Electronic and Optical Properties

The PL spectra of the PFs show well-resolved structural features with maxima at 420,445, and 475 nm assigned to the 0-0, 0-1, and 0-2 intrachain singlet transition, respectively (the 0-0 transition, the most intense) [247]. Due to the tail emission spectrum of PFs, the thin films emit bright sky-blue light. The QE of the PFs is very high, typically in the range of 40 to 80% and, as shown for PFO 196, it depends substantially on the morphology of the polymer [248]. 

FIGURE 2.9 Typical absorption and emission spectra of polyfluorene in thin films (shown for poly(9,9-dioctylfluorene) 196). (From Gong, X., Iyer, P.K., Moses, D., Bazan, G.C., Heeger, A.J., and Xiao, S.S., Adv. Fund. Mater., 13, 325, 2003. With permission.) 

The ultraviolet and x-ray photoelectron spectroscopy (UPS and XPS) measurements are used to calculate /P of PFO at — 5.6 +0.05 eV, and the band gap at 3.1 +0.1 eV, which is also much closer to the optical band gap than to the value deduced from the electrochemistry in films [254]. Thus, the HOMO LUMO levels of PF can be reasonably well-matched by work functions of ITO/PEDOT (—5.1eV) and Ca electrode (ca. — 2.9 eV), respectively. However, 

FIGURE 2.11 Cyclic voltammetry of PFO 196 in thin film (potentials vs. Ag/AgCl). (From Janietz, S., Bradley, D.D.C., Grell, M., Giebeler, C., Inbasekaran, M., and Woo, E.P., Appl. Phys. Lett., 73, 2453, 1998. With permission.) 

Beside the excellent optical properties and suitable HOMO-LUMO energy levels, the PFs possess great charge-transport properties. Time-of-flight (TOF) measurements of PFO showed nondispersive hole transport with a room temperature mobility of holes of fi+ = 4 x 10-4 cm2/(V s) at a field of li 5 x 105 V/cm that is about one order of magnitude higher than that in PPV [259]. The polymer revealed only a weak-field dependence of the mobility, from /r+ = 3 x 1(U4 cm2/(V s) at E= 4 x 104 V/cm to /r+ = 4.2 x 1(V4 cm2/(V s) at E= 8 x 105 V/cm. 

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PZT (lead zirconate titanate) and PLZT (lead lanthanum zirconate titanate) combine ferroelectic, optical, and electronic properties and are used in optoelectronic and piezoelectric devices. Powders for hot pressing produced by CVD are being investigated. 

High-Temperature Crystallization The size-tunable optical and electronic properties of semiconductor nanocrystals are attractive for a variety of optoelectronic applications. In solution-phase crystallization, precursors undergo chemical reaction to form nuclei, and particle growth is arrested with capping ligands that... 

Valence Band Spectroscopy. Optical and electronic properties of UPD metal flms on metal electrodes have been studied in situ by means of differential- and electroreflectance spectroscopy [98], Optical absorption bands, however, reflect a combined density of electronic states at a photon energy which is the energetic difference of... 

As a result of their unique optical and electronic properties, particularly their ability to fluoresce at discrete wavelengths directly proportional to their sizes and material compositions, QDs have found use in many fields, including electronics, biology, medicine, and even cosmetics. The first attempts to modify their surface characteristics to make them water-soluble and biocompatible eventually led to their use as fluorescent labels for biomolecules in many applications (Rogach et al., 1996 Bruchez et al., 1998 Chan and Nie, 1998). 

The initial stages, notably the formation of a monolayer on a foreign substrate at underpotentials, were mainly studied by classical electrochemical techniques, such as cyclic voltammetry [8, 9], potential-step experiments or impedance spectroscopy [10], and by optical spectroscopies, e.g., by differential reflectance [11-13] or electroreflectance [14] spectroscopy, in an attempt to evaluate the optical and electronic properties of thin metal overlayers as function of their thickness. Competently written reviews on the classic approach to metal deposition, which laid the basis of our present understanding and which still is indispensable for a thorough investigation of plating processes, are found in the literature [15-17]. 

The fundamental physical properties of nanowire materials can be improved even more to surpass their bulk counterpart using precisely engineered NW heterostructures. It has been recently demonstrated that Si/Ge/Si core/shell nanowires exhibit electron mobility surpassing that of state-of-the-art technology.46 Group III-V nitride core/shell NWs of multiple layers of epitaxial structures with atomically sharp interfaces have also been demonstrated with well-controlled and tunable optical and electronic properties.47,48 Together, the studies demonstrate that semiconductor nanowires represent one of the best-defined nanoscale building block classes, with well-controlled chemical composition, physical size, and superior electronic/optical properties, and therefore, that they are ideally suited for assembly of more complex functional systems. 

Due to higher variety of possible structures, copolymers allow a better control of the HOMO LUMO levels necessary to optimize the EL properties of the PPV, compared to homopolymers. Often the optical and electronic properties in copolymers can be finely tuned by simply changing the feed ratio of comonomers (although the structure-property relationship in these systems is even more complex than in homo-PPV polymers). Using different comonomer units, various PPV-based materials with tuned optical and electronic properties have been prepared. 

Because of the great importance of PF as a class of conjugated polymers with excellent optical and electronic properties, several theoretical studies were performed to better understand the electronic structure and the photophysical processes, which occur in these materials [260-265],... 

SCHEME 2.47 Fluorene-phenylene vinylene copolymers and their optical and electronic properties in the solid state. 

Optical and Electronic Properties of the Fluorene-Perylene Copolymers... 

The heteroaromatic stannanes undergo the normal electrophilic substitution reactions of their protic precursors, and often to an enhanced degree. They are often prepared with the aim of a subsequent Stille cross-coupling reaction, and oligothiophenes with potentially useful optical and electron properties have been prepared by coupling between stannyl- and bromo-thiophenes, for example, Equation (63).204... 

It is clear that the combination of different architectures and the precise localization of functionalities within a single macromolecule provide unique opportunities for the control of molecular shape as well as molecular, optical, and electronic properties. A significant hurdle that still remains today is the relatively demanding multistep process used to prepare dendrons and hybrids. This, in turn, translates into limited availability but, as high added-value applications emerge, it is clear that current, as well as yet-to-be-developed, syntheses will be used to prepare specialty materials that benefit from the unique properties derived from the combination of dendritic and linear architectures. 

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