Application and characterization of ferrolectric materials: An atomic force microscopy approach

Abstract

Ferroelectric materials are an important class of functional materials that show unique properties such as polarization-electric field hysteresis and charge release under pressure and temperature change. These unique properties have been exploited in a wide variety of applications. In this thesis, I have explored the use of atomic force microscope to gain deeper understanding in both the characterisation and applications of functional materials. The strong depolarization field at the surface of ferroelectrics has long been used to grow nanoparticles. However, the influence of crystallographic orientation, crystal purity and poling procedure are not known to a satisfactory level. In this thesis, it is found that the crystallographic orientation as well as the direction and magnitude of the voltage used for domain switching strongly affects the amount and spatial distribution of silver photodeposition. While the effect of crystallographic orientation is understood with reference to band bending and electron tunnelling at the ferroelectric surface, the impact of scanning direction and poling voltage required analysis of surface potential dynamics of a ferroelectric Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT). Through understanding that the initial surface potential is a function of the applied bias, as it reflects the interplay between the polarisation and screen charges, it was deduced that tip induced charge injection during domain switching was responsible for the enhanced and reduced photodeposition at particular domain boundaries. Moreover, this knowledge was applied to create arbitrary spatial distribution of photodeposited metal. The photodeposited metal created in this manner demonstrates broadband enhancement of quantum dot (QD) photoluminescence due to confinement of the excitation field near the silver nanoparticle and coupling of the multiple plasmonic modes with the QD emission. Moreover. this technique promises exception spatial control only limited to an atomic force microscope tip geometry. Local characterisation of functional materials was also studied. In particular, the impact of synthesis procedure of poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) on morphology, polarization switching, and local piezoresponse hysteresis loops was evaluated. Ferroelectric switchable domains were identified by piezoresponse force microscopy (PFM) for all films. Small grains, with weak piezoresponse character, were observed for films annealed just above the Curie temperature. Acicular grains were obtained when the annealing temperature approached the melting point and the piezoresponse increased. Annealing above the melting point decreased the piezoresponse and the morphology changed dramatically into plate-like structures. Finally, the electromechanical properties of a novel organoruthenium complex trans-[Ru(CCC6H4-4-NO2)(CCPh)(dppe)2] [dppe = 1,2-bis(diphenylphosphino)ethane] was studied with atomic force microscopy. This crystal displayed a large electric-field-induced strain behaving differently from conventional piezoelectric materials that must, structurally, be noncentrosymmetric. Further studies of related systematically varied crystalline organoruthenium complexes reveal that the strong electromechanical coupling effect is not from classical ferroelectricity, electrostriction, flexoelectricity or electrochemical strain. It was, instead, attributed to the disorder in the molecular packing, which facilitates reorientation of the molecular dipoles under the action of an applied electric field. Show more