Electrical Modes for AFM

Electric Force Microscopy

EFM oscillates a conducting AFM tip to sense electric force gradients.  Usually, this is done in 2 passes – one to measure the topography in a standard dynamic mode and the second to “lift” a set amount above the recorded profile to measure electric field strength.  Using the phase signal, a map of the gradient of the electric field is created.

Schematic of EFM operation in lift mode
Schematic of EFM operation in lift mode with a second pass at a defined height above the surface to sense electric field strength

Kelvin Probe Force Microscopy

Like EFM, KFM also is an oscillating technique that requires a conducting probe. Kelvin Probe Force Microscopy (KFM) mode measures the work function of the surface.  It is also known as surface potential microscopy.

This mode can be done as a lift technique (dual pass) or as a single-pass method. The single-pass method, referred to as High Definition KFM (HD KFM), offers higher sensitivity for measuring the surface potential plus higher spatial resolution. HD KFM requires a second lock-in amplifier as shown below.  Because the single pass does not require tip to “lift” above the surface, resolution of the electrical signal is much stronger and provides the added resolution.

Schematic of Single Pass Kelvin Probe Microscopy
Single Pass Kelvin Probe Microscopy

Conductive AFM

Conducting AFM, or Scanning Spreadding Resistance (SSR), is used to image a material in contact mode with a conductive AFM tip.  By biasing the tip (or sample) the current is measured passing from one to the other and a map ofthe conductivity (or inversely, resistivity) can be obtained.

Conducting mode AFM measuring the current between tip and sample
Conducting mode AFM measuring current between tip and sample

Piezoresponse Force Microscopy

Piezoresponse Force Microscopy (PFM) mode is a Contact mode technique that maps piezoelectric domains. PFM uses the inverse piezoelectric effect by applying a voltage to use the resulting deformation in the material to map the piezoelectrical properties of a surface. This method requires a conductive probe and an applied AC signal. The amplitude and phase signals are used to measure piezoelectric orientation.

Schematic of Piezoresponse force microscopy
Piezoresponse force microscopy schematic