H₂ At The Tip Of STM Boosts Resolution

If you want to enhance the resolution of your scanning tunneling microscope, stick a hydrogen molecule on the tip. A team led by Ruslan Temirov of the Jülich Research Center, in Germany, reported in 2008 that STM resolution of complex organic molecules can be greatly enhanced by modifying the tip with hydrogen. But until now, the researchers didn’t understand why the trick works or what exactly the STM measures in that experiment. By repeating the experiments with deuterium and using quantum-mechanical calculations to analyze perylenetetra­carboxylic anhydride supported on gold, the team obtained the answer (Phys. Rev. Lett. 2010, 105, 086103). As a result of its electronic structure and small size, a single molecule of deuterium or hydrogen confined between the probe tip and the organic molecule serves double duty, Temirov and coworkers say. They explain that D₂ or H₂ functions as a nanoscale force sensor because of short-range electronic repulsion between its electrons and those in the surface; it also acts as a transducer that converts this repulsive force into variations of the tunneling conductance.