bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrochemical Characteristics of Vertically-Aligned Ti3C2 Nanosheet Electrode for High-Speed Charge/Discharge Supercapacitors
Authors: Hideshima Sho , Kawasaki Yusuke , Takimoto Daisuke , Gogotsi Yury , Sugimoto Wataru
Published: February 2020

Two-dimensional titanium carbide (Ti3C2) nanosheets have recently received attention as a supercapacitor electrode material capable of achieving high capacity and rapid charge-discharge due to psudocapacitance associated with redox reaction on the surface1. To effectively use the surface of each single Ti3C2 nanosheet, nanosheet electrodes with well-controlled porous structures are necessary to increase ion and electron transfer pathway. Here, electrochemical performance of vertically-aligned Ti3C2 nanosheet electrodes, fabricated based on our proposed nanosheet vertical alignment method combined with electrophoretic deposition and freeze drying2, was examined.

To check the importance of the vertically aligned structure to improve electrochemical performance, a vertically-aligned and a horizontally-stacked nanosheet electrode were fabricated by using Ti3C2 nanosheets with lateral size of 200 nm. Cyclic voltammograms of the vertically-aligned electrode show redox peaks at approximately 1.0 V and 1.6 V versus Li/ Li+ at a scan rate of 2 mV s−1, whereas that of the horizontally-stacked electrode gives no significant peaks from Ti3C2 (Figure 1). The capacity obtained from the vertically-aligned electrode was larger than that obtained from horizontally-stacked electrode, suggesting that the vertically-aligned structure improved ion diffusion path to the surface. To investigate the effect of the sheet size on the electrochemical characteristics, vertically-aligned electrodes composed of three different-sized Ti3C2 nanosheets (100, 200 and 600 nm) were prepared and evaluated. It should be noted that the amount of the nanosheets loaded on each electrode was kept constant at 0.68-0.70 mg cm-2. The capacity reached approximately 153 (100 nm), 140 (200 nm) and 136 mAh g−1 (600 nm), at 0.2C current rate, respectively, suggesting that small Ti3C2 nanosheets could effectively improve Li ion accessibility to their active sites compared with large nanosheets. Thus the vertically-aligned Ti3C2 nanosheet electrode could be useful for supercapacitors with high capacity and high-speed charge/discharge ability.

This research was partly supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) (JPMJAL1008) from Japan Science and Technology Agency (JST).

References

M. Naguib, V. N. Mochalin, M. W. Barsoum and Y. Gogotsi, MXenes: A new family of two-dimensional materials, Adv. Mater., 26, 992 (2014).

D. Mochizuki, R. Tanaka, Y. Ayato, S. Makino and W. Sugimoto, Vertically Aligned Reduced Graphite Oxide Nanosheet Film and its Application in a High-Speed Charge/Discharge Electrochemical Capacitor, ACS Appl. Energy Mater., 2, 1033 (2019).

Figure 1
Flexible Transparent Molybdenum Trioxide Nanopapers for Supercapacitors
Authors: Yao Bin , Huang Liang , Li Yat
Published: February 2020

Free-standing paper-like film with excellent transparency and flexibility holds great potential to serve as a transparent electrode for wearable electronics and energy storage. Transition metal oxide have shown their great potential for energy storage due to their high capacitance and stability. Nevertheless, the fabrication of ultrathin transparent paper electrode via the assembly of 1D transition metal oxide nanostructure has never been reported. Orthorhombic molybdenum trioxide (α-MoO3), transition metal oxide with layered structures, has been recognized as a promising electrode for energy storage. In this talk, I will talk about the fabrication of flexible and transparent MoO3 nanopapers via assembly of ultralong molybdenum trioxide nanobelts (MoO3 NBs) and their application for high-performance supercapacitor electrodes. The well-dispersed, ultralong MoO3 NBs with an average length of 200 µm have been synthesized via a hydrothermal method. A flexible transparent molybdenum trioxide nanopaper via the assembly of these ultralong nanobelts displays an excellent average transmittance of 90% in the visible region. Significantly, the freestanding nanopaper electrode delivers an outstanding specific capacitance of 1198 F g-1 at 2 mV s-1, which is just slightly below the theoretical value of molybdenum trioxide (1256 F g-1). The excellent specific capacitance can be ascribed to the efficient ion diffusion in the 3D structure and the very thin nanobelts, as well as the reduced distance of electron transport from transparent nanopaper electrode to current collector. The nanopaper-based supercapacitor device also shows an excellent long-term stability performance over 20,000 cycles with a retention rate of 96.5%.

Figure 1
(Invited) Triboelectric Nanogenerators for Internet of Things and Self-Powered Systems
Authors: Wang Zhong Lin
Published: February 2020

Self-powered system is a system that can sustainably operate without an external power supply for sensing, detection, data processing and data transmission. Nanogenerators (NG) were first developed for self-powered systems based on piezoelectric effect and triboelectrification effect for converting tiny mechanical energy into electricity, which have applications in internet of things, environmental/infrastructural monitoring, medical science, environmental science and security. NGs are the applications of Maxwell’s displacement current in energy and sensors. NGs have three major application fields: micro/nano-power source, self-powered sensors and blue energy. We will present the applications of the NGs for harvesting all kind mechanical energy that is available but wasted in our daily life, such as human motion, walking, vibration, mechanical triggering, rotating tire, wind, flowing water and more. Then, we will illustrate the networks based on triboelectric NGs for harvesting ocean water wave energy, for exploring its possibility as a sustainable large-scale power supply. Lastly, we will show that NGs as self-powered sensors for actively detecting the static and dynamic processes arising from mechanical agitation using the voltage and current output signals. This presentation will cover not only the fundamental science of triboelectric nanogenerators, but also will focus on the technology innovation and perspective industrial applications as well as business opportunities.

Z.L. Wang, Materials Today, 20 (2017) 74-82.

“Nanogenerators for Self-Powered Devices and Systems”, by Z.L. Wang, published by Georgia Institute of Technology (first book for free online down load): http://smartech.gatech.edu/handle/1853/39262

Z.L. Wang, L. Lin, J. Chen. S.M. Niu, Y.L. Zi “Triboelectric Nanogenerators”, Springer, 2016. http://www.springer.com/us/book/9783319400389

L. Wang “Triboelectric Nanogenerators as New Energy Technology for Self-Powered Systems and as Active Mechanical and Chemical Sensors”, ACS Nano 7 (2013) 9533-9557.

Z.L. Wang, J. Chen, L. Lin “Progress in triboelectric nanogenertors as new energy technology and self-powered sensors”, Energy & Environmental Sci, 8 (2015) 2250-2282.
(Invited) Creating All 2D Ultrathin Opto-Electronic Devices Using CVD Grown Materials
Authors: Warner Jamie H
Published: February 2020

2D materials come with a variety of properties including metallic, semiconducting and insulating. This enables them to be assembled into ultrathin devices that have lateral and vertical geometries. Advances in materials chemistry over the recent years means that the building blocks of 2D crystals can be synthesized by bottom up methods to achieve large scale area coverage using chemical vapour deposition. In this talk, I will discuss how we have created ultrathin all 2D opto-electronic devices using graphene as the semi-metal electrode, MoS2 and WS2 as the active semiconductor, and hBN as the insulator. Atomic resolution structural studies are performed using scanning transmission electron microscopy to understand defects, grain boundaries, dopants and interlayer stacking. Lateral devices are constructed to achieve transistor operation, and double up as photodetectors. Vertical devices that utilize tunneling interfaces are assembled and operated as photodetectors and light emitting devices. I will discuss how the thickness of the semiconductor layers and using type II heterojunctions of MoS2 and WS2 can lead to photovoltaic responses. Recent work on the scalable production of arrays of red light emitting devices using only CVD grown 2D crystals will be presented.
Large-Scale Manufacturing of Functionally-Encoded Semiconductor Nanowires for Electronics and Photonics
Authors: Mujica Maritza , Mohabir Amar T , Tutuncuoglu Gozde , Behrens Sven , Breedveld Victor , Filler Michael A
Published: February 2020

Semiconductor nanowires can be produced with exquisite spatial control of composition and morphology using the vapor-liquid-solid (VLS) mechanism. This capability has permitted, for example, demonstrations of transistors, photodetectors, solar cells, and biosensors. Central to our process is the use of sacrificial, porous-walled, seed particle-lined silica microcapsules, whose interior surface serves as a high-surface area growth substrate. These microcapsules protect the growing nanostructures, are produced with a scalable emulsion templating technique, and are compatible with large-scale chemical reactors. We will show how microcapsule structure and drying is influenced by silica nanoparticle type and concentration, emulsification parameters, and nanoparticle cross-linking agent. We will also demonstrate the synthesis of Si nanowires with axially-programmed dopant profiles on the microcapsule interior.
(Invited) Intrinsically Stretchable Semiconductors and Electronics Enabled By Polymeric Nanostructures
Authors: Wang Sihong
Published: February 2020

The vast amount of biological mysteries and biomedical challenges faced by human provide a prominent drive for seamlessly merging electronics with biological living systems (e.g. human bodies) to achieve long-term stable functions. Towards this trend, the main bottlenecks are the huge mechanical mismatch between the current form of rigid electronics and the soft biological tissues.

In this talk, I will describe a new form of electronics with skin-like softness and stretchability, which is built upon a new class of intrinsically stretchable polymer materials and a new set of fabrication technology. As the core material basis, intrinsically stretchable polymer semiconductors have been developed through the physical engineering of polymer chain dynamics and crystallization based on the nanoconfinement effect. This fundamentally-new and universally-applicable methodology enables conjugated polymers to possess both high electrical-performance and extraordinary stretchability.[1] Then, proceeding towards building electronics with this new class of polymer materials, the first polymer-applicable fabrication platform has been designed for large-scale intrinsically stretchable transistor arrays.[2] As a whole, these renovations in the material basis and technology foundation have led to the realization of circuit-level functionalities for the processing of biological signals, with unprecedented mechanical deformability and skin conformability. Equipping electronics with human-compatible form-factors has opened a new paradigm for wearable and implantable bio-electronic tools for biological studies, personal healthcare, medical diagnosis and therapeutics.[3]

References

[1] J. Xu#, S. Wang#, et al. Highly stretchable polymer semiconductor films through the nanoconfinement effect,
Science
, 2017, 355, 59-64.

[2] S. Wang, et al. Skin electronics from scalable fabrication of an intrinsically stretchable transistor array,
Nature
, 2018, 555, 83-88.

[3] S. Wang, J. Y. Oh, J. Xu, H. Tran, Z. Bao. Skin-inspired electronics: an emerging paradigm,
Accounts of Chemical Research
, 2018, 51, 1033–1045.
Transparent and Flexible Self-Charging Power Film and Its Application in a Sliding Unlock System in Touchpad Technology
Authors: Luo Jianjun , Tang Wei , Fan Feng Ru , Liu Chaofeng , Pang Yaokun , Cao Guozhong , Wang Zhong Lin
Published: February 2020

Portable and wearable personal electronics and smart security systems are accelerating the development of transparent, flexible, and thin-film electronic devices. Here, we report a transparent and flexible selfcharging power film (SCPF) that functions either as a power generator integrated with an energy storage unit or as a self-powered information input matrix. The SCPF possesses the capability of harvesting mechanical energy from finger motions, based on the coupling between the contact electrification and electrostatic induction effects, and meanwhile storing the generated energy. Under the fast finger sliding, the film can be charged from 0 to 2.5 V within 2094 s and discharge at 1 μA for approximately 1630 s. Furthermore, the film is able to identify personal characteristics during a sliding motion by recording the electric signals related to the person’s individual bioelectricity, applied pressing force, sliding speed, and so on, which shows its potential applications in security systems in touchpad technology.
(Invited) Tunable Elastomeric Optical Gratings Based on Triboelectric Nanogenerators
Authors: Zheng Li
Published: February 2020

For people in life, the existence of triboelectrification usually is a kind of negative effect, especially in winter. However, the team of professor Zhong Lin Wang of Georgia Institute of Technology in the United states invented Triboelectric nanogenerator (TENG) to fully utilize this kind of effect. TENG can convert mechanical energy in the environment into electrical energy, which based on the principle of triboelectrification and charge induction, such as waves, sounds, muscle contractions, mechanical vibrations, etc. The charge separation between the two layers of polymer films will lead to the formation of potential difference. In addition, not only the fabrication process of TENG is simple and low cost, it also can be used in large-scale production. At the same time, it has excellent durability and plasticity, so TENG can be easily combined with other devices. Therefore, TENG and its applications are an important research direction of nanoenergy. Here we have designed TENG for different applications. Listed below is two applications in electrostatic manipulations based on TENG: 1. Integrate a TENG with electrostatic actuation system (EAS) to manipulate the movement of micro-droplets and fine solids. The mechanical triggering of the TENG produces a very high electrostatic field inside the electrostatic actuator, producing a Coulomb force that drives tiny objects (liquid or solid) in the electrostatic actuator. Therefore, the tribomotion of TENG can be used as a driving source and control signals of electrostatic actuators.2. Based on dielectric elastomer actuator (DEA), tunable optical gratings (TOGs) and TENG combined so that the grating period can be controlled by dielectric elastomer actuator. Therefore, a self-powered TOGs system can be implemented, with TENG as the power and control module for its system. In combination with unique output performance of the TENG, three different TOGs systems were designed and used to compress or expand the spacing of the grating array by driving the dielectric elastomer actuator.
Electrical Conductivity of n-Type Behavior in Black BiOCl
Authors: Myung Yoon , Na Chan Woong , Kim Yonghwan , Park Jeunghee , Browning Nigel D , Banerjee Parag
Published: February 2020

We studied n-type conduction characteristics using p-type BiOCl by oxygen vacancy formation at the exposed (001) surface. This formation of oxygen vacancy at the surface of BiOCl nanosheets, confirmed by ACTEM, XPS and Raman spectra that oxygen vacancy gradually increasing as a function of UV illumination time. The low temperature PL result shows decreasing band edge emission of BiOCl due to high density oxygen vacancies were placed under conduction band edge of BiOCl. The conductivity dependence on oxygen partial pressure reveals n-type characteristic behavior as oxygen vacancies were present.
(Invited) Investigation of Interfacial Charge Transfer Doping of 2D MoS2
Authors: Khondaker Saiful I. , Chamlagain Bhim
Published: February 2020

Layered two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) have attracted a great deal of attention because of their exciting electrical and optical properties and their potential applications in electronic and optoelectronic devices. Because of ultra low volume and chemically inert surface of 2D materials, it is challenging to achieve controlled doping of 2D materials. Recently, interfacial charge transfer induced doping of 2D materials have gained significant attention. In this talk, I will present electrical transport studies of 2D molybdenum disulfide (MoS2) field effect transistors decorated with zero dimensional (0D) gold (Au) nanoislands. I will discuss how the Au nanoisland coverage affects the interfacial charge transfer based doping and tailor the electrical properties of the host material. From the detailed low temperature electron transport measurements, I will elucidate the role of 2D-0D interface on the transport properties of MoS2 FETs.
(Invited) Engineering Au/TiO2 Interface for Electron-Driven Heterogeneous Catalysis
Authors: Wei Wei David
Published: February 2020

Interfacial electronic properties of Metal/oxide have been extensively studied due to its importance for heterogeneous catalysis, however, to date, the exact role of interfacial atomic structures in governing catalytic processes still remains elusive. Herein, we demonstrate how the manipulation of atomic structures at the Au/TiO2 interface significantly alters the interfacial electron distribution and prompts O2 activation. It is discovered that at the defect-free Au/TiO2 interface, electrons transfer from Ti3+ species into Au nanoparticles (NPs) and further migrate into adsorbed perimeter O2 molecules (i.e., in the form of Au-O-O-Ti), facilitating O2 activation and leading to a 34 times higher CO oxidation activity than that on the oxygen vacancy (V
o)-rich interface, at which electrons from Ti centers are trapped by interfacial V
o ­on TiO2 and hardly interact with perimeter O2 molecules. Collectively, our results establish an atomic-level description of the underlying mechanism regulating the atomic structure at metal/oxide interfaces for optimizing heterogeneous catalysis.
(Invited) Electrocatalysts for Lithium-Oxygen and Li-Sulfur Batteries
Authors: Lu Yi-Chun
Published: February 2020

Electrocatalysis plays a critical role in metal-oxygen and metal-sulfur batteries. Rational catalyst design for nonaqueous oxygen evolution reaction (OER) in Li-O2 batteries and polysulfide reduction reaction in Li-S batteries is required to further improve the battery performances. In this presentation, we will discuss the catalyst effects and design strategies to improve the electrode kinetics and reversibility of Li-O2 and Li-S batteries. We will discuss promoting the fraction of the high-surface-energy facet of the catalyst is an effective approach to increase the Li-O2 OER activity. We designed and synthesized a series of chromium oxide (Cr2O3) with varying fractions of high-energy facet by inhibiting the growth of the low-energy facet, i.e. (012) of Cr2O3. We will discuss the correlation between the Li-O2 OER activity and the fraction of high-energy facet. Catalyst design consideration for polysulfide reduction to lithium sulfide will also be discussed. We will combine electrochemical characterizations and spectroscopic techniques to provide insights into the catalytic processes in these battery systems.

Acknowledgement:

This work was supported by two grants from the Research Grants Council (RGC) of the Hong Kong Special Administrative Region, China, under T23-601/17-R and NSFC/RGC Joint Research Scheme 2018/19 Project No. N_CUHK435/18.
(Invited) Programming Thermal Energy Transport in Semiconductor Nanowires
Authors: Filler Michael A
Published: February 2020

The confinement of mid-infrared light in deep subwavelength volumes offers exciting opportunities to control the transport of thermal energy. This talk will describe our recent efforts to program the synthesis and understand the properties of semiconductor nanowires containing chains of mid-infrared plasmonic resonators. I will discuss (i) how the programming of dopant profile enables designer thermal photon absorbers, (ii) that combining the dielectric properties of semiconductors and the 1-D geometry of nanowires leads to very strong near-field interactions, and (iii) that these interactions allow photons to carry as much heat as electrons and phonons in the solid state.
Influence of Metal Dopants on MoS2 Crystallization Investigated through in Situ Electron Microscopy
Authors: Kondekar Neha , Boebinger Matthew G , Tian Mengkun , Kirmani Mohammad Hamza , McDowell Matthew T
Published: February 2020

Few-layered MoS2 has electronic properties that can be tuned with the material thickness and crystal structure, making it attractive for a multitude of applications including next-generation optical and electronic devices. Doping MoS2 crystals through substitution or adsorption of transition metals has been shown to improve carrier concentration and mobility values while lowering the contact resistance. [1-3] Prior work has investigated the altered physicochemical properties caused by additions of metal atoms into the MoS2 lattice, but there is a significant lack of understanding and control of how dopants affect the crystallization and growth processes of the materials during synthesis. For manufacturing of highly-controlled MoS2 and related materials, improved knowledge of how to control the material structure during the growth process itself is essential. In this study, we combine in situ transmission electron microscopy (TEM) with ex situ investigation to determine how the addition of transition metal dopants influences the crystallization and growth of MoS2 materials from a solid-state amorphous precursor. Low concentrations of Ni are observed to cause significant differences in the MoS2 crystallization and growth process, leading to an increase in resulting MoS2 crystal size. This is likely a result of the observed altered mobility of interfaces between crystals in the presence of Ni, which is known to substitute at edge sites, during heating-induced growth. Addition of other metallic dopants, such as Cu, Fe and Co, did not result in similar increases in MoS2 crystallite size. Ongoing experiments building on the insights from this study aim to understand the influence of these metal atoms on the electronic and optical properties of few-layer MoS2 films. These findings demonstrate the important role of additional elements in controlling the evolution of TMDCs during synthesis, which should be considered when designing these materials for a variety of applications.

References

[1] Zhao, Y., Xu, K., Pan, F., Zhou, C., Zhou, F., & Chai, Y. Doping, Contact and Interface Engineering of Two-Dimensional Layered Transition Metal Dichalcogenides Transistors. Advanced Functional Materials, 2017, 27.

[2] Gao, J., Kim, Y. D., Liang, L., Idrobo, J. C., Chow, P., Tan, J., et. al. Transition-Metal Substitution Doping in Synthetic Atomically Thin Semiconductors. Advanced Materials, 2016, 28, 9735–9743.

[3] Lockhart De La Rosa, C. J., Arutchelvan, G., Leonhardt, A., Huyghebaert, C., Radu, I., Heyns, M., & De Gendt, S. (2018). Relation between Film Thickness and Surface Doping of MoS2 Based Field Effect Transistors. APL Materials,
2018, 6, 58301.
Analytical Model for the Breakdown Voltage in Punch-through IGBTs
Authors: Zhu Chen , Andrei Petru
Published: February 2020

Insulated-gate bipolar-transistors (IGBTs) are becoming the main switching devices in medium- and high-power applications, such as switched-mode power supplies, power switches, variable frequency driver circuits, traction motors, and inducting heating. Although the switching speed of IGBTs is lower than in the case of metal-oxide-semiconductor field-effect-transistors (MOSFETs), the forward voltage drop is significantly lower, particularly in higher blocking voltage rated devices, which provides major advantage for using IGBTs in many applications. In addition, IGBTs have higher breakdown voltage and currant handling capacity than traditional bipolar-junction-transistors (BJTs), which makes them attractive for a variety of applications related to electric power switches.

In this presentation, we introduce two new analytical models for the breakdown voltage of the punch-through IGBTs. The models are based on the accurate analytical estimation of the ionization integral and the solution of the Poisson equation in the drift region, which lead to a nonlinear equation for the breakdown voltage as a function of the doping concentrations in the drift region, buffer layer, and gate and emitter junctions, as well as of the widths of these regions and junctions. In general, this equation does not have an analytical solution and needs to be solved numerically in order to compute the breakdown voltage. Analytical solutions can be found only in a number of particular cases that will be discussed at the conference. The numerical and analytical solutions of the breakdown voltages are compared to more accurate results obtained using Synopsys’s Sentaurus Device Simulator [1], which is a commercial-grade finite element semiconductor simulator. We observe a very good agreement between our models and the finite element simulations for a wide range doping concentration and geometric dimensions of the IGBTs. In addition, at the conference, we will also compare our analytical results with previous analytical results published in the literature [2].

The proposed analytical model provides an efficient way to design the geometry and doping concentration profiles in punch-through IGBTs and helps the optimization process of the breakdown voltage in these devices [3]. More details about the approximations used in our analytical calculations, the results of the comparison with the more precise finite element simulations, and the existing literature will be presented at the conference.

[1] 2016 Sentaurus Device Simulator User Manual, www.synopsys.com

[2] J. Baliga, Fundamentals of Power Semiconductor Devices: Springer, 2008.

[3] Zhu, C., & Andrei, P. (2017). Sensitivity of Breakdown Voltage of Power Transistors to Dopant Impurities. ECS Transactions, 80(7), 171-179.
Safe Operation of Maximum Temperature for Planar Gate SiC Mosfet under Avalanche Stress Shock
Authors: Li Xuan , Hou Zijie , Tong Xing , Deng Xiaochuan , Jiang Junning , Zhang Yourun , Zhang Bo
Published: February 2020

The silicon carbide (SiC) metal-oxide-semiconductor-field-effect transistors (MOSFET) exhibits its superior dynamic characteristic due to significant shrink of chip size as a result of SiC wide band gap. This advantage transfers to the fast switching speed (i.e., higher dI/dt, dV/dt) of device, promoting high efficient and high power dense applications. When turn-off process, however, the sharp current variation may result in SiC MOSFET suffering avalanche stress. This shock becomes one of reliability issues for SiC MOSFET, influencing the reliability of the converter further. Recently, the avalanche related studies of SiC MOSFET raise extensive investigations and discussions. It is found that the achieved avalanche energy (E
av) of a specific SiC MOSFET is varied based on different load in the unclamped inductive switching (UIS) test and the underlying avalanche breakdown cause is relevant to thermal runaway. Therefore, it is of necessity to fully understand the role of temperature in the SiC MOSFET during avalanche mode. Meanwhile, a relatively direct and concise indicator to estimate the avalanche failure of SiC MOSFET is expected in practical scenario, even though various components are entangled inside the subject during avalanche shock. In this paper, avalanche capability of 1200V CREE SiC MOSFET characterizing planar gate structure is assessed using various load (3.6mH, 1.1mH, 0.3mH) UIS and heat induced avalanche failure is analyzed.

Through the SiC MOSFET suitable UIS test bench, the withstanding avalanche energy of 1200V rated CREE MOSFET (planar gate) under various avalanche shocks are achieved. Furthermore, verified by the experimentally calibrated TCAD simulation based-on the subject, the underlying mechanism for the avalanche breakdown is revealed. Through the investigation of temperature distribution inside device during UIS process, it confirms that the maximum temperature play predominant role in avalanche damage. When the temperature located at interface of Pbase/N drift region rises up to melting temperature of metal system of source, i.e., around 900K, it results in avalanche catastrophic failure of device. Based on this failure mechanism, accordingly we establish a maximum temperature limit (T
av(max)) of SiC MOSFET to guarantee safe operation in terms of avalanche shock. The indicator T
av(max) is defined as a critical limit of avalanche damage inside planar gate SiC MOSFET.
(Invited) Basic Ammonothermal Growth of Bulk GaN Single Crystal Using Sodium Mineralizers
Authors: Lee Young Kuk
Published: February 2020

GaN is a promising semiconductor of wide band gap of 3.4 eV at room temperature and can be used as a substrate material for blue/green/UV light-emitting diode (LED) and laser diode (LD) [1]. Sapphire has been the most widely used substrate for manufacturing GaN-based LEDs. However, GaN films grown on sapphire substrates include many critical defects owing to the large lattice mismatch (~15%) and large difference of thermal expansion coefficients (~25%) between GaN and sapphire. Thus, GaN substrates are needed for fabrication of the high power LEDs and LDs.

Two typical routes, ammono-basic and ammono-acidic, have been studied in the ammonothermal growth of GaN crystals. Addition of alkali (Li, Na, and K) elements to ammonia typically results in a basic ammonia solution, whereas addition of halogens (F, Cl, Br, and I) to ammonia typically results in an acidic ammonia solution. The acidic ammonothermal method has a fast crystal growth rate but requires installation of Pt liner due to the corrosive environment. Thus, the basic ammonothermal method is regarded as more appropriate for mass production of GaN crystals [2].

Only a few types of basic mineralizer including NaNH2 [3] and KNH2 [4] have been used previously for the crystal growth of GaN. The reports of comparison with other sodium-based mineralizers for ammonothermally grown GaN crystals have not to be achieved. Potassium mineralizers were not included in this study because they are more sensitive to oxygen and moisture and corrosive to autoclave compared to sodium mineralizers.

In this study, we demonstrate the growth rate, crystal quality, and surface morphology of the as-grown GaN single crystals according to sodium amide, sodium azide, and sodium metal as mineralizer. Also, we describe the morphology and properties of approximately 2 inch GaN crystals grown by ammonothermal method using sodium metal mineralizer.

Reference

[1] S. Nakamura, G. Fosol, The Blue Laser Diode, Springer, Berlin, 1997, chap. 1.

[2] R. Dwilinski, R. Doradzinski, J. Garczynski, L. P. Sierzputowski, A. Puchalski, Y. Kanbara, K. Yagi, H. Minakuchi, H. Hayashi, J. Cryst. Growth 311 (2009) 3015.

[3] T. Hashimoto, F. Wu, J. S. Speck, S. Nakamura, J. Cryst. Growth 310 (2008) 3907.

[4] R. Dwilinski, R. Doradzinski, J. Garczynski, L. P. Sierzputowski, A. Puchalski, Y. Kanbara, K. Yagi, H. Minakuchi, H. Hayashi, J. Cryst. Growth 310 (2008) 3911.
Field Plating for Improved Ga2O3 Schottky Rectifier Performance
Authors: Carey Patrick H , Yang Jiancheng , Sharma Ribhu , Ren Fan , Pearton Stephen J. , Law Mark E
Published: February 2020

Ga2O3­ rectifiers have the potential to surpass SiC and GaN diodes due to the ultrawide-bandgap of 4.9 eV. In this context to reach the maximum potential for Schottky rectifiers, electric field mitigation techniques must be used to optimize performance. In this study, the effects of various dielectric field plates (SiO2/SiN, Al2O3/SiN, HfO2/SiN) on device performance of circular Ga2O3 Schottky rectifiers was investigated. The rectifiers were fabricated on 10 µm, lightly doped (n = 2.8 × 1016 cm-3) β-Ga2O3 epitaxial layers grown by Hydride Vapor Phase Epitaxy on conducting substrate (n = 4.8 × 1018 cm-3) grown by Edge-Defined, Film-Fed growth. Temperature dependent forward current-voltage characteristics were used to extract the average Schottky barrier height of 1.14 eV ± 0.03 eV for Ni, average ideality factor of 1.02 ± 0.02, and the Richardson’s constant of 48.1 A/cm2K2. The reverse breakdown and leakage current were the two characteristics which were predominantly affected by the field plate dielectrics. The highest reverse breakdown reached was 730 V for Al2O3/SiN, which was significantly higher than SiO2/SiN and HfO2/SiN, 562 V and 401 V, respectively. Electric field modeling was used to determine the location of peak electric field and was found to occur at the edge of the field plate, which agreed with experimental observations of breakdown induced defects occurring on the edge of the device. The on-resistance ranged from 3.8-5.0 × 10-3 Ω-cm2 which was dependent on diode size, diameters from 50 to 200 µm. This led to a power figure-of-merit (VB
2/RON) of 1.4 × 108 W-cm2.
Band Offsets of Insulating & Semiconducting Oxides on (AlxGa1-x)O3
Authors: Fares Chaker , Kneiss Max , von Wenckstern Holger , Grundmann Marius , Tadjer Marko J , Ren Fan , Hays David , Gila Brent P , Pearton Stephen J.
Published: February 2020

β-Ga2O3 is a wide bandgap semiconductor (~4.8 eV) with properties suited to power electronics, truly solar-blind UV detection, and extreme environment applications. Additional bandgap tunability can be achieved through incorporation of Al into β-Ga2O3, leading to β-(Al,Ga)2O3 monoclinic phase alloys with the bandgap tuned from 4.8- 6 eV. This enables heterostructure designs such as modulation-doped electron channels, quantum wells, and superlattices.

Out of these possible structures, β-(AlxGa1-x)2O3/Ga2O3 heterostructure field-effect transistors (HFETs) have shown impressive and improving performance compared to Ga2O3 based metal semiconductor field effect transistor (MESFETs). For these heterostructures, other wide bandgap oxides can be used as gates dielectrics, and narrow energy bandgap conductive oxides can be utilized to reduce contact resistance between the ohmic contacts and semiconductor surface. For gate dielectrics on (AlxGa1-x)2O3, one must utilize oxide materials with a bandgap of 7 eV of higher to have adequate offsets in both the conduction and valence bands. In this study, we utilize x-ray photoelectron spectroscopy (XPS) to report on the determination of the band alignment of various insulating and semiconducting dielectrics onto (AlxGa1-x)2O3 at various aluminum concentrations. The dielectrics were deposited using atomic layer deposition (ALD) or sputtering. For SiO2 and Al2O3, there are differences of up to 1 eV in band alignments on single crystal (AlxGa1-x)2O3, depending on whether they are deposited by sputtering or ALD. For the conductive oxides, AZO/ β-(AlxGa1-x)2O3 and ITO/ β-(AlxGa1-x)2O3 heterojunctions have a nested gap (type I) band offset. The valence and conduction band offsets are negative in both cases and can be used to enhance carrier transport across the heterointerface.
Catalyst Particle Scale Analysis By Transport and Reaction Simulation on 3D Nano-Scale Images for PEMFC
Authors: Ogawa Shohei , Padgett Elliot , Muller David A. , Kongkanand Anusorn , Litster Shawn
Published: February 2020

Optimizing the structure of the catalyst layer is critical to improving the proton exchange membrane fuel cell (PEMFC) performance at low Platinum (Pt) loading. A cell with reduced cathode Pt-loading for the cost reduction suffers from a limited performance at high current density, and it is a barrier to practical applications of PEMFC [1]. In addition to Pt-loadings, the types of carbon supports affect cell performance [2]. In the prior work, the nano-scale scanning transmission electron microscopy computed tomography (STEM-CT) was applied to visualize the individual Pt particle distributions in 3D at two Pt-loadings on two types of carbon supports [3]. This imaging showed that with medium surface area, low porosity carbon support (e.g., Vulcan), a large majority of Pt particles are on the surface of the carbon support. In contrast, STEM-CT showed that with a high-surface-area carbon (HSC), a majority of Pt particles are within the porous carbon support. These internal Pt particles are not in contact with the acidic electrolyte, therefore it is unclear how protons are transported to the Pt surface and how the oxygen reduction reaction (ORR) occurs on these Pt particles. In addition, prior experimental results suggest that the utilization of inner Pt particles is improved at high relative humidity with HSC since the micropores in the carbon support are likely to be filled with condensed water [3].

Here, to investigate the characteristics of those catalyst materials, we developed a transport and ORR model on 3D structures obtained from the STEM-CT images. We solve the Poisson-Nernst-Planck equations to model the electromigration and surface charge effects on the proton concentration in the porous carbon supports with a continuum approximation. Figure 1 shows the ORR current density on the Pt particles in the HSC support at the electrode potential, 0.7 V. The simulation result suggests that negative surface charge on Pt particles may be necessary for a high proton concentration and activity of inner particles in water-filled micropores at high relative humidity. This model can be used to characterize the performance of the catalyst layers in different configurations.

Acknowledgement

This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271.

References

[1] A. Kongkanand and M. F. Mathias, “The Priority and Challenge of High-Power Performance of Low-Platinum Proton-Exchange Membrane Fuel Cells,” J. Phys. Chem. Lett., vol. 7, no. 7, pp. 1127–1137, Apr. 2016.

[2] V. Yarlagadda et al., “Boosting Fuel Cell Performance with Accessible Carbon Mesopores,” ACS Energy Lett., vol. 3, no. 3, pp. 618–621, 2018.

[3] E. Padgett et al., “Editors’ Choice—Connecting Fuel Cell Catalyst Nanostructure and Accessibility Using Quantitative Cryo-STEM Tomography,” J. Electrochem. Soc., vol. 165, no. 3, pp. F173–F180, 2018.

Figure 1
Macroscopic Modeling of Liquid Water in Channel and GDL Based on Detailed Two-Phase CFD
Authors: Tanaka Shoichi , Takayama Ryo , Takayama Tsutomu , Motegi Haruki , Tsukamoto Takayuki , Yoneda Masakazu
Published: February 2020

In order to perform efficient simulations of full-stack PEFC systems, the behavior of liquid water in channels and GDL is modeled using detailed two-phase CFD with VOF method. The relationship between liquid water volume fraction, pressure drop and liquid water velocity are evaluated in a macroscopic control volume of well­-resolved two-phase CFD of the flow channel, resulting in the database of gas and liquid relative permeability for water saturation ratio. The exhaust velocity of liquid water from GDL to channel is estimated by another detailed CFD, where the structure of GDL is measured by X-ray CT and directly converted to computational mesh. The effect of contact angle of separator attached to GDL is also studied from the viewpoint of characteristics about water exhaust.

For evaluating the models obtained, simulations of PEFC are performed. Coupled with electrochemical reactions, all relevant transport phenomena (mass, chemical species and heat) are solved on the coarse-grained mesh. The mass transport equation is solved by Darcy’s law, using the distribution of the equivalent hydraulic parameter in flow channels estimated by detailed gas phase CFD. Here mass transport is coupled with heat and chemical species transport equations. Those all transport phenomena are coupled with electrochemical reactions in the MEA. Transport of chemical species and water through the MEA are also considered. Many engineering models are employed in order to consider various transport phenomena, such as water uptake into electrolytes and effective oxygen transport resistance from gas phase to reaction sites in catalyst layers, etc. Electrochemical reactions are modeled by Butler-Volmer equation, in the manner of lumped parameter models. The results of simulations of 25 cm2 serpentine flow cell under several operating conditions are compared with experimental data, showing that the model we propose is useful for the purpose of PEFC design.
Membrane Electrode Assembly Fabrication Method Effects on Catalyst Layer Structure, Interfaces, and Performance
Authors: Medina Samantha , Wang Min , Van Cleve Timothy , Mauger Scott A , Ulsh Michael , Neyerlin K C , Pylypenko Svitlana
Published: February 2020

The membrane electrode assembly (MEA) is a critical component in polymer electrolyte membrane fuel cells. MEAs are often prepared by depositing the catalyst layer directly on to the membrane. In lab-scale research, catalyst coated membranes or CCMs can be prepared by hand painting or ultrasonic spraying. Catalyst-coated diffusion media (CCDM), also known as gas diffusion electrodes (GDE) have the catalyst layer deposited on the gas diffusion media. For the large-scale manufacturability of MEAs, GDEs may be easier to manufacture in a roll-to-roll (R2R) fashion because the challenges associated with membrane swelling are avoided. In addition, for R2R fabrication, there are many ink deposition methods. For example, in gravure coating, a patterned gravure roller can be used in combination with a doctor blade and a pressure roller to coat the catalyst layer. The coating quality can be optimized by modifying the rotation speed of the gravure roller.1 Alternatively, in slot die coating modification of the distance from the slot die to the substrate can be used to improve coating quality, effectively changing the sheer rate experienced by the ink. With all fabrication methods (ultrasonically sprayed, slot die R2R, micro gravure R2R, etc.) several processing parameters can be varied, impacting the structure of the electrode and its performance. Additionally, ink composition (solvent ratio, ionomer to catalyst ratio) will also have significant impact on the catalyst layer structure.2 Thus, the MEA fabrication method, the processing conditions, ink composition, and chemistry and morphology of the catalyst will all affect the surfaces and interfaces that exist in an MEA.

It is important to understand these material interfaces and the structure, property, performance relationship between the catalyst layer components. Electron microscopy plays a key role in these studies. This work will highlight the importance of assessing MEAs at different scales and from different perspectives: catalyst morphology, top-down electrode view, cross-sectional MEA view. Microscopy can be a powerful tool for visualizing all the components within an electrode structure. Extraction of quantifiable parameters from digital image processing provides direct correlations with performance metrics, furthering the utility of electron microcopy for electrode studies.

[1] S.A. Mauger et. al, Gravure Coating for Roll-to-Roll Manufacturing of Proton-Exchange-Membrane Fuel Cell Catalyst Layers, J. Electrochem. Soc. 165 (2018) F1012–F1018. doi:10.1149/2.0091813jes.

[2] Khandavalli, S. et al. Rheological Investigation on the Microstructure of Fuel Cell Catalyst Inks. ACS Appl. Mater. Interfaces (2018). doi:10.1021/acsami.8b15039

Figure 1
(Invited) A New View of Fuel Cell Electrocatalysts through Multimodal Analytical Electron Microscopy
Authors: Cullen David A. , Cao Shaohong , Chi Miaofang , More Karren L.
Published: February 2020

Significant advancements have been made in the performance of oxygen reduction reaction (ORR) electrocatalysts for polymer electrolyte membrane fuel cells. State-of-the-art Pt-alloy catalysts show exceptionally high mass activity and recent research into ordered nanoparticles shows promising improvements in durability. The activity of platinum group metal-free (PGM-free) catalysts, which will be required as world-wide demand for fuel cell stacks increases, have also improved substantially, although significant durability issues persist. Accelerated materials discovery and development is required to keep pace with ever-changing market and performance demands, such as the shift in focus towards heavy duty transportation where highly durable, cost-effective catalysts are required.

Scanning transmission electron microscopy (STEM) has long played an important role in assessing the quality and durability of electrocatalysts and electrode structures, and the increasing demands on catalyst performance require a corresponding improvement in both the resolution and quantitative precision of analytical methods. Fortunately, recent advances in high-speed cameras and spectrometers allow for an unprecedented view of catalyst structure and composition in two, and in some cases, three dimensions. The newly installed JEOL NEOARM atomic-resolution STEM at the Center for Nanophase Materials Sciences combines the latest generation spectrometers with new imaging modes such as electron ptychography. The ability to acquire multiple signals simultaneously at high data acquisition speeds and across a range of accelerating voltages (30-200 kV) allows for a fresh look at the atomic structure of both Pt-based and PGM-free catalysts.

The simultaneous application of electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy to study the beam sensitive surfaces of sub-5nm Pt-alloy nanoparticles (<5nm) will be discussed. The combination of dual EELS-EDS analysis using ultra-low voltage (30kV) electron beams under cryogenic cooling will also be demonstrated on atomically dispersed transition metal sites in PGM-free electrodes. Finally, recent developments in pixelated cameras and ptychographic analysis of 2D materials will be extended to both Pt-based and PGM-free electrocatalyst systems to explore the opportunities and limits of low-dose, high-speed imaging.

Acknowledgements

Research sponsored by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (DOE), as part of the FC-PAD and ElectroCat Consortia, which is part of the Energy Materials Network. Microscopy performed as part of a user project at ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a U.S. DOE Office of Science User Facility.