Surface plasmon enhanced Förster resonant energy transfer (FRET) between CdTe nanocrystal quantum dots (QDs) has been observed in a multilayer acceptor QD-gold nanoparticle-donor QD sandwich structure. Compared to a donor-acceptor QD bilayer structure without gold nanoparticles, the FRET rate is enhanced by a factor of 80 and the Förster radius increases by 103%. Furthermore, a strong impact of the donor QD properties on the surface plasmon mediated FRET is reported.

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The influences of donor and acceptor concentrations on Förster resonant energy transfer (FRET) in a separated donor-acceptor quantum dot bilayer structure have been investigated. Donor intra-ensemble energy transfer is shown to have an impact on the donor-acceptor FRET efficiency in the bilayer structure. At high donor concentrations the FRET distance dependence and the acceptor concentration dependence in the separated donor-acceptor layer structure agree well with theories developed for FRET between randomly distributed, homogeneous donor and acceptor ensembles. However, discrepancies between measurement and theory are found at low donor concentrations. A donor concentration study shows that the FRET efficiency decreases with increasing donor concentration even though a donor concentration-independent FRET efficiency is predicted by standard theory. The observed dependence of the FRET efficiency on the donor concentration can be explained within the FRET rate model, for a constant, donor concentration independent FRET rate, by taking into account the concentration dependent donor reference lifetime arising from intra-donor ensemble FRET. This shows that the decrease in the FRET efficiency with increasing donor concentration is not a signature of a change in the donor-acceptor FRET rate, but due to the competition of the donor-acceptor and donor-donor energy transfer for the higher energy donors. As the intra-donor ensemble FRET represents another decay mechanism, the donor quantum yield for the higher energy donors decreases with increasing donor quantum dot (QD) concentration, as can also be seen from the redshift of the donor emission spectrum. Using this concentration dependent donor quantum yield in the calculation of the Förster radius, the FRET theory for homogeneous donor and acceptor ensembles can be modified to include the effect of the donor intra-ensemble transfer and to correctly describe the trends and absolute values of the measured FRET efficiencies as a function of the donor and the acceptor concentrations. These results show that in QD systems where intra-donor ensemble FRET is as important as the radiative and nonradiative donor decay mechanisms, the FRET rate rather than the FRET efficiency more appropriately characterizes the donor-acceptor FRET. By fitting with the rate model, FRET rates as high as (1.2 ns) −1 have been determined for the structures presented here.

The distance dependence of localized surface plasmon (LSP) coupled Förster resonance energy transfer (FRET) is experimentally and theoretically investigated using a trilayer structure composed of separated monolayers of donor and acceptor quantum dots with an intermediate Au nanoparticle layer. The dependence of the energy transfer efficiency, rate, and characteristic distance, as well as the enhancement of the acceptor emission, on the separations between the three constituent layers is examined. A d –4 dependence of the energy transfer rate is observed for LSP-coupled FRET between the donor and acceptor planes with the increased energy transfer range described by an enhanced Förster radius. The conventional FRET rate also follows a d –4 dependence in this geometry. The conditions under which this distance dependence is valid for LSP-coupled FRET are theoretically investigated. The influence of the placement of the intermediate Au NP is investigated, and it is shown that donor–plasmon coupling has a greater influence on the characteristic energy transfer range in this LSP-coupled FRET system. The LSP-enhanced Förster radius is dependent on the Au nanoparticle concentration. The potential to tune the characteristic energy transfer distance has implications for applications in nanophotonic devices or sensors.

The quantum dot ͑QD͒ concentration dependence of the optical properties of QD monolayers is shown to be dominated by Förster resonant energy transfer ͑FRET͒ from smaller to larger QDs in the ensemble. With increasing QD concentration a redshift of the peak emission wavelength, a shortening of the photoluminescence lifetime of the QDs on the high-energy side of the ensemble emission spectrum as well as increased difference in the lifetimes on the high-and low-energy sides are observed in the layer-by-layer deposited QD monolayers. There is also evidence of an increased rise time in the time-resolved photoluminescence decays on the low-energy side of the QD emission for two of the three samples presented in most detail. A theory of FRET in two dimensions is applied to explain the lifetime decrease on the high-energy side of the ensemble emission and confirms that the impact of the QD concentration on the optical properties is primarily due to FRET from the smaller to larger QDs in the ensemble. The concentration effects are stronger in QD samples which have a broader emission peak compared to the Stokes shift. Based on good agreement with FRET theory, the QD concentration and the overlap of the QD emission and absorption peaks can both be used to control the efficiency of the FRET process in monodispersed QD layers.

Surface plasmon ͑SP͒ enhanced photoluminescence ͑PL͒ from CdTe quantum dots ͑QDs͒ on monolayers of Au nanoparticles is investigated under both resonant and nonresonant conditions. Enhancement of the QD PL intensity is observed when the emission spectrum is redshifted with respect to the SP absorption resonance. Coupling to the SPs results in a redshift and broadening of the PL spectrum, and an increase in the PL decay rate. The largest coupling is observed for QD monolayers with peak emission at 667 nm, producing a ten fold increase in PL intensity. No change in PL intensity and decay rate is observed at the SP resonance. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2422906͔ Surface plasmons ͑SPs͒ excited on thin metallic structures/nanoparticles play a major role in photoluminescence ͑PL͒ enhancement 1-4 and PL quenching. 5,6 These effects have been attributed to enhanced excitation field, 2-5 modified radiative and nonradiative decay rates, 7-9 and coupling to SPs. 3,4,7,8 In a metal-emitter system, the exact conditions under which enhancement or quenching of the PL will occur are still under debate. We report on the offresonance enhancement of the quantum dot ͑QD͒ emission and the importance of SP scattering.The structures, grown using a layer-by-layer technique, 10,11 are comprised of 3 monolayers ͑ML͒ of Au NPs ͑diameter ϳ7 nm͒ on a quartz substrate, followed by a polyelectrolyte ͑PE͒ spacer layer and capped by 2 ML of CdTe thioglycolic acid stabilized QDs.12-14 The 3 ML of Au NPs and 2 ML of QDs on quartz substrates are deposited using positively charged polyethyleneimine as the counterpart. The spacer is comprised of bilayers of positively charged poly͑diallyldimethylammonium chloride͒ and negatively charged poly͑sodium 4-styrene sulfonate͒, with thicknesses of approximately 1.4 and 11.7 nm for one and nine PE bilayers, respectively.15 Room temperature PL spectra are recorded with the excitation wavelength of 400 nm using a Perkin-Elmer fluorescence spectrometer. The time-resolved PL decays were measured using a PicoQuant Microtime200 time-resolved confocal microscope system with 150 ps resolution. Excitation is provided by 480 nm picosecond pulses at a 10 MHz repetition rate from an LDH-480 laser head controlled by a PDL-800B driver ͑PicoQuant͒.The PL spectra for CdTe QD monolayers, with peak emissions at 556, 612, 667, and 757 nm, were recorded as a function of QD-metal separation. The PL intensity enhancement factor is determined by a comparison with the PL peak intensity in the absence of the Au NP layer. A tenfold PL peak intensity enhancement was observed for QDs emitting at 667 nm, whereas a fourfold increase was noted for those emitting at 612 and 757 nm and essentially no discernible change was observed for the 556 nm QDs, as shown in Fig. 1. The PL peak wavelength and the full width at half maximum ͑FWHM͒ as a function of the number of PE bilayers, for the 667 and 556 nm QDs, are shown in Fig. 2 ͑the data for the 612 and 757 nm QDs, are not shown for clarity but are discussed below͒. R...

Time-controlled plasma treatment of MoS2 FETs improves carrier transport due to the presence of a two-dimensional oxide phase.

Förster resonance energy transfer ͑FRET͒ between CdTe quantum dots ͑QDs͒ at nanoscale proximity to gold nanoparticle ͑Au NP͒ layers is investigated experimentally. We have observed the enhancement in the acceptor QDs' photoluminescence lifetime intensities. The decrease in donor QDs' exciton lifetime from 5.74 to 2.06 ns, accompanied by an increase in acceptor QDs' exciton lifetime from 3.38 to 7.52 ns, provided evidence for enhanced FRET between the QDs near Au NPs. The Au NPs' surface plasmon dipole fields are assisted to overcome the weak electronic coupling between the emitting ͑donor͒ and absorbing ͑acceptor͒ transition exciton dipoles in the homogeneous medium. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2981209͔The Förster resonance energy transfer ͑FRET͒ process is an important energy transfer mechanism at the nanoscale distance for transportation of excitation energy between the two chemical species. 1 The FRET process between semiconductor colloidal nanocrystallites or quantum dots ͑QDs͒ has been investigated in closely packed mixed layers and in separated donor and acceptor layers. 2 A cascaded FRET in artificial solids formed by layers of QDs has also been demonstrated. 3 Efforts to improve the FRET process have been focused on structures with reduced donor-acceptor QDs' separation, improved spectral overlap of the donor emission and acceptor absorption spectra, and increased packing density of the layers. 4