Microscopic description and uncertainty of the Stokes shift in semiconductors.

Using GaAs as a test material, we investigated the redshift between emission and the optical bandgap. The knowledge of the energy difference, referred to as the Stokes shift, is of considerable importance for solid-state light sources because its magnitude defines the centered monochromaticity of the emission. Employing Fan's theory, we reveal the basic parameters, which determine the Stokes shift, and provide an uncertainty analysis, considering both uncorrelated and correlated variables. We disclose that the considerable scatter of the dielectric constants in the literature causes uncertainties comparable to or even exceeding the mean. The work stresses that the high-frequency and static dielectric constants might be closer than the currently promoted numbers.

Optical Bandgap Definition via a Modified Form of Urbach's Rule.

We are reporting an esoteric method to determine the optical bandgap of direct gap materials by employing Urbach's rule. The latter, which describes the slope of the band tail absorption in semiconductors, in its original version, cannot be employed to pinpoint the optical bandgap. Herein, however, we show that a modified form of Urbach's rule defines the optical bandgap, and therefore, enables the accurate determination of the optical bandgap energy, which turns out to be identical with the threshold energy for the band tail absorption. The model further produces an explicit expression for the absorption coefficient at the optical bandgap energy.

Using Interaction of Nano Dipoles to Control the Growth of Nanorods.

Charged facets of a nanocrystal can form an intrinsic nanometer-size electric dipole. When the spacing between these nano dipoles is adjusted, the dipolar interaction energy is tuned from a fraction to a multiple of the thermal energy. Consequently, the one-dimensional oriented attachment can be switched on or off, as is the growth of nanorods. This kinetically controlled growth is achieved at relatively low reaction temperatures while the thermodynamically controlled growth dominates at higher temperatures. The synthesized PbSe nanorods are branchless, exhibiting a single-exponential photoluminescence decay trace with an e-folding lifetime of 1.3 µs and a photoluminescence quantum yield of 35%.

Inherent photoluminescence Stokes shift in GaAs.

The intrinsic photoluminescence Stokes shift, i.e., the energy difference between optical band gap and emission peak, of 350 µm thick semi-insulating GaAs wafers is found to be 4 meV at room temperature. The result is based on the determination of the optical bulk band gap from the transmission trend via modified Urbach rule whose result is confirmed with the transmission derivative method. The findings reveal the detailed balance of the optically evoked transitions and disclose the intrinsic link between Stokes shift and the Urbach tail slope parameter.

Photocurrent limit in nanowires.

Square root photocurrent dependences of nanowires on light intensity were reported in the literature without clarification of the limiting effect. In this Letter, we derived a relation excellently fitting the observed nonlinearities and, intensifying the significance of the result, we demonstrated that the fit parameters involved can be employed to determine the impurity concentration and electronic response time of nano-sized semiconductors.

Photocurrent theory based on coordinate dependent lifetime.

Recent work by Pejova [Mater. Res. Bull. 43, 2887 (2008)] showed that the widely cited classical photocurrent theory of DeVore [Phys. Rev. 102, 86 (1956)] does not necessarily apply for photocurrent experiments carried out on thin-film semiconductors. In this Letter, we theoretically and experimentally justify the successful use of the photocurrent model published by Bouchenaki et al. [J. Opt. Soc. Am. B 8, 691 (1991)].