A simple trick allows a highly sensitive microscope to capture the details of bulky molecules’ chemical structures.
An atomic-force microscope forms images of single molecules and their internal bonds by scanning the molecules with a probe and measuring the repulsive force experienced by the probe’s tip. But bulky molecules that protrude towards the tip alter the force readings, which has an effect similar to throwing the image out of focus.
Daniel Ebeling and his colleagues at Justus Liebig University Giessen, Germany, operated the microscope in a mode that allows an electric current to flow from the tip to the sample. The amount of current corresponds to the tip’s distance from the sample. In the researchers’ protocol, the current is held constant; as a result, the tip’s height changes as it follows a molecule’s contours. This keeps the image of the bonds in focus while simultaneously acquiring data about the molecule’s 3D shape.
Test images of a structurally complex molecule — a triphenylene radical — showed that this technique could keep more of the structure in focus than conventional methods can.