Valence tautomers are electronically labile compounds that can switch between two distinct energy states with two different electronic distributions, which modifies the physical properties of the compound. This interconversion is triggered by external stimuli, such as temperature increase, pressure, application of an electric field, light incidence, and others [1,2]. The present study aims to investigate valence tautomerism (VT) characteristics in trans pyridine-solvated Co(dioxolene)2(Py)2 complexes, where dioxolene represents 3,5-di-tert-butyl semiquinone and 3,5- di-tert-butyl catecholate, and Py stands for pyridine. Each dioxolene form is associated with a specific energy state: 3,5-di-tert-butyl catecholate corresponds to the low-spin (LS) state, while 3,5-di-tert-butyl semiquinone is in the high-spin (HS) state, both of which are stable in given temperatures. In this compound, the interconversion is directly linked to a change in cobalt's oxidation state from LS-Co(III) to HS-Co(II), allowing the spin state to be identified via single-crystal X-ray diffraction (SCXRD) by analyzing cobalt's bond lengths. However, the chemical environment appears to be crucial for the occurrence of valence tautomerism in solvated Co(dioxolene)2(Py)2 crystals [3]. Different complex/solvent crystal ratios (1:0, 2:1, and 1:2) were analyzed using SCXRD across a wide temperature range (100-300 K) to assess the influence of crystal packing and solvation on VT interconversion. Among these, only the 2:1 complex/solvent ratio (Co(dioxolene)2(Py)2⋅0.5Py) exhibited VT , while the other two remained in the low-spin state throughout the entire investigated temperature range. Our findings demonstrate that the torsion angle (θ in Figure 1) between pyridine planes and dioxolene centroids in Co(dioxolene)2(Py)2⋅0.5Py crystal directly correlates with the LS-Co(III) ⇔ HS-Co(II) VT interconversion. The HS-Co(II) state can be observed when θ exceeds ∼17.5°. A search of the CCDC database revealed that this pattern extends to all compounds structurally similar to Co(dioxolene)2(Py)2, suggesting a direct correlation between molecular torsion and spin state. In the 2:1 complex/solvent ratio, only half of the molecules in the unit cell undergo the phase transition. This partial conversion appears to result from π-stacking interactions between the solvent and pyridine ligands, which restrict pyridine rotation and consequently inhibit the transition. To characterize the valence tautomerism properties of these compounds, we also performed DFT calculations using the structures obtained from SCXRD measurements at different temperatures as starting points using the SIESTA software package (version 4.1.5) [4]. Electronic structure calculations have become increasingly important for characterizing and even predicting the occurrence of valence tautomerism [5]. The experimental magnetic susceptibility curve of Co(dioxolene)2(Py)2⋅0.5Py served as a reference to validate and select the appropriate functional for the calculations. Among the tested functionals, the Local Density Approximation (LDA) [6] functional proved most suitable, as it generally reproduced the main features of the experimental magnetic susceptibility curve. After selecting the functional, we performed atomic position optimizations for different temperature values. For starting points below the transition temperature, we observed a significant reduction in both bond lengths and torsion angles, with both parameters converging to values characteristic of the LS state. In contrast, calculations using starting points above the transition temperature maintained bond lengths consistent with the HS state, while the pyridine plane torsion angles remained above the critical value (∼17.5°). These results reinforce the existence of a correlation between torsion of Pyridine’s plans and spin state of these molecules and suggests a need of a closer look at the role of the so called innocent ligands in the VT observation.

In this work, we report anab initiostudy of the structural and thermodynamic properties of two-dimensional transition-metal dichalcogenides (2D-TMDC) alloys, Mo(1-x)Wx(S, Se, Te)2, using the cluster expansion framework to compute the Helmholtz free energy of alloys as a function of alloy composition and temperature, in the framework of the generalized quasi-chemical approximation. We consider alloying only on the metal sublayer. Our results indicate a weak dependence of the structural properties (lattice constants, nearest-neighbor bond lengths, and layer width) on the alloy composition (i.e. concentrations of W and Mo atoms), in line with the very similar values of the atomic radii of Mo and W atoms. A stronger dependence on the chalcogen is obtained, a trend that reflects the larger variations in atomic radii among the three chalcogen species. As a function of composition, the structural parameters we examined show similar trends, with negligible bowing (i.e. deviations from a Vegard's law interpolation between end compounds), for the three alloys. Moreover, already at 300 K the behavior of these structural features as a function of composition is very similar to that of the standard-regular-solution (SRS) high-temperature limit. In contrast, the electronic band gaps of the the three alloys as a function of composition show small but significant bowing, as high as -1% to -2% near thex= 0.5 alloy composition. Similarly to the structural features, the band gaps attain the high-temperature SRS limit already at 300 K. Regarding thermodynamic properties, we obtain negative values of the internal energy of mixing for the three alloys over the full range of compositions. Therefore, the theoretical alloying phase diagram for the three alloys is featureless, with stability of a fully-mixed alloy at all temperatures and compositions, with no miscibility gap (hence no bimodal nor spinodal decomposition lines). The thermodynamic potentials (mixing internal energy, mixing entropy, and mixing free energy) reach the high-temperature limit at ∼1000 K, the temperature range of synthesis of 2D-TMDC alloys. These trends of structural and electronic properties of the 2D-TMDC alloys are due to the very similar atomic radii and the nearly identical coordination chemistry of Mo and W. Our results are in agreement with experimental work on the alloying of Mo and W atoms, for samples of Mo(1-x)WxS2monolayer alloys, that found that the random mixed alloy is the thermodynamically stable state for this alloy, with no segregation or phase separation.

The development and investigation of smart materials, which present bistability when exposed to external stimuli is a key challenge to material physics and chemistry.Among the various types of these materials, the valence tautomers are compounds which switch between different electronic and spin states and can be used as sensors, signal processors and memory storage [1] since their solid structure does not present substantial rupture during the valence tautomerism (VT) interconversion.The VT has been studied in molecules with a cobalt metal center, nitrogen based ancillary ligands and semiquinone radicals [2-3], and it was observed that it is modulated by the ancillary ligand.For these cobalt complexes, the VT takes place in a reversible fashion [4], in both liquid state and solid state, as single crystals, being possibly dependent on the solid-state arrangement of the complexes and on solvation [5][6].The VT in such molecules can be induced by temperature as first and second order transitions with a wide range of characteristic T1/2 according to the ancillary ligand.In the low temperature regime, the VT is also shown to be induced with photo irradiation in multiple wavelengths.Interestingly, it can also be induced with soft and hard X-rays irradiation with high yield of metastable isomers [7-8].Among the cobalt complexes that display VT, the cobalt 3,5-di-tert-butyl semiquinone pyridine complex is a particularly interesting tautomer, because not only its valence tautomerism can be thermo and photo-induced, but also turned on or off by the presence of solvent molecules in the crystal lattice [5].It can be crystallized in two different forms, with and without a solvent molecule in the crystal lattice.The first shows no temperature dependence of its magnetic susceptibility, and in the second, the same dependence indicates that only half of the cobalt centers in the unit cell present VT, which we confirmed in X-ray diffraction (XRD) experiments.This, along with results of density functional theory (DFT) calculations, raised an interesting possibility of studying the behavior of particular sites of the crystal separately, utilizing X-ray energies around the cobalt K-edge to understand how each particular site responds to the temperature and how the total VT interconversion takes place within the crystal lattice.In our work we combine the site selectivity of XRD and the characteristic resonant X-ray absorption by cobalt atoms in different oxidation states, in order to spatially map the thermoinduced valence tautomerism within the crystal, and also within the cobalt complexes.

In this work, we explain the origin and the mechanism responsible for the strong enhancement of the Raman signal of sulfur chains encapsulated by single-wall carbon nanotubes by running resonance Raman measurements in a wide range of excitation energies for two nanotube samples with different diameter distributions. The Raman signal associated with the vibrational modes of the sulfur chain is observed when it is confined by small-diameter metallic nanotubes. Moreover, a strong enhancement of the Raman signal is observed for excitation energies corresponding to the formation of excited nanotube-chain-hybrid electronic states. Our hypothesis was further tested by high pressure Raman measurements and confirmed by density functional theory calculations of the electronic density of states of hybrid systems formed by sulfur chains encapsulated by different types of single-wall carbon nanotubes.

We report the results of a survey applied to students and professionals in the area of physics in Brazil, pursuing to draw a portrait of the composition of this community in terms of the social markers of difference age, race, ethnicity, geographical origin, sex, gender, sexual orientation, and disabilities. The main goal was to quantify the representativeness of different groups in the community and to detect motivations and difficulties encountered by each group throughout their studies and career. This survey was open to the members of the Brazilian Physics Society from July to September 2018. Our outcomes reveal that (i) the Brazilian physicists community is poorly diverse even in comparison with the population composition, (ii) the main obstacle to pursue the career is socioeconomic vulnerability, and (iii) harassment is high in our society, being more pronounced among women. We hope that these results will be useful to scientific and educational institutions to develop different strategies and policies to change this current situation.

Root growth is reduced in soils with low pH [H+] and abundant soluble aluminum [Al3+], which can be a consequence of the interaction between Al3+ and cell wall composition. The competition between Al3+ and Ca2+ toward binding to pectin molecules was evaluated in roots of Urochloa decumbens, an African grass highly adapted to acidic Al-rich soils. Variations in the composition and distribution of pectins can change the extensibility, rigidity, porosity, and adhesive properties of plant cell walls, which were tested in seedlings of U. decumbens exposed to pH 3.5, 4.5 and 5.8 and to 0, 80, 160 and 320 μM of Al3+ for 80h. Root growth corroborated that U. decumbens is very tolerant to soil acidity, with effective reduction of root growth only at pH 3.5. Immunocytochemical approaches demonstrated variations in pectin composition induced both by Al3+ and by H+ in root tissues and zones. Based on the usual linkage between Ca2+ and pectins, Density Functional Theory (DFT) analyses indicated that Al3+ bound easier to pectins than Ca2+ did, leading to the formation of more Al3+-pectate complexes than Ca2+-pectate complexes, which resulted in higher rigidity of cell walls, and hampered cell extension.

Jacutingaite (Pt2HgSe3) is a recently discovered layered platinum-group mineral. Recent experimental studies have shown that it displays the properties of a quantum spin Hall insulator (QSHI), and theoretical studies indicate that its two-dimensional monolayer is a QSHI with a robust topological gap of similar to 0.5 eV. Jacutingaite is thus promising for potential applications to nanoelectronics and spintronics. The Raman spectrum of three-dimensional bulk jacutingaite and the symmetries of its vibrational modes, fundamental for understanding structural modifications of this material, are still unexplored. Here, we address the zone-center Raman optical phonons of bulk jacutingaite by experiments, symmetry, and first-principles calculations. The improved synthesis used here provided crystals of higher purity and of micrometer size, allowing the study of single crystals. Polarized Raman spectroscopy was used to assign the symmetries of nine out of the 11 Raman-active modes expected by group theory and their respective selection rules. The calculated wavenumbers of the Raman-active modes, in addition to their atomic displacements, are in very good agreement with experiments. In addition, we discuss the use of different exchange correlation functionals within density functional theory, as local functionals and nonlocal functionals that best describe van der Waals interactions. The influence of the inclusion of spin-orbit coupling on calculated vibrational phonon wavenumbers and lattice parameters is commented, and it was found that the local density approximation provides a good description. Our results are of paramount importance to further exploitation of the effects of jacutingaite's structural modifications to tune its properties, as well as for its structural, optical, electronic, mechanical, and thermal applications.

Abasic sites (AP site) in a DNA duplex have been experimentally used to produce fluorescent Ag nanoclusters (NC) with a small number of atoms (n ≤ 6). These AP-DNA:NC complexes act as biological makers that help to locate genes associated with diseases related to single nucleotide polymorphisms (SNP), for example. Abasic sites are the most common SNP genetic variation, and their detection may help predict a host of genetically determined diseases. In this work, we report a theoretical study of the optical absorption spectra of AP-DNA:Ag4 and AP-DNA:Au4 complexes using a fully ab initio methodology. We consider several different base environments for the noble-metal nanocluster occupying the AP site, and compute the absorption spectra of sixteen AP-DNA:Ag4 and sixteen AP-DNA:Au4 complexes. We find that optical absorption in the AP-DNA:Ag4 complexes tends to concentrate in the green-to-violet range of frequencies (2.50 eV ≤ ħω ≤ 3.2 eV) and that AP-DNA:Au4 complexes display absorption peaks in the violet-to-ultraviolet interval (ħω ≥ 3.0 eV). An analysis of the optical absorption mechanisms in these complexes shows that they can be of local, charge-transfer, or hybrid nature, i.e., AP-DNA:NC complexes display the full variety of optical absorption processes in molecular systems. In particular, we identify both charge-transfer and hybrid processes involving several DNA bases surrounding the NC. Importantly, we find that even sequences where the Ag4 cluster is not in a guanine rich neighborhood display absorption peaks in the visible-light spectrum. Moreover, we obtain that the maximum intensities of the absorption peaks in complexes with pyrimidine vacancies are generally higher than those in complexes with purine vacancies. Regarding the selectivity of single-vacancy AP-DNA to specific noble-metal nanocluster sizes, our calculations show that the four-atom Ag4 (Au4) species fits naturally and binds into the AP-site in a single-vacancy AP-DNA.

The combined effects of defect-defect interaction and strains of up to 10% on the onset of magnetic states in the quasi-one-dimensional electronic states generated by the so-called 558 linear defect in graphene monolayers are investigated by means of ab initio calculations. Results are analyzed on the basis of the heuristics of the Stoner criterion. We find that conditions for the emergence of magnetic states on the 558 defect can be tuned by uniaxial tensile parallel strains (along the defect direction) as well as by uniaxial compressive perpendicular strains, at both limits of isolated and interacting 558 defects. Parallel tensile strains and perpendicular compressive strains are shown to give rise to two cooperative effects that favor the emergence of itinerant magnetism on the 558 defect in graphene: enhancement of the density of states (DOS) of the resonant defect states in the region of the Fermi level and tuning of the Fermi level to the maximum of the related DOS peak. On the other hand, parallel compressive strains and perpendicular tensile strains are shown to be detrimental to the development of magnetic states in the 558 defect, because in these cases the Fermi level is found to shift away from the maximum of the DOS of the defect states. Effects of isotropic and unisotropic biaxial strains are also analyzed in terms of the conditions encoded in the Stoner criterion.

Optical properties of DNA:Ag-nanoclusters complexes have been successfully applied experimentally in Chemistry, Physics, and Biology. Nevertheless, the mechanisms behind their optical activity remain unresolved. In this work, we present a time-dependent density functional study of optical absorption in DNA:Ag_{4}. In all 23 different complexes investigated, we obtain new absorption peaks in the visible region that are not found in either the isolated Ag_{4} or isolated DNA base pairs. Absorption from red to green are predominantly of charge-transfer character, from the Ag_{4} to the DNA fragment, while absorption in the blue-violet range are mostly associated to electronic transitions of a mixed character, involving either DNA-Ag_{4} hybrid orbitals or intracluster orbitals. We also investigate the role of exchange-correlation functionals in the calculated optical spectra. Significant differences are observed between the calculations using the PBE functional (without exact exchange) and the CAM-B3LYP functional (which partly includes exact exchange). Specifically, we observe a tendency of charge-transfer excitations to involve purines bases, and the PBE spectra error is more pronounced in the complexes where the Ag cluster is bound to the purines. Finally, our results also highlight the importance of adding both the complementary base pair and the sugar-phosphate backbone in order to properly characterize the absorption spectrum of DNA:Ag complexes.

We present a tight-binding model that allows a quantitative fitting of the ab initio band structure, near the Fermi energy, of several 2D metal-organic and covalent-organic honeycomb-like frameworks. The model is based on the kagome-honeycomb lattice, defined as the superposition of the named lattices. The full spectrum of the model Hamiltonian is analytically solvable in the case of one orbital per site and nearest-neighbor hopping, and at selected points of the Brillouin zone for hopping up to second neighbors. With proper choices of parameters and band occupation, the model describes five types of electronic structure within this class of materials. All five types are obtained in explicit fittings of the model to first-principles calculations of 2D frameworks. The model also permits the identification of crystallographic point group broken symmetries that lead to band gap openings in Cu-3(HITP)(2). Spin orbit effects are also investigated in model and first-principles calculations of Ni3C12S12.

We investigate the electronic structure and lattice stability of pristine and functionalized (with either hydrogen or oxygen) $\alpha$-graphyne systems. We identify lattice instabilities due to soft-phonon modes, and describe two mechanisms leading to gap opening in the Dirac-fermion electronic spectrum of these systems: symmetry breaking, connected with the lattice instabilities, and partial incorporation of an $sp^3$-hybrid character in the covalent-bonding network of a buckled hydrogenated $\alpha$-graphyne lattice that retains the symmetries of the parent pristine $\alpha$-graphyne. In the case of an oxygen-functionalized $\alpha$-graphyne structure, each O atom binds asymmetrically to two twofold-coordinated C atoms, breaking inversion and mirror symmetries, and leading to the opening of a sizeable gap of 0.22 eV at the Dirac point. Generally, mirror symmetries are found to suffice for the occurrence of gapless Dirac cones in these $\alpha$-graphyne systems, even in the absence of inversion symmetry centers. Moreover, we analyze the gapless and gapped Dirac cones of pristine and functionalized $\alpha$-graphynes from the perspective of the dispersion relations for massless and massive free Dirac fermions. We find that mirror-symmetry breaking mimics a Dirac-fermion mass-generation mechanism in the oxygen-functionalized $\alpha$-graphyne, leading to gap opening and to isotropic electronic dispersions with a rather small electron-hole asymmetry. In the hydrogen-functionalized case, we find that carriers show a remarkable anisotropy, behaving as massless fermions along the M-K line in the Brillouin zone and as massive fermions along the $\Gamma$-K line.

In this work we have synthetized and characterized by X-ray diffraction five cobalt complexes with 6-thioguanine (6-ThioGH), 6-thioguanosine (6-ThioGuoH), or 2'-deoxy-6-thioguanosine (2'-d-6-ThioGuoH) ligands. In all cases, these ligands coordinate to cobalt via N7 and S6 forming a chelate ring. However, independently of reagents ratio, 6-ThioGH provided monodimensional cobalt(II) coordination polymers, in which the 6-ThioG(-) acts as bridging ligand. However, for 2'-d-6-ThioGuoH and 6-TliioGuoH, the structure directing effect of the sugar residue gives rise to mononuclear cobalt complexes which form extensive H-bond interactions to generate 3D supramolecular networks. Furthermore, with 2'-d-6-ThioGuoH the cobalt ion remains in the divalent state, whereas with 6-ThioGuoH oxidation occurs and Co(III) is found. The electrical and magnetic properties of the coordination polymers isolated have been studied and the results discussed with the aid of DFT calculations, in the context of molecular wires.

We perform ab initio calculations that indicate that the relative stability of antiphase boundaries (APB) with armchair and zigzag chiralities in monolayer boron nitride (BN) is determined by the chemical potentials of the boron and nitrogen species in the synthesis process. In an N-rich environment, a zigzag APB with N-rich core is the most stable structure, while under B-rich or intrinsic growth conditions, an armchair APB with stoichiometric core is the most stable. This stability transition is shown to arise from a competition between homopolar-bond (B-B and N-N) and elastic energy costs in the core of the APBs. Moreover, in the presence of a carbon source we find that a carbon-doped zigzag APB becomes the most stable boundary near the N-rich limit. The electronic structure of the two types of APBs in BN is shown to be particularly distinct, with the zigzag APB depicting defect-like deep electronic bands in the band gap, while the armchair APB shows bulk-like shallow electronic bands.

Kelvin probe force microscopy images show that the surface potential of graphite changes with time as the contamination covers its surface. Using mass spectrometry we identify the molecular mass of the contaminants to be compatible with that of tetracene, a polycyclic aromatic hydrocarbon (PAH), and its isomers. A combination of desorption and Kelvin probe force microscopy experiments plus theoretical calculations confirms that these molecules are the main contaminant for graphitic surfaces in air ambient conditions. Interestingly, when the sample temperature is increased above ∼50°C the molecules are desorbed and the surface potential becomes fairly homogeneous, suggesting that graphitic surfaces should be almost atomically clean above this temperature. PAHs are potent atmospheric pollutants, potentially carcinogenic, that consist of fused aromatic rings. Incomplete combustion of organic materials can increase the concentration of PAHs in the atmosphere, which in urban regions is enough to totally cover the surface of graphite in a time period that varies from minutes to a few hours. One of the consequences of the adsorption of molecules on graphene is the doping of its surface and the variation of the charge neutrality point originated by the charge transfer between graphene and the contamination layer.

The reaction between 1,3,5-tris(4-hydroxyphenyl)benzene and benzene-1,3,5-tricarbonyl trichloride leads to polyester condensation and formation of a novel COF on an Au(111) surface. The characterization performed in situ by means of variable temperature STM and XPS reveals the formation of an array of hexagonal cavities with ca. 2 nm size.

Ab initio calculations indicate that while the electronic states introduced by grain boundaries in graphene are only partially confined to the defect core, a domain boundary introduces states near the Fermi level that are very strongly confined to the core of the defect, and that display a ferromagnetic ground state. The domain boundary is fully immersed within the graphene matrix, hence this magnetic state is protected from reconstruction effects that have hampered experimental detection in the case of ribbon edge states. Furthermore, our calculations suggest that charge transfer between one-dimensional extended defects and the bulk in graphene is short ranged for both grain and domain boundaries.