Excitonic and Confinement Effects of 2D Layered (C₁₀H₂₁NH₃)₂PbBr₄ Single Crystals

Abstract

Recognition of unusual optoelectronic properties for twodimensional (2D) layered organic−inorganic lead(II) halide materials (C n H 2n+1 NH 3 ) 2 PbX 4 (X = I, Br, and Cl) has attracted intense renewed interest in this class of materials. Single crystals of the 2D layered materials (C 10 H 21 NH 3 ) 2 PbBr 4 and pseudo-alloy (C 10 H 21 NH 3 ) 2 PbI 2 Br 2 were grown for photophysical evaluation. A 10-carbon alkylammonium cation was selected for investigation to provide strong dielectric screening in order to highlight quantum confinement effects of the anionic (PbX 4 2− ) semiconductor layer. Single crystals of the 2D layered (C 10 H 21 NH 3 ) 2 PbBr 4 compound display a characteristic free exciton with a binding energy of ca. 280 meV. Observation of a short photoluminescence lifetime of 2.8 ± 0.2 ns suggests that this electronic transition for the PbBr 4 -based layered material has only singlet character. Sheets of (C 10 H 21 NH 3 ) 2 PbBr 4 with thicknesses of a few layers were fabricated, and the dimensions were verified by AFM experiments. Excitonic emissions from (C 10 H 21 NH 3 ) 2 PbBr 4 and (C 10 H 21 NH 3 ) 2 PbI 4 exhibit relatively small spectral shifts from the bulk down to a thickness of five layers indicative of the strong confinement effect of the 10-carbon alkylammonium spacers. Single crystals of the pseudo-alloy (C 10 H 21 NH 3 ) 2 PbBr 2 I 2 give an excitonic absorption peak close to that of the tetrabromide (C 10 H 21 NH 3 ) 2 PbBr 4 and an emission peak with a large Stokes shift to a position similar to that of the tetraiodide (C 10 H 21 NH 3 ) 2 PbI 4 .

Green-Emitting Powders of Zero-Dimensional Cs4PbBr6: Delineating the Intricacies of the Synthesis and the Origin of Photoluminescence

“…In pure 2D perovskites such as 2D MA 2 PbI 4 , the difference between the band gaps of a monolayer and bulk material is theoretically calculated to be only 0.01 eV . Experimentally, unlike the quasi-2D, for example, (BA) 2 (MA) n −1 Pb n I 3 n + 1 (BA = butylammonium) that possesses a considerable blueshift in the PL peak position as the number of layers ( n ) decreases, pure 2D perovskites showed insignificant differences. , Hence, a 2D perovskite structure with a small Cs cation as an interlayer spacer could explain the narrow range of PL (515 to 524 nm) , reported for the emissive 0D single crystals, powders, and nanocrystals. The proposed Cs 2 PbBr 4 is different from 3–5 layered CsPbBr 3 nanosheets or nanoplatelets that usually emit between 440 and 460 nm. , This discrepancy in the PL peak position could be attributed to the fact that, in the case of nanosheets and nanoplatelets, their top and bottom surfaces are covered with large organic cations such as oleylammonium or octylammonium, and hence, they could be considered as quasi-2D perovskites, leading to the pronounced blueshift in their emissions.…”

Strong Triplet-Exciton–LO-Phonon Coupling in Two-Dimensional Layered Organic–Inorganic Hybrid Perovskite Single Crystal Microflakes

“…When the number of layers reduces from seven to one, the PL peak is blue-shifted, and the exciton peak energy is increased by ∼9.76 meV, as shown in Figure b. The blue shift with reduced layers is similar to that of (C 10 H 21 NH 3 ) 2 PbBr 4 , which can be ascribed to the increased optical band gap caused by the expansion of the lattice as the number of layers decreases. , Figure c displays the normalized PL spectra of the perovskite crystal microflake with different layer numbers at low temperature (7 K); the blue shift of P1 (3.042 eV) with a fwhm of 9.32 meV can still be observed clearly. However, it is worth noting that the other emission peak (P2) with fwhm of 4.30 meV appears at 3.018 eV for the seven-layer perovskite, and the intensity of the PL peak gradually decreases as the number of layers decreases.…”

Directional Anisotropy of the Vibrational Modes in 2D-Layered Perovskites

“…Two-dimensional (2D) Ruddlesden–Popper lead halide perovskites have emerged as a highly versatile material for optoelectronic applications , due to their bright emission, , strong excitonic and dielectric confinement, − and photostability in solar energy harvesting under ambient conditions. − This class of 2D layered perovskites can be fabricated using simple and low-cost strategies in the form of atomically thin layers, single crystals, and ensembles. , They consist of layers of corner-sharing [PbX 6 ] 4– octahedra that are separated by organic ammonium-based cations ( e.g ., phenethylammonium (PEA) and butylammonium (BA)), which are too large to fit into a three-dimensional (3D) octahedral structure, ,, and form a natural superlattice of octahedral planes linked by interdigitated bilayers of the organic moieties. This hierarchical architecture has a variety of highly appealing properties that are different from their 3D framework: the inorganic octahedral layers form a quantum well potential for the electronic carriers, in which the confinement can be tuned by the number n of adjacent octahedral planes, ,,, an aspect that is also extremely interesting for fundamental studies; ,, the electron–phonon coupling (and distance) between the inorganic layers can be modified by the choice of the organic moiety; ,− and the electronic level structure as well as the band gap can be tailored by choice of the halide anion and via lattice deformations. ,− Furthermore, the presence of long hydrophobic organic moieties intercalated between the octahedral layers protects the layered perovskites from moisture permeation, conferring them structural and functional stability. ,, …”