Atom Manipulation Crosses New Threshold

In a teeny tiny display of patriotism, a team led by researchers at the University of Basel has created the world’s smallest Swiss national flag out of ions (Nat. Commun. 2014, DOI: 10.1038/ncomms5403). To generate the flag’s iconic cross, the scientists used the tip of an atomic force microscope (AFM) to pick up bromine ions scattered across a sodium chloride surface and move them one by one into formation. With the hope of one day creating atomic-scale electromechanical devices, physicists have been dragging single atoms around and repositioning them on surfaces with scanning probe tips since the 1990s. Usually, the atomic manipulations are carried out on metallic or semiconducting surfaces at very low temperatures. The Swiss cross, however, was generated on an insulating surface—sodium chloride—at room temperature. The researchers accomplished the more difficult task, says team leader Ernst Meyer, because of “tremendous progress” in the sensitivity and stability of AFMs. The team believes it succeeded in building the 20-ion cross because the NaCl-coated AFM tip it used had sodium ions at its apex: These positively charged particles were able to pick up the negatively charged bromines.