Inhomogeneous Broadening of Photoluminescence Spectra and Kinetics of Nanometer-Thick (Phenethylammonium)2PbI4 Perovskite Thin Films: Implications for Optoelectronics

An outstanding potentiality of layered two-dimensional (2D) organic-inorganic hybrid perovskites (2DHPs) is in the development of solar cells, photodetectors, and light-emitting diodes. In 2DHPs, an exciton is localized in an atomically thin lead(II) halide inorganic layer of sub-nanometer thickness as in a quantum well sandwiched between organic layers as energetic and dielectric barriers. In previous years, versatile optical characterization of 2DHPs has been carried out mainly for thin flakes of single crystals and ultrathin (of the order of 20 nm) polycrystalline films, whereas there is a lack of optical characterization of thick (hundreds of nanometers) polycrystalline films, fundamentals for fabrication of devices. Here, with the use of photoluminescence (PL) and absorption spectroscopies, we studied the exciton behavior in ∼200 nm polycrystalline thin films of 2D perovskite (PEA)₂PbI₄, where PEA is phenethylammonium. Contrary to the case of ultrathin films, we have found that peak energies and line width of the excitonic bands in our films demonstrate unusual extremely weak sensitivity to temperature in 20-300 K diapason. The excitonic PL band is characterized by a significant (∼30 meV) Stokes shift with respect to the corresponding absorption band as well as by a full absence of the exciton fine structure at cryogenic temperatures. We suggest that the observed effects are due to the large inhomogeneous broadening of the excitonic PL and absorption bands resulting from the (PEA)₂PbI₄ band gap energy dependence on the number of lead(II) halide layers of individual crystallites. The characteristic time of the exciton energy funneling from higher- to lower-energy crystallites within (PEA)₂PbI₄ polycrystalline thin films is about 100 ps.

Interplay between spin proximity effect and charge-dependent exciton dynamics in MoSe2/CrBr3 van der Waals heterostructures

Semiconducting ferromagnet-nonmagnet interfaces in van der Waals heterostructures present a unique opportunity to investigate magnetic proximity interactions dependent upon a multitude of phenomena including valley and layer pseudospins, moiré periodicity, or exceptionally strong Coulomb binding. Here, we report a charge-state dependency of the magnetic proximity effects between MoSe2 and CrBr3 in photoluminescence, whereby the valley polarization of the MoSe2 trion state conforms closely to the local CrBr3 magnetization, while the neutral exciton state remains insensitive to the ferromagnet. We attribute this to spin-dependent interlayer charge transfer occurring on timescales between the exciton and trion radiative lifetimes. Going further, we uncover by both the magneto-optical Kerr effect and photoluminescence a domain-like spatial topography of contrasting valley polarization, which we infer to be labyrinthine or otherwise highly intricate, with features smaller than 400 nm corresponding to our optical resolution. Our findings offer a unique insight into the interplay between short-lived valley excitons and spin-dependent interlayer tunneling, while also highlighting MoSe2 as a promising candidate to optically interface with exotic spin textures in van der Waals structures.

Spin-layer locking of interlayer excitons trapped in moiré potentials

Van der Waals heterostructures offer attractive opportunities to design quantum materials. For instance, transition metal dichalcogenides (TMDs) possess three quantum degrees of freedom: spin, valley index and layer index. Furthermore, twisted TMD heterobilayers can form moiré patterns that modulate the electronic band structure according to the atomic registry, leading to spatial confinement of interlayer excitons (IXs). Here we report the observation of spin-layer locking of IXs trapped in moiré potentials formed in a heterostructure of bilayer 2H-MoSe₂ and monolayer WSe₂. The phenomenon of locked electron spin and layer index leads to two quantum-confined IX species with distinct spin-layer-valley configurations. Furthermore, we observe that the atomic registries of the moiré trapping sites in the three layers are intrinsically locked together due to the 2H-type stacking characteristic of bilayer TMDs. These results identify the layer index as a useful degree of freedom to engineer tunable few-level quantum systems in two-dimensional heterostructures.

Strongly Coupled Coherent Phonons in Single-Layer MoS2

We present a transient absorption setup combining broadband detection over the visible-UV range with high temporal resolution (∼20 fs) which is ideally suited to trigger and detect vibrational coherences in different classes of materials. We generate and detect coherent phonons (CPs) in single-layer (1L)-MoS₂, as a representative semiconducting 1L-transition metal dichalcogenide (TMD), where the confined dynamical interaction between excitons and phonons is unexplored. The coherent oscillatory motion of the out-of-plane A'₁ phonons, triggered by the ultrashort laser pulses, dynamically modulates the excitonic resonances on a time scale of few tens of fs. We observe an enhancement by almost 2 orders of magnitude of the CP amplitude when detected in resonance with the C exciton peak, combined with a resonant enhancement of CP generation efficiency. Ab initio calculations of the change in the 1L-MoS₂ band structure induced by the A'₁ phonon displacement confirm a strong coupling with the C exciton. The resonant behavior of the CP amplitude follows the same spectral profile of the calculated Raman susceptibility tensor. These results explain the CP generation process in 1L-TMDs and demonstrate that CP excitation in 1L-MoS₂ can be described as a Raman-like scattering process.

Out-of-plane orientation of luminescent excitons in two-dimensional indium selenide

Van der Waals materials offer a wide range of atomic layers with unique properties that can be easily combined to engineer novel electronic and photonic devices. A missing ingredient of the van der Waals platform is a two-dimensional crystal with naturally occurring out-of-plane luminescent dipole orientation. Here we measure the far-field photoluminescence intensity distribution of bulk InSe and two-dimensional InSe, WSe2 and MoSe2. We demonstrate, with the support of ab-initio calculations, that layered InSe flakes sustain luminescent excitons with an intrinsic out-of-plane orientation, in contrast with the in-plane orientation of dipoles we find in two-dimensional WSe2 and MoSe2 at room-temperature. These results, combined with the high tunability of the optical response and outstanding transport properties, position layered InSe as a promising semiconductor for novel optoelectronic devices, in particular for hybrid integrated photonic chips which exploit the out-of-plane dipole orientation.

Many-body perturbation theory calculations using the yambo code

yambo is an open source project aimed at studying excited state properties of condensed matter systems from first principles using many-body methods. As input, yambo requires ground state electronic structure data as computed by density functional theory codes such as Quantum ESPRESSO and Abinit. yambo's capabilities include the calculation of linear response quantities (both independent-particle and including electron-hole interactions), quasi-particle corrections based on the GW formalism, optical absorption, and other spectroscopic quantities. Here we describe recent developments ranging from the inclusion of important but oft-neglected physical effects such as electron-phonon interactions to the implementation of a real-time propagation scheme for simulating linear and non-linear optical properties. Improvements to numerical algorithms and the user interface are outlined. Particular emphasis is given to the new and efficient parallel structure that makes it possible to exploit modern high performance computing architectures. Finally, we demonstrate the possibility to automate workflows by interfacing with the yambopy and AiiDA software tools.

Optical Contrast and Raman Spectroscopy Techniques Applied to Few-Layer 2D Hexagonal Boron Nitride

The successful integration of few-layer thick hexagonal boron nitride (hBN) into devices based on two-dimensional materials requires fast and non-destructive techniques to quantify their thickness. Optical contrast methods and Raman spectroscopy have been widely used to estimate the thickness of two-dimensional semiconductors and semi-metals. However, they have so far not been applied to two-dimensional insulators. In this work, we demonstrate the ability of optical contrast techniques to estimate the thickness of few-layer hBN on SiO2/Si substrates, which was also measured by atomic force microscopy. Optical contrast of hBN on SiO2/Si substrates exhibits a linear trend with the number of hBN monolayers in the few-layer thickness range. We also used bandpass filters (500-650 nm) to improve the effectiveness of the optical contrast methods for thickness estimations. We also investigated the thickness dependence of the high frequency in-plane E2g phonon mode of atomically thin hBN on SiO2/Si substrates by micro-Raman spectroscopy, which exhibits a weak thickness-dependence attributable to the in-plane vibration character of this mode. Ab initio calculations of the Raman active phonon modes of atomically thin free-standing crystals support these results, even if the substrate can reduce the frequency shift of the E2g phonon mode by reducing the hBN thickness. Therefore, the optical contrast method arises as the most suitable and fast technique to estimate the thickness of hBN nanosheets.

Exciton-Phonon Coupling in the Ultraviolet Absorption and Emission Spectra of Bulk Hexagonal Boron Nitride

We present an ab initio method to calculate phonon-assisted absorption and emission spectra in the presence of strong excitonic effects. We apply the method to bulk hexagonal BN, which has an indirect band gap and is known for its strong luminescence in the UV range. We first analyze the excitons at the wave vector q[over ¯] of the indirect gap. The coupling of these excitons with the various phonon modes at q[over ¯] is expressed in terms of a product of the mean square displacement of the atoms and the second derivative of the optical response function with respect to atomic displacement along the phonon eigenvectors. The derivatives are calculated numerically with a finite difference scheme in a supercell commensurate with q[over ¯]. We use detailed balance arguments to obtain the intensity ratio between emission and absorption processes. Our results explain recent luminescence experiments and reveal the exciton-phonon coupling channels responsible for the emission lines.

Intravalley Spin-Flip Relaxation Dynamics in Single-Layer WS2

In monolayer (1L) transition metal dichalcogenides (TMDs) the valence and conduction bands are spin-split because of the strong spin-orbit interaction. In tungsten-based TMDs the spin-ordering of the conduction band is such that the so-called dark excitons, consisting of electrons and holes with opposite spin orientation, have lower energy than A excitons. The transition from bright to dark excitons involves the scattering of electrons from the upper to the lower conduction band at the K point of the Brillouin zone, with detrimental effects for the optoelectronic response of 1L-TMDs, since this reduces their light emission efficiency. Here, we exploit the valley selective optical selection rules and use two-color helicity-resolved pump-probe spectroscopy to directly measure the intravalley spin-flip relaxation dynamics in 1L-WS₂. This occurs on a sub-ps time scale, and it is significantly dependent on temperature, indicative of phonon-assisted relaxation. Time-dependent ab initio calculations show that intravalley spin-flip scattering occurs on significantly longer time scales only at the K point, while the occupation of states away from the minimum of the conduction band significantly reduces the scattering time. Our results shed light on the scattering processes determining the light emission efficiency in optoelectronic and photonic devices based on 1L-TMDs.

Ab Initio Calculations of Ultrashort Carrier Dynamics in Two-Dimensional Materials: Valley Depolarization in Single-Layer WSe2

In single-layer WSe₂, a paradigmatic semiconducting transition metal dichalcogenide, a circularly polarized laser field can selectively excite electronic transitions in one of the inequivalent K^± valleys. Such selective valley population corresponds to a pseudospin polarization. This can be used as a degree of freedom in a "valleytronic" device provided that the time scale for its depolarization is sufficiently large. Yet, the mechanism behind the valley depolarization still remains heavily debated. Recent time-dependent Kerr experiments have provided an accurate way to visualize the valley dynamics by measuring the rotation of a linearly polarized probe pulse applied after a circularly polarized pump pulse. We present here a clear, accurate and parameter-free description of the valley dynamics. By using an atomistic, ab initio approach, we fully disclose the elemental mechanisms that dictate the depolarization effects. Our results are in excellent agreement with recent time-dependent Kerr experiments. We explain the Kerr dynamics and its temperature dependence in terms of electron-phonon-mediated processes that induce spin-flip intervalley transitions.

Fermi energy dependence of the optical emission in core/shell InAs nanowire homostructures

InAs nanowires grown by vapor-liquid-solid (VLS) method are investigated by photoluminescence. We observe that the Fermi energy of all samples is reduced by ∼20 meV when the size of the Au nanoparticle used for catalysis is increased from 5 to 20 nm. Additional capping with a thin InP shell enhances the optical emission and does not affect the Fermi energy. The unexpected behavior of the Fermi energy is attributed to the differences in the residual donor (likely carbon) incorporation in the axial (low) and lateral (high incorporation) growth in the VLS and vapor-solid (VS) methods, respectively. The different impurity incorporation rate in these two regions leads to a core/shell InAs homostructure. In this case, the minority carriers (holes) diffuse to the core due to the built-in electric field created by the radial impurity distribution. As a result, the optical emission is dominated by the core region rather than by the more heavily doped InAs shell. Thus, the photoluminescence spectra and the Fermi energy become sensitive to the core diameter. These results are corroborated by a theoretical model using a self-consistent method to calculate the radial carrier distribution and Fermi energy for distinct diameters of Au nanoparticles.

Quantum Interference Effects in Resonant Raman Spectroscopy of Single- and Triple-Layer MoTe2 from First-Principles

We present a combined experimental and theoretical study of resonant Raman spectroscopy in single- and triple-layer MoTe₂. Raman intensities are computed entirely from first-principles by calculating finite differences of the dielectric susceptibility. In our analysis, we investigate the role of quantum interference effects and the electron-phonon coupling. With this method, we explain the experimentally observed intensity inversion of the A₁^' vibrational modes in triple-layer MoTe₂ with increasing laser photon energy. Finally, we show that a quantitative comparison with experimental data requires the proper inclusion of excitonic effects.

Unified Description of the Optical Phonon Modes in N-Layer MoTe2

N-layer transition metal dichalcogenides provide a unique platform to investigate the evolution of the physical properties between the bulk (three-dimensional) and monolayer (quasi-two-dimensional) limits. Here, using high-resolution micro-Raman spectroscopy, we report a unified experimental description of the Γ-point optical phonons in N-layer 2H-molybdenum ditelluride (MoTe2). We observe series of N-dependent low-frequency interlayer shear and breathing modes (below 40 cm(-1), denoted LSM and LBM) and well-defined Davydov splittings of the mid-frequency modes (in the range 100-200 cm(-1), denoted iX and oX), which solely involve displacements of the chalcogen atoms. In contrast, the high-frequency modes (in the range 200-300 cm(-1), denoted iMX and oMX), arising from displacements of both the metal and chalcogen atoms, exhibit considerably reduced splittings. The manifold of phonon modes associated with the in-plane and out-of-plane displacements are quantitatively described by a force constant model, including interactions up to the second nearest neighbor and surface effects as fitting parameters. The splittings for the iX and oX modes observed in N-layer crystals are directly correlated to the corresponding bulk Davydov splittings between the E2u/E1g and B1u/A1g modes, respectively, and provide a measurement of the frequencies of the bulk silent E2u and B1u optical phonon modes. Our analysis could readily be generalized to other layered crystals.

Probing quantum confinement within single core-multishell nanowires

Theoretically core-multishell nanowires under a cross-section of hexagonal geometry should exhibit peculiar confinement effects. Using a hard X-ray nanobeam, here we show experimental evidence for carrier localization phenomena at the hexagon corners by combining synchrotron excited optical luminescence with simultaneous X-ray fluorescence spectroscopy. Applied to single coaxial n-GaN/InGaN multiquantum-well/p-GaN nanowires, our experiment narrows the gap between optical microscopy and high-resolution X-ray imaging and calls for further studies on the underlying mechanisms of optoelectronic nanodevices.

Anisotropic optical response of GaN and AlN nanowires

We present a theoretical study of the electronic structure and optical properties of free-standing GaN and AlN nanowires. We have implemented the empirical tight-binding method, with an orbital basis sp(3), that includes the spin-orbit interaction. The passivation of the dangling bonds at the free surfaces is also studied, together with the effects on the electronic structure of the nanowire. For both GaN and AlN nanowires, we have found a remarkable anisotropy of the optical absorption when the light-polarization changes, showing in the case of GaN a dependence on the nanowire size.

[Lactic acidosis in recovered asphyctic newborn infants]

Thirty newborn babies delivered by elective caesarean section were studied. They were randomly placed in two groups. The first group consisted of newborns who scored 3 or less on the Apgar scale at one minute and 8 or more after five minutes. The other group included newborns which scored 8 or higher at one minute and 9 at five minutes. Blood samples were obtained from the umbilical cord which allowed for the measuring of the infants' pH, serum gases and pyruvic lactic acid levels. An increased level lactate was found in asphyctic infants. Five cases presented lactic acidosis which continued to be persistent in four of them an hour after they were born. Although increased levels of lactate were found also in the control group, they never presented lactic acidosis. No correlation was found between the pH and lactate levels nor between lactate and bicarbonate. A clear correlation between lactic acidosis and fetal distress was documented. It was concluded that lactate quantification is a useful indicator of neonatal asphyxia. Lactic acidosis; Apgar scores; asphyxia.