Atomic Force Microscopy Acquisition Rates Accelerate
Because they are mechanical devices, conventional atomic force microscopes (AFMs) can see fine details but not fast ones. The instruments make nanoscale measurements by dragging a sharp stylus across a sample, and their readout involves tracking the motion of a light beam reflected from a microcantilever that bears the stylus.
As a result, it takes approximately 30 s to move the microscope's head across a surface and collect an image. This is too slow to enable the user to follow molecular processes, and it impacts the application of AFMs in nanolithography and data storage. Now researchers at the University of Bristol in the UK have boosted the image acquisition rates of these systems a thousandfold.
Behind the rate increase lie both mechanical and photonic enhancements. To increase the mechanical scanning speed, the researchers mounted samples on a quartz crystal tuning fork that vibrated from side to side at a few tens of kilohertz. They also moved the tuning fork back and forth at approximately 30 Hz, using a piezoactuator. Together these motions ensured that the stylus traversed the sample quickly.
In another mechanical innovation, they kept the tip in contact with the sample by applying force to the back of the stylus. They coated the back of the microcantilever with a thin layer of polymer to damp and dissipate the resulting high-frequency oscillations. These two changes enabled the stylus to travel up and down at much higher speeds than it would normally.
On the photonic and signal processing front, the researchers used a conventional optical arrangement. They constructed a measurement detection system with bandwidth in the tens of megahertz in collaboration with Infinitesima Ltd., also of Bristol.
In a demonstration of the instrument's high-speed capabilities, the group collected images of soft crystalline and molten polymers with nanometer resolution at 14.3 ms per frame. As for the future of high-speed atomic force microscopy, Infinites-ima is working on a video-rate instrument.
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