Surface exciton absorption, intrinsic

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. 

In the last subsection, we invoked phonons to explain the nonradiative broadening of the surface structures. However, at very low temperature, the surface state at the bottom of the excitonic band cannot undergo broadening either by phonon absorption or by phonon creation the phonon bath at 2 K does not suffice to account for the 3- to 4-cm 1 nonradiative width of the first surface resonance. Nevertheless, we assume the intrinsic nature of this broadening, since it is observed, constant, for all our best crystals.67120... 

From a comparison of the optical absorption and excitation data for the oxides (Table V), it is clear that the energy decreases with increasing cation size along the series Mg to Ba. The bulk exciton transitions of these oxides also decrease in a similar manner (Table VI). It is possible to make a semi-quantitative calculation of the intrinsic surface energy states using the approach of Levine and Mark (151) where the ions in an ideal surface are considered to be equivalent to bulk ions except for their reduced Madelung... 

Another unambiguous piece of evidence that exceeds the S, 5o transition energy is provided by intrinsic photoconductivity measurements. The problem with such experiments is to avoid or, at least, reduce spurious effects due to exciton dissociation at the surface (see below). The best way to do this is to insert a spacer — which may be a vacuum gap — between sample and contact. Applying this technique demonstrated that in crystalline anthracene the onset of intrinsic photoconductivity starts at 3.9 eV, i.e., 0.8 eV above the singlet exciton eno-gy. The energy gap between absorption and onset of intrinsic photoconductivity becomes smaUa" in the series naphthalene to pentacene. 

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