Spectroscopie électronique | EPFL Graph Search

La spectroscopie électronique est une famille de méthodes spectroscopiques qui étudie les transitions, en absorption ou en émission, entre les états électroniques d'un atome ou d'une molécule. Les principales spectroscopies électroniques sont présentées ci-dessous. Spectroscopie électromagnétique La source de bombardement est un rayonnement électromagnétique. Les rayonnements électromagnétiques exploités en spectroscopie électronique vont du rayonnement visible jusqu'aux rayons X en passant par les ultraviolets proches, moyens et lointains. Les rayons visibles et ultraviolets interviennent au niveau des électrons de valence (externes) alors que les rayons X interviennent au niveau des électrons de cœur (proches du noyau). Spectroscopie de transition électronique Transition électronique Sous l’effet des rayons, une transition électronique a lieu au niveau des atomes et des molécules. Les principales méthodes spectroscopiques concernées sont les suivantes :

Transfer of Graphene under Ultra-High Vacuum

Darius Constantin Merk

This thesis reports on a novel method for graphene transfer that is fully UHV compatible, which has remained a challenge for a long time due to the necessity of supporting graphene for transfer and most often requiring supporting layer removal post-transfer. Our graphene transfer method is based on the wafer-bonding approach, where graphene is stamped onto a target surface, from a support to which it is weakly bound. We use a bilayer of graphene supported by a teflon tape for the low adhesion between graphene layers and the flexibility of teflon tape to allow adaptability to the target surface.We demonstrate successful transfer onto Cu(100) and Ir(111) single crystal surfaces as well as on gr/Ir(111), thus forming a (partial) graphene bilayer on Ir(111). For in-situ characterization, we use Auger electron spectroscopy and STM. Auger measurements confirm that we are able to transfer an average 0.8-0.9 ML of carbon, by comparison to CVD grown samples. After transfer onto Ir(111), we show that by annealing to 1000 °C, we are able to observe by STM the characteristic moiré pattern formed by CVD grown graphene on Ir(111). Auger spectra show that the graphene-Iridium interaction undergoes a change from physisorption to chemisorption upon annealing. XAS and Raman spectroscopy demonstrate that the annealing step taken post transfer to observe the moiré by STM is only necessary to induce a change in interaction strength but not to heal graphene defects, to increase its domain size, or to make it flat. After transfer onto Cu(100), Raman spectra show that the transferred graphene is of the same quality as the CVD grown graphene before transfer, without further annealing. Comparison of synchrotron based high resolution XAS spectra of transferred gr/Ir(111) to CVD grown samples confirms that the transferred graphene is flat on top of the Ir(111) crystal surface.This enables sample and device fabrication, usually hampered by impurities, to be carried out in UHV leading to fully reproducible results. Clean twisted bilayer graphene samples can be made and studied in UHV. Further, surface science can be extended to the third dimension, by capping layers and continuing growth, etc., all under clean conditions.

EPFL2022

Near-Atomic-Scale Mapping of Electronic Phases in Rare Earth Nickelate Superlattices

Duncan Alexander, Bernat Mundet, Jean-Marc Triscone

Nanoscale mapping of the distinct electronic phases characterizing the metal-insulator transition displayed by most of the rare-earth nickelate compounds is fundamental for discovering the true nature of this transition and the possible couplings that are established at the interfaces of nickelate-based heterostructures. Here, we demonstrate that this can be accomplished by using scanning transmission electron microscopy in combination with electron energy-loss spectroscopy. By tracking how the O K and Ni L edge fine structures evolve across two different NdNiO3/SmNiO3 superlattices, displaying either one or two metal- insulator transitions depending on the individual layer thickness, we are able to determine the electronic state of each of the individual constituent materials. We further map the spatial configuration associated with their metallic/insulating regions, reaching unit cell spatial resolution. With this, we estimate the width of the metallic/insulating boundaries at the NdNiO3/SmNiO3 interfaces, which is measured to be on the order of four unit cells.

AMER CHEMICAL SOC2021

Strain-driven oxygen deficiency in multiferroic SrMnO3 thin films

Rolf Erni, Cécile Hébert

The structural and electronic properties of a series of SrMnO3 thin films grown on different substrates leading to different biaxial strains are examined by means of aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and electron energy-loss spectroscopy (EELS). Here, we report a comprehensive theoretical investigation on the impact of oxygen nonstoichiometry and strain on the O-K and Mn-L-2,L-3 electron energy-loss near-edge structures of SrMnO3 with the aid of the all-electron density functional theory code WIEN2K. Our experimental and theoretical EELS data reveal pronounced peak-height changes and energy shifts as a function of strain, which are ascribed to differences in the oxygen vacancy density rather than to a direct effect of lattice distortions. In addition, we find that the formation of oxygen vacancies in SrMnO3 is likely to be enhanced by tensile strain. Compared to the unstrained material, the vacancy formation energy is reduced by 0.25 eV under 3.78% tensile strain.

Amer Physical Soc2016

Transfer of Graphene under Ultra-High Vacuum

Darius Constantin Merk

EPFL2022