Valeria del Campo

The Many Faces of Graphene as Protection Barrier. Performance under Microbial Corrosion and Ni Allergy Conditions

In this work we present a study on the performance of CVD (chemical vapor deposition) graphene coatings grown and transferred on Ni as protection barriers under two scenarios that lead to unwanted metal ion release, microbial corrosion and allergy test conditions. These phenomena have a strong impact in different fields considering nickel (or its alloys) is one of the most widely used metals in industrial and consumer products. Microbial corrosion costs represent fractions of national gross product in different developed countries, whereas Ni allergy is one of the most prevalent allergic conditions in the western world, affecting around 10% of the population. We found that grown graphene coatings act as a protective membrane in biological environments that decreases microbial corrosion of Ni and reduces release of Ni2+ ions (source of Ni allergic contact hypersensitivity) when in contact with sweat. This performance seems not to be connected to the strong orbital hybridization that Ni and graphene interface present, indicating electron transfer might not be playing a main role in the robust response of this nanostructured system. The observed protection from biological environment can be understood in terms of graphene impermeability to transfer Ni2+ ions, which is enhanced for few layers of graphene grown on Ni. We expect our work will provide a new route for application of graphene as a protection coating for metals in biological environments, where current strategies have shown short-term efficiency and have raised health concerns.

Electrical Percolation and Aging of Gold Films

Electric transport in ultrathin metallic films can be either “percolative” or “conductive” depending on the links between the islands that constitute the film. Once the formation of long-range connections is established within the film, the overlayer reaches the so-called percolation threshold. This work describes a quantitative study of the electrical resistance of Au films, as a function of coverage. Film resistance displays a universal scaling law dependence with a critical exponent of 1.9 before percolation, which changes to 1.5 after percolation. These values are between the theoretical predictions for the evolution of growth as 2D or 3D systems. Results also indicate deposition parameters have a defining role in the evolution of the resistance during fabrication. A rise in pressure or deposition rate results in a lowering of the thicknesses at which percolation occurs. A decrease in the substrate temperature modified the typical resistance behavior of the Volmer–Weber growth mode to a trend of 2D growth mode. Finally, results describing the effect of film’s aging on the electrical resistance are presented. Aging is responsible for an important reduction in the film resistance after percolation, a process mainly mediated by material diffusion.

Unoccupied interface and molecular states in thiols and dithiols monolayers

The electronic structure of self-assembled monolayers (SAMs) formed by thiols of different lengths and dithiol molecules bound to Au(111) has been characterized. Inverse photoemission spectroscopy (IPES) and density functional theory have been used to describe the molecule/Au substrate system. All molecular layers display a clear signal in the IPES data at the edge of the lowest unoccupied system orbital (LUSO), roughly 3 eV above the Fermi level. There is also evidence, in both the experimental data and the calculation, of a finite density of states just below the LUSO edge, which has been recognized as localized at the Au-substrate interface. Regardless of the molecular lengths and in addition to this induced density of interface states, an apparent antibonding Au-S state has been identified in the IPES data for both molecular systems. The main difference between the electronic structures of thiol and dithiol SAMs is a shift in the energy of the antibonding state.