Modem materials semiconductors

The importance of materials science to U.S. competitiveness can hardly be overstated. Key materials science areas underlie virtually every facet of modem life. Semiconductors underpin our electronics industry. Optical fibers are essential for communications. Superconducting materials will probably affect many areas ceramics, composites, and thin films are having a big impact now in transportation, construction, manufacturing, and even in sports—tennis rackets are an example. 

One of the points made in Schwenz and Moore was that the physical chemistry laboratory should better reflect the range of activities found in current physical chemistry research. This is reflected in part by the inclusion of modem instrumentation and computational methods, as noted extensively above, but also by the choice of topics. A number of experiments developed since Schwenz and Moore reflect these current topics. Some are devoted to modem materials, an extremely active research area, that I have broadly construed to include semiconductors, nanoparticles, self-assembled monolayers and other supramolecular systems, liquid crystals, and polymers. Others are devoted to physical chemistry of biological systems. I should point out here, that with rare exceptions, I have not included experiments for the biophysical chemistry laboratory in this latter category, primarily because the topics of many of these experiments fall out of the range of a typical physical chemistry laboratory or lecture syllabus. Systems of environmental interest were well represented as well. 

In this brief review we illustrated on selected examples how combinatorial computational chemistry based on first principles quantum theory has made tremendous impact on the development of a variety of new materials including catalysts, semiconductors, ceramics, polymers, functional materials, etc. Since the advent of modem computing resources, first principles calculations were employed to clarify the properties of homogeneous catalysts, bulk solids and surfaces, molecular, cluster or periodic models of active sites. Via dynamic mutual interplay between theory and advanced applications both areas profit and develop towards industrial innovations. Thus combinatorial chemistry and modem technology are inevitably intercoimected in the new era opened by entering 21 century and new millennium. 

The predictions made by Mendeleev provide an excellent example of how a scientific theory allows far-reaching predictions of as-yet-undiscovered phenomena. Today s chemists still use the periodic table as a predictive tool. For example, modem semiconductor materials such as gallium arsenide were developed in part by predicting that elements in the appropriate rows and columns of the periodic table should have the desired properties. At present, scientists seeking to develop new superconducting materials rely on the periodic table to identify elements that are most likely to confer superconductivity. 

One application of modem solid-state electronic devices is semiconductor materials that convert electrical energy into light. These light-emitting diodes (LEDs) are used for visual displays and solid-state lasers. Many indicator lights are LEDs, and diode lasers read compact discs in a CD player. The field of diode lasers is expanding particularly rapidly, driven by such applications as fiber optic telephone transmission. 

Oxides play many roles in modem electronic technology from insulators which can be used as capacitors, such as the perovskite BaTiOs, to the superconductors, of which the prototype was also a perovskite, Lao.sSro CutT A, where the value of x is a function of the temperature cycle and oxygen pressure which were used in the preparation of the material. Clearly the chemical difference between these two materials is that the capacitor production does not require oxygen partial pressure control as is the case in the superconductor. Intermediate between these extremes of electrical conduction are many semiconducting materials which are used as magnetic ferrites or fuel cell electrodes. The electrical properties of the semiconductors depend on the presence of transition metal ions which can be in two valence states, and the conduction mechanism involves the transfer of electrons or positive holes from one ion to another of the same species. The production problem associated with this behaviour arises from the fact that the relative concentration of each valence state depends on both the temperature and the oxygen partial pressure of the atmosphere. 

Apart from the endeavor to optimize the stmcture property relations of materials used in modem optoelectronic devices there is the desire to understand the conceptual premises of charge transport in random organic solids. The use of amorphous, instead of crystalline, organic semiconductor materials is favored... 

The list in Table 7.2 may appear incomplete to the modem chemist utilizing or studying chemical deposition e.g., only thioacetamide is noted as a sulphide source and selenides are not included. However, when we reflect that the vast bulk of the work carried out on CD concerned just sulphides, selenides and oxides, this old table might point the way to a major expansion of the CD technique, both for semiconductors and for other compounds. Further processing may be expected to extend the types of material even further. For example, arsenates and phosphates may be reducible in some cases to the better-known (to the semiconductor community) arsenides and phosphides. 

For many years, during and after the development of the modem band theory of electronic conduction in crystalline solids, it was not considered that amorphous materials could behave as semiconductors. The occurrence of bands of allowed electronic energy states, separated by forbidden ranges of energy, to become firmly identified with the interaction of an electronic waveform with a periodic lattice. Thus, it proved difficult for physicists to contemplate the existence of similar features in materials lacking such long-range order. 

The most familiar application of amorphous semiconductors will, for many readers, be in the field of replication of printed matter. The xerography process, npon which many modem photocopiers are based, involves the ability of an electrostatically charged plate of amorphous chalcogenide (or similar material) to discharge under illn-mination. Residual charging of illuminated areas is employed in the transfer of ink onto the duplicator paper. Naturally, the mobility of photoinduced carriers in the amorphons semiconductor photoreceptor is of central importance in the validity of the process, and considerable commercial effort has been (and is being) devoted to the study of transport in disordered materials suitable for the process. 

Early solid-state devices relied on observing the ionization in intrinsic semiconductors. Early devices were impractical due to the requirement of extremely pure material. Modem devices are based on semiconductor junction diodes. These diodes have a rectifying junction that only allows the flow of current in one direction. Incident radiation creates ionization inside the bulk of the diode and creates a pulse of current in the opposite direction to the normal current flow through a diode that is straightforward to detect. 

Semiconductor A generic term for a device that controls electrical signals. It specifically refers to a material (such as silicon, germanium or gallium arsenide) that can be altered either to conduct electrical current or to block its passage. Carbon nanotubes may eventually be used as semiconductors. Semiconductors are partly responsible for the miniaturization of modem electronic devices, as they are vital components in computer memory and processor chips. The manufacture of semiconductors is carried out by small firms, and by industry giants such as Intel and Advanced Micro Devices. 

Defects play an important part in both the chemical and physical behavior of solids and much of modem science and technology centers upon the exploitation or suppression of the properties that defects confer upon a solid. The chemical and physical aspects of a defect are intimately connected, but for simplicity, defects are often portrayed as influencing only chemical or physical properties. Thus, defects such as dislocations, which have a profound effect upon the mechanical properties of a solid, also influence chemical properties such as rates of dissolution and reaction, although these chemical aspects are not always mentioned. Similarly, defects that can be considered as more chemical in nature, such as impurity atoms, have profound effects on the physical properties of materials, as witnessed by doped semiconductors see Semiconductors, Semiconductor Interfaces). 

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