While single-molecule manipulation using a scanning tunneling microscope offers excellent control, this technique is limited to conducting surfaces only. Application-oriented systems on the other hand usually require non-conductive surfaces. For these surfaces, non-contact atomic force microscopy (NC-AFM) is the method of choice for atomic resolution imaging and manipulation with utmost sensitivity and control.

Here, there are difficulties to overcome. Compared with scanning tunneling microscopy, the NC-AFM option is far less mature and poses a greater challenge to experimentalists.

Room temperature result

Using non-contact AFM, Jens Schütte and co-workers from Angelika Kühnle's group at Johannes Gutenberg-Universität Mainz, Germany, have manipulated single organic molecules on a TiO2(110) surface under ultrahigh vacuum conditions at room temperature. The molecules were tailor-made 3,4,9,10-perylene tetracarboxylic diimide (PTCDI) derivatives functionalized with alkyl side chains provided by Heinz Langhals from Ludwig-Maximilians-Universität München.

The molecules adsorb in a tilted configuration on the bridging oxygen rows of the substrate. By taking advantage of this adsorption position and by using the tilt between the imaged substrate and the imaging plane, a simple manipulation protocol allows controlled and reproducible switching of single molecules from one side of the bridging oxygen rows to the other.

Estimating the energy barrier

Thanks to density functional theory calculations performed by Michael Rohlfing at Osnabrück Universität, Schütte obtained molecular-level details of the manipulation process. By considering the adsorption energies of different adsorptions positions, the energy barrier for the switching process can be estimated.

This barrier is high enough to prevent spontaneous switching at room temperature, while at the same time it is low enough to be overcome when approaching the microscope tip.

Proof of principle

This work provides the proof of principle for single-molecule switching on an insulating surface. It paves the way for exploiting single-molecule switching in future molecular electronic applications. For example, as a switch in an electronic circuit or within a logic storage device.

Further details can be found in the journal Nanotechnology.