Fine-Tuning of the Excitonic Response in Monolayer WS2 Domes via Coupled Pressure and Strain Variation.

We present a spectroscopic investigation of the vibrational and optoelectronic properties of WS2 domes in the 0-0.65 GPa range. The pressure evolution of the system morphology, deduced by the combined analysis of Raman and photoluminescence spectra, revealed a significant variation in the dome's aspect ratio. The modification of the dome shape caused major changes in the mechanical properties of the system resulting in a sizable increase of the out-of-plane compressive strain while keeping the in-plane tensile strain unchanged. The variation of the strain gradients drives a nonlinear behavior in both the exciton energy and radiative recombination intensity, interpreted as the consequence of a hybridization mechanism between the electronic states of two distinct minima in the conduction band. Our results indicate that pressure and strain can be efficiently combined in low dimensional systems with unconventional morphology to obtain modulations of the electronic band structure not achievable in planar crystals.

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. Here we present a technique based on hydrogen irradiation to induce the formation of wrinkles and bubbles in hBN, resulting in remarkably high strains of ∼2%. By combining infrared (IR) near-field scanning optical microscopy and micro-Raman measurements with numerical calculations, we characterize the response to strain for both IR-active and Raman-active modes, revealing the potential of the vibrational properties of hBN as highly sensitive strain probes.

Gap Opening in Double-Sided Highly Hydrogenated Free-Standing Graphene.

Conversion of free-standing graphene into pure graphane─where each C atom is sp3 bound to a hydrogen atom─has not been achieved so far, in spite of numerous experimental attempts. Here, we obtain an unprecedented level of hydrogenation (≈90% of sp3 bonds) by exposing fully free-standing nanoporous samples─constituted by a single to a few veils of smoothly rippled graphene─to atomic hydrogen in ultrahigh vacuum. Such a controlled hydrogenation of high-quality and high-specific-area samples converts the original conductive graphene into a wide gap semiconductor, with the valence band maximum (VBM) ∼ 3.5 eV below the Fermi level, as monitored by photoemission spectromicroscopy and confirmed by theoretical predictions. In fact, the calculated band structure unequivocally identifies the achievement of a stable, double-sided fully hydrogenated configuration, with gap opening and no trace of π states, in excellent agreement with the experimental results.

Hydrogen-Induced Conversion of SnS2 into SnS or Sn: A Route to Create SnS2 /SnS Heterostructures.

The family of van der Waals (vdW) materials is large and diverse with applications ranging from electronics and optoelectronics to catalysis and chemical storage. However, despite intensive research, there remains significant knowledge-gaps pertaining to their properties and interactions. One such gap is the interaction between these materials and hydrogen, a potentially vital future energy vector and ubiquitous processing gas in the semiconductor industry. This work reports on the interaction of hydrogen with the vdW semiconductor SnS2 , where molecular hydrogen (H2 ) and H-ions induce a controlled chemical conversion into semiconducting-SnS or to ß-Sn. This hydrogen-driven reaction is facilitated by the different oxidation states of Sn and is successfully applied to form SnS2 /SnS heterostructures with uniform layers, atomically flat interfaces and well-aligned crystallographic axes. This approach is scalable and offers a route for engineering materials at the nanoscale for semiconductor technologies based on the earth-abundant elements Sn and S, a promising result for a wide range of potential applications.

Strain-Induced Exciton Hybridization in WS_{2} Monolayers Unveiled by Zeeman-Splitting Measurements.

Mechanical deformations and ensuing strain are routinely exploited to tune the band gap energy and to enhance the functionalities of two-dimensional crystals. In this Letter, we show that strain leads also to a strong modification of the exciton magnetic moment in WS_{2} monolayers. Zeeman-splitting measurements under magnetic fields up to 28.5 T were performed on single, one-layer-thick WS_{2} microbubbles. The strain of the bubbles causes a hybridization of k-space direct and indirect excitons resulting in a sizable decrease in the modulus of the g factor of the ground-state exciton. These findings indicate that strain may have major effects on the way the valley number of excitons can be used to process binary information in two-dimensional crystals.

Experimental Adhesion Energy in van der Waals Crystals and Heterostructures from Atomically Thin Bubbles.

The formation of gas-filled bubbles on the surface of van der Waals crystals provides an ideal platform whereby the interplay of the elastic parameters and interlayer forces can be suitably investigated. Here, we combine experimental and numerical efforts to study the morphology of the bubbles at equilibrium and highlight unexpected behaviors that contrast with the common assumptions. We exploit such observations to develop an accurate analytical model to describe the shape and strain of the bubbles and exploit it to measure the adhesion energy between a variety of van der Waals crystals, showing sizable material-dependent trends.

Tailoring the optical properties of dilute nitride semiconductors at the nanometer scale.

We report on the innovative approaches we developed for the fabrication of site-controlled semiconductor nanostructures [e.g. quantum dots (QDs), nanowires], based on the spatially selective incorporation and/or removal of hydrogen in dilute nitride semiconductor alloys [e.g. Ga(AsN) and (InGa)(AsN)]. In such systems, the formation of stable nitrogen-hydrogen complexes removes the effects nitrogen has on the alloy properties, which in turn paves the way to the direct engineering of the material's electronic-and, thus, optical-properties: not only the bandgap energy, but also the refractive index and the polarization properties of the system can indeed be tailored with high precision and in a reversible manner. Here, lithographic approaches and/or plasmon-assisted optical irradiation-coupled to the ultra-sharp diffusion profile of hydrogen in dilute nitrides-are employed to control the hydrogen implantation and/or removal process at a nanometer scale. This results in a highly deterministic control of the spatial and spectral properties of the fabricated nanostructures, eventually obtaining semiconductor nanowires with controlled polarization properties, as well as site-controlled QDs with an extremely high control on their spatial and spectral properties. The nanostructures fabricated with these techniques, whose optical properties have also been simulated by finite-element-method calculations, are naturally suited for a deterministic coupling in optical nanocavities (i.e. photonic crystal cavities and circular Bragg resonators) and are therefore of potential interest for emerging quantum technologies.

Towards free-standing graphane: atomic hydrogen and deuterium bonding to nano-porous graphene.

Graphane is formed by bonding hydrogen (and deuterium) atoms to carbon atoms in the graphene mesh, with modification from the pure planar sp2 bonding towards an sp3 configuration. Atomic hydrogen (H) and deuterium (D) bonding with C atoms in fully free-standing nano porous graphene (NPG) is achieved, by exploiting low-energy proton (or deuteron) non-destructive irradiation, with unprecedented minimal introduction of defects, as determined by Raman spectroscopy and by the C 1s core level lineshape analysis. Evidence of the H- (or D-) NPG bond formation is obtained by bringing to light the emergence of a H- (or D-) related sp3-distorted component in the C 1s core level, clear fingerprint of H-C (or D-C) covalent bonding. The H (or D) bonding with the C atoms of free-standing graphene reaches more than 1/4 (or 1/3) at% coverage. This non-destructive H-NPG (or D-NPG) chemisorption is very stable at high temperatures up to about 800 K, as monitored by Raman and x-ray photoelectron spectroscopy, with complete healing and restoring of clean graphene above 920 K. The excellent chemical and temperature stability of H- (and D-) NPG opens the way not only towards the formation of semiconducting graphane on large-scale samples, but also to stable graphene functionalisation enabling futuristic applications in advanced detectors for the ß-spectrum analysis.

The Interaction of Hydrogen with the van der Waals Crystal γ-InSe.

The emergence of the hydrogen economy requires development in the storage, generation and sensing of hydrogen. The indium selenide ( γ -InSe) van der Waals (vdW) crystal shows promise for technologies in all three of these areas. For these applications to be realised, the fundamental interactions of InSe with hydrogen must be understood. Here, we present a comprehensive experimental and theoretical study on the interaction of γ -InSe with hydrogen. It is shown that hydrogenation of γ -InSe by a Kaufman ion source results in a marked quenching of the room temperature photoluminescence signal and a modification of the vibrational modes of γ -InSe, which are modelled by density functional theory simulations. Our experimental and theoretical studies indicate that hydrogen is incorporated into the crystal preferentially in its atomic form. This behaviour is qualitatively different from that observed in other vdW crystals, such as transition metal dichalcogenides, where molecular hydrogen is intercalated in the vdW gaps of the crystal, leading to the formation of "bubbles" for hydrogen storage.

Addressing the Fundamental Electronic Properties of Wurtzite GaAs Nanowires by High-Field Magneto-Photoluminescence Spectroscopy.

At ambient conditions, GaAs forms in the zincblende (ZB) phase with the notable exception of nanowires (NWs) where the wurtzite (WZ) lattice is also found. The WZ formation is both a complication to be dealt with and a potential feature to be exploited, for example, in NW homostructures wherein ZB and WZ phases alternate controllably and thus band gap engineering is achieved. Despite intense studies, some of the fundamental electronic properties of WZ GaAs NWs are not fully assessed yet. In this work, by using photoluminescence (PL) under high magnetic fields (B = 0-28 T), we measure the diamagnetic shift, ΔEd, and the Zeeman splitting of the band gap free exciton in WZ GaAs formed in core-shell InGaAs-GaAs NWs. The quantitative analysis of ΔEd at different temperatures (T = 4.2 and 77 K) and for different directions of B⃗ allows the determination of the exciton reduced mass, µexc, in planes perpendicular (µexc = 0.052 m0, where m0 is the electron mass in vacuum) and parallel (µexc = 0.057 m0) to the c axis of the WZ lattice. The value and anisotropy of the exciton reduced mass are compatible with the electron lowest-energy state having Γ7C instead of Γ8C symmetry. This finding answers a long discussed issue about the correct ordering of the conduction band states in WZ GaAs. As for the Zeeman splitting, it varies considerably with the field direction, resulting in an exciton gyromagnetic factor equal to 5.4 and ∼0 for B⃗//c and B⃗⊥c, respectively. This latter result provides fundamental insight into the band structure of wurtzite GaAs.

Laser Level Scheme of Self-Interstitials in Epitaxial Ge Dots Encapsulated in Si.

Recently, it was shown that lasing from epitaxial Ge quantum dots (QDs) on Si substrates can be obtained if they are partially amorphized by Ge ion bombardment (GIB). Here, we present a model for the microscopic origin of the radiative transitions leading to enhanced photoluminescence (PL) from such GIB-QDs. We provide an energy level scheme for GIB-QDs in a crystalline Si matrix that is based on atomistic modeling with Monte Carlo (MC) analysis and density functional theory (DFT). The level scheme is consistent with a broad variety of PL experiments performed on as-grown and annealed GIB-QDs. Our results show that an extended point defect consisting of a split-[110] self-interstitial surrounded by a distorted crystal lattice of about 45 atoms leads to electronic states at the Γ-point of the Brillouin zone well below the conduction band minimum of crystalline Ge. Such defects in Ge QDs allow direct transitions of electrons localized at the split-interstitial with holes confined in the Ge QD. We identify the relevant growth and annealing parameters that will let GIB-QDs be employed as an efficient laser active medium.

Bandgap Energy of Wurtzite InAs Nanowires.

InAs nanowires (NWs) have been grown on semi-insulating InAs (111)B substrates by metal-organic chemical vapor deposition catalyzed by 50, 100, and 150 nm-sized Au particles. The pure wurtzite (WZ) phase of these NWs has been attested by high-resolution transmission electron microscopy and selected area diffraction pattern measurements. Low temperature photoluminescence measurements have provided unambiguous and robust evidence of a well resolved, isolated peak at 0.477 eV, namely 59 meV higher than the band gap of ZB InAs. The WZ nature of this energy band has been demonstrated by high values of the polarization degree, measured in ensembles of NWs both as-grown and mechanically transferred onto Si and GaAs substrates, in agreement with the polarization selection rules for WZ crystals. The value of 0.477 eV found here for the bandgap energy of WZ InAs agrees well with theoretical calculations.

Body composition in late preterm infants according to percentile at birth.

BACKGROUND: The data on body composition of late preterm infants, evaluated according to percentile at birth, are scarce. The study aimed to investigate body composition of late preterm infants, according to percentile at birth, and to compare their body composition with that of term newborns. METHODS: A total of 122 (99 appropriate and 23 small for gestational age (SGA)) late preterm infants underwent growth and body composition assessment using an air displacement plethysmography system on the fifth day of life and at term. The reference group was composed of 42 healthy, term, breast-fed infants. RESULTS: At birth, appropriate and SGA late preterm infants had lower fat mass and fat-free mass indexes than term newborns. The fat mass and fat-free mass content increased significantly throughout the study, irrespective of percentile at birth. At term, fat mass index, but not fat-free mass index, was higher in both appropriate and SGA late preterm infants than in term newborns. CONCLUSION: Late preterm infants, irrespective of their percentile at birth, show postnatal growth characterized by predominant fat mass accretion. The potential long-term health clinical implications of these findings need to be further elucidated.

Polarized light absorption in wurtzite InP nanowire ensembles.

We investigate the absorption properties of ensembles of wurtzite (WZ) InP nanowires (NWs) by high-resolution polarization-resolved photoluminescence excitation (PLE) spectroscopy at T = 10 K. The degree of linear polarization of absorbed light, ρ(abs), resulting from the PLE spectra is governed by a competition between the dielectric mismatch effect and the WZ selection rules acting differently on different optical transitions. These two contributions are deconvoluted with the help of finite-difference time-domain simulations, thus providing information about the symmetry of the three highest valence bands (A, B, and C) of WZ InP and the extent of the spin-orbit interaction on these states. Moreover, ρ(abs) shows two characteristic dips corresponding to the two sharp A and B exciton resonances in the PLE spectra. A model developed for the dip in A provides the first experimental evidence of an enhancement in the dielectric mismatch effect originating from the Coulomb interaction between electron and hole.

Single photons on demand from novel site-controlled GaAsN/GaAsN:H quantum dots.

We demonstrate triggered single-photon emission from a novel system of site-controlled quantum dots (QDs), fabricated by exploiting the hydrogen-assisted, spatially selective passivation of N atoms in dilute nitride semiconductors. Evidence of this nonclassical behavior is provided by the observation of strong antibunching in the autocorrelation histogram of the QD exciton emission line. This class of site-controlled quantum emitters can be exploited for the fabrication of new hybrid QD-nanocavity systems of interest for future quantum technologies.

Exceptional Hydrogen Uptake in Crystalline InxGa1-xN Semiconductors.

The irradiation of InN and InxGa1-xN samples with low-energy H ions results in exceptionally high hydrogen uptake in a crystalline semiconductor. This phenomenon is attributed to specific In-H complex formation. By exploiting spectral fingerprints of the In-H complexes observable in In L3-edge X-ray absorption spectroscopy, we provide direct evidence of complex formation. Density functional theory calculations assist in interpreting the X-ray absorption spectra and offer insights into the energetics of complex formation. We quantify the total amount of reversibly incorporated hydrogen in these semiconductors and discuss their strengths and weaknesses as innovative materials for hydrogen storage.

Imaging the Quantum Capacitance of Strained MoS2 Monolayers by Electrostatic Force Microscopy.

We implemented radio frequency-assisted electrostatic force microscopy (RF-EFM) to investigate the electric field response of biaxially strained molybdenum disulfide (MoS2) monolayers (MLs) in the form of mesoscopic bubbles, produced via hydrogen (H)-ion irradiation of the bulk crystal. MoS2 ML, a semiconducting transition metal dichalcogenide, has recently attracted significant attention due to its promising optoelectronic properties, further tunable by strain. Here, we take advantage of the RF excitation to distinguish the intrinsic quantum capacitance of the strained ML from that due to atomic scale defects, presumably sulfur vacancies or H-passivated sulfur vacancies. In fact, at frequencies fRF larger than the inverse defect trapping time, the defect contribution to the total capacitance and to transport is negligible. Using RF-EFM at fRF = 300 MHz, we visualize simultaneously the bubble topography and its quantum capacitance. Our finite-frequency capacitance imaging technique is noninvasive and nanoscale and can contribute to the investigation of time- and spatial-dependent phenomena, such as the electron compressibility in quantum materials, which are difficult to measure by other methods.

Localisation-to-delocalisation transition of moiré excitons in WSe2/MoSe2 heterostructures.

Moiré excitons (MXs) are electron-hole pairs localised by the periodic (moiré) potential forming in two-dimensional heterostructures (HSs). MXs can be exploited, e.g., for creating nanoscale-ordered quantum emitters and achieving or probing strongly correlated electronic phases at relatively high temperatures. Here, we studied the exciton properties of WSe2/MoSe2 HSs from T = 6 K to room temperature using time-resolved and continuous-wave micro-photoluminescence also under a magnetic field. The exciton dynamics and emission lineshape evolution with temperature show clear signatures that MXs de-trap from the moiré potential and turn into free interlayer excitons (IXs) for temperatures above 100 K. The MX-to-IX transition is also apparent from the exciton magnetic moment reversing its sign when the moiré potential is not capable of localising excitons at elevated temperatures. Concomitantly, the exciton formation and decay times reduce drastically. Thus, our findings establish the conditions for a truly confined nature of the exciton states in a moiré superlattice with increasing temperature and photo-generated carrier density.

Exciton-Phonon Interactions in Strained Domes of Monolayer MoS2 Studied by Resonance Raman Spectroscopy.

This work describes a resonance Raman study performed in the domes of monolayer MoS2 using 23 different laser excitation energies covering the visible and near-infrared (NIR) ranges. The multiple excitation results allowed us to investigate the exciton-phonon interactions of different phonons (A'1, E', and LA) with different excitonic optical transitions in biaxially strained monolayer MoS2. The analysis of the intensities of the two first-order peaks, A'1 and E', and the double-resonance 2LA Raman band as a function of the laser excitation furnished the values of the energies of the indirect exciton and the direct excitonic transitions in the strained MoS2 domes. It was noticed that the out-of-plane A'1 phonon mode is significantly enhanced only by the indirect exciton I and the C exciton, whereas the in-plane E' mode is only enhanced by the C exciton of the MoS2 dome, thus revealing the weak interaction of these phonons with the A and B excitons in the strained MoS2 domes. On the other hand, the 2LA Raman band is significantly enhanced at the indirect exciton I and by the A (or B) exciton but not enhanced by the C exciton, thus showing that the LA edge phonons that participate in the double-resonance process in MoS2 have a weak interaction with the C exciton.