A metal-free organic-inorganic aqueous flow battery.

As the fraction of electricity generation from intermittent renewable sources--such as solar or wind--grows, the ability to store large amounts of electrical energy is of increasing importance. Solid-electrode batteries maintain discharge at peak power for far too short a time to fully regulate wind or solar power output. In contrast, flow batteries can independently scale the power (electrode area) and energy (arbitrarily large storage volume) components of the system by maintaining all of the electro-active species in fluid form. Wide-scale utilization of flow batteries is, however, limited by the abundance and cost of these materials, particularly those using redox-active metals and precious-metal electrocatalysts. Here we describe a class of energy storage materials that exploits the favourable chemical and electrochemical properties of a family of molecules known as quinones. The example we demonstrate is a metal-free flow battery based on the redox chemistry of 9,10-anthraquinone-2,7-disulphonic acid (AQDS). AQDS undergoes extremely rapid and reversible two-electron two-proton reduction on a glassy carbon electrode in sulphuric acid. An aqueous flow battery with inexpensive carbon electrodes, combining the quinone/hydroquinone couple with the Br2/Br(-) redox couple, yields a peak galvanic power density exceeding 0.6 W cm(-2) at 1.3 A cm(-2). Cycling of this quinone-bromide flow battery showed >99 per cent storage capacity retention per cycle. The organic anthraquinone species can be synthesized from inexpensive commodity chemicals. This organic approach permits tuning of important properties such as the reduction potential and solubility by adding functional groups: for example, we demonstrate that the addition of two hydroxy groups to AQDS increases the open circuit potential of the cell by 11% and we describe a pathway for further increases in cell voltage. The use of π-aromatic redox-active organic molecules instead of redox-active metals represents a new and promising direction for realizing massive electrical energy storage at greatly reduced cost.

Extending the Lifetime of Organic Flow Batteries via Redox State Management.

Redox flow batteries based on quinone-bearing aqueous electrolytes have emerged as promising systems for energy storage from intermittent renewable sources. The lifetime of these batteries is limited by quinone stability. Here, we confirm that 2,6-dihydroxyanthrahydroquinone tends to form an anthrone intermediate that is vulnerable to subsequent irreversible dimerization. We demonstrate quantitatively that this decomposition pathway is responsible for the loss of battery capacity. Computational studies indicate that the driving force for anthrone formation is greater for anthraquinones with lower reduction potentials. We show that the decomposition can be substantially mitigated. We demonstrate that conditions minimizing anthrone formation and avoiding anthrone dimerization slow the capacity loss rate by over an order of magnitude. We anticipate that this mitigation strategy readily extends to other anthraquinone-based flow batteries and is thus an important step toward realizing renewable electricity storage through long-lived organic flow batteries.

Obtaining a Low and Wide Atomic Layer Deposition Window (150-275 °C) for In2 O3 Films Using an InIII Amidinate and H2 O.

Indium oxide is a major component of many technologically important thin films, most notably the transparent conductor indium tin oxide (ITO). Despite being pyrophoric, homoleptic indium(III) alkyls do not allow atomic layer deposition (ALD) of In2 O3 using water as a co-precursor at substrate temperatures below 200 °C. Several alternative indium sources have been developed, but none allows ALD at lower temperatures except in the presence of oxidants such as O2 or O3 , which are not compatible with some substrates or alloying processes. We have synthesized a new indium precursor, tris(N,N'-diisopropylformamidinato)indium(III), compound 1, which allows ALD of pure, carbon-free In2 O3 films using H2 O as the only co-reactant, on substrates in the temperature range 150-275 °C. In contrast, replacing just the H of the anionic iPrNC(H)NiPr ligand with a methyl group (affording the known tris(N,N'-diisopropylacetamidinato)indium(III), compound 2) results in a considerably higher and narrower ALD window in the analogous reaction with H2 O (225-300 °C). Kinetic studies demonstrate that a higher rate of surface reactions in both parts of the ALD cycle gives rise to this difference in the ALD windows.

UV-Vis spectrophotometry of quinone flow battery electrolyte for in situ monitoring and improved electrochemical modeling of potential and quinhydrone formation.

Quinone-based aqueous flow batteries provide a potential opportunity for large-scale, low-cost energy storage due to their composition from earth abundant elements, high aqueous solubility, reversible redox kinetics and their chemical tunability such as reduction potential. In an operating flow battery utilizing 9,10-anthraquinone-2,7-disulfonic acid, the aggregation of an oxidized quinone and a reduced hydroquinone to form a quinhydrone dimer causes significant variations from ideal solution behavior and of optical absorption from the Beer-Lambert law. We utilize in situ UV-Vis spectrophotometry to establish (a), quinone, hydroquinone and quinhydrone molar attenuation profiles and (b), an equilibrium constant for formation of the quinhydrone dimer (KQHQ) ∼ 80 M-1. We use the molar optical attenuation profiles to identify the total molecular concentration and state of charge at arbitrary mixtures of quinone and hydroquinone. We report density functional theory calculations to support the quinhydrone UV-Vis measurements and to provide insight into the dimerization conformations. We instrument a quinone-bromine flow battery with a Pd-H reference electrode in order to demonstrate how complexation in both the negative (quinone) and positive (bromine) electrolytes directly impacts measured half-cell and full-cell voltages. This work shows how accounting for electrolyte complexation improves the accuracy of electrochemical modeling of flow battery electrolytes.

Epitaxial Growth of MgxCa1-xO on GaN by Atomic Layer Deposition.

We demonstrate for the first time that a single-crystalline epitaxial MgxCa1-xO film can be deposited on gallium nitride (GaN) by atomic layer deposition (ALD). By adjusting the ratio between the amounts of Mg and Ca in the film, a lattice matched MgxCa1-xO/GaN(0001) interface can be achieved with low interfacial defect density. High-resolution X-ray diffraction (XRD) shows that the lattice parameter of this ternary oxide nearly obeys Vegard's law. An atomically sharp interface from cross-sectional transmission electron microscopy (TEM) confirmed the high quality of the epitaxy. High-temperature capacitance-voltage characterization showed that the film with composition Mg0.25Ca0.75O has the lowest interfacial defect density. With this optimal oxide composition, a Mg0.25Ca0.75O/AlGaN/GaN metal-oxide-semiconductor high-electron-mobility (MOS-HEMT) device was fabricated. An ultrahigh on/off ratio of 1012 and a near ideal SS of 62 mV/dec were achieved with this device.

Synthesis of Calcium(II) Amidinate Precursors for Atomic Layer Deposition through a Redox Reaction between Calcium and Amidines.

We have prepared two new Ca(II) amidinates, which comprise a new class of ALD precursors. The syntheses proceed by a direct reaction between Ca metal and the amidine ligands in the presence of ammonia. Bis(N,N'-diisopropylformamidinato)calcium(II) (1) and bis(N,N'-diisopropylacetamidinato)calcium(II) (2) adopt dimeric structures in solution and in the solid state. X-ray crystallography revealed asymmetry in one of the bridging ligands to afford the structure [(η(2) -L)Ca(µ-η(2) :η(2) -L)(µ-η(2) :η(1) -L)Ca(η(2) -L)]. These amidinate complexes showed unprecedentedly high volatility as compared to the widely employed and commercially available Ca(II) precursor, [Ca3 (tmhd)6 ]. In CaS ALD with 1 and H2 S, the ALD window was approximately two times wider and lower in temperature by about 150 °C than previously reported with [Ca3 (tmhd)6 ] and H2 S. Complexes 1 and 2, with their excellent volatility and thermal stability (up to at least 350 °C), are the first homoleptic Ca(II) amidinates suitable for use as ALD precursors.

Heteroepitaxy of La2O3 and La(2-x)Y(x)O3 on GaAs (111)A by atomic layer deposition: achieving low interface trap density.

GaAs metal-oxide-semiconductor devices historically suffer from Fermi-level pinning, which is mainly due to the high trap density of states at the oxide/GaAs interface. In this work, we present a new way of passivating the interface trap states by growing an epitaxial layer of high-k dielectric oxide, La(2-x)Y(x)O(3), on GaAs(111)A. High-quality epitaxial La(2-x)Y(x)O(3) thin films are achieved by an ex situ atomic layer deposition (ALD) process, and GaAs MOS capacitors made from this epitaxial structure show very good interface quality with small frequency dispersion and low interface trap densities (D(it)). In particular, the La(2)O(3)/GaAs interface, which has a lattice mismatch of only 0.04%, shows very low D(it) in the GaAs bandgap, below 3 × 10(11) cm(-2) eV(-1) near the conduction band edge. The La(2)O(3)/GaAs capacitors also show the lowest frequency dispersion of any dielectric on GaAs. This is the first achievement of such low trap densities for oxides on GaAs.

Creation and control of two-dimensional electron gas using Al-based amorphous oxides/SrTiO3 heterostructures grown by atomic layer deposition.

The formation of a two-dimensional electron gas (2-DEG) using SrTiO(3) (STO)-based heterostructures provides promising opportunities in oxide electronics. We realized the formation of 2-DEG using several amorphous layers grown by the atomic layer deposition (ALD) technique at 300 °C which is a process compatible with mass production and thereby can provide the realization of potential applications. We found that the amorphous LaAlO(3) (LAO) layer grown by the ALD process can generate 2-DEG (∼1 × 10(13)/cm(2)) with an electron mobility of 4-5 cm(2)/V·s. A much higher electron mobility was observed at lower temperatures. More remarkably, amorphous YAlO(3) (YAO) and Al(2)O(3) layers, which are not polar-perovskite-structured oxides, can create 2-DEG as well. 2-DEG was created by means of the important role of trimethylaluminum, Me(3)Al, as a reducing agent for STO during LAO and YAO ALD as well as the Al(2)O(3) ALD process at 300 °C. The deposited oxide layer also plays an essential role as a catalyst that enables Me(3)Al to reduce the STO. The electrons were localized very near to the STO surface, and the source of carriers was explained based on the oxygen vacancies generated in the STO substrate.

Glass-encapsulated light harvesters: more efficient dye-sensitized solar cells by deposition of self-aligned, conformal, and self-limited silica layers.

A major loss mechanism in dye-sensitized solar cells (DSCs) is recombination at the TiO(2)/electrolyte interface. Here we report a method to reduce greatly this loss mechanism. We deposit insulating and transparent silica (SiO(2)) onto the open areas of a nanoparticulate TiO(2) surface while avoiding any deposition of SiO(2) over or under the organic dye molecules. The SiO(2) coating covers the highly convoluted surface of the TiO(2) conformally and with a uniform thickness throughout the thousands of layers of nanoparticles. DSCs incorporating these selective and self-aligned SiO(2) layers achieved a 36% increase in relative efficiency versus control uncoated cells.

Low energy carbon capture via electrochemically induced pH swing with electrochemical rebalancing.

We demonstrate a carbon capture system based on pH swing cycles driven through proton-coupled electron transfer of sodium (3,3'-(phenazine-2,3-diylbis(oxy))bis(propane-1-sulfonate)) (DSPZ) molecules. Electrochemical reduction of DSPZ causes an increase of hydroxide concentration, which absorbs CO2; subsequent electrochemical oxidation of the reduced DSPZ consumes the hydroxide, causing CO2 outgassing. The measured electrical work of separating CO2 from a binary mixture with N2, at CO2 inlet partial pressures ranging from 0.1 to 0.5 bar, and releasing to a pure CO2 exit stream at 1.0 bar, was measured for electrical current densities of 20-150 mA cm-2. The work for separating CO2 from a 0.1 bar inlet and concentrating into a 1 bar exit is 61.3 kJ molCO2-1 at a current density of 20 mA cm-2. Depending on the initial composition of the electrolyte, the molar cycle work for capture from 0.4 mbar extrapolates to 121-237 kJ molCO2-1 at 20 mA cm-2. We also introduce an electrochemical rebalancing method that extends cell lifetime by recovering the initial electrolyte composition after it is perturbed by side reactions. We discuss the implications of these results for future low-energy electrochemical carbon capture devices.

In situ electrochemical recomposition of decomposed redox-active species in aqueous organic flow batteries.

Aqueous organic redox flow batteries offer a safe and potentially inexpensive solution to the problem of storing massive amounts of electricity produced from intermittent renewables. However, molecular decomposition represents a major barrier to commercialization-and although structural modifications can improve stability, it comes at the expense of synthetic cost and molecular weight. Now, utilizing 2,6-dihydroxy-anthraquinone (DHAQ) without further structural modification, we demonstrate that the regeneration of the original molecule after decomposition represents a viable route to achieve low-cost, long-lifetime aqueous organic redox flow batteries. We used in situ (online) NMR and electron paramagnetic resonance, and complementary electrochemical analyses to show that the decomposition compound 2,6-dihydroxy-anthrone (DHA) and its tautomer, 2,6-dihydroxy-anthranol (DHAL) can be recomposed to DHAQ electrochemically through two steps: oxidation of DHA(L)2- to the dimer (DHA)24- by one-electron transfer followed by oxidation of (DHA)24- to DHAQ2- by three-electron transfer per DHAQ molecule. This electrochemical regeneration process also rejuvenates the positive electrolyte-rebalancing the states of charge of both electrolytes without introducing extra ions.

Functioning Water-Insoluble Ferrocenes for Aqueous Organic Flow Battery via Host-Guest Inclusion.

Ferrocene (Fc) is one of the very limited organic catholyte options for aqueous organic flow batteries (AOFBs), a potential electrochemical energy storage solution to the intermittency of renewable electricity. Commercially available Fc derivatives are barely soluble in water, while existing methods for making water-soluble Fc derivatives by appending hydrophilic or charged moieties are tedious and time-consuming, with low yields. Here, a strategy was developed based on host-guest inclusion to acquire water-soluble Fc-based catholytes by simply mixing Fc derivatives with ß-cyclodextrins (ß-CDs) in water. Factors determining the stability and the electrochemical behavior of the inclusion complexes were identified. When adopted in a neutral pH AOFB, the origin of capacity loss was identified to be a chemical degradation caused by the nucleophilic attack on the center FeIII atom of the oxidized Fc derivatives. By limiting the state of charge, a low capacity fade rate of 0.0073 % h-1 (or 0.0020 % per cycle) was achieved. The proposed strategy may be extended to other families of electrochemically active water-insoluble organic compounds, bringing more electrolyte options for practical AOFB applications.

Surface and interface processes during atomic layer deposition of copper on silicon oxide.

The initial surface chemistry and growth mechanisms of the atomic layer deposition (ALD) of metallic copper on SiO(2) surfaces are investigated using an amidinate precursor (copper(I) di-sec-butylacetamidinate, [Cu((s)Bu-amd)](2)) and molecular hydrogen. Using in situ Fourier transform infrared spectroscopy together with calculations based on density functional theory, we show that the initial surface reaction of [Cu((s)Bu-amd)](2) with hydroxylated SiO(2) takes place by displacement of one of the sec-butylacetamidinate ligands at a surface -OH site, thus forming a Si-O-Cu-((s)Bu-amd) surface species, evident by the stretching vibrations of Si-O-Cu and the chelating -NCN- bonds. Molecular hydrogen exposure during a subsequent pulse dissociates most of the sec-butylacetamidinate ligands bound to surface Cu, which releases free amidine vapor, leaving Cu atoms free to agglomerate on the surface and thus opening more reactive sites for the next [Cu((s)Bu-amd)](2) pulse. Copper agglomeration is evident in the IR absorbance spectra through the partial recovery of the intensity of SiO(2) optical phonon modes upon H(2) reduction, which was lost after the reaction of [Cu((s)Bu-amd)](2) with the initial SiO(2) surface. The thermally activated ligand rearrangement from a bridging to a monodentate structure occurs above 220 degrees C through hydrogenation of the ligand by surface hydroxyl groups after exposure to a [Cu((s)Bu-amd)](2) pulse. As Cu particles grow with further ALD cycles, the activation temperature is lowered to 185 degrees C, and hydrogenation of the ligand takes place after H(2) pulses, catalyzed by Cu particles on the surface. The surface ligand rearranged into a monodentate structure can be removed during subsequent Cu precursor or H(2) pulses. Finally, we postulate that the attachment of dissociated ligands to the SiO(2) surface during the [Cu((s)Bu-amd)](2) pulse can be responsible for carbon contamination at the surface during the initial cycles of growth, where the SiO(2) surface is not yet completely covered by copper metal.

Electrostatically Doped Silicon Nanowire Arrays for Multispectral Photodetectors.

Nanowires have promising applications as photodetectors with superior ability to tune absorption with morphology. Despite their high optical absorption, the quantum efficiencies of these nanowire photodetectors remain low due to difficulties in fabricating a shallow junction using traditional doping methods. As an alternative, we report nonconventional radial heterojunction photodiodes obtained by conformal coating of an indium oxide layer on silicon nanowire arrays. The indium oxide layer has a high work function which induces a strong inversion in the silicon nanowire and creates a virtual p-n junction. The resulting nanowire photodetectors show efficient carrier separation and collection, leading to an improvement of quantum efficiency up to 0.2. In addition, by controlling the nanowire radii, the spectral responses of the In2O3/Si nanowire photodetectors are tuned over several visible light wavelengths, creating a multispectral detector. Our approach is promising for the development of highly efficient wavelength-selective photodetectors.

Synthesis of volatile, reactive coinage metal 5,5-bicyclic amidinates with enhanced thermal stability for chemical vapor deposition.

Many microelectronic devices require thin films of silver or gold as wiring layers. We report silver(i) and gold(i) bicyclic amidinate complexes, wherein the constrained ligand geometry lessens the propensity for thermal decomposition. These new volatile compounds provide metallic films of silver and gold during CVD with hydrogen below 230 °C.

Atomic Layer Deposition of Tin Monosulfide Using Vapor from Liquid Bis(N,N'-diisopropylformamidinato)tin(II) and H2S.

The oxide and sulfide of divalent tin show considerable promise for sustainable thin-film optoelectronics, as transparent conducting and light absorbing p-type layers, respectively. Chemical vapor deposition (CVD) and atomic layer deposition (ALD) provide attractive routes to these layers. The literature on volatile tin(II) compounds used as CVD or ALD precursors shows that new compounds can provide different growth rates, film morphologies, preferred crystallographic orientations, and other material properties. We report here the synthesis and characterization of a new liquid tin(II) precursor, bis(N,N'-diisopropylformamidinato)tin(II) (1), which is effective in ALD of SnS in combination with H2S between 65 and 180 °C. Like other highly reactive tin(II) precursors, the growth per cycle linearly decreases from 0.82 Å/cycle at 65 °C to 0.4 Å/cycle at 180 °C. This is obviously different from the case of previously reported SnS ALD using bis(2,4-pentanedionato)tin(II), Sn(acac)2, and H2S; films grow at 0.22-0.24 Å/cycle almost independent of the substrate temperature (125-225 °C, J. Phys. Chem. C 2010, 114, 17597). Quartz crystal microbalance (QCM) experiments for SnS ALD using 1 at 80, 120, and 160 °C were carried out to study the linear decrease of the growth per cycle with increasing substrate temperature. On the basis of these QCM studies, although the mechanism of chemisorption-loss of one ligand or two-can be manipulated by changing the exposure of 1, the purging time, or the temperature, only the temperature changes the growth per cycle. We therefore attribute the decreasing growth per cycle with increasing temperature to a decreasing surface thiol density. Photovoltaic devices prepared from 1-derived SnS have a performance similar to those of the best devices prepared from other precursors, and the device yield and replicability of J-V properties are substantially increased by using 1.

Direct-Liquid-Evaporation Chemical Vapor Deposition of Nanocrystalline Cobalt Metal for Nanoscale Copper Interconnect Encapsulation.

In advanced microelectronics, precise design of liner and capping layers become critical, especially when it comes to the fabrication of Cu interconnects with dimensions lower than its mean free path. Herein, we demonstrate that direct-liquid-evaporation chemical vapor deposition (DLE-CVD) of Co is a promising method to make liner and capping layers for nanoscale Cu interconnects. DLE-CVD makes pure, smooth, nanocrystalline, and highly conformal Co films with highly controllable growth characteristics. This process allows full Co encapsulation of nanoscale Cu interconnects, thus stabilizing Cu against diffusion and electromigration. Electrical measurements and high-resolution elemental imaging studies show that the DLE-CVD Co encapsulation layer can improve the reliability and thermal stability of Cu interconnects. Also, with the high conductivity of Co, the DLE-CVD Co encapsulation layer have the potential to further decrease the power consumption of nanoscale Cu interconnects, paving the way for Cu interconnects with higher efficiency in future high-end microelectronics.

Novel phase diagram behavior and materials design in heterostructural semiconductor alloys.

Structure and composition control the behavior of materials. Isostructural alloying is historically an extremely successful approach for tuning materials properties, but it is often limited by binodal and spinodal decomposition, which correspond to the thermodynamic solubility limit and the stability against composition fluctuations, respectively. We show that heterostructural alloys can exhibit a markedly increased range of metastable alloy compositions between the binodal and spinodal lines, thereby opening up a vast phase space for novel homogeneous single-phase alloys. We distinguish two types of heterostructural alloys, that is, those between commensurate and incommensurate phases. Because of the structural transition around the critical composition, the properties change in a highly nonlinear or even discontinuous fashion, providing a mechanism for materials design that does not exist in conventional isostructural alloys. The novel phase diagram behavior follows from standard alloy models using mixing enthalpies from first-principles calculations. Thin-film deposition demonstrates the viability of the synthesis of these metastable single-phase domains and validates the computationally predicted phase separation mechanism above the upper temperature bound of the nonequilibrium single-phase region.

A Two-Step Absorber Deposition Approach To Overcome Shunt Losses in Thin-Film Solar Cells: Using Tin Sulfide as a Proof-of-Concept Material System.

As novel absorber materials are developed and screened for their photovoltaic (PV) properties, the challenge remains to reproducibly test promising candidates for high-performing PV devices. Many early-stage devices are prone to device shunting due to pinholes in the absorber layer, producing "false-negative" results. Here, we demonstrate a device engineering solution toward a robust device architecture, using a two-step absorber deposition approach. We use tin sulfide (SnS) as a test absorber material. The SnS bulk is processed at high temperature (400 °C) to stimulate grain growth, followed by a much thinner, low-temperature (200 °C) absorber deposition. At a lower process temperature, the thin absorber overlayer contains significantly smaller, densely packed grains, which are likely to provide a continuous coating and fill pinholes in the underlying absorber bulk. We compare this two-step approach to the more standard approach of using a semi-insulating buffer layer directly on top of the annealed absorber bulk, and we demonstrate a more than 3.5× superior shunt resistance Rsh with smaller standard error σRsh. Electron-beam-induced current (EBIC) measurements indicate a lower density of pinholes in the SnS absorber bulk when using the two-step absorber deposition approach. We correlate those findings to improvements in the device performance and device performance reproducibility.