Optically Detected Magnetic Resonance Spectroscopy of Cu-Doped CdSe/CdS and CuInS₂ Colloidal Quantum Dots

Abstract

Copper-doped II−VI and copper-based I−III−VI 2 colloidal quantum dots (CQDs) have been at the forefront of interest in nanocrystals over the past decade, attributable to their optically activated copper states. However, the related recombination mechanisms are still unclear. The current work elaborates on recombination processes in such materials by following the spin properties of copper-doped CdSe/CdS (Cu@CdSe/CdS) and of CuInS 2 and CuInS 2 /(CdS, ZnS) core/shell CQDs using continuous-wave and time-resolved optically detected magnetic resonance (ODMR) spectroscopy. The Cu@CdSe/CdS ODMR showed two distinct resonances with different g factors and spin relaxation times. The best fit by a spin Hamiltonian simulation suggests that emission comes from recombination of a delocalized electron at the conduction band edge with a hole trapped in a Cu 2+ site with a weak exchange coupling between the two spins. The ODMR spectra of CuInS 2 CQDs (with and without shells) differ significantly from those of the copper-doped II−VI CQDs. They are comprised of a primary resonance accompanied by another resonance at half-field, with a strong correlation between the two, indicating the involvement of a triplet exciton and hence stronger electron−hole exchange coupling than in the doped core/shell CQDs. The spin Hamiltonian simulation shows that the hole is again associated with a photogenerated Cu 2+ site. The electron resides near this Cu 2+ site, and its ODMR spectrum shows contributions from superhyperfine coupling to neighboring indium atoms. These observations are consistent with the occurrence of a self-trapped exciton associated with the copper site. The results presented here support models under debate for over a decade and help define the magneto-optical properties of these important materials.

Low-Temperature Photoluminescence Dynamics Reveal the Mechanism of Light Emission by Colloidal CuInS2 Quantum Dots

“…As discussed above, the strong hole localization quenches the orbital magnetic moment and the hole state becomes a pure-spin state. Indeed, recent optically detected magnetic resonance studies on CuInS 2 QDs showed that g e ≈ g h . Thus, the observed insensitivity of the PL lifetimes to magnetic field is indeed expected for the free-to-bound PL mechanism.…”

“…Indeed, recent optically detected magnetic resonance studies on CuInS 2 QDs showed that g e ≈ g h . 62 Thus, the observed insensitivity of the PL lifetimes to magnetic field is indeed expected for the free-tobound PL mechanism.…”

The impact of spatially heterogeneous chemical doping on the electronic properties of CdSe quantum dots: insights from ab initio computation

“…S6 †). 67,68 These small molecules represent the experimentally used long-alkyl-chain passivating ligands. 17,21 The geometry optimization of these ligand-passivated QDs exhibits minimal modifications on the surface structure compared to their bare counterparts (ESI, section S4 and Table S1 †).…”

“…29,30 Radiative recombination of delocalized conduction band electrons and localized holes at the Cu site gives rise to distinct photoluminescence (PL) bands and long excited state lifetimes. [31][32][33][34] These chemically modi-fied QDs also provide functional advantages such as reduced reabsorption, efficient light conversion, and tunability, proposing their unique applications. [35][36][37] The creation of localized defect states indicates well-defined and indistinguishable photon emission.…”

Investigating the Mechanism by Which Intragap States Tailor Light Emission: Case of Copper Deficient and Zn Doped Cu−In−Se Quantum Dots**

“…[7] For example, Fuhr et al [3] developed the Cu 1 + /Cu 2 + -defect model, and showed that band-edge optical properties are strongly influenced by the native optically active defects. Harchol et al [8] recently explored the luminescence mechanism, further indicating that this model also involves recombination of a self-trapped exciton localized around the copper site. Meanwhile, multi-state relaxation pathway, [9] and donor-acceptor pair emission [10] have also been proposed.…”