MAPbBr3 Nanocrystals Research Articles

In-situ fabricated anisotropic halide perovskite nanocrystals in polyvinylalcohol nanofibers: Shape tuning and polarized emission

We report an in-situ fabrication of halide perovskite (CH3NH3PbX3, CH3NH3 = methylammonium, MA, X= Cl, Br, I) nanocrystals in polyvinylalcohol (PVA) nanofibers (MAPbX3@PVA nanofibers) through electrospinning a perovskite precursor solution. With the content of the precursors increased, the resulting MAPbBr3 nanocrystals in PVA matrix changed the shape from ellipsoidal to pearl-like, and finely into rods-like. Optimized MAPbBr3@PVA nanofibers show strong polarized emission with the photoluminescence quantum yield of up to 72%. We reveal correlations between the shape of in-situ fabricated perovskite nanocrystals and the polarization degree of their emission by comparing experimental data from the single nanofiber measurements with theoretical calculations. Polarized emission of MAPbBr3@PVA nanofibers can be attributed to the dielectric confinement and quantum confinement effects. Moreover, nanofibers can be efficiently aligned by using parallel positioned conductor strips with an air gap as collector. A polarization ratio of 0.42 was achieved for the films of well-aligned MAPbBr3@PVA nanofibers with a macroscale size of 0.5 cm × 2 cm, which allows potential applications in displays, lasers, waveguides, etc.

UV-Green Emission from Organolead Bromide Perovskite Nanocrystals

Ultraviolet (UV)-green emitting MAPbBr3 nanocrystals were synthesized at room temperature, employing a cost-effective solution-based method. The size control of nanocrystals was achieved through varying ligand and solute concentrations, which resulted in a tunable band gap and emission spectrum. The growth mechanism as well as the effect of ligand concentration on the structural and optical properties were studied in detail. The excitation spectra extended from the blue to UV region. This indicates that these perovskites are promising photon down conversion materials, which can combine with III-nitride UV/blue light-emitting diodes (LEDs) to emit white light. This work may bring III-nitride-based white LEDs one step closer to widespread adoption in general illumination market because the large emission range that has been produced with the ligand-assisted reprecipitation process is an important milestone in the path to justifying commercialization.

Unconventional solution-phase epitaxial growth of organic-inorganic hybrid perovskite nanocrystals on metal sulfide nanosheets

Epitaxial heterostructures based on organic-inorganic hybrid perovskites and two-dimensional materials hold great promises in optoelectronics, but they have been prepared only via solid-state methods that restricted their practical applications. Herein, we report cubic-phased MAPbBr3 (MA=CH3NH3+) nanocrystals were epitaxially deposited on trigonal/hexagonal-phased MoS2 nanosheets in solution by facilely tuning the solvation conditions. In spite of the mismatched lattice symmetry between the square MAPbBr3 (001) overlayer and the hexagonal MoS2 (001) substrate, two different aligning directions with lattice mismatch of as small as 1% were observed based on the domain-matching epitaxy. This was realized most likely due to the flexible nature and absence of surface dangling bonds of MoS2 nanosheets. The formation of the epitaxial interface affords an effective energy transfer from MAPbBr3 to MoS2, and as a result, paper-based photodetectors facilely fabricated from these solution-dispersible heterostructures showed better performance compared to those based on MoS2 or MAPbBr3 alone. In addition to the improved energy transfer and light adsorption, the use of MoS2 nanosheets provided flexible and continuous substrates to connect the otherwise discrete MAPbBr3 nanocrystals and achieved the better film forming ability. Our work suggests that the scalable preparation of heterostructures based on organic-inorganic hybrid perovskites and 2D materials via solution-phase epitaxy may bring about more opportunities for expanding their optoelectronic applications.

Room-Temperature Synthesis of Widely Tunable Formamidinium Lead Halide Perovskite Nanocrystals

Colloidal perovskite nanocrystals based on formamidinium lead halide (FAPbX3) have been synthesized by the ligand-assisted reprecipitation method using PbX2–dimethyl sulfoxide complexes as precursors at room temperature. Well-defined cubic-shaped FAPbX3 nanocrystals have been obtained with a size d of ∼10 nm. The synthesized FAPbX3 nanocrystals show bright photoluminescence with a high photoluminescence quantum yield (75% for FAPbBr3). The lifetimes of FAPbBr3 nanocrystals were measured for the samples isolated at several different centrifugal speeds. The photoluminescence can be tuned from the blue to near-infrared region (λpeak = 408–784 nm) by changing either the amount of oleylamine or the composition of X. The color expression range is 135% of the NTSC standard. The bandwidth of the photoluminescence spectra of FAPbX3 nanocrystals is narrow (full width at half-maximum of 18–48 nm). FAPbX3 nanocrystals show thermal stability that is better than that of MAPbBr3 nanocrystals.

In-Situ Formed Type I Nanocrystalline Perovskite Film for Highly Efficient Light-Emitting Diode.

Excellent color purity with a tunable band gap renders organic-inorganic halide perovskite highly capable of performing as light-emitting diodes (LEDs). Perovskite nanocrystals show a photoluminescence quantum yield exceeding 90%, which, however, decreases to lower than 20% upon formation of a thin film. The limited photoluminescence quantum yield of a perovskite thin film has been a formidable obstacle for development of highly efficient perovskite LEDs. Here, we report a method for highly luminescent MAPbBr3 (MA = CH3NH3) nanocrystals formed in situ in a thin film based on nonstoichiometric adduct and solvent-vacuum drying approaches. Excess MABr with respect to PbBr2 in precursor solution plays a critical role in inhibiting crystal growth of MAPbBr3, thereby forming nanocrystals and creating type I band alignment with core MAPbBr3 by embedding MAPbBr3 nanocrystals in the unreacted wider band gap MABr. A solvent-vacuum drying process was developed to preserve nanocrystals in the film, which realizes a fast photoluminescence lifetime of 3.9 ns along with negligible trapping processes. Based on a highly luminescent nanocrystalline MAPbBr3 thin film, a highly efficient green LED with a maximum external quantum efficiency of 8.21% and a current efficiency of 34.46 cd/A was demonstrated.

Colloidal Organohalide Perovskite Nanoplatelets Exhibiting Quantum Confinement.

We prepare colloidal nanoplatelets of methylammonium lead bromide (MAPbBr3) perovskite and compare the optical signatures of excitons in these two-dimensional systems to spherical perovskite nanocrystals and the corresponding bulk phase. We find that excitonic features that had previously been attributed to quantum confinement in MAPbBr3 nanocrystals are in fact a property of the bulk perovskite phase. Furthermore, we find that higher-energy absorption features originate from two-dimensional nanoplatelets, which are present in the nanocrystal reaction product. Upon further purification, we obtain colloidal nanoplatelets with predominantly single unit cell thickness and submicron lateral dimensions, which are stable in solution and exhibit a sharp excitonic absorption feature 0.5 eV blue-shifted from that of the three-dimensional bulk MAPbBr3 phase, representing a new addition to the growing family of colloidal two-dimensional nanostructures.