Publications

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.

Exciton Dynamics in CdS−Ag₂S Nanorods with Tunable Composition Probed by Ultrafast Transient Absorption Spectroscopy

Electron relaxation dynamics in CdS−Ag2S nanorods have been measured as a function of the relative fraction of the two semiconductors, which can be tuned via cation exchange between Cd2+ and Ag+. The transient bleach of the first excitonic state of the CdS nanorods is characterized by a biexponential decay corresponding to fast relaxation of the excited electrons into trap states. This signal completely disappears when the nanorods are converted to Ag2S but is fully recovered after a second exchange to convert them back to CdS, demonstrating annealing of the nonradiative trap centers probed and the robustness of the cation exchange reaction. Partial cation exchange produces heterostructures with embedded regions of Ag2S within the CdS nanorods. Transient bleaching of the CdS first excitonic state shows that increasing the fraction of Ag2S produces a greater contribution from the fast component of the biexponential bleach recovery, indicating that new midgap relaxation pathways are created by the Ag2S material. Transient absorption with a mid-infrared probe further confirms the presence of states that preferentially trap electrons on a time scale of 1 ps, 2 orders of magnitude faster than that of the parent CdS nanorods. These results suggest that the Ag2S regions within the heterostucture provide an efficient relaxation pathway for excited electrons in the CdS conduction band.

Multiscale Characterization of the Influence of the Organic–Inorganic Interface on the Dielectric Breakdown of Nanocomposites

Nanoscale engineered materials such as nanocomposites can display or be designed to enhance their material properties through control of the internal interfaces. Here, we unveil the nanoscale origin and important characteristics of the enhanced dielectric breakdown capabilities of gold nanoparticle/polymer nanocomposites. Our multiscale approach spans from the study of a single chemically designed organic/inorganic interface to micrometer-thick films. At the nanoscale, we relate the improved breakdown strength to the interfacial charge retention capability by combining scanning probe measurements and density functional theory calculations. At the meso- and macroscales, our findings highlight the relevance of the nanoparticle concentration and distribution in determining and enhancing the dielectric properties, as well as identifying this as a crucial limiting factor for the achievable sample size.

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

The formation of extremely high aspect ratio CdSe nanorods (30:1), as well as arrow-, teardrop-, tetrapod-, and branched tetrapod-shaped nanocrystals of CdSe, has been achieved by growth of the nanoparticles in a mixture of hexylphosphonic acid and trioctylphosphine oxide. The most influential factors in shape control are the ratio of surfactants, injection volume, and time-dependent monomer concentration.

Electronic spectra and phase transitions in semiconductor nanocrystals

A summary was not available at time of publication.

Using Graphene Liquid Cell Transmission Electron Microscopy to Study <em>in Situ</em> 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.

Synthesis of Colloidal Cobalt Nanoparticles with Controlled Size and Shapes

Luminescent Cavity Design for High Ambient Contrast Ratio, High Efficiency Displays

<title>Abstract</title> A key display characteristic is its efficiency (emitted light power divided by input power). While display efficiencies are being improved through emissive (e.g., quantum dot and organic light emitting display (OLED) designs1,2, which remove the highly inefficient color filters found in traditional liquid crystal displays (LCDs)3,4, polarization filters, which block about 50% of the light, remain required to inhibit ambient light reflection. We introduce a luminescent cavity design to replace both the color and polarization filters. Narrow-band, large Stokes shift, CdSe/CdS quantum dot emitters are embedded in a reflective cavity pixel element with a small top aperture. The remainder of the top surface is coated black reducing ambient light reflection. A single pixel demonstrates an extraction efficiency of 40.9% from a cavity with an 11% aperture opening. A simple proof-of-concept multi-pixel array is demonstrated.

Scaling law for structural metastability in semiconductor nanocrystals

Abstract In contrast to extended solids, nanometer size crystals convert from one structure to another by single nucleation events, and the transformation obey simple kinetics. Barrier heights are observed to increase in nanocrystals of larger size, but also depend on the nature of the nanocrystal surface. These results are analogous to magnetic phase transitions in nanocrystals and suggest some general rules which may be of use in the discovery of new metastable phases.

Critical differences in 3D atomic structure of individual ligand-protected nanocrystals in solution

Seeing subtle nanoparticle differences A challenge in the fabrication of nanoparticles is that even for particles of uniform size, there will still be a distribution in the atomic arrangements and surface capping ligands from one particle to the next. Using liquid-cell transmission electron microscopy, Kim et al. reconstructed the structure of individual nanocrystals synthesized in one batch while they were still in solution. A comparison of multiple particles showed structural heterogeneity and differences between the interior and the outer shell of the individual nanoparticles, as well as nanoparticles containing extended defects and thus differences in internal strain, all of which can affect the physical and chemical properties of each particle. Science , this issue p. 60

Ultrahigh stress and strain in hierarchically structured hollow nanoparticles

Se Exafs Study of the Elevated Wurtzite to Rock Salt Structural Phase Transition in Cdse Nanocrystals

Elucidating the Weakly Reversible Cs-Pb-Br Perovskite Nanocrystal Reaction Network with High-Throughput Maps and Transformations

&lt;p&gt;Advances in automation and data analytics can aid exploration of the complex chemistry of nanoparticles. Lead halide perovskite colloidal nanocrystals provide an interesting proving ground: there are reports of many different phases and transformations, which has made it hard to form a coherent conceptual framework for their controlled formation through traditional methods. In this work, we systematically explore the portion of Cs-Pb-Br synthesis space in which many optically distinguishable species are formed using high-throughput robotic synthesis to understand their formation reactions. We deploy an automated method that allows us to determine the relative amount of absorbance that can be attributed to each species in order to create maps of the synthetic space. These in turn facilitate improved understanding of the interplay between kinetic and thermodynamic factors that underlie which combination of species are likely to be prevalent under a given set of conditions. Based on these maps, we test potential transformation routes between perovskite nanocrystals of different shapes and phases. We find that shape is determined kinetically, but many reactions between different phases show equilibrium behavior. We demonstrate a dynamic equilibrium between complexes, monolayers and nanocrystals of lead bromide, with substantial impact on the reaction outcomes. This allows us to construct a chemical reaction network that qualitatively explains our results as well as previous reports and can serve as a guide for those seeking to prepare a particular composition and shape. &lt;/p&gt;

Raman studies on C60 at high pressures and low temperatures

Nanotechnology and the Environment

Advanced nanotechnology, which goes beyond our understanding of natural systems and cycles, offers enormous promise for furthering environmental protection technologies. As different aspects of nanotechnology are developed, the broader environmental impacts must be discussed, including determining the potential benefits of reducing or preventing pollutants from industrial sources, as well as monitoring or controlling unintended negative effects through proper and comprehensive risk assessment and life cycle analysis. Although the use of nanotechnology offers many major benefits for industrial growth and environmental sustainability and protection, the energy needs of creating nanoparticles, the considerable amounts of potentially hazardous waste created from their manufacture, and the ecotoxic behavior of many nanoparticles cause societal questions that nanotechnology and new technical advances provide cleaner answers to existing ecological issues (energy).

Organization of 'nanocrystal molecules' using DNA

Nanoscale Science, Engineering and Technology Research Directions

This report describes important future research directions in nanoscale science, engineering and technology. It was prepared in connection with an anticipated national research initiative on nanotechnology for the twenty-first century. The research directions described are not expected to be inclusive but illustrate the wide range of research opportunities and challenges that could be undertaken through the national laboratories and their major national scientific user facilities with the support of universities and industry.

Formation of hollow nanocrystals through the nanoscale kirkendall \neffect

Hollow nanocrystals can be synthesized through a mechanism analogous to the Kirkendall Effect, in which pores form because of the difference in diffusion rates between two components in a diffusion couple. Starting with cobalt nanocrystals, we show that their reaction in solution with oxygen and either sulfur or selenium leads to the formation of hollow nanocrystals of the resulting oxide and chalcogenides. This process provides a general route to the synthesis of hollow nanostructures of a large number of compounds. A simple extension of the process yielded platinum-cobalt oxide yolk-shell nanostructures, which may serve as nanoscale reactors in catalytic applications.