Lithuanian Researchers Boost Atomic Force Microscopy with Faster, Wider Scanning

Scientists in Lithuania have developed a breakthrough method that dramatically speeds up atomic force microscopy (AFM) and expands its scanning range to the millimetre scale. Developed by researchers at Vilnius University (VU) and the Center for Physical Sciences and Technology, this innovation, featured in the journal “Measurement”, marks a significant advancement in nanoscale imaging, with potential applications across materials science, biotechnology, and semiconductor research.

“The atomic force microscope, invented in the mid-1980s, relies on an ultra-sharp tip mounted on a flexible cantilever to probe surface structures. It operates in the following way: a small cantilever is fixed at one end to a massive holder and has a tip sharpened to a few atoms at the other end; this tip, when dragged across a surface, can “feel” very fine surface irregularities by falling into tiny pits and climbing over small bumps. The movements of the tip are detected using a laser that shines on the reflective surface at the end of the cantilever, and the displacement of the reflected beam is measured. Due to the long optical path, even very small (nanometer-scale) vertical movements of the tip cause a significant shift in the reflected beam’s position,” explains Dr Viktor Novičenko from VU.

The standard AFM scanning protocol involves moving the tip in a “snake-like” trajectory across the XY plane. The tip moves from left to right across the entire surface, then shifts slightly downward before sweeping back from right to left, repeating this pattern continuously. Known as raster scanning, this method is effective but limited by slow speeds and precision constraints, making it less suitable for large-scale imaging.

“When scanning large areas, the issue of tip positioning can be illustrated with the following analogy: a blindfolded person is asked to walk one kilometre north, then take one step west, and return south along a path perfectly parallel to the first. Clearly, such a scenario is nearly impossible in practice,” states Dr V. Novičenko.

In 2017, scientists Šarūnas Vaitekonis and Artūras Ulčinas from the Center for Physical Sciences and Technology demonstrated a different scanning protocol — a sample glued to a rotating base is spun at a constant angular velocity, while the tip is moved radially outward from the rotation axis. During scanning, the tip traces a spiral trajectory in the XY plane. Inspired by the mechanics of a gramophone, this rotational scanning method leverages constant acceleration to achieve scanning speeds reaching centimetres per second.

However, this method introduces another challenge: the sample glued to the rotating base is always tilted with respect to the axis of rotation. “In other words, the normal vector of the sample’s plane forms a non-zero angle with the axis of rotation. This means the measured periodic signal contains a strong parasitic first harmonic, which conveys no useful information except for the fact that the sample is tilted,” says Dr Š. Vaitekonis. As the tip moves farther from the axis of rotation, the dominance of the first harmonic increases “until eventually, the tip’s oscillation — due to the amplified first harmonic — reaches the limits of its measurement range,” he adds.

According to the physicists, to prevent tip oscillation caused by the sample’s tilt, the sample can be moved up and down in real time to compensate for the tilt effect. However, this introduces yet another complication — every electromechanical device has inertia, and at rotational speeds of 40–50 Hz, this inertial effect acts as a delay with uncertain latency.

“With experience in control theory and periodic systems, I immediately recognised that the problem was mathematically solvable. The real challenge was developing a controller that could eliminate the first harmonic in real time, even with the uncertainty caused by the system’s inertia at high rotational speeds,” explains Dr V. Novičenko.

Dr A. Ulčinas, head of the laboratory at the Centre for Physical Sciences and Technology, emphasises the significance of the discovery: “Scanning large surface areas with nanometre precision can have substantial practical value. AFM can be applied to a wide variety of samples, ranging from semiconductors, novel battery materials, and polymers to biological objects. With our method, we can collect in hours the amount of information that would take days, if not weeks, to obtain conventionally. Seeing the commercial potential of this technology, we decided to patent our discovery.”

Currently, the Center for Physical Sciences and Technology, together with Vilnius University, holds an approved patent application for this novel AFM scanning technology.