Ultrafast carrier dynamics of epitaxial silicene

The recent integration of silicene in field-effect transistors (FET) opened new challenges in the comprehension of the chemical and physical properties of this elusive two-dimensional allotropic form of silicon. Intense efforts have been devoted to the study of the epitaxial Silicene/Ag(111) system in order to elucidate the presence of Dirac fermion in analogy with graphene; strong hybridization effects in silicene superstructures on silver have been invoked as responsible for the disruption of \u3c0 and \u3c0* bands. In this framework, the measured ambipolar effect in silicene-based FET characterized by a relatively high mobility, points out to a complex physics at the silicene-silver interface, demanding for a deeper comprehension of its details on the atomic scale. Here we elucidate the role of the metallic support in determining the physical properties of the Si/Ag interface, by means of optical techniques combined with theoretical calculations of the optical response of the supported system. The silicene/Ag(111) spectra, which turn out to be strongly non-additive, are analyzed in the framework of theoretical density functional based calculations allowing us to single out contributions arising from different localization. Electronic transitions involving silver states are found to provide a huge contribution to the optical absorption of silicene on silver, compatible with a strong Si-Ag hybridization. The results point to a dimensionality-driven peculiar dielectric response of the two-dimensional-silicon/silver interface, which is confirmed by means of Transient-Reflectance spectroscopy. The latter shows a metallic-like carrier dynamics, (both for silicene and amorphous silicon), hence providing an optical demonstration of the strong hybridization arising in silicene/Ag(111) systems

Coulomb drag in anisotropic systems: a theoretical study on a double-layer phosphorene

We theoretically study the Coulomb drag resistivity in a double-layer electron system with highly anisotropic parabolic band structure using Boltzmann transport theory. As an example, we consider a double-layer phosphorene on which we apply our formalism. This approach, in principle, can be tuned for other double-layered systems with paraboloidal band structures. Our calculations show the rotation of one layer with respect to another layer can be considered a way of controlling the drag resistivity in such systems. As a result of rotation, the off-diagonal elements of drag resistivity tensor have non-zero values at any temperature. In addition, we show that the anisotropic drag resistivity is very sensitive to the direction of momentum transfer between two layers due to highly anisotropic inter-layer electron-electron interaction and also the plasmon modes. In particular, the drag anisotropy ratio, \r{ho}yy/\r{ho}xx, can reach up to ~ 3 by changing the temperature. Furthermore,our calculations suggest that including the local field correction in dielectric function changes the results significantly. Finally, We examine the dependence of drag resistivity and its anisotropy ratio on various parameters like inter-layer separation, electron density, short-range interaction and insulating substrate/spacer.Comment: 10 pages, 9 figure

Why all the fuss about 2D semiconductors?

Graphene is no longer alone; a family of atomically thin 2D semiconductors has emerged. Optoelectronics and photonics applications are in their experimental infancy but the future holds much promise.Comment: Commentary article, 1 figure, 1 tabl

Hydrophilic Character of Single-Layer MoS2 Grown on Ag(111)

The study of MoS2/metal interfaces is crucial for engineering efficient semiconductor−metal contacts in 2D MoS2-based devices. Here we investigate a MoS2/Ag heterostructure fabricated by growing a single MoS2 layer on Ag(111) by pulsed laser deposition under ultrahigh vacuum (UHV) conditions. The surface structure is observed in situ by scanning tunneling microscopy, revealing the hexagonal moiré pattern characteristic of the clean MoS2/Ag(111) interface. Ex situ Raman spectroscopy reveals an anomalous behavior of vibrational modes, induced by the strong MoS2−Ag interaction. After few-hours exposure to ambient conditions the Raman response significantly changes and the formation of molybdenum oxysulfides is revealed by X-ray photoelectron spectroscopy. These effects are due to the interplay with water vapor and can be reversed by a moderate UHV annealing. A polymeric (PMMA) capping is demonstrated to hinder water-induced modifications, preserving the original interface quality for months