Optoelectronic Properties of Inorganic Compounds

Materials exhibiting third-order nonlinear optical (NLO) properties have applications in a number of important technologies including power limiting for sensor protection and optically addressed optical switches for photonics switching, all optical signal processing and optical computing.^1–5 Because of the potential importance of these technologies, there is currently intense research interest in developing new third-order NLO materials with large effective third-order NLO susceptibilities, χ⁽³⁾, and the appropriate properties for the various applications.

The cathode ray tube (CRT) is currently the most widely used electronic display technology. The luminescent images of the CRT are generated by independently exciting the red, green, and blue (RGB) phosphors of each pixel, whose emissions add to give the desired color. While CRTs have excellent picture quality, their size, weight, and low shock resistance prevent them from being used in most mobile applications such as laptop computers and other small consumer electronic devices. The technology that has been applied almost universally in mobile applications involves the use of liquid crystal displays (LCDs). The individual pixels in these displays consist of a liquid crystalline material sandwiched between two electrodes, which in turn is sandwiched between crossed polarizers. The individual pixels of the LCD act as electrically activated light valves, allowing light to be transmitted from a light source behind the LCDpanel. The light source can be a fluorescent back light or a mirror that reflects the incident light. Color images are generated by placing magenta, yellow, and cyan color-pass filters in front of selected pixels, and driving the individual color elements of each pixel to generate the desired spectrum.

Significant advances in the fields of colloid and sol-gel chemistry in the last two decades now allow fabrication of micro- and nano-sized structures using finely divided monodispersed colloidal particles. ^1–7 As we approach the 21st century, there is a growing trend on the part of the scientific community to apply these concepts to develop systems of smaller dimensions. Homogeneous solid (3-D) is giving way to multilayers with quasi-2-D structures and quasi-1-D structures such as nanowires or clusters in an insulating matrix, and finally to porous nanocrystalline films. Nanocrystalline semiconductor films are constituted by a network of mesoscopic oxide or chalocogenide particles such as Ti0₂, ZnO, Nb₂O₅, WO₃, Ta₂O₅, CdS, or CdSe, which are interconnected to allow electronic conduction to take place. The pores between the particles are filled with a semiconducting or a conducting medium, such as a p-type semiconductor, a hole transmitter, or an electrolyte, forming a junction of extremely large contact area. In this fashion, the negatively and positively charged contacts of the electric cell become interdigitated on a length scale as small as a few nanometers. Nanostructured materials offer many new opportunities to study fundamental processes in a controlled manner and this in turn leads to fabrication of new devicesm some of which are summarized in Fig. The unique optical and electronic features of these are being exploited to develop optoelectroinc devices such as photoelectrodes in solar cells, photochromic displays/switches, optical switches, chemical sensors, intercallation batteries, capacitor dielectrice/supercapacitors, heat-reflecting and UV-absorbing layers, coatings to improve chemical and mechanical stability of glass, etc. In some recent articles^8–10 we have outlined some of these novel applications.

Transition metal complexes with an open-shell configuration (d ⁿ , n < 10) have received considerable attention for their interesting photophysical properties. The binuclear metal-metal bonded complexes of Rh^I, Ir^I, and Pt^II display rich photochemistry.¹ One example of particular interest is the Pt^II complex, [Pt₂(H₂P₂O₅)₄]⁴⁻, which has an excited state that is better oxidant and reductant than its ground state,² and undergoes facile atom-transfer reactions with a variety of organic substrates.³ However, perhaps the most extensively studied transition metal complexes in this area are based on [Ru(bpy)₃]²⁺, bpy = bipyridine. The complexes have been used in the study of both electron and energy transfer reactions and contributed a great deal to the understanding of photochemistry in inorganic systems.⁴

The investigation of the properties of substances under high pressures has emerged as a major multidisciplinary research endeavor embracing a diverse arsenal of spectroscopic, physical, and chemical probes. High pressure NMR, ESR, IR, Raman, Brillouin, electronic absorption, electronic emission, X-ray, and Mössbauer spectroscopic experiments are now commonplace.¹ The vigorous state of high pressure research is attested to by a number of excellent books^2–5 and review articles,^1,6–8 to which the reader is referred to gain insight into the historical origins and current breadth of high pressure studies. Holzapfel’s review provides a thorough, up-to-date compendium of high pressure references.⁸ The exhaustive compilation of earlier high pressure literature (1900–1968) by Merrill also should be noted.⁹ Many have contributed to the development of high pressure science. However, particular mention should be made of the pioneering high pressure work of Bridgman,^10,11 rightly called the father of high pressure science, and the thorough and richly diverse high pressure spectroscopic studies of Drickamer.^12–15

Over the last few decades there has been a remarkable growth in applications of chemical sensors. This growth stems from the increased need for sensitive and selective sensors in many technological aspects of life such as robotics, automation, enviromental science, information technology, and medicine.¹ Semiconductor-based sensors and photoluminescent sensors have attracted much attention in this regard.^2,3 The known electronic properties of semiconductor materials and the contactless nature of photoluminescence (PL) spectroscopy make inorganic semiconductors an attractive approach for chemical sensing.

Optical sensors are materials that potentially have a wide range of uses and applications in both medical and environmental situations among others. Optical sensors can be designed to make use of changes in the wavelengths or extinction coefficients of the sensing material. Alternately for emissive materials, it is possible to use changes in the emission wavelengths or intensities to monitor the presence or absence of chemical species. These chemical species can be cations, anions, or organic molecules. For a sensor to be useful it is necessary for the device to be selective for the specific chemical species of interest, and that the change in the property of the sensing material be responsive in a consistent manner to changes in concentration of the chemical species being detected or analyzed.^1–8 This chapter is focused on optical sensors incorporating metals, and one feature of such sensors is their use to detect metal ions in solutions. For the metal binding site in such a sensor it is usual to employ chelate or macrocyclic ligands because they can be tailored to selectively complex a variety of different metal ions. For the detection of uncharged molecules a host will usually be selected such that its cavity matches the shape and size of the chosen guest. More recently metal-containing optical sensors are being developed that can function as anion selective receptors, and again the receptor must be specifically designed to meet the requirements of the individual anions.⁹

Metallo-organic compounds are just one class of “molecular materials” currently attracting intense interest for their potential use in telecommunications devices. Other types of molecular material include wholly organic polymers containing push—pull, electron donor—acceptor combinations in either the main-chain or most often as side-chain substituents¹; highly conjugated main-chain polymers,² e.g., polyacetylenes, polypyrroles, polyphenylenevinylenes, etc.; and more complex macrocyclics such as C₆₀ and related fullerenes.^3,4 In each case the principal property of interest is the optical nonlinearity, either x ² or x ³ , where the susceptibility may have both real and imaginary components.