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Barium disilicide (BaSi 2) is a semiconductor with promising photovoltaic properties. To build efficient devices, a good understanding of its defects is required. In this article, we present the first systematic study of such defects in a series of BaSi 2 samples synthetized in Barium-rich and Silicon-rich conditions. We combined electron paramagnetic resonance (EPR), Photoluminescence (PL) and density functional theory (DFT) to complete our analysis. With these techniques, we spectroscopically characterized five paramagnetic defect centers (defect 1 to defect 5) and four PL centers (P0 to P3). We identified several of them: in Ba-rich conditions, defect 5 corresponds to the negatively charged silicon vacancy V Si −1. In Si-rich conditions, defect 1 and defect 2 correspond to a negatively charged complex defect O Si −1, consisting of a pseudotetrahedron of 3 silicon and 1 oxygen in two slightly different environments. For PL, we attributed two bands (P0 and P1) to transitions involving O Si once again; P2 and P3 were attributed to transitions involving another complex defect [O i S i ], an interstitial silicon in close proximity to an interstitial oxygen. The existence of these defects rationalizes previous reports in the literature, in particular the increased photoresponse upon hydrogen passivation. [Display omitted] [ABSTRACT FROM AUTHOR]

Yttrium aluminum borate (YAB) powders prepared by sol-gel process have been investigated to understand their photoluminescence (PL) mechanism. The amorphous YAB powders exhibit bright visible PL from blue emission for powders calcined at 450 °C to broad white PL for higher calcination temperature. Thanks to 13C labelling, NMR and EPR studies show that propionic acid initially used to solubilize the yttrium nitrate is decomposed into aromatic molecules confined within the inorganic matrix. DTA-TG-MS analyses show around 2 wt % of carbogenic species. The PL broadening corresponds to the apparition of a new band at 550 nm, associated with the formation of aromatic species. Furthermore, pulsed ENDOR spectroscopy combined with DFT calculations enables us to ascribe EPR spectra to free radicals derived from small (2 to 3 rings) polycyclic aromatic hydrocarbons (PAH). PAH molecules are thus at the origin of the PL as corroborated by slow afterglow decay and thermoluminescence experiments. [ABSTRACT FROM AUTHOR]

Silicon nanoparticles (NPs) serve a wide range of optical, electronic, and biological applications. Chemical grafting of various molecules to Si NPs can help to passivate their reactive surfaces, 'fine-tune' their properties, or even give them further interesting features. In this work, 1H, 13C, and 29Si solid-state NMR spectroscopy has been combined with density functional theory calculations to study the surface chemistry of hydride-terminated and alkyl-functionalized Si NPs. This combination of techniques yields assignments for the observed chemical shifts, including the contributions resulting from different surface planes, and highlights the presence of physisorbed water. Resonances from near-surface 13C nuclei were shown to be substantially broadened due to surface disorder and it is demonstrated that in an ambient environment hydride-terminated Si NPs undergo fast back-bond oxidation, whereas long-chain alkyl-functionalized Si NPs undergo slow oxidation. Furthermore, the combination of NMR spectroscopy and DFT calculations showed that the employed hydrosilylation reaction involves anti-Markovnikov addition of the 1-alkene to the surface of the Si NPs. [ABSTRACT FROM AUTHOR]

Radical S-adenosine-L-methionine (SAM or AdoMet) proteins are involved in chemically difficult reactions including the synthesis of cofactors, the generation of protein radicals, and the maturation of complex organometallic catalytic sites. In the first and common step of the reaction, a conserved [Fe4S4] cluster donates an electron to perform the reductive cleavage of AdoMet into methionine and a reactive radical 5′-dA· species. The latter extracts a hydrogen atom from substrate eliciting one of the about 40 reactions so far characterized for this family of proteins. It has been suggested that the radical-generating mechanism differs depending on whether AdoMet is a cofactor or a substrate. It has also been speculated that electron transfer from the [Fe4S4] cluster to AdoMet is sulfur-based. Here we have used protein crystallography and theoretical calculations to show that regardless whether AdoMet serves as a cofactor or a substrate, the 5′-dA· generating mechanism should be common to the radical SAM proteins studied so far, and that electron transfer is mediated by a unique Fe from the conserved [Fe4S4] cluster. This unusual electron transfer is determined by the sulfonium ion in AdoMet. [ABSTRACT FROM AUTHOR]