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.
Figure 2: The process of single atom manipulation; (a) when the tip is far from the surface the system is in state A; (b) as the tip moves closer the potential barrier is reduced and the system moves to lower energy state B, giving a more attractive tip-surface interaction (Trevethan et al., 2007)
STM in Manipulation of Single Atoms and Molecules
In STM there is a high electron field which is created by the bias voltage which is applied with the technique between the tip and the sample (Kawai & Kawakatsu, 2006). STM is particularly useful for single molecule manipulation as the tunnelling current which is used is able to selectively break chemical bonds within molecules as well as being able to induce chemical association between atoms (Tseng et al., 2008). This was the first type of SPM which was identified as being suitable for atom manipulation, back in the 1990s. The technique was found to be useful for moving atoms to a desired location as well as building new structures atom-by-atom (Stroscio & Eigler, 1991). STM has also more recently been used in construction of quantum-level devices such as single molecule switches (Hla, 2008). There is relatively little discussion given in the literature as to the limitations of STM as an atom manipulation tool, although there is some suggestion that SFM may produce more desirable results.
SFM in the Manipulation of Single Atoms and Molecules
Noncontact AFM, a type of SFM (NC-AFM), also known as dynamic force microscopy (DFM) has also been shown to be useful in manipulation of atoms, with the additional benefit of not requiring sample conductivity which is a noted limitation of STM. This is due to DFM not requiring the applied bias voltage that is needed in STM (Kawai & Kawakatsu, 2006). Oyabu et al. (2005) showed that NC-AFM was suitable for manipulation of both intrinsic adatoms of semiconductor surfaces and deposited adsorbates on the surface. This used short-range interaction force which created a pulling process by the tip of the microscope on the atoms.
Both STM and frequency modulation AFM (FM-AFM), another type of SFM, have shown to be useful at low and room temperature (Morita et al., 2005; Sugimoto et al., 2008). Oyabu et al. (2005) showed NC-AFM manipulations to be useful in atom-by-atom creation at room temperature. Kawai and Kawakatsu (2006) demonstrated that at room temperature adatoms could be extracted and attached using small amplitude DFM. There have however been suggested to be some issues with stability and sensitivity in the use of SFM in atom manipulation (Trevethan et al., 2007). This may indicate that it would be preferable to use STM if the manipulations are to be used at room temperatures, and SFM only if lower temperature conditions are available.
There are two common set-ups for the manipulation of single atoms using NC-AFM. The most conventional method is that of a large amplitude silicon cantilever (Trevethan et al., 2007). Reducing the amplitude of DFM has been shown to be effective in improving the resolution when used in imaging applications (Giessibl et al., 2000). For this, a Kawai and Kawakatsu (2006) demonstrated that adatoms could be laterally manipulated in and over the half unit cell using this same strategy. This level of manipulation was associated with causing a repulsive interaction force between the tip and the sample, while adatoms could be moved to vacant stable sites using strong attractive force.
Trevethan et al. (2007) showed that a small amplitude quartz tuning fork would be an appropriate set-up for reducing the amplitude in NC-AFM. This method is able to improve sensitivity as it increases the signal-to-noise ratio thereby creating better resolution. This may also then allow for a better degree of control to be attained in manipulation (Kawai & Kawakatsu, 2006). It would therefore appear to be the case that lowering the frequency in this manner may produce preferable results.
Vertical manipulation is that which has been predominantly experimented with. For example Morita et al. (2005) reported removal of Si adatoms from a Si(111)-(7×7) surface using vertical manipulation. Lateral manipulation using SPM has so far been not been thoroughly explored. In addition to the Kawai and Kawakatsu (2006) study, there have been a small number of other studies which have examined lateral manipulation. For example lateral manipulation has been successfully recorded with small molecules such as Xe atoms on Ni(110), CO molecules and Pt atoms on Pt(111), and a number of small molecules and atoms on Cu(211) (Bartels, Meyer & Reider, 1997; Meyer et al., 1999).
Nanotubes and Buckyballs
In addition to the use in manipulation of single atoms and molecules, SPM is also implicated in manipulation of nanotubes. For example Kashiwase et al. (2008) used SFM to separate a carbon nanotube (CNT) bundle into two separate CNT bundles. They also were able to manipulate the CNT bundles into desirable positions to allow them to be bound to another bundle.
STM has also been used to successfully manipulate a C60 molecule, also known as a buckyball, on the clean 2×1-reconstructed Si(001) surface (Keeling et al., 2005). This is a complex process which has been suggested to be associated with long-range periodicity, notably 3a0 and 4a0 periodicities. This means that the molecule passes through a sequence of adsorption configurations until it arrives as an equivalent configuration. STM is exceedingly useful in this application as the complex nature of the periodicity in movement means that it is important to understand the atomic-scale details in the process. The use of STM allows for not only manipulation of the molecule, but also imaging of the processes so that a greater understanding of these may be gained (Martsinovich et al., 2006). This has allowed for the construction of the mechanism for C60 rolling, which is given in Figure 3.
Gaining a better understanding of the behaviour of these nanotubes and buckyballs has important implications for nano-biotechnology applications, as they are classified as ‘smart materials’ which may hold potential for applications in improving drug delivery technologies (Rakesh et al., 2008). This is due to certain characteristics of the molecules, such as their size, geometry and surface structure, which make them ideal for applications as nanocarrier systems. They are also very stable molecules, which is further indicative of their potential for use in biological systems (Khosravi-Darani et al., 2007).
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.
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.