Browsing by Subject "Giant oscillator strength"

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Open Access
Low-threshold optical gain and lasing of colloidal nanoplatelets
(IEEE, 2014-10) Keleştemur, Yusuf; Güzeltürk, Burak; Olutaş, Murat; Delikanlı, Savaş; Demir, Hilmi Volkan
Semiconductor nanocrystals, which are also known as colloidal quantum dots (CQDs), are highly attractive materials for high performance optoelectronic device applications such as lasers. With their size, shape and composition tunable electronic structure and optical properties, CQDs are highly desired for achieving full-color, temperature-insensitive, low-threshold and solution-processed lasers [1, 2]. However, due to their small size, they suffer from the nonradiative multiexciton Auger Recombination (AR), where energy of a bound electron-hole pair is transferred to a third particle of either an electron or a hole instead of radiative recombination. Therefore, CQDs having suppressed AR are strongly required for achieving high quality CQD-based lasers. To address this issue, CQDs having different size, shape and electronic structure have been synthesized and studied extensively [3-5]. Generally, suppression of AR and lower optical gain thresholds are achieved via reducing the wavefunction overlap of the electron and hole in a CQD. However, the separation of the electron and hole wavefunctions will dramatically decrease the oscillator strength and optical gain coefficient, which is highly critical for achieving high performance lasers. Therefore, colloidal materials with suppressed AR and high gain coefficients are highly welcomed. Here, we study optical gain performance of colloidal quantum wells [6] of CdSe-core and CdSe/CdS core/crown nanoplatelets (NPLs) that demonstrate remarkable optical properties with ultra-low threshold one- and two-photon optical pumping. As a result of their giant oscillator strength, superior optical gain and lasing performance are achieved from these colloidal NPLs with greatly enhanced gain coefficient [7]. © 2014 IEEE.
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 Volkan
Surface 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.