Mn(2+)-Doped CdSe/CdS Core/Multishell Colloidal Quantum Wells Enabling Tunable Carrier-Dopant Exchange Interactions

Observation of spin-splitting energies on sp-d exchange interactions tailored in colloidal CdSe/CdMnS core/shell nanoplatelets: an atomistic tight-binding model

Using the atomistic tight-binding model plus sp-d exchange term, the embedding of magnetic ions into CdSe/CdMnS core/shell nanoplatelets (NPLs) at different effective temperatures resulted in sp-d exchange interactions, which in turn cause modifications in electronic and magnetic characteristics. The influence of CdMnS monolayers on single-particle spectra, optical band gaps, wave function overlaps and exciton binding energies is more pronounced than that of the effective temperature. Due to the electron, hole and Zeeman splitting energies, with the growth of CdMnS shell monolayers, electron g-factor values are unchanged, but hole and exciton g-factor values are enhanced. Additionally, all g values decrease with increasing temperature, thus representing decreased magnetization of the paramagnetic system. By changing nanoplatelet architectures and temperatures, manipulation of s-d and p-d exchange interactions is accomplished. Overall, studied materials combine the merits of NPLs and magnetic ions, hence leading to alternate possibilities for active applications in spin-based devices.

Transition Metal Ions Influence the Performance of Photodetector of Two-Dimensional CdS Nanoplatelets

Transition metal-doped two-dimensional (2D) semiconductor nanoplatelets (NPLs) with atomically precise thickness have attracted much research interest due to their inherent photo-physical properties. In this work, we have synthesized 2D Cu-doped CdS NPLs, investigated the charge transfer dynamics using ultrafast transient absorption spectroscopy, and fabricated an efficient photodetector device. A large Stoke's shifted emission at ~685 nm with an average lifetime of about ~1.45 μs is observed in Cu-doped CdS NPLs. Slower bleach recovery kinetics leads to large charge carrier separation in Cu-doped NPLs which is beneficial for photodetector applications. Cu-doped NPLs-based photodetectors exhibit high photocurrent, fast response (~120 ms), ~600 times higher photoresponsivity, and ~300 times higher detectivity (~4.1×1013 Jones) than undoped CdS NPLs. These excellent properties of Cu-doped CdS NPLs make this material an efficient alternative for next-generation optoelectronic devices.

Optically detected magnetic resonance spectroscopic analyses on the role of magnetic ions in colloidal nanocrystals

Incorporating magnetic ions into semiconductor nanocrystals has emerged as a prominent research field for manipulating spin-related properties. The magnetic ions within the host semiconductor experience spin-exchange interactions with photogenerated carriers and are often involved in the recombination routes, stimulating special magneto-optical effects. The current account presents a comparative study, emphasizing the impact of engineering nanostructures and selecting magnetic ions in shaping carrier-magnetic ion interactions. Various host materials, including the II-VI group, halide perovskites, and I-III-VI2 in diverse structural configurations such as core/shell quantum dots, seeded nanorods, and nanoplatelets, incorporated with magnetic ions such as Mn2+, Ni2+, and Cu1+/2+ are highlighted. These materials have recently been investigated by us using state-of-the-art steady-state and transient optically detected magnetic resonance (ODMR) spectroscopy to explore individual spin-dynamics between the photogenerated carriers and magnetic ions and their dependence on morphology, location, crystal composition, and type of the magnetic ion. The information extracted from the analyses of the ODMR spectra in those studies exposes fundamental physical parameters, such as g-factors, exchange coupling constants, and hyperfine interactions, together providing insights into the nature of the carrier (electron, hole, dopant), its local surroundings (isotropic/anisotropic), and spin dynamics. The findings illuminate the importance of ODMR spectroscopy in advancing our understanding of the role of magnetic ions in semiconductor nanocrystals and offer valuable knowledge for designing magnetic materials intended for various spin-related technologies.

High-Frequency EPR and ENDOR Spectroscopy of Mn2+ Ions in CdSe/CdMnS Nanoplatelets

Semiconductor colloidal nanoplatelets based of CdSe have excellent optical properties. Their magneto-optical and spin-dependent properties can be greatly modified by implementing magnetic Mn²⁺ ions, using concepts well established for diluted magnetic semiconductors. A variety of magnetic resonance techniques based on high-frequency (94 GHz) electron paramagnetic resonance in continuous wave and pulsed mode were used to get detailed information on the spin structure and spin dynamics of Mn²⁺ ions in core/shell CdSe/(Cd,Mn)S nanoplatelets. We observed two sets of resonances assigned to the Mn²⁺ ions inside the shell and at the nanoplatelet surface. The surface Mn demonstrates a considerably longer spin dynamics than the inner Mn due to lower amount of surrounding Mn²⁺ ions. The interaction between surface Mn²⁺ ions and ¹H nuclei belonging to oleic acid ligands is measured by means of electron nuclear double resonance. This allowed us to estimate the distances between the Mn²⁺ ions and ¹H nuclei, which equal to 0.31 ± 0.04, 0.44 ± 0.09, and more than 0.53 nm. This study shows that the Mn²⁺ ions can serve as atomic-size probes for studying the ligand attachment to the nanoplatelet surface.

2D II-VI Semiconductor Nanoplatelets: From Material Synthesis to Optoelectronic Integration

The field of colloidal synthesis of semiconductors emerged 40 years ago and has reached a certain level of maturity thanks to the use of nanocrystals as phosphors in commercial displays. In particular, II-VI semiconductors based on cadmium, zinc, or mercury chalcogenides can now be synthesized with tailored shapes, composition by alloying, and even as nanocrystal heterostructures. Fifteen years ago, II-VI semiconductor nanoplatelets injected new ideas into this field. Indeed, despite the emergence of other promising semiconductors such as halide perovskites or 2D transition metal dichalcogenides, colloidal II-VI semiconductor nanoplatelets remain among the narrowest room-temperature emitters that can be synthesized over a wide spectral range, and they exhibit good material stability over time. Such nanoplatelets are scientifically and technologically interesting because they exhibit optical features and production advantages at the intersection of those expected from colloidal quantum dots and epitaxial quantum wells. In organic solvents, gram-scale syntheses can produce nanoparticles with the same thicknesses and optical properties without inhomogeneous broadening. In such nanoplatelets, quantum confinement is limited to one dimension, defined at the atomic scale, which allows them to be treated as quantum wells. In this review, we discuss the synthetic developments, spectroscopic properties, and applications of such nanoplatelets. Covering growth mechanisms, we explain how a thorough understanding of nanoplatelet growth has enabled the development of nanoplatelets and heterostructured nanoplatelets with multiple emission colors, spatially localized excitations, narrow emission, and high quantum yields over a wide spectral range. Moreover, nanoplatelets, with their large lateral extension and their thin short axis and low dielectric surroundings, can support one or several electron-hole pairs with large exciton binding energies. Thus, we also discuss how the relaxation processes and lifetime of the carriers and excitons are modified in nanoplatelets compared to both spherical quantum dots and epitaxial quantum wells. Finally, we explore how nanoplatelets, with their strong and narrow emission, can be considered as ideal candidates for pure-color light emitting diodes (LEDs), strong gain media for lasers, or for use in luminescent light concentrators.

Ultrafast Quenching of Excitons in the ZnxCd1-xS/ZnS Quantum Dots Doped with Mn2+ through Charge Transfer Intermediates Results in Manganese Luminescence

For the first time, a specific time-delayed peak was registered in the femtosecond transient absorption (TA) spectra of Zn_xCd_1−xS/ZnS (x~0.5) alloy quantum dots (QDs) doped with Mn²⁺, which was interpreted as the electrochromic Stark shift of the band-edge exciton. The time-delayed rise and decay kinetics of the Stark peak in the manganese-doped QDs significantly distinguish it from the kinetics of the Stark peak caused by exciton–exciton interaction in the undoped QDs. The Stark shift in the Mn²⁺-doped QDs developed at a 1 ps time delay in contrast to the instantaneous appearance of the Stark shift in the undoped QDs. Simultaneously with the development of the Stark peak in the Mn²⁺-doped QDs, stimulated emission corresponding to ⁴T₁-⁶A₁ Mn²⁺ transition was detected in the subpicosecond time domain. The time-delayed Stark peak in the Mn²⁺-doped QDs, associated with the development of an electric field in QDs, indicates the appearance of charge transfer intermediates in the process of exciton quenching by manganese ions, leading to the ultrafast Mn²⁺ excitation. The usually considered mechanism of the nonradiative energy transfer from an exciton to Mn²⁺ does not imply the development of an electric field in a QD. Femtosecond TA data were analyzed using a combination of empirical and computational methods. A kinetic scheme of charge transfer processes is proposed to explain the excitation of Mn²⁺. The kinetic scheme includes the reduction of Mn²⁺ by a 1Se electron and the subsequent oxidation of Mn¹⁺ with a hole, leading to the formation of an excited state of manganese.

Orientation of Individual Anisotropic Nanocrystals Identified by Polarization Fingerprint

The polarization of photoluminescence emitted from anisotropic nanocrystals directly reflects the symmetry of the eigenstates involved in the recombination process and can thus be considered as a characteristic feature of a nanocrystal. We performed polarization resolved magneto-photoluminescence spectroscopy on single colloidal Mn²⁺:CdSe/CdS core-shell quantum dots of wurtzite crystal symmetry. At zero magnetic field, a distinct linear polarization pattern is observed, while applying a magnetic field enforces circularly polarized emission with a characteristic saturation value below 100%. These polarization features are shown to act as a specific fingerprint of each individual nanocrystal. A model considering the orientation of the crystal c⃗ axis with respect to the optical axis and the magnetic field and taking into account the impact of magnetic doping is introduced and quantitatively explains our findings. We demonstrate that a careful analysis of the polarization state of single nanocrystal emission using the full set of Stokes parameters allows for identification of the complete three-dimensional orientation of the crystal anisotropy axis of an individual nanoobject in lab coordinates.

Light-Induced Paramagnetism in Colloidal Ag+-Doped CdSe Nanoplatelets

We describe a study of the magneto-optical properties of Ag⁺-doped CdSe colloidal nanoplatelets (NPLs) that were grown using a novel doping technique. In this work, we used magnetic circularly polarized luminescence and magnetic circular dichroism spectroscopy to study light-induced magnetism for the first time in 2D solution-processed structures doped with nominally nonmagnetic Ag⁺ impurities. The excitonic circular polarization (P_X) and the exciton Zeeman splitting (ΔE_Z) were recorded as a function of the magnetic field (B) and temperature (T). Both ΔE_Z and P_X have a Brillouin-function-like dependence on B and T, verifying the presence of paramagnetism in Ag⁺-doped CdSe NPLs. The observed light-induced magnetism is attributed to the transformation of nonmagnetic Ag⁺ ions into Ag²⁺, which have a nonzero magnetic moment. This work points to the possibility of incorporating these nanoplatelets into spintronic devices, in which light can be used to control the spin injection.

Optically detected magnetic resonance in CdSe/CdMnS nanoplatelets

Core/shell CdSe/(Cd,Mn)S colloidal nanoplatelets containing magnetic Mn2+ ions are investigated by the optically detected magnetic resonance technique, combining 60 GHz microwave excitation and photoluminescence detection. Resonant heating of the Mn spin system is observed. We identify two mechanisms of optical detection, via variation of either the photoluminescence polarization or its intensity in an external magnetic field. The spin-lattice relaxation dynamics of the Mn spin system is measured and used for evaluation of the Mn concentration. In CdSe/(Cd,Zn,Mn)S nanoplatelets the addition of Zn in the shells significantly broadens the magnetic resonance, evidencing local strain.

Insight into the Spin Properties in Undoped and Mn-Doped CdSe/CdS-Seeded Nanorods by Optically Detected Magnetic Resonance

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 spin-based technologies. The current work explores spin properties in colloidal CdSe/CdS:Mn seeded-nanorod structures doped with a dilute concentration of Mn²⁺ 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-to-band, 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' g-factors 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 Mn²⁺ 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.

Magneto-Optics of Excitons Interacting with Magnetic Ions in CdSe/CdMnS Colloidal Nanoplatelets

Excitons in diluted magnetic semiconductors represent excellent probes for studying the magnetic properties of these materials. Various magneto-optical effects, which depend sensitively on the exchange interaction of the excitons with the localized spins of the magnetic ions can be used for probing. Here, we study core/shell CdSe/(Cd,Mn)S colloidal nanoplatelets hosting diluted magnetic semiconductor layers. The inclusion of the magnetic Mn²⁺ ions is evidenced by three magneto-optical techniques using high magnetic fields up to 15 T: polarized photoluminescence, optically detected magnetic resonance, and spin-flip Raman scattering. We show that the holes in the excitons play the dominant role in exchange interaction with magnetic ions. We suggest and test an approach for evaluation of the Mn²⁺ concentration based on the spin-lattice relaxation dynamics of the Mn²⁺ spin system.

Emergence of Impurity-Doped Nanocrystal Light-Emitting Diodes

In recent years, impurity-doped nanocrystal light-emitting diodes (LEDs) have aroused both academic and industrial interest since they are highly promising to satisfy the increasing demand of display, lighting, and signaling technologies. Compared with undoped counterparts, impurity-doped nanocrystal LEDs have been demonstrated to possess many extraordinary characteristics including enhanced efficiency, increased luminance, reduced voltage, and prolonged stability. In this review, recent state-of-the-art concepts to achieve high-performance impurity-doped nanocrystal LEDs are summarized. Firstly, the fundamental concepts of impurity-doped nanocrystal LEDs are presented. Then, the strategies to enhance the performance of impurity-doped nanocrystal LEDs via both material design and device engineering are introduced. In particular, the emergence of three types of impurity-doped nanocrystal LEDs is comprehensively highlighted, namely impurity-doped colloidal quantum dot LEDs, impurity-doped perovskite LEDs, and impurity-doped colloidal quantum well LEDs. At last, the challenges and the opportunities to further improve the performance of impurity-doped nanocrystal LEDs are described.

Surface spin magnetism controls the polarized exciton emission from CdSe nanoplatelets

The surface of nominally diamagnetic colloidal CdSe nanoplatelets can demonstrate paramagnetic behaviour owing to the uncompensated spins of dangling bonds, as we reveal here by optical spectroscopy in high magnetic fields up to 15 T using the exciton spin as a probe of the surface magnetism. The strongly nonlinear magnetic field dependence of the circular polarization of the exciton emission is determined by the magnetization of the dangling-bond spins (DBSs), the exciton spin polarization as well as the spin-dependent recombination of dark excitons. The sign of the exciton-DBS exchange interaction depends on the nanoplatelet growth conditions.

Directed Exciton Magnetic Polaron Formation in a Single Colloidal Mn2+:CdSe/CdS Quantum Dot

One of the most prominent signatures of transition-metal doping in colloidal nanocrystals is the formation of charge carrier-induced magnetization of the dopant spin sublattice, called exciton magnetic polaron (EMP). Understanding the direction of EMP formation, however, is still a major obstacle. Here, we present a series of temperature-dependent photoluminescence studies on single colloidal Mn²⁺:CdSe/CdS core/shell quantum dots (QDs) performed in a vector magnetic field providing a unique insight into the interaction between individual excitons and numerous magnetic impurities. The energy of the QD emission and its full width at half-maximum are controlled by the interplay of EMP formation and statistical magnetic fluctuations, in excellent agreement with theory. Most important, we give the first direct demonstration that anisotropy effects-hypothesized for more than a decade-dominate the direction of EMP formation. Our findings reveal a pathway for directing the orientation of optically induced magnetization in colloidal nanocrystals.

Energetic hot electrons from exciton-to-hot electron upconversion in Mn-doped semiconductor nanocrystals

Generation of hot electrons and their utilization in photoinduced chemical processes have been the subjects of intense research in recent years mostly exploring hot electrons in plasmonic metal nanostructures created via decay of optically excited plasmon. Here, we present recent progress made in generation and utilization of a different type of hot electrons produced via biphotonic exciton-to-hot electron "upconversion" in Mn-doped semiconductor nanocrystals. Compared to the plasmonic hot electrons, those produced via biphotonic upconversion in Mn-doped semiconductor nanocrystals possess much higher energy, enabling more efficient long-range electron transfer across the high energy barrier. They can even be ejected above the vacuum level creating photoelectrons, which can possibly produce solvated electrons. Despite the biphotonic nature of the upconversion process, hot electrons can be generated with weak cw excitation equivalent to the concentrated solar radiation without requiring intense or high-energy photons. This perspective reviews recent work elucidating the mechanism of generating energetic hot electrons in Mn-doped semiconductor nanocrystals, detection of these hot electrons as photocurrent or photoelectron emission, and their utilization in chemical processes such as photocatalysis. New opportunities that the energetic hot electrons can open by creating solvated electrons, which can be viewed as the longer-lived and mobile version of hot electrons more useful for chemical processes, and the challenges in practical utilization of energetic hot electrons are also discussed.

Persuasive Evidence for Electron-Nuclear Coupling in Diluted Magnetic Colloidal Nanoplatelets Using Optically Detected Magnetic Resonance Spectroscopy

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 magneto-optical 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-xMn_xS, 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.

Probing the light hole/heavy hole switching with correlated magneto-optical spectroscopy and chemical analysis on a single quantum dot

A whole series of complementary studies have been performed on the same single nanowire containing a quantum dot: cathodoluminescence spectroscopy and imaging, micro-photoluminescence spectroscopy under magnetic field and as a function of temperature, and energy-dispersive x-ray spectrometry and imaging. The ZnTe nanowire was deposited on a Si₃N₄ membrane with Ti/Al patterns. The complete set of data shows that the CdTe quantum dot features the heavy-hole state as a ground state, although the compressive mismatch strain promotes a light-hole ground state as soon as the aspect ratio is larger than unity (elongated dot). A numerical calculation of the whole structure shows that the transition from the heavy-hole to the light-hole configuration is pushed toward values of the aspect ratio much larger than unity by the presence of a (Zn, Mg)Te shell, and that the effect is further enhanced by a small valence band offset between the semiconductors in the dot and around it.

Light Emitting Spin Active Electronic States in Ultra-Thin Mn Doped CdSe Layered Nanosheets

The layered nanosheets exhibit a variety of physical and optical properties originating from amalgamation of intra- and inter- layer electronic interactions, which makes them promising materials for advanced devices with varsatile controlling channels. In particular, the dilute magnetic semiconductor multilayered nanosheets have promising optical, electrical and magnetic properties that have been less explored so far. Here, the spin permissible optical properties from solvothermally grown Mn doped CdSe (thickness ~2.26 nm) multilayered nanosheets are reported on. The presence of multi-phase magnetic orderings with a sharp ferromagnetic transition at temperature ~48 K pertinent to the stabilization and co-existence of Mn²⁺ and Mn³⁺ based local phases have been observed from the (Cd,Mn)Se layered nanosheets corroborating to the x-ray absorption near edge structure, electron paramagnetic resonance, Raman scattering and magnetic measurements. The optical absorption and photoluminescence (PL) studies at room temperature affirm wide array of optical properties in the visible regime corresponding to the band edge and intriguing dopant-phase mediated spin approved transitions. The circularly polarized magneto-PL and life time analysis exhibits the spin-polarized fast radiative transitions confirming the presence of spin-active electronic states.

Ultrahigh-efficiency aqueous flat nanocrystals of CdSe/CdS@Cd1-xZnxS colloidal core/crown@alloyed-shell quantum wells

Colloidal semiconductor nanoplatelets (NPLs) are highly promising luminescent materials owing to their exceptionally narrow emission spectra. While high-efficiency NPLs in non-polar organic media can be obtained readily, NPLs in aqueous media suffer from extremely low quantum yields (QYs), which completely undermines their potential, especially in biological applications. Here, we show high-efficiency water-soluble CdSe/CdS@Cd1-xZnxS core/crown@shell NPLs formed by layer-by-layer grown and composition-tuned gradient Cd1-xZnxS shells on CdSe/CdS core/crown seeds. Such control of shell composition with monolayer precision and effective peripheral crown passivation, together with the compact capping density of short 3-mercaptopropionic acid ligands, allow for QYs reaching 90% in water, accompanied by a significantly increased photoluminescence lifetime (∼35 ns), indicating the suppression of nonradiative channels in these NPLs. We also demonstrate the controlled attachment of these NPLs without stacking at the nanoscale by taking advantage of their 2D geometry and hydrophilicity. This is a significant step in achieving controlled assemblies and overcoming the stacking process, which otherwise undermines their film formation and performance in optoelectronic applications. Moreover, we show that the parallel orientation of such NPLs achieved by the controlled attachment enables directed emission perpendicular to the surface of the NPL films, which is highly advantageous for light extraction in light-emitting platforms.

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.

Frontier challenges in doping quantum dots: synthesis and characterization

Impurity doping in semiconductor quantum dots (QDs) has numerous prospects in implementing and altering their properties and technologies. Herein, we review the state-of-the-art doping techniques arising from colloidal synthesis methods. We first discuss the advantages and challenges involved in doping; we then discuss various doping techniques, including clustering of dopants as well as expulsion out of the lattice due to self-purification. Some of these techniques have been shown to open up a new generation of robust doped semiconductor quantum dots with cluster-free doping which will be suitable for various spin-based solid-state device technologies and overcome the longstanding challenges of controlled impurity doping. Further, we discuss inhibitors such as defects, clustering and interfaces, followed by current open questions. These include pathways to obtain uniform doping in the required radial position with unprecedented control over the dopant concentration and the size of the QDs.

We discuss state-of-the-art doping strategies for colloidal quantum dots, their principle, advantages and challenges in implementing the strategies.

Cadmium Chalcogenide Nano-Heteroplatelets: Creating Advanced Nanostructured Materials by Shell Growth, Substitution, and Attachment

The current direction in the evolution of 2D semiconductor nanocrystals involves the combination of metal and semiconductor components to form new nanoengineered materials called nano-heteroplatelets. This Review covers different heterostructure architectures that can be applied to cadmium chalcogenide nanoplatelets, including variously shaped shell, metal nanoparticle decoration, and doped and alloy systems. Here, for the first time a complete classification of nano-heteroplatelet types is provided with recommended notations and a systematization of the existing knowledge and experience concerning heterostructure formation techniques, addressing the morphology, optoelectronic and magnetic properties, and novel features of different heterostructures. This Review is also devoted to possible applications of these heterostructures and of one-component nanoplatelets in multiple fields, including light-emitting devices and biological imaging.

Ultrathin One- and Two-Dimensional Colloidal Semiconductor Nanocrystals: Pushing Quantum Confinement to the Limit

Research on ultrathin nanomaterials is one of the fastest developing areas in contemporary nanoscience. The field of ultrathin one- (1D) and two-dimensional (2D) colloidal nanocrystals (NCs) is still in its infancy, but offers the prospect of production of ultrathin nanomaterials in liquid-phase at relatively low costs, with versatility in terms of composition, size, shape, and surface control. In this Perspective, the state of the art in the field is concisely outlined and critically discussed to highlight the essential concepts and challenges. We start by presenting a brief overview of the ultrathin colloidal 1D and 2D semiconductor NCs prepared to date, after which the synthesis strategies and formation mechanisms of both 1D and 2D NCs are discussed. The properties of these low-dimensional materials are then reviewed, with emphasis on the optical properties of luminescent NCs. Finally, the future prospects for the field are addressed.

Synthesis and Characterization of Ultrathin Silver Sulfide Nanoplatelets

We report the synthesis of ultrathin silver sulfide (Ag₂S) nanoplatelets (NPLs) synthesized via a one-pot method in ethylene glycol with 3-mercaptopropionic acid serving as both the sulfur precursor and the platelet ligand. The colloidally synthesized nanoplatelets are exceptionally thin, with a thickness of only 3.5 ± 0.2 Å and a 1S exciton Bohr diameter to confinement ratio of ∼12.6. The NPL growth is shown to be quantized by layer thickness using absorption and photoluminescence (PL) spectroscopy. Transmission electron microscopy, atomic force microscopy, and X-ray diffraction analyses of the NPLs show that they correspond to the (202) plane of the β-Ag₂S structure. The PL quantum yield of these NPLs is ∼30%, suggesting their potential use in biomedical imaging. Optoelectronic properties were evaluated via sensitized photocurrent spectroscopy with the resulting spectra closely matching the distinctive absorption spectral shape of the Ag₂S NPLs.

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.

Giant Excitonic Exchange Splittings at Zero Field in Single Colloidal CdSe Quantum Dots Doped with Individual Mn2+ Impurities

Replacing a single atom of a host semiconductor nanocrystal with a functional dopant can introduce completely new properties potentially valuable for "solotronic" information-processing applications. Here, we report successful doping of colloidal CdSe quantum dots with a very small number of manganese ions-down to the ultimate limit of one. Single-particle spectroscopy reveals spectral fingerprints of the spin-spin interactions between individual dopants and quantum-dot excitons. Spectrally well-resolved emission peaks are observed that can be related to the discrete spin projections of individual Mn²⁺ ions. In agreement with theoretical predictions, the exchange splittings are enhanced by more than an order of magnitude in these quantum dots compared to their epitaxial counterparts, opening a path for solotronic applications at elevated temperatures.

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 Mn(2+) 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 Mn(2+) ions in bidoped clusters couple antiferromagnetically and are magneto-optically passive. Mn(2+)-doped clusters therefore represent a system where magneto-optical functionality is caused by solitary dopants, which might be beneficial for future solotronic applications.