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
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. (digital.library.unt.edu/ark:/67531/metadc115113/m1/83/: accessed February 23, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; .