Singlet–Triplet Splittings in the Luminescent Excited States of Colloidal Cu⁺:CdSe, Cu⁺:InP, and CuInS₂ Nanocrystals: Charge-Transfer Configurations and Self-Trapped Excitons

Abstract

The electronic and magnetic properties of the luminescent excited states of colloidal Cu(+):CdSe, Cu(+):InP, and CuInS2 nanocrystals were investigated using variable-temperature photoluminescence (PL) and magnetic circularly polarized luminescence (MCPL) spectroscopies. The nanocrystal electronic structures were also investigated by absorption and magnetic circular dichroism (MCD) spectroscopies. By every spectroscopic measure, the luminescent excited states of all three materials are essentially indistinguishable. All three materials show very similar broad PL line widths and large Stokes shifts. All three materials also show similar temperature dependence of their PL lifetimes and MCPL polarization ratios. Analysis shows that this temperature dependence reflects Boltzmann population distributions between luminescent singlet and triplet excited states with average singlet-triplet splittings of ∼1 meV in each material. These similarities lead to the conclusion that the PL mechanism in CuInS2 NCs is fundamentally different from that of bulk CuInS2 and instead is the same as that in Cu(+)-doped NCs, which are known to luminesce via charge-transfer recombination of conduction-band electrons with copper-localized holes. The luminescence of CuInS2 nanocrystals is explained well by invoking exciton self-trapping, in which delocalized photogenerated holes contract in response to strong vibronic coupling at lattice copper sites to form a luminescent excited state that is essentially identical to that of the Cu(+)-doped semiconductor nanocrystals.

Highly Luminescent Water-Dispersible NIR-Emitting Wurtzite CuInS2/ZnS Core/Shell Colloidal Quantum Dots

“…46 However, recent work has unambiguously demonstrated that the PL in CP CIS QDs originates from the radiative recombination of a delocalized CB electron with a hole localized on a Cu + ion. 31,47,48 Nevertheless, the nature of the hole localization process (i.e., self-trapping onto a regular Cu + ion 48 or capture by a native defect, such as Cu In 2– ) has yet to be unravelled. 31 …”

Optoelectronic Properties in Near‐Infrared Colloidal Heterostructured Pyramidal “Giant” Core/Shell Quantum Dots

“…Upon photoexcitation, the 1S holes nonradiatively moved from the valence band to the impurity in the core 30. The luminescence of the g‐QDs is attributed to the radiative recombination of the conduction band 1S electrons with the impurity holes 30.…”

Highly Emissive Divalent-Ion-Doped Colloidal CsPb1–xMxBr3 Perovskite Nanocrystals through Cation Exchange

“…24,25 Furthermore, impurity doping of perovskite thin films has been shown to improve their performance in solar cells. 26−28 Recently, Mn 2+ doping in colloidal CsPbCl 3 perovskite NCs has been achieved by a direct synthesis method, in which PbCl 2 and MnCl 2 precursors were mixed in the desired ratio, leading to NCs with the characteristic Mn 2+ emission.…”