Low-Threshold Lasing from 2D Homologous Organic–Inorganic

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Low Threshold Lasing from 2D Homologous OrganicInorganic Hybrid Ruddlesden-Popper Perovskite Single-Crystals Raghavan Murugesan Chinnambedu, Tzu-Pei Chen, Shao-Sian Li, Wei-Liang Chen, Chao-Yuan Lo, Yu-Ming Liao, Golam Haider, Cheng-Chieh Li, Chia-Chun Chen, Raman Sankar, Yu-Ming Chang, Fang-Cheng Chou, and Chun-Wei Chen Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b00990 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 25, 2018

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Nano Letters

Low Threshold Lasing from 2D Homologous Organic-Inorganic Hybrid Ruddlesden-Popper Perovskite Single-Crystals Chinnambedu Murugesan Raghavan,†,⊥ Tzu-Pei Chen, †,‡,‖,⊥ Shao-Sian Li,*, ⁋ Wei-Liang Chen, § Chao-Yuan Lo, § Yu-Ming Liao, ‡,‖ Golam Haider, ‡ Cheng-Chieh Lin, ± Chia-Chun Chen,± Raman Sankar, §, ¶ Yu-Ming Chang, §,△ Fang-Cheng Chou, §,△ and Chun-Wei Chen*,†, #,⸸,△

†

Department of Materials Science and Engineering, National Taiwan University, Taipei 106,

Taiwan ‡

‖

Department of Physics, National Taiwan University, Taipei 106, Taiwan

Nano Science and Technology Program, Taiwan International Graduate Program, Academia

Sinica and National Taiwan University, Taipei 115, Taiwan ⁋

Graduate Institute of Biomedical Optomechatronics, Taipei Medical University, Taipei 110,

Taiwan §

¶

Center for Condensed Matter Sciences, National Taiwan University, Taipei 106, Taiwan

Institute of Physics, Academia Sinica, Taipei 115, Taiwan

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±

#

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Department of Chemistry, National Taiwan Normal University, Taipei 116, Taiwan

Taiwan Consortium of Emergent Crystalline Materials (TCECM), Ministry of Science and

Technology, Taipei 106, Taiwan △

Center of Atomic Initiative for New Materials (AI-MAT), National Taiwan University, Taipei

106, Taiwan ⸸

International Graduate Program of Molecular Science and Technology, National Taiwan

University (NTU-MST), Taipei 106, Taiwan

KEYWORDS. 2D RPP single crystals, solution growth, photoluminescence, lasing

ABSTRACT: Organic-inorganic hybrid two dimensional (2D) perovskites have recently attracted great attention in optical and optoelectronic applications due to their inherent natural quantum-well structure. We report the growth of high quality millimeter-sized single crystals belonging to homologous two-dimensional (2D) hybrid organic-inorganic Ruddelsden-Popper perovskites (RPPs) of (BA)2(MA)n-1PbnI3n+1 (n=1, 2, and 3) by a slow evaporation at constant temperature (SECT) solution-growth strategy. The as-grown 2D hybrid perovskite single crystals exhibit excellent crystallinity, phase purity and spectral uniformity. Low-threshold lasing behaviors with different emission wavelengths at room temperature have been observed from the homologous 2D hybrid RPP single crystals. Our result demonstrates that solution-growth homologous organic-inorganic hybrid 2D perovskite single crystals open up a new window as a promising candidate for optical gain media.

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Main text Organic-inorganic hybrid halide perovskite semiconductors is an emerging class of solutionprocessable optoelectronic material for solar cells with remarkable power conversion efficiencies, and also for light emitting diodes (LEDs) with high quantum yields in the past few years.1-7 In particular, this class of materials possess excellent physical properties for photovoltaic applications such as high optical absorption coefficient, low exciton binding energy and long and balanced electron-hole diffusion length.8-11 Organic-inorganic hybrid perovskite compounds typically have an ABX3 three dimensional (3D) lattice framework (ex. A=MA, B=Pb, and X=Cl, Br, I) and their bandgaps can be tuned by ion modifications.12 However, the key challenge in the commercialization of 3D organic-inorganic hybrid perovskite photovoltaics is related to the well-known long term instability of devices under ambient condition.13,14 Another class of two-dimensional (2D) organic-inorganic hybrid perovskite counterparts have recently attracted great attention owing to their superior ambient stability as well as promising optoelectronic properties.15-17 Remarkable progresses both on the performance of solar cells and LEDs based on the 2D organic-inorganic hybrid perovskites have been demonstrated.1520

These 2D Ruddlesden-Popper perovskites (RPPs) have the generic chemical formula A2A’n-

1MnX3n+1,

where A represents organic spacers such as long chain alkyl ammonium or phenyl

alkylammonium cation and A′ is a small organic (CH3NH3+) or inorganic (Cs+) cation, M is a metal, and X is a halide.21,22 The 2D RPPs have a naturally formed “quantum well (QW)-like” structure consisting of inorganic perovskite layers of corner-sharing PbX6 octahedra sandwiched between organic spacers.22 The value of n is the number of inorganic perovskite layers per unit cell that determines the width of QW and consequently the tunability of the optoelectronic

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properties of the 2D RPPs. For example, variations in the bandgap energies can be achieved by controlling the layer number (n) of the inorganic slabs.23,24 Due to their inherent QW structure, the 2D layered perovskites have shown intriguing physical properties, such as strong excitons with large binding energies and anisotropic transport of charge carriers compared to their 3D counterparts.21,25-27 Although 2D perovskites have attracted enormous attention because of their potential applications in solar cells and light emitting diodes, the synthesis of homologous 2D perovskites with high purity remains a big challenge, due to the complexity in the wet-chemical processes. In particular, the difference in solubility between the organic spacer and methyl ammonium cation leads to the difficulty in obtaining pure phase homologous compounds in stoichiometric compositions.28 Recently, pure phase (purified with only single n value) samples of the homologous series of 2D perovskites have been successfully synthesized22,28 and these as-synthesized phase-pure 2D perovskites have led to the demonstration of stable and efficient thin film solar cells.15 It was previously demonstrated that large 3D perovskite MAPbX3 single crystals exhibit exceptionally long carrier diffusion lengths, resulting from higher carrier mobility and much smaller trap densities in the single crystals than in polycrystalline thin films.9,29 In this work, we were motivated to grow high-quality homologous 2D hybrid RPP single crystals with phase purity and millimeter-sized dimension to obtain deep understanding of the fundamental physical properties of these promising optoelectronic materials. The growth of millimeter-sized high-quality organic-inorganic homologous 2D hybrid RPP (BA)2(MA)n-1PbnI3n+1 single crystals with different n values (n=1, 2 and 3) was achieved by a slow evaporation at constant temperature (SECT) solution-growth technique. These homologous 2D organic-inorganic hybrid RPP single crystals with different n values (n=1, 2 and 3) exhibit superior optical properties with tunable

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emission wavelengths and exemplary uniformity. Intriguingly, low threshold lasing behaviors at room-temperature for these homologous 2D hybrid RPP single crystals were observed for the first time, suggesting that 2D hybrid organic-inorganic perovskite may be a potential candidate for optical gain media. The phase-pure homologous 2D hybrid RPP (C4H9NH3)2(CH3NH3)n-1PbnI3n+1 (n=1, 2, 3) compounds were synthesized from an optimized reaction between PbI2, CH3NH3I (MAI) and C4H9NH3I (BA) using I2 free hydrogen iodide (HI) solution according to a previous report.22 H3PO2 was used as a stabilizing agent to control the I2 evaporation from the HI acid. Because BA acts as a kinetic barrier in this reaction, it was crucial to optimize the BA concentrations to obtain the as-synthesized phase-pure 2D perovskites. We optimized the concentrations of the BA cations based on the earlier report.22 The product obtained by this reaction was dried under reduced pressure, and stored in a glove box. The as-synthesized well-dried pure phase compounds of (BA)2PbI4 (n=1), (BA)2(MA)Pb2I7 (n=2) and (BA)2(MA)2Pb3I10 (n=3) were then used to grow high-quality single crystals. There are several reported growth strategies for 3D organic-inorganic hybrid perovskite materials, including inverse temperature crystallization (ITC),30 anti-solvent vapor diffusion (ASVD)29 and top-seeded solution growth.9 However, the reports on the growth of homologous 2D organic-inorganic hybrid perovskite single crystals are highly limited.

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(a)

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(b)

5 mm

Figure 1. (a) Growth process of homologous 2D RPP single crystals (b) as-grown crystals of n=1,2 and 3 (c) X-ray diffraction patterns of the grown crystals. The insets in (c) represent the corresponding crystal structures with different perovskite layer thickness. For the growth of 2D hybrid RPP single crystals, we have attempted different growth techniques such as slow cooling, inverse temperature crystallization(ITC) and anti-solvent vapor diffusion (AVSD) techniques (described in supporting information S1) that are commonly employed to grow the bulk 3D single crystals. However, as compared to 3D hybrid perovskites, the growth of 2D RPPs in the form of large size single crystals are more complicated, which might be attributed to the presence of large organic moiety and long chain alkyl amine such as BA in their lattice framework. We found that the ITC and ASVD methods are not feasible to grow 2D hybrid RPP single crystals using HI/H3PO2 solvent. Whereas, the growth of 2D hybrid RPP single crystals using the slow cooling technique leads to randomly stacked multi-crystalline flakes (Figure S1 in supporting information), similar to the recent report on the growth of

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homologous 2D hybrid perovskites containing C6H5C2H4NH3+ (PEA) as an organic spacer.28 Here, instead of using the slow cooling method, we developed a slow evaporation at constant temperature (SECT) solution-growth technique to grow millimeter-sized high-quality 2D organic-inorganic hybrid RPP (BA)2(MA)n-1PbnI3n+1 single crystals with different n values (n=1, 2 and 3). As compared to the slow cooling technique, our SECT solution-growth method effectively controls the rapid nucleation and reduces the multi-crystalline stacking of 2D perovskite flakes in order to obtain well-shaped single crystals with millimeter scale and high phase purity. The schematic illustration in Figure 1(a) reveals the step-by-step process in the growth of homologous 2D RPP single crystals. In order to grow single crystals, the HI solution was saturated by dissolving the pure individual compounds in separate beakers at 60 °C (Step 1). The saturated solution was then subjected to slow evaporation at a constant temperature. Once the solvent started to evaporate slowly, the saturated solution became supersaturated (Step 2) which drove the crystallization. The excess solute in the supersaturated solution became crystallized at the bottom of the beakers by spontaneous nucleation process at different positions, followed by the growth of platelet-like crystals (Step 3). After carefully controlling the slow evaporation process for 24 hours, as-grown well-faceted rectangular shape crystals were obtained (Step 4). The corresponding photo images of the as-grown millimeter-sized 2D RPP single crystals with n=1, 2 and 3 are shown in Figure 1(b). The crystal structure and purity of the as-grown 2D perovskite single crystals were confirmed by X-ray diffraction (XRD) analysis. The XRD patterns of n = 1, 2 and 3 single crystals were indexed with respect to the orthorhombic 2D RPP crystal structures as shown in Figure 1(c). The periodic repetitions of Miller planes for all the crystals confirm the single crystalline nature. For the homologous 2D hybrid RPP crystals, the thickness of 2D inorganic perovskite units increases

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by the addition of small organic methyl ammonium cations into the crystal structure. An expansion of the unit cell occurs by the addition of a perovskite layer which can be revealed at an additional low angle reflection below 2θ = 14° in the XRD patterns of 2D perovskite single crystals with n=2 and n=3. The compounds (BA)2PbI4 (n=1), (BA)2(MA)Pb2I7 (n=2) and (BA)2(MA)2Pb3I10 (n=3) show one, two and three evenly spaced reflections below 2θ = 14° respectively, indicating the formation of pure phase homologous 2D RPP single crystals.22 The corresponding crystal structures and orientations of 2D perovskite crystals with n=1, 2 and 3 are shown in the insets of XRD patterns (Figure 1(c)) where the in-plane orientations of the PbI6 perovskite sheets are along the crystal surface. The preferred crystal growth orientation for the 2D perovskite crystals with n=1 is along the c-axis, while the ones with n=2 and n=3 exhibit the preferred growth orientation along the b-axis. The estimated lattice distance of the first diffraction peak in the 2D perovskite crystals with n=1,2 and 3 are well correlated with the stepwise increase of a single PbI6 perovskite layer, which is approximately equal to 0.6 nm. The observed XRD results for these homologous hybrid organic-inorganic 2D RPP crystals are well consistent with the reports in literature.22,24 In addition to the high crystallinity confirmed by the XRD, the high purity and uniformity of the as-grown large single crystals were further revealed by probing the spatially correlated photoluminescence (PL) and time-resolved photoluminescence (TRPL) properties. Figures 2(a) and (b) show the optical images of the bottom surface and the top surface of the n=1 crystal with lateral dimensions around 3mm x 2mm respectively, where the surface morphology reveals obvious rectangular plateaus in the 2D perovskite single crystal. In particular, a step-pyramidlike growth pattern on the top crystal surface as shown in Figure 2(b) indicates that the formation of these rectangular plateaus might be resulted from the step-growth nucleation mechanism31 at

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the growth of 2D single crystals by the SECT solution-growth technique. In order to examine the information of purity and uniformity of the as-grown 2D perovskite crystal, we measured the PL emission spectra over the entire millimeter-sized crystal (n=1). The high-resolution spatial PL spectral mapping image was obtained with a total mapping pixel number of 128 x 128 as shown in Figure 2(c), where the color code represents the distribution of peak wavelengths from the collected PL spectra. A very uniform spectral distribution over the entire crystal was clearly observed. The inset of Figure 2(d) shows the typical PL emission spectrum of the 2D perovskite single crystal (n=1), with the emission peak centered at 523.5 nm and the FHWM of 20.8 nm. The sharp PL emission peak with a narrow bandwidth accounts for the excellent optical properties of our 2D perovskite single crystals. The histogram of the peak wavelengths collected from all the PL spectra over the entire crystal is narrowly centered at 523.0±0.6 nm (Figure 2(d)), indicating the superior uniformity of the as-grown 2D perovskite crystal. To gain more insight on the correlation between the crystal quality and PL emission, spatially resolved timeresolved PL spectroscopy was further performed by the fluorescence lifetime image mapping (FLIM) technique to reveal the spatial distribution of photoexcited carrier dynamics for the entire crystal as presented in Figure 2(e). Each pixel in the FLIM image represents a single TRPL curve and the color code represents the distribution of carrier lifetime obtained by TRPL curves fitted with a bi-exponential decay model. (also refer to Supporting Information) The histogram of the average lifetime distribution is shown in Figure 2(f) where the center of Gaussian distribution for the photoexcited carrier lifetime in the n=1 single crystal is around 1.4 ns with a narrow FWHM of 0.76 ns. The majority of photoexcited carriers radiatively recombined with a shorter lifetime around 1 to 2 ns (the green areas in FLIM image of Figure 2(e) and the green curve in the inset of Figure 2(f)). Only those near the edges of the rectangular plateaus on the crystal surface

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exhibit longer lifetime around 3.0 to 3.5 ns (yellow and red areas in FLIM image of Figure 2(e) and the red curve in the inset of Figure 2(f)). Beside the color contrast showing the PL decay lifetime, the FLIM image of the as-grown single crystal (n=1) as shown in Figure 2 (e) also clearly revealed the well-defined rectangular plateaus corresponding to the step-growth pattern as observed in the optical microscope of Figure 2(a). The result of highly uniform PL and TRPL distribution over the millimeter-sized 2D perovskite crystal well supports that the as-grown single crystal by SECT solution-growth technique exhibits excellent quality and homogeneity. For the as-grown 2D hybrid RPP single crystals with n=2 and n=3, the corresponding PL spectral mapping images also show high-quality and uniformity over the large area of the crystal surface as shown in the supporting information (Figure S4).

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PL intensity (a.u.)

1.0

single pixel total pixel averaged

480

0.5

510 540 570 Wavelength (nm)

0.0 521 522 523 524 525 526 527

0.5 mm

PL Intensity (a.u.)

Peak center (nm)

Frequency (kCount)

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Frequency (kCount)

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100

edge 3.48 ns center 1.81 ns

10-1

2.0

10-2 10-3

10-4

1.0

0.0 0

1

2

0

3

20 40 Time (ns)

4

5

6

Lifetime (ns)

Figure 2. Optical characterization of (BA)2PbI4 (n=1) single crystal. Optical images of (a) the crystal bottom surface and (b) the top surface. The formation of a “step-pyramid-like” growth pattern is observed at the crystal top surface. (c) Spectral mapping image of PL and (d) the histogram of the peak wavelengths of the entire crystal measured from the bottom surface. The

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inset shows the typical PL emission spectrum. (e) FLIM image for the entire crystal. (f) Histogram of lifetime distribution obtained from FLIM measurement. The inset exhibits two types of TRPL decay curves corresponding to different locations on crystal surface, where the green curve was obtained at the center and the red curve was obtained from the edge of the rectangular plateau.

As compared to the 3D counterparts, the organic-inorganic 2D perovskite materials exhibit unique optical properties due to naturally formed quantum-well (QW) structure where the inorganic perovskite slabs form the 2D QW layers and they are sandwiched between organic barrier layers. Moreover, these materials exhibit remarkable excitonic and tunable optoelectronic properties with the quantum mechanically additional degree of freedom by varying the inorganic perovskite slab thickness.22,24 Figures 3 (a) and (b) exhibit the optical absorption and PL spectra of the homologous 2D hybrid perovskite single crystals with n=1, 2 and 3, respectively. The optical bandgap values estimated from the excitonic absorption spectra decrease monotonically with increasing n from 1 to 3 as a result of the quantum confinement effect. The optical bandgap values of the single crystals with n=1, 2 and 3 are estimated to be 2.4 eV, 2.2 eV, and 2.0 eV respectively. The sharp nature of the excitonic absorption and PL emission peaks have been observed in all the three single crystals containing different ratios of organic and inorganic components. Due to strong electron-hole Coulomb interaction in the QW-like 2D perovskite system, large exciton binding energies (> 200 meV) at room-temperature were observed.24 Recently, many 3D hybrid ABX3 perovskites in the form of thin films, nanowires and quantum dots have been demonstrated as efficient optical gain media for lasing.32-36 In contrast, observations of lasing from homologous 2D hybrid organic-inorganic hybrid perovskites are still

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rare. Inspired by excellent optical properties, such as strong absorption and photoluminescence, large exciton binding energies, and tunable emission wavelengths of these homologous 2D hybrid RPP single crystals, we further explored the possible lasing behavior of these solutiongrown high-quality single crystals. Figure 3(c) shows the evolution of PL spectra as a function of laser pumping fluences for the 2D hybrid perovskite single crystal with n=1. The 374 nm pulse laser with a frequency of 40 MHz and a pulse width of 55 ps was used as the pumping source. At a low pumping level, the emission spectra are similar to the above PL spectrum with a peak center at 523 nm and a FWHM of ~ 20 nm. As the pumping energy gradually increased above the threshold pumping fluence (Eth) of 2.85 µJ/cm2 as shown in Figures 3 (c) and (d), a steep rise in the emission intensity accompanying with an emergence of multiple pronounced sharp peaks with a linewidth