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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

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

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 Fe

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.

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.

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 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.

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 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

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.

Using atomic force microscopy and spot-profile analyzing low energy electron diffraction, we have observed the existence of a striking faceting instability in Al(110) homoepitaxy, characterized by the formation of nanocrystals with well-defined facets. These hut-shaped nanocrystals are over tenfold higher than the total film coverage, and coexist in a bimodal growth mode with much shallower and more populous surface mounds. We further use density functional theory calculations to elucidate the microscopic origin of the faceting instability, induced by surprisingly low activation barriers for adatom ascent at step edges and island corners.

Surface morphologies of nanostructured carbon films, produced by supersonic cluster beam deposition, have been investigated by atomic force microscopy. From topographical images, we evaluated the surface roughness corresponding to different scan lengths and thicknesses, and deduced that the images examined are self-affine. Experimental scaling exponents and the hypothesis of non-destructive primeval cluster aggregation suggest that quenched noise effects may be important in the process of growth of the nanostructured surfaces.

We present experimental results concerning the Friction Force Microscopy (FFM) investigation of smooth mica and Si surfaces lubricated by a model lubricant. We have studied the friction force as a function of normal load and sliding velocity, analyzing results in the framework of interfacial liquid structuring and drainage effects.

The first stages of aggregation of low energy deposited carbon clusters on Ag(1 1 0) surface have been observed in situ by scanning tunnelling microscopy (STM). Sub-monolayer coverages of the substrate at room temperature have been obtained by using a supersonic neutral cluster beam with a mass distribution centered at an average of 900 atoms/cluster. We observe that clusters are able to diffuse on the surface and along steps, and organise themselves in compact islands principally located at step edges. Size of supported islands ranges from a few nm up to 40 nm. A statistical analysis based on several STM images shows that the island size follows a lognormal distribution law. Possible mechanisms involved in the diffusion and coalescence of carbon clusters on metallic surfaces are discussed.

In recent years there has been a rapid increase in nanotechnology applications to medicine in order to prevent and treat diseases in the human body. The established and future applications have the potential to dramatically change medical science. The present paper will give a few examples that could transform common medical procedures.