Browsing by Author "Hernandez-Martinez, Pedro L."

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    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).
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    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.
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    Excitonics of hybrid nanostructures arranged with mixed dimensionality
    (American Physical Society, 2012) Hernandez-Martinez, Pedro L.; Govorov, A. O.; Demir, Hilmi Volkan
    We present a complete study of the F\"{o}rster-type nonradiative energy transfer in hybrid nanostructures composed of nanoparticles, nanowires and quantum wells, and investigate the effects of quantum confinement in different dimensions. We systematically consider all possible combinations in terms of dimensionality for exciton-exciton interactions in these hybrid architectures, and analyze the resulting energy transfer rates for item-to-item excitonic coupling as a function of dimensionality. We derive a full set of analytical expressions and show that the exciton transfer strongly depends on the dimensionality and geometry of the hybrid system. Arrangements of such nanostructures with mixed dimensionality ranging from the low dimensionality to the high offer important high-efficiency applications in photovoltaics [1,2], while in the reciprocal case (from the high dimensionality to the low) in light generation [3] and LEDs [4]. [1] J. Sambur, et al., Science 330, 63 (2010). [2] M. D. Kelzenberg, et al., Nature Materials 9, 239--244 (2010). [3] R. Yan, et al., Nature Photonics 3, 569-576 (2009). [4] H.V. Demir, et al., Nano Today (2011) doi:10.1016/j.nantod.2011.10.006
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    Optically excited exciton transfer of spherical quantum dots via optical near-field ınteractions
    (American Physical Society, 2012) Togay, Amirahmadov; Hernandez-Martinez, Pedro L.; Demir, Hilmi Volkan
    We study a system composed of a mixture of different-sized spherical quantum dots (QDs) involving optical near-field (ONF) interactions to induce effective optical excitation transfer. Here energy transfer was explained by resonant energy transfer via the optical near-field interaction between the first excited state of small QDs and the second excited state of large QDs. The energy transfer in a film of different-sized QDs made of CdTe and CdSe were experimentally demonstrated. An analysis between the optical near field transfer rate [1] and F¨orster type transfer rate was made. The proper understanding of the exciton transfer between these QDs is important for the design and implementation of near-field photonic devices employing them. [1] M. Ohtsu, et al., Principles of Nanophotonics, CRC Press (2008).
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    Phonon-assisted nonradiative energy transfer from colloidal quantum dots to monocrystalline bulk silicon
    (IEEE, 2012) Yeltik, Aydan; Güzeltürk, Burak; Hernandez-Martinez, Pedro L.; Demir, Volkan Demir
    Silicon is one of the most dominant materials in photovoltaics. To increase optical absorption of silicon solar cells, colloidal quantum dots (QDs) have been proposed as a good sensitizer candidate owing to their favorably high absorption cross-section and tunable emission and absorption properties. To this end, QD sensitization of silicon has previously been studied by mostly facilitating radiative energy transfer (RET) [1,2]. Although RET based sensitization has achieved a considerable increase in conversion efficiencies in silicon photovoltaics, RET is fundamentally limited due to the effective coupling problem of emitted photons to silicon. Alternatively, nonradiative energy transfer (NRET), which relies on near field dipole-dipole coupling [3], has been shown to be feasible in sensitizer-silicon hybrid systems [4-8]. Although colloidal QDs as a sensitizer have been used to facilitate NRET into silicon, the detailed mechanisms of NRET to an indirect bandgap nonluminecent material, together with the role of phonon assistance and temperature activation, have not been fully understood to date. In this study, we propose a QD-silicon nanostructure hybrid platform to study the NRET dynamics as a function of temperature for distinct separation thicknesses between the donor QDs and the acceptor silicon plane. Here, we show NRET from colloidal QDs to bulk Si using phonon assisted absorption, developing its physical model to explain temperature-dependent lifetime dynamics of NRET in these QD-Si hybrids. © 2012 IEEE.