ICTON 2010 We.B2.5 978-1-4244-779 - /10/$26.00 ©2010 IEEE Influence of Intra-Ensemble Energy Transfer on the Properties of Nanocrystal Quantum Dot Structures and Devices Manuela Lunz 1 , A. Louise Bradley 1* , Wei-Yu Chen 2 , Valerie A. Gerard 2 , Stephen J. Byrne 2 Yurii K. Gun’ko 2 , Vladimir Lesnyak 3 and Nikolai Gaponik 3 1 Semiconductor Photonics Group, School of Physics, Trinity College Dublin, Dublin 2, Ireland 2 School of Chemistry, Trinity College Dublin, Dublin 2, Ireland 3 Physical Chemistry, TU Dresden, Bergstr. 66b, 01062 Dresden, Germany * Tel: 00353 (0)1 896 3595, Fax: 00353 (0)1 67 11759, e-mail: bradlel@tcd.ie ABSTRACT The impact of intra-ensemble Förster resonant energy transfer (FRET) on the optical properties of monodispersed quantum dot (QD) monolayers and a donor/acceptor FRET bilayer structure are presented. The QD structures are characterized by steady-state absorption and photoluminescence (PL) spectroscopy as well as time-resolved PL measurements. The optical properties of the monodispersed monolayers, such as peak emission wavelength and PL decays, are strongly influenced by FRET from smaller to larger QDs within the ensemble. Comparing several QD samples, the spectral overlap of the QD ensemble and the QD concentration were identified as parameters that allow for tuning of FRET in monodispersed QD structures. For the donor/acceptor QD bilayer structure an unexpected decrease of the FRET efficiency between donor and acceptor layers is observed with increasing donor QD concentration. The concentration-dependent donor lifetime and a constant donor-acceptor FRET rate can explain this decrease within the FRET rate model. Even though the donor-acceptor FRET rate is donor-concentration independent – as expected from theory – its competition with donor-donor energy transfer leads to a concentration dependence of the FRET efficiency from donors to acceptors. This shows that intra-ensemble FRET can have an important impact on device performance. Keywords: Förster resonant energy transfer, colloidal quantum dots, photoluminescence spectroscopy, time- resolved emission decay, inhomogeneous broadening, spectral overlap. 1. INTRODUCTION Due to their unique optical properties semiconductor colloidal nanocrystals or quantum dots (QDs) have attracted a lot of interest in fundamental and applied research during the last decades. They show broad absorption bands and narrow, tuneable emission lines that make them ideal components for sensing devices or photovoltaics [1]. Most solid state QD devices contain QD layers that interact with other components of the structures as, for example, in graded energy structures [2], photovoltaic devices [3] and sensors [4]. In closely packed, monodispersed nanocrystal QD structures energy transfer between QDs can occur based on the overlap of emitting and absorbing states of QDs with different sizes, which is due to the inhomogeneous broadening of the QD ensemble and the Stokes shift between QD absorption and emission peak. This intra-ensemble energy transfer has been observed in QD solids [5, 6] and layers [7] for CdSe as well as PbS QDs, that have been suggested for applications in the visible and infrared wavelength region respectively. It is important to investigate energy transfer within monodispersed QD structures more closely in order to identify ways to control intra-ensemble energy transfer other than by temperature tuning [6]. Here we analyse the intra-ensemble energy transfer in monodispersed CdTe QD monolayers in detail for one QD sample and show that it is dominated by the Förster resonant energy transfer (FRET) mechanism [8]. Then, the properties of QD monolayers prepared from eleven different green-emitting CdTe QDs are compared at one concentration. The QD concentration and the self-overlap of the spectral features of the QD ensemble both influence the intra-ensemble process significantly [9]. As an example, a donor/acceptor QD bilayer structure is investigated to highlight the impact of intra-donor ensemble FRET on the FRET efficiency in this structure and therefore, in more general terms, on the performance of QD based devices. 2. EXPERIMENTAL METHODS Negatively charged CdTe quantum dots (QDs), stabilized by thioglycolic acid in aqueous solution [10], were deposited into layer structures. A Layer-by-layer electrostatic assembly technique [11] was used to prepare structures with monodispersed QD monolayers only or with donor and acceptor QD layers separated by a poly(diallyldimethylammonium chloride) spacer layer. The QD layers were deposited by immersion in a micromolar QD solution and the QD concentration in the layers was changed by varying the immersion time. Further details about the sample preparation can be found elsewhere [9, 12]. The absorption spectra of the QD structures were measured using a double beam UV-Vis Recording Spectrometer (Shimadzu UV-2401 PC). The steady-state photoluminescence (PL) spectra were recorded with a Perkin-Elmer LS 55 fluorescence spectrometer using an excitation wavelength of 400 nm. A PicoQuant Microtime200 time-resolved confocal microscope system with 150 ps resolution was used to measure the time-