(440d) Enhancing Light Absorption and Emission in Quantum Dot Solids Using Al Nanostructures

Nanocrystals such as CdSe/CdS core/shell quantum dots (QDs) are desirable optical materials with tunable band gaps, high quantum yields, and large Stokes shifts. These materials are typically synthesized in solution, but integration into optoelectronic devices requires the creation of nanocrystal solids. These solids must preserve the isolation between individual nanocrystals in order to maintain the benefits of quantum confinement, while simultaneously allowing for the transport of excitons or electrical currents. Nanocrystal solids exhibit distinctly different optical properties from their solution-phase counterparts, and complete engineering of these devices requires a detailed understanding of these changes in optical properties. Here we aim to manipulate the optical properties of QD solids by incorporating plasmonic nanostructures that enhance and direct light absorption as well as tune the local density of optical states, which influences light emission.

We study the influence of arrays of plasmonic nanostructures on the absorption and the emission from QD solids. The arrays consist of aluminum disks and rings, which both scatter incident light and possess strongly enhanced local electromagnetic fields. To avoid quenching and to provide a uniform surface for the deposition of the QD solid, the plasmonic nanostructures are completely embedded within a thin film of alumina. Aluminum is chosen over silver or gold by matching the plasmonic nanostructures to the absorption of the semiconductor nanocrystals. We study the absorption and the emission from both a continuous QD solid and from patterned QD solid regions.

Electromagnetic simulations are performed using finite-difference time-domain numerical simulation software to optimize the designs and understand the interactions between the Al nanostructures and the QD solids. The Al nanostructures are 30 nm tall and embedded in 34 nm of alumina to provide a thin barrier that prevents direct contact of the QD solid with the aluminum nanostructures. The optical properties of the QD solid are modeled using data from variable angle spectroscopic ellipsometry.

For a 60 nm thick QD film deposited on top of the alumina layer, we observe greater than a threefold increase in absorption within the continuous QD solid and greater than a ninefold increase in absorption within the patterned QD solid regions due to the presence of the embedded aluminum nanostructure array. Furthermore, we find that the magnitude of the scattering cross section of the plasmonic nanostructure contributes more to the overall absorption enhancement than the perfectly matched resonance of the individual aluminum nanostructure. This indicates the importance of the scattering properties of these embedded arrays to redistribute the incident light and increase the path length within the absorbing QD solid. The addition of a back reflector prevents the transmission of light through the system and increases the absorption within the QD solid. However for certain nonabsorbing spacer layer thicknesses between the nanostructure array and the back reflector, the reflected light destructively interferes and greatly reduces the electric field and thus the absorption within the QD solid.

In conjunction with absorption simulations, the emission from the QD solid is important to consider as the experimentally measurable luminescence consists of both the absorption of incident light and the emission from the individual QDs in the QD solid. Using a dipole to model the emission from the QDs, the emission is averaged over the entire periodic array to determine the emission enhancement from the embedded aluminum nanostructure array. Thus with these two simulations, the overall luminescence enhancement from the embedded nanostructured array is determined.

To support these simulation results, we fabricate aluminum nanostructured arrays embedded in alumina through a combination of electron beam lithography, thermal evaporation, and RF sputtering. We use a home-built fluorescence lifetime imaging microscopy setup with both time-resolved lifetime and steady state emission spectra capabilities to characterize the changes in optical properties due to the presence of the embedded aluminum nanostructured array. To create patterned QD solids, we require a ligand exchange from nonpolar ligands common during the synthesis process including oleate and myristate to polar ligands like 3-mercaptopropionic acid. In doing so, a blueshift in the emission wavelength from 588 to 584 nm is observed along with a significant drop in luminescence. A more drastic change in the fluorescence lifetime is observed from over 15 ns to less than 1 ns. These changes in the emission wavelength and the fluorescence lifetime are expected from the ligand exchange process due to the addition of surface defects and decrease in surface passivation of the QDs. This insight will help guide the efficient integration of plasmonic nanostructured arrays into optoelectronic devices.