DMA Dynamic characterization of viscoelastic solids by AFM: The nano DMA mode

Quantitative measurement at nano scale of complex modulus of materials submitted to dynamic sollicitation through a surface tip interaction is a challenge. Experimental difficulties come from the relevance of measurement on the full frequency domain. Other phenomenoms must be also be taken in account like the adhesion during approach and unloading, non linear effects during measurement and multidimentional cantilever solicitation which happens during a conventional AFM approach. These experimental difficulties must be taken into account in order to link the material properties traditionaly obtained in dynamical mechanical analysis, meaning the storage and loss moduli, to the data generated by the nano scale instruments. In order to achieve these requirements, the AFM nanoDMA approach use different algorithms : dual channel demodulation, phase drift correction, reference frequency tracking, enabling a small strain measurement in the rheologically relevant 0,1 Hz to 20 kHz at a spatial resolution only an AFM can provide. This technique complements the peak force QNM mode ( quantitative nanomechanical characterization) which extract the necessary informations coming from systematic analysis of the approach retreat curves during each tip surface interaction during measurements. It’s then possible to create viscoelastic properties mapping ( loss, storage moduli, tan delta ) on a large material range in frequency and temperature with the unattainable resolution of the AFM technique. This viscoelastic analysis technique allows also to plot time temperature master curves and to measure activation energy using the arhenius law. Comparative measurements between bulk DMA and nanoDMA obtained in nanoindentation and in AFM are proposed on different materials as PDMS ( Polydimethylsiloxane ), FEP ( fluorinated ethylene propylene) or peek ( PolyEtherEtherKetone) which is a semi crystalline thermoplastic. It allows for example to analyse the material evolution on both sides of the Tg and to quantify the volume of material which has been submitted to an irreversible recrystallization process. We will also present the experimental procedure associate to these results

Controlled Porosity in Ferroelectric BaTiO₃ Photoanodes

The use of ferroelectric polarization to promote electron-hole separation has emerged as a promising strategy to improve photocatalytic activity. Although ferroelectric thin films with planar geometry have been largely studied, nanostructured and porous ferroelectric thin films have not been commonly used in photo-electrocatalysis. The inclusion of porosity in ferroelectric thin films would enhance the surface area and reactivity, leading to a potential improvement of the photoelectrochemical (PEC) performance. Herein, the preparation of porous barium titanate (pBTO) thin films by a soft template-assisted sol-gel method is reported, and the control of porosity using different organic/inorganic ratios is verified by the combination of scanning electron microscopy and ellipsometry techniques. Using piezoresponse force microscopy, the switching of ferroelectric domains in pBTO thin films is observed, confirming that the ferroelectric polarization is still retained in the porous structures. In addition, the presence of porosity in pBTO thin films leads to a clear improvement of the PEC response. By electrochemical poling, we also demonstrated the tuning of the PEC performance of pBTO thin films via ferroelectric polarization. Our work offers a simple and low-cost approach to control the morphology optimization of ferroelectric thin films, which could open up the development of materials with great potential for PEC applications

Torsional Force Microscopy of Van der Waals Moir\'es and Atomic Lattices

In a stack of atomically-thin Van der Waals layers, introducing interlayer twist creates a moir\'e superlattice whose period is a function of twist angle. Changes in that twist angle of even hundredths of a degree can dramatically transform the system's electronic properties. Setting a precise and uniform twist angle for a stack remains difficult, hence determining that twist angle and mapping its spatial variation is very important. Techniques have emerged to do this by imaging the moir\'e, but most of these require sophisticated infrastructure, time-consuming sample preparation beyond stack synthesis, or both. In this work, we show that Torsional Force Microscopy (TFM), a scanning probe technique sensitive to dynamic friction, can reveal surface and shallow subsurface structure of Van der Waals stacks on multiple length scales: the moir\'es formed between bilayers of graphene and between graphene and hexagonal boron nitride (hBN), and also the atomic crystal lattices of graphene and hBN. In TFM, torsional motion of an AFM cantilever is monitored as the it is actively driven at a torsional resonance while a feedback loop maintains contact at a set force with the surface of a sample. TFM works at room temperature in air, with no need for an electrical bias between the tip and the sample, making it applicable to a wide array of samples. It should enable determination of precise structural information including twist angles and strain in moir\'e superlattices and crystallographic orientation of VdW flakes to support predictable moir\'e heterostructure fabrication.Comment: 28 pages, 14 figures including supplementary material