Measurement of the surface potential and work functions of materials are important in analyzing their electronic structures and surface residual charges. Initially, the concept of macroscopic work function measurement was pioneered by Lord Kelvin in 1898. Innovations in the atomic force microscope led to the invention of Kelvin probe force microscopy (KPFM) in the 1990s. KPFM has since become a popular tool for nanoscale electrical characterizations of materials. In conventional KPFM techniques, an AC voltage is applied between the AFM probe and the sample. Differences in the work functions between the probe and the sample result in a measurable electrostatic force which is then nullified to provide the surface potential image. The requirement of an AC voltage intrinsically limits the spatial resolution to between 30-100 nm in ambient conditions due to the stray-capacitance effect.To address the limited spatial resolution, we have invented Pulsed Force Kelvin Probe Force Microscopy (PF-KPFM) in 2019. PF-KPFM is a robust technique based on the pulsed force mode of AFM, and can routinely achieve < 10 nm spatial resolution images in ambient conditions on a range of samples.
The operational principle of PF-KPFM deviates from the measurement paradigm of conventional KPFM techniques. By using a field-effect transistor (FET), electrical contact between the probe and the sample is achieved. Spatially separated charges, which form through Fermi-level alignment, result in Coulombically-driven cantilever oscillations, which are used as the basis for the real-time surface potential measurements. Thus, the need for an AC voltage is removed, effectively eliminating the stray-capacitance effect. Furthermore, the electrical connections are temporally synchronized to when the probe is only a few nanometers away from the sample surface, further increasing the spatial resolution.PF-KPFM offers the following improvements over conventional KPFMs:
PF-KPFM avoids the need for AC voltages between the probe and the sample,PF-KPFM minimizes the stray-capacitance effect,PF-KPFM is a single-pass technique, in which the AFM is always under feedback,PF-KPFM delivers an improved spatial resolution (<10 nm) in ambient conditions.