Electrochemical STM

The teclmique of scaiming electrochemical microscopy (SECM) [62] uses the same apparatus as in electrochemical STM, but instead of measuring tunnelling currents, the reaction O + ue —> R (where O and R... 

Figure 6.2-2 Simplified circuit of an electrochemical STM setup. In addition to the potentiostat...

In this method the creation of defects is achieved by the application of ultrashort (10 ns) voltage pulses to the tip of an electrochemical STM arrangement. The electrochemical cell composed of the tip and the sample within a nanometer distance is small enough that the double layers may be polarized within nanoseconds. On applying positive pulses to the tip, the electrochemical oxidation reaction of the surface is driven far from equilibrium. This leads to local confinement of the reactions and to the formation of nanostructures. For every pufse applied, just one hole is created directly under the tip. This overcomes the restrictions of conventional electrochemistry (without the ultrashort pulses), where the formation of nanostructures is not possible. The holes generated in this way can then be filled with a metal such as Cu by... 

While the first STM studies of electrode surfaces were performed with self-built instruments, scanning tunneling microscopes for electrochemical use are nowadays commercially available at a price that hardly justifies the effort of homemade equipment. Nevertheless, new instrumental designs are now and then discussed in the literature, which are still worthwhile to be considered for special applications. There is, however, additional equipment required for the operation of an electrochemical STM, for which homemade designs may be advantageous over commercially available ones and hence is briefly mentioned here in terms of tip preparation and isolation, the electrochemical cell, and vibration damping. 

The first STM experiments were performed under UHV conditions, and so the bias potential was simply applied as a difference across the tip and sample. However, introducing an electrolyte above the sample brought with it some particular problems. It is no longer sufficient simply to apply a bias voltage equal to the potential difference between tip and sample as this means that the potentials of the tip and sample are undefined with respect to any fixed reference, a wholly undesirable situation. Consequently, modern electrochemical STM systems operate under bipotentiostatic control with the tip and sample controlled and monitored independently with respect to the reference electrode. The bias potential is then still given by (Fs — FT), but VT and Fs are now potentials with respect to the reference electrode. 

Pobelov I, Li Z, Wandlowski T (2008) Electrolyte gating in redox-active tunneling junctions -an electrochemical STM approach. J Am Chem Soc 130 16045-16054... 

The Scanning Tunneling Microscope has demonstrated unique capabilities for the examination of electrode topography, the vibrational spectroscopic imaging of surface adsorbed species, and the high resolution electrochemical modification of conductive surfaces. Here we discuss recent progress in electrochemical STM. Included are a comparison of STM with other ex situ and in situ surface analytic techniques, a discussion of relevant STM design considerations, and a semi-quantitative examination of faradaic current contributions for STM at solution-covered surfaces. Applications of STM to the ex situ and in situ study of electrode surfaces are presented. 

The design criteria for an in situ electrochemical STM include the above outlined considerations as well as several needs peculiar to an electrochemical environment. Sonnenfeld and Hansma (57) constructed the first STM to operate under solution. Their work highlights two important design considerations. Firstly, the tip and sample should be the only electrically active parts of the microscope exposed to solution. This first solution microscope was... 

Lithography With the STM Electrochemical Techniques. The nonuniform current density distribution generated by an STM tip has also been exploited for electrochemical surface modification schemes. These applications are treated in this paper as distinct from true in situ STM imaging because the electrochemical modification of a substrate does not a priori necessitate subsequent imaging with the STM. To date, all electrochemical modification experiments in which the tip has served as the counter electrode, the STM has been operated in a two-electrode mode, with the substrate surface acting as the working electrode. The tip-sample bias is typically adjusted to drive electrochemical reactions at both the sample surface and the STM tip. Because it has as yet been impossible to maintain feedback control of the z-piezo (tip-substrate distance) in the presence of significant faradaic current (vide infra), all electrochemical STM modification experiments to date have been performed in the absence of such feedback control. 

Figure 6.2-2 Simplified circuit of an electrochemical STM setup. In addition to the potentiostat (see Figure 6.2.1), an STM preamplifier is added, to which the tip is connected. Ul potentiosta-tic setpoint, U2 tunneling voltage, l(t) tunneling current, U3 = -R l(t).

This technology has been extended to measurements in electrolyte solutions by independently controlling the potential of the substrate and the tip with respect to a reference electrode located in the solution. This electrochemical STM allows the progress of electrochemical reactions to be monitored in situ under potential control. The instrument uses a four-electrode configuration in which the potentials are controlled so that the current flowing between the substrate and tip is dominated by the tunneling current, while a predominantly Faradaic current flows between the substrate and the counter electrodes. 

Characterization of surfaces and thin films has been revolutionized by the invention of scanning probe microscopes, i,e, scanning force microscopy, scanning tunnelling microscopy, and scanning near field optical microscopy [262-264], These methods not only allow imaging of molecular and supramolecular details, but can also be employed to probe and to manipulate chemical properties on a nanoscopic or molecular scale, e,g., mechanical SFM [265], chemical SFM [266], electrochemical STM [267,268],... 

Reducing the distance between the electrodes into the micro-to nanometer range, e.g., by employing the tip of an electrochemical STM as a local counter electrode, decreases the time constant of the electrochemical cell well into the... 

The experiments were conducted in a conventional electrochemical STM [11,35]. Au films with a thickness of 250 nm on a Cr-coated (2-nm) glass substrate were used as samples. After repeated cycles of rinsing with triply distilled water and subsequent flame annealing, these films exhibited (111) terraces up to lOOnm wide. High-resolution images of these surfaces show the (v x 22) reconstruction of the clean Au(lll) surface in accordance with [36]. 

Figure 4 Sketch of the electrochemical STM for short-pulse surface modifications. The potential of the working electrode (WE) is controlled by a low-frequency potentiostat (Pot) versus the reference electrode (RE) via the counterelectrode (CE). The tunneling voltage (Ut) is supplied via the I/U converter of the STM (A) and a low-pass filter (LP). To apply the short pulses to the STM tip, a high-frequency pulse generator (Pulse) is switched onto the tip for a few milliseconds.

Redox (Bio)molecules in Electrochemical STM and Other Nanogap Configurations... 

CO adsorption on Ru(OOOl) shows contributions from both COl and multiple-bonded CO (COh) Wliile a certain amount of activity towards CO oxidation to CO2 was seen on the surface of polyciystalline Ru, Ru(OOOl) exhibited almost none, judged by the absence of the FTIR peak around 2350 cm . At low CO doses, the STM image showed a (V 3 x 3) R 30° CO overlayer with a coverage of 0.33 ML, similar to the structures found with UHV a further increase in CO doses produced a new c(2 x 2)-2CO stmc-ture as the saturation phase, where CO occupied both the on-top and the three-fold hollow sites, with coverage of 0.5 ML. A combined electrochemical, STM and FTIR study of CO on bare and Pt-modrfiedRu(OOOl) and Rut 1010) surfaces followed. ... 

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