Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

where the constant A and the temperature dependence of g depend on thedetails of the disorder that is present. This model (for systems where thedisorder has a Gaussian width of about 0.1 eV) corresponds well withexperiments in molecularly doped polymers [26], although it is difficult tomeasure over a large enough temperature range to distinguish the temperaturedependence predicted by Eq. (12) from the simple Arrehnius behaviorwhich might be expected if structural reorganization energies controlled thetransport. Similar behavior to that predicted by Eq. (12) might be expected innanocrystalline systems if disorder is the dominant effect.In summary, we have shown that in addition to the electronic tunnelingprocess, both internal and dielectric relaxation energies are likely to beimportant in determining electron transfer dynamics in nanocrystals. Bothof these relaxation energies are size dependent and can be in the range 50–100meV for small CdSe particles. We have also identified surface trapping,higher-lying states, and disorder as factors likely to complicate the transportin real samples, and we will return to these issues in the light of the experimentalresults discussed in Section V.III.EXPERIMENTAL TECHNIQUESIn this section, we give a brief introduction to some of the experimentaltechniques used in characterizing electron transfer and electron transport innanocrystalline materials.Measurement of the luminescence spectrum is a standard technique forcharacterizing nanocrystals, because it gives information about the particlesize in quantum-confined systems. Less standard, especially in solid films, ismeasuring the quantum efficiency of luminescence (i.e., the ratio of the numberof photons emitted to the number of photons absorbed). This measurementis particularly useful in studying charge transfer to or from a nanocrystal,because charge separation typically quenches the luminescence ofthe photoexcited species, which may be either the nanocrystal itself or someneighboring fluorescent molecule or polymer [5,27]. In solution, luminescenceefficiency can be measured by comparison with a known standard solutionwhere the geometry, absorbance, and solution refractive indices are known.In a solid film, however, it is difficult to determine the total luminescence froma measurement in a particular direction. To overcome this problem, it is necessaryto use an integrating sphere to collect the emitted light. An integratingsphere is a hollow sphere coated with a diffusely reflecting coating, which hasthe property that the intensity measured by a detector in the wall of the sphereis proportional to the total amount of light generated inside the sphere, irrespectiveof its direction. Using a laser to excite a solid film placed inside anCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.