An accurate total energy calculation is essential in materials computation. To date, many tight-binding (TB) approaches based on parameterized hopping can produce electronic structures comparable to those obtained using first-principles calculations. However, TB approaches still have limited applicability for determining material properties derived from the total energy. That is, the predictive power of the TB total energy is impaired by an inaccurate evaluation of the repulsive energy. The complexity associated with the parametrization of TB repulsive potentials is the weak link in this evaluation. In this study, we propose a new method for obtaining the pairwise TB repulsive potential for crystalline materials by employing the Chen-Möbius inversion theorem. We show that the TB-based phonon dispersions, calculated using the resulting repulsive potential, compare well with those obtained by first-principles calculations for various systems, including covalent and ionic bulk materials and two-dimensional materials. The present approach only requires the first-principles total energy and TB electronic band energy as input and does not involve any parameters. This striking feature enables us to generate repulsive potentials programmatically.

We investigate the adsorption of molecular hydrogen on pristine zinc oxide (ZnO) platelets. The volumetric and gravimetric hydrogen storage capacities of the ZnO monolayers are evaluated in a broad range of thermodynamic conditions (i.e., for temperatures in the range 77 K < T < 450 K, and for external gas pressures up to 200 bar). The thermodynamic properties and the microscopic spatial distribution of the adsorbed hydrogen fluid are assessed within the density functional theory of liquids for quantum fluids at finite temperature (QLDFT), and the adsorption enthalphies are obtained by fitting the computed adsorption densities to the Toth model isotherm. Compared to graphene platelets, the ZnO sheets impose a rather tighter confinement to the motion of the hydrogen molecules parallel to the surface. The isosteric heat of adsorption approaches 3.2 kJ/mol in the low density regime. This quantity shows a fairly smooth dependence on the hydrogen uptake for temperatures below 100 K, while it is shown to depend quite sensitively on the adsorbate density above this temperature.

We elucidate the flexoelectricity of materials in the high strain gradient regime, of which the underlying mechanism is less understood. By using the generalized Bloch theorem, we uncover a strong flexoelectric-like effect in bent thinfilms of Si and Ge due to a high strain gradient-induced insulator-to-metal transition. We show that an unusual type-II band alignment is formed between the compressed and elongated sides of the bent film, resulting in a spatial separation of electron and hole. Therefore, upon the insulator-to-metal transition, electrons transfer from the compressed side to the elongated side to reach the thermodynamic equilibrium, leading to pronounced polarization along the film thickness dimension. The obtained transverse flexoelectric coefficients are unexpectedly high, with a quadratic dependence on the film thickness. This new mechanism is extendable to other semiconductor materials with moderate energy gaps. Our findings have important implications for the future applications of flexoelectricity in semiconductor materials.

The pertechnetate ion TcVIIO4 - is a nuclear fission product whose major issue is the high mobility in the environment. Experimentally, it is well known that Fe3O4 can reduce TcVIIO4 - to TcIV species and retain such products quickly and completely, but the exact nature of the redox process and products is not completely understood. Therefore, we investigated the chemistry of TcVIIO4 - and TcIV species at the Fe3O4(001) surface through a hybrid DFT functional (HSE06) method. We studied a possible initiation step of the TcVII reduction process. The interaction of the TcVIIO4 - ion with the magnetite surface leads to the formation of a reduced TcVI species without any change in the Tc coordination sphere through an electron transfer that is favored by the magnetite surfaces with a higher FeII content. Furthermore, we explored various model structures for the immobilized TcIV final products. TcIV can be incorporated into a subsurface octahedral site or adsorbed on the surface in the form of TcIVO2·xH2O chains. We propose and discuss three model structures for the adsorbed TcIVO2·2H2O chains in terms of relative energies and simulated EXAFS spectra. Our results suggest that the periodicity of the Fe3O4(001) surface matches that of the TcO2·2H2O chains. The EXAFS analysis suggests that, in experiments, TcO2·xH2O chains were probably not formed as an inner-shell adsorption complex with the Fe3O4(001) surface.

We elucidate the flexoelectricity of semiconductors in the high strain gradient regime, the underlying mechanism of which is less understood. By using the generalized Bloch theorem, we uncover a strong flexoelectric-like effect in bent thinfilms of Si and Ge due to a high-strain-gradient-induced band gap closure. We show that an unusual type-II band alignment is formed between the compressed and elongated sides of the bent film. Therefore, upon the band gap closure, electrons transfer from the compressed side to the elongated side to reach the thermodynamic equilibrium, leading to a pronounced change of polarization along the film thickness dimension. The obtained transverse flexoelectric coefficients are unexpectedly high with a quadratic dependence on the film thickness. This new mechanism is extendable to other semiconductor materials with moderate energy gaps. Our findings have important implications for the future applications of flexoelectricity in semiconductor materials.

Platelets and ribbons of monolayer molybdenum disulfide () exhibit characteristic electronic states along the edges, which tend to decouple from the extended states in the interior of the 2D extended structure. Under sulfur excess specifically triangular platelets are formed, which exhibit 1D extended electronic states with a pronounced catalytic activity along the edges. The present density‐functional‐based investigation shows that very similar electronic states occur also along the edges of holes inside an otherwise perfect, extended monolayer. For triangular defects, the edge states can delocalize around the hole. Density functional molecular dynamics simulations on 150 different nanoscale defect geometries and termination variants prove the high structural stability of hole defects in against further reconstructions and a thermodynamic propensity to participate in sulfur exchange reactions.

More than 150 different structural defects in periodic molybdenum disulfide (MoS2) monolayers were investigated systematically for the present study by density-functional calculations. All defects were chosen to be triangular, but of different size and distance from one another. Molecular-dynamics simulations proved the defect stability against temperature effects. The energetic properties of the defective MoS2 monolayers-especially the density of states-can be viewed as a combination of edge and bulk properties with the defect size being of dominating influence. A dependence on the defect-edge termination was present only in the very smallest defects. The definition of a localization parameter helps to characterize the grade of localization of the states. Finally, a strong influence of the defect distance on the electronic properties could not be observed, except for very small distances.

Fewest-switches surface hopping (FSSH) is employed in order to investigate the nonadiabatic excited-state dynamics of thiophene and related compounds and hence to establish a connection between the electronic system, the critical points in configuration space and the deactivation dynamics. The potential-energy surfaces of the studied molecules were calculated with complete active space self-consistent field and time-dependent density-functional theory. They are analyzed thoroughly to locate and optimize minimum-energy conical intersections, which are essential to the dynamics of the system. The influence of decoherence on the dynamics is examined by employing different decoherence schemes. We find that irrespective of the employed decoherence algorithm, the population dynamics of thiophene give results which are sound with the expectations grounded on the analysis of the potential-energy surface. A more detailed look at single trajectories as well as on the excited-state lifetimes, however, reveals a substantial dependence on how decoherence is accounted for. In order to connect these findings, we describe how ensemble averaging cures some of the overcoherence problems of uncorrected FSSH. Eventually, we identify carbon–sulfur bond cleavage as a common feature accompanying electronic transitions between different states in the simulations of all thiophene-related compounds studied in this work, which is of interest due to their relevance in organic photovoltaics.

Light-weight materials such as graphene have attracted a great interest for hydrogen storage applications. In this study, a metal-on graphene nanocomposite was synthesized for hydrogen storage for on-board applications. The graphene used as support was obtained from precursor graphite oxide, which was synthesized using the Improved Tours method and later reduced by L-Ascorbic acid and ammonia. The precursor graphite flakes used in the synthesis of graphite oxide were 100 mu m in size. The XRD patterns depicted a low angle shift in GO, 11.3 degrees from graphite 30 degrees proved successful oxidation of graphite. The FTIR and Raman band showed L-Ascorbic reduced the GO better than ammonia through the observed decrease on the functional bands after reduction. The elemental analysis carried out using XRF showed the successful loading of Ca on the graphene matrix and TEM images also confirmed this. The hydrogen storage capacity of the nanocomposite was tested using TGA and TPD equipped with Mass Spectrometer. The Ca/graphene nanocomposite reduced by ammonia exhibited a large hydrogen storage uptake at 4.98 wt. % compared to L-Ascorbic acid.

We present a realistic phenomenological description of liquid transport through defective, layered membranes. We derive general expressions based on conventional models of laminar flow and extend the formalism to accommodate slip flow. We consider different types of defects including in-layer vacancies that provide an activation-free tortuous path through the membrane. Of the many factors that affect flow, the most important is the radius of in-layer vacancy defects, which enters in the fourth power in expressions for the flux density. We apply our formalism to water transport through defective multilayer graphene oxide membranes and find that the flow remains in the laminar regime. Our results show that observed high water permeability in this system can be explained quantitatively by a sufficient density of in-layer pores that shorten the effective diffusion path.

In this work, we present a quantum mechanical investigation, based on the self-consistent charge density functional tight-binding (SCC-DFTB) method, of the functionalization with silane-type ligands (TETT) of a spherical TiO2 nanoparticle of realistic size (2.2 nm containing 700 atoms) to create an efficient nanosystem for simultaneous photodynamic therapy and drug transport. We determine the mechanism of the TETT ligand anchoring and its stability under thermal treatment, through molecular dynamics simulations at 300 K. Then, we build a medium and a full coverage model (22 and 40 TETTs, respectively) and analyze the interaction among TETT ligands and between the ligands and the surface. Finally, on the fully covered nanoparticle, we succeed in localizing two minimum energy structures for an attached doxorubicin anticancer molecule (DOX) and provide the atomistic details for both the covalent and the non-covalent (electrostatic) types of interaction. A future development of this work will be the investigation of the loading capacity of this drug delivery system and of the pH effect of the surrounding aqueous environment.

The recent discovery of borophene, a two-dimensional allotrope of boron, raises many questions about its structure and its chemical and physical properties. Boron has a high chemical affinity to oxygen but little is known about the oxidation behaviour of borophene. Here we use first principles calculations to study the phase diagram of free-standing, two-dimensional B 1− x O x for compositions ranging from x = 0 to x = 0.6, which correspond to borophene and B 2 O 3 sheets, respectively. Our results indicate that no stable compounds except borophene and B 2 O 3 sheets exist. Intermediate compositions are heterogeneous mixtures of borophene and B 2 O 3 . Other hypothetical crystals such as B 2 O are unfavorable and some of them underwent spontaneous disproportionation into borophene and B 2 O 3 . It is also shown that oxidizing borophene inside the flakes is thermodynamically unfavorable over forming B 2 O 3 at the edges. All findings can be rationalized by oxygen’s preference of two-fold coordination which is incompatible with higher in-plane coordination numbers preferred by boron. These results agree well with recent experiments and pave the way to better understand the process of oxidation of borophene and other two-dimensional materials.

Realization of half-metallicity (HM) in low dimensional materials is a fundamental challenge for nano spintronics and a critical component for developing alternative generations of information technology. Using first-principles calculations, we reveal an unconventional deformation potential for zigzag nanoribbons (NRs) of 2D-Xenes. Both the conduction band minimum (CBM) and valence band maximum (VBM) of the edge states have negative deformation potentials. This unique property, combined with the localization and spin-polarization of the edge states, enables us to induce spin-splitting and HM using an inhomogeneous strain pattern, such as simple in-plane bending. Indeed, our calculation using the generalized Bloch theorem reveals the predicted HM in bent zigzag silicene NRs. Furthermore, the magnetic stability of the long range magnetic order for the spin-polarized edge states is maintained well against the bending deformation. These aspects indicate that it is a promising approach to realize HM in low dimensions with the zigzag 2D-Xene NRs.

Recently, several theoretical predictions have been made about 2D planar FeC, CoC, NiC, and CuC, while their bulk phases still remain unknown. Here, we present a generalization of the 2D family of 3d transition metal monocarbides (TMC) by searching for their stable configurations with DFT methods and an evolutionary algorithm. It is found that in the TMC row (TM = Sc-Cu) the tendency of 3D rocksalt phase formation is monotonously interchanged with 2D phase appearance, namely, planar orthorhombic TMC characterized by carbon dimers inside metal hexagons. Among them, orthorhombic CoC and FeC monocarbides would likely be formed rather than any other 2D metal carbide phase or metal/graphene interface.

2D soft nanomaterials are an emerging research field due to their versatile chemical structures, easily tunable properties, and broad application potential. In this study, a benzene-bridged polypyrrole film with a large area, up to a few square centimeters, is synthesized through an interfacial polymerization approach. As-prepared semiconductive films exhibit a bandgap of approximate to 2 eV and a carrier mobility of approximate to 1.5 cm(2) V-1 s(-1), inferred from time-resolved terahertz spectroscopy. The samples are employed to fabricate in-plane micro-supercapacitors (MSCs) by laser scribing and exhibit an ultrahigh areal capacitance of 0.95 mF cm(-2), using 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) as an electrolyte. Importantly, the maximum energy and power densities of the developed MSCs reach values up to 50.7 mWh cm(-3) and 9.6 kW cm(-3), respectively; the performance surpassing most of the 2D material-based MSCs is reported to date.

External fields are a powerful tool to probe optical excitations in a material. The linear energy shift of an excitation in a magnetic field is quantified by its effective g factor. Here we show how exciton g factors and their sign can be determined by converged first-principles calculations. We apply the method to monolayer excitons in semiconducting transition metal dichalcogenides and to interlayer excitons in MoSe2/WSe2 heterobilayers and obtain good agreement with recent experimental data. The precision of our method allows us to assign measured g factors of optical peaks to specific transitions in the band structure and also to specific regions of the samples. This revealed the nature of various, previously measured interlayer exciton peaks. We further show that, due to specific optical selection rules, g factors in van der Waals heterostructures are strongly spin and stacking-dependent. The calculation of orbital angular momenta requires the summation over hundreds of bands, indicating that for the considered two-dimensional materials the basis set size is a critical numerical issue. The presented approach can potentially be applied to a wide variety of semiconductors.

The creation of heterogeneous analogs to homogeneous catalysts is of great importance to many industrial processes. Acetic acid synthesis via the carbonylation of methanol is one such process and it relies on a difficult-to-separate homogeneous Ir-based catalyst. Using a combination of density functional theory (DFT) and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, we determine the structure and mechanism for methanol carbonylation over a promising single-site Ir-La/C heterogeneous catalyst replacement. Here, the Ir center is the active site with the acetyl-Ir complex being a rate controlling intermediate. Furthermore, the La both atomically disperses the Ir and acts as a Lewis acid site. In fact, the La promoter in the Ir-La/C catalyst was found to behave similarly to homogeneous promoters by abstracting an iodine from the It center and accelerating the CO insertion step. Overall, this work provides key insight into the atornistic nature of the Ir-La/C single-site catalyst and allows for the further design and optimization of single-site heterogeneous catalysts. (C) 2018 Elsevier Inc. All rights reserved.

We report the experimental observation of radiative recombination from Rydberg excitons in a two-dimensional semiconductor, monolayer WSe2, encapsulated in hexagonal boron nitride. Excitonic emission up to the 4s excited state is directly observed in photoluminescence spectroscopy in an out-of-plane magnetic field up to 31 T. We confirm the progressively larger exciton size for higher energy excited states through diamagnetic shift measurements. This also enables us to estimate the 1s exciton binding energy to be about 170 meV, which is significantly smaller than most previous reports. The Zeeman shift of the 1s to 3s states, from both luminescence and absorption measurements, exhibits a monotonic increase of the g-factor, reflecting nontrivial magnetic-dipole-moment differences between ground and excited exciton states. This systematic evolution of magnetic dipole moments is theoretically explained from the spreading of the Rydberg states in momentum space.

Correction for ‘Synthetic 2-D lead tin sulfide nanosheets with tuneable optoelectronic properties from a potentially scalable reaction pathway’ by Kane Norton et al., Chem. Sci., 2019, 10, 1035–1045.

Magnetite has attracted increasing attention in recent years due to its promising and diverse applications in biomedicine. Theoretical modelling can play an important role in understanding magnetite-based nanomaterials at the atomic scale for a deeper insight into the experimental observations. However, calculations based on density functional theory (DFT) are too costly for realistically large models of magnetite nanoparticles. Classical force field methods are very fast but lack of precision and of the description of electronic effects. Therefore, a cheap and efficient quantum mechanical simulation method with comparable accuracy to DFT is highly desired. Here, a less computationally demanding DFT-based method, i.e., self-consistent charge density functional tight-binding (SCC-DFTB), is adopted to investigate magnetite bulk and low-index (001) surfaces with newly proposed parameters for Fe-O interactions. We report that SCC-DFTB with on-site Coulomb correction provides results in quantitatively comparable agreement with those obtained by DFT + U and hybrid functional methods. Therefore, SCC-DFTB is valued as an efficient and reliable method for the description magnetite. This assessment will promote SCC-DFTB computational studies on magnetite-based nanostructures that attract increasing attention for medical applications.