Researchers have shown off the first images of the “charge distribution” in a single molecule, showing an intricate dance of electrons at tiny scales. Charges on single atoms have been measured before, but capturing the dance within a complex molecule is significantly more difficult. The pioneering measurement could shed light on a range of “charge-transfer” processes that are common in nature. Details are reported in the journal Nature Nanotechnology. The work comes from a group at IBM Research Zurich that specialises in examining the world at the infinitesimal scale of atoms and molecules. The same team is responsible for the measurement of charge on single atoms, as well as the first image of a single molecule - in a sense the new work is a combination of those two views. However, it makes use of a different technique, called Kelvin probe microscopy. It is a variant of the atomic force microscopy that allowed the first molecular image in 2009. It requires a tiny bar just billionths of a metre across and with a sharp tip that ends in a single small molecule. This bar, or cantilever, is held at a small voltage while it is scanned across the surface of a much larger, X-shaped molecule, naphthalocyanine. As the charged tip encounters charges within the naphthalocyanine, the cantilever begins wagging in a way that shows up precisely where the electrons are. The trick of naphthalocyanine, though, is that by applying a voltage to the molecule directly, two hydrogen atoms at its centre swap places, and the electrons reshuffle to opposite arms of the “X”. With the team's technique, they were able to observe this change in charge distribution. In combination with more established techniques, the approach will shed light on the nanoscale world that is promising not only for fundamental science, but also for future applications in which electric behaviour at such scales will be exploited. “It will now be possible to investigate at the single-molecule level how charge is redistributed when individual chemical bonds are formed between atoms and molecules on surfaces,” said lead author of the research Fabian Mohn. “This is essential as we seek to build atomic and molecular scale devices.”
