Colloidal optoelectronics of self-assembled quantum well superstructures

Abstract

Advances in the colloidal nanocrystal synthesis enabled creation of quasi twodimensional colloidal quantum wells (CQWs) in the last decade. These CQWs possess similar properties to those of epitaxially grown quantum wells while at the same time offering the benefits of low-cost synthesis and solubility in various solvents. Their atomically precise thickness and one-dimensional quantum confinement grant them favorable properties such as narrow emission linewidth, reduced inhomogeneous broadening and giant oscillator strength. In addition, due to their quasi-two dimensional shape, they display intrinsic anisotropy. Because of this anisotropy, the particle interactions in closely packed films depend greatly on the orientation of these CQWs. To fully utilize the interaction of CQWs with each other or with other particles in their proximity, we develop a selfassembly technique, which is used to deposit highly uniform thin CQW films onto various solid substrates. This self-assembly technique allows us to deposit CQWs as a continuous monolayer while at the same time controlling their orientation throughout the substrate, thereby modifying their packing factor as well as nearfield dipole-dipole interactions. This self-assembly technique is also employed to create large-area CQW films of any desired thickness, simply by applying the same deposition technique on the same substrate as many times as desired. We use these self-assembled CQW films to study the two main aspects of nanocrystal optoelectronics, namely, Förster resonance energy transfer (FRET) and optical gain, with CQWs. By using the orientation-controlled CQW monolayers, we show that the rate of FRET from colloidal quantum dots (QDs) to a monolayer of CQWs can be tuned via dipole-dipole interactions between QDs and CQWs. We use Förster’s theory of nonradiative energy transfer while taking into account the anisotropy of the excitonic CQW excitonic state as well as its delocalization throughout the CQW to account for our results. Next, we show that our multilayered CQW films display optical gain in uncharacteriscally low thicknesses (as small as 40 nm) due to the tight packing and extremely uniform deposition of the CQWs. We furthermore study systematically the observed threshold of amplified spontaneous emission (ASE) in these CQW multilayers as a function of the film thickness (i.e., the number of monolayers), and demonstrate that the gain threshold drops with increasing thickness, accompanied by the red-shift of the ASE peak. These trends can be explained by the varying degree of optical mode confinement, which is a function of both the film thickness as well as the wavelength of propagating mode. Our self-assembly technique allows to study and make use of the favorable properties of the CQWs including anisotropy and enhanced optical gain. Since this technique enables us to produce large-area films displaying excellent homogeneity, it can be a benchmark building block for creating device-scale 2- or 3-dimensional superstructures from CQWs as well as from other types of colloidal nanocrystals to be utilized in both in- and out-of-plane optical applications.