C. Ottaviani

Evidence of graphene-like electronic signature in silicene nanoribbons

We report on the electronic properties of straight, 1.6 nm wide, silicene nanoribbons on Ag(110), arranged in a one-dimensional grating with a pitch of 2 nm, whose high-resolution scanning tunneling microscopy images reveal a honeycomb geometry. Angle-resolved photoemission shows quantum confined electronic states of one-dimensional character. The silicon band dispersion along the direction of the nanoribbons suggests a behavior analogous to the Dirac cones of graphene on different substrates.

Evidence of Dirac fermions in multilayer silicene

Multilayer silicene, the silicon analogue of multilayer graphene, grown on silver (111) surfaces, possesses a honeycomb (√3 × √3)R30° reconstruction, observed by scanning tunnelling microscopy at room temperature, past the initial formation of the dominant, 3×3 reconstructed, silicene monolayer. For a few layers silicene film we measure by synchrotron radiation photoelectron spectroscopy, a cone-like dispersion at the Brillouin zone centre due to band folding. π* and π states meet at ∼0.25 eV below the Fermi level, providing clear evidence of the presence of gapless Dirac fermions.

1D graphene-like silicon systems: silicene nano-ribbons

Through this review we can follow the various phases that have led to the discovery of the new allotrope form of silicon: silicene. This is a one-atom thick silicon sheet arranged in a honeycomb lattice, similar to graphene. For silicon, which usually is sp3 hybridized, it represents an unusual and rare structure. First, silicene was theoretically hypothesized and subsequently its structure calculated as a possible candidate for nano-ribbons of Si grown on the anisotropic Ag(110) surface. It was only later, when the physical and chemical properties of this peculiar form of silicon, demonstrating the presence of π and π* bands giving the so-called Dirac cones at the K corners of the Brillouin zone, the sp2-like nature of the valence orbitals of the Si-Si bonds and its strong resistance towards oxygen were reported, that the real existence of silicene became recognized in the scientific community. This review is essentially focused on the experimental work performed on 1D isolated silicene nano-ribbons and their 1D dense array grown on Ag(110) surfaces.

24 h stability of thick multilayer silicene in air

Thick epitaxial multilayer silicene films with a root 3 x root 3R(30 degrees) surface structure show only mild surface oxidation after 24 h in air, as measured by Auger electron spectroscopy. X-ray diffraction and Raman spectroscopy measurements performed in air without any protective capping, as well as, for comparison, with a thin Al2O3 cap, showed the (002) reflection and the G, D and 2D Raman structures, which are unique fingerprints of thick multilayer silicene.

The fate of the 2√3 × 2√3R(30°) silicene phase on Ag(111)

Silicon atoms deposited on Ag(111) produce various single layer silicene sheets with different buckling patterns and periodicities. Low temperature scanning tunneling microscopy reveals that one of the silicene sheets, the hypothetical √7 × √7 silicene structure, on 2√3 × 2√3 Ag(111), is inherently highly defective and displays no long-range order. Moreover, Auger and photoelectron spectroscopy measurements reveal its sudden death, to end, in a dynamic fating process at ∼300 °C. This result clarifies the real nature of the 2√3 × 2√3R(30°) silicene phase and thus helps to understand the diversity of the silicene sheets grown on Ag(111).

Multilayer silicene: clear evidence

One year after the publication of the seminal paper on monolayer 3 by 3 reconstructed silicene grown on a silver (111) substrate, evidence of the synthesis of epitaxial root3 by root3 reconstructed multilayer silicene hosting Dirac fermions was presented. Although a general consensus was immediately reached in the former case, in the latter one, the mere existence of multilayer silicene was questioned and strongly debated. Here, we demonstrate by means of a comprehensive x-ray crystallographic study, that multilayer silicene is effectively realized upon growth at rather low growth temperatures (~200{\deg}C), while, instead, 3D growth of silicon crystallites takes place at higher temperatures, (~300{\deg}C). This transition to bulk like silicon perfectly explains the various data presented and discussed in the literature and solves their conflicting interpretations.

Silicon Nanosheets: Crossover between Multilayer Silicene and Diamond-like Growth Regime

The structural and electronic properties of nanoscale Si epitaxially grown on Ag(111) can be tuned from a multilayer silicene phase, where the constitutive layers incorporate a mixed sp2/sp3 bonding, to other ordinary Si phases, such as amorphous and diamond-like Si. Based on comparative scanning tunneling microscopy and Raman spectroscopy investigations, a key role in determining the nanoscale Si phase is played by the growth temperature of the epitaxial deposition on Ag(111) substrate and the presence or absence of a single-layer silicene as a seed for the successive growth. Furthermore, when integrated into a field-effect transistor device, multilayer silicene exhibits a characteristic ambipolar charge carrier transport behavior that makes it strikingly different from other conventional Si channels and suggestive of a Dirac-like character of the electronic bands of the crystal. These findings spotlight the interest in multilayer silicene as a different nanoscale Si phase for advanced nanotechnology applications such as ultrascaled nanoelectronics and nanomembranes, as well as for fundamental exploration of quantum properties.

Mn-silicide nanostructures aligned on massively parallel silicon nano-ribbons

The growth of Mn nanostructures on a 1D grating of silicon nano-ribbons is investigated at atomic scale by means of scanning tunneling microscopy, low energy electron diffraction and core level photoelectron spectroscopy. The grating of silicon nano-ribbons represents an atomic scale template that can be used in a surface-driven route to control the combination of Si with Mn in the development of novel materials for spintronics devices. The Mn atoms show a preferential adsorption site on silicon atoms, forming one-dimensional nanostructures. They are parallel oriented with respect to the surface Si array, which probably predetermines the diffusion pathways of the Mn atoms during the process of nanostructure formation.

Synchrotron radiation photoelectron spectroscopy of the O(2s) core level as a tool for monitoring the reducing effects of ion bombardment on SnO2 thin films

Abstract Synchrotron radiation ultraviolet photoemission spectroscopy (UPS) of O(2s) core levels, valence band (VB) and Sn(4d) spectroscopy was used to monitor the effect of Ar + sputtering on SnO 2 films. The decrease of the O(2s) peak intensity and the increase of the Sn 2+ component in the Sn(4d) peak could be satisfactorily correlated with the enhancement of the band gap feature, occurring at 2.9 eV in the VB spectrum, never been due to the formation of Sn(5s–5p) states.

H-Induced Si-Rich 3C-Si(100) 3x2 Surface Metallization

Atomic hydrogen interaction onto the 3C-SiC(100) 3x2 surface is investigated by synchrotron radiation based photoemission spectroscopies, atom resolved scanning tunneling microscopy and spectroscopy, and infrared absorption spectroscopy. Contrary to its wellknown role in semiconductor surface passivation, atomic hydrogen is found to metallize the 3C-SiC(100) 3x2 surface. This unexpected behavior results from an asymmetric attack of the Si-dimers located below the surface, leading to charge transfer and to metallization. Interestingly, the H-covered 3C-SiC(100) 3x2 surface metallization is not removed by oxygen.