María Vela - Academia.edu (original) (raw)

Papers by María Vela

Langmuir, 1998

The structure and dynamics of dodecanethiol and butanethiol monolayers adsorbed on Au(111) at 298... more The structure and dynamics of dodecanethiol and butanethiol monolayers adsorbed on Au(111) at 298 K have been followed by in situ and ex situ scanning tunneling microscopy (STM). Results show that a gaslike initial adsorption stage where monatomic deep pit clustering occurs is followed by an advanced adsorption stage in which adlayer structural changes p(n×1) w (x3×x3)R30°S c(4×2) take place. The first change is interpreted as the motion of adsorbed molecules from fcc to hcp sites, and the second one is explained by the fluctuations of adsorbed molecules from hollow to bridge sites. The structural fluctuations at adsorbate domains occur simultaneously with fluctuations in the size of monatomic deep pits. These processes reveal the complexity of surface dynamics of alkanethiol adlayers on Au(111) at 298 K.

Electrochimica Acta, 1998

The dynamics of 1-dodecanethiol self-assembled monolayers (SAMs) on Au(111) produced by contactin... more The dynamics of 1-dodecanethiol self-assembled monolayers (SAMs) on Au(111) produced by contacting pure thiol was followed by in situ STM at 298 K. Initially, these SAMs constituted a heterogeneous surface consisting of disordered and ordered adsorbate domains at terraces, step edges and pits, forming a p(6  1) superlattice which later changed to the (3 p  3 p)R308 structure. Subsequently, the c(4  2) superlattice was also observed. Surface dynamics involves the coalescence and Ostwald ripening phenomena at pits occurring simultaneously with adsorbate ordering at dierent surface domains. The dierence in surface mobility between adsorbate-free and 1dodecanethiol-covered Au(111) can be explained taking into account both gold lattice relaxation due to adsorption and alkyl chain interaction at SAM. Data analysis in terms of clustering theory allowed us to conclude that a single mechanism is likely involved in the overall surface phenomena in which the mass transport is either a nonsteady state surface diusion or an interface transfer along step edges as rate-determining step.

Journal of Physical Chemistry C, 2007

The Journal of Physical Chemistry B, 2006

Raman and surface-enhanced Raman scattering (SERS) spectroscopies, complemented with scanning tun... more Raman and surface-enhanced Raman scattering (SERS) spectroscopies, complemented with scanning tunnel microscopy and electrochemical techniques, have been used to obtain information about the amount and spatial distribution of methylene blue (MB) molecules immobilized on sulfur and four ultrathin molecular alkanethiolate films self-assembled on Au(111) and rough Au electrodes. The intensity of the Raman signals allow one to estimate the amount of immobilized MB at different organic films, whereas the decrease in the SERS intensity as a function of distance for the rough Au electrodes is used to locate the average position of the MB species with respect to the Au substrate. We found that significant amounts of cationic MB species are able to diffuse into methyl-terminated thiols, but they are stopped at the outer plane of the self-assembled monolayer (SAM) by negatively charged carboxylate groups. The relative shift of C-N stretching Raman modes indicates that the binding of MB to S is different from that found for MB on thiols. Most of the molecules immobilized on methyl-and carboxylate-terminated thiols are electrochemically inactive, suggesting that strong coupling between the Au electrode and the MB molecules is needed for charge transfer. Our results are consistent with a small population of electrochemically active MB species very close to the Au surface that reach this position driven by their lipophilic (hydrophobic) character through defects at SAMs.

The Journal of Physical Chemistry C, 2008

High coverage S phases (surface coverage g0.33), spontaneously formed by immersion of Au(111) in ... more High coverage S phases (surface coverage g0.33), spontaneously formed by immersion of Au(111) in Na 2 S aqueous solutions at room temperature, have been studied by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), surface enhanced Raman spectroscopy (SERS), electrochemistry, and density functional theory (DFT) calculations. XPS data show no evidence of a AuS phase, as no oxidized gold is detected. Voltammetric data are also inconsistent with the formation of a AuS phase with 0.5 stoichiometry. In situ and ex situ SERS measurements of S-covered nanostructured gold substrates demonstrate that the surface species present at the gold surface consist of a mixture of chemisorbed S and polysulfide species, as already proposed based on in situ STM images. A DFT surface model that is energetically feasible and reproduces well the experimental STM images is presented. The proposed model involves only a small rearrangement of the upper Au layer and coexistence of monomeric and polymeric S. Therefore, the high coverage S phase should be described as a mixture of monomeric and polymeric chemisorbed sulfur rather than as an extended 2D AuS phase.

The Journal of Physical Chemistry C, 2008

In situ Raman spectroscopy and electrochemical experiments are used to study different methylene ... more In situ Raman spectroscopy and electrochemical experiments are used to study different methylene blue (MB) species immobilized in molecular films (sulfur, propanethiol, dodecanethiol, mercaptopropionic acid, and mercaptoundecanoic acid monolayers), self-assembled on SERS-active Au surfaces in aqueous electrolyte under potential control. The presence of three different MB species along reduction-oxidation cycles is inferred, irrespective of the nature of the molecular film. Residual Raman intensity in the spectroscopically silent leuco methylene blue (LMB) potential range demonstrate a dissimilar degree of connectivity for electron transfer between MB molecules and substrate depending on the spacer. In addition, the intensity of the Raman signal depends on the monolayer chemistry where hydrophobic and/or electrostatic interactions tend to stabilize the MB species against desorption from the molecular film driven by concentration gradients.

The Journal of Physical Chemistry B, 2006

The surface structure of dodecanethiolate self-assembled monolayers (SAMs) on Au(111) surfaces, f... more The surface structure of dodecanethiolate self-assembled monolayers (SAMs) on Au(111) surfaces, formed from the liquid phase, have been studied by grazing incidence X-ray diffraction (GIXRD), scanning tunneling microscopy (STM), and electrochemical techniques. STM images show that the surface structure consists of (square root 3 x square root 3)-R30 degrees domains with only a few domains of the c(4 x 2) lattice. The best fitting of GIXRD data for the (square root 3 x square root 3)-R30 degrees lattice is obtained with alkanethiolate adsorption at the top sites, although good fittings are also obtained for the fcc and hcp hollow sites. On the basis of this observation, STM data, electrochemical measurements, and previously reported data, we propose a two-site model that implies the formation of incoherent domains of alkanethiolate molecules at top and fcc hollow sites. This model largely improves the fitting of the GIXRD data with respect to those observed for single adsorption sites and, also, for the other possible two-site combinations. The presence of alkanethiolate molecules adsorbed at the less favorable top sites could result from the adsorption pathway that involves an initial physisorption step which, for steric reasons, takes place at on top sites. Once the molecules are chemisorbed, the presence of energy barriers for alkanethiolate surface diffusion, arising mostly from chain-chain interactions, "freezes" some of them at the on top sites, hindering their movement toward fcc hollow sites. By considering the length of the hydrocarbon chain and the adsorption time, the two-site model could be a tool to explain most of the controversial results on this matter reported in the literature.

The Journal of Physical Chemistry B, 2000

The potential (E)-dependent transformations of adsorbed sulfur on Au(111) in 0.1 M NaOH + 3 × 10-... more The potential (E)-dependent transformations of adsorbed sulfur on Au(111) in 0.1 M NaOH + 3 × 10-3 M Na 2 S have been followed by in situ STM imaging. When E is changed from-0.6 to-0.8 V, the transformation from rectangular S 8 surface structures to a 3 × 3 R30°S lattice takes place. This process involves sulfur atom desorption, the formation of rectangular tetramerical surface structures, and the displacement of sulfur atoms to nearest hollow sites. When E moves from-0.8 to-1.0 V fast desorption of the 3 × 3 R 30°l attice from Au (111) terraces is observed while sulfur atoms become progressively bonded to step edges. Sulfur atom readsorption to form the 3 × 3 R 30°lattice takes place by returning to E)-0.8 V. Experimental data provide an estimation of the excess of binding energy close to step edges.

The Journal of Physical Chemistry B, 2002

The charge transfer of methylene blue molecules immobilized on different monolayer covered-Au(111... more The charge transfer of methylene blue molecules immobilized on different monolayer covered-Au(111) surfaces has been studied by electrochemical techniques complemented by scanning tunneling microscopy. The positively charged methylene blue ions (MB+) are effectively immobilized on negatively charged S and I adlayers. The amount of charge related to the (MB+/MBH) surface redox couple (qsurf) depends on the surface concentration of the adsorbed S and I atoms and on their geometric arrangement. When methylene blue immobilization is made on COO--terminated thiols (mercaptopropionic acid, MPA, and mercaptoundecanoic acid, MUA), the value of qsurf is markedly reduced, indicating that the electron transfer through hydrocarbon chains is very difficult, although the MB+ is effectively immobilized. Sulfide contamination introduces defects in the MPA and MUA layers and, thus, markedly increases the number of immobilized methylene blue molecules that are active for charge transfer. Electrochemical runs made with meth...

Physical Review Letters, 2003

Combining electrochemical methods, in situ scanning tunneling microscopy, and surface x-ray diffr... more Combining electrochemical methods, in situ scanning tunneling microscopy, and surface x-ray diffraction allowed study of the structure and kinetics of S=Au111 electrodes in aqueous electrolytes under potential control. Integrated intensities of a particular crystal truncation rod at anti-Bragg conditions were used to trace the sulfur adsorption and desorption as a function of electrode potential in real time. The S desorption is a first order process and the adsorption follows a Langmuir isotherm. A weakly bound S layer is found on the surface before charge transfer, and then specific adsorption occurs.

Physical Chemistry Chemical Physics, 2005

The surface structures, defects and dynamics of self-assembled monolayers (SAMs) on Au(111) are r... more The surface structures, defects and dynamics of self-assembled monolayers (SAMs) on Au(111) are reviewed. In the case of the well-known c(4 x 2) and radical 3 x radical 3 R30 degrees surface structures, the present discussion is centered on the determination of the adsorption sites. A more complex scenario emerges for the striped phases, where a variety of surface structures that depends on surface coverage are described. Recently reported surface structures at non-saturation coverage show the richness of the self-assembly process. The study of surface dynamics sheds light on the relative stability of some of these surface structures. Typical defects at the alkanethiol monolayer are shown and discussed in relation to SAMs applications.

Physical Chemistry Chemical Physics, 2009

The engineering of cavity void metallic arrays allows to vary the plasmon-polariton mode energies... more The engineering of cavity void metallic arrays allows to vary the plasmon-polariton mode energies from the near infrared to the ultraviolet through the tuning of the void height and diameter, and the selection of the appropriate material. Typically Au nanocavity substrates can be grown with better reproducibility, homogeneity, and stability, while Ag structures display significantly larger SERS enhancements. To exploit these two apparently excluding aspects, quality and enhancement, we report a detailed study of 500 nm Au-nanocavity templates modified by the controlled electrochemical deposition of 100 Ag layers, a thickness similar to the visible light skin-depth of bulk Ag. The SERS amplification of the ordered cavity-arrays is determined using 4-mercaptopyridine as a non-electronic resonant molecular probe. The ultrathin Ag layer modification of the Au substrates results in a strong amplification of the SERS signal both in the red and the green part of the spectrum, and in a spectral shift of the Raman resonance scans. These observations are assigned to Ag-induced changes in the plasmon-polariton response of the nanostructure. The reported results provide a general platform for the preparation of renewable SERS-active substrates that combine the durability and higher quality of Au nanotemplates, with the enhanced SERS amplification factors of Ag.

physica status solidi (c), 2009

ABSTRACT A monoenergetic positron beam was employed to characterize the uniformity and the nanost... more ABSTRACT A monoenergetic positron beam was employed to characterize the uniformity and the nanostructures of gold surfaces layers grown using an electrochemical treatment. The results obtained indicate that positrons are sensitive to the roughness process and their different levels of roughness. They are discussed in terms of the morphological changes in the columnar-like structure formed during the roughening process. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Nanoscale, 2012

All starting materials for the dye synthesis were purchased from commercial sources and were used... more All starting materials for the dye synthesis were purchased from commercial sources and were used as received. 1 H NMR (200 MHz) spectra have been measured in a Varian Bruker AC-200 spectrometer. Mass spectra were measured with a Bruker Daltonik microTOF mass spectrometer (ESI+, ESI-). Fluorescence measurements were performed on a Cary Eclipse fluorometer. Absorption spectra were measured on a Cary 50 spectrophotometer. Scheme S1

Langmuir, 2007

We have studied the self-assembly of thiol monolayers on high-area nanostructured gold surfaces. ... more We have studied the self-assembly of thiol monolayers on high-area nanostructured gold surfaces. These surfaces are highly irregular with a fractal dimension close to 2.5. Auger electron spectroscopy and voltammetric data indicate that thiol self-assembly with a maximum surface coverage ≈ 1 / 3 takes place, the same result as that found for smooth gold surfaces. Therefore, neither curvature effects, which would promote higher coverage, nor excluded volume effects, which would result in lower coverage, are present in these irregular surfaces. The high surface area of the bare electrodes exhibits a rapid surface decay in different liquid media that is hindered by alkanethiolate chemisorption. The presence of thiolate SAMs reduces markedly the mass transport surface diffusion of gold adatoms, hindering surface area decay and freezing the system in a metastable state for days. This effect cannot be explained by considering only hydrocarbon-hydrocarbon chain interactions, because it is also observed for ordered arrays of adsorbed S atoms. Therefore, interactions between ordered chemisorbed species at high coverage seem to be responsible for the observed behavior. The thiol-covered high-area metallic substrates can be used to efficiently anchor a large number of molecules, biomolecules, or nanostructures, improving the performance of SAM-based optical and electrochemical devices.

Langmuir, 2004

A comparative study of charge-transfer processes from/to methyl-terminated and carboxylate-termin... more A comparative study of charge-transfer processes from/to methyl-terminated and carboxylate-terminated thiolate-covered Au(111) surfaces to/from immobilized methylene blue (MB) molecules is presented. Scanning tunneling microscopy images with molecular resolution reveal the presence of molecular-sized defects, missing rows, and crystalline domains with different tilts that turn the thickness of the alkanethiolate SAM (the spacer) uncertain. The degree of surface heterogeneity at the SAMs increases as the number of C units (n) in the hydrocarbon chain decreases from n) 6. Defective regions act as preferred paths for MB incorporation into the methyl-terminated SAMs, driven by hydrophobic forces. The presence of negativecharged terminal groups at the SAMs reduces the number of molecules that can be incorporated, immobilizing them at the outer plane of the monolayer. Only MB molecules incorporated into the SAMs close to the Au(111) surface (at a distance < 0.5 nm) are electrochemically active. MB molecules trapped in different defects explain the broad shape and humps observed in the voltammogram of the redox couple. The heterogeneous charge-transfer rate constants for MB immobilized into methyl-terminated thiolate SAMs are higher than those estimated for carboxylate-terminated SAMs, suggesting a different orientation of the immobilized molecule in the thiolate environment.

Langmuir, 2001

The stripping of the underpotential deposited (3× 3)R30°sulfur lattice from Au(111) surfaces is s... more The stripping of the underpotential deposited (3× 3)R30°sulfur lattice from Au(111) surfaces is studied by electrochemical techniques complemented with in situ scanning tunneling microscopy. The kinetics of the stripping process fits a model where adsorption/desorption competes with nucleation and growth of two-dimensional holes under charge transfer control. We propose that two phases compete for the electrode surface: a condensed phase consisting of holes nucleated at defects of the adsorbed layer and a diluted phase consisting of randomly distributed vacancies produced by desorption. The relative contribution of each process depends on the applied potential and the step density of the substrate.

Langmuir, 2009

Degradation of thiolate self-assembled monolayers (SAMs) in ambient conditions and liquid environ... more Degradation of thiolate self-assembled monolayers (SAMs) in ambient conditions and liquid environments seriously limits the fabrication of thiol-based devices. Here, we demonstrate that nanostructured gold exhibits higher resistance to SAM degradation and increased electrochemical stability against thiolate desorption in relation to polycrystalline preferred oriented Au(111). The increased stability can be related to the presence of a large number of defects, such as adatoms, vacancies, and steps where the thiolate binding energy is stronger than at terraces. The nanostructured Au is an interesting platform because it can be easily prepared, has surface enhanced Raman spectroscopy (SERS) activity, and exhibits a high signal/noise ratio for amperometric detection because of its large real surface area.

Langmuir, 2001

The electroadsorption of S on Au(111) from 0.1 M NaOH + 3 × 10-3 M Na2S solutions has been studie... more The electroadsorption of S on Au(111) from 0.1 M NaOH + 3 × 10-3 M Na2S solutions has been studied by in situ scanning tunneling microscopy (STM), electrochemical methods, and ex situ X-ray photoemission spectrocopy (XPS). By analyzing STM images, we have observed that S adsorbs on Au(111) forming a 3× 3R30°superstructure. Under potential control this lattice slowly and continuously transforms into S octomers (S8) in the range-0.7/-0.5 V (i.e., at typical potentials observed under open circuit conditions). In this potential range, mixtures of both structures are present on the Au(111) surface. An XPS study of the S 2p peak from the adlayers reveals the presence of three components that can be assigned to S forming a 3× 3R30°structure, S8, and bulk S at surface defects. The most important component is that corresponding to S8, in good agreement with the STM images. Furthermore, XPS spectra recorded for 3× 3R30°thiol adlayers on Au(111), characterized by STM and atomic force microscopy, lead to similar S 2p XPS spectra. A comparison between these cases allows us to conclude that S in spontaneously formed S8 on Au(111) exhibits the same binding energy of the core electronic levels (i.e., same chemical state) as S in 3× 3R30°spontaneously formed thiol lattices, although the adsorption sites are different.

Langmuir, 1998

The structure and dynamics of dodecanethiol and butanethiol monolayers adsorbed on Au(111) at 298... more The structure and dynamics of dodecanethiol and butanethiol monolayers adsorbed on Au(111) at 298 K have been followed by in situ and ex situ scanning tunneling microscopy (STM). Results show that a gaslike initial adsorption stage where monatomic deep pit clustering occurs is followed by an advanced adsorption stage in which adlayer structural changes p(n×1) w (x3×x3)R30°S c(4×2) take place. The first change is interpreted as the motion of adsorbed molecules from fcc to hcp sites, and the second one is explained by the fluctuations of adsorbed molecules from hollow to bridge sites. The structural fluctuations at adsorbate domains occur simultaneously with fluctuations in the size of monatomic deep pits. These processes reveal the complexity of surface dynamics of alkanethiol adlayers on Au(111) at 298 K.

Electrochimica Acta, 1998

The dynamics of 1-dodecanethiol self-assembled monolayers (SAMs) on Au(111) produced by contactin... more The dynamics of 1-dodecanethiol self-assembled monolayers (SAMs) on Au(111) produced by contacting pure thiol was followed by in situ STM at 298 K. Initially, these SAMs constituted a heterogeneous surface consisting of disordered and ordered adsorbate domains at terraces, step edges and pits, forming a p(6  1) superlattice which later changed to the (3 p  3 p)R308 structure. Subsequently, the c(4  2) superlattice was also observed. Surface dynamics involves the coalescence and Ostwald ripening phenomena at pits occurring simultaneously with adsorbate ordering at dierent surface domains. The dierence in surface mobility between adsorbate-free and 1dodecanethiol-covered Au(111) can be explained taking into account both gold lattice relaxation due to adsorption and alkyl chain interaction at SAM. Data analysis in terms of clustering theory allowed us to conclude that a single mechanism is likely involved in the overall surface phenomena in which the mass transport is either a nonsteady state surface diusion or an interface transfer along step edges as rate-determining step.

Journal of Physical Chemistry C, 2007

The Journal of Physical Chemistry B, 2006

Raman and surface-enhanced Raman scattering (SERS) spectroscopies, complemented with scanning tun... more Raman and surface-enhanced Raman scattering (SERS) spectroscopies, complemented with scanning tunnel microscopy and electrochemical techniques, have been used to obtain information about the amount and spatial distribution of methylene blue (MB) molecules immobilized on sulfur and four ultrathin molecular alkanethiolate films self-assembled on Au(111) and rough Au electrodes. The intensity of the Raman signals allow one to estimate the amount of immobilized MB at different organic films, whereas the decrease in the SERS intensity as a function of distance for the rough Au electrodes is used to locate the average position of the MB species with respect to the Au substrate. We found that significant amounts of cationic MB species are able to diffuse into methyl-terminated thiols, but they are stopped at the outer plane of the self-assembled monolayer (SAM) by negatively charged carboxylate groups. The relative shift of C-N stretching Raman modes indicates that the binding of MB to S is different from that found for MB on thiols. Most of the molecules immobilized on methyl-and carboxylate-terminated thiols are electrochemically inactive, suggesting that strong coupling between the Au electrode and the MB molecules is needed for charge transfer. Our results are consistent with a small population of electrochemically active MB species very close to the Au surface that reach this position driven by their lipophilic (hydrophobic) character through defects at SAMs.

The Journal of Physical Chemistry C, 2008

High coverage S phases (surface coverage g0.33), spontaneously formed by immersion of Au(111) in ... more High coverage S phases (surface coverage g0.33), spontaneously formed by immersion of Au(111) in Na 2 S aqueous solutions at room temperature, have been studied by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), surface enhanced Raman spectroscopy (SERS), electrochemistry, and density functional theory (DFT) calculations. XPS data show no evidence of a AuS phase, as no oxidized gold is detected. Voltammetric data are also inconsistent with the formation of a AuS phase with 0.5 stoichiometry. In situ and ex situ SERS measurements of S-covered nanostructured gold substrates demonstrate that the surface species present at the gold surface consist of a mixture of chemisorbed S and polysulfide species, as already proposed based on in situ STM images. A DFT surface model that is energetically feasible and reproduces well the experimental STM images is presented. The proposed model involves only a small rearrangement of the upper Au layer and coexistence of monomeric and polymeric S. Therefore, the high coverage S phase should be described as a mixture of monomeric and polymeric chemisorbed sulfur rather than as an extended 2D AuS phase.

The Journal of Physical Chemistry C, 2008

In situ Raman spectroscopy and electrochemical experiments are used to study different methylene ... more In situ Raman spectroscopy and electrochemical experiments are used to study different methylene blue (MB) species immobilized in molecular films (sulfur, propanethiol, dodecanethiol, mercaptopropionic acid, and mercaptoundecanoic acid monolayers), self-assembled on SERS-active Au surfaces in aqueous electrolyte under potential control. The presence of three different MB species along reduction-oxidation cycles is inferred, irrespective of the nature of the molecular film. Residual Raman intensity in the spectroscopically silent leuco methylene blue (LMB) potential range demonstrate a dissimilar degree of connectivity for electron transfer between MB molecules and substrate depending on the spacer. In addition, the intensity of the Raman signal depends on the monolayer chemistry where hydrophobic and/or electrostatic interactions tend to stabilize the MB species against desorption from the molecular film driven by concentration gradients.

The Journal of Physical Chemistry B, 2006

The surface structure of dodecanethiolate self-assembled monolayers (SAMs) on Au(111) surfaces, f... more The surface structure of dodecanethiolate self-assembled monolayers (SAMs) on Au(111) surfaces, formed from the liquid phase, have been studied by grazing incidence X-ray diffraction (GIXRD), scanning tunneling microscopy (STM), and electrochemical techniques. STM images show that the surface structure consists of (square root 3 x square root 3)-R30 degrees domains with only a few domains of the c(4 x 2) lattice. The best fitting of GIXRD data for the (square root 3 x square root 3)-R30 degrees lattice is obtained with alkanethiolate adsorption at the top sites, although good fittings are also obtained for the fcc and hcp hollow sites. On the basis of this observation, STM data, electrochemical measurements, and previously reported data, we propose a two-site model that implies the formation of incoherent domains of alkanethiolate molecules at top and fcc hollow sites. This model largely improves the fitting of the GIXRD data with respect to those observed for single adsorption sites and, also, for the other possible two-site combinations. The presence of alkanethiolate molecules adsorbed at the less favorable top sites could result from the adsorption pathway that involves an initial physisorption step which, for steric reasons, takes place at on top sites. Once the molecules are chemisorbed, the presence of energy barriers for alkanethiolate surface diffusion, arising mostly from chain-chain interactions, &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;freezes&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot; some of them at the on top sites, hindering their movement toward fcc hollow sites. By considering the length of the hydrocarbon chain and the adsorption time, the two-site model could be a tool to explain most of the controversial results on this matter reported in the literature.

The Journal of Physical Chemistry B, 2000

The potential (E)-dependent transformations of adsorbed sulfur on Au(111) in 0.1 M NaOH + 3 × 10-... more The potential (E)-dependent transformations of adsorbed sulfur on Au(111) in 0.1 M NaOH + 3 × 10-3 M Na 2 S have been followed by in situ STM imaging. When E is changed from-0.6 to-0.8 V, the transformation from rectangular S 8 surface structures to a 3 × 3 R30°S lattice takes place. This process involves sulfur atom desorption, the formation of rectangular tetramerical surface structures, and the displacement of sulfur atoms to nearest hollow sites. When E moves from-0.8 to-1.0 V fast desorption of the 3 × 3 R 30°l attice from Au (111) terraces is observed while sulfur atoms become progressively bonded to step edges. Sulfur atom readsorption to form the 3 × 3 R 30°lattice takes place by returning to E)-0.8 V. Experimental data provide an estimation of the excess of binding energy close to step edges.

The Journal of Physical Chemistry B, 2002

The charge transfer of methylene blue molecules immobilized on different monolayer covered-Au(111... more The charge transfer of methylene blue molecules immobilized on different monolayer covered-Au(111) surfaces has been studied by electrochemical techniques complemented by scanning tunneling microscopy. The positively charged methylene blue ions (MB+) are effectively immobilized on negatively charged S and I adlayers. The amount of charge related to the (MB+/MBH) surface redox couple (qsurf) depends on the surface concentration of the adsorbed S and I atoms and on their geometric arrangement. When methylene blue immobilization is made on COO--terminated thiols (mercaptopropionic acid, MPA, and mercaptoundecanoic acid, MUA), the value of qsurf is markedly reduced, indicating that the electron transfer through hydrocarbon chains is very difficult, although the MB+ is effectively immobilized. Sulfide contamination introduces defects in the MPA and MUA layers and, thus, markedly increases the number of immobilized methylene blue molecules that are active for charge transfer. Electrochemical runs made with meth...

Physical Review Letters, 2003

Combining electrochemical methods, in situ scanning tunneling microscopy, and surface x-ray diffr... more Combining electrochemical methods, in situ scanning tunneling microscopy, and surface x-ray diffraction allowed study of the structure and kinetics of S=Au111 electrodes in aqueous electrolytes under potential control. Integrated intensities of a particular crystal truncation rod at anti-Bragg conditions were used to trace the sulfur adsorption and desorption as a function of electrode potential in real time. The S desorption is a first order process and the adsorption follows a Langmuir isotherm. A weakly bound S layer is found on the surface before charge transfer, and then specific adsorption occurs.

Physical Chemistry Chemical Physics, 2005

The surface structures, defects and dynamics of self-assembled monolayers (SAMs) on Au(111) are r... more The surface structures, defects and dynamics of self-assembled monolayers (SAMs) on Au(111) are reviewed. In the case of the well-known c(4 x 2) and radical 3 x radical 3 R30 degrees surface structures, the present discussion is centered on the determination of the adsorption sites. A more complex scenario emerges for the striped phases, where a variety of surface structures that depends on surface coverage are described. Recently reported surface structures at non-saturation coverage show the richness of the self-assembly process. The study of surface dynamics sheds light on the relative stability of some of these surface structures. Typical defects at the alkanethiol monolayer are shown and discussed in relation to SAMs applications.

Physical Chemistry Chemical Physics, 2009

The engineering of cavity void metallic arrays allows to vary the plasmon-polariton mode energies... more The engineering of cavity void metallic arrays allows to vary the plasmon-polariton mode energies from the near infrared to the ultraviolet through the tuning of the void height and diameter, and the selection of the appropriate material. Typically Au nanocavity substrates can be grown with better reproducibility, homogeneity, and stability, while Ag structures display significantly larger SERS enhancements. To exploit these two apparently excluding aspects, quality and enhancement, we report a detailed study of 500 nm Au-nanocavity templates modified by the controlled electrochemical deposition of 100 Ag layers, a thickness similar to the visible light skin-depth of bulk Ag. The SERS amplification of the ordered cavity-arrays is determined using 4-mercaptopyridine as a non-electronic resonant molecular probe. The ultrathin Ag layer modification of the Au substrates results in a strong amplification of the SERS signal both in the red and the green part of the spectrum, and in a spectral shift of the Raman resonance scans. These observations are assigned to Ag-induced changes in the plasmon-polariton response of the nanostructure. The reported results provide a general platform for the preparation of renewable SERS-active substrates that combine the durability and higher quality of Au nanotemplates, with the enhanced SERS amplification factors of Ag.

physica status solidi (c), 2009

ABSTRACT A monoenergetic positron beam was employed to characterize the uniformity and the nanost... more ABSTRACT A monoenergetic positron beam was employed to characterize the uniformity and the nanostructures of gold surfaces layers grown using an electrochemical treatment. The results obtained indicate that positrons are sensitive to the roughness process and their different levels of roughness. They are discussed in terms of the morphological changes in the columnar-like structure formed during the roughening process. (© 2009 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Nanoscale, 2012

All starting materials for the dye synthesis were purchased from commercial sources and were used... more All starting materials for the dye synthesis were purchased from commercial sources and were used as received. 1 H NMR (200 MHz) spectra have been measured in a Varian Bruker AC-200 spectrometer. Mass spectra were measured with a Bruker Daltonik microTOF mass spectrometer (ESI+, ESI-). Fluorescence measurements were performed on a Cary Eclipse fluorometer. Absorption spectra were measured on a Cary 50 spectrophotometer. Scheme S1

Langmuir, 2007

We have studied the self-assembly of thiol monolayers on high-area nanostructured gold surfaces. ... more We have studied the self-assembly of thiol monolayers on high-area nanostructured gold surfaces. These surfaces are highly irregular with a fractal dimension close to 2.5. Auger electron spectroscopy and voltammetric data indicate that thiol self-assembly with a maximum surface coverage ≈ 1 / 3 takes place, the same result as that found for smooth gold surfaces. Therefore, neither curvature effects, which would promote higher coverage, nor excluded volume effects, which would result in lower coverage, are present in these irregular surfaces. The high surface area of the bare electrodes exhibits a rapid surface decay in different liquid media that is hindered by alkanethiolate chemisorption. The presence of thiolate SAMs reduces markedly the mass transport surface diffusion of gold adatoms, hindering surface area decay and freezing the system in a metastable state for days. This effect cannot be explained by considering only hydrocarbon-hydrocarbon chain interactions, because it is also observed for ordered arrays of adsorbed S atoms. Therefore, interactions between ordered chemisorbed species at high coverage seem to be responsible for the observed behavior. The thiol-covered high-area metallic substrates can be used to efficiently anchor a large number of molecules, biomolecules, or nanostructures, improving the performance of SAM-based optical and electrochemical devices.

Langmuir, 2004

A comparative study of charge-transfer processes from/to methyl-terminated and carboxylate-termin... more A comparative study of charge-transfer processes from/to methyl-terminated and carboxylate-terminated thiolate-covered Au(111) surfaces to/from immobilized methylene blue (MB) molecules is presented. Scanning tunneling microscopy images with molecular resolution reveal the presence of molecular-sized defects, missing rows, and crystalline domains with different tilts that turn the thickness of the alkanethiolate SAM (the spacer) uncertain. The degree of surface heterogeneity at the SAMs increases as the number of C units (n) in the hydrocarbon chain decreases from n) 6. Defective regions act as preferred paths for MB incorporation into the methyl-terminated SAMs, driven by hydrophobic forces. The presence of negativecharged terminal groups at the SAMs reduces the number of molecules that can be incorporated, immobilizing them at the outer plane of the monolayer. Only MB molecules incorporated into the SAMs close to the Au(111) surface (at a distance < 0.5 nm) are electrochemically active. MB molecules trapped in different defects explain the broad shape and humps observed in the voltammogram of the redox couple. The heterogeneous charge-transfer rate constants for MB immobilized into methyl-terminated thiolate SAMs are higher than those estimated for carboxylate-terminated SAMs, suggesting a different orientation of the immobilized molecule in the thiolate environment.

Langmuir, 2001

The stripping of the underpotential deposited (3× 3)R30°sulfur lattice from Au(111) surfaces is s... more The stripping of the underpotential deposited (3× 3)R30°sulfur lattice from Au(111) surfaces is studied by electrochemical techniques complemented with in situ scanning tunneling microscopy. The kinetics of the stripping process fits a model where adsorption/desorption competes with nucleation and growth of two-dimensional holes under charge transfer control. We propose that two phases compete for the electrode surface: a condensed phase consisting of holes nucleated at defects of the adsorbed layer and a diluted phase consisting of randomly distributed vacancies produced by desorption. The relative contribution of each process depends on the applied potential and the step density of the substrate.

Langmuir, 2009

Degradation of thiolate self-assembled monolayers (SAMs) in ambient conditions and liquid environ... more Degradation of thiolate self-assembled monolayers (SAMs) in ambient conditions and liquid environments seriously limits the fabrication of thiol-based devices. Here, we demonstrate that nanostructured gold exhibits higher resistance to SAM degradation and increased electrochemical stability against thiolate desorption in relation to polycrystalline preferred oriented Au(111). The increased stability can be related to the presence of a large number of defects, such as adatoms, vacancies, and steps where the thiolate binding energy is stronger than at terraces. The nanostructured Au is an interesting platform because it can be easily prepared, has surface enhanced Raman spectroscopy (SERS) activity, and exhibits a high signal/noise ratio for amperometric detection because of its large real surface area.

Langmuir, 2001

The electroadsorption of S on Au(111) from 0.1 M NaOH + 3 × 10-3 M Na2S solutions has been studie... more The electroadsorption of S on Au(111) from 0.1 M NaOH + 3 × 10-3 M Na2S solutions has been studied by in situ scanning tunneling microscopy (STM), electrochemical methods, and ex situ X-ray photoemission spectrocopy (XPS). By analyzing STM images, we have observed that S adsorbs on Au(111) forming a 3× 3R30°superstructure. Under potential control this lattice slowly and continuously transforms into S octomers (S8) in the range-0.7/-0.5 V (i.e., at typical potentials observed under open circuit conditions). In this potential range, mixtures of both structures are present on the Au(111) surface. An XPS study of the S 2p peak from the adlayers reveals the presence of three components that can be assigned to S forming a 3× 3R30°structure, S8, and bulk S at surface defects. The most important component is that corresponding to S8, in good agreement with the STM images. Furthermore, XPS spectra recorded for 3× 3R30°thiol adlayers on Au(111), characterized by STM and atomic force microscopy, lead to similar S 2p XPS spectra. A comparison between these cases allows us to conclude that S in spontaneously formed S8 on Au(111) exhibits the same binding energy of the core electronic levels (i.e., same chemical state) as S in 3× 3R30°spontaneously formed thiol lattices, although the adsorption sites are different.