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The fast-inertial-relaxation engine (FIRE) has proven to efficiently find local minima of potential energies or related penalty functions although its implementation requires only few, additional lines of code in a molecular-dynamics or steepest-descent program. So far, FIRE has been predominantly applied to particle-based or low-dimensional problems. In this work, we demonstrate that it can also benefit the solution of boundary-value problems. Towards this end, we study the mechanical contact between an elastic body and a rigid indenter of varying complexity by augmenting Green's function molecular dynamics (GFMD) with FIRE. We find a rather remarkable speedup, which can be further enhanced when choosing the masses associated with the eigenmodes of the free elastic solid appropriately. For the investigated adhesive and randomly rough indenter with typical system size, 100 mass-weighted FIRE-GFMD iterations suffice to relax the excess energy to ${10}^{\ensuremath{-}3}$ of its original value. The standard GFMD method needs 25 times more iterations. For the investigated problems, FIRE-GFMD even appears to slightly outperform conjugate-gradient based optimization.

This work elucidates friction in Poly-Ether-Ether-Ketone (PEEK) sliding contacts through multiscale simulations. At the nanoscale, non-reactive classical molecular dynamics (MD) simulations of dry and water-lubricated amorphous PEEK-PEEK interfaces are performed. During a short running-in phase, we observe structural transformations at the sliding interface that result in flattening of the initial nanotopographies accompanied by strong polymer chain alignment in the shearing direction. Our MD simulations reveal a linear pressure-dependence of the shear stress τMD (P,σH2o) [MPa]=0.18P + 50.5 - 1.25σH2o, where σH2o [nm-2] is the surface number density of adsorbed water molecules. This constitutive law is of central importance for our multiscale approach, since it forms a link between MD and elastoplastic contact mechanics calculations. An integration of τMD (P,σH2o) over the real area of contact yields a macroscopic friction coefficient μmacro (σH2o) that allows for a meaningful comparison with friction coefficients μexp≈0.5-0.7 which are in good agreement with the calculated dry friction coefficients μmacro(σH2o=0).For milder experimental loads, our multiscale model suggests that the lower friction states with μexp≈0.2 originate in the presence of physisorbed molecules (e.g. water), which significantly reduce interfacial adhesion.

Friction and wear reduction by diamond-like carbon (DLC) in automotive applications can be affected by zinc-dialkyldithiophosphate (ZDDP), which is widely used in engine oils. Our experiments show that DLC’s tribological behaviour in ZDDP-additivated oils can be optimised by tailoring its stiffness, surface nano-topography and hydrogen content. An optimal combination of ultralow friction and negligible wear is achieved using hydrogen-free tetrahedral amorphous carbon (ta-C) with moderate hardness. Softer coatings exhibit similarly low wear and thin ZDDP-derived patchy tribofilms but higher friction. Conversely, harder ta-Cs undergo severe wear and sub-surface sulphur contamination. Contact-mechanics and quantum-chemical simulations reveal that shear combined with the high local contact pressure caused by the contact stiffness and average surface slope of hard ta-Cs favour ZDDP fragmentation and sulphur release. In absence of hydrogen, this is followed by local surface cold welding and sub-surface mechanical mixing of sulphur resulting in a decrease of yield stress and wear., Wear reduction in diamond-like carbon interacting with ZDDP-additivated oils is essential for current automotive applications. Here, the authors present an atomic-scale study revealing that this can be achieved by tailoring diamond-like carbon’s stiffness, surface nano-topography, and hydrogen content.

A stable passivation of surface dangling bonds underlies the outstanding friction properties of diamond and diamond-like carbon (DLC) coatings in boundary lubrication. While hydrogen is the simplest termination of a carbon dangling bond, fluorine can also be used as a monoatomic termination, providing an even higher chemical stability. However, whether and under which conditions a substitution of hydrogen with fluorine can be beneficial to friction is still an open question. Moreover, which of the chemical differences between C-H and C-F bonds are responsible for the change in friction has not been unequivocally understood yet. In order to shed light on this problem, we develop a density functional theory-based, nonreactive force field that describes the relevant properties of hydrogen- and fluorine-terminated diamond and DLC tribological interfaces. Molecular dynamics and nudged elastic band simulations reveal that the frictional stress at such interfaces correlates with the corrugation of the contact potential energy, thus ruling out a significant role of the mass of the terminating species on friction. Furthermore, the corrugation of the contact potential energy is almost exclusively determined by steric factors, while electrostatic interactions only play a minor role. In particular, friction between atomically flat diamond surfaces is controlled by the density of terminations, by the C-H and C-F bond lengths, and by the H and F atomic radii. For sliding DLC/DLC interfaces, the intrinsic atomic-scale surface roughness plays an additional role. While surface fluorination decreases the friction of incommensurate diamond contacts, it can negatively affect the friction performance of carbon surfaces that are disordered and not atomically flat. This work provides a general framework to understand the impact of chemical structure of surfaces on friction and to generate design rules for optimally terminated low-friction systems.

This work discusses the use of slipping domains to improve load support in water-lubricated microsystems through a multi-scale approach. A semi-analytical continuum formulation is developed to calculate changes in pressure distribution for a line contact with added slip. It is found that the slipping region maximizing hydrodynamic lift should favor inlet flow, but not be located at the contact outlet. Estimations for its positioning are provided. Slip on flat surfaces gives the largest gains in load support and friction reduction, but moderate improvements are also possible on circular geometries featuring high convergence ratios. Finally, Molecular Dynamics simulations are employed to quantify slip on hydrophobic surfaces. Among the considered topologies, graphene is an excellent candidate to generate slip in water-lubricated microsystems.

The nanoscale lubrication properties of graphene depend on chemical functionalization, adsorbants, and bonding to the substrate. The dependence of friction on the rotational orientation of graphene on a Pt(111) surface was studied by high-resolution friction force microscopy in ultra-high vacuum and interpreted through complementary simulations. Lateral forces reveal an atomic-scale stick-slip motion with the periodicity of the graphene structure. Additionally, the lateral forces were modulated by a Moire pattern, which depends on the rotation of the respective domain. Comparing the experimental results to simulations of the friction process based on the Prandtl-Tomlinson model and to atomistic simulations of the tip-sample interactions, it was found that the modulation of lateral forces originated from a structural mismatch between Pt(111) and graphene domains that were locally pinned to the substrate. Domains with preferred orientations, such as the R1° and the R24°, appeared to exhibit lower average friction than those with orientations less frequently observed, such as the R10° domain.

Amorphous carbon (a-C) occurs as a tribologically induced phase in diamond or diamond-like carbon coatings. The interaction of ambient gases (${\mathrm{H}}_{2}$, ${\mathrm{N}}_{2}$, ${\mathrm{O}}_{2}$, ${\mathrm{H}}_{2}\mathrm{O}$, ${\mathrm{CO}}_{2}$) with a-C of varying mass density is studied by means of ab initio thermodynamics simulations. Different scenarios such as moist air or pure gases are investigated under different pressure and temperature conditions. Equilibrium concentrations of chemisorbed and fragmented final states are found to only exhibit a minor dependence on the specific conditions of the gas phase reservoir. The differences in local structure of the a-C samples with varying mass density such as pore size and coordination and dangling bonds, as well as the competition among different gas molecules for a-C atoms as reactants, affect the equilibrium concentrations to a greater degree. The availability and reactivity of a-C atoms are thus found to mainly control the chemical composition of a-C interacting with ambient gases in thermodynamic equilibrium. Trends found in the analysis of chemical groups occurring in equilibrium can possibly be transferred to a more realistic nonequilibrium tribological scenario.

We present 2p core-level spectra of size-selected aluminum and silicon cluster cations from soft X-ray photoionization efficiency curves and density functional theory. The experimental and theoretical results are in very good quantitative agreement and allow for geometric structure determination. New ground state geometries for Al12+, Si15+, Si16+, and Si19+ are proposed on this basis. The chemical shifts of the 2p electron binding energies reveal a substantial difference for aluminum and silicon clusters: while in aluminum the 2p electron binding energy decreases with increasing coordination number, no such correlation was observed for silicon. The 2p binding energy shifts in clusters of both elements differ strongly from those of the corresponding bulk matter. For aluminum clusters, the core-level shifts between outer shell atoms and the encapsulated atom are of opposite sign and one order of magnitude larger than the corresponding core-level shift between surface and bulk atoms in the solid. For silicon clusters, the core-level shifts are of the same order of magnitude in clusters and in bulk silicon but no obvious correlation of chemical shift and bond length, as present for reconstructed silicon surfaces, are observed.

The two-stage mechanochemical amination of graphite by dry ball milling of graphite in a planetary ball mill under Ar followed by NH3 yields aminated multilayer graphene (AMFG) as intermediates for carbon/polymer hybrids and nanocomposites. Opposite to efficient edge-selective graphene functionalization under Ar, CO2 and N2 pressure, the onestage ball milling under NH3 pressure affords rather low N content (

Inorganic conductometric gas sensors struggle to overcome limitations in high power consumption and poor selectivi-ty. Herein, recent advances in developing self-powered gas sensors with tunable selectivity are introduced. Alternative general approaches for powering gas sensors were realized via proper integration of complementary functionalities (namely; powering and sensing) in a singular heterostructure. These solar light driven gas sensors operating at room temperature without applying any additional external powering sources are comparatively discussed. The TYPE-1 gas sensor based on integration of pure inorganic interfaces (e.g. CdS/n-ZnO/p-Si) is capable of delivering a self-sustained sensing response, while it shows a non-selective interaction towards oxidizing and reducing gases. The structural and the optical merits of TYPE-1 sensor are investigated giving more insights into the role of light activation on the modu-lation of the self-powered sensing response. In the TYPE-2 sensor, the selectivity of inorganic materials is tailored through surface functionalization with self-assembled organic monolayers (SAMs). Such hybrid interfaces (e.g. SAMs/ZnO/p-Si) have specific surface interactions with target gases compared to the non-specific oxidation-reduction interactions governing the sensing mechanism of simple inorganic sensors. The theoretical modeling using density functional theory (DFT) has been used to simulate the sensing behavior of inorganic/organic/gas interfaces, revealing that the alignment of organic/gas frontier molecular orbitals with respect to the inorganic Fermi level is the key factor for tuning selectivity. These platforms open new avenues for developing advanced energy-neutral gas sensing devices and concepts.

The efficient separation of photogenerated electron–hole pairs and stability against corrosion are critical preconditions for a photoelectrode to achieve a high photoelectrochemical performance. In this work it is shown how both criteria can be met by employing a heterostructure of bismuth vanadate (BiVO4) and titanium dioxide (TiO2) as the photocatalyst. Using electronic structure calculations, an alteration of the band alignment is predicted at the heterojunction from type I to type II by hydrogen treatment of the top TiO2 layer. Guided by this idea, we have fabricated heterostructures of BiVO4 and TiO2 and studied the effect of hydrogen treatment. The achieved band engineering results in a significant improvement in photocurrent density, up to 4.44 mA cm–2 at 1.23 V vs RHE, and a low onset potential, −0.14 V vs RHE, under visible light illumination. The enhanced photoelectrochemical performance originates in facilitated hole transport to the electrode surface and enhanced photoabsorption in the TiO2 la...

Despite the pronounced polarity of C-F bonds, many fluorinated carbon compounds are hydrophobic: a controversial phenomenon known as "polar hydrophobicity". Here, its underlying microscopic mechanisms are explored by ab initio calculations of fluorinated and hydrogenated diamond (111) surfaces interacting with single water molecules. Gradient- and van, der Waals-corrected density functional theory simulations reveal that "polar hydrophobicity" of the fully fluorinated surfaces is caused by a negligible surface/water electrostatic interaction. The densely packed C-F surface dipoles generate a short-range electric field that decays within the core repulsion zone of the surface and hence vanishes in regions accessible by adsorbates. As a result, water physisorption on fully F-terminated surfaces is weak (adsorption energies E-ad < 0.1 eV) and dominated by van der Waals interactions. Conversely, the near-surface electric field generated by loosely packed dipoles on mixed F/H-terminated surfaces has a considerably longer range, resulting in a stronger water physisorption (E-ad > 0.2 eV) that is dominated by electrostatic interactions. The suppression of electrostatic interactions also holds for perfluorinated molecular carbon compounds, thus explaining the prevalent hydrophobicity of fluorocarbons. In general, densely packed polar terminations do not always lead to short-range electric fields. For example, surfaces with substantial electron density spill-out give rise to electric fields with a much slower decay. However, electronic spill-out is limited in F/H-terminated carbon materials. Therefore, our ab initio results, can be reproduced and rationalized by a simple classical point-charge model. Consequently, classical force fields can be used to study the wetting of F/H-terminated diamond, revealing a pronounced correlation between adsorption energies of single H2O molecules and water contact angles.

Hydrogenated TiO2 (H:TiO2) is intensively investigated due to its improvement in solar absorption, but there are major issues related to its structural, optical and electronic properties and therefore an easily compatible method of preparation is much needed. In order to clarify this issue we studied TiO2 nanocrystals under the partial pressure of hydrogen to modify the structural, optical and electrical properties and to significantly improve the photocatalytic and photoelectrochemical performance. The hydrogen treated TiO2 nanocrystals contained paramagnetic Ti3+ centers and exhibited a higher visible light absorption cross-section as was confirmed by electron paramagnetic resonance diffuse reflectance spectra measurements and X-ray photoelectron spectroscopy. The hydrogen annealed samples showed a noticeable improvement in photocatalytic activity under visible light (λ > 380 nm) which was demonstrated by the degradation of methylene blue dye and an improved photoelectrochemical response in terms of high photocurrent density. Ab initio simulations of TiO2 were performed in order to elucidate the conditions under which localized Ti3+ centres rather than delocalized shallow donor states are created upon the reduction of TiO2. Randomly distributed oxygen vacancies lead to localized deep donor states while the occupation of the oxygen vacancies by atomic hydrogen favours the delocalized shallow donor solution. Furthermore, it was found that localization is stabilized at high defect concentrations and destabilized under external pressures. In those cases where localized Ti3+ states are present, the DFT simulations showed a considerable enhancement of the visible light absorption as well as a pronounced broadening of the localized Ti3+ energy levels with increasing defect concentration.

Large-scale quantum molecular dynamics simulations unveil eight friction regimes of water-lubricated low-index diamond surfaces. Four of these friction regimes are universal, i.e., they occur on diamond (111), (001), as well as (110). Dry sliding leads to immediate cold welding accompanied by amorphization (regime I). Small amounts of water (less than $8{\mathrm{H}}_{2}\mathrm{O}$ per $\mathrm{n}{\mathrm{m}}^{2})$ can preserve crystallinity and lower friction by localizing shear to interfacial ether groups (regime II). A further increase in water surface density results in passivating hydrogen/hydroxyl layers (regime IV) and finally (for more than $20{\mathrm{H}}_{2}\mathrm{O}$ per $\mathrm{n}{\mathrm{m}}^{2})$ in free water layers between hydrogen/hydroxyl passivated diamond surfaces (regime V). The other four friction regimes are special, i.e., they occur only on certain surfaces. An ultralow friction regime is established by aromatic Pandey surface passivation on diamond (111) surfaces (regime III). On diamond (110) surfaces, regime II coexists with three other regimes: while partial cold welding via C--C bonds (regime VI) or C--O--C bonds (regime VII) leads to frictional shear stresses that are in-between the cold-welding regimes (I and II) and the non-cold-welding regimes (IV and V), the formation of an oxidized carbon monolayer consisting of keto and ether groups results in ultralow friction (regime VIII). Regime VIII is also observed for diamond (001) surfaces. These findings are rationalized by the structural and energetic peculiarities of the different low-index surfaces. Our study provides guidelines for nanoscale control and manipulation of oxygen functional groups on carbon surfaces in boundary lubrication with water or other oxygen-containing lubricants.

Methanol formation from CO2 and molecular hydrogen on ZnO/Cu catalysts is studied by gradient corrected density functional theory. The catalytically active region is modeled as a minimum size inverse catalyst represented by ZnxO gamma(H) clusters of different size and a ZnO nano-ribbon on an extended Cu (1 1 1) surface. These systems are chosen as a representative of thermodynamically stable catalyst structures under typical reaction conditions. Comparison to a high level wave function method reveals that density functional theory systematically underestimates reaction barriers, but nevertheless conserves their energetic ordering. In contrast to other metal-supported oxides like ceria and zirconia, the reaction proceeds through the formation of formate on ZnOx/Cu, thus avoiding the CO intermediate. The difference between the oxides is attributed to variance in the initial activation of CO2. The energetics of the formate reaction pathway is insensitive to the exact environment of undercoordinated Zn active sites, which points to a general mechanism for Cu-Zn based catalysts. (C) 2018 Elsevier Inc. All rights reserved.

Molecular dynamics simulations of diamond-cubic silicon and carbon under combined shear and compression show the formation of an amorphous solid with liquidlike structure at room temperature. Consistent with the opposite density changes of the two crystals upon melting, the amorphous material is denser than the crystal in silicon and less dense than the crystal in carbon. As a result, its rate of formation is enhanced by pressure in silicon but suppressed in carbon. These results are particularly unexpected for silicon, whose amorphous structure is supposed to be liquidlike only when hydrostatically compressed above the polyamorphic transition pressure (∼14GPa). Below this pressure, amorphous silicon is expected to have a low-density structure with density close to that of the diamond-cubic crystal. Our simulations show that this polyamorphic transition disappears under shear and high-density, liquidlike amorphous silicon with metallic ductility forms even at low pressure. These results are potentially transferable to other diamond-cubic crystals, like germanium and ice Ih, and provide insights into nonequilibrium materials transformations that govern friction and wear in tribological systems.

This work encompasses a facile method for tailoring surface defects in electrospun TiO2 nanofibers by employing hydrogen plasma treatments. This amiable processing method was proven with SQUID, EPR, and XPS to be highly effective in generating oxygen vacancies, accompanied by the reduction of Ti4+ centers to Ti3+, resulting in the formation of black titania. The treatment temperature was found to affect the Ti3+/Ti4+ ratios and surface valence, while preserving the original 1D morphology of the titania fibers. Ab initio DFT calculations showed that a high concentration of oxygen vacancies is highly efficient in producing midgap states that enhance the system absorption over the whole visible range, as observed with UV/vis/NIR diffuse reflectance spectroscopy. Pristine TiO2 nanofibers produced a photocurrent density of 0.02 mA/cm2 at 1.23 V vs RHE, whereas the hydrogen plasma treatment resulted in up to a 10-fold increase in the photoelectrochemical performance.

Simulations of magnetorheological fluids are performed with different models for the hydrodynamic drag law. The shear stress predictions from two coupled discrete element – smoothed particle hydrodynamics models with different drag laws are compared to pure discrete element simulations for a wide range of Mason numbers. The discrete element model has a higher computational efficiency but the treatment of the hydrodynamic drag force involves some rough approximations. Based on the results of this study, a criterion is proposed for the applicability of the pure discrete element model in the simulation of sheared magnetorheological suspensions.

Large-scale quantum molecular dynamics of water-lubricated diamond (111) surfaces in sliding contact reveals multiple friction regimes. While water starvation causes amorphization of the tribological interface, small H_{2}O traces are sufficient to preserve crystallinity. This can result in high friction due to cold welding via ether groups or in ultralow friction due to aromatic surface passivation triggered by tribo-induced Pandey reconstruction. At higher water coverage, Grotthuss-type diffusion and H_{2}O dissociation yield dense H/OH surface passivation leading to another ultralow friction regime.

Selectivity and low power consumption are major challenges in the development of sophisticated gas sensor devices. A sensor system is presented that unifies selective sensor-gas interactions and energy-harvesting properties, using defined organic-inorganic hybrid materials. Simulations of chemical-binding interactions and the consequent electronic surface modulation give more insight into the complex sensing mechanism of selective gas detection.

We use density functional calculations to determine the binding sites and binding energies of Li + at graphene edges and prismatic graphite surfaces. Binding is favorable at bare and carbonyl terminated surfaces, but not favorable at hydrogen terminated surfaces. These findings have implications for the exfoliation of graphitic anodes in lithium-ion batteries that happens if solute and solvent co-intercalate. First, specific adsorption facilitates desolvation of Li + . Second, chemisorption lowers the surface energy by about 1 J m −2 prismatic surface area, and gives graphite additional stability against exfoliation. The results offer an explanation for experiments that consistently show exfoliation for hydrogenated graphite, but show no exfoliation for oxygenated graphite.

The collision of two cylindrical hydrogen-free diamond-like carbon (DLC) asperities with approximately 60 % sp3 hybridization has been studied using classical molecular dynamics. The severity of the collision can be controlled by the impact parameter b that measures the width of the projected overlap of the two cylinders. For a cylinder radius of R = 23 nm, three collisions with b = 0.5 nm, b = 1 nm and b = 2.0 nm are compared. While for the two small b a single shear band between the collision partners and a strongly localized sp2/sp1 hybridised third-body zone between the asperities is observed, the b = 2 nm collision is accompanied by pronounced plastic deformation in both asperities that destabilize the metastable sp3-rich phase leading to a drastic increase in the amount of rehybridized tribomaterial. In addition, pronounced roughening of the cylinder surfaces, asymmetric material transfer and the generation of wear debris are found in this case. For the b = 0.5 and 1 nm collision, the evolution of third-body volume can be quantitatively described by a simple geometric overlap model that assumes a sliding-induced phase transformation localized between both asperities. For b = 2 nm, this model underestimates the third-body volume by more than 150 % indicating that plasticity has to be taken into account in simple geometric models of severe DLC/DLC asperity collisions.

The long-term stability of InGaN photoanodes in liquid environments is an essential requirement for their use in photoelectrochemistry. In this paper, we investigate the relationships between the compositional changes at the surface of n-type In(x)Ga(1-x)N (x ∼ 0.10) and its photoelectrochemical stability in phosphate buffer solutions with pH 7.4 and 11.3. Surface analyses reveal that InGaN undergoes oxidation under photoelectrochemical operation conditions (i.e., under solar light illumination and constant bias of 0.5 VRHE), forming a thin amorphous oxide layer having a pH-dependent chemical composition. We found that the formed oxide is mainly composed of Ga-O bonds at pH 7.4, whereas at pH 11.3 the In-O bonds are dominant. The photoelectrical properties of InGaN photoanodes are intimately related to the chemical composition of their surface oxides. For instance, after the formation of the oxide layer (mainly Ga-O bonds) at pH 7.4, no photocurrent flow was observed, whereas the oxide layer (mainly In-O bonds) at pH 11.3 contributes to enhance the photocurrent, possibly because of its reported high photocatalytic activity. Once a critical oxide thickness was reached, especially at pH 7.4, no significant changes in the photoelectrical properties were observed for the rest of the test duration. This study provides new insights into the oxidation processes occurring at the InGaN/liquid interface, which can be exploited to improve InGaN stability and enhance photoanode performance for biosensing and water-splitting applications.

Diamond surfaces wear when sliding against silica – a much softer material. Although this plays a major role in chemical mechanical planarization of diamond or machining of rocks, the underlying microscopic mechanisms are hardly understood. Here, the stability of diamond (110) surfaces in sliding contact with amorphous silica and with amorphous silicon (a mechanically similar reference system) is studied by quantum-chemical simulations. Interfacial C–Si bonds with amorphous silicon are too weak to harm diamond (110) surfaces. Conversely, strong C–O and C–Si bonds form between silica and diamond. However, this alone is insufficient to challenge interfacial diamond-like C–C bonds during sliding. Analysis of local forces and electronic structure shows that bonding to silica can chemically activate C–C bonds between terminating carbon zigzag chains and bulk diamond when the zigzag chains stay aromatic. Mechanochemical breaking of these weak bonds and subsequent lifting of C–C–C zigzag units mark the onset of wear on diamond (110) surfaces. This is in contrast to the silicon case, where the higher interfacial bond density renders the diamond surface non-aromatic and diamond-like.

Photoelectrochemical (PEC) properties of nanostructured hematite (Fe2O3) thin films prepared using plasma-enhanced chemical vapor deposition (PE-CVD) were investigated against the influence of processing parameters and post-synthesis heat-treatment procedures. Annealing at high temperatures (>500 °C) was found to substantially affect the micro-structure (grain growth and densification) and electronic (interdiffusion at the film/substrate interface) concomitantly manifested in an enhancement in the PEC behavior. The Sn impurity level in hematite films was found to increase with the annealing temperature with highest values achieved in samples heat-treated at 750 °C, due to the interdiffusion and substitution of Sn(IV) species at Fe(III) sites. Sn:Fe2O3 films exhibited significantly high photocurrent density of 1.33 mA cm−2 at the water oxidation level of 1.23 V vs. RHE. The diffusion of Sn ions into iron oxide lattice altered the electronic properties of hematite films due to electron–donor behavior of the dopants that was verified by X-ray photoelectron spectroscopy and secondary ion mass spectroscopy (SIMS) analyses. Deposition of a thin overlayer of TiO2 (10 nm) on hematite films by atomic layer deposition (ALD) was found to further improve the photocurrent density to 1.8 mA cm−2 at 1.23 V vs. RHE. Ab-initio calculations on the effect of substitutional Sn(IV) dopants in the Fe2O3 lattice on the electronic structure and the band alignment between hematite and the TiO2 over layer revealed that Sn-dopants led to the generation of localized Fe(II) centers augmenting the n-type behavior of hematite. No effect of the Sn-doping on the electrostatic potential was found on a macroscopic scale. However, the charge transfer from the Sn-doping to the Fe(II) centers would cause high electric fields on the nanometer scale and might hence play an important role in the efficient separation of electron and holes. The simulations showed that the hematite band edges are enclosed by the TiO2 band edges and therefore electron depletion at the surface–liquid interface is enhanced. This might lead to reduced recombination rates near the surface and consequently to increased photocurrents, since the Fe2O3/TiO2 interface constitutes a barrier for hole transport.

Linear sp-hybridized carbon chains have recently been proposed as one-dimensional nanostructures for electronic applications and as intermediate products in many nanoscale processes. However, their...

We carry out molecular statics simulations of the indentation of bare and graphene-covered Pt (111) surfaces with smooth and rough indenters of radius 1.5 to 10 nm. Our simulations show that the plastic yield of bare surfaces strongly depends on atomic-scale indenter roughness such as terraces or amorphous disorder. Covering surfaces with graphene regularizes this response to the results obtained for ideally smooth indenters. Our results suggest that graphene monolayers and other 2D materials mitigate the effect of roughness, which could be exploited to improve the fidelity of experiments that probe the mechanical properties of interfaces.

Tribological shearing of polycrystalline metals typically leads to grain refinement at the sliding interface. This study, however, shows that nanocrystalline metals exhibit qualitatively different behavior. Using large-scale atomistic simulations, we demonstrate that during sliding, contact interface nanocrystalline grains self-organize through extensive grain coarsening and lattice rotation until the optimal plastic slip orientation is established. Subsequently, plastic deformation is frequently confined to localized nanoshear bands aligned with the shearing direction and emanating from voids and other defects in the vicinity of the sliding interface.

Organic-inorganic hybrid gas sensors can offer outstanding performance in terms of selectivity and sensitivity towards single gas species. The enormous variety of organic functionalities enables novel flexibility of active sensor surfaces compared to commonly used pure inorganic materials, but goes along with an increase of system complexity that usually hinders a predictable sensor design. In this work, an ultra-selective NO2 sensor is realized based on self-assembled monolayer (SAM)-modified semiconductor nanowires (NWs). The crucial chemical and electronic parameters for an effective interaction between the sensor and different gas species are identified using density functional theory simulations. The theoretical findings are consistent with the experimentally observed extraordinary selectivity and sensitivity of the amine-terminated SnO2 NW towards NO2. The energetic position of the SAM-gas frontier orbitals with respect to the NW Fermi level is the key to ensure or impede an efficient charge transfer between the NW and the gas. As this condition strongly depends on the gas species and the sensor system, these insights into the charge transfer mechanisms can have a substantial impact on the development of highly selective hybrid gas sensors.

We demonstrate the effect of hydrogen plasma treatment on hematite films as a simple and effective strategy for modifying the existing substrate to improve significantly the band edge positions and photoelectrochemical (PEC) performance. Plasma treated hematite films were consist of mixed phases (Fe 3 O 4 :α-Fe 2 O 3 ) which was confirmed by XPS and Raman analysis, treated films also showed higher absorption cross-section and were found to be a promising photoelectrode material. The treated samples showed enhance photocurrent densities with maximum of 3.5 mA/cm 2 at 1.8 V/RHE and the photocurrent onset potentials were shifted from 1.68 V RHE (untreated) to 1.28 V RHE (treated). Hydrogen plasma treatment under non-equilibrium conditions induced a valence dynamics among Fe centers in the sub-surface region that was sustained by the incorporation of hydrogen in the hematite lattice as supported by the density functional theory calculations.

Typical empirical bond-order potentials are short ranged and give ductile instead of brittle behavior for materials such as crystalline silicon or diamond. Screening functions can be used to increase the range of these potentials. We outline a general procedure to combine screening functions with bond-order potentials that does not require to refit any of the potential's properties. We use this approach to modify Tersoff's [Phys. Rev. B 39, 5566 (1989)], Erhart & Albe's [Phys. Rev. B 71, 35211 (2005)] and Kumagai et al.'s [Comp. Mater. Sci. 39, 457 (2007)] Si, C and Si-C potentials. The resulting potential formulations correctly reproduce brittle materials response, and give an improved description of amorphous phases.

Coulson’s bond order is a chemically intuitive quantity that measures the difference in the occupation of bonding and anti-bonding orbitals. Both empirical and rigorously derived bond order expressions have evolved in the course of time and proven very useful for atomistic modeling of materials. The latest generation of empirical formulations has recently been augmented by screening-function approaches. Using friction and wear of diamond and diamond-like carbon as examples, we demonstrate that such a screened bond order scheme allows for a faithful description of dynamical bond-breaking processes in materials far from equilibrium. The rigorous bond order expansions are obtained by systematic coarse-graining of the tight binding approximation and form a bridge between the electronic structure and the atomistic modeling hierarchies. They have enabled bottom-up derivations of bond order potentials for covalently bonded semiconductors, transition metals, and multicomponent intermetallics. The recently developed magnetic bond order potential gives a correct description of both directional covalent bonds and magnetic interactions in iron and is able to correctly predict the stability of bulk Fe polymorphs as well as the intricate properties of dislocation cores. The bond order schemes hence represent a family of reliable and powerful models that can be applied in large-scale simulations of complex processes involving fracture, wear, and plasticity.

We study the restructuring of solid nickel catalyst nanoparticles during carbon nanotube growth by environmental transmission electron microscopy and multiscale modeling. Our molecular dynamics/continuum transport calculations of surface-diffusion-mediated restructuring are in quantitative agreement with the experimentally observed catalyst shape evolutions. The restructuring time scale is determined by reduced Ni diffusion through the stepped Ni-C interface region where the catalyst surface strongly anchors to the growing nanotube.

We have re-investigated the structural identity of the famous gold-phosphine-halide Au:PR3:X compound having 55–69 gold atoms and core size of 1.4 nm (similar to “Schmid gold” or Nanogold®) from the viewpoint of the Superatom-Complex (SAC) model for ligand protected metal clusters, and in consideration of the ligand-adatom groups observed previously for the structurally known 39-atom cluster [Au39(PR3)14Cl6]−1. Density functional theory is used to define the formation energy of various compositions and structures, enabling a comparison of the stability of different cluster-sizes. In agreement with the SAC model, we find a strong correlation between optimal energy and delocalized electron shell closings: The 58 electron shell closing is a driving force behind the energetics. Of all compositions studied here, the energetically best one is [Au69(PR3)20Cl12]−1 anion, which has a truncated decahedral 37-atom core encapsulated by 20 Au:PR3 and 12 Au–Cl groups. It is energetically and chemically far superior to the standard models based on Au55(PR3)12X6. Critical comparisons are made to recent experiments (NMR and mass spectrometry).

We deform representative volume elements of amorphous carbon obtained from melt-quenches in molecular dynamics calculations using bond-order and machine learning interatomic potentials. A Drucker-Prager law with a zero-pressure flow stress of $41.2$~GPa and an internal friction coefficient of $0.39$ describes the deviatoric stress during flow as a function of pressure. We identify the mean coordination number as the order parameter describing this flow surface. However, a description of the dynamical relaxation of the quenched samples towards steady-state flow requires an additional order parameter. We suggest an intrinsic strain of the samples as a possible order parameter and present equations for its evolution. Our results provide insights into rehybridization and pressure dependence of friction between coated surfaces as well as routes towards the description of amorphous carbon in macroscale models of deformation., Comment: 12 pages, 3 figures