Browsing by Author "Gungor K."

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Open Access
Alloyed heterostructures of CdSexS1-x nanoplatelets with highly tunable optical gain performance
(American Chemical Society, 2017) Kelestemur Y.; Dede, D.; Gungor K.; Usanmaz, C. F.; Erdem, O.; Demir, Hilmi Volkan
Here, we designed and synthesized alloyed heterostructures of CdSexS1-x nanoplatelets (NPLs) using CdS coating in the lateral and vertical directions for the achievement of highly tunable optical gain performance. By using homogeneously alloyed CdSexS1-x core NPLs as a seed, we prepared CdSexS1-x/CdS core/crown NPLs, where CdS crown region is extended only in the lateral direction. With the sidewall passivation around inner CdSexS1-x cores, we achieved enhanced photoluminescence quantum yield (PL-QY) (reaching 60%), together with increased absorption cross-section and improved stability without changing the emission spectrum of CdSexS1-x alloyed core NPLs. In addition, we further extended the spectral tunability of these solution-processed NPLs with the synthesis of CdSexS1-x/CdS core/shell NPLs. Depending on the sulfur composition of the CdSexS1-x core and thickness of the CdS shell, CdSexS1-x/CdS core/shell NPLs possessed highly tunable emission characteristics within the spectral range of 560-650 nm. Finally, we studied the optical gain performances of different heterostructures of CdSexS1-x alloyed NPLs offering great advantages, including reduced reabsorption and spectrally tunable optical gain range. Despite their decreased PL-QY and reduced absorption cross-section upon increasing the sulfur composition, CdSexS1-x based NPLs exhibit highly tunable amplified spontaneous emission performance together with low gain thresholds down to ∼53 μJ/cm2.
Open Access
CdSe/CdSe1-xTex core/crown heteronanoplatelets: tuning the excitonic properties without changing the thickness
(American Chemical Society, 2017) Kelestemur Y.; Guzelturk, B.; Erdem, O.; Olutas M.; Erdem, T.; Usanmaz, C. F.; Gungor K.; Demir, Hilmi Volkan
Here 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 ∼570 nm (for CdSe1-xTex crown with x = 0.02) and ∼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 ∼95%) owing to the suppression of nonradiative hole trap sites. Also, we found significantly increased fluorescence lifetimes from ∼49 up 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.
Open Access
Flexible and fragmentable tandem photosensitive nanocrystal skins
(Royal Society of Chemistry, 2016) Akhavan S.; Uran, C.; Bozok, B.; Gungor K.; Kelestemur Y.; Lesnyak, V.; Gaponik N.; Eychmüller A.; Demir, Hilmi Volkan
We proposed and demonstrated the first account of large-area, semi-transparent, tandem photosensitive nanocrystal skins (PNSs) constructed on flexible substrates operating on the principle of photogenerated potential buildup, which avoid the need for applying an external bias and circumvent the current-matching limitation between junctions. We successfully fabricated and operated the tandem PNSs composed of single monolayers of colloidal water-soluble CdTe and CdHgTe nanocrystals (NCs) in adjacent junctions on a Kapton polymer tape. Owing to the usage of a single NC layer in each junction, noise generation was significantly reduced while keeping the resulting PNS films considerably transparent. In each junction, photogenerated excitons are dissociated at the interface of the semi-transparent Al electrode and the NC layer, with holes migrating to the contact electrode and electrons trapped in the NCs. As a result, the tandem PNSs lead to an open-circuit photovoltage buildup equal to the sum of those of the two single junctions, exhibiting a total voltage buildup of 128.4 mV at an excitation intensity of 75.8 μW cm-2 at 350 nm. Furthermore, we showed that these flexible PNSs could be bent over 3.5 mm radius of curvature and cut out in arbitrary shapes without damaging the operation of individual parts and without introducing any significant loss in the total sensitivity. These findings indicate that the NC skins are promising as building blocks to make low-cost, flexible, large-area UV/visible sensing platforms with highly efficient full-spectrum conversion.
Open Access
High-stability, high-efficiency organic monoliths made of oligomer nanoparticles wrapped in organic matrix
(American Chemical Society, 2016) Soran-Erdem Z.; Erdem, T.; Gungor K.; Pennakalathil, J.; Tuncel, D.; Demir, Hilmi Volkan
Oligomer nanoparticles (OL NPs) have been considered unsuitable for solid-state lighting due to their low quantum yields and low temperature stability of their emission. Here, we address these problems by forming highly emissive and stable OL NPs solids to make them applicable in lighting. For this purpose, we incorporated OL NPs into sucrose matrix and then prepared their all-organic monoliths. We show that wrapping the OL NPs in sucrose significantly increases their quantum yield up to 44%, while the efficiency of their dispersion and direct solid-film remain only at ∼6%. We further showed ∼3-fold improved temperature stability of OL NP emission within these monoliths. Our experiments revealed that a physical passivation mechanism is responsible from these improvements. As a proof-of-concept demonstration, we successfully employed these high-stability, high-efficiency monoliths as color converters on a blue LED chip. Considering the improved optical features, low cost, and simplicity of the presented methodology, we believe that this study holds great promise for a ubiquitous use of organic OL NPs in lighting and possibly in other photonic applications.
Open Access
Highly efficient nonradiative energy transfer from colloidal semiconductor quantum dots to wells for sensitive noncontact temperature probing
(Wiley-VCH Verlag, 2016) Olutas M.; Guzelturk, B.; Kelestemur Y.; Gungor K.; Demir, Hilmi Volkan
This 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 ≈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. Â© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Open Access
Nanocrystal light-emitting diodes based on type II nanoplatelets
(Elsevier BV, 2018) Liu, B.; Delikanli S.; Gao, Y.; Dede, D.; Gungor K.; Demir, Hilmi Volkan
Colloidal semiconductor nanoplatelets (NPLs) have recently emerged as a new family of semiconductor nanocrystals with distinctive structural and electronic properties originating from their atomically flat architecture. To date, type II NPLs have been demonstrated to possess great potential to optoelectronic applications, such as solar cells and lasers. Herein, nanocrystal light-emitting diodes (LEDs) based on type II NPLs have been developed. The photoluminescence quantum yield of these used type II NPL (CdSe/CdSe0.8Te0.2 core/crown) is close to 85%. By exploring an effective inverted structure with the dual hole transport layer, the NPL-LEDs exhibit i) a turn-on voltage of 1.9 V, ii) a maximum luminance of 34520 cd m−2, iii) an EQE of 3.57% and a PE of 9.44 lm W−1. Compared with previous NPL-based LEDs, the performance of our devices is remarkably enhanced. For example, the luminance is 350-fold higher than the best inverted NPL-based LED. The findings may not only represent a significant step for NPL-based LEDs, but also unlock a new opportunity that this class of type II NPLs materials are promising for developing high-performance LEDs.
Open Access
Near-unity efficiency energy transfer from colloidal semiconductor quantum wells of CdSe/cdS nanoplatelets to a monolayer of MoS2
(American Chemical Society, 2018) Taghipour, N.; Martinez, P. L. H.; Ozden, A.; Olutas M.; Dede, D.; Gungor K.; Erdem, O.; Perkgoz, N. K.; Demir, Hilmi Volkan
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 Förster resonance energy transfer (FRET). Here, we show ultrahigh-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 ∼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 Förster radius of ∼33 nm.
Open Access
Near-unity emitting copper-doped colloidal semiconductor quantum wells for luminescent solar concentrators
(Wiley-VCH Verlag, 2017) Sharma, M.; Gungor K.; Yeltik A.; Olutas M.; Guzelturk, B.; Kelestemur Y.; Erdem, T.; Delikanli S.; McBride, J. R.; Demir, Hilmi Volkan
Doping of bulk semiconductors has revealed widespread success in optoelectronic applications. In the past few decades, substantial effort has been engaged for doping at the nanoscale. Recently, doped colloidal quantum dots (CQDs) have been demonstrated to be promising materials for luminescent solar concentrators (LSCs) as they can be engineered for providing highly tunable and Stokes-shifted emission in the solar spectrum. However, existing doped CQDs that are aimed for full solar spectrum LSCs suffer from moderately low quantum efficiency, intrinsically small absorption cross-section, and gradually increasing absorption profiles coinciding with the emission spectrum, which together fundamentally limit their effective usage. Here, the authors show the first account of copper doping into atomically flat colloidal quantum wells (CQWs). In addition to Stokes-shifted and tunable dopant-induced photoluminescence emission, the copper doping into CQWs enables near-unity quantum efficiencies (up to ≈97%), accompanied by substantially high absorption cross-section and inherently step-like absorption profile, compared to those of the doped CQDs. Based on these exceptional properties, the authors have demonstrated by both experimental analysis and numerical modeling that these newly synthesized doped CQWs are excellent candidates for LSCs. These findings may open new directions for deployment of doped CQWs in LSCs for advanced solar light harvesting technologies.
Open Access
Platelet-in-Box Colloidal Quantum Wells: CdSe/CdS@CdS Core / Crown@Shell Heteronanoplatelets
(Wiley-VCH Verlag, 2016) Kelestemur Y.; Guzelturk, B.; Erdem, O.; Olutas M.; Gungor K.; Demir, Hilmi Volkan
Here, 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 (≈20 μJ 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. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Open Access
Simple and complex metafluids and metastructures with sharp spectral features in a broad extinction spectrum: particle-particle interactions and testing the limits of the Beer-Lambert law
(American Chemical Society, 2017) Besteiro, L. V.; Gungor K.; Demir, Hilmi Volkan; Govorov, A. O.
Metallic nanocrystals (NCs) are useful instruments for light manipulation around the visible spectrum. As their plasmonic resonances depend heavily on the NC geometry, modern fabrication techniques afford a great degree of control over their optical responses. We take advantage of this fact to create optical filters in the visible-near IR. Our systems show an extinction spectrum that covers a wide range of wavelengths (UV to mid-IR) while featuring a narrow transparency band around a wavelength of choice. We achieve this by carefully selecting the geometries of a collection of NCs with narrow resonances that cover densely the spectrum from the UV to the mid-IR except for the frequencies targeted for transmission. This fundamental design can be executed in different kinds of systems, including a solution of colloidal metal NCs (metafluids), a structured planar metasurface, or a combination of both. Along with the theory, we report experimental results, showing metasurface realizations of the system, and we discuss the strengths and weaknesses of these different approaches, paying particular attention to particle-particle interaction and to what extent it hinders the intended objective by shifting and modifying the profile of the planned resonances through the hybridization of their plasmonic modes. We found that the Beer-Lambert law is very robust overall and is violated only upon aggregation or in configurations with nearly touching NCs. This striking property favors the creation of metafluids with a narrow transparency window, which are investigated here.