NanotechnologyNanotechnology developed from a purely theoretical vision pioneered by R. Feynman and popularized, for example, by E. Drexler into a large, scientifically and commercially active field. In the course of this development the meaning of the word nanotechnology broadened, now including essentially all activities involving structures smaller than 100 nm. However, the original idea proposed by Feynman in his famous lecture ``There is plenty of room at the bottom'', delivered to the American Physical Society in Pasadena in December 1959, was to manipulate and control matter at small length scales and thus to establish a miniaturized manufacturing and fabrication technology in analogy to the macroscopic world, but ultimately at the atomic scale. For example, Feynman suggested a cascade of master and slave hands to carry out operations at smaller and smaller length scales.
The development of the (low-temperature) scanning tunneling microscope, the (LT-)STM, an instrument with the capability to image and controllably move atoms, and the iconic IBM logo `written' with single Xe atoms on a Ni surface can be seen as the beginning of the experimental realization of atomic-scale nanotechnology in the sense proposed by Feynman. Yet, three decades since Eigler's groundbreaking experiments the question whether the technological paradigm of manufacturing can be extrapolated down to the molecular limit is still open. It is obvious that this would be extremely useful: Molecular biology illustrates that, in principle, devices and materials with remarkable properties can be made of functional molecules, and it would be extremely prospective if such intricate molecular structures as found in living systems could be manufactured in the laboratory.
Harnessing artificial functional nanostructures made of molecules in non-biological contexts is a process involving many challenges: Biological examples of such nanostructures need to be studied, since these provide the essential inspiration; this includes solving their structures and developing a mechanistic understanding of their functionalities. Next, moving beyond natural systems, artificial structures with certain desired functionalities have to be designed, using a mix of inspiration by nature, physical and chemical intuition and predictive simulations. Based on this, chemists need to synthesize appropriate molecules and supramolecular assemblies. But at the very end, there is one challenge that has not really been addressed yet: How can several complex molecular building blocks be assembled into the final functional structure? Of course, one could rely entirely on self-assembly, as does nature in biological systems. But this would restrict the range of accessible structures, since it can be difficult to obtain non-equilibrium structures. Moreover, in artificial self-assembled systems it has turned out to be difficult to create deep structural hierarchies. In contrast, directed manufacture, if at all possible, may allow for the creation of arbitrary metastable and strongly hierarchical molecular structures.
Cast in modern language, the challenge laid down by Feynman in his scale-bridging concept of master and slave hands is: Can we use the tip of a scanning probe microscope as the robot arm of a molecular assembly machine or, in yet other words, carry out `3D printing' at the single molecule level with the help of the SPM tip? This capability of directed molecular fabrication, if ever achieved, would be a breakthrough for molecular nanotechnology.
Feynman's ultimate vision was synthesizing arbitrary materials and structures atom-by-atom, placing the individual atoms at just the right positions. Interestingly, first steps in this direction have been realized in the meantime. However, for making delicate supramolecular structures that are held together by weak bonds, one cannot start at the level of atoms, since the exothermic formation of individual covalent bonds will be disruptive in such a delicate environment. Hence, one needs to handle prefabricated molecules and arrange them into the desired structures.
Today, we are still far away from a directed manufacture of intricate molecular nanostructures, despite the impressive success of atomic manipulation experiments with the LT-STM. Although the target molecules are considerably larger than atoms, their controlled manipulation is in fact more difficult than that of atoms and small, e.g. diatomic, molecules. The reason lies in the complexity of the molecular building blocks themselves, juxtaposed with a `slave hand' that has only a small number of mechanical degrees of freedom: the tip of the STM is a more or less rigid rod, not a hand with complex fine-motoric capabilities. If a proactive and directed manufacture of molecular nanostructures is to be realized, flexible objects with anisotropic shapes and many internal degrees of freedom would have to be controlled with this simple rod. This discrepancy creates a complexity gap in controlled molecular manipulation and explains why only a few examples of controlled large-molecule manipulations are reported in the literature, while there are many cases of atom manipulations, which even include the assembly of large arrays of logic gates and the making of a movie.
Being able to create arbitrary structures is the ultimate goal of materials design. However, even if we will never succeed in making large quantities of material with the SPMs, it is still conceivable that we may fabricate, possibly utilizing multiple tips, nearly arbitrary nanostructures and then study emergent phenomena at the quantum scale on individual (molecular) nanostructures. Of course, this opportunity was already foreseen by Feynman, who in the above mentioned lecture pointed out that manipulating and controlling things on a small scale ``might tell us much of great interest about the strange phenomena that occur in complex situations''.
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