Scanning Probe Techniques

Scanning probe techniques have been a valuable tool since their invention in the 1980s [19, 20]. In these experimental methods distance-dependent interactions like tunneling current, force, or light transmission between a sharp needle ("tip" or "probe") in close proximity to a surface ("sample") are utilized to produce an image of the sample. Two principal measurement modes were implemented: (i) to maintain a constant vertical probe position while measuring the interaction change often due to surface topography ("constant distance mode"), and (ii) to maintain constant interaction while adjusting the distance with the feedback signal reflecting surface topography ("constant interaction mode"). Both modes offer the possibility to characterize surfaces down to the atomic scale in a great variety of environments from ultrahigh vacuum to aqueous solutions. It is as well possible to characterize time-dependent reactions like crystallization or corrosion processes, as it can be done continuously and hence on-line. To scan a probe over a surface in the desired way while reacting to the topography of the sample, positioning tools with spatial resolution in the 0.1-nm regime are necessary. The latter is fulfilled by piezoelectric actuators made of ceramics like PZT (lead zirconate titanate) or PMN (lead magnesium niobate), which in different directions can be extended by less than the size of one crystal unit cell. With a suitable detection unit to measure small interaction changes connected to a distance feedback circuit and digital data representation, an image of the surface can be portrayed. Experimental adaptations and extensions based on the interaction mechanisms originally used for imaging purposes lead the way to monitor more complex features than the topography as described below.

In scanning tunneling microscopy (STM) the current of electrons tunneling between a conductive wire (preferentially heavy metals like tungsten, platinum, or iridium) with an atomically sharp tip and a (semi-) conductive surface across vacuum is measured (Fig. 2). The tunneling current decays exponentially with increasing distance between tip

Figure 2. Schematic drawing of the STM principle.

and surface and additionally depends on the local densities of electronic states of the tunneling partners and the work function of the sample. Because of its strong distance behavior only the atom at the very end of the tip and the nearest surface atom are involved within the tunneling event. A slight change in distance according to progression along rows of surface atoms alters the tunneling current in a measurable manner so that true atomic resolution can be achieved.

Experimentally the onset of a tunneling current is obtained by approaching tip and surface to less than a few tenths of a nanometer under a constant bias voltage. The sign of the bias determines the direction of the tunneling current; this means unoccupied electronic surface states are probed with occupied electronic tip states or vice versa. Scanning the surface row by row either at constant height or constant current (see above) reveals the surface topography.

Additionally, STM can be applied to probe electronic structures. Modulating the tip-surface distance and measuring the change of the tunneling current at constant bias allows to extract the local work function of the sample. Modulating the bias and measuring the corresponding current changes at a constant tip-surface distance allows to extract the electronic states around the Fermi level of the sample (STS, scanning tunneling spectroscopy). This can lead to some chemical information about the surface, but for more detailed information electronic core levels have to be sensed, which is not possible with the STM (see [21] and especially references therein).

Scanning electrochemical microscopy (SECM) measures highly localized electrochemical currents associated with charge transfer reactions on metallic sample surfaces under liquid environment [22, 23]. In macroscopic measurements it can be compared with cyclovoltammetry. The reactions occur in a four-electrode electrochemical cell under bipo-tentiostatic control. There are two mechanisms respectively pathways of image production. Electron tunneling and electrochemical reactions via a water bridge occur according to the applied voltage. It can be used for the detection of localized electrochemical reactions at surfaces. It can also be used for microstructuring biomaterials like titanium [24]. In addition SECM is also capable of probing the kinetics of solution reactions and adsorption phenomena and monitoring heterogeneous electron transfer kinetics associated with processes on conducting surfaces [25].

The invention of the scanning force microscopy (SFM) was a breakthrough for these techniques as it is possible to image nonconducting substrates with a resolution of 0.2 nm laterally and 0.001 nm vertically. It does not require specimens to be metal coated or stained. Noninvasive imaging can be performed on surfaces in their native states and under near physiological conditions. It has been proven to be particularly successful for imaging biological samples, such as proteins, nucleic acids, and whole cells. By scanning, dynamic processes can be imaged, such as erosion, hydration, physicochemical changes, and adsorption at interfaces. Therefore the SFM is currently the SPM technique with the widest applicability for biomaterial research.

In SFM a small tip attached to a micro beam (cantilever) is scanned across the surface of the specimen and deflected by topographic features. The force of interaction may be repulsive or attractive, giving rise to different modes of operation. Moving the cantilever from the interaction free zone far above the surface, it snaps into contact (Fig. 3) due to the attractive force between the tip and the sample, which can be described in a simple way by the Lennard Jones potential. The piezo pushes the tip further towards the sample and the positive repulsive force reaches a maximum. As the piezo is retracted the repulsive force is reduced and the force changes sign. If the bending force of the cantilever gets larger than the attractive force towards the surface, the tip loses contact. The tip can be held in the repulsive regime of the Lennard Jones potential or oscillated in the attractive or repulsive regime, resulting in different interactions [26, 27]. These differences are important as biomolecules are deformed by applying a load of some nanonewtons as present in contact mode [28].

The deflection is usually monitored by a laser beam that is reflected and detected with a four split photodiode. This signal is used to maintain a constant force via a feedback loop and to monitor the height data. The SFM can be operated in a variety of modes that can provide different information about the sample. Usually the z-deflection is monitored and interpreted according to the parameters under investigation. An easy to read overview covering many scanning probe

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