Électron de valence | EPFL Graph Search

Un électron de valence est un électron situé dans la couche de valence d'un atome. Les propriétés physiques d'un élément sont largement déterminées par leur configuration électronique, notamment la configuration de la couche de valence. La présence d'un ou plusieurs électrons de valence joue un rôle important dans cette définition des car elle permet de déterminer la valence . Lorsqu'un atome a une couche de valence incomplète, il peut partager ou donner des électrons de valence avec d'autres atomes pour remplir sa couche externe et former une liaison chimique stable. Le nombre d'électrons de valence d'un élément peut être déterminé à partir de sa position dans le tableau périodique. Les éléments situés dans la même colonne ont le même nombre d'électrons de valence, et les éléments ayant le même nombre d'électrons de valence ont tendance à avoir des propriétés chimiques similaires. Par exemple, tous les éléments du groupe des halogènes ont sept électrons de valence, ce qui leur pe

Interface Investigations on Titanium Nitride Bilayer Systems

Dominik Anwar Jaeger

Nanocomposite coatings composed of two phases with atomically sharp phase boundaries, show interesting mechanical properties. These properties are often originating from their high interface to volume ratio. Composites of nanocrystalline titanium nitride (TiN) grains surrounded by a one to two monolayer thick interlayer of silicon nitride (Si3N4) show an enhanced nanohardness. The central theme of this thesis is concernedwith the interfacial properties of two-dimensional bilayer systems, which are used as model systems to describe the interfaces occurring in nanocomposite coatings. The systems under investigation are TiN interfaces in contact with silicon (Si), silicon nitride (Si3N4) and aluminum nitride (AlN). The primary tool used to analyze the interfaces of bilayer systems is X-ray Photoelectron Spectroscopy (XPS) with emphasis put on the shake-up feature of the Ti 2p photoelectron line. Shake-ups in TiN are observed as an additional peak on the lower binding energy side of the energy lines of the Ti 2p orbitals. Shake-ups are strongly influenced by valence electrons and electron density distributions. This makes them a powerful tool to probe the chemical and electronic structure of TiN interfaces. The aim of this study is to utilize the shake-up energy and its intensity to gain insight into interfacial structures and correlate their changes to interfacial polarization and macroscopic mechanical properties. Single crystalline (sc-) and oxygen-free TiN as well as oxygen-free bilayer systems were deposited by unbalanced magnetron sputtering and analyzed by Angle Resolved (AR-)XPS. Bilayer samples were deposited and their quality was controlled using X-ray diffraction (crystallinity), Rutherford back scattering (elemental composition), and atomic force microscopy (roughness). All XPS samples were fabricated, transfered and analyzed whilst maintaining ultra high vacuum. A precise and self-consistent XPS data processing method was developed to evaluate Ti 2p spectra. This method accounts for the correct photoelectron line shape, background subtraction and photoelectron peak area intensity. Binding energy, shake-up energy and intensity ratios of shake-ups taken frompristine TiN surfaces are precisely determined, and the influence of oxygen on the information content in peak positions and intensities was investigated. The shake-up energy and intensity of bulk sc-TiN and its origin of the shake-up are discussed. An analytical description for the XPS signal ratio of bilayer systems is derived to separate the interfacial signals from the bulk information. The results obtained by this analytical description are strongly influenced by the interface thickness that has been found to be proportional to the overlayer thickness. The revealed interface properties show a correlation between the shake-up intensity and the interface morphology, oxygen content, overlayer material and overlayer thickness. AR-XPS and X-ray Photoelectron Diffraction (XPD) results were used to interpret the crystalline structure of the different TiN/AlN and TiN/Si3N4 bilayer systems. AlN shows XPD patterns indicating a crystalline growth of AlN on sc-TiN. The electrically insulating AlN overlayer creates a charge accumulation at the TiN interface, which results in an enhanced shake-up intensity. XPD patterns of Si3N4 systems revealed a crystalline growth of Si3N4 in the first 0.6nm. The intensity of the diffraction patterns reduces with increasing Si3N4 overlayer thickness due to a change in the growth behavior from crystalline to amorphous structures. Si3N4 films show, in comparison to AlN, reduced interface charging and hence a lower shakeup intensity. The crystalline growth of Si3N4 in the initial stages is hindered in systems where a bias voltage is applied to the substrate during the deposition process. In contrast to the unbiased systems, which have crystalline interfacial structures, the biased systems no longer show XPD patterns due to a loss of crystallinity. Additionally the shake-up intensity of biased systems is thickness-independent, which is in contrast to unbiased systems. The difference in the shake-up intensity of biased and unbiased Si3N4 is explained by a different band gap of the Si3N4 structure in the first two monolayers. This thesis shows that the increase in the shake-up intensity is correlated to intrinsic and extrinsic interface charging. The obtained results, in combination with theoretical structure models from literature, show that in one to two monolayer thick interlayers a build-up of interface polarization is unlikely. The observed nanohardness enhancement in TiN/Si3N4 systems is explained with already known hardness effects.

EPFL2012

Kinks in the angle resolved photoemission and inelastic x-ray spectra of high temperature superconductors

Jeff Graf

Though the high superconducting transition temperature (Tc) is the most interesting technological aspect of high temperature superconductors, the complex way in which the spin, lattice and electronic degrees of freedom interplay, makes them of the highest scientific interest. This is illustrated by their rich phase diagram characterized by a variety of exotic groundstate including; Mott insulating, pseudogap and high temperature superconductivity. One of the most serious barriers that has prevented our understanding of the mechanism of high temperature superconductors thus far is the lack of information on how low energy Landau quasiparticles result from an organization of real particles. This organization, often colloquially referred to as "dressing", is a fundamental general concept in physics that explains a variety of physical phenomena, such as exotic particle formations and phase transitions. The role of the lattice in this dressing is particularly controversial. Phonons are quanta of lattice vibration energy, and play a crucial role in conventional superconductivity. They provide an attractive interaction allowing the electrons to condensate in superconducting Cooper pairs. However, high temperature superconductivity in the cuprates in achieved through hole doping in an antiferromagnetic Mott insulator. In this case, the antiferromagnetic background, the strong coulomb repulsion and the anisotropic superconducting gap all suggest a marginal role of the phonons. In order to assess experimentally the exact role of the phonon, angle resolved photoemission spectroscopy (ARPES) is the weapon of choice given its unique ability to probe the electronic structure of solids in an energy- and momentum-resolved manner. In this thesis, I will use ARPES to characterize the various form of dressing of the quasiparticles in the high temperature superconductor from the Fermi level all the way to the valence band complex. I'll show that when combined with isotope substitution and inelastic x-rays scattering, the strong electron-phonon interaction can be probed in vivid details. This thesis presents three fundamental results. 1) Using the isotope effect, the important and unusual role of the phonon is demonstrated, as well as the strong interplay between the magnetic and phonic degrees of freedom. 2) Using inelastic scattering, the intriguing interplay between the Fermi surface and the bond stretching phonon is exposed. And 3) By exploring the ARPES data up to high energy, two new energy scales at high binding energy as well as the spectral waterfalls phenomenon are revealed. When all these new elements are considered together, they clearly show the need to consider simultaneously the spin, lattice, Fermi surface topology and electronic degrees of freedom. The spectacular progress in ARPES techniques over the past two decades was critical for these results, and I will present in this thesis our current effort in pushing the techniques even further. In particular I'll present my efforts in building a laser based ARPES setup with a hemispherical analyzer and a spin resolved time of flight analyzer.

EPFL2008

Quadrupolar Benzobisthiazole-Cored Arylamines as Highly Efficient Two-Photon Absorbing Fluorophores

Vladislav Semak

A computer-aided design of novel D-pi-A-pi-D styrylamines containing five isomeric benzobisthiazole moieties as the electron-accepting core has revealed the linear centrosymmetric benzo[1,2-d:4,5-d']bisthiazole as the most promising building block for engineering chromophores displaying high two-photon absorption (TPA) in the near-IR region, as also confirmed experimentally. The ease of synthesis of quadrupolar derivatives thereof, combined with extraordinarly high TPA action cross sections (delta(TPA)Phi(f) > 1500 GM), makes these heteroaromatic systems particularly attractive as diagnostic agents in 3D fluorescence imaging.

Amer Chemical Soc2014