Two Biexciton Types Coexisting in Coupled Quantum Dot Molecules

Abstract

Coupled colloidal quantum dot molecules are an emerging class of nanomaterials, introducing new degrees of freedom for designing quantum dot-based technologies. The properties of multiply excited states in these materials are crucial to their performance as quantum light emitters but cannot be fully resolved by existing spectroscopic techniques. Here we study the characteristics of biexcitonic species, which represent a rich landscape of different configurations, such as segregated and localized biexciton states. To this end, we introduce an extension of Heralded Spectroscopy to resolve different biexciton species in the prototypical CdSe/CdS coupled quantum dot dimer system. We uncover the coexistence and interplay of two distinct biexciton species: A fast-decaying, strongly-interacting biexciton species, analogous to biexcitons in single quantum dots, and a long-lived, weakly-interacting species corresponding to two nearly-independent excitons separated to the two sides of the coupled quantum dot pair. The two biexciton types are consistent with numerical simulations, assigning the strongly-interacting species to two excitons localized at one side of the quantum dot molecule and the weakly-interacting species to excitons segregated to the two quantum dot molecule sides. This deeper understanding of multiply excited states in coupled quantum dot molecules can support the rational design of tunable single- or multiple-photon quantum emitters.

Figures (5)

• Figure 1: Multiple BX States in CQDMs and the Heralded Spectroscopy Method. a)(i) Top: Two photons are emitted sequentially by a radiative relaxation in a CQDM from a biexciton (BX) state of two possible spatial configurations, to the exciton (1X) state and eventually to the ground state (GS). Bottom: Scheme of the heralded spectroscopy method that uses photon correlations to resolve the arrival time and energy of the photon pairs. Only two-photon cascades that were detected following the same excitation pulse are registered as heralded events. (ii) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and energy-dispersive spectroscopy (EDS) images of a fused dimer. b) 2D spectrum-lifetime histogram of all the post-selected BX emissions from a 5-min measurement of a single CQDM. On top is the full vertical binning in logarithmic scale and to the left is the full horizontal binning of the 2D BX histogram, showcasing the BX decay lifetime and spectrum, respectively. c) The two-component fit of the BX population in (b), each component with an independent exponential decay in time and an independent Voigt profile distribution in energy. The black horizontal line in (b) and (c) is due to a 'hot’ excluded pixel in the detector (see Methods section).
• Figure 2: 2D Heralded Analysis of Single Particles. The BX population from a 5-min measurement of (i) a monomer, a fused dimer with (ii) a high $g^{2}(0)$ contrast and (iii) a low $g^{(2)}(0)$ contrast, and (iv) a non-fused dimer. The particles feature a $g^{2}(0)$ contrast of approximately 0.09, 0.13, 0.37, and 0.45, respectively. Orange, blue, and green boxes distinguish between the different types of particles: monomers, fused dimers, and non-fused dimers, respectively. Schematics of the particle types are shown in the inset of (a) and transmission electron microscopy (TEM) images of the different particle types are shown in the inset of (b). The image of the fused dimers sample in panel (b) (ii) features two fused dimers that differ in the extent of fusion and filling of their interfacial area, the "neck". a) The bright gray bars are the full vertical binning (FVB) of the 2D BX population histogram (as the one shown in \ref{['fig:method']}b), showcasing the BX fluorescence decay lifetime. The blue and orange areas correspond to the FVB of the fast and the slow fitted BX components, respectively. A lifetime of $1ns$ acts as a threshold between "fast" and "slow". b) The bright gray area is the full horizontal binning (FHB) of the 2D BX population histogram, showcasing the BX spectrum. The blue and orange lines correspond to the FHB of the fast and the slow fitted BX components, respectively. In red asterisks and red dashed line are the 1X spectrum and its fitted Voigt profile, respectively. In dark gray, the normalized spectrum of all detections from the measurement. The gap in the gray areas is due to a 'hot' excluded pixel in the detector (see Methods section).
• Figure 3: BX Shifts According to Particle Type. BX shifts ($\Delta_{BX}$) of (a) the fast and (b) the slow fitted BX components of all the single particles, according to type. Monomers and ${\sim}68\%$ of the fused dimers did not exhibit a component with a lifetime of 1ns or higher and therefore do not appear in panel (b). Black dashed lines represent zero BX shift (equal energy of BX and 1X emissions).
• Figure 4: BX Components as a Function of $g^{(2)}(0)$. Weighted mean of (a) BX lifetimes and (b) BX shifts of the two fitted BX components of single particles and (c) the relative contribution of the "slow" BX component, as a function of $g^{(2)}(0)$, colored according to particle type. The particles shown in \ref{['fig:singleParticles']} are marked with their corresponding number. Lines to the left and above the axes represent the marginal distributions as kernel density plots, with colors matching the particle type. In panel (c), the particles centered at 0 contribution are those that exhibited a sub-ns decay in both BX components.
• Figure 5: Electronic Structures and Calculated Fluorescence Spectra of BX and 1X States in CQDMs. a) Illustration of the homodimer (center) and heterodimers (left, right) under study. b) BX binding energy of the lowest-energy BX state as a function of the asymmetry between the sizes of the cores forming the CQDMs. Dots are calculated values and the dotted line is a guide to the eye. A small departure from the homodimer limit ($\Delta{r}{\approx}0.1nm$) leads to a highly negative BX shift. c) Low-energy states of BXs and 1Xs in (i) homodimers ($\Delta{r}{=}0$) and (ii) heterodimers ($\Delta{r}{=}0.2nm$). The blue arrows label the BX optical transitions, and the schematics illustrate the main charge carrier spatial configuration in the CI expansion. d) Simulated emission spectra of the BX (bright gray) and the 1X (red) in (i) homodimers and in (ii) heterodimers, as the ones shown in (c), at $T{=}300K$. The reference energy (shift${=}0meV$) is that of the 1X in the homodimers. The black dashed arrows indicate for transitions 3, 4 and 5 their respective resulting 1X state.