Kelvin Probe Force Microscopy (KPFM / KFM)

Kelvin Probe Force Microscopy (KPFM), also called Kelvin Force Microscopy (KFM), is an AFM-based technique that maps the surface potential and work function of materials at the nanoscale. A conductive AFM tip is brought close to the sample surface, and an AC voltage is applied to the tip. A feedback loop measures and nullifies the electrostatic force between tip and sample, directly yielding the contact potential difference (VCPD) — the local surface potential. KPFM provides simultaneous topography and surface potential maps, making it essential for semiconductor, photovoltaic, and 2D materials research.

KPFM is used across a wide range of nanoscale research applications: (1) Semiconductor characterization — mapping dopant distributions, p-n junctions, charge trapping, and device interfaces; (2) Photovoltaics — evaluating surface potential and built-in electric fields in perovskite, organic, and thin-film solar cells to optimize device efficiency; (3) 2D materials — analyzing work function variations in graphene, MoS₂, hBN, and other van der Waals materials; (4) Organic electronics — studying charge injection barriers and carrier transport in organic semiconductor films; (5) Corrosion science — detecting surface oxidation and electrochemical potential variations at the nanoscale.

In dual-pass (lift mode) KFM, the AFM tip scans each line twice: first at the surface to record topography, then at a fixed lift height (typically 20–100 nm) to record surface potential. This separation reduces topographic crosstalk but the increased tip-sample distance degrades electrostatic sensitivity and spatial resolution. In single-pass KFM, topography and surface potential are recorded simultaneously in a single scan using bimodal or multi-frequency excitation. Single-pass KFM offers higher sensitivity (shorter tip-sample distance), better spatial resolution, and faster acquisition. CSInstruments' HD-KFM III implements an advanced single-pass mode operating at 0.1–0.5 nm effective tip-sample distance, delivering 100–10,000× greater electrostatic sensitivity than standard lift-mode KFM.

Work function is the minimum energy required to remove an electron from the surface of a material to vacuum. In KPFM, the measured quantity is the contact potential difference (VCPD) between the AFM tip and sample: VCPD = (φ_tip − φ_sample) / e, where φ is the work function and e is the electron charge. By calibrating the tip work function using a known reference material (e.g., HOPG or Au(111)), absolute work function values of the sample can be extracted. Work function mapping reveals local variations in surface composition, doping level, oxidation state, and charge distribution — information not accessible by standard topographic AFM.

The spatial resolution of KPFM depends on the measurement mode, tip geometry, and tip-sample distance. Standard lift-mode KFM typically achieves 20–50 nm lateral resolution due to the large tip-sample distance (20–100 nm lift height). Single-pass KPFM achieves significantly better resolution — typically 5–20 nm — because measurements are performed at minimal tip-sample distance. CSInstruments' HD-KFM III, operating in true single-pass mode with bimodal dual-frequency excitation at 0.1–0.5 nm tip-sample distance, achieves sub-10 nm lateral resolution and sub-millivolt potential sensitivity, enabling visualization of individual grain boundaries, monolayer steps, and nm-scale surface features in 2D materials and semiconductors.

Yes, KPFM is one of the most important techniques for perovskite solar cell research. It directly maps the built-in electric field distribution, surface potential variations across grain boundaries, trap state density, and local open-circuit voltage (Voc) variations at the nanoscale. KPFM illumination experiments — performed with and without light — reveal photogenerated carrier dynamics and charge separation efficiency at each layer interface (ETL/perovskite/HTL). CSInstruments' Nano-Observer II with HD-KFM III is specifically optimized for this application, offering glove-box compatibility for air-sensitive perovskite samples and optical coupling for illuminated KPFM measurements.

Both KPFM and EFM (Electrostatic Force Microscopy) use a conductive AFM tip to detect electrostatic interactions with the sample. The key difference is in quantification: EFM measures the phase shift or amplitude change caused by electrostatic forces, providing qualitative contrast of charge or potential differences. KPFM uses an additional feedback loop to null the electrostatic force by applying a DC bias to the tip, directly yielding the quantitative contact potential difference (VCPD) in millivolts. KPFM is therefore preferred when absolute surface potential values or work function mapping is needed, while EFM is used for faster qualitative screening of charged domains or dielectric contrasts.

CSInstruments offers KPFM measurements on the Nano-Observer II and Nano-Observer One Plus AFM systems. The standard KFM module uses dual-pass (lift mode) operation and is available on both systems. For researchers requiring maximum sensitivity and resolution, the HD-KFM III module — available exclusively on the Nano-Observer II — provides true single-pass operation with bimodal dual-frequency excitation, dC/dZ dielectric mapping, KFM Moiré measurements, and Electrical Field Compensation (EFC) for combined KPFM/MFM measurements. The Galaxy Dual Controller also enables HD-KFM on legacy Bruker Multimode, 5100, 5500, and 5600 AFM systems.

HD-KFM (High-Definition Kelvin Force Microscopy) is the single-pass KPFM mode of CSInstruments. It measures topography and surface potential in the same pass by exciting the second eigenmode of the cantilever electrically, with the tip 0.1–0.5 nm from the surface, which gives higher lateral resolution and sensitivity than lift-mode KPFM.

Single-pass HD-KFM can. Because the tip stays very close to the surface and the signal is amplified by the second eigenmode, it reveals charges and structures below the surface, for example carbon nanotubes embedded in a polymer.

dC/dZ measures how the tip–sample capacitance changes with distance. Recorded together with the surface potential in HD-KFM III, it maps dielectric properties and thin-film variations at the nanoscale.