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    Near-infrared-emitting five-monolayer thick copper-doped CdSe nanoplatelets
    (Wiley-V C H Verlag Gmbh, 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 approximate to 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.
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    Near-unity efficiency energy transfer from colloidal semiconductor quantum wells of CdSe/CdS nanoplatelets to a monolayer of MoS2
    (Amer Chemical Soc, 2018) Taghipour, Nima; Martinez, Pedro Ludwig Hernandez; Özden, Ayberk; Olutaş, Murat; Dede, Didem
    A hybrid structure of the quasi-2D colloidal semiconductor quantum wells assembled with a single layer of 2D transition metal dichalcogenides offers the possibility of highly strong dipole-to-dipole coupling, which may enable extraordinary levels of efficiency in Forster resonance energy transfer (FRET). Here, we show ultra-high-efficiency FRET from the ensemble thin films of CdSe/CdS nanoplatelets (NPLs) to a MoS2 monolayer. From time-resolved fluorescence spectroscopy, we observed the suppression of the photoluminescence of the NPLs corresponding to the total rate of energy transfer from similar to 0.4 to 268 ns(-1). Using an Al2O3 separating layer between CdSe/CdS and MoS2 with thickness tuned from 5 to 1 nm, we found that FRET takes place 7- to 88-fold faster than the Auger recombination in CdSe-based NPLs. Our measurements reveal that the FRET rate scales down with d(-2) for the donor of CdSe/CdS NPLs and the acceptor of the MoS2 monolayer, d being the center-to-center distance between this FRET pair. A full electromagnetic model explains the behavior of this d(-2) system. This scaling arises from the delocalization of the dipole fields in the ensemble thin film of the NPLs and full distribution of the electric field across the layer of MoS2. This d(-2) dependency results in an extraordinarily long Forster radius of similar to 33 nm.
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    Orientation-controlled nonradiative energy transfer to colloidal nanoplatelets: Engineering dipole orientation factor
    (Amer Chemical Soc, 2019) Erdem, Onur; Güngör, Kıvanç; Güzeltürk, Burak; Tanrıöver, İbrahim; Sak, Mustafa; Olutaş, Murat; Dede, Didem
    We proposed and showed strongly orientation-controlled Forster resonance energy transfer (FRET) to highly anisotropic CdSe nanoplatelets (NPLs). For this purpose, we developed a liquidair interface self-assembly technique specific to depositing a complete monolayer of NPLs only in a single desired orientation, either fully stacked (edge-up) or fully nonstacked (face-down), with near-unity surface coverage and across large areas over 20 cm(2). These NPL monolayers were employed as acceptors in an energy transfer working model system to pair with CdZnS/ZnS core/shell quantum dots (QDs) as donors. We found the resulting energy transfer from the QDs to be significantly accelerated (by up to 50%) to the edge-up NPL monolayer compared to the face-down one. We revealed that this acceleration of FRET is accounted for by the enhancement of the dipoledipole interaction factor between a QD-NPL pair (increased from 1/3 to 5/6) as well as the closer packing of NPLs with stacking. Also systematically studying the distance-dependence of FRET between QDs and NPL monolayers via varying their separation (d) with a dielectric spacer, we found out that the FRET rate scales with d(-4) regardless of the specific NPL orientation. Our FRET model, which is based on the original Forster theory, computes the FRET efficiencies in excellent agreement with our experimental results and explains well the enhancement of FRET to NPLs with stacking. These findings indicate that the geometrical orientation of NPLs and thereby their dipole interaction strength can be exploited as an additional degree of freedom to control and tune the energy transfer rate.