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Energy transfer and wavelength tunable lasing of single perovskite alloy nanowire
Journal Pre-proof Energy Transfer and Wavelength Tunable Lasing of Single Perovskite Alloy Nanowire Bing Tang, Yingjie Hu, Jian Lu, Hongxing Dong, Nanli Mou, Xinyu Gao, Hu Wang, Xiongwei Jiang, Long Zhang PII:
S2211-2855(20)30198-1
DOI:
https://doi.org/10.1016/j.nanoen.2020.104641
Reference:
NANOEN 104641
To appear in:
Nano Energy
Received Date: 30 December 2019 Revised Date:
10 February 2020
Accepted Date: 21 February 2020
Please cite this article as: B. Tang, Y. Hu, J. Lu, H. Dong, N. Mou, X. Gao, H. Wang, X. Jiang, L. Zhang, Energy Transfer and Wavelength Tunable Lasing of Single Perovskite Alloy Nanowire, Nano Energy, https://doi.org/10.1016/j.nanoen.2020.104641. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier Ltd. All rights reserved.
Graphical Abstract We realized a broadly and continuously wavlength-tunable lasers (480-525 nm) in single perosvkite alloy nanowire. An anion-diffusion process was proposed to construct the nanowire with a widely tunable bandgap (2.41-2.82 eV), the kinetics and atomic-scale mechanism of anion diffusion were analyzed. The carrier dynamics were systematically studied to clarify the energy transfer of single perovskite alloy nanowire.
Energy Transfer and Wavelength Tunable Lasing of Single Perovskite Alloy Nanowire Bing Tang†ǁ+€, Yingjie Hu‡€, Jian Lu+§€, Hongxing Dong†ǁ*, Nanli Mou†, Xinyu Gao†, Hu Wang+§#, Xiongwei Jiang†, Long Zhang†ǁ* † Laboratory of Micro-Nano Optoelectronic Materials and Devices, Laboratory of Laser and Infrared Materials, Key Laboratory of Materials for High-Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai, 201800, China ǁ Hangzhou Institute for Advanced Study, University of Chinese Academy of Sciences, No.1, Sub-Lane Xiangshan, Xihu District, Hangzhou 310024, China + Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, China ‡ Nanjing Key Laboratory of Advanced Functional Materials, Nanjing Xiaozhuang University, Nanjing 211171, China. § Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai, 201210, China. # School of Physical Science and Technology, ShanghaiTech University, Shanghai, 201210, China
Corresponding Author: [email protected]; [email protected]
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ABSTRACT Single perovskite alloy nanowire capable of emitting lasing broadly and continuously is highly desirable for the miniaturization and integration of all-photonic devices. However, due to the limitation of a soft and dynamic crystal lattice and the synthesized strategy, single perovskite nanowire with single-band lasing emission is mostly observed. Here, we propose a solid-solid anion-diffusion process to construct single CsPbCl3-3xBr3x perovskite alloy nanowire with a widely tunable bandgap from 2.41 to 2.82 eV and a regularly geometrical structure. We realized a broadly and continuously tunable nanolaser from 480 to 525 nm in single CsPbCl3-3xBr3x nanowire, as different spots along the length serve as a gain medium and microcavity simultaneously. The kinetics and atomic-scale mechanism of solid-solid anion diffusion were analyzed by a quantitative study and theoretical calculations, giving a small activation energy of halide migrations. The dynamics of carrier transportations revealed the energy transfer in single CsPbCl3-3xBr3x alloy nanowire, that’s why broadly tunable lasers are difficult to realize in single isolated nanowire. Our work proposes a new strategy to construct single perovskite alloy nanowire for achieving a broadly and continuously tunable laser, clarifies the mechanism and kinetics of anion diffusion in perovskite alloy nanowires.
Keywords: Perovskite Alloy Nanowire; Anion Diffusion Mechanism; Energy Transfer; Broadly Tunable Nanolaser
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1. Introduction Semiconductor nanowire lasers with a broadband and continuous wavelength tunability are highly desirable for integrated photonic and optoelectronic devices.[1–9] Although many broadly tunable lasers have been reported, it is still rely on the corporation of several spatially separating different nanowires on a single device.[10–17] This largely restricts the further miniaturization and integration of on-chip photonic devices. Single alloy nanowire capable of emitting lasing broadly and continuously in a large spectral range, which has attracted great attention, and is proposed to be an ideal candidate for constructing miniaturized light sources of a highly integrated all-photonic circuit. Tremendous efforts have been paid for the development of broadly tunable lasers in single alloy nanowire. However, this has proven particularly challenging due to the strict requirements on single isolated nanowire, such as the regularly geometrical structure, a widely and continuously tunable gain profile, and high optical gain to overcome the strong self-absorption of low-energy side. In fact, single CdSSe alloy nanowire has been demonstrated to show a tunable photoluminescence (PL) emission.[18–20] Nevertheless, broadly tunable lasers are seriously surpassed by the highly absorption of the low-energy side and
thus
largely
dependent
on
the
assistance
of
a
complex
and
sophisticated
micromanipulation.[21–23] Therefore, the selection of a high gain material and the construction of high-quality single alloy nanowire with a gradually changed bandgap are very crucial for realizing a broadly and continuously tunable laser. Perovskite alloy nanowires may provide such a way. Low threshold lasers were widely demonstrated in hybrid and all-inorganic perovskite nanowires.[10,11,24–29] Moreover, Yan et al. recently observed simultaneous two-color lasing from single CsPbBrxI3-x nanowire fabricated by a chemical vapor deposition (CVD) method.[30] These reports identify the high optical gain
3
of perovskite compounds, and thus indicates the potential of realizing a broadly tunable laser in single perovskite nanowire. Nevertheless, due to the limitation of a soft and dynamic lattice, most fabricated perovskite nanowires usually exhibit a homogeneous halide distribution and PL emission, some structures may even have serious phase separations.[27,31,32] A new mechanism or strategy should be exploited to construct and optimize the geometrical structure, crystal quality and gain profile of single perovskite nanowire for the realization of a broadly and continuously tunable laser. Notably, the small activation energies of halide migrations enable a slow solid-solid anion diffusion between perovskite nanostructures, this allows us to realize a well controllability in halide distribution, PL emission and structural morphology of single perovskite nanowire.[33–35] Therefore, single perovskite nanowire with a broadly and continuously tunable PL emission along the length can be constructed through an anion-diffusion process while maintaining its regularly geometrical structure, high quality crystal and excellent optical gain property. Such perovskite alloy nanowire not only offers a promising route to explore a broadly and continuously wavelength-tunable laser, but also provides a better platform to study the mechanism and kinetics of solid-solid anion-diffusion process.[1,3,18,36] The deep investigations of anion diffusion may provide insight for analyzing the unique properties and performance of perovskite materials. Herein, we realize a broadly and continuously tunable laser in single CsPbCl3-3xBr3x perovskite alloy nanowire, where different positions along the alloy nanowire function as both a spatially tunable gain medium and microcavity. The nanowire showing a regularly geometrical structure and a gradually changed bandgap along the length was constructed through a solidsolid anion-diffusion process. A quantitative study using the confocal PL measurements was performed to analyze the detailed evolution property of perovskite alloy nanowire and the
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kinetics of the anion-diffusion process. The results show a time-dependent behavior of halide distribution along the nanowire, which can be used to fabricate single perovskite alloy nanowire with PL and lasing emission over different spectral range. The atomic-scale mechanism of Branions diffusion was further clarified by the density functional theory (DFT) calculations, revealing a low activation energy (Ea~0.36 eV for P1; Ea~0.22 eV for P2) of halide migrations in perovskites. Time-resolved photoluminescence measurements were carried out to study the energy transfer in single alloy nanowire, which is responsible for higher lasing thresholds of Clrich regions. 2. Experiments 2.1 The synthesis of CsPbCl3, CsPbBr3, composition-graded CsPbCl3-3xBr3x nanowires and CsPbBr3 microspheres Both the CsPbCl3 and CsPbBr3 nanowires were synthesized by using a home-built chemical vapor deposition system. For the synthesis of CsPbCl3 nanowires, a 0.2 g mixture of powerded cesium chloride (CsCl, 99.999%, Sigma-Aldrich) and lead chloride (PbCl2, 99.999%, SigmaAldrich) with molar ratio 1:1 was placed in the heating center of the tube furnace as the vapor sources. The mixture of cesium bromide (CsBr, 99.999%, Sigma-Aldrich) and lead bromide (PbBr2, 99.999%, Sigma-Aldrich) powers in the boat was used as the vapor sources for the preparation of CsPbBr3 nanowires. Three piecs of freshly cleaved mica substrateds (11 mm × 14 mm) were mounted ~4-6 cm away from the heating center in the down stream. Prior to the deposition, the system was pumped to be a vacuum enviroment and then a high-purity nitrogen flow at 60 sccm was guided inside. The temeprature of the heating center quickly rose to 550 ℃ for the deposition of CsPbCl3 nanowires. The reaction temperature for CsPbB3 nanowires is 570 ℃. The temperature of whole system keep constant for 30-60 minutes before cooling down
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to room temeprature. Then mica substrates with as-synthesized CsPbCl3 or CsPbBr3 nanowires can be taken out. The CsPbBr3 microspheres were synthesized according to our previous reports. To obtain the composition-graded CsPbCl3-3xBr3x nanowires, an anion-exchange reaction at 220 ℃ was carried out between CsPbBr3 mciropsheres and CsPbCl3 nanowires. The characterizations of as-prepared CsPbCl3-3xBr3x nanowires are highly dependent on the reaction time and temperature, and the amount of CsPbBr3 microspheres. In our experiments, CsPbCl3 nanowires with L>30 µm are usually used to construct composition-graded nanowires with a widely tunable bandgap. The reaction time varies for different CsPbCl3 nanowires, about 8-16 minutes is optimal. 2.2 Structural,
Optical
characterizations
and
Time-Resolved
Photoluminescence
Measurements The scanning electron microscopy (SEM) images of as-prepared perovskite nanowires were collected using the field-emission scanning electron microscopy (FE-SEM; Auriga S40, Zeiss, Oberkochen, Germany) operated at 1 kV to identify their physical structures. The X-ray diffraction data of as-synthsized perovskite samples were obtained through a X-ray diffractometer with Cu-Kα radiation (PANalytical Empyrean) to analyze their crystal structures. The real-color images of perovskite nanostructures were recorded using Horiba Raman spectroscopy (LabRAM HR Evolution) under the illumination of a 405 CW laser. All the PL and lasing spectra of as-grown samples were perfromed using the confocla micro-photoluminescence system equipped with Horiba Raman spectroscopy, a 405 CW laser and a femtosecond laser (Libra, Coherent, ~40 fs, 10 kHz). The Horiba Raman spectroscopy consists of a 1800 grating, a multichannel ari-cooled CCD detector (Syncerity OE), a 50X and 10X objective; its spectral resolution is as small as~0.0166 nm. For the PL measurements of composition-graded CsPbCl3-
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3xBr3x
nanowires, a 405 CW or 400 nm fs laser was used as the excitation source. A BBO crystal
was placed behind the 800 nm amplifier laser soure to generate 400 nm fs-laser as the pumping soure of lasing experiments. In the lifetime experiments, a Ti:sapphire system (Chameleon Vision, Coherent) with a repetition rate of 80 MHz and tunable wavelength from 680 to 1080 nm is used, which delivers pulses of ~150 fs duration at a center wavelength of 800 nm. Together with a frequency doubler, the laser source provided a wavelength range from 340 to 490 nm for excitation. A spectrometer (iHR550, Jobin Yvon) was used to collect the fluorescence of NGQDs. By employing the time-correlated single photon counting (TCSPC, HydraHarp 400, PicoQuant) technique, we carried out the measurements of the fluorescence lifetime on single CsPbCl3-3xBr3x alloy nanowire with a single photon detector (PMA Hybrid 06, PicoQuant). 2.3 Theoretical Calculations Density functional theory (DFT) calculations were performed with CASTEP package on the basis of the plane-wave-pseudo-potential approach. The exchange and correlation interactions were modeled using the generalized gradient approximation (GGA) and the Perdew-BurkeErnzerhof (PBE) functional. The interactions between the atomic core and the valence electrons were described by the norm-conserving pseudopotential for energy band calculations and ultrasoft pseudopotential for the transition states search, respectively. The norm-conserving and ultrasoft pseudopotentials were used with a cutoff energy of 820 and 290 eV, respectively. During the geometrical optimization, all atoms were allowed to relax without any constraints until the convergence thresholds of maximum displacement, maximum force and energy were smaller than 0.001 Å, 0.03 eV/Å and 1.0 × 10–5 eV/atom, respectively. The diffusion of Br(Cl) vacancy was investigated by searching the possible diffusion route and identifying the migration transition state with the lowest diffusion energy barrier. We assumed that the migration of Br(Cl)
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vacancy is the self-diffusion from one defective lattice site to its nearest neighboring Br(Cl) site. The diffusion energy barrier is the energy difference between the total energies of transition state and the initial defective structure. The transition state is searched by the generalized synchronous transit (LST/QST) method implemented in the CASTEP code. The algorithm starts from a linear synchronous transit (LST) optimization, and continues with a quadratic synchronous transit (QST) maximization process. Thereafter, the conjugate gradient (CG) minimization is conducted from the obtained LST/QST structure to refine the geometry of transition state. The LST/QST/CG calculations are repeated till a stable transition state is obtained. 3. Results and Discussion The construction of single perovskite alloy nanowire with a gradually changed bandgap is the critical for achieving a broadly tunable laser. The strong ionic bonding nature of perovskite structures enables a soft and dynamic crystal lattice, and thus we propose to construct single CsPbCl3-3xBr3x perovskite alloy nanowire via a solid-solid anion-diffusion process, as shown in Figure 1a. Single CsPbCl3-3xBr3x alloy nanowire with a gradually changed halide distribution along the length can be induced through the anion-diffusion process at 220 ℃. Detailed information about preparation methods can be found in experimental section. Theoretically, the electronic structures of the alloy nanowire were analyzed in Figure 1b. The results show that the bandgap continuously narrows with the increasing of Br- anions, resulting in a continuously changed PL emission along the length. Figure 1c gives the real-color image of one typical CsPbCl3-3xBr3x alloy nanowire under the illumination of a 405 CW laser. The alloy nanowire clearly presents a continuous color transition from violet to blue, cyan and green; this indicates the formation of a gradually changed bandgap along the length. Spatially PL measurements were performed to further clarify that, as shown in the bottom inset of Figure 1c. It can be clearly seen
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that PL spectra collected from every single spot along the length shows a strong and single-peak emission; the typical PL emission wavelength covers the range from 440 nm (2.41 eV) to 515 nm (2.82 eV) as the excited spot moves from one end to the other. This tendency agrees well with the theoretical calculations in Figure 1b. Single CsPbCl3-3xBr3x perovskite alloy nanowire with a continuously and widely tunable bandgap is constructed here via a solid-solid aniondiffusion process. To clarify the construction of CsPbCl3-3xBr3x alloy nanowire, the kinetics of solid-solid anion-diffusion process at 220 ℃ were systematically analyzed using the confocal PL measurements. As shown in Figure 2a and 2b, CsPbCl3 perovskite nanowires with a homogeneous PL emission of ~ 415 nm were first constructed through a CVD process. In Figure 2a, the evolution phase of CsPbCl3 nanowire is depicted by three-stage process. At stage I (see Figure 2c and 2d), an anion-diffusion process initiates at the interface between CsPbCl3 nanowire and CsPbBr3 sources; with time increases, Br- anions diffuse into one part of the nanowire and induce the redshift of PL emission wavelength, the other part keeps constant. Finally, the Branions have diffused through the whole nanowire, resulting in a purely gradient nanowire (λ: 415-λ1 nm, λ1<525). At stage II (see Figure 2f, 2g and 2h), the Br- anions continually diffuse along the length but slows down; the PL emission wavelength of the whole nanowire redshifts. In this phase, the whole nanowire is also composition-graded along the length (λ: λ2-λ3 nm, λ2>415, λ1<λ3<525). At a certain time, the PL emission wavelength of one side reaches the maximum (near the CsPbBr3 source). At stage III (see Figure 2j, 2k, 2l), the halide distribution and PL emission wavelength of the whole nanowire comes to be identical along the length (λ: λ4 nm, λ3>λ4>λ2). Thus, single CsPbCl3-3xBr3x alloy nanowire covering different spectral range can be fabricated by carefully controlling the reaction time. Taking one CsPbCl3 nanowire as an
9
example, the kinetics of the anion-diffusion process is quantitatively analyzed. The composition profiles of single alloy nanowire recorded at different time are plotted in Figure 2e, 2i and 2m, which agrees well with the three-stage process. In Figure 2e, the Br- content and PL emission wavelength of one part grows and the other part far away from the CsPbBr3 source keeps constant with the increasing of time. In Figure 2i, the Br- content and PL emission wavelength of the whole alloy nanowire grows. Finally, in Figure 2m, the PL emission wavelength of the whole nanowire comes to be identical with time increases. The diffusion length versus the square root of reaction time is well fitted by a linear curve in Figure S1a, indicating a constant diffusion coefficient in this process.[33,35,37] And a diffusion coefficient of 3.40×10-10 cm2/s can be extracted from the composition-distance curve in Figure S1b, this agrees well with previous reports by Jin and co-workers.[33] The kinetics of the anion-diffusion process now have been quantitatively analyzed using the confocal PL measurements. The atomic-scale mechanism of the solid-solid anion-diffusion process is analyzed theoretically using DFT. A vacancy-assisted mechanism is mainly considered here in lead halide perovskites.[38–40] Figure 3a schematically depicts the mechanism of the whole anion-diffusion process. The picture is divided into three parts: CsPbCl3 regions, where provides the Cl- anions (left part indicated by pink dash); diffusion regions, where the anion diffusion mainly occurs; CsPbBr3 regions, where provides the Br- anions (right part signaled by green dash). The cyan and pink arrows represent the migrations of VBr- and VCl-, respectively, they both consist of P1 and P2 paths (see Figure 3b and 3c). An anion migration easily initiates when CsPbCl3 and CsPbBr3 perovskites contact, due to the low activation energy (see Figure 3d, 3e, S2a and S2b). With time increases, the Br- and Cl- anions diffuse deeply through two paths of P1 and P2; the diffusion region moves and broadens, resulting in a gradient halide distribution along the length of the
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alloy nanowire. As shown in Figure 3b and 3c, the P1 and P2 paths represent anion migrations along <100> and <110> directions, respectively. The energy profiles and typical transition states of VBr- migrations along P1 and P2 paths are presented in Figure 3d and 3e, the results about VClmigrations are provided in Figure S2. It can be seen that the activation energies Ea of VBrmigrations are both very small, while the Ea~0.36 eV of <100> direction is higher than 0.22 eV of <110> direction. To clarify the origin of this difference, we analyze the cubic crystal structure of CsPbX3, and find the VBr- migrations along <100> direction break one more Pb-Br bonds than <110> direction. The X-ray diffraction patterns of CsPbCl3 and CsPbBr3 nanowires with cubic phase are presented in Figure S3. Compared with conventional III-V semiconductors such as 3-5 eV of GaAs, the lower activation energy allows a feasible and fast anion diffusion between different halide perovskites.[41] Here, the anion-diffusion process follows a vacancy-mediated mechanism, thus a diffusion coefficient of 1.43×10-9 cm2s-1 (p1 path) and 3.86×10-8 cm2s-1 (p2 path) can be estimated from the Arrhenius equation.[33,38] The result is closer to the experimental value obtained from Figure S1b, the difference may originate from vacancy concentrations or the variation of Br-/Cl- ratio.[42,43] Herein, we analyzed the atomic-scale mechanism of the anion-diffusion process, revealed the low activation energy of halide migrations in perovskite structures. The dynamics of carrier transportation are the critical to decouple the optical properties of micro/nanostructures, here, the PL dynamics of single CsPbCl3-3xBr3x alloy nanowire were analyzed. Theoretically, we calculated the typical energy-level structures of different positions along single CsPbCl3-3xBr3x alloy nanowire using DFT. The results show that the bandgap of the alloy nanowire continuously narrows with the increasing of Br- content along the axial direction. Consequently, an energy transfer is expected between the Cl-rich and the Br-rich regions, as
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shown in Figure 4a. We employed the time-resolved photoluminescence spectroscopy (TRPL) to study the dynamics of carrier transportation, by measuring the spatially separated carrier lifetime from high-bandgap Cl-rich region to the low-bandgap Br-rich region. The PL decay curves for emission peak from 415 to 515 nm are shown in Figure 4b. Here, the data of 415 and 515 nm comes from two uniform nanowires with a homogeneous PL emission as a comparison; the other decay curves from 450 to 500 nm are measured on single CsPbCl3-3xBr3x alloy nanowire. These curves were all well fitted by a bi-exponential function, y(t) = A0 + A1e-t/τ1 + A2e-t/τ2, where τ1 and τ2 represent fast decay time and slow decay time, A1 and A2 are their percentage contribution respectively. The fitting parameters were summarized in Table S1 and plotted in Figure 4c and 4d. With increasing the ingredient of bromide, we found that the fast decay (0.14-3.2 ns) and slow decay (0.36-8.8 ns) both raise. However, the fast component ratio reduces from 89 % to 16 %, corresponding to the slow component ratio dilates from 11 % to 84 % significantly. These variation tendencies are consistent with the previous reports.[44,45] Therefore, we attributed the fast decay to carrier diffusion processes and the slow decay to radiative recombination respectively. The fast component ratio reduces rapidly from 450 to 500 nm, this trend indicates the carrier flow formed from Cl-rich region to Br-rich region, due to the graded bandgap level. In the Cl-rich region, most carrier transfer the energy to lower bandgap region rather than radiative recombination, which cause lower quantum yield than Br-rich region.[44–47] In Br-rich region, it’s longer recombination time and higher quantum yield would enable lower lasing threshold. We noticed that Fu et al. reported a negative component ratio which indicates the rise kinetics, whereas positive component ratio indicates decay kinetics. Here, all the component ratio we got are positive. This difference may come from sample structure, Fu et al. investigated twodimensional layered halide perovskites, and the energy transfer direction was same with the laser
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propagation, so the higher carrier could concentrate in the lowest energy layer.[44] Additionally, the PL intensity of single perovskite nanowire also supports the presence of energy transfer (Figure S4). There are two channels to cause the photon propagation in nanowire, one is optical waveguide, the other is carrier transportation.[45] For uniform nanowire, the optical waveguide is dominant; the PL intensity of both two ends is much higher. For alloy nanowire, the carrier diffusion driven by the gradient bandgap is dominant. It can be seen that the PL intensity of the gradient nanowire increases gradually from blue region to green region. Similar behavior is also observed in organic-inorganic perovskite nanowire.[45] Single perovskite alloy nanowire with a regularly geometrical structure (see Figure S5) and a widely tunable PL emission is an ideal candidate for constructing a broadly wavelength-tunable laser. We study the lasing performance of single CsPbCl3-3xBr3x alloy nanowire using a confocal micro-photoluminescence system. As shown in Figure 5a, an individual CsPbCl3-3xBr3x alloy nanowire was excited by a femtosecond laser of 400 nm; the CsPbCl3-3xBr3x alloy nanowire has a widely tunable PL emission and a regular structure with a triangular cross section. At a high pump density, such a nanowire can behave as both a gain medium and microcavity to realize a broadly tunable lasing. We can see the typical lasing from 480 to 525 nm is successively observed as the excitation spot moves from one side to the other. Unlike CdSSe nanowire, single isolated perovskite nanowire can emit lasing broadly and continuously without cutting the lowenergy side or adding a coupling cavity.[21,22] This can be attributed to the high optical gain of perovskite materials and the unique advantages of solid-solid anion diffusion to construct a highquality perovskite nanowire with a gradually changed bandgap. Figure 5b shows the typical lasing spectra of single CsPbCl3-3xBr3x nanowire recorded at a high pump density, a widely tunable lasing at 480, 500, 510, 520 and 525 nm can be clearly observed. In contrary, single
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CsPbCl3-3xBr3x nanowire with a homogeneous halide distribution only exhibits single-band lasing emission, as shown in Figure S6. Figure 5c-g shows the power-dependent spectra for typical lasing at 480, 500, 510, 520 and 525 nm, respectively. It can be clearly observed that all these spectra undergo a transition from a broadband spontaneous emission to the stimulated emission with several sharp peaks with the increasing of pump density. At a relatively low pump density (below the lasing threshold), broadband emission of 467, 493, 502, 509, and 512 nm can be observed. When pump density reaches or exceeds their lasing thresholds, several new sharp peaks with narrow linewidth abruptly appears above the background of spontaneous emission. Further increasing the pump density, the intensity of lasing peak grows rapidly and finally the simulated emission dominates the whole process. A set of regular resonant peaks was clearly observed from these lasing spectra, which is attributed to a Fabry−Perot cavity. Taking lasing at ~500 nm as an example, its mode spacing can be simply given by: ∆λ = λ2/2ngL, where ∆λ and λ are the mode spacing and lasing wavelength respectively, ng represents the group refractive index, and L is the effective cavity length.[48−51] The effective cavity length L can be determined to be ~21 µm with a mode spacing (λ1=498.1 nm, λ2=499.2 nm) ∆λ~1.1 nm, which means that the lasing signal is attributed to the FP mode. The localized FP cavity is formed by the back scattering from the defects.[30,52] And, a detailed simulation of the electromagnetic field distribution using Comsol Multiphysics is presented in Figure S7, this further demonstrates the formation of a FP cavity lasing. It can be clearly seen that the optical field is well confined inside, thus the resonant modes are reflected back and forth along the length. As shown in Figure S8 and 5h, the lasing thresholds of 480, 500, 510, 520 and 525 nm are defined to be 35.0, 29.3, 26.5, 17.5, and 11.7 µJ/cm2, respectively. As the output lasing wavelength blueshifts, the lasing threshold increases from 11.7 to 35.0 µJ/cm2. This can be attributed to the energy transfer in
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single CsPbCl3-3xBr3x alloy nanowire, which has been clarified above. Similar behavior also has been reported in CdSSe nanostructures.[53–55] These results clearly demonstrate the capability of single CsPbCl3-3xBr3x perovskite alloy nanowire to realize a broadly and continuously tunable lasing over a large spectral range. 4. Conclusions In conclusion, we have proposed and constructed single perovskite alloy nanowire via a solid-solid anion-diffusion process for realizing a broadly and continuously tunable laser. Single CsPbCl3-3xBr3x alloy nanowire has a regularly geometrical structure and a broadly tunable PL emission from 440 to 515 nm along the length due to the spatially graded halide distribution. This alloy nanowire enabled us to explore a broadly tunable laser in single nanowire, as the different spots along the nanowire can serve as optical cavity and a widely tunable gain medium simultaneously. A quantitative study was carried out to analyze the dynamics of the solid-solid anion-diffusion process and the three-stage evolution property of perovskite alloy nanowire. DFT calculations of the anion-diffusion process clarified the intrinsic mechanism and low activation energies of both two halide-transport routes. The TRPL measurements revealed the energy transfer, which is the critical to analyze the unique properties of single perovskite alloy nanowire such as the higher lasing threshold of short-wavelength emission. Our work provides an avenue to realize a broadly and continuously tunable lasing in single perovskite nanowire, and an insight into understanding the atomic-scale mechanism and kinetics of the elemental anion diffusion of perovskites which are very important to analyze their unique properties and performance.
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Figure 1. (a) A schematic of CsPbCl3-3xBr3x perovskite alloy nanowire with a widely tunable PL emission constructed through a solid-solid anion-diffusion process. (b) The electronic sturtcures of different spots along single CsPbCl3-3xBr3x alloy nanowire with the typical halide content, showing a gradually changed bandgap. (c) The real-color image and typical PL spectra of single CsPbCl3-3xBr3x perovskite alloy nanowire.
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Figure 2. The kinetics of the solid-solid anion-diffusion process. (a) A schmatic of the whole process of fabricating CsPbCl3-3xBr3x perovskite alloy nanowire. (b) The optical mages of CsPbCl3 nanowires. The typical PL images and spectra of CsPbCl3-3xBr3x alloy nanowires of stage I (c, d), stage II (f, g, h), stage III (j, k, l), their typical PL emission wavelength is listed in the top-left part. A quantitative analysis of composition evolution at stage I (e), stage II (i), stage III (m) on single perovskite nanowire.
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Figure 3. Atomic-scale mechanism of the solid-solid anion-diffusion process. (a) A schematic of the anion-diffuison process between CsPbCl3 and CsPbBr3 perovskites. The cyan and pink arrows represent by the migrations of VBr- and VCl- anions, respectively. The anion migrations along <100> and <110> directions are signaled by P1 (b) and P2 (c), respectively. The corresponding energy profiles of Br- anion migrations along P1 (d) and P2 (e) paths.
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Figure 4. The energy transfer along single CsPbCl3-3xBr3x perovskite alloy nanowire. (a) Schematic illustration of the energy transfer along the length of the nanowire, the typical molecular orbitals of different spots along the nanowire are presented in the top and bottom parts. (b) Typical time-resolved photoluminescence curves (hollow circle) of single CsPbCl3-3xBr3x alloy nanowire in various peak positions.The fitted curves (solid line) are calculated using a biexponential function. (c) The change trend of fast decay time and slow decay time with the lightemitting peak varying from 415 nm to 515 nm. (d) The decay probability change trend of fast decay component and slow decay component with the light-emitting peak varying from 415 nm to 515 nm. It is noted that the data of 415 nm and 515 nm is recorded from two nanowires with a uniform PL emission.
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Figure 5. A broadly and continuously wavelength-tunable laser in single CsPbCl3-3xBr3x perovskite alloy nanowire. (a) The schematic of optically pumping lasing experiments with a 400 nm fs-laser (~40 fs, 10 kHz) as the excitation source. (b) Typical lasing spectra obtained from single CsPbCl3-3xBr3x alloy nanowire at a high pump density. The power-dependent PL spectra of typical lasing at 480 nm (c), 500 nm (d), 510 nm (e), 520 nm (f) and 525 nm (g). (h) The lasing thresholds are plotted versus the lasing wavelength.
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Acknowledgments This work was supported financially by the NSFC (61675219, 61875256, 91950201). H. Dong acknowledges the Youth Top-notch Talent Support Program in Shanghai. Y. Hu acknowledges the support of the Excellent Science and Technology Innovation Group of Jiangsu Province. We thank Q. Si for helping design and draw the schematic picture of Figure 5a. Author Contributions B.T. prepared the samples, performed the sample characterizations and designed the experiment, guided by H.D. and L.Z., Y.H. carried out the theoretical calculations of electronic structures and anion-diffusion mechanism, J.L. and H.W. carried out the time-resolved photoluminescence measurements and the data analysis, N.M. performed the lasing mechanism analysis and numerical simulations, X.J. and X.G. carried out the optical experimental measurements. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. €These authors contributed equally. Declaration of competing interest The authors declare no competing financial interest. Notes Color should be used for figures1-5 in print. Appendix A. Supplementary data Supplementary data to this article can be found online at Elsevier website.
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Vitae
Bing Tang received his Bachelor's and Master’s degrees from Central South University and Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences, respectively. He is now a doctor candidate in SIOM, Chinese Academy of Sciences. His research interests mainly focus on the controllable growth of novel perovskite microcavity structures and the study of high quality nanolasing in low-dimensional perovskite micro/nanostructures.
Yingjie Hu received his Ph.D. degree from Nanjing University of Science and Technology in 2015. He joined Nanjing Xiaozhuang University as an associate professor since 2018. His interests focus on studying the relationship between materials structures and properties with applications in material design using theory, modeling and computation as tools.
Jian Lu received his optics Ph.D. degree from Fudan University in 2012. He joined Shanghai Advanced Research Institute, Chinese Academy of Sciences as an associate professor since 2013. His researches mainly focus on the optical properties of novel semiconductor, VCSEL laser and single photon source.
Hongxing Dong is currently a professor in Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences. He received his Ph.D. degree from Fudan University in 2011. His research interests mainly focus on the study of high quality microcavity lasers.
Nanli Mou received her Bachelor's degrees from Sichuan University, she is now a doctor
26
candidate in Shanghai Institute of Optics and Fine Mechanics (SIOM). Her research interests mainly focus on the metamaterials and plasmonics.
Xinyu Gao received the Bachelor's degree from the Department of Materials Science and Engineering of China Jiliang University in 2019. She is now a postgraduate student in Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, China. Her research interests mainly focus on the study of high-performance microcavity structure design and the growth mechanism.
Hu Wang is a master candidate at School of Physical Science and Technology of ShanghaiTech University. His research interests mainly focus on the light-matter interaction in perovskite semiconductor optical microcavity.
Xiongwei Jiang is now a professor in Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences. He received his Bachelor's and Master’s degrees both from East China University of Science and Technology. His research interests mainly focus on the growth of multifunctional and optical materials.
Long Zhang received his Ph.D. Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences in 2000. Afterwards, he worked one year as a Postdoctoral Fellow in SIOM and then worked as a Research Assistant Professor in University of Münster, Germany from 2001 to 2006. In 2006, he came back to China and join SIOM as a professor. His research interests focus on advanced infrared materials, laser ceramics and the high quality microcavity structures.
27
Highlights A broadly and continuously (480-525 nm) tunable laser is realized in single CsPbCl3-3xBr3x nanowire. Single CsPbCl3-3xBr3x nanowire with a widely tunable bandgap (2.41-2.82 eV) is constructed through an anion-diffusion process. The kinetics and atomic-scale mechanism are clarified in theory and experiment. The
energy
transfer
is
revealed
by
time-resolved
measurements in single perovskite alloy nanowire.
photoluminescence
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Tang Bing: Visualization, Conceptualization, Methodology, Writing - Original draft preparation Hu Yingjie: Writing - Original Draft, Visualization, Software, Methodology Lu Jian: Writing - Original Draft, Visualization, Methodology Dong Hongxing: Conceptualization, Writing-Review & Editing, Resources, Funding acquisition Mou Nanli: Formal analysis, Visualization, Software, Methodology Gao Xinyu: Investigation, Visualization Wang Hu: Investigation, Visualization Jiang Xiongwei: Data Curation, Resources, Supervision, Project administration Zhang Long: Supervision, Resources, Funding acquisition
S2211-2855(20)30198-1
DOI:
https://doi.org/10.1016/j.nanoen.2020.104641
Reference:
NANOEN 104641
To appear in:
Nano Energy
Received Date: 30 December 2019 Revised Date:
10 February 2020
Accepted Date: 21 February 2020
Please cite this article as: B. Tang, Y. Hu, J. Lu, H. Dong, N. Mou, X. Gao, H. Wang, X. Jiang, L. Zhang, Energy Transfer and Wavelength Tunable Lasing of Single Perovskite Alloy Nanowire, Nano Energy, https://doi.org/10.1016/j.nanoen.2020.104641. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier Ltd. All rights reserved.
Graphical Abstract We realized a broadly and continuously wavlength-tunable lasers (480-525 nm) in single perosvkite alloy nanowire. An anion-diffusion process was proposed to construct the nanowire with a widely tunable bandgap (2.41-2.82 eV), the kinetics and atomic-scale mechanism of anion diffusion were analyzed. The carrier dynamics were systematically studied to clarify the energy transfer of single perovskite alloy nanowire.
Energy Transfer and Wavelength Tunable Lasing of Single Perovskite Alloy Nanowire Bing Tang†ǁ+€, Yingjie Hu‡€, Jian Lu+§€, Hongxing Dong†ǁ*, Nanli Mou†, Xinyu Gao†, Hu Wang+§#, Xiongwei Jiang†, Long Zhang†ǁ* † Laboratory of Micro-Nano Optoelectronic Materials and Devices, Laboratory of Laser and Infrared Materials, Key Laboratory of Materials for High-Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai, 201800, China ǁ Hangzhou Institute for Advanced Study, University of Chinese Academy of Sciences, No.1, Sub-Lane Xiangshan, Xihu District, Hangzhou 310024, China + Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, China ‡ Nanjing Key Laboratory of Advanced Functional Materials, Nanjing Xiaozhuang University, Nanjing 211171, China. § Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai, 201210, China. # School of Physical Science and Technology, ShanghaiTech University, Shanghai, 201210, China
Corresponding Author: [email protected]; [email protected]
1
ABSTRACT Single perovskite alloy nanowire capable of emitting lasing broadly and continuously is highly desirable for the miniaturization and integration of all-photonic devices. However, due to the limitation of a soft and dynamic crystal lattice and the synthesized strategy, single perovskite nanowire with single-band lasing emission is mostly observed. Here, we propose a solid-solid anion-diffusion process to construct single CsPbCl3-3xBr3x perovskite alloy nanowire with a widely tunable bandgap from 2.41 to 2.82 eV and a regularly geometrical structure. We realized a broadly and continuously tunable nanolaser from 480 to 525 nm in single CsPbCl3-3xBr3x nanowire, as different spots along the length serve as a gain medium and microcavity simultaneously. The kinetics and atomic-scale mechanism of solid-solid anion diffusion were analyzed by a quantitative study and theoretical calculations, giving a small activation energy of halide migrations. The dynamics of carrier transportations revealed the energy transfer in single CsPbCl3-3xBr3x alloy nanowire, that’s why broadly tunable lasers are difficult to realize in single isolated nanowire. Our work proposes a new strategy to construct single perovskite alloy nanowire for achieving a broadly and continuously tunable laser, clarifies the mechanism and kinetics of anion diffusion in perovskite alloy nanowires.
Keywords: Perovskite Alloy Nanowire; Anion Diffusion Mechanism; Energy Transfer; Broadly Tunable Nanolaser
2
1. Introduction Semiconductor nanowire lasers with a broadband and continuous wavelength tunability are highly desirable for integrated photonic and optoelectronic devices.[1–9] Although many broadly tunable lasers have been reported, it is still rely on the corporation of several spatially separating different nanowires on a single device.[10–17] This largely restricts the further miniaturization and integration of on-chip photonic devices. Single alloy nanowire capable of emitting lasing broadly and continuously in a large spectral range, which has attracted great attention, and is proposed to be an ideal candidate for constructing miniaturized light sources of a highly integrated all-photonic circuit. Tremendous efforts have been paid for the development of broadly tunable lasers in single alloy nanowire. However, this has proven particularly challenging due to the strict requirements on single isolated nanowire, such as the regularly geometrical structure, a widely and continuously tunable gain profile, and high optical gain to overcome the strong self-absorption of low-energy side. In fact, single CdSSe alloy nanowire has been demonstrated to show a tunable photoluminescence (PL) emission.[18–20] Nevertheless, broadly tunable lasers are seriously surpassed by the highly absorption of the low-energy side and
thus
largely
dependent
on
the
assistance
of
a
complex
and
sophisticated
micromanipulation.[21–23] Therefore, the selection of a high gain material and the construction of high-quality single alloy nanowire with a gradually changed bandgap are very crucial for realizing a broadly and continuously tunable laser. Perovskite alloy nanowires may provide such a way. Low threshold lasers were widely demonstrated in hybrid and all-inorganic perovskite nanowires.[10,11,24–29] Moreover, Yan et al. recently observed simultaneous two-color lasing from single CsPbBrxI3-x nanowire fabricated by a chemical vapor deposition (CVD) method.[30] These reports identify the high optical gain
3
of perovskite compounds, and thus indicates the potential of realizing a broadly tunable laser in single perovskite nanowire. Nevertheless, due to the limitation of a soft and dynamic lattice, most fabricated perovskite nanowires usually exhibit a homogeneous halide distribution and PL emission, some structures may even have serious phase separations.[27,31,32] A new mechanism or strategy should be exploited to construct and optimize the geometrical structure, crystal quality and gain profile of single perovskite nanowire for the realization of a broadly and continuously tunable laser. Notably, the small activation energies of halide migrations enable a slow solid-solid anion diffusion between perovskite nanostructures, this allows us to realize a well controllability in halide distribution, PL emission and structural morphology of single perovskite nanowire.[33–35] Therefore, single perovskite nanowire with a broadly and continuously tunable PL emission along the length can be constructed through an anion-diffusion process while maintaining its regularly geometrical structure, high quality crystal and excellent optical gain property. Such perovskite alloy nanowire not only offers a promising route to explore a broadly and continuously wavelength-tunable laser, but also provides a better platform to study the mechanism and kinetics of solid-solid anion-diffusion process.[1,3,18,36] The deep investigations of anion diffusion may provide insight for analyzing the unique properties and performance of perovskite materials. Herein, we realize a broadly and continuously tunable laser in single CsPbCl3-3xBr3x perovskite alloy nanowire, where different positions along the alloy nanowire function as both a spatially tunable gain medium and microcavity. The nanowire showing a regularly geometrical structure and a gradually changed bandgap along the length was constructed through a solidsolid anion-diffusion process. A quantitative study using the confocal PL measurements was performed to analyze the detailed evolution property of perovskite alloy nanowire and the
4
kinetics of the anion-diffusion process. The results show a time-dependent behavior of halide distribution along the nanowire, which can be used to fabricate single perovskite alloy nanowire with PL and lasing emission over different spectral range. The atomic-scale mechanism of Branions diffusion was further clarified by the density functional theory (DFT) calculations, revealing a low activation energy (Ea~0.36 eV for P1; Ea~0.22 eV for P2) of halide migrations in perovskites. Time-resolved photoluminescence measurements were carried out to study the energy transfer in single alloy nanowire, which is responsible for higher lasing thresholds of Clrich regions. 2. Experiments 2.1 The synthesis of CsPbCl3, CsPbBr3, composition-graded CsPbCl3-3xBr3x nanowires and CsPbBr3 microspheres Both the CsPbCl3 and CsPbBr3 nanowires were synthesized by using a home-built chemical vapor deposition system. For the synthesis of CsPbCl3 nanowires, a 0.2 g mixture of powerded cesium chloride (CsCl, 99.999%, Sigma-Aldrich) and lead chloride (PbCl2, 99.999%, SigmaAldrich) with molar ratio 1:1 was placed in the heating center of the tube furnace as the vapor sources. The mixture of cesium bromide (CsBr, 99.999%, Sigma-Aldrich) and lead bromide (PbBr2, 99.999%, Sigma-Aldrich) powers in the boat was used as the vapor sources for the preparation of CsPbBr3 nanowires. Three piecs of freshly cleaved mica substrateds (11 mm × 14 mm) were mounted ~4-6 cm away from the heating center in the down stream. Prior to the deposition, the system was pumped to be a vacuum enviroment and then a high-purity nitrogen flow at 60 sccm was guided inside. The temeprature of the heating center quickly rose to 550 ℃ for the deposition of CsPbCl3 nanowires. The reaction temperature for CsPbB3 nanowires is 570 ℃. The temperature of whole system keep constant for 30-60 minutes before cooling down
5
to room temeprature. Then mica substrates with as-synthesized CsPbCl3 or CsPbBr3 nanowires can be taken out. The CsPbBr3 microspheres were synthesized according to our previous reports. To obtain the composition-graded CsPbCl3-3xBr3x nanowires, an anion-exchange reaction at 220 ℃ was carried out between CsPbBr3 mciropsheres and CsPbCl3 nanowires. The characterizations of as-prepared CsPbCl3-3xBr3x nanowires are highly dependent on the reaction time and temperature, and the amount of CsPbBr3 microspheres. In our experiments, CsPbCl3 nanowires with L>30 µm are usually used to construct composition-graded nanowires with a widely tunable bandgap. The reaction time varies for different CsPbCl3 nanowires, about 8-16 minutes is optimal. 2.2 Structural,
Optical
characterizations
and
Time-Resolved
Photoluminescence
Measurements The scanning electron microscopy (SEM) images of as-prepared perovskite nanowires were collected using the field-emission scanning electron microscopy (FE-SEM; Auriga S40, Zeiss, Oberkochen, Germany) operated at 1 kV to identify their physical structures. The X-ray diffraction data of as-synthsized perovskite samples were obtained through a X-ray diffractometer with Cu-Kα radiation (PANalytical Empyrean) to analyze their crystal structures. The real-color images of perovskite nanostructures were recorded using Horiba Raman spectroscopy (LabRAM HR Evolution) under the illumination of a 405 CW laser. All the PL and lasing spectra of as-grown samples were perfromed using the confocla micro-photoluminescence system equipped with Horiba Raman spectroscopy, a 405 CW laser and a femtosecond laser (Libra, Coherent, ~40 fs, 10 kHz). The Horiba Raman spectroscopy consists of a 1800 grating, a multichannel ari-cooled CCD detector (Syncerity OE), a 50X and 10X objective; its spectral resolution is as small as~0.0166 nm. For the PL measurements of composition-graded CsPbCl3-
6
3xBr3x
nanowires, a 405 CW or 400 nm fs laser was used as the excitation source. A BBO crystal
was placed behind the 800 nm amplifier laser soure to generate 400 nm fs-laser as the pumping soure of lasing experiments. In the lifetime experiments, a Ti:sapphire system (Chameleon Vision, Coherent) with a repetition rate of 80 MHz and tunable wavelength from 680 to 1080 nm is used, which delivers pulses of ~150 fs duration at a center wavelength of 800 nm. Together with a frequency doubler, the laser source provided a wavelength range from 340 to 490 nm for excitation. A spectrometer (iHR550, Jobin Yvon) was used to collect the fluorescence of NGQDs. By employing the time-correlated single photon counting (TCSPC, HydraHarp 400, PicoQuant) technique, we carried out the measurements of the fluorescence lifetime on single CsPbCl3-3xBr3x alloy nanowire with a single photon detector (PMA Hybrid 06, PicoQuant). 2.3 Theoretical Calculations Density functional theory (DFT) calculations were performed with CASTEP package on the basis of the plane-wave-pseudo-potential approach. The exchange and correlation interactions were modeled using the generalized gradient approximation (GGA) and the Perdew-BurkeErnzerhof (PBE) functional. The interactions between the atomic core and the valence electrons were described by the norm-conserving pseudopotential for energy band calculations and ultrasoft pseudopotential for the transition states search, respectively. The norm-conserving and ultrasoft pseudopotentials were used with a cutoff energy of 820 and 290 eV, respectively. During the geometrical optimization, all atoms were allowed to relax without any constraints until the convergence thresholds of maximum displacement, maximum force and energy were smaller than 0.001 Å, 0.03 eV/Å and 1.0 × 10–5 eV/atom, respectively. The diffusion of Br(Cl) vacancy was investigated by searching the possible diffusion route and identifying the migration transition state with the lowest diffusion energy barrier. We assumed that the migration of Br(Cl)
7
vacancy is the self-diffusion from one defective lattice site to its nearest neighboring Br(Cl) site. The diffusion energy barrier is the energy difference between the total energies of transition state and the initial defective structure. The transition state is searched by the generalized synchronous transit (LST/QST) method implemented in the CASTEP code. The algorithm starts from a linear synchronous transit (LST) optimization, and continues with a quadratic synchronous transit (QST) maximization process. Thereafter, the conjugate gradient (CG) minimization is conducted from the obtained LST/QST structure to refine the geometry of transition state. The LST/QST/CG calculations are repeated till a stable transition state is obtained. 3. Results and Discussion The construction of single perovskite alloy nanowire with a gradually changed bandgap is the critical for achieving a broadly tunable laser. The strong ionic bonding nature of perovskite structures enables a soft and dynamic crystal lattice, and thus we propose to construct single CsPbCl3-3xBr3x perovskite alloy nanowire via a solid-solid anion-diffusion process, as shown in Figure 1a. Single CsPbCl3-3xBr3x alloy nanowire with a gradually changed halide distribution along the length can be induced through the anion-diffusion process at 220 ℃. Detailed information about preparation methods can be found in experimental section. Theoretically, the electronic structures of the alloy nanowire were analyzed in Figure 1b. The results show that the bandgap continuously narrows with the increasing of Br- anions, resulting in a continuously changed PL emission along the length. Figure 1c gives the real-color image of one typical CsPbCl3-3xBr3x alloy nanowire under the illumination of a 405 CW laser. The alloy nanowire clearly presents a continuous color transition from violet to blue, cyan and green; this indicates the formation of a gradually changed bandgap along the length. Spatially PL measurements were performed to further clarify that, as shown in the bottom inset of Figure 1c. It can be clearly seen
8
that PL spectra collected from every single spot along the length shows a strong and single-peak emission; the typical PL emission wavelength covers the range from 440 nm (2.41 eV) to 515 nm (2.82 eV) as the excited spot moves from one end to the other. This tendency agrees well with the theoretical calculations in Figure 1b. Single CsPbCl3-3xBr3x perovskite alloy nanowire with a continuously and widely tunable bandgap is constructed here via a solid-solid aniondiffusion process. To clarify the construction of CsPbCl3-3xBr3x alloy nanowire, the kinetics of solid-solid anion-diffusion process at 220 ℃ were systematically analyzed using the confocal PL measurements. As shown in Figure 2a and 2b, CsPbCl3 perovskite nanowires with a homogeneous PL emission of ~ 415 nm were first constructed through a CVD process. In Figure 2a, the evolution phase of CsPbCl3 nanowire is depicted by three-stage process. At stage I (see Figure 2c and 2d), an anion-diffusion process initiates at the interface between CsPbCl3 nanowire and CsPbBr3 sources; with time increases, Br- anions diffuse into one part of the nanowire and induce the redshift of PL emission wavelength, the other part keeps constant. Finally, the Branions have diffused through the whole nanowire, resulting in a purely gradient nanowire (λ: 415-λ1 nm, λ1<525). At stage II (see Figure 2f, 2g and 2h), the Br- anions continually diffuse along the length but slows down; the PL emission wavelength of the whole nanowire redshifts. In this phase, the whole nanowire is also composition-graded along the length (λ: λ2-λ3 nm, λ2>415, λ1<λ3<525). At a certain time, the PL emission wavelength of one side reaches the maximum (near the CsPbBr3 source). At stage III (see Figure 2j, 2k, 2l), the halide distribution and PL emission wavelength of the whole nanowire comes to be identical along the length (λ: λ4 nm, λ3>λ4>λ2). Thus, single CsPbCl3-3xBr3x alloy nanowire covering different spectral range can be fabricated by carefully controlling the reaction time. Taking one CsPbCl3 nanowire as an
9
example, the kinetics of the anion-diffusion process is quantitatively analyzed. The composition profiles of single alloy nanowire recorded at different time are plotted in Figure 2e, 2i and 2m, which agrees well with the three-stage process. In Figure 2e, the Br- content and PL emission wavelength of one part grows and the other part far away from the CsPbBr3 source keeps constant with the increasing of time. In Figure 2i, the Br- content and PL emission wavelength of the whole alloy nanowire grows. Finally, in Figure 2m, the PL emission wavelength of the whole nanowire comes to be identical with time increases. The diffusion length versus the square root of reaction time is well fitted by a linear curve in Figure S1a, indicating a constant diffusion coefficient in this process.[33,35,37] And a diffusion coefficient of 3.40×10-10 cm2/s can be extracted from the composition-distance curve in Figure S1b, this agrees well with previous reports by Jin and co-workers.[33] The kinetics of the anion-diffusion process now have been quantitatively analyzed using the confocal PL measurements. The atomic-scale mechanism of the solid-solid anion-diffusion process is analyzed theoretically using DFT. A vacancy-assisted mechanism is mainly considered here in lead halide perovskites.[38–40] Figure 3a schematically depicts the mechanism of the whole anion-diffusion process. The picture is divided into three parts: CsPbCl3 regions, where provides the Cl- anions (left part indicated by pink dash); diffusion regions, where the anion diffusion mainly occurs; CsPbBr3 regions, where provides the Br- anions (right part signaled by green dash). The cyan and pink arrows represent the migrations of VBr- and VCl-, respectively, they both consist of P1 and P2 paths (see Figure 3b and 3c). An anion migration easily initiates when CsPbCl3 and CsPbBr3 perovskites contact, due to the low activation energy (see Figure 3d, 3e, S2a and S2b). With time increases, the Br- and Cl- anions diffuse deeply through two paths of P1 and P2; the diffusion region moves and broadens, resulting in a gradient halide distribution along the length of the
10
alloy nanowire. As shown in Figure 3b and 3c, the P1 and P2 paths represent anion migrations along <100> and <110> directions, respectively. The energy profiles and typical transition states of VBr- migrations along P1 and P2 paths are presented in Figure 3d and 3e, the results about VClmigrations are provided in Figure S2. It can be seen that the activation energies Ea of VBrmigrations are both very small, while the Ea~0.36 eV of <100> direction is higher than 0.22 eV of <110> direction. To clarify the origin of this difference, we analyze the cubic crystal structure of CsPbX3, and find the VBr- migrations along <100> direction break one more Pb-Br bonds than <110> direction. The X-ray diffraction patterns of CsPbCl3 and CsPbBr3 nanowires with cubic phase are presented in Figure S3. Compared with conventional III-V semiconductors such as 3-5 eV of GaAs, the lower activation energy allows a feasible and fast anion diffusion between different halide perovskites.[41] Here, the anion-diffusion process follows a vacancy-mediated mechanism, thus a diffusion coefficient of 1.43×10-9 cm2s-1 (p1 path) and 3.86×10-8 cm2s-1 (p2 path) can be estimated from the Arrhenius equation.[33,38] The result is closer to the experimental value obtained from Figure S1b, the difference may originate from vacancy concentrations or the variation of Br-/Cl- ratio.[42,43] Herein, we analyzed the atomic-scale mechanism of the anion-diffusion process, revealed the low activation energy of halide migrations in perovskite structures. The dynamics of carrier transportation are the critical to decouple the optical properties of micro/nanostructures, here, the PL dynamics of single CsPbCl3-3xBr3x alloy nanowire were analyzed. Theoretically, we calculated the typical energy-level structures of different positions along single CsPbCl3-3xBr3x alloy nanowire using DFT. The results show that the bandgap of the alloy nanowire continuously narrows with the increasing of Br- content along the axial direction. Consequently, an energy transfer is expected between the Cl-rich and the Br-rich regions, as
11
shown in Figure 4a. We employed the time-resolved photoluminescence spectroscopy (TRPL) to study the dynamics of carrier transportation, by measuring the spatially separated carrier lifetime from high-bandgap Cl-rich region to the low-bandgap Br-rich region. The PL decay curves for emission peak from 415 to 515 nm are shown in Figure 4b. Here, the data of 415 and 515 nm comes from two uniform nanowires with a homogeneous PL emission as a comparison; the other decay curves from 450 to 500 nm are measured on single CsPbCl3-3xBr3x alloy nanowire. These curves were all well fitted by a bi-exponential function, y(t) = A0 + A1e-t/τ1 + A2e-t/τ2, where τ1 and τ2 represent fast decay time and slow decay time, A1 and A2 are their percentage contribution respectively. The fitting parameters were summarized in Table S1 and plotted in Figure 4c and 4d. With increasing the ingredient of bromide, we found that the fast decay (0.14-3.2 ns) and slow decay (0.36-8.8 ns) both raise. However, the fast component ratio reduces from 89 % to 16 %, corresponding to the slow component ratio dilates from 11 % to 84 % significantly. These variation tendencies are consistent with the previous reports.[44,45] Therefore, we attributed the fast decay to carrier diffusion processes and the slow decay to radiative recombination respectively. The fast component ratio reduces rapidly from 450 to 500 nm, this trend indicates the carrier flow formed from Cl-rich region to Br-rich region, due to the graded bandgap level. In the Cl-rich region, most carrier transfer the energy to lower bandgap region rather than radiative recombination, which cause lower quantum yield than Br-rich region.[44–47] In Br-rich region, it’s longer recombination time and higher quantum yield would enable lower lasing threshold. We noticed that Fu et al. reported a negative component ratio which indicates the rise kinetics, whereas positive component ratio indicates decay kinetics. Here, all the component ratio we got are positive. This difference may come from sample structure, Fu et al. investigated twodimensional layered halide perovskites, and the energy transfer direction was same with the laser
12
propagation, so the higher carrier could concentrate in the lowest energy layer.[44] Additionally, the PL intensity of single perovskite nanowire also supports the presence of energy transfer (Figure S4). There are two channels to cause the photon propagation in nanowire, one is optical waveguide, the other is carrier transportation.[45] For uniform nanowire, the optical waveguide is dominant; the PL intensity of both two ends is much higher. For alloy nanowire, the carrier diffusion driven by the gradient bandgap is dominant. It can be seen that the PL intensity of the gradient nanowire increases gradually from blue region to green region. Similar behavior is also observed in organic-inorganic perovskite nanowire.[45] Single perovskite alloy nanowire with a regularly geometrical structure (see Figure S5) and a widely tunable PL emission is an ideal candidate for constructing a broadly wavelength-tunable laser. We study the lasing performance of single CsPbCl3-3xBr3x alloy nanowire using a confocal micro-photoluminescence system. As shown in Figure 5a, an individual CsPbCl3-3xBr3x alloy nanowire was excited by a femtosecond laser of 400 nm; the CsPbCl3-3xBr3x alloy nanowire has a widely tunable PL emission and a regular structure with a triangular cross section. At a high pump density, such a nanowire can behave as both a gain medium and microcavity to realize a broadly tunable lasing. We can see the typical lasing from 480 to 525 nm is successively observed as the excitation spot moves from one side to the other. Unlike CdSSe nanowire, single isolated perovskite nanowire can emit lasing broadly and continuously without cutting the lowenergy side or adding a coupling cavity.[21,22] This can be attributed to the high optical gain of perovskite materials and the unique advantages of solid-solid anion diffusion to construct a highquality perovskite nanowire with a gradually changed bandgap. Figure 5b shows the typical lasing spectra of single CsPbCl3-3xBr3x nanowire recorded at a high pump density, a widely tunable lasing at 480, 500, 510, 520 and 525 nm can be clearly observed. In contrary, single
13
CsPbCl3-3xBr3x nanowire with a homogeneous halide distribution only exhibits single-band lasing emission, as shown in Figure S6. Figure 5c-g shows the power-dependent spectra for typical lasing at 480, 500, 510, 520 and 525 nm, respectively. It can be clearly observed that all these spectra undergo a transition from a broadband spontaneous emission to the stimulated emission with several sharp peaks with the increasing of pump density. At a relatively low pump density (below the lasing threshold), broadband emission of 467, 493, 502, 509, and 512 nm can be observed. When pump density reaches or exceeds their lasing thresholds, several new sharp peaks with narrow linewidth abruptly appears above the background of spontaneous emission. Further increasing the pump density, the intensity of lasing peak grows rapidly and finally the simulated emission dominates the whole process. A set of regular resonant peaks was clearly observed from these lasing spectra, which is attributed to a Fabry−Perot cavity. Taking lasing at ~500 nm as an example, its mode spacing can be simply given by: ∆λ = λ2/2ngL, where ∆λ and λ are the mode spacing and lasing wavelength respectively, ng represents the group refractive index, and L is the effective cavity length.[48−51] The effective cavity length L can be determined to be ~21 µm with a mode spacing (λ1=498.1 nm, λ2=499.2 nm) ∆λ~1.1 nm, which means that the lasing signal is attributed to the FP mode. The localized FP cavity is formed by the back scattering from the defects.[30,52] And, a detailed simulation of the electromagnetic field distribution using Comsol Multiphysics is presented in Figure S7, this further demonstrates the formation of a FP cavity lasing. It can be clearly seen that the optical field is well confined inside, thus the resonant modes are reflected back and forth along the length. As shown in Figure S8 and 5h, the lasing thresholds of 480, 500, 510, 520 and 525 nm are defined to be 35.0, 29.3, 26.5, 17.5, and 11.7 µJ/cm2, respectively. As the output lasing wavelength blueshifts, the lasing threshold increases from 11.7 to 35.0 µJ/cm2. This can be attributed to the energy transfer in
14
single CsPbCl3-3xBr3x alloy nanowire, which has been clarified above. Similar behavior also has been reported in CdSSe nanostructures.[53–55] These results clearly demonstrate the capability of single CsPbCl3-3xBr3x perovskite alloy nanowire to realize a broadly and continuously tunable lasing over a large spectral range. 4. Conclusions In conclusion, we have proposed and constructed single perovskite alloy nanowire via a solid-solid anion-diffusion process for realizing a broadly and continuously tunable laser. Single CsPbCl3-3xBr3x alloy nanowire has a regularly geometrical structure and a broadly tunable PL emission from 440 to 515 nm along the length due to the spatially graded halide distribution. This alloy nanowire enabled us to explore a broadly tunable laser in single nanowire, as the different spots along the nanowire can serve as optical cavity and a widely tunable gain medium simultaneously. A quantitative study was carried out to analyze the dynamics of the solid-solid anion-diffusion process and the three-stage evolution property of perovskite alloy nanowire. DFT calculations of the anion-diffusion process clarified the intrinsic mechanism and low activation energies of both two halide-transport routes. The TRPL measurements revealed the energy transfer, which is the critical to analyze the unique properties of single perovskite alloy nanowire such as the higher lasing threshold of short-wavelength emission. Our work provides an avenue to realize a broadly and continuously tunable lasing in single perovskite nanowire, and an insight into understanding the atomic-scale mechanism and kinetics of the elemental anion diffusion of perovskites which are very important to analyze their unique properties and performance.
15
Figure 1. (a) A schematic of CsPbCl3-3xBr3x perovskite alloy nanowire with a widely tunable PL emission constructed through a solid-solid anion-diffusion process. (b) The electronic sturtcures of different spots along single CsPbCl3-3xBr3x alloy nanowire with the typical halide content, showing a gradually changed bandgap. (c) The real-color image and typical PL spectra of single CsPbCl3-3xBr3x perovskite alloy nanowire.
16
Figure 2. The kinetics of the solid-solid anion-diffusion process. (a) A schmatic of the whole process of fabricating CsPbCl3-3xBr3x perovskite alloy nanowire. (b) The optical mages of CsPbCl3 nanowires. The typical PL images and spectra of CsPbCl3-3xBr3x alloy nanowires of stage I (c, d), stage II (f, g, h), stage III (j, k, l), their typical PL emission wavelength is listed in the top-left part. A quantitative analysis of composition evolution at stage I (e), stage II (i), stage III (m) on single perovskite nanowire.
17
Figure 3. Atomic-scale mechanism of the solid-solid anion-diffusion process. (a) A schematic of the anion-diffuison process between CsPbCl3 and CsPbBr3 perovskites. The cyan and pink arrows represent by the migrations of VBr- and VCl- anions, respectively. The anion migrations along <100> and <110> directions are signaled by P1 (b) and P2 (c), respectively. The corresponding energy profiles of Br- anion migrations along P1 (d) and P2 (e) paths.
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Figure 4. The energy transfer along single CsPbCl3-3xBr3x perovskite alloy nanowire. (a) Schematic illustration of the energy transfer along the length of the nanowire, the typical molecular orbitals of different spots along the nanowire are presented in the top and bottom parts. (b) Typical time-resolved photoluminescence curves (hollow circle) of single CsPbCl3-3xBr3x alloy nanowire in various peak positions.The fitted curves (solid line) are calculated using a biexponential function. (c) The change trend of fast decay time and slow decay time with the lightemitting peak varying from 415 nm to 515 nm. (d) The decay probability change trend of fast decay component and slow decay component with the light-emitting peak varying from 415 nm to 515 nm. It is noted that the data of 415 nm and 515 nm is recorded from two nanowires with a uniform PL emission.
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Figure 5. A broadly and continuously wavelength-tunable laser in single CsPbCl3-3xBr3x perovskite alloy nanowire. (a) The schematic of optically pumping lasing experiments with a 400 nm fs-laser (~40 fs, 10 kHz) as the excitation source. (b) Typical lasing spectra obtained from single CsPbCl3-3xBr3x alloy nanowire at a high pump density. The power-dependent PL spectra of typical lasing at 480 nm (c), 500 nm (d), 510 nm (e), 520 nm (f) and 525 nm (g). (h) The lasing thresholds are plotted versus the lasing wavelength.
20
Acknowledgments This work was supported financially by the NSFC (61675219, 61875256, 91950201). H. Dong acknowledges the Youth Top-notch Talent Support Program in Shanghai. Y. Hu acknowledges the support of the Excellent Science and Technology Innovation Group of Jiangsu Province. We thank Q. Si for helping design and draw the schematic picture of Figure 5a. Author Contributions B.T. prepared the samples, performed the sample characterizations and designed the experiment, guided by H.D. and L.Z., Y.H. carried out the theoretical calculations of electronic structures and anion-diffusion mechanism, J.L. and H.W. carried out the time-resolved photoluminescence measurements and the data analysis, N.M. performed the lasing mechanism analysis and numerical simulations, X.J. and X.G. carried out the optical experimental measurements. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. €These authors contributed equally. Declaration of competing interest The authors declare no competing financial interest. Notes Color should be used for figures1-5 in print. Appendix A. Supplementary data Supplementary data to this article can be found online at Elsevier website.
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Vitae
Bing Tang received his Bachelor's and Master’s degrees from Central South University and Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences, respectively. He is now a doctor candidate in SIOM, Chinese Academy of Sciences. His research interests mainly focus on the controllable growth of novel perovskite microcavity structures and the study of high quality nanolasing in low-dimensional perovskite micro/nanostructures.
Yingjie Hu received his Ph.D. degree from Nanjing University of Science and Technology in 2015. He joined Nanjing Xiaozhuang University as an associate professor since 2018. His interests focus on studying the relationship between materials structures and properties with applications in material design using theory, modeling and computation as tools.
Jian Lu received his optics Ph.D. degree from Fudan University in 2012. He joined Shanghai Advanced Research Institute, Chinese Academy of Sciences as an associate professor since 2013. His researches mainly focus on the optical properties of novel semiconductor, VCSEL laser and single photon source.
Hongxing Dong is currently a professor in Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences. He received his Ph.D. degree from Fudan University in 2011. His research interests mainly focus on the study of high quality microcavity lasers.
Nanli Mou received her Bachelor's degrees from Sichuan University, she is now a doctor
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candidate in Shanghai Institute of Optics and Fine Mechanics (SIOM). Her research interests mainly focus on the metamaterials and plasmonics.
Xinyu Gao received the Bachelor's degree from the Department of Materials Science and Engineering of China Jiliang University in 2019. She is now a postgraduate student in Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, China. Her research interests mainly focus on the study of high-performance microcavity structure design and the growth mechanism.
Hu Wang is a master candidate at School of Physical Science and Technology of ShanghaiTech University. His research interests mainly focus on the light-matter interaction in perovskite semiconductor optical microcavity.
Xiongwei Jiang is now a professor in Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences. He received his Bachelor's and Master’s degrees both from East China University of Science and Technology. His research interests mainly focus on the growth of multifunctional and optical materials.
Long Zhang received his Ph.D. Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences in 2000. Afterwards, he worked one year as a Postdoctoral Fellow in SIOM and then worked as a Research Assistant Professor in University of Münster, Germany from 2001 to 2006. In 2006, he came back to China and join SIOM as a professor. His research interests focus on advanced infrared materials, laser ceramics and the high quality microcavity structures.
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Highlights A broadly and continuously (480-525 nm) tunable laser is realized in single CsPbCl3-3xBr3x nanowire. Single CsPbCl3-3xBr3x nanowire with a widely tunable bandgap (2.41-2.82 eV) is constructed through an anion-diffusion process. The kinetics and atomic-scale mechanism are clarified in theory and experiment. The
energy
transfer
is
revealed
by
time-resolved
measurements in single perovskite alloy nanowire.
photoluminescence
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Tang Bing: Visualization, Conceptualization, Methodology, Writing - Original draft preparation Hu Yingjie: Writing - Original Draft, Visualization, Software, Methodology Lu Jian: Writing - Original Draft, Visualization, Methodology Dong Hongxing: Conceptualization, Writing-Review & Editing, Resources, Funding acquisition Mou Nanli: Formal analysis, Visualization, Software, Methodology Gao Xinyu: Investigation, Visualization Wang Hu: Investigation, Visualization Jiang Xiongwei: Data Curation, Resources, Supervision, Project administration Zhang Long: Supervision, Resources, Funding acquisition