Multiexcitons in type-II colloidal semiconductor quantum dots

Dan Oron, Miri Kazes, and Uri Banin

Phys. Rev. B 75, 035330 – Published 22 January 2007

Abstract

The spectroscopy and dynamics of multiple excitations on colloidal type-II $Cd Te ∕ Cd Se$ core-shell quantum dots (QDs) are explored via quasi-cw multiexciton spectroscopy. The charge separation induced by the band offset redshifts the exciton emission and increases the radiative lifetime. In addition, we observe a significant modification of multiexciton properties compared with core-only or type-I QDs. In particular, the Auger recombination lifetimes are significantly increased, up to a nanosecond time scale. While in type-I QDs the Auger lifetime scales with the volume, we find for type-II QDs a scaling law that introduces a linear dependence also on the radiative lifetime. We observe a blueshift of the biexciton emission and extract biexciton repulsion of up to $30 meV$ in type-II QDs. This is assigned to the dominance of the Coulomb repulsion as the positive and negative charges become spatially separated, which overwhelms the correlation binding term. Higher electronic excited states can remain type I even when the lowest transition is already type II, resulting in a different size dependence of the triexciton emission. Finally, we discuss the possibilities of “multiexciton band gap engineering” using colloidal type-II QDs.

Received 14 November 2006

DOI:https://doi.org/10.1103/PhysRevB.75.035330

Authors & Affiliations

Dan Oron, Miri Kazes, and Uri Banin

The Institute of Chemistry and the Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel*

^*URL: http://chem.ch.huji.ac.il/~nano/

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Vol. 75, Iss. 3 — 15 January 2007

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Images

Figure 1
Absorption spectra of colloidal $Cd Te ∕ Cd Se$ core-shell nanoparticles with a $3.9 − nm$ core and shell thicknesses of 0.2, 0.5, 0.9, 1.5, 2.15, and $2.5 nm$ (total diameter 4.3, 4.9, 5.7, 6.9, 8.2, and $8.9 nm$ , respectively). Consecutive spectra are shifted vertically for clarity. The inset shows a TEM image of nanoparticles with a $0.9 − nm$ shell. The bar length is $20 nm$ .Reuse & Permissions
Figure 2
(Color online) (a) Calculated radial electron (top) and hole (bottom, inverted) probability distribution functions for both the lowest $S$ state (dashed line) and the lowest $P$ state (dash-dotted line) for a shell thickness of 0.9 nm. For clarity, wave functions are shifted by the level energy (dashed and dash-dotted horizontal lines, respectively). (b) Measured $X$ emission lifetime (× symbols) and the effective mass model prediction (dashed line), based on the $e − h$ overlap integral.Reuse & Permissions
Figure 3
(Color online) Methods for extraction of the Auger recombination lifetime, demonstrated on $3.9 − nm$ CdTe core, $0.9 − nm$ -thick CdSe shell nanoparticles. (a) Emission transient measured at the $X$ peak $(700 nm)$ at a $600 μ J$ pulse energy (solid line) and its decomposition into an instantaneous component (dashed line) and a slowly decaying $(35 − ns)$ component (dash-dotted line). (b) Translation of pulse energies to average number of absorbed photons by fitting the integrated delayed emission intensity (from $5 ns$ after the excitation pulse to $75 ns$ , plus signs) to a saturation curve (dashed line). The vertical line at $\approx 25 μ J$ indicates the energy for absorption of one photon per dot. According to this, the curve shown in (a) corresponds to about 25 absorbed photons per dot. (c) The measured ratio of the instantaneous component and the slowly decaying one as a function of the number of absorbed photons per dot (plus signs) along with a saturation curve (dashed line).Reuse & Permissions
Figure 4
(Color online) The derivative of the measured ratio of the instantaneous component and the slowly decaying one with respect to the absorbed number of photons (left axis) and a corresponding approximate BX Auger lifetime (right axis) as a function of (a) QD volume (dashed line is a linear dependence, corresponding to the type-I scaling) and (b) $V τ_{rad}$ . This fits very well with a linear dependence (dashed line).Reuse & Permissions
Figure 5
Transient spectra measured at the peak of the excitation pulse at increasing pulse energies (40, 150, 600, 1500, and $6000 μ J$ ) for (a) QDs with a $0.9 − nm$ CdSe shell, just at the transition from type I to type II. No shift is observed in the first few spectra, corresponding to nearly zero BX shift. (b) QDs with a $2.15 − nm$ CdSe shell, already in the type-II regime. The blueshift indicates a repulsive BX state.Reuse & Permissions
Figure 6
(Color online) Multiexciton shifts as a function of shell thickness. (a) Measured $Δ_{X X}$ in $Cd Te ∕ Cd Se$ QDs (plus signs) and in CdSe QDs of similar diameter (× symbols). While type-I QDs (CdSe) show binding; upon transition to type II a shift to repulsion is observed. A theoretical prediction (solid line) for the type-II QDs based on a weighted average between the measured binding energy of type-I QDs (dashed line) and the model prediction for the repulsion energy (dash-dotted line) are also shown. (b) Measured TX shift in $Cd Te ∕ Cd Se$ QDs (plus signs) and in CdSe QDs of similar diameter (× symbols), compared with the calculated $1 P_{e}$ to $1 S_{e}$ shift (solid line and dashed line, respectively). The measured $1 P$ to $1 S$ shift for CdSe QDs (extrapolated from Ref. 35) is shown as a dash-dotted black line.Reuse & Permissions

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