Marcos Flores

Size effects in thin gold films: Discrimination between electron-surface and electron-grain boundary scattering by measuring the Hall effect at 4 K

We report the Hall effect measured in gold films evaporated onto mica substrates, the samples having an average grain diameter D that ranges between 12 and 174 nm, and a thickness t of approximately 50 nm and 100 nm. The Hall mobility was determined at low temperatures T (4 K ≤ T ≤ 50 K). By tuning the grain size during sample preparation, we discriminate whether the dominant collision mechanism controlling the resistivity of the samples at 4 K is electron-surface or electron-grain boundary scattering, based upon whether the Hall mobility depends linearly on film thickness t or on grain diameter D.

Size effects on the Hall constant in thin gold films

We report the Hall constant RH, drift mobility μD, and Hall mobility μH measured at 4 K in thin gold films deposited on mica substrates, where the dominant electron scattering mechanism is electron-surface scattering. RH increases with increasing film thickness and decreases with increasing magnetic field. For high magnetic fields B≥6 T, RH turns out to be approximately independent of magnetic field, and its value is close to that of the free electron model. We use the high magnetic field values of RH to determine film thickness. This nondestructive method leads to a determination of film thickness that agrees to within 10% with the thickness measured by other techniques. The theoretical predictions, based upon the theory of Fuchs–Sondheimer and the theory of Calecki, are at variance with experimental observations.

Resistivity of thin gold films on mica induced by electron-surface scattering from a self-affine fractal surface

We present a rigorous comparison between resistivity data and theoretical predictions involving the theory of Palasantzas [G. Palasantzas and J. Barnas, Phys. Rev. B 56, 7726 (1997)], and the modified Sheng, Xing, and Wang-fractal theory [R. C. Munoz et al., Phys. Rev. B 66, 205401 (2002)], regarding the resistivity arising from electron scattering by a self-affine fractal surface on gold films using no adjustable parameters. We find that both theories lead to an approximate description of the temperature dependence of the resistivity data. However, the description of charge transport based upon fractal scaling seems oversimplified, and the predicted increase in resistivity arising from electron-surface scattering seems at variance with other experimental results. If the samples are made up of grains such that the mean grain diameter D > l0(300), the electronic mean free path in the bulk at 300 K, then the predicted increase in resistivity at 4 K is of the order of a few percent. This contradicts publishe...

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.

Molecular ion emission from alkanethiol-SAMs by HCI bombardment

In this work, we employ highly charged ions to study the sputtering of positive molecular fragments from two different alkanethiol self-assembled monolayers (SAMs) on gold surfaces: undecanethiol and dodecanethiol. The SAMs are bombarded with a pulsed Arq+ beam (3<q<12) with kinetic energies ranging from 1 to 15 keV. The desorbed positive molecular ions were detected and analyzed from time-of-flight spectra, and thereby the masses and yields of secondary ions were obtained. The proton yields are dependent on the charge state of the incident ion. On the other hand, the positive molecular ion yields, such as CnHm+, are charge state independent. The positive molecular ion yields decay with the molecular size n.

A top-down approach to produce protein-functionalized and highly thermally stable cellulose fibrils

Protein-functionalized cellulose fibrils, having various amounts of covalently bonded proteins at their surface, were successfully extracted from the tunic of Pyura chilensis tunicates using successive alkaline extractions. Pure cellulose fibrils were also obtained by further bleaching and were used as reference material. Extraction yields of protein-functionalized cellulose fibrils were within the range of 62-76% by weight based on the dry initial tunic powder. Fourier-transform infrared and Raman spectroscopy confirmed the preservation of residual protein at the surface of cellulose fibrils, which was then quantified by X-ray photoelectron spectroscopy. The protein-functionalized cellulose fibrils were found to have relatively high crystallinity and their cellulose I crystalline structure was preserved upon applying alkaline treatments. The extracted cellulosic materials were found to be constituted of fibrils having a ribbon-like morphology with widths ranging from ∼30 nm up to ∼400 nm. These protein-functionalized cellulose fibrils were found to have outstanding thermal stability with one of them having onset and peak degradation temperatures of ∼350 and 374 °C, respectively. These values were found to be 24 and 41 °C higher than for bleached cellulose.

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.

Magnetic properties of Co/Cu/Py antidot films with different pore diameters

The magnetic properties of ordered nanoscale Co/Cu/Py multilayer antidot films with different pore sizes prepared on top of nanoporous alumina membranes (NAMs) are presented. Using Co and Py films separated by a Cu thin layer, and tuning the pore diameters of the NAMs, we were able to play with the coercivity of the films and observe stepped magnetization curves, as a consequence of the different coercivities of the Co and Py films. The magnetic properties of the multilayer antidots have been measured and compared with results obtained for antidots of Cu/Py. The magnetization reversal process that occurs in each individual layer and in the multilayer was studied by means of micromagnetic simulations.