The stability of two-dimensional (2D) layers and membranes is subject of a long standing theoretical debate. According to the so called Mermin-Wagner theorem, long wavelength fluctuations destroy the long-range order for 2D crystals. Similarly, 2D membranes embedded in a 3D space have a tendency to be crumpled. These dangerous fluctuations can, however, be suppressed by anharmonic coupling between bending and stretching modes making that a two-dimensional membrane can exist but should present strong height fluctuations. The discovery of graphene, the first truly 2D crystal and the recent experimental observation of ripples in freely hanging graphene makes these issues especially important. Beside the academic interest, understanding the mechanisms of stability of graphene is crucial for understanding electronic transport in this material that is attracting so much interest for its unusual Dirac spectrum and electronic properties. Here we address the nature of these height fluctuations by means of straightforward atomistic Monte Carlo simulations based on a very accurate many-body interatomic potential for carbon. We find that ripples spontaneously appear due to thermal fluctuations with a size distribution peaked around 70 \AA which is compatible with experimental findings (50-100 \AA) but not with the current understanding of stability of flexible membranes. This unexpected result seems to be due to the multiplicity of chemical bonding in carbon.

Intrinsic ripples in graphene

A novel crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2 the top layer of selenium atoms are substituted by sulfur atoms while the bottom selenium layer remains intact. The peculiar structure of this new material is systematically investigated by Raman, photoluminescence and X-ray photoelectron spectroscopy and confirmed by transmission-electron microscopy and time-of-flight secondary ion mass spectrometry. Density-functional theory calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction (HER) activity is discovered for the Janus monolayer and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.

Janus Monolayer Transition Metal Dichalcogenides

Silicene, a promising new 2D material

We present first-principles calculations of silicene/graphene and germanene/graphene bilayers. Various supercell models are constructed in the calculations in order to reduce the strain of the lattice-mismatched bilayer systems. Our energetics analysis and electronic structure results suggest that graphene can be used as a substrate to synthesize monolayer silicene and germanene. Multiple phases of single crystalline silicene and germanene with different orientations relative to the substrate could coexist at room temperature. The weak interaction between the overlayer and the substrate preserves the low-buckled structure of silicene and germanene, as well as their linear energy bands. The gap induced by breaking the sublattice symmetry in silicene on graphene can be up to 57 meV.

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Stability and Electronic Properties of Two-Dimensional Silicene and Germanene on Graphene

The monolayer of black phosphorus, or ``phosphorene,'' has recently emerged as a two-dimensional semiconductor with intriguing highly anisotropic transport properties. Existing calculations of its intrinsic phonon-limited electronic transport properties so far rely on the deformation potential approximation, which is in general not directly applicable to anisotropic materials since the deformation along one specific direction can scatter electrons traveling in all directions. We perform a first-principles calculation of the electron-phonon interaction in phosphorene based on density functional perturbation theory and Wannier interpolation. Our calculation reveals that (1) the high anisotropy provides extra phase space for electron-phonon scattering, and (2) optical phonons have appreciable contributions. Both effects cannot be captured by the deformation potential calculations. Our simulation predicts carrier mobilities $\ensuremath{\sim}170\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{V}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$ for both electrons and holes at $300\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and a thermoelectric figure of merit $zT$ of up to 0.14 in $p$-type impurity-free phosphorene at $500\phantom{\rule{0.16em}{0ex}}\mathrm{K}$.

Ab initio study of electron-phonon interaction in phosphorene

A DFT study of electronic, optical and thermoelectric properties of Ge-halide perovskites CsGeX3 (X=F, Cl and Br)

The Janus two-dimensional (2D) materials have recently demonstrated excellent physical and chemical properties. The electronic, thermoelectric, and optical properties of the Janus In2SeTe monolayer have been investigated using density functional theory (DFT) calculations. Our results revealed the Janus In2SeTe monolayer has a direct bandgap of 1.07 eV, which is desirable and smaller than that of InSe and InTe. The studied optical properties indicate that Janus In2SeTe monolayer has a very good absorbing capability of light from the infrared region to ultraviolet one with a large absorption coefficient. Moreover, the thermoelectric property calculations suggest that the In2SeTe Janus monolayer can be a potential thermoelectric material, with a great electronic figure of merit (
 $${ZT}_{\mathrm{e}}$$
 ) of 0.97 at 300 K. In addition, the $${ZT}_{\mathrm{e}}$$
 increases with temperature up to a value of 1.42 at 800 K. Our results revealed the Janus In2SeTe monolayer possesses good properties that could offer the possibility of optoelectronic and energy conversion applications.

A First-Principles Investigation on Electronic Structure, Optical and Thermoelectric Properties of Janus In2SeTe Monolayer

Silicene and germanene freestanding layers are usually described as a honeycomb lattice formed by two hexagonal sub-lattices presenting a height difference, namely the layer buckling. In this work, first-principles calculations show that silicene and germanene can be rippled at 0 K with various wavelengths, without any compressive strain of the layer. For germanene, the height difference between two Ge atoms from the same sub-lattice can be as high as 4.7 [Formula: see text] for an undulation length of 81 [Formula: see text]. The deformations are related to slight (lower than 1.7°) bond angle modifications, and the energy cost is remarkably low, lying between 0.1 and 0.8 meV per atom. These undulations modify the electronic structure, opening a gap of 15 meV.

https://iopscience.iop.org/article/10.1088/1361-648X/ab6ae8/pdf

Undulated silicene and germanene freestanding layers: why not?

We report the epitaxial growth of one Silicon monolayer on the LaAlO3(111) substrate, a high-κ crystalline oxide Structural and chemical properties were investigated in-situ using reflection high energy electron diffraction (RHEED) and X-ray photoelectron spectroscopy (XPS) The deposition was achieved by molecular beam epitaxy in the temperature range RT-500°C A two-dimensional epitaxial growth mode is observed for a deposition at temperature between 300°C and 500°C The deposited single layer is formed by two dimensional (2D) structures of Si

https://iopscience.iop.org/article/10.1088/1742-6596/491/1/012003/pdf

Two dimensional Si layer epitaxied on LaAlO3(111) substrate: RHEED and XPS investigations

The electronic, optical and thermoelectric properties of the Ge 2SeS monolayer were detailed using the first-principle calculations. The dynamic stability of the Ge2SeS structure monolayer has been confirmed by the phonon dispersion curve. Our electronic calculations indicate that the Ge2 SeS monolayer is a semiconductor at equilibrium state with an indirect bandgap lower/larger than that of the GeS/GeSe monolayer calculated by generalized gradient approximation (GGA) of Perdew Burke Ernzerhof (PBE) functional. The optical properties such as dielectric constant, reflectivity , extinction coefficient, and absorption coefficient versus the energy were investigated. In addition, the thermoelectric properties are obtained using the semi-classical Boltzmann transport theory. The results show a large Seebeck coefficient of 2470 μV/K at 300K, the electronic figure of merit ( ZTe) is 0.91 in Ge2SeS monolayer at 300 K. The predicted optical and thermoelectric properties could make the Ge2SeS monolayer a potential candidate for applications in optoelectronic and energy conversion technologies at room temperature.

Mohamed Zanouni

Papers

A DFT study of electronic, optical and thermoelectric properties of Ge-halide perovskites CsGeX3 (X=F, Cl and Br)

A First-Principles Investigation on Electronic Structure, Optical and Thermoelectric Properties of Janus In2SeTe Monolayer

Undulated silicene and germanene freestanding layers: why not?

Two dimensional Si layer epitaxied on LaAlO3(111) substrate: RHEED and XPS investigations

Electronic structure, optical and thermoelectric properties of Ge2SeS monolayer via first-principles study