Browsing by Author "Mutlugün, Evren"

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Open Access
CdSe/ZnS core-shell nanocrystal based scintillators for enhanced detection in UV
(IEEE, 2006) Demir, Hilmi Volkan; Soğancı, İbrahim Murat; Mutlugün, Evren
In this work, we present a novel scintillator that incorporates CdSe/ZnS core-shell nanocrystals to utilize both their high quantum fluorescence efficiency in the visible and high optical absorption in the UV for the purpose of facilitating UV imaging on a Si platform. Here we demonstrate enhancement in UV detection up to 800% with respect to the host polymer in the UV.
Open Access
Cesium–lead based inorganic perovskite quantum-dots as interfacial layer for highly stable perovskite solar cells with exceeding 21% efficiency
(Elsevier, 2019) Akın, S.; Altıntaş, Y.; Mutlugün, Evren; Sonmezoglu, S.
Despite the excellent photovoltaic performances of perovskite solar cells (PSCs), the instability of PSCs under severe environment (e.g. humidity, light-induced, etc.) limits further commercialization of such devices. Therefore, in recent years, research on the long-term stability improvement of PSCs has been actively carried out in perovskite field. To address these issues, we demonstrated the incorporation of ultra-thin interfacial layer of inorganic CsPbBr1.85I1.15 perovskite quantum-dots (PQDs) that can effectively passivate defects at or near to the perovskite/hole transport material (HTM) interface, significantly suppressing interfacial recombination. This passivation layer increased the open circuit voltage (Voc) of triple-cation perovskite cells by as much as 50 mV, with champion cells achieving Voc ∼ 1.14 V. As a result, we obtained hysteresis-free cells with the efficiency beyond 21%. More importantly, devices based on such architecture are capable of resisting humidity and lightinduced. Remarkably, the device employing CsPbBr1.85I1.15 demonstrated a superb shelf-stability aganist to humidity under ambient conditions (R.H.≥40%), retaining nearly 91% of initial efficiency after 30 days, while the efficiency of control device rapidly dropped to 45% from its initial value under the same conditions. Besides benefiting from the high moisture resistivity as well as supressed ion migration, PSCs based on PQDs showed better operational stability (retaining 94% of their initial performance) than that of the PQDs-free one under continuous light irradiation over 400 h. In addition, a faster PL decay time of 4.66 ns was attained for perovskite/PQDs structure (5.77 ns for only PQDs structure) due to the favorable energy transfer at the interface, indicating a Förster resonance energy transfer (FRET) mechanism. This work indicates that inorganic PQDs are important materials as interlayer in PSCs to supremely enhance the device stability and efficiency.
Open Access
Efficient exciton transfer from an epitaxial quantum well to an energy gradient structure composed of layer-by-layer assembled colloidal quantum dots
(American Physical Society, 2012) Nizamoğlu, Sedat; Hernandez-Martinez, Pedro L.; Mutlugün, Evren; Demir, Hilmi Volkan
Efficient Exciton Transfer from an Epitaxial Quantum Well to an Energy Gradient Structure Composed of Layer-by-Layer Assembled Colloidal Quantum Dots SEDAT NIZAMOGLU1 , Bilkent University, PEDRO LUDWIG HERNANDEZ MARTINEZ, Bilkent University, Nanyang Technological University, EVREN MUTLUGUN, Bilkent University, HILMI VOLKAN DEMIR, Bilkent University, Nanyang Technological University — In this work, we study exciton migration from a violet-emitting epitaxial quantum well (QW) to an energy gradient structure that consists of layer-by-layer assembled, green- and red-emitting quantum dot (QD) bilayer. In the experimental study, the energy gradient of these green and red QDs provides an increase of 64.2% in the exciton transfer efficiency with respect to the bilayer of only red-emitting QDs. These results suggest that the energy difference between the QD layers significantly boosts the QW-QD exciton transfer rate compared to the mono-dispersed case. To support this experimental observation, we propose a theoretical model based on optical near field and density matrix to investigate the effects of energy difference between the QD layers. The strong exciton transfer from the epitaxial QWs to the colloidal QDs is essential to the energy efficiency of hybrid optoelectronic devices [1-3]. [1] A. Ruland, et al., Adv. Mater. 23, 4573–4577 (2011). [2] M. Naruse, et al., Phys. Rev. 82, 125417 (2010). [3] S. Nizamoglu, et al., Appl. Phys. Lett. 98, 163108 (2011).
Open Access
Exciton harvesting systems of nanocrystals
(2011) Mutlugün, Evren
Semiconductor nanocrystals, also known as colloidal quantum dots, have gained substantial scientific interest for innovative light harvesting applications including those in biolabeling. Organic dyes and fluorescent proteins are widely used in biotargeting and live cell imaging, but their intrinsic optical properties, such as narrow excitation windows, limit their potential for advanced applications, e.g., spectral multiplexing. Compared to these organic fluorophores, favorable properties of the quantum dots including high photoluminescence quantum yields together with tunable emission peaks and narrow spectral emission widths, high extinction coefficients, and broad absorption bands enable us to discover and innovate light harvesting composites. In such systems, however, the scientific challenge is to achieve high levels of energy transfer from one species to the other, with additional features of versatility and tunability. To address these problems, as a conceptual advancement, this thesis proposes and demonstrates a new class of versatile light harvesting systems of semiconductor nanocrystals mediated by excitonic interactions based on Förstertype nonradiative energy transfer. In this thesis, we synthesized near-unity efficiency colloidal quantum dots with as-synthesized photoluminescence quantum yields of >95%. As proof-of-concept demonstrations, we studied and achieved highly efficient exciton harvesting systems of quantum dots bound to fluorescent proteins, where the excitons are zipped from the dots to the proteins in the composite. This led to many folds of light harvesting (tunable up to 15 times) in the case of the green fluorescent protein. Using organic dye molecules electrostatically interacting with quantum dots, we showed high levels of exciton migration from the dots to the molecules (up to 94%). Furthermore, we demonstrated stand-alone, flexible membranes of nanocrystals in unprecedentedly large areas (> 50 cm × 50 cm), which paves the way for highend, large-scale applications. In the thesis, we also developed exciton-exciton coupling models to support the experimental results. This thesis opens up new possibilities for exciton-harvesting in biolabeling and optoelectronics.
Open Access
Giant alloyed hot injection shells enable ultralow optical gain threshold in colloidal quantum wells
(American Chemical Society, 2019) Altıntaş, Yemliha; Güngör, Kıvanç; Gao, Y.; Sak, Mustafa; Quliyeva, Ulviyya; Bappi, G.; Mutlugün, Evren; Sargent, E. H.; Demir, Hilmi Volkan
As an attractive materials system for high-performance optoelectronics, colloidal nanoplatelets (NPLs) benefit from atomic-level precision in thickness, minimizing emission inhomogeneous broadening. Much progress has been made to enhance their photoluminescence quantum yield (PLQY) and photostability. However, to date, layer-by-layer growth of shells at room temperature has resulted in defects that limit PLQY and thus curtail the performance of NPLs as an optical gain medium. Here, we introduce a hot-injection method growing giant alloyed shells using an approach that reduces core/shell lattice mismatch and suppresses Auger recombination. Near-unity PLQY is achieved with a narrow full-width-at-half-maximum (20 nm), accompanied by emission tunability (from 610 to 650 nm). The biexciton lifetime exceeds 1 ns, an order of magnitude longer than in conventional colloidal quantum dots (CQDs). Reduced Auger recombination enables record-low amplified spontaneous emission threshold of 2.4 μJ cm–2under one-photon pumping. This is lower by a factor of 2.5 than the best previously reported value in nanocrystals (6 μJ cm–2 for CdSe/CdS NPLs). Here, we also report single-mode lasing operation with a 0.55 mJ cm–2 threshold under two-photoexcitation, which is also the best among nanocrystals (compared to 0.76 mJ cm–2 from CdSe/CdS CQDs in the Fabry–Pérot cavity). These findings indicate that hot-injection growth of thick alloyed shells makes ultrahigh performance NPLs.
Open Access
Highly flexible, full-color, top-emitting quantum dot light-emitting diode tapes
(IEEE, 2013) Yang X.; Mutlugün, Evren; Gao, Y.; Zhao, Y.; Tan, S.T.; Sun X.W.; Demir, Hilmi Volkan
We report flexible tapes of high-performance, top-emitting, quantum dot based, light-emitting diodes (QLEDs) with multicolor emission, actively working even when flexed. The resulting QLED tapes reach a high peak luminance level of 19,265 cd/m2. © 2013 IEEE.
Open Access
Highly luminescent ZnCdTeS nanocrystals with wide spectral tunability for efficient color-conversion white-light-emitting-diodes
(Institute of Physics Publishing Ltd., 2021-09-28) Soheyli, Ehsan; Zargoush, Sirous; Yazıcı, Ahmet Faruk; Sahraei, Reza; Mutlugün, Evren
CdTe-based semiconductor nanocrystals (NCs) with size and composition-dependent efficient bandgap properties are historically mature nanomaterials for colloidal optoelectronic applications. In this work, we present the highly luminescent quaternary ZnCdTeS NCs with tunable emission across a wide visible spectrum from green to red spectral range. Prepared via a direct aqueous-based approach, a second capping agent of trisodium citrate (TSC) was used to enhance the photoluminescence (PL) emission efficiency, the chemical stability, and to spectrally widen the coverage of the emission spectra of ZnCdTeS NCs. Adding TCS created a remarkable blue shift from 572 nm in the absence of TSC, to 548 nm. On the other hand, upon optimization of experimental parameters, superior ZnCdTeS NCs with a narrow PL profile typically less than 50 nm, the high quantum efficiency of 76%, and tunable emission from 515-to-645 nm were synthesized in an aqueous solvent. The keynotes were the superior and reproducible luminescent properties for the core only NCs, without shell and using relatively low reaction temperatures. It was shown that in the suggested synthesis method, the high efficiency emitted color of ZnCdTeS NCs can be easily controlled from 515-to-650 nm with excellent stability against harsh conditions. The biexponential decay profiles of samples prepared at different reaction temperatures demonstrated that the average recombination lifetime is below 40 ns and increases with the growth of the ZnCdTeS NCs. Results reveal that the excitonic energy levels have the main role in the recombination process. Finally, to demonstrate the functional advantages of the prepared NCs in optoelectronics, the NCs were used to fabricate color-conversion white light-emitting diodes. The color coordinate of the device is recorded as (0.4951, 0.3647) with CRI of 91, CCT of 1954 K, and LER of 251 lm W−1 by employing only two distinct emitters for color conversion.
Open Access
Highly stable, near-unity efficiency atomically flat semiconductor nanocrystals of CdSe/ZnS hetero-nanoplatelets enabled by ZnS-Shell hot-injection growth
(WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2019) Yemliha, Yemliha; Quliyeva, Ulviyya; Güngör, Kıvanç; Erdem, Onur; Kelestemur, Yusuf; Mutlugün, Evren; Kovalenko, M.; Demir, Hilmi Volkan
Colloidal semiconductor nanoplatelets (NPLs) offer important benefits in nanocrystal optoelectronics with their unique excitonic properties. For NPLs, colloidal atomic layer deposition (c‐ALD) provides the ability to produce their core/shell heterostructures. However, as c‐ALD takes place at room temperature, this technique allows for only limited stability and low quantum yield. Here, highly stable, near‐unity efficiency CdSe/ZnS NPLs are shown using hot‐injection (HI) shell growth performed at 573 K, enabling routinely reproducible quantum yields up to 98%. These CdSe/ZnS HI‐shell hetero‐NPLs fully recover their initial photoluminescence (PL) intensity in solution after a heating cycle from 300 to 525 K under inert gas atmosphere, and their solid films exhibit 100% recovery of their initial PL intensity after a heating cycle up to 400 K under ambient atmosphere, by far outperforming the control group of c‐ALD shell‐coated CdSe/ZnS NPLs, which can sustain only 20% of their PL. In optical gain measurements, these core/HI‐shell NPLs exhibit ultralow gain thresholds reaching ≈7 µJ cm−2. Despite being annealed at 500 K, these ZnS‐HI‐shell NPLs possess low gain thresholds as small as 25 µJ cm−2. These findings indicate that the proposed 573 K HI‐shell‐grown CdSe/ZnS NPLs hold great promise for extraordinarily high performance in nanocrystal optoelectronics.
Open Access
Improving performance and stability in quantum dot-sensitized solar cell through single layer graphene/Cu2S nanocomposite counter electrode
(Elsevier, 2020) Akman, E.; Altıntaş, Y.; Gülen, M.; Yılmaz, M.; Mutlugün, Evren; Sönmezoğlu, S.
In this work, we presented an effective nanocomposite to modify the Cu2S film by employing single layer graphene (SLG) frameworks via chemical vapor deposition, and utilized this nanocomposite as counter electrode (CE) with CdSe/ZnS core/shell quantum dots for highly stable and efficient quantum dot-sensitized solar cell (QDSSC). Furthermore, Cu2S film is directly synthesized on SLG framework by electrodeposition method. Using this nanocomposite as CE, we have achieved the high efficiency as high as 3.93% with fill factor of 0.63, which is higher than those with bare Cu2S CE (3.40% and 0.57). This remarkable performance is attributed to the surface area enhancement by creating nanoflower-shape, the reduction of charge transfer resistance, improvement of catalytic stability, and the surface smoothness as well as good adhesion. More importantly, no visible color change and detachment from surface for the Cu2S@SLG nanocomposite was observed, demonstrating that the SLG framework is critical role in shielding the Cu2S structure from sulphur ions into electrolyte, and increasing the adhesion of the Cu2S structure on surface, thus preventing its degradation. Consequently, the Cu2S@SLG nanocomposite can be utilized as an effective agent to boost up the performance of QDSSCs.
Open Access
Large-area (> 50 cm × 50 cm), freestanding, flexible, optical membranes of Cd-free nanocrystal quantum dots
(IEEE, 2012) Mutlugün, Evren; Hernandez Martinez, Pedro L.; Eroğlu, Cüneyt; Coşkun, Yasemin; Erdem, Talha; Sharma, Vijay K.; Ünal, Emre; Panda, S. K.; Hickey, S. G.; Gaponik, N.; Eychmuller, A.; Demir, Hilmi Volkan
Colloidal semiconductor quantum dots (QDs) have been extensively explored for numerous applications ranging from optoelectronics to biotechnology. This strong demand for the colloidal QDs arises because of their favorable optical and electronic properties. From the application points of view, QDs typically need to be used in their solid form, as opposed to their as-synthesized dispersion form. For immobilization of QDs and homogeneity of their films, various polymers have been used to host QDs within solid media. However, the integration of QDs into a polymeric medium is commonly complex, which requires a high level of understanding to provide optical quality. © 2012 IEEE.
Open Access
Light-harvesting positively-charged nanocrystals for strong energy transfer to dye molecules
(IEEE, 2008-11) Mutlugün, Evren; Demir, Hilmi Volkan
In this work, water-soluble positively-charged CdSe/ZnS core/shell nanocrystals were presented, serving as light harvester (donor) for strong energy transfer to rhodamine B dye molecules (acceptor) in solution. In accordance with photoluminescence (PL) characterization, the emission of rhodamine B was enhanced at the laser pump wavelength, which may open up possibilities for lasers based on dye molecules pumped at different wavelengths.
Open Access
Localized plasmon-engineered spontaneous emission of CdSe/ZnS nanocrystals closely-packed in the proximity of Ag nanoislands
(2007) Soğancı, İbrahim Murat; Nizamoğlu, Sedat; Mutlugün, Evren; Demir, Hilmi Volkan
As a proof-of-concept demonstration, we show that the localized plasmons of metal nanoisland films provide the ability to modify and control the collective spontaneous emission of nanocrystals in their proximity (including emission peak wavelength and linewidth in addition to intensity). Using randomly-distributed Ag nanoparticles, we demonstrate plasmonic resonance tuned for the proximal CdSe/ZnS NC emitters to shift PL peak wavelength (by 14 nm) and reduce the FWHM (by 10 nm), while enhancing PL intensity by 15.1 and 21.6 times compared to the control groups of nanocrystals alone and those with nanoAg but no dielectric spacer, respectively.
Open Access
Multi-layered CdSe/ZnS/CdSe heteronanocrystals to generate and tune white light
(2008-11) Nizamoğlu, Sedat; Mutlugün, Evren; Özel, Tuncay; Demir, Hilmi Volkan; Sapra, S.; Gaponik, N.; Eychmüller, A.
In this study, tuneable white light generation by controlling CdSe/ZnS/CdSe core/shell/shell heteronanocrystals integrated on InGaN/GaN light emitting diodes was presented. These multilayered quantum dots, also known as onion-like heterostructures, were designed and synthesized to emit in red (around 600 nm) from the CdSe core and in green (around 550 nm) from the CdSe shell. By designing and hybridizing these red-green emitting heterostructures on blue emitting LEDs, an integrated WLEDs on a single chip was demonstrated. By controlling the number of integrated heteronanocrystals, their (x,y) tristimulus coordinates were tuned from (0.26,0.23) to (0.37,0.36), along with their corresponding correlated color temperature tuned from 27413 K to 4192 K and the luminous efficacy of their optical radiation (the ratio of the emitted luminous flux to the radiant flux) tuned from 258 lm/W to 375 lm/W. Further investigation on the change of in-film optical properties of these heteronanocrystals with respect to their in-solution emission was performed.
Open Access
Multi-material specific, targeted self-assembly of nanocrystal emitters using genetically engineered peptides on optoelectronic microchips
(IEEE, 2008-11) Zengin, Güliş; Şeker, U. O. S.; Koç, Aslı; Mutlugün, Evren; Akyüz, Özgün; Sarı, Emre; Sarıkaya, M.; Tamerler, C.; Demir, Hilmi Volkan
We demonstrated material-specific binding of the quantum dot emitters hybridized with GEPI on multi-material patterned microchips. These proof-of-concept results open up new opportunities in nanophotonics, allowing for more specific and controlled assembly of quantum dots in optoelectronic devices and building of novel molecular organic-inorganic hybrid devices
Open Access
Nanocrystal emitters for enhanced photovoltaics in UV
(IEEE, 2008-11) Mutlugün, Evren; Soğancı, İbrahim Murat; Demir, Hilmi Volkan
[No abstract available]
Open Access
Nanocrystal LEDs with enhanced external quantum efficiency enabled by the use of phosphorescent molecules
(IEEE, 2013) Mutlugün, Evren; Abiyasa, A.P.; Güzeltürk, Burak; Gao, Y.; Leck, K.S.; Sun X.W.; Demir, Hilmi Volkan
We report efficiency enhancement in quantum dot (QD) based LEDs with the aid of excitonic energy transfer from co-doped TCTA:Ir(ppy)3 layer to CdSe/ZnS QDs while providing spectrally pure emission. © 2013 IEEE.
Open Access
Natural wax-stabilized perovskite nanocrystals as pen-on-paper inks and doughs
(American Chemical Society, 2022-05-27) Karabel Ocal, S.; Kiremitler, N. B.; Yazici, A. F.; Çelik, N.; Önses, M. Serdar; Mutlugün, Evren
Perovskite nanocrystals (PNCs) are emerging luminescent materials for a wide range of technological applications. The broad adaptation of PNCs will be greatly improved by addressing their intrinsically low stability and developing processes for their assembly into 2D and 3D structures using facile approaches. Inspired by the mechanism of natural protection of leaves, this paper proposes natural carnauba wax (CW) as an encapsulation material for PNCs. The synthesis of PNCs is performed in the presence of CW, which is derived from the leaves of Copernicia prunifera palm. CW acts as a solvent and replaces the commonly used octadecene in the preparation of PNCs. The facile synthesis in CW results in PNCs with greatly improved thermal, water, and air stability. Furthermore, the thermal and mechanical properties make PNC-Wax a highly suitable solid ink for versatile processing of these materials into 2D and 3D architectures. PNC-Wax can be printed via a pen-on-paper approach by heating at modest temperatures. The rapid plasticization of PNC-Wax by mechanical agitation enables hand-shaping of the material in a manner similar to playdoughs, which would possibly enable the versatile use of this material for various applications. © 2022 American Chemical Society. All rights reserved.
Open Access
Novel ultraviolet scintillators based on semiconductor quantum dot emitters for significantly enhanced photodetection and photovoltaics
(2007) Mutlugün, Evren
Silicon photonics opens opportunities to realize optoelectronic devices directly on large-scale integrated electronics, leveraging advanced Si fabrication and computation capabilities. However, silicon is constrained in different aspects for use in optoelectronics. Such one limitation is observed in Si based photodetectors, cameras, and solar cells that exhibit very poor responsivity in the ultraviolet (UV) spectral range. Si CMOS photodetectors and CCD cameras cannot be operated in UV, despite the strong demand for UV detection and imaging in security applications. Also, although 95% of the photovoltaics market is dominated by Si based solar cells, silicon is not capable of using UV radiation of the solar spectrum for solar energy conversion, as required especially in space applications. In this thesis for the first time, we demonstrate novel UV scintillators made of semiconductor quantum dot emitters hybridized on Si detectors and cameras to detect and image in UV with significantly improved responsivity and on Si solar cells to generate electrical energy from UV radiation with significantly improved solar conversion efficiency. We present the device conception, design, fabrication, experimental characterization, and theoretical analysis of these UV nanocrystal scintillators. Integrating highly luminescent CdSe/ZnS core-shell nanocrystals, we demonstrate hybrid photodetectors that exhibit two-orders-of-magnitude peak enhancement in their responsivity. We also develop photovoltaic nanocrystal scintillators to enhance open-circuit voltage, short-circuit current, fill factor, and solar conversion efficiency in UV. Hybridizing CdSe/ZnS quantum dots on Si photovoltaic devices, we show that the solar conversion efficiency is doubled under white light illumination (Xe lamp). Such UV scintillator nanocrystals hold great promise to enable photodetection and imaging in UV and extend photovoltaic activity to UV.
Open Access
Record high external quantum efficiency of 19.2% achieved in light-emitting diodes of colloidal quantum wells enabled by hot-Injection shell growth
(WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2020) Liu, B.; Altıntaş, Yemliha; Wang, L.; Shendre, S.; Sharma, Manoj; Sun, H.; Mutlugün, Evren; Demir, Hilmi Volkan
Colloidal quantum wells (CQWs) are regarded as a highly promising class of optoelectronic materials, thanks to their unique excitonic characteristics of high extinction coefficients and ultranarrow emission bandwidths. Although the exploration of CQWs in light‐emitting diodes (LEDs) is impressive, the performance of CQW‐LEDs lags far behind other types of soft‐material LEDs (e.g., organic LEDs, colloidal‐quantum‐dot LEDs, and perovskite LEDs). Herein, high‐efficiency CQW‐LEDs reaching close to the theoretical limit are reported. A key factor for this high performance is the exploitation of hot‐injection shell (HIS) growth of CQWs, which enables a near‐unity photoluminescence quantum yield (PLQY), reduces nonradiative channels, ensures smooth films, and enhances the stability. Remarkably, the PLQY remains 95% in solution and 87% in film despite rigorous cleaning. Through systematically understanding their shape‐, composition‐, and device‐engineering, the CQW‐LEDs using CdSe/Cd0.25Zn0.75S core/HIS CQWs exhibit a maximum external quantum efficiency of 19.2%. Additionally, a high luminance of 23 490 cd m−2, extremely saturated red color with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.715, 0.283), and stable emission are obtained. The findings indicate that HIS‐grown CQWs enable high‐performance solution‐processed LEDs, which may pave the path for future CQW‐based display and lighting technologies.
Open Access
Size effect in optical activation of TiO2 nanoparticles in photocatalytic process
(IEEE, 2007) Soğancı, İbrahim Murat; Mutlugün, Evren; Tek, Sümeyra; Demir, Hilmi Volkan; Yücel, D.; Çeliker, G.
In this work, we optically investigate and characterize the photocatalytic recovery of contaminated TiO 2 nanoparticles of different sizes that are incorporated in solgel films to study the size effect. We demonstrate significant improvement in the optical efficiency of the photocatalytic nanoparticles as we reduce the particle size.