doc. Antonio Cammarata, Ph.D.

Reaching near-zero friction is one of the jewels on the crown of tribology, and structural superlubricity is a crucial mechanism to achieve it. Previous works focus mainly on the structural superlubricity at incommensurate crystalline interfaces. However, realizing such interfaces on a large scale without defects and contaminations is a formidable challenge. Here, we report a charge-induced robust macroscale superlubricity between graphite and atomically flat surfaces in the ambient condition. We transferred graphite flakes on Si3N4 balls and used them to measure the friction properties on pristine and charged atomically flat surfaces such as 300 nm SiO2/Si and sapphire. We found that the surface charge can dramatically reduce the coefficient of friction between graphite and substrates by two orders of magnitude to 10−4, and the sliding is wearless even under harsh contact conditions (∼1.1 GPa center pressure and >100 m). We demonstrate that the surface charge is critical in achieving superlubricity possibly because it can reduce adhesion between graphite and substrate surfaces and make the substrate surfaces resistant to contaminations. Our method offers a ready-to-use solution to superlubricity alternative to achieve incommensurate crystalline interfaces. Thus, it can reduce the difficulty of realizing macroscale superlubricity for applications.

The most critical processes in designing a potential catalyst for the production of ethane are activating methane and suppressing further dehydrogenation. In the present study, using first- principle calculations, we predict that the BNG-Ir13 cluster can efficiently activate methane and promote the Csingle bondC coupling reactions. Ir in the BNG-Ir13 cluster's top site is found to be the most stable adsorption site of methane with stable adsorption energy of −0.45 eV. Methane is activated with a low activation energy barrier of 0.16 eV, and the reaction energy is −0.54 eV. It is the most facile step and is thermodynamically favorable, likely to occur at low-temperature conditions. The fourth dehydrogenation step is the rate-determining step, which shows the highest activation energy barrier (1.24 eV) in the methane dehydrogenation process on the BNG- Ir13 cluster. Based on the DFT calculations, selective dehydrogenation of methane and self- coupling reactions of methyl groups formed ethane with a low kinetic barrier assisted by the BNG-Ir13 cluster. Furthermore, hydrogen molecules are likely to form, implying that the BNG-Ir13 cluster is a potential and bi-functional catalyst in selective conversion of methane to ethane and in the production of hydrogen molecules at optimum conditions.

Diamond-based compounds are ideal materials to build nanoengineered devices with wide applicability in nanophotonics, optomechanics, photovoltaics and electronics, where tuning the width and the character of the electronic band gap is paramount. While the available information is focused on specific aspects of the material preparation and response, a general understanding of how the entangled geometric and electronic properties determine the characteristics of the band gap is missing. The present work aims to tackle this challenge by means of first principle simulations. We show that specific charge distributions in the ion environment determine the width of the band gap; in order to control it, we suggest how to select suitable dopant atomic types and how to impose specific structural deformations. We also propose different routes to switch the character of the band gap from indirect to direct. The results pave new avenues aimed to design diamond-based nanostructured materials with targeted optical and electronic properties. The outcomes of the present work are general and can therefore be promptly applied to the study of optical and electronic materials irrespective of their chemical composition and atomic topology.

Sliding of the WSC coated ceramics (ball-on-disc configuration) was investigated under vacuum condition at multiple loads (2–18 N), and each experiment was followed by detailed Raman and SEM analysis of both the wear track and the wear scar (on the ball). Main finding is that under low loads (up to 8 N), wear is polishing, carbon structure becomes more ordered and the number of WS2 monolayer increases. Above 8 N, wear transitions into an abrasive regime interrupting the aforementioned processes increasing both friction and wear. Furthermore, roles of the coating components were differentiated: WS component is responsible for the low friction, whereas the carbon part is responsible for excellent wear properties.

The control of friction at the atomic scale is fundamental to optimize the exfoliation of layered materials. To this aim, we report a density-functional investigation of how intercalated molecules affect the nanoscale friction of van derWaals transition-metal dichalcogenides.We find that the molecule does not interact with the electronic density of the layers directly; nonetheless it determines the features of the valence band of the system. In particular, the valence-band width appears to be a promising parameter to correlate the electronic properties with the nanofrictional response; it then constitutes a guide for the automatic search of intercalation molecules suitable for layer exfoliation. The present outcomes also constitute a theoretical tool for future investigations of the effect that intercalated species have on the nanoscale friction in layered materials.

Optimal regulation of lattice thermal conductivity in low-dimensional materials is fundamental to obtain highly efficient miniaturized devices. To this aim, we use quantum-mechanical based analyses to understand how atomic type and structural geometry determine electron density and lattice dynamic features ruling the thermal conduction. As a case study, we consider layered van der Waals transition metal dichalcogenides with a finite number of layers. We find that a large thermal conductivity is realized when the atomic bonds display highly covalent character, promoting fast motions of the cations in correspondence of the low-frequency phonon band. Such an effect is the result of the entangled electronic and phonon features, which are captured by the covalency and cophonicity metric. The investigation protocol that we present has general applicability and can be used to design novel thermal low-dimensional materials irrespective of the kind of atomic topology and chemical composition.

In dry sliding, the coefficient of friction depends on the material pair and contact conditions. If the material and operating conditions remain unchanged, the coefficient of friction is constant. Obviously, we can tune friction by surface treatments, but it is a nonreversible process. Here, we report active control of friction forces on TiO2 thin films under UV light. It is reversible and stable and can be tuned/controlled with the light wavelength. The analysis of atomic force microscopy signals by wavelet spectrograms reveals different mechanisms acting in the darkness and under UV. Ab initio simulations on UV light-exposed TiO2 show a lower atomic orbital overlapping on the surface, which leads to a friction reduction of up to 60%. We suggest that photocontrol of friction is due to the modification of atomic orbital interactions from both surfaces at the sliding interface.

Nanodiamonds, commonly described as fragments of diamond, have been theoretically found to have lower HOMO-LUMO energy splitting compared to the bandgap of bulk diamond. This apparent lack of correlation between theory and experiment is caused by the position of the LUMO, which is placed in the surface of the ND. An eventual enlargement of the ND towards a macroscopic size will turn the LUMO into the unoccupied surface states, which are not accounted if the bandgap of a bulk material is measured. Here, the electron structure of the nanodiamonds is evaluated, demonstrating that due their nature they should be described as discrete systems instead of bulk materials. Hence, the word bandgap should be avoided in the case of the nanodiamonds, using HOMO-LUMO gap instead. Additionally, our obtained ionization potentials show a satisfactory degree of correlation with the experiment, while the electron affinities are found to be positive. Although this feature fits the estimation performed from experimental data, it opposes the generally accepted idea of a negative electron affinity for hydrogenated nanodiamonds. The present article clarifies common misunderstandings regarding the electronic nature of the NDs, and provides some guidelines for the correct computation of this systems. Finally, as a helpful tool, an estimation of the content of carbon atoms and its surface to volume ratio is provided starting from the diamond unit cell.

Frictional forces acting during the relative motion of nanosurfaces are the cause of energy loss and wear which limit an efficient assembly and yield of atomic-scale devices. In this research, we investigate the microscopic origin of the dissipative processes as a result of the frictional response, with the aim to control them in a subtle way. We recast the study of friction in terms of phonon modes of the system at the equilibrium, with no need to resort to dynamics simulations. As a case study, we here consider layer sliding in transition metal dichalcogenides thin films. We find that the population of specific atomic orbitals and the relative contribution of the atomic type to selected system vibrations are the crucial quantities which determine the frictional response in tribological conditions. A reduced amount of energy dissipation is found when the bond character is more ionic and the layer sliding is realized by a faster motion of the chalcogen atoms. The individuated relevant parameters governing the energy dissipation can be used as descriptors in high-throughput calculations or machine learning engines to screen databases of frictional materials. The presented framework is general and can be promptly extended to the design of tribological materials with targeted frictional response, irrespective of the chemistry and atomic topology.

A proper control of nanoscale friction is mandatory for the fabrication and operation of optimal nanoengineered devices. In this respect, the use of electric fields looks to be promising, since they are able to alter the frictional response without imprinting permanent deformations into the structure. To this aim, we perform ab initio simulations to study the microscopic mechanisms governing friction in low-dimensional materials in the presence of electrostatic fields. We consider MX2 transition metal dichalcogenides as a case study. By applying an electric field along an axis orthogonal to the atom layers, we induce a transfer of charge along the same axis; this transfer modifies the interatomic forces, leading, in general, to easier relative layer motion. The reported outcomes constitute a starting point to study the effect of the field direction on the intrinsic friction in future investigations. Finally, the present results can be used to predict the preferential electronic redistribution in nanostructured devices where metal-to-insulator transitions may occur in working conditions.

Study of friction and energy dissipation always relied on direct observations. Actual theories provide limited prediction on the frictional and dissipative properties if only the material chemistry and geometry are known. We here develop a framework to study intrinsic friction and energy dissipation based on the only knowledge of the normal modes of the system at equilibrium. We derive an approximated expression for the first anharmonic term in the potential energy expansion which does not require the computation of the third-order force constants. Moreover, we show how to characterize the frequency content of observed physical quantities and individuate the dissipative processes active during experimental measurements. As a case study, we consider the relative sliding motion of atomic layers in molybdenum disulfide dry lubricant, and we discuss how to extract information on the energetics of sliding from atomic force microscopy signals. The presented framework switches the investigation paradigm on friction and energy dissipation from dynamic to static studies, paving avenues to explore for the design of alternative anisotropic tribological and thermal materials.

Phonon–phonon scattering processes are the crucial phenomena which account for phonon decay, thermal expansion, heat transfer, protein dynamics, spin relaxation and related quantities. In this work, we show how the symmetries of the system determine which scattering processes are allowed at any order of anharmonic approximation, irrespective of the chemical composition. We also discuss how to control the system symmetries to switch on and off any single scattering process. We apply the presented results to the study and control of nanoscale intrinsic friction and thermal transport in lamellar van der Waals transition metal dichalcogenides. Thanks to its general formulation, the presented framework expands the materials science tool set for the design of nanoengineered thermally-active materials, irrespective of the specific chemical composition and atomic topology.

Nanostructured materials are essential building blocks for the fabrication of new devices for energy harvesting/storage, sensing, catalysis, magnetic, and optoelectronic applications. However, because of the increase of technological needs, it is essential to identify new functional materials and improve the properties of existing ones. The objective of this Viewpoint is to examine the state of the art of atomic-scale simulative and experimental protocols aimed to the design of novel functional nanostructured materials, and to present new perspectives in the relative fields. This is the result of the debates of Symposium I “Atomic-scale design protocols towards energy, electronic, catalysis, and sensing applications”, which took place within the 2018 European Materials Research Society fall meeting.

We investigated themetal-insulator transition for epitaxial thin films of the perovskite CaFeO3, a material with a significant oxygen ligand hole contribution to its electronic structure.We find that biaxial tensile and compressive strain suppress the metal-insulator transition temperature. By combining hard x-ray photoelectron spectroscopy, soft x-ray absorption spectroscopy, and density functional calculations, we resolve the element-specific changes to the electronic structure across the metal-insulator transition. We demonstrate that the Fe sites undergo no observable spectroscopic change between themetallic and insulating states, whereas theOelectronic configuration undergoes significant changes. This strongly supports the bond-disproportionation model of the metal-insulator transition for CaFeO3 and highlights the importance of ligand holes in its electronic structure. By sensitively measuring the ligand hole density, however, we find that it increases by ∼5–10% in the insulating state, which we ascribe to a further localization of electron charge on the Fe sites. These results provide detailed insight into the metal-insulator transition of negative charge transfer compounds and should prove instructive for understanding metal-insulator transitions in other late transition metal compounds such as the nickelates.

This review summarizes recent advances in the area of tribology based on the outcome of a Lorentz Center workshop surveying various physical, chemical and mechanical phenomena across scales. Among the main themes discussed were those of rough surface representations, the breakdown of continuum theories at the nano- and micro-scales, as well as multiscale and multiphysics aspects for analytical and computational models relevant to applications spanning a variety of sectors, from automotive to biotribology and nanotechnology. Significant effort is still required to account for complementary nonlinear effects of plasticity, adhesion, friction, wear, lubrication and surface chemistry in tribological models. For each topic, we propose some research directions.

We study the atomic contributions to the nanoscale friction in layered MX2 (M = Mo, W; X = S, Se, Te) transitionmetal dichalcogenides by combining ab initio techniques with group-theoretical analysis. Starting from stable atomic configurations, we propose a computational method, named normal-modes transition approximation (NMTA), to individuate possible sliding paths from only the analysis of the phonon modes of the stable geometry. The method provides a way to decompose the atomic displacements realizing the layer sliding in terms of phonon modes of the stable structure, so as to guide the selection and tuning of specific atomic motions promoting MX2 sheets gliding, and to adjust the corresponding energy barrier. The present results show that main contributions to the nanoscale friction are due to few low frequency phonon modes, corresponding to rigid shifts of MX2 layers. We also provide further evidences that a previously reported Ti-doped MoS2 phase is a promising candidate as new material with enhanced tribologic properties. The NMTA approach can be exploited to tune the energetic and the structural features of specific phonon modes, and, thanks to its general formulation, can also be applied to any solid state system, irrespective of the chemical composition and structural topology.

Vibrational contributions to intrinsic friction in layered transition metal dichalcogenides (TMD) have been studied at different charge content. We find that any deviation from charge neutrality produces complex rearrangements of atomic positions and electronic distribution, and consequent phase transitions. Upon charge injection, cell volume expansion is observed, due to charge accumulation along an axis orthogonal to the layer planes. Such accumulation is accounted by the d3z2-r2 orbital of the transition metal and it is regulated by the Pt2g,eg orbital polarization. The latter, in turn, determines the frequency of the phonon modes related to the intrisic friction through non-trivial electro-vibrational coupling. The bond covalency and atom pair cophonicity can be exploited as a knob to control such coupling, ruling subtle charge flows through atomic orbitals hence determining vibrational frequencies at specific charge content. The results can be exploited to finely tune vibrational contributions to intrinsic friction in TMD structures, in order to facilitate assembly and operation of nanoelectromechanical systems and, ultimately, to govern electronic charge distribution in TMD-based devices for applications beyond nanoscale tribology.

CaFeO3 is a prototypical negative charge transfer oxide that undergoes electronic metal-insulator transition concomitant with a dilation and contraction of nearly rigid octahedra. Altering the charge neutrality of the bulk system destroys the electronic transition, while the structure is significantly modified at high charge content. Using density functional theory simulations, we predict an alternative avenue to modulate the structure and the electronic transition in CaFeO3. Charge distribution can be modulated using strain-rotation coupling and thin film engineering strategies, proposing themselves as a promising avenue for fine tuning electronic features in transition metal-oxide perovskite.

n-Layered (n = 2, 3, 4) MX2 transition metal dichalcogenides (M = Mo, W; X = S, Se, Te) have been studied using DFT techniques. Long-range van der Waals forces have been modeled using the Grimme correction to capture interlayer interactions. We study the dynamic and electronic dependence of atomic displacement on the number of layers. We find that the displacement patterns mainly affected by a change in the layer number are low-frequency modes at Γ and A k-points; such modes are connected with the intrinsic tribological response. We disentangle electro–phonon coupling by combining orbital polarization, covalency and cophonicity analysis with phonon band calculations. We find that the frequency dependence on the number of layers and the atomic type has a non-trivial relation with the electronic charge distribution in the interlayer region. We show that the interlayer electronic density can be adjusted by appropriately tuning M–X cophonicity, acting as a knob to control vibrational frequencies, hence the intrinsic frictional response. The present results can be exploited to study the electro–phonon coupling effects in TMD-based materials beyond tribological applications.

Ab initio and group theoretical techniques have been used to investigate the microscopic interactions that govern phase matchability in nonlinear optical materials. Li2CdMS4 (M = Ge, Sn) diamond-like semiconductors (DLSs) have been considered as a case study for their peculiarity: despite the similar geometry and stoichiometry, the former is type I phase matchable unlike the latter. We disentangle the electronic and structural features that determine the dielectric tensor into contributions that can be singularly adjusted, in order to tune the refractive index, and thus the phase matching behavior. We suggest possible experimental routes to modulate the refractive index and hence the phase matchability in DLSs. Finally we propose a new DLS material with low phase matching threshold. Such approach can be extended to harness the optical response in other classes of nonlinear optical materials.

We propose a protocol to disentangle the electro-vibrational structural coupling contributing to the intrinsic tribologic properties of layered MX2 transition metal dichalcogenides (M = Mo, W; X = S, Se, Te) under load. We employ ab initio techniques to model how changing the interlayer distance affects the electronic distribution and the vibrational properties of the system. We analyze the electro-vibrational coupling features by combining orbital polarization and mode Gruneisen parameters analyses with the recently developed bond covalency descriptor and the lattice dynamic metric named cophonicity. We find that intralayer charge distribution depends on the interlayer distance, determining, in turn, a shift of specific vibrational frequencies. We finally suggest a route to control the frequency shift, thus the bulk response to the load, in transition metal dichalcogenides through a proper selection of the atomic type.

Density functional theory and group-theoretical methods are used to explore the origin for ferroelectricity in cation ordered LaSrMnO4 with the Ruddlesden-Popper structure. The equilibrium phase exhibits the polar Pca21 space group where small polar displacements of d4Mn3+ coexist with antiferrodistortive octahedral rotations and Jahn-Teller distortions. We find that the octahedral rotations and Jahn-Teller distortion stabilize the polar structure and induce polar displacements through high-order anharmonic interactions among the three modes, making LaSrMnO4 a hybrid-improper ferroelectric material. The rotations result from the ionic size mismatch between A cations and Mn whereas the Jahn-Teller distortions are energetically favored owing to the coupling between the local eg orbital polarization of the two nearest-neighboring Mn cations in the two-dimensional MnO2 sheets. Our results indicate that anharmonic interactions among multiple centric modes can be activated by cation ordering to induce polar displacements in layered oxides, making it a reliable approach for realizing acentric properties in artificially constructed materials.

Lattice dynamics of MX2 transition metal dichalcogenides (M = Mo, W; X = S, Se, Te) have been studied with density functional theory techniques to control the macroscopic tribological behavior. Long-range van der Waals forces have been modeled with Grimme correction to capture the interlayer interactions. A new lattice dynamic metric, named cophonicity, is proposed and used in combination with electronic and geometric descriptors to relate the stability of the lattice distortions with the electro-structural features of the system. The cophonicity analysis shows that the distortion modes relevant to the microscopic friction can be controlled by tuning the relative MIX atomic contributions to the phonon density of states. Guidelines on how to engineer macroscopic friction at nanoscale are formulated, and finally applied to design a new Ti-doped MoS2 phase with enhanced tribologic properties.

The compositional dependence of metal-oxygen BO6 octahedral distortions, including bond elongations and rotations, is frequently discussed in the ABO3 perovskite literature; structural distortions alleviate internal stresses driven by under- or over-coordinated bond environments. Here we identify the dependence of octahedral rotations from changes in metal-oxygen bond covalency in orthorhombic perovskites. Using density functional theory we formulate a covalency metric, which captures both the real and k-space interactions between the magnitude and sense, i.e., in-phase or out-of-phase, octahedral rotations, to explore the link between the ionic-covalent Fe-O bond and the interoctahedral Fe-O-Fe bond angles in Pbnm ferrates. Our survey finds that the covalency of the metal-oxygen bond is correlated with the rotation amplitude: We find the more covalent the Fe-O bond, the less distorted is the structure and the more important the long-range inter-octahedral (Fe-O-Fe bond angle) interactions. Finally, we show how to indirectly tune the B-O bond covalency by A-cation induced BO6 rotations independent of ionic size, facilitating design of targeted bonding interactions in complex perovskites.

We use a symmetry-based structural analysis combined with an electronic descriptor for bond covalency to explain the origin of the second-order nonlinear optical response (second harmonic generation, SHG) in noncentrosymmetric nonpolar ATeMoO6 compounds (where A = Mg, Zn, or Cd). We show that the SHG response has a complex dependence on the asymmetric geometry of the AO6 and AO4 functional units and the orbital character at the valence band edge, which we are able to distinguish using an A–O bond covalency descriptor. The degree of covalency between the divalent A-site cation and the oxygen ligands dominates over the geometric contributions to the SHG arising from the acentric polyhedra, and this can be understood from considerations of the local static charge density distribution. The use of a local dipole model for the polyhedral moieties (AO4/AO6, MoO4, and TeO4) can account for a nonzero SHG response, even though the materials exhibit nonpolar structures; however, it is insufficient to explain the change in the magnitude of the SHG response upon A-cation substitution. The atomic scale and electronic structure understanding of the macroscopic SHG behavior is then used to identify hypothetical HgTeMoO6 as a candidate telluromolybdate with an enhanced nonlinear optical response.