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Applications of STM/AFM Beyond Imaging

Introduction
Scanning probe microscopy (SPM) has been used for the last decade for applications in imaging. In particular it has been identified as a means of atom discrimination and identification (Sugimoto et al., 2009). There are however a number of other applications for SPM techniques which have been developed over recent years. Specific types of SPM, including scanning tunnelling microscopy (STM) and also scanning force microscopy (SFM) have been used in such applications as single atom and molecule manipulation, C60 buckyball manipulation, local melting, the IBM millipede and electrostatic patterning. This essay further discusses some of the applications outside of imaging to which SPM has been applied.

Single Atom and Molecule Manipulation
One of the main areas in which SPM has been implicated for functions other than simply visualization purposes is in the area of material manipulations. This is an area which has seen great advancement over recent years, with an ever increasing focus on quantum applications which require material manipulation at a very high level. In particular, the manipulation of single atoms has been suggested to be critical in the advancement of nanoscience and particularly nanotechnology development (Sugimoto et al., 2009). The SPM techniques STM and SFM have both been implicated as useful techniques in this process due to their ability to interact with individual adsorbed nanoparticles within a larger material. The SPM techniques are particularly beneficial as the combination of manipulation and imaging functions which they are able to provide increases precision at the atomic level (Otero, Rosei & Besenbacher, 2006). This therefore renders them more useful for material modifications than other tools such as micro-electromagnets, as these may be limited in the materials with which they may be used (Drndic et al., 1998).
To switch to the function of manipulation over imaging the oscillating tip is brought closer to the surface than it would be for imaging purpose. This is demonstrated in Figure 1, where the distance Z is calculated to give the maximum attractive van der Waals force. This then allows the tip to interact with the atom to a stronger extent than would usually occur (Pizzagalli & Baratoff, 2003). Figure 2 shows that this is due to a change in the energy state which occurs as the tip moves closer to the atom (Trevethan et al., 2007). This then allows the user to manipulate atoms, molecules and nano-clusters with nanoscale precision (Tseng et al., 2008). Although this holds potential benefits as a technique specifically for atom and molecule manipulation, it also highlights that there is a necessity for those wishing to use SPM as a purely visual tool to ensure that the system in place does not result in inadvertent atomic changes.

Local Melting with STM
In addition to the movement of atoms and molecules, SPM may also hold applications for creating nano-scale alterations in materials through melting. The use of these to achieve local melting at the nano-scale level may allow for extremely high precision melting which may be used to remove specific areas of target material. This then allows for a slightly faster level of molecular moderation than removing a small molecule at a time. For example Grigoropoulos, Hwang and Chimmalgi (2007) describe achieving subtractive material modification at the 10nm level. This localized melting may be used to etch the surface of the material, and SFM has been used to create patterns on the surface of polymers via melting and deformation (Loos, 2005).
It is possible that using SPM different structures may also be created on the surface of materials. For example Gangopadhyay, Kar and Mathur (2007) investigated the formation of mounds and pits on a gold surface using a tungsten SMP tip and found that the production of these structures could be highly controlled, with reproducibility up to 90% for mounds and 100% for pits. This has potentially useful implications again for building nanostructures. This high level of reproducibility has also been reported in other studies. For example Grigoropoulos, Chimmalgi and Hwang (2007) also found there to be high reproducibility in the nanostructures which could be constructed via localized melting on thin gold surfaces. Their study also provided further suggestions as to how the precision of these techniques could be even further improved, for example by improving specialized probe design and integrating digital micromirror arrays. The reproducibility of these structures indicates that this local melting would be a valid way for construction of desirable structures using SMP techniques.

Conclusions
It may therefore be seen that SPM has evolved far from its original application as a mere imaging technique, with ever-increasing applications being found in the nanotechnology field. This is not to say that it is not still useful as an imaging technique however, and many of the other applications to which it is currently being developed have benefited from the ability to use imaging processes of the techniques too. The integration of SPM imaging with the manipulation properties has allowed for greater precision to be achieved in these techniques, which is why SMP possibly holds greater potential in experimental nano-scale manipulation than some other techniques available.
Although the manipulations which are possible by SPM are useful for research they are however limited in terms of industrial applications, as they would not be time- or cost-efficient. Instead, self-construction techniques are more suitable to these applications and SPM is more suitable for gaining insight into chemical structures, as well as experimental construction of molecules. It is taken as a given that further developments will be made in this field over the coming years, and one area in which focus would definitely be beneficial would be in investigation of improvements to the actual SPM techniques and set-ups so that the full potential as a research tool may be maximized. For example suggestions have already been made as to how reproducibility of results may be improved through changes to the equipment design, and this may also led to further improvements in precision.

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