p-State Luminescence in CdSe Nanoplatelets: Role of Lateral Confinement and a Longitudinal Optical Phonon Bottleneck

Abstract

We evidence excited state emission from p states well below ground state saturation in CdSe nanoplatelets. Size-dependent exciton ground and excited state energies and population dynamics are determined by four independent methods: time-resolved PL, time-integrated PL, rate equation modeling, and Hartree renormalized k·p calculations-all in very good agreement. The ground state-excited state energy spacing strongly increases with the lateral platelet quantization. Depending on its detuning to the LO phonon energy, the PL decay of CdSe platelets is governed by a size tunable LO phonon bottleneck, related to the low exciton-phonon coupling, very large oscillator strength, and energy spacing of both states. This is, for instance, ideal to tune lasing properties. CdSe platelets are perfectly suited to control the exciton-phonon interaction by changing their lateral size while the optical transition energy is determined by their thickness.

Excitons in core-only, core-shell and core-crown CdSe nanoplatelets: Interplay between in-plane electron-hole correlation, spatial confinement, and dielectric confinement

“…This is especially the case in NPLs, because Coulomb interactions are expected to be magnified by dielectric mismatch. Previous theoretical studies of NPLs with finite lateral size missed this requirement, as Coulomb interaction was either overlooked [23] or dealt with using a mean-field (Hartree) model [24]. The latter has been tested in analytically solvable 2D systems and found to be insufficient, especially in the weak confinement regime [25].…”

“…[20] (∼25meV) and Ref. [24] (∼40 meV). Theoretical estimates using Hartree-renormalized multiband k · p theory yielded 35 meV blueshift [24], in reasonable agreement with ours.…”

“…[24] (∼40 meV). Theoretical estimates using Hartree-renormalized multiband k · p theory yielded 35 meV blueshift [24], in reasonable agreement with ours. By contrast, calculations neglecting electron-hole interaction obtained (for square plates) blueshifts of 250-300 meV [23], about three times larger than our estimates for such geometry.…”

Consequence of shape elongation on emission asymmetry for colloidal CdSe/CdS nanoplatelets

“…The overgrowth of a shell (typically of a wider band gap such as CdS or ZnS on CdSe) on the core material has been proven as an efficient way for 1) increasing both the quantum yield and stability of the emitters, and 2) decreasing their emission blinking [21][22][23][24][25]. The luminescence dynamics have been studied, for example, as a function of the lateral dimensions of the nanoplatelets [26][27][28][29], and partial emission polarization has been found for elongated platelets [30,31]. However, the dipolar nature of these emitters has not yet been investigated.…”

Sideband pump-probe technique resolves nonlinear modulation response of PbS/CdS quantum dots on a silicon nitride waveguide

“…A most promising development in this direction is the establishment of colloidal nanostructures with two-dimensional confinement, so-called nanoplatelets, which show a greatly reduced GS exciton lifetime compared to QDs. 48 To enhance the modulation depth reached in photonic devices, the QDs or nanoplatelets can be implanted into the silicon nitride waveguide, as has been demonstrated in Ref. 36.…”