Dynamic Behavior in Piezoresponse Force Microscopy

Frequency dependent dynamic behavior in Piezoresponse Force Microscopy (PFM) implemented on a beam-deflection atomic force microscope (AFM) is analyzed using a combination of modeling and experimental measurements. The PFM signal comprises contributions from local electrostatic forces acting on the tip, distributed forces acting on the cantilever, and three components of the electromechanical response vector. These interactions result in the bending and torsion of the cantilever, detected as vertical and lateral PFM signals. The relative magnitudes of these contributions depend on geometric parameters of the system, the stiffness and frictional forces of tip-surface junction, and operation frequencies. The dynamic signal formation mechanism in PFM is analyzed and conditions for optimal PFM imaging are formulated. The experimental approach for probing cantilever dynamics using frequency-bias spectroscopy and deconvolution of electromechanical and electrostatic contrast is implemented.Comment: 65 pages, 15 figures, high quality version available upon reques

Physical properties of BFO thin film heterostructures under ansiotropic epitaxial strain

This thesis represents an effort to understand the structure of anisotropically strained Bismuth Ferrite (BiFeO3 BFO). This is executed by using anisotropic epitaxy and exploring the structure, magnetism, and electromechanical response in anisotropically strained BFO at various levels of average in-plane strain. This includes in the vicinity of the strain-induced morphotropic phase boundary where large enhancements to the electromechanical performance are identified. Bismuth ferrite (BFO) is a room-temperature magnetoelectric material that is able to easily adapt its crystal structure to accomodate any strain that is applied to it. By utilising high-index crystallographic substrates the effect anisotropic epitaxial strain has been explored using three different substrate materials (SrTiO3, (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT), and LaAlO3) each with four orientations. The unit cell parameters of the BFO films behave linearly when weakly compressively strained on SrTiO3, and become more non-linear on LSAT. The strain-driven morphotropic phase boundary in BFO films grown on tilted LaAlO3(310) surfaces is able to stabilise a low-symmetry bridging phase between the well known M_A and M_C symmetries of BFO when deposited on SrTiO3 and LaAlO3 respectively. The anisotropic strain conditions of the substrate miscut force the BFO film to maintain strain along a high-symmetry in-plane direction whilst partially relaxing in the orthogonal low-symmetry in-plane direction. Interferometric displacement sensor (IDS) measurements indicate that the intrinsic piezoresponse of this new phase of BFO is double that of the R'-like version. Moreover we see spectroscopic indications through IDS and band-excitation frequency response measurements that there is a field-induced phase transition occurring under electric field wherein the low-symmetry phase is reversibly interconverted into the tetragonal-like phase creating a giant effective electromechanical response. These observations are fully supported by density functional theory and effective Hamiltonian calculations. When growing thicker films of this soft low-symmetry phase, a rich and detailed phase coexistence between the R', T', and bridging phase arise that is reminiscent of a highly tilted mixed-phase BFO. The topography of these samples also exhibit domain-like periodic stripes that evolve with the crystallography and are intimately linked together. \\ At the end of this thesis a number of neutron scattering experiments are presented on BFO films on YAlO3, LaAlO3, LSAT, and SrTiO3 substrates. Despite calculations and some experimental hints of a C-type antiferromagnetic phase in T'-BFO, there appears to be no evidence of this magnetic phase in BFO//YAO and BFO//LAO. Additionally, a cycloid model has been developed and implemented in order to fit ambiguous cycloidal peaks with a constrained model. This model is applied to two different systems of BFO with the results and interpretations discussed

High Resolution Scanning Probes for Ferroelectric Thin Films

Advances in materials growth techniques enable precise control over the growth of novel functional materials such as ferroelectric thin films, which are interesting from both a physics and applications perspective. Physical properties of ferroelectric thin films differ a lot from their bulk counterparts, mainly due to the lattice mismatch at the film-substrate interface and differential thermal contraction experienced during growth. Those property anomalies are confined to a narrow range usually thinner than 1000 nm. High-resolution probes are important for understanding the spatial and temporal properties of these systems. We have developed mechanical and optical scanning probe techniques and used them to investigate various strain-engineered ferroelectric thin films. These optical and scanning probe techniques are designed to detect ferroelectric domain dynamics. Our experimental results either give direct evidence to verify material functionality, or reveal the relation between nano-scale dynamics to their macroscopic properties

Electromechanical response of saddle points in twisted hBN moir\'e superlattices

In twisted layered materials (t-LMs), an inter-layer rotation can break inversion symmetry and create an interfacial array of staggered out-of-plane polarization due to AB/BA stacking registries. This symmetry breaking can also trigger the formation of edge in-plane polarizations localized along the perimeter of AB/BA regions (i.e., saddle point domains). However, a comprehensive experimental investigation of these features is still lacking. Here, we use piezo force microscopy to probe the electromechanical behavior of twisted hexagonal boron nitride (t-hBN). For a parallel stacking alignment of t-hBN, we reveal very narrow (width ~ 20 nm) saddle point polarizations, which we also measure in the anti-parallel configuration. These localized polarizations can still be found on a multiply-stacked t-hBN structure, determining the formation of a double moir\'e. We also visualize a t-hBN moir\'e superlattice in the topography maps with atomic force microscopy, related to the strain accumulated at the saddle point domains. Our findings imply that polarizations in t-hBN do not only point in the out-of-plane direction, but also show an in-plane component, giving rise to a much more complex 3D polarization field

Piezoresponse in ferroelectric materials under uniform electric field of electrodes

The analytical solution for the displacements of an anisotropic piezoelectric material in the uniform electric field is presented for practical use in the “global excitation mode” of piezore-sponse force microscopy. The solution is given in the Wolfram Mathematica interactive program code, allowing the derivation of the expression of the piezoresponse both in cases of the anisotropic and isotropic elastic properties. The piezoresponse’s angular dependencies are analyzed using model lithium niobate and barium titanate single crystals as examples. The validity of the isotropic approximation is verified in comparison to the fully anisotropic solution. The approach developed in the paper is important for the quantitative measurements of the piezoelectric response in nano-materials as well as for the development of novel piezoelectric materials for the sensors/actuators applications. © 2021 by the author. Licensee MDPI, Basel, Switzerland.This research was funded by the Russian Science Foundation, grant number 19-72-10076

Nanoscale mechanical and electrical properties of low-dimensional structures

In this thesis, we mainly study the mechanical, electrical and electromechanical properties of low-dimensional structures of advanced materials, in particular two-dimensional (2D) materials and compound semiconductor (CS) structures and devices. Given the scarcity of methods for direct nano-mapping of physical properties of complex three-dimensional (3D) multilayer CS and 2D materials heterostructures, we adapted and developed suitable optical methods and functional scanning probe microscopies (SPM) approaches based in atomic force microscopy (AFM). These allowed us to successfully investigate the behaviour of one- and two dimensional (1D and 2D) free oscillating structures, such as AFM cantilevers, tuning forks (TF), Si3N4 membranes and graphene drums using the optical laser Doppler vibrometry (LDV) and dynamic AFM modes, finding governing relations of the dynamic behaviour in real-life systems and comparing these with modelling. In addition to the existing ultrasonic SPM, such as force modulation and ultrasonic force microscopy (FMM and UFM), we developed a new method called modulation ultrasonic force microscopy (M-UFM), which allows for nonlinear local excitation and the probing of membrane vibrations. Furthermore, we probe mechanical, electrical and thermal properties of supported layers and heterostructures of diverse transition metal dichalcogenides (TMDCs) and franckeite, understanding their intrinsic surface and subsurface nanostructure. In the final part of this thesis, we explored the feasibility of combining nano-sectioning via Beam Exit Cross-sectional Polishing (BEXP) and the material sensitive SPM analysis for the investigation of defects in CS structures, such as multiple quantum wells (MQW) and nanowires (NWs), and 2D material heterostructures. We applied this methodology to investigate the propagation of material defects, such as antiphase domains in CS, and their effects on the morphology, nanomechanics and electric properties in MQW structures, and to directly observe reverse piezoelectric domains inside individual GaN NWs