Absorption spectroscopy | EPFL Graph Search

Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of electromagnetic radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating field. The intensity of the absorption varies as a function of frequency, and this variation is the absorption spectrum. Absorption spectroscopy is performed across the electromagnetic spectrum. Absorption spectroscopy is employed as an analytical chemistry tool to determine the presence of a particular substance in a sample and, in many cases, to quantify the amount of the substance present. Infrared and ultraviolet–visible spectroscopy are particularly common in analytical applications. Absorption spectroscopy is also employed in studies of molecular and atomic physics, astronomical spectroscopy and remote sensing. There is a wide range of experimental approaches for measuring absorption spectra. The most common arrangement i

Etude de l'origine et de la croissance de particules submicrométriques dans les plasmas radiofréquence réactifs

Christian Deschenaux

The formation of sub-micron sized particulates has been studied in a low pressure (0.1 mbar) radio-frequency (13.56 MHz) capacitively coupled plasma discharge. Particles are studied in situ by infrared absorption spectroscopy, laser illumination and Cavity Ring Down Absorption Spectroscopy. Transmission Electron Microscopy was applied ex situ for particulates collected during the plasma. We propose that two principal mechanisms may be involved in the formation of these particulates: formation and trapping of negative ions in the gaseous phase, and the recycling of material deposited on the electrodes during the discharge, by film erosion or delamination. The relative importance of these two mechanisms was studied in silane, methane, ethylene and acetylene plasmas. The growth of the particulates in silane plasmas, monitored with Infrared and Cavity Ring Down Absorption Spectroscopy, allowed us to distinguish an increasing volume growth phase followed by an agglomeration phase at constant volume, in agreement with the literature. A slight powder and film recycling effect is detected as the experiments are repeated, as observed by an increasing particle size. The hydrogen content of powder formed in silane plasmas was estimated in situ by infrared absorption spectroscopy and changes in the infrared absorption spectrum could be observed for silane diluted with hydrogen and argon. In particular, the porous structure of the powder was observed from the high content of surface related vibration bonds relative to bulk related bonds. Photoluminescence was detected for the particles at 2.15eV that could possibly be attributed to the crystalline structure of the particles. Infrared Absorption Spectroscopy and mass spectrometry applied in situ showed that acetylenic compounds are preferentially produced in methane, ethylene and acetylene plasmas and that the hydrogen content of the other species produced in these plasmas depends on the initial gas hydrogen content. Negative ions were evidenced in these plasmas and a polymerization reaction is proposed between acetylene and C2nHx- negative ions to explain the formation of particle's precursors. The formation of aromatic structures is suggested for n>3. The different relative abundance of C2Hx- ions is proposed to explain the different powder production rates with different initial gases or different fluxes. Hydrogen may inhibit the formation of high-mass negative ions and particles. Fast growth of spherical amorphous carbon particles (∼100s) was observed for a low pressure (0.1 mbar) acetylene plasma. The sp3 structure was confirmed in situ by infrared absorption spectroscopy. Slow growth (∼1000s) of irregularly shaped particles is observed for methane plasmas at the same pressure and power. Particle heterogeneity was studied with Transmission Electron Microscopy and showed graphitic onions, multiwalled nanotubes, flakes and agglomerates attributed to the erosion or the delamination of the deposited carbon layer. The influence of the recycling of the deposited layer on the powder formation was observed in situ from enhanced particle formation as the experiments were repeated. A similar behavior was observed for discharges containing mixtures of hexamethyldisiloxane, oxygen and helium. Keeping in mind that the control of powder homogeneity implies a control over the reactor contamination and its history, we conclude that an important part of the particulates formed in the discharges studied depends on the gas phase reactions, determined by the plasma chemistry.

EPFL2002

Magnetism and metal-organic self-assembly of rare-earth atoms on decoupling layers

Sébastien Reynaud

This thesis presents investigations of magnetic and structural properties of two dimensional metal-organic frameworks and of single atoms, both adsorbed on decoupling layers grown on metal surfaces, with focus on rare earth elements.We first report on a series of attempts at realizing self-assembled metal-organic networks on different decoupling layers, probed by scanning tunneling microscopy. The aim is to produce regular structures with high surface filling factors such that the magnetic properties of the rare earth atoms in the metal-organic networks can be investigated by X-ray ensemble measurements. The combinations of three molecular ligands (QDC, BDA, ZnTPyP) on three insulating layers (MgO, NaCl, graphene) with Tb as rare earth are reported. Of the three decoupling layers, only graphene is found to allow the synthesis of metal-organic structures. We successfully synthesized large islands of a rare earth-QDC coordination complex, with a majority of fivefold coordination of the rare earth atoms. The same structure is reproduced with Dy and Er.X-ray absorption spectroscopy, X-ray magnetic circular and linear dichroism measurements are then carried out on the Dy- and Er-QDC/gr/Ir(111) networks. Compared to single atoms on gr/Ir(111), both Dy and Er change their

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occupancy from 4

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and change their easy axis of magnetization. Both species are paramagnetic when incorporated into the metal-organic structure. The last part of the thesis is devoted to the study of Dy, Ho, and Gd on NaCl thin films. All three atoms display

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occupancy of the gas phase. The top-Cl adsorption site is determined with scanning tunneling microscopy. The magnetic properties are probed with X-ray absorption spectroscopy, X-ray magnetic circular and linear dichroism. Ho and Gd are paramagnetic. Dy has out-of-plane anisotropy with open hysteresis at non zero magnetic field. However, thermally assisted quantum tunneling of the magnetization prevents remanence at zero field.

EPFL2022

Long Spin Lifetime in Rare-Earth Single-Atom Magnets at Surfaces

Boris Sorokin

Further miniaturisation of magnetic storage devices requires an advent of new types of magnets, since classical ferromagnetic materials show lack of remanence at nano- and subnanoscale. A single atom can represent the smallest possible bit of information. Even though the isolated single atoms are paramagnetic, the interaction of surface adsorbed atoms with a substrate can result in high magnetic anisotropy energy and long magnetisation lifetime. In this thesis the magnetic properties of surface supported rare-earth single atoms are investigated with X-ray absorption spectroscopy, X-ray magnetic circular dichroism and multiplet simulations. The first goal of the project was to find new adatom/substrate combinations that exhibit a long adatom magnetic lifetime, possibly extending the current limits of the atomic magnetic moment stability to higher temperatures and longer spin relaxation times. The second objective was to implement the external electric field control of the adatom magnetic properties.Firstly, we present the experimental and theoretical research on Dy and Ho single atoms deposited on BaO. A comparison of our results with similar studies sheds light on the impact of the substrate crystal field, in particular Dy-O bond covalency, on the magnetic stability of the rare-earth adatoms. Next, the detailed experimental investigation of Dy single atoms on ZnO is presented. This adatom/substrate pair is used as first attempt to implement the electric field control of the adatom magnetic properties. Lastly, we study the Dy adatoms on graphene layers grown on various metals. The vibrational modes of the metallic substrates are shown to strongly affect the Dy spin lifetime, while addition of a second graphene layer does not allow to significantly improve the adatom magnetic stability.

EPFL2022

Magnetism and metal-organic self-assembly of rare-earth atoms on decoupling layers

Sébastien Reynaud

4f

occupancy from 4

f^n

4f^{n-1}

4f

EPFL2022