Bright Excitonic Fine Structure in Metal-Halide Perovskites: From Two-Dimensional to Bulk.

The optical response of two-dimensional (2D) perovskites, often referred to as natural quantum wells, is primarily governed by excitons, whose properties can be readily tuned by adjusting the perovskite layer thickness. We have investigated the exciton fine structure splitting in the archetypal 2D perovskite (PEA)2(MA)n-1PbnI3n+1 with varying numbers of inorganic octahedral layers n = 1, 2, 3, and 4. We demonstrate that the in-plane excitonic states exhibit splitting and orthogonally oriented dipoles for all confinement regimes. The evolution of the exciton states in an external magnetic field provides further insights into the g-factors and diamagnetic coefficients. With increasing n, we observe a gradual evolution of the excitonic parameters characteristic of a 2D to three-dimensional transition. Our results provide valuable information concerning the evolution of the optoelectronic properties of 2D perovskites with the changing confinement strength.

Polaron Vibronic Progression Shapes the Optical Response of 2D Perovskites.

The optical response of 2D layered perovskites is composed of multiple equally-spaced spectral features, often interpreted as phonon replicas, separated by an energy Δ ≃ 12 - 40 meV, depending upon the compound. Here the authors show that the characteristic energy spacing, seen in both absorption and emission, is correlated with a substantial scattering response above ≃ 200 cm-1 (≃ 25 meV) observed in resonant Raman. This peculiar high-frequency signal, which dominates both Stokes and anti-Stokes regions of the scattering spectra, possesses the characteristic spectral fingerprints of polarons. Notably, its spectral position is shifted away from the Rayleigh line, with a tail on the high energy side. The internal structure of the polaron consists of a series of equidistant signals separated by 25-32 cm-1 (3-4 meV), depending upon the compound, forming a polaron vibronic progression. The observed progression is characterized by a large Huang-Rhys factor (S > 6) for all of the 2D layered perovskites investigated here, indicative of a strong charge carrier - lattice coupling. The polaron binding energy spans a range ≃ 20-35 meV, which is corroborated by the temperature-dependent Raman scattering data. The investigation provides a complete understanding of the optical response of 2D layered perovskites via the direct observation of polaron vibronic progression. The understanding of polaronic effects in perovskites is essential, as it directly influences the suitability of these materials for future opto-electronic applications.

Approaching the Intrinsic Properties of Moiré Structures Using Atomic Force Microscopy Ironing.

Stacking monolayers of transition metal dichalcogenides (TMDs) has led to the discovery of a plethora of new exotic phenomena, resulting from moiré pattern formation. Due to the atomic thickness and high surface-to-volume ratio of heterostructures, the interfaces play a crucial role. Fluctuations in the interlayer distance affect interlayer coupling and moiré effects. Therefore, to access the intrinsic properties of the TMD stack, it is essential to obtain a clean and uniform interface between the layers. Here, we show that this is achieved by ironing with the tip of an atomic force microscope. This post-stacking procedure dramatically improves the homogeneity of the interfaces, which is reflected in the optical response of the interlayer exciton. We demonstrate that ironing improves the layer coupling, enhancing moiré effects and reducing disorder. This is crucial for the investigation of TMD heterostructure physics, which currently suffers from low reproducibility.

Layered BiOI single crystals capable of detecting low dose rates of X-rays.

Detecting low dose rates of X-rays is critical for making safer radiology instruments, but is limited by the absorber materials available. Here, we develop bismuth oxyiodide (BiOI) single crystals into effective X-ray detectors. BiOI features complex lattice dynamics, owing to the ionic character of the lattice and weak van der Waals interactions between layers. Through use of ultrafast spectroscopy, first-principles computations and detailed optical and structural characterisation, we show that photoexcited charge-carriers in BiOI couple to intralayer breathing phonon modes, forming large polarons, thus enabling longer drift lengths for the photoexcited carriers than would be expected if self-trapping occurred. This, combined with the low and stable dark currents and high linear X-ray attenuation coefficients, leads to strong detector performance. High sensitivities reaching 1.1 × 103 µC Gyair-1 cm-2 are achieved, and the lowest dose rate directly measured by the detectors was 22 nGyair s-1. The photophysical principles discussed herein offer new design avenues for novel materials with heavy elements and low-dimensional electronic structures for (opto)electronic applications.

Fine Structure Splitting of Phonon-Assisted Excitonic Transition in (PEA)2PbI4 Two-Dimensional Perovskites.

Two-dimensional van der Waals materials exhibit particularly strong excitonic effects, which causes them to be an exceptionally interesting platform for the investigation of exciton physics. A notable example is the two-dimensional Ruddlesden-Popper perovskites, where quantum and dielectric confinement together with soft, polar, and low symmetry lattice create a unique background for electron and hole interaction. Here, with the use of polarization-resolved optical spectroscopy, we have demonstrated that the simultaneous presence of tightly bound excitons, together with strong exciton-phonon coupling, allows for observing the exciton fine structure splitting of the phonon-assisted transitions of two-dimensional perovskite (PEA)2PbI4, where PEA stands for phenylethylammonium. We demonstrate that the phonon-assisted sidebands characteristic for (PEA)2PbI4 are split and linearly polarized, mimicking the characteristics of the corresponding zero-phonon lines. Interestingly, the splitting of differently polarized phonon-assisted transitions can be different from that of the zero-phonon lines. We attribute this effect to the selective coupling of linearly polarized exciton states to non-degenerate phonon modes of different symmetries resulting from the low symmetry of (PEA)2PbI4 lattice.

Thickness-Dependent Dark-Bright Exciton Splitting and Phonon Bottleneck in CsPbBr3-Based Nanoplatelets Revealed via Magneto-Optical Spectroscopy.

The optimized exploitation of perovskite nanocrystals and nanoplatelets as highly efficient light sources requires a detailed understanding of the energy spacing within the exciton manifold. Dark exciton states are particularly relevant because they represent a channel that reduces radiative efficiency. Here, we apply large in-plane magnetic fields to brighten optically inactive states of CsPbBr3-based nanoplatelets for the first time. This approach allows us to access the dark states and directly determine the dark-bright splitting, which reaches 22 meV for the thinnest nanoplatelets. The splitting is significantly less for thicker nanoplatelets due to reduced exciton confinement. Additionally, the form of the magneto-PL spectrum suggests that dark and bright state populations are nonthermalized, which is indicative of a phonon bottleneck in the exciton relaxation process.

Quantification of Exciton Fine Structure Splitting in a Two-Dimensional Perovskite Compound.

Applications of two-dimensional (2D) perovskites have significantly outpaced the understanding of many fundamental aspects of their photophysics. The optical response of 2D lead halide perovskites is dominated by strongly bound excitonic states. However, a comprehensive experimental verification of the exciton fine structure splitting and associated transition symmetries remains elusive. Here we employ low temperature magneto-optical spectroscopy to reveal the exciton fine structure of (PEA)2PbI4 (here PEA is phenylethylammonium) single crystals. We observe two orthogonally polarized bright in-plane free exciton (FX) states, both accompanied by a manifold of phonon-dressed states that preserve the polarization of the corresponding FX state. Introducing a magnetic field perpendicular to the 2D plane, we resolve the lowest energy dark exciton state, which although theoretically predicted, has systematically escaped experimental observation (in Faraday configuration) until now. These results corroborate standard multiband, effective-mass theories for the exciton fine structure in 2D perovskites and provide valuable quantification of the fine structure splitting in (PEA)2PbI4.

Spectral Dependence of the Energy Transfer from Photosynthetic Complexes to Monolayer Graphene.

Fluorescence excitation spectroscopy at cryogenic temperatures carried out on hybrid assemblies composed of photosynthetic complexes deposited on a monolayer graphene revealed that the efficiency of energy transfer to graphene strongly depended on the excitation wavelength. The efficiency of this energy transfer was greatly enhanced in the blue-green spectral region. We observed clear resonance-like behavior for both a simple light-harvesting antenna containing only two chlorophyll molecules (PCP) and a large photochemically active reaction center associated with the light-harvesting antenna (PSI-LHCI), which pointed towards the general character of this effect.

Brightening of dark excitons in 2D perovskites.

Optically inactive dark exciton states play an important role in light emission processes in semiconductors because they provide an efficient nonradiative recombination channel. Understanding the exciton fine structure in materials with potential applications in light-emitting devices is therefore critical. Here, we investigate the exciton fine structure in the family of two-dimensional (2D) perovskites (PEA)2SnI4, (PEA)2PbI4, and (PEA)2PbBr4. In-plane magnetic field mixes the bright and dark exciton states, brightening the otherwise optically inactive dark exciton. The bright-dark splitting increases with increasing exciton binding energy. Hot photoluminescence is observed, indicative of a non-Boltzmann distribution of the bright-dark exciton populations. We attribute this to the phonon bottleneck, which results from the weak exciton­acoustic phonon coupling in soft 2D perovskites. Hot photoluminescence is responsible for the strong emission observed in these materials, despite the substantial bright-dark exciton splitting.

Nonradiative Energy Transfer and Selective Charge Transfer in a WS2/(PEA)2PbI4 Heterostructure.

van der Waals heterostructures are currently the focus of intense investigation; this is essentially due to the unprecedented flexibility offered by the total relaxation of lattice matching requirements and their new and exotic properties compared to the individual layers. Here, we investigate the hybrid transition-metal dichalcogenide/2D perovskite heterostructure WS2/(PEA)2PbI4 (where PEA stands for phenylethylammonium). We present the first density functional theory (DFT) calculations of a heterostructure ensemble, which reveal a novel band alignment, where direct electron transfer is blocked by the organic spacer of the 2D perovskite. In contrast, the valence band forms a cascade from WS2 through the PEA to the PbI4 layer allowing hole transfer. These predictions are supported by optical spectroscopy studies, which provide compelling evidence for both charge transfer and nonradiative transfer of the excitation (energy transfer) between the layers. Our results show that TMD/2D perovskite (where TMD stands for transition-metal dichalcogenides) heterostructures provide a flexible and convenient way to engineer the band alignment.

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.

Detection of the Faraday Chiral Anisotropy.

The connection between chirality and electromagnetism has attracted much attention through the recent history of science, allowing the discovery of crucial nonreciprocal optical phenomena within the context of fundamental interactions between matter and light. A major phenomenon within this family is the so-called Faraday chiral anisotropy, the long-predicted but yet unobserved effect which arises due to the correlated coaction of both natural and magnetically induced optical activities at concurring wavelengths in chiral systems. Here, we report on the detection of the elusive anisotropic Faraday chiral phenomenon and demonstrate its enantioselectivity. The existence of this fundamental effect reveals the accomplishment of envisioned nonreciprocal electromagnetic metamaterials referred to as Faraday chiral media, systems where novel electromagnetic phenomena such as negative refraction of light at tunable wavelengths or even negative reflection can be realized. From a more comprehensive perspective, our findings have profound implications for the general understanding of parity-violating photon-particle interactions in magnetized media.

Tuning the Excitonic Properties of the 2D (PEA)2(MA)n-1PbnI3n+1 Perovskite Family via Quantum Confinement.

In atomically thin two-dimensional (2D) crystals, the excitonic properties and band structure scale strongly with the thickness, providing a new playground for the investigation of exciton physics in the ultimate confinement regime. Here, we demonstrate the evolution of the fundamental excitonic properties, such as reduced mass, wave function extension, and exciton binding energy, in the 2D perovskite (PEA)2(MA)n-1PbnI3n+1, for n = 1, 2, 3. These parameters are experimentally determined using optical spectroscopy in a high magnetic field up to 65 T. The observation of the interband Landau level transitions provides direct access to the reduced effective mass µ and band gap Eg. We show that µ increases with the number of inorganic sheets n, reaching the value of three-dimensional (3D) MAPbI3 already for n = 3. Our experimental observations contradict the general expectation that quantum confinement leads to an enhanced carrier mass, showing another aspect of the unprecedented flexibility in the design of the electronic properties of 2D perovskites.

Microscopic Picture of Electron-Phonon Interaction in Two-Dimensional Halide Perovskites.

Perovskites have attracted much attention due to their remarkable optical properties. While it is well established that excitons dominate their optical response, the impact of higher excitonic states and formation of phonon sidebands in optical spectra still need to be better understood. Here, we perform a theoretical study of excitonic properties of monolayered hybrid organic perovskites-supported by temperature-dependent photoluminescence measurements. Solving the Wannier equation, we obtain microscopic access to the Rydberg-like series of excitonic states including their wave functions and binding energies. Exploiting the generalized Elliot formula, we calculate the photoluminescence spectra demonstrating a pronounced contribution of a phonon sideband for temperatures up to 50 K, in agreement with experimental measurements. Finally, we predict temperature-dependent line widths of the three energetically lowest excitonic transitions and identify the underlying phonon-driven scattering processes.

Excitation efficiency determines the upconversion luminescence intensity of ß-NaYF4:Er3+,Yb3+ nanoparticles in magnetic fields up to 70 T.

Lanthanide-doped nanoparticles enable conversion of near-infrared photons to visible ones. This property is envisioned as a basis of a broad range of applications: from optoelectronics, via energy conversion, to bio-sensing and phototherapy. The spectrum of applications can be extended if magnetooptical properties of lanthanide dopants are well understood. However, at present, there are many conflicting reports on the influence of the magnetic field on the upconverted luminescence. In this work, we resolve this discrepancy by performing a comprehensive study of ß-NaYF4:Er3+,Yb3+ nanoparticles. Crucially, we show that the magnetic field impacts the luminescence only via a Zeeman-driven detuning between the excitation laser and the absorption transition. On the other hand, the energy transfer and multiphonon relaxation rates are unaffected. We propose a phenomenological model, which qualitatively reproduces the experimental results. The presented results are expected to lead to design of novel, dual-mode opto-magnetic upconverting nanomaterials.

Revealing Excitonic Phonon Coupling in (PEA)2(MA)n-1PbnI3n+1 2D Layered Perovskites.

The family of 2D Ruddlesden-Popper perovskites is currently attracting great interest of the scientific community as highly promising materials for energy harvesting and light emission applications. Despite the fact that these materials are known for decades, only recently has it become apparent that their optical properties are driven by the exciton-phonon coupling, which is controlled by the organic spacers. However, the detailed mechanism of this coupling, which gives rise to complex absorption and emission spectra, is the subject of ongoing controversy. In this work we show that the particularly rich, absorption spectra of (PEA)2(CH3NH3)n-1PbnI3n+1 (where PEA stands for phenylethylammonium and n = 1, 2, 3), are related to a vibronic progression of excitonic transition. In contrast to other two-dimensional perovskites, we observe a coupling to a high-energy (40 meV) phonon mode probably related to the torsional motion of the NH3+ head of the organic spacer.

Symmetry Breakdown in Franckeite: Spontaneous Strain, Rippling, and Interlayer Moiré.

Franckeite is a naturally occurring layered mineral with a structure composed of alternating stacks of SnS2-like and PbS-like layers. Although this superlattice is composed of a sequence of isotropic two-dimensional layers, it exhibits a spontaneous rippling that makes the material structurally anisotropic. We demonstrate that this rippling comes hand in hand with an inhomogeneous in-plane strain profile and anisotropic electrical, vibrational, and optical properties. We argue that this symmetry breakdown results from a spatial modulation of the van der Waals interaction between layers due to the SnS2-like and PbS-like lattices incommensurability.

Giant Fine Structure Splitting of the Bright Exciton in a Bulk MAPbBr3 Single Crystal.

Exciton fine structure splitting in semiconductors reflects the underlying symmetry of the crystal and quantum confinement. Because the latter factor strongly enhances the exchange interaction, most work has focused on nanostructures. Here, we report on the first observation of the bright exciton fine structure splitting in a bulk semiconductor crystal, where the impact of quantum confinement can be specifically excluded, giving access to the intrinsic properties of the material. Detailed investigation of the exciton photoluminescence and reflection spectra of a bulk methylammonium lead tribromide single crystal reveals a zero magnetic field splitting as large as ∼200 µeV. This result provides an important starting point for the discussion of the origin of the large bright exciton fine structure splitting observed in perovskite nanocrystals.

Determining Interaction Enhanced Valley Susceptibility in Spin-Valley-Locked MoS2.

Two-dimensional transition metal dichalcogenides (TMDCs) are recently emerged electronic systems with various novel properties, such as spin-valley locking, circular dichroism, valley Hall effect, and superconductivity. The reduced dimensionality and large effective masses further produce unconventional many-body interaction effects. Here we reveal strong interaction effects in the conduction band of MoS2 by transport experiment. We study the massive Dirac electron Landau levels (LL) in high-quality MoS2 samples with field-effect mobilities of 24 000 cm2/(V·s) at 1.2 K. We identify the valley-resolved LLs and low-lying polarized LLs using the Lifshitz-Kosevitch formula. By further tracing the LL crossings in the Landau fan diagram, we unambiguously determine the density-dependent valley susceptibility and the interaction enhanced g-factor from 12.7 to 23.6. Near integer ratios of Zeeman-to-cyclotron energies, we discover LL anticrossings due to the formation of quantum Hall Ising ferromagnets, the valley polarizations of which appear to be reversible by tuning the density or an in-plane magnetic field. Our results provide evidence for many-body interaction effects in the conduction band of MoS2 and establish a fertile ground for exploring strongly correlated phenomena of massive Dirac electrons.