Counting Electrons in Electrides.

The selection and design of charge integration methods remain an outstanding challenge in materials chemistry. In complex materials like electrides, this challenge is amplified by the small charge and complex shape of electride wave functions. For these reasons, popular integration methods, such as the Bader method, usually fail to assign any charge to the bare electrons in an electride. To address this challenge, we developed an algorithm that instead partitions the charge based on the electron localization function (ELF), a popular scheme for visualizing chemically important features in molecules and solids. The algorithm uses Bader segmentation of the ELF to find the electride electrons and Voronoi segmentation of the ELF to identify atoms. We apply this method, "BadELF", to the quantification of atomic radii and oxidation states in both ionic compounds and electrides. For ionic compounds, we find that the BadELF method yields radii that agree closely with Shannon crystal radii, while the oxidation states agree closely with the Bader method. When they are applied to electrides, however, only the BadELF algorithm yields chemically meaningful charges. We argue that the BadELF method provides a useful strategy to identify electrides and obtain new insight into their most essential property: the quantity of electrons within them.

Sc2C, a 2D Semiconducting Electride.

Electrides are exotic materials that typically have electrons present in well-defined lattice sites rather than within atoms. Although all known electrides have an electropositive metal cation adjacent to the electride site, the effect of cation electronegativity on the properties of electrides is not yet known. Here, we examine trivalent metal carbides with varying degrees of electronegativity and experimentally synthesize Sc2C. Our studies identify the material as a two-dimensional (2D) electride, even though Sc is more electronegative than any metal previously found adjacent to an electride site. Further, by exploring Sc2C and Al2C computationally, we find that higher electronegativity of the cation drives greater hybridization between metal and electride orbitals, which opens a band gap in these materials. Sc2C is the first 2D electride semiconductor, and we propose a design rule that cation electronegativity drives the change in its band structure.

Electrolyte-Free Spectroscopy and Imaging of Graphite Intercalation.

Engineering electrode materials for optoelectronic and energy storage applications requires a fundamental understanding of intercalation using spatially-resolved techniques. However, spectroscopic methods can have limited spatial resolution and low intensity since the signal passes through electrolyte. Here, a device geometry is presented in which the electrolyte is laterally separated from the area probed spectroscopically, so that the signal does not pass through the electrolyte. This geometry enables us to visualize ion transport with optical microscopy and monitor charge transfer with Raman and visible reflectance spectroscopies. In addition, vibrational changes are probed in the mid-IR, a region previously difficult to access due to electrolyte absorption. This geometry will allow many layered electrodes to be probed in situ using time- and spatially-resolved techniques, including photon and electron spectroscopies.

Bonding in 2D Donor-Acceptor Heterostructures.

The ability to alter distances between atoms is among the most important tools in materials design. Despite this importance, controlling the interlayer distance in stacks of 2D materials remains a challenge. Here we show from first-principles that stacking electrenes-a new class of electron-donating 2D materials-with other 2D materials provides this control. The resulting donor-acceptor heterostructures have interlayer distances 1 Å less than van der Waals layered materials but 1 Å more than covalent or ionic bonds. This yields a class of quasi-bonds that exhibit characteristics of both ordinary chemical bonds and van der Waals interactions. We show how quasi-bonds have tunable polarities and strengths and that these bonds can be understood by drawing on familiar concepts from molecular orbital theory. We also demonstrate several useful properties of 2D donor-acceptor heterostructures, including superlubricity, ultralow work functions, and greatly improved voltages for lithium-ion batteries.

Band Gap Engineering in a 2D Material for Solar-to-Chemical Energy Conversion.

The electronic structure of 2D semiconductors depends on their thickness, providing new opportunities to engineer semiconductors for energy conversion, electronics, and catalysis. Here we show how a 3D semiconductor, black phosphorus, becomes active for solar-to-chemical energy conversion when it is thinned to a 2D material. The increase in its band gap, from 0.3 eV (3D) to 2.1 eV (2D monolayer), is accompanied by a 40-fold enhancement in the formation of chemical products. Despite this enhancement, smaller flakes also have shorter excited state lifetimes. We deduce a mechanism in which recombination occurs at flake edges, while the "van der Waals" surface of black phosphorus bonds to chemical intermediates and facilitates electron transfer. The unique properties of black phosphorus highlight its potential as a customizable material for solar energy conversion and catalysis, while also allowing us to identify design rules for 2D photocatalysts that will enable further improvements in these materials.

Experimental Demonstration of an Electride as a 2D Material.

Because of their loosely bound electrons, electrides offer physical properties useful in chemical synthesis and electronics. For these applications and others, nanosized electrides offer advantages, but to-date no electride has been synthesized as a nanomaterial. We demonstrate experimentally that Ca2N, a layered electride in which layers of atoms are separated by layers of a 2D electron gas (2DEG), can be exfoliated into two-dimensional (2D) nanosheets using liquid exfoliation. The 2D flakes are stable in a nitrogen atmosphere or in select organic solvents for at least one month. Electron microscopy and elemental analysis reveal that the 2D flakes retain the crystal structure and stoichiometry of the parent 3D Ca2N. In addition, the 2D flakes exhibit metallic character and an optical response that agrees with DFT calculations. Together these findings suggest that the 2DEG is preserved in the 2D material. With this work, we bring electrides into the nanoregime and experimentally demonstrate a 2D electride, Ca2N.

Tunneling Electrical Connection to the Interior of Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) are typically poor electrical conductors, which limits their uses in sensors, fuel cells, batteries, and other applications that require electrically conductive, high surface area materials. Although metal nanoclusters (NCs) are often added to MOFs, the electrical properties of these hybrid materials have not yet been explored. Here, we show that adding NCs to a MOF not only imparts moderate electrical conductivity to an otherwise insulating material but also renders it photoconductive, with conductivity increasing by up to 4 orders of magnitude upon light irradiation. Because charge transport occurs via tunneling between spatially separated NCs that occupy a small percent of the MOF's volume, the pores remain largely open and accessible. While these phenomena are more pronounced in single-MOF crystals (here, Rb-CD-MOFs), they are also observed in films of smaller MOF crystallites (MIL-53). Additionally, we show that in the photoconductive MOFs, the effective diffusion coefficients of electrons can match the typical values of small molecules diffusing through MOFs; this property can open new vistas for the development of MOF electrodes and, in a wider context, of electroactive and light-harvesting MOFs.

Identifying champion nanostructures for solar water-splitting.

Charge transport in nanoparticle-based materials underlies many emerging energy-conversion technologies, yet assessing the impact of nanometre-scale structure on charge transport across micrometre-scale distances remains a challenge. Here we develop an approach for correlating the spatial distribution of crystalline and current-carrying domains in entire nanoparticle aggregates. We apply this approach to nanoparticle-based α-Fe2O3 electrodes that are of interest in solar-to-hydrogen energy conversion. In correlating structure and charge transport with nanometre resolution across micrometre-scale distances, we have identified the existence of champion nanoparticle aggregates that are most responsible for the high photoelectrochemical activity of the present electrodes. Indeed, when electrodes are fabricated with a high proportion of these champion nanostructures, the electrodes achieve the highest photocurrent of any metal oxide photoanode for photoelectrochemical water-splitting under 100 mW cm(-2) air mass 1.5 global sunlight.

Storage of electrical information in metal-organic-framework memristors.

Single crystals of a cyclodextrin-based metal-organic framework (MOF) infused with an ionic electrolyte and flanked by silver electrodes act as memristors. They can be electrically switched between low and high conductivity states that persist even in the absence of an applied voltage. In this way, these small blocks of nanoporous sugar function as a non-volatile RRAM memory elements that can be repeatedly read, erased, and re-written. These properties derive from ionic current within the MOF and the deposition of nanometer-thin passivating layers at the anode flanking the MOF crystal. The observed phenomena are crucially dependent on the sub-nanometer widths of the channels in the MOF, allowing the passage of only smaller ions. Conversely, with the electrolyte present but no MOF, there are no memristance or memory effects.

A silica sol-gel design strategy for nanostructured metallic materials.

Batteries, fuel cells and solar cells, among many other high-current-density devices, could benefit from the precise meso- to macroscopic structure control afforded by the silica sol-gel process. The porous materials made by silica sol-gel chemistry are typically insulators, however, which has restricted their application. Here we present a simple, yet highly versatile silica sol-gel process built around a multifunctional sol-gel precursor that is derived from the following: amino acids, hydroxy acids or peptides; a silicon alkoxide; and a metal acetate. This approach allows a wide range of biological functionalities and metals--including noble metals--to be combined into a library of sol-gel materials with a high degree of control over composition and structure. We demonstrate that the sol-gel process based on these precursors is compatible with block-copolymer self-assembly, colloidal crystal templating and the Stöber process. As a result of the exceptionally high metal content, these materials can be thermally processed to make porous nanocomposites with metallic percolation networks that have an electrical conductivity of over 1,000 S cm(-1). This improves the electrical conductivity of porous silica sol-gel nanocomposites by three orders of magnitude over existing approaches, opening applications to high-current-density devices.

Reconsidering Anode Materials for Fluoride-Ion Batteries-The Unexpected Roles of Carbide Formation.

Carbon is a ubiquitous additive to enhance the electrical conductivity of battery electrodes. Although carbon is generally assumed to be inert, the poor reversibility seen in some fluoride-ion battery electrodes has not been explained or systematically explored. Here, we utilize the Materials Project database to assess electrode deactivation reactions that result in the formation of a metal carbide. Specifically, we compare the theoretical potentials of MFy reduction to either the corresponding metal M or metal carbide MCx . We find that the formation of MCx is unlikely to be important in anodes that operate at modest reduction potentials, such as those made from electronegative metals like Zn, Sn, or Pb. However, in anodes that operate at extreme reduction potentials, such as alkaline earths or lanthanides, we find that formation of MCx is relevant and can emerge as a mechanism for capacity loss. Thus, side reactions of metals with carbon additives that form metal carbides possibly explain the poor reversibility of lanthanide or alkaline earth metal-based electrode materials. Finally, we highlight that the carbide formation process might be exploited for designing cheap anode systems with improved reversibility.

Assessing ternary materials for fluoride-ion batteries.

Although lithium-ion batteries have transformed energy storage, there is a need to develop battery technologies with improved performance. Fluoride-ion batteries (FIBs) may be promising alternatives in part due to their high theoretical energy density and natural elemental abundance. However, electrode materials for FIBs, particularly cathodes, have not been systematically evaluated, limiting rapid progress. Here, we evaluate ternary fluorides from the Materials Project crystal structure database to identify promising cathode materials for FIBs. Structures are further assessed based on stability and whether fluorination/defluorination occurs without unwanted disproportionation reactions. Properties are presented for pairs of fluorinated/defluorinated materials including theoretical energy densities, cost approximations, and bandgaps. We aim to supply a dataset for extracting property and structural trends of ternary fluoride materials that may aid in the discovery of next-generation battery materials.

Plasmoelectronics: coupling plasmonic excitation with electron flow.

Explorations of the coupling of light and charge via localized surface plasmons have led to the discovery that plasmonic excitation can influence macroscopic flows of charge and, conversely, that charging events can change the plasmonic excitation. We discuss recent theory and experiments in the emerging field of plasmoelectronics, with particular emphasis on the application of these materials to challenges in nanotechnology, energy use, and sensing.

Influence of plasmonic Au nanoparticles on the photoactivity of Fe2O3 electrodes for water splitting.

An experimental study of the influence of gold nanoparticles on α-Fe(2)O(3) photoanodes for photoelectrochemical water splitting is described. A relative enhancement in the water splitting efficiency at photon frequencies corresponding to the plasmon resonance in gold was observed. This relative enhancement was observed only for electrode geometries with metal particles that were localized at the semiconductor-electrolyte interface, consistent with the observation that minority carrier transport to the electrolyte is the most significant impediment to achieving high efficiencies in this system.

Block copolymer directed nanoporous metal thin films.

Porous metal thin films have high potential for use in applications such as catalysis, electrical contacts, plasmonics, as well as energy storage and conversion. Structuring metal thin films on the nanoscale to generate high surface areas poses an interesting challenge as metals have high surface energy. In this communication, we demonstrate direct access to nanostructured metal nanoparticle hybrid thin films with high nanoparticle loadings through spin coating of a mixture of block copolymer and ligand stabilized platinum and palladium nanoparticles. Plasma cleaning to remove the organics results in a conductive metal thin film. We expect that the methods described here can be generalized to other metals, mixtures of metal nanoparticles, and intermetallics.

Integrating structure control over multiple length scales in porous high temperature ceramics with functional platinum nanoparticles.

High temperature ceramics with porosity on multiple length scales offer great promise in high temperature catalytic applications for their high surface area and low flow resistance in combination with thermal and chemical stability. We have developed a bottom-up approach to functional, porous, high-temperature ceramics structured on eight distinct length scales integrating functional Pt nanoparticles from the near-atomic to the macroscopic level. Structuring is achieved through a combination of micromolding and multicomponent colloidal self-assembly. The resulting template is filled with a solution containing a solvent, a block copolymer, a ceramic precursor, and a nanoparticle catalyst precursor as well as a radical initiator. Heat treatment results in three-dimensionally interconnected, high-temperature ceramic materials functionalized with well-dispersed 1-2 nm Pt catalyst nanoparticles and very high porosity.

First-Principles Prediction of Electrochemical Electron-Anion Exchange: Ion Insertion without Redox.

It is widely assumed that the gain or loss of electrons in a material must be accompanied by its reduction or oxidation. Here, we report a system in which the insertion/deinsertion of an electron occurs without any reduction or oxidation. Using first-principles methods, we demonstrate this effect in the Y2CF2-[Y2C]2+(e-)2 material system, where (e-) indicates a lattice site containing a bare electron. We present a model in which Y2CF2 is in contact with a fluoride-containing electrolyte and the application of a positive voltage drives fluorination while a negative voltage reverses the process. We show that this chemistry does not change the oxidation states of the host lattice, causes no significant volume expansion, and occurs rapidly at room temperature. Finally, we demonstrate that this mechanism of ion insertion may enable a broad class of anion shuttle battery electrodes, some with gravimetric capacities nearly double those employed in intercalation-type Li-ion batteries.

Ratcheting quasi-ballistic electrons in silicon geometric diodes at room temperature.

Ratcheting effects play an important role in systems ranging from mechanical socket wrenches to biological motor proteins. The underlying principle is to convert a fluctuating, unbiased force into unidirectional motion. Here, we report the ratcheting of electrons at room temperature using a semiconductor nanowire with precisely engineered asymmetry. Modulation of the nanowire diameter creates a cylindrical sawtooth geometry with broken inversion symmetry on a nanometer-length scale. In a two-terminal device, this structure responded as a three-dimensional geometric diode that funnels electrons preferentially in one direction through specular reflection of quasi-ballistic electrons at the nanowire surface. The ratcheting effect causes charge rectification at frequencies exceeding 40 gigahertz, demonstrating the potential for applications such as high-speed data processing and long-wavelength energy harvesting.

Organization of nanoparticles in polymer brushes.

We have demonstrated a facile infiltration process, in which gold nanoparticles are assembled into block copolymer brushes. After solvent annealing, the polymer-covered nanoparticles are either sequestered into the corresponding block copolymer domain or expulsed from the brush, depending on the shell density of the nanoparticles.

Direct access to thermally stable and highly crystalline mesoporous transition-metal oxides with uniform pores.

Even after a decade or so of research, the direct synthesis of highly crystalline mesoporous transition-metal oxides that are thermally stable and well ordered still constitutes a major challenge. Although various soft- and hard-templating approaches have been developed in the past, they usually suffer from multiple, tedious steps and often result in poor structure control. For many applications including power generation and energy conversion, however, high crystallinity and controlled mesoporosity are a prerequisite. To this end, here we report on an approach established for group-IV (titanium) and group-V (niobium) oxides, with potential applications to photovoltaic cells and fuel cells, respectively, which overcomes previous limitations. It gives direct access to the desired materials in a 'one-pot' synthesis using block copolymers with an sp2-hybridized carbon-containing hydrophobic block as structure-directing agents which converts to a sturdy, amorphous carbon material under appropriate heating conditions. This in situ carbon is sufficient to act as a rigid support keeping the pores of the oxides intact while crystallizing at temperatures as high as 1,000 degrees C.