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consistent with the observed data. 2-4 Quantum confinement occurs when electron-hole pairs are generated in a particle, wire, or film that is smaller than the volume these electrons and holes would normally occupy in the bulk material. In porous silicon, the quantum confinement apparently occurs in silicon fibers or particles that are on the order of 2 nm in diameter. 3 The quantum confinement model predicts that the emission energy of porous silicon will shift to the blue as these silicon domains get smaller. Although this model adequately explains much of the data, there are still details that need to be worked out. For instance, it is known that many crystal faces will undergo reconstruction to lower the surface free energy associated with dangling bonds or sterically demanding surface species. Because the nature of the bonding is altered relative to the bulk solid, reconstruction can result in introduction of new energy states distinct from those arising from the crystalline solid. In very small particles or wires, the fraction of atoms residing on the surface is significant, and so the density of surface states in these materials can be large enough to significantly alter the electronic structure of the material as a whole. Thus, even a first approximation analysis of so-called quantum particles should take into account the possibility that the "quantum-box" states are in resonance with surface states. One of the conclusions that we have drawn from our work on luminescence quenching of porous silicon is that a lot is happening at the surface, and chemical transformations of the surface can generate surface states on porous silicon that are more or less accessible energetically. HI H ,,,\

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Si Si Si Surface Species

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Si Si Si "Dangling Bonds

Figure 1. Possible CarrierTraps Presenton the Porous Si Surface In addition to the higher relative density of surface electronic states, which may have a distinctive chemical nature as depicted in Figure 1, there are other photophysical mechanisms that may be operative in nanoparticles that are not available to bulk solids. Since the surface free energy is a significant term in determining the overall thermodynamics of nanoparticles, surface strain can have a large effect on the electronic properties of nanoparticles. For example, Alivasatos and coworkers have shown that the phase diagram for II-VI nanoparticles is very different from the bulk 508

material, and that both the kinetics and thermodynamics of phase 5 transitions can be size-dependent. ,6 Available De-excitation Pathways We are just beginning to work out the interplay of porous silicon's light emission process with its surface. There are a variety of relaxation pathways available to porous silicon-or for that matter, any semiconductor. Figure 2 depicts some de-excitation mechanisms that we have been interested in probing with porous silicon. 2. Bulk Trapping, Nonradiative Recombination Trapping/ Recombination

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4. Interfacial Energy Transfer

1. Photoluminescence Ox 5. Interfacial Ch