Clément Riedel

Determination of the nanoscale dielectric constant by means of a double pass method using electrostatic force microscopy

We present a method to determine the local dielectric permittivity of thin insulating layers. The measurement is based on the detection of force gradients in electric force microscopy by means of a double pass method. The proposed experimental protocol is simple to implement and does not need any modification of standard commercial devices. Numerical simulations based on the equivalent charge method make it possible to carry out quantification whatever the thickness of film, the radius of the tip, and the tip-sample distance. This method has been validated on a thin SiO2 sample for which the dielectric permittivity at the nanoscale has been characterized in the literature. We also show how we can quantitatively measure the local dielectric permittivity for ultrathin polymer film of poly(vinyl acetate) and polystyrene.

Imaging dielectric relaxation in nanostructured polymers by frequency modulation electrostatic force microscopy

We have developed a method for imaging the temperature-frequency dependence of the dynamics of nanostructured polymer films with spatial resolution. This method provides images with dielectric compositional contrast well decoupled from topography. Using frequency-modulation electrostatic-force-microscopy, we probe the local frequency-dependent (0.1–100 Hz) dielectric response through measurement of the amplitude and phase of the force gradient in response to an oscillating applied electric field. When the phase is imaged at fixed frequency, it reveals the spatial variation in dielectric losses, i.e., the spatial variation in molecular/dipolar dynamics, with 40 nm lateral resolution. This is demonstrated by using as a model system; a phase separated polystyrene/polyvinyl-acetate (PVAc) blend. We show that nanoscale dynamic domains of PVAc are clearly identifiable in phase images as those which light-up in a band of temperature, reflecting the variations in the molecular/dipolar dynamics approaching the glass transition temperature of PVAc.

Broadband nanodielectric spectroscopy by means of amplitude modulation electrostatic force microscopy (AM-EFM)

In this work we present a new AFM based approach to measure the local dielectric response of polymer films at the nanoscale by means of Amplitude Modulation Electrostatic Force Microscopy (AM-EFM). The proposed experimental method is based on the measurement of the tip-sample force via the detection of the second harmonic component of the photosensor signal by means of a lock-in amplifier. This approach allows reaching unprecedented broad frequency range (2-3 × 10(4)Hz) without restrictions on the sample environment. The method was tested on different poly(vinyl acetate) (PVAc) films at several temperatures. Simple analytical models for describing the electric tip-sample interaction semi-quantitatively account for the dependence of the measured local dielectric response on samples with different thicknesses and at several tip-sample distances.

Nanoscale dielectric properties of insulating thin films: From single point measurements to quantitative images

Dielectric relaxation (DR) has shown to be a very useful technique to study dielectric materials like polymers and other glass formers, giving valuable information about the molecular dynamics of the system at different length and time scales. However, the standard DR techniques have a fundamental limitation: they have no spatial resolution. This is of course not a problem when homogeneous and non-structured systems are analyzed but it becomes an important limitation for studying the local properties of heterogeneous and/or nano-structured materials. To overcome this constrain we have developed a novel approach that allows quantitatively measuring the local dielectric permittivity of thin films at the nanoscale by means of Electrostatic Force Microscopy. The proposed experimental method is based on the detection of the local electric force gradient at different values of the tip-sample distance. The value of the dielectric permittivity is then calculated by fitting the experimental points using the Equivalent Charge Method. Even more interesting, we show how this approach can be extended in order to obtain quantitative dielectric images of insulating thin films with an excellent lateral resolution.

On the use of electrostatic force microscopy as a quantitative subsurface characterization technique: A numerical study

We present a numerical study on the use of electrostatic force microscopy (EFM) as a non invasive subsurface characterization technique. We discuss the ability to resolve a buried object in a dielectric matrix considering two parameters: the detectability (i.e., signal superior to the noise) and the lateral resolution. The effects of the dielectric constant, thickness of the sample, and depth at which the object is buried are quantified. We show that the sensitivity reached in EFM permits to characterize subsurface objects in a dielectric matrix. We demonstrate that both lateral resolution and detectability decreases when the tip object distance increases. On the other hand, these two quantities increase with the dielectric constant of the matrix. A first step toward EFM tomography is proposed for objects creating non correlated signals.

Polymer Rheology by Dielectric Spectroscopy

The aim of this chapter is to discuss the main models describing the polymer dynamics at macroscopic scale and present some of the experimental techniques that permit to test these models. We will notably show how Broadband Dielectric Spectroscopy (BDS) permits to obtain rheological information about polymers, i.e. permits to understand how the matter flows and moves. We will focus our study the whole chain motion of cis-polyisoprene 1,4 (PI).