Browsing by Subject "Colloidal Quantum Wells"

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    Colloidal photonics of semiconductor nanocrystals: from polarized color conversion to efficient solar concentration
    (2018-07) Güngör, Kıvanç
    Effective photon management is pivotal to the success of future photonic applications. The demand for high-performance electronic displays and solar light harvesters has been increasingly growing, ever with high expectations in advancing their power efficiencies. Semiconductor nanocrystals are highly promising for use in such advanced photonic applications. However, conventional device architectures and fabrication methods cannot fully exploit their potential. To address the need for their effective utilization, in this thesis, we proposed and demonstrated novel photon managing methods for colloidal nanocrystals to target polarized color conversion and efficient solar concentration. Nanocrystals possess an unmatched color purity for next-generation displays but color enrichment in displays suffers from the inherent random polarization in their photoluminescence. Instead of clipping the undesired polarization, we show a new class of v-shaped backlight unit (v-BLU) creating Fano resonances to enforce isotropic quantum emitters of the integrated color-conversion nanocrystals to emit polarized light. While enabling a front-panel configuration, the proposed v-BLU of nanocrystals allows for a strong modification of the density of optical states via resonance coupling. This control over the density of states for isotropic quantum dots empowers the realization of high polarization contrast ratios while sustaining their optical transmission. Similar to color conversion, colloidal nanocrystals are also instrumental to light harvesting, in particular using atomically at nanocrystals with their step-like absorption profile making them potentially ideal candidates for luminescent solar concentrators (LSCs). Nevertheless, practically zero Stokes shift in their photoluminescence fundamentally limits their utilization. Here we overcame this limitation by proposing the doping of such colloidal quantum wells inducing a large Stokes shift with near-unity photoluminescence quantum efficiency. We developed and demonstrated high-performance LSC panels of the copper-doped quantum wells outperforming the LSCs of their undoped counterparts and doped quantum dots. The LSCs of such Cu-doped quantum wells offer record optical flux gain compared to other colloids. We believe that the findings presented in this thesis will advance the applications of colloidal nanocrystals boosting the performance of their next-generation photonic devices to unprecedented levels.
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    Synthesis and characterization of colloidal quantum wells: from simple size-tuned core to complex multi-crown structures
    (2018-08) Dede, Didem
    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 CdSe1􀀀xTex 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.