Top-down technique

FIGURE 15.50 The images show four stages in the construction of a ring of iron atoms on a copper substrate. The scientists used top-down" techniques to place 48 iron atoms into a ring. The circular waves seen in the final image show the density of the surface electrons inside the ring, whit h acts as a "i orral" lor the pint trons. 

Since the first demonstration that DNA can act as a scaffold for metallization to produce conducting wires [270], the proposal to create circuits on a nanometre size scale has appeared achievable. With CMOS technology predicted to reach a miniaturization limit by 2012 [271], and the high cost and low throughput associated with nanometre scale scanning probe and electron beam lithographic top-down techniques, the DNA-templated synthesis of wires with widths typically less than 50 nm may provide a scheme to cir-... 

Nanotechnology requires a different approach to fabrication from that of microtechnology. Whereas microscale structures are typically formed by top-down techniques (lithography, deposition, etc.), the practical formation of structures at nanoscale dimensions involves additional components, that is, bottom-up self assembly. 

Hierarchical clustering procedures iteratively partition the item set into disjointed subsets. There are top-down and bottom-up techniques. The top-down techniques partition can be into two or more subsets, and the number of subsets can be fixed or variable. The aim is to maximize the similarity of the items within the subset or to maximize the difference of the items between subsets. The bottom-up techniques work the other way around and build a hierarchy by assembling iteratively larger clusters from smaller clusters until the whole item set is contained in a single cluster. A popular hierarchical technique is nearest-neighbor clustering, a technique that works bottom up by iteratively joining two most similar clusters to a new cluster. 

One-dimensional (1-D) nanosbuctures (b) are composed mainly of nanowires, nanorods and nanobelts. Spontaneous growth, template-based synthesis and electrospinning are considered bottom-up approaches, while lithography is a top-down technique [7, 84]. Two-dimensional (2-D) nanostructures (c) involve thin films, which have been the object of intensive study for almost a century and for which many methods have been developed and improved. 

The overall handling of nanoparticles in terms of safety becomes also more and more an important issue. Thus, the preparation of safe and easy to handle dispersions is of great technical importance. Those particles being synthesized in I Ls may be used direcdy, being inherently safe [13]. On the other hand, nanoparticles that are manufactured by other chemical bottom-up-techniques or via top-down techniques can be dispersed by using specific ILs, also because of their generally versatile surface active properties [14]. 

HEA analysis is a top-down technique used to model the relationship between critical human failures and hazards, and the mitigating aspects of the system design. 

The first of the four tests causation asks whether harm could occur because of some unsafe matter on which a charge of negligence could be based. Top-down techniques are appropriate at this phase of a functional safety assessment. The worst case scenario of destruction of a fully occupied platform would be 500 fatalities. 

There are now three top-down techniques developed to realize porous silicon membranes from solid silicon electrochemical etching (anodization), micromachining, and thin film deposition/annealing. These techniques create different pore morphologies and are suited to different membrane thiek-nesses and porosity ranges. In this regard they are quite complementary. 

Porous silicon has been fabricated by both top-down techniques from solid silicon and bottom-up routes from silicon atoms and silicon-based molecules. Over the last 50 years, electrochemical etching has been the most investigated approach for chip-based apphcations and has been utilized to create highly directional mesoporosity and macroporosity. Chemical conversion of porous or solid silica is now receiving increasing attention for applications that require inexpensive mesoporous silicon in powder form. Very few techniques are currently available for creating wholly microporous silicon with pore size below 2 nm. This review summarizes, from a chronological perspective, how more than 30 fabrication routes have now been developed to create different types of porous silicon. 

Note that there are currently very few techniques to make wholly microporous silicon (see handbook chapter Microporous Silicon ) where the average pore diameter is under 2 nm. For virtually all top-down techniques, the porous silicon created is poly crystalline. For some bottom-up techniques such as sputtering/dealloying (Fukatani et al. 2005), electrodeposition (Krishnamurthy et al. 2011), or sodiothermic reduction (Wang et al. 2013), it is reported to be amorphous. Choice of fabrication technique for both mesoporous and macroporous silicon is very much dictated by application area, which in turn has differing requirements on porosity levels, pore morphology, skeleton purity, physical form, cost, and volume. 

Within the last decade the use of nanoclusters for novel (bio)electronic and (bio)optical devices gained world-wide attention due to a number of key-developments describe in this article. Furthermore, attempts to reduee the size of electronic devices with top down techniques will encounter insurmountable barriers due to limits of UV-lithography. The use of colloidal particles as elements and building blocks for new devices is a novel and cost efficient route. Bio-nano-assemblies will enable us to construct three-dimensional electronic circuits of ultra-high packing density. Manufacturing of bio-nano-devices will require the development of a completely new set of techniques to arrange, manipulate and couple colloidal particles. 

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