Research Article www.acsami.org
Low Threshold Two-Photon-Pumped Amplified Spontaneous Emission in CH3NH3PbBr3 Microdisks Bin Yang,†,‡ Xin Mao,†,‡ Songqiu Yang,† Yajuan Li,†,‡ Yanqiu Wang,† Meishan Wang,§ Weiqiao Deng,† and Keli Han*,† †
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023, P. R. China ‡ University of the Chinese Academy of Sciences, Beijing 100039, P. R. China § School of Physics and Optoelectronics Engineering, Ludong University, Yantai 264025, China S Supporting Information *
ABSTRACT: Two-photon-pumped amplified spontaneous emission (ASE) of CH3NH3PbBr3 microdisks (MDs) were investigated by using femtosecond laser system. Low threshold at 2.2 mJ cm−2 was obtained. Also, emission spectral tunability from 500 to 570 nm was demonstrated by synthesis the mixed halide perovskite MDs. The spatial effect of photoluminescence (PL) properties under onephoton and two-photon excitation were also studied by means of two-photon laser scanning microscope (TPLSM) and time-resolved PL spectroscopy. It was found that the band to band emission of near-surface regions and photocarriers’ diffusion from near-surface regions to interior regions is significant for one-photon excitation. By contrast, reabsorption of emission under two-photon excitation plays a major role in the emission properties of the MDs. These results will give a more comprehensive understanding of the nonlinear effect of CH3NH3PbBr3 single crystals. KEYWORDS: amplified spontaneous emission, CH3NH3PbBr3, perovskite, recombination, solution processed
■
CsPbBr3 QDs in solution.17 Xu et al. achieved two-photonpumped lasing at a remarkably low threshold by coupling CsPbBr3 nanocrystals with microtubule resonators.18 For microdisks, Zhang et al. fabricated the 1D microwires and 2D microplates of lead bromide perovskite controllably by selectively binding two surfactants to different crystal planes and found that they have low-threshold for two-photonpumped lasing.19 To date, however, two-photon-pumped dynamic PL performances of CH3NH3PbBr3 single crystals is still not clear. On the other hand, photocarriers’ recombination process is believed to have a significant impact on the PL properties of perovskite semiconductors.20−27 However, the multiphoton-pumped photocarriers‘ recombination processes are seldom studied. In the present work, we have developed a rapid method of synthesizing CH3NH3PbBr3 microdisks (MDs). Two-photonpumped ASE with low threshold at 2.2 mJ cm−2 was obtained. Emission spectral tunability from 500 to 570 nm was demonstrated by synthesis of the mixed halide perovskite MDs. The spatial effect of PL properties under one-photon and two-photon excitation were also studied by means of a twophoton laser scanning microscope (TPLSM) and time-resolved PL spectroscopy. This new cheap synthesis method and the
INTRODUCTION Multiphoton-pumped absorption of semiconductor is of current interest toward applications such as nonlinear gain media.1 For example, two-photon-pumped amplified spontaneous emission (ASE) has significant application in frequency up conversion lasing,2 in which gain medium can be excited at red or near-infrared wavelengths without phase-matching requirements.3 Until now, conjugated polymers, fluorescent dyes, and small organic molecules were proposed for nonlinear gain media; however, it is diffucult to achieve a strong nonlinear optical response for them.4 Quantum dots (QDs) are favorable light-emitting materials, which have been widely investigated in recent years.5 Unfortunately, these colloidal nanocrystals suffer from high trap densities and rapid auger recombination,6,7 and the optical gain is severely limited. These caused the high threshold value for two-photon-pumped ASE. Recently, organolead halide perovskite single crystals have been widely investigated for their low trap density, long carrier diffusion length, and superior optical properties.8−15 It was reported that CH3NH3PbBr3 single crystal has excellent photoluminscence (PL) properties, which suggested CH3NH3PbBr3 single crystals as potential optical devices.9 More recently, much progress has been made in two-photonpumped ASE and lasing in perovskite semiconductors. For nanocrystals, Pan et al. studied the two-photon-pumped ASE of CsPbBr3 QDs films, and the threshold value is 12 mJ cm−2.16 Wang et al. achieved a lower threshold value (2.5 mJ cm−2) for © XXXX American Chemical Society
Received: April 10, 2016 Accepted: July 8, 2016
A
DOI: 10.1021/acsami.6b04246 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Sample growth and characterization. (a) Schematic of the growth process of CH3NH3PbBr3 single crystals without disturbing. (b) A stirring bar is placed in the inner dish of the precursor solution. (c) SEM image of the CH3NH3PbBr3 MDs. (d) A selected individual CH3NH3PbBr3 MD. Element mapping images of Pb (e) and Br (f) for the selected individual MD. (g) XRD spectrum of the CH3NH3PbBr3 MDs. (h) Steady state absorption and PL spectra of CH3NH3PbBr3 MDs under one-photon excitation. Inset: calculation of the optical band gap using the Tauc method.
nucleation site is necessary. So we introduced a stirring force into the beaker to stir the precursor solution (Figure 1b).15 Micrometer sized CH3NH3PbBr3 MDs were obtained within 5 min when the precursor solution was heated from room temperature to 100 °C. The growth rate was several orders of magnitude faster than the previously reported growth methods, which lasted from 1 day to several days.9,28 Figure 1c shows the scanning electron microscope (SEM) image of CH3NH3PbBr3 MDs with length scales of the order of 10−60 μm. Figure 1d shows a selected individual MD. To confirm the chemical composition of the MD, we carried out energy-dispersive X-ray spectroscopy (EDS) analysis. Figure 1e,f indicates that Pb and Br elements were distributed uniformly within the CH3NH3PbBr3 MD. Furthermore, powder X-ray diffraction (XRD) indicated pure perovskite phase for CH3NH3PbBr3 (Figure 1g).29,30 The XRD spectrum of the CH3NH3PbBr3 MDs was dominated by the (001) series of peaks, which
good nonlinear optical properties of CH3NH3PbBr3 MDs suggest the potential for applications.
■
RESULTS AND DISCUSSION CH3NH3PbBr3 MDs were synthesized by a modified solution inverse temperature crystallization method. It was already reported that CH3NH3PbBr3 perovskite can exhibit inverse temperature solubility behavior in N,N-dimethylformamide (DMF) solvent.13 First, the CH3NH3PbBr3 precursor solution was obtained by mixing equal moles of CH3NH3Br and PbBr2 in DMF at room temperature, and the solution concentration was maintained at 1.0 M. Heating this precursor solution at 100 °C led to the generation of seeds, which subsequently grew to form three-dimensional crystals of size in the order of millimeter, as shown in Figure 1a. To produce micrometer sized single crystals, a considerable increase in the number of B
DOI: 10.1021/acsami.6b04246 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
CH3NH3PbBr3‑xIx (x = 0.5) MDs, we also observed the ASE properties, as shown in Figure 2d. The component-dependent PL intensity vs excitation intensity is shown in the Supporting Information (SI). However, on further increase in x, no ASE peaks could be observed, which may be due to the increase of defects of the mixed halide perovskites.34 A PL spectra tunability was achieved from 500 to 570 nm. These results suggested that CH3NH3PbBr3 MD is an ideal material as the gain medium in high-performance frequency-upconversion lasers. Inspired by the good two-photon pumped ASE properties, we attempt to explore the three-photon-pumped PL from CH3NH3PbBr3 MDs. However, there was no ASE peak that appeared when the pump density increased from 0.7 to 3.0 mJ cm−2, as shown in Figure S5. The MDs were damaged on further increase in the pump intensity. Notably, threephoton-pumped PL was located around 559 nm, which is in agreement with that under two-photon excitation and different from that under one-photon excitation. This observation indicates that the difference between one-photon and twophoton PL does not originate from different selection rules.35 To understand the redshift of PL spectra under two-photon excitation and the photocarriers’ recombination processes, we studied the PL properties of CH3NH3PbBr3 MDs by using TPLSM. The PL images and local PL spectra excited by onephoton and two-photon pulses are shown in Figure 3. The red
suggested a lamellar structure with PbBr2 2D-layers parallel to the substrate.9 The steady state absorption and PL of the CH3NH3PbBr3 MDs (Figure 1h) were further investigated. The band edge appeared at 2.17 eV and the one-photon excitation PL showed a strong peak around 550 nm, which was very close to the band edge. These results are in good agreement with the reported CH3NH3PbBr3 single crystals.12,13 Also, the PL spectra overlapped with the absorption spectra, indicating a small vibronic relaxation. We further studied the two-photon-pumped PL properties of CH3NH3PbBr3 MDs. A femtosecond laser system was used as the light source. The output wavelength was 800 nm, with a pulse width of
