Valence-Band Mixing Effects in the Upper-Excited-State Magneto-Optical Responses of Colloidal Mn²⁺-Doped CdSe Quantum Dots

We present an experimental study of the magneto-optical activity of multiple excited excitonic states of manganese-doped CdSe quantum dots chemically prepared by the diffusion doping method. Giant excitonic Zeeman splittings of each of these excited states can be extracted for a series of quantum dot sizes and are found to depend on the radial quantum number of the hole envelope function involved in each transition. As seven out of eight transitions involve the same electron energy state, 1S_e, the dominant hole character of each excitonic transition can be identified, making use of the fact that the <i>g</i>-factor of the pure heavy-hole component has a different sign compared to pure light hole or split-off components. Because the magnetic exchange interactions are sensitive to hole state mixing, the giant Zeeman splittings reported here provide clear experimental evidence of quantum-size-induced mixing among valence-band states in nanocrystals

Chemical Synthesis, Doping, and Transformation of Magic-Sized Semiconductor Alloy Nanoclusters

Nanoclusters are important prenucleation intermediates for colloidal nanocrystal synthesis. In addition, they exhibit many intriguing properties originating from their extremely small size lying between molecules and typical nanocrystals. However, synthetic control of multicomponent semiconductor nanoclusters remains a daunting goal. Here, we report on the synthesis, doping, and transformation of multielement magic-sized clusters, generating the smallest semiconductor alloys. We use Lewis acid–base reactions at room temperature to synthesize alloy clusters containing three or four types of atoms. Mass spectrometry reveals that the alloy clusters exhibit “magic-size” characteristics with chemical formula of Zn_<i>x</i>Cd_{13–<i>x</i>}Se₁₃ (<i>x</i> = 0–13) whose compositions are tunable between CdSe and ZnSe. Successful doping of these clusters creates a new class of diluted magnetic semiconductors in the extreme quantum confinement regime. Furthermore, the important role of these alloy clusters as prenucleation intermediates is demonstrated by low temperature transformation into quantum alloy nanoribbons and nanorods. Our study will facilitate the understanding of these novel diluted magnetic semiconductor nanoclusters, and offer new possibilities for the controlled synthesis of nanomaterials at the prenucleation stage, consequently producing novel multicomponent nanomaterials that are difficult to synthesize

Current-Induced Magnetic Polarons in a Colloidal Quantum-Dot Device

Electrical spin manipulation remains a central challenge for the realization of diverse spin-based information processing technologies. Motivated by the demonstration of confinement-enhanced sp–d exchange interactions in colloidal diluted magnetic semiconductor (DMS) quantum dots (QDs), such materials are considered promising candidates for future spintronic or spin-photonic applications. Despite intense research into DMS QDs, electrical control of their magnetic and magneto-optical properties remains a daunting goal. Here, we report the first demonstration of electrically induced magnetic polaron formation in any DMS, achieved by embedding Mn²⁺-doped CdSe/CdS core/shell QDs as the active layer in an electrical light-emitting device. Tracing the electroluminescence from cryogenic to room temperatures reveals an anomalous energy shift that reflects current-induced magnetization of the Mn²⁺ spin sublattice, that is, excitonic magnetic polaron formation. These electrically induced magnetic polarons exhibit an energy gain comparable to their optically excited counterparts, demonstrating that magnetic polaron formation is achievable by current injection in a solid-state device

Digital Doping in Magic-Sized CdSe Clusters

Magic-sized semiconductor clusters represent an exciting class of materials located at the boundary between quantum dots and molecules. It is expected that replacing single atoms of the host crystal with individual dopants in a one-by-one fashion can lead to unique modifications of the material properties. Here, we demonstrate the dependence of the magneto-optical response of (CdSe)₁₃ clusters on the discrete number of Mn²⁺ ion dopants. Using time-of-flight mass spectrometry, we are able to distinguish undoped, monodoped, and bidoped cluster species, allowing for an extraction of the relative amount of each species for a specific average doping concentration. A giant magneto-optical response is observed up to room temperature with clear evidence that exclusively monodoped clusters are magneto-optically active, whereas the Mn²⁺ ions in bidoped clusters couple antiferromagnetically and are magneto-optically passive. Mn²⁺-doped clusters therefore represent a system where magneto-optical functionality is caused by solitary dopants, which might be beneficial for future solotronic applications

sp–d Exchange Interactions in Wave Function Engineered Colloidal CdSe/Mn:CdS Hetero-Nanoplatelets

In two-dimensional (2D) colloidal semiconductor nanoplatelets, which are atomically flat nanocrystals, the precise control of thickness and composition on the atomic scale allows for the synthesis of heterostructures with well-defined electron and hole wave function distributions. Introducing transition metal dopants with a monolayer precision enables tailored magnetic exchange interactions between dopants and band states. Here, we use the absorption based technique of magnetic circular dichroism (MCD) to directly prove the exchange coupling of magnetic dopants with the band charge carriers in hetero-nanoplatelets with CdSe core and manganese-doped CdS shell (CdSe/Mn:CdS). We show that the strength of both the electron as well as the hole exchange interactions with the dopants can be tuned by varying the nanoplatelets architecture with monolayer accuracy. As MCD is highly sensitive for excitonic resonances, excited level spectroscopy allows us to resolve and identify, in combination with wave function calculations, several excited state transitions including spin–orbit split-off excitonic contributions. Thus, our study not only demonstrates the possibility to expand the extraordinary physical properties of colloidal nanoplatelets toward magneto-optical functionality by transition metal doping but also provides an insight into the excited state electronic structure in this novel two-dimensional material

Photogating through Unidirectional Charge Carrier Funneling in Two-Dimensional Transition Metal Dichalcogenide/Perovskite Heterostructure Photodetectors

Two-dimensional (2D) van der Waals (vdW) semiconductors such as transition metal dichalcogenides (TMDCs) or 2D halide perovskites receive increasing attention as active materials in photosensing applications due to their high oscillator strength, large electronic mobility, and mechanical flexibility. For triggering an efficient separation of optically generated charge carriers and hence improving the photodetectivity, different vdW semiconductors are combined into functional heterostructures, i.e., TMDCs and 2D Ruddlesden–Popper perovskite. However, despite their increasing usage in devices, energy and charge carrier transfer between TMDCs and 2D Ruddlesden–Popper materials is still controversially discussed, and the underlying mechanisms of device operation are ambiguous. Here, in molybdenum disulfide/butylammonium lead iodide (MoS2/BA2PbI4) heterostructures, we demonstrate a unidirectional hole transfer from MoS2 to BA2PbI4 and an electron blocking through the butylammonium ions. MoS2/BA2PbI4 photodetectors show drastically improved responsivities, reduced dark current, and an increased detectivity compared to MoS2- and BA2PbI4-only devices. We provide evidence that this improvement is related to a gain mechanism due to photogating in the MoS2 channel caused by the unidirectional hole transfer between MoS2 and the BA2PbI4