bluuebunny/crossref_metadata_2025 huggingface dataset:
Synaptic Behavior and Conduction Mechanisms in Core-Shell Nanowires for Neuromorphic Hardware Applications
Authors: Eswara Vasisth Shangradhanva , Nino Juan Claudio
Published: July 2021

To tackle the ever-increasing demand for faster computation time and energy efficiency, analog neuromorphic architectures have been proposed in place of current artificial neural networks (ANNs) to emulate the computational abilities of the brain. Non-volatile, two terminal devices like memristors have been proven as viable candidates as an artificial synapse in these architectures. Presently, these devices are fabricated as crossbar, thin-film or nanowires. Nanowire-based architectures can play an important role in enhancing sparsity and randomness of a network, particularly when built using core-shell structures. We have previously shown that, depending on the forming process, a Pt core and HfO2 shell nanowire with a Ti top electrode can display eightwise (8W) or counter eightwise (C8W) bipolar resistive switching (BRS) at low potential ranges (±1 V). We will show that the symmetry of the core-shell nanowires combined with the 8W and C8W switching observed, are indicative of anti-serial memristor arrangement. Moreover, this configuration allows for the individual nanowires to exhibit complementary resistive switching (CRS) at higher potentials (±2 V). We will also show the current conduction mechanisms of this system, where thermionic emission dominates in the low resistance state while ohmic conduction dominates in the high resistance state, when the device is operated in BRS mode; and hopping conduction dominates in state 0 while space-charge-limited conduction dominates in state 1 when, the device is operated in CRS mode. When in BRS mode, the LRS and HRS states are stable for at least 1000 cycles and retain memory for 104 s. Finally, when testing for synaptic plasticity via long-term potentiation and depression (LTP and LTD), non-linear weight update behavior was observed. The effect of voltage pulse schemes on the synaptic response will be discussed in detail.
(Invited) Lactate Fueled Wearable Power Suppliers: From Enzymatic Biofuel Cell to Biosupercapacitor and Photobiobattery
Authors: Xiao Xinxin , Magner Edmond , Ulstrup Jens
Published: July 2021

Nowadays, growing research in wearable biosensors for personal healthcare spurs the interest in flexible and miniaturized power suppliers. Enzymatic biofuel cells (EBFCs) [1] utilize oxidoreductases as catalysts, allowing to generate electricity from the chemical energy in sugars. Lactate exists in tear or sweat with a relatively high concentration, which can be the fuel for EBFC. Those applications require flexible, biocompatible and high-surface-area electrodes. Nanoporous gold (NPG) can be an excellent candidate meeting all the requirements [2]. We fabricate a mechanically stable and flexible NPG by electrochemically dealloying Au/Ag alloy films that are sputtered on plastic membranes [3]. The NPG has been biofunctionalized for EBFC applications. Flexible NPG based lactate/O2 EBFCs on commercial contact lenses register 1.7 ± 0.1 μW cm-2 in air-equilibrated artificial tear solutions, which is slightly lower than those obtained in PBS (2.4 ± 0.2 μW cm-2) [3]. A quasi-solid-state and flexible EBFC can be skin-worn consuming the stocking fuel in a hydrogel electrolyte, avoiding the constrain of the required sweat sugars [4]. The device can also function as a self-powered biosupercapacitor delivering pulses for over 600 cycles, with a power density over 10 times higher than that from the EBFC alone.

The combination of bioelectrodes and photoelectrode leads to a new-type photobiobattery. We will also describe a lactate/O2 photobiobattery using a dioxygen-reducing biocathode and a lactate-oxidising photocatalysts on the anode.

References

[1] X. Xiao, H.-q. Xia, R. Wu, L. Bai, L. Yan, E. Magner, S. Cosnier, E. Lojou, Z. Zhu, A. Liu, Tackling the Challenges of Enzymatic (Bio)Fuel Cells, Chem. Rev., 119 (2019) 9509-9558.

[2] X. Xiao, J. Ulstrup, H. Li, J. Zhang, P. Si, Nanoporous gold assembly of glucose oxidase for electrochemical biosensing, Electrochimica Acta, 130 (2014) 559-567.

[3] X. Xiao, T. Siepenkoetter, P.Ó. Conghaile, D. Leech, E. Magner, Nanoporous Gold-Based Biofuel Cells on Contact Lenses, ACS Appl. Mater. Interfaces, 10 (2018) 7107-7116.

[4] X. Xiao, E. Magner, A quasi-solid-state and self-powered biosupercapacitor based on flexible nanoporous gold electrodes, Chem. Commun., 54 (2018) 5823-5826.
(Invited) Analysis of the Heterogeneous Charge-Transfer Kinetics for Adsorbed Redox Monolayers on Semiconductor
Authors: Maldonado Stephen , Vasquez Robert , Waelder Jacob
Published: July 2021

This presentation will describe our latest results in comprehensive modeling of the voltammetric response of redox active monolayers on semiconductor electrodes. The motivation is to provide more utility in voltammetric data that pertain to redox molecules adsorbed on semiconductors, as common in dye-sensitized photoelectrochemistry and artificial photosynthesis systems. First, a background on the prior art on kinetic analysis of redox monolayers on metal and semiconductor electrodes will be provided. Second, the mathematical model used in this work will be detailed, including a brief primer on the derivation of the expression that relates semiconductor surface energetics, surface traps, electrostatics, charge-transfer kinetics, and Marcus theory. Third, the voltammetric responses of several redox-active species physisorbed and chemisorbed on Si electrodes will be presented. The focus will be on responses collected in the dark and in non-aqueous electrolytes. Fourth, the extent of kinetic information that can be extracted from individual and sets of voltammograms will be described.
Advanced Redox Flow Battery Chemistries
Authors: Wang Wei
Published: July 2021

Redox flow batteries (RFBs) have increasingly being recognized as a prominent candidate for large-scale energy storage due to their unique advantages of high safety, decoupling of power and energy, long lifespan, quick response, and potentially low cost. Electrolyte is de facto the most critical component in a RFB system. This presentation describes development of new electrolyte chemistries at Pacific Northwest National Laboratory. Solvation chemistry of the different electrolyte systems will be discussed, which provide a greater understanding of dynamic interactions between solvent-solvent, ion-solvent, and ion-ion at the molecular level. Such understanding is pivotal in developing new redox flow system with higher energy density and temperature stability. New RFB chemistries based on a variety of redox active materials will also be presented.
Gas Evolution from Mixed-Acid Vanadium Redox Flow Batteries
Authors: Wittman Reed M , Pratt Harry , Anderson Travis , Preger Yuliya
Published: July 2021

Mixed-Acid (MA) Vanadium Redox Flow Batteries (VRFB) improve on standard VRFBs’ energy density by adding hydrochloric acid (HCl) to the standard sulfuric acid (H2SO4) electrolyte. This improves the stability of the vanadium (V) in the solution, which allows for electrolytes with higher concentrations of V ions and increased operating temperatures. The addition of HCl to the system introduces a new safety risk, the evolution of chlorine gas (Cl2) from the catholyte.

In this study, we cycled an MA VRFB under a range of conditions to understand the fundamental causes of gas evolution. We constructed a single cell MA VRFB using a standard electrolyte composition of 2M H2SO4, 5M HCl and 2M V, with a gas analyzer sampling the head space of the catholyte chamber. Cl2 gas quantity was measured under systematically varied SOCs, cycling rates, electrolyte flow rates, electrode materials and electrolyte temperatures. Mechanisms of gas evolution at each condition were determined from analysis of the cycling data and the fundamental chemistry of the system. Conditions that lead to high rates of gas evolution were identified. This new understanding of gas evolution will aid development of safer MA VRFB systems that mitigate Cl2 formation during operation.

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.
Plasma Oxygen Interaction with Organic Polymer Films: A Mechanistic Study
Authors: Longo Roberto , Ventzek Peter , Ranjan Alok
Published: July 2021

In this presentation, first principles calculations combined with advanced surface diagnostics are used to unravel the mechanisms of plasma oxygen interaction with organic films, of interest for advanced patterning in semiconductor device manufacturing. Results from a combination of X-ray photoelectron spectroscopy (XPS) diagnosed oxygen plasma-exposed polystyrene films and first principles modeling of organic films (polystyrene, polyethylene and derivatives) provide insights into how organic films are oxidized by oxygen atoms. XPS measurements show the rapid formation of C-O structures and their saturation after oxygen exposure on both pristine and argon bombarded polystyrene samples. Quantum mechanics calculations confirm that C-OH formation can be immediate without recourse to previously formed dangling bonds. Multiple oxygen impacts are required for scission of pristine ethylene carbon strands. Therefore, ethylene films can be converted to polyols that are stable, whereas more likely strands are broken before polyol formation through the formation of water and C=O. On the contrary, intermediate compounds with adjacent C=O bonds are not likely to form stable structures. The combination of XPS measurements and modeling imply that the oxidation of polystyrene and polyethylene is self-limiting on both hydrogen saturated and de-hydrogenated (after argon ion plasma bombardment) surfaces.
Low Fluorine Colorless Polyimide Substrate for Flexible OLED Display
Authors: Kim Yong-Hae , Lee Won-Jae , Kim Su-Jung , Cho Nam-Sung , Kim Dong-Min , Kim Gi Heon
Published: July 2021

Flexible display has become the main stream for the next innovative display. To build the flexible electronics, polyimide varnish is usually coated on the carrier glass with the thickness of a few micrometer and various devices such as flexible solar cell, flexible display, flexible printed circuit board, flexible terahertz sensor, and smart window [1]. Currently available colorless polyimide varnish is usually made of fluorine contained 6-FDA, which has good characteristics such as high transmittance, low yellow index, excellent solubility, but also has bad characteristics such as low decomposition temperature, high coefficient of thermal expansion, high cost, and environmentally unfriendliness due to fluorine. In this study, we make low fluorine contained polyimide varnish by substituting 6-FDA to BPDA + ODPA.

Fig. 1 shows the molecular structure of 6-FDA, BPDA, and ODPA. Low fluorine contained polyimide varnish is composed of 40% 6-FDA, 40% BPDA, 20% ODPA and it showed the excellent characteristics such as the glass transition temperature of 300 deg, the decomposition temperature of 366 deg, the yellow index of 2.2, the coefficient of thermal expansion of 20 ppm/deg, and the transmittance at 550 nm wavelength of 98.1%.

Because polyimide substrate made from coating polyimide varnish on carrier glass have to be protected using inorganic barrier material such as SiO2 and Si3N4. Also, transparent electrode is also required to apply voltage to the OLED display. Because the color of the polyimide substrate can be changed with the variation of barrier material thickness and transparent electrode thickness, we used optical simulator, Macleod, to access the transmittance change with the thickness change of SiO2 and Si3N4 as shown in fig. 2. Optimal thickness is selected at SiO2 of 400 nm and Si3N4 of 50 nm. The experimental transmittance is well matched with the simulation transmittance as shown in fig. 3.

Fabricated device will be detached using innovative laser lift-off (LLO) that utilized excimer laser which scan the interface between the polyimide substrate and the carrier glass from the backside [2]. Fig, 4 shows the laser irradiation and the detached wrinkle free polyimide substrate. Before the delamination, the polyimide substrate is clear, but shows yellowish color during delamination process. Optimum laser energy for the delamination is compared with the color index as shown in fig. 5.

We will make the OLED display on the polyimide substrate.

Yi et al., Solar Energy 195, 340 (2020),

Bian et al., App. Sur. Sci. 499, 143910 (2020).

Figure 1
(Invited) Probing Charge-Carrier Transport in Chemically Doped Polymeric Semiconductors
Authors: Ferguson Andrew , Aubry Taylor , Blackburn Jeff
Published: July 2021

With organic semiconductors demonstrating potential for use in various electronic and optoelectronic applications, some attention has turned to an understanding of charge-carrier doping and transport processes in such material systems. Here we demonstrate a novel electrical transport measurement technique, based on microwave spectroscopy, to probe redox-based chemical p-type doping in conjugated semiconducting polymers. Through a comparison of these measurements with macroscopic charge-transport probes, we illustrate that charge injection from a prototypical dopant like 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) initially results in charge-carriers that are bound to the reduced F4-TCNQ counterion, but that the coulomb attraction can be overcome by doping to high charge-carrier concentrations. In contrast, a series of novel charge-transfer dopants based on dodecaborane clusters appended with various organic moieties results in injected charge-carriers that are spatially separated from the counterion. This behavior results from the unique chemical and electronic structure of these dopants, which localizes the negative counter-charge on the center of the dodecaborane cluster, thereby reducing the coulomb attraction for all measurable charge-carrier concentrations and resulting in enhanced charge-carrier transport. These observations point to chemical control over the charge-injection and transport processes that can yield highly doped organic semiconductors with impressive electrical transport properties, in spite of the limitations that their low dielectric constants would suggest.
(Invited) Achieving Stable Organic Transistors By Eliminating the Major Degradation Pathways
Authors: Jurchescu Oana D
Published: July 2021

Organic semiconductors (OSCs) have attracted a great deal of attention over the past decades given their unique properties, which provide an opportunity to incorporate electronics in non-traditional areas such as bio-integrated devices, flexible and rollable displays or clothing. Coupled with low-cost processing and chemical versatility, these materials are gradually becoming an integral part of various components in contemporary optoelectronic applications. For example, the organic field-effect transistors (OFETs) represent the basic building blocks of displays, radio frequency identification tags and conformable sensors. Along with a performance that should match the application specific requirements, maintaining this performance during operation is a prerequisite for any technology. Therefore, identifying the sources and mechanisms leading to operational instabilities are crucial in eliminating their impact, to ultimately achieve the much-needed stability to make these devices components in real-life applications. In this presentation, I will discuss the sources of instabilities in organic transistors and how they manifest in device operation. I will then describe a technique which provides a reliable diagnostic tool to identify with high accuracy the environmental and operational degradation pathways occurring in OFETs during operation. By identifying the most probable degradation pathways, we developed new device design rules which resulted in OFETs with unparalleled operational stability in both staggered and coplanar configurations regardless of the active material (small molecule and polymeric OSCs), as confirmed by the constant mobility and exceptionally low threshold voltage shift of ΔVth
= 0.1 eV achieved under aggressive bias stress conditions for 500 min in ambient air.

The work was done in collaboration with Hamna F. Iqbal, Qianxiang Ai, Karl J. Thorley, Hu Chen, Iain McCulloch, Chad Risko, and John E. Anthony
Development of Extremely Stable Anthraquinone Negolytes for Aqueous Flow Batteries
Authors: Wu Min , Gordon Roy , Aziz Michael J.
Published: July 2021

The greatest challenges for the commercialization of aqueous organic redox flow batteries are long lifetime and low mass-production cost of organic active species. Here we report several advances in this regard. We developed a new synthetic route for extremely stable anthraquinones from inexpensive precursor anthracene. Further, we demonstrated that high pH suppresses the decomposition of this anthraquinone negolyte, leading to a demonstrated capacity fade rate of <1%/year, which represents the lowest fade rate, in the absence of rebalancing techniques, of any flow battery ever published – organic or inorganic. Additionally, we report progress in reducing the production cost of high-performance anthraquinones with one-pot synthesis methods in aqueous conditions.
Mediated Lithium-Sulfur Flow Batteries for Grid-Scale Energy Storage
Authors: Meyerson Melissa , Small Leo
Published: July 2021

With the growth of intermittent renewable power sources, such as solar and wind energy, there is an increasing need for inexpensive, high capacity, grid-scale energy storage. The high capacity (1675 mAh/g) and low cost (~0.1 $/kg) of sulfur make Li-S batteries an ideal candidate to fill this need. However, sulfur has poor electrical conductivity and Li-S batteries are prone to polysulfide shuttling that decreases the battery life. In an effort to address these problems, we are developing a highly-scalable mediated Li-S flow battery using a lithium metal anode, solid state Li ion conductor separator, sulfur cathode, and a redox mediator. During discharge, the Li metal is oxidized to Li+ and the redox mediator (RM) is reduced. The RM will then react with the S returning to its oxidized form and enabling the formation of Li2S without the need for electrical contact between the S and counter electrode. The oxidized RM is then reduced again, and the process is repeated until all available S has reacted. A second RM with a potential lower than that of the Li-S reaction is used to enable the charge process. Two such mediators are decamethylferrocene (2.86 V vs Li/Li+) and cobaltocene (2.06 V vs. Li/Li+), Figure 1. The flow battery design has multiple benefits. First, the use of a solid state Li ion conductor mitigates polysulfide shuttling, thereby increasing the cycle life. Second, the issue of sulfur conductivity is addressed through use of the redox mediator, where the mediator can easily diffuse through the electrolyte reaching the sulfur and eliminating the need for excess conductive carbon additives. Third, the flow battery design enables physical separation of the anode and cathode, mitigating the potential for short circuiting and thermal runaway even if the Li does puncture the solid state electrolyte. Using this cell design, we examine the role of the solvent, anion, and polysulfide concentration on Li+ transport across the solid electrolyte interphase as well as the role of the mediation potential and interactions between the polysulfide and mediator.

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Figure 1
Supercapacitors with Prussian Blue Derived Carbon Encapsulated Fe/Fe3c Nanocomposites
Authors: Kumar Ankit , Das Debanjan , Sarkar Debasish , Patil Satish , Shukla Ashok
Published: July 2021

A one-step synthesis of carbon encapsulated Fe/Fe3C nanoparticles by pyrolyzing single source precursor of Prussian Blue (Iron (III) ferrocyanide) for its usage as anode material in high-performance supercapacitors is reported. The synthetic method produces 3D doughnut shaped porous structures comprising numerous interconnected Fe/Fe3C nanoparticles entirely encapsulated within layers of graphitic carbon. Such a porous structure facilitates electrolytic ion diffusion during charge storage on Fe/Fe3C nanoparticles through surface or near surface- based faradaic reactions, while the metallic iron helps enhancing the electronic conductivity of the electroactive material. Accordingly, the charge storage in such carbon encapsulated Fe/Fe3C nanoparticles is governed by capacitive as well as diffusion-controlled processes at lower scan rates, and is taken over by capacitive processes at higher scan rates. The material achieves a specific capacitance of 223 F/g at a scan rate of 10 mV/s along with compelling cycling performance exhibiting a little decay in capacitance over 20000 cycles. When coupled with activated-carbon cathode, Fe/Fe3C//activated-carbon asymmetric supercapacitor out-performs many recently reported supercapacitors, which reflects their potential in next-generation storage applications.
Computational Analysis of Joule Heating Effect in Triple Material Gate AlGaN/GaN High Electron Mobility Transistor
Authors: Kashem Md Tashfiq Bin , Subrina Samia
Published: July 2021

Aluminum gallium nitride (AlGaN)/gallium nitride (GaN) high electron mobility transistor (HEMT) has shown great potential and superiority over other devices in high power and high speed-based systems [1-4]. Recently, a novel AlGaN/GaN HEMT structure named triple material gate (TMG) HEMT has been proposed in [5] in which the gate contact is composed of three metals with different work functions. TMG HEMT ensures high carrier velocity and better suppression of short channel effects. In this work, a two-dimensional electro-thermal model has been developed to investigate the joule heating effect in TMG AlGaN/GaN HEMT using the software COMSOL [6]. The current continuity and heat transport equations are solved self-consistently to obtain the temperature distribution profile in various layers of the device. Thermal efficiency of TMG HEMT is compared with dual material gate and single material gate HEMT. It has been found that, besides providing better current carrying capability, TMG HEMT shows optimum thermal performance. Moreover, the variation of peak temperature in the channel with drain and gate voltages is analyzed to study the bias voltage dependence of the device temperature rise. Effects of several device parameters like total channel length, spontaneous and piezoelectric polarization induced charges and ratio of lengths of control and screen gates, on the thermal performance have also been explored. Simulations are carried out for the device lying on different substrate materials (Sapphire, GaN, Si and SiC) with varying thermal conductivity to demonstrate their performance in heat removal capacity. This simulation model can serve as a very useful tool to comprehend the thermal behavior of the device for high power and high frequency applications.

References:

[1] Fletcher, AS Augustine, et al. "Analysis of AlGaN/GaN HEMT using discrete field plate technique for high power and high frequency applications." AEU-International Journal of Electronics and Communications 99 (2019): 325-330.

[2] Nirmal, D., et al. "Current collapse modeling in AlGaN/GaN HEMT using small signal equivalent circuit for high power application." Superlattices and Microstructures 113 (2018): 810-820.

[3] Chung, Jinwook W., et al. "AlGaN/GaN HEMT With 300-GHz fmax." IEEE Electron Device Letters 31.3 (2010): 195-197.

[4] Shen, L., et al. "AlGaN/AlN/GaN high-power microwave HEMT." IEEE Electron Device Letters 22.10 (2001): 457-459.

[5] Kashem, MTB, and Subrina, S. "Analytical modeling of channel potential and threshold voltage of triple material gate AlGaN/GaN HEMT including trapped and polarization‐induced charges." International Journal of Numerical Modelling: Electronic Networks, Devices and Fields 32.1 (2019): e2476.

[6] COMSOL Multiphysics® (Version 3.5a) [Computer software]. Burlington, MA.
Reducing the Threshold Voltage Instability of β-Ga2O3 Nanobelt Field-Effect Transistors Using Ozone Treatment
Authors: Cho Cheolhee , Park Hyunik , Kim Jihyun
Published: July 2021

β-Ga2O3 is a promising semiconducting material for next-generation power devices owing to its wide bandgap (4.8 eV), high breakdown electric field (8 MV/cm), and good thermal stability. β-Ga2O3 has a monoclinic crystal structure with weak covalent bonding along the direction of (100), so bulk β-Ga2O3 can be exfoliated into ultra-thin flakes, which can be integrated with van der Waals materials. An example of the application is metal-insulator-semiconductor field-effect transistors (MISFETs) using h-BN as a gate dielectric. H-BN has an atomically flat surface without dangling bond, minimizing interfacial states between the channel and insulator layer. Moreover, the high thermal conductivity of h-BN (30 W/m·K along the c-plane) makes heat management easier (cf. SiO2: 1.4 W/m·K). Therefore, β-Ga2O3/h-BN MISFETs are expected to enhance device performance with a synergetic effect of both materials.

However, there are remaining reliability issues about the gate-bias stress instability (BSI), the temporary or permanent degradation of electrical properties from gate bias stress (ex. threshold voltage, field-effect mobility, on/off ratio). β-Ga2O3 FETs have been developed for high voltage operation in harsh environments, so that suppression of BSI is required. BSI is mainly attributed to charge trapping in the channel, channel/dielectric interface, or dielectric layer under gate-bias stress. In this work, we tried to reduce bias-stress instability of β-Ga2O3/h-BN MISFETs by UV-O3 treatment. Numerous reports demonstrated the annealing of oxygen-related defects by UV-O3 treatment in oxide-based materials. Therefore, UV-O3 treatment was expected to also work on β-Ga2O3.

First, β-Ga2O3 (200-300 nm) and h-BN (40-50 nm) flakes were exfoliated from each bulk crystal with adhesive tape. H-BN flake was dry-transferred on SiO2/p++Si substrate, and subsequently β-Ga2O3 flake was stacked on h-BN. The source, drain, and gate electrodes (Ti/Au 50 nm/100 nm) were defined by e-beam lithography, followed by e-beam evaporation and lift-off processes. Bias-stress characteristics were systematically measured by Measure-Stress-Measure (MSM) method—repeating the sequence of 1) IDS-VGS, 2) VGS stress, 3) IDS-VGS sweep. β-Ga2O3/h-BN MISFET was UV-O3 treated using a commercial UV-O3 cleaning system (PSD-UV, Novascan Technologies, Ames, IA, USA). By following bias-stress characteristics measurement, we could observed reduced shifts of electrical properties regarding as annealing effect on oxygen-related defect by UV-O3 treatment. This work grants facile way to achieve β-Ga2O3 FETs stable to stress and open up new opportunities for the practical implementation of β-Ga2O3 power device. The details of methods and results will be presented at the conference.
Structure, Optical and Electrical Properties of the High Doped ZnO Thin Films Deposited By RF Magnetron Sputtering of Powder Target in Methane Ambient
Authors: Nazarov Alexei N. , Vasin Andriy V , Rusavsky Andriy V. , Gomeniuk Yuri V. , Tyagulskii Igor P. , Yatskiv Roman , Grym Jan , Lorinčik J. , Vorliček V. , Mamykin Sergii V. , Gudymenko O. I. , Kladko Vasil P.
Published: July 2021

The ZnO thin films were deposited by RF-magnetron sputtering of ZnO powder target using pure argon, argon with hydrogen and argon with methane as reactive gas. Surface morphology and crystalline structure of the films were studied by SEM, XRD and XRR techniques. Chemical composition and optical properties were investigated by SIMS, Raman scattering spectroscopy (RSS), PL spectroscopy and spectral ellipsometry. Electrical measurements were performed on the ZnO films deposited on SiO2-Si and silicon wafers n- and p-type with Ni contacts by TLM, IV, CV characteristics, admittance spectroscopy, Van der Pauw and Hall methods. It was found that growth morphology, crystalline structure, optical and electrical properties of the films are strongly affected by adding methane to argon during the deposition process. Adding of methane resulted in a destruction of hexagonal phase texture and considerable increase a size of ZnO nanocrystallites (from 50 nm for argon to 200 nm for argon/methane working gas). SIMS measurements demonstrated that methane promoted hydrogen incorporation into the ZnO film that was more than an order of magnitude larger in comparison with adding molecular hydrogen in the gas mixture at the same concentration. Adding of methane resulted in a high energy shift of near band edge ultraviolet photoluminescence band, quenching of deep level emission in the visible spectral range and increase of optical band gap from 3.24 to 3.45 eV. The strongest effect of methane has been found for electrical resistivity that reduced by 3 orders of magnitude in comparison with films deposited in pure argon. Additionally, electrical conductance of the ZnO film deposited at high methane concentration does not depend on measurement temperature. Hall method with combination with Van der Pauw resistivity measurements shows: strong n-type doping of the film; donor concentration more than 1020 cm-3 slightly depending on methane concentration; maximum electron mobility about 7 cm2/V s. The direct comparison of resistivity and donor concentration of the films deposited using methane and molecular hydrogen as doping precursors has demonstrated that doping efficiency of methane is about an order of magnitude larger than that of molecular hydrogen under similar deposition conditions. On the other hand, methane addition in working gas resulted in an intensity decrease of the E2L (100 cm-1) and E2H (438 cm-1) lines in the RSS that can be associated with some disturbance of the ZnO structure. Additionally a peak at 71 cm-1 was observed in some films that can be related to the metallic Zn. The possible mechanisms of the ZnO film formation, nature of the doping and electron transport mechanisms will be discussed.
Optical Properties and HR-XRD Study of /InGaAs/Al Ga As/GaAs Structures with InAs Quantum Dots and Different Capping Layers
Authors: Polupan Georgiy v. , Cisneros-Tamayo Ricardo , Torchynska Tetyana
Published: July 2021

The Al0.30Ga0.70As/InGaAs/AlGaInAs/Al0.30Ga0.70As structures with embedded InAs quantum dots (QDs) covered by strain reduced AlGaInAs capping layer have been investigated in as grown state by means of a photoluminescence (PL) and high resolution X ray diffraction (HR-XRD) methods. Two types of QD structures with different quantum well (QW) capping layers: Al0.10In0.15Ga0.75As (#1) and Al0.40In0 .15Ga0.45As (#2) are compared. It is revealed that the QD emission in the structure with Al0.10In0.15Ga0.75As capping is characterized by the highest PL intensity of the ground state (GS) band and smaller the full with at half maximum (FWHM), compared to #2 structures. The variation of the GS emission peak versus temperature has been monitored within the range of 10-400K for the as grown film states and compared with shrinkage of the energy bandgaps in the InAs and GaAs bulk crystals. The results show that the efficiency of Ga/Al/In intermixing in #2 is less than in #1. HR-XRD study has shown that the both QD structures are characterized by best quality of QW interfaces with the high numbers of Pendellösung peaks. The HR-XRD scans were numerically modelled for the analysis of QW layer compositions. The peculiarities of PL spectra of the studied QD structures are analyzed and the advances of studied QD structures have been discussed.
(Invited) Shallow Donor Impurities in GaN
Authors: Freitas Jaime A.
Published: July 2021

Significant advances have been achieved during the last three decades in heteroepitaxial, homoepitaxial, and bulk growth of nitride semiconductors. Despite this remarkable progress, the full realization of the functionality of this material system is often limited by the presence of pervasive residual impurities inherent to the high-temperature growth process, and/or by the presence of lattice point and extended defects. A number of important fundamental aspects of these properties must be understood and controlled in order to realize optimum device performance.

We review here on a systematic investigation, employing a combination of defect-sensitive techniques, carried out on freestanding HVPE-GaN films to identify the chemical nature of the shallow donors. High-resolution, variable temperature PL experiments were performed in the spectral region associated with recombination processes involving the ground and excited states of the neutral donor bound excitons and their two-electron-satellites in GaN substrates and homoepitaxial films. High-sensitivity SIMS and high-resolution FTIR measurements unambiguously identify Si and O shallow donors with ground state binding energies of 30.18 meV and 33.20 meV, respectively. These binding energies are in excellent agreement with values obtained by the analyses of the two-electron-satellite PL spectra considering the participation of ground and excited state donor bound excitons. High-resolution PL studies of homoepitaxial GaN films confirm the SIMS and the FTIR impurity identifications. These results were conveniently employed to verify the incorporation and activation of shallow donors in high crystalline quality GaN substrates.

This work supported by the Office of Naval Research
(Invited) Additive Manufacturing of Functional Circuits on 3D Freeform Surfaces
Authors: Yi Ning , Cheng Huanyu
Published: July 2021

Deployment of functional circuits on a 3D freeform surface is of significant interest to wearable devices on curvilinear skin/tissue surfaces or smart Internet-of-Things with sensors on 3D objects. We've recently presented a new fabrication strategy that can directly print functional circuits either transient (biodegradable) or long-lasting onto freeform surfaces by intense pulsed light-induced mass transfer of zinc nanoparticles (Zn NPs). The intense pulsed light can locally raise the temperature of ZnNPs to cause evaporation. A conformal lamination of a kirigami-patterned soft semi-transparent polymer film with ZnNPs to a 3D surface results in condensation of ZnNPs to form conductive yet degradable Zn patterns onto a 3D freeform surface for constructing transient electronics. Immersing the Zn patterns into a copper sulfate or silver nitrate solution can further convert the transient device to a long-lasting device with copper or silver. Functional circuits with integrated sensors and a wireless communication component on 3D glass beakers and seashells with complex surface geometries demonstrate the viability of this manufacturing strategy.
Topological Supramolecular Network Enabled Intrinsically Stretchable Organic Electronics for Multimodal and Seamless Biointerfaces
Authors: Jiang Yuanwen , Bao Zhenan
Published: July 2021

Multielectrode arrays that can seamlessly integrate with electrogenic tissues provide critical information to diagnose disease pathophysiology and even augment human functions. Despite extensive efforts to merge rigid inorganic electrodes with soft and curvilinear tissues, the inherent misalignment between their mechanical properties (e.g., stiffness vs stretchability) inevitably leads to a trade-off between the device density and tissue conformability.

Organic electronic materials, on the other hand, offer exciting new opportunities in the formation of minimally invasive and seamlessly integrated biointerfaces due to its potential combination of intrinsic softness, stretchability and electric conductivity. Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), as one of the most prominent conducting polymer, has been widely used to reduce interfacial impedance of bioelectronic devices due to its ionic-electronic dual conducting mechanism. However, pristine PEDOT:PSS thin film is stiff and brittle that cannot be stretched to sustain natural tissue deformations.

In this talk, I will elaborate our effort on the development of stretchable, conductive PEDOT:PSS with chronic stability. Through a rationally-designed supramolecule-based topological network, we achieved for the first time simultaneously enhanced conductivity (up to 3000 S/cm) and stretchability (100% strain without cracks) of PEDOT:PSS with even direct photopatternability (down to 2 µm feature size). The as-fabricated stretchable PEDOT:PSS electrode array exhibits low interfacial impedance with excellent conformability with human skin and animal tissue, allowing for high-density mapping of electrophysiological activities as well as precise neuromodulation through unprecedented brainstem based neural interfaces.
(Invited) Wearable Sweat Sensors - Towards Big Data for Human Health
Authors: Javey Ali
Published: July 2021

Wearable sensor technologies play a significant role in realizing personalized medicine through continuously monitoring an individual’s health state. To this end, human sweat is an excellent candidate for non-invasive monitoring as it contains physiologically rich information. In this talk, I will present our recent advancements on fully-integrated perspiration analysis system that can simultaneously measure sweat rate, metabolites, electrolytes, drugs and heavy metals, as well as the skin temperature to calibrate the sensors' response. Our work bridges the technological gap in wearable biosensors by merging plastic-based sensors that interface with the skin, and silicon integrated circuits consolidated on a flexible circuit board for complex signal processing. This wearable system is used to measure the detailed sweat profile of subjects engaged in prolonged physical activities, and infer real-time assessment of physiological state of the subjects. Case studies on the correlation of sweat analytes with those of blood and various physiological conditions will be presented, including for applications in dehydration studies, diabetes monitoring, drug metabolism rate studies, and detection and monitoring of cystic fibrosis. Finally, a general roadmap for the technology will be presented, with focus on opportunities and challenges.
(Invited) Flexible Organic Sensors and Actuators in Human-Machine Interfaces
Authors: Ng Tse Nga
Published: July 2021

My research aims to develop low-cost, flexible, large-area organic devices for different human-machine interfaces.1,2 In this presentation, we discuss case studies for two different point-of-use applications:

1) Motor skills characterization. There is no objective metric for evaluating motor skill training progress, and current assessments rely on qualitative surveys. We have fabricated an instrumented glove with pressure and motion sensors for motor characterization. This glove is useful for characterizing motor skills of people suffering from spasticity, a neuromuscular disorder that causes muscle stiffness/resistance and jerky movement. Analyses of force versus velocity show movement-dependent muscle resistance in a patient with spasticity. Through the flexible sensor systems, the shift from subjective scores to objective measurement will promote better diagnosis and dramatically improve the accuracy in tracking patient response to therapy.

2) Haptic interaction. Haptic actuators generate touch sensations and provide realism and depth in human-machine interactions. A new generation of soft haptic interfaces is desired to produce the distributed signals over large areas that are required to mimic natural touch interactions. We demonstrated the printing integration of multi-material actuators, with an eye toward improving the fidelity and robustness of the printing process. We achieved a compact tactile display by using liquid-crystal elastomers patterned by extrusion.3 Printing with organic materials is a versatile approach that can lead to devices that are ergonomic, readily customizable, and economical for everyone to explore potential benefits and create new haptic applications.

1 M. Amit, L. Chukoskie, A. J. Skalsky, H. Garudadri and T. N. Ng, Advanced Functional Materials, 2019, 1905241.

2 M. Amit, R. K. Mishra, Q. Hoang, A. M. Galan, J. Wang and T. N. Ng, Materials Horizons, 2019, 6, 604–611.

3 Y. Zhai, Z. Wang, K. S. Kwon, S. Cai, D. J. Lipomi, T. N. Ng, Advanced Materials (2020) 2002541.
Liquid Metals Distribution in Polymers for Soft Electronics
Authors: Zhang Jiuyang
Published: July 2021

Growing interests have been received for conductive fibers due to their electrical conductivity with one-dimensional structures. Fibers with liquid metal (LM) as conductive fillers exhibit distinctive advantages, such as metallic conductivity and superior stretchability. So far, the unique characters of LM have been not fully used in the one-dimensional materials. This work successfully created conductive fibers with LM fillers gradient dispersed in polymer matrix. Such LM fiber showed distinguished thermally programmable shapes and electrical conductivities. The fiber can be programed into bending or helical shapes with controlled bending degree depended on the heat absorbed by the LM inside the fiber. Interestingly, the resistance of the LM fiber could be also tuned by the thermal energy. Upon heating, the LM fiber showed a transition from an insulator (108 Ω) to a conductor (10 Ω) with programmed conductivity along the axial direction. More importantly, both the shape programmability and conductivity transition were highly reversible by heating and cooling. The programmed helical LM fiber successfully mimicked a nature plant tendril with excellent mechanical performance and showed unique strain-induced electrical conductivity. The LM fiber could be also applied as flexible temperature electrical switches in high-tech devices, such as lunar rovers.
(Invited) Programmable Gold Nanowire Tattoo for Multimodal Wearable Sensors
Authors: Cheng Wenlong
Published: July 2021

Sensitive, specific yet multifunctional tattoo-like electronics are ideal wearable systems for health monitoring anytime anywhere because they are virtually becoming parts of Human skins offering burdenless “unfeelable” wearing experience. Here, I will present a new skin-like, multifunctional electronic tattoo entirely from gold using standing enokitake mushroom-like vertically aligned nanowire membrane in conjunction with programmable local cracking technology. Only a single material type is needed for our integrated gold circuits involved in interconnects and multiplexed specific sensors, thereby avoiding complex multi-materials interface. We can fine-tune the properties of elastic gold conductors from strain-insensitive to highly strain-sensitive simply by programming localized crack size, shape and orientations. Furthermore, we demonstrate in-plane integration of strain/pressure sensor, anisotropic orientation-specific sensors, strain-insensitive stretchable interconnects, temperature sensors, glucose sensors, and lactate sensors without the need of soldering or gluing.
(Invited) Solid-Liquid Contact Electrification and Its Use for Energy Harvesting and Self-Powered Applications
Authors: Lin Zong-Hong
Published: July 2021

The abundance of water on earth provides a large window to utilize the mechanical energy within river currents and ocean waves. In this regard, hydropower harvesting through solid–liquid contact electrification has received considerable interest in the recent past. Despite advancements in nanotechnology, liquid energy harvesting devices, especially solid–liquid triboelectric nanogenerators (S–L TENGs), require efficient engineering of the interfacial properties of their substrates to transfer liquid mass and momentum rapidly with the effective generation/transfer of surface charges. To face this challenge, several parameters such as the selection of material, surface morphology and surface properties are currently being studied to develop a better system architecture for energy harvesting and self-powered application platforms with three different interacting modes of liquid contact. Moreover, several parameters of the contact solvents such as the ionic activity and polarity have been studied to understand the practical applicability of S–L TENGs to harvest energy from different natural and artificial resources. In addition, the scope of harvesting mechanical energy from other volatile organic compounds has been studied recently. Self-powered applications of S–L TENGs in various fields have also been demonstrated by different research groups. This work indicates recent progress in the development of S–L TENGs for the first time in terms of the different properties of solid and liquid contact materials along with their respective applications. Furthermore, the work concludes with perspectives, future opportunities, and major challenges of fabricating S–L TENGs as an efficient energy harvester.

References

Chatterjee, S. Saha, S. R. Barman, I. Khan, Y.-P. Pao, S. Lee, D. Choi, Z.-H. Lin (2020) Nano Energy, 69, 105092.

Z‐H. Lin, G. Zhu, Yu S. Zhou, Y. Yang, P. Bai, J. Chen, Z. L. Wang (2013) Angew. Chem. Int. Ed., 52, 5065-5069.

Z-H. Lin, Y. Xie, Y. Yang, S. Wang, G. Zhu, Z. L. Wang (2013) ACS Nano, 7, 4554-4560.

Z‐H. Lin, G. Cheng, L. Lin, S. Lee, Z. L. Wang (2013) Angew. Chem. Int. Ed., 52, 12545-12549.

Z-H. Lin, G. Cheng, S. Lee, K. C Pradel, Z. L. Wang (2014) Adv. Mater., 26, 4690-4698.
(Invited) Closed-Loop Electrostimulation Enabled by Nanogenerator Technology
Authors: Wang Xudong
Published: July 2021

Electrostimulation (ES) has long been utilized as a versatile therapeutic strategy for treating human diseases. The nanogenerator technology stands at a unique position for connecting biological ES to body motions by converting biomechanical energy to electric pulses. This process forms a closed-loop ES in both energy flow and function feedback. This talk will first elaborate how the closed-loop ES operates as a close analogue to how the body controls its own functions. The most recent progresses of flexible and complex piezoelectric material innovations in our group will be viewed for implantable nanogenerator development, including 3D printing techniques and degradable piezoelectric biomaterials. These materials exhibited tissue-matching mechanical property as well as excellent electromechanical coupling properties, ideal for in situ electrostimulation generation. In our recent research, we also successfully implemented the closed-loop electrostimulation concept for skin wound healing, hair growth, bone healing and vagus nerve stimulation for obesity control with superb performance. An electrical stimulation bandage was developed that could accelerate skin wounds recovery to within 3 days as compared to 12 days of usual contraction-based healing processes in rodents. Via a similar design, the electrical pulses were applied to stimulate hair growth. In another work, an implanted vagus nerve stimulation system was developed in responsive to the peristalsis of stomach. The electric signals generated by this device stimulates the vagal afferent fibers to reduce food intake and achieve weight control. With the advanced electromechanical material and device design and engineering, the closed-loop electostimulation concept will quickly grow into a very impactful application direction of nanogenerator technology for human wellbeing.