Unlocking the Potential of Nanoribbon-Based Sb2S3/Sb2Se3 van-der-Waals Heterostructure for Solar-Energy-Conversion and Optoelectronics Applications.

High-performance nanosized optoelectronic devices based on van der Waals (vdW) heterostructures have significant potential for use in a variety of applications. However, the investigation of nanoribbon-based vdW heterostructures are still mostly unexplored. In this study, based on first-principles calculations, we demonstrate that a Sb2S3/Sb2Se3 vdW heterostructure, which is formed by isostructural nanoribbons of stibnite (Sb2S3) and antimonselite (Sb2Se3), possesses a direct band gap with a typical type-II band alignment, which is suitable for optoelectronics and solar energy conversion. Optical absorption spectra show broad profiles in the visible and UV ranges for all of the studied configurations, indicating their suitability for photodevices. Additionally, in 1D nanoribbons, we see sharp peaks corresponding to strongly bound excitons in a fashion similar to that of other quasi-1D systems. The Sb2S3/Sb2Se3 heterostructure is predicted to exhibit a remarkable power conversion efficiency (PCE) of 28.2%, positioning it competitively alongside other extensively studied two-dimensional (2D) heterostructures.

Franckeite as an Exfoliable Naturally Occurring Topological Insulator.

Franckeite is a natural superlattice composed of two alternating layers of different composition which has shown potential for optoelectronic applications. In part, the interest in franckeite lies in its layered nature which makes it easy to exfoliate into very thin heterostructures. Not surprisingly, its chemical composition and lattice structure are so complex that franckeite has escaped screening protocols and high-throughput searches of materials with nontrivial topological properties. On the basis of density functional theory calculations, we predict a quantum phase transition originating from stoichiometric changes in one of franckeite composing layers (the quasihexagonal one). While for a large concentration of Sb, franckeite is a sequence of type-II semiconductor heterojunctions, for a large concentration of Sn, these turn into type-III, much alike InAs/GaSb artificial heterojunctions, and franckeite becomes a strong topological insulator. Transmission electron microscopy observations confirm that such a phase transition may actually occur in nature.

Structural and magnetic properties of a defective graphene buffer layer grown on SiC(0001): a DFT study.

Understanding the role of defects in the magnetic properties of the graphene buffer layer (BL) grown on substrates should be important to provide hints for manufacturing future graphene-based spintronic devices in a controlled fashion. Herein, density functional theory was applied to assess the structure and magnetic properties of defective BL on 6H-SiC(0001). Particularly, we conducted a thorough study of one and two vacancies and Stone-Wales defects in the BL. Our results reveal that the removal of a carbon atom in the BL framework that was originally bonded to a Si atom in the substrate is preferred over that of a sp2-bonded atom. As a result, a hexacoordinated silicon atom is formed with a slightly deviated octahedral geometry. A stable antiferromagnetic (AF) state was verified for the single vacancy system, with a quite different spin-density distribution to the one obtained for the perfect BL. Also, this AF state is nearly degenerate with the non-magnetic and low magnetic states. As for the Stone-Wales defect, the AF sate is almost degenerate with the most stable M = 2 µB magnetic configuration. However, the introduction of two vacancies in the carbon network of BL causes the loss of magnetism of the BL-SiC system. Our theoretical calculations support experimental predictions favoring the BL as the site for single vacancy formation rather than the epitaxial monolayer graphene, by 4.3 eV.

Layer breathing and shear modes in multilayer graphene: a DFT-vdW study.

In this work, we study structural and vibrational properties of multilayer graphene using density-functional theory with van der Waals (vdW) functionals. Initially, we analyze how different vdW functionals compare by evaluating the lattice parameters, elastic constants and vibrational frequencies of low energy optical modes of graphite. Our results indicate that the vdW-DF1-optB88 functional has the best overall performance on the description of vibrational properties. Next, we use this functional to study the influence of the vdW interactions on the structural and vibrational properties of multilayer graphene. Specifically, we evaluate binding energies, interlayer distances and phonon frequencies of layer breathing and shear modes. We observe excellent agreement between our calculated results and available experimental data, which suggests that this functional has truly predictive power for layer-breathing and shear frequencies that have not been measured yet. This indicates that careful selected vdW functionals can describe interlayer bonding in graphene-related systems with good accuracy.

Electronic properties and vibrational spectra of (NH4)2M″(SO4)2·6H2O (M = Ni, Cu) Tutton's salt: DFT and experimental study.

The complex crystals of the family of the Tutton's salt have been investigated through the numerous experimental and theoretical studies to understand their physical properties and their potential technological applications. In spite of the more than 60 years of research, there are very few studies about the electronic properties of Tutton's salt. In our present work, we have calculated the stability, electronic properties and the first theoretical study of band structure of the three different crystals of the Tutton's salt, ammonium nickel sulfate hexahydrate ((NH4)2Ni(SO4)2·6H2O), ammonium nickel-copper sulfate hexahydrate ((NH4)2Ni0.5Cu0.5(SO4)2·6H2O) and ammonium copper sulfate hexahydrate ((NH4)2Ni(SO4)2·6H2O) with the help of periodic ab-initio calculations based on density functional theory (DFT). In addition to this, the internal Raman and FTIR modes of the ionic fragments [Ni(H2O)6]2+, [Cu(H2O)6]2+ NH4+ and SO42- of the sample crystals were obtained by employing the ab initio and the orientation of the molecular vibrations of the ionic fragments have been presented in picturized form. Furthermore, the Raman and FTIR spectroscopy of the sample crystals were measured in the range 100-4000 cm-1 and 400-4000 cm-1 respectively, and the internal vibrational modes obtained from experimental measurement have been compared with those obtained from DFT calculations.

Giant and Tunable Anisotropy of Nanoscale Friction in Graphene.

The nanoscale friction between an atomic force microscopy tip and graphene is investigated using friction force microscopy (FFM). During the tip movement, friction forces are observed to increase and then saturate in a highly anisotropic manner. As a result, the friction forces in graphene are highly dependent on the scanning direction: under some conditions, the energy dissipated along the armchair direction can be 80% higher than along the zigzag direction. In comparison, for highly-oriented pyrolitic graphite (HOPG), the friction anisotropy between armchair and zigzag directions is only 15%. This giant friction anisotropy in graphene results from anisotropies in the amplitudes of flexural deformations of the graphene sheet driven by the tip movement, not present in HOPG. The effect can be seen as a novel manifestation of the classical phenomenon of Euler buckling at the nanoscale, which provides the non-linear ingredients that amplify friction anisotropy. Simulations based on a novel version of the 2D Tomlinson model (modified to include the effects of flexural deformations), as well as fully atomistic molecular dynamics simulations and first-principles density-functional theory (DFT) calculations, are able to reproduce and explain the experimental observations.

Electronic and structural properties of vacancies and hydrogen adsorbates on trilayer graphene.

Using ab initio calculations, we study the electronic and structural properties of vacancies and hydrogen adsorbates on trilayer graphene. Those defects are found to share similar low-energy electronic features, since they both remove a p(z) electron from the honeycomb lattice and induce a defect level near the Fermi energy. However, a vacancy also leaves unpaired σ electrons on the lattice, which lead to important structural differences and also contribute to magnetism. We explore both ABA and ABC stackings and compare properties such as formation energies, magnetic moments, spin density and the local density of states (LDOS) of the defect levels. These properties show a strong sensitivity to the layer in which the defect is placed and smaller sensitivities to sublattice placing and stacking type. Finally, for the ABC trilayer, we also study how these states behave in the presence of an external field, which opens a tunable gap in the band structure of the non-defective system. The p(z) defect states show a strong hybridization with band states as the field increases, with reduction and eventually loss of magnetization, and a non-magnetic, midgap-like state is found when the defect is at the middle layer.