Browsing by Author "Nizamoğlu, Sedat"

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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
Electric field dependence of radiative recombination lifetimes in polar InGaN/GaN quantum heterostructures
(IEEE, 2009) Sarı, Emre; Nizamoğlu, Sedat; Lee I.-H.; Baek J.-H.; Demir, Hilmi Volkan
We report on external electric field dependence of recombination lifetimes in polar InGaN/GaN quantum heterostructures. In our study, we apply external electric fields one order of magnitude less than and in opposite direction to the polarization-induced electrostatic fields inside the well layers. Under the increasing external electric field, we observe a decrease in carrier lifetimes (τ) and radiative recombination lifetimes (τr), latter showing a weaker dependence. Our results on τr show an agreement with our transfer matrix method based simulation results and demonstrate Fermi's golden rule in polar InGaN/GaN quantum heterostructures dependent on electric field. For our study, we grew 5 pairs of 2.5 nm thick In0.15Ga 0.85N quantum well and 7.5 nm thick GaN barrier layers in a p-i-n diode architecture using metal-organic chemical vapor deposition (MOCVD) on a c-plane sapphire substrate. Devices with 300 μm × 300 μm mesa size were fabricated using standard photolithography, reactive ion etching and metallization steps. We used indium-tin oxide (ITO) based semi-transparent contacts in top (p-GaN) layer for uniform application of electric field across the well layers. The fabricated devices were diced and mounted on a TO-can for compact testing. © 2009 IEEE.
Open Access
Enhanced spontaneous emission in semiconductor nanocrystal solids using resonant energy transfer for integrated devices
(IEEE, 2008-11) Nizamoğlu, Sedat; Demir, Hilmi Volkan
Size-tuneable optical properties of semiconductor nanocrystal (NC) quantum dots make them attractive for a wide range of device applications. However, in these device applications, nanocrystals typically suffer from relatively low quantum efficiency (QE) when they are cast into solid form. To reduce the effect of this problem, we propose and demonstrate the enhancement of spontaneous emission in nanocrystal solids by recycling their trapped excitons through resonant nonradiative Forster energy transfer (ET) for hybrid integrated devices. For this purpose, we designed closely packed CdSe/ZnS core/shell nanocrystal emitters with an energy gradient of approximately 160 meV integrated on LEDs.
Open Access
Förster-type nonradiative energy transfer directed from colloidal quantum dots to epitaxial quantum wells for light harvesting applications
(Optical Society of America, 2011) Nizamoğlu, Sedat; Sarı, Emre; Baek J.-H.; Lee I.-H.; Demir, Hilmi Volkan
We report on Frster-type nonradiative energy transfer directed from CdSe/ZnS core/shell quantum dots to InGaN/GaN quantum wells with 69.6% efficiency at 1.527 ns-1 rate at room temperature for potential light harvesting and solar cells applications. © 2011 OSA.
Open Access
High-quality white light generation using dually hybridized nanocrystals and conjugated polymers
(IEEE, 2007) Nizamoğlu, Sedat; Özel, Tuncay; Mutlugun, Evren; Huyal, Ilkem Ozge; Sarı, Emre; Tian, N.; Holder, E.; Demir, Hilmi Volkan
Open Access
InGaN/GaN based LEDs with electroluminescence in violet, blue, and green tuned by epitaxial growth temperature
(IEEE, 2007) Sarı, Emre; Nizamoğlu, Sedat; Özel, Tuncay; Demir, Hilmi Volkan; İnal, Ayşe; Ülker, Erkin; Özbay, Ekmel; Dikme, Y.; Heuken, M.
In this work, we present a full set of InGaN LEDs based on a single optimal InGaN/GaN quantum design with emission wavelengths spanning from green to blue to violet by tuning the active layer growth temperature to precisely control InN incorporation into the quantum structures.
Open Access
Investigation of excitonic effects in polar InGaN/GaN quantum heterostructures for enhanced quantum electroabsorption in blue
(IEEE, 2007) Sarı, Emre; Nizamoğlu, Sedat; Özel, Tuncay; Koç, Aslı; Lee I.-H.; Baek J.H.; Demir, Hilmi Volkan
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
Nanocrystal hybridized white light sources integrated on near UV leds
(World Scientific Publishing, 2007) Nizamoğlu, Sedat; Demir, Hilmi Volkan
We report on CdSe/ZnS core-shell nanocrystal (NC) based white light sources integrated on near-UV InGaN/GaN light emitting diodes (LEDs). We present the design, epitaxial growth, fabrication, integration and characterization of such hybrid NC-LEDs that incorporate the dual combinations of cyan and yellow nanocrystals (λPL=500-580 nm) and the trio combinations of cyan, green, and red nanocrystals (λPL=500-540-620 nm).
Open Access
Nanocrystal integrated light emitting diodes based on radiative and nonradiative energy transfer for the green gap
(IEEE, 2009) Nizamoğlu, Sedat; Sarı, Emre; Baek J.-H.; Lee I.-H.; Demir, Hilmi Volkan
Recently the photometric conditions for ultra-efficient solid-state lighting have been discussed [1-2]. These studies show that a luminous efficacy of optical radiation at 408 lm/Wopt and a color rendering index (CRI) of 90 at a correlated color temperature (CCT) of 3000 K are achievable at the same time. For this purpose light emitting diodes (LEDs) emitting in blue, green, yellow, and red colors at 463, 530, 573, and 614 nm with relative optical power levels of 1/8, 2/8, 2/8, and 3/8, are required, respectively [1-2]. Although InxGa1-xN material system is capable to cover the whole visible by changing the In composition (x), it is technically extremely challenging to obtain efficient green/yellow light emitting diodes especially at those wavelengths (i.e., at 530 nm and 573 nm, respectively) due to reduced internal quantum efficiency [2-4]. Furthermore, by using the (Al xGa1-x)1-yInyP quaternary alloy it is also possible to cover from 650 nm to 580 nm. However, the efficiencies significantly decrease towards green. Therefore, there exists a significant gap in the green-yellow spectral regions (known as "the green gap") to make efficient light emitting diodes. To address this green gap problem, we propose and demonstrate proof-of-concept nanocrystal (NCs) hybridized green/yellow light emitting diodes that rely on both radiative energy transfer and nonradiative energy transfer (i.e., FRET-Förster resonance energy transfer) for color conversion on near-ultraviolet (near-UV) LEDs.
Open Access
Near-UV InGaN/GaN-based dual-operation quantum optoelectronic devices
(SPIE, 2007) Özel, Tuncay; Sarı, Emra; Nizamoğlu, Sedat; Demir, Hilmi Volkan
We present a novel dual-operation InGaN/GaN based quantum optoelectronic device (QOD) that operates as a quantum electroabsorption modulator in reverse bias and as a light emitter in forward bias in the spectral range of near-ultraviolet (UV). Here we report the design, epitaxial growth, fabrication, and characterization of such QODs that incorporate ∼2-3 nm thick InGaN/GaN quantum structures for operation between 380 nm and 400 nm. In reverse bias, our QODs show an optical absorption coefficient change of ∼14000 cm -1 with a reverse bias of 9 V (corresponding to ∼40 cm -1 absorption coefficient change for 1 V/μm field swing) at 385 nm, reported for the first time for InGaN/GaN quantum structures in the near-UV range. In forward bias, though, our QODs exhibit optical electroluminescence spectrum centered around 383 nm with a full width at half maximum of 20 nm and photoluminescence spectrum centered around 370 nm with a full width at half maximum of 12 nm. This dual operation makes such quantum optoelectronic devices find a wide range of optoelectronics applications both as an electroabsorption modulator and a light emitting diode (LED).
Open Access
Non-radiative energy-transfer-driven quantum dot LEDs
(IEEE, 2010) Güzeltürk, Burak; Erdem, Talha; Ünal, Emre; Nizamoğlu, Sedat; Tuncel, Donus; Demir, Hilmi Volkan
Open Access
Novel hybrid light emitting diodes with multiple assemblies of nanocrystals to generate and tune white light
(2007) Nizamoğlu, Sedat
Today approximately one third of the world population (about two billion people) in under-developed countries has no access to electricity and relies on unhealthy, costly and low-quality fuel-based lighting for home illumination. In the rest of the world, lighting consumes a large portion (20%) of the total electricity production, which significantly contributes to global warming problem. Also given limited resources, such large energy consumption needs to be reduced for sustainable economic growth. Solid state lighting provides remedy to these problems for the entire globe. Therefore, the advancement of white light emitting diodes (WLEDs) becomes a key point for development of human civilization. To this end, the strong demand for the development of high quality WLEDs around the globe motivates our research work on the investigation of white light generation with high color rendering index. In this thesis, we develop and demonstrate nanocrystal hybridized light emitting diodes with high color rendering index. By the hybridization of multiple layer-by-layer assemblies of CdSe/ZnS core-shell nanocrystals on blue and near ultraviolet (n-UV) InGaN/GaN light emitting diodes, we show white light generation with highly tunable optical properties such as tristimulus color coordinates, correlated color temperature, and color rendering index. Additionally, by using dual hybridization of nanocrystals in combination with conjugated polymers, we obtain white light sources with high color rendering indices exceeding the requirements of the future solid state lighting applications. In this thesis, we present design, growth, fabrication, experimental characterization and theoretical analysis of these hybrid white LEDs.
Open Access
Novel nanocrystal-integrated LEDs utilizing radiative and nonradiative energy transfer for high-quality efficient light generation
(2011) Nizamoğlu, Sedat
To combat environmental issues escalating with the increasing carbon footprint, combined with the energy problem of limited resources, innovating fundamentally new ways of raising energy efficiency and level of energy utilization is essential to our energy future. Today, to this end, achieving lighting efficiency is an important key because artificial lighting consumes about 19% of total energy generation around the globe. There is a large room for improving lighting efficacy for potential carbon emission cut. However, the scientific challenge is to reach simultaneously high-quality photometric performance. To address these problems, we proposed, developed and demonstrated a new class of color-conversion light emitting diodes (LEDs) integrated with nanophosphors of colloidal quantum dots. The favorable properties of these semiconductor nanocrystal quantum dots, including size-tuneable and narrow-band emission with high photostability, have provided us with the ability of achieving highquality, efficient lighting. Via using custom-design combinations of such nanocrystal emitters, we have shown that targeted white luminescence spectra can be generated with desired high photometric performance, which is important for obtaining application-specific white LEDs, e.g., for indoors lighting, street lighting, and LED-TV backlighting. Furthermore, dipole-dipole coupling capability of these semiconductor nanocrystals has allowed us to realize novel device designs based on Förster-type nonradiative energy transfer. By mastering exciton-exciton interactions in color-conversion LEDs, we have demonstrated enhanced color conversion via recycling of trapped excitons and white light generation based on nonradiative pumping of nanocrystal quantum dots for color conversion. This research work has led to successful demonstrators of semiconductor nanocrystal quantum dots that photometrically outperform conventional rareearth phosphor powders in terms of color rendering, luminous efficacy of optical radiation, color temperature and scotopic/photopic ratio for the first time.
Open Access
Plasmon-coupled photocapacitor neuromodulators
(American Chemical Society, 2020) Melikov, R.; Srivastava, S. B.; Karatum, O.; Doğru-Yüksel, I. B.; Jalali, H. B.; Sadeghi, S.; Dikbaş, U. M.; Ülgüt, Burak; Kavaklı, İ. H.; Çetin, A. E.; Nizamoğlu, Sedat
Efficient transduction of optical energy to bioelectrical stimuli is an important goal for effective communication with biological systems. For that, plasmonics has a significant potential via boosting the light−matter interactions. However, plasmonics has been primarily used for heat-induced cell stimulation due to membrane capacitance change (i.e., optocapacitance). Instead, here, we demonstrate that plasmonic coupling to photocapacitor biointerfaces improves safe and efficacious neuromodulating displacement charges for an average of 185% in the entire visible spectrum while maintaining the faradic currents below 1%. Hotelectron injection dominantly leads the enhancement of displacement current in the blue spectral window, and the nanoantenna effect is mainly responsible for the improvement in the red spectral region. The plasmonic photocapacitor facilitates wireless modulation of single cells at three orders of magnitude below the maximum retinal intensity levels, corresponding to one of the most sensitive optoelectronic neural interfaces. This study introduces a new way of using plasmonics for safe and effective photostimulation of neurons and paves the way toward ultrasensitive plasmon-assisted neurostimulation devices.
Open Access
Polar vs. nonpolar InGaN/GaN quantum heterostructures: Opposite quantum confined electroabsorption and carrier dynamics behavior
(IEEE, 2010) Sarı, Emre; Nizamoğlu, Sedat; Choi J.H.; Lee, S.J.; Baik, K.H.; Lee I.H.; Baek J.H.; Hwang, S.-M.; Demir, Hilmi Volkan
We present a study of quantum confined electroabsorption and carrier dynamics in polar c-plane and nonpolar a-plane InGaN/GaN quantum heterostructures. We demonstrate red-shifting absorption edge, due to quantum confined Stark effect, in nonpolar InGaN/GaN quantum structures in response to increased electric field, while we show the opposite effect with blue-shifting absorption spectra in polar quantum structures. Moreover, confirmed by time-resolved photoluminescence measurements, we prove that carrier lifetimes increase with increasing electric field for nonpolar structures, whereas the opposite occurs for polar ones.
Open Access
Quantum efficiency enhancement optimization in colloidal semiconductor quantum dot solids using nonradiative energy transfer
(Optical Society of America, 2010) Nizamoğlu, Sedat; Akın, Onur; Demir, Hilmi Volkan
We investigate quantum efficiency (QE) enhancement via recycling of trapped excitons in energy-gradient of quantum dot solids using nonradiative energy transfer. The maximum QE increase of 17% is achieved when the donor-acceptor ratio is 1:1. © 2010 Optical Society of America.
Open Access
Room-temperature, high-efficiency conversion of mott-wannier excitons to frenkel excitons in hybrid semiconductor quantum dot/polymer composites
(OSA, 2011) Nizamoğlu, Sedat; Sun, X. W.; Demir, Hilmi Volkan
Efficient conversion from Mott-Wannier to Frenkel excitons at room temperature is observed in hybrid inorganic/organic composites of CdSe/ZnS core/shell heteronanocrystals in MDMO-PPV homopolymers at a rate of 0.2628 ns-1 with an efficiency of 80.9%. © 2011 Optical Society of America.
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Superior warm-White light-emitting diodes integrated with quantum dot nanophosphors for high luminous efficacy and color rendering
(Optical Society of America, 2011) Nizamoğlu, Sedat; Erdem, Talha; Sun, X. W.; Demir, Hilmi Volkan
Quantum dot nanophoshor hybridized warm-white LEDs are reported to exhibit high photometric performance of luminous efficacy exceeding 350 lm/Wopt and color rendering index close to 90 at correlated color temperatures <3000 K. ©2011 Optical Society of America.