A group of scientists from the University of Aarhus in Denmark, in collaboration with CEMES-CNRS in Toulouse, France, have recently reported that so-called Lander molecules (C90H98) can cause the rearrangement of atoms on a copper surface, a crucial step toward practical nanofabrication. Speaking for the group, Federico Rosei says, "We have observed Lander acting as a template to build metallic nanostructures 0.75nm wide and 1.85nm long at step edges of a copper substrate; the step-edge adapts to the structure and shape of the molecule, leading to nanostructures two copper atoms wide and seven copper atoms long."
The electronic connection of single molecules to nanoelectrodes on a surface is a basic, unsolved problem in the emerging field of molecular electronics. The presented results, which were recently published in Science (F. Rosei, et al., Science 296, 328, 2002), may point to a new way of electronically connecting single molecules to the outer world.
"Previous studies had shown that complex molecules may cause surface restructuring," Rosei says. "But for the first time the process is directly linked to the shape of the molecules. This could point to developments in which specifically designed molecules induce predefined patterns on substrates, with clear implications in surface engineering."
The Lander molecule can be visualized as a table where the table's top acts as a wire and is separated from the substrate by four legs. The cavity under the table traps copper atoms. The dimensions of the resulting copper structure are directly related to the dimensions of the table. The researchers were able to directly observe the nanostructure formed using scanning tunneling microscopy to image and move the molecules (see figure).
A Lander molecule pushed (arrow) from the edge of copper revealing (circle) fabricated tooth-like structure where the molecule was docked. Image is 13 x 13nm.
Lander molecules are deposited on the surface with an organic molecular beam evaporated from a glass crucible under ultrahigh vacuum. Upon submonolayer deposition of the Lander at room temperature, the molecules adsorb on the surface and diffuse toward step edges, where their complex interaction with the surface causes them to act as templates, imposing a peculiar step restructuring.
Rosei says, "Currently we are studying the behavior of other molecules from the Lander family; for example, those with eight legs instead of four and those with a longer table board or a wider separation of the legs. We are applying these to the same surface and other metal surfaces."
This root work is being done within the framework of a European Network called "Bottom-up Nanomachines" (BUN). Rosei says, "Our ultimate goal is to design, simulate, synthesize, interconnect, assemble and test molecular nano-devices and nano-machines starting from their molecular or atomic parts, eventually achieving practical devices and applications."
Flemming Besenbacher, director of the Interdisciplinary Nanoscience Center at Aarhus University (iNANO), says, "Our results point to a new self-fabrication process at the nanoscale. This may be of utmost importance when it comes to interconnecting molecules with atomic scale precision." P.B.