الفهرس 
THEORITICAL BACKGROUND AND LITERATUREOnly 14 pages are availabe for public view
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Abstract
Lead chalcogenide semiconductor quantum dots embedded in
glasses have various photonic applications because they absorb photons
at wavelengths from near-infrared to visible. For practical applications,
quantum dots should be incorporated into chemically and mechanically
stable solid matrices and glasses. Among these matrices, germanate
glasses superior to other glasses by its high thermal, chemical and
mechanical stability. Also they can prevent quantum dots from
agglomerating. Compared with many traditional II–VI and III–V
materials, the IV–VI lead chalcogenide nanocrystals (NCs) have smaller
band gaps (0 – 0.32 eV) and larger Bohr radius (18 – 46 nm). These large
radii allow strong confinement to be achieved in relatively large
structures. Thus, QDs of IV-VI materials may have properties reflecting
all of the benefits of strong quantum confinement, with reduced influence
from surface effects; for the same level of confinement as that of QDs of
II-VI or III-V materials, the surface-to-volume ratio can be quite low in
IV-VI materials. Furthermore, these large Bohr radii also allow for these
materials to be fabricated with much stronger quantum confinement than
is possible with II-VI or III-V materials.
The objective of our study was to inspect the optical properties of
glasses containing sources of semiconductor particle PbO2, Te, Se. So for
the present work, two base germanate glass systems (as a host materials)
[x ZnO – (45 –x) Li2O – 55 GeO2 where x = 0, 10, 15, 20 and 25 mole %]
and [x NaF – (100-x) GeO2 where x = 5, 10, 15, 20 and 30 mole %] were
prepared and characterized.
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Formation, optical and electrical properties of a novel system of PbTe
quantum dots (QDs) embedded in zinc lithium germanate glass matrix
[20 ZnO – 25 Li2O – 55 GeO2: 0.5 wt % (PbO2 – Te)] were investigated.
The existence of PbTe QDs is confirmed by X-ray diffraction (XRD),
optical absorption (OA), transmission electron microscopy (TEM) and
Raman spectroscopy. We have investigated the effect of heat-treatment
temperatures (340 - 400 °C) and annealing times (15 - 180 min) on the
growth of PbTe semiconductor QDs. It is found that the increase in heattreatment
temperatures is strongly affecting the nanoparticles growth and
capping surface of the PbTe nanoparticles. Heat-treatment of doped glass
samples at these temperatures leads to formation of PbTeO3 layer around
the PbTe QDs which can be ascribed to the interaction of oxygen atoms
from the base glass matrix with Te at the nanoparticle-matrix interface.
On the other hand, the PbTe QDs sizes have been calculated by effective
mass approximation (EMA) are found to be 3.3, 3.6, 4.8, 5.6 and 5.9 nm
for the glass samples annealed at 340, 360, 370, 380 and 400 °C for 15
min, respectively. However, for the glass samples annealed at 340 °C for
15, 60, 120 and 180 min, the estimated PbTe QDs sizes are 3.3, 3.8, 4 and
4.7 nm, respectively. Size dispersion for the QDs is around 5% as
estimated by the method of Wu et al. Also, the PbTe QDs sizes were
calculated by Scherrer’s formula from XRD analysis (are found to be lie
in the range (3.6 – 8.7 nm) and obtained from TEM images (their values
6.5 and 7.5 for the glass samples annealed at 400˚C for 15 min and 340
°C for 120 min, respectively) which are very close to that calculated by
(EMA). The electrical activation energy and optical band gap showed an
increase with decreasing the QDs sizes which may be due to the splitting
of both valence and conduction bands into a set of discrete energy levels.
x
PbSe nanocrystals were grown in fluorogermanate glasses having
composition, 10 NaF – 90 GeO2: 1 wt % (PbO2 – Se), the effect of
annealing temperature (475 - 550 °C) and time (30-180 min) on the
growth and optical properties of PbSe QDs are investigated. PbSe QDs
were confirmed by XRD, OA and TEM and the obtained values of PbSe
QDs sizes are found in the range (4.3 – 10.8 nm). The optical band gap
and thermoelectric power values showed a decrease with the increasing in
PbSe nanocrystals sizes. Raman spectroscopy confirmed the formation of
PbSe nanoparticles by the appearance of surface and longitudinal optical
phonons of PbSe.
Finally, PbTe quantum dots were grown in fluorogermanate glasses
having composition, 10 NaF – 90 GeO2: 0.5 wt % (PbO2 – Te). At
constant annealing temperature 500 °C, the effect of annealing times (30-
180 min) on the growth and optical properties of PbTe QDs are
investigated. The formation of PbTe nanoparticles were confirmed by
XRD, OA and TEM analysis. The first exciton transition was tuned from
0.796 to 2.19 μm by the heat treatment of doped fluorogermanate glasses
at 500 ˚C and duration times ranged from 30 to 180 min. It is found that
PbTe QDs have sizes in the range (3.2 – 7.3 nm). The obtained optical
energy gap for the present study showed size dependence.