Photo-induced lattice distortion in 2H-MoTe2 probed by time-resolved core level photoemission.

The technological interest in MoTe2 as a phase engineered material is related to the possibility of triggering the 2H-1T' phase transition by optical excitation, potentially allowing for an accurate patterning of metallic areas into a semiconducting canvas via laser irradiation. In this paper, we investigate the photo-induced modifications of a bulk 2H-MoTe2 crystal by means of time-resolved X-ray photoemission spectroscopy. We observe that in the microsecond timescale, the core levels shift to higher kinetic energies due to surface photovoltage fields, while in the sub-nanosecond range, the photoemission peaks shift in the opposite direction. With the support of DFT calculations, we ascribe the latter effect to the deformation of the lattice in the out-of-plane direction, which is along the pathway for the 2H-1T' phase transition. Our data indicate an intermediate lattice excitation state with a measured lifetime in the order of 600 ps, in which the displacement of Mo and Te atoms causes the Te 4d electrons to shift towards higher binding energies.

First-Principles Nonequilibrium Green's Function Approach to Ultrafast Charge Migration in Glycine.

We investigate the photoinduced ultrafast charge migration phenomenon in the glycine molecule using a recently proposed nonequilibrium Green's functions (NEGF) approach. We first consider the dynamics resulting from the sudden removal of an electron in the valence shells, finding a satisfactory agreement with available data. Then we explicitly simulate the laser-induced photoionization process and study the evolution of the system after the pulse. We disentangle polarization and correlation effects in the electron dynamics and assign the main frequencies to specific elements of the reduced one-particle density matrix. We show that electronic correlations renormalize the bare frequencies, redistribute the spectral weights, and give rise to new spectral features.

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.

Tailoring the optical properties of MoS2 and WS2 single layers via organic functionalization.

Tailoring the structural and electronic properties of 2D materials is fundamental to boost their use in a wide range of technological applications. In this paper, by means of first principles simulations, we show how methyl functionalization of MoS2 and WS2 monolayers can be employed to change their energy gap, tune their optoelectronic properties and modify the relative stability of their structural phases (or polytypes). In particular for both compound monolayers, we find that the most stable semiconducting H phase becomes metallic upon methyl functionalization, while in the metastable T' phase the band gap increases as a function of the -CH3 coverage; correspondingly the phase stability is reversed and the on-set of the optical absorption is blue-shifted.

Strongly bound excitons in anatase TiO2 single crystals and nanoparticles.

Anatase TiO2 is among the most studied materials for light-energy conversion applications, but the nature of its fundamental charge excitations is still unknown. Yet it is crucial to establish whether light absorption creates uncorrelated electron-hole pairs or bound excitons and, in the latter case, to determine their character. Here, by combining steady-state angle-resolved photoemission spectroscopy and spectroscopic ellipsometry with state-of-the-art ab initio calculations, we demonstrate that the direct optical gap of single crystals is dominated by a strongly bound exciton rising over the continuum of indirect interband transitions. This exciton possesses an intermediate character between the Wannier-Mott and Frenkel regimes and displays a peculiar two-dimensional wavefunction in the three-dimensional lattice. The nature of the higher-energy excitations is also identified. The universal validity of our results is confirmed up to room temperature by observing the same elementary excitations in defect-rich samples (doped single crystals and nanoparticles) via ultrafast two-dimensional deep-ultraviolet spectroscopy.Here the authors combine steady-state angle-resolved photoemission spectroscopy, ellipsometry and ultrafast two-dimensional ultraviolet spectroscopy to examine the role of many-body correlations in anatase TiO2, revealing the existence of strongly bound excitons in single crystals and nanoparticles.

Coexistence of negatively and positively buckled isomers on n+-doped Si(111) − 2 × 1.

A long-standing puzzle regarding the Si(111) − 2 × 1 surface has been solved. The surface energy gap previously determined by photoemission on heavily n-doped crystals was not compatible with a strongly bound exciton known from other considerations to exist. New low-temperature angle-resolved photoemission and scanning tunneling microscopy data, together with theory, unambiguously reveal that isomers with opposite bucklings and different energy gaps coexist on such surfaces. The subtle energetics between the isomers, dependent on doping, leads to a reconciliation of all previous results.

Comparative bioavailability of diltiazem in prolonged-release oral preparations.

This study was conducted to compare the bioavailability of two prolonged-release pharmaceutical forms containing 300 mg of diltiazem. The test formulation is a new design of tablets with a hydrophilic matrix, and the reference formulation is capsules containing prolonged liberation microgranules, in the same dose, that are commercially available in the pharmaceutical market. Diltiazem plasma concentrations were analyzed by high-performance liquid chromatography (HPLC), which involves solid-phase extraction for plasma sample preparation. Twelve healthy volunteers participated in the study, which had a single-dose, two-treatment, two-sequence-crossover, randomized design. The preparations were compared using pharmacokinetic parameters such as the area under the plasma concentration-time curve AUC(0-36), peak plasma concentration Cmax, and Cmax/AUC(0-36) ratio as a measure for the absorption rate. No statistically significant difference was observed for any of the parameters, and the 90% confidence intervals calculated for the ratio of the logarithmically transformed AUC(0-36) and Cmax/AUC(0-36) values of both formulations were within the bioequivalence limit of 0.80-1.25. Moreover, an in vitro study of dissolution according to USP 23 was conducted, and the in vitro parameters were calculated.

Stability of capsules containing omeprazole in enteric coated pellets.

The aim of the present study was to comparatively evaluate the stability of capsules containing 20 mg of Omeprazole, in enteric coated pellets, from seven pharmaceutical laboratories on Argentine market. The stability test was performed under the conditions indicated by the ICH: 40 degrees C, 75% HR, with and without light, during a six month period. The remaining content of Omeprazole, total percentage of impurities and percentage of released active principle in vitro, were determined by HPLC. The organoleptic characteristics of the pellets were visually examined. The results obtained at six months indicate that, from the seven products studied, four were found to have a content of Omeprazole higher than 90% of the labeled amount, in both lighting conditions tested, and also comply with the USP23 specifications with respect to the release in vitro. We conclude that the progressive darkening of the pellets indicates, qualitatively, the level of degradation of the product and that the stability of Omeprazole depends on the correct formulation and the primary container.

Structural and optical properties of the Ge(111)-(2 x 1) surface.

We study the two lowest-energy isomers of the Ge(111)-(2 x 1) surface, by a state-of-the-art first-principles calculation of their optical spectra, including the electron-hole interaction effects. A comparison of our results with the available experimental data suggests that, at difference with the silicon case, the stablest isomer differs from the standard "buckled Pandey chains" reconstruction. This conclusion is supported by accurate total-energy results.