Scanning probe microscopies for analytical studies at the nanometer scale

The scanning probe microscopies (SPM) have transformed the way of studying the structure and the properties of a wide variety of systems. Without doubt, they have exerted a pivotal role in many scientific disciplines like physics, chemistry, biology and engineering and have helped to give birth to novel fields such as the nanoscience and nanotechnology. This review attempts to highlight the versatility and high sensitivity of these techniques for capturing analytical information at the nanometer scale. In this context we will provide a survey of scanning probe evolution from the capabilities to image topography, atomic/molecular structure and in-situ dynamic processes to the mapping or local probing of physical and chemical properties. A selection of illustrative SPM studies is presented covering several areas of science.Les microscòpies locals de rastreig han transformat la manera d'estudiar l'estructura i les propietats d'una gran varietat de sistemes. Sens dubte, han tingut un paper essencial en moltes disciplines, com ara la física, la química, la biologia i l'enginyeria, i han contribuït al naixement de nous camps, com ara la nanociència i la nanotecnologia. El present article intenta destacar la versatilitat i l'alta sensibilitat d'aquestes tècniques per tal de capturar informació analítica a escala nanomètrica. En aquest context, s'intentarà examinar l'evolució d'aquestes tècniques nanoscòpiques des de la seva capacitat per a recollir informació topogràfica, estructura atomicomolecular i processos dinàmics in situ fins a determinar localment propietats físiques i químiques. Es presenta una selecció d'estudis il·lustratius basats en aquestes tècniques que abraça diverses àrees de la ciència

Electrophoretic origin of long-range repulsion of colloids near water/Nafion interfaces

The ICN2 is funded by the CERCA program/Generalitat de Catalunya.One of the most striking properties of Nafion is the formation of a long-range solute exclusion zone (EZ) in contact with water. The mechanism of formation of this EZ has been the subject of a controversial and long-standing debate. Previous studies by Schurr et al. and Florea et al. root the explanation of this phenomenon in the ion-exchange properties of Nafion, which generates ion diffusion and ion gradients that drive the repulsion of solutes by diffusiophoresis. Here we have evaluated separately the electrophoretic and chemiphoretic contributions to multi-ionic diffusiophoresis using differently charged colloidal tracers as solutes to identify better their contribution in the EZ formation. Our experimental results, which are also supported by numerical simulations, show that the electric field, built up due to the unequal diffusion coefficients of the exchanged ions, is the dominant parameter behind such interfacial phenomenon in the presence of alkali metal chlorides. The EZ formation depends on the interplay of the electric field with the zeta potential of the solute and can be additionally modulated by changing ion diffusion coefficients or adding salts. As a consequence, we show that not all solutes can be expelled from the Nafion interface and hence the EZ is not always formed. This study thus provides a more detailed description of the origin and dynamics of this phenomenon and opens the door to the rational use of this active interface for many potential applications

Sequential tasks performed by catalytic pumps for colloidal crystallization

Gold-platinum catalytic pumps immersed in a chemical fuel are used to manipulate silica colloids. The manipulation relies on the electric field and the fluid flow generated by the pump. Catalytic pumps perform various tasks, such as the repulsion of colloids, the attraction of colloids, and the guided crystallization of colloids. We demonstrate that catalytic pumps can execute these tasks sequentially over time. Switching from one task to the next is related to the local change of the proton concentration, which modifies the colloid zeta potential and consequently the electric force acting on the colloids

Synthesis of polydopamine at the femtoliter scale and confined fabrication of Ag nanoparticles on surfaces

Nanoscale polydopamine motives are fabricated on surfaces by deposition of precursor femtolitre droplets with an AFM tip and used as confined reactors to fabricate Ag nanoparticles patterns by in-situ reduction of an Ag+ salt

Key Parameters Controlling the Performance of Catalytic Motors

The development of autonomous micro/nanomotors driven by self-generated chemical gradients is a topic of high interest given their potential impact in medicine and environmental remediation. Although impressive functionalities of these devices have been demonstrated, a detailed understanding of the propulsion mechanism is still lacking. In this work, we perform a comprehensive numerical analysis of the key parameters governing the actuation of bimetallic catalytic micropumps. We show that the fluid motion is driven by self-generated electro-osmosis where the electric field originates by a proton current rather than by a lateral charge asymmetry inside the double layer. Hence, the surface potential and the electric field are the key parameters for setting the pumping strength and directionality. The proton flux that generates the electric field stems from the proton gradient induced by the electrochemical reactions taken place at the pump. Surprisingly the electric field and consequently the fluid flow are mainly controlled by the ionic strength and not by the conductivity of the solution, as one could have expected. We have also analyzed the influence of the chemical fuel concentration, electrochemical reaction rates, and size of the metallic structures for an optimized pump performance. Our findings cast light on the complex chemomechanical actuation of catalytic motors and provide important clues for the search, design, and optimization of novel catalytic actuators

Imaging the Proton Concentration and Mapping the Spatial Distribution of the Electric Field of Catalytic Micropumps

Catalytic engines can use hydrogen peroxide as a chemical fuel in order to drive motion at the microscale. The chemo-mechanical actuation is a complex mechanism based on the interrelation between catalytic reactions and electro-hydrodynamics phenomena. We studied catalytic micropumps using fluorescence confocal microscopy to image the concentration of protons in the liquid. In addition, we measured the motion of particles with different charges in order to map the spatial distributions of the electric field, the electrostatic potential and the fluid flow. The combination of these two techniques allows us to contrast the gradient of the concentration of protons against the spatial variation in the electric field. We present numerical simulations that reproduce the experimental results. Our work sheds light on the interrelation between the different processes at work in the chemomechanical actuation of catalytic pumps. Our experimental approach could be used to study other electrochemical systems with heterogeneous electrodes

Mechanical detection of carbon nanotube resonator vibrations

Bending-mode vibrations of carbon nanotube resonator devices were mechanically detected in air at atmospheric pressure by means of a novel scanning force microscopy method. The fundamental and higher order bending eigenmodes were imaged at up to 3.1GHz with sub-nanometer resolution in vibration amplitude. The resonance frequency and the eigenmode shape of multi-wall nanotubes are consistent with the elastic beam theory for a doubly clamped beam. For single-wall nanotubes, however, resonance frequencies are significantly shifted, which is attributed to fabrication generating, for example, slack. The effect of slack is studied by pulling down the tube with the tip, which drastically reduces the resonance frequency

Nanoelectrode Scanning Probes from Fluorocarbon-Coated Single-Walled Carbon Nanotubes

We have developed a method to coat single-walled carbon nanotubes attached to AFM tips with conformal fluorocarbon polymer films formed in an inductively coupled plasma reactor. The polymer provides a chemically inert and electrically insulating outer layer and mechanically stabilizes the attached nanotube sufficiently to enable imaging in liquids without the need for an intervening adhesive. Electrical pulse etching of the insulating coating exclusively at the nanotube tip end results in well-defined highly conductive nanoelectrodes. For these probes, the conductive properties of the nanotubes are not affected by the coating. Some nanoelectrodes behave as rectifying diodes, which may be developed into novel molecular devices integrated onto scanning probes