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Colloidal probe Atomic Force Microscopy (AFM) allows to explore sliding friction phenomena in graphite contacts of nominal lateral size up to hundreds of nanometers. It is known that contact formation involves tribo-induced material transfer of graphite flakes from the graphitic substrate to the colloidal probe. In this context, sliding states with nearly-vanishing friction, i.e. superlubricity, may set in. A comprehensive investigation of the transfer layer properties is mandatory to ascertain the origin of superlubricity. Here we explore the friction response of micrometric beads, of different size and pristine surface roughness, sliding on graphite under ambient conditions. We show that such tribosystems undergo a robust transition towards a low-adhesion, low-friction state dominated by mechanical interactions at one dominant tribo-induced nanocontact. Friction force spectroscopy reveals that the nanocontact can be superlubric or dissipative, in fact undergoing a load-driven transition from dissipative stick-slip to continuous superlubric sliding. This behavior is excellently described by the thermally-activated, single-asperity Prandtl-Tomlinson model. Our results indicate that upon formation of the transfer layer, friction depends on the energy landscape experienced by the topographically-highest tribo-induced nanoasperity. Consistently we find larger dissipation when the tribo-induced nanoasperity is sled against surfaces with higher atomic corrugation than graphite, like MoS2 and WS2, in prototypical Van der Waals layered hetero-junctions., 35 pages, 6 figures, to be published in Langmuir

We develop a phase-space ab-initio formalism to compute Ballistic Electron Emission Spectroscopy current-voltage I(V)'s in a metal-semiconductor interface. We consider injection of electrons into the conduction band for direct bias ($V>0$) and injection of holes into the valence band or injection of secondary Auger electrons into the conduction band for reverse bias ($V, 35 pages, 11 figures. Appl. Surf. Sci (2022)

In strongly correlated electron materials, the electronic, spin, and charge degrees of freedom are closely intertwined. This often leads to the stabilization of emergent orders that are highly sensitive to external physical stimuli promising opportunities for technological applications. In perovskite ruthenates, this sensitivity manifests in dramatic changes of the physical properties with subtle structural details of the RuO

Colloidal probe Atomic Force Microscopy (AFM) allows to explore sliding states of vanishing friction, i.e. superlubricity, in mesoscopic graphite contacts. In this respect, superlubricity is known to appear upon formation of a triboinduced transfer layer, originated by material transfer of graphene flakes from the graphitic substrate to the colloidal probe. It was suggested that friction vanishes due to crystalline incommensurability at the sliding interface thus formed. However several details are missing, including the roles of tribolayer roughness and of loading conditions. Hereafter we gain deeper insight into the tribological response of micrometric silica beads sliding on graphite under ambient conditions. We show that the tribotransferred flakes increase interfacial roughness from tenths to several nanometers, in fact causing a breakdown of adhesion and friction by one order of magnitude. Furthermore, they behave as protruding asperities dissipating mechanical energy via atomic-scale stick-slip instabilities. Remarkably, such contact junctions can undergo a load-driven transition from continuous superlubric sliding to dissipative stick-slip, that agrees with the single-asperity Prandtl-Tomlinson model. Our results indicate that friction at mesoscopic silica-graphite junctions depends on the specific energy landscape experienced by the topographically-highest triboinduced nanoasperity. Superlubricity may arise from the load-controlled competition between interfacial crystalline incommensurability and contact pinning effects., 34 pages, 5 figures, to be published in Carbon

International audience; Macroscopic current-voltage measurements and nanoscopic ballistic electron emission spectroscopy (BEES) have been used to probe the Schottky barrier height (SBH) at metal/Ge(100) junctions for two metal electrodes (Au and Pt) and different metallization methods, specifically, thermal-vapor and laser-vapor deposition. Analysis of macroscopic current-voltage characteristics indicates that a SBH of 0.61-0.63 eV controls rectification at room temperature. On the other hand, BEES measured at 80 K reveals the coexistence of two distinct barriers at the nanoscale, taking values in the ranges 0.61-0.64 and 0.70-0.74 eV for the cases studied. For each metal-semiconductor junction, the macroscopic measurement agrees well with the lower barrier found with BEES. Ab initio modeling of BEES spectra ascribes the two barriers to two different atomic registries between the metals and the Ge(100) surface, a significant relevant insight for next-generation highly miniaturized Ge-based devices.

In strongly correlated electron materials, the electronic, spin, and charge degrees of freedom are closely intertwined. This often leads to the stabilization of emergent orders that are highly sensitive to external physical stimuli promising opportunities for technological applications. In perovskite ruthenates, this sensitivity manifests in dramatic changes of the physical properties with subtle structural details of the RuO$_6$ octahedra, stabilizing enigmatic correlated ground states, from a hotly debated superconducting state via electronic nematicity and metamagnetic quantum criticality to ferromagnetism. Here, it is demonstrated that the rotation of the RuO$_6$ octahedra in the surface layer of Sr$_2$RuO$_4$ generates new emergent orders not observed in the bulk material. Through atomic-scale spectroscopic characterization of the low-energy electronic states, four van Hove singularities are identified in the vicinity of the Fermi energy. The singularities can be directly linked to intertwined nematic and checkerboard charge order. Tuning of one of these van Hove singularities by magnetic field is demonstrated, suggesting that the surface layer undergoes a Lifshitz transition at a magnetic field of ~32T. The results establish the surface layer of Sr$_2$RuO$_4$ as an exciting 2D correlated electron system and highlight the opportunities for engineering the low-energy electronic states in these systems., Comment: 26 pages including supplementary material, replaced with published version

We report high-resolution surface morphology and friction force maps of polycrystalline organic thin films derived by deposition of the n-type perylene diimide semiconductor PDI8-CN2. We show that the in-plane molecular arrangement into ordered, cofacial slip-stacked rows results in a largely anisotropic surface structure, with a characteristic sawtooth corrugation of a few Ångstroms wavelength and height. Load-controlled experiments reveal different types of friction contrast between the alternating sloped and stepped regions, with transitions from atomic-scale dissipative stick-slip to smooth sliding with ultralow friction within the surface unit cell. Notably, such a rich phenomenology is captured under ambient conditions. We demonstrate that friction contrast is well reproduced by numerical simulations assuming a reduced corrugation of the tip-molecule potential nearby the step edges. We propose that the side alkyl chains pack into a compact low-surface-energy overlayer, and friction modulation reflects periodic heterogeneity of chains bending properties and subsurface anchoring to the perylene cores.

Ab initio nonequilibrium Keldysh formalism based on an $N$-order renormalization technique is used to compute $I(V)$ ballistic electron emission microscopy characteristics at the Au/Ge(001) interface. Such a formalism quantitatively reproduces precise experimental measurements under ultrahigh vacuum and low-temperature conditions. At $T=0$ K, the ballistic current follows the law ${(V\ensuremath{-}{V}_{\mathrm{SB}})}^{2.1}$, ${V}_{\mathrm{SB}}$ being the Schottky barrier. At $Tg0$ K, temperature effects become significant near the onset and must be taken into account to identify an accurate value for ${V}_{\mathrm{SB}}$ from a best-fit procedure. We find two values for ${V}_{\mathrm{SB}}$, 0.67 and 0.75 eV, which we associate with two different atomic registries at the interface.

We report a combined X-ray photoelectron spectroscopy and atomic force microscopy study of ultrafine (

We report high-resolution surface morphology and friction force maps of polycrystalline organic thin films derived by deposition of the n-type perylene diimide semiconductor PDI8-CN

We investigate with scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT) calculations the surface structures and the electronic properties of Fe

We describe a computational approach for the automatic recognition and classification of atomic species in scanning tunnelling microscopy images. The approach is based on a pipeline of image processing methods in which the classification step is performed by means of a Fuzzy Clustering algorithm. As a representative example, we use the computational tool to characterize the nanoscale phase separation in thin films of the Fe-chalcogenide superconductor FeSex Te1-x , starting from synthetic data sets and experimental topographies. We quantify the stoichiometry fluctuations on length scales from tens to a few nanometres.

Among the various families of Fe-based superconductors, iron chalcogenides, while presenting a transition temperature not particularly high, show great advantages for potential applications at high fields, albeit at liquid helium temperature. In fact, the critical temperature can be increased by stress up to 21 K in thin films, moreover stress can push the critical field up to more than 50 T, and the irreversibility field up to of 45 T. Also critical current densities higher than 1 MA/cm 2 can be reached in self field and at 4.2 K, with a very weak dependence on the magnetic field. Interestingly, the J C anisotropy depends on the mode of growth and substrate used. These high values of J C can be achieved without appreciable anisotropy. In this work, we present an overview of Fe(Se 0.5 Te 0.5 ) thin film deposition by pulsed laser ablation, and a study of their superconducting properties. STM and TEM techniques have been used to characterise structural and morphological properties and, thus, pinning centres. On different substrates the films grow with different types of defects, hence giving totally different J C anisotropy. Our preliminary measurements on a Josephson junction show that in Fe(Se,Te), the J C at the grain boundary is not severely depressed, which makes this material even more interesting from the practical point of view.

We investigate with scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT) calculations the surface structures and the electronic properties of Fe1+y Te thin films grown by pulsed laser deposition. Contrary to the regular arrangement of antiferromagnetic nanostripes previously reported on cleaved single-crystal samples, the surface of Fe1+y Te thin films displays a peculiar distribution of spatially inhomogeneous nanostripes. Both STM and DFT calculations show the bias-dependent nature of such features and support the interpretation of spin-polarized tunneling between the FeTe surface and an unintentionally magnetized tip. In addition, the spatial inhomogeneity is interpreted as a purely electronic effect related to changes in hybridization and Fe-Fe bond length driven by local variations in the concentration of excess interstitial Fe cations. Unexpectedly, the surface density of states measured by STS strongly evolves with temperature in close proximity to the antiferromagnetic-paramagnetic first-order transition, and reveals a large pseudogap of 180-250 meV at about 50-65 K. We believe that in this temperature range a phase transition takes place, and the system orders and locks into particular combinations of orbitals and spins because of the interplay between excess interstitial magnetic Fe and strongly correlated d-electrons.

The paper presents an analysis of the reliability of the method adopted in measuring techniques (particularly in AFM) for determining the fractal properties of rough surfaces. It is established that the parameter α (0 < α < 1) and, correspondingly, the Lipα class cannot be reliably defined using the finite part of the Fourier series and Lorentz theorem. Mandelbrot’s definition and the Lorentz theorem are shown to be insufficient as a theoretical basis for defining fractality experimentally. As a result of the mixing of the two concepts of nonidentity forming the basis of the modern understanding of fractality, there appear inaccuracies. The first concept implies that the fractal function should be perceived as a “broken line”, i.e., a nondifferentiable line. The second obligatory property of the fractal function is the geometrical self-similarity of the function at different scale levels. The examples cited in the paper indicate that these properties do not imply one another as they are different in their essence. A new method is proposed for identification of fractality in experimental research. It is based on checking the geometrical similarity of the additive components of the distribution function of asperity heights. This approach may serve as a basis for developing a new theory of the interactions of fractal rough surfaces.

The critical fluctuations at second order structural transitions in a bulk crystal may affect the dissipation of mechanical probes even if completely external to the crystal surface. Here, we show that noncontact force microscope dissipation bears clear evidence of the antiferrodistortive phase transition of SrTiO_{3}, known for a long time to exhibit a unique, extremely narrow neutron scattering "central peak." The noncontact geometry suggests a central peak linear response coupling connected with strain. The detailed temperature dependence reveals for the first time the intrinsic central peak width of order 80 kHz, 2 orders of magnitude below the established neutron upper bound.

We probe electron transport across the Au/organic interface based on oriented thin films of the high-performance n-type perylene diimide semiconductor PDI8-CN2. To this purpose, we prepared organic-on-inorganic Schottky diodes, with Au directly evaporated onto PDI8-CN2 grown on n-Si. Temperature-dependent current–voltage characteristics and complementary ballistic electron emission microscopy studies reveal that rectification at the Au/PDI8-CN2 interface is controlled by a spatially inhomogeneous injection barrier, that varies on a length scale of tens of nanometers according to a Gaussian distribution with mean value ∼0.94 eV and standard deviation ∼100 meV. The former gradually shifts to ∼1.04 eV on increasing PDI8-CN2 thickness from 5 nm to 50 nm. Experimental evidences and general arguments further allow to establish the energetics at the Au/PDI8-CN2 interface. Our work indicates injection-limited current flow in PDI8-CN2-based devices with evaporated Au electrodes. Furthermore, it suggests chemical reactivity of PDI8-CN2 with both Au and Si, driven by the lateral isocyano groups.

We report experimental results on the morphological characterization of WC coatings deposited by High-Velocity-Oxygen-Fuel (HVOF) thermal spraying technique. Optical microscopy has been used to characterize film microstructure from the millimeter down to the micrometer scale; surface roughness and correlation length have been estimated as a function of film thickness. We have observed that analysed specimens display a self-affine morphology with Hurst exponent H = (0.33 ± 0.05), in agreement with the scaling behaviour associated to the Kardar–Parisi–Zhang stochastic growth equation and to discrete ballistic deposition models. Our findings suggest that the introduction of scaling arguments within refined theoretical frameworks, realistically treating local splats interactions, could allow to directly predict the morphological evolution of coatings, partially overcoming the intrinsic complexity of the HVOF thermal spray process.

We investigated the contact mechanics and friction forces between atomic force microscope (AFM) probes and self-affine fractal carbon films. We studied single-asperity contacts by means of conventional nanometric conical tips whilst custom-designed micrometric flat tips were adopted to form multiple junctions between the probe and the sample. By varying the externally applied load we found that the average frictional force follows a power-law behavior in the single-asperity regime and a linear behavior in the multi-asperity regime. The friction coefficient was the same for carbon specimens having different fractality. We also acquired quasi-static load–displacement curves on micrometric scale, revealing a strong dependence of the average indentation depth on the values of fractal parameters. A comparison of experimental data with contact theories for randomly rough surfaces is provided. © 2003 Elsevier Science B.V. All rights reserved.

We present a novel experimental technique devoted to the investigation of contact mechanics on mesoscopic scale. It consists of an atomic force microscope (AFM) equipped with custom-designed probes with integrated flat micrometric tips. Samples are normally compressed by the flat tips and load–displacement curves are acquired. The latter allow to investigate the mechanical response under a multi-asperity regime not accessible by conventional AFM. Preliminary results are reported for the contact mechanics of nanostructured carbon-based films having a self-affine fractal morphology.

High-resolution microscopy techniques such as electron microscopy, scanning tunnelling microscopy and atomic force microscopy represent well-established, powerful tools for the structural characterization of adsorbed DNA molecules at the nanoscale. Notably, the analysis of DNA contours allows mapping intrinsic curvature and flexibility along the molecular backbone. This is particularly suited to address the impact of the base-pairs sequence on the local conformation of the strands and plays a pivotal role for investigations relating the inherent DNA shape and flexibility to other functional properties. Here, we introduce novel chain descriptors aimed to characterize the local intrinsic curvature and flexibility of adsorbed DNA molecules with unknown orientation. They consist of stochastic functions that couple the curvatures of two nanosized segments, symmetrically placed on the DNA contour. We show that the fine mapping of the ensemble-averaged functions along the molecular backbone generates characteristic patterns of variation that highlight all pairs of tracts with large intrinsic curvature or enhanced flexibility. We demonstrate the practical applicability of the method for DNA chains imaged by atomic force microscopy. Our approach paves the way for the label-free comparative analysis of duplexes, aimed to detect nanoscale conformational changes of physical or biological relevance in large sample numbers.

Accessing, controlling and understanding nanoscale friction and dissipation is a crucial issue in nanotechnology, where moving elements are central. Recently, ultra-sensitive noncontact pendulum Atomic Force Microscope (AFM) succeeded in detecting the electronic friction drop caused by the onset of superconductivity in Nb, raising hopes that a wider variety of mechanisms of mechanical dissipation arising from electron organization into different collective phenomena will become accessible through this unconventional surface probe. Among them, the driven phase dynamics of charge-density-waves (CDWs) represents an outstanding challenge as a source of dissipation. Here we report a striking multiplet of AFM dissipation peaks arising at nanometer distances above the surface of NbSe$_2$ - a layered compound exhibiting an incommensurate CDW. Each peak appears at a well defined tip-surface interaction force of the order of a nN, and persists until T=70K where CDW short-range order is known to disappear. A theoretical model is presented showing that the peaks are connected to tip-induced local 2$\pi$ CDW phase slips. Under the attractive potential of the approaching tip, the local CDW surface phase landscape deforms continuously until a series of 2$\pi$ jumps occur between different values of the local phase. As the tip oscillates to and fro, each slip gives rise to a hysteresis cycle, appearing at a selected distance, the dissipation corresponding to "pumping" in and out a local slip in the surface CDW phase of NbSe$_2$.

We report our results on the use of micromechanical structures, such as microcantilevers (MCs), entirely based on transition metal oxides (TMOs) for the development of sensors and devices with materials at the verge of different physical states. We will discuss details of the fabrication processes and show three applications that originate from the possibility of stretching, heating and set into oscillation free-standing crystalline films or heterostructures made with these functional materials.

We propose a method for the characterization of the local intrinsic curvature of adsorbed DNA molecules. It relies on a novel statistical chain descriptor, namely the ensemble averaged product of curvatures for two nanosized segments, symmetrically placed on the contour of atomic force microscopy imaged chains. We demonstrate by theoretical arguments and experimental investigation of representative samples that the fine mapping of the average product along the molecular backbone generates a characteristic pattern of variation that effectively highlights all pairs of DNA tracts with large intrinsic curvature. The centrosymmetric character of the chain descriptor enables targetting strands with unknown orientation. This overcomes a remarkable limitation of the current experimental strategies that estimate curvature maps solely from the trajectories of end-labeled molecules or palindromes. As a consequence our approach paves the way for a reliable, unbiased, label-free comparative analysis of bent duplexes, aimed to detect local conformational changes of physical or biological relevance in large sample numbers. Notably, such an assay is virtually inaccessible to the automated intrinsic curvature computation algorithms proposed so far. We foresee several challenging applications, including the validation of DNA adsorption and bending models by experiments and the discrimination of specimens for genetic screening purposes.

We report on the magnetic field and angular dependence of the critical current density of epitaxial FeTe0.5Se0.5 thin films. The films exhibit high critical current values and weak dependence on the applied magnetic field. The Jc is larger for field parallel to the c-axis, which is the opposite behavior of what expected from the critical field anisotropy. The analysis of the activation energy for vortex motion indicates that the single pinning regime holds up to 9 T, suggesting that correlated pinning centers are more effective than the vortex-vortex interaction even at the largest applied fields. Scanning tunneling microscope analysis indicates threading dislocations as possible pinning centers.

Nanopatterns on metal surfaces can be easily created by ion sputtering. However, due to the fast diffusion processes characterizing these materials, the nanostructures are often unstable at room temperature and above. This effect prevents the use of such patterned substrates in nanotechnology applications. In this letter, we present a simple oxidation process able to stabilize these features durably. The method has been tested on Cu, but its generality suggests that it can be applied to many other metals.

A quantitative surface reconstruction technique has been developed for the geometric characterization of three-dimensional structures by using a combined focused ion beam-scanning electron microscopy (FIB-SEM) instrument. A regular pattern of lines is milled at normal incidence on the sample to be characterized and an image is acquired at a large tilt angle. By analyzing the pattern under the tilted view, a quantitative estimation of surface heights is obtained. The technique has been applied to a test sample and nanoscale resolution has been achieved. The reported results are validated by a comparison with atomic force microscopy measurements.

One problem related to the actuation principle of macroscopic dielectric elastomer actuators is the high voltage required, typically in the Kilovolt range, that imposes particular care in the insulation of the whole actuator from the surrounding environment. This high actuation voltage, however, can be drastically reduced if a thin film of dielectric elastomer is used. Despite this, the manufacture of a macroscopic stack-like actuator, starting from thin films of dielectric elastomer can present many manufacture difficulties, like the handling and the assembly of the films, the power distribution to hundreds or thousands of layers, the presence of defects in one single layer that can cause the complete failure of the whole actuator. In this paper, a fast, semi-automatic process is proposed for the manufacture of modular units of dielectric elastomer, each of them consisting of many layers of rolled thin dielectric film. All the manufactured units are independent and take their power from a lateral, compliant supply rail that contacts the sides the electroded layers. This design is very suitable for industrial production: each module can be independently tested and then assembled in a complete macroscopic actuator composed by an unlimited number of these modules. The simple assembly methodology and the semi-automatic manufacture process described in this paper allows the fabrication of multilayer stacked devices, that can be used both as contractile or expanding actuators.

We report on contact mechanics experiments addressing the role of surface roughness on interfacial stiffness and adhesion. Colloidal atomic force microscopy probes, based on poly(dimethylsiloxane) microparticles, are pressed against ceramic substrates with different roughnesses; the applied load, deformation rate and dwell time being separately controlled. We observe a clear dependence of load–deformation curves and pull-off forces on roughness values, likely arising from morphological modulation of the contact area; remarkably this affects the contact stiffness, which is found to decrease for rougher junctions. The emergence of purely geometrical effects for poly(dimethylsiloxane) rough contacts extends previous findings on plastically deformed self-affine surfaces and demonstrates the efficient tuning of contact response through a proper design of surface morphology.

Films of the molecular nanomagnet Mn12ac with thicknesses spanning the nm to mm range have been deposited using a customised vacuum spray deposition technique. The films have been characterized by AFM, XPS, magnetic and magneto-optical measurements, indicating that the deposition procedure does not damage the Mn12ac molecules. As a result, the films show magnetic properties similar to those of the parent molecular material down to the nm thickness range, as revealed by magneto-optical methods. Vacuum spray deposition therefore represents a promising approach for the preparation under controlled vacuum conditions of ultrathin films of molecular nanomagnets supported on transparent dielectric substrates, making possible the magneto-optical readout of the magnetisation state of the Mn12ac film.

We report for the first time on muscovite mica surfaces nanostructured by a low-energy defocused Ar ion beam: ripple structures self-organize on macroscopic areas, with wavelength and roughness in the range 40-140 nm and 0.5-15 nm respectively, according to ions dose. In detail we address structural and chemical variations of the surface layer induced by sputtering. X-ray Photoelectron Spectroscopy (XPS) survey spectra reveal selective sputtering and Al surface enrichment whereas Atomic Force Microscopy (AFM) force-spectroscopy experiments indicate reduced charging of irradiated specimens under aqueous electrolyte solutions. Such experimental evidences contribute to clarify the chemical and physical properties of nanostructured mica samples, in view of their potential use as templates for aligned deposition of organic molecules and investigations on nanolubrication phenomena. (C) 2006 Elsevier B.V. All rights reserved.

Using a quartz crystal microbalance technique, we have measured the interfacial viscosity of nitrogen monolayers deposited on very homogeneous Pb(111) surfaces. At temperatures below 15?K, no dissipation is detected, suggesting that the N2 films are rigidly coupled to the oscillating electrode. By raising the temperature close to 20?K, we find sliding of the nitrogen film for coverages above about 0.5?nominal layers. The observed temperature dependence is in good qualitative agreement with recent calculations on the static friction of a nitrogen slab in contact with a crystalline Pb substrate.

Morphological information can be related to significant properties of solid bodies, like their friction, adhesion and wear. The primary aim of the present contribution is to provide evidences of the crucial role played by roughness in contact mechanics, based on Atomic Force Microscopy investigations at the nanoscale. We report experimental results concerning poly(dimethylsiloxane) colloidal probes indenting smooth substrates and discuss the dependence of load-penetration curves and pull-off forces on system details. We suggest their use to perform novel contact mechanics experiments on nanostructured rough surfaces.

We have studied the nanofriction of Ne monolayers with a quartz-crystal microbalance technique at temperatures below 6.5 K and in ultrahigh-vacuum conditions. Very homogeneous and smooth lead electrodes have been physically deposited on a quartz blank at 150 K and then annealed at room temperatures. With such a Pb-plated quartz-crystal microbalance, we have observed a pronounced depinning transition separating a low-coverage region, where the film is nearly locked to the oscillating electrode, from a high-coverage region characterized by slippage at the solid-fluid boundary. Such a behavior has been found to be very reproducible. These data are suggestive of a structural depinning of the solid Ne film when it becomes incommensurate with the lead substrate, in agreement with the results of an extensive molecular-dynamics study.

The most striking and impressive feature of AFM is probably related to its flexibility, with particular emphasis on detailed morphological, contact mechanics, friction and adhesion studies routinely performed under different environments. These capabilities have led the AFM to extend our fundamental knowledge on friction phenomena.

We investigated the plasmonic response of arrays of Al nanowires fabricated in high-vacuum and embedded within a transparent protective medium. The nanostructures exhibited a strongly-birefringent plasmonic response which, depending on the mutual orientation of the incident-field polarization and the nanowire axis, allowed the plasmon resonance to span the whole spectral range from the visible to the deep-ultraviolet regime. Comparing the experimental data with theoretical calculations allowed to rationalize the optical response in terms of non-ideal nanowire morphologies arising from the bottom-up character of the nanofabrication method. The broadband plasmonic response suggests the potential application of these systems in plasmon-enhanced photovoltaics, exploiting the great advantage of the low-cost of aluminium.

We report on the growth of {111}-faceted Co nanocrystals endowed with fcc structure during multilayer epitaxial deposition on a flat Cu(110) substrate. Lateral ordering of the Co nanocrystals into a periodic array is achieved if the Co deposition is performed on a Cu(110) substrate, prepatterned by ion sculpting. While for low coverages the Co film growth is conformal, reproducing the underlying arrangement of the Cu substrate pattern, for film thicknesses above 16 monolayers, size-selected {111}-faceted Co nanocrystals nucleate in registry with the underlying pattern periodicity.

Metal surfaces can be easily nanopatterned via ion sputtering: mounds or ripples can be created depending on the surface symmetry and temperature. However, in many cases these structures are unstable at room temperature and above, due to the adatom fast diffusion. This fact prevents the use of such systems as substrate or nanostamps for a technological implementation. In this paper we present a spot profile analysis low energy electron diffraction (SPA-LEED) study on the nanopatterning of a Rh(1 1 0) single crystal. Like the other (1 1 0) metal surfaces, previously investigated, also Rh(1 1 0) shows for increasing temperatures a transition between different rippled morphologies. The main advantage of this system is its stability at room temperature. From SPA-LEED data we can measure the structural features (average periodicity and local faceting) of the observed rippled structures.

We demonstrate that a combination of ion sputtering and soft lithography is an alternative and effective way of nanostructuring soft matter. We create self-organized nanoscale structures on a glass template by irradiating the surface with a defocused, low energy Ar ion beam. Capillary force lithography is then used to transfer the pattern, exploiting the glass transition of polymeric layers. In particular, we demonstrate the pattern transfer of a periodic 150 nm ripple structure onto an organic compound. This new, unconventional combination is then a low-cost strategy that opens the way to a variety of applications in the field of organic-based devices.

The nonstationary character of roughness is a widely recognized property of surface morphology and suggests modeling several solid surfaces by fractal geometry. In the field of contact mechanics, this demands novel investigations attempting to clarify the role of multiscale roughness during physical contact. Here we review the results we recently obtained in the characterization of the contact mechanics of fractal surfaces by depth-sensing indentation. One class of experiments was conducted on organic thin films, load-displacement curves being acquired by atomic force microscopy using custom-designed tips. Another class of experiments focused on well-defined crystalline and mechanically polished ceramic substrates probed by a traditional nanoindenter. We observed the first-loading cycle to be considerably affected by surface roughness. Plastic failure was found to dominate incipient contact while contact stiffness increased on decreasing fractal dimension and roughness. Our findings suggest fractal parameters to drive contact mechanics whenever the penetration depth is kept below the interface width.

We demonstrate that the mechanical properties of self-affine fractal thin films can be investigated on mesoscopic scale with an atomic force microscope. Sexithienyl films have been studied by acquiring load-displacement curves with flat micrometric tips. It is shown that the mechanical response of these samples strongly depends on their surface morphology, the contact stiffness varying an order of magnitude upon small but significative changes of fractal parameters. This indicates a general route to tailor films properties at the stage of their deposition and growth.

The non-stationary character of roughness is a widely recognized property of surface morphology and suggests modelling several solid surfaces by fractal geometry. In the field of contact mechanics this demands for novel investigations attempting to clarify the role of multi-scale roughness during physical contact. Here we propose an experimental investigation of the mechanical response of SrTiO3 substrates probed by a commercial nanoindentor. Load-displacement curves have been acquired respectively on well-defined crystalline surfaces and on mechanically lapped surfaces. We observe the first-loading cycles to be considerably affected by surface roughness whenever the penetration depth is kept below the interface width. The obtained results are analyzed within an elasto-plastic deformation model for fractal surfaces and a picture is developed to describe the deformation process with respect to surface roughness and structural parameters.Copyright © 2003 by ASME

Frictional properties of nanostructured carbon-based films, obtained by deposition of supersonic carbon clusters, have been investigated by friction force microscopy under ambient conditions. The experiment was performed at low loads to avoid plastic deformation and wear of materials. By analysing the load-dependent measurements acquired on samples with different composition, we deduced that the Hertzian-plus-offset model can take into account the frictional behaviour of these materials. A strong dependence of adhesive forces on the specific surface location was observed. A quantitative comparison among these films and atom-assembled carbon compounds is finally presented.

We report on the fabrication, mechanical characterization and electrostatic actuation of dielectric STO(0?0?1) thin film microcantilevers (MCs). Finite element analysis (FEA) is used for mechanical analysis and for calculating the distribution and the magnitude of the dielectric forces on the actual devices. The actuation of insulating oxide microstructures is of potential interest in the field of ferroelectric/multiferroic materials as well as for developing novel detecting schemes on dielectric oxides.

We used scanning tunnelling microscopy to study the morphology of superconducting FeSe0.5Te0.5 thin films epitaxially grown by pulsed laser deposition. Samples with critical temperature Tc above the bulk value (>16 K) show large atomic terraces, and a square lattice of periodicity 3.8 A associated with the Se/Te surface termination. Differences in the height coordinate of the chalcogenide atoms are clearly visible at the atomic level. On the contrary, samples with lower Tc (11 K) show hillocks generated by a spiral surface growth driven by threading dislocations of screw character. A comparative x-ray diffraction analysis reveals differences of compressive strain for the two classes of specimens. Variations in the deposition rate are found to affect film growth and inner strain, which ultimately tune Tc.

International audience; Thin films of head-tail (H-T) regio-regular poly[3-(4-octyloxyphenyl) thiophene] (POOPT) were grown using the MAPLE (Matrix Assisted Pulsed Laser Evaporation) technique in which the target is a frozen solution of the polymer in chloroform. Target evaporation was obtained by laser irradiation at 1064 nm and substrates were kept at different temperatures. Information concerning the preservation of the polymer local chemical structure following laser irradiation was obtained by FTIR (Fourier Transform InfraRed) spectroscopy. Based on FTIR data, the chemical structure of the deposited polymer seemed to undergo little or no damage. From UV-Visible spectroscopic analysis, it turned out that the degree of order of the film is strongly affected by the substrate temperature: the polymer was deposited in a disordered form on the substrate at room temperature whereas on the hot surface we locally obtained the -stacked structure characteristic of polythiophenes. Atomic force microscopy (AFM) images showed that the polymer formed aggregates of different dimensions (1 m to 5 m) with a columnar shape and showed micrometric domains with sharp and apparently regular edges for the film grown on a hot substrate. Electrical measurements performed by a standard two-probe technique confirm that the structural order degree strongly affects the film charge transport properties.