Biexciton Binding Energy and Line width of Single Quantum Dots at Room Temperature

Abstract

Broadening of multiexciton emission from colloidal quantum dots (QDs) at room temperature is important for their use in high-power applications, but an in-depth characterization has not been possible until now. We present and apply a novel spectroscopic method to quantify the biexciton line width and biexciton binding energy of single CdSe/CdS/ZnS colloidal QDs at room temperature. In our method, which we term “cascade spectroscopy”, we select emission events from the biexciton cascade and reconstruct their spectrum. The biexciton has an average emission line width of 86 meV on the single-QD scale, similar to that of the exciton. Variations in the biexciton repulsion (E b = 4.0 ± 3.1 meV; mean ± standard deviation of 15 QDs) are correlated with but are more narrowly distributed than variations in the exciton energy (10.0 meV standard deviation). Using a simple quantum-mechanical model, we conclude that inhomogeneous broadening in our sample is primarily due to variations in the CdS shell thickness.

Resolving the Controversy in Biexciton Binding Energy of Cesium Lead Halide Perovskite Nanocrystals through Heralded Single-Particle Spectroscopy

“…As is evident from Figure c and Figure S5, the CsPbBr 3 NCs BX spectrum is broader by a factor of 1.4 ± 0.4 compared to the 1X spectrum (measurements of CsPbI 3 NCs suggest a similar factor, but at a lower confidence). A possible explanation for this broadening is the presence of multiple decay routes from the BX to the 1X state, possibly through thermally excited hot BX states which are close energetically to the lowest BX state. , We note that this is qualitatively different from observations in core/​shell/​shell CdSe/​CdS/​ZnS NCs. , However, II-VI semiconductor NCs feature more pronounced spectral diffusion compared to LHP NCs. , As demonstrated in ref , attenuation of the BX spectral diffusion as compared to the 1X spectral diffusion can overwhelm BX broadening effects, if they exist, and lead to a BX spectrum that is narrower than the 1X.…”

“… 31 , 34 We note that this is qualitatively different from observations in core/shell/shell CdSe/CdS/ZnS NCs. 16 , 17 However, II-VI semiconductor NCs feature more pronounced spectral diffusion compared to LHP NCs. 35 , 36 As demonstrated in ref ( 16 ), attenuation of the BX spectral diffusion as compared to the 1X spectral diffusion can overwhelm BX broadening effects, if they exist, and lead to a BX spectrum that is narrower than the 1X.…”

“…Room temperature single-particle heralded spectroscopy has been recently introduced as a way to overcome these shortcomings of ensemble approaches. , This is achieved by temporally isolating photon pairs originating from the BX→1X→G cascade of single particles and is hence free of all the aforementioned biases and ambiguities. In this work, we utilize this technique to unambiguously determine the BX binding energies of the prototypical LHP NCs CsPbBr 3 and CsPbI 3 .…”

Resolving the Triexciton Recombination Pathway in CdSe/CdS Nanocrystals through State-Specific Correlation Measurements

“…We include spectral resolution in order to differentiate between S-like and P-like emission without employing any assumptions of the lifetime or scaling of the relative states. Similar strategies have been employed recently by Sander et al to measure the biexciton binding energy of single nanocrystals; however, in their case, more complex wavelength filtering methods were needed to isolate biexciton emission energies from the exciton as they are significantly spectrally overlapped . Unlike the results demonstrated by Fisher et al, we conduct our experiments at low-flux where we maintain state specificity and directly measure both possible pathways to compare branching ratios.…”

Shape‐Tuned Multiphoton‐Emitting InP Nanotetrapods

“…This fluctuates the local electric‐field and instantaneously tilt the band alignment to alter the emission energy. [ 34,52,53 ] Here, we could rule out the biexciton effect in spectral analysis due to i) very small spectral shift between a single exciton and biexciton at room temperature regarding their emission linewidths, [ 54,55 ] ii) mild single‐NC excitation fluence (< N X > < 1) that safely lie at the single‐exciton‐generation regime (Figure S17, Supporting Information), and iii) very low biexciton emissivity (relative biexciton quantum yield ≈0.03%) of tetrapods (Note S6, Supporting Information). To quantitatively compare the QCSE in the NCs, the statistics of the shifts in the spectra of single NCs were collected (Figure 4d).…”