Kelvin Probe Force Microscopy
Kelvin probe force microscopy (Kelvin mode of Scanning Probe Microscopy) was invented for measuring contact potential difference between the probe and the sample . At present time Kelvin mode is based on the two-pass technique. In the first pass the topography is acquired using standard Semicontact mode (mechanical excitation of the cantilever). In the second pass this topography is retraced at a set lift height from the sample surface to detect the electric surface potential Ф(x). During this second pass the cantilever is no longer excited mechanically but electrically by applying to the tip the voltage Vtip containing dc and ac components
Vtip = Vdc + Vac sin(wt)
The resulting capacitive force Fcap between the tip and a surface at potential Vs is
Fcap =(1/2) (Vtip - Ф(x))2(dC/dz)
where C(z) is the tip-surface capacitance. The first harmonic force
Fcap w = (dC/dz (Vdc - Ф(x)Vac)sin(wt)
leads to suitable cantilever oscillations. The feedback then changes the dc tip potential Vdc until the w component of the cantilever (and accordingly w component of the tip-force) vanishes, e.g. Vdc(x) became equal to Ф(x). So mapping Vdc(x) reflects distribution of the surface potential along the sample surface. If no special tip-sample bias voltage is applied this distribution is Contact Potential Difference distribution.
- Appl. Phys. Lett. 58, 2921 (1991).