The development of the scanning microscope in the early 1980s brought a breakthrough in imaging, opening a window into the world. The main idea is to scan a very sharp tip on a substrate and record the strength of the interaction between the tip and the surface at each location.
In scanning force microscopy, this interaction is – as the name implies – the force between the tip and the structures on the surface. This force is usually determined by measuring how the dynamics of a vibrating tip changes as it scans on objects deposited on a substrate. A common analogy is tapping a finger on a table and sensing objects placed on a surface.
Prof. in the Physics Department of ETH Zurich. This paradigm has been reversed by a team led by Alexander Eichler, senior scientist in the group of Christian Degen. Writing in Physical Review Applied, they first report a scanning force microscope in which the tip rests while the substrate vibrates with the specimens on it.
Performing force microscopy by “moving the table under the finger” can make the process more complicated. In a sense, it does. But mastering the complexity of this inverted approach is a big payoff.
The new method promises to extend the sensitivity of force microscopy to its fundamental limits, beyond what might be expected from further improvements of the traditional “finger harness” approach.
The key to increased sensitivity is the choice of substrate. The ‘table’ in the experiments of Eckler, Dagen and his colleagues is a porous membrane made of silicon nitride, which is only 41 nm in thickness.
Collaborators from the group ETH physicists of Albert Schlisser at the University of Copenhagen, Denmark established these low-mass membranes as excellent nanomechanical resonators with excellent quality factors.
Once tapped on a membrane, it vibrates millions of times or more before coming to rest. Given these exquisite mechanical properties, it becomes beneficial, at least in principle, to vibrate the table instead of the finger.
New concept to practice
Converting this theoretical promise into experimental potential is the aim of the ongoing project between the groups of Dagen and Schlisser, including Drs. Ramasubramanian Chitra and ETH is with the theory support of Professor Oled Zilberg of the Institute for Theoretical Physics at Zurich. As a milestone on that journey, experimental teams have now demonstrated that the concept of membrane-based scanning force microscopy works in an actual device.
In particular, they showed that neither loading the membrane with the sample nor bringing the tip within a few nanometers distance compromises the membrane’s exceptional mechanical properties. However, once the tip reaches even closer to the sample, the frequency or amplitude of the membrane changes.
To be able to quantify these changes, the membrane represents an island where the tip and sample interact, as well as mechanically pre-coupled each other, providing a sensitive optical interferometer to provide a The laser beam can be partially reflected.
To make this setup work, the team successfully solved gold nanoparticles and tobacco mosaic virus. These images serve as proof of principle for the novel microscopy concept, although they do not yet push capabilities into the new field. But the goal is in reach.
Researchers have developed their novel approach known as magnetic resonance force microscopy (MRFM), to enable magnetic resonance imaging with a resolution of single atoms, thus providing unique insights into, for example, viruses in.
Another breakthrough in MRI imaging at the atomic scale would be highly specialized physical and chemical information about atoms in combination with ultimate spatial resolution. To achieve that vision, a sensitivity close to the fundamental limit given by quantum mechanics is required.
The team is confident that they can realize such quantum-limiting force sensors through advances in membrane engineering and measurement methods. With the demonstration that membrane-based scanning force microscopy is possible, the ambitious goal has now come a big step closer.