KPFM

KPFM, or Kelvin Probe Force Microscopy, is a scanning probe technique that maps the surface potential or work function variations of a sample at nanometer-scale resolution. It extends atomic force microscopy (AFM) by measuring electrostatic interactions between a conductive tip and the sample, providing electrical information in addition to topography.

Principle and modes: A conductive AFM tip, biased relative to the sample, experiences an electrostatic force that depends on the contact potential difference (CPD) between the tip and surface. An alternating voltage is applied to the tip, and a feedback loop adjusts a DC bias to cancel or minimize the electrostatic response at the modulation frequency. The DC bias that achieves this cancellation equals the negative local CPD, yielding a surface potential map when recorded across the surface. KPFM is commonly implemented in two variants: amplitude-modulation (AM-KPFM), which detects the electrostatic response in the first harmonic of the cantilever signal, and frequency-modulation (FM-KPFM), which derives CPD from shifts in the cantilever resonance frequency, often with higher sensitivity and reduced cross-talk.

Measurement procedure: Experiments are typically conducted in two passes. The first pass records the surface topography. In a second pass, often in lift mode, the tip retraces the contour at a fixed height while the bias is swept to determine the CPD, allowing separation of electrostatic forces from short-range forces tied to topography.

Applications and limitations: KPFM is used to study work function variations, charge distribution, and electronic properties in semiconductors, photovoltaics, polymers, graphene and other two-dimensional materials, and nanoelectronic devices. Limitations include dependence on tip geometry and calibration for quantitative CPD, sensitivity to environmental conditions, and reduced spatial resolution governed by the tip radius and stray capacitances.