Investigation of lateral forces in dynamic mode using combined AFM/STM

Ankara : The Department of Physics and The Institute of Engineering and Science of Bilkent University, 2007.Thesis (Ph.D.) -- Bilkent University, 2007.Includes bibliographical references leaves 114-126.In 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.Atabak, MehrdadPh.D

Noncontact lateral-force gradient measurement on Si(111)-7×7 surface with small-amplitude off-resonance atomic force microscopy

In this work, the authors report on a quantitative investigation of lateral-force gradient and lateral force between a tungsten tip and Si(111)-(7×7) surface using combined noncontact lateral-force microscopy and scanning tunneling microscopy. Simultaneous lateral-force gradient and scanning tunneling microscopy images of single and multiatomic step are obtained. In our measurement, tunnel current is used as feedback. The lateral-stiffness contrast has been observed to be 2.5 N/m at a single atomic step, in contrast to 13 N/m at a multiatomic step on Si (111) surface. They also carried out a series of lateral stiffness-distance spectroscopy, which show a sharp increase in tip-surface interaction stiffness as the sample is approached toward the surface

Effect of Smear Layer on the Push-Out Bond Strength of Two Different Compositions of White Mineral Trioxide Aggregate

Introduction: The aim of this in vitro study was to evaluate the effect of smear layer on the push-out bond strength of white mineral trioxide aggregate (WMTA) with and without disodium hydrogen phosphate (Na2HPO4). Materials and Methods: Dentin discs with standard cavities were obtained from extracted human single-rooted teeth and divided to 4 groups (n=15) according to the irrigation regimen and the canal filling material. In groups 1 and 3, canals were irrigated with normal saline; in groups 2 and 4, irrigation method included sodium hypochlorite (NaOCl) and then ethylenediaminetetra-acetic acid (EDTA). The canals were filled with WMTA in first and second groups and with WMTA+Na2HPO4; in groups 3 and 4. The samples were wrapped in wet gauze and incubated in 37°C for 3 days. The push-out bond strength was then measured by means of the Universal Testing Machine and the failure modes were examined under stereomicroscope at 40× magnification. Tow-way ANOVA was used to evaluate the effect of material type and smear layer removal. Post hoc Tukey test was used for the two-by-two comparison of the groups. Results: The greatest and lowest mean±standard deviation for push-out bond strength were observed in groups 4 (4.54±1.14 MPa) and 1 (1.44±0.96 MPa), respectively. The effect of removing the smear layer on the push-out bond strength of WMTA+Na2HPO4 was significant (P=0.01), but not for WMTA (P=0.52). Interestingly, there was significant difference between groups 1, 3 and 2, 4 (P<0.05). The failure mode for all experimental groups was of mixed type. Conclusion: Under circumstances of this in vitro study, removal of smear layer increases push-out bond strength when Na2HPO4 is added to WMTA

Ultra-small oscillation amplitude nc-AFM/STM imaging, force and dissipation spectroscopy of Si(100)(2 × 1)

Si(100)(2 × 1) surface is imaged using a new nc-AFM (non-contact atomic force microscopy)/STM with sub-Ångstrom oscillation amplitudes using stiff hand-made tungsten levers. Simultaneous force gradient and scanning tunneling microscopy images of individual dimers and atomic scale defects are obtained. We measured force-distance and dissipation-distance curves with different tips. Some of the tips show long-range force interactions, whereas some others resolve short-range interatomic force interactions. We observed that the tips showing short-range force interaction give atomic resolution in force gradient scans. This result suggests that short-range force interactions are responsible for atomic resolution in nc-AFM. We also observed an increase in the dissipation as the tip is approached closer to the surface, followed by an unexpected decrease as we pass the inflection point in the energy-distance curve. © 2002 Elsevier Science Ltd. All rights reserved

Measurement of energy dissipation between tungsten tip and Si(1 0 0)-( 2 × 1 ) using sub-Ångström oscillation amplitude non-contact atomic force microscope

Energy dissipation plays an important role in non-contact atomic force microscopy (nc-AFM), atomic manipulation and friction. In this work, we studied atomic scale energy dissipation between a tungsten tip and Si(1 0 0)-(2×1) surface. Dissipation measurements are performed with a high sensitivity nc-AFM using sub-Ångström oscillation amplitudes below resonance. We observed an increase in the dissipation as the tip is approached closer to the surface, followed by an unexpected decrease as we pass the inflection point in the energy–distance curve. This dissipation is most probably due to transformation of the kinetic energy of the tip into phonons and heat

Simultaneous non-contact atomic force microscopy (nc-AFM)/STM imaging and force spectroscopy of Si(1 0 0)(2 × 1) with small oscillation amplitudes

Si(1 0 0)(2 x 1) surface is imaged using a new non-contact atomic force microscopy (nc-AFM)/STM with sub-Angstrom oscillation amplitudes using stiff tungsten levers. Simultaneous force gradient and STM images of individual dimers and atomic scale defects are obtained. We measured force-distance (f-d) curves with different tips. Some of the tips show long force interactions, whereas some others resolve short-range interatomic force interactions. We observed that the tips showing short-range force interaction give atomic resolution in force gradient scans. This result suggests that short-range force interactions are responsible for atomic resolution in nc-AFM

Sub-Angstrom oscillation amplitude non-contact atomic force microscopy for lateral force gradient measurement

We report the first results from novel sub-Angstrom oscillation amplitude non-contact atomic force microscopy developed for lateral force gradient measurements. Quantitative lateral force gradients between a tungsten tip and Si(1 1 1)-(7 x 7) surface can be measured using this microscope. Simultaneous lateral force gradient and scanning tunnelling microscope images of single and multi atomic steps are obtained. In our measurement, tunnel current is used as feedback. The lateral stiffness contrast has been observed to be 2.5 N/mat single atomic step, in contrast to 13 N/m at multi atomic step on Si(1 1 1) surface. We also carried out a series of lateral stiffness-distance spectroscopy. We observed lateral stiffness-distance curves exhibit sharp increase in the stiffness as the sample is approached towards the surface. We usually observed positive stiffness and sometimes going into slightly negative region