61 research outputs found
Epitaxial Synthesis of Blue Phosphorene
Phosphorene is a new two-dimensional material composed of a single or few
atomic layers of black phosphorus. Phosphorene has both an intrinsic tunable
direct band gap and high carrier mobility values, which make it suitable for a
large variety of optical and electronic devices. However, the synthesis of
single-layer phosphorene is a major challenge. The standard procedure to obtain
phosphorene is by exfoliation. More recently, the epitaxial growth of
single-layer phosphorene on Au(111) has been investigated by molecular beam
epitaxy and the obtained structure has been described as a blue-phosphorene
sheet. In the present study, large areas of high-quality monolayer phosphorene,
with a band gap value at least equal to 0.8 eV, have been synthesized on
Au(111). Our experimental investigations, coupled with DFT calculations, give
evidence of two distinct phases of blue phosphorene on Au(111), instead of one
as previously reported, and their atomic structures have been determined.Comment: This paper reports on the epitaxial synthesis of blue phosphoren
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Preventing carbon contamination of optical devices for X-rays: the effect of oxygen on photon-induced dissociation of CO on platinum
Platinum is one of the most common coatings used to optimize mirror
reflectivity in soft X-ray beamlines. Normal operation results in optics
contamination by carbon-based molecules present in the residual vacuum of
the beamlines. The reflectivity reduction induced by a carbon layer at the mirror
surface is a major problem in synchrotron radiation sources. A time-dependent
photoelectron spectroscopy study of the chemical reactions which take place at
the Pt(111) surface under operating conditions is presented. It is shown that the
carbon contamination layer growth can be stopped and reversed by low partial
pressures of oxygen for optics operated in intense photon beams at liquidnitrogen
temperature. For mirrors operated at room temperature the carbon
contamination observed for equivalent partial pressures of CO is reduced and
the effects of oxygen are observed on a long time scale
Asymmetric hysteresis of N\'eel caps in flux-closure magnetic dots
We investigated with XMCD-PEEM magnetic imaging the magnetization reversal
processes of N\'eel caps inside Bloch walls in self-assembled, micron-sized
Fe(110) dots with flux-closure magnetic state. In most cases the
magnetic-dependent processes are symmetric in field, as expected. However, some
dots show pronounced asymmetric behaviors. Micromagnetic simulations suggest
that the geometrical features (and their asymmetry) of the dots strongly affect
the switching mechanism of the N\'eel caps.Comment: Proceeding for MMM-Intermag 2010 (Washington
Formation of one-dimensional self-assembled silicon nanoribbons on Au(110)-(2x1)
We report results on the self-assembly of silicon nanoribbons on the (2x1)
reconstructed Au(110) surface under ultra-high vacuum conditions. Upon
adsorption of 0.2 monolayer (ML) of silicon the (2x1) reconstruction of Au(110)
is replaced by an ordered surface alloy. Above this coverage a new
superstructure is revealed by low electron energy diffraction (LEED) which
becomes sharper at 0.3 Si ML. This superstructure corresponds to Si nanoribbons
all oriented along the [-110] direction as revealed by LEED and scanning
tunneling microscopy (STM). STM and high-resolution photoemission spectroscopy
indicate that the nanoribbons are flat and predominantly 1.6 nm wide. In
addition the silicon atoms show signatures of two chemical environments
corresponding to the edge and center of the ribbons.Comment: Under publication in Applied Physics Letter
Silicon Sheets By Redox Assisted Chemical Exfoliation
In this paper, we report the direct chemical synthesis of silicon sheets in
gram-scale quantities by chemical exfoliation of pre-processed calcium
di-silicide (CaSi2). We have used a combination of X-ray photoelectron
spectroscopy, transmission electron microscopy and Energy-dispersive X-ray
spectroscopy to characterize the obtained silicon sheets. We found that the
clean and crystalline silicon sheets show a 2-dimensional hexagonal graphitic
structure.Comment: Accepted in J. Phys.: Condens. Matte
Single-layer graphene on epitaxial FeRh thin films
Graphene is a 2D material that displays excellent electronic transport properties with prospective applications in many fields. Inducing and controlling magnetism in the graphene layer, for instance by proximity of magnetic materials, may enable its utilization in spintronic devices. This paper presents fabrication and detailed characterization of single-layer graphene formed on the surface of epitaxial FeRh thin films. The magnetic state of the FeRh surface can be controlled by temperature, magnetic field or strain due to interconnected order parameters. Characterization of graphene layers by X-ray Photoemission and X-ray Absorption Spectroscopy, Low-Energy Ion Scattering, Scanning Tunneling Microscopy, and Low-Energy Electron Microscopy shows that graphene is single-layer, polycrystalline and covers more than 97% of the substrate. Graphene displays several preferential orientations on the FeRh(0 0 1) surface with unit vectors of graphene rotated by 30 degrees, 15 degrees, 11 degrees, and 19 degrees with respect to FeRh substrate unit vectors. In addition, the graphene layer is capable to protect the films from oxidation when exposed to air for several months. Therefore, it can be also used as a protective layer during fabrication of magnetic elements or as an atomically thin spacer, which enables incorporation of switchable magnetic layers within stacks of 2D materials in advanced devices
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