Browsing by Subject "Excitons"

Now showing 1 - 20 of 41

Open Access
Absorption enhancement of molecules in the weak plasmon-exciton coupling regime
(Optical Society of American (OSA), 2014) Balci, S.; Karademir, E.; Kocabas, C.; Aydınlı, Atilla
We report on the experimental and theoretical investigations of enhancing the optical absorption of organic molecules in the weak plasmon-exciton coupling regime. A metal-organic hybrid structure consisting of dye molecules embedded in the polymer matrix is placed in close vicinity to thin metal films. We have observed a transition from a weak coupling regime to a strong coupling one as the thickness of the metal layer increases. The results indicate that absorption of the self-assembled J-aggregate nanostructures can be increased in the weak plasmon-exciton coupling regime and strongly quenched in the strong coupling regime. A theoretical model based on the transfer-matrix method qualitatively confirms the experimental results obtained from polarization-dependent spectroscopic reflection measurements.
Open Access
An ab initio study of vertical heterostructures formed by CdO and SnC monolayers
(Elsevier, 2024-01-30) Seyedmohammadzadeh, Mahsa; Mobaraki, Arash; Tanatar, B.; Gülseren, Oğuz
Assembling two dimensional (2D) materials in vertical heterostructures is one of the main techniques for tuning electronic and optical properties. In most cases, known as van der Waals heterostructures (vdWHs), the interlayer distances are larger than typical covalent bond lengths resulting in weak interlayer interactions. It has been shown that reducing the distance between the layers can significantly alter the properties of separated layers, which is not so noticeable in vdWHs and thus creates a new platform for controlling the physical properties of 2D materials. Motivated by enhanced properties of 2D vertical heterostructures, employing ab-initio calculations based on density functional theory we examined CdO/SnC systems in four different configurations. Our results reveal that in spite of thermodynamic and mechanical stabilities of all considered structures, according to the calculated phonon frequencies, only the structure formed by placing the Sn atom on the O atom and the C atom on the Cd atom is dynamically stable at zero temperature. This structure has an interlayer distance of 2.52 Å which is smaller than the interlayer distance in typical vdWHs. We investigated the electronic and optical properties of this dynamically stable structure utilizing GW approximation and solving Bethe–Salpeter equation. Unlike the monolayer CdO which possesses a single optical absorption peak close to the red light energy, the considered CdO/SnC structure has an optical band gap of 1.14 eV, and it can absorb 13% of incident light in the blue light region.
Open Access
Anisotropic Emission from Multilayered Plasmon Resonator Nanocomposites of Isotropic Semiconductor Quantum Dots
(American Chemical Society, 2011-01-19) Ozel, T.; Nizamoglu, S.; Sefunc, M.A.; Samarskaya, O.; Ozel, I. O.; Mutlugun, E.; Lesnyak, V.; Gaponik N.; Eychmuller, A.; Gaponenko, S. V.; Demir, Hilmi Volkan
We propose and demonstrate a nanocomposite localized surface plasmon resonator embedded into an artificial three-dimensional construction. Colloidal semiconductor quantum dots are assembled between layers of metal nanoparticles to create a highly strong plasmon-exciton interaction in the plasmonic cavity. In such a multilayered plasmonic resonator architecture of isotropic CdTe quantum dots, we observed polarized light emission of 80% in the vertical polarization with an enhancement factor of 4.4, resulting in a steady-state anisotropy value of 0.26 and reaching the highest quantum efficiency level of 30% ever reported for such CdTe quantum dot solids. Our electromagnetic simulation results are in good agreement with the experimental characterization data showing a significant emission enhancement in the vertical polarization, for which their fluorescence decay lifetimes are substantially shortened by consecutive replication of our unit cell architecture design. Such strongly plasmon-exciton coupling nanocomposites hold great promise for future exploitation and development of quantum dot plasmonic biophotonics and quantum dot plasmonic optoelectronics.
Open Access
Broadband optical transparency in plasmonic nanocomposite polymer films via exciton-plasmon energy transfer
(OSA - The Optical Society, 2016) Dhama R.; Rashed, A. R.; Caligiuri V.; El Kabbash M.; Strangi, G.; De Luca A.
Inherent absorptive losses affect the performance of all plasmonic devices, limiting their fascinating applications in the visible range. Here, we report on the enhanced optical transparency obtained as a result of the broadband mitigation of optical losses in nanocomposite polymeric films, embedding core-shell quantum dots (CdSe@ZnS QDs) and gold nanoparticles (Au-NPs). Exciton-plasmon coupling enables non-radiative energy transfer processes from QDs to metal NPs, resulting in gain induced transparency of the hybrid flexible systems. Experimental evidences, such as fluorescence quenching and modifications of fluorescence lifetimes confirm the presence of this strong coupling between plexcitonic elements. Measures performed by means of an ultra-fast broadband pump-probe setup demonstrate loss compensation of gold NPs dispersed in plastic network in presence of gain. Furthermore, we compare two films containing different concentrations of gold NPs and same amount of QDs, to investigate the role of acceptor concentration (Au-NPs) in order to promote an effective and efficient energy transfer mechanism. Gain induced transparency in bulk systems represents a promising path towards the realization of loss compensated plasmonic devices. © 2016 Optical Society of America.
Open Access
Colloidal nanoplatelet/conducting polymer hybrids: excitonic and material properties
(American Chemical Society, 2016) Guzelturk, B.; Menk, F.; Philipps, K.; Kelestemur Y.; Olutas M.; Zentel, R.; Demir, Hilmi Volkan
Here 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.
Open Access
Colloidal quantum dot light-emitting diodes employing phosphorescent small organic molecules as efficient exciton harvesters
(American Chemical Society, 2014) Mutlugun, E.; Guzelturk, B.; Abiyasa, A. P.; Gao, Y.; Sun X. W.; Demir, Hilmi Volkan
Nonradiative energy transfer (NRET) is an alternative excitation mechanism in colloidal quantum dot (QD) based electroluminescent devices (QLEDs). Here, we develop hybrid highly spectrally pure QLEDs that facilitate energy transfer pumping via NRET from a phosphorescent small organic molecule-codoped charge transport layer to the adjacent QDs. A partially codoped exciton funnelling electron transport layer is proposed and optimized for enhanced QLED performance while exhibiting very high color purity of 99%. These energy transfer pumped hybrid QLEDs demonstrate a 6-fold enhancement factor in the external quantum efficiency over the conventional QLED structure, in which energy transfer pumping is intrinsically weak.
Open Access
Conditions for observation of Rabi oscillations in an exciton-polariton system
(Elsevier BV, 1995) Shumovsky, A. S.; Müstecaplioǧlu, Ö. E.
The pure quantum model describing Rabi oscillations of exciton-polaritons (EP) in a micro-cavity with finite quality is considered. It is shown that the oscillations can be observed if the cavity damping rate does not exceed some critical value depending on the EP coupling constant and the detuning of the cavity mode. An explicit expression for the renormalized Rabi frequency is found. Comparison of the results with experimental data shows a good agreement. © 1995.
Open Access
Critical role of CdSe nanoplatelets in color-converting CdSe/ZnS nanocrystals for InGaN/GaN light-emitting diodes
(OSA - The Optical Society, 2016) Hasanov N.; Sharma, V. K.; Martinez, P. L. H.; Tan S.T.; Demir, Hilmi Volkan
Here we report CdSe nanoplatelets that are incorporated into color-converting CdSe/ZnS nanocrystals for InGaN/GaN light-emitting diodes. The critical role of CdSe nanoplatelets as an exciton donor for the color conversion was experimentally investigated. The power conversion efficiency of the hybrid light-emitting diode was found to increase by 23% with the incorporation of the CdSe nanoplatelets. The performance enhancement is ascribed to efficient exciton transfer from the donor CdSe nanoplatelet quantum wells to the acceptor CdSe/ZnS nanocrystal quantum dots through Fï¿½rster-type nonradiative resonance energy transfer.
Open Access
Determination of energy-band offsets between GaN and AlN using excitonic luminescence transition in AlGaN alloys
(American Institute of Physics, 2006) Westmeyer, A. N.; Mahajan, S.; Bajaj, K. K.; Lin J. Y.; Jiang, H. X.; Koleske, D. D.; Senger, R. T.
We report the determination of the energy-band offsets between GaN and AlN using the linewidth (full width at half maximum) of an extremely sharp excitonic luminescence transition in Alx Ga1-x N alloy with x=0.18 at 10 K. Our sample was grown on C -plane sapphire substrate by metal-organic chemical-vapor deposition at 1050 °C. The observed value of the excitonic linewidth of 17 meV is the smallest ever reported in literature. On subtracting a typical value of the excitonic linewidth in high-quality GaN, namely, 4.0 meV, we obtain a value of 13.0 meV, which we attribute to compositional disorder. This value is considerably smaller than that calculated using a delocalized exciton model [S. M. Lee and K. K. Bajaj, J. Appl. Phys. 73, 1788 (1993)]. The excitons are known to be strongly localized by defects and/or the potential fluctuations in this alloy system. We have simulated this localization assuming that the hole, being much more massive than the electron, is completely immobile, i.e., the hole mass is treated as infinite. Assuming that the excitonic line broadening is caused entirely by the potential fluctuations experienced by the conduction electron, the value of the conduction-band offset between GaN and AlN is determined to be about 57% of the total-band-gap discontinuity. Using our model we have calculated the variation of the excitonic linewidth as a function of Al composition in our samples with higher Al content larger than 18% and have compared it with the experimental data. We also compare our value of the conduction-band offset with those recently proposed by several other groups using different techniques.
Open Access
Electronic structure of conducting organic polymers: insights from time-dependent density functional theory
(John Wiley & Sons Ltd., 2014) Salzner, U.
Conducting organic polymers (COPs) became an active field of research after it was discovered how thin films rather than insoluble infusible powders can be produced. The combination of the properties of plastics with those of semiconductors opened the research field of organic electronics. COPs share many electronic properties with inorganic semiconductors, but there are also major differences, e.g., the nature of the charge carriers and the amount of the exciton binding energy. Theoretical analysis has been used to interpret experimental observations early on. The polaron model that was developed from one-electron theories is still the most widely used concept. In the 1990s, time-dependent density functional theory (TDDFT) became available for routine calculations. Using TDDFT, electronic states of long oligomers can be calculated. Now UV spectra of neutral and oxidized or reduced species can be compared with in situ UV spectra recorded during doping. Likewise states of cations can be used to model photoelectron spectra. Analysis of states has resolved several puzzles which cannot be understood with the polaron model, e.g., the origin of the dual absorption band of green polymers and the origin of a 'vestigial neutral band' upon doping of long oligomers. DFT calculations also established that defect localization is not crucial for spectral changes observed during doping and that there are no bound bipolarons in COPs.
Open Access
Enhanced exciton transfer from the cascaded bilayer of green-and red-emitting CdTe quantum dots into bulk silicon
(Optical Society of America, 2013) Yeltik, Aydan; Güzeltürk, Burak; Demir, Hilmi Volkan
We show enhanced transfer of excitons from the energy-gradient of bilayered green/red-emitting quantum dots into silicon using cascaded nonradiative energy with an overall enhancement factor of 1.3 at room temperature for solar cell sensitization. © 2013 The Optical Society.
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
Exciton induced directed motion of unconstrained atoms in an ultracold gas
(Institute of Physics Publishing, 2017) Leonhardt, K.; Wüster, Sebastian; Rost, Jan Michael
We demonstrate that through localised Rydberg excitation in a three-dimensional cold atom cloud atomic motion can be rendered directed and nearly confined to a plane, without spatial constraints for the motion of individual atoms. This enables creation and observation of non-adiabatic electronic Rydberg dynamics in atoms accelerated by dipole-dipole interactions under natural conditions. Using the full l = 0, 1 m = 0, ±1 angular momentum state space, our simulations show that conical intersection crossings are clearly evident, both in atomic position information and excited state spectra of the Rydberg system. Hence, flexible Rydberg aggregates suggest themselves for probing quantum chemical effects in experiments on length scales much inflated as compared to a standard molecular situation. © 2017 IOP Publishing Ltd.
Open Access
Excitonic condensation under spin-orbit coupling and BEC-BCS crossover
(The American Physical Society, 2007) Hakioǧlu T.; Şahin, M.
The condensation of electron-hole pairs is studied at zero temperature and in the presence of a weak spin-orbit coupling (SOC) in coupled quantum wells. Under realistic conditions, a perturbative SOC can have observable effects in the order parameter of the condensate. First, the fermion exchange symmetry is absent. As a result, the condensate spin has no definite parity. Additionally, the excitonic SOC breaks the rotational symmetry yielding a complex order parameter in an unconventional way; i.e., the phase pattern of the order parameter is a function of the condensate density. This is manifested through finite off-diagonal components of the static spin susceptibility, suggesting a new experimental method to confirm an excitonic condensate.
Open Access
Excitonic energy transfer dynamics in hybrid organic/inorganic nanocomposites at high loading levels
(Optical Society of America, 2012) Güzeltürk, Burak; Hernandez-Martinez, Pedro L.; Tuncel, Dönüş; Demir, Hilmi Volkan
Temperature dependent exciton migration in the hybrid nanocomposites of conjugated polymers chemically integrated with quantum dots is studied at high loading levels. The underlying interplay between the exciton transfer and diffusion is revealed. © OSA 2012.
Open Access
Green stimulated emission boosted by nonradiative resonant energy transfer from blue quantum dots
(American Chemical Society, 2016) Gao, Y.; Yu, G.; Wang Y.; Dang C.; Sum, T. C.; Sun, H.; Demir, Hilmi Volkan
Thanks to their tunability and versatility, the colloidal quantum dots (CQDs) made of II-VI semiconductor compound offer the potential to bridge the "green gap" in conventional semiconductors. However, when the CQDs are pumped to much higher initial excitonic states compared to their bandgap, multiexciton interaction is enhanced, leading to a much higher stimulated emission threshold. Here, to circumvent this drawback, for the first time, we show a fully colloidal gain in green enabled by a partially indirect pumping approach assisted by Förster resonance energy transfer process. By introducing the blue CQDs as exciton donors, the lasing threshold of the green CQDs, is reduced dramatically. The blue CQDs thus serve as an energy-transferring buffer medium to reduce excitation energy from pumping photons in a controlled way by injecting photoinduced excitons into green CQDs. Our newly developed colloidal pumping scheme could enable efficient CQD lasers of full visible colors by a single pump source and cascaded exciton transfer. This would potentially pave the way for an efficient multicolor laser for lighting and display applications.
Open Access
Increased quantum efficiency and reduced red-shift in polymer nanoparticle luminophors
(IEEE, 2008-11) Huyal, İlkem Özge; Özel, Tuncay; Tuncel, Dönüş; Demir, Hilmi Volkan
In this paper, using a polyfluorene derivative, increased fluorescence quantum efficiency and reduced red-shift in the film form of polymer nanoparticle luminophors is achieved, when compared to directly spin coated polymer thin films.
Open Access
Large-Area (over 50 cm × 50 cm) Freestanding Films of Colloidal InP/ZnS Quantum Dots
(American Chemical Society, 2012) Mutlugun, E.; Hernandez Martinez, P. L.; Eroglu, C.; Coskun, Y.; Erdem, T.; Sharma, V. K.; Unal, E.; Panda, S. K.; Hickey, S. G.; Gaponik, N.; Eychmuller, A.; Demir, Hilmi Volkan
We propose and demonstrate the fabrication of flexible, freestanding films of InP/ZnS quantum dots (QDs) using fatty acid ligands across very large areas (greater than 50 cm x 50 cm), which have been developed for remote phosphor applications in solid-state lighting. Embedded in a poly(methyl methacrylate) matrix, although the formation of stand alone films using other QDs commonly capped with trioctylphosphine oxide (TOPO) and oleic acid is not efficient, employing myristic acid as ligand in the synthesis of these QDs, which imparts a strongly hydrophobic character to the thin film, enables film formation and ease of removal even on surprisingly large areas, thereby avoiding the need for ligand exchange. When pumped by a blue LED, these Cd-free QD films allow for high color rendering, warm white light generation with a color rendering index of 89.30 and a correlated color temperature of 2298 K. In the composite film, the temperature-dependent emission kinetics and energy transfer dynamics among different-sized InP/ZnS QDs are investigated and a model is proposed. High levels of energy transfer efficiency (up to 80%) and strong donor lifetime modification (from 18 to 4 ns) are achieved. The suppression of the nonradiative channels is observed when the hybrid film is cooled to cryogenic temperatures. The lifetime changes of the donor and acceptor InP/ZnS QDs in the film as a result of the energy transfer are explained well by our theoretical model based on the exciton-exciton interactions among the dots and are in excellent agreement with the experimental results. The understanding of these excitonic interactions is essential to facilitate improvements in the fabrication of photometrically high quality nanophosphors. The ability to make such large-area, flexible, freestanding Cd-free QD films pave the way for environmentally friendly phosphor applications including flexible, surface-emitting light engines.
Open Access
Light matter interaction in plexcitonic crystals and moiré cavities
(2015-01) Karademir, Ertuğrul
Surface plasmon polaritons (SPPs) are quanta of electromagnetic excitations at the interface between metal and dielectric media. SPPs with an evanescent tail in the perpendicular direction, thus their properties are sensitive to variations in the optical properties of the dielectrics film. If SPPs are created near excitonic media, coupling between excitons and SPs can be achieved. In this thesis, interaction dynamics of SPP-exciton coupling is investigated. In weak coupling case, properties of SPPs and excitons are perturbed as the enhancement of the optical absorption in excitonic matrices. In the strong coupling, coupled pairs (plexcitons) causes Rabi splitting in SPP dispersion curves. By patterning the metal-dielectric interface with sine profile grating, it is possible to form a band gap on the dispersion curve, width of which can be tuned by the groove depth and SPP-Exciton coupling can be engineered. Using this, a new type of crystal, plexcitonic crystal, is proposed and demonstrated that exhibit directional dependent coupling on square and triangular lattices. Superposing an additional grating on the initial one but with a slight difference in pitch, results in Moir e cavities, in which, slow plasmon modes can be confined. We show that we can directly image these modes using dark field microscopy. Further, the slow cavity mode in contact with an excitonic source, where SPPs are coupled with near field coupling, results in ampliffed light signal. Various Moire cavities are shown to exhibit plasmonic lasing when slow plasmon modes in Ag coated cavities are excited inside a suitable gain medium.
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.