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arXiv2019

Laboratory-based X-ray Absorption Spectroscopy on a Working Pouch Cell Battery at Industrially-Relevant Charging Rates

Li-ion battery (LIB) research has continuing importance for the entire range of applications from consumer products to vehicle electrification and grid stabilization. In many cases, standard electrochemical methods only provide an overall voltage or specific capacity, giving an inadequate description of parallel redox processes or chemical gradients at the particle and pack level. X-ray absorption fine structure (XAFS) is frequently used to augment bulk electrochemical data, as it provides element-specific changes in oxidation state and local atomic structure. Such microscopic descriptors are crucial for elucidating charge transfer and structural changes associated with bonding or site mixing, two key factors in evaluating state of charge and modes of cell failure. However, the impact of XAFS on LIB research has been significantly constrained by a logistical barrier: contemporary XAFS work is performed almost exclusively at synchrotron x-ray light sources, where beamtime is infrequent and experiment time-frames are limited. Here we show that modern laboratory based XAFS can not only be applied to, e.g., characterization of ex situ LIB electrode materials, but can also be used for operando studies at industrially-relevant charging rates in a standard pouch cell preparation. Such capability enables accelerated discovery of new materials and improved operation modes for LIBs.

physics.app-ph↗

arXiv2019

Large Polaron Generation and Dynamics in 3D Metal-Halide Perovskites

In recent years, metal halide perovskites have generated tremendous interest for optoelectronic applications and their underlying fundamental properties. Due to the large electron-phonon coupling characteristic of soft lattices, self-trapping phenomena are expected to dominate hybrid perovskite photoexcitation dynamics. Yet, while the photogeneration of small polarons was proven in low dimensional perovskites, the nature of polaron excitations in technologically relevant 3D perovskites, and their influence on charge carrier transport, remain elusive. In this study, we used a combination of first principle calculations and advanced spectroscopy techniques spanning the entire optical frequency range to pin down polaron features in 3D metal halide perovskites. Mid-infrared photoinduced absorption shows the photogeneration of states associated to low energy intragap electronic transitions with lifetime up to the ms time scale, and vibrational mode renormalization in both frequency and amplitude. Density functional theory supports the assignment of the spectroscopic features to large polarons leading to new intra gap transitions, hardening of phonon mode frequency, and renormalization of the oscillator strength. Theory provides quantitative estimates of the charge carrier masses and mobilities increase upon polaron formation, confirming experimental results. Overall, this work contributes to complete the scenario of elementary photoexcitations in metal halide perovskites and highlights the importance of polaronic transport in perovskite-based optoelectronic devices.

physics.app-ph↗

arXiv2019

Morphology and kinetics of asphalt binder microstructure at gas, liquid, and solid interfaces

We combined optical and atomic force microscopy to observe morphology and kinetics of microstructures that formed at free surfaces of unmodified pavement-grade 64-22 asphalt binders upon cooling from 150 $^{\circ}$C to room temperature (RT) at 5 $^{\circ}$C/min, and changes in these microstructures when the surface was terminated with a transparent solid (glass) or liquid (glycerol) over-layer. The main findings are: (1) At free binder surfaces, wrinkled microstructures started to form near the wax crystallization temperature ($\sim$45 $^{\circ}$C), then grew to $\sim$5 $μ$m diameter, $\sim$25 nm wrinkle amplitude and 10-30$\%$ surface area coverage upon cooling to RT, where they persisted indefinitely without observable change in shape or density. (2) Glycerol coverage of the binder surface during cooling reduced wrinkled area and wrinkle amplitude three-fold compared to free binder surfaces upon initial cooling to RT; continued glycerol coverage at RT eliminated most surface microstructures within $\sim$4 hours. (3) No surface microstructures were observed to form at binder surfaces covered with glass. (4) Sub-micron bulk microstructures were observed by near-infrared microscopy beneath the surfaces of all binder samples, with size, shape and density independent of surface coverage. No tendency of such structures to float to the top or sink to the bottom of mm-thick samples was observed. (5) We attribute the dependence of surface wrinkling on surface coverage to variation in interface tension, based on a thin-film continuum mechanics model.

cond-mat.soft↗

arXiv2019

Effect of UV radiation on the structure of graphene oxide in water and its impact on cytotoxicity and As(III) adsorption

Graphene oxide (GO) is widely used in different applications, however once release into the environment it can change its structure and affect the transport of important contaminants such as arsenic. In this work we show that UV radiation, even in the range of 28-74 uW/cm2 of irradiance up to 120 h of exposure, can induce important changes in the structure of graphene oxide, by eliminating -OH and C=O functional groups. This reduction affected the stability of graphene oxide in water by decreasing its zeta potential from 41 to 37 mV with the increase of the exposure time. Our results showed that after 24 h of UV exposure, As(III) adsorption capacity decreased from 5 mg/g to 4.7 mg/g, however after 48 h of irradiation the adsorption increased with time, reaching 5.1 mg/g at 120 h under 74 uW/cm2 of irradiation. Computer modelling showed that even a degraded GO structure can have an interaction energy of 53 kcal/mol with H3AsO3. Furthermore, we observed that despite clear changes in surface composition and particle size, the reduction of graphene oxide maintained a high degree of cytotoxicity since cell viability decreased to 60% with a 50 ug/ml dose; except for the sample irradiated at 74 uW/cm2 for five days, which showed 20% with the same concentration.

cond-mat.mtrl-sci↗

arXiv2019

Effect of the degree of oxidation of graphene oxide on As(III) adsorption

The study of the interaction between graphene oxide and arsenic is of great relevance towards the development of adsorbent materials and as a way to understand how these two materials interact in the environment. In this work we show that As(III) adsorption, primarily H3AsO3, by graphene oxide is dependent on its degree of oxidation. Variations in the concentration of potassium permanganate resulted in an increase on the C/O ratio from 1.98 to 1.35 with C-OH and C-O-C concentrations of 18 and 32%, respectively. Three oxidation degrees were studied, the less oxidized material reached a maximum As(III) adsorption capacity of 124 mg/g, whereas the graphene with the highest degree of oxidation reached a value of 288 mg/g at pH 7, to the authors knowledge, the highest reported in the literature. The interaction between graphene oxide and As(III) was also studied by Density Functional Theory (DFT) computer models showing that graphene oxide interacts with As(III) primarily through hydrogen bonds, having interaction energies with the hydroxyl and epoxide groups of 378 and 361 kcal/mol, respectively. Finally, cytotoxicity tests showed that the graphene oxide had a cellular viability of 57% with 50 μg/ml, regardless of its degree of oxidation.

cond-mat.mtrl-sci↗

arXiv2019

Microstructure and corrosion evolution of additively manufactured aluminium alloy AA7075 as a function of ageing

Additively manufactured high strength aluminium alloy AA7075 was prepared using selective laser melting. High strength aluminium alloys prepared by selective laser melting have not been widely studied to date. The evolution of microstructure and hardness, with the attendant corrosion, were investigated. Additively manufactured AA7075 was investigated both in the as-produced condition and as a function of artificial ageing. The microstructure of specimens prepared was studied using electron microscopy. Production of AA7075 by selective laser melting generated a unique microstructure, which was altered by solutionising and further altered by artificial ageing - resulting in microstructures distinctive to that of wrought AA7075-T6. The electrochemical response of additively manufactured AA7075 was dependent on processing history, and unique to wrought AA7075-T6, whereby dissolution rates were generally lower for additively manufactured AA7075. Furthermore, immersion exposure testing followed by microscopy, indicated different corrosion morphology for additively manufactured AA7075, whereby resultant pit size was notably smaller, in contrast to wrought AA7075-T6.

physics.app-ph↗

arXiv2019

Development and characterization of detectors for large area application in neutron scattering and small area application in neutron reflectometry

Neutron scattering techniques offer a unique combination of structural and the dynamic information of atomic and molecular systems over a wide range of distances and times. The increasing complexity in science investigations driven by technological advances is reflected in the studies of neutron scattering science, which enforces a diversification and an improvement of experimental tools, from the instrument design to the detector performance. It calls as well for more advanced data analysis and modelling. The improvements in resolution, count rate and signal-to-background ratio, achievable with the new instrumentations, also drive the research of alternative technologies to replace the 3He-based detector technology unable to fulfil the requirement of increasing performance. Two solution have been studied: a boron-10-based gaseous detector, the Multi-Blade and a solid-state Si-Gd detector. Both solution are suitable alternatives for neutron detection, able to meet the demands of high performance. It has been shown not only the technical characteristic of the devices, but how the science can profit from the better performance of these new detector technologies in real experimental condition.

physics.ins-det↗

arXiv2019

Battery Diagnostics with Sensitive Magnetometry

The ever-increasing demand for high-capacity rechargeable batteries highlights the need for sensitive and accurate diagnostic technology for determining the state of a cell, for identifying and localizing defects, or for sensing capacity loss mechanisms. Here, we demonstrate the use of atomic magnetometry to map the weak induced magnetic fields around a Li-ion battery cell as a function of state of charge and upon introducing mechanical defects. These measurements provide maps of the magnetic susceptibility of the cell, which follow trends characteristic for the battery materials under study upon discharge. In addition, the measurements reveal hitherto unknown long time-scale transient internal current effects, which were particularly pronounced in the overdischarged regime. The diagnostic power of this technique is promising for the assessment of cells in research, quality control, or during operation, and could help uncover details of charge storage and failure processes in cells.

physics.app-ph↗

arXiv2019

Field emission: Why converting LAFE voltages to macroscopic fields before making a Fowler-Nordheim plot has often led to spurious characterization results

An important parameter used to characterize large-area field electron emitters (LAFEs) is the characteristic apex field enhancement factor γ_C. This parameter is normally extracted from the slope of a Fowler-Nordheim (FN) plot. Several years ago, the development of an "orthodoxy test" allowed a sample of 19 published FN plots relating to LAFEs to be tested, and it was found that about 40% of the related papers were reporting spuriously high values for γ_C. In technological papers relating to LAFE characterization, common practice is to pre-convert the measured voltage into an (apparent) value of macroscopic field before making and analyzing a FN plot. This paper suggests that the cause of the "spurious-FEF-value" problem is the widespread use of a pre-conversion equation that is defective (for example, not compatible with ordinary electrical circuit theory) when it is applied to so-called "non-ideal" field emission devices/systems. Many real devices/ systems are non-ideal. The author argues that FN plots should be made using raw experimental current-voltage data, and that an orthodoxy test should be applied to the resulting FN plot before any more-detailed analysis, and that (in view of growing concerns over the reliability of published "scientific" results) reviewers should scrutinize field emission materials-characterization papers with enhanced care.

physics.app-ph↗

arXiv2019

Optical cooling of lead halide perovskite nanoparticles enhanced by Mie resonances

Halide perovskites is a family of semiconductor materials demonstrating prospective properties for optical cooling owing to efficient luminescence at room temperature and strong electron-phonon interaction. On the other hand, perovskite based nanophotonic designs would allow for efficient optical cooling at nanoscale. Here we propose a novel strategy for enhancement of optical cooling at nanoscale based on optical resonances engineering in halide perovskite nanoparticles. Namely, photoluminescence up-conversion efficiency in a nanoparticle is optimized via excitation of Mie-resonances both at emission and absorption wavelengths. The optimized theoretically photo-induced temperature decrease is achieved for a hybrid halide perovskite (CH$_3$NH$_3$PbI$_3$) 530 nm nanoparticle on a glass substrate by more than 100 K under CW illumination at wavelength 980 nm and moderate intensities (7*10$^6$ W/cm2). The optimized regime originates from simultaneous excitation of magnetic quadrupole and magnetic octopole at pump and emission wavelengths, respectively. The combination of thermally sensitive photoluminescence signal and simplicity in fabrication of halide perovskite nanocavity will pave the way for implementation of nanoscale optical coolers for advanced applications.

physics.app-ph↗

arXiv2019

Tuning Water Slip Behavior in Nanochannels using Self-Assembled Monolayers

Water slip at solid surfaces is important for a wide range of micro/nano-fluidic applications. While it is known that water slip behavior depends on surface functionalization, how it impacts the molecular level dynamics and mass transport at the interface is still not thoroughly understood. In this paper, we use nonequilibrium molecular dynamics simulations to investigate the slip behavior of water confined between gold surfaces functionalized by self-assembled monolayer (SAM) molecules with different polar functional groups. We observe a positive-to-negative slip transition from hydrophobic to hydrophilic SAM functionalizations, which is found related to the stronger interfacial interaction between water molecules and more hydrophilic SAM molecules. The stronger interaction increases the surface friction and local viscosity, making water slip more difficult. More hydrophilic functionalization also slows down the interfacial water relaxation and leads to more pronounced water trapping inside the SAM layer, which both impede water slip. The results from this work will provide useful insights to the understanding of the water slip at functionalized surfaces and design guidelines for various applications.

physics.app-ph↗

arXiv2019

Investigating Short-term and Long-term Binder Performance of High-RAP Mixtures Containing Waste Cooking Oil

The environmental and economic benefits of recycling asphalt pavements have received much attention in recent years. Because of the increase in the cost of raw materials and energy carriers, the reuse of large portions of reclaimed asphalt pavement (RAP) is critical in reducing both the cost and environmental footprint of asphalt pavements. High-RAP mixtures are more prone to low temperature cracking and poor mixture workability because of the higher stiffness of RAP binder. Recycling agents are one of the additives which are used to improve these deficiencies. However, there is some ambiguity about the optimum content of recycling agent to assure proper performance of recycled asphalt pavement during its service life. The current study used 60% and 100% fractionated RAP with waste cooking oil as a recycling agent and crumb rubber to alleviate the aforementioned problems. Laboratory evaluation showed that increasing the amount of recycling agent in the high-RAP mixtures improved their workability and low temperature performance while decreasing moisture damage and rutting resistance. The long-term susceptibility to aging of recycled binder with the organically-based recycling agent was also investigated. A procedure to obtain the optimum percentage of recycling agent was devised to strike a balance between the performance characteristics of mixtures with a high-RAP content.

physics.app-ph↗

arXiv2019

ITO-based Electro-absorption Modulator for Photonic Neural Activation Function

Recently integrated optics has become an intriguing platform for implementing machine learning algorithms and inparticular neural networks. Integrated photonic circuits can straightforwardly perform vector-matrix multiplicationswith high efficiency and low power consumption by using weighting mechanism through linear optics. Although,this can not be said for the activation function which requires either nonlinear optics or an electro-optic module withan appropriate dynamic range. Even though all-optical nonlinear optics is potentially faster, its current integrationis challenging and is rather inefficient. Here we demonstrate an electro-absorption modulator based on an IndiumTin Oxide layer, whose dynamic range is used as nonlinear activation function of a photonic neuron. The nonlinearactivation mechanism is based on a photodiode, which integrates the weighed products, and whose photovoltage drivesthe elecro-absorption modulator. The synapse and neuron circuit is then constructed to execute a 200-node MNISTclassification neural network used for benchmarking the nonlinear activation function and compared with an equivalentelectronic module.

physics.app-ph↗

arXiv2019

Graphene epsilon-near-zero plasmonic crystals

Plasmonic crystals are a class of optical metamaterials that consist of engineered structures at the sub-wavelength scale. They exhibit optical properties that are not found under normal circumstances in nature, such as negative-refractive-index and epsilon-near-zero (ENZ) behavior. Graphene-based plasmonic crystals present linear, elliptical, or hyperbolic dispersion relations that exhibit ENZ behavior, normal or negative-index diffraction. The optical properties can be dynamically tuned by controlling the operating frequency and the doping level of graphene. We propose a construction approach to expand the frequency range of the ENZ behavior. We demonstrate how the combination of a host material with an optical Lorentzian response in combination with a graphene conductivity that follows a Drude model leads to an ENZ condition spanning a large frequency range.

physics.app-ph↗

arXiv2019

Topological bands and localized vibration modes in quasiperiodic beams

We investigate a family of quasiperiodic continuous elastic beams, the topological properties of their vibrational spectra, and their relation to the existence of localized modes. We specifically consider beams featuring arrays of ground springs at locations determined by projecting from a circle onto an underlying periodic system. A family of periodic and quasiperiodic structures is obtained by smoothly varying a parameter defining such projection. Numerical simulations show the existence of vibration modes that first localize at a boundary, and then migrate into the bulk as the projection parameter is varied. Explicit expressions predicting the change in the density of states of the bulk define topological invariants that quantify the number of modes spanning a gap of a finite structure. We further demonstrate how modulating the phase of the ground springs distribution causes the topological states to undergo an edge-to-edge transition. The considered configurations and topological studies provide a framework for inducing localized modes in continuous elastic structural components through globally spanning, deterministic perturbations of periodic patterns defined by the considered projection operations.

cond-mat.mes-hall↗

arXiv2019

Molecular monolayer stabilizer for multilayer 2D materials

2D van der Waals materials have rich and unique functional properties, but many are susceptible to corrosion under ambient conditions. Here we show that linear alkylamines are highly effective in protecting the optoelectronic properties of these materials such as black phosphorous (BP) and transitional metal dichalcogenides. As a representative example, n-hexylamine can be applied in the form of thin molecular monolayers on BP flakes with less-than-2nm thickness and can prolong BP's lifetime from a few hours to several weeks and even months in ambient environments. Characterizations combined with our theoretical analysis show that the thin monolayers selectively sift out water molecules, forming a drying layer to achieve the passivation of the protected 2D materials. The monolayer coating is also stable in air, hydrogen annealing, and organic solvents, but can be removed by certain organic acids.

physics.app-ph↗

arXiv2019

Superlubricity-Based Electrostatic Microgenerators

Although electrostatic microgenerators (ESMGs) have promised for nearly two decades extensive applications in wireless, self-powered microscale devices and sensors for security, personal health systems, communication, infrastructure and environmental monitoring, commercialized ESMGs are still scarce, mainly due to very low current densities and short product life for most applications. Here we demonstrate that a combination of structural superlubricity, a state of nearly zero friction and wear between two contacted solid surfaces, and nanotechnology can endow ESMGs with superlong life and high performances such as transferred charge, current, and power (at least three orders of magnitude higher than those of conventional ESMGs). Among a few significant advantages of superlubricity-based ESMGs, two are particularly noticeable. First, they can be driven by much lower external loads than conventional ESMGs; second, they can be massively fabricated by using micro-machining processes. The quantitative relationships, experimental proof of concept, and optimization results reported in this Letter can guide future design and facilitate the commercialization of ESMGs.

physics.app-ph↗

arXiv2019

Efficient Nano Antenna for Photonic Devices

This paper presents the efficient Yagi-Uda nanoantenna with the chain of directors, reflector and a feed element, where these elements have been optimized to fulfil the requirement of high directivity and the gain of antenna. The proposed design consists of six core and cladding with silver core and silicon cladding is used to achieve high directivity. The design is analyzed by FIT based CST Software package by which Directivity and Gain of the antenna are computed. The enhancement of this directivity is mainly contributed by the increase in the number of directors from the four to five and the optimized length of the directors which is scaled by 0.9. The simulated results show that the proposed Yagi antenna provides good performance in terms of directivity. The suggested antenna design shows the improved directivity of 17.62 at 500 nm wavelength. The proposed design has wide range of applications including as solar cells.

physics.app-ph↗