H. Özgür Özer

Direct measurement of molecular stiffness and damping in confined water layers

We present direct and linear measurements of the normal stiffness and damping of a confined, few molecule thick water layer. The measurements were obtained by use of a small amplitude

Quantitative atom-resolved force gradient imaging using noncontact atomic force microscopy

(0.36\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})

Direct measurement of interatomic force gradients using an ultra-low-amplitude atomic force microscope

, off-resonance atomic force microscopy technique. We measured stiffness and damping oscillations revealing up to seven molecular layers separated by

Local force gradients on Si(111) during simultaneous scanning tunneling/atomic force microscopy

2.526\ifmmode\pm\else\textpm\fi{}0.482\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}

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

. Relaxation times could also be calculated and were found to indicate a significant slow-down of the dynamics of the system as the confining separation was reduced. We found that the dynamics of the system is determined not only by the interfacial pressure, but more significantly by solvation effects which depend on the exact separation of tip and surface. The dynamic forces reflect the layering of the water molecules close to the mica surface and are enhanced when the tip-surface spacing is equivalent to an integer multiple of the size of the water molecules. We were able to model these results by starting from the simple assumption that the relaxation time depends linearly on the film stiffness.

Dissipation Imaging with Low Amplitude off-Resonance Atomic Force Microscopy

~Received 4 April 2001; accepted for publication 11 June 2001! Quantitative force gradient images are obtained using a sub-angstrom amplitude, off-resonance lever oscillation method during scanning tunneling microscopy imaging. We report the direct observation of short-range bonds, and the measured short-range force interaction agrees well in magnitude and length scale with theoretical predictions for single bonds. Atomic resolution is shown to be associated with the presence of a prominent short-range contribution to the total force interaction. It is shown that the background longer-range interaction, whose relative magnitude depends on the tip structure, has a significant effect on the contrast observed at the atomic scale.

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

Interatomic force gradients between a W tip and a 7×7 reconstructed Si(111) surface were measured using an off–resonance, ultra–low–amplitude atomic force microscope (AFM) technique. The amplitudes used were less than 1 Å (peak–to–peak), which allowed direct measurement of the interaction force gradients as a function of separation. The force gradient curves are shown to consist of an attractive van der Waals part and short–range attractive and repulsive interactions. The van der Waals background can be subtracted, leaving a short–range interaction with an energy parameter of 1.9–3.4 eV and an interaction length–scale of 0.54–1.26 Å, characteristic of a single atomic bond. This correlates well with our observation of single–atom resolved force gradient images. In general, the interaction is reversible up to the zero intercept of the force gradient (inflection point of the energy). Beyond this point hysteresis tends to be observed and the onset of inelastic deformation can be clearly discerned. An analysis of the atomic scale contact gives reasonable values for the interfacial energy, yield strength, and the energy per atom needed to initiate plastic deformation.

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

The authors report simultaneous scanning tunneling and force imaging of Si(111) 7×7 with sub-angstrom oscillation amplitudes. Both constant height and constant current scans with tungsten tips/levers always showed larger attractive stiffness over corner holes than over adatoms, the opposite of theoretical expectations. Constant height scans show that this cannot be explained by interaction of tip motion with long range forces. Silicon levers, however, sometimes exhibited inversions of force contrast following local tip changes. The authors suggest that there may be charge variations between atomic sites on the surface, which produce electrostatic tip forces additional to the covalent forces usually regarded as dominant.

Nanomechanics Using an Ultra-Small Amplitude AFM

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-Angstrom 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. # 2003 Elsevier Science B.V. All rights reserved. PACS: 61.16.Ch

Radiation pressure excitation of a low temperature atomic force/magnetic force microscope for imaging in 4-300 K temperature range

A small amplitude non-contact atomic force microscope/scanning tunnelling microscope (nc-AFM/STM) is used to study dissipative interactions at atomic resolution on Cu(100) and Si(111) surfaces. For Cu(100) atomic resolution images of phase contrast are obtained, showing energy dissipation as high as 100 meV/cycle at each atomic site during constant tunnel current scans. In contrast, the Si(111) 7×7 surface usually did not exhibit significant phase contrast during normal STM operating conditions. However, when the driving oscillation frequency was set to a sub-harmonic of the lever resonant frequency, atomic contrast in phase could be readily observed. We believe this harmonic coupling is due to the nonlinearity of the tip-sample interaction, and at these frequencies part of the energy is dissipated via the lever Q.