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Öğe CdSe/CdSe1-xTex core/crown heteronanoplatelets: tuning the excitonic properties without changing the thickness(Amer Chemical Soc, 2017) Keleştemur, Yusuf; Güzeltürk, Burak; Erdem, Onur; Olutaş, Murat; Erdem, TalhaHere we designed and synthesized CdSe/CdSe1-xTex core/crown nanoplatelets (NPLs) with controlled crown compositions by using the core-seeded-growth approach. We confirmed the uniform growth of the crown regions with well-defined shape and compositions by employing transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. By precisely tuning the composition of the CdSe1-xTex crown region from pure CdTe (x = 1.00) to almost pure CdSe doped with several Te atoms (x = 0.02), we achieved tunable excitonic properties without changing the thickness of the NPLs and demonstrated the evolution of type-II electronic structure. Upon increasing the Te concentration in the crown region, we obtained continuously tunable photoluminescence peaks within the range of similar to 570 nm (for CdSe1-xTex crown with x = 0.02) and similar to 660 nm (for CdSe1-xTex crown with x = 1.00). Furthermore, with the formation of the CdSe1-xTex crown region, we observed substantially improved photoluminescence quantum yields (up to similar to 95%) owing to the suppression of nonradiative hole trap sites. Also, we found significantly increased fluorescence lifetimes from similar to 49 up to similar to 326 ns with increasing Te content in the crown, suggesting the transition from quasi-type-II to type-II electronic structure. With their tunable excitonic properties, this novel material presented here will find ubiquitous use in various efficient light-emitting and-harvesting applications.Öğe Colloidal nanoplatelet/conducting polymer hybrids: excitonic and material properties(Amer Chemical Soc, 2016) Güzeltürk, Burak; Menk, Florian; Philipps, Kai; Keleştemur, Yusuf; Olutaş, MuratHere we present the first account of conductive polymer/colloidal nanoplatelet hybrids. For this, we developed DEH-PPV-based polymers with two different anchor groups (sulfide and amine) acting as surfactants for CdSe nanoplatelets, which are atomically flat semiconductor nanocrystals. Hybridization of the polymers with the nanoplatelets in the solution phase was observed to cause strong photoluminescence quenching in both materials. Through steady-state photoluminescence and excitation spectrum measurements, photoluminescence quenching was shown to result from dominant exciton dissociation through charge transfer at the polymer/nanoplatelet interfaces that possess a staggered (i.e., type II) band alignment. Importantly, we found out that sulfide-based anchors enable a stronger emission quenching than amine-based ones, suggesting that the sulfide anchors exhibit more efficient binding to the nanoplatelet surfaces. Also, shorter surfactants were found to be more effective for exciton dissociation as compared to the longer ones. In addition, we show that nanoplatelets are homogeneously distributed in the hybrid films owing to the functional polymers. These nanocomposites can be used as building blocks for hybrid optoelectronic devices, such as solar cells.Öğe Continuously tunable emission in inverted type-I CdS/CdSe core/crown semiconductor nanoplatelets(Wiley-V C H Verlag Gmbh, 2015) Delikanlı, Savaş; Güzeltürk, Burak; Hernandez-Martinez, Pedro L.; Erdem, Talha; Keleştemur, Yusuf; Olutaş, MuratThe synthesis and unique tunable optical properties of core/crown nanoplatelets having an inverted Type-I heterostructure are presented. Here, colloidal 2D CdS/CdSe heteronanoplatelets are grown with thickness of four monolayers using seed-mediated method. In this work, it is shown that the emission peak of the resulting CdS/CdSe heteronanoplatelets can be continuously spectrally tuned between the peak emission wavelengths of the core only CdS nanoplatelets (421 nm) and CdSe nanoplatelets (515 nm) having the same vertical thickness. In these inverted Type-I nanoplatelets, the unique continuous tunable emission is enabled by adjusting the lateral width of the CdSe crown, having a narrower bandgap, around the core CdS nanoplatelet, having a wider bandgap, as a result of the controlled lateral quantum confinement in the crown region additional to the pure vertical confinement. As a proof-of-concept demonstration, a white light generation is shown by using color conversion with these CdS/CdSe heteronanoplatelets having finely tuned thin crowns, resulting in a color rendering index of 80. The robust control of the electronic structure in such inverted Type-I heteronanoplatelets achieved by tailoring the lateral extent of the crown coating around the core template presents a new enabling pathway for bandgap engineering in solution-processed quantum wells.Öğe High-efficiency optical gain in Type-II semiconductor nanocrystals of alloyed colloidal quantum wells(Amer Chemical Soc, 2017) Güzeltürk, Burak; Keleştemur, Yusuf; Olutaş, Murat; Li, Quyang; Lian, TianquanColloidal nanocrystals having controlled size, tailored shape, and tuned composition have been explored for optical gain and lasing. Among these, nanocrystals having Type-II electronic structure have been introduced toward low-threshold gain. However, to date, their performance has remained severely limited due to diminishing oscillator strength and modest absorption cross-section. Overcoming these problems, here we realize highly efficient optical gain in Type-II nanocrystals by using alloyed colloidal quantum wells. With composition-tuned core/alloyed-crown CdSe/CdSexTe1-x quantum wells, we achieved amplified spontaneous emission thresholds as low as 26 mu J/cm(2), long optical gain lifetimes (tau(gain) approximate to 400 ps), and high modal gain coefficients (g(modal) approximate to 930 cm(-1)). We uncover that the optical gain in these Type-II quantum wells arises from the excitations localized to the alloyed-crown region that are electronically coupled to the charge-transfer state. These alloyed heteronanostructures exhibiting remarkable optical gain performance are expected to be highly appealing for future display and lighting technologies.Öğe Highly efficient nonradiative energy transfer from colloidal semiconductor quantum dots to wells for sensitive noncontact temperature probing(Wiley-V C H Verlag Gmbh, 2016) Olutaş, Murat; Güzeltürk, Burak; Keleştemur, Yusuf; Güngör, Kıvanç; Demir, Hilmi VolkanThis study develops and shows highly efficient exciton-transferring hybrid semiconductor nanocrystal films of mixed dimensionality comprising quasi 0D and 2D colloids. Through a systematic study of time-resolved and steady-state photoluminescence spectroscopy as a function of the donor-to-acceptor molar concentration ratio and temperature, a high-efficiency nonradiative energy transfer (NRET) process from CdZnS/ZnS core/shell quantum dots (QDs) directed to atomically flat CdSe nanoplatelets (NPLs) in their solid-state thin films is uncovered. The exciton funneling in this system reaches transfer efficiency levels as high as 90% at room temperature. In addition, this study finds that with decreasing temperature exciton transfer efficiency is increased to a remarkable maximum level of approximate to 94%. The enhancement in the dipole-dipole coupling strength with decreasing temperature is well accounted by increasing photoluminescence quantum yield of the donor and growing spectral overlap between the donor and the acceptor. Furthermore, NRET efficiency exhibits a highly linear monotonic response with changing temperature. This makes the proposed QD-NPL composites appealing for noncontact sensitive temperature probing based on NRET efficiencies as a new metric. These findings indicate that combining colloidal nanocrystals of different dimensionality enables efficient means of temperature probing at an unprecedented sensitivity level at nanoscale through almost complete exciton transfer.Öğe 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 SavaşHere, 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.Öğe Low-threshold optical gain and lasing of colloidal nanoplatelets(IEEE, 2014) Keleştemur, Yusuf; Güzeltürk, Burak; Olutaş, Murat; Delikanlı, Savaş; Demir, Hilmi VolkanÖğe Nonradiative energy transfer in colloidal CdSe nanoplatelet films(Royal Soc Chemistry, 2015) Güzeltürk, Burak; Olutaş, Murat; Delikanlı, Savaş; Keleştemur, Yusuf; Erdem, OnurNonradiative energy transfer (NRET) has been extensively studied in colloidal nanocrystal (quantum dots) and nanorod (quantum wires) assemblies. In this work, we present the first account of spectroscopic evidence of NRET in solid thin films of CdSe based colloidal nanoplatelets (NPLs), also known as colloidal quantum wells. The NRET was investigated as a function of the concentration of two NPL populations with different vertical thicknesses via steady state and time resolved spectroscopy. NRET takes place from the NPLs with smaller vertical thickness (i.e., larger band gap) to the ones with a larger vertical thickness (i.e., smaller band gap) with efficiency up to similar to 60%. Here, we reveal that the NRET efficiency is limited in these NPL solid film assemblies due to the self-stacking of NPLs within their own population causing an increased distance between the donor-acceptor pairs, which is significantly different to previously studied colloidal quantum dot based architectures for nonradiative energy transfer.Öğe Platelet-in-Box Colloidal Quantum Wells: CdSe/CdS@CdS Core/Crown@Shell Heteronanoplatelets(Wiley-V C H Verlag Gmbh, 2016) Keleştemur, Yusuf; Güzeltürk, Burak; Erdem, Onur; Olutaş, Murat; Güngör, KıvançHere, the CdSe/CdS@CdS core/crown@shell heterostructured nanoplatelets (NPLs) resembling a platelet-in-box structure are developed and successfully synthesized. It is found that the core/crown@shell NPLs exhibit consistently substantially improved photoluminescence quantum yield compared to the core@shell NPLs regardless of their CdSe-core size, CdS-crown size, and CdS-shell thickness. This enhancement in quantum yield is attributed to the passivation of trap sites resulting from the critical peripheral growth with laterally extending CdS-crown layer before the vertical shell growth. This is also verified with the disappearance of the fast nonradiative decay component in the core/crown NPLs from the time-resolved fluorescence spectroscopy. When compared to the core@shell NPLs, the core/crown@shell NPLs exhibit relatively symmetric emission behavior, accompanied with suppressed lifetime broadening at cryogenic temperatures, further suggesting the suppression of trap sites. Moreover, constructing both the CdS-crown and CdS-shell regions, significantly enhanced absorption cross-section is achieved. This, together with the suppressed Auger recombination, enables the achievement of the lowest threshold amplified spontaneous emission (approximate to 20 mu J cm(-2)) from the core/crown@shell NPLs among all different architectures of NPLs. These findings indicate that carefully heterostructured NPLs will play a critical role in building high-performance colloidal optoelectronic devices, which may even possibly challenge their traditional epitaxially grown thin-film based counterparts.Öğe Temperature-dependent emission kinetics of colloidal semiconductor nanoplatelets strongly modified by stacking(Amer Chemical Soc, 2016) Erdem, Onur; Olutaş, Murat; Güzeltürk, Burak; Keleştemur, Yusuf; Demir, Hilmi VolkanWe systematically studied temperature-dependent emission kinetics in solid films of solution-processed CdSe nanoplatelets (NPLs) that are either intentionally stacked or nonstacked. We observed that the steady-state photoluminescence (PL) intensity of nonstacked NPLs considerably increases with decreasing temperature, whereas there is only a slight increase in stacked NPLs. Furthermore, PL decay time of the stacked NPL ensemble is comparatively much shorter than that of the nonstacked NPLs, and this result is consistent at all temperatures. To account for these observations, we developed a probabilistic model that describes excitonic processes in a stack using Markov chains, and we found excellent agreement between the model and experimental results. These findings develop the insight that the competition between the radiative channels and energy transfer assisted hole trapping leads to weakly temperature-dependent PL intensity in the case of the stacked NPL ensembles as compared to the nonstacked NPLs lacking strong energy transfer. This study shows that it is essential to account for the effect of NPL stacking to understand their resulting PL emission properties.Öğe 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, AshmaUnique 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.
