Figure 1.

Obstacles in measuring the 1X–BX energy ladder. (a) The energy diagram of the 1X and BX states in a nanoparticle. The energy difference between the BX and 1X states (E_BX) is smaller than the difference between the 1X and ground states (E_1X) by the biexciton binding energy (ε_b). (b) Schematic of the thermal broadening of spectral lines (intensity normalized for clarity). At room temperature, the 1X-ground (blue) and BX–1X (red) transitions emission lines are broadened to approximately ∼k_BT ≈ 26 mev > ε_b. As a result, the two emission spectra substantially overlap. (c) Schematic dependence of the 1X energy on nanoparticle size. An ensemble measurement (with a ± 5% size variance) at room temperature roughly includes a mixture of the depicted spectra. (d) Scheme of spectral drift in the emission lines of a single nanocrystal throughout the measurement time. Apart from the stochastic random walk of the spectral lines (diffusion), discrete spectral jumps typically accompany blinking events.