With the aim of manipulating the mechanical properties of the recently discussed two-dimensional material MXene, we investigate the effect of alloying. We consider substitutional doping of B and V at Ti and C sites of Ti 2 C. Calculations of quantities such as in-plane stiffness, Young's modulus, and critical strain through rigorous first-principles technique establish that B doping is highly effective in improving the elastic properties. Oxygen passivation of B-doped Ti 2 C in addition to improved elastic properties also exhibits reasonably high critical strains making them ideally suited for applications in flexible devices. Our study further reveals the presence of strong spin-phonon coupling in unpassivated Ti 2 C compounds which influences the mechanical behavior. The damage of Ti 2 C in its magnetic ground state of A-type antiferromagnetic structure is found to occur at much higher strain than that of the nonmagnetic Ti 2 C.
The out-of-plane electric polarization at the surface of SrTiO 3 (STO), an archetypal perovskite oxide, may stabilize new electronic states and/or host novel device functionality. This is particularly significant in proximity to atomically thin membranes, such as graphene, although a quantitative understanding of the polarization across graphene-STO interface remains experimentally elusive. Here, we report direct observation and measurement of a large intrinsic out-of-plane polarization at the interface of singlelayer graphene and TiO 2-terminated STO (100) crystal. Using a unique temperature dependence of anti-hysteretic gate-transfer characteristics in dual-gated graphene-on-STO field-effect transistors, we estimate the polarization to be as large as ≈12 μC cm −2 , which is also supported by the density functional theory calculations and low-frequency noise measurements. The anti-hysteretic transfer characteristics is quantitatively shown to arise from an interplay of band bending at the STO surface and electrostatic potential due to interface polarization, which may be a generic feature in hybrid electronic devices from two-dimensional materials and perovskite oxides.
With the aim of manipulating the mechanical properties of the recently discussed two-dimensional material MXene, we investigate the effect of alloying. We consider substitutional doping of B and V at Ti and C sites of Ti 2 C. Calculations of quantities such as in-plane stiffness, Young's modulus, and critical strain through rigorous first-principles technique establish that B doping is highly effective in improving the elastic properties. Oxygen passivation of B-doped Ti 2 C in addition to improved elastic properties also exhibits reasonably high critical strains making them ideally suited for applications in flexible devices. Our study further reveals the presence of strong spin-phonon coupling in unpassivated Ti 2 C compounds which influences the mechanical behavior. The damage of Ti 2 C in its magnetic ground state of A-type antiferromagnetic structure is found to occur at much higher strain than that of the nonmagnetic Ti 2 C.
From a careful analysis of existing data as well as new measurements, we show that the size dependence of the lattice parameters in metal nanoparticles with face-centered cubic (fcc) and body-centered cubic (bcc) symmetries display opposite trends: nanoparticles with fcc structure generally contract with decreasing particle size, while those with bcc structure expand. We present a microscopic explanation for this apparently puzzling behavior based on first-principles simulations. Our results, obtained from a comparison of density functional theory calculations with experimental data, indicate that the nanoparticles are capped by a surface monolayer of oxygen atoms, which is routinely detected by surface-sensitive techniques. The bcc- and fcc-based nanoparticles respond in contrasting fashion to the presence of the oxygen capping layer, and this dictates whether the corresponding lattice parameter would increase or decrease with size reduction. The metal-oxygen bonds at the surface, being shorter and stronger than typical metal-metal bonds, pull the surface metal atoms outward. This outward movement of surface atoms influences the core regions to a larger extent in the relatively open bcc geometry, producing a rather large overall expansion of the cluster, compared to the bulk. In case of fcc clusters, on the other hand, the outward movement of surface metal atoms does not percolate too far inside, resulting in either a smaller net expansion or contraction of the cluster depending on the extent of surface oxygen coverage. Our study therefore provides a convincing physicochemical basis for the correlation between the underlying geometry and the nature of change of the lattice parameters under size reduction.
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