Matteo Aramini

Decoration of graphene with nickel nanoparticles: study of the interaction with hydrogen

Graphene obtained from thermal exfoliation of graphite oxide was chemically functionalized with nickel nanoparticles (NPs) without exposing the system to oxidizing agents. Its structural, physical and chemical properties have been studied by means of TEM, X-ray photoelectron and Raman spectroscopies, and SQuID magnetometry. The formation of 17 nm super-paramagnetic (SPM) monodispersed Ni NPs was observed. Nitrogen sorption experiments at 77 K yield a Brunauer–Emmet–Teller specific surface area (BET-SSA) of 505 m2 g−1 and helium adsorption at room temperature gives a skeletal density of 2.1 g cm−3. The interaction with atomic hydrogen was investigated by means of Muon Spin Relaxation (μSR) showing a considerable fraction of captured muonium (∼38%), indicative of strong hydrogen–graphene interactions. Hydrogen adsorption has been measured via pressure concentration isotherms demonstrating a maximum of 1.1 mass% of adsorbed hydrogen at 77 K and thus a 51% increased hydrogen adsorption compared to other common carbon based materials.

Hydrogen motions in defective graphene: the role of surface defects

Understanding the mobility of H at the surface of carbon nanostructures is one of the essential ingredients for a deep comprehension of the catalytic formation of H2 in interstellar clouds. In this paper, we combine neutron vibrational spectroscopy with DFT molecular dynamics simulations to study the local environment of H structures chemisorbed at the surface of disordered graphene sheets. At 5 K, the ground state is composed of large clusters of hydrogen chemisorbed at sp2 carbon sites, on the edges and in voids of the graphene sheets. At temperatures of ∼300 K, a high degree of dispersion of the clusters is observed, involving the breaking and reforming of covalent bonds which, at low temperatures, is mediated by incoherent tunnelling of hydrogen.

Muons probe magnetism and hydrogen interaction in graphene

Muon spin resonance (μSR) is a powerful technique for investigating the local magnetic fields in materials through implanted muons. Here we report a μSR study of chemically produced thermally exfoliated graphene. Our results provide an experimental answer to the many theoretical investigations of magnetic properties of graphene. The observed muon spin precession is attributed to a localized muon–hydrogen nuclear dipolar interaction rather than to a hyperfine interaction with magnetic electrons. This proves the absence of magnetism in chemically produced thermally exfoliated graphene.

NMR investigation of the pressure induced Mott transition to superconductivity in Cs3C60 isomeric compounds

The discovery in 1991 of high temperature superconductivity (SC) in A3C60 compounds, where A is an alkali ion, has been initially ascribed to a BCS mechanism, with a weak incidence of electron correlations. However various experimental evidences taken for compounds with distinct alkali content established the interplay of strong correlations and Jahn Teller distortions of the C60 ball. The importance of electronic correlations even in A3C60 has been highlighted by the recent discovery of two expanded fulleride Cs3C60 isomeric phases that are Mott insulators at ambient pressure. Both phases undergo a pressure induced first order Mott transition to SC with a (p, T) phase diagram displaying a dome shaped SC, a common situation encountered nowadays in correlated electron systems. NMR experiments allowed us to establish that the bipartite A15 phase displays Neel order at 47K, while magnetic freezing only occurs at lower temperature in the fcc phase. NMR data do permit us to conclude that well above the critical pressure, the singlet superconductivity found for light alkalis is recovered. However deviations from BCS expectations linked with electronic correlations are found near the Mott transition. So, although SC involves an electron-phonon mechanism, correlations have a significant incidence on the electronic properties, as had been anticipated from DMFT calculations.