Electrostatic Mechanism of Emission Enhancement in Hybrid Metal-semiconductor Light-emitting Heterostructures Page: 73
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If we now include the fact that the exciton orientations are randomly distributed,
we find that carriers with E > 0 will be strongly attracted at significantly further distances
than for those with E < 0. This means that even if the diffusion mentioned before were
insufficient to negate the weak repulsion experienced by those excitons, there would still
be a net migration inward as carriers with E > 0 are pulled in from further out and
attracted more effectively than the others are repelled. Thus one must conclude that the
image charge effect produces a net attraction for carriers even when accounting for
dipole-dipole interactions instead of the Coulomb attraction experienced by individual
carriers. I also note that attractive forces have been predicted for excitons in CdS
nanorods interacting with images induced in Au nanoparticles attached to the end of the
nanorod. [64].
5.6 Angle Dependent PL measurements
One of the consequences of the image charge effect as applied to dipoles is that
the maximum acceleration for the e-h pairs occurs when the pair is oriented normal to
the NP. This makes sense from a minimization of energy standpoint, as the energy of a
system of two dipoles is minimized when the two are aligned in the same direction. This
suggests that a majority of carriers in the vicinity of NPs should be oriented normal to
the surface. Any change in the distrubtion of carrier orientations should produce a
change in the angular distribution of emitted photons. In order to investigate this, the
samples were subjected to angle-dependent PL measurements, the results of which are
presented in Figure 5.9. Since the emission is travelling from a high-index material (nGaN
22.5) to a lower index material (nAir=l) I have normalized the emission intensities to
account for the transmission as a function of angle (see Appendix B). I present both the
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Llopis, Antonio. Electrostatic Mechanism of Emission Enhancement in Hybrid Metal-semiconductor Light-emitting Heterostructures, dissertation, May 2012; Denton, Texas. (https://digital.library.unt.edu/ark:/67531/metadc115113/m1/83/?rotate=270: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; .