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We explore the oxidation of a single layer of black phosphorus using ab initio density functional theory calculations. We search for the equilibrium structures of phosphorene oxides, POx with various oxygen concentrations x (0 ≤ x ≤ 1). By evaluating the formation energies with diverse configurations and their vibrational properties for each of various x values, we identify a series of stable oxidized structures with x and confirm that the oxidation occurs naturally. We also find that oxidation makes some modes from the P-O bonds and P-P bonds IR-active implying that the infrared spectra can be used to determine the degree of oxidation of phosphorene. Our electronic structure calculations reveal that the fully oxidized phosphorene (PO) has a direct band gap of 0.83 eV similar to the pristine phosphorene. Intriguingly, the PO possesses two nonsymmorphic symmetries with the inversion symmetry broken, guaranteeing symmetry-protected band structures including the band degeneracy and four-fold degenerate Dirac points. Our results provide an important guide in the search for the rare example of a Dirac semimetal with a higher level of degeneracy, giving significant insight into the relations between the symmetry of the lattice and band topology of electrons.

International audience

International audience; Edge roughness is expected to play a major role in narrow ribbons obtained from two-dimensional materials, due to the large length/surface ratio of the disordered edges with respect to the whole system surface. In the case of semiconducting transition metal dichalcogenides, a physical and quantitative understanding of the impact of edge roughness on the transport properties of ribbons with nanometer widths is essential in view of their potential applications in ultrascaled nanoelectronics. By means of atomistic quantum transport simulations, we show that the conductance due to edge states within the bulk gap is strongly suppressed by roughness. The corresponding localization length is found to be in the order of few nanometers. At low carrier energies outside the gap, edge roughness drives the system into the diffusive transport regime. The study of the mean free path, under different conditions of roughness and for different ribbon widths, shows that the conductance is moderately affected for widths above 10 nm and lengths in the order of 100 nm, with a more significant degradation for ultra-narrow ribbons

International audience; Thanks to their thinness, self-passivated surface and large variety, two-dimensional materials have attracted much interest for their possible application in nanoelectronics. In particular, semiconducting transition metal dichalcogenides and their van der Waals heterostructures are very promising for the realization of low-power tunnel fieldeffect transistors. By means of self-consistent quantum transport simulations, we explore the performances of two alternative architectures for the devices: the planar architecture and the vertical architecture. While for the former, which is based on a p-i-n junction, the tunneling occurs laterally within the same two-dimensional material layer, in the latter the tunneling occurs through the vertical heterostructure between two different materials, which are chosen to have a convenient band alignment. Our results enable a comparison of the performance of two architectures in the ideal case, and can serve as a first guide for the choice of the transistor design based on the desired application.

The extraordinary properties of graphene have spurred huge interest in the experimental realization of a two-dimensional honeycomb lattice of silicon, namely, silicene. However, its synthesis on supporting substrates remains a challenging issue. Recently, strong doubts against the possibility of synthesizing silicene on metallic substrates have been brought forward because of the non-negligible interaction between silicon and metal atoms. To solve the growth problems, we directly deposited silicon on a chemically inert graphite substrate at room temperature. Based on atomic force microscopy, scanning tunneling microscopy, and ab initio molecular dynamics simulations, we reveal the growth of silicon nanosheets where the substrate-silicon interaction is minimized. Scanning tunneling microscopy measurements clearly display the atomically resolved unit cell and the small buckling of the silicene honeycomb structure. Similar to the carbon atoms in graphene, each of the silicon atoms has three nearest and six second nearest neighbors, thus demonstrating its dominant sp