molecular physics Physics sector which deals with both the study of the spatial configuration, the dynamic properties of isolated molecules and the determination of their characteristic parameters, and the interactions between molecules and their dynamics in dense systems.
IN-DEPTH ABSTRACTfrom Molecular Physics ofDanilo De Rossi (Encyclopedia of Science and Technology)
PHYSICS OF MOLECULAR FILMS
The idea that matter, which to our senses manifests itself as a continuum, instead consists of innumerable particles dates back to very ancient times, albeit in the form of a philosophical conjecture. Molecules (Latin diminutive form of the word mass) are the smallest particles that maintain the fundamental chemical properties of the substance they constitute. Science has long shown how these particles that make up a particular substance interact continuously with each other, through forces of attraction and repulsion. Theintermolecular forcesand the continuous thermal agitation with consequent chaotic motion govern the aggregation states of matter. The study of molecular physics therefore constitutes a basic chapter in the physics of matter. In recent decades the interest in molecular and supramolecular systems in condensed form has grown enormously, giving rise to a considerable number of both theoretical and applicative studies. In particular, molecular films of an inorganic, organic and biological nature are the subject of a huge amount of research, which embrace different aspects of the physico-chemical and molecular film technologies.
The materials constituting molecular films can be of different types, even if the current interest is mainly directed towards organic and biological systems, and the techniques used by nature or man for their formation are very diversified. The framework of studies of a theoretical nature and of those carried out with application intents is very vast: in synthetic form it goes from research on the structure and properties of soap bubbles and the lipid double layer of the cell membrane, to investigations on molecular films organized as elements primordial at the origin of life, up to the recent technical developments of themolecular electronicsand the achievements, in perspective, of organic and biological processors. Research relating to films and systems at the molecular scale converges different scientific disciplines (physics, chemistry, biology and engineering) and is at the origin of multiple applications. In recent years the science of nanometric systems and nanotechnologies / “> nanotechnologies have developed enormously, crossing the boundaries of individual disciplines in a joint effort destined to revolutionize many technological sectors in the near future. How much of the expectations and resources invested will actually be realized it is to be seen, but there is no doubt that the field of organized molecular and supramolecular systems of artificial origin constitutes one of the fundamental sectors of scientific and technological cognitive development.
The current research is essentially aimed at the development of molecular electronics, the most significant result of which has been obtained so far concerns display devices through liquid crystals. The ambition for the future is the creation of applications consisting of single molecules or single molecular complexes. Potentially the field is very wide because it includes solar cells, optical memories, holographic systems and others. Prototype models of these devices already exist which reveal interesting application perspectives.
The current studies and the realizations that make use of molecular films are mainly framed in the developments of the so-called molecular electronics. From the application point of view, the most relevant result in this sector is the liquid crystal display devices, characterized today by high performance and very wide use. Molecular electronics addresses the study of molecular materials for electronic and electro-optical applications, and the study of the so-called molecular electronics which concerns devices made up of single molecules or single molecular complexes. The use of supramolecular structures in the form of ordered films straddles the two lines and, although for at least two decades there has been intense activity aimed at studying possible applications of molecular films, the use of these systems outside the laboratories is limited today. The main problems that limit the development of a real technology of these films are the reduced environmental resistance and thermal stability, the inadequate mechanical properties, the great difficulties in preparing films free from defects and micropores (pin – holes ), the limited reproducibility in the preparation phase.
Despite these problems, which are however assiduously studied, the proposed applications are very varied. The main fields of study for application purposes are related to: ( a ) molecular films with nonlinear optical properties for the optical treatment of information; ( b ) interface films for the development of sensors and biosensors; ( c ) photoelectric devices and systems; ( d ) resist for microlithography in the field of integrated electronics; ( e ) controllable chromatic devices (electrically, thermally, light) in the context of information display systems (display); ( f ) ferroelectric molecular films; ( g) ferromagnetic organic films; ( h ) semiconductor, conductive and superconductive molecular films. The use of molecular films for the construction of devices is essentially motivated by the prospect of creating properly structured and organized molecular architectures, capable of performing specific and coordinated functions, especially in the fields of signal transduction and treatment and energy conversion.
An important example to clarify the potential of technologies related to the use of molecular films is the system based on the use of the photosynthetic protein bacteriorhodopsin / “> bacteriorhodopsin (BR). This protein represents the key to the photosynthetic process of the halobacteria, obtained from the purple membrane. of Halobacterium salinarium (HS) and suitably mutated by genetic route. The membrane is made up of lipids and BR trimers which, activated by light, allows pumping protons. A functional graphic scheme of the photosynthetic activity of the HS membrane is shown in fig.The different photoelectric processes related to the absorption of photons by the BR, which are the prerequisites for its technological use, are schematically shown in fig. Instead of the non-mutant form ( wild type) of BR, mutant forms are used, as some of them, suitably selected, have much better photoelectric properties. Molecular BR mutant films enclosed in lipid layers, obtained by fragmentation of the bacterial cell membrane, are used today for the realization of various molecular-based electrooptical devices, with revolutionary perspectives. Solar cells, optical memories, photochromic and photoelectronic devices, holographic and optical correlation systems have already been made in the form of prototypes and appear to be of great application interest. At the same volume, BR-based three-dimensional optical memories are capable of storing data in quantities three hundred times higher than current solid-state devices, focused on silicon technologies.