Two-dimensional Organic-inorganic Perovskites Research Articles

Halogen atom tuning the fluorescence emission of atomically thin two-dimensional organic-inorganic perovskites

Organic-inorganic hybrid perovskite shows great application prospects in the field of luminescent material due to relatively high chemical stability and regulation of efficient luminescence performance. Here, a series of organic-inorganic hybrid perovskites (R-XPMA)2PbCl4 (R = 2,3,4 substituted, PMA = benzylamine, X = Cl, Br) were prepared. Through the change from the molecular level two-dimensional perovskite “thickness”, as well as the preparation of heterojunction composites, tunning of the optical properties of the perovskite was achieved. The experimental results show that the shift of the fluorescence emission was related to the species and positions of halogen atoms to a certain degree, providing new insight into the optical properties regulation of layered perovskite materials.

Unusual symmetry breaking in high-temperature enantiomeric ferroelectrics with large spontaneous polarization.

Chiral organic-inorganic hybrid perovskites have gained extensive research interest due to their combination of chirality and the excellent optical, electrical and spin properties of perovskite materials, especially in two-dimensional hybrid perovskites. Herein, we report two-dimensional organic-inorganic perovskite enantiomeric ferroelectric [(R)-β-MPA]2CdCl4 (1) and [(S)-β-MPA]2CdCl4 (2) (MPA+ =methylphenethylammonium). Their mirror relationships are verified by both circular dichroism (CD) and crystal structures. At the same time, the two exhibit very similar ferroelectricity and related properties, including high Curie temperature (343 K), large spontaneous polarization (4.65 μC cm-2), and low coercive force field (13 kV cm-1). Unusually, at room temperature the crystal phase is monoclinic with the space group C2 and above the phase transition temperature it is triclinic with the space group P1, which means that the symmetry decreases with the increase of temperature. In addition, it exhibits a flexible switchable SHG response, while [(R)-β-MPA]2CdCl4 and [(S)-β-MPA]2CdCl4 have wide band gaps of 4.21 and 4.26 eV, respectively, mainly contributed by inorganic CdCl6 octahedra. This discovery opens a new way for the construction of two-dimensional enantiomeric molecular ferroelectrics.

Conformational disorder of organic cations tunes the charge carrier mobility in two-dimensional organic-inorganic perovskites

The chemical nature of the organic cations governs the optoelectronic properties of two-dimensional organic-inorganic perovskites. But its mechanism is not fully understood. Here, we apply femtosecond broadband sum frequency generation vibrational spectroscopy to investigate the molecular conformation of spacer organic cations in two-dimensional organic-inorganic perovskite films and establish a correlation among the conformation of the organic cations, the charge carrier mobility, and broadband emission. Our study indicates that both the mobility and broadband emission show strong dependence on the molecular conformational order of organic cations. The gauche defect and local chain distortion of organic cations are the structural origin of the in-plane mobility reduction and broad emission in two-dimensional organic-inorganic perovskites. Both of the interlayer distance and the conformational order of the organic cations affect the out-of-plane mobility. This work provides molecular-level understanding of the conformation of organic cations in optimizing the optoelectronic properties of two-dimensional organic-inorganic perovskites.

A multiaxial lead-free two-dimensional organic-inorganic perovskite ferroelectric.

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) have recently gained tremendous interest because of their unique features in contrast to three-dimensional counterparts and traditional 2D materials. However, although some 2D HOIP ferroelectrics have been achieved, the issue of toxic Pb and uniaxial nature impede their further application. Herein, for the first time, we report a lead-free 2D HOIP multiaxial ferroelectric, [3,3-difluorocyclobutylammonium]2CuCl4 (1), which shows four ferroelectric axes and eight equivalent polarization directions, more than those of the other 2D HOIP ferroelectrics and even the inorganic perovskite ferroelectric BaTiO3 (three ferroelectric axes and six equivalent polarization directions). 1 also features a high Curie temperature of 380 K and exhibits remarkable thermochromism of color change from green-yellow to dark brown. To our knowledge, 1 is the first multiaxial lead-free 2D HOIP ferroelectric. This work sheds light on the exploration of better lead-free 2D HOIP ferroelectrics.

Fine Structural Details Matter: A Lesson from Seven-Layered 2D Hybrid Perovskites

What is the maximum thickness limit of inorganic layers in two-dimensional organic-inorganic perovskites? This has recently been a key question to fill the stability-efficiency gap of perovskite optoelectronics. In this issue of Chem, Mao et al. push the limit: they synthesize and characterize seven-layers-thick two-dimensional Dion-Jacobson perovskite single crystals.

Temperature-Dependent Band Gap in Two-Dimensional Perovskites: Thermal Expansion Interaction and Electron-Phonon Interaction.

Two-dimensional organic-inorganic perovskites have attracted considerable interest recently. Here, we present a systematic study of the temperature-dependent photoluminescence on phase pure (n-BA)2(MA) n-1Pb nI3 n+1 ( n = 1-5) and (iso-BA)2(MA) n-1Pb nI3 n+1 ( n = 1-3) microplates obtained by mechanical exfoliation. The photoluminescence peak position gradually changes from a red-shift for n = 1 to a blue-shift for n = 5 with an increase in temperature in the (n-BA)2(MA) n-1Pb nI3 n+1 ( n = 1-5) series, while only a monotonous blue-shift has been observed for the (iso-BA)2(MA) n-1Pb nI3 n+1 ( n = 1-3) series, which can be attributed to the competition between the thermal expansion interaction and electron-phonon interaction. In the (n-BA)2(MA) n-1Pb nI3 n+1 ( n = 1-5) series, the thermal expansion interaction and electron-phonon interaction are both gradually enhanced and the former progressively dominates the latter from n = 1 to n = 5, resulting in the band gap versus temperature changing from a red-shift to a blue-shift. In contrast, both of these factors show a weaker layer thickness dependence, leading to the monotonous blue-shift in the (iso-BA)2(MA) n-1Pb nI3 n+1 ( n = 1-3) series.

Room Temperature Coherently Coupled Exciton-Polaritons in Two-Dimensional Organic-Inorganic Perovskite.

Two-dimensional (2D) organic-inorganic perovskite semiconductors with natural multiquantum well structures and confined 2D excitons are intriguing for the study of strong exciton-photon coupling, due to their large exciton binding energy and oscillation strength. This strong coupling leads to a formation of the half-light half-matter bosonic quasiparticle called exciton-polariton, consisting of a linear superposition state between photonic and excitonic states. Here, we demonstrate room temperature strong coupling in exfoliated wavelength-tunable 2D organic-inorganic perovskite semiconductors embedded into a planar microcavity, exhibiting large energetic splitting-to-line width ratios (>34.2). Angular-dependent spectroscopy measurements reveal that hybridized polariton states act as an ultrafast and reversible energy oscillation, involving 2D perovskite exciton, cavity modes (CM), and Bragg modes of the distributed Bragg reflector. Meanwhile, sizable hybrid particles dominantly couple to the measured optical field through the CMs. Our findings advocate a considerable promise of 2D organic-inorganic perovskite to explore fundamental quantum phenomena such as Bose-Einstein condensation, superfluidity, and exciton-polariton networks.

(C6H5C2H4NH3)2GeI4: A Layered Two-Dimensional Perovskite with Potential for Photovoltaic Applications.

Recently, two-dimensional organic-inorganic perovskites have attracted increasing attention due to their unique photophysical properties and high stability. Here we report a lead-free, two-dimensional perovskite, (PEA)2GeI4 (PEA = C6H5(CH2)2NH3+). Structural characterization demonstrated that this 2D perovskite structure is formed with inorganic germanium iodide planes separated by organic PEAI layers. (PEA)2GeI4 has a direct band gap of 2.12 eV, in agreement with 2.17 eV obtained by density functional theory (DFT) calculations, implying that it is suitable for a tandem solar cell. (PEA)2GeI4 luminesces at room-temperature with a moderate lifetime, exhibiting good potential for photovoltaic applications. In addition, 2D (PEA)2GeI4 is more stable than 3D CH3NH3GeI3 in air, owing to the presence of a hydrophobic organic long chain. This work provides a direction for the development of 2D Ge-based perovskites with potential for photovoltaic applications.

The FRET performance and aggregation-induced emission of two-dimensional organic-inorganic perovskite, and its application to the determination of Hg(II)

The authors describe the synthesis of a two-dimensional organic-inorganic perovskite of the type NH3(CH2)10NH3)PbBr4 by a fast precipitation method. The resulting powder under photoecitation at 270 nm exhibits yellow luminescence (peaking at 550 nm) in DMSO/water mixtures if the ratio is bigger than 4/6. On addition of Hg(II) and a rhodamine spirolactam, ring opening occurs and colorimetric and fluorescent responses can be seen. Specifically, the emission at 550 nm gradually decreases and a new emission peak appears at 585 nm (corresponding to the emission peak of RBEA-Hg(II) complex) while the color of the solution gradually turns to pink under visible light. The emission peak of the modified perovskite overlaps the 585 nm absorption of rhodamine B, and this results in a FRET effect whose Forster radius (donor-acceptor distance) is calculated from decay time data to be 10.4 A. Based on these findings, a FRET-based assay for Hg(II) was worked that has a 2.4 μM detection limit.

Mechanism for Broadband White-Light Emission from Two-Dimensional (110) Hybrid Perovskites.

The recently discovered phenomenon of broadband white-light emission at room temperature in the (110) two-dimensional organic-inorganic perovskite (N-MEDA)[PbBr4] (N-MEDA = N(1)-methylethane-1,2-diammonium) is promising for applications in solid-state lighting. However, the spectral broadening mechanism and, in particular, the processes and dynamics associated with the emissive species are still unclear. Herein, we apply a suite of ultrafast spectroscopic probes to measure the primary events directly following photoexcitation, which allows us to resolve the evolution of light-induced emissive states associated with white-light emission at femtosecond resolution. Terahertz spectra show fast free carrier trapping and transient absorption spectra show the formation of self-trapped excitons on femtosecond time-scales. Emission-wavelength-dependent dynamics of the self-trapped exciton luminescence are observed, indicative of an energy distribution of photogenerated emissive states in the perovskite. Our results are consistent with photogenerated carriers self-trapped in a deformable lattice due to strong electron-phonon coupling, where permanent lattice defects and correlated self-trapped states lend further inhomogeneity to the excited-state potential energy surface.

Synthesis, optical properties and photostability of novel fluorinated organic–inorganic hybrid (R–NH3)2PbX4 semiconductors

We report on the synthesis and the optical properties of several novel semiconductors (R–NH3)2PbX4(X = Br−, I− or Cl−). These semiconductors are two-dimensional organic–inorganic perovskite (2DOIP) materials and have multiple quantum-well energy level structures. We vary the organic components , introducing fluorine atoms into the organic part, on the phenyl ring of the amine. We discuss its influence on the self-organization ability and long-term photostability of the 2DOIPs. The trends of introducing fluorine atoms on the self-organization and long-term photostability of 2DOIPs are obtained by analysing the optical experimental results, and show that the influence of the fluorine position on the benzene ring is quite important. The most promising compounds seem to be the ones with the fluorine atom sitting on the para position of the phenyl group.