In Situ SECM Characterization

The SECM theories were developed for various tip shapes to enable the in situ quantitative characterization of a nanotip by approach curve measurement in solution. The fit of an experimental approach curve with a theoretical curve provides quantitative information about the shape and size of an SECM tip as originally proposed in 

FIGURE 1.24 (a) Experimental (symbols) approach curve for tetraethylammonium transfer 

DCE-fllled nanopipette with an inner radius of 8.1 nm. The best fit was obtained with RG = 1.9, while the experimental curve significantly deviates from theoretical curves with larger or smaller RG values of 1.1 or 2.5, respectively. This analysis also gave an extremely short distance of 0.8 nm at tip-substrate contact, which confirmed that the orifice of the polished nanopipette is very flat. The flatness of a tip can be checked more precisely by a positive approach curve, which is more sensitive to the tip-substrate distance. A positive approach curve with an inlaid disk tip with any RG is empirically given by Reference [150] 

FIGURE 1.25 (a) Experimental approach curve (squares) as obtained with an etched Pt 

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The shape and size of a nanotip must be accurately determined for nanoscale SECM, especially when data is quantitatively analyzed or spatial resolution is quantitatively assessed. It shonld be emphasized that, without the knowledge of tip shape, tip size can be readily nnderestimated or overestimated from a limiting current in the bulk solution, which can be conveniently measured by amperometry and voltanunetry. The shape and shape of a nanotip can be much more reliably characterized by SECM. An important advantage of the SECM-based characterization is that the effective shape and size of a nanotip can be determined in solution. This in situ SECM characterization contrasts to ex situ SEM, FIB, and TEM characterization. Recently, AFM was introduced as a unique method for both in situ and ex situ tip characterization. 

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