Bright magnetic dipole radiation from two-dimensional lead-halide perovskites

Anisotropic Electron-Phonon Interactions in 2D Lead-Halide Perovskites

Two-dimensional (2D) hybrid organic-inorganic metal halide perovskites offer enhanced stability for perovskite-based applications. Their crystal structure's soft and ionic nature gives rise to strong interaction between charge carriers and ionic rearrangements. Here, we investigate the interaction of photogenerated electrons and ionic polarizations in single-crystal 2D perovskite butylammonium lead iodide (BAPI), varying the inorganic lamellae thickness in the 2D single crystals. We determine the directionality of the transition dipole moments (TDMs) of the relevant phonon modes (in the 0.3-3 THz range) by the angle- and polarization-dependent THz transmission measurements. We find a clear anisotropy of the in-plane photoconductivity, with a ∼10% reduction along the axis parallel with the transition dipole moment of the most strongly coupled phonon. Detailed calculations, based on Feynman polaron theory, indicate that the anisotropy originates from directional electron-phonon interactions.

The role of Pb oxidation state of the precursor in the formation of 2D perovskite microplates

Two-dimensional (2D) lead halide perovskites are an exciting class of materials currently being extensively explored for photovoltaics and other optoelectronic applications. Their ionic nature makes them ideal candidates for solution processing into both thin films and nanostructured crystals. Understanding how 2D lead halide perovskite crystals form is key towards full control over their physical properties, which may enable new physical phenomena and devices. Here, we investigate the effects of the Pb oxidation state of the initial inorganic precursor on the growth of pure-phase (n = 1) - Popper 2D perovskite BA₂PbI₄ in single-step synthesis. We examine the different crystallisation routes in exposing PbO₂ and PbI₂ powders to a BAI : IPA organo-halide solution, by combining in situ optical microscopy, UV-VIS spectroscopy and time-resolved high performance liquid chromatography. So far, works using PbO₂ to synthesise 3D LHPs introduce a preceding step to reduce PbO₂ into either PbO or PbI₂. In this work, we find that BA₂PbI₄ is directly formed when exposing PbO₂ to BAI : IPA without the need for an external reducing agent. We explain this phenomenon by the spontaneous reduction/oxidation of PbO₂/BAI that occurs under iodine-rich conditions. We observe differences in the final morphology (rectangles vs. octagons) and nanocrystal growth rate, which we explain through the different chemistry and iodoplumbate complexes involved in each case. As such, this work spans the horizon of usable lead precursors and offers a new turning knob to control crystal growth in single-step LHP synthesis.

Size-dependent chiro-optical properties of CsPbBr3 nanoparticles

Chiral metal halide perovskites have garnered substantial interest because of their promising properties for application in optoelectronics and spintronics. Understanding the mechanism of chiral imprinting is paramount for optimizing their utility. To elucidate the nature of the underlying chiral imprinting mechanism, we investigated how the circular dichroism (CD) intensity varies with nanoparticle size for quantum confined sizes of colloidal CsPbBr₃ perovskite nanoparticles (NPs) capped by chiral β-methylphenethylammonium bromide ligands. We find that the CD intensity decreases strongly with increasing NP size, which, along with the shape of the CD spectra, points to electronic interactions between ligand and NP as the dominant mechanism of chiral imprinting in smaller NPs. We observe that as the NP size increases and crosses the quantum confinement threshold, the dominant mechanism of chirality transfer switches and is dominated by surfaces effects, e.g., structural distortions. These findings provide a benchmark for quantitative models of chiral imprinting.

Managing Growth and Dimensionality of Quasi 2D Perovskite Single-Crystalline Flakes for Tunable Excitons Orientation

Hybrid perovskites are among the most promising materials for optoelectronic applications. Their 2D crystalline form is even more interesting since the alternating inorganic and organic layers naturally forge a multiple quantum-well structure, leading to the formation of stable excitonic resonances. Nevertheless, a controlled modulation of the quantum well width, which is defined by the number of inorganic layers (n) between two organic ones, is not trivial and represents the main synthetic challenge in the field. Here, a conceptually innovative approach to easily tune n in lead iodide perovskite single-crystalline flakes is presented. The judicious use of potassium iodide is found to modulate the supersaturation levels of the precursors solution without being part of the final products. This allows to obtain a fine tuning of the n value. The excellent optical quality of the as synthesized flakes guarantees an in-depth analysis by Fourier-space microscopy, revealing that the excitons orientation can be manipulated by modifying the number of inorganic layers. Excitonic out-of-plane component, indeed, is enhanced when "n" is increased. The combined advances in the synthesis and optical characterization fill in the picture of the exciton behavior in low-dimensional perovskite, paving the way to the design of materials with improved optoelectronic characteristics.

Optical-Frequency Magnetic Polarizability in a Layered Semiconductor

The optical response of crystals is most commonly attributed to electric dipole interactions between light and matter. Although metamaterials support "artificial" magnetic resonances supported by mesoscale structuring, there are no naturally occurring materials known to exhibit a nonzero optical-frequency magnetic polarizability. Here, we experimentally demonstrate and quantify a naturally occurring nonzero magnetic polarizability in a layered semiconductor system: two-dimensional (Ruddlesden-Popper phase) hybrid organic-inorganic perovskites. These results demonstrate the only known material with an optical-frequency permeability that differs appreciably from vacuum, informing future efforts to find, synthesize, or exploit atomic-scale optical magnetism for novel optical phenomena such as negative index of refraction and electromagnetic cloaking.

Exciton-Photonics: From Fundamental Science to Applications

Semiconductors in all dimensionalities ranging from 0D quantum dots and molecules to 3D bulk crystals support bound electron-hole pair quasiparticles termed excitons. Over the past two decades, the emergence of a variety of low-dimensional semiconductors that support excitons combined with advances in nano-optics and photonics has burgeoned an advanced area of research that focuses on engineering, imaging, and modulating the coupling between excitons and photons, resulting in the formation of hybrid quasiparticles termed exciton-polaritons. This advanced area has the potential to bring about a paradigm shift in quantum optics, as well as classical optoelectronic devices. Here, we present a review on the coupling of light in excitonic semiconductors and previous investigations of the optical properties of these hybrid quasiparticles via both far-field and near-field imaging and spectroscopy techniques. Special emphasis is given to recent advances with critical evaluation of the bottlenecks that plague various materials toward practical device implementations including quantum light sources. Our review highlights a growing need for excitonic material development together with optical engineering and imaging techniques to harness the utility of excitons and their host materials for a variety of applications.

Temperature-Dependent Photoluminescence of Hexafluorobenzene-Intercalated Phenethylammonium Tin Iodide 2D Perovskite

Tin halide perovskites are potential alternatives of lead halide perovskites. However, the easy oxidation of Sn²⁺ to Sn⁴⁺ brings in a challenge. Recently, layered two-dimensional hybrid tin halide perovskites have been shown to partially resist the oxidation process because of the presence of hydrophobic organic molecules. Consequently, such layered hybrid perovskites are being explored for optoelectronic applications. The optical properties of layered tin halide perovskites depend on the interlayer separation and the dielectric mismatch between the organic and inorganic layers. Intercalation (insertion) of a molecular species between the layers modifies the interlayer interactions affecting the optical properties of layered hybrid perovskites. We investigated the effect of hexafluorobenzene (HFB) intercalation in phenethylammonium tin iodide [(PEA)₂ SnI₄ ] using temperature-dependent (6 K to 300 K) photoluminescence (PL). HFB intercalation increases the bandgap. A strong PL quenching is observed in pristine (PEA)₂ SnI₄ below 150 K, probably because of the presence of non-emissive states. HFB intercalation suppresses the influence of such non-emissive states resulting in an increase in PL intensity at the cryogenic temperatures. Our results highlight that a simple molecular intercalation (non-covalent interaction) into layered hybrid perovskites can significantly tailor the electronic and optical properties.

Self-Hybridized Polaritonic Emission from Layered Perovskites

Light-matter coupling in excitonic materials has been the subject of intense recent investigations due to emergence of new materials. Two-dimensional layered hybrid organic/inorganic perovskites (2D HOIPs) support strongly bound excitons at room temperature with some of the highest oscillator strengths and electric loss tangents among the known excitonic materials. Here, we report strong light-matter coupling in Ruddlesden-Popper phase 2D HOIP crystals without the necessity of an external cavity. We report the concurrent occurrence of multiple orders of hybrid light-matter states via both reflectance and luminescence spectroscopy in thick (>100 nm) crystals and near-unity absorption in thin (<20 nm) crystals. We observe resonances with quality factors of >250 in hybridized exciton-polaritons and identify a linear correlation between exciton-polariton mode splitting and extinction coefficient of the various 2D HOIPs. Our work opens the door to studying polariton dynamics in self-hybridized and open cavity systems with broad applications in optoelectronics and photochemistry.

Manganese doping for enhanced magnetic brightening and circular polarization control of dark excitons in paramagnetic layered hybrid metal-halide perovskites

Materials combining semiconductor functionalities with spin control are desired for the advancement of quantum technologies. Here, we study the magneto-optical properties of novel paramagnetic Ruddlesden-Popper hybrid perovskites Mn:(PEA)2PbI4 (PEA = phenethylammonium) and report magnetically brightened excitonic luminescence with strong circular polarization from the interaction with isolated Mn2+ ions. Using a combination of superconducting quantum interference device (SQUID) magnetometry, magneto-absorption and transient optical spectroscopy, we find that a dark exciton population is brightened by state mixing with the bright excitons in the presence of a magnetic field. Unexpectedly, the circular polarization of the dark exciton luminescence follows the Brillouin-shaped magnetization with a saturation polarization of 13% at 4 K and 6 T. From high-field transient magneto-luminescence we attribute our observations to spin-dependent exciton dynamics at early times after excitation, with first indications for a Mn-mediated spin-flip process. Our findings demonstrate manganese doping as a powerful approach to control excitonic spin physics in Ruddlesden-Popper perovskites, which will stimulate research on this highly tuneable material platform with promise for tailored interactions between magnetic moments and excitonic states.

The red light emission in 2D (C4SH3CH2NH3)2SnI4 and (C4OH7CH2NH3)2SnI4 perovskites

Two-dimensional (2D) perovskites have a large exciton binding energy due to the structure of the quantum confinement, which produces a faster radiative recombination, and so are promising potential materials for light-emitting diodes. However, most of the highly efficient hybrid halide perovskites are based on the toxic Pb-based materials, so the replacement of Pb with less toxic and suitable substitute elements has been investigated for environmental efficient materials. Herein, we report the Sn-based 2D perovskites, which include (TPM)2SnI4 (TPM = C4SH3CH2NH3) and (TFF)2SnI4 (TFF = C4OH7CH2NH3), as red emission materials. Structural characterization by single crystal X-ray diffraction reveals that (TPM)2SnI4 undergoes a structural evolution from the orthorhombic space group Cmc21 (100 K) to Pbca (298 K), while the (TFF)2SnI4 perovskite exhibits the monoclinic space group P21/c at 100 K and 298 K. The inorganic framework of (TFF)2SnI4 was separated by the bilayer TFF chains with an empty space, which is an effective structure to increase the quantum confinement effect. The band gaps of the (TPM)2SnI4 (1.80 eV) and (TFF)2SnI4 (1.73 eV) compounds indicate the direct band gap semiconductor materials. From the time-resolved photoluminescence results, it can be seen that (TPM)2SnI4 produces uniform short emission (0.73 ns) throughout the entire powder crystals, whereas (TFF)2SnI4 has a uniform and long emission life time (47 ns). Temperature-dependent photoluminescence (PL) studies indicate that the (TPM)2SnI4 and (TFF)2SnI4 perovskites have a strong split red emission at low temperature due to the vibration of the inorganic framework. As the temperature increases, the PL spectra shift to the high energy region and the emission intensity decreases. The PL spectra of (TPM)2SnI4 and (TFF)2SnI4 perovskites have maximum peak wavelengths at 622 nm and 640 nm, and show the photoluminescence quantum yields of 0.30% and 1.71%, respectively.

Even-Parity Self-Trapped Excitons Lead to Magnetic Dipole Radiation in Two-Dimensional Lead Halide Perovskites

Recently, unconventional bright magnetic dipole (MD) radiation was observed from two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs). According to commonly accepted HOIP band structure calculations, such MD light emission from the ground-state exciton should be strictly symmetry forbidden. These results suggest that MD emission arises in conjunction with an as-yet unidentified symmetry-breaking mechanism. In this paper, we show that MD light emission originates from a self-trapped p-like exciton stabilized at energies below the primary electric dipole (ED)-emitting 1s exciton. Using suitable combinations of sample and collection geometries, we isolate the distinct temperature-dependent properties of the ED and MD photoluminescence (PL). We show that the ED emission wavelength is nearly constant with temperature, whereas the MD emission wavelength exhibits substantial red shifts with heating. To explain these results, we derive a microscopic model comprising two distinct parity exciton states coupled to lattice distortions. The model explains many experimental observations, including the thermal red shift, the difference in emission wavelengths, and the relative intensities of the ED and MD emission. Thermodynamic analysis of temperature-dependent PL reveals that the MD emission originates from a locally distorted structure. Finally, we demonstrate unusual hysteresis effects of the MD-emitting state near structural phase transitions. We hypothesize that this is another manifestation of the local distortions, indicating that they are insensitive to phase changes in the equilibrium lattice structure.