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We predict that specific group-IV elements and IV-IV, III-V, and II-VI compounds can form stable, freestanding two-dimensional (2D) monolayers consisting of octagon, hexagon, and square rings (ohs), in which the threefold coordination of atoms is preserved to allow sp2-type hybridization. These monolayers can also construct bilayers, multilayers, three-dimensional (3D) layered van der Waals solids, and 3D crystals with strong vertical bonds between layers as well as quasi-one-dimensional nanotubes and nanoribbons with diverse edge geometries. All these ohs structures can constitute a large class of 2D materials ranging from good metals to wide bandgap semiconductors and display physical and chemical properties rather different from those of their counterparts in the hexagonal (honeycomb) network. The metallic state of freestanding 2D C, Si, and Ge ohs monolayers and 3D C ohs bulk contrast, respectively, with graphene, silicene, germanene, and graphite.

In this paper we investigated the interaction between a graphene nanoflake anchored to the 2D graphene monolayer. This interaction is attractive but weak and is capable of setting a well defined registry in equilibrium. Rotational and linear displacements from equilibrium registry generate restoring forces, which can be controlled by external agents. Similar flakes can be self-assembled and can also execute simple harmonic motion as if a physical pendulum. Oscillation of a nanoflake about their equilibrium registries resulting in a characteristic libration frequency is predicted. This frequency depends on the size and geometry of the flake. Moreover, the libration frequency, as well as the electronic and magnetic properties of the flake+monolayer systems, can be tuned by a foreign molecule anchored to the flake, by electric charging and applied parallel and perpendicular electric and magnetic fields. When the sliding of the flake is combined with rotation, the friction force can be reduced dramatically. It is surprising that weak interaction can offer such features at nanoscale, which may offer potential applications. Our predictions are obtained by first-principles calculations based on density functional theory.

A recent study predicted that a 2D single layer of antimony in buckled honeycomb as well as asymmetric washboard structures, named antimonene, is stable at high temperature and displays semiconducting properties. Based on first-principles, spin-polarized density functional calculations, we investigated chemisorption of selected adatoms and physisorption of molecules on two antimonene phases. Since adspecies-adspecies interaction is minimized by using large supercells, our results mimic the effects of isolated, single adatoms or molecules. We found that molecules such as ${\mathrm{H}}_{2},\phantom{\rule{0.16em}{0ex}}{\mathrm{O}}_{2}$, and ${\mathrm{H}}_{2}\mathrm{O}$ neither form strong chemical bonds nor dissociate; they are physisorbed with a weak binding energy without affecting the properties of antimonene. The adatoms, such as H, Li, B, C, N, O, Al, In, Si, P, Cl, Ti, As, and Sb, are chemisorbed with significant binding energy, whereby the atomic and electronic structures are modified locally. Boron and carbon adatoms are implemented into buckled antimonene crystal leading to a local reconstruction of the crystal. Nitrogen gives rise to Stone-Wales type defects. The localized states originating from adatoms give rise to diversity of electronic structure. The lowest conduction and highest valence bands of antimonene in asymmetric washboard structures have very high curvature. Once combined with adatom states, these bands offer a variety of features. Specific adatoms lead to spin polarization, attain magnetic moments, and can attribute a half-metallic character to antimonene.

In addition to stable single-layer buckled honeycomb and washboard structures of group-V elements (or pnictogens P, As, Sb, and Bi) we show that these elements can also form two-dimensional, single-layer structures consisting of buckled square and octagon rings. An extensive analysis comprising the calculation of mechanical properties, vibration frequencies, and finite-temperature ab initio molecular dynamics confirms that these structures are dynamically and thermally stable and suitable for applications at room temperature and above. All these structures are semiconductors with a fundamental band gap, which is wide for P but decreases with increasing row number. The effect of the spin-orbit coupling decreases the band gap and is found to be crucial for Sb and Bi. These results are obtained from first-principles calculations based on density functional theory.

Recent studies show that several metal oxides and dichalcogenides $(M{X}_{2})$, which exist in nature, can be stable in two-dimensional (2D) form and each year several new $M{X}_{2}$ structures are explored. The unstable structures in $H$ (hexagonal) or $T$ (octahedral) forms can be stabilized through Peierls distortion. In this paper, we propose new 2D forms of ${\mathrm{RuS}}_{2}$ and ${\mathrm{RuSe}}_{2}$ materials. We investigate in detail the stability, electronic, magnetic, optical, and thermodynamic properties of 2D $\mathrm{Ru}{X}_{2}$ $(X=\text{S},\text{Se})$ structures from first principles. While their $H$ and $T$ structures are unstable, the distorted $T$ structures $({T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{Ru}{X}_{2})$ are stable and have a nonmagnetic semiconducting ground state. The molecular dynamic simulations also confirm that ${T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{Ru}{X}_{2}$ systems are stable even at 500 K without any structural deformation. ${T}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{RuS}}_{2}$ and ${T}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{RuSe}}_{2}$ have indirect band gaps with 0.745 eV (1.694 eV with HSE) and 0.798 eV (1.675 eV with HSE) gap values, respectively. We also examine their bilayer and trilayer forms and find direct and smaller band gaps. We find that AA stacking is more favorable than the AB configuration. The new 2D materials obtained can be good candidates with striking properties for applications in semiconductor electronic, optoelectronic devices, and sensor technology.

Based on first-principles density functional calculations, we predict that nitrogen atoms can form a single-layer, buckled honeycomb structure called nitrogene, which is rigid and stable even above room temperature. This 2D crystalline phase of nitrogen, which corresponds to a local minimum in the Born-Oppenheimer surface, is a nonmagnetic insulator with saturated $\ensuremath{\pi}$ bonds. When grown on a substrate like Al(111) surface and graphene, nitrogene binds weakly to substrates and hence preserves its free-standing properties, but it can easily be pealed off. Zigzag and armchair nanoribbons of nitrogene have fundamental band gaps derived from reconstructed edge states. These band gaps are tunable with size and suitable for the emerging field of 2D electronics. Nitrogene forms not only bilayer, but also 3D graphitic multilayer structures. Single-layer nitrogene can nucleate and grow on the armchair edges of hexagonal boron nitride.

The pseudolayered character of 3D bulk crystals of antimony has led us to predict its 2D single-layer crystalline phase named antimonene in a buckled honeycomb structure like silicene. Sb atoms also form an asymmetric washboard structure like black phospherene. Based on an extensive analysis comprising ab initio phonon and finite-temperature molecular dynamics calculations, we show that these two single-layer phases are robust and can remain stable at high temperatures. They are nonmagnetic semiconductors with band gaps ranging from 0.3 eV to 1.5 eV, and are suitable for 2D electronic applications. The washboard antimonene displays strongly directional mechanical properties, which may give rise to a strong influence of strain on the electronic properties. Single-layer antimonene phases form bilayer and trilayer structures with wide interlayer spacings. In multilayers, this spacing is reduced and eventually the structure changes to 3D pseudolayered bulk crystals. The zigzag and armchair nanoribbons of the antimonene phases have fundamental band gaps derived from reconstructed edge states and display a diversity of magnetic and electronic properties depending on their width and edge geometry. Their band gaps are tunable with the widths of the nanoribbons. When grown on substrates, such as germanene or Ge(111), the buckled antimonene attains a significant influence of substrates.

In this work, we present the adsorption of ${\text{Au}}_{n}{\text{Pt}}_{n}$ clusters on graphene surface by using the density-functional theory within localized density approximation (LDA), and generalized gradient approximation (GGA). We calculate the adsorption energy, geometries, band structure, charge differences, and magnetization up to $n=3$. We observe that graphene can be metallic or semiconducting depending on number of Au and Pt atoms in the cluster and the charge transfer between cluster and the graphene. LDA and GGA results are given and compared for all cases. We find that ${\text{Au}}_{3}{\text{Pt}}_{3}$ and AuPt graphene can have half metallic properties within GGA.

This paper presents our study on the atomic, electronic, magnetic structures, and phonon modes of the low-temperature orthorhombic phase of undoped ${\text{BaFe}}_{2}{\text{As}}_{2}$ crystal. The electronic structure is characterized by a sharp $\text{Fe-}3d$ peak close to the Fermi level and is dominated by $\text{Fe-}3d$- and $\text{As-}4p$-hybridized states. Ba contribution occurs only at lower energies. The spin ordering of the magnetic ground state, which is determined by minimizing the total energy of different spin alignments on Fe atoms in the conventional cell, is in agreement with experimental findings but is different from the antiferromagnetic spin ordering obtained by assigning antiparallel spin directions on two Fe atoms in the primitive unit cell. Valuable information about the charge transfer and bonding is revealed through the analysis of the charge density. Electrons are transferred from Ba to Fe-As layers and also from Fe to As atoms. The magnetic phonon calculations of the ground state are carried out to predict Raman and infrared-active modes. Softening of some calculated spin-dependent phonon modes corroborates the contribution of spin-lattice coupling to the structural phase transition from $I4/mmm$ to $Fmmm$.