Although very similar in many technological applications, graphene and MoS 2 bear significant differences if exposed to humid environments. As an example, lubrication properties of graphene are reported to improve while those of MoS 2 to deteriorate: it is unclear whether this is due to oxidation from disulfide to oxide or to water adsorption on the sliding surface. By means of ab initio calculations we show here that these two layered materials have similar adsorption energies for water on the basal planes. They both tend to avoid water intercalation between their layers and to display only mild reactivity of defects located on the basal plane. It is along the edges where marked differences arise: graphene edges are more reactive at the point that they immediately prompt water splitting. MoS 2 edges are more stable and consequently water adsorption is much less favoured than in graphene. We also show that water-driven oxidation of MoS 2 layers is unfavoured with respect to adsorption.

We derive a connection between the intrinsic tribological properties and the electronic properties of a solid interface. In particular, we show that the adhesion and frictional forces are dictated by the electronic charge redistribution occurring due to the relative displacements of the two surfaces in contact. We define a figure of merit to quantify such a charge redistribution and show that simple functional relations hold for a wide series of interactions including metallic, covalent, and physical bonds. This suggests unconventional ways of measuring friction by recording the evolution of the interfacial electronic charge during sliding. Finally, we explain that the key mechanism to reduce adhesive friction is to inhibit the charge flow at the interface and provide examples of this mechanism in common lubricant additives.

Among the members of the transition metal dichalcogenides (TMD) family, molybdenum disulfide has the most consolidated application outcomes in tribological fields. However, despite the growing usage as nanostructured solid lubricant due to its lamellar structure, little is known about the atomistic interactions taking place at the interface between two MoS2 sliding layers, especially at high loads. By means of ab initio modeling of the static potential energy surface and charge distribution analysis, we demonstrate how electrostatic interactions, negligible in comparison with van der Waals and Pauli contributions at zero load, progressively affect the sliding motion at increasing loads. As such, they discriminate the relative stability and the frictional behavior of bilayers where the two monolayers defining the interface have a different relative orientation. In particular, for antiparallel sliding layers we observed a load-induced increase of both the depth of the minima and the height of the energy barriers compared to parallel ones, which may have important consequences for the fabrication of more efficient ultralow friction devices at the nanoscale.Among the members of the transition metal dichalcogenides (TMD) family, molybdenum disulfide has the most consolidated application outcomes in tribological fields. However, despite the growing usage as nanostructured solid lubricant due to its lamellar structure, little is known about the atomistic interactions taking place at the interface between two MoS2 sliding layers, especially at high loads. By means of ab initio modeling of the static potential energy surface and charge distribution analysis, we demonstrate how electrostatic interactions, negligible in comparison with van der Waals and Pauli contributions at zero load, progressively affect the sliding motion at increasing loads. As such, they discriminate the relative stability and the frictional behavior of bilayers where the two monolayers defining the interface have a different relative orientation. In particular, for antiparallel sliding layers we observed a load-induced increase of both the depth of the minima and the height of the energy barriers compared to parallel ones, which may have important consequences for the fabrication of more efficient ultralow friction devices at the nanoscale

Electron transport through metal-molecule contacts greatly affects the operation and performance of electronic devices based on organic semiconductors [1][2][3][4] and is at the heart of molecular electronics exploiting single-molecule junctions [5][6][7][8] . Much of our understanding of the charge injection and extraction processes in these systems relies on our knowledge of the potential barrier at the contact. Despite significant experimental and theoretical advances a clear rationale of the contact barrier at the single-molecule level is still missing. Here, we use scanning tunnelling microscopy to probe directly the nanocontact between a single molecule and a metal electrode in unprecedented detail. Our experiments show a significant variation on the submolecular scale. The local barrier modulation across an isolated 4-[trans-2-(pyrid-4-yl-vinyl)] benzoic acid molecule bound to a copper(111) electrode exceeds 1 eV. The giant modulation reflects the interaction between specific molecular groups and the metal and illustrates the critical processes determining the interface potential. Guided by our results, we introduce a new scheme to locally manipulate the potential barrier of the molecular nanocontacts with atomic precision.The electronic structure at the interface between a bulk metal and an organic semiconductor thin film has been extensively studied [9][10][11] and is commonly described in the framework of a band alignment model at the interface. In the single-molecule case, however, this model faces its limits. Chemical bonding between an organic molecule and a metal surface can result in significant charge transfer and rearrangement, which depend critically on the local atomic geometry 5,12 . In this situation of a strongly hybridized electronic system, a good indicator of the physical and chemical processes determining the molecule-metal contact is the work function, which for metal substrates is defined as the energy difference between the vacuum level far above the surface and the Fermi level (see also Fig. 1a; ref. 13).The formation of induced dipoles at the interface owing to the bonding of molecules can substantially modify the work function, which thereby provides valuable information on the degree of charge reorganization at the interface [13][14][15] . Photoemission experiments, often used to determine the averaged, coverage-dependent work function of a surface 11,16,17 , have generated considerable progress in understanding the formation of the interface built-in potential. However, these experiments cannot provide any information at the molecular length scale. Lateral resolution can be achieved by photoemission of adsorbed Xe, scanning tunnelling or Kelvin probe measurements 13,[18][19][20] . revealed local modifications of the work function but could not probe a single-molecule contact with submolecular resolution. A quantity closely related to the work function is the local potential-energy barrier experienced by tunnelling electrons during scanning tunnelling microscopy (STM) measurement...