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

Thin Film Approaches to The Srf Cavity Problem: Fabrication and Characterization of Superconducting Thin Films

Superconducting Radio Frequency (SRF) cavities are responsible for the acceleration of charged particles to relativistic velocities in most modern linear accelerators, such as those employed at high-energy research facilities like Thomas Jefferson National Laboratory’s CEBAF and the LHC at CERN. Recognizing SRF as primarily a surface phenomenon enables the possibility of applying thin films to the interior surface of SRF cavities, opening a formidable tool chest of opportunities by combining and designing materials that offer greater benefit. Thus, while improvements in radio frequency cavity design and refinements in cavity processing techniques have improved accelerator performance and efficiency – 1.5 GHz bulk niobium SRF cavities have achieved accelerating gradients in excess of 35 MV/m – there exist fundamental material bounds in bulk superconductors limiting the maximally sustained accelerating field gradient (approximately 45 MV/m for Niobium) where inevitable thermodynamic breakdown occurs. With state of the art niobium based cavity design fast approaching these theoretical limits, novel material innovations must be sought in order to realize next generation SRF cavities. One proposed method to improve SRF performance is to utilize thin film superconducting-insulating-superconducting (SIS) multilayer structures to effectively magnetically screen a bulk superconducting layer such that it can operate at higher field gradients before suffering critically detrimental SRF losses. This dissertation focuses on the production and characterization of thin film superconductors for such SIS layers for radio-frequency applications

Algorithmic approaches to high speed atomic force microscopy

Thesis (Ph.D.)--Boston UniversityThe atomic force microscope (AFM) has a unique set of capabilities for investigating biological systems, including sub-nanometer spatial resolution and the ability to image in liquid and to measure mechanical properties. Acquiring a high quality image, however, can take from minutes to hours. Despite this limited frame rate, researchers use the instrument to investigate dynamics via time-lapse imaging, driven by the need to understand biomolecular activities at the molecular level. Studies of processes such as DNA digestion with DNase, DNA-RNA polymerase binding and RNA transcription from DNA by RNA polymerase redefined the potential of AFM in biology. As a result of the need for better temporal resolution, advanced AFMs have been developed. The current state of the art in high-speed AFM (HS-AFM) for biological studies is an instrument developed by Toshio Ando at Kanazawa University in Japan. This instrument can achieve 12 frames/sec and has successfully visualized the motion of protein motors at the molecular level. This impressive instrument as well as other advanced AFMs, however, comes with tradeoffs that include a small scan size, limited imaging modes and very high cost. As a result, most AFM users still rely on standard commercial AFMs. The work in this thesis develops algorithmic approaches that can be implemented on existing instruments, from standard commercial systems to cutting edge HS-AFM units, to enhance their capabilities. There are four primary contributions in this thesis. The first is an analysis of the signals available in an AFM with respect to the information they carry and their suitability for imaging at different scan speeds. The next two are algorithmic approaches to HS-AFM that take advantage of these signals in different ways. The first algorithm involves a new sample profile estimator that yields accurate topology at speeds beyond the bandwidth of the limiting actuator. The second involves more efficient sampling, using the data in real time to steer the tip. Both algorithms yield at least an order of magnitude improvement in imaging rate but with different tradeoffs. The first operates beyond the bandwidth of the controller managing the tip-sample interaction and therefore the applied force is not well-regulated. The second keeps this control intact but is effective only on a limited set of samples, namely biopolymers or other string-like samples. Experiments on calibration samples and λ-DNA show that both of the algorithms improve the imaging rate by an order of magnitude. In the fourth contribution, extended applications of AFMs equipped with the algorithmic approaches are the tracking of a macromolecule moving along a string-like sample and a time optimal path for repetitive non-raster scans along string-like samples

Hybrid structures for molecular level sensing

With substantial molecular mobility and segment dynamics relative to metals and ceramics, all polymeric materials, to some extent, are stimuli-responsive by exhibiting pronounced chemical and physical changes in the backbone, side chains, segments, or end groups induced by changes in the local environment. Thus, the push to incorporate polymeric materials as sensing/responsive nanoscale layers into next-generation miniaturized sensor applications is a natural progression. The significance and impact of this research is wide-ranging because it offers design considerations and presents results in perhaps two of the most critical broad areas of nanotechnology: ultrathin multifunctional polymer coatings and miniaturized sensors. In this work, direct evidence is given showing that polymer coatings comprised of deliberately selected molecular segments with very different chemistry can have switchable properties, and that the surface composition can be precisely controlled, and thus properties can be tuned: all in films on the order of 20 nm and less. Furthermore, active sensing layers in the form of plasma-polymerized polymers are successfully incorporated into actual silicon based microsensors resulting in a novel hybrid organic/inorganic materials platform for microfabricated MEMS sensors with record performance far beyond contemporary sensors in terms of detection sensitivity to various environments. The results produced in this research show thermal sensors with more than two orders of magnitude better sensitivity than what is attainable currently. In addition, a humidity response on the order of parts per trillion, which is four orders of magnitude more sensitive than current designs is achieved. Molecular interactions and forces for organic molecules are characterized at the picoscale to optimize polymeric nanoscale layer design that in turn optimize and lead to microscale hybrid sensors with unprecedented sensitivities

Investigation of Wear Mechanism of Gallium Nitride

The optoelectronic properties of gallium nitride (GaN) has been extensively studied for decades, which has facilitated its application in many different areas, cementing it as one of the most important semiconductor materials in the world. However, in comparison to the study of its optoelectronic properties, there are few studies of its mechanical properties - especially the tribological performance. Knowing the tribological properties of GaN, such as friction and wear, is crucial for understanding its machinability, the implementation of GaN in MEMS, solar cells, and other devices, as well as the wear performance of these GaN-based devices when working under harsh environments. In our study, we reveal that GaN has an ultralow wear nature, and that its wear rate can approach that of diamond. We also discover that the wear rate of GaN is affected by its crystallographic orientation, humidity, and composition.For the crystallographic orientation dependence, we look into the physics by both experimental and computational methods. We demonstrate that both the friction coefficient and wear rate of GaN exhibits a 60° periodicity. We conclude that these periodic variations of wear rate and friction coefficient are the results of a periodic variation of the energy barrier.The moisture dependent wear mechanism of GaN has been investigated under dry, low humidity, and high humidity environments. The results show that the wear rate of GaN perfectly follows an increasing of the humidity which spans over two orders of magnitude when the testing environment switches from dry nitrogen to humid lab air. On the contrary, the friction coefficient gave a contrary response, i.e., the lowest friction coefficient was found under low humidity environment, dry nitrogen had the highest friction coefficient, and the humid environment had its friction in the middle. Various characterization techniques, including SEM/EDS, AFM and TEM were employed to interrogate the worn surfaces under each condition. Based on the results, we hypothesize that the wear under dry nitrogen environment is adhesive in nature whereas grooving abrasive wear dominates the wear behavior of GaN under a humid environment.The compositional study of GaN wear revealed that by alloying different elements into the GaN system, one can not only tune the bandgap, but also modify the wear rate. This finding can be useful for applications and design that require suitable electronic properties while keep the wear rate within an acceptable range.Furthermore, during the investigation of the GaN wear mechanism, we discovered that the tribological sliding can also alter the surface band bending of this material. Our results demonstrate that the environment, number of sliding cycles, and normal loads can effectively tune the surface band bending of GaN. This finding shows the capability of mechanical dynamic contact for surface electronic property modification, which can be used in various applications, such as gas sensing, photocatalysis, and photochemistry.Understanding of the wear mechanism of GaN as well as the shear-induced band bending on GaN can remarkably promote the applications of GaN in various fields other than the optoelectronic area. This also reinforces the important message that tribology is not only a discipline that focuses on investigation of protective coating and lubrication but also can be used in device design and fabrication

Understanding the interfacial processes of reactive nanobubbles toward agricultural applications

There is a growing interest in nanobubble (NB) technology because of its diverse applications (e.g., detergent-free cleaning, water aeration, ultra-sound imaging and intracellular drug delivery, and mineral processing). NBs have a higher efficiency of mass transfer compared to bulk scale bubbles due to the high specific surface areas. The high specific surface also facilitates physical adsorption and chemical reactions in the gas liquid interface. Furthermore, the collapse of NBs creates shock waves and the formation of hydroxyl radicals (OH). However, it remains elusive why or how NBs are stabilized in water and particularly, the states of internal pressures of NBs are difficult to measure. This thesis employs the injection of high-pressure gases through a hydrophobized ceramic membrane to produce different gaseous NBs in water. The results indicate that increasing the injection gas pressure (60–80 psi) and solution temperatures (6–40 oC) both reduce bubble sizes, which are validated by two independent models develop from the Young-Laplace equation and contact mechanics. Both models yield consistent prediction of the internal pressures of various NBs (120 psi-240 psi). The developed methods and model framework are useful to unravel properties of NBs and support engineering applications of NBs. In addition, Atomic Force Microscopy-Scanning Electrochemical Microscopy (AFM-SECM) has evolved to be a powerful tool for simultaneous topographical-electrochemical measurements at local material surfaces with high spatial resolution. Such measurements are crucial for understanding structure-activity relationships relevant to a wide range of applications in material science, life science and chemical processes. The electrochemical behavior of surface NBs on gold substrate is measured by AFM-SECM, to better understand the chemical properties of NBs. Moreover, this study investigates the effects of four types of NBs (air, oxygen, nitrogen, and carbon dioxide) on seed germination and plant growth. Nitrogen NBs exhibit considerable effects in the seed germination, whereas air and carbon dioxide NBs do not significantly promote germination. The growth of stem length and diameter, leave numbers, and leave width are promoted by NBs (except air). Furthermore, the promotion effect is primarily ascribed to the generation of exogenous reactive oxygen species (ROS) by NBs and higher efficiency of nutrient fixation or utilization

Determination of nanoscale ferroelectric domain distribution in multilayer piezoelectric actuators

Caratterizzazione in scala nanometrica della distribuzione di domini ferroelettrici in attuatori piezoelettrici multistrato, utilizzati negli iniettori dei motori diesel. Le analisi sono state fatte attraverso la tecnica: Piezoresponse Force Microscopy (PFM). Essa consiste in un microscopio a forza ionica (AFM),in cui viene applicata una differenza di potenziale alla punta che entra in contatto con il materiale.ope