The scanning tunneling microscope

The scanning tunneling microscope (STM) is an excellent device to obtain topographic images of an electrode surface [1], The principal part of this apparatus is a metal tip with a very fine point (see Fig. 15.1), which can be moved in all three directions of space with the aid of piezoelectric crystals. All but the very end of the tip is insulated from the solution in order to avoid tip currents due to unwanted electrochemical reactions. The tip is brought very close, up to a few Angstroms, to the electrode surface. When a potential bias AF, usually of the order 

Sometimes the tip is moved at a constant height x, while the current is recorded as a function of position. However, this technique can be 

As an example we consider the Au(100) surface of a single crystal Au electrode [3]. This is one of the few surfaces that reconstruct in the vacuum. The perfect surface with its quadratic structure is not thermodynamically stable it rearranges to form a denser lattice with a hexagonal structure (see Fig. 15.3), which has a lower surface energy. In an aqueous solution the surface structure depends on the electrode potential. In sulfuric acid the reconstructed surface is observed at potentials below about 0.36 V vs. SCE, while at higher potentials the reconstruction disappears, and the perfect quadratic structure is ob- 

Gold oxidahon starts at electrode potenhals -rl200 mV vs. AI/AICI3, first at the steps between different terraces. At higher potenhals pits are formed, rapidly resulting in complete disintegrahon of the substrate. 

These results are quite interesting. The initial stages of Al deposition result in nanosized deposits. Indeed, from the STM studies we recently succeeded in making bulk deposits of nanosized Al with special bath compositions and special electrochemical techniques [10]. Moreover, the preliminary results on tip-induced nanostructuring show that nanosized modifications of electrodes by less noble elements are possible in ionic liquids, thus opening access to new structures that cannot be made in aqueous media. 

Copper electrodeposition on Au(111) Copper is an interesting metal and has been widely investigated in electrodeposition studies from aqueous solutions. There are numerous pubhcations in the literature on this topic. Furthermore, technical processes to produce Cu interconnects on microchips have been established in aqueous solutions. In general, the quahty of the deposits is strongly influenced by the bath composition. On the nanometer scale, one finds different superstructures in the underpotential deposition regime if different counter-ions are used in the solutions. A co-adsorption between the metal atoms and the anions has been reported. In the underpotential regime, before the bulk deposition begins, one Cu mono-layer forms on Au(lll) [66]. 

The integrated charge would correspond to 0.7 0.1 Cu monolayers. Thus, either a less closely packed Cu layer or an anion co-adsorption that can both lead to a Moire superstructure are probed in the solution investigated [Al2Cl7] is the predominant anion. At 4-200 mV vs. Cu/Cu the superstructure disappears and a completely closed Cu monolayer is observed, with a charge corresponding to 1.0 0.1 Cu monolayers. 

This result is quite surprising, as no second Cu monolayer has yet been reported in aqueous solutions, nor have clusters up to 1 nm in height in the UPD regime. It 

In the overpotential deposition regime we observed that nanosized Al was deposited in the initial stages. Eurthermore, a transfer of Al from the scanning tip to the Al covered substrate was observed. We accidentally succeeded in an indirect tip-induced nanostructuring of Al on growing Al (Eigure 6.2-6). 

Au(m) nanoclusters with heights of up to nm form at-FlOO mV vs. AI/AICI3 (a) a typical height profile is shown in (b). Upon a potential step to -fITOO mV vs. AI/AICI3 the clusters dissolve immediately and leave holes in the surfaces as well as small Au islands (c) alloying between Al and Au is very likely. 

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Fig. VIII-1. Schematic illustration of the scanning tunneling microscope (STM) and atomic force microscope (AFM). (From Ref. 9.)...

Marrian C R K, Perkins F K, Brandow S L, Koloski T S, Dobisz E A and Calvert J M 1994 Low voltage electron beam lithography in self-assembled ultrathin films with the scanning tunneling microscope Appi. Rhys. Lett. 64 390... 

Stroscio J A and Eigler D M 1991 Atomic and molecular manipulation with the scanning tunneling microscope Science 254 319... 

Hamers R, Avouris P and Boszo F 1987 Imaging of chemical-bond formation with the scanning tunnelling microscope NH, dissociation on Si(OOI) Rhys. Rev. Lett. 59 2071... 

Tang S L, McGhie A J and Suna A 1993 Molecular-resolution imaging of insulating macromolecules with the scanning tunnelling microscope via a nontunnelling, electric-field-induced mechanism Phys. Rev. B 47 3850... 

Maaloum M, Chretien D, Karsenti E and FIdrber J K FI 1994 Approaching microtubule structure with the scanning tunnelling microscope (STM) J. Ceii Sc/. 107 part II 3127... 

Fig. 4. Atom manipulation by the scanning tunneling microscope (STM). Once the STM tip has located the adsorbate atom, the tip is lowered such that the attractive interaction between the tip and the adsorbate is sufficient to keep the adsorbate "tethered" to the tip. The tip is then moved to the desired location on the surface and withdrawn, leaving the adsorbate atom bound to the surface at a new location. The figure schematically depicts the use of this process in the formation of a "quantum corral" of 48 Fe atoms arranged in a circle of about 14.3 nm diameter on a Cu(lll) surface at 4 K.

This slow diffusion of a crucial new technique can be compared with the invention of the scanning tunnelling microscope (STM) by Binnig and Rohrer, first made public in 1983, like X-ray diffraction rewarded with the Nobel Prize 3 years later, but unlike X-ray diffraction quickly adopted throughout the world. That invention, of comparable importance to the discoveries of 1912,now(2 decades later) has sprouted numerous variants and has virtually created a new branch of surface science. With it, investigators can not only see individual surface atoms but they can also manipulate atoms singly (Eigler and Schweitzer 1990). This rapid adoption of... 

The bundle of MWCNT can be released in ultrasonic cleaner using ethanol as the solvent. The scanning tunnelling microscope (STM) image of thus released MWCNT is shown in Fig. 2. 

G- Binning and H. Rohrer (Zurich) design of the scanning tunneling microscope. 

The main technique employed for in situ electrochemical studies on the nanometer scale is the Scanning Tunneling Microscope (STM), invented in 1982 by Binnig and Rohrer [62] and combined a little later with a potentiostat to allow electrochemical experiments [63]. The principle of its operation is remarkably simple, a typical simplified circuit being shown in Figure 6.2-2. 

Three scanning probe techniques are described in more detail below the scanning tunneling microscope, the atomic force microscope, and the friction force microscope. 

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... 

The basis of the scanning tunnelling microscope, illustrated schematically in Figure 3.5, lies in the ability of electronic wavefunctions to penetrate a potential barrier which classically would be forbidden. Instead of ending abruptly at a... 

Hamers RJ (1989) Atomic-resolution surface spectroscopy with the scanning tunneling microscope. Ann Rev Phys Chem 40 531-559... 

Tersoff J, Hamann DR (1983) Theory and application for the scanning tunneling microscope. Phys Rev Lett 50 1998... 

Eigler D, Lutz CP, Rudge WE (1991) An atomic switch realized with the scanning tunneling microscope. Nature 352 600... 

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