When trying to see objects that are too small for optical microscopes to image, scientists often turn to scanning probe microscopes. Instead of a lens, these instruments have a tiny suspended tip, that moves up and down as it makes contact with the object's surface. An image, which can reveal details as small as one millionth of a millimeter, is obtained by scanning that probe back and forth across the object. Scientists from Germany's Universitaet Tübingen have now taken scanning probe microscopy a step farther, by creating a probe made not from a solid material, but from a gas of atoms - this "quantum tip" is said to increase the resolution of images beyond what has so far been possible.
The researchers started with a pure gas of rubidium atoms, and cooled it to less than a millionth of a degree above absolute zero (-273.15C/−459.67F). The atoms were then captured in a magnetic trap, holding them in place for use as the quantum tip on Tübingen's cold-atom scanning probe microscope.
In a test of the technology, one of the tips was scanned across a surface containing vertically-grown carbon nanotubes. As the tip touched the surface, individual atoms were stripped from it as they stuck to the surface. By measuring these atom losses, the location and height of the tubes could be established.
Upon being cooled further, the atomic gas formed into what is known as a Bose-Einstein condensate, in which individual atoms are no longer discernible from one another and essentially become one big "super-atom." Using the tip in that state, individual freestanding nanotubes could be imaged.
This was achieved in "contact mode," in which the atoms actually touch the object. The tips can also be used in "dynamical mode," in which the Bose-Einstein condensate oscillates perpendicular to the object's surface. By tracking the frequency and amplitude of the oscillations, which are affected by the surface topography, the appearance of that surface can be determined. The advantage of this mode is that no atoms get stripped from the condensate and stuck to the object, which could throw off subsequent measurements.
The Tübingen scientists believe that future improvements to the cold-atom scanning probe microscope could increase its current resolution of eight micrometers by a factor of a thousand.
The research was recently published in the journal Nature Nanotechnology.
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