A multidisciplinary approach for the production and characterization of colloidal quantum dots, which show great promise for implementation in modern optoelectronic applications, is described. The approach includes the design and formation of unique core/shell structures with alloy-composed layers between the core and the shell. Such structures eliminate interfacial defects and suppress the Auger process, thus reducing the known fluorescence blinking and endowing the quantum dots with robust chemical and spectral stability. The unique design enables the generation and sustained existence of single and multiple excitons with a defined spin-polarized emission recombination. The studies described herein implement the use of single-dot magneto-optical measurements and optically detected magnetic resonance spectroscopy, for direct identification of interfacial defects and for resolving exciton fine structure. The results are of paramount importance for a fundamental understanding of optical transitions in colloidal quantum dots, with an impact on appropriate materials design for practical applications.

The incorporation of magnetic impurities into semiconductor nanocrystals with size confinement promotes enhanced spin exchange interaction between photogenerated carriers and the guest spins. This interaction stimulates new magnetooptical properties with significant advantages for emerging spin-based technologies. Here we observe and elaborate on carrier−guest interactions in magnetically doped colloidal nanoplatelets with the chemical formula CdSe/Cd 1−x Mn x S, explored by optically detected magnetic resonance and magneto-photoluminescence spectroscopy. The host matrix, with a quasi-type II electronic configuration, introduces a dominant interaction between a photogenerated electron and a magnetic dopant. Furthermore, the data convincingly presents the interaction between an electron and nuclear spins of the doped ions located at neighboring surroundings, with consequent influence on the carrier's spin relaxation time. The nuclear spin contribution by the magnetic dopants in colloidal nanoplatelets is considered here for the first time.

Controlling the spin degrees of freedom of photogenerated species in semiconductor nanostructures via magnetic doping is an emerging scientific field that may play an important role in the development of new spinbased technologies. The current work explores spin properties in colloidal CdSe/ CdS:Mn seeded-nanorod structures doped with a dilute concentration of Mn 2+ ions across the rods. The spin properties were determined using continuous-wave optically detected magnetic resonance (ODMR) spectroscopy recorded under variable microwave chopping frequencies. These experiments enabled the deconvolution of a few different radiative recombination processes: band-toband, trap-to-band, and trap-to-trap emission. The results uncovered the major role of carrier trapping on the spin properties of elongated structures. The magnetic parameters, determined through spin-Hamiltonian simulation of the steady-state ODMR spectra, reflect anisotropy associated with carrier trapping at the seed/rod interface. These observations unveiled changes in the carriers' gfactors and spin-exchange coupling constants as well as extension of radiative and spin−lattice relaxation times due to magnetic coupling between interface carriers and neighboring Mn 2+ ions. Overall, this work highlights that the spin degrees of freedom in seeded nanorods are governed by interfacial trapping and can be further manipulated by magnetic doping. These results provide insights into anisotropic nanostructure spin properties relevant to future spin-based technologies.

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.

Magnetic doping in halide perovskite semiconductors is of timely interest in the pursuit of new optical and magnetic properties that surpass those of the existing undoped materials. Here, we report a thorough investigation of the optical and magneto-optical properties of Ni 2+ -doped cesium lead halide perovskite with a chemical formula CsPb(Br 1−x Cl x ) 3 , implementing steady-state and transient photoluminescence (PL), polarized magneto-PL, and optically detected magnetic resonance (ODMR) spectroscopies. The magneto-PL measurements revealed three PL features with different degrees of circular polarization, associated with recombination from band-edge and trapping states. The ODMR measurements probed magnetic resonance transitions of photogenerated electrons and holes with phenomenological g-factors that deviate from those of band-edge states. Simulations of the ODMR spectra suggested carriers' trapping in shallow traps with a slight anisotropic surrounding and with weak electron−hole exchange coupling. Furthermore, we observed substantial broadening of the hole resonance, due to its spin-exchange coupling with the Ni 2+ unpaired spins. Overall, these ODMR measurements uncovered the role of the dopant in localizing photogenerated carriers by stiffening (becoming more rigid by decreasing the structural dynamics) the crystal structure and, for the first time, provide a direct observation of carrier-dopant spin exchange interactions in metal-halide perovskite nanocrystals. These results offer insight into the influence of magnetic dopants on the electronic structures of metal-halide perovskites, with a view toward emerging spin-based devices made from perovskites.