Browsing by Subject "CdSe nanoplatelets"

Now showing 1 - 5 of 5

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Bright-dark exciton interplay evidenced by spin polarization in cdse/cdmns nanoplatelets for spin-optronics
(American Chemical Society, 2025-01-08) Shornikova, Elena V.; Yakovlev, Dmitri R.; Tolmachev, Danil O.; Prosnikov, Mikhail A.; Christianen, Peter C. M.; Shendre, Sushant; Işık, Furkan; Delikanlı, Savaş; Demir, Hilmi Volkan; Bayer, Manfred
Diluted magnetic semiconductor (DMS) colloidal nanocrystals demonstrate remarkable magneto-optical properties. The ability to control their magnetization and, consequently, the circular polarization of exciton emission holds significant potential for spintronic applications. However, the interplay between bright and dark exciton recombination and its impact on the polarization of emission are not yet fully understood. We measure the magneto-optical properties of colloidal CdSe/CdMnS nanoplatelets at cryogenic temperatures in high magnetic fields up to 30 T. The degree of circular polarization of photoluminescence demonstrates nonmonotonous behavior in a magnetic field. In low magnetic fields, the polarization degree is positive due to an exchange interaction of excitons with localized spins of magnetic Mn2+ ions. After reaching a maximum, the polarization degree starts to decrease and reverses the sign to negative in high magnetic fields, which is unusual in DMSs. The critical magnetic field, in which the sign is reversed, increases when the temperature is elevated. We develop a model that explains this behavior by an interplay of bright and dark exciton recombination. In high magnetic fields, the dark exciton radiative recombination rate accelerates due to mixing with the bright state, and the intrinsic Zeeman splitting of dark exciton overcomes the exchange with Mn2+ ions. As a result, the lowest |-2 > exciton energy level dominates in emission, providing negatively polarized photoluminescence.
Embargo
Demystifying trion emission in cdse nanoplatelets
(American Chemical Society, 2024-08-19) Riesner, Maurizio; Shabani, Farzan; Van Emmichoven, Levin Zeylmans; Klein, Julian; Delikanlı, Savaş; Fainblat, Rachel; Demir, Hilmi Volkan; Bacher, Gerd
At cryogenic temperatures, the photoluminescence spectrum of CdSe nanoplatelets (NPLs) usually consists of multiple emission lines, the origin of which is still under debate. While there seems to be consensus that both neutral excitons and trions contribute to the NPL emission, the prominent role of trions is rather puzzling. In this work, we demonstrate that Förster resonant energy transfer in stacks of NPLs combined with hole trap states in specific NPLs within the stack trigger trion formation, while single NPL spectra are dominated by neutral excitonic emission. This interpretation is verified by implementing copper (Cu+) dopants into the lattice as intentional hole traps. Trion emission gets strongly enhanced, and due to the large amount of hole trapping Cu+ states in each single NPL, trion formation does not necessarily require stacking of NPLs. Thus, the ratio between trion and neutral exciton emission can be controlled by either changing the amount of stacked NPLs during sample preparation or implementing copper dopants into the lattice which act as additional hole traps.
Open Access
High-frequency EPR and ENDOR spectroscopy of Mn2+ ions in CdSe/CdMnS nanoplatelets
(American Chemical Society, 2023-02-20) Babunts, Roman A.; Uspenskaya, Yulia A.; Romanov, Nikolai G.; Orlinskii, Sergei B.; Mamin, Georgy V.; Shornikova, Elena V.; Yakovlev, Dmitri R.; Bayer, M.; Işık, Furkan; Shendre, S.; Delikanlı, Savaş; Demir, Hilmi Volkan
Semiconductor colloidal nanoplatelets based of CdSe have excellent optical properties. Their magneto-optical and spin-dependent properties can be greatly modified by implementing magnetic Mn2+ ions, using concepts well established for diluted magnetic semiconductors. A variety of magnetic resonance techniques based on high-frequency (94 GHz) electron paramagnetic resonance in continuous wave and pulsed mode were used to get detailed information on the spin structure and spin dynamics of Mn2+ ions in core/shell CdSe/(Cd,Mn)S nanoplatelets. We observed two sets of resonances assigned to the Mn2+ ions inside the shell and at the nanoplatelet surface. The surface Mn demonstrates a considerably longer spin dynamics than the inner Mn due to lower amount of surrounding Mn2+ ions. The interaction between surface Mn2+ ions and 1H nuclei belonging to oleic acid ligands is measured by means of electron nuclear double resonance. This allowed us to estimate the distances between the Mn2+ ions and 1H nuclei, which equal to 0.31 ± 0.04, 0.44 ± 0.09, and more than 0.53 nm. This study shows that the Mn2+ ions can serve as atomic-size probes for studying the ligand attachment to the nanoplatelet surface.
Open Access
Near-Infrared-Emitting five-monolayer thick copper-doped CdSe nanoplatelets
(WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2019) Sharma, Ashma; Sharma, Manoj; Güngör, Kıvanç; Olutaş, Murat; Dede, Didem; Demir, Hilmi Volkan
Doped nanocrystals are instrumental to the high‐performance luminescent solar concentrators (LSCs) and the color conversion devices. Recently, copper (Cu)‐doped three and four monolayer (ML) thick CdSe nanoplatelets (NPLs) have been shown superior to the existing Cu‐doped quantum dots (QDs) for their use in LSCs. However, additional improvement in the LSC performance can be achieved by further redshifting the emission into the near‐infrared (NIR) region of electromagnetic spectrum and increasing the absorbed portion of the solar irradiation. Cu‐doping into higher thicknesses of these atomically flat NPLs (e.g., ≥5 ML) can achieve these overarching goals. However, addition of the dopant ions during the nucleation stage disturbs this high‐temperature growth process and leads to multiple populations of NPLs and QDs. Here, by carefully controlling the precursor chemistry the successful doping of Cu in five ML thick NPLs by high‐temperature nucleation doping method is demonstrated. The optimized synthesis method shows nearly pure population of doped five ML thick NPLs, which possess ≈150 nm Stokes‐shifted NIR emission with high quantum yield of 65 ± 2%. Structural, elemental, and optical studies are conducted to confirm the successful doping and understand the detailed photophysics. Finally, these materials are tested experimentally and theoretically for their performance as promising LSC materials.
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.