Synthesis and characterization of colloidal quantum wells: from simple size-tuned core to complex multi-crown structures
Author
Dede, Didem
Advisor
Demir, Hilmi Volkan
Date
2018-08Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
As a new class of semiconductor nanocrystals, colloidal quantum wells (CQWs),
also commonly known as nanoplatelets (NPLs), exhibit remarkable electronic and
optical properties that will potentially nd a wide range of use from nanophotonics
to optoelectronics. NPLs feature step-like absorption pro les and discrete
emission spectra with giant oscillator strength resulting in high recombination
rates. All these features make these atomically-
at structures highly attractive
for light-harvesting and -generating applications. In this thesis, to understand
the size-tuned properties of their two-dimensional architecture, we conducted a
systematic study on the core-only NPLs by using a set of 4 monolayer (ML) CdSe
cores as our working model and carefully altered their aspect ratio while keeping
their lateral area constant. In such a core-only NPL structure, electron and hole
are both con ned in the core resulting in type-I electronic band alignment. By
decreasing the width of these NPLs to a value comparable to or less than their
exciton Bohr radius, we observe additional con nement e ects emerge. Subsequently,
by growing CdSe1xTex alloyed crown around these starting 4 ML CdSe
cores, we nd type-II electronic band alignment is obtained. Thanks to their
spatially indirect excitons, these core crown NPLs show extraordinarily long radiative
lifetimes. Moreover, with the increased absorption cross-section owing to
their added crown, high-performance optical gain is achieved via their core/crown
heterostructure. However, in this form, their usage is limited since they are unstable
in solution forming gels and they exhibit strong tendency to form stacks in
lms. To address this problem, here we proposed and developed a multi-crown architecture
by additionally growing a CdS crown around the periphery of the type-
II heterostructure, enabling excellent optical gain media with enhanced stability.
The structural and optical characterizations of the synthesized multi-crown NPLs indicate that this complex architecture holds great promise for making devices in
colloidal nanophotonics and optoelectronics.
Keywords
Colloidal SynthesisSemiconductor Nanocrystals
Colloidal Quantum Wells
Nanoplatelets
Optical Gain