Othmar Marti

An atomic-resolution atomic-force microscope implemented using an optical lever

We present the first atomic‐resolution image of a surface obtained with an optical implementation of the atomic‐force microscope (AFM). The native oxide on silicon was imaged with atomic resolution, and ≊5‐nm resolution images of aluminum, mechanically ground iron, and corroded stainless steel were obtained. The relative merits of an optical implementation of the AFM as opposed to a tunneling implementation are discussed.

Atomic Force Microscopy of Liquid-Covered Surfaces: Atomic Resolution Images.

Images of graphite surfaces that are covered with oil reveal the hexagonal rings of carbon atoms. Images of a sodium chloride surface, protected from moisture by oil, exhibit a monoatomic step. Together, these images demonstrate the potential of atomic force microscopy (AFM) for studying both conducting and nonconducting surfaces, even surfaces covered with liquids. Our AFM uses a cross of double wires with an attached diamond stylus as a force sensor. The force constant is ≊40 N/m. The resonant frequency is ≊3 kHz. The lateral and vertical resolutions are 0.15 nm and 5 pm.

The simultaneous measurement of elastic, electrostatic and adhesive properties by scanning force microscopy: pulsed-force mode operation

We describe the pulsed-force mode, a new measuring mode for the scanning force microscope to image elastic, electrostatic and adhesive properties simultaneously with topography. The pulsed-force mode reduces lateral shear forces between the tip and the sample. Even very delicate samples can be mapped at high lateral resolution with full control over the force applied to the sample. The achieved scanning speed is comparable to that in contact-mode operation. The pulsed-force mode electronics can easily be added to many microscopes without much alteration of the original set-up. No change of the data acquisition software or of the feedback circuit is necessary.

Combined scanning force and friction microscopy of mica

A scanning force microscope using the optical lever detection method was modified to measure simultaneously the force normal to the sample surface and the friction force arising from scanning. The bending of sheet-like cantilevers is used to detect the normal force whereas the twisting of the same cantilever measures the friction force. The two effects cause, to first order, orthogonal deflections of the light beam and can therefore be measured simultaneously and independently. The relationship between normal and frictional forces and the resulting deflection angles is discussed. The authors present constant-force topographs and friction images of the surface unit-cell structure of mica and of single-layer steps on mica.

Regulation of a microcantilever response by force feedback

A feedback mechanism is used to control the forces incident on a mechanical microcantilever as a function of the monitored cantilever motion. The control is effected by modifying the intensity of an auxiliary laser beam that generates a thermally induced stress. The feedback is designed to reduce the effective resonance quality factor of the cantilever. The resultant regulation of the cantilever motion is shown to improve the measurement dynamics in atomic force microscopy, without significantly degrading the signal to noise ratio.

Laser Fabrication of Large-Scale Nanoparticle Arrays for Sensing Applications

A novel method for high-speed fabrication of large scale periodic arrays of nanoparticles (diameters 40-200 nm) is developed. This method is based on a combination of nanosphere lithography and laser-induced transfer. Fabricated spherical nanoparticles are partially embedded into a polymer substrate. They are arranged into a hexagonal array and can be used for sensing applications. An optical sensor with the sensitivity of 365 nm/RIU and the figure of merit of 21.5 in the visible spectral range is demonstrated.

Tunneling microscopy, lithography, and surface diffusion on an easily prepared, atomically flat gold surface

We show that a gold surface with atomically flat terraces as large as (150 nm)2 can be easily prepared in air by melting a gold wire with an oxyacetylene torch. Features with characteristic dimensions as low as 10 nm can be written and observed on these terraces with a scanning tunneling microscope. The features are appreciably distorted by diffusion within an hour.

Scanning tunneling microscope combined with a scanning electron microscope

We have developed a small scanning tunneling microscope (STM) to be incorporated into a scanning electron microscope (SEM). Vibration isolation and damping is achieved solely with Viton dampers. As a stand-alone unit, a tunnel-gap stability of about 1 A is reached at atmospheric air pressure without additional sound protection. Stability improves by at least an order of magnitude when incorporated into a SEM.

Pulsed force mode : a new method for the investigation of surface properties

Scanning force microscopy is extended by the pulsed force mode from simple imaging of topography to measuring elastic, electrostatic and adhesive sample properties. Lateral forces are virtually eliminated so that mapping of delicate samples with high resolution in air and fluids is easily possible. Scanning speed is comparable to that in contact mode. The new opportunities for scanning force microscopy given by the pulsed force mode is demonstrated in selected applications.

Forces affecting the substrate in resonant tapping force microscopy

We propose a simple model to describe the interaction of a forced cantilever oscillation with a specimen in a tapping-mode scanning force microscope experiment in order to make a rough estimation of the forces affecting the surface with each touch down of the tip. Assuming weak damping of the cantilever (quality factor of the cantilever between 100 and 1000) and of the surface, we can estimate the forces to be in the range of those in the contact mode. These forces can vary by orders of magnitude, e.g. 10-6 to 10-11 N. To reduce the interaction force we suggest scanning on the low-frequency side of the resonance frequency of the non-contact cantilever oscillation. Increasing the difference of phase between the non-contact oscillation of the cantilever in air and the oscillation during contact introduces strong variations of the force. The improvement in resolution which can be achieved for soft samples by using the tapping-mode system results from the elimination of shear forces and the possibility of minimizing the force on the surface by varying the set-point of the scanning amplitude. Forces on the substrate will be enhanced by a large substrate stiffness.