Influence of intra-ensemble energy transfer on the properties of nanocrystal quantum dot structures and devices

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, timeresolved emission decay, inhomogeneous broadening, spectral overlap. . 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]. INTRODUCTIONHere 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.