Publications

Berichtigung: Highly Luminescent Nanocrystals From Removal of Impurity Atoms Residual From Ion‐Exchange Synthesis

Die Autoren dieser Zuschrift möchten darauf hinweisen, dass im oberen Teil von Abbildung 1 B die x-Achsen von Diagramm und Einschub um 80 nm verschoben wiedergegeben wurden: Die x-Achse des Diagramms muss von 420 nm bis 670 nm reichen und nicht von 500 nm bis 750 nm. Diagramm und Einschub mit den korrekten x-Achsen sind hier gezeigt. Alle weiteren Resultate und Schlussfolgerungen des Beitrags sind von diesem Versehen nicht betroffen. […︁] B) Photoluminescence spectra of CdS nanorods (top), […︁] obtained from hot injection (red), cation exchange (green), and post-change purification (blue). […︁] Inset of B: Normalized photoluminescence showing similarity in the photoluminescence peak positions between samples obtained from the three different methods. […︁]

Energy Frontier Research Centers (EFRCs): A Response to Five Challenges for Science and the Imagination (2011 EFRC Summit, panel session)

A distinguished panel of speakers at the 2011 EFRC Summit looks at the EFRC Program and how it serves as a response to "Five Challenges for Science and the Imagination”, the culminating report that arose from a series of Basic Research Needs workshops. The panel members are Paul Alivisatos, the Director of Lawrence Berkeley National Laboratory, George Crabtree, Distinguished Fellow at Argonne National Laboratory, Mildred Dresselhause, Institute Professor at the Massachusetts Institute of Technology, and Mark Ratner, Professor at Northwestern University. The 2011 EFRC Summit and Forum brought together the EFRC community and science and policy leaders from universities, national laboratories, industry and government to discuss "Science for our Nation's Energy Future." In August 2009, the Office of Science established 46 Energy Frontier Research Centers. The EFRCs are collaborative research efforts intended to accelerate high-risk, high-reward fundamental research, the scientific basis for transformative energy technologies of the future. These Centers involve universities, national laboratories, nonprofit organizations, and for-profit firms, singly or in partnerships, selected by scientific peer review. They are funded at $2 to $5 million per year for a total planned DOE commitment of $777 million over the initial five-year award period, pending Congressional appropriations. These integrated, multi-investigator Centers are conducting fundamental research focusing on one or more of several “grand challenges” and use-inspired “basic research needs” recently identified in major strategic planning efforts by the scientific community. The purpose of the EFRCs is to integrate the talents and expertise of leading scientists in a setting designed to accelerate research that transforms the future of energy and the environment.

Studies of the dynamics of biological macromolecules using Au nanoparticle–DNA artificial molecules

The recent development of graphene liquid cells, a nanoscale version of liquid bubble wrap, is a breakthrough for <italic>in situ</italic> liquid phase electron microscopy (EM). Using ultrathin graphene sheets as the liquid sample container, graphene liquid cells have allowed the unprecedented atomic resolution observation of solution phase growth and dynamics of nanocrystals. Here we explore the potential of this technique to probe nanoscale structure and dynamics of biomolecules <italic>in situ</italic>, using artificial Au nanoparticle–DNA artificial molecules as model systems. The interactions of electrons with both the artificial molecules and the liquid environment have been demonstrated and discussed, revealing both the opportunities and challenges of using graphene liquid cell EM as a new method of bio-imaging.

Nanotools for Neuroscience and Brain Activity Mapping

Neuroscience is at a crossroads. Great effort is being invested into deciphering specific neural interactions and circuits. At the same time, there exist few general theories or principles that explain brain function. We attribute this disparity, in part, to limitations in current methodologies. Traditional neurophysiological approaches record the activities of one neuron or a few neurons at a time. Neurochemical approaches focus on single neurotransmitters. Yet, there is an increasing realization that neural circuits operate at emergent levels, where the interactions between hundreds or thousands of neurons, utilizing multiple chemical transmitters, generate functional states. Brains function at the nanoscale, so tools to study brains must ultimately operate at this scale, as well. Nanoscience and nanotechnology are poised to provide a rich toolkit of novel methods to explore brain function by enabling simultaneous measurement and manipulation of activity of thousands or even millions of neurons. We and others refer to this goal as the Brain Activity Mapping Project. In this Nano Focus, we discuss how recent developments in nanoscale analysis tools and in the design and synthesis of nanomaterials have generated optical, electrical, and chemical methods that can readily be adapted for use in neuroscience. These approaches represent exciting areas of technical development and research. Moreover, unique opportunities exist for nanoscientists, nanotechnologists, and other physical scientists and engineers to contribute to tackling the challenging problems involved in understanding the fundamentals of brain function.

Graphene Veils and Sandwiches

We report a new and highly versatile approach to artificial layered materials synthesis which borrows concepts of molecular beam epitaxy, self-assembly, and graphite intercalation compounds. It readily yields stacks of graphene (or other two-dimensional sheets) separated by virtually any kind of "guest" species. The new material can be "sandwich like", for which the guest species are relatively closely spaced and form a near-continuous inner layer of the sandwich, or "veil like", where the guest species are widely separated, with each guest individually draped within a close-fitting, protective yet atomically thin graphene net or veil. The veils and sandwiches can be intermixed and used as a two-dimensional platform to control the movements and chemical interactions of guest species.

Semiconductor Nanocrystals as Fluorescent Biological Labels

Semiconductor nanocrystals were prepared for use as fluorescent probes in biological staining and diagnostics. Compared with conventional fluorophores, the nanocrystals have a narrow, tunable, symmetric emission spectrum and are photochemically stable. The advantages of the broad, continuous excitation spectrum were demonstrated in a dual-emission, single-excitation labeling experiment on mouse fibroblasts. These nanocrystal probes are thus complementary and in some cases may be superior to existing fluorophores.

Cation Exchange: A Versatile Tool for Nanomaterials Synthesis

The development of nanomaterials for next generation photonic, optoelectronic, and catalytic applications requires a robust synthetic toolkit for systematically tuning composition, phase, and morphology at nanometer length scales. While de novo synthetic methods for preparing nanomaterials from molecular precursors have advanced considerably in recent years, postsynthetic modifications of these preformed nanostructures have enabled the stepwise construction of complex nanomaterials. Among these postsynthetic transformations, cation exchange reactions, in which the cations ligated within a nanocrystal host lattice are substituted with those in solution, have emerged as particularly powerful tools for fine-grained control over nanocrystal composition and phase. In this feature article, we review the fundamental thermodynamic and kinetic basis for cation exchange reactions in colloidal semiconductor nanocrystals and highlight its synthetic versatility for accessing nanomaterials intractable by direct synthetic methods from molecular precursors. Unlike analogous ion substitution reactions in extended solids, cation exchange reactions at the nanoscale benefit from rapid reaction rates and facile modulation of reaction thermodynamics via selective ion coordination in solution. The preservation of the morphology of the initial nanocrystal template upon exchange, coupled with stoichiometric control over the extent of reaction, enables the formation of nanocrystals with compositions, morphologies, and crystal phases that are not readily accessible by conventional synthetic methods.

Charge transport in hybrid nanorod-polymer composite photovoltaic cells

Charge transport in composites of inorganic nanorods and aconjugated polymer is investigated using a photovoltaic device structure.We show that the current-voltage (I-V) curves in the dark can be modelledusing the Shockley equation modified to include series and shuntresistance at low current levels, and using an improved model thatincorporates both the Shockley equation and the presence of a spacecharge limited region at high currents. Under illumination, theefficiency of photocurrent generation is found to be dependent on appliedbias. Furthermore, the photocurrent-light intensity dependence was foundto be sublinear. An analysis of the shunt resistance as a function oflight intensity suggests that the photocurrent as well as the fill factoris diminished as a result of increased photoconductivity of the activelayer at high light intensity. By studying the intensity dependence ofthe open circuit voltage for nanocrystals with different diameters andthus ! band gaps, it was inferred that Fermi-level pinning occurs at theinterface between the aluminum electrode and the nanocrystal.

Resonance Raman studies of the ground and lowest electronic excited state in CdS nanocrystals

The size dependence of the resonance Raman spectrum of CdS nanocrystals ranging in size from 10 to 70 Å radius has been studied. We find that while the lowest electronic excited state is coupled strongly to the lattice, this coupling decreases as the nanocrystal size is decreased. We demonstrate that the lifetime of the initially prepared excited state can influence the apparent strength of electron-vibration coupling. Absolute resonance Raman cross section measurements can be used to determine the value of the excited state lifetime, thus removing this parameter. The coupling to the lattice, while less in nanocrystals than in the bulk, is still greater than what is predicted assuming an infinite confining potential. The width of the observed LO mode broadens with decreasing size, indicating that the resonance Raman process is intrinsically multimode in its nature. The frequency of the observed longitudinal optic (LO) mode has a very weak dependence on size, in contrast to results obtained from multiple quantum well systems. The temperature dependence of the frequency and linewidth of the observed LO mode is similar to the bulk and indicates that the LO mode decays into acoustic vibrations in 2.5 ps.

Size Dependence of a Temperature-Induced Solid–Solid Phase Transition in Copper(I) Sulfide

Determination of the phase diagrams for the nanocrystalline forms of materials is crucial for our understanding of nanostructures and the design of functional materials using nanoscale building blocks. The ability to study such transformations in nanomaterials with controlled shape offers further insight into transition mechanisms and the influence of particular facets. Here we present an investigation of the size-dependent, temperature-induced solid-solid phase transition in copper sulfide nanorods from low- to high-chalcocite. We find the transition temperature to be substantially reduced, with the high chalcocite phase appearing in the smallest nanocrystals at temperatures so low that they are typical of photovoltaic operation. Size dependence in phase trans- formations suggests the possibility of accessing morphologies that are not found in bulk solids at ambient conditions. These other- wise-inaccessible crystal phases could enable higher-performing materials in a range of applications, including sensing, switching, lighting, and photovoltaics.

Directed Assembly of Discrete Gold Nanoparticle Groupings Using\nBranched DNA Scaffolds

The concept of self-assembled dendrimers is explored for the creation of discrete nanoparticle assemblies.\nHybridization of branched DNA trimers and nanoparticle−DNA conjugates results in the synthesis of\nnanoparticle trimer and tetramer complexes. Multiple tetramer architectures are investigated, utilizing\nAu−DNA conjugates with varying secondary structural motifs. Hybridization products are analyzed by\ngel electrophoresis, and discrete bands are observed corresponding to structures with increasing numbers\nof hybridization events. Samples extracted from each band are analyzed by transmission electron\nmicroscopy, and statistics compiled from micrographs are used to compare assembly characteristics for\neach architecture. Asymmetric structures are also produced in which both 5- and 10-nm Au particles are\nassembled on branched scaffolds.

High-Resolution Transmission Electron Microscopy Observation of Colloidal Nanocrystal Growth Mechanisms using Graphene Liquid Cells

We introduce a new type of liquid cell for in-situ electron microscopy based upon entrapment of a liquid film between layers of graphene. We employ this cell to achieve high-resolution imaging of colloidal platinum nanocrystal growth. The ability to directly image and resolve critical steps at atomic resolution provides new insights into nanocrystal coalescence and reshaping during growth.

Photoexcited Small Polaron Formation in Goethite (α-FeOOH) Nanorods Probed by Transient Extreme Ultraviolet Spectroscopy

Small polaron formation limits the mobility and lifetimes of photoexcited carriers in metal oxides. As the ligand field strength increases, the carrier mobility decreases, but the effect on the photoexcited small polaron formation is still unknown. Extreme ultraviolet transient absorption spectroscopy is employed to measure small polaron formation rates and probabilities in goethite (α-FeOOH) crystalline nanorods at pump photon energies from 2.2 to 3.1 eV. The measured polaron formation time increases with excitation photon energy from 70 ± 10 fs at 2.2 eV to 350 ± 30 fs at 2.6 eV, whereas the polaron formation probability (85 ± 10%) remains constant. By comparison to hematite (α-Fe₂O₃), an oxide analogue, the role of ligand composition and metal center density in small polaron formation time is discussed. This work suggests that incorporating small changes in ligands and crystal structure could enable the control of photoexcited small polaron formation in metal oxides.

Recent advances in the study of colloidal nanocrystals enabled by in situ liquid-phase transmission electron microscopy

Electrical studies of individual colloidal inorganic nanocrystals

It is possible to make colloidal nanocrystals of many different materials with very well controlled size, shape, and topology (branched, nested, etc.). These materials have been studied optically extensively. However, electrical studies of colloidal nanocrystals have been more limited due to the difficulties associated with making good electrical contacts. This task will show new ways of making such electrical contacts reliably and also will discuss the resulting three-terminal electrical device properties.

Impact of Uniform Facets on the Thermodynamics of Ligand Exchanges on Colloidal Quantum Dots

Nanocrystal surfaces play a critical role in dictating their physical and chemical properties. Colloidal semiconductor nanocrystals, or quantum dots, are generally synthesized with insulating aliphatic ligands, but these ligands can be post-synthetically exchanged with other molecular species for a variety of applications. While previous work has investigated the thermodynamics of ligand exchanges on quasi-spherical quantum dots through sensitive isothermal titration calorimetry and 1H NMR measurements, the influence of well-defined crystal facets on the propensity of these ligand exchanges is yet to be explored using these techniques. We studied the exchange of native cadmium carboxylate ligands for zinc chloride ligands on (111) faceted cadmium selenide tetrahedrons and (100) faceted cadmium selenide cubes. From these measurements, we find the unexpected result that the thermodynamics of this ligand exchange between these facets is nearly identical. Further, a modified Ising model was used to fit both the calorimetric and 1H NMR data to extract thermodynamic parameters for the exchange. These simulations suggest that the observed enthalpic differences can be mostly attributed to varying inter-ligand interactions on the (111) and (100) facets.

CdSe Nanocrystal Rods/Poly(3-hexylthiophene) Composite Photovoltaic Devices

Photovoltaic devices remain an important aim for thin films of conjugated polymers. Here is reported the construction of devices with improved photovoltaic performance, which is achieved by blending elongated CdSe nanocrystals (see Figure) with regioregular poly(3-hexylthiophene). Improved transport arising from denser aggregation between the elongated particles is a probable source of the enhanced energy conversion.

Electric field modulation studies of optical absorption in CdSe nanocrystals: Dipolar character of the excited state

The Stark effect on the electronic absorption spectrum of CdSe nanocrystals has been studied for nanocrystals ranging in size from 80 to 20 Å in diameter. For all but the smallest clusters, a second derivative line shape is observed, indicative of a dipole moment in the excited state. This result is independent of the surface modification and appears in both CdS and CdSe systems. The Δμ ranges from 15±10 D in the smallest clusters and up to 100±10 D in the largest; however, the increase is not monotonic, and in the very largest clusters studied (d≳70 Å), the dipole moment decreases. The dipolar character is lost in clusters less than 25 Å. These results can be explained by a model in which there is resonance of an interior state with a surface state at a particular size, with the mixing occurring on a preferred axis.