Showing papers by "Maria C. Asensio published in 2012"

Silicene: Compelling Experimental Evidence for Graphenelike Two-Dimensional Silicon

Abstract: Because of its unique physical properties, graphene, a 2D honeycomb arrangement of carbon atoms, has attracted tremendous attention. Silicene, the graphene equivalent for silicon, could follow this trend, opening new perspectives for applications, especially due to its compatibility with Si-based electronics. Silicene has been theoretically predicted as a buckled honeycomb arrangement of Si atoms and having an electronic dispersion resembling that of relativistic Dirac fermions. Here we provide compelling evidence, from both structural and electronic properties, for the synthesis of epitaxial silicene sheets on a silver (111) substrate, through the combination of scanning tunneling microscopy and angular-resolved photoemission spectroscopy in conjunction with calculations based on density functional theory.

Band-gap expansion in the surface-localized electronic structure of MoS 2 (0002)

Abstract: The electronic band structure of MoS${}_{2}$ single crystals has been investigated using angle-resolved photoelectron spectroscopy and first-principles calculations. The orbital symmetry and $k$ dispersion of these electronic states responsible for the direct and the indirect electronic band gaps have been unambiguously determined. By experimentally probing an increase of the electronic band gap, we conclude that a MoS${}_{2}$ (0002) surface localized state exists just below the valence band maximum at the $\ensuremath{\Gamma}$ point. This electronic state originates from the sulfur planes within the topmost layer. Our comprehensive study addresses the surface electronic structure of MoS${}_{2}$ and the role of van der Waals interlayer interactions.

ANTARES, a scanning photoemission microscopy beamline at SOLEIL

Abstract: As one of the latest beamline built at the SOLEIL synchrotron source, ANTARES beamline offers a spectroscopic non-destructive nano-probe to study advanced materials. This innovative scanning photoemission microscopy combines linear and angle sweeps to perform precise electronic band structure determination by Nano Angle Resolved Photoelectron Spectroscopy (nanoARPES) and chemical imaging by core level detection. The beamline integrates effectively insertion devices and a high transmission beamline optics. This photon source has been combined with an advanced microscope, which has precise sample handling abilities. Moreover, it is fully compatible with a high resolution R4000 Scienta hemispherical analyzer and a set of Fresnel Zone Plates (FZP) able to focalize the beam spot up to a few tenths of nanometers, depending on the spatial resolution of the selected FZP. We present here the main conceptual design of the beamline and endstation, together with some of the firsts commissioning results.

Zooming in on Electronic Structure: NanoARPES at SOLEIL and ALS

Abstract: In today's state-of-the-art ARPES, probe sizes are routinely in the range from tens to hundreds of microns. However, in complex materials, length scales from the nano- to the mesoscopic scales control and characterize electronic and other properties. Thus it is not possible to access relevant length scales for many complex engineered materials using ARPES. In the last 15 years, the focusing of the photon beam down to 10's of nm has been routinely accomplished in techniques such as scanning transmission X-ray microscopy (STXM) and scanning photoemission microscopy (SPEM). Application of these techniques to ARPES to create a new “nanoARPES” (or nARPES for short) will for the first time allow the interplay between geometric and electronic structure to be probed directly in arbitrary sample geometries.

Chemical imaging and angle-resolved photoemission study of well-ordered thermally reduced SrTiO 3 (100)

Abstract: The structural and electronic properties of thermally reduced SrTiO${}_{3}$(100) single crystals have been investigated using a probe with real- and reciprocal-space sensitivity: a synchrotron radiation microsopic setup which offers the possibility of scanning photoemission microscopy and angle-resolved photoelectron spectroscopy (ARPES) down to the nanometric scale. We have spectroscopically imaged the chemical composition of samples presenting reproducible and suitable low-energy diffraction patterns after following well-established thermal reduction protocols. At the micrometric scale, Ca-rich areas have been directly imaged using high-energy resolution core-level photoemission. Moreover, we have monitored the effect of Ca segregation on different features of the SrTiO${}_{3}$(100) electronic band structure, measuring ARPES inside, outside, and at the interface of surface inhomogeneities with the Ca-rich identified areas. In particular, the interaction of Ca with the well-known intragap localized state, previously attributed to oxygen vacancies, has been investigated. Moreover, the combination of direct imaging and spectroscopic techniques with high spatial resolution has clarified the long-standing dilemma related to the bulk or surface character of Ca segregation in SrTiO${}_{3}$. Our results present solid evidence that the penetration depth of Ca segregation is very small. In contrast to what has been previously proposed, the origin of the long-range surface reconstructions can unlikely be associated to Ca due to strong local variations of its surface concentration.

Interferometer-controlled soft X-ray scanning photoemission microscope at SOLEIL

Abstract: ANTARES beamline (BL), a new soft X-ray scanning photoemission microscope located at the SOLEIL synchrotron storage ring has been recently designed, built and commissioned. The implemented interferometer control allows the accurate measurement of the transverse position of the Fresnel zone plate (FZP) relative to the sample. An effective sample position feedback has been achieved during experiments in static mode, with a fixed FZP position required to perform nano Angle-Resolved Photoelectron Spectroscopy (Nano-ARPES) measurements. Likewise, long-term stability has been attained for the FZP position relative to the sample during the translation of the FZP when performing typical X-ray absorption experiments around the absorption edges of light elements. Moreover, a fully automatic feedback digital control of the interferometric system provides extremely low orthogonal distortion of the recorded two-dimensional images. The microscope is diffraction limited with the resolution set to several tens of nanometers by the quality of the zone plates. Details on the design of the interferometric system and a brief description of the first commissioning results are presented here.