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    Experimental determination of the absorption cross-section and molar extinction coefficient of colloidal CdSe nanoplatelets
    (Amer Chemical Soc, 2015) Yeltik, Aydan; Delikanlı, Savaş; Olutaş, Murat; Kelestemur, Yusuf; Güzeltürk, Burak
    There has been a strong interest in solution-processed two-dimensional nanomaterials because of their great potential in optoelectronics. Here, the absorption cross-section and molar extinction coefficient of four and five monolayer thick colloidal CdSe nanoplatelets (NPLs) having various lateral sizes are reported. The absorption cross-section of these NPLs and their corresponding molar extinction coefficients are found to strongly depend on the lateral area. An excellent agreement is observed between the experimental results and the calculated values based on the small-particle light absorption model. With these optical properties, NPLs hold great promise for optoelectronic applications.
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    Lateral size-dependent spontaneous and stimulated emission properties in colloidal CdSe nanoplatelets
    (Amer Chemical Soc, 2015) Olutaş, Murat; Güzeltürk, Burak; Keleştemur, Yusuf; Yeltik, Aydan; Delikanlı Savaş
    Here, we systematically investigated the spontaneous and stimulated emission performances of solution-processed atomically flat quasi-2D nanoplatelets (NPLs) as a function of their lateral size using colloidal CdSe core NPLs. We found that the photoluminescence quantum efficiency of these NPLs decreases with increasing lateral size while their photoluminescence decay rate accelerates. This strongly suggests that nonradiative channels prevail in the NPL ensembles having extended lateral size, which is well-explained by the increasing number of the defected NPL subpopulation. In the case of stimulated emission the role of lateral size in NPLs influentially emerges both in the single- and two-photon absorption (1PA and 2PA) pumping. In the amplified spontaneous emission measurements, we uncovered that the stimulated emission thresholds of 1PA and 2PA exhibit completely opposite behavior with increasing lateral size. The NPLs with larger lateral sizes exhibited higher stimulated emission thresholds under 1PA pumping due to the dominating defected subpopulation in larger NPLs. On the other hand, surprisingly, larger NPLs remarkably revealed lower 2PA-pumped amplified spontaneous emission thresholds. This is attributed to the observation of a "giant" 2PA cross-section overwhelmingly growing with increasing lateral size and reaching record levels higher than 10(6) GM, at least an order of magnitude stronger than colloidal quantum dots and rods. These findings suggest that the lateral size control in the NPLs, which is commonly neglected, is essential to high-performance colloidal NPL optoelectronic devices in addition to the vertical monolayer control.
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    Nonradiative energy transfer between doped and undoped flat semiconductor nanocrystals of colloidal quasi-2D nanoplatelets
    (Amer Chemical Soc, 2019) Yeltik, Aydan; Olutaş, Murat; Sharma, Manoj; Güngör, Kıvanç; Demir, Hilmi Volkan
    Atomically flat colloidal semiconductors such as nanoplatelets (NPLs) promise great potential for different optoelectronic applications. Here, we systematically investigate the excitonic energy transfer from colloidal Cu-doped CdSe to undoped core/shell CdSe/CdS nanoplatelets via steady-state and time-resolved photoluminescence spectros-copy techniques. We show the strong quenching in photoluminescence emission of the doped NPL donors together with significant modifications in the time-resolved kinetics by changing the concentration of the undoped NPL acceptors in close proximity. This newly presented all-colloidal and all-quasi-2D doped-undoped NPL-NPL hybrid system shows near-unity room-temperature energy transfer efficiency (99%) in solid films. We strongly believe that such highly efficient energy transfer in doped-undoped hybrid films will create more interest in the scientific community to further explore different donor/acceptor combinations with these newly reported doped NPLs for next-generation energy harvesting applications.
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    Understanding the journey of dopant copper ions in atomically flat colloidal nanocrystals of CdSe nanoplatelets using partial cation exchange reactions
    (Amer Chemical Soc, 2018) Sharma, Manoj; Olutaş, Murat; Yeltik, Aydan; Keleştemur, Yusuf; Sharma, Ashma
    Unique electronic and optical properties of doped semiconductor nanocrystals (NCs) have widely stimulated a great deal of interest to explore new effective synthesis routes to achieve controlled doping for highly efficient materials. In this work, we show copper doping via postsynthesis partial cation exchange (CE) in atomically flat colloidal semiconductor nanoplatelets (NPLs). Here chemical reactivity of different dopant precursors, reaction kinetics, and shape of seed NPLs were extensively elaborated for successful doping and efficient emission. Dopant-induced Stokes shifted and tunable photoluminescence emission (640 to 830 nm) was observed in these Cu-doped CdSe NPLs using different thicknesses and heterostructures. High quantum yields (reaching 63%) accompanied by high absorption cross sections (>2.5 times) were obtained in such NPLs compared to those of Cu-doped CdSe colloidal quantum dots (CQDs). Systematic tuning of the doping level in these two-dimensional NPLs provides an insightful understanding of the chemical dopant based orbital hybridization in NCs. The unique combination of doping via the partial CE method and precise control of quantum confinement in such atomically flat NPLs originating from their magic-sized vertical thickness exhibits an excellent model platform for studying photophysics of doped quantum confined systems.