Observation of Electron Shakeup in CdSe/CdS Core/Shell Nanoplatelets

Abstract

While ensembles of CdSe nanoplatelets (NPLs) show remarkably narrow photoluminescence line widths at room temperature, adding a CdS shell to increase their fluorescence efficiency and photostability causes line width broadening. Moreover, ensemble emission spectra of CdSe/CdS core/shell NPLs become strongly asymmetric at cryogenic temperatures. If the origin of these effects were understood, this could potentially lead to stable core/shell NPLs with narrower emission, which would be advantageous for applications. To move in this direction, we report timeresolved emission spectra of individual CdSe/CdS core/shell NPLs at 4 K. We observe surprisingly complex emission spectra that contain multiple spectrally narrow emission features that change during the experiment. With machine-learning algorithms, we can extract characteristic peak energy differences in these spectra. We show that they are consistent with electron "shakeup lines" from negatively charged trions. In this process, an electron-hole pair recombines radiatively, but gives part of its energy to the remaining electron by exciting it into a higher single-electron level. This "shakeup" mechanism is enabled in our NPLs due to strong exciton binding and weak lateral confinement of the charge carriers. Time-resolved single-photon-counting measurements and numerical calculations suggest that spectral jumps in the emission features originate from fluctuations in the confinement potential caused by microscopic structural changes on the NPL surface (e.g., due to mobile surface charges). Our results provide valuable insights into line width broadening mechanisms in colloidal NPLs.

Tuning exciton diffusion, mobility and emission line width in CdSe nanoplatelets via lateral size

“…Figure 1 shows temperature dependent PL emission spectra of exemplary sizes at 4 and 200 K under off-resonant 2.95 eV (420 nm) excitation. Clearly a double emission is observed, which has been attributed to trion (lower energy emission T − ) and exciton (upper energy emission X) 21,[25][26][27] PL. We remark that due to the low coupling to LOphonons in the CdSe platelets, e.g.…”

“…Robust excitons even at room temperature with high exciton binding energies (of ∼ 200 meV) have been found. 2,[22][23][24] The low temperature double peak emission of CdSe NPLs has been attributed to exciton and additional trion states, 21,25,26 where the latter exhibit a fine structure due to shakeup lines, 27 that originate from the quantized nature of electron states in the laterally weakly confined system.…”

Emission State Structure and Linewidth Broadening Mechanisms in Type-II CdSe/CdTe Core–Crown Nanoplatelets: A Combined Theoretical–Single Nanocrystal Optical Study

“…36 We are thus led to discard any shake-up like mechanism, that would be at play and responsible for the multiplication of peaks in the emission. 37 The regular energy spacing discards as well simultaneous emission from different states within a stack of NPLs. 38 Further experimental evidences are given below.…”

Optically driving the radiative Auger transition

“…Radiative Auger emission has been observed over a large spectral range: in the X-ray emission of atoms 9 ; close to visible frequencies on donors in semiconductors 10 and quantum emitters 11 , 12 ; and at infrared frequencies as shake-up lines in two-dimensional systems 13 – 17 . Furthermore, radiative Auger connects carrier dynamics to the quantum optical properties of the emitted photons 11 , making it a powerful probe of multi-particle systems.…”