ABSTRACT
The emerging generation of quantum dot optoelectronic devices offers an appealing prospect of a size-tunable band gap. The confinement-enabled control over electronic properties, however, requires nanoparticles to be sufficiently small, which leads to a large area of interparticle boundaries in a film. Such interfaces lead to a high density of surface traps which ultimately increase the electrical resistance of a solid. To address this issue, we have developed an inverse energy-gradient core/shell architecture supporting the quantum confinement in nanoparticles larger than the exciton Bohr radius. The assembly of such nanostructures exhibits a relatively low surface-to-volume ratio, which was manifested in this work through the enhanced conductance of solution-processed films. The reported core/shell geometry was realized by growing a narrow gap semiconductor layer (CdSe) on the surface of a wide-gap core material (CdS) promoting the localization of excitons in the shell domain, as was confirmed by ultrafast transient absorption and emission lifetime measurements. The band gap emission of fabricated nanoshells, ranging from 15 to 30 nm in diameter, has revealed a characteristic size-dependent behavior tunable via the shell thickness with associated quantum yields in the 4.4-16.0% range.
ABSTRACT
Tracking the energy flow in nanoscale materials is an important yet challenging goal. Experimental methods for probing the intermolecular energy transfer (ET) are often burdened by the spectral crosstalk between donor and acceptor species, which complicates unraveling their individual contributions. This issue is particularly prominent in inorganic nanoparticles and biological macromolecules featuring broad absorbing profiles. Here, we demonstrate a general spectroscopic strategy for measuring the ET efficiency between nanostructured or molecular dyes exhibiting a significant donor-acceptor spectral overlap. The reported approach is enabled through spectral shaping of the broadband excitation light with solutions of donor molecules, which inhibits the excitation of respective donor species in the sample. The resulting changes in the acceptor emission induced by the spectral modulation of the excitation beam are then used to determine the quantum efficiency and the rate of ET processes between arbitrary fluorophores (molecules, nanoparticles, polymers) with high accuracy. The feasibility of the reported method was demonstrated using a control donor-acceptor system utilizing a protein-bridged Cy3-Cy5 dye pair and subsequently applied for studying the energy flow in a CdSe560-CdSe600 binary nanocrystal film.
