Silicon dominates the electronics industry, but its poor optical properties mean that III-V compound semiconductors are preferred for photonics applications. Photoluminescence at visible wavelengths ...was observed from porous Si at room temperature in 1990, but the origin of these photons (do they arise from highly localized defect states or quantum confinement effects?) has been the subject of intense debate ever since. Attention has subsequently shifted from porous Si to Si nanocrystals, but the same fundamental question about the origin of the photoluminescence has remained. Here we show, based on measurements in high magnetic fields, that defects are the dominant source of light from Si nanocrystals. Moreover, we show that it is possible to control the origin of the photoluminescence in a single sample: passivation with hydrogen removes the defects, resulting in photoluminescence from quantum-confined states, but subsequent ultraviolet illumination reintroduces the defects, making them the origin of the light again.
Two types of graphene nanoribbons: (a) potassium-split graphene nanoribbons (GNRs), and (b) oxidative unzipped and chemically converted graphene nanoribbons (CCGNRs) were investigated for their ...magnetic properties using the combination of static magnetization and electron spin resonance measurements. The two types of ribbons possess remarkably different magnetic properties. While a low-temperature ferromagnet-like feature is observed in both types of ribbons, such room-temperature feature persists only in potassium-split ribbons. The GNRs show negative exchange bias, but the CCGNRs exhibit a “positive exchange bias”. Electron spin resonance measurements suggest that the carbon-related defects may be responsible for the observed magnetic behavior in both types of ribbons. Furthermore, information on the proton hyperfine coupling strength has been obtained from hyperfine sublevel correlation experiments performed on the GNRs. Electron spin resonance finds no evidence for the presence of potassium (cluster) related signals, pointing to the intrinsic magnetic nature of the ribbons. Our combined experimental results may indicate the coexistence of ferromagnetic clusters with antiferromagnetic regions leading to disordered magnetic phase. We discuss the possible origin of the observed contrast in the magnetic behaviors of the two types of ribbons studied.
•Chemical functionalization produces (in)direct gap with effective carrier masses of 10–100×10−3m0.•‘Alt’-configured adatom-monolayer exhibits anisotropic effective carrier mass.•‘Top’-configured ...adatom-monolayer results in planar structure, possibly linked to relative electron negativity and pseudo Jahn–Teller effect.•‘Hollow’-configured adatom-monolayer results in opening of a gap at the Dirac cone; however, a planar surface state pins the Fermi level.
This study presents first-principles results on the electronic functionalization of silicene and germanene monolayers by means of chemisorption of adatom species H, Li, F, Sc, Ti, V. Three general adatom-monolayer configurations are considered, each having its distinct effect on the electronic structure, yielding metallic or semiconducting dispersions depending on the adatom species and configuration. The induced bandgap is a (in)direct Γ gap ranging from 0.2 to 2.3eV for both silicene and germanene. In general the alternating configuration was found to be the most energetically stable. The boatlike and chairlike conformers are degenerate with the former having anisotropic effective carrier masses. The top configuration leads to the planar monolayer and predominately to a gapped dispersion. The hollow configuration with V adatoms retains the Dirac cone, but with strong orbital planar hybridization at the Fermi level. We also observe a planar surface state the Fermi level for the latter systems.
•We calculated the electronic and vibrational properties of three different reconstructions of silicene on Ag(111).•The silicon structures modeled reproduce three 2D epitaxial silicon layers ...experimentally observed on 111 Ag, namely the (4×4), (√13×√13) and (2√3×2√3) phase.•Few peaks in the experimental Raman spectrum are identified and attributed to the silicene layers.•The trend of the intensity and frequency of the silicene Raman peaks is linked to the semimetallic or semiconductive behavior of the layers.•The band structures of the three silicene layers are calculated and confirm the indications of the Raman spectrum
The electronic and vibrational properties of three different reconstructions of silicene on Ag(111) are calculated and compared to experimental results. The 2D epitaxial silicon layers, namely the (4×4), (√13×√13) and (2√3×2√3) phases, exhibit different electronic and vibrational properties. Few peaks in the experimental Raman spectrum are identified and attributed to the vibrational modes of the silicene layers. The position and behavior of the Raman peaks with respect to the excitation energy are shown to be a fundamental tool to investigate and discern different phases of silicene on Ag(111).
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