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

Calculating the complex dielectric function for optical interband transitions we show that the\ud two-dimensional crystals silicene and germanene possess the same low-frequency absorbance as\ud graphene. It is determined by the Sommerfeld finestructure constant. Deviations occur for higher\ud frequencies when the first interband transitions outsideKorK\ud 0\ud contribute. The low-frequency results are a consequence of the honeycomb geometry but do not depend on the group-IV atom, the sheet buckling,\ud and the orbital hybridization. The two-dimensional crystals may be useful as absorption normals in\ud silicon technolog

PACS 71.35.Cc-Intrinsic properties of excitons; optical absorption spectra PACS 73.22.-f-Electronic structure of nanoscale materials and related systems PACS 78.67.Wj-Optical properties of graphene Abstract-We show by first-principles calculations that, due to depressed screening and enhanced two-dimensional confinement, excitonic resonances with giant oscillator strength appear in hydrogenated Si and Ge layers, which qualitatively and quantitatively differ from those of graphane. Their large exciton binding energies and oscillator strengths make them promising for observation of novel physical effects and application in optoelectronic devices on the nanoscale.

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The recently reported synthesis of silicene in the form of nanoribbons on Ag(110) or 2D epitaxial\ud sheets on Ag(111) aroused considerable interest in the scientific community. Both overlayers were\ud reported to display signatures of Dirac fermions with linearly dispersing electronic bands. In this\ud work, we study the electronic structure of these adsorbate systems within density functional theory.\ud We show that the conical features apparent in angle-resolved photoelectron spectroscopy\ud measurements are not due to silicon but to the silver substrate, as an effect of band folding induced\ud by the Si overlayer periodicity

The exotic electrodynamics properties of graphene come from the linearly dispersive electronic bands that host massless Dirac electrons. A similar behavior was predicted to manifest in freestanding silicene, the silicon counterpart of graphene, thereby envisaging a new route for a silicon photonics. However, the access to silicene exploitation in photonics was hindered so far by the use of optically inappropriate substrates in experimentally realized silicene. Here we report on the optical conductivity of silicon nanosheets epitaxially grown on the optically transparent Al2O3(0001) from a thickness of a few tens of nanometers down to the extreme twodimensional (2D) limit. When approaching a 2D regime, a Dirac-like electrodynamics can be deduced from the observation of a low-energy optical conductivity feature owing to a silicene-based interfacing to the substrate.

We study the electronic properties of two-dimensional (2D) group-III nitrides BN, AlN, GaN, InN, and TlN by first-principles approaches. With increasing group-III atomic number, a decrease of the electronic gap from 6.7 eV to 0 eV takes place. 2D GaN and 2D InN in honeycomb geometry present a direct gap at Γ, while the honeycomb structures of BN and AlN tend to be indirect semiconductors with the valence band maximum at K. Alloying of the nitrides allows tuning the gap with cation composition. Interestingly, Inx Ga1-xN and Inx Tl1-xN alloys enable, with varying x, to construct type I or type II heterostructures. We demonstrate that it is possible to tailor the electronic and optical response from UV to IR. We suggest that 2D InGaN and InTlN heterostructures may efficiently harvest light and serve as building blocks for a future generation of III-V solar cells. Finally, 2D InTlN with a low In content is eligible as the emitter and detector for THz applications

We present ellipsometry data of the dielectric function of wurtzite ZnO in a wide energy range ͑2.5-32 eV͒. The ordinary and extraordinary components show a strong anisotropy above 10 eV, a feature for which ZnO deviates from the other II-VI wurtzite compounds. With the aid of ab initio calculations, performed within many-body perturbation theory ͑MBPT͒ and within time-dependent density-functional theory ͑TDDFT͒, we analyze the origin of the measured optical structures. TDDFT, with the use of a static long-range exchangecorrelation kernel, proves to be a cheaper computational tool than MBPT to yield a good description of the whole spectrum. Theoretical results for the zinc-blende phase are also presented.