Molecular electronics definition

In view of current state of the art, namely, silicon-based technology, molecular electronics definitely exceed the expectations of a single product line. Going back, for instance, to 1960 silicon-based electronics were nearly exclusively considered as a simple replacement for the vacuum tube. However, it would have been myopic to limit the potential of silicon in that field of research. In fact, silicon constituted a technological platform, which evolved into the development of various products, most of them unfathomable at the time. Similarly, molecular electronics may be considered as a platform technology, rather than a single product line, which may give rise to many industrial products which are currently unforeseeable. 

Clearly the form of a deformation density depends crucially on the definition of the reference state used in its calculation. A deformation density is therefore meaningful only in terms of its reference state, which must be taken into account in its interpretation. As we will see shortly, the theory of AIM provides information on bonding directly from the total molecular electron density, thereby avoiding a reference density and its associated problems. But first we discuss experimentally obtained electron densities. 

As the analytical, synthetic, and physical characterization techniques of the chemical sciences have advanced, the scale of material control moves to smaller sizes. Nanoscience is the examination of objects—particles, liquid droplets, crystals, fibers—with sizes that are larger than molecules but smaller than structures commonly prepared by photolithographic microfabrication. The definition of nanomaterials is neither sharp nor easy, nor need it be. Single molecules can be considered components of nanosystems (and are considered as such in fields such as molecular electronics and molecular motors). So can objects that have dimensions of >100 nm, even though such objects can be fabricated—albeit with substantial technical difficulty—by photolithography. We will define (somewhat arbitrarily) nanoscience as the study of the preparation, characterization, and use of substances having dimensions in the range of 1 to 100 nm. Many types of chemical systems, such as self-assembled monolayers (with only one dimension small) or carbon nanotubes (buckytubes) (with two dimensions small), are considered nanosystems. 

Molecular electronics currently is defined as the use of organic molecular materials to perform an active function in the processing of information and its transmission and storage. An alternative definition has been suggested, namely, the achievement of switching on a molecular scale. As observed by G.G. Roberts (University of Oxford), It is interesting to note that only a modest diminution in the size of electronic circuit components is required before the scale of individual molecules is reached in fact many existing ciicuit elements could alieady be accommodated within the aiea occupied by a leukemia virus. ... 

The presence of overcrowding in triphenylene has been demonstrated by Clar (1950) from an examination of the absorption spectra at 18°C and — 170°C. At — 170°C the / -band spectra of such aromatic hydrocarbons as benzene, naphthalene, anthracene, and pyrene become more distinct, showing much more fine structure than at 18°C. This is explained by the cessation at low temperature of thermal collisions which produce molecular deformations, thereby improving the definition of the molecular electronic orbitals. Where this change in spectra does not occur, permanent deformation at both low and high temperatures... 

The demonstration of molecular electronics has been really difficult, as can be seen for the Aviram-Ratner rectifier. The only molecular device that has been demonstrated experimentally is the molecular photodiode, by Fujihira et al. [5-7]. In the course of time, the term molecular electronics has acquired a broader definition, such as that of being the field in which organic molecular materials perform an active function in the processing and its transmission and storage [8]. 

Considerable progress has been made in this area since the pertinent section of MBBMA was written. At that time only the Mo2 molecule had been well characterized both experimentally and theoretically, although some results were available on other molecules that have only weak bonds in which the d orbitals play essentially no role. In such molecules, of which Fe2, Co2, Ni2 and Cu2 seem typical, the bond is formed by overlap of the valence shell 4s orbitals and the two cF configurations then interact only weakly. However, these d" + d systems give rise to an enormous number of molecular electronic states close in energy to the ground state. These molecules are therefore very difficult to characterize definitively by either theoretical or experimental means. 

The field of molecular electronics may be considered to encompass much more than molecular electronic devices. In its broadest context, molecular electronics may be regarded as simply the application of molecules, primarily organic molecules, to electronics. This definition would include such areas as liquid crystalline materials, piezoelectric materials such as poly(vinylidine fluoride), chemically sensitive field-eflFect transistors (CHEMFET), and the whole range of electroactive polymers. These applications are beyond the scope of this book and are covered in other reviews 34, 33). However, given the basic tenet of molecular electronics, namely, the ability to engineer and assemble molecular structures into a useful device, the broader definition raises the question of whether organic molecules can be specifically assembled or engineered for unique applications in electronics. 

Primary process" has been used in accordance with this recently suggested definition (2) "Any continuous sequence of one or more primary steps which starts with the light absorption step." In this sense a primary step is "any one of the elementary transformations of an excited state molecule of the species which absorbs light. The absorption step Itself is also a primary step" (2). Important primary processes of OTM compounds which are described here include (1) absorption, (ii) dissociative reactions, (iii) intramolecular "twisting" isomerizations, (iv) intermolecular energy transfer, (v) inter-molecular electron transfer, (vi) luminescence. Reactions involving OTM compounds as quenchers have also been included. 

Within the quantum chemical description of molecular electron density clouds, a natural criterion, the Density Domain criterion, provides a quantum chemical definition for functional groups [14-18]. Furthermore, techniques that generate fuzzy electron density contributions for local molecular moieties that are analogous to the fuzzy electron density clouds of complete molecules, determined by the analytic Additive Fuzzy Density Fragmentation (AFDF) method [19-21], or the earlier numerical-grid MEDLA method [22,23], are also... 

On the basis of these definitions one can describe chemical bonding in molecules containing noble gas elements with the aid of the properties of p(r). One starts by searching for the bond paths 2uid their associated bond critical points Tg in the molecular electron density distribution. If all bond paths are found, then the properties of p(r) along the bond paths will be used to characterize the chemical bonds. For example, the value of can be used to determine a bond order, the anisotropy of Pp can be related to the n character of a bond, the position of the bond critical point is a measure of the bond polarity and the curvature of the bond path reveals the bent-bond character of a bond [17, 19]. 

While semiempirical models which can be applied to molecules the size of 1 and 2 are necessarily only approximate, we were searching for trends rather than absolute values. In concept, the design of semiempirical quantum mechanical models of molecular electronic structure requires the definition of the electronic wavefunction space by a basis set of atomic orbitals representing the valence shells of the atoms which constitute the molecule. A specification of quantum mechanical operators in this function space is provided by means of parameterized matrices. Specification of the number of electrons in the system completes the information necessary for a calculation of electronic energies and wavefunctions if the molecular geometry is known. The selection of the appropriate functional forms for the parameterization of matrices is based on physical intuition and analogy to exact quantum mechanics. The numerical values of the parameters are obtained by fitting to selected experimental results, typically atomic properties. 

Recently many quantum chemists have dedicated a lot of elforts to the calculation and treatment of the electronic structures of polyatomic systems including heavy elements, which are involved in many interesting chemical and physical phenomena. They still present unique difficulties to the theoretical study. Until recently, the relativistic effect had ever been thought less important for chemical properties because the relativity appears primarily in the core electrons, which had been believed to be unlikely to affect chemically active valence regions dramatically. Recent studies, however, have revealed not only quantitatively but also quahtatively that the relativistic effect plays essential and comprehensive roles in total natures of molecular electronic structures for heavy-element systems. We are nowadays convinced that the relativistic effect is definitely important for the accurate theoretical treatment of heavy-element systems as well as the electron correlation effect. 

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. 

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