Colour AFM generates fast results
Image of silicon using novel colour atomic force microscopy with on-the-fly parameter mapping [2017 Hideki Kawakatsu, Kawakatsu Laboratory, Institute of Industrial Science, The University of Tokyo]
Japan-based researchers have developed a method to convert the data scanned by an atomic force microscope into clear, colour images.
The method allows researchers to quickly acquire images of many materials with chemical contrast in order to extract information about the surface more easily.
Researchers often perform AFM measurements by moving the AFM tip up and down - constant frequency shift mode - so the frequency of its vibrations stays the same.
But while this operating mode is widely used for imaging, large portions of information of the potential landscape can be lost.
Alternatively, the tip can be kept at a fixed height above a sample surface - constant height mode - while measuring changes in tip vibrations as it interacts with the surface.
This mode yields more information but is limited to operating in low drift conditions.
In a bid to optimise information acquisition, Professor Hideki Kawakatsu from the University of Tokyo's Institute of Industrial Science (IIS) and colleagues developed a new method in which the AFM stays above the sample, but in a position where the vibrational frequency is strongly influenced by the surface.
Converting the data scanned by an atomic force microscope into clear, colour images. [2017 Hideki Kawakatsu, Kawakatsu Laboratory, Institute of Industrial Science, The University of Tokyo.]
"With a view to implementing a versatile, simple to use AFM with chemical contrast, we introduce here a control scheme that sets the working point to the vicinity of the bottom of the frequency curve," explains Kawakatsu.
According to the researcher, their mathematical treatment of the data allows surface parameters to be calculated at comparable rates to the scan.
Crucially, the approach also yields three independent Morse parameters per pixel, allowing the researchers to assign the colours - red, green and blue - to the variables and produce full-colour images.
Morse parameters mapping on quenched silicon. [P. E. Allain et al Applied Physics Letters, 111, 123104 (2017)]
The researchers have successfully tested their scheme on a quenched silicon surface, using a commercially-available cantilever, with results indicating the method is robust and provides atomic resolution in UHV. Scan rates are comparable with normal imaging.
"The power of the method is the use of a robust modulation technique to define the operating point of imaging at the bottom of the frequency curve, and the use of the same position modulation to sample the profile of the potential landscape with an excursion of a few angstroms," says Kawakatsu.
"By the use of faster demodulation circuitry, the increase in frequency of modulation will allow an even faster mapping of the potential on-the-fly," he adds. "Imaging of quasi-crystals and tracking of colour change of a migrating atom are interesting demonstrations for the future."
Zoomed out RGB combined picture with distinguishable atoms. Image size is 6 × 5 nm² [P. E. Allain et al Applied Physics Letters, 111, 123104 (2017)]
"If the colors in the image are the same, we can say the signals come from the same type of atom and surroundings," highlights fellow postdoctoral researcher Denis Damiron. "This new way of representing complex chemical and physical information from a surface could let us probe the movements and behavior of atoms in unprecedented detail."
Research is published in Applied Physics Letters.