Browsing by Author "Erdem, Onur"

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Open Access
Anomalous spectral characteristics of ultrathin sub-nm colloidal CdSe nanoplatelets
(Optical Society of America, 2017) Bose S.; Delikanlı, Savaş; Yeltik, Aydan; Sharma, Manoj; Erdem, Onur; Dang C.; Fan W.; Zhang D.H.; Demir, Hilmi Volkan
We demonstrate high quantum yield broad photoluminescence emission of ultrathin sub-nanometer CdSe nanoplatelets (two-monolayer). They also exhibit polarization-characterized lateral size dependent anomalous heavy hole and light/split-off hole absorption intensities. © 2017 IEEE.
Open Access
Cd-free Cu-doped ZnInS/ZnS Core/Shell nanocrystals: Controlled synthesis and photophysical properties
(SpringerOpen, 2018) Kaur, Manpreet; Sharma, Ashma; Olutaş, Murat; Erdem, Onur; Kumar, A.; Sharma, Manoj; Demir, Hilmi Volkan
Here, we report efficient composition-tunable Cu-doped ZnInS/ZnS (core and core/shell) colloidal nanocrystals (CNCs) synthesized by using a colloidal non-injection method. The initial precursors for the synthesis were used in oleate form rather than in powder form, resulting in a nearly defect-free photoluminescence (PL) emission. The change in Zn/In ratio tunes the percentage incorporation of Cu in CNCs. These highly monodisperse Cu-doped ZnInS CNCs having variable Zn/In ratios possess peak emission wavelength tunable from 550 to 650 nm in the visible spectrum. The quantum yield (QY) of these synthesized Cd-free CNCs increases from 6.0 to 65.0% after coating with a ZnS shell. The CNCs possessing emission from a mixed contribution of deep trap and dopant states to only dominant dopant-related Stokes-shifted emission are realized by a careful control of stoichiometric ratio of different reactant precursors during synthesis. The origin of this shift in emission was understood by using steady state and time-resolved fluorescence (TRF) spectroscopy studies. As a proof-of-concept demonstration, these blue excitable Cu-doped ZnInS/ZnS CNCs have been integrated with commercial blue LEDs to generate white-light emission (WLE). The suitable combination of these highly efficient doped CNCs results led to a Commission Internationale de l’Enclairage (CIE) color coordinates of (0.33, 0.31) at a color coordinate temperature (CCT) of 3694 K, with a luminous efficacy of optical radiation (LER) of 170 lm/Wopt and a color rendering index (CRI) of 88.
Open Access
Colloidal optoelectronics of self-assembled quantum well superstructures
(2020-06) Erdem, Onur
Advances in the colloidal nanocrystal synthesis enabled creation of quasi twodimensional colloidal quantum wells (CQWs) in the last decade. These CQWs possess similar properties to those of epitaxially grown quantum wells while at the same time oﬀering the beneﬁts of low-cost synthesis and solubility in various solvents. Their atomically precise thickness and one-dimensional quantum conﬁnement grant them favorable properties such as narrow emission linewidth, reduced inhomogeneous broadening and giant oscillator strength. In addition, due to their quasi-two dimensional shape, they display intrinsic anisotropy. Because of this anisotropy, the particle interactions in closely packed ﬁlms depend greatly on the orientation of these CQWs. To fully utilize the interaction of CQWs with each other or with other particles in their proximity, we develop a selfassembly technique, which is used to deposit highly uniform thin CQW ﬁlms onto various solid substrates. This self-assembly technique allows us to deposit CQWs as a continuous monolayer while at the same time controlling their orientation throughout the substrate, thereby modifying their packing factor as well as nearﬁeld dipole-dipole interactions. This self-assembly technique is also employed to create large-area CQW ﬁlms of any desired thickness, simply by applying the same deposition technique on the same substrate as many times as desired. We use these self-assembled CQW ﬁlms to study the two main aspects of nanocrystal optoelectronics, namely, Förster resonance energy transfer (FRET) and optical gain, with CQWs. By using the orientation-controlled CQW monolayers, we show that the rate of FRET from colloidal quantum dots (QDs) to a monolayer of CQWs can be tuned via dipole-dipole interactions between QDs and CQWs. We use Förster’s theory of nonradiative energy transfer while taking into account the anisotropy of the excitonic CQW excitonic state as well as its delocalization throughout the CQW to account for our results. Next, we show that our multilayered CQW ﬁlms display optical gain in uncharacteriscally low thicknesses (as small as 40 nm) due to the tight packing and extremely uniform deposition of the CQWs. We furthermore study systematically the observed threshold of ampliﬁed spontaneous emission (ASE) in these CQW multilayers as a function of the ﬁlm thickness (i.e., the number of monolayers), and demonstrate that the gain threshold drops with increasing thickness, accompanied by the red-shift of the ASE peak. These trends can be explained by the varying degree of optical mode conﬁnement, which is a function of both the ﬁlm thickness as well as the wavelength of propagating mode. Our self-assembly technique allows to study and make use of the favorable properties of the CQWs including anisotropy and enhanced optical gain. Since this technique enables us to produce large-area ﬁlms displaying excellent homogeneity, it can be a benchmark building block for creating device-scale 2- or 3-dimensional superstructures from CQWs as well as from other types of colloidal nanocrystals to be utilized in both in- and out-of-plane optical applications.
Open Access
Colloidal semiconductor nanocrystals
(Springer Singapore, 2022-10-28) Erdem, Onur; Demir, Hilmi Volkan
In this chapter, we review colloidal semiconductor nanocrystals (NCs) and their remarkable size-dependent properties. We emphasize on colloidal nanoplatelets and explain how they differ from NCs of other classes.
Open Access
Conclusions and future outlook
(Springer Singapore, 2022-10-28) Erdem, Onur; Demir, Hilmi Volkan
Open Access
Highly directional highly efficient solution processed light emitting diodes of all face down
(Wiley-VCH Verlag GmbH & Co. KGaA, 2023-04-06) Baruj, Hamed Dehghanpour; Bozkaya, İklim; Canımkurbey, Betül; Işık, Ahmet Tarık; Shabani, Farzan; Delikanlı, Savaş; Shendre, S.; Erdem, Onur; Işık, Furkan; Demir, Hilmi Volkan
Semiconductor colloidal quantum wells (CQWs) provide anisotropic emission behavior originating from their anisotropic optical transition dipole moments (TDMs). Here, solution-processed colloidal quantum well light-emitting diodes (CQW-LEDs) of a single all-face-down oriented self-assembled monolayer (SAM) film of CQWs that collectively enable a supreme level of IP TDMs at 92% in the ensemble emission are shown. This significantly enhances the outcoupling efficiency from 22% (of standard randomly-oriented emitters) to 34% (of face-down oriented emitters) in the LED. As a result, the external quantum efficiency reaches a record high level of 18.1% for the solution-processed type of CQW-LEDs, putting their efficiency performance on par with the hybrid organic-inorganic evaporation-based CQW-LEDs and all other best solution-processed LEDs. This SAM-CQW-LED architecture allows for a high maximum brightness of 19,800 cd m$^{−2}$ with a long operational lifetime of 247 h at 100 cd m$^{−2}$ as well as a stable saturated deep-red emission (651 nm) with a low turn-on voltage of 1.7 eV at a current density of 1 mA cm$^{−2}$ and a high J$_{90}$ of 99.58 mA cm$^{−2}$. These findings indicate the effectiveness of oriented self-assembly of CQWs as an electrically-driven emissive layer in improving outcoupling and external quantum efficiencies in the CQW-LEDs.
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
Introduction
(Springer Singapore, 2022-10-28) Erdem, Onur; Demir, Hilmi Volkan
We begin by giving a short overview of colloidal nanoplatelets, and we explain why their orientation-controlled assemblies are of particular interest. We finalize the chapter by giving an outline of our brief.
Open Access
Light-induced paramagnetism in colloidal Ag+-doped CdSe nanoplatelets
(American Chemical Society, 2021-03-25) Najafi, A.; Sharma, Manoj; Delikanlı, Savaş; Bhattacharya, A.; Murphy, J. R.; Pientka, J.; Sharma, A.; Quinn, A. P.; Erdem, Onur; Kattel, S.; Kelestemur, Y.; Kovalenko, M. V.; Rice, W. D.; Demir, Hilmi Volkan; Petrou, A.
We describe a study of the magneto-optical properties of Ag+-doped CdSe colloidal nanoplatelets (NPLs) that were grown using a novel doping technique. In this work, we used magnetic circularly polarized luminescence and magnetic circular dichroism spectroscopy to study light-induced magnetism for the first time in 2D solution-processed structures doped with nominally nonmagnetic Ag+ impurities. The excitonic circular polarization (PX) and the exciton Zeeman splitting (ΔEZ) were recorded as a function of the magnetic field (B) and temperature (T). Both ΔEZ and PX have a Brillouin-function-like dependence on B and T, verifying the presence of paramagnetism in Ag+-doped CdSe NPLs. The observed light-induced magnetism is attributed to the transformation of nonmagnetic Ag+ ions into Ag2+, which have a nonzero magnetic moment. This work points to the possibility of incorporating these nanoplatelets into spintronic devices, in which light can be used to control the spin injection.
Open Access
Liquid interface self-assembly of colloidal nanoplatelets for optoelectronics
(Springer Singapore, 2022-10-28) Erdem, Onur; Demir, Hilmi Volkan
In this chapter, we discuss how liquid interface self-assembly can contribute to the utilization of colloidal semiconductor nanoplatelets in optoelectronics. Self-assembled nanoplatelet mono- or multilayers can be used as two-dimensional optically active waveguides, gain media of ultra-thin lasers, or energy transfer-based photosensitizers.
Open Access
Liquid interface self-assembly with colloidal quantum wells
(Springer Singapore, 2022-10-28) Erdem, Onur; Demir, Hilmi Volkan
In this chapter, we discuss our methodologies for the self-assembly of colloidal nanoplatelets (NPLs) at the liquid interface. We also review other recent studies on orientation-controlled platelet assembly on liquid interfaces. We compare the results of the reported studies and discuss the parameters that affect the NPL orientation at the liquid interface.
Open Access
Mechanosynthesis of polymer-stabilized lead bromide perovskites: insight into the formation and phase conversion of nanoparticles
(Tsinghua University Press, 2021-04) Jiang, G.; Erdem, Onur; Hübner, R.; Georgi, M.; Wei, W.; Fan, X.; Wang, J.; Demir, Hilmi Volkan
The application of polymers to replace oleylamine (OLA) and oleic acid (OA) as ligands for perovskite nanocrystals is an effective strategy to improve their stability and durability especially for the solution-based processing. Herein, we report a mechanosynthesis of lead bromide perovskite nanoparticles (NPs) stabilized by partially hydrolyzed poly(methyl methacrylate) (h-PMMA) and high-molecular-weight highly-branched poly(ethylenimine) (PEI-25K). The as-synthesized NP solutions exhibited green emission centered at 516 nm, possessing a narrow full-width at half-maximum of 17 nm and as high photoluminescence quantum yield (PL QY) as 85%, while showing excellent durability and resistance to polar solvents, e.g., methanol. The colloids of polymer-stabilized NPs were directly processable to form stable and strongly-emitting thin films and solids, making them attractive as gain media. Furthermore, the roles of h-PMMA and PEI-25K in the grinding process were studied in depth. The h-PMMA can form micelles in the grinding solvent of dichloromethane to act as size-regulating templates for the growth of NPs. The PEI-25K with large amounts of amino groups induced significant enrichment of PbBr2 in the reaction mixture, which in turn caused the formation of CsPb2Br5-mPbBr2 and CsPbBr3-Cs4PbBr6-nCsBr NPs. The presence of CsPbBr3-Cs4PbBr6-nCsBr NPs was responsible for the high PL QY, as the Cs4PbBr6 phase with a wide energy bandgap can passivate the surface defects of the CsPbBr3 phase. This work describes a direct and facile mechanosynthesis of polymer-coordinated perovskite NPs and promotes in-depth understanding of the formation and phase conversion for perovskite NPs in the grinding process.
Open Access
Near-field energy transfer into silicon inversely proportional to distance using quasi-2D colloidal quantum well donors
(Wiley-VCH Verlag GmbH & Co. KGaA, 2021-09-12) Humayun, Muhammad Hamza; Hernandez-Martinez, Pedro Ludwig; Gheshlaghi, Negar; Erdem, Onur; Altıntaş, Yemliha; Shabani, Farzan; Demir, Hilmi Volkan
Silicon is the most prevalent material system for light-harvesting applications; however, its inherent indirect bandgap and consequent weak absorption limits its potential in optoelectronics. This paper proposes to address this limitation by combining the sensitization of silicon with extraordinarily large absorption cross sections of quasi-2D colloidal quantum well nanoplatelets (NPLs) and to demonstrate excitation transfer from these NPLs to bulk silicon. Here, the distance dependency, d, of the resulting Förster resonant energy transfer from the NPL monolayer into a silicon substrate is systematically studied by tuning the thickness of a spacer layer (of Al2O3) in between them (varied from 1 to 50 nm in thickness). A slowly varying distance dependence of d−1 with 25% efficiency at a donor–acceptor distance of 20 nm is observed. These results are corroborated with full electromagnetic solutions, which show that the inverse distance relationship emanates from the delocalized electric field intensity across both the NPL layer and the silicon because of the excitation of strong in-plane dipoles in the NPL monolayer. These findings pave the way for using colloidal NPLs as strong light-harvesting donors in combination with crystalline silicon as an acceptor medium for application in photovoltaic devices and other optoelectronic platforms.
Open Access
Observation of optical gain from aqueous quantum well heterostructures in water
(Royal Society of Chemistry, 2022-09-25) Delikanlı, Savaş; Durmuşoğlu, E.G.; Erdem, Onur; Shabani, Farzan; Kumar, Satish; Barujb, Hamed Dehghanpour; Demir, Hilmi Volkan; Işık, Furkan; Canımkurbey, Betül
Although achieving optical gain using aqueous solutions of colloidal nanocrystals as a gain medium is exceptionally beneficial for bio-optoelectronic applications, the realization of optical gain in an aqueous medium using solution-processed nanocrystals has been extremely challenging because of the need for surface modification to make nanocrystals water dispersible while still maintaining their gain. Here, we present the achievement of optical gain in an aqueous medium using an advanced architecture of CdSe/CdS@CdxZn1−xS core/crown@gradient-alloyed shell colloidal quantum wells (CQWs) with an ultralow threshold of ∼3.4 μJ cm−2 and an ultralong gain lifetime of ∼2.6 ns. This demonstration of optical gain in an aqueous medium is a result of the carefully heterostructured CQWs having large absorption cross-section and gain cross-section in addition to inherently slow Auger recombination in these CQWs. Furthermore, we show low-threshold in-water amplified spontaneous emission (ASE) from these aqueous CQWs with a threshold of 120 μJ cm−2. In addition, we demonstrate a whispering gallery mode laser with a low threshold of ∼30 μJ cm−2 obtained by incorporating films of CQWs by exploiting layer-by-layer approach on a fiber. The observation of low-threshold optical gain with ultralong gain lifetime presents a significant step toward the realization of advanced optofluidic colloidal lasers and their continuous-wave pumping.
Open Access
Optical gain in ultrathin self‐assembled bi‐layers of colloidal quantum wells enabled by the mode confinement in their high‐index dielectric waveguides
(Wiley-VCH Verlag, 2020) Foroutan-Barenji, Sina; Erdem, Onur; Gheshlaghi, Negar; Altıntaş, Yemliha; Demir, Hilmi Volkan
This study demonstrates an ultra‐thin colloidal gain medium consisting of bi‐layers of colloidal quantum wells (CQWs) with a total film thickness of 14 nm integrated with high‐index dielectrics. To achieve optical gain from such an ultra‐thin nanocrystal film, hybrid waveguide structures partly composed of self‐assembled layers of CQWs and partly high‐index dielectric material are developed and shown: in asymmetric waveguide architecture employing one thin film of dielectric underneath CQWs and in the case of quasi‐symmetric waveguide with a pair of dielectric films sandwiching CQWs. Numerical modeling indicates that the modal confinement factor of ultra‐thin CQW films is enhanced in the presence of the adjacent dielectric layers significantly. The active slabs of these CQW monolayers in the proposed waveguide structure are constructed with great care to obtain near‐unity surface coverage, which increases the density of active particles, and to reduce the surface roughness to sub‐nm scale, which decreases the scattering losses. The excitation and propagation of amplified spontaneous emission (ASE) along these active waveguides are experimentally demonstrated and numerically analyzed. The findings of this work offer possibilities for the realization of ultra‐thin electrically driven colloidal laser devices, providing critical advantages including single‐mode lasing and high electrical conduction.
Open Access
Orientation-controlled nonradiative energy transfer to colloidal nanoplatelets: engineering dipole orientation factor
(American Chemical Society, 2019) Erdem, Onur; Güngör, Kıvanç; Güzeltürk, Burak; Tanrıöver, İbrahim; Sak, Mustafa; Olutaş, Murat; Dede, Didem; Kelestemur, Yusuf; Demir, Hilmi Volkan
We proposed and showed strongly orientation-controlled Förster resonance energy transfer (FRET) to highly anisotropic CdSe nanoplatelets (NPLs). For this purpose, we developed a liquid–air 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 cm2. 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 dipole–dipole 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 Förster 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.
Open Access
Self-assembly of colloidal nanocrystals
(Springer Singapore, 2022-10-28) Erdem, Onur; Demir, Hilmi Volkan
We describe different approaches to colloidal nanocrystal self-assembly and review the reports where different NC superstructures were constructed using these techniques.
Open Access
Self-resonant microlasers of colloidal quantum wells constructed by direct deep patterning
(American Chemical Society, 2021-06-09) Gheshlaghi, Negar; Foroutan-Barenji, Sina; Erdem, Onur; Altintas, Yemliha; Shabani, Farzan; Humayun, Muhammad Hamza; Demir, Hilmi Volkan
Here, the first account of self-resonant fully colloidal μ-lasers made from colloidal quantum well (CQW) solution is reported. A deep patterning technique is developed to fabricate well-defined high aspect-ratio on-chip CQW resonators made of grating waveguides and in-plane reflectors. The fabricated waveguide-coupled laser, enabling tight optical confinement, assures in-plane lasing. CQWs of the patterned layers are closed-packed with sharp edges and residual-free lifted-off surfaces. Additionally, the method is successfully applied to various nanoparticles including colloidal quantum dots and metal nanoparticles. It is observed that the patterning process does not affect the nanocrystals (NCs) immobilized in the attained patterns and the different physical and chemical properties of the NCs remain pristine. Thanks to the deep patterning capability of the proposed method, patterns of NCs with subwavelength lateral feature sizes and micron-scale heights can possibly be fabricated in high aspect ratios.
Open Access
Single-mode lasing from a single 7 nm thick monolayer of colloidal quantum wells in a monolithic microcavity
(Wiley-VCH Verlag, 2021-03-03) Foroutan-Barenji, Sina; Erdem, Onur; Delikanlı, Savaş; Yağcı, Hüseyin Bilge; Gheshlaghi, Negar; Altıntaş, Yemliha; Demir, Hilmi Volkan
In this work, monolithically-fabricated vertical cavity surface emitting lasers (VCSELs) of densely-packed, orientation-controlled, atomically flat colloidal quantum wells (CQWs) using a self-assembly method is reported and single-mode lasing from a record thin colloidal gain medium with a film thickness of 7 nm under femtosecond optical excitation is demonstrated. Specially engineered CQWs are used to demonstrate these hybrid CQW-VCSELs consisting of only a few layers to a single monolayer of CQWs and are achieved the lasing from these thin gain media by thoroughly modeling and implementing a vertical cavity consisting of distributed Bragg reflectors with an additional dielectric layer for mode tuning. Accurate spectral and spatial alignment of the cavity mode with the CQW films is secured with the help of full electromagnetic computations. While overcoming the long-pending problem of limited electrical conductivity in thicker colloidal films, such ultrathin colloidal gain media can be helpful to enable fully electrically-driven colloidal lasers.
Open Access
Strongly polarized color conversion of isotropic colloidal quantum dots coupled to fano resonances
(Royal Society of Chemistry, 2024-07-24) Güngör, Kıvanç; Erdem, Onur; Güzeltürk, Burak; Ünal, Emre; Jun, Shinae; Jang, Eunjoo; Demir, Hilmi Volkan
Colloidal quantum dots (QDs) offer high color purity essential to high-quality liquid crystal displays (LCDs), which enables unprecedented levels of color enrichment in LCD-TVs today. However, for LCDs requiring polarized backplane illumination in operation, highly polarized light generation using inherently isotropic QDs remains a fundamental challenge. Here, we show strongly polarized color conversion of isotropic QDs coupled to Fano resonances of v-grooved surfaces compatible with surface-normal LED illumination for next-generation QD-TVs. This architecture overcomes the critically oblique excitation of surface plasmon coupled emission by using v-shapes imprinted on the backlight unit (BLU). With isotropic QDs coated on the proposed v-BLU surface, we experimentally measured a far-field polarization contrast ratio of similar to 10. Full electromagnetic solution shows Fano line-shape transmission in transverse magnetic polarization allowing for high transmission as an indication for forward-scattering configuration. Of these QDs coupled to the surface plasmon-polariton modes, we observed strong modifications in their emission kinetics revealed by time-resolved photoluminescence spectroscopy and via dipole orientations identified by back focal plane imaging. This collection of findings indicates conclusively that these isotropic QDs are forced to radiate in a linearly polarized state from the patterned planar surface under surface-normal excitation. For next-generation QD-TVs, the proposed polarized color-converting isotropic QDs on such v-BLUs can be deployed in bendable electronic displays. We demonstrate a novel backlight unit utilizing the plasmonic interaction of quantum dots with v-shaped metallic grooves, which are capable of enhancing the emission in the desired polarization while suppressing the unwanted polarization component.