Extrinsic photoluminescence properties of individual micro-particle of Cs4PbBr6 perovskite with “defect” structure

Optical performance of the lead halide perovskites with zero-dimension (0D) structure has been in a hot debate for optoelectronic applications. Here, Cs4PbBr6 hexagonalmicro-particles with a remarkable green emission are first fabricated via a low-temperature solution-process employed ethanol as solvent. Our results underline that the existence of bromine vacancies and the introduction of hydroxyl induce a narrowed band gap with the formation of a defect level, which contributes to the extrinsic photoluminescence (PL) properties synergistically. Thanks to the high exciton binding energy and the unique morphology with a regular geometric structure of the as-obtained micro-particles, two-photon pumped amplified spontaneous emission (ASE) and single mode lasing from an individual Cs4PbBr6 particle are realized. Our results not only provide an insight into the origin of optical emission from Cs4PbBr6, but also demonstrate that the versatile Cs4PbBr6 offers a new opportunity for novel nonlinear photonics applications as an up-conversion laser. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Even through extensive studies have been made on the PL of 0D perovskite within recent years, the exact original mechanism of the green emission is still being discussed controversially. Notably, non-luminescent Cs 4 PbBr 6 NCs have been explored and a transformation of nonluminescent Cs 4 PbBr 6 to highly luminescent CsPbBr 3 as well as a decomposition of CsPbBr 3 into non-emissive Cs 4 PbBr 6 has been demonstrated [10,11]. More specific, no particular crystalline phase due to the identical electron diffraction patterns of emissive Cs 4 PbX 6 (X = Cl, Br, I) and non-emissive Cs 4 PbX 6 making the PL origin even more intriguing. Chen et al. reported the bright green emission of Cs 4 PbBr 6 is originated from exciton recombination confined in [PbBr 6 ] 4− octahedra [12]. However, the majority of studies agree that Cs 4 PbBr 6 have a large bandgap (>3.2 eV), such a high band-gap material cannot show PL in the visible regions for band to band pure exciton transitions [13][14][15][16]. An impurity phase of CsPbBr 3 in Cs 4 PbBr 6 crystals is believed to be the occurrence of green luminescence [17]. Nonetheless, Mohammed and Bakr group ascribed the origin of the PL to the presence of intrinsic defects, such as bromide vacancies, via synthesizing Cs 4 PbBr 6 single crystals to rule out the morphological defects of CsPbBr 3 [18]. Thus, a plausible interpretation for the origin of the PL is the presence of shallow or deep energy defects with the formation of sub-bandgap states whose energies are related to the formation energy of the defect itself. Among the possible types of defects, the most prevalent defects are halogen vacancies, which behave as radiative recombination centers by forming sub-bandgap states where excitons get trapped [19][20][21][22].
Moreover, Zhang et al. described those emissive Cs 4 PbBr 6 phases, including powers and NCs, were generated in the presence of a polar solvent (i.e., dimethylformamide) [7], whereas those non-emissive phases were synthesized in an environment of nonpolar solvent (i.e., octadecene) [14]. Notably, although polar solvents like water give rise to chemical instability of perovskites [23,24], water [25] molecules improve the quality of perovskite films and promote the device performance reversibly have also been reported [26]. Wenke Zhou et al. believed the hydrogen bonding between the hydroxyl in water and the iodide ion in perovskite induced the deactivation process, changing the deep defect states to shallow ones [27]. Therefore, some problematic advantages of hydroxyl would not be possible overlooked during the PL process of 0D perovskite.
Herein, the stable 0D Cs 4 PbBr 6 micro-particles with "defect" structures were synthesized by a simple polar solution-based method employed ethyl alcohol as solution for the first time. The origin of the intriguing PL is clarified in details by using experimental analysis and theoretical calculation, which verifies the introduction of hydroxyl induces the PL process with the assistant of Br vacancies. In addition, an amplified spontaneous emission (ASE) and a single-mode lasing from an individual Cs 4 PbBr 6 micro-particle is readily achieved with the excitation of 800 nm femtosecond (fs) laser. All these results evidently suggest that stable Cs 4 PbBr 6 micro-particles provide a promising application for integrated photonic devices.

Experimental
Cesium (I) bromide (CsBr, Sigma-Aldrich), lead (II) bromide (PbBr 2 , Sigma-Aldrich), and dimethyl sulfoxide (DMSO) were used without further purification. The precursor solution was prepared by dissolving 0.05 M CsBr, 0.2 M PbBr 2 in 10 mL DMSO. The as-obtained solution was magnetic stirring for 12 h, and 0.9 mL of HBr was added afterward and stirred for 0.5 h for fully dissolved. The mixed solution was then filtered by using an injection filter. Simple drop-immersion of ethanol with a dosage of 200 µL was employed in the obtained filtrate until 3 mL ethanol was added. The yellow precipitate was finally obtained after the solution became limpidity from an appreciably turbid condition following the addition of 3 mL ethanol. After centrifuged with 3 min, the limpidity solution was washed with acetone and centrifuged to obtain the deposition, and then the final powder was collected after drying in a vacuum oven for 24 h. The experimental process is described in Fig. 1.
The powder X-ray diffraction (XRD) patterns were collected using a Bruker AXS D8 diffractometer. The microstructures were recorded by transmission electron microscopy (TEM) and high-resolution field transmission electron microscopy (HRTEM) using U.S. FEI TecnaiG2 F20 operating at 200 kV. The absorption spectra were measured on a HITACHI U-4100 type spectrophotometer (Hitachi, Tokyo, Japan). The photo-luminescence excitation (PLE) and PL spectra were recorded by a HITACHI-F-7000 spectrophotometer. For the lasing measurement, a homemade micro-PL system (OlympusBX-52 microscope and a 20×0.8 NA objective lens) was used to focus 800 nm femtosecond laser beam to be a 5 µm diameter spot at different area of microplates. A Ti: sapphire femtosecond laser (Coherent Libra) integrated with an optical parametric amplifier (Coherent OPerA Solo), which generated femtosecond pulses (50 fs,1 kHz) was used as the excitation source. Cs 4 PbBr 6 hexagonal micro-particles located inside the low-temperature (77 K) chamber was excited by the laser spot. Emission observed from the microcavity was then collected by the same objective lens. The received optical signal emitted from the microcavity was either coupled to a conventional charge-coupled device (CCD) camera for the recording of the near-field image or attached to a monochromator (Princeton SpectraPro 2750 integrated with a ProEM EMCCD camera with spectral resolution less than 0.1 nm) for spectrum analysis. Fourier transform infrared (FT-IR) spectra were measured by a Nicolet is 10 Fourier transform infrared spectrometer. The chemical states of Cs 4 PbBr 6 micro-particles were characterized by X-ray photoelectron spectroscopy (XPS) (200 W) with Al Kα radiation under the vacuum conditions. The first principles calculations were performed by the planewave pseudopotential method implemented in the CASTEP package based on the density functional theory (DFT) calculations [28]. The ion-electron interactions were modeled by ultrasoft pseudopotentials [29] for all elements. The generalized gradient approximation (GGA) [30] was adopted to describe the exchange and correlation potentials. The kinetic energy cutoff of 380 eV and Monkhorst-Pack k-point meshes [31] with a density of (3×3×3) points in the Brillouin zone were chosen.

Results and discussion
The XRD patterns of the sample indicate that the perovskite structure is formed ( Fig. 2(a)), in which a series of diffraction peaks can be indexed to the planes of hexagonal space group R3c of perovskite structure (JCPDS No.73-2478) [7]. SEM characterizations ( Fig. 2(b)) confirm that the as-obtained yellow samples are micro-meter sized particles with an average particle size of ∼ 30 µm. The mapping images of the particles demonstrate the well dispersion of elements of Cs, Pb and Br in the as-obtained Cs 4 PbBr 6 particles as exhibited in Fig. 2(c). A strong green emission located at 538 nm is recorded under 310 nm excitation, and the photograph of the green emission originated from an individual particle is presented in the inset of Fig. 2(d). The observed green emission is believed to be derived from individual Cs 4 PbBr 6 particle rather than CsPbBr 3 . It should be pointed out that a sharp absorption (310 nm) is observed, along with a strong absorption edgy appears around 514 nm in the absorption spectrum as shown in Fig. 2(d). This result infers that the two absorption band corresponds to the band gap absorption of Cs 4 PbBr 6 and a mid-band gap related to the introduction of defect level.  Fig. 1(b) exhibits the morphology of a single particle and the histogram of the inset describes the size and thickness distribution of the as-obtained Cs 4 PbBr 6 particles). (d) PL and absorption spectra of Cs 4 PbBr 6 particles (the inset of Fig. 2(d) shows the bright green emission from a single particle).
The FT-IR spectrum illustrates that a broad peak of hydroxide (-OH) appears in the synthesized Cs 4 PbBr 6 via the polar solution-based synthesis method, compared with its counterpart synthesized via a hot-injection process [32] (Fig. 3(a)). Moreover, the XPS spectrum displays the co-existence of Pb-Br bonds and Pb-O bonds (Fig. 3(b)), further demonstrating that the fabricated microcrystals contain a mass of bromine vacancies, some of which would be occupied by -OH to form Pb-O bond. The band structure and density of states properties of the 0D perovskite is investigated by the first-principles calculations. Cs 4 PbBr 6 has a bandgap with 3.76 eV (Fig. 3(c)) which is consistent with the results reported by Manna et al. [14], so the absorption at around 310 nm corresponds to the band gap for the GGA calculation underestimates the band gap in some extent. As shown in Fig. 3(c), the valence band (VB) is mainly formed by the p orbitals of bromine, and conduction band (CB) is made up of Pb-p orbitals with a little Br-s and Br-p orbitals, which means [PbBr 6 ] 4− octahedra dictate the optical transition behavior of perovskite materials. It should be pointed out that pure Cs 4 PbBr 6 with an energy gap of 3.76 eV should not exhibit PL at 550 nm for a interband transition. Thus, the green emission is possible only when defects are introduced to the band gap [18]. Moreover, although it is concluded that high V Br concentrations result in a lower radiative recombination efficiency, and a consequent decrease in PLQY, the recombination from mid-bandgap states due to crystal defects is identified as being the origin of the green PL in Cs 4 PbBr 6 [33] . The existence of a bromide vacancy introduce a sub-bandgap state 2.75 eV located in Cs 4 PbBr 6 as shown in Fig. 3(d). Unfortunately, the existence of bromide vacancy bring the green emission seems illogical. The incorporation of -OH groups as a defect is considerate to be introduced into Cs 4 PbBr 6 [34], however, through DFT calculations, the bandgap decreases with the introduction of -OH, the bandgap decreases from 3.75 to 3.50 eV (Fig. 3(e)) without the presence of the sub-bandgap state . This result is inconsistent with the report which proposed the incorporation of -OH groups form a 2.6 eV sub-bandgap state in Cs 4 PbBr 6 [35]. Furthermore, it is found that the incorporation of -OH and bromide vacancies contributes to a sub-bandgap at 2.44 eV in Cs 4 PbBr 6 with a narrowed energy gap (3.50 eV) (Fig. 3(f)). It is revealed that bromide vacancies are the dominant defects that responsible for the intrinsic n-type conduction in Cs 4 PbBr 6 , while the introduction of -OH contributes to the decrease of the band-gap, the synergistic effect of which induces the quite unambiguously green emission. The above results demonstrate the introduction of bromine vacancies and hydroxyl groups, contribute to the extrinsic luminescence properties synergistically.
In general, trap state density as a key factor seriously deteriorates the stimulated emission properties by causing the loss of photon-generated carriers through fast nonradiative recombination [36]. However, what unexpected in this work is that Cs 4 PbBr 6 micro-particles with "defect" structures of bromine vacancies and hydroxyl groups exhibit excellent stimulated emission properties under two-photon pumping by using Ti: sapphire laser system from Coherent (800 nm, 45 fs, 1 kHz repetition rate). During the lasing measurement, we perform a confocal microphotoluminescence spectrometer to detect the PL spectra of a single particle (see Figs. 4(c) and 5(c)). When the pump light source is focused at the edge of the particle, the intensity of the green emission increases with increasing pump power, and subsequently a bright green emission can be observed with naked eyes, as shown in Figs. 4(a) and 4(b). Figure 4(d) plots the total PL intensity and full width at half-maximum (FWHM) under different excitation density of Cs 4 PbBr 6 micro-particles. Under 800 nm pumping with a low excitation density (<125.00 µJ/cm 2 ), the emission intensity of Cs 4 PbBr 6 micro-particles follow a quadratic dependence on the pump power with a slope K (K = log I PL /logI exc ) of ∼1.62 [37], suggesting the domination of the two-photon absorption mechanism. When the excitation density increases above 125.00 µJ/cm 2 , the PL intensity is drastically increased with the excitation density, which presents a non-linear dependence of the PL intensity on the pumping energy. Concerning the FWHM (Fig. 4(d)), a clear line narrowing, from 12.6 nm down to about 3.2 nm as the excitation density increases is observed. The strong PL intensity increases and the line narrowing occur with the increase of excitation density, ambiguously demonstrating the appearance of ASE phenomenon. For the unique morphology of our obtained particles as shown in Fig. 4(c), it is hard to achieve a cavity for supporting laser loop feedback for excited from the edge of the micro-particle because of the non-parallel geometry.
For the unique morphology of our obtained particles which has a parallel upper and lower surface at the center of the particle and unparalleled surface around the edge of the particle, the PL spectra of these macro-particles were recorded for the pump light source focused at the center of the particle as presented in Figs. 5(a) and 5(b). A bright green emission and optical waveguide phenomenon is observed under 800 nm fs pumping as well. Moreover, it can be seen that a single sharp peak abruptly appears above the spontaneous emission background and grows drastically with further increasing pump density. Importantly, no other resonant peaks were found during this process. The corresponding changes of the integrated emission intensity and FWHM as a function of pump intensity are plotted in Fig. 5(d). When the pumping power increases over 153.5 µJ/cm 2 , a very sharp lasing peak centered at 538 nm in combination with a sharp decrease in FWHM to 0.4 nm emerges, and the lasing intensity grows rapidly with the increase of pump intensity. A clear evolvement from above optical characteristics indicates the achievement of a   single-mode laser. Single-mode lasing achieved from a particle if the free spectral range of the cavity is larger than the band-width of the gain material, and the latter is evaluated to be 3.2 nm from the FWHM of the ASE spectra of single particle. Here, we estimate the mode spacing, ∆λ, of plate microcavity [37], where λ is the center of the emission wavelength, n g is the group refractive index of ≈3.7 and 2L is the cavity length of microcrystal. ∆λ is estimated to be 11.5 nm for a Fabry-perot cavity provided with the particle for the thickness of 3.4 µm, which is clearly larger than the gain bandwidth value. Therefore, a single mode lasing could be realized for a natural feedback cavity for the F-P laser model with the micro-particles, when the excitation focused on the parallel symmetry of the upper and lower surface of our particle. In fact, the WGM mode is excluded for the absence of the observation of the excited light around the edge of Cs 4 PbBr 6 micro-particle. Moreover, ∆λ is calculated to be 1.6 nm when the cavity length is 48.56 µm of the micro-particle for a WGM mode, which indicates that a multi rather than single mode lasing should appear, inconsistent with our observation. Thus, a single mode F-P lasing output could be readily achieved in this work thanks to the laser loop feedback provided with the macro-particles for the thickness below 3.4 µm.

Conclusion
In summary, Cs 4 PbBr 6 micro-particles, which possess a 0D perovskite structure, are first fabricated via a low-temperature solution-process employed ethanol as solvent. Fluorescence image studies down to the single-particle level confirm the PL of bright green emission from an individual particle. Our results underline that the introduction of hydroxyl and the existence of bromine vacancies results in a narrowed band gap with the formation of a defect level, which contributes to the extrinsic PL properties synergistically. Because of the high exciton binding energy and the unique morphology with a regular geometric structure, two-photon pumped ASE and a single-mode lasing based on an individual Cs 4 PbBr 6 micro-particle are readily achieved by an 800 nm femtosecond laser. These results suggest that 0D perovskite Cs 4 PbBr 6 with "defect" structures could be promising candidates for the future development of perovskite materials based light-emitting diodes and lasers.