Beyond the predictions routinely achievable by first-principles calculations and using metal-organic chemical vapor deposition (MOCVD), we report a GaN monolayer in a buckled geometry obtained in confinement at the graphene/SiC interface. Conductive atomic force microscopy (C-AFM) was used to investigate vertical current injection across the graphene/SiC interface and to establish the uniformity of the intercalated regions. Scanning transmission electron microscopy (S/TEM) was used for atomic resolution imaging and spectroscopy along the growth direction. The experimentally obtained value of the buckling parameter, 1.01 & PLUSMN; 0.11 & ANGS;, adds to the existing knowledge of buckled GaN monolayers, which is based solely on predictive first-principles calculations. Our study reveals a discontinuity in the anticipated stacking sequence attributed to a few-layer graphitic-like GaN structure. Instead, we identify an atomic order suggestive of ultrathin gallium oxide Ga2O3, whose formation is apparently mediated by dissociative adsorption of oxygen onto the GaN monolayer. An atomic resolution image of an intercalated structure at a graphene/SiC interface along the growth direction which is determined as a buckled GaN monolayer at the immediate interface with an underlying SiC substrate and ultrathin Ga2O3 on top.

Correction for Discovering atomistic pathways for supply of metal atoms from methyl-based precursors to graphene surface by Davide G. Sangiovanni et al., Phys. Chem. Chem. Phys., 2023, 25, 829-837, https://doi.org/10.1039/D2CP04091C.

Zinc aluminogallate, Zn(AlxGa1−x)2O4 (ZAGO), a single-phase spinel structure, offers considerable potential for high-performance electronic devices due to its expansive compositional miscibility range between aluminum (Al) and gallium (Ga). Direct growth of high-quality ZAGO epilayers however remains problematic due to the high volatility of zinc (Zn). This work highlights a novel synthesis process for high-quality epitaxial quaternary ZAGO thin films on sapphire substrates, achieved through thermal annealing of a ZnGa2O4 (ZGO) epilayer on sapphire in an ambient air setting. In-situ annealing x-ray diffraction measurements show that the incorporation of Al in the ZGO epilayer commenced at 850 °C. The Al content (x) in ZAGO epilayer gradually increased up to around 0.45 as the annealing temperature was raised to 1100 °C, which was confirmed by transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy. X-ray rocking curve measurement revealed a small full width at half maximum value of 0.72 °, indicating the crystal quality preservation of the ZAGO epilayer with a high Al content. However, an epitaxial intermediate �–(AlxGa1−x)2O3 layer (� - AGO) was formed between the ZAGO and sapphire substrate. This is believed to be a consequence of the interdiffusion of Al and Ga between the ZGO thin film and sapphire substrate. Using density functional theory, the substitution cost of Ga in sapphire was determined to be about 0.5 eV lower than substitution cost of Al in ZGO. Motivated by this energetically favorable substitution, a formation mechanism of the ZAGO and AGO layers was proposed. Spectroscopic ellipsometry studies revealed an increase in total thickness of the film from 105.07 nm (ZGO) to 147.97 nm (ZAGO/AGO) after annealing to 1100 °C, which were corroborated using TEM. Furthermore, an observed increase in the direct (indirect) optical bandgap from 5.06 eV (4.7 eV) to 5.72 eV (5.45 eV) with an increasing Al content in the ZAGO layer further underpins the formation of a quaternary ZAGO alloy with a tunable composition.

By addressing precursor prevalence and energetics using the DFT-based synthetic growth concept (SGC), the formation mechanism of self-induced InAlN core–shell nanorods (NRs) synthesized by reactive magnetron sputter epitaxy (MSE) is explored. The characteristics of In- and Al-containing precursor species are evaluated considering the thermal conditions at a typical NR growth temperature of around 700 °C. The cohesive and dissociation energies of In-containing precursors are consistently lower than those of their Al-containing counterparts, indicating that In-containing precursors are more weakly bonded and more prone to dissociation. Therefore, In-containing species are expected to exhibit lower abundance in the NR growth environment. At increased growth temperatures, the depletion of In-based precursors is even more pronounced. A distinctive imbalance in the incorporation of Al- and In-containing precursor species (namely, AlN/AlN+, AlN2/AlN2+, Al2N2/Al2N2+, and Al2/Al2+ vs InN/InN+, InN2/InN2+, In2N2/In2N2+, and In2/In2+) is found at the growing edge of the NR side surfaces, which correlates well with the experimentally obtained core–shell structure as well as with the distinctive In-rich core and vice versa for the Al-rich shell. The performed modeling indicates that the formation of the core–shell structure is substantially driven by the precursors’ abundance and their preferential bonding onto the growing edge of the nanoclusters/islands initiated by phase separation from the beginning of the NR growth. The cohesive energies and the band gaps of the NRs show decreasing trends with an increment in the In concentration of the NRs’ core and with an increment in the overall thickness (diameter) of the NRs. These results reveal the energy and electronic reasons behind the limited growth (up to ∼25% of In atoms of all metal atoms, i.e., InxAl1–xN, x ∼ 0.25) in the NR core and may be qualitatively perceived as a limiting factor for the thickness of the grown NRs (typically <50 nm).

Identification and synthesis of 2D topological insulators is particularly elusive. According to previous ab initio predictions 2D InBi (Indium Bismide) is a material exhibiting topological properties which are combined with a band gap suitable for practical applications. We employ ab initio molecular dynamics (AIMD) simulations to assess the thermal stability as well as the mechanical properties such as elastic modulus and stress-strain curves of 2D InBi. The obtained new knowledge adds further characteristics appealing to the feasibility of its synthesis and its potential applications. We find that pristine 2D InBi, H-InBi (hydrogenated 2D InBi) as well as 2D InBi heterostructures with graphene are all stable well above room temperature, being the calculated thermal stability for pristine 2D InBi 850 K and for H-InBi in the range above 500 K. The heterostructures of 2D InBi with graphene exhibit thermal stability exceeding 1000 K. In terms of mechanical properties, pristine 2D InBi exhibits similarities with another 2D material, stanene. The fracture stress for 2D InBi is estimated to be similar to 3.3 GPa (similar to 3.6 GPa for stanene) while elastic modulus of 2D InBi reads similar to 34 GPa (to compare with similar to 23 GPa for stanene). Overall, the thermal stability, elastic, and fracture resistant properties of 2D InBi and its heterostructures with graphene appear as high enough to motivate future attempts directed to its synthesis and characterization.

Conceptual 2D group III nitrides and oxides (e.g., 2D InN and 2D InO) in heterostructures with graphene have been realized by metal-organic chemical vapor deposition (MOCVD). MOCVD is expected to bring forth the same impact in the advancement of 2D semiconductor materials as in the fabrication of established semiconductor materials and device heterostructures. MOCVD employs metal-organic precursors such as trimethyl-indium, -gallium, and -aluminum, with (strong) metal-carbon bonds. Mechanisms that regulate MOCVD processes at the atomic scale are largely unknown. Here, we employ density-functional molecular dynamics - accounting for van der Waals interactions - to identify the reaction pathways responsible for dissociation of the trimethylindium (TMIn) precursor in the gas phase as well as on top-layer and zero-layer graphene. The simulations reveal how collisions with hydrogen molecules, intramolecular or surface-mediated proton transfer, and direct TMIn/graphene reactions assist TMIn transformations, which ultimately enables delivery of In monomers or InH and CH3In admolecules, on graphene. This work provides knowledge for understanding the nucleation and intercalation mechanisms at the atomic scale and for carrying out epitaxial growth of 2D materials and graphene heterostructures.

Tritantalum pentanitride (Ta₃N₅) semiconductor is a promising material for photoelectrolysis of water with high efficiency. Ta₃N₅ is a metastable phase in the complex system of TaN binary compounds. Growing stabilized single-crystal Ta₃N₅ films is correspondingly challenging. Here, we demonstrate the growth of a nearly single-crystal Ta₃N₅ film with epitaxial domains on c-plane sapphire substrate, Al₂O₃(0001), by magnetron sputter epitaxy. Introduction of a small amount ~2% of O₂ into the reactive sputtering gas mixed with N₂ and Ar facilitates the formation of a Ta₃N₅ phase in the film dominated by metallic TaN. In addition, we indicate that a single-phase polycrystalline Ta₃N₅ film can be obtained with the assistance of a Ta₂O₅ seed layer. With controlling thickness of the seed layer smaller than 10 nm and annealing at 1000 °C, a crystalline β phase Ta₂O₅ was formed, which promotes the domain epitaxial growth of Ta₃N₅ films on Al₂O₃(0001). The mechanism behind the stabilization of the orthorhombic Ta₃N₅ structure resides in its stacking with the ultrathin seed layer of orthorhombic β-Ta₂O₅, which is energetically beneficial and reduces the lattice mismatch with the substrate.

The emergence of specific and outstanding 2D-structure-related material performance has motivated a search for 2D atomic structures that can even be described as non-van-der-Waals-type materials. This has been exemplified with materials from group IV and group III-V which naturally crystallize in diamond, zincblende or wurtzite crystal structures. Here, we give insight into various atomic structures of indium oxide at the 2D limit featuring different stoichiometry, including 2D InO and 2D In2O3. We find that 2D InO with an InSe-type structure and its characteristic In-In distances compare closely with available first-time experimental results. An as yet unexplored 2D structure of indium oxide is found to be a planar hexagonal monolayer of h-In2O3.

Realization of semiconductor materials at the two-dimensional (2D) limit can elicit exceptional and diversified performance exercising transformative influence on modern technology. We report experimental evidence for the formation of conceptually new 2D indium oxide (InO) and its material characteristics. The formation of 2D InO was harvested through targeted intercalation of indium (In) atoms and deposition kinetics at graphene/SiC interface using a robust metal organic chemical vapor deposition (MOCVD) process. A distinct structural configuration of two sub-layers of In atoms in "atop" positions was imaged by scanning transmission electron microscopy (STEM). The bonding of oxygen atoms to indium atoms was indicated using electron energy loss spectroscopy (EELS). A wide bandgap energy measuring a value of 4.1 eV was estimated by conductive atomic force microscopy measurements (C-AFM) for the 2D InO.

The initial stages of metal organic chemical vapor deposition (MOCVD) of AlN on epitaxial graphene at temperatures in excess of 1200 degrees C have been rationalized. The use of epitaxial graphene, in conjunction with high deposition temperatures, can deliver on the realization of nanometer thin AlN whose material quality is characterized by the appearance of luminescent centers with narrow spectral emission at room temperature. It has been elaborated, based on our previous comprehensive ab initio molecular dynamics simulations, that the impact of graphene on AlN growth consists in the way it promotes dissociation of the trimethylaluminum, (CH3)(3)Al, precursor with subsequent formation of Al adatoms during the initial stages of the deposition process. The high deposition temperatures ensure adequate surface diffusion of the Al adatoms which is an essential factor in material quality enhancement. The role of graphene in intervening with the dissociation of another precursor, trimethylgallium, (CH3)(3)Ga, has accordingly been speculated by presenting a case of propagation of ultrathin GaN of semiconductor quality. A lower deposition temperature of 1100 degrees C in this case has better preserved the structural integrity of epitaxial graphene. Breakage and decomposition of the graphene layers has been deduced in the case of AlN deposition at temperatures in excess of 1200 degrees C.

We investigate theoretically the electronic and optical absorption properties of two sub-classes of oligosilanes: (i) Si(CH3)(4), Si-4(CH3)(8), and Si-8(CH3)(8) that contain Si dot, ring and cage, respectively, and exhibit typical Si-C and Si-Si bonds; and (ii) persilastaffanes Si7H6(CH3)(6) and Si12H6(CH3)(12), which contain extended delocalized s-electrons in Si-Si bonds over three-dimensional Si frameworks. Our modeling is performed within the GW approach up to the partially self-consistent GW(0) approximation, which is more adequate for reliably predicting the optical band gaps of materials. We examine how the optical properties of these organosilicon compounds depend on their size, geometric features, and Si/C composition. Our results indicate that the present methodology offers a viable way of describing the optical excitations of tailored functional Si-C-based clusters and molecular optical tags with potential use as efficient light absorbers/emitters in molecular optical devices. (C) 2020 The Author(s). Published by Elsevier B.V.

The possibility for kinetic stabilization of prospective 2D AlN was explored by rationalizing metal organic chemical vapor deposition (MOCVD) processes of AlN on epitaxial graphene. From the wide range of temperatures which can be covered in the same MOCVD reactor, the deposition was performed at the selected temperatures of 700, 900, and 1240 degrees C. The characterization of the structures by atomic force microscopy, electron microscopy and Raman spectroscopy revealed a broad range of surface nucleation and intercalation phenomena. These phenomena included the abundant formation of nucleation sites on graphene, the fragmentation of the graphene layers which accelerated with the deposition temperature, the delivery of excess precursor-derived carbon adatoms to the surface, as well as intercalation of sub-layers of aluminum atoms at the graphene/SiC interface. The conceptual understanding of these nanoscale phenomena was supported by our previous comprehensiveab initiomolecular dynamics (AIMD) simulations of the surface reaction of trimethylaluminum, (CH3)(3)Al, precursor with graphene. A case of applying trimethylindium, (CH3)(3)In, precursor to epitaxial graphene was considered in a comparative way.

We deposit CSx thin solid films by reactive direct current magnetron sputtering of a C target in an argon plasma, using carbon disulfide (CS2) as a precursor to film growth. We investigate the influence of the partial pressure of the CS2 vapor introduced into the plasma on the composition, the chemical bonding structure, the structural, and the mechanical properties as determined by x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, scanning electron microscopy (SEM), and nanoindentation for films deposited at 150 and 300 degrees C. The Raman and the XPS results indicate that S atoms are incorporated in mostly sp(2) bonded C network. These results agree with previous ab-initio theoretical findings obtained by modeling of the CSx compound by the Synthetic Growth Concept. The microstructure of the films as well as the results of their Raman characterization and the nano mechanical testing results all point out that with the increasing S content some spa bonding is admixed in the predominantly sp(2) bonded CSx network, leading to typical amorphous structure with short and interlocked graphene-like planes for S contents between 2% and 8%. We conclude that CSx thin solid films deposited by using CS2 as a precursor would be CSx films deposited at low temperature of similar to 150 degrees C and with an S content in the region of 6 at.% may be interesting candidates for applications as hard/elastic protective coatings.

Metal organic chemical vapor deposition (MOCVD) of group III nitrides on graphene heterostructures offers new opportunities for the development of flexible optoelectronic devices and for the stabilization of conceptually-new two-dimensional materials. However, the MOCVD of group III nitrides is regulated by an intricate interplay of gas-phase and surface reactions that are beyond the resolution of experimental techniques. We use density-functional ab initio molecular dynamics (AIMD) with van der Waals corrections to identify atomistic pathways and associated electronic mechanisms driving precursor/surface reactions during metal organic vapor phase epitaxy at elevated temperatures of aluminum nitride on graphene, considered here as model case study. The results presented provide plausible interpretations of atomistic and electronic processes responsible for delivery of Al, C adatoms, and C-Al, CHx, AlNH2 admolecules on pristine graphene via precursor/surface reactions. In addition, the simulations reveal C adatom permeation across defect-free graphene, as well as exchange of C monomers with graphene carbon atoms, for which we obtain rates of approximate to 0.3 THz at typical experimental temperatures (1500 K), and extract activation energies Eexca = 0.28 +/- 0.13 eV and attempt frequencies A(exc) = 2.1 (x1.7(+/- 1)) THz via Arrhenius linear regression. The results demonstrate that AIMD simulations enable understanding complex precursor/surface reaction mechanisms, and thus propose AIMD to become an indispensable routine prediction-tool toward more effective exploitation of chemical precursors and better control of MOCVD processes during synthesis of functional materials.

All known materials wear under extended mechanical contacting. Superlubricity may present solutions, but is an expressed mystery in C-based materials. We report negative wear of carbon nitride films; a wear-less condition with mechanically induced material inflation at the nanoscale and friction coefficient approaching ultralow values (0.06). Superlubricity in carbon nitride is expressed as C-N bond breaking for reduced coupling between graphitic-like sheets and eventual N-2 desorption. The transforming surface layer acts as a solid lubricant, whereas the film bulk retains its high elasticity. The present findings offer new means for materials design at the atomic level, and for property optimization in wear-critical applications like magnetic reading devices or nanomachines.

We propose hybrid molecular systems containing small carbon atomic chains interconnected by graphene-like flakes, theoretically predicted as true energy minima, as low-dimensional structures that may be useful in electronic devices at the limit of the atomic miniaturization. The effects of an external electric field applied along the direction of the carbon chains indicate that it is possible to control energy gap and spin polarization with sufficiently high strength, within the limit of the structural restoring of the systems. In this sense, by applying electric fields with magnitudes in the 1-5 V/nm range, we obtain semiconductor-to-metallic transitions for all odd-numbered carbon-chain systems proposed here. Furthermore, high-spin-to-low-spin transitions are determined for these systems as a function of the electric-field magnitude. In the case of the even-numbered carbon-chain systems, the overall electric field effect is pushing electron density near the Fermi level, leading to a gapless or metallic regime at 3.0 V/nm. An electric-field control of the spin-polarization of these latter systems is only achieved by doping the extremities of the graphene-like terminations with sulfur atoms. This finding, however, is beneficial for applications of these systems in spin controlled carbon-based devices connected by gold electrodes, even in the presence of a weak spin-orbit coupling.

A theoretical and experimental study on the growth and properties of a ternary carbon-based material, CSxFy, synthesized from SF6 and C as primary precursors is reported. The synthetic growth concept was applied to model the possible species resulting from the fragmentation of SF6 molecules and the recombination of S-F fragments with atomic C. The possible species were further evaluated for their contribution to the film growth. Corresponding solid CSxFy thin films were deposited by reactive direct current magnetron sputtering from a C target in a mixed Ar/SF6 discharge with different SF6 partial pressures (P-SF6). Properties of the films were determined by x-ray photoelectron spectroscopy, x-ray reflectivity, and nanoindentation. A reduced mass density in the CSxFy films is predicted due to incorporation of precursor species with a more pronounced steric effect, which also agrees with the low density values observed for the films. Increased P-SF6 leads to decreasing deposition rate and increasing density, as explained by enhanced fluorination and etching on the deposited surface by a larger concentration of F/F-2 species during the growth, as supported by an increment of the F relative content in the films. Mechanical properties indicating superelasticity were obtained from the film with lowest F content, implying a fullerene-like structure in CSxFy compounds.

A new carbon-based compound: CS_xF_y was addressed by density functional theory calculations and synthesized by reactive magnetron sputtering. Geometry optimizations and energy calculations were performed on graphene-like model systems containing sulfur and fluorine atoms. It is shown that [S+F] concentrations in the range of 0−10 at.%, structural ordered characteristics similar to graphene pieces containing ring defects are energetically feasible. The modeling predicts that CS_xF_y thin films with graphite and fullerene-like characteristics may be obtained for the mentioned concentration range. Accordingly, thin films were synthesized from a graphite solid target and sulfur hexafluoride as reactive gas. In agreement with the theoretical prediction, transmission electron microscopy characterization and selected area electron diffraction confirmed the presence of small ordered clusters with graphitic features in a sample containing 0.4 at.% of S and 3.4 at.% of F.

First-principles calculations are employed to investigate structural, electronic and topological insulating properties of XBi (X = B, Al, Ga, and In) monolayers upon halogenation. It is known that Y-XBi (X = Ga, In, Tl; Y = F, Cl, Br, I) can originate inversion-asymmetric topological insulators with large bulk band gaps. Our results suggest that Y-XBi (X = B, Al; Y = F, Cl, Br, I) may also result in nontrivial topological insulating phases. Despite the lower atomic number of B and Al, the spin-orbit coupling opens a band gap of about 400 meV in Y-XBi (X = B, Al), exhibiting an unusual electronic behavior for practical applications in spintronics. The nature of the bulk band gap and Dirac-cone edge states in their nanoribbons depends on the group-III elements and Y chemical species. They lead to a chemical tunability, giving rise to distinct band inversion symmetries and exhibiting Rashba-type spin splitting in the valence band of these systems. These findings indicate that a large family of Y-XBi sheets can exhibit nontrivial topological characteristics, by a proper tuning, and open a new possibility for viable applications at room temperature.

Graphite-like hexagonal AlN (h-AlN) multilayers have been experimentally manifested and theoretically modeled. The development of any functional electronics applications of h-AlN would most certainly require its integration with other layered materials, particularly graphene. Here, by employing vdW-corrected density functional theory calculations, we investigate structure, interaction energy, and electronic properties of van der Waals stacking sequences of few-layer h-AlN with graphene. We find that the presence of a template such as graphene induces enough interlayer charge separation in h-AlN, favoring a graphite-like stacking formation. We also find that the interface dipole, calculated per unit cell of the stacks, tends to increase with the number of stacked layers of h-AlN and graphene.

We perform a theoretical study of the electronic properties of polyaromatic hydrocarbon (PAH) molecules, as well as benzene and graphene, adsorbed on copper and gold. The PAH molecules studied are coronene (C24H12), circumcoronene (C54H18) and circumcircumcoronene (C96H24), which we consider as gradual approximations to an infinite graphene layer. In order to understand how the size of the adsorbed PAH molecules influences the adsorbate-metal interactions, we generalize the approach used in our earlier study [Phys Rev B, 85 (2012), p. 205423] to decompose the binding energies and net charge transfers into separate contributions from specific groups of atoms, and we then show that the zigzag edges of the PAH molecules interact stronger with the metal surfaces than the armchair ones. We discuss the nature of binding in our model systems as well as the formation of interface dipoles. We show that for all model systems studied here, the charge rearrangement contribution to the interface dipoles can be expressed as the product of the charge involved in the formation of the dipole and the distance between well-defined centers of charge for electron accumulation and depletion. This distance is only marginally dependent on the specific PAH molecules, decreasing slowly with their size.

We have investigated gas-phase reactions driven by silane (SiH4), which is the dopant precursor in the metalorganic chemical vapor deposition (MOCVD) of aluminum nitride (AlN) doped by silicon, with prime focus on determination of the associated energy barriers. Our theoretical strategy is based on combining density-functional methods with minimum energy path calculations. The outcome of these calculations is suggestive for kinetically plausible and chemically stable reaction species with Al-Si bonding such as (CH3)(2)AlSiH3 and N-Si bonding such as H2NSiH3. Within this theoretical perspective, we propose a view of these reaction species as relevant for the actual MOCVD of Si-doped AlN, which is otherwise known to be contributed by the reaction species (CH3)(2)AlNH2 with Al-N bonding. By reflecting on experimental evidence in the MOCVD of various doped semiconductor materials, it is anticipated that the availability of dopant species with Al-Si, and alternatively N-Si bonding near the hot deposition surface, can govern the incorporation of Si atoms, as well as other point defects, at the AlN surface.

We investigate, by means of first-principles calculations, the impact of a gold surface on the proton-transfer of the guanine-cytosine (GC) DNA base pair. Our calculations employ density functional improvements to correct van der Waals interactions and properly treat a weakly bound GC pair at an Au(111) surface. We adopted the simultaneous double proton-transfer (SDPT) mechanism proposed by Lowdin, which may lead to a spontaneous mutation in the structure of DNA from specific tautomerization involving the base pairs. Our calculated differences in the energetics and kinetics of the SDPT in the GC pair, when in contact with an inert gold surface, indicate a reduction of about 31% in the activation energy barrier of the GC/Au(111) tautomeric equilibrium. This finding gives strong evidence that tautomerism of DNA base pairs, binding to a noble surface, may be indeed relevant for the assessment of a possible point mutation, which could be induced by the presence of gold nanoparticles during DNA replication.

We employ ab initio calculations to predict the equilibrium structure, stability, reactivity, and Raman scattering properties of sixteen different (H3C)(n)X(SiH3)(3-n) compounds (X = B, Al, Ga, In) with n = 0-3. Among this methylsilylmetal family, only the (H3C)(3)X members, i.e., trimethylboron (TMB), trimethylaluminum (TMA), trimethylgallium (TMG), and trimethylindium (TMI), are currently well-studied. The remaining twelve compounds proposed here open up a two-dimensional array of new possibilities for precursors in various deposition processes, and evoke potential applications in the chemical synthesis of other compounds. We infer that within the (H3C)(n)X(SiH3)(3-n) family, the compounds with fewer silyl groups (and consequently with more methyl groups) are less reactive and more stable. This trend is verified from the calculated cohesive energy, Gibbs free energy of formation, bond strength, and global chemical indices. Furthermore, we propose sequential reaction routes for the synthesis of (H3C)(n)X(SiH3)(3-n) by substitution of methyl by silyl groups, where the silicon source is the silane gas. The corresponding reaction barriers for these chemical transformations lie in the usual energy range typical for MOCVD processes. We also report the Raman spectra and light scattering properties of the newly proposed (H3C)(n)X(SiH3)(3-n) compounds, in comparison with available data of known members of this family. Thus, our computational experiment provides useful information for a systematic understanding of the stability/reactivity and for the identification of these compounds.

The band structure and stability of XBi and XBi3 (X = B, Al, Ga, and In) single sheets are predicted using first-principles calculations. It is demonstrated that the band gap values of these new classes of two-dimensional (2D) materials depend on both the spin-orbit coupling (SOC) and type of group-III elements in these hetero-sheets. Thus, topological properties can be achieved, allowing for viable applications based on coherent spin transport at room temperature. The spin-orbit effects are proved to be essential to explain the tunability by group-III atoms. A clear effect of including SOC in the calculations is lifting the spin degeneracy of the bands at the Gamma point of the Brillouin zone. The nature of the band gaps, direct or indirect, is also tuned by SOC, and by the appropriate X element involved. It is observed that, in the case of XBi single sheets, band inversions naturally occur for GaBi and InBi, which exhibit band gap values around 172 meV. This indicates that these 2D materials are potential candidates for topological insulators. On the contrary, a similar type of band inversion, as obtained for the XBi, was not observed in the XBi3 band structure. In general, the calculations, taking into account SOC, reveal that some of these buckled sheets exhibit sizable gaps, making them suitable for applications in room-temperature spintronic devices.

Two-dimensional (2D) binary XBi compounds, where X belongs to group III elements (B, Al, Ga, and In), in a buckled honeycomb structure may originate sizable gap Z₂ topological insulators (TIs). These are characterized by exhibiting single band inversion at the Γ point as well as nontrivial edge states in their corresponding nanoribbons. By using first-principles calculations, we demonstrate that hydrogenation of XBi single layers leads to distinct and stable crystal structures, which can preserve their topological insulating properties. Moreover, hydrogenation opens a band gap in this new class of 2D Z₂ TIs, with distinct intensities, exhibiting an interesting electronic behavior for viable room-temperature applications of these 2D materials. The nature of the global band gap (direct or indirect) and topological insulating properties depend on the X element type and spatial configuration of the sheet, as well as the applied strain. Our results indicate that the geometric configuration can be crucial for preserving totally the topological characteristics of the hydrogenated sheets. We identify sizable band inversions in the band structure for the relaxed hydrogenated GaBi and InBi in their chairlike configurations and for hydrogenated BBi and AlBi under strain. Based on these findings, hydrogenation gives rise to a flexible chemical tunability and can preserve the band topology of the pristine XBi phases.

Fluorinated carbon-based thin films offer a wide range of properties for many technological applications that depend on the microstructure of the films. To gain a better understanding of the role of fluorine in the structural formation of these films, CFx systems based on graphene-like fragments were studied by first-principles calculations. Generally, the F concentration determines the type of film that can be obtained. For low F concentrations (up to similar to 5 at. %), films with fullerene-like as well as graphite-like features are expected. Larger F concentrations (greater than= 10 at. %) give rise to increasingly amorphous carbon films. Further increasing the F concentration in the films leads to formation of a polymer-like microstructure. To aid the characterization of CFx systems generated by computational methods, a statistical approach is developed.

A systematic study of the optical and magnetic excitations of 12 MSi12 and four MSi10 transition metal encapsulating Si cages has been carried out by employing real time time-dependent density functional theory. Criteria for the choice of transition metals (M) are clusters stability, synthesizability, and diversity. It was found that both the optical absorption and the spin-susceptibility spectra are mainly determined by, in decreasing order of importance, (1) the cage shape, (2) the group in the Periodic Table to which M belongs, and (3) the period of M in the Periodic Table. Cages with similar structures and metal species that are close to each other in the Periodic Table possess spectra sharing many similarities; for example, the optical absorption spectra of the MSi12 (M = V, Nb, Ta, Cr, Mo, and W), which are highly symmetric and belong to groups 4 and 5 of the Periodic Table, all share a very distinctive peak at around 4 eV. In all cases, although some of the observed transitions are located at the Si skeleton of the cages, the transition metal species is always significant for the optical absorption and the spin-susceptibility spectra. Our results provide fingerprint data for identification of gas-phase MSi12 and MSi10 by optical absorption spectroscopy.

Semiconductors assembled upon nanotemplates consisting of metal-encapsulating Si cage clusters (M@Si-n) have been proposed as prospective materials for nanodevices. To make an accurate and systematic prediction of the optical properties of such M@Si-n clusters, which represent a new type of metal silicon hybrid material for components in nanoelectronics, we have performed first-principles calculations of the electronic properties and quasiparticle band gaps for a variety of M@Si-12 (M Ti, Cr, Zr, Mo, Ru, Pd, Hf, and Os) and M@Si-16 (M = Ti, Zr, and Hf) clusters. At first stage, the electronic structure calculations have been performed within plane-wave density functional theory in order to predict equilibrium geometries, polarizabilities, and optical absorption spectra of these endohedral cagelike clusters. The quasiparticle calculations were performed within the GW approximation, which predict that all of these systems are semiconductors exhibiting large band gaps. The present results have demonstrated that the independent-particle absorption spectra of M@Si-n, calculated within the local density or generalized gradient approximations to density functional theory, are dramatically influenced by many-body effects. On average, the quasiparticle band gaps were significantly increased, in comparison with the independent-particle gaps, giving values in the 2.45-5.64 eV range. Consequently, the inclusion of many-body effects in the electron electron interaction, and going beyond the mean-field approximation of independent particles, might be essential to realistically describe the optical spectra of isolated M@Si-n clusters, as well as their cluster-assembled materials.

The reactive high power impulse magnetron sputtering processes of carbon in argon/tetrafluoromethane (CF4) and argon/octafluorocyclobutane (c-C4F8) have been characterized. Amorphous carbon fluoride (CFx) films were synthesized at deposition pressure and substrate temperature of 400 mPa and 110 degrees C, respectively. The CFx film composition was controlled in the range of 0.15 andlt; x andlt; 0.35 by varying the partial pressure of the F-containing gases from 0 mPa to 110 mPa. The reactive plasma was studied employing time averaged positive ion mass spectrometry and the resulting thin films were characterized regarding their composition, chemical bonding and microstructure as well as mechanical properties by elastic recoil detection analysis, X-ray photoelectron spectroscopy, transmission electron microscopy, nanoindentation, and water droplet contact angle measurements, respectively. The experimental results were compared to results obtained by first-principles calculations based on density functional theory. The modeling of the most abundant precursor fragment from the dissociation of CF4 and C4F8 provided their relative stability, abundance, and reactivity, thus permitting to evaluate the role of each precursor during film growth. Positive ion mass spectrometry of both fluorine plasmas shows an abundance of CF+, C+, CF2+, and CF3+ (in this order) as corroborated by first-principles calculations. Only CF3+ exceeded the Ar+ signal in a CF4 plasma. Two deposition regimes are found depending on the partial pressure of the fluorine-containing reactive gas, where films with fluorine contents below 24 at.% exhibit a graphitic nature, whereas a polymeric structure applies to films with fluorine contents exceeding 27 at.%. Moreover, abundant precursors in the plasma are correlated to the mechanical response of the different CFx thin films. The decreasing hardness with increasing fluorine content can be attributed to the abundance of CF3+ precursor species, weakening the carbon matrix.

First-principles calculations, which also implement the nudged elastic band (NEB) code, are performed to investigate (i) the stability of the (C2H5)(3)B:NH3 adduct formed by the initial precursor molecules triethylborane (C2H5)(3)B and ammonia NH3 in the metal-chemical-vapor-deposition (MOCVD) of hexagonal BN, and (ii) the energy barrier to the first ethane elimination through consistent unimolecular, ammonia-assisted, and adduct-assisted reaction pathways. Comparison is done with the reference case of the (CH3)(3)Al:NH3 adduct, notoriously known for its high degree of stability and reactivity, which determines an overall severe parasitic gas-phase chemical reaction mechanism in the deposition of AlN.

Phosphorus-carbide (CP_x) thin films have been deposited by unbalanced reactive magnetron sputtering and investigated by TEM, XPS, SEM, ERDA, Raman scattering spectroscopy, nanoindentation testing, and four-point electrical probe techniques. As-deposited films with x=0.1 are electron amorphous with elements of FL structure and high mechanical resiliency with hardness of 34.4 GPa and elastic recovery of 72%. The electrical resistivity of the films are in the range 0.4-1.7 Ωcm for CP_0.027, 1.4-22.9 Ωcm for CP_0.1, and lower than the minimal value the four-point probe is able to detect for CP_x with x≥0.2.

Density functional theory (DFT) calculations were performed in order to investigate the stability and the electronic structure of graphene-gold interfaces. Two configurations were studied: a gold cluster interacting with graphene and different polycyclic aromatic hydrocarbon (PAH) molecules, namely, C6H6 (benzene), C24H12 (coronene), and C54H18 (circumcoronene) adsorbed on an Au(111) surface. Nonlocal interactions were accounted for by using the semiempirical DFT-D2 method of Grimme. A limited set of calculations were also performed by using the first-principles van der Waals density functional method (vdW-DF). Adsorption distances around 3 angstrom and electronic charge transfer values of about (3-13) x 10(-3)e(-) per carbon atom were predicted for all systems. No major changes resulting from the adsorption of the gold cluster were detected in the graphenes density of states. The DFT-D2 results involving the adsorption of the PAH molecules on gold show an estimated binding energy of 73 meV per carbon atom, as well as an electronic charge loss of 0.10 x 10(-2) e(-), also per carbon atom, for an extended graphene sheet adsorbed on a gold surface. The modeling of the adsorption of C6H6 molecule on a gold surface suggests that the vdW-DF method provides more accurate results for the binding energies of such systems, in comparison to pure DFT calculations, which do not take the nonlocal interactions into account, as well as to simulations employing the DFT-D2 method.

Theoretical calculations focused on the stability of an infinite hexagonal AlN (h-AlN) sheet and its structural and electronic properties were carried out within the framework of DFT at the GGA-PBE level of theory. For the simulations, an h-AlN sheet model system consisting in 96 atoms per super-cell has been adopted. For h-AlN, we predict an Al-N bond length of 1.82 angstrom and an indirect gap of 2.81 eV as well as a cohesive energy which is by 6% lower than that of the bulk (wurtzite) AlN which can be seen as a qualitative indication for synthesizability of individual h-AlN sheets. Besides the study of a perfect h-AlN sheet, also the most typical defects, namely, vacancies, anti-site defects and impurities were also explored. The formation energies for these defects were calculated together with the total density of states and the corresponding projected states were also evaluated. The charge density in the region of the defects was also addressed. Energetically, the anti-site defects are the most costly, while the impurity defects are the most favorable, especially so for the defects arising from Si impurities. Defects such as nitrogen vacancies and Si impurities lead to a breaking of the planar shape of the h-AlN sheet and in some cases to the formation of new bonds. The defects significantly change the band structure in the vicinity of the Fermi level in comparison to the band structure of the perfect h-AlN which can be used for deliberately tailoring the electronic properties of individual h-AlN sheets.

Corannulene has been a useful prototype for studying C-based nanostructures as well as surface chemistry and reactivity of sp(2)-hybridized carbon-based materials. We have investigated fluorination and hydrogenation of corannulene carrying out density functional theory calculations. In general, the fluorination is energetically more favorable than hydrogenation of corannulene. The substitution of the peripheral H atoms in the corannulene molecule by F atoms leads to a larger cohesive energy gain than when F (or H) atoms are bonded to the hub carbon and bridge carbon sites of this molecule. As expected for doped C-based nanostructures, the hydrogenation or fluorination significantly changes the HOMO-LUMO gap of the system. We have obtained HOMO-LUMO gap variations of 0.13-3.46 eV for F-doped and 0.38-1.52 eV for H-doped systems. These variations strongly depend on the concentration and position of the incorporated F/H atoms, instead of the structural stability of the doped systems. Considering these calculations, we avoid practical difficulties associated with the addition/substitution reactions of larger curved two-dimensional (2D) carbon nanostructures, and we obtain a comprehensive and systematic understanding of a variety of F/H 2D doped systems.

Carbon-based fullerene-like (FL) solid compounds are a new class of materials with extraordinary mechanical properties, which can be tuned by the dopant choice and its concentration. In this work, FL sulfocarbide (CSx) was studied by DFT simulations during synthetic growth with CmSn (m, n andlt;= 2). The energetic and structural effects of S atoms at C sites in a graphene-like network were addressed by geometry optimizations and cohesive energy calculations. Results showed that for S concentrations lower than 10 at. %, smoothly bent pure hexagonal networks predominate. For higher S concentrations, the higher defect concentration leads to stronger deformation of the graphene-like sheets. It was determined that FL-CSx is well-structured (not amorphous) for S contents between 10 and 20 at. %. In contrast to other FL materials, bond rotation mechanisms are not expected to play any significant role during FL-CSx formation, and cross-linking sites are less frequent and may be assimilated in the planar structure during growth. Both quasi-planar networks and cage-like conformations were found to form during the synthetic growth of CSx. The detailed analysis of how CSx structural patterns form during its synthetic growth provides a realistic picture for the deposition of this novel compound by magnetron sputtering.

Structural and bonding patterns arising from the incorporation of fluorine atoms in a graphene-like network relevant to the deposition of carbon fluoride (CF(x)) films were addressed by first-principles calculations. We find that large N-member (N = 8-12) rings, defects by sheet branching, and defects associated with bond rotation pertain to CF(x). The cohesive energy gains associated with these patterns are similar to 0.2-0.4 eV/at., which is similar to those for a wide range of defects in other C-based nanostructured solids. Fullerene-like CF(x) is predicted for F concentrations below similar to 10 at.%, while CF(x) compounds with higher F content are predominantly amorphous or polymeric.

Fluorine containing amorphous carbon films (CF(x), 0.16 andlt;= x andlt;= 0.35) have been synthesized by reactive high power impulse magnetron sputtering (HiPIMS) in an Ar/CF(4) atmosphere. The fluorine content of the films was controlled by varying the CF(4) partial pressure from 0 mPa to 110 mPa at a constant deposition pressure of 400 mPa and a substrate temperature of 110 degrees C. The films were characterized regarding their composition, chemical bonding and microstructure as well as mechanical properties by applying elastic recoil detection analysis, X-ray photoelectron spectroscopy, Raman spectroscopy, transmission electron microscopy, and nanoindentation. First-principles calculations were carried out to predict and explain F-containing carbon thin film synthesis and properties. By geometry optimizations and cohesive energy calculations the relative stability of precursor species including C(2), F(2) and radicals, resulting from dissociation of CF4, were established. Furthermore, structural defects, arising from the incorporation of F atoms in a graphene-like network, were evaluated. All as-deposited CF(x) films are amorphous. Results from X-ray photoelectron spectroscopy and Raman spectroscopy indicate a graphitic nature of CF(x) films with x andlt;= 0.23 and a polymeric structure for films with x andgt;= 0.26. Nanoindentation reveals hardnesses between similar to 1 GPa and similar to 16 GPa and an elastic recovery of up to 98%.

arbon nanostructures consisting of corannulene/coronene-like pieces connected by atomic chains and doped with nitrogen atoms have been addressed by carrying out first-principles calculations within the framework of the spin-polarized density functional theory. Our results show that the conformation, charge distributions, and spin states are significantly influenced by the nitrogen incorporation in comparison to these characteristics of similar pure carbon structures. Higher concentration of incorporated nitrogen leads to a smaller highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap and different conductive states near the Fermi level. In turn the different location of the N-incorporation sites allows switching on and off of the pi-electron magnetism in these systems. We found that the rotational deformation of the terminations with respect to the carbon chain depends on the number and the location of the incorporated N atoms. The most stable N-doped structures exhibit a relative rotation of the terminations of approximately 90 degrees. These findings indicate that by controllable N doping one can tune the conducting channel of carbon chains connected to sp(2) terminations; thus obtaining low band-gap nano-units.

Fullerene-Like (FL) Sulpho-Carbide (CSx) compounds have been addressed by first principles calculations. Geometry optimization and cohesive energy results are presented for the relative stability of precursor species such as C2S, CS2, and C2S2 in isolated form. The energy cost for structural defects, arising from the substitution of C by S is also reported. Similar to previously synthesized FL-CNx and FL-CPx compounds, the pentagon, the double pentagon defects as well as the Stone-Wales defects are confirmed as energetically feasible in CSx compounds.

The energy cost for P atom intercalation and corresponding structural implications during formation of Fullerene-like Phosphorus carbide (FL-CPx) were evaluated within the framework of Density Functional Theory. Single P atom interstitial defects in FL-CPx are energetically feasible and exhibit energy cost of 0.93-1.21 eV, which is comparable to the energy cost for experimentally confirmed tetragon defects and dangling bonds in CPx. A single P atom intercalation event in FL-CPx can increase the inter-sheet distance from 3.39-3.62 angstrom to 5.81-7.04 angstrom. These theoretical results are corroborated by Selected Area Electron Diffraction characterization of FL-CPx samples.

Carbon nanowires made of long linear atomic chains have attracted considerable interest due to their potential applications in nanoelectronics. We report a theoretical characterization of assemblies with good prospects for chemical synthesis made of two coronene molecules (graphene-like pieces) bridged by carbon linear chains with distinct sizes and parities. Our calculations are performed within all-electron density functional theory. We examine the effects of two conformations (syn and anti) of the terminal anchor pieces, representing energy minima for these systems, on the properties of the carbon chains. The calculated electronic states reveal that simplified chemical models such as those based on cumulenes or polyynes are not appropriate to describe the linear chains with sp(2) terminations. For these types of atomic chains, we find that the electronic ground state of the odd-numbered chains is spin polarized. Vibrational properties of all these chains are studied by calculating Raman scattering and infrared spectra. We show that syn and anti conformations of the graphene-like terminations lead to important effects in the vibrational features of the chains, detectable by the Raman light-scattering depolarization.

The energy cost for dangling bond formation in Fullerene-like Carbon Nitride (FL-CNx) and Phosphorus carbide (FL-CPx) as well as their amorphous counterparts: a-CNx, a-CPx, and a-C has been calculated within the framework of Density Functional Theory and compared with surface water adsorption measurements. The highest energy cost is found in the FL-CNx ( about 1.37 eV) followed by FL-CPx compounds (0.62-1.04 eV). (C) 2009 Elsevier B. V. All rights reserved.

Amorphous phosphorous-carbide films have been considered as a new tribological coating material with unique electrical properties. However, such CPx films have not found practical use until now because they tend to oxidize/hydrolyze rapidly when in contact with air. Recently, we demonstrated that CPx thin films with a fullerene-like structure can be deposited by magnetron sputtering, whereby the structural incorporation of P atoms induces the formation of strongly bent and inter-linked graphene planes. Here, we compare the uptake of water in fullerene-like phosphorous-carbide (FL-CPx) thin films with that in amorphous phosphorous-carbide (a-CPx), and amorphous carbon (a-C) thin films. Films of each material were deposited on quartz crystal substrates by reactive DC magnetron sputtering to a thickness in the range 100-300 nm. The film microstructure was characterized by X-ray photoelectron spectroscopy, and high resolution transmission electron microscopy. A quartz crystal microbalance placed in a vacuum chamber was used to measure their water adsorption. Measurements indicate that FL-CPx films adsorbed less water than the a-CPx and a-C ones. To provide additional insight into the atomic structure of defects in the FL-CPx and a-CPx compounds, we performed first-principles calculations within the framework of density functional theory. Cohesive energy comparison reveals that the energy cost formation for dangling bonds in different configurations is considerably higher in FL-CPx than for the amorphous films. Thus, the modeling confirms the experimental results that dangling bonds are less likely in FL-CPx than in a-CPx and a-C films.

By using DFT calculations we show that nano-wires consisting of endohedral MSi12 cage-like molecules are stable especially for light transition metal elements (M = Fe, Ni, Co, Ti, V, and Cu). The nano-wire assemblies are stabilized by the metal atoms located along their principal axes and can be seen as close Si-based analogues of C nanotubes, but with hexagonal cross-section due to the D6h-symmetry of their MSi12 building blocks. Independently on M, with the increase in the length of a (MSi12)m nano-wire, its HOMO-LUMO gap decreases gradually. The metallic behavior of (MSi12)m defines them as possible conductive components for self-assembled nano-devices. © 2008 Elsevier B.V. All rights reserved.

Phosphorus-carbide, CP_x (0.025≤x≤0.1) thin films have beensynthesized by magnetron sputtering from pressed graphite-phosphorustargets. The films were characterized by X-ray photoelectron spectroscopy,transmission electron microscopy and diffraction, andnanoindentation. CP_0.02 exhibits C-P bonding in an amorphous structure with elements of curved grapheneplanes, yielding a material with unique short range order. These features are consistent with what has been predicted by our results of theoreticallymodeled synthetic growth of CP_x. The films are mechanicallyresilient with hardness up to 24 GPa and elastic recovery upto 72%.

Humidity influences the tribological performance of the head-disk interface in magnetic data storage devices. In this work we compare the uptake of water of amorphous carbon nitride (a-CN_x) films, widely used as protective overcoats in computer disk drive systems, with fullerene-like carbon nitride (FL-CN_x) and amorphous carbon (a-C) films. Films with thickness in the range 10-300 run were deposited on quartz crystal substrates by reactive DC magnetron sputtering. A quartz crystal microbalance placed in a vacuum chamber was used to measure the water adsorption. Electron paramagnetic resonance (EPR) has been used to correlate water adsorption with film microstructure and surface defects (dangling bonds). Measurements indicate that the amount of adsorbed water is highest for the pure a-C films and that the FL-CN_x films adsorbed less than a-CN_x. EPR data correlate the lower water adsorption on FL-CNx films with a possible lack of dangling bonds on the film surface. To provide additional insight into the atomic structure of defects in the FL-CN_x, a-CNx and a-C compounds, we performed first-principles calculations within the framework of Density Functional Theory. Emphasis was put on the energy cost for formation of vacancy defects and dangling bonds in relaxed systems. Cohesive energy comparison reveals that the energy cost formation for dangling bonds in different configurations is considerably higher in FL-CN_x than for the amorphous films. These simulations thus confirm the experimental results showing that dangling bonds are much less likely in FL-CN_x than in a-CNx and a-C films.

Direct impact of H2 and N2 diluents on the metal-organic-chemical-vapor-deposition gas-phase chemistry in M(CH3)3/NH3 (M = Al, Ga, In) systems is identified in the framework of Density Functional Theory in terms of cohesive energy differences. While both diluents destabilize model reaction species, i.e. adducts, transition states and chain complexes, the effect is particularly strong with respect to N2 in the Al(CH3)3/NH3 system, and can be a factor to restrain the expansion of chain complexes that deplete the gas-phase from precursors. Theoretical results are supported by experimental evidences of higher growth rate and superior optical properties of AlN grown in N2 vs. H2 diluent. © 2006 Elsevier B.V. All rights reserved.

The formation and structural evolution of fullerene-like (FL) carbon phosphide (CP_x) during synthetic growth were studied by first-principles calculations. Geometry optimizations and comparison between the cohesive energies suggest stability for solid FL-CP_x compounds. In comparison with fullerene-like carbon nitride, higher curvature of the graphene sheets and higher density of cross-linkages between them is predicted and explained by the different electronic properties of P and N. Cage-like and onion-like structures, both containing tetragons, are found to be typical for fullerene-like CP_x. Segregation of P is predicted at fractions exceeding ~20 at.%.

Inherently nanostructured CP_x compounds were studied by first-principles calculations. Geometry optimizations and cohesive energy comparisons show stability for C₃P, C₂P, C₃P₂, CP, and P₄ (P₂) species in isolated form as well as incorporated in graphene layers. The energy cost for structural defects, arising from the substitution of C for P and intercalation of P atoms in graphene, was also evaluated. We find a larger curvature of the graphene sheets and a higher density of cross-linkage sites in comparison to fullerene-like (FL) CN_x, which is explained by differences in the bonding between P and N. Thus, the computational results extend the scope of fullerene-like thin film materials with FL-CP_x and provide insights for its structural properties.