Transient photoluminescence and simultaneous amplified spontaneous emission from multiexciton states in $\mathrm{CdSe}$ quantum dots

We use transient photoluminescence to spectrally resolve the emission from 1, 2, and 3 electron-hole pairs states in $\mathrm{CdSe}$ colloidal quantum dots with radii ranging between 2.3 and $5.2\mathrm{nm}$. The power dependence of the triexciton, biexciton and single exciton emission intensities are reported. We incorporate $\mathrm{CdSe}∕\mathrm{ZnS}$ core-shell nanocrystals into a ${\mathrm{TiO}}_2$ host matrix and observe simultaneous amplified spontaneous emission from the $1{\mathrm{S}}_{3∕2}\text{−}1\mathrm{Se}$ and $1{\mathrm{P}}_{3∕2}\text{−}1{\mathrm{P}}_{\mathrm{e}}$ quantum dot transitions. The achievement of amplified spontaneous emission from the $1{\mathrm{P}}_{\mathrm{e}}$ QD state suggests that the $1{\mathrm{P}}_{\mathrm{e}}$ state sixfold degeneracy might be lifted by the $\mathrm{e}\text{−}\mathrm{h}$ exchange interaction.

Figure 1

(a) 3D transient PL spectrum recorded on a $1.1\mathrm{ns}$timescale for a $2.3\mathrm{nm}$ radius QDs sample; the PL intensity is coded in white color; brighter area corresponds to higher PL intensity. (b) PL spectra $20\mathrm{ps}$ (stars) and $800\mathrm{ps}$ (circles) after excitation extracted from Fig. 1a. (c) $4.3\mathrm{nm}$ QD radius PL spectra $20\mathrm{ps}$ (stars) and $800\mathrm{ps}$ (circles) after excitation. (d) $2.3\mathrm{nm}$ radius QD band edge PL spectra $60\mathrm{ps}$ (open triangle) and $800\mathrm{ps}$ (solid triangle) after excitation, the blue shift is due to negative biexciton binding energy.

Figure 2

(a) $\Delta$ versus the QD radius; (b) a comparison between $\Delta$ (squares) and the $1{\mathrm{P}}_{3∕2}\text{−}1{\mathrm{P}}_{\mathrm{e}}$ energy transition (triangles) relative to the $1{\mathrm{S}}_{3∕2}\text{−}1\mathrm{Se}$ band edge transition; the notation $1{\mathrm{S}}_{3∕2}$, $1{\mathrm{P}}_{3∕2}$, $1{\mathrm{S}}_{\mathrm{e}}$ and $1{\mathrm{P}}_{\mathrm{e}}$ of Ref. 20 designates the first two hole and electron levels, respectively; measured $1{\mathrm{P}}_{3∕2}\text{−}1{\mathrm{P}}_{\mathrm{e}}$ energy position (diamonds) for three QD samples at room temperature.

Figure 3

(a) Single exciton saturation curve; single exciton spectra are taken $1\mathrm{ns}$ after excitation. Solid line (least square fit) shows that Eq. (2) reproduces the saturation curve. (b) Biexciton emission power dependence; the solid line shows the least square fit result using Eq. (3). (c) Multiexcitonic band power dependence (circles) and least square fit results using Eq. (3) (crosses) and Eq. (4) (solid line), respectively. (d) Normalized power dependence of the single exciton (squares) biexciton (triangles) and the multiexcitonic band (circles); the 3 different slopes show that the 3 bands originate from 3 different emission processes.

Figure 4

(a) $2.3\mathrm{nm}$ (circles) and $3.4\mathrm{nm}$ (diamonds) radius QD biexciton PL decay curves. (b) PL decay curve (circles) of the multiexcitonic band for a $3.4\mathrm{nm}$ radius QD sample.

Figure 5

(a) Linear PL spectrum of a $3.1\mathrm{nm}$ radius QD doped titania thin film. (b) PL spectrum for $2.2\mathrm{e}\text{−}\mathrm{h}$ pairs injected on average into the QDs (triangles); simultaneous ASE (stars) for $4\mathrm{e}\text{−}\mathrm{h}$ pairs injected on average into the QDs; FWHM for the two peaks are on the order of $30\mathrm{meV}\left(10\mathrm{nm}\right)$. (c) Power dependence of the integrated area under the first (triangle) and second peak (circle); the inset is a zoom around the threshold regions. (d) Simultaneous ASE (stars) for a $5.2\mathrm{nm}$ radius QDs doped titania thin film, the square curve represents the PL spectrum at low pump intensity.

Figure 6

(a) PL spectra $20\mathrm{ps}$ after excitation before (stars) and after (circles) subtraction of the single excitation contribution. (b) Contribution of the single exciton emission to the PL spectrum $20\mathrm{ps}$ (diamonds) and $300\mathrm{ps}$ (circles) after excitation and rescaled Poissonian probability (crosses) to generate more than $1\mathrm{e}\text{−}\mathrm{h}$ pair per QD right after an excitation pulse.