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substrate using two different self-assembly methods [6].
Various surface chemistries can be applied to the colloidal form
of QDs, enabling precise attachment on confined locations. Three-
dimensional electron
SiO
2
−
hole pair confinement results in sharp
emission peaks controlled by composition and size. In core/shell
structures, the energy gap between the core and the shell materials
forms a potential well, which controls the maximum number of
electron and hole states. Thus, the spectral characteristics can be
tailored to a given application by controlling the QD composition.
The energy band gap of the core, given by the core diameter or
effective length, mainly determines the wavelength of photons
emitted during excitation recombination. Smaller dimensions result
in higher quantum energies between the conduction (or valence
band) edge and electron (or hole) energy levels. Electron
−
hole pairs,
created in nanoparticles (NPs) by a pump source, recombine under
the introduction of energy such as laser light, and emit photons
with energy levels corresponding to the first excited state. Light
propagation in the waveguide is then controlled by the interaction
of electric fields of neighboring QDs [5].
Figure 12.7
QD waveguides. (a) Schematic of operation. Electrons in
the QDs are excited by external pump source. Signal light
at waveguide edge stimulates emission of photons, which
are transmitted along the waveguide with certain gain. The
output signal is imparted by interdot coupling. Reprinted
with permission from Ref. [6]. Copyright 2006 American
Chemical Society. (b) Schematic representation of QD as
core/shell quantum box. (c) 1D energy diagram of QDs.
Reprinted with permission from Ref. [7]. Copyright 2005
IEEE.
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