New Scanning Microscopy Techniques: Scanning Noise Microscopy Scanning Tunneling Microscopy Assisted by Surface Plasmons

Abstract

The present paper will be divided into two sections: In the first part the principle of the scanning noise microscopy i.e. the combination with potentiometry will be discussed. The second part will describe the investigation of the interaction of surface plasmons with the tunneling process of electrons.

The analysis of the current noise in scanning tunneling microscopy experiments reveals that at zero bias white thermal noise is observed. Its amplitude is given by the Nyquist formula for the tunneling resistance. Since the latter depends exponentially from the gap distance it is possible to operate a scanning tunneling microscope at exactly zero bias between sample and tunneling tip. The electronic feedback for the gap distance maintains a constant current noise instead of operating at constant direct current. Tests on a polycrystalline silver surface show that the obtained resolution is comparable to the one in conventional STM. As already a minor deviation from zero bias between tunneling tip and sample surface leads to a significant dc-component of the average current scanning noise microscopy can be very well combined with potentiometry. For any point on the sample the potential can be accurately measured by adjusting the potential such that the dc-component of the average tunneling current vanishes. The resolution obtained in our experiment is in the order of 1 µV.

The influence of surface plasmons excited in a polycrystalline silver film on the tunneling current of a scanning tunneling microscope has been analyzed. The plasmons cause an additional flow of electrons from the tungsten tip to the silver surface on the order of up to 50 pA. This process is independent of the polarity of the applied bias voltage, thereby excluding effects of thermal expansion. The different nature of the ordinary tunneling current and the surface plasmon induced current is clearly revealed by their different dependence on the gap distance. The local distribution of the intensity of the surface plasmon induced signal reveals structures on a nanometer scale.