Tip-sample Interaction Force Research Articles

Nanoguitar: Oscillating string as force sensor

A force sensor is presented that makes use of an oscillating string. In a conventional scanning force microscopy setup comprising a tip and a sample consisting of a flexible cantilever beam, a string is attached to the free end of the beam along its deflection axis. Changes of the tip–sample interaction force modify the string tension and hence the resonance frequency of transverse oscillations. These oscillations can have amplitudes of the order of microns without causing noticeable wavering of the cantilever beam. This kind of sensor is particularly suitable for applications that require a stiff sensor. A prototype was built using a carbon fiber 5 μm in diameter and 4 mm in length, oscillating at 4 kHz. A force resolution of 2.5 nN was achieved in vacuum for a response time of 1 ms and a sensor stiffness of 160 N/m. For a conventional beam deflection sensor of equal stiffness this corresponds to a deflection sensitivity of 0.015 nm.

On the nature of nanometer-scale ring structures in the scanning tunneling microscopy images of tungsten diselenide WSe2

Scanning tunneling microscopy (SIM) measurements of tungsten diselenide at a large resistance gap show bright image imperfections on a nanometer scale. These bright spots are observed to be reversibly converted into ring structures as the resistance gap is decreased. This phenomenon was explained on the basis of the trapped electrons around donor dopant atoms and the tip-sample force interactions.

Combined scanning tunneling and force microscopy

A combined scanning tunneling and force microscope (STM/FM) was built which allows for the simultaneous measurement of forces and force gradients along STM constant current contours. Measuring Au(111) surfaces with this combined STM/FM, very low tip–sample interaction forces and variations of the local stiffness (force gradient) in the vicinity of single adsorbates and domain boundaries of the Au(111) (√3×23) surface reconstruction were found. On Au(111) substrates covered by a compact thin organic film the interaction force is higher, indicating repulsive contact between tip and sample. An elastic theory for the deconvolution of real topography from local electronic and sample deformation effects on STM constant current contours of soft samples was developed.

Tip–sample interaction forces in scanning tunneling microscopy: Effects of contaminants

We have made simultaneous measurements of the tunnel current and the force of interaction as a gold-coated cantilever approaches towards and recedes from a gold sample in atmosphere. We will describe the effects arising from the surface contaminant layer. We also have made an estimation of the minimum value of the contact (repulsive) force that should be present for tunneling between gold and gold in air to be observed.

Direct force measurement in scanning tunneling microscopy

A novel force measurement using a scanning tunneling microscope as a forced oscillator is described. Results obtained from tunneling between a tungsten tip and a graphite substrate show that a maximum tip-sample force about 10−6 N exists during the constant current mode of operation. These results are in agreement with a previous model where large contact areas insulated by contaminants between tip and substrate were suggested as a cause of large tip-sample interaction forces. This method can achieve a force sensitivity of 10−8 N and for conductive substrates provide a simple, versatile alternative to existing methods of atomic force microscopy.