Scanning probe method

Scanning the surface with a laser beam in a confocal arrangement can result in topographic information about the investigated surface since the confocal arrangement and further optical features dominate this experimental approach, whereas the scanning feature is of lesser importance, this microscopy is treated in the Sect. 7.3. 

Probing the LDOS can be done in two fundamentally different ways  

The tip can be scanned at constant height (distance) over the surface. This is possible only when the surface is smooth on an atomic scale, otherwise the tip might crash into surface features. 

Alternatively the tip can be scanned in a constant tunneling current mode. In this case the actually measured current is fed into an electronic regulation circuit which adjusts the actual tip-surface distance to a value resulting in a constant value of the tunneling current at all probed places. The signal fed into the z-axis piezodrive provides information about the local elevation. This mode works 

Instrumentation. In order to operate a STM under in situ conditions, i.e. in the presence of an electrolyte solution, some conditions have to be fulfilled. The design of the STM must allow investigation of a horizontal surface at the bottom of the microscope. The tip has to be coated as completely as possible in order to minimize the Faradaic current. Since the potential of the electrode surface under investigation has to be maintained at a fixed, controlled potential with respect to a reference electrode, a four-electrode arrangement requiring a corresponding bipotentiostat is necessary. The schematic drawing of the electrochemical cell as depicted in Fig. 7.3 shows the major components. 

3 SCANNING PROBE METHODS 27.3.1 Scanning Tunneling Microscopy 

The scanning tunneling microscope (STM) was invented by Binnig and Rohrer in 1982. This quickly led to the award of a Nobel prize in 1986. Initially, STM proved 

In in sitn STM experiments the tip is immersed in the cell electrolyte. Electrochemical reactions can occnr on all the exposed metal snrface of the tip. It is important to snppress these electrochemical currents to valnes that are considerably less than the tunnehng current. This is done by sheathing all bnt a few micrometers at the end of the tip, with an insnlating layer of glass or polymer (Bach et al., 1993). 

The great advantage of STM is the ability to do in situ structural studies on electrode surfaces. However, STM does not yield chemical information. STM-derived structural information is confined to a very small area on the electrode, typically 1000 Ax 1000 A, so meaningful correlations between the stmctnral data and electrochemical data can be done only on single-crystal surfaces. The close proximity of the tip to the electrode surface can distort kinetic data acqnired by an STM. 

FIGURE 27.18 (A) Weakly ordered Pt nanoclusters deposited spontaneously on a Ru(OOOl) 

---

Salmeron M, Liu G-Y and Ogletree D F 1995 Molecular arrangement and mechanical stability of self-assembled monolayers on Au(111) under applied load Force in Scanning Probe Methods ed H-J Guntherodt et al (Amsterdam Kluwer)... 

Bhushan, B., Fuchs, H., and Kawata, S., Applied Scanning Probe Methods V, Springer-Verlag, Berlin, Heidelberg, 2007. 

The STM method was developed by Binnig and Rohrer, who received the Nobel Prize for their invention. STM is the most mature of the scanning probe methods. A sharp tip (curvature of the order 100 A) is brought close to a surface and a low potential difference (the bias voltage) is applied between sample and tip. 

Scanning Probe Methods The main problem with scanning probe methods is the lack of chemical information. Methods have to be devised to get chemical information simnltaneonsly with the images. 

Despite the enormous impact that scanning probe methods have had on our understanding of reactions at oxide surfaces, both STM and AFM suffer from the lack of chemical specificity. The application of STM-inelastic electron tunneling spectroscopy is a potential solution as it can be used to measure the vibrational spectrum of individual molecules at the surface [69, 70]. 

Bhushan B, Fuchs H, Hosaka S (eds) (2004) Applied scanning probe methods. Springer, Berlin... 

M. Fujihira in H.-J. Guntherodt, D. Anselmetti, and E. Meyer (eds.), Forces in Scanning Probe Methods, NATO ASI Series, Series E Applied Sciences, Vol.286, Kluwer Academic Publishers, Dordrecht, 1995, pp.567-591. 

R. Wiesendanger and H.-J. Guntherodt (eds) Scanning Tunneling Microscopy III Theory of STM and Related Scanning Probe Methods, Springer, Berlin, 1993. 

Although the detailed information on the polymorphic phases has to be obtained with e.g., diffraction and scanning probe methods, our eyes are attracted by the beauty of optical microscope images often encountered with polymorphs under transformation. Figure 5.17 shows an optical microscopy image of a / -NPNN thin film (thickness 2 pm) on a glass substrate, exhibiting a transformation at... 

Hayazawa, N., and Saito, Y. 2007. Tip-enhanced spectroscopy for nano investigation of molecular vibrations. In Applied scanning probe methods VI, eds. B. Bhushan and S. Kawata, 257-85. Berlin Springer. 

Jarvis SP, Pethica JB (1995) In Guntherodt H-J, Anselmetti D,Meyer E (eds) Forces in Scanning Probe Methods. NATO ASI Series, Ser E, vol 286. Kluwer, Dordrecht, p 105... 

Like all scanning probe methods, STM experiments are limited by the rate at which images can be recorded. Again, limitations in the imaging rate arise from mechanical instrument design issues (i.e., resonances of the microscope itself). Further limitations arise from the small scan range usually employed in STM... 

Using myosin manipulation or scanning probe methods, these ATP hydrolysis coupled steps have been resolved into multiple stochastic substeps (Fig. 12.Id). The laser trap method, however, has not been able to resolve substeps. We attribute this to the large scanning probe slowing the substeps to a time within our system s time resolution. The size of the substeps was 5.5 nm, corresponding to the interval of actin monomers on a protofilament in a two-stranded filament. Substeps occurred both in the forward and back-... 

However small the structures under study are, the STM tip always consists of atoms, which have a finite dimension. Hence, the structure of the tip has an influence of what is observed with STM. If one assumes in a Gedanken-experiment that an infinitesimally sharp object is standing on the surface (mathematically a delta-function), then the tip will be convoluted at the object (Fig. 10.16, far right) and visible in the STM image will be the front most tip-end (up-side down) and not the sharp object itself. Hence, sharp tips are crucial for high-resolution in STM and, more general, in all Scanning Probe Methods (SPM). 

B. Bhushan, H. Fuchs, S. Hosaka, Applied Scanning Probe Methods I—IV, Springer, Berlin, Heidelberg, 2004—06. E. Meyer, H. Hug, R. Bennewitz, Scanning Probe Microscopy the Lab on a Tip, Springer, Berlin, Heidelberg 2004. 

An ideal tool for the investigation of the electronic structure of metal clusters is offered by scanning probe methods which have been used in three pioneering studies of nanometer-size clusters of Au on GaAs(llO) [229], Fe clusters on the same surface [230], and of size-selected Siio clusters on a reconstructed Au(OOl) surface [231], which all have been published already in 1989. In the first investigation [229], a characteristic spectrum of band-gap states was observed for the Au particles grown on GaAs. Both donor and acceptor states... 

---