Selected


Journal of the American Chemical Society 133 (2), 279–285 (2011)

posted 13 Feb 2011, 06:00 by Pavlos Lagoudakis   [ updated 13 Feb 2011, 06:23 ]


"Gauging the flexibility of fluorescent markers for the interpretation of fluorescence resonance energy transfer"

Journal of the American Chemical Society 133 (2), 279–285 (2011)

Abstract: Intramolecular distances in proteins and other biomolecules can be studied in the living cell by means of fluorescence resonance energy transfer (FRET) in steady state or pulsed-excitation experiments. The major uncertainty originates from the unknown orientation between the optical dipole moments of the fluorescent markers, especially when the molecule undergoes thermal fluctuations in physiological conditions. We introduce a statistical method for the interpretation of fluorescence decay dynamics in donor-acceptor FRET pairs that allows us to retrieve both the orientation and the extent of directional fluctuations of the involved dipole moments. We verify the method by applying it to donor-acceptor pairs controllably attached to DNA helices and find that common assumptions such as complete rotational freedom or fully hindered rotation of the dipoles fail a physical interpretation of the fluorescence decay dynamics. This methodology is applicable in single molecule and ensemble measurements of FRET to derive more accurate distance estimates from optical experiments, without the need for more complex and expensive NMR studies.    


Advanced Materials 22, (5) 602 (2010)

posted 13 Feb 2011, 06:00 by Pavlos Lagoudakis   [ updated 13 Feb 2011, 06:23 ]

"Increased color conversion efficiency in hybrid light emitting diodes utilizing non-radiative energy transfer"

Advanced Materials 22, (5) 602 (2010)

Abstract: An efficient hybrid color-conversion light-emitting device consisting of colloidal nanocrystal quantum dots (NQDs) and a surface-patterned GaN-based LED is demonstrated (see figure). Excitation in a surface-patterned LED is efficiently transferred to NQD emitters via non-radiative energy transfer. A twofold enhancement of the NQD emission is achieved.    


Physical Review Letters, 102, 077402 (2009)

posted 13 Feb 2011, 05:59 by Pavlos Lagoudakis   [ updated 13 Feb 2011, 06:09 ]

“Photocurrent enhancement in hybrid nanocrystal quantum dot/p-i-n photovoltaic devices”  

Physical Review Letters, 102, 077402 (2009) 
Abstract:We fabricate a hybrid nanocrystal quantum-dot patterned p-i-n structure that utilizes nonradiative energy transfer from highly absorbing colloidal nanocrystal quantum dots to a patterned semiconductor slab to demonstrate a sixfold increase of the photocurrent conversion efficiency compared to the bare p-i-n semiconductor device.


On the news: New ScientistPhysics WorldOptics.orgScienceDaily, university's  press release   



Appl. Phys. Lett. 94, 233502 (2009)

posted 13 Feb 2011, 05:59 by Pavlos Lagoudakis   [ updated 13 Feb 2011, 06:09 ]

"Efficient light harvesting in hybrid CdTe nanocrystal/bulk GaAs p-i-n photovoltaic devices"

Appl. Phys. Lett. 94, 233502 (2009)

Abstract:A hybrid colloidal CdTe nanocrystal/bulk GaAs p-i-n heterostructure is demonstrated to have potential for highly efficient light harvesting photovoltaic devices. An array of rectangular channels is fabricated on the surface of the GaAs heterostructure penetrating through its active layer and subsequently filled with water soluble CdTe nanocrystals emitting in the near infrared. Photogenerated carriers in the highly absorbing colloidal nanocrystals are efficiently transferred by means of nonradiative energy transfer to the patterned heterostructure possessing high carrier mobility and converted to electrical current. A threefold enhancement of both photocurrent and monochromatic power conversion efficiency has been achieved.

Applied Physics Letters 91, 092126 (2007)

posted 13 Feb 2011, 05:58 by Pavlos Lagoudakis   [ updated 13 Feb 2011, 06:21 ]

“Temperature Dependence of Exciton Transfer in Hybrid Quantum well/Nanocrystal Heterostructures” 
Applied Physics Letters 91, 092126 (2007)
Abstract: The authors investigate the temperature dependence of exciton transfer from a single InGaN quantum well (QW) donor to colloidal CdS nanocrystal quantum dot acceptors and obtain an optimum transfer efficiency of 65% at 60 K. Time and spectrally resolved measurements reveal that the transfer efficiency is dominated by the interplay between exciton localization and nonradiative recombination intrinsic to the QW. 

Phys. Rev. Lett. 98, 017401 (2007)

posted 13 Feb 2011, 05:58 by Pavlos Lagoudakis   [ updated 13 Feb 2011, 06:21 ]

“Room temperature exciton storage in elongated semiconductor nanocrystals”

Phys. Rev. Lett. 98, 017401 (2007)

Abstract:The excited state of colloidal nanoheterostructures consisting of a spherical CdSe nanocrystal with an epitaxially attached CdS rod can be perturbed effectively by electric fields. Field-induced fluorescence quenching coincides with a conversion of the excited state species from the bright exciton to a metastable trapped state (dark exciton) characterized by a power-law luminescence decay. The conversion is reversible so that up to 10% of quenched excitons recombine radiatively post turn-off of a 1  μs field pulse, increasing the delayed luminescence by a factor of 80. Excitons can be stored for up to 105 times the natural lifetime, opening up applications in optical memory elements. 


Phys. Rev. Lett. 98, 126405 (2007)

posted 13 Feb 2011, 05:51 by Pavlos Lagoudakis   [ updated 13 Feb 2011, 06:21 ]

“Room temperature polariton lasing in semiconductor microcavities”

Phys. Rev. Lett. 98, 126405 (2007)

Abstract:We observe a room-temperature low-threshold transition to a coherent polariton state in bulk GaN microcavities in the strong-coupling regime. Nonresonant pulsed optical pumping produces rapid thermalization and yields a clear emission threshold of 1 mW, corresponding to an absorbed energy density of 29 u.J cm.2, 1 order of magnitude smaller than the best optically pumped (In,Ga)N quantumwell surface-emitting lasers (VCSELs). Angular and spectrally resolved luminescence show that the polariton emission is beamed in the normal direction with an angular width of .10 deg. and spatial size around 5 u.m. 


Angewandte Chemie Int. Ed. 46, 3450 (2007)

posted 13 Feb 2011, 05:42 by Pavlos Lagoudakis   [ updated 13 Feb 2011, 06:21 ]

“Exciton accumulation in π-conjugated wires encapsulated by light-harvesting macrocycles”

Angewandte Chemie Int. Ed. 46, 3450 (2007)    

Nature Materials 4, 340-346 (2005)

posted 13 Feb 2011, 05:41 by Pavlos Lagoudakis   [ updated 13 Feb 2011, 06:20 ]

“Spin-conserving carrier recombination in conjugated polymers” 

Nature Materials 4, 340-346 (2005)

Abstract:The ultimate efficiency of polymer light-emitting diodes is limited by the fraction of charges recombining in the molecular singlet manifold. We address the question of whether this fraction can principally exceed the fundamental limit set down by spin statistics, which requires the possibility of spin changes during exciton formation. Sensitized phosphorescence at 4–300 K enables a direct quantification of spin conversion in coulombically bound electron–hole pairs, the precursors to exciton formation. These are stabilized in external electric fields over times relevant to carrier transport, capture and recombination in devices. No interconversion of exciton intermediates between singlet and triplet configurations is observed. Static magnetic fields are equally unable to induce spin mixing in electroluminescence. Our observations imply substantial exchange splitting at all times during carrier capture. Prior statements regarding increased singlet yields above 25% merely on the basis of higher singlet than triplet formation rates should therefore be re-examined.  


Nano Letters 5, 2044 (2005)

posted 13 Feb 2011, 05:36 by Pavlos Lagoudakis   [ updated 13 Feb 2011, 06:20 ]

“Wavefunction engineering in elongated semiconductor quantum dots”

Nano Letters 5, 2044 (2005)

Abstract:We explore two routes to wave function engineering in elongated colloidal CdSe/CdS quantum dots, providing deep insight into the intrinsic physics of these low-dimensional heterostructures. Varying the aspect ratio of the nanoparticle allows control over the electron−hole overlap (radiative rate), and external electric fields manipulate the interaction between the delocalized electron and the localized hole. In agreement with theory, this leads to an exceptional size dependent quantum confined Stark effect with field induced intensity modulations, opening applications as electrically switchable single photon sources. 


 



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