Structural, elastic, electronic, optical and vibrational properties of single-layered, bilayered and bulk molybdenite MoS2-2H

In recent years, transition metal dichalcogenides have received great attention since they can be prepared as two-dimensional semiconductors, presenting heterodesmic structures incorporating strong in-plane covalent bonds and weak out-of-plane interactions, with an easy cleavage/exfoliation in single or multiple layers. In this context, molybdenite, the mineralogical name of molybdenum disulfide, MoS2, has drawn much attention because of its very promising physical properties for optoelectronic applications, in particular a band gap that can be tailored with the material's thickness, optical absorption in the visible region and strong light-matter interactions due to the planar exciton confinement effect. Despite this wide interest and the numerous experimental and theoretical articles in the literature, these report on just one or two specific features of bulk and layered MoS2 and sometimes provide conflicting results. For these reasons, presented here is a thorough theoretical analysis of the different aspects of bulk, monolayer and bilayer MoS2 within the density functional theory (DFT) framework and with the DFT-D3 correction to account for long-range interactions. The crystal chemistry, stiffness, and electronic, dielectric/optical and phonon properties of single-layered, bilayered and bulk molybdenite have been investigated, to obtain a consistent and detailed set of data and to assess the variations and cross correlation from the bulk to single- and double-layer units. The simulations show the indirect-direct transition of the band gap (K-K' in the first Brillouin zone) from the bulk to the single-layer structure, which however reverts to an indirect transition when a bilayer is considered. In general, the optical properties are in good agreement with previous experimental measurements using spectroscopic ellipsometry and reflectivity, and with preliminary theoretical simulations

The effect of long-range interactions on the infrared and Raman spectra of aragonite (CaCO3, Pmcn) up to 25 GPa

Long-range interactions are relevant in the physical description of materials, even for those where other stronger bonds give the leading contributions. In this work, we demonstrate this assertion by simulating the infrared and Raman spectra of aragonite, an important calcium carbonate polymorph (space group Pmcn) in geological, biological and materials science fields. To this aim, we used Density Functional Theory methods and two corrections to include long-range interactions (DFT-D2 and DFT-D3). The results were correlated to IR spectroscopy and confocal Raman spectrometry data, finding a very good agreement between theory and experiments. Furthermore, the evolution of the IR/Raman modes up to 25 GPa was described in terms of mode-Grüneisen's parameters, which are useful for geological and materials science applications of aragonite. Our findings clearly show that weak interactions are of utmost importance when modelling minerals and materials, even when they are not the predominant forces

Structural and Elastic Behaviour of Sodalite Na8(Al6Si6O24)Cl2 at High-Pressure by First-Principle Simulations

Sodalite Na8(Al6Si6O24)Cl2 (space group P-43n) is an important mineral belonging to the zeolite group, with several and manyfold fundamental and technological applications. Despite the interest in this mineral from different disciplines, very little is known regarding the high-pressure elastic properties. The present study aims at filling this knowledge gap, reporting the equation of state and the elastic moduli of sodalite calculated in a wide pressure range, from –6 GPa to 22 GPa. The results were obtained from Density Functional Theory simulations carried out with Gaussian-type basis sets and the well-known hybrid functional B3LYP. The DFT-D3 a posteriori correction to include the van der Waals interactions in the physical treatment of the mineral was also applied. The calculated equation of state parameters at 0 GPa and absolute zero (0 K), i.e., K0 = 70.15(7) GPa, K’ = 4.46(2) and V0 = 676.85(3) Å3 are in line with the properties derived from the stiffness tensor, and in agreement with the few experimental data reported in literature. Sodalite was found mechanically instable when compressed above 15.6 GPa

FINITE ELEMENT ANALYSIS FOR THE PREDICTION OF THE CIRCUMFERENTIAL BAMBOO STRENGTH

Reported theoretical analyses have not explained the variation with radius of the circumferential strength of bamboo. This study aims to analyze this issue using finite element analysis. A rectangular image representative of the culm cross section of Phyllostachys edulis was divided along the radial direction into ten parts. Next, ten FE rectangular models representing the fibers, matrix, and void content at each radial position were generated. The matrix and the fibers were assumed to be isotropic, having elastic moduli of 1800 MPa and 18000 MPa, respectively. The strength predicted with the fiber first principal stress is parabolic along the radial direction, consistent with experimental findings

Thermal treatment of bamboo with flame: influence on the mechanical characteristics

The mechanical properties of bamboo are susceptible to degradation due to both physical and biological agents. Among the non-chemical treatments, we studied the influence of a short-time heat treatment, using an LPG-gas torch, on the mechanical properties of a bamboo (Phyllostachys viridiglaucescens) growing in Italy. The response was very encouraging as we found no significant reduction in either elastic modulus or tensile, compressive and bending strength. Several samples were subject to tension, compression and bending tests to compare the responses of the treated and untreated culms. The average tensile elastic modulus was slightly greater for the untreated culms. The average tensile strength of the untreated culms was only slightly greater, and the differences can be assumed to be insignificant from a structural point of view. The average value of the treated culms compressive elastic modulus was slightly greater than that of the untreated ones. The compressive strength was essentially the same. The bending mechanical behaviour was barely influenced by the thermal treatment. A microscopic investigation (optical and electron microscopy) was undertaken to investigate the possible deterioration of the bamboo microstructure due to the heat treatment. No appreciable damage was detectable in the treated material. The proposed heat treatments can be considered as a reliable and sustainable protection practice for bamboo culms

Crystal-chemical, vibrational and electronic properties of 1M-phlogopite K(Mg,Fe)3Si3AlO10(OH)2 from Density Functional Theory simulations

Trioctahedral micas are peculiar minerals that may present interesting electronic properties that can be modulated by specific cationic substitutions. In the present work, a detailed characterization of the structural, vibrational, and electronic properties of 1M-phlogopite as a function of the FeII/MgII substitutions, with Mg/Fe ratio ≥ 2, is reported. The results were obtained from density functional theory simulations at the B3LYP-D* level of theory, which included the effect of long-range interactions, and also using all-electron Gaussian-type orbitals to describe the atoms in the mineral. The crystal structures of the different phlogopite models were in good agreement with previous X-ray and neutron diffraction data reported in the literature. In addition, the simulated Raman spectra well described the experimental ones obtained from confocal Raman micro-spectrometry, providing additional information on the atomic motions. The electronic band structure and the atom- and orbital-projected density of states were also discussed, describing the nature of the band gap and electronic transitions, and how they vary with the iron content

Density functional investigation of the thermophysical and thermochemical properties of talc [Mg3Si4O10(OH)2]

The knowledge of the P, T behavior of talc is very important in mineralogical–petrological and geophysical research fields because talc can be considered a hydrous phase that can recycle water into the Earth’s mantle and also an important mineral in both industrial and technological applications. However, very few works have been presented to fully characterize the thermodynamic properties of this mineral, especially at atomic scale. In a previous work, we modeled the structural and mechanical properties of talc using the B3LYP-D* hybrid density functional, which included a correction for the dispersive forces and all-electron Gaussian-type orbital basis sets. The results were in good agreement with single-crystal X-ray and neutron diffraction experimental data. Here, we extend the investigation to the thermochemical and thermophysical properties of talc using the same density functional approach and the quasi-harmonic approximation, providing the thermal equation of state, the heat capacity and the entropy of the mineral at different P, T conditions

SEISMIC, a Python-based code of the Quantas package to calculate the phase and group acoustic velocities in crystals

The present work reports the theoretical background and the capabilities of SEISMIC, a Python code specifically developed to calculate the propagation of the sound waves inside crystalline materials. SEISMIC is a tool integrated in the QUANTAS package and provides a series of useful information for engineers and geophysicists, such as the phase and group velocities, the power flow angle, the enhancement factors, and the polarization vectors, using as input the elastic moduli and the density of the material. Numerical treatments of the derivatives were avoided, using analytical methods to obtain numerically stable results. The code relies only on Python numerical and graphical libraries to ensure a full cross-platform usability

Preface to StSPM2019EV special issue

For the first year, the Journal of Microscopy publishes a special \u2018StSPM2019EV\u2019 issue with selected contributions invited at the 3rd workshop Science through Scanning Probe Microscopy 2019 \u2013 Extended Version (StSPM19EV). The Italian Society for Microscopical Sciences (SISM) organised this event, in collaboration with the Institute for the Study of Nanostructured Materials (ISMN) of the CNR and the Department of Biological, Geological and Environmental Sciences (BiGeA) of the University of Bologna. Every 3 years, the event meets up Italian scientists working with Scanning Probe Microscopy. This year StSPM is joined to the workshop \u2018Scanning probe microscopy and spectroscopy for mineral, biological and material sciences\u2019, organised by Prof. Giovanni Valdr\ue8 of BiGeA every 2 years, becoming StSPM19EV. The workshop aimed to illustrate the scientific progresses achieved in Italy thanks to SPM techniques. The scientific program reported alternate sessions on \u2018Materials Science\u2019 and \u2018Life Science\u2019, highlighting how SPM contributes to such branches. This Extended Version included two didactic lectures for degree/PhD students, accordingly to the educational approach of the \u2018Scanning probe microscopy and spectroscopy for mineral, biological and material sciences\u2019 workshop