Dissertations / Theses on the topic 'AFM cantilever'

The dynamic properties of several rectangular and V-shaped microcantilevers were investigated. Particular attention was paid to the higher flexural eigenmodes of oscillation. The potential of the higher flexural modes was demonstrated through the use of cantilevers as standalone sensors for adsorbed mass. The mass adsorbed on the surface of a cantilever was in the form of a homogeneous water layer measured as a function of relative humidity. The minimum detectable water layer thicknesses were 13.7 Å, 3.2 Å, 1.1 Å, and 0.7 Å for the first four modes of a rectangular cantilever, clearly demonstrating enhanced accuracy for the higher eigenmodes of oscillation. These thicknesses correspond to minimum detectable masses of 33.5 pg, 7.8 pg, 2.7 pg and 1.7 pg for the first four modes. For quantitative applications the spring constants of each cantilever must be determined. Many methods exist but only a small number can calibrate the higher flexural eigenmodes. A method was developed to simultaneously calibrate all flexural modes of microcantilever sensors. The method was demonstrated for the first four eigenmodes of several rectangular and V-shaped cantilevers with nominal fundamental spring constants in the range of 0.03 to 1.75 N/m. The spring constants were determined with accuracies of 5-10 %. Spring constants of the fundamental mode were generally in agreement with those determined using the Sader method. The method is compatible with existing AFM systems. It relies on a flow of gas from a microchannel and as such poses no risk of damage to the cantilever beam, its tip, or any coating. A related method was developed for the torsional modes of oscillation. Preliminary results are shown for the fundamental mode of a rectangular cantilever. The method can be easily extended to the higher torsional modes, V-shaped cantilevers, and potentially, the flapping modes of the legs of V-shaped microcantilevers.

This Thesis relates to the continually advancing field of microelectromechanical systems (MEMS). With MEMS technology, there are many different areas of concentration available for research. This Thesis addresses analysis and preliminary characterization of a cantilever-type MEMS chemical sensor for detection of chemicals and organic components operating at room temperature (20˚C and sea level pressure of 1 atm). Such sensors can be useful in a wide variety of applications. There currently exist several different types of MEMS chemical sensors. Each is based on a different detection method, e.g., capacitive, thermal, resistive, etc., and is used for specific tasks. Out of all currently available detection methods, the most common is the gravimetric method. The gravimetric sensor works by absorbing the chemical in a special material, usually a polymer, which alters the overall mass of the sensing element that can then be measured, or detected, to identify the chemical absorbed. One of the more exciting developments in the field of gravimetric chemical MEMS has been with the advancement of cantilever-type sensors. These cantilevers are small and usually on the order of only about 300 m in length. In order to utilize the gravimetric method, a cantilever is coated with a polymer that allows an analyte to bond to it and change its mass, which in turn changes the resonant frequency of the cantilever. The change in frequency can then be measured and analyzed and from it, the amount of absorbed mass can be calculated. Current research in the cantilever-type resonating sensors for the detection of hydrogen is developing measurement capabilities of 1 ppm (part-per-million). In this Thesis number of sample cantilevers were qualitatively assessed and their dimensional geometry measured. Based on these measurements, frequency data were obtained. In addition, the overall uncertainty in the resonant frequency results was calculated and the contributing factors to this uncertainty were investigated. Experimental methods that include laser vibrometry, optoelectronic laser interferometric microscopy (OELIM), and atomic force microscopy (AFM), were utilized to measure the frequency responses of the samples. The analytically predicted natural frequencies were compared to the experimental data to determine correlation subject to the uncertainty analysis. Parametric analyses involving chemical absorption processes were also conducted. Such analyses considered different parameters, e.g., damping and stiffness as well as changes in their values, to determine contributions they make to the quality of the frequency data and the effect they have on sensitivity of the MEMS cantilever-type chemical sensors. Once these parametric analyses were completed, it was possible to estimate the sensitivity of the cantilever, or the ability for the cantilever to detect frequency shifts due to absorption of the target chemical. Results of the parametric analyses of the fundamental resonant frequency were then correlated with the sensitivity results based on the chemical absorption. This Thesis correlates many results and ideas and probes problems revolving around the analysis and characterization of a MEMS cantilever-type chemical sensor.

Parmi les polytypes les plus connus du carbure de silicium (SiC), le polytype cubique (3C-SiC), est le seul qui peut croitre sur des substrats silicium. L’hétérostructure 3C-SiC/Si est intéressante non seulement pour son faible coût de production mais aussi pour la conception de Systèmes Micro-Electro-Mécaniques (« MEMS »). La valeur élevée du module de Young du 3C-SiC, comparé à celui du silicium, permettrait à des cantilevers submicroniques, fabriqués à partir de films minces de 3C-SiC, de vibrer à ultra-hautes fréquences (>100MHz). Cette haute fréquence de résonance est la clé pour obtenir un système AFM non-contact ultra-sensible et rapide. Cependant, il n’existe pas de cantilever en SiC disponible sur le marché en raison de la difficulté à élaborer des films minces de 3C-SiC de bonne qualité, la technique de synthèse la plus utilisée étant le Dépôt Chimique en phase Vapeur (CVD). La raison première de cette difficulté à obtenir un matériau de bonne qualité réside essentiellement dans l’important désaccord de maille et la différence de dilatation thermique entre le 3C-SiC et Si qui génèrent des défauts cristallins à l’interface et jusqu’à la surface du film de 3C-SiC, la zone la plus défectueuse se localisant auprès de l’interface……
Among aIl the well known polytypes ofihe silicon carbide (SiC), the cubic polytype (3C-SiC) is the only one that min be grown on silicon substrates. This heterostructure 3C SiC/Si ta interesting not only for its low production cost but also for the design of tise Micro-Electro-Mechanical Systems (MEMS). The high value ofthe Young’s modulis the 3C-SiC, compared to the silicon, allows submicronic cantilevers, fabrmcated from tIse 3C-SiC thin filins, to resonate at ultra-high frequency (>100MHz). The high resonant frequency is the key to obtain s fast, ultra-sensitive non-contact AFM systein.However, there isn’t any SiC cantilevers available on the market because of the difficulty to elaborate gond quality 3C-SiC thin films, with tIse Chemical Vapor Deposition (CVD) technique being tIse most frequently used synthesis technology. Tise first reason of tIse difficulty with the CVD technology to obtain gond quality thin film rests essentially in the important lattice mismatch and the difference in thermal expansion coefficient existing between 3C SiC and Si which generate crystalline defects at the interface and propagating tilI the 3C-SiC filin surface, with the inost defective zone localizing near the interface……

The design and implementation of a micro-force displacement system was completed to test the force-displacement characteristics of square silicon diaphragms with side lengths of 4mm, 5mm, and 7mm with a thickness of 10um. The system utilizes a World Precision Instruments Fort 10g force transducer attached to a World Precession Instruments TBM4M amplifier. A Keithley 2400 source meter provided data acquisition of the force component of the system. A micro prober tip was utilized as the testing probe attached to the force transducer with a tip radius of 5um. The displacement of samples was measured using a Newport M433 linear stage driven by a Newport ESP300 motion controller (force readings at constant displacement intervals). An additional 3 linear stages were used to provide X and Y-axis positioning of samples beneath the probe tip. The system components were mounted to an optical bench to provide stability during testing. C# was used to deliver the code to the individual components of the system. In addition the software provides a graphic user interface for future users that includes a calibration utility (both X/Y and force calibration), live force-displacement graph, motion control, and a live video feed for sample alignment. Calibration of the force transducer was accomplished using an Adam Equipment PGW153e precision balance to assign force values to the voltage data produced from the transducer. Displacement calibration involved the use of a microscope calibration micrometer. The system was characterized with an equipment variability of ±1.02mg at 1.75um, and ±1.86mg at 3.5um with the ability to characterize samples with stiffness less than 279 mg/um. The displacement resolution of the system was determined to be 35 nm per step of the linear stages. The diaphragms created to test the machine were fabricated from 10um thick device layer SOI wafers. An etch consisting of 38g/l silicic acid, 7g/l ammonium persulfate, and 5% TMAH was used to reduce the formation of hillocks, and provide a consistent etch rate. A Gage R&R study was performed on the fabricated diaphragms, indicating that the deflection produced by the 4mm, 5mm, and 7mm diaphragms was resolvable by the machine. A model was developed to correlate theoretical results to the observed measured values.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005.
Includes bibliographical references (p. 47-52).
Atomic force microscopy (AFM) has been a powerful instrument that provides nanoscale imaging of surface features, mainly of rigid metal or ceramic surfaces that can be insulators as well as conductors. Since it has been demonstrated that AFM could be used in aqueous environment such as in water or various buffers from which physiological condition can be maintained, the scope of the application of this imaging technique has been expanded to soft biological materials. In addition, the main usage of AFM has been to image the material and provide the shape of surface, which has also been diversified to molecular-recognition imaging - functional force imaging through force spectroscopy and modification of AFM cantilevers. By immobilizing of certain molecules at the end of AFM cantilever, specific molecules or functionalities can be detected by the combination of intrinsic feature of AFM and chemical modification technique of AFM cantilever. The surface molecule that is complementary to the molecule at the end of AFM probe can be investigated via specificity of molecule-molecule interaction.
(cont.) Thus, this AFM cantilever chemistry, or chemical functionalization of AFM cantilever for the purpose of chemomechanical surface characterization, can be considered as an infinite source of applications important to understanding biological materials and material interactions. This thesis is mainly focused on three parts: (1) AFM cantilever chemistry that introduces specific protocols in details such as adsorption method, gold chemistry, and silicon nitride cantilever modification; (2) validation of cantilever chemistry such as X-ray photoelectron spectroscopy (XPS), AFM blocking experiment, and fluorescence microscopy, through which various AFM cantilever chemistry is verified; and (3) application of cantilever chemistry, especially toward the potential of force spectroscopy and the imaging of biological material surfaces.
by Sunyoung Lee.
S.M.

Atomic Force Microscopy (AFM) is perhaps the most significant member of the scanning probe microscopes family and, because of its capability of working in air and liquid environments with virtually no limitations on imaging conditions and types of samples, it is definitely one of the most widely used. It has become an indispensable tool to measure mechanical properties at the nanoscale in various research contexts. Scanning probes used in AFM are micromechanical oscillators (typically cantilevers) and the theory of AFM dynamics is based on the analysis of the oscillating modes of beam resonators or the simpler spring-mass model. Cantilevers can be driven by the thermal excitation and/or an external driver. Usually cantilevers are driven near resonances corresponding to flexural eigenmodes that can be described as damped harmonic oscillators. Advanced techniques consider multifrequency excitation or band excitation to broaden the measurable events in tip-sample interactions, thus expanding the variety of sample properties that can be accessed. Multifrequency methods imply excitation and/or detection of several frequencies of the cantilever oscillations and concern the associated nonlinear cantilever dynamics. Such excitation/detection schemes provide higher resolution and sensitivity to materials properties such as the elastic constants and the sample chemical environment with lateral resolution in the nanometer range. In order to measure these parameters, information on peak force of interaction, energy dissipation and contact dynamics is required. Techniques to measure the parameters of the cantilever in the stationary regime are well established. In dynamics methods the external driver (thermal noise, piezoelectric driver, etc.) excites the cantilever and a number of techniques have been implemented to gain information from the tip-sample interactions, but usually the interaction of the tip with the surface is revealed by the modification of the average value of the amplitude, frequency or phase shift over many oscillation cycles. Reconstruction of the complete evolution of the interaction force between the tip and the sample surface during a single interaction event is not even considered. As an alternative to these well established techniques and to push further the AFM possibilities, it is important to examine the possibility of analyzing single-event or impulsive interactions. This opens the possibility to capture the information conveyed by the sensing tip in a single interaction, in contrast to the cycle average used in many dynamic techniques. The single-event interactions are basically of the impact kind, with the simultaneous excitation of many cantilever eigenmodes and/or harmonics. The averaging techniques provide superior sensibility, allowing to probe the details of force interactions down to the molecular level, but to study single-event interactions it is mandatory to provide analysis techniques that are able to characterize all excited cantilever oscillation modes at once without averaging. The temporal evolution of the amplitude, phase and frequency during few oscillation cycles of the cantilever provides information that cannot be obtained with standard methods. In the present thesis a data analysis method allowing to retrieve these quantities during an impulsive cantilever excitation is proposed. This thesis concentrates on the dynamics of the flexural modes of the thermally driven cantilever in air when its tip is excited by a single impact on the sample surface. The signal analysis is based on the combination of wavelet and Fourier transforms that can be applied to a broad class of AFM impulsive measurements. To exemplify the method, a short time interval around the jump-to-contact (JTC) transition in ambient conditions is investigated, with the aim to characterize the transient excitation of the cantilever eigenmodes before and after the impact. The experimental evidences that high-order flexural modes are excited in air upon a single impact tip–sample interaction induced by the JTC transition are presented. The way to retrieve information about the instantaneous total force act ing on the cantilever tip, contact dynamics and energy dissipation at all frequencies simultaneously, without averaging or interruption, is developed. The exploration of these transient conditions of the cantilever is not possible with dynamic techniques based on the resonant driving or using Fourier transform analysis alone. The analysis presented in this work is useful to deal with nonrepeatable experiments and to determine the exact single interaction dynamics in terms of the full cantilever spectral excitations, features that are not normally considered in dynamical AFM techniques.

The human body is an intricate network of many highly regulated biochemical processes and cell adhesion is one of them. Cell adhesion is mediated by specific interactions between molecules on apposing cell surfaces and is critical to many physiological and pathological processes like inflammation and cancer metastasis. During inflammation, blood-borne circulating leukocytes regularly stick to and roll on the vessel walls, which consist in part, adhesive contacts mediated by the selectin family of adhesion receptors (P-, E- and L-selectin). This is the beginning of a multi-step cascade that ultimately leads to leukocyte recruitment in areas of injury or infection. In vivo, selectin-mediated interactions take place in a hydrodynamic milieu and hence, it becomes imperative to study these interactions under very similar conditions in vitro. The goal of this project was to characterize the kinetic and mechanical properties of selectin interactions with different physiologically relevant ligands and selectin-specific monoclonal antibodies (mAbs) under a mechanically stressful milieu, using atomic force microscopy (AFM). Elasticity studies revealed that bulk of the complex compliance came from the selectins, with the ligands or mAbs acting as relatively stiffer components in the stretch experiments. Furthermore, molecular elasticity was inversely related to selectin length with the Consensus Repeats (CRs) behaving as Hookean springs in series. Besides, monomeric vs. dimeric interactions could be clearly distinguished from the elasticity measurements. L-selectin dissociation studies with P-selectin Glycoprotein Ligand 1 (PSGL-1) and Endoglycan revealed that catch bonds operated at low forces while slip bonds were observed at higher forces. These results were consistent with previous P-selectin studies and suggested that catch bonds could contribute to the shear threshold for L-selectin-mediated rolling By contrast, only slip bonds were observed for L-selectin-antibody interactions, suggesting that catch bonds could be a common characteristic of selectin-ligand interactions. Force History studies revealed that off-rates of L-selectin-sPSGL-1 (or 2-GSP-6) interactions were not just dependent on applied force, as has been widely accepted but in fact, depended on the entire history of force application, thus providing a new paradigm for how force could regulate bio-molecular interactions. Characterizing selectin-ligand interactions at the molecular level, devoid of cellular contributions, is essential in understanding the role played by molecular properties in leukocyte adhesion kinetics. In this aspect, data obtained from this project will not only add to the existing body of knowledge but also provide new insights into mechanisms by which selectins initiate leukocyte adhesion in shear.

La force de Casimir, du nom du physicien qui prédit en 1948 l'existence de cette force attractive dans la configuration idéale de deux miroirs parfaits, plans et parallèles, est d'origine purement quantique. Elle résulte de l'existence des fluctuations quantiques de point zéro du champ électromagnétique et joue un rôle prépondérant dans le fonctionnement des nanosystèmes mécaniques en cours de développement et qui pourraient dans les années futures révolutionner l'industrie de la microélectronique. Pourtant, les effets correctifs liés à la conductivité des surfaces ou à l'empilement diélectrique des couches commencent seulement à être étudiés. L'objet du travail expérimental développé durant cette thèse a été la réalisation d'un appareil de mesure de forces faibles entre deux surfaces de tailles micrométriques en vue de l'étude de la force de Casimir. Nous avons adapté la technique mise au point au cours de la thèse de G. Jourdan à un environnement cryogénique afin d'atteindre les sensibilités en force requises pour l'étude de surfaces complexes. Ce manuscrit décrit ce nouvel appareil et ses performances. Nous avons ainsi étudié l'amortissement visqueux de notre sonde de force (un microlevier AFM avec une sphère collée à son extrémité) engendré par le confinement du fluide environnant. A 4 K, nous avons mis en évidence des effets opto-mécaniques induits par le bruit laser. Enfin, nos premiers tests entre deux surfaces d'or de faible rugosité nous ont amenés à lutter contre une force parasite. Nous avons déterminé l'origine de cette force et réussi à la contrer. La force de Casimir entre une sphère en or et un échantillon de silicium a alors pu être mesurée
The Casimir force, named after the Dutch physicist who predicted in 1948 the existence of this attractive force in an ideal conguration of two perfectly conducting and parallel plates, is due to quantum-mechanical effects. It results from electromagnetic fluctuations of vacuum and cannot be neglected anymore in the performance of the nano-mechanical systems (NEMS), which are currently developed and could also revolutionize the microelectronics industry. However, the study of the corrections brought by the surface conductivity or by the dielectric stacks has just started. This thesis work presents the achievement of a new instrument dedicated to the measurement of the Casimir force between two surfaces at the micron scale. We have adapted the technique performed during the G. Jourdan's PhD thesis to a cryogenic environment in order to reach the sensitivity needed to investigate complex surfaces. The viscous damping of the force probe (AFM cantilever) due to the environmental fluid confined between the sphere glued at the tip of the AFM cantilever and a sample has been analysed. Measurements at 4K have revealed an opto-mechanical effect induced by laser noise. Finally, our first tests between two low rough gold surfaces have showed the presence of a parasite force. We have found out the origin of this force and succeeded to remove it. The first measurement of the Casimir force between a gold sphere and a silicon sample has been performed

This mémoire presents my current research interests in micro and nano-mechanics in a comprehensive manuscript. Our experimental device is first presented: this atomic force microscope, designed and realized in the Laboratoire de Physique de l'ENS Lyon, is based on a quadrature phase differential interferometer. It features a very high resolution (down to 10 fm/rtHz) in the measurement of deflexion, down to low frequencies and on a huge input range. The dual output of the interferometer implies a specific handling to interface common scanning probe microscope controllers. We developed analog circuitries to tackle static (contact mode) and dynamic (tapping mode) operations, and we demonstrate their performance by imaging a simple calibration sample. As a first application, we used the high sensitivity of our interferometer to study the mechanical behavior of micro-cantilevers from their fluctuations. The keystone of the analysis is the Fluctuation-Dissipation Theorem (FDT), relating the thermal noise spectrum to the dissipative part of the response. We apply this strategy to confront Sader's model for viscous dissipation with measurements on raw silicon cantilevers in air, demonstrating an excellent agreement. When a gold coating is added, the thermal noise is strongly modified, presenting a 1/f like trend at low frequencies: we show that this behavior is due to a viscoelastic damping, and we provide a quantitative phenomenological model. We also characterize the mechanical properties of cantilevers (stiffness and Elastic Moduli) from a mapping of the thermal noise on their surface. This analysis validates the description of the system in term of its normal modes of oscillations in an Euler-Bernoulli framework for flexion and in Saint-Venant approach for torsion, but points toward a refined model for the dispersion relation of torsional modes. Finally, we present peeling experiments on a single wall carbon nanotube attached to the cantilever tip. It is pushed against a flat substrate, and we measure the quasi-static force as well as the dynamic stiffness using an analysis of the thermal noise during this process. The most striking feature of these two observables is a plateau curve for a large range of compression, the values of which are substrate dependent. We use the Elastica to describe the shape of the nanotube, and a simple energy of adhesion per unit length Ea to describe the interaction with the substrate. We analytically derive a complete description of the expected behavior in the limit of long nanotubes. The analysis of the experimental data within this simple framework naturally leads to every quantity of interest in the problem: the force plateau is a direct measurement of the energy of adhesion Ea for each substrate, and we easily determine the mechanical properties of the nanotube itself.

Atomic force microscopy (AFM) can be used for atomic and nanoscale surface characterization in both air and liquid environments. AFM is basically used to measure the mechanical, chemical and biological properties of the sample under investigation. AFM contains basically a base-excited microcantilever with nano tip along with a sensing circuit for scanning of images. Design and analysis of this microcantilevers is a challenging task in real time practice. In the present work, design and dynamic analysis of rectangular microcantilevers in tapping mode with tip-mass effect is considered. Computer simulations are performed with both lumped-parameter and distributed parameter models. The interatomic forces between the nano tip mass and substrate surfaces are treated using Lennard Jones (LJ) model and DMT model. The equations of motion are derived for both one-degree of freedom lumped parameter model with squeeze-film damping and distributed parameter model under the harmonic base excitation. Also the nonlinearity of the cantilever is investigated by considering cubic stiffness. The distributed parameter model is simplified with one mode approximation using Galerkin’s scheme. The resulting nonlinear dynamic equations are solved using in numerical Runge-Kutta method using a MATLAB program. The natural frequencies of the microcantilever and dynamic response are obtained. Dynamic stability issues are studied using phase diagrams and frequency responses. An experimental work is carried out to understand the variations in dynamic characteristics of a chromium plated steel microcantilever specimen fabricated using wire-cut EDM process. An electrodynamic exciter is attached at the cantilever base and laser Doppler Vibrometer (LDV) is used to provide sensing signal at the oscilloscope. The sine sweep excitation is provided by a signal generator and power amplifier set-up. The frequency response obtained manually is used to arrive-at the natural frequencies and damping factors.

The interferometric cantilever displacement detection in non-contact atomic force microscopy (NC-AFM) is in fundamental aspects explored and optimized. Furthermore, the opto-mechanical interaction of the light field with the cantilever is investigated in detail. Cantilevers are harmonic oscillators that are designed to have a high sensitivity for the detection of minute external forces typically originating from tip-sample interaction. In this work, however, the high sensitivity is used for detailed studies of opto-mechanical forces due to the radiation pressure of the light interacting with the cantilever. The interferometer in the NC-AFM setup consists of an optical cavity working similar to a Fabry-Pérot interferometer in combination with a reference interference arm working similar to a Michelson interferometer combining multi-beam interference with a reference beam resulting in a complex superposition of beams forming the interferometric intensity modulation signal. The character of the interferometer can be adjusted from predominant Michelson to predominant Fabry-Pérot characteristics by the optical loss inside the cavity. A systematic approach for accurate alignment, by using 3D intensity maps and intensity-over-distance curves, as well as the implications of deficient fiber-cantilever configurations are explored and the impact of the interferometer configuration on the detection system noise floor is investigated. A new physical property, namely, the Fabry-Perot enhancement factor is introduced that is a direct measure for the light intensity interacting with the cantilever compared to the reference beam intensity reflected back inside the fiber. The quantification of the optical loss yields an exact knowledge of the amount of light interacting with the cantilever that is crucial to understand opto-mechanical effects. The resulting opto-mechanical force varies sinusoidally during the course of one oscillation cycle. It is a key result of this work that the sinusoidal modification of the cantilever restoring force can be described analogue to the restoring force of a pendulum. This results in an observable amplitude dependent frequency shift of the cantilever oscillation, allowing a calculation of the ratio of the opto-mechanical force relative to the cantilever restoring force and thus allows an in-situ measurement of the cantilever stiffness with remarkable precision. Further investigation of the cantilever oscillation yields that other characteristic properties of the oscillation are significantly modified by the opto-mechanical interaction. The observed effective fundamental mode Q-factor drops significantly while the cantilever amplitude response to a certain excitation voltage increases. A discrete numerical model describing the cantilever as a 1D linear chain of mass points is implemented, yielding that the additional opto-mechanical force results in a partial pinning of the cantilever at the edges of the interferometric fringes. Pinning efficiently shifts energy from the fundamental mode to higher modes and modes of a pinned cantilever, resulting in a complex modal structure.

abstract: Near-field thermal radiation occurs when the distance between two surfaces at different temperatures is less than the characteristic wavelength of thermal radiation. While theoretical studies predict that the near-field radiative heat transfer could exceed Planck’s blackbody limit in the far-field by orders of magnitudes depending on the materials and gap distance, experimental measurement of super-Planckian near-field radiative heat flux is extremely challenging in particular at sub-100-nm vacuum gaps and few has been demonstrated. The objective of this thesis is to develop a novel thermal metrology based on AFM bi-material cantilever and experimentally measure near-field thermal radiation. The experiment setup is completed and validated by measuring the near-field radiative heat transfer between a silica microsphere and a silica substrate and comparing with theoretical calculations. The bi-material AFM cantilever made of SiNi and Au bends with temperature changes, whose deflection is monitored by the position-sensitive diode. After careful calibration, the bi-material cantilever works as a thermal sensor, from which the near-field radiative conductance and tip temperature can be deduced when the silica substrate approaches the silica sphere attached to the cantilever by a piezo stage with a resolution of 1 nm from a few micrometers away till physical contact. The developed novel near-field thermal metrology will be used to measure the near-field radiative heat transfer between the silica microsphere and planar SiC surface as well as nanostructured SiC metasurface. This research aims to enhance the fundamental understandings of radiative heat transfer in the near-field which could lead to advances in microelectronics, optical data storage and thermal systems for energy conversion and thermal management.
Dissertation/Thesis
Masters Thesis Mechanical Engineering 2019

Different methods for the determination of cantilever properties in non-contact atomic force microscopy (NC-AFM) are under investigation. A key aspect is the determination of the cantilever stiffness being essential for a quantitative NC-AFM data analysis including the extraction of the tip-surface interaction force and potential. Furthermore, a systematic analysis of the displacement noise in the cantilever oscillation detection is performed with a special focus on the thermally excited cantilever oscillation. The propagation from displacement noise to frequency shift noise is studied under consideration of the frequency response of the PLL demodulator. The effective Q-factor of cantilevers depends on the internal damping of the cantilever as well as external influences like the ambient pressure and the quality of the cantilever fixation. While the Q-factor has a strong dependence on the ambient pressure between vacuum and ambient pressure yielding a decrease by several orders of magnitude, the pressure dependence of the resonance frequency is smaller than 1% for the same pressure range. On the other hand, the resonance frequency highly depends on the mass of the tip at the end of the cantilever making its reliable prediction from known cantilever dimensions difficult. The cantilever stiffness is determined with a high-precision static measurement method and compared to dimensional and dynamic methods. Dimensional methods suffer from the uncertainty of the measured cantilever dimensions and require a precise knowledge its material properties. A dynamic method utilising the measurement of the thermally excited cantilever displacement noise to obtain cantilever properties allows to characterise unknown cantilevers but requires an elaborative measurement equipment for spectral displacement noise analysis. Having the noise propagation in the NC-AFM system fully characterised, a proposed method allows for spring constant determination from the frequency shift noise at the output of the PLL demodulator with equipment already being available in most NC-AFM setups.

Low-temperature magnetic force microscopy was used to study the phase diagram of a La1/3Pr1/3Ca1/3MnO3 thin film grown on a (110) NdGaO3 (NGO) substrate by pulsed laser deposition. Traditionally, one can observe the phase change at the nanoscale level as the sample is cooled from room temperature through the transition temperature to liquid nitrogen temperatures, but in this case a fixed voltage ranging from 0 V to 31 V was applied before each cooling cycle. From in and ex situ transport measurements, it is observed that the temperature of the peak of the transition increases with applied field; however, the MFM images show that the magnetic transition begins at a lower temperature with the same increase in field. Thus, this dissertation shows that a new voltage control exists for the phase transition in certain manganites.
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My doctoral research has focused on the characterization of dynamic response of atomic force microscope (AFM) cantilevers for thermofluidic and biophysical sensors, a novel scanning thermal microscopy technique development using a tipless microcantilever to investigate micro/nanoscale transport properties in liquid, and the characterization of the surface nanomechanical properties of biocompatible polyelectrolyte hydrogels with AFM for biomedical applications. The temperature effects on Sader‟s viscous model for multilayered microcantilevers immersed in an aqueous medium were experimentally verified as a preliminary work. Next, temperature dependence of the near-wall oscillation of microcantilevers submerged in an aqueous medium was investigated to explore the possibility of a near-wall thermometry sensor. By correlating the frequency response of a microcantilever immersed in an aqueous medium near a solid surface (within the width of a microcantilever) with the surrounding liquid temperature, the near-wall region microscale temperature distributions at the probing site were successfully determined. For biological applications, this work has been extended to examine the effect of adsorption-induced surface stress change on the stiffness of a microcantilever immersed in saline solution with varying salt concentrations. It was found that adsorption-induced surface stress change increased the stiffness of a microcantilever in saline solution with increasing salt concentration ranging from 0 to 2 molality. The surface nanomechanical properties of 2-hydroxyethyl methacrylate (HEMA) and 2-methacryloxyethyl trimethyl ammonium chloride (MAETAC) copolymer hydrogels were probed using AFM. The HEMA-MAETAC polyelectrolyte hydrogels with increasing positive charge concentrations ranging from 0 to 400 mM in increments of 40 mM, were fabricated using different proportions of HEMA and MAETAC monomers. Increasing proportions of positively charged MAETAC monomers produced hydrogels with increasingly swollen states and correspondingly decreasing measures of surface elasticity, or Young‟s modulus. The attachment of porcine pulmonary artery endothelial cells (PPAECs) increased with increasing prepared hydrogel charge concentration and subsequently decreasing surface elasticity. Keywords: Atomic Force Microscope (AFM), Microcantilever, Thermofluidic sensor, Hydrogel, Surface elasticity

Atomic Force Microscopy (AFM) has been combined with many optical microscopy techniques to expand the applications and capabilities of both techniques. However, the intense illumination of AFM cantilevers can give rise to a significant temperature increase due to the light power absorption on the gold coating, which is undesirable, especially for live cell imaging. Computational simulations based on Finite-Difference Time-Domain (FDTD) simulations for electromagnetic wave propagation and absorption combined with a finite element method implementation for thermal simulations were used to study the temperature increase of AFM cantilevers exposed to wide-field and confocal fluorescence illumination. Our analysis is consistent with a previous study based on a 2D model of a bimetallic strip. It is also observed a strong dependency of the temperature increase on the confocal spot vertical position. In some simulation’s conditions close to experimental set-up configurations we predict with our simulations a temperature increase up to 13 degrees Celcius, a temperature increase which undoubtedly will have an impact on live cell imaging. We conclude that temperature increases induced by fluorescence excitation light in combined AFM-fluorescence microscopy set-ups can be significant in the sense that they might affect the biological sample under investigation. A proper design of the combined microscopy system is therefore highly recommendable to avoid any potential sample heating artefact.