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arXiv2019

Acoustically Driven and Modulation Inducible Radiating Elements

The low propagation loss of electromagnetic radiation below 1 MHz offers significant opportunities for low power, long range communication systems to meet growing demands for IoT applications. Especially in the very low frequency (VLF: 3-30 kHz) range and lower, propagation through tens of meters of seawater, hundreds of meters of earth, and hundreds of kilometers of air with only 2-3 dB of loss is possible. However, the fundamental reduction in efficiency as the size of electrical antennas decreases below a wavelength (30 m at 1 MHz) has made portable communication systems in the VLF and low frequency (LF: 30-300 kHz) ranges impractical for decades. A paradigm shift from electrical to piezoelectric antennas utilizing strain-driven currents at resonant wavelengths up to five orders of magnitude smaller than electrical antennas offers the promise for orders of magnitude efficiency improvement over the electrical state-of-the-art. This work demonstrates a lead zirconate titanate transmitter >6000 times more efficient than a comparably sized electrical antenna and capable of bit rates up to 60 bit/s using frequency-shift keying. Detailed analysis of design parameters offers a roadmap for significant future improvement in both radiation efficiency and data rate in the new field of acoustically driven antennas.

physics.app-ph↗
arXiv2019

Scalable, green fabrication of single-crystal noble metal films and nanostructures for low-loss nanotechnology applications

High quality metal thin films and nanostructures are critical building blocks for next generation nanotechnologies. They comprise low-loss circuit elements in nanodevices, provide new catalytic pathways for water splitting and $CO_2$ reduction technologies, and enable the confinement of spatially extended electromagnetic waves to be harnessed for application in information processing, energy harvesting, engineered metamaterials, and new technologies that will operate in the quantum plasmonics limit. However, the controlled fabrication of high-definition single-crystal subwavelength metal nanostructures remains a significant hurdle, due to the tendency for polycrystalline metal growth using conventional physical vapor deposition methods, and the challenges associated with placing solution-grown nanocrystals in desired orientations and locations on a surface to fabricate functional devices. Here, we introduce a new scalable, green, wet chemical approach to monocrystalline noble metals that enables the fabrication of ultrasmooth, epitaxial, single-crystal films of controllable thickness. They are ideal for the subtractive manufacture of nanostructure through ion beam milling, and additive crystalline nanostructure via lithographic patterning to enable large area, single-crystal metamaterials and high aspect ratio nanowires. Our single-crystal nanostructures demonstrate improved feature quality and pattern transfer yield, reduced optical and resistive losses, tailored local fields, and greatly improved stability compared to polycrystalline structures, supporting greater local field enhancements and enabling new practical advances at the nanoscale.

physics.app-ph↗
arXiv2019

Understanding anti-parity-time symmetric systems with a conventional heat transfer framework -- comment on "Anti-parity-time symmetry in diffusive systems"

Inspired by non-Hermitian physics, Li et al. (Science 364, 170-173) theoretically predicted and experimentally demonstrated a stationary temperature profile in a diffusive heat transfer system - seemingly indicating that heat "stops" diffusing. By analogy to the wave physics framework, the motionless and moving temperature profiles are manifestations of the anti-parity-time APT symmetry and symmetry breaking states, respectively. Their experimental setup consists of two thermally coupled rings rotating in the opposite direction. At a particular rotation speed, known as the exceptional point, the APT symmetry of the system changes, resulting in the temperature profile switching between stationary and moving states. In fact, this seemingly unusual and exotic behavior can be elegantly captured and predicted using a conventional heat transfer framework with similarity and scaling analysis. In this work, we show the system behavior can be characterized into three zones by two widely-used dimensionless parameters on a regime map. The exceptional point, discovered using wave physics, is located precisely on the zone boundary on the regime map, indicating a balance between the contribution of thermal coupling and mechanical motion. Furthermore, the observed cessation of thermal diffusion is merely a result of the long diffusion time constant of the experimental setup. The unfamiliarity of concepts in another scientific field as well as the remarkable equivalence of the two points of view prompts this in-depth discussion of the analogy between wave physics and heat transfer. We believe that this work can help bridge the gap and promote new developments in the two distinctly different disciplines.

physics.app-ph↗
arXiv2019

Micro/nanomaterials for improving solar still and solar evaporation -- A review

In last decades, solar stills, as one of the solar desalination technologies, have been well studied in terms of their productivity, efficiency and economics. Recently, to overcome the bottleneck of traditional solar still, improving solar still by optimizing the solar evaporation process based on micro/nanomaterials have been proposed as a promising strategy. In this review, the recent development for achieving high-performance of solar still and solar evaporation are discussed, including materials as well as system configurations. Meanwhile, machine learning was used to analyze the importance of different factors on solar evaporation, where thermal design was founded to be the most significant parameter that contributes in high-efficiency solar evaporation. Moreover, several important points for the further investigations of solar still and solar evaporation were also discussed, including the temperature of the air-water interface, salt rejecting and durability, the effect of solid-liquid interaction on water phase change.

physics.app-ph↗
arXiv2019

Two-dimensional plasmonic waveguides for nanolasing and four-wave mixing

Plasmonic waveguides are an essential element of nanoscale coherent sources, including nanolasers and four-wave mixing (FWM) devices. Here we report how the design of the plasmonic waveguide needs to be guided by the ultimate application. This contrasts with traditional approaches in which the waveguide is considered in isolation. We find that hybrid plasmonic waveguides, with a nonlinear material sandwiched between the metal substrate and a high-index layer, are best suited for FWM applications, whereas metallic wedges are preferred in nanolasers. We also find that in plasmonic nanolasers high-index buffer layers perform better than more traditional low-index buffers.

physics.optics↗
arXiv2019

Broadband nonreciprocal acoustic propagation using programmable boundary conditions: from analytical modelling to experimental implementation

In this paper, we theoretically, numerically and experimentally demonstrate the acoustic isolator effect in a 1D waveguide with direction dependent controlled boundary conditions. A theoretical model is used to explain the principle of non reciprocal propagation in boundary controlled waveguides. Numerical simulations are carried out on a reduced model to show the non-reciprocity as well as the passivity of the system, through the computation of the scattering matrix and the power delivered by the system. Finally, an experimental implementation validate the potential of programmable boundary conditions for non reciprocal propagation.

physics.app-ph↗
arXiv2019

Automated Droplet Size Distribution Measurements Using Digital Inline Holography

Droplet generation through spray breakup is an unsteady and non-linear process which produces a relatively dense, highly polydisperse aerosol containing non-spherical droplets with sizes spanning several orders of magnitude. Such variability in size and shape can lead to significant sources of error for conventional measurements based on laser scattering. Although direct imaging of droplets can potentially overcome these limitations, imaging suffers from a shallow depth of field as well as occlusions, which prevents the complete spray from being analyzed. In comparison, digital inline holography (DIH), a low-cost coherent imaging technique, can enable high-resolution imaging of the sample over an extended depth of field, typically several orders of magnitude larger than traditional imaging. In this study, we showcase an automated DIH imaging system for characterizing monodisperse and polydisperse aerosol droplet size and shape distributions in the 20 um-3 mm diameter range, over a large sample volume. The high accuracy of the technique is demonstrated by measurements of monodisperse droplets generated by a vibrating orifice droplet generator, achieving a resolution of ~14.2. Measurements of a polydisperse spray from a flat fan nozzle serve to establish the versatility of DIH in extracting a two-dimensional size-eccentricity distribution function, which indicates a strong semilogarithmic scaling between the two parameters that decays as the droplet migrates away from the nozzle. Due to its low cost and compact setup as well as the high density of data obtained, DIH can serve as a promising approach for future aerosol characterization.

physics.app-ph↗
arXiv2019

On the spatial resolution of EBSD in magnesium

We measured the physical lateral resolution of the electron backscatter diffraction (EBSD) technique for the case of pure magnesium and tungsten. Spatial resolution, among other parameters, depends significantly on the accelerating voltage and the atomic number of the material. For the case of lighter metals, it is supposed to be lower than in the case of heavier metals for a given accelerating voltage. In the present work, lateral resolution was measured in dependence of accelerating voltage on a straight high angle grain boundary which was positioned parallel (horizontal boundary) and perpendicular (vertical boundary) to the tilt axis of the specimen. For magnesium the best lateral resolution of 240 nm was obtained at an accelerating voltage of 5 kV. The resolution dramatically worsened to values as high as 3500 nm as the voltage was increased from 15 kV to 30 kV. The aspect ratio of horizontal and vertical lateral resolution tended to 1.0 at the accelerating voltage of 5 kV and to 2.5 at the accelerating voltage of 30 kV. These values as function of accelerating voltages were compared with those obtained on the high atomic number metal tungsten. Here resolution at 5 kV was about a quarter of that of magnesium. With increasing voltage, the value almost didnt change. For all voltages the resolution aspect ratio stayed close to 1.0.

cond-mat.mtrl-sci↗
arXiv2019

MoS2 Field Effect Transistors based sensors for low concentration acetone detection

The measurement of acetone in human breath, a known biomarker for diabetes, can act as an effective screening method for diabetes. While gas chromatography and mass spectroscopy based methods can detect gases in low concentration, they are costly, bulky, need complex sample preparation and are slow. Solid state devices such as Field Effect Transistors (FETs) can perform this faster and at a lower cost. In this work, we present the results of acetone gas sensing by molybdenum disulfide (MoS2) based FETs. MoS2 channel was functionalized with polymethyl methacrylate (PMMA). When acetone molecules bind to PMMA, an electrostatic gating effect takes place and changes the electrical characteristics of the FET. More the number of acetone molecules that bind to the PMMA layer, the higher the shift in the drain current-voltage (Id-Vg) curve of the FET. This shift is modeled as a change in the threshold voltage of the FET. The Id-Vg curve was observed to shift downwards, as the acetone concentration increases. The limit of detection was observed to be 0.4 parts per million (ppm). This is the first sub-ppm concentration acetone detection at room temperature using a solid state device.

physics.app-ph↗
arXiv2019

Experimental and Numerical Investigation of Corona Discharge Induced Flow on a Flat Plate

Electrohydrodynamic (EHD) flow induced by planar corona discharge in the wall boundary layer region is investigated experimentally and via a multiphysics computational model. The EHD phenomena has many potential engineering applications, its optimization requires a mechanistic understanding of the ion and flow transport. The corona EHD actuator consisting of two electrodes located in the wall boundary layer creates an EHD driven wall jet. The applied voltage between the electrodes is varied and the resulting effects in the charge density and flow field are measured. Constant current hotwire anemometry is used to measure velocity profile. The airflow near the wall acts a jet and it reaches a maximum of 1.7 m/s with an energy conversion efficiency of ~2%. The velocity decreases sharply in the normal direction. Multiphysics numerical model couples ion transport equation and the Navier Stokes equations to solve for the spatiotemporal distribution of electric field, charge density and flow field. The numerical results match experimental data shedding new insights into mass, charge and momentum transport phenomena. The EHD driven flow can be applied to flow control strategies and design of novel particle collectors.

physics.flu-dyn↗
arXiv2019

Library based identification and characterisation of polymers with nano-FTIR and IR-sSNOM imaging

AFM is a technique widely applied in the nanoscale characterisation of polymers and their surface properties. With nano-FTIR and IR-sSNOM imaging an optical dimension is added to this technique that allows for straightforward high resolution characterisation and spectroscopy of polymers. As the volume sampled by these near-field techniques depends mostly on the radius of the cantilever tip, typically 10 nm, it is orders of magnitude smaller than in conventional techniques. Nevertheless, comparability of nano-FTIR near-field spectra and data from macroscopic methods has been shown. Some relevant polymers such as polystyrene however, prove to be more difficult to detect than others. Furthermore, the small sampled volume suggests lower signal quality of nano-FTIR data and proof of its suitability for a reliable library search identification is lacking. To evaluate the techniques especially towards automatic and higher throughput identification of nanoscale polymers, for example in blends or environmental samples, we examined domain distributions in a PS-LDPE film and spectral responses of foils of the most relevant commercial polymers. We demonstrate the successful library search identification of all samples with nano-FTIR data measured in less than seven minutes/spectrum. We discuss aspects affecting the accuracy of the identification and show that already the small spectral range of 1700-1300 cm$^{-1}$ leads to similar success in differentiating between polymer types with near-field data as with conventional far-field FTIR spectroscopy.

physics.app-ph↗
arXiv2019

Validation of a temperature-dependent elasto-viscoplastic material model for a talcum-filled polypropylene/polyethylene co-polymer using glove box flap component tests

In the automotive industry, thermoplastic polymers are used for a significant number of interior and exterior parts. These components have to pass all underlying crash and safety relevant tests, where a proper performance is desired in the range of low to high ambient temperatures. Today, the vehicle design is heavily aided by numerical simulation methods for advancing towards a prototype free vehicle development. This requires an accurate modeling of the temperature- and ratedependent, elasto-viscoplastic mechanical response of the polymer structures. In this work, the validation of a novel elasto-viscoplastic temperature-dependent material model is performed using glove box flap segments subjected to impact loading by a spherical punch in a custom-build loading frame. The proposed material model shows a very good prediction of the experimental results.

physics.app-ph↗
arXiv2019

Graphene/Polyelectrolyte Layer-by-Layer Coatings for Electromagnetic Interference Shielding

Electromagnetic interference (EMI) shielding coating materials with thicknesses in the microscale are required in many sectors, including communications, medical, aerospace and electronics, to isolate the electromagnetic radiation emitted from electronic equipment. We report a spray, layer-by-layer (LbL) coating approach to fabricate micron thick, highly-ordered and electrically-conductive coatings with exceptional EMI shielding effectiveness (EMI SE >4830 dB/mm), through the alternating self-assembly of negatively-charged reduced graphene oxide (RGO) and a positively-charged polyelectrolyte (PEI). The microstructure and resulting electrical properties of the (PEI/RGO)n LbL structures are studied as function of increasing mass of graphene deposited per cycle (keeping the PEI content constant), number of deposited layers (n), flake diameter and type of RGO. A strong effect of the lateral flake dimensions on the electrical properties is observed, which also influences the EMI SE. A maximum EMI SE of 29 dB is obtained for a 6 um thick (PEI/RGO)10 coating with 19 vol.% loading of reduced electrochemically-exfoliated graphene oxide flakes with diameters ~3um. This SE performance exceeds those previously reported for thicker graphene papers and bulk graphene/polymer composite films with higher RGO or graphene nanoplatelets contents, which represents an important step towards the fabrication of thin and light-weight high-performance EMI shielding structures.

physics.app-ph↗
arXiv2019

An Explicit Model for Ultra-thin Gate-All-Around Junctionless Nanowire FETs, Including 2D Quantum Confinement

In this paper, we develop an explicit model to predict the DC electrical behavior in ultra-thin surrounding gate junctionless nanowire FET. The proposed model takes into account 2D electrical and geometrical confinements of carrier charge density within few discrete sub-bands. Combining a parabolic approximation of the Poisson equation, first order perturbation theory for the Schrodinger subband energy eigenvalues, and Fermi-Dirac statistics for the confined carrier density leads to an explicit solution of the DC characteristic in ultra-thin junctionless devices. Validity of the model has been verified with technology computer-aided design simulations. The results confirms its validity for all regions of operation, i.e., from deep depletion to accumulation and from linear to saturation. This represents an essential step toward analysis of circuits based on junctionless nanowire devices.

physics.app-ph↗
arXiv2019

Effects of Filler Configuration and Moisture on Dissipation Factor and Critical Electric Field of Epoxy Composites for HV-ICs Encapsulation

Molding compounds (MCs) have been used extensively as an encapsulation material for integrated circuits, however, MCs are susceptible to moisture and charge spreading over time. The increase in dissipation factor due to the increase of parasitic electrical conductivity (σ) and the decrease in dielectric strength (E_MC^Crit) restrict their applications. Thus, a fundamental understanding of moisture transport will suggest strategies to suppress moisture diffusion and broaden their applications. In this paper, we 1) propose a generalized effective medium and solubility (GEMS) Langmuir model to quantify water uptake as a function of filler configuration and relative humidity; 2) investigate the dominant impact of reacted-water on σ through numerical simulations, mass-uptake, and DC conductivity measurements; 3) investigate electric field distribution to explain how moisture ingress reduces E_MC^Crit; and finally 4) optimize the filler configuration to lower the dissipation factor, and enhance E_MC^Crit. The GEMS-Langmuir model can be used for any application (e.g., photovoltaics, biosensors) where moisture diffusion leads to reliability challenges.

physics.app-ph↗
arXiv2019

Temperature dependence of normalized sensitivity of Love wave sensor with unidirectional carbon fiber epoxy composite/Mn-doped 0.24PIN-0.46PMN-0.30PT ternary single crystal configuration

We have derived a general formula for sensitivity optimization of gravimetric sensors and use it to design a high precision and high sensitivity gravimetric sensor using unidirectional carbon fiber epoxy composite (CFEC) guiding layer on single crystal Mn-doped yPb(In1/2Nb1/2)O3-(1-x-y)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (Mn: PIN-PMN-PT) piezoelectric substrate. The normalized maximum sensitivity exhibits a decreasing tendency with temperature up to 55 degrees Celsius. For the CFEC-on-Mn: PIN-PMN-PT sensor configuration with wavelength 24 {mu}m at 25 degrees Celsius, the maximum sensitivity can reach as high as 760.88 cm2/g, which is nearly twice that of traditional SiO2/ST quartz configuration gravimetric sensor.

physics.app-ph↗
arXiv2019

Spectral responsivity and photoconductive gain in thin film black phosphorus photodetectors

We have fabricated black phosphorus photodetectors and characterized their full spectral responsivity. These devices, which are effectively in the bulk thin film limit, show broadband responsivity ranging from <400 nm to the ~3.8 $μ$m bandgap. In the visible, an intrinsic responsivity >7 A/W can be obtained due to internal gain mechanisms. By examining the full spectral response, we identify a sharp contrast between the visible and infrared behavior. In particular, the visible responsivity shows a large photoconductive gain and gate-voltge dependence, while the infrared responsivity is nearly independent of gate voltage and incident light intensity under most conditions. This is attributed to a contribution from the surface oxide. In addition, we find that the polarization anisotropy in responsivity along armchair and zigzag directions can be as large as 103 and extends from the band edge to 500 nm. The devices were fabricated in an inert atmosphere and encapsulated by Al$_2$O$_3$ providing stable operation for more than 6 months.

physics.app-ph↗