Polaronic Mass Enhancement and Polaronic Excitons in Metal Halide Perovskites.

In metal halide perovskites, the complex dielectric screening together with low energy of phonon modes leads to non-negligible Fröhlich coupling. While this feature of perovskites has already been used to explain some of the puzzling aspects of carrier transport in these materials, the possible impact of polaronic effects on the optical response, especially excitonic properties, is much less explored. Here, with the use of magneto-optical spectroscopy, we revealed the non-hydrogenic character of the excitons in metal halide perovskites, resulting from the pronounced Fröhlich coupling.

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.

Thickness-Dependent Dark-Bright Exciton Splitting and Phonon Bottleneck in CsPbBr-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 CsPbBr-based nanoplatelets for the first time.

Interlayer excitons in MoSe/2D perovskite hybrid heterostructures - the interplay between charge and energy transfer.

van der Waals crystals have opened a new and exciting chapter in heterostructure research, removing the lattice matching constraint characteristics of epitaxial semiconductors. They provide unprecedented flexibility for heterostructure design. Combining two-dimensional (2D) perovskites with other 2D materials, in particular transition metal dichalcogenides (TMDs), has recently emerged as an intriguing way to design hybrid opto-electronic devices.

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.

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.

Vibrational Properties in Highly Strained Hexagonal Boron Nitride Bubbles.

Hexagonal boron nitride (hBN) is widely used as a protective layer for few-atom-thick crystals and heterostructures (HSs), and it hosts quantum emitters working up to room temperature. In both instances, strain is expected to play an important role, either as an unavoidable presence in the HS fabrication or as a tool to tune the quantum emitter electronic properties. Addressing the role of strain and exploiting its tuning potentiality require the development of efficient methods to control it and of reliable tools to quantify it.

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 SnS-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.

Moiré Intralayer Excitons in a MoSe/MoS Heterostructure.

Spatially periodic structures with a long-range period, referred to as a moiré pattern, can be obtained in van der Waals bilayers in the presence of a small stacking angle or of lattice mismatch between the monolayers. Theoretical predictions suggest that the resulting spatially periodic variation of the band structure modifies the optical properties of both intra- and interlayer excitons of transition metal dichalcogenide heterostructures. Here, we report on the impact of the moiré pattern formed in a MoSe/MoS heterobilayer encapsulated in hexagonal boron nitride.

Intervalley Scattering of Interlayer Excitons in a MoS/MoSe/MoS Heterostructure in High Magnetic Field.

Degenerate extrema in the energy dispersion of charge carriers in solids, also referred to as valleys, can be regarded as a binary quantum degree of freedom, which can potentially be used to implement valleytronic concepts in van der Waals heterostructures based on transition metal dichalcogenides. Using magneto-photoluminescence spectroscopy, we achieve a deeper insight into the valley polarization and depolarization mechanisms of interlayer excitons formed across a MoS/MoSe/MoS heterostructure. We account for the nontrivial behavior of the valley polarization as a function of the magnetic field by considering the interplay between exchange interaction and phonon-mediated intervalley scattering in a system consisting of Zeeman-split energy levels.

Site-selective measurement of coupled spin pairs in an organic semiconductor.

From organic electronics to biological systems, understanding the role of intermolecular interactions between spin pairs is a key challenge. Here we show how such pairs can be selectively addressed with combined spin and optical sensitivity. We demonstrate this for bound pairs of spin-triplet excitations formed by singlet fission, with direct applicability across a wide range of synthetic and biological systems.

Observation of A Raman mode splitting in few layer black phosphorus encapsulated with hexagonal boron nitride.

We investigate the impact of encapsulation with hexagonal boron nitride (h-BN) on the Raman spectrum of few layer black phosphorus. The encapsulation results in a significant reduction of the line width of the Raman modes of black phosphorus, due to a reduced phonon scattering rate. We observe a so far elusive peak in the Raman spectra ∼4 cm above the A mode in trilayer and thicker flakes, which had not been observed experimentally.

Probing the Interlayer Exciton Physics in a MoS/MoSe/MoS van der Waals Heterostructure.

Stacking atomic monolayers of semiconducting transition metal dichalcogenides (TMDs) has emerged as an effective way to engineer their properties. In principle, the staggered band alignment of TMD heterostructures should result in the formation of interlayer excitons with long lifetimes and robust valley polarization. However, these features have been observed simultaneously only in MoSe/WSe heterostructures.

Defect Healing and Charge Transfer-Mediated Valley Polarization in MoS/MoSe/MoS Trilayer van der Waals Heterostructures.

Monolayer transition metal dichalcogenides (TMDCs) grown by chemical vapor deposition (CVD) are plagued by a significantly lower optical quality compared to exfoliated TMDCs. In this work, we show that the optical quality of CVD-grown MoSe is completely recovered if the material is sandwiched in MoS/MoSe/MoS trilayer van der Waals heterostructures. We show by means of density functional theory that this remarkable and unexpected result is due to defect healing: S atoms of the more reactive MoS layers are donated to heal Se vacancy defects in the middle MoSe layer.

Highly Oriented Atomically Thin Ambipolar MoSe Grown by Molecular Beam Epitaxy.

Transition metal dichalcogenides (TMDCs), together with other two-dimensional (2D) materials, have attracted great interest due to the unique optical and electrical properties of atomically thin layers. In order to fulfill their potential, developing large-area growth and understanding the properties of TMDCs have become crucial. Here, we have used molecular beam epitaxy (MBE) to grow atomically thin MoSe on GaAs(111)B.

Revealing Large-Scale Homogeneity and Trace Impurity Sensitivity of GaAs Nanoscale Membranes.

III-V nanostructures have the potential to revolutionize optoelectronics and energy harvesting. For this to become a reality, critical issues such as reproducibility and sensitivity to defects should be resolved. By discussing the optical properties of molecular beam epitaxy (MBE) grown GaAs nanomembranes we highlight several features that bring them closer to large scale applications.

Unraveling the Exciton Binding Energy and the Dielectric Constant in Single-Crystal Methylammonium Lead Triiodide Perovskite.

We have accurately determined the exciton binding energy and reduced mass of single crystals of methylammonium lead triiodide using magneto-reflectivity at very high magnetic fields. The single crystal has excellent optical properties with a narrow line width of ∼3 meV for the excitonic transitions and a 2s transition that is clearly visible even at zero magnetic field. The exciton binding energy of 16 ± 2 meV in the low-temperature orthorhombic phase is almost identical to the value found in polycrystalline samples, crucially ruling out any possibility that the exciton binding energy depends on the grain size.

Spatially resolved studies of the phases and morphology of methylammonium and formamidinium lead tri-halide perovskites.

The family of organic-inorganic tri-halide perovskites including MA (MethylAmmonium)PbI, MAPbICl, FA (FormAmidinium)PbI and FAPbBr are having a tremendous impact on the field of photovoltaic cells due to the combination of their ease of deposition and high energy conversion efficiencies. Device performance, however, is known to be still significantly affected by the presence of inhomogeneities. Here we report on a study of temperature dependent micro-photoluminescence which shows a strong spatial inhomogeneity related to the presence of microcrystalline grains, which can be both bright and dark.

Integrated III-V Photonic Crystal--Si waveguide platform with tailored optomechanical coupling.

Optomechanical systems, in which the vibrations of a mechanical resonator are coupled to an electromagnetic radiation, have permitted the investigation of a wealth of novel physical effects. To fully exploit these phenomena in realistic circuits and to achieve different functionalities on a single chip, the integration of optomechanical resonators is mandatory. Here, we propose a novel approach to heterogeneously integrate arrays of two-dimensional photonic crystal defect cavities on top of silicon-on-insulator waveguides.

Ordered systems of site-controlled pyramidal quantum dots incorporated in photonic crystal cavities.

The coupling of a prescribed number of site-controlled pyramidal quantum dots (QDs) with photonic crystal (PhC) cavities was studied by polarization and power-dependent photoluminescence measurements. The energy of the cavity mode could be readily tuned, making use of the high spectral uniformity of the QDs and designing PhC cavities with different hole radii. Efficient coupling of the PhC cavity modes both to the ground state and to the excited state transitions of the QDs was observed, whereas no evidence for far off-resonant coupling was found.