HYPERFINE TECHNIQUES STUDIES OF SURFACES AND INTERFACES USING ISOLATED AD-ATOM PROBES
by Abel Fenta
Friday, 12 April 2013 from to (Europe/Zurich)
at CERN ( 26-1-022 )
at CERN ( 26-1-022 )
In the last two decades, thin film and nanostructured materials have assumed a prominent role in physics and materials engineering, dedicated to the miniaturization of devices in a multitude of applications. Electrical and magnetic properties and molecular functionalization strongly depend on surface and interface nanoscale interactions, where new physical models should apply aiming their understanding and control. Where the human imagination fails, Nature surprises; such is the case of the Mermin-Wagner Theorem, for many years supporting the idea that a two-dimensional structure would be unstable and thus impossible to obtain, while the contrary was proven to be true by A. Geim and K. Novoselov in 2004 when they isolated successfully graphite monolayers by a method of mechanical exfoliation, hence, the first 2D material was found, the graphene. This one-atom-thick manufacture of carbon, uniquely, as well being hidden behind the scratching of a pencil-also discovered in England for over 400 years, combines extreme mechanical strength, exceptionally high electronic and thermal conductivities, as well as many other supreme properties, all of which make it highly attractive for numerous applications. In the present work - still debuting experiments – we investigate the mechanisms of adhesion of ad-atoms on the surface, alone or when forming clusters, preferably in regions of structural defects (of different kinds), their capture processes, adsorption and migration of atoms. The aim is to investigate electronic, magnetic or catalytic properties. Understanding how the adsorption could be controlled would contribute to the development of innovative devices based on graphene. Experimental works are accompanied by theory and computational models generally based on density functional theory and/or molecular dynamics calculations, providing an important support for studying the electronic properties. In this context, where theory was first denying that 2D materials could exist, now theory try to repent itself by a multitude of enthusiastic predictions, hoping for new discoveries of unique properties, being a major driver of nanotechnology for the development of devices such as quantum detectors of single molecules with applications in catalysis and nucleation of clusters in nanostructures, ballistic transistors and spintronic devices. Our experimental observables are the hyperfine parameters of add-atoms on graphene, measured with the nuclear spectroscopy PAC (Perturbed Angular Correlations) technique. PAC allows to probe at the atomic scale the add-atoms interactions without interfering with the graphene electronic structure, thereby providing unique information, which is impossible to obtain by electron spectroscopy and electron microscopy techniques such as, AFM or STM, not exempted from interactions between the tip and the surface test or ad-atoms therein. By PAC measurements it can be determined the electric field gradient (EFG) and magnetic hyperfine field (MHF) at atomic scale, electronic structure and magnetic environment of ad-atoms. The EFG provides structural information, location of the probe, stability, and bond (ionic, covalent bonding, van der Waals). The MHF translates properties correlated with the electronic spin configuration. We intend, in this brief presentation, to present the first results of the hyperfine parameters in graphene on different substrates, obtained by PAC measurements, using as probes the radioactive isotopes 111mCd, 111In and 199mHg. These are preliminary results of a large portfolio of experiments and ideas, with envisaged complementary studies of other experimental techniques the whole accompanied by the theoretical background based on ab initio simulations, aiming to achieve consistent models.