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ChemInform Abstract: Cation Exchange: A Versatile Tool for Nanomaterials Synthesis

Abstract Review: 82 refs.

Synthesis of PbS Nanorods and Other Ionic Nanocrystals of Complex Morphology by Sequential Cation Exchange Reactions

We show that nanocrystals (NCs) with well-established synthetic protocols for high shape and size monodispersity can be used as templates to independently control the NC composition through successive cation exchange reactions. Chemical transformations like cation exchange reactions overcome a limitation in traditional colloidal synthesis, where the NC shape often reflects the inherent symmetry of the underlying lattice. Specifically we show that full or partial interconversion between wurtzite CdS, chalcocite Cu(2)S, and rock salt PbS NCs can occur while preserving anisotropic shapes unique to the as-synthesized materials. The exchange reactions are driven by disparate solubilites between the two cations by using ligands that preferentially coordinate to either monovalent or divalent transition metals. Starting with CdS, highly anisotropic PbS nanorods are created, which serve as an important material for studying strong two-dimensional quantum confinement, as well as for optoelectronic applications. In NC heterostructures containing segments of different materials, the exchange reaction can be made highly selective for just one of the components of the heterostructure. Thus, through precise control over ion insertion and removal, we can obtain interesting CdS|PbS heterostructure nanorods, where the spatial arrangement of materials is controlled through an intermediate exchange reaction.

Efficiency of Hole Transfer from Photoexcited Quantum Dots to Covalently Linked Molecular Species

Hole transfer from high photoluminescence quantum yield (PLQY) CdSe-core CdS-shell semiconductor nanocrystal quantum dots (QDs) to covalently linked molecular hole acceptors is investigated. (1)H NMR is used to independently calibrate the average number of hole acceptor molecules per QD, N, allowing us to measure PLQY as a function of N, and to extract the hole transfer rate constant per acceptor, kht. This value allows for reliable comparisons between nine different donor-acceptor systems with variant shell thicknesses and acceptor ligands, with kht spanning over 4 orders of magnitude, from single acceptor time constants as fast as 16 ns to as slow as 0.13 ms. The PLQY variation with acceptor coverage for all kht follows a universal equation, and the shape of this curve depends critically on the ratio of the total hole transfer rate to the sum of the native recombination rates in the QD. The dependence of kht on the CdS thickness and the chain length of the acceptor is investigated, with damping coefficients β measured to be (0.24 ± 0.025) Å(-1) and (0.85 ± 0.1) Å(-1) for CdS and the alkyl chain, respectively. We observe that QDs with high intrinsic PLQYs (>79%) can donate holes to surface-bound molecular acceptors with efficiencies up to 99% and total hole transfer time constants as fast as 170 ps. We demonstrate the merits of a system where ill-defined nonradiative channels are suppressed and well-defined nonradiative channels are engineered and quantified. These results show the potential of QD systems to drive desirable oxidative chemistry without undergoing oxidative photodegradation.

Real-Time Observation of Water-Soluble Mineral Precipitation in Aqueous Solution by <i>In Situ</i> High-Resolution Electron Microscopy

The precipitation and dissolution of water-soluble minerals in aqueous systems is a familiar process occurring commonly in nature. Understanding mineral nucleation and growth during its precipitation is highly desirable, but past in situ techniques have suffered from limited spatial and temporal resolution. Here, by using in situ graphene liquid cell electron microscopy, mineral nucleation and growth processes are demonstrated in high spatial and temporal resolution. We precipitate the mineral thenardite (Na2SO4) from aqueous solution with electron-beam-induced radiolysis of water. We demonstrate that minerals nucleate with a two-dimensional island structure on the graphene surfaces. We further reveal that mineral grains grow by grain boundary migration and grain rotation. Our findings provide a direct observation of the dynamics of crystal growth from ionic solutions.

Ion Exchange Synthesis of III–V Nanocrystals

III-V nanocrystals displaying high crystallinity and low size dispersity are difficult to access by direct synthesis from molecular precursors. Here, we demonstrate that cation exchange of cadmium pnictide nanocrystals with group 13 ions yields monodisperse, crystalline III-V nanocrystals, including GaAs, InAs, GaP, and InP. This report highlights the versatility of cation exchange for accessing nanocrystals with covalent lattices.

Nanocrystal Diffusion in a Liquid Thin Film Observed by in Situ Transmission Electron Microscopy

We have directly observed motion of inorganic nanoparticles during fluid evaporation using a transmission electron microscope. Tracking real-time diffusion of both spherical (5-15 nm) and rod-shaped (5 x 10 nm) gold nanocrystals in a thin film of water-15% glycerol reveals complex movements, such as rolling motions coupled to large-step movements and macroscopic violations of the Stokes-Einstein relation for diffusion. As drying patches form during the final stages of evaporation, particle motion is dominated by the nearby retracting liquid front.

Photovoltaic Performance of Ultrasmall PbSe Quantum Dots

We investigated the effect of PbSe quantum dot size on the performance of Schottky solar cells made in an ITO/PEDOT/PbSe/aluminum structure, varying the PbSe nanoparticle diameter from 1 to 3 nm. In this highly confined regime, we find that the larger particle bandgap can lead to higher open-circuit voltages (∼0.6 V), and thus an increase in overall efficiency compared to previously reported devices of this structure. To carry out this study, we modified existing synthesis methods to obtain ultrasmall PbSe nanocrystals with diameters as small as 1 nm, where the nanocrystal size is controlled by adjusting the growth temperature. As expected, we find that photocurrent decreases with size due to reduced absorption and increased recombination, but we also find that the open-circuit voltage begins to decrease for particles with diameters smaller than 2 nm, most likely due to reduced collection efficiency. Owing to this effect, we find peak performance for devices made with PbSe dots with a first exciton energy of ∼1.6 eV (2.3 nm diameter), with a typical efficiency of 3.5%, and a champion device efficiency of 4.57%. Comparing the external quantum efficiency of our devices to an optical model reveals that the photocurrent is also strongly affected by the coherent interference in the thin film due to Fabry-Pérot cavity modes within the PbSe layer. Our results demonstrate that even in this simple device architecture, fine-tuning of the nanoparticle size can lead to substantial improvements in efficiency.

In Situ TEM Etching of Gold Nanocrystals: Elucidating the Shape Transformation Mechanisms and Chemistry of the Graphene Liquid Cell

Size Dependence of a First Order Solid-Solid Phase Transition: The Wurtzite to Rock Salt Transformation in CdSe Nanocrystals

Measurements of the size dependence of a solid-solid phase transition are presented. High-pressure x-ray diffraction and optical absorption are used to study the wurtzite to rock salt structural transformation in CdSe nanocrystals. These experiments show that both the thermodynamics and kinetics of this transformation are altered in finite size, as compared to bulk CdSe. An explanation of these results in the context of transformations in bulk systems is presented. Insight into the kinetics of transformations in both bulk and nanocrystal systems can be gained.

Ligand-Controlled Colloidal Synthesis and Electronic Structure Characterization of Cubic Iron Pyrite (FeS₂) Nanocrystals

Iron pyrite (FeS2) is a promising photovoltaic absorber because of its Earth abundance, high optical extinction, and infrared band gap (Eg = 0.95 eV), but its use has been hindered because of the difficulty of phase pure synthesis. Pyrite phase purity is a paramount concern, as other phases of iron sulfide have undesirable electronic properties. Here we report the synthesis of phase pure iron pyrite nanocrystals with cubic morphology and a mean dimension of 80 nm. Control over the nanocrystal shape was achieved using an unusual ligand, 1-hexadecanesulfonate. The particles were characterized via synchrotron X-ray spectroscopy, indicating an indirect band gap of 1.00 ± 0.11 eV and a valence bandwidth of nearly 1 eV. Transmission electron microscopy from early reaction stages suggests a nucleation and growth mechanism similar to solution precipitation syntheses typical of metal oxide nanocrystals, rather than the diffusion-limited growth process typical of hot-injection metal chalcogenide nanocrystal syntheses.

Extracting structured seed-mediated gold nanorod growth procedures from scientific text with LLMs

The synthesis of gold nanorods remains largely heuristically understood. Large language models provide a route for extracting their structured synthesis procedures from scientific articles to accelerate investigation into synthesis pathways.

From Molecules to Materials: Current Trends and Future Directions

The development, characterization, and exploitation of novel materials based on the assembly of molecular components is an exceptionally active and rapidly expanding field. For this reason, the topic of molecule-based materials (MBMs) was chosen as the subject of a workshop sponsored by the Chemical Sciences Division of the United States Department of Energy. The purpose of the workshop was to review and discuss the diverse research trajectories in the field from a chemical perspective, and to focus on the critical elements that are likely to be essential for rapid progress. The MBMs discussed encompass a diverse set of compositions and structures, including clusters, supramolecular assemblies, and assemblies incorporating biomolecule-based components. A full range of potentially interesting materials properties, including electronic, magnetic, optical, structural, mechanical, and chemical characteristics were considered. Key themes of the workshop included synthesis of novel components, structural control, characterization of structure and properties, and the development of underlying principles and models. MBMs, defined as “useful substances prepared from molecules or molecular ions that maintain aspects of the parent molecular framework” are of special significance because of the capacity for diversity in composition, structure, and properties, both chemical and physical. Key attributes are the ability in MBMs to access the additional dimension of multiple length scales and available structural complexity via organic chemistry synthetic methodologies and the innovative assembly of such diverse components. The interaction among the assembled components can thus lead to unique behavior. A consequence of the complexity is the need for a multiplicity of both existing and new tools for materials synthesis, assembly, characterization, and theoretical analysis. For some technologically useful properties, e.g., ferro- or ferrimagnetism and superconductivity, the property is not a property of a molecule or ion; it is a cooperative solid-state (bulk) property—a property of the entire solid. Hence, the desired properties are a consequence of the interactions between the molecules or ions, and understanding the solid-state structure as well as methods to predict, control, and modulate the structure are essential to understanding and manipulating such behaviors. As challenging as this is, molecules enable a substantially greater ability of control than atoms as building blocks for new materials and thus are well positioned to contribute significantly to new materials. The diversity of components and processes leads to the recognition of the critical role of cross-disciplinary research, including not only that between traditionally different areas within chemistry, but also between chemistry and biochemistry, physics, and a number of engineering disciplines. Enhancing communication and active collaboration between these groups was seen as a critical goal for the research area.

An artificial intelligence’s interpretation of complex high-resolution in situ transmission electron microscopy data

<title>Abstract</title> <italic>In situ</italic> transmission electron microscopy (TEM) has enabled researchers to visualize complicated nano- and atomic-scale processes with sub-Angstrom spatial resolution and millisecond time resolution. These processes are often highly dynamical and can be time-consuming to analyze and interpret. Here, we report how variational autoencoders (VAEs), a deep learning algorithm, can provide an artificial intelligence’s interpretation of high-resolution <italic>in situ</italic> TEM data by condensing and deconvoluting complicated atomic-scale dynamics into a latent space with reduced dimensionality. In this work, we designed a VAEs model with high latent dimensions capable of deconvoluting information from complex high-resolution TEM data. We demonstrate how VAEs with high latent dimensions trained on atomically resolved TEM images of lead sulfide (PbS) nanocrystals is able to capture movements and perturbations of periodic lattices in both simulated and real <italic>in situ</italic> TEM data. The VAEs model shows capability of detecting and deconvoluting dynamical nanoscale physical processes, such as the rotation of crystal lattices and intraparticle ripening during the annealing of semiconductor nanocrystals. With the help of the VAEs model, we can identify an <italic>in situ</italic> observation that can serve as a direct experimental evidence of the existence of intraparticle ripening, a process so far has not been recorded under atomically-resolved <italic>in situ</italic> TEM for colloidal semiconductor nanocrystals. The VAEs model provides a potent tool for facilitating the analysis and interpretation of complex <italic>in situ</italic> TEM data as a part of an autonomous experimental workflow. It also shows potential to identify unexpected phenomena during the data analysis.

Size-Dependent Polar Ordering in Colloidal GeTe Nanocrystals

The question of the nature and stability of polar ordering in nanoscale ferroelectrics is examined with colloidal nanocrystals of germanium telluride (GeTe). We provide atomic-scale evidence for room-temperature polar ordering in individual nanocrystals using aberration-corrected transmission electron microscopy and demonstrate a reversible, size-dependent polar-nonpolar phase transition of displacive character in nanocrystal ensembles. A substantial linear component of the distortion is observed, which is in contrast with theoretical reports predicting a toroidal state.

Using Graphene Liquid Cell Transmission Electron Microscopy to Study &lt;em&gt;in Situ&lt;/em&gt; Nanocrystal Etching

Graphene liquid cell electron microscopy provides the ability to observe nanoscale chemical transformations and dynamics as the reactions are occurring in liquid environments. This manuscript describes the process for making graphene liquid cells through the example of graphene liquid cell transmission electron microscopy (TEM) experiments of gold nanocrystal etching. The protocol for making graphene liquid cells involves coating gold, holey-carbon TEM grids with chemical vapor deposition graphene and then using those graphene-coated grids to encapsulate liquid between two graphene surfaces. These pockets of liquid, with the nanomaterial of interest, are imaged in the electron microscope to see the dynamics of the nanoscale process, in this case the oxidative etching of gold nanorods. By controlling the electron beam dose rate, which modulates the etching species in the liquid cell, the underlying mechanisms of how atoms are removed from nanocrystals to form different facets and shapes can be better understood. Graphene liquid cell TEM has the advantages of high spatial resolution, compatibility with traditional TEM holders, and low start-up costs for research groups. Current limitations include delicate sample preparation, lack of flow capability, and reliance on electron beam-generated radiolysis products to induce reactions. With further development and control, graphene liquid cell may become a ubiquitous technique in nanomaterials and biology, and is already being used to study mechanisms governing growth, etching, and self-assembly processes of nanomaterials in liquid on the single particle level.

Effect of Thermal Fluctuations on the Radiative Rate in Core/Shell Quantum Dots

The effect of lattice fluctuations and electronic excitations on the radiative rate is demonstrated in CdSe/CdS core/shell spherical quantum dots (QDs). Using a combination of time-resolved photoluminescence spectroscopy and atomistic simulations, we show that lattice fluctuations can change the radiative rate over the temperature range from 78 to 300 K. We posit that the presence of the core/shell interface plays a significant role in dictating this behavior. We show that the other major factor that underpins the change in radiative rate with temperature is the presence of higher energy states corresponding to electron excitation into the shell. These effects should be present in other core/shell samples and should also affect other excited state rates, such as the rate of Auger recombination or the rate of charge transfer.

Dielectric Core–Shell Optical Antennas for Strong Solar Absorption Enhancement

We demonstrate a new light trapping technique that exploits dielectric core-shell optical antennas to strongly enhance solar absorption. This approach can allow the thickness of active materials in solar cells lowered by almost 1 order of magnitude without scarifying solar absorption capability. For example, it can enable a 70 nm thick hydrogenated amorphous silicon (a-Si:H) thin film to absorb 90% of incident solar radiation above the bandgap, which would otherwise require a thickness of 400 nm in typical antireflective coated thin films. This strong enhancement arises from a controlled optical antenna effect in patterned core-shell nanostructures that consist of absorbing semiconductors and nonabsorbing dielectric materials. This core-shell optical antenna benefits from a multiplication of enhancements contributed by leaky mode resonances (LMRs) in the semiconductor part and antireflection effects in the dielectric part. We investigate the fundamental mechanism for this enhancement multiplication and demonstrate that the size ratio of the semiconductor and the dielectric parts in the core-shell structure is key for optimizing the enhancement. By enabling strong solar absorption enhancement, this approach holds promise for cost reduction and efficiency improvement of solar conversion devices, including solar cells and solar-to-fuel systems. It can generally apply to a wide range of inorganic and organic active materials. This dielectric core-shell antenna can also find applications in other photonic devices such as photodetectors, sensors, and solid-state lighting diodes.

A New Nonhydrolytic Single-Precursor Approach to Surfactant-Capped Nanocrystals of Transition Metal Oxides

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTA New Nonhydrolytic Single-Precursor Approach to Surfactant-Capped Nanocrystals of Transition Metal OxidesJörg Rockenberger, Erik C. Scher, and A. Paul AlivisatosView Author Information Department of Chemistry, Box 101 University of California, Berkeley, Berkeley California 94720 Cite this: J. Am. Chem. Soc. 1999, 121, 49, 11595–11596Publication Date (Web):November 24, 1999Publication History Received10 September 1999Published online24 November 1999Published inissue 1 December 1999https://pubs.acs.org/doi/10.1021/ja993280vhttps://doi.org/10.1021/ja993280vrapid-communicationACS PublicationsCopyright © 1999 American Chemical SocietyRequest reuse permissionsArticle Views7246Altmetric-Citations652LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-AlertscloseSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Metals,Nanocrystals,Nanoparticles,Oxides,Transmission electron microscopy Get e-Alerts