Browsing by Subject "Cadmium selenide"

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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 Volkan
As 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.
Open Access
Light-induced paramagnetism in colloidal Ag+-doped CdSe nanoplatelets
(American Chemical Society, 2021-03-25) Najafi, A.; Sharma, Manoj; Delikanlı, Savaş; Bhattacharya, A.; Murphy, J. R.; Pientka, J.; Sharma, A.; Quinn, A. P.; Erdem, Onur; Kattel, S.; Kelestemur, Y.; Kovalenko, M. V.; Rice, W. D.; Demir, Hilmi Volkan; Petrou, A.
We describe a study of the magneto-optical properties of Ag+-doped CdSe colloidal nanoplatelets (NPLs) that were grown using a novel doping technique. In this work, we used magnetic circularly polarized luminescence and magnetic circular dichroism spectroscopy to study light-induced magnetism for the first time in 2D solution-processed structures doped with nominally nonmagnetic Ag+ impurities. The excitonic circular polarization (PX) and the exciton Zeeman splitting (ΔEZ) were recorded as a function of the magnetic field (B) and temperature (T). Both ΔEZ and PX have a Brillouin-function-like dependence on B and T, verifying the presence of paramagnetism in Ag+-doped CdSe NPLs. The observed light-induced magnetism is attributed to the transformation of nonmagnetic Ag+ ions into Ag2+, which have a nonzero magnetic moment. This work points to the possibility of incorporating these nanoplatelets into spintronic devices, in which light can be used to control the spin injection.
Open Access
Ultraefficient förster-type nonradiative energy transfer enabled by the complex dielectric medium with tuned permittivity
(American Chemical Society, 2021-06-10) Hernandez-Martinez, P. L.; Yücel, A. C.; Demir, Hilmi Volkan
Förster-type nonradiative energy transfer (FRET) is one of the primary near-field phenomena and is a useful, fundamental mechanism allowing us to control the excitation energy flow. Using carefully chosen pairs of quantum emitters/absorbers (donors/acceptors), FRET has proved to be essential in a variety of light-generating and -harvesting systems. However, FRET takes place only in a limited spatial range, and its efficiency suffers from an adversely rapidly decreasing profile over the increasing distance between the donor and acceptor. To foster FRET, reaching ultimate levels of efficiency and extending its range, we systematically studied the FRET mechanism by tuning the background medium’s permittivity. The FRET rates of donor–acceptor pairs consisting of a point-like, quasi-0-dimensional quantum dot and quasi-2-dimensional quantum well nanostructures are analytically derived to characterize the change of FRET rates with respect to the medium’s permittivity. The analysis reveals that the FRET rate becomes singular when the permittivity approaches zero and there is a fixed value for the point-like and all other nanostructures, respectively. By setting the medium’s relative permittivity to realistic values near the singular point, which can be realized by a digital metamaterial approach, ultrahigh FRET rates and thereby ultraefficient FRET-based systems are achievable.
Open Access
Ultrahigh green and red optical gain cross sections from solutions of colloidal quantum well heterostructures
(American Chemical Society, 2021-03-11) Delikanli, Savaş; Erdem, Onur; Işık, Furkan; Dehghanpour Baruj, Hamed; Shabani, Farzan; Yağcı, Hüseyin Bilge; Durmuşoğlu, E. G.; Demir, Hilmi Volkan
We demonstrate amplified spontaneous emission (ASE) in solution with ultralow thresholds of 30 μJ/cm2 in red and of 44 μJ/cm2 in green from engineered colloidal quantum well (CQW) heterostructures. For this purpose, CdSe/CdS core/crown CQWs, designed to hit the green region, and CdSe/CdS@CdxZn1–xS core/crown@gradient-alloyed shell CQWs, further tuned to reach the red region by shell alloying, were employed to achieve high-performance ASE in the visible range. The net modal gain of these CQWs reaches 530 cm–1 for the green and 201 cm–1 for the red, 2–3 orders of magnitude larger than those of colloidal quantum dots (QDs) in solution. To explain the root cause for ultrahigh gain coefficient in solution, we show for the first time that the gain cross sections of these CQWs is ≥3.3 × 10–14 cm2 in the green and ≥1.3 × 10–14 cm2 in the red, which are two orders of magnitude larger compared to those of CQDs.