Cadmium selenide nanoplatelets (CdSe-NPLs) are solution processable nanostructures showing narrow absorption and emission excitonic features at room temperature [1]. These result from a giant oscillator strength and weak phonon-coupling [2], offering the opportunity to create robust, long-lived, excitons upon photoexcitation. 

In comparison with quantum dots (strong lateral QC) and wells (absent lateral QC), the characteristic CdSe-NPL lateral dimensions (𝐿_𝑥 and 𝐿_𝑦), ranging from a few to tens of nm, offer the chance to exploit and study excitons in different lateral confinement regimes. Detailed calculations showed that in the strong QC regime (NPL areas < 40 nm) excitonic states can be 100 meV apart, while in the weak/intermediate QC regime (NPL areas between ~120 and ~500 nm²) this difference can be as small as tens of meV [3]. In optical experiments, resolving multiple excitonic states depends on their relative energy difference as well as on the linewidth of the respective absorption and emission features. To date, the experimental and direct observation of multiple excitonic states in CdSe-NPLs have been reported from photoluminescence measurements at low temperature (200 K) [4]. 

Here, we report the first (direct) room temperature observation of multiple excitonic states in CdSe-NPLs from transient absorption (TA) experiments. As shown in Fig. (a), the differential absorption change 𝛥𝐴 is measured, after pumping resonantly at the HH excitonic peak, as a function of the pump-probe delay, 𝑡. By adding 𝛥𝐴 to the steady state absosorption 𝐴₀, see Fig. (b), a clear splitting of the HH excitonic peak is observed at around 𝑡=0 ps, which corresponds to the time when the pump-pulse ends to excite the NPLs. This splitting, altough with different strength, is observed indipendently of the used excitation energy, therefore ruling out any coherent artifact that can eventually happen in pump-probe experiments. 

In addition, fits of the decay kinetics of the TA biexciton (XX) feature with a multi-exponential decay function [Fig. (c)] resolve a 0.9 THz (3.7 meV) oscillation with a decay constant of ~1.5 ps [Fig. (d)]. This frequency does not depends on the excitation energy and well compares with the LA phonon mode reported in similar CdSe-NPLs [5], indicating a weak, but appreciable, XX-phonon coupling. Both frequency and decay constants are not found to change with the NPL area, further confirming the interaction with the LA phonon mode, as the latter is not expected to change along samples with an identical thickness.