D4 Photoluminescence and Absorption Spectroscopy

Topic

The energy of the photon emitted or absorbed by this semiconductor material is inversely related to the photon's wavelength—utilizing the same relationship between photon energy and wavelength employed by models for semiconductor nanoparticles of other material sets: the shorter the wavelength of the emitted or absorbed photon, the higher its energy, in accordance with that inverse relationship. The fact that this semiconductor material characteristically emits or absorbs a photon of a specific wavelength means that the photon's energy is consequently determined by that characteristic wavelength.

The absorption coefficient of this semiconductor material, measured near its band edge, is fitted using the Tauc model to determine the material's bandgap; the Tauc model extrapolates the portion of the absorption coefficient—which rises sharply just above the band edge—down to the point of zero absorption, and the photon energy at which this extrapolation intersects the zero-absorption axis directly reveals the semiconductor material's bandgap. A steeper rise in the absorption coefficient just above the band edge allows the Tauc model to extrapolate the bandgap value with greater precision. Thus, this model—utilizing the relationship between photon energy and wavelength, along with the bandgap derived via the Tauc model—combines photoluminescence and near-band-edge absorption to jointly determine both the bandgap and the optical quality of the semiconductor material (a quality described in greater detail by another model in this set). While near-band-edge absorption and the Tauc model reveal the bandgap based on the minimum energy the material still absorbs, photoluminescence—viewed from the complementary perspective of the photon energy actually emitted during recombination—reveals whether defect states exist below the bandgap that might broaden or shift the emission relative to Tauc model predictions. The fact that the photoluminescence emission matches the bandgap derived from near-band-edge absorption allows the two techniques to jointly confirm that the semiconductor material is free of the defect states that would otherwise degrade its optical quality.

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