Extended model for the interaction of dielectric thin films with an electrostatic force microscope probe

To improve measurements of the dielectric permittivity of nanometric portions by means of Local Dielectric Spectroscopy (LDS), we introduce an extension to current analytical models for the interpretation of the interaction between the probe tip of an electrostatic force microscope (EFM) and a thin dielectric film covering a conducting substrate. Using the proposed models, we show how more accurate values for the dielectric constant can be obtained from single-frequency measurements at various probe/substrate distances, not limited to a few tip radii

Dynamics of poly(vinyl butyral) studied using dielectric spectroscopy and 1H NMR relaxometry.

Dielectric spectroscopy and NMR relaxometry unveiled the PVB segmental dynamics across the glass transition temperature

Electrostatic force microscopy and potentiometry of realistic nanostructured systems

We investigate the dependency of electrostatic interaction forces on applied potentials in Electrostatic Force Microscopy (EFM) as well as in related local potentiometry techniques like Kelvin Probe Microscopy (KPM). The approximated expression of electrostatic interaction between two conductors, usually employed in EFM and KPM, may loose its validity when probe-sample distance is not very small, as often realized when realistic nanostructured systems with complex topography are investigated. In such conditions, electrostatic interaction does not depend solely on the potential difference between probe and sample, but instead it may depend on the bias applied to each conductor. For instance, electrostatic force can change from repulsive to attractive for certain ranges of applied potentials and probe-sample distances, and this fact cannot be accounted for by approximated models. We propose a general capacitance model, even applicable to more than two conductors, considering values of potentials applied to each of the conductors to determine the resulting forces and force gradients, being able to account for the above phenomenon as well as to describe interactions at larger distances. Results from numerical simulations and experiments on metal stripe electrodes and semiconductor nanowires supporting such scenario in typical regimes of EFM investigations are presented, evidencing the importance of a more rigorous modelling for EFM data interpretation. Furthermore, physical meaning of Kelvin potential as used in KPM applications can also be clarified by means of the reported formalism.Comment: 20 pages, 7 figures, 1 tabl

Design, fabrication and characterization of composite piezoelectric ultrafine fibers for cochlear stimulation

Sensorineural hearing loss, primed by dysfunction or death of hair cells in the cochlea, is the main cause of severe or profound deafness. Piezoelectric materials work similarly to hair cells, namely, as mechano-electrical transducers. Polyvinylidene fluoride (PVDF) films have demonstrated potential to replace the hair cell function, but the obtained piezoresponse was insufficient to stimulate effectively the auditory neurons. In this study, we reported on piezoelectric nanocomposites based on ultrafine PVDF fibers and barium titanate nanoparticles (BTNPs), as a strategy to improve the PVDF performance for this application. BTNP/PVDF fiber meshes were produced via rotating-disk electrospinning, up to 20/80 weight composition. The BTNP/PVDF fibers showed diameters ranging in 0.160-1.325 μm. Increasing collector velocity to 3000 rpm improved fiber alignment. The piezoelectric β phase of PVDF was well expressed following fabrication and the piezoelectric coefficients increased according to the BTNP weight ratio. The BTNP/PVDF fibers were not cytotoxic towards cochlear epithelial cells. Neural-like cells adhered to the composite fibers and, upon mechanical stimulation, showed enhanced viability. Using BTNP filler for PVDF matrices, in the form of aligned ultrafine fibers, increased the piezoresponse of PVDF transducers and favored neural cell contact. Piezoelectric nanostructured composites might find application in next generation cochlear implants

Enhanced crystallization kinetics in poly(ethylene terephthalate) thin films evidenced by infrared spectroscopy

The cold crystallization process in poly(ethylene terephthalate) (PET) spin-coated ultrathin films was studied by infrared spectroscopy. The conformational change associated to the formation of crystal phase during annealing at 107 °C was measured in real time, by monitoring both intensity and frequency shift of trans and gauche conformer bands of the PET glycol segment. Enhancement of crystallization kinetics was observed in thin films deposited on amorphous silicon, with respect to a 20 μm thick free standing film used as reference, where the fastest kinetics was observed for the thinnest (35. nm) film. Experimental findings were interpreted in terms of scarce interaction between PET films and silicon substrate, which does not provide slowing down of crystallization kinetics as observed on different substrates. This results in a dominant effect of the polymer/air interface, where faster kinetics is observed, as also confirmed by atomic force microscopy imaging, particularly on the thinnest film. Additionally, Avrami and Avramov analyses evidence a decrease of both the Avrami exponent, related to growth dimensionality, and induction time, related to delay of nucleation start, when decreasing film thickness. Therefore, the reported results enrich the description of confinement and substrate interaction effects on the cold crystallization process taking place in PET ultrathin film

Local Piezoelectric Response of Polymer/Ceramic Nanocomposite Fibers

Effective converse piezoelectric coefficient (d33,eff) mapping of poly(vinylidene fluoride) (PVDF) nanofibers with ceramic BaTiO3 nanoparticle inclusions obtained by electrospinning was carried out by piezoresponse force microscopy (PFM) in a peculiar dynamic mode, namely constant-excitation frequency-modulation (CE-FM), particularly suitable for the analysis of compliant materials. Mapping of single nanocomposite fibers was carried out to demonstrate the ability of CE-FM-PFM to investigate the nanostructure of semicrystalline polymers well above their glass transition temperature, such as PVDF, by revealing the distribution of piezoelectric activity of the nanofiber, as well as of the embedded nanoparticles employed. A decreased piezoelectric activity at the nanoparticle site compared to the polymeric fiber was found. This evidence can be rationalized in terms of a tradeoff between the dielectric constants and piezoelectric coefficients of the component materials, as well as on the mutual orientation of polar axes

Irreversibly Adsorbed Layer in Supported Ultrathin Polymer Film Investigated by Local Dielectric Spectroscopy

Polymer chains can adsorb onto a solid substrate without the formation of chemical bonds. Because this mechanism of adsorption is driven by the weak dipolar interactions and requires simultaneous pinning of many repeating units of the chain, its kinetics can be extremely slow, especially for polymer melt . As a consequence, polymer chains at the interface with a substrate can reside for very long times in non-equilibrium states, before reaching the equilibrium configuration. Remarkably, recent works verified that the deviations from the bulk behavior in the dynamics of nanoconfined polymers are strongly affected by those non-equilibrium configurations assumed in adsorbed layers. In this Chapter, we report experimental evidences on the existence of an irreversibly adsorbed layer in poly(vinyl acetate) (PVAc) films in contact with different substrates. The presence of such a layer is proved through atomic force microscopy imaging of the residual layer remaining on the substrate after washing the polymer film in a good solvent. Moreover, we demonstrate that the evolution of the irreversibly adsorbed layer is unambiguously related to the change in relaxation dynamics of polymer films under annealing at a high temperature (~T g + 60 K). Finally, we demonstrate the direct effect of this adsorbed layer on the maximum moisture uptake of supported ultrathin PVAc films, hence providing a simple approach for controlling the moisture absorption of the nanosized polymer films

Electrostatic force microscopy and potentiometry of realistic nanostructured systems

We investigate the dependency of electrostatic interaction forces on applied potentials in electrostatic force microscopy (EFM) as well as in related local potentiometry techniques such as Kelvin probe microscopy (KPM). The approximated expression of electrostatic interaction between two conductors, usually employed in EFM and KPM, may loose its validity when probe-sample distance is not very small, as often realized when realistic nanostructured systems with complex topography are investigated. In such conditions, electrostatic interaction does not depend solely on the potential difference between probe and sample, but instead it may depend on the bias applied to each conductor. For instance, electrostatic force can change from repulsive to attractive for certain ranges of applied potentials and probe-sample distances, and this fact cannot be accounted for by approximated models. We propose a general capacitance model, even applicable to more than two conductors, considering values of potentials applied to each of the conductors to determine the resulting forces and force gradients, being able to account for the above phenomenon as well as to describe interactions at larger distances. Results from numerical simulations and experiments on metal stripe electrodes and semiconductor nanowires supporting such scenario in typical regimes of EFM investigations are presented, evidencing the importance of a more rigorous modeling for EFM data interpretation. Furthermore, physical meaning of Kelvin potential as used in KPM applications can also be clarified by means of the reported formalism

Effect of Confinement on Structural Relaxation in Ultrathin Polymer Films Investigated by Local Dielectric Spectroscopy

The effect of confinement on structural relaxation in ultrathin poly(vinyl acetate) films has been studied by local dielectric spectroscopy. This scanning probe method allows the investigation of dielectric relaxation at nanometer scale of supported ultrathin films having a free surface. Measurements have been performed at ambient pressure and controlled atmosphere on films with decreasing thickness. A deviation of dynamic properties from the bulk behavior, showing up as an increase of the relaxation rate, was observed starting from film thickness of 35 nm, which corresponds to about 3 times the gyration radius of polymer chains. A 2-fold increase of relaxation rate was measured for the thinnest investigated film of 18 nm. Local dielectric spectroscopy is therefore an effective method to elucidate confinement effects on relaxation dynamics in ultrathin polymer films with a free upper surface