Molecular electronics macroscopic

The maintenance of a connection to experiment is essential in that reliability is only measurable against experimental results. However, in practice, the computational cost of the most reliable conventional quantum chemical methods has tended to preclude their application to the large, low-symmetry molecules which form liquid crystals. There have however, been several recent steps forward in this area and here we will review some of these newest developments in predictive computer simulation of intramolecular properties of liquid crystals. In the next section we begin with a brief overview of important molecular properties which are the focus of much current computational effort and highlight some specific examples of cases where the molecular electronic origin of macroscopic properties is well established. 

To extract useful results from a molecular electronic device, or just to measure its electronic characteristics, connections must be made to macroscopic probes. That is, metallic electrodes must interface to different ends of the molecule of interest. An experiment may interrogate a single molecule, or may measure a one-molecule-thick layer, i.e., a monolayer, of the molecules of interest, provided all the molecules are oriented in the same direction. In either case, several questions arise. What is the nature of the contact between metal and molecule(s) What metal should be chosen, and what should be the form or shape of this electrode ... 

In particular, poly(amidoamine) dendrimers were peripherally modified with diimide moieties (see the structure shown in Scheme 1.43). After rednction with dithionite, this dendrimer was cast into a film, the electronic properties of which were isotropic. (This means that on the molecular and macroscopic levels, there is a three-dimensional (3-D) electron delocalization.) The conductivity was humidity dependent. Water molecules integrate into the material s crystal structure and take part in long-distance electron transfer. Such an effect of water was also observed to enhance electric... 

In addition to characterization of molecular and macroscopic electro-optic activity, it is important to define optical loss. Optical loss can be influenced both by absorption and by scattering effects. In order to minimize overall loss, it is important to understand the independent contributions made by scattering and absorption. To separate these effects, we need to determine the contributions made by both chromophore and polymer host to the optical absorption at device operating wavelengths. Chromophore interband electronic absorption can be measured on resonance by traditional UV-Visible spectrometry however, we will typically be concerned with optical absorption at telecommunication wavelengths of 1.3 and 1.55 microns where such techniques do not provide accurate information. Total optical absorption at 1.3 microns is occasionally determined by both the interband electronic absorption of the chromophore and by C-H vi-... 

As the limits of silicon are approached, molecular electronics offers an alternative route to the design of materials for use not only at the macroscopic scale but also... 

Provided the intriguing concept of molecule-based electronics could be fully realized, we can expect an enormous increase in the integration density and storage capacity of computers (and, eventually, sensors) by many orders of magnitude, compared with present technology. However, several critical issues must be addressed in order to realize the promise of molecular electronics. These include the development of functional units, the simplest of which is the conductor (or wire), access to these units from the macroscopic world, and... 

All aspects of molecular shape and size are fully reflected by the molecular electron density distribution. A molecule is an arrangement of atomic nuclei surrounded by a fuzzy electron density cloud. Within the Born-Oppenheimer approximation, the location of the maxima of the density function, the actual local maximum values, and the shape of the electronic density distribution near these maxima are fully sufficient to deduce the type and relative arrangement of the nuclei within the molecule. Consequently, the electronic density itself contains all information about the molecule. As follows from the fundamental relationships of quantum mechanics, the electronic density and, in a less spectacular way, the nuclear distribution are both subject to the Heisenberg uncertainty relationship. The profound influence of quantum-mechanical uncertainty at the molecular level raises important questions concerning the legitimacy of using macroscopic analogies and concepts for the description of molecular properties. ... 

Kinetic Theory of Fracture. Catastrophic failure of a polymeric material is a complex process in which a sequence of partially understood events occurs at both the molecular and macroscopic levels. The stress-induced cleavage of the main-chain polymer bond is one event occurring on the molecular level which has been studied by both stress MS and electron spin resonance spectroscopy (ESR). 

We must rather consider molecular electronics in a wider sense. Every electronic unit whose function is based on a molecular property can be considered to be a molecular-electronics device. Thus, we can define a macroscopic molecular electronics. A first success story on the road to molecular electronics have been the colour displays based on OLEDs (see Chap. 11). The pixels of these displays consist... 

The path to single molecules as switching elements, and thereby from mono-molecular to macroscopic molecular electronics, is long. After finding molecules with the desired properties, one will have to investigate ... 

Furthermore, a way must be found to permit the transfer of information between the molecules and the macroscopic world. An information exchange with other molecular-electronic subunits and, via an interface, with conventional electronic devices must be possible. Research in these areas is stiU in its infancy. In any case, molecular electronics will require molecules as switching elements and as connecting wires . These applications are discussed in the next Sects. 12.2 and 12.3. 

Finally, aside from the OLEDs and solar cells described in Chap. 11, a series of other devices for macroscopic molecular electronics have been demonstrated, in which numbers of organic molecules carry out a specific function. We mention rectifiers, transistors, and storage elements. These will be discussed in Sects. 12.5, 12.6, and 12.7. Instead of a crystal, most of the conceivable applications in molecular electronics will presumably make use of organised and structured films. The preparation and characterisation of such films is therefore particularly important [21, 22]. We shall, however, not treat it in detail here. 

The ability to connect by means of wiring is an important precondition for the fabrication of networks of molecular-electronic elements. For this purpose, as well as for the connection to macroscopic devices, one requires connecting wires on a molecular scale. The search for suitable molecular conductors has up to now been more a goal for molecular physics than for solid-state physics. We refer in this connection to Sects. 21.5 and 22.4, as well as 22.5, in the textbook by Haken and Wolf[l]. 

Molecular electronics" (ME) (sensii stricto), or molecular-scale electronics" or unimolecular electronics" (UE) is the study of electrical and electronic processes measured or controlled om a molecular scale or on the nanometer scaleJ A wider definition of molecular electronics sensu lato), or "molecule-based electronics" encompasses electronic p ocesses by molecular assemblies of any scale, including macroscopic crystals and conducting polymers/ This article deals with UE and focuses on electrical conduction (asymmetric or not), through single molecules or through a monolayer of molecules measured in parallel. 

Molecular electronics (ME) is so named because it uses molecules to function as switches and wires . ME is a term that refers both to the use of molecular materials in electronics and to electronics at molecular level. It is as yet not very clear how molecular electronic devices will operate, but it is conjectured that active molecules are needed, either in isolation or becoming active by association with other molecules. It is thought that electronics is likely to imitate some of the basic functions of macroscopic devices such as memories, sensors and logic circuits. 

The extraction of the molecidar parameters requires an assumption on the orientation distribution for the p angle and on the relative magnitude of the different elements of the molecular hyperpolarizabihty. This is achieved with the knowledge of the symmetry of the molecular electronic transitions lying in the vicinity of the fundamental and the harmonic wavelengths. Finally, working with relative intensities on the macroscopic susceptibihty tensor, one can extract the orientation angle 9, usually determined for a narrow distribution, and the ratio of the two dominant hyperpolarizability tensor elements. 

---