Carlos Pimentel

Atomic-Scale Sliding Friction on Graphene in Water.

The sliding of a sharp nanotip on graphene completely immersed in water is investigated by molecular dynamics (MD) and atomic force microscopy. MD simulations predict that the atomic-scale stick-slip is almost identical to that found in ultrahigh vacuum. Furthermore, they show that water plays a purely stochastic role in sliding (solid-to-solid) friction. These observations are substantiated by friction measurements on graphene grown on Cu and Ni, where, oppositely of the operation in air, lattice resolution is readily achieved. Our results promote friction force microscopy in water as a robust alternative to ultra-high-vacuum measurements.

Sub-nanometer resolution of an organic semiconductor crystal surface using friction force microscopy in water

Organic semiconductors (OSC) are attracting much interest for (opto)electronic applications, such as photovoltaics, LEDs, sensors or solid state lasers. In particular, crystals formed by small π-conjugated molecules have shown to be suitable for constructing OSC devices. However, the (opto)electronic properties are complex since they depend strongly on both the mutual orientation of molecules as well as the perfection of bulk crystal surfaces. Hence, there is an urgent need to control nano-topographic OSC features in real space. Here we show that friction force microscopy in water is a very suitable technique to image the free surface morphology of an OSC single crystal (TDDCS) with sub-nanometer resolution. We demonstrate the power of the method by direct correlation to the structural information extracted from combined single crystal (SC-) and specular (s-) XRD studies, which allows us to identify the pinning centers encountered in the stick-slip motion of the probing tip with the topmost methyl groups on the TDDCS surface.

Estimation of interaction energy and contact stiffness in atomic-scale sliding on a model sodium chloride surface in ethanol

Friction force microscopy (FFM) in aqueous environments has recently proven to be a very effective method for lattice-resolution imaging of crystal surfaces. Here we demonstrate the use of ethanol for similar measurements on water-soluble materials. Lattice resolved frictional stick-slip traces of a cleaved NaCl(100) surface submerged in ethanol are compared with previous obtained FFM results in ultrahigh vacuum (UHV). We use the Prandtl-Tomlinson framework to estimate the amplitude of the corrugation potential and the contact stiffness. The surface potential amplitude scales with the applied normal loads are in good agreement with data obtained for NaCl measured under UHV conditions, but demonstrates deviations from the ideal periodic potential given by the Prandtl-Tomlinson model. An additional finding is that the use of ethanol allows us to explore higher load ranges without detectable evidence of surface wear. The contact stiffness does not vary significantly with the normal load up to 38 nN, while above it a sudden increase by almost one order of magnitude was observed. Comparing this to previous results suggests that considerable atom rearrangements may occur in the contact region, although the (100) surface structure is preserved by ethanol-assisted diffusion of Na and Cl ions.

First occurrence of the rare mineral slavikite in Spain

The magnesium and iron sulfate mineral slavikite has been found in Pastora Mine, Aliseda, Cáceres, Spain, in association with a number of other sulfate minerals such as alunogen, fibroferrite and tschermigite. Remarkably, slavikite is the major phase on some walls of Pastora Mine. To the best of our knowledge, this is the first reported occurrence of slavikite in Spain. Here, we present a mineralogical characterisation of the slavikite found in Pastora Mine using X-ray powder diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, electron probe micro-analysis and thermogravimetry. This characterisation also includes an analysis of the morphology of slavikite crystals based on the Bravais-Friedel-Donnay-Harker method, and the determination of the following slavikite formula from microprobe analyses and thermogravimetric data: Mg4Fe10(SO4)15(OH)8·13(H2O). Finally, we discuss the possible formation conditions of slavikite in Pastora Mine on the basis of available geological information and water analyses. We hypothesise that the presence of slavikite as major sulfate phase in some sites of Pastora Mine is partially the result of a process of mineral enrichment in which the moderate solubility of slavikite and wet and dry weathering cycles play a major role.ResumenEl mineral eslavikita, sulfato de hierro y magnesio, se ha encontrado en la Mina Pastora, Aliseda, Cáceres, España, asociado con otros sulfatos como alunógeno, fibroferrita y tschermigita. La eslavikita es la fase más abundante en algunas de las paredes de la Mina Pastora. Hasta donde tenemos conocimiento, ésta es la primera vez que se documenta la presencia de eslavikita en España. En este trabajo se presenta la caracterización mineralógica de la eslavikita encontrada en la Mina Pastora utilizando difracción de rayos X por método de polvo, microscopía electrónica de barrido, espectroscopia de rayos X por energía dispersiva, micro sonda electrónica y análisis termogravimétrico. Esta caracterización incluye también un análisis de la morfología de los cristales de eslavikita, utilizando el método Bravais-Fiedel-Donnay–Harker, y la determinación de la fórmula de la eslavikita a partir de los análisis químicos y termogravimétricos: Mg4Fe10(SO4)15(OH)8·13(H2O). Finalmente, se discuten las posibles condiciones de formación de la eslavikita en la Mina Pastora a partir de los datos geológicos disponibles y de los análisis de aguas locales. El hecho de que la eslavikita sea el sulfato más abundante en algunos lugares de la Mina Pastora indica la actuación de procesos de enriquecimiento mineral en los que la moderada solubilidad de la eslavikita y los ciclos de meteorización secos y húmedos juegan un papel importante.

Friction and Wear of Mineral Surfaces in Liquid Environments

Lateral Force Microscopy (LFM) is a very suitable technique to investigate the structure and reactivity of mineral surfaces in liquids. Studies performed in the last two decades have shown that the dissolution and growth of mineral surfaces immersed in water and aqueous solutions can be monitored by recording friction signals with LFM. Moreover, the sensitivity of lateral forces to both structure and chemistry makes possible to use LFM to obtain information about monolayers formed on mineral faces. Finally, numerous mineral surfaces are excellent substrates on which nanoparticles and complex organic molecules can be deposited and subsequently imaged and manipulated. This opens the way to future applications in molecular electronics. This chapter presents an overview of the recent use of LFM in liquid to investigate mineral surfaces and processes occurring on them.