Photophysics nanoclusters

Uosaki K, Okamura M, Ebina K (2004) Photophysical and photoelectrochemical characteristics of multilayers of CdS nanoclusters. Faraday Discuss 125 39-53... 

The photophysical processes of semiconductor nanoclusters are discussed in this section. The absorption of a photon by a semiconductor cluster creates an electron-hole pair bounded by Coulomb interaction, generally referred to as an exciton. The peak of the exciton emission band should overlap with the peak of the absorption band, that is, the Franck-Condon shift should be small or absent. The exciton can decay either nonradiatively or radiative-ly. The excitation can also be trapped by various impurities states (Figure 10). If the impurity atom replaces one of the constituent atoms of the crystal and provides the crystal with additional electrons, then the impurity is a donor. If the impurity atom provides less electrons than the atom it replaces, it is an acceptor. When the impurity is lodged in an interstitial position, it acts as a donor. A missing atom in the crystal results in a vacancy which deprives the crystal of electrons and makes the vacancy an acceptor. In a nanocluster, there may be intrinsic surface states which can act as either donors or acceptors. Radiative transitions can occur from these impurity states, as shown in Figure 10. The spectral position of the defect-related emission band usually shows significant red-shift from the exciton absorption band. 

The discussion so far has concentrated on the fundamental spectroscopic and photophysical properties of semiconductor nanoclusters. These nanoclusters represent a new class of novel materials and many potential applications are being evaluated. In the next several chapters I discuss several topics that are of interest in the photoscience area nonlinear optical properties, photoconductivity, and photochemical conversion. 

Different parameters are required to characterize the resonant and the nonresonant optical nonlinearity. This has often been a source of confusion in the literature, even today. For nonresonant processes, the magnitude of the nonlinearity is measured by either x(3) or n2. However, for resonant processes, x(3)> a2> or ni alone cannot measure the magnitude of the nonlinearity. For example, a different x(3) value can result from the same material when lasers with different pulse widths are used for the measurement. A complete characterization of the nonlinearity requires a set of parameters, including % 3), the ground state absorption coefficient, the laser pulse width, and the excited state relaxation time. In a simple two- or three-level system, once all these factors are properly taken into account, the best parameter for measuring the resonant nonlinearity is simply the ground state absorption cross section of the material. In the following section I focus on the resonant nonlinearity only as this is closely related to the photophysical properties. The discussion of nonresonant nonlinearity of semiconductor nanoclusters can be found elsewhere [17, 84-86],... 

In slightly over a decade, the study of semiconductor nanoclusters and nanostructured materials has evolved into an important discipline. It has attracted attention from scientists in such different disciplines as physics, chemistry, and material science. In this chapter, I have focused only on their photophysical and photochemical properties. However, other properties and potential applications have also been explored [160,161]. 

It is the purpose of this chapter to review the photophysical properties of some supramolecular assemblies of nanoclusters and complementary organic molecules. 

---