Graduate Student Devon Jakob

"Pulsed Force Kelvin Probe Force Microscopy: A New Paradigm for Surface Potential Measurements"

In recent years, a global push towards carbon neutrality has driven the research and development of novel optoelectronic materials such as inorganic-organic lead-halide perovskites. Measurement of the contact potential difference (CPD) and work functions of these materials are crucial steps in the continued push for more efficient, highly stable perovskite materials and devices. Kelvin probe force microscopy (KPFM), an imaging technique based on atomic force microscopy (AFM), is a robust and popular tool for CPD and work function mapping at the nanoscale. However, the conventional KPFM variants are typically limited in their spatial resolution to between 30 – 100 nm under ambient conditions.(1) The continually decreasing size and increasing complexity of perovskite materials presents challenges in uncovering their important electrical properties through KPFM. In this talk, a recently developed KPFM technique based on the pulsed force mode of AFM is presented. The signal generation mechanism of PF-KPFM is based on an entirely unique paradigm which utilizes the intrinsic Fermi level alignment between the AFM tip and the sample without the need for an external oscillating voltage.(2) As a result, PF-KPFM avoids many of the intrinsic limitations associated with other KPFM variants which limit their achievable spatial resolution, most notably the stray capacitance effect and the requirement for lift mode.

During the talk, the operating principle of PF-KPFM is explained and compared to that of conventional KPFM variants. PF-KPFM is shown to be suitable for probing the interfaces between metals and semiconductors, for mapping individual ferroelectric domains, and imaging nanoscale perovskite domains with ~10 nm spatial resolution. From these measurements, new insights into the nanoscale electrical properties are established. Furthermore, integration of PF-KPFM with peak force infrared (PFIR) microscopy has been achieved to establish a high throughput measurement platform for simultaneous electrical, chemical, mechanical, and topographic maps, all at ~10 nm.(3) As a result, insights into the nanoscale relationships between physical properties can be explored in great detail. Lastly, the extension of the PF-KPFM signal-generation framework to the tapping mode of AFM is introduced, greatly increasing the accessibility of the PF-KPFM technique for super-resolution CPD mapping.(4)

1) Glatzel, T., Sadewasser, S. & Lux-Steiner, M. C. Amplitude or Frequency Modulation-Detection in Kelvin Probe Force Microscopy. Appl. Surf. Sci. 210, 84-89 (2003).

2) Jakob, D. S., Wang, H. & Xu, X. G. Pulsed Force Kelvin Probe Force Microscopy. ACS Nano 14, 4839-4848 (2020).

3) Jakob, D. S. et al. Peak Force Infrared–Kelvin Probe Force Microscopy. Angew. Chem. 132, 16217-16224 (2020).

4) Jakob, D. S., Li, N., Zhou, H., & Xu, X. G. Integrated Tapping Mode Kelvin Probe Force Microscopy with Photo-induced Force Microscopy for Correlative Chemical and Surface Potential Imaging. Under Review