Electronic properties of several two dimensional halides from ab initio calculations.

Using density functional theory, we study the electronic properties of several halide monolayers. We show that their electronic bandgaps, as obtained with the HSE hybrid functional, range between 3.0 and 7.5 eV and that their phonon spectra are dynamically stable. Additionally, we show that under an external electric field some of these systems exhibit a semiconductor-to-metal transition.

Vibrational Properties of a Two-Dimensional Silica Kagome Lattice.

Kagome lattices are structures possessing fascinating magnetic and vibrational properties, but in spite of a large body of theoretical work, experimental realizations and investigations of their dynamics are scarce. Using a combination of Raman spectroscopy and density functional theory calculations, we study the vibrational properties of two-dimensional silica (2D-SiO2), which has a kagome lattice structure. We identify the signatures of crystalline and amorphous 2D-SiO2 structures in Raman spectra and show that, at finite temperatures, the stability of 2D-SiO2 lattice is strongly influenced by phonon-phonon interaction. Our results not only provide insights into the vibrational properties of 2D-SiO2 and kagome lattices in general but also suggest a quick nondestructive method to detect 2D-SiO2.

Van der Waals interactions and the limits of isolated atom models at interfaces.

Van der Waals forces are among the weakest, yet most decisive interactions governing condensation and aggregation processes and the phase behaviour of atomic and molecular matter. Understanding the resulting structural motifs and patterns has become increasingly important in studies of the nanoscale regime. Here we measure the paradigmatic van der Waals interactions represented by the noble gas atom pairs Ar-Xe, Kr-Xe and Xe-Xe with a Xe-functionalized tip of an atomic force microscope at low temperature. Individual rare gas atoms were fixed at node sites of a surface-confined two-dimensional metal-organic framework. We found that the magnitude of the measured force increased with the atomic radius, yet detailed simulation by density functional theory revealed that the adsorption induced charge redistribution strengthened the van der Waals forces by a factor of up to two, thus demonstrating the limits of a purely atomic description of the interaction in these representative systems.

Reproducibility in density functional theory calculations of solids.

The widespread popularity of density functional theory has given rise to an extensive range of dedicated codes for predicting molecular and crystalline properties. However, each code implements the formalism in a different way, raising questions about the reproducibility of such predictions. We report the results of a community-wide effort that compared 15 solid-state codes, using 40 different potentials or basis set types, to assess the quality of the Perdew-Burke-Ernzerhof equations of state for 71 elemental crystals. We conclude that predictions from recent codes and pseudopotentials agree very well, with pairwise differences that are comparable to those between different high-precision experiments. Older methods, however, have less precise agreement. Our benchmark provides a framework for users and developers to document the precision of new applications and methodological improvements.

Single-Layer ReS2: Two-Dimensional Semiconductor with Tunable In-Plane Anisotropy.

Rhenium disulfide (ReS2) and diselenide (ReSe2), the group 7 transition metal dichalcogenides (TMDs), are known to have a layered atomic structure showing an in-plane motif of diamond-shaped-chains (DS-chains) arranged in parallel. Using a combination of transmission electron microscopy and transport measurements, we demonstrate here the direct correlation of electron transport anisotropy in single-layered ReS2 with the atomic orientation of the DS-chains, as also supported by our density functional theory calculations. We further show that the direction of conducting channels in ReS2 and ReSe2 can be controlled by electron beam irradiation at elevated temperatures and follows the strain induced to the sample. Furthermore, high chalcogen deficiency can induce a structural transformation to a nonstoichiometric phase, which is again strongly direction-dependent. This tunable in-plane transport behavior opens up great avenues for creating nanoelectronic circuits in 2D materials.

Three-fold rotational defects in two-dimensional transition metal dichalcogenides.

As defects frequently govern the properties of crystalline solids, the precise microscopic knowledge of defect atomic structure is of fundamental importance. We report a new class of point defects in single-layer transition metal dichalcogenides that can be created through 60° rotations of metal-chalcogen bonds in the trigonal prismatic lattice, with the simplest among them being a three-fold symmetric trefoil-like defect. The defects, which are inherently related to the crystal symmetry of transition metal dichalcogenides, can expand through sequential bond rotations, as evident from in situ scanning transmission electron microscopy experiments, and eventually form larger linear defects consisting of aligned 8-5-5-8 membered rings. First-principles calculations provide insights into the evolution of rotational defects and show that they give rise to p-type doping and local magnetic moments, but weakly affect mechanical characteristics of transition metal dichalcogenides. Thus, controllable introduction of rotational defects can be used to engineer the properties of these materials.

Testing several recent van der Waals density functionals for layered structures.

Six recently developed exchange functionals for pairing with two different versions of van der Waals density functionals (vdW-DF) are tested for weakly bonded solids. The test, using 26 layered weakly bonded compounds, benchmarks the lattice constants against experimental data and the interlayer binding energies against reference data from the random-phase approximation (RPA). The investigated functionals tend to give interlayer binding energies higher than the RPA benchmark, and the overall performance for lattice constants is good. The exchange functionals optB86b and cx13 paired with the original vdW-DF and the B86R functional paired with vdW-DF2 are found to give particularly good results for equilibrium geometries.

Triazine-based graphitic carbon nitride: a two-dimensional semiconductor.

Graphitic carbon nitride has been predicted to be structurally analogous to carbon-only graphite, yet with an inherent bandgap. We have grown, for the first time, macroscopically large crystalline thin films of triazine-based, graphitic carbon nitride (TGCN) using an ionothermal, interfacial reaction starting with the abundant monomer dicyandiamide. The films consist of stacked, two-dimensional (2D) crystals between a few and several hundreds of atomic layers in thickness. Scanning force and transmission electron microscopy show long-range, in-plane order, while optical spectroscopy, X-ray photoelectron spectroscopy, and density functional theory calculations corroborate a direct bandgap between 1.6 and 2.0 eV. Thus TGCN is of interest for electronic devices, such as field-effect transistors and light-emitting diodes.

Defects in bilayer silica and graphene: common trends in diverse hexagonal two-dimensional systems.

By combining first-principles and classical force field calculations with aberration-corrected high-resolution transmission electron microscopy experiments, we study the morphology and energetics of point and extended defects in hexagonal bilayer silica and make comparison to graphene, another two-dimensional (2D) system with hexagonal symmetry. We show that the motifs of isolated point defects in these 2D structures with otherwise very different properties are similar, and include Stone-Wales-type defects formed by structural unit rotations, flower defects and reconstructed double vacancies. The morphology and energetics of extended defects, such as grain boundaries have much in common as well. As both sp(2)-hybridised carbon and bilayer silica can also form amorphous structures, our results indicate that the morphology of imperfect 2D honeycomb lattices is largely governed by the underlying symmetry of the lattice.

In situ growth of cellular two-dimensional silicon oxide on metal substrates.

Crystalline hexagonally ordered silicon oxide layers with a thickness of less than a nanometer are grown on transition metal surfaces in an in situ electron microscopy experiment. The nucleation and growth of silica bilayers and monolayers, which represent the thinnest possible ordered structures of silicon oxide, are monitored in real time. The emerging layers show structural defects reminiscent of those in graphene and can also be vitreous. First-principles calculations provide atomistic insight into the energetics of the growth process. The interplay between the gain in silica-metal interaction energy due to their epitaxial match and energy loss associated with the mechanical strain of the silica network is addressed. The results of calculations indicate that both ordered and vitreous mono/bilayer structures are possible, so that the actual morphology of the layer is defined by the kinetics of the growth process.