A Selective Cation Exchange Strategy for the Synthesis of Colloidal Yb3+-Doped Chalcogenide Nanocrystals with Strong Broadband Visible Absorption and Long-Lived Near-Infrared Emission

Kinetically controlled morphology and composition of colloidal nanoparticles: cation exchange reactions from copper sulfide to transition metal (Mn, Zn, Fe, and Co) sulfides

The cation exchange reaction is a powerful method for generating nanomaterials with unique structures because of the easy control of the size, morphology, composition, and crystal structure of the nanoparticles. This study investigated the kinetically controlled morphology and composition of colloidal nanoparticles (NPs) through cation exchange reactions, specifically focusing on variations from copper sulfide to transition metal sulfides, including Co, Fe, Zn, and Mn sulfides. In the cation exchange reaction, Co exhibited the fastest exchange rate, followed by Fe, Mn, and Zn. The difference in kinetics rates affected the change in morphology; Co, with the fastest rate, was immediately and uniformly distributed in the NPs. For Fe, a sandwich structure was initially formed and this gradually transformed into a solid-solution phase. After exchanging Cu with Mn and Zn, a heterostructure was formed, which became increasingly clear as the reaction progressed. The transformation of the morphology and crystal structure were confirmed using XRD, TEM, and SEM analyses. The findings of this study suggest that the morphology and distinct structures of the exchanged particles can be controlled by manipulating the kinetics rates of cations through cation exchange reactions. This process offers a powerful tool for the tailored synthesis of colloidal nanoparticles and provides a design principle for enabling predictable outcomes through cation exchange reactions.

An electrochemiluminescence sensor based on lanthanide bimetallic MOFs with a "cascade sensitization mechanism" for the sensitive detection of CA242

Lanthanide metal-organic frameworks (Ln-MOFs) broaden the optical sensing applications of lanthanide ions due to the antenna effect between organic ligands and metals. However, the sensitization ability of the ligand to metal ions is limited, and maximizing the sensitization of the electrochemiluminescence behavior of Eu3+ is still a challenge for the application of Ln-MOFs. Therefore, under the guidance of the "cascade sensitization mechanism" based on the antenna effect sensitizing the electrochemiluminescence of bimetallic Ln-MOFs, we proposed Eu/Tb-MOFs with high luminescence intensity as a signal probe. According to the antenna effect, the conjugated structure and high extinction coefficient of the benzene ring of 2-amino terephthalic acid (NH2-BDC) can enhance the ECL luminescence intensity of Eu/Tb-MOFs. Tb3+ can act as an energy bridge between NH2-BDC and Eu3+, buffering the energy gap. The bimetallic sensitization is formed between Tb3+ and Eu3+, which can inhibit the reverse internal flow of energy and ensure the high luminous efficiency of Eu3+. In addition, the nanosphere mixed valence Fe3O4 as a co-reactant accelerator promotes the formation of transient free radical SO4•- through the valence change of Fe2+/Fe3+. The ECL immunosensor constructed by luminophores Eu/Tb-MOFs and nanosphere Fe3O4 provided a new explanation for the ECL self-luminous of Eu/Tb-MOFs.

Synthesis and Modulation of Low-Dimensional Transition Metal Chalcogenide Materials via Atomic Substitution

In recent years, low-dimensional transition metal chalcogenide (TMC) materials have garnered growing research attention due to their superior electronic, optical, and catalytic properties compared to their bulk counterparts. The controllable synthesis and manipulation of these materials are crucial for tailoring their properties and unlocking their full potential in various applications. In this context, the atomic substitution method has emerged as a favorable approach. It involves the replacement of specific atoms within TMC structures with other elements and possesses the capability to regulate the compositions finely, crystal structures, and inherent properties of the resulting materials. In this review, we present a comprehensive overview on various strategies of atomic substitution employed in the synthesis of zero-dimensional, one-dimensional and two-dimensional TMC materials. The effects of substituting elements, substitution ratios, and substitution positions on the structures and morphologies of resulting material are discussed. The enhanced electrocatalytic performance and photovoltaic properties of the obtained materials are also provided, emphasizing the role of atomic substitution in achieving these advancements. Finally, challenges and future prospects in the field of atomic substitution for fabricating low-dimensional TMC materials are summarized.

Shedding Light on Host-to-Yb3+ Energy Transfer in Cs2AgBiBr6:Yb3+ (nano)crystals

The optical properties of Cs2AgBiBr6 double perovskite nanocrystals have attracted considerable attention as lead-free alternatives to lead halide perovskites. A promising strategy to create additional flexibility in the emission color is doping lanthanide ions into Cs2AgBiBr6. Incorporating Yb3+ in the lattice has been shown to give rise to near-infrared (NIR) emission, but the energy transfer mechanism remained unclear. Here, we report on the luminescence and sensitization mechanism of Yb3+ in Cs2AgBiBr6 nano- and microcrystals. We observe that the incorporation of Yb3+ in the host lattice does not strongly affect the broadband red emission of the Cs2AgBiBr6 host but does give rise to an additional and characteristic ∼1000 nm NIR line emission from Yb3+. Temperature-dependent and time-resolved photoluminescence studies of undoped and Yb-doped Cs2AgBiBr6 reveal that the energy transfer does not take place through the red emissive state of the Cs2AgBiBr6 host. Instead, there is a competition between relaxation to the red-emitting state and trapping of the photoexcited charge carriers on Yb3+. Trapping on Yb3+ subsequently results in a charge transfer state that relaxes to the 2F5/2 excited state of Yb3+, followed by NIR narrow line f-f emission to the 2F7/2 ground state.

Anion Exchange in Semiconductor Magic-Size Clusters

Ion exchange is an effective postsynthesis strategy for the design of colloidal nanomaterials with unique structures and properties. In contrast to the rapid development of cation exchange (CE), the study of anion exchange is still in its infancy and requires an in-depth understanding. Magic-size clusters (MSCs) are important reaction intermediates in quantum dot (QD) synthesis, and studying the ion exchange processes can provide valuable insights into the transformations of QDs. Here, we achieved anion exchange in Cd-based MSCs and elucidated the reaction pathways. We demonstrated that the anion exchange was a stepwise intermolecular transition mediated by covalent inorganic complexes (CICs). We proposed that this transition involved three essential steps: the disassembly of CdE1-MSCs into CdE1-CICs (step 1), an anion exchange reaction from CdE1-CICs to CdE2-CICs (step 2), and assembly of CdE2-CICs to CdE2-MSCs (step 3). Step 3 was the rate-determining step and followed first-order reaction kinetics (kobs = 0.01 min-1; from CdSe-MSCs to CdS-MSCs). Further studies revealed that the activity of foreign anions only affected the reaction kinetics without altering the reaction pathway. The present study provides a deeper insight into the anion exchange mechanisms of MSCs and will further shed light on the synthesis of QDs.

Controlled Sequential Doping of Metal Nanocluster

Atomically precise doping of metal nanoclusters provides excellent opportunities not only for subtly tailoring their properties but also for in-depth understanding of composition (structure)-property correlation of metal nanoclusters and has attracted increasing interest partly due to its significance for fundamental research and practical applications. Although single and multiple metal atom doping of metal nanoclusters (NCs) has been achieved, sequential single-to-multiple metal atom doping is still a big challenge and has not yet been reported. Herein, by introducing a second ligand, a novel multistep synthesis method was developed, controlled sequential single-to-multiple metal atom doping was successfully achieved for the first time, and three doped NCs Au25Cd1(p-MBT)17(PPh3)2, Au18Cd2(p-MBT)14(PPh3)2, and [Au19Cd3(p-MBT)18]- (p-MBTH: para-methylbenzenethiol) were obtained, including two novel NCs that were precisely characterized via mass spectrometry, single-crystal X-ray crystallography, and so forth. Furthermore, sequential doping-induced evolutions in the atomic and crystallographic structures and optical and catalytic properties of NCs were revealed.

Ultra-wideband-responsive photon conversion through co-sensitization in lanthanide nanocrystals

Distinctive upconversion or downshifting of lanthanide nanocrystals holds promise for biomedical and photonic applications. However, either process requires high-energy lasers at discrete wavelengths for excitation. Here we demonstrate that co-sensitization can break this limitation with ultrawide excitation bands. We achieve co-sensitization by employing Nd³⁺ and Ho³⁺ as the co-sensitizers with complementary absorptions from the ultraviolet to infrared region. Symmetric penta-layer core-shell nanostructure enables tunable fluorescence in the visible and the second near-infrared window when incorporating different activators (Er³⁺, Ho³⁺, Pr³⁺, and Tm³⁺). Transient spectra confirm the directional energy transfer from sensitizers to activators through the bridge of Yb³⁺. We validate the features of the nanocrystals for low-powered white light-emitting diode-mediated whole-body angiography of mice with a signal-to-noise ratio of 12.3 and excitation-regulated encryption. This co-sensitization strategy paves a new way in lanthanide nanocrystals for multidirectional photon conversion manipulation and excitation-bandwidth-regulated fluorescence applications.

Cesium Manganese Bromide Nanocrystal Sensitizers for Broadband Vis-to-NIR Downshifting

Simultaneously achieving both broad absorption and sharp emission in the near-infrared (NIR) is challenging. Coupling of an efficient absorber such as lead halide perovskites to lanthanide emissive species is a promising way to meet the demands for visible-to-NIR spectral conversion. However, lead-based perovskite sensitizers suffer from relatively narrow absorption in the visible range, poor stability, and toxicity. Herein, we introduce a downshifting configuration based on lead-free cesium manganese bromide nanocrystals acting as broad visible absorbers coupled to sharp emission in the NIR-I and NIR-II spectral regions. To achieve this, we synthesized CsMnBr₃ and Cs₃MnBr₅ nanocrystals and attempted to dope them with a series of lanthanides, achieving success only with CsMnBr₃. The correlation of the lanthanide emission to the CsMnBr₃ visible absorption was confirmed with steady-state excitation spectra and time-resolved photoluminescence measurements, whereas the mechanism of downconversion from the CsMnBr₃ matrix to the lanthanides was understood by density functional theory calculations. This study shows that lead-free metal halides with an appropriate phase are effective sensitizers for lanthanides and offer a route to efficient downshifting applications.

Reversible Transformation between Cs3Cu2I5 and CsCu2I3 Perovskite Derivatives and Its Anticounterfeiting Application

Lead halide perovskites have promising values in photoelectronic and photovoltaic applications, but the toxicity of lead is a hard barrier. Copper halide perovskite derivatives (CHPDs), as a lead-free substitution of lead halide perovskites, also exhibit excellent photoelectric properties. Here, we present a facile one-step route for the synthesis of blue-emissive Cs₃Cu₂I₅ (emission at 440 nm) and yellow-emissive CsCu₂I₃ (emission at 552 nm) CHPDs in ethanol at room temperature. Triggered by ethanol or CsI, a reversible chemical transformation accompanied by emissive color change between Cs₃Cu₂I₅ and CsCu₂I₃ CHPDs was achieved. The reversible transformation mechanism was discussed, and this transformation was employed for effective anticounterfeiting.

Synthesis of Mixed-Valent Lanthanide Sulfide Nanoparticles

In targeting reduced valent lanthanide chalcogenides, we report the first nanoparticle synthesis of the mixed-valent ferromagnets Eu₃ S₄ and EuSm₂ S₄ . Using divalent lanthanide halides with bis(trimethylsilyl)sulfide and oleylamine, we prepared nanoparticles of EuS, Eu₃ S₄ , EuSm₂ S₄ , SmS_1.9 , and Sm₃ S₄ . All nanoparticle phases were identified using powder X-ray diffraction, transmission electron microscopy was used to confirm morphology and nanoparticle size, and magnetic susceptibility measurements for determining the ordering temperatures and valence. The UV/Vis, Raman and X-ray photoelectron spectroscopies for each phase were compared. Surprisingly, the phase is influenced by the halide and the reaction temperature, where EuCl₂ formed EuS while EuI₂ formed Eu₃ S₄ , highlighting the role of kinetics in phase stabilization. Interestingly, at lower temperatures EuI₂ initially forms EuS, and converts over time to Eu₃ S₄ .

In Vivo Near-Infrared Imaging Using Ternary Selenide Semiconductor Nanoparticles with an Uncommon Crystal Structure

The implementation of in vivo fluorescence imaging as a reliable diagnostic imaging modality at the clinical level is still far from reality. Plenty of work remains ahead to provide medical practitioners with solid proof of the potential advantages of this imaging technique. To do so, one of the key objectives is to better the optical performance of dedicated contrast agents, thus improving the resolution and penetration depth achievable. This direction is followed here and the use of a novel AgInSe₂ nanoparticle-based contrast agent (nanocapsule) is reported for fluorescence imaging. The use of an Ag₂ Se seeds-mediated synthesis method allows stabilizing an uncommon orthorhombic crystal structure, which endows the material with emission in the second biological window (1000-1400 nm), where deeper penetration in tissues is achieved. The nanocapsules, obtained via phospholipid-assisted encapsulation of the AgInSe₂ nanoparticles, comply with the mandatory requisites for an imaging contrast agent-colloidal stability and negligible toxicity-and show superior brightness compared with widely used Ag₂ S nanoparticles. Imaging experiments point to the great potential of the novel AgInSe₂ -based nanocapsules for high-resolution, whole-body in vivo imaging. Their extended permanence time within blood vessels make them especially suitable for prolonged imaging of the cardiovascular system.

Near-Infrared Emissive Cyanido-Bridged {YbFe2} Molecular Nanomagnets Sensitive to the Nitrile Solvents of Crystallization

One of the pathways toward luminescent single-molecule magnets (SMMs) is realized by the self-assembly of lanthanide(3+) ions with cyanido transition metal complexes. We report a novel family of emissive SMMs, {YbIII(4-pyridone)4[FeII(phen)2(CN)2]2}(CF3SO3)3·solv (solv = 2MeCN, 1·MeCN; 2AcrCN, 1·AcrCN; 2PrCN, 1·PrCN; 2MalCN·1MeOH; 1·MalCN; MeCN = acetonitrile, AcrCN = acrylonitrile, PrCN = propionitrile, MalCN = malononitrile). They are based on paramagnetic YbIII centers coordinating diamagnetic [FeII(phen)2(CN)2] metalloligands but differ in the nitrile solvents of crystallization. They exhibit a field-induced slow magnetic relaxation dominated by a Raman process, without an Orbach relaxation as indicated by AC magnetic data and the ab initio calculations. The Raman relaxation is solvent-dependent as represented by the power “n” of the BRamanTn contribution varying from 3.07(1), to 2.61(1), 2.37(1), and 1.68(4) for 1·MeCN, 1·PrCN, 1·AcrCN, and 1·MalCN, respectively, while the BRaman parameter adopts the opposite trend. This was correlated with the variation of phonon modes schemes, including the number of available vibrational modes and their energies, dependent on the increasing complexity of the applied nitrile. 1·MeCN and 1·MalCN show the additional T-independent relaxation assignable to dipole-dipole interactions as confirmed by its suppression in 1·AcrCN and 1·PrCN revealing longer Yb–Yb distances and the disappearance in the LuIII-diluted 1·MeCN@Lu. All compounds exhibit YbIII–centered near-infrared photoluminescence sensitized by organic ligands.

Rare-Earth Sulfide Nanocrystals from Wet Colloidal Synthesis: Tunable Compositions, Size-Dependent Light Absorption, and Sensitized Rare-Earth Luminescence

We report the synthesis of colloidal EuS, La₂S₃, and LaS₂ nanocrystals between 150 and 255 °C using rare-earth iodides in oleylamine. The sulfur source dictates phase selection between La₂S₃ and LaS₂, which are stabilized for the first time as colloidal nanocrystals. The indirect bandgap absorption of LaS₂ shifts from 635 nm for nanoellipsoids to 365 nm for square-based nanoplates. Er³⁺ photoluminescence in La₂S₃:Er³⁺ (10%) is sensitized by the semiconducting host in the 390-450 nm range. The synthetic route yields tunable compositions of rare-earth sulfide nanocrystals. Interaction of light with these novel semiconducting nanostructures hosting rare-earth emitters should be attractive for applications that require broadband sensitization of RE emitters.

Networking State of Ytterbium Ions Probing the Origin of Luminescence Quenching and Activation in Nanocrystals

At the organic-inorganic interface of nanocrystals, electron-phonon coupling plays an important but intricate role in determining the diverse properties of nanomaterials. Here, it is reported that highly doping of Yb3+ ions within the nanocrystal host can form an energy-migration network. The networking state Yb3+ shows both distinct Stark splitting peak ratios and lifetime dynamics, which allows quantitative investigations of quenching and thermal activation of luminescence, as the high-dimensional spectroscopy signatures can be correlated to the attaching and de-attaching status of surface molecules. By in-situ surface characterizations, it is proved that the Yb-O coordination associated with coordinated water molecules has significantly contributed to this reversible effect. Moreover, using this approach, the prime quencher -OH can be switched to -CH in the wet-chemistry annealing process, resulting in the electron-phonon coupling probability change. This study provides the molecular level insights and dynamics of the surface dark layer of luminescent nanocrystals.

Switching to the brighter lane: pathways to boost the absorption of lanthanide-doped nanoparticles

Lanthanide-doped nanoparticles (LNPs) are speedily colonizing several research fields, such as biological (multimodal) imaging, photodynamic therapy, volumetric encoding displays, and photovoltaics. Yet, the electronic transitions of lanthanide ions obey the Laporte rule, which dramatically hampers their light absorption capabilities. As a result, the brightness of these species is severely restricted. This intrinsic poor absorption capability is the fundamental obstacle for untapping the full potential of LNPs in several of the aforementioned fields. Among others, three of the most promising physicochemical approaches that have arisen during last two decades to face the challenges of increasing LNP absorption are plasmonic enhancement, organic-dye sensitization, and coupling with semiconductors. The fundamental basis, remarkable highlights, and comparative achievements of each of these pathways for absorption enhancement are critically discussed in this minireview, which also includes a detailed discussion of the exciting perspectives ahead.

Quo Vadis, Nanoparticle-Enabled In Vivo Fluorescence Imaging?

The exciting advancements that we are currently witnessing in terms of novel materials and synthesis approaches are leading to the development of colloidal nanoparticles (NPs) with increasingly greater tunable properties. We have now reached a point where it is possible to synthesize colloidal NPs with functionalities tailored to specific societal demands. The impact of this new wave of colloidal NPs has been especially important in the field of biomedicine. In that vein, luminescent NPs with improved brightness and near-infrared working capabilities have turned out to be optimal optical probes that are capable of fast and high-resolution in vivo imaging. However, luminescent NPs have thus far only reached a limited portion of their potential. Although we believe that the best is yet to come, the future might not be as bright as some of us think (and have hoped!). In particular, translation of NP-based fluorescence imaging from preclinical studies to clinics is not straightforward. In this Perspective, we provide a critical assessment and highlight promising research avenues based on the latest advances in the fields of luminescent NPs and imaging technologies. The disillusioned outlook we proffer herein might sound pessimistic at first, but we consider it necessary to avoid pursuing "pipe dreams" and redirect the efforts toward achievable-yet ambitious-goals.

Recent Advances of Near-Infrared (NIR) Emissive Metal Complexes Bridged by Ligands with N- and/or O-Donor Sites

Near-infrared (NIR) emissive metal complexes have shown potential applications in optical communication, chemosensors, bioimaging, and laser and organic light-emitting diodes (OLEDs) due to their structural tunability and luminescence stability. Among them, complexes with bridging ligands that exhibit unique emission behavior have attracted extensive interests in recent years. The target performance can be easily achieved by NIR light-emitting metal complexes with bridging ligands through molecular structure design. In this review, the luminescence mechanism and design strategies of NIR luminescent metal complexes with bridging ligands are described firstly, and then summarize the recent advance of NIR luminescent metal complexes with bridging ligands in the fields of electroluminescence and biosensing/bioimaging. Finally, the development trend of NIR luminescent metal complexes with bridging ligands are proposed, which shows an attractive prospect in the field of photophysical and photochemical materials.

Manganese-Mediated Growth of ZnS Shell on KMnF3:Yb,Er Cores toward Enhanced Up/Downconversion Luminescence

Epitaxially growing a semiconductor shell on the surface of upconversion nanocrystals to form a core/shell structure is believed to be a promising strategy to improve the luminescent efficiency of lanthanide ions doped in particle cores and, meanwhile, enriches the optical properties of the resulting nanocrystals. However, liquid-phase synthesis of such core/shell-structured nanocrystals comprised of a lanthanide ion-doped core and semiconductor shell remains challenging because of the chemical incompatibilities between lanthanides and the most intermediate gap semiconductors. In this context, the successful growth of ZnS shell on a KMnF₃ core codoped with Yb³⁺/Er³⁺ ions is reported to enhance the upconversion luminescence of Er³⁺ ions. The underlying core/shell formation mechanism is elucidated in detail combining the hard-soft acid-base theory with structural analysis of the resulting nanocrystals. Quite unexpectedly, Mn²⁺ diffusion across the core/shell interface occurs during ZnS shell growth, giving rise to Mn²⁺ emission from the ZnS shell. Thus, the resulting core/shell particles exhibited unique up/downconversion luminescence from doped lanthanide metal ions and transition-metal ions, respectively. By manipulating the ion diffusion and shell growth kinetics, the upconversion and downconversion luminescent performance of KMnF₃:Yb,Er@ZnS nanocrystals are further optimized and the related mechanisms are discussed. Further, temperature-dependent upconversion and downconversion photoluminescence properties of KMnF₃:Yb,Er@ZnS nanocrystals show potential for ratiometric luminescence temperature sensing.

Heating-up synthesis of cesium bismuth bromide perovskite nanocrystals with tailored composition, morphology, and optical properties

This study represents the heating-up synthesis of lead-free cesium bismuth bromide perovskite nanocrystals (NCs). CsBr and BiBr3 precursors are used to synthesize uniform and phase-pure cesium bismuth bromide NCs, and the reaction is performed via an injection-free, heating-up method in the presence of a solvent mixture with a high boiling point. The size and composition of cesium bismuth bromide NCs are readily controlled by changing the reaction time, temperature, and amount of surfactant added to the reaction mixture. Upon heating, sequential phase evolution occurs, resulting in the formation of kinetically stable BiOBr in the early reaction stages, which transformed into the thermodynamically stable Cs3BiBr6 and Cs3Bi2Br9 with an increase in either the reaction time or the reaction temperature. Furthermore, the absorption and photoluminescence properties of Cs3BiBr6 and Cs3Bi2Br9 NCs are characterized to investigate their composition-dependent optical properties. This work provides the potential to synthesize various types of lead-free perovskite NCs by tailoring the size and compositions.

Sensitized Yb3+ Luminescence in CsPbCl3 Film for Highly Efficient Near-Infrared Light-Emitting Diodes

Near-infrared (NIR) light emitting diodes (LEDs) with the emission wavelength over 900 nm are useful in a wide range of optical applications. Narrow bandgap NIR emitters have been widely investigated using organic compounds and colloidal quantum dots. However, intrinsically low charge mobility and luminescence efficiency of these materials limit improvement of the external quantum efficiency (EQE) of NIR LEDs, which is far from practical applications. Herein, a highly efficient NIR LED is demonstrated, which is based on an energy transfer from wide bandgap all inorganic perovskite (CsPbCl3) to ytterbium ions (Yb3+) as an NIR emitter doped in the perovskite crystalline film. High mobility of electrically excited carriers in the perovskite crystalline film provides a long carrier diffusion and enhances radiative recombination of an emission center due to minimized charge trapping losses, resulting in high EQE value in LEDs. The NIR emission of Yb3+ at around 1000 nm is found to be sensitized by CsPbCl3 thin film with a photoluminescence quantum yield over 60%. The LED based on Yb3+-doped CsPbCl3 film exhibits a high EQE of 5.9% with a peak wavelength of 984 nm, achieved by high carrier transporting ability and effective sensitized emission property in the solid-film structure.

Modulation of lanthanide luminescence via an electric field

The modulation of luminescence via external stimuli such as temperature, mechanical stress, hydrostatic pressure, as well as electric and/or magnetic fields, has witnessed great progress, enabled the disclosure of new principles and energy transfer pathways, and widened applications. However, investigations on the luminescence modulation of lanthanide ions doped in semiconductors via an applied electric field are still absent. Herein, for the first time, we have demonstrated the in situ, real-time, and reversible modulation of the luminescence of Eu³⁺ doped in SnO₂ nanocrystals by manipulating the recombination rate of photo-generated electrons and holes, and the accompanied energy transfer mode in terms of linear and quasi-sinusoidal, from semiconductor to lanthanide ions. Following the same principle, the modulation of near infrared responsive Er³⁺ in SnO₂ and the visible luminescence of perovskite nanocrystals is further realized. This study offers extra methodologies for luminescence modulation, in addition to those already reported for ferro- and/or piezoelectric-hosted luminescent materials.

Yb- and Mn-Doped Lead-Free Double Perovskite Cs2AgBiX6 (X = Cl-, Br-) Nanocrystals

Lead-free double perovskite nanocrystals (NCs) have emerged as a new category of materials that hold the potential for overcoming the instability and toxicity issues of lead-based counterparts. Doping chemistry represents a unique avenue toward tuning and optimizing the intrinsic optical and electronic properties of semiconductor materials. In this study, we report the first example of doping Yb³⁺ ions into lead-free double perovskite Cs₂AgBiX₆ (X = Cl^-, Br^-) NCs via a hot injection method. The doping of Yb³⁺ endows the double perovskite NCs with a newly emerged near-infrared emission band (sensitized from the NC hosts) in addition to their intrinsic trap-related visible photoluminescence. By controlling the Yb-doping concentration, the dual emission profiles and photon relaxation dynamics of the double perovskite NCs can be systematically tuned. Furthermore, we have successfully inserted divalent Mn²⁺ ions in Cs₂AgBiCl₆ NCs and observed emergence of dopant emission. Our work illustrates an effective and facile route toward modifying and optimizing optical properties of double perovskite Cs₂AgBiX₆ (X = Cl^-, Br^-) NCs with an indirect bandgap nature, which can broaden a range of their potential applications in optoelectronic devices.

Host sensitized lanthanide photoluminescence from post-synthetically modified semiconductor nanoparticles depends on reactant identity

This work investigates the photoluminescence characteristics where cadmium selenide (CdSe) and zinc sulfide (ZnS) nanoparticles are treated post-synthetically by the trivalent lanthanide cations (Ln³⁺) [Ln = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb] separately to form either CdSe/Ln or ZnS/Ln nanoparticles. Host sensitized Ln³⁺ emission was found to be present only in CdSe/Eu, CdSe/Tb, ZnS/Eu, ZnS/Tb and ZnS/Yb nanoparticles. In all the cases tuning of emission of the nanoparticles has been observed, irrespective of the presence or absence of host sensitization. The elemental compositions of CdSe and ZnS nanoparticles upon post-synthetic treatment show a remarkable difference. Incorporation of lanthanides in the nanoparticles is evident with significant alteration in the anionic content, and complete cation exchange of either Cd²⁺ or Zn²⁺ by Ln³⁺ has not been detected; as evaluated from energy dispersive X-ray spectroscopy. Further evaluation on this comes from considering thermodynamic parameters of inter cation interaction. In cases where the host sensitized Ln³⁺ emission have been observed, luminescence lifetime measurements reveal significant protection of Ln³⁺ in the nanoparticles. Noticeable difference in photophysical properties for a given Ln³⁺ has been realized in the two hosts. The photophysical observations have been rationalized using (i) charge trapping mediated host sensitized dopant emission, (ii) autoionization of excited electrons, and (iii) environment induced photoluminescence quenching. The post-synthetic modification discussed in the present work provides an easy and less synthetically demanding room temperature based protocol to avail lanthanide incorporated (doped) semiconductor nanoparticles that can potentially use the unique emission properties of the lanthanide cations.

Anion Exchange and the Quantum-Cutting Energy Threshold in Ytterbium-Doped CsPb(Cl1- xBr x)3 Perovskite Nanocrystals

Colloidal halide perovskite nanocrystals of CsPbCl₃ doped with Yb³⁺ 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 Yb³⁺ dopants. This quantum-cutting process is thought to involve a charge-neutral defect cluster within the nanocrystal's internal volume. We demonstrate that Yb³⁺-doped CsPbCl₃ nanocrystals can be converted postsynthetically to Yb³⁺-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 CsPbCl₃ down to E_g ≈ 2.53 eV (∼490 nm) in CsPb(Cl_0.25Br_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 Yb³⁺²F_7/2 → ²F_5/2 absorption threshold. These data demonstrate that very high quantum-cutting energy efficiencies can be achieved in Yb³⁺-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 Yb³⁺ 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.

Enhancing the upconversion luminescence properties of Er3+–Yb3+ doped yttrium molybdate through Mg2+ incorporation: effect of laser excitation power on temperature sensing and heat generation

Upconversion luminescence was enhanced by incorporating Mg²⁺ into Er³⁺–Yb³⁺-doped yttrium molybdate and the effect of laser excitation power on temperature sensing and nanoheating was investigated.

Yb3+ and Yb3+/Er3+ Doping for Near-Infrared Emission and Improved Stability of CsPbCl3 Nanocrystals

Lead halide perovskite nanocrystals (NCs) exhibit excellent tunable emissions covering the entire visible spectral region, but they do not emit near-infrared (NIR) light. We synthesized rare earth element doped CsPbCl3 NCs for NIR emission. The Yb3+ doped CsPbCl3 NCs emitted strong 986 nm NIR light; the Yb3+/Er3+ co-doped CsPbCl3 NCs emitted at 1533 nm. The total photoluminescence quantum yield (PL QY) of the CsPbCl3 NCs changed from 5.0% to 127.8% upon incorporating 2.0% Yb3+, a factor of 25.6 times enhancement. The material's stability was tested under continuous ultraviolet (365 nm) irradiation. The doped CsPbCl3 NCs exhibited a better stability than the undoped one. The PL intensity of the undoped CsPbCl3 NCs dropped to 20% of the initial value in 27 h, while the doped one took 85 h.

Picosecond Quantum Cutting Generates Photoluminescence Quantum Yields Over 100% in Ytterbium-Doped CsPbCl3 Nanocrystals

Recent advances in the ytterbium doping of CsPbX₃ (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 Yb³⁺:CsPbCl₃ nanocrystals that reproducibly yields sensitized Yb³⁺²F_5/2 → ²F_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 Yb³⁺-induced defects play a critical role in facilitating a picosecond nonradiative energy-transfer process that de-excites the photoexcited nanocrystal and simultaneously excites two Yb³⁺ 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 Yb³⁺ luminescence in CsPbX₃ nanocrystals, with ramifications for future applications of high-efficiency spectral-conversion nanomaterials in solar technologies.

Synthesis of tailor-made colloidal semiconductor heterostructures

This feature article provides an account of the various bottom-up and top-down methods that have been developed to prepare colloidal heterostructures and highlights the benefits of a seeded assembly approach for greater control and customizability.