Time-resolved electrostatic force microscopy using tip-synchronized charge generation with pulsed laser excitation

Takuya Matsumoto,Yutaka Ie,Kento Araki,Hiroshi Ohoyama,Yoshio Aso

doi:10.1038/s42005-019-0108-x

Abstract

Nanoscale observation of charge distribution and electric polarization is crucial for understanding and controlling functional materials and devices. In particular, the importance of charge dynamics is well recognized, and direct methods to observe charge generation, transfer, and recombination processes are required. Here, we describe tip-synchronized time-resolved electrostatic force microscopy. Numerical modeling clarifies that the tip-synchronized method provides temporal resolution with the timescale of the cantilever oscillation cycle. This method enables us to resolve sub-microsecond charge migration on the surface. The recombination of photo-excited carriers in a bilayer organic photovoltaic thin film is observed as a movie with a 0.3 µs frame step time resolution. Analysis of the images shows that the carrier lifetime is 2.3 µs near the donor/acceptor interface. The tip-synchronized method increases the range of time-resolved electrostatic force microscopy, paving the way for studies of nanoscale charge dynamics.

Highlights

Nanoscale observation of charge distribution and electric polarization is crucial for understanding and controlling functional materials and devices
High-speed scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have provided real-time observations of crystal growth and biological molecular motion[8,9], but these observations are limited to the millisecond timescale
We show that the method is useful for observing charge dynamics, including charge relaxation at the donor–acceptor interface, and present a movie of a recombination process inside an organic photovoltaics (OPVs) film

Summary

Results

The time response of frequency shift mode EFM is usually limited by the bandwidth of the phase-locked loop (PLL) circuit To overcome this limitation, we developed tip-synchronized tr-EFM, which is a pump-probe method enabling us to detect microsecond lifetime local charges directly with the repetition rate of the cantilever vibration frequency. Fint(t) involves large variations in Fvdw(t) depending on the tip position in the scanning area This problem can be avoided by constant amplitude feedback under constant cantilever excitation energy conditions with self-oscillation. This setup keeps Fvdw(t) constant and gives conventional topography that is similar to usual tapping mode measurements, while simultaneously obtaining frequency shift information reflecting mainly attractive forces in the non-contact region.

V Sample

Discussion

Methods