"The possibility of breaking and restoring molecular junctions between a semiconductor nanocrystal and a metal substrate was discovered by accident," Zeger Hens of Gent University told nanotechweb.org. "Our purpose was to develop a universal way of linking quantum dots to metal substrates by chemical to probe the electronic structure of the quantum dots with scanning tunnelling spectroscopy."
According to Hens, to avoid a "molecular mess" on the gold surface, the scientists added bifunctional molecular links to the nanocrystals rather than to the gold surface. Then they used room-temperature STM to check if the dithiol molecules they chose had attached cadmium-selenium (CdSe) quantum dots to the substrate.
The researchers found that, with a negative potential applied to the gold substrate, they were able to image the nanocrystals, indicating that they were attached to the gold and weren't shunted by the STM tip. But, under a positive surface potential of more than 250-500 mV, the STM tip acted as a vacuum cleaner. "All nanocrystals were simply transferred from the surface to the STM tip," Hens explained.
"The gold-sulphur bond is widely used in experimental nanodevices to attach nanocrystals to surfaces or electrodes," said Hens. "Hence its failure to withstand positive substrate potentials under atmospheric conditions is an important drawback."
The scientists also discovered that they could deliberately displace the nanocrystals, picking them up and repositioning them at another place on the substrate - a function that "opens the possibility of making quantum dot architectures using a scanning tunnelling microscope".
"In the end, we could image this process as a game of basketball", said Hens, "where a quantum dot was picked up from the surface - breaking the molecular link using positive substrate potentials - and put back on a predefined spot on the surface, restoring the molecular link using large negative substrate potentials."
To check whether it was the unique properties of the gold-sulphur (Au-S) link causing this effect, the researchers also tested a system using ZnO quantum dots linked via a mercaptoacid. This arrangement contained Au-S links but was different in every other respect. The scientists found it easy to pick up individual quantum dots from the gold surface, but harder to replace them because it was tricky to evaluate the negative potential large enough to put back the dots without damaging the gold surface.
The scientists reported their work in Applied Physics Letters.