Synthesis of Soluble and Processable Rod-, Arrow-, Teardrop-, and Tetrapod-Shaped CdSe Nanocrystals

Morphology Control of Au-Ni Hybrid Nanoparticles: Exploring Heterostructures and Optical Tuning

Hybrid nanoparticles (NPs) have attracted considerable attention because of their ability to provide diverse properties by integrating the inherent properties of multiple components; however, synthetic strategies to control their morphology remain unexplored. In this study, a new method was used to control the morphology and optical properties of Au-Ni heterostructure (ANH) NPs. Unique morphological changes were observed by varying the Au/Ni precursor ratio from 2:1 to 1:4, exhibiting a shape transformation from dumbbell-like to quasi-spherical owing to the Ni NP size expansion, whereas the Au NP maintained their size. Moreover, increasing the Ni ratio induced plasmonic band broadening and wavelength redshift, resulting in color changes from red to navy and black. In terms of the structure, the atomic orientation of the crystallite showed that even a large lattice mismatch can result in heterojunctions at the NPs. In addition, the reaction aliquots uncovered heterogeneous nucleation and growth of ANH NPs in the colloidal system, demonstrating Ni reduction on the preformed Au NP owing to the reduction in potential gap. This study provides new insights into controlling the morphology of hybrid NPs using colloidal synthesis and the design of optimized materials for various applications.

Long-Chain Lipids Facilitate Insertion of Large Nanoparticles into Membranes of Small Unilamellar Vesicles

Insertion of hydrophobic nanoparticles into phospholipid bilayers is limited to small particles that can incorporate into a hydrophobic membrane core between two lipid leaflets. Incorporation of nanoparticles above this size limit requires the development of challenging surface engineering methodologies. In principle, increasing the long-chain lipid component in the lipid mixture should facilitate incorporation of larger nanoparticles. Here, we explore the effect of incorporating very long phospholipids (C24:1) into small unilamellar vesicles on the membrane insertion efficiency of hydrophobic nanoparticles that are 5-11 nm in diameter. To this end, we improve an existing vesicle preparation protocol and utilized cryogenic electron microscopy imaging to examine the mode of interaction and evaluate the insertion efficiency of membrane-inserted nanoparticles. We also perform classical coarse-grained molecular dynamics simulations to identify changes in lipid membrane structural properties that may increase insertion efficiency. Our results indicate that long-chain lipids increase the insertion efficiency by preferentially accumulating near membrane-inserted nanoparticles to reduce the thermodynamically unfavorable disruption of the membrane.

Proton Shuttle-Assisted Surface Reconstruction toward Nonpolar Facets-Terminated Zinc-Blende CdSe/CdS Core/Shell Quantum Dots

Surface reconstruction can rearrange the surface atoms of a crystal without the need of growth processes and has the potential to synthesize crystals with novel morphologies and facets that cannot be obtained through regular synthesis. However, little is known about the molecular mechanisms of the surface reconstruction process. Here, utilizing surface reconstruction, we report the synthesis of nonpolar facets (110) facets)-terminated dodecahedral zinc-blende CdSe/CdS core/shell quantum dots. The morphology transformation is achieved by first fully exchanging the cadmium carboxylate ligand with oleylamine and then undergoing surface reconstruction. The surface reconstruction-induced morphology transformation is confirmed by transmission electron microscopy and absorption spectroscopy. Details of kinetic experiments and simulation results demonstrated that successful surface reconstruction must be assisted by a proton shuttle. Except for the first report on zinc-blende quantum dots terminated with (110) facets, the surface reconstruction aided by the proton shuttle offers valuable insights for devising methods to regulate the properties of nanocrystals.

Riboflavin secreted by Shewanella sp. FDL-2 facilitates its reduction of Se(iv) and Te(iv) by promoting electron transfer

The biological reduction of selenite (Se(iv)) or tellurite (Te(iv)) to Se0 or Te0 has received increasing attention, as related studies have favored the development of Se/Te pollution control methods. In the presence of the electron donor, the microbes acquired energy and transferred electrons to Se(iv) or Te(iv) to achieve their detoxication. However, the microbial electron transfer pathways involved in this process are still not fully understood. In this study, we reported that marine Shewanella sp. FDL-2 (FDL-2) was capable of reducing Se(iv) and Te(iv) through a novel riboflavin-involved pathway. The results showed that FDL-2 can effectively reduce 10 mM Se(iv) and 5 mM Te(iv) to Se0 and Te0, which was further confirmed by XPS and XRD analyses. RT-qPCR results indicate the upregulation of genes coding flavin-related proteins, and the production of flavin-related substances by strain FDL-2 during Se(iv)/Te(iv) bioreduction was proven by fluorescence chromatography analysis. In addition, the presence of riboflavin enhanced the electron transfer efficiency, indicating its promoting effect on the bioreduction of Se(iv)/Te(iv). Overall, our results highlight a riboflavin-involved electron transfer pathway during Se(iv)/Te(iv) bioreduction and thus deepen our understanding of the corresponding mechanism.

Make Selenium Reactive Again: Activating Elemental Selenium for Synthesis of Metal Selenides Ranging from Nanocrystals to Large Single Crystals

The inertness of elemental selenium is a significant obstacle in the synthesis of selenium-containing materials at low reaction temperatures. Over the years, several recipes have been developed to overcome this hurdle; however, most of the methods are associated with the use of highly toxic, expensive, and environmentally harmful reagents. As such, there is an increasing demand for the design of cheap, stable, and nontoxic reactive selenium precursors usable in the low-temperature synthesis of transition metal selenides with vast applications in nanotechnology, thermoelectrics, and superconductors. Herein, a novel synthetic route has been developed for activating elemental selenium by using a solvothermal approach. By comprehensive 77Se NMR, Raman, and infrared spectroscopies and gas chromatography-mass spectrometry, we show that the activated Se solution contained HSe-, [Se-Se]2-, and Se2- ions, as well as dialkyl selenide (R2Se) and dialkyl diselenide (R-Se-Se-R) species in dynamic equilibrium. This also corresponded to the first observation of naked Se22- in solution. The versatility of the developed Se precursor was demonstrated by the successful synthesis of (i) the polycrystalline room-temperature modification of the β-Ag2Se thermoelectric material; (ii) large single crystals of superconducting β-FeSe; (iii) CdSe nanocrystals with different particle sizes (3-10 nm); (iv) nanosheets of PtSe2; and (v) mono- and dibenzyl selenides and diselenides at room temperature. The simplicity and diversity of the developed Se activation method holds promise for applied and fundamental research.

Colloidal Semiconductor Nanocrystal Lasers and Laser Diodes

Lasers and optical amplifiers based on solution-processable materials have been long-desired devices for their compatibility with virtually any substrate, scalability, and ease of integration with on-chip photonics and electronics. These devices have been pursued across a wide range of materials including polymers, small molecules, perovskites, and chemically prepared colloidal semiconductor nanocrystals, also commonly referred to as colloidal quantum dots. The latter materials are especially attractive for implementing optical-gain media as in addition to being compatible with inexpensive and easily scalable chemical techniques, they offer multiple advantages derived from a zero-dimensional character of their electronic states. These include a size-tunable emission wavelength, low optical gain thresholds, and weak sensitivity of lasing characteristics to variations in temperature. Here we review the status of colloidal nanocrystal lasing devices, most recent advances in this field, outstanding challenges, and the ongoing progress toward technological viable devices including colloidal quantum dot laser diodes.

Advancements in Perovskite Nanocrystal Stability Enhancement: A Comprehensive Review

Over the past decade, perovskite technology has been increasingly applied in solar cells, nanocrystals, and light-emitting diodes (LEDs). Perovskite nanocrystals (PNCs) have attracted significant interest in the field of optoelectronics owing to their exceptional optoelectronic properties. Compared with other common nanocrystal materials, perovskite nanomaterials have many advantages, such as high absorption coefficients and tunable bandgaps. Owing to their rapid development in efficiency and huge potential, perovskite materials are considered the future of photovoltaics. Among different types of PNCs, CsPbBr₃ perovskites exhibit several advantages. CsPbBr₃ nanocrystals offer a combination of enhanced stability, high photoluminescence quantum yield, narrow emission bandwidth, tunable bandgap, and ease of synthesis, which distinguish them from other PNCs, and make them suitable for various applications in optoelectronics and photonics. However, PNCs also have some shortcomings: they are highly susceptible to degradation caused by environmental factors, such as moisture, oxygen, and light, which limits their long-term performance and hinders their practical applications. Recently, researchers have focused on improving the stability of PNCs, starting with the synthesis of nanocrystals and optimizing (i) the external encapsulation of crystals, (ii) ligands used for the separation and purification of nanocrystals, and (iii) initial synthesis methods or material doping. In this review, we discuss in detail the factors leading to instability in PNCs, introduce stability enhancement methods for mainly inorganic PNCs mentioned above, and provide a summary of these approaches.

Noble metal nanodendrites: growth mechanisms, synthesis strategies and applications

Inorganic nanodendrites (NDs) have become a kind of advanced nanomaterials with broad application prospects because of their unique branched architecture. The structural characteristics of nanodendrites include highly branched morphology, abundant tips/edges and high-index crystal planes, and a high atomic utilization rate, which give them great potential for usage in the fields of electrocatalysis, sensing, and therapeutics. Therefore, the rational design and controlled synthesis of inorganic (especially noble metals) nanodendrites have attracted widespread attention nowadays. The development of synthesis strategies and characterization methodology provides unprecedented opportunities for the preparation of abundant nanodendrites with interesting crystallographic structures, morphologies, and application performances. In this review, we systematically summarize the formation mechanisms of noble metal nanodendrites reported in recent years, with a special focus on surfactant-mediated mechanisms. Some typical examples obtained by innovative synthetic methods are then highlighted and recent advances in the application of noble metal nanodendrites are carefully discussed. Finally, we conclude and present the prospects for the future development of nanodendrites. This review helps to deeply understand the synthesis and application of noble metal nanodendrites and may provide some inspiration to develop novel functional nanomaterials (especially electrocatalysts) with enhanced performance.

Anisotropic Heavy-Metal-Free Semiconductor Nanocrystals: Synthesis, Properties, and Applications

Heavy-metal (Cd, Hg, and Pb)-containing semiconductor nanocrystals (NCs) have been explored widely due to their unique optical and electrical properties. However, the toxicity risks of heavy metals can be a drawback of heavy-metal-containing NCs in some applications. Anisotropic heavy-metal-free semiconductor NCs are desirable replacements and can be realized following the establishment of anisotropic growth mechanisms. These anisotropic heavy-metal-free semiconductor NCs can possess lower toxicity risks, while still exhibiting unique optical and electrical properties originating from both the morphological and compositional anisotropy. As a result, they are promising light-emitting materials in use various applications. In this review, we provide an overview on the syntheses, properties, and applications of anisotropic heavy-metal-free semiconductor NCs. In the first section, we discuss hazards of heavy metals and introduce the typical heavy-metal-containing and heavy-metal-free NCs. In the next section, we discuss anisotropic growth mechanisms, including solution-liquid-solid (SLS), oriented attachment, ripening, templated-assisted growth, and others. We discuss mechanisms leading both to morphological anisotropy and to compositional anisotropy. Examples of morphological anisotropy include growth of nanorods (NRs)/nanowires (NWs), nanotubes, nanoplatelets (NPLs)/nanosheets, nanocubes, and branched structures. Examples of compositional anisotropy, including heterostructures and core/shell structures, are summarized. Third, we provide insights into the properties of anisotropic heavy-metal-free NCs including optical polarization, fast electron transfer, localized surface plasmon resonances (LSPR), and so on, which originate from the NCs' anisotropic morphologies and compositions. Finally, we summarize some applications of anisotropic heavy-metal-free NCs including catalysis, solar cells, photodetectors, lighting-emitting diodes (LEDs), and biological applications. Despite the huge progress on the syntheses and applications of anisotropic heavy-metal-free NCs, some issues still exist in the novel anisotropic heavy-metal-free NCs and the corresponding energy conversion applications. Therefore, we also discuss the challenges of this field and provide possible solutions to tackle these challenges in the future.

Reversible Facet Reconstruction of CdSe/CdS Core/Shell Nanocrystals by Facet-Ligand Pairing

Synthesis of colloidal semiconductor nanocrystals with defined facet structures is challenging, though such nanocrystals are essential for fully realizing their size-dependent optical and optoelectronic properties. Here, for the mostly developed colloidal wurtzite CdSe/CdS core/shell nanocrystals, facet reconstruction is investigated under typical synthetic conditions, excluding nucleation, growth, and interparticle ripening. Within the reaction time window, two reproducible sets of facets─each with a specific group of low-index facets─can be reversibly reconstructed by switching the ligand system, indicating thermodynamic stability of each set. With a unique <0001> axis, atomic structures of the low-index facets of wurtzite nanocrystals are diverse. Experimental and theoretical studies reveal that each facet in a given set is paired with a common ligand in the solution, namely, either fatty amine and/or cadmium alkanoate. The robust bonding modes of ligands are found to be strongly facet-dependent and often unconventional, instead of following Green's classification. Results suggest that facet-controlled nanocrystals can be synthesized by optimal facet-ligand pairing either in synthesis or after-synthesis reconstruction, implying semiconductor nanocrystal formation with size-dependent properties down to an atomic level.

Electronic Structure and Excited State Dynamics of Cadmium Chalcogenide Nanorods

The cylindrical quasi-one-dimensional shape of colloidal semiconductor nanorods (NRs) gives them unique electronic structure and optical properties. In addition to the band gap tunability common to nanocrystals, NRs have polarized light absorption and emission and high molar absorptivities. NR-shaped heterostructures feature control of electron and hole locations as well as light emission energy and efficiency. We comprehensively review the electronic structure and optical properties of Cd-chalcogenide NRs and NR heterostructures (e.g., CdSe/CdS dot-in-rods, CdSe/ZnS rod-in-rods), which have been widely investigated over the last two decades due in part to promising optoelectronic applications. We start by describing methods for synthesizing these colloidal NRs. We then detail the electronic structure of single-component and heterostructure NRs and follow with a discussion of light absorption and emission in these materials. Next, we describe the excited state dynamics of these NRs, including carrier cooling, carrier and exciton migration, radiative and nonradiative recombination, multiexciton generation and dynamics, and processes that involve trapped carriers. Finally, we describe charge transfer from photoexcited NRs and connect the dynamics of these processes with light-driven chemistry. We end with an outlook that highlights some of the outstanding questions about the excited state properties of Cd-chalcogenide NRs.

Rich Landscape of Colloidal Semiconductor-Metal Hybrid Nanostructures: Synthesis, Synergetic Characteristics, and Emerging Applications

Nanochemistry provides powerful synthetic tools allowing one to combine different materials on a single nanostructure, thus unfolding numerous possibilities to tailor their properties toward diverse functionalities. Herein, we review the progress in the field of semiconductor-metal hybrid nanoparticles (HNPs) focusing on metal-chalcogenides-metal combined systems. The fundamental principles of their synthesis are discussed, leading to a myriad of possible hybrid architectures including Janus zero-dimensional quantum dot-based systems and anisotropic quasi 1D nanorods and quasi-2D platelets. The properties of HNPs are described with particular focus on emergent synergetic characteristics. Of these, the light-induced charge-separation effect across the semiconductor-metal nanojunction is of particular interest as a basis for the utilization of HNPs in photocatalytic applications. The extensive studies on the charge-separation behavior and its dependence on the HNPs structural characteristics, environmental and chemical conditions, and light excitation regime are surveyed. Combining the advanced synthetic control with the charge-separation effect has led to demonstration of various applications of HNPs in different fields. A particular promise lies in their functionality as photocatalysts for a variety of uses, including solar-to-fuel conversion, as a new type of photoinitiator for photopolymerization and 3D printing, and in novel chemical and biomedical uses.

Synthesis of Colloidal GaN and AlN Nanocrystals in Biphasic Molten Salt/Organic Solvent Mixtures under High-Pressure Ammonia

Group III nitrides are of great technological importance for electronic devices. These materials have been widely manufactured via high-temperature methods such as physical vapor transport (PVT), chemical vapor deposition (CVD), and hydride vapor phase epitaxy (HVPE). The preparation of group III nitrides by colloidal synthesis methods would provide significant advantages in the form of optical tunability via size and shape control and enable cost reductions through scalable solution-based device integration. Solution syntheses of III-nitride nanocrystals, however, have been scarce, and the quality of the synthesized products has been unsatisfactory for practical use. Here, we report that incorporating a molten salt phase in solution synthesis can provide a viable option for producing crystalline III-nitride nanomaterials. Crystalline GaN and AlN nanomaterials can be grown in a biphasic molten-salt/organic-solvent mixture under an ammonia atmosphere at moderate temperatures (less than 300 °C) and stabilized under ambient conditions by postsynthetic treatment with organic surface ligands. We suggest that microscopic reversibility of monomer attachment, which is essential for crystalline growth, can be achieved in molten salt during the nucleation and the growth of the III-nitride nanocrystals. We also show that increased ammonia pressure increases the size of the GaN nanocrystals produced. This work demonstrates that use of molten salt and high-pressure reactants significantly expands the chemical scope of solution synthesis of inorganic nanomaterials.

Indium arsenide quantum dots: an alternative to lead-based infrared emitting nanomaterials

Colloidal quantum dots (QDs) emitting in the infrared (IR) are promising building blocks for numerous photonic, optoelectronic and biomedical applications owing to their low-cost solution-processability and tunable emission. Among them, lead- and mercury-based QDs are currently the most developed materials. Yet, due to toxicity issues, the scientific community is focusing on safer alternatives. In this regard, indium arsenide (InAs) QDs are one of the best candidates as they can absorb and emit light in the whole near infrared spectral range and they are RoHS-compliant, with recent trends suggesting that there is a renewed interest in this class of materials. This review focuses on colloidal InAs QDs and aims to provide an up-to-date overview spanning from their synthesis and surface chemistry to post-synthesis modifications. We provide a comprehensive overview from initial synthetic methods to the most recent developments on the ability to control the size, size distribution, electronic properties and carrier dynamics. Then, we describe doping and alloying strategies applied to InAs QDs as well as InAs based heterostructures. Furthermore, we present the state-of-the-art applications of InAs QDs, with a particular focus on bioimaging and field effect transistors. Finally, we discuss open challenges and future perspectives.

Design Principles of Colloidal Nanorod Heterostructures

Anisotropic heterostructures of colloidal nanocrystals embed size-, shape-, and composition-dependent electronic structure within variable three-dimensional morphology, enabling intricate design of solution-processable materials with high performance and programmable functionality. The key to designing and synthesizing such complex materials lies in understanding the fundamental thermodynamic and kinetic factors that govern nanocrystal growth. In this review, nanorod heterostructures, the simplest of anisotropic nanocrystal heterostructures, are discussed with respect to their growth mechanisms. The effects of crystal structure, surface faceting/energies, lattice strain, ligand sterics, precursor reactivity, and reaction temperature on the growth of nanorod heterostructures through heteroepitaxy and cation exchange reactions are explored with currently known examples. Understanding the role of various thermodynamic and kinetic parameters enables the controlled synthesis of complex nanorod heterostructures that can exhibit unique tailored properties. Selected application prospects arising from such capabilities are then discussed.

Colloidal nanomaterials for water quality improvement and monitoring

Water is the most important resource for all kind forms of live. It is a vital resource distributed unequally across different regions of the globe, with populations already living with water scarcity, a situation that is spreading due to the impact of climate change. The reversal of this tendency and the mitigation of its disastrous consequences is a global challenge posed to Humanity, with the scientific community assuming a major obligation for providing solutions based on scientific knowledge. This article reviews literature concerning the development of nanomaterials for water purification technologies, including collaborative scientific research carried out in our laboratory (nanoLAB@UA) framed by the general activities carried out at the CICECO-Aveiro Institute of Materials. Our research carried out in this specific context has been mainly focused on the synthesis and surface chemical modification of nanomaterials, typically of a colloidal nature, as well as on the evaluation of the relevant properties that arise from the envisaged applications of the materials. As such, the research reviewed here has been guided along three thematic lines: 1) magnetic nanosorbents for water treatment technologies, namely by using biocomposites and graphite-like nanoplatelets; 2) nanocomposites for photocatalysis (e.g., TiO2/Fe3O4 and POM supported graphene oxide photocatalysts; photoactive membranes) and 3) nanostructured substrates for contaminant detection using surface enhanced Raman scattering (SERS), namely polymers loaded with Ag/Au colloids and magneto-plasmonic nanostructures. This research is motivated by the firm believe that these nanomaterials have potential for contributing to the solution of environmental problems and, conversely, will not be part of the problem. Therefore, assessment of the impact of nanoengineered materials on eco-systems is important and research in this area has also been developed by collaborative projects involving experts in nanotoxicity. The above topics are reviewed here by presenting a brief conceptual framework together with illustrative case studies, in some cases with original research results, mainly focusing on the chemistry of the nanomaterials investigated for target applications. Finally, near-future developments in this research area are put in perspective, forecasting realistic solutions for the application of colloidal nanoparticles in water cleaning technologies.

Dimensional Transformation of Molecular Magnetic Materials

Two-dimensional (2D) magnetic layered materials have revolutionized size dependent magnetism to manipulate spin-based devices. However, it has been challenging to artificially create 2D magnetic materials from three-dimensional (3D) crystal structures with a variety of material groups. Here, we present the dimensionality manipulation via cation exchange of a 3D Prussian blue analogue [RbMnFe(CN)₆] toward a 2D magnetic sheet [(K,Rb)(V,Mn)(Cr,Fe)(CN)₆] with the magnetic ordering temperature rising from 12 to 330 K. Such a 2D magnetic sheet achieves crystalline V-Cr coordination in the Prussian blue lattice with pronounced anisotropy and stimuli responsiveness. The pressure dependent magnetic tunability of such 2D networks is predicted using first-principles calculations and demonstrated using the phase transitions of the hydrogel. This previously unobserved phenomenon of dimensional manipulation of a bulk crystal structure provides a rational strategy to expand the diversity and chemical compositions of 2D molecular magnetic material libraries.

Photoluminescent, "ice-cream cone" like Cu-In-(Zn)-S/ZnS nanoheterostructures

Copper based ternary and quaternary quantum confined nanostructures have attracted huge attention over recent years due to their potential applications in photonics, photovoltaics, imaging, sensing and other areas. However, anisotropic nanoheterostructures of this type are still poorly explored to date, despite numerous predictions of the distinctive optical properties of these highly fluorescent heavy metal free nanostructures. Here, we report new fluorescent multicomponent Cu-In-(Zn)-S/ZnS nanoheterostructures with a unique anisotropic "ice-cream cone" like morphology. These nanostructures have been prepared with a seeded growth technique and exhibit distinct photophysical properties with maximum emission in the visible range (≈ 640 nm) and long photoluminescence lifetimes (τ_average ≥ 300 ns). In depth time interval studies have been carried out to better understand the step by step growth mechanism of this distinct "ice-cream cone" like geometry. We have demonstrated that the crystal structure evolution from the zinc blende Cu-In-S core to the wurtzite "ice cream cone" like Cu-In-(Zn)-S/ZnS nanocrystals plays a key role in the origin of this morphology. This research opens new possibilities to produce unique fluorescent Cu-based multicomponent anisotropic heteronanostructures, while also offering a distinctive insight into the design of bespoke nanostructures, which could find a range of potential applications.

A microfluidic approach for synthesis and kinetic profiling of branched gold nanostructures

Automatized approaches for nanoparticle synthesis and characterization represent a great asset to their applicability in the biomedical field by improving reproducibility and standardization, which help to meet the selection criteria of regulatory authorities. The scaled-up production of nanoparticles with carefully defined characteristics, including intrinsic morphological features, and minimal intra-batch, batch-to-batch, and operator variability, is an urgent requirement to elevate nanotechnology towards more trustable biological and technological applications. In this work, microfluidic approaches were employed to achieve fast mixing and good reproducibility in synthesizing a variety of gold nanostructures. The microfluidic setup allowed exploiting spatial resolution to investigate the growth evolution of the complex nanoarchitectures. By physically isolating intermediate reaction fractions, we performed an advanced characterization of the shape properties during their growth, not possible with routine characterization methods. Employing an in-house developed method to assign a specific identity to shapes, we followed the particle growth/deformation process and identified key reaction parameters for more precise control of the generated morphologies. Besides, this investigation led to the optimization of a one-pot multi-size and multi-shape synthesis of a variety of gold nanoparticles. In summary, we describe an optimized platform for highly controlled synthesis and a novel approach for the mechanistic study of shape-evolving nanomaterials.

Enhanced visible light-driven photocatalytic performance of CdSe nanorods

Cadmium selenide (CdSe) semiconductor nanorods are prepared in hydrothermal process using hydrazine hydrate (N₂H₄.H₂O) and ammonia (NH₃.H₂O) as reducing agents. The reaction time is increased to 7 h and the amount of hydrazine hydrate used is also increased to 15 mL which have resulted in diminished stacking faults in the CdSe nanorods prepared. The crystal structure, morphological variations, and size of the prepared CdSe nanorods are examined by XRD analysis. The crystalline size of the CdSe nanorods is 20-30 nm in diameter. HRTEM images reveal the formation of high order CdSe nanorods of the length about 25-40 nm. The bandgap in the CdSe nanoparticles is determined to be 2.17 eV. The peak at 595 nm in photoluminescence (PL) spectrum indicates oxygen vacancy defects in the prepared CdSe sample. The variation of dielectric properties with respect to temperature and frequency of pelletized CdSe is studied. High photocatalytic efficiency (98%) of catalyst/H₂O₂ is also achieved for decomposition of Rhodamine-B dye.

Multi-step atomic mechanism of platinum nanocrystals nucleation and growth revealed by in-situ liquid cell STEM

The understanding of crystal growth mechanisms has broadened substantially. One significant advancement is based in the conception that the interaction between particles plays an important role in the growth of nanomaterials. This is in contrast to the classical model, which neglects this process. Direct imaging of such processes at atomic-level in liquid-phase is essential for establishing new theoretical models that encompass the full complexity of realistic scenarios and eventually allow for tailoring nanoparticle growth. Here, we investigate at atomic-scale the exact growth mechanisms of platinum nanocrystals from single atom to final crystals by in-situ liquid phase scanning transmission electron microscopy. We show that, after nucleation, the nanocrystals grow via two main stages: atomic attachment in the first stage, where the particles initially grow by attachment of the atoms until depletion of the surrounding zone. Thereafter, follows the second stage of growth, which is based on particle attachment by different atomic pathways to finally form mature nanoparticles. The atomic mechanisms underlying these growth pathways are distinctly different and have different driving forces and kinetics as evidenced by our experimental observations.

Catalyzed Growth for Atomic-Precision Colloidal Chalcogenide Nanowires and Heterostructures: Progress and Perspective

One-dimensional colloidal semiconductor nanowires are of wide interest in nanoscale electronics and photonics. As compared to the zero-dimensional counterparts, their geometrical anisotropy offers an additional degree of freedom to tailor the electronic and optical properties and enables customized heterostructures with increased complexity. The colloidal synthetic chemistry developed over past decades has fueled the emergence of diverse one-dimensional nanocrystals and heterostructures, whereas the synthetic pursuit for compositionally and structurally defining them at the atomic-level precision remains yet a giant challenge. Catalyzed growth, wherein nanowires grow at the catalyst-nanowire interfaces in a layer-by-layer manner, offers a promising path toward such an ultimate goal. In this Perspective, we will take a close look at how catalyzed growth would enable the on-demand, atomic-precision control of colloidal nanowires and their heterostructures. We then further highlight their potentials for constructing higher-order heteroarchitectures with new and/or enhanced performances. Finally, we conclude with a forward-looking perspective on future challenges.

Mapping the effect of geometry on the radiative rate in core/shell QDs: core size dictates the conduction band offset

Computational models have been developed that can accurately predict the electronic structure and thus optical properties of a variety of quantum dot (QD) materials. However, the application of these models to core/shell and other heterostructured QDs has received less experimental corroboration owing to the difficulty in systematically synthesizing and characterizing large ranges of geometries. In the current work, we synthesized a library of core/shell CdSe/CdS QDs with varying core sizes and shell thicknesses, and have characterized their radiative recombination rates. We find that the core size has only a modest effect on the radiative recombination rates, far less than is predicted by conventional effective mass models. In order to theoretically describe the experimental data, we performed an empirical modification of an effective mass model. We find that the conduction band offset between CdSe and CdS must be empirically altered based on QD core size in order to match our experimental data. This is hypothesized to be a result of reduced interfacial strain in core/shell QDs with smaller cores. The resultant relationship between conduction band offset and core size is used to create a predictive map of radiative lifetime as a function of core size and shell thickness. This map will be useful to researchers implementing CdSe/CdS core/shell QDs for a variety of applications since it can provide geometry specific excited state lifetimes.

Predicting the radiative rate in CdSe/CdS core/shell quantum dots is made possible by using a core-size-dependent conduction band offset.

Mapping Composition-Selectivity Relationships of Supported Sub-10 nm Cu-Ag Nanocrystals for High-Rate CO2 Electroreduction

Colloidal Cu-Ag nanocrystals measuring less than 10 nm across are promising candidates for integration in hybrid CO₂ reduction reaction (CO₂RR) interfaces, especially in the context of tandem catalysis and selective multicarbon (C₂-C₃) product formation. In this work, we vary the synthetic-ligand/copper molar ratio from 0.1 to 1.0 and the silver/copper atomic ratio from 0 to 0.7 and study the variations in the nanocrystals' size distribution, morphology and reactivity at rates of ≥100 mA cm^-2 in a gas-fed recycle electrolyzer operating under neutral to mildly basic conditions (0.1-1.0 M KHCO₃). High-resolution electron microscopy and spectroscopy are used in order to characterize the morphology of sub-10 nm Cu-Ag nanodimers and core-shells and to elucidate trends in Ag coverage and surface composition. It is shown that Cu-Ag nanocrystals can be densely dispersed onto a carbon black support without the need for immediate ligand removal or binder addition, which considerably facilitates their application. Although CO₂RR product distribution remains an intricate function of time, (kinetic) overpotential and processing conditions, we nevertheless conclude that the ratio of oxygenates to hydrocarbons (which depends primarily on the initial dispersion of the nanocrystals and their composition) rises 3-fold at moderate Ag atom % relative to Cu NCs-based electrodes. Finally, the merits of this particular Cu-Ag/C system and the recycling reactor employed are utilized to obtain maximum C₂-C₃ partial current densities of 92-140 mA cm^-2 at -1.15 V_RHE and liquid product concentrations in excess of 0.05 wt % in 1 M KHCO₃ after short electrolysis periods.

Decoupling Radiative and Auger Processes in Semiconductor Nanocrystals by Shape Engineering

One of the most challenging aspects of semiconductor nanotechnology is the presence of extremely efficient nonradiative decay pathways (known as Auger processes) that hinder any attempt at creating population inversion and obtaining gain in nanocrystals. What is even more frustrating is that, in most cases, the strategies adopted to slow down Auger in these nanostructures also lead to a comparable increase in the radiative recombination times, so that there is no overall improvement from the point of view of their applicability as emissive media. Here we present a comprehensive theoretical characterization of CdTe tetrapods and show that in these versatile nanostructures it is possible to achieve a complete decoupling between radiative and Auger processes, where the latter can be strongly suppressed compared to spherical structures, by careful shape engineering, without affecting the efficiency of radiative recombination.

Inverse-designed semiconductor nanocatalysts for targeted CO2 reduction in water

The most commonly used photocatalyst for CO₂ reduction is TiO₂. However, this semiconductor material is far from being ideally suited for this purpose, owing to its inefficient energy harvesting (it absorbs in the UV), low reduction rates (it exhibits short carrier lifetimes), and lack of selectivity with respect to competing reactions (such as the nearly isoenergetic and kinetically more favourable water reduction). In this work we compile a wish-list of properties for the ideal photocatalyst (including high reaction selectivity, availability of multiple redox equivalents at one time, large contact area for CO₂ adsorption with independently tunable band gap, and availability of electrons and holes at different locations on the surface for the two redox reactions to take place), and, using the principles of inverse design, we engineer a semiconductor nanostructure that not only meets all the necessary fundamental criteria to act as a catalyst for CO₂ reduction, but also exhibits all the wish-list properties, as confirmed by our state-of-the-art atomistic semi-empirical pseudopotential modelling. The result is a potentially game-changing material.

Understanding Discrete Growth in Semiconductor Nanocrystals: Nanoplatelets and Magic-Sized Clusters

ConspectusSemiconductor nanocrystals (NCs) fluoresce with a color that strongly depends on their size and shape. Thus, to obtain homogeneous optical properties, researchers have strived to synthesize particles that are uniform. However, because NCs typically grow through continuous, incremental addition of material, slight differences in the growth process between individual crystallites yield statistical distributions in size and shape, leading to inhomogeneities in their optical characteristics. Much work has focused on improving synthetic protocols to control these distributions and enhance performance. Interestingly, during these efforts, several syntheses were discovered that exhibit a different type of growth process. The NCs jump from one discrete size to the next. Through purification methods, one of these sizes can then be isolated, providing a different approach to uniform NCs. Unfortunately, the fundamental mechanism behind such discrete growth and how it differs from the conventional continuous process have remained poorly understood.Discrete growth has been observed in two major classes of NCs: semiconductor nanoplatelets (NPLs) and magic-sized clusters (MSCs). NPLs are quasi-two-dimensional crystallites that exhibit a precise thickness of only a few atomic layers but much larger lateral dimensions. During growth, NPLs slowly appear with an increasing number of monolayers. By halting this process at a specific time, NPLs with a desired thickness can then be isolated (e.g., four monolayers). Because the optical properties are primarily governed by this thickness, which is uniform, NPLs exhibit improved optical properties such as narrower fluorescence line widths.While NPLs have highly anisotropic shapes and show discrete growth only in one dimension (thickness), MSCs are isotropic particles. The name "magic" arose because a specific set of NC sizes appear during synthesis. They have been believed to represent special atomic arrangements that possess enhanced structural stability. Historically, they were very small, hence molecular-scale "clusters." Isolation of one of the MSC sizes can then, in principle, provide a uniform sample of NCs. More recently, MSC growth has been extended to larger sizes, beyond what is commonly considered to be the "cluster" regime, challenging the conventional explanation for these materials.This Account summarizes recent work by our group to understand the mechanism that governs discrete growth in semiconductor NCs. We begin by describing the synthesis of NPLs. Next, we discuss the mechanism behind the highly anisotropic shape of NPLs. We build on this by examining the ripening process in NPLs. We show that NPLs slowly appear with increasing thickness, counterintuitively through lateral growth. Then, we turn to the synthesis of MSCs, in particular focusing on their growth mechanism. Our findings indicate a strong connection between NPLs and MSCs. Finally, we review several remaining challenges for the growth of NPLs and MSCs and give a brief outlook on the future of discrete growth. By understanding the underlying process, we believe that it can be exploited more broadly, potentially moving us toward more uniform nanomaterials.

Manipulation of Interfacial Diffusion for Controlling Nanoscale Transformation

The unprecedented development of inorganic nanostructure synthesis has paved the way toward their broad applications in areas such as food science, agroforestry, energy conversion, and biomedicine. The precise manipulation of the nucleation and subsequent growth has been recognized as the central guiding principle for controlling the size and morphology of the nanostructures. However, conventional colloid syntheses based on direct precipitation reactions still have limitations in their versatility and extendibility. The crystal structure of a material determines the limited number of possible morphologies that its nanostructures can adopt. Further, as nucleation and growth kinetics are sensitive to not only the nature of the precipitation reactions but also ligands and ripening effect, rigorous control of reaction conditions must be established for every specific synthesis. In addition, multiple experimental parameters are entangled with each other, thereby requiring rigorous control of all reaction conditions. As a result, it is usually challenging to extend a synthetic recipe from one material to another. As an alternative method, the direct transformation of existing nanostructures into target ones has become an effective and robust approach capable of creating various complex nanostructures that are otherwise challenging to obtain using conventional methods. To this end, an in-depth understanding of nanoscale transformation toward the synthesis of inorganic nanostructures with diverse properties and applications is highly desirable.In this Account, we aim to reveal the critical effect of the interfacial diffusion on controlled nanoscale transformation. We first discuss how the interdiffusion rates determine the morphology and properties of bimetallic nanostructures. While equal interdiffusion rates lead to perfect mixing and generate fully alloyed nanostructures, interdiffusion at unequal rates creates vacancies in the fast diffusion side, which may cause dramatic morphological transformation to the nanostructures. Then, we introduce interfacial reactions, including the Kirkendall cavitation process, elimination reaction, and solid-state reaction, to promote the unbalanced interdiffusion and generalize nanoscale transformations in materials of various compositions, morphologies, and crystal structures. Finally, we discuss the use of capping ligands to inhibit the diffusion of atoms on one side of the interface in order to enable selective etching or transformation of the nanostructures. By modifying the nanostructured surface with specific capping ligands, the diffusion of surface atoms is restricted. When nanoparticles undergo chemical reactions (such as etching or heating), the outward diffusion of substances dominates, thereby successfully achieving chemical and morphological transformations. We believe that controlled interfacial diffusion can effectively manipulate nanoscale transformations, thus providing new strategies for the custom synthesis of multifunctional nanomaterials for various specific applications.

Shape controlled iron oxide nanoparticles: inducing branching and controlling particle crystallinity

Branched or multiply branched iron oxide nanoparticles are synthesized, the crystal domains rearrange forming single crystalline structures, that is crucial for efficient magnetic hyperthermia.

Heterogeneity induced dual luminescence properties of AgInS2 and AgInS2–ZnS alloyed nanocrystals

In the PL spectra of heterogeneous nanocrystals (In₂S₃–AgInS₂ and In₂S₃–AgInS₂–ZnS) two distinctly different peaks could be found at 430 and 710–515 nm.

Tolman's Electronic Parameter of the Ligand Predicts Phase in the Cation Exchange to CuFeS2Nanoparticles

The metastable and thermodynamically favored phases of CuFeS₂ are shown to be alternatively synthesized during partial cation exchange of hexagonal Cu₂S using various phosphorus-containing ligands. Transmission electron microscopy and energy dispersive spectroscopy mapping confirm the retention of the particle morphology and the approximate CuFeS₂ stoichiometry. Powder X-ray diffraction patterns and refinements indicate that the resulting phase mixtures of metastable wurtzite-like CuFeS₂ versus tetragonal chalcopyrite are correlated with the Tolman electronic parameter of the tertiary phosphorus-based ligand used during the cation exchange. Strong L-type donors lead to the chalcopyrite phase and weak donors to the wurtzite-like phase. To our knowledge, this is the first demonstration of phase control in nanoparticle synthesis using solely L-type donors.

Fast Neutron Imaging with Semiconductor Nanocrystal Scintillators

Fast neutrons offer high penetration capabilities for both light and dense materials due to their comparatively low interaction cross sections, making them ideal for the imaging of large-scale objects such as large fossils or as-built plane turbines, for which X-rays or thermal neutrons do not provide sufficient penetration. However, inefficient fast neutron detection limits widespread application of this technique. Traditional phosphors such as ZnS:Cu embedded in plastics are utilized as scintillators in recoil proton detectors for fast neutron imaging. However, these scintillation plates exhibit significant light scattering due to the plastic-phosphor interface along with long-lived afterglow (on the order of minutes), and therefore alternative solutions are needed to increase the availability of this technique. Here, we utilize colloidal nanocrystals (NCs) in hydrogen-dense solvents for fast neutron imaging through the detection of recoil protons generated by neutron scattering, demonstrating the efficacy of nanomaterials as scintillators in this detection scheme. The light yield, spatial resolution, and neutron-vs-gamma sensitivity of several chalcogenide (CdSe and CuInS₂)-based and perovskite halide-based NCs are determined, with only a short-lived afterglow (below the order of seconds) observed for all of these NCs. FAPbBr₃ NCs exhibit the brightest total light output at 19.3% of the commercial ZnS:Cu(PP) standard, while CsPbBrCl₂:Mn NCs offer the best spatial resolution at ∼2.6 mm. Colloidal NCs showed significantly lower gamma sensitivity than ZnS:Cu; for example, 79% of the FAPbBr₃ light yield results from neutron-induced radioluminescence and hence the neutron-specific light yield of FAPbBr₃ is 30.4% of that of ZnS:Cu(PP). Concentration and thickness-dependent measurements highlight the importance of increasing concentrations and reducing self-absorption, yielding design principles to optimize and foster an era of NC-based scintillators for fast neutron imaging.

Red Light-Emitting Diodes with All-Inorganic CsPbI3/TOPO Composite Nanowires Color Conversion Films

This work presents a method for obtaining a color-converted red light source through a combination of a blue GaN light-emitting diode and a red fluorescent color conversion film of a perovskite CsPbI3/TOPO composite. High-quality CsPbI3 quantum dots (QDs) were prepared using the hot-injection method. The colloidal QD solutions were mixed with different ratios of trioctylphosphine oxide (TOPO) to form nanowires. The color conversion films prepared by the mixed ultraviolet resin and colloidal solutions were coated on blue LEDs. The optical and electrical properties of the devices were measured and analyzed at an injection current of 50 mA; it was observed that the strongest red light intensity was 93.1 cd/m2 and the external quantum efficiency was 5.7% at a wavelength of approximately 708 nm when CsPbI3/TOPO was 1:0.35.

Resonant plasmon enhancement of light emission from CdSe/CdS nanoplatelets on Au nanodisk arrays

Semiconducting nanoplatelets (NPLs) have attracted great attention due to the superior photophysical properties compared to their quantum dot analogs. Understanding and tuning the optical and electronic properties of NPLs in a plasmonic environment is a new paradigm in the field of optoelectronics. Here, we report on the resonant plasmon enhancement of light emission including Raman scattering and photoluminescence from colloidal CdSe/CdS nanoplatelets deposited on arrays of Au nanodisks fabricated by electron beam lithography. The localized surface plasmon resonance (LSPR) of the Au nanodisk arrays can be tuned by varying the diameter of the disks. In the case of surface-enhanced Raman scattering (SERS), the Raman intensity profile follows a symmetric Gaussian shape matching the LSPR of the Au nanodisk arrays. The surface-enhanced photoluminescence (SEPL) profile of NPLs, however, follows an asymmetric Gaussian distribution highlighting a compromise between the excitation and emission enhancement mechanisms originating from energy transfer and Purcell effects. The SERS and SEPL enhancement factors depend on the nanodisk size and reach maximal values at 75 and 7, respectively, for the sizes, for which the LSPR energy of Au nanodisks coincides with interband transition energies in the semiconductor platelets. Finally, to explain the origin of the resonant enhancement behavior of SERS and SEPL, we apply a numerical simulation to calculate plasmon energies in Au nanodisk arrays and emission spectra from NPLs in such a plasmonic environment.

Chiral 3D CdSe Nanotetrapods

Three-dimensional (3D) nanomaterials have been intensively investigated because of their unique properties and wide range of potential applications; however, the ligand-induced chirality in 3D semiconductor nanocrystals has been scarcely studied. In this paper, we report the synthesis of hydrophobic 3D CdSe nanotetrapods (Tps) with a high degree of uniformity in their morphology by using the hot-injection method. The core and arms of Tps are distinct in their crystal structure, thus creating an intracrystal heterojunction. The size of Tps, primarily the length of four arms, is controlled by changing the amount of didecyldimethylammonium bromide and reaction time. Next, enantiopure cysteine ligands were introduced to replace the hydrophobic native stabilizers to prepare chiral l- and d-cysteine-capped CdSe Tps. Importantly, the circular dichroism (CD) line shapes of l/d-cysteine-capped CdSe Tps are assigned to the different excitonic transitions of the core and arms, respectively. In addition, the observed CD activities are found to be sensitive to the size of the CdSe Tps, where the anisotropic g factors have increased and reached the maximum value at a moderate aspect ratio (AR) and a further increase of the AR leads to a decrease of the g factor. Because of charge transfer between the core and arms, we propose a plausible mechanism potentially responsible for the induced CD line shapes in terms of the excitonic states of Tps with two different crystal structures. We believe that chiral 3D nanomaterials with anisotropic morphologies could offer new opportunities for relevant applications.

Synthesis and Photocatalytic Activity of β-Ga2O3 Nanostructures for Decomposition of Formaldehyde under Deep Ultraviolet Irradiation

An attempt to degrade volatile organic compounds (VOCs) and sterilize air simultaneously is highly desirable to improve indoor air quality. With the help of deep ultraviolet (UVC) lighting, harmful bacteria that exists in the air can be destroyed. Thus, a new photocatalytic substance that can break down VOCs under UVC irradiation is required. Here, we demonstrate the photocatalytic activity of β-Ga2O3 nanostructures, synthesized via the solvothermal method for removing formaldehyde (HCHO) under deep ultraviolet irradiation. The Raman and XRD results indicated that as-synthesized nanostructures showed β-crystalline phase with a monoclinic structure. The photoluminescence spectrum exhibited a broad and strong green emission peak at 510 nm, which was likely attributed to a considerable amount of oxygen and gallium vacancies formed during the calcinating process. The photocatalytic efficiency for decomposing HCHO at room temperature under deep ultraviolet irradiation (278 nm) of the synthesized β-Ga2O3 nanoparticles is higher than that of the β-Ga2O3 nanorods. Both nanoparticles and nanorods obeyed the pseudo-first-order Langmuir-Hinshelwood kinetic model with a degradation rate constant of 0.057 and 0.033 min−1, corresponding to the efficiency of 82% and 62% in the formaldehyde removal, respectively.

Atomically Precise Nanocrystals

Nanocrystals are a state-of-matter in the border area between molecules and bulk materials. Unlike bulk materials, nanocrystals have size-dependent properties, yet the question remains whether nanocrystal properties can be analyzed, understood, and controlled with atomic precision, a key characteristic of molecules. Acknowledging the inclination of nanocrystals to form defect structures, we first outline the prospects of atomically precise analysis. A broad spectrum of analytical methods has become available over the last five years, such that for heterogeneous nanocrystal ensembles, a single, atomically precise representative structure can be determined to explore structure-property relations. Atomically precise synthesis, on the other hand, remains an outstanding challenge that may well face fundamental limitations. However, to amplify properties and prepare nanocrystals for specific applications, full atomic precision may not be needed. Examples of an atomic precision light approach, focusing on exact thickness or facet control, exist and can inspire scientists to explore atomic precision in nanocrystal research further.

Bio-assisted preparation of efficiently architectured nanostructures of γ-Fe2O3 as a molecular recognition platform for simultaneous detection of biomarkers

Magnetic nanoparticles of iron oxide (γ-Fe2O3) have been prepared using bio-assisted method and their application in the field of biosensors is demonstrated. Particularly in this work, different nanostructures of γ-Fe2O3 namely nanospheres (NS), nanograsses (NG) and nanowires (NW) are prepared using a bio-surfactant namely Furostanol Saponin (FS) present in Fenugreek seeds extract through co-precipitation method by following "green" route. Three distinct morphologies of iron oxide nanostructures possessing the same crystal structure, magnetic properties, and varied size distribution are prepared and characterized. The resultant materials are analyzed using field emission scanning electron microscopy, transmission electron microscopy, powder X-ray diffraction, X-ray photoelectron spectroscopy, vibrating sample magnetometer and Fourier transform infrared spectroscopy. Moreover, the effect of reaction time and concentration of FS on the resultant morphologies of γ-Fe2O3 nanostructures are systematically investigated. Among different shapes, NWs and NSs of γ-Fe2O3 are found to exhibit better sensing behaviour for both the individual and simultaneous electrochemical detection of most popular biomarkers namely dopamine (DA) and uric acid (UA). Electrochemical studies reveal that γ-Fe2O3 NWs showed better sensing characteristics than γ-Fe2O3 NSs and NGs in terms of distinguishable voltammetric signals for DA and UA with enhanced oxidation current values. Differential pulse voltammetric studies exhibit linear dependence on DA and UA concentrations in the range of 0.15-75 µM and 5 μM - 0.15 mM respectively. The detection limit values for DA and UA are determined to be 150 nM and 5 µM. In addition γ-Fe2O3 NWs modified electrode showed higher sensitivity, reduced overpotential along with good selectivity towards the determination of DA and UA even in the presence of other common interferents. Thus the proposed biosensor electrode is very easy to fabricate, eco-friendly, cheaper and possesses higher surface area suggesting the unique structural patterns of γ-Fe2O3 nanostructures to be a promising candidate for electrochemical bio-sensing and biomedical applications.

Ligand-Induced Chirality in Asymmetric CdSe/CdS Nanostructures: A Close Look at Chiral Tadpoles

Ligand-induced chirality in asymmetric CdSe/CdS core-shell nanocrystals (NCs) has been extensively applied in chiral biosensors, regioselective syntheses and assemblies, circularly polarized luminescence (CPL), and chiroptic-based devices due to their excellent physiochemical properties, such as the tunable quantum confinement effects, surface functionality, and chemical stability. Herein, we present CdSe/CdS NCs with various morphologies such as nanoflowers, tadpoles, and dot/rods (DRs) with chirality induced by surface chiral ligands. The observed circular dichroism (CD) and CPL activities are closely associated with the geometrical characteristics of the nanostructures, such as the shell thickness and the aspect ratio of the CdSe/CdS NCs. Furthermore, in situ observations of the growth of tadpoles with a single tail indicate that the CD response is mainly attributed to the CdS shell, which has a maximum tail length of ∼45 nm (approximately λ/10 of the incident light wavelength). On the other hand, the CPL activity is only related to the CdSe core, and the activity benefits from a thin CdS shell with a relatively high photoluminescence quantum yield (QY). Further theoretical models demonstrated the aspect-ratio-dependent g-factor and QY variations in these asymmetric nanostructures. These findings provide insights into not only the asymmetric synthesis of CdSe/CdS NCs, but also the rational design of CdSe/CdS nanostructures with tunable CD and CPL activities.

Library Design of Ligands at the Surface of Colloidal Nanocrystals

Surfaces-and interfaces-are ubiquitous at the nanoscale. Their relevance to nanoscience and nanotechnology is therefore inherent. Colloidal inorganic nanocrystals (NCs), which can show more than a half of their atoms at the surface, are paradigmatic of the role of surfaces in determining materials' form and functions. Therefore, colloidal NCs may be regarded as soluble surfaces, allowing convenient study of ensemble structure and properties in the solution phase.Colloidal NCs commonly bear chemical species at their surface. Such species (generally referred to as ligands) are introduced already in the synthetic procedures and are added postsynthesis in surface chemistry modification (ligand exchange) reactions. Ligands (i) affect the reactivity and diffusion of the synthetic precursors, (ii) mediate NC interactions with the surroundings, and (iii) contribute to the overall electronic structure. In principle, a vast amount of ligands, as large as our imagination, could be used to coordinate the surface of colloidal NCs. In practice and despite the plethora of studies on NC surface chemistry, a relatively limited number of ligands have been explored. In addition, the importance of designing a set of ligands with tailored features (a ligand library), which may permit comprehensive discussion and explanation of the role of surfaces in the NC structure and properties, is often overlooked. Ligand libraries may also foster heuristic access to novel, unexpected observations.Here, the rational design of ligand libraries is discussed, suggesting that it may be a general method to advance knowledge on colloidal NCs and nanomaterials at large.First, a general ligand framework is introduced. The main subunits are identified: ligands are constituted by a binding group and a pendant moiety, bearing functional substituent groups. On this basis, ligand binding at the NC surface is discussed borrowing concepts from coordination chemistry. Dynamic equilibria at the NC surface are highlighted, revealing the compromise between forming and breaking bonds at interfaces and its intricate interplay with the surroundings. Tailoring of the ligand subunits may impart functions to the whole ligand, eventually transposable to the ligated NC.On these bases, it is shown how ligand design may be exploited to (i) exert control on the size and shape of the NCs, (ii) determine NCs' dispersibility in a solvent and affect their self-assembly, and (iii) tune the NCs' optical and electronic properties. These observations point to a description of colloidal NCs as un-decomposable species: ligands may be conceived as an integral part of the overall chemical and electronic structure of the colloidal NC and should not be considered as mere appendages that weakly perturb the inorganic core features.Finally, a perspective on the ligand library design is given. Function-oriented design of the ligand subunits is foreseen as an effective strategy to explore the chemical diversity space. High-throughput screening processes by using computation may represent a valuable tool for such an exploration. The whole ligand features, which depend on the subunits, can be implemented in the final NCs, providing feedback for refined design, toward a priori materials design. Ligand libraries can be fundamental to enabling colloidal NCs as reliable luminophores and (photo)catalysts.

Long-armed hexapod nanocrystals of cesium lead bromide

Colloidal lead halide perovskite nanocrystals (LHP NCs) assume a variety of morphologies (e.g. cubes, sheets, and wires). Their labile structural and surface characters allow them to undergo post-synthetic evolution of shape and crystallographic characters. Such transformations can be advantageous or deleterious, and it is therefore vital to both understand and exert control over these processes. In this study, we report novel long-armed hexapod structures of cesium lead bromide nanocrystals. These branched structures evolve from quantum-confined CsPbBr3 nanosheets to Cs4PbBr6 hexapods over a period of 24 hours. Time-resolved optical and structural characterization reveals a post-synthesis mechanism of phase transformation, oriented attachment and branch elongation. More generally, the study reveals important processes associated with LHP NC aging and demonstrates the utility of slow reaction kinetics in obtaining complex morphologies.

Colloidal-ALD-Grown Core/Shell CdSe/CdS Nanoplatelets as Seen by DNP Enhanced PASS-PIETA NMR Spectroscopy

Ligand exchange and CdS shell growth onto colloidal CdSe nanoplatelets (NPLs) using colloidal atomic layer deposition (c-ALD) were investigated by solid-state nuclear magnetic resonance (NMR) experiments, in particular, dynamic nuclear polarization (DNP) enhanced phase adjusted spinning sidebands-phase incremented echo-train acquisition (PASS-PIETA). The improved sensitivity and resolution of DNP enhanced PASS-PIETA permits the identification and study of the core, shell, and surface species of CdSe and CdSe/CdS core/shell NPLs heterostructures at all stages of c-ALD. The cadmium chemical shielding was found to be proportionally dependent on the number and nature of coordinating chalcogen-based ligands. DFT calculations permitted the separation of the the ^111/113Cd chemical shielding into its different components, revealing that the varying strength of paramagnetic and spin-orbit shielding contributions are responsible for the chemical shielding trend of cadmium chalcogenides. Overall, this study points to the roughening and increased chemical disorder at the surface during the shell growth process, which is not readily captured by the conventional characterization tools such as electron microscopy.

The non-stationary case of the Maxwell-Garnett theory: growth of nanomaterials (2D gold flakes) in solution

The solution-based growth mechanism is a common process for nanomaterials. The Maxwell-Garnett theory (for light-matter interactions) describes the solution growth in an effective medium, homogenized by a mean electromagnetic field, which applies when materials are in a stationary phase. However, the charge transitions (inter- and intra-transitions) during the growth of nanomaterials lead to a non-stationary phase and are associated with time-dependent permittivity constant transitions (for nanomaterials). Therefore, time-independence in the standard Maxwell-Garnett theory is lost, resulting in time dependence, ε _i(t). This becomes important when the optical spectrum of a solution needs to be deconvoluted at different reaction times since each peak represents a specific charge/energy transfer with a specific permittivity constant. Based on this, we developed a time-resolved deconvolution approach, f(t) ∝ ε _i(t), which led us to identify the transitions (inter- and intra-transitions) with their dominated growth regimes. Two gold ion peaks were precisely measured (322 nm and 367 nm) for the inter-transition, and three different polyaniline oxidation states (PAOS) for the intra-transition, including A (372 nm), B (680 nm), and C (530 nm). In the initial reaction time regime (0-90 min), the permittivity constant of gold was found to be highly dependent on time, i.e. f _E ∝ ε _i(t), since charge transfer takes place from the PAOS to gold ions (i.e. inter-transition leads to a reduction reaction). In the second time regime (90-180 min), the permittivity constant of gold changes as the material deforms from 3D to 2D (f _S ∝ ε _3D-2D), i.e. intra-transition (combined with thermal reduction). Our approach provides a new framework for the time-dependent modelling of (an)isotropic solutions of other nanomaterials and their syntheses.

Polymer Coated Semiconducting Nanoparticles for Hybrid Materials

This paper reviews synthetic concepts for the functionalization of various inorganic nanoparticles with a shell consisting of organic polymers and possible applications of the resulting hybrid materials. A polymer coating can make inorganic nanoparticles soluble in many solvents as individual particles and not only do low molar mass solvents become suitable, but also polymers as a solid matrix. In the case of shape anisotropic particles (e.g., rods) a spontaneous self-organization (parallel orientation) of the nanoparticles can be achieved, because of the formation of lyotropic liquid crystalline phases. They offer the possibility to orient the shape of anisotropic nanoparticles macroscopically in external electric fields. At least, such hybrid materials allow semiconducting inorganic nanoparticles to be dispersed in functional polymer matrices, like films of semiconducting polymers. Thereby, the inorganic nanoparticles can be electrically connected and addressed by the polymer matrix. This allows LEDs to be prepared with highly fluorescent inorganic nanoparticles (quantum dots) as chromophores. Recent works have aimed to further improve these fascinating light emitting materials.