Orientational imaging in polar polymers by piezoresponse force microscopy

Abstract

We report orientational imaging of the polarization distribution in nanostructured ferroelectric copolymer of polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) and collagen fibrils using vertical and lateral modes of piezoresponse force microscopy (PFM). In PVDF-TrFE, detection of azimuthal variations in the lateral PFM signal is attributed to the alignment of the molecular chains along different directions. Local switching in PVDF-TrFE is shown to proceed via 120 or 180 rotation of dipoles around the molecular chain, depending upon the strength of the applied electric field. Analysis of the vertical and lateral PFM signals in collagen reveals polar anisotropy of the electromechanical properties along the axes of the fibrils. The surface plots of the piezoelectric response are constructed for both materials based on their piezoelectric tensors and are shown to be consistent with the observed vertical and lateral PFM maps.

Effect of thermal annealing on ferroelectric domain structures in poly(vinylidene-fluoride-trifluorethylene) Langmuir-Blodgett thin films

“…Piezo-response force microscopy (PFM) has proven to be a powerful tool to probe the ferroelectric domain structures and polarization switching dynamics at the nanoscale in ferroelectric polymers. 7,[15][16][17][18][19][20][21] In this work, we report the PFM study of the effect of thermal annealing on ferroelectric domain structures in 6 to 20 monolayer (ML) polycrystalline ferroelectric PVDF-TrFE thin films. Stripe-shape domain structures were written at room temperature and subjected to thermal annealing at progressively higher temperatures up to the ferroelectric Curie temperature T C of approximately 110 C. The static configuration of the domain walls (DWs) does not exhibit appreciable changes in characteristics after thermal annealing, showing an temperature-independent roughness exponent f of 0.4-0.5, which is qualitatively different from what has been observed in ferroelectric oxides.…”

Domain wall roughness and creep in nanoscale crystalline ferroelectric polymers

“…We also observe a fractal dimensionality of 1.5, which is attributed to the correlation between the in-plane and out-of-plane polarization. As the DW width in these polymers is at the subnanometer scale, 27,28 we expect that the domains can be stabilized at sizes as small as 10 nm, 18 which promises device applications with a lateral density exceeding Tera-bit/in, 2 competitive with perovskite ferroelectrics, while the thickness scaling limit is the physical thickness of one monolayer. …”

“…[11][12][13][14][15] However, only few scanning probe studies have been carried out on polymeric ferroelectric thin films. [16][17][18][19] Questions such as the critical length and time scales for domain nucleation and propagation in crystalline polymer thin films as the system approaches lower dimensions, especially in the presence of disorder, remain to be answered.…”

“…1(b)), while the in-plane polarization in different grains varies in orientation. 18 To study the static configuration of the DWs, we wrote stripe domains by scanning the film while applying voltage higher than the coercive voltage (<1 V/ML) with alternating polarities to the conducting tip. Figures 1(d) and 1(e) show the phase and amplitude images of oppositely polarized stripe domains on a 10 ML film.…”

Two-Step Polarization Switching in Ferroelectric Polymers

“…A close look at the lamellar crystals indicates that they have an elliptical shape. Considering the in-plane x-ray diffraction patterns [27] and combined vertical and horizontal PFM observations [28], the arcs in the SAED pattern obtained from the nanostrips are indexed to both (200) and (110) with the f010g plane contacting the substrate. This indicates that the c axis of the P(VDF-TrFE) lamellae is parallel to the nanostripes and substrate, while the a axis is aligned preferentially perpendicular to the nanostripes.…”