The isolation of graphene in 2004 from graphite was a defining moment for the “birth” of a field: two-dimensional (2D) materials In recent years, there has been a rapidly increasing number of papers focusing on non-graphene layered materials, including transition-metal dichalcogenides (TMDs), because of the new properties and applications that emerge upon 2D confinement Here, we review significant recent advances and important new developments in 2D materials “beyond graphene” We provide insight into the theoretical modeling and understanding of the van der Waals (vdW) forces that hold together the 2D layers in bulk solids, as well as their excitonic properties and growth morphologies Additionally, we highlight recent breakthroughs in TMD synthesis and characterization and discuss the newest families of 2D materials, including monoelement 2D materials (ie, silicene, phosphorene, etc) and transition metal carbide- and carbon nitride-based MXenes We then discuss the doping and functionalization of 2

Recent Advances in Two-Dimensional Materials beyond Graphene

A process for the growth, transfer and fabrication of silicene field-effect transistors enables devices that have mobility of about 100 cm2  V−1  s–1.

Silicene field-effect transistors operating at room temperature

VASPKIT: A user-friendly interface facilitating high-throughput computing and analysis using VASP code

We have grown an atom-thin, ordered, two-dimensional multi-phase film in situ through germanium molecular beam epitaxy using a gold (111) surface as a substrate. Its growth is similar to the formation of silicene layers on silver (111) templates. One of the phases, forming large domains, as observed in scanning tunneling microscopy, shows a clear, nearly flat, honeycomb structure. Thanks to thorough synchrotron radiation core-level spectroscopy measurements and advanced density functional theory calculations we can identify it as a ?3????3 R(30?) germanene layer in conjunction with a ?7????7 R(19.1?) Au(111) supercell, presenting compelling evidence of the synthesis of the germanium-based cousin of graphene on gold.

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Germanene: a novel two-dimensional germanium allotrope akin to graphene and silicene

First-principles band theory, properly augmented by topological considerations, has provided a remarkably successful framework for predicting new classes of topological materials. This Colloquium discusses the underpinnings of the topological band theory and its materials applications.

Colloquium : Topological band theory

We present first-principles studies of the optical absorbance of the group IV honeycomb crystals graphene, silicene, germanene, and tinene. We account for many-body effects on the optical properties by using the non-local hybrid functional HSE06. The optical absorption peaks are blueshifted due to quasiparticle corrections, while the influence on the low-frequency absorbance remains unchanged and reduces to a universal value related to the Sommerfeld fine structure constant. At the Dirac points spin-orbit interaction opens fundamental band gaps; parabolic bands with a very small effective mass emerge. Consequently, the low-frequency absorbance is modified with a spin-orbit-induced transparency region and an increase of the absorbance at the fundamental absorption edge.

https://iopscience.iop.org/article/10.1088/0953-8984/25/39/395305/pdf

Massive Dirac quasiparticles in the optical absorbance of graphene, silicene, germanene, and tinene.

We compute the optical conductivity of 2D honeycomb crystals beyond the usual Dirac-cone approximation. The calculations are mainly based on the independent-quasiparticle approximation of the complex dielectric function for optical interband transitions. The full band structures are taken into account. In the case of silicene, the influence of excitonic effects is also studied. Special care is taken to derive converged spectra with respect to the number of k points in the Brillouin zone and the number of bands. In this way both the real and imaginary parts of the optical conductivity are correctly described for small and large frequencies. The results are applied to predict the optical properties reflection, transmission and absorption in a wide range of photon energies. They are discussed in the light of the available experimental data.

https://iopscience.iop.org/article/10.1088/1367-2630/16/10/105007/pdf

Optical properties of two-dimensional honeycomb crystals graphene, silicene, germanene, and tinene from first principles

We show that the low-frequency absorbance of undoped graphene, silicene, and germanene has a universal value, only determined by the Sommerfeld fine-structure constant. This result is derived by means of ab initio calculations of the complex dielectric function for optical interband transitions applied to two-dimensional (2D) crystals with honeycomb geometry. The assumption of chiral massless Dirac fermions is not necessary. The low-frequency absorbance does not depend on the group-IV atom, neither on the sheet buckling nor on the orbital hybridization. We explain these findings via an analytical derivation of the relationship between absorbance and fine-structure constant for 2D Bloch electrons. The effect of deviations of the electronic bands from linearity is also discussed. DOI: 10.1103/PhysRevB.87.035438

Universal infrared absorbance of two-dimensional honeycomb group-IV crystals

Calculating the complex dielectric function for optical interband transitions we show that the two-dimensional crystals silicene and germanene possess the same low-frequency absorbance as graphene. It is determined by the Sommerfeld finestructure constant. Deviations occur for higher frequencies when the first interband transitions outside K or K′ contribute. The low-frequency results are a consequence of the honeycomb geometry but do not depend on the group-IV atom, the sheet buckling, and the orbital hybridization. The two-dimensional crystals may be useful as absorption normals in silicon technology.

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Infrared absorbance of silicene and germanene

The ab initio calculation of optical spectra of sheet crystals usually arranges them in a three-dimensional superlattice with a sufficiently large interlayer distance. We show how the resulting frequency-dependent dielectric tensor is related to the anisotropic optical conductivity of an individual sheet or to the dielectric tensor of a corresponding film with thickness $d$. Their out-of-plane component is taken into account, in contrast to usual treatments. We demonstrate that the generalized transfer-matrix method to model the optical properties of a layer system containing a sheet crystal accounts for all tensor components. As long as $d\ensuremath{\ll}\ensuremath{\lambda}$ ($\ensuremath{\lambda}$-wavelength of light) this generalized formulation of the optical properties for anisotropic two-dimensional (2D) systems of arbitrary thickness reproduces the limits found in literature that are based either on electromagnetic boundary conditions for a conducting surface or on an isotropic dielectric tensor. For $s$-polarized light, the results are independent of the sheet description. For oblique incidence of $p$-polarized light, the tensor nature of the optical conductivity (or the dielectric function) of the sheet crystal strongly impacts on reflectance, transmittance, and absorbance due to the out-of-plane optical conductivity. The limit $d\ensuremath{\rightarrow}0$ should be taken in the final expressions. Example spectra are given for the group-IV honeycomb 2D crystals graphene and silicene.

Lars Matthes

Papers

Massive Dirac quasiparticles in the optical absorbance of graphene, silicene, germanene, and tinene.

Optical properties of two-dimensional honeycomb crystals graphene, silicene, germanene, and tinene from first principles

Universal infrared absorbance of two-dimensional honeycomb group-IV crystals

Infrared absorbance of silicene and germanene

Influence of out-of-plane response on optical properties of two-dimensional materials: First principles approach