Optical Properties of Silicon Nanoparticles

For biophotonic applications, the optical properties of the silicon nanoparticles are, of course, paramount. These include the absorption spectrum, PL emission spectrum, photoluminescence excitation (PLE) spectrum, quantum yield (ratio of photons emitted to photons absorbed), and PL lifetime. Even though silicon nanoparticles can be quite efficient light emitters, they retain much of the indirect band gap behavior of bulk silicon. In bulk silicon, the indirect band gap has an energy of about 1.1 eV (corresponding to a wavelength of ~1100 nm). The first direct transition (quantum-mechanically allowed, momentum-conserving absorption or emission of a photon without participation of a phonon) occurs at about 3.4 eV (corresponding to a wavelength of about 365 nm), but the change from indirect to direct band gap behavior is not sharply defined. In silicon nanoparticles, the energy of the indirect band gap increases dramatically with decreasing particle size, but the energy of the first direct transition does not change much. Figure 4.1 shows typical absorbance, PL, and PLE spectra for silicon nanoparticles with orange-red emission. The PL spectrum is broad, with a full-width at half-maximum of about 130 nm (0.39 eV) and a maximum intensity near 645 nm. The width of the PL spectrum results primarily from polydispersity in particle size. There is also some blue emission from this sample, between 400 and 500 nm. Although the maximum PL intensity is in the red region of the spectrum, the sample appears orange because the eye is more sensitive to the shorter wavelength portion of the broad emission. The wavelength of peak PL intensity can be varied by changing the particle size. With the methods used to prepare this sample (see below), the PL peak can be varied from about 500 nm to above 800 nm [13]. The width of the PL spectrum can be reduced by using size-selective precipitation

[14] or chromatographic methods [10,15] to narrow the particle size distribution. The effect of size on the PL emission wavelength is a result of quantum confinement effects that increase the band gap energy as the nanocrystal size decreases. This is essentially the same mechanism that leads to size-dependent PL in CdSe and other direct band gap materials.

The absorbance spectrum, in the region where the nanoparticles are reasonably strong absorbers, mimics that of crystalline silicon thin films with little dependence on particle size. The inset of Figure 4.1 compares absorption from the nanocrystal sample with literature data for a single-crystalline

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