from the linear dependence of radiative



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lifetime on temperature. By contrast, the NPLs with lateral size close to the exciton Bohr diameter exhibit 0D excitonic behaviors due to the strong quantum confinement. Based on the 2D behaviors of excitons, we further determine a short intrinsic radiative lifetime of 8.2 ps, a large oscillator strength of 1.18 × 104, and exciton binding energy of ~120 meV for 2D excitons in quasi-2D CsPbBr3 NPLs, predicting their fantastic performance for light-emitting device applications.

The NPLs were synthesized by mildly modifying the synthetic procedure reported by Bekenstein et al.7 For synthesis of NPLs with an average lateral size of 20.2 nm, a solution containing lead bromide (PbBr2), oleylamine (OAm), oleic acid (OA), and octadecene (ODE) in a 20 mL vial was preheated at 120 oC for 10 minutes under magnetic stirring to dissolve PbBr2, and then the preformed cesium-oleate solution was injected. After the reaction proceeded for 5 seconds, it was quenched by immersing the vial into ice water. The NPLs were then collected by centrifugation with n-hexane for several times. The X-ray diffraction (XRD) pattern of the as-obtained products (Fig. 1a) shows characteristically double peaks around 30o, indicating the synthesis of orthorhombic CsPbBr3 (space group Pbnm, a = 8.202 Å, b = 8.244 Å, c = 11.748 Å).5 Figure 1b shows the representative transmission electron microscopy (TEM) image, clearly demonstrating the formation of NPLs with high uniformity in terms of both shape and size. Notably, the NPLs can be assembled to lie perpendicular to the TEM copper grids by the deposition of an increased concentration of the NPLs solution, as illustrated in Fig. 1c. On the basis of these high-quality TEM images, the lateral size and thickness of NPLs were measured by counting more than 100 of typical NPLs. An average lateral size of 20.2 nm and thickness of 3.4 nm were then determined from the size distribution histograms shown in Fig. 1d. Furthermore, the thickness of the NPLs was also probed by atomic force microscopy (AFM). The AFM topographic phase image and the AFM height profile recorded along the black line (Fig. S1) validates the 3.1 nm thickness of the NPLs, which is well consistent with the TEM results. For simplification, we denote the NPLs as 20.2-nm NPLs. The atomic-resolution high-angle annular dark-filed scanning transmission electron microscopy (HAADF-STEM) images (Fig. 1e,f) were recorded from a single 20.2-nm NPL. It is clear that a number of bright nanoparticles were generated (Fig. 1e) when the NPL was exposed to high-intensity electron beam due to the occurrence of in-situ reduction process.8 As shown in Fig. 1f, the periodic lattice


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fringes with spacing of 0.82 nm and 0.83 nm could be clearly distinguished, which correspond to the (100) and (010) planes of orthorhombic CsPbBr3, respectively. The (001) plane could be therefore deduced as the crystal plane exposing on surface of CsPbBr3 NPLs. Considering the thickness of 3.1 nm and lattice spacing of 1.17 nm between (001) planes, we can conclude that thickness

of

such

20.2-nm

NPLs

is

about

three

unit

cells.

The

corresponding

fast-Fourier-transformation (FFT) pattern (the inset of Fig. 1f) is also well consistent with the corresponding atomic-resolution HAADF-STEM image. To study the dependence of lateral size on the excitonic behaviors later, we also prepared CsPbBr3 NPLs with different lateral sizes by adjusting the reaction temperature in a proper range. The NPLs with average lateral sizes of 7.3 nm and 14.8 nm (Fig. S2) were produced by decreasing the reaction temperature to 100 and 110oC while maintaining other conditions the same, respectively. Notably, the thickness of NPLs almost kept the same while the lateral size changed. Likewise, we denote the NPLs with average lateral sizes of 7.3 nm and 14.8 nm as 7.3-nm NPLs and 14.8-nm NPLs for simplification.

Figure 1. Structural characterizations for 20.2-nm CsPbBr3 NPLs. (a) XRD pattern. (b, c) The representative TEM images of the NPLs with two types of assembled configurations: (b) edge-to-edge configuration, and (c) face-to-face configuration. Scale bars: 50 nm. (d) Histogram of the NPLs showing the distributions of thickness and lateral size. (e) High-magnification 5 ACS Paragon Plus Environment


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HAADF-STEM image. Scale bar: 5 nm. (f) Atomic-resolution HAADF-STEM image and the corresponding FFT pattern in the inset. Scale bar: 1 nm.

Fig. 2a shows the room temperature PL spectra of the 7.3-nm, 14.8-nm, and 20.2-nm NPLs. The emission peaks exhibit apparent blue-shifts from 2.54 to 2.59 eV with decreasing lateral size, indicating increase of quantum confinement effect. We note that the thicknesses of the three samples are very close (Fig. S2b,d), suggesting that lateral size is the dominant factor for the blue shift. Time-resolved PL decays of the NPLs shown in Fig. 2b clearly indicate their excellent mono-exponential decay at room temperature. Such a decay feature is generally indicative of either excitonic or trap-assisted recombination.8 The latter can be safely excluded due to the low density of trap states in our NPLs, as evidence by the excitation-density dependent PLQY measurements in Fig. 2c. From the plot, the PLQY of the drop-casted NPL films remain almost unchanged when the excitation density varies for more than 2 orders of magnitude within the low-level excitation regime (1-300 mW/cm2, ~1013-1015 cm-3). If the trap density in the NPL films is similar to or higher than the corresponding photocarrier density, a trap-filling effect resulting in increase of PLQY with increasing excitation density should be observed.8, 24 We note that in perovskites the trap-mediated nonradiative recombination is characterized by slow decay due to accumulation of charges and slow depopulation,25 which is different from those thermally activated nonradiative channels that decay very fast. Therefore, the low trap density is not contradictory to the PLQY of 50-70%. The slower decay of the 7.3-nm NPLs suggests higher trap density than that in other two samples, in consistent with its lower PLQY. Such trap sites should be mainly from surface defects due to the highly ionic nature and good stoichiometry of perovskite materials, in which point defects are not likely to reside within the volume of the nanocrystals. The excitonic nature of the emissions is also supported by excitation density-dependent PL experiments of the NPL films (Fig. S3). The pure exciton emission, low trap density, high luminescence efficiency, and narrow size distribution of the quasi-2D CsPbBr3 NPLs make them an ideal system for the dynamics studies of exciton recombination.


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Figure 2. PL properties of the CsPbBr3 NPLs. (a) PL spectra of the 7.3-nm, 14.8-nm, and 20.2-nm NPLs. (b) PL decay traces of the 7.3-nm, 14.8-nm, and 20.2-nm NPLs. The system response is also plotted as the gray line. (c) PLQY as a function of excitation density. The measurements are repeated through an increase-decrease cycle of the excitation density from 1 to 400 mW cm-2. All the measurements are taken at room temperature.

We further studied the recombination dynamics of excitons for the produced quasi-2D CsPbBr3 NPLs. Temperature-dependent PL decay measurements (Fig. S4) show prolonged PL lifetime with increasing temperature, which is a typical feature of giant oscillator strength effect in 2D system.16 The radiative lifetime (τr) was then extracted from temperature dependencies of both the PL intensity (Fig. S5) and total lifetime via τ r = τ PL / ηint , following the well-developed method.26 In this method, the internal quantum efficiency of PL, ηint, is assumed to be unit at low temperature limit because the nonradiative recombination channels are thermally activated.27 This is reasonable in our case considering the measured high PLQY at room temperature and the relative PL intensity between the low temperature and room temperature. The calculated radiative lifetimes of excitons for the NPLs are shown in Fig. 3a. For the 14.8-nm and 20.2-nm NPLs, the radiative lifetimes show similar temperature dependence. Specifically, the radiative lifetimes are almost constant at temperatures below 100 K, while increasing linearly with a slope of ~11.8 ps/K when the temperature is above 100 K. The log-log plot further confirms the T1 dependence of radiative lifetime (Fig. S6), which clearly corresponds the thermalization of 2D excitons based on the previously reported features for III-V and II-VI semiconductor quantum wells.28-29 The observations of such 2D excitons also verify the low trap densities of the NPLs. For the 7.3-nm NPLs, however, the radiative lifetime keeps almost unchanged till room temperature, suggesting a 0D nature.28 The characteristics is also consistent



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with the early reports for quantum dots.28 The shorter radiative lifetime for the 14.8-nm and 20.2-nm NPLs than that of the 7.3-nm NPLs could be due to the giant oscillator strength of the ground exciton states in quasi-2D NPLs.16 Taken together, the results clearly demonstrate a transition from 0D excitons to 2D excitons in the NPLs when the lateral size increases from 7.3 to 14.8 nm. Such transition can be attributed to the weaker quantum confinement effect when the lateral size increases from 7.3 to 14.8 nm considering the exciton Bohr diameter for bulk CsPbBr3 is about 7 nm.1

Figure 3. 2D and localization behaviors of excitons in the CsPbBr3 NPLs. (a) Radiative lifetime as a function of temperature. For 14.8-nm and 20.2-nm NPLs, the radiative lifetimes show linear dependence on temperature (characteristic of 2D excitons) when T>100 K. The solid curve represents linear fit with a slope of 11.8 ps/K. For 7.3-nm NPLs, the radiative lifetime is almost independent on temperature within the entire temperature range. (b) PL decay traces of the 20.2-nm NPLs recorded at different emission energies. Inset: The PL decay time as a function of emission energy, revealing localization of excitons.

The almost constant radiative lifetime at temperatures below 100 K is similar to that reported in epitaxial quantum wells,29-31 which has been attributed to localization effect of excitons and the change of density of states from 0D to 2D with increasing temperature.31-32 We note that the effect of phase transitions on the change in 0D to 2D exciton behavior around 100 K can be excluded, because no abrupt change of band gap energy was observed in temperature-dependent PL spectra (Fig. S5d). To confirm the localization of excitons we measured the 8 ACS Paragon Plus Environment

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spectral-dependent PL decay at low temperature. Fig. 3b shows the PL decay spectra of the 20.2-nm NPLs monitored at different emission energies. One can observe a decrease of lifetime with increasing emission energy (inset of Fig. 3b). Moreover, as the emission energy increases, the spectrum gradually changes from mono-exponential to bi-exponential decay, and the weight of the fast decay becomes higher. These features are typical for exciton localization as those have been frequently reported in semiconductor quantum wells.33-35 Notably, the decay of 14.8-nm NPLs also shows similar features (Fig. S7). From the slope of 11.8 ps/K in Fig. 3a, we can then obtain the intrinsic radiative lifetime, τ0, of the free exciton in perfect quasi-2D CsPbBr3 NPLs. For 2D quantum structures, in the limit h 2k 02

2D Behaviors of Excitons in Cesium Lead Halide Perovskite

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