Interplay of Phonon Directionality and Emission Polarization in Two-Dimensional Layered Metal Halide Perovskites.

ConspectusLayered metal halide perovskites represent a natural quantum well system for charge carriers that provides rich physics, and the organic encapsulation of the inorganic metal halide layers not only increases their stability in devices but also provides an immense freedom to design their functionality. Intriguingly, these organic moieties strongly impact the optical, electrical, and mechanical properties, not only through their dielectric, elastic, and chemical properties but also because of induced mechanical distortions in the inorganic lattice. This tunability makes two-dimensional layered perovskites (2DLPs) highly attractive as light emitters. Common consensus is that exciton-phonon coupling plays an important role in radiative recombination. For bulk and some two-dimensional (2D) materials, the band edge emission broadening can be described by the classic models for polar inorganic semiconductors, while for the temperature dependence of the self-trapped exciton emission, an analysis developed for color centers has been successfully applied. For many 2DLPs these approaches do not work because of the complexity of their vibrational spectra. However, their emission is still strongly determined by phonons, and therefore, an adequate understanding of the electron-phonon coupling needs to be developed.With polarized and angle-resolved Raman spectroscopy studies on single 2DLP flakes based on different ammonium molecules as organic cations, in 2020 we revealed very rich phonon spectra in the low-frequency regime. Although the phonon bands at low frequency can generally be attributed to the vibrations of the inorganic lattice, we found very different responses by only changing the type of organic cations. In addition, the intensity of the different phonon modes depended strongly on the angle of the linearly polarized excitation beam with respect to the in-plane axes of the octahedron lattice. In 2022, we mapped this angular dependence of the phonon modes, which allowed identification of the directionality of the different lattice vibrations. By correlating the phonon spectra with the temperature-dependent emission for a set of 2DLPs that featured very different self-trapped exciton (STE) emission, we demonstrated that the exciton relaxation cannot be related to coupling with a single (longitudinal-optical) phonon band and that several phonon bands should be involved in the emission process. To gain insights into the exciton-phonon coupling effects on the band edge emission, we performed both angle-resolved polarized emission and Raman spectroscopy on single 2D lead iodide perovskite microcrystals. These experiments revealed the impact of the organic cations on the linear polarization of the emission and corroborated that multiple phonon bands should be involved in the radiative recombination process. Analysis of the temperature-dependent line width broadening of the band edge emission showed that for many systems, the behavior cannot be described by assuming the involvement of only one phonon mode in the electron-phonon coupling process. Our studies revealed a wealth of highly directional low-frequency phonons in 2DLPs from which several bands are involved in the emission process, which leads to diverse optical and vibrational properties depending on the type of organic cation in the material.

Phonon Directionality Impacts Electron-Phonon Coupling and Polarization of the Band-Edge Emission in Two-Dimensional Metal Halide Perovskites.

Two-dimensional metal halide perovskites are highly versatile for light-driven applications due to their exceptional variety in material composition, which can be exploited for the tunability of mechanical and optoelectronic properties. The band-edge emission is defined by the structure and composition of both organic and inorganic layers, and electron-phonon coupling plays a crucial role in the recombination dynamics. However, the nature of the electron-phonon coupling and what kind of phonons are involved are still under debate. Here we investigate the emission, reflectance, and phonon response from single two-dimensional lead iodide microcrystals with angle-resolved polarized spectroscopy. We find an intricate dependence of the emission polarization with the vibrational directionality in the materials, which reveals that several bands of low-frequency phonons with nonorthogonal directionality contribute to the band-edge emission. Such complex electron-phonon coupling requires adequate models to predict the thermal broadening of the emission and provides opportunities to design polarization properties.

Observation of phonon Stark effect.

Stark effect, the electric-field analogue of magnetic Zeeman effect, is one of the celebrated phenomena in modern physics and appealing for emergent applications in electronics, optoelectronics, as well as quantum technologies. While in condensed matter it has prospered only for excitons, whether other collective excitations can display Stark effect remains elusive. Here, we report the observation of phonon Stark effect in a two-dimensional quantum system of bilayer 2H-MoS2. The longitudinal acoustic phonon red-shifts linearly with applied electric fields and can be tuned over ~1 THz, evidencing giant Stark effect of phonons. Together with many-body ab initio calculations, we uncover that the observed phonon Stark effect originates fundamentally from the strong coupling between phonons and interlayer excitons (IXs). In addition, IX-mediated electro-phonon intensity modulation up to ~1200% is discovered for infrared-active phonon A2u. Our results unveil the exotic phonon Stark effect and effective phonon engineering by IX-mediated mechanism, promising for a plethora of exciting many-body physics and potential technological innovations.

Size Dependent Specific Heat Capacity of PbSe Nanocrystals.

Specific heat capacity is one of the most fundamental thermodynamic properties of materials. In this work, we measured the specific heat capacity of PbSe nanocrystals with diameters ranging from 5 to 23 nm, and its value increases significantly from 0.2 to 0.6 J g-1 °C-1. We propose a mass assignment model to describe the specific heat capacity of nanocrystals, which divides it into four parts: electron, inner, surface, and ligand. By eliminating the contribution of ligand and electron specific heat capacity, the specific heat capacity of the inorganic core is linearly proportional to its surface-to-volume ratio, showing the size dependence. Based on this linear relationship, surface specific heat capacity accounts for 40-60% of the specific heat capacity of nanocrystals with size decreasing. It can be attributed to the uncoordinated surface atoms, which is evidenced by the appearance of extra surface phonons in Raman spectra and ab initio molecular dynamics (AIMD) simulations.

Identification and Structural Characterization of Twisted Atomically Thin Bilayer Materials by Deep Learning.

Two-dimensional materials are expected to play an important role in next-generation electronics and optoelectronic devices. Recently, twisted bilayer graphene and transition metal dichalcogenides have attracted significant attention due to their unique physical properties and potential applications. In this study, we describe the use of optical microscopy to collect the color space of chemical vapor deposition (CVD) of molybdenum disulfide (MoS2) and the application of a semantic segmentation convolutional neural network (CNN) to accurately and rapidly identify thicknesses of MoS2 flakes. A second CNN model is trained to provide precise predictions on the twist angle of CVD-grown bilayer flakes. This model harnessed a data set comprising over 10,000 synthetic images, encompassing geometries spanning from hexagonal to triangular shapes. Subsequent validation of the deep learning predictions on twist angles was executed through the second harmonic generation and Raman spectroscopy. Our results introduce a scalable methodology for automated inspection of twisted atomically thin CVD-grown bilayers.

Interlayer bond polarizability model for interlayer phonons in van der Waals heterostructures.

Raman scattering provides essential insights into phonons, electronic structures and electron-phonon coupling within solids through the intensity of Raman peaks, which cannot be easily quantified using the classical bond polarizability model. The interlayer bond polarizability model (IBPM) had been developed to understand the Raman intensity of layer-breathing modes (LBMs) in two-dimensional materials. However, the quantitative understanding of the LBM intensity of a van der Waals heterostructure (vdWH) remains challenging. Here, in polynary vdWHs comprising twisted multilayer graphene (tMLG), MoS2 and hBN, we observed a series of LBMs, whose intensity is markedly dependent on the excitation energy and twist angle of the tMLG constituent. An improved IBPM is proposed to quantitatively understand the Raman intensity of LBMs in the tMLG-based vdWHs, including the emergence or absence of a specific LBM when the excitation energy is resonant with the electronic states of tMLG or MoS2 constituents. This work underscores the significant potential of the improved IBPM in accurately understanding and predicting the intensity profile of LBM in polynary vdWHs, even for the case of Raman scattering with excitation energies selectively resonant with the electronic states of the corresponding specific constituents.

Berry Curvature Dipole Induced Giant Mid-Infrared Second-Harmonic Generation in 2D Weyl Semiconductor.

Due to its inversion-broken triple helix structure and the nature of Weyl semiconductor, 2D Tellurene (2D Te) is promising to possess a strong nonlinear optical response in the infrared region, which is rarely reported in 2D materials. Here, a giant nonlinear infrared response induced by large Berry curvature dipole (BCD) is demonstrated in the Weyl semiconductor 2D Te. Ultrahigh second-harmonic generation response is acquired from 2D Te with a large second-order nonlinear optical susceptibility (χ(2) ), which is up to 23.3 times higher than that of monolayer MoS2 in the range of 700-1500 nm. Notably, distinct from other 2D nonlinear semiconductors, χ(2) of 2D Te increases extraordinarily with increasing wavelength and reaches up to 5.58 nm V-1 at ≈2300 nm, which is the best infrared performance among the reported 2D nonlinear materials. Large χ(2) of 2D Te also enables the high-intensity sum-frequency generation with an ultralow continuous-wave (CW) pump power. Theoretical calculations reveal that the exceptional performance is attributed to the presence of large BCD located at the Weyl points of 2D Te. These results unravel a new linkage between Weyl semiconductor and strong optical nonlinear responses, rendering 2D Te a competitive candidate for highly efficient nonlinear 2D semiconductors in the infrared region.

Impact of the organic cation on the band-edge emission of two-dimensional lead-bromide perovskites.

Organic-inorganic low-dimensional layered metal-halide perovskites are semiconductors in which the optoelectronic properties can be tuned by the material composition and the design of the layered architecture. While the electronic band structure is mainly determined by the inorganic octahedra lattice, the binding and conformation of the organic cations induces related lattice distortions that can break the symmetry and lead to the splitting of the exciton energy levels, and influence the dielectric confinement. Furthermore, organic-induced lattice deformations lead to offsets in k-space (where k is the wavevector) that go along with the exciton energy level splitting. Hence, the electronic transitions between these levels require the momentum contribution of phonons, and contributions of phonons in the exciton recombination dynamics result in thermal broadening of the emission linewidth. In this work, we investigate the band-edge emission of two-dimensional Ruddlesden-Popper lead-bromide perovskites synthesized with different organic cations that vary in their binding head group and their alkyl chain length. We find several peaks in the low-temperature photoluminescence spectra, and the number of peaks in the band-edge emission and their decay dynamics depend strongly on the type of organic cation in the material, which we relate to the difference in the inorganic lattice distortions that the cations induce. For two-dimensional layered perovskites with mainly in-plane distortions, induced by short primary ammonium molecules, we find a two-fold splitting of the band edge emission at low temperatures. If also out-of-plane distortions are present, as for the long-chain primary ammoniums, a three-fold splitting is observed. Interestingly, the low-energy peaks of the split series merge into the highest energy peak with increasing temperature. Thermal broadening analysis of the temperature-dependent photoluminescence linewidth in the structures with out-of-plane distortions yields energies that are larger than those reported for the inorganic lattice phonons. This indicates the involvement of either high-frequency oscillations involving the organic cations, or the broadening might be related to higher order phonon scattering processes in the excitonic recombination process. The strong directionality of the phonon modes in the octahedral lattice could promote the involvement of multiple electron-phonon scattering processes in the exciton relaxation dynamics, for example involving modes with orthogonal directionality.

Diagnostic Efficacy of Carotid Ultrasound for Predicting the Risk of Perioperative Hypotension or Fluid Responsiveness: A Meta-Analysis.

Despite the acceptance of carotid ultrasound for predicting patients' fluid responsiveness in critical care and anesthesia, its efficacy for predicting hypotension and fluid responsiveness remains unclear in the perioperative setting. Electronic databases were searched from inception to May 2023 to identify observational studies focusing on the use of corrected blood flow time (FTc) and respirophasic variation in carotid artery blood flow peak velocity (ΔVpeak) for assessing the risks of hypotension and fluid responsiveness. Using FTc as a predictive tool (four studies), the analysis yielded a pooled sensitivity of 0.82 (95% confidence interval (CI): 0.72 to 0.89) and specificity of 0.94 (95% CI: 0.88 to 0.97) for the risk of hypotension (area under curve (AUC): 0.95). For fluid responsiveness, the sensitivity and specificity of FTc were 0.79 (95% CI: 0.72 to 0.84) and 0.81 (95% CI: 0.75 to 0.86), respectively (AUC: 0.87). In contrast, the use of ΔVpeak to predict the risk of fluid responsiveness showed a pooled sensitivity of 0.76 (95% CI: 0.63 to 0.85) and specificity of 0.74 (95% CI: 0.66 to 0.8) (AUC: 0.79). The current meta-analysis provides robust evidence supporting the high diagnostic accuracy of FTc in predicting perioperative hypotension and fluid responsiveness, which requires further studies for verification.

Enhanced Layer-Breathing Modes in van der Waals Heterostructures Based on Twisted Bilayer Graphene.

The characterization of interlayer coupling in two-dimensional van der Waals heterostructures (vdWHs) is essential to understand their quantum behaviors and structural functionalities. Interlayer shear and layer-breathing (LB) phonons carry rich information on interlayer interaction, but they are usually too weak to be detected via standard Raman spectroscopy due to the weak electron-phonon coupling (EPC). Here, we report a universal strategy to enhance LB modes of vdWHs based on twisted bilayer graphene (tBLG). In both tBLG/hBN and tBLG/MoS2 vdWHs, the resonantly excited electrons in tBLG can strongly couple to LB phonons extended over the entire layers in the vdWHs, whose resonance condition is tunable by the twist angle of tBLG. In vdWHs containing twisted graphene layers with multiple twisted interfaces, the EPC of LB phonons coming from the collective LB vibrations of entire heterostructure layers can be tuned by resonant excitation of programmable van Hove singularities according to each twisted interface. The universality and tunability of enhanced LB phonons by tBLG make it a promising method to investigate EPC and interlayer interaction in related vdWHs.

Abnormal Out-of-Plane Vibrational Raman Mode in Electrochemically Intercalated Multilayer MoS2.

Raman spectroscopy is a powerful technique to probe structural and doping behaviors of two-dimensional (2D) materials. In MoS2, the always coexisting in-plane (E2g1) and out-of-plane (A1g) vibrational modes are used as reliable fingerprints to distinguish the number of layers, strains, and doping levels. In this work, however, we report an abnormal Raman behavior, i.e., the absence of the A1g mode in cetyltrimethylammonium bromide (CTAB)-intercalated MoS2 superlattice. This unusual behavior is quite different from the softening of the A1g mode induced by surface engineering or electric-field gating. Interestingly, under a strong laser illumination, heating, or mechanical indentation, an A1g peak gradually appears, accompanied by the migration of intercalated CTA+ cations. The abnormal Raman behavior is mainly attributed to the constraint of the out-of-plane vibration due to intercalations and resulting severe electron doping. Our work renews the understanding of Raman spectra of 2D semiconducting materials and sheds light on developing next-generation devices with tunable structures.

Control of Raman Scattering Quantum Interference Pathways in Graphene.

Graphene is an ideal platform to study the coherence of quantum interference pathways by tuning doping or laser excitation energy. The latter produces a Raman excitation profile that provides direct insight into the lifetimes of intermediate electronic excitations and, therefore, on quantum interference, which has so far remained elusive. Here, we control the Raman scattering pathways by tuning the laser excitation energy in graphene doped up to 1.05 eV. The Raman excitation profile of the G mode indicates its position and full width at half-maximum are linearly dependent on doping. Doping-enhanced electron-electron interactions dominate the lifetimes of Raman scattering pathways and reduce Raman interference. This will provide guidance for engineering quantum pathways for doped graphene, nanotubes, and topological insulators.

Quantum interference between dark-excitons and zone-edged acoustic phonons in few-layer WS2.

Fano resonance which describes a quantum interference between continuum and discrete states, provides a unique method for studying strongly interacting physics. Here, we report a Fano resonance between dark excitons and zone-edged acoustic phonons in few-layer WS2 by using the resonant Raman technique. The discrete phonons with large momentum at the M-point of the Brillouin zone and the continuum dark exciton states related to the optically forbidden transition at K and Q valleys are coupled by the exciton-phonon interactions. We observe rich Fano resonance behaviors across layers and modes defined by an asymmetry-parameter q: including constructive interference with two mirrored asymmetry Fano peaks (weak coupling, q > 1 and q < - 1), and destructive interference with Fano dip (strong coupling, ∣q∣ < < 1). Our results provide new insight into the exciton-phonon quantum interference in two-dimensional semiconductors, where such interferences play a key role in their transport, optical, and thermodynamic properties.

Long-range ordered porous carbons produced from C60.

Carbon structures with covalent bonds connecting C60 molecules have been reported1-3, but their production methods typically result in very small amounts of sample, which restrict the detailed characterization and exploration necessary for potential applications. We report the gram-scale preparation of a new type of carbon, long-range ordered porous carbon (LOPC), from C60 powder catalysed by α-Li3N at ambient pressure. LOPC consists of connected broken C60 cages that maintain long-range periodicity, and has been characterized by X-ray diffraction, Raman spectroscopy, magic-angle spinning solid-state nuclear magnetic resonance spectroscopy, aberration-corrected transmission electron microscopy and neutron scattering. Numerical simulations based on a neural network show that LOPC is a metastable structure produced during the transformation from fullerene-type to graphene-type carbons. At a lower temperature, shorter annealing time or by using less α-Li3N, a well-known polymerized C60 crystal forms owing to the electron transfer from α-Li3N to C60. The carbon K-edge near-edge X-ray absorption fine structure shows a higher degree of delocalization of electrons in LOPC than in C60(s). The electrical conductivity is 1.17 × 10-2 S cm-1 at room temperature, and conduction at T < 30 K appears to result from a combination of metallic-like transport over short distances punctuated by carrier hopping. The preparation of LOPC enables the discovery of other crystalline carbons starting from C60(s).

Emerging Roles of Type-I Interferons in Neuroinflammation, Neurological Diseases, and Long-Haul COVID.

Interferons (IFNs) are pleiotropic cytokines originally identified for their antiviral activity. IFN-α and IFN-ß are both type I IFNs that have been used to treat neurological diseases such as multiple sclerosis. Microglia, astrocytes, as well as neurons in the central and peripheral nervous systems, including spinal cord neurons and dorsal root ganglion neurons, express type I IFN receptors (IFNARs). Type I IFNs play an active role in regulating cognition, aging, depression, and neurodegenerative diseases. Notably, by suppressing neuronal activity and synaptic transmission, IFN-α and IFN-ß produced potent analgesia. In this article, we discuss the role of type I IFNs in cognition, neurodegenerative diseases, and pain with a focus on neuroinflammation and neuro-glial interactions and their effects on cognition, neurodegenerative diseases, and pain. The role of type I IFNs in long-haul COVID-associated neurological disorders is also discussed. Insights into type I IFN signaling in neurons and non-neuronal cells will improve our treatments of neurological disorders in various disease conditions.

Polymer-like Inorganic Double Helical van der Waals Semiconductor.

In high-performance flexible and stretchable electronic devices, conventional inorganic semiconductors made of rigid and brittle materials typically need to be configured into geometrically deformable formats and integrated with elastomeric substrates, which leads to challenges in scaling down device dimensions and complexities in device fabrication and integration. Here we report the extraordinary mechanical properties of the newly discovered inorganic double helical semiconductor tin indium phosphate. This spiral-shape double helical crystal shows the lowest Young's modulus (13.6 GPa) among all known stable inorganic materials. The large elastic (>27%) and plastic (>60%) bending strains are also observed and attributed to the easy slippage between neighboring double helices that are coupled through van der Waals interactions, leading to the high flexibility and deformability among known semiconducting materials. The results advance the fundamental understanding of the unique polymer-like mechanical properties and lay the foundation for their potential applications in flexible electronics and nanomechanics disciplines.

Double-Cavity Modulation of Exciton Polaritons in CsPbBr3 Microwire.

The lead halide perovskite has become a promising candidate for the study of exciton polaritons due to their excellent optical properties. Here, both experimental and simulated results confirm the existence of two kinds of Fabry-Pérot microcavities in a single CsPbBr3 microwire with an isosceles right triangle cross section, and we experimentally demonstrate that confined photons in a straight and a folded Fabry-Pérot microcavity are strongly coupled with excitons to form exciton polaritons. Furthermore, we reveal the polarization characteristic and double-cavity modulation of exciton polaritons emission by polarization-resolved fluorescence spectroscopy. Our results not only prove that the modulation of exciton polaritons emission can occur in this simple double-cavity system but also provide a possibility to develop related polariton devices.

Two-Dimensional Wedge-Shaped Magnetic EuS: Insight into the Substrate Step-Guided Epitaxial Synthesis on Sapphire.

Rare earth chalcogenides (RECs) with novel luminescence and magnetic properties offer fascinating opportunities for fundamental research and applications. However, controllable synthesis of RECs down to the two-dimensional (2D) limit still has a great challenge. Herein, 2D wedge-shaped ferromagnetic EuS single crystals are successfully synthesized via a facile molten-salt-assisted chemical vapor deposition method on sapphire. Based on the theoretical simulations and experimental measurements, the mechanisms of aligned growth and wedge-shaped growth are systematically proposed. The wedge-shaped growth is driven by a dual-interaction mechanism, where the coupling between EuS and the substrate steps impedes the lateral growth, and the strong bonding of nonlayered EuS itself facilitates the vertical growth. Through temperature-dependent Raman and photoluminescence characterization, the nanoflakes show a large Raman temperature coefficient of -0.030 cm-1 K-1 and uncommon increasing band gap with temperature. More importantly, by low-temperature magnetic force microscopy characterization, thickness variation of the magnetic signal is revealed within one sample, indicating the great potential of the wedge-shaped nanoflake to serve as a platform for highly efficient investigation of thickness-dependent magnetic properties. This work sheds new light on 2D RECs and will offer a deep understanding of 2D wedge-shaped materials.