Highly Efficient Forster Energy Transfer between Quantum Dots and J/aggerate

The integration of organic and inorganic semiconductors on the nanoscale offers the possibility of developing new photonic devices that combine the best features of these two distinct classes of material. We describe a novel hybrid organic/inorganic nanocomposite in which alternating monolayers of J-aggregates of cyanine dye and crystalline semiconductor quantum dots are grown by a layer-by-layer self-assembly technique. We demonstrate near-field photon-mediated coupling of vastly dissimilar optical excitations in the two materials that can reach efficiencies of up to 98% at room temperature. Unlike prior demonstrations of the energy transfer in heterostructures involving semiconductor quantum wells, the optical attributes of our QD/J-aggregate hybrids are highlighted by their robust excitonic transitions at room temperature with spectrally concentrated absorption and emission strengths.

Using layer-by-layer (LBL) assembly approach, a single monolayer J-aggregate of cyanine dye (TDBC) was sandwiched between two monolayers of CdSe–ZnS core–shell structured QDs, with polyelectrolyte (PDDA) acting as the ultrathin 'molecular glue'.

 

a, Schematic of the hybrid film layer structure: a monolayer J-aggregate of 5,6-dichloro-2-[3-[5,6-dichloro-1-ethyl-3-(3-sulphopropyl)-2(3H)-benzimidazolidene]-1-propenyl]-1-ethyl-3-(3-sulphopropyl) benzimidazolium hydroxide (TDBC) is sandwiched between two monolayers of CdSe–ZnS QDs, joined by monolayers of poly(diallyldimethylammonium chloride) (PDDA). Molecular structures of TDBC, PDDA and MUA are illustrated in the lower panel. bd, AFM images of the LBL-grown films, consisting of a monolayer of QDs (lamdaem = 653 nm) (PDDA/QD) (b), a monolayer of J-aggregate (PDDA/TDBC/PDDA) (c), and the hybrid film II (d). The scale bars in bd are 100 nm.

Photoexcitation at lambdaex = 380 nm to ensure a dominant input excitation to the QDs alone. The resulting photoluminescence (PL) spectrum of film exhibited a pronounced quenching effect of the QD emission by nearly 98%, with a simultaneous boost of the J-aggregate luminescence. Such luminescence contrast is consistent with the quantum yields of the QDs (50%) and J-aggregate (15%). Next, time-resolved PL was studied. The discrepancy between the temporal behaviours of QDs and J-aggregates in the hybrid film was attributed to the presence of multiple energy transfer rates and small-residue J-aggregate absorption at the excitation wavelength. The overall behaviours can be well described by a simple rate equation model, which yielded a fast energy transfer rate of 0.05 ps-1 accounting for 76% of the total QD population.

 

 

a, Absorption (blue) and emission (orange) spectra of the TDBC J-aggregate film, as well as the emission spectrum of the QDs (lambdaem = 548 nm) (green). b, PL spectra of the hybrid film I (red), QD control film (green) and the J-aggregate control film (blue) with excitation wavelength at 380 nm. The inset shows the absorbance spectrum of the hybrid film I (solid line) in semi-logarithmic scale. c, Time-resolved QD PL decay traces (normalized) from the QD control film (red) and the hybrid film I (green), detected in the neighbourhood of 546 nm. d, PL spectrum of the hybrid film I (red) was fitted by a linear combination (green) of the QD and the J-aggregate emissions (dashed line). It is estimated that more than 97% of the photons collected within the detection band from hybrid film I were contributed by the QD emission. e, Time-resolved PL decay traces measured by streak camera at picosecond timescale, from the QD in hybrid film I (green), the J-aggregate in hybrid film I (orange) and the J-aggregate control film (purple). The model assumed the presence of three different energy transfer time constants, fitted to be 20 ps, 110 ps and 1,100 ps, respectively.

 In a role reversal, efficient energy transfer from J-aggregates to QDs could lead to a hybrid material that brings together the benefits from both constituents, namely large absorption strength and high quantum efficiency. We explored this in hybrid film II, where the QD emission was offset to the lower energy side of the J-aggregate emission.

a, The emission spectrum of TDBC J-aggregate (emission at 594 nm) (orange) spectrally overlaps with the QD absorption (blue), but the QD emission (purple) is centred at 653 nm. b, PL spectra of the hybrid film II (blue) and the QD control film (black). The excitation wavelengths were set to be at excitation at 589 nm, the peak of J-band absorption. Contributions of the QD emission (purple dashed line) and the J-aggregate emission (orange dashed line) to the PL of hybrid film II are separated. d, The raw PLE spectrum of hybrid film II (red) and QD control (black) when detecting at the QD emission peak. e, J-aggregate to QD energy transfer efficiency calculated using the data shown in c. The arrow indicates the peak of J-band absorption.

 We observed an order of magnitude increase in the effective absorption of QD at the J-aggregate absorption peak, where energy transfer was determined to be about 22%. Thus we demonstrate the potency of using an organic aggregate as the light-harvesting antenna to significantly enlarge the absorption cross-section of an inorganic semiconductor. we recently demonstrated energy transfer of 38% from J-aggregate to QD through the implementation of a different type of J-aggregate with blueshifted emission at 470 nm.

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