Perovskites (ABO3) with high electronic and ionic conductivity and catalytic characteristics [1] have been studied intensively for solid oxide fuel cells (SOFCs) [2] and oxygen permeation membranes [3] at high temperatures. A major barrier that limits the efficiency of SOFCs and oxygen permeation flux is the slow kinetics of surface oxygen exchange on the oxide surface.

Ruddlesden-Popper (RP) oxides such as La2NiO4+δ (LNO) can lead to higher oxygen ionic conductivity relative to ABO3 perovskites [4] and are interesting alternative cathode materials for SOFCs. By substituting of a larger cation such as Sr2+ (1.31 Ã…) for La3+ (1.22 Ã…) in LNO, the structural stresses are released, which leads to decrease in the amount of additional oxygen to stabilize the structure. Consequently, modifying the rock salts layer where the oxygen diffuse results in the reduction of the oxygen diffusion coefficients. [5] However, the influence of Sr substitution on the in-plane and out-of-plane surface oxygen exchange kinetics of La2-xSrxNiO4±δ (LSNO) is poorly understood due to difficulties in the growth of single crystals of LSNO with high Sr substitution.

In this study, we investigate how Sr substitution can affect the surface oxygen exchange kinetics of LSNO thin films with a wide range of Sr content (xSr = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) grown on (001)cubic-Y2O3-stabilized ZrO2 (YSZ). The structural orientation of the epitaxial LSNO thin films can be modified from the in-plane to the out-of-plane orientation by increasing the Sr content from 0.0 to 1.0. Such a change in the film orientation can be explained by the reduction in the surface energy of the (001)tetra. surface with increasing Sr as revealed from Density function theory (DFT) calculations. Ex situ Auger electron spectroscopy (AES) indicates no formation of secondary phases across the Sr contents.

We show the strong dependence of the surface oxygen exchange rate (kq) of LSNO thin films on the Sr substitution, which can be attributed to the structural reorientation and the adsorption energy change of molecular oxygen on La-La bridge sites.

References:

[1] Adler, S. B. Chem. Rev. 2004, 104, 4791.

[2] Adler, S. B.; Chen, X. Y.; Wilson, J. R. J. Catal. 2007, 245, 91.

[3] Hashim, S. M.; Mohamed, A. R.; Bhatia, S. Adv. Colloid Interface Sci. 2010, 160, 88.

[4] Amow, G.; Skinner, S. J. J. Solid State Electrochem. 2006, 10, 538.

[5] Skinner, S. J.; Kilner, J. A. Ionics 1999, 5, 171.

Hydrogenated amorphous silicon carbide (a-SiC:H) has shown promising activities as a photocathode for photoelectrochemical (PEC) water splitting. This material has many promising advantages for large-scale utilization since it is compromised entirely of earth abundant materials and can be fabricated in industrial processing techniques. Therefore, it is of paramount importance to identify and overcome the performance limitations for this material in order to address the global environmental and energy demands.

One limitation for a-SiC:H photocathodes is the non-ideal alignment of the conduction and valence band edge positions. This requires a bias voltage to be applied to drive water splitting, which can be overcome by integrating a PV cell under the photocathode. The challenges for this PV/PEC integration require matching the Vop and Jsc of the PV cell with the Vonset and Jplateua of the photocathode, while at the same time managing the spectral utilization of the sun. To improve the PV matching with the PEC films, we have fabricated several unique single and tandem junction PV cells with both amorphous silicon and nano-crystalline silicon, showing enhanced current matching and performance.

In addition, we have utilized several surface passivation techniques to reduce corrosion during the PEC testing. Using both ALD and RF sputtering depositions, we deposited thin transparent conducting layers on the surface of the a-SiC:H photocathode, which showed improved onset potentials, saturated photocurrent densities and enhanced stability.

Finally, we have investigated various hydrogen evolution catalysts deposited on the passivated a-SiC:H photocathodes, showing significantly enhanced water splitting capabilities at reduced bias potentials. Electronic band diagrams have been developed to explain the activity (or non-activity) of different catalysts.Overall, we have been able to identify and address significant hurdles in the development a-SiC:H photocathodes for solar water splitting, and herein report our recent advances with regards to PV integration, surface passivation, and hydrogen evolution catalysis.

The creation and exploration of artificial graphene structures has recently become the focus of great interest. In particular, controlling the interlayer rotation (or twist) angles in multilayer graphene stacks allows one to modulate the overall band structure. However, producing such a structure remains difficult due to the random distribution of twist angles in as-grown samples. Here we report a novel way for creating large area graphene stacks with a pre-determined twist angle. We first grow single layer graphene whose orientation is aligned over a few cm length scale on copper foil using chemical vapor deposition (CVD) method. The overall and local angle alignment of this graphene sample is confirmed using low energy electron microscopy (LEED), dark-field transmission electron microscopy (DF-TEM) and selected area electron diffraction (SAED) techniques. Since the graphene is well aligned over a few centimeters, we can create large area graphene stacks with known twist angle by transferring these graphene layers while controlling the orientation of each layer during transfer. We confirm that the layers are coupled with the designed twist angle, by probing the resulting band structure using angle resolved photoemission spectroscopy (ARPES) and examining their interlayer optical resonance features using spatially resolved hyperspectral (DUV-Vis-NIR wavelengths) measurements. This new method is scalable, controllable, and uses commercially available copper foil, and thus paves the way to explore and exploit the novel properties of two-dimensional crystals in artificial stacks with controlled interlayer structures.

Scientists are often posed with the problem of finding a good choice of experimental parameters that optimize a particular quantity in the presence of ambiguity about the underlying system. This ambiguity arises from unknown physical constants, competing (and often poorly understood) mechanisms, and measurement or calibration error.

Instead of an exhaustive and potentially expensive search over all such parameters, the method of Optimal Learning uses Bayesian statistics and heuristics to guide an experimenter through parameter space in an efficient manner, with the goal of learning about the underlying physical system through each experiment.

We give an overview of the Optimal Learning method and the heuristics involved. We present how a meaningful baseline prior belief is elicited, using the domain knowledge of the experimenter. We then present a real-world application of the Optimal Learning technique to characterizing the stability of nano-emulsions stabilized by gold nanorods.

The new generation of non volatile memories for data storage is based on a unique property of systems known as phase change materials, i.e. the super fast (ns) and reversible phase transition between the disordered and the crystalline phases. One of the reasons of the fast crystallization of GeTe-based phase change materials is the very high atomic mobility of the supercooled liquid phase, even close to the glass transition temperature. This feature is in turn a consequence of the fact that GeTe is a fragile liquid, i.e. it shows a breakdown of the Stokes-Einstein relation (SER) that relates viscosity and diffusion in the hydrodynamic regime.

In this work, we investigate by large scale molecular dynamics simulations the microscopic origin of the breakdown of the SER in GeTe. To this end we employed an interatomic potential based onto a Neural Network framework that allows to overcome the limitations of conventional first principles calculations in terms of system size and timescale.

Our findings demonstrate that the breakdown of the SER is due to the presence of dynamical heterogeneities in the atomic motion. We quantified as a function of temperature the spatial extent of domains of slow/fast moving particles. The most mobile particles tend to cluster in domains that contain a significant number of chains of homopolar Ge-Ge bonds. The number and length of these chains increases with supercooling, boosting the atomic mobility and then the fast crystallization of GeTe-based phase change materials. We also found a certain degree of cooperative motion in this system, which is due to both first-shell correlated and string-like motion. Finally, we investigated the role of the domains of most immobile and mobile particles during crystallization, finding that mobile particles tend not to crystallize, but instead to flow around immobile (crystalline) nuclei facilitating the atomic rearrangement at the liquid-crystal interface.

Yali Liu, Hao Zheng, Dongdong Xiao, Yingchun Lyu, Jiayue Peng, Rui Wang, Yongsheng Hu, Lin Gu, Hong Li*, Liquan Chen Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, P.R. ChinaE-mail: hli@iphy.ac.cn

Rechargeable nonaqueous lithium air battery has attracted wide attention due to its very high theoretical energy density. It is still very challenge for operating the batteries in air, partially due to influences of moisture and carbon dioxide. It has been demonstrated that there would be Li2CO3 in the discharge products when the reactive gas contains CO2. It has been thought that Li2CO3 is very difficult to be decomposed during charging.

Therefore, most reported lithium air batteries are investigated under high pure oxygen with CO2 less than 5 ppm. In 2011, Takechi et al reported a Li/CO2:O2 (from 0 to 100% volume CO2) battery, which didn’t show a reversible charge capacity with a cut-off voltage of 4.5 V even in the first cycle. McCloskey et al reported a Li/O2 battery with CO2 as a contamination gas (10% volume).

The battery employed LiTFSI-DME as electrolyte and a sloped charging voltage profile up to 4.8 V was reported in the first cycle. A reversible Li/CO2:O2 (1:1, volume ratio) battery with DME based and DMSO based electrolyte was reported by Kang et al recently. They pointed that Li2CO3 was the main discharge product in this battery and can form reversibly.

We have also reported that Li2CO3 can be decomposed after mixing with NiO as catalyst. Accordingly, there is no doubt that formed Li2CO3 can be decomposed under suitable conditions. Therefore, it is plausible that a rechargeable Li/CO2 battery could be also developed. According to thermodynamic calculation, the specific energy density of Li/CO2 batteries is almost three fourths of Li/O2 battery. It also can be calculated that the theoretical voltage is about 2.8 V based on the equation: 4Li + 3CO2 → 2Li2CO3 + C. The Li/CO2 battery could be attractive especially when CO2 is enriched in atmosphere.

Previously, Archer et al reported a primary Li/CO2 battery which cannot be recharged and only discharge in the high temperature. In this report, we will show that a Li/CO2:O2 (2:1, volume ratio) battery and a Li/CO2 battery can operate reversibly at room temperature when lithium triflate (LiCF3SO3)-TEGDME is used as the electrolyte, various carbon as air electrodes. The polarization of the reactions, products formed under different conditions with different volume ratio of O2: CO2, and the relationship between the electrochemical performances and the structure, morphology and the composition of the products are analyzed based on electrochemical measurements combining with in situ and ex situ STEM, SEM, AFM, FTIR, Raman, XRD, SIMS techniques and DFT calculations.

With the continuous miniaturization of semiconductor materials and devices, high aspect ratio semiconductor nanostructures, such as nanowires, have become a primary candidate for next generation electronic and optoelectronic devices. As- and P-based III-V compound semiconductor nanowires are of particular interest for optoelectronic devices due to their high carrier mobility and optical emission efficiency compared to indirect-bandgap group IV materials. In this report, various III-V semiconductor nanowires, including GaAs, InP, InAs nanowires and related nanowire heterostructures, were grown epitaxially on GaAs, InP, InAs (111)B or Si (111) substrates by metalorganic chemical vapor deposition (MOCVD) using Au nanoparticles as catalyst. Some challenging issues related to the growth of III-V semiconductor nanowires by MOCVD and their implications on optical properties will be reviewed. In addition, some prototype nanowire devices such as GaAs/AlGaAs nanowire lasers and solar cell devices have been demonstrated.

Firstly, GaAs nanowires with high optical and crystal quality were demonstrated by choosing an appropriate V/III ratio together with growth temperature. By passivating the GaAs nanowires with a AlGaAs shell, 1.9 ns of minority carrier lifetime has been obtained at room temperature. Precise control of crystal structure either in zincblende (ZB) crystal or wurtzite (WZ) crystal phase or mixed phases was also demonstrated in various III-V semiconductor nanowires. This unique phenomenon in nanowires which can not be realized in their bulk counterpart opens new possibilities for engineering nanowire devices.Prototype nanowire solar cell devices were fabricated by planarizing the GaAs/AlGaAs/GaAs nanowire structures. The devices exhibit a spectrally broad photo-response and the conversion efficiency can be over 4%. Modeling of Nanowire lasers has also been carried out by calculating the threshold gain for nanowire guided modes as a function of nanowire diameter and length. Gain spectrum for GaAs nanowires as a function of injected carrier density was modeled using microscopic gain theory. Based on these calculations, we have optimized the structure design for nanowire laser devices. The prototype GaAs/AlGaAs/GaAs nanowire laser structures were grown and optically pumped laser operation was demonstrated at room temperature.

Nanostructured forms of carbon have extraordinary mechanical and electrical properties. The fact that carbon is considered inherently biocompatible means that these more recently discovered forms have attracted the attention of those of us interested in the development of more effective electromaterials for medical bionics.

A number of different carbon nanotube based electromaterial platforms have been shown to provide effective electrical communication with both nerve and muscle cells. These advances will be presented here. More recently, a structure consisting of graphene layers deposited on a biopolymer substrate has proven to be effective in nerve cell communication. A striking feature of this electrode structure is that the very thein layer of (bilayer) graphene used has minimal effective on the mechanical properties of the underlying biopolymer yet provides sufficient electronic conductivity for electrical stimulation of nerve cells.

Triboelectrification is an effect that is known to each and every one probably ever since the ancient Greek time, but it is usually taken as a negative effect and is avoided in many technologies. We have recently invented a triboelectric nanogenerator (TENG) that is used to convert mechanical energy into electricity by a conjunction of triboelectrification and electrostatic induction. As for this power generation unit, in the inner circuit, a potential is created by the triboelectric effect due to the charge transfer between two thin organic/inorganic films that exhibit opposite tribo-polarity; in the outer circuit, electrons are driven to flow between two electrodes attached on the back sides of the films in order to balance the potential. Ever since the first report of the TENG in January 2012, the output power density of TENG has been improved for five orders of magnitude within 12 months. The area power density reaches 313 W/m2, volume density reaches 490 kW/m3, and a conversion efficiency of ~50% has been demonstrated. The TENG can be applied to harvest all kind mechanical energy that is available but wasted in our daily life, such as human motion, walking, vibration, mechanical triggering, rotating tire, wind, flowing water and more. Alternatively, TENG can also be used as a self-powered sensor for actively detecting the static and dynamic processes arising from mechanical agitation using the voltage and current output signals of the TENG, respectively, with potential applications for touch pad and smart skin technologies. The TENG is possible not only for self-powered portable electronics, but also as a new energy technology with a potential of contributing to the world energy in the near future.

[1] Z.L. Wang “Triboelectric Nanogenerators as New Energy Technology for Self-Powered Systems and as Active Mechanical and Chemical Sensors”, ACS Nano, DOI 10.1021/nn404614z.