Indirect Exciton Formation due to Inhibited Carrier Thermalization in Single CdSe/CdS Nanocrystals

Abstract

Temperature-dependent single-particle spectroscopy is used to study interfacial energy transfer in model light-harvesting CdSe/CdS core-shell tetrapod nanocrystals. Using alternating excitation energies, we identify two thermalized exciton states in single nanoparticles that are attributed to a strain-induced interfacial barrier. At cryogenic temperatures, emission from both states exemplifies the effects of intraparticle disorder and enables their simultaneous characterization, revealing that the two states are distinct in regards to emission polarization, spectral diffusion, and blinking.

Using shape to turn off blinking for two-colour multiexciton emission in CdSe/CdS tetrapods

“…As we describe in the introduction, dual red PL has been attributed to the existence of two distinct processes: direct core emission and spatially indirect (quasi type II) core/shell emission resulting from the presence of a potential energy barrier at the core/arm interface (Supplementary Figure 4)1415. We assign the peak at ∼600 nm to the direct emission process (Supplementary Note 2)15, which becomes more competitive with increasing pump power. We find that a longer arm length is needed to observe dual-red emission at room-temperature and speculate that the added strain induced by arm lengthening enhances the ‘barrier' effect that underlies this process.…”

“…The ability of the direct exciton state to coexist with the indirect exciton state was attributed to the presence of a strain-induced interfacial barrier inhibiting electron relaxation to the CdS shell (type II structure) or delocalization into the CdS shell (quasi type II structure). The two red emissions (direct, 2.045 and indirect, 2.020 eV) were observed at all excitation pump fluences and afforded typical random spectral diffusion and fluorescence blinking behaviours15.…”

“…Barrier-mediated dual emission in tetrapods involves two red-emission bands associated with the CdSe core1415. Specifically, in single-tetrapod studies conducted at cryogenic temperatures, two red-colour emissions were observed and attributed to emission from direct and indirect exciton states, respectively.…”

“…Specifically, in single-tetrapod studies conducted at cryogenic temperatures, two red-colour emissions were observed and attributed to emission from direct and indirect exciton states, respectively. The former involves radiative recombination of a CdSe-localized electron–hole pair, or exciton, whereas the latter involves recombination of either a CdSe-localized hole and a CdS-localized electron (type II band alignment) or a CdSe-localized hole and a delocalized electron (quasi type II band alignment)15. The ability of the direct exciton state to coexist with the indirect exciton state was attributed to the presence of a strain-induced interfacial barrier inhibiting electron relaxation to the CdS shell (type II structure) or delocalization into the CdS shell (quasi type II structure).…”

Ultrafast Photoluminescence from the Core and the Shell in CdSe/CdS Dot‐in‐Rod Heterostructures

“…In all cases in Figure , only two PL features are resolved. Although CdSe/CdS nano‐heterostructures have previously demonstrated spatially indirect emission, those samples were tetrapods which contained zinc blende CdSe cores and wurtzite CdS “legs” and spatially indirect emission was characterized by both a distinct energy and longer lifetime. Spatially indirect emission has also been claimed in CdSe/CdS dot‐in‐rod structures, but this claim was not accompanied by an energetically distinct spectral feature with distinct lifetime .…”