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Postsynthetic vapor-phase anion exchange in the ferromagnetic two-dimensional (2D) hybrid metal-halide perovskite, PEA 2 CrCl 4 (PEA + = phenethylammonium), is reported. Anion exchange using vaporous trimethylsilyl bromide (TMS-Br) is shown to drive complete conversion of solution-processed PEA 2 CrCl 4 polycrystalline thin films to PEA 2 CrBr 4 . Low-temperature magnetic circular dichroism spectroscopy indicates ferromagnetic ordering in these PEA 2 CrCl 4 and PEA 2 CrBr 4 films. Via partial anion exchange of exfoliated flakes of PEA 2 CrCl 4 single crystals, we demonstrate that it is possible to generate abrupt lateral PEA 2 CrCl 4 /PEA 2 CrBr 4 magneto-heterointerfaces. Kinetic studies reveal that lateral heterostructure formation is dictated by rapid edge-site halide exchange followed by slower intralayer bromide diffusion, and there is negligible interlayer (3D) bromide or TMS-Br diffusion. The importance of the bulky PEA + interlayer cation in suppressing 3D diffusion is highlighted by parallel anion-exchange experiments on MA 2 CrCl 4 (MA + = methylammonium), which instead show 3D exchange. Comparison of anion-exchange reactions in PEA 2 CrCl 4 , PEA 2 MnCl 4 , and PEA 2 PbCl 4 shows that 2D bromide diffusion is slowest in PEA 2 CrCl 4 , attributed to the antiferrodistortive ordering found in this composition. In addition to demonstrating both postsynthetic composition control and heterostructure formation in ferromagnetic Cr-based 2D perovskites for the first time, these results also advance our fundamental understanding of ion-exchange processes in this relatively unexplored family of 2D perovskites, broadening opportunities for investigation and control of novel spin effects in low-dimensional metal-halide perovskites.

Moiré superlattices of layered transition metal dichalcogenides are proven to host periodic electron crystals due to strong correlation effects. These electron crystals can also be intertwined with intricate magnetic phenomena. In this Letter, we present our findings on the moiré exchange effect, resulting from the modulation of local magnetic moments by electron crystals within well-aligned WSe_{2}/WS_{2} heterobilayers. Employing polarization-resolved magneto-optical spectroscopy, we unveil a high-energy excitonic resonance near one hole per moiré unit cell (v=-1), which possesses a giant g factor several times greater than the already very large g factor of the WSe_{2} A exciton in this heterostructure. Supported by continuum model calculations, these high-energy states are found to be dark excitons brightened through Umklapp scattering from the moiré mini-Brillouin zone. When the carriers form a Mott insulating state near v=-1, the Coulomb exchange between doped carriers and excitons forms an effective magnetic field with moiré periodicity. This moiré exchange effect gives rise to the observed giant g factor for the excitonic Umklapp state.

Cesium lead halide nanostructures have highly tunable optical and optoelectronic properties. Establishing precise control in forming perovskite single-crystal nanostructures is key to unlocking the full potential of these materials. However, studying the growth kinetics of colloidal cesium lead halides is challenging due to their sensitivity to light, electron beam, and environmental factors like humidity. In this study, in situ observations of CsPbBr 3 -particle dynamics were made possible through extremely low dose liquid cell transmission electron microscopy, showing that oriented attachment is the dominant pathway for the growth of single-crystal CsPbBr 3 architectures from primary nanocubes. In addition, oriented assembly and fusion of ligand-stabilized cubic CsPbBr 3 nanocrystals are promoted by electron beam irradiation or introduction of polar additives that both induce partial desorption of the original ligands and polarize the nanocube surfaces. These findings advance our understanding of cesium lead halide growth mechanisms, aiding the controlled synthesis of other perovskite nanostructures.

Ytterbium-doped all-inorganic lead-halide perovskites (Yb 3+ :CsPb(Cl 1- x Br x ) 3 ) show broadband absorption and exceptionally high near-infrared photoluminescence quantum yields, providing opportunities for solar spectral shaping to improve photovoltaic power conversion efficiencies. Here, we report that Yb 3+ :CsPb(Cl 1- x Br x ) 3 NCs also show extremely strong negative thermal quenching of the Yb 3+ luminescence, with intensities at room temperature >100 times those at 5 K for some compositions. Analysis of this temperature dependence as a function of x shows that it stems from thermally activated quantum cutting related to the temperature dependence of the spectral overlap between the PL of the perovskite (donor) and the simultaneous-pair absorption of two Yb 3+ ions (acceptor). In the Yb 3+ :CsPbBr 3 limit, this spectral overlap goes to zero at 5 K, such that only single-Yb 3+ sensitization requiring massive phonon emission occurs. At room temperature, Yb 3+ PL in this composition is enhanced ∼135-fold by thermally activated quantum cutting, highlighting the extreme efficiency of quantum cutting relative to single-Yb 3+ sensitization. These results advance the fundamental mechanistic understanding of quantum cutting in doped perovskites, with potential ramifications for solar and photonics technologies.

This paper describes coinage-metal-doped InP quantum dots (QDs) as a platform for enhanced electron transfer to molecular acceptors relative to undoped QDs. A synthetic strategy is developed to prepare doped InP/ZnSe QDs. First-principles DFT calculations show that Ag + and Cu + dopants localize photoexcited holes while leaving electrons delocalized. This charge carrier wave function modulation is leveraged to enhance electron transfer to molecular acceptors by up to an order of magnitude. Examination of photoluminescence quenching data suggests that larger electron acceptors, such as anthraquinone and methyl viologen, bind to the QD surface in two ways: by direct adsorption to the surface and by adsorption following displacement of a weakly bound surface cation-ligand complex. Reactions with larger acceptors show the greatest increases in electron transfer between doped and undoped quantum dots, while smaller acceptors show smaller enhancements. Specifically, benzoquinone shows the smallest, followed by naphthoquinone and then methyl viologen and anthraquinone. These results demonstrate the benefits of dopant-induced excited-state carrier localization on photoinduced charge transfer and highlight design principles for improved implementation of quantum dots in photoredox catalysis., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

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., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)

The layered 2D van der Waals ferromagnets CrX 3 (X = Cl, Br, I) show broad d-d photoluminescence (PL). Here we report preparation, structural characterization, and spectroscopic studies of all three CrX 3 compounds doped with the optical impurity, Yb 3+ . EXAFS measurements show very similar Cr K-edge and Yb L-edge data for each doped compound, and good fits of the latter are obtained for structures having Yb 3+ occupying substitutional octahedral sites. Yb-X bond lengths are systematically ∼0.25 Å larger than their Cr-X counterparts. 4 K PL measurements show efficient sensitization of Yb 3+ luminescence upon photoexcitation into lattice absorption bands [Cr 3+ d-d and ligand-to-metal charge-transfer (LMCT)] for all three compounds, converting their nondescript broadband d-d PL into sharp f-f emission. The PL of CrCl 3 :Yb 3+ and CrBr 3 :Yb 3+ occurs at energies typical for [YbX 6 ] 3- with these halides, with PL decay times of 0.5-1.0 ms at 4 K, but CrI 3 :Yb 3+ displays anomalously low-energy Yb 3+ emission and an unusually short PL decay time of only 8 μs at 4 K. Data analysis and angular overlap model (AOM) calculations show that Yb 3+ in CrI 3 :Yb 3+ has a lower spin-orbit splitting energy than reported for any other Yb 3+ in any other compound. We attribute these observations to exceptionally high covalency of the Yb 3+ f orbitals in CrI 3 :Yb 3+ stemming primarily from the shallow valence-shell ionization potentials of the iodide anions.

Solution-processed semiconductors are in demand for present and next-generation optoelectronic technologies ranging from displays to quantum light sources because of their scalability and ease of integration into devices with diverse form factors. One of the central requirements for semiconductors used in these applications is a narrow photoluminescence (PL) line width. Narrow emission line widths are needed to ensure both color and single-photon purity, raising the question of what design rules are needed to obtain narrow emission from semiconductors made in solution. In this review, we first examine the requirements for colloidal emitters for a variety of applications including light-emitting diodes, photodetectors, lasers, and quantum information science. Next, we will delve into the sources of spectral broadening, including "homogeneous" broadening from dynamical broadening mechanisms in single-particle spectra, heterogeneous broadening from static structural differences in ensemble spectra, and spectral diffusion. Then, we compare the current state of the art in terms of emission line width for a variety of colloidal materials including II-VI quantum dots (QDs) and nanoplatelets, III-V QDs, alloyed QDs, metal-halide perovskites including nanocrystals and 2D structures, doped nanocrystals, and, finally, as a point of comparison, organic molecules. We end with some conclusions and connections, including an outline of promising paths forward.

Semiconductor quantum dots (QDs) are efficient organic photoredox catalysts due to their high extinction coefficients and easily tunable band edge potentials. Despite the majority of the surface being covered by ligands, our understanding of the effect of the ligand shell on organic photocatalysis is limited to steric effects. We hypothesize that we can increase the activity of QD photocatalysts by designing a ligand shell with targeted electronic properties, namely, redox-mediating ligands. Herein, we functionalize our QDs with hole-mediating ferrocene (Fc) derivative ligands and perform a reaction where the slow step is hole transfer from QD to substrate. Surprisingly, we find that a hole-shuttling Fc inhibits catalysis, but confers much greater stability to the catalyst by preventing a build-up of destructive holes. We also find that dynamically bound Fc ligands can promote catalysis by surface exchange and creation of a more permeable ligand shell. Finally, we find that trapping the electron on a ligand dramatically increases the rate of reaction. These results have major implications for understanding the rate-limiting processes for charge transfer from QDs and the role of the ligand shell in modulating it., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)

A solution-phase synthesis of colloidally stable A 2 BF 6 nanocrystals is reported for the first time, focusing on A + = Cs + , NH 4 + and B 4+ = Zr 4+ . Handling hypertoxic HF is avoided by using NH 4 F and a low-boiling-point alcohol, representing the first synthesis of any A 2 BF 6 nanocrystals without HF addition. The chemical incompatability of Zr 4+ with other common fluoride sources is discussed.

Excitons, Coulomb-bound electron-hole pairs, play a crucial role in both optical excitation and correlated phenomena in solids. When excitons interact with other quasiparticles, few- and many-body excited states can appear. Here we report an interaction between exciton and charges enabled by unusual quantum confinement in two-dimensional moiré superlattices, which results in many-body ground states composed of moiré excitons and correlated electron lattices. In an H-stacked (60 o -twisted) WS 2 /WSe 2 heterobilayer, we found an interlayer moiré exciton whose hole is surrounded by its partner electron's wavefunction distributed among three adjacent moiré traps. This three-dimensional excitonic structure enables large in-plane electrical quadrupole moments in addition to the vertical dipole. Upon doping, the quadrupole facilitates the binding of interlayer moiré excitons to the charges in neighbouring moiré cells, forming intercell charged exciton complexes. Our work provides a framework for understanding and engineering emergent exciton many-body states in correlated moiré charge orders., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

The two-dimensional (2D) van der Waals ferromagnet CrI 3 has been doped with the magnetic optical impurity Yb 3+ to yield materials that display sharp multiline Yb 3+ photoluminescence (PL) controlled by the magnetism of CrI 3 . Magneto-PL shows that Yb 3+ magnetization is pinned to the magnetization of CrI 3 . An effective internal field of ∼10 T at Yb 3+ is estimated, attributed to strong in-plane Yb 3+ -Cr 3+ superexchange coupling. The anomalously low energy of Yb 3+ PL in CrI 3 reflects relatively high Yb 3+ -I - covalency, contributing to Yb 3+ -Cr 3+ superexchange coupling. The Yb 3+ PL energy and line width both reveal the effects of spontaneous zero-field CrI 3 magnetic ordering within 2D layers below T C , despite the absence of net magnetization in multilayer samples. These results illustrate the use of optical impurities as "designer defects" to introduce unique functionality to 2D magnets.

Crystallographic defects are essential to the functional properties of semiconductors, controlling everything from conductivity to optical properties and catalytic activity. In nanocrystals, too, defect engineering with extrinsic dopants has been fruitful. Although intrinsic defects like vacancies can be equally useful, synthetic strategies for controlling their generation are comparatively underdeveloped. Here, we show that intrinsic defect concentration can be tuned during the synthesis of colloidal metal oxide nanocrystals by the addition of metal salts. Although not incorporated in the nanocrystals, the metal salts dissociate at high temperatures, promoting the dissociation of carboxylate ligands from metal precursors, leading to the introduction of oxygen vacancies. For example, the concentration of oxygen vacancies can be controlled up to 9% in indium oxide nanocrystals. This method is broadly applicable as we demonstrate by generating intrinsic defects in metal oxide nanocrystals of various morphologies and compositions.

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.

Overcoming the challenges of patterning luminescent materials will unlock additive and more sustainable paths for the manufacturing of next-generation on-chip photonic devices. Electrohydrodynamic (EHD) inkjet printing is a promising method for deterministically placing emitters on these photonic devices. However, the use of this technique to pattern luminescent lead halide perovskite nanocrystals (NCs), notable for their defect tolerance and impressive optical and spin coherence properties, for integration with optoelectronic devices remains unexplored. In this work, we additively deposit nanoscale CsPbBr 3 NC features on photonic structures via EHD inkjet printing. We perform transmission electron microscopy of EHD inkjet printed NCs to demonstrate that the NCs' structural integrity is maintained throughout the printing process. Finally, NCs are deposited with sub-micrometer control on an array of parallel silicon nitride nanophotonic cavities and demonstrate cavity-emitter coupling via photoluminescence spectroscopy. These results demonstrate EHD inkjet printing as a scalable, precise method to pattern luminescent nanomaterials for photonic applications.

Many-body interactions between carriers lie at the heart of correlated physics. The ability to tune such interactions would allow the possibility to access and control complex electronic phase diagrams. Recently, two-dimensional moiré superlattices have emerged as a promising platform for quantum engineering such phenomena 1-3 . The power of the moiré system lies in the high tunability of its physical parameters by adjusting the layer twist angle 1-3 , electrical field 4-6 , moiré carrier filling 7-11 and interlayer coupling 12 . Here we report that optical excitation can highly tune the spin-spin interactions between moiré-trapped carriers, resulting in ferromagnetic order in WS 2 /WSe 2 moiré superlattices. Near the filling factor of -1/3 (that is, one hole per three moiré unit cells), as the excitation power at the exciton resonance increases, a well-developed hysteresis loop emerges in the reflective magnetic circular dichroism signal as a function of magnetic field, a hallmark of ferromagnetism. The hysteresis loop persists down to charge neutrality, and its shape evolves as the moiré superlattice is gradually filled, indicating changes of magnetic ground state properties. The observed phenomenon points to a mechanism in which itinerant photoexcited excitons mediate exchange coupling between moiré-trapped holes. This exciton-mediated interaction can be of longer range than direct coupling between moiré-trapped holes 9 , and thus magnetic order arises even in the dilute hole regime. This discovery adds a dynamic tuning knob to the rich many-body Hamiltonian of moiré quantum matter 13-19 ., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)

The emergence of next-generation spintronic and spin-photonic technologies will be aided by the development of materials showing strongly coupled magnetic, electronic, and optical properties. Through a combination of magneto-photoluminescence and magnetic circular dichroism spectroscopies we demonstrate strong magneto-optical responses from CsEuCl 3 perovskite nanocrystals and thin films in the near-UV/visible region, stemming from the f-d transitions centered at the B-site Eu 2+ cations. We show that this material undergoes a ferromagnetic phase transition at ∼3 K in both the nanocrystal and thin-film samples, resulting in complete spin alignment and indicating intrinsic ferromagnetism. We also report the observation of spin-polarized photoluminescence in the presence of a magnetic field at cryogenic temperatures, saturating with a large polarization ratio (Δ I / I = ( I L - I R )/( I L + I R )) of nearly 30% at modest magnetic fields (∼2 T). These results highlight CsEuCl 3 as an intrinsically ferromagnetic, luminescent metal-halide perovskite with potentially interesting implications for future spin-based technologies using perovskites.

We develop a framework powered by machine learning (ML) and high-throughput density functional theory (DFT) computations for the prediction and screening of functional impurities in groups IV, III-V, and II-VI zinc blende semiconductors. Elements spanning the length and breadth of the periodic table are considered as impurity atoms at the cation, anion, or interstitial sites in supercells of 34 candidate semiconductors, leading to a chemical space of approximately 12,000 points, 10% of which are used to generate a DFT dataset of charge dependent defect formation energies. Descriptors based on tabulated elemental properties, defect coordination environment, and relevant semiconductor properties are used to train ML regression models for the DFT computed neutral state formation energies and charge transition levels of impurities. Optimized kernel ridge, Gaussian process, random forest, and neural network regression models are applied to screen impurities with lower formation energy than dominant native defects in all compounds., Competing Interests: The authors declare no competing interests., (© 2022 The Authors.)

This tutorial review presents our perspective on designing organic molecules for the functionalization of inorganic nanomaterial surfaces, through the model of an "anchor-functionality" paradigm. This "anchor-functionality" paradigm is a streamlined design strategy developed from a comprehensive range of materials ( e.g. , lead halide perovskites, II-VI semiconductors, III-V semiconductors, metal oxides, diamonds, carbon dots, silicon, etc. ) and applications ( e.g. , light-emitting diodes, photovoltaics, lasers, photonic cavities, photocatalysis, fluorescence imaging, photo dynamic therapy, drug delivery, etc. ). The structure of this organic interface modifier comprises two key components: anchor groups binding to inorganic surfaces and functional groups that optimize their performance in specific applications. To help readers better understand and utilize this approach, the roles of different anchor groups and different functional groups are discussed and explained through their interactions with inorganic materials and external environments.

Spin-orbit coupling (SOC) is responsible for a range of spintronic and topological processes in condensed matter. Here, we show photonic analogs of SOCs in exciton-polaritons and their condensates in microcavities composed of birefringent lead halide perovskite single crystals. The presence of crystalline anisotropy coupled with splitting in the optical cavity of the transverse electric and transverse magnetic modes gives rise to a non-Abelian gauge field, which can be described by the Rashba-Dresselhaus Hamiltonian near the degenerate points of the two polarization modes. With increasing density, the exciton-polaritons with pseudospin textures undergo phase transitions to competing condensates with orthogonal polarizations. Unlike their pure photonic counterparts, these exciton-polaritons and condensates inherit nonlinearity from their excitonic components and may serve as quantum simulators of many-body SOC processes.

Moiré superlattices of 2D materials with a small twist angle are thought to exhibit appreciable flexoelectric effect, though unambiguous confirmation of their flexoelectricity is challenging due to artifacts associated with commonly used piezoresponse force microscopy (PFM). For example, unexpectedly small phase contrast (≈8°) between opposite flexoelectric polarizations is reported in twisted bilayer graphene (tBG), though theoretically predicted value is 180°. Here a methodology is developed to extract intrinsic moiré flexoelectricity using twisted double bilayer graphene (tDBG) as a model system, probed by lateral PFM. For small twist angle samples, it is found that a vectorial decomposition is essential to recover the small intrinsic flexoelectric response at domain walls from a large background signal. The obtained threefold symmetry of commensurate domains with significant flexoelectric response at domain walls is fully consistent with the theoretical calculations. Incommensurate domains in tDBG with relatively large twist angles can also be observed by this technique. A general strategy is provided here for unraveling intrinsic flexoelectricity in van der Waals moiré superlattices while providing insights into engineered symmetry breaking in centrosymmetric materials., (© 2021 Wiley-VCH GmbH.)

Transition metal dichalcogenide moiré bilayers with spatially periodic potentials have emerged as a highly tunable platform for studying both electronic 1-6 and excitonic 4,7-13 phenomena. The power of these systems lies in the combination of strong Coulomb interactions with the capability of controlling the charge number in a moiré potential trap. Electronically, exotic charge orders at both integer and fractional fillings have been discovered 2,5 . However, the impact of charging effects on excitons trapped in moiré potentials is poorly understood. Here, we report the observation of moiré trions and their doping-dependent photoluminescence polarization in H-stacked MoSe 2 /WSe 2 heterobilayers. We find that as moiré traps are filled with either electrons or holes, new sets of interlayer exciton photoluminescence peaks with narrow linewidths emerge about 7 meV below the energy of the neutral moiré excitons. Circularly polarized photoluminescence reveals switching from co-circular to cross-circular polarizations as moiré excitons go from being negatively charged and neutral to positively charged. This switching results from the competition between valley-flip and spin-flip energy relaxation pathways of photo-excited electrons during interlayer trion formation. Our results offer a starting point for engineering both bosonic and fermionic many-body effects based on moiré excitons 14 ., (© 2021. The Author(s), under exclusive licence to Springer Nature Limited.)

Carrier-doped semiconductor nanocrystals (NCs) offer strong plasmonic responses at frequencies beyond those accessible by conventional plasmonic nanoparticles. Like their noble metal analogues, these emerging materials can harness free space radiation and confine it to the nanoscale but at resonance frequencies that are natively infrared and spectrally tunable by carrier concentration. In this work we combine monochromated STEM-EELS and theoretical modeling to investigate the capability of colloidal indium tin oxide (ITO) NC pairs to form hybridized plasmon modes, providing an additional route to influence the IR plasmon spectrum. These results demonstrate that ITO NCs may have greater coupling strength than expected, emphasizing their potential for near-field enhancement and resonant energy transfer in the IR region.

Degenerately doped metal-oxide nanocrystals (NCs) show localized surface plasmon resonances (LSPRs) that are tunable via their tunable excess charge-carrier densities. Modulation of excess charge carriers has also been used to control magnetism in colloidal doped metal-oxide NCs. The addition of excess delocalized conduction-band (CB) electrons can be achieved through aliovalent doping or by postsynthetic techniques such as electrochemistry or photodoping. Here, we examine the influence of charge-compensating aliovalent dopants on the potentials of excess CB electrons in free-standing colloidal degenerately doped oxide NCs, both experimentally and through modeling. Taking Sn 4+ :In 2 O 3 (ITO) NCs as a model system, we use spectroelectrochemical techniques to examine differences between aliovalent doping and photodoping. We demonstrate that whereas photodoping introduces excess CB electrons by raising the Fermi level relative to the CB edge, aliovalent impurity substitution introduces excess CB electrons by stabilizing the CB edge relative to an externally defined Fermi level. Significant differences are thus observed electrochemically between spectroscopically similar delocalized CB electrons compensated by aliovalent dopants and those compensated by surface cations (e.g., protons) during photodoping. Theoretical modeling illustrates the very different potentials that arise from charge compensation via aliovalent substitution and surface charge compensation. Spectroelectrochemical titrations allow the ITO NC band-edge stabilization as a function of Sn 4+ doping to be quantified. Extremely large capacitances are observed in both In 2 O 3 and ITO NCs, making these NCs attractive for reversible charge-storage applications.

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 Mn 2+ :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.

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Two-dimensional (2D) semiconductors enable the investigation of light-matter interactions in low dimensions 1,2 . Yet, the study of elementary photoexcitations in 2D semiconductors with intrinsic magnetic order remains a challenge due to the lack of suitable materials 3,4 . Here, we report the observation of excitons coupled to zigzag antiferromagnetic order in the layered antiferromagnetic insulator NiPS 3 . The exciton exhibits a narrow photoluminescence linewidth of roughly 350 μeV with near-unity linear polarization. When we reduce the sample thickness from five to two layers, the photoluminescence is suppressed and eventually vanishes for the monolayer. This suppression is consistent with the calculated bandgap of NiPS 3 , which is highly indirect for both the bilayer and the monolayer 5 . Furthermore, we observe strong linear dichroism (LD) over a broad spectral range. The optical anisotropy axes of LD and of photoluminescence are locked to the zigzag direction. Furthermore, their temperature dependence is reminiscent of the in-plane magnetic susceptibility anisotropy. Hence, our results indicate that LD and photoluminescence could probe the symmetry breaking magnetic order parameter of 2D magnetic materials. In addition, we observe over ten exciton-A 1g -phonon bound states on the high-energy side of the exciton resonance, which we interpret as signs of a strong modulation of the ligand-to-metal charge-transfer energy by electron-lattice interactions. Our work establishes NiPS 3 as a 2D platform for exploring magneto-exciton physics with strong correlations.

Metal chalcogenide magic-sized nanoclusters have shown intriguing photophysical and chemical properties, yet ambient instability has hampered their extensive applications. Here we explore the periodic assembly of these nanoscale building blocks through organic linkers to overcome such limitations and further boost their properties. We designed a diamine-based heat-up self-assembly process to assemble Mn 2+ :(CdSe) 13 and Mn 2+ :(ZnSe) 13 magic-sized nanoclusters into three- and two-dimensional suprastructures, respectively, obtaining enhanced stability and solid-state photoluminescence quantum yields (from <1% for monoamine-based systems to ~72% for diamine-based suprastructures). We also exploited the atomic-level miscibility of Cd and Zn to synthesize Mn 2+ :(Cd 1-x Zn x Se) 13 alloy suprastructures with tunable metal synergy: Mn 2+ :(Cd 0.5 Zn 0.5 Se) 13 suprastructures demonstrated high catalytic activity (turnover number, 17,964 per cluster in 6 h; turnover frequency, 2,994 per cluster per hour) for converting CO 2 to organic cyclic carbonates under mild reaction conditions. The enhanced stability, photoluminescence and catalytic activity through combined cluster-assembly and metal synergy advance the usability of inorganic semiconductor nanoclusters.

Carrier spins in semiconductor nanocrystals are promising candidates for quantum information processing. Using a combination of time-resolved Faraday rotation and photoluminescence spectroscopies, we demonstrate optical spin polarization and coherent spin precession in colloidal CsPbBr 3 nanocrystals that persists up to room temperature. By suppressing the influence of inhomogeneous hyperfine fields with a small applied magnetic field, we demonstrate inhomogeneous hole transverse spin-dephasing times ( T 2 * ) that approach the nanocrystal photoluminescence lifetime, such that nearly all emitted photons derive from coherent hole spins. Thermally activated LO phonons drive additional spin dephasing at elevated temperatures, but coherent spin precession is still observed at room temperature. These data reveal several major distinctions between spins in nanocrystalline and bulk CsPbBr 3 and open the door for using metal-halide perovskite nanocrystals in spin-based quantum technologies.

Leveraging recent advances in electron energy monochromation and aberration correction, we record the spatially resolved infrared plasmon spectrum of individual tin-doped indium oxide nanocrystals using electron energy-loss spectroscopy (EELS). Both surface and bulk plasmon responses are measured as a function of tin doping concentration from 1-10 atomic percent. These results are compared to theoretical models, which elucidate the spectral detuning of the same surface plasmon resonance feature when measured from aloof and penetrating probe geometries. We additionally demonstrate a unique approach to retrieving the fundamental dielectric parameters of individual semiconductor nanocrystals via EELS. This method, devoid from ensemble averaging, illustrates the potential for electron-beam ellipsometry measurements on materials that cannot be prepared in bulk form or as thin films.

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 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-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 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.

Using magnetic circular dichroism (MCD) spectroscopy, we demonstrate giant temperature- and field-dependent conduction-band splittings in colloidal EuS and EuSe nanocrystals.

Cyclic voltammetry is demonstrated as a useful method to model equilibrium binding between quantum dots and redox active small molecules. Specifically, the interaction of a library of ferrocene derivatives with CdSe quantum dots is examined. For the strongly interacting systems, ferrocene carboxylic acid (FcCOOH) and ferrocene hexanethiol (Fc-hexSH), the binding equilibria can be quantitatively deduced by modeling the cyclic voltammetry data. This modeling allows extraction of the diffusion coefficients, equilibrium constants associated with both the reduced and oxidized species, and forward and reverse rates associated with binding for both the reduced and oxidized species. Taken together these data give direct insight into the binding of small molecules to quantum-dot surfaces as a function of oxidation state, critical information for the design of quantum dots as photoredox catalysts and charge transfer mediators.

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 Mn 2+ :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.

We have synthesized unique colloidal nanoplatelets of the two-dimensional (2D) van der Waals ferromagnet CrI 3 and have characterized these nanoplatelets structurally, magnetically, and by magnetic circular dichroism spectroscopy. The CrI 3 nanoplatelets have lateral dimensions of ∼25 nm and thicknesses of only ∼4 nm, corresponding to just a few CrI 3 monolayers. Magnetic and magneto-optical measurements demonstrate robust 2D ferromagnetic ordering with Curie temperatures similar to bulk CrI 3 , despite their small size. These data also show magnetization steps akin to those observed in micron-sized few-layer 2D sheets associated with concerted spin-reversal of individual CrI 3 layers within few-layer van der Waals stacks. Similar data have also been obtained for CrBr 3 and anion-alloyed Cr(I 1- x Br x ) 3 nanoplatelets. These results represent the first example of lateral nanostructures of 2D van der Waals ferromagnets of any composition. The demonstration of robust ferromagnetism at nanometer lateral dimensions opens new doors for miniaturization in spintronics devices based on van der Waals ferromagnets.

Semiconducting nanocrystals have been the subject of intense research due to the ability to modulate the electronic and magnetic properties by controlling the size of the crystal, introducing dopants, and surface modification. While relatively simple models such as a particle in a sphere can work well to describe moderately sized quantum dots, this approximation becomes less accurate for very small nanocrystals that are strongly confined. In this work, we report all-electron, relativistic ab initio electronic structure calculations for a series of ZnO quantum dots in order to study the modulation of the Rashba effect. The impact and magnitude of spin-orbit coupling and crystalline anisotropy on the fine structure of the band-edge excitonic manifold are discussed.

Indium phosphide (InP) semiconductor nanocrystals (NCs) provide a promising alternative to traditional heavy-metal-based luminescent materials for lighting and display technologies, and implementation of InP NCs in consumer products is rapidly increasing. As-synthesized InP NCs typically have very low photoluminescence quantum yields (PLQY), however. Although empirical methods have led to NCs with near-unity PLQYs, a fundamental understanding of how specific synthetic and post-synthetic protocols can alter the electronic landscape of InP NCs is still lacking. Here, we have studied a series of homologous InP NCs prepared from InP clusters using a combination of room-temperature and low-temperature time-resolved spectroscopies to elucidate how specific charge-carrier trapping processes are affected when various surface modifications are performed. The data allow identification of large PLQY increases that occur specifically through elimination of surface electron traps and provide a rationale for understanding the microscopic origins of this trap suppression in terms of elimination of undercoordinated surface In 3+ ions. Despite essentially complete elimination of surface electron trapping when surface In 3+ is addressed, hole trapping still exists. This hole trapping is shown to be partially suppressed by even very thin shell growth, attributable to elimination of undercoordinated surface phosphides. We also observe signatures of bright-dark excitonic splitting in InP NCs with only submonolayer surface coverage of select additives (divalent Lewis acids or fluoride anions)-signatures that have only been previously observed in thick-shelled InP NCs. Together, these synthetic and spectroscopic results improve our understanding of relationships between specific InP NC surface chemistries and the resulting NC photophysics.

A series of Ag 1- x Cu x InS 2 nanocrystals (NCs) spanning from 0 ≤ x ≤ ∼1 was synthesized by partial cation exchange to identify copper's contributions to the electronic structure and spectroscopic properties of these NCs. Discrete midgap states appear above the valence band upon doping AgInS 2 NCs with Cu + (small x). Density functional theory calculations confirm that these midgap states are associated with the 3d valence orbitals of the Cu + impurities. With increasing x, these impurity d levels gradually evolve to become the valence-band edge of CuInS 2 NCs, but the highest-occupied orbital's description does not change significantly across the entire range of x. In contrast with this gradual evolution, Ag 1- x Cu x InS 2 NC photoluminescence shifts rapidly with initial additions of Cu + (small x) but then becomes independent of x beyond x > ∼0.20, all the way to CuInS 2 ( x = 1.00). Data analysis suggests small but detectable hole delocalization in the luminescent excited state of CuInS 2 NCs, estimated by Monte Carlo simulations to involve at most about four copper ions. These results provide unique insights into the luminescent excited states of these materials and they reinforce the description of CuInS 2 NCs as "heavily copper-doped NCs" in which photogenerated holes are rapidly localized in copper 3d-based orbitals.

Colloidal halide perovskite nanocrystals of CsPbCl 3 doped with Yb 3+ have demonstrated remarkably high sensitized photoluminescence quantum yields (PLQYs), approaching 200%, attributed to a picosecond quantum-cutting process in which one photon absorbed by the nanocrystal generates two photons emitted by the Yb 3+ dopants. This quantum-cutting process is thought to involve a charge-neutral defect cluster within the nanocrystal's internal volume. We demonstrate that Yb 3+ -doped CsPbCl 3 nanocrystals can be converted postsynthetically to Yb 3+ -doped CsPb(Cl 1- x Br x ) 3 nanocrystals without compromising the desired high PLQYs. Nanocrystal energy gaps can be tuned continuously from E g ≈ 3.06 eV (405 nm) in CsPbCl 3 down to E g ≈ 2.53 eV (∼490 nm) in CsPb(Cl 0.25 Br 0.75 ) 3 while retaining a constant PLQY above 100%. Reducing E g further causes a rapid drop in PLQY, interpreted as reflecting an energy threshold for quantum cutting at approximately twice the energy of the Yb 3+ 2 F 7/2 → 2 F 5/2 absorption threshold. These data demonstrate that very high quantum-cutting energy efficiencies can be achieved in Yb 3+ -doped CsPb(Cl 1- x Br x ) 3 nanocrystals, offering the possibility to circumvent thermalization losses in conventional solar technologies. The presence of water during anion exchange is found to have a deleterious effect on the Yb 3+ PLQYs but does not affect the nanocrystal shapes or morphologies, or even reduce the excitonic PLQYs of analogous undoped CsPb(Cl 1- x Br x ) 3 nanocrystals. These results provide valuable information relevant to the development and application of these unique materials for spectral-shifting solar energy conversion technologies.

The transfer of protons and electrons is key to energy conversion and storage, from photosynthesis to fuel cells. Increased understanding and control of these processes are needed. A new anthracene-phenol-pyridine molecular triad was designed to undergo fast photoinduced multiple-site concerted proton-electron transfer (MS-CPET), with the phenol moiety transferring an electron to the photoexcited anthracene and a proton to the pyridine. Fluorescence quenching and transient absorption experiments in solutions and glasses show rapid MS-CPET (3.2 × 10 10 s -1 at 298 K). From 5.5 to 90 K, the reaction rate and kinetic isotope effect (KIE) are independent of temperature, with zero Arrhenius activation energy. From 145 to 350 K, there are only slight changes with temperature. This MS-CPET reaction thus occurs by tunneling of both the proton and electron, in different directions. Since the reaction proceeds without significant thermal activation energy, the rate constant indicates the magnitude of the electron/proton double tunneling probability.

Recent advances in the ytterbium doping of CsPbX 3 (X = Cl or Cl/Br) nanocrystals have presented exciting new opportunities for their application as downconverters in solar-energy-conversion technologies. Here, we describe a hot-injection synthesis of Yb 3+ :CsPbCl 3 nanocrystals that reproducibly yields sensitized Yb 3+ 2 F 5/2 → 2 F 7/2 luminescence with near-infrared photoluminescence quantum yields (PLQYs) well over 100% and almost no excitonic luminescence. Near-infrared PLQYs of 170% have been measured. Through a combination of synthesis, variable-temperature photoluminescence spectroscopy, and transient-absorption and time-resolved photoluminescence spectroscopies, we show that the formation of shallow Yb 3+ -induced defects play a critical role in facilitating a picosecond nonradiative energy-transfer process that de-excites the photoexcited nanocrystal and simultaneously excites two Yb 3+ dopant ions, i.e., quantum cutting. Energy transfer is very efficient at all temperatures between 5 K and room temperature but only grows more efficient as the temperature is elevated in this range. Our results provide insights into the microscopic mechanism behind the extremely efficient sensitization of Yb 3+ luminescence in CsPbX 3 nanocrystals, with ramifications for future applications of high-efficiency spectral-conversion nanomaterials in solar technologies.

We present a spectroscopic study of colloidal PbSe quantum dots (QDs) that have been photodoped to introduce excess delocalized conduction-band (CB) electrons. High-quality absorption spectra are obtained for these degenerately doped QDs with excess electron concentrations up to ∼10 20 cm -3 . At the highest doping levels, electrons have completely filled the 1S e orbitals of the CB and partially populated the higher-energy 1P e orbitals. Spectroscopic changes observed as a function of carrier concentration permit an unambiguous assignment of the second excitonic absorption maximum to 1P h -1P e transitions. At intermediate doping levels, a clear absorption feature appears between the first two excitonic maxima that is attributable to parity-forbidden 1S h,e -1P e,h excitations, which become observable because of electrostatic symmetry breaking. Redshifts of the main excitonic absorption features with increased carrier concentration are also analyzed. The Coulomb stabilization energies of both the 1S h -1S e and 1P h -1P e excitons in n-doped PbSe QDs are remarkably similar to those observed for multiexcitons with the same electron count. The origins of these redshifts are discussed.

Colloidal ZnO semiconductor nanocrystals have previously been shown to accumulate multiple delocalized conduction-band electrons under chemical, electrochemical, or photochemical reducing conditions, leading to emergent semimetallic characteristics such as quantum plasmon resonances and raising prospects for application in multielectron redox transformations. Here, we demonstrate a dramatic enhancement in the capacitance of colloidal ZnO nanocrystals through aliovalent Fe 3+ -doping. Very high areal and volumetric capacitances (33 μF cm -2 , 233 F cm -3 ) are achieved in Zn 0.99 Fe 0.01 O nanocrystals that rival those of the best supercapacitors used in commercial energy-storage devices. The redox properties of these nanocrystals are probed by potentiometric titration and optical spectroscopy. These data indicate an equilibrium between electron localization by Fe 3+ dopants and electron delocalization within the ZnO conduction band, allowing facile reversible charge storage and removal. As "soluble supercapacitors", colloidal iron-doped ZnO nanocrystals constitute a promising class of solution-processable electronic materials with large charge-storage capacity attractive for future energy-storage applications.

Copper-sulfide nanocrystals can accommodate considerable densities of delocalized valence-band holes, introducing localized surface plasmon resonances (LSPRs) attractive for infrared plasmonic applications. Chemical control over nanocrystal shape, composition, and charge-carrier densities further broadens their scope of potential properties and applications. Although a great deal of control over LSPRs in these materials has been demonstrated, structural complexities have inhibited detailed descriptions of the microscopic chemical processes that transform them from nearly intrinsic to degenerately doped semiconductors. A comprehensive understanding of these transformations will facilitate use of these materials in emerging technologies. Here, we apply spectroelectrochemical potentiometry as a quantitative in situ probe of copper-sulfide nanocrystal Fermi-level energies ( E F ) during redox reactions that switch their LSPR bands on and off. We demonstrate spectroscopically indistinguishable LSPR bands in low-chalcocite copper-sulfide nanocrystals with and without lattice cation vacancies and show that cation vacancies are much more effective than surface anions at stabilizing excess free carriers. The appearance of the LSPR band, the shift in E F , and the change in crystal structure upon nanocrystal oxidation are all fully reversible upon addition of outer-sphere reductants. These measurements further allow quantitative comparison of the coupled and stepwise oxidation/cation-vacancy-formation reactions associated with LSPRs in copper-sulfide nanocrystals, highlighting fundamental thermodynamic considerations relevant to technologies that rely on reversible or low-driving-force plasmon generation in semiconductor nanostructures.

Concerns about the toxicity and instability of lead-halide perovskites have driven a recent surge in research toward alternative lead-free perovskite materials, including lead-free double perovskites with the elpasolite structure and visible bandgaps. Synthetic approaches to this class of materials remain limited, however, and no examples of heterometallic elpasolites as nanomaterials have been reported. Here, we report the synthesis and characterization of colloidal nanocrystals of Cs 2 AgBiX 6 (X = Cl, Br) elpasolites using a hot-injection approach. We further show that postsynthetic modification through anion exchange and cation extraction can be used to convert these nanocrystals to new materials including Cs 2 AgBiI 6 , which was previously unknown experimentally. Nanocrystals of Cs 2 AgBiI 6 , synthesized via a novel anion-exchange protocol using trimethylsilyl iodide, have strong absorption throughout the visible region, confirming theoretical predictions that this material could be a promising photovoltaic absorber. The synthetic methodologies presented here are expected to be broadly generalizable. This work demonstrates that nanocrystal ion-exchange reactivity can be used to discover and develop new lead-free halide perovskite materials that may be difficult or impossible to access through direct synthesis.