Functional devices, assembling

Multilayer assemblies of nanocrystals of CdSe are generally fragile and are not suited for use in functional devices. Assemblies involving alternate layers of nanocrystals and linkers are relatively robust. The method of layer-by-layer deposition has received a good deal of attention over the last few years, since it provides a convenient low-cost means to prepare ultrathin films of controlled thickness, suited for device applications. In a typical experiment, one end of a monolayer forming a bifunctional spacer, is tethered to a flat substrate such... 

Finally, to produce the structural and functional devices of the cell, polypeptides are synthesized by ribosomal translation of the mRNA. The supramolecular complex of the E. coli ribosome consists of 52 protein and three RNA molecules. The power of programmed molecular recognition is impressively demonstrated by the fact that aU of the individual 55 ribosomal building blocks spontaneously assemble to form the functional supramolecular complex by means of noncovalent interactions. The ribosome contains two subunits, the 308 subunit, with a molecular weight of about 930 kDa, and the 1590-kDa 50S subunit, forming particles of about 25-nm diameter. The resolution of the well-defined three-dimensional structure of the ribosome and the exact topographical constitution of its components are still under active investigation. Nevertheless, the localization of the multiple enzymatic domains, e.g., the peptidyl transferase, are well known, and thus the fundamental functions of the entire supramolecular machine is understood [24]. 

Research on semiconductor nanoparticle technology by chemists, materials scientists, and physicists has already led to the fabrication of a number of devices. Initially, Alivisatos and co-workers developed an electroluminescence device from a dispersion of CdSe nanoparticles capped with a conducting polymer349 and then improved on this by replacing the polymer with a layer of CdS, producing a device with efficiency and lifetime increased by factors of 8 and 10, respectively. 0 Chemical synthetic methods for the assembly of nanocrystal composites, consisting of II-VI quantum dot polymer composite materials,351 represent one important step towards the fabrication of new functional devices that incorporate quantum dots. 

Recently, Whitesides et al. [88, 164, 165] have replaced Hg with an In/Ga eutectic alloy (E-Gain) (Fig. 5c). In/Ga alloy-based electrodes present few advantages related to (1) the lower affinity for the bottom Au or Ag electrode, so that the junction can be assembled in air, (2) low toxicity and (3) good processability and mouldability. These characteristics indicate E-Gain electrodes as possible candidates for incorporation into functional devices. Some disadvantages are related to the surface of the In/Ga alloy (1) unlike Hg, it is not atomically flat and (2) it forms in few minutes a discontinuous layer of oxide [81]. 

The ability to synthesize NWs rationally with controlled electronic properties and to assemble NWs into regular arrays readily enables exploration of these NW structures for a variety of functional device arrays. 

Cui, C. and Lieber, C.M., Functional nanoscale electronic devices assembled using silicon nanowire building blocks, Science, 291, 851,2001. 

For many applications such as catalysis and possible functional devices, SAMs are simply too thin, the organized structure not flexible enough or the sterical situation within the layer too confined in order to incorporate a desired function or respond to changes in the environment in a dynamic and reversible way. One approach to increase the layer thickness of well-ordered self-assembled stractures of up to 100 nm is the formation of SAM and LB multilayers by means of consecutive preparation steps (Fig. 9.1 (3)) [5, 108]. This strategy was successfully applied by several research groups, but requires the constant intervention of the experimenter to put one type of monomolecular layer on top of the other. The dynamic behavior of the layer is limited by the crystal-like organization of the system and the extreme confinement of all surface-bonded molecules. Hence, surface... 

In this book we will take a closer look at a great variety of different strategies that are pursued to assemble and organize nanostructures into larger assemblies and even into functional devices or materials. 

In this chapter, we describe three different systems with which to construct electro- and photo-functional molecular assemblies on electrode surfaces. The first is the bottom-up fabrication of redox-conducting metal complex oligomers on an electrode surface and their characteristic redox conduction behavior, distinct from conventional redox polymers.11-13 The second is a photoelectric conversion system using a porphyrin and redoxconducting metal complex.14 The third is the use of a cyanobacterial photosystem I with molecular wires for a biophotosensor and photoelectrode.15 16 These systems will be the precursors of new types of molecular devices working in electrolyte solution. 

Y. Cui, C. Lieber, Functional Nanoscale Electronic Devices Assembled Using Silicon Nanowire Building Blocks, Science 291 (2001) 851. 

A similar functional device can be constructed through the orientation-controlled assembly of molecules with donor, acceptor and sensitizer parts. Three kinds of dye - ferrocene, viologen and pyrene - were covalently con-... 

Transition metal polypyridine complexes are highly redox-active, both in their electronic ground- and excited states. Their electron transfer reactivity and properties can be fine-tuned by variations in the molecular structure and composition. They are excellent candidates for applications in redox-catalysis and photocatalysis, conversion of light energy into chemical or electrical energy, as sensors, active components of functional supramolecular assemblies, and molecular electronic and photonic devices. 

In addition, the subject of assembly looms as an apparently insurmountable obstacle. It is not at all clear how researchers would go about assembling individual molecular components into a functioning device, although bioengineering offers a potential solution to this problem. Examples of self-assembled structures exist everywhere in nature from the helical secondary structure of DNA to the human brain. Current knowledge of such systems is simply inadequate to allow scientists to employ similar forces to create synthetic molecular electronic devices. Clearly, an enormous amount of groundwork needs to be laid if the concept of the molecular electronic device or biochip computer is ever to become a reality. 

Multilayer assemblies using monothiols such as those of CdSe (see Figure 4.15) are generally fragile and are not suited for use in functional devices. One of the... 

So far, we have shown that redox-modulated recognition, a prevalent feature in biological systems, can be employed in the design of functional devices. As mentioned earher, one challenge to the efficient application of such systems is the ability to immobilize, order and thus individually address them. One way to provide the desired anisotropy is through the use of colloids functionalized with self-assembled monolayers (SAMs). " In a recent model smdy, a diacyl diaminopyri-dine-functionahzed gold colloid (DAP-Au), capable of binding flavin, has been... 

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