Ultrafast AFM delivered
Image: Ultrafast mapping of ion migration induced by an electric field in a perovskite solar-cell device. [Liam Collins/ORNL]
US-based Oak Ridge National Laboratory researchers have developed a technique for making ultrafast measurements using atomic force microscopy, which previously could only investigate slow or static material structures and functions.
In AFM, a rastering probe maps a material's surface and captures physical and chemical properties; but the probe is slow to respond to what it detects.
As Dr Liam Collins from ORNL points out: "AFM offers unparalleled insight into structure and materials functionality across nanometre length scales, but the spatial resolution afforded by the AFM tip is counterpoised by slow detection speeds compared to other common microscopy techniques, such as optical and SEM."
Given this, Collins and colleagues developed an ultrafast AFM imaging approach - 'fast free force recovery' - which uses advanced machine learning algorithms to analyse instantaneous tip motion and overcome the mechanical bandwidth of the cantilever.
The method allows direct reconstruction of the tip-sample forces and enables time-resolved imaging at sub-bandwidth speeds.
According to the researchers, the method produces high-resolution images some 3,500 times faster than standard AFM detection methods.
Applying the method to Kelvin probe force microscopy, the researchers could explore ion migration in organonetallic halide perovskite materials and went on to image charged ion migration under an applied electric field.
"This new approach can probe fast processes, such as charge screening, ionic transport and electrochemical phenomena, which were previously inaccessible with traditional AFM," highlights Collins.
The researchers expect their approach to be valid for all modes of non-contact AFM operation.
Research is published in ACS Nano.