Browsing by Subject "Atomic force microscope"

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Open Access
Effects of thermal annealing on the morphology of the AlxGa(1x)N films
(Elsevier, 2010-01-08) Corekci, S.; Tekeli, Z.; Cakmak, M.; Ozcelik, S.; Dinc, Y.; Zeybek, O.; Özbay, Ekmel
Effects of thermal annealing on the morphology of the AlxGa(1-x)N films with two different high Al-contents (x=0.43 and 0.52) have been investigated by atomic force microscopy (AFM). The annealing treatments were performed in a nitrogen (N-2) gas ambient as short-time (4 min) and long-time (30 min). Firstly, the films were annealed as short-time in the range of 800-950 degrees C in steps of 50-100 degrees C. The surface root-mean-square (rms) roughness of the films reduced with increasing temperature at short-time annealing (up to 900 degrees C), while their surface morphologies were not changed. At the same time, the degradation appeared on the surface of the film with lower Al-content after 950 degrees C. Secondly, the Al0.43Ga0.57N film was annealed as long-time in the range of 1000-1200 degrees C in steps of 50 degrees C. The surface morphology and rms roughness of the film with increasing temperature up to 1150 degrees C did not significantly change. Above those temperatures, the surface morphology changed from step-flow to grain-like and the rms roughness significantly increased.
Open Access
High-resolution imaging of elastic properties using harmonic cantilevers
(Elsevier, 2004) Sahin, O.; Yaralioglu, G.; Grow, R.; Zappe, S. F.; Atalar, Abdullah; Quate, C.; Solgaard, O.
We present a micromachined scanning probe cantilever, in which a specific higher-order flexural mode is designed to be resonant at an exact integer multiple of the fundamental resonance frequency. We have fabricated such cantilevers by reducing the stiffness of the third order flexural mode relative to the fundamental mode, and we have demonstrated that these cantilevers enable sensing of non-linear mechanical interactions between the atomically sharp tip at the free end of the cantilever and a surface with unknown mechanical properties in tapping-mode atomic force microscopy. Images of surfaces with large topographical variations show that for such samples harmonic imaging has better resolution than standard tapping-mode imaging.
Open Access
Nanomechanical characterization of materials by enhanced higher harmonics of a tapping cantilever
(2005) Balantekin, Müjdat
In 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 imaging
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
(2010) Selvi, Şeref Burak
Dynamic 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.