Thesis

Colloidal semiconductor nanocrystals for colour conversion and self-assembled lasers

Creator
Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 2023
Thesis identifier
  • T16454
Person Identifier (Local)
  • 201752886
Qualification Level
Qualification Name
Department, School or Faculty
Abstract
  • Nanocrystals have been at the forefront of technological developments in photonics thanks to their unique optoelectronic properties. In this thesis, different types of luminescent semiconductor nanocrystals have been studied, improved, and implemented towards novel applications. The topics of research discussed are: (i) II-V colloidal quantum dots as the building blocks of novel lasers via self-assembly from the bottom up (the fabrication and study of these are in fact the main focus of the thesis), and (ii) a perovskite quantum dot-based structure as a robust colour-converter of LEDs.(i) II-VI alloyed core-shell CdxS1-xSe/ZnS quantum dots are nanosized colloids (dispersed in solution) with excellent optical properties in the visible spectrum. These Cd-based colloidal quantum dots represent the most mature colloidal quantum dot technology and their use as light emitters and laser gain material is being intensely pursued. To date, colloidal quantum dot (and related nanocrystal) lasers have been made from the top down with the quantum dots deposited into an optical resonator fabricated separately. Departing from this standard approach and fully capitalizing on the solution processability of colloids, this work uses quantum dots as nanobricks to create supracrystal/supraparticle microspheres that self-assemble from a bottom-up approach. These supraparticles act simultaneously as the gain material and the optical microcavity. In addition to that they are capable of laser emission under optical excitation. Using red-emitting CdxS1-xSe/ZnS quantum dots, laser oscillation between the 625 and the 655 nm is obtained from single quantum dot spheres with diameters of 5.6 ± 3.2 μm and energy threshold of 4.7 ± 2.1 nJ for a 532 nm pump source with a beam spot size of approximately 6 μm in diameter.The possibility of making hetero-supraparticles by selecting and self-assembling together quantum dots with different emission and absorption spectra is also demonstrated. When carefully selected, these combinations can enhance the quality factor of the sphere. As an example, microspheres of red quantum dots and green quantum dots had an increase in the quality factor from 135±19 to 340±60, when compared to red quantum dot microspheres of the same size (6.0 ± 0.5 μm in diameter). Microspheres with quantum dots of red and higher band gap species also maintain similar laser threshold energies to their red quantum dot microsphere counterparts. In the example mentioned above, both microspheres reached laser threshold at 12 – 14 mJ.cm-2, for a beam spot size of 2.88×10-7 cm2. In addition to that, microspheres with higher bandgap quantum dots in their composition have also reported laser for cavities of sizes 3 – 4 times bigger, further suggesting that the increase in the quality factor and decrease in self-absorption is promoted by the addition of higher bandgap quantum dots.Synthesis of microspheres with different bandgap quantum dots also allow for simultaneous multicolor lasing in a single sphere. Stable dual laser emission of yellow (575 nm) and red (630 nm) is shown in a microsphere of 6.0 ± 0.5 μm in diameter, for energy thresholds between 13.3 and 45.6 nJ and for a spot size of approximately 4.85 × 10-7 cm2.The integration of supraparticle lasers to other devices is demonstrated via transfer printing. This method can move them reliably between substrates, and this was done to successfully couple them to waveguides. This demonstration paves the way to more complex designs and applications in integrated photonics.In addition to quantum dots, the self-assembly procedure was also tested and adapted to other types of semiconductor nanocrystals, including nanoplatelets and tetrapods, in a collaboration work with Nanyang Technological University (LUMINOUS! group).(ii) A different material was studied for colour conversion. Ceasium lead bromide perovskite nanocrystals have up to date the fastest luminescent dynamics of all known nanocrystals and are therefore appealing for light communication applications. However, they are not stable in the presence of heat and humidity. Different coatings using two different polymers (PDMS and PMMA) have been studied as a way of protecting and increasing the stability of CsPbBr3@Cs4PbBr6 crystals. While PDMS samples were not stable upon immersion in water, PMMA composites showed little to no trace of degradation when immersed in water under vigorous stirring for up to 72 hours. Bandwidth measurements with PMMA samples have given similar results to the current state of the art, showing that PMMA is an effective matrix host for CsPbBr3@Cs4PbBr6 against moisture and water.
Advisor / supervisor
  • Dawson, Martin
Resource Type
DOI
Date Created
  • 2022

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