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

Alloyed Zinc Chalcogenide Magic-Sized Nanoclusters and Their Transformation to Alloyed Quantum Dots

Alloying provides the opportunity to widen the physical and chemical properties of quantum dots (QDs); however, the precise controlled composition of alloyed QDs is still a challenge. In this work, a few quaternary alloyed zinc chalcogenide magic-sized nanoclusters (MSCs) were synthesized using the active chalcogen precursors of tri(dimethylamine)phosphine chalcogen, such as Zn21S4Se3Te4 (MSCs-348), Zn14S4Se4Te7 (MSCs-350), Zn15S1Se4Te6 (MSCs-349), and Zn17S2Se2Te7 (MSCs-355) MSCs. The composition of alloyed zinc chalcogenide MSCs was tuned with the different amounts of added chalcogen precursors. Finally, the produced alloyed zinc chalcogenide MSCs can be used as precursors to synthesize alloyed zinc chalcogenide QDs, and the composition of zinc chalcogenide QDs can be adjusted with different alloyed MSCs. This work provides methods to alloy MSCs with controlled composition, providing efficient precursors for alloyed QDs.

Intrinsic Chiroptical Activity and Excitonic Properties of Group II-VI Magic-Size Clusters

Semiconductor magic-size clusters (MSCs), lying in the local minima of the potential landscape, are important intermediates that emerge during the synthesis of colloidal quantum dots. They have definite geometrical and electronic structures, thus serving as atomically precise building blocks for assembling supramolecular structures and devices with unprecedented functionalities. Here we report the intrinsic chiroptical activity in the magic-size cadmium and zinc chalcogenide clusters with magic numbers of 13, 33, and 34 possessing unique core-shell structures. They are responsive to circularly polarized light from the ultraviolet to visible region, with size-tunable energy gap, absorption wavelength, and excitonic characteristics. The origin of the chiroptical activity and the evolution of excitonic states with magic size are disclosed by time-dependent density functional theory calculations within a correlated electron-hole picture. This molecular-level understanding of the photophysical properties of group II-VI MSCs provides essential guidelines for utilizing them for chiral optoelectronics and photonics.

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.

Ligand-Induced Divergent Evolution of ZnSe Magic Sized Clusters

Alkylamine ligand-induced evolutions of ZnSe magic sized clusters (MSCs) toward divergent products have been discovered for the first time. With correspondingly assigned molecular structures, the same ZnSe MSC was found to undergo either single-atom growth or dissolution through the elaborate tailoring of alkylamine ligands.

CdTe magic-size cluster synthesis via a cation exchange method and conversion mechanism

The quasi-metallic nature of Te is not conducive to telluride formation and crystallization, which makes the synthesis of CdTe magic-size clusters (MSCs) in a single-ensemble form still challenging. CdTe MSCs are usually synthesized by direct synthesis, a method that must avoid the formation of quantum dots by selecting suitable active precursors and precisely controlling the reaction temperature. In addition, the organic Cd compounds and superhydrogenated precursors used are air-sensitive. Herein, CdTe MSC-448 in a single-ensemble form was synthesized for the first time via a cation exchange method using ZnTe MSC-389 as a template and Cd2+ as an exchange ion. In situ absorption spectroscopy characterization combined with the two-pathway model proposed by Yu's group reveals that the conversion of ZnTe MSC-389 into CdTe MSC-448 is assisted by their corresponding precursor compounds (PCs). After the addition of Cd precursors to ZnTe MSC-389 solution, ZnTe MSC-389 is transformed into ZnTe PC-389, which then undergoes a rapid cation exchange reaction with Cd2+ to yield CdTe PC-448, and CdTe PC-448 is finally converted into CdTe MSC-448. CdTe MSCs in single-ensemble form were obtained by cation exchange in air at room temperature, avoiding the formation of quantum dots (QDs) at high temperatures in the direct synthesis method conducted without the use of toxic and expensive active precursors, which provides a new route to the synthesis of CdTe MSCs.

Synthesis and characterization of octopamine sulfate, norfenefrine sulfate and etilefrine sulfate reference materials for doping control

BACKGROUND

Doping is the use of prohibited substances by athletes to improve their performance. World Anti‐Doping Agency (WADA)‐accredited laboratories require various metabolite reference standards of the prohibited chemical substances or drugs for routine quality control. Therefore, it was proposed to develop efficient synthetic methodologies for highly pure reference materials of Phase II metabolites of octopamine, norfenefrine and etilefrine, which are prohibited in sports by WADA under the S6 stimulant category. The reference materials were characterized using various analytical techniques. New high‐performance liquid chromatography with diode‐array detection (HPLC‐DAD) methods were developed for purity assessment.

RESULTS

The synthesized Phase II metabolite reference standards, i.e. octopamine sulfate, norfenefrine sulfate and etilefrine sulfate, were confirmed by 1H NMR, 13C NMR, liquid chromatography–high‐resolution mass spectrometry (LC‐HRMS), attenuated total reflectance Fourier transform infrared and thermogravimetric analysis. In the LC‐HRMS study, the mass error value of synthesized compounds was less than 10 ppm (error) which confirms the identity of the reference materials. New HPLC‐DAD method were developed to ensure the purity of the reference materials. We used the HILIC column as metabolite reference standards are highly polar. The mobile phase was composed of water and acetonitrile in fixed composition. The HPLC‐DAD purity of the developed reference materials was observed as 100%.

CONCLUSION

We have developed reproducible synthetic routes for octopamine sulfate, norfenefrine sulfate and etilefrine sulfate, which are prohibited in sports by WADA. The synthesized metabolites were characterized using different advanced analytical techniques. These reference standards will be helpful to all WADA‐accredited laboratories in routine anti‐doping testing. © 2023 Society of Chemical Industry (SCI).

Thermal Effect on Photoelectrochemical Water Splitting Toward Highly Solar to Hydrogen Efficiency

Photoelectrochemical (PEC) hydrogen production is an emerging technology that uses renewable solar light aimed to establish a sustainable carbon-neutral society. The barriers to commercialization are low efficiency and high cost. To date, researchers have focused on materials and systems. However, recent studies have been conducted to utilize thermal effects in PEC hydrogen production. This Review provides a fresh perspective to utilize the thermal effects for PEC performance enhancement while delineating the underlying principles and equations associated with efficiency. The fundamentals of the thermal effect on the PEC system are summarized from various perspectives: kinetics, thermodynamics, and empirical equations. Based on this, materials are classified as plasmonic metals, quantum dot-based semiconductors, and photothermal organic materials, which have an inherent response to photothermal irradiation. Finally, the economic viability and challenges of these strategies for PEC are explained, which can pave the way for the future progress in the field.

Synthesis, Assembly, and Applications of Magic-Sized Semiconductor (CdSe)13 Cluster

ConspectusAtomically precise metal chalcogenide clusters (MCCs) are model molecular compounds of scientifically and technologically important semiconductor nanocrystals, which are known as quantum dots (QDs). The significantly high ambient stability of MCCs of particular sizes, as compared to that of slightly smaller or larger sizes, made them be termed "magic-sized clusters" (MSCs). In other words, MSCs with specific sizes between sizes of precursors (typically, metal-ligand complexes) and nanocrystals (typically, QDs) appear sequentially during the colloidal synthesis of nanocrystals, while the other cluster species decompose to precursor monomers or are consumed during the growth of the nanocrystals. Unlike nanocrystals with an ambiguous atomic-level structure and a substantial size distribution, MSCs possess atomically monodisperse size, composition, and distinct atomic arrangement. Chemical synthesis and exploration of properties of MSCs are of great significance since they help systematically understand the evolution of fundamental properties as well as build structure-activity relationships at distinct molecular levels. Furthermore, MSCs are anticipated to offer atomic-level insights into the growth mechanism of the semiconductor nanocrystals, which is highly desirable in the design of advanced materials with new functions. In this Account, we cover our recent efforts in the advancement of one of the most important stoichiometric CdSe MSCs, (CdSe)₁₃. In particular, we present its molecular structure derived from a single crystal X-ray crystallographic study of the closest MSC, Cd₁₄Se₁₃. The crystal structure of MSC enables not only the understanding of the electronic structure and prediction of the potential sites for heteroatom dopants (e.g., Mn²⁺ and Co²⁺) but also the identification of favorable synthetic conditions for the selective synthesis of desired MSCs. Next, we focus on enhancing the photoluminescence quantum yield and stability of Mn²⁺ doped (CdSe)₁₃ MSCs through their self-assembly, which is facilitated by the rigid diamines. In addition, we show how atomic-level synergistic effects and functional groups of the assemblies of alloy MSCs can be utilized for a highly enhanced catalytic CO₂ fixation with epoxides. Benefiting from the intermediate stability, the MSCs are explored as single-source precursors to low-dimensional nanostructures, such as nanoribbons and nanoplatelets, through the controlled transformation. Distinct differences in the outcome of the solid-state and colloidal-state conversion of MSCs suggest the need for careful consideration of the phase and reactivity of MSCs as well as the type of dopant to achieve novel structured multicomponent semiconductors. Finally, we summarize the Account and provide future perspectives on the fundamental and applied scientific research of MSCs.

Direct Observation of Off-Stoichiometry-Induced Phase Transformation of 2D CdSe Quantum Nanosheets

Crystal structures determine material properties, suggesting that crystal phase transformations have the potential for application in a variety of systems and devices. Phase transitions are more likely to occur in smaller crystals; however, in quantum-sized semiconductor nanocrystals, the microscopic mechanisms by which phase transitions occur are not well understood. Herein, the phase transformation of 2D CdSe quantum nanosheets caused by off-stoichiometry is revealed, and the progress of the transformation is directly observed by in situ transmission electron microscopy. The initial hexagonal wurtzite-CdSe nanosheets with atomically uniform thickness are transformed into cubic zinc blende-CdSe nanosheets. A combined experimental and theoretical study reveals that electron-beam irradiation can change the stoichiometry of the nanosheets, thereby triggering phase transformation. The loss of Se atoms induces the reconstruction of surface atoms, driving the transformation from wurtzite-CdSe(11 2 ¯ $\bar{2}$ 0) to zinc blende-CdSe(001) 2D nanocrystals. Furthermore, during the phase transformation, unconventional dynamic phenomena occur, including domain separation. This study contributes to the fundamental understanding of the phase transformations in 2D quantum-sized semiconductor nanocrystals.

Room-temperature formation of alloy ZnxCd13-xSe13 magic-size clusters via cation exchange in diamine solution

Magic-size clusters (MSCs) are molecular materials with unique properties at the border between molecules and solids, providing important insights into the nanocrystal formation process. However, the synthesis of multicomponent alloy MSCs in a single-ensemble form remains challenging due to their tiny size and difficult doping control. Herein, for the first time, we successfully synthesized alloy Zn_xCd_13-xSe₁₃ MSCs (x = 1-12) with a unique sharp absorption peak at 352 nm by cation exchange between Cd²⁺ ions and pre-synthesized (ZnSe)₁₃ MSCs in a diamine solution at room temperature. The experimental results show that the use of diamines is crucial to the formation of stable Zn_xCd_13-xSe₁₃ MSCs, which may be attributed to two amine groups that can coordinate to the surface of MSCs simultaneously. Limited by the robust interaction between diamine ligands and MSCs, the partial cation exchange results in the formation of alloy Zn_xCd_13-xSe₁₃ MSCs. In contrast, complete cation exchange occurs in a monoamine solution, giving (CdSe)₃₄ MSCs. Besides, a lower reaction temperature and a higher concentration of diamine favor the formation of Zn_xCd_13-xSe₁₃ MSCs. Our study provides an important basis for further understanding of the transformation of MSCs and a new approach to the controllable synthesis of alloyed MSCs.

One-Pot Heat-Up Synthesis of ZnSe Magic-Sized Clusters Using Thiol Ligands

The synthesis of two new families of ZnSe magic-sized clusters (MSCs) is achieved using the thiol ligand 1-dodecanethiol in a simple one-pot heat-up approach. The sizes of the MSCs are controlled with the thiol ligand concentration and reaction temperature.

Theoretical studies on the two-photon absorption of II-VI semiconductor nano clusters

Semiconductor clusters, Zn_nO_n, Zn_nS_n, and Cd_nS_n (n = 2–8), were optimized and the corresponding stable structures were acquired. The symmetry, bond length, bond angle, and energy gap between HOMO and LUMO were analyzed. According to reasonable calculation and comparative analysis for small clusters Zn₂O₂, Zn₂S₂, and Cd₂S₂, an effective method based on density function theory (DFT) and basis set which lay the foundation for the calculation of the large clusters have been obtained. The two-photon absorption (TPA) results show that for the nano clusters with planar configuration, sizes play important role on the TPA cross section, while symmetries determine the TPA cross section under circumstance of 3D stable structures. All our conclusions provide theoretical support for the development of related experiments.

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.

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.

Highly luminescent and catalytically active suprastructures of magic-sized semiconductor nanoclusters

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 Mn²⁺:(CdSe)₁₃ and Mn²⁺:(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 Mn²⁺:(Cd_1-xZn_xSe)₁₃ alloy suprastructures with tunable metal synergy: Mn²⁺:(Cd_0.5Zn_0.5Se)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 CO₂ 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.

Mass spectrometry for multi-dimensional characterization of natural and synthetic materials at the nanoscale

Characterization of materials at the nanoscale plays a crucial role in in-depth understanding the nature and processes of the substances. Mass spectrometry (MS) has characterization capabilities for nanomaterials (NMs) and nanostructures by offering reliable multi-dimensional information consisting of accurate mass, isotopic, and molecular structural information. In the last decade, MS has emerged as a powerful nano-characterization technique. This review comprehensively summarizes the capabilities of MS in various aspects of nano-characterization that greatly enrich the toolbox of nano research. Compared with other characterization techniques, MS has unique capabilities for real-time monitoring and tracking reaction intermediates and by-products. Moreover, MS has shown application potential in some novel aspects, such as MS imaging of the biodistribution and fate of NMs in animals and humans, stable isotopic tracing of NMs, and risk assessment of NMs, which deserve update and integration into the current knowledge framework of nano-characterization.

Transformations Among Colloidal Semiconductor Magic-Size Clusters

A knowledge of colloidal semiconductor magic-size clusters (MSCs) is essential for understanding how fundamental properties evolve during transformations from individual molecules to semiconductor quantum dots (QDs). Compared to QDs, MSCs display much narrower optical absorption bands; the higher cluster stability gives rise to a narrower size distribution. During the production of binary QDs such as II-VI metal (M) chalcogenide (E) ones, binary ME MSCs observed were interpreted as side products and/or the nuclei of QDs. Prior to the current development of our two-step approach followed by our two-pathway model, it had been extremely challenging to synthesize MSCs as a unique product without the nucleation and growth of QDs. With the two-step approach, we have demonstrated that MSCs can be readily engineered as a sole product at room temperature from a prenucleation stage sample, also called an induction period (IP) sample. It is important that we were able to discover that the evolution of the MSCs follows first-order reaction kinetics behavior. Accordingly, we proposed that a new type of compound, termed as "precursor compounds" (PCs) of MSCs, was produced in an IP sample. Such PCs are optically transparent at the absorption peak positions of their MSC counterparts as well as to longer wavelengths. It is thought that quasi isomerization of a single PC results in the development of one MSC.In this Account, we provide an overview of our latest advances regarding the transformations among binary CdE MSCs as well as from binary CdTe to ternary CdTeSe MSCs. Optical absorption spectroscopy has been employed to study these transformations, all of which display well-defined isosbestic points. We have proposed that these MSC to MSC transformations occur via their corresponding PCs, also called immediate PCs. It is reasonable that the as-synthesized PC (in an IP sample) and the immediate PC (in an incubated and/or diluted sample) probably have different configurations. A transformation between two PCs may involve an intermolecular reaction, with either first-order reaction kinetics or a more complicated time profile. A transformation between one immediate PC and its counterpart MSC may contain an intramolecular reaction. The present Account, which addresses the PC-enabled MSC transformations with isosbestic points probed by optical absorption spectroscopy, calls for more experimental and theoretical attention to understand these magic species and their transformation processes more precisely.

Exploiting Functional Impurities for Fast and Efficient Incorporation of Manganese into Quantum Dots

The incorporation of manganese (Mn) ions into Cd(Zn)-chalcogenide QDs activates strong spin-exchange interactions between the magnetic ions and intrinsic QD excitons that have been exploited for color conversion, sunlight harvesting, electron photoemission, and advanced imaging and sensing. The ability to take full advantage of novel functionalities enabled by Mn dopants requires accurate control of doping levels over a wide range of Mn contents. This, however, still represents a considerable challenge. Specific problems include the difficulty in obtaining high Mn contents, considerable broadening of QD size dispersion during the doping procedure, and large batch-to-batch variations in the amount of incorporated Mn. Here, we show that these problems originate from the presence of unreacted cadmium (Cd) complexes whose abundance is linked to uncontrolled impurities participating in the QD synthesis. After identifying these impurities as secondary phosphines, we modify the synthesis by introducing controlled amounts of "functional" secondary phosphine species. This allows us to realize a regime of nearly ideal QD doping when incorporation of magnetic ions occurs solely via addition of Mn-Se units without uncontrolled deposition of Cd-Se species. Using this method, we achieve very high per-dot Mn contents (>30% of all cations) and thereby realize exceptionally strong exciton-Mn exchange coupling with g-factors of ∼600.

Magnetic Circular Dichroism in Nanomaterials: New Opportunity in Understanding and Modulation of Excitonic and Plasmonic Resonances

The unique capability of magnetic circular dichroism (MCD) in revealing geometry and electronic information has provided new opportunities in exploring the relationship between structure and magneto-optical properties in nanomaterials with extraordinary optical absorption. Here, the representative studies referring to application of the MCD technique in semiconductor and noble metal nanomaterials are overviewed. MCD is powerful in elucidating the structural information of the excitonic transition in semiconductor nanocrystals, electronic transitions in noble metal nanoclusters, and plasmon resonance in noble metal nanostructures. By virtue of these advantages, the MCD technique shows its unrivalled ability in evaluating the magnetic modulation of excitonic and plasmonic optical activity of nanomaterials with varied chemical composition, geometry, assembly conformation, and coupling effect. Knowledge of the key factors in manipulating magneto-optical properties at the nanoscale acquired with the MCD technique will largely boost the application of semiconductor and noble nanomaterials in the fields of sensing, spintronic, nanophotonics, etc.

Exciton-driven change of phonon modes causes strong temperature dependent bandgap shift in nanoclusters

The fundamental bandgap E_g of a semiconductor—often determined by means of optical spectroscopy—represents its characteristic fingerprint and changes distinctively with temperature. Here, we demonstrate that in magic sized II-VI clusters containing only 26 atoms, a pronounced weakening of the bonds occurs upon optical excitation, which results in a strong exciton-driven shift of the phonon spectrum. As a consequence, a drastic increase of dE_g/dT (up to a factor of 2) with respect to bulk material or nanocrystals of typical size is found. We are able to describe our experimental data with excellent quantitative agreement from first principles deriving the bandgap shift with temperature as the vibrational entropy contribution to the free energy difference between the ground and optically excited states. Our work demonstrates how in small nanoparticles, photons as the probe medium affect the bandgap—a fundamental semiconductor property.

The bandgap of nanostructures usually follows the bulk value upon temperature change. Here, the authors find that in small nanocrystals a weakening of the bonds due to optical excitation causes a pronounced phonon shift, leading to a drastic enhancement of the bandgap’s temperature dependence.

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

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

First Synthesis of Mn-Doped Cesium Lead Bromide Perovskite Magic Sized Clusters at Room Temperature

Mn-doped CsPbBr₃ perovskite magic sized clusters (PMSCs) are synthesized for the first time using benzoic acid and benzylamine as passivating ligands and MnCl₂·4H₂O and MnBr₂ as the Mn²⁺ dopant sources at room temperature. The same approach is used to prepare Mn-doped CsPbBr₃ perovskite quantum dots (PQDs). The concentration of MnX₂ (X = Cl or Br) affects the excitonic absorption of the PMSCs and PQDs. A higher concentration of MnX₂ favors PMSCs over PQDs as well as higher photoluminescence (PL) quantum yields (QYs) and PL stability. The large ratio between the characteristic Mn emission (∼590 nm) and the host band-edge emission shows efficient energy transfer from the host exciton to the Mn²⁺ dopant. PL excitation, electron paramagnetic resonance, and time-resolved PL results all support Mn²⁺ doping in CsPbBr₃, which likely replaces Pb²⁺ ions. This study establishes a new method for synthesizing Mn-doped PMSCs with good PL stability, high PLQY and highly effective passivation.

Efficient White LEDs Using Liquid-state Magic-sized CdSe Quantum Dots

Magic clusters have attracted significant interest to explore the dynamics of quantum dot (QD) nucleation and growth. At the same time, CdSe magic-sized QDs reveal broadband emission in the visible wavelength region, which advantageously offer simple integration of a single-type of nanomaterial and high color rendering ability for white light-emitting diodes (LEDs). Here, we optimized the quantum yield of magic-sized CdSe QDs up to 22% via controlling the synthesis parameters without any shelling or post-treatment process and integrated them in liquid-state on blue LED to prevent the efficiency drop due to host-material effect. The fabricated white LEDs showed color-rendering index and luminous efficiency up to 89 and 11.7 lm/W, respectively.

Formation of colloidal alloy semiconductor CdTeSe magic-size clusters at room temperature

Alloy semiconductor magic-size clusters (MSCs) have received scant attention and little is known about their formation pathway. Here, we report the synthesis of alloy CdTeSe MSC-399 (exhibiting sharp absorption peaking at 399 nm) at room temperature, together with an explanation of its formation pathway. The evolution of MSC-399 at room temperature is detected when two prenucleation-stage samples of binary CdTe and CdSe are mixed, which are transparent in optical absorption. For a reaction consisting of Cd, Te, and Se precursors, no MSC-399 is observed. Synchrotron-based in-situ small angle X-ray scattering (SAXS) suggests that the sizes of the two samples and their mixture are similar. We argue that substitution reactions take place after the two binary samples are mixed, which result in the formation of MSC-399 from its precursor compound (PC-399). The present study provides a room-temperature avenue to engineering alloy MSCs and an in-depth understanding of their probable formation pathway.

Alloy magic-size clusters (MSCs) are difficult to synthesize, in part because so little is known about how they form. Here, the authors produce single-ensemble alloy CdTeSe MSCs at room temperature by mixing prenucleation-stage solutions of CdTe and CdSe, uncovering a formation pathway that may extend to the synthesis of other alloy MSCs.

Facile fabrication of configuration controllable self-assembled Al nanostructures as UV SERS substrates

The resonantly enhanced near-surface electromagnetic field of Al nanostructures (NSs) excited with UV light offers an effective and precise approach to obtain vibration information in complex mixtures up to a single-molecule magnitude. In this work, we propose a facile, cost-effective and large-scale way to fabricate self-assembled Al NSs with systematically controllable configuration and size for the preparation of SERS substrates. The stalagmite-shaped Al nanoparticles (NPs) are directly utilized based on the Volmer-Weber model and drastically develop as a function of the annealing temperature with tunable optical properties, resulting in a maximum enhancement factor of the vibrational Raman signal from adenine molecules up to 3.49 × 10⁷. With a variation of the deposition thickness at an identical temperature, the self-assembled Al NSs sensitively evolve from stalagmite-shaped NPs to worm-like nano-mounds with increased light consumption induced by scattering, which inherently leads to a noticeable reduction in the EF due to the reduced plasmon coupling between NSs with the sparse distribution. The enhancement effect of the SERS substrates is theoretically discussed with the FDTD simulation, providing an instructive reference for promising applications in the bio-sensing technique.

Precursor Self-Assembly Identified as a General Pathway for Colloidal Semiconductor Magic-Size Clusters

Little is known about the formation pathway of colloidal semiconductor magic-size clusters (MSCs). Here, the synthesis of the first single-ensemble ZnSe MSCs, which exhibit a sharp optical absorption singlet peaking at 299 nm, is reported; their formation is independent of Zn and Se precursors used. It is proposed that the formation of MSCs starts with precursor self-assembly followed by Zn and Se covalent bond formation to result in immediate precursors (IPs) which can transform into the MSCs. It is demonstrated that the IPs in cyclohexane appear transparent in optical absorption, and become visible as MSCs exhibiting one sharp optical absorption peak when a primary amine is added at room temperature. It is shown that when the preparation of the IP is controlled to be within the induction period, which occurs prior to nucleation and growth of conventional quantum dots (QDs), the resulting MSCs can be produced without the complication of the simultaneous coproduction of conventional QDs. The present study reveals the existence of precursor self-assembly which leads to the formation of colloidal semiconductor MSCs and provides insights into a multistep nucleation process in cluster science.

Conversion Reactions of Atomically Precise Semiconductor Clusters

Clusters are unique molecular species that can be viewed as a bridge between phases of matter and thus between disciplines of chemistry. The structural and compositional complexity observed in cluster chemistry serves as an inspiration to the material science community and motivates our search for new phases of matter. Moreover, the formation of kinetically persistent cluster molecules as intermediates in the nucleation of crystals makes these materials of great interest for determining and controlling mechanisms of crystal growth. Our lab developed a keen interest in clusters insofar as they relate to the nucleation of nanoscale semiconductors and the modeling of postsynthetic reaction chemistry of colloidal materials. In particular, our discovery of a structurally unique In₃₇P₂₀X₅₁ (X = carboxylate) cluster en route to InP quantum dots has catalyzed our interest in all aspects of cluster conversion, including the use of clusters as precursors to larger nanoscale colloids and as platforms for examining postsynthetic reaction chemistry. This Account is presented in four parts. First, we introduce cluster chemistry in a historical context with a focus on main group, metallic, and semiconductor clusters. We put forward the concept of rational, mechanism-driven design of colloidal semiconductor nanocrystals as the primary motivation for the studies we have undertaken. Second, we describe the role of clusters as intermediates both in the synthesis of well-known material phases and in the discovery of unprecedented nanomaterial structures. The primary distinction between these two approaches is one of kinetics; in the case of well-known phases, we are often operating under high-temperature thermolysis conditions, whereas for materials discovery, we are discovering strategies to template the growth of kinetic phases as dictated by the starting cluster structure. Third, we describe reactions of clusters as model systems for their larger nanomaterial progeny with a primary focus on cation exchange. In the case of InP, cation exchange in larger nanostructures has been challenging due to the covalent nature of the crystal lattice. However, in the higher energy, strained cluster intermediates, cation exchange can be accomplished even at room temperature. This opens opportunities for accessing doped and alloyed nanomaterials using postsynthetically modified clusters as single-source precursors. Finally, we present surface chemistry of clusters as the gateway to subsequent chemistry and reactivity, and as an integral component of cluster structure and stability. Taken as a whole, we hope to make a compelling case for using clusters as a platform for mechanistic investigation and materials discovery.

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.

Effect of Small Molecule Additives in the Prenucleation Stage of Semiconductor CdSe Quantum Dots

For the addition of small molecules in the prenucleation stage of colloidal CdSe conventional quantum dots (QDs), there is insufficient knowledge regarding what are advantageous circumstances. Here, we present a study about such addition. When CH₃COOH or Zn(OOCCH₃)₂ is added in the prenucleation stage (at 120 °C) of a reaction consisting of cadmium myristate (Cd(OOC(CH₂)₁₂CH₃)₂, Cd(MA)₂ made from CdO) and Se powder in 1-octadecene (ODE), CdSe magic-size clusters (MSCs) exhibiting a single sharp absorption doublet at 433/460 nm are synthesized in a single-ensemble form (around 220 °C). We demonstrate that such small molecule addition suppresses the nucleation and growth of QDs and thus directs a competitive process to the formation of MSCs. The present study provides insight into the individual but linked pathways to forming CdSe QDs and MSCs and introduces new avenues to improve the production of MSCs through the addition of small molecules in the prenucleation stage.

Evolution of Two Types of CdTe Magic-Size Clusters from a Single Induction Period Sample

There are two types of colloidal semiconductor nanocrystals (NCs) that exhibit band gap absorption that is relatively sharp compared to conventional quantum dots (QDs). One type displays an absorption doublet, while the other displays an absorption singlet. Here, we report the evolution of the two types of NCs at room temperature from a single CdTe sample extracted during the induction period (IP) prior to nucleation and growth of conventional QDs. The resulting NCs exhibit band gap absorption peaking at ∼371 nm and are magic-size clusters (MSCs), labeled as dMSC-371 and sMSC-371 for the doublet and singlet cases, respectively. We demonstrate that dMSC-371 (with another peak at ∼350 nm) evolves when the sample is incubated. When the sample is dispersed without incubation into a toluene and octylamine mixture, dMSC-371 or sMSC-371 grows depending on the amine amount. We propose that dMSC-371 and sMSC-371 are a pair of polymorphs (with identical CdTe core compositions). The present study brings insight into the formation relationship between dMSCs and sMSCs.

Thermally-induced reversible structural isomerization in colloidal semiconductor CdS magic-size clusters

Structural isomerism of colloidal semiconductor nanocrystals has been largely unexplored. Here, we report one pair of structural isomers identified for colloidal nanocrystals which exhibit thermally-induced reversible transformations behaving like molecular isomerization. The two isomers are CdS magic-size clusters with sharp absorption peaks at 311 and 322 nm. They have identical cluster masses, but slightly different structures. Furthermore, their interconversions follow first-order unimolecular reaction kinetics. We anticipate that such isomeric kinetics are applicable to a variety of small-size functional nanomaterials, and that the methodology developed for our kinetic study will be helpful to investigate and exploit solid–solid transformations in other semiconductor nanocrystals. The findings on structural isomerism should stimulate attention toward advanced design and synthesis of functional nanomaterials enabled by structural transformations.

Few structural isomers of colloids, with identical masses but different structures, have been identified. Here, the authors observe an interesting example of structural isomerism in a pair of semiconductor magic-size clusters, which reversibly transform between one another with first-order unimolecular reaction kinetics.

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.

Effect of sulfonating agent and ligand chemistry on structural and optical properties of CuSbS2 particles prepared by heat-up method

CuSbS₂ particles were prepared by a facile heat-up method to investigate the effect of sulfur source and ligand chemistry.

Liquid-Phase Transmission Electron Microscopy for Studying Colloidal Inorganic Nanoparticles

For the past few decades, nanoparticles of various sizes, shapes, and compositions have been synthesized and utilized in many different applications. However, due to a lack of analytical tools that can characterize structural changes at the nanoscale level, many of their growth and transformation processes are not yet well understood. The recently developed technique of liquid-phase transmission electron microscopy (TEM) has gained much attention as a new tool to directly observe chemical reactions that occur in solution. Due to its high spatial and temporal resolution, this technique is widely employed to reveal fundamental mechanisms of nanoparticle growth and transformation. Here, the technical developments for liquid-phase TEM together with their application to the study of solution-phase nanoparticle chemistry are summarized. Two types of liquid cells that can be used in the high-vacuum conditions required by TEM are discussed, followed by recent in situ TEM studies of chemical reactions of colloidal nanoparticles. New findings on the growth mechanism, transformation, and motion of nanoparticles are subsequently discussed in detail.