Revealing Two Distinct Formation Pathways of 2D Wurtzite-CdSe Nanocrystals Using In Situ X-Ray Scattering.

Understanding the mechanism underlying the formation of quantum-sized semiconductor nanocrystals is crucial for controlling their synthesis for a wide array of applications. However, most studies of 2D CdSe nanocrystals have relied predominantly on ex situ analyses, obscuring key intermediate stages and raising fundamental questions regarding their lateral shapes. Herein, the formation pathways of two distinct quantum-sized 2D wurtzite-CdSe nanocrystals - nanoribbons and nanosheets - by employing a comprehensive approach, combining in situ small-angle X-ray scattering techniques with various ex situ characterization methods is studied. Although both nanostructures share the same thickness of ≈1.4 nm, they display contrasting lateral dimensions. The findings reveal the pivotal role of Se precursor reactivity in determining two distinct synthesis pathways. Specifically, highly reactive precursors promote the formation of the nanocluster-lamellar assemblies, leading to the synthesis of 2D nanoribbons with elongated shapes. In contrast, mild precursors produce nanosheets from a tiny seed of 2D nuclei, and the lateral growth is regulated by chloride ions, rather than relying on nanocluster-lamellar assemblies or Cd(halide)2 -alkylamine templates, resulting in 2D nanocrystals with relatively shorter lengths. These findings significantly advance the understanding of the growth mechanism governing quantum-sized 2D semiconductor nanocrystals and offer valuable guidelines for their rational synthesis.

Critical roles of metal-ligand complexes in the controlled synthesis of various metal nanoclusters.

Metal nanoclusters (NCs), an important class of nanoparticles (NPs), are extremely small in size and possess quasi-molecular properties. Due to accurate stoichiometry of constituent atoms and ligands, NCs have strong structure-property relationship. The synthesis of NCs is seemingly similar to that of NPs as both are formed by colloidal phase transitions. However, they are considerably different because of metal-ligand complexes in NC synthesis. Reactive ligands can convert metal salts to complexes, actual precursors to metal NCs. During the complex formation, various metal species occur, having different reactivity and fraction depending on synthetic conditions. It can alter their degree of participation in NC synthesis and the homogeneity of final products. Herein, we investigate the effects of complex formation on the entire NC synthesis. By controlling the fraction of various Au species showing different reactivity, we find that the extent of complex formation alters reduction kinetics and the uniformity of Au NCs. We demonstrate that this concept can be universally applied to synthesize Ag, Pt, Pd, and Rh NCs.

Atom-Precise Heteroatom Core-Tailoring of Nanoclusters for Enhanced Solar Hydrogen Generation.

While core-shell nanomaterials are highly desirable for realizing enhanced optical and catalytic properties, their synthesis with atomic-level control is challenging. Here, the synthesis and crystal structure of [Au12 Ag32 (SePh)30 ]4- , the first example of selenolated Au-Ag core-shell nanoclusters, comprising a gold icosahedron core trapped in a silver dodecahedron, which is protected by an Ag12 (SePh)30 shell, is presented. The gold core strongly modifies the overall electronic structure and induces synergistic effects, resulting in high enhancements in the stability and near-infrared-II photoluminescence. The Au12 Ag32 and its homometal analog Ag44 , show strong interactions with oxygen vacancies of TiO2 , facilitating the interfacial charge transfer for photocatalysis. Indeed, the Au12 Ag32 /TiO2 exhibits remarkable solar H2 production (6810 µmol g-1  h-1 ), which is ≈6.2 and ≈37.8 times higher than that of Ag44 /TiO2 and TiO2 , respectively. Good stability and recyclability with minimal catalytic activity loss are additional features of Au12 Ag32 /TiO2 . The experimental and computational results reveal that the Au12 Ag32 acts as an efficient cocatalyst by possessing a favorable electronic structure that aligns well with the TiO2 bands for the enhanced separation of photoinduced charge carriers due to the relatively negatively charged Au12 core. These atomistic insights will motivate uncovering of the structure-catalytic activity relationships of other nanoclusters.

Stretchable colour-sensitive quantum dot nanocomposites for shape-tunable multiplexed phototransistor arrays.

High-performance photodetecting materials with intrinsic stretchability and colour sensitivity are key requirements for the development of shape-tunable phototransistor arrays. Another challenge is the proper compensation of optical aberrations and noises generated by mechanical deformation and fatigue accumulation in a shape-tunable phototransistor array. Here we report rational material design and device fabrication strategies for an intrinsically stretchable, multispectral and multiplexed 5 × 5 × 3 phototransistor array. Specifically, a unique spatial distribution of size-tuned quantum dots, blended in a semiconducting polymer within an elastomeric matrix, was formed owing to surface energy mismatch, leading to highly efficient charge transfer. Such intrinsically stretchable quantum-dot-based semiconducting nanocomposites enable the shape-tunable and colour-sensitive capabilities of the phototransistor array. We use a deep neural network algorithm for compensating optical aberrations and noises, which aids the precise detection of specific colour patterns (for example, red, green and blue patterns) both under its flat state and hemispherically curved state (radius of curvature of 18.4 mm).

Structural Insights into Multi-Metal Spinel Oxide Nanoparticles for Boosting Oxygen Reduction Electrocatalysis.

Multi-metal oxide (MMO) materials have significant potential to facilitate various demanding reactions by providing additional degrees of freedom in catalyst design. However, a fundamental understanding of the (electro)catalytic activity of MMOs is limited because of the intrinsic complexity of their multi-element nature. Additional complexities arise when MMO catalysts have crystalline structures with two different metal site occupancies, such as the spinel structure, which makes it more challenging to investigate the origin of the (electro)catalytic activity of MMOs. Here, uniform-sized multi-metal spinel oxide nanoparticles composed of Mn, Co, and Fe as model MMO electrocatalysts are synthesized and the contributions of each element to the structural flexibility of the spinel oxides are systematically studied, which boosts the electrocatalytic oxygen reduction reaction (ORR) activity. Detailed crystal and electronic structure characterizations combined with electrochemical and computational studies reveal that the incorporation of Co not only increases the preferential octahedral site occupancy, but also modifies the electronic state of the ORR-active Mn site to enhance the intrinsic ORR activity. As a result, nanoparticles of the optimized catalyst, Co0.25 Mn0.75 Fe2.0 -MMO, exhibit a half-wave potential of 0.904 V (versus RHE) and mass activity of 46.9 A goxide -1 (at 0.9 V versus RHE) with promising stability.

Recent Advances and Prospects in Colloidal Nanomaterials.

Colloidal nanomaterials of metals, metal oxides, and metal chalcogenides have attracted great attention in the past decade owing to their potential applications in optoelectronics, catalysis, and energy conversion. Introduction of various synthetic routes has resulted in diverse colloidal nanostructured materials with well-controlled size, shape, and composition, enabling the systematic study of their intriguing physicochemical, optoelectronic, and chemical properties. Furthermore, developments in the instrumentation have offered valuable insights into the nucleation and growth mechanism of these nanomaterials, which are crucial in designing prospective materials with desired properties. In this perspective, recent advances in the colloidal synthesis and mechanism studies of nanomaterials of metal chalcogenides, metals, and metal oxides are discussed. In addition, challenges in the characterization and future direction of the colloidal nanomaterials are provided.

High photoluminescence from self-assembled Ag2Cl2(dppe)2 clusters through metallophilic interactions.

Ligand protected metal nanoclusters (NCs) are an emerging class of functional materials with intriguing photophysical and chemical properties. The size and molecular structure play an important role in endowing NCs with characteristic optical and electronic properties. Modulation of these properties through the chemical reactivity of NCs is largely unexplored. Here, we report on the synthesis of self-assembled Ag2Cl2(dppe)2 clusters through the ligand-exchange-induced transformation of [Pt2Ag23Cl7(PPh3)10] NCs [(dppe): 1,2-bis(diphenylphosphino)ethane; (PPh3): triphenylphosphine]. The single crystal x-ray structure reveals that two Ag atoms are bridged by one dppe and two Cl ligands, forming a Ag2Cl2(dppe) cluster, which is subsequently self-assembled through dppe ligands to form [Ag2Cl2(dppe)2]n. Importantly, the Ag2Cl2(dppe)2 cluster assembly exhibits high photoluminescence quantum yield: ∼18%, which is attributed to the metallophilic interactions and rigidification of the ligand shell. We hope that this work will motivate the exploitation of the chemical reactivity of NCs as a new path to attain cluster assemblies endowed with enhanced photophysical properties.

Role of the Precursor Composition in the Synthesis of Metal Ferrite Nanoparticles.

Ternary oxide nanoparticles have attracted much interest because of their intriguing properties, which are not exhibited by binary oxide nanoparticles. However, the synthesis of ternary oxide nanoparticles is not trivial and requires a fundamental understanding of the complicated precursor chemistry that governs the formation mechanism. Herein, we investigate the role of the chemical composition of precursors in the formation of ternary oxide nanoparticles via a combination of mass spectrometry, electron microscopy with elemental mapping, and thermogravimetric analysis. Mn2+, Co2+, and Ni2+ ions easily form bimetallic-oxo clusters with Fe3+ ions with a composition of MFe2O(oleate)6 (M = Mn, Co, Ni). The use of clusters as precursors leads to the successful synthesis of monodisperse metal ferrite nanoparticles (MFe2O4). On the contrary, zinc- or copper-containing complexes are formed independently from iron-oxo clusters in the precursor synthesis. The mixture of complexes without a bimetallic-oxo core yields a mixture of two different nanoparticles. This study reveals the importance of the precursor composition in the synthesis of ternary oxide nanoparticles.

Highly Fluorescent Gold Cluster Assembly.

The photoluminescence (PL) of metal nanoclusters (NCs), originating from their molecule-like electronic structure, is one of the most intriguing properties of NCs. Although various strategies such as tailoring the size, structure, and chemical environment of NCs have shown to improve the PL, their quantum yields (QYs) are still lagging far behind those of conventional luminescent materials, including quantum dots and organic fluorophores. Herein, we report the synthesis of highly luminescent gold cluster assembly (GCA) from Zn2+-ion-mediated assembly of Au4(SRCOO-)4 clusters using mercaptocarboxylic acid as a protective ligand and reductant as well as a growth suppressor. The synergetic combination of unique aurophilic interactions among Au4 clusters and the rigidified chemical environment induced by metal ion chelation through carboxylate groups is responsible for the ultrabright greenish-blue fluorescence with a QY up to 90%. Furthermore, the unique flexibility of dis/reassembly and the aggregation-dependent strong fluorescence of GCA offer a great potential for applications in biodegradable and trackable drug delivery systems.

Magic-Sized Stoichiometric II-VI Nanoclusters.

Metal chalcogenide nanomaterials have gained widespread interest in the past two decades for their potential optoelectronic, energy, and catalytic applications. The colloidal growth of various forms of these materials, such as nanowires, platelets, and lamellar assemblies, proceeds through certain thermodynamically stable, ultrasmall (<2 nm) intermediates called magic-sized nanoclusters (MSCs). Due to quantum confinement and its resultant intriguing properties, isolation or direct synthesis of MSCs and their structure characterization, which is very much challenging, are current topics of fundamental and applied scientific research. By comprehensive understanding of the structure-activity relationships in MSCs, the nucleation and growth processes can be manipulated, resulting in the synthesis of novel metal chalcogenide materials for various applications. This review focuses on recent advances in the chemical synthesis, characterization, and theoretical calculations of CdSe and its related II-VI nanoclusters. It highlights the studies of photophysical and magneto-optical properties as well as heteroatom doping of MSCs followed by their chemical transformation to high-dimensional nanostructures. At the end of the review, future directions and possible ways to overcome the challenges in the research of semiconductor MSCs are also presented.

Cd12Ag32(SePh)36: Non-Noble Metal Doped Silver Nanoclusters.

While there are numerous recent reports on doping of a ligand-protected noble metal nanocluster (e.g., Au and Ag) with another noble metal, non-noble metal (e.g., Cd) doping remains challenging. Here, we design a phosphine-assisted synthetic strategy and synthesize a Cd doped Ag nanocluster, Cd12Ag32(SePh)36 (SePh: selenophenolate), which exhibits characteristic UV-vis absorption features and rare near-infrared (NIR) photoluminescence at ∼1020 nm. The X-ray single crystal structure reveals an asymmetric two-shell Ag4@Ag24 metal kernel protected by four nonplanar Cd3Ag(SePh)9 metal-ligand frameworks. Furthermore, the electronic structure analysis shows that the cluster is a 20-electron "superatom" and density functional theory predicts that its chiral optical response is comparable to the well-known Au38(SR)24 cluster. Our synthetic approach will pave a new path for introducing other non-noble metals into noble metal nanoclusters for exploring their effect on optical and chemical properties.

Molecular-Level Understanding of Continuous Growth from Iron-Oxo Clusters to Iron Oxide Nanoparticles.

The formation of inorganic nanoparticles has been understood based on the classical crystallization theory described by a burst of nucleation, where surface energy is known to play a critical role, and a diffusion-controlled growth process. However, this nucleation and growth model may not be universally applicable to the entire nanoparticle systems because different precursors and surface ligands are used during their synthesis. Their intrinsic chemical reactivity can lead to a formation pathway that deviates from a classical nucleation and growth model. The formation of metal oxide nanoparticles is one such case because of several distinct chemical aspects during their synthesis. Typical carboxylate surface ligands, which are often employed in the synthesis of oxide nanoparticles, tend to continuously remain on the surface of the nanoparticles throughout the growth process. They can also act as an oxygen source during the growth of metal oxide nanoparticles. Carboxylates are prone to chemical reactions with different chemical species in the synthesis such as alcohol or amine. Such reactions can frequently leave reactive hydroxyl groups on the surface. Herein, we track the entire growth process of iron oxide nanoparticles synthesized from conventional iron precursors, iron-oleate complexes, with strongly chelating carboxylate moieties. Mass spectrometry studies reveal that the iron-oleate precursor is a cluster comprising a tri-iron-oxo core and carboxylate ligands rather than a mononuclear complex. A combinatorial analysis shows that the entire growth, regulated by organic reactions of chelating ligands, is continuous without a discrete nucleation step.

Co2+-Doping of Magic-Sized CdSe Clusters: Structural Insights via Ligand Field Transitions.

Magic-sized clusters represent materials with unique properties at the border between molecules and solids and provide important insights into the nanocrystal formation process. However, synthesis, doping, and especially structural characterization become more and more challenging with decreasing cluster size. Herein, we report the successful introduction of Co2+ ions into extremely small-sized CdSe clusters with the intention of using internal ligand field transitions to obtain structural insights. Despite the huge mismatch between the radii of Cd2+ and Co2+ ions (>21%), CdSe clusters can be effectively synthesized with a high Co2+ doping concentration of ∼10%. Optical spectroscopy and mass spectrometry suggest that one or two Co2+ ions are substitutionally embedded into (CdSe)13 clusters, which is known as one of the smallest CdSe clusters. Using magnetic circular dichroism spectroscopy on the intrinsic ligand field transitions between the different 3d orbitals of the transition metal dopants, we demonstrate that the Co2+ dopants are embedded on pseudotetrahedral selenium coordinated sites despite the limited number of atoms in the clusters. A significant shortening of Co-Se bond lengths compared to bulk or nanocrystals is observed, which results in the metastability of Co2+ doping. Our results not only extend the doping chemistry of magic-sized semiconductor nanoclusters, but also suggest an effective method to characterize the local structure of these extremely small-sized clusters.

Large-Scale Synthesis and Medical Applications of Uniform-Sized Metal Oxide Nanoparticles.

Thanks to recent advances in the synthesis of high-quality inorganic nanoparticles, more and more types of nanoparticles are becoming available for medical applications. Especially, metal oxide nanoparticles have drawn much attention due to their unique physicochemical properties and relatively inexpensive production costs. To further promote the development and clinical translation of these nanoparticle-based agents, however, it is highly desirable to reduce unwanted interbatch variations of the nanoparticles because characterizing and refining each batch are costly, take a lot of effort, and, thus, are not productive. Large-scale synthesis is a straightforward and economic pathway to minimize this issue. Here, the recent achievements in the large-scale synthesis of uniform-sized metal oxide nanoparticles and their biomedical applications are summarized, with a focus on nanoparticles of transition metal oxides and lanthanide oxides, and clarifying the underlying mechanism for the synthesis of uniform-sized nanoparticles. Surface modification steps to endow hydrophobic nanoparticles with water dispersibility and biocompatibility are also briefly described. Finally, various medical applications of metal oxide nanoparticles, such as bioimaging, drug delivery, and therapy, are presented.

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

Nanoclusters are important prenucleation intermediates for colloidal nanocrystal synthesis. In addition, they exhibit many intriguing properties originating from their extremely small size lying between molecules and typical nanocrystals. However, synthetic control of multicomponent semiconductor nanoclusters remains a daunting goal. Here, we report on the synthesis, doping, and transformation of multielement magic-sized clusters, generating the smallest semiconductor alloys. We use Lewis acid-base reactions at room temperature to synthesize alloy clusters containing three or four types of atoms. Mass spectrometry reveals that the alloy clusters exhibit "magic-size" characteristics with chemical formula of ZnxCd13-xSe13 (x = 0-13) whose compositions are tunable between CdSe and ZnSe. Successful doping of these clusters creates a new class of diluted magnetic semiconductors in the extreme quantum confinement regime. Furthermore, the important role of these alloy clusters as prenucleation intermediates is demonstrated by low temperature transformation into quantum alloy nanoribbons and nanorods. Our study will facilitate the understanding of these novel diluted magnetic semiconductor nanoclusters, and offer new possibilities for the controlled synthesis of nanomaterials at the prenucleation stage, consequently producing novel multicomponent nanomaterials that are difficult to synthesize.

Iron oxide nanoclusters for T 1 magnetic resonance imaging of non-human primates.

Iron-oxide-based contrast agents for magnetic resonance imaging (MRI) had been clinically approved in the United States and Europe, yet most of these nanoparticle products were discontinued owing to failures to meet rigorous clinical requirements. Significant advances have been made in the synthesis of magnetic nanoparticles and their biomedical applications, but several major challenges remain for their clinical translation, in particular large-scale and reproducible synthesis, systematic toxicity assessment, and their preclinical evaluation in MRI of large animals. Here, we report the results of a toxicity study of iron oxide nanoclusters of uniform size in large animal models, including beagle dogs and the more clinically relevant macaques. We also show that iron oxide nanoclusters can be used as T 1 MRI contrast agents for high-resolution magnetic resonance angiography in beagle dogs and macaques, and that dynamic MRI enables the detection of cerebral ischaemia in these large animals. Iron oxide nanoclusters show clinical potential as next-generation MRI contrast agents.

Digital Doping in Magic-Sized CdSe Clusters.

Magic-sized semiconductor clusters represent an exciting class of materials located at the boundary between quantum dots and molecules. It is expected that replacing single atoms of the host crystal with individual dopants in a one-by-one fashion can lead to unique modifications of the material properties. Here, we demonstrate the dependence of the magneto-optical response of (CdSe)13 clusters on the discrete number of Mn(2+) ion dopants. Using time-of-flight mass spectrometry, we are able to distinguish undoped, monodoped, and bidoped cluster species, allowing for an extraction of the relative amount of each species for a specific average doping concentration. A giant magneto-optical response is observed up to room temperature with clear evidence that exclusively monodoped clusters are magneto-optically active, whereas the Mn(2+) ions in bidoped clusters couple antiferromagnetically and are magneto-optically passive. Mn(2+)-doped clusters therefore represent a system where magneto-optical functionality is caused by solitary dopants, which might be beneficial for future solotronic applications.

Role of hyaluronic acid and phospholipid in the lubrication of a cobalt-chromium head for total hip arthroplasty.

The tribological performance of total hip arthroplasty has an important influence on its success rate. This study examined the concentration-dependent role of hyaluronic acid (HA) and phospholipid (dipalmitoylphosphatidylcholine, DPPC) in the boundary lubricating ability of retrieved cobalt-chromium femoral heads. The microscale frictional coefficients (µ) were measured by atomic force microscopy using a rectangular silicon cantilever integrated with sharp silicon tips. In the case of HA lubricant, the frictional coefficients decreased significantly at concentrations of 2.0 (0.16 ± 0.03) and 3.5 mg/ml (0.11 ± 0.01) while increased at 5.0 mg/ml (0.15 ± 0.01), compared to that with phosphate buffer saline (0.25 ± 0.03). The concentration-dependent lubrication behavior of DPPC was most effective when DPPC was in the physiological concentration range, showing µ = 0.16 ± 0.01 in polypropylene glycol, and 0.05 ± 0.01, 0.02 ± 0.01, and 0.03 ± 0.01 at a DPPC concentration of 0.05, 0.2, and 3.0 mg/ml, respectively. Results obtained show significant differences between the DPPC concentration groups. Conclusively, the microscale frictional response of the retrieved CoCr femoral head has a significant dependence on the concentrations of HA and DPPC. Moreover, observed optimal concentration of HA and DPPC for effective lubrication is similar to that observed in normal human synovial fluid. Therefore, a retrieval of the synovia may be considered during total hip replacement surgeries in an effort for reduction of friction between head and liner of total hip replacement implants.