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The quenching of emission in proximity to metallic surfaces via non-radiative energy transfer is studied for sensing applications. It can also be used for the measurement of distances on the nanoscale. We report on energy transfer from a plane of CdTe quantum dots (QDs) plane to a plane of gold (Au) nanoparticles. Both photoluminescence (PL) and luminescence lifetime measurements demonstrate that energy transfer efficiency not only depends on plane separation but also Au nanoparticle concentration. Energy transfer to the plane of metal nanoparticles can be considered within the standard Forster resonant energy transfer (FRET) model or the nano-metal surface energy transfer (NSET) model. It is found that both dependences are well described within the model of FRET which shows a 1/d4 distance dependence and a 1/C Au Au concentration dependence. However, surprisingly large Forster radii of 10 nm, larger than expected from the spectral overlap of the QD emission and gold localised surface plasmon absorption, are obtained.

Forster resonant energy transfer (FRET) can be applied to create energy flow on the nano-scale for lightharvesting, colour conversion or sensing applications. The performance of such devices depends on the efficiency of the energy transfer process between the donors and acceptors. In order to achieve high FRET efficiencies, the FRET rate has to dominate over the other donor decay rates. The FRET rate depends on the donor-acceptor separation, the acceptor concentration and it has also been proposed that it can be strongly enhanced by localized surface plasmons supported by metal nanoparticles. The impact of these different parameters on the FRET rate in a CdTe quantum dot donor-acceptor bilayer structure is presented. The quantum dot structures, prepared by a layer-by-layer deposition technique, were characterized by steady-state photoluminescence (PL) and absorption spectroscopy as well as time-resolved PL measurements. The FRET rate of the different structures was determined from the time-resolved donor PL decays and its separation and concentration dependence was compared with FRET theory.

Colloidal nanocrystal quantum dots have attracted interest for use in light-harvesting and emitting devices. One of the key parameters for such applications is the quantum efficiency. Nanocrystal quantum dots exhibit reduced quantum yield in layers or solids compared with solutions. Optimizing the overall efficiency of the emission is an important goal. Here we report on a large enhancement of the quantum efficiency of mixed TGA-stabilised CdTe quantum dot in closely packed monolayers formed with nanocrystal QDs of two sizes. Energy is transferred from the smaller donor quantum dots to the larger acceptor quantum dots via Forster resonant energy transfer (FRET). The increased quantum efficiency is attributed to exciton recycling from the donor defect states. The acceptor emission enhancement and relative quantum efficiency are strongly dependent on the donor:acceptor concentration ratio. A relative quantum efficiency of 160% is achieved for the mixed quantum dot layer with a donor:acceptor ratio of 11∶1.

Spatial light modulator (SLM) is a key component of modern video projectors, beam shaping devices, laser beam combiners and optical information processors. Challenging tasks in SLM are optoelectronic integration, all-optical operation and phase modulation with low absorption. A possible improvement along these lines could be the use of new nano-materials with large and controllable optical nonlinearities, e.g. the quantum dots (QDs).

The effects of surface plasmons (SPs) on Forster resonant energy transfer (FRET) in colloidal quantum dot (QD) structures have been investigated. CdTe QDs of two different sizes acted as donors and acceptors in a mixed donor-acceptor monolayer on top of a gold nanoparticle layer and an acceptor-gold-donor sandwich structure. The structures were prepared by a layer-by-layer technique and characterized by absorption and photoluminescence (PL) spectroscopy as well as time-resolved PL measurements. For the case of the mixed QD monolayer a recovery of the acceptor emission is observed when increasing the acceptor-gold separation. The reduction of the donor lifetime indicates that the FRET process is enhanced by 30%. In the sandwich structure a clear modification of the FRET distance dependence towards a typical SP influenced dependence on the QD-gold separation is observed. The maximum acceptor QD emission occurs at approximately 12 nm from the intermediate gold layer, corresponding to a donor-acceptor separation at which no FRET is observed in structures without gold. By varying the gold concentration the acceptor PL enhancement can be increased and an absolute enhancement of 25% compared to structures without gold is achieved.

The impact of intra-ensemble Forster 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.

In this work, we demonstrate the functionality of optical limiting obtained with CdTe nanocrystals embedded in polystyrene. This functionality relies on our experimentally observed large, saturable, and controlled nonlinear optical properties of CdTe nanocrystals with sizes around 2 nm, at laser wavelength of 532 nm. Our investigation is presenting an experimental proof of concept. The limiting threshold occurs at an average laser power of 10 mW. For laser beam diameter of 1 mm, the threshold intensity for this optical limiting device is ∼ 1 W/cm2.

Colloidal semiconducting quantum dots ( QDs), also known as nanocrystals, offer a number of advantages in light generation, e.g., for solid-state lighting, biol ogical labeling, and bio -imaging.3 These applications typically necessitate high quantum eff iciencies in film, preferably with polarized emission in certain applications. One of the major problems encountered when working with films of these QDs is the decrease in their fluorescence quantum efficiency in the solid form. Another technical challenge is to obtain anis otropic emission from these QDs. Recently plasmonics has enabled a wide range of important applications related to nanocrystals including metal enhanced light generation and plasmonic biosensing.1,2 In this work, to address the se difficulties encountered with QD solids, we investigate and demonstrate anisotropic enhanced emission from semiconductor quantum dots placed in multilayered plasmon resonator nanocomposite s using strong plasmon-exciton interactions. In these metal nanoparticle (MNP) resonator architectures, we show that highly isotropic nature of QD emission c an desirably be altered through modifying emission kinetics by plasmon coupling, while also enhancing the overall QD emission.

The processes of ageing, chemical and photochemical treatment leading to 3D networks (gels) in aqueous colloidal solutions of CdTe nanocrystals were investigated. The gels were dried to form highly porous aerogels. Potential applicability of the approaches to other nanoparticle types (e.g. noble metals) was demonstrated. The great potential of such kinds of aerogels for application in optoelectronics and catalysis is discussed.