Biomedical Engineering Reference
In-Depth Information
color of luminescent emission changes from
green to red. This change is attributed to the
effect of quantum confinement
[53]
. Since the
structure of energy levels of a quantum dot is
dependent on its size, the energy of the emitted
photon is also determined by the diameter of
the quantum dot.
11.3.2 Bragg Phenomenon
The concept of the Bragg phenomenon
emerged in 1912 from studies of X-ray dif-
fraction from crystalline solids; it also applies
to electromagnetics. If white light impinges
obliquely on a photonic structure that can be
represented as a set of discrete and identical
parallel planes, each separated from its near-
est neighbors by a distance
d
, as shown in
Figure 11.7
, and the wave propagation vector
of the incident light is inclined at an angle
θ
with respect to those planes, then light of free-
space wavelength
FIGURE 11.5
Rayleigh scattering in opalescent glass.
The scattered light appears blue from the side, but orange
light shines through. From
www.lickr.com/photos/
optick/112909824/
. (For interpretation of the references
to color in this figure legend, the reader is referred to the
web version of this topic.)
are a subclass of quantum dots. The size
dependence of the luminescence upon excita-
tion of quantum dots with ultraviolet light is
manifested as follows: On increasing the diam-
eter of the quantum dots from 3 to 8.3 nm, the
λ
=
(2
D
SINθ)/
M
,
(
M
=
1, 2, 3, ...),
(11.2)
is specularly reflected due to constructive inter-
ference between neighboring planes. This angu-
lar selectivity of the Bragg phenomenon (often
called
diffraction
) is a major cause of structural
FIGURE 11.6
Quantum dots, each with a core of CdSe
and a shell of ZnS, suspended in toluene. The color of a
monodisperse suspension depends on the diameter of the
quantum dots. A polydisperse suspension can even be white.
Courtesy of Jian Xu (Pennsylvania State University, USA)
and Ocean NanoTech LLC (Springdale, AR, USA). (For inter-
pretation of the references to color in this figure legend, the
reader is referred to the web version of this topic.)
FIGURE 11.7
To explain Bragg's law.
