Kelvin controller

The Kelvin probe module is an add-on module to either the OC4.5-Station or the full Nanonis control system (with OC4) and is intended for the mapping of surface charge distribution or contact potential difference (CPD) between tip and sample. Its fully digital implementation allows for maximum flexibility and does not require any additional wiring. Any channel can for instance be selected as input signal for either amplitude or frequency shift modulation modes. An internal lock-in demodulates the input signal and a PI-loop regulates the bias voltage to compensate for the change in contact potential. The modulating AC-component of the bias voltage, defined by the user, is internally added to the regulated DC bias voltage.

Generally, a change in the bias voltage results in a change of the force gradient between the tip and the sample due to electrostatic interactions (measured  through  the  frequency  shift  in  ncAFM). In a very good approximation this change is parabolic. The bias voltage at the maximum of the parabola, i.e. the point at which a change in bias does not influence the frequency shift, lies at the relative work function  between  tip  and  sample  material.  To  measure  this  work  function  (also  known  as  contact potential) is the scope of Kelvin probe microscopy. In  a  Kelvin  probe  experiment  the  bias  voltage  applied  between  tip  and  sample  is modulated  at  afrequency that is higher than the cut-off frequency of the z-feedback controller, i.e. an AC-component of a few kHz is added to the DC-offset of the bias. The response of the modulation on the frequencyshift is measured with a built-in fully digital lock-in detector. A separate PI-controller regulates the DC-offset such that the response of the frequency shift is zero, i.e. it is at the maximum of the parabola. Now the DC-offset is a direct measure of the work function/contact potential.

FM-Mode:

In the FM mode the bias is modulated at a frequency that is higher than the bandwidth of the  z-feedback,  but  not  commensurate with  the  oscillatory  motion  of  the  cantilever.  This  mode  is directly supported by the Nanonis Kelvin probe module.

AM-Mode:

In  the  AM  mode  the  bias  is  modulated  at  a  frequency  that  is  commensurate  with  the oscillatory  motion  of  the  cantilever, usually  at  a  higher  resonance  of  the  cantilever.  This  mode  is supported by the Nanonis Kelvin probe module but requires an dual-OC4 or an additional external lock-in detector capable of operating at the desired frequencies.

The Kelvin controller module is tightly integrated with the control system, especially the scan control, spectroscopy  and  z-feedback  module. The  KPFM  data  can  therefore  be  acquired, displayed and analysed in exactly the same way as all other data channels.