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arXiv2019

Artificial Intelligence Enabled Material Behavior Prediction

Artificial Intelligence and Machine Learning algorithms have considerable potential to influence the prediction of material properties. Additive materials have a unique property prediction challenge in the form of surface roughness effects on fatigue behavior of structural components. Traditional approaches using finite element methods to calculate stress risers associated with additively built surfaces have been challenging due to the computational resources required, often taking over a day to calculate a single sample prediction. To address this performance challenge, Deep Learning has been employed to enable low cycle fatigue life prediction in additive materials in a matter of seconds.

cs.LG↗

arXiv2019

Unconventional Optical Response in Engineered Au-Ag Nanostructures

This article describes the optical properties of nanostructures composed of silver particles embedded into a gold matrix. In previous studies these materials were shown to exhibit temperature dependent transitions to a highly conductive and strongly diamagnetic state. Here we describe the anomalous optical properties of these nanostructures. Most notably, these materials fail to obey Mie theory and exhibit an unconventional resonance with a maximum at about 4 eV, while the usual gold and silver localized surface plasmon resonances are suppressed. This effect implies a significant deviation from the bulk dielectric functions of gold and silver. We further resolved this resonance into its absorbance and scattering sub-parts. It is observed that the resonance is largely comprised of scattering, with negligible losses even at ultraviolet frequencies.

cond-mat.mtrl-sci↗

arXiv2019

Thermal conductivity of molybdenum disulfide nanotube from molecular dynamics simulations

Single layer molybdenum disulfide (SLMoS2), a semiconductor possesses intrinsic bandgap and high electron mobility, has attracted great attention due to its unique electronic, optical, mechanical and thermal properties. Although thermal conductivity of SLMoS2 has been widely investigated recently, less studies focus on molybdenum disulfide nanotube (MoS2NT). Here, the comprehensive temperature, size and strain effect on thermal conductivity of MoS2NT are investigated. A chirality-dependent strain effect is identified in thermal conductivity of zigzag nanotube, in which the phonon group velocity can be significantly reduced by strain. Besides, results show that thermal conductivity has a ~T-1 and a ~L\b{eta} relation with temperature from 200 to 400 K and length from 10 to 320 nm, respectively. This work not only provides feasible strategies to modulate the thermal conductivity of MoS2NT, but also offers useful insights into the fundamental mechanisms that govern the thermal conductivity, which can be used for the thermal management of low dimensional materials in optical, electronic and thermoelectrical devices. Introduction.

cond-mat.mes-hall↗

arXiv2019

An image-driven machine learning approach to kinetic modeling of a discontinuous precipitation reaction

Micrograph quantification is an essential component of several materials science studies. Machine learning methods, in particular convolutional neural networks, have previously demonstrated performance in image recognition tasks across several disciplines (e.g. materials science, medical imaging, facial recognition). Here, we apply these well-established methods to develop an approach to microstructure quantification for kinetic modeling of a discontinuous precipitation reaction in a case study on the uranium-molybdenum system. Prediction of material processing history based on image data (classification), calculation of area fraction of phases present in the micrographs (segmentation), and kinetic modeling from segmentation results were performed. Results indicate that convolutional neural networks represent microstructure image data well, and segmentation using the k-means clustering algorithm yields results that agree well with manually annotated images. Classification accuracies of original and segmented images are both 94\% for a 5-class classification problem. Kinetic modeling results agree well with previously reported data using manual thresholding. The image quantification and kinetic modeling approach developed and presented here aims to reduce researcher bias introduced into the characterization process, and allows for leveraging information in limited image data sets.

physics.app-ph↗

arXiv2019

High-efficiency thermophotovoltaic system that employs an emitter based on a silicon rod-type photonic crystal

Thermophotovoltaic systems in principle enable utilization of heat that is usually regarded as wasted energy. However, the wavelength selectivity of the thermal emitter required for high efficiencies is rather difficult to control with conventional designs. Here, we design a thermophotovoltaic system comprising silicon rods as thermal emitter with a relatively narrow emission spectrum and a photovoltaic cell with a band gap corresponding to 1.76 $μ$m, and verify efficient power generation. By accurately measuring the heat flux that enters the emitter, the emitter temperature, and the electrical output power of the photovoltaic cell, we find that the actual system efficiency (ratio of ingoing heat flux to output power) is 11.2% at an emitter temperature of 1338 K, and that the output power density footprint is 0.368 W/cm2. The obtained efficiency is relatively high, i.e., 1.65 times that of the previously reported record value (6.8%). Further efficiency improvements in the future may lead to development of distributed energy supplies using combustion heat.

physics.app-ph↗

arXiv2019

Active meta-optics and nanophotonics with halide perovskites

Meta-optics based on optically resonant all-dielectric structures is a rapidly developing research area driven by its potential applications for low-loss efficient metadevices. Active, light-emitting subwavelengh nanostructures and metasurfaces are of a particular interest for meta-optics, as they offer unique opportunities for novel types of compact light sources and nanolasers. Recently, the study of halide perovskites has attracted an enormous attention due to their exceptional optical and electrical properties. As a result, this family of materials can provide a prospective platform for modern nanophotonics and meta-optics, allowing to overcome many obstacles associated with the use of conventional semiconductor materials. Here we review the recent progress in the field of halide-perovskite meta-optics with the central focus on light-emitting nanoantennas and metasurfaces for the emerging field of active metadevices.

physics.app-ph↗

arXiv2019

XeF$_2$-Enhanced Focused Ion Beam Etching and Passivation of GaSb

Focused ion beams (FIBs) are widely used to modify optoelectronic devices, but their utility is severely restricted in some III-V materials due to adverse effects such as nanofibre growth and contamination. This study describes an effective machining method for the modification and passivation of GaSb using a focused Ga$^+$ beam with the addition of XeF$_2$ gas. The added gas suppressed the nanofibre growth and allowed for controlled etching. Moreover, the milled surface exhibited much lower roughness and contained much lower levels of carbon and oxygen contaminants. These effects are attributed to the formation of a gallium fluoride layer, which inhibits the catalytic vapour-liquid-solid (VLS) nanofibre growth. The fluoride layer prevents the surface oxidation, acting as an effective passivation layer. This gas-assisted FIB process eliminates the idiosyncratic challenges of modifying GaSb with an ion beam and it can be implemented cost-effectively and rapidly in conventional FIB systems---allowing for site-specific modification of bespoke GaSb-based devices.

physics.app-ph↗

arXiv2019

Enhancing the Open-Circuit Voltage of Perovskite Solar Cells by up to 120 mV using π-Extended Phosphoniumfluorene Electrolytes as Hole Blocking Layers

Four π-extended phosphoniumfluorene electrolytes (π-PFEs) are introduced as hole-blocking layers (HBL) in inverted architecture planar perovskite solar cells (PVSCs) with the structure of ITO/PEDOT:PSS/MAPbI3/PCBM/HBL/Ag. The deep-lying highest occupied molecular orbital (HOMO) energy level of the π-PFEs effectively blocks holes, decreasing contact recombination. We demonstrate that the incorporation of π-PFEs introduces a dipole moment at the PCBM/Ag interface, resulting in a significant enhancement of the built-in potential of the device. This enhancement results in an increase in the open-circuit voltage of the device by up to 120 mV, when compared to the commonly used bathocuproine HBL. The results are confirmed both experimentally and by numerical simulation. Our work demonstrates that interfacial engineering of the transport layer/contact interface by small molecule electrolytes is a promising route to suppress non-radiative recombination in perovskite devices and compensate for a non-ideal energetic alignment at the hole-transport layer/perovskite interface.

physics.app-ph↗

arXiv2019

From planar junction to local junction: A new structure design of amorphous/crystalline silicon heterojunction solar cells for high efficiency and low cost

In order to obtain higher conversion efficiency and to reduce production cost for amorphous silicon/crystalline silicon (a-Si/c-Si) based heterojunction solar cells, a Heterojunction of Amorphous silicon and Crystalline silicon with Localized pn structure (HACL) has been designed. The potential performance of the HACL solar cell has been assessed by ATLAS simulation program. Its potential performance is compared with that of the Heterojunction with Intrinsic Thin film (HIT) and Heterojunction of Amorphous silicon and Crystalline silicon with Diffused junction (HACD) solar cells. The simulated results indicated that the conversion efficiency and the short-circuit current density of the HACL cell can reach to 28.18% and 43.06 mA/cm2, respectively, and are higher than that of the HIT and HACD cells. The main reasons for the great improvement are (1) to increase the light utilization rate on both sides of the cell and (2) to enhance the collection efficiency of the photocarriers. Moreover, the HACL structure can decrease the consumption of rare materials, such as indium, since the transparent conductive oxide (TCO) can be free in this structure. It is concluded that the HACL solar cell is a promising structure for high efficiency and low cost.

physics.app-ph↗

arXiv2019

Route to Achieving Enhanced Quantum Capacitance in Functionalized Graphene based Supercapacitor Electrodes

We have investigated the quantum capacitance ($C_Q$) in functionalized graphene, modified with ad-atoms from different groups in the periodic table. Changes in the electronic band structure of graphene upon functionalization and subsequently the quantum capacitance ($C_Q$) of the modified graphene were systematically analyzed using density functional theory(DFT) calculations. We observed that the quantum capacitance can be enhanced significantly by means of controlled doping of N, Cl and P ad-atoms in the pristine graphene surface. These ad-atoms are behaving as magnetic impurities in the system, generates a localized density of states near the Fermi energy, which intern increases charge(electron/hole) carrier density in the system. As a result, a very high quantum capacitance was observed. Finally, the temperature dependent study of $C_Q$ for Cl and N functionalized graphene shows that the CQ remains very high in a wide range of temperature near the room temperature.

cond-mat.mtrl-sci↗

arXiv2019

Neutron diffraction strain tomography: 2D axisymmetric sample geometry

We consider a method for neutron residual strain/stress analysis combining features of transmission strain tomography (e.g. through Braggs edge imaging) and neutron diffraction techniques. Analogous to transmission strain tomography, our approach observes integrals of strain along the path of a beam, however, rather than transmitted beams, we observe a diffracted beam. The practical feasibility of neutron diffraction tomography is demonstrated in case of a 2D axisymmetric sample where the reconstruction of the stress field is determined from the measurement of a strain profile. The reconstruction is compared to a set of measurements made using a traditional neutron stress scanning technique in order to validate the new method. Similar strain accuracy was achieved by the new technique within a reasonable measurement time. The practical and theoretical challenges regarding the technique are also discussed.

physics.app-ph↗

arXiv2019

Conduction and valence band offsets of Ga2O3/h-BN heterojunction

h-BN and Ga2O3 are two promising semiconductor materials. However, the band alignment of the Ga2O3/h-BN heterojunction has not been identified, hindering device development. In this study, the heterojunction was prepared by metalorganic chemical vapor deposition and pulsed laser deposition. Transmission electron microscopy confirmed sharp heterointerface and revealed structural evolution as amorphous-Ga2O3 grew thicker on lattice mismatched h-BN. The valence and conduction band offsets were determined by high-resolution X-ray photoemission spectroscopy to be 1.75 and 3.35-3.65 eV, respectively, corresponding to a type-II heterojunction. The extremely large type-II band offsets along with indirect bandgap of Ga2O3 may be leveraged for exceptional electron confinement and storage.

physics.app-ph↗

arXiv2019

Determination of band offsets of Ga2O3/FTO heterojunction for current spreading for high temperature and UV applications

Because of relatively low electron mobility of Ga2O3, it is important to identify proper current spreading materials. Fluorine-doped SnO2 (FTO) offers superior properties to those of indium tin oxide (ITO) including higher thermal stability, larger bandgap, and lower cost. However, the Ga2O3/FTO heterojunction including the important band offset and the I-V characteristics have not been reported. In this work, we have grown the Ga2O3/FTO heterojunction and performed X-ray photoelectron spectroscopy (XPS) measurement. The conduction and valence band offsets were determined to be 0.11 and 0.42 eV, indicating a minor barrier for electron transport and type-I characteristics. The subsequent I-V measurement of the Ga2O3/FTO heterojunction exhibited ohmic behavior. The results of this work manifests excellent candidacy of FTO for current spreading layers of Ga2O3 devices for high temperature and UV applications.

physics.app-ph↗

arXiv2019

Capturing Strain Stiffening Using Volume Controlled Cavity Expansion

Strain-stiffening is a well-documented behavior in soft biological materials such as liver and brain tissue. Measuring and characterizing this nonlinear response, which is commonly considered as a mechanism for damage prevention, is of great interest to engineers for design of better biomimetic materials, and to physicians for diagnostic purposes. However, probing the elastic response of soft or biological materials at large deformation in their natural habitat, is an arduous task. Here, we present the Volume Controlled Cavity Expansion (VCCE) technique as an in-vivo measurement method that offers the ability of characterizing the stiffening response of materials in addition to identifying their shear modulus. By employing minimal constitutive representations involving only two constants (Mooney-Rivlin, Gent, and Ogden) we show that for the conventional PDMS samples, this technique and an accompanying data analysis method capture the shear modulus, as well as providing reliable measures of the stiffening behavior of the samples.

cond-mat.soft↗

arXiv2019

Sound Reception System by an Acoustic Luneburg Lens

Applying the directionality of a Luneburg lens, a sound reception system for ships is designed. A two-dimensional acoustic Luneburg lens which is made of 253 acrylic pipes of different radii which control the refractive index is used. It has a large disk shape; a diameter of 180 cm and a height of 20 cm. The 8 microphones were installed around the edges of the lens. The sound reception ability of the lens was tested on the sea using the horn of a university training ship as an emitter and the lens loaded on a small boat as a receiver. Both are moving at 20 ~ 40 km/h at a distance of between 300 ~ 700 m. The lens could focus to recognize the direction of the input sound in the frequency range between 300 ~ 900 Hz.

physics.app-ph↗

arXiv2019

A Multi-Modal Miniature Surface Forces Apparatus (uSFA) for Interfacial Science Measurements

Advances in the research of intermolecular and surface interactions result from the development of new and improved measurement techniques and combinations of existing techniques. Here, we present a new miniature version of the Surface Force Apparatus (the uSFA) that has been designed for ease of use and multi-modal capabilities with retention of the capabilities of other SFA models including accurate measurement of surface separation distance and physical characterization of dynamic and static physical forces (i.e., normal, shear, and friction) and interactions (e.g., van der Waals, electrostatic, hydrophobic, steric, biospecific). The small physical size of the uSFA, compared to previous SFA models, makes it portable and suitable for integration into commercially available optical and fluorescence light microscopes, as demonstrated here. The large optical path entry and exit ports make it ideal for concurrent force measurements and spectroscopy studies. Examples of the use of the uSFA in combination with surface plasmon resonance (SPR) and Raman spectroscopy measurements are presented. Due to the short working distance constraints associated with Raman spectroscopy, an interferometric technique was developed and applied for calculating the inter-surface separation distance based on Newtons rings. The introduction of the \muSFA will mark a transition in SFA usage from primarily physical characterization to concurrent physical characterization with in situ chemical and biological characterization to study interfacial phenomena, including (but not limited to) molecular adsorption, fluid flow dynamics, determination of surface species and morphology, and (bio) molecular binding kinetics.

physics.ins-det↗

arXiv2019

Simultaneous Magnetic Particle Imaging and Navigation of large superparamagnetic nanoparticles in bifurcation flow experiments

Magnetic Particle Imaging (MPI) has been successfully used to visualize the distribution of superparamagnetic nanoparticles within 3D volumes with high sensitivity in real time. Since the magnetic field topology of MPI scanners is well suited for applying magnetic forces on particles and micron-sized ferromagnetic devices, MPI has been recently used to navigate micron-sized particles and micron-sized swimmers. In this work, we analyze the magnetophoretic mobility and the imaging performance of two different particle types for Magnetic Particle Imaging/Navigation (MPIN). MPIN constantly switches between imaging and magnetic modes, enabling quasi-simultaneous navigation and imaging of particles. We determine the limiting flow velocity to be 8.18 mL/s using a flow bifurcation experiment, that allows all particles to flow only through one branch of the bifurcation. Furthermore, we have succeeded in navigating the particles through the branch of a bifurcation phantom narrowed by either 60% or 100% stenosis, while imaging their accumulation on the stenosis. The particles in combination with therapeutic substances have a high potential for targeted drug delivery and could help to reduce the dose and improve the efficacy of the drug, e.g. for specific tumor therapy and ischemic stroke therapy.

physics.app-ph↗

arXiv2019

Simple physical models for the partially transparent radiative windows, comparison to the radiative coolers

In this work we solve approximately the radiative heat transfer problem in one dimension to perform a comparative analysis of the time averaged performance of the partially transparent radiative windows and radiative coolers. Our physical model includes the atmosphere, the window, and the backwall that are all in the thermal equilibrium with each other, and that can exchange energy via radiative heat transfer or convection. Moreover, we use a simplified two-state model for the optical properties of an atmosphere and a window material which assumes two distinct sets of the optical reflection/absorption/transmission parameters in the visible/near-IR versus mid-IR spectral ranges. Furthermore, we have distinguished the design goals for the partially transparent windows and radiative coolers and provided optimal choice for the material parameters to realize these goals. Thus, radiative coolers are normally non-transparent in the visible, and the main goal is to design a cooler with the temperature of its dark side as low as possible compared to that of the atmosphere. For the radiative windows, however, their surfaces are necessarily partially transparent in the visible. In the cooling mode, therefore, the main question is about the maximal visible light transmission through the window at which the temperature on the window somber side does not exceed that of the atmosphere. We believe that our simple, yet in-depth comparative analysis of the radiative windows and coolers can be useful for a large number of scientists and engineers pursuing research in these disciplines.

physics.app-ph↗