Nanostructures experimental procedures

The second procedure is different from the previous one in several aspects. First, the metallic substrate employed is Au, which does not show a remarkable dissolution under the experimental conditions chosen, so that no faradaic processes are involved at either the substrate or the tip. Second, the tip is polarized negatively with respect to the surface. Third, the potential bias between the tip and the substrate must be extremely small (e.g., -2 mV) otherwise, no nanocavity formation is observed. Fourth, the potential of the substrate must be in a region where reconstruction of the Au(lll) surface occurs. Thus, when the bias potential is stepped from a significant positive value (typically, 200 mV) to a small negative value and kept there for a period of several seconds, individual pits of about 40 nm result, with a depth of two to four atomic layers. According to the authors, this nanostructuring procedure is initiated by an important electronic (but not mechanical) contact between tip and substrate. As a consequence of this interaction, and stimulated by an enhanced local reconstruction of the surface, some Au atoms are mobilized from the Au surface to the tip, where they are adhered. When the tip is pulled out of the surface, a pit with a mound beside it is left on the surface. The formation of the connecting neck between the tip and surface is similar to the TILMD technique described above but with a different hnal result a hole instead of a cluster on the surface (Chi et al., 2000). 

In addition to structural analysis and purity evaluation, Raman spectroscopy can also be used to estimate the crystal size of nanostructured solids. In most cases size characterization using Raman spectroscopy is based on the phonon confinement model (PCM), which uses changes in Raman frequency and Raman peak shape to estimate the crystal size. Although several attempts have been made to relate confinement-induced changes in the Raman spectrum of ND to the crystal size, the agreement between calculated and experimental data and the accuracy of the fitting procedure are still unsatisfactory. A detailed discussion of the limitations of the PCM and the accuracy of previous studies on ND powders is given in Ref [86]. 

There are many experimental techniques for the preparation of nanowires from the liquid phase. A considerable research effort has been expended in developing template-free methods for the deposition of one-dimensional nanostructures in a liquid environment the most important procedures are hydrothermal methods, electrospinning,sonochemical and surfactant assisted. 

As specified at the beginning of the chapter, the number of papers focusing on the use of nanostructures in amperometric sensing is increasing rapidly. In many cases, the nanosized materials involved and the techniques proposed for the deposition onto electrode surfaces are actually variants of well-known systems and procedures. On the other hand, some of the most iimovative experimental approaches are so complex to realize, or require so much expensive instrumentation, that they are very rarely adopted. Moreover, novel systems, such as multimetallic nano-objects and some multicomponent composite materials, still need to be adequately studied with respect to fundamental properties. In addition, some nano-objects developed in different contexts, e.g., organic electronics and catalysis, might be of great interest in the frame of amperometric sensors. 

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