Design and implementation of high-bandwidth, high-resolution imaging in atomic force microscopy

Video-rate imaging with subnanometer resolution without compromising on the scan range has been a long-awaited goal in Atomic Force Microscopy (AFM). The past decade saw significant advances in hardware used in atomic force microscopes, which further enable the feasibility of high-speed Atomic Force Microscopy. Control design in AFMs plays a vital role in realizing the achievable limits of the device hardware. Almost all AFMs in use today use Proportional-Integral-Derivative(PID) control designs, which can be majorly improved upon for performance and robustness. We address the problem of AFM control design through a systems approach to design model-based control laws that can give major improvements in the performance and robustness of AFM imaging. First, we propose a cascaded control design approach to tapping mode imaging, which is the most common mode of AFM imaging. The proposed approach utilizes the vertical positioning sensor in addition to the cantilever deflection sensor in the feedback loop. The control design problem is broken down into that of an inner control loop and an outer control loop. We show that by appropriate control design, unwanted effects arising out of model uncertainties and nonlinearities of the vertical positioning system are eliminated. Experimental implementation of the proposed control design shows improved imaging quality at up to 30% higher speeds. Secondly, we address a fundamental limitation in tapping mode imaging by proposing a novel transform-based imaging mode to achieve an order of magnitude improvement in AFM imaging bandwidth. We introduce a real-time transform that effects a frequency shift of a given signal. We combine model-based reference generation along with the real-time transform. The proposed method is shown to have linear dynamical characteristics, making it conducive for model-based control designs, thus paving the way for achieving superior performance and robustness in imaging

Magnetoelectric Coupling in BaTiO3-BiFeO3 Multilayers: Growth Optimization and Characterization

The presented thesis explores the magnetoelectric (ME) coupling in multiferroic thin film multilayers of BaTiO3 (BTO) and BiFeO3 (BFO). Multiferroics possess more than one ferroic order parameter, in this case ferroelectricity and anti-ferromagnetism. Cross-coupling between these otherwise separate order parameters promises great advantages in the fields of multistate memory, spintronics and even medical applications. The first major challenge in this field of study is the rarity of multiferroics. Second, most known multiferroics, both intrinsic and extrinsic in nature, possess very low ME coupling coefficients. In previous studies conducted by our group, BTO-BFO multilayers deposited by pulsed laser deposition (PLD) showed a ME coupling coefficient αME enhanced by one order of magnitude, when compared to single-layers of the intrinsic multiferroic BFO. However, the mechanism of ME coupling in such heterostructures is poorly understood until now. In this thesis, we used a selection of structural, chemical, electrical and magnetic measurements to maximize the αME-coefficient and shed light on the origin of this enhanced ME effect. The comparison of BTO-BFO multilayers over single-layers revealed not only enhanced ME-coupling, but also reduced mosaicity, roughness and leakage current density in multilayers. Following a parametric sample optimization, we achieved an atomically smooth interface roughness and vast improvements in the ferroelectric properties by introducing a shadow mask in the PLD process. We measured the highest αME-value so far of 480 Vcm-1Oe-1 for a multilayer with a double-layer thickness of only 4.6 nm, two orders of magnitude larger than the coefficient of 4 Vcm-1Oe-1 measured for BFO single-layers. The αME-coefficient in these multilayers stands in an inverse correlation with the double-layer thickness ddl. The influence of oxygen pressure during growth and BTO-BFO ratio on αME was shown to be neglible in comparison to that of ddl. From the characteristic dependencies of αME on magnetic bias field, temperature and ddl, we concluded the existence of an interface-driven coupling mechanism in BTO-BFO multilayers.:1 Introduction 2 Theory of Multiferroic Magnetoelectrics 2.1 Primary Ferroic Properties 2.2 Magnetoelectric Coupling 3 Materials 3.1 The General Structure of Perovskites ABX3 3.2 Strontium Titanate SrTiO3 3.3 Barium Titanate BaTiO3 3.4 Bismuth Ferrite BiFeO3 3.5 Heterostructures Based on BiFeO3 4 Experimental Section 4.1 Thin Film Fabrication 4.2 X–Ray Diffraction 4.3 Microscopic Techniques 4.4 Chemical Analysis Techniques 4.5 Ferroelectric Characterization 4.6 Magnetic Property Measurements 4.7 Measurement of the Magnetoelectric Coupling Coefficient 5 BaTiO3–BiFeO3 Heterostructures 5.1 General Properties of Single-Layers and Multilayers of BTO and BFO 5.2 PLD–Growth of BaTiO3–BiFeO3 Multilayers 5.3 Manipulation of Multilayer Properties through Design 5.4 Effectiveness of Eclipse–PLD 5.5 Enhanced ME Effect in BaTiO3–BiFeO3 Multilayers 6 Summary and Outlook A Magnetoelectric Measurement Setup B Magnetic Background Measurements C Polarized Neutron Reflectometry Literature Own and Contributed Work Acknowledgement Erratu

Quantum transport in encapsulated graphene "p-n" junctions

Two dimensional electron gases (2DEGs) have been an exceptional platform and a constant source of new discoveries in quantum physics during the last decades. While for a long time 2DEGs fabricated by molecular beam epitaxy have been the working-horse of quantum transport measurements, with the discovery of graphene in 2004 a new, truly two-dimensional material entered the field. Within the few years since the experimental discovery of graphene it has risen from relative obscurity to the status of an exciting and promising model for 2D solids. The great interest in graphene can be attributed to its exceptional band structure which is described at low energies by the massless Dirac Hamiltonian, where the valence- and conduction-band touch each other at a single point (Dirac point). Being a zero-gap semi-conductor separates graphene from conventional metals and semi-conductors, making it unique of its kind. The ability to combine graphene with various 2D materials in so called Van der Waals heterostructures allows to taylor its properties almost at will. The nearly defect-free grapene lattice holds the potential for ballistic transport over long distances. Futhermore, the refraction index across an n-n’ (unipolar) junction or p-n (bipolar) junction can be tuned seamlessly from positive to negative which is unique for graphene. Combining the ballistic transport with the tunability of the refraction index across an interface makes clean graphene an excellent platform for the investigation of various electron optical experiments. This Thesis focuses on quantum transport phenomena in two-terminal graphene p-n junction, as this combines two bench-mark signatures in graphene, namely the observation of massless Dirac-fermions and the ability to establish gapless p-n junction. The Thesis starts with chapter 2 where important concepts related to the unique electronic band structure of graphene are introduced. This includes the ability to establish gapless p-n junctions, approaches how to characterize clean graphene, the possibility to form superlattices with other layered materials such as hexagonal boron-nitride (hBN) or the possibility to address additional degrees of freedom such as the valley-isospin. In chapter 3 a short comparison between suspension and encapsulation of graphene is given, since these two techniques are the most common ones to fabricate ultra-clean graphene. However, the fabricational details in chapter 4 are restricted to the encapsulation. Furthermore, details on how to fabricate local top- and bottom-gates, which are needed to establish p-n junctions, are given. The currently most common method to establish electrical contact with hBN/graphene/hBN heterostructures is via so called side-contacts. In chapter 5 an alternative approach is introduced to establish inner point contacts, being compatible with the encapsulation-technique. The latter might be of special interest if an isolated electrical contact has to be established in the middle of a hBN/graphene/hBN heterostructure. With chapter 6 the experimental part of the Thesis involving quantum transport in p-n junctions starts. In this chapter Fabry-Pérot resonances in a p-n-p device in the absence and presence of a Moiré superlattice are discussed. Fabry-Pérot resonances can be used to gain information about the exact position of the p-n junction as a function of charge carrier doping and on the yet not fully known band-reconstruction due to the Moirésuperlattice. In chapter 7 we report on three types of magnetoconductance oscillations which can occur along a graphene p-n junction. While several previous studies have tried to explain the observation of individual magnetoconductance oscillations, none of them describes all at the same time. On the contrary, we present experimental results where three different kinds of oscillations are observed within the same device/measurement. The latter allows for a more direct comparison between the different types of mangetoconductance oscillations and we can rule out differences in various device architectures. Finally, we can describe the underlying physics of the different types of magnetoconductance oscillations with a consistent model. Upon further increasing the magnetic field to very high values, the transport is governed by the lowest Landau level. In combination with a p-n junction, which is located perpendicular to the transport direction, conductance oscillations resulting from valley-isospin physics are expected. In chapter 8 experimental results are presented which show signatures of this effect for the first time. By tuning the position of the p-n junction this allows to locally probe the relative edge configuration, giving rise to conductance oscillations in the order of e^2/h. In the last chapter, chapter 9, preliminary experimental results and theoretical calculations on the electrical counterpart of the Michelson Morley interferometer are presented