Electrostatic Force Microscopy with the Nano-RT AFM

The Electrostatic Force Microscope is a modified AFM where the cantilever is connected to an independently controlled bias. The bias is used to create an electrostatic field between the tip and the substrate. It is intuitively obvious that the size and form of the tip is expected to be a crucial factor limiting the detection and observation of images. However in previously published theoretical studies of EFM (in all its forms), the tip characteristics do not appear explicitly in the imaging process. In an alternative, reciprocal, approach to understanding force microscopy originally applied to magnetic force microscopy (MFM) systems, the role of the tip is quite obvious. The high-resolution capability makes the EFM a powerful tool for characterizing electronic nanosystems.

EFM can be used to distinguish conductive and insulating regions in a sample. There are two different types of measurements that an AFM can perform in EFM mode. The first images electric field gradients on the surface of the sample, and the second images the surface potential of the sample. The electric field gradient method measures the changes in the internal electric field of a sample by monitoring the force on the scanning tip. In this case, no feedback is required and a constant voltage is maintained on the tip. As the tip moves over an attractive electric field gradient, it is pulled toward the sample. When the tip traverses a repulsive gradient, it is pushed away from the sample. Both these effects can clearly be observed by measuring the deflection of the cantilever as it is scanned at some height above the surface.

The surface potential method measures the surface voltage on the sample by adjusting the voltage on the tip. In order to maintain feedback, the applied voltage on the cantilever is adjusted such that a constant amplitude or deflection is maintained. Although the Nano-R™ is capable of imaging with the surface potential method by using LabView, for simplicity all these studies were conducted in the field gradient mode.

The method is similar to MFM. Images can be collected in DC (contact mode) by recording the deflection of the cantilever or by AC mode (close contact mode) where the cantilever is oscillated above the surface and either the phase or amplitude of the cantilever is recorded. In this work we chose to do DC imaging. However, there is no reason that AC imaging cannot be performed and by following the MFM application note this can easily be done.