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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.

In the present paper, we provide a detailed theoretical investigation on fundamental thermodynamic, thermomechanical, and electronic properties of wurtzite ZnS between 0–20 GPa and 0–2000 K, obtained by ab initio density functional theory and the B3LYP functional. Several properties, such as phonon dispersion relations, elastic and piezoelectric constants, and thermodynamic and thermoelastic behaviors were calculated and reported. The analysis of the data via volume-integrated third-order Birch-Murnaghan fitting resulted in K0 = 72.17(4) GPa, K′ = 3.87(1), and V0 = 85.781(1) Å3 at T = 0 K. The Born criteria for the mechanical stability of the mineral phase showed that wurtzite is unstable above about 19 GPa in static conditions. We calculated a direct bandgap for wz-ZnS of 4.86 eV at zero compression, which became an indirect one by increasing pressure above 17 GPa. The results are in good agreement with the experimental and theoretical ones reported in the literature, and further extend the knowledge of an important zinc sulfide phase, for both geological and industrial applications.

The recently described monodimensional hybrid pseudo-perovskite ((CH3)3SO)PbI3 exhibits complete Pb2+/Bi3+ miscibility in the B site. Heterovalent substitution imposes that charge-compensating defects are introduced in the lattice as well. This paper reports the energetics of point defects and the interaction between charge-compensating defects that occur upon Bi3+ doping in ((CH3)3SO)PbI3. Periodic DFT simulations were employed to analyze the formation energy of Pb2+ vacancies, (CH3)3SO+ vacancies, iodide vacancies, or the insertion of interstitial iodide ions. Solid solutions with a large Bi3+ content were modeled considering different charge compensation defects (Pb2+ vacancies, (CH3)3SO+ vacancies, and interstitial iodide) to consider electrical neutrality. Both the formation energy evaluation and the structural parameters after relaxation are in very good agreement with available experimental data (both X-ray diffraction and absorption spectroscopy), confirming that Pb2+ vacancies are indeed the primary ionic charge-compensation mechanism leading to solid solutions described by the formula ((CH3)3SO)3Pb3xBi2(1-x)V(1-x)I9 (0 < x < 1), where V indicates a B site vacancy. As a consequence of the complete miscibility, the defect concentration at intermediate compositions is considerable, and it is probably responsible for various peculiar structural effects observed, e.g., in ((CH3)3SO)3Pb0.99Bi1.34I9, which are related to short-range order phenomena in the chain composition. Atomistic simulations were then carried out to estimate defect-defect interactions as a function of their respective positions inside the linear PbI6/BiI6 octahedral chains. The resulting bond energies for atom pairs and triads effectively explain the relative stability of defect clusters. This approach is expected to shed light on the short-range order in this class of doped materials, but it can be extended to other structures as well. The understanding of defect structures and control of structural and electronic properties have implications for the engineering of doped materials, which find vast applications in optoelectronic devices.

In this study, we present a full characterization of the electronic properties of phase change material (PCM) double-layered heterostructures deposited on silicon substrates. Thin films of amorphous Ge-rich Ge-Sb-Te (GGST) alloys were grown by physical vapor deposition on Sb2Te3 and on Ge2Sb2Te5 layers. The two heterostructures were characterized in situ by X-ray and ultraviolet photoemission spectroscopies (XPS and UPS) during the formation of the interface between the first and the second layer (top GGST film). The evolution of the composition across the heterostructure interface and information on interdiffusion were obtained. We found that, for both cases, the final composition of the GGST layer was close to Ge2SbTe2 (GST212), which is a thermodynamically favorable off-stoichiometry GeSbTe alloy in the Sb-GeTe pseudobinary of the ternary phase diagram. Density functional theory calculations allowed us to calculate the density of states for the valence band of the amorphous phase of GST212, which was in good agreement with the experimental valence bands measured in situ by UPS. The same heterostructures were characterized by X-ray diffraction as a function of the annealing temperature. Differences in the crystallization process are discussed on the basis of the photoemission results.

Classical molecular dynamics simulations are used to investigate the 3D evolution of stacking faults (SFs), including the partial dislocation (PD) loops enclosing them, during growth of 3C-SiC layers on Si(001). It is shown that the evolution of single PD loops releasing tensile strain during the initial carbonization stage, commonly preceding 3C-SiC deposition, leads to the formation of experimentally observed V- or Δ-shaped SFs, the key role being played by the differences in the mobilities between Si- and C-terminated PD segments. Nucleation in the adjacent planes of PD loops takes place at later stage of 3C-SiC deposition, when slightly compressive-strain conditions are present. It is shown that such a process very efficiently decreases the elastic energy of the 3C-SiC crystal. The maximum energy decrease is obtained via the formation of triple SFs with common boundaries made up by PD loops yielding a zero total Burgers vector. Obtained results explain the experimentally observed relative abundance of compact microtwin regions in 3C-SiC layers as compared with the other SF-related defects.

International audience; Molybdenum dithiocarbamates (MoDTCs) are a class of lubricant additives widely employed in automotives. Most of the studies concerning MoDTC take into account the dimeric structures because of their industrial relevance, with the mononuclear compounds usually neglected, because isolating and characterizing subgroups of MoDTC molecules are generally difficult. However, the byproducts of the synthesis of MoDTC can impact the friction reduction performance at metallic interfaces, and the effect of mononuclear MoDTC (mMoDTC) compounds in the lubrication has not been considered yet in the literature. In this study, we consider for the first time the impurities of MoDTC consisting of mononuclear compounds and combine experimental and computational techniques to elucidate the interaction of these impurities with binuclear MoDTC in commercial formulations. We present a preliminary strategy to separate a commercial MoDTC product in chemically different fractions. These fractions present different tribological behaviors depending on the relative amount of mononuclear and binuclear complexes. The calculations indicate that the dissociation mechanism of mMoDTC is similar to the one observed for the dimeric structures. However, the different chemical properties of mMoDTC impact the kinetics for the formation of the beneficial molybdenum disulfide (MoS2) layers, as shown by the tribological experiments. These results help to understand the functionality of MoDTC lubricant additives, providing new insights into the complex synergy between the different chemical structures.

The crossing of stacking faults (SFs) (namely dislocation forests) in 3C-SiC is experimentally probed to cause detrimental effects on the electronic properties of the layer. By means of classical molecular dynamics simulations, for the first time the evolution of two crossing partial dislocation loops is shown, revealing the mechanism that leads to the formation of a complex defect at their intersection. The obtained atomistic structure is then further refined by ab initio calculations, revealing that this defect does not cause defect states within the gap of the pristine material. The case of the intersection of two double-dislocation loops, that is, involving double SFs, in the hypothesis that the evolution follows the same mechanism found for single-dislocation loops is also investigated. Contrary to the single-plane intersection, the simulations reveal that the junction formed by double-dislocation loops does cause intragap states, thus suggesting detrimental effects on the electronic properties of the 3C-SiC layers.

Conversion of CO2 to reduced products is a promising route to alleviate irreversible climate change. Here we report the synthesis of a Co-based phthalocyanine with pyridine moieties (CoPc-Pyr), which is supported on a carbon electrode and shows Faradaic efficiency ∼90% for CO at 490 mV of overpotential (-0.6 V vs reversible hydrogen electrode (RHE)). In addition, its catalytic activity at -0.7 V versus RHE surpasses other Co-based molecular and metal-organic framework catalysts for CO2 reduction at this bias. Density functional theory calculations show that pyridine moieties enhance CO2 adsorption and electron affinity of the Co center by an inductive effect, thus lowering the overpotential necessary for CO2 conversion. Our study shows that CoPc-Pyr reduces CO2 at lower overpotential and with higher activity than noble metal electrodes, such as silver.

Since its discovery, graphene has been the object of study for several and manifold applications, ranging from photocatalysis to mechanics and electronics of materials. However, the use of pure graphene in certain opto- and microelectronic applications is sometimes limited because of its zero band gap. Among the different methods to widen the band gap, in the present work the attention is focused on the so-called van der Waals composites (or heterostructures, or heterojunctions), namely the stacking of two monolayers of different materials that are held together by weak van der Waals interactions. An interesting composite is the graphene/molybdenite (MoS2) heterostructure, where the second material is a bidimensional semiconductor characterized by strong in-plane covalent bonds and weak out-of-plane interactions, which means that it is possible to exfoliate MoS2 into monolayers of atomic thickness. Also, from the crystallographic point of view, both graphene and the monolayer of molybdenite (MoS2-1H) have a 2D hexagonal lattice and their stacking could be structurally favourable. In this work, the electronic band structure and density of states, complex dielectric function and optical properties of the stacked van der Waals bilayer heterojunction graphene/MoS2-1H were calculated and compared to both the single monolayers of graphene and molybdenite, to understand how the interaction between the two materials may alter the above cited properties. The analysis of the band structure in this van der Waals composite clearly showed a small direct band gap related to the transition π → π ∗ (ca. 2.5 meV) in correspondence of the high symmetry point K. The heterojunction showed also, as expected, some important variations in the complex dielectric function and related properties in the visible-light spectral region. The results obtained in the present work could be of use for future development and applications of this kind and similar 2D composite materials with tailored electronic and optical properties.

In this article, we study the modification of the optical, chemical and electronic properties of graphene oxide (GO) during thermal reduction in ultra-high-vacuum by combining the results of several electron spectroscopies. We find that the fraction of oxygen moieties on the surface, as deduced from the evolution of C 1s core level in photoemission, is progressively reduced upon increasing the annealing temperature from 150 to 650 °C. The intensity of the CH stretching mode, associated with CH defects on GO surface and measured in the low energy region of electron energy loss spectra (EELS), decreases as a function of the annealing temperature. The removal or the reduction of such hydrogen or oxygen defects induces a restoration of sp2 carbon hybridization. The presence of such hybridization is confirmed by the capability to excite π -plasmon as observed in the EELS spectra. In particular we find a critical annealing temperature (Tann = 300 °C) at which π -plasmon excitation via electron scattering is accessible suggesting the formation of graphene-like domains with size comparable with the plasmon wavelength ( λ p ~ 5 nm). The linear dispersion of π -band close to Fermi level, as measured in UPS, confirms the formation of graphene-like domains.

Potentiostatic polarization of different stainless steel grades with a mirror surface finishing was performed in order to investigate the role of Mo on stainless steels corrosion behaviour. In the attempt to gain more insight into transpassive dissolution mechanism, passive films were grown in neutral aqueous solution (pH ∼ 7) at different polarizing potentials ranging from passive to transpassive conditions. According to the photoelectrochemical characterization, Cr dissolution occurs even at low polarizing potentials in the case of AISI 304L, while higher band gap values were obtained for passive films grown on austenitic 316L and duplex SSs. Capacitance measurements suggest that the flat band potential values estimated for passive films grown on austenitic 316L and duplex SS are more noble if compared to that of AISI 304L at all polarizing potentials. This is rationalized by considering that passive films on 316L and duplex are doped with Mo during film growth, changing their electronics properties (band gap and flat band potential) with a consequent shift to more positive value of the potential for chromate formation and dissolution.

The purpose of this work is to present a learning path aimed at deepening student understanding of the fundamental concepts underlying the electronic properties of new materials, graphene in particular. To achieve this task, we propose a five-week long workshop where students may be introduced to fundamental concepts of advanced physics, rarely used in learning paths, such as the symmetry properties of the crystal lattice, the group theory , the features of the free electron wave functions and energy levels, the relativistic Dirac equation. Particular emphasis is given to the manner of introducing these concepts, since an essential knowledge of solid state physics, quantum physics and relativity is first necessary. We here present and discuss these concepts as preliminary steps towards a learning sequence that may guide physics/engineering undergraduates to reach a deeper understanding of the physics underlying the complex world of graphene and its properties. The conceptual framework might support both instructors and students toward further scientific investigations.

A few layers of functionalized graphene (FLG) have been obtained by the oxidative unzipping reaction of single-walled carbon nanotubes. A detailed Scanning Tunneling Microscopy and Transmission Electron Microscopy study shows that several different morphologies are observed, depending on the substrate upon which they are deposited, thus providing useful hints for a real world application of these carbon nanostructures. In addition, a combined Scanning Tunneling Spectroscopy investigation has shown that the electronic properties of these FLG are affected by their functionalization degree, opening interesting perspectives for their use in sensor and biosensor devices. © Taylor & Francis Group, LLC.

Understanding and controlling the chemical reactivity of carbon nanotubes (CNTs) is a fundamental requisite to prepare novel nanoscopic structures with practical uses in materials applications. Here, we present a comprehensive microscopic and spectroscopic characterization of carbon nanotubes which have been chemically modified. Specifically, scanning tunneling microscopy ( STM) investigations of short-oxidized single-walled carbon nanotubes (SWNTs) functionalized with aliphatic chains via amide reaction reveal the presence of bright lumps both on the sidewalls and at the tips. The functionalization pattern is consistent with the oxidation reaction which mainly occurs at the nanotube tips. Thermogravimetric analysis (TGA), steady-state electronic absorption (UV-vis-NIR), and Raman spectroscopic studies confirm the STM observations.

The epitaxial growth of graphene on catalytically active metallic surfaces via chemical vapor deposition (CVD) is known to be one of the most reliable routes toward high-quality large-area graphene. This CVD-grown graphene is generally coupled to its metallic support resulting in a modification of its intrinsic properties. Growth on oxides is a promising alternative that might lead to a decoupled graphene layer. Here, we compare graphene on a pure metallic to graphene on an oxidized copper surface in both cases grown by a single step CVD process under similar conditions. Remarkably, the growth on copper oxide, a high-k dielectric material, preserves the intrinsic properties of graphene; it is not doped and a linear dispersion is observed close to the Fermi energy. Density functional theory calculations give additional insight into the reaction processes and help explaining the catalytic activity of the copper oxide surface.

Carbon nanostructures, or nanocarbons (CNSs), i.e. the low-dimensional nanomaterials that comprise 0D fullerenes, 1D nanotubes and 2D graphene, have emerged over the last two decades in the field of new molecular materials for their unique structural and electronic properties, prompting a huge interest worldwide both from the viewpoint of fundamental research and for their application in molecular electronics, materials science, energy storage and conversion, biomedicine, sensing and biosensing. Because of the several and diverse technological applications so far proposed, and of the many more expected in the near future, the present volume is addressed to researchers from several different areas: chemistry and materials chemistry, nanotech- nology, medicine and industrial research.

The understanding of the electronic properties of thin oxide film is an important step toward the understanding of the mechanisms of film dissolution and breakdown as well as for their application in the field of electrolytic capacitors and solar energy conversion. From this point of view the correct location of the characteristic energy levels (flat band potential, Ufb, and conduction (valence) band edge EC (EV)), of a passive film/electrolyte junction is the preliminary task for a deeper understanding of the mechanism of charge transfer at oxide/electrolyte interface. At this aim the most frequently employed method to locate such characteristic energy levels of semiconductor oxide/electrolyte junction is by means of differential capacitance measurements and their representation in the classical Mott-Schottky plots. Yet, as frequently reported in literature, passive films are usually very thin and with a variable degree of lattice disorder (from amorphous to polycrystalline phase) and both these characteristics should invalidate the use of the simple M-S theory (1,2,3). In order to highlight the role of film thickness on the differential admittance behaviour of thin anodic oxide/electrolyte interface we will present, in this work, the results of an extensive study on the electronic properties of amorphous niobia (a-Nb2O5) film anodically grown up to different thickness by anodizing in H2SO4 1 N solution at different formation potentials, namely 5, 10, and 20 V. The results of an electrochemical impedance spectroscopy (EIS) and differential admittance (DA) study in a large range of electrode potentials and ac frequency will be presented and discussed. The experimental results strongly evidence the limits of the traditional approach based on the M-S theory. Moreover a new model is proposed, to take into consideration both the amorphous nature of the film as well the effect of the metal back contact, when the space charge region in the oxide is approaching the metal/oxide interface. The experimental results evidenced that to cope with the simulated theoretical behaviour it is necessary to introduce a field-dependent dielectric constant for the a-Nb2O5 film as previously reported by Ord et al. and attributed to the electrostriction effect (2).

We have measured bulk sensitive photoemission spectra of pure vanadium sesquioxide (V2O3) in its metallic phase as a function of different surface preparations. We observe the presence of a clear coherent peak in the vicinity of the Fermi level and of satellites intensities at the V 2p core level. After scraping, the coherent intensities are almost completely suppressed in both core level and valence band. Our results suggest a remarkable change of the screening properties in strongly correlated systems when going from the surface to the volume.

We have measured bulk sensitive photoemission spectra of pure vanadium sesquioxide (V2O3) in its metallic phase as a function of different surface preparations. We observe the presence of a clear coherent peak in the vicinity of the Fermi level and of satellites intensities at the V 2p core level. After scraping, the coherent intensities are almost completely suppressed in both core level and valence band. Our results suggest a remarkable change of the screening properties in strongly correlated systems when going from the surface to the volume.

We investigated the temperature dependence (10-180 K) of the photoluminescence (Pi,) emission spectrum of self-organized InAs/GaAs quantum dots grown under different conditions. The temperature dependence of the PL intensity is determined by two thermally activated processes: (i) quenching due to the escape of carriers from the quantum dots and (ii) carrier transfer between dots via wetting layer states. The existence of different dot families is confirmed by the deconvolution of the spectra in gaussian components with full width half maxima of 20-30 meV. The transfer of excitation is responsible for the sigmoidal temperature dependence of the peak energies of undeconvoluted PL bands. (C) 1996 American Institute of Physics.

We have performed a comparative study of structural, electronic and optical properties of Ru 2 Si 3 , Ru 2 Ge 3 , Os 2 Si 3 and Os 2 Ge 3 by means of ultrasoft pseudopotential and full-potential linearized augmented plane wave methods. The estimated difference in the cohesion energy between the low-temperature orthorhombic phase and the high temperature tetragonal one for all these compounds indicates that the former phase is lower in energy with respect to the latter one. All materials in the orthorhombic structure are found to be direct band-gap semiconductors, still some of them in the tetragonal structure display an indirect nature (Os 2 Si 3 ) or a competitive direct-indirect character (Ru 2 Ge 3 ) of the gap. Optical properties are discussed by analyzing the imaginary part of the dielectric function and the dipole matrix elements corresponding to different interband transitions indicating for osmium silicide and germanide the presence of low-energy transitions with an appreciable value of the oscillator strength.

In this letter, we discuss the interesting changes in the band structure of two ternary (FeOs)Si-2 configurations with respect to the isostructural binary beta -FeSi2 and OsSi2 compounds. Our first-principle calculations also demonstrate that one of the ternaries is a direct-bandgap semiconductor with an appreciable value of the oscillator strength of the first direct transition at 0.78 eV. (C) 2001 American Institute of Physics

Reflection electron energy loss spectroscopy and x-ray photoemission spectroscopy results obtained from laser-ablated amorphous CN x thin films have been analyzed in order to quantitatively deduce the change of the threefold/fourfold carbon coordination ratio induced by the nitrogenation. It has been found that nitrogenation induces an increase of the threefold-coordinated carbon atoms with respect to the fourfold-coordinated ones, evidencing the graphitization of the material. All the bond concentrations of the threefold and fourfold coordinations have been evaluated.

We present a comprehensive computational study on the properties of face-centered cubic and hexagonal chalcogenide Ge2Sb2Te5. We calculate the electronic structure using density functional theory (DFT); the obtained density of states (DOS) compares favorably with experiments, also looking suitable for transport analysis. Optical constants including refraction index and absorption coefficient capture major experimental features, aside from an energy shift owed to an underestimate of the band gap that is typical of DFT calculations. We also compute the phonon DOS for the hexagonal phase, obtaining a speed of sound and thermal conductivity in good agreement with the experimental lattice contribution. The calculated heat capacity reaches ~ 1.4 x 106 J/(m3 K) at high temperature, in agreement with experimental data, and provides insight into the low-temperature range (< 150 K), where data are unavailable., 19 pages, 8 figures

The structural and the optical properties of InAs layers grown on high index GaAs surfaces by molecular beam epitaxy are investigated in order to understand the formation and the self-organization of quantum dots (QDs) on novel index surfaces. Four different GaAs substrate orientations have been examined, namely, (111)B, (311)A, (311)B and (100). The (100) surface was used as a reference sample. STM pictures exhibit a uniform QD coverage for all the samples with the exception of (111)B, which displays a surface characterized by very large islands and where STM pictures give no evidence of QD formation. The photoluminescence (PL) spectra of GaAs (100) and 311 samples show typical QD features with PL peaks in the energy range 1.15-1.35 eV with comparable efficiency. No significant quenching of PL up to temperatures as high as 70 K was observed. These results suggest that the high index substrates are promising candidates for production of high quality self-assembled QD materials for application to photonics. (C) 1997 Elsevier Science Ltd.