Exploiting Functional Impurities for Fast and Efficient Incorporation of Manganese into Quantum Dots

Stability of photoelectrochemical cells based on colloidal quantum dots

Solar-driven photoelectrochemical (PEC) cells, sensitized by colloidal quantum dots (QDs), are emerging as a promising approach for solar-to-fuel conversion, including hydrogen evolution and peroxide production. The high absorption coefficient and customizable size/composition/shape of QDs can effectively enhance and broaden the light absorption capabilities of the system. Additionally, QD-based heterostructures can facilitate carrier transfer, thereby enhancing the overall performance. To date, the photocurrent density of QD based photoelectrodes for water splitting has significantly surpassed that of conventional metal oxides and sulfides. However, despite recent advances in enhancing the photocurrent density of QD-based photoelectrodes, long-term operational stability remains a key challenge for their practical applications. Few studies so far have investigated in depth the stability mechanism of QD-based PEC cells alongside potential fabrication improvements. In this Review, we first discuss the dominant factors and mechanisms responsible for the deterioration of both QDs and QD-based PEC devices. Subsequently, we outline the prevalent processing techniques and effective strategies for the fabrication of durable PEC cells. Finally, future perspectives and research directions in this field are proposed.

Highly efficient carrier multiplication in inverted CdSe/HgSe quantum dots mediated by magnetic impurities

Incorporating manganese (Mn) impurities into II-VI semiconductors alters their properties through strong exchange interactions with the host material. In colloidal quantum dots (QDs), these interactions enable ultrafast bidirectional energy transfer between the magnetic impurity and the QD intrinsic states, with rates exceeding the rate of energy loss via phonon emission. This suggests that Mn-QD interactions could harness hot carrier energy before dissipation. Here, we demonstrate that by using Mn-doped CdSe/HgSe core/shell QDs, we can efficiently convert the kinetic energy of a hot exciton into an additional electron-hole pair. This spin-exchange carrier multiplication occurs through the rapid capture of a hot exciton by a Mn ion, which then undergoes spin-flip relaxation, producing two excitons near the QD band edge. Due to the inverted band structure of CdSe/HgSe QDs, where the shell has a lower bandgap than the core, both electrons and holes produced via carrier multiplication localize in the shell. This facilitates their efficient extraction, making these QDs promising for applications in electro-optical devices and photochemical reactions.

Origin of Dopant-Carrier Exchange Coupling and Excitonic Zeeman Splitting in Mn2+-Doped Lead Halide Perovskite Nanocrystals

Low-dimensional metal halide perovskites have unique optical and electrical properties that render them attractive for the design of diluted magnetic semiconductors. However, the nature of dopant-exciton exchange interactions that result in spin-polarization of host-lattice charge carriers as a basis for spintronics remains unexplored. Here, we investigate Mn2+-doped CsPbCl3 nanocrystals using magnetic circular dichroism spectroscopy and show that Mn2+ dopants induce excitonic Zeeman splitting which is strongly dependent on the nature of the band-edge structure. We demonstrate that the largest splitting corresponds to exchange interactions involving the excited state at the M-point along the spin-orbit split-off conduction band edge. This splitting gives rise to an absorption-like C-term excitonic MCD signal, with the estimated effective g-factor (geff) of ca. 70. The results of this work help resolve the assignment of absorption transitions observed for metal halide perovskite nanocrystals and allow for a design of new diluted magnetic semiconductor materials for spintronics applications.

Induced Circularly Polarized Luminescence and Exciton Fine Structure Splitting in Magnetic-Doped Chiral Perovskites

Magnetic impurity doping in semiconductors has emerged as an important strategy to endow exotic photophysical and magnetic properties. While most reported hosts are centrosymmetric semiconductors, doping magnetic ions into a noncentrosymmetric chiral semiconductor can offer additional control of photonic and spin polarization. In this work, we synthesized a Mn2+-doped chiral two-dimensional (2D) perovskite, Mn2+:(R-MPA)2PbBr4 (R-MPA+ = R-methyl phenethylammonium). We found that the optical activity of chiral 2D perovskites is enhanced with an increased concentration of Mn2+ ions. Additionally, efficient energy transfer from the chiral host to the Mn2+ dopants is observed. This energy transfer process gives rise to circularly polarized luminescence from the excited state of Mn2+ (4T1 → 6A1), exhibiting a photoluminescence quantum yield up to 24% and a dissymmetry factor of 11%. The exciton fine structures of undoped and Mn2+-doped (R-MPA)2PbBr4 are further studied through magnetic circular dichroism (MCD) spectroscopy. Our analysis shows that chiral organic cations lead to an exciton fine structure splitting energy as large as 5.0 meV, and the splitting is further increased upon Mn2+ doping. Our results reveal the strong impacts of molecular chirality and magnetic dopants on the exciton structures of halide perovskites.

Spin-exchange carrier multiplication in manganese-doped colloidal quantum dots

Carrier multiplication is a process whereby a kinetic energy of a carrier relaxes via generation of additional electron-hole pairs (excitons). This effect has been extensively studied in the context of advanced photoconversion as it could boost the yield of generated excitons. Carrier multiplication is driven by carrier-carrier interactions that lead to excitation of a valence-band electron to the conduction band. Normally, the rate of phonon-assisted relaxation exceeds that of Coulombic collisions, which limits the carrier multiplication yield. Here we show that this limitation can be overcome by exploiting not 'direct' but 'spin-exchange' Coulomb interactions in manganese-doped core/shell PbSe/CdSe quantum dots. In these structures, carrier multiplication occurs via two spin-exchange steps. First, an exciton generated in the CdSe shell is rapidly transferred to a Mn dopant. Then, the excited Mn ion undergoes spin-flip relaxation via a spin-conserving pathway, which creates two excitons in the PbSe core. Due to the extremely fast, subpicosecond timescales of spin-exchange interactions, the Mn-doped quantum dots exhibit an up-to-threefold enhancement of the multiexciton yield versus the undoped samples, which points towards the considerable potential of spin-exchange carrier multiplication in advanced photoconversion.