Novel Janus gamma-Pb 2 XY monolayers with high thermoelectric performance X=S, Se and Y=Se, Te X ≠ Y.

The quest for efficient thermoelectric materials has intensified with the advent of novel Janus monolayers exhibiting exceptional thermoelectric parameters. In this work, we comprehensively investigate the structural, electronic, transport, phonon, and thermoelectric properties of novel Janus γ -Pb 2 XY (X=S, Se; Y=Se, Te; X ≠ Y) monolayers using density functional theory combined with the Boltzmann transport equation. Our findings unveil the energetic, dynamic, thermal, and mechanical stability of these monolayers, along with their remarkable thermoelectric performance. Remarkably, the p-type γ -Pb 2 SeTe monolayer exhibits an outstanding figure of merit (ZT) of 6.88 at 800 K, attributed to its intrinsically low lattice thermal conductivity of 0.162 Wm - 1 K - 1 arising from strong phonon scattering, low group velocity, low phonon relaxation time, and a high Grüneisen parameter. Furthermore, these monolayers demonstrate high Seebeck coefficients and electrical conductivities, making them promising for efficient charge transport and thermoelectric energy conversion. Our results highlight the immense potential of Janus γ -Pb 2 XY monolayers as promising candidates for high-temperature thermoelectric applications and open up exciting avenues for further exploration of these novel two-dimensional materials in energy-related technologies.

Covalent functionalization of germanene employing computational simulations.

Computational simulations through density functional theory in conjunction with M06-L and HSE functional have been carried out to investigate the chemical reactivity of the germanene monolayer. It is exceptionally reactive, with an average reaction energy of -60.4 kcal mol-1 for the nineteen functional groups considered: H, F, Cl, Br, O, S, Se, Ge, OH, SH, CH3, CF3, NH, NH2, C6H5, C6H4, CCl2, CBr2, and the azomethine ylide. The results indicate that oxygen is the most reactive reagent (-96.9 kcal mol-1), followed by fluorine (-83.1 kcal mol-1). Germanene presents a rich organic chemistry, and functionalization with azomethine ylides, benzynes, and carbenes can be easily accomplished as indicated by the reaction energies computed, which lie between -45 and -65 kcal mol-1. Furthermore, germanene is significantly more reactive than graphene and hexagonal boron nitride monolayers since the reaction energy for germanene is more than 40 kcal mol-1 lower. Although, in general, germanene is slightly more reactive than black and blue phosphorene and less prone to oxidation, but its oxidation when exposed to air occurs spontaneously. The addition of functional groups works cooperatively. The reaction energies become lower as the number of functional groups increases, thus favouring the agglomeration of functional groups attached unless the steric effect alters this pattern. Finally, we analyzed the electronic properties of functionalized germanene. It is possible to fine-tune the band gap of germanene from 0.1 to 2 eV using different functional groups and coverages. For O-50% and S-50% functionalized germanene, we found that carrier recombination is the most difficult due to the considerable differences between the effective masses of holes and electrons, which is promising for optical applications.

Theoretical Characterization of Germanene Doped with Main Group Elements.

Herein, using density functional calculations, we studied the substitutional doping in germanene with B, C, N, O, Al, Si, P, S, Ga, As, and Se. Nitrogen is the element that can be more easily incorporated into the germanene lattice, followed by silicon, carbon, and boron. Almost all dopants were efficient in opening a band-gap. Yet, caution should be taken because this opening strongly depends on the dopant concentration. Carbon and sulfur were the most effective elements for band-gap opening. C-doping generates the lowest effective masses (me*/m0=mh*/m0=0.09). The equal me and mh values indicate an intrinsic semiconductor behavior, a characteristic shared by the chalcogenides-doped systems. Additionally, we performed a detailed analysis of the preferred disposition of dopants in the germanene lattice. In contrast with the results obtained for graphene, when multiple atoms are introduced in the germanene framework, they do not prefer to be agglomerated, adopting a random disposition, except in the case of sulfur and nitrogen, which favored specific dopant arrangement. Two sulfur dopants showed a notorious preference for replacing a Ge-Ge bond but without forming an S-S linkage, thus adopting a thiophene-like structure that may impart germanene exciting properties, as observed for S and N codoped graphene.

Novel octa-graphene-like structures based on GaP and GaAs.

CONTEXT: The discovery of graphene gave way to the search for new two-dimensional structures. In this regard, octa-graphene is a carbon allotrope consisting of 4- and 8-membered rings in a single planar sheet, drawing the research community's attention to study their inorganic analogs. Considering the promising properties of octa-graphene-like structures and the role of GaAs and GaP in semiconductor physics, this study aims to propose, for the first time, two novel inorganics buckled nanosheets based on the octa-graphene structure, the octa-GaAs and octa-GaP. This work investigated the structural, electronic, and vibrational properties of these novel octa-graphene-based materials. The octa-GaP and octa-GaAs have an indirect band gap transition with a valence band maximum between M and Г points and a conduction band minimum at Г point with energy of 3.05 eV and 2.56 eV, respectively. The QTAIMC analysis indicates that both structures have incipient covalent in their bonds. The vibrational analysis demonstrates the occurrence of ΓRaman = 6Ag + 6Bg and ΓRaman = 12A' + 12B″ for octa-GaP and octa-GaAs, respectively. The symmetry reduction of octa-GaAs leads to activating inactive modes observed in the octa-GaP structure. The frontier crystalline orbitals are composed by Ga(px) and P(py and pz) orbitals for octa-GaP and Ga(px and py) and As(s, py, and pz) for octa-GaAs in the valence bands while in the conduction bands by Ga(py, pz, and s) for both compounds and P(px and pz) and As(py). The phonon bands demonstrate the absence of the negative frequency modes and the structural stability of these new nanosheets. This report aims to reveal the fundamental properties of both newfound materials for stimulating experimental research groups in the search for synthesis routes to obtain this structure. METHODS: This work used the DFT/B3LYP approach implemented in the CRYSTAL17 computational package. Ga, As, and P atomic centers were described by triple-zeta valence with polarization (TZVP) basis set. The vibrational analysis was carried out via coupled-perturbed Hartree-Fock/Kohn Sham (CPHF/KS) method, and the chemical bonds were evaluated via the quantum theory of atoms in molecules and crystals (QTAIMC).

TopIso3D Viewer: Enhancing Topological Analysis through 3D Isosurfaces.

We present TopIso3D Viewer, a software with a user-friendly graphical interface, that generates three-dimensional maps to analyze descriptors based on the Quantum Theory of Atoms in Molecules (QTAIM), applied in periodic and nonperiodic systems. The software also automates the launching of topological analysis calculations through the Topond package and generates a report that facilitates the identification of the values of the calculated descriptors, in the Bond Critical Points (BCP) and Critical Points of the Laplacian of the electron density (LCP), facilitating the classification of chemical interactions. The map projects created can be stored in the form of HTML files, for later consultation through any type of browser. For validation of the software, several systems with 0-3D dimensions were studied. In addition, the topology of urea molecular crystal and its isolated molecule were revisited.

Modeling of BN-Doped Carbon Nanotube as High-Performance Thermoelectric Materials.

Ternary BNC nanotubes were modeled and characterized through a periodic density functional theory approach with the aim of investigating the influence on the structural, electronic, mechanical, and transport properties of the quantity and pattern of doping. The main energy band gap is easily tunable as a function of the BN percentage, the mechanical stability is generally preserved, and an interesting piezoelectric character emerges in the BNC structures. Moreover, C@(BN)1-xCx double-wall presents promising values of the thermoelectric coefficients due to the combined lowering of the thermal conductivity and increase of charge carriers. Computed results are in qualitative agreement with the little experimental evidence and therefore can provide insights on an atomic scale of the real samples and direct the synthesis towards increasingly performing hybrid nanomaterials.

Unconventional Disorder by Femtosecond Laser Irradiation in Fe2O3.

This paper demonstrates that femtosecond laser-irradiated Fe2O3 materials containing a mixture of α-Fe2O3 and ε-Fe2O3 phases showed significant improvement in their photoelectrochemical performance and magnetic and optical properties. The absence of Raman-active vibrational modes in the irradiated samples and the changes in charge carrier emission observed in the photocurrent density results indicate an increase in the density of defects and distortions in the crystalline lattice when compared to the nonirradiated ones. The magnetization measurements at room temperature for the nonirradiated samples revealed a weak ferromagnetic behavior, whereas the irradiated samples exhibited a strong one. The optical properties showed a reduction in the band gap energy and a higher conductivity for the irradiated materials, causing a higher current density. Due to the high performance observed, it can be applied in dye-sensitized solar cells and water splitting processes. Quantum mechanical calculations based on density functional theory are in accordance with the experimental results, contributing to the elucidation of the changes caused by femtosecond laser irradiation at the molecular level, evaluating structural, energetic, and vibrational frequency parameters. The surface simulations enable the construction of a diagram that elucidates the changes in nanoparticle morphologies.

Unveiling the Structural Behavior under Pressure of Filled M0.5Co4Sb12 (M = K, Sr, La, Ce, and Yb) Thermoelectric Skutterudites.

Skutterudite-type compounds based on □Co4Sb12 pnictide are promising for thermoelectric application due to their good Seebeck values and high carrier mobility. Filling the 8a voids (in the cubic space group Im3̅) with different elements (alkali, alkali earth, and rare earth) helps to reduce the thermal conductivity and thus increases the thermoelectric performance. A systematic characterization by synchrotron X-ray powder diffraction of different M-filled Co4Sb12 (M = K, Sr, La, Ce, and Yb) skutterudites was carried out under high pressure in the range ∼0-12 GPa. The isothermal equations of state (EOS) were obtained in this pressure range and the Bulk moduli (B0) were calculated for all the filled skutterudites, yielding unexpected results. A lattice expansion due to the filler elements fails in the description of the Bulk moduli. Topochemical studies of the filler site environment exhibited a slight disturbance and an increased ionic character when the filler is incorporated. The mechanical properties by means of Bulk moduli resulted in being sensitive to the presence of filler atoms inside the skutterudite voids, being affected by the covalent/ionic exchange of the Co-Sb and Sb-Sb bonds.

Probing the Site-Selective Doping in SrSnO3:Eu Oxides and Its Impact on the Crystal and Electronic Structures Using Synchrotron Radiation and DFT Simulations.

The impact of Eu3+ doping at the Sr2+ and Sn4+ sites in SrSnO3 on its structural and electronic properties was studied and correlated with the photocatalytic efficiency. The compounds were synthesized using a modified Pechini method. Refinement of the synchrotron X-ray diffraction (S-XRD) data showed that the samples had an orthorhombic Pbnm symmetry. The incorporation of Eu into the lattice led to increased short- and long-range disorder, inducing additional distortion in the SnO6. XANES measurements revealed that mixed Eu valences (Eu3+ and Eu2+) were present in Eu-doped samples, and DFT calculations confirmed the presence of these ions at Sr2+/Sr4+ sites in the SrSnO3, resulting in changes in the electronic behavior. The catalytic performance toward Remazol yellow dye photodegradation and the catalysts' surface properties were also evaluated. The catalytic efficiency followed the order of Sr(Sn0.99Eu0.01)SnO3 > (Sr0.99Eu0.01)SnO3 > SrSnO3. The order was clearly related to selected-site doping that changed the degree of the inter- and intraoctahedral distortion and the introduction of different Eu midgap states, which apparently favor charge separation upon photoexcitation during photocatalysis. The results shown here are of great importance to the functionalization of SrSnO3 and other perovskite materials by lanthanoid ions, especially Eu3+, for effective applications as photocatalysts.

Quantitative evaluation of the surface stability and morphological changes of Cu2O particles.

Cu2O low-index surfaces periodic models have been simulated based on density functional theory. The calculated surfaces energies allowed estimating the morphology by means of the Wulff theorem as well as the investigation of possible paths of morphological changes. Therefore, systematic morphology diagrams and change paths according to the energy modulation in relation to the surfaces stabilizations were elaborated. The applicability of this strategy was exemplified by comparing the obtained results with experimental available data from the literature. The morphology diagrams with the quantitative energetic point of view can be used as a guide to support experimental works in order to understand the relation between surface interactions and crystal growth.

Piezoelectric Response of Porous Nanotubes Derived from Hexagonal Boron Nitride under Strain Influence.

A computational study via periodic density functional theory of porous nanotubes derived from single-layer surfaces of porous hexagonal boron nitride nanotubes (PBNNTs) and inorganic graphenylene-like boron nitride nanotubes (IGP-BNNTs) has been carried out with the main focus in its piezoelectric behavior. The simulations showed that the strain provides a meaningful improve in the piezoelectric response on the zigzag porous boron nitride nanotubes. Additionally, its stability, possible formation, elastic, and electronic properties were analyzed, and for comparison purpose, the porous graphene and graphenylene nanotubes were studied. From the elastic properties study, it was found that IGP-BNNTs exhibited a higher rigidity because of the influence of the superficial porous area, as compared to PBNNTs. The present study provides evidence that the strain is a way to maximize the piezoelectric response and make this material a good candidate for electromechanical devices.

α-Ag2-2xZnxWO4 (0 ≤ x ≤ 0.25) Solid Solutions: Structure, Morphology, and Optical Properties.

A theoretical study was elaborated to support the experimental results of the Zn-doped α-Ag2WO4. Theses α-Ag2-2xZnxWO4 (0 ≤ x ≤ 0.25) solid solutions were obtained by coprecipitation method. X-ray diffraction data indicated that all α-Ag2-2xZnxWO4 (0 ≤ x ≤ 0.25) microcrystals presented an orthorhombic structure. The experimental values of the micro-Raman frequencies were in reasonable agreement with both previously reported and calculated results. Microscopy images showed that the replacement of Ag+ by Zn2+ promoted a reduction in the average crystal size and modifications in the morphology, from rod-like with hexagonal shape to roll-like with a curved surface. A theoretical methodology based on the surfaces calculations and Wulff constructions was applied to study the particle shapes transformations and the surface energy variations in α-Ag2-2xZnxWO4 (0 ≤ x ≤ 0.25) system. The decrease in the band gap value (from 3.18 to 3.08 eV) and the red shift in photoluminescence with the Zn2+ addition were associated with intermediary energy levels between the valence and conduction bands. First-principles calculations with density functional theory associated with B3LYP hybrid functional were conducted. The calculated band structures revealed an indirect band gap for the α-Ag2-2xZnxWO4 models. The electronic properties of α-Ag2WO4 and α-Ag2-2xZnxWO4 microcrystals were linked to distortion effects and oxygen vacancies (VOx) present in the clusters, respectively. Finally, photoluminescence properties of α-Ag2WO4 and α-Ag2-2xZnxWO4 microcrystals were explained by means of distortional effects and oxygen vacancies (VOx) in [AgOy] (y = 2, 4, 6, and 7) and [WO6] clusters, respectively, causing a red shift. Calculations revealed that the substitution for Ag+ with Zn2+ occurred randomly in the α-Ag2WO4 lattice, and it was more favorable on the Ag4 site, where the local coordination of Ag+ cations was four.

Computational study of Th(4+) and Np(4+) hydration and hydrolysis of Th(4+) from first principles.

The aqueous solvation of Th and Np in the IV oxidation state was examined using cluster models generated by Monte Carlo simulations and density functional theory embedded within the COSMO continuum model to approximate the effect of bulk water. Our results suggest that the coordination number (CN) for both Th(IV) and NP(IV) should be 9, in accordance to some experimental and theoretical results from the literature. The structural values for average oxygen-metal distances are within 0.01 Å compared to experimental data, and also within the experimental error. The calculated ΔG Sol0 are in very good agreement with experimental reported values, with deviations at CN = 9 lower than 1% for both Th(IV) and Np(IV). The hydrolysis constants are also in very good agreement with experimental values. Finally, this [corrected] methodology has the advantage of using a GGA functional (BP86) that not only makes the calculations more affordable computationally than hybrid functional or ab initio molecular dynamics simulations (Car-Parrinello) calculations, but also opens the perspective to use resolution of identity (RI) calculations for more extended systems.

Europium doped zinc sulfide: a correlation between experimental and theoretical calculations.

This paper presents the correlation among electronic and optical property effects induced by the addition of different concentrations of europium (Eu3+) in zinc sulfide (ZnS) by microwave-assisted solvothermal (MAS) method. A shift of the photoluminescence (PL) emission was observed with the increase of Eu3+. The periodic DFT calculations with the B3LYP hybrid functional were performed using the CRYSTAL computer code. The UV-vis spectra and theoretical results indicate a decrease in behavior of the energy gap as a function of dopant concentration. Therefore, new localized states are generated in the forbidden band gap region, the new states increase the probability of less energy transitions which may be responsible for a red shift in the PL bands spectrum.

Preparation of TiO2/SnO2 thin films by sol-gel method and periodic B3LYP simulations.

Titanium dioxide (TiO2) thin films are grown by the sol-gel dip-coating technique, in conjunction with SnO2 in the form of a heterostructure. It was found that the crystalline structure of the most internal layer (TiO2) depends on the thermal annealing temperature and the substrate type. Films deposited on glass substrate submitted to thermal annealing until 550 °C present anatase structure, whereas films deposited on quartz substrate transform to rutile structure at much higher temperatures, close to 1000 °C, unlike powder samples where the phase transition takes place at about 780 °C. When structured as rutile, the oxide semiconductors TiO2/SnO2 have very close lattice parameters, making the heterostructure assembling easier. The SnO2 and TiO2 have their electronic properties evaluated by first-principles calculations by means of DFT/B3LYP. Taking into account the calculated band structure diagram of these materials, the TiO2/SnO2 heterostructure is qualitatively investigated and proposed to increase the detection efficiency as gas sensors. This efficiency can be further improved by doping the SnO2 layer with Sb atoms. This assembly may be also useful in photoelectrocatalysis processes.

Photocatalytic activity of semiconductor sulfide heterostructures.

This paper reports a theoretical and experimental study of the heterostructure photocatalytic activity in a CdS or ZnS and CdS@ZnS decorated system prepared by a microwave assisted solvothermal (MAS) method. A theoretical model of the decorated system was created in order to analyze the electronic transition mainly in their interface. The results show that CdS and ZnS interfaces produce an electron charge transfer from the CdS electron-populated clusters to the ZnS hole-populated clusters which helps to enhance the photocatalytic activity of the CdS@ZnS decorated system.

DFT study with inclusion of the Grimme potential on anatase TiO2: structure, electronic, and vibrational analyses.

Periodic DFT/B3LYP calculations for TiO(2) anatase bulk were carried out including semiempirical dispersive interactions (DFT-D2) to pure Khon-Sham DFT energy. From this standard methodology, van der Waals (vdW) radii were optimized to 1.4214 Å for titanium and 1.3420 Å for oxygen centers in order to achieve a better structural description. The results from this approach (here named DFT/B3LYP-D*) showed a better description for lattice constants, vibrational IR and Raman, energy band gaps, and bulk modulus than default DFT/B3LYP and DFT/B3LYP-D. The dispersion correction showed more reliable results and was necessary to achieve a good agreement with reported single crystal results, without new formalism or additional computational cost.

Different origins of green-light photoluminescence emission in structurally ordered and disordered powders of calcium molybdate.

A strong greenish-light photoluminescence (PL) emission was measured at room temperature for disordered and ordered powders of CaMoO4 prepared by the polymeric precursor method. The structural evolution from disordered to ordered powders was accompanied by XRD, Raman spectroscopy, and TEM imagery. High-level quantum mechanical calculations in the density functional framework were used to interpret the formation of the structural defects of disorder powders in terms of band diagram and density of states. Complex cluster vacancies [MoO3 x V(O(z))] and [CaO7 x V(O(z))] (where V(O(z)) = V(O(X)), V(O(*)), V(O(**))) were suggested to be responsible to the appearance of new states shallow and deeply inserted in the band gap. These defects give rise to the PL in disordered powders. The natural PL emission of ordered CaMoO4 was attributed to an intrinsic slight distortion of the [MoO4] tetrahedral in the short range.