Building Molecular Machines on Surfaces

The design, synthesis, and operation of multicomponent molecular systems capable of performing specific, directional mechanical movements under the action of a defined energy input – namely, molecular machines – constitute a fascinating challenge in the field of nanoscience. In the past ten years a great variety of artificial molecular machines have been constructed and very interesting concepts concerning molecular-level movements controlled by external inputs have been developed. Most of these studies have been performed in solution, where the investigated systems contain a huge number of molecules which behave independently from one another because they cannot be addressed individually. Before such systems can find applications in many fields of technology, they have to be interfaced with the macroscopic world by ordering them in some way so that they can behave coherently and can be addressed in space. The problem of obtaining ordered arrays of molecular machines can be approached by a variety of techniques which include deposition on surfaces, incorporation into polymers, organization at interfaces, or immobilization into membranes or porous materials. In the last few years, the development of scanning probe techniques has also enabled direct observation and manipulation of single machine-like molecules on surfaces. Techniques of this kind have opened novel directions to the study of molecular machines, and have also contributed to better understand the differences between movement at the macroscopic and at the molecular level. Here we discuss recent achievements on molecular machines working at surfaces, as single molecules or ordered arrays.