Browsing by Subject "Atomic force microscopy."
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Item Open Access Atomic scale investigation of clean and epi-grown Si(001) surfaces using scanning tunneling microscopy(Bilkent University, 1996) Özer, H. ÖzgürIn this thesis, clean and epi-grown Si(001)(2x1)surfaces are analyzed by Scanning Tunneling Microscopy (STM). The STM and Ultra High Vacuum System (UHV) in which the microscope is installed, are described. A brief history of the studies on the reconstruction and fundamental features of the Si(001) surface is also given. First, the sample and tip preparation techniques were optimized. Sample preparation method, which includes both ex situ chemical and in situ heating cleaning procedures, was found not to give routinely the clean and atomically flat surfaces, because of the criticality of the temperature values used during heat treatments. The monoatomic steps, dimer rows, defects such as missing dimer and dimer groups, were observed on clean Si(001) surfaces. Double height step formation due to contamination was also detected on a few samples. Buckling of dimers, which is believed to be due mainly to either the high defect density or tip-surface interaction, was observed on one sample. Si and Ge were grown epitaxially on the silicon substrate, with 0.11 ML and 3.2 ML coverages, respectively. The Si growth on Si(001) was found to occur as island formation because of the low substrate temperature (ca. 300 degrees C). Strong shape anisotropy and diffusional anistropy in the growth have been observed. On the other hand, the large coverage of Ge on Si(001) at a relatively high substrate temperature (ca. 500 degrees C) resulted in step flow growth rather than individual island formation on the terraces.Item Open Access Construction of an atomic force microscope operating in air and liquid(Bilkent University, 2003) Karaköse, MustafaIn this thesis, a new atomic force microscope capable of measuring in liquid and air has been introduced. The highly improved design of the instrument eliminates some of the technical problems which are occurred before. The instrument makes use of the small amplitude AC-Mode technique to detect the interaction forces between the tip and the surface. Some results of initial test scans of the instrument are displayed.Item Open Access Design of driver electronics for 32 cantilevers in atomic force microscopy(Bilkent University, 2001) Balantekin, MüjdatItem Open Access Force spectroscopy using bimodal atomic force microscopy(Bilkent University, 2010) Aksoy, Mehmet DenizIn atomic force microscopy (AFM) achieving compositional contrast while mapping topographical features is a challenging task. Conventional single mode frequency and amplitude modulation AFM techniques are sensitive to the properties of the tip sample interaction, however in the absence of additional information channels, compositional features such as elasticity and density cannot be distinguished from topographical variations. To tackle this problem bimodal excitation techniques are introduced. In bimodal amplitude modulation AFM, sensitivity to compositional features improves by recording the phase of the higher order vibrations, while the topography is acquired using the amplitude of the first order vibrations. Increased sensitivity to mechanical properties allows imaging delicate samples such as organic molecules using gentle forces. In this thesis we propose a force spectroscopy technique in which two modes of a cantilever are excited in such a way that the amplitudes of the components of the vibration stay constant. Presence of the force field modulates the properties of the primarily bi-harmonic vibration of the cantilever, which is, in our case, the instantaneous frequencies of vibration modes. The frequency shift of the first mode remains sensitive to topographical variation, whereas the frequency shift of the higher mode samples the gradient of the tip sample forces and allows us to extract the tip sample interaction as a function of separation within a single cycle of the slow oscillation. We provide an analytic treatment of the proposed scheme and confirm our predictions by numerical simulations. We present an analysis of the sensitivity of higher mode frequency shifts to compositional features in the presence of thermal and sensor noise. We demonstrate that the method is suitable for the fast acquisition of contact properties, especially in vacuum environment where the large quality factor of the cantilever limits the available bandwidth of the amplitude modulation techniques. Finally we investigate phase shifts in bimodal amplitude modulation AFM using the developed formalism and show that phase contrast can be optimized by solving a simpler problem in single mode amplitude modulation AFM.Item Open Access Investigation of lateral forces in dynamic mode using combined AFM/STM(Bilkent University, 2007) Atabak, MehrdadIn this Ph.D. work, we constructed a ¯ber optic interferometer based non-contact Atomic Force Microscope (nc-AFM) combined with Scanning Tunneling Micro- scope(STM) to study lateral force interactions on Si(111)-(77) surface. The in- terferometer has been built in such a way that its sensitivity surpasses that of the earlier versions used in normal force measurements. The improvement in the resolution of the interferometer has allowed us to use sub-Angstrom oscillation amplitudes to obtain quantitative lateral force measurements. We have observed single and double atomic steps on Si(111)-(77) surface in topography and lat- eral sti®ness images. This information allowed us to measure the lateral forces directly and quantitatively. We have also carried out lateral force-distance spec- troscopy experiments, in which we simultaneously measured the force gradient and tunneling current, as the sample is approached towards the tip. The lateral force?distance curves exhibit a sharp increase of the force gradient, just after the tunnel current starts to increase, while the sample is approaching to the tip. We observed only positive force gradients. In separate experiments, we imaged the Cu-TBPP molecules deposited on Cu(100) surface in normal and torsional mode in dynamic force microscope us- ing STM feedback, with a homemade tungsten cantilever. Our experiments have shown the possibility of manipulating molecules on surface using a vibrating can- tilever. However the forces involved in these experiments are not quantitatively measured due to limitations of the method.Item Open Access Nanomechanical characterization of materials by enhanced higher harmonics of a tapping cantilever(Bilkent University, 2005) Balantekin, MüjdatIn a tapping-mode atomic force microscope, the periodic interaction of the tip with the sample surface creates a tip-sample interaction force, and the pure sinusoidal motion of the cantilever is disturbed. Hence, the frequency spectrum of the oscillating cantilever contains higher harmonics at integer multiples of the excitation frequency. In this thesis, we utilize one of the higher harmonics of a vibrating cantilever to investigate the material properties at the nanoscale. We show analytically that the amplitudes of the higher harmonics increase monotonically for a range of sample stiffness, if the interaction is dominated by elastic force. We propose a method in which the cantilever is excited at a submultiple of its resonant frequency (w1/n) to enhance the nth harmonic. The numerical simulations are performed to obtain the response of the tip-sample system for the proposed method. The proposed method is modified to eliminate the chaotic system response observed in the very high harmonic distortion case. The experiments are carried out to see if the enhanced higher harmonic can discriminate the material variations in heterogeneous samples and to find how it is related to the topography changes on the homogeneous sample surfaces. We show that the enhanced higher harmonic can be utilized to map material heterogeneity in polymer blends with a very high signal-to-noise ratio. The surface features ca. 100 nm in size are clearly resolved. A comparison is also made to conventional tapping-mode topography and phase imagingItem Open Access Noise analysis of interdigital cantilevers for atomic force microscopy(Bilkent University, 1998) Yaralıoğlu, G. GökseninAtomic force microscoiDe (AFM) is proved to be a powerful tool for atomic resolution surface imaging. The most crucial parts of an AFM system are the cantilever with an integrated tip and the deflection detection sensor. AFM systems measure deflections that are comparable to atomic dimensions using technicpies such as tunneling, interferometry, piezoresistive sensing and optical lever detection. Interdigital (ID) cantilevers are the most recently introduced method which makes use of its interferometric nature to improve deflection detection sensitivity. Basicallj^ ID cantilever is composed of two sets of interleaving fingers which create an optical phase grating. In this thesis, a detailed analysis of ID cantilevers will be presented. The theory underlying the o[)eration of the phase gratings with the response curves curd confirming e.xperimental results will be formulated. The noise performance of the ID cantilever will be compared to the optical lever detection method. We will present a new method for the mechaniccd noise calculation by using the analogy between electrical circuits and mechanical structures. This new method will be applied to the AFM cantilevers to calculate the noise correlation on the cantilever surface. We will also present the signal to noise ratio (SNR) calculation method on the cantilever. One of the basic problem of the all AF'M systems is the speed limitation due to single AF'M tip scanning at relatively low frequencies yielding low throughput. A direct approach to this problem is the operation of cantilever arrays instead of one cantilever. In this thesis, we will also present the electronics for cantilever arrays which increases the throughput of the AFM systems.Item Open Access Novel techniques in multi-frequency atomic force microscopy and spectroscopy(Bilkent University, 2009) Abak, Musa KurtuluşThe capability of measuring material properties of nanostructures simultaneously with their size and shape is very desirable for characterization of novel materials and devices at the nanoscale Here we present two novel techniques for imaging and spectroscopy of mechanical and electrical properties of surface nanostructures simultaneously with topographic imaging. First we present a scanning probe technique that can be used to measure charging of localized states on conducting or partially insulating substrates at room temperature under ambient conditions. Electrostatic interactions in the presence of a charged particle between the tip and the sample is monitored by the second order flexural mode, while the fundamental mode is used for stabilizing the tip-sample separation. Cycling the bias voltage between two limits, it is possible to observe hysteresis of the second order mode amplitude due to charging. Results are presented on silicon nitride films containing silicon nanocrystals. Second we report use of nonlinear tip-sample interactions to convert the frequency components of periodic tip-sample interaction forces to frequencies where they can be resonantly detected by resonant heterodyne mixing. One flexural mode of a cantilever is used for tapping-mode imaging and another flexural mode is used for detection of forces converted in presence of an externally injected mechanical oscillation at the difference frequency of the detecting mode and a harmonic of the tapping mode. Material contrast in attractive and repulsive regimes are demonstrated on samples with polymethyl methacrylate patterns and with deoxyribonucleic acid strands on silicon. The techniques can be implemented using standard force microscopy systemsand cantilevers, which make them potentially useful to a greater scientific community.Item Open Access Simulation of steady-state response of tip-sample interaction for a torsional cantilever in tapping mode atomic force microscopy for material characterization in nanoscale(Bilkent University, 2010) Selvi, Şeref BurakDynamic atomic force microscopy (AFM) techniques involving multifrequency excitation or detection schemes offer improved compositional sensitivity and quantitative material property imaging. A correct interpretation of cantilever vibrations in multifrequency excitation and detection schemes demands an improved understanding of the effects of enhanced high frequency vibrations on the steady-state dynamics of the cantilever and in particular, on the tip-sample interaction force. In this thesis, a simulation background is developed with proper modelling of tip-sample ensemble for accurate simulation of tip-sample interaction when multifrequency excitation and detection schemes are utilized. The simulation results are analyzed and used for material characterization. The tip-sample ensemble is modelled as a multiple degree of freedom system that includes torsional mode and higher order flexural modes of the cantilever. The nonlinear behavior of sample surface is also included in the model. This mechanical model is transformed into an electrical circuit and an electrical circuit simulator is used to find steady-state of the circuit. Thereby, a simulation of steady-state dynamics of multifrequency imaging schemes is achieved.Using the developed simulation tool, the effect of torsional vibrations and higher order flexural vibrations on steady-state of tip-sample interaction is investigated. The tip trajectory and tip-sample interaction force are calculated for torsional harmonic cantilevers. The potential of torsional harmonic cantilevers in reconstruction of tip-sample interaction force for the quantitative estimation of material properties is verified. Change in amplitude of torsional harmonics with respect to elastic modulus (sensitivity) is investigated. It is shown that sensitivity of a particular torsional harmonic changes with sample stiffness and higher harmonics are more sensitive to change in stiffness. Additionally, a noise analysis of torsional harmonic cantilevers is made and included in the simulations. The tip-sample interaction force is recovered from the simulated torsional vibration signal and the effective elastic modulus of the sample is estimated. It is observed that accuracy of the estimation is affected by number of torsional harmonics used in the recovery of interaction force.