Single molecule manipulation techniques

In the following section, the synthesis of a biphenyl molecule from two iodobenzene molecules adsorbed on a Cu(lll) surface using single molecule manipulation techniques with the STM-tip is presented as example. 

Experimental techniques based on the application of mechanical forces to single molecules in small assemblies have been applied to study the binding properties of biomolecules and their response to external mechanical manipulations. Among such techniques are atomic force microscopy (AFM), optical tweezers, biomembrane force probe, and surface force apparatus experiments (Binning et al., 1986 Block and Svoboda, 1994 Evans et ah, 1995 Israelachvili, 1992). These techniques have inspired us and others (see also the chapters by Eichinger et al. and by Hermans et al. in this volume) to adopt a similar approach for the study of biomolecules by means of computer simulations. 

Advances in nanochemistry have been fuelled by advances in instrumentation for visualisation and manipulation on the nanoscale, particularly scanning probe microscopic techniques such as STM and AFM, and observing and manipulating the chemistry of single molecules is now a reality. 

The techniques involved are single-molecule spectroscopy including optical (fluorescence) methods and single-molecule mechanical manipulation including scanning probe techniques (e.g., force measurements). 

Using a combination of manipulation techniques together with tunneling spectroscopy measurements, a number of chemical reactions have been induced on single molecules leading to the synthesis of new chemical products [39,47,48]. 

Molecules with specific functions, to be used in nanoelectronic and nanomechanical devices, can be constructed, and their physical and chemical properties can be studied in situ with STM spectroscopy techniques on an individual basis. Even though the direct industrial application of single molecule construction may not be possible in the near future, the knowledge can help initiate a mass scale production. Thus, with these achievements in molecular manipulation possibilities with the STM, a new dimension for future nanoscience and technology is now open. 

In summary of this section, it can be said that experimental manipulations and conductance measurements on single molecules are still a big scientific challenge, and a lot of the progress that has been recently made has been achieved for particular substrate/molecule systems and can not be easily transferred to other surface materials or types of molecules. The STM is certainly the most versatile instrument for manipulations and measurements on the nanoscale but it is not very suitable for an integration into nano-electronic devices. New techniques such as mechanically controlled break-junctions will have to be further developed for this purpose in the future. 

In the last few years, the development of scanning probe techniques41 3 has enabled direct observation and manipulation of single 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. 

Scanning probe techniques Tip manipulation Atomic-scale precision, no resist, solvents, etc. Very slow for single molecule control (serial) Parallel methods, micromachining... 

One of the big problems in single-molecule experiments is how to keep the molecule in place long enough to make observations on it, as well as how to grab hold of different parts of the molecule in order to manipulate it. In AFM and similar techniques the molecules are immobilized by being adsorbed on or attached to macroscopic surfaces. Another method relies on the properties of finely focused laser beams to act as optic il twcezersor optical trap-s to manipulate microscopic particles suspended in solution. 

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