Defect nanostructuring

FIGURE 36.1 Schematic illustration of some electrochemical techniques employed for surface nanostructuring (a) tip-induced local metal deposition (b) defect nanostructuring (c) localized electrochemical nucleation and growth d) electronic contact nanostructuring. 

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 

FIGURE 36.2 Height of the Cu nanostructure generated in a hole on a Au(l 11) surface as a function of time when a sequence of potential steps running in the negative direction are applied to the electrode. (From Xia et ah, 1999, with permission from Elsevier.) 

It is possible to generate holes on the surface of the substrate through the application of very short negative voltage pulses to the STM tip. This procedure only succeeds using highly concentrated electrolytes. 

If the potential applied to the substrate is controlled carefully, it is possible to confine the deposition of copper from the solution to the volume inside the hole. This is so because metal deposition on the Cu(l X 1) stmcture outside the hole is disfavored with respect to metal deposition on the hole, where the lattice parameter of Cu should be close to that of the bulk metal. 

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The first successful attempts of electrochemical nanostructuring, pioneered by Penner et al. [69], involved the generation of surface defects by the tip at predetermined positions, which were created either by a mechanical contact between tip and substrate (tip crash) or by some sort of sputtering process, initiated by high-voltage... 

Bioactive materials can be used as powders for filling small defects and as coatings that enhance metallic prosthesis fixation. However, when considering bioactive materials for bone regeneration in medium and large defects, bioactive pieces with appropriate mechanical properties are required. At this point, the key is to keep the properties provided by the nanostructure when processing a piece at the macroscopic... 

Self-assembly processes in nature are sometimes catalyzed by enzymes. Zeolites are, in many ways, the inorganic counterparts of enzymes, with their ability to selectively bind other substances and perform catalysis. Can templates or catalysts be effective in increasing rates and reducing defects in a wide range of nanostructured materials ... 

Characterization of the surface impurities on the catalyst is also essential, and photoreactivity data should be analyzed in terms of active and accessible surface area. The defect state of the surface and nanostructure are also important aspects to understand. Current advances in the synthesis allow preparing Titania or titanate nanorods with different diameter and aspect ratio, and different surface nanostructure as well. Limiting the discussion here to only preparations by hydrothermal treatment (for reasons of conciseness), various mechanisms of growing of the nanorods has been reported. The differences in the mechanism of formation would imply differences in the surface characteristics of the nanorods, but there is no literature available on this topic. 

These novel carbon nanostructures can also be modified by (a) doping, that is the addition of foreign atoms into the carbon nanostructure, (b) by the introduction of structural defects that modify the arrangement of the carbon atoms and (c) by functionalization involving covalent or noncovalent bonding with other molecules. These modifications opened up new perspectives in developing novel composite materials with different matrices (ceramic, polymer and metals). For example, polymer composites containing carbon nanostructures have attracted considerable attention due to... 

In this section, different nanocarbons and their chemical and physical properties are discussed (for more details see Chapters 1 and 2). Furthermore, the types of defects that can be embedded within these carbon nanostructures are explained, as well as their resulting chemical and physical properties. 

Defects in carbon nanostructures can be classified into (a) structural defects, (b) topological defects, (c) high curvature and (d) non-sp2 carbon defects. Even slight changes within the carbon nanostructure can modify the chemical and physical properties. Some defects in carbon systems results in high chemical reactivity, mainly due to the accumulation of electrons in the vicinity of the dopant. These defects can be used as anchoring sites in order to make the carbon nanostructures more compatible with ceramic or polymer matrices, thus enhancing interactions between carbon structures (filler) and the host matrices. 

In the beginning, functionalization reactions were applied to fullerenes [1], later to CNTs [4,3], and recently to graphene [5]. Although both functionalization approaches have clear differences, they share the same intrinsic objective the creation of defects or doping within the surface of the carbon nanostructures in order to facilitate the interactions between the matrix and the filler. 

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