Non-contact lateral force microscopy.

The goal of atomic force microscopy (AFM) is to measure the short-range forces that act between the tip and the surface. The signal recorded, however, includes long-range forces that are often an unwanted background. Lateral force microscopy (LFM) is a branch of AFM in which a component of force perpendicular to the surface normal is measured. If we consider the interaction between tip and sample in terms of forces, which have both direction and magnitude, then we can make a very simple yet profound observation: over a flat surface, long-range forces that do not yield topographic contrast have no lateral component. Short-range interactions, on the other hand, do. Although contact-mode is the most common LFM technique, true non-contact AFM techniques can be applied to perform LFM without the tip depressing upon the sample. Non-contact lateral force microscopy (nc-LFM) is therefore ideal to study short-range forces of interest. One of the first applications of nc-LFM was the study of non-contact friction. A similar setup is used in magnetic resonance force microscopy to detect spin flipping. More recently, nc-LFM has been used as a true microscopy technique to systems unsuitable for normal force microscopy.

Intramolecular Force Contrast and Dynamic Current-Distance Measurements at Room Temperature.

Scanning probe microscopy can be used to probe the internal atomic structure of flat organic molecules. This technique requires an unreactive tip and has, until now, been demonstrated only at liquid helium and liquid nitrogen temperatures. We demonstrate intramolecular and intermolecular force contrast at room temperature on PTCDA molecules adsorbed on a Ag/Si(111)-(√[3]×√[3]) surface. The oscillating force sensor allows us to dynamically measure the vertical decay constant of the tunneling current. The precision of this method is increased by quantifying the transimpedance of the current to voltage converter and accounting for the tip oscillation. This measurement yields a clear contrast between neighboring molecules, which we attribute to the different charge states.

Atomic structure affects the directional dependence of friction.

Friction between two objects can be understood by the making, stretching, and breaking of thousands of atomic-scale asperities. We have probed single atoms in a nonisotropic surface [the H-terminated Si(100) surface] with a lateral force microscope operating in noncontact mode. We show that these forces are measurably different, depending upon the direction. Experimentally, these differences are observable in both the line profiles and the maximum stiffnesses. Density functional theory calculations show a concerted motion of the whole Si dimer during the tip-sample interaction. These results demonstrate that on an asperity-by-asperity basis, the surface atomic structure plays a strong role in the directional dependence of friction.

Phantom force induced by tunneling current: a characterization on Si(111).

Simultaneous measurements of tunneling current and atomic forces provide complementary atomic-scale data of the electronic and structural properties of surfaces and adsorbates. With these data, we characterize a strong impact of the tunneling current on the measured force on samples with limited conductivity. The effect is a lowering of the effective gap voltage through sample resistance which in turn lowers the electrostatic attraction, resulting in an apparently repulsive force. This effect is expected to occur on other low-conductance samples, such as adsorbed molecules, and to strongly affect Kelvin probe measurements when tunneling occurs.

Interstitial ultrasound heating applicator for MR-guided thermal therapy.

The ability to control the shape of thermal coagulation was investigated for various interstitial heating applicators incorporating planar transducers and device rotation. Magnetic-resonance-compatible interstitial ultrasound applicators were constructed and the effects of ultrasound power, frequency, scan rate and heating time on lesion radius were studied in heating experiments in excised liver tissue. Continuous thermal lesions were generated by scanning heating applicators over a 180 angular sector. The region of thermal coagulation was restricted to the prescribed sector. Lesion radius increased with acoustic power and heating time and decreased with increasing frequency. The relationship between the temperature distribution generated by the applicator and the resulting thermal lesion was assessed with MRI. Analysis of MR temperature maps revealed that the temperature distribution could be measured accurately within 2 mm from the surface of the applicator, and the boundary of thermal coagulation was defined by a temperature of 54 +/- 12 degrees C. Calculations of temperature distributions indicated that slower scan rates can overcome the tendency of perfusion to reduce the radius of thermal lesion. This applicator design and delivery strategy make conformal interstitial heating possible.