Scanning probe microscopy feedback

Figure 5.2 The scanning tunneling microscope (STM) system. The scanner moves the tip over the sample surface and the feedback loop keeps the tunneling current constant. (Reproduced with kind permission of Springer Science and Business Media from E. Meyer, J.H. Hug, andR. Bennewitz, Scanning Probe Microscopy the Lab on a Tip, Springer-Verlag, Berlin. 2004 Springer-Verlag GmbH).

Since the pioneering work of Binnig157 in the 1980s, the family of microscopic techniques collectively known as scanned probe microscopy (or SPM) has become widely available and is extensively used in conducting polymer research. SPM consists of a number of related techniques in which a fine probe is rastered across a sample surface. Interaction between the probe tip and the sample drives a feedback system that allows topographical mapping of the sample surface. Scanning tunnel-... 

Fast scan measurements, i.e. for investigations of the dynamics of surface diffusion or reconstruction are done preferably in constant height instead of constant current mode because no electronic feedback circuit, limiting response time and scan speed, is involved in this mode. Obviously this works only with very smooth electrode surfaces. An electronic setup (bipotentiostat) that allows fast transient methods combined with scanning probe microscopies has been reported [21]. 

SPM Scanning probe microscopy (a general term encompassing all types of microscopy which utilize a scanned micro-sharpened probe and feedback circuitry to image nanometric phenomena, including AFM, ECAFM, LFM, STM, and many others)... 

Scanning probe microscopy techniques commonly used for electrochemical measurements include SICM, SECM, and SECM-SICM. Each technique has distinct advantages and disadvantages. Important considerations are type(s) of signal detected, feedback mechanism, imaging conditions, relative ease of probe fabrication and resolution, many of which are interrelated. In the following section, SICM, SECM, and SECM-SICM will be considered relative to one another. 

In scanning electrochemical microscopy (SECM) a microelectrode probe (tip) is used to examine solid-liquid and liquid-liquid interfaces. SECM can provide information about the chemical nature, reactivity, and topography of phase boundaries. The earlier SECM experiments employed microdisk metal electrodes as amperometric probes [29]. This limited the applicability of the SECM to studies of processes involving electroactive (i.e., either oxidizable or reducible) species. One can apply SECM to studies of processes involving electroinactive species by using potentiometric tips [36]. However, potentio-metric tips are suitable only for collection mode measurements, whereas the amperometric feedback mode has been used for most quantitative SECM applications. 

As described in the introduction, submicrometer disk electrodes are extremely useful to probe local chemical events at the surface of a variety of substrates. However, when an electrode is placed close to a surface, the diffusion layer may extend from the microelectrode to the surface. Under these conditions, the equations developed for semi-infinite linear diffusion are no longer appropriate because the boundary conditions are no longer correct [97]. If the substrate is an insulator, the measured current will be lower than under conditions of semi-infinite linear diffusion, because the microelectrode and substrate both block free diffusion to the electrode. This phenomena is referred to as shielding. On the other hand, if the substrate is a conductor, the current will be enhanced if the couple examined is chemically stable. For example, a species that is reduced at the microelectrode can be oxidized at the conductor and then return to the microelectrode, a process referred to as feedback. This will occur even if the conductor is not electrically connected to a potentiostat, because the potential of the conductor will be the same as that of the solution. Both shielding and feedback are sensitive to the diameter of the insulating material surrounding the microelectrode surface, because this will affect the size and shape of the diffusion layer. When these concepts are taken into account, the use of scanning electrochemical microscopy can provide quantitative results. For example, with the use of a 30-nm conical electrode, diffusion coefficients have been measured inside a polymer film that is itself only 200 nm thick [98]. 

Scanning electrochemical microscopy (SECM the same abbreviation is also used for the device, i.e., the microscope) is often compared (and sometimes confused) with scanning tunneling microscopy (STM), which was pioneered by Binning and Rohrer in the early 1980s [1]. While both techniques make use of a mobile conductive microprobe, their principles and capabilities are totally different. The most widely used SECM probes are micrometer-sized ampero-metric ultramicroelectrodes (UMEs), which were introduced by Wightman and co-workers 1980 [2]. They are suitable for quantitative electrochemical experiments, and the well-developed theory is available for data analysis. Several groups employed small and mobile electrochemical probes to make measurements within the diffusion layer [3], to examine and modify electrode surfaces [4, 5], However, the SECM technique, as we know it, only became possible after the introduction of the feedback concept [6, 7],... 

A further spatially resolved method, also based on work function contrast, is scanning Kelvin probe microscopy (SKPM). As an extended version of atomic force microscopy (AFM), additional information on the local surface potential is revealed by a second feedback circuit. The method delivers information depending on the value (p (p(x) + A x). Here, A(zS(x) is the difference in work function between the sample and the AFM tip and cp(x) is the local electric potential [12]. (p x) itself gives information on additional surface charges due to... 

Characterize nitride layer microstructure and composition by x-ray diffraction, electron probe microanalysis, scanning electron microscopy, and transmission electron microscopy. Use this information in a feedback loop to modify alloy chemistry and nitridation processing conditions to optimize the protectiveness of the nitride surface layer. 

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