Judicious Ligand Design in Ruthenium Polypyridyl CO2 Reduction Catalysts to Enhance Reactivity by Steric and Electronic Effects

A series of RuII polypyridyl complexes of the structural design [RuII(R−tpy)(NN)(CH3CN)]2+ (R−tpy=2,2′:6′,2′′-terpyridine (R=H) or 4,4′,4′′-tri-tert-butyl-2,2′:6′,2′′-terpyridine (R=tBu); NN=2,2′-bipyridine with methyl substituents in various positions) have been synthesized and analyzed for their ability to function as electrocatalysts for the reduction of CO2 to CO. Detailed electrochemical analyses establish how substitutions at different ring positions of the bipyridine and terpyridine ligands can have profound electronic and, even more importantly, steric effects that determine the complexes’ reactivities. Whereas electron-donating groups para to the heteroatoms exhibit the expected electronic effect, with an increase in turnover frequencies at increased overpotential, the introduction of a methyl group at the ortho position of NN imposes drastic steric effects. Two complexes, [RuII(tpy)(6-mbpy)(CH3CN)]2+ (trans-[3]2+; 6-mbpy=6-methyl-2,2′-bipyridine) and [RuII(tBu−tpy)(6-mbpy)(CH3CN)]2+ (trans-[4]2+), in which the methyl group of the 6-mbpy ligand is trans to the CH3CN ligand, show electrocatalytic CO2 reduction at a previously unreactive oxidation state of the complex. This low overpotential pathway follows an ECE mechanism (electron transfer–chemical reaction–electron transfer), and is a direct result of steric interactions that facilitate CH3CN ligand dissociation, CO2 coordination, and ultimately catalytic turnover at the first reduction potential of the complexes. All experimental observations are rigorously corroborated by DFT calculations

Ultrafast spin-currents and charge conversion at \u3ci\u3e3d-5d\u3c/i\u3e interfaces probed by time-domain terahertz spectroscopy

Spintronic structures are extensively investigated for their spin-orbit torque properties, required for magnetic commutation functionalities. Current progress in these materials is dependent on the interface engineering for the optimization of spin transmission. Here, we advance the analysis of ultrafast spin-charge conversion phenomena at ferromagnetic-Transition metal interfaces due to their inverse spin-Hall effect properties. In particular, the intrinsic inverse spin-Hall effect of Pt-based systems and extrinsic inverse spin-Hall effect of Au:W and Au:Ta in NiFe/Au:(W,Ta) bilayers are investigated. The spin-charge conversion is probed by complementary techniques-ultrafast THz time-domain spectroscopy in the dynamic regime for THz pulse emission and ferromagnetic resonance spin-pumping measurements in the GHz regime in the steady state-to determine the role played by the material properties, resistivities, spin transmission at metallic interfaces, and spin-flip rates. These measurements show the correspondence between the THz time-domain spectroscopy and ferromagnetic spin-pumping for the different set of samples in term of the spin mixing conductance. The latter quantity is a critical parameter, determining the strength of the THz emission from spintronic interfaces. This is further supported by ab initio calculations, simulations, and analysis of the spin-diffusion and spin-relaxation of carriers within the multilayers in the time domain, permitting one to determine the main trends and the role of spin transmission at interfaces. This work illustrates that time-domain spectroscopy for spin-based THz emission is a powerful technique to probe spin-dynamics at active spintronic interfaces and to extract key material properties for spin-charge conversion

Spintronic terahertz emitters with integrated metallic terahertz cavities

Spintronic terahertz emitters (STEs), based on optical excitation of nanometer thick ferromagnetic/heavy metal (FM/HM) heterojunctions, have become important sources for the generation of terahertz (THz) pulses. However, the efficiency of the optical-to-THz conversion remains limited. Although optical techniques have been developed to enhance the optical absorption, no investigations have studied the application of THz cavities. Here, to enhance the THz efficiency of STEs in a selected THz spectral range, FM/HM structures are realized on ultra-thin sapphire layers with metallic mirrors to create lambda/4 THz resonant cavities. THz emission time domain spectroscopy of these STE/sapphire/mirror heterostructures, with sapphire thicknesses ranging from 110 mu m to 25 mu m, shows enhancement of the emitted THz field that fits the lambda/4 cavity resonance with up to a doubling of the field in the spectrum, and in agreement with temporal simulations of the emitted THz pulse. By taking advantage of birefringent materials, we further show the potential of control of the polarization state of the emitted THz pulse. This work shows the potential of enhancing and engineering THz emission from STEs using THz cavities that can be controlled over a broad spectral range, which can be easily combined with optical cavities.Web of Science13101907189

Apport de l’IRM dans le diagnostic des kystes et tumeurs bénignes des maxillaires

Médecine. Chirurgie dentaireLes lésions bénignes des maxillaires constituent un ensemble nosologique varié. Alors qu’une classification internationale détaillée existe, il n’est pas toujours aisé de bien identifier le type de lésion. De ce fait, il peut être difficile de poser avec certitude un diagnostic avant d’obtenir la pièce opératoire complète. En vue d’améliorer l’identification du type de lésion l’apport de l’IRM peut être envisagé comme une imagerie de deuxième, voire de troisième intention. Associant de multiples séquences d’acquisition et l’utilisation d’un produit de contraste le diagnostic adéquat peut être proposé au patient avec davantage de certitude. Dans une première partie une synthèse de du fonctionnement technique de l’IRM est réalisée. Par la suite, les caractéristiques des lésions kystiques et tumorales en radiographie conventionnelle seront étudiées. Une troisième puis quatrième parties s’attacheront à décrire l’aspect de ces lésions et leurs différences en imagerie IRM