Nanofriction with few layers of graphene as lubrication is an interesting issue recently, and it provides a quite important guide for modeling the nanofriction properties of nanodevice. Based on the molecular dynamics (MD) simulations, nanofriction properties of a silicon tip sliding on different graphene layers with or without substrate were studied systemically. We revealed that the friction of these systems exhibits clearly the even-odd oscillations with different thickness of graphene, and we further demonstrated that such even-odd oscillations behavior is totally independent of the size of the silicon tips, as well as applying normal loadings. The underlying physics of this intriguing phenomenon is attributed to the oscillations of indirect-contact-atom-number between top and sublayers of suspended graphene. Furthermore, we showed that such indirect contact oscillations would be reflected by the direct contact oscillations between the tip and the top-layer graphene when graphene lubrication layers on a rigid substrate. Overall, our new findings not only enrich the nanofriction mechanism of graphene lubrication systems, but also introduce a new way to design the nanofriction systems with two-dimensional (2D) van der Waals materials as lubrications.

On the basis of density functional theory (DFT) calculations, we propose a stable two-dimensional (2D) monolayer phosphorus carbide (PC) with a GaSe-like structure, which has intriguing electronic and optical properties. Our calculated results show that this 2D monolayer structure is more stable than the other allotropes predicted by Tománek et al. [Nano Lett., 2016, 16, 3247–3252]. More importantly, this structure exhibits superb optical absorption, which can be mainly attributed to its direct band gap of 2.65 eV. The band edge alignments indicate that the 2D PC monolayer structure can be a promising candidate for photocatalytic water splitting. Furthermore, we found that strain is an effective method used to tune the electronic structures varying from direct to indirect band-gap semiconductor or even to metal. In addition, the introduction of one carbon vacancy in such a 2D PC structure can induce a magnetic moment of 1.22 µB. Our findings add a new member to the 2D material family and provide a promising candidate for optoelectronic devices in the future.

Using first-principles density functional theory (DFT) calculations, we systematically investigate the strain effects on the adsorption energies, magnetic ordering and electronic properties of 3d transition metal (TM) atoms (from Sc to Co) adsorbed on phosphorene (P). We find that the adsorption energy of TM can be enhanced by compressive strain whereas weakened by tensile strain. Our results show that strain plays a decisive role in the magnetic moments as well as the magnetic coupling states of TM adatoms. Importantly, the transitions from antiferromagnetic (AFM) state to ferromagnetic (FM) state or to another different AFM ordering can be induced by strain effect. In addition, we observe the semiconductor to metal or half-metal transitions in some TM@P systems by applying strain. Our findings shed a new light on precisely engineering the magnetic properties and electronic properties of the TM@P systems, which will have great potential applications in spin electronics and other related fields. (C) 2017 Elsevier B.V. All rights reserved.

Manipulating magnetism of low-dimensional materials is of great importance for their practical applications. Here, using first-principles calculations, we report a systematic investigation of the magnetic properties of C-doped H saturated zigzag phosphorene nanoribbons (H-ZPNRs), which are rather different from those of 2D periodic systems due to the quantum size effect. First of all, we observed a greatly enhanced magnetic moment locating mainly on the C atom and also slightly on its surrounding P atoms. Our results also indicated a strong dependence of the magnetic moment of the C atom on its location, which decays from the edge to the center site of the nanoribbons with an odd-even oscillating behavior originating from Friedel oscillation in low-dimensional materials. As for the C atom on a specific location, its magnetic moment decreases gradually with increasing width of H-ZPNRs, degenerating to the 2D case. What is more, we found that both the magnitude and the oscillating behavior of the magnetic moment on the C atom can be tuned by the edge saturation atoms. In addition, the case of two C atoms co-doping H-ZPNRs was also studied, showing non-magnetic (NM), ferromagnetic (FM) and antiferromagnetic (AFM) states depending on the locations of the two C atoms. Our findings suggest a plausible route for manipulating magnetism of the sp element doped H-ZPNRs, which are expected to have potential applications in spintronics.

Employing first-principles approach within density functional theory (DFT), spin friction properties of Co monolayer sliding on Mn/W(110) substrate were investigated (Wolter et al., 2012) [22]. The magnitude of spin friction is obtained by calculating the friction difference between the spin-polarized and non-spin-polarized cases. Our results reveal that the magnetic system exhibits significantly lower friction than the non-magnetic system, which can be explained by an electronic level mechanism of spins on friction properties. Additionally, this study provides an approach to estimate the spin friction under the formwork of DFT.

Understanding and controlling nanofriction are important in practical applications of nanotechnology. Our first-principles calculations reveal that interlayer nanofriction between two graphene layers can be tuned by applying an external electric field; the tuned magnitude of the coefficient of friction ranges from −30 to 30 %, which is attributed to the increased disparity of electronic structures between AA and AB stackings. This effect is significantly observed in boron- or nitrogen-doped systems compared with a pristine graphene system. Our findings present a feasible and precise strategy to tune the frictional properties of graphene systems.

The van der Waals corrected first-principles approach within density functional theory was used to investigate the nanofriction properties of phosphorus-doped nanocrystalline diamond films. Our results demonstrate that substitutional phosphorus atoms can significantly decrease the coefficient of friction between two hydrogenated diamond films, and the doping effects are affected by the interfacial environment. These nanofriction modulations can be elucidated by interfacial charge redistribution induced by doping atoms. Our study presents an electronic level mechanism of the doping effects on friction properties of doped interfaces.

Density functional theory calculations including dispersion correction (DFT-D2) were used to investigate the relationship between charge distribution and nanofriction characteristics of graphene-based material systems. In our calculations, the single-side-hydrogenated graphene (SSHGraphene) system exhibits lower coefficient of friction, whereas the graphane system exhibits larger one compared with graphene system. These results are attributed to the adjustments of interfacial charge distribution that are induced by different hydrogen passivations. The charge distribution is smooth along the sliding direction for the SSHGraphene sheet, which yields a small potential barrier. Corrugation of the charge distribution in graphane system is much steeper than that in graphene system, which leads to a larger potential barrier. Comparative investigations reveal that the interfacial charge distributions determine the nanofriction performance, which may be helpful for friction modulation and design of new controlling lubricant material.