SECM with Other Techniques

In many cases, the electrochemical signal at the tip is not sufficient to assess the tip-substrate distance independently from the interfacial process. In consequence, a variety of instrumental concepts have been investigated to maintain the tip-substrate distance constant, irrespective of the topography, and to operate the SECM in constant-distance mode. In practice, another measurement is performed - either alternatively or simultaneously, and usually with a separate sensor - to determine the tip-substrate distance and continually to adjust the tip position along the z-axis while scanning along the x- and y-axes over the sample. 

The picking mode involves using a hydrodynamic enhancement of the amperometric response to control the distance [122]. This approach was attractive because it only required an electrochemical signal from the tip, but was slow because the desired diffusion-controlled feedback tip current could only be recorded after the tip had been retracted far away (200 pm) then moved quickly (50 trm s ) back towards the sample to reach a preset convective enhancement of the current and, after a rest time sufficient for the signal to decay to its steady state. Moreover, this sequence had to be repeated at every point along the scan. 

Adapted from the technology implemented in scanning near field optical microscopy, SECM with shear-force detection involves vibrating the SECM tip with a piezoelectric acmator and recording variations in resonance frequency when the tip is within a few hundred nanometers from the sample surface [124,125]. This is a fast method when operated in real time during the scan, but it is tricky to implement in practice. The tip must be long and narrow to be appropriately flexible but, more importantly, the control loop parameters need to be frequently adjusted as the resonance and shear force properties vary with the tip dimensions, solution viscosity, and sometimes with the elasticity of the sample surface. Moreover, the parameters need to be readjusted if the tip is removed for polishing. 

The performance of SECM experiments in association with atomic force microscopy (AFM) is typically carried out using a commercial instrument. The concept involves converting the AFM tip into an electrode to operate as the SECM tip, and using the force signal to control its position above the sample surface [127-131]. The tip design (see discussion in Section 12.2.3) is critical and its fabrication technically challenging these points currently limit the take-up of this method. 

SECM associated with scanning ion conductance microscopy (SICM) requires a double tip, on one side of which is a conventional microdisc electrode and on the other side is a narrow pipette filled with electrolyte and an electrode that measures ionic conductance through the mouth of the pipette with respect to another electrode in the bulk solution. When the pipette mouth is within one pipette tip radius away from the sample surface, the conductance varies sufficiently to be used as a control signal to maintain the z-position of the tip during the scans, thereby affording constant-distance SECM operations [133,134]. This methodology is fast and apparently less-challenging to implement than shear force SECM, but it requires the fabrication of double-barrel tips in which one channel is left empty and the other is filled with a conventional microdisc. 

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Ring UME tips (Figure 12.6d) have received attention only recently because of the increased interest in the combination of SECM with other techniques such as AFM (6) and optical miCToscopy (10). Fabrication of ring UMEs involves several steps ... 

Although SECM is a mature technique, there is still room for further developments in different directions, such as new SECM probes and new fabrication methods, new SECM operation modes, combination of SECM with other techniques, and the application of SECM to novel interfaces and systems under extreme conditions. These are discussed in the following sections. 

COMBINING SECM WITH OTHER TECHNIQUES Photodiode... 

The quartz crystal nanobalance (QCN) can be combined with practically any electrochemical methods, such as cyclic voltammetry, chronoamperometry, chronocoulometry, potentiostatic, galvanostatic, rotating disc electrode [11], or potentiometric measurements. The EQCN can be further combined with other techniques, e.g., with UV-Vis spectroscopy [12], probe beam deflection (PBD) [13], radiotracer [14], atomic force microscopy (AEM) [15], and scanning electrochemical microscopy (SECM) [16]. The concept and the instrumentation of... 

In Chapter 17, the combination of SECM with other techniqnes has been presented. Hybrid systems, like SECM-AFM, are useful for obtaining more information abont processes at an interface. In addition to these techniques, SECM can be combined with other methodologies that allow further sample analysis and afford the extraction of sample structural information, for instance, by coupling SECM with mass spectrometry (MS). Several approaches for scanning mass spectrometry have been reported for the spatial detection of different species at solid-liqnid, liquid-liquid, or air-solid interface. Both chemical " and electrochemical reactions have been characterized by these means. These approaches are based on the use of concentric capillaries that allow the flux of carrier gas or redox components to the substrate (i.e., through the outer capillary) and the extraction... 

In addition, one can envisage the coupling of SECM with other analytical techniques such as liquid chromatography (LC) or capillary electrophoresis (CE). Therefore, relevant analytes detected in a complex sample by SECM can be extracted and separated by a secondary analytical technique. As a consequence, online monitoring of electrochemical and chemical surface information can be imagined (see Figure 18.7). 

Complementary with other imaging techniques, such as electron microscopy and other scanning probe microscopies (SPMs), SECM routinely offers /xm resolution of the surface structures in bulk electrolyte solutions. SECM also... 

The major advantage of the SECM as a patterning tool lies in its ability to drive a wide range of chemical and electrochemical surface reactions. As compared with other scanning probe techniques, e.g., STM and AFM, in an SECM experiment the mechanism of the surface process is usually known and involves electrochemical and chemical processes. Moreover, the con-... 

A highly significant recent development is the integration of atomic force microscopy with SECM (AFM-SECM) [220]. Further details are described in Chapter 3.2 of this volume. An etched Pt wire was flattened and then insulated with electrophoretic paint. This probe acts as the cantilever in AFM and as the SECM tip providing dual force-sensing and electrochemical capabilities. Submicron resolution for SECM and for topography by AEM was achieved for test samples of track-etched polycarbonate membranes and ionic crystal surfaces. As the resolution of the technique improves and it is combined with other scanning probes, it seems likely that SECM can be applied in an ever-wider spectrum of scientific fields. 

The essential components of a basic SECM setup are shown in Fig. 2. As with other scanning probe techniques, an SECM is equipped with a positioning system to allow for lateral scanning as well the vertical approach of the tip as close as possible to the surface to be probed. 

Distinguishing between ECE and DISPl mechanisms by many conventional electrochemical methods is difficult. However, in common with generation-collection measurements at other double electrode geometries, and earlier applications of double potential step transient methods, generation-collection and feedback measurements with SECM have been shown to allow an unequivocal mechanistic assignment. Moreover, SECM allows the measurement of larger rate constants, for Equation 7.29, than these other techniques. 

While the speed of patterning is probably the major reason that limits the wide application of scanning probe techniques as surface modification tools, their resolution capabilities is the driving force that keeps them as future competitors of nowadays methods. The resolution of the patterns formed by the SECM depends on the size and geometry of the UME as well as on the distance it is held above the surface. At the same time, the resolution of the patterns can easily be controlled (as compared with other SPM techniques) by different approaches such as the chemical lens. This will enable forming patterns with different widths just by controlling the electrolyte and surface-UME distance rather than by changing the UME. 

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