Ultrahigh Areal Capacity Li Electrodeposition at Metal-Solid Electrolyte Interfaces under Minimal Stack Pressures Enabled by Interfacial Na-K Liquids

The need for higher energy density rechargeable batteries has generated interest in metallic electrodes paired with solid electrolytes. However, impedance growth at the Li metal-solid electrolyte interface due to void formation during cycling at practical current densities and areal capacities, e.g., greater than 0.5 mA cm^-2 and 1.5 mAh cm^-2 respectively, remains a significant barrier. Here, we show that introducing a wetting interfacial film of Na-K liquid between the Li metal and the Li_6.75La₃Zr_1.75Ta_0.25O₁₂ (LLZTO) solid electrolyte permits reversible stripping and plating of up to 150 μm of Li (30 mAh cm^-2), approximately 10 times the areal capacity of today's lithium-ion batteries, at current densities above 0.5 mA cm^-2 and stack pressures below 75 kPa, all with minimal changes in cell impedance. We further show that this increase in the accessible areal capacity at high stripping current densities is due to the presence of Na-K liquid at the Li stripping interface; this performance improvement is not enabled in the absence of the Na-K liquid. This design approach holds promise for overcoming interfacial stability issues that have heretofore limited the performance of solid-state metal batteries.

Bayesian-Optimization-Assisted Laser Reduction of Poly(acrylonitrile) for Electrochemical Applications

Laser reduction of polymers has recently been explored to rapidly and inexpensively synthesize high-quality graphitic and carbonaceous materials. However, in past work, laser-induced graphene has been restricted to semiaromatic polymers and graphene oxide; in particular, poly(acrylonitrile) (PAN) is claimed to be a polymer that cannot be laser-reduced successfully to form electrochemically active material. In this work, three strategies to surmount this barrier are employed: (1) thermal stabilization of PAN to increase its sp² content for improved laser processability, (2) prelaser treatment microstructuring to reduce the effects of thermal stresses, and (3) Bayesian optimization to search the parameter space of laser processing to improve performance and discover morphologies. Based on these approaches, we successfully synthesize laser-reduced PAN with a low sheet resistance (6.5 Ω sq^-1) in a single lasing step. The resulting materials are tested electrochemically, and their applicability as membrane electrodes for vanadium redox flow batteries is demonstrated. This work demonstrates electrodes that are processed in air, below 300 °C, which are cycled stably over 2 weeks at 40 mA cm^-2, motivating further development of laser reduction of porous polymers for membrane electrode applications such as RFBs.

Synthesis and Characterization of Dense Carbon Films as Model Surfaces to Estimate Electron Transfer Kinetics on Redox Flow Battery Electrodes

Redox flow batteries (RFBs) are a promising electrochemical technology for the efficient and reliable delivery of electricity, providing opportunities to integrate intermittent renewable resources and to support unreliable and/or aging grid infrastructure. Within the RFB, porous carbonaceous electrodes facilitate the electrochemical reactions, distribute the flowing electrolyte, and conduct electrons. Understanding electrode reaction kinetics is crucial for improving RFB performance and lowering costs. However, assessing reaction kinetics on porous electrodes is challenging as their complex structure frustrates canonical electroanalytical techniques used to quantify performance descriptors. Here, we outline a strategy to estimate electron transfer kinetics on planar electrode materials of similar surface chemistry to those used in RFBs. First, we describe a bottom-up synthetic process to produce flat, dense carbon films to enable the evaluation of electron transfer kinetics using traditional electrochemical approaches. Next, we characterize the physicochemical properties of the films using a suite of spectroscopic methods, confirming that their surface characteristics align with those of widely used porous electrodes. Last, we study the electrochemical performance of the films in a custom-designed cell architecture, extracting intrinsic heterogeneous kinetic rate constants for two iron-based redox couples in aqueous electrolytes using standard electrochemical methods (i.e., cyclic voltammetry, electrochemical impedance, and spectroscopy). We anticipate that the synthetic methods and experimental protocols described here are applicable to a range of electrocatalysts and redox couples.

Dynamics of Hydroxyl Anions Promotes Lithium Ion Conduction in Antiperovskite Li2OHCl

Li2OHCl is an exemplar of the antiperovskite family of ionic conductors, for which high ionic conductivities have been reported, but in which the atomic-level mechanism of ion migration is unclear. The stable phase is both crystallographically defective and disordered, having ∼1/3 of the Li sites vacant, while the presence of the OH- anion introduces the possibility of rotational disorder that may be coupled to cation migration. Here, complementary experimental and computational methods are applied to understand the relationship between the crystal chemistry and ionic conductivity in Li2OHCl, which undergoes an orthorhombic to cubic phase transition near 311 K (≈38 °C) and coincides with the more than a factor of 10 change in ionic conductivity (from 1.2 × 10-5mS/cm at 37 °C to 1.4 × 10-3 mS/cm at 39 °C). X-ray and neutron experiments conducted over the temperature range 20-200 °C, including diffraction, quasi-elastic neutron scattering (QENS), the maximum entropy method (MEM) analysis, and ab initio molecular dynamics (AIMD) simulations, together show conclusively that the high lithium ion conductivity of cubic Li2OHCl is correlated to "paddlewheel" rotation of the dynamic OH- anion. The present results suggest that in antiperovskites and derivative structures a high cation vacancy concentration combined with the presence of disordered molecular anions can lead to high cation mobility.

Non-Solvent Induced Phase Separation Enables Designer Redox Flow Battery Electrodes

Porous carbonaceous electrodes are performance-defining components in redox flow batteries (RFBs), where their properties impact the efficiency, cost, and durability of the system. The overarching challenge is to simultaneously fulfill multiple seemingly contradictory requirements-i.e., high surface area, low pressure drop, and facile mass transport-without sacrificing scalability or manufacturability. Here, non-solvent induced phase separation (NIPS) is proposed as a versatile method to synthesize tunable porous structures suitable for use as RFB electrodes. The variation of the relative concentration of scaffold-forming polyacrylonitrile to pore-forming poly(vinylpyrrolidone) is demonstrated to result in electrodes with distinct microstructure and porosity. Tomographic microscopy, porosimetry, and spectroscopy are used to characterize the 3D structure and surface chemistry. Flow cell studies with two common redox species (i.e., all-vanadium and Fe^2+/3+ ) reveal that the novel electrodes can outperform traditional carbon fiber electrodes. It is posited that the bimodal porous structure, with interconnected large (>50 µm) macrovoids in the through-plane direction and smaller (<5 µm) pores throughout, provides a favorable balance between offsetting traits. Although nascent, the NIPS synthesis approach has the potential to serve as a technology platform for the development of porous electrodes specifically designed to enable electrochemical flow technologies.

Order-disorder transition in nano-rutile TiO2 anodes: a high capacity low-volume change Li-ion battery material

Nano-sized particles of rutile TiO₂ is a promising material for cheap high-capacity anodes for Li-ion batteries. It is well-known that rutile undergoes an irreversible order-disorder transition upon deep discharge. However, in the disordered state, the Li_xTiO₂ material retains a high reversible ion-storage capacity of >200 mA h g^-1. Despite the promising properties of the material, the structural transition and evolution during the repeated battery operation has so far been studied only by diffraction-based methods, which only provide insight into the part that retains some long-range order. Here, we utilize a combination of ex situ and operando total scattering with pair distribution function analysis and transmission electron microscopy to investigate the atomic-scale structures of the disordered Li_xTiO₂ forming upon the discharge of nano-rutile TiO₂ as well as to elucidate the phase behavior in the material during the repeated charge-discharge process. Our investigation reveals that nano-rutile upon Li-intercalation transforms into a composite of ∼5 nm domains of a layered Li_xTiO₂α-NaFeO₂-type structure with ∼1 nm Li_xTiO₂ grain boundaries with a columbite-like structural motif. During repeated charge-discharge cycling, the structure of this composite is retained and stores Li through a complete solid-solution transition with a remarkably small volume change of only 1 vol%.

Apparatus for operando x-ray diffraction of fuel electrodes in high temperature solid oxide electrochemical cells

Characterizing electrochemical energy conversion devices during operation is an important strategy for correlating device performance with the properties of cell materials under real operating conditions. While operando characterization has been used extensively for low temperature electrochemical cells, these techniques remain challenging for solid oxide electrochemical cells due to the high temperatures and reactive gas atmospheres these cells require. Operando X-ray diffraction measurements of solid oxide electrochemical cells could detect changes in the crystal structure of the cell materials, which can be useful for understanding degradation process that limit device lifetimes, but the experimental capability to perform operando X-ray diffraction on the fuel electrodes of these cells has not been demonstrated. Here we present the first experimental apparatus capable of performing X-ray diffraction measurements on the fuel electrodes of high temperature solid oxide electrochemical cells during operation under reducing gas atmospheres. We present data from an example experiment with a model solid oxide cell to demonstrate that this apparatus can collect X-ray diffraction patterns during electrochemical cell operation at high temperatures in humidified H₂ gas. Measurements performed using this apparatus can reveal new insights about solid oxide fuel cell and solid oxide electrolyzer cell degradation mechanisms to enable the design of durable, high performance devices.

Net-zero emissions energy systems

Some energy services and industrial processes-such as long-distance freight transport, air travel, highly reliable electricity, and steel and cement manufacturing-are particularly difficult to provide without adding carbon dioxide (CO₂) to the atmosphere. Rapidly growing demand for these services, combined with long lead times for technology development and long lifetimes of energy infrastructure, make decarbonization of these services both essential and urgent. We examine barriers and opportunities associated with these difficult-to-decarbonize services and processes, including possible technological solutions and research and development priorities. A range of existing technologies could meet future demands for these services and processes without net addition of CO₂ to the atmosphere, but their use may depend on a combination of cost reductions via research and innovation, as well as coordinated deployment and integration of operations across currently discrete energy industries.

Molecular understanding of polyelectrolyte binders that actively regulate ion transport in sulfur cathodes

Polymer binders in battery electrodes may be either active or passive. This distinction depends on whether the polymer influences charge or mass transport in the electrode. Although it is desirable to understand how to tailor the macromolecular design of a polymer to play a passive or active role, design rules are still lacking, as is a framework to assess the divergence in such behaviors. Here, we reveal the molecular-level underpinnings that distinguish an active polyelectrolyte binder designed for lithium-sulfur batteries from a passive alternative. The binder, a cationic polyelectrolyte, is shown to both facilitate lithium-ion transport through its reconfigurable network of mobile anions and restrict polysulfide diffusion from mesoporous carbon hosts by anion metathesis, which we show is selective for higher oligomers. These attributes allow cells to be operated for >100 cycles with excellent rate capability using cathodes with areal sulfur loadings up to 8.1 mg cm-2.

Two-dimensional lithium diffusion behavior and probable hybrid phase transformation kinetics in olivine lithium iron phosphate

Olivine lithium iron phosphate is a technologically important electrode material for lithium-ion batteries and a model system for studying electrochemically driven phase transformations. Despite extensive studies, many aspects of the phase transformation and lithium transport in this material are still not well understood. Here we combine operando hard X-ray spectroscopic imaging and phase-field modeling to elucidate the delithiation dynamics of single-crystal lithium iron phosphate microrods with long-axis along the [010] direction. Lithium diffusivity is found to be two-dimensional in microsized particles containing ~3% lithium-iron anti-site defects. Our study provides direct evidence for the previously predicted surface reaction-limited phase-boundary migration mechanism and the potential operation of a hybrid mode of phase growth, in which phase-boundary movement is controlled by surface reaction or lithium diffusion in different crystallographic directions. These findings uncover the rich phase-transformation behaviors in lithium iron phosphate and intercalation compounds in general and can help guide the design of better electrodes.

Lithium transport and phase transformation kinetics in olivine LiFePO₄ electrode remain not fully understood. Here the authors show that microsized olivine particles possess 2D lithium diffusivity and exhibit a possible hybrid mode of phase boundary migration upon cycling.

Low-profile self-sealing sample transfer flexure box

A flexural bearing mechanism has enabled the development of a self-sealing box for protecting air sensitive samples during transfer between glove boxes, micro-machining equipment, and microscopy equipment. The simplicity and self-actuating feature of this design makes it applicable to many devices that operate under vacuum conditions. The models used to design the flexural mechanism are presented in detail. The device has been tested in a Zeiss Merlin GEMINI II scanning electron microscope with Li₃PS₄ samples, showing effective isolation from air and corrosion prevention.

Accommodating High Transformation Strains in Battery Electrodes via the Formation of Nanoscale Intermediate Phases: Operando Investigation of Olivine NaFePO4

Virtually all intercalation compounds exhibit significant changes in unit cell volume as the working ion concentration varies. Na_xFePO₄ (0 < x < 1, NFP) olivine, of interest as a cathode for sodium-ion batteries, is a model for topotactic, high-strain systems as it exhibits one of the largest discontinuous volume changes (∼17% by volume) during its first-order transition between two otherwise isostructural phases. Using synchrotron radiation powder X-ray diffraction (PXD) and pair distribution function (PDF) analysis, we discover a new strain-accommodation mechanism wherein a third, amorphous phase forms to buffer the large lattice mismatch between primary phases. The amorphous phase has short-range order over ∼1nm domains that is characterized by a and b parameters matching one crystalline end-member phase and a c parameter matching the other, but is not detectable by powder diffraction alone. We suggest that this strain-accommodation mechanism may generally apply to systems with large transformation strains.

Engineering the Transformation Strain in LiMnyFe1-yPO4 Olivines for Ultrahigh Rate Battery Cathodes

Alkali ion intercalation compounds used as battery electrodes often exhibit first-order phase transitions during electrochemical cycling, accompanied by significant transformation strains. Despite ∼30 years of research into the behavior of such compounds, the relationship between transformation strain and electrode performance, especially the rate at which working ions (e.g., Li) can be intercalated and deintercalated, is still absent. In this work, we use the LiMnyFe1-yPO4 system for a systematic study, and measure using operando synchrotron radiation powder X-ray diffraction (SR-PXD) the dynamic strain behavior as a function of the Mn content (y) in powders of ∼50 nm average diameter. The dynamically produced strain deviates significantly from what is expected from the equilibrium phase diagrams and demonstrates metastability but nonetheless spans a wide range from 0 to 8 vol % with y. For the first time, we show that the discharge capacity at high C-rates (20-50C rate) varies in inverse proportion to the transformation strain, implying that engineering electrode materials for reduced strain can be used to maximize the power capability of batteries.

Identification of Li-Ion Battery SEI Compounds through (7)Li and (13)C Solid-State MAS NMR Spectroscopy and MALDI-TOF Mass Spectrometry

Solid-state (7)Li and (13)C MAS NMR spectra of cycled graphitic Li-ion anodes demonstrate SEI compound formation upon lithiation that is followed by changes in the SEI upon delithiation. Solid-state (13)C DPMAS NMR shows changes in peaks associated with organic solvent compounds (ethylene carbonate and dimethyl carbonate, EC/DMC) upon electrochemical cycling due to the formation of and subsequent changes in the SEI compounds. Solid-state (13)C NMR spin-lattice (T1) relaxation time measurements of lithiated Li-ion anodes and reference poly(ethylene oxide) (PEO) powders, along with MALDI-TOF mass spectrometry results, indicate that large-molecular-weight polymers are formed in the SEI layers of the discharged anodes. MALDI-TOF MS and NMR spectroscopy results additionally indicate that delithiated anodes exhibit a larger number of SEI products than is found in lithiated anodes.

Three-Dimensional Growth of Li2S in Lithium-Sulfur Batteries Promoted by a Redox Mediator

During the discharge of a lithium-sulfur (Li-S) battery, an electronically insulating 2D layer of Li2S is electrodeposited onto the current collector. Once the current collector is enveloped, the overpotential of the cell increases, and its discharge is arrested, often before reaching the full capacity of the active material. Guided by a new computational platform known as the Electrolyte Genome, we advance and apply benzo[ghi]peryleneimide (BPI) as a redox mediator for the reduction of dissolved polysulfides to Li2S. With BPI present, we show that it is now possible to electrodeposit Li2S as porous, 3D deposits onto carbon current collectors during cell discharge. As a result, sulfur utilization improved 220% due to a 6-fold increase in Li2S formation. To understand the growth mechanism, electrodeposition of Li2S was carried out under both galvanostatic and potentiostatic control. The observed kinetics under potentiostatic control were modeled using modified Avrami phase transformation kinetics, which showed that BPI slows the impingement of insulating Li2S islands on carbon. Conceptually, the pairing of conductive carbons with BPI can be viewed as a vascular approach to the design of current collectors for energy storage devices: here, conductive carbon "arteries" dominate long-range electron transport, while BPI "capillaries" mediate short-range transport and electron transfer between the storage materials and the carbon electrode.

Mechanism and Kinetics of Li2S Precipitation in Lithium-Sulfur Batteries

The kinetics of Li2 S electrodeposition onto carbon in lithium-sulfur batteries are characterized. Electrodeposition is found to be dominated by a 2D nucleation and growth process with rate constants that depend strongly on the electrolyte solvent. Nucleation is found to require a greater overpotential than growth, which results in a morphology that is dependent on the discharge rate.

Packaging Design and Assembly for Ultrahigh Energy Density Microbatteries

For commercially available lithium ion batteries, the package structures can account for as much as 70% of its volume. In a multiyear effort, we have developed designs, fabrications, and assembly processes for ultrahigh energy density microbatteries of 5 and 1 mm3. Over the course of the research, the packaging system evolved due to changes or limitation specific to the battery system itself. During the study, improvements in the construction of the housing for the cathodes were made. Anodized aluminum cans fabricated through our in-house micromachining facility replaced the electroformed gold cans used in earlier experiments. Throughout the iteration of the can design, a flange around its open end, which would be used for sealing, was present, and both designs were fabricated as arrays. The ability to fabricate the housings in an array format lends it to be implemented into a scalable production assembly process. Materials evaluated for the cover ranged from copper foil to the final design of metalized sapphire with a plated through via. One area which needed to be addressed was reducing the temperature of the assembly process due to a change in the cell materials. This resulted in developing a low-temperature hermetic sealing process. The seal between the cover and can was made using a novel composite ring of indium and epoxy. The indium provided the hermeticity needed for the battery chemistry and the epoxy provided mechanical robustness. Most of our experience was with sealing temperatures of 100°C, but the sealing temperature is only limited by the cure temperature of the epoxy. One experiment was conducted to look at the reliability of the seal. A dozen 1-mm3 cells were filled with lithium and sealed. They were aged in laboratory ambient environment and periodically weighed. There was no weight gain in any of the cells over the course of several months, but one cell that was opened at the end of the experiment rapidly gained weight as the lithium corroded. The microbattery cell developed in this program used a lithium cobalt oxide cathode in the form of a porous sintered compact. Its bottom surface was coated with a gold film so that it could be thermocompression bonded to a gold bump on the bottom of the can. Following bonding, the cathode assembly was spray coated with a polymer separator. The anode was a piece of lithium metal bonded to the battery cover. After the cover was sealed to the battery can, the cell was filled with electrolyte and charged. Then a plug was inserted in the via through the cover to seal the battery. Batteries assembled in this manner exhibited energy densities in excess of 200 Wh/L for the smaller volumetric package.

Improving the capacity of sodium ion battery using a virus-templated nanostructured composite cathode

In this work we investigated an energy-efficient biotemplated route to synthesize nanostructured FePO4 for sodium-based batteries. Self-assembled M13 viruses and single wall carbon nanotubes (SWCNTs) have been used as a template to grow amorphous FePO4 nanoparticles at room temperature (the active composite is denoted as Bio-FePO4-CNT) to enhance the electronic conductivity of the active material. Preliminary tests demonstrate a discharge capacity as high as 166 mAh/g at C/10 rate, corresponding to composition Na0.9FePO4, which along with higher C-rate tests show this material to have the highest capacity and power performance reported for amorphous FePO4 electrodes to date.

Mitigating mechanical failure of crystalline silicon electrodes for lithium batteries by morphological design

A novel strategy is developed to mitigate lithiation-induced fracture in crystalline Si anodes by deliberately designing anisometric anode morphologies to counteract the anisotropy in the crystalline/amorphous interface velocity.

Operando studies of nanoscale olivine cathodes for Li-ion batteries

Compounds of interest for ion storage in advanced batteries frequently exhibit phase transformations, driven by large and variable electrochemical driving forces inherent to practical use. Understanding how materials variables (e.g. composition, nanoscale-crystallite size and dynamic electrochemical conditions) affect the phase transition is of vital importance for practical applications as the reversibility and stability of these structural transformations determine the energy, power, and lifetime of the system. Due to its outstanding power, safety and cycle-life olivine LiFePO4 (LFP) has during the past decade become a widely used, and one of the most well-studied, lithium ion battery cathode materials. It is well-established that for LiFePO4 the storage/release of lithium is accompanied by a first-order phase transition between lithiated and delithiated states. However, it would be a mistake to conclude that the behavior of pure LFP is representative of all olivines, in particular the vast range of doped and mixed-metal olivines that are also of interest for their advantageous electrochemical properties.1,2 Utilizing operando synchrotron radiation powder X-ray diffraction (SR-PXD), we demonstrate here, by systematic screening of the electrochemical driven phase transitions in a series of LiMnyFe1-yPO4 (y =0.1-0.8) powders, a completely different phase transformation mode dominated by formation of metastable solid solutions for nanoscale LMFP compared to the binary lithiation states within the extremely well-studied case of LFP. Through Rietveld refinement the misfit strains during phase transformations are examined, revealing small elastic misfits between phases within the extended solid solution regime. On the basis of the time- and state-of-charge dependence of the olivine structure parameters, we propose a coherent transformation mechanism, and finally, we bring evidence that the observed metastability is enabled by particle size reduction to the nanoscale.

In situ observation of random solid solution zone in LiFePO₄ electrode

Nanostructured LiFePO4 (LFP) electrodes have attracted great interest in the Li-ion battery field. Recently there have been debates on the presence and role of metastable phases during lithiation/delithiation, originating from the apparent high rate capability of LFP batteries despite poor electronic/ionic conductivities of bulk LFP and FePO4 (FP) phases. Here we report a potentiostatic in situ transmission electron microscopy (TEM) study of LFP electrode kinetics during delithiation. Using in situ high-resolution TEM, a Li-sublattice disordered solid solution zone (SSZ) is observed to form quickly and reach 10-25 nm × 20-40 nm in size, different from the sharp LFP|FP interface observed under other conditions. This 20 nm scale SSZ is quite stable and persists for hundreds of seconds at room temperature during our experiments. In contrast to the nanoscopically sharp LFP|FP interface, the wider SSZ seen here contains no dislocations, so reduced fatigue and enhanced cycle life can be expected along with enhanced rate capability. Our findings suggest that the disordered SSZ could dominate phase transformation behavior at nonequilibrium condition when high current/voltage is applied; for larger particles, the SSZ could still be important as it provides out-of-equilibrium but atomically wide avenues for Li(+)/e(-) transport.

Polysulfide flow batteries enabled by percolating nanoscale conductor networks

A new approach to flow battery design is demonstrated wherein diffusion-limited aggregation of nanoscale conductor particles at ∼1 vol % concentration is used to impart mixed electronic-ionic conductivity to redox solutions, forming flow electrodes with embedded current collector networks that self-heal after shear. Lithium polysulfide flow cathodes of this architecture exhibit electrochemical activity that is distributed throughout the volume of flow electrodes rather than being confined to surfaces of stationary current collectors. The nanoscale network architecture enables cycling of polysulfide solutions deep into precipitation regimes that historically have shown poor capacity utilization and reversibility and may thereby enable new flow battery designs of higher energy density and lower system cost. Lithium polysulfide half-flow cells operating in both continuous and intermittent flow mode are demonstrated for the first time.

Spin-On thin films of YBa2Cu3O7-y and La2−xSrxCuO4-y from Citrate-Polymer Precursors

Thin films of La2−xSrxCuO4 and YBa2Cu3O7-y superconductors have been prepared from citrate-polymer precursors. By a simple spin-coating and pyrolysis process, films which are dense and continuous at greater than 0.5 μm thickness have been prepared. For YBa2Cu3O7-y films deposited on SrTiO3, a large degree of epitaxial orientation has been observed, with the a-b plane of the superconductor parallel to the (100) plane of the substrate. Microstructural characterization and electrical properties measurements of these films are presented.

Equilibrium Configuration of Bi-Doped ZnO Grain Boundaries: Intergranular Amorphous Films

It is shown that the solid state equilibrium configuration of ZnO grain boundaries saturated with Bi-doping is a nanometer-thick amorphous film. Polycrystalline ZnO samples doped with Bi2O3 were studied using high resolution transmission electron microscopy (HRTEM) and dedicated scanning transmission electron microscopy (STEM). Samples were equilibrated below the eutectic temperature (Teutectic = 740°C) and at 1 atmosphere pressure, starting from three different initial states: one was cooled from above the eutectic temperature; a second was processed entirely below the eutectic temperature; and the third was de-segregated by applying high pressure (1 GPa) followed by annealing at 1 atmospheric pressure. In all instances, ZnO grain boundaries contain an amorphous film 1.0–1.3 ran in thickness, corresponding to a Bi excess equivalent to approximately one monolayer.

Particulate mobility in vertical deposition of attractive monolayer colloidal crystals

In the colloidal self-assembly of charged particles on surfaces with opposite polarity, disorder often dominates. In this report, we show that ionic strength, volume fraction, and solvent evaporation temperature can be optimized in the vertical deposition method to yield hexagonal close-packed monolayer arrays with positively charged colloids on negatively charged bare glass. We further extend our study to form well-defined binary two-dimensional superlattices with oppositely charged monolayers grown layer-by-layer. Our results suggest that the lack of particulate mobility in oppositely charged systems is the main cause of disorder, and maximum mobility is attained when all three growth parameters are finely adjusted to increase the time scale for the particles to stabilize and order during crystal growth in these attractive systems. A clear understanding and control of the collective behavior of highly mobile colloids could lead to the creation of greater diversity of nanoarchitectures.

Stamped microbattery electrodes based on self-assembled M13 viruses

The fabrication and spatial positioning of electrodes are becoming central issues in battery technology because of emerging needs for small scale power sources, including those embedded in flexible substrates and textiles. More generally, novel electrode positioning methods could enable the use of nanostructured electrodes and multidimensional architectures in new battery designs having improved electrochemical performance. Here, we demonstrate the synergistic use of biological and nonbiological assembly methods for fabricating and positioning small battery components that may enable high performance microbatteries with complex architectures. A self-assembled layer of virus-templated cobalt oxide nanowires serving as the active anode material in the battery anode was formed on top of microscale islands of polyelectrolyte multilayers serving as the battery electrolyte, and this assembly was stamped onto platinum microband current collectors. The resulting electrode arrays exhibit full electrochemical functionality. This versatile approach for fabricating and positioning electrodes may provide greater flexibility for implementing advanced battery designs such as those with interdigitated microelectrodes or 3D architectures.

Particle and substrate charge effects on colloidal self-assembly in a sessile drop

By direct video monitoring of dynamic colloidal self-assembly during solvent evaporation in a sessile drop, we investigated the effect of surface charge on the ordering of colloidal spheres. The in situ observations revealed that the interaction between charged colloidal spheres and substrates affects the mobility of colloidal spheres during convective self-assembly, playing an important role in the colloidal crystal growth process. Both ordered and disordered growth was observed depending on different chemical conditions mediated by surface charge and surfactant additions to the sessile drop system. These different self-assembly behaviors were explained by the Coulombic and hydrophobic interactions between surface-charged colloidal spheres and substrates.

Kinetic stages of single-component colloidal crystallization

To harness the full potential of colloidal self-assembly, the dynamics of the transition between colloids in suspension to a colloidal crystalline film should be better understood. In this report, the structural changes during the self-assembly process in a vertical configuration for colloids in the size range 200-400 nm are monitored in situ, using the transmission spectrum of the colloidal assembly treated as an emergent photonic crystal. It is found that there are several sequential stages of colloidal ordering: in suspension, with a larger lattice parameter than the solid state, in a close-packed wet state with solvent in the interstices, and, finally, in a close-packed dry state with air in the interstices. Assuming that these stages lead continuously from one to another, we can interpret colloidal crystallization as being initiated by interparticle forces in suspension first, followed by capillary forces. This result has implications for identifying the optimum conditions to obtain high-quality nanostructures of submicrometer-sized colloidal particles.

Nanometer-scale wetting of the silicon surface by its equilibrium oxide

Despite the extremely broad technical applications of the Si/SiO2 structure, the equilibrium wetting properties of silicon oxide on silicon are poorly understood. Here, we produce new results in which a solid-state buffer method is used to systematically titrate oxygen activity about the Si/SiO2 coexistence value. The equilibrium morphology at the Si(001) surface over >8 decades of PO2 about coexistence is revealed to be a uniform sub-stoichiometric SiOx film of sub-nanometer thickness, coexisting with secondary island structures which coarsen with annealing time. A new thermodynamic method using chemical potential to stabilize and control surficial oxides in nanoscale devices is suggested.

Layer transfer approach to opaline hetero photonic crystals

By taking advantage of the hydrophobicity of dry polystyrene colloidal crystal (opal) films and the large surface tension of water, a convectively self-assembled polystyrene opal film on a hydrophilic glass substrate can be peeled off from the substrate and floated on the water surface. A layer transfer technique was developed to sequentially stack floating opal films of different sphere sizes, resulting in opaline hetero photonic crystals. The feasibility of this technique to planar defect engineering in a self-assembled colloidal photonic crystal was also demonstrated. Both structural observation and optical characterization confirmed the crystalline integrity of the resultant opaline heterostructures.

Assembly of metal nanoparticles into nanogaps

The directed assembly of nanoparticles and nanoscale materials onto specific locations of a surface is one of the major challenges in nanotechnology. Here we present a simple and scalable method and model for the assembly of nanoparticles in between electrical leads. Gold nanoparticles, 20 nm in diameter, were assembled inside electrical gaps ranging from 15 to 150 nm with the use of positive ac dielectrophoresis. In this method, an alternating current is used to create a gradient of electrical field that attracts particles in between the two leads used to create the potential. Assembly is achieved when dielectrophoretic forces exceed thermal and electrostatic forces; the use of anchoring molecules, present in the gap, improves the final assembly stability. We demonstrate with both experiment and theory that nanoparticle assembly inside the gap is controlled by the applied voltage and the gap size. Experimental evidence and modeling suggest that a gap-size-dependent threshold voltage must be overcome before particle assembly is realized. Assembly results as a function of frequency and time are also presented. Assembly of fewer than 10 isolated particles in a gap is demonstrated. Preliminary electrical characterization reveals that stable conductance of the assembled particles can be achieved.

Virus-enabled synthesis and assembly of nanowires for lithium ion battery electrodes

The selection and assembly of materials are central issues in the development of smaller, more flexible batteries. Cobalt oxide has shown excellent electrochemical cycling properties and is thus under consideration as an electrode for advanced lithium batteries. We used viruses to synthesize and assemble nanowires of cobalt oxide at room temperature. By incorporating gold-binding peptides into the filament coat, we formed hybrid gold-cobalt oxide wires that improved battery capacity. Combining virus-templated synthesis at the peptide level and methods for controlling two-dimensional assembly of viruses on polyelectrolyte multilayers provides a systematic platform for integrating these nanomaterials to form thin, flexible lithium ion batteries.

Ionic colloidal crystals: Ordered, multicomponent structures via controlled heterocoagulation

We propose a new type of ordered colloid, the "ionic colloidal crystal" (ICC), which is stabilized by attractive electrostatic interactions analogous to those in atomic ionic materials. The rapid self-organization of colloids via this method should result in a diversity of orderings that are analogous to ionic compounds. Most of these complex structures would be difficult to produce by other methods. We use a Madelung summation approach to evaluate the conditions where ICC's are thermodynamically stable. Using this model, we compare the relative electrostatic energies of various structures showing that the regions of ICC stability are determined by two dimensionless parameters representing charge balance and the spatial extent of the electrostatic interactions. Parallels and distinctions between ICC's and classical ionic crystals are discussed. Monte Carlo simulations are utilized to examine the glass transition and melting temperatures, between which crystallization can occur, of a model system having the rocksalt structure. These tools allow us to make a first-order prediction of the experimentally accessible regions of surface charge, particle size, ionic strength, and temperature where ICC formation is probable.

Nanometer-thick surficial films in oxides as a case of prewetting

Stable, nanometer-thick films are observed to form at the {1120} facets of Bi(2)O(3)-doped ZnO in several bulk-phase stability fields. Electron microscopy shows these surficial films to exhibit some degree of partial order in quenched samples. The equilibrium film thickness, corresponding to the Gibbs excess solute, decreases monotonically with decreasing temperature until vanishing at a dewetting temperature, well below the eutectic. Assuming that perfect wetting occurs at some higher temperature above the eutectic, as is observed on polycrystal surfaces and at grain boundaries in the same system, the adsorption and wetting events in this system illustrate temperature- and composition-dependent prewetting. The observation of a second class of thicker films coexisting with nanodroplets and a numerical evaluation of thickness versus temperature elucidate the critical role of volumetric thermodynamic terms in determining film stability and thickness. Analogous temperature-dependent surface films involving adsorbed MoO(3) on Al(2)O(3) were also observed.

Peptides with selective affinity for carbon nanotubes

Because of their extraordinary electronic and mechanical properties, carbon nanotubes have great potential as materials for applications ranging from molecular electronics to ultrasensitive biosensors. Biological molecules interacting with carbon nanotubes provide them with specific chemical handles that would make several of these applications possible. Here we use phage display to identify peptides with selective affinity for carbon nanotubes. Binding specificity has been confirmed by demonstrating direct attachment of nanotubes to phage and free peptides immobilized on microspheres. Consensus binding sequences show a motif rich in histidine and tryptophan, at specific locations. Our analysis of peptide conformations shows that the binding sequence is flexible and folds into a structure matching the geometry of carbon nanotubes. The hydrophobic structure of the peptide chains suggests that they act as symmetric detergents.