Nitrure | EPFL Graph Search

vignette|redresse=1.5|Mèche en acier durcie par un revêtement de nitrure de titane. Les nitrures sont des composés où l'azote est au nombre d'oxydation –III. Ils constituent une large famille dont certains représentants ont des applications concrètes comme le nitrure de titane dont la dureté est mise à profit pour renforcer certains outils. L'ion N est isoélectronique de l'ion oxyde O et de l'ion fluorure F. Il existe des nitrures ioniques ( où M = Be, Mg, Ca) et des nitrures covalents (, ). L'ion nitrure est également un ligand qui se rencontre dans les complexes des éléments de transition, le nom du ligand est nitrido : ex . L'azote est alors considéré comme triplement lié au métal. L'anion nitrure se rencontre à l'état solide avec un rayon ionique de 140 pm (picomètre, 1 × 10 m). Il n'existe jamais en solution aqueuse, car c'est une base très forte qui s'hydrolyse immédiatement au contact de l'eau en captant successivement trois protons pour donner de l'ammoniac (). Il est formell

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

Reactivity of Multimetallic Thorium Nitrides Generated by Reduction of Thorium Azides

Luciano Barluzzi, Fang-Che Hsueh, Marinella Mazzanti, Rosario Scopelliti

Thorium nitrides are likely intermediates in the reported cleavage and functionalization of dinitrogen by molecular thorium complexes and are attractive compounds for the study of multiple bond formation in f-element chemistry, but only one example of thorium nitride isolable from solution was reported. Here, we show that stable multimetallic azide/nitride thorium complexes can be generated by reduction of thorium azide precursors─a route that has failed so far to produce Th nitrides. Once isolated, the thorium azide/nitride clusters, M3Th═N═Th (M = K or Cs), are stable in solutions probably due to the presence of alkali ions capping the nitride, but their synthesis requires a careful control of the reaction conditions (solvent, temperature, nature of precursor, and alkali ion). The nature of the cation plays an important role in generating a nitride product and results in large structural differences with a bent Th═N═Th moiety found in the K-bound nitride as a result of a strong K–nitride interaction and a linear arrangement in the Cs-bound nitride. Reactivity studies demonstrated the ability of Th nitrides to cleave CO in ambient conditions yielding CN–.

2022

Design, synthesis, reactivity, and magnetic properties of uranium based oxide and nitride complexes

Luciano Barluzzi

The activation of small molecules is a paramount challenge in modern chemistry. The use of cheap and abundant molecules such as N2, H2, CO2, or CO as energy supplies or as precursors for fine chemicals production is highly desirable. In particular, the only industrial process known so far that uses the ubiquitous molecule N2 is the Haber-Bosch process for the production of NH3, which consumes about 2% of the world energy yearly. Low valent uranium complexes have been proven to be ideal candidates in the field of small molecule activation such as N2. The activation of N2 in the Haber-Bosch process is thought to proceed through the formation of metal nitrides. The study of molecular nitride species, thus, is extremely important in light of the possibility to achieve N-functionalization processes. Uranium nitride species, indeed, are able to effect the activation of small molecules by N-functionalization. Low valent uranium species are of interest also for their magnetic properties. The magnetic anisotropy and the unpaired electron in U(III) species make them, indeed, ideal candidates as Single Molecule Magnets (SMM). The main objective of this thesis is to synthesize novel uranium species for the activation of small molecules and to synthesize low valent uranium species of interest for their magnetic properties. In the first part the synthesis of a dimeric U(III) complex bridged by an oxo linker and its reactivity with N2 will be reported. The functionalization of the activated N2 shows striking differences compared to previously reported complexes. The difference in reactivity is accompanied by a difference in the magnetic properties of the complexes, which reflects their different electronic structure and binding scheme. Since the full splitting of N2 could not be achieved so far with U(III) complexes, the synthesis of a bis-µ-nitride complex, which would serve as a model for cleaved N2, was targeted. Its synthesis and characterization are reported in Chapter 3. We proved that such a bis nitride complex is reactive and is able to activate small molecules, forming N-C bonds by reaction with molecules like CO2, CS2, or CO. More importantly, H2 is activated by the bis nitride complex in an unprecedented oxidative cleavage. While uranium nitride complexes supported by the siloxide ligand were reported to exhibit nucleophilic reactivity, other nitride complexes are instead unreactive or even electrophilic. In order to rationalize this behaviour, the synthesis of different uranium nitride complexes bearing different coligands has been targeted and their synthesis is described in Chapter 4. The importance of the coligand has been experimentally proven in closely related systems. Despite several bridging nitride complexes have been reported, a paucity of terminal uranium nitride complexes are, instead, isolable. In particular, the photochemical reactivity of terminal uranium azide complexes has so far failed to yield the desired terminal nitride species. In this work we report the first photochemical synthesis of an isolable uranium terminal nitride. The terminal nitride complex is remarkably stable and its functionalization with CO is reported. Lastly, the synthesis of low valent heteroleptic complexes is reported. These compounds can serve as starting materials for the synthesis of novel uranium complexes and are of interest for their magnetic properties.

EPFL2021