The properties of tasked nanocrystals in energy-related devices are strongly dependent on the presence and chemical nature of ligands at their surface, and the architectures they assume in electroactive layers. Here I will describe an exceptionally versatile class of reagents for native ligand stripping of carboxylate-, phosphonate- and amine- passivated nanocrystals, resulting in either bare or BF4-/DMF-passivated surfaces depending on the material used. These reagents are effective both for thin films of nanocrystals as well as their dispersions. I will also show that dispersions of ligand stripped nanocrystals are useful as nanoinks for making devices and are further amenable to architecturing into mesoporous materials using a new class of macromolecular structure directing agents that make particular use of specific and dynamic molecular interactions at bare nanocrystal surfaces to overcome entropic barriers for co-assembly. This new class of materials with their independently tunable metrics (pore size, wall thickness, crystallite size, shape and composition, etc) is well poised to address murky structure-property correlations in porous materials for energy storage, catalysis, separations, and beyond.

The power of imaging to “see” what our eyes cannot otherwise see has been made clear in the history of the telescope, the microscope, night vision and, most recently, atomic scale microscopes. Transport imaging combines the resolution of near-field optics with the charge generation control of a scanning electron microscope. The technique is related to, but different from standard cathodoluminescence, since it maintains the spatial information of the emitted light. Light is collected in either the far-field or the near-field from a CCD camera or near field scanning optical microscope (NSOM). It is possible to determine minority carrier or exciton diffusion lengths from a single optical image, without any electrical contact to the sample, and to map spatial variations in transport properties in real materials that are difficult to access in any other way. Transport can be imagined in all dimensions - bulk materials, thin films and nanostuctures. This approach can be applied with any luminescent material and will be illustrated with examples from the study of GaN and ZnO nanowires, thin film solar cell materials and TlBr and CdZnTe for nuclear radiation detectors.

Discussion of a nanowire formation mechanism, in which axial screw dislocations provide the self-perpetuating steps to enable 1-dimensional (1D) crystal growth. Different from the well-known vapor-liquid-solid (VLS) growth, this mechanism was initially found in hierarchical nanowire structures with helically rotating branches resembling “pine trees”. Dislocations can further result in the spontaneous formation of nanotubes, 2D plates, and other morphologies. Many new examples have been recently established to show that dislocation-driven growth is general to many materials grown in vapor or solution phase. We have used classical crystal growth theory to guide the rational design of dislocation-driven nanowire growth. These discoveries can create a new dimension in the rational design and synthesis of nanomaterials. Furthermore, it will enable the scalable and low-cost synthesis of earth-abundant nanomaterials for large scale renewable energy applications, such as in solar and thermoelectric energy conversion, and nanostructured battery electrodes. This will be illustrated by the growth of 1D nanomaterials of earth abundant and inexpensive semiconductors, such as hematite (a-Fe2O3), pyrite (FeS2), and cuprous oxide (Cu2O). The photoelectrochemical investigations of these nanomaterials and various doping and nanostructuring strategies using 3D hierarchical nanocomposites or branching nanostructures are investigated to overcome the conflicting requirements by light harvesting and carrier collection.

Nanoparticles and nanomaterials possess size- and shape-tunable properties, which can be greatly altered compared to their bulk values. The new properties can be harnessed for energy applications. To realize the effective development of nanomaterials and devices our group is researching each stage of the process. We are: 1) exploring new synthetic nanochemistry techniques to control the shape, size, and composition of nanomaterials, 2) integrating nanoparticles into practical devices, and 3) developing new nanoscale characterization methods. Our materials and methods target the thermoelectric, battery, and catalytic applications.

In this talk I will discuss our latest results from each stage toward realizing a nanomaterial device. I will discuss our colloidal work on cobalt phosphides where we have cracked the normally inert tri-n- octylphosphine oxide (TOPO) and use it as a phosphorous source. In this system we’ve also investigated the nanoscale Kirkendall effect and found that the process occurs in distinct stages: initial inward anion diffusion, followed by crystallization, and then the characteristic outward cation diffusion of Kirkendall hollowing. I’ll talk about our novel method to produce nanosheets of the thermoelectric metal oxide NaxCoO2, using sol-gel chemistry and kinetic demixing. The nanosheets are uniform in length and shape with highly anisotropic dimensions of 20 nanometer thickness and 2 millimeter lateral lengths (aspect ratio of 100,000:100,000:1). For devices I will discuss our work on nanoparticle surfactant ligand removal and replacement with inorganic groups, and our binderless nanoparticle battery electrodes – cobalt oxide battery anodes made without binders or carbon black. And finally, to characterize nanoscale phonon transport I will present our work on a microfabricated phonon spectrometer. Non-thermal distributions of phonons are locally excited and detected in silicon micro- and nanostructures by decay of quasiparticles injected into an adjacent superconducting tunnel junction. Using this technique, narrow frequency bands of phonons may be isolated and applied to investigate phonon transport through nanostructures at sub-kelvin temperatures. In our prototype phonon spectrometer we have demonstrated spatial resolution below 1 micron and frequency resolution of ~10 GHz. Frequency range is governed by emission and absorption behaviors in the
superconducting tunnel junctions and is expected to extend from ~80 to ~800 GHz.

Metal oxide nanostructures are emerging as a unique class of electrode materials for energy conversion and storage. In comparison to bulk materials, they have larger semiconductor /electrolyte interfacial area, shorter diffusion length for minority carriers, equally good charge transport and lower surface reflectivity. Our goal is to develop chemically-modified semiconductor metal oxide electrodes, with enhanced photoelectrochemical and electrochemical properties. In this talk, I will review our recent efforts in using hydrogen thermal treatment as a general strategy to improve the performance of metal oxide electrodes, such as TiO2 for photoelectrochemical water oxidation and charge storage in a supercapacitor device.

Self-assembly of molecules is an attractive materials construction strategy due to its simplicity in application. By considering peptidic molecules in the bottom-up materials self-assembly design process, one can take advantage of inherently biomolecular attributes; intramolecular folding events, secondary structure, and electrostatic interactions; in addition to more traditional self-assembling molecular attributes such as amphiphilicty, to define hierarchical material structure and consequent properties. These self-assembled materials range from hydrogels for biomaterials to nanostructures with defined morphology and chemistry display for inorganic materials templating. Hydrogels and hybrid materials will both be discussed.

The local nano- and overall network structure, and resultant viscoelastic and cell-level biological properties, of hydrogels that are formed via beta-hairpin self-assembly will be presented. These peptide hydrogels are potentially excellent scaffolds for tissue repair and regeneration due to inherent cytocompatibility, porous morphology, and shear-thinning but instant recovery viscoelastic properties. The 20 amino acid parent peptide MAX1 (H2N-VKVKVKVKVDPPTKVKVKVKV-CONH2), has been shown to fold and self-assemble into a rigid hydrogel based on environmental cues such as pH, salt, and temperature including physiological conditions. The hydrogel is composed of a network of fibrils that are 3 nm wide and heavily branched and entangled with no covalent crosslinking required for gel stiffness. In addition, slight design variations of the MAX1 sequence allow for tunability of the self-assembly/hydrogelation kinetics as well as the tunability of the local peptide nanostructure and hierarchical network structure. In turn, by controlling hydrogel self-assembly kinetics, one dictates the ultimate stiffness of the resultant network and the kinetics through which gelation occurs. Importantly, once formed into a solid, self-supporting gel the network can be disrupted by the introduction of a shear stress. The system can shear thin but immediately reheal to preshear stiffness on the cessation of the shear stress. This shear thinning behavior of these physical networks makes them interesting candidates for injectable delivery in vivo where no post injection chemistry is required to set up the network. Peptide structure for folding and self-assembly, self-assembly characterization, gel material properties (particularly the flow and re-healing behavior), and cell-level biological properties of these peptide hydrogels will be discussed.

In addition, peptide fibrils can be used to template the growth of inorganic materials as well as the assembly of inorganic nanoparticles. Both long polypeptides, synthesized through recombinant techniques, as well as short peptides have also been designed for fibrillar assembly. On forming fibrils, the long polypeptides display chemistry at desired distances along the fibrils, thus allowing desired inorganic particle assembly with great fidelity. Short, beta-hairpin peptides can be used in the gel state to template the growth of inorganic sol-gel phases on the surface of the fibrils or can be used to template inorganic nanoparticle assembly with excellent precision.

Cryo transmission electron microscopy (cryoTEM), transmission electron microscopy (TEM), small angle neutron or x-ray scattering (SANS, ,SAXS), atomic force microscopy, oscillatory rheology, spectroscopy, in vitro cell culturing and preliminary in vivo animal experiments have all been used to characterize the nano-through-microstructure and material properties of the above self-assembled systems.