Background

The potential of the scanning tunnelling microscope as a tool for manipulating single atoms with atomic precision was first demonstrated by Don Eigler and Erhard Schweizer at IBM's Almaden research centre in 1990 [1]. Subsequent work by Karl-Heinz Rieder's group at the Free University in Berlin went on to show that small molecules could be pushed, pulled and slid across copper surfaces [2].

A drawback to working with metal surfaces is that atoms and molecules diffuse across them very easily. This means that experiments must often be carried out in cryogenic conditions and also that severe constraints may be imposed on potential spin-off technologies.

Room temperature manipulation

Here in Nottingham we reported the first demonstration of molecular manipulation at room temperature in 1995 [3]. Buckminsterfullerene, often called C₆₀ or the buckyball, was investigated on the silicon (111) and (100) surfaces on which it adsorbs strongly, making a number of covalent bonds.

The NottSPM controller

A key tool that we use for molecular manipulation is the NottSPM microscope controller. This was developed in-house specifically for single molecule experiments and contains many useful features [4].

The results of manipulation can be seen in tip traces produced by the STM when it is operated in constant current mode. If the adsorbate hops away from the tip, then the STM control system detects a drop in the tunnel current and lowers the tip height to compensate. Pushing molecules in this way is known as repulsive mode manipulation.

Attractive vs. repulsive mode manipulation

Attractive manipulation is also possible for fullerenes on silicon surfaces. We have shown that the height of the STM tip above the molecule is the critical factor in determining which mode of manipulation will occur. As the tip is lowered we see a change from an attractive regime to predominately repulsive mode manipulations [5].

Rolling Buckminsterfullerene on the Si(100) surface

A careful analysis of C₆₀ manipulation traces shows that the molecule rolls along the troughs of the Si(100) surface. Periodic signatures recorded by the NottSPM controller indicate that as the molecule is translated by the tip, it repeatedly adopts the same series of binding configurations on the surface.

A mechanism was proposed for this movement involving bond breaking, pivotting (about a pair of bonds), and also bond formation. Our experimental results were confirmed by ab initio DFT calculations performed by Dr. Lev Kantorovich's group at King's College London [6,7].

Current projects

The group maintains a keen interest in single molecule studies. As part of the EU-funded NANOMAN project we are currently developing a non-contact Atomic Force Microscope (AFM) for the purposes of extending our manipulation studies to insulating surfaces. The recent delivery of two cryogenic SPMs within the group also presents new opportunites to study manipulation in low temperature systems.

This project seeks to develop novel surface-bound building blocks for positional assembly. These will be driven via computer-controlled techniques such as SPM manipulation.

Work will be carried out by a consortium of research groups from Nottingham, Brighton, Glasgow, Sheffield and Southampton Universities.

For more information about the origins of this project, please have a look at the EPSRC Ideas Factory "Software Control of Matter" website.