The boson peak in silicate glasses: insight from molecular dynamics.

In the low-frequency regime, ≈1 THz, glasses show an anomalous excess in their vibrational density of states called the boson peak (BP). The origin of BP has been a subject of debate since its first discovery a few decades ago. Although BP has been the focus of numerous studies, no conclusive answers have been found about its origins, which remained elusive to date. Here, we present results based on molecular dynamics of several binary and ternary silicate glasses with different network intermediates and modifier oxides. The vibrational density of states and the BP are reported for all the studied glasses. Their correlation with the elastic constant C44, structural, and dynamical properties are extensively discussed in terms of Voronoi atomic volume and the vibrational mean square displacement of Q4 species specifically. We also question the classical classification of alkali oxides as modifiers, and we suggest that Li2O plays the role of pseudo-intermediate oxide in lithium silicate glasses. This claim is supported by the effect of Li on various vibrational modes, and this effect differs from the other alkali metals. Furthermore, we demonstrate a correlation between the BP intensities and both the Voronoi volume of the Q4 and Q3 units and vibrational mean square displacements.

Potential of nanostructured carbon materials for iodine detection in realistic environments revealed by first-principles calculations.

In the context of effective detection of iodine species (I2, CH3I) formed in nuclear power plants and nuclear fuel reprocessing facilities, we perform a comparative study of the potential sensing performance of four expectedly promising 2D materials (8-Pmmn borophene, BC3, C3N, and BC6N) towards the iodine-containing gases and, with the view of checking selectivity, towards common inhibiting gases in the containment atmosphere (H2O and CO), applying methods of dispersion-corrected density functional theory with periodic boundary conditions. A covalent bond is formed between the CO molecule and boron in BC3 or in 8-Pmmn borophene, compromising the anticipated applicability of these materials for iodine detection. The presence of nitrogen atoms in BC6N-2 prevents the formation of a covalent bond with CO; however, the closeness of adsorption energies for all the four gases studied does not distinguish this material as specifically sensitive to iodine species. Finally, the energies of adsorption on C3N yield a significant and promising discrimination between the adsorption energies of (I2, CH3I) vs. (CO, H2O), revealing possibilities for this material's use as an iodine sensor. The conclusions are supported by simulations at finite temperature; underlying electronic structures are also discussed.

Enhanced Photocatalytic Water Splitting of SrTiO3 Perovskite through Cobalt Doping: Experimental and Theoretical DFT Understanding.

Throughout extensive research endeavors, SrTiO3 has emerged as a promising photocatalytic material for utilizing solar energy and facilitating hydrogen production via water splitting. Yet, the pursuit of enhanced efficiency and amplified hydrogen generation has prompted researchers to delve into the realm of advanced doping strategies. In this work, using experimental characteristics and DFT calculations, we studied the effect of cobalt substitution on the structural, electronic, optical, and magnetic properties as well as the photocatalytic activity of SrTi1-xCoxO3-δ (x = 0, 0.125, 0.25, 0.375, and 0.5) perovskites. The samples were successfully prepared by using the solid-state reaction method. Based on X-ray diffraction and the Rietveld refinement method, the elaborated samples were shown to preserve the absorption range up to the visible region. Moreover, the position of band edge levels after cobalt doping becomes more appropriate for water splitting. Our findings report that all cobalt-doped compounds exhibit good photocatalytic activities and could be used as suitable photocatalyst materials for hydrogen production.

Kinetic Monte Carlo simulation of polycrystalline silver metal electrodeposition: scaling of roughness and effects of deposition parameters.

In this work, a kinetic Monte Carlo (KMC) technique was used to simulate the growth morphology of electrodeposited polycrystalline Ag thin films under a galvanostatic condition (current density). The many-body Embedded Atom Method (EAM) potential has been used to describe the Ag-Ag atomic interaction. Herein, the surface morphology is affected by the kinetic diffusion of adatoms where four jump processes are considered, namely hopping, exchange, step-edge exchange and grain boundary. The results have shown that the surface roughness follows a power law behavior versus film thickness (∝Lα) and time (∝tß), with the roughness and growth exponents α and ß found to be α = 1.14 ± 0.01 and ß = 0.57 ± 0.01. The surface morphology under different deposition parameters (current density and substrate temperature) has been discussed in detail. The surface roughness increases where the current density increases due to high deposition rates, which can accelerate the growth of island mode, especially on the (111) surface. In contrast, the surface roughness decreases the temperature of the substrate increases due to thermal agitation, allowing to transform nearly columnar grains to grains with a flat and smooth surface. Finally, the simulations provided information on the subsurface deposition rate of each grain that is not directly available for experimental investigations. It was observed that the (111) grain has a faster deposition rate compared to the (100) and (110) grains due to the low surface energy of the (111) grain.

The effect of Sr substitution on the structural and physical properties of manganite perovskites Ca1-xSrxMnO3-δ (0 ≤ x ≤ 1).

Calcium manganite (CaMnO3-δ) has been extensively utilized in many applications due to its unique physical and chemical properties. In this study, the effect of Sr-substitution at the Ca-site on the structural, magnetic, electronic and electrical properties of CaMnO3 manganite perovskites is investigated in detail. The perovskite compounds Ca1-xSrxMnO3-δ (x = 0, 0.25, 0.5, 0.75 and 1) were synthesized through the sol-gel method at 1200 °C. From the patterns of X-ray diffraction, it was observed that all of the synthesized compounds show a pure perovskite phase at room temperature. The refinement results of the perovskite series suggest that a structural transformation from an orthorhombic (Pnma) to a hexagonal (P63/mmc) system occurred for 0.50 < x ≤ 0.75. We note however that the sample with the composition x = 0.50 showed a phase mixture of orthorhombic (Pnma) and hexagonal (P63/mmc). Based on DFT calculations, we have demonstrated the energetic stability of all compounds by negative formation energy and confirmed the semiconductor behavior by the presence of a band gap. The change in the band gap value with the Sr content suggests the potential tuning of the electronic behavior of CaMnO3-SrMnO3 solid solution. Furthermore, as the temperature increases from 300 to 1000 K, the electrical resistivity exhibits a reduction while the Seebeck coefficient (S) shows an augmentation. The negative values of S indicated the n-type-semiconductor nature of all compounds. The obtained values of the activation energy from thermal evolution of resistivity suggested that the electrical transport behavior of all the compounds followed the mechanism of small polaron hopping. Power factor is greatly affected by the Sr amount and reached a maximum value at x = 0.50. Overall, introducing Sr into the CaMnO3-δ matrix improved the power factor and reduced electrical resistivity. According to the obtained results, the studied manganite perovskites could be proposed as suitable materials for photocatalytic and thermoelectric applications.

Adsorption of NO, NO2 and H2O in divalent cation faujasite type zeolites: a density functional theory screening approach.

Emissions of diesel exhaust gas in confined work environments are a major health and safety concern, because of exposition to nitrogen oxides (NOx). Removal of these pollutants from exhaust gas calls for engineering of an optimum sorbent for the selective trapping of NO and NO2 in the presence of water. To this end, periodic density functional theory calculations along with a recent dispersion correction scheme, namely the Tkatchenko-Scheffler scheme coupled with iterative Hirshfeld partitioning TS/HI, were performed to investigate the interactions between NO, NO2, H2O and a series of divalent cation (Be2+, Mg2+, Ca2+, Sr2+, Ba2+, Fe2+, Cu2+, Zn2+, Pd2+, and Pt2+) faujasites. This enabled the identification of the optimum zeolites to selectively capture NOx in the presence of H2O, with respect to two important criteria, such as thermodynamic affinity and regeneration. Our results revealed that Pt2+ and Pd2+ containing faujasites are the best candidates for effective capture of both NO and NO2 molecules, which paves the way towards the use of these sorbents to address this challenging application.

Atomistic insights into the structure and elasticity of densified 45S5 bioactive glasses.

Glasses have applications in regenerative medicine due to their bioactivity, enabling interactions with hard and soft tissues. Soda-lime phosphosilicate glasses, such as 45S5, represent a model system of bioactive glasses. Regardless of their importance as bioactive materials, the relationship between the structure, density, and cooling process has not been studied in detail. This hinders further development of glasses as biomaterials. We used molecular dynamics simulations to study the elastic and structural properties of densified 45S5 bioactive glass and liquids over a wide range of densities. We performed a systematic analysis of the glass structure to density relationship to correlate the change in the properties with the structural change to enhance the mechanical properties of bioactive glasses while preserving their bioactive nature. The results show that the glass structure tends to be repolymerized, as indicated by increased network connectivity and a tetrahedral to octahedral polyhedral transition. We were able to tailor the elastic properties while keeping the bioactivity of the glass. The results presented here will provide some guidance to develop bioactive glasses with enhanced mechanical properties.

Atomic simulation of adsorption of SO2 pollutant by metal (Zn, Be)-oxide and Ni-decorated graphene: a first-principles study.

Due to the impact of toxic gases on human health, considerable interest has been shown in detecting noxious air pollutants, particularly sulfur dioxide (SO2), both experimentally and theoretically. This work provides new insights into the adsorbing (SO2) molecules on the surface of metal-oxide graphitic structures, i.e., Beryllium-Oxide (BeO), Zinc-Oxide (ZnO), and Ni-decorated graphene applying a first-principles study. Computational analyses suggest that the type of binding of SO2 molecule on BeO and ZnO sheets is physisorption so that binding energies of -0.405 and -0.154 eV were assigned to ZnO and BeO nanosheets in that order. The adsorption energy of SO2 on metal oxide sheets was much higher than the pristine graphene. Taking pristine graphene as an adsorbent for SO2 molecule, it was found that such nanomaterial is not an efficient adsorbent due to the weak interactions (-0.157 eV) and low electron charge transfer (0.042 e) present in SO2/graphene complex. To overcome this issue, graphene nanosheets decorated with nickel atoms were studied for interaction with SO2 molecules; the results indicate that the SO2 molecules were chemisorbed on Ni-decorated graphene sheets with an adsorption energy of -2.297 eV. Chemisorption of SO2 molecules on Ni-decorated graphene sheets was proven by the strong orbital hybridization between Ni 3d and sulfur 3p orbitals in the Projected Density of States (PDOS) plot. This work provides useful information about SO2 adsorption on Ni-decorated graphene sheets in order to develop a new class of gas sensing devices. Superior chemisorption of SO2 on Ni-decorated graphene sheets compared to the physical adsorption on BeO and ZnO sheets makes Ni-decorated graphene a potential candidate for detecting SO2 molecules.

Structure-Elasticity Relationship of Potassium Silicate Glasses from Brillouin Light Scattering Spectroscopy and Molecular Dynamics Simulations.

Brillouin light scattering (BLS) spectroscopy and molecular dynamic (MD) simulations allowed the identification of a relationship between the elastic properties and the structure of K-containing glasses of formula (K2O)x-(SiO2)1-x, having different K2O concentrations. Excellent agreement was observed between experimental data and simulations. The peculiar elastic properties observed for these potassium silicate glasses have been extensively discussed in terms of structural and energetic features of the materials. Elastic properties were shown to be strongly dependent on the asymmetry of potential energy in the K-BO interactions and the K-NBO interactions. A low K2O content (below 10-15% K2O) appeared to be in favor of K+-BO interactions and high asymmetry of potential energy, whereas a high K2O content (from 10 to 15% K2O) was in favor of K+-NBO interactions with lower asymmetry. Our results suggest a possible explanation to the observed anomalous dependence of elastic properties of potassium silicate glasses with K2O amount.

Ionic self-diffusion and the glass transition anomaly in aluminosilicates.

The glass transition temperature (Tg) is the temperature after which a supercooled liquid undergoes a dynamical arrest. Usually, glass network modifiers (e.g., Na2O) affect the behavior of Tg. However, in aluminosilicate glasses, the effect of different modifiers on Tg is still unclear and shows an anomalous behavior. Here, based on molecular dynamics simulations, we show that Tg decreases with increasing charge balancing cation field strength (FS) in the aluminosilicate glasses, which is an anomalous behavior as compared to other oxide glasses. The results show that the origins of this anomaly come from the dynamics of the supercooled liquid above Tg, which in turn is correlated to pair excess entropy. Our results deepen our understanding of the effect of different modifiers on the properties of the aluminosilicate glasses.

DNA Nucleobase under Ionizing Radiation: Unexpected Proton Transfer by Thymine Cation in Water Nanodroplets.

The effect of ionizing radiation on DNA constituents is a widely studied fundamental process using experimental and computational techniques. In particular, radiation effects on nucleobases are usually tackled by mass spectrometry in which the nucleobase is embedded in a water nanodroplet. Here, we present a multiscale theoretical study revealing the effects and the dynamics of water droplets towards neutral and ionized thymine. In particular, by using both hybrid quantum mechanics/molecular mechanics and full ab initio molecular dynamics, we reveal an unexpected proton transfer from thymine cation to a nearby water molecule. This leads to the formation of a neutral radical thymine and a Zundel structure, while the hydrated proton localizes at the interface between the deprotonated thymine and the water droplet. This observation opens entirely novel perspectives concerning the reactivity and further fragmentation of ionized nucleobases.

Effect of Sodium Oxide Modifier on Structural and Elastic Properties of Silicate Glass.

Molecular dynamics (MD) simulations and Brillouin light scattering (BLS) spectroscopy experiments have been carried to study the structure of sodium silicate glasses (SiO2)(100-X)(Na2O)X, where X ranges from 0 to 45 at room temperature. The MD-obtained glass structures have been subjected to energy minimization at zero temperature to extract the elastic constants also obtained by BLS spectroscopy. The structures obtained are in good agreement with the structural experimental data realized by different techniques. The simulations show that the values of the elastic constants as a function of X (i.e., Na2O mol %) agree well with those measured by BLS spectroscopy. The variations of elastic constants C11 and C44 as a function of Na2O mol % are discussed and correlated to structural results and potential energies of oxygen atoms.