Synthesis and optical properties of PbS/MnS core shell quantum dots
Optical characteristics of colloidal quantum dots (QDs) are influenced by the dot size, composition and the capping molecules. This thesis focuses on the study of PbS/MnS core shell (CS) QDs where PbS serves as the core and MnS is the shell. The colloidal chemistry method was used to synthesise P...
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| Format: | Thesis |
| Language: | English |
| Published: |
2019
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| Subjects: | |
| Online Access: | http://psasir.upm.edu.my/id/eprint/90519/1/FS%202019%2085%20-%20IR.pdf |
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| Summary: | Optical characteristics of colloidal quantum dots (QDs) are influenced by the dot
size, composition and the capping molecules. This thesis focuses on the study
of PbS/MnS core shell (CS) QDs where PbS serves as the core and MnS is the
shell. The colloidal chemistry method was used to synthesise PbS QDs at room
temperature. The growth of MnS shell by a simple ions substitution called cation
exchange was successfully applied to fabricate the PbS/MnS core shell QDs in
this research. This is a unique chemistry reaction which only can be occurred in
chalcogenide material.
This research also focuses on structural and optical properties of the PbS QDs
and PbS/MnS core shell QDs. The structural properties were characterised by
using High Resolution Transmission Electron Microscopy (HRTEM) and Energy
Dispersive X-ray (EDX). The HRTEM results show that the average size of the
PbS QDs was 6.00 ± 1.00 nm with spherical in shape. For PbS/MnS core shell
QDs, its average size was enlarged to 7.00 ± 0.50 nm and 7.40 ± 0.60 nm for
PbS/MnS CS 0.3 and PbS/MnS CS 0.6, respectively. The size was increased
due to the growth of MnS shell after the cation exchange. Besides that, the
analysis of elemental composition via EDX has confirmed the presence of MnS
composition. From the spectrum, an EDX peak associated with Mn was
observed at 5.9 keV.
The optical properties and the behaviour of the charge carriers inside the PbS
and PbS/MnS QDs were investigated by photoluminescence (PL) at different
temperature (10-300 K), power excitation (10-200 mW) and exposure time (3-
40 minutes). At room temperature, the PL peak energies of PbS/MnS core shell
QDs were blue shifted upon the increasing shell thickness due to strong
confinement effect induced by the shell.
Temperature dependent PL shows the effect on the PL peak energy, full width
half maximum (FWHM) and PL intensity. In general, the PL peak energy and
FWHM were monotonically increased as the temperature increased which
related to the interaction of charge carriers with phonons. In contrast, the PL
peak intensities were quenched as the temperature increased which associated
with the excitation of carriers out of the QDs into non-radiative recombination
centres. Photoenhancement phenomenon was studied by exposed the samples
under illumination up to 40 minutes and we found that the PL intensity was
increased over period of time due to the presence of oxygen. In power dependent
PL, the integrated PL intensity (IPL) was increased with the increasing power
density. From the graph of IPL as a function of power density, generally gradient
of the graph was close to unity where the radiative recombination is dominant.
The development of PbS/MnS core shell QDs would be useful in the future
especially in the application of photovoltaic devices. |
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