Browsing by Author "Quliyeva, Ulviyya"
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Item Open Access Giant alloyed hot injection shells enable ultralow optical gain threshold in colloidal quantum wells(American Chemical Society, 2019) Altıntaş, Yemliha; Güngör, Kıvanç; Gao, Y.; Sak, Mustafa; Quliyeva, Ulviyya; Bappi, G.; Mutlugün, Evren; Sargent, E. H.; Demir, Hilmi VolkanAs an attractive materials system for high-performance optoelectronics, colloidal nanoplatelets (NPLs) benefit from atomic-level precision in thickness, minimizing emission inhomogeneous broadening. Much progress has been made to enhance their photoluminescence quantum yield (PLQY) and photostability. However, to date, layer-by-layer growth of shells at room temperature has resulted in defects that limit PLQY and thus curtail the performance of NPLs as an optical gain medium. Here, we introduce a hot-injection method growing giant alloyed shells using an approach that reduces core/shell lattice mismatch and suppresses Auger recombination. Near-unity PLQY is achieved with a narrow full-width-at-half-maximum (20 nm), accompanied by emission tunability (from 610 to 650 nm). The biexciton lifetime exceeds 1 ns, an order of magnitude longer than in conventional colloidal quantum dots (CQDs). Reduced Auger recombination enables record-low amplified spontaneous emission threshold of 2.4 μJ cm–2under one-photon pumping. This is lower by a factor of 2.5 than the best previously reported value in nanocrystals (6 μJ cm–2 for CdSe/CdS NPLs). Here, we also report single-mode lasing operation with a 0.55 mJ cm–2 threshold under two-photoexcitation, which is also the best among nanocrystals (compared to 0.76 mJ cm–2 from CdSe/CdS CQDs in the Fabry–Pérot cavity). These findings indicate that hot-injection growth of thick alloyed shells makes ultrahigh performance NPLs.Item Open Access Highly stable multicrown heterostructures of type-II nanoplatelets for ultralow threshold optical gain(American Chemical Society, 2019) Dede, Didem; Taghipour, Nima; Quliyeva, Ulviyya; Sak, Mustafa; Kelestemur, Yusuf; Güngör, Kıvanç; Demir, Hilmi VolkanSolution-processed type-II quantum wells exhibit outstanding optical properties, which make them promising candidates for light-generating applications including lasers and LEDs. However, they may suffer from poor colloidal stability under ambient conditions and show strong tendency to assemble into face-to-face stacks. In this work, to resolve the colloidal stability and uncontrolled stacking issues, we proposed and synthesized CdSe/CdSe1–xTex/CdS core/multicrown heteronanoplatelets (NPLs), controlling the amount of Te up to 50% in the crown without changing their thicknesses, which significantly increases their colloidal and photostability under ambient conditions and at the same time preserving their attractive optical properties. Confirming the final lateral growth of CdS sidewalls with X-ray photoelectron spectroscopy, energy-dispersive analysis, and photoelectron excitation spectroscopy, we found that the successful coating of this CdS crown around the periphery of conventional type-II NPLs prevents the unwanted formation of needle-like stacks, which results in reduction of the undesired scattering losses in thin-film samples of these NPLs. Owing to highly efficient exciton funneling from the outmost CdS crown accompanied by the reduced scattering and very low waveguide loss coefficient (∼18 cm–1), ultralow optical gain thresholds of multicrown type-II NPLs were achieved to be as low as 4.15 μJ/cm2 and 2.48 mJ/cm2 under one- and two-photon absorption pumping, respectively. These findings indicate that the strategy of using engineered advanced heterostructures of nanoplatelets provides solutions for improved colloidal stability and enables enhanced photonic performance.Item Open Access Highly stable, near-unity efficiency atomically flat semiconductor nanocrystals of CdSe/ZnS hetero-nanoplatelets enabled by ZnS-Shell hot-injection growth(WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2019) Yemliha, Yemliha; Quliyeva, Ulviyya; Güngör, Kıvanç; Erdem, Onur; Kelestemur, Yusuf; Mutlugün, Evren; Kovalenko, M.; Demir, Hilmi VolkanColloidal semiconductor nanoplatelets (NPLs) offer important benefits in nanocrystal optoelectronics with their unique excitonic properties. For NPLs, colloidal atomic layer deposition (c‐ALD) provides the ability to produce their core/shell heterostructures. However, as c‐ALD takes place at room temperature, this technique allows for only limited stability and low quantum yield. Here, highly stable, near‐unity efficiency CdSe/ZnS NPLs are shown using hot‐injection (HI) shell growth performed at 573 K, enabling routinely reproducible quantum yields up to 98%. These CdSe/ZnS HI‐shell hetero‐NPLs fully recover their initial photoluminescence (PL) intensity in solution after a heating cycle from 300 to 525 K under inert gas atmosphere, and their solid films exhibit 100% recovery of their initial PL intensity after a heating cycle up to 400 K under ambient atmosphere, by far outperforming the control group of c‐ALD shell‐coated CdSe/ZnS NPLs, which can sustain only 20% of their PL. In optical gain measurements, these core/HI‐shell NPLs exhibit ultralow gain thresholds reaching ≈7 µJ cm−2. Despite being annealed at 500 K, these ZnS‐HI‐shell NPLs possess low gain thresholds as small as 25 µJ cm−2. These findings indicate that the proposed 573 K HI‐shell‐grown CdSe/ZnS NPLs hold great promise for extraordinarily high performance in nanocrystal optoelectronics.Item Open Access Semiconductor nanoplatelet heterostructures enhanced via combinations of colloidal atomic layer deposition and hot injection shell growths(Bilkent University, 2019-07) Quliyeva, UlviyyaOne of the most promising families of semiconductor nanocrystals in colloidal optoelectronics and nanophotonics is considered to be colloidal quantum wells, also commonly referred to as nanoplatelets (NPLs). Possessing an atomically flat structure, NPLs feature unique properties including spectrally-resolved and tunable light-holeand heavy-hole transitions accompanied by their respective giant oscillator strengths. CdSe, CdS and CdTe, making the first colloidal NPLs synthesized in core-only structure, portray distinct qualities necessary for light-harvesting and -generating applications. However, going beyond the core structure, there are many properties that are highly enhanced by growing crown and/or shell layers around core NPLs. While the crown growth takes place anisotropically in lateral directions, the shell layer covers the entire NPL surface, combinations of which enable NPL heterostructures in new architectures. Depending on the electronic alignment of parts of the NPL heterostructure and the resulting confinement of electron-hole wave functions, these hetero-NPLs can be type-I or type-II. In type-I electron-hole pairs are confined in the core-NPL and recombination occurs in a direct pathway. In type-II electron-hole wave function is separated into different semiconductor layers, resulting in spatially indirect recombination. In this thesis, we synthesized and showed thin- and thick-shell grown heterostructures of type-I CdSe/ZnS NPLs using hot-injection (HI) for the first time particularly for these semiconductor NPLs. Unlike the typical colloidal atomic layer deposition (c-ALD) technique, which produces NPL heterostructures with low quantum yield (QY) and low chemical and optical stability, our approach yields CdSe/ZnS NPLs of almost unity (100%) quantum yield (QY) and improved chemical stability, tested by washing the same samples rigorously up to 6 times with ethanol with little change observed in the QY. Additionally, unparalleled thermal and optical aging endurances is achieved in aging tests. These tests experimentally demonstrated that, elevated to 400 K, HI thick-shelled NPLs can retain up to 65% of their emission intensity in the colloidal form and 52% of that in the film. This level of high stability creates a great opportunity for employing these NPLs for high-temperature applications. Also, in the thesis, we synthesized and studied CdS/CdSe core/crown, CdS/CdZnS core/c-ALD shell-grown and CdS/CdSe/CdZnS core/crown/c-ALD shell-grown heterostructures of NPLs. Here the starting-template CdS NPLs are considered to be unique in terms of their emission in the blue region, which may open up new opportunities for NPL lasing in this spectral region. Nominally CdS NPLs are folded due to great lateral sizes. However, in this research work, when coated with crown and shell layer, these particles unfold. The unrolled CdS/CdSe core/crown NPLs are found to exhibit relatively higher QY up to 15-20% in its class of CdS core-seeded NPLs. The findings of this thesis reveal that such heterostructures of the NPLs are very rich in terms of variety of the quantum architectures one can achieve using them as working model systems.Item Open Access Ultrathin highly luminescent two-monolayer colloidal CdSe nanoplatelets(WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2019) Delikanlı, Savaş; Yu, G.; Yeltik, Aydan; Bose, S.; Erdem, Talha; Yu, J.; Erdem, Onur; Sharma, Manoj; Sharma, Vijay Kumar; Quliyeva, Ulviyya; Shendre, S.; Dang, C.; Zhang, D.; Sum, T.; Fan, W.; Demir, Hilmi VolkanSurface effects in atomically flat colloidal CdSe nanoplatelets (NLPs) are significantly and increasingly important with their thickness being reduced to subnanometer level, generating strong surface related deep trap photoluminescence emission alongside the bandedge emission. Herein, colloidal synthesis of highly luminescent two‐monolayer (2ML) CdSe NPLs and a systematic investigation of carrier dynamics in these NPLs exhibiting broad photoluminescence emission covering the visible region with quantum yields reaching 90% in solution and 85% in a polymer matrix is shown. The astonishingly efficient Stokes‐shifted broadband photoluminescence (PL) emission with a lifetime of ≈100 ns and the extremely short PL lifetime of around 0.16 ns at the bandedge signify the participation of radiative midgap surface centers in the recombination process associated with the underpassivated Se sites. Also, a proof‐of‐concept hybrid LED employing 2ML CdSe NPLs is developed as color converters, which exhibits luminous efficacy reaching 300 lm Wopt−1. The intrinsic absorption of the 2ML CdSe NPLs (≈2.15 × 106 cm−1) reported in this study is significantly larger than that of CdSe quantum dots (≈2.8 × 105 cm−1) at their first exciton signifying the presence of giant oscillator strength and hence making them favorable candidates for next‐generation light‐emitting and light‐harvesting applications.