We report high optical enhancement in Ag/Au alloys and porous gold nanostructures using Surface Enhanced Raman Spectroscopy (SERS) technique. Scanning electron microscopy investigation shows the formation of Ag/Au alloys particles during irradiation of Ag-Au bilayer deposited on FTO (SnO:F) substrate by laser fluency equal to 0.5 J/cm or 1.0 J/cm with 12 ns laser pulse duration. The dealloying process of these Au-Ag alloy particles leads to the formation of Au nanoporous particles. The obtained nanostructures were studied with SERS and revealed a promising enhancement factor in porous Au nanostructure and tunability of localized surface plasmon resonance. The highly dense strong hot spots and large specific area in porous structure of gold nanostructures is the origin of the highly enhancement factor observed experimentally and theoretically. A very good agreement between simulation and experimental results was found confirming the potential of Au/Ag alloys and particularly porous gold nanostructure in SERS application.

This work reports on the effect of annealing temperature on the size, shape and wetting of particles obtained on 4H-SiC substrate by the dewetting process of a deposited nanoscale-thick Au film, with focus on the difference between solid-state dewetting (below Au melting temperature) and liquid-state dewetting (above Au melting temperature). After depositing nanoscale-thick Au film on the SiC substrate, annealings are perfomed so to induce the solid-state or liquid-state dewetting process of the film with the consequent formation of particles. Plan-view and cross-view scanning electron microscopy analyses are carried out to quantify the evolution of the average planar size and vertical size of the particles and of the average contact angle of the particles to the SiC surface versus the annealing temperature. These analyses allow us to extract quantitative information on the wetting behaviour of the particles on the SiC surface by calculating the adhesion work versus the annealing temperature. Energy dispersive x-ray analyses are, also, performed on the dewetted particles to analyze their composition in the various annealing conditions. Overall, we set a general framework connecting process parameters to the nano-shape of the dewetted particles towards specific shape design for selected applications.

We report on the effect of the surface roughness of Pd nanostructures on the electrical characteristics (Schottky barrier height) of Pd nanostructures/SiC nano-contacts as probed by conductive atomic force microscopy. We produced Pd nanostructures on 6H-SiC surface by thermal-induced dewetting of a nanoscale-thick deposited Pd film. We changed the Pd nanostructures mean diameter <D> and mean surface roughness <?> by controlling the annealing temperature. Scanning Electron Microscopy and Atomic Force Microscopy allowed us to quantify <D> and <?> in various annealing conditions. In addition, current-voltage characteristics were acquired on single Pd nanostructure/SiC contacts by placing the tip of Conductive Atomic Force Microscopy on the Pd nanostructure. Typical rectifying Schottky characteristics were recorded from which the Schottky barrier heights ? were extracted. For ballistic Pd nanostructures (<D>?25 nm), ? was found to increase by increasing the nanostructures surface roughness above a critical value. For non-ballistic Pd nanostructures (<D> in the range 150-200 nm), no effect of the surface roughness was observed on ?. Regarding the ballistic Pd/SiC contact, the observed phenomenon is ascribed to the conducting electrons surface scattering relaxation mechanism and the surface roughness scattering mean free path for electrons is evaluated in the range 1.5-3.5 nm.

Nowadays, gold nanoparticles Au nanoparticles (AuNPs) capture great interest due to their chemical stability, optical properties and biocompatibility. The success of technologies based on the use of AuNPs implies the development of simple synthesis methods allowing, also, the fine control over their properties (shape, sizes, structure). Here, we present the AuNPs fabrication by nanosecond pulsed laser ablation in citrate-solution, that has the advantage of being a simple, economic and eco-sustainable method to fabricate colloidal solutions of NPs. We characterized the stability and the absorbance of the solutions by Ultraviolet-Visible (UV-Vis) spectroscopy and the morphology of the AuNPs by Transmission Electron Microscopy. In addition, we used the AuNPs solutions as colorimetric sensor to detect the amount of glyphosate in liquid. Indeed, glyphosate is one of the most widely used herbicides which intensive use represents a risk to human health. The glyphosate presence in the colloidal AuNPs solutions determines the aggregation of the AuNPs causing the change in the color of the solution. The variation of the optical properties of the colloidal solutions versus the concentration of glyphosate is studied.

In recent years, the field of nanoporous metals has undergone accelerated developments as these materials possess high specific surface areas, well-defined pore sizes, functional sites, and a wide range of functional properties. Nanoporous gold (NPG) is, surely, the most attractive system in the class of nanoporous metals: it combines several desired characteristics as occurrence of surface plasmon resonances, enormous surface area, electrochemical activity, biocompatibility, in addition to feasibility in preparation. All these properties concur in the exploitatiton of NPG as an efficient and versatile sensong platform. In this regard, NPG-based sensors have shown exceptional sensitivity and selectivity to a wide range of analytes ranging from molecules to biomolecules (and until the single molecule detection) and the enormous surface/volume ratio was shown to be crucial in determining these performances. Thanks to these characteristics, NPG-based sensors are finding applications in medical, biological, and safety fields so as in medical diagnostics and monitoring processes. So, a rapidly growing literature is currently investigating the properties of NPG systems toward the detection of a multitude of classes of analytes highlighting strengths and limits. Due to the extension, complexity, and importance of this research field, in the present review we attempt, starting from the discussion of specific cases, to focus our attention on the basic properties of NPG in connection to the main sensing applications, i.e., surface enhanced Raman spectroscopy-based and electrochemical-based sensing. Owing to the nano-sized pore channels and Au ligaments, which are much smaller than the wavelength of visible light (400-700 nm), surface plasmon resonances of NPG can be effectively excited by visible light and presents unique features compared with other nanostructured metals, such as nanoparticles, nanorods, and nanowires. This characteristics leads to optical sensors exploiting NPG through unique surface plasmon resonance properties that can be monitored by UV-Vis, Raman, or fluorescence spectroscopy. On the other hand, the catalytic properties of NPG are exploited electrochemical sensors are on the electrical signal produced by a specific analyte adsorbed of the NPG surface. In this regard, the enourmous NPG surface area is crucial in determining the sensitivity enhancement. Due to the extension, complexity, and importance of the NPG-based sensing field, in the present review we attempt, starting from the discussion of specific cases, to focus our attention on the basic properties of NPG in connection to the main sensing applications, i.e., surface enhanced Raman spectroscopy-based and electrochemical-based sensing. Starting from the discussion of the basic morphological/structural characteristics of NPG as obtained during the fabrication step and post-fabrication processes, the review aims to a comprehensive schematization of the main classes of sensing applications highlighting the basic involved physico-chemical properties and mechanisms. In each discussed specific example, the main involved parameters and processes governing the sensing mechanism are elucidated. In this way, the review aims at establishing a general framework connecting the processes parameters to the characteristics (pore size, etc.) of the NPG. Some examples are discussed concerning surface plasmon enhanced Uv-Vis, Raman, fluorescence spectroscopy in order to realize efficient NPG-based optical sesnors: in this regard, the underlaying connections between NPG structural/morphological properties and the optical response and, hence, the optical-based sensing performances are described and analyzed. Some other examples are discussed concerning the exploitation of the electrochemical characteristics of NPG for ultra-high sensitivity detection of analytes: in this regard, the key parameters determing the NPG activity and selectivity selectivity toward a variety of reactants are discussed, as high surface-to-volume ratio and the low coordination of surface atoms. In addition to the use of standard NPG films and leafs as sensing platforms, also the role of hybrid NPG-based nanocomposites and of nanoporous Au nanostructures is discussed due to the additional increase of the electrocatalytic acticvity and of exposed surface area resulting in the possible further sensitivity increase.

There is a huge demand for rapid, reliable and low-cost methods for the analysis of heavy metals in drinking water, particularly in the range of sub-part per billion (ppb). In the present work, we describe the preparation, characterization and analytical performance of the disposable sensor to be employed in Square Wave Anodic Stripping Voltammetry (SWASV) for ultra-trace simultaneous determination of cadmium and lead. The electrode consists of graphene paper-perfluorosulfonic ionomer-bismuth nano-composite material. The electrode preparation implies a key step aimed to enhance the Bi adsorption into nafion film, prior to the bismuth electro-deposition. Finely dispersed bismuth nanoparticles embedded in the ionomer film are obtained. The electrode was characterized by Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX), Atomic Force Microscopy (AFM), X-ray Photoelectron Spectroscopy (XPS) and Electrochemical Impedance Spectroscopy (EIS). The electrode shows a linear response in the 5-100 ppb range, a time-stability tested up to almost three months, and detection limits up to 0.1 ppb for both Pb and Cd. The electrode preparation method is simple and low in cost and the obtained analytical performance is very competitive with the state of art for the SWASV determination of Pb and Cd in solution.

Nanostructured WO represents a promising material for electrochromic and sensing devices. In this scenario, electrodeposition is a promising low-cost approach for careful production. The electrodeposition of tungsten oxide film from a peroxo-tungstic-acid (PTA) solution is investigated. WO is synthetized onto Indium doped Tin Oxide (ITO) substrates, in a variety of shapes, from a fragmentary, thin layer up to a thick continuous film. Samples were investigated by scanning electron (SEM) and atomic force microscopy (AFM), Rutherford backscattering spectrometry (RBS), X-ray Diffraction analysis (XRD), energy gap measurement. Electrodeposition current curves are compared with characterization results to model the growth process. Early stages of electrodeposition are characterized by a transient cathodic current revealing an instantaneous nucleation followed by a diffusion limited process. A quantitative analysis of W deposition rate and current at working electrode validates a microscopic model for WO electrodeposition driving the process towards nanostructured versus continuous WO film.

We exploit the thermal-induced dewetting process of a deposited Pt film to produce two-dimensional arrays of Pt nanostructures on SiC surface. After depositing 7.5 nm-thick Pt film on SiC, we performed annealings increasing the temperature from 773 to 1073 K. Energy dispersive X-ray spectroscopy, atomic force microscopy, and scanning electron microscopy analyses were crossed to draw information on the chemical and morphological evolution of the Pt film increasing the annealing temperature. The gradual evolution of the Pt film, due to the ongoing dewetting process, from a holed film to a web of Pt filaments connecting Pt agglomerates and to, finally, isolated shaped nanostructures was observed. The Pt film surface roughness, fraction of uncovered surface area by the metal film, mean height and mean planar size of the dewetted nanostructures were quantified versus the annealing temperature to extract quantitative information on the main parameters characterizing the dewetting process and the nanostructures morphology.

Bimetallic Au/Pd nanoscale-thick films were sputter-deposited at room temperature on a silicon carbide (SiC) surface, and the surface-morphology evolution of the films versus thickness was studied with scanning electron microscopy. This study allowed to elucidate the Au/Pd growth mechanism by identifying characteristic growth regimes, and to quantify the characteristic parameters of the growth process. In particular, we observed that the Au/Pd film initially grew as three-dimensional clusters; then, increasing Au/Pd film thickness, film morphology evolved from isolated clusters to partially coalesced wormlike structures, followed by percolation morphology, and, finally, into a continuous rough film. The application of the interrupted coalescence model allowed us to evaluate a critical mean cluster diameter for partial coalescence, and the application of Vincent's model allowed us to quantify the critical Au/Pd coverage for percolation transition.

Composites fabricated by metal nanoparticles supported on graphene sheets are promising for many technological applications ranging from sensing, energy production and storage, catalysis to electronics and optoelectronics. This is due to the benefits offered by the properties of both components. In particular, charge transfer effects at graphene-nanoparticles interface are the basis for the operation of several devices. Here we present an approach to fabricate hybrid composites consisting of graphene-noble metal nanoparticles. Mono- and bimetallic platinum and palladium nanoparticles are produced by laser ablation in a liquid environment and their morphology is characterized by transmission electron microscopy. In addition, in order to fabricate low-dimensional hybrid composites, the nanoparticles are loaded on graphene layer by spin coating of colloidal solutions and the density of the nanoparticles on graphene was evaluated by scanning electron microscopy. The nanoparticles-graphene charge transfer was studied by employing Raman spectroscopy and conductive atomic force microscopy. In particular, the degree of the G and 2D Raman peak shifts (evidenced by the Raman Spectroscopy measurements) and the characteristics of the current-voltage curves (evidenced by conductive atomic force microscopy) allowed inferring an n-type doping effect of the metal nanoparticles on the graphene properties, i. e. electrons transfer from the nanoparticles to the graphene layers. A combined analysis of these results allow us to demonstrate that the chemical composition of the metal nanoparticles is a key parameter in determining the charge transfer phenomenon in low-dimensional systems.

Pd and Pt nanoparticles on Fluorine-doped tin oxide (FTO) are produced. This outcome is reached by processing nanoscale-thick Pd and Pt films deposited on the FTO surface by nanosecond laser pulse. Such laser processes are demonstrated to initiate a dewetting phenomenon in the deposited metal films and lead to the formation of the nanoparticles. In particular, the effect of the film's thickness on the mean size of the nanoparticles, when fixed the laser fluence, is studied. Our results indicate that the substrate topography influences the dewetting process of the metal films and, as a consequence, impacts on the nanoparticle characteristics. The results concerning the Pd and Pt nanoparticles' sizes versus starting films thickness and substrate topography are discussed. In particular, the presented discussion is based on the elucidation of the effect of the substrate topography effect on the dewetting process through the excess of chemical potential. Finally, Raman analysis on the fabricated samples are presented. They show, in particular for the case of the Pd nanoparticles on FTO, a pronounced Raman signal enhancement imputable to plasmonic effects.

The assembly behaviour of amphiphilic poly(styrene)-block-poly(t-butyl acrylate) block copolymer at air/water interface has been studied in view of its ability to form condensed phases. The film, transferred by Horizontal Precipitation Langmuir-Blodgett method without modifications, has been studied. Chemical imaging unravels the orientation of the condensed phases and, once defined their chemical orientation, the morphology of the block copolymer film and the pressure-induced growth processes have been investigated by Atomic Force Microscopy. Micelles and flat regions, 1 nm thick, were found. The micelles growth mechanism is interpreted as a coalescence process, allowing to predict the pressure-induced evolution of the chemical microphases.

The morphological evolution of surface supported Au nanorods, induced by thermal heating, has been examined. This study is focused on the establishment of the connection of the nanorods morphology to the annealing temperature, on the elucidation of the acting microscopic mechanisms and quantitative evaluation of the involved parameters leading to the morphology evolution. In particular, after depositing the nanorods on SiO surface, thermal processes were performed to induce their morphological evolution and we pointed out our attention on: a) the study of the morphological evolution of single isolated Au nanorods, which was identified as a reshaping process towards a spherical shape. The morphological analysis led us to establish the correlation between the annealing temperature and the nanorods aspect ratio and to elucidate the basic atomic driving mechanism for the reshaping process;b) the study of the morphological evolution of closely spaced nanorods (forming a film over the substrate surface). The analysis led us to the identification of the mechanisms governing the nanorods joining and evolution in larger architectures which, then, operate a reshaping process.On the basis of the results, we set a general framework for the design of complex morphology nanostructures on surfaces with desired shape and aspect ratio.

Non-enzymatic electrochemical glucose sensing was obtained by gold nanostructures on graphene paper, produced by laser or thermal dewetting of 1.6 and 8 nm-thick Au layers, respectively. Nanosecond laser annealing produces spherical nanoparticles (AuNPs) through the molten-phase dewetting of the gold layer and simultaneous exfoliation of the graphene paper. The resulting composite electrodes were characterized by X-ray photoelectron spectroscopy, cyclic voltammetry, scanning electron microscopy, micro Raman spectroscopy and Rutherford back-scattering spectrometry. Laser dewetted electrode presents graphene nanoplatelets covered by spherical AuNPs. The sizes of AuNPs are in the range of 10-150 nm. A chemical shift in the XPS Au4f core-level of 0.25-0.3 eV suggests the occurrence of AuNPs oxidation, which are characterized by high stability under the electrochemical test. Thermal dewetting leads to electrodes characterized by faceted not oxidized gold structures. Glucose was detected in alkali media at potential of 0.15-0.17 V vs. saturated calomel electrode (SCE), in the concentration range of 2.5 mu M-30 mM, exploiting the peak corresponding to the oxidation of two electrons. Sensitivity of 1240 mu A mM(-1) cm(-2), detection limit of 2.5 mu M and quantifications limit of 20 mu M were obtained with 8 nm gold equivalent thickness. The analytical performances are very promising and comparable to the actual state of art concerning gold based electrodes.

Mono-metallic and bi-metallic Palladium and Platinum nanoparticles find important applications for sensing detection, catalysis, hydrogen storage, fuel cell etc. However, the success of such technologies is subjected to the development of simple, versatile, low-cost, high-throughput methods for the production of the nanoparticles with desired sizes or composition. In particular, we present a laser-assisted synthesis method for the production of stable mono- and bi-metallic Pd and Pt nanoparticles. It is based on the nanosecond-pulsed laser ablation, in liquid environment, of pure Pd or Pt targets and of PtPd composite target at different ablation times. We characterized nanoparticles' morphology and crystalline structure by Transmission Electron Microscopy, Selected Area Electron Diffraction, X-Ray Diffraction and Energy Dispersive X-Ray measurements. In particular, the microscopic analysis showed that the average diameter of the nanoparticles is around 10-15 nm. The stability of the solutions was checked by UV-vis spectroscopy. Furthermore, by Scanning Transmission Electron Microscopy we found that, in bimetallic nanoparticles, Pt and Pd are homogeneously distributed on the whole volume of particle. In addition, we used the produced nanoparticles to decorate graphene layers by simple spin coating of the colloidal solutions onto the substrates, obtaining nanoparticles/Graphene nanocomposites with a "starry sky" type morphology. Also, trough Scanning Electron Microscopy images, the surface density of the nanoparticles on graphene was evaluated. We establish, therefore, a general working framework for the controlled nanofabrication of nanoparticles/graphene nanocomposites that could find interesting applications in catalysis and in electronic devices.

Metal nanostructures are, nowadays, extensively used in applications such as catalysis, electronics, sensing, optoelectronics and others. These applications require the possibility to design and fabricate metal nanostructures directly on functional substrates, with specifically controlled shapes, sizes, structures and reduced costs. A promising route towards the controlled fabrication of surface-supported metal nanostructures is the processing of substrate-deposited thin metal films by fast and ultrafast pulsed lasers. In fact, the processes occurring for laser-irradiated metal films (melting, ablation, deformation) can be exploited and controlled on the nanoscale to produce metal nanostructures with the desired shape, size, and surface order. The present paper aims to overview the results concerning the use of fast and ultrafast laser-based fabrication methodologies to obtain metal nanostructures on surfaces from the processing of deposited metal films. The paper aims to focus on the correlation between the process parameter, physical parameters and the morphological/structural properties of the obtained nanostructures. We begin with a review of the basic concepts on the laser-metal films interaction to clarify the main laser, metal film, and substrate parameters governing the metal film evolution under the laser irradiation. The review then aims to provide a comprehensive schematization of some notable classes of metal nanostructures which can be fabricated and establishes general frameworks connecting the processes parameters to the characteristics of the nanostructures. To simplify the discussion, the laser types under considerations are classified into three classes on the basis of the range of the pulse duration: nanosecond-, picosecond-, femtosecond-pulsed lasers. These lasers induce different structuring mechanisms for an irradiated metal film. By discussing these mechanisms, the basic formation processes of micro-and nano-structures is illustrated and justified. A short discussion on the notable applications for the produced metal nanostructures is carried out so as to outline the strengths of the laser-based fabrication processes. Finally, the review shows the innovative contributions that can be proposed in this research field by illustrating the challenges and perspectives.

Electrochemical non-enzymatic detections of glucose and fructose were based on gold nanoparticles (AuNPs) onto graphene paper. Electrodes based on AuNPs have been obtained inducing dewetting, by thermal (furnace) or by laser, of sputter deposited 8 nm-thick Au layer onto graphene paper. The electrodes were characterized by Scanning Electron Microscopy, Micro Raman Spectroscopy, X-ray Diffraction, Rutherford back-scattering Spectroscopy and Cyclic Voltammetry. The main difference exhibited by thermal and laser dewetting processes lies in the size and shape of the resulting gold nanoparticles. Laser dewetting originates smaller particles than that obtained by thermal dewetting. The particles are almost spherical and mainly localized onto graphene nanoplatelets. The size of AuNPs is in the ranges 10-150 nm. Electrodes obtained by thermal process present gold nanostructures characterized by faceted AuNPs. Typical sizes are in the range of 20-40 and 200-400 nm. The electrocatalytic activity toward glucose and fructose oxidation in alkaline phosphate buffer solution are presented and discussed. Glucose was detected at a potential of 0.17 V (laser dewetting) or 0.19 V (thermal dewetting) vs SCE, which corresponds to the intense peak of two electrons oxidation. Fructose was detected at potential of 0.4 V vs SCE. Sensitivity up to 1240 ?A mM cm for glucose detection was obtained. The resulting analytical performances for glucose and fructose detection are very promising since comparable to the actual state of art for nanostructured gold electrodes which are, however, produced by complex multi-steps wet processes and/or enzymes.

The surface roughness of nanoscale metal systems plays a key role in determining the systems properties and, therefore, the electrical, optical, etc. response of nanodevices based on them. In this work, we experimentally analyze the roughness evolution in dewetting Ag and Pt films deposited on SiO substrate. In particular, after depositing 15 nm-thick Ag or Pt films on the SiO substrate, standard annealing processes were performed below the melting temperatures of the metals so to induce the solid-state dewetting of the films. The surface morphology evolution of the Ag and Pt films was studied by means of Atomic Force Microscopy analysis as a function of the annealing temperature T and of the annealing time t. In particular, these analysis allowed to quantify the roughness ? of the Ag and Pt films versus the annealing temperature T and the annealing time t. The analysis of these plots allowed us to draw combined insights on the dewetting process characteristics, on the dewetting-induced roughening properties, and on the material-dependent parameters by the comparison of the results obtained for the Ag film and the Pt film. These analysis, in addition, open perspectives towards the development of a method to produce supported metal films with controlled surface roughness for designed applications.

Bimetallic PtPd nanoparticles are of special interest for their tunable properties in a wide range of applications as plasmonics, energy storage, catalysis. So, in this work we present a simple and versatile method for the production of bimetallic PtPd nanoparticles on a transparent and conductive substrate, such as fluorine-doped tin oxide/glass (FTO/glass) substrate. The method is based on the deposition of thin Pt/Pd bilayers on the FTO substrate. Then, we induced the melting, alloying, and dewetting process of the Pt and Pd layers by nanosecond laser irradiation with the consequent formation of the bimetallic PtPd nanoparticles. We characterized the nanoparticles by Scanning Electron Microscopy, Rutherford backscattering spectrometry, and X-Ray Diffraction measurements. In particular, the microscopic analysis showed that the average diameter of the nanoparticles is independent on the thickness of the deposited bilayers and on the layers sequence. On the other hand, the X-ray diffraction measurements confirmed that the structure of the nanoparticles consists in a PtPd alloy structure. The formation process of the nanoparticles is, finally, discussed on the basis of the general microscopic mechanisms involved in the laser-induced melting, alloying, and dewetting of the metallic films.

Deposited Au films and coatings are, nowadays, routinely used as active or passive elements in several innovative electronic, optoelectronic, sensing, and energy devices. In these devices, the physical properties of the Au films are strongly determined by the films nanoscale structure. In addition, in these devices, often, a layer of Ti is employed to promote adhesion and, so, influencing the nanoscale structure of the deposited Au film. In this work, we present experimental analysis on the nanoscale cross-section and surface morphology of Au films deposited on Ti. In particular, we sputter-deposited thick ( > 100 nm thickness) Au films on Ti foils and we used Scanning Electron Microscopy to analyze the films cross-sectional and surface morphology as a function of the Au film thickness and deposition angle. In addition, we analyzed the Au films surface morphology by Atomic Force Microscopy which allowed quantifying the films surface roughness versus the film thickness and deposition angle. The results establish a relation between the Au films cross-sectional and surface morphologies and surface roughness to the film thickness and deposition angle. These results allow setting a general working framework to obtain Au films on Ti with specific morphological and topographic properties for desired applications in which the Ti adhesion layer is needed for Au.