Video-rate atomic force microscopy has been of significant interest in recent years as an enabling tool for observing dynamic processes at the nanoscale in a myriad of fields. The acquisition of conventional atomic force microscopy images is a slow process with the majority of commercial AFMs operating at very low speeds. A small number of groups have set out to develop video-rate AFMs, and scan speeds as high as 20 frames per second have been reported with image sizes as large as 0.6 μm × 0.6 μm. While this is adequate for imaging some biologically relevant samples, the small imaging window prevents this technology from being used to observe larger dynamic processes, as needed in many applications.
The LDCN team has been systematically developing a suite of technologies that enables them to successfully perform video-rate atomic force microscopy on dynamic samples with an image size of the order of several microns. The success of our work is due to a number of breakthrough technologies developed by LDCN researchers over the past few years. These include: invention of non-raster scanning methods for high-speed AFM, design and implementation of advanced high-bandwidth feedback controllers for the AFM z-axis feedback loop, design and fabrication of high-stroke, high-speed nanopositioners, development of a framework for robust control of quality factor in AFM microcantilevers, and establishment of a number of methods for fast estimation of AFM microcantilever amplitude and phase. These methods have collectively led to the video-rate imaging of a moving sample with well-established non-raster scan patterns such as spiral and cycloid.
Video-rate AFM imaging of a circular-shaped area of 1.5 μm diameter with the spiral pattern at the rate of 37.5 frames per second.
Left: The experimental setup used to perform video-rate AFM imaging, comprising a Nanosurf easyScan2 AFM, a custom-designed XYZ nanopositioner, a tapping-mode cantilever, and a calibration grating.
Right: Block diagram of the closed-loop system.
Video-rate AFM imaging of a rectangular-shaped area of 5 μm × 10 μm with the cycloid pattern at the rate of 20 frames per second.
To perform sequential scanning continuously without the necessity of large closed-loop bandwidth, a novel non-raster scan pattern known as rosette has been proposed and implemented for video-rate AFM imaging. To generate the rosette pattern, the lateral axes of the nanopositioner are forced to follow a high frequency sinusoid superimposed on a low frequency one. In this project, a 2DOF MEMS nanopositioner, previously designed by LDCN team, has been employed to implement scanning at high frame rate.
Video-rate AFM imaging of a circular-shaped area 6 μm with the rosette pattern at the rate of 10 frames per second.
The experimental setup used to perform rosette scanning, comprising a 2-DOF MEMS nanopositioner, a custom-designed PCB and AFM.