Electronic Structure and Exciton–Phonon Interaction in Two-Dimensional Colloidal CdSe Nanosheets

Abstract

We study the electronic structure of ultrathin zinc-blende two-dimensional (2D)-CdSe nanosheets both theoretically, by Hartree-renormalized k·p calculations including Coulomb interaction, and experimentally, by temperature-dependent and time-resolved photoluminescence measurements. The observed 2D-heavy hole exciton states show a strong influence of vertical confinement and dielectric screening. A very weak coupling to phonons results in a low phonon-contribution to the homogeneous line-broadening. The 2D-nanosheets exhibit much narrower ensemble absorption and emission linewidths as compared to the best colloidal CdSe nanocrystallites ensembles. Since those nanoplatelets can be easily stacked and tend to roll up as they are large, we see a way to form new types of multiple quantum wells and II-VI nanotubes, for example, for fluorescence markers.

Stacking in Colloidal Nanoplatelets: Tuning Excitonic Properties

“…Therefore, quantum confinement along lateral dimensions is expected to be very weak in these NPLs since the lateral size is much larger than the size (∼5À7 nm) of the first exciton. 15,25 Previously, Dubertret et al demonstrated that addition of a polar solvent such as ethanol into the solution of the NPLs, which is dissolved in apolar solvents (e.g., hexane and toluene), stacks the NPLs on top of each other, resulting in column-like assemblies. 19 When ethanol is introduced, NPLs tend to minimize their surface energy by drawing away ethanol since ethanol is an antisolvent for the ligands (oleic acid) of the NPLs.…”

“…Multiexponential decay (three or four exponential decay functions) in the NPLs was also previously observed. 9,15,26 Figure 3c depicts the amplitude-averaged photoluminescence lifetimes of the NPLs as a function of total added ethanol amount (see Figure S3 for the individual lifetime components). The NPLs have a ∼3.38 ns average photoluminescence lifetime before stacking is induced, which is in accordance with the photoluminescence lifetime of the 4 ML CdSe NPLs.…”

“…22,23 In the case of NPLs, the density of states is not inhomogeneously broadened since the magic-sized vertical thickness (e.g., 3, 4, or 5 monolayers) of the NPLs dictates their band gap energy. 15,17,25 Quantum confinement due to the lateral direction in the NPLs is very weak because lateral sizes are typically larger than the exciton Bohr radius. 15 Therefore, one should expect to observe different signatures for the exciton transfer via FRET in stacked NPL assemblies as compared to ones in close-packed QD assemblies.…”

“…15,17,25 Quantum confinement due to the lateral direction in the NPLs is very weak because lateral sizes are typically larger than the exciton Bohr radius. 15 Therefore, one should expect to observe different signatures for the exciton transfer via FRET in stacked NPL assemblies as compared to ones in close-packed QD assemblies.…”

Colloidal synthesis and optical properties of type-II CdSe–CdTe and inverted CdTe–CdSe core–wing heteronanoplatelets

“…Such NPLs exhibit electronic and optical properties that differ from those for QDs, such as much narrower absorption and PL bands (FWHM < 10 nm), a giant oscillator strength, lower thresholds for amplified spontaneous emission and higher optical gain saturation. [33][34][35] However, the synthesis of heteronanostructures based on such nanoplatelets is limited to the preparation of CdSe-CdS or CdSe-CdZnS nanoparticles. [36][37][38] Recently, we have shown that overgrowth of CdSe NPLs with CdS in the presence of acetate species results in the formation of NPLs with a "corewings" architecture.…”